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maxillo-faciale et stomatologie

Safety of uninterrupted direct oral anticoagulant versus

uninterrupted vitamin-K antagonist for ambulatory atrial flutter catheter ablation: a propensity score-matched cohort study

A ma soeur préférée, pour ta présence plus qu'indispensable pour moi. Vingt deux ans que je te connais, avec toujours cette entente inébranlable. Tu ne t'en rends peut être pas compte, mais tu es une force pour moi. Grace à toi j'ai pu avancer et j'espère de tout coeur que tu connaisses le même bonheur que moi.

A Camille, tu es celle qui à fait que mon expérience à Caen celle qu'elle est aujourd'hui. Tu as rendu les moments difficiles supportables et les bons moments merveilleux. Ton soutien a été indispensable pour moi. Ta personnalité et ton esprit d'équité m'ont permis de devenir meilleur. Je ne te le montre pas souvent (car je suis un male alpha) mais tu compte énormément pour moi. Qui sait, peut être avec le temps je deviendrai un male écolo.

A Fadi, pour m'avoir aidé à grandir en tant qu'homme puis médecin. Data are expressed as adjusted odds ratio and 95 % confidence interval. Abbreviations: see Table 1. V. CONCLUSION……………………………………………………………….9

Introduction

Radiofrequency catheter ablation is the first-line therapy for symptomatic cavotricuspid isthmusdependent (i.e. common) atrial flutter (CAF), and represents one of the most frequent procedures performed in invasive electrophysiology laboratories [START_REF] Pekka Raatikainen | A decade of information on the use of cardiac implantable electronic devices and interventional electrophysiology procedures in the European Society of Cardiology countries: 2017 report from the European Heart Rhythm Association[END_REF] . Its high acute success rate is associated with a complication rate ranging from 2 to 5%, mostly bleeding events, favored by the necessity of periprocedural anticoagulation for thromboembolic prevention 2 . The majority of tertiary medical centers practices CAF catheter ablation on an overnight stay-basis 2 , although the advantages of sameday home discharge (i.e. ambulatory catheter ablation) are well described in the literature [START_REF] Vora | Safety and feasibility of same day discharge in patients undergoing radiofrequency catheter ablation[END_REF][START_REF] Marijon | Feasibility and safety of same-day home discharge after radiofrequency catheter ablation[END_REF][START_REF] Theodoreson | Same-day cardiac catheter ablation if safe and cost-effective: experience from a UK tertiary center[END_REF] . There is a predictable growth of rate of ambulatory catheter ablation procedures in the future because of the increased financial pressure from healthcare authorities [START_REF] Theodoreson | Same-day cardiac catheter ablation if safe and cost-effective: experience from a UK tertiary center[END_REF][START_REF] Pasquié | Ambulatory management for interventional cardiology[END_REF] . Direct oral anticoagulants (DOAs) have emerged as the preferred choices over VKAs for thromboembolic prevention due to a better efficacysafety profile [START_REF] Ha | Continuous and minimallyinterrupted direct oral anticoagulant are both safe compared with vitamin K antagonist for atrial fibrillation ablation: an updated meta-analysis[END_REF] . There is currently no data on the safety profile of uninterrupted DOA for ambulatory CAF catheter ablation, as most of the aforementioned studies only included patients with a vitamin-K antagonist (VKA) regimens [START_REF] Marijon | Feasibility and safety of same-day home discharge after radiofrequency catheter ablation[END_REF][START_REF] Theodoreson | Same-day cardiac catheter ablation if safe and cost-effective: experience from a UK tertiary center[END_REF] .

Hence, we aimed to evaluate the safety of uninterrupted DOA regimen in ambulatory CAF catheter ablation, compared with uninterrupted VKA regimen. As secondary objectives, we aimed to evaluate feasibility of CAF catheter ablation in the modern era of DOA, and identity predictive factors of early postoperative complications (bleeding and total).

Methods

Eligibility and inclusion

Same-day home discharge has been a standard of care in our tertiary university center for every patient scheduled for radiofrequency catheter ablation for CAF for the last 10 years. We performed a retrospective analysis of our prospective protocol for ambulatory CAF catheter ablation, in order to evaluate feasibility, safety and cost-effectiveness of such practice. We report here the results of the safety and feasibility analysis. Between November 2009 and March 2019, 1251 patients  18 year-old underwent elective CAF catheter ablation in our Regional University Hospital. Patients were not eligible to ambulatory catheter ablation if procedure was scheduled too late to allow same-day discharge (i.e. after 1.00 the afternoon), or if CAF catheter ablation was associated to any other catheter ablation in the atria (e.g. atrial fibrillation, atypical atrial flutter, etc.).

Procedural aspects

All patients underwent a complete preoperative visit with the referring physician and/or the electrophysiologist, including clinical examination, twelve-lead electrocardiogram (ECG) and transthoracic echocardiography (TTE). Blood analysis was drawn at least 48-hours before the procedure, with hemostasis, platelet count, hemoglobin, electrolytes and creatinine. Individual bleeding and thromboembolic risks were integrated in HAS-BLED and CHA 2 DS 2 -VASc scores respectively. Obesity was defined as body mass index (BMI)  30kg/m 2 . Chronic kidney disease was defined as an estimated glomerular filtration rate (eGFR)  60ml/mn/m 2 using CKD-EPI formula. All procedures were performed under either uninterrupted VKA (target INR 2 to 3) or uninterrupted DOA (apixaban, dabigatran or rivaroxaban, dose-adjusted if indicated) for at least three weeks before the procedure. In DOA patients, last dose was administered on the evening before or in the morning, depending on the scheduled time of the procedure. Preoperative transesophageal echography (TEE) was only performed in patients presenting with insufficient anticoagulation during the past three weeks, in order to rule-out intra-atrial thrombus. Patients were admitted in the ambulatory unit at 7.45 in the morning the day of the procedure, with a previous fasting time of at least 6 hours. All procedures were performed by a skilled senior electrophysiologist, eventually assisted by a junior and/or resident physician. Transfemoral venous access with two venous sheaths was performed under local anesthesia, potentialized by IV midazolam and morphine if necessary. Utilization of ultrasound guidance for venous puncture was at the discretion of the electrophysiologist. A quadri-or decapolar diagnostic catheter was placed within the coronary sinus through a 6Fr sheath, and eventually a halo duodecapolar catheter in the right atrium in order to confirm the isthmus complete block (through a second 6Fr sheath). Fluoroscopy-guided linear ablation between tricuspid annulus and inferior vena cava was performed using an 8-mm non-irrigated tip or a 4-mm irrigated tip catheter, at the discretion of the electrophysiologist (settings at 60W-60°C or 40W-35°C respectively), through a single 8Fr or 8,5Fr sheath. Procedural success was defined as conversion to sinus rhythm during ablation (in patients presenting in flutter) associated with a bidirectional cavotricuspid isthmus block, persistent after at least 20 minutes after last radiofrequency application. Prolonged procedure duration was defined as superior to 70mn (mean procedure duration in the cohort). In case of conversion from CAF to atrial fibrillation or atypical atrial flutter procedure was aborted and re-scheduled under general anesthesia if necessary. No patient underwent electrical cardioversion because of the absence of anesthesiologist during procedure. After the procedure, catheters and sheaths were removed and manual compression was applied. A pressure bandage was applied during the next 6 hours after complete hemostasis and bed-resting was mandatory in declive position in the dedicated ambulatory unit. Patients were monitored by a trained nurse during 6 hours and, before discharge, assessed by the attending physician, including neurological and hemodynamic status, groin examination, 12-lead ECG and TTE in case of clinical suspicion of pericardial effusion. Patient were discharged from hospital at least 30 minutes after the pressure bandage was taken off and patient stood up without complication.

In DOA patients, the next post-ablation dose was administered at least 6 hours following complete hemostasis. Patients were given both written and verbal instruction to contact if necessary the cardiology department in case of any alert sign (groin or limb pain, hematoma, neurological and cardiovascular symptoms, etc.). In case of suspected complication, patient was kept overnight and until resolution of the complication.

Follow-up and outcomes

All patients were contacted by phone at one-week by a clinical nurse for systematic complication screening. Primary safety endpoint was the occurrence of any early and clinically significant bleeding.

Early bleeding was defined as occurring in a delay ranging from the beginning of the procedure (first attempt of vascular access) to the end of the first postoperative week, as the majority of complications occurs in the early postoperative time [START_REF] Marijon | Feasibility and safety of same-day home discharge after radiofrequency catheter ablation[END_REF] . Clinically significant bleeding was defined as type 2 or greater bleeding according to the Bleeding Academic Research Consortium (BARC) classification [START_REF] Mehran | Standardized bleeding definitions for cardiovascular clinical trials. A consensus report from the bleeding academic research consortium[END_REF] .

Secondary safety endpoint was the occurrence of any early and clinically significant complication in a delay ranging from the beginning of the procedure to the end of the first postoperative week, including : bleeding (BARC  2), transient sinus node dysfunction (SND) or atrioventricular block (AVB), persistent SND or AVB requiring pacemaker implantation, congestive heart failure or recurrent symptomatic atrial flutter or fibrillation. Effective same-day discharge was defined as a patient discharged from hospital, and without any of the primary or the secondary safety endpoint at the end of the first postoperative week.

Statistical analysis

Data are presented as mean  standard deviation and categorical variables are given as number of patients with the attribute (percentage). For continuous variables, a Student's t test or a Mann-Whitney U test, as appropriate, was performed for comparison between two groups. The Chi-square test was used for analysis of categorical variables. To limit the effect of potential selection bias, we performed a propensity score matching considering the confounding variables associated with the type of anticoagulation regimen in univariate analysis (age, gender, body mass index, HAS-BLED and CHA 2 DS 2 -VASc scores, chronic kidney disease, associated antiplatelet treatment, procedure duration and number of femoral venipunctures/sheaths for catheter ablation). Primary and secondary safety outcomes were then compared between VKA and DOA patients in the matched cohort. Odds ratio (OR) and 95% confidence intervals (95% CI) were calculated. Quality of the propensity score matching was evaluated using standardized differences before and after matching (Figure S1).

Multivariate analysis for prediction of primary and secondary safety outcomes was performed using a stepwise logistic regression analysis. Since matching reduces the sample size and may limit external validity, we performed the analysis in the global population (n=981). We computed two models for both outcomes using or not the year of procedure (respectively model 1 and 2). Variables associated with outcomes with a p < 0.20 were included, but only variables with p <0.10 were conserved in the final model. A p < 0.05 denoted statistical significance. Analyses and figures were conducted using IBM SPSS Statistics for Macintosh (Version 23.0, IBM, Chicago, Illinois) and R (Version 3.1.1, R Foundation for Statistical Computing, Vienna, Austria) for propensity score-matching.

Ethics

Approval for this study was obtained from the Ethics Committee of Caen University Hospital and was in accordance with the declaration of Helsinki. The study protocol was compliant with the STROBE statement. All patients provided written informed consent before catheter ablation.

Results

Baseline characteristics of patients undergoing ambulatory CAF catheter ablation

Study flow chart is displayed in Figure 1. Nine hundred and eighty one patients were eligible to ambulatory CAF catheter ablation and therefore included in the study. Their baseline and procedural characteristics are detailed in Table 1. Sinus rhythm was obtained in 93% of patients at the end of the procedure (with or without complete and bidirectional isthmus block).

Safety and feasibility of ambulatory CAF catheter ablation in patients with uninterrupted DOA

Propensity score matching yielded 820 patients, 410 in each group. Patients receiving parenteral anticoagulation only (subcutaneous low molecular weight heparin or intravenous heparin; n =27) or without any anticoagulant treatment (n = 3 patients) were excluded from the propensity score matching analysis. DOA and VKA patients had similar baseline characteristics (Table 2), except for a higher prevalence of obesity in the DOA group (30% versus 23%; p = 0.03).

Outcomes in the propensity matched-cohort are detailed in Table 3. Occurrence of the primary endpoint (defined as early and clinically significant bleeding) was 3.3% (27/810) at one-week, and occurrence of the secondary endpoint (defined as any early and clinically significant complication) was 6.5% (53/810) at one-week. Complications were immediate (i.e. before discharge) in 96% (51/53) of patients or during the first postoperative week (i.e. readmission after hospital discharge) in 3.7%

(2/53). Seventy percent (19/27) of bleeding events were classified as BARC 2; no fatal bleeding nor death occurred at the end of the first week. Pacemaker implantation rate was 1%. Mean lengths of hospital stay were 2.9  6 and 3.5  5 days for patients with bleeding and any complication respectively.

Considering anticoagulant regimen, occurrence of the primary safety endpoint was 2.9% (12/410) versus 3.7% (15/410) respectively for DOA and VKA patients (OR = 0.79; 95% CI 0.37 -1.71; p = 0,7; Figure 2). Occurrence of the secondary safety endpoint was 5.1% (21/410) versus 7.8%

(32/410) respectively for DOA and VKA patients (OR = 0.64; 95% CI 0.36 -1.13; p = 0.15; Figure 3). Feasibility of same-day home discharge was comparable between DOA and VKA patients (effective same-day discharge 94.9% versus 92.2%; p = 0.15).

Predictors of early complications after ambulatory CAF catheter ablation

Table 4 summarizes the multivariate analysis for prediction of the primary and secondary safety endpoints in the study population (n=981). Female gender, high HAS-BLED score and prolonged procedure duration were independently associated with both safety endpoints, in both models. Year of the procedure was independently associated with the secondary safety endpoint, with a higher complication rate in the first years (driven by non-bleeding complications). Relationships between HAS-BLED score and probability of bleeding and complication at the end of the first postoperative week are illustrated in Figure 4.

Discussion

This propensity score matched-cohort analysis from a high-experienced tertiary center shows that uninterrupted DOA is a safe periprocedural anticoagulation regimen in patients undergoing ambulatory CAF catheter ablation, compared to uninterrupted VKA. We found a high effective sameday discharge rate in this setting. We also identified female gender, high HAS-BLED score and prolonged procedure duration as independent risk factors for adverse outcomes at the end of the first postoperative week.

Safety of uninterrupted DOAs in ambulatory CAF catheter ablation

The main finding of our study is that uninterrupted DOA regimen has a similar safety profile compared with uninterrupted VKA regimen in patients undergoing ambulatory CAF catheter ablation.

DOA use in atrial arrhythmias at-risk of thromboembolic events has been increasing in the last years, because of a better long-term efficacy-safety profile compared with VKA [START_REF] Ruff | Comparison of the efficacy and safety of new oral anticoagulants with warfarin in patients with atrial fibrillation: a meta-analysis of randomized trials[END_REF] . In their recent and updated meta-analysis, Ha et al. found that continuous and minimally-interrupted DOA were safe periprocedural anticoagulation regimens in atrial fibrillation catheter ablation, with a lower risk of major bleeding compared with uninterrupted VKA [START_REF] Ha | Continuous and minimallyinterrupted direct oral anticoagulant are both safe compared with vitamin K antagonist for atrial fibrillation ablation: an updated meta-analysis[END_REF] . Preliminary data from Amara et al. recently suggested that CAF catheter ablation could also be performed safely in patients taking continuous or minimally-interrupted DOA, but in a small cohort and in the non-ambulatory setting [START_REF] Fromentin | New oral anticoagulants in patients undergoing atrial flutter radiofrequency catheter ablation: an observational study[END_REF] . Still, a recent European survey found that DOA were interrupted before CAF ablation in 65% of patients [START_REF] Glover | Catheter ablation for atrial flutter: a survey by the European Heart Rhythm Association and Canadian Heart Rhythm Society[END_REF] . In the present study the bleeding rate is similar as previously reported in the non-ambulatory setting 2 , supporting the safety of uninterrupted DOA for ambulatory CAF catheter ablation. One can note that despite propensity score matching, patients in the DOA group were more frequently obese than patients in the VKA group (30% versus 23%; p = 0.03). Obesity is a known risk factor for bleeding after catheter ablation [START_REF] Winkle | The impact of obesity on atrial fibrillation ablaiton: patient characteristics, long-term outcomes and complications[END_REF] and, if this difference has a significant consequence, it would be in the way of an overestimation of bleeding risk in the DOA group. We therefore believe that it is not altering the main finding of the present study.

Feasibility of ambulatory CAF catheter ablation

This study shows that ambulatory CAF catheter ablation is feasible, even in patients with DOA, with a nearly 95% effective same-day discharge rate. Data concerning ambulatory catheter ablation are scarce in the current literature, and we describe here the largest cohort of ambulatory patients under a DOA regimen in the specific context of CAF catheter ablation. Previous studies on this topic included various types of electrophysiological procedures with different risk profiles, including CAF but also atrioventricular node, slow-pathway and accessory-pathway catheter ablations [START_REF] Vora | Safety and feasibility of same day discharge in patients undergoing radiofrequency catheter ablation[END_REF][START_REF] Marijon | Feasibility and safety of same-day home discharge after radiofrequency catheter ablation[END_REF][START_REF] Theodoreson | Same-day cardiac catheter ablation if safe and cost-effective: experience from a UK tertiary center[END_REF] . Moreover, none of the aforementioned studies specifically accounted for the type of anticoagulation regimen. In this context, our work shows a an effective same-day discharge rate similar to previously reported (between 90 and 95%). However, one can note a higher procedural failure rate in our cohort compared with the recent European survey [START_REF] Glover | Catheter ablation for atrial flutter: a survey by the European Heart Rhythm Association and Canadian Heart Rhythm Society[END_REF] . Conversion from CAF to atrial fibrillation (or atypical atrial flutter) and unrelievable pain were responsible for nearly a half and a fifth of all procedural failures respectively. Because of the absence of anesthesiologist during the procedure, we weren't able to perform an electrical cardioversion in the first case, or to convert from local to general anesthesia in the second case. Therefore, our higher procedural failure rate might be the consequence of the lack anesthetic support rather than anticoagulant-or ambulatory-dependent.

Risk factors of complications after ambulatory CAF catheter ablation

There is currently no data in the literature concerning risks factors for complications after ambulatory CAF catheter ablation, and the present study gives some new insights for risk stratification of ambulatory patients. Similarly to non-ambulatory patients, we found that high HAS-BLED score was a risk factor for bleeding and non-bleeding complications [START_REF] Clementy | Outcomes after ablaiton for typical atrial flutter (from the Loire Valey Atrial Fibrillation Project)[END_REF] , as well as prolonged procedure duration [START_REF] Shim | Long duration of radiofrequency energy delivery is an independent predictor of clinical recurrence after catheter ablation of atrial fibrillation: over 500 cases experience[END_REF] .

There is accumulating evidence suggesting that women are at higher risk of developing complications after catheter ablation, and our findings are consistent with previous studies [START_REF] Brembilla-Perrot | Is ablation of atrial flutter always safe?[END_REF][START_REF] Brembilla-Perrot | Sex-related differences in peri-and post-ablation clinical data for patients with atrial flutter[END_REF][START_REF] Cheung | Sex-based differences in outcomes, 30-day readmissions, and costs following catheter ablation of atrial fibrillation: the United States Nationwide Readmissions Database 2010-2014[END_REF] . This association might reflect disparities in anatomical and electrophysiological substrate, response to medications/anticoagulants, referral for catheter ablation, etc. compared with men. Identification of such risk factors at the preoperative visit of patients eligible to ambulatory catheter ablation might help lowering the risk of postoperative adverse outcomes (e.g. by reducing procedure time when feasible or increasing the postoperative monitoring period to at-least an overnight stay).

Approximately two third of medical centers practice CAF ablation in an overnight stay-basis 2 , although the advantages of same-day home discharge in terms of patient-reported outcomes, feasibility, safety, cost and resources consumption are well known [START_REF] Marijon | Feasibility and safety of same-day home discharge after radiofrequency catheter ablation[END_REF][START_REF] Theodoreson | Same-day cardiac catheter ablation if safe and cost-effective: experience from a UK tertiary center[END_REF] . There are numerous incentives for the development of ambulatory catheter ablation, with a predictable growth of ambulatory practice to every medical sector in the future [START_REF] Pasquié | Ambulatory management for interventional cardiology[END_REF] . It is likely that ambulatory catheter ablation will expand to atrial fibrillation management in selected patients someday, but caution as to be taken for now. Even though promising results were recently published by Opel et al., numerous procedural aspects differ between CAF and atrial fibrillation catheter ablation [START_REF] Opel | Comparison of a high throughput day case atrial fibrillation ablation service in a local hospital with standard regional tertiary cardiac centre care[END_REF] . A dedicated prospective study is therefore needed to evaluate the safety of such practice.

Limitations

Our study is an observational registry and not a randomized prospective trial. However, the propensity score analysis included risk factors associated with the anticoagulation regimen (individually or included in the HAS-BLED and CHA 2 DS 2 -VASc scores such hypertension, diabetes, chronic heart failure, left ventricular ejection fraction, etc.), in order to limit the impact of such bias. One can note that a prospective randomized design in this setting would be hardly justifiable, because of the preferred long-term safety profile of DOA over VKA [START_REF] Ha | Continuous and minimallyinterrupted direct oral anticoagulant are both safe compared with vitamin K antagonist for atrial fibrillation ablation: an updated meta-analysis[END_REF][START_REF] Ruff | Comparison of the efficacy and safety of new oral anticoagulants with warfarin in patients with atrial fibrillation: a meta-analysis of randomized trials[END_REF] . One can note that exhaustive anti-arrhythmic medication status (dose, duration and eventual interruption of treatment before procedure) was not collected for all patients, and it is known that the presence of such medication is a risk factor for intermittent and persistent conductive disorders [START_REF] Clementy | Outcomes after ablaiton for typical atrial flutter (from the Loire Valey Atrial Fibrillation Project)[END_REF] . We believe that it might not have impact on the primary safety endpoint of early and clinically significant bleeding, but caution as to be made concerning non-bleeding complications.

Conclusions

Uninterrupted DOA is a safe periprocedural anticoagulation regimen for ambulatory CAF catheter, in a high-volume center with a dedicated ambulatory unit, trained paramedical staff and clear patient information about alert signs. Effective same-day discharge is obtained in 95% of patients, and identification of high-risk patients at the preoperative visit might help in lowering the risk of postoperative adverse outcomes in this setting.

Annexes

Table 1

Baseline characteristics of the study population (n=981).

Data are expressed as mean  standard deviation and number (percentage). Data are expressed as adjusted odds ratio and 95 % confidence interval. Abbreviations: see Table 1.

RESUME DE LA THESE EN ANGLAIS :

Aims. Same-day home discharge after common atrial flutter (CAF) catheter ablation is a feasible, safe and cost-effective practice, but there is currently no data concerning patients treated by uninterrupted direct oral anticoagulant (DOA). We evaluated safety and feasibility of ambulatory CAF catheter ablation in patients treated by DOA compared with vitamin-K antagonist (VKA). Methods and results. Patients scheduled for isolated ambulatory CAF catheter ablation to our tertiary university center between 2009 and 2019 were included. Propensity score for anticoagulant type was calculated from age, gender, body mass index, baseline characteristics and risk factors (included in HAS-BLED and CHA 2 DS 2 -VASc scores), chronic kidney disease, associated antiplatelet treatment, procedure duration and number of femoral venipunctures. Propensity score matching yielded 820 patients (age 67  11 years). Catheter ablation was performed under uninterrupted VKA (n=410; INR = 2.5  0.6) or uninterrupted DOA (n=410). Occurrence of the primary safety outcome, defined as any early and clinically significant bleeding (BARC  2) at one-week, was similar between DOA and VKA patients (2.9 vs 3.7%; p = 0.70), as well as the rate of effective same-day discharge (94.9 vs 92.2%; p = 0.15). Female gender, high HAS-BLED score and prolonged procedure duration were risk factors for bleeding at one-week. Conclusion. Uninterrupted DOA regimen is safe for selected patients undergoing ambulatory CAF catheter ablation, in a high-volume center with a dedicated ambulatory unit and a standardized procedural and postoperative management.

KEY WORDS : atrial flutter; catheter ablation; direct oral anticoagulant; ambulatory care; outcomes.

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  DOCTEUR EN MÉDECINE Présentée et soutenue publiquement le : 5 septembre 2019 par Mr Patrick BITTAR Né (e) le 10 octobre 1991 à St Denis (93200) : Safety of uninterrupted direct oral anticoagulant versus uninterrupted vitamin-K antagonist for ambulatory atrial flutter catheter ablation: a propensity score-matched cohort study.Monsieur le Professeur Paul MILLIEZ, Vous me faites l'honneur de présider ce jury et de juger mon travail. Je vous témoigne toute ma reconnaissance et vous remercie pour le temps, votre accessibilité et la formation que vous m'avez accordée. À Monsieur le Professeur Farzin BEYGUI, Pour avoir accepté de juger mon travail et pour votre enseignement durant ces années d'internat. Veuillez trouver ici toute ma reconnaissance et l'expression de mon profond respect. A Messieurs les Docteurs Pierre OLLITRAULT et Mathieu CHEQUEL, Merci d'avoir accepté de diriger ma thèse. Vous m'avez aidé à me perfectionner et à adorer une spécialité qui me plaisait déjà. Malgré une étrange passion à aimer me cacher mes vêtements, ces temps passés auprès de vous ont été un réel plaisir. Vous avez été pour moi des collègues, des mentors et enfin des amis. À Monsieur le Docteur Damien LEGALLOIS, Pour avoir accepté de juger mon travail et pour ton enseignement. Je n'oublierai jamais ces moments autour d'une table à discuter et s'amuser, qui ont permis de rendre moins durs ces 1eres années d'internat. À Laure et Arnaud, pour votre enseignement et les connaissances que vous m'avez apportées. A toutes les équipes médicales et paramédicales CHU et à l'ensemble de mes co-internes. A Monsieur le Docteur Benoit POTIER, pour ses moments à travailler ensemble en discutant de médecine et surtout d'autre chose. En espérant continuer ainsi malgré les péripéties administratives. A mes Parents, vous m'avez poussé et soutenu. Vous avez cru en moi et c'est ce qui m'a permis de réussir. Votre soutien infaillible est une force. Vous êtes pour moi une source d'exemple et je suis devenu ce que je suis grâce à vous. Même si je ne vous le dis pas souvent (voir jamais), je vous aime.

Enfin à ma famille

  et amis que je n'ai pu revoir depuis presque 10ans, bientôt je l'espère nous nous retrouverons Abréviations APT = antiplatelet therapy; BMI = body mass index; CAF = common atrial flutter CKD = chronic kidney disease; DOA = direct oral anticoagulant; INR = international normalized ratio; IV = intravenous; LMWH = low molecular weight heparin; o.d. = once daily; RF = radiofrequency; t.d. = twice daily; VKA = vitamin-K antagonist Tableaux et figures

  Abbreviations: APT = antiplatelet therapy; BMI = body mass index; CKD = chronic kidney disease;

Figure 1 Figure 2 Figure 3 Figure 4

 1234 Figure 1

  Abbreviations: APT = antiplatelet therapy; BMI = body mass index; CKD = chronic kidney disease; DOA = direct oral anticoagulant; INR = international normalized ratio; IV = intravenous; LMWH = low molecular weight heparin; o.d. = once daily; RF = radiofrequency; t.d. = twice daily; VKA = vitamin-K antagonist.

Figure 1 :

 1 Figure 1: Study flow-chart.

Figure 2 (

 2 Figure 2 (Representative Figure): No difference in early and clinically significant bleeding occurrence after ambulatory CAF catheter ablation in patient with DOA, compared with uninterrupted VKA. Abbreviations: see Table 1.

  Ton amour du métier et ton influence m'auront guidé jusqu'à là et je t'en suis reconnaissant.A mon fwewe de coeur Florent, tu as été mon 1 er ami ici et néanmoins le meilleur. Ces années avec toi n'ont pas fait de nous les meilleurs, mais à priori ont été bien marrant ! Je te dis à bientôt pour notre

prochaine collaboration, avec bien sur une piste de ski dans la salle d'attente ! A scaffy, scaf ou ScaffGodLike, toi qui a été l'un de mes premiers chefs et maintenant un proche ami.

Avec ton enseignement j'ai appris à corriger mes erreurs, me poussant à me perfectionner en Cardiologie. Maintenant tu me pousses à me perfectionner dans le cyclisme et le sport médiéval. J'ai hâte d'être dans quelques années pour découvrir ton nouveau hobby ! En espérant continuer de te voir toi et PAF A Thibaut et Laurine, avec qui je ne compte plus les bons moments passé ensemble (soirées jeux, apéro plage, 2030, etc.). Je vous souhaite bon courage pour la fin également ! A mes amis citadins : Hugues, Cyril, Guillaume, 6 gnous et sens du nuage. Malgré la distance notre amitié est restée inchangé. Chaque trajet à Paris est pour moi une occasion de vous revoir et bientôt c'est moi qui vais vous accueillir à Caen. A mes collègues néanmoins amis caennais. Martin et Nico pour ces bons moments passés à Cherbourg qui ont continué par la suite. A Yoann pour notre 1 er semestre à écumer le sunset et le blackbird. A mes amis cardiologues sans qui cet internat ne se serait pas aussi bien passé : Lin, Clémence, Charline et Eric.

Table 1 Demographics Age

 1Age

	, year-old Male gender, n (%) BMI, kg/m 2 Obesity, n (%) Thromboembolic and bleeding risks CHA 2 DS 2 -VASc score HAS-BLED score CKD, n (%) Antithrombotic therapy, n (%) VKA DOA LMWH or IV Heparin None Associated APT, n (%) INR (for VKA patients) DOA type/dose (for DOA patients) Apixaban 5mg t.d. Apixaban 2.5mg t.d. Rivaroxaban 20mg o.d. Rivaroxaban 15mg o.d. Dabigatran 150mg t.d. Dabigatran 110mg t.d. Procedural aspects Sheaths 3, n (%) Procedural success, n (%) Reason of procedural failure, n (%) Conversion to atrial fibrillation or atypical flutter Unrelievable pain Failure to obtain complete isthmus block Unknown Baseline characteristics of the study population (n=981). Number of RF sessions, n (%) Procedure duration, minutes Data are expressed as mean  standard deviation and number (percentage).	67  11 784 (80) 28  7 234 (24) 2.3  1.4 2.5  1.1 195 (20) 541 (55) 410 (42) 27 (3) 3 (0.3) 132 (14) 2.5  0.6 180 (44) 9 (2.2) 135 (33) 11 (2.7) 70 (17) 5 (1.2) 300 (31) 72  31 865 (88) 54 (5.5) 22 (2.2) 15 (1.5) 25 (2.5)

Table 2 Before matching After matching VKA

 2

			DOA	P-value	VKA	DOA	P-value
		(n=541)	(n=410)		(n= 410)	(n = 410)
	Age, year-old	67  11	67  11	0.586	67  11	67  11	0.845
	Male gender, n	445 (82)	315 (77)	0.047	326 (80)	315 (77)	0.398
	(%)
	Obesity, n (%)	104 (19)	122 (30)	<0.001	93 (23)	122 (30)	0.026
	HAS-BLED score	2.6  1.1	2.3  1.1	<0.001	2.4  1.2	2.3  1.1	0.148
	CHA 2 DS 2 -VASc	2.5  1.4	2.2  1.5	0.008	2.3  1.4	2.2  1.5	0.154
	score
	CKD, n (%)	123 (23)	64 (16)	0.008	71 (17)	64 (16)	0.572
	Associated APT,	87 (16)	38 (9,3)	0.003	49 (12)	38 (9,3)	0.257
	n(%)
	Procedure	75  33	69  29	0.016	72  28	68  28	0.063
	duration, mn
	Sheaths	2.33  0.52 2.25  0.45	0.014	2.29  0.50 2.25  0.45	0.240
	Baseline characteristics of patients according to antithrombotic therapy (VKA or DOA), before and
	after propensity score-matching .
	Data are expressed as mean  standard deviation or number (percentage). Abbreviations: see

Table 1 .

 1

Table 3 Table 3 :

 33 Safety outcomes at one-week.

	*Same patient had both CHF and symptomatic AF.
	Data are expressed as number (percentage). Abbreviations: see

text. Total (n=820) VKA (n=410) DOA (n=410) OR (95% CI) P- value Complications (all-type) 53 (6.5) 32 (7.8) 21 (5.1) 0.64 (0.36 -1.13) 0.15 Bleeding 27 (3.3) 15 (3.7) 12 (2.9) 0.79 (0.37 -1.72) 0.70

	BARC 2	19 (2.3)	10 (2.4)	9 (2.2)	0.90 (0.36 -2.23)	0.82
	BARC 3a	2 (0.2)	1 (0.2)	1 (0.2)	1 (0.06 -16.04)	1
	BARC 3b	5 (0.6)	3 (0.7)	2 (0.5)	0.67 (0.11 -4.00)	0.65
	BARC 3c	1 (0.1)	1 (0.2)	0	1 (0.99 -1.00)	0.32
	BARC 5	0	0	0	-	-

Non-bleeding complications 26 (3.2) 17 (4.1) 9 (2.2) 0.53 (0.36 -1.25) 0.40

	Table 4 HAS-BLED Score	2.144 1.648 -2.788	< 0.001
	A Procedure year	Bleeding 0.753 0.663 -0.856	< 0.001
	Model 1 Procedure duration 70mn 8.476 3.838 -18.721 < 0.001 aOR 95% CI P-value
	Male gender	0.271 0.135 -0,545	< 0.001
	HAS-BLED score	1.865 1.372 -2.536	< 0.001
	Associated APT	2.125 0.942 -4.793	0.069
	Procedure duration 70mn 3.017 1.351 -6.739	0.007
	B				Bleeding
	Model 2	aOR	95% CI	P-value
	Male gender	0.315 0.158 -0.627	0.001
	Transient SND or AVB HAS-BLED Score	11 (1.3) 1.924 1.416 -2.615 7 (1.7)	4 (1.0) < 0.001	0.57 (0.17 -1.95)	0.36
	Permanent SND or AVB Procedure year	8 (1) 0.881 0.763 -1.019 5 (1.2)	3 (0.7) 0.087	0.60 (0.14 -2.52)	0.48
	requiring pacemaker Procedure duration 70mn 3.717 1.593 -8.673	0.002
		Congestive heart failure	5 (0.6)*	3 (0.7)	2 (0.5)	0.67 (0.11 -4.00)	0.65
	C	Symptomatic atrial	3 (0.4)* Complication (all-type) 2 (0.5) 1 (0.2)	0.50 (0.05 -5.52)	0.56
	fibrillation or flutter Model 1	aOR		95% CI	P-value
	Male gender	0.349 0.196 -0.619	< 0.001
	HAS-BLED Score	2.122 1.647 -2.734	< 0.001
	Sheaths 3	1.743 0.969 -3.136	0.064
	Procedure duration 70mn 5.878 2.785 -12.406 < 0.001
	D			Complication (all-type)
	Model 2	aOR		95% CI	P-value
	Male gender	0.378 0.210 -0.679	0.001
	Obesity	1.724 0.948 -3.135	0.074

Table 4 :

 4 Predictors of the safety endpoints using logistic regression multivariate analysis for the

primary (A: model 1; B: model 2) and the secondary safety endpoint (C: model 1; D: model 2) in the study population (n=981).

Table 2

 2 Baseline characteristics of patients according to antithrombotic therapy (VKA or DOA), before and after propensity score-matching .

Data are expressed as mean  standard deviation or number (percentage). Abbreviations: see Table

1

.

Table 3 :

 3 Safety outcomes at one-week.

*Same patient had both CHF and symptomatic AF.

Data are expressed as number (percentage). Abbreviations: see text.

Table 4 :

 4 Predictors of the safety endpoints using logistic regression multivariate analysis for the

primary (A: model 1; B: model 2) and the secondary safety endpoint (C: model 1; D: model 2) in the study population (n=981).

Table 1 .

 1 Figure 3: No difference in early and clinically significant complication occurrence after ambulatoryCAF catheter ablation in patient with uninterrupted DOA compared with uninterrupted VKA. Sécurité des anticoagulants oraux directs ininterrompus par rapport aux antivitamine K ininterrompus dans l'ablation par radiofréquence des flutters atriaux communs en ambulatoire: étude de cohorte avec score de propension. La sortie effective à domicile le jour même après une ablation de flutter commun par radiofréquence est une pratique réalisable, sûre et rentable, mais il n'existe actuellement aucune donnée concernant les patients traités par un anticoagulant oral direct (AOD) ininterrompu. Nous avons évalué la sécurité et la faisabilité de l'ablation de flutter en ambulatoire chez des patients traités par AOD par rapport à un antagoniste de la vitamine K (AVK). Méthodes et résultats. Les patients devant subir une ablation isolée de flutter en ambulatoire dans notre centre universitaire entre 2009 et 2019 ont été inclus. Le score de propension pour le type d'anticoagulant a été calculé à partir de l'âge, du sexe, de l'indice de masse corporelle, des caractéristiques initiales et des facteurs de risque (inclus dans les scores HAS-BLED et CHA2DS2-VASc), de l'insuffisance rénale chronique, du traitement antiplaquettaire associé, de la durée de procédure et du nombre de ponction fémorale. L'appariement par score de propension a donné 820 patients (âgés de 67  11 ans). L'ablation a été réalisée sous AVK ininterrompu (n = 410; INR = 2,5  0,6) ou AOD ininterrompu (n = 410). La survenue du résultat principal de tolérance, définie comme tout saignement précoce et cliniquement significatif (BARC 2) à une semaine, était similaire chez les patients sous AOD et sous AVK (2,9 vs 3,7%; p = 0,70), ainsi que le taux de sortie effective le jour même (94,9% contre 92,2%; p = 0,15). Le sexe féminin, le score HAS-BLED élevé et la durée de procédure prolongée étaient des facteurs de risque de saignement à une semaine. Conclusion. Les AOD ininterrompus sont un traitement anticoagulant sans danger pour l'ablation du flutter en ambulatoire chez les patients sélectionnés, dans un habitué, avec une unité ambulatoire dédiée et une gestion standardisée de la procédure et du post-opératoire.

	« Par délibération de son Conseil en date du 10 Novembre 1972, l'Université
	n'entend donner aucune approbation ni improbation aux opinions émises dans les
	thèses ou mémoires. Ces opinions doivent être considérées comme propres à leurs
	auteurs ».

Abbreviations: see Table

1

. Figure 4: Association between outcomes and HAS-BLED score in the study population using ANOVA. Abbreviations: see Table 1. MOTS CLES : flutter atrial; ablation par cathéter; anticoagulant oral direct; soins ambulatoires; résultats. TITRE DE LA THESE EN ANGLAIS : Safety of uninterrupted direct oral anticoagulant versus uninterrupted vitamin-K antagonist for ambulatory atrial flutter catheter ablation: a propensity scorematched cohort study.

Remerciements


Laura Prioretti

Frédéric Carriere

Ben Field

Luisana Avilan

Marie-Hélène Montané

Benoît Menand
email: benoit.menand@univ-amu.fr

Brigitte Gontero

Targeting TOR signaling for enhanced lipid productivity in algae

Microalgae can be used for many applications, including the development of third generation biofuels. However, the trade-off between biomass production and lipid accumulation and productivity still impairs the viable production of sustainable biofuel from microalgae because neutral lipids accumulate under stress conditions. Recently, in the model marine diatom Phaeodactylum tricornutum, it was shown that inhibiting the target of rapamycin (TOR) kinase using specific ATP-competitive inhibitors could overcome this issue. We believe that basic knowledge in this rather new field is required to develop innovative strategies to improve neutral lipid accumulation in oleaginous microalgae. In this minireview, we therefore focused on the current research on the TOR signaling pathway with a focus on its control of lipid homeostasis.

Since TOR has been well-studied in animals, we first describe the effectors of TOR in animal lipogenesis, adipogenesis and lipolysis. We then propose a tentative model of TOR regulation of TAG accumulation in algae, a process that has hitherto been overlooked.

Importance of lipids in 3 rd generation biofuels from microalgae

Lipids, together with carbohydrates, proteins and nucleic acids are essential components of living cells. While glycerophospholipids, sphingolipids, glycolipids and sterol lipids are the main components of cell membranes and can also function as signal transducers, triacylglycerols (TAG) constitute a universal form of transient energy storage in the form of fatty acids esterified with glycerol. They are found for instance in the core of lipid droplets in mammalian adipose tissues, in plant seeds, oleaginous yeasts and microalgae. They are the most abundant neutral lipid species in oils of plant origin. Stored TAGs can fuel the cell in times of nutrient limitation. The pathway leading to neutral lipid synthesis is fairly well conserved among eukaryotes: acetyl-CoA derived from carbon-metabolism is the substrate for de novo fatty acid (FA) synthesis, FAs are then conjugated to existing lipophilic alcohols through a series of acylation reactions in which diacylglycerol (DAG) is the last intermediate before TAG [START_REF] Ruggles | Making, baking, and breaking: the synthesis, storage, and hydrolysis of neutral lipids[END_REF]. A particularity of plant and algae is that the chloroplast is the major site of de novo FA synthesis [START_REF] Li-Beisson | The lipid biochemistry of eukaryotic algae[END_REF].

Cellular TAGs and other neutral storage lipids such as steryl esters are stored in lipid droplets, and their content can vary considerably depending on cell type and external environmental cues. For example, plants store TAGs in seeds in order to support the germination and vegetative growth of the next generation, while microalgae and yeast accumulate TAGs during starvation.

In the modern world the increasing energy demand from human activities threatens the availability of finite natural resources and the environment due to the release of CO2 and other pollutants. Against this background, neutral lipids represent a promising renewable resource for the biofuel industry, as well as for human and animal nutrition and health. Oleaginous microorganisms, including certain microalgae, yeasts and bacteria, can accumulate up to 80% of their biomass in the form of TAGs under appropriate conditions [START_REF] Shields-Menard | A review on microbial lipids as a potential biofuel[END_REF][START_REF] Chisti | Biodiesel from microalgae[END_REF]. These organisms therefore represent a major resource for the development of third generation biofuels. Algae are a polyphyletic group of photosynthetic organisms that evolved from multiple endosymbiotic events. The endosymbiosis of a cyanobacterium by a non-photosynthetic eukaryote led to the Archaeplastida group of chloroplast-containing eukaryotes which includes red algae, glaucophytes, green algae and land plants. Further endosymbiosis of green or red algae by other non-photosynthetic eukaryotes led to the evolution of diverse groups of algae including euglenoids and diatoms [START_REF] Cavalier-Smith | Principles of protein and lipid targeting in secondary symbiogenesis: euglenoid, dinoflagellate, and sporozoan plastid origins and the eukaryote family tree[END_REF]. Illustrating this diversity, diatoms have retained unique characteristics from different domains of life [START_REF] Benoiston | The evolution of diatoms and their biogeochemical functions[END_REF] such as the presence of the urea cycle [START_REF] Allen | Evolution and metabolic significance of the urea cycle in photosynthetic diatoms[END_REF], which is typical of animals, and a sterol biosynthetic pathway that uses intermediates from both plant and fungal sterol biosynthesis [START_REF] Fabris | Tracking the sterol biosynthesis pathway of the diatom Phaeodactylum tricornutum[END_REF]. The use of microalgae for lipid stock production has many advantages that have been widely explored and described [START_REF] Hu | Microalgal triacylglycerols as feedstocks for biofuel production: perspectives and advances[END_REF]: a broad range of microalgal strains can be exploited for oil production [START_REF] Chisti | Biodiesel from microalgae[END_REF][START_REF] Liang | Advancing oleaginous microorganisms to produce lipid via metabolic engineering technology[END_REF]; they can reach high growth rates (1 to 4 divisions per day for the model green alga, Chlamydomonas reinhardtii [START_REF] Wang | Algal lipid bodies: stress induction, purification, and biochemical characterization in wild-type and starchless Chlamydomonas reinhardtii[END_REF]); and by consuming CO2 they contribute to reducing greenhouse gas emissions. Moreover, because microalgae can use a variety of other inorganic nutrients they can even be exploited for bio-remediation from wastewaters [START_REF] Shields-Menard | A review on microbial lipids as a potential biofuel[END_REF]. Since microalgae are able to grow in controlled environments like that of photobioreactors as well as under extreme conditions (high salinity, and across a broad range of temperature and light intensities), they are not subject to seasonal changes and do not necessarily compete with crops for land and water.

Microalgae typically produce large amounts of lipids under stress conditions such as nutrient limitation, and most notably under nitrogen (N) starvation. However, these stresses interrupt cell proliferation and therefore reduce total biomass and limit the overall lipid yield. In the last decade there has been a gold rush for the selection of strains or the creation of engineered microorganisms able to accumulate more lipids. In this regard a path frequently beaten by researchers is to knockout or overexpress genes involved in carbon and lipid metabolism [START_REF] Liang | Advancing oleaginous microorganisms to produce lipid via metabolic engineering technology[END_REF]. However, the growth of these strains is often compromised and a better knowledge of the regulation of the balance between growth and lipid accumulation is still required. In this context, a new route is represented by the study of the TOR (Target of Rapamycin) kinase pathway [START_REF] Loewith | Target of rapamycin (TOR) in nutrient signaling and growth control[END_REF], which is a central hub in the regulation of various cell cycle processes including the regulation of nutrient and energy homeostasis, lipid synthesis and is conserved in eukaryotes [START_REF] Caron | The Roles of mTOR Complexes in Lipid Metabolism[END_REF].

We will first present evidence for the importance of TOR signaling in the regulation of lipid homeostasis from the wealth of studies in animals, and then we will present how manipulation of TOR signaling can improve TAG productivity in algae. Finally, we will discuss how recent studies in photosynthetic eukaryotes, in parallel with discoveries on TOR signaling in animals, can be used to understand how TOR regulates TAG accumulation in algae and, ultimately, to target TOR signaling for increased lipid productivity.

The principal TOR signaling effectors that control lipid metabolism in animals

TOR (Target of Rapamycin) is a large (280 kDa) multi-domain kinase belonging to the phosphatidylinositol kinase-related kinase (PIKK) family. TOR is inhibited by rapamycin (Fig. 1) through interaction with the 12 kDa TOR-FRB domain (FKBP12rapamycin-binding domain). Two canonical TOR containing complexes (TORC), TORC1 and TORC2, exist in both mammals and yeast. Recently, different forms of a TORC3 that differ in structure, function and subcellular localization have been discovered in mammals [START_REF] Harwood | ETV7 is an essential component of a rapamycin-insensitive mTOR complex in cancer[END_REF][START_REF] Kazyken | The nuclear import of ribosomal proteins is regulated by mTOR[END_REF][START_REF] Luo | Rapamycin inhibits mSin1 phosphorylation independently of mTORC1 and mTORC2[END_REF][START_REF] Smithson | Proteomic analysis reveals GIT1 as a novel mTOR complex component critical for mediating astrocyte survival[END_REF]. The TORC1 core, which is composed of the TOR kinase, RAPTOR (regulatory associated protein of TOR) and LST8 (lethal with SEC13 protein 8), is found in all eukaryotes [18] and is sensitive to the allosteric inhibitor rapamycin, which binds to the FRB domain of TOR. TORC1 is regulated by nutrients such as glucose, amino acids, as well as by growth factors and is inhibited by stressrelated signals such as starvation and hypoxia. Conserved functions of TORC1 include the activation of protein, lipid and nucleotide synthesis, as well as promoting progression of the cell cycle. At the same time, TORC1 also inhibits stress-responsive genes and catabolic processes like autophagy [19,[START_REF] Saxton | mTOR Signaling in Growth[END_REF]. Upstream signals are funneled towards TORC1 via different repressors and activators that regulate TOR intracellular localization and activity. One of the major upstream repressors of TORC1 is the AMPactivated protein kinase (AMPK), a major sensor of cellular energy status via the AMP/ATP and ADP/ATP ratio in mammals [START_REF] Crozet | Mechanisms of regulation of SNF1/AMPK/SnRK1 protein kinases[END_REF]. The orthologues of AMPK in yeast (SNF1, sucrose non fermenting 1) and plants (SnRK1, SNF1-related kinase 1) [START_REF] Jamsheer | The FCS-like zinc finger scaffold of the kinase SnRK1 is formed by the coordinated actions of the FLZ domain and intrinsically disordered regions[END_REF][START_REF] Sanz | AMPK in Yeast: The SNF1 (Sucrose Non-fermenting 1) Protein Kinase Complex[END_REF] are not responsive to AMP, indicating that there are distinct strategies for energy sensing in different organisms.

As a general rule, active TOR in mammals promotes lipogenesis and adipogenesis and inhibits lipolysis [START_REF] Ricoult | The multifaceted role of mTORC1 in the control of lipid metabolism[END_REF]. Lipid synthesis and storage are promoted by the SREBP (sterol regulatory element-binding protein) transcription factor, a master regulator of the transcription of lipo-and sterolgenic genes such as fatty acid synthase, acetyl-CoA carboxylase alpha, stearoyl CoA-desaturase, HMG-CoA reductase, and farnesyl diphosphate synthase [START_REF] Caron | The Roles of mTOR Complexes in Lipid Metabolism[END_REF]. SREBP is retained in the membrane of the endoplasmic reticulum where it is unable to activate transcription. In response to sterol depletion SREBP is cleaved and the SREBP DNA-binding domain is imported into the nucleus where it promotes the transcription of lipogenic enzymes [START_REF] Jeon | SREBPs: metabolic integrators in physiology and metabolism[END_REF]. In mammals, TORC1 regulates the activation of SREBP via two main mechanisms: the phosphorylation of (i) the ribosomal S6 kinase 1 (S6K1) to promote lipogenic processes including the processing of SREBP, and (ii) of lipin-1, a phosphatidic acid phosphatase (PAH) that converts phosphatidic acid (PA) into DAG, an important precursor of membrane and neutral lipids. Upon TORC1 inhibition, lipin-1 becomes dephosphorylated and is imported into the nucleus where it inhibits SREBP to downregulate lipid synthesis. Interestingly, TORC stability and activity is itself dependent on PA, which binds the FRB domain of TOR and acts as an indicator of nutrient sufficiency [START_REF] Menon | Lipid sensing by mTOR complexes via de novo synthesis of phosphatidic acid[END_REF][START_REF] Yoon | Phosphatidic acid activates mammalian target of rapamycin complex 1 (mTORC1) kinase by displacing FK506 binding protein 38 (FKBP38) and exerting an allosteric effect[END_REF]. Another S6K1 target, the Serine/Arginine (SR)-rich protein kinase 2 (SRPK2), which controls the expression of lipogenic enzymes was recently reported [START_REF] Lee | Post-transcriptional Regulation of De Novo Lipogenesis by mTORC1-S6K1-SRPK2 Signaling[END_REF]. It mediates the phosphorylation of SR proteins and their interaction with the spliceosomal protein U1-70K, which is responsible for the splicing of lipogenic pre-mRNAs. The phosphorylation of another mTORC1-S6K target, the glutamylprolyl tRNA synthetase (EPRS), induces EPRS release from the amino acyl tRNA multisynthetase complex and its interaction with the fatty acid transport protein 1, promoting its translocation to the plasma membrane and importing fatty acid to the cells [START_REF] Arif | EPRS is a critical mTORC1-S6K1 effector that influences adiposity in mice[END_REF]. In addition to inducing lipogenesis and controlling cellular lipid homeostasis in different cell types in the brain and liver, mammalian TORC1 also has a positive effect on the differentiation of stem cells into mature adipocytes and in the formation of adipose tissue [START_REF] Wipperman | Mammalian Target of Rapamycin: A Metabolic Rheostat for Regulating Adipose Tissue Function and Cardiovascular Health[END_REF]. TORC1 controls lipid metabolism in white and brown adipose tissues that respectively favor lipid storage or lipid use. The mobilization of the energy stored in the form of TAG, through the action of several lipases [START_REF]Lipid droplet mobilization: The different ways to loosen the purse strings[END_REF] and -oxidation of FA can generate either ATP or heat through the uncoupling protein 1 (UCP1) that decouples the mitochondrial electrochemical gradient. These processes, which are still under study, illustrate the fundamental role of TOR in regulating lipid and energy metabolism at the cell and organism level.

TOR inhibition can induce a "get fat growth" regime in algae

Despite the large diversity and the complex evolution of photosynthetic eukaryotes, all plants and algae investigated contain a conserved TORC1 with at least TOR, RAPTOR and LST8 [START_REF] Van Dam | Evolution of the TOR pathway[END_REF]. No other TOR complexes have yet been described in these groups of organisms. TOR is an essential regulator of proliferation in both plants and algae as shown by the embryo-lethal phenotype of tor null mutants in plants, and the repression of both plant and algal growth by the allosteric TOR inhibitor rapamycin or by ATP-competitive TOR inhibitors [START_REF] Montané | ATP-competitive mTOR kinase inhibitors delay plant growth by triggering early differentiation of meristematic cells but no developmental patterning change[END_REF][START_REF] Prioretti | The inhibition of TOR in the model diatom Phaeodactylum tricornutum promotes a get-fat growth regime, Algal Research -Biomass[END_REF][START_REF] Crespo | Inhibition of target of rapamycin signaling by rapamycin in the unicellular green alga Chlamydomonas reinhardtii[END_REF][START_REF] Menand | Expression and disruption of the Arabidopsis TOR (target of rapamycin) gene[END_REF].

Two independent studies conducted in plants and algae demonstrated the link between TOR repression and neutral lipid accumulation [START_REF] Caldana | Systemic analysis of inducible target of rapamycin mutants reveal a general metabolic switch controlling growth in Arabidopsis thaliana[END_REF][START_REF] Lee | Metabolomic response of Chlamydomonas reinhardtii to the inhibition of target of rapamycin (TOR) by rapamycin[END_REF]. Repression of tor expression by inducible RNA silencing in Arabidopsis seedlings led to major metabolic changes including the massive accumulation of storage lipids and starch [START_REF] Caldana | Systemic analysis of inducible target of rapamycin mutants reveal a general metabolic switch controlling growth in Arabidopsis thaliana[END_REF].

Likewise, repression of TOR activity by rapamycin treatment led to increased levels in TAG synthesis-related genes, such as glycerol-3-phosphate acyltransferase and acyl-CoA:diacylglycerol acyltransferase (DGAT), and consequently to TAG accumulation in Cyanidioschyzon merolae (red algae) and Chlamydomonas reinhardtii (green algae) [START_REF] Imamura | Target of rapamycin (TOR) plays a critical role in triacylglycerol accumulation in microalgae[END_REF]. These studies were pioneering in that they opened the way to targeting TOR signaling for improving plant and algal lipid production. More recent studies conducted on other diverse groups of microalgae, including the diatom Phaeodactylum tricornutum and the euglenoid Euglena gracilis, showed that TOR inhibition results in the accumulation of large lipid droplets rather similar to those that appear in response to N starvation [START_REF] Prioretti | The inhibition of TOR in the model diatom Phaeodactylum tricornutum promotes a get-fat growth regime, Algal Research -Biomass[END_REF][START_REF] Imamura | Target of rapamycin (TOR) plays a critical role in triacylglycerol accumulation in microalgae[END_REF][START_REF] Madeira | TORC1 inhibition induces lipid droplet replenishment in yeast[END_REF][START_REF] Imamura | TOR (target of rapamycin) is a key regulator of triacylglycerol accumulation in microalgae[END_REF][START_REF] Mukaida | The effect of rapamycin on biodiesel-producing protist Euglena gracilis[END_REF] (Fig. 2). TOR inhibition also induces lipid droplet accumulation in budding yeast, suggesting that this phenotype might be a common feature rather than being specific to photosynthetic eukaryotes [START_REF] Madeira | TORC1 inhibition induces lipid droplet replenishment in yeast[END_REF]. Although TAG accumulation seems to be a general response to TOR inhibition in algae and plants, other associated phenotypes are not always similar. For example, while we observed that P. tricornutum treatment with the ATP-competitive TOR inhibitor AZD-8055 (Fig. 1) could lead to a stronger decrease in chlorophyll fluorescence than N starvation, no significant effect of rapamycin on chlorophyll content was observed in C. reinhardtii by Roustan and Weckwerth [START_REF] Roustan | Quantitative Phosphoproteomic and System-Level Analysis of TOR Inhibition Unravel Distinct Organellar Acclimation in Chlamydomonas reinhardtii[END_REF]. However, while ATP-competitive TOR inhibitors can efficiently inhibit TOR, rapamycin is less efficient, at least in animals where it only partially inhibits TOR function [START_REF] Kang | mTORC1 phosphorylation sites encode their sensitivity to starvation and rapamycin[END_REF]. This is probably also the case in algae since rapamycin, even at high concentrations, only partially represses algal growth to 40-50% of that of untreated cells [START_REF] Prioretti | The inhibition of TOR in the model diatom Phaeodactylum tricornutum promotes a get-fat growth regime, Algal Research -Biomass[END_REF][START_REF] Crespo | Inhibition of target of rapamycin signaling by rapamycin in the unicellular green alga Chlamydomonas reinhardtii[END_REF][START_REF] Juppner | The target of rapamycin kinase affects biomass accumulation and cell cycle progression by altering carbon/nitrogen balance in synchronized Chlamydomonas reinhardtii cells[END_REF][START_REF] Roustan | Quantitative in vivo phosphoproteomics reveals reversible signaling processes during nitrogen starvation and recovery in the biofuel model organism Chlamydomonas reinhardtii[END_REF]. On the other hand, ATP-competitive TOR inhibitors can almost completely stop algal proliferation [18,[START_REF] Prioretti | The inhibition of TOR in the model diatom Phaeodactylum tricornutum promotes a get-fat growth regime, Algal Research -Biomass[END_REF][START_REF] Imamura | TOR (target of rapamycin) is a key regulator of triacylglycerol accumulation in microalgae[END_REF]. More comparative analyses with similar TOR inhibitors and at similar effective doses are therefore required to accurately compare the extent and diversity of TOR functions in different algae.

As discussed above, a major bottleneck in the development of algae for biofuel production is the fact that most conditions that induce TAG accumulation also repress growth, which strongly limits productivity. Obviously, this is also a concern with TOR inhibitors considering the essential and conserved positive function of TOR in the regulation of cell growth and proliferation. However, analysis of the dose-dependent effects of AZD-8055 in the diatom P. tricornutum allowed the identification of a dose that induced a 3 to 4 fold higher lipid productivity than N starvation. This enhanced lipid productivity was due to a moderated impairment of proliferation that allowed cells to grow and divide while accumulating TAG, in what was coined a "get fat-growth regime" [START_REF] Prioretti | The inhibition of TOR in the model diatom Phaeodactylum tricornutum promotes a get-fat growth regime, Algal Research -Biomass[END_REF]. Notably, it is not possible to titrate N to achieve the same effect, because N will be rapidly consumed by the cells. These results show that fine tuning of TOR activity can improve lipid productivity, and underline the need to fully decipher the TOR signaling axis involved in lipid homeostasis to improve the balance between proliferation and TAG accumulation.

How could the TOR signaling pathway regulate neutral lipid accumulation in algae?

Recent phosphoproteomics and pharmacogenetics analyses have identified potential members of the TOR pathway in photosynthetic eukaryotes. Although some of these data were obtained in Arabidopsis and have not yet been confirmed in Chlamydomonas or other more distantly related algae, this section presents an overview of the potentially conserved TOR signaling axis involved in lipid homeostasis, and a tentative model of how TOR might regulate TAG accumulation in algae (Fig. 3).

S6Ks are conserved TOR targets found in plants and algae that are known to regulate translation [START_REF] Dobrenel | The Arabidopsis TOR Kinase Specifically Regulates the Expression of Nuclear Genes Coding for Plastidic Ribosomal Proteins and the Phosphorylation of the Cytosolic Ribosomal Protein S6[END_REF][START_REF] Serfontein | Evolution of the TSC1/TSC2-TOR signaling pathway[END_REF]. Phosphoproteomic studies of Chlamydomonas cells treated with rapamycin or AZD-8055 identified TOR-dependent phosphorylation sites in S6K and ribosomal S6 protein [START_REF] Roustan | Quantitative Phosphoproteomic and System-Level Analysis of TOR Inhibition Unravel Distinct Organellar Acclimation in Chlamydomonas reinhardtii[END_REF][START_REF] Werth | Investigating the effect of target of rapamycin kinase inhibition on the Chlamydomonas reinhardtii phosphoproteome: from known homologs to new targets[END_REF]. In mammals, S6K regulates SREBP processing and other lipogenic genes. However, SREBP homologs are not found in plants and algae. Interestingly, one of the two Arabidopsis S6Ks localizes to the nucleus, suggesting its potential involvement in transcriptional regulation [START_REF] Mahfouz | Arabidopsis TARGET OF RAPAMYCIN interacts with RAPTOR, which regulates the activity of S6 kinase in response to osmotic stress signals[END_REF]. Orthologues of the SREBP repressor and PAH (lipin-1) are also found in plants and algae [START_REF] Nakamura | Arabidopsis lipins mediate eukaryotic pathway of lipid metabolism and cope critically with phosphate starvation[END_REF].

Arabidopsis has two PAHs (PAH1 and 2), whose loss of function induces increase in phospholipid content [START_REF] Nakamura | Arabidopsis lipins mediate eukaryotic pathway of lipid metabolism and cope critically with phosphate starvation[END_REF]. In algae species, transcriptomics, proteomics or metabolomics did not report PAHs or PA but this might reflect that such signaling proteins and PA are hard to detect in the nutrient conditions devoted to increase cellular lipid contents [START_REF] Arora | Leveraging algal omics to reveal potential targets for augmenting TAG accumulation[END_REF]. In the green microalga Chlorella vulgaris, proteomics studies reported that S6K increased under nutrient repleted conditions [START_REF] Guarnieri | Proteomic analysis of Chlorella vulgaris: potential targets for enhanced lipid accumulation[END_REF] and the phosphatidic acid phosphatase PAP was identified [START_REF] Guarnieri | Examination of triacylglycerol biosynthetic pathways via de novo transcriptomic and proteomic analyses in an unsequenced microalga[END_REF]. Strikingly, PAH2 is phosphorylated in a TOR dependent manner in Arabidopsis, suggesting a conservation of the link between TOR and PAH in photosynthetic eukaryotes [START_REF] Van Leene | Capturing the phosphorylation and protein interaction landscape of the plant TOR kinase[END_REF]. PAH1 is also phosphorylated by the cyclin-dependent kinases (CDKs) in Arabidopsis suggesting a further link between lipid homeostasis and cell cycle progression [START_REF] Craddock | Cyclindependent kinase activity enhances phosphatidylcholine biosynthesis in Arabidopsis by repressing phosphatidic acid phosphohydrolase activity[END_REF]. Hence, although a direct link between S6K/PAH and TAG accumulation has not yet been demonstrated in algae, their conservation strongly suggests that they may play a significant role in algal lipid homeostasis (Fig. 3).

Other potential effectors of TOR signaling were directly identified in genetic screens on algae. First, two members of the DYRK (dual-specificity tyrosinephosphorylation regulated kinase) family of protein kinases were isolated in screens for Chlamydomonas insertional mutants defective for oil and starch accumulation [START_REF] Kajikawa | Algal dual-specificity tyrosine phosphorylation-regulated kinase, triacylglycerol accumulation regulator1, regulates accumulation of triacylglycerol in nitrogen or sulfur deficiency[END_REF][START_REF] Schulz-Raffelt | Hyper-accumulation of starch and oil in a Chlamydomonas mutant affected in a plant-specific DYRK kinase[END_REF].

One, named TAR1 (triacylglycerol accumulation regulator 1) is the homolog of the yeast and Arabidopsis YAK1 (yet another kinase 1), and the other, named DYRKP, belongs to a plant and algal DYRK subfamily. A link between YAK1/DYRK and TOR signaling was revealed recently in plants during a screen for Arabidopsis mutants resistant to the TOR inhibitor AZD-8055. Arabidopsis YAK1 is a negative regulator of growth that is repressed by TOR [START_REF] Barrada | A TOR-YAK1 signaling axis controls cell cycle, meristem activity and plant growth in Arabidopsis[END_REF], interacts with TOR, and is phosphorylated in a TOR-dependent manner [START_REF] Van Leene | Capturing the phosphorylation and protein interaction landscape of the plant TOR kinase[END_REF] but it is not yet clear how TOR-YAK/DYRK signaling affects TAG levels in algae. Another screen aimed at identifying TOR signaling effectors in algae led to the identification of the rapamycin-hypersensitive mutant vip1-1 [START_REF] Couso | Synergism between Inositol Polyphosphates and TOR Kinase Signaling in Nutrient Sensing, Growth Control, and Lipid Metabolism in Chlamydomonas[END_REF]. VIP1 encodes an inositol hexakisphosphate kinase that pyrophosphorylates the inositol phosphate InsP6 to produce the signaling molecules InsP7 and InsP8. The fact that InsP7 and InsP8 levels drop in rapamycin-treated wild-type cells confirmed a link with TOR signaling [START_REF] Couso | Synergism between Inositol Polyphosphates and TOR Kinase Signaling in Nutrient Sensing, Growth Control, and Lipid Metabolism in Chlamydomonas[END_REF]. Interestingly, the higher levels of TAGs in the vip1-1 mutant led to the conclusion that InsP7 and InsP8 activate growth at the expense of TAG accumulation [START_REF] Couso | Synergism between Inositol Polyphosphates and TOR Kinase Signaling in Nutrient Sensing, Growth Control, and Lipid Metabolism in Chlamydomonas[END_REF]. In Arabidopsis, AZD-8055 leads to increased phosphorylation of the VIP1 homolog VIP2 suggesting an indirect regulation by TOR [START_REF] Van Leene | Capturing the phosphorylation and protein interaction landscape of the plant TOR kinase[END_REF]. Interestingly, both tar1 and vip1 mutant phenotypes are dependent on the presence of a carbon source. tar1 mutant fails to accumulate TAGs when N starvation is coupled to acetate supply (photomixotrophy) but accumulates high level of TAGs in photoautotrophic N-deficient conditions [START_REF] Shinkawa | Algal Protein Kinase, Triacylglycerol Accumulation Regulator1, Modulates Cell Viability and Gametogenesis in Carbon/Nitrogen Imbalanced Conditions[END_REF]. Similarly, vip1 is hypersensitive to TOR inhibitors on acetate supplied media but not in photoautotrophic conditions [START_REF] Couso | Synergism between Inositol Polyphosphates and TOR Kinase Signaling in Nutrient Sensing, Growth Control, and Lipid Metabolism in Chlamydomonas[END_REF]. This highlights the importance of the TOR signaling pathway in integrating different nutrient signals to regulate growth and reserve accumulation in algae.

Last but not least, autophagy was the first intracellular process connected to TOR signaling to be reported in algae. Rapamycin treatment was shown to induce vacuolization [START_REF] Crespo | Inhibition of target of rapamycin signaling by rapamycin in the unicellular green alga Chlamydomonas reinhardtii[END_REF] and post-translational modification of the key autophagy-related protein ATG8 in C. reinhardtii [START_REF] Perez-Perez | The TOR Signaling Network in the Model Unicellular Green Alga Chlamydomonas reinhardtii[END_REF]. More recently, ATG7 phosphorylation was shown to be dependent on TOR [START_REF] Roustan | Quantitative in vivo phosphoproteomics reveals reversible signaling processes during nitrogen starvation and recovery in the biofuel model organism Chlamydomonas reinhardtii[END_REF], suggesting that TOR may regulate autophagy via this protein. In addition, a recent report showed that treatment of cells with concanamycin A, an inhibitor of the autophagic flux, repressed TAG accumulation in response to N starvation. This work indicates that autophagy and vacuolar function are therefore required for lipid droplet accumulation in Chlamydomonas [START_REF] Couso | Autophagic flux is required for the synthesis of triacylglycerols and ribosomal protein turnover in Chlamydomonas[END_REF]. It is therefore likely that the activation of autophagy plays an important role in lipid droplet accumulation in response to TOR inhibition.

Perspectives

This minireview shows the importance of measuring proliferation and/or growth in connection with TAG productivity whatever the context. Manipulation of specific TOR signaling axis and the use of different ATP competitive inhibitors of TOR, at different concentrations, may help to achieve a better TAG productivity. Identifying the downstream targets of TOR needs further investigations to decipher the top secret of TOR in different organisms and especially in algae that are still overlooked for TOR function. More comparative studies are indeed required, particularly between different algae due to their diversity and because most TOR effectors were studied in the model green alga Chlamydomonas reinhardtii, which is not necessarily the best choice for biotechnological applications. It is obvious that there are still many remaining questions to understand the relationships between TOR and TAG accumulation in algae and many studies to perform in that fascinating field.

Figure legends

Fig. 1 .

 1 Fig. 1. Chemical structures of the TOR inhibitors rapamycin (A) and AZD-8055 (B).

Fig. 2 .

 2 Fig. 2. Effect of TOR inhibition on lipid storage in microalgae. A) Nile Red staining of lipid droplets in Phaeodactylum tricornutum after five days of treatment with DMSO (left) or AZD-8055 2 µM (right), scale bars 10 µm (modified with permission from Prioretti et al. 2017). B) BODIPY and AC-202 merged staining of lipid droplets in Chlamydomonas reinhardtii after 48h of treatment with DMSO (left) or AZD-8055 2 µM (right), scale bar 5 µm (modified from Harchouni et al. 2018). C) BODIPY staining of lipid droplets in a rapamycin-sensitive strain of Cyanidioschyzon merolae expressing

Fig. 3 .

 3 Fig. 3. Model of effectors of TOR-dependent regulation of growth and lipid homeostasis in microalgae.

Acknowledgments

This work was supported by the ANR SIGNAUX BioNRJ (ANR-15-CE05-0021-03)


Lionel Cordesses

Benoit Thuilot

Philippe Martinet

Christophe Cariou
email: christophe.cariou@cemagref.fr

Curved Path Following of a Farm Tractor Using a CP-DGPS

Keywords: guidance systems, autonomous mobile robots, robot control, nonlinear control systems, agriculture, implementation

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Precision farming can be described as a three steps process. Land surveying is the first one, while agricultural maps recording is the second one. According to the previous steps, the last one could be variable rate seeding, fertilizing,. . . We will focus on the third step, as it requires an accurate vehicle guidance. Let us illustrate this concept in the particular case of a farm tractor. The need for automatic vehicle guidance systems seems obvious on this machine: the driver would only have to finely tune different functions of this machine. Meanwhile, the guidance system would achieve a very accurate positioning, thus insuring neither overlapping nor missing areas.

Many agricultural situations could be reported. Most of them could be classified as two categories. The first one is a "previously-recorded" path-following task: this trajectory could have been stored during the seeding for instance, and replayed during weeding. In the second one, the machine must follow a new curve. This curve is computed from the last passage plus an offset (the tool length for instance). We could also mention new possibilities, such as automatic half-turn for driver assistance. All these situations require a control along curved paths.

Many experiments have been conducted and reported in the literature. Most approaches can be classified into two categories, as they rely upon two kinds of sensors and information.

Relative information and guidance systems

The first one uses relative information. Researchers have focused for instance on the video camera sensor. Mounted at the end of the implement or at another appropriate location, it provides the operator with a set of "eyes" near the reference line, for instance, a sugar beets row. This contact-free sensing system presents the same offset information as earlier furrow following or edgedetecting sensors: the reference line to be followed by the vehicle is not the one extracted in the video image. Moreover, it has a challenging feature from a control point of vue: it provides anticipation to the control system, since the camera is scanning ahead of the vehicle. Various approaches have been tested. Some rely on 2D information extracted from the image plane [START_REF] Debain | Lois de commande pour le contrôle et la mobilité de machines agricoles[END_REF], [START_REF] Khadraoui | Vision based control in driving assistance of agricultural vehicles[END_REF] and [START_REF] Ollis | Vision based perception for an automated harvester[END_REF]. Other are based on 3D sensors, for instance made of a laser scanning and a video camera [START_REF] Chateau | Fusion d'attributs: application au guidage d'engins agricoles[END_REF]. Two kinds of difficulties arise: the first one is the reference extraction. The whole guidance system relies upon the quality and robustness of the image processing algorithms. Extreme conditions of dust, for example, reveal the weakness of the detection side. The other problem arises when the new reference is computed in an iterative way from the previous one. The automatic guidance system can lead to oscillations. In the same situation, human operators frequently and periodically sacrifice for efficiency. They do not exactly follow the previous path, and often "straighten out" the edge of the worked area.

Absolute information and guidance systems

The second one uses absolute information. A recent technological development allows accurate three dimensional (3D) positioning of the vehicle in a field without the need for buried cables, field-installed beacons or video sensors: the Global Positioning Systems (GPS). For instance, during the harvest, corn weight measurement devices are coupled to a differential GPS to produce a yield map. It allows farmers to adjust seed, pesticide, and fertilizer to each field location.

State of the art DGPS (Carrier Phase DGPS or CP-DGPS) reach the real-time centimeter accuracy. This allows the design and implementation of an absolute vehicle guidance system with the availability of a very accurate positioning system. This technology can be used in special operations, in which the vision system is unable to proceed, for instance, in a spraying or fertilizing operation with no visible markers such as boom wheel tracks or foam marks. Researches have been carried out in this area, using fiber optic gyroscopes (FOG) and GPS [START_REF] Nagasaka | Automated operation in paddy fields with a fiber optic gyro sensor and GPS[END_REF]. The influence of navigation sensors in GPS based land navigation has also been studied in [START_REF] Abbot | Land -vehicle navigation using GPS[END_REF]. Promising results have been reported where an " all-GPS " solution has been preferred: with multiple GPS [START_REF] O'connor | Automatic steering of a farm vehicle using GPS[END_REF] or with one GPS [START_REF] Cordesses | GPS-based control of a land vehicle[END_REF].

We will first introduce the reader with the kinematic model of the farm tractor, with respect to a curved reference. We will then propose a non linear, velocity independent control law designed for curved paths following. A brief description of the straight line case will be derived. Lastly, we will describe our implementation and experimental setup on a farm tractor. Experimental results will be presented and some conclusions will be drawn.

VELOCITY INDEPENDENT CONTROL LAW FOR CURVE FOLLOWING

In the scope of this paper, we will focus on the particular case of a farm tractor. Most agricultural tasks are performed at constant velocity.

Unfortunately, this velocity relies upon various factors, such as crop variety and soil properties, and is tuned by the driver. This is the reason why we decided to design a velocity independent control law. Such an approach has been addressed in [START_REF] Elkaim | System identification and robust control of farm vehicles using CDGPS[END_REF]. We will present here a different solution.

After a brief presentation of the non linear kinematic model of the farm tractor, we will derive our velocity independent control law for curved path following. We have to mention that we will focus on the path following aspect of the guidance system, while not dealing with the path planning step. In fact, the reference is assumed to exist, as previously stated.

2.1 Kinematic model of the tractor with respect to a curved reference.

Let (zW x) be an absolute frame. The reference path is assumed to be a curve C in this frame. We will denote s the curvilinear abscissa along C, c(s) the path curvature, and g(s) = dc(s) ds . The farm tractor model is assumed to match a tricycle model of length l with a controlled front wheel (figure 1). We chose to put the tractor control point at the center of the rear axle, denoted O. Many reasons have led us to use this point. Most of them derive from practical implementation issues :

• O is the tractor closest point to the rear implement. We are highly interested in gathering data sensed near the implement, since it performs the agricultural task. • The GPS antenna antenna must be on the top of the tractor cabin, to see more satellites. Since the control point has to be straight down the antenna, point O suits well.

The last reason is a theoretical one. This point O shows special control properties, and therefore is widely used to deal with the control of car-like systems [START_REF] Samson | Control of chained systems. Application to path following and time-varying point stabilization of mobile robots[END_REF], [START_REF] Tilbury | A multisteering trailer system: conversion into chained form using dynamic feedback[END_REF].

Finally, as in [START_REF] Samson | Control of chained systems. Application to path following and time-varying point stabilization of mobile robots[END_REF], we will denote y the distance between O and the closest point M of C. We assume that point O location is always described by a unique couple (s, y). This assumption is valid in real world experiments as long as the tractor remains close to the reference path. The configuration of the tractor is thus described without any ambiguity by the state vector X = (s, y, θ) T with :

s : the curvilinear abscissa of point M -y : the lateral deviation of the tractor with respect to the reference path. -θ : the heading of the tractor with respect to the reference path. It can be described as the angle between the tractor body and the reference tangent at point M .

Two control variables are available, namely U = (v, δ) T with:

-v: linear velocity at point O (the control variable is actually the rotation velocity of the rear wheels, from which the value of v can be inferred). -δ: orientation of the front wheels (assumed to be superposed, see figure 1).

The vehicle kinematic equations are derived according to celebrated pure rolling and nonslipping assumptions, and rigid body assumptions. The former imposes that the linear velocity vector at point O is directed along the vehicle axle. The latter implies that an instantaneous rotation center exists. It has then been proved (see [START_REF] Samson | Control of chained systems. Application to path following and time-varying point stabilization of mobile robots[END_REF]) that the model can write as (for the sake of simplicity, c(s) and g(s) are written hereafter c and g):

           ṡ = v cos θ 1 -cy ẏ = v sin θ θ = v( tan δ l - c cos θ 1 -cy ) (1)
The equations are clearly non linear. They become singular when y = 1 c . This means the point O is located at the center of curvature of the reference path. This problem is not encountered in practical situations: the path curvature is small and the farm tractor remains close to the reference.

Design of a velocity independent control law

Farm tractor control along curved paths has already been addressed in some particular cases [START_REF] Bell | Realistic autofarming closed-loop tractor control over irregular paths using kinematic GPS[END_REF]. We use hereafter a more general approach: we convert the non linear model ( 1) into a chained form. Such an approach is interesting as it allows us to use, for a large part, linear systems theory, while still relying upon the non linear model of the plant.

A three dimensional chained form writes as:

   ȧ1 = m 1 ȧ2 = a 3 m 1 ȧ3 = m 2 (2)
with A = (a 1 , a 2 , a 3 ) T and M = (m 1 , m 2 ) T respectively the state and control vectors. In order to convert the farm tractor model ( 1) into chained form (2), let us impose:

a 1 = s
In order to match (2), the new control m 1 necessarily writes as:

m 1 ∆ = ȧ1 = v cos θ 1 -cy (3)
Moreover, let us impose:

a 2 = y
We have: ȧ2 = v sin θ ∆ = a 3 m 1 Therefore we must choose:

a 3 = (1 -c y) tan θ
and the last control variable m 2 is given by:

m 2 ∆ = ȧ3 = d dt ((1 -c y)tan θ) = -c v sin θ tan θ -g v cos θ 1 -c y tan θ y + (1 -c y)v cos 2 θ tan δ l -c cos θ 1 -c y (4)
In conclusion, the non linear tractor model ( 1) can be converted into chained form (2) according to the state transformations:

A = Θ(X) with Θ(X) = (s, y, (1 -cy) tan θ) T
and the control transformations: M = Υ(U , X) defined by ( 3) and ( 4)

This chained form can be used to design a control law as long as these transformations are invertible, i.e. v = 0 and θ = π 2 [π]. From a practical point of view, once properly initialized, the guided tractor respects these conditions.

In order to get a velocity independent control law, we now replace the time derivative in (2) by a derivation with respect to the curvilinear abscissa s. The normalized model becomes:

   a 1 = 1 a 2 = a 3 a 3 = m 3 with m 3 = m 2 m 1 (5)
The lower part of model ( 5) is completely linear, so it can be brought to zero using the following virtual control law:

m 3 = -K d a 3 -K p a 2 (K p , K d ) ∈ R +2 (6)
As a matter of fact, reporting (6) in ( 5), leads to:

a 2 + K d a 2 + K p a 2 = 0 (7)
which implies that both a 2 and a 3 , and therefore y and θ converge to zero.

The above error dynamics is expressed with respect to s. So, for any given (K p , K d ), the resulting trajectories remain identical, even if v is varying. This control law is then clearly velocity independent.

Reporting ( 6) in ( 3) and ( 4), the real control is:

δ(y, θ) = arctan l cos 3 θ (1 -c 2 g y tan θ -K d (1 -c y) tan θ -K p y + c (1 -c y) tan 2 θ + c cos θ 1 -c y (8)
The above δ(y, θ) is not bounded, and thus, the control law does not prevent from actuator saturation. It is quite simple to bound the virtual control law m 3 to any arbitrary value. Nevertheless, it leads to a bounded δ only if the trajectory to be followed exhibits some good properties. This is true, for instance, for a circle, where g = 0 and c is constant. The theoretical stability of the closed loop system is still insured, since the chained system is a double integrator (see [START_REF] Sussmann | A general result on the stabilization of linear systems using bounded controls[END_REF]).

The straight line case

This curved path control law can be gracefully degraded to the case of straight line following. A straight line reference imposes both c = g = 0.

The control law (8) then becomes:

δ(y, θ) = arctan(l cos 3 θ(-K d tan θ -K p y)) (9)
When we saturate 3 using a sigmoid, the expression of the control law turns to:

δ(y, θ) = arctan K l cos 3 θ 1-e -k(K d tan θ+Kp y) 1+e -k(K d tan θ+Kp y) (10)
This control law is clearly saturated: K can be tuned so as to ensure: max ∀y,∀ θ |δ(y, θ)| = δ max . Such a control law has been previously designed and used to control a combine harvester along straight lines in a field [START_REF] Cordesses | GPS-based control of a land vehicle[END_REF].

EXPERIMENTAL RESULTS

Experiments have been carried out on a farm tractor in our experimental farm in Montoldre, France.

The CP-DGPS dual frequency receiver is a Dassault-Sercel unit: Aquarius 5002 system. Its claimed accuracy is about 1 cm, and its upper sampling frequency is 10 Hz. Three dimensional velocities are computed thanks to Doppler measurements. Its short initialization time allows its use in many farm applications.

The controller has been implemented in high level language (C++) on a Pentium based computer.

Implementation of a state measurement unit

Both control laws (8) and ( 9) require lateral deviation y and heading error θ.

Since the GPS antenna is straight up the point O, the receiver only provides us with the 3D position and velocity of that point.

The y measurement is easily obtained from the knowledge of the reference path and the O coordinates. The θ is more complex to derive. A common and costly solution makes use of additional hardware and sensor (either a gyroscope or an attitude measurement unit). We have preferred a cheaper approach based upon our only exteroceptive sensor, the CP-DGPS receiver. Under the previously mentioned pure rolling and non-slipping assumptions, one can easily infer the heading of the farm tractor thanks to realtime velocity data.

Step response of the system

Although a step path is not common in agriculture, we have used the step response to check the behaviour of our closed loop system. All the experiments have been conducted on an almost flat ground with the straight line control law (10).

The very first experiments have been carried out with a 2 meters step at 8 km/h. One can compare on figure 2 simulation and experimental results. I. One can notice that the results appear quite satisfactory (σ x < 3.1 cm and µ x < 2.7 cm). 1.9 3.1 3.1

Curve following

Up to now, we have only validated the straight line following law. In the sequel of these experiments, we now investigate the curved path following control law (8).

We first recorded in open loop a reference path, roughly made of straight lines and a quarter of circle (figure 4).

The closed loop experiments with respect to this new path started with a 0.8 m lateral deviation (see figure 5 at initial time). One can notice that the farm vehicle has converged to the reference path (both curves overlap on figure 5). The bias is about 3 cm and the standard deviation from the mean less than 5 cm. Moreover, the maximum lateral deviation along the whole path is 18.4 cm (see figure 5 at time t = 30 s). The curvature of the reference path at this time is the highest one, thus leading to a δ varying very quickly. Such a behaviour can be explained. Our investigations have led us to incriminate the low level servoing system. First of all, one must keep in mind this system does have a response time (a few hundreds of milliseconds). Moreover, it has not been optimized for fast tracking: neither the hydraulic components nor the controller -off the shelves devices -can achieve very high performances.

We have recorded a second reference path: it is longer than the previous one, and crosses different types of ground (figure 6). Thorough analysis of the lateral deviation leads to the same conclusions. We first built a non linear control law using a chained form of the kinematic model of the farm tractor. Moreover, it has been designed so as to be velocity independent for practical reasons.

We have implemented this control law on a farm tractor. Curved path following has been demonstrated. We have also checked the velocity independence property.

Although results are quite satisfying on an almost flat ground, further research has to be done to take into account for slope. One can also mention the use of other sensors to insure a highest level of reliability, and thus security.

Another important issue is to build a better hydraulic servoing system, in order to decrease both latency time and response time. One could also take into account for the latency of the whole vehicle thanks to an anticipatory system.

Fig. 1 .

 1 Fig. 1. Kinematic model of the farm tractor

Fig. 2 .Fig. 3 .

 23 Fig. 2. Simulation and experimental trajectory at 8 km/hWe have also checked the velocity independence property. Experiments have been conducted at 4 and 8 km/h (constant velocity) and from 4 to 8 km/h (varying velocity). One can check on the upper curve of figure3that the three step responses almost perfectly overlap. The lower curve plots the raw velocity as computed by the GPS receiver. One can notice the sound behaviour of the farm tractor during all these experiments.

Fig. 5 .

 5 Fig. 4. Reference path and closed loop, farm tractor trajectory

  Fig. 6. Second reference path 4. CONCLUSION The purpose of this work was to control a farm tractor along a curved path, using a CP-DGPS sensor.

Table I .

 I Bias and standard deviation from mean

	velocity (km.h -1 )	4	8	4 to 8
	µ x (cm)	2.7 0.3	2.1
	σ x (cm)