Luca Arpaia
email: l.arpaia@brgm.fr

Jeremy Rohmer

Anne Lemoine

Rodrigo Pedreros

Generating random waveforms for stochastic tsunami simulations

come

Generating random waveforms for stochastic tsunami simulations

L. Arpaia 1* , J. Rohmer 1 , A. Lemoine 2 and R. Pedreros Numerous near-field earthquake-induced tsunami simulations have shown that the spatial distribution of the fault slip can have a significant impact on the nearshore wave propagation and ultimately on the wave height in some sensitive areas. However fault rupture is a complex phenomenon which depends on pre-rupture stress condition, geometrical setting and frictional property of the fault, which are largely unknown. In the present study, we model the slip spatial field on the fault as a random field [Mai and Beroza, 2002]. Instead of resorting to the classical Gaussian assumption, random slip patterns are modeled using a joint lognormal distribution to account for the fact that the probability of negative values is zero. The Karhunen-Loeve (KL) expansion is used to generate the slip patterns [Gonzalez et al., 2016] while the free surface ocean displacement is computed by means of the Okada solution of an elastic dislocation problem. We investigate the relationship between the slip spectrum and initial waveform spectrum. We show that, in the context of Probabilistic Tsunami Hazard Analysis, we can effectively reduce the number of terms retained in the slip expansion. The interest is to compactly represent the mapping between a variable of interest at coast Y and the vector of the input random variables (the coefficients of the KL expansion) through the generalized Polynomial Chaos Expansion (gPCE) implemented in the Openturns software [Baudin et al., 2015].

The gPCE surrogate is tested on idealized one-dimensional benchmarks as well as on the historical event of the 1755 Lisbon tsunami.

  1 1 French Geological Survey, Coastal Risk and Climate Change

2 French Geological Survey, Seismic risk Unit February 19, 2020


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Lise Bankir
email: lise.bankir@inserm.fr

Lucile Figueres

Caroline Prot-Bertoye

Nadine Bouby

Gilles Crambert

J Howard Pratt

Pascal Houillier
email: pascal.houillier@inserm.fr

MEDULLARY AND CORTICAL THICK ASCENDING LIMB: SIMILARITIES AND DIFFERENCES

Keywords: NKCC2, Cortex, Medulla, Hormonal regulation, Genetic disease

whether they are published or not. The documents may come

List of Abbreviations

Foot note

Arginine vasopressin and AVP are the internationally accepted term and abbreviation that designate the 9 amino acid natural hormone produced in the hypothalamus of almost all mammals. When synthetic vasopressin was not yet available, purified extracts from pig neurohypophysis were used as a source of vasopressin for experimentatal purposes. A genetic shift that occurred during evolution in the suborder of Suina (to which the pig belongs) replaced an arginine in position 8 by a lysine. It was thus necessary to distinguish between "lysine-vasopressin" and "arginine-vasopressin" found in all other mammals. This is why this hormone is abreviated AVP. This single amino acid substitution does not alter the affinity of the hormone for its receptors, nor the functional response of the target tissues.

Introduction

In mammals, the thick ascending limb (TAL) plays a crucial role in major kidney functions. Because of the location of the macula densa close to the distal extremity of this nephron segment, the fluid composition in the late TAL lumen may influence the tubulo-glomerular feedback [TGF] system and thus the glomerular filtration rate (GFR). The active pumping of sodium chloride (NaCl) in the TAL allows the formation of hypo-osmotic urine and thus the ability to excrete solute-free water. The TAL also generates the "single effect" initiating the accumulation of solutes in the medulla by counter-current multiplication, a crucial step in the ability to produce hyperosmotic urine. Additionally, the TAL plays a significant role in calcium, magnesium, bicarbonate and ammonium homeostasis. Therefore the TAL is critical in determining the composition of the medullary interstitium, the environment of the medullary collecting duct, and the composition of the tubular fluid that is delivered to the distal convoluted tubule and collecting duct where the composition of final urine is ultimately determined.

The severity of several monogenic diseases stemming from this tubular segment illustrates the importance of the TAL in the overall renal functions. The impairment in the ability to reabsorb NaCl, which characterizes the various types of Bartter syndrome and HELIX syndrome, results in symptomatic, sometimes lethal, decreases in extracellular fluid volume, with secondary hyperaldosteronism and polyuria. The loss-of-function mutations of claudin (CLDN)-16 and -19 cause familial hypomagnesemia with hypercalciuria and nephrocalcinosis (FHHNC, 248190), a massive renal loss of calcium and magnesium and, eventually, end-stage renal disease. Mutations in the UMOD gene, encoding uromodulin, are responsible for medullary cystic kidney disease-2 (MCKD2; 603860) and familial juvenile hyperuricemic nephropathy (HNJF1; 162000). In addition, common variants in the UMOD gene are associated with an increased risk of progression of chronic kidney disease in GWAS [START_REF] Devuyst | The UMOD Locus: Insights into the Pathogenesis and Prognosis of Kidney Disease[END_REF].

A considerable number of articles have been devoted to the study of this nephron segment, especially focussing on its transport properties and their regulation by various hormones. Many studies were based on in vivo micropuncture of the loop of Henle of superficial nephrons (at the late proximal and early distal tubules accessible at the kidney surface). Other studies used microperfusion of single portions of TAL isolated by microdissection.

Investigations addressing integrative aspects of renal function often consider the TAL as a whole without taking into account the fact that it includes at least two subsegments, the medullary and the cortical portions (M-TAL and C-TAL, respectively) which lie in very different environments and exhibit somewhat different anatomical and functional characteristics. The purpose of the present review is to draw the attention on multiple differences in the anatomical and functional characteristics of these two subsegments, and to highlight the need to take these differences into account when interpreting results collected during in vivo studies under normal or pathological conditions. Finally, some yet unanswered issues will be considered. Several other recent reviews have recapitulated a number of features regarding the TAL [START_REF] Gonzalez-Vicente | Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension[END_REF]136,195] including one that also focusses on the axial heterogeneity of the TAL [START_REF] Dimke | Axial and cellular heterogeneity in electrolyte transport pathways along the thick ascending limb[END_REF]. The anatomy and functional characteristics of the macula densa cells will not be addressed here (see reviews in [109,143]).

Role of the TAL in urine concentration and dilution

Along its whole length, the TAL epithelium actively pumps sodium chloride out of the lumen into the surrounding extracellular space, and exhibits an extremely low permeability to water. Thus, it dilutes the fluid present in the lumen below the osmolarity of the surrounding extracellular fluid. Nevertheless, the TAL participates in two different functions regarding water handling by the kidney. In its early part, in the medulla (especially in the inner stripe (IS)), it contributes to the building of the hyperosmotic gradient, and in its more distal part, in the cortex, it produces hypo-osmotic urine. Accordingly, it plays crucial roles in both urine concentration and urine dilution, or in other words, in both water conservation and free-water excretion. These two apparently opposite functions, based on the same transport properties, are due to the different environment of two subsegments of the TAL. In the IS, the thick ascending limbs are running close to the thin descending limbs, especially those of the long looped-nephrons which extend over a variable length in the inner medulla. In the cortex, the thick ascending limbs (at least those of short looped-and cortical nephrons) are running in the medullary rays, among the descending pars recta of the proximal tubules and cortical collecting ducts. Both M-TAL and C-TAL reabsorb NaCl but M-TAL can create only a small transepithelial concentration difference, whereas C-TAL creates a larger transepithelial concentration difference that permits lowering of tubular fluid osmolarity to less than 150 mosm/L. These differences are schematized in Figure 1.

In the IS, the close proximity of IS-TALs and long loop thin limbs allows the "single effect" of the counter-current multiplication mechanism [START_REF] Burg | Thick ascending limb of Henle's loop[END_REF][START_REF] Hebert | The medullary thick limb: function and modulation of the single-effect multiplier[END_REF]160]. This allows sodium, reabsorbed by the IS-TAL, to enter into the thin descending limbs of long loops of Henle that are highly permeable to sodium [START_REF] Imai | Heterogeneity of the descending thin limb of Henle's loop[END_REF]. As a result, sodium can be driven down towards the inner medulla [113]. This single effect is restricted to the IS because it requires TALs to run in close proximity of thin limbs. Interestingly, the IS was shown to elongate more than other kidney zones (along the cortico-papillary axis) in situations that involve a high urine concentrating activity (see Section 5 below).

In the C-TAL, the intense sodium reabsorption by a water-tight epithelium results in a large transepithelial difference in sodium concentration and osmolarity.

Along the C-TAL, the luminal fluid reaches a lower limit of dilution that cannot be exceeded because of the backflux of sodium and/or the leakage of water (no epithelium can be 100 % water-tight). This limit has been named by Maurice Burg the "static head" [START_REF] Burg | Thick ascending limb of Henle's loop[END_REF] (Figure 1). This limit is probably reached at a variable distance along the C-TAL. Most micropuncture studies of the earliest accessible site of the distal tubule report a relatively narrow range of luminal fluid osmolarity (or tubular fluid-to-plasma osmolarity ratio) under various experimental conditions. In contrast, in the same studies, the sodium concentration in the early distal tubule fluid (or the tubular fluid-to-plasma sodium concentration ratio) may vary within a much larger range [START_REF] Bankir | Is the process of urinary urea concentration responsible for a high glomerular filtration rate?[END_REF]119,166,167]. These observations suggest that the "static head" reached along the C-TAL is determined by the maximum achievable transepithelial osmolarity difference, not by the maximum achievable sodium concentration difference [START_REF] Bankir | Protein-and diabetes-induced glomerular hyperfiltration: role of glucagon, vasopressin, and urea[END_REF][START_REF] Bankir | Is the process of urinary urea concentration responsible for a high glomerular filtration rate?[END_REF].

The localization of the static head may vary according to several factors. A shift in this static head toward a more superficial part of this segment may provide a "functional reserve" of TAL transport capacity.

The major role of the TAL in both urine concentration and urine dilution is well illustrated by the fact that urine, in healthy humans, becomes iso-osmotic to plasma after the total inhibition of NaCl transport in this segment by the administration of a large dose of furosemide (40 mg i.v.). With such abolition of TAL function, urine is no longer concentrated nor diluted [START_REF] Chun | Ethnic differences in renal responses to furosemide[END_REF].

Anatomy and vascular and tubular environment

The TAL is a part of the loop of Henle. It ascends through the inner and outer stripes of the outer medulla (IS and OS, respectively) and through the medullary rays of the cortex. Although this term is rarely used, the TAL is the "pars recta" of the distal tubule [114,115]. In rodents, the M-TAL exhibits a similar length for all nephrons, whether they have short or long loops of Henle. In contrast, the C-TAL exhibits a variable length, depending on the location of the parent glomerulus in the cortex (Figure 2). It is important to note that juxtamedullary nephrons have almost no C-TAL, while the most superficial nephrons have the longest C-TAL. Consequently, the dilution of tubular fluid is very limited in juxtamedullary nephrons whereas it is more intense in superficial nephrons. In primates, including humans, and in some mammals living in fresh water (beavers), the transition from the thin descending limb to the TAL occurs at variable depths into the OM. Moreover, a relatively large fraction of superficial nephrons exhibit a very short loop of Henle that does not penetrate at all in the medulla (called "cortical nephrons") and thus totally lack a M-TAL [START_REF] Bankir | Urinary concentrating ability: insights from comparative anatomy[END_REF][START_REF] Bankir | Renal medulla][END_REF]. In carnivores, all nephrons have a long loop that penetrates, at least for a short distance, into the inner medulla.

Although no abrupt subsegmentation is visible along the TAL, several anatomical and functional differences between the medullary and cortical TAL have been described [116]. Actually, it might be appropriate to further subdivide the M-TAL into IS-TAL and OS-TAL because the vascular and tubular environments of these two subsegments are quite different. These differences are summarized in Table 1, and beautifully illustrated in two movies about the human and dog kidney made by R.

Beeuwkes and available on the ISN Academy website [19,20]. When considering only two subsegments in the TAL, the limit between the two should be located at the IS/OS transition rather than at the OS/cortex border because the descending structures facing the IS-TAL and the OS-TAL are markedly different: thin descending limbs in the IS, and proximal pars recta in the OS. The tubular environment of the TALs in the OS resembles that in the medullary rays of the cortex, thus making the OS-TAL environment more similar to that of the C-TAL, although their vascular environment is different. Note that many in vitro studies of M-TALs isolated by microdissection or by selective sieving, probably concern exclusively IS-TALs because this subsegment is much easier to isolate from surrounding structures than the OS-TAL [180].

The IS-TALs are surrounded by a dense capillary network issued from lateral branches of descending arterial vasa recta (Figure 3). This blood has undergone filtration in the juxtamedullary glomeruli and thus, most likely, has a higher oncotic pressure and hematocrit than cortical blood because proteins and blood cells are not filtered. The surrounding interstitium is hyper-osmotic, due to the accumulation of solutes (mainly NaCl) by counter-current multiplication. The IS-TALs lie at a distance from the vascular bundles, which supply blood to, and return blood from the inner medulla (Figure 3). The venous vasa recta issued from the IS do not join the vascular bundles but ascend directly through the OS where they surround the descending pars recta and ascending OS-TALs. They empty directly into the arcuate veins [START_REF] Beeuwkes | The vascular organization of the kidney[END_REF][START_REF] Beeuwkes | Tubular organization and vasculartubular relations in the dog kidney[END_REF]113,116]. There are almost no arterial capillaries in the OS because the efferent arterioles of the juxtamedullary glomeruli give very few lateral branches before breaking up into numerous parallel descending vasa recta (Figure 3). The "nutrient" blood flow of OS-TALs is mainly provided by vasa recta ascending from the inner medulla and the IS. In some desert-adapted species, these vasa recta may even ascend in the medullary rays of the deep cortex and merge with the interlobular veins, thus bringing the OS environment up to the deep part of the cortex [START_REF] Bankir | The vascular organization of the kidney of Psammomys obesus[END_REF]104].

In the cortex, the C-TALs lie in the medullary rays and are surrounded by capillaries issued from the efferent arterioles of superficial or mid-cortical glomeruli [19][20][START_REF] Beeuwkes | The vascular organization of the kidney[END_REF][START_REF] Beeuwkes | Tubular organization and vasculartubular relations in the dog kidney[END_REF]. Due to the very high blood flow in the cortex, the cortical interstitium is isoosmotic, at least in the "labyrinth" regions (comprising convoluted proximal and distal tubules, and glomeruli).

The total cross-sectional surface area of the tubule does not change much from M-TAL to C-TAL but the epithelium becomes thinner and thus, the lumen wider [116]. Because of the very low permeability to water, the flow of luminal fluid remains fairly unchanged along the whole TAL but the velocity of the fluid should decline progressively since the lumen widens. The contact time of the tubular fluid with the epithelium should thus be longer in the C-TAL than in the M-TAL.

The TAL epithelium is characterized along its whole length by an extremely high density of mitochondria, and very abundant basolateral infoldings interdigitating between adjacent cells, and closely associated with the mitochondria (Figure 4) [116]. In contrast, the luminal membrane shows no infoldings and very few microvilli, at least in the M-TAL. The ratio of luminal-to-basolateral membrane surface area in the IS-TAL is probably the lowest of all nephron segments [116,151]. The surface area of basolateral membrane of epithelial cells in the OS-TAL is 25% smaller than that in the IS-TAL and the mitochondrial inner membrane is 40% shorter, suggesting a less intense solute transport activity in the OS-TAL [151].

A few studies described two different cell types in the TAL based on either their morphology or their electrical properties (Figure 5) [128,192]. Allen and Tisher [START_REF] Allen | Morphology of the ascending thick limb of Henle[END_REF] described that some cells have a rough luminal surface with prominent microvilli (R cells) while others have a smooth surface, generally devoid of extensive microvilli except along the lateral cell margins (S cells). S cells are more abundant than R cells in the IS-TAL, but the proportion of R cells increases progressively in the OS-TAL and C-TAL. In the study of Yoshitomi et al [194], microelectrode empalement of cells in microdissected M-TAL of hamster kidneys revealed two cell populations with either a high or a low basal conductance (High-BC and Low-BC, respectively). Tsuruoka et al [182], also studying the TAL of the hamster kidney, brought evidence that the Rand S-cells correspond to the Low-BC and High-BC cells and secrete or reabsorb K + , respectively [182].

In summary, IS-TAL, OS-TAL and C-TAL do not share the same environment and differ by some characteristics of their cells. In addition, the epithelium of these subsegments comprises in various proportions cells with distinct morphological and functional features.

Molecular characteristics

Enzymes, channels, transporters and transport properties

Na,K-ATPase activity is detected in all nephron segments, with a much higher intensity in the TAL than in the proximal tubule or collecting duct. No difference between the M-TAL and the C-TAL regarding the Na,K-ATPase activity has been reported in the rat or mouse, whereas the activity is lower in the C-TAL than in the M-TAL in the rabbit [106].

A major membrane ion transporter in the TAL is the Na-K-2Cl cotransporter NKCC2 that allows entry of NaCl from the tubular fluid into the cells [START_REF] Ares | Molecular regulation of NKCC2 in the thick ascending limb[END_REF]. In human, rat, mouse and rabbit, SLC12A1, the gene encoding NKCC2, is translated into various NKCC2 transcripts: three full-length NKCC2 isoforms (NKCC2A, NKCC2B, and NKCC2F); in the mouse, additional isoforms with truncated carboxy-terminal ends have been reported [START_REF] Castrop | Isoforms of renal Na-K-2Cl cotransporter NKCC2: expression and functional significance[END_REF]. In the rat, mouse and rabbit kidney, the F isoform is expressed in the M-TAL, the B isoform in the C-TAL and the A isoform in both subsegments, with few interspecies differences [137,149,164,193] (Figure 7). In humans, the NKCC2A transcript is the most intensely expressed [START_REF] Carota | Localization and functional characterization of the human NKCC2 isoforms[END_REF]. The three transcripts can be detected throughout the TAL; however, NKCC2 -A and -F isoforms are mostly expressed in the OS-OM, whereas NKCC2B is mostly expressed in the cortex [START_REF] Carota | Localization and functional characterization of the human NKCC2 isoforms[END_REF]. In rabbit and rodents, the order of apparent affinities for chloride is NKCC2B > NKCC2A > NKCC2F [START_REF] Castrop | Isoforms of renal Na-K-2Cl cotransporter NKCC2: expression and functional significance[END_REF][START_REF] Gagnon | Functional comparison of renal Na-K-Cl cotransporters between distant species[END_REF][START_REF] Gimenez | Spatially distributed alternative splice variants of the renal Na-K-Cl cotransporter exhibit dramatically different affinities for the transported ions[END_REF]153], whereas in human the chloride affinity declines from NKCC2B to NKCC2F then to NKCC2A [START_REF] Carota | Localization and functional characterization of the human NKCC2 isoforms[END_REF]. In all species investigated so far, the isoform(s) with the lowest affinity is more abundantly expressed in the M-TAL, consistent with the high chloride concentration of the fluid delivered to this segment; conversely, the isoform with the highest affinity is expressed in the C-TAL, allowing chloride concentration to decrease below that of the surrounding extracellular fluid.

The ATP-sensitive, inwardly rectifying potassium channel ROMK (Kir1.1) is the low conductance potassium channel expressed at the apical membrane of TAL cells, from the IS-OM up to the macula densa, allowing potassium to recycle across the apical membrane [192]. No obvious difference in ROMK expression between M-TAL and C-TAL can be seen; however, a few C-TAL cells do not express ROMK, whereas all express NKCC2 [START_REF] Ecelbarger | Regulation of potassium channel Kir 1.1 (ROMK) abundance in the thick ascending limb of Henle's loop[END_REF]128,192], as shown in Figure 5.

Chloride exits the cell via CLC-K channels inserted in the basolateral membrane. The TAL expresses two distinct CLC-K channels, named CLC-K1 and CLC-K2 in the mouse, and CLC-KA and CLC-KB in humans [186]. CLC-K2 is expressed along the entire TAL, whereas CLC-K1 is expressed only in the M-TAL

[108]. CLC-K channel function requires the presence of a subunit named Barttin, the absence of which creates a renal loss of NaCl referred to as Bartter's syndrome type 4 [START_REF] Birkenhager | Mutation of BSND causes Bartter syndrome with sensorineural deafness and kidney failure[END_REF], whereas inactivating mutations of CLC-KB in humans cause Bartter's syndrome type 3.

The TAL reabsorbs about 15 % of filtered bicarbonate. The Na/H exchanger NHE2 is highly expressed at the apical membrane of C-TAL cells, and less in the M-TAL [START_REF] Chambrey | Immunolocalization of the Na+/H+ exchanger isoform NHE2 in rat kidney[END_REF]; in the latter, the main apical transporter involved in bicarbonate absorption is the Na/H exchanger NHE3. At the basolateral membrane, bicarbonate is extruded mainly via the Cl/HCO 3 exchanger AE2 [START_REF] Bourgeois | Basolateral membrane Cl(-)-, Na(+)-, and K(+)-coupled base transport mechanisms in rat MTALH[END_REF] in the M-TAL, whereas a robust Na-HCO 3 cotransport activity has been reported in the C-TAL [111].

The functional properties of the TAL rely heavily on the specificities of the paracellular pathway. The backbone of this paracellular route of transport is the multimeric tight junction (TJ) complex that is composed of structural-and anchoring proteins. Most important for the permeability properties are members of the claudin family [START_REF] Gunzel | Claudins and the modulation of tight junction permeability[END_REF]. In the TAL, several claudin proteins are expressed under basal conditions: claudin 3, 10b, 16 and 19 [START_REF] Gunzel | Claudins and the modulation of tight junction permeability[END_REF]. However, these claudin proteins exhibit distinct patterns of expression along the mouse TAL. Cldn 10b is highly expressed in the IS and is also detected in OS and cortex whereas Cldn 16 is virtually absent from the IS but highly expressed in both OS and cortex [START_REF] Gong | ILDR1 is important for paracellular water transport and urine concentration mechanism[END_REF]. Mice lacking the Ig-like domain containing receptor one (Ildr1), a protein expressed at the tricellular junction of the distal nephron, including TAL, display a significantly higher permeability to water in C-TAL than normal mice [START_REF] Gong | ILDR1 is important for paracellular water transport and urine concentration mechanism[END_REF]. M-TAL was not studied.

The main transcellular and paracellular transport characteristics of the different subsegments of the TAL are schematized in Figure 6, mostly reflecting what was observed in rats and/or mice. However, the electrolyte transport properties of the M-TAL and C-TAL differ significantly among species.

NaCl transport

The main solute absorbed in both M-TAL and C-TAL is NaCl. In both subsegments, NaCl absorption creates a lumen-positive transepithelial potential difference PD te . Although the molecular mechanisms involved in NaCl absorption in both subsegments are quite similar, the properties of M-TAL and C-TAL differ. When isolated tubules are studied under symmetrical conditions, PD te is consistently higher in C-TAL than in MTAL, both in rat and mouse [START_REF] Breiderhoff | Deletion of claudin-10 rescues claudin-16-deficient mice from hypomagnesemia and hypercalciuria[END_REF][START_REF] Friedman | Basal and hormone-activated calcium absorption in mouse renal thick ascending limbs[END_REF][START_REF] Hebert | NaCl transport in mouse medullary thick ascending limbs. I. Functional nephron heterogeneity and ADHstimulated NaCl cotransport[END_REF]122,124,152,183]. In both species, the M-TAL and C-TAL are more permeable to sodium than to chloride, as the ratio of sodium to chloride permeability (P) is greater than unity, whatever the experimental method used to determine it [START_REF] Breiderhoff | Deletion of claudin-10 rescues claudin-16-deficient mice from hypomagnesemia and hypercalciuria[END_REF][START_REF] Breiderhoff | Deletion of claudin-10 (Cldn10) in the thick ascending limb impairs paracellular sodium permeability and leads to hypermagnesemia and nephrocalcinosis[END_REF][START_REF] Burg | Function of the thick ascending limb of Henle's loop[END_REF][START_REF] Greger | Cation selectivity of the isolated perfused cortical thick ascending limb of Henle's loop of rabbit kidney[END_REF][START_REF] Hebert | NaCl transport in mouse medullary thick ascending limbs. I. Functional nephron heterogeneity and ADHstimulated NaCl cotransport[END_REF][START_REF] Himmerkus | AVP dynamically increases paracellular Na(+) permeability and transcellular NaCl transport in the medullary thick ascending limb of Henle's loop[END_REF]110,131,156]. However, the ratio between P Na and P Cl is highly variable, ranging from 1.4 to more than 6 without any obvious link to the medullary or cortical origin of the tubular segment. In the rabbit, the M-TAL reabsorbs more NaCl than the C-TAL [START_REF] Burg | Function of the thick ascending limb of Henle's loop[END_REF]156,171] whereas the converse is reported for the rat and mouse [START_REF] Stefano | Effects of glucagon on Na+, Cl-, K+, Mg2+ and Ca2+ transports in cortical and medullary thick ascending limbs of mouse kidney[END_REF][START_REF] Friedman | CO2-stimulated NaCl absorption in the mouse renal cortical thick ascending limb of Henle. Evidence for synchronous Na +/H+ and Cl-/HCO3-exchange in apical plasma membranes[END_REF][START_REF] Hebert | NaCl transport in mouse medullary thick ascending limbs. I. Functional nephron heterogeneity and ADHstimulated NaCl cotransport[END_REF]122,124,154,172]. Of note, the C-TAL can lower the luminal NaCl concentration to as low as 65 mmol/L [156], showing a greater ability to create and sustain a larger transepithelial osmotic gradient than the M-TAL [START_REF] Burg | Function of the thick ascending limb of Henle's loop[END_REF].

Ammonium and bicarbonate transport

The TAL is running between two structures secreting ammonium, the pars recta of the proximal tubule and the collecting duct. More than thirty years ago, Good [START_REF] Good | Effects of potassium on ammonia transport by medullary thick ascending limb of the rat[END_REF] and Garvin et coll. [START_REF] Garvin | Active NH4+ absorption by the thick ascending limb[END_REF] reported that ammonium is actively absorbed across the M-TAL in the rat and rabbit (see review in [START_REF] Good | The thick ascending limb as a site of renal bicarbonate reabsorption[END_REF]). The ammonium absorption by the M-TAL is necessary for the concentration of ammonium in the medullary interstitium and subsequent secretion by the medullary collecting duct [START_REF] Bourgeois | NHE4 is critical for the renal handling of ammonia in rodents[END_REF]. The absorption of ammonia from lumen to interstitium by the M-TAL and secretion into the nearby OM-CD create a short-circuit within the IS, and thus limit the access of this toxic compound to the cortical portions of the nephron where it could diffuse into the cortical blood and get access to the general circulation. The rat C-TAL is able to reabsorb ammonium, but at a lower rate (~50 %) than the M-TAL [START_REF] Good | Ammonia and bicarbonate transport by thick ascending limb of rat kidney[END_REF]. A low ammonium transport in the C-TAL is likely to be protective as ammonium is highly toxic to the renal cortex [142].

In the rat, both the C-TAL and M-TAL reabsorb bicarbonate at a similar rate accounting for most of the bicarbonate absorption in the loop of Henle [START_REF] Good | Ammonia and bicarbonate transport by thick ascending limb of rat kidney[END_REF]. By contrast, the rabbit C-TAL does not reabsorb bicarbonate [START_REF] Iino | Effect of acid-base status in vivo on bicarbonate transport by rabbit renal tubules in vitro[END_REF], likely because it does not contain any carbonic anhydrase activity [START_REF] Dobyan | Renal carbonic anhydrase[END_REF]. Likely, both ammonium and bicarbonate transport in vivo is higher in the M-TAL than in the C-TAL because the load delivered to the M-TAL is higher.

Divalent cation transport

In the rabbit, Suki et al reported a quantitatively similar net calcium absorption by the M-TAL and the C-TAL [175]. In both subsegments, most investigators found that calcium transport was inhibited by luminal furosemide and was passive, driven by the lumen-positive PD te [START_REF] Bourdeau | Voltage dependence of calcium transport in the thick ascending limb of Henle's loop[END_REF][START_REF] Bourdeau | Inhibition of calcium absorption in the cortical thick ascending limb of Henle's loop by furosemide[END_REF][START_REF] Imai | Effects of parathyroid hormone and N6,O2'-dibutyryl cyclic AMP on Ca2+ transport across the rabbit distal nephron segments perfused in vitro[END_REF]169,174].

Several studies reported that the mouse M-TAL does not absorb calcium [START_REF] Bailly | Isoproterenol increases Ca, Mg, and NaCl reabsorption in mouse thick ascending limb[END_REF][START_REF] Stefano | Effects of parathyroid hormone and calcitonin on Na+, Cl-, K+, Mg2+ and Ca2+ transport in cortical and medullary thick ascending limbs of mouse kidney[END_REF][START_REF] Stefano | Effects of glucagon on Na+, Cl-, K+, Mg2+ and Ca2+ transports in cortical and medullary thick ascending limbs of mouse kidney[END_REF]190]. Only one study, to our knowledge, reported a significant absorption of calcium by the mouse M-TAL [START_REF] Friedman | Basal and hormone-activated calcium absorption in mouse renal thick ascending limbs[END_REF]. In both rat and mouse C-TAL, the activation of the basolateral calcium-sensing receptor CaSR decreases the paracellular permeability to calcium [START_REF] Desfleurs | Calcium-sensing receptor: regulation of electrolyte transport in the thick ascending limb of Henle's loop[END_REF]124]. In aggregate, these studies suggest that calcium transport occurs mostly in the C-TAL. Functionally, it makes sense to limit calcium absorption across the IS-TAL to avoid an excessive increase in interstitial medullary calcium content and thus limit the risk of nephrocalcinosis. Magnesium absorption has not been assessed in the rabbit M-TAL. In the rat and mouse M-TAL studied under control (symmetrical) conditions, and whatever the values of lumen-positive voltage, the net magnesium flux was always very limited, most often not significantly different from zero [START_REF] Bailly | Isoproterenol increases Ca, Mg, and NaCl reabsorption in mouse thick ascending limb[END_REF][START_REF] Stefano | Effects of parathyroid hormone and calcitonin on Na+, Cl-, K+, Mg2+ and Ca2+ transport in cortical and medullary thick ascending limbs of mouse kidney[END_REF][START_REF] Stefano | Effects of glucagon on Na+, Cl-, K+, Mg2+ and Ca2+ transports in cortical and medullary thick ascending limbs of mouse kidney[END_REF]126,190]. In contrast, some alterations in the movement of divalent cations may be observed under conditions in which net NaCl transport and PD te are suppressed [189]. Likely, the secretion is accounted for by a transcellular process, the mechanism of which remains unsettled.

In the rabbit, rat and mouse C-TAL, magnesium absorption is strongly correlated to PD te , as already described for calcium; when the lumen is positive with respect to the peritubular space, a significant absorption of magnesium occurs [START_REF] Stefano | Transepithelial Ca2+ and Mg2+ transport in the cortical thick ascending limb of Henle's loop of the mouse is a voltage-dependent process[END_REF]126,169]. This observation supports the view that magnesium transport in the C-TAL is passive, along the paracellular pathway.

Water transport

In most studies, no net fluid transport was observed in either C-TAL or M-TAL isolated from rats and perfused under symmetrical conditions [START_REF] Good | Ammonia and bicarbonate transport by thick ascending limb of rat kidney[END_REF]107,163], or from mice or rabbits [START_REF] Burg | Thick ascending limb of Henle's loop[END_REF]. However, when perfused under asymmetrical conditions in the presence of peritubular AVP, a significant water transport (~15 % of the perfusion rate) was measured across the rat M-TAL [START_REF] Good | Effects of osmolality on bicarbonate absorption by medullary thick ascending limb of the rat[END_REF], showing that this tubular segment is not fully impermeable to water. It is however unlikely that the experimental conditions set in this study (a large transepithelial osmotic gradient) exist in vivo in the M-TAL. A larger osmotic gradient exists across the C-TAL epithelium. However, to our knowledge, no study has measured the water flux across the C-TAL.

Urea transport

Several facilitated urea transporters are expressed in specific structures within the kidney (terminal collecting duct, thin descending limb of Henle's loop, and arterial vasa recta) [START_REF] Fenton | Urea transporter knockout mice and their renal phenotypes[END_REF]. None of these transporters is expressed in the TAL. To our knowledge, only one study evaluated the permeability of the TAL to urea [107]: the permeability coefficient was significantly greater by about 50% in the C-TAL and OS-TAL than in the IS-TAL. Several micropuncture studies in various rodent species have reported urea concentrations and flux in the early distal tubule fluid (thus, at the exit of the C-TAL) far greater than those in the late proximal tubule, demonstrating a large addition of urea in the loop of Henle [START_REF] De Rouffignac | Micropuncture study of water, electrolytes, and urea movements along the loops of henle in psammomys[END_REF]118,158]. Thus, even if some urea is reabsorbed in the C-TAL, the concentration of urea at the macula densa (as judged from its concentration in early distal fluid) is far higher than that in the blood. It has been proposed that this may indirectly play a role in the regulation of the glomerular filtration rate by influencing inversely the minimum concentration of NaCl reaching the macula densa [START_REF] Bankir | Protein-and diabetes-induced glomerular hyperfiltration: role of glucagon, vasopressin, and urea[END_REF].

Hormonal control of ion transport in the TAL

Receptors to peptidic hormones

The main biochemical and enzymatic differences that have been observed between the M-TAL and C-TAL are summarized in Table 1. The stimulation of adenylyl cyclase activity by various hormones in M-and C-TAL has been carefully studied, mostly by the group of François Morel, with single-tubule adenylyl cyclase microassays [101,132,133]. Both segments of the TAL express adenylyl cyclase sensitive to several peptidic hormones but the responsiveness may be different between the C-TAL and the M-TAL and may differ among species (Table 2). Figure 7 shows the intensity of hormone-induced increase in adenylyl cyclase activity of the rat kidney [132]. AVP stimulates this enzyme activity in both M-TAL and C-TAL in the rabbit, rat and mouse kidney [132-135] but not in the human kidney [START_REF] Chabardes | Adenylate cyclase responsiveness to hormones in various portions of the human nephron[END_REF]157] (Table 2). In the rabbit at least, the concentration of AVP necessary to stimulate adenylyl cyclase is one order of magnitude higher in the TAL than in the collecting duct system [100, 134, 135]. A functional study in vivo also showed that a 5-fold higher AVP infusion rate is needed to stimulate solute transport in the TAL than water permeability in the collecting duct [START_REF] Elalouf | Sensitivities of rat kidney thick ascending limbs and collecting ducts to vasopressin in vivo[END_REF]. This suggests that the effect of AVP in the TAL occurs only at relatively high concentrations of the hormone.

PTH has no effect on adenylyl cyclase activity in the rabbit and rat M-TAL but elicits a 15-fold increase in activity in the C-TAL [START_REF] Chabardes | PTH sensitive adenyl cyclase activity in different segments of the rabbit nephron[END_REF] (Figure 7). In humans, PTH increases adenylyl cyclase activity in both subsegments [START_REF] Chabardes | Adenylate cyclase responsiveness to hormones in various portions of the human nephron[END_REF].

Calcitonin-sensitive adenylate cyclase in the rabbit nephron was found to be more intense in the M-TAL than in the C-TAL [START_REF] Chabardes | Distribution of calcitonin-sensitive adenylate cyclase activity along the rabbit kidney tubule[END_REF], whereas in the rat kidney, glucagon-sensitive adenylate cyclase activity is very intense in the M-TAL and less intense in the C-TAL (Figure 7) [START_REF] Bailly | The distal nephron of rat kidney: a target site for glucagon[END_REF].

In the rat kidney, the G protein alpha S subunit (G s  is intensely expressed at the basolateral membrane of M-TAL and C-TAL cells [173]. Note that the GNAS1 gene encoding G s  is not imprinted in the TAL, at variance with the proximal tubule where the maternal allele is preferentially expressed. No expression of G i  2 is found in M-TAL or C-TAL whereas G i  1 is apically expressed by M-TAL in OS and C-TAL, but absent from the IS; G i  3 is intensely expressed in macula densa cells [173].

Finally, adenylyl cyclase VI and IX genes are expressed in the rat TAL, with no evidence for a different expression between C-TAL and M-TAL [START_REF] Bek | Differential expression of adenylyl cyclases in the rat nephron[END_REF].

The similarities and differences regarding the expression of 6 distinct receptors to peptidic hormone are summarized in Table 3 [4, 44, 45, 51, 54, 70, 71, 92, 120, 121, 125, 138-141, 147, 148, 155, 162, 168, 176, 177]. The most striking differential expression is that of parathyroid hormone receptor type 1, which is highly expressed in C-TAL but not detectable in IS-TAL (at least in rodents).

Control of ion transport in the TAL: Peptidic hormones

Following the studies revealing that several peptidic hormones stimulate adenylate cyclase activity in the TAL [132], a number of micropuncture studies and in vitro studies of isolated microperfused tubules have been performed to evaluate the influence of these hormones on ion transport in the TAL. Studies in the mouse have shown that the regulation differs between the two TAL subsegments. In the M-TAL, AVP, glucagon and insulin stimulate NaCl but not magnesium or calcium transport [START_REF] Stefano | Effects of parathyroid hormone and calcitonin on Na+, Cl-, K+, Mg2+ and Ca2+ transport in cortical and medullary thick ascending limbs of mouse kidney[END_REF][START_REF] Stefano | Effects of glucagon on Na+, Cl-, K+, Mg2+ and Ca2+ transports in cortical and medullary thick ascending limbs of mouse kidney[END_REF]126,127,188,190]. PTH has no effect on the M-TAL [187]. In contrast, in the C-TAL, PTH, calcitonin, glucagon, and AVP stimulate NaCl, as well as magnesium and calcium transport [START_REF] Stefano | Effects of parathyroid hormone and calcitonin on Na+, Cl-, K+, Mg2+ and Ca2+ transport in cortical and medullary thick ascending limbs of mouse kidney[END_REF][START_REF] Stefano | Effects of glucagon on Na+, Cl-, K+, Mg2+ and Ca2+ transports in cortical and medullary thick ascending limbs of mouse kidney[END_REF]188,190]. However, the effect on calcium and magnesium absorption is proportionaly greater than that on Na and Cl transport, suggesting that the four hormones are able to increase the paracellular pathway permeability to divalent cations. In the rabbit C-TAL, PTH and calcitonin stimulate the divalent cation transport but not that of NaCl [169, 174]. In the rat M-TAL, glucagon decreases net bicarbonate absorption. The molecular basis of the differences between species and between M-TAL and C-TAL are relatively easy to understand, based on the studies that quantified the formation of cAMP upon exposure to various hormones (Figure 7 and Table 2) [132]. Table 4 summarizes the main results of these studies [START_REF] Bourdeau | Effect of PTH on calcium transport across the cortical thick ascending limb of Henle's loop[END_REF][START_REF] Stefano | Effects of parathyroid hormone and calcitonin on Na+, Cl-, K+, Mg2+ and Ca2+ transport in cortical and medullary thick ascending limbs of mouse kidney[END_REF][START_REF] Stefano | Effects of glucagon on Na+, Cl-, K+, Mg2+ and Ca2+ transports in cortical and medullary thick ascending limbs of mouse kidney[END_REF][START_REF] Friedman | Basal and hormone-activated calcium absorption in mouse renal thick ascending limbs[END_REF][START_REF] Good | Inhibition of bicarbonate absorption by peptide hormones and cyclic adenosine monophosphate in rat medullary thick ascending limb[END_REF][START_REF] Good | Angiotensin II inhibits HCO-3 absorption via a cytochrome P-450-dependent pathway in MTAL[END_REF][START_REF] Hall | Effect of vasopressin on electrical potential difference and chloride transport in mouse medullary thick ascending limb of Henle's loop[END_REF][START_REF] Hebert | NaCl transport in mouse medullary thick ascending limbs. I. Functional nephron heterogeneity and ADHstimulated NaCl cotransport[END_REF][START_REF] Imai | Effects of parathyroid hormone and N6,O2'-dibutyryl cyclic AMP on Ca2+ transport across the rabbit distal nephron segments perfused in vitro[END_REF]120,122,127,169,175,188,190].

Control of ion transport in the TAL: Prostaglandins

In the rabbit, Stokes showed that PGE 2 decreases by about 40 % both net chloride absorption and PD te in the M-TAL but not in the C-TAL [171]. In the rat, endogenous PGE 2 was shown to inhibit chloride reabsorption in the M-TAL [150].

Further studies showed that PGE 2 decreases the maximal velocity but not the affinity of the cotransporter NKCC2 in the mouse M-TAL [105]. In the medulla, interstitial cells, not tubular cells, are the site of intense COX-2 expression (reviewed in [START_REF] Harris | Physiological regulation of cyclooxygenase-2 in the kidney[END_REF]) and COX-2 expression increases in response to a high salt diet and to water deprivation [START_REF] Chen | Increased dietary NaCl induces renal medullary PGE2 production and natriuresis via the EP2 receptor[END_REF][START_REF] Harris | Cyclooxygenase-2 and the kidney: functional and pathophysiological implications[END_REF].

Metabolic activity

TALs exhibit an intense metabolic activity for supporting the active NaCl pumping and the synthesis of uromodulin. The oxygen and nutrient supply is abundant in the cortex, which receives a high blood flow per unit tissue mass, but it is much more limited in the outer medulla. In the OM, OS-TALs are the nephron segments that mostly contribute to the metabolic activity whereas in the cortex, the C-TALs are only a small fraction of the metabolic demand because of the large volume occupied by the proximal tubules.

In the 1980's, the group of Frank Epstein performed a number of studies intended to evaluate the vulnerability of the kidney to the lack of energetic substrates [START_REF] Brezis | Selective glutathione depletion on function and structure of the isolated perfused rat kidney[END_REF][START_REF] Epstein | Improved function with amino acids in the isolated perfused kidney[END_REF]. The urine concentrating ability and the histology of the isolated perfused rat kidney were assessed when dDAVP, a vasopressin V2 receptor agonist, was added to the infusion fluid. In kidneys perfused with glucose as the sole energetic substrate, the urine concentrating ability declined over time, and degenerative changes of the M-TALs appeared and worsened throughout the following 80 min. When glutathion or its precursors, cystein, glutamic acid and glycine, were added to the perfusate, these abnormalities were ameliorated, showing how important is the energetic supply for the M-TALs [START_REF] Brezis | Selective glutathione depletion on function and structure of the isolated perfused rat kidney[END_REF][START_REF] Epstein | Improved function with amino acids in the isolated perfused kidney[END_REF]. The lesions (chromatin margination, mitochondrial swelling and even cell fragmentation) appeared only in M-TALs and mainly in those remote from vascular bundles and near the inner medulla, areas most likely to be anoxic.

Supplementation of the perfusion medium with hemoglobin or with erythrocytes

prevented the M-TAL injury [START_REF] Brezis | Selective vulnerability of the medullary thick ascending limb to anoxia in the isolated perfused rat kidney[END_REF]123], thus showing the importance of both appropriate substractes and proper oxygenation.

A subsequent in vivo study evaluated the development of acute renal failure in rats submitted to several simultaneous insults (uninephrectomy + salt depletion + indomethacin + administration of radiocontrast media). Injury was confined to the OM with selective necrosis of the M-TALs. The lesions were most severe in the deepest part of the OM and away from the vasa recta. Other nephron segments were free of damage except for some vacuolization of the proximal tubules [START_REF] Heyman | Acute renal failure with selective medullary injury in the rat[END_REF]. Altogether, these findings suggest that the M-TALs are exquisitely susceptible to anoxic damage because of their high metabolic rate and their relatively low blood supply. In contrast, the C-TALs are well preserved because they lie in the highly perfused cortical tissue.

In addition to its transport activity, the TAL is also involved in an intense metabolic activity for the synthesis of several important compounds, namely uromodulin (UMOD, or Tamm-Horsfall protein = THP), epidermal growth factor, glycogen, etc.... In situ hybridization revealed a progressively increasing labeling of UMOD mRNA in the TAL from its early portion in the IS to its end, beyond the macula densa [START_REF] Bachmann | Tamm-Horsfall protein-mRNA synthesis is localized to the thick ascending limb of Henle's loop in rat kidney[END_REF]. A recent study of the mouse and rat transcriptomes showed that Umod expression is greater in C-TAL than in M-TAL in rodent kidneys, with a similar trend in the human kidney [178]. The different levels of uromodulin expression along the (cortico-medullary) TAL may explain differential responses to loop diuretics and water deprivation observed in mouse models and human subjects harbouring mutations of uromodulin, responsible for autosomal dominant tubulointerstitial kidney disease (ADTKD) [START_REF] Bernascone | A transgenic mouse model for uromodulin-associated kidney diseases shows specific tubulo-interstitial damage, urinary concentrating defect and renal failure[END_REF]117]. Epidermal growth factor mRNA and protein are abundantly expressed in both the M-TAL and C-TAL of the rat kidney, as detected by in situ hybridization and immuno-histochemistry, respectively, but no quantitative data is available [159].

The M-TAL of the rat exhibits a very high ability to deaminate and metabolize branched-chain amino acids that the liver cannot do, and to use them as a metabolic substrate [START_REF] Burch | Branched-chain amino acid aminotransferase along the rabbit and rat nephron[END_REF]179]. This may be useful for supporting the energetic cost of active NaCl transport and UMOD synthesis and secretion. No information is available regarding the C-TAL.

Evolution and adaptation to the environment

Adaptations among different orders or species

Marine elasmobranchs (or cartilaginous fish, including sharks, skates, rays and sawfish) possess a rectal gland with characteristics and transport properties that strongly resemble those of the mammalian TAL, except that the cell polarity is reversed. This gland secretes NaCl into the lumen, thus allowing NaCl excretion. The rectal gland epithelium has been used for the cloning of the first NaCl co-transporter [191], allowing the subsequent cloning of the mammalian homologues [START_REF] Gamba | Molecular cloning, primary structure, and characterization of two members of the mammalian electroneutral sodium-(potassium)-chloride cotransporter family expressed in kidney[END_REF].

In lower vertebrates, the early part of the kidney's distal tubule, called the "diluting segment", has the same characteristics as the mammalian TAL including the ability to reabsorb NaCl while being impermeable to water. However the nephrons are not folded in a loop. The final urine can be diluted below plasma value or reconcentrated at most to the same osmolarity as that of plasma under the influence of the antidiuretic hormone vasotocin, acting on the collecting system and bladder; but urine cannot become hyper-osmotic. This segment is thus similar to the mammalian C-TAL [START_REF] Bankir | Urinary concentrating ability: insights from comparative anatomy[END_REF][START_REF] Bankir | Thick ascending limb--anatomy and function: role in urine concentrating mechanisms[END_REF]. This diluting segment has been used as a model of the TAL in a number of experiments and also served for the expression cloning of some of the membrane molecules found in the TAL.

Among mammals, differences in habitat and in fresh water availability lead to wide differences in the need for urine concentration or dilution. These differences in habitat, along with wide differences in body size, are reflected in the strong allometric relationship of cortical thickness to body size (Figure 8) [START_REF] Beuchat | Structure and concentrating ability of the mammalian kidney: correlations with habitat[END_REF]. They are associated with structural adaptations in M-TAL and/or C-TAL length and volume and in their respective proportions. Marine mammals (dolphins, whales) which permanently live in sea water, a strongly hyperosmotic environment, never need to excrete solute-free water. Their kidney cortex is much thinner than that in other mammals (Figure 8), presumably reflecting shorter C-TAL length and lower transport activity [START_REF] Beuchat | Structure and concentrating ability of the mammalian kidney: correlations with habitat[END_REF].

Accordingly, as shown by Beuchat [START_REF] Beuchat | Structure and concentrating ability of the mammalian kidney: correlations with habitat[END_REF], marine mammals show an overall lower cortical thickness-to-body size ratio, along with a smaller allometric constant compared to terrestrial or fresh-water mammals (Figure 8).

Within the rodent order, a strong specialization is apparent in the vascularization and the vascular-tubular relationships of the kidney medulla, as illustrated in Figure 9 and described in several reviews [START_REF] Bankir | Urinary concentrating ability: insights from comparative anatomy[END_REF]112,113]: the alternance of vascular bundles and interbundle regions is clearly visible in the kidneys of the three rodents (see also pictures in ISN Academy website [START_REF] Bankir | Vascular-tubular relationships in the mammalian kidney[END_REF]). Vascular bundles are much less apparent in the kidney of the gundi (a lagomorph, the same order as that of the rabbit), a species that exhibits a much lower ability to concentrate urine [START_REF] Bankir | Urinary concentrating ability: insights from comparative anatomy[END_REF].

The relative IS thickness, and thus the length of the IS-TALs differ markedly from species to species [START_REF] Bankir | Urinary concentrating ability: insights from comparative anatomy[END_REF], suggesting specific functional adaptations of the urine concentrating mechanism to diet and habitat. In contrast there are no obvious species differences in the vascularization of the cortex, thus in the C-TAL environment, except in some desert-adapted rodents in which the ascending vasa recta penetrate for some distance into the medullary rays and empty in the interlobular veins [START_REF] Bankir | The vascular organization of the kidney of Psammomys obesus[END_REF]. This extends to the deep cortex the possibility of countercurrent exchanges present in the OS.

Individual adaptations within a given species

In addition to evolution-based differences in cortical thickness, the balance between M-TAL and C-TAL activity (involved in urine concentration and in urine dilution, respectively) is somewhat plastic within a given species. For instance, as shown in Figure 10 [START_REF] Bankir | Adaptation of the rat kidney to altered water intake and urine concentration[END_REF][START_REF] Bouby | Selective ADH-induced hypertrophy of the medullary thick ascending limb in Brattleboro rats[END_REF], prolonged infusion of the selective V2 receptor agonist dDAVP in Brattleboro rats with central diabetes insipidus, or partial restriction in fluid intake in normal rats resulted in a significant increase in kidney mass that was characterized by a preferential thickening of the IS, a selective hypertrophy of the IS-TAL and a concomitant atrophy of the C-TAL [START_REF] Bankir | Adaptation of the rat kidney to altered water intake and urine concentration[END_REF][START_REF] Bouby | Selective ADH-induced hypertrophy of the medullary thick ascending limb in Brattleboro rats[END_REF]. Conversely, providing a waterrich food abolished the usual heterogeneity in volume of epithelium observed between the M-TAL and the C-TAL (Figure 10). In line with these morphological changes, in dDAVP-treated Brattleboro rats, the stimulation of adenylate cyclase activity by AVP or glucagon was strongly enhanced in M-TAL but reduced in C-TAL [181].

The elongation of the IS in dDAVP-treated rats provides a longer distance over which the counter-current multiplication process between the M-TAL and the thin descending limbs can operate. Feeding rats a high protein diet imposes an increased need for water economy in the excretion of an increased load of urea and other protein-derived wastes (that are much more concentrated in urine than in plasma).

High protein intake in rats (from 10 to 32% casein) induced a very similar pattern of kidney hypertrophy (Figures 10 and11) to that induced by the stimulation of urine concentrating activity by dDAVP or partial water deprivation [START_REF] Bankir | Adaptation of the kidney to protein intake and to urine concentrating activity: similar consequences in health and CRF[END_REF][START_REF] Bouby | Role of the urinary concentrating process in the renal effects of high protein intake[END_REF]. In response to a high protein diet, the abundance of Na,K-ATPase was increased in the IS-TAL but not in the OS-TAL (C-TAL not measured) (Table 5) [START_REF] Bouby | Effect of high protein intake on sodium, potassiumdependent adenosine triphosphatase activity in the thick ascending limb of Henle's loop in the rat[END_REF]. Such a preferential hypertrophy of the IS and of the M-TAL is not observed during the compensatory hypertrophy of the kidney following a uninephrectomy [START_REF] Bouby | Role of the urinary concentrating process in the renal effects of high protein intake[END_REF]. Thus, it is specific to an enhanced need to concentrate urine, whether induced directly by vasopressin or secondarily by a high protein intake (which is actually known to increase vasopressin secretion [START_REF] Bankir | Adaptation of the kidney to protein intake and to urine concentrating activity: similar consequences in health and CRF[END_REF][START_REF] Bouby | Role of the urinary concentrating process in the renal effects of high protein intake[END_REF]).

Adaptations among different ethnic groups

Recent observations suggest that some adaptations may also exist in humans, among different ethnic groups. For example, African Americans (AAs) have higher plasma vasopressin concentration and a greater ability to concentrate urine than Caucasians (CAs). A better ability to conserve water may result from evolutionary adaptations to the ancestral hot climate in Africa [START_REF] Bankir | Ethnic differences in urine concentration: possible relationship to blood pressure[END_REF] and reveal a possible survival and reproductive disadvantage in those with a lower ability to conserve water (rather than salt [START_REF] Blaustein | The pathogenesis of hypertension: black-white differences[END_REF]102]) during the transatlantic slave transport and hard work in hot climate.

A recent report of the effects of furosemide on urinary electrolyte excretion in AAs and CAs concluded that TAL transport activity is greater in the former [START_REF] Chun | Ethnic differences in renal responses to furosemide[END_REF] as might be expected when water conservation is a priority. However, another study examining the effects of a water load in subjects of these two ethnic groups came to the opposite conclusion, i.e., the TAL activity is less in AAs than CAs [184]. These apparently divergent observations are actually not in conflict because each of them reflects a different function of the TAL. The water load tested the diluting activity permitted by the C-TAL whereas the furosemide tested the contribution of the M-TAL to the urine concentrating activity. This duality is also illustrated by findings in rats, as shown in Figure 10 B: the stimulation of the urine concentrating activity by a chronic dDAVP infusion induced a hypertrophy of the M-TAL and an atrophy of the C-TAL. In contrast, a chronic increase in fluid intake reduced the volume of M-TAL and induced a small but distinct increase in the volume of the C-TAL [START_REF] Bankir | Adaptation of the rat kidney to altered water intake and urine concentration[END_REF][START_REF] Bouby | Selective ADH-induced hypertrophy of the medullary thick ascending limb in Brattleboro rats[END_REF]. Most probably, AAs have a higher M-TAL transport activity that allows them to increase urine concentration in times of low water availability or more abundant extra-renal water loss. But they also display a lower capacity for solute-free water generation by the C-TAL that delays the excretion of acute large water loads. In general outline, this theory parallels that of Neel's "thrifty genotype" [144] which was proposed to explain the human tendency to develop "diseases of civilization" when humans transitioned from a traditional hunter-gatherer lifestyle to the modern environment of excess.

Consequences of TAL heterogeneity for the understanding of pathophysiology

During chronic kidney disease (CKD), the ability to concentrate urine declines more rapidly than the ability to dilute urine. This relative better preservation of solutefree water generation may be explained by the fact that urine dilution depends largely on sodium reabsorption by the C-TAL which lies in a kidney region where blood flow is extremely abundant. Even if kidney function is partially impaired, oxygen supply for the transport activity in tubules lying in the cortex is still in large excess to the needs (because of the much higher perfusion rate of the kidney than that in other organs).

In contrast, the blood supply to the medulla is much less abundant, even in the healthy kidney. During CKD, juxtamedullary nephrons, those which provide blood to the medulla via their efferent arterioles, undergo more glomerulosclerosis than superficial or mid-cortical nephrons [145,170]. This may be explained by the higher blood pressure to which their glomeruli are exposed compared to that in more superficial nephrons, because their afferent arterioles arise from the earliest part of the interlobular arteries. Moreover, there is a strong drop in the local blood supply along the medullary vasculature due to the high resistance of the long vasa recta.

Accordingly, the highly energy-demanding M-TALs are exposed to partial anoxia resulting in cell damage and death [START_REF] Bankir | Vascular contributions to pathogenesis of acute renal failure[END_REF][START_REF] Brezis | Hypoxia of the renal medulla--its implications for disease[END_REF]165]. This compromises the transport activity of the M-TAL and thus the building up and maintenance of the osmotic gradient in the inner medulla. With less solutes accumulated in the medullary interstitium, the urine concentrating ability declines.

Several studies suggest that an excessive reabsorption of sodium chloride in the TAL may contribute to hypertension [START_REF] Ares | Molecular regulation of NKCC2 in the thick ascending limb[END_REF]103,195]. This additional reabsorption may occur when the load of NaCl delivered to the M-TAL is increased. This extrareabsorption should improve the ability to conserve not only sodium, but also water by powering more efficiently the urine concentrating mechanism. This scenario probably plays a role mainly in salt-sensitive hypertension, as concluded from the studies comparing African Americans and Caucasians [3, 55, 184] [185]. By contrast, an isolated increase in NKCC2 activity and/or expression, which moves the "static head" backward but does not change the overall amount of NaCl reabsorbed in the TAL, is not likely to be a cause of hypertension.

The different functions of the M-TAL and C-TAL may explain the paradoxical response to furosemide reported in a case of tubulo-interstitial disease due to a mutation in the UMOD gene [START_REF] Devuyst | Uromodulin: from physiology to rare and complex kidney disorders[END_REF]. Devuyst et al performed an investigation in a proband with a missense mutation of UMOD, with still a relatively preserved renal function, and her unaffected sister [117]. They were submitted to an oral dose of furosemide. Contrary to what could be expected, the diuretic response to furosemide and the fall in urine osmolality were more important and detected earlier in the proband than in the control sibling. The authors suggested that, in the proband, NKCC2 transport capacity was reduced in the M-TAL, but this defect was compensated for by an additional NaCl reabsorption along the C-TAL due to a shift forward of the "static head" providing some additional length for this reabsorption [START_REF] Bankir | Thick ascending limb--anatomy and function: role in urine concentrating mechanisms[END_REF][START_REF] Burg | Thick ascending limb of Henle's loop[END_REF].

Patients with HELIX syndrome (OMIM#617671) have a severe renal NaCl wasting stemming from a defect in paracellular Na transport in the TAL that can be traced to a loss of function mutation in the CLDN10B gene [START_REF] Hadj-Rabia | Multiplex epithelium dysfunction due to CLDN10 mutation: the HELIX syndrome[END_REF]. In that respect, patients with the HELIX syndrome resemble patients with Bartter syndrome. However, they have an increased, not decreased, reabsorption of magnesium in the TAL, causing hypermagnesemia. One reason could be that the defect in Na absorption in the M-TAL results in a higher Na concentration in the lumen of the C-TAL and a higher PD te in the cortex, thereby increasing the driving force for Mg absorption in this subsegment. A not exclusive explanation is the higher expression of CLDN16/CLDN19 at the tight junction of C-TAL when CLDN10 is missing, which increases the paracellular permeability to magnesium [129].

Conclusion and perspectives

As highlighted above, the TAL should not be considered as a single unit in our understanding of its functions. Besides some characteristics that are common to the entire TAL, there are many differences at the molecular level and transport properties between the medullary and the cortical parts of this segment. Moreover, these two subsegments have quite different peritubular environments in the outer medulla and cortex.

The most significant functional limit between the two subsegments of the TAL lies at the junction between the inner and outer stripes of the outer medulla, not at the cortico-medullary border. The descending segments facing the TALs below and above this limit are the thin descending limbs and the pars recta of the proximal tubule, respectively. This difference is obviously important because the "single effect" of the concentrating mechanism, allowing sodium reabsorbed by the M-TAL, to enter the thin limbs, thus feeding the counter-current multiplication system, can occur only in the IS. In addition, the vascular environment of the TAL also differs markedly between the IS (capillaries issued from descending vasa recta) and OS (ascending vasa recta running in parallel with the tubules). As a consequence of the different vasculo-tubular relationships in the IS and OS + cortex, the composition of the IS interstitium is largely determined by the ion transport across the M-TAL; in contrast, the ion transport across the C-TAL epithelium has little influence on the composition of the cortical interstitium because of the very high blood flow in this kidney zone.

Besides the description of the similarities and differences between M-TAL and C-TAL summarized in this review, we are facing a gigantic gap in knowledge with regard to the factors responsible for the basal differences between the two subsegments and the differential adaptation of C-TAL and M-TAL to various metabolic conditions. A few examples are given below. i) In rodents, IS-TAL reabsorbs very little calcium and magnesium, as compared to C-TAL. Cldn16 is not detected in the mouse IS-TAL (but Cldn19 is). The factors that prevent Cldn16 expression in the IS-TAL (but allow expression in OS-TAL and C-TAL) are unknown.

ii) In the mouse C-TAL, the mosaic pattern of claudin expression at the tight junction is intriguing and not understood; in addition, the functional consequence of the difference in the composition of tight junctions has not been assessed in detail.

Its potential plasticity, under various metabolic conditions, and the mechanisms therof should be studied.

iii) Overall, the heterogeneity within the TAL cells is higher than initially considered. For example, some cells of the TAL have no detectable ROMK expression at the apical membrane; some cells have a low basolateral potassium conductance whereas others have a high conductance. Again, the factors that determine, and potentially modify this heterogeneity, and the detailed functional consequences are unknown. iv) Independently of adaptations resulting from evolution between species, the factors responsible for the simultaneous hypertrophy of the M-TAL and relative atrophy of the C-TAL in adult animals submitted to a chronic increase in urine concentrating activity (by AVP infusion or high protein diet) have not been determined. v) A few conditions (summarized in section 5), specifically affect the morphology of the C-TAL or the M-TAL. The molecular mechanisms involved in these changes are elusive and the consequences on the various functions of the TAL have not been systematically assessed.

In summary, the functional and environmental differences between the medullary and cortical portions of the TAL mentioned above should be taken into account when interpreting the results of experimental and clinical studies. In addition, they should stimulate further research to improve our understanding of the factors that determine (and potentially alter) these differences. the transepithelial difference remains relatively small because the osmolality of the interstitium also declines from deep inner stripe to cortico-medullary junction (thus, only a small space between the two curves in the medulla). In contrast, in the cortex, where the interstitial osmolality is fairly uniform and very close to plasma osmolality, the active NaCl pumping creates a much larger transepithelial difference in osmolality (thus a wider distance between the two curves), until a limit is reached (called "static head" [START_REF] Burg | Thick ascending limb of Henle's loop[END_REF]) at a certain distance before the end of the TAL. At this limit and downstream, the passive paracellular Na secretion offsets the transcellular active Na absorption and a net flux is no longer present. The distance along the TAL where this limit is reached may probably vary according to various regulatory factors. different along their course through three different regions of the kidney. In the IS, the nutrient blood supply of the TALs is provided by a dense capillary plexus issued from descending arterial vasa recta emerging from the nearby vascular bundles (not visible on this picture). The TALs lie in the interbundle bundle regions. In the OS, the nutrient blood supply of the TALs originates from numerous ascending veins (not visible on this picture). In the cortex, the blood supply of the TALs is provided by the peritubular capillaries issued from the efferent arterioles of superficial and mid-cortical glomeruli (not visible on this picture). The TALs run in "medullary rays" (MR) that also comprise the descending pars recta of proximal tubules, and collecting ducts. Note the lack of cortical TAL in the long-looped nephron. The relative height of the different medullary zones is not similar in all species.

The alternance of vascular bundles and interbundle regions in the inner stripe of outer medulla, and the paucity or arterial blood supply in the outer stripe are visible in all species. Adapted after [START_REF] Bankir | Urinary concentrating ability: insights from comparative anatomy[END_REF]. A. Rats with a high water intake (HWI) received a water-rich food and 5% glucose solution as drinking fluid, while dDAVP rats were infused with this vasopressin V2 receptor agonist via minipumps. After 6 weeks, kidneys were perfusion-fixed and the height of the different renal zones evaluated. As expected, urine osmolality was largely reduced in HWI and markedly increased in dDAVP rats. Increased urine concentration was associated with a marked increase in kidney weight (1362 vs 914 mg/100g body weight), but all renal zones where not affected in the same way. The height of the IS was increased by 66% and that of the OS was reduced by 18%. This means that the IS-TALs were markedly lengthened while the OS-TALs were shortened. B. The volume of epithelium per unit length of tubule was calculated from measurements of surface area of TAL cross sections in serial kidney sections. In control rats, the epithelium of the TALs was somewhat thicker in the IS than in the OS and cortex. In rats treated with dDAVP, this axial heterogeneity was strongly reinforced : the IS-TALs were thickened and the C-TAL thinned. Conversely, this axial heterogeneity was totally abolished in rats receiving a high water intake. In response to chronic stimulation by ADH, TALs in the IS were thus both lengthened and thickened, providing much "power" for the "single effect" of the urinary concentrating mechanism. A, modified, and B, drawn from data in [START_REF] Bankir | Adaptation of the rat kidney to altered water intake and urine concentration[END_REF]. C. Cross sections at mid-inner stripe level in the kidneys of Brattleboro rats with central diabetes insipidus (DI) and DI rats treated with chronic infusion of dDAVP for 6 weeks. The hypertrophy of the epithelium of the TALs (shown by asterisks) is very prominent. Collecting ducts (C) show only a modest increase in epithelium thickness. Adapted from [START_REF] Bouby | Selective ADH-induced hypertrophy of the medullary thick ascending limb in Brattleboro rats[END_REF]. g body weight, in the two groups, respectively. The high protein-induced hypertrophy was characterized by a preferential increase in thickness of the inner stripe of the outer medulla (+54%) while total kidney height, from cortex to papillary tip, increased by only 17%. TAL epithelium was thickened in the IS but not in the OS and cortex (not shown). Adapted from [START_REF] Bouby | Role of the urinary concentrating process in the renal effects of high protein intake[END_REF]. The right panel shows two representative cross sections of corresponding kidneys stained by in vivo infusion of Alcian Blue.

  PTH1R parathyroid hormone receptor type 1 ROMK renal outer medullary potassium channel TAL thick ascending limb (of Henle's loop) • C-TAL cortical thick ascending limb • M-TAL medullary thick ascending limb • OS-TAL thick ascending limb in the outer stripe of the outer medulla • IS-TAL thick ascending limb in the inner stripe of the outer medulla TGF tubulo-glomerular feedback (control of GFR) V2R vasopressin V2 receptor GWAS genome-wide association studies

  [129]. Cldn 19 is expressed in TJ and detected intracellularly in OS-and C-TAL whereas it is only detected intracellularly in IS-TAL [152]. Cldn 14 is detected in few TJ in OS-TAL and cortex under control conditions [152]; on a high calcium-containing diet, Cldn 14 is highly expressed at TJ of OS-TAL and cortex, but not in IS-TAL [152]. Whether a similar pattern exists in human and rat is unknown. The mechanisms underlying this distinct pattern of expression are unknown. However, the heterogeneous expression of claudins along the TAL parallels the distinct paracellular transport properties of the C-TAL and the M-TAL (vide infra). Both M-TAL and C-TAL express very low water permeability. Several. aquaporins, expressed in various segments of the mammalian nephron and collecting duct (AQP1, AQP2, AQP3) [130, 146], have not been detected in the thick ascending limb [146, 161], a finding in good agreement with the ability of this segment to strongly dilute the tubular fluid. A recent study by Gong et al shed some light on the molecular basis of the « impermeability » of the TAL to water

Figure 1 .

 1 Figure 1. A. The TAL actively reasbsorbs NaCl along its entire length and exhibits a negligeable permeability to water. However, with this same functional properties, it participates to both urine concentration (in the medulla) and urine dilution (in the cortex). Counter-current multiplication of a "single effect", created by active NaCl reabsorption (red arrows), can occur only in the inner stripe where the sodium permeable thin descending limbs (TDL) of long loops run close to the ascending thick limbs. In the cortex, active NaCl reabsorption (blue arrows) leads to dilution of the tubular fluid. B. In the medulla, NaCl pumping dilutes the tubular fluid below the osmolality of the surrounding interstitium, but

Figure 2 .

 2 Figure 2. Different types of kidneys in mammals, based on nephron length. A. The typical kidney observed in rodents exhibits a combination of numerous shortlooped and fewer long-looped nephrons. The loops of all short-looped nephrons turn close to the inner stripe-inner medullary border while the loops of longlooped nephrons turn at various levels of the inner medulla. B.In species with a moderate or low urine concentrating ability (human, pig, beaver), many nephrons have a shorter loop of Henle that may turn at various levels through the cortex and outer medulla. A clear distinction between OS and IS within the outer medulla (OM) is difficult to identify. The nephrons whose loop does not penetrate at all in the medulla are called "cortical nephrons". They can participate to urine dilution, but not to urine concentration. C. In carnivores, the loop of all nephrons penetrates in the inner medulla, at least for a short distance.

Figure 3 .

 3 Figure 3. A. Rat kidney vasculature. Arterial vessels and capillaries have been filled with Microfil© silicone rubber. The venous vessels are not visible. Co = cortex; OS and IS = outer and inner stripe of the outer medulla, respectively; IM = inner medulla. B. Diagram of a long-looped and a short-looped nephron. The three different subsegments of the TAL (in the inner stripe, outer stripe and cortex) are shown by different colors. The vascular environment of the TALs is very

Figure 4 .

 4 Figure 4. Survey on location and ultrastructure of the thick ascending limb of Henle's loop. The direction of the urinary flow is indicated by white arrows. C = cortex; OS and IS = outer and inner stripe of the outer medulla, respectively; IZ = inner medulla (inner zone); glomeruli are shown as open circles. Ultrastructure of TAL cells is depicted on the right. a = M-TAL; b = C-TAL; c = macula densa. Note the highly interdigitated cells with large mitochondria, enclosed in lateral processes. Reproduced from [116].

Figure 5 .

 5 Figure 5. Two cell types in the thick ascending limb of the rat kidney. Indirect immunofluorescence micrographs with double staining of the same section. A. Labelling with anti-rBSC1/NKCC2 antibody, showing linear apical membrane staining of C-TAL. B. Labeling with anti-ROMK antibody, showing uneven apical staining (magnifications 3600). C. High-power magnification (31,000) of C-TAL cross section stained with anti-ROMK antibody (rhodamine) showing the heterogeneous staining; open arrow, a ROMK-negative cell, between two ROMK-labeled cells. Reproduced from [192].

Figure 6 .

 6 Figure 6. Diagram illustrating the different transcellular and paracellular transports occuring in the three subsegments of the TAL. The main transporters and channels present in the apical and basolateral membranes are listed on the right, as well as claudins expressed at the tight junctions. Vertical arrows indicate the direction of the tubular fluid flow. Horizontal arrows indicate the mouvements of electrolytes through (black arrows) or between (blues arrows) the cells in each subsegment.

Figure 7 .

 7 Figure 7. Left. Adenylate cyclase activity along the rat nephron in response to stimulation by parathyroid hormone (PTH), salmon calcitonin (SCT), vasopressin (AVP) and glucagon. Dot density indicates the intensity of the responses. Reproduced from [132]. Right. Localisation of the expression of insulin and angiotensin II receptors along the nephron. Grey scale parallels the intensity of the expression. Drawn after data in [148, 176] for angiotensin II, and in [45, 141] for insulin receptors.

Figure 8 .

 8 Figure 8. Thickness of the renal cortex as a function of body mass. A. In 149 different mammals. B. In the same mammals, separated by habitat. Marine mammals show a much thinner cortex (and thus much shorter C-TALs) than other mammals. Reproduced from [25].

Figure 9 .

 9 Figure 9. Arterial vasculature of the kidney in four different mammals illustrated in midsagittal sections of kidneys injected via the renal artery with Microfil Silicone Rubber (©). Enlargement of each picture has been adapted to facilitate comparison between kidneys of different species. Mo.Ger. = Mongolian Gerbil.

Figure 10 .

 10 Figure 10. Influence of vasopressin and the urinary concentrating activity on the height of the different kidney zones and the thickness of the thick ascending limb epithelium. A. Rats with a high water intake (HWI) received a water-rich

Figure 11 .

 11 Figure 11. Influence of a low or a high protein diet on the height of the different kidney zones in Sprague Dawley rats. Kidney weight was 424 and 497 mg/100

Acknowledgements

This work is supported by grants from Agence Nationale de la Recherche (ANR-12-BSV1-0031-01 and ANR-17-CE14-0032 to PH).

Lise Bankir wants to thank Alan B. Weder (University of Michigan, Ann Arbor, MI, USA, now retired) for stimulating discussions about ethnic differences in kidney function and blood pressure regulation.

Reference list

See at the end of the document.