Daniel Romero
email: daniel.romero_arrieta@nokia.com

Almonacil Sylvain

Jean-Marc Conan

Laurie Paillier

Vincent Michau

Eric Dutisseuil

Sébastien Bigo

Jérémie Renaudier

Rajiv Boddeda

Block Interleaver Dimensioning and Real-Time Demonstration for Ground-to-Satellite Optical Communications

We provide a methodology to dimension the interleaver duration for free-space optical links. We validate it on an FPGA transceiver by emulating ground-geostationary strong turbulence conditions. 150ms interleaver reduces launch power by two orders of magnitude, making it compliant with commercially-available amplifiers to transmit 10Gbit/s.

Introduction

The continuous increase in the demand for data transmission has pushed the state-of-the-art radio frequency satellite links to their theoretical ceiling. Optical ground-satellite links can bring a large capacity leap but suffer from channel specific impairments, such as frequency selective molecular absorption [START_REF] Arrieta | Ca a y Limits of Optical Satellite C n a n[END_REF][START_REF] Almonacil | Experimental Study of the Impact of Molecular Absorption on Coherent Free Space Optical Links[END_REF], cloud attenuation and atmospheric turbulence induced power variations. The refractive index variations due to turbulence create time-distributed attenuation peaks of received optical power (ROP). Due to the finite receiver sensitivity and the optical amplifier output power limitations (few tens of watts [START_REF] Varona | Allfiber, single-frequency, and single-mode Er 3+ :Yb 3+ fiber a f r a 6 n re-d a 8 n[END_REF]), the attenuation peaks create error bursts. These bursts can be partially mitigated by time diversity techniques [START_REF] Trisno | Characterization of time delayed diversity to mitigate fading in atmospheric turbulence channels[END_REF] but at the expense of duplicating the communication hardware. A more promising approach is to use Adaptive Optics (AO) [START_REF] Vedrenne | Performance analysis of an adaptive optics based optical feeder link ground station[END_REF], complemented with channel encoding and a data interleaver [START_REF] Shi | Interleaving for combating bursts of errors[END_REF][START_REF] Greco | Design of the high-speed framing, FEC, and interleaving hardware used in a 5.4km free-space optical communication experiment[END_REF].

In this article, we propose a methodology for dimensioning a block interleaver for that purpose. As a case study, we consider a ground to geostationary (GEO) satellite uplink while accounting for point-ahead anisoplanatism [START_REF] Vedrenne | Performance analysis of an adaptive optics based optical feeder link ground station[END_REF] in strong turbulence. We use a numerical tool to generate time series of the received optical power after adaptive optics pre-compensation and ground-satellite propagation. We then apply the proposed methodology to assess the required interleaver duration to ensure that the errors after the deinterleaver are adequately distributed. Finally, we validate the technique experimentally by emulating a 10 Gbit/s free space optical chain incorporating a real-time interleaving system on FPGAs.

Dimensioning Block Interleavers

In Fig. 1a, we depict an uplink communication system through the atmosphere along with data encoding and decoding blocks. The blocks in solid line are implemented experimentally except for the FEC coding and codeword interleaving (𝜋) to correct residual bursts (~100s bits).

As the Forward Error Correction (FEC) blocks cannot correct burst errors of ~ms, interleaving is used to distribute the errors over a long duration, called the interleaver time. Here we use a block interleaver with M rows and N columns [START_REF] Kenneth | A Theory of Interleavers[END_REF]. As it is impractical to implement bit per bit interleavers at high data rates, we consider an interleaver which interleaves blocks of bits. We denote 𝐼 𝑟𝑒𝑠 the number of bits per block, as shown in Fig. 1b. The input/output blocks are written/read in/from the columns/rows of the matrix, respectively. At the reception, the received bits are deinterleaved and forwarded to the error correction stage.

Without loss of generality, we assume that any interleaver matrix contains 𝑒 𝑏 burst errors while the other bits have a mean bit error rate (BER) of 𝐵𝐸𝑅 ̅̅̅̅̅̅ . To distribute the errors uniformly over all the FEC codewords in the interleaver matrix, we set the value of M as follows Where 𝐹𝐸𝐶 𝑁 is the size of the FEC codeword and ⌈ ⌉ denotes the ceiling function. Assuming a continuous 𝑒 𝑏 , the maximum number of burst errors per FEC codeword is given by

M = ⌈ 𝐹𝐸𝐶 𝑁 𝐼 𝑟𝑒𝑠 ⌉ (1)
𝑒 𝐹𝐸𝐶 𝑏 = 𝐼 𝑟𝑒𝑠 × ⌈ 𝑒 𝑏 𝑁 ⌉ (2)
Assuming a constant 𝐵𝐸𝑅 ̅̅̅̅̅̅ for the remaining bits, the total number of errors per FEC codeword can be estimated using

𝑒 𝐹𝐸𝐶 = 𝑒 𝐹𝐸𝐶 𝑏 + 𝐵𝐸𝑅 ̅̅̅̅̅̅ (𝐹𝐸𝐶 𝑁 -𝑒 𝐹𝐸𝐶 𝑏 ) (3)
Thus, for a given FEC, with pre-FEC BER threshold of BERFEC, 𝑁 must satisfy the following condition

N ≥ ⌈ 𝐼 𝑟𝑒𝑠 × 𝑒 𝑏 (1 -𝐵𝐸𝑅 ̅̅̅̅̅̅ ) 2𝐹𝐸𝐶 𝑁 (BER 𝐹𝐸𝐶 -𝐵𝐸𝑅 ̅̅̅̅̅̅ ) ⌉ (4)
Which is valid when 𝐵𝐸𝑅 ̅̅̅̅̅̅ is lower than BERFEC threshold. For a bit rate of 𝑅 𝑏 , the interleaver time is given by (𝑀 × 𝑁)/𝑅 𝑏 . The ratio between the interleaver time and the fading time (eb/Rb) depends only on the FEC limit and the 𝐵𝐸𝑅 ̅̅̅̅̅̅ . We consider the DVB-S2 andard' FEC a reference [START_REF] Jtc | E 3 2 3 7-1 -V1.4.1 -Digital Video Broadcasting (DVB); Second generation framing structure, channel coding and modulation systems for Broadcasting, Interactive[END_REF] (𝐹𝐸𝐶 𝑁 = 64800) with hard decision decoding yielding pre-FEC BER limits of 0.08, 0.035, 0.02 for code rates 1/2, 2/3 and 3/4 at a post-FEC BER of 10 -9 . In Fig. 2a, we present the interleaver time as a function of the 𝐵𝐸𝑅 ̅̅̅̅̅̅ for various code rates using a 1 ms fading time. As expected, higher code rates require longer interleaver times and we use a code rate of 1/2 as a baseline to minimize the interleaver length.

Channel modelling

In this section, we describe how we generate a representative set of timeseries expected in ground to geostationary satellite links to illustrate our methodology. In optical atmospheric propagation, the spatial strength of the atmospheric turbulence is characterized by two parameters: the coherence length (𝑟 0 also called Fried parameter) and the anisoplanatic angle (𝜃 0 ). The smaller these values, the more severe the turbulence. We use the measurements presented in [START_REF] Sprung | Characterization of optical turbulence at the solar observatory at the Mount Teide[END_REF][START_REF] Osborn | Optical turbulence profiling with Stereo-SCIDAR for VLT and ELT[END_REF] to generate turbulence profiles as show in [START_REF] Vedrenne | Performance analysis of an adaptive optics based optical feeder link ground station[END_REF]. Assuming 𝑟 0 and 𝜃 0 as independent stochastic variables, we selected a turbulence profile which comprises 99.9% of the measured cases. We obtained a 𝑟 0 of 4 cm and a 𝜃 0 of 4 8 𝜇𝑟𝑎𝑑, along the line of sight at 1550 nm.

The temporal behaviour of the turbulence can be modelled by the vertical windspeed profile. Here as a challenging scenario, we selected a large windspeed at ground level (10 𝑚/𝑠) which requires rapid adaptive optics response (4.7 kHz, ~0.5 ms delay correcting up to 90 Zernike radial modes in our simulations), while keeping low tropospheric windspeed (15 𝑚/𝑠.) which creates long coherence times and large error bursts.

To obtain the received optical power time series we use an experimentally validated simulation tool [START_REF] Védrenne | T r ulence effects on bidirectional ground-to-satellite laser communication[END_REF], fed with the obtained wind and turbulence profiles. We generated multiple time series of 1 second duration for a ground to geostationary satellite link with a 40 and 25 cm emission and reception pupil, respectively, with the link budget model presented in [START_REF] Vedrenne | Performance analysis of an adaptive optics based optical feeder link ground station[END_REF].

Experimental measurements

We use an NRZ-OOK based transceiver for the optical modulation of the data at 10 Gbps. The experimental setup is shown in Fig. 2b. We implemented a real-time block interleaver and deinterleaver on two VC709 FPGA boards each equipped with 1 DDR3 SDRAM memory [START_REF][END_REF] with a capacity of 32 Gbits and a data transfer rate of ~100 Gbps. At the transmitter, a pseudorandom binary sequence (PRBS) bit stream is generated and interleaved before modulation. We use 𝐼 𝑟𝑒𝑠 = 512 bits which is sufficiently small (< 1% of the FEC codeword) while allowing us to sufficiently expedite the memory write/read times. As the deinterleaver at the receiver needs to recover the exact position of the incoming data in the interleaving matrix, a frame with headers containing the block order information is created at the transmission. An additional 1% overhead is used for this synchronization.

The modulated signal is then sent to a variable optical attenuator (VOA) which emulates the static channel losses. Then, a fast VOA with a bandwidth (5 MHz) higher than the power variations (<1 kHz) is used to emulate the simulated timeseries. At the reception, the signal is pre-amplified using Erbium Doped Fiber Amplifier (EDFA) #1, filtered (50 GHz) to perform amplifier noise rejection and amplified by EDFA#2 to reach the power level required by the receiver photodiode. At its output, the bit stream is aligned, deinterleaved and uncoded BER is measured for each FEC codeword length.

In the static regime, the BER of the received data was measured as a function of ROP (Fig. 3a). We found that when the ROP falls below a threshold of -45 dBm the receiver loses the clock reference and typically recovers around ~0.7 ms after it rises above this threshold.

To calculate the interleaver time, we first estimate the fading time and the 𝐵𝐸𝑅 ̅̅̅̅̅̅ from the generated time series. To do this, we look for fadings in a 100 ms window as shown in Fig. 3b. Then, using the measured ROP to BER curve, we calculate the BER in the window and add the timing recovery as shown in Fig. 3c. The fading time corresponds to the segments which are shown in red (solid and line pattern) and the 𝐵𝐸𝑅 ̅̅̅̅̅̅ (dashed line) was calculated in the segments with BER less than 0.5. We found that 𝐵𝐸𝑅 ̅̅̅̅̅̅ typically lies within 0.001 and 0.01. By increasing the launch power at the transmitter and consequently the received power, the fading times decrease. Even for a launch power of 60 W, we observed that fading times vary between 1 and 15 ms.

We now measure experimentally the required interleaver time by varying the interleaver time until all the codewords after the interleaver have a BER lower than pre-FEC limit. In Fig. 4a, we show a snippet of the ROP series along with the measured BER in time after the de-interleaver. We observe that for the chosen interleaver time (100 ms) all the codewords satisfy the FEC condition. This whole process is repeated for each fading event and varying its position in the interleaver window. In Fig. 4b, we show the measured required interleaver times (crosses) for various fading times along with the calculated ones (blue circles). The measurements fall within +/-10% of the calculations in up to 95% cases. The residual error could be attributed to the presence of multiple fadings or fading position in the interleaver window for the measurements whereas the simulations were done by assuming the fading event in the middle of the window.

For various launch powers, we calculate the minimum interleaver time for the worst fading event over all the time series, and the results are shown in Fig. 4c. We can see that even for large launch powers an interleaver is necessary for error-free transmissions under strong turbulence. We observe that an interleaver of 150 ms can bring a reduction of two orders of magnitude, making it feasible to use a commercially available amplifier of 30 W [START_REF] Elr-Sf | Erbium Single-frequency CW Lasers[END_REF] to establish 10 Gbps links.

Conclusion

In this article, we proposed a methodology for dimensioning the block interleaver size for optical satellite links. We developed a real-time FPGA interleaver and validated our methodology under strong turbulence conditions. The predictions agree within +/-10% with the measurements. With commercial amplifiers, we showed that an interleaver of 150 ms is sufficient to transmit 10 Gbps ground-geostationary satellite links.

Fig. 1 :

 1 Fig. 1: (a) The optical communication chain for ground-satellite communication, solid line blocks were experimentally emulated. (b) MxN block interleaver with blocks of size Ires at the transmitter.

Fig. 2 :

 2 Fig. 2: (a) Interleaver time as a function of 𝐵𝐸𝑅 ̅̅̅̅̅̅ for different FEC code rates for a fading time of 1 ms. (b) The experimental apparatus emulating a free space optical link communication system.

Fig. 3 :

 3 Fig.3: (a) The dependence of the measured BER with ROP. BER reaches 0.5 for ROP <-45 dBm (b) A snippet of a time series of 100 ms with the fading zone in red. (c) BER of the time series.

Fig. 4 :

 4 Fig.4: (a) The measured BER after the deinterleaver with all the received blocks below the FEC threshold (0.08). (b) The measured and calculated interleaver time for different fading times from the time series. (c) The minimum simulated interleaver time as a function of the launch power.

Code rate = 1/2 Code rate = 2/3 Code rate = 3/4 -10 -5 0


Shubhayan Ghosal
email: shubhayang.math.ug@jadavpuruniversity.in

Invoking Fractional Calculus to extend the classic Riemann Zeta-Bernoulli relation

In this paper,we explore and extend the classical definition of the zeta function and the Riemann Lioville Fractional calculus on the entire complex plane, except the origin where it has a simple pole and residue 1, through the analytic continuation of Bernoulli Numbers, as a fractional differintegral.

1 Some less noticed properties of a Bernoulli Polynomial

Defining Appell Sequence

A sequence s n (x) is said to be Appell [START_REF] Tempesta | On appell sequences of polynomials of bernoulli and euler type[END_REF] for a function g(t), provided that it is not identically 0, if the following holds

g(t)e xt = ∞ k=0 s k (x) • t k k!
where y ∈ C with field characteristic 0 and which upon Maclaurian expansion, Cauchy product of series and comparison of L.H.S and R.H.S gives us

s k (x) = n k=0 n k g k (0) • x n-k
where g k (0) is the k th derivative of g at t = 0.Basically, it is a Sheffer sequence in (g(t), t). Another property which is often used to describe the Appell sequence is

s ′ n (x) = ns n-1 (x)
which is also one of the properties inherited by Bernoulli Polynomials. A wellknown property of Appell sequence can be derived from the Binomial theorem for natural index

s n (x + y) = n r=0 n r s n (x) • y n-r (1)
.

Bernoulli Polynomials as a Special case of Appell

A Bernoulli Polynomial [START_REF] Srivastava | Fractional calculus and the sum of powers of natural numbers[END_REF] is an Appell sequence with g(t) = e t -1 t and the coefficients hence derived are called Bernoulli Numbers. The Bernoulli Polynomial is given as

t e t -1 e xt = ∞ k=0 B k (x) • t k k!
and explicitly, as before

B n (x) = n r=0 n r B r • x n-r
Here, the coefficients of the function in the Maclaurian expansion have been termed as the Bernoulli Numbers. The Bernoulli Numbers can thus be defined as

x e x -1 = ∞ n=0 B n x n
n! , thus yielding

B n = lim x→0 D n x x e x -1
We are now to extend the Bernoulli Numbers as a new function B(x) with x ∈ R [START_REF] Sc Woon | Analytic continuation of bernoulli numbers, a new formula for the riemann zeta function, and the phenonmenon of scattering of zeros[END_REF]. The function will return the standard Bernoulli numbers at positive integer arguments. Note that, to extend the function, we resort to the standard fractional calculus definition of Riemann-Liouville. The Riemann-Liouville fractional integral is defined on the complex plane as

I s f (x) = 1 Γ(n) x a (x -t) s • f (t) dt We intend to show the relation ζ(-n) = -Bn+1
n+1 [START_REF] Sury | Bernoulli numbers and the riemann zeta function[END_REF] by proving this relation ∀ s ∈ C on the half planes Re(s) ≤ 0, Re(s) ≥ 0, s ̸ = 0. Our approach would be fractional calculus. First we define a few terms related to fractional calculus. The fractional calculus concept mainly deals with viewing the differentiation and integration as operators and inverses of each other, with the ability to extend the number of times the operator acting to any non-integer order, perhaps the complex numbers too! However, in this paper, we would be mostly dealing with the Riemann-Liouville concepts and some preliminary ideas like

I α I β f = I α+β f and [1]D s f = D ⌊Re(s)⌋ I -({Re(s)}+i•Im(s)) f, Re(s) > 0. (2)
Indeed, we intend to present the proof of how a generic operator can be extended on the complex plane, thanks to S.C. Woon. [START_REF] Sc Woon | Analytic continuation of operators-operators acting complex stimes-applications: from number theory and group theory to quantum field and string theories[END_REF] 2 Analytical Continuation of an Operator Using Nested Series Expansion

Thinking of continuing an operator some natural number 'n' times is quite usual as we think. But to continue it to some complex 'z' times is what is somewhat off path. With reference to [START_REF] Sc Woon | Analytic continuation of bernoulli numbers, a new formula for the riemann zeta function, and the phenonmenon of scattering of zeros[END_REF], we assume a generic operator is allowed to continue n times, denoting it as A n . Then we have

D z = (w1 -(w1 -D)) z = w z 1 -1 - D w z = w z 1 + ∞ n=0 (-1) n z n 1 - D w n = w z 1 + ∞ n=0 (-1) n z n 1 + n k=0 -1 w k n k D k (3)
where z ∈ C, w ∈ R + , Re(z) > 1 w and 1 is the identity operator. Also,

x n = 1 n! • n-1 k=0 (x -k) ∀ x ∈ R.
B(s) = lim x→0 D x s x e x -1 = - ∞ n=0 lim x→0 D s x (xe nx ) = - 1 Γ(-s) ∞ n=0 lim x→0 x a (x -t) -s-1 • te nt dt = 1 Γ(-s) ∞ n=0 lim x→0 0 x-a t -s-1 • (x -t)e n(x-t) dt = 1 Γ(-s) lim x→0 0 ∞ t -s-1 • (x -t)dt + ∞ n=1 lim x→0 0 ∞ t -s-1 (x -t)e n(x-t) dt = 1 Γ(-s) ∞ n=1 lim x→0 x 0 ∞ t -s-1 e -nt dt - 0 ∞ t -s e -nt dt = 1 Γ(-s) ∞ n=1 1 n s-1 ∞ 0 k -s e k dk = 1 Γ(-s) ∞ n=1 1 n s-1 • Γ(-s + 1) = ζ(1 -s) • (-s) =⇒ ζ(1 -s) = - B(s) s (4) ∀ Re(s) ⩽ 0, s ∈ C, s ̸ = 0
And now, given that Re(s) > 0, we use (5) to complete the rest of the proof. Let m = ⌊Re(s)⌋, s ′ = {Re(s)} + i • Im(s), then s = m + s ′ . On the 6 th line, the quantity

C = 0 ∞ t -s ′ -1 dt is already 0 if m = 1 or else D m x (Cx) = 0. B(s) = lim x→0 D s x x e x -1 = lim x→0 D (m+s ′ ) x x e x -1 = - 1 Γ(-s ′ ) ∞ n=0 lim x→0 D m x x a (x -t) -s ′ -1 te nt dt = 1 Γ(-s ′ ) ∞ n=0 lim x→0 D m x 0 x-a t -s ′ -1 (x -t)e n(x-t) dt = 1 Γ(-s ′ ) lim x→0 D m x 0 ∞ t -s ′ -1 (x -t)dt + ∞ n=1 0 ∞ t -s ′ -1 (x -t)e n(x-t) dt = 1 Γ(-s ′ ) ∞ n=1 lim x→0 D x m 0 ∞ t -s ′ -1 (x -t)e n(x-t) dt = 1 Γ(-s ′ ) ∞ n=1 lim x→0 D x m e nx 0 ∞ t -s ′ -1 (x -t)e -nt dt = 1 Γ(-s ′ ) ∞ n=1 lim x→0 D m x xe nx 0 ∞ t -s ′ -1 e -nt dt -D m x e nx 0 ∞ t -s ′ e -nt dt = 1 Γ(s ′ ) ∞ n=1 mn m-1 0 ∞ t -s ′ -1 e -nt dt -n m 0 ∞ t -s ′ e -nt dt = - 1 Γ(-s ′ ) ∞ n=1 n (m+s ′ -1) (mΓ(-s ′ ) -Γ(-s ′ + 1)) = ζ(1 -s) • (-s) (5)
Hence, the result

ζ(1 -s) = - B(s) s holds ∀ s ∈ C -{0}.[2] But then, ζ(-n) = - B(n + 1) n + 1 = - lim x→0 D x n+1 x e x -1 n + 1 = - B n+1 n + 1
which are the standard Bernoulli numbers. Hence B(s) is indeed the analytical continuation of Bernoulli numbers, more precisely Bernoulli Polynomials at argument 0 i.e, B n (0). It is not very difficult to realise the reflection formula of the Bernoulli numbers from the analytic relation of the Riemann-zeta function.

B(1 -s) = 1 -s s 2 s • π s-1 sin πs 2 • Γ(1 -s) • B(s) (6)

Conclusion

It can be noted that not only we have extended the relation, we made an extension to a particular class of polynomials and invoked a new method involving fractional calculus to derive a classical formula. May be this method turns out to somehow throw some light on the age old Riemann hypothesis, owing to its unique representation as a fractional (complex!) differintegral. In the next paper, we will explore the intriguing properties of these extended numbers and their applications in various areas of mathematics, including Hurwitz and Lerch Zeta Functions and Distribution of primes.

3 0 x s- 1 e x - 1

 011 Analytical Continuation of Bernoulli Numbers as a Fractional Derivative and the proof Intutively speaking, the integral representation ζ(s)•Γ(s) = ∞ dx gives us some hint of the connection of the Riemann-Lioville integral, but not rigorously. Let us start with a function B(s) = lim x→0 D s x x e x -1 , s ∈ C, Re(s) < 0. In the subsequent steps we change t → x -t and then nt = k, also we set a → -∞.

Acknowledgment

I would like to express my gratitude to my friend/batch-mate Sagnik Saha, Department of Mathematics, Jadavpur University, who provided valuable assistance in compiling the document in LaTeX. His technical expertise and attention to detail were instrumental in ensuring the quality and readability of this research paper. I am deeply appreciative of his dedication and commitment to this project, and I am honored to have worked with him. Sagnik's contribution to this project will undoubtedly serve as a valuable asset to his future academic pursuits and professional endeavors.


M Boudali

J-Y Yap

K Quntero

M El-Zeenny

A Garcia Guerra

J Ibanez-Guzman

J Dauwels

M Tlig

Crossing Road Intersections by an Autonomous Vehicle: Evaluation via Simulation

Keywords: Autonomous driving systems, scenariobased validation, simulation, decision-making planner evaluation

Intersection crossing by an autonomous vehicle (AV) is a difficult task due to multiple situations that may occur in an area with convergence of multiple traffic entities. The vehicle needs to understand its situation with regards to its spatio-temporal relationship with respect to the perceived traffic agents, the context imposed by the intersection geometry, and traffic rules.

The testing and evaluation of this manoeuvre in real conditions is difficult even in proving grounds. For this purpose, simulation techniques are used. This paper presents the evaluation of this function using two open-source simulation-AV software stacks, namely Carla-AutoWare & SVL Apollo-Baidu. The purpose is to test their performance using different metrics and as a first step, in this paper the focus is in collision results.

Introduction

The metric performance evaluation of intelligent vehicles with high levels of autonomy, that implement the sense-act-act paradigm is considered as a strategic endeavour. This can be done at component level or at system level either on proving grounds or using simulation techniques. Testing in public roads is done only after a rigorous evaluation process and approval by various stakeholders.

The results presented in this paper are the product of a joint study between industry and academia that seeks to develop evaluation methods based on simulation technologies whilst leveraging in the strong field experience of the different partners.

The evaluation of an AV can be partitioned into two phases: components and systems. Whilst ideally most of the work can be done in a proving ground under controlled scenarios, these evaluations require much resources with hazardous situations being difficult to recreate or simply forbidden. Component level evaluations imply the formulation of different assumptions and constraints in the different tests.

For the purposes of this study the decision-making and planning function has been chosen, with the use case of crossing road intersections. Choosing the decision-making & planning function implies that several assumptions need to be made in particular with regards to the perception, localisation and map functions. These assumptions will be detailed along the paper.

Road intersections have been identified as one of the most challenging scenarios within the Operational Design Domain (ODD) for autonomous vehicles. It is a part of the road network entailing the co-existence of different traffic agents with different behaviours, and different types of traffic rules. It is thus a configuration where numerous accidents occur [START_REF] Li | Drivers' visual scanning behavior at signalized and unsignalized intersections: A naturalistic driving study in China[END_REF], [START_REF] Richard | A random parameters probit model of urban and rural intersection crashes[END_REF] and therefore these scenarios have been widely studied due to the complexity for AVs to maneuver safely and efficiently, see [START_REF] Astarit | Surrogate Safetu Measures from traffic Simulation Models a Comparison of different Models for Intersection Safety Evaluation[END_REF], [START_REF] Bonela | Review of traffic safety evaluation at T-intersections using surrogate safety measures in developing countries context[END_REF] [START_REF] Ali Pirdavani | Evaluation of Traffic Safety at un-signalized intersections using microsimulation[END_REF], [START_REF] Zaid Tahir | Intersection focussed situation Coverage-Based Verification and Validation Framework of Autonomous Vehicles Implemented in CARLA[END_REF].

In order to cross autonomously, the vehicle needs to make several decisions including adjusting its speed and direction accordingly with its situation. Simple questions such as Does it have sufficient time to cross the intersection in the presence of nearby vehicles? Does it need to stop due to the traffic signs? What will be the attitude of the upcoming vehicle who has no priority at the intersection? Or What would be the manoeuvre executed by the pedestrian close to the intersection? These questions are related to decisionmaking and path planning which are at the core of AV functions and control its's behaviour according to the mission to accomplish and its situation within its navigation environment. Figure 1 shows a typical functional diagram of an AV.

The approach taken for the evaluation of the decisionmaking and path planning function as applied to the crossing of intersections is based on two major considerations: the selection of the software stack and the selection of the simulators.

The software stack implements the functions of the architecture of an AV (shown in Figure 1) and the simulators shall enable the evaluation of the system under test.

The Software Stack

Among the most known open-source software stacks Autoware and Apollo baidu have been used by large companies and governmental organizations (e.g., Lincoln, Volvo, Ford, Intel, Hitachi, LG, and the US Department of Transportation). These are the autonomous driving systems considered in this work; however, the methodology is in no way restrictive and can be deployed on other platforms.

Very few studies compare the performance of these AVs pipelines (i.e. Apollo Baidu and Autoware). Among the existing work, in [START_REF] Raju | Performance of open autonomous vehicle platforms: Autoware and Apollo[END_REF] the comparison has been made in terms of architecture and algorithms used. Another interesting work [START_REF] Garcia | A Comprehensive Study of Autonomous Vehicle Bugs[END_REF] has compared the two AVs in terms of the number of bugs in the software. In this study, the proposal is to compare the two AVs performance in terms of behaviour and decision making.

The latest version of Autoware can be found in [START_REF] Source | Github[END_REF] and for Apollo-Baidu in [START_REF] Source | Github[END_REF].

The Simulators

Virtual environments are often used for the development and testing of Autonomous vehicle systems (AVs) before exposing them to their actual physical deployment environments, especially when actual physical actuation is involved. Generic simulation environments are popular amongst researchers as it promotes and democratizes the development of AVs.

SVL Simulator (formerly known as LGSVL Simulator) [START_REF]LGSVL simulator: A high fidelity simulator for autonomous driving[END_REF] and CARLA [START_REF] Dosovitskiy | CARLA: An Open Urban Driving Simulator[END_REF] are two of the most well-known open-sourced simulation tools. These are notable in their use of well-known gaming engines such as the Unity Engine [START_REF]Unity Engine[END_REF] and the Unreal Engine [START_REF] Games | [END_REF] respectively. Furthermore, they provide bridges towards open-source Automated Driving System (ADS) such as AutoWare [START_REF] Kato | Autoware on Board: Enabling Autonomous Vehicles with Embedded Systems[END_REF] or Apollo [START_REF]Baidu apollo[END_REF]. In this study, both simulators would be adopted, where Apollo would be tested in SVL Simulator while AutoWare would be tested in CARLA. Both platforms enable the researchers to develop specific scenarios to be tested and generate synthetic data which would be used to analyze and compare the AVs behavior and decision making.

Paper Structure

The remainder of the paper is structured as follows:

Section 2 provides an overview on metrics for this problem type and in the larger scope of the project. Section 3 describes the approach applied to evaluate both platforms, from the experiment design to the analysis methodology and the metrics considered. Section 4 presents the results which are mainly focused on collisions, and Section 5 concludes the paper and outlines perspectives.

Overview on Metrics

This section aims at providing a state of the art on metrics related to the addressed problem in the larger scope of the project, however the results presented in this paper are collision-oriented. The safety validation of AD (Autonomous Driving) Systems is still an unsolved issue. Metrics play an essential role in the validation process and must be selected and/or adapted for different applications, some examples include [START_REF] Project Pegasus | Requirements and conditions-Critical scenarios for human driverscritically metric[END_REF] [18]: -Identification of safety critical test scenarios derived from recorded data, supporting the scenario-based testing approach in simulation or proving grounds. This application is interesting for both AV developers and certification bodies. -Estimation of the safety performance of a specific AD:

for AV developers to improve the decision making. Live monitoring of the AV state thought appropriate metrics will allow developers to identify a safe trajectory and to execute the appropriate reactions, and for certification bodies to evaluate and assess the AVs behaviour

In this research, we will focus on the last purpose using a scenario-based testing approach and a set of metrics to evaluate through simulation the driving performance of two ADS (Autonomous Driving Systems), Apollo and Autoware, on an intersection scenario.

AV stakeholders agree on the need of metrics to assess safety performance. Several ways and metrics have been put forward, but not widely adopted. It is assumed that a combination of metrics will be needed to support the evidence of safety performance.

In addition, there is a subjectivity to classify acceptable and non-acceptable situations produced by different metrics. Metrics and specific thresholds to distinguish dangerous interactions from safe interactions are strongly dependent on the scope of application and scenarios.

Driving safety metrics are classified according to distinct characteristics (e.g. data source, accessibility, applicability, among others).

-Lagging metrics based on the frequency of the event measured (e.g. collision frequency) versus Leading metrics involving assumptions and models being able to predict future outcomes (e.g. TTC and PET) -Black box metrics using data that can be obtained without any access to the ADS versus Grey and White box metrics needing access to the ADS information.

As an overview, in the larger scope of the ASV project, a set of Black-box-Leading metrics has been identified as essential and are described in the internal publications [START_REF] Guerra | ASV-D1.11 -Metrics for behavioural safety validation of Automated vehicles-A literature review[END_REF] "Metrics for behavioural safety validation of Automated vehicles-A literature review" and [START_REF] Zeenny | ASV-D2.8 -Definition of Metrics for Intersection Evaluation[END_REF] "Definition of Metrics for Intersection Evaluation". These metrics are based on the safety envelope and exhibit contextually safe vehicle motion concepts [START_REF]AVSC Best Practices for Metrics and Methods for Assessing Safety Performance of Automated Driving Systems (ADS)[END_REF]. Thresholds related to these metrics can and will be used namely deriving from naturalistic driving data widely used in the literature [START_REF]Basic theory of Driving[END_REF] [23] [START_REF] S_Como | _Russo, Driving Safety Performance Assessment Metrics for ADS-Equipped Vehicles[END_REF] [25] [START_REF] _Schindler | Validation: Metrics& KPIs for Autonomous vehicles[END_REF].

In the scope of this paper a subset of metrics is used since the purpose of the results is collision-oriented. The metrics used include classic dynamics such as speed, acceleration, relative distance and distance to the intersection for each actor. These are considered sufficient for the results presented herein and allow the early identification of interesting differences between the software stacks.

Evaluation Approach

The following subsections describe the process of generating the data necessary for the first building blocks of the evaluation task in the ASV project. Firstly, simulations were carried out on both frameworks: Autoware running on Carla simulator and Apollo running on SVL simulator (where the simulator offers the 3D environment and the planning module of the AD controls the ego vehicle), data quality was then assessed in order to ensure homogeneous implementations on both frameworks (i.e. metrics, vehicles' geometry, test plan parameters) and, once quality was satisfactory, then the evaluation phase took place.

In order to evaluate the planning module, the localization of the vehicle and the environment detection using sensors [START_REF] Kato | Autoware on Board: Enabling Autonomous Vehicles with Embedded Systems[END_REF] are necessary information. Vehicle dynamics characteristics could also influence the planning module evaluation.

To avoid any impact on evaluation in this work, we propose to limit the influence of other modules (see Figure 1) by exploiting the ground truth signals and implementing similar vehicles on both simulators.

In addition, to ensure the consistency of the scenarios in different simulation platforms such as the map and behavior of the actors, the same map is shared between simulators and the synchronization of the actor behaviors by a distance-time based trigger are done on the same way.

The following subsections describe the definition process of the test campaign (i.e. experiment design), and the analysis methodology based on the selected metrics.

Experiment design

The planning module remains among the most complex autonomous driving tasks to perform as it determines vehicle actions and therefore safety. The evaluation of this module is carried out in the functional scenario of a cross intersection in an urban area.

In the studied intersection configuration 2 actors interact. The first actor is considered as the ego vehicle controlled by the AD while the second one is considered as the intruder controlled by the simulator.

In order to evaluate the AD and how it reacts to the traffic sign and its behaviour in the presence of other vehicles, two scenarios are proposed, see Figure 2.

The first scenario, called SC_1.2, includes a stop traffic sign in the roadside of the ego vehicle. The objective of the ego vehicle is to respect the stop sign and to cross the intersection safely as shown in the figure bellow. The second scenario, called SC_1.1, includes a stop traffic sign in the roadside of the intruder vehicle but it is considered that the intruder commits a violation and ignores the stop sign to engage in the intersection. The purpose of the ego vehicle is to cross the intersection safely and to adapt its velocity given the trajectory and velocity of the intruder.

For the two proposed scenarios, several test cases are generated by varying: speed of the ego, speed of the intruder and the triggering event for the meeting between the ego and the intruder vehicle. The triggering events are controlled by the distance between the bumper of the intruder and the intersection.

SC_1.2 SC_1.1

Figure 2: Simulation Scenarios Three distances are chosen for the meeting between actors: the first one is computed between the bumper of the intruder and a point A located before the intersection, the second one is computed related to a point B in the middle of the intersection and the third one is related to a point C located after the intersection. Using these distances, the intruder vehicle should be at the selected point (A, B, or C) when the ego vehicle intends to cross the intersection, see Figure 3. This synchronization between the ego and the intruder vehicle is theoretically performed according to the time necessary by the intruder to reach the given point (A, B, or C) and the time necessary for ego to reach the intersection without considering the deceleration of the ego due to the presence of the intruder. The speed of the ego and that of the intruder vary, respectively, between 20 km/h and 30 km/h, and between 25 km/h and 40 km/h as summarized on Table 1. To reduce the impact of using two different simulators, we adopt similar configurations by using the same vehicle configuration and the same map to perform test cases.

Simulation analysis methodology

The method is used to generate the simulation dataset, perform quality assessment on the data and focus on the results of the decision-making module of each AD stack. The global workflow is presented on Figure 4. As the analysis is done in black-box mode, the method focuses on defining the following steps of the analysis strategy.

1. Scenario validation criteria: Automatically validate the simulation results of both AD stacks for each specific scenario, i.e. ensuring that positions and dynamics are respected. Scenario specifications are verified for the simulation campaign by Pass/Fail criteria.

Separate analysis: Analyze the AD stacks results

separately in order to understand the behaviour of the AD system. At this stage, the focus is on the decision-making of each system in relation to the pre-defined scenario criteria. 3. Combined analysis: Compare the decision making of Autoware with the results of Apollo.

Once the separate analysis is done, results can be compared between both stacks allowing to identify key differences, and the risk levels on each AD stack by means of comparison.

The first results of the application of the method are focused on collision analysis. For this purpose (i.e. collision identification and characterization of global behaviour in both platforms) only basic metrics are used, namely: speed for both actors in both scenarios, relative distance and distance to the intersection for each actor.

However, in the broader scope of the project and out of the scope of this paper, some of the metrics identified as essential include: clearance, Post Encroachment Time (PET), Proportion of Stopping Distance (PSD), and Delta Time To Collision (DTTC).

Results

From a general point of view, results analysis on both platforms should be done for situations in nominal conditions (no risk associated), safety critical conditions 1 , and for collision situations. This work focusses specifically on the results for collision cases and lays the ground for further work aiming at the study of safety critical events and the comparison of nominal behaviour to identify prevention strategies and accepted risk levels on both AD stacks. However, even though the focus herein will be collision-oriented, for introductory purposes, the nominal behaviour will be firstly presented in both platforms. Subsequent subsections describe the collision profile for the dataset, and then a zoom on collision in both platforms and a zoom on collisions on one framework which does not occur on the other one.

In what follows we will refer to Autoware and Apollo when presenting results for simplicity; however, it is reiterated that the two frameworks studied in this work are Autoware running on Carla's simulator and Apollo running on SVL's simulator2 .

Nominal Behaviour

Figure 5 and Figure 6 show the behaviour for both platforms in the absence of risk, i.e. nominal behaviour, for test case 3B, B being the riskiest configuration where the ego vehicle should encounter the intruder the closest by definition right inside the intersection. This nominal behaviour exemplifies how the ego vehicle reacts to the intruder vehicle by braking and no collision occurs. In can be seen in Figure 6 that nominal behaviour in Apollo's case implies that the ego vehicle starts braking as soon as the intruder starts moving (in all test cases studied in this work). In Autoware's case, ego braking occurs in the intersection, see Figure 5. This particular behaviour in Apollo's nominal case is observed systematically in all test cases in the dataset.

1 Also referred to as near-misses which could result into collisions if parameters were slightly

Collisions Profile in the Dataset

Collision situations are the main focus of the results presented herein and they lay the ground for the work currently addressed in the scope of the project and out of the scope of this paper. As a reminder, a total of 36 test cases were simulated in each platform for which 18 test cases corresponded to a scenario with no stop line and the other 18 corresponded to a scenario with a stop line. The global distribution for the detected collisions is displayed on Figure 7. Table 2 summarizes the collisions encountered in this first dataset. In general, the following points are identified:

-Collisions in the scenario with a stop line on ego's path do arise which could be unexpected since the intruder is not performing any unexpected manoeuvres and the stop line should in theory serve as a protection mechanism for ego, -More collisions are observed in Autoware in the scenario without the stop line which is intuitive since the stop line is expected to act as a prevention measure; however, the opposite behaviour is observed in Apollo, -Some collisions observed in Autoware correspond to the ego vehicle stopping in the middle of the intersection (i.e. speed at collision = 0 km/h in the table) as the intruder approaches. This behaviour is observed for both scenario types (i.e. with and without the stop line in ego's path).

Collisions on Both Frameworks

For this first dataset, two test cases proved to result in a collision both in Autoware as well as in Apollo (test cases 2A and 6A). The overall behaviour for both of these cases is presented in the following figures for case 2A for illustration purposes.

In both platforms, the behaviour is such that: -Ego restarts after the stop line even though intruder is approaching at constant speed -Ego slows down in the middle of the intersection and then the collision occurs. In the case of Autoware the ego comes to a full stop. The hypotheses for these first preliminary results for the ADs are: -Apparent lack of consideration of the intruder's dynamics.

-Apparent lack of consideration of the intruder's geometry. Ego seems to either slow down or even stop in Autoware's case aiming to avoid the collision which would have potentially happened if it were not for the size of the intruder. This will be further analyzed with an extensive test campaign in which not only patterns could emerge but also a clearer view on the robustness of the simulations. It is however interesting that for this first exploratory phase collisions are found in both platforms specifically for the scenario providing theoretically a safer context given the stop line.

Collisions on Autoware vs No Collisions in Apollo

As described in Table 2 for Autoware, collisions arise, as expected, mostly in the scenario without a stop line and specifically for case B which is the case in which the ego vehicle encounters the intruder the nearest in the intersection. For all these cases of collision without a stop line for ego, the observed behaviour is as the one presented in Figure 10 for case 4B. This is, for all of these cases the ego vehicle slows down and stops in 2 out of the 3 cases and as a result it gets hit by the intruder. In all 3 cases the ego vehicle enters the intersection before the intruder as displayed in the figure .  Conversely, in Apollo, for these particular test cases the behaviour is as the one presented on Figure 11. Namely, since Apollo slows down a lot earlier, from the moment the intruder starts moving regardless of the distance, the ego vehicle then crosses the intersection systematically after the intruder and no collision can occur. At this stage, the absence of collision is then judged to be a consequence of the very much anticipated behaviour of the ego vehicle.

Collisions on Apollo vs No Collisions in Autoware

In the cases where collisions occur in Apollo and do not arise in Autoware, the behaviour is summarized by the ego vehicle systematically crossing the intersection before the intruder vehicle and starting to slow down but failing to avoid the collision if indeed this was the intention. This behaviour is illustrated with one example for Apollo in Figure 12 for the scenario with a stop line. The ego vehicle stops at the stop line, restarts, then slows down as the intruder engages in the intersection and the collision occurs.

Conversely, for Autoware, the behavior is consistent with the example provided on Figure 13. This is, ego also reaches the intersection before the intruder, stops at the stop line, restarts and stops again as the intruder vehicle engages in the intersection successfully avoiding the collision. Ego then is able to restart again.

Figure 12: Apollo 3Ascenario with stop line Figure 13: Autoware 3Ascenario with stop line

Conclusion and Perspectives

For this first phase, analysis of collision situations was performed for a scenario of a cross intersection with and without a stop line on the ego vehicle's path. This first dataset, even though restrictive in size but rich in configuration and interaction possibilities, allowed acquiring preliminary comprehension of the behavior in both AD stacks in order to better guide the following test campaigns. The following general results emerge:

-Apollo seems to react to the intruder from the moment it starts moving. In further phases the pertinence of such behavior will be evaluated with larger datasets and statistical representation. -Autoware seems to react to the intruder as the interaction is likely to occur in the intersection as human driving would also suggest.

-

Collisions do occur in scenarios with a stop line with both stacks, contrary to what could be expected. Causes cannot be stated in this paper and a subsequent repeatability study should shed a light on this subject. More broadly it could be argued that the interactions between actors are coded differently in both ADs and that the behavior should be similar for most scenario types (other than intersections). This should be confirmed with proper testing. The theoretical ground has been laid for the next analysis phase regarding the fundamental set of metrics to be analyzed in order to state on the prevention strategies and risk levels accepted on each AD stack. The subject of new metrics more suitable to the new studied situations could arise when having to address nominal behavior and safety critical cases. At this point it is envisaged to study the PET in order to verify if evident differences can be found in terms of the risk taken when crossing the intruder in the intersection. It is foreseen to explore the subject of repeatability and verify how the responses of the systems may vary and induce different outcomes and in what proportion.

New scenarios are also envisaged, namely by considering the addition of Vulnerable Road Users (VRU) and subsequently situations where the ego's mission is inherently riskier such as left turns for example.

Finally, further studies envisage the use of the same simulator for both ADs in order to homogenize the testing environments.

Figure 1 :

 1 Figure 1: Functional architecture of an AV (courtesy of Renault S.A.)

Figure 3 :Figure 4 :

 34 Figure 3: Overview of test case definition -Carla

Figure 5 :Figure 6 :

 56 Figure 5: Nominal behaviour test case 3B -Autoware

Figure 7 :

 7 Figure 7: Detected collisions

Figure 8 :Figure 9 :

 89 Figure 8: Test case 2A -Autoware

Figure 10 :Figure 11 :

 1011 Figure 10: Test case 4B -Autoware

Table 1 : Test campaign Ego max speed (km/h)

 1

	Intruder	30	20
	speed (Km/h)
	40	Test 1A,1B,1C Test 4A,4B,4C
	25	Test 2A,2B,2C Test 5A,5B,5C
	35	Test 3A,3B,3C Test 6A,6B,6C

Table 2 :

 2 Collision Results

		EgoSpeed	IntruderSpeed
		AtCollision	AtCollision
		(Km/h)	(Km/h)
	No Stop Line
	Apollo_Test3A	30.3	35
	Autoware_Test4B 0	40
	Autoware_Test5B 3.6	25
	Autoware_Test6B 0	35
	Stop Line
	Apollo_Test4A	9.7	40
	Apollo_Test6A	9.7	35
	Apollo_Test3A	12.3	35
	Apollo_Test2A	8.7	25
	Autoware_Test2A 0	25
	Autoware_Test6A 0	35

These frameworks are in no way restrictive and can be adapted if judged useful.


Morgane Corre

Alice Lebreton
email: alice.lebreton@ens.psl.eu

Regulation of cold-inducible RNA-binding protein (CIRBP) in response to cellular stresses

Keywords: CIRBP, alternative splicing, stress, post-translational modification, subcellular localization, translation

Highlights

-CIRBP is a conserved, stress-responsive RNA-binding protein.

-CIRBP acts as a regulator of key pathways such as cell cycle and inflammation.

-CIRBP regulations range from transcription to protein modification and localization.

-Alternative splicing mechanisms modulate CIRBP expression and function.

Introduction

The cold-inducible RNA-binding protein (CIRBP, also referred to as CIRP or hnRNP A18) is part of the family of glycine-rich RNA-binding proteins (GRP) carrying an RNA recognition motif (RRM) [START_REF] Sheikh | Identification of several human homologs of hamster DNA damage-inducible transcripts: cloning and characterization of a novel UV-inducible cDNA that codes for a putative RNA-binding protein[END_REF]. CIRBP orthologs are extremely conserved across vertebrate species, and also share strong homologies with regulators of the same family in plants (Fig. 1A) [START_REF] Zhong | Recent progress in the research of cold-inducible RNA-binding protein[END_REF] In vertebrates, CIRBP is expressed in a wide variety of tissues [START_REF] Nishiyama | Diurnal change of the cold-inducible RNA-binding protein (Cirp) expression in mouse brain[END_REF] and usually encodes several protein isoforms, the most representative one being usually a 18-21 kDa polypeptide. It was first identified as a DNA-Damageinduced transcript [START_REF] Sheikh | Identification of several human homologs of hamster DNA damage-inducible transcripts: cloning and characterization of a novel UV-inducible cDNA that codes for a putative RNA-binding protein[END_REF][START_REF] Fornace | DNA damage-inducible transcripts in mammalian cells[END_REF] and hypothermia-induced protein [START_REF] Nishiyama | A glycine-rich RNA-binding protein mediating cold-inducible suppression of mammalian cell growth[END_REF] and later described as a key response factor to a broad panel of both extracellular and intracellular stresses [START_REF] Zhong | Recent progress in the research of cold-inducible RNA-binding protein[END_REF], which is also involved in the development of various diseases [START_REF] Lujan | Cold-inducible RNA binding protein in cancer and inflammation[END_REF]. The RRM spans the 90 first residues, the RG/RGG domain goes from the 90 th to the 137 th residues and then the RSY domain extends until the end of the protein sequence. (C) Crystal structure of the RRM domain of human CIRBP [START_REF] Coburn | Crystal structure of the human heterogeneous ribonucleoprotein A18 RNA-recognition motif[END_REF]. (D) Four main classes of human CIRBP transcripts isoforms, collapsed as the most representative transcript(s) for each group. Blue, stable transcripts encoding the canonical 18-kDa protein; pink, unstable, NMD-sensitive transcripts also encoding the canonical protein; purple, transcripts encoding a long form of the protein; and green, processed non-coding transcripts. (E) Nonexhaustive representation of CIRBP-stimulating conditions (pink bubbles) and subsequent CIRBP-dependent responses (blue bubbles).

CIRBP preferentially binds the 3'-untranslated regions (UTRs) of its target mRNA transcripts and regulates their polyadenylation site selection [START_REF] Liu | Cold-induced RNA-binding proteins regulate circadian gene expression by controlling alternative polyadenylation[END_REF], stability and translation [START_REF] Yang | Post-transcriptional regulation of thioredoxin by the stress inducible heterogenous ribonucleoprotein A18[END_REF], thus impacting key cellular pathways, such as apoptosis, cell cycle or inflammation [START_REF] Zhong | Recent progress in the research of cold-inducible RNA-binding protein[END_REF]. The polyvalence of CIRBP response to different contexts could be linked to the multiple layers of regulation of the gene itself.

For instance, while the structure of the protein appears quite simple, several post-translational modifications have been described [START_REF] Yang | Functional significance for a heterogenous ribonucleoprotein A18 signature RNA motif in the 3'-untranslated region of ataxia telangiectasia mutated and Rad3-related (ATR) transcript[END_REF][START_REF] De Leeuw | The cold-inducible RNA-binding protein migrates from the nucleus to cytoplasmic stress granules by a methylation-dependent mechanism and acts as a translational repressor[END_REF], which tune the role of CIRBP essentially by affecting its sub-cellular localization. Additionally, regulations of the transcription and alternative splicing (AS) of CIRBP impact its transcript stability and translation levels, thereby affecting downstream CIRBP regulatory functions.

CIRBP regulation is thus multileveled and multifactorial. This versatility allows its regulatory role to be co-opted for coordinating responses to various types of stressors across a broad range of tissues and species, although a thorough landscape of CIRBP relevance in vital cellular functions is still missing. In this review, we aim at summarizing CIRBP known regulation mechanisms, from transcription to protein relocalization, in the context of intracellular and extracellular stresses or physiological variations.

CIRBP is a conserved RNA-binding stress response regulator

CIRBP sequence conservation and functional domains

The CIRBP locus is located on chromosome 19 in humans and on chromosome 10 in mice, and encodes a 18.5 kDa protein (for the main isoform in these species). The protein amino-acid sequence is very conserved across species; human CIRBP shares 95% identity with murine, rat and chicken CIRBP, 74 to 78% with Xenopus xCIRBP or with its homolog in zebrafish, and under 50% with members of the GRP family in Arabidopsis thaliana (GRP7 and GRP8. [START_REF] Van Nocker | Two cDNAs from Arabidopsis thaliana encode putative RNA binding proteins containing glycine-rich domains[END_REF], arguing of an early origin of this family of regulators in Eukaryotes (Fig. 1A). The CIRBP protein comprises three main domains (Fig. 1B). The RNA-binding domain (RBD), also called RNA-recognition motif (RRM), is located in the N-terminal region of the protein and carries two consensus and highly conserved ribonucleoprotein sequences (RNP1 and RNP2) [START_REF] Sheikh | Identification of several human homologs of hamster DNA damage-inducible transcripts: cloning and characterization of a novel UV-inducible cDNA that codes for a putative RNA-binding protein[END_REF]. It adopts the classical fold of this domain consisting of four antiparallel beta-sheets stacked between two alpha helices (Fig. 1C), which creates an interaction platform with its targets. It has been shown to bind specific mRNA targets, preferentially in their 3'-UTRs [START_REF] Yang | The UV-inducible RNA-binding protein A18 (A18 hnRNP) plays a protective role in the genotoxic stress response[END_REF]. A putative 51-nucleotide consensus binding motif containing 6 invariant nucleotides in positions 5, 13, 14, 19, 29 and 51 has been proposed, based on the comparative analysis of the sequence of CIRBP mRNA targets identified by an immunotube-based strategy [START_REF] Yang | Functional significance for a heterogenous ribonucleoprotein A18 signature RNA motif in the 3'-untranslated region of ataxia telangiectasia mutated and Rad3-related (ATR) transcript[END_REF]. However, to date, the high number of putative CIRBP targets identified in RNA immunoprecipitation-or cross-linking and immunoprecipitation assays followed by sequencing argues for a rather low specificity in their recognition [START_REF] Yang | Functional significance for a heterogenous ribonucleoprotein A18 signature RNA motif in the 3'-untranslated region of ataxia telangiectasia mutated and Rad3-related (ATR) transcript[END_REF][START_REF] Indacochea | Cold-inducible RNA binding protein promotes breast cancer cell malignancy by regulating Cystatin C levels[END_REF][START_REF] Morf | Cold-inducible RNA-binding protein modulates circadian gene expression posttranscriptionally[END_REF][START_REF] Corre | Alternative splicing induced by bacterial pore-forming toxins sharpens CIRBPmediated cell response to Listeria infection[END_REF]. In the C-terminal region, the unstructured arginine-glycine (RG)-rich domain is characterized by its decoration with several RG/RGG motifs. This type of domain, common to CIRBP and other stress-induced proteins, is generally involved in mRNA processing and transport [START_REF] Yang | The UV-inducible RNA-binding protein A18 (A18 hnRNP) plays a protective role in the genotoxic stress response[END_REF]. As a target of various post-translational modifications, the RGG domain also serves as protein regulation platform [START_REF] Chong | RGG/RG motif regions in RNA binding and phase separation[END_REF]. Some authors consider that this domain is subdivided into an N-terminal RGG-rich portion containing three RGG repeats [START_REF] Aoki | Methylation of Xenopus CIRP2 regulates its arginineand glycine-rich region-mediated nucleocytoplasmic distribution[END_REF] and a C-terminal arginine, serine, tyrosine (RSY)-rich domain [START_REF] Bourgeois | Nonclassical nuclear localization signals mediate nuclear import of CIRBP[END_REF], both serving adjacent roles for protein cellular relocalization.

If the structure of the main form of the protein is rather simple, the transcription of the CIRBP loci and further maturation of its pre-mRNA can result in multiple mRNA transcripts. So far, 46 transcript variants have been identified in humans, among which 22 stable isoforms and 17 unstable isoforms (targeted for degradation by the nonsense-mediated decay -NMD-, see below) are coding, all in all resulting in at least 17 distinct CIRBP-encoded polypeptides differing in their amino-acid sequence. AS of CIRBP RNA isoforms thus represents both a quantitative regulatory layer and a potential reservoir of diversification for CIRBP protein sequence, structure and function around a common core.

CIRBP expression landscape across tissues

CIRBP displays a broad range of constitutive expression patterns across tissues as assessed in mice [START_REF] Nishiyama | Diurnal change of the cold-inducible RNA-binding protein (Cirp) expression in mouse brain[END_REF], with stronger levels in the brain, testis, lungs and heart and milder expression in the liver, thymus, spleen, stomach, intestine, colon kidney and uterus. Upon mild hypothermia, for instance during hibernation or circadian variations of body temperatures, CIRBP expression is amplified via the regulation of AS of its pre-mRNAs towards the shortest protein-coding isoform in the brain of mice [START_REF] Nishiyama | Diurnal change of the cold-inducible RNA-binding protein (Cirp) expression in mouse brain[END_REF][START_REF] Horii | Hypothermia induces changes in the alternative splicing pattern of cold-inducible RNA-binding protein transcripts in a nonhibernator, the mouse[END_REF], in the heart of Syrian hamsters [START_REF] Horii | Mild hypothermia causes a shift in the alternative splicing of cold-inducible RNA-binding protein transcripts in Syrian hamsters[END_REF] and in Xenopus oocytes [START_REF] Matsumoto | CIRP2, a major cytoplasmic RNA-binding protein in Xenopus oocytes[END_REF]. CIRBP expression is also largely impacted in cancer development, where it can act as an oncogene or as a tumor suppressor [START_REF] Lujan | Cold-inducible RNA binding protein in cancer and inflammation[END_REF], depending on the tissue. CIRBP localization is predominantly nuclear except in Xenopus oocytes [START_REF] Aoki | Methylation of Xenopus CIRP2 regulates its arginineand glycine-rich region-mediated nucleocytoplasmic distribution[END_REF], but the protein can be translocated to the cytoplasm and even released into the extracellular space upon specific stimulations, such as mild hypothermia, inflammation or exposure to ultraviolet (UV) light [START_REF] Zhong | Recent progress in the research of cold-inducible RNA-binding protein[END_REF]. The spatiotemporal regulation of CIRBP functions is thus dictated by its shuttling across cell membranes. Although ubiquitously expressed, CIRBP protein-coding isoforms, localization and functions can thus differ depending on the tissue, or in specific pathophysiological contexts.

Functions of CIRBP in stress responses and inflammation

Despite its name, CIRBP responds to a plethora of stresses beyond hypothermia. It is also positively regulated by UV exposure [START_REF] Sheikh | Identification of several human homologs of hamster DNA damage-inducible transcripts: cloning and characterization of a novel UV-inducible cDNA that codes for a putative RNA-binding protein[END_REF] and mild hypoxia [START_REF] Wellmann | Oxygen-regulated expression of the RNA-binding proteins RBM3 and CIRP by a HIF-1-independent mechanism[END_REF], and negatively regulated by inflammatory cytokines [START_REF] Lopez | Tumor necrosis factor and transforming growth factor β regulate clock genes by controlling the expression of the cold inducible RNA-binding protein (CIRBP)[END_REF], heat shock [START_REF] Haltenhof | A conserved kinase-based bodytemperature sensor globally controls alternative splicing and gene expression[END_REF], bacterial infection [START_REF] Corre | Alternative splicing induced by bacterial pore-forming toxins sharpens CIRBPmediated cell response to Listeria infection[END_REF], low circulating glucose [START_REF] Zhao | RNA-Binding Proteins and Alternative Splicing Genes Are Coregulated in Human Retinal Endothelial Cells Treated with High Glucose[END_REF] and oxidative stress [START_REF] Xue | Effects of ischemia and H 2 O 2 on the cold stress protein CIRP expression in rat neuronal cells[END_REF] (Fig. 1D). In response to these stressors, CIRBP acts itself as a gene expression regulator.

In rodents, mild hypothermia induces a change in CIRBP transcript isoform proportions by AS, which in turn favors CIRBP protein production [START_REF] Nishiyama | Diurnal change of the cold-inducible RNA-binding protein (Cirp) expression in mouse brain[END_REF][START_REF] Horii | Hypothermia induces changes in the alternative splicing pattern of cold-inducible RNA-binding protein transcripts in a nonhibernator, the mouse[END_REF][START_REF] Horii | Mild hypothermia causes a shift in the alternative splicing of cold-inducible RNA-binding protein transcripts in Syrian hamsters[END_REF][START_REF] Gotic | Temperature regulates splicing efficiency of the cold-inducible RNA-binding protein gene Cirbp[END_REF]. CIRBP binding, preferentially on a 51-base pair motif located in the 3'-UTRs of mRNAs, has been shown to stabilize its targets, inhibit their deadenylation and increase their translation by interacting with eIF4G1 [START_REF] Yang | Functional significance for a heterogenous ribonucleoprotein A18 signature RNA motif in the 3'-untranslated region of ataxia telangiectasia mutated and Rad3-related (ATR) transcript[END_REF]. Conversely, the recruitment of CIRBP to membraneless organelles named stress granules (SGs) upon certain stresses has been shown to repress translation [START_REF] De Leeuw | The cold-inducible RNA-binding protein migrates from the nucleus to cytoplasmic stress granules by a methylation-dependent mechanism and acts as a translational repressor[END_REF].

CIRBP targets are mostly involved in stress responses, but also in development, translation and circadian clock. CIRBP activation thus affects a large panel of cellular pathways, such as cell survival, cell death, proliferation, differentiation, tumorigenesis, telomere maintenance, etc. [START_REF] Zhong | Recent progress in the research of cold-inducible RNA-binding protein[END_REF] (Fig. 1D). Recent evidence also points CIRBP as a player in responses to viral and bacterial infections, with a rather pro-infectious role [START_REF] Corre | Alternative splicing induced by bacterial pore-forming toxins sharpens CIRBPmediated cell response to Listeria infection[END_REF][START_REF] Gokhale | Altered m 6 A modification of specific cellular transcripts affects flaviviridae infection[END_REF]. Independently of its RNA-binding characteristics, extracellular CIRBP can also act as damageassociated molecular pattern (DAMP) upon its release, and then as a sensor of sepsis or hemorrhagic shock, thereby playing an important role as a mediator of inflammatory responses in macrophages, neutrophils, lymphocytes and dendritic cells [START_REF] Aziz | Extracellular CIRP (eCIRP) and inflammation[END_REF].

The large panel of stressors to which CIRBP responds and the multiple pathways it impacts require a fine-tuning of its expression and function depending on conditions (Fig. 2).

Quantitative and qualitative regulations of CIRBP transcripts

A number of signaling pathways impact CIRBP expression. It is inhibited by TGF-β and TNF signaling in human and murine neurons [START_REF] Lopez | Tumor necrosis factor and transforming growth factor β regulate clock genes by controlling the expression of the cold inducible RNA-binding protein (CIRBP)[END_REF], while the GSK3-β kinase not only phosphorylates the CIRBP protein, but also increases CIRBP mRNA levels during the development of Alzheimer's disease [START_REF] Liu | Activation of glycogen synthase kinase-3 inhibits protein phosphatase-2A and the underlying mechanisms[END_REF]. CIRBP expression is also inhibited by intracellular calcium influx in U-2 OS human cells [START_REF] Fujita | TRPV4-dependent induction of a novel mammalian cold-inducible protein SRSF5 as well as CIRP and RBM3[END_REF]. In contrast, upon hypothermia, CIRBP expression is induced via the transient receptor potential vanilloid 4 TRPV4 receptor, although in a mechanism independent of this receptor ion channel activity [START_REF] Fujita | TRPV4-dependent induction of a novel mammalian cold-inducible protein SRSF5 as well as CIRP and RBM3[END_REF]. Similar results were recapitulated with receptors TRPV3 and 8 of the same family [START_REF] Fujita | Involvement of TRPV3 and TRPM8 ion channel proteins in induction of mammalian cold-inducible proteins[END_REF]. CIRBP mRNA levels were also found upregulated in response to porcine reproductive and respiratory syndrome virus (PRRSV) [START_REF] Xiao | Cold-inducible RNA-binding protein (CIRBP) promotes porcine reproductive and respiratory syndrome virus (PRRSV)-induced inflammatory response[END_REF], and by avian erythroblastosis virus E26 oncogene homolog-1 ETS-1 in Thp1 cells [START_REF] He | ETS-1 facilitates Th1 cell-mediated mucosal inflammation in inflammatory bowel diseases through upregulating CIRBP[END_REF] as driver of inflammation.

Regulation by CIRBP transcription

The first evidence of CIRBP regulation at the transcription level were brought by the observation that CIRBP could be transcribed from two alternative transcription start sites (TSS) upon cold shock in murine neuronal cells [START_REF] Al-Fageeh | Cold-inducible RNA binding protein (CIRP) expression is modulated by alternative mRNAs[END_REF]. The isoform with the longest 5'-UTR also harbored an internal ribosome entry site (IRES), possibly promoting CIRBP cap-independent translation in a context where overall cellular translation is diminished. Further studies showed that these isoforms were under the control of two promoters, bound by a different set of transcription factors in response to cold [START_REF] Al-Fageeh | Alternative promoters regulate cold inducible RNA-binding (CIRP) gene expression and enhance transgene expression in mammalian cells[END_REF]. Another study detected the presence of a temperature sensitive element within the first intron of the canonical isoform, which allowed response to circadian temperature oscillations [START_REF] Gotic | Temperature regulates splicing efficiency of the cold-inducible RNA-binding protein gene Cirbp[END_REF]. This study also showed that neither CIRBP pre-mRNA levels, nor RNA polymerase II occupancy on the CIRBP locus were affected by cold. This suggests a splicing-dependent regulation of CIRBP levels upon temperature variations. In HEK293T cells, another temperature-responsive element was identified in the 5' region upstream of the promoter, called mild-cold responsive element (MCRE) [START_REF] Sumitomo | Identification of a novel enhancer that binds Sp1 and contributes to induction of cold-inducible RNA-binding protein (cirp) expression in mammalian cells[END_REF]. This GC-rich octanucleotide, found within 1,000 bp of the transcription start site, acts as a transcriptional enhancer responsive to Sp1 binding. While the interaction in normothermia and mild hypothermia conditions is low, it is enhanced upon cold stress, when Sp1 accumulates in the nucleus, thereby increasing CIRBP transcription.

Altogether, these results suggest diverse mechanisms of regulation of CIRBP transcription, through alternative promoters, transcription start sites, enhancers or other regulatory motifs (Fig. 2). Yet, since most studies were performed in the context of hypothermia, transcriptional regulation of CIRBP in the context of stressors that repress CIRBP expression, like heat shock, would deserve further investigation.

Alternative splicing in favor of the canonical CIRBP transcript increases its stability and protein production

Beyond usage of alternative TSSs as described above, alternative splicing events have also been shown to finely tune the representation of CIRBP mRNA isoforms and subsequent protein production (Fig. 2). Two CIRBP mRNA isoforms with different expression pattern were initially described in the context of circadian cycles [START_REF] Nishiyama | Diurnal change of the cold-inducible RNA-binding protein (Cirp) expression in mouse brain[END_REF] and hibernation [START_REF] Sano | Hibernation-specific alternative splicing of the mRNA encoding cold-inducible RNA-binding protein in the hearts of hamsters[END_REF]. In the hearts of hibernating animals, long isoforms of CIRBP that harbor a retained intron and a premature stop codon are predominantly expressed in euthermia, but are spliced in favor of a shorter, protein-coding isoform in hibernation condition. This splicing results in a rapid increase in CIRBP protein expression as part of the response to cold. Splicing is also involved in CIRBP expression regulation upon daily temperature oscillations, as addition of the splicing inhibiting drug spliceostatin A abolished CIRBP accumulation upon mild hypothermia [START_REF] Gotic | Temperature regulates splicing efficiency of the cold-inducible RNA-binding protein gene Cirbp[END_REF].

Broadened use of high-depth RNA-sequencing resulted in more detailed insight into CIRBP splicing patterns in different conditions. In the context of viral infection of human Huh7 cells, m 6 A-RNAimmunoprecipitaion followed by sequencing has revealed another splicing-dependent regulation of CIRBP [START_REF] Gokhale | Altered m 6 A modification of specific cellular transcripts affects flaviviridae infection[END_REF], where an isoform encoding a 32-kDa version of CIRBP was spliced in favor of the shorter, canonical protein coding transcript. A methylation mark, placed in the coding sequence near the last exon-exon junction of the canonical isoform, was lost upon infection, resulting in reduced intron retention. This resulted in sustained expression of the short, canonical form of CIRBP despite general translation inhibition upon infection. A similar loss in methylation and effect on CIRBP AS was also observed upon Thapsigargin-induced endoplasmic reticulum (ER) stress [START_REF] Gokhale | Altered m 6 A modification of specific cellular transcripts affects flaviviridae infection[END_REF].

Alternative splicing towards non-canonical CIRBP transcripts decreases their stability and protein expression

In the context of heat shock, where CIRBP overall expression is decreased [START_REF] Gotic | Temperature regulates splicing efficiency of the cold-inducible RNA-binding protein gene Cirbp[END_REF], an opposite mechanism of regulation by AS has been recently described. High-resolution RNA-sequencing data has revealed that, upon hyperthermia (38°C and over), reduced CIRBP expression was most likely due to a preferential splicing of its pre-mRNAs into an unstable mRNA isoform instead of the canonical one [START_REF] Haltenhof | A conserved kinase-based bodytemperature sensor globally controls alternative splicing and gene expression[END_REF]. The two isoforms encode almost identical proteins, but they diverge in their 3'-UTRs, in which the unstable isoform harbors an exon-exon junction, rendering it sensitive to degradation by NMD [START_REF] Haltenhof | A conserved kinase-based bodytemperature sensor globally controls alternative splicing and gene expression[END_REF][START_REF] Neumann | Alternative splicing coupled mRNA decay shapes the temperature-dependent transcriptome[END_REF]. The reduced stability of this transcript results in decreased overall CIRBP mRNA and protein levels. This regulation of AS has been linked the stress responsive kinase CLK1. Upon heat shock, inhibition of the activity of CLK1 leads to the dephosphorylation of serinearginine-rich splicing factors (SRSFs). In turn, modulation of the activity of SRSFs leads to a switch in the splicing of many genes, including CIRBP.

A very similar regulation of CIRBP has recently been identified in the context of bacterial infection of human epithelial cells [START_REF] Corre | Alternative splicing induced by bacterial pore-forming toxins sharpens CIRBPmediated cell response to Listeria infection[END_REF]. The invasive bacterium Listeria monocytogenes secretes a poreforming toxin, listeriolysin O (LLO), the cytolytic activity of which inhibits CLK1 and SRSF phosphorylation, then leading to CIRBP isoform switch towards the above-mentioned NMDsensitive transcript and to a subsequent decrease in overall CIRBP expression. The switch impacts the cell response to bacterial infection, as expression of the canonical isoform favors infection, while the unstable one appears to be anti-bacterial, although the limited difference in the sequence of the two protein products (4 C-terminal amino-acids are missing in the product of translation from the unstable isoform) does not in principle give much latitude for extensive functional divergence between the two protein isoforms. Coincidentally, a shift in CIRBP-bound targets from housekeeping genes to stress-response targets was observed upon Listeria infection, in line with CIRBP contributing to cellular responses to infection-induced stress.

In cervical and uterine cancer cells, AS of CIRBP affecting its polyadenylation site in favor of less stable isoforms with shorter 3'-UTRs, and subsequent decrease in protein expression, negatively impact the prognosis of patients [START_REF] Shang | The prognostic value of DNA methylation, posttranslational modifications and correlated with immune infiltrates in gynecologic cancers[END_REF].

Alternative splicing of CIRBP thus allows a fine tuning of its expression, mostly centered around the intertwining of the production of the canonical isoform with that of other isoforms of different stability and/or coding potential (Fig. 2). In response to opposite types of stresses (cold versus heat, or high versus low circulating glucose levels [START_REF] Zhao | RNA-Binding Proteins and Alternative Splicing Genes Are Coregulated in Human Retinal Endothelial Cells Treated with High Glucose[END_REF]), fine-tuned modulation of CIRBP splicing and expression can craft an appropriate response for each condition.

Subcellular localization of CIRBP governs its functions

Intracellular CIRBP is decorated by post-translational modifications

The canonical CIRBP protein has been shown to be the target of several post-translational modifications, mostly affecting its cellular localization and subsequent function (Fig. 2). The homolog of CIRBP in Xenopus, xCIRBP, can interact in vitro with the arginine methyltransferase xPRMT1 [START_REF] Aoki | Methylation of Xenopus CIRP2 regulates its arginineand glycine-rich region-mediated nucleocytoplasmic distribution[END_REF]. Further experiments have shown that, in HeLa cells overexpressing xPRMT1 and transfected with xCIRBP, the latter was methylated in its RGG domain by xPRMT1, which induced its translocation from the nucleus to the cytoplasm. The methylation-dependent shuttling of CIRBP towards the cytoplasm has since been observed in other cells lines (Simian COS, human HeLa and murine NIH3T3) upon heat shock, oxidative, osmotic or ER stress [START_REF] De Leeuw | The cold-inducible RNA-binding protein migrates from the nucleus to cytoplasmic stress granules by a methylation-dependent mechanism and acts as a translational repressor[END_REF], and is enhanced upon PRMT1 overexpression [START_REF] Aoki | Methylation of Xenopus CIRP2 regulates its arginineand glycine-rich region-mediated nucleocytoplasmic distribution[END_REF]. In these stress conditions, following relocalization of methylated CIRBP to the cytoplasm, it is targeted to SGs, which act as mRNA-sequestering platform upon certain stresses. This relocalization requires an intact RRM, suggesting that it is dependent on CIRBP RNA-binding properties [START_REF] De Leeuw | The cold-inducible RNA-binding protein migrates from the nucleus to cytoplasmic stress granules by a methylation-dependent mechanism and acts as a translational repressor[END_REF]. When recruited to SGs, CIRBP acts as a translational repressor via the interaction of its unstructured RGG region, but not its RRM, with sequestered mRNAs. As mentioned above, this repressive function inside SGs contrasts with the previously-described role of CIRBP as facilitating the translation of its target mRNAs in the cytoplasm [START_REF] Yang | Post-transcriptional regulation of thioredoxin by the stress inducible heterogenous ribonucleoprotein A18[END_REF], emphasizing the strong impact of CIRBP localization on its function. Detailed investigation of CIRBP nucleo-cytoplasmic shuttling has revealed that its nuclear localization is dictated by the interaction between its RGG and RSY domains with the transportins TNPO1 and TNPO3 respectively [START_REF] Bourgeois | Nonclassical nuclear localization signals mediate nuclear import of CIRBP[END_REF]. Through the disruption of CIRBP liquid-liquid phase separation (LLPS) in presence of RNA, the transportins outcompete the recruitment of CIRBP to SGs. When CIRBP is methylated in its RGG domain, in contrast, the transportins, and especially TNPO1, dissociate, which opposes nuclear import, LLPS and recruitment to SGs. A following study showed that ATP binding to CIRBP was also able to disrupt LLPS and recruitment to SGs, probably by competing with RNA binding to the RGG [START_REF] Zhou | ATP regulates RNA-driven cold inducible RNA binding protein phase separation[END_REF]. Recently, the SRPK1 kinase was shown to phosphorylate CIRBP on the RGG domain, thereby impeding CIRBP LLPS in vitro and its recruitment to SGs in Hela cells, maybe by competing with PRMT1 for CIRBP availability [START_REF] Lenard | Phosphorylation regulates CIRBP arginine methylation, transportin-1 binding and liquid-liquid phase separation[END_REF]. Moreover, CIRBP phosphorylation by SRPK1 also impedes its recruitment by TNPO1.

The control of CIRBP localization by phosphorylation is substantiated by other studies. Indeed, in human RKO cells exposed to UV light, CIRBP is phosphorylated in its RGG, first by the CK2 kinase and then, on multiple sites by the GSK3β kinase [START_REF] Yang | Functional significance for a heterogenous ribonucleoprotein A18 signature RNA motif in the 3'-untranslated region of ataxia telangiectasia mutated and Rad3-related (ATR) transcript[END_REF]. These modifications induce its translocation from the nucleus to the cytoplasm, where it interacts with many target mRNAs, including that of the genotoxic stress associated kinase ATR, and increases its expression. Phosphorylation of CIRBP by GSK3β upon UV exposure additionally impacts it binding activity to targets such as thioredoxin (TRX) [START_REF] Yang | Post-transcriptional regulation of thioredoxin by the stress inducible heterogenous ribonucleoprotein A18[END_REF]. Interestingly, although SGs are formed upon UV exposure, CIRBP is not recruited to these compartments in this context [START_REF] Yang | The UV-inducible RNA-binding protein A18 (A18 hnRNP) plays a protective role in the genotoxic stress response[END_REF]. Neither is it translocated to the cytoplasm in response to mild hypothermia [START_REF] De Leeuw | The cold-inducible RNA-binding protein migrates from the nucleus to cytoplasmic stress granules by a methylation-dependent mechanism and acts as a translational repressor[END_REF], emphasizing yet again the versatility of CIRBP regulation mechanisms as a means to adapt its function to various stresses. Various redundant or complementary mechanisms are thus deployed, in an intricated network of regulations, to adjust the sub-cellular localization of CIRBP and thereby tightly control its function within the cells (Fig. 2).

Extracellular CIRBP

In addition to its intracellular role, CIRBP has been characterized as a DAMP that is translocated to the extracellular space in patients and murine models undergoing hemorrhagic shock or sepsis, and in macrophages under hypoxic stress [START_REF] Qiang | Cold-inducible RNA-binding protein (CIRP) triggers inflammatory responses in hemorrhagic shock and sepsis[END_REF] (Fig. 2). How exactly CIRBP is translocated from the cytoplasm to the extracellular space remains unclear, but as CIRBP lacks a signal peptide, its translocation through the canonical ER-Golgi route has been ruled out. Cell fractionation experiments detected CIRBP enriched in the lysosomal fraction, indicating that CIRBP could be release through lysosomal exocytosis. Another hypothesis suggests that cell necrosis releases CIRBP in the circulation where it can bind as a DAMP to the TLR4 receptors of neighbor cell and induce the release of inflammatory cytokines through activation of the NF-κB pathway, of the inflammasome through NLRP3, or through NETosis [START_REF] Aziz | Extracellular CIRP (eCIRP) and inflammation[END_REF]. After its release, extracellular CIRBP is mostly involved in organizing inflammatory responses.

Conclusions and future challenges

CIRBP regulation is multileveled, tissue-, cell type-and species-dependent, with sometimes opposed roles liable to a specific context. Although large parts of its regulatory pathways have been deciphered in various stress conditions, the precise molecular mechanism that govern CIRBP regulation at the transcript, protein or cellular localization level in a given context are still incomplete. The rise in the use of long-read sequencing technologies should help to clarify the diverse AS events affecting CIRBP expression and to pinpoint their consequences on transcript stability and protein expression. They could also shed light on apparent contradictory functions of CIRBP in distinct contexts, in case they depend on the expression of alternative isoforms.

Another argument in favor of systematically reviewing CIRBP expression patterns in any given condition also lies in its therapeutic potential. Indeed, it is already thoroughly described as a key factor in the development of many cancers [START_REF] Zhong | Recent progress in the research of cold-inducible RNA-binding protein[END_REF], inflammatory diseases [START_REF] Lujan | Cold-inducible RNA binding protein in cancer and inflammation[END_REF], degenerative pathologies [START_REF] Liu | Activation of glycogen synthase kinase-3 inhibits protein phosphatase-2A and the underlying mechanisms[END_REF], in responding to viral [START_REF] Gokhale | Altered m 6 A modification of specific cellular transcripts affects flaviviridae infection[END_REF] and bacterial [START_REF] Corre | Alternative splicing induced by bacterial pore-forming toxins sharpens CIRBPmediated cell response to Listeria infection[END_REF] infections, and more. Its role in the development of cancers is dual; while CIRBP is mostly described as a strong oncogene relevant in aggressive prognosis for melanoma, breast, colon and prostate cancers as well as in the induction of metastasis [START_REF] Zhong | Recent progress in the research of cold-inducible RNA-binding protein[END_REF], it can also act as a tumor suppressor in other pathological conditions, such as gynecological and nasopharyngeal cancers [START_REF] Hamid | Expression of cold-inducible RNA-binding protein in the normal endometrium, endometrial hyperplasia, and endometrial carcinoma[END_REF][START_REF] Lin | Loss of Cirbp expression is correlated with the malignant progression and poor prognosis in nasopharyngeal carcinoma[END_REF]. Enhancing CIRBP expression through so-called therapeutic hypothermia has launched promising anti-apoptotic effects resulting in protection against organ injury in animal models of ischemia and cardiac failure [START_REF] Sun | Therapeutic hypothermia protects photoreceptors through activating Cirbp pathway[END_REF][START_REF] Wu | Therapeutic hypothermia enhances cold-inducible RNA-binding protein expression and inhibits mitochondrial apoptosis in a rat model of cardiac arrest[END_REF]. Some strategies, like CIRBP neutralizing antibodies [START_REF] Godwin | Blocking cold-inducible RNA-binding protein protects liver from ischemia-reperfusion injury[END_REF], the use of the small CIRBP-derived peptide C23 [START_REF] Denning | C23, an oligopeptide derived from coldinducible RNA-binding protein, suppresses inflammation and reduces lung injury in neonatal sepsis[END_REF], or using CIRBP as a vaccination platform [START_REF] Silva | Cold-inducible RNA binding protein as a vaccination platform to enhance immunotherapeutic responses against hepatocellular carcinoma[END_REF] already yield promising results in treating hepatic ischemia, sepsis or tumoral targeting. Given the utmost significance of this factor in disease outcome, research on its regulation represents a tempting track of development relevant to public health.

Fig. 1 .

 1 Fig. 1. Structure and conservation of CIRBP. (A) Evolution of the CIRBP sequence across model organisms. Tree branch lengths represent genetic distance and percentages represent protein sequence identities, determined by BLASTP analysis on the RefSeq protein database. (B-C) Structure of CIRBP. (B) Map of the three described CIRBP domains in humans. The RRM spans the 90 first residues, the RG/RGG domain goes from the 90 th to the 137 th residues and then the RSY domain extends until the end of the protein sequence. (C) Crystal structure of the RRM domain of human CIRBP [7]. (D) Four main classes of human CIRBP transcripts isoforms, collapsed as the most representative transcript(s) for each group. Blue, stable transcripts encoding the canonical 18-kDa protein; pink, unstable, NMD-sensitive transcripts also encoding the canonical protein; purple, transcripts encoding a long form of the protein; and green, processed non-coding transcripts. (E) Nonexhaustive representation of CIRBP-stimulating conditions (pink bubbles) and subsequent CIRBP-dependent responses (blue bubbles).

Fig. 2 .

 2 Fig. 2. CIRBP regulation levels. Schematic representation of CIRBP regulation events and their associated stresses. CIRBP is regulated at the transcriptional, splicing, translational and post-translational levels. Regulation is stress-type-dependent and affects CIRBP cellular localization and molecular function in cells.

[START_REF] Corre | Alternative splicing induced by bacterial pore-forming toxins sharpens CIRBPmediated cell response to Listeria infection[END_REF]

Acknowledgements

We are grateful to the members of the group Bacterial infection, response & dynamics for their team spirit and careful proofreading of the manuscript, as well as to Alexandra Louis for helping with phylogeny.

Funding

MC received a doctoral fellowship from the MESRI (Ministère de l'enseignement supérieur, de la recherche et de l'innovation) and from LabEx MemoLife (ANR-10-LABX-54) under the program "Investissements d'Avenir" managed by the French national research agency.

Declaration of competing interest

The authors declare that they have no conflict of interest.