Several factors contribute to the failure of the central nervous system (CNS) to regenerate after injury. These include inhibition of axonal growth by myelin and glial scar associated molecules, as well as the intrinsic inability of adult CNS neurons to grow long axons in environments that are permissive for younger neurons. Neurons in the peripheral nervous system (PNS) display a much higher capacity to regenerate after injury than CNS neurons, as shown by conditioning lesion experiments and by microtransplantation of dorsal root ganglia neurons into CNS white matter tracts. Our central hypothesis is that neurons of the PNS express specific regeneration associated genes that mediate their enhanced growth response after injury. We have employed a combination of subtractive hybridization, microarray comparison and promoter analysis to probe for genes specific to neurons of the dorsal root ganglia (DRG), using cerebellar granule neurons (CGN) as a reference. We have identified over a thousand different genes, many of whose products form interaction networks and signaling pathways. Moreover, we have identified several dozen transcription factors that may play a role in establishing DRG neuron identity and shape their responses after injury. One of these transcription factors is Signal Transducer and Activator of Transcription 3 (STAT3), previously known to be upregulated in the PNS after a conditioning lesion but not known to be specific to the PNS. Using a real time PCR and immunochemical approaches we have shown that STAT3 is constitutively expressed and selectively active in DRG neurons both in culture and in vivo. We show that the overexpression of wild type STAT3 in cerebellar granule neurons leads to the formation of supernumerary neurites, whereas the overexpression of constitutively active STAT3-C leads to a 20% increase in total neurite outgrowth. It is hoped that the genetic delivery of STAT3-C, potentially combined with co-activators of transcription, will improve functional regeneration of CNS axons in vivo.

La douleur est perçue par des neurones spécialisés, les nocicepteurs, dont le corps cellulaire est localisé, au niveau du tronc, dans les ganglions de la racine dorsale (DRG). Ces neurones détectent les informations sensorielles en périphérie (peau, muscles ou viscères) et les transmettent aux neurones spinaux qu'ils connectent au niveau de la corne dorsale de la moelle épinière. D'un point de vue morphologique, anatomique, physiologique, mais également moléculaire, une hétérogénéité importante de ces neurones est observée. Le but principal du laboratoire est de trouver de nouvelles molécules impliquées dans les mécanismes moléculaires qui spécifient les différentes sous-populations neuronales des DRG. Dans ce cadre, il a été possible d'identifier et valider plusieurs gènes présentant un profil d'expression très particulier et spécifiant des populations neuronales bien distinctes au sein des DRG. Parmi ces gènes, tafa4 est principalement exprimé dans des neurones non-peptidergiques de type C. Tafa4 est une petite protéine sécrétée proche des chemokines de type CC dont la fonction est jusqu'à présent inconnue, et dont l'expression dans les DRG n'a encore jamais été décrite. Au cours de ce travail, j'ai pu identifier Tafa4 comme un nouveau marqueur d'une sous-population de neurones sensoriels des DRG : les C-LTMRs (C-Low Threshold MechanoReceptor). La génération d'une lignée de souris Tafa4 KO dans laquelle le gène tafa4 a été remplacé par la protéine fluorescente Vénus, nous a permis de mettre en évidence que la population de neurones tafa4+ projette en central dans la lamina II interne de la moelle épinière et en périphérique exclusivement au niveau de la peau poilue. / The perception of pain is initiated by the detection of noxious stimuli by the peripheral endings of primary nociceptive neurons. They are a specialized group of small-diameter pseudounipolar neurons with cell bodies in the dorsal roots ganglia (DRG). They give rise to thinly myelinated (Ad-fibers) or unmyelinated (C-fibers) afferent fibers, which convey the signal from the periphery to the dorsal horn of the spinal cord. Our laboratory is interested in molecular mechanisms which underlie the specification of somatic sensory neurons and their properties. In order to find novel molecular factors involved in this process, we identified several new nociceptor subtype specific genes by microarray experiments. Among these genes, tafa4 which encodes a small secreted protein distantly related to CC chemokine with unknown function, appears to have a DRG-specific expression from early developmental stages and becomes restricted to a subset of C-fibers non-peptidergic nociceptors in adult DRG. By using transgenic mice, we show that Tafa4 neurons specifically project to the dorsal horn lamina IIi and innervate the hairy skin. They have electrophysiological signature of C-Low-threshold mechanoreceptors (C-LTMRs), a population of sensory neurons implicated in the injury-induced mechanical hyper-sensitivity as well as in the affective component of touch. Mutant mice lacking Tafa4 do not present developmental defects and specify Tafa4 population correctly. However, despite no obvious molecular changes in Tafa4 mutants, these mice display significant increase in tissue injury induced hyper-sensitivity which could be reduced by intrathecally applied Tafa4 protein.