Ca2+ influx through ionotropic glutamate receptors regulates a variety of developmental processes including neurite outgrowth and naturally occurring cell death. In the CNS, NMDA receptors were originally thought to be the sole source of Ca2+ influx through glutamate receptors; however, AMPA receptors also allow a significant influx of Ca2+ ions. The Ca2+ permeability of AMPA receptors is regulated by the insertion of one or more edited GluR2 subunits into the receptors. Although Ca2+-permeable AMPA receptors are a familiar feature in developing neurons, the developmental function of these receptors during the formation of the nervous system has yet to be established. This study was designed to investigate the expression and functional role of Ca2+-permeable AMPA receptors in developing chicken spinal motoneurons. Our results demonstrate that chicken lumbar motoneurons express functional AMPA receptors as early as embryonic day (E) 5. Electrophysiological recordings of kainate-evoked currents indicate a significant reduction in the Ca2+ permeability of AMPA receptors between E6 and E11. During this developmental period, the Ca2+ permeability of AMPA receptors decreases three-fold. Reduction in the Ca2+ permeability of AMPA receptors is accompanied by increased expression of GluR2 mRNA in the spinal motoneuron pool. Changes in GluR2 mRNA expression occur in parallel to changes in GluR2 protein expression in the chicken ventral spinal cord. Changes in the Ca2+-permeability of AMPA receptors are not mediated by age-dependent changes in the editing pattern of GluR2 subunits. At early stages of development, functional AMPA receptors were composed of a combination of GluR3 and GluR4 subunits. mRNA analysis indicates that GluR4 is the most abundant subunit in the chicken ventral spinal cord between E6 and E11. Immunohistochemistry analysis of spinal cord sections also demonstrated that both GluR3 and GluR4 proteins are expressed at E6 and E11. Expression of Ca2+-permeable AMPA receptors regulates the maturation of dendritic outgrowth in developing spinal motoneurons. Measurements of dendritic length and branching pattern demonstrate significant changes in the dendritic morphology of spinal motoneurons between E6 and E11. Blockade of AMPA receptor activation with CNQX between E5 and E8 causes a significant increase in dendritic outgrowth in lumbar motoneurons, when compared with vehicle-treated embryos. Treatment of chicken embryos with CNQX between E8 and E11, when AMPA receptors become Ca2+-impermeable, has no affect on dendritic morphology. However, blockade of NMDA receptor activation with MK-801 causes a significant reduction in dendritic outgrowth of lumbar motoneurons by E11. These findings indicate that AMPA receptor activation between E5 and E8 limits dendritic outgrowth in developing motoneurons, whereas NMDA receptor activation is involved in dendritic remodeling after the establishment of synaptic contacts with sensory afferents.

Le développement cérébral est une succession d'étapes aboutissant à l'établissement d'un réseau neuronal. Il fait intervenir de nombreuses molécules comme des protéines d'adhésion permettant l'interaction des neurones avec leur environnement. L'implication de nombreux gènes codant des protéines d'adhésion dans la physiopathologie de maladies neuropsychiatriques comme l'autisme souligne l'intérêt à en identifier de nouveaux. Pendant ma thèse, j'ai pu caractériser deux nouveaux gènes, Susd2 et Susd4, codant des protéines contenant des domaines CCP (Complement Control Protein), classiquement connus pour leur présence dans les protéines participant à la régulation du système du Complément. Récemment, des protéines à domaines CCP ont été décrites chez la souris comme ayant une fonction dans le développement neuronal. L'existence de nombreuses protéines prédites à domaines CCP sans fonction connue m'ont conduit à tenter de caractériser Susd2 et Susd4 qui en font partie.Susd2 est exprimé dans les neurones au sein de cultures de cellules d'hippocampe de rat. Son expression atteint un pic à un stade post natal précoce, suggérant une fonction développementale. La protéine Susd2 recombinante a une localisation neuronale diffuse, mais est particulièrement enrichie dans les synapses excitatrices. La diminution de l'expression de Susd2 a pour conséquences un défaut de croissance axonale, une augmentation de la croissance dendritique, et une inhibition spécifique de la synaptogénèse excitatrice. Susd4 est également exprimé dans les neurones, avec un pic d'expression au stade embryonnaire, et semble jouer un rôle de régulation du développement dendritique. / During brain development, several steps precisely coordinated lead to establishment of a functional neuronal network. Many molecules participate to this process, including adhesion proteins mediating interactions between neurons and their environment. Involvement of numerous genes coding for adhesion proteins in neuropsychiatric diseases such as autism argue for usefulness of identifying new ones. During my PhD, I characterized two new genes, Sud2 and Susd4, coding for proteins containing CCP domains (Complement Control Protein), classically described in proteins involved in Complement regulation system. Recently, in mammals, CCP containing proteins were shown to be involved in neuronal development. Identification of several predicted CCP containing proteins without a known function prompted me to characterize Susd2 and Susd4 which are part of them.Susd2 is expressed in neurons from hippocampal cell cultures. Its peak of expression takes place in early post natal period, suggesting a developmental function. Susd2 recombinant protein has a diffuse neuronal localization, but is particularly enriched in excitatory synapses. Decreased expression of Susd2 leads to decreased axonal growth, increased dendritic growth, and specific inhibition of excitatory synaptogenesis. Susd4 is also expressed in neurons, with a peak of expression during embryonic development, and seems to act as a regulator of dendritic growth.