Functions of membrane deforming-proteins during neuronal development

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DESCRIPTION (provided by applicant): During brain development, proper neuronal migration and morphogenesis is critical for the establishment of functional circuits. Both neuronal migration as well as axon and dendrite differentiation requires extensive membrane remodeling and cytoskeleton dynamics. Until recently, most studies in this field have focused on proteins directly regulating microtubules and actin cytoskeletal dynamics. However, recent evidence suggests that a new class of molecules directly controlling membrane deformation and dynamics (BAR-like superfamily subdivided into BAR / N-BAR, F-BAR, and I-BAR domains) regulate important cell biological processes ranging from membrane invagination (endocytosis) to membrane protrusion (filopodia formation). The most recently identified, the F-BAR subfamily, has mostly been studied in cell lines or more reductionist in vitro systems and the 23 human genes of this sub-family have poorly characterized functions in vivo. Recently, a large deletion in one of these genes called srGAP3 or MEGAP was shown to cause a familial form of severe mental retardation called 3p- syndrome suggesting that some F-BAR containing proteins might play important functions during brain development. Furthermore, several other genes containing either GAP domains or BAR domains such as Oligophrenin-1 (Billuart et al., 1998) and OCRL-1 (Nussbaum et al. 1997) have been found to be mutated in severe forms of mental retardation. Therefore, our work on srGAP2 will provide a framework to improve our understanding of the function of proteins sharing similar domains during cortical development. We recently initiated the study of a F-BAR domain-containing protein called srGAP2 and found that this protein is a novel regulator of neurite growth and branching (Guerrier et al., 2009). This function requires its N- terminal F-BAR domain. Surprisingly, unlike previously characterized F-BAR domains which regulate membrane invagination and endocytosis, the F-BAR domain of srGAP2 induces filopodia-like membrane protrusions in COS7 cells and cortical neurons resembling those induced by I-BAR domains in vitro and in vivo. Importantly our structure- function analysis led us hypothesize that the regulation of membrane deformation by F-BAR domain-containing proteins plays critical roles during neuronal morphogenesis. We further hypothesize that srGAP2 activation involves release from an autoinhibitory conformation through binding of protein interactors to its SH3 domain. We will test this hypothesis using multiple approaches divided in three specific aims: in Aim1, we will identify the molecular mechanisms regulating the function of srGAP2 in cortical neurons by using combinations of biochemical and cell biological approaches in order to identify and characterize the binding partners of its SH3 domain which is critical for the regulation of srGAP2 activity during neuronal migration and morphogenesis. In Aim 2, we will explore the function of srGAP2 in axon/dendrite branching induced by well-characterized branching factors such as Slit1/2 and BDNF in vitro and in vivo. In Aim 3, we will test the requirement of srGAP2 for proper neuronal migration and morphogenesis in vivo using a recently characterized gene-trap allele of srGAP2. We will also test the functional redundancy of srGAP2 and srGAP3 which share both structural organization and developmental expression patterns. PUBLIC HEALTH RELEVANCE: During brain development, proper neuronal migration and morphogenesis is critical for the establishment of functional neuronal circuits. Here we propose to study the function of a novel family of proteins regulating membrane deformation (F-BAR proteins) in neuronal migration and morphogenesis. This work will provide important new insights into the developmental mechanisms leading to a wide range of pathologies including severe mental retardation (3p- syndrome).