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. We have accumulated evidence demonstrating that a F-BAR containing protein called srGAP2 is a novel negative regulator of neuronal migration and morphology. This function requires its N-terminal F-BAR domain and surprisingly, unlike previously characterized domains of this family, we found that the F-BAR domain of srGAP2 induces filopodia-like membrane protrusions resembling those induced by I-BAR domains in cell lines and in cortical neurons in vitro and in vivo. Previous work has demonstrated that in non-neuronal cell lines, induction of filopodia decreases the rate of cell migration and the persistence of leading edge protrusions. We found that knockdown of srGAP2 expression reduces leading process morphology and increases the rate of neuronal migration in vivo. Overexpression of srGAP2 or its F-BAR domain have the opposite effects, increasing leading process dynamics and blocking migration. Importantly, expression of the F-BAR domain with a 49aa truncation in its C-terminal domain which localizes to the membrane but fails to elicit filopodia-like membrane protrusions, does not inhibit neuronal migration. Finally, we found that the two other functional domains of srGAP2 (a Rac1-specific GAP domain and a SH3 domain) also participate to srGAP2 function in neuronal migration. These results led us to formulate the hypothesis that direct regulation of membrane deformation by F-BAR-like proteins plays critical roles in regulating neuronal migration and morphogenesis. 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 neuronal migration and morphogenesis in vivo using a genetic loss-of- function approach taking advantage of a srGAP2 gene trap mouse line that we recently acquired.
In Aim 3, we will test the function of three other uncharacterized, brain-specific F-BAR containing proteins (srGAP3/MEGAP, Gas7, and FCHo1) in neuronal migration and morphogenesis using combinations of in vitro and in vivo approaches.
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).