Linking WRP/srGAP3 to the Cognitive Deficits in 3p- Syndrome and Its Role in the Regulation of Dendritic Filopodia Formation
Rho GTPase signaling regulates a wide variety of cellular functions in the developing and adult central nervous system. These molecular switches are in turn spatially and temporally regulated by an over abundance of positive and negative regulatory proteins: the activating guanine nucleotide exchange factors (GEFs) and the deactivating GTPase activating proteins (GAPs). The WAVE-associated Rac GAP (WRP) is thought to regulate key aspects of synapse development and function, and has been implicated in a form of mental retardation in humans called 3p- Syndrome. WRP is a member of the srGAP family of Rho GAP domain containing proteins, which share a characteristic domain organization and are expressed throughout the brain. Recently, one of the members of this family was found to contain a newly described inverse F-BAR (IF-BAR) domain of unknown function and to regulate cortical migration in developing neurons. This study focuses on the regulatory capacity of WRP during the development of neuronal connections in the central nervous system, and what role its loss may have on cognitive functions.
To assess these roles, biochemical studies were performed to characterize the way in which WRP's novel IF-BAR domain interacted with lipid membranes. Additionally, WRP's role in regulating neuronal function was assessed both in vitro and in vivo through the use of mouse model systems for critical genes in the WAVE complex pathway, including a conditional WRP KO mouse developed in our lab. Finally, because WRP is implicated in mental retardation, behaviors of WRP heterozygous and null mice have been evaluated.
This study shows that WRP's IF-BAR domain senses, or facilitates, outward membrane protrusions through a convex lipid-binding surface of the dimerized WRP IF-BAR domain. WRP localizes to the membranes of dendritic shafts via its IF-BAR domain where it is enriched in filopodia like projections. During dendritic filopodia formation, WRP functions to regulate the WAVE-1 complex and its downstream effectors, including the Arp2/3 complex. Loss of WRP in vivo and in vitro results in a reduction of dendritic spines, and that this is a function of WRP's role in the initiation of dendritic filopodia, not during the maturation of dendritic filopodia into mature dendritic spines. Finally, this study demonstrates that the loss of WRP results in deficits in learning and memory, linking WRP to the cognitive deficits seen in 3p- syndrome.
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