Regulation of astrocyte migration occurs in a GEF-Rho GTPase dependent manner : with a discussion of Rho GTPase-effector choice

Abstract

I believe that these data indicate that Solo activation of Rac1 and RhoA GTPases induces GTPase-effector preference in a conformationally dependent manner. I propose that particular GEFs binding particular GTPases causes activation as well as a slight conformational change which could predispose the GTPase to preferentially bind downstream effectors due to altered protein binding affinity; I have dubbed this the "GEF-induced-GTPase preference" theory. I believe that the effects on astrocyte migration and morphology upon Solo knock down indicate that Solo interaction with Rac1 could predispose Rac1 to activate the PAK/Erk1/2 pathway and that Solo interaction with RhoA could predispose RhoA to activate the ROCK/LIMK pathway. Additionally, Solo's interaction with Filamin A and localization to the cell periphery in migratory cells indicates that Solo plays a role in the spatial activation of Rho GTPases and could be involved in formation of signaling scaffolds which bring cellular receptors, actin binding proteins, GTPases and GEFs in close proximity to influence spatiotemporal signaling.

My study indicates that astrocytes migrate robustly on aligned poly-L-lactic acid and when cultured on collagen IV, preferentially adhere to laminin 1, 1, 1, and spread most on fibronectin. Additionally, C8-S astrocytes migrate in a GTP-binding protein dependent manner which is facilitated by the GEF Solo. Solo co-precipitates with Rac1, RhoA, and Filamin A and relocates to regions of actin dynamics in migrating astrocytes indicating that it takes part in GTPase activation and possibly in forming signaling scaffolds on actin fibers through Filamin A binding. Additionally, astrocyte migration is significantly curtailed when Solo expression is knocked down via siRNA. Astrocyte cell length, cell area, and stress fiber numbers decreased as well and this indicates the possibility that Solo plays a crucial role in astrocyte migratory signaling.

Astrocyte migration is integral to maintenance of the adult central nervous system, guiding neuronal development, and is involved in a variety of glial pathologies. Understanding astrocyte behavior when cultured on various extracellular materials and matrix proteins gives insight into outside-in signaling and presents avenues for biomedical exploration. Astrocytes migrate by manipulating the actin cytoskeleton through activation of Rho GTPases and subsequent effectors and this activation is accomplished by guanine nucleotide exchange factors (GEFs). Of particular interest to regulating actin dynamics is the primarily migratory GEF Solo, a short isoform of Trio, which is expected to activate Rac1 and bind Filamin A based on structural similarity between protein family members. This study aims to determine how C8-S astrocytes alter migration and morphology on electrospun fibers and fibrous extracellular matrix proteins, explore the expression of Dbl family GEFs in a serum induced migratory context, define which downstream proteins Solo interacts with, and elucidate the role which Solo plays in astrocyte migratory signaling.