The GTPase Rab21 is required for neuronal development and migration in the cerebral cortex.

Development of the mammalian neocortex requires proper inside-out migration of developing cortical neurons from the germinal ventricular zone toward the cortical plate. The mechanics of this migration requires precise coordination of different cellular phenomena including cytoskeleton dynamics, membrane trafficking, and cell adhesion. The small GTPases play a central role in all these events. The small GTPase Rab21 regulates migration and neurite growth in developing neurons. Moreover, regulators and effectors of Rab21 have been implicated in brain pathologies with cortical malformations, suggesting a key function for the Rab21 signaling pathway in cortical development. Mechanistically, it has been posited that Rab21 influences cell migration by controlling the trafficking of endocytic vesicles containing adhesion molecules. However, direct evidence of the participation of Rab21 or its mechanism of action in the regulation of cortical migration is still incomplete. In this study, we demonstrate that Rab21 plays a critical role in the differentiation and migration of pyramidal neurons by regulating the levels of the amyloid precursor protein on the neuronal cell surface. Rab21 loss of function increased the levels of membrane-exposed APP, resulting in impaired cortical neuronal differentiation and migration. These findings further our understanding of the processes governing the development of the cerebral cortex and shed light onto the molecular mechanisms behind cortical development disorders derived from the malfunctioning of Rab21 signaling effectors.

BARS Influences Neuronal Development by Regulation of Post-Golgi Trafficking.

Neurons are highly polarized cells requiring precise regulation of trafficking and targeting of membrane proteins to generate and maintain different and specialized compartments, such as axons and dendrites. Disruption of the Golgi apparatus (GA) secretory pathway in developing neurons alters axon/dendritic formation. Therefore, detailed knowledge of the mechanisms underlying vesicles exiting from the GA is crucial for understanding neuronal polarity. In this study, we analyzed the role of Brefeldin A-Ribosylated Substrate (CtBP1-S/BARS), a member of the C-terminal-binding protein family, in the regulation of neuronal morphological polarization and the exit of membrane proteins from the Trans Golgi Network. Here, we show that BARS is expressed during neuronal development in vitro and that RNAi suppression of BARS inhibits axonal and dendritic elongation in hippocampal neuronal cultures as well as largely perturbed neuronal migration and multipolar-to-bipolar transition during cortical development in situ. In addition, using plasma membrane (PM) proteins fused to GFP and engineered with reversible aggregation domains, we observed that expression of fission dominant-negative BARS delays the exit of dendritic and axonal membrane protein-containing carriers from the GA. Taken together, these data provide the first set of evidence suggesting a role for BARS in neuronal development by regulating post-Golgi membrane trafficking.