Hyperactivity of Rac1-GTPase pathway impairs neuritogenesis of cortical neurons by altering actin dynamics

Abstract

The small-GTPase Rac1 is a key molecular regulator linking extracellular signals to actin cytoskeleton dynamics. Loss-of-function mutations in RAC1 and other genes of the Rac signaling pathway have been implicated in the pathogenesis of Intellectual Disability (ID). The Rac1 activity is negatively controlled by GAP proteins, however the effect of Rac1 hyperactivity on neuronal networking in vivo has been poorly studied. ArhGAP15 is a Rac-specific negative regulator, expressed in the main subtypes of pyramidal cortical neurons. In the absence of ArhGAP15, cortical pyramidal neurons show defective neuritogenesis, delayed axonal elongation, reduced dendritic branching, both in vitro and in vivo. These phenotypes are associated with altered actin dynamics at the growth cone due to increased activity of the PAK-LIMK pathway and hyperphosphorylation of ADF/cofilin. These results can be explained by shootin1 hypo-phosphorylation and uncoupling with the adhesion system. Functionally, ArhGAP15−/− mice exhibit decreased synaptic density, altered electroencephalographic rhythms and cognitive deficits. These data suggest that both hypo- and hyperactivation of the Rac pathway due to mutations in Rac1 regulators can result in conditions of ID, and that a tight regulation of Rac1 activity is required to attain the full complexity of the cortical networks.

Conditional Deletion of CC2D1A Reduces Hippocampal Synaptic Plasticity and Impairs Cognitive Function through Rac1 Hyperactivation

“…It is noteworthy that CC2D1A is abundantly expressed in both excitatory and inhibitory interneurons in the CA1 region of the mouse hippocampus. Recent studies have provided evidence that hyperactive Rac1 can affect the architecture and function of cortical and hippocampal inhibitory interneurons (Zamboni et al, 2016(Zamboni et al, , 2018. The functional role of CC2D1A in inhibitory interneurons remains to be determined.…”

“…Given that PAKs and cofilin are major downstream effectors of Rac1 in the regulation of actin cytoskeleton dynamics in dendritic spines (Pyronneau et al, 2017;Zamboni et al, 2018), we therefore examined whether Rac1-PAK-cofilin signaling is increased in cKO mice by assessing the phosphorylation state of PAK1-3 and cofilin. There were significantly higher levels of phospho-PAK1-3 (t (10) ϭ 2.27, p ϭ 0.04; n ϭ 6 in each group; two-tailed unpaired Student's t test) and phospho-cofilin (t (10) ϭ 3.03, p ϭ 0.01; n ϭ 6 in each group; two-tailed unpaired Student's t test) in hippocampal CA1 tissue homogenates in cKO mice compared with WT mice, whereas the total expression levels of PAK1 and cofilin were unchanged in cKO mice compared with WT mice (Fig.…”

Rho GTPases in Intellectual Disability: From Genetics to Therapeutic Opportunities

“…Actin polymerization, increased myosin II motor function and active association of actin-binding proteins with adhesion complexes promote neurite elongation and leading edge progression. The phosphorylation of shootin1 downstream of Rac has been recognized as a key mechanism to couple enhanced actin flow with cell adhesion via a linking bridge, known as a “clutch” [ 22 , 24 , 25 ].…”

“…ArhGAP15 is a Rac1-specific brain-specific GAP and its loss leads to an overall reduced efficiency of neurite elongation and branching, and a simpler morphology of pyramidal and hippocampal neurons [ 21 , 25 ]. Finally, ALS2/Alsin is a Rac1 GEF, that colocalizes with Rac1 within growth cones and promotes neurite outgrowth [ 46 ].…”

“…The loss of ArhGAP15 accompanies the loss of the recognized disease gene Zeb2 [ 173 ], nonetheless ARHGAP15 might contribute to the severity of these conditions or, alternatively, could act as a modifier gene. In mice loss of ArhGAP15 results in increased Rac1/Rac3 activity, reduced spine density, reduced axonal and dendritic complexity and cognitive deficits [ 21 , 25 ].…”