TC10β/CDC42 GTPase activating protein is required for the growth of cortical neuron dendrites

Dendritic spine actin cytoskeleton in autism spectrum disorder

Dendritic spines are small actin-rich protrusions from neuronal dendrites that form the postsynaptic part of most excitatory synapses. Changes in the shape and size of dendritic spines correlate with the functional changes in excitatory synapses and are heavily dependent on the remodeling of the underlying actin cytoskeleton. Recent evidence implicates synapses at dendritic spines as important substrates of pathogenesis in neuropsychiatric disorders, including autism spectrum disorder (ASD). Although synaptic perturbations are not the only alterations relevant for these diseases, understanding the molecular underpinnings of the spine and synapse pathology may provide insight into their etiologies and could reveal new drug targets. In this review, we will discuss recent findings of defective actin regulation in dendritic spines associated with ASD.

Rho GTPase-activating proteins: Regulators of Rho GTPase activity in neuronal development and CNS diseases

The Rho family of small GTPases was considered as molecular switches in regulating multiple cellular events, including cytoskeleton reorganization. The Rho GTPase-activating proteins (RhoGAPs) are one of the major families of Rho GTPase regulators. RhoGAPs were initially considered negative mediators of Rho signaling pathways via their GAP domain. Recent studies have demonstrated that RhoGAPs also regulate numerous aspects of neuronal development and are related to various neurodegenerative diseases in GAP-dependent and GAP-independent manners. Moreover, RhoGAPs are regulated through various mechanisms, such as phosphorylation. To date, approximately 70 RhoGAPs have been identified; however, only a small portion has been thoroughly investigated. Thus, the characterization of important RhoGAPs in the central nervous system is crucial to understand their spatiotemporal role during different stages of neuronal development. In this review, we summarize the current knowledge of RhoGAPs in the brain with an emphasis on their molecular function, regulation mechanism and disease implications in the central nervous system.

Emerging roles of ARHGAP33 in intracellular trafficking of TrkB and pathophysiology of neuropsychiatric disorders

Intracellular trafficking of receptor proteins is essential for neurons to detect various extracellular factors during the formation and refinement of neural circuits. However, the precise mechanisms underlying the trafficking of neurotrophin receptors to synapses remain elusive. Here, we demonstrate that a brain-enriched sorting nexin, ARHGAP33, is a new type of regulator for the intracellular trafficking of TrkB, a high-affinity receptor for brain-derived neurotrophic factor. ARHGAP33 knockout (KO) mice exhibit reduced expression of synaptic TrkB, impaired spine development and neuropsychiatric disorder-related behavioural abnormalities. These deficits are rescued by specific pharmacological enhancement of TrkB signalling in ARHGAP33 KO mice. Mechanistically, ARHGAP33 interacts with SORT1 to cooperatively regulate TrkB trafficking. Human ARHGAP33 is associated with brain phenotypes and reduced SORT1 expression is found in patients with schizophrenia. We propose that ARHGAP33/SORT1-mediated TrkB trafficking is essential for synapse development and that the dysfunction of this mechanism may be a new molecular pathology of neuropsychiatric disorders.

The molecular mechanisms of neurotrophin receptor trafficking are only partially understood. Here the authors show that ARHGAP33 interacts with SORT1 to regulate TrkB trafficking, the dysfunction of which impairs synapse development and leads to schizophrenia-related behavioural abnormalities in mice.

NOMA-GAP/ARHGAP33 regulates synapse development and autistic-like behavior in the mouse

Neuropsychiatric developmental disorders, such as autism spectrum disorders (ASDs) and schizophrenia, are typically characterized by alterations in social behavior and have been linked to aberrant dendritic spine and synapse development. Here we show, using genetically engineered mice, that the Cdc42 GTPase-activating multiadaptor protein, NOMA-GAP, regulates autism-like social behavior in the mouse, as well as dendritic spine and synapse development. Surprisingly, we were unable to restore spine morphology or autism-associated social behavior in NOMA-GAP-deficient animals by Cre-mediated deletion of Cdc42 alone. Spine morphology can be restored in vivo by re-expression of wild-type NOMA-GAP or a mutant of NOMA-GAP that lacks the RhoGAP domain, suggesting that other signaling functions are involved. Indeed, we show that NOMA-GAP directly interacts with several MAGUK (membrane-associated guanylate kinase) proteins, and that this modulates NOMA-GAP activity toward Cdc42. Moreover, we demonstrate that NOMA-GAP is a major regulator of PSD-95 in the neocortex. Loss of NOMA-GAP leads to strong upregulation of serine 295 phosphorylation of PSD-95 and moreover to its subcellular mislocalization. This is associated with marked loss of surface α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor and defective synaptic transmission, thereby providing a molecular basis for autism-like social behavior in the absence of NOMA-GAP.

SNX26, a GTPase-activating protein for Cdc42, interacts with PSD-95 protein and is involved in activity-dependent dendritic spine formation in mature neurons

SNX26, a brain-enriched RhoGAP, plays a key role in dendritic arborization during early neuronal development in the neocortex. In mature neurons, it is localized to dendritic spines, but little is known about its role in later stages of development. Our results show that SNX26 interacts with PSD-95 in dendritic spines of cultured hippocampal neurons, and as a GTPase-activating protein for Cdc42, it decreased the F-actin content in COS-7 cells and in dendritic spines of neurons. Overexpression of SNX26 resulted in a GTPase-activating protein activity-dependent decrease in total protrusions and spine density together with dramatic inhibition of filopodia-to-spine transformations. Such effects of SNX26 were largely rescued by a constitutively active mutant of Cdc42. Consistently, an shRNA-mediated knockdown of SNX26 significantly increased total protrusions and spine density, resulting in an increase in thin or stubby type spines at the expense of the mushroom spine type. Moreover, endogenous expression of SNX26 was shown to be bi-directionally modulated by neuronal activity. Therefore, we propose that in addition to its key role in neuronal development, SNX26 also has a role in the activity-dependent structural change of dendritic spines in mature neurons.