Disruption of wave-associated Rac GTPase-activating protein (Wrp) leads to abnormal adult neural progenitor migration associated with hydrocephalus

Sculpting Dendritic Spines during Initiation and Maintenance of Neuropathic Pain

Accumulating evidence has established a firm role for synaptic plasticity in the pathogenesis of neuropathic pain. Recent advances have highlighted the importance of dendritic spine remodeling in driving synaptic plasticity within the CNS. Identifying the molecular players underlying neuropathic pain induced structural and functional maladaptation is therefore critical to understanding its pathophysiology. This process of dynamic reorganization happens in unique phases that have diverse pathologic underpinnings in the initiation and maintenance of neuropathic pain. Recent evidence suggests that pharmacological targeting of specific proteins during distinct phases of neuropathic pain development produces enhanced antinociception. These findings outline a potential new paradigm for targeted treatment and the development of novel therapies for neuropathic pain. We present a concise review of the role of dendritic spines in neuropathic pain and outline the potential for modulation of spine dynamics by targeting two proteins, srGAP3 and Rac1, critically involved in the regulation of the actin cytoskeleton.

Beyond Axon Guidance: Roles of Slit-Robo Signaling in Neocortical Formation

The formation of the neocortex relies on intracellular and extracellular signaling molecules that are involved in the sequential steps of corticogenesis, ranging from the proliferation and differentiation of neural progenitor cells to the migration and dendrite formation of neocortical neurons. Abnormalities in these steps lead to disruption of the cortical structure and circuit, and underly various neurodevelopmental diseases, including dyslexia and autism spectrum disorder (ASD). In this review, we focus on the axon guidance signaling Slit-Robo, and address the multifaceted roles of Slit-Robo signaling in neocortical development. Recent studies have clarified the roles of Slit-Robo signaling not only in axon guidance but also in progenitor cell proliferation and migration, and the maturation of neocortical neurons. We further discuss the etiology of neurodevelopmental diseases, which are caused by defects in Slit-Robo signaling during neocortical formation.

Mind the (sr)GAP - roles of Slit-Robo GAPs in neurons, brains and beyond

The Slit-Robo GTPase-activating proteins (srGAPs) were first identified as potential Slit-Robo effectors that influence growth cone guidance. Given their N-terminal F-BAR, central GAP and C-terminal SH3 domains, srGAPs have the potential to affect membrane dynamics, Rho family GTPase activity and other binding partners. Recent research has clarified how srGAP family members act in distinct ways at the cell membrane, and has expanded our understanding of the roles of srGAPs in neuronal and non-neuronal cells. Gene duplication of the human-specific paralog of srGAP2 has resulted in srGAP2 family proteins that may have increased the density of dendritic spines and promoted neoteny of the human brain during crucial periods of human evolution, underscoring the importance of srGAPs in the unique sculpting of the human brain. Importantly, srGAPs also play roles outside of the nervous system, including during contact inhibition of cell movement and in establishing and maintaining cell adhesions in epithelia. Changes in srGAP expression may contribute to neurodevelopmental disorders, cancer metastasis and inflammation. As discussed in this Review, much remains to be discovered about how this interesting family of proteins functions in a diverse set of processes in metazoans and the functional roles srGAPs play in human disease.

Ulk4 Is Essential for Ciliogenesis and CSF Flow

Ciliopathies are an emerging class of devastating disorders with pleiotropic symptoms affecting both the central and peripheral systems and commonly associated with hydrocephalus. Even though ciliary components and three master transcriptional regulators have been identified, little is known about the signaling molecules involved. We previously identified a novel gene, Unc51-like-kinase 4 (ULK4), as a risk factor of neurodevelopmental disorders. Here we took multidisciplinary approaches and uncovered essential roles of Ulk4 in ciliogenesis. We show that Ulk4 is predominantly expressed in the ventricular system, and Ulk4(tm1a/tm1a) ependymal cells display reduced/disorganized cilia with abnormal axonemes. Ulk4(tm1a/tm1a) mice exhibit dysfunctional subcommissural organs, obstructive aqueducts, and impaired CSF flow. Mechanistically, we performed whole-genome RNA sequencing and discovered that Ulk4 regulates the Foxj1 pathway specifically and an array of other ciliogenesis molecules. This is the first evidence demonstrating that ULK4 plays a vital role in ciliogenesis and that deficiency of ULK4 can cause hydrocephalus and ciliopathy-related disorders.

SIGNIFICANCE STATEMENT

Ciliopathies are an emerging class of devastating disorders with pleiotropic symptoms affecting both the central and peripheral systems. Ciliopathies are commonly associated with hydrocephalus, and Unc51-like-kinase 4 (Ulk4) has been identified as one of 12 genes causing hydrocephalus in mutants. Here we uncover an essential role of Ulk4 in ciliogenesis. Ulk4 is predominantly expressed in the ventricles, and mutant ependymal cells display reduced/disorganized/nonfunctional motile cilia with abnormal axonemes and impaired CSF flow. Ulk4 modulates expression of the master regulator of ciliogenesis, Foxj1, and other ciliogenesis molecules. This is the first report demonstrating a vital role of Ulk4 in ciliogenesis. ULK4 deficiency may be implicated in human hydrocephalus and other ciliopathy-related disorders.

Dynamic expression of srGAP2 in cell nuclei and cytoplasm during the differentiation of rat neural stem cells in vitro

Different SLIT-ROBO Rho GTPase-activating proteins (srGAPs) have different levels of expression and diverse functions during neural development. Although srGAP2 is expressed in developmental brain tissue, little is known about its influence on cellular development of the nervous system. In the current study, dynamic expression of endogenous srGAP2 during neural stem cell/progenitor cell (NSC/NPC) differentiation in vitro was investigated in order to elucidate the association between the dynamic expression of srGAP2 and neural development. srGAP2 was expressed in undifferentiated NSCs/NPCs, and differentiated neurons and astrocytes with distinct expression patterns. In conjunction with the differentiation of NSCs/NPCs in vitro, the number of srGAP2+ cells markedly reduced. The percentage of srGAP2+ cells in the population of nestin+ and β‑tubulin III+ cells was significantly downregulated while in the population of glial fibrillary acidic protein‑positive cells, almost all cells were srGAP2+. srGAP2 was predominantly expressed in the cell nucleus in all cell types. srGAP2 was also weakly expressed in the cytoplasm of nestin+ and β‑tubulin III+ cells at 3 and 7 days in vitro. However levels were gradually downregulated during the process of differentiation and almost disappeared in β‑tubulin III+ cells at 14 days. The results from the present study suggest that srGAP2 is involved in regulating NSC/NPC differentiation during neural development. The translocation of srGAP2 in the cytoplasm and cell nucleus in different cell types may function as a director in decisions regarding cell fate.

Morphological and behavioral characterization of adult mice deficient for SrGAP3

SrGAP3 belongs to the family of Rho GTPase proteins. These proteins are thought to play essential roles in development and in the plasticity of the nervous system. SrGAP3-deficient mice have recently been created and approximately 10 % of these mice developed a hydrocephalus and died shortly after birth. The others survived into adulthood, but displayed neuroanatomical alteration, including increased ventricular size. We now show that SrGAP3-deficient mice display increased brain weight together with increased hippocampal volume. This increase was accompanied by an increase of the thickness of the stratum oriens of area CA1 as well as of the thickness of the molecular layer of the dentate gyrus (DG). Concerning hippocampal adult neurogenesis, we observed no significant change in the number of proliferating cells. The density of doublecortin-positive cells also did not vary between SrGAP3-deficient mice and controls. By analyzing Golgi-impregnated material, we found that, in SrGAP3-deficient mice, the morphology and number of dendritic spines was not altered in the DG. Likewise, a Sholl-analysis revealed no significant changes concerning dendritic complexity as compared to controls. Despite the distinct morphological alterations in the hippocampus, SrGAP3-deficient mice were relatively inconspicuous in their behavior, not only in the open-field, nest building but also in the Morris water-maze. However, the SrGAP3-deficient mice showed little to no interest in burying marbles; a behavior that is seen in some animal models related to autism, supporting the view that SrGAP3 plays a role in neurodevelopmental disorders.

SrGAP3 knockout mice display enlarged lateral ventricles and specific cilia disturbances of ependymal cells in the third ventricle

In several mouse models of mental retardation, ventricular enlargements have been observed. Mutation in the SrGAP3 gene residing on chromosome 3p25 has previously been associated with intellectual disability in humans. In addition, SrGAP3 is related to Rho-GAPs signaling pathways, which play essential roles in the development and plasticity of the nervous system. About 10 % of postnatal homozygous SrGAP3-deficient mice die due to hydrocephalus, whereas the remaining mice survive into adulthood but display enlarged ventricles. We analyze the ventricular enlargement of these mice by performing a post-mortem MRI approach. We found a more than 15-fold enlargement of the lateral ventricles of homozygous SrGAP3-deficient mice. Moreover, we demonstrate that this phenotype was not accompanied by a stenosis of the aqueduct. Instead, SrGAP3 knockout mice displayed reduced densities of cilia of ependymal cells in These third ventricle compared to age-matched controls. This results indicate that the ventricular enlargement may be due to ciliopathy.

A single PXXP motif in the C-terminal region of srGAP3 mediates binding to multiple SH3 domains

The Slit-Robo GTPase-activating protein 3 (srGAP3) has been implicated in different critical aspects of neuronal development. These findings have mainly been based on the characterisation of the three conserved globular N-terminal domains, while the function of the C-terminal region (CTR) is still unknown. We show that this predicted unstructured region acts as an adaptor by binding to the endocytic proteins Amphiphysin, Endophilin-A2, Endophilin-A1, as well as the Ras signalling protein Grb2. All these interactions depend on a single proline-rich motif in the CTR and the Src-homology 3 domains of the binding partners. Via these interactions srGAP3 could link receptor signalling events to the endocytic machinery.

Regulating Rac in the nervous system: molecular function and disease implication of Rac GEFs and GAPs

Rho family GTPases, including RhoA, Rac1, and Cdc42 as the most studied members, are master regulators of actin cytoskeletal organization. Rho GTPases control various aspects of the nervous system and are associated with a number of neuropsychiatric and neurodegenerative diseases. The activity of Rho GTPases is controlled by two families of regulators, guanine nucleotide exchange factors (GEFs) as the activators and GTPase-activating proteins (GAPs) as the inhibitors. Through coordinated regulation by GEFs and GAPs, Rho GTPases act as converging signaling molecules that convey different upstream signals in the nervous system. So far, more than 70 members of either GEFs or GAPs of Rho GTPases have been identified in mammals, but only a small subset of them have well-known functions. Thus, characterization of important GEFs and GAPs in the nervous system is crucial for the understanding of spatiotemporal dynamics of Rho GTPase activity in different neuronal functions. In this review, we summarize the current understanding of GEFs and GAPs for Rac1, with emphasis on the molecular function and disease implication of these regulators in the nervous system.

Dynamic shaping of cellular membranes by phospholipids and membrane-deforming proteins

All cellular compartments are separated from the external environment by a membrane, which consists of a lipid bilayer. Subcellular structures, including clathrin-coated pits, caveolae, filopodia, lamellipodia, podosomes, and other intracellular membrane systems, are molded into their specific submicron-scale shapes through various mechanisms. Cells construct their micro-structures on plasma membrane and execute vital functions for life, such as cell migration, cell division, endocytosis, exocytosis, and cytoskeletal regulation. The plasma membrane, rich in anionic phospholipids, utilizes the electrostatic nature of the lipids, specifically the phosphoinositides, to form interactions with cytosolic proteins. These cytosolic proteins have three modes of interaction: 1) electrostatic interaction through unstructured polycationic regions, 2) through structured phosphoinositide-specific binding domains, and 3) through structured domains that bind the membrane without specificity for particular phospholipid. Among the structured domains, there are several that have membrane-deforming activity, which is essential for the formation of concave or convex membrane curvature. These domains include the amphipathic helix, which deforms the membrane by hemi-insertion of the helix with both hydrophobic and electrostatic interactions, and/or the BAR domain superfamily, known to use their positively charged, curved structural surface to deform membranes. Below the membrane, actin filaments support the micro-structures through interactions with several BAR proteins as well as other scaffold proteins, resulting in outward and inward membrane micro-structure formation. Here, we describe the characteristics of phospholipids, and the mechanisms utilized by phosphoinositides to regulate cellular events. We then summarize the precise mechanisms underlying the construction of membrane micro-structures and their involvements in physiological and pathological processes.

A link between the nuclear-localized srGAP3 and the SWI/SNF chromatin remodeler Brg1

The Slit-Robo GTPase activating protein 3 (srGAP3) is an important modulator of actin cytoskeletal dynamics and has an important influence on a variety of neurodevelopmental processes. Mutations in the SRGAP3 gene on chromosome 3p25 have been found in patients with intellectual disability. Genome-wide association studies and behavioral assays of knockout mice had also revealed SRGAP3 as a risk gene for schizophrenia. We have recently shown that srGAP3 protein undergoes regulated shuttling between the cytoplasm and the nucleus during neuronal development. It is shown here that nuclear-localized srGAP3 interacts with the SWI/SNF remodeling factor Brg1. This interaction is mediated by the C-terminal of srGAP3 and the ATPase motif of Brg1. In the primary cultured rat cortical neurons, the levels of nuclear-localized srGAP3 and its interaction with Brg1 have a significant impact on dendrite complexity. Furthermore, the interaction between srGAP3 and Brg1 was also involved in valproic acid (VPA) -induced neuronal differentiation of Neuro2a cells. We then show that GTP-bound Rac1 and GAP-43 may be potential mediators of nuclear srGAP3 and Brg1. Our results not only indicate a novel signaling pathway that contributes to neuronal differentiation and dendrite morphology, but also implicate a novel molecular mechanism underlying srGAP3 regulation of gene expression.

The inverse F-BAR domain protein srGAP2 acts through srGAP3 to modulate neuronal differentiation and neurite outgrowth of mouse neuroblastoma cells

The inverse F-BAR (IF-BAR) domain proteins srGAP1, srGAP2 and srGAP3 are implicated in neuronal development and may be linked to mental retardation, schizophrenia and seizure. A partially overlapping expression pattern and highly similar protein structures indicate a functional redundancy of srGAPs in neuronal development. Our previous study suggests that srGAP3 negatively regulates neuronal differentiation in a Rac1-dependent manner in mouse Neuro2a cells. Here we show that exogenously expressed srGAP1 and srGAP2 are sufficient to inhibit valporic acid (VPA)-induced neurite initiation and growth in the mouse Neuro2a cells. While ectopic- or over-expression of RhoGAP-defective mutants, srGAP1^R542A and srGAP2^R527A exert a visible inhibitory effect on neuronal differentiation. Unexpectedly, knockdown of endogenous srGAP2 fails to facilitate the neuronal differentiation induced by VPA, but promotes neurite outgrowth of differentiated cells. All three IF-BAR domains from srGAP1-3 can induce filopodia formation in Neuro2a, but the isolated IF-BAR domain from srGAP2, not from srGAP1 and srGAP3, can promote VPA-induced neurite initiation and neuronal differentiation. We identify biochemical and functional interactions of the three srGAPs family members. We propose that srGAP3-Rac1 signaling may be required for the effect of srGAP1 and srGAP2 on attenuating neuronal differentiation. Furthermore, inhibition of Slit-Robo interaction can phenocopy a loss-of-function of srGAP3, indicating that srGAP3 may be dedicated to the Slit-Robo pathway. Our results demonstrate the interplay between srGAP1, srGAP2 and srGAP3 regulates neuronal differentiation and neurite outgrowth. These findings may provide us new insights into the possible roles of srGAPs in neuronal development and a potential mechanism for neurodevelopmental diseases.

Under lock and key: spatiotemporal regulation of WASP family proteins coordinates separate dynamic cellular processes

WASP family proteins are nucleation promoting factors that bind to and activate the Arp2/3 complex in order to stimulate nucleation of branched actin filaments. The WASP family consists of WASP, N-WASP, WAVE1-3, WASH, and the novel family members WHAMM and JMY. Each of the family members contains a C-terminus responsible for their nucleation promoting activity and unique N-termini that allow for them to be regulated in a spatiotemporal manner. Upon activation they reorganize the cytoskeleton for different cellular functions depending on their subcellular localization and regulatory protein interactions. Emerging evidence indicates that WASH, WHAMM, and JMY have functions that require the coordination of both actin polymerization and microtubule dynamics. Here, we review the mechanisms of regulation for each family member and their associated in vivo functions including cell migration, vesicle trafficking, and neuronal development.