SrGAP3 interacts with lamellipodin at the cell membrane and regulates Rac-dependent cellular protrusions

Structuring and validating a prognostic model for low-grade gliomas based on the genes for plasma membrane tension

Background

Recent studies indicate that cell mechanics are associated with malignancy through its impact on cell migration and adhesion. Gliomas are the most common primary malignant brain tumors. Low-grade gliomas (LGGs) include diffuse LGGs (WHO grade II) and intermediate-grade gliomas (WHO grade III). Few studies have focused on membrane tension in LGGs. Herein, we assessed the prognostic value of plasma membrane tension-related genes (MTRGs) in LGGs.

Methods

We selected plasma MTRGs identified in previous studies for analysis. Based on LGG RNA sequencing (RNA-seq) data in The Cancer Genome Atlas, a prognostic signature containing four genes was constructed via log-rank testing, LASSO regression and stepwise multivariate Cox regression and was validated with other datasets. Additionally, functional annotation, pathway enrichment and immune and molecular characteristics of the prognostic model defined subgroups were analyzed. Thereafter, a predictive nomogram that integrated baseline characteristics was constructed to determine the 3, 5, and 10-year overall survival (OS) of patients with LGG. Differentially expressed genes were confirmed via quantitative reverse-transcription polymerase chain reaction (qRT-PCR) and immunohistochemistry (IHC).

Results

Our MTRG prognostic signature was based on ARFIP2, PICK1, SH3GL2, and SRGAP3 expression levels. The high-risk group was more positively associated with apoptosis and cell adhesion pathways and exhibited a low IDH1 mutation rate, high TP53 mutation rate and a low 1p19q co-deletion rate. The high-risk group also exhibited incremental infiltration of immune cells, more forceful immune activities and high expression of immune checkpoints as well as benefited less from immune therapy compared with the low-risk group. Our prognostic model had better forecasting ability than other scoring systems. We found that the nomogram was a better tool for predicting outcomes for patients with LGG. Finally, qRT-PCR confirmed that SH3GL2 and SRGAP3 expression levels in glioma tissues were significantly lower than those in normal brain tissues. The results of IHC analysis confirmed that SH3GL2 protein expression was higher in patients with longer survival.

Conclusion

Our plasma membrane tension-related gene prognostic signature is a prospective tool that can differentiate between prognosis, gene mutation landscape, immune microenvironment, immune infiltration and immunotherapeutic efficacy in LGG.

C. elegans srGAP is an α-catenin M domain-binding protein that strengthens cadherin-dependent adhesion during morphogenesis

The cadherin-catenin complex (CCC) is central to embryonic development and tissue repair, yet how CCC binding partners function alongside core CCC components remains poorly understood. Here, we establish a previously unappreciated role for an evolutionarily conserved protein, the slit-robo GTPase-activating protein SRGP-1/srGAP, in cadherin-dependent morphogenetic processes in the Caenorhabditis elegans embryo. SRGP-1 binds to the M domain of the core CCC component, HMP-1/α-catenin, via its C terminus. The SRGP-1 C terminus is sufficient to target it to adherens junctions, but only during later embryonic morphogenesis, when junctional tension is known to increase. Surprisingly, mutations that disrupt stabilizing salt bridges in the M domain block this recruitment. Loss of SRGP-1 leads to an increase in mobility and decrease of junctional HMP-1. In sensitized genetic backgrounds with weakened adherens junctions, loss of SRGP-1 leads to late embryonic failure. Rescue of these phenotypes requires the C terminus of SRGP-1 but also other domains of the protein. Taken together, these data establish a role for an srGAP in stabilizing and organizing the CCC during epithelial morphogenesis by binding to a partially closed conformation of α-catenin at junctions.

Revisiting I-BAR Proteins at Central Synapses

Dendritic spines, the distinctive postsynaptic feature of central nervous system (CNS) excitatory synapses, have been studied extensively as electrical and chemical compartments, as well as scaffolds for receptor cycling and positioning of signaling molecules. The dynamics of the shape, number, and molecular composition of spines, and how they are regulated by neural activity, are critically important in synaptic efficacy, synaptic plasticity, and ultimately learning and memory. Dendritic spines originate as outward protrusions of the cell membrane, but this aspect of spine formation and stabilization has not been a major focus of investigation compared to studies of membrane protrusions in non-neuronal cells. We review here one family of proteins involved in membrane curvature at synapses, the BAR (Bin-Amphiphysin-Rvs) domain proteins. The subfamily of inverse BAR (I-BAR) proteins sense and introduce outward membrane curvature, and serve as bridges between the cell membrane and the cytoskeleton. We focus on three I-BAR domain proteins that are expressed in the central nervous system: Mtss2, MIM, and IRSp53 that promote negative, concave curvature based on their ability to self-associate. Recent studies suggest that each has distinct functions in synapse formation and synaptic plasticity. The action of I-BARs is also shaped by crosstalk with other signaling components, forming signaling platforms that can function in a circuit-dependent manner. We discuss another potentially important feature-the ability of some BAR domain proteins to impact the function of other family members by heterooligomerization. Understanding the spatiotemporal resolution of synaptic I-BAR protein expression and their interactions should provide insights into the interplay between activity-dependent neural plasticity and network rewiring in the CNS.

Dendritic Spine Initiation in Brain Development, Learning and Diseases and Impact of BAR-Domain Proteins

Dendritic spines are small, bulbous protrusions along neuronal dendrites where most of the excitatory synapses are located. Dendritic spine density in normal human brain increases rapidly before and after birth achieving the highest density around 2-8 years. Density decreases during adolescence, reaching a stable level in adulthood. The changes in dendritic spines are considered structural correlates for synaptic plasticity as well as the basis of experience-dependent remodeling of neuronal circuits. Alterations in spine density correspond to aberrant brain function observed in various neurodevelopmental and neuropsychiatric disorders. Dendritic spine initiation affects spine density. In this review, we discuss the importance of spine initiation in brain development, learning, and potential complications resulting from altered spine initiation in neurological diseases. Current literature shows that two Bin Amphiphysin Rvs (BAR) domain-containing proteins, MIM/Mtss1 and SrGAP3, are involved in spine initiation. We review existing literature and open databases to discuss whether other BAR-domain proteins could also take part in spine initiation. Finally, we discuss the potential molecular mechanisms on how BAR-domain proteins could regulate spine initiation.

Identification of SRGAP2 as a potential oncogene and a prognostic biomarker in hepatocellular carcinoma

BACKGROUND

Hepatocellular carcinoma (HCC) is one of the common malignancies worldwide. Slit-Robo GTPase-activating proteins (SRGAPs) have been shown to regulate the occurrence and development of various tumors. However, their specific roles in HCC remain elusive.

METHODS

The expression pattern, genetic alteration and prognostic value of SRGAPs in HCC are analyzed by bioinformatics tools. The biological functions of SRGAP2 in HCC cells are demonstrated by in vitro experiments. The high-throughput RNA sequencing is conducted to explore the underlying molecular mechanisms of SRGAP2 in HCC cells.

RESULTS

The expression levels of SRGAP1 and SRGAP2 are significantly elevated in HCC tissues compared to the normal both in Oncomine and TCGA datasets, and SRGAP2 are dramatically upregulated both in mRNA and protein levels. Moreover, higher SRGAP2 is significantly related to the clinical stages of HCC. Meanwhile, SRGAP2 might be an independent prognostic indicator, as it correlates negatively with the clinical outcomes of HCC patients. Further SRGAP2-silencing experiments imply that SRGAP2 might remarkably promote the migration and invasion of HCC cells in an EMT-independent pattern. Based on the high-throughput RNA sequencing of SRGAP2-knockdown HCC cells, enrichment and network analyses demonstrate that SRGAP2 is closely associated with cellular metabolic signaling.

CONCLUSIONS

Our study firstly illustrates the crucial role of SRGAP2 in the metastasis of HCC and explores its underlying molecular mechanisms. We identify SRGAP2 as a promising prognostic biomarker and a novel therapeutic target for HCC patients.

Rac1-GTPase regulates compression-induced actin protrusions (CAPs) of mesenchymal stem cells in 3D collagen micro-tissues

Cells can sense mechanical signals through cytoskeleton reorganization. We previously discovered the formation of omni-directional actin protrusions upon compression loading, namely compression-induced actin protrusions (CAPs), in human mesenchymal stem cells (MSCs) in 3D micro-tissues. Here, the regulatory roles of three RhoGTPases (CDC42, Rac1 and RhoA) in the formation of CAPs were investigated. Upon compression loading, extensive formation of CAPs was found, significantly associated with an upregulated mRNA expression of Rac1 only, but not CDC42, nor RhoA. Upon chemical inhibition of these RhoGTPase activity during compression, only Rac1 activity was significantly suppressed, associating with the reduced CAP formation. Silencing the upstream regulators of these RhoGTPase pathways including Rac1 by specific siRNA dramatically disrupted actin cytoskeleton, distorted cell morphology and aborted CAP formation. Silencing cortactin (CTTN), a downstream effector of the Rac1 pathway, induced a compensatory upregulation of Rac1, enabling the MSCs to respond to the compression loading stimulus in terms of CAP formation, although at a reduced number. The importance of Rac1 signalling in CAP formation and the corresponding upregulation of lamellipodial markers also suggest that these CAPs are lamellipodia in nature. This study delineates the mechanism of compression-induced cytoskeleton reorganization, contributing to rationalizing mechanical loading regimes for functional tissue engineering.

Language Impairment with a Partial Duplication of DOCK8

Duplications of the distal region of the short arm of chromosome 9 are rare, but are associated with learning disabilities and behavioral disturbances. We report in detail the cognitive and language features of a child with a duplication in the 9p24.3 region, arr[hg19] 9p24.3(266,045-459,076)×3. The proband exhibits marked expressive and receptive problems, which affect both structural and functional aspects of language. These problems might result from a severe underlying deficit in working memory. Regarding the molecular causes of the observed symptoms, they might result from the altered expression of selected genes involved in procedural learning, particularly some of components of the SLIT/ROBO/FOXP2 network, strongly related to the development and evolution of language. Dysregulation of specific components of this network can result in turn from an altered interaction between DOCK8, affected by the microduplication, and CDC42, acting as the hub component of the network encompassing language-related genes.

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.

Using Phosphatidylinositol Phosphorylation as Markers for Hyperglycemic Related Breast Cancer

Studies have suggested that type 2 diabetes (T2D) is associated with a higher incidence of breast cancer and related mortality rates. T2D postmenopausal women have an ~20% increased chance of developing breast cancer, and women with T2D and breast cancer have a 50% increase in mortality compared to breast cancer patients without diabetes. This correlation has been attributed to the general activation of insulin receptor signaling, glucose metabolism, phosphatidylinositol (PI) kinases, and growth pathways. Furthermore, the presence of breast cancer specific PI kinase and/or phosphatase mutations enhance metastatic breast cancer phenotypes. We hypothesized that each of the breast cancer subtypes may have characteristic PI phosphorylation profiles that are changed in T2D conditions. Therefore, we sought to characterize the PI phosphorylation when equilibrated in normal glycemic versus hyperglycemic serum conditions. Our results suggest that hyperglycemia leads to: 1) A reduction in PI3P and PIP3, with increased PI4P that is later converted to PI(3,4)P2 at the cell surface in hormone receptor positive breast cancer; 2) a reduction in PI3P and PI4P with increased PIP3 surface expression in human epidermal growth factor receptor 2-positive (HER2+) breast cancer; and 3) an increase in di- and tri-phosphorylated PIs due to turnover of PI3P in triple negative breast cancer. This study begins to describe some of the crucial changes in PIs that play a role in T2D related breast cancer incidence and metastasis.

In vivo proximity proteomics of nascent synapses reveals a novel regulator of cytoskeleton-mediated synaptic maturation

Excitatory synapse formation during development involves the complex orchestration of both structural and functional alterations at the postsynapse. However, the molecular mechanisms that underlie excitatory synaptogenesis are only partially resolved, in part because the internal machinery of developing synapses is largely unknown. To address this, we apply a chemicogenetic approach, in vivo biotin identification (iBioID), to discover aspects of the proteome of nascent synapses. This approach uncovered sixty proteins, including a previously uncharacterized protein, CARMIL3, which interacts in vivo with the synaptic cytoskeletal regulator proteins SrGAP3 (or WRP) and actin capping protein. Using new CRISPR-based approaches, we validate that endogenous CARMIL3 is localized to developing synapses where it facilitates the recruitment of capping protein and is required for spine structural maturation and AMPAR recruitment associated with synapse unsilencing. Together these proteomic and functional studies reveal a previously unknown mechanism important for excitatory synapse development in the developing perinatal brain.

FBP17 and CIP4 recruit SHIP2 and lamellipodin to prime the plasma membrane for fast endophilin-mediated endocytosis

Endocytosis mediates the cellular uptake of micronutrients and the turnover of plasma membrane proteins. Clathrin-mediated endocytosis is the major uptake pathway in resting cells¹, but several clathrin-independent endocytic routes exist in parallel^2,3. One such pathway, fast endophilin-mediated endocytosis (FEME), is not constitutive but triggered upon activation of certain receptors, including the β₁ adrenergic receptor⁴. FEME activates promptly following stimulation as endophilin is pre-enriched by the phosphatidylinositol-3,4-bisphosphate-binding protein lamellipodin^4,5. However, in the absence of stimulation, endophilin foci abort and disassemble after a few seconds. Looking for additional proteins involved in FEME, we found that 20 out of 65 BAR domain-containing proteins tested colocalized with endophilin spots. Among them, FBP17 and CIP4 prime the membrane of resting cells for FEME by recruiting the 5'-lipid phosphatase SHIP2 and lamellipodin to mediate the local production of phosphatidylinositol-3,4-bisphosphate and endophilin pre-enrichment. Membrane-bound GTP-loaded Cdc42 recruits FBP17 and CIP4, before being locally deactivated by RICH1 and SH3BP1 GTPase-activating proteins. This generates the transient assembly and disassembly of endophilin spots, which lasts 5-10 seconds. This mechanism periodically primes patches of the membrane for prompt responses upon FEME activation.

Anterior Pituitary Transcriptome Suggests Differences in ACTH Release in Tame and Aggressive Foxes

Domesticated species exhibit a suite of behavioral, endocrinological, and morphological changes referred to as "domestication syndrome." These changes may include a reduction in reactivity of the hypothalamic-pituitary-adrenal (HPA) axis and specifically reduced adrenocorticotropic hormone release from the anterior pituitary. To investigate the biological mechanisms targeted during domestication, we investigated gene expression in the pituitaries of experimentally domesticated foxes (Vulpes vulpes). RNA was sequenced from the anterior pituitary of six foxes selectively bred for tameness ("tame foxes") and six foxes selectively bred for aggression ("aggressive foxes"). Expression, splicing, and network differences identified between the two lines indicated the importance of genes related to regulation of exocytosis, specifically mediated by cAMP, organization of pseudopodia, and cell motility. These findings provide new insights into biological mechanisms that may have been targeted when these lines of foxes were selected for behavior and suggest new directions for research into HPA axis regulation and the biological underpinnings of domestication.

Rho GTPase signaling complexes in cell migration and invasion

Cell migration is dependent on the dynamic formation and disassembly of actin filament-based structures, including lamellipodia, filopodia, invadopodia, and membrane blebs, as well as on cell-cell and cell-extracellular matrix adhesions. These processes all involve Rho family small guanosine triphosphatases (GTPases), which are regulated by the opposing actions of guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). Rho GTPase activity needs to be precisely tuned at distinct cellular locations to enable cells to move in response to different environments and stimuli. In this review, we focus on the ability of RhoGEFs and RhoGAPs to form complexes with diverse binding partners, and describe how this influences their ability to control localized GTPase activity in the context of migration and invasion.

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.

Structural History of Human SRGAP2 Proteins

In the development of the human brain, human-specific genes are considered to play key roles, conferring its unique advantages and vulnerabilities. At the time of Homo lineage divergence from Australopithecus, SRGAP2C gradually emerged through a process of serial duplications and mutagenesis from ancestral SRGAP2A (3.4–2.4 Ma). Remarkably, ectopic expression of SRGAP2C endows cultured mouse brain cells, with human-like characteristics, specifically, increased dendritic spine length and density. To understand the molecular mechanisms underlying this change in neuronal morphology, we determined the structure of SRGAP2A and studied the interplay between SRGAP2A and SRGAP2C. We found that: 1) SRGAP2A homo-dimerizes through a large interface that includes an F-BAR domain, a newly identified F-BAR extension (Fx), and RhoGAP-SH3 domains. 2) SRGAP2A has an unusual inverse geometry, enabling associations with lamellipodia and dendritic spine heads in vivo, and scaffolding of membrane protrusions in cell culture. 3) As a result of the initial partial duplication event (∼3.4 Ma), SRGAP2C carries a defective Fx-domain that severely compromises its solubility and membrane-scaffolding ability. Consistently, SRGAP2A:SRAGP2C hetero-dimers form, but are insoluble, inhibiting SRGAP2A activity. 4) Inactivation of SRGAP2A is sensitive to the level of hetero-dimerization with SRGAP2C. 5) The primal form of SRGAP2C (P-SRGAP2C, existing between ∼3.4 and 2.4 Ma) is less effective in hetero-dimerizing with SRGAP2A than the modern SRGAP2C, which carries several substitutions (from ∼2.4 Ma). Thus, the genetic mutagenesis phase contributed to modulation of SRGAP2A’s inhibition of neuronal expansion, by introducing and improving the formation of inactive SRGAP2A:SRGAP2C hetero-dimers, indicating a stepwise involvement of SRGAP2C in human evolutionary history.

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.

Regulating Rho GTPases and their regulators

Rho GTPases regulate cytoskeletal and cell adhesion dynamics and thereby coordinate a wide range of cellular processes, including cell migration, cell polarity and cell cycle progression. Most Rho GTPases cycle between a GTP-bound active conformation and a GDP-bound inactive conformation to regulate their ability to activate effector proteins and to elicit cellular responses. However, it has become apparent that Rho GTPases are regulated by post-translational modifications and the formation of specific protein complexes, in addition to GTP-GDP cycling. The canonical regulators of Rho GTPases - guanine nucleotide exchange factors, GTPase-activating proteins and guanine nucleotide dissociation inhibitors - are regulated similarly, creating a complex network of interactions to determine the precise spatiotemporal activation of Rho GTPases.

Lamellipodin-Deficient Mice: A Model of Rectal Carcinoma

During a survey of clinical rectal prolapse (RP) cases in the mouse population at MIT animal research facilities, a high incidence of RP in the lamellipodin knock-out strain, C57BL/6-Raph1tm1Fbg (Lpd-/-) was documented. Upon further investigation, the Lpd-/- colony was found to be infected with multiple endemic enterohepatic Helicobacter species (EHS). Lpd-/- mice, a transgenic mouse strain produced at MIT, have not previously shown a distinct immune phenotype and are not highly susceptible to other opportunistic infections. Predominantly male Lpd-/- mice with RP exhibited lesions consistent with invasive rectal carcinoma concomitant to clinically evident RP. Multiple inflammatory cytokines, CD11b+Gr1+ myeloid-derived suppressor cell (MDSC) populations, and epithelial cells positive for a DNA damage biomarker, H2AX, were elevated in affected tissue, supporting their role in the neoplastic process. An evaluation of Lpd-/- mice with RP compared to EHS-infected, but clinically normal (CN) Lpd-/- animals indicated that all of these mice exhibit some degree of lower bowel inflammation; however, mice with prolapses had significantly higher degree of focal lesions at the colo-rectal junction. When Helicobacter spp. infections were eliminated in Lpd-/- mice by embryo transfer rederivation, the disease phenotype was abrogated, implicating EHS as a contributing factor in the development of rectal carcinoma. Here we describe lesions in Lpd-/- male mice consistent with a focal inflammation-induced neoplastic transformation and propose this strain as a mouse model of rectal carcinoma.

Fasting and Systemic Insulin Signaling Regulate Phosphorylation of Brain Proteins That Modulate Cell Morphology and Link to Neurological Disorders

Diabetes is strongly associated with cognitive decline, but the molecular reasons are unknown. We found that fasting and peripheral insulin promote phosphorylation and dephosphorylation, respectively, of specific residues on brain proteins including cytoskeletal regulators such as slit-robo GTPase-activating protein 3 (srGAP3) and microtubule affinity-regulating protein kinases (MARKs), in which deficiency or dysregulation is linked to neurological disorders. Fasting activates protein kinase A (PKA) but not PKB/Akt signaling in the brain, and PKA can phosphorylate the purified srGAP3. The phosphorylation of srGAP3 and MARKs were increased when PKA signaling was activated in primary neurons. Knockdown of PKA decreased the phosphorylation of srGAP3. Furthermore, WAVE1, a protein kinase A-anchoring protein, formed a complex with srGAP3 and PKA in the brain of fasted mice to facilitate the phosphorylation of srGAP3 by PKA. Although brain cells have insulin receptors, our findings are inconsistent with the down-regulation of phosphorylation of target proteins being mediated by insulin signaling within the brain. Rather, our findings infer that systemic insulin, through a yet unknown mechanism, inhibits PKA or protein kinase(s) with similar specificity and/or activates an unknown phosphatase in the brain. Ser⁸⁵⁸ of srGAP3 was identified as a key regulatory residue in which phosphorylation by PKA enhanced the GAP activity of srGAP3 toward its substrate, Rac1, in cells, thereby inhibiting the action of this GTPase in cytoskeletal regulation. Our findings reveal novel mechanisms linking peripheral insulin sensitivity with cytoskeletal remodeling in neurons, which may help to explain the association of diabetes with neurological disorders such as Alzheimer disease.

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.

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.

Steering cell migration: lamellipodium dynamics and the regulation of directional persistence

Membrane protrusions at the leading edge of cells, known as lamellipodia, drive cell migration in many normal and pathological situations. Lamellipodial protrusion is powered by actin polymerization, which is mediated by the actin-related protein 2/3 (ARP2/3)-induced nucleation of branched actin networks and the elongation of actin filaments. Recently, advances have been made in our understanding of positive and negative ARP2/3 regulators (such as the SCAR/WAVE (SCAR/WASP family verprolin-homologous protein) complex and Arpin, respectively) and of proteins that control actin branch stability (such as glial maturation factor (GMF)) or actin filament elongation (such as ENA/VASP proteins) in lamellipodium dynamics and cell migration. This Review highlights how the balance between actin filament branching and elongation, and between the positive and negative feedback loops that regulate these activities, determines lamellipodial persistence. Importantly, directional persistence, which results from lamellipodial persistence, emerges as a critical factor in steering cell migration.

The focal adhesion-localized CdGAP regulates matrix rigidity sensing and durotaxis

Motile cells are capable of sensing the stiffness of the surrounding extracellular matrix through integrin-mediated focal adhesions and migrate towards regions of higher rigidity in a process known as durotaxis. Durotaxis plays an important role in normal development and disease progression, including tumor invasion and metastasis. However, the signaling mechanisms underlying focal adhesion-mediated rigidity sensing and durotaxis are poorly understood. Utilizing matrix-coated polydimethylsiloxane gels to manipulate substrate compliance, we show that cdGAP, an adhesion-localized Rac1 and Cdc42 specific GTPase activating protein, is necessary for U2OS osteosarcoma cells to coordinate cell shape changes and migration as a function of extracellular matrix stiffness. CdGAP regulated rigidity-dependent motility by controlling membrane protrusion and adhesion dynamics, as well as by modulating Rac1 activity. CdGAP was also found to be necessary for U2OS cell durotaxis. Taken together, these data identify cdGAP as an important component of an integrin-mediated signaling pathway that senses and responds to mechanical cues in the extracellular matrix in order to coordinate directed cell motility.

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.

srGAP1 regulates lamellipodial dynamics and cell migratory behavior by modulating Rac1 activity

srGAP1 limits Rac1 activity at lamellipodia in a negative feedback manner, allowing concomitant activation of Rac1 and RhoA at lamellipodia. Rho signaling causes membrane ruffling through actomyosin contractility and removes the protrusive structures. Such coordination of Rac and Rho determines migratory behavior through lamellipodial dynamics.

The distinct levels of Rac activity differentially regulate the pattern of intrinsic cell migration. However, it remains unknown how Rac activity is modulated and how the level of Rac activity controls cell migratory behavior. Here we show that Slit-Robo GAP 1 (srGAP1) is a modulator of Rac activity in locomotive cells. srGAP1 possesses a GAP activity specific to Rac1 and is recruited to lamellipodia in a Rac1-dependent manner. srGAP1 limits Rac1 activity and allows concomitant activation of Rac1 and RhoA, which are mutually inhibitory. When both GTPases are activated, the protrusive structures caused by Rac1-dependent actin reorganization are spatially restricted and periodically destabilized, causing ruffling by RhoA-induced actomyosin contractility. Depletion of srGAP1 overactivates Rac1 and inactivates RhoA, resulting in continuous spatiotemporal spreading of lamellipodia and a modal shift of intrinsic cell motility from random to directionally persistent. Thus srGAP1 is a key determinant of lamellipodial dynamics and cell migratory behavior.

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.

The mental retardation-associated protein srGAP3 regulates survival, proliferation, and differentiation of rat embryonic neural stem/progenitor cells

The mental retardation-associated protein, srGAP3 is highly expressed in neurogenic sites. It is thought to regulate the key aspects of neuronal development and functions. Little is known about the interaction between srGAP3 and immature neural stem cells/neural progenitor cells (NSCs/NPCs). In the current study, the expression of srGAP3 in NSCs/NPCs was detected. Then, survival, proliferation, differentiation, and morphological alteration of NSCs/NPCs were assessed after a lentivirus-mediated knockdown of srGAP3. The results showed that srGAP3 is highly expressed in NSCs/NPCs both in vitro and in vivo. After knockdown of srGAP3 (LV3-srGAP3 infection), viability and proliferation of NSCs/NPCs dramatically decreased, approximately 85% displayed a similar morphology with type I cells that have no or only few indistinguishable processes. After 7 days culture in a differentiation medium, 62.5%±8.3% of cells in the srGAP3 knockdown group were nestin-positive and 24.8%±5.8% of them were β-tubulin III-positive, which are significantly higher (30.2%±9.9% and 14.6%±2.7%) than in the control group (LV3-NC infection). In addition, cells in the knockdown group had significantly fewer, but longer processes. Our results demonstrate that srGAP3 knockdown negatively regulates NSCs/NPCs survival, proliferation, differentiation, and morphological alteration, particularly, process formation. Taken together, our results provide strong evidence that srGAP3 is involved in the regulation of biological behavior and the morphological features in rat NSCs/NPCs in vitro.

The Rac1 hypervariable region in targeting and signaling: a tail of many stories

Cellular signaling by small GTPases is critically dependent on proper spatio-temporal orchestration of activation and output. In addition to their core G (guanine nucleotide binding)-domain, small GTPases comprise a hypervariable region (HVR) and a lipid anchor that are generally accepted to control subcellullar localization. The HVR encodes in many small GTPases a polybasic region (PBR) that permits charge-mediated association to the inner leaflet of the plasma membrane or to intracellular organelles. Over the past 15-20 years, evidence has accumulated for specific protein-protein interactions, mediated by the HVR, that control both targeting and signaling specificity of small GTPases. Using the RhoGTPase Rac1 as a paradigm we here review a series of protein partners that require the Rac1 HVR for association and that control various aspects of localized Rac1 signaling. Some of these proteins represent Rac1 activators, whereas others mediate Rac1 inactivation and degradation and yet others potentiate Rac1 downstream signaling. Finally, evidence is discussed which shows that the HVR of Rac1 also contributes to effector interactions, co-operating with the N-terminal effector domain. The complexity of localized Rac1 signaling, reviewed here, is most likely exemplary for many other small GTPases as well, representing a challenge to identify and define similar mechanisms controlling the specific signaling induced by small GTPases.

The cellular function of srGAP3 and its role in neuronal morphogenesis

The Slit-Robo GTPase activating protein 3 (srGAP3) dynamically regulates cytoskeletal reorganisation through inhibition of the Rho GTPase Rac1 and interaction with actin remodelling proteins. SrGAP3-mediated reorganisation of the actin cytoskeleton is crucial for the normal development of dendritic spines and loss of srGAP3 leads to abnormal synaptic activity and impaired cognitive behaviours in mice, which is reminiscent of an association between disrupted srGAP3 and intellectual disability in humans. Additionally, srGAP3 has been implicated to act downstream of Slit-Robo signalling in commissural axons of the spinal cord. Thus, srGAP3-mediated cytoskeletal reorganisation has an important influence on a variety of neurodevelopmental processes, which may be required for normal cognitive function.

A tumor suppressor role for srGAP3 in mammary epithelial cells

srGAP3, a member of the Slit-Robo sub-family of Rho GTPase-activating proteins (Rho GAPs), controls actin and microtubule dynamics through negative regulation of Rac. Here, we describe a potential role for srGAP3 as a tumor suppressor in mammary epithelial cells. We show that RNAi-mediated depletion of srGAP3 promotes Rac dependent, anchorage-independent growth of partially transformed human mammary epithelial cells (HMECs). Furthermore, srGAP3 expression is absent, or significantly reduced in 7/10 breast cancer cell lines compared with normal HMECs. Re-expression of srGAP3 in a subset of these cell lines inhibits both anchorage-independent growth and cell invasion in a GAP-dependent manner, and this is accompanied by an increase in phosphorylation of the ezrin/radixin/moesin (ERM) family proteins and myosin light chain 2 (MLC2). Inhibition of the Rho regulated kinase, ROCK, reduces ERM and MLC2 phosphorylation and restores invasion. We conclude that srGAP3 has tumor suppressor-like activity in HMECs, likely through its activity as a negative regulator of Rac1.

The MRL proteins: adapting cell adhesion, migration and growth

MIG-10, RIAM and Lamellipodin (Lpd) are the founding members of the MRL family of multi-adaptor molecules. These proteins have common domain structures but display distinct functions in cell migration and adhesion, signaling, and in cell growth. The binding of RIAM with active Rap1 and with talin provides these MRL molecules with important regulatory roles on integrin-mediated cell adhesion and migration. Furthermore, RIAM and Lpd can regulate actin dynamics through their binding to actin regulatory Ena/VASP proteins. Recent data generated with the Drosophila MRL ortholog called Pico and with RIAM in melanoma cells indicate that these proteins can also regulate cell growth. As MRL proteins represent a relatively new family, many questions on their structure-function relationships remain unanswered, including regulation of their expression, post-translational modifications, new interactions, involvement in signaling and their knockout mice phenotype.

The F-BAR domains from srGAP1, srGAP2 and srGAP3 regulate membrane deformation differently

Coordination of membrane deformation and cytoskeletal dynamics lies at the heart of many biological processes critical for cell polarity, motility and morphogenesis. We have recently shown that Slit-Robo GTPase-activating protein 2 (srGAP2) regulates neuronal morphogenesis through the ability of its F-BAR domain to regulate membrane deformation and induce filopodia formation. Here, we demonstrate that the F-BAR domains of two closely related family members, srGAP1 and srGAP3 [designated F-BAR(1) and F-BAR(3), respectively] display significantly different membrane deformation properties in non-neuronal COS7 cells and in cortical neurons. F-BAR(3) induces filopodia in both cell types, though less potently than F-BAR(2), whereas F-BAR(1) prevents filopodia formation in cortical neurons and reduces plasma membrane dynamics. These three F-BAR domains can heterodimerize, and they act synergistically towards filopodia induction in COS7 cells. As measured by fluorescence recovery after photobleaching, F-BAR(2) displays faster molecular dynamics than F-BAR(3) and F-BAR(1) at the plasma membrane, which correlates well with its increased potency to induce filopodia. We also show that the molecular dynamic properties of F-BAR(2) at the membrane are partially dependent on F-Actin. Interestingly, acute phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] depletion in cells does not interfere with plasma membrane localization of F-BAR(2), which is compatible with our result showing that F-BAR(2) binds to a broad range of negatively-charged phospholipids present at the plasma membrane, including phosphatidylserine (PtdSer). Overall, our results provide novel insights into the functional diversity of the membrane deformation properties of this subclass of F-BAR-domains required for cell morphogenesis.