GEFT, a Rho family guanine nucleotide exchange factor, regulates neurite outgrowth and dendritic spine formation

Epigenetically upregulated GEFT-derived invasion and metastasis of rhabdomyosarcoma via epithelial mesenchymal transition promoted by the Rac1/Cdc42-PAK signalling pathway

Background

Metastasis of rhabdomyosarcoma (RMS) is the primary cause of tumour-related deaths. Previous studies have shown that overexpression of the guanine nucleotide exchange factor T (GEFT) is correlated with a poorer RMS prognosis, but the mechanism remains largely unexplored.

Methods

We focused on determining the influence of the GEFT-Rho-GTPase signalling pathway and the epithelial–mesenchymal transition (EMT) or mesenchymal–epithelial transition (MET) on RMS progression and metastasis by using RMS cell lines, BALB/c nude mice and cells and molecular biology techniques.

Findings

GEFT promotes RMS cell viability, migration, and invasion; GEFT also inhibits the apoptosis of RMS cells and accelerates the growth and lung metastasis of RMS by activating the Rac1/Cdc42 pathways. Interestingly, GEFT upregulates the expression levels of N-cadherin, Snail, Slug, Twist, Zeb1, and Zeb2 and reduces expression level of E-cadherin. Thus, GEFT influences the expression of markers for EMT and MET in RMS cells via the Rac1/Cdc42-PAK1 pathways. We also found that the level of GEFT gene promoter methylation in RMS is lower than that in normal striated muscle tissue. Significant differences were observed in the level of GEFT gene methylation in different histological subtypes of RMS.

Interpretation

These findings suggest that GEFT accelerates the tumourigenicity and metastasis of RMS by activating Rac1/Cdc42-PAK signalling pathway-induced EMT; thus, it may serve as a novel therapeutic target.

Fund

This work was supported by grants from the National Natural Science Foundation of China (81660441, 81460404, and 81160322) and Shihezi University Initiative Research Projects for Senior Fellows (RCZX201447). Funders had no role in the design of the study, data collection, data analysis, interpretation, or the writing of this report.

GEFT protein expression in digestive tract malignant tumors and its clinical significance

Guanine nucleotide exchange factor T (GEFT), a member of the Rho guanine nucleotide exchange factor family, is expressed in a variety of tumors. In the present study, the expression and clinical significance of GEFT in malignant digestive tract tumors was assessed. Tumor and adjacent control samples from 180 patients were tested. Positive GEFT expression rates were 80, 83.33 and 86.67% in esophageal squamous carcinoma (ESCC), gastric carcinoma (GC) and colorectal cancer (CRC), respectively. GEFT expression was associated with diffuse type carcinoma according to the Lauren classification (χ²=12.525, P=0.002) and tumor-node-metastasis (TNM) stages III/IV (χ²=4.033, P=0.045) in GC, and with vessel carcinoma embolus (χ²=7.890, P=0.005) and lymph node metastasis (χ²=5.455, P=0.020) in CRC, but was not associated with other clinicopathological parameters. Patients with high levels of GEFT protein expression had a less favorable outcome compared with patients with low levels of GEFT expression in patients with CRC (χ²=3.876, P=0.049). However, a significant association was not found between GEFT expression and overall survival in patients with ESCC (χ²=0.040, P=0.842) or GC (χ²=0.501, P=0.479). The rate of human epidermal growth factor receptor 2 upregulation in patients with GC was 13.33% and it was associated with nerve invasion (χ²=4.005, P=0.045) and TNM stages III/IV (χ²=5.600, P=0.018). Mismatch repair protein (MMRP) defect was observed in six cases, and the KRAS mutation rate was 26.67% in patients with CRC. GEFT expression was significantly correlated with MMRP (r=-0.285, P=0.027) and KRAS mutation in patients with CRC (r=0.697, P<0.001). These findings revealed frequent GEFT upregulation in malignant digestive tract tumors, which may have promoted tumor development. GEFT expression in CRC may be associated with microsatellite instability and KRAS mutation status, suggesting that GEFT may be a potential therapeutic target for patients with CRC.

GEFT aberrant expression in soft tissue sarcomas

Background

Guanine nucleotide exchange factor T (GEFT) exhibits high amplification level using high-resolution array comparative genomic hybridization in rhabdomyosarcoma. The overexpression rate of GEFT protein is higher in rhabdomyosarcoma than in normal striated muscle tissues. This study evaluated the aberrant expression of GEFT in multiple subtypes of soft tissue sarcoma (STS) and compared the differences in clinical pathology, histological feature and expression levels of GEFT protein and mRNA between chromosomal translocation-associated sarcomas (CTAS) and non-chromosomal translocation-associated sarcomas (NCTAS).

Methods

GEFT protein expression was detected using immunohistochemistry (IHC) and tissue microarrays. Quantitative reverse transcriptase polymerase chain reaction (qRT-PCR) was used to detect the expression of GEFT mRNA.

Results

The rates of GEFT positive expression (196/219, 89.50%) and overexpression (113/219, 51.60%) were higher in multiple subtypes of STS than in normal striated muscle tissues. The rates of GEFT positive expression and overexpression in all subtypes of STS detected were significantly higher than that in the controls. No difference of GEFT expression was detected between CTAS and NCTAS.

Conclusions

The abnormal expression of GEFT exists in various subtypes of STS, which may play an important role in tumorigenesis of STS.

Src-mediated phosphorylation of βPix-b regulates dendritic spine morphogenesis

PAK-interacting guanine nucleotide exchange factor (βPix) has been implicated in many actin-based cellular processes including spine morphogenesis in neurons. However, the molecular mechanisms by which βPix controls spine morphology remain elusive. Previously, we have reported the expression of several alternative spliced βPix isoforms in the brain. Here, we report a novel finding that the b isoform of βPix (βPix-b) mediates regulation of spine and synapse formation. We found that βPix-b, which is mainly expressed in neurons, enhances spine and synapse formation through preferential localization at spines. In neurons, glutamate treatment efficiently stimulates Rac1 GEF activity of βPix-b. The glutamate stimulation also promotes Src kinase-mediated phosphorylation of βPix-b in both AMPA receptor- and NMDA receptor-dependent manner. Tyrosine 598 (Y598) of βPix-b is identified as the major Src-mediated phosphorylation site. Finally, Y598 phosphorylation of βPix-b enhances its Rac1 GEF activity that is critical for spine and synapse formation. In conclusion, we provide a novel mechanism by which βPix-b regulates activity-dependent spinogenesis and synaptogenesis via Src-mediated phosphorylation.

Dendritic spine loss caused by AlCl3 is associated with inhibition of the Rac 1/cofilin signaling pathway

Aluminum (Al) has neurotoxicity that can result in cognitive dysfunction. Hippocampal dendritic spine loss is a pathological characteristic of cognitive dysfunction. Our previous study reported that Al exposure caused dendritic spine loss in the hippocampus, but the underlying mechanism remains unclear. In this study, rats were orally administered 50, 150 or 450 mg/kg of AlCl₃ for 90 days. The dendritic spine density of the CA1 and DG regions was detected by Golgi-Cox staining. The F-actin/G-actin ratio, the expression of drebrin A and the components of the Rac 1/cofilin pathway were measured in the hippocampus. The results obtained showed that AlCl₃ caused dendritic spine loss and decreased the F-actin/G-actin ratio. In addition, it was found that AlCl₃ downregulated the expression of Rac 1, p-PAK, p-LIMK, p-cofilin and drebrin A and upregulated cofilin expression. Altogether, these results demonstrated that Al inactivated the Rac 1/cofilin pathway by inhibiting the phosphorylation of cofilin and the polymerization of F-actin, resulting in dendritic spine loss in the hippocampus.

Neuronal adaptor FE65 stimulates Rac1-mediated neurite outgrowth by recruiting and activating ELMO1

Neurite outgrowth is a crucial process in developing neurons for neural network formation. Understanding the regulatory mechanisms of neurite outgrowth is essential for developing strategies to stimulate neurite regeneration after nerve injury and in neurodegenerative disorders. FE65 is a brain-enriched adaptor that stimulates Rac1-mediated neurite elongation. However, the precise mechanism by which FE65 promotes the process remains elusive. Here, we show that ELMO1, a subunit of ELMO1-DOCK180 bipartite Rac1 guanine nucleotide exchange factor (GEF), interacts with the FE65 N-terminal region. Overexpression of FE65 and/or ELMO1 enhances, whereas knockdown of FE65 or ELMO1 inhibits, neurite outgrowth and Rac1 activation. The effect of FE65 alone or together with ELMO1 is attenuated by an FE65 double mutation that disrupts FE65-ELMO1 interaction. Notably, FE65 is found to activate ELMO1 by diminishing ELMO1 intramolecular autoinhibitory interaction and to promote the targeting of ELMO1 to the plasma membrane, where Rac1 is activated. We also show that FE65, ELMO1, and DOCK180 form a tripartite complex. Knockdown of DOCK180 reduces the stimulatory effect of FE65-ELMO1 on Rac1 activation and neurite outgrowth. Thus, we identify a novel mechanism by which FE65 stimulates Rac1-mediated neurite outgrowth by recruiting and activating ELMO1.

Sex-dependent effects of chromogranin B P413L allelic variant as disease modifier in amyotrophic lateral sclerosis

Recent genetic studies yielded conflicting results regarding a role for the variant chromogranin B (CHGB)P413L allele as a disease modifier in ALS. Moreover, potential deleterious effects of the CHGBP413L variant in ALS pathology have not been investigated. Here we report that in transfected cultured cells, the variant CHGBL413 protein exhibited aberrant properties including mislocalization, failure to interact with mutant superoxide dismutase 1 (SOD1) and defective secretion. The CHGBL413 transgene in SOD1G37R mice precipitated disease onset and pathological changes related to misfolded SOD1 specifically in female mice. However, the CHGBL413 variant also slowed down disease progression in SOD1G37R mice, which is in line with a very slow disease progression that we report for a Swedish woman with ALS who is carrier of two mutant SOD1D90A alleles and two variant CHGBP413L and CHGBR458Q alleles. In contrast, overexpression of the common CHGBP413 allele in SOD1G37R mice did not affect disease onset but significantly accelerated disease progression and pathological changes. As in transgenic mice, the CHGBP413L allele conferred an earlier ALS disease onset in women of Japanese and French Canadian origins with less effect in men. Evidence is presented that the sex-dependent effects of CHGBL413 allelic variant in ALS may arise from enhanced neuronal expression of CHGB in females because of a sex-determining region Y element in the gene promoter. Thus, our results suggest that CHGB variants may act as modifiers of onset and progression in some ALS populations and especially in females because of higher expression levels compared to males.

Expression of p21-activated kinases 1 and 3 is altered in the brain of subjects with depression

The p21-activated kinases (PAKs) of group I are the main effectors for the small Rho GTPases, critically involved in neurodevelopment, plasticity and maturation of the nervous system. Moreover, the neuronal complexity controlled by PAK1/PAK3 signaling determines the postnatal brain size and synaptic properties. Stress induces alterations at the level of structural and functional synaptic plasticity accompanied by reductions in size and activity of the hippocampus and the prefrontal cortex (PFC). These abnormalities are likely to contribute to the pathology of depression and, in part, reflect impaired cytoskeleton remodeling pointing to the role of Rho GTPase signaling. Thus, the present study assessed the expression of the group I PAKs and their activators in the brain of depressed subjects. Using quantitative polymerase chain reaction (qPCR), mRNA levels and coexpression of the group I PAKs: PAK1, PAK2, and PAK3 as well as of their activators: RAC1, CDC42 and ARHGEF7 were examined in postmortem samples from the PFC (n=25) and the hippocampus (n=23) of subjects with depression and compared to control subjects (PFC n=24; hippocampus n=21). Results demonstrated that mRNA levels of PAK1 and PAK3, are significantly reduced in the brain of depressed subjects, with PAK1 being reduced in the PFC and PAK3 in the hippocampus. No differences were observed for the ubiquitously expressed PAK2. Following analysis of gene coexpression demonstrated disruption of coordinated gene expression in the brain of subjects with depression. Abnormalities in mRNA expression of PAK1 and PAK3 as well as their altered coexpression patterns were detected in the brain of subjects with depression.

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.

Geft is dispensable for the development of the second heart field

Geft is a guanine nucleotide exchange factor, which can specifically activate Rho family of small GTPase by catalyzing the exchange of bound GDP for GTP. Geft is highly expressed in the excitable tissue as heart and skeletal muscle and plays important roles in many cellular processes, such as cell proliferation, migration, and cell fate decision. However, the in vivo role of Geft remains unknown. Here, we generated a Geft conditional knockout mouse by flanking exons 5-17 of Geft with loxP sites. Cre-mediated deletion of the Geft gene in heart using Mef2c-Cre transgenic mice resulted in a dramatic decrease of Geft expression. Geft knockout mice develop normally and exhibit no discernable phenotype, suggesting Geft is dispensable for the development of the second heart field in mouse. The Geft conditional knockout mouse will be a valuable genetic tool for uncovering the in vivo roles of Geft during development and in adult homeostasis.

PAK4-6 in cancer and neuronal development

PAKs 4, 5 and 6 are members of the group B family of p21-activated kinases. Among this group, PAK4 has been most extensively studied. While it has essential roles in embryonic development, in adults high levels of PAK4 are frequently associated with cancer. PAK4 is overexpressed in a variety of cancers, and the Pak4 gene is amplified in some cancers. PAK4 overexpression is sufficient to cause oncogenic transformation in cells and in mouse models. The tight connection between PAK4 and cancer make it a promising diagnostic tool as well as a potential drug target. The group B PAKs also have important developmental functions. PAK4 is important for many early developmental processes, while PAK5 and PAK6 play roles in learning and memory in mice. This chapter provides an overview of the roles of the group B PAKs in cancer as well as development, and includes a discussion of PAK mediated signaling pathways and cellular functions.

Deubiquitinating activity of CYLD is impaired by SUMOylation in neuroblastoma cells

CYLD is a deubiquitinating (DUB) enzyme that has a pivotal role in modulating nuclear factor kappa B (NF-κB) signaling pathways by removing the lysine 63- and linear-linked ubiquitin chain from substrates such as tumor necrosis factor receptor-associated factor 2 (TRAF2) and TRAF6. Loss of CYLD activity is associated with tumorigenicity, and levels of CYLD are lost or downregulated in different types of human tumors. In the present study, we found that high CYLD expression was associated with better overall survival and relapse-free neuroblastoma patient outcome, as well as inversely correlated with the stage of neuroblastoma. Retinoic acid-mediated differentiation of neuroblastoma restored CYLD expression and promoted SUMOylation of CYLD. This posttranslational modification inhibited deubiquitinase activity of CYLD against TRAF2 and TRAF6 and facilitated NF-κB signaling. Overexpression of non-SUMOylatable mutant CYLD in neuroblastoma cells reduced retinoic acid-induced NF-κB activation and differentiation of cells, but instead promoted cell death.

An oncogenic kinase: putting PAK5 forward

INTRODUCTION

Overexpression of p21-activated kinase 5 (PAK5) is discovered in many tumors, probably due to its regulation in cytoskeleton, antiapoptosis and proliferation. A better understanding of the modulation mechanisms of PAK5 is needed for the development of tumor treatment where current therapeutics is inadequate.

AREAS COVERED

This review discusses the current understanding of PAK5 functions as an oncogenic kinase in tumor cellular regulation. Mechanisms of action and molecular pathways involved in cytoskeleton regulation, antiapoptosis and proliferation of tumors are discussed.

EXPERT OPINION

PAKs are serine/threonine kinases and downstream effectors for Cdc42 and Rac, the subfamilies of Rho small GTPases. PAK5 shares sequence identities in p21-GTPase-binding domain and kinase domain and is completely different in other regions compared with other PAKs. Overexpression of PAK5 has been found in several tumors, probably due to its contribution to proliferation, cytoskeleton and anti-apoptosis. Additional regulation mechanisms which are independent of Rho GTPases also indicate that PAK5 functions as a special signal molecule in cellular signaling pathways of tumor progression.

Dendritic spines: the locus of structural and functional plasticity

The introduction of high-resolution time lapse imaging and molecular biological tools has changed dramatically the rate of progress towards the understanding of the complex structure-function relations in synapses of central spiny neurons. Standing issues, including the sequence of molecular and structural processes leading to formation, morphological change, and longevity of dendritic spines, as well as the functions of dendritic spines in neurological/psychiatric diseases are being addressed in a growing number of recent studies. There are still unsettled issues with respect to spine formation and plasticity: Are spines formed first, followed by synapse formation, or are synapses formed first, followed by emergence of a spine? What are the immediate and long-lasting changes in spine properties following exposure to plasticity-producing stimulation? Is spine volume/shape indicative of its function? These and other issues are addressed in this review, which highlights the complexity of molecular pathways involved in regulation of spine structure and function, and which contributes to the understanding of central synaptic interactions in health and disease.

Bisphenol A promotes dendritic morphogenesis of hippocampal neurons through estrogen receptor-mediated ERK1/2 signal pathway

Bisphenol A (BPA), an environmental endocrine disruptor, has attracted increasing attention to its adverse effects on brain developmental process. The previous study indicated that BPA rapidly increased motility and density of dendritic filopodia and enhanced the phosphorylation of N-methyl-d-aspartate (NMDA) receptor subunit NR2B in cultured hippocampal neurons within 30min. The purpose of the present study was further to investigate the effects of BPA for 24h on dendritic morphogenesis and the underlying mechanisms. After cultured for 5d in vitro, the hippocampal neurons from 24h-old rat were infected by AdV-EGFP to indicate time-lapse imaging of living neurons. The results demonstrated that the exposure of the cultured hippocampal neurons to BPA (10, 100nM) or 17β-estradiol (17β-E2, 10nM) for 24h significantly promoted dendritic development, as evidenced by the increased total length of dendrite and the enhanced motility and density of dendritic filopodia. However, these changes were suppressed by an ERs antagonist, ICI182,780, a non-competitive NMDA receptor antagonist, MK-801, and a mitogen-activated ERK1/2-activating kinase (MEK1/2) inhibitor, U0126. Meanwhile, the increased F-actin (filamentous actin) induced by BPA (100nM) was also completely eliminated by these blockers. Furthermore, the result of western blot analyses showed that, the exposure of the cultures to BPA or 17β-E2 for 24h promoted the expression of Rac1/Cdc42 but inhibited that of RhoA, suggesting Rac1 (Ras related C3 botulinum toxinsubstrate 1)/Cdc42 (cell divisioncycle 42) and RhoA (Ras homologous A), the Rho family of small GTPases, were involved in BPA- or 17β-E2-induced changes in the dendritic morphogenesis of neurons. These BPA- or 17β-E2-induced effects were completely blocked by ICI182,780, and were partially suppressed by U0126. These results reveal that, similar to 17β-E2, BPA exerts its effects on dendritic morphogenesis by eliciting both nuclear actions and extranuclear-initiated actions that are integrated to influence the development of dendrite in hippocampal neurons.

Role of p-21-activated kinases in cancer progression

The p-21-activated kinases (PAKs) are downstream effectors of Rho GTPases Rac and Cdc42. The PAK family consists of six members which are segregated into two subgroups (Group I and Group II) based on sequence homology. Group I PAKs (PAK1-3) are the most extensively studied but there is increasing interest in the functionality of Group II PAKs (PAK4-6). The PAK family proteins are thought to play an important role in many different cellular processes, some of which have particular significance in the context of cancer progression. This review explores established and more recent data, linking the PAK family kinases to cancer progression including expression profiles, evasion of apoptosis, promotion of cell survival, and regulation of cell invasion. Finally, we discuss attempts to therapeutically target the PAK family and outline the major obstacles that still need to be overcome.

Strike a pose: Gαq complexes at the membrane

The heterotrimeric G protein Gαq is a central player in signal transduction, relaying signals from activated G-protein-coupled receptors (GPCRs) to effectors and other proteins to elicit changes in intracellular Ca(2+), the actin cytoskeleton, and gene transcription. Gαq functions at the intracellular surface of the plasma membrane, as do its best-characterized targets, phospholipase C-β, p63RhoGEF, and GPCR kinase 2 (GRK2). Recent insights into the structure and function of these signaling complexes reveal several recurring themes, including complex multivalent interactions between Gαq, its protein target, and the membrane, that are likely essential for allosteric control and maximum efficiency in signal transduction. Thus, the plasma membrane is not only a source of substrates but also a key player in the scaffolding of Gαq-dependent signaling pathways.

p21-Activated kinase 5: a pleiotropic kinase

The PAKs (p21-activated kinases) are highly conserved serine/threonine protein kinases which comprise six mammalian PAKs. PAK5 (p21-activated kinase 5) is the least understood member of PAKs that regulate many intracellular processes when they are stimulated by activated forms of the small GTPases Cdc42 and Rac. PAK5 takes an important part in multiple signal pathways in mammalian cells and controls a variety of cellular functions including cytoskeleton organization, cell motility and apoptosis. The main goal of this review is to describe the structure, mechanisms underlying its activity regulation, its role in apoptosis and the likely directions of further research.

FE65 interacts with ADP-ribosylation factor 6 to promote neurite outgrowth

FE65 is an adaptor protein that binds to the amyloid precursor protein (APP). As such, FE65 has been implicated in the pathogenesis of Alzheimer's disease. In addition, evidence suggests that FE65 is involved in brain development. It is generally believed that FE65 participates in these processes by recruiting various interacting partners to form functional complexes. Here, we show that via its first phosphotyrosine binding (PTB) domain, FE65 binds to the small GTPase ADP-ribosylation factor 6 (ARF6). FE65 preferentially binds to ARF6-GDP, and they colocalize in neuronal growth cones. Interestingly, FE65 stimulates the activation of both ARF6 and its downstream GTPase Rac1, a regulator of actin dynamics, and functions in growth cones to stimulate neurite outgrowth. We show that transfection of FE65 and/or ARF6 promotes whereas small interfering RNA knockdown of FE65 or ARF6 inhibits neurite outgrowth in cultured neurons as compared to the mock-transfected control cells. Moreover, knockdown of ARF6 attenuates FE65 stimulation of neurite outgrowth and defective neurite outgrowth seen in FE65-deficient neurons is partially corrected by ARF6 overexpression. Notably, the stimulatory effect of FE65 and ARF6 on neurite outgrowth is abrogated either by dominant-negative Rac1 or knockdown of Rac1. Thus, we identify FE65 as a novel regulator of neurite outgrowth via controlling ARF6-Rac1 signaling.

Rho GEFs and cancer: linking gene expression and metastatic dissemination

Guanine nucleotide exchange factors (GEFs) that promote GTP loading onto the guanosine triphosphatases (GTPases) Rho and Rac are prominent players in cancer progression. Recent studies have highlighted the relevance of several GEFs, including the phosphatidylinositol 3,4,5-trisphosphate Rac exchangers P-Rex1 and P-Rex2a, in breast tumorigenesis. New evidence suggests that the exchange factors Vav2 and Vav3 play synergistic roles in breast cancer by sustaining tumor growth, neoangiogenesis, and metastasis. The identification of a Vav-regulated transcriptome and Vav-related genes that control specific steps of metastatic dissemination of breast cancer cells to the lungs highlights the complexities of the signaling networks regulated by Rho/Rac GTPases and may lead to novel therapeutic targets.

Methods for treating neurological conditions (WO2011159945)

This patent application claims that inhibition of p21-activated kinases (PAK) reverses, partially reverses or delays clinical signs in neurological conditions (main claim for Huntington's disease (HD), substance abuse and addiction, Parkinson's disease, depression, bipolar disorder, anxiety disorder, posttraumatic stress disorder and neurofibromatosis). Several compounds with a pyrido-[2,3-d]pyrimidine-7(8H)-one core and high affinity to the catalytic domain of PAK1-4 are described in the patent. These PAK inhibitors are hypothesized to exert beneficial effects on clinical symptoms via modulation of dendritic spine morphology and/or synaptic function. Preliminary preclinical data suggest that PAK inhibition may be an interesting approach for the treatment of HD, neurofibromatosis and fragile X syndrome, while data for other neurological conditions are missing. Current limitations call for a comprehensive characterization of the role of PAK dysfunction in neurological disorders before further testing the effect of PAK inhibitors in relevant preclinical models. If ever, it will probably take many years before the most promising compounds will head to the clinic for further assessment in patients with neurological disorders.

The role of small GTPases in neuronal morphogenesis and polarity

The highly dynamic remodeling and cross talk of the microtubule and actin cytoskeleton support neuronal morphogenesis. Small RhoGTPases family members have emerged as crucial regulators of cytoskeletal dynamics. In this review we will comprehensively analyze findings that support the participation of RhoA, Rac, Cdc42, and TC10 in different neuronal morphogenetic events ranging from migration to synaptic plasticity. We will specifically address the contribution of these GTPases to support neuronal polarity and axonal elongation.

Deconstructing signal transduction pathways that regulate the actin cytoskeleton in dendritic spines

Dendritic spines are the sites of most excitatory synapses in the central nervous system. Recent studies have shown that spines function independently of each other, and they are currently the smallest known processing units in the brain. Spines exist in an array of morphologies, and spine structure helps dictate synaptic function. Dendritic spines are rich in actin, and actin rearrangements are critical regulators of spine morphology and density. In this review, we discuss the importance of actin in regulating dendritic spine morphogenesis, and discuss the upstream signal transduction pathways that either foster or inhibit actin polymerization. The understanding of actin regulatory pathways is best conceptualized as a hierarchical network in which molecules function in discrete levels defined by their molecular distance to actin. To this end, we focus on several classes of molecules, including guanine nucleotide exchange factors, small GTPases, small GTPase effectors, and actin binding proteins. We discuss how individual proteins in these molecular classes impact spine morphogenesis, and reveal the biochemical interactions in these networks that are responsible for shaping actin polymerization. Finally, we discuss the importance of these actin regulatory pathways in neuropsychiatric disorders.

The RhoG/ELMO1/Dock180 signaling module is required for spine morphogenesis in hippocampal neurons

Dendritic spines are actin-rich structures, the formation and plasticity of which are regulated by the Rho GTPases in response to synaptic input. Although several guanine nucleotide exchange factors (GEFs) have been implicated in spine development and plasticity in hippocampal neurons, it is not known how many different Rho GEFs contribute to spine morphogenesis or how they coordinate the initiation, establishment, and maintenance of spines. In this study, we screened 70 rat Rho GEFs in cultured hippocampal neurons by RNA interference and identified a number of candidates that affected spine morphogenesis. Of these, Dock180, which plays a pivotal role in a variety of cellular processes including cell migration and phagocytosis, was further investigated. We show that depletion of Dock180 inhibits spine morphogenesis, whereas overexpression of Dock180 promotes spine morphogenesis. ELMO1, a protein necessary for in vivo functions of Dock180, functions in a complex with Dock180 in spine morphogenesis through activating the Rac GTPase. Moreover, RhoG, which functions upstream of the ELMO1/Dock180 complex, is also important for spine formation. Together, our findings uncover a role for the RhoG/ELMO1/Dock180 signaling module in spine morphogenesis in hippocampal neurons.

p63RhoGEF--a key mediator of angiotensin II-dependent signaling and processes in vascular smooth muscle cells

The purpose of our study was to investigate the role of endogenous p63RhoGEF in G(q/11)-dependent RhoA activation and signaling in rat aortic smooth muscle cells (RASMCs). Therefore, we studied the expression and subcellular localization in freshly isolated RASMCs and performed loss of function experiments to analyze its contribution to RhoGTPase activation and functional responses such as proliferation and contraction. By this, we could show that p63RhoGEF is endogenously expressed in RASMCs and acts there as the dominant mediator of the fast angiotensin II (ANG II)-dependent but not of the sphingosine-1-phosphate (S(1)P)-dependent RhoA activation. p63RhoGEF is not an activator of the concomitant Rac1 activation and functions independently of caveolae. The knockdown of endogenous p63RhoGEF significantly reduced the mitogenic response of ANG II, abolished ANG II-induced stress fiber formation and cell elongation in 2-D culture, and impaired the ANG II-driven contraction in a collagen-based 3-D model. In conclusion, our data provide for the first time evidence that p63RhoGEF is an important mediator of ANG II-dependent RhoA activation in RASMCs and therewith a leading actor in the subsequently triggered cellular processes, such as proliferation and contraction.

Retrograde BMP signaling controls synaptic growth at the NMJ by regulating trio expression in motor neurons

Retrograde signaling is essential for coordinating the growth of synaptic structures; however, it is not clear how it can lead to modulation of cytoskeletal dynamics and structural changes at presynaptic terminals. We show that loss of retrograde bone morphogenic protein (BMP) signaling at the Drosophila larval neuromuscular junction (NMJ) leads to a significant reduction in levels of Rac GEF Trio and a diminution of transcription at the trio locus. We further find that Trio is required in motor neurons for normal structural growth. Finally, we show that transgenic expression of Trio in motor neurons can partially restore NMJ defects in larvae mutant for BMP signaling. Based on our findings, we propose a model in which a retrograde BMP signal from the muscle modulates GTPase activity through transcriptional regulation of Rac GEF trio, thereby regulating the homeostasis of synaptic growth at the NMJ.

The emerging importance of group II PAKs

The Rho-family GTPases Rho Rac and Cdc42 regulate many intracellular processes through their interaction with downstream effector proteins. The PAKs (p21-activated kinases) are a family of effector proteins for Rac and Cdc42. PAKs are important regulators of actin cytoskeletal dynamics, neurite outgrowth, cell survival, hormone signalling and gene transcription. There are six mammalian PAKs that can be divided into two groups: group I PAKs (PAK1-3) and group II PAKs (PAK4-6). Although the two PAK groups are architecturally similar, there are differences in their mode of regulation, suggesting that their cellular functions are likely to be different. Whereas much is known about group I PAKs, less is known about the more recently discovered PAK4, PAK5 and PAK6. This review will focus on the latest structural and functional results relating to the group II PAKs and discuss the emerging importance of group II PAKs in disease progression.

Identification of a novel Bves function: regulation of vesicular transport

Blood vessel/epicardial substance (Bves) is a transmembrane protein that influences cell adhesion and motility through unknown mechanisms. We have discovered that Bves directly interacts with VAMP3, a SNARE protein that facilitates vesicular transport and specifically recycles transferrin and beta-1-integrin. Two independent assays document that cells expressing a mutated form of Bves are severely impaired in the recycling of these molecules, a phenotype consistent with disruption of VAMP3 function. Using Morpholino knockdown in Xenopus laevis, we demonstrate that elimination of Bves function specifically inhibits transferrin receptor recycling, and results in gastrulation defects previously reported with impaired integrin-dependent cell movements. Kymographic analysis of Bves-depleted primary and cultured cells reveals severe impairment of cell spreading and adhesion on fibronectin, indicative of disruption of integrin-mediated adhesion. Taken together, these data demonstrate that Bves interacts with VAMP3 and facilitates receptor recycling both in vitro and during early development. Thus, this study establishes a newly identified role for Bves in vesicular transport and reveals a novel, broadly applied mechanism governing SNARE protein function.

Bves: ten years after

Bves was discovered in 1999 by two independent laboratories using screens to identify novel genes that were highly expressed in the developing heart (Reese et al., 1999; Andree et al., 2000). As an evolutionarily conserved transmembrane protein, Bves is postulated to play a role in cell adhesion and cell motility. In studies of Bves protein disruption, there have been multiple phenotypes, but few molecular mechanisms have been advanced to explain the underlying cause of these phenotypes. As the molecular function of Bves protein begins to be uncovered, it is now time to review the literature to examine the significance of this work and future directions of study. This review summarizes the literature on this unique protein and explores new and exciting data that support emerging themes on its molecular function.

P-Rex1 - a multidomain protein that regulates neurite differentiation

The Rac-GEF P-Rex1 promotes membrane ruffling and cell migration in response to Rac activation, but its role in neuritogenesis is unknown. Rac1 promotes neurite differentiation; Rac3, however, may play an opposing role. Here we report that in nerve growth factor (NGF)-differentiated rat PC12 cells, P-Rex1 localised to the distal tips of developing neurites and to the axonal shaft and growth cone of differentiating hippocampal neurons. P-Rex1 expression inhibited NGF-stimulated PC12 neurite differentiation and this was dependent on the Rac-GEF activity of P-Rex1. P-Rex1 inhibition of neurite outgrowth was rescued by low-dose cytochalasin D treatment, which prevents actin polymerisation. P-Rex1 activated Rac3 GTPase activity when coexpressed in PC12 cells. In the absence of NGF stimulation, targeted depletion of P-Rex1 in PC12 cells by RNA interference induced the spontaneous formation of beta-tubulin-enriched projections. Following NGF stimulation, enhanced neurite differentiation, with neurite hyper-elongation correlating with decreased F-actin at the growth cone, was demonstrated in P-Rex1 knockdown cells. Interestingly, P-Rex1-depleted PC12 cells exhibited reduced Rac3 and Rac1 GTPase activity. This study has identified P-Rex1 as a Rac3-GEF in neuronal cells that localises to, and regulates, actin cytoskeletal dynamics at the axonal growth cone to in turn regulate neurite differentiation.

Targeted disruption of the Pak5 and Pak6 genes in mice leads to deficits in learning and locomotion

PAK6 is a member of the group B family of PAK serine/threonine kinases, and is highly expressed in the brain. The group B PAKs, including PAK4, PAK5, and PAK6, were first identified as effector proteins for the Rho GTPase Cdc42. They have important roles in filopodia formation, the extension of neurons, and cell survival. Pak4 knockout mice die in utero, and the embryos have several abnormalities, including a defect in the development of motor neurons. In contrast, Pak5 knockout mice do not have any noticeable abnormalities. So far nothing is known about the biological function of Pak6. To address this, we have deleted the Pak6 gene in mice. Since Pak6 and Pak5 are both expressed in the brain, we also generated Pak5/Pak6 double knockout mice. These mice were viable and fertile, but had several locomotor and behavioral deficits. Our results indicate that Pak5 and Pak6 together are not required for viability, but are required for a normal level of locomotion and activity as well as for learning and memory. This is consistent with a role for the group B PAKs in the nervous system.

Phosphorylation of Homer3 by calcium/calmodulin-dependent kinase II regulates a coupling state of its target molecules in Purkinje cells

Homer proteins are components of postsynaptic density (PSD) and play a crucial role in coupling diverse target molecules. However, the regulatory aspect of Homer-mediated coupling has been addressed only about a dominant-negative effect of Homer1a, which requires de novo gene expression. Here, we present evidence that Homer-mediated coupling is regulated by its phosphorylation state. We found that Homer3, the predominant isoform in Purkinje cells, is phosphorylated by calcium/calmodulin-dependent protein kinase II (CaMKII) both in vitro and in vivo. Biochemical fractionation with phosphor-specific antibodies revealed the presence of phosphorylated Homer3 in the cytosolic fraction in contrast to high levels of nonphosphorylated Homer3 in PSD. In P/Q-type voltage-gated-Ca2+ channel knock-out mice, in which CaMKII activation was reduced, the levels of Homer3 phosphorylation and the soluble form of Homer 3 were markedly lower. Furthermore, both robust phosphorylation of Homer3 and its dissociation from metabotropic glutamate receptor 1alpha (mGluR1alpha) were triggered by depolarization in primary cultured Purkinje cells, and these events were inhibited by CaMKII inhibitor. An in vitro binding kinetic analysis revealed that these phosphorylation-dependent events were attributable to a decrease in the affinity of phosphorylated Homer3 for its ligand. In a heterologous system, the Ca2+ signaling pattern induced by mGluR1alpha activation was modulated by the Homer3 phosphorylation state. Together, these findings suggested that Homer3 in Purkinje cells might function as a reversible coupler regulated by CaMKII phosphorylation and that the phosphorylation is capable of regulating the postsynaptic molecular architecture in response to synaptic activity.

Bves directly interacts with GEFT, and controls cell shape and movement through regulation of Rac1/Cdc42 activity

Bves is an integral membrane protein with no determined function and no homology to proteins outside of the Popdc family. It is widely expressed throughout development in myriad organisms. Here, we demonstrate an interaction between Bves and guanine nucleotide exchange factor T (GEFT), a GEF for Rho-family GTPases. This interaction represents the first identification of any protein that has a direct physical interaction with any member of the Popdc family. Bves and GEFT are shown to colocalize in adult skeletal muscle. We also demonstrate that exogenous expression of Bves reduces Rac1 and Cdc42 activity levels while not affecting levels of active RhoA. Consistent with a repression of Rac1 and Cdc42 activity, we show changes in speed of cell locomotion and cell roundness also result from exogenous expression of Bves. Modulation of Rho-family GTPase signaling by Bves would be highly consistent with previously described phenotypes occurring upon disruption of Bves function in a wide variety of model systems. Therefore, we propose Bves as a novel regulator of the Rac1 and Cdc42 signaling cascades.

P-Rex2 regulates Purkinje cell dendrite morphology and motor coordination

The small GTPase Rac controls cell morphology, gene expression, and reactive oxygen species formation. Manipulations of Rac activity levels in the cerebellum result in motor coordination defects, but activators of Rac in the cerebellum are unknown. P-Rex family guanine-nucleotide exchange factors activate Rac. We show here that, whereas P-Rex1 expression within the brain is widespread, P-Rex2 is specifically expressed in the Purkinje neurons of the cerebellum. We have generated P-Rex2(-/-) and P-Rex1(-/-)/P-Rex2(-/-) mice, analyzed their Purkinje cell morphology, and assessed their motor functions in behavior tests. The main dendrite is thinned in Purkinje cells of P-Rex2(-/-) pups and dendrite structure appears disordered in Purkinje cells of adult P-Rex2(-/-) and P-Rex1(-/-)/P-Rex2(-/-) mice. P-Rex2(-/-) mice show a mild motor coordination defect that progressively worsens with age and is more pronounced in females than in males. P-Rex1(-/-)/P-Rex2(-/-) mice are ataxic, with reduced basic motor activity and abnormal posture and gait, as well as impaired motor coordination even at a young age. We conclude that P-Rex1 and P-Rex2 are important regulators of Purkinje cell morphology and cerebellar function.

The myelin-associated glycoprotein inhibitor BENZ induces outgrowth and survival of rat dorsal root ganglion cell cultures

The novel myelin-associated glycoprotein (MAG) inhibitor BENZ binds to the N-acetylneuraminic acid (Neu5Ac) portion of the N-terminal Ig-like domain of MAG. Treatment of rat dorsal root ganglion (DRG) cell cultures with BENZ-induced outgrowth of neurofilament 200-positive neurites improved survival of neurons and increased the number of GFAP-positive cells, as determined by fluorescence and confocal laser microscopy and by Western immunoblotting. Furthermore, treatment of DRG cell cultures with BENZ repressed gene and protein expression of the small GTPase RhoA but induced expression of Rho GTP-activating proteins 5 and 24, likely to counteract protein kinase A activity. Specifically, expression of inhibitors of neurite outgrowth, for example, Rock2 and PAK4, was repressed, but cofilin 1, a promoter of axonal growth, was induced. We propose that the MAG inhibitor BENZ abrogates the RhoA-ROCK-cofilin pathway to promote neurite outgrowth. Our findings require confirmation by in vivo animal studies.

Dlx transcription factors promote migration through repression of axon and dendrite growth

In the mouse telencephalon, Dlx homeobox transcription factors are essential for the tangential migration of subpallial-derived GABAergic interneurons to neocortex. However, the mechanisms underlying this process are poorly understood. Here, we demonstrate that Dlx1/2 has a central role in restraining neurite growth of subpallial-derived immature interneurons at a stage when they migrate tangentially to cortex. In Dlx1-/-;Dlx2-/- mutants, neurite length is increased and cells fail to migrate. In Dlx1-/-;Dlx2+/- mutants, while the tangential migration of immature interneurons appears normal, they develop dendritic and axonal processes with increased length and decreased branching, and have deficits in their neocortical laminar positions. Thus, Dlx1/2 is required for coordinating programs of neurite maturation and migration. In this regard, we provide genetic evidence that in immature interneurons Dlx1/2 repression of the p21-activated serine/threonine kinase PAK3, a downstream effector of the Rho family of GTPases, is critical in restraining neurite growth and promoting tangential migration.

Galphaq directly activates p63RhoGEF and Trio via a conserved extension of the Dbl homology-associated pleckstrin homology domain

The coordinated cross-talk from heterotrimeric G proteins to Rho GTPases is essential during a variety of physiological processes. Emerging data suggest that members of the Galpha(12/13) and Galpha(q/11) families of heterotrimeric G proteins signal downstream to RhoA via distinct pathways. Although studies have elucidated mechanisms governing Galpha(12/13)-mediated RhoA activation, proteins that functionally couple Galpha(q/11) to RhoA activation have remained elusive. Recently, the Dbl-family guanine nucleotide exchange factor (GEF) p63RhoGEF/GEFT has been described as a novel mediator of Galpha(q/11) signaling to RhoA based on its ability to synergize with Galpha(q/11) resulting in enhanced RhoA signaling in cells. We have used biochemical/biophysical approaches with purified protein components to better understand the mechanism by which activated Galpha(q) directly engages and stimulates p63RhoGEF. Basally, p63RhoGEF is autoinhibited by the Dbl homology (DH)-associated pleckstrin homology (PH) domain; activated Galpha(q) relieves this autoinhibition by interacting with a highly conserved C-terminal extension of the PH domain. This unique extension is conserved in the related Dbl-family members Trio and Kalirin and we show that the C-terminal Rho-specific DH-PH cassette of Trio is similarly activated by Galpha(q).

Tumorhead distribution to cytoplasmic membrane of neural plate cells is positively regulated by Xenopus p21-activated kinase 1 (X-PAK1)

Tumorhead (TH) regulates neural plate cell proliferation during Xenopus early development, and gain or loss of function prevents neural differentiation. TH shuttles between the nuclear and cytoplasmic/cortical cell compartments in embryonic cells. In this study, we show that subcellular distribution of TH is important for its functions. Targeting TH to the cell cortex/membrane potentiates a TH gain of function phenotype and results in neural plate expansion and inhibition of neuronal differentiation. We have found that TH subcellular localization is regulated, and that its shuttling between the nucleus and the cell cortex/cytoplasm is controlled by the catalytic activity of p21-activated kinase, X-PAK1. The phenotypes of embryos that lack, or have excess, X-PAK1 activity mimic the phenotypes induced by loss or gain of TH functions, respectively. We provide evidence that X-PAK1 is an upstream regulator of TH and discuss potential functions of TH at the cell cortex/cytoplasmic membrane and in the nucleus.

Cdk5 regulates EphA4-mediated dendritic spine retraction through an ephexin1-dependent mechanism

The development of dendritic spines is thought to be crucial for synaptic plasticity. Dendritic spines are retracted upon Eph receptor A4 (EphA4) activation, but the mechanisms that control this process are not well understood. Here we report an important function of cyclin-dependent kinase 5 (Cdk5) in EphA4-dependent spine retraction in mice. We found that blocking Cdk5 activity inhibits ephrin-A1-triggered spine retraction and reduction of mEPSC frequency at hippocampal synapses. The activation of EphA4 resulted in the recruitment of Cdk5 to EphA4, leading to the tyrosine phosphorylation and activation of Cdk5. EphA4 and Cdk5 then enhanced the activation of ephexin1, a guanine-nucleotide exchange factor that regulates activation of the small Rho GTPase RhoA. The association between EphA4 and ephexin1 was significantly reduced in Cdk5(-/-) brains and Cdk5-dependent phosphorylation of ephexin1 was required for the ephrin-A1-mediated regulation of spine density. These findings suggest that ephrin-A1 promotes EphA4-dependent spine retraction through the activation of Cdk5 and ephexin1, which in turn modulates actin cytoskeletal dynamics.

Signaling from MARK to Tau: Regulation, Cytoskeletal Crosstalk, and Pathological Phosphorylation

The hyperphosphorylation of tau is an early step in the degeneration of neurons in Alzheimer's disease and other tauopathies. Of particular importance is the phosphorylation of tau in the repeat domain which detaches tau from microtubules. This makes microtubules dynamic for their role in differentiation and neurite outgrowth, and it controls the level of tau on the microtubule surface which keeps the tracks clear for axonal transport. However, the detachment of tau from microtubules can also initiate the reactions that lead to pathological aggregation into neurofibrillary tangles. Phosphorylation of tau in the repeat domain is achieved by the kinase MARK/Par-1, a member of the calcium/calmodulin-dependent protein kinase group of kinases. In this report, we focus on the modes of MARK regulation. MARK contains several domains which offer multiple ways of regulation by posttranslational modification (e.g. phosphorylation), interactions with scaffolding proteins and subcellular targeting (e.g. 14-3-3), and interactions with other proteins. We consider in particular the interactions between MARK and other kinases, notably MARKK/TAO-1 and PAK5. MARKK (a member of the Ste20 family of kinases) activates MARK by phosphorylating it at a critical threonine residue within the activation loop. Activated MARK in turn phosphorylates tau, causes its detachment from microtubules and renders them labile. PAK5 inactivates MARK, not by phosphorylation, but by binding to the catalytic domain. PAK5 contributes to microtubule stability by preventing the MARK-induced phosphorylation of tau; conversely, PAK5 contributes to actin dynamics, presumably through the activation of cofilin, an F-actin severing protein. Thus, MARK and its regulators MARKK and PAK5 appear to mediate the crosstalk between the actin and microtubule cytoskeleton in an antagonistic fashion.

The Rho-family guanine nucleotide exchange factor GEFT enhances retinoic acid- and cAMP-induced neurite outgrowth

The Rho GTPases are important regulators of neurite outgrowth and pathfinding. We have recently reported that a Rho-family guanine nucleotide exchange factor, GEFT, modulates dendrite spine morphology and basal neurite outgrowth in primary hippocampal neurons and Neuro2A cells, respectively. Here we demonstrate that GEFT protein is highly expressed in all regions of the brain and is highly up-regulated upon treatment of Neuro2A cells with retinoic acid and dibutyric cAMP, which promote dendrite and axon-like neurite extensions, respectively. Within retinoic acid-induced neurite extensions, GEFT is localized to actin-enriched regions in the primary neurites, with little or no expression from secondary branches. Dibutyric cAMP-induced neurite extensions are highly concentrated for GEFT at the actin-rich distal tip of the growth cone. Additionally, we demonstrate that GEFT promotes neurite outgrowth in undifferentiated as well as differentiated Neuro2A cells. Together, our data provide new evidence suggesting that GEFT is an important regulator of multiple processes involved in axon and dendrite formation.

The 3-hydroxy-3-methylglutaryl co-enzyme A reductase inhibitor pravastatin enhances neurite outgrowth in hippocampal neurons

Epidemiological studies demonstrate a relationship between statin [3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitor] usage and reduced risk of developing Alzheimer's disease. To determine whether statins affect neuronal development, we treated cultured rat hippocampal neurons with pravastatin. After 4-48 h of treatment, pravastatin significantly increased the number of neurites produced by each cell and caused a corresponding increase in levels of the membrane phospholipid phosphatidylcholine. Pravastatin treatment also significantly increased neurite length and branching but did not affect cellular cholesterol levels. Co-incubation with mevalonate, but not cholesterol, abolished the stimulatory effect of pravastatin on neurite outgrowth. Treatment of neurons with isoprenoids also abolished the effect of pravastatin on neurite growth, suggesting that pravastatin may stimulate neuritogenesis by preventing isoprenylation of signaling molecules such as the Rho family of small GTPases. A specific inhibitor of geranylgeranylation, but not farnesylation, mimicked the stimulatory effect of pravastatin on neuritogenesis. Pravastatin treatment significantly decreased levels of membrane-associated RhoA. These data suggest that pravastatin treatment increases neurite outgrowth and may do so via inhibiting the activity of geranylgeranylated proteins such as RhoA.

Rac1 induces the clustering of AMPA receptors during spinogenesis

Glutamatergic synapses switch from nonspiny synapses to become dendritic spines during early neuronal development. Here, we report that the lack of sufficient Rac1, a small RhoGTPase, contributes to the absence of spinogenesis in immature neurons. The overexpression of green fluorescence protein-tagged wild-type Rac1 initiated the formation of dendritic spines in cultured dissociated hippocampal neurons younger than 11 d in vitro, indicating that Rac1 is likely one of the missing pieces responsible for the lack of spines in immature neurons. The overexpression of wild-type Rac1 also induced the clustering of AMPA receptors (AMPARs) and increased the amplitude of miniature EPSCs (mEPSCs). The expression of constitutively active Rac1 induced the formation of unusually large synapses with large amounts of AMPAR clusters. Also, our live imaging experiments revealed that the contact of an axon induced the clustering of Rac1, and subsequent morphological changes led to spinogenesis. Additionally, the overexpression of wild-type Rac1 and constitutively active Rac1 increased the size of preexisting spines and the amplitude of mEPSCs in mature neurons (>21 d in vitro) within 24 h after transfection. Together, these results indicate that activation of Rac1 enhances excitatory synaptic transmission by recruiting AMPARs to synapses during spinogenesis, thus providing a mechanistic link between presynaptic and postsynaptic developmental changes. Furthermore, we show that Rac1 has two distinct roles at different stages of neuronal development. The activation of Rac1 initiates spinogenesis at an early stage and regulates the function and morphology of preexisting spines at a later stage.