Signal transduction in neuronal migration: roles of GTPase activating proteins and the small GTPase Cdc42 in the Slit-Robo pathway

Slit stimulation recruits Dock and Pak to the roundabout receptor and increases Rac activity to regulate axon repulsion at the CNS midline

Drosophila Roundabout (Robo) is the founding member of a conserved family of repulsive axon guidance receptors that respond to secreted Slit proteins. Here we present evidence that the SH3-SH2 adaptor protein Dreadlocks (Dock), the p21-activated serine-threonine kinase (Pak), and the Rac1/Rac2/Mtl small GTPases can function during Robo repulsion. Loss-of-function and genetic interaction experiments suggest that limiting the function of Dock, Pak, or Rac partially disrupts Robo repulsion. In addition, Dock can directly bind to Robo's cytoplasmic domain, and the association of Dock and Robo is enhanced by stimulation with Slit. Furthermore, Slit stimulation can recruit a complex of Dock and Pak to the Robo receptor and trigger an increase in Rac1 activity. These results provide a direct physical link between the Robo receptor and an important cytoskeletal regulatory protein complex and suggest that Rac can function in both attractive and repulsive axon guidance.

GC-GAP, a Rho family GTPase-activating protein that interacts with signaling adapters Gab1 and Gab2

Gab1 and Gab2 are scaffolding proteins acting downstream of cell surface receptors and interact with a variety of cytoplasmic signaling proteins such as Grb2, Shp-2, phosphatidylinositol 3-kinase, Shc, and Crk. To identify new binding partners for GAB proteins and better understand their functions, we performed a yeast two-hybrid screening with hGab2-(120-587) as bait. This work led to identification of a novel GTPase-activating protein (GAP) for Rho family GTPases. The GAP domain shows high similarity to the recently cloned CdGAP and displays activity toward RhoA, Rac1, and Cdc42 in vitro. The protein was named GC-GAP for its ability to interact with GAB proteins and its activity toward Rac and Cdc42. GC-GAP is predominantly expressed in the brain with low levels detected in other tissues. Antibodies directed against GC-GAP recognized a protein of approximately 200 kDa. Expression of GC-GAP in 293T cells led to a reduction in active Rac1 and Cdc42 levels but not RhoA. Suppression of GC-GAP expression by siRNA inhibited proliferation of C6 astroglioma cells. In addition, GC-GAP contains several classic proline-rich motifs, and it interacts with the first SH3 domain of Crk and full-length Nck in vitro. We propose that Gab1 and Gab2 in cooperation with other adapter molecules might regulate the cellular localization of GC-GAP under specific stimuli, acting to regulate precisely Rac and Cdc42 activities. Given that GC-GAP is specifically expressed in the nervous system and that it is localized to the dendritic processes of cultured neurons, GC-GAP may play a role in dendritic morphogenesis and also possibly in neural/glial cell proliferation.

Neuronal migration from the forebrain to the olfactory bulb requires a new attractant persistent in the olfactory bulb

Interneurons in the olfactory bulb (OB) are generated not only in the developing embryo but also throughout the postnatal life of mammals from neuronal precursor cells migrating from the anterior subventricular zone (SVZa) of the mammalian forebrain. We discovered that the OB secretes a diffusible activity that attracts these neuronal precursor cells. The attractive activity is present in specific layers in the OB, including the glomerular layer but not the granule cell layer. The attractive activity and the neuronal responsiveness persist from embryonic through neonatal to adult stages. Removal of the rostral OB significantly reduces SVZa migration toward the OB, an effect that can be rescued by a transplant of the OB but not by that of the neocortex. The activity in the OB is not mimicked by the known attractants. These results provide an explanation for the continuous migration of SVZa neurons toward the OB, demonstrate an important role of the OB in neuronal migration, and reveal the existence of a new chemoattractant.

Robo4 is a vascular-specific receptor that inhibits endothelial migration

Guidance and patterning of axons are orchestrated by cell-surface receptors and ligands that provide directional cues. Interactions between the Robo receptor and Slit ligand families of proteins initiate signaling cascades that repel axonal outgrowth. Although the vascular and nervous systems grow as parallel networks, the mechanisms by which the vascular endothelial cells are guided to their appropriate positions remain obscure. We have identified a putative Robo homologue, Robo4, based on its differential expression in mutant mice with defects in vascular sprouting. In contrast to known neuronal Robo family members, the arrangement of the extracellular domains of Robo4 diverges significantly from that of all other Robo family members. Moreover, Robo4 is specifically expressed in the vascular endothelium during murine embryonic development. We show that Robo4 binds Slit and inhibits cellular migration in a heterologous expression system, analogous to the role of known Robo receptors in the nervous system. Immunoprecipitation studies indicate that Robo4 binds to Mena, a known effector of Robo-Slit signaling. Finally, we show that Robo4 is the only Robo family member expressed in primary endothelial cells and that application of Slit inhibits their migration. These data demonstrate that Robo4 is a bona fide member of the Robo family and may provide a repulsive cue to migrating endothelial cells during vascular development.

Four-dimensional migratory coordinates of GABAergic interneurons in the developing mouse cortex

We have used time-lapse multiphoton microscopy to map the migration and settling pattern of GABAergic interneurons that originate in the ganglionic eminence of the ventral forebrain and incorporate into the neocortex of the cerebral hemispheres. Imaging of the surface of the cerebral hemispheres in both explant cultures and brains of living mouse embryos revealed that GABAergic interneurons migrating within the marginal zone originate from three different sources and migrate via distinct and independent streams. After reaching their areal destination, interneurons descend into the underlying cortex to assume positions with isochronically generated, radially derived neurons. The dynamics and pattern of cell migration in the marginal zone (see movies, available at www.jneurosci.org) suggest that the three populations of interneurons respond selectively to distinct local cues for directing their migration to the appropriate areas and layers of the neocortex. This approach opens a new avenue for study of normal and abnormal neuronal migration in their native environment and indicate that interneurons have specific programs for their areal and laminar deployment.

Distinguishing between directional guidance and motility regulation in neuronal migration

Although neuronal migration is an essential process in development, how neural precursors reach their final destination in the nervous system is not well understood. Secreted molecules that are known to be involved in axon guidance are likely to play important roles in regulating neuronal migration, but an important issue that remains unclear is whether such molecules act as directional guidance cues or as motility regulators in neuronal migration. The secreted protein Slit was initially suggested to be a repellent for migrating neurons (Wu et al., 1999). However, it was concluded recently that Slit plays an inhibitory rather than a repulsive role in neuronal migration (Mason et al., 2001). We have developed a series of assays that allow us to differentiate between repulsive and inhibitory effects of secreted molecules, and we demonstrate that Slit is a repellent capable of reversing the direction of neurons migrating either in culture or in their native pathways. We also show that although Slit reduces migratory speed under certain conditions, it can function as a repellent without concurrent inhibition of neuronal migration. This is the first study to clearly demonstrate that migrating neurons can be directionally guided by secreted molecules. These findings provide a basis to understand the physiological roles of secreted molecules in the developing nervous system and have implications on how they could be applied therapeutically. Our results also indicate that it should be possible to determine the specific action of other molecules as directional guidance cues or as motility regulators of cell migration.

Frequent epigenetic inactivation of the SLIT2 gene in gliomas

The SLIT family of genes consists of large extracellular matrix-secreted and membrane-associated glycoproteins. The Slits (Slit1-3) are ligands for the repulsive guidance receptors, the robo gene family. The Slit-Robo interactions mediate the repulsive cues on axons and growth cones during neural development. In a recent report, we demonstrated that promoter region CpG island of human SLIT2 was frequently hypermethylated in lung, breast and colorectal tumours and the silenced gene transcript suppressed the malignant phenotype in in vitro assays. In this report we undertook epigenetic, genetic and expression analysis of SLIT2 gene in a large series of gliomas and glioma cell lines. Promoter region CpG island of SLIT2 was found to be methylated in 71% (5/7) of glioma cell lines and was unmethylated in five DNA samples from normal brain tissues. The hypermethylation of the SLIT2 promoter region in glioma cell lines correlated with loss of expression and treatment with the demethylating agent 5-aza-2'-deoxycytidine reactivated SLIT2 gene expression. In primary gliomas, SLIT2 was methylated in 59% (37/63) of tumours analysed. In addition, SLIT2 expression was downregulated in methylated gliomas relative to unmethylated tumour samples, as demonstrated by quantitative real-time RT-PCR. Loss of heterozygosity analysis revealed that SLIT2 methylated gliomas retained both alleles of a microsatellite marker within 100 kb of the SLIT2 gene at 4p15.2. Exogenous expression of SLIT2 in a glioma cell line that was heavily methylated for SLIT2 decreased in vitro colony formation. Our data indicate that SLIT2 is frequently inactivated by promoter region CpG island hypermethylation in gliomas and may be a good candidate for a glioma tumour suppressor gene (TSG) located at 4p15.2. Furthermore, our data suggest that a detailed analysis of both the cancer genome and epigenome will be required to identify key TSGs involved in glioma development.

Induction of tumor angiogenesis by Slit-Robo signaling and inhibition of cancer growth by blocking Robo activity

Slit is a secreted protein known to function through the Roundabout (Robo) receptor as a chemorepellent in axon guidance and neuronal migration, and as an inhibitor in leukocyte chemotaxis. Here we show Slit2 expression in a large number of solid tumors and Robo1 expression in vascular endothelial cells. Recombinant Slit2 protein attracted endothelial cells and promoted tube formation in a Robo1- and phosphatidylinositol kinase-dependent manner. Neutralization of Robo1 reduced the microvessel density and the tumor mass of human malignant melanoma A375 cells in vivo. These findings demonstrate the angiogenic function of Slit-Robo signaling, reveal a mechanism in mediating the crosstalk between cancer cells and endothelial cells, and indicate the effectiveness of blocking this signaling pathway in treating cancers.

Actin cytoskeleton regulation in neuronal morphogenesis and structural plasticity

The actin cytoskeleton plays a major role in morphological development of neurons and in structural changes of adult neurons. This article reviews the myriad functions of actin and myosin in axon initiation, growth, guidance and branching, in morphogenesis of dendrites and dendritic spines, in synapse formation and stability, and in axon and dendrite retraction. Evidence is presented that signaling pathways involving the Rho family of small GTPases are key regulators of actin polymerization and myosin function in the context of different aspects of neuronal morphogenesis. These studies support an emerging theme: Different aspects of neuronal morphogenesis may involve regulation of common core signaling pathways, in particular the Rho GTPases.

A genetic model for a central (septum transversum) congenital diaphragmatic hernia in mice lacking Slit3

Congenital diaphragmatic hernia (CDH) is a significant cause of pediatric mortality in humans with a heterogeneous and poorly understood etiology. Here we show that mice lacking Slit3 developed a central (septum transversum) CDH. Slit3 encodes a member of the Slit family of guidance molecules and is expressed predominantly in the mesothelium of the diaphragm during embryonic development. In Slit3 null mice, the central tendon region of the diaphragm fails to separate from liver tissue because of abnormalities in morphogenesis. The CDH progresses through continuous growth of the liver into the thoracic cavity. This study establishes the first genetic model for CDH and identifies a previously unsuspected role for Slit3 in regulating the development of the diaphragm.

Control of axon elongation via an SDF-1alpha/Rho/mDia pathway in cultured cerebellar granule neurons

Rho-GTPase has been implicated in axon outgrowth. However, not all of the critical steps controlled by Rho have been well characterized. Using cultured cerebellar granule neurons, we show here that stromal cell-derived factor (SDF)-1alpha, a neural chemokine, is a physiological ligand that can turn on two distinct Rho-dependent pathways with opposite consequences. A low concentration of the ligand stimulated a Rho-dependent pathway that mediated facilitation of axon elongation. In contrast, Rho/ROCK activation achieved by a higher concentration of SDF-1alpha caused repression of axon formation and induced no more increase in axon length. However, even at this higher concentration a Rho-dependent axon elongating activity could be recovered upon removal of ROCK activity using Y-27632. SDF-1alpha-induced axon elongating activity under ROCK inhibition was replicated by the dominant-active form of the mammalian homologue of the Drosophila gene Diaphanous (mDia)1 and counteracted by its dominant-negative form. Furthermore, RNAi knockdown of mDia1 abolished SDF-1alpha-induced axon elongation. Together, our results support a critical role for an SDF-1alpha/Rho/mDia1 pathway in mediating axon elongation.

Assembly of cell regulatory systems through protein interaction domains

The sequencing of complete genomes provides a list that includes the proteins responsible for cellular regulation. However, this does not immediately reveal what these proteins do, nor how they are assembled into the molecular machines and functional networks that control cellular behavior. The regulation of many different cellular processes requires the use of protein interaction domains to direct the association of polypeptides with one another and with phospholipids, small molecules, or nucleic acids. The modular nature of these domains, and the flexibility of their binding properties, have likely facilitated the evolution of cellular pathways. Conversely, aberrant interactions can induce abnormal cellular behavior and disease. The fundamental properties of protein interaction domains are discussed in this review and in detailed reviews on individual domains at Science's STKE at http://www.sciencemag.org/cgi/content/full/300/5618/445/DC1.

Distribution of p120 catenin during rat brain development: potential role in regulation of cadherin-mediated adhesion and actin cytoskeleton organization

p120 catenin (p120ctn) is implicated in the regulation of cadherin-mediated adhesion and actin cytoskeleton remodeling. The interaction of cytoplasmic p120ctn with the guanine exchange factor Vav2 is one of the signaling pathways implicated in cytoskeleton dynamics. We show here that p120ctn is regulated during rat brain development and is distributed at the membrane and within the cytoplasm where it associates with N-cadherin and Vav2, respectively. p120ctn shifts progressively from an axonal expression to a punctuate staining localized to a subset of synapses. In cultured hippocampal neurons, p120ctn redistributes from growth cones to synapses, where it partly colocalizes with N-cadherin or Vav2 and filamentous actin. In the adult forebrain, we show that p120ctn and Vav2 are highly expressed by neuroblasts migrating from the lateral subventricular zone to the olfactory bulb. The dynamic expression pattern of p120ctn and the biochemical evidences of its association with N-cadherin and Vav2 strongly suggest that p120ctn plays a major role in neuronal migration, neurite outgrowth and synapse formation, and plasticity.

GAPs galore! A survey of putative Ras superfamily GTPase activating proteins in man and Drosophila

Typical members of the Ras superfamily of small monomeric GTP-binding proteins function as regulators of diverse processes by cycling between biologically active GTP- and inactive GDP-bound conformations. Proteins that control this cycling include guanine nucleotide exchange factors or GEFs, which activate Ras superfamily members by catalyzing GTP for GDP exchange, and GTPase activating proteins or GAPs, which accelerate the low intrinsic GTP hydrolysis rate of typical Ras superfamily members, thus causing their inactivation. Two among the latter class of proteins have been implicated in common genetic disorders associated with an increased cancer risk, neurofibromatosis-1, and tuberous sclerosis. To facilitate genetic analysis, I surveyed Drosophila and human sequence databases for genes predicting proteins related to GAPs for Ras superfamily members. Remarkably, close to 0.5% of genes in both species (173 human and 64 Drosophila genes) predict proteins related to GAPs for Arf, Rab, Ran, Rap, Ras, Rho, and Sar family GTPases. Information on these genes has been entered into a pair of relational databases, which can be used to identify evolutionary conserved proteins that are likely to serve basic biological functions, and which can be updated when definitive information on the coding potential of both genomes becomes available.

The Rho/ROCK pathway mediates neurite growth-inhibitory activity associated with the chondroitin sulfate proteoglycans of the CNS glial scar

Axons fail to regenerate in the central nervous system after injury. Chondroitin sulfate proteoglycans (CSPG) expressed in the scar significantly contribute to the nonpermissive properties of the central nervous system environment. To examine the inhibitory activity of a CSPG mixture on retina ganglion cell (RGC) axon growth, we employed both a stripe assay and a nerve fiber outgrowth assay. We show that the inhibition exerted by CSPGs in vitro can be blocked by application of either C3 transferase, a specific inhibitor of the Rho GTPase, or Y27632, a specific inhibitor of the Rho kinase. These results demonstrate that CSPG-associated inhibition of neurite outgrowth is mediated by the Rho/ROCK signaling pathway. Consistent with these results, we found that retina ganglion cell axon growth on glial scar tissue was enhanced in the presence of C3 transferase and Y27632, respectively. In addition, we show that the recently identified inhibitory CSPG Te38 is upregulated in the lesioned spinal cord.

Loss of WAVE-1 causes sensorimotor retardation and reduced learning and memory in mice

The Scar/WAVE family of scaffolding proteins organize molecular networks that relay signals from the GTPase Rac to the actin cytoskeleton. The WAVE-1 isoform is a brain-specific protein expressed in variety of areas including the regions of the hippocampus and the Purkinje cells of the cerebellum. Targeted disruption of the WAVE-1 gene generated mice with reduced anxiety, sensorimotor retardation, and deficits in hippocampal-dependent learning and memory. These sensorimotor and cognitive deficits are analogous to the symptoms of patients with 3p-syndrome mental retardation who are haploinsufficient for WRP/MEGAP, a component of the WAVE-1 signaling network. Thus WAVE-1 is required for normal neural functioning.

Neurons take shape

To construct the intricate network of connections that supports the functions of an adult nervous system, neurons must form highly elaborate processes, extending in the appropriate direction across long distances to form synapses with their partners. As the nervous system takes shape, the process of neuronal morphogenesis is controlled by a broad repertoire of cellular signals. These extracellular cues and cellular interactions are translated by receptors at the cell surface into physical forces that control the dynamic architecture of the neuron as it explores the surrounding terrain. The interpretation of these cues involves a large set of intracellular proteins, whose functional logic we are just beginning to appreciate. We shall consider the basic mechanics of neuronal morphogenesis and some of the emerging pathways that seem to link the outer and inner worlds of the neuron.

Characterization of a brain-specific Rho GTPase-activating protein, p200RhoGAP

The Rho GTPase-activating proteins (RhoGAPs) are a family of multifunctional molecules that transduce diverse intracellular signals by regulating Rho GTPase activities. A novel RhoGAP family member, p200RhoGAP, is cloned in human and mouse. The murine p200RhoGAP shares 86% sequence identity with the human homolog. In addition to a conserved RhoGAP domain at the N terminus, multiple proline-rich motifs are found in the C-terminal region of the molecules. Northern blot analysis revealed a brain-specific expression pattern of p200RhoGAP. The RhoGAP domain of p200RhoGAP stimulated the GTPase activities of Rac1 and RhoA in vitro and in vivo, and the conserved catalytic arginine residue (Arg-58) contributed to the GAP activity. Expression of the RhoGAP domain of p200RhoGAP in Swiss 3T3 fibroblasts inhibited actin stress fiber formation stimulated by lysophosphatidic acid and platelet-derived growth factor-induced membrane ruffling but not Bradykinin-induced filopodia formation. Endogenous p200RhoGAP was localized to cortical actin in naive N1E-115 neuroblastoma cells and to the edges of extended neurites of differentiated N1E-115 cells. Transient expression of the RhoGAP domain and the full-length molecule, but not the catalytic arginine mutants, readily induced a differentiation phenotype in N1E-115 cells. Finally, p200RhoGAP was capable of binding to the Src homology 3 domains of Src, Crk, and phospholipase Cgamma in vitro and became tyrosine-phosphorylated upon association with activated Src in cells. These results suggest that p200RhoGAP is involved in the regulation of neurite outgrowth by exerting its RhoGAP activity and that its cellular activity may be regulated through interaction with Src-like tyrosine kinases.

Quantification of expression of netrins, slits and their receptors in human prostate tumors

Recently, DCC (Deleted in Colorectal Cancer) protein has been forwarded as a receptor for netrin. The Netrin/DCC complex is critical for axon guidance and cell migration. In the developing nervous system, netrin protein secreted by midline cells attracts commissural axons by activating the DCC receptor on growth cones. This attraction can be switched to repulsion or silenced completely, depending on the DCC binding partner. The potential suppressor function of DCC in prostate tumorigenesis, through a still unknown mechanism, prompted us to quantify the expression of several genes involved in this axon guidance pathway. The relative expression levels of DCC, NEO1, NTN1, NTN2L, NTN4, UNC5C, Slit1, Slit2, Slit3, Robo1 and Robo2 were simultaneous quantified in 48 tumors and 7 normal prostate tissues by using real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR). A reduction in DCC, NEO1, NTN1 and NTN4 expression was observed in prostate tumors, while many of the same prostate tumors over-expressed either Slit genes or their receptors, Robo.

The netrin receptor UNC-40/DCC stimulates axon attraction and outgrowth through enabled and, in parallel, Rac and UNC-115/AbLIM

Netrins promote axon outgrowth and turning through DCC/UNC-40 receptors. To characterize Netrin signaling, we generated a gain-of-function UNC-40 molecule, MYR::UNC-40. MYR::UNC-40 causes axon guidance defects, excess axon branching, and excessive axon and cell body outgrowth. These defects are suppressed by loss-of-function mutations in ced-10 (a Rac GTPase), unc-34 (an Enabled homolog), and unc-115 (a putative actin binding protein). ced-10, unc-34, and unc-115 also function in endogenous unc-40 signaling. Our results indicate that Enabled functions in axonal attraction as well as axon repulsion. UNC-40 has two conserved cytoplasmic motifs that mediate distinct downstream pathways: CED-10, UNC-115, and the UNC-40 P2 motif act in one pathway, and UNC-34 and the UNC-40 P1 motif act in the other. Thus, UNC-40 might act as a scaffold to deliver several independent signals to the actin cytoskeleton.

Movement through Slits: cellular migration via the Slit family

First isolated in the fly and now characterised in vertebrates, the Slit proteins have emerged as pivotal components controlling the guidance of axonal growth cones and the directional migration of neuronal precursors. As well as extensive expression during development of the central nervous system (CNS), the Slit proteins exhibit a striking array of expression sites in non-neuronal tissues, including the urogenital system, limb primordia and developing eye. Zebrafish Slit has been shown to mediate mesodermal migration during gastrulation, while Drosophila slit guides the migration of mesodermal cells during myogenesis. This suggests that the actions of these secreted molecules are not simply confined to the sphere of CNS development, but rather act in a more general fashion during development and throughout the lifetime of an organism. This review focuses on the non-neuronal activities of Slit proteins, highlighting a common role for the Slit family in cellular migration.

Signalling and crosstalk of Rho GTPases in mediating axon guidance

Axon extension during development of the nervous system is guided by many factors, but the signalling mechanisms responsible for triggering this extension remain mostly unknown. Here we have examined the role of Rho family small guanosine triphosphatases (GTPases) in mediating axon guidance by diffusible factors. Expression of either dominant-negative or constitutively active Cdc42 in cultured Xenopus laevis spinal neurons, at a concentration that does not substantially affect filopodial formation and neurite extension, abolishes the chemoattractive growth cone turning induced by a gradient of brain-derived neurotrophic factor that can activate Cdc42 and Rac in cultured neurons. Chemorepulsion induced by a gradient of lysophosphatidic acid is also abolished by the expression of dominant-negative RhoA. We also show that an asymmetry in Rho kinase or filopodial initiation across the growth cone is sufficient to trigger the turning response and that there is a crosstalk between the Cdc42 and RhoA pathways through their converging actions on the myosin activity essential for growth cone chemorepulsion.

Molecular mechanisms of axon guidance

Axons are guided along specific pathways by attractive and repulsive cues in the extracellular environment. Genetic and biochemical studies have led to the identification of highly conserved families of guidance molecules, including netrins, Slits, semaphorins, and ephrins. Guidance cues steer axons by regulating cytoskeletal dynamics in the growth cone through signaling pathways that are still only poorly understood. Elaborate regulatory mechanisms ensure that a given cue elicits the right response from the right axons at the right time but is otherwise ignored. With such regulatory mechanisms in place, a relatively small number of guidance factors can be used to generate intricate patterns of neuronal wiring.

Slit proteins, potential endogenous modulators of inflammation

Recent studies suggest that molecules important for guiding neuronal migration and axon path-finding also play a role in modulating leukocyte chemotaxis. Neuronal migration and leukocyte chemotaxis may share some common regulatory mechanisms. Intracellular signal transduction mechanisms guiding neuronal migration and leukocyte chemotaxis are beginning to be elucidated. Studying molecular mechanisms modulating cell migration may provide new insights into understanding of endogenous inhibitors of inflammation.

Neuronal migration and molecular conservation with leukocyte chemotaxis

Cell migration is essential in species ranging from bacteria to humans (for recent reviews, see Lauffenburger and Horwitz 1996; Mitchison and Cramer 1996; Montell 1999). In the amoebae Dictyostelium discoideum, cell migration is involved in chemotaxis toward food sources and in aggregation (for review, see Devreotes and Zigmond 1988; Parent and Devreotes 1999; Chung et al. 2001). In higher vertebrates, cell migration plays crucial roles in multiple physiological and pathological processes. During embryonic and neonatal development, cell migration is crucial in morphogenetic processes such as gastrulation, cardiogenesis, and the formation of the nervous system (for review, see Hatten and Mason 1990; Rakic 1990; Hatten and Heintz 1998; Bentivoglio and Mazzarello 1999). In adult animals, cell migration is required for leukocyte trafficking and inflammatory responses (for review, see McCutcheon 1946; Harris 1954; Devreotes and Zigmond 1988). In tumoriogenesis, tumor-induced angiogenesis and tumor metastasis both involve cell migration. Although it is well known that cell migration is necessary for all these processes, our understanding of mechanisms controlling cell migration is still limited. Here we briefly review the significance of neuronal migration and focus on recent studies on the directional guidance of neuronal migration, discussing the possibility that guidance mechanisms for neurons are conserved with those for other somatic cells.

Grit, a GTPase-activating protein for the Rho family, regulates neurite extension through association with the TrkA receptor and N-Shc and CrkL/Crk adapter molecules

Neurotrophins are key regulators of the fate and shape of neuronal cells and act as guidance cues for growth cones by remodeling the actin cytoskeleton. Actin dynamics is controlled by Rho GTPases. We identified a novel Rho GTPase-activating protein (Grit) for Rho/Rac/Cdc42 small GTPases. Grit was abundant in neuronal cells and directly interacted with TrkA, a high-affinity receptor for nerve growth factor (NGF). Another pool of Grit was recruited to the activated receptor tyrosine kinase through its binding to N-Shc and CrkL/Crk, adapter molecules downstream of activated receptor tyrosine kinases. Overexpression of the TrkA-binding region of Grit inhibited NGF-induced neurite elongation. Further, we found some tendency for neurite promotion in full-length Grit-overexpressing PC12 cells upon NGF stimulation. These results suggest that Grit, a novel TrkA-interacting protein, regulates neurite outgrowth by modulating the Rho family of small GTPases.

Regulation of rho family GTPases is required to prevent axons from crossing the midline

Rho family GTPases are ideal candidates to regulate aspects of cytoskeletal dynamics downstream of axon guidance receptors. To examine the in vivo role of Rho GTPases in midline guidance, dominant negative (dn) and constitutively active (ct) forms of Rho, Drac1, and Dcdc42 are expressed in the Drosophila CNS. When expressed alone, only ctDrac and ctDcdc42 cause axons in the pCC/MP2 pathway to cross the midline inappropriately. Heterozygous loss of Roundabout enhances the ctDrac phenotype and causes errors in embryos expressing dnRho or ctRho. Homozygous loss of Son-of-Sevenless (Sos) also enhances the ctDrac phenotype and causes errors in embryos expressing either dnRho or dnDrac. CtRho suppresses the midline crossing errors caused by loss of Sos. CtDrac and ctDcdc42 phenotypes are suppressed by heterozygous loss of Profilin, but strongly enhanced by coexpression of constitutively active myosin light chain kinase (ctMLCK), which increases myosin II activity. Expression of ctMLCK also causes errors in embryos expressing either dnRho or ctRho. Our data confirm that Rho family GTPases are required for regulation of actin polymerization and/or myosin activity and that this is critical for the response of growth cones to midline repulsive signals. Midline repulsion appears to require down-regulation of Drac1 and Dcdc42 and activation of Rho.

Attractive and repulsive functions of Slit are mediated by different receptors in the Drosophila trachea

Oxygen delivery in many animals is enabled by the formation of unicellular capillary tubes that penetrate target tissues to facilitate gas exchange. We show that the tortuous outgrowth of tracheal unicellular branches towards their target tissues is controlled by complex local interactions with target cells. Slit, a phylogenetically conserved axonal guidance signal, is expressed in several tracheal targets and is required both for attraction and repulsion of tracheal branches. Robo and Robo2 are expressed in different branches, and are both necessary for the correct orientation of branch outgrowth. At the CNS midline, Slit functions as a repellent for tracheal branches and this function is mediated primarily by Robo. Robo2 is necessary for the tracheal response to the attractive Slit signal and its function is antagonized by Robo. We propose that the attractive and repulsive tracheal responses to Slit are mediated by different combinations of Robo and Robo2 receptors on the cell surface.

Signaling mechanisms that regulate actin-based motility processes in the nervous system

Actin-based motility is critical for nervous system development. Both the migration of neurons and the extension of neurites require organized actin polymerization to push the cell membrane forward. Numerous extracellular stimulants of motility and axon guidance cues regulate actin-based motility through the rho GTPases (rho, rac, and cdc42). The rho GTPases reorganize the actin cytoskeleton, leading to stress fiber, filopodium, or lamellipodium formation. The activity of the rho GTPases is regulated by a variety of proteins that either stimulate GTP uptake (activation) or hydrolysis (inactivation). These proteins potentially link extracellular signals to the activation state of rho GTPases. Effectors downstream of the rho GTPases that directly influence actin polymerization have been identified and are involved in neurite development. The Arp2/3 complex nucleates the formation of new actin branches that extend the membrane forward. Ena/VASP proteins can cause the formation of longer actin filaments, characteristic of growth cone actin morphology, by preventing the capping of barbed ends. Actin-depolymerizing factor (ADF)/cofilin depolymerizes and severs actin branches in older parts of the actin meshwork, freeing monomers to be re-incorporated into actively growing filaments. The signaling mechanisms by which extracellular cues that guide axons to their targets lead to direct effects on actin filament dynamics are becoming better understood.

Cloning of rat ARHGAP4/C1, a RhoGAP family member expressed in the nervous system that colocalizes with the Golgi complex and microtubules

The Rho GTPase family of intracellular molecular switches control multiple cellular functions via the regulation of the actin cytoskeleton. Increasing evidence implicates a critical involvement of these molecules in the nervous system, particularly during neuronal migration and polarity, axon and growth cone guidance, dendritic arborization and synaptic formation. However, the molecules regulating Rho GTPase activities in the nervous system are less known. Here, we present the cloning of rat ARHGAP4, a member of the Rho GTPase activating protein family, and also demonstrate its close linkage to the vasopressin 2 receptor gene. In vitro, recombinant ARHGAP4 stimulated the GTPase activity of three members of Rho GTPases, Rac1, Cdc42 and RhoA. ARHGAP4 mRNA expression was observed in multiple tissues with marked expression throughout the developing and adult nervous systems. On closer analysis of protein levels, ARHGAP4 was significantly restricted to specific regions in the nervous system. These included the stratum lucidem in the CA3 area of the hippocampus, neuronal fibers in the ventral region of the brainstem and striatum, and in the cerebellar granule cells. Subcellularly, endogenous ARHGAP4 expression localized to the Golgi complex and could redistribute to the microtubules, for example during mitosis. In addition, distinct protein expression was observed in the tips of differentiating neurites of PC12 cells. Collectively, these results demonstrate that ARHGAP4 is more widely expressed than previously thought but potentially possesses specialized activity in regulating members of the Rho GTPase family in specific cellular compartments of the nervous system.

How do Rho family GTPases direct axon growth and guidance? A proposal relating signaling pathways to growth cone mechanics

For a neuron to play its assigned role in a neural circuit, it has to extend elaborate projections, dendrites and axons, to make precise connections with specific target cells. The past decade has seen the identification of a vast diversity of molecules that assist in the guidance of axons toward their intended targets: guidance cues, growth cone receptors, signaling proteins (Tessier-Lavigne and Goodman, 1996; Song and Poo, 2001). But just how do all of these proteins work together to cause the axon to grow, stop, or turn in a specific direction? In this review, we examine this process from several different perspectives - cytoskeletal dynamics; biochemistry of intracellular signaling proteins; molecular analysis of axon guidance receptors - to try to collapse some of the apparent complexity of axon guidance into a more coherent picture. In particular, we will see how relatively simple and consistent manipulations of the kinetic constants of Rho family GTPases could account for many aspects of the cycle of actin dynamics that underlies axon growth and guidance. This review will intentionally be highly selective in its treatment of this subject in order to synthesize a simplified view that may be of value in directing further thinking and experiments.

Activation of the repulsive receptor Roundabout inhibits N-cadherin-mediated cell adhesion

The formation of axon trajectories requires integration of local adhesive interactions with directional information from attractive and repulsive cues. Here, we show that these two types of information are functionally integrated; activation of the transmembrane receptor Roundabout (Robo) by its ligand, the secreted repulsive guidance cue Slit, inactivates N-cadherin-mediated adhesion. Loss of N-cadherin-mediated adhesion is accompanied by tyrosine phosphorylation of beta-catenin and its loss from the N-cadherin complex, concomitant with the formation of a supramolecular complex containing Robo, Abelson (Abl) kinase and N-cadherin. Local formation of such a receptor complex is an ideal mechanism to steer the growth cone while still allowing adhesion and growth in other directions.

The semaphorin receptor plexin-B1 signals through a direct interaction with the Rho-specific nucleotide exchange factor, LARG

Semaphorins are axon guidance molecules that signal through the plexin family of receptors. Semaphorins also play a role in other processes such as immune regulation and tumorigenesis. However, the molecular signaling mechanisms downstream of plexin receptors have not been elucidated. Semaphorin 4D is the ligand for the plexin-B1 receptor and stimulation of the plexin-B1 receptor activates the small GTPase RhoA. Using the intracellular domain of plexin-B1 as an affinity ligand, two Rho-specific guanine nucleotide exchange factors, leukemia-associated Rho GEF (LARG; GEF, guanine nucleotide exchange factors) and PSD-95/Dlg/ZO-1 homology (PDZ)-RhoGEF, were isolated from mouse brain as plexin-B1-specific interacting proteins. LARG and PDZ-RhoGEF contain several functional domains, including a PDZ domain. Biochemical characterizations showed that the PDZ domain of LARG is directly involved in the interaction with the carboxy-terminal sequence of plexin-B1. Mutation of either the PDZ domain in LARG or the PDZ binding site in plexin-B1 eliminates the interaction. The interaction between plexin-B1 and LARG is specific for the PDZ domain of LARG and LARG does not interact with plexin-A1. A LARG-interaction defective mutant of the plexin-B1 receptor was created and was unable to stimulate RhoA activation. The data in this report suggest that LARG plays a critical role in plexin-B1 signaling to stimulate Rho activation and cytoskeletal reorganization.

Constitutively active myosin light chain kinase alters axon guidance decisions in Drosophila embryos

Conventional myosin II activity provides the motile force for axon outgrowth, but to achieve directional movement during axon pathway formation, myosin activity should be regulated by the attractive and repulsive guidance cues that guide an axon to its target. Here, evidence for this regulation is obtained by using a constitutively active Myosin Light Chain Kinase (ctMLCK) to selectively elevate myosin II activity in Drosophila CNS neurons. Expression of ctMLCK pan-neurally or in primarily pCC/MP2 neurons causes these axons to cross the midline incorrectly. This occurs without altering cell fates and is sensitive to mutations in the regulatory light chains. These results confirm the importance of regulating myosin II activity during axon pathway formation. Mutations in the midline repulsive ligand Slit, or its receptor Roundabout, enhance the number of ctMLCK-induced crossovers, but ctMLCK expression also partially rescues commissure formation in commissureless mutants, where repulsive signals remain high. Overexpression of Frazzled, the receptor for midline attractive Netrins, enhances ctMLCK-dependent crossovers, but crossovers are suppressed when Frazzled activity is reduced by using loss-of-function mutations. These results confirm that proper pathway formation requires careful regulation of MLCK and/or myosin II activity and suggest that regulation occurs in direct response to attractive and repulsive cues.

Human RhoGAP domain-containing proteins: structure, function and evolutionary relationships

Proteins containing a RhoGAP (Rho GTPase activating protein) domain usually function to catalyze the hydrolysis of GTP that is bound to Rho, Rac and/or Cdc42, inactivating these regulators of the actin cytoskeleton. Using database searches, at least 53 distinct RhoGAP domain-containing proteins are likely to be encoded in human DNA. Phylogenetic analysis of only the RhoGAP domains divides these proteins into distinct families that appear to be functionally related. We also review the current understanding of the structure and likely functions of these human proteins. The presence of RhoGAP domains in a number of different human proteins suggests that cytoskeletal changes, regulated by Rho GTPase, may be integrated with many different signaling pathways.

The novel Rho-GTPase activating gene MEGAP/ srGAP3 has a putative role in severe mental retardation

In the last few years, several genes involved in X-specific mental retardation (MR) have been identified by using genetic analysis. Although it is likely that additional genes responsible for idiopathic MR are also localized on the autosomes, cloning and characterization of such genes have been elusive so far. Here, we report the isolation of a previously uncharacterized gene, MEGAP, which is disrupted and functionally inactivated by a translocation breakpoint in a patient who shares some characteristic clinical features, such as hypotonia and severe MR, with the 3p(-) syndrome. By fluorescence in situ hybridization and loss of heterozygosity analysis, we demonstrated that this gene resides on chromosome 3p25 and is deleted in 3p(-) patients that present MR. MEGAP/srGAP3 mRNA is predominantly and highly expressed in fetal and adult brain, specifically in the neurons of the hippocampus and cortex, structures known to play a pivotal role in higher cognitive function, learning, and memory. We describe several MEGAP/srGAP3 transcript isoforms and show that MEGAP/srGAP3a and -b represent functional GTPase-activating proteins (GAP) by an in vitro GAP assay. MEGAP/srGAP3 has recently been shown to be part of the Slit-Robo pathway regulating neuronal migration and axonal branching, highlighting the important role of MEGAP/srGAP3 in mental development. We propose that haploinsufficiency of MEGAP/srGAP3 leads to the abnormal development of neuronal structures that are important for normal cognitive function.

Molecular control of neuronal migration

Our understanding of neuronal migration has been advanced by multidisciplinary approaches. At the cellular level, tangential and radial modes of neuronal migration contribute to different populations of neurons and have differential dependence on glial cells. At the molecular level, extracellular guidance cues have been identified and intracellular signal transduction pathways are beginning to be revealed. Interestingly, mechanisms guiding axon projection and neuronal migration appear to be conserved with those for chemotactic leukocytes.

GAPs in Slit-Robo signaling

Neuronal migration requires the integration of a number of diverse environmental cues and subsequent translation to specific responses such as directed cytoskeletal remodeling. Accurate knowledge of the signal transduction pathways linking activation of surface receptors to actin dynamics is necessary in order to understand the regulation of such processes. Activation of the Roundabout (Robo) receptor mediates a repulsive response in certain pioneering axons and migratory neurons. Recently, Wong et al have described a signaling link between Robo activation and specific GTPase activity that appears to regulate neuronal migration. A novel family of GTPase regulators, responsive to Slit-Robo engagement, has been identified and convincingly shown to alter the migration of neuronal cells. This study not only delineates a specific signaling route from guidance receptors to directed neuronal movement, but also offers clues towards potential regulatory mechanisms that ensure specificity of the Slit-Robo response.

The plexin-B1/Rac interaction inhibits PAK activation and enhances Sema4D ligand binding

The small GTPase Rac has been implicated in growth cone guidance mediated by semaphorins and their receptors. Here we demonstrate that plexin-B1, a receptor for Semaphorin4D (Sema4D), and p21-activated kinase (PAK) can compete for the interaction with active Rac and plexin-B1 can inhibit Rac-induced PAK activation. We have also demonstrated that expression of active Rac enhances the ability of plexin-B1 to interact with Sema4D. Active Rac stimulates the localization of plexin-B1 to the cell surface. The enhancement in Sema4D binding depends on the ability of Rac to bind plexin-B1. These observations support a model where signaling between Rac and plexin-B1 is bidirectional; Rac modulates plexin-B1 activity and plexin-B1 modulates Rac function.

Axon guidance: the cytoplasmic tail

Recent advances in the study of axon guidance have begun to clarify the intricate signalling mechanisms utilised by receptors that mediate path-finding. Many of these axon guidance receptors, including Plexin B, EphA, ephrin B and Robo, regulate the Rho family of GTPases, to effect changes in motility. Recent studies demonstrate a critical role for the cytoplasmic tails of guidance receptors in signalling and also reveal the potential for a great deal of crosstalk between the various receptor-signalling pathways.

Growth cone pathfinding and filopodial dynamics are mediated separately by Cdc42 activation

Although evidence exists that activation of the Rho family GTPase Cdc42 affects axonal development, its specific roles within a growth cone are not well delineated. To evaluate the model that Cdc42 activation regulates growth cone navigation by promoting filopodial activity, we adopted a live analysis strategy that uses transgenic Drosophila lines in which neurons coexpressed constitutively active Cdc42 (Cdc42(V12)) and membrane-targeted green fluorescent protein. We found that growth cones that displayed pathfinding defects exhibited little change in their filopodial activity, whereas others without pathfinding defects exhibited an similar50% increase in their filopodial activity. Moreover, effector loop mutations that were added to the constitutively active Cdc42 (Cdc42(V12C40) and Cdc42(V12A37)) exerted little influence over filopodial activity caused by Cdc42 activation but suppressed the pathfinding defects of the growth cones. Together, these data suggest that Cdc42 controls filopodial activity in axonal growth cones independently of its effects on their pathfinding.