Expression of Tiam-1 in the developing brain suggests a role for the Tiam-1-Rac signaling pathway in cell migration and neurite outgrowth

Rac1 modulates sphingosine 1-phosphate-mediated activation of phosphoinositide 3-kinase/Akt signaling pathways in vascular endothelial cells

Sphingosine 1-phosphate (S1P) is a platelet-derived sphingolipid that activates G protein-coupled S1P receptors and initiates a broad range of responses in vascular endothelial cells. The small GTPase Rac1 is implicated in diverse S1P-modulated cellular responses in endothelial cells, yet the molecular mechanisms involved in S1P-mediated Rac1 activation are incompletely understood. We studied the pathways involved in S1P-mediated Rac1 activation in bovine aortic endothelial cells (BAEC) and found that S1P-induced Rac1 activation is impaired following chelation of G protein betagamma subunits by transfection of betaARKct. Treatment with the Src tyrosine kinase inhibitor PP2 completely attenuated S1P-mediated Rac1 activation; however, pretreatment of BAEC with wortmannin, an inhibitor of phosphoinositide (PI) 3-kinase, had no effect on Rac1 activation while completely blocking S1P-induced Akt phosphorylation. We used Rac1-specific small interfering RNA (siRNA) duplexes to "knock down" endogenous Rac1 expression and found that siRNA-mediated Rac1 knockdown significantly impaired basal as well as S1P-induced phosphorylation of protein kinase Akt, as well as several downstream targets of Akt including endothelial nitric-oxide synthase and glycogen synthase kinase 3beta. By contrast, S1P-induced phosphorylation of the mitogen-activated protein kinases ERK1/2 was unperturbed by siRNA-mediated Rac1 knockdown. We found that overexpression of the Rac1 guanine nucleotide exchange factor (GEF) Tiam1 markedly enhanced Rac1 activity, whereas a dominant negative Tiam1 mutant significantly attenuated S1P-mediated Rac1 activation. Taken together, these studies identify G protein betagamma subunits, Src kinase and the GEF Tiam1 as upstream modulators of S1P-mediated Rac1 activation, and establish a central role for Rac1 in S1P-mediated activation of PI 3-kinase/Akt/endothelial nitric-oxide synthase signaling in vascular endothelial cells.

The guanine nucleotide exchange factor Tiam1 increases colon carcinoma growth at metastatic sites in an orthotopic nude mouse model

Alterations in migration and adhesion are critical to invasion and metastasis. To examine signaling pathways important for colon tumor metastasis, cells of increased migratory potential from the low migratory SW480 human colorectal carcinoma parental cell line were biologically selected by serial migration through modified Boyden chambers. Several sublines were obtained with statistically significantly increased migration relative to the parental cell line. One highly migratory population was single-cell cloned and characterized. The migratory clones exhibit a four- to five-fold increase in protein and mRNA expression of T-lymphoma invasion and metastasis gene 1 (Tiam1), a guanine nucleotide exchange factor. To determine directly the role of Tiam1 in the migration of these migratory sublines, the parental SW480 cell line was transfected with a plasmid encoding the Tiam1 protein, and single cell clones were established. Ectopic expression of Tiam1 in these clones led to morphologic changes identical to biologically selected clones and increased migration. Finally, the implantation of clones that overexpress Tiam1 into the cecum of athymic mice resulted in tumor growth in the spleen, liver, and lung, whereas parental cells do not form tumors by this route of injection. These results demonstrate that overexpression of Tiam1 contributes to the metastatic phenotype of colon cancer cells.

The Rac1-GEF Tiam1 couples the NMDA receptor to the activity-dependent development of dendritic arbors and spines

NMDA-type glutamate receptors play a critical role in the activity-dependent development and structural remodeling of dendritic arbors and spines. However, the molecular mechanisms that link NMDA receptor activation to changes in dendritic morphology remain unclear. We report that the Rac1-GEF Tiam1 is present in dendrites and spines and is required for their development. Tiam1 interacts with the NMDA receptor and is phosphorylated in a calcium-dependent manner in response to NMDA receptor stimulation. Blockade of Tiam1 function with RNAi and dominant interfering mutants of Tiam1 suggests that Tiam1 mediates effects of the NMDA receptor on dendritic development by inducing Rac1-dependent actin remodeling and protein synthesis. Taken together, these findings define a molecular mechanism by which NMDA receptor signaling controls the growth and morphology of dendritic arbors and spines.

A pivotal role of calcineurin signaling in development and maturation of postnatal cerebellar granule cells

Primary culture of postnatal cerebellar granule cells provides a model system that recapitulates many molecular events of developing granule cells in vivo. Depolarization of cultured granule cells increases intracellular Ca(2+) and activates Ca(2+)/calmodulin-dependent calcineurin (CaN) phosphatase. This Ca(2+) signaling mimics some of the signaling events for proliferation, migration, and differentiation of granule cells in vivo. We investigated the genome-wide expression profiles of depolarization- and CaN-regulated genes in cultured mouse granule cells and addressed their relevance to gene regulation in developing granule cells in vivo. Granule cells were cultured under a nondepolarization condition (5 mM KCl) and a depolarization condition (25 mM KCl) with and without the CaN inhibitor FK506. Gene expression profiles between depolarization and nondepolarization and between FK506 treatment and untreatment were analyzed by microarray techniques. Both depolarization and FK506 treatment influence expression levels of a large number of genes, most of which are overlapping, however, are conversely regulated by these two treatments. Importantly, many of the FK506-responsive genes are up- or down-regulated in parallel with gene expression in postnatal granule cells in vivo. The FK506-down-regulated genes are highly expressed in proliferating/premigratory granule cells and many of these genes encode cellular components involved in cell proliferation, migration, and differentiation. In contrast, the FK506-up-regulated genes are predominantly expressed in postmigratory granule cells, including many functional molecules implicated in synaptic transmission and modulation. This investigation demonstrates that the CaN signaling plays a pivotal role in development and synaptic organization of granule cells during the postnatal period.

Expression of Trio, a member of the Dbl family of Rho GEFs in the developing rat brain

Trio, a member of the Dbl family of guanine nucleotide exchange factors (GEFs), has a series of spectrin repeats, two GEF domains, protein interaction domains, and a putative kinase domain, potentially important in neuronal axon guidance and cell migration. Most knowledge about Trio is based on studies of Caenorhabditis elegans and Drosophila, while the function of Trio in vertebrates is unclear. The aim of these experiments was to establish the patterns of Trio expression in the postnatal rat brain. During postnatal (P) development, high levels of Trio mRNA are found in the cerebral cortex, hippocampus, thalamus, caudate/putamen, and olfactory bulb, with lower levels in the septal nucleus, nucleus accumbens, amygdala, and hypothalamus. Except for the cerebellum, Trio mRNA in major brain areas is highest at P1, decreasing gradually during development, with low but detectable levels at P30. In P14 cerebral cortex, pyramidal neurons with strongly staining soma and dendrites are observed primarily in layer 5. In hippocampus, strong staining is observed in pyramidal neurons, granule cells, and isolated interneurons. Cerebellar Purkinje neurons exhibit intense staining in the soma and in extensive dendritic arbors at P7 and P14. Levels of Trio mRNA and the intensity of Trio immunostaining in cerebellar Purkinje cells increase from P1 to P30. Consistent with the in situ hybridization pattern, Western blot analyses show that Trio levels in the hippocampus and cortex are high at P1, decreasing until P30. The data suggest that Trio plays an important role during neuronal development.

The role of the Rho GTPases in neuronal development

Our brain serves as a center for cognitive function and neurons within the brain relay and store information about our surroundings and experiences. Modulation of this complex neuronal circuitry allows us to process that information and respond appropriately. Proper development of neurons is therefore vital to the mental health of an individual, and perturbations in their signaling or morphology are likely to result in cognitive impairment. The development of a neuron requires a series of steps that begins with migration from its birth place and initiation of process outgrowth, and ultimately leads to differentiation and the formation of connections that allow it to communicate with appropriate targets. Over the past several years, it has become clear that the Rho family of GTPases and related molecules play an important role in various aspects of neuronal development, including neurite outgrowth and differentiation, axon pathfinding, and dendritic spine formation and maintenance. Given the importance of these molecules in these processes, it is therefore not surprising that mutations in genes encoding a number of regulators and effectors of the Rho GTPases have been associated with human neurological diseases. This review will focus on the role of the Rho GTPases and their associated signaling molecules throughout neuronal development and discuss how perturbations in Rho GTPase signaling may lead to cognitive disorders.

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

The Rho family of small GTPases controls a wide range of cellular processes in eukaryotic cells, such as normal cell growth, proliferation, differentiation, gene regulation, actin cytoskeletal organization, cell fate determination, and neurite outgrowth. The activation of Rho-GTPases requires the exchange of GDP for GTP, a process catalyzed by the Dbl family of guanine nucleotide exchange factors. We demonstrate that a newly identified guanine nucleotide exchange factor, GEFT, is widely expressed in the brain and highly concentrated in the hippocampus, and the Purkinje and granular cells of the cerebellum. Exogenous expression of GEFT promotes dendrite outgrowth in hippocampal neurons, resulting in spines with larger size as compared with control spines. In neuroblastoma cells, GEFT promotes the active GTP-bound state of Rac1, Cdc42, and RhoA and increases neurite outgrowth primarily via Rac1. Furthermore, we demonstrated that PAK1 and PAK5, both downstream effectors of Rac1/Cdc42, are necessary for GEFT-induced neurite outgrowth. AP-1 and NF-kappaB, two transcriptional factors involved in neurite outgrowth and survival, were up-regulated in GEFT-expressing cells. Together, our data suggest that GEFT enhances dendritic spine formation and neurite outgrowth in primary neurons and neuroblastoma cells, respectively, through the activation of Rac/Cdc42-PAK signaling pathways.

The role of the guanine nucleotide exchange factor Tiam1 in cellular migration, invasion, adhesion and tumor progression

While advances in molecular genetics have provided new insights into molecular alterations that lead to the development of many tumors, including breast carcinoma, the genetic and epigenetic alterations that result in metastatic spread of the disease, from which afflicted patients ultimately succumb, are much more poorly understood. Important biologic processes in the development of metastasis include increased migration and invasion of tumor cells. While the regulation of these processes is complex, they are controlled in part by small G proteins of the Rho family, including Rho, Rac, and Cdc42, that are involved in cytoskeletal organization. These proteins, active when bound to GTP, are, in turn, regulated by guanine nucleotide exchange factors (GNEFs) and guanine nucleotide activating proteins. The GNEF Tiam1 catalyzes nucleotide exchange for Rac in vivo, and Rac, Cdc42 and Rho in vitro. Tiam1 was identified first in 1994 by in vitro selection for invasiveness in T-lymphoma cells. Accordingly, Tiam1 has been shown to increase invasion in T-lymphoma cells, as well as to increase cellular migration in fibroblasts, and to promote motility in some neuronal cells. In contrast, Tiam1 has been demonstrated to increase cellular adhesion in some epithelial cell populations. Thus, Tiam1 has multiple roles in regulating cellular functions, likely dependent on the cell type, the substratum, transformation status of the cells, and the activation state of small G proteins in a given cell. Increasing evidence has focused on Tiam1's regulation, as well as Tiam1's role in cancer progression and metastasis. Recent results from other laboratories and ours have demonstrated that increased Tiam1 expression correlates with grade of breast cancer in humans and metastatic potential of human breast carcinoma cell lines in nude mice. This review will discuss Tiam1's cellular functions and methods of regulation, and will highlight Tiam1's contribution to cancer progression and metastasis.

Membrane/cytoskeleton communication

Accumulations of particular lipids in ordered arrays in the membrane (termed microdomains or lipid rafts) can attract proteins with specific targeting domains. Both the lipid and protein components of rafts communicate with the cytoskeleton directly thereby regulating cellular responses. Recent evidence implicating phosphoinositide 1,5 bisphosphate (PIP2) in cytoskeletal regulation shows that agonist sensitive regulation of PIP2 homoeostasis occurs specifically rafts, which appear to provide a major structural substrate for its function. The crucial role of PIP2 in generating cytoskeletal responses is chiefly achieved by regulating proteins that control actin dynamics directly. Many of these regulatory proteins are also specifically enriched in rafts either directly (by insertion into the lipid bilayer via acetylation motifs), or indirectly via interactions with other raft components. The notion that rafts form membrane platforms or modules that mediate signaling responses has been most extensively demonstrated in the immune synapse (IS) of T cells, a complex assemblage of rafts that integrates signaling cascades originating from the simultaneous activation of a wide variety of receptors. The IS is essential for both the amplification and maintenance of T-cell activation, and its assembly at the antigen presenting site depends on the interactions between rafts and the actin cytoskeleton that regulates coalescence of smaller raft components into the larger IS complex. Likewise the neuron, which represents the most highly polarized cell in the body, utilizes the regulation of actin dynamics in response to a plethora of extracellular signals to control axon pathfinding thereby sculpting nervous system cytoarchitecture with utmost precision. It is now becoming clear, that as in the T-cell, lipid rafts in the growing axon can assemble into highly specific, yet malleable and dynamic, signaling modules that regulate actin dynamics in a fashion that is also PIP2-dependent and that utilizes both familiar and novel regulatory mechanisms. It seems clear that raft mediated cytoskeletal regulation represents a highly conserved mechanism to integrate cellular responses to diverse signals.

Roles of STEF/Tiam1, guanine nucleotide exchange factors for Rac1, in regulation of growth cone morphology

Rho family GTPases are suggested to be pivotal for growth cone behavior, but regulation of their activities in response to environmental cues remains elusive. Here, we describe roles of STEF and Tiam1, guanine nucleotide exchange factors for Rac1, in neurite growth and growth cone remodeling. We reveal that, in primary hippocampal neurons, STEF/Tiam1 are localized within growth cones and essential for formation of growth cone lamellipodia, eventually contributing to neurite growth. Furthermore, experiments using a dominant-negative form demonstrate that STEF/Tiam1 mediate extracellular laminin signals to activate Rac1, promoting neurite growth in N1E-115 neuroblastoma cells. STEF/Tiam1 are revealed to mediate Cdc42 signal to activate Rac1 during lamellipodial formation. We also show that RhoA inhibits the STEF/Tiam1-Rac1 pathway. These data are used to propose a model that extracellular and intracellular information is integrated by STEF/Tiam1 to modulate the balance of Rho GTPase activities in the growth cone and, consequently, to control growth cone behavior.

The in vivo roles of STEF/Tiam1, Rac1 and JNK in cortical neuronal migration

The coordinated migration of neurons is a pivotal step for functional architectural formation of the mammalian brain. To elucidate its molecular mechanism, gene transfer by means of in utero electroporation was applied in the developing murine brain, revealing the crucial roles of Rac1, its activators, STEF/Tiam1, and its downstream molecule, c-Jun N-terminal kinase (JNK), in the cerebral cortex. Functional repression of these molecules resulted in inhibition of radial migration of neurons without affecting their proper differentiation. Interestingly, distinct morphological phenotypes were observed; suppression of Rac1 activity caused loss of the leading process, whereas repression of JNK activity did not, suggesting the complexity of the signaling cascade. In cultured neurons from the intermediate zone, activated JNK was detected along microtubules in the processes. Application of a JNK inhibitor caused irregular morphology and increased stable microtubules in processes, and decreased phosphorylation of microtubule associated protein 1B, raising a possibility of the involvement of JNK in controlling tubulin dynamics in migrating neurons. Our data thus provide important clues for understanding the intracellullar signaling machinery for cortical neuronal migration.

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.

The role of Rho GTPases and associated kinases in regulating neurite outgrowth

Neurones are highly specialised cells that can extend over great distances, enabling the complex networking of the nervous system. We are beginning to understand in detail the molecular mechanisms that control the shape of neurones during development. One family of proteins that are clearly essential are the Rho GTPases which have a pivotal role in regulating the actin cytoskeleton in all cell types. The Rho GTPases are responsible for the activation and downregulation of many downstream kinases. This review discusses individual kinases that are regulated by three members of the Rho GTPases, Rac, Rho and Cdc42 and their function during neurite outgrowth and remodelling.

Mice deficient in the Rac activator Tiam1 are resistant to Ras-induced skin tumours

Proteins of the Rho family control signalling pathways that regulate the actin cytoskeleton and gene transcription. In vitro studies have implicated Rho-like GTP-hydrolysing enzymes (GTPases) in cell migration, cell-cycle progression, and Ras-induced focus formation, suggesting a role for these GTPases in the formation and progression of tumours in vivo. To study this, we have generated mice lacking the Rac-specific activator Tiam1, a T-lymphoma invasion and metastasis inducing protein. Here we show that such Tiam1(-/-) mice are resistant to the development of Ras-induced skin tumours initiated with 7,12-dimethylbenzanthracene and promoted with 12-O-tetradecanoylphorbol-13-acetate. Moreover, the few tumours produced in Tiam1(-/-) mice grew much slower than did tumours in wild-type mice. Tiam1-deficient primary embryonic fibroblasts were also resistant to Ras(V12)-induced focus formation. Analysis of Tiam1 heterozygotes indicated that both tumour initiation and promotion were dependent on the Tiam1 gene dose. Tiam1 deficiency was associated with increased apoptosis during initiation, and with impeded proliferation during promotion. Although the number of tumours in Tiam1(-/-) mice was small, a greater proportion progressed to malignancy, suggesting that Tiam1 deficiency promotes malignant conversion. Our studies identify the Rac activator Tiam1 as a critical regulator of different aspects of Ras-induced tumour formation.

Expression of stef, an activator of Rac1, correlates with the stages of neuronal morphological development in the mouse brain

STEF (Sif- and Tiam1-like exchange factor), a guanine nucleotide exchange factor, was identified as a candidate molecule in regulation of neural development. The STEF gene product specifically activates Rac1, a member of the Rho-like small G proteins. Here we report the detailed examination of the expression profile of the stef gene in the mouse brain. In situ hybridization revealed that the stef gene was expressed in a stage- and region-specific manner in the mouse brain; it was expressed during certain developmental stages in the cerebral cortex, the olfactory bulb, the rostral migratory pathway (RMP) and the hippocampus. In the cerebral cortex, stef transcripts were detected in migrating cells in the intermediate zone as well as neurons in the cortical plate. While the expression in the cerebral cortex was reduced at adult stages, considerable expression was found to be maintained in other regions (RMP, olfactory bulb, hippocampal formation), which are the tissues where neurons continue to undergo morphological remodeling including cellular migration, neurite extension and synapse formation even in adults. Thus, stef gene expression appears to correspond to neuronal morphological changes.

Characterization of STEF, a guanine nucleotide exchange factor for Rac1, required for neurite growth

Accumulating evidence suggests that Rho family GTPases play critical roles in the organization of the nervous system. We previously identified a guanine nucleotide exchange factor of Rac1, STEF (SIF and Tiam 1-like exchange factor), which can induce ruffling membrane in KB cells and is predominantly expressed in the brain during development. Here, we characterize the molecular nature of STEF and its involvement in neurite growth. Deletion analyses revealed distinct roles for individual domains: PHnTSS for membrane association, DH for enzymatic activity, and PHc for promoting catalytic activity. Ectopic expression of STEF in N1E-115 neuroblastoma cells induced neurite-like processes containing F-actin, betaIII tubulin, MAP2, and GAP43 in a Rac1-dependent manner even under the serum-containing neurite-inhibiting conditions. We further found that a PHnTSS STEF fragment specifically inhibited the function of both STEF and Tiam1, a closely related Rac1 guanine nucleotide exchange factor. Suppression of endogenous STEF and Tiam1 activities in N1E-115 cells by ectopically expressed PHnTSS STEF resulted in inhibition of neurite outgrowth in serum-starved conditions, which usually induce neurite formation. Furthermore, these inhibitory effects were rescued by exogenously expressed STEF or Tiam1, suggesting that STEF and Tiam1 are involved in neurite formation through the activation of Rac1 and successive cytoskeletal reorganization of neuronal cells during development.

Novel mechanism for gonadotropin-releasing hormone neuronal migration involving Gas6/Ark signaling to p38 mitogen-activated protein kinase

Gonadotropin-releasing hormone (GnRH) is the central regulator of the reproductive axis. Normal sexual maturation depends on the migration of GnRH neurons from the olfactory placode to the hypothalamus during development. Previously, we showed restricted expression of the membrane receptor adhesion-related kinase (Ark) in immortalized cell lines derived from migratory but not postmigratory GnRH neurons. In addition, Ark and GnRH transcripts were detected along the GnRH neuron migratory route in the E13 mouse cribriform plate. In the present study, we examined the role of Ark and its ligand, Gas6 (encoded by growth arrest-specific gene 6), in GnRH neuron migration. Gas6 stimulated lamellipodial extension, membrane ruffling, and chemotaxis of immortalized NLT GnRH neuronal cells via the Ark receptor. Gas6/Ark signaling promoted activation of the Rho family GTPase Rac, and adenoviral-mediated expression of dominant negative N17Rac abolished Gas6/Ark-induced actin cytoskeletal reorganization and migration of GnRH neuronal cells. In addition, p38 MAPK was activated downstream of Ark and Rac, and inhibition of p38 MAPK with either SB203580 or adenoviral dominant negative p38alpha also blocked Gas6/Ark-mediated migration. Finally, downstream of Rac and p38 mitogen-activated protein kinase (MAPK), Gas6/Ark signaling promoted activation of MAPK-activated protein kinase 2 and induced phosphorylation of HSP25, a known regulator of cortical actin remodeling. The data are the first to demonstrate a migratory signaling pathway downstream of Ark/Axl family receptors and suggest a previously unidentified role for p38 MAPK in neuronal migration. Furthermore, these studies support a potential role for Ark in the regulation of GnRH neuronal migration.

alpha2-chimaerin, a Cdc42/Rac1 regulator, is selectively expressed in the rat embryonic nervous system and is involved in neuritogenesis in N1E-115 neuroblastoma cells

Neuronal differentiation involves Rac and Cdc42 GTPases. alpha-Chimaerin, a Rac/Cdc42 regulator, occurs as alpha1- and alternatively spliced Src homology 2 (SH2) domain-containing alpha2-isoforms. alpha2-chimaerin mRNA was highly expressed in the rat embryonic nervous system, especially in early postmitotic neurons. alpha1-chimaerin mRNA was undetectable before embryonic day 16.5. Adult alpha2-chimaerin mRNA was restricted to neurons within specific brain regions, with highest expression in the entorhinal cortex. alpha2-chimaerin protein localized to neuronal perikarya, dendrites, and axons. The overall pattern of alpha2-chimaerin mRNA expression resembles that of cyclin-dependent kinase regulator p35 (CDK5/p35) which participates in neuronal differentiation and with which chimaerin interacts. To determine whether alpha2-chimaerin may have a role in neuronal differentiation and the relevance of the SH2 domain, the morphological effects of both chimaerin isoforms were investigated in N1E-115 neuroblastoma cells. When plated on poly-lysine, transient alpha2-chimaerin but not alpha1-chimaerin transfectants formed neurites. Permanent alpha2-chimaerin transfectants generated neurites whether or not they were stimulated by serum starvation, and many cells were enlarged. Permanent alpha1-chimaerin transfectants displayed numerous microspikes and contained F-actin clusters, a Cdc42-phenotype, but generated few neurites. In neuroblastoma cells, alpha2-chimaerin was predominantly soluble with some being membrane-associated, whereas alpha1-chimaerin was absent from the cytosol, being membrane- and cytoskeleton-associated, paralleling their subcellular distribution in brain. Transient transfection with alpha2-chimaerin mutated in the SH2 domain (N94H) generated an alpha1-chimaerin-like phenotype, protein partitioned in the particulate fraction, and in NGF-stimulated pheochromocytoma cell line 12 (PC12) cells, neurite formation was inhibited. These results indicate a role for alpha2-chimaerin in morphological differentiation for which its SH2 domain is vital.

Distribution of transcripts for the brain-specific GDP/GTP exchange factor collybistin in the developing mouse brain

The dbl-like GDP/GTP exchange factor, collybistin, binds to the receptor anchoring protein gephyrin and activates the Rho-like GTPase Cdc42. Collybistin was found in two splice variants I and II, both of which share a tandem Dbl homology/pleckstrin homology (DH/PH) domain. In heterologous expression systems, collybistin II induces the formation of submembraneous gephyrin aggregates and therefore has been implicated in inhibitory synapse formation. Expression of collybistin is restricted to neuronal tissues and is predominantly found in brain. Here, we investigated the spatio-temporal distribution of collybistin transcripts in the embryonic mouse brain and compared it to gephyrin and glycine receptor mRNA patterns. Our data show that collybistin expression is upregulated when neurons become postmitotic and start to differentiate.

Expression of kalirin, a neuronal GDP/GTP exchange factor of the trio family, in the central nervous system of the adult rat

Kalirin is a multifunctional protein identified by its interaction with peptidylglycine alpha-amidating monooxygenase, an enzyme essential for neuropeptide biosynthesis. Several forms of Kalirin exist, all containing spectrin-like repeats, a Dbl homology (DH) domain, and an adjacent pleckstrin homology (PH) domain; several different COOH-termini provide additional DH/PH domains and a putative protein kinase. Kalirin binds Rac1 and affects cytoskeletal organization, neuropeptide secretion, and iNOS activity. By in situ hybridization, the highest levels of Kalirin mRNA were found in the cerebral cortex, hippocampal formation, and Purkinje cells, with high levels also in thalamus, caudate putamen, septal nucleus, nucleus accumbens, amygdala, and anterior olfactory nucleus. Low levels of Kalirin mRNA were detected in the paraventricular, supraoptic, and reticular thalamic nuclei and in the ventromedial hypothalamic nucleus. Brain areas with high levels of Kalirin mRNA showed strong Kalirin-like immunoreactivity. Pyramidal neurons with strongly staining soma and long dendrites were observed primarily in layer 5 of the cerebral cortex. In the hippocampus, a uniform distribution of neurons with fine dendritic staining was observed in the pyramidal cell layer, in the granule cell layer, and in the hilar cells of the dentate gyrus as well as in isolated interneurons. Cerebellar Purkinje neurons exhibited intense staining in the soma and in extensive dendritic arbors extending to the surface of the molecular layer. During embryonic development, Trio, the Drosophila orthologue of Kalirin, plays an essential role in axon guidance; localization of Kalirin to the somatodendritic region of adult neurons provides the basis for future studies of regulation and function.

Synaptic development is controlled in the periactive zones of Drosophila synapses

A cell-adhesion molecule fasciclin 2 (FAS2), which is required for synaptic growth and still life (SIF), an activator of RAC, were found to localize in the surrounding region of the active zone, defining the periactive zone in Drosophila neuromuscular synapses. BetaPS integrin and discs large (DLG), both involved in synaptic development, also decorated the zone. However, shibire (SHI), the Drosophila dynamin that regulates endocytosis, was found in the distinct region. Mutant analyses showed that sif genetically interacted with Fas2 in synaptic growth and that the proper localization of SIF required FAS2, suggesting that they are components in related signaling pathways that locally function in the periactive zones. We propose that neurotransmission and synaptic growth are primarily regulated in segregated subcellular spaces, active zones and periactive zones, respectively.

Molecular cloning of neuronally expressed mouse betaPix isoforms

Pix, a Pak-interacting exchange factor, is known to be involved in the regulation of Cdc42/Rac GTPases and Pak kinase activity. In this study, we cloned the cDNAs encoding two betaPix isoforms from mouse brain cDNA library. Both of the cloned genes, designated betaPix-b and betaPix-c (GenBank Accession Nos. AF247654 and AF247655, respectively), have a novel insert region consisting of 59 amino acid residues. In betaPix-c, 75 amino acid residues are deleted in the proline-rich region at the carboxyl-terminus of betaPix. In situ hybridization studies with insert region-specific probe in rat embryo show that insert region-containing isoforms are expressed mainly in the central nervous system. Moreover, temporal expression pattern of isoforms is correlated with the active neurogenesis period in the cerebral cortex and cerebellum. These results strongly suggest that betaPix isoforms may play important roles in the cellular events required for brain development such as neuronal migration.

An isoform of kalirin, a brain-specific GDP/GTP exchange factor, is enriched in the postsynaptic density fraction

Communication between membranes and the actin cytoskeleton is an important aspect of neuronal function. Regulators of actin cytoskeletal dynamics include the Rho-like small GTP-binding proteins and their exchange factors. Kalirin is a brain-specific protein, first identified through its interaction with peptidylglycine-alpha-amidating monooxygenase. In this study, we cloned rat Kalirin-7, a 7-kilobase mRNA form of Kalirin. Kalirin-7 contains nine spectrin-like repeats, a Dbl homology domain, and a pleckstrin homology domain. We found that the majority of Kalirin-7 protein is associated with synaptosomal membranes, but a fraction is cytosolic. We also detected higher molecular weight Kalirin proteins. In rat cerebral cortex, Kalirin-7 is highly enriched in the postsynaptic density fraction. In primary cultures of neurons, Kalirin-7 is detected in spine-like structures, while other forms of Kalirin are visualized in the cell soma and throughout the neurites. Kalirin-7 and its Dbl homology-pleckstrin homology domain induce formation of lamellipodia and membrane ruffling, when transiently expressed in fibroblasts, indicative of Rac1 activation. Using Rac1, the Dbl homology-pleckstrin homology domain catalyzed the in vitro exchange of bound GDP with GTP. Kalirin-7 is the first guanine-nucleotide exchange factor identified in the postsynaptic density, where it is positioned optimally to regulate signal transduction pathways connecting membrane proteins and the actin cytoskeleton.

Enhancement of the migration of metastatic human breast cancer cells by phosphatidic acid

Phosphatidic acid (PA), lysophosphatidic acid (LPA), and sphingosine 1-phosphate (SPP) are naturally occurring phospholipids which induce a variety of effects as extracellular messengers. In this study, we compared the effects of these phospholipid signaling molecules on the migration of invasive and noninvasive breast cancer cell lines, an index of the metastatic potential of these cells. As previously demonstrated, invasive MDA-MB-231 breast cancer cells exhibited increased constitutive (nonstimulated) migration in comparison to poorly invasive MCF-7 cells. Phosphatidic acid employed at nanomolar concentrations markedly potentiated migration of the invasive cells but had no effect on migration of either the noninvasive MCF-7 cells or nonneoplastic human epithelial cells. Lysophosphatidic acid and sphingosine 1-phosphate inhibited both the directed (chemotactic) and random (chemokinetic) migration of MDA-MB-231 cells. Experiments were undertaken to characterize the signaling pathway involved in constitutive and PA-stimulated migration of MDA-MB-231 cells. The tyrosine kinase inhibitors staurosporine and genistein inhibited constitutive and PA-induced migration in a dose-dependent manner, consistent with a role for tyrosine phosphorylation in the migratory response. In addition, the phosphatidylinositol (PI) 3' kinase inhibitors wortmannin and LY294002 strongly inhibited both the constitutive and PA-stimulated migration of the invasive breast cancer cells, indicating that PI-3' kinase plays an important role in the metastatic migration of breast cancer cells. Finally, PA-induced migration of MDA-MB-231 was markedly attenuated by pretreatment of cells with Clostridium difficile Toxin B, pertussis toxin and suramin, implying a role for a Gi receptor-dependent process involving activation of the small GTP-binding protein Rho. Since an enhanced ability to migrate heightens the metastatic potential of cells within solid tumors, our results suggest that the metastatic capabilities of breast cancer cells may be enhanced by a receptor-driven cellular process initiated by phosphatidic acid or related lipid phosphate messengers.

Cloning and characterization of T-cell lymphoma invasion and metastasis 2 (TIAM2), a novel guanine nucleotide exchange factor related to TIAM1

TIAM1 is a guanine nucleotide exchange factor that was identified in a screen for genes that increase the invasiveness of T lymphoma cell lines (Habets et al., 1994, Cell 77(4): 537-549). We have identified a gene, T-cell lymphoma invasion and metastasis 2 (HGMW-approved symbol TIAM2), with significant identity to the carboxyl-terminal region of the TIAM1 and mapped it to 6q25. TIAM2 is expressed as an approximately 3.3-kb transcript in cerebrum and as an approximately 4.4-kb transcript in the cerebellum and testis. The approximately 4. 4-kb message encodes a longer form of the approximately 3.3-kb mRNA predicted protein, and both contain homology to the Dbl-homologous region (70%) and Pleckstrin-homologous (54%) regions of TIAM1. We have purified TIAM2 and shown it to have GDP-GTP exchange activity. In situ hybridizations demonstrate TIAM2 expression in the E13.5 telencephalon of mouse embryos and in the cerebral cortex, hippocampus, and ependyma of adult mouse brains.

Identification of the stef gene that encodes a novel guanine nucleotide exchange factor specific for Rac1

The Rho family GTPases are involved in a variety of cellular events by changing the organization of actin cytoskeletal networks in response to extracellular signals. However, it is not clearly known how their activities are spatially and temporally regulated. Here we report the identification of a novel guanine nucleotide exchange factor for Rac1, STEF, which is related in overall amino acid sequence and modular structure to mouse Tiam1 and Drosophila SIF proteins. STEF protein contains two pleckstrin homology domains, a PDZ domain and a Dbl homology domain. The in vitro assay showed that STEF protein specifically enhanced the dissociation of GDP from Rac1 but not that from either RhoA or Cdc42. Expression of a truncated STEF protein in culture cells induced membrane ruffling with altered actin localization, which implies that this protein also activates Rac1 in vivo. The stef transcript was observed in restricted parts of mice, including cartilaginous tissues and the cortical plate of the central nervous system during embryogenesis. These findings suggested that STEF protein participates in the control of cellular events in several developing tissues, possibly changing the actin cytoskeletal network by activating Rac1.

Myelin and collapsin-1 induce motor neuron growth cone collapse through different pathways: inhibition of collapse by opposing mutants of rac1

Precise growth cone guidance is the consequence of a continuous reorganization of actin filament structures within filopodia and lamellipodia in response to inhibitory and promoting cues. The small GTPases rac1, cdc42, and rhoA are critical for regulating distinct actin structures in non-neuronal cells and presumably in growth cones. Collapse, a retraction of filopodia and lamellipodia, is a typical growth cone behavior on contact with inhibitory cues and is associated with depolymerization and redistribution of actin filaments. We examined whether small GTPases mediate the inhibitory properties of CNS myelin or collapsin-1, a soluble semaphorin, in chick embryonic motor neuron cultures. As demonstrated for collapsin-1, CNS myelin-evoked growth cone collapse was accompanied by a reduction of rhodamine-phalloidin staining most prominent in the growth cone periphery, suggesting actin filament disassembly. Specific mutants of small GTPases were capable of desensitizing growth cones to CNS myelin or collapsin-1. Adenoviral-mediated expression of constitutively active rac1 or rhoA abolished CNS myelin-induced collapse and allowed remarkable neurite extension on a CNS myelin substrate. In contrast, expression of dominant negative rac1 or cdc42 negated collapsin-1-induced growth cone collapse and promoted neurite outgrowth on a collapsin-1 substrate. These findings suggest that small GTPases can modulate the signaling pathways of inhibitory stimuli and, consequently, allow the manipulation of growth cone behavior. However, the fact that opposite mutants of rac1 were effective against different inhibitory stimuli speaks against a universal signaling pathway underlying growth cone collapse.

Cdc42: An essential Rho-type GTPase controlling eukaryotic cell polarity

Cdc42p is an essential GTPase that belongs to the Rho/Rac subfamily of Ras-like GTPases. These proteins act as molecular switches by responding to exogenous and/or endogenous signals and relaying those signals to activate downstream components of a biological pathway. The 11 current members of the Cdc42p family display between 75 and 100% amino acid identity and are functional as well as structural homologs. Cdc42p transduces signals to the actin cytoskeleton to initiate and maintain polarized gorwth and to mitogen-activated protein morphogenesis. In the budding yeast Saccharomyces cerevisiae, Cdc42p plays an important role in multiple actin-dependent morphogenetic events such as bud emergence, mating-projection formation, and pseudohyphal growth. In mammalian cells, Cdc42p regulates a variety of actin-dependent events and induces the JNK/SAPK protein kinase cascade, which leads to the activation of transcription factors within the nucleus. Cdc42p mediates these processes through interactions with a myriad of downstream effectors, whose number and regulation we are just starting to understand. In addition, Cdc42p has been implicated in a number of human diseases through interactions with its regulators and downstream effectors. While much is known about Cdc42p structure and functional interactions, little is known about the mechanism(s) by which it transduces signals within the cell. Future research should focus on this question as well as on the detailed analysis of the interactions of Cdc42p with its regulators and downstream effectors.

The guanine nucleotide exchange factor Tiam1 affects neuronal morphology; opposing roles for the small GTPases Rac and Rho

The invasion-inducing T-lymphoma invasion and metastasis 1 (Tiam1) protein functions as a guanine nucleotide exchange factor (GEF) for the small GTPase Rac1. Differentiation-dependent expression of Tiam1 in the developing brain suggests a role for this GEF and its effector Rac1 in the control of neuronal morphology. Here we show that overexpression of Tiam1 induces cell spreading and affects neurite outgrowth in N1E-115 neuroblastoma cells. These effects are Rac-dependent and strongly promoted by laminin. Overexpression of Tiam1 recruits the alpha 6 beta 1 integrin, a laminin receptor, to specific adhesive contacts at the cell periphery, which are different from focal contacts. Cells overexpressing Tiam1 no longer respond to lysophosphatidic acid- induced neurite retraction and cell rounding, processes mediated by Rho, suggesting that Tiam1-induced activation of Rac antagonizes Rho signaling. This inhibition can be overcome by coexpression of constitutively active RhoA, which may indicate that regulation occurs at the level of Rho or upstream. Conversely, neurite formation induced by Tiam1 or Rac1 is further promoted by inactivating Rho. These results demonstrate that Rac- and Rho-mediated pathways oppose each other during neurite formation and that a balance between these pathways determines neuronal morphology. Furthermore, our data underscore the potential role of Tiam1 as a specific regulator of Rac during neurite formation and illustrate the importance of reciprocal interactions between the cytoskeleton and the extracellular matrix during this process.