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

Incarvine C and its analogues inhibit the formation of cell cytoskeleton by targeting Rac1

Ras-related C3 botulinum toxin substrate 1 (Rac1) has emerged as a key regulator in the treatment of cancer metastasis because of its involvement in the formation of cell plate pseudopods and effects on cell migration. In this study, we found that incarvine C, a natural product isolated from Incarvillea sinensis, and its seven analogues exhibited antitumour activity by inhibiting cell cytoskeleton formation, with moderate cytotoxicity. Accordingly, these compounds inhibited the cytoskeleton-mediated migration and invasion of MDA-MB-231 cells, with inhibition rates ranging from 37.30 % to 69.72 % and 51.27 % to 70.90 % in vitro, respectively. Moreover, they induced G2/M phase cell cycle arrest in MDA-MB-231 cells. A pull-down assay revealed that the interaction between Rac1 and its downstream effector protein PAK1 was inhibited by these compounds and that the compound Ano-6 exhibited substantial activity, with an inhibition rate of more than 90 %. Molecular docking showed that incarvine C and its analogues could bind to the nucleotide-binding pocket of Rac1, maintaining high levels of inactivated Rac1. As Ano-6 exhibited significant activity in vitro, its anti-cancer activity was tested in vivo. Four weeks of oral treatment with Ano-6 was well-tolerated in mice, and it induced a potential anti-tumour response in xenografts of MDA-MB-231 cells. Further studies demonstrated that Ano-6 was enriched in tumour tissues after 2 h of administration and induced an increase in the number of dead tumour cells. In summary, these findings not only reveal the mechanism of incarvine C but also provide a new molecular template for Rac1 inhibitors and identify a promising candidate for breast cancer treatment.

Advances in Understanding the Molecular Mechanisms of Neuronal Polarity

The establishment and maintenance of neuronal polarity are important for neural development and function. Abnormal neuronal polarity establishment commonly leads to a variety of neurodevelopmental disorders. Over the past three decades, with the continuous development and improvement of biological research methods and techniques, we have made tremendous progress in the understanding of the molecular mechanisms of neuronal polarity establishment. The activity of positive and negative feedback signals and actin waves are both essential in this process. They drive the directional transport and aggregation of key molecules of neuronal polarity, promote the spatiotemporal regulation of ordered and coordinated interactions of actin filaments and microtubules, stimulate the specialization and growth of axons, and inhibit the formation of multiple axons. In this review, we focus on recent advances in these areas, in particular the important findings about neuronal polarity in two classical models, in vitro primary hippocampal/cortical neurons and in vivo cortical pyramidal neurons, and discuss our current understanding of neuronal polarity.​.

Mechanisms and consequences of dysregulation of the Tiam family of Rac activators in disease

The Tiam family proteins - Tiam1 and Tiam2/STEF - are Rac1-specific Guanine Nucleotide Exchange Factors (GEFs) with important functions in epithelial, neuronal, immune and other cell types. Tiam GEFs regulate cellular migration, proliferation and survival, mainly through activating and directing Rac1 signalling. Dysregulation of the Tiam GEFs is significantly associated with human diseases including cancer, immunological and neurological disorders. Uncovering the mechanisms and consequences of dysregulation is therefore imperative to improving the diagnosis and treatment of diseases. Here we compare and contrast the subcellular localisation and function of Tiam1 and Tiam2/STEF, and review the evidence for their dysregulation in disease.

TIAM2S as a novel regulator for serotonin level enhances brain plasticity and locomotion behavior

TIAM2S, the short form of human T-cell lymphoma invasion and metastasis 2, can have oncogenic effects when aberrantly expressed in the liver or lungs. However, it is also abundant in healthy, non-neoplastic brain tissue, in which its primary function is still unknown. Here, we examined the neurobiological and behavioral significance of human TIAM2S using the human brain protein panels, a human NT2/D1-derived neuronal cell line model (NT2/N), and transgenic mice that overexpress human TIAM2S (TIAM2S-TG). Our data reveal that TIAM2S exists primarily in neurons of the restricted brain areas around the limbic system and in well-differentiated NT2/N cells. Functional studies revealed that TIAM2S has no guanine nucleotide exchange factor (GEF) activity and is mainly located in the nucleus. Furthermore, whole-transcriptome and enrichment analysis with total RNA sequencing revealed that TIAM2S-knockdown (TIAM2S-KD) was strongly associated with the cellular processes of the brain structural development and differentiation, serotonin-related signaling, and the diseases markers representing neurobehavioral developmental disorders. Moreover, TIAM2S-KD cells display decreased neurite outgrowth and reduced serotonin levels. Moreover, TIAM2S overexpressing TG mice show increased number and length of serotonergic fibers at early postnatal stage, results in higher serotonin levels at both the serum and brain regions, and higher neuroplasticity and hyperlocomotion in latter adulthood. Taken together, our results illustrate the non-oncogenic functions of human TIAM2S and demonstrate that TIAM2S is a novel regulator of serotonin level, brain neuroplasticity, and locomotion behavior.

Apical polarity proteins recruit the RhoGEF Cysts to promote junctional myosin assembly

Silver et al. show that the RhoGEF Cysts links apical polarity proteins to Rho1 and myosin activation at adherens junctions to support junctional and epithelial integrity in the Drosophila ectoderm.

The spatio-temporal regulation of small Rho GTPases is crucial for the dynamic stability of epithelial tissues. However, how RhoGTPase activity is controlled during development remains largely unknown. To explore the regulation of Rho GTPases in vivo, we analyzed the Rho GTPase guanine nucleotide exchange factor (RhoGEF) Cysts, the Drosophila orthologue of mammalian p114RhoGEF, GEF-H1, p190RhoGEF, and AKAP-13. Loss of Cysts causes a phenotype that closely resembles the mutant phenotype of the apical polarity regulator Crumbs. This phenotype can be suppressed by the loss of basolateral polarity proteins, suggesting that Cysts is an integral component of the apical polarity protein network. We demonstrate that Cysts is recruited to the apico-lateral membrane through interactions with the Crumbs complex and Bazooka/Par3. Cysts activates Rho1 at adherens junctions and stabilizes junctional myosin. Junctional myosin depletion is similar in Cysts- and Crumbs-compromised embryos. Together, our findings indicate that Cysts is a downstream effector of the Crumbs complex and links apical polarity proteins to Rho1 and myosin activation at adherens junctions, supporting junctional integrity and epithelial polarity.

RhoGTPases at the synapse: An embarrassment of choice

Activity-dependent modifications in the strength of excitatory synapses are considered to be major cellular mechanisms that contribute to the plasticity of neuronal networks underlying learning and memory. Key mechanisms for the regulation of synaptic efficacy involve the dynamic changes in size and number of dendritic spines, as well as the synaptic incorporation and removal of AMPA-type glutamate receptors (AMPAr). As key regulators of the actin cytoskeleton, the Rho subfamily of GTP-binding proteins play a critical role in synaptic development and plasticity. They shuttle between the active GTP-bound form and the inactive GDP-bound form under the regulation of dedicated guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). More than 80 human GEFs and 70 GAPs have been identified, most of which are expressed in the brain with a specific spatial and temporal expression pattern. However, the function of most GEFs and GAPs in the brain has not been elucidated. In this review, we highlight the novel neuronal function of the synaptic RhoGAP ARHGAP12 and the ID-associated RhoGEF TRIO and further propose 3 possible approaches of neurons utilizing Rho GTPase regulatory proteins to accurately modulate synaptic function.

Clinical implication of Tiam1 overexpression in the prognosis of patients with serous ovarian carcinoma

T lymphoma invasion and metastasis 1 (Tiam1), a guanine nucleotide exchange factor, was originally identified as an invasion- and metastasis-inducing gene in T lymphoma cells. High expression levels of the human Tiam1 gene have been found in numerous human malignancies, suggesting a potential role as a modifier of tumor initiation and progression. However, little is known about the status of Tiam1 in ovarian carcinoma. The present study aimed to investigate the clinicopathological significance of high Tiam1 expression in serous ovarian carcinoma. Immunohistochemical staining for Tiam1 was performed in 182 patients with serous ovarian carcinoma, in 76 patients with ovarian borderline tumors and in 72 patients with benign ovarian tumors. Immunofluorescence staining was also performed to detect the subcellular localization of Tiam1 protein in SK-OV-3 ovarian carcinoma cells. The correlations between high Tiam1 expression and the clinicopathological features of the ovarian carcinomas were evaluated by the χ² test and Fisher's exact test. The overall survival (OS) rates were calculated by the Kaplan-Meier method, and the association between prognostic factors and patient survival was analyzed by the Cox proportional hazard model. Tiam1 protein showed a cytoplasmic and nuclear staining pattern in ovarian carcinoma. Strongly-positive Tiam1 protein expression was observed in 59.3% (108/182) of ovarian carcinomas, which was significantly higher than in benign serous tumors (12.5%; 9/72). Moreover, the rate of strongly-positive Tiam1 expression in borderline serous tumors (31.6%; 24/76) was also significantly higher than that in benign serous tumors. High Tiam1 protein expression was closely associated with a high histological grade, metastasis, advanced clinical stage and lower OS rates in ovarian carcinoma. Multivariate analysis indicated that Tiam1 was an independent prognostic factor, along with metastasis and clinical stage, in patients with ovarian carcinoma. In conclusion, Tiam1 expression is strongly associated with grade and outcome in ovarian carcinoma, and may serve as a useful molecular marker for clinical management.

Cytoskeletal regulation by AUTS2 in neuronal migration and neuritogenesis

Mutations in the Autism susceptibility candidate 2 gene (AUTS2), whose protein is believed to act in neuronal cell nuclei, have been associated with multiple psychiatric illnesses, including autism spectrum disorders, intellectual disability, and schizophrenia. Here we show that cytoplasmic AUTS2 is involved in the regulation of the cytoskeleton and neural development. Immunohistochemistry and fractionation studies show that AUTS2 localizes not only in nuclei, but also in the cytoplasm, including in the growth cones in the developing brain. AUTS2 activates Rac1 to induce lamellipodia but downregulates Cdc42 to suppress filopodia. Our loss-of-function and rescue experiments show that a cytoplasmic AUTS2-Rac1 pathway is involved in cortical neuronal migration and neuritogenesis in the developing brain. These findings suggest that cytoplasmic AUTS2 acts as a regulator of Rho family GTPases to contribute to brain development and give insight into the pathology of human psychiatric disorders with AUTS2 mutations.

NG2 regulates directional migration of oligodendrocyte precursor cells via Rho GTPases and polarity complex proteins

The transmembrane proteoglycan NG2 is expressed by oligodendrocyte precursor cells (OPC), which migrate to axons during developmental myelination and remyelinate in the adult after migration to injured sites. Highly invasive glial tumors also express NG2. Despite the fact that NG2 has been implicated in control of OPC migration, its mode of action remains unknown. Here, we show in vitro and in vivo that NG2 controls migration of OPC through the regulation of cell polarity. In stab wounds in adult mice we show that NG2 controls orientation of OPC toward the wound. NG2 stimulates RhoA activity at the cell periphery via the MUPP1/Syx1 signaling pathway, which favors the bipolar shape of migrating OPC and thus directional migration. Upon phosphorylation of Thr-2256, downstream signaling of NG2 switches from RhoA to Rac stimulation. This triggers process outgrowth through regulators of front-rear polarity and we show using a phospho-mimetic form of NG2 that indeed NG2 recruits proteins of the CRB and the PAR polarity complexes to stimulate Rac activity via the GEF Tiam1. Our findings demonstrate that NG2 is a core organizer of Rho GTPase activity and localization in the cell, which controls OPC polarity and directional migration. This work also reveals CRB and PAR polarity complexes as new effectors of NG2 signaling in the establishment of front-rear polarity.

RNAi mediated Tiam1 gene knockdown inhibits invasion of retinoblastoma

T lymphoma invasion and metastasis protein (Tiam1) is up-regulated in variety of cancers and its expression level is related to metastatic potential of the type of cancer. Earlier, Tiam1 was shown to be overexpressed in retinoblastoma (RB) and we hypothesized that it was involved in invasiveness of RB. This was tested by silencing Tiam1 in RB cell lines (Y79 and Weri-Rb1) using siRNA pool, targeting different regions of Tiam1 mRNA. The cDNA microarray of Tiam1 silenced cells showed gene regulations altered by Tiam1 were predominantly on the actin cytoskeleton interacting proteins, apoptotic initiators and tumorogenic potential targets. The silenced phenotype resulted in decreased growth and increased apoptosis with non-invasive characteristics. Transfection of full length and N-terminal truncated construct (C1199) clearly revealed membrane localization of Tiam1 and not in the case of C580 construct. F-actin staining showed the interaction of Tiam1 with actin in the membrane edges that leads to ruffling, and also imparts varying invasive potential to the cell. The results obtained from our study show for the first time that Tiam1 modulates the cell invasion, mediated by actin cytoskeleton remodeling in RB.

Neuronal Rho GEFs in synaptic physiology and behavior

In the mammalian brain, the majority of excitatory synapses are housed in micron-sized dendritic protrusions called spines, which can undergo rapid changes in shape and number in response to increased or decreased synaptic activity. These dynamic alterations in dendritic spines require precise control of the actin cytoskeleton. Within spines, multidomain Rho guanine nucleotide exchange factors (Rho GEFs) coordinate activation of their target Rho GTPases by a variety of pathways. In this review, we focus on the handful of disease-related Rho GEFs (Kalirin; Trio; Tiam1; P-Rex1,2; RasGRF1,2; Collybistin) localized at synapses and known to affect electrophysiology, spine morphology, and animal behavior. The goal is to integrate structure/function studies with measurements of synaptic function and behavioral phenotypes in animal models.

Balanced Tiam1-rac1 and RhoA drives proliferation and invasion of pancreatic cancer cells

Tiam1 is a rac1-specific guanine nucleotide exchange factor, and Tiam1-rac1 is involved in a number of cellular processes. Rac1 and RhoA act as molecular switches that cycle between GTP- and GDP-bound states to balance the activities of rac1 and RhoA. The downregulation of rac1 activity leads to upregulation of RhoA activity, which promotes invasion and migration of pancreatic cancers cells. At present, however, the role of Tiam1-rac1 and RhoA in pancreatic cancers is not fully understood. We found that Tiam1 was upregulated in pancreatic cancers and was significantly expressed in tumors without lymph node involvement or distant metastasis compared with cancers where there was involvement. Although Tiam1-rac1 signaling promoted pancreatic cancer cell proliferation and tumor growth via the Wnt signaling pathway in vitro and in vivo, inhibiting Tiam1-rac1 signaling did not prolong the overall survival time in vivo. This provided evidence that there was a balance between rac1 and RhoA activities in pancreatic cancers. Furthermore, only the combined inhibition of Tiam1-rac1 and RhoA had a beneficial effect on the growth of pancreatic cancers in vivo. Taken together, these results suggest that the progression of pancreatic tumors is partially controlled by the balance between Tiam1-rac1 and RhoA.

Calcineurin-dependent cofilin activation and increased retrograde actin flow drive 5-HT-dependent neurite outgrowth in Aplysia bag cell neurons

Neurite outgrowth in response to soluble growth factors often involves changes in intracellular Ca(2+); however, mechanistic roles for Ca(2+) in controlling the underlying dynamic cytoskeletal processes have remained enigmatic. Bag cell neurons exposed to serotonin (5-hydroxytryptamine [5-HT]) respond with a threefold increase in neurite outgrowth rates. Outgrowth depends on phospholipase C (PLC) → inositol trisphosphate → Ca(2+) → calcineurin signaling and is accompanied by increased rates of retrograde actin network flow in the growth cone P domain. Calcineurin inhibitors had no effect on Ca(2+) release or basal levels of retrograde actin flow; however, they completely suppressed 5-HT-dependent outgrowth and F-actin flow acceleration. 5-HT treatments were accompanied by calcineurin-dependent increases in cofilin activity in the growth cone P domain. 5-HT effects were mimicked by direct activation of PLC, suggesting that increased actin network treadmilling may be a widespread mechanism for promoting neurite outgrowth in response to neurotrophic factors.

The role of small GTPases in neuronal morphogenesis and polarity

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

The Rac GTP exchange factor TIAM-1 acts with CDC-42 and the guidance receptor UNC-40/DCC in neuronal protrusion and axon guidance

The mechanisms linking guidance receptors to cytoskeletal dynamics in the growth cone during axon extension remain mysterious. The Rho-family GTPases Rac and CDC-42 are key regulators of growth cone lamellipodia and filopodia formation, yet little is understood about how these molecules interact in growth cone outgrowth or how the activities of these molecules are regulated in distinct contexts. UNC-73/Trio is a well-characterized Rac GTP exchange factor in Caenorhabditis elegans axon pathfinding, yet UNC-73 does not control CED-10/Rac downstream of UNC-6/Netrin in attractive axon guidance. Here we show that C. elegans TIAM-1 is a Rac-specific GEF that links CDC-42 and Rac signaling in lamellipodia and filopodia formation downstream of UNC-40/DCC. We also show that TIAM-1 acts with UNC-40/DCC in axon guidance. Our results indicate that a CDC-42/TIAM-1/Rac GTPase signaling pathway drives lamellipodia and filopodia formation downstream of the UNC-40/DCC guidance receptor, a novel set of interactions between these molecules. Furthermore, we show that TIAM-1 acts with UNC-40/DCC in axon guidance, suggesting that TIAM-1 might regulate growth cone protrusion via Rac GTPases in response to UNC-40/DCC. Our results also suggest that Rac GTPase activity is controlled by different GEFs in distinct axon guidance contexts, explaining how Rac GTPases can specifically control multiple cellular functions.

Axons extend great distances to make precise synaptic connections in the developing nervous system. Axons are guided to their targets by the growth cone, a dynamic structure at the axon distal tip that senses extracellular cues telling the axon where to go. In response to guidance cues, growth cones alter their shape and motility resulting in outgrowth and turning. The cytoskeleton (actin and microtubules) underlies growth cone motility and guidance. The signaling mechanisms linking guidance receptors to cytoskeletal change remain mysterious. Here, we define a new signaling mechanism downstream of the guidance receptor UNC-40/DCC involving the GTPases CDC-42 and Rac, which have long been known to control growth cone protrusion. We show that CDC-42 and Rac act in a linear pathway in axon guidance; CDC-42 acts upstream of the GTPase regulatory molecule TIAM-1, which is a GTP exchange factor specific for Rac and which activates Rac signaling. We also show that TIAM-1 acts with UNC-40/DCC signaling in protrusion and axon guidance. Our results imply that Rac GTPase function in axon guidance is complex and that distinct GEFs (TIAM-1 and UNC-73/Trio) might control Rac GTPases in different aspects of axon guidance.

ROCK1 feedback regulation of the upstream small GTPase RhoA

Rho-associated coiled-coil containing protein kinase 1 (ROCK1) is a key downstream effector of the small GTPase RhoA. Targeting ROCK1 has shown promising clinical potential in cancer, cardioprotection, hypertension, diabetes, neuronal regeneration, and stem cell biology. General working hypothesis in previous studies has centered on the function of ROCK1 as a downstream sequence in the RhoA signaling pathway. In this study, the effects of the direct inhibition of ROCK1 on the activity of upstream RhoA and Rac1 were examined using a combined pharmacological and genetic approach. We report an intriguing mechanism by which the inhibition of ROCK1 indirectly diminishes the activity of upstream RhoA through the stimulation of Tiam1-induced Rac1 activity. This novel feedback mechanism, in which ROCK1 mediates upstream Rac1 and RhoA activity, offers considerable insight into the diverse effects of ROCK1 on the functional balance of the Rho family of small GTPases, which regulates actin cytoskeleton reorganization processes and the resulting overall behavior of cells.

Sequencing of neuroblastoma identifies chromothripsis and defects in neuritogenesis genes

Neuroblastoma is a childhood tumour of the peripheral sympathetic nervous system. The pathogenesis has for a long time been quite enigmatic, as only very few gene defects were identified in this often lethal tumour. Frequently detected gene alterations are limited to MYCN amplification (20%) and ALK activations (7%). Here we present a whole-genome sequence analysis of 87 neuroblastoma of all stages. Few recurrent amino-acid-changing mutations were found. In contrast, analysis of structural defects identified a local shredding of chromosomes, known as chromothripsis, in 18% of high-stage neuroblastoma. These tumours are associated with a poor outcome. Structural alterations recurrently affected ODZ3, PTPRD and CSMD1, which are involved in neuronal growth cone stabilization. In addition, ATRX, TIAM1 and a series of regulators of the Rac/Rho pathway were mutated, further implicating defects in neuritogenesis in neuroblastoma. Most tumours with defects in these genes were aggressive high-stage neuroblastomas, but did not carry MYCN amplifications. The genomic landscape of neuroblastoma therefore reveals two novel molecular defects, chromothripsis and neuritogenesis gene alterations, which frequently occur in high-risk tumours.

Elevated expression of RGS19 impairs the responsiveness of stress-activated protein kinases to serum

Regulators of G protein signaling (RGS proteins) serve as GTPase activating proteins for the signal transducing Gα subunits. RGS19, also known as Gα-interacting protein (GAIP), has been shown to subserve other functions such as the regulation of macroautophagy and growth factor signaling. We have recently demonstrated that the expression of RGS19 in human embryonic kidney (HEK) 293 cells resulted in the disruption of serum-induced mitogenic response along the classical Ras/Raf/MEK/ERK pathway. Here, we further examined the effect of RGS19 expression on the stress-activated protein kinases (SAPKs). Both c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) became non-responsive to serum in 293/RGS19 cells, yet the two SAPKs responded to UV irradiation or osmotic stress induced by sorbitol. Kinases upstream of JNK and p38 MAPK, including MKK3/6, MKK4, and MLK3, also failed to respond to serum stimulation in 293/RGS19 cells. Serum-induced activation of the small GTPases Rac1 and Cdc42 was similarly suppressed in these cells. Our results indicate that elevated expression of RGS19 can severely disrupt the regulation of MAPKs by small GTPases.

The role of Rho GTPase proteins in CNS neuronal migration

The architectonics of the mammalian brain arise from a remarkable range of directed cell migrations, which orchestrate the emergence of cortical neuronal layers and pattern brain circuitry. At different stages of cortical histogenesis, specific modes of cell motility are essential to the stepwise formation of cortical architecture. These movements range from interkinetic nuclear movements in the ventricular zone, to migrations of early-born, postmitotic polymorphic cells into the preplate, to the radial migration of precursors of cortical output neurons across the thickening cortical wall, and the vast, tangential migrations of interneurons from the basal forebrain into the emerging cortical layers. In all cases, actomyosin motors act in concert with cell adhesion receptor systems to provide the force and traction needed for forward movement. As key regulators of actin and microtubule cytoskeletons, cell polarity, and adhesion, the Rho GTPases play critical roles in CNS neuronal migration. This review will focus on the different types of migration in the developing neocortex and cerebellar cortex, and the role of the Rho GTPases, their regulators and effectors in these CNS migrations, with particular emphasis on their involvement in radial migration.

Phosphorylation of STEF/Tiam2 by protein kinase A is critical for Rac1 activation and neurite outgrowth in dibutyryl cAMP-treated PC12D cells

Although cAMP-induced neuritogenesis was described in 1975, the signaling pathways from cAMP to cytoskeletal regulation remain elusive. Here we report that the protein kinase A (PKA)-Sif-and Tiam1-like exchange factor (STEF)-Rac1 pathway plays a central role in cytoskeletal regulation during neurite outgrowth in PC12D cells. This result could be extrapolated to axon guidance depending on cAMP signals.

The second messenger cAMP plays a pivotal role in neurite/axon growth and guidance, but its downstream pathways leading to the regulation of Rho GTPases, centrally implicated in neuronal morphogenesis, remain elusive. We examined spatiotemporal changes in Rac1 and Cdc42 activity and phosphatidylinositol 3,4,5-triphosphate (PIP₃) concentration in dibutyryl cAMP (dbcAMP)-treated PC12D cells using Förster resonance energy transfer–based biosensors. During a 30-min incubation with dbcAMP, Rac1 activity gradually increased throughout the cells and remained at its maximal level. There was no change in PIP₃ concentration. After a 5-h incubation with dbcAMP, Rac1 and Cdc42 were activated at the protruding tips of neurites without PIP₃ accumulation. dbcAMP-induced Rac1 activation was principally mediated by protein kinase A (PKA) and Sif- and Tiam1-like exchange factor (STEF)/Tiam2. STEF depletion drastically reduced dbcAMP-induced neurite outgrowth. PKA phosphorylates STEF at three residues (Thr-749, Ser-782, Ser-1562); Thr-749 phosphorylation was critical for dbcAMP-induced Rac1 activation and neurite extension. During dbcAMP-induced neurite outgrowth, PKA activation at the plasma membrane became localized to neurite tips; this localization may contribute to local Rac1 activation at the same neurite tips. Considering the critical role of Rac1 in neuronal morphogenesis, the PKA—STEF–Rac1 pathway may play a crucial role in cytoskeletal regulation during neurite/axon outgrowth and guidance, which depend on cAMP signals.

Distinct ligand specificity of the Tiam1 and Tiam2 PDZ domains

Guanine nucleotide exchange factor proteins of the Tiam family are activators of the Rho GTPase Rac1 and critical for cell morphology, adhesion, migration, and polarity. These proteins are modular and contain a variety of interaction domains, including a single post-synaptic density-95/discs large/zonula occludens-1 (PDZ) domain. Previous studies suggest that the specificities of the Tiam1 and Tiam2 PDZ domains are distinct. Here, we sought to conclusively define these specificities and determine their molecular origin. Using a combinatorial peptide library, we identified a consensus binding sequence for each PDZ domain. Analysis of these consensus sequences and binding assays with peptides derived from native proteins indicated that these two PDZ domains have overlapping but distinct specificities. We also identified residues in two regions (S(0) and S(-2) pockets) of the Tiam1 PDZ domain that are important determinants of ligand specificity. Site-directed mutagenesis of four nonconserved residues in these two regions along with peptide binding analyses confirmed that these residues are crucial for ligand affinity and specificity. Furthermore, double mutant cycle analysis of each region revealed energetic couplings that were dependent on the ligand being investigated. Remarkably, a Tiam1 PDZ domain quadruple mutant had the same specificity as the Tiam2 PDZ domain. Finally, analysis of Tiam family PDZ domain sequences indicated that the PDZ domains segregate into four distinct families based on the residues studied here. Collectively, our data suggest that Tiam family proteins have highly evolved PDZ domain-ligand interfaces with distinct specificities and that they have disparate PDZ domain-dependent biological functions.

Structural and thermodynamic analysis of PDZ-ligand interactions

Tiam-family guanine exchange proteins are activators of the Rho GTPase Rac1 and critical for cell morphology, adhesion, migration, and polarity. These modular proteins contain a variety of signaling domains, including a single postsynaptic density-95/discs large/zonula occludens-1 (PDZ) domain. Here, we show how structural and thermodynamic approaches applied to the Tiam1 PDZ domain can be used to gain unique insights into the affinity and specificity of PDZ-ligand interactions with peptides derived from Syndecan1 and Caspr4 proteins. First, we describe a fluorescence anisotropy-based assay that can be used to determine PDZ-ligand interactions, and describe important considerations in designing binding experiments. Second, we used site-specific mutagenesis in combination with double-mutant cycle analysis to probe the binding energetics and cooperativity of residues in two ligand binding pockets (S(0) and S(-2)) that are involved in Tiam1 PDZ-ligand interactions. Peptide ligand binding results and double-mutant cycle analysis revealed that the S(0) pocket was important for Syndecan1 and Caspr4 peptide interactions and that the S(-2) pocket provided selectivity for the Syndecan1 ligand. Finally, we devised a "peptide evolution" strategy whereby a Model consensus peptide was "evolved" into either the Syndecan1 or Caspr4 peptide by site-directed mutagenesis. These results corroborated the PDZ mutational analysis of the S(0) pocket and identified the P(-4) position in the ligand as critical for Syndecan1 affinity and selectivity. Together, these studies show that a combined structural and thermodynamic approach is powerful for obtaining insights into the origin of Tiam1 PDZ-ligand domain affinity and specificity.

Altered expression of miRNA-21 and its targets in the hippocampus after traumatic brain injury

Traumatic brain injury (TBI) initiates many different signaling cascades throughout the brain that impact both pathophysiological and neuroprotective processes. Cellular mechanisms that can modulate these processes may play an important role in determining the nature and extent of the damage suffered after TBI and therefore influence overall outcome after injury. MicroRNAs (miRNAs) are an important class of noncoding regulatory RNAs providing an epigenetic mechanism for the regulation of protein expression levels of target genes. We report that miR-21 expression is significantly up-regulated in the hippocampus after rodent TBI, with expression levels peaking by 3 days postinjury and returning to near sham levels by 15 days postinjury. In situ localization of miR-21 transcripts indicates widespread expression in normal brain, with a pronounced increase in expression after TBI evident throughout the cortex and hippocampus, including the dentate gyrus and CA3 cell layer. We used a combination of the miRanda, TargetScan, and PicTar prediction algorithms to identify 99 potential target genes that possess miR-21 binding sites within their 3' untranslated regions. Analysis of these genes' annotated Gene Ontology molecular function and biological process terms revealed an overrepresentation of genes involved in enzyme-linked receptor signaling, transcriptional regulation, and developmental processes. These results suggest that increased miR-21 expression in the hippocampus may influence multiple components of TBI pathophysiology.

RHO GTPase signaling for axon extension: is prenylation important?

Many lines of evidence indicate the importance of the Rho family guanine nucleotide triphosphatases (GTPases) in directing axon extension and guidance. The signaling networks that involve these proteins regulate actin cytoskeletal dynamics in navigating neuronal growth cones. However, the intricate patterns that regulate Rho GTPase activation and signaling are not yet fully defined. Activity and subcellular localization of the Rho GTPases are regulated by post-translational modification. The addition of a geranylgeranyl group to the carboxy (C-) terminus targets Rho GTPases to the plasma membrane and promotes their activation by facilitating interaction with guanine nucleotide exchange factors and allowing sequestering by association with guanine dissociation inhibitors. However, it is unclear how these modifications affect neurite extension or how subcellular localization alters signaling from the classical Rho GTPases (RhoA, Rac1, and Cdc42). Here, we review recent data addressing this issue and propose that Rho GTPase geranylgeranylation regulates outgrowth.

Hippocampal region-specific regulation of NF-kappaB may contribute to learning-associated synaptic reorganisation

Activity of the transcription factor NF-kappaB is required for memory formation, but the identity and function of the genes it may regulate in this context remain obscure. Here, we comprehensively characterise NF-kappaB throughout the rat hippocampus following passive avoidance training and report significant subregion-specific increased activity across the dorsoventral axis 3h post-learning. Moreover, putative NF-kappaB binding motifs predominated in structural genes previously shown to regulate 3h following avoidance conditioning, the protein products of which may be involved in the subsequent synaptic remodelling required for consolidation. Finally, we assessed the influence of NF-kappaB-mediated transcription on neuritic structure and report that inhibition of NF-kappaB significantly decreases growth and branching of primary hippocampal neurons. These results suggest that NF-kappaB activity following hippocampal learning may contribute to consolidation-associated synaptic reorganisation.

Inhibitory and excitatory subtypes of cochlear nucleus neurons are defined by distinct bHLH transcription factors, Ptf1a and Atoh1

The cochlear nucleus (CN), which consists of dorsal and ventral cochlear nuclei (DCN and VCN), plays pivotal roles in processing and relaying auditory information to the brain. Although it contains various types of neurons, the origins of the distinct subtypes and their developmental molecular machinery are still elusive. Here we reveal that two basic helix-loop-helix transcription factors play crucial roles in specifying neuron subtypes in the CN. Pancreatic transcription factor 1a (Ptf1a) and atonal homolog 1 (Atoh1) were found to be expressed in discrete dorsolateral regions of the embryonic neuroepithelia of the middle hindbrain (rhombomeres 2-5). Genetic lineage tracing using mice that express Cre recombinase from the Ptf1a locus or under the control of the Atoh1 promoter revealed that inhibitory (GABAergic and glycinergic) or excitatory (glutamatergic) neurons of both DCN and VCN are derived from the Ptf1a- and Atoh1-expressing neuroepithelial regions, respectively. In the Ptf1a or Atoh1 null embryos, production of inhibitory or excitatory neurons, respectively, was severely inhibited in the CN. These findings suggest that inhibitory and excitatory subtypes of CN neurons are defined by Ptf1a and Atoh1, respectively and, furthermore, provide important insights into understanding the machinery of neuron subtype specification in the dorsal hindbrain.

uPAR promotes formation of the p130Cas-Crk complex to activate Rac through DOCK180

The urokinase-type plasminogen activator receptor (uPAR) drives tumor cell membrane protrusion and motility through activation of Rac; however, the pathway leading from uPAR to Rac activation has not been described. In this study we identify DOCK180 as the guanine nucleotide exchange factor acting downstream of uPAR. We show that uPAR cooperates with integrin complexes containing β₃ integrin to drive formation of the p130Cas–CrkII signaling complex and activation of Rac, resulting in a Rac-driven elongated-mesenchymal morphology, cell motility, and invasion. Our findings identify a signaling pathway underlying the morphological changes and increased cell motility associated with uPAR expression.

Comparing quantitative trait Loci and gene expression data

We develop methods to compare the positions of quantitative trait loci (QTL) with a set of genes selected by other methods, such as microarray experiments, from a sequenced genome. We apply our methods to QTL for addictive behavior in mouse, and a set of genes upregulated in a region of the brain associated with addictive behavior, the nucleus accumbens (NA). The association between the QTL and NA genes is not significantly stronger than expected by chance. However, chromosomes 2 and 16 do show strong associations suggesting that genes on these chromosomes might be associated with addictive behavior. The statistical methodology developed for this study can be applied to similar studies to assess the mutual information in microarray and QTL analyses.

Convergent CaMK and RacGEF signals control dendritic structure and function

Structural plasticity of excitatory synapses is a vital component of neuronal development, synaptic plasticity and behavior, and its malfunction underlies many neurodevelopmental and psychiatric disorders. However, the molecular mechanisms that control dendritic spine morphogenesis have only recently emerged. We summarize recent work that has revealed an important connection between calcium/calmodulin-dependent kinases (CaMKs) and guanine-nucleotide-exchange factors (GEFs) that activate the small GTPase Rac (RacGEFs) in controlling dendritic spine morphogenesis. These two groups of molecules function in neurons as a unique signaling cassette that transduces calcium influx into small GTPase activity and, thence, actin reorganization and spine morphogenesis. Through this pathway, CaMKs and RacGEFs amplify calcium signals and translate them into spatially and temporally regulated structural remodeling of dendritic spines.

Rho-kinase phosphorylates PAR-3 and disrupts PAR complex formation

A polarity complex of PAR-3, PAR-6, and atypical protein kinase C (aPKC) functions in various cell polarization events. PAR-3 directly interacts with Tiam1/Taim2 (STEF), Rac1-specific guanine nucleotide exchange factors, and forms a complex with aPKC-PAR-6-Cdc42*GTP, leading to Rac1 activation. RhoA antagonizes Rac1 in certain types of cells. However, the relationship between RhoA and the PAR complex remains elusive. We found here that Rho-kinase/ROCK/ROK, the effector of RhoA, phosphorylated PAR-3 at Thr833 and thereby disrupted its interaction with aPKC and PAR-6, but not with Tiam2. Phosphorylated PAR-3 was observed in the leading edge, and in central and rear portions of migrating cells having front-rear polarity. Knockdown of PAR-3 by small interfering RNA (siRNA) impaired cell migration, front-rear polarization, and PAR-3-mediated Rac1 activation, which were recovered with siRNA-resistant PAR-3, but not with the phospho-mimic PAR-3 mutant. We propose that RhoA/Rho-kinase inhibits PAR complex formation through PAR-3 phosphorylation, resulting in Rac1 inactivation.

Origin of climbing fiber neurons and their developmental dependence on Ptf1a

Climbing fiber (CF) neurons in the inferior olivary nucleus (ION) extend their axons to Purkinje cells, playing a crucial role in regulating cerebellar function. However, little is known about their precise place of birth and developmental molecular machinery. Here, we describe the origin of the CF neuron lineage and the involvement of Ptf1a (pancreatic transcription factor 1a) in CF neuron development. Ptf1a protein was found to be expressed in a discrete dorsolateral region of the embryonic caudal hindbrain neuroepithelium. Because expression of Ptf1a is not overlapping other transcription factors such as Math1 (mouse atonal homolog 1) and Neurogenin1, which are suggested to define domains within caudal hindbrain neuroepithelium (Landsberg et al., 2005), we named the neuroepithelial region the Ptf1a domain. Analysis of mice that express beta-galactosidase from the Ptf1a locus revealed that CF neurons are derived from the Ptf1a domain. In contrast, retrograde labeling of precerebellar neurons indicated that mossy fiber neurons are not derived from Ptf1a-expressing progenitors. We could observe a detailed migratory path of CF neurons from the Ptf1a domain to the ION during embryogenesis. In Ptf1a null mutants, putative immature CF neurons produced from this domain were unable to migrate or differentiate appropriately, resulting in a failure of ION formation. Apoptotic cells were observed in the mutant hindbrain. Furthermore, the fate of some cells in the Ptf1a lineage were changed to mossy fiber neurons in Ptf1a null mutants. These findings clarify the precise origin of CF neurons and suggest that Ptf1a controls their fate, survival, differentiation, and migration during development.

Protein kinase Czeta and glycogen synthase kinase-3beta control neuronal polarity in developing rodent enteric neurons, whereas SMAD specific E3 ubiquitin protein ligase 1 promotes neurite growth but does not influence polarity

Enteric nervous system (ENS) precursors migrate extensively before differentiating to form uni-axonal or multi-axonal neurons. ENS precursor survival, neurite growth, and cell migration are all directed by Ret kinase, but downstream signaling pathways are incompletely understood. We now demonstrate that proteins regulating polarity in other cells including partitioning defective 3 (PAR3), PAR6, protein kinase Czeta (PKCzeta), and glycogen synthase kinase 3beta (GSK3beta) are expressed in developing enteric neurons with a polarized distribution. Blocking PKCzeta or GSK3beta reduces ENS precursor migration and induces the formation of multi-axonal neurons. Axon elongation also depends on SMURF1 (SMAD specific E3 ubiquitin protein ligase 1), which promotes RhoA degradation and associates with polarity proteins. SMURF1 inhibition, however, does not increase the number of multi-axonal neurons in ENS precursors. These data link cell surface Ret activation with molecular machinery controlling cytoskeletal dynamics and suggest that polymorphisms influencing PKCzeta or GSK3beta might alter Hirschsprung disease penetrance or expressivity by affecting ENS precursor migration.

Rho-kinase modulates the function of STEF, a Rac GEF, through its phosphorylation

Rho family GTPases are key regulators of various physiological processes. Several recent studies indicated that the antagonistic relationship between Rho and Rac is essential for cell polarity and that the Rac activity is negatively regulated by Rho. In this study, we found that Rho-kinase, an effector of Rho, counteracted the Rac GEF STEF-induced Rac1 activation in COS7 cells. Rho-kinase phosphorylated STEF at Thr1662 in vitro, and Y-27632, a Rho-kinase inhibitor, suppressed lysophosphatidic acid-induced phosphorylation of STEF in PC12D cells. STEF interacted with specific molecules such as microtubule-associated protein 1B, and the phosphorylation of STEF by Rho-kinase diminished its interaction with these molecules. STEF promoted nerve growth factor-induced neurite outgrowth in PC12D cells, while the phosphomimic mutant of STEF had a weakened ability to enhance neurite outgrowth. Taken together, these results suggest that the phosphorylation of STEF by Rho-kinase exerts the inhibitory effect on the function of STEF.

Rac is a dominant regulator of cadherin-directed actin assembly that is activated by adhesive ligation independently of Tiam1

Classic cadherins function as adhesion-activated cell signaling receptors. On adhesive ligation, cadherins induce signaling cascades leading to actin cytoskeletal reorganization that is imperative for cadherin function. In particular, cadherin ligation activates actin assembly by the actin-related protein (Arp)2/3 complex, a process that critically affects the ability of cells to form and extend cadherin-based contacts. However, the signaling pathway(s) that activate Arp2/3 downstream of cadherin adhesion remain poorly understood. In this report we focused on the Rho family GTPases Rac and Cdc42, which can signal to Arp2/3. We found that homophilic engagement of E-cadherin simultaneously activates both Rac1 and Cdc42. However, by comparing the impact of dominant-negative Rac1 and Cdc42 mutants, we show that Rac1 is the dominant regulator of cadherin-directed actin assembly and homophilic contact formation. To pursue upstream elements of the Rac1 signaling pathway, we focused on the potential contribution of Tiam1 to cadherin-activated Rac signaling. We found that Tiam1 or the closely-related Tiam2/STEF1 was recruited to cell-cell contacts in an E-cadherin-dependent fashion. Moreover, a dominant-negative Tiam1 mutant perturbed cell spreading on cadherin-coated substrata. However, disruption of Tiam1 activity with dominant-negative mutants or RNA interference did not affect the ability of E-cadherin ligation to activate Rac1. We conclude that Rac1 critically influences cadherin-directed actin assembly as part of a signaling pathway independent of Tiam1.

Involvement of a Rac activator,P-Rex1, in neurotrophin-derived signaling and neuronal migration

Rho-family GTPases play key roles in regulating cytoskeletal reorganization, contributing to many aspects of nervous system development. Their activities are known to be regulated by guanine nucleotide exchange factors (GEFs), in response to various extracellular cues. P-Rex1, a GEF for Rac, has been mainly investigated in neutrophils, in which this molecule contributes to reactive oxygen species formation. However, its role in the nervous system is essentially unknown. Here we describe the expression profile and a physiological function of P-Rex1 in nervous system development. In situ hybridization revealed that P-Rex1 is dynamically expressed in a variety of cells in the developing mouse brain, including some cortical and DRG neurons. In migrating neurons in the intermediate zone, P-Rex1 protein was found to localize in the leading process and adjacent cytoplasmic region. When transfected in pheochromocytoma PC12 cells, P-Rex1 can be activated by NGF, causing an increase in GTP-bound Rac1 and cell motility. Deletion analyses suggested roles for distinct domains of this molecule. Experiments using a P-Rex1 mutant lacking the Dbl-homology domain, a dominant-negative-like form, and small interfering RNA showed that endogenous P-Rex1 was involved in cell migration of PC12 cells and primary cultured neurons from the embryonic day 14 cerebral cortices, induced by extracellular stimuli (NGF, BDNF, and epidermal growth factor). Furthermore, in utero electroporation of the mutant protein into the embryonic cerebral cortex perturbed radial neuronal migration. These findings suggest that P-Rex1, which is expressed in a variety of cell types, is activated by extracellular cues such as neurotrophins and contributes to neuronal migration in the developing nervous system.

Cdk5 phosphorylates and stabilizes p27kip1 contributing to actin organization and cortical neuronal migration

p27(kip1), a cyclin-dependent kinase (CDK) inhibitor (CKI), generally suppresses CDK activity in proliferating cells. Although another role of p27 in cell migration has been recently suggested in vitro, the physiological importance of p27 in cell migration remains elusive, as p27-deficient mice have not shown any obvious migration-defect-related phenotypes. Here, we show that Cdk5, an unconventional neuronal CDK, phosphorylates and stabilizes p27 as an upstream regulator, maintaining the amount of p27 in post-mitotic neurons. In vivo RNA interference (RNAi) experiments showed that reduced amounts of p27 caused inhibition of cortical neuronal migration and decreased the amount of F-actin in the processes of migrating neurons. The Cdk5-p27 pathway activates an actin-binding protein, cofilin, which is also shown to be involved in cortical neuronal migration in vivo. Our findings shed light on a previously unknown new relationship between CDK and CKI in G0-arrested cells that regulates cytoskeletal reorganization and neuronal migration during corticogenesis.

Ptf1a, a bHLH transcriptional gene, defines GABAergic neuronal fates in cerebellum

The molecular machinery governing glutamatergic-GABAergic neuronal subtype specification is unclear. Here we describe a cerebellar mutant, cerebelless, which lacks the entire cerebellar cortex in adults. The primary defect of the mutant brains was a specific inhibition of GABAergic neuron production from the cerebellar ventricular zone (VZ), resulting in secondary and complete loss of external germinal layer, pontine, and olivary nuclei during development. We identified the responsible gene, Ptf1a, whose expression was lost in the cerebellar VZ but was maintained in the pancreas in cerebelless. Lineage tracing revealed that two types of neural precursors exist in the cerebellar VZ: Ptf1a-expressing and -nonexpressing precursors, which generate GABAergic and glutamatergic neurons, respectively. Introduction of Ptf1a into glutamatergic neuron precursors in the dorsal telencephalon generated GABAergic neurons with representative morphological and migratory features. Our results suggest that Ptf1a is involved in driving neural precursors to differentiate into GABAergic neurons in the cerebellum.

Kalirin: a dual Rho guanine nucleotide exchange factor that is so much more than the sum of its many parts

A large number of Rho guanine nucleotide exchange factors (GEFs) and Rho GTPase activating proteins (GAPs) are used in the CNS to activate specific Rho GTPase family members, thereby inducing various signaling mechanisms that regulate neuronal shape, growth, and plasticity, in part through their effects on the actin cytoskeleton. Kalirin is a large neuronal dual Rho GEF that activates Rac1, RhoA, and RhoG via its two Rho GEF domains. This activation, which is spatially and temporally regulated, allows Kalirin to influence neurite initiation, axonal growth, and dendritic morphogenesis. In addition, this alternatively spliced gene generates developmentally regulated transcripts that yield proteins localized to the postsynaptic density (PSD). Kalirin-7, which interacts with PSD-95, is necessary for dendritic spine formation. In addition, Kalirins have the ability to regulate and influence other aspects of neuronal morphogenesis via protein-protein interactions with their other domains, including many spectrins, other protein and lipid interaction domains, and a potential kinase. These interactions have implications not only for neuronal morphogenesis but also for vesicle trafficking, secretion, neuronal maintenance, and neurodegenerative disease.

MAP1B phosphorylation is differentially regulated by Cdk5/p35, Cdk5/p25, and JNK

Mode I phosphorylated MAP1B is observed in developing and pathogenic brains. Although Cdk5 has been believed to phosphorylate MAP1B in the developing cerebral cortex, we show that a Cdk5 inhibitor does not suppress mode I phosphorylation of MAP1B in primary and slice cultures, while a JNK inhibitor does. Coincidently, an increase in phosphorylated MAP1B was not observed in COS7 cells when Cdk5 was cotransfected with p35, but this did occur with p25 which is specifically produced in pathogenic brains. Our primary culture studies showed an involvement of Cdk5 in regulating microtubule dynamics without affecting MAP1B phosphorylation status. The importance of regulating microtubule dynamics in neuronal migration was also demonstrated by in utero electroporation experiments. These findings suggest that mode I phosphorylation of MAP1B is facilitated by JNK but not Cdk5/p35 in the developing cerebral cortex and by Cdk5/p25 in pathogenic brains, contributing to various biological events.

PAR-6–PAR-3 mediates Cdc42-induced Rac activation through the Rac GEFs STEF/Tiam1

A polarity complex of PAR-3, PAR-6 and atypical protein kinase C (aPKC) functions in various cell-polarization events, including neuron specification. The small GTPase Cdc42 binds to PAR-6 and regulates cell polarity. However, little is known about the downstream signals of the Cdc42-PAR protein complex. Here, we found that PAR-3 directly interacted with STEF/Tiam1, which are Rac-specific guanine nucleotide-exchange factors, and that STEF formed a complex with PAR-3-aPKC-PAR-6-Cdc42-GTP. Cdc42 induces lamellipodia in a Rac-dependent manner in N1E-115 neuroblastoma cells. Disruption of Cdc42-PAR-6 or PAR-3-STEF binding inhibited Cdc42-induced lamellipodia but not filopodia. The isolated STEF-binding PAR-3 fragment was sufficient to induce lamellipodia independently of Cdc42 and PAR-6. PAR-3 is required for Cdc42-induced Rac activation, but is not essential for lamellipodia formation itself. In cultured hippocampal neurons, STEF accumulated at the tip of the growing axon and colocalized with PAR-3. The spatio-temporal activation and signalling of Cdc42-PAR-6-PAR-3-STEF/Tiam1-Rac seem to be involved in neurite growth and axon specification. We propose that the PAR-6-PAR-3 complex mediates Cdc42-induced Rac activation by means of STEF/Tiam1, and that this process seems to be required for the establishment of neuronal polarity.

PIP3 is involved in neuronal polarization and axon formation

Recent experiments in various cell types such as mammalian neutrophils and Dictyostelium discoideum amoebae point to a key role for the lipid product of PI 3-kinase, PIP(3), in determining internal polarity. In neurons, as a consequence of the elongation of one neurite, the axon is specified and the cell acquires its polarity. To test the hypothesis that PI 3-kinase and PIP(3) may play a role in neuronal polarity, and especially in axon specification, we observed localization of PIP(3) visualized by Akt-PH-GFP in developing hippocampal neurons. We found that PIP(3) accumulates in the tip of the growing processes. This accumulation is inhibited by addition of PI 3-kinase inhibitors. Those inhibitors, consistently with a role of PIP(3) in process formation and elongation, delay the transition from stage 1 neurons to stage 3 neurons, and both axon formation and elongation. Moreover, when the immature neurite contacts a bead coated with laminin, a substrate known to induce axon specification, PIP(3) accumulates in its growth cone followed by a rapid elongation of the neurite. In such conditions, the addition of PI 3-kinase inhibitors inhibits both PIP(3) accumulation and future axon elongation. These results suggest that PIP(3) is involved in axon specification, possibly by stimulating neurite outgrowth. In addition, when a second neurite contacted the beads, this neurite rapidly elongates whereas the elongation of the first laminin-contacting neurite stops, consistently with the hypothesis of a negative feedback mechanism from the growing future axon to the other neurites.

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.