Overexpression of a neural-specific rho family GTPase, cRac1B, selectively induces enhanced neuritogenesis and neurite branching in primary neurons

Gain-of-function p.F28S variant in RAC3 disrupts neuronal differentiation, migration and axonogenesis during cortical development, leading to neurodevelopmental disorder

BACKGROUND

RAC3 encodes a Rho family small GTPase that regulates the behaviour and organisation of actin cytoskeleton and intracellular signal transduction. Variants in RAC3 can cause a phenotypically heterogeneous neurodevelopmental disorder with structural brain anomalies and dysmorphic facies. The pathomechanism of this recently discovered genetic disorder remains unclear.

METHODS

We investigated an early adolescent female with intellectual disability, drug-responsive epilepsy and white matter abnormalities. Through exome sequencing, we identified the novel de novo variant (NM_005052.3): c.83T>C (p.Phe28Ser) in RAC3. We then examined the pathophysiological significance of the p.F28S variant in comparison with the recently reported disease-causing p.Q61L variant, which results in a constitutively activated version of RAC3.

RESULTS

In vitro analyses revealed that the p.F28S variant was spontaneously activated by substantially increased intrinsic GTP/GDP-exchange activity and bound to downstream effectors tested, such as PAK1 and MLK2. The variant suppressed the differentiation of primary cultured hippocampal neurons and caused cell rounding with lamellipodia. In vivo analyses using in utero electroporation showed that acute expression of the p.F28S variant caused migration defects of excitatory neurons and axon growth delay during corticogenesis. Notably, defective migration was rescued by a dominant negative version of PAK1 but not MLK2.

CONCLUSION

Our results indicate that RAC3 is critical for brain development and the p.F28S variant causes morphological and functional defects in cortical neurons, likely due to the hyperactivation of PAK1.

Variant-specific changes in RAC3 function disrupt corticogenesis in neurodevelopmental phenotypes

Variants in RAC3, encoding a small GTPase RAC3 which is critical for the regulation of actin cytoskeleton and intracellular signal transduction, are associated with a rare neurodevelopmental disorder with structural brain anomalies and facial dysmorphism. We investigated a cohort of 10 unrelated participants presenting with global psychomotor delay, hypotonia, behavioural disturbances, stereotyped movements, dysmorphic features, seizures and musculoskeletal abnormalities. MRI of brain revealed a complex pattern of variable brain malformations, including callosal abnormalities, white matter thinning, grey matter heterotopia, polymicrogyria/dysgyria, brainstem anomalies and cerebellar dysplasia. These patients harboured eight distinct de novo RAC3 variants, including six novel variants (NM_005052.3): c.34G > C p.G12R, c.179G > A p.G60D, c.186_188delGGA p.E62del, c.187G > A p.D63N, c.191A > G p.Y64C and c.348G > C p.K116N. We then examined the pathophysiological significance of these novel and previously reported pathogenic variants p.P29L, p.P34R, p.A59G, p.Q61L and p.E62K. In vitro analyses revealed that all tested RAC3 variants were biochemically and biologically active to variable extent, and exhibited a spectrum of different affinities to downstream effectors including p21-activated kinase 1. We then focused on the four variants p.Q61L, p.E62del, p.D63N and p.Y64C in the Switch II region, which is essential for the biochemical activity of small GTPases and also a variation hot spot common to other Rho family genes, RAC1 and CDC42. Acute expression of the four variants in embryonic mouse brain using in utero electroporation caused defects in cortical neuron morphology and migration ending up with cluster formation during corticogenesis. Notably, defective migration by p.E62del, p.D63N and p.Y64C were rescued by a dominant negative version of p21-activated kinase 1. Our results indicate that RAC3 variants result in morphological and functional defects in cortical neurons during brain development through variant-specific mechanisms, eventually leading to heterogeneous neurodevelopmental phenotypes.

Pathophysiological Mechanisms in Neurodevelopmental Disorders Caused by Rac GTPases Dysregulation: What's behind Neuro-RACopathies

Rho family guanosine triphosphatases (GTPases) regulate cellular signaling and cytoskeletal dynamics, playing a pivotal role in cell adhesion, migration, and cell cycle progression. The Rac subfamily of Rho GTPases consists of three highly homologous proteins, Rac 1-3. The proper function of Rac1 and Rac3, and their correct interaction with guanine nucleotide-exchange factors (GEFs) and GTPase-activating proteins (GAPs) are crucial for neural development. Pathogenic variants affecting these delicate biological processes are implicated in different medical conditions in humans, primarily neurodevelopmental disorders (NDDs). In addition to a direct deleterious effect produced by genetic variants in the RAC genes, a dysregulated GTPase activity resulting from an abnormal function of GEFs and GAPs has been involved in the pathogenesis of distinctive emerging conditions. In this study, we reviewed the current pertinent literature on Rac-related disorders with a primary neurological involvement, providing an overview of the current knowledge on the pathophysiological mechanisms involved in the neuro-RACopathies.

Expression analyses of Rac3, a Rho family small GTPase, during mouse brain development

Rac3 is a member of Rho family small GTPases which regulates cellular signaling and cytoskeletal dynamics. The RAC3 gene abnormalities have been shown to cause neurodevelopmental disorders with structural brain anomalies, including polymicrogyria/dysgyria, callosal abnormalities, brainstem anomalies, and cerebellar dysplasia. Although this evidence indicates that Rac3 is essential in brain development, not only its molecular mechanism but also the expression profile is yet to be elucidated. In this study, we carried out expression analyses of Rac3 with mouse brain tissues. In immunoblotting, Rac3 exhibited a tissue-dependent expression profile in the young adult mouse and was expressed in a developmental stage-dependent manner in brain. In primary cultured hippocampal neurons, while Rac3 was distributed mainly in the cytoplasm, it was visualized in axon and dendrites with partial localization at synapses, in consistent with the observation in biochemical fractionation analyses. In immunofluorescence analyses with brain slices, Rac3 was distributed strongly and moderately in the axon and cytoplasm, respectively, of cerebral cortex at postnatal day (P) 2 and P18. Similar distribution profile was also observed in hippocampus. Taken together, the results obtained strongly suggest that Rac3 plays an important physiological role in neuronal tissues during corticogenesis, and defects in the Rac3 function induce structural brain anomalies leading to pathogenesis of neurodevelopmental disorders.

Gangliosides in the differentiation process of primary neurons: the specific role of GM1-oligosaccharide

It has been recently reported by our group that GM1-oligosaccharide added to neuroblastoma cells or administered to mouse experimental model mimics the neurotrophic and neuroprotective properties of GM1 ganglioside. In addition to this, differently from GM1, GM1-oligosaccharide is not taken up by the cells, remaining solubilized into the extracellular environment interacting with cell surface proteins. Those characteristics make GM1-oligosaccharide a good tool to study the properties of the endogenous GM1, avoiding to interfere with the ganglioside natural metabolic pathway. In this study, we show that GM1-oligosaccharide administered to mice cerebellar granule neurons by interacting with cell surface induces TrkA-MAP kinase pathway activation enhancing neuron clustering, arborization and networking. Accordingly, in the presence of GM1-oligosaccharide, neurons show a higher phosphorylation rate of FAK and Src proteins, the intracellular key regulators of neuronal motility. Moreover, treated cells express increased level of specific neuronal markers, suggesting an advanced stage of maturation compared to controls. In parallel, we found that in the presence of GM1-oligosaccharide, neurons accelerate the expression of complex gangliosides and reduce the level of the simplest ones, displaying the typical ganglioside pattern of mature neurons. Our data confirms the specific role of GM1 in neuronal differentiation and maturation, determined by its oligosaccharide portion. GM1-oligosacchairide interaction with cell surface receptors triggers the activation of intracellular biochemical pathways responsible for neuronal migration, dendrites emission and axon growth.

The Rac3 GTPase in Neuronal Development, Neurodevelopmental Disorders, and Cancer

Rho family small guanosine triphosphatases (GTPases) are important regulators of the cytoskeleton, and are critical in many aspects of cellular and developmental biology, as well as in pathological processes such as intellectual disability and cancer. Of the three members of the family, Rac3 has a more restricted expression in normal tissues compared to the ubiquitous member of the family, Rac1. The Rac3 polypeptide is highly similar to Rac1, and orthologues of the gene for Rac3 have been found only in vertebrates, indicating the late appearance of this gene during evolution. Increasing evidence over the past few years indicates that Rac3 plays an important role in neuronal development and in tumor progression, with specificities that distinguish the functions of Rac3 from the established functions of Rac1 in these processes. Here, results highlighting the importance of Rac3 in distinct aspects of neuronal development and tumor cell biology are presented, in support of the non-redundant role of different members of the two Rac GTPases in physiological and pathological processes.

Rac1 and Rac3 GTPases differently influence the morphological maturation of dendritic spines in hippocampal neurons

The Rac1 and Rac3 GTPases are co-expressed in the developing nervous system, where they are involved in different aspects of neuronal development, including the formation of synapses. The deletion of both Rac genes determines a stronger reduction of dendritic spines in vitro compared to the knockout of either gene, indicating that Rac1 and Rac3 play a synergistic role in the formation of these structures. Here, we have addressed the role of each GTPase in the formation of dendritic spines by overexpressing either Rac1 or Rac3 in wildtype neurons, or by re-expressing either GTPase in double knockout hippocampal cultures. We show that the Rac3 protein is expressed with Rac1 in developing hippocampal neurons. Overexpression of either GTPase in WT neurons increases the density of dendritic spines, suggesting the involvement of both GTPases in their formation. We also found that the re-expression of either Rac1 or Rac3 in double knockout neurons is sufficient to restore spinogenesis. Rac1 is significantly more efficient than Rac3 in restoring the formation of spines. On the other hand the quantitative analysis in neurons overexpressing or re-expressing either GTPase shows that Rac3 induces a more pronounced increase in the size of the spines compared to Rac1. These enlarged spines form morphological synapses identified by the juxtaposition of postsynaptic and presynaptic markers. Thus, while Rac1 appears more efficient in inducing the formation of mature spines, Rac3 is more efficient in promoting their enlargement. Our study highlights specific roles of Rac1 and Rac3, which may be functionally relevant also to synaptic plasticity.

RhoA Signaling and Synaptic Damage Occur Within Hours in a Live Pig Model of CNS Injury, Retinal Detachment

PURPOSE

The RhoA pathway is activated after retinal injury. However, the time of onset and consequences of activation are unknown in vivo. Based on in vitro studies we focused on a period 2 hours after retinal detachment, in pig, an animal whose retina is holangiotic and contains cones.

METHODS

Under anesthesia, retinal detachments were created by subretinal injection of a balanced salt solution. Two hours later, animals were sacrificed and enucleated for GTPase activity assays and quantitative Western blot and confocal microscopy analyses.

RESULTS

RhoA activity with detachment was increased 1.5-fold compared to that in normal eyes or in eyes that had undergone vitrectomy only. Increased phosphorylation of myosin light chain, a RhoA effector, also occurred. By 2 hours, rod cells had retracted their terminals toward their cell bodies, disrupting the photoreceptor-to-bipolar synapse and producing significant numbers of spherules with SV2 immunolabel in the outer nuclear layer of the retina. In eyes with detachment, distant retina that remained attached also showed significant increases in RhoA activity and synaptic disjunction. Increases in RAC1 activity and glial fibrillary acidic protein (GFAP) were not specific for detachment, and sprouting of bipolar dendrites, reported for longer detachments, was not seen. The RhoA kinase inhibitor Y27632 significantly reduced axonal retraction by rod cells.

CONCLUSIONS

Activation of the RhoA pathway occurs quickly after injury and promotes synaptic damage that can be controlled by RhoA kinase inhibition. We suggest that retinal detachment joins the list of central nervous system injuries, such as stroke and spinal cord injury, that should be considered for rapid therapeutic intervention.

Zebrafish diras1 Promoted Neurite Outgrowth in Neuro-2a Cells and Maintained Trigeminal Ganglion Neurons In Vivo via Rac1-Dependent Pathway

The small GTPase Ras superfamily regulates several neuronal functions including neurite outgrowth and neuron proliferation. In this study, zebrafish diras1a and diras1b were identified and were found to be mainly expressed in the central nervous system and dorsal neuron ganglion. Overexpression of green fluorescent protein (GFP)-diras1a or GFP-diras1b triggered neurite outgrowth of Neuro-2a cells. The wild types, but not the C terminus truncated forms, of diras1a and diras1b elevated the protein level of Ras-related C3 botulinum toxin substrate 1 (Rac1) and downregulated Ras homologous member A (RhoA) expression. Glutathione S-transferase (GST) pull-down assay also revealed that diras1a and diras1b enhanced Rac1 activity. Interfering with Rac1, Pak1, or cyclin-dependent kinase 5 (CDK5) activity or with the Arp2/3 inhibitor prevented diras1a and diras1b from mediating the neurite outgrowth effects. In the zebrafish model, knockdown of diras1a and/or diras1b by morpholino antisense oligonucleotides not only reduced axon guidance but also caused the loss of trigeminal ganglion without affecting the precursor markers, such as ngn1 and neuroD. Co-injection with messenger RNA (mRNA) derived from mouse diras1 or constitutively active human Rac1 restored the population of trigeminal ganglion. In conclusion, we provided preliminary evidence that diras1 is involved in neurite outgrowth and maintains the number of trigeminal ganglions through the Rac1-dependent pathway.

Loss of Either Rac1 or Rac3 GTPase Differentially Affects the Behavior of Mutant Mice and the Development of Functional GABAergic Networks

Rac GTPases regulate the development of cortical/hippocampal GABAergic interneurons by affecting the early development and migration of GABAergic precursors. We have addressed the function of Rac1 and Rac3 proteins during the late maturation of hippocampal interneurons. We observed specific phenotypic differences between conditional Rac1 and full Rac3 knockout mice. Rac1 deletion caused greater generalized hyperactivity and cognitive impairment compared with Rac3 deletion. This phenotype matched with a more evident functional impairment of the inhibitory circuits in Rac1 mutants, showing higher excitability and reduced spontaneous inhibitory currents in the CA hippocampal pyramidal neurons. Morphological analysis confirmed a differential modification of the inhibitory circuits: deletion of either Rac caused a similar reduction of parvalbumin-positive inhibitory terminals in the pyramidal layer. Intriguingly, cannabinoid receptor-1-positive terminals were strongly increased only in the CA1 of Rac1-depleted mice. This increase may underlie the stronger electrophysiological defects in this mutant. Accordingly, incubation with an antagonist for cannabinoid receptors partially rescued the reduction of spontaneous inhibitory currents in the pyramidal cells of Rac1 mutants. Our results show that Rac1 and Rac3 have independent roles in the formation of GABAergic circuits, as highlighted by the differential effects of their deletion on the late maturation of specific populations of interneurons.

Roles of Rac1 and Rac3 GTPases during the development of cortical and hippocampal GABAergic interneurons

Rac GTPases are regulators of the cytoskeleton that play an important role in several aspects of neuronal and brain development. Two distinct Rac GTPases are expressed in the developing nervous system, the widely expressed Rac1 and the neural-specific Rac3 proteins. Recent experimental evidence supports a central role of these two Rac proteins in the development of inhibitory GABAergic interneurons, important modulatory elements of the brain circuitry. The combined inactivation of the genes for the two Rac proteins has profound effects on distinct aspects of interneuron development, and has highlighted a synergistic contribution of the two proteins to the postmitotic maturation of specific populations of cortical and hippocampal interneurons. Rac function is modulated by different types of regulators, and can influence the activity of specific effectors. Some of these proteins have been associated to the development and maturation of interneurons. Cortical interneuron dysfunction is implicated in several neurological and psychiatric diseases characterized by cognitive impairment. Therefore the description of the cellular processes regulated by the Rac GTPases, and the identification of the molecular networks underlying these processes during interneuron development is relevant to the understanding of the role of GABAergic interneurons in cognitive functions.

Cytoskeletal and signaling mechanisms of neurite formation

The formation of a neurite, the basis for axons and dendrites, begins with the concerted accumulation and organization of actin and microtubules. Whereas much is known about the proteins that play a role in these processes, because they perform similar functions in axon branching and filopodia formation, much remains to be discovered concerning the interaction of these individual cytoskeletal regulators during neurite formation. Here, we review the literature regarding various models of filopodial formation and the way in which proteins that control actin organization and polymerization induce neurite formation. Although several different regulators of actin polymerization are involved in neurite initiation, redundancy occurs between these regulators, as the effects of the loss of a single regulator can be mitigated by the addition of neurite-promoting substrates and proteins. Similar to actin dynamics, both microtubule stabilizing and destabilizing proteins play a role in neurite initiation. Furthermore, interactions between the actin and microtubule cytoskeleton are required for neurite formation. Several lines of evidence indicate that the interactions between these two components of the cytoskeleton are needed for force generation and for the localization of microtubules at sites of nascent neurites. The general theme that emerges is the existence of several central regulatory pathways on which extracellular cues converge to control and organize both actin and microtubules to induce the formation of neurites.

Convergence of genes and cellular pathways dysregulated in autism spectrum disorders

Rare copy-number variation (CNV) is an important source of risk for autism spectrum disorders (ASDs). We analyzed 2,446 ASD-affected families and confirmed an excess of genic deletions and duplications in affected versus control groups (1.41-fold, p = 1.0 × 10⁻⁵) and an increase in affected subjects carrying exonic pathogenic CNVs overlapping known loci associated with dominant or X-linked ASD and intellectual disability (odds ratio = 12.62, p = 2.7 × 10⁻¹⁵, ∼3% of ASD subjects). Pathogenic CNVs, often showing variable expressivity, included rare de novo and inherited events at 36 loci, implicating ASD-associated genes (CHD2, HDAC4, and GDI1) previously linked to other neurodevelopmental disorders, as well as other genes such as SETD5, MIR137, and HDAC9. Consistent with hypothesized gender-specific modulators, females with ASD were more likely to have highly penetrant CNVs (p = 0.017) and were also overrepresented among subjects with fragile X syndrome protein targets (p = 0.02). Genes affected by de novo CNVs and/or loss-of-function single-nucleotide variants converged on networks related to neuronal signaling and development, synapse function, and chromatin regulation.

Rac-GTPases Regulate Microtubule Stability and Axon Growth of Cortical GABAergic Interneurons

Cortical interneurons are characterized by extraordinary functional and morphological diversity. Although tremendous progress has been made in uncovering molecular and cellular mechanisms implicated in interneuron generation and function, several questions still remain open. Rho-GTPases have been implicated as intracellular mediators of numerous developmental processes such as cytoskeleton organization, vesicle trafficking, transcription, cell cycle progression, and apoptosis. Specifically in cortical interneurons, we have recently shown a cell-autonomous and stage-specific requirement for Rac1 activity within proliferating interneuron precursors. Conditional ablation of Rac1 in the medial ganglionic eminence leads to a 50% reduction of GABAergic interneurons in the postnatal cortex. Here we examine the additional role of Rac3 by analyzing Rac1/Rac3 double-mutant mice. We show that in the absence of both Rac proteins, the embryonic migration of medial ganglionic eminence-derived interneurons is further impaired. Postnatally, double-mutant mice display a dramatic loss of cortical interneurons. In addition, Rac1/Rac3-deficient interneurons show gross cytoskeletal defects in vitro, with the length of their leading processes significantly reduced and a clear multipolar morphology. We propose that in the absence of Rac1/Rac3, cortical interneurons fail to migrate tangentially towards the pallium due to defects in actin and microtubule cytoskeletal dynamics.

Involvement of Rho-family GTPases in axon branching

Development of the nervous system requires efficient extension and guidance of axons and dendrites culminating in synapse formation. Axonal growth and navigation during embryogenesis are controlled by extracellular cues. Many of the same extracellular signals also regulate axonal branching. The emergence of collateral branches from the axon augments the complexity of nervous system innervation and provides an additional mechanism for target selection. Rho-family GTPases play an important role in regulating intracellular cytoskeletal and signaling pathways that facilitate axonal morphological changes. RhoA/G and Rac1 GTPase functions are complex and they can induce or inhibit branch formation, depending on neuronal type, cell context or signaling mechanisms. Evidence of a role of Cdc42 in axon branching is mostly lacking. In contrast, Rac3 has thus far been implicated in the regulation of axon branching. Future analysis of the upstream regulators and downstream effectors mediating the effects of Rho-family GTPase will provide insights into the cellular processes effected, and shed light on the sometimes opposing roles of these GTPases in the regulation of axon branching.

Next-generation sequencing analysis of gene regulation in the rat model of retinopathy of prematurity

PURPOSE

The purpose of this study was to identify the genes, biochemical signaling pathways, and biological themes involved in the pathogenesis of retinopathy of prematurity (ROP).

METHODS

Next-generation sequencing (NGS) was performed on the RNA transcriptome of rats with the Penn et al. (Pediatr Res 36:724-731, 1994) oxygen-induced retinopathy model of ROP at the height of vascular abnormality, postnatal day (P) 19, and normalized to age-matched, room-air-reared littermate controls. Eight custom-developed pathways with potential relevance to known ROP sequelae were evaluated for significant regulation in ROP: The three major Wnt signaling pathways, canonical, planar cell polarity (PCP), and Wnt/Ca(2+); two signaling pathways mediated by the Rho GTPases RhoA and Cdc42, which are, respectively, thought to intersect with canonical and non-canonical Wnt signaling; nitric oxide signaling pathways mediated by two nitric oxide synthase (NOS) enzymes, neuronal (nNOS) and endothelial (eNOS); and the retinoic acid (RA) signaling pathway. Regulation of other biological pathways and themes was detected by gene ontology using the Kyoto Encyclopedia of Genes and Genomes and the NIH's Database for Annotation, Visualization, and Integrated Discovery's GO terms databases.

RESULTS

Canonical Wnt signaling was found to be regulated, but the non-canonical PCP and Wnt/Ca(2+) pathways were not. Nitric oxide signaling, as measured by the activation of nNOS and eNOS, was also regulated, as was RA signaling. Biological themes related to protein translation (ribosomes), neural signaling, inflammation and immunity, cell cycle, and cell death were (among others) highly regulated in ROP rats.

CONCLUSIONS

These several genes and pathways identified by NGS might provide novel targets for intervention in ROP.

Rac1 and rac3 GTPases control synergistically the development of cortical and hippocampal GABAergic interneurons

The intracellular mechanisms driving postmitotic development of cortical γ-aminobutyric acid (GABA)ergic interneurons are poorly understood. We have addressed the function of Rac GTPases in cortical and hippocampal interneuron development. Developing neurons express both Rac1 and Rac3. Previous work has shown that Rac1 ablation does not affect the development of migrating cortical interneurons. Analysis of mice with double deletion of Rac1 and Rac3 shows that these GTPases are required during postmitotic interneuron development. The number of parvalbumin-positive cells was affected in the hippocampus and cortex of double knockout mice. Rac depletion also influences the maturation of interneurons that reach their destination, with reduction of inhibitory synapses in both hippocampal CA1 and cortical pyramidal cells. The decreased number of cortical migrating interneurons and their altered morphology indicate a role of Rac1 and Rac3 in regulating the motility of cortical interneurons, thus interfering with their final localization. While electrophysiological passive and active properties of pyramidal neurons including membrane capacity, resting potential, and spike amplitude and duration were normal, these cells showed reduced spontaneous inhibitory currents and increased excitability. Our results show that Rac1 and Rac3 contribute synergistically to postmitotic development of specific populations of GABAergic cells, suggesting that these proteins regulate their migration and differentiation.

Rho-independent stimulation of axon outgrowth and activation of the ERK and Akt signaling pathways by C3 transferase in sensory neurons

Peripheral nerve injury triggers the activation of RhoA in spinal motor and peripheral sensory neurons. RhoA activates a number of effector proteins including the Rho-associated kinase, ROCK, which targets the cytoskeleton and leads to inhibition of neurite outgrowth. Blockade of the Rho/ROCK pathway by pharmacological means improves axon regeneration after experimental injury. C3_bot transferase, an exoenzyme produced by Clostridium botulinum, inactivates RhoA by ADP-ribosylation. It has been successfully applied in experimental CNS lesions to facilitate axon regeneration. Up to now it was not investigated thoroughly whether C3_bot exerts positive effects on peripheral axon regeneration as well. In the present study, recombinant membrane permeable C3_bot produced a small, but significant, axon outgrowth effect on peripheral sensory neurons dissociated from adult dorsal root ganglia (DRG) of the rat. Neuronal overexpression of C3, however, did not enhance axonal growth. Moreover, transfection of plasmids encoding dominant negative RhoA or RhoA specific shRNAs failed to increase axonal growth. Furthermore, we show that the C3_bot mutant, C3_E174Q, which lacks RhoA inhibitory activity, still stimulates axonal growth. When analyzing possible signaling mechanisms we found that extracellular signal-regulated kinase (ERK) and Akt are activated by C3_bot and ERK is induced by the C3_E174Q mutant. Upregulation of kinase activities by C3_bot occurs significantly faster than inactivation of RhoA indicating a RhoA-independent pathway of action by C3_bot. The induction of ERK signaling by C3_bot was detected in embryonic hippocampal neurons, too. Taken together, although RhoA plays a central role for inhibition of axon outgrowth by myelin-derived inhibitors, it does not interfere with axonal growth of sensory neurons on a permissive substrate in vitro. C3_bot blocks neuronal RhoA activity, but its positive effects on axon elongation and branching appear to be mediated by Rho independent mechanisms involving activation of axon growth promoting ERK and Akt kinases.

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.

Minimal essential length of Clostridium botulinum C3 peptides to enhance neuronal regenerative growth and connectivity in a non-enzymatic mode

C3 ADP-ribosyltransferase is a valuable tool to study Rho-dependent cellular processes. In the current study we investigated the impact of enzyme-deficient peptides derived from Clostridium botulinum C3 transferase in the context of neuronal process elongation and branching, synaptic connectivity, and putative beneficial effects on functional outcome following traumatic injury to the CNS. By screening a range of peptidic fragments, we identified three short peptides from C3bot that promoted axon and dendrite outgrowth in cultivated hippocampal neurons. Furthermore, one of these fragments, a 26-amino acid peptide covering the residues 156-181 enhanced synaptic connectivity in primary hippocampal culture. This peptide was also effective to foster axon outgrowth and re-innervation in organotypical brain slice culture. To evaluate the potential of the 26mer to foster repair mechanisms after CNS injury we applied this peptide to mice subjected to spinal cord injury by either compression impact or hemisection. A single local administration at the site of the lesion improved locomotor recovery. In addition, histological analysis revealed an increased serotonergic input to lumbar motoneurons in treated compared with control mice. Pull-down assays showed that lesion-induced up-regulation of RhoA activity within the spinal cord was largely blocked by C3bot peptides despite the lack of enzymatic activity.

Normal levels of Rac1 are important for dendritic but not axonal development in hippocampal neurons

BACKGROUND INFORMATION

Rho family GTPases are required for cytoskeletal reorganization and are considered important for the maturation of neurons. Among these proteins, Rac1 is known to play a crucial role in the regulation of actin dynamics, and a number of studies indicate the involvement of this protein in different steps of vertebrate neuronal maturation. There are two distinct Rac proteins expressed in neurons, namely the ubiquitous Rac1 and the neuron-specific Rac3. The specific functions of each of these GTPases during early neuronal development are largely unknown.

RESULTS

The combination of the knockout of Rac3 with Rac1 down-regulation by siRNA (small interfering RNA) has been used to show that down-regulation of Rac1 affects dendritic development in mouse hippocampal neurons, without affecting axons. F-actin levels are strongly decreased in neuronal growth cones following down-regulation of Rac1, and time-lapse analysis indicated that the reduction of Rac1 levels decreases growth-cone dynamics.

CONCLUSIONS

These results show that normal levels of endogenous Rac1 activity are critical for early dendritic development, whereas dendritic outgrowth is not affected in hippocampal neurons from Rac3-null mice. On the other hand, early axonal development appears normal after Rac1 down-regulation. Our findings also suggest that the initial establishment of neuronal polarity is not affected by Rac1 down-regulation.

Targeting and localized signalling by small GTPases

Polarized cellular responses, for example, cell migration, require the co-ordinated assembly of signalling complexes at a particular subcellular location, such as the leading edge of cells. Small GTPases of the Ras superfamily play central roles in many (polarized) responses to growth factors, chemokines or integrin ligands. These small GTPases are functionally distinct, yet remarkably homologous in their primary sequence and especially in their effector domains. Therefore it has long been unclear how GTPase signalling specificity is regulated. Small GTPases carry a lipid anchor, in the context of a hypervariable region, which mediates membrane association. However, whereas the lipid has long been proposed to be the critical regulator of subcellular GTPase targeting, there is now increasing evidence that specific protein-protein interactions are important as well. This review discusses recent findings on GTPase targeting and proposes a revised model for GTPase signalling. In this model, the hypervariable domain acts in conjunction with the lipid tail to target the GTPase to specific membrane-associated protein complexes. Here, local GTPase activation occurs, leading to subsequent exposure of the effector domain, binding to effector proteins and the initiation of downstream signalling.

The cytoskeletal and signaling mechanisms of axon collateral branching

During development, axons are guided to their appropriate targets by a variety of guidance factors. On arriving at their synaptic targets, or while en route, axons form branches. Branches generated de novo from the main axon are termed collateral branches. The generation of axon collateral branches allows individual neurons to make contacts with multiple neurons within a target and with multiple targets. In the adult nervous system, the formation of axon collateral branches is associated with injury and disease states and may contribute to normally occurring plasticity. Collateral branches are initiated by actin filament– based axonal protrusions that subsequently become invaded by microtubules, thereby allowing the branch to mature and continue extending. This article reviews the current knowledge of the cellular mechanisms of the formation of axon collateral branches. The major conclusions of this review are (1) the mechanisms of axon extension and branching are not identical; (2) active suppression of protrusive activity along the axon negatively regulates branching; (3) the earliest steps in the formation of axon branches involve focal activation of signaling pathways within axons, which in turn drive the formation of actin-based protrusions; and (4) regulation of the microtubule array by microtubule-associated and severing proteins underlies the development of branches. Linking the activation of signaling pathways to specific proteins that directly regulate the axonal cytoskeleton underlying the formation of collateral branches remains a frontier in the field.

Rac1 and Rac3 GTPases regulate the development of hilar mossy cells by affecting the migration of their precursors to the hilus

We have previously shown that double deletion of the genes for Rac1 and Rac3 GTPases during neuronal development affects late developmental events that perturb the circuitry of the hippocampus, with ensuing epileptic phenotype. These effects include a defect in mossy cells, the major class of excitatory neurons of the hilus. Here, we have addressed the mechanisms that affect the loss of hilar mossy cells in the dorsal hippocampus of mice depleted of the two Rac GTPases. Quantification showed that the loss of mossy cells was evident already at postnatal day 8, soon after these cells become identifiable by a specific marker in the dorsal hilus. Comparative analysis of the hilar region from control and double mutant mice revealed that synaptogenesis was affected in the double mutants, with strongly reduced presynaptic input from dentate granule cells. We found that apoptosis was equally low in the hippocampus of both control and double knockout mice. Labelling with bromodeoxyuridine at embryonic day 12.5 showed no evident difference in the proliferation of neuronal precursors in the hippocampal primordium, while differences in the number of bromodeoxyuridine-labelled cells in the developing hilus revealed a defect in the migration of immature, developing mossy cells in the brain of double knockout mice. Overall, our data show that Rac1 and Rac3 GTPases participate in the normal development of hilar mossy cells, and indicate that they are involved in the regulation of the migration of the mossy cell precursor by preventing their arrival to the dorsal hilus.

Rho GTPases as regulators of morphological neuroplasticity

GTPases function as intracellular, bimolecular switches by adopting different conformational states in response to binding GDP or GTP. Their activation is mediated through cell-surface receptors. Rho GTPases act on several downstream effectors involved in cellular morphogenesis, cell polarity, migration and cell division. In neurons, Rho GTPases regulate various features of dendritic and axonal outgrowth during development and regeneration mainly through their effects on the cytoskeleton. This review summarizes the main functions of Rho, Rac and Cdc42 GTPases as key regulators of morphological neuroplasticity under normal and pathological conditions.

Rho and Ras GTPases in axon growth, guidance, and branching

The establishment of precise neuronal cell morphology provides the foundation for all aspects of neurobiology. During development, axons emerge from cell bodies after an initial polarization stage, elongate, and navigate towards target regions guided by a range of environmental cues. The Rho and Ras families of small GTPases have emerged as critical players at all stages of axonogenesis. Their ability to coordinately direct multiple signal transduction pathways with precise spatial control drives many of the activities that underlie this morphogenetic program: the dynamic assembly, disassembly, and reorganization of the actin and microtubule cytoskeletons, the interaction of the growing axon with other cells and extracellular matrix, the delivery of lipids and proteins to the axon through the exocytic machinery, and the internalization of membrane and proteins at the leading edge of the growth cone through endocytosis. This article highlights the contribution of Rho and Ras GTPases to axonogenesis.

Rac1 regulates neuronal polarization through the WAVE complex

Neuronal migration and axon growth, key events during neuronal development, require distinct changes in the cytoskeleton. Although many molecular regulators of polarity have been identified and characterized, relatively little is known about their physiological role in this process. To study the physiological function of Rac1 in neuronal development, we have generated a conditional knock-out mouse, in which Rac1 is ablated in the whole brain. Rac1-deficient cerebellar granule neurons, which do not express other Rac isoforms, showed impaired neuronal migration and axon formation both in vivo and in vitro. In addition, Rac1 ablation disrupts lamellipodia formation in growth cones. The analysis of Rac1 effectors revealed the absence of the Wiskott-Aldrich syndrome protein (WASP) family verprolin-homologous protein (WAVE) complex from the plasma membrane of knock-out growth cones. Loss of WAVE function inhibited axon growth, whereas overexpression of a membrane-tethered WAVE mutant partially rescued axon growth in Rac1-knock-out neurons. In addition, pharmacological inhibition of the WAVE complex effector Arp2/3 also reduced axon growth. We propose that Rac1 recruits the WAVE complex to the plasma membrane to enable actin remodeling necessary for axon growth.

Balanced Vav2 GEF activity regulates neurite outgrowth and branching in vitro and in vivo

We have investigated the role of Vav2, a reported Rac1/Cdc42 GEF, on the development of Xenopus spinal neurons in vitro and in vivo. Both gain and loss of Vav2 function inhibited the rate neurite extension on laminin (LN), while only GFP-Vav2 over-expression enhanced process formation and branching. Vav2 over-expression protected neurons from RhoA-mediated growth cone collapse, similar to constitutively active Rac1, suggesting that Vav2 activates Rac1 in spinal neurons. Enhanced branching on LN required both Vav2 GEF activity and N-terminal tyrosine residues, but protection from RhoA-mediated collapse only required GEF activity. Interestingly, wild-type spinal neurons exhibited increased branching on the cell adhesion molecule L1, which required Vav2 GEF function, but not N-terminal tyrosine residues. Finally, we find that Vav2 differentially affects the Rohon-Beard peripheral and central process extension but promotes neurite branching of commissural interneurons near the ventral midline. Together, we suggest that balanced Vav2 activity is necessary for optimal neurite outgrowth and promotes branching by targeting GEF activity to branch points.

Essential role of Rac1 and Rac3 GTPases in neuronal development

Rac GTPases are members of the Rho family regulating the actin cytoskeleton and implicated in neuronal development. Ubiquitous Rac1 and neuron-specific Rac3 GTPases are coexpressed in the developing mammalian brain. We used Cre-mediated conditional deletion of Rac1 in neurons combined with knockout of neuron-specific Rac3 to study the role of these GTPases in neural development. We found that lack of both genes causes motor behavioral defects, epilepsy, and premature death of mice. Deletion of either GTPase does not produce evident phenotypes. Double-knockout mice show specific defects in the development of the hippocampus. Selective impairment of the dorsal hilus of double-knockout animals is associated with alteration in the formation of the hippocampal circuitry. Axonal pathways to and from the dorsal hilus are affected because of the deficit of hilar mossy cells. Moreover, analysis of Rac function in hippocampal cultures shows that spine formation is strongly hampered only in neurons lacking both Rac proteins. These findings show for the first time that both Rac1 and Rac3 are important for the development of the nervous system, wherein they play complementary roles during late stages of neuronal and brain development.

Identification of a developmentally regulated pathway of membrane retrieval in neuronal growth cones

The growth-cone plasma membrane constantly reconfigures during axon navigation and upon target recognition. The identity and regulation of the membrane pathway(s) participating in remodeling of the growth-cone surface remain elusive. Here, we identify a constitutive, high-capacity plasma-membrane-recycling activity in the axonal growth cones, which is mediated by a novel bulk endocytic pathway that is mechanistically related to macropinocytosis. This pathway generates large compartments at sites of intense actin-based membrane ruffling through the actions of phosphatidylinositol 3-kinase, the small GTPase Rac1 and the pinocytic chaperone Pincher. At early developmental stages, bulk endocytosis is the primary endocytic pathway for rapid retrieval of the growth-cone plasma membrane. At later stages, during the onset of synaptogenesis, an intrinsic program of maturation leads to downregulation of basal bulk endocytosis and the emergence of depolarization-induced synaptic-vesicle exo-endocytosis. We propose that the control of bulk membrane retrieval contributes to the homeostatic regulation of the axonal plasma membrane and to growth-cone remodeling during axonal outgrowth. In addition, we suggest that the downregulation of bulk endocytosis during synaptogenesis might contribute to the preservation of synaptic-vesicle specificity.

Mammalian Rho GTPases: new insights into their functions from in vivo studies

Rho GTPases are key regulators of cytoskeletal dynamics and affect many cellular processes, including cell polarity, migration, vesicle trafficking and cytokinesis. These proteins are conserved from plants and yeast to mammals, and function by interacting with and stimulating various downstream targets, including actin nucleators, protein kinases and phospholipases. The roles of Rho GTPases have been extensively studied in different mammalian cell types using mainly dominant negative and constitutively active mutants. The recent availability of knockout mice for several members of the Rho family reveals new information about their roles in signalling to the cytoskeleton and in development.

P-Rex1 - a multidomain protein that regulates neurite differentiation

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

Functions of Rac GTPases during neuronal development

The small GTPases of the Rho family are important regulators of the actin cytoskeleton and are critical for several aspects of neuronal development including the establishment of neuronal polarity, extension of axon and dendrites, neurite branching, axonal navigation and synapse formation. The aim of this review is to present evidence supporting the function of Rac and Rac-related proteins in different aspects of neuronal maturation, based on work performed with organisms including nematodes, Drosophila, Xenopus and mice, and with primary cultures of developing neurons. Three of the 4 vertebrate Rac-related genes, namely Rac1, Rac3 and RhoG, are expressed in the nervous system, and several data support an essential role of all 3 GTPases in distinct aspects of neuronal development and function. Two important points emerge from the analysis presented: highly homologous Rac-related proteins may perform different functions in the developing nervous system; on the other hand, the data also indicate that similar GTPases may perform redundant functions in vivo.

PX-RICS, a novel splicing variant of RICS, is a main isoform expressed during neural development

In our previous study, we identified RICS, a novel beta-catenin-interacting protein with the GAP activity toward Cdc42 and Rac1, and found that RICS plays an important role in the regulation of neural functions, including postsynaptic NMDA signaling and neurite outgrowth. Here we report the characterization of an N-terminal splicing variant of RICS, termed PX-RICS, which has additional phox homology (PX) and src homology 3 (SH3) domains in its N-terminal region. The PX domain of PX-RICS interacted specifically with phosphatidylinositol 3-phosphate [PtdIns(3)P], PtdIns(4)P and PtdIns(5)P. Consistent with this binding affinity, PX-RICS was found to be localized at the endoplasmic reticulum (ER), Golgi and endosomes. We also found that wild-type PX-RICS possessed much lower GAP activity than RICS, whereas a mutant form of PX-RICS whose PX domain lacks the binding ability to phosphoinositides (PIs) exhibited the GAP activity comparable to that of RICS. However, PX-RICS and RICS exhibited similar inhibitory effects on neurite elongation of Neuro-2a cells. Furthermore, we demonstrate that PX-RICS is a main isoform expressed during neural development. Our results suggest that PX-RICS is involved in early brain development including extension of axons and dendrites, and postnatal remodeling and fine-tuning of neural circuits.

Rac1 and Rac3 have opposing functions in cell adhesion and differentiation of neuronal cells

Rac1 and Rac3 are highly homologous members of the Rho small GTPase family. Rac1 is ubiquitously expressed and regulates cell adhesion, migration and differentiation in various cell types. Rac3 is primarily expressed in brain and may therefore have a specific function in neuronal cells. We found that depletion of Rac1 by short interference RNA leads to decreased cell-matrix adhesions and cell rounding in neuronal N1E-115 cells. By contrast, depletion of Rac3 induces stronger cell adhesions and dramatically increases the outgrowth of neurite-like protrusions, suggesting opposite functions for Rac1 and Rac3 in neuronal cells. Consistent with this, overexpression of Rac1 induces cell spreading, whereas overexpression of Rac3 results in a contractile round morphology. Rac1 is mainly found at the plasma membrane, whereas Rac3 is predominantly localized in the perinuclear region. Residues 185-187, present in the variable polybasic rich region at the carboxyl terminus are responsible for the difference in phenotype induced by Rac1 and Rac3 as well as for their different intracellular localization. The Rac1-opposing function of Rac3 is not mediated by or dependent on components of the RhoA signaling pathway. It rather seems that Rac3 exerts its function through negatively affecting integrin-mediated cell-matrix adhesions. Together, our data reveal that Rac3 opposes Rac1 in the regulation of cell adhesion and differentiation of neuronal cells.

betaPIX controls cell motility and neurite extension by regulating the distribution of GIT1

Cell motility entails the reorganization of the cytoskeleton and membrane trafficking for effective protrusion. GIT1/p95-APP1 is a member of a family of GTPase-activating proteins for ARF GTPases that affect endocytosis, adhesion and migration. GIT1 associates with paxillin and a complex including the Rac/Cdc42 exchanging factors PIX/Cool and the kinase PAK. In this study, we show that overexpression of betaPIX induces the accumulation of endogenous and overexpressed GIT1 at large structures similar to those induced by an ArfGAP-defective mutant of GIT1 (p95-C2). Immunohistochemical analysis and immunoelectron microscopy reveal that these structures include the endogenous transferrin receptor. Time-lapse analysis during motogenic stimuli shows that the formation and perinuclear accumulation of the p95-C2-positive structures is paralleled by inhibition of lamellipodium formation and cell retraction. Both dimerization and a functional SH3 domain of betaPIX are required for the accumulation of GIT1 in fibroblasts, which is prevented by the monomeric PIX-PG-DeltaLZ. This mutant also prevents the formation of endocytic aggregates and inhibition of neurite outgrowth in retinal neurons expressing p95-C2. Our results indicate that betaPIX is an important regulator of the subcellular distribution of GIT1, and suggest that alteration in the level of expression of the complex affects the endocytic compartment and cell motility.

Role of the Rho GTPase-activating protein RICS in neurite outgrowth

The Rho family of small GTPases, including RhoA, Rac1 and Cdc42, are critical regulators of the actin cytoskeleton. In neuronal systems, Rho GTPase-activating proteins (RhoGAPs) and their substrates, Rho GTPases, have been implicated in regulating multiple processes in the morphological development of neurons, including axonal growth and guidance, dendritic elaboration and formation of synapses. RICS is mainly expressed in the brain and functions as a RhoGAP protein for Cdc42 and Rac1 in vitro. To examine the biological function of RICS, we disrupted the RICS gene in mice. RICS knockout mice developed normally and were fertile. However, when cultured in vitro, Cdc42 activity in RICS(-/-) neurons was higher than that in wild-type neurons. Consistent with this finding, hippocampal and cerebellar granule neurons derived from RICS(-/-) mice bore longer neurites than those from wild-type mice. These findings suggest that RICS plays an important role in neurite extension by regulating Cdc42 in vivo.

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

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

Rac3-induced neuritogenesis requires binding to Neurabin I

Rac3, a neuronal GTP-binding protein of the Rho family, induces neuritogenesis in primary neurons. Using yeast two-hybrid analysis, we show that Neurabin I, the neuronal F-actin binding protein, is a direct Rac3-interacting molecule. Biochemical and light microscopy studies indicate that Neurabin I copartitions and colocalizes with Rac3 at the growth cones of neurites, inducing Neurabin I association to the cytoskeleton. Moreover, Neurabin I antisense oligonucleotides abolish Rac3-induced neuritogenesis, which in turn is rescued by exogenous Neurabin I but not by Neurabin I mutant lacking the Rac3-binding domain. These results show that Neurabin I mediates Rac3-induced neuritogenesis, possibly by anchoring Rac3 to growth cone F-actin.

Rho GTPases regulate rhabdom morphology in octopus photoreceptors

In the cephalopod retina, light/dark adaptation is accompanied by a decrease/increase in rhabdom size and redistribution of rhodopsin and retinochrome. Rearrangements in the actin cytoskeleton probably govern changes in rhabdom size by regulating the degradation/formation of rhabdomere microvilli. Photopigment movements may be directed by microtubules present in the outer segment core cytoplasm. We believe that rhodopsin activation by light stimulates Rho and Rac signaling pathways, affecting these cytoskeletal systems and their possible functions in controlling rhabdom morphology and protein movements. In this study, we localized cytoskeletal and signaling proteins in octopus photoreceptors to determine their concurrence between the lighting conditions. We used toxin B from Clostridium difficile to inhibit the activity of Rho/Rac and observed its effect on the location of signaling proteins and actin and tubulin. In both lighting conditions, we found Rho in specific sets of juxtaposed rhabdomeres in embryonic and adult retinas. In the light, Rho and actin were localized along the length of the rhabdomere, but, in the dark, both proteins were absent from a space beneath the inner limiting membrane. Rac colocalized with tubulin in the outer segment core cytoplasm and, like Rho, the two proteins were also absent beneath the inner limiting membrane in the dark. The distribution of actin and Rho was affected by toxin B and, in dark-adapted retinas, actin and Rho distribution was similar to that observed in the light. Our results suggest that the Rho/Rac GTPases are candidates for the regulation of rhabdomere size and protein movements in light-dark-adapted octopus photoreceptors.

Generation and characterization of Rac3 knockout mice

Rac proteins are members of the Rho family of GTPases involved in the regulation of actin dynamics. The three highly homologous Rac proteins in mammals are the ubiquitous Rac1, the hematopoiesis-specific Rac2, and the least-characterized Rac3. We show here that Rac3 mRNA is widely and specifically expressed in the developing nervous system, with highest concentration at embryonic day 13 in the dorsal root ganglia and ventral spinal cord. At postnatal day 7 Rac3 appears particularly abundant in populations of projection neurons in several regions of the brain, including the fifth layer of the cortex and the CA1-CA3 region of the hippocampus. We generated mice deleted for the Rac3 gene with the aim of analyzing the function of this GTPase in vivo. Rac3 knockout animals survive embryogenesis and show no obvious developmental defects. Interestingly, specific behavioral differences were detected in the Rac3-deficient animals, since motor coordination and motor learning on the rotarod was superior to that of their wild-type littermates. No obvious histological or immunohistological differences were observed at major sites of Rac3 expression. Our results indicate that, in vivo, Rac3 activity is not strictly required for normal development in utero but may be relevant to later events in the development of a functional nervous system.

Effect of thymosin β15 on the branching of developing neurons

The thymosin betas (Tbetas) are polypeptide regulators of actin dynamics that are critical for the growth and branching of neurites in developing neurons. We found that mRNAs for Tbeta4, Tbeta10, and Tbeta15 were highly expressed in the developing rat brain during neuritogenesis, supporting a role for the Tbetas in this process. Overexpression of the Tbetas increased the number of neurite branches per neuron in cultured hippocampal and cerebral cortex neurons, and Tbeta15 had the greatest effect. Actin binding activity appears to be essential for the branch-promoting activity of Tbetas because two mutants of Tbeta15 lacking monomeric actin binding activity failed to stimulate branch formation. We also found that transfection of siRNA against Tbeta15 reduced branching. Taken together, these data suggest that the three Tbetas, and especially Tbeta15, stimulate neurite branching during brain 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.

Small GTP-binding protein RhoB is expressed in glial müller cells in the vertebrate retina

Among several small Rho GTPases observed in the chick retina, RhoB was transiently expressed during development and mainly present in glial Müller cells in the adult. The aim of this study was to compare the distribution of RhoB in the chick and mouse adult retinas and to study its potential role in the maintenance of cell morphology. The distribution of RhoB was studied in situ and pure Müller cell cultures were submitted to Clostridium difficile toxin A and lysophosphatidic acid (LPA) treatment in order to inactivate and activate Rho proteins, respectively. Cell morphology, F-actin arrangement, RhoB, and vimentin distribution were studied by immunofluorescence and confocal microscopy. The results showed that, in both species, all vimentin-containing cells also expressed RhoB in situ and in vitro. Toxin A promoted cell rounding and detachment due to actin depolymerization, changing the distribution of RhoB only in chick cells. In serum-starved cells LPA stimulated actin polymerization and cell spreading, but only in chick cells was RhoB distribution recruited to expanding cellular processes and newly formed focal adhesions. These data suggest that, although RhoB is expressed by Müller cells in chick and mouse, its role in the maintenance of cellular morphology and regulation may be different. In addition, we show that RhoB may be an interesting Müller cell marker in the adult retina.

Assay and Properties of the GIT1/p95‐APP1 ARFGAP

GIT1/p95-APP1 is an adaptor protein with an aminoterminal ARFGAP domain involved in the regulation of ARF6 function. GIT1/p95-APP1 forms stable complexes with a number of proteins including downstream effectors and exchanging factors for members of the Rho family of small GTPases. This protein can also interact with other adaptor proteins implicated in the regulation of cell adhesion and synapse formation. The stability of the endogenous and reconstituted complexes after cell lysis allows the biochemical identification and characterization of the GIT1 complexes that can be isolated from different cell types. This article presents methods for the identification of the endogenous and reconstituted GIT1 complexes that can be utilized for the biochemical and functional characterization of the complexes from different tissue and cell types.

MOCA induces membrane spreading by activating Rac1

The modifier of cell adhesion protein (MOCA), or Dock3, initially identified as presenilin-binding protein (PBP), belongs to the Dock180 family of proteins and is localized specifically in neurons. Here we demonstrate that MOCA binds to Rac1 and enhances its activity, which leads to the activation of c-Jun NH(2)-terminal kinase (JNK) and causes changes in cell morphology. Farnesylated MOCA, which is localized in the plasma membrane, enhances the activation of Rac1 and JNK more markedly than wild-type MOCA, and cells expressing farnesylated MOCA show flattened morphology similar to those expressing a constitutive active mutant of Rac1, Rac1Q61L. On poly-d-lysine-coated dishes, endogenous MOCA is concentrated on the leading edge of broad membrane protrusions (lamellipodia) where actin filaments are co-localized. MOCA is also concentrated with actin on the growth cone in primary cultures of cortical neurons. These observations suggest that MOCA may induce cytoskeletal reorganization and changes in cell adhesion by regulating the activity of Rac1.

Myosin IIA drives neurite retraction

Neuritic extension is the resultant of two vectorial processes: outgrowth and retraction. Whereas myosin IIB is required for neurite outgrowth, retraction is driven by a motor whose identity has remained unknown until now. Preformed neurites in mouse Neuro-2A neuroblastoma cells undergo immediate retraction when exposed to isoform-specific antisense oligonucleotides that suppress myosin IIB expression, ruling out myosin IIB as the retraction motor. When cells were preincubated with antisense oligonucleotides targeting myosin IIA, simultaneous or subsequent addition of myosin IIB antisense oligonucleotides did not elicit neurite retraction, both outgrowth and retraction being curtailed. Even during simultaneous application of antisense oligonucleotides against both myosin isoforms, lamellipodial spreading continued despite the complete inhibition of neurite extension, indicating an uncoupling of lamellipodial dynamics from movement of the neurite. Significantly, lysophosphatidate- or thrombin-induced neurite retraction was blocked not only by the Rho-kinase inhibitor Y27632 but also by antisense oligonucleotides targeting myosin IIA. Control oligonucleotides or antisense oligonucleotides targeting myosin IIB had no effect. In contrast, Y27632 did not inhibit outgrowth, a myosin IIB-dependent process. We conclude that the conventional myosin motor, myosin IIA, drives neurite retraction.

Differential distribution of Rac1 and Rac3 GTPases in the developing mouse brain: implications for a role of Rac3 in Purkinje cell differentiation

Rac3 is one of the three known Rac GTPases in vertebrates. Rac3 shows high sequence homology to Rac1, and its transcript is specifically expressed in the developing nervous system, where its localization and function are unknown. By using Rac3-specific antibodies, we show that the endogenous Rac3 protein is differentially expressed during mouse brain development, with a peak of expression at times of neuronal maturation and synaptogenesis. Comparison with Rac1 shows clear-cut differences in the overall distribution of the two GTPases in the developing brain, and in their subcellular distribution in regions of the brain where both proteins are expressed. At P7, Rac3 staining is particularly marked in the deep cerebellar nuclei and in the pons, where it shows a discontinuous distribution around the neuronal cell bodies, in contrast with the diffuse staining of Rac1. Rac3 does not evidently co-localize with pre- and post-synaptic markers, nor with GFAP-positive astrocytes, but it clearly co-localizes with actin filaments, and with the terminal portions of calbindin-positive Purkinje cell axons in the deep cerebellar nuclei. Our data implicate Rac3 in neuronal differentiation, and support a specific role of this GTPase in actin-mediated remodelling of Purkinje cell neuritic terminals at time of synaptogenesis.

ARNO and ARF6 regulate axonal elongation and branching through downstream activation of phosphatidylinositol 4-phosphate 5-kinase alpha

In the developing nervous system, controlled neurite extension and branching are critical for the establishment of connections between neurons and their targets. Although much is known about the regulation of axonal development, many of the molecular events that regulate axonal extension remain unknown. ADP-ribosylation factor nucleotide-binding site opener (ARNO) and ADP-ribosylation factor (ARF)6 have important roles in the regulation of the cytoskeleton as well as membrane trafficking. To investigate the role of these molecules in axonogenesis, we expressed ARNO and ARF6 in cultured rat hippocampal neurons. Expression of catalytically inactive ARNO or dominant negative ARF6 resulted in enhanced axonal extension and branching and this effect was abrogated by coexpression of constitutively active ARF6. We sought to identify the downstream effectors of ARF6 during neurite extension by coexpressing phosphatidyl-inositol-4-phosphate 5-Kinase alpha [PI(4)P 5-Kinase alpha] with catalytically inactive ARNO and dominant negative ARF6. We found that PI(4)P 5-Kinase alpha plays a role in neurite extension and branching downstream of ARF6. Also, expression of inactive ARNO/ARF6 depleted the actin binding protein mammalian ena (Mena) from the growth cone leading edge, indicating that these effects on axonogenesis may be mediated by changes in cytoskeletal dynamics. These results suggest that ARNO and ARF6, through PI(4)P 5-Kinase alpha, regulate axonal elongation and branching during neuronal development.