Distribution of Rho family GTPases in the adult rat hippocampus and cerebellum

Cdc42 signaling regulated by dopamine D2 receptor correlatively links specific brain regions of hippocampus to cocaine addiction

BACKGROUND

Hippocampus plays critical roles in drug addiction. Cocaine-induced modifications in dopamine receptor function and the downstream signaling are important regulation mechanisms in cocaine addiction. Rac regulates actin filament accumulation while Cdc42 stimulates the formation of filopodia and neurite outgrowth. Based on the region specific roles of small GTPases in brain, we focused on the hippocampal subregions to detect the regulation of Cdc42 signaling in long-term morphological and behavioral adaptations to cocaine.

METHODS

Genetically modified mouse models of Cdc42, dopamine receptor D1 (D1R) and D2 (D2R) and expressed Cdc42 point mutants that are defective in binding to and activation of its downstream effector molecules PAK and N-WASP were generated, respectively, in CA1 or dentate gyrus (DG) subregion.

RESULTS

Cocaine induced upregulation of Cdc42 signaling activity. Cdc42 knockout or mutants blocked cocaine-induced increase in spine plasticity in hippocampal CA1 pyramidal neurons, leading to a decreased conditional place preference (CPP)-associated memories and spatial learning and memory in water maze. Cdc42 knockout or mutants promoted cocaine-induced loss of neurogenesis in DG, leading to a decreased CPP-associated memories and spatial learning and memory in water maze. Furthermore, by using D1R knockout, D2R knockout, and D2R/Cdc42 double knockout mice, we found that D2R, but not D1R, regulated Cdc42 signaling in cocaine-induced neural plasticity and behavioral changes.

CONCLUSIONS

Cdc42 acts downstream of D2R in the hippocampus and plays an important role in cocaine-induced neural plasticity through N-WASP and PAK-LIMK-Cofilin, and Cdc42 signaling pathway correlatively links specific brain regions (CA1, dentate gyrus) to cocaine-induced CPP behavior.

Exploration of potential role of Rho GTPase in nicotine dependence-induced withdrawal syndrome in mice

The RhoA gene showed an important genotypic association with nicotine dependence and smoking initiation. The current study aims to investigate the effect of the Rho GTPase inhibitor ML141 in the progression of nicotine dependence in a mice model of precipitated nicotine withdrawal syndrome by mecamylamine.The experimental procedure involved administration of 2.5 mg/kg nicotine dissolved in normal saline subcutaneously (s.c) four times a day consecutively for 7 days and last single dose in the morning on 8th day. ML-141 was dissolved in dimethyl sulfoxide (DMSO) and was administered daily with nicotine as corrective treatment at a dose of 1,5 and 10 mg/kg (p < 0.05). An injection of 3 mg/kg of mecamylamine intraperitoneal (ip) was given an hour later than the last nicotine dose on the day 8 to precipitate withdrawal of nicotine and withdrawal severity was assessed by measuring hyperalgesia, piloerection, jumping frequency, tremors, and withdrawal severity score (WSS). Various behavioural changes such as hyperalgesia, piloerection, jumping frequency, and tremors were monitored and WSS was calculated. ML-141 a selective Rho GTPase inhibitor was found to show dose-dependent effect on all these parameters. Inhibition of Rho GTPase was found to reduce the severity of withdrawal syndrome; therefore, it can be concluded that Rho GTPase would serve as a suitable biological target by regulating the reward system in brain and could be used as new target for drug discovery.

Synaptic Potentiation at Basal and Apical Dendrites of Hippocampal Pyramidal Neurons Involves Activation of a Distinct Set of Extracellular and Intracellular Molecular Cues

In the central nervous system, several forms of experience-dependent plasticity, learning and memory require the activity-dependent control of synaptic efficacy. Despite substantial progress in describing synaptic plasticity, mechanisms related to heterogeneity of synaptic functions at local circuits remain elusive. Here we studied the functional and molecular aspects of hippocampal circuit plasticity by analyzing excitatory synapses at basal and apical dendrites of mouse hippocampal pyramidal cells (CA1 region) in acute brain slices. In the past decade, activity of metalloproteinases (MMPs) has been implicated as a widespread and critical factor in plasticity mechanisms at various projections in the CNS. However, in the present study we discovered that in striking contrast to apical dendrites, synapses located within basal dendrites undergo MMP-independent synaptic potentiation. We demonstrate that synapse-specific molecular pathway allowing MMPs to rapidly upregulate function of NMDARs in stratum radiatum involved protease activated receptor 1 and intracellular kinases and GTPases activity. In contrast, MMP-independent scaling of synaptic strength in stratum oriens involved dopamine D1/D5 receptors and Src kinases. Results of this study reveal that 2 neighboring synaptic systems differ significantly in extracellular and intracellular cascades that control synaptic gain and provide long-searched transduction pathways relevant for MMP-dependent synaptic plasticity.

Dissociative role for dorsal hippocampus in mediating heroin self-administration and relapse through CDK5 and RhoB signaling revealed by proteomic analysis

Addiction is characterized by drug craving, compulsive drug taking and relapse, which is attributed to aberrant neuroadaptation in brain regions implicated in drug addiction, induced by changes in gene and protein expression in these regions after chronic drug exposure. Accumulating evidence suggests that the dorsal hippocampus (DH) plays an important role in mediating drug-seeking and drug-taking behavior and relapse. However, the molecular mechanisms underlying these effects of the DH are unclear. In the present study, we employed a label-free quantitative proteomic approach to analyze the proteins altered in the DH of heroin self-administering rats. A total of 4015 proteins were quantified with high confidence, and 361 proteins showed significant differences compared with the saline control group. Among them, cyclin-dependent kinase 5 (CDK5) and ras homolog family member B (RhoB) were up-regulated in rats with a history of extended access to heroin. Functionally, inhibition of CDK5 in the DH enhanced heroin self-administration, indicating that CDK5 signaling in the DH acts as a homeostatic compensatory mechanism to limit heroin-taking behavior, whereas blockade of the Rho-Rho kinase (ROCK) pathway attenuated context-induced heroin relapse, indicating that RhoB signaling in the DH is required for the retrieval (recall) of addiction memory. Our findings suggest that manipulation of CDK5 signaling in the DH may be essential in determining vulnerability to opiate taking, whereas manipulation of RhoB signaling in the DH may be essential in determining vulnerability to relapse. Overall, the present study suggests that the DH can exert dissociative effects on heroin addiction through CDK5 and RhoB signaling.

MiR-194 is involved in morphogenesis of spiral ganglion neurons in inner ear by rearranging actin cytoskeleton via targeting RhoB

Many microRNAs participate in the development, differentiation and function preservation of the embryonic and adult inner ear, but many details still need to be elucidated regarding the numerous microRNAs in the inner ear. Based on previous investigations on the microRNA profile in the inner ear, we confirmed that several microRNAs are expressed in the inner ear, and we detected the spatial expression of these microRNAs in the neonatal mouse inner ear. Then we focused on miR-194 for its specific expression with a dynamic spatiotemporal pattern during inner ear development. Overexpression of miR-194 in cultured spiral ganglion cells significantly affected the dendrites of differentiated neurons, with more branching and obviously dispersed nerve fibres. Furthermore, the cytoskeleton of cultured cells was markedly affected, as disordered actin filaments resulting from miR-194 overexpression and enhanced filaments resulting from miR-194 knockdown were observed. Together with the bioinformatic methods, the RT-qPCR and western blot results showed that RhoB is a candidate target of miR-194 in the morphogenesis of spiral ganglion neurons. Additionally, the double luciferase reporter system was used to identify RhoB as a novel target of miR-194. Finally, the inhibition of RhoB activation by Clostridium difficile toxin B disturbed the organization of the actin filament, similar to the effects of miR-194 overexpression. In summary, we investigated microRNA expression in the mouse inner ear, and demonstrated that miR-194 is dynamically expressed during inner ear development; importantly, we found that miR-194 affects neuron morphogenesis positively through Rho B-mediated F-actin rearrangement.

Dependence receptor involvement in subtilisin-induced long-term depression and in long-term potentiation

The serine protease subtilisin induces a form of long-term depression (LTD) which is accompanied by a reduced expression of the axo-dendritic guidance molecule Unco-ordinated-5C (Unc-5C). One objective of the present work was to determine whether a loss of Unc-5C function contributed to subtilisin-induced LTD by using Unc-5C antibodies in combination with the pore-forming agents Triton X-100 (0.005%) or streptolysin O in rat hippocampal slices. In addition we have assessed the effect of subtilisin on the related dependence receptor Deleted in Colorectal Cancer (DCC) and used antibodies to this protein for functional studies. Field excitatory postsynaptic potentials (fEPSPs) were analyzed in rat hippocampal slices and protein extracts were used for Western blotting. Subtilisin produced a greater loss of DCC than of Unc-5C, but the antibodies had no effect on resting excitability or fEPSPs and did not modify subtilisin-induced LTD. However, antibodies to DCC but not Unc-5C did reduce the amplitude of theta-burst long-term potentiation (LTP). In addition, two inhibitors of endocytosis - dynasore and tat-gluR2(3Y) - were tested and, although the former compound had no effect on neurophysiological responses, tat-gluR2(3Y) did reduce the amplitude of subtilisin-induced LTD without affecting the expression of DCC or Unc-5C but with some loss of PostSynaptic Density Protein-95. The results support the view that the dependence receptor DCC may be involved in LTP and suggest that the endocytotic removal of a membrane protein or proteins may contribute to subtilisin-induced LTD, although it appears that neither Unc-5C nor DCC are involved in this process.

Association of N-cadherin levels and downstream effectors of Rho GTPases with dendritic spine loss induced by chronic stress in rat hippocampal neurons

Chronic stress promotes cognitive impairment and dendritic spine loss in hippocampal neurons. In this animal model of depression, spine loss probably involves a weakening of the interaction between pre- and postsynaptic cell adhesion molecules, such as N-cadherin, followed by disruption of the cytoskeleton. N-cadherin, in concert with catenin, stabilizes the cytoskeleton through Rho-family GTPases. Via their effector LIM kinase (LIMK), RhoA and ras-related C3 botulinum toxin substrate 1 (RAC) GTPases phosphorylate and inhibit cofilin, an actin-depolymerizing molecule, favoring spine growth. Additionally, RhoA, through Rho kinase (ROCK), inactivates myosin phosphatase through phosphorylation of the myosin-binding subunit (MYPT1), producing actomyosin contraction and probable spine loss. Some micro-RNAs negatively control the translation of specific mRNAs involved in Rho GTPase signaling. For example, miR-138 indirectly activates RhoA, and miR-134 reduces LIMK1 levels, resulting in spine shrinkage; in contrast, miR-132 activates RAC1, promoting spine formation. We evaluated whether N-cadherin/β-catenin and Rho signaling is sensitive to chronic restraint stress. Stressed rats exhibit anhedonia, impaired associative learning, and immobility in the forced swim test and reduction in N-cadherin levels but not β-catenin in the hippocampus. We observed a reduction in spine number in the apical dendrites of CA1 pyramidal neurons, with no effect on the levels of miR-132 or miR-134. Although the stress did not modify the RAC-LIMK-cofilin signaling pathway, we observed increased phospho-MYPT1 levels, probably mediated by RhoA-ROCK activation. Furthermore, chronic stress raises the levels of miR-138 in accordance with the observed activation of the RhoA-ROCK pathway. Our findings suggest that a dysregulation of RhoA-ROCK activity by chronic stress could potentially underlie spine loss in hippocampal neurons.

Trio gene is required for mouse learning ability

Trio is a guanine nucleotide exchange factor with multiple guanine nucleotide exchange factor domains. Trio regulates cytoskeleton dynamics and actin remodeling and is involved in cell migration and axonal guidance in neuronal development. The null allele of the Trio gene led to embryonic lethality, and Trio null embryos displayed aberrant organization in several regions of the brain at E18.5, including hippocampus. Nestin-Trio-/- mice, in which the Trio gene was deleted specifically in the neuronal system by the Nestin-Cre system, displayed severe phenotypes, including low survival rate, ataxia and multiple developmental defects of the cerebellum. All Nestin-Trio-/- mice died before reaching adulthood, which hinders research on Trio gene function in adult mice. Thus, we generated EMX1-Trio-/- mice by crossing Trio-floxed mice with EMX1-Cre mice in which Cre is expressed in the brain cortex and hippocampus. EMX1-Trio-/- mice can survive to adulthood. Trio gene deletion results in smaller brains, an abnormal hippocampus and disordered granule cells in the dentate gyrus (DG) and cornu ammonis (CA). Behavior tests showed that Trio deletion interfered with the hippocampal-dependent spatial learning in the mice, suggesting that Trio plays critical roles in the learning ability of adult mice. We conclude that the Trio gene regulates the neuronal development of the hippocampus and that it affects the intelligence of adult mice.

Host response profile of human brain proteome in toxoplasma encephalitis co-infected with HIV

Background

Toxoplasma encephalitis is caused by the opportunistic protozoan parasite Toxoplasma gondii. Primary infection with T. gondii in immunocompetent individuals remains largely asymptomatic. In contrast, in immunocompromised individuals, reactivation of the parasite results in severe complications and mortality. Molecular changes at the protein level in the host central nervous system and proteins associated with pathogenesis of toxoplasma encephalitis are largely unexplored. We used a global quantitative proteomic strategy to identify differentially regulated proteins and affected molecular networks in the human host during T. gondii infection with HIV co-infection.

Results

We identified 3,496 proteins out of which 607 proteins were differentially expressed (≥1.5-fold) when frontal lobe of the brain from patients diagnosed with toxoplasma encephalitis was compared to control brain tissues. We validated differential expression of 3 proteins through immunohistochemistry, which was confirmed to be consistent with mass spectrometry analysis. Pathway analysis of differentially expressed proteins indicated deregulation of several pathways involved in antigen processing, immune response, neuronal growth, neurotransmitter transport and energy metabolism.

Conclusions

Global quantitative proteomic approach adopted in this study generated a comparative proteome profile of brain tissues from toxoplasma encephalitis patients co-infected with HIV. Differentially expressed proteins include previously reported and several new proteins in the context of T. gondii and HIV infection, which can be further investigated. Molecular pathways identified to be associated with the disease should enhance our understanding of pathogenesis in toxoplasma encephalitis.

Electronic supplementary material

The online version of this article (doi:10.1186/1559-0275-11-39) contains supplementary material, which is available to authorized users.

Novel treatment strategies for schizophrenia from improved understanding of genetic risk

Recent years have seen significant advances in our understanding of the genetic basis of schizophrenia. In particular, genome-wide approaches have suggested the involvement of many common genetic variants of small effect, together with a few rare variants exerting relatively large effects. While unequivocal identification of the relevant genes has, for the most part, remained elusive, the genes revealed as potential candidates can in many cases be clustered into functionally related groups which are potentially open to therapeutic intervention. In this review, we summarise this information, focusing on the accumulating evidence that genetic dysfunction at glutamatergic synapses and post-synaptic signalling complexes contributes to the aetiology of the disease. In particular, there is converging support for involvement of post-synaptic JNK pathways in disease aetiology. An expansion of our neurobiological knowledge of the basis of schizophrenia is urgently needed, yet some promising novel pharmacological targets can already be discerned.

The small GTPases RhoA and Rac1 regulate cerebellar development by controlling cell morphogenesis, migration and foliation

The small GTPases RhoA and Rac1 are key cytoskeletal regulators that function in a mutually antagonistic manner to control the migration and morphogenesis of a broad range of cell types. However, their role in shaping the cerebellum, a unique brain structure composed of an elaborate set of folia separated by fissures of different lengths, remains largely unexplored. Here we show that dysregulation of both RhoA and Rac1 signaling results in abnormal cerebellar ontogenesis. Ablation of RhoA from neuroprogenitor cells drastically alters the timing and placement of fissure formation, the migration and positioning of granule and Purkinje cells, the alignment of Bergmann glia, and the integrity of the basement membrane, primarily in the anterior lobules. Furthermore, in the absence of RhoA, granule cell precursors located at the base of fissures fail to undergo cell shape changes required for fissure initiation. Many of these abnormalities can be recapitulated by deleting RhoA specifically from granule cell precursors but not postnatal glia, indicating that RhoA functions in granule cell precursors to control cerebellar morphogenesis. Notably, mice with elevated Rac1 activity due to loss of the Rac1 inhibitors Bcr and Abr show similar anterior cerebellar deficits, including ectopic neurons and defects in fissure formation, Bergmann glia organization and basement membrane integrity. Together, our results suggest that RhoA and Rac1 play indispensable roles in patterning cerebellar morphology.

The potential role of rho GTPases in Alzheimer's disease pathogenesis

Alzheimer's disease (AD) is characterized by a wide loss of synapses and dendritic spines. Despite extensive efforts, the molecular mechanisms driving this detrimental alteration have not yet been determined. Among the factors potentially mediating this loss of neuronal connectivity, the contribution of Rho GTPases is of particular interest. This family of proteins is classically considered a key regulator of actin cytoskeleton remodeling and dendritic spine maintenance, but new insights into the complex dynamics of its regulation have recently determined how its signaling cascade is still largely unknown, both in physiological and pathological conditions. Here, we review the growing evidence supporting the potential involvement of Rho GTPases in spine loss, which is a unanimously recognized hallmark of early AD pathogenesis. We also discuss some new insights into Rho GTPase signaling framework that might explain several controversial results that have been published. The study of the connection between AD and Rho GTPases represents a quite unchartered avenue that holds therapeutic potential.

RhoG protein regulates glycoprotein VI-Fc receptor γ-chain complex-mediated platelet activation and thrombus formation

We investigated the mechanism of activation and functional role of a hitherto uncharacterized signaling molecule, RhoG, in platelets. We demonstrate for the first time the expression and activation of RhoG in platelets. Platelet aggregation, integrin αIIbβ3 activation, and α-granule and dense granule secretion in response to the glycoprotein VI (GPVI) agonists collagen-related peptide (CRP) and convulxin were significantly inhibited in RhoG-deficient platelets. In contrast, 2-MeSADP- and AYPGKF-induced platelet aggregation and secretion were minimally affected in RhoG-deficient platelets, indicating that the function of RhoG in platelets is GPVI-specific. CRP-induced phosphorylation of Syk, Akt, and ERK, but not SFK (Src family kinase), was significantly reduced in RhoG-deficient platelets. CRP-induced RhoG activation was consistently abolished by a pan-SFK inhibitor but not by Syk or PI3K inhibitors. Interestingly, unlike CRP, platelet aggregation and Syk phosphorylation induced by fucoidan, a CLEC-2 agonist, were unaffected in RhoG-deficient platelets. Finally, RhoG(-/-) mice had a significant delay in time to thrombotic occlusion in cremaster arterioles compared with wild-type littermates, indicating the important in vivo functional role of RhoG in platelets. Our data demonstrate that RhoG is expressed and activated in platelets, plays an important role in GPVI-Fc receptor γ-chain complex-mediated platelet activation, and is critical for thrombus formation in vivo.

Rho GTPases mediate the mechanosensitive lineage commitment of neural stem cells

Adult neural stem cells (NSCs) play important roles in learning and memory and are negatively impacted by neurological disease. It is known that biochemical and genetic factors regulate self-renewal and differentiation, and it has recently been suggested that mechanical and solid-state cues, such as extracellular matrix (ECM) stiffness, can also regulate the functions of NSCs and other stem cell types. However, relatively little is known of the molecular mechanisms through which stem cells transduce mechanical inputs into fate decisions, the extent to which mechanical inputs instruct fate decisions versus select for or against lineage-committed blast populations, or the in vivo relevance of mechanotransductive signaling molecules in native stem cell niches. Here we demonstrate that ECM-derived mechanical signals act through Rho GTPases to activate the cellular contractility machinery in a key early window during differentiation to regulate NSC lineage commitment. Furthermore, culturing NSCs on increasingly stiff ECMs enhances RhoA and Cdc42 activation, increases NSC stiffness, and suppresses neurogenesis. Likewise, inhibiting RhoA and Cdc42 or downstream regulators of cellular contractility rescues NSCs from stiff matrix- and Rho GTPase-induced neurosuppression. Importantly, Rho GTPase expression and ECM stiffness do not alter proliferation or apoptosis rates indicating that an instructive rather than selective mechanism modulates lineage distributions. Finally, in the adult brain, RhoA activation in hippocampal progenitors suppresses neurogenesis, analogous to its effect in vitro. These results establish Rho GTPase-based mechanotransduction and cellular stiffness as biophysical regulators of NSC fate in vitro and RhoA as an important regulatory protein in the hippocampal stem cell niche.

Molecular changes associated with hippocampal long-lasting depression induced by the serine protease subtilisin-A

The serine protease subtilisin-A (SubA) induces a form of long-term depression (LTD) of synaptic transmission in the rat hippocampus, and molecular changes associated with SubA-induced LTD (SubA-LTD) were explored by using recordings of evoked postsynaptic potentials and immunoblotting. SubA-LTD was prevented by a selective inhibitor of SubA proteolysis, but the same inhibitor did not affect LTD induced by electrical stimulation or activation of metabotropic glutamate receptors. SubA-LTD was reduced by the protein kinase inhibitors genistein and lavendustin A, although not by inhibitors of p38 mitogen-activated protein kinase, glycogen synthase kinase-3, or protein phosphatases. It was also reduced by (RS)-α-methyl-4-carboxyphenylglycine, a broad-spectrum antagonist at metabotropic glutamate receptors. Inhibition of the Rho kinase enzyme Rho-associated coiled-coil kinase reduced SubA-LTD, although inhibitors of the RhoGTPase-activating enzymes farnesyl transferase and geranylgeranyl transferase did not. In addition, a late phase of SubA-LTD was dependent on new protein synthesis. There was a small, non-significant difference in SubA-LTD between wild-type and RhoB(-/-) mice. Marked decreases were seen in the levels of Unc-5H3, a protein that is intimately involved in the development and plasticity of glutamatergic synapses. Smaller changes were noted, at higher concentrations of SubA, in Unc-5H1, vesicle-associated membrane protein-1 (synaptobrevin), and actin, with no changes in the levels of synaptophysin, synaptotagmin, RhoA, or RhoB. None of these changes was associated with LTD induced electrically or by the metabotropic glutamate receptor agonist (RS)-3,5-dihydroxyphenylglycine. These results indicate that SubA induces molecular changes that overlap with other forms of LTD, but that the overall molecular profile of SubA-LTD is quite different.

Ovarian steroids increase spinogenetic proteins in the macaque dorsal raphe

Dendritic spines are the basic structural units of neuronal plasticity. Intracellular signaling cascades that promote spinogenesis have centered on RhoGTPases. We found that ovarian steroids increase gene expression of RhoGTPases [Ras homolog gene family member A (RhoA), cell division control protein 42 homolog (Cdc42), and ras-related C3 botulinum toxin substrate (Rac)] in laser-captured serotonin neurons. We sought to confirm that the increases observed in gene expression translate to the protein level. In addition, a preliminary study was conducted to determine whether an increase in spines occurs via detection of the spine marker protein, postsynaptic density-95 (PSD-95). Adult ovariectomized (Ovx) monkeys were treated with estradiol (E), progesterone (P), or E+P for 1 month. Sections through the dorsal raphe nucleus were immunostained for RhoA and Cdc42 (n=3-4/group). The number and positive pixel area of RhoA-positive cells and the positive pixel area of Cdc42-positive fibers were determined. On combining E- and E+P-treated groups, there was a significant increase in the average and total cell number and positive pixel area of RhoA-positive cells. E, P, and E+P treatments, individually or combined, also increased the average and total positive pixel area of Cdc42-positive fibers. With remaining sections from two animals in each group, we conducted a preliminary examination of the regulation of PSD-95 protein expression. PSD-95, a postsynaptic scaffold protein, was examined with immunogold silver staining (n=2/group), and the total number of PSD-95-positive puncta was determined with stereology across four levels of the dorsal raphe. E, P, and E+P treatment significantly increased the total number of PSD-95-positive puncta. Together, these findings indicate that ovarian steroids act to increase gene and protein expression of two pivotal RhoGTPases involved in spinogenesis and preliminarily indicate that an increased number of spines and/or synapses result from this action. Increased spinogenesis on serotonin dendrites would facilitate excitatory glutamatergic input and in turn, increase serotonin neuronal activity throughout the brain.

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.

The serine protease subtilisin suppresses epileptiform activity in rat hippocampal slices and neocortex in vivo

Serine proteases of the S8A family and those belonging to the subtilase group generate a long-lasting inhibition of hippocampal evoked potentials, which shows little recovery and resembles long-term depression. The present work investigates the effects of subtilisin A on epileptiform activity induced in hippocampal slices. Interictal bursts were generated by perfusion with 4-aminopyridine in magnesium-free medium, whereas ictal bursts were produced by the addition of baclofen. Subtilisin A superfused for 10 min at concentrations of 50 nM and above reduced the duration of ictal bursts, whereas higher concentrations reduced the frequency of interictal activity with little or no recovery, indicating similarity with the long-term depression reported previously. The anti-epileptiform activity was not prevented by inhibitors of phosphatases or several kinases, but the inhibition of ictal activity was selectively reduced by the tyrosine kinase inhibitor genistein. The rho-activated coiled-coil kinase (ROCK) inhibitor Y-27632 had no effect on the suppression of ictal or interictal bursts. Subtilisin applied at nanomolar concentrations to the surface of the cerebral cortex in vivo also suppressed epileptiform spikes induced by bicuculline. It is concluded that serine proteases of the subtilase group are highly potent inhibitors of epileptiform activity, especially ictal bursts, and that tyrosine kinases may be involved in that inhibition. The mechanism of inhibition is different from the long-lasting depression of evoked potentials, which is partly mediated via ROCK.

Altered apoptotic responses in neurons lacking RhoB GTPase

Caspase 3 activation has been linked to the acute neurotoxic effects of central nervous system damage, as in traumatic brain injury or cerebral ischaemia, and also to the early events leading to long-term neurodegeneration, as in Alzheimer's disease. However, the precise mechanisms activating caspase 3 in neuronal injury are unclear. RhoB is a member of the Rho GTPase family that is dramatically induced by cerebral ischaemia or neurotrauma, both in preclinical models and clinically. In the current study, we tested the hypothesis that RhoB might directly modulate caspase 3 activity and apoptotic or necrotic responses in neurons. Over-expression of RhoB in the NG108-15 neuronal cell line or in cultured corticohippocampal neurons elevated caspase 3 activity without inducing overt toxicity. Cultured corticohippocampal neurons from RhoB knockout mice did not show any differences in sensitivity to a necrotic stimulus - acute calcium ionophore exposure - compared with neurons from wild-type mice. However, corticohippocampal neurons lacking RhoB exhibited a reduction in the degree of DNA fragmentation and caspase 3 activation induced by the apoptotic agent staurosporine, in parallel with increased neuronal survival. Staurosporine induction of caspase 9 activity was also suppressed. RhoB knockout mice showed reduced basal levels of caspase 3 activity in the adult brain. These data directly implicate neuronal RhoB in caspase 3 activation and the initial stages of programmed cell death, and suggest that RhoB may represent an attractive target for therapeutic intervention in conditions involving elevated caspase 3 activity in the central nervous system.

A role for RhoB in synaptic plasticity and the regulation of neuronal morphology

Actin-rich dendritic spines are the locus of excitatory synaptic transmission and plastic events such as long-term potentiation (LTP). Morphological plasticity of spines accompanies activity-dependent changes in synaptic strength. Several Rho GTPase family members are implicated in regulating neuronal and, in particular, spine structure via actin and the actin-binding protein cofilin. However, despite expression in hippocampus and cortex, its ability to modulate actin-regulatory proteins, and its induction during aging, RhoB has been relatively neglected. We previously demonstrated that LTP is associated with specific RhoB activation. Here, we further examined its role in synaptic function using mice with genetic deletion of the RhoB GTPase (RhoB(-/-) mice). Normal basal synaptic transmission accompanied reduced paired-pulse facilitation and post-tetanic potentiation in the hippocampus of RhoB(-/-) mice. Early phase LTP was significantly reduced in RhoB(-/-) animals, whereas the later phase was unaffected. In wild-type mice (RhoB(+/+)), Western blot analysis of potentiated hippocampus showed significant increases in phosphorylated cofilin relative to nonpotentiated slices, which were dramatically impaired in RhoB(-/-) slices. There was also a deficit in phosphorylated Lim kinase levels in the hippocampus from RhoB(-/-) mice. Morphological analysis suggested that lack of RhoB resulted in increased dendritic branching and decreased spine number. Furthermore, an increase in the proportion of stubby relative to thin spines was observed. Moreover, spines demonstrated increased length along with increased head and neck widths. These data implicate RhoB in cofilin regulation and dendritic and spine morphology, highlighting its importance in synaptic plasticity at a structural and functional level.

The pericyte: cellular regulator of microvascular blood flow

The vascular system - through its development, response to injury, and remodeling during disease - constitutes one of the key organ systems sustaining normal human physiology; conversely, its dysregulation also underlies multiple pathophysiologic processes. Regulation of vascular endothelial cell function requires the integration of complex signals via multiple cell types, including arterial smooth muscle, capillary and post-capillary pericytes, and other perivascular cells such as glial and immune cells. Here, we focus on the pericyte and its roles in microvascular remodeling, reviewing current concepts in microvascular pathophysiology and offering new insights into the specific roles that pericyte-dependent signaling pathways may play in modulating endothelial growth and microvascular tone during pathologic angiogenesis and essential hypertension.

Morphogenesis and regulation of Bergmann glial processes during Purkinje cell dendritic spine ensheathment and synaptogenesis

Astrocytes have an important role in synaptic formation and function but how astrocytic processes become associated with synaptic structures during development is not well understood. Here we analyzed the pattern of growth of the processes extending off the main Bergmann glial (BG) shafts during synaptogenesis in the cerebellum. We found that during this period, BG process outgrowth was correlated with increased ensheathment of dendritic spines. In addition, two-photon time-lapse imaging revealed that BG processes were highly dynamic, and processes became more stable as the period of spine ensheathment progressed. While process motility was dependent on actin polymerization, activity of cytoskeletal regulators Rac1 and RhoG did not play a role in glial process dynamics or density, but was critical for maintaining process length. We extended this finding to probe the relationship between process morphology and ensheathment, finding that shortened processes result in decreased coverage of the spine. Furthermore, we found that areas in which BG expressed dn-Rac1, and therefore had a lower level of synaptic ensheathment, showed an overall increase in synapse number. These analyses reveal how BG processes grow to surround synaptic structures, elucidate the importance of BG process structure for proper development of synaptic ensheathment, and reveal a role for ensheathment in synapse formation.

On the Rho'd: the regulation of membrane protrusions by Rho-GTPases

Cell migration entails the formation of cellular protrusions such as lamellipodia or filopodia, the growth of which is powered by the polymerisation of actin filaments abutting the plasma membrane. Specific Rho-GTPase subfamilies are able to drive different types of protrusions. However, significant crosstalk between Rho-family members and the interplay of distinct Rho-effectors regulating or modulating actin reorganization in protrusions complicate the picture of how precisely they are initiated and maintained. Here, we briefly sketch our current knowledge on structure and dynamics of different protrusions as well as their regulation by Rho-GTPases. We also comment on topical, unresolved controversies in the field, with special emphasis on the interrelation of different protrusion types, and on the composition of the nanomachineries driving them.

Dexamethasone suppresses infiltration of RhoA+ cells into early lesions of rat traumatic brain injury

Inflammatory cell infiltration is a major part of secondary tissue damage in traumatic brain injury (TBI). RhoA is an important member of Rho GTPases and is involved in leukocyte migration. Inhibition of RhoA and its downstream target, Rho-associated coiled kinase (ROCK), has been proven to promote axon regeneration and function recovery following injury in the central nervous system (CNS). Previously, we showed that dexamethasone, an immunosuppressive corticosteroid, attenuated early expression of three molecules associated with microglia/macrophages activation following TBI in rats. Here, the effects of dexamethasone on the early expression of RhoA have been investigated in brains of TBI rats by immunohistochemistry. In brains of rats treated with TBI alone, significant RhoA+ cell accumulation was observed at 18 h post-injury and continuously increased during our observed time period. The accumulated RhoA+ cells were distributed to the areas of pannecrosis and selective neuronal loss. Most accumulated RhoA+ cells were identified as active microglia/macrophages by double-labelling. Dexamethasone (1 mg/kg body weight) was intraperitoneally injected on day 0 and 2 immediately following brain injury. Numbers of RhoA+ cells were significantly reduced on day 1 and 2 following administration of dexamethasone but returned to vehicle control level on day 4. However, dexamethasone treatment did not change the proportion of RhoA+ cells. These observations suggest that dexamethasone has only a transient effect on early leukocyte recruitment.

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.

Pericyte Rho GTPase mediates both pericyte contractile phenotype and capillary endothelial growth state

Pericytes regulate microvascular development and maturation through the control of endothelial cell motility, proliferation, and differentiation. The Rho GTPases have recently been described as key regulators of pericyte shape and contractile phenotype by signaling through the actin cytoskeleton in an isoactin-specific manner. In this report, we reveal that Rho GTPase-dependent signal transduction not only influences pericyte shape and contractile potential but also modulates capillary endothelial proliferative status and pericyte-endothelial interactions in vitro. We provide evidence that overexpression of mutant Rho GTPases, but not other Ras-related small GTPases, significantly alters pericyte shape, contractility, and endothelial growth state in microvascular cell co-cultures. In particular, we describe the use of a silicon substrate deformation assay to demonstrate that pericyte contractility is Rho GTP- and Rho kinase-dependent; further, we describe a novel in vitro system for examining pericyte-mediated endothelial growth arrest and show that control pericytes are capable of growth-arresting capillary endothelial cells in a cell contact-dependent manner, whereas pericytes overexpressing dominant-active and -negative Rho GTPase are comparably incompetent. These data strongly suggest that signaling through the pericyte Rho GTPase pathway may provide critical cues to the processes of microvascular stabilization, maturation, and contractility during development and disease.

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.

Gene Transfer of Hepatocyte Growth Factor Gene Improves Learning and Memory in the Chronic Stage of Cerebral Infarction

There is no specific treatment to improve the functional recovery in the chronic stage of ischemic stroke. To provide the new therapeutic options, we examined the effect of overexpression of hepatocyte growth factor (HGF) in the chronic stage of cerebral infarction by transferring the HGF gene into the brain using hemagglutinating virus of Japan envelope vector. Sixty rats were exposed to permanent middle cerebral artery occlusion (day 1). Based on the sensorimotor deficits at day 7, the rats were divided equally into control vector or HGF-treated rats. At day 56, rats transfected with the HGF gene showed a significant recovery of learning and memory in Morris water maze tests (control vector 50±4 s; HGF 33±5 s; P <0.05) and passive avoidance task (control vector 132.4±37.5 s; HGF 214.8±26.5 s; P <0.05). Although the total volume of cerebral infarction was not related to the outcome, immunohistochemical analysis for Cdc42 and synaptophysin in the peri-infarct region revealed that HGF enhanced the neurite extension and increased synapses. Immunohistochemistry for glial fibriary acidic protein revealed that the formation of glial scar was also prevented by HGF gene treatment. Additionally, the number of the arteries was increased in the HGF group at day 56. These data demonstrated that HGF has a pivotal role for the functional recovery after cerebral infarction through neuritogenesis, improved microcirculation, and the prevention of gliosis. Our results also provide evidence for the feasibility of gene therapy in the chronic stage of cerebral infarction.

NMDA receptor activation induces translocation and activation of Rac in mouse hippocampal area CA1

Neuronal development requires several discrete morphological steps that are believed to involve the small GTPase Rac. For example, neural activity, through NMDA receptors and/or AMPA receptors, activates Rac leading to elaboration of dendritic arbors. In the current study, we have conducted studies which indicate that Rac might be an important molecule involved in morphological plasticity in the adult mouse. We demonstrate that Rac is expressed at synapses in the adult mouse hippocampus. We also demonstrate that treatment of hippocampal slices with NMDA induces membrane translocation and activation of Rac in area CA1. Interestingly, we also find that there is an increase in Rac that is associated with NMDA receptor complexes following NMDA receptor activation. Taken together, our data are consistent with the idea that Rac could be participating in NMDA receptor-dependent changes in morphology that occur during synaptic plasticity and memory formation in the adult mouse hippocampus.

5-HT7 receptor is coupled to G alpha subunits of heterotrimeric G12-protein to regulate gene transcription and neuronal morphology

The neurotransmitter serotonin (5-HT) plays an important role in the regulation of multiple events in the CNS. We demonstrated recently a coupling between the 5-HT4 receptor and the heterotrimeric G13-protein resulting in RhoA-dependent neurite retraction and cell rounding (Ponimaskin et al., 2002). In the present study, we identified G12 as an additional G-protein that can be activated by another member of serotonin receptors, the 5-HT7 receptor. Expression of 5-HT7 receptor induced constitutive and agonist-dependent activation of a serum response element-mediated gene transcription through G12-mediated activation of small GTPases. In NIH3T3 cells, activation of the 5-HT7 receptor induced filopodia formation via a Cdc42-mediated pathway correlating with RhoA-dependent cell rounding. In mouse hippocampal neurons, activation of the endogenous 5-HT7 receptors significantly increased neurite length, whereas stimulation of 5-HT4 receptors led to a decrease in the length and number of neurites. These data demonstrate distinct roles for 5-HT7R/G12 and 5-HT4R/G13 signaling pathways in neurite outgrowth and retraction, suggesting that serotonin plays a prominent role in regulating the neuronal cytoarchitecture in addition to its classical role as neurotransmitter.

Expression analysis of neuroleukin, calmodulin, cortactin, and Rho7/Rnd2 in the intact and injured mouse brain

Subtracted cDNA libraries from the mouse developing inferior colliculus were previously constructed between postnatal day (P) 6 and 10. In the P10-P6 subtracted library, neuroleukin, calmodulin I, cortactin, and Rho7 were identified. The goal of the present study was to analyze their distribution, at the mRNA and protein levels, in both the adult and the developing mouse brain. The four molecules showed a wide expression throughout the brain, with a neuronal-enriched localization in structures such as the cortex, the hippocampus, the cerebellum, and the inferior colliculus. The level of expression of their corresponding mRNAs increased during brain postnatal development. The expression of these molecules was also investigated 2 weeks after a mechanical lesion in the adult cerebral cortex. Neuroleukin and cortactin were found to be expressed by reactive astrocytes, while there were no changes in the expression of calmodulin and Rho7. The expression of neuroleukin, calmodulin, cortactin, and Rho7 is discussed in the context of their putative role in the maturation of the brain and in the axonal regeneration process.

Regulation of myosin light chain phosphorylation by RhoB in neuronal cells

The phosphorylation of myosin light chain (MLC) is a key regulatory point in the control of cellular morphology. Evidence suggests that RhoA-a member of the Rho GTPase family-regulates MLC phosphorylation via Rho kinase (ROK). Neurones display subtle alterations in their cytoarchitecture during the synaptic plasticity following high-frequency stimulation. We have recently demonstrated that RhoB, and not RhoA, is activated in neurones by high-frequency stimulation. However, the downstream consequences of RhoB activation in cells are unclear. In this study, we tested the hypothesis that RhoB might stimulate neuronal MLC phosphorylation. Transfection of PC12 cells with constitutively active RhoB increased MLC phosphorylation. Conversely, dominant-negative RhoB vectors reduced MLC phosphorylation. The effect of RhoB was attenuated by pretreatment with a selective ROK inhibitor. This confirms that Rho GTPases are important regulators of MLC phosphorylation, but suggests that, in neuronal cells, the control is exerted via RhoB rather than RhoA.

Neurotropic action of androgens: principles, mechanisms and novel targets

The importance of androgen signaling is well recognized for numerous aspects of central nervous system (CNS) function, ranging from sex-specific organization of neuroendocrine and behavioral circuits to adaptive capacity, resistance and repair. Nonetheless, concepts for the therapeutic use of androgens in neurological and mental disorders are far from being established. This review outlines some critical issues which interfere with decisions on the suitability of androgens as therapeutic agents for CNS conditions. Among these, sex-specific organization of neural substrates and resulting differential responsiveness to endogenous gonadal steroids, convergence of steroid hormone actions on common molecular targets, co-presence of different sex steroid receptors in target neuronal populations, and in situ biotransformation of natural androgens apparently pose the principal obstacles for the characterization of specific neurotropic effects of androgens. Additional important, albeit less explored aspects consist in insufficient knowledge about molecular targets in the CNS which are under exclusive or predominant androgen control. Own experimental data illustrate the variability of pharmacological effects of natural and synthetic androgens on CNS functions of adaptive relevance, such as sexual behavior, anxiety and endocrine responsiveness to stress. Finally, we present results from an analysis of the consequences of aging for the rat brain transcriptome and examination of the influence of androgens on differentially expressed genes with presumable significance in neuropathology.

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

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

Increased long-term potentiation in the CA1 region of rat hippocampus via modulation of GTPase signalling or inhibition of Rho kinase

There is accumulating evidence that Ras, and Ras-related GTPases of the Rho family, such as RhoA, RhoB and Rac1, are involved in synaptic plasticity in brain regions such as the hippocampus. We have recently shown that Rho family GTPases are activated by synaptic transmission in the CA1 region of the hippocampus. Since the function of these GTPases is dependent on post-translational isoprenylation by either farnesyl or geranylgeranyl transferases, we tested the hypothesis that inhibition of isoprenylation would modify long-term potentiation (LTP). Farnesyl transferase inhibition, which suppressed activation of RhoB and Ras but not RhoA or Rac1, reduced the magnitude of LTP, while geranylgeranyl transferase inhibition, which inhibited RhoA and Rac1 but not RhoB, increased the magnitude of LTP. In addition, Y-27632, a specific inhibitor of a downstream effector of Rho GTPases-Rho-kinase-also increased the magnitude of LTP. This provides strong evidence that GTPases are important mediators of synaptic plasticity, and demonstrates that Rho-kinase acts to reduce the degree of plasticity at hippocampal synapses during LTP. Rho-kinase inhibitors have the unusual property of increasing the magnitude of LTP, and so may be potential cognitive enhancers.

Immunological function in mice lacking the Rac-related GTPase RhoG

RhoG is a low-molecular-weight GTPase highly expressed in lymphocytes that activates gene transcription and promotes cytoskeletal reorganization in vitro. To study the in vivo function of RhoG, we generated mice homozygous for a targeted disruption of the RhoG gene. Despite the absence of RhoG, the development of B and T lymphocytes was unaffected. However, there was an increase in the level of serum immunoglobulin G1 (IgG1) and IgG2b as well as a mild increase of the humoral immune response to thymus-dependent antigens. In addition, B- and T-cell proliferation in response to antigen receptor cross-linking was slightly increased. Although RhoG deficiency produces a mild phenotype, our experiments suggest that RhoG may contribute to the negative regulation of immune responses. The lack of a strong phenotype could indicate a functional redundancy of RhoG with other Rac proteins in lymphocytes.

Activation of Rho GTPases by synaptic transmission in the hippocampus

Ras-related GTPases of the Rho family, such as RhoA and RhoB, are well-characterised mediators of morphological change in peripheral tissues via their effects on the actin cytoskeleton. We tested the hypothesis that Rho family GTPases are involved in synaptic transmission in the CA1 region of the hippocampus. We show that GTPases are activated by synaptic transmission. RhoA and RhoB were activated by low frequency stimulation, while the induction of long-term potentiation (LTP) by high frequency stimulation was associated with specific activation of RhoB via NMDA receptor stimulation. This illustrates that these GTPases are potential mediators of synaptic transmission in the hippocampus, and raises the possibility that RhoB may play a role in plasticity at hippocampal synapses during LTP.