Rho GTPases, dendritic structure, and mental retardation

Dendritic spines and their role in the pathogenesis of neurodevelopmental and neurological disorders

Since Cajal introduced dendritic spines in the 19th century, they have attained considerable attention, especially in neuropsychiatric and neurologic disorders. Multiple roles of dendritic spine malfunction and pathology in the progression of various diseases have been reported. Thus, it is inevitable to consider these structures as new therapeutic targets for treating neuropsychiatric and neurologic disorders such as autism spectrum disorders, schizophrenia, dementia, Down syndrome, etc. Therefore, we attempted to prepare a narrative review of the literature regarding the role of dendritic spines in the pathogenesis of aforementioned diseases and to shed new light on their pathophysiology.

The Wingless planar cell polarity pathway is essential for optimal activity-dependent synaptic plasticity

From fly to man, the Wingless (Wg)/Wnt signaling molecule is essential for both the stability and plasticity of the nervous system. The Drosophila neuromuscular junction (NMJ) has proven to be a useful system for deciphering the role of Wg in directing activity-dependent synaptic plasticity (ADSP), which, in the motoneuron, has been shown to be dependent on both the canonical and the noncanonical calcium Wg pathways. Here we show that the noncanonical planar cell polarity (PCP) pathway is an essential component of the Wg signaling system controlling plasticity at the motoneuron synapse. We present evidence that disturbing the PCP pathway leads to a perturbation in ADSP. We first show that a PCP-specific allele of disheveled (dsh) affects the de novo synaptic structures produced during ADSP. We then show that the Rho GTPases downstream of Dsh in the PCP pathway are also involved in regulating the morphological changes that take place after repeated stimulation. Finally, we show that Jun kinase is essential for this phenomenon, whereas we found no indication of the involvement of the transcription factor complex AP1 (Jun/Fos). This work shows the involvement of the neuronal PCP signaling pathway in supporting ADSP. Because we find that AP1 mutants can perform ADSP adequately, we hypothesize that, upon Wg activation, the Rho GTPases and Jun kinase are involved locally at the synapse, in instructing cytoskeletal dynamics responsible for the appearance of the morphological changes occurring during ADSP.

Early chronic fasudil treatment rescues hippocampal alterations in the Ts65Dn model for down syndrome

Down syndrome (DS) is the most common genetic disorder associated with intellectual disability. To study this syndrome, several mouse models have been developed. Among the most common is the Ts65Dn model, which mimics most of the alterations observed in DS. Ts65Dn mice, as humans with DS, show defects in the structure, density, and distribution of dendritic spines in the cerebral cortex and hippocampus. Fasudil is a potent inhibitor of the RhoA kinase pathway, which is involved in the formation and stabilization of dendritic spines. Our study analysed the effect of early chronic fasudil treatment on the alterations observed in the hippocampus of the Ts65Dn model. We observed that treating Ts65Dn mice with fasudil induced an increase in neural plasticity in the hippocampus: there was an increment in the expression of PSA-NCAM and BDNF, in the dendritic branching and spine density of granule neurons, as well as in cell proliferation and neurogenesis in the subgranular zone. Finally, the treatment reduced the unbalance between excitation and inhibition present in this model. Overall, early chronic treatment with fasudil increases cell plasticity and eliminates differences with euploid animals.

Balancing act of small GTPases downstream of plexin-A4 signaling motifs promotes dendrite elaboration in mammalian cortical neurons

The precise development of neuronal morphologies is crucial to the establishment of synaptic circuits and, ultimately, proper brain function. Signaling by the axon guidance cue semaphorin 3A (Sema3A) and its receptor complex of neuropilin-1 and plexin-A4 has multifunctional outcomes in neuronal morphogenesis. Downstream activation of the RhoGEF FARP2 through interaction with the lysine-arginine-lysine motif of plexin-A4 and consequent activation of the small GTPase Rac1 promotes dendrite arborization, but this pathway is dispensable for axon repulsion. Here, we investigated the interplay of small GTPase signaling mechanisms underlying Sema3A-mediated dendritic elaboration in mouse layer V cortical neurons in vitro and in vivo. Sema3A promoted the binding of the small GTPase Rnd1 to the amino acid motif lysine-valine-serine (LVS) in the cytoplasmic domain of plexin-A4. Rnd1 inhibited the activity of the small GTPase RhoA and the kinase ROCK, thus supporting the activity of the GTPase Rac1, which permitted the growth and branching of dendrites. Overexpression of a dominant-negative RhoA, a constitutively active Rac1, or the pharmacological inhibition of ROCK activity rescued defects in dendritic elaboration in neurons expressing a plexin-A4 mutant lacking the LVS motif. Our findings provide insights into the previously unappreciated balancing act between Rho and Rac signaling downstream of specific motifs in plexin-A4 to mediate Sema3A-dependent dendritic elaboration in mammalian cortical neuron development.

Genomic and Reverse Translational Analysis Discloses a Role for Small GTPase RhoA Signaling in the Pathogenesis of Schizophrenia: Rho-Kinase as a Novel Drug Target

Schizophrenia is one of the most serious psychiatric disorders and is characterized by reductions in both brain volume and spine density in the frontal cortex. RhoA belongs to the RAS homolog (Rho) family and plays critical roles in neuronal development and structural plasticity via Rho-kinase. RhoA activity is regulated by GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs). Several variants in GAPs and GEFs associated with RhoA have been reported to be significantly associated with schizophrenia. Moreover, several mouse models carrying schizophrenia-associated gene variants involved in RhoA/Rho-kinase signaling have been developed. In this review, we summarize clinical evidence showing that variants in genes regulating RhoA activity are associated with schizophrenia. In the last half of the review, we discuss preclinical evidence indicating that RhoA/Rho-kinase is a potential therapeutic target of schizophrenia. In particular, Rho-kinase inhibitors exhibit anti-psychotic-like effects not only in Arhgap10 S490P/NHEJ mice, but also in pharmacologic models of schizophrenia (methamphetamine- and MK-801-treated mice). Accordingly, we propose that Rho-kinase inhibitors may have antipsychotic effects and reduce cognitive deficits in schizophrenia despite the presence or absence of genetic variants in small GTPase signaling pathways.

Rho kinase inhibitors ameliorate cognitive impairment in a male mouse model of methamphetamine-induced schizophrenia

Schizophrenia (SCZ) is a severe psychiatric disorder characterized by positive symptoms, negative symptoms, and cognitive deficits. Current antipsychotic treatment in SCZ improves positive symptoms but has major side effects and little impact on negative symptoms and cognitive impairment. The pathoetiology of SCZ remains unclear, but is known to involve small GTPase signaling. Rho kinase, an effector of small GTPase Rho, is highly expressed in the brain and plays a major role in neurite elongation and neuronal architecture. This study used a touchscreen-based visual discrimination (VD) task to investigate the effects of Rho kinase inhibitors on cognitive impairment in a methamphetamine (METH)-treated male mouse model of SCZ. Systemic injection of the Rho kinase inhibitor fasudil dose-dependently ameliorated METH-induced VD impairment. Fasudil also significantly suppressed the increase in the number of c-Fos-positive cells in the infralimbic medial prefrontal cortex (infralimbic mPFC) and dorsomedial striatum (DMS) following METH treatment. Bilateral microinjections of Y-27632, another Rho kinase inhibitor, into the infralimbic mPFC or DMS significantly ameliorated METH-induced VD impairment. Two proteins downstream of Rho kinase, myosin phosphatase-targeting subunit 1 (MYPT1; Thr696) and myosin light chain kinase 2 (MLC2; Thr18/Ser19), exhibited increased phosphorylation in the infralimbic mPFC and DMS, respectively, after METH treatment, and fasudil inhibited these increases. Oral administration of haloperidol and fasudil ameliorated METH-induced VD impairment, while clozapine had little effect. Oral administration of haloperidol and clozapine suppressed METH-induced hyperactivity, but fasudil had no effect. These results suggest that METH activates Rho kinase in the infralimbic mPFC and DMS, which leads to cognitive impairment in male mice. Rho kinase inhibitors ameliorate METH-induced cognitive impairment, perhaps via the cortico-striatal circuit.

Rac1 GTPase activation impairs fear conditioning-induced structural changes in basolateral amygdala neurons and long-term fear memory formation

Long-term memory formation leads to enduring alterations in synaptic efficacy and neuronal responses that may be created by changes in neuronal morphology. We show that fear conditioning leads to a long-lasting increase in the volume of the primary and secondary dendritic branches, but not of distal branches, of neurons located at the basolateral amygdala (BLA). The length of the dendritic branches is not affected by fear conditioning. Fear conditioning leads to an enduring increase in the length and volume of dendritic spines, especially in the length of the spine neck and the volume of the spine head. Fear conditioning does not affect dendritic spine density. We further reveal that activation of Rac1 in BLA during fear conditioning impairs long-term auditory, but not contextual, fear conditioning memory. Activation of Rac1 during fear conditioning prevents the enduring increase in the dendritic primary branch volume and dendritic spines length and volume. Rac1 activation per se has no effect on neuronal morphology. These results show that fear conditioning induces changes known to reduce the inhibition of signal propagation along the dendrite and the increase in synaptic efficacy whereas preventing these changes, by Rac1 activation, impairs fear memory formation.

The Evaluation of Rac1 Signaling as a Potential Therapeutic Target of Alzheimer's Disease

The Small GTPase Rac1 is critical for various fundamental cellular processes, including cognitive functions. The cyclical activation and inactivation of Rac1, mediated by Rac guanine nucleotide exchange factors (RacGEFs) and Rac GTPase-activating proteins (RacGAPs), respectively, are essential for activating intracellular signaling pathways and controlling cellular processes. We have recently shown that the Alzheimer's disease (AD) therapeutic drug donepezil activates the Rac1-PAK pathway in the nucleus accumbens (NAc) for enhanced aversive learning. Also, PAK activation itself in the NAc enhances aversive learning. As aversive learning allows short-term preliminary AD drug screening, here we tested whether sustained Rac1 activation by RacGAP inhibition can be used as an AD therapeutic strategy for improving AD-learning deficits based on aversive learning. We found that the RacGAP domain of breakpoint cluster region protein (Bcr) (Bcr-GAP) efficiently inhibited Rac1 activity in a membrane ruffling assay. We also found that, in striatal/accumbal primary neurons, Bcr knockdown by microRNA mimic-expressing adeno-associated virus (AAV-miRNA mimic) activated Rac1-PAK signaling, while Bcr-GAP-expressing AAV inactivated it. Furthermore, conditional knockdown of Bcr in the NAc of wild-type adult mice enhanced aversive learning, while Bcr-GAP expression in the NAc inhibited it. The findings indicate that Rac1 activation by RacGAP inhibition enhances aversive learning, implying the AD therapeutic potential of Rac1 signaling.

Effects of heterozygous deletion of autism-related gene Cullin-3 in mice

Autism Spectrum Disorder (ASD) is a developmental disorder in which children display repetitive behavior, restricted range of interests, and atypical social interaction and communication. CUL3, coding for a Cullin family scaffold protein mediating assembly of ubiquitin ligase complexes through BTB domain substrate-recruiting adaptors, has been identified as a high-risk gene for autism. Although complete knockout of Cul3 results in embryonic lethality, Cul3 heterozygous mice have reduced CUL3 protein, demonstrate comparable body weight, and display minimal behavioral differences including decreased spatial object recognition memory. In measures of reciprocal social interaction, Cul3 heterozygous mice behaved similarly to their wild-type littermates. In area CA1 of hippocampus, reduction of Cul3 significantly increased mEPSC frequency but not amplitude nor baseline evoked synaptic transmission or paired-pulse ratio. Sholl and spine analysis data suggest there is a small yet significant difference in CA1 pyramidal neuron dendritic branching and stubby spine density. Unbiased proteomic analysis of Cul3 heterozygous brain tissue revealed dysregulation of various cytoskeletal organization proteins, among others. Overall, our results suggest that Cul3 heterozygous deletion impairs spatial object recognition memory, alters cytoskeletal organization proteins, but does not cause major hippocampal neuronal morphology, functional, or behavioral abnormalities in adult global Cul3 heterozygous mice.

Neuroproteomic mapping of kinases and their substrates downstream of acetylcholine: finding and implications

INTRODUCTION

Since the emergence of the cholinergic hypothesis of Alzheimer's disease (AD), acetylcholine has been viewed as a mediator of learning and memory. Donepezil improves AD-associated learning deficits and memory loss by recovering brain acetylcholine levels. However, it is associated with side effects due to global activation of acetylcholine receptors. Muscarinic acetylcholine receptor M1 (M1R), a key mediator of learning and memory, has been an alternative target. The importance of targeting a specific pathway downstream of M1R has recently been recognized. Elucidating signaling pathways beyond M1R that lead to learning and memory holds important clues for AD therapeutic strategies.

AREAS COVERED

This review first summarizes the role of acetylcholine in aversive learning, one of the outputs used for preliminary AD drug screening. It then describes the phosphoproteomic approach focused on identifying acetylcholine intracellular signaling pathways leading to aversive learning. Finally, the intracellular mechanism of donepezil and its effect on learning and memory is discussed.

EXPERT OPINION

The elucidation of signaling pathways beyond M1R by phosphoproteomic approach offers a platform for understanding the intracellular mechanism of AD drugs and for developing AD therapeutic strategies. Clarifying the molecular mechanism that links the identified acetylcholine signaling to AD pathophysiology will advance the development of AD therapeutic strategies.

Roles of Rac1-Dependent Intrinsic Forgetting in Memory-Related Brain Disorders: Demon or Angel

Animals are required to handle daily massive amounts of information in an ever-changing environment, and the resulting memories and experiences determine their survival and development, which is critical for adaptive evolution. However, intrinsic forgetting, which actively deletes irrelevant information, is equally important for memory acquisition and consolidation. Recently, it has been shown that Rac1 activity plays a key role in intrinsic forgetting, maintaining the balance of the brain's memory management system in a controlled manner. In addition, dysfunctions of Rac1-dependent intrinsic forgetting may contribute to memory deficits in neurological and neurodegenerative diseases. Here, these new findings will provide insights into the neurobiology of memory and forgetting, pathological mechanisms and potential therapies for brain disorders that alter intrinsic forgetting mechanisms.

Crosstalk between the Rho and Rab family of small GTPases in neurodegenerative disorders

Neurodegeneration is associated with defects in cytoskeletal dynamics and dysfunctions of the vesicular trafficking and sorting systems. In the last few decades, studies have demonstrated that the key regulators of cytoskeletal dynamics are proteins from the Rho family GTPases, meanwhile, the central hub for vesicle sorting and transport between target membranes is the Rab family of GTPases. In this regard, the role of Rho and Rab GTPases in the induction and maintenance of distinct functional and morphological neuronal domains (such as dendrites and axons) has been extensively studied. Several members belonging to these two families of proteins have been associated with many neurodegenerative disorders ranging from dementia to motor neuron degeneration. In this analysis, we attempt to present a brief review of the potential crosstalk between the Rab and Rho family members in neurodegenerative pathologies such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington disease, and amyotrophic lateral sclerosis (ALS).

Molecular, Circuit, and Stress Response Characterization of Ventral Pallidum Npas1-Neurons

Altered activity of the ventral pallidum (VP) underlies disrupted motivation in stress and drug exposure. The VP is a very heterogeneous structure composed of many neuron types with distinct physiological properties and projections. Neuronal PAS 1-positive (Npas1+) VP neurons are thought to send projections to brain regions critical for motivational behavior. While Npas1+ neurons have been characterized in the globus pallidus external, there is limited information on these neurons in the VP. To address this limitation, we evaluated the projection targets of the VP Npas1+ neurons and performed RNA-sequencing on ribosome-associated mRNA from VP Npas1+ neurons to determine their molecular identity. Finally, we used a chemogenetic approach to manipulate VP Npas1+ neurons during social defeat stress (SDS) and behavioral tasks related to anxiety and motivation in Npas1-Cre mice. We used a similar approach in females using the chronic witness defeat stress (CWDS). We identified VP Npas1+ projections to the nucleus accumbens, ventral tegmental area, medial and lateral habenula, lateral hypothalamus, thalamus, medial and lateral septum, and periaqueductal gray area. VP Npas1+ neurons displayed distinct translatome representing distinct biological processes. Chemogenetic activation of hM3D(Gq) receptors in VP Npas1+ neurons increased susceptibility to a subthreshold SDS and anxiety-like behavior in the elevated plus maze and open field while the activation of hM4D(Gi) receptors in VP Npas1+ neurons enhanced resilience to chronic SDS and CWDS. Thus, the activity of VP Npas1+ neurons modulates susceptibility to social stressors and anxiety-like behavior. Our studies provide new information on VP Npas1+ neuron circuitry, molecular identity, and their role in stress response.SIGNIFICANCE STATEMENT The ventral pallidum (VP) is a structure connected to both reward-related and aversive brain centers. It is a key brain area that signals the hedonic value of natural rewards. Disruption in the VP underlies altered motivation in stress and substance use disorder. However, VP is a very heterogeneous area with multiple neuron subtypes. This study characterized the projection pattern and molecular signatures of VP Neuronal PAS 1-positive (Npas1+) neurons. We further used tools to alter receptor signaling in VP Npas1+ neurons in stress to demonstrate a role for these neurons in stress behavioral outcomes. Our studies have implications for understanding brain cell type identities and their role in brain disorders, such as depression, a serious disorder that is precipitated by stressful events.

Inhibition of Rho-kinase ameliorates decreased spine density in the medial prefrontal cortex and methamphetamine-induced cognitive dysfunction in mice carrying schizophrenia-associated mutations of the Arhgap10 gene

Copy-number variations in the ARHGAP10 gene encoding Rho GTPase-activating protein 10 are associated with schizophrenia. Model mice (Arhgap10 S490P/NHEJ mice) that carry "double-hit" mutations in the Arhgap10 gene mimic the schizophrenia in a Japanese patient, exhibiting altered spine density, methamphetamine-induced cognitive dysfunction, and activation of RhoA/Rho-kinase signaling. However, it remains unclear whether the activation of RhoA/Rho-kinase signaling due to schizophrenia-associated Arhgap10 mutations causes the phenotypes of these model mice. Here, we investigated the effects of fasudil, a brain permeable Rho-kinase inhibitor, on altered spine density in the medial prefrontal cortex (mPFC) and on methamphetamine-induced cognitive impairment in a touchscreen‑based visual discrimination task in Arhgap10 S490P/NHEJ mice. Fasudil (20 mg/kg, intraperitoneal) suppressed the increased phosphorylation of myosin phosphatase-targeting subunit 1, a substrate of Rho-kinase, in the striatum and mPFC of Arhgap10 S490P/NHEJ mice. In addition, daily oral administration of fasudil (20 mg/kg/day) for 7 days ameliorated the reduced spine density of layer 2/3 pyramidal neurons in the mPFC. Moreover, fasudil (3-20 mg/kg, intraperitoneal) rescued the methamphetamine (0.3 mg/kg)-induced cognitive impairment of visual discrimination in Arhgap10 S490P/NHEJ mice. Our results suggest that Rho-kinase plays significant roles in the neuropathological changes in spine morphology and in the vulnerability of cognition to methamphetamine in mice with schizophrenia-associated Arhgap10 mutations.

Altered pain sensitivity in 5×familial Alzheimer disease mice is associated with dendritic spine loss in anterior cingulate cortex pyramidal neurons

ABSTRACT

Chronic pain is highly prevalent. Individuals with cognitive disorders such as Alzheimer disease are a susceptible population in which pain is frequently difficult to diagnosis. It is still unclear whether the pathological changes in patients with Alzheimer disease will affect pain processing. Here, we leverage animal behavior, neural activity recording, optogenetics, chemogenetics, and Alzheimer disease modeling to examine the contribution of the anterior cingulate cortex (ACC) neurons to pain response. The 5× familial Alzheimer disease mice show alleviated mechanical allodynia which can be regained by the genetic activation of ACC excitatory neurons. Furthermore, the lower peak neuronal excitation, delayed response initiation, as well as the dendritic spine reduction of ACC pyramidal neurons in 5×familial Alzheimer disease mice can be mimicked by Rac1 or actin polymerization inhibitor in wild-type (WT) mice. These findings indicate that abnormal of pain sensitivity in Alzheimer disease modeling mice is closely related to the variation of neuronal activity and dendritic spine loss in ACC pyramidal neurons, suggesting the crucial role of dendritic spine density in pain processing.

Stimulation of Neurite Outgrowth in Cerebrocortical Neurons by Sodium Channel Activator Brevetoxin-2 Requires Both N-Methyl-D-aspartate Receptor 2B (GluN2B) and p21 Protein (Cdc42/Rac)-Activated Kinase 1 (PAK1)

N-methyl-D-aspartate (NMDA) receptors play a critical role in activity-dependent dendritic arborization, spinogenesis, and synapse formation by stimulating calcium-dependent signaling pathways. Previously, we have shown that brevetoxin 2 (PbTx-2), a voltage-gated sodium channel (VGSC) activator, produces a concentration-dependent increase in intracellular sodium [Na+]I and increases NMDA receptor (NMDAR) open probabilities and NMDA-induced calcium (Ca2+) influxes. The objective of this study is to elucidate the downstream signaling mechanisms by which the sodium channel activator PbTx-2 influences neuronal morphology in murine cerebrocortical neurons. PbTx-2 and NMDA triggered distinct Ca2+-influx pathways, both of which involved the NMDA receptor 2B (GluN2B). PbTx-2-induced neurite outgrowth in day in vitro 1 (DIV-1) neurons required the small Rho GTPase Rac1 and was inhibited by both a PAK1 inhibitor and a PAK1 siRNA. PbTx-2 exposure increased the phosphorylation of PAK1 at Thr-212. At DIV-5, PbTx-2 induced increases in dendritic protrusion density, p-cofilin levels, and F-actin throughout the dendritic arbor and soma. Moreover, PbTx-2 increased miniature excitatory post-synaptic currents (mEPSCs). These data suggest that the stimulation of neurite outgrowth, spinogenesis, and synapse formation produced by PbTx-2 are mediated by GluN2B and PAK1 signaling.

Trio and Kalirin as unique enactors of Rho/Rac spatiotemporal precision

Rac1 and RhoA are among the most widely studied small GTPases. The classic dogma surrounding their biology has largely focused on their activity as an "on/off switch" of sorts. However, the advent of more sophisticated techniques, such as genetically-encoded FRET-based sensors, has afforded the ability to delineate the spatiotemporal regulation of Rac1 and RhoA. As a result, there has been a shift from this simplistic global view to one incorporating the precision of spatiotemporal modularity. This review summarizes emerging data surrounding the roles of Rac1 and RhoA as cytoskeletal regulators and examines how these new data have led to a revision of the traditional dogma which placed Rac1 and RhoA in antagonistic pathways. This more recent evidence suggests that rather than absolute activity levels, it is the tight spatiotemporal regulation of Rac1 and RhoA across multiple roles, from oppositional to complementary, that is necessary to execute coordinated cytoskeletal processes affecting cell structure, function, and migration. We focus on how Kalirin and Trio, as dual GEFs that target Rac1 and RhoA, are uniquely designed to provide the spatiotemporally-precise shifts in Rac/Rho balance which mediate changes in neuronal structure and function, particularly by way of cytoskeletal rearrangements. Finally, we review how alterations in Trio and/or Kalirin function are associated with cellular abnormalities and neuropsychiatric disease.

Chronic Physical and Vicarious Psychosocial Stress Alter Fentanyl Consumption and Nucleus Accumbens Rho GTPases in Male and Female C57BL/6 Mice

Chronic stress can increase the risk of developing a substance use disorder in vulnerable individuals. Numerous models have been developed to probe the underlying neurobiological mechanisms, however, most prior work has been restricted to male rodents, conducted only in rats, or introduces physical injury that can complicate opioid studies. Here we sought to establish how chronic psychosocial stress influences fentanyl consumption in male and female C57BL/6 mice. We used chronic social defeat stress (CSDS), or the modified vicarious chronic witness defeat stress (CWDS), and used social interaction to stratify mice as stress-susceptible or resilient. We then subjected mice to a 15 days fentanyl drinking paradigm in the home cage that consisted of alternating forced and choice periods with increasing fentanyl concentrations. Male mice susceptible to either CWDS or CSDS consumed more fentanyl relative to unstressed mice. CWDS-susceptible female mice did not differ from unstressed mice during the forced periods, but showed increased preference for fentanyl over time. We also found decreased expression of nucleus accumbens Rho GTPases in male, but not female mice following stress and fentanyl drinking. We also compare fentanyl drinking behavior in mice that had free access to plain water throughout. Our results indicate that stress-sensitized fentanyl consumption is dependent on both sex and behavioral outcomes to stress.

Phosphoproteomic of the acetylcholine pathway enables discovery of the PKC-β-PIX-Rac1-PAK cascade as a stimulatory signal for aversive learning

Acetylcholine is a neuromodulator critical for learning and memory. The cholinesterase inhibitor donepezil increases brain acetylcholine levels and improves Alzheimer's disease (AD)-associated learning disabilities. Acetylcholine activates striatal/nucleus accumbens dopamine receptor D2-expressing medium spiny neurons (D2R-MSNs), which regulate aversive learning through muscarinic receptor M1 (M1R). However, how acetylcholine stimulates learning beyond M1Rs remains unresolved. Here, we found that acetylcholine stimulated protein kinase C (PKC) in mouse striatal/nucleus accumbens. Our original kinase-oriented phosphoproteomic analysis revealed 116 PKC substrate candidates, including Rac1 activator β-PIX. Acetylcholine induced β-PIX phosphorylation and activation, thereby stimulating Rac1 effector p21-activated kinase (PAK). Aversive stimulus activated the M1R-PKC-PAK pathway in mouse D2R-MSNs. D2R-MSN-specific expression of PAK mutants by the Cre-Flex system regulated dendritic spine structural plasticity and aversive learning. Donepezil induced PAK activation in both accumbal D2R-MSNs and in the CA1 region of the hippocampus and enhanced D2R-MSN-mediated aversive learning. These findings demonstrate that acetylcholine stimulates M1R-PKC-β-PIX-Rac1-PAK signaling in D2R-MSNs for aversive learning and imply the cascade's therapeutic potential for AD as aversive learning is used to preliminarily screen AD drugs.

Rac-maninoff and Rho-vel: The symphony of Rho-GTPase signaling at excitatory synapses

Synaptic connections between neurons are essential for every facet of human cognition and are thus regulated with extreme precision. Rho-family GTPases, molecular switches that cycle between an active GTP-bound state and an inactive GDP-bound state, comprise a critical feature of synaptic regulation. Rho-GTPases are exquisitely controlled by an extensive suite of activators (GEFs) and inhibitors (GAPs and GDIs) and interact with many different signalling pathways to fulfill their roles in orchestrating the development, maintenance, and plasticity of excitatory synapses of the central nervous system. Among the mechanisms that control Rho-GTPase activity and signalling are cell surface receptors, GEF/GAP complexes that tightly regulate single Rho-GTPase dynamics, GEF/GAP and GEF/GEF functional complexes that coordinate multiple Rho-family GTPase activities, effector positive feedback loops, and mutual antagonism of opposing Rho-GTPase pathways. These complex regulatory mechanisms are employed by the cells of the nervous system in almost every step of development, and prominently figure into the processes of synaptic plasticity that underlie learning and memory. Finally, misregulation of Rho-GTPases plays critical roles in responses to neuronal injury, such as traumatic brain injury and neuropathic pain, and in neurodevelopmental and neurodegenerative disorders, including intellectual disability, autism spectrum disorder, schizophrenia, and Alzheimer's Disease. Thus, decoding the mechanisms of Rho-GTPase regulation and function at excitatory synapses has great potential for combatting many of the biggest current challenges in mental health.

A Kalirin missense mutation enhances dendritic RhoA signaling and leads to regression of cortical dendritic arbors across development

Normally, dendritic size is established prior to adolescence and then remains relatively constant into adulthood due to a homeostatic balance between growth and retraction pathways. However, schizophrenia is characterized by accelerated reductions of cerebral cortex gray matter volume and onset of clinical symptoms during adolescence, with reductions in layer 3 pyramidal neuron dendritic length, complexity, and spine density identified in multiple cortical regions postmortem. Nogo receptor 1 (NGR1) activation of the GTPase RhoA is a major pathway restricting dendritic growth in the cerebral cortex. We show that the NGR1 pathway is stimulated by OMGp and requires the Rho guanine nucleotide exchange factor Kalirin-9 (KAL9). Using a genetically encoded RhoA sensor, we demonstrate that a naturally occurring missense mutation in Kalrn, KAL-PT, that was identified in a schizophrenia cohort, confers enhanced RhoA activitation in neuronal dendrites compared to wild-type KAL. In mice containing this missense mutation at the endogenous locus, there is an adolescent-onset reduction in dendritic length and complexity of layer 3 pyramidal neurons in the primary auditory cortex. Spine density per unit length of dendrite is unaffected. Early adult mice with these structural deficits exhibited impaired detection of short gap durations. These findings provide a neuropsychiatric model of disease capturing how a mild genetic vulnerability may interact with normal developmental processes such that pathology only emerges around adolescence. This interplay between genetic susceptibility and normal adolescent development, both of which possess inherent individual variability, may contribute to heterogeneity seen in phenotypes in human neuropsychiatric disease.

A familiar study on self-limited childhood epilepsy patients using hIPSC-derived neurons shows a bias towards immaturity at the morphological, electrophysiological and gene expression levels

Background

Self-limited Childhood Epilepsies are the most prevalent epileptic syndrome in children. Its pathogenesis is unknown. In this disease, symptoms resolve spontaneously in approximately 50% of patients when maturity is reached, prompting to a maturation problem. The purpose of this study was to understand the molecular bases of this disease by generating and analyzing induced pluripotent stem cell-derived neurons from a family with 7 siblings, among whom 4 suffer from this disease.

Methods

Two affected siblings and, as controls, a healthy sister and the unaffected mother of the family were studied. Using exome sequencing, a homozygous variant in the FYVE, RhoGEF and PH Domain Containing 6 gene was identified in the patients as a putative genetic factor that could contribute to the development of this familial disorder. After informed consent was signed, skin biopsies from the 4 individuals were collected, fibroblasts were derived and reprogrammed and neurons were generated and characterized by markers and electrophysiology. Morphological, electrophysiological and gene expression analyses were performed on these neurons.

Results

Bona fide induced pluripotent stem cells and derived neurons could be generated in all cases. Overall, there were no major shifts in neuronal marker expression among patient and control-derived neurons. Compared to two familial controls, neurons from patients showed shorter axonal length, a dramatic reduction in synapsin-1 levels and cytoskeleton disorganization. In addition, neurons from patients developed a lower action potential threshold with time of in vitro differentiation and the amount of current needed to elicit an action potential (rheobase) was smaller in cells recorded from NE derived from patients at 12 weeks of differentiation when compared with shorter times in culture. These results indicate an increased excitability in patient cells that emerges with the time in culture. Finally, functional genomic analysis showed a biased towards immaturity in patient-derived neurons.

Conclusions

We are reporting the first in vitro model of self-limited childhood epilepsy, providing the cellular bases for future in-depth studies to understand its pathogenesis. Our results show patient-specific neuronal features reflecting immaturity, in resonance with the course of the disease and previous imaging studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-021-02658-2.

Rho Signaling in Synaptic Plasticity, Memory, and Brain Disorders

Memory impairments are associated with many brain disorders such as autism, Alzheimer's disease, and depression. Forming memories involves modifications of synaptic transmission and spine morphology. The Rho family small GTPases are key regulators of synaptic plasticity by affecting various downstream molecules to remodel the actin cytoskeleton. In this paper, we will review recent studies on the roles of Rho proteins in the regulation of hippocampal long-term potentiation (LTP) and long-term depression (LTD), the most extensively studied forms of synaptic plasticity widely regarded as cellular mechanisms for learning and memory. We will also discuss the involvement of Rho signaling in spine morphology, the structural basis of synaptic plasticity and memory formation. Finally, we will review the association between brain disorders and abnormalities of Rho function. It is expected that studying Rho signaling at the synapse will contribute to the understanding of how memory is formed and disrupted in diseases.

Dendritic Spine Initiation in Brain Development, Learning and Diseases and Impact of BAR-Domain Proteins

Dendritic spines are small, bulbous protrusions along neuronal dendrites where most of the excitatory synapses are located. Dendritic spine density in normal human brain increases rapidly before and after birth achieving the highest density around 2-8 years. Density decreases during adolescence, reaching a stable level in adulthood. The changes in dendritic spines are considered structural correlates for synaptic plasticity as well as the basis of experience-dependent remodeling of neuronal circuits. Alterations in spine density correspond to aberrant brain function observed in various neurodevelopmental and neuropsychiatric disorders. Dendritic spine initiation affects spine density. In this review, we discuss the importance of spine initiation in brain development, learning, and potential complications resulting from altered spine initiation in neurological diseases. Current literature shows that two Bin Amphiphysin Rvs (BAR) domain-containing proteins, MIM/Mtss1 and SrGAP3, are involved in spine initiation. We review existing literature and open databases to discuss whether other BAR-domain proteins could also take part in spine initiation. Finally, we discuss the potential molecular mechanisms on how BAR-domain proteins could regulate spine initiation.

Neuroplasticity and Multilevel System of Connections Determine the Integrative Role of Nucleus Accumbens in the Brain Reward System

A growing body of evidence suggests that nucleus accumbens (NAc) plays a significant role not only in the physiological processes associated with reward and satisfaction but also in many diseases of the central nervous system. Summary of the current state of knowledge on the morphological and functional basis of such a diverse function of this structure may be a good starting point for further basic and clinical research. The NAc is a part of the brain reward system (BRS) characterized by multilevel organization, extensive connections, and several neurotransmitter systems. The unique role of NAc in the BRS is a result of: (1) hierarchical connections with the other brain areas, (2) a well-developed morphological and functional plasticity regulating short- and long-term synaptic potentiation and signalling pathways, (3) cooperation among several neurotransmitter systems, and (4) a supportive role of neuroglia involved in both physiological and pathological processes. Understanding the complex function of NAc is possible by combining the results of morphological studies with molecular, genetic, and behavioral data. In this review, we present the current views on the NAc function in physiological conditions, emphasizing the role of its connections, neuroplasticity processes, and neurotransmitter systems.

Connecting the Neurobiology of Developmental Brain Injury: Neuronal Arborisation as a Regulator of Dysfunction and Potential Therapeutic Target

Neurodevelopmental disorders can derive from a complex combination of genetic variation and environmental pressures on key developmental processes. Despite this complex aetiology, and the equally complex array of syndromes and conditions diagnosed under the heading of neurodevelopmental disorder, there are parallels in the neuropathology of these conditions that suggest overlapping mechanisms of cellular injury and dysfunction. Neuronal arborisation is a process of dendrite and axon extension that is essential for the connectivity between neurons that underlies normal brain function. Disrupted arborisation and synapse formation are commonly reported in neurodevelopmental disorders. Here, we summarise the evidence for disrupted neuronal arborisation in these conditions, focusing primarily on the cortex and hippocampus. In addition, we explore the developmentally specific mechanisms by which neuronal arborisation is regulated. Finally, we discuss key regulators of neuronal arborisation that could link to neurodevelopmental disease and the potential for pharmacological modification of arborisation and the formation of synaptic connections that may provide therapeutic benefit in the future.

Fe65: A Scaffolding Protein of Actin Regulators

The scaffolding protein family Fe65, composed of Fe65, Fe65L1, and Fe65L2, was identified as an interaction partner of the amyloid precursor protein (APP), which plays a key function in Alzheimer’s disease. All three Fe65 family members possess three highly conserved interaction domains, forming complexes with diverse binding partners that can be assigned to different cellular functions, such as transactivation of genes in the nucleus, modulation of calcium homeostasis and lipid metabolism, and regulation of the actin cytoskeleton. In this article, we rule out putative new intracellular signaling mechanisms of the APP-interacting protein Fe65 in the regulation of actin cytoskeleton dynamics in the context of various neuronal functions, such as cell migration, neurite outgrowth, and synaptic plasticity.

Rac1 Signaling in Amygdala Astrocytes Regulates Fear Memory Acquisition and Retrieval

The importance of astrocytes in behavior control is increasingly appreciated, but little is known about the effects of their dynamic activity in regulating learning and memory. In the present study, we constructed AAVs of photoactivatable and photoinactivatable Ras-related C3 botulinum toxin substrate 1 (Rac1) under the mGFAP promoter, which enabled the manipulation of Rac1 activity in astrocytes by optical stimulation in free-moving mice. We found that both up-regulation and down-regulation of astrocytic Rac1 activity in the basolateral amygdala (BLA) attenuated memory acquisition in a fear conditioning mouse model. Meanwhile, neuronal activation in the BLA induced by memory acquisition was inhibited under both the up- and down-regulation of astrocytic Rac1 activity during training. In terms of the impact on fear memory retrieval, we found both up- and down-regulation of BLA astrocytic Rac1 activity impaired memory retrieval of fear conditioning and memory retrieval-induced neuronal activation. Notably, the effect of astrocytic Rac1 on memory retrieval was reversible. Our results demonstrate that the normal activity of astrocytic Rac1 is necessary for the activation of neurons and memory formation. Both activation and inactivation of astrocytic Rac1 activity in the BLA reduced the excitability of neurons, and thereby impaired fear memory acquisition and retrieval.

The Rac-GEF Tiam1 Promotes Dendrite and Synapse Stabilization of Dentate Granule Cells and Restricts Hippocampal-Dependent Memory Functions

The dentate gyrus (DG) controls information flow into the hippocampus and is critical for learning, memory, pattern separation, and spatial coding, while DG dysfunction is associated with neuropsychiatric disorders. Despite its importance, the molecular mechanisms regulating DG neural circuit assembly and function remain unclear. Here, we identify the Rac-GEF Tiam1 as an important regulator of DG development and associated memory processes. In the hippocampus, Tiam1 is predominantly expressed in the DG throughout life. Global deletion of Tiam1 in male mice results in DG granule cells with simplified dendritic arbors, reduced dendritic spine density, and diminished excitatory synaptic transmission. Notably, DG granule cell dendrites and synapses develop normally in Tiam1 KO mice, resembling WT mice at postnatal day 21 (P21), but fail to stabilize, leading to dendrite and synapse loss by P42. These results indicate that Tiam1 promotes DG granule cell dendrite and synapse stabilization late in development. Tiam1 loss also increases the survival, but not the production, of adult-born DG granule cells, possibly because of greater circuit integration as a result of decreased competition with mature granule cells for synaptic inputs. Strikingly, both male and female mice lacking Tiam1 exhibit enhanced contextual fear memory and context discrimination. Together, these results suggest that Tiam1 is a key regulator of DG granule cell stabilization and function within hippocampal circuits. Moreover, based on the enhanced memory phenotype of Tiam1 KO mice, Tiam1 may be a potential target for the treatment of disorders involving memory impairments.SIGNIFICANCE STATEMENT The dentate gyrus (DG) is important for learning, memory, pattern separation, and spatial navigation, and its dysfunction is associated with neuropsychiatric disorders. However, the molecular mechanisms controlling DG formation and function remain elusive. By characterizing mice lacking the Rac-GEF Tiam1, we demonstrate that Tiam1 promotes the stabilization of DG granule cell dendritic arbors, spines, and synapses, whereas it restricts the survival of adult-born DG granule cells, which compete with mature granule cells for synaptic integration. Notably, mice lacking Tiam1 also exhibit enhanced contextual fear memory and context discrimination. These findings establish Tiam1 as an essential regulator of DG granule cell development, and identify it as a possible therapeutic target for memory enhancement.

Maternal Exposure to Di-(2-ethylhexyl) Phthalate Impairs Hippocampal Synaptic Plasticity in Male Offspring: Involvement of Damage to Dendritic Spine Development

Exposure to di-(2-ethylhexyl) phthalate (DEHP), a widely used kind of plasticizer, can result in neurodevelopment impairments and learning and memory disorders. We studied the effects and possible mechanisms of maternal DEHP treatment on hippocampal synaptic plasticity in offspring. Pregnant Wistar rats were randomly divided into four groups and received 0, 30, 300, 750 (mg/kg)/d DEHP by gavage from gestational day (GD) 0 to postnatal day (PN) 21. Our data showed that DEHP exposure impaired hippocampal synaptic plasticity, damaged synaptic ultrastructure, and decreased synaptic protein levels in male pups. Furthermore, DEHP decreased the density of dendritic spines, affected F-actin polymerization, and downregulated the Rac1/PAK/LIMK1/cofilin signaling pathway in male offspring. However, the alterations in the hippocampi of female offspring were not observed. These results illustrate that maternal DEHP exposure could impair hippocampal synaptic plasticity by affecting synaptic structure and dendritic spine development in male offspring, which may be attributed to altered cytoskeleton construction induced by downregulation of the Rac1/PAK/LIMK1/cofilin signaling pathway.

Myosin 18Aα targets the guanine nucleotide exchange factor β-Pix to the dendritic spines of cerebellar Purkinje neurons and promotes spine maturation

Myosin 18Aα is a myosin 2-like protein containing unique N- and C-terminal protein interaction domains that co-assembles with myosin 2. One protein known to bind to myosin 18Aα is β-Pix, a guanine nucleotide exchange factor (GEF) for Rac1 and Cdc42 that has been shown to promote dendritic spine maturation by activating the assembly of actin and myosin filaments in spines. Here, we show that myosin 18A⍺ concentrates in the spines of cerebellar Purkinje neurons via co-assembly with myosin 2 and through an actin binding site in its N-terminal extension. miRNA-mediated knockdown of myosin 18A⍺ results in a significant defect in spine maturation that is rescued by an RNAi-immune version of myosin 18A⍺. Importantly, β-Pix co-localizes with myosin 18A⍺ in spines, and its spine localization is lost upon myosin 18A⍺ knockdown or when its myosin 18A⍺ binding site is deleted. Finally, we show that the spines of myosin 18A⍺ knockdown Purkinje neurons contain significantly less F-actin and myosin 2. Together, these data argue that mixed filaments of myosin 2 and myosin 18A⍺ form a complex with β-Pix in Purkinje neuron spines that promotes spine maturation by enhancing the assembly of actin and myosin filaments downstream of β-Pix's GEF activity.

Bioinformatics Investigation and Contribution of Other Chromosomes Besides Chromosome 21 in the Risk of Down Syndrome Development

INTRODUCTION

Down syndrome as a genetic disorder is a popular research topic in molecular studies. One way to study Down syndrome is via bioinformatics.

METHODS

In this study, a gene expression profile from a microarray study was selected for more investigation.

RESULTS

The study of Down syndrome patients shows specific genes with differential expression and network centrality properties. These genes are introduced as RHOA, FGF2, FYN, and CD44, and their level of expression is high in these patients.

CONCLUSION

This study suggests that besides chromosomes 21, there are additional contributing chromosomes to the risk of Down syndrome development. Besides, these genes could be used for clinical studies after more analysis.

Functional repertoire of protein kinases and phosphatases in synaptic plasticity and associated neurological disorders

Protein phosphorylation and dephosphorylation are two essential and vital cellular mechanisms that regulate many receptors and enzymes through kinases and phosphatases. Ca²⁺- dependent kinases and phosphatases are responsible for controlling neuronal processing; balance is achieved through opposition. During molecular mechanisms of learning and memory, kinases generally modulate positively while phosphatases modulate negatively. This review outlines some of the critical physiological and structural aspects of kinases and phosphatases involved in maintaining postsynaptic structural plasticity. It also explores the link between neuronal disorders and the deregulation of phosphatases and kinases.

Effects of 7,8-Dihydroxyflavone on Lipid Isoprenoid and Rho Protein Levels in Brains of Aged C57BL/6 Mice

Synaptic impairment may be the main cause of cognitive dysfunction in brain aging that is probably due to a reduction in synaptic contact between the axonal buttons and dendritic spines. Rho proteins including the small GTPase Rac1 have become key regulators of neuronal morphogenesis that supports synaptic plasticity. Small Rho- and Ras-GTPases are post-translationally modified by the isoprenoids geranylgeranyl pyrophosphate (GGPP) and farnesyl pyrophosphate (FPP), respectively. For all GTPases, anchoring in the plasma membrane is essential for their activation by guanine nucleotide exchange factors (GEFs). Rac1-specific GEFs include the protein T lymphoma invasion and metastasis 1 (Tiam1). Tiam1 interacts with the TrkB receptor to mediate the brain-derived neurotrophic factor (BDNF)-induced activation of Rac1, resulting in cytoskeletal rearrangement and changes in cellular morphology. The flavonoid 7,8-dihydroxyflavone (7,8-DHF) acts as a highly affine-selective TrkB receptor agonist and causes the dimerization and autophosphorylation of the TrkB receptor and thus the activation of downstream signaling pathways. In the current study, we investigated the effects of 7,8-DHF on cerebral lipid isoprenoid and Rho protein levels in male C57BL/6 mice aged 3 and 23 months. Aged mice were daily treated with 100 mg/kg b.w. 7,8-DHF by oral gavage for 21 days. FPP, GGPP, and cholesterol levels were determined in brain tissue. In the same tissue, the protein content of Tiam1 and TrkB in was measured. The cellular localization of the small Rho-GTPase Rac1 and small Rab-GTPase Rab3A was studied in total brain homogenates and membrane preparations. We report the novel finding that 7,8-DHF restored levels of the Rho proteins Rac1 and Rab3A in membrane preparations isolated from brains of treated aged mice. The selective TrkB agonist 7,8-DHF did not affect BDNF and TrkB levels, but restored Tiam1 levels that were found to be reduced in brains of aged mice. FPP, GGPP, and cholesterol levels were significantly elevated in brains of aged mice but not changed by 7,8-DHF treatment. Hence, 7,8-DHF may be useful as pharmacological tool to treat age-related cognitive dysfunction although the underlying mechanisms need to be elucidated in detail.

The Role of ADF/Cofilin in Synaptic Physiology and Alzheimer's Disease

Actin-depolymerization factor (ADF)/cofilin, a family of actin-binding proteins, are critical for the regulation of actin reorganization in response to various signals. Accumulating evidence indicates that ADF/cofilin also play important roles in neuronal structure and function, including long-term potentiation and depression. These are the most extensively studied forms of long-lasting synaptic plasticity and are widely regarded as cellular mechanisms underlying learning and memory. ADF/cofilin regulate synaptic function through their effects on dendritic spines and the trafficking of glutamate receptors, the principal mediator of excitatory synaptic transmission in vertebrates. Regulation of ADF/cofilin involves various signaling pathways converging on LIM domain kinases and slingshot phosphatases, which phosphorylate/inactivate and dephosphorylate/activate ADF/cofilin, respectively. Actin-depolymerization factor/cofilin activity is also regulated by other actin-binding proteins, activity-dependent subcellular distribution and protein translation. Abnormalities in ADF/cofilin have been associated with several neurodegenerative disorders such as Alzheimer’s disease. Therefore, investigating the roles of ADF/cofilin in the brain is not only important for understanding the fundamental processes governing neuronal structure and function, but also may provide potential therapeutic strategies to treat brain disorders.

Dysregulation of Neurite Outgrowth and Cell Migration in Autism and Other Neurodevelopmental Disorders

Despite decades of study, elucidation of the underlying etiology of complex developmental disorders such as autism spectrum disorder (ASD), schizophrenia (SCZ), intellectual disability (ID), and bipolar disorder (BPD) has been hampered by the inability to study human neurons, the heterogeneity of these disorders, and the relevance of animal model systems. Moreover, a majority of these developmental disorders have multifactorial or idiopathic (unknown) causes making them difficult to model using traditional methods of genetic alteration. Examination of the brains of individuals with ASD and other developmental disorders in both post-mortem and MRI studies shows defects that are suggestive of dysregulation of embryonic and early postnatal development. For ASD, more recent genetic studies have also suggested that risk genes largely converge upon the developing human cerebral cortex between weeks 8 and 24 in utero. Yet, an overwhelming majority of studies in autism rodent models have focused on postnatal development or adult synaptic transmission defects in autism related circuits. Thus, studies looking at early developmental processes such as proliferation, cell migration, and early differentiation, which are essential to build the brain, are largely lacking. Yet, interestingly, a few studies that did assess early neurodevelopment found that alterations in brain structure and function associated with neurodevelopmental disorders (NDDs) begin as early as the initial formation and patterning of the neural tube. By the early to mid-2000s, the derivation of human embryonic stem cells (hESCs) and later induced pluripotent stem cells (iPSCs) allowed us to study living human neural cells in culture for the first time. Specifically, iPSCs gave us the unprecedented ability to study cells derived from individuals with idiopathic disorders. Studies indicate that iPSC-derived neural cells, whether precursors or "matured" neurons, largely resemble cortical cells of embryonic humans from weeks 8 to 24. Thus, these cells are an excellent model to study early human neurodevelopment, particularly in the context of genetically complex diseases. Indeed, since 2011, numerous studies have assessed developmental phenotypes in neurons derived from individuals with both genetic and idiopathic forms of ASD and other NDDs. However, while iPSC-derived neurons are fetal in nature, they are post-mitotic and thus cannot be used to study developmental processes that occur before terminal differentiation. Moreover, it is important to note that during the 8-24-week window of human neurodevelopment, neural precursor cells are actively undergoing proliferation, migration, and early differentiation to form the basic cytoarchitecture of the brain. Thus, by studying NPCs specifically, we could gain insight into how early neurodevelopmental processes contribute to the pathogenesis of NDDs. Indeed, a few studies have explored NPC phenotypes in NDDs and have uncovered dysregulations in cell proliferation. Yet, few studies have explored migration and early differentiation phenotypes of NPCs in NDDs. In this chapter, we will discuss cell migration and neurite outgrowth and the role of these processes in neurodevelopment and NDDs. We will begin by reviewing the processes that are important in early neurodevelopment and early cortical development. We will then delve into the roles of neurite outgrowth and cell migration in the formation of the brain and how errors in these processes affect brain development. We also provide review of a few key molecules that are involved in the regulation of neurite outgrowth and migration while discussing how dysregulations in these molecules can lead to abnormalities in brain structure and function thereby highlighting their contribution to pathogenesis of NDDs. Then we will discuss whether neurite outgrowth, migration, and the molecules that regulate these processes are associated with ASD. Lastly, we will review the utility of iPSCs in modeling NDDs and discuss future goals for the study of NDDs using this technology.

Nitric oxide controls excitatory/inhibitory balance in the hypoglossal nucleus during early postnatal development

Synaptic remodeling during early postnatal development lies behind neuronal networks refinement and nervous system maturation. In particular, the respiratory system is immature at birth and is subjected to significant postnatal development. In this context, the excitatory/inhibitory balance dramatically changes in the respiratory-related hypoglossal nucleus (HN) during the 3 perinatal weeks. Since, development abnormalities of hypoglossal motor neurons (HMNs) are associated with sudden infant death syndrome and obstructive sleep apnea, deciphering molecular partners behind synaptic remodeling in the HN is of basic and clinical relevance. Interestingly, a transient expression of the neuronal isoform of nitric oxide (NO) synthase (NOS) occurs in HMNs at neonatal stage that disappears before postnatal day 21 (P21). NO, in turn, is a determining factor for synaptic refinement in several physiopathological conditions. Here, intracerebroventricular chronic administration (P7–P21) of the broad spectrum NOS inhibitor l-NAME (N(ω)-nitro-l-arginine methyl ester) differentially affected excitatory and inhibitory rearrangement during this neonatal interval in the rat. Whilst l-NAME led to a reduction in the number of excitatory structures, inhibitory synaptic puncta were increased at P21 in comparison to administration of the inactive stereoisomer d-NAME. Finally, l-NAME decreased levels of the phosphorylated form of myosin light chain in the nucleus, which is known to regulate the actomyosin contraction apparatus. These outcomes indicate that physiologically synthesized NO modulates excitatory/inhibitory balance during early postnatal development by acting as an anti-synaptotrophic and/or synaptotoxic factor for inhibitory synapses, and as a synaptotrophin for excitatory ones. The mechanism of action could rely on the modulation of the actomyosin contraction apparatus.

Disruption of CTNND2, encoding delta-catenin, causes a penetrant attention deficit disorder and myopia

Attention deficit hyperactivity disorder (ADHD) is a common and highly heritable neurodevelopmental disorder with poorly understood pathophysiology and genetic mechanisms. A balanced chromosomal translocation interrupts CTNND2 in several members of a family with profound attentional deficit and myopia, and disruption of the gene was found in a separate unrelated individual with ADHD and myopia. CTNND2 encodes a brain-specific member of the adherens junction complex essential for postsynaptic and dendritic development, a site of potential pathophysiology in attentional disorders. Therefore, we propose that the severe and highly penetrant nature of the ADHD phenotype in affected individuals identifies CTNND2 as a potential gateway to ADHD pathophysiology similar to the DISC1 translocation in psychosis or AUTS2 in autism.

Rho GTPases in the Amygdala-A Switch for Fears?

Fear is a fundamental evolutionary process for survival. However, excess or irrational fear hampers normal activity and leads to phobia. The amygdala is the primary brain region associated with fear learning and conditioning. There, Rho GTPases are molecular switches that act as signaling molecules for further downstream processes that modulate, among others, dendritic spine morphogenesis and thereby play a role in fear conditioning. The three main Rho GTPases—RhoA, Rac1, and Cdc42, together with their modulators, are known to be involved in many psychiatric disorders that affect the amygdala′s fear conditioning mechanism. Rich2, a RhoGAP mainly for Rac1 and Cdc42, has been studied extensively in such regard. Here, we will discuss these effectors, along with Rich2, as a molecular switch for fears, especially in the amygdala. Understanding the role of Rho GTPases in fear controlling could be beneficial for the development of therapeutic strategies targeting conditions with abnormal fear/anxiety-like behaviors.

[Intrinsic and extrinsic mechanisms regulating neuronal dendrite morphogenesis]

Neurons are the structural and functional unit of the nervous system. Precisely regulated dendrite morphogenesis is the basis of neural circuit assembly. Numerous studies have been conducted to explore the regulatory mechanisms of dendritic morphogenesis. According to their action regions, we divide them into two categories: the intrinsic and extrinsic regulators of neuronal dendritic morphogenesis. Intrinsic factors are cell type-specific transcription factors, actin polymerization or depolymerization regulators and regulators of the secretion or endocytic pathways. These intrinsic factors are produced by neuron itself and play an important role in regulating the development of dendrites. The extrinsic regulators are either secreted proteins or transmembrane domain containing cell adhesion molecules. They often form receptor-ligand pairs to mediate attractive or repulsive dendritic guidance. In this review, we summarize recent findings on the intrinsic and external molecular mechanisms of dendrite morphogenesis from multiple model organisms, including Caenorhabditis elegans, Drosophila and mice. These studies will provide a better understanding on how defective dendrite development and maintenance are associated with neurological diseases.

Dendritic remodeling of D1 neurons by RhoA/Rho-kinase mediates depression-like behavior

Depression alters the structure and function of brain reward circuitry. Preclinical evidence suggests that medium spiny neurons (MSNs) in the nucleus accumbens (NAc) undergo structural plasticity; however, the molecular mechanism and behavioral significance is poorly understood. Here we report that atrophy of D1, but not D2 receptor containing MSNs is strongly associated with social avoidance in mice subject to social defeat stress. D1-MSN atrophy is caused by cell-type specific upregulation of the GTPase RhoA and its effector Rho-kinase. Pharmacologic and genetic reduction of activated RhoA prevents depressive outcomes to stress by preventing loss of D1-MSN dendritic arbor. Pharmacologic and genetic promotion of activated RhoA enhances depressive outcomes by reducing D1-MSN dendritic arbor and is sufficient to promote depressive-like behaviors in the absence of stress. Chronic treatment with Rho-kinase inhibitor Y-27632 after chronic social defeat stress reverses depression-like behaviors by restoring D1-MSN dendritic complexity. Taken together, our data indicate functional roles for RhoA and Rho-kinase in mediating depression-like behaviors via dendritic remodeling of NAc D1-MSNs and may prove a useful target for new depression therapeutics.

Synaptic Plasticity on Motoneurons After Axotomy: A Necessary Change in Paradigm

Motoneurons axotomized by peripheral nerve injuries experience profound changes in their synaptic inputs that are associated with a neuroinflammatory response that includes local microglia and astrocytes. This reaction is conserved across different types of motoneurons, injuries, and species, but also displays many unique features in each particular case. These reactions have been amply studied, but there is still a lack of knowledge on their functional significance and mechanisms. In this review article, we compiled data from many different fields to generate a comprehensive conceptual framework to best interpret past data and spawn new hypotheses and research. We propose that synaptic plasticity around axotomized motoneurons should be divided into two distinct processes. First, a rapid cell-autonomous, microglia-independent shedding of synapses from motoneuron cell bodies and proximal dendrites that is reversible after muscle reinnervation. Second, a slower mechanism that is microglia-dependent and permanently alters spinal cord circuitry by fully eliminating from the ventral horn the axon collaterals of peripherally injured and regenerating sensory Ia afferent proprioceptors. This removes this input from cell bodies and throughout the dendritic tree of axotomized motoneurons as well as from many other spinal neurons, thus reconfiguring ventral horn motor circuitries to function after regeneration without direct sensory feedback from muscle. This process is modulated by injury severity, suggesting a correlation with poor regeneration specificity due to sensory and motor axons targeting errors in the periphery that likely render Ia afferent connectivity in the ventral horn nonadaptive. In contrast, reversible synaptic changes on the cell bodies occur only while motoneurons are regenerating. This cell-autonomous process displays unique features according to motoneuron type and modulation by local microglia and astrocytes and generally results in a transient reduction of fast synaptic activity that is probably replaced by embryonic-like slow GABA depolarizations, proposed to relate to regenerative mechanisms.

The Role of Rac GTPase in Dendritic Spine Morphogenesis and Memory

The ability to form memories in the brain is needed for daily functions, and its impairment is associated with human mental disorders. Evidence indicates that long-term memory (LTM)-related processes such as its consolidation, extinction and forgetting involve changes of synaptic efficacy produced by alterations in neural transmission and morphology. Modulation of the morphology and number of dendritic spines has been proposed to contribute to changes in neuronal transmission mediating such LTM-related processes. Rac GTPase activity is regulated by synaptic activation and it can affect spine morphology by controlling actin-regulatory proteins. Recent evidence shows that changes in Rac GTPase activity affect memory consolidation, extinction, erasure and forgetting and can affect spine morphology in brain areas that mediate these behaviors. Altered Rac GTPase activity is associated with abnormal spine morphology and brain disorders. By affecting Rac GTPase activity we can further understand the roles of spine morphogenesis in memory. Moreover, manipulation of Rac GTPase activity may serve as a therapeutic tool for the treatment of memory-related brain diseases.

The intellectual disability protein Oligophrenin-1 controls astrocyte morphology and migration

Astrocytes are involved in several aspects of neuronal development and properties which are altered in intellectual disability (ID). Oligophrenin-1 is a RhoGAP protein implicated in actin cytoskeleton regulation, and whose mutations are associated with X-linked ID. Oligophrenin-1 is expressed in neurons, where its functions have been widely reported at the synapse, as well as in glial cells. However, its roles in astrocytes are still largely unexplored. Using in vitro and in vivo models of oligophrenin1 disruption in astrocytes, we found that oligophrenin1 regulates at the molecular level the RhoA/ROCK/MLC2 pathway in astroglial cells. We also showed at the cellular level that oligophrenin1 modulates astrocyte morphology and migration both in vitro and in vivo, and is involved in glial scar formation. Altogether, these data suggest that oligophrenin1 deficiency alters not only neuronal but also astrocytic functions, which might contribute to the development of ID.

Genetic Dissection of Alzheimer's Disease Using Drosophila Models

Alzheimer’s disease (AD), a main cause of dementia, is the most common neurodegenerative disease that is related to abnormal accumulation of the amyloid β (Aβ) protein. Despite decades of intensive research, the mechanisms underlying AD remain elusive, and the only available treatment remains symptomatic. Molecular understanding of the pathogenesis and progression of AD is necessary to develop disease-modifying treatment. Drosophila, as the most advanced genetic model, has been used to explore the molecular mechanisms of AD in the last few decades. Here, we introduce Drosophila AD models based on human Aβ and summarize the results of their genetic dissection. We also discuss the utility of functional genomics using the Drosophila system in the search for AD-associated molecular mechanisms in the post-genomic era.

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.

Dendritic spines: Revisiting the physiological role

Dendritic spines are small, thin, specialized protrusions from neuronal dendrites, primarily localized in the excitatory synapses. Sophisticated imaging techniques revealed that dendritic spines are complex structures consisting of a dense network of cytoskeletal, transmembrane and scaffolding molecules, and numerous surface receptors. Molecular signaling pathways, mainly Rho and Ras family small GTPases pathways that converge on actin cytoskeleton, regulate the spine morphology and dynamics bi-directionally during synaptic activity. During synaptic plasticity the number and shapes of dendritic spines undergo radical reorganizations. Long-term potentiation (LTP) induction promote spine head enlargement and the formation and stabilization of new spines. Long-term depression (LTD) results in their shrinkage and retraction. Reports indicate increased spine density in the pyramidal neurons of autism and Fragile X syndrome patients and reduced density in the temporal gyrus loci of schizophrenic patients. Post-mortem reports of Alzheimer's brains showed reduced spine number in the hippocampus and cortex. This review highlights the spine morphogenesis process, the activity-dependent structural plasticity and mechanisms by which synaptic activity sculpts the dendritic spines, the structural and functional changes in spines during learning and memory using LTP and LTD processes. It also discusses on spine status in neurodegenerative diseases and the impact of nootropics and neuroprotective agents on the functional restoration of dendritic spines.

Dystrobrevin is required postsynaptically for homeostatic potentiation at the Drosophila NMJ

Evolutionarily conserved homeostatic systems have been shown to modulate synaptic efficiency at the neuromuscular junctions of organisms. While advances have been made in identifying molecules that function presynaptically during homeostasis, limited information is currently available on how postsynaptic alterations affect presynaptic function. We previously identified a role for postsynaptic Dystrophin in the maintenance of evoked neurotransmitter release. We herein demonstrated that Dystrobrevin, a member of the Dystrophin Glycoprotein Complex, was delocalized from the postsynaptic region in the absence of Dystrophin. A newly-generated Dystrobrevin mutant showed elevated evoked neurotransmitter release, increased bouton numbers, and a readily releasable pool of synaptic vesicles without changes in the function or numbers of postsynaptic glutamate receptors. In addition, we provide evidence to show that the highly conserved Cdc42 Rho GTPase plays a key role in the postsynaptic Dystrophin/Dystrobrevin pathway for synaptic homeostasis. The present results give novel insights into the synaptic deficits underlying Duchenne Muscular Dystrophy affected by a dysfunctional Dystrophin Glycoprotein complex.