Aberrant Rac1-cofilin signaling mediates defects in dendritic spines, synaptic function, and sensory perception in fragile X syndrome

Destabilization of fear memory by Rac1-driven engram-microglia communication in hippocampus

Rac1 is a key regulator of the cytoskeleton and neuronal plasticity, and is known to play a critical role in psychological and cognitive brain disorders. To elucidate the engram specific Rac1 signaling in fear memory, a doxycycline (Dox)-dependent robust activity marking (RAM) system was used to label dorsal dentate gyrus (DG) engram cells in mice during contextual fear conditioning. Rac1 mRNA and protein levels in DG engram cells were peaked at 24 h (day 1) after fear conditioning and were more abundant in the fear engram cells than in the non-engram cells. Optogenetic activation of Rac1 in a temporal manner in DG engram cells before memory retrieval decreased the freezing level in the fear context. Optogenetic activation of Rac1 increased autophagy protein 7 (ATG7) expression in the DG engram cells and activated DG microglia. Microglia-specific transcriptomics and fluorescence in situ hybridization revealed that overexpression of ATG7 in the fear engram cells upregulated the mRNA of Toll-like receptor TLR2/4 in DG microglia. Knockdown of microglial TLR2/4 rescued fear memory destabilization induced by ATG7 overexpression or Rac1 activation in DG engram cells. These results indicate that Rac1-driven communications between engram cells and microglia contributes to contextual fear memory destabilization, and is mediated by ATG7 and TLR2/4, and suggest a novel mechanistic framework for the cytoskeletal regulator in fear memory interference.

FLNA regulates neuronal maturation by modulating RAC1-Cofilin activity in the developing cortex

Periventricular nodular heterotopia (PNH), the most common brain malformation diagnosed in adulthood, is characterized by the presence of neuronal nodules along the ventricular walls. PNH is mainly associated with mutations in the FLNA gene - encoding an actin-binding protein - and patients often develop epilepsy. However, the molecular mechanisms underlying the neuronal failure still remain elusive. It has been hypothesized that dysfunctional cortical circuitry, rather than ectopic neurons, may explain the clinical manifestations. To address this issue, we depleted FLNA from cortical pyramidal neurons of a conditional Flnaflox/flox mice by timed in utero electroporation of Cre recombinase. We found that FLNA regulates dendritogenesis and spinogenesis thus promoting an appropriate excitatory/inhibitory inputs balance. We demonstrated that FLNA modulates RAC1 and cofilin activity through its interaction with the Rho-GTPase Activating Protein 24 (ARHGAP24). Collectively, we disclose an uncharacterized role of FLNA and provide strong support for neural circuit dysfunction being a consequence of FLNA mutations.

Tactile sensory processing deficits in genetic mouse models of autism spectrum disorder

Altered sensory processing is a common feature in autism spectrum disorder (ASD), as recognized in the Diagnostic and Statistical Manual of Mental Disorders (DSM-5). Although altered responses to tactile stimuli are observed in over 60% of individuals with ASD, the neurobiological basis of this phenomenon is poorly understood. ASD has a strong genetic component and genetic mouse models can provide valuable insights into the mechanisms underlying tactile abnormalities in ASD. This review critically addresses recent findings regarding tactile processing deficits found in mouse models of ASD, with a focus on behavioral, anatomical, and functional alterations. Particular attention was given to cellular and circuit-level functional alterations, both in the peripheral and central nervous systems, with the objective of highlighting possible convergence mechanisms across models. By elucidating the impact of mutations in ASD candidate genes on somatosensory circuits and correlating them with behavioral phenotypes, this review significantly advances our understanding of tactile deficits in ASD. Such insights not only broaden our comprehension but also pave the way for future therapeutic interventions.

Alteration of synaptic protein composition during developmental synapse maturation

The postsynaptic density (PSD) is a collection of specialized proteins assembled beneath the postsynaptic membrane of dendritic spines. The PSD proteome comprises ~1000 proteins, including neurotransmitter receptors, scaffolding proteins and signalling enzymes. Many of these proteins have essential roles in synaptic function and plasticity. During brain development, changes are observed in synapse density and in the stability and shape of spines, reflecting the underlying molecular maturation of synapses. Synaptic protein composition changes in terms of protein abundance and the assembly of protein complexes, supercomplexes and the physical organization of the PSD. Here, we summarize the developmental alterations of postsynaptic protein composition during synapse maturation. We describe major PSD proteins involved in postsynaptic signalling that regulates synaptic plasticity and discuss the effect of altered expression of these proteins during development. We consider the abnormality of synaptic profiles and synaptic protein composition in the brain in neurodevelopmental disorders such as autism spectrum disorders. We also explain differences in synapse development between rodents and primates in terms of synaptic profiles and protein composition. Finally, we introduce recent findings related to synaptic diversity and nanoarchitecture and discuss their impact on future research. Synaptic protein composition can be considered a major determinant and marker of synapse maturation in normality and disease.

Dysfunctions of cellular context-sensitivity in neurodevelopmental learning disabilities

Pyramidal neurons have a pivotal role in the cognitive capabilities of neocortex. Though they have been predominantly modeled as integrate-and-fire point processors, many of them have another point of input integration in their apical dendrites that is central to mechanisms endowing them with the sensitivity to context that underlies basic cognitive capabilities. Here we review evidence implicating impairments of those mechanisms in three major neurodevelopmental disabilities, fragile X, Down syndrome, and fetal alcohol spectrum disorders. Multiple dysfunctions of the mechanisms by which pyramidal cells are sensitive to context are found to be implicated in all three syndromes. Further deciphering of these cellular mechanisms would lead to the understanding of and therapies for learning disabilities beyond any that are currently available.

Remodeling of the postsynaptic proteome in male mice and marmosets during synapse development

Postsynaptic proteins play crucial roles in synaptic function and plasticity. During brain development, alterations in synaptic number, shape, and stability occur, known as synapse maturation. However, the postsynaptic protein composition changes during development are not fully understood. Here, we show the trajectory of the postsynaptic proteome in developing male mice and common marmosets. Proteomic analysis of mice at 2, 3, 6, and 12 weeks of age shows that proteins involved in synaptogenesis are differentially expressed during this period. Analysis of published transcriptome datasets shows that the changes in postsynaptic protein composition in the mouse brain after 2 weeks of age correlate with gene expression changes. Proteomic analysis of marmosets at 0, 2, 3, 6, and 24 months of age show that the changes in the marmoset brain can be categorized into two parts: the first 2 months and after that. The changes observed in the first 2 months are similar to those in the mouse brain between 2 and 12 weeks of age. The changes observed in marmoset after 2 months old include differential expression of synaptogenesis-related molecules, which hardly overlap with that in mice. Our results provide a comprehensive proteomic resource that underlies developmental synapse maturation in rodents and primates.

Role of Cyclins and Cytoskeletal Proteins in Endometriosis: Insights into Pathophysiology

Endometriosis is a gynecological condition where endometrium-like tissue grows outside the uterus, posing challenges in understanding and treatment. This article delves into the deep cellular and molecular processes underlying endometriosis, with a focus on the crucial roles played by cyclins and cytoskeletal proteins in its pathogenesis, particularly in the context of Epithelial-Mesenchymal Transition (EMT). The investigation begins by examining the activities of cyclins, elucidating their diverse biological roles such as cell cycle control, proliferation, evasion of apoptosis, and angiogenesis among ectopic endometrial cells. A comprehensive analysis of cytoskeletal proteins follows, emphasizing their fundamental biological roles and their specific significance to endometriotic cell features. This review sheds light on the interconnected pathways through which cyclins and cytoskeletal proteins converge, contributing to the genesis and progression of endometriosis. Understanding these molecular complexities not only provides insight into the underlying causes of the disease but also holds promise for the development of specific therapeutic approaches, ushering in a new era in the management of this devastating disorder.

Conditional Astrocyte Rac1KO Attenuates Hyperreflexia after Spinal Cord Injury

Spasticity is a hyperexcitability disorder that adversely impacts functional recovery and rehabilitative efforts after spinal cord injury (SCI). The loss of evoked rate-dependent depression (RDD) of the monosynaptic H-reflex is indicative of hyperreflexia, a physiological sign of spasticity. Given the intimate relationship between astrocytes and neurons, that is, the tripartite synapse, we hypothesized that astrocytes might have a significant role in post-injury hyperreflexia and plasticity of neighboring neuronal synaptic dendritic spines. Here, we investigated the effect of selective Rac1KO in astrocytes (i.e., adult male and female mice, transgenic cre-flox system) on SCI-induced spasticity. Three weeks after a mild contusion SCI, control Rac1wt animals displayed a loss of H-reflex RDD, that is, hyperreflexia. In contrast, transgenic animals with astrocytic Rac1KO demonstrated near-normal H-reflex RDD similar to pre-injury levels. Reduced hyperreflexia in astrocytic Rac1KO animals was accompanied by a loss of thin-shaped dendritic spine density on α-motor neurons in the ventral horn. In SCI-Rac1wt animals, as expected, we observed the development of dendritic spine dysgenesis on α-motor neurons associated with spasticity. As compared with WT animals, SCI animals with astrocytic Rac1KO expressed increased levels of the glial-specific glutamate transporter, glutamate transporter-1 in the ventral spinal cord, potentially enhancing glutamate clearance from the synaptic cleft and reducing hyperreflexia in astrocytic Rac1KO animals. Taken together, our findings show for the first time that Rac1 activity in astrocytes can contribute to hyperreflexia underlying spasticity following SCI. These results reveal an opportunity to target cell-specific molecular regulators of H-reflex excitability to manage spasticity after SCI.Significance Statement Spinal cord injury leads to stretch reflex hyperexcitability, which underlies the clinical symptom of spasticity. This study shows for the first time that astrocytic Rac1 contributes to the development of hyperreflexia after SCI. Specifically, astrocytic Rac1KO reduced SCI-related H-reflex hyperexcitability, decreased dendritic spine dysgenesis on α-motor neurons, and elevated the expression of the astrocytic glutamate transporter-1 (GLT-1). Overall, this study supports a distinct role for astrocytic Rac1 signaling within the spinal reflex circuit and the development of SCI-related spasticity.

A common single nucleotide variant in the cytokine receptor-like factor-3 (CRLF3) gene causes neuronal deficits in human and mouse cells

Single nucleotide variants in the general population are common genomic alterations, where the majority are presumed to be silent polymorphisms without known clinical significance. Using human induced pluripotent stem cell (hiPSC) cerebral organoid modeling of the 1.4 megabase Neurofibromatosis type 1 (NF1) deletion syndrome, we previously discovered that the cytokine receptor-like factor-3 (CRLF3) gene, which is co-deleted with the NF1 gene, functions as a major regulator of neuronal maturation. Moreover, children with NF1 and the CRLF3L389P variant have greater autism burden, suggesting that this gene might be important for neurologic function. To explore the functional consequences of this variant, we generated CRLF3L389P-mutant hiPSC lines and Crlf3L389P-mutant genetically engineered mice. While this variant does not impair protein expression, brain structure, or mouse behavior, CRLF3L389P-mutant human cerebral organoids and mouse brains exhibit impaired neuronal maturation and dendrite formation. In addition, Crlf3L389P-mutant mouse neurons have reduced dendrite lengths and branching, without any axonal deficits. Moreover, Crlf3L389P-mutant mouse hippocampal neurons have decreased firing rates and synaptic current amplitudes relative to wild type controls. Taken together, these findings establish the CRLF3L389P variant as functionally deleterious and suggest that it may be a neurodevelopmental disease modifier.

A PAK1 Mutational Hotspot Within the Regulatory CRIPaK Domain is Associated With Severe Neurodevelopmental Disorders in Children

BACKGROUND

P-21-activated kinases (PAKs) are protein serine/threonine kinases, part of the RAS/mitogen-activated protein kinase pathway. PAK1 is highly expressed in the central nervous system and crucially involved in neuronal migration and brain developmental processes. Recently, de novo heterozygous missense variants in PAK1 have been identified as an ultrarare cause of pediatric neurodevelopmental disorders.

METHODS

We report a series of children affected with postnatal macrocephaly, neurodevelopmental impairment, and drug-resistant epilepsy. Repeated electroencephalographic (EEG) and video-EEG evaluations were performed over a two- to 10-year period during follow-up to delineate electroclinical histories. Genetic sequencing studies and computational evaluation of the identified variants were performed in our patient cohort.

RESULTS

We identified by whole-exome sequencing three novel de novo variants in PAK1 (NM_001128620: c.427A>G, p.Met143Val; c.428T>C, p.Met143Thr; c.428T>A, p.Met143Lys) as the underlying cause of the disease in our families. The three variants affected the same highly conserved Met143 residue within the cysteine-rich inhibitor of PAK1 (CRIPaK) domain, which was identified before as a PAK1 inhibitor target. Computational studies suggested a defective autoinhibition presumably due to impaired PAK1 autoregulation as a result of the recurrent substitution.

CONCLUSIONS

We delineated the electroclinical phenotypes of PAK1-related neurological disorders and highlight a novel mutational hotspot that may involve defective autoinhibition of the PAK1 protein. The three novel variants affecting the same hotspot residue within the CRIPaK domain highlight potentially impaired PAK1-CRIPaK interaction as a novel disease mechanism. These findings shed light on possible future treatments targeted at the CRIPaK domain, to modulate PAK1 activity and function.

Mitochondrial leak metabolism induces the Spemann-Mangold Organizer via Hif-1α in Xenopus

An instructive role for metabolism in embryonic patterning is emerging, although a role for mitochondria is poorly defined. We demonstrate that mitochondrial oxidative metabolism establishes the embryonic patterning center, the Spemann-Mangold Organizer, via hypoxia-inducible factor 1α (Hif-1α) in Xenopus. Hypoxia or decoupling ATP production from oxygen consumption expands the Organizer by activating Hif-1α. In addition, oxygen consumption is 20% higher in the Organizer than in the ventral mesoderm, indicating an elevation in mitochondrial respiration. To reconcile increased mitochondrial respiration with activation of Hif-1α, we discovered that the "free" c-subunit ring of the F1Fo ATP synthase creates an inner mitochondrial membrane leak, which decouples ATP production from respiration at the Organizer, driving Hif-1α activation there. Overexpression of either the c-subunit or Hif-1α is sufficient to induce Organizer cell fates even when β-catenin is inhibited. We propose that mitochondrial leak metabolism could be a general mechanism for activating Hif-1α and Wnt signaling.

Co-exposure to low-dose lead, cadmium, and mercury promotes memory deficits in rats: Insights from the dynamics of dendritic spine pruning in brain development

Lead (Pb), cadmium (Cd), and mercury (Hg) are environmentally toxic heavy metals that can be simultaneously detected at low levels in the blood of the general population. Although our previous studies have demonstrated neurodevelopmental toxicity upon co-exposure to these heavy metals at these low levels, the precise mechanisms remain largely unknown. Dendritic spines are the structural foundation of memory and undergo significant dynamic changes during development. This study focused on the dynamics of dendritic spines during brain development following Pb, Cd, and Hg co-exposure-induced memory impairment. First, the dynamic characteristics of dendritic spines in the prefrontal cortex were observed throughout the life cycle of normal rats. We observed that dendritic spines increased rapidly from birth to their peak value at weaning, followed by significant pruning and a decrease during adolescence. Dendritic spines tended to be stable until their loss in old age. Subsequently, a rat model of low-dose Pb, Cd, and Hg co-exposure from embryo to adolescence was established. The results showed that exposure to low doses of heavy metals equivalent to those detected in the blood of the general population impaired spatial memory and altered the dynamics of dendritic spine pruning from weaning to adolescence. Proteomic analysis of brain and blood samples suggested that differentially expressed proteins upon heavy metal exposure were enriched in dendritic spine-related cytoskeletal regulation and axon guidance signaling pathways and that cofilin was enriched in both of these pathways. Further experiments confirmed that heavy metal exposure altered actin cytoskeleton dynamics and disturbed the dendritic spine pruning-related LIM domain kinase 1-cofilin pathway in the rat prefrontal cortex. Our findings demonstrate that low-dose Pb, Cd, and Hg co-exposure may promote memory impairment by perturbing dendritic spine dynamics through dendritic spine pruning-related signaling pathways.

Circuit-level theories for sensory dysfunction in autism: convergence across mouse models

Individuals with autism spectrum disorder (ASD) exhibit a diverse range of behavioral features and genetic backgrounds, but whether different genetic forms of autism involve convergent pathophysiology of brain function is unknown. Here, we analyze evidence for convergent deficits in neural circuit function across multiple transgenic mouse models of ASD. We focus on sensory areas of neocortex, where circuit differences may underlie atypical sensory processing, a central feature of autism. Many distinct circuit-level theories for ASD have been proposed, including increased excitation-inhibition (E-I) ratio and hyperexcitability, hypofunction of parvalbumin (PV) interneuron circuits, impaired homeostatic plasticity, degraded sensory coding, and others. We review these theories and assess the degree of convergence across ASD mouse models for each. Behaviorally, our analysis reveals that innate sensory detection behavior is heightened and sensory discrimination behavior is impaired across many ASD models. Neurophysiologically, PV hypofunction and increased E-I ratio are prevalent but only rarely generate hyperexcitability and excess spiking. Instead, sensory tuning and other aspects of neural coding are commonly degraded and may explain impaired discrimination behavior. Two distinct phenotypic clusters with opposing neural circuit signatures are evident across mouse models. Such clustering could suggest physiological subtypes of autism, which may facilitate the development of tailored therapeutic approaches.

The translation initiating factor eIF4E and arginine methylation underlie G3BP1 function in dendritic spine development of neurons

Communication between neurons relies on neurotransmission that takes place at synapses. Excitatory synapses are located primarily on dendritic spines that possess diverse morphologies, ranging from elongated filopodia to mushroom-shaped spines. Failure in the proper development of dendritic spines has detrimental consequences on neuronal connectivity, but the molecular mechanism that controls the balance of filopodia and mushroom spines is not well understood. G3BP1 is the key RNA-binding protein that assembles the stress granules in non-neuronal cells to adjust protein synthesis upon exogenous stress. Emerging evidence suggests that the biological significance of G3BP1 extends beyond its role in stress response, especially in the nervous system. However, the mechanism underlying the regulation and function of G3BP1 in neurons remains elusive. Here we found that G3BP1 suppresses protein synthesis and binds to the translation initiation factor eIF4E via its NTF2-like domain. Notably, the over-production of filopodia caused by G3BP1 depletion can be alleviated by blocking the formation of the translation initiation complex. We further found that the interaction of G3BP1 with eIF4E is regulated by arginine methylation. Knockdown of the protein arginine methyltransferase PRMT8 leads to elevated protein synthesis and filopodia production, which is reversed by the expression of methylation-mimetic G3BP1. Our study, therefore, reveals arginine methylation as a key regulatory mechanism of G3BP1 during dendritic spine morphogenesis and identifies eIF4E as a novel downstream target of G3BP1 in neuronal development independent of stress response.

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.

Altered striatal actin dynamics drives behavioral inflexibility in a mouse model of fragile X syndrome

The proteome of glutamatergic synapses is diverse across the mammalian brain and involved in neurodevelopmental disorders (NDDs). Among those is fragile X syndrome (FXS), an NDD caused by the absence of the functional RNA-binding protein FMRP. Here, we demonstrate how the brain region-specific composition of postsynaptic density (PSD) contributes to FXS. In the striatum, the FXS mouse model shows an altered association of the PSD with the actin cytoskeleton, reflecting immature dendritic spine morphology and reduced synaptic actin dynamics. Enhancing actin turnover with constitutively active RAC1 ameliorates these deficits. At the behavioral level, the FXS model displays striatal-driven inflexibility, a typical feature of FXS individuals, which is rescued by exogenous RAC1. Striatal ablation of Fmr1 is sufficient to recapitulate behavioral impairments observed in the FXS model. These results indicate that dysregulation of synaptic actin dynamics in the striatum, a region largely unexplored in FXS, contributes to the manifestation of FXS behavioral phenotypes.

Early 7,8-Dihydroxyflavone Administration Ameliorates Synaptic and Behavioral Deficits in the Young FXS Animal Model by Acting on BDNF-TrkB Pathway

Fragile X syndrome (FXS) is the leading inherited form of intellectual disability and the most common cause of autism spectrum disorders. FXS patients exhibit severe syndromic features and behavioral alterations, including anxiety, hyperactivity, impulsivity, and aggression, in addition to cognitive impairment and seizures. At present, there are no effective treatments or cures for FXS. Previously, we have found the divergence of BDNF-TrkB signaling trajectories is associated with spine defects in early postnatal developmental stages of Fmr1 KO mice. Here, young fragile X mice were intraperitoneal injection with 7,8-Dihydroxyflavone (7,8-DHF), a high affinity tropomyosin receptor kinase B (TrkB) agonist. 7,8-DHF ameliorated morphological abnormities in dendritic spine and synaptic structure and rescued synaptic and hippocampus-dependent cognitive dysfunction. These observed improvements of 7,8-DHF involved decreased protein levels of BDNF, p-TrkB^Y816, p-PLCγ, and p-CaMKII in the hippocampus. In addition, 7,8-DHF intervention in primary hippocampal neurons increased p-TrkB^Y816 and activated the PLCγ1-CaMKII signaling pathway, leading to improvement of neuronal morphology. This study is the first to account for early life synaptic impairments, neuronal morphological, and cognitive delays in FXS in response to the abnormal BDNF-TrkB pathway. Present studies provide novel evidences about the effective early intervention in FXS mice at developmental stages and a strategy to produce powerful impacts on neural development, synaptic plasticity, and behaviors.

PLPP/CIN-mediated NF2 S10 dephosphorylation distinctly regulates kainate-induced seizure susceptibility and neuronal death through PAK1-NF-κB-COX-2-PTGES2 signaling pathway

BACKGROUND

Pyridoxal-5'-phosphate phosphatase/chronophin (PLPP/CIN) selectively dephosphorylates serine (S) 10 site on neurofibromin 2 (NF2, also known as merlin (moesin-ezrin-radixin-like protein) or schwannomin). p21-activated kinase 1 (PAK1) is a serine/threonine protein kinase, which is involved in synaptic activity and plasticity in neurons. NF2 and PAK1 reciprocally regulate each other in a positive feedback manner. Thus, the aim of the present study is to investigate the effects of PLPP/CIN-mediated NF2 S10 dephosphorylation on PAK1-related signaling pathways under physiological and neuroinflammatory conditions, which are largely unknown.

METHODS

After kainate (KA) injection in wild-type, PLPP/CIN^-/- and PLPP/CIN^Tg mice, seizure susceptibility, PAK1 S204 autophosphorylation, nuclear factor-κB (NF-κB) p65 S276 phosphorylation, cyclooxygenase-2 (COX-2) upregulation, prostaglandin E synthase 2 (PTGES2) induction and neuronal damage were measured. The effects of 1,1'-dithiodi-2-naphthtol (IPA-3, a selective inhibitor of PAK1) pretreatment on these responses to KA were also validated.

RESULTS

PLPP/CIN overexpression increased PAK1 S204 autophosphorylation concomitant with the enhanced NF2 S10 dephosphorylation in hippocampal neurons under physiological condition. Following KA treatment, PLPP/CIN overexpression delayed the seizure on-set and accelerated PAK1 S204 phosphorylation, NF-κB p65 S276 phosphorylation, COX-2 upregulation and PTGES2 induction, which were ameliorated by PLPP/CIN deletion or IPA-3. Furthermore, IPA-3 pretreatment shortened the latency of seizure on-set without affecting seizure severity (intensity) and ameliorated CA3 neuronal death induced by KA.

CONCLUSIONS

These findings indicate that PLPP/CIN may regulate seizure susceptibility (the latency of seizure on-set) and CA3 neuronal death in response to KA through NF2-PAK1-NF-κB-COX-2-PTGES2 signaling pathway.

LIM Kinases, LIMK1 and LIMK2, Are Crucial Node Actors of the Cell Fate: Molecular to Pathological Features

LIM kinase 1 (LIMK1) and LIM kinase 2 (LIMK2) are serine/threonine and tyrosine kinases and the only two members of the LIM kinase family. They play a crucial role in the regulation of cytoskeleton dynamics by controlling actin filaments and microtubule turnover, especially through the phosphorylation of cofilin, an actin depolymerising factor. Thus, they are involved in many biological processes, such as cell cycle, cell migration, and neuronal differentiation. Consequently, they are also part of numerous pathological mechanisms, especially in cancer, where their involvement has been reported for a few years and has led to the development of a wide range of inhibitors. LIMK1 and LIMK2 are known to be part of the Rho family GTPase signal transduction pathways, but many more partners have been discovered over the decades, and both LIMKs are suspected to be part of an extended and various range of regulation pathways. In this review, we propose to consider the different molecular mechanisms involving LIM kinases and their associated signalling pathways, and to offer a better understanding of their variety of actions within the physiology and physiopathology of the cell.

Mouse models of fragile X-related disorders

The fragile X-related disorders are an important group of hereditary disorders that are caused by expanded CGG repeats in the 5' untranslated region of the FMR1 gene or by mutations in the coding sequence of this gene. Two categories of pathological CGG repeats are associated with these disorders, full mutation alleles and shorter premutation alleles. Individuals with full mutation alleles develop fragile X syndrome, which causes autism and intellectual disability, whereas those with premutation alleles, which have shorter CGG expansions, can develop fragile X-associated tremor/ataxia syndrome, a progressive neurodegenerative disease. Thus, fragile X-related disorders can manifest as neurodegenerative or neurodevelopmental disorders, depending on the size of the repeat expansion. Here, we review mouse models of fragile X-related disorders and discuss how they have informed our understanding of neurodegenerative and neurodevelopmental disorders. We also assess the translational value of these models for developing rational targeted therapies for intellectual disability and autism disorders.

Fmr1-KO mice failure to detect object novelty associates with a post-test decrease of structural and synaptic plasticity upstream of the hippocampus

Mice with deletion of the FMR1 gene show episodic memory impairments and exhibit dendritic spines and synaptic plasticity defects prevalently identified in non-training conditions. Based on evidence that synaptic changes associated with normal or abnormal memory emerge when mice are cognitively challenged, here we examine whether, and how, fragile entorhinal and hippocampal synapses are remodeled when mice succeed or fail to learn. We trained Fmr1 knockout (KO) and wild-type C57BL/6J (WT) mice in the novel object recognition (NOR) paradigm with 1 h or 24 h training-to-test intervals and then assessed whether varying the time between the presentation of similar and different objects modulates NOR performance and plasticity along the entorhinal cortex-hippocampus axis. At the 1 h-interval, KO mice failed to discriminate the novel object, showed a collapse of spines in the lateral entorhinal cortex (LEC), and of long-term potentiation (LTP) in the lateral perforant path (LPP), but a normal increase in hippocampal spines. At the 24 h, they exhibited intact NOR performance, typical LEC and hippocampal spines, and exaggerated LPP-LTP. Our findings reveal that the inability of mice to detect object novelty primarily stands in their impediment to elaborate, and convey to the hippocampus, sensory/perceptive object representations.

Rac1/PAK1 signaling contributes to bone cancer pain by Regulation dendritic spine remodeling in rats

Bone cancer pain (BCP) is severe chronic pain caused by tumor metastasis to the bones, often resulting in significant skeletal remodeling and fractures. Currently, there is no curative treatment. Therefore, insight into the underlying mechanisms could guide the development of mechanism-based therapeutic strategies for BCP. We speculated that Rac1/PAK1 signaling plays a critical role in the development of BCP. Tumor cells implantation (TCI) into the tibial cavity resulted in bone cancer-associated mechanical allodynia. Golgi staining revealed changes in the excitatory synaptic structure of WDR (Wide-dynamic range) neurons in the spinal cord, including increased postsynaptic density (PSD) length and thickness, and width of the cleft. Behavioral and western blotting test revealed that the development and persistence of pain correlated with Rac1/PAK1 signaling activation in primary sensory neurons. Intrathecal injection of NSC23766, a Rac1 inhibitor, reduced the persistence of BCP as well as reversed the remodeling of dendrites. Therefore, we concluded that activation of the Rac1/PAK1 signaling pathway in the spinal cord plays an important role in the development of BCP through remodeling of dendritic spines. Modulation of the Rac1/PAK1 pathway may be a potential strategy for BCP treatment.

Social deficits via dysregulated Rac1-dependent excitability control of prefrontal cortical neurons and increased GABA/glutamate ratios

Identifying symptom-specific convergent mechanisms for neurodevelopmental disorders is a promising strategy in advancing therapies. Here, we show that bidirectional dysregulation of Rac1 activity in the medial prefrontal cortex (mPFC) dictates shared social deficits in mice. Selective upregulation or downregulation of Rac1 activity in glutamatergic or fast-spiking GABAergic neurons results in excessive or inadequate control of excitability combined with a decrease in glutamate or an increase in GABA concentrations and an increase in the GABA/glutamate ratio, which is responsible for social deficits. Notably, the autism model of Shank3B knockout mice exhibits aberrantly enhanced Rac1 activity, reduced glutamate concentrations, and pyramidal neuron excitability in mPFC accompanied with social deficits, which were corrected by either excitatory-neuron-specific downregulation of Rac1 activity or upregulation of neuronal excitability. Thus, this work shows a convergence between genetic autism risk factors, dysregulation of Rac1 signaling, and excitation-inhibition imbalance, enabling mechanism-based stratification of patients with social deficits.

Inka2, a novel Pak4 inhibitor, regulates actin dynamics in neuronal development

The actin filament is a fundamental part of the cytoskeleton defining cell morphology and regulating various physiological processes, including filopodia formation and dendritic spinogenesis of neurons. Serine/threonine-protein kinase Pak4, an essential effector, links Rho GTPases to control actin polymerization. Previously, we identified the Inka2 gene, a novel mammalian protein exhibiting sequence similarity to Inka1, which serves as a possible inhibitor for Pak4. Although Inka2 is dominantly expressed in the nervous system and involved in focal-adhesion dynamics, its molecular role remains unclear. Here, we found that Inka2-iBox directly binds to Pak4 catalytic domain to suppress actin polymerization. Inka2 promoted actin depolymerization and inhibited the formation of cellular protrusion caused by Pak4 activation. We further generated the conditional knockout mice of the Inka2 gene. The beta-galactosidase reporter indicated the preferential Inka2 expression in the dorsal forebrain neurons. Cortical pyramidal neurons of Inka2^-/- mice exhibited decreased density and aberrant morphology of dendritic spines with marked activation/phosphorylation of downstream molecules of Pak4 signal cascade, including LIMK and Cofilin. These results uncovered the unexpected function of endogenous Pak4 inhibitor in neurons. Unlike Inka1, Inka2 is a critical mediator for actin reorganization required for dendritic spine development.

Actin filaments are an essential part of the cytoskeleton defining cell morphology and regulating various cellular processes, such as cell migration and synapse formation in the brain. Actin polymerization is controlled by the kinase activity of the Pak4 signaling cascade, including LIMK and Cofilin. Previously, we identified the Inka2 gene, which is strongly expressed in the mammalian central nervous system and a similar sequence as Inka1. Inka1 was reported to serve as a Pak4 inhibitor in cancer cell lines; however, the physiological function of Inka2 is unclear. In this study, we found that (i) Inka2 overexpression inhibits the formation of cell-protrusion caused by Pak4 activation; (ii) Inka2 directly binds to the catalytic domain of Pak4 to inhibit intracellular actin polymerization; (iii) Inka2 is specifically expressed in neurons in the forebrain region, including the cerebral cortex and hippocampus that are known to be essential for brain plasticity, such as learning and memory; and (iv) cortical neurons of Inka2-deficient mice showed decreased synapse formation and abnormal spine morphology, probably due to the marked phosphorylation of LIMK and Cofilin. These results indicate that Inka2 is an endogenous Pak4 inhibitor in neurons required for normal synapse formation through the modulation of actin reorganization.

Functional interdependence of the actin regulators CAP1 and cofilin1 in control of dendritic spine morphology

The vast majority of excitatory synapses are formed on small dendritic protrusions termed dendritic spines. Dendritic spines vary in size and density that are crucial determinants of excitatory synaptic transmission. Aberrations in spine morphogenesis can compromise brain function and have been associated with neuropsychiatric disorders. Actin filaments (F-actin) are the major structural component of dendritic spines, and therefore, actin-binding proteins (ABP) that control F-actin dis-/assembly moved into the focus as critical regulators of brain function. Studies of the past decade identified the ABP cofilin1 as a key regulator of spine morphology, synaptic transmission, and behavior, and they emphasized the necessity for a tight control of cofilin1 to ensure proper brain function. Here, we report spine enrichment of cyclase-associated protein 1 (CAP1), a conserved multidomain protein with largely unknown physiological functions. Super-resolution microscopy and live cell imaging of CAP1-deficient hippocampal neurons revealed impaired synaptic F-actin organization and dynamics associated with alterations in spine morphology. Mechanistically, we found that CAP1 cooperates with cofilin1 in spines and that its helical folded domain is relevant for this interaction. Moreover, our data proved functional interdependence of CAP1 and cofilin1 in control of spine morphology. In summary, we identified CAP1 as a novel regulator of the postsynaptic actin cytoskeleton that is essential for synaptic cofilin1 activity.

Sortilin deletion in the prefrontal cortex and hippocampus ameliorates depressive-like behaviors in mice via regulating ASM/ceramide signaling

Major depressive disorder (MDD) is a common psychiatric disorder characterized by persistent mood despondency and loss of motivation. Although numerous hypotheses have been proposed, the possible pathogenesis of MDD remains unclear. Several recent studies show that a classic transporter protein, sortilin, is closely associated with depression. In the present study, we investigated the role of sortilin in MDD using a well-established rodent model of depression. Mice were subjected to chronic unpredictable mild stress (CUMS) for 6 weeks. We showed that the expression levels of sortilin were significantly increased in the prefrontal cortex and hippocampus of CUMS mice. The depressive-like behaviors induced by CUMS were alleviated by specific knockdown of sortilin in the prefrontal cortex and hippocampus. We revealed that sortilin facilitated acid sphingomyelinase (ASM)/ceramide signaling, which activated RhoA/ROCK2 signaling, ultimately causing the transformation of dendritic spine dynamics. Specific overexpression of sortilin in the prefrontal cortex and hippocampus induced depressive-like behaviors, which was mitigated by injection of ASM inhibitor SR33557 (4 µg/μL) into the prefrontal cortex and hippocampus. In conclusion, sortilin knockdown in the prefrontal cortex and hippocampus plays an important role in ameliorating depressive-like behavior induced by CUMS, which is mainly evidenced by decreasing the trafficking of ASM from the trans-Golgi network to the lysosome and reducing the ceramide levels. Our results provide a new insight into the pathology of depression, and demonstrate that sortilin may be a potential therapeutic target for MDD.

NMDA receptors mediate synaptic plasticity impairment of hippocampal neurons due to arsenic exposure

Endemic arsenism is a worldwide health problem. Chronic arsenic exposure results in cognitive dysfunction due to arsenic and its metabolites accumulating in hippocampus. As the cellular basis of cognition, synaptic plasticity is pivotal in arsenic-induced cognitive dysfunction. N-methyl-D-aspartate receptors (NMDARs) serve physiological functions in synaptic transmission. However, excessive NMDARs activity contributes to exitotoxicity and synaptic plasticity impairment. Here, we provide an overview of the mechanisms that NMDARs and their downstream signaling pathways mediate synaptic plasticity impairment due to arsenic exposure in hippocampal neurons, ways of arsenic exerting on NMDARs, as well as the potential therapeutic targets except for water improvement.

mTOR-Dependent Spine Dynamics in Autism

Autism Spectrum Conditions (ASC) are a group of neurodevelopmental disorders characterized by deficits in social communication and interaction as well as repetitive behaviors and restricted range of interests. ASC are complex genetic disorders with moderate to high heritability, and associated with atypical patterns of neural connectivity. Many of the genes implicated in ASC are involved in dendritic spine pruning and spine development, both of which can be mediated by the mammalian target of rapamycin (mTOR) signaling pathway. Consistent with this idea, human postmortem studies have shown increased spine density in ASC compared to controls suggesting that the balance between autophagy and spinogenesis is altered in ASC. However, murine models of ASC have shown inconsistent results for spine morphology, which may underlie functional connectivity. This review seeks to establish the relevance of changes in dendritic spines in ASC using data gathered from rodent models. Using a literature survey, we identify 20 genes that are linked to dendritic spine pruning or development in rodents that are also strongly implicated in ASC in humans. Furthermore, we show that all 20 genes are linked to the mTOR pathway and propose that the mTOR pathway regulating spine dynamics is a potential mechanism underlying the ASC signaling pathway in ASC. We show here that the direction of change in spine density was mostly correlated to the upstream positive or negative regulation of the mTOR pathway and most rodent models of mutant mTOR regulators show increases in immature spines, based on morphological analyses. We further explore the idea that these mutations in these genes result in aberrant social behavior in rodent models that is due to these altered spine dynamics. This review should therefore pave the way for further research on the specific genes outlined, their effect on spine morphology or density with an emphasis on understanding the functional role of these changes in ASC.

Proteomic Identification of Phosphorylation-Dependent Septin 7 Interactors that Drive Dendritic Spine Formation

Septins are a family of cytoskeletal proteins that regulate several important aspects of neuronal development. Septin 7 (Sept7) is enriched at the base of dendritic spines in excitatory neurons and mediates both spine formation and spine and synapse maturation. Phosphorylation at a conserved C-terminal tail residue of Sept7 mediates its translocation into the dendritic spine head to allow spine and synapse maturation. The mechanistic basis for postsynaptic stability and compartmentalization conferred by phosphorylated Sept7, however, is unclear. We report herein the proteomic identification of Sept7 phosphorylation-dependent neuronal interactors. Using Sept7 C-terminal phosphopeptide pulldown and biochemical assays, we show that the 14-3-3 family of proteins specifically interacts with Sept7 when phosphorylated at the T426 residue. Biochemically, we validate the interaction between Sept7 and 14-3-3 isoform gamma and show that 14-3-3 gamma is also enriched in the mature dendritic spine head. Furthermore, we demonstrate that interaction of phosphorylated Sept7 with 14-3-3 protects it from dephosphorylation, as expression of a 14-3-3 antagonist significantly decreases phosphorylated Sept7 in neurons. This study identifies 14-3-3 proteins as an important physiological regulator of Sept7 function in neuronal development.

Stress Elicits Contrasting Effects on Rac1-Cofilin Signaling in the Hippocampus and Amygdala

There is accumulating evidence for contrasting patterns of stress-induced morphological and physiological plasticity in glutamatergic synapses of the hippocampus and amygdala. The same chronic stress that leads to the formation of dendritic spines in the basolateral amygdala (BLA) of rats, leads to a loss of spines in the hippocampus. However, the molecular underpinnings of these divergent effects of stress on dendritic spines are not well understood. Since the activity of the Rho GTPase Rac1 and the actin-depolymerizing factor cofilin are known to play a pivotal role in spine morphogenesis, we investigated if alterations in this signaling pathway reflect the differential effects of stress on spine plasticity in the hippocampus and amygdala. A day after the end of chronic immobilization stress (2 h/day for 10 days), we found a reduction in the activity of Rac1, as well as its effector p21-activated kinase 1 (PAK1), in the rat hippocampus. These changes, in turn, decreased cofilin phosphorylation alongside a reduction in the levels of profilin isoforms. In striking contrast, the same chronic stress increased Rac1, PAK1 activity, cofilin phosphorylation, and profilin levels in the BLA, which is consistent with enhanced actin polymerization leading to spinogenesis in the BLA. In the hippocampus, on the other hand, the same stress caused the opposite changes, the functional consequences of which would be actin depolymerization leading to the elimination of spines. Together, these findings reveal a role for brain-region specific differences in the dysregulation of Rac1-to-cofilin signaling in the effects of repeated stress on two brain areas that are implicated in the emotional and cognitive symptoms of stress-related psychiatric disorders.

Towards Kinase Inhibitor Therapies for Fragile X Syndrome: Tweaking Twists in the Autism Spectrum Kinase Signaling Network

Absence of the Fragile X Mental Retardation Protein (FMRP) causes autism spectrum disorders and intellectual disability, commonly referred to as the Fragile X syndrome. FMRP is a negative regulator of protein translation and is essential for neuronal development and synapse formation. FMRP is a target for several post-translational modifications (PTMs) such as phosphorylation and methylation, which tightly regulate its cellular functions. Studies have indicated the involvement of FMRP in a multitude of cellular pathways, and an absence of FMRP was shown to affect several neurotransmitter receptors, for example, the GABA receptor and intracellular signaling molecules such as Akt, ERK, mTOR, and GSK3. Interestingly, many of these molecules function as protein kinases or phosphatases and thus are potentially amendable by pharmacological treatment. Several treatments acting on these kinase-phosphatase systems have been shown to be successful in preclinical models; however, they have failed to convincingly show any improvements in clinical trials. In this review, we highlight the different protein kinase and phosphatase studies that have been performed in the Fragile X syndrome. In our opinion, some of the paradoxical study conclusions are potentially due to the lack of insight into integrative kinase signaling networks in the disease. Quantitative proteome analyses have been performed in several models for the FXS to determine global molecular processes in FXS. However, only one phosphoproteomics study has been carried out in Fmr1 knock-out mouse embryonic fibroblasts, and it showed dysfunctional protein kinase and phosphatase signaling hubs in the brain. This suggests that the further use of phosphoproteomics approaches in Fragile X syndrome holds promise for identifying novel targets for kinase inhibitor therapies.

Transgenerational Effects of Prenatal Ethanol Exposure in Prepubescent Mice

Background: Fetal alcohol spectrum disorders (FASD) represent a leading cause of non-genetic neuropathologies. Recent preclinical evidence from suggests that prenatal ethanol exposure (PrEE), like other environmental exposures, may have a significant, transgenerational impact on the offspring of directly exposed animals, including altered neocortical development at birth and behavior in peri-pubescent mice. How these adverse behavioral outcomes are manifested within the brain at the time of behavioral disruption remains unknown.

Methods: A transgenerational mouse model of FASD was used to generate up to a third filial generation of offspring to study. Using a multi-modal battery of behavioral assays, we assessed motor coordination/function, sensorimotor processing, risk-taking behavior, and depressive-like behavior in postnatal day (P) 20 pre-pubescent mice. Additionally, sensory neocortical area connectivity using dye tracing, neocortical gene expression using in situ RNA hybridization, and spine density of spiny stellate cells in the somatosensory cortex using Golgi-Cox staining were examined in mice at P20.

Results: We found that PrEE induces behavioral abnormalities including abnormal sensorimotor processing, increased risk-taking behavior, and increased depressive-like behaviors that extend to the F3 generation in 20-day old mice. Assessment of both somatosensory and visual cortical connectivity, as well as cortical RZRβ expression in pre-pubescent mice yielded no significant differences among any experimental generations. In contrast, only directly-exposed F1 mice displayed altered cortical expression of Id2 and decreased spine density among layer IV spiny stellate cells in somatosensory cortex at this pre-pubescent, post weaning age.

Conclusion: Our results suggest that robust, clinically-relevant behavioral abnormalities are passed transgenerationally to the offspring of mice directly exposed to prenatal ethanol. Additionally, in contrast to our previous findings in the newborn PrEE mouse, a lack of transgenerational findings within the brain at this later age illuminates the critical need for future studies to attempt to discover the link between neurological function and the described behavioral changes. Overall, our study suggests that multi-generational effects of PrEE may have a substantial impact on human behavior as well as health and well-being and that these effects likely extend beyond early childhood.

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.

The molecular view of mechanical stress of brain cells, local translation, and neurodegenerative diseases

The assumption that chronic mechanical stress in brain cells stemming from intracranial hypertension, arterial hypertension, or mechanical injury is a risk factor for neurodegenerative diseases was put forward in the 1990s and has since been supported. However, the molecular mechanisms that underlie the way from cell exposure to mechanical stress to disturbances in synaptic plasticity followed by changes in behavior, cognition, and memory are still poorly understood. Here we review (1) the current knowledge of molecular mechanisms regulating local translation and the actin cytoskeleton state at an activated synapse, where they play a key role in the formation of various sorts of synaptic plasticity and long-term memory, and (2) possible pathways of mechanical stress intervention. The roles of the mTOR (mammalian target of rapamycin) signaling pathway; the RNA-binding FMRP protein; the CYFIP1 protein, interacting with FMRP; the family of small GTPases; and the WAVE regulatory complex in the regulation of translation initiation and actin cytoskeleton rearrangements in dendritic spines of the activated synapse are discussed. Evidence is provided that chronic mechanical stress may result in aberrant activation of mTOR signaling and the WAVE regulatory complex via the YAP/TAZ system, the key sensor of mechanical signals, and influence the associated pathways regulating the formation of F actin filaments and the dendritic spine structure. These consequences may be a risk factor for various neurological conditions, including autistic spectrum disorders and epileptic encephalopathy. In further consideration of the role of the local translation system in the development of neuropsychic and neurodegenerative diseases, an original hypothesis was put forward that one of the possible causes of synaptopathies is impaired proteome stability associated with mTOR hyperactivity and formation of complex dynamic modes of de novo protein synthesis in response to synapse-stimulating factors, including chronic mechanical stress.

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.

Identification of GOLPH3 Partners in Drosophila Unveils Potential Novel Roles in Tumorigenesis and Neural Disorders

Golgi phosphoprotein 3 (GOLPH3) is a highly conserved peripheral membrane protein localized to the Golgi apparatus and the cytosol. GOLPH3 binding to Golgi membranes depends on phosphatidylinositol 4-phosphate [PI(4)P] and regulates Golgi architecture and vesicle trafficking. GOLPH3 overexpression has been correlated with poor prognosis in several cancers, but the molecular mechanisms that link GOLPH3 to malignant transformation are poorly understood. We recently showed that PI(4)P-GOLPH3 couples membrane trafficking with contractile ring assembly during cytokinesis in dividing Drosophila spermatocytes. Here, we use affinity purification coupled with mass spectrometry (AP-MS) to identify the protein-protein interaction network (interactome) of Drosophila GOLPH3 in testes. Analysis of the GOLPH3 interactome revealed enrichment for proteins involved in vesicle-mediated trafficking, cell proliferation and cytoskeleton dynamics. In particular, we found that dGOLPH3 interacts with the Drosophila orthologs of Fragile X mental retardation protein and Ataxin-2, suggesting a potential role in the pathophysiology of disorders of the nervous system. Our findings suggest novel molecular targets associated with GOLPH3 that might be relevant for therapeutic intervention in cancers and other human diseases.

Arhgap22 Disruption Leads to RAC1 Hyperactivity Affecting Hippocampal Glutamatergic Synapses and Cognition in Mice

Rho GTPases are a class of G-proteins involved in several aspects of cellular biology, including the regulation of actin cytoskeleton. The most studied members of this family are RHOA and RAC1 that act in concert to regulate actin dynamics. Recently, Rho GTPases gained much attention as synaptic regulators in the mammalian central nervous system (CNS). In this context, ARHGAP22 protein has been previously shown to specifically inhibit RAC1 activity thus standing as critical cytoskeleton regulator in cancer cell models; however, whether this function is maintained in neurons in the CNS is unknown. Here, we generated a knockout animal model for arhgap22 and provided evidence of its role in the hippocampus. Specifically, we found that ARHGAP22 absence leads to RAC1 hyperactivity and to an increase in dendritic spine density with defects in synaptic structure, molecular composition, and plasticity. Furthermore, arhgap22 silencing causes impairment in cognition and a reduction in anxiety-like behavior in mice. We also found that inhibiting RAC1 restored synaptic plasticity in ARHGAP22 KO mice. All together, these results shed light on the specific role of ARHGAP22 in hippocampal excitatory synapse formation and function as well as in learning and memory behaviors.

Role of hypoxia in inhibiting dendritic cells by VEGF signaling in tumor microenvironments: mechanism and application

The tumor microenvironment (TME) plays a central role in tumor initiation, development, immune escape, and clinical treatment. Hypoxia, an important characteristic of the TME, mediates vascular endothelial factor (VEGF) signaling through direct or indirect mechanisms. Directly, hypoxia promotes the expression of VEGF through hypoxia-inducible factor (HIF) induction. Indirectly, VEGF inhibits dendritic cell (DC) maturation and function by binding to VEGF receptors (VEGFRs) and co-receptors expressed on cell membranes. Additionally, HIF can bypass VEGF/VEGFR and activate downstream signaling factors to promote tumor development. Currently, DC vaccine, anti-HIF and anti-VEGF therapies are widely used in clinical treatment, but their long-term effects remain limited. Therefore, a further understanding of the effects of hypoxia and VEGF signaling on DCs will help in the development of innovative combination therapies and the identification of new targets.

LIM-Kinases in Synaptic Plasticity, Memory, and Brain Diseases

Learning and memory require structural and functional modifications of synaptic connections, and synaptic deficits are believed to underlie many brain disorders. The LIM-domain-containing protein kinases (LIMK1 and LIMK2) are key regulators of the actin cytoskeleton by affecting the actin-binding protein, cofilin. In addition, LIMK1 is implicated in the regulation of gene expression by interacting with the cAMP-response element-binding protein. Accumulating evidence indicates that LIMKs are critically involved in brain function and dysfunction. In this paper, we will review studies on the roles and underlying mechanisms of LIMKs in the regulation of long-term potentiation (LTP) and depression (LTD), the most extensively studied forms of long-lasting synaptic plasticity widely regarded as cellular mechanisms underlying learning and memory. We will also discuss the involvement of LIMKs in the regulation of the dendritic spine, the structural basis of synaptic plasticity, and memory formation. Finally, we will discuss recent progress on investigations of LIMKs in neurological and mental disorders, including Alzheimer’s, Parkinson’s, Williams–Beuren syndrome, schizophrenia, and autism spectrum disorders.

Infantile spasms-linked Nedd4-2 mediates hippocampal plasticity and learning via cofilin signaling

Individuals affected by infantile spasms (IS), such as those carrying mutations in an IS-linked gene, neural precursor cell expressed developmentally downregulated gene 4-like (Nedd4-2), exhibit developmental delays and learning disabilities, but the underlying mechanism is unknown. Using conditional Nedd4-2 knockout mice, we uncover that Nedd4-2 functions to maintain the excitatory synapses in hippocampal neurons and allows for late-phase long-term synaptic potentiation (L-LTP) at Schaffer collateral synapses in the hippocampus. We also find that Nedd4-2 is required for multiple forms of hippocampus-dependent learning and memory. Mechanistically, we show that loss of Nedd4-2 leads to a decrease in actin polymerization caused by reduced phosphorylation of the actin depolymerizing protein cofilin. A cell-permeable peptide promoting phosphorylation of endogenous cofilin in Nedd4-2 knockout neurons restores the number of hippocampal excitatory synapses and hippocampal L-LTP and partially restores hippocampus-dependent learning in mice. Taken together, our results reveal a novel mechanism underlying IS-associated learning disabilities and may provide information for future therapeutic strategies for IS.

Loss of CC2D1A in Glutamatergic Neurons Results in Autistic-Like Features in Mice

Biallelic loss-of-function mutations in Coiled-coil and C2 domain containing 1A (CC2D1A) cause autosomal recessive intellectual disability, sometimes comorbid with other neurodevelopmental disabilities, such as autism spectrum disorder (ASD) and seizures. We recently reported that conditional deletion of Cc2d1a in glutamatergic neurons of the postnatal mouse forebrain leads to impaired hippocampal synaptic plasticity and cognitive function. However, the pathogenic origin of the autistic features of CC2D1A deficiency remains elusive. Here, we confirmed that CC2D1A is highly expressed in the cortical zones during embryonic development. Taking advantage of Cre-LoxP-mediated gene deletion strategy, we generated a novel line of Cc2d1a conditional knockout (cKO) mice by crossing floxed Cc2d1a mice with Emx1-Cre mice, in which CC2D1A is ablated specifically in glutamatergic neurons throughout all embryonic and adult stages. We found that CC2D1A deletion leads to a trend toward decreased number of cortical progenitor cells at embryonic day 12.5 and alters the cortical thickness on postnatal day 10. In addition, male Cc2d1a cKO mice display autistic-like phenotypes including self-injurious repetitive grooming and aberrant social interactions. Loss of CC2D1A also results in decreased complexity of apical dendritic arbors of medial prefrontal cortex (mPFC) layer V pyramidal neurons and increased synaptic excitation/inhibition (E/I) ratio in the mPFC. Notably, chronic treatment with minocycline rescues behavioral and morphological abnormalities, as well as E/I changes, in male Cc2d1a cKO mice. Together, these findings indicate that male Cc2d1a cKO mice recapitulate autistic-like phenotypes of human disorder and suggest that minocycline has both structural and functional benefits in treating ASD.

Astrocytes-derived extracellular vesicles in motion at the neuron surface: Involvement of the prion protein

Astrocytes-derived extracellular vesicles (EVs) are key players in glia-neuron communication. However, whether EVs interact with neurons at preferential sites and how EVs reach these sites on neurons remains elusive. Using optical manipulation to study single EV-neuron dynamics, we here show that large EVs scan the neuron surface and use neuronal processes as highways to move extracellularly. Large EV motion on neurites is driven by the binding of EV to a surface receptor that slides on neuronal membrane, thanks to actin cytoskeleton rearrangements. The use of prion protein (PrP)-coated synthetic beads and PrP knock out EVs/neurons points at vesicular PrP and its receptor(s) on neurons in the control of EV motion. Surprisingly, a fraction of large EVs contains actin filaments and has an independent capacity to move in an actin-mediated way, through intermittent contacts with the plasma membrane. Our results unveil, for the first time, a dual mechanism exploited by astrocytic large EVs to passively/actively reach target sites on neurons moving on the neuron surface.

Local Translation in Nervous System Pathologies

Dendrites and axons can extend dozens to hundreds of centimeters away from the cell body so that a single neuron can sense and respond to thousands of stimuli. Thus, for an accurate function of dendrites and axons the neuronal proteome needs to be asymmetrically distributed within neurons. Protein asymmetry can be achieved by the transport of the protein itself or the transport of the mRNA that is then translated at target sites in neuronal processes. The latter transport mechanism implies local translation of localized mRNAs. The role of local translation in nervous system (NS) development and maintenance is well established, but recently there is growing evidence that this mechanism and its deregulation are also relevant in NS pathologies, including neurodegenerative diseases. For instance, upon pathological signals disease-related proteins can be locally synthesized in dendrites and axons. Locally synthesized proteins can exert their effects at or close to the site of translation, or they can be delivered to distal compartments like the nucleus and induce transcriptional responses that lead to neurodegeneration, nerve regeneration and other cell-wide responses. Relevant key players in the process of local protein synthesis are RNA binding proteins (RBPs), responsible for mRNA transport to neurites. Several neurological and neurodegenerative disorders, including amyotrophic lateral sclerosis or spinal motor atrophy, are characterized by mutations in genes encoding for RBPs and consequently mRNA localization and local translation are impaired. In other diseases changes in the local mRNA repertoire and altered local protein synthesis have been reported. In this review, we will discuss how deregulation of localized translation at different levels can contribute to the development and progression of nervous system pathologies.

Reaction-Diffusion Model-Based Research on Formation Mechanism of Neuron Dendritic Spine Patterns

The pattern abnormalities of dendritic spine, tiny protrusions on neuron dendrites, have been found related to multiple nervous system diseases, such as Parkinson's disease and schizophrenia. The determination of the factors affecting spine patterns is of vital importance to explore the pathogenesis of these diseases, and further, search the treatment method for them. Although the study of dendritic spines is a hot topic in neuroscience in recent years, there is still a lack of systematic study on the formation mechanism of its pattern. This paper provided a reinterpretation of reaction-diffusion model to simulate the formation process of dendritic spine, and further, study the factors affecting spine patterns. First, all four classic shapes of spines, mushroom-type, stubby-type, thin-type, and branched-type were reproduced using the model. We found that the consumption rate of substrates by the cytoskeleton is a key factor to regulate spine shape. Moreover, we found that the density of spines can be regulated by the amount of an exogenous activator and inhibitor, which is in accordance with the anatomical results found in hippocampal CA1 in SD rats with glioma. Further, we analyzed the inner mechanism of the above model parameters regulating the dendritic spine pattern through Turing instability analysis and drew a conclusion that an exogenous inhibitor and activator changes Turing wavelength through which to regulate spine densities. Finally, we discussed the deep regulation mechanisms of several reported regulators of dendritic spine shape and densities based on our simulation results. Our work might evoke attention to the mathematic model-based pathogenesis research for neuron diseases which are related to the dendritic spine pattern abnormalities and spark inspiration in the treatment research for these diseases.

The Protein Arginine Methyltransferase PRMT8 and Substrate G3BP1 Control Rac1-PAK1 Signaling and Actin Cytoskeleton for Dendritic Spine Maturation

Excitatory synapses of neurons are located on dendritic spines. Spine maturation is essential for the stability of synapses and memory consolidation, and overproduction of the immature filopodia is associated with brain disorders. The structure and function of synapses can be modulated by protein post-translational modification (PTM). Arginine methylation is a major PTM that regulates chromatin structure, transcription, and splicing within the nucleus. Here we find that the protein arginine methyltransferase PRMT8 is present at neuronal synapses and its expression is upregulated in the hippocampus when dendritic spine maturation occurs. Depletion of PRMT8 leads to overabundance of filopodia and mis-localization of excitatory synapses. Mechanistically, PRMT8 promotes dendritic spine morphology through methylation of the dendritic RNA-binding protein G3BP1 and suppression of the Rac1-PAK1 signaling pathway to control synaptic actin dynamics. Our findings unravel arginine methylation as a crucial regulatory mechanism for actin cytoskeleton during synapse development.

Altered hippocampal dendritic spine maturation after hypoxia-induced seizures in neonatal rats

Cognitive comorbidities often follow early-life seizures (ELS), especially in the setting of autism and other neurodevelopmental syndromes. However, there is an incomplete understanding of whether neuronal and synaptic development are concomitantly dysregulated. We have previously shown that hypoxia-induced seizures (HS) in postnatal day (P)10 rats increase acute and later-life hippocampal glutamatergic neurotransmission and spontaneous recurrent seizures, and impair cognition and behavior. As dendritic spines critically regulate synaptic function, we hypothesized that ELS can induce developmentally specific changes in dendritic spine maturation. At intervals during one month following HS in P10 rats, we assessed dendritic spine development on pyramidal neurons in the stratum radiatum of hippocampal area CA1. Compared to control rats in which spine density significantly decreased from P10 to early adulthood (P38), post-seizure rats failed to show a developmental decrease in spine density, and spines from P38 post-seizure rats appeared more immature-shaped (long, thin). In addition, compared to P38 control rats, post-seizure P38 rats expressed significantly more synaptic PSD-95, a marker of mature synapses. These changes were preceded by a transient increase in hippocampal expression of cofilin phosphorylated at Ser3, representing a decrease in cofilin activity. These results suggest that early-life seizures may impair normal dendritic spine maturation and pruning in CA1 during development, resulting in an excess of less efficient synapses, via activity-dependent modification of actin-regulating proteins such as cofilin. Given that multiple neurodevelopmental disorders show similar failures in developmental spine pruning, the current findings may represent a deficit in structural plasticity that could be a component of a mechanism leading to later-life cognitive consequences associated with early-life seizures.

RAB1A regulates glioma cellular proliferation and invasion via the mTOR signaling pathway and epithelial-mesenchymal transition

Aim: We aimed at investigating the mechanism of RAB1A proliferation and invasion in gliomas. Materials & methods: Genome-wide expression profile data and immunohistochemistry were analyzed to assess RAB1A expression in gliomas. The Transwell assay, wound healing assay, brain slice coculture model, cellular fluorescence and intracranial xenograft model of nude mice were used to determine the proliferation and invasion of glioma cells. Results & conclusion: RAB1A was highly expressed in gliomas compared with normal brain tissue. The overall survival time of glioma patients with high RAB1A expression was significantly shortened. RAB1A regulated the activity of RAC1 by inhibiting the mTOR signaling pathway, affecting actin polymerization, cell morphology and cell polarity. RAB1A downregulation inhibited the epithelial-mesenchymal transition, proliferation and invasion of glioma cells.

The Role of Protein Arginine Methylation as Post-Translational Modification on Actin Cytoskeletal Components in Neuronal Structure and Function

The brain encompasses a complex network of neurons with exceptionally elaborated morphologies of their axonal (signal-sending) and dendritic (signal-receiving) parts. De novo actin filament formation is one of the major driving and steering forces for the development and plasticity of the neuronal arbor. Actin filament assembly and dynamics thus require tight temporal and spatial control. Such control is particularly effective at the level of regulating actin nucleation-promoting factors, as these are key components for filament formation. Arginine methylation represents an important post-translational regulatory mechanism that had previously been mainly associated with controlling nuclear processes. We will review and discuss emerging evidence from inhibitor studies and loss-of-function models for protein arginine methyltransferases (PRMTs), both in cells and whole organisms, that unveil that protein arginine methylation mediated by PRMTs represents an important regulatory mechanism in neuritic arbor formation, as well as in dendritic spine induction, maturation and plasticity. Recent results furthermore demonstrated that arginine methylation regulates actin cytosolic cytoskeletal components not only as indirect targets through additional signaling cascades, but can also directly control an actin nucleation-promoting factor shaping neuronal cells—a key process for the formation of neuronal networks in vertebrate brains.