Modulation of dendritic spines and synaptic function by Rac1: a possible link to Fragile X syndrome pathology

Disrupted Hippocampal-Prefrontal Networks in a Rat Model of Fragile X Syndrome: A Study Linking Neural Dynamics to Autism-Like Behavioral Impairments

Fragile X Syndrome (FXS) is associated with autism spectrum disorder (ASD) symptoms that are associated with cognitive, learning, and behavioral challenges. We investigated how known molecular disruptions in the Fmr1 knockout (FMR-KO) rat model of FXS negatively impact hippocampal-prefrontal cortex (H-PFC) neural network activity and consequent behavior.

Methods

FMR-KO and control rats underwent a battery of behavioral tests assessing sociability, memory, and anxiety. Single-unit electrophysiology recordings were then conducted to measure patterns of neural activity in H-PFC circuit. Advanced mathematical models were used to characterize the patterns that were then compared between groups using generalized linear mixed models.

Results

FMR-KO rats demonstrated significant behavioral deficits in sociability, spatial learning, and anxiety, aligning with symptoms of ASD. At the neural level, these rats exhibited abnormal firing patterns in the H-PFC circuit that is critical for learning, memory, and social behavior. The neural networks in FMR-KO rats were also less densely connected and more fragmented, particularly in hippocampal-PFC correlated firing. These findings suggest that disruptions in neural network dynamics underlie the observed behavioral impairments in FMR-KO rats.

Conclusion

FMR-KO significantly disrupts several characteristics of action potential firing in the H-PFC network, leading to deficits in social behavior, memory, and anxiety, as seen in FXS. This disruption is characterized by less organized and less resilient hippocampal-PFC networks. These findings suggest that therapeutic strategies aimed at normalizing neural dynamics, such as with brain stimulation, could potentially improve behavior and cognitive functions in autistic individuals.

HIGHLIGHTS

Fragile X Syndrome is associated with autism, cognitive challenges and anxietyThe loss of Fmr1 protein disrupts processes involved in building neural networksThe consequence is abnormal neural dynamics in hippocampal-prefrontal cortex networksNormalization of dynamics could improve outcomes in FXS and ASD.

Dendritic Spines and Pain Memory

Neuropathic pain is a debilitating form of pain arising from injury or disease of the nervous system that affects millions of people worldwide. Despite its prevalence, the underlying mechanisms of neuropathic pain are still not fully understood. Dendritic spines are small protrusions on the surface of neurons that play an important role in synaptic transmission. Recent studies have shown that dendritic spines reorganize in the superficial and deeper laminae of the spinal cord dorsal horn with the development of neuropathic pain in multiple models of disease or injury. Given the importance of dendritic spines in synaptic transmission, it is possible that studying dendritic spines could lead to new therapeutic approaches for managing intractable pain. In this review article, we highlight the emergent role of dendritic spines in neuropathic pain, as well as discuss the potential for studying dendritic spines for the development of new therapeutics.

Characterization of a Clinically and Biologically Defined Subgroup of Patients with Autism Spectrum Disorder and Identification of a Tailored Combination Treatment

Autism spectrum disorder (ASD) is a heterogeneous group of neurodevelopmental disorders (NDDs) with a high unmet medical need. The diagnosis of ASD is currently based on behavior criteria, which overlooks the diversity of genetic, neurophysiological, and clinical manifestations. Failure to acknowledge such heterogeneity has hindered the development of efficient drug treatments for ASD and other NDDs. DEPI® (Databased Endophenotyping Patient Identification) is a systems biology, multi-omics, and machine learning-driven platform enabling the identification of subgroups of patients with NDDs and the development of patient-tailored treatments. In this study, we provide evidence for the validation of a first clinically and biologically defined subgroup of patients with ASD identified by DEPI, ASD Phenotype 1 (ASD-Phen1). Among 313 screened patients with idiopathic ASD, the prevalence of ASD-Phen1 was observed to be ~24% in 84 patients who qualified to be enrolled in the study. Metabolic and transcriptomic alterations differentiating patients with ASD-Phen1 were consistent with an over-activation of NF-κB and NRF2 transcription factors, as predicted by DEPI. Finally, the suitability of STP1 combination treatment to revert such observed molecular alterations in patients with ASD-Phen1 was determined. Overall, our results support the development of precision medicine-based treatments for patients diagnosed with ASD.

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.

Synaptic dysfunction in schizophrenia

Schizophrenia is a chronic disease presented with psychotic symptoms, negative symptoms, impairment in the reward system, and widespread neurocognitive deterioration. Disruption of synaptic connections in neural circuits is responsible for the disease's development and progression. Because deterioration in synaptic connections results in the impaired effective processing of information. Although structural impairments of the synapse, such as a decrease in dendritic spine density, have been shown in previous studies, functional impairments have also been revealed with the development of genetic and molecular analysis methods. In addition to abnormalities in protein complexes regulating exocytosis in the presynaptic region and impaired vesicle release, especially, changes in proteins related to postsynaptic signaling have been reported. In particular, impairments in postsynaptic density elements, glutamate receptors, and ion channels have been shown. At the same time, effects on cellular adhesion molecular structures such as neurexin, neuroligin, and cadherin family proteins were detected. Of course, the confusing effect of antipsychotic use in schizophrenia research should also be considered. Although antipsychotics have positive and negative effects on synapses, studies indicate synaptic deterioration in schizophrenia independent of drug use. In this review, the deterioration in synapse structure and function and the effects of antipsychotics on the synapse in schizophrenia will be discussed.

Axonal and presynaptic FMRP: Localization, signal, and functional implications

Fragile X mental retardation protein (FMRP) binds a selected set of mRNAs and proteins to guide neural circuit assembly and regulate synaptic plasticity. Loss of FMRP is responsible for Fragile X syndrome, a neuropsychiatric disorder characterized with auditory processing problems and social difficulty. FMRP actions in synaptic formation, maturation, and plasticity are site-specific among the four compartments of a synapse: presynaptic and postsynaptic neurons, astrocytes, and extracellular matrix. This review summarizes advancements in understanding FMRP localization, signals, and functional roles in axons and presynaptic terminals.

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.

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.

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.

Cortactin Contributes to Activity-Dependent Modulation of Spine Actin Dynamics and Spatial Memory Formation

Postsynaptic structures on excitatory neurons, dendritic spines, are actin-rich. It is well known that actin-binding proteins regulate actin dynamics and by this means orchestrate structural plasticity during the development of the brain, as well as synaptic plasticity mediating learning and memory processes. The actin-binding protein cortactin is localized to pre- and postsynaptic structures and translocates in a stimulus-dependent manner between spines and the dendritic compartment, thereby indicating a crucial role for synaptic plasticity and neuronal function. While it is known that cortactin directly binds F-actin, the Arp2/3 complex important for actin nucleation and branching as well as other factors involved in synaptic plasticity processes, its precise role in modulating actin remodeling in neurons needs to be deciphered. In this study, we characterized the general neuronal function of cortactin in knockout mice. Interestingly, we found that the loss of cortactin leads to deficits in hippocampus-dependent spatial memory formation. This impairment is correlated with a prominent dysregulation of functional and structural plasticity. Additional evidence shows impaired long-term potentiation in cortactin knockout mice together with a complete absence of structural spine plasticity. These phenotypes might at least in part be explained by alterations in the activity-dependent modulation of synaptic actin in cortactin-deficient neurons.

Neuronal Cytoskeleton in Intellectual Disability: From Systems Biology and Modeling to Therapeutic Opportunities

Intellectual disability (ID) is a pathological condition characterized by limited intellectual functioning and adaptive behaviors. It affects 1–3% of the worldwide population, and no pharmacological therapies are currently available. More than 1000 genes have been found mutated in ID patients pointing out that, despite the common phenotype, the genetic bases are highly heterogeneous and apparently unrelated. Bibliomic analysis reveals that ID genes converge onto a few biological modules, including cytoskeleton dynamics, whose regulation depends on Rho GTPases transduction. Genetic variants exert their effects at different levels in a hierarchical arrangement, starting from the molecular level and moving toward higher levels of organization, i.e., cell compartment and functions, circuits, cognition, and behavior. Thus, cytoskeleton alterations that have an impact on cell processes such as neuronal migration, neuritogenesis, and synaptic plasticity rebound on the overall establishment of an effective network and consequently on the cognitive phenotype. Systems biology (SB) approaches are more focused on the overall interconnected network rather than on individual genes, thus encouraging the design of therapies that aim to correct common dysregulated biological processes. This review summarizes current knowledge about cytoskeleton control in neurons and its relevance for the ID pathogenesis, exploiting in silico modeling and translating the implications of those findings into biomedical research.

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.

Sculpting Dendritic Spines during Initiation and Maintenance of Neuropathic Pain

Accumulating evidence has established a firm role for synaptic plasticity in the pathogenesis of neuropathic pain. Recent advances have highlighted the importance of dendritic spine remodeling in driving synaptic plasticity within the CNS. Identifying the molecular players underlying neuropathic pain induced structural and functional maladaptation is therefore critical to understanding its pathophysiology. This process of dynamic reorganization happens in unique phases that have diverse pathologic underpinnings in the initiation and maintenance of neuropathic pain. Recent evidence suggests that pharmacological targeting of specific proteins during distinct phases of neuropathic pain development produces enhanced antinociception. These findings outline a potential new paradigm for targeted treatment and the development of novel therapies for neuropathic pain. We present a concise review of the role of dendritic spines in neuropathic pain and outline the potential for modulation of spine dynamics by targeting two proteins, srGAP3 and Rac1, critically involved in the regulation of the actin cytoskeleton.

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 small GTPase Arf6 is dysregulated in a mouse model for fragile X syndrome

Fragile X syndrome (FXS), the most common inherited cause of intellectual disability, results from silencing of the fragile X mental retardation gene 1 (FMR1). The analyses of FXS patients' brain autopsies revealed an increased density of immature dendritic spines in cortical areas. We hypothesize that the small GTPase Arf6, an actin regulator critical for the development of glutamatergic synapses and dendritic spines, is implicated in FXS. Here, we determined the fraction of active, GTP-bound Arf6 in cortical neuron cultures and synaptoneurosomes from Fmr1 knockout mice, measured actin polymerization in neurons expressing Arf6 mutants with variant GTP- or GDP-binding properties, and recorded hippocampal long-term depression induced by metabotropic glutamate receptors (mGluR-LTD) in acute brain slices. We detected a persistently elevated Arf6 activity, a loss of Arf6 sensitivity to synaptic stimulation and an increased Arf6-dependent dendritic actin polymerization in mature Fmr1 knockout neurons. Similar imbalances in Arf6-GTP levels and actin filament assembly were caused in wild-type neurons by RNAi-mediated depletion of the postsynaptic Arf6 guanylate exchange factors IQSEC1 (BRAG2) or IQSEC2 (BRAG1). Targeted deletion of Iqsec1 in hippocampal neurons of 3-week-old mice interfered with mGluR-LTD in wild-type, but not in Fmr1 knockout mice. Collectively, these data suggest an aberrant Arf6 regulation in Fmr1 knockout neurons with consequences for the actin cytoskeleton, spine morphology, and synaptic plasticity. Moreover, FXS and syndromes caused by genetic variants in IQSEC1 and IQSEC2 share intellectual disabilities and developmental delay as main symptoms. Therefore, dysregulation of Arf6 may contribute to the cognitive impairment in FXS.

Stress-Sensitive Protein Rac1 and Its Involvement in Neurodevelopmental Disorders

Ras-related C3 botulinum toxin substrate 1 (Rac1) is a small GTPase that is well known for its sensitivity to the environmental stress of a cell or an organism. It senses the external signals which are transmitted from membrane-bound receptors and induces downstream signaling cascades to exert its physiological functions. Rac1 is an important regulator of a variety of cellular processes, such as cytoskeletal organization, generation of oxidative products, and gene expression. In particular, Rac1 has a significant influence on certain brain functions like neuronal migration, synaptic plasticity, and memory formation via regulation of actin dynamics in neurons. Abnormal Rac1 expression and activity have been observed in multiple neurological diseases. Here, we review recent findings to delineate the role of Rac1 signaling in neurodevelopmental disorders associated with abnormal spine morphology, synaptogenesis, and synaptic plasticity. Moreover, certain novel inhibitors of Rac1 and related pathways are discussed as potential avenues toward future treatment for these diseases.

FMRP Modulates Activity-Dependent Spine Plasticity by Binding Cofilin1 mRNA and Regulating Localization and Local Translation

Multiple variants of intellectual disability, e.g., the Fragile X Syndrome are associated with alterations in dendritic spine morphology, thereby pointing to dysregulated actin dynamics during development and processes of synaptic plasticity. Surprisingly, although the necessity of spine actin remodeling was demonstrated repeatedly, the importance and precise role of actin regulators is often undervalued. Here, we provide evidence that structural and functional plasticity are severely impaired after NMDAR-dependent LTP in the hippocampus of Fmr1 KO mice. We can link these defects to an aberrant activity-dependent regulation of Cofilin 1 (cof1) as activity-dependent modulations of local cof1 mRNA availability, local cof1 translation as well as total cof1 expression are impaired in the absence of FMRP. Finally, we can rescue activity-dependent structural plasticity in KO neurons by mimicking the regulation of cof1 observed in WT cells, thereby illustrating the potential of actin modulators to provide novel treatment strategies for the Fragile X Syndrome.

Association between thiopurine exposure and depression in patients with inflammatory bowel disease and rheumatoid arthritis

BACKGROUND

Ras-related C3 botulinum substrate 1 (Rac1) is a member of the small molecule family Rho guanosine triphosphate (GTP)ases. Recent findings reveal epigenetic downregulation of Rac1 is a mechanism of depression.

AIMS

The purpose of this study was to evaluate Rac1 as a therapeutic target for depression we examine the association between thiopurines, which inhibit Rac1, and the risk of depression among US veterans.

METHODS

This study uses data spanning January 2000-May 2019, comparing thiopurine exposure (no exposure, less than one year, 1-2.9 years, 3-5 years, and greater than five years) in two separate cohorts, a rheumatoid arthritis cohort and inflammatory bowel disease cohort. We estimate the hazard of depression using a time dependent cox proportional hazards model.

RESULTS

A total of 76,763 rheumatoid arthritis and 46,787 inflammatory bowel disease patients met all inclusion criteria. Patients exposed to thiopurines less than one year have a 27% (hazard ratio=1.272; 95% confidence interval=(1.038-1.559)) and 67% (hazard ratio=1.667 95% confidence interval=(1.501-1.850)) higher risk of depression in the rheumatoid arthritis and inflammatory bowel disease cohorts, respectively. In the inflammatory bowel disease cohort, we find the risk of depression is increased for up to five years of thiopurine exposure.

CONCLUSION

These results provide evidence that Rac1 regulation is a viable therapeutic target for depression. Further research into therapeutics targeting Rac1 for the treatment of depression is warranted.

Cytoskeletal regulation of synaptogenesis in a model of human fetal brain development

Excitatory synapse formation begins in mid-fetal gestation. However, due to our inability to image fetal synaptogenesis, the initial formation of synapses remains understudied. The recent development of human fetal brain spheroids provides access to this critical period of synapse formation. Using human neurons and brain spheroids, we address how altered actin regulation impacts the formation of excitatory synapses during fetal brain development. Prior to synapse formation, inhibition of RhoA kinase (ROCK) signaling promotes neurite elongation and branching. In addition to increasing neural complexity, ROCK inhibition increases the length of protrusions along the neurite, ultimately promoting excitatory synapse formation in human cortical brain spheroids. A corresponding increase in Rac1-driven actin polymerization drives this increase in excitatory synaptogenesis. Using STORM super-resolution microscopy, we demonstrate that actomyosin regulators, including the Rac1 regulator, α-PIX, and the RhoA regulator, p115-RhoGEF, localize to nascent excitatory synapses, where they preferentially localize to postsynaptic compartments. These results demonstrate that coordinated RhoGTPase activities underlie the initial formation of excitatory synapses and identify critical cytoskeletal regulators of early synaptogenic events.

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.

TRPM7 Is Required for Normal Synapse Density, Learning, and Memory at Different Developmental Stages

The TRPM7 chanzyme contributes to several biological and pathological processes in different tissues. However, its role in the CNS under physiological conditions remains unclear. Here, we show that TRPM7 knockdown in hippocampal neurons reduces structural synapse density. The synapse density is rescued by the α-kinase domain in the C terminus but not by the ion channel region of TRPM7 or by increasing extracellular concentrations of Mg²⁺ or Zn²⁺. Early postnatal conditional knockout of TRPM7 in mice impairs learning and memory and reduces synapse density and plasticity. TRPM7 knockdown in the hippocampus of adult rats also impairs learning and memory and reduces synapse density and synaptic plasticity. In knockout mice, restoring expression of the α-kinase domain in the brain rescues synapse density/plasticity and memory, probably by interacting with and phosphorylating cofilin. These results suggest that brain TRPM7 is important for having normal synaptic and cognitive functions under physiological, non-pathological conditions.

Association between thiopurine medication exposure and Alzheimer's disease among a cohort of patients with inflammatory bowel disease

Introduction

Ras-related C3 botulinum toxin substrate 1 (Rac1), a member of the Rho-GTPase family of proteins, could be an Alzheimer's disease (AD) triggering co-factor due to its effect on both amyloid precursor protein (APP) and tau. Thiopurine medications, such as azathioprine and mercaptopurine, are immunosuppressants that suppress Rac1 activation. We hypothesize that due to their ability to suppress Rac1, thiopurines are associated with a lower risk of AD.

Methods

To explore the relationship between thiopurines and incident AD diagnosis, we conducted a national retrospective cohort study among U.S. Veterans with inflammatory bowel disease (IBD), including Crohn's disease (CD) or ulcerative colitis (UC), as well as a non-IBD control. We created propensity score-matched cohorts and estimated the hazard ratio via the time-dependent Cox proportional hazards model.

Results

The study sample size was 66,312 patients and consisted of 24,057 IBD patients (4354 thiopurine exposed and 19,703 unexposed) and 42,255 patients without IBD or thiopurine exposure. Patients exposed to thiopurines have the lowest rate of AD, and our results demonstrate for each additional year of thiopurine exposure risk of AD is reduced by 8.3%% (adjusted HR = 0.917; 95% CI = [0.851–0.989]).

Discussion

Our results support the preclinical findings implicating Rac1 in the AD disease process. A national cohort study demonstrated that Rac1 is associated with the AD process consistent with the preclinical evidence. Further exploration and evaluation of Rac1 inhibition are needed.

The Ras Superfamily of Small GTPases in Non-neoplastic Cerebral Diseases

The small GTPases from the Ras superfamily play crucial roles in basic cellular processes during practically the entire process of neurodevelopment, including neurogenesis, differentiation, gene expression, membrane and protein traffic, vesicular trafficking, and synaptic plasticity. Small GTPases are key signal transducing enzymes that link extracellular cues to the neuronal responses required for the construction of neuronal networks, as well as for synaptic function and plasticity. Different subfamilies of small GTPases have been linked to a number of non-neoplastic cerebral diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), intellectual disability, epilepsy, drug addiction, Huntington’s disease (HD), amyotrophic lateral sclerosis (ALS) and a large number of idiopathic cerebral diseases. Here, we attempted to make a clearer illustration of the relationship between Ras superfamily GTPases and non-neoplastic cerebral diseases, as well as their roles in the neural system. In future studies, potential treatments for non-neoplastic cerebral diseases which are based on small GTPase related signaling pathways should be explored further. In this paper, we review all the available literature in support of this possibility.

HACE1 deficiency leads to structural and functional neurodevelopmental defects

Objective

We aim to characterize the causality and molecular and functional underpinnings of HACE1 deficiency in a mouse model of a recessive neurodevelopmental syndrome called spastic paraplegia and psychomotor retardation with or without seizures (SPPRS).

Methods

By exome sequencing, we identified 2 novel homozygous truncating mutations in HACE1 in 3 patients from 2 families, p.Q209* and p.R332*. Furthermore, we performed detailed molecular and phenotypic analyses of Hace1 knock-out (KO) mice and SPPRS patient fibroblasts.

Results

We show that Hace1 KO mice display many clinical features of SPPRS including enlarged ventricles, hypoplastic corpus callosum, as well as locomotion and learning deficiencies. Mechanistically, loss of HACE1 results in altered levels and activity of the small guanosine triphosphate (GTP)ase, RAC1. In addition, HACE1 deficiency results in reduction in synaptic puncta number and long-term potentiation in the hippocampus. Similarly, in SPPRS patient-derived fibroblasts, carrying a disruptive HACE1 mutation resembling loss of HACE1 in KO mice, we observed marked upregulation of the total and active, GTP-bound, form of RAC1, along with an induction of RAC1-regulated downstream pathways.

Conclusions

Our results provide a first animal model to dissect this complex human disease syndrome, establishing the first causal proof that a HACE1 deficiency results in decreased synapse number and structural and behavioral neuropathologic features that resemble SPPRS patients.

A Synaptic Perspective of Fragile X Syndrome and Autism Spectrum Disorders

Altered synaptic structure and function is a major hallmark of fragile X syndrome (FXS), autism spectrum disorders (ASDs), and other intellectual disabilities (IDs), which are therefore classified as synaptopathies. FXS and ASDs, while clinically and genetically distinct, share significant comorbidity, suggesting that there may be a common molecular and/or cellular basis, presumably at the synapse. In this article, we review brain architecture and synaptic pathways that are dysregulated in FXS and ASDs, including spine architecture, signaling in synaptic plasticity, local protein synthesis, (m)RNA modifications, and degradation. mRNA repression is a powerful mechanism for the regulation of synaptic structure and efficacy. We infer that there is no single pathway that explains most of the etiology and discuss new findings and the implications for future work directed at improving our understanding of the pathogenesis of FXS and related ASDs and the design of therapeutic strategies to ameliorate these disorders.

Rac1 Modulates Excitatory Synaptic Transmission in Mouse Retinal Ganglion Cells

Ras-related C3 botulinum toxin substrate 1 (Rac1), a member of the Rho GTPase family which plays important roles in dendritic spine morphology and plasticity, is a key regulator of cytoskeletal reorganization in dendrites and spines. Here, we investigated whether and how Rac1 modulates synaptic transmission in mouse retinal ganglion cells (RGCs) using selective conditional knockout of Rac1 (Rac1-cKO). Rac1-cKO significantly reduced the frequency of AMPA receptor-mediated miniature excitatory postsynaptic currents, while glycine/GABA_A receptor-mediated miniature inhibitory postsynaptic currents were not affected. Although the total GluA1 protein level was increased in Rac1-cKO mice, its expression in the membrane component was unchanged. Rac1-cKO did not affect spine-like branch density in single dendrites, but significantly reduced the dendritic complexity, which resulted in a decrease in the total number of dendritic spine-like branches. These results suggest that Rac1 selectively affects excitatory synaptic transmission in RGCs by modulating dendritic complexity.

Fragile X Mental Retardation Protein positively regulates PKA anchor Rugose and PKA activity to control actin assembly in learning/memory circuitry

Recent work shows Fragile X Mental Retardation Protein (FMRP) drives the translation of very large proteins (>2000 aa) mediating neurodevelopment. Loss of function results in Fragile X syndrome (FXS), the leading heritable cause of intellectual disability (ID) and autism spectrum disorder (ASD). Using the Drosophila FXS disease model, we discover FMRP positively regulates the translation of the very large A-Kinase Anchor Protein (AKAP) Rugose (>3000 aa), homolog of ASD-associated human Neurobeachin (NBEA). In the central brain Mushroom Body (MB) circuit, where Protein Kinase A (PKA) signaling is necessary for learning/memory, FMRP loss reduces Rugose levels and targeted FMRP overexpression elevates Rugose levels. Using a new in vivo transgenic PKA activity reporter (PKA-SPARK), we find FMRP loss reduces PKA activity in MB Kenyon cells whereas FMRP overexpression elevates PKA activity. Consistently, loss of Rugose reduces PKA activity, but Rugose overexpression has no independent effect. A well-established PKA output is regulation of F-actin cytoskeleton dynamics. In the FXS disease model, F-actin is aberrantly accumulated in MB lobes and single MB Kenyon cells. Consistently, Rugose loss results in similar F-actin accumulation. Moreover, targeted FMRP, Rugose and PKA overexpression all result in increased F-actin accumulation in the MB circuit. These findings uncover a FMRP-Rugose-PKA mechanism regulating actin cytoskeleton. This study reveals a novel FMRP mechanism controlling neuronal PKA activity, and demonstrates a shared mechanistic connection between FXS and NBEA associated ASD disease states, with a common link to PKA and F-actin misregulation in brain neural circuits. SIGNIFICANCE STATEMENT: Autism spectrum disorder (ASD) arises from a wide array of genetic lesions, and it is therefore critical to identify common underlying molecular mechanisms. Here, we link two ASD states; Neurobeachin (NBEA) associated ASD and Fragile X syndrome (FXS), the most common inherited ASD. Using established Drosophila disease models, we find Fragile X Mental Retardation Protein (FMRP) positively regulates translation of NBEA homolog Rugose, consistent with a recent advance showing FMRP promotes translation of very large proteins associated with ASD. FXS exhibits reduced cAMP induction, a potent activator of PKA, and Rugose/NBEA is a PKA anchor. Consistently, we find brain PKA activity strikingly reduced in both ASD models. We discover this pathway regulation controls actin cytoskeleton dynamics in brain neural circuits.

Epigenetic modifications by polyphenolic compounds alter gene expression in the hippocampus

In this study, we developed an experimental protocol leveraging enhanced reduced representation bisulphite sequencing to investigate methylation and gene expression patterns in the hippocampus in response to polyphenolic compounds. We report that the administration of a standardized bioavailable polyphenolic preparation (BDPP) differentially influences methylated cytosine patterns in introns, UTR and exons in hippocampal genes. We subsequently established that dietary BDPP-mediated changes in methylation influenced the transcriptional pattern of select genes that are involved in synaptic plasticity. In addition, we showed dietary BDPP mediated changes in the transcriptional pattern of genes associated with epigenetic modifications, including members of the DNA methyl transferase family (DNMTs) and the Ten-eleven translocation methylcytosine dioxygenases family (TETs). We then identified the specific brain bioavailable polyphenols effective in regulating the transcription of DNMTs, TETs and a subset of differentially methylated synaptic plasticity-associated genes. The study implicates the regulation of gene expression in the hippocampus by epigenetic mechanisms as a novel therapeutic target for dietary polyphenols.

Summary: The health benefits of dietary polyphenols may be due to their ability to change epigenetic marks in the brain. More studies will clarify how polyphenols from diet can improve mental health.

Modeling the neuropsychiatric manifestations of Lowe syndrome using induced pluripotent stem cells: defective F-actin polymerization and WAVE-1 expression in neuronal cells

Background

Lowe syndrome (LS) is a rare genetic disorder caused by loss of function mutations in the X-linked gene, OCRL, which codes for inositol polyphosphate 5-phosphatase. LS is characterized by the triad of congenital cataracts, neurodevelopmental impairment (primarily intellectual and developmental disabilities [IDD]), and renal proximal tubular dysfunction. Studies carried out over the years have shown that hypomorphic mutations in OCRL adversely affect endosome recycling and actin polymerization in kidney cells and patient-derived fibroblasts. The renal problem has been traced to an impaired recycling of megalin, a multi-ligand receptor that plays a key role in the reuptake of lipoproteins, amino acids, vitamin-binding proteins, and hormones. However, the neurodevelopmental aspects of the disorder have been difficult to study because the mouse knockout (KO) model does not display LS-related phenotypes. Fortunately, the discovery of induced pluripotent stem (iPS) cells has provided an opportunity to grow patient-specific neurons, which can be used to model neurodevelopmental disorders in vitro, as demonstrated in the many studies that have been published in the past few years in autism spectrum disorders (ASD), schizophrenia (SZ), bipolar disorder (BD), and IDD.

Methods

We now report the first findings in neurons and neural progenitor cells (NPCs) generated from iPS cells derived from patients with LS and their typically developing male siblings, as well as an isogenic line in which the OCRL gene has been incapacitated by a null mutation generated using CRISPR-Cas9 gene editing.

Results

We show that neuronal cells derived from patient-specific iPS cells containing hypomorphic variants are deficient in their capacity to produce F-filamentous actin (F-actin) fibers. Abnormalities were also found in the expression of WAVE-1, a component of the WAVE regulatory complex (WRC) that regulates actin polymerization. Curiously, neuronal cells carrying the engineered OCRL null mutation, in which OCRL protein is not expressed, did not show similar defects in F-actin and WAVE-1 expression. This is similar to the apparent lack of a phenotype in the mouse Ocrl KO model, and suggests that in the complete absence of OCRL protein, as opposed to producing a dysfunctional protein, as seen with the hypomorphic variants, there is partial compensation for the F-actin/WAVE-1 regulating function of OCRL.

Conclusions

Alterations in F-actin polymerization and WRC have been found in a number of genetic subgroups of IDD and ASD. Thus, LS, a very rare genetic condition, is linked to a more expansive family of genes responsible for neurodevelopmental disorders that have shared pathogenic features.

Electronic supplementary material

The online version of this article (10.1186/s13229-018-0227-3) contains supplementary material, which is available to authorized users.

Thrombospondin receptor α2δ-1 promotes synaptogenesis and spinogenesis via postsynaptic Rac1

Astrocytes promote synapse formation during development via secreted factors including thrombospondin family proteins, which act through the neuronal calcium channel subunit α2δ-1. Risher et al. demonstrate that this process requires signaling via the Rho GTPase Rac1 to facilitate the maturation of dendritic spine synapses in the cortex.

Astrocytes control excitatory synaptogenesis by secreting thrombospondins (TSPs), which function via their neuronal receptor, the calcium channel subunit α2δ-1. α2δ-1 is a drug target for epilepsy and neuropathic pain; thus the TSP–α2δ-1 interaction is implicated in both synaptic development and disease pathogenesis. However, the mechanism by which this interaction promotes synaptogenesis and the requirement for α2δ-1 for connectivity of the developing mammalian brain are unknown. In this study, we show that global or cell-specific loss of α2δ-1 yields profound deficits in excitatory synapse numbers, ultrastructure, and activity and severely stunts spinogenesis in the mouse cortex. Postsynaptic but not presynaptic α2δ-1 is required and sufficient for TSP-induced synaptogenesis in vitro and spine formation in vivo, but an α2δ-1 mutant linked to autism cannot rescue these synaptogenesis defects. Finally, we reveal that TSP–α2δ-1 interactions control synaptogenesis postsynaptically via Rac1, suggesting potential molecular mechanisms that underlie both synaptic development and pathology.

An Intellectual Disability-Related Missense Mutation in Rac1 Prevents LTP Induction

The small GTPase Rac1 promotes actin polymerization and plays a critical and increasingly appreciated role in the development and plasticity of glutamatergic synapses. Growing evidence suggests that disruption of the Rac1 signaling pathway at glutamatergic synapses contributes to Autism Spectrum Disorder/intellectual disability (ASD/ID)-related behaviors seen in animal models of ASD/ID. Rac1 has also been proposed as a strong candidate of convergence for many factors implicated in the development of ASD/ID. However, the effects of ASD/ID-related mutations in Rac1 itself have not been explored in neurons. Here, we investigate a recently reported de novo missense mutation in Rac1 found in an individual with severe ID. Our modeling predicts that this mutation will strongly inhibit Rac1 activation by occluding Rac1's GTP binding pocket. Indeed, we find that this de novo mutation prevents Rac1 function and results in a selective reduction in synaptic AMPA receptor function. Furthermore, this mutation prevents the induction of long-term potentiation (LTP), the cellular mechanism underlying learning and memory formation. Together, our findings strongly suggest that this mutation contributes to the development of ID in this individual. This research demonstrates the importance of Rac1 in synaptic function and plasticity and contributes to a growing body of evidence pointing to dysregulation of actin polymerization at glutamatergic synapses as a contributing factor to ASD/ID.

Imbalance of synaptic actin dynamics as a key to fragile X syndrome?

Our experiences and memories define who we are, and evidence has accumulated that memory formation is dependent on functional and structural adaptations of synaptic structures in our brain. Especially dendritic spines, the postsynaptic compartments of synapses show a strong structure-to-function relationship and a high degree of structural plasticity. Although the molecular mechanisms are not completely understood, it is known that these modifications are highly dependent on the actin cytoskeleton, the major cytoskeletal component of the spine. Given the crucial involvement of actin in these mechanisms, dysregulations of spine actin dynamics (reflected by alterations in dendritic spine morphology) can be found in a variety of neurological disorders ranging from schizophrenia to several forms of autism spectrum disorders such as fragile X syndrome (FXS). FXS is caused by a single mutation leading to an inactivation of the X-linked fragile X mental retardation 1 gene and loss of its gene product, the RNA-binding protein fragile X mental retardation protein 1 (FMRP), which normally can be found both pre- and postsynaptically. FMRP is involved in mRNA transport as well as regulation of local translation at the synapse, and although hundreds of FMRP-target mRNAs could be identified only a very few interactions between FMRP and actin-regulating proteins have been reported and validated. In this review we give an overview of recent work by our lab and others providing evidence that dysregulated actin dynamics might indeed be at the very base of a deeper understanding of neurological disorders ranging from cognitive impairment to the autism spectrum.

Rho GTPases in Intellectual Disability: From Genetics to Therapeutic Opportunities

Rho-class small GTPases are implicated in basic cellular processes at nearly all brain developmental steps, from neurogenesis and migration to axon guidance and synaptic plasticity. GTPases are key signal transducing enzymes that link extracellular cues to the neuronal responses required for the construction of neuronal networks, as well as for synaptic function and plasticity. Rho GTPases are highly regulated by a complex set of activating (GEFs) and inactivating (GAPs) partners, via protein:protein interactions (PPI). Misregulated RhoA, Rac1/Rac3 and cdc42 activity has been linked with intellectual disability (ID) and other neurodevelopmental conditions that comprise ID. All genetic evidences indicate that in these disorders the RhoA pathway is hyperactive while the Rac1 and cdc42 pathways are consistently hypoactive. Adopting cultured neurons for in vitro testing and specific animal models of ID for in vivo examination, the endophenotypes associated with these conditions are emerging and include altered neuronal networking, unbalanced excitation/inhibition and altered synaptic activity and plasticity. As we approach a clearer definition of these phenotype(s) and the role of hyper- and hypo-active GTPases in the construction of neuronal networks, there is an increasing possibility that selective inhibitors and activators might be designed via PPI, or identified by screening, that counteract the misregulation of small GTPases and result in alleviation of the cognitive condition. Here we review all knowledge in support of this possibility.

Dendritic spine actin cytoskeleton in autism spectrum disorder

Dendritic spines are small actin-rich protrusions from neuronal dendrites that form the postsynaptic part of most excitatory synapses. Changes in the shape and size of dendritic spines correlate with the functional changes in excitatory synapses and are heavily dependent on the remodeling of the underlying actin cytoskeleton. Recent evidence implicates synapses at dendritic spines as important substrates of pathogenesis in neuropsychiatric disorders, including autism spectrum disorder (ASD). Although synaptic perturbations are not the only alterations relevant for these diseases, understanding the molecular underpinnings of the spine and synapse pathology may provide insight into their etiologies and could reveal new drug targets. In this review, we will discuss recent findings of defective actin regulation in dendritic spines associated with ASD.

Crosstalk between Rac1-mediated actin regulation and ROS production

The small RhoGTPase Rac1 is implicated in a variety of events related to actin cytoskeleton rearrangement. Remarkably, another event that is completely different from those related to actin regulation has the same relevance; the Rac1-mediated production of reactive oxygen species (ROS) through NADPH oxidases (NOX). Each outcome involves different Rac1 downstream effectors; on one hand, events related to the actin cytoskeleton require Rac1 to bind to WAVEs proteins and PAKs that ultimately promote actin branching and turnover, on the other, NOX-derived ROS production demands active Rac1 to be bound to a cytosolic activator of NOX. How Rac1-mediated signaling ends up promoting actin-related events, NOX-derived ROS, or both is poorly understood. Rac1 regulators, including scaffold proteins, are known to exert tight control over its functions. Hence, evidence of Rac1 regulatory events leading to both actin remodeling and NOX-mediated ROS generation are discussed. Moreover, cellular functions linked to physiological and pathological conditions that exhibit crosstalk between Rac1 outcomes are analyzed, while plausible roles in neuronal functions (and dysfunctions) are highlighted. Together, discussed evidence shed light on cellular mechanisms which requires Rac1 to direct either actin- and/or ROS-related events, helping to understand crucial roles of Rac1 dual functionality.

Fragile X Mental Retardation Protein Regulates Activity-Dependent Membrane Trafficking and Trans-Synaptic Signaling Mediating Synaptic Remodeling

Fragile X syndrome (FXS) is the leading monogenic cause of autism and intellectual disability. The disease arises through loss of fragile X mental retardation protein (FMRP), which normally exhibits peak expression levels in early-use critical periods, and is required for activity-dependent synaptic remodeling during this transient developmental window. FMRP canonically binds mRNA to repress protein translation, with targets that regulate cytoskeleton dynamics, membrane trafficking, and trans-synaptic signaling. We focus here on recent advances emerging in these three areas from the Drosophila disease model. In the well-characterized central brain mushroom body (MB) olfactory learning/memory circuit, FMRP is required for activity-dependent synaptic remodeling of projection neurons innervating the MB calyx, with function tightly restricted to an early-use critical period. FMRP loss is phenocopied by conditional removal of FMRP only during this critical period, and rescued by FMRP conditional expression only during this critical period. Consistent with FXS hyperexcitation, FMRP loss defects are phenocopied by heightened sensory experience and targeted optogenetic hyperexcitation during this critical period. FMRP binds mRNA encoding Drosophila ESCRTIII core component Shrub (human CHMP4 homolog) to restrict Shrub translation in an activity-dependent mechanism only during this same critical period. Shrub mediates endosomal membrane trafficking, and perturbing Shrub expression is known to interfere with neuronal process pruning. Consistently, FMRP loss and Shrub overexpression targeted to projection neurons similarly causes endosomal membrane trafficking defects within synaptic boutons, and genetic reduction of Shrub strikingly rescues Drosophila FXS model defects. In parallel work on the well-characterized giant fiber (GF) circuit, FMRP limits iontophoretic dye loading into central interneurons, demonstrating an FMRP role controlling core neuronal properties through the activity-dependent repression of translation. In the well-characterized Drosophila neuromuscular junction (NMJ) model, developmental synaptogenesis and activity-dependent synaptic remodeling both require extracellular matrix metalloproteinase (MMP) enzymes interacting with the heparan sulfate proteoglycan (HSPG) glypican dally-like protein (Dlp) to restrict trans-synaptic Wnt signaling, with FXS synaptogenic defects alleviated by both MMP and HSPG reduction. This new mechanistic axis spanning from activity to FMRP to HSPG-dependent MMP regulation modulates activity-dependent synaptogenesis. We discuss future directions for these mechanisms, and intersecting research priorities for FMRP in glial and signaling interactions.

Mapping pathologic circuitry in schizophrenia

Schizophrenia is a complex disorder lacking an effective treatment option for the pervasive and debilitating cognitive impairments experienced by patients. Working memory is a core cognitive function impaired in schizophrenia that depends upon activation of distributed neural network, including the circuitry of the dorsolateral prefrontal cortex (DLPFC). Accordingly, individuals diagnosed with schizophrenia show reduced DLPFC activation while performing working-memory tasks. This lower DLPFC activation appears to be an integral part of the disease pathophysiology, and not simply a reflection of poor performance. Thus, the cellular and circuitry alterations that underlie lower DLPFC neuronal activity in schizophrenia must be determined in order to identify appropriate therapeutic targets. Studies using human postmortem brain tissue provide a robust way to investigate and characterize these cellular and circuitry alterations at multiple levels of resolution, and such studies provide essential information that cannot be obtained either through in vivo studies in humans or through experimental animal models. Studies examining neuronal morphology, protein expression and localization, and transcript levels indicate that a microcircuit composed of excitatory pyramidal cells and inhibitory interneurons containing the calcium-binding protein parvalbumin is altered in the DLPFC of subjects with schizophrenia and likely contributes to DLPFC dysfunction.

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

Fragile X syndrome (FXS) is the most common inherited cause of intellectual disabilities and a leading cause of autism. FXS is caused by a trinucleotide expansion in the gene FMR1 on the X chromosome. The neuroanatomical hallmark of FXS is an overabundance of immature dendritic spines, a factor thought to underlie synaptic dysfunction and impaired cognition. We showed that aberrantly increased activity of the Rho GTPase Rac1 inhibited the actin-depolymerizing factor cofilin, a major determinant of dendritic spine structure, and caused disease-associated spine abnormalities in the somatosensory cortex of FXS model mice. Increased cofilin phosphorylation and actin polymerization coincided with abnormal dendritic spines and impaired synaptic maturation. Viral delivery of a constitutively active cofilin mutant (cofilinS3A) into the somatosensory cortex of Fmr1-deficient mice rescued the immature dendritic spine phenotype and increased spine density. Inhibition of the Rac1 effector PAK1 with a small-molecule inhibitor rescued cofilin signaling in FXS mice, indicating a causal relationship between PAK1 and cofilin signaling. PAK1 inhibition rescued synaptic signaling (specifically the synaptic ratio of NMDA/AMPA in layer V pyramidal neurons) and improved sensory processing in FXS mice. These findings suggest a causal relationship between increased Rac1-cofilin signaling, synaptic defects, and impaired sensory processing in FXS and uncover a previously unappreciated role for impaired Rac1-cofilin signaling in the aberrant spine morphology and spine density associated with FXS.

PTEN: Local and Global Modulation of Neuronal Function in Health and Disease

Phosphatase and tensin homolog deleted on chromosome ten (PTEN) was recently revealed to be a synaptic player during plasticity events in addition to its well-established role as a general controlling factor in cell proliferation and neuronal growth during development. Alterations of these direct actions of PTEN at synapses may lead to synaptic dysfunction with behavioral and cognitive consequences. A recent paradigmatic example of this situation, Alzheimer's disease (AD), is associated with excessive recruitment of PTEN into synapses leading to pathological synaptic depression. By contrast, some forms of autism are characterized by failure to weaken synaptic connections, which may be related to insufficient PTEN signaling. Understanding the modulation of synaptic function by PTEN in these pathologies may contribute to the development of new therapies.

A Subset of Autism-Associated Genes Regulate the Structural Stability of Neurons

Autism spectrum disorder (ASD) comprises a range of neurological conditions that affect individuals’ ability to communicate and interact with others. People with ASD often exhibit marked qualitative difficulties in social interaction, communication, and behavior. Alterations in neurite arborization and dendritic spine morphology, including size, shape, and number, are hallmarks of almost all neurological conditions, including ASD. As experimental evidence emerges in recent years, it becomes clear that although there is broad heterogeneity of identified autism risk genes, many of them converge into similar cellular pathways, including those regulating neurite outgrowth, synapse formation and spine stability, and synaptic plasticity. These mechanisms together regulate the structural stability of neurons and are vulnerable targets in ASD. In this review, we discuss the current understanding of those autism risk genes that affect the structural connectivity of neurons. We sub-categorize them into (1) cytoskeletal regulators, e.g., motors and small RhoGTPase regulators; (2) adhesion molecules, e.g., cadherins, NCAM, and neurexin superfamily; (3) cell surface receptors, e.g., glutamatergic receptors and receptor tyrosine kinases; (4) signaling molecules, e.g., protein kinases and phosphatases; and (5) synaptic proteins, e.g., vesicle and scaffolding proteins. Although the roles of some of these genes in maintaining neuronal structural stability are well studied, how mutations contribute to the autism phenotype is still largely unknown. Investigating whether and how the neuronal structure and function are affected when these genes are mutated will provide insights toward developing effective interventions aimed at improving the lives of people with autism and their families.

Co-regulation of mRNA translation by TDP-43 and Fragile X Syndrome protein FMRP

For proper mammalian brain development and functioning, the translation of many neuronal mRNAs needs to be repressed without neuronal activity stimulations. We have discovered that the expression of a subclass of neuronal proteins essential for neurodevelopment and neuron plasticity is co-regulated at the translational level by TDP-43 and the Fragile X Syndrome protein FMRP. Using molecular, cellular and imaging approaches, we show that these two RNA-binding proteins (RBP) co-repress the translation initiation of Rac1, Map1b and GluR1 mRNAs, and consequently the hippocampal spinogenesis. The co-repression occurs through binding of TDP-43 to mRNA(s) at specific UG/GU sequences and recruitment of the inhibitory CYFIP1-FMRP complex by its glycine-rich domain. This novel regulatory scenario could be utilized to silence a significant portion of around 160 common target mRNAs of the two RBPs. The study establishes a functional/physical partnership between FMRP and TDP-43 that mechanistically links several neurodevelopmental disorders and neurodegenerative diseases.

Statins and the brain: New perspective for old drugs

Statins are one of the most popular lipid-lowering drugs (LLDs). Upon oral administration, these drugs are well absorbed by the intestine and effectively used for the treatment of dyslipidemias. Recently, statins are becoming also well-known for their cholesterol-independent effects and their potential use in brain diseases and different types of cancers. While still controversial, recent research has suggested that statin's cholesterol-independent activities work possibly through alterations on isoprenoid levels. This reduction of isoprenoids in the central nervous system might result in effective biochemical and behavioral improvements on certain neurological disorders. This manuscript aims to highlight current research describing the use of statin therapy in the brain and discuss whether statins might affect neuronal dynamics and function independently of their cholesterol regulatory role.

Enlarged dendritic spines and pronounced neophobia in mice lacking the PSD protein RICH2

Background

The majority of neurons within the central nervous system receive their excitatory inputs via small, actin-rich protrusions called dendritic spines. Spines can undergo rapid morphological alterations according to synaptic activity. This mechanism is implicated in learning and memory formation as it is ultimately altering the number and distribution of receptors and proteins at the post-synaptic membrane, thereby regulating synaptic input. The Rho-family GTPases play an important role in regulating this spine plasticity by the interaction with cytoskeletal components and several signaling pathways within the spine compartment. Rho-GAP interacting CIP4 homologue2/RICH2 is a Rho-GAP protein regulating small GTPases and was identified as an interaction partner of the scaffolding protein SHANK3 at post-synaptic densities.

Results

Here, we characterize the loss of RICH2 in a novel mouse model. Our results show that RICH2 KO animals display a selective and highly significant fear of novel objects and increased stereotypic behavior as well as impairment of motor learning. We found an increase in multiple spine synapses in the hippocampus and cerebellum along with alterations in receptor composition and actin polymerization. Furthermore, we observed that the loss of RICH2 leads to a disinhibition of synaptic RAC1 in vivo.

Conclusions

The results are in line with the reported role of RAC1 activity being essential for activity-dependent spine enlargement. Since SHANK3 mutations are known to be causative for neuropsychiatric diseases of the Autism Spectrum (ASD), a disintegrated SHANK3/RICH2 complex at synaptic sites might at least in part be responsible for abnormal spine formation and plasticity in ASDs.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-016-0206-6) contains supplementary material, which is available to authorized users.

Activity-dependent FMRP requirements in development of the neural circuitry of learning and memory

The activity-dependent refinement of neural circuit connectivity during critical periods of brain development is essential for optimized behavioral performance. We hypothesize that this mechanism is defective in fragile X syndrome (FXS), the leading heritable cause of intellectual disability and autism spectrum disorders. Here, we use optogenetic tools in the Drosophila FXS disease model to test activity-dependent dendritogenesis in two extrinsic neurons of the mushroom body (MB) learning and memory brain center: (1) the input projection neuron (PN) innervating Kenyon cells (KCs) in the MB calyx microglomeruli and (2) the output MVP2 neuron innervated by KCs in the MB peduncle. Both input and output neuron classes exhibit distinctive activity-dependent critical period dendritic remodeling. MVP2 arbors expand in Drosophila mutants null for fragile X mental retardation 1 (dfmr1), as well as following channelrhodopsin-driven depolarization during critical period development, but are reduced by halorhodopsin-driven hyperpolarization. Optogenetic manipulation of PNs causes the opposite outcome--reduced dendritic arbors following channelrhodopsin depolarization and expanded arbors following halorhodopsin hyperpolarization during development. Importantly, activity-dependent dendritogenesis in both neuron classes absolutely requires dfmr1 during one developmental window. These results show that dfmr1 acts in a neuron type-specific activity-dependent manner for sculpting dendritic arbors during early-use, critical period development of learning and memory circuitry in the Drosophila brain.