Fragile X mental retardation protein is required for synapse elimination by the activity-dependent transcription factor MEF2

Pre- and Postsynaptic MEF2C Promotes Experience-Dependent, Input-Specific Development of Cortical Layer 4 to Layer 2/3 Excitatory Synapses and Regulates Activity-Dependent Expression of Synaptic Cell Adhesion Molecules

Experience- and activity-dependent transcription is a candidate mechanism to mediate development and refinement of specific cortical circuits. Here, we demonstrate that the activity-dependent transcription factor myocyte enhancer factor 2C (MEF2C) is required in both presynaptic layer (L) 4 and postsynaptic L2/3 mouse (male and female) somatosensory (S1) cortical neurons for development of this specific synaptic connection. While postsynaptic deletion of Mef2c weakens L4 synaptic inputs, it has no effect on inputs from local L2/3, contralateral S1, or the ipsilateral frontal/motor cortex. Similarly, homozygous or heterozygous deletion of Mef2c in presynaptic L4 neurons weakens L4 to L2/3 excitatory synaptic inputs by decreasing presynaptic release probability. Postsynaptic MEF2C is specifically required during an early postnatal, experience-dependent, period for L4 to L2/3 synapse function, and expression of transcriptionally active MEF2C (MEF2C-VP16) rescues weak L4 to L2/3 synaptic strength in sensory-deprived mice. Together, these results suggest that experience- and/or activity-dependent transcriptional activation of MEF2C promotes development of L4 to L2/3 synapses. Additionally, MEF2C regulates the expression of many pre- and postsynaptic genes in postnatal cortical neurons. Interestingly, MEF2C was necessary for activity-dependent expression of many presynaptic genes, including those that function in transsynaptic adhesion and neurotransmitter release. This work provides mechanistic insight into the experience-dependent development of specific cortical circuits.

Neural Circuit Remodeling: Mechanistic Insights from Invertebrates

As nervous systems mature, neural circuit connections are reorganized to optimize the performance of specific functions in adults. This reorganization of connections is achieved through a remarkably conserved phase of developmental circuit remodeling that engages neuron-intrinsic and neuron-extrinsic molecular mechanisms to establish mature circuitry. Abnormalities in circuit remodeling and maturation are broadly linked with a variety of neurodevelopmental disorders, including autism spectrum disorders and schizophrenia. Here, we aim to provide an overview of recent advances in our understanding of the molecular processes that govern neural circuit remodeling and maturation. In particular, we focus on intriguing mechanistic insights gained from invertebrate systems, such as the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster. We discuss how transcriptional control mechanisms, synaptic activity, and glial engulfment shape specific aspects of circuit remodeling in worms and flies. Finally, we highlight mechanistic parallels across invertebrate and mammalian systems, and prospects for further advances in each.

Key Synaptic Pathology in Autism Spectrum Disorder: Genetic Mechanisms and Recent Advances

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impaired social interactions and verbal communication, accompanied by symptoms of restricted and repetitive patterns of behavior or interest. Over the past 30 years, the morbidity of ASD has increased in most areas of the world. Although the pathogenesis of ASD is not fully understood, it has been associated with over 1000 genes or genomic loci, indicating the importance and complexity of the genetic mechanisms involved. This review focuses on the synaptic pathology of ASD and particularly on genetic variants involved in synaptic structure and functions. These include SHANK, NLGN, NRXN, FMR1, and MECP2 as well as other potentially novel genes such as CHD8, CHD2, and SYNGAP1 that could be core elements in ASD pathogenesis. Here, we summarize several pathological pathways supporting the hypothesis that synaptic pathology caused by genetic mutations may be the pathogenic basis for ASD.

Synaptic cell adhesion molecules contribute to the pathogenesis and progression of fragile X syndrome

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability and a monogenic cause of autism spectrum disorders. Deficiencies in the fragile X messenger ribonucleoprotein, encoded by the FMR1 gene, lead to various anatomical and pathophysiological abnormalities and behavioral deficits, such as spine dysmorphogenesis and learning and memory impairments. Synaptic cell adhesion molecules (CAMs) play crucial roles in synapse formation and neural signal transmission by promoting the formation of new synaptic contacts, accurately organizing presynaptic and postsynaptic protein complexes, and ensuring the accuracy of signal transmission. Recent studies have implicated synaptic CAMs such as the immunoglobulin superfamily, N-cadherin, leucine-rich repeat proteins, and neuroligin-1 in the pathogenesis of FXS and found that they contribute to defects in dendritic spines and synaptic plasticity in FXS animal models. This review systematically summarizes the biological associations between nine representative synaptic CAMs and FMRP, as well as the functional consequences of the interaction, to provide new insights into the mechanisms of abnormal synaptic development in FXS.

RhoGEF Tiam2 Regulates Glutamatergic Synaptic Transmission in Hippocampal CA1 Pyramidal Neurons

Glutamatergic synapses exhibit significant molecular diversity, but circuit-specific mechanisms that underlie synaptic regulation are not well characterized. Prior reports show that Rho-guanine nucleotide exchange factor (RhoGEF) Tiam1 regulates perforant path→dentate gyrus granule neuron synapses. In the present study, we report Tiam1's homolog Tiam2 is implicated in glutamatergic neurotransmission in CA1 pyramidal neurons. We find that Tiam2 regulates evoked excitatory glutamatergic currents via a postsynaptic mechanism mediated by the catalytic Dbl-homology domain. Overall, we present evidence for RhoGEF Tiam2's role in glutamatergic synapse function at Schaffer collateral→CA1 pyramidal neuron synapses.

Current Update on Categorization of Migraine Subtypes on the Basis of Genetic Variation: a Systematic Review

Migraine is a complex neurovascular disorder that is characterized by severe behavioral, sensory, visual, and/or auditory symptoms. It has been labeled as one of the ten most disabling medical illnesses in the world by the World Health Organization (Aagaard et al Sci Transl Med 6(237):237ra65, 2014). According to a recent report by the American Migraine Foundation (Shoulson et al Ann Neurol 25(3):252-9, 1989), around 148 million people in the world currently suffer from migraine. On the basis of presence of aura, migraine is classified into two major subtypes: migraine with aura (Aagaard et al Sci Transl Med 6(237):237ra65, 2014) and migraine without aura. (Aagaard K et al Sci Transl Med 6(237):237ra65, 2014) Many complex genetic mechanisms have been proposed in the pathophysiology of migraine but specific pathways associated with the different subtypes of migraine have not yet been explored. Various approaches including candidate gene association studies (CGAS) and genome-wide association studies (Fan et al Headache: J Head Face Pain 54(4):709-715, 2014). have identified the genetic markers associated with migraine and its subtypes. Several single nucleotide polymorphisms (Kaur et al Egyp J Neurol, Psychiatry Neurosurg 55(1):1-7, 2019) within genes involved in ion homeostasis, solute transport, synaptic transmission, cortical excitability, and vascular function have been associated with the disorder. Currently, the diagnosis of migraine is majorly behavioral with no focus on the genetic markers and thereby the therapeutic intervention specific to subtypes. Therefore, there is a need to explore genetic variants significantly associated with MA and MO as susceptibility markers in the diagnosis and targets for therapeutic interventions in the specific subtypes of migraine. Although the proper characterization of pathways based on different subtypes is yet to be studied, this review aims to make a first attempt to compile the information available on various genetic variants and the molecular mechanisms involved with the development of MA and MO. An attempt has also been made to suggest novel candidate genes based on their function to be explored by future research.

EF1α-associated protein complexes affect dendritic spine plasticity by regulating microglial phagocytosis in Fmr1 knock-out mice

Fragile X syndrome (FXS) is the most common inherited cause of intellectual disability. There is no specific treatment for FXS due to the lack of therapeutic targets. We report here that Elongation Factor 1α (EF1α) forms a complex with two other proteins: Tripartite motif-containing protein 3 (TRIM3) and Murine double minute (Mdm2). Both EF1α-Mdm2 and EF1α-TRIM3 protein complexes are increased in the brain of Fmr1 knockout mice as a result of FMRP deficiency, which releases the normal translational suppression of EF1α mRNA and increases EF1α protein levels. Increased EF1α-Mdm2 complex decreases PSD-95 ubiquitination (Ub-PSD-95) and Ub-PSD-95-C1q interaction. The elevated level of TRIM3-EF1α complex is associated with decreased TRIM3-Complement Component 3 (C3) complex that inhibits the activation of C3. Both protein complexes thereby contribute to a reduction in microglia-mediated phagocytosis and dendritic spine pruning. Finally, we created a peptide that disrupts both protein complexes and restores dendritic spine plasticity and behavioural deficits in Fmr1 knockout mice. The EF1α-Mdm2 and EF1α-TRIM3 complexes could thus be new therapeutic targets for FXS.

The Mef2c Gene Dose-Dependently Controls Hippocampal Neurogenesis and the Expression of Autism-Like Behaviors

Mutations in the activity-dependent transcription factor MEF2C have been associated with several neuropsychiatric disorders. Among these, autism spectrum disorder (ASD)-related behavioral deficits are manifested. Multiple animal models that harbor mutations in Mef2c have provided compelling evidence that Mef2c is indeed an ASD gene. However, studies in mice with germline or global brain knock-out of Mef2c are limited in their ability to identify the precise neural substrates and cell types that are required for the expression of Mef2c-mediated ASD behaviors. Given the role of hippocampal neurogenesis in cognitive and social behaviors, in this study we aimed to investigate the role of Mef2c in the structure and function of newly generated dentate granule cells (DGCs) in the postnatal hippocampus and to determine whether disrupted Mef2c function is responsible for manifesting ASD behaviors. Overexpression of Mef2c (Mef2cOE ) arrested the transition of neurogenesis at progenitor stages, as indicated by sustained expression of Sox2+ in Mef2cOE DGCs. Conditional knock-out of Mef2c (Mef2ccko ) allowed neuronal commitment of Mef2ccko cells; however, Mef2ccko impaired not only dendritic arborization and spine formation but also synaptic transmission onto Mef2ccko DGCs. Moreover, the abnormal structure and function of Mef2ccko DGCs led to deficits in social interaction and social novelty recognition, which are key characteristics of ASD behaviors. Thus, our study revealed a dose-dependent requirement of Mef2c in the control of distinct steps of neurogenesis, as well as a critical cell-autonomous function of Mef2c in newborn DGCs in the expression of proper social behavior in both sexes.

Synaptic and circuit functions of multitransmitter neurons in the mammalian brain

Neurons in the mammalian brain are not limited to releasing a single neurotransmitter but often release multiple neurotransmitters onto postsynaptic cells. Here, we review recent findings of multitransmitter neurons found throughout the mammalian central nervous system. We highlight recent technological innovations that have made the identification of new multitransmitter neurons and the study of their synaptic properties possible. We also focus on mechanisms and molecular constituents required for neurotransmitter corelease at the axon terminal and synaptic vesicle, as well as some possible functions of multitransmitter neurons in diverse brain circuits. We expect that these approaches will lead to new insights into the mechanism and function of multitransmitter neurons, their role in circuits, and their contribution to normal and pathological brain function.

Directly reprogrammed fragile X syndrome dorsal forebrain precursor cells generate cortical neurons exhibiting impaired neuronal maturation

Introduction

The neurodevelopmental disorder fragile X syndrome (FXS) is the most common monogenic cause of intellectual disability associated with autism spectrum disorder. Inaccessibility to developing human brain cells is a major barrier to studying FXS. Direct-to-neural precursor reprogramming provides a unique platform to investigate the developmental profile of FXS-associated phenotypes throughout neural precursor and neuron generation, at a temporal resolution not afforded by post-mortem tissue and in a patient-specific context not represented in rodent models. Direct reprogramming also circumvents the protracted culture times and low efficiency of current induced pluripotent stem cell strategies.

Methods

We have developed a chemically modified mRNA (cmRNA) -based direct reprogramming protocol to generate dorsal forebrain precursors (hiDFPs) from FXS patient-derived fibroblasts, with subsequent differentiation to glutamatergic cortical neurons and astrocytes.

Results

We observed differential expression of mature neuronal markers suggesting impaired neuronal development and maturation in FXS- hiDFP-derived neurons compared to controls. FXS- hiDFP-derived cortical neurons exhibited dendritic growth and arborization deficits characterized by reduced neurite length and branching consistent with impaired neuronal maturation. Furthermore, FXS- hiDFP-derived neurons exhibited a significant decrease in the density of pre- and post- synaptic proteins and reduced glutamate-induced calcium activity, suggesting impaired excitatory synapse development and functional maturation. We also observed a reduced yield of FXS- hiDFP-derived neurons with a significant increase in FXS-affected astrocytes.

Discussion

This study represents the first reported derivation of FXS-affected cortical neurons following direct reprogramming of patient fibroblasts to dorsal forebrain precursors and subsequently neurons that recapitulate the key molecular hallmarks of FXS as it occurs in human tissue. We propose that direct to hiDFP reprogramming provides a unique platform for further study into the pathogenesis of FXS as well as the identification and screening of new drug targets for the treatment of FXS.

FMRP phosphorylation and interactions with Cdh1 regulate association with dendritic RNA granules and MEF2-triggered synapse elimination

Fragile X Messenger Ribonucleoprotein (FMRP) is necessary for experience-dependent, developmental synapse elimination and the loss of this process may underlie the excess dendritic spines and hyperconnectivity of cortical neurons in Fragile X Syndrome, a common inherited form of intellectual disability and autism. Little is known of the signaling pathways that regulate synapse elimination and if or how FMRP is regulated during this process. We have characterized a model of synapse elimination in CA1 neurons of organotypic hippocampal slice cultures that is induced by expression of the active transcription factor Myocyte Enhancer Factor 2 (MEF2) and relies on postsynaptic FMRP. MEF2-induced synapse elimination is deficient in Fmr1 KO CA1 neurons, and is rescued by acute (24 h), postsynaptic and cell autonomous reexpression of FMRP in CA1 neurons. FMRP is an RNA binding protein that suppresses mRNA translation. Derepression is induced by posttranslational mechanisms downstream of metabotropic glutamate receptor signaling. Dephosphorylation of FMRP at S499 triggers ubiquitination and degradation of FMRP which then relieves translation suppression and promotes synthesis of proteins encoded by target mRNAs. Whether this mechanism functions in synapse elimination is not known. Here we demonstrate that phosphorylation and dephosphorylation of FMRP at S499 are both necessary for synapse elimination as well as interaction of FMRP with its E3 ligase for FMRP, APC/Cdh1. Using a bimolecular ubiquitin-mediated fluorescence complementation (UbFC) assay, we demonstrate that MEF2 promotes ubiquitination of FMRP in CA1 neurons that relies on activity and interaction with APC/Cdh1. Our results suggest a model where MEF2 regulates posttranslational modifications of FMRP via APC/Cdh1 to regulate translation of proteins necessary for synapse elimination.

The gut-brain connection: Exploring the influence of the gut microbiota on neuroplasticity and neurodevelopmental disorders

Neuroplasticity refers to the ability of brain circuits to reorganize and change the properties of the network, resulting in alterations in brain function and behavior. It is traditionally believed that neuroplasticity is influenced by external stimuli, learning, and experience. Intriguingly, there is new evidence suggesting that endogenous signals from the body's periphery may play a role. The gut microbiota, a diverse community of microorganisms living in harmony with their host, may be able to influence plasticity through its modulation of the gut-brain axis. Interestingly, the maturation of the gut microbiota coincides with critical periods of neurodevelopment, during which neural circuits are highly plastic and potentially vulnerable. As such, dysbiosis (an imbalance in the gut microbiota composition) during early life may contribute to the disruption of normal developmental trajectories, leading to neurodevelopmental disorders. This review aims to examine the ways in which the gut microbiota can affect neuroplasticity. It will also discuss recent research linking gastrointestinal issues and bacterial dysbiosis to various neurodevelopmental disorders and their potential impact on neurological outcomes.

The RGG motif proteins: Interactions, functions, and regulations

Proteins with motifs rich in arginines and glycines were discovered decades ago and are functionally involved in a staggering range of essential processes in the cell. Versatile, specific, yet adaptable molecular interactions enabled by the unique combination of arginine and glycine, combined with multiplicity of molecular recognition conferred by repeated di-, tri-, and multiple peptide motifs, allow RGG motif proteins to interact with a broad range of proteins and nucleic acids. Furthermore, posttranslational modifications at the arginines in the motif extend the RGG protein's capacity for a fine-tuned regulation. In this review, we focus on the biochemical properties of the RGG motif, its molecular interactions with RNAs and proteins, and roles of the posttranslational modification in modulating their interactions. We discuss current knowledge of the RGG motif proteins involved in mRNA transport and translation, highlight our merging understanding of their molecular functions in translational regulation and summarize areas of research in the future critical in understanding this important family of proteins. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications Translation > Mechanisms.

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.

Upregulation of Cell Division Cycle 20 Expression Alters the Morphology of Neuronal Dendritic Spines in the Nucleus Accumbens by Promoting FMRP Ubiquitination

The nucleus accumbens (NAc) is the key area of the reward circuit, but its heterogeneity has been poorly studied. Using single-cell RNA sequencing, we revealed a subcluster of GABAergic neurons characterized by cell division cycle 20 (Cdc20) mRNA expression in the NAc of adult rats. We studied the coexpression of Cdc20 and Gad1 mRNA in the NAc neurons of adult rats and assessed Cdc20 protein expression in the NAc during rat development. Moreover, we microinjected AAV2/9-hSyn-Cdc20 with or without the dual-AAV system into the bilateral NAc for sparse labelling to observe changes in the synaptic morphology of mature neurons and assessed rat behaviours in open field and elevated plus maze tests. Furthermore, we performed the experiments with a Cdc20 inhibitor, Cdc20 overexpression AAV vector, and Cdc20 conditional knockout primary striatal neurons to understand the ubiquitination-dependent degradation of fragile X mental retardation protein (FMRP) in vitro and in vivo. We confirmed the mRNA expression of Cdc20 in the NAc GABAergic neurons of adult rats, and its protein level was decreased significantly 3 weeks post-birth. Upregulated Cdc20 expression in the bilateral NAc decreased the dendritic spine density in mature neurons and induced anxiety-like behaviour in rats. Cdc20-APC triggered FMRP degradation through K48-linked polyubiquitination in Neuro-2a cells and primary striatal neurons and downregulated FMRP expression in the NAc of adult rats. These data revealed that upregulation of Cdc20 in the bilateral NAc reduced dendritic spine density and led to anxiety-like behaviours, possibly by enhancing FMRP degradation via K48-linked polyubiquitination.

Amyloid beta induces Fmr1 ‐dependent translational suppression and hyposynchrony of neural activity via phosphorylation of eIF2α and eEF2

Alzheimer's disease (AD) is the most common cause of dementia, with the accumulation of amyloid beta peptide (Aβ) being one of the main causes of the disease. Fragile X mental retardation protein (FMRP), encoded by fragile X mental retardation 1 (Fmr1), is an RNA‐binding protein that represses translation of its bound mRNAs or exerts other indirect mechanisms that result in translational suppression. Because the accumulation of Aβ has been shown to cause translational suppression resulting from the elevated cellular stress response, in this study we asked whether and how Fmr1 is involved in Aβ‐induced translational regulation. Our data first showed that the application of synthetic Aβ peptide induces the expression of Fmr1 in cultured primary neurons. We followed by showing that Fmr1 is required for Aβ‐induced translational suppression, hyposynchrony of neuronal firing activity, and loss of excitatory synapses. Mechanistically, we revealed that Fmr1 functions to repress the expression of phosphatases including protein phosphatase 2A (PP2A) and protein phosphatase 1 (PP1), leading to elevated phosphorylation of eukaryotic initiation factor 2‐α (eIF2α) and eukaryotic elongation factor 2 (eEF2), and subsequent translational suppression. Finally, our data suggest that such translational suppression is critical to Aβ‐induced hyposynchrony of firing activity, but not the loss of synapses. Altogether, our study uncovers a novel mechanism by which Aβ triggers translational suppression and we reveal the participation of Fmr1 in altered neural plasticity associated with Aβ pathology. Our study may also provide information for a better understanding of Aβ‐induced cellular stress responses in AD.

Hyperexcitability and Homeostasis in Fragile X Syndrome

Fragile X Syndrome (FXS) is a leading inherited cause of autism and intellectual disability, resulting from a mutation in the FMR1 gene and subsequent loss of its protein product FMRP. Despite this simple genetic origin, FXS is a phenotypically complex disorder with a range of physical and neurocognitive disruptions. While numerous molecular and cellular pathways are affected by FMRP loss, there is growing evidence that circuit hyperexcitability may be a common convergence point that can account for many of the wide-ranging phenotypes seen in FXS. The mechanisms for hyperexcitability in FXS include alterations to excitatory synaptic function and connectivity, reduced inhibitory neuron activity, as well as changes to ion channel expression and conductance. However, understanding the impact of FMR1 mutation on circuit function is complicated by the inherent plasticity in neural circuits, which display an array of homeostatic mechanisms to maintain activity near set levels. FMRP is also an important regulator of activity-dependent plasticity in the brain, meaning that dysregulated plasticity can be both a cause and consequence of hyperexcitable networks in FXS. This makes it difficult to separate the direct effects of FMR1 mutation from the myriad and pleiotropic compensatory changes associated with it, both of which are likely to contribute to FXS pathophysiology. Here we will: (1) review evidence for hyperexcitability and homeostatic plasticity phenotypes in FXS models, focusing on similarities/differences across brain regions, cell-types, and developmental time points; (2) examine how excitability and plasticity disruptions interact with each other to ultimately contribute to circuit dysfunction in FXS; and (3) discuss how these synaptic and circuit deficits contribute to disease-relevant behavioral phenotypes like epilepsy and sensory hypersensitivity. Through this discussion of where the current field stands, we aim to introduce perspectives moving forward in FXS research.

Mechanisms governing activity-dependent synaptic pruning in the developing mammalian CNS

Almost 60 years have passed since the initial discovery by Hubel and Wiesel that changes in neuronal activity can elicit developmental rewiring of the central nervous system (CNS). Over this period, we have gained a more comprehensive picture of how both spontaneous neural activity and sensory experience-induced changes in neuronal activity guide CNS circuit development. Here we review activity-dependent synaptic pruning in the mammalian CNS, which we define as the removal of a subset of synapses, while others are maintained, in response to changes in neural activity in the developing nervous system. We discuss the mounting evidence that immune and cell-death molecules are important mechanistic links by which changes in neural activity guide the pruning of specific synapses, emphasizing the role of glial cells in this process. Finally, we discuss how these developmental pruning programmes may go awry in neurodevelopmental disorders of the human CNS, focusing on autism spectrum disorder and schizophrenia. Together, our aim is to give an overview of how the field of activity-dependent pruning research has evolved, led to exciting new questions and guided the identification of new, therapeutically relevant mechanisms that result in aberrant circuit development in neurodevelopmental disorders.

Experience-dependent weakening of callosal synaptic connections in the absence of postsynaptic FMRP

Reduced structural and functional interhemispheric connectivity correlates with the severity of Autism Spectrum Disorder (ASD) behaviors in humans. Little is known of how ASD-risk genes regulate callosal connectivity. Here, we show that Fmr1, whose loss-of-function leads to Fragile X Syndrome (FXS), cell autonomously promotes maturation of callosal excitatory synapses between somatosensory barrel cortices in mice. Postnatal, cell-autonomous deletion of Fmr1 in postsynaptic Layer (L) 2/3 or L5 neurons results in a selective weakening of AMPA receptor- (R), but not NMDA receptor-, mediated callosal synaptic function, indicative of immature synapses. Sensory deprivation by contralateral whisker trimming normalizes callosal input strength, suggesting that experience-driven activity of postsynaptic Fmr1 KO L2/3 neurons weakens callosal synapses. In contrast to callosal inputs, synapses originating from local L4 and L2/3 circuits are normal, revealing an input-specific role for postsynaptic Fmr1 in regulation of synaptic connectivity within local and callosal neocortical circuits. These results suggest direct cell autonomous and postnatal roles for FMRP in development of specific cortical circuits and suggest a synaptic basis for long-range functional underconnectivity observed in FXS patients.

Criss-crossing autism spectrum disorder and adult neurogenesis

Autism spectrum disorder (ASD) comprises a group of multifactorial neurodevelopmental disorders primarily characterized by deficits in social interaction and repetitive behavior. Although the onset is typically in early childhood, ASD poses a lifelong challenge for both patients and caretakers. Adult neurogenesis (AN) is the process by which new functional neurons are created from neural stem cells existing in the post-natal brain. The entire event is based on a sequence of cellular processes, such as proliferation, specification of cell fate, maturation, and ultimately, synaptic integration into the existing neural circuits. Hence, AN is implicated in structural and functional brain plasticity throughout life. Accumulating evidence shows that impaired AN may underlie some of the abnormal behavioral phenotypes seen in ASD. In this review, we approach the interconnections between the molecular pathways related to AN and ASD. We also discuss existing therapeutic approaches targeting such pathways both in preclinical and clinical studies. A deeper understanding of how ASD and AN reciprocally affect one another could reveal important converging pathways leading to the emergence of psychiatric disorders.

Temporal differences of neurodevelopment processes between species

The ontogeny programs are highly conserved across all vertebrates, although there are significant temporal variations in interspecies developmental processes. Changing the timing and rate of developmental processes could affect subsequent organogenesis profoundly and may also have been critical factors in evolutionary diversity. However, despite their potential importance, the cellular and molecular mechanisms that control interspecies differences in developmental timescale remain unclear. This review highlights recent advances in the experimental models to compare interspecies differences in neurodevelopmental processes, neurogenesis, and neuronal maturation and discusses the possible mechanisms that could generate species-specific timescales.

ER stress-induced modulation of neural activity and seizure susceptibility is impaired in a fragile X syndrome mouse model

Imbalanced neuronal excitability homeostasis is commonly observed in patients with fragile X syndrome (FXS) and the animal model of FXS, the Fmr1 KO. While alterations of neuronal intrinsic excitability and synaptic activity at the steady state in FXS have been suggested to contribute to such a deficit and ultimately the increased susceptibility to seizures in FXS, it remains largely unclear whether and how the homeostatic response of neuronal excitability following extrinsic challenges is disrupted in FXS. Our previous work has shown that the acute response following induction of endoplasmic reticulum (ER) stress can reduce neural activity and seizure susceptibility. Because many signaling pathways associated with ER stress response are mediated by Fmr1, we asked whether acute ER stress-induced reduction of neural activity and seizure susceptibility are altered in FXS. Our results first revealed that acute ER stress can trigger a protein synthesis-dependent prevention of neural network synchronization in vitro and a reduction of susceptibility to kainic acid-induced seizures in vivo in wild-type but not in Fmr1 KO mice. Mechanistically, we found that acute ER stress-induced activation of murine double minute-2 (Mdm2), ubiquitination of p53, and the subsequent transient protein synthesis are all impaired in Fmr1 KO neurons. Employing a p53 inhibitor, Pifithrin-α, to mimic p53 inactivation, we were able to blunt the increase in neural network synchronization and reduce the seizure susceptibility in Fmr1 KO mice following ER stress induction. In summary, our data revealed a novel cellular defect in Fmr1 KO mice and suggest that an impaired response to common extrinsic challenges may contribute to imbalanced neuronal excitability homeostasis in FXS.

An Autism-Associated Neuroligin-3 Mutation Affects Developmental Synapse Elimination in the Cerebellum

Neuroligin is a postsynaptic cell-adhesion molecule that is involved in synapse formation and maturation by interacting with presynaptic neurexin. Mutations in neuroligin genes, including the arginine to cystein substitution at the 451st amino acid residue (R451C) of neuroligin-3 (NLGN3), have been identified in patients with autism spectrum disorder (ASD). Functional magnetic resonance imaging and examination of post-mortem brain in ASD patients implicate alteration of cerebellar morphology and Purkinje cell (PC) loss. In the present study, we examined possible association between the R451C mutation in NLGN3 and synaptic development and function in the mouse cerebellum. In NLGN3-R451C mutant mice, the expression of NLGN3 protein in the cerebellum was reduced to about 10% of the level of wild-type mice. Elimination of redundant climbing fiber (CF) to PC synapses was impaired from postnatal day 10–15 (P10–15) in NLGN3-R451C mutant mice, but majority of PCs became mono-innervated as in wild-type mice after P16. In NLGN3-R451C mutant mice, selective strengthening of a single CF relative to the other CFs in each PC was impaired from P16, which persisted into juvenile stage. Furthermore, the inhibition to excitation (I/E) balance of synaptic inputs to PCs was elevated, and calcium transients in the soma induced by strong and weak CF inputs were reduced in NLGN3-R451C mutant mice. These results suggest that a single point mutation in NLGN3 significantly influences the synapse development and refinement in cerebellar circuitry, which might be related to the pathogenesis of ASD.

Involvement of myocyte enhancer factor 2c in the pathogenesis of autism spectrum disorder

Myocyte enhancer factor 2 (MEF2), a family of transcription factor of MADS (minichromosome maintenance 1, agamous, deficiens and serum response factor)-box family needed in the growth and differentiation of a variety of human cells, such as neural, immune, endothelial, and muscles. As per existing literature, MEF2 transcription factors have also been associated with synaptic plasticity, the developmental mechanisms governing memory and learning, and several neurologic conditions, like autism spectrum disorders (ASDs). Recent genomic findings have ascertained a link between MEF2 defects, particularly in the MEF2C isoform and the ASD. In this review, we summarized a concise overview of the general regulation, structure and functional roles of the MEF2C transcription factor. We further outlined the potential role of MEF2C as a risk factor for various neurodevelopmental disorders, such as ASD, MEF2C Haploinsufficiency Syndrome and Fragile X syndrome.

FMRP regulates the subcellular distribution of cortical dendritic spine density in a non-cell-autonomous manner

Fragile X syndrome (FXS) is the most common form of intellectual disability that arises from the dysfunction of a single gene-Fmr1. The main neuroanatomical correlate of FXS is elevated dendritic spine density on cortical pyramidal neurons, which has been modeled in Fmr1-/Y mice. However, the cell-autonomous contribution of Fmr1 on cortical dendritic spine density has not been assessed. Even less is known about the role of Fmr1 in heterozygous female mosaic mice, which are a putative model for human Fmr1 full mutation carriers (i.e., are heterozygous for the full Fmr1-silencing mutation). In this neuroanatomical study, spine density in cortical pyramidal neurons of Fmr1+/- and Fmr1-/Y mice was studied at multiple subcellular compartments, layers, and brain regions. Spine density in Fmr1+/- mice is higher than WT but lower than Fmr1-/Y. Not all subcellular compartments in layer V Fmr1+/- and Fmr1-/Y cortical pyramidal neurons are equally affected: the apical dendrite, a key subcellular compartment, is principally affected over basal dendrites. Within apical dendrites, spine density is differentially affected across branch orders. Finally, identification of FMRP-positive and FMRP-negative neurons within Fmr1+/- permitted the study of the cell-autonomous effect of Fmr1 on spine density. Surprisingly, layer V cortical pyramidal spine density between FMRP-positive and FMRP-negative neurons does not differ, suggesting that the regulation of the primary neuroanatomical defect of FXS-elevated spine density-is non-cell-autonomous.

Right Place at the Right Time: How Changes in Protocadherins Affect Synaptic Connections Contributing to the Etiology of Neurodevelopmental Disorders

During brain development, neurons need to form the correct connections with one another in order to give rise to a functional neuronal circuitry. Mistakes during this process, leading to the formation of improper neuronal connectivity, can result in a number of brain abnormalities and impairments collectively referred to as neurodevelopmental disorders. Cell adhesion molecules (CAMs), present on the cell surface, take part in the neurodevelopmental process regulating migration and recognition of specific cells to form functional neuronal assemblies. Among CAMs, the members of the protocadherin (PCDH) group stand out because they are involved in cell adhesion, neurite initiation and outgrowth, axon pathfinding and fasciculation, and synapse formation and stabilization. Given the critical role of these macromolecules in the major neurodevelopmental processes, it is not surprising that clinical and basic research in the past two decades has identified several PCDH genes as responsible for a large fraction of neurodevelopmental disorders. In the present article, we review these findings with a focus on the non-clustered PCDH sub-group, discussing the proteins implicated in the main neurodevelopmental disorders.

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.

Genes influenced by MEF2C contribute to neurodevelopmental disease via gene expression changes that affect multiple types of cortical excitatory neurons

Myocyte enhancer factor 2 C (MEF2C) is an important transcription factor during neurodevelopment. Mutation or deletion of MEF2C causes intellectual disability (ID), and common variants within MEF2C are associated with cognitive function and schizophrenia risk. We investigated if genes influenced by MEF2C during neurodevelopment are enriched for genes associated with neurodevelopmental phenotypes and if this can be leveraged to identify biological mechanisms and individual brain cell types affected. We used a set of 1055 genes that were differentially expressed in the adult mouse brain following early embryonic deletion of Mef2c in excitatory cortical neurons. Using genome-wide association studies data, we found these differentially expressed genes (DEGs) to be enriched for genes associated with schizophrenia, intelligence and educational attainment but not autism spectrum disorder (ASD). For this gene set, genes that overlap with target genes of the Fragile X mental retardation protein (FMRP) are a major driver of these enrichments. Using trios data, we found these DEGs to be enriched for genes containing de novo mutations reported in ASD and ID, but not schizophrenia. Using single-cell RNA sequencing data, we identified that a number of different excitatory glutamatergic neurons in the cortex were enriched for these DEGs including deep layer pyramidal cells and cells in the retrosplenial cortex, entorhinal cortex and subiculum, and these cell types are also enriched for FMRP target genes. The involvement of MEF2C and FMRP in synapse elimination suggests that disruption of this process in these cell types during neurodevelopment contributes to cognitive function and risk of neurodevelopmental disorders.

MEF2C Hypofunction in Neuronal and Neuroimmune Populations Produces MEF2C Haploinsufficiency Syndrome-like Behaviors in Mice

BACKGROUND

Microdeletions of the MEF2C gene are linked to a syndromic form of autism termed MEF2C haploinsufficiency syndrome (MCHS). MEF2C hypofunction in neurons is presumed to underlie most of the symptoms of MCHS. However, it is unclear in which cell populations MEF2C functions to regulate neurotypical development.

METHODS

Multiple biochemical, molecular, electrophysiological, behavioral, and transgenic mouse approaches were used to characterize MCHS-relevant synaptic, behavioral, and gene expression changes in mouse models of MCHS.

RESULTS

We showed that MCHS-associated missense mutations cluster in the conserved DNA binding domain and disrupt MEF2C DNA binding. DNA binding-deficient global Mef2c heterozygous mice (Mef2c-Het) displayed numerous MCHS-related behaviors, including autism-related behaviors, changes in cortical gene expression, and deficits in cortical excitatory synaptic transmission. We detected hundreds of dysregulated genes in Mef2c-Het cortex, including significant enrichments of autism risk and excitatory neuron genes. In addition, we observed an enrichment of upregulated microglial genes, but this was not due to neuroinflammation in the Mef2c-Het cortex. Importantly, conditional Mef2c heterozygosity in forebrain excitatory neurons reproduced a subset of the Mef2c-Het phenotypes, while conditional Mef2c heterozygosity in microglia reproduced social deficits and repetitive behavior.

CONCLUSIONS

Taken together, our findings show that mutations found in individuals with MCHS disrupt the DNA-binding function of MEF2C, and DNA binding-deficient Mef2c global heterozygous mice display numerous MCHS-related phenotypes, including excitatory neuron and microglia gene expression changes. Our findings suggest that MEF2C regulates typical brain development and function through multiple cell types, including excitatory neuronal and neuroimmune populations.

The Fragile X Mental Retardation Protein Regulates Striatal Medium Spiny Neuron Synapse Density and Dendritic Spine Morphology

The fragile X mental retardation protein (FMRP), an RNA-binding protein that mediates the transport, stability, and translation of hundreds of brain RNAs, is critically involved in regulating synaptic function. Loss of FMRP, as in fragile X syndrome (FXS), is a leading monogenic cause of autism and results in altered structural and functional synaptic plasticity, widely described in the hippocampus and cortex. Though FXS is associated with hyperactivity, impaired social interaction, and the development of repetitive or stereotyped behaviors, all of which are influenced by striatal activity, few studies have investigated the function of FMRP here. Utilizing a cortical-striatal co-culture model, we find that striatal medium spiny neurons (MSNs) lacking FMRP fail to make normal increases in PSD95 expression over a short time period and have significant deficits in dendritic spine density and colocalized synaptic puncta at the later measured time point compared to wildtype (WT) MSNs. Acute expression of wtFMRP plasmid in Fmr1 KO co-cultures results in contrasting outcomes for these measures on MSNs at the more mature time point, reducing spine density across multiple spine types but making no significant changes in colocalized puncta. FMRP’s KH2 and RGG RNA-binding domains are required for normal elimination of PSD95, and interruption of these domains slightly favors elimination of immature spine types. Further, KH2 is required for normal levels of colocalized puncta. Our data are largely consistent with a basal role for FMRP and its RNA-binding domains in striatal synapse stabilization on developing MSNs, and in light of previous findings, suggest distinct regional and/or cell type-specific roles for FMRP in regulating synapse structure.

Myocyte Enhancer Factor 2A (MEF2A) Defines Oxytocin-Induced Morphological Effects and Regulates Mitochondrial Function in Neurons

The neuropeptide oxytocin (OT) is a well-described modulator of socio-emotional traits, such as anxiety, stress, social behavior, and pair bonding. However, when dysregulated, it is associated with adverse psychiatric traits, such as various aspects of autism spectrum disorder (ASD). In this study, we identify the transcription factor myocyte enhancer factor 2A (MEF2A) as the common link between OT and cellular changes symptomatic for ASD, encompassing neuronal morphology, connectivity, and mitochondrial function. We provide evidence for MEF2A as the decisive factor defining the cellular response to OT: while OT induces neurite retraction in MEF2A expressing neurons, OT causes neurite outgrowth in absence of MEF2A. A CRISPR-Cas-mediated knockout of MEF2A and retransfection of an active version or permanently inactive mutant, respectively, validated our findings. We also identified the phosphatase calcineurin as the main upstream regulator of OT-induced MEF2A signaling. Further, MEF2A signaling dampens mitochondrial functioning in neurons, as MEF2A knockout cells show increased maximal cellular respiration, spare respiratory capacity, and total cellular ATP. In summary, we reveal a central role for OT-induced MEF2A activity as major regulator of cellular morphology as well as neuronal connectivity and mitochondrial functioning, with broad implications for a potential treatment of disorders based on morphological alterations or mitochondrial dysfunction.

Autophagy in Neurodevelopmental Disorders

Neurodevelopmental diseases are a class of neurodevelopmental disorders characterized by cognitive impairment and behavioral abnormalities and are mainly manifested as developmental disorders of the brain and nervous system. The pathological mechanism is not fully understood and may be related to hereditary or environmental factors. The elevation of autophagy during neural development suggests that autophagy may be involved in the process of neurodevelopment. This chapter focuses on the important functions of autophagy in all aspects of neurodevelopment and the role and mechanism of autophagy in neurodevelopmental disorders, especially in autism spectrum disorder.

Xenotransplanted Human Cortical Neurons Reveal Species-Specific Development and Functional Integration into Mouse Visual Circuits

How neural circuits develop in the human brain has remained almost impossible to study at the neuronal level. Here, we investigate human cortical neuron development, plasticity, and function using a mouse/human chimera model in which xenotransplanted human cortical pyramidal neurons integrate as single cells into the mouse cortex. Combined neuronal tracing, electrophysiology, and in vivo structural and functional imaging of the transplanted cells reveal a coordinated developmental roadmap recapitulating key milestones of human cortical neuron development. The human neurons display a prolonged developmental timeline, indicating the neuron-intrinsic retention of juvenile properties as an important component of human brain neoteny. Following maturation, human neurons in the visual cortex display tuned, decorrelated responses to visual stimuli, like mouse neurons, demonstrating their capacity for physiological synaptic integration in host cortical circuits. These findings provide new insights into human neuronal development and open novel avenues for the study of human neuronal function and disease.

Video Abstract

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Cell-intrinsic mechanisms of human neoteny in mouse-human chimeric cerebral cortex

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Human neurons show prolonged maturation and single-cell integration in mouse cortex

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Stable dendritic spines and long-term synaptic plasticity in xenotransplanted neurons

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Human neurons show decorrelated activity and tuned responses to visual stimuli

NEXMIF/KIDLIA Knock-out Mouse Demonstrates Autism-Like Behaviors, Memory Deficits, and Impairments in Synapse Formation and Function

Autism spectrum disorder (ASD) is a heterogeneous neurodevelopmental disability that demonstrates impaired social interactions, communication deficits, and restrictive and repetitive behaviors. ASD has a strong genetic basis and many ASD-associated genes have been discovered thus far. Our previous work has shown that loss of expression of the X-linked gene NEXMIF/KIDLIA is implicated in patients with autistic features and intellectual disability (ID). To further determine the causal role of the gene in the disorder, and to understand the cellular and molecular mechanisms underlying the pathology, we have generated a NEXMIF knock-out (KO) mouse. We find that male NEXMIF KO mice demonstrate reduced sociability and communication, elevated repetitive grooming behavior, and deficits in learning and memory. Loss of NEXMIF/KIDLIA expression results in a significant decrease in synapse density and synaptic protein expression. Consistently, male KO animals show aberrant synaptic function as measured by excitatory miniatures and postsynaptic currents in the hippocampus. These findings indicate that NEXMIF KO mice recapitulate the phenotypes of the human disorder. The NEXMIF KO mouse model will be a valuable tool for studying the complex mechanisms involved in ASD and for the development of novel therapeutics for this disorder.SIGNIFICANCE STATEMENT Autism spectrum disorder (ASD) is a heterogeneous neurodevelopmental disorder characterized by behavioral phenotypes. Based on our previous work, which indicated the loss of NEXMIF/KIDLIA was associated with ASD, we generated NEXMIF knock-out (KO) mice. The NEXMIF KO mice demonstrate autism-like behaviors including deficits in social interaction, increased repetitive self-grooming, and impairments in communication and in learning and memory. The KO neurons show reduced synapse density and a suppression in synaptic transmission, indicating a role for NEXMIF in regulating synapse development and function. The NEXMIF KO mouse faithfully recapitulates the human disorder, and thus serves as an animal model for future investigation of the NEXMIF-dependent neurodevelopmental disorders.

Neuronal Myocyte-Specific Enhancer Factor 2D (MEF2D) Is Required for Normal Circadian and Sleep Behavior in Mice

The transcription factor, myocyte enhancer factor-2 (MEF2), is required for normal circadian behavior in Drosophila; however, its role in the mammalian circadian system has not been established. Of the four mammalian Mef2 genes, Mef2d is highly expressed in the suprachiasmatic nucleus (SCN) of the hypothalamus, a region critical for coordinating peripheral circadian clocks. Using both conventional and brain-specific Mef2d KO (Mef2d ^-/-) mouse lines, we demonstrate that MEF2D is essential for maintaining the length of the circadian free-running period of locomotor activity and normal sleep patterns in male mice. Crossing Mef2d ^-/- with Per2::luc reporter mice, we show that these behavioral changes are achieved without altering the endogenous period of the master circadian oscillator in the SCN. Together, our data suggest that alterations in behavior in Mef2d ^-/- mice may be the result of an effect on SCN output, rather than an effect on timekeeping within the SCN itself. These findings add to the growing body of evidence that MEF2 proteins play important roles in the brain.SIGNIFICANCE STATEMENT These studies are the first to show a role for MEF2 proteins in the brain outside of the hippocampus, and our findings suggest that these proteins may play diverse roles in the CNS. It is important to continue to build on our understanding of the roles of proteins acting in the SCN because SCN dysfunction underlies jet lag in humans and influences the response to shift work schedules, which are now known as risk factors for the development of cancer. Our work on MEF2D could be the basis for opening new lines of research in the development and regulation of circadian rhythms.

Reward-Related Expectations Trigger Dendritic Spine Plasticity in the Mouse Ventrolateral Orbitofrontal Cortex

An essential aspect of goal-directed decision-making is selecting actions based on anticipated consequences, a process that involves the orbitofrontal cortex (OFC) and potentially, the plasticity of dendritic spines in this region. To investigate this possibility, we trained male and female mice to nose poke for food reinforcers, or we delivered the same number of food reinforcers non-contingently to separate mice. We then decreased the likelihood of reinforcement for trained mice, requiring them to modify action-outcome expectations. In a separate experiment, we blocked action-outcome updating via chemogenetic inactivation of the OFC. In both cases, successfully selecting actions based on their likely consequences was associated with fewer immature, thin-shaped dendritic spines and a greater proportion of mature, mushroom-shaped spines in the ventrolateral OFC. This pattern was distinct from spine loss associated with aging, and we identified no effects on hippocampal CA1 neurons. Given that the OFC is involved in prospective calculations of likely outcomes, even when they are not observable, constraining spinogenesis while preserving mature spines may be important for solidifying durable expectations. To investigate causal relationships, we inhibited the RNA-binding protein fragile X mental retardation protein (encoded by Fmr1), which constrains dendritic spine turnover. Ventrolateral OFC-selective Fmr1 knockdown recapitulated the behavioral effects of inducible OFC inactivation (and lesions; also shown here), impairing action-outcome conditioning, and caused dendritic spine excess. Our findings suggest that a proper balance of dendritic spine plasticity within the OFC is necessary for one's ability to select actions based on anticipated consequences.SIGNIFICANCE STATEMENT Navigating a changing environment requires associating actions with their likely outcomes and updating these associations when they change. Dendritic spine plasticity is likely involved, yet relationships are unconfirmed. Using behavioral, chemogenetic, and viral-mediated gene silencing strategies and high-resolution microscopy, we find that modifying action-outcome expectations is associated with fewer immature spines and a greater proportion of mature spines in the ventrolateral orbitofrontal cortex (OFC). Given that the OFC is involved in prospectively calculating the likely outcomes of one's behavior, even when they are not observable, constraining spinogenesis while preserving mature spines may be important for maintaining durable expectations.

Emerging roles for MEF2 in brain development and mental disorders

The MEF2 family of transcription factors regulate large programs of gene expression important for the development and maintenance of many tissues, including the brain. MEF2 proteins are regulated by neuronal synaptic activity, and they recruit several epigenetic enzymes to influence chromatin structure and gene expression during development and throughout adulthood. Here, we provide a brief review of the recent literature reporting important roles for MEF2 during early brain development and function, and we highlight emerging roles for MEF2 as a risk factor for multiple neurodevelopmental disorders and mental illnesses, such as autism, intellectual disability, and schizophrenia.

Antagonizing Increased miR-135a Levels at the Chronic Stage of Experimental TLE Reduces Spontaneous Recurrent Seizures

Mesial temporal lobe epilepsy (mTLE) is a chronic neurological disease characterized by recurrent seizures. The antiepileptic drugs currently available to treat mTLE are ineffective in one-third of patients and lack disease-modifying effects. miRNAs, a class of small noncoding RNAs which control gene expression at the post-transcriptional level, play a key role in the pathogenesis of mTLE and other epilepsies. Although manipulation of miRNAs at acute stages has been reported to reduce subsequent spontaneous seizures, it is uncertain whether targeting miRNAs at chronic stages of mTLE can also reduce seizures. Furthermore, the functional role and downstream targets of most epilepsy-associated miRNAs remain poorly understood. Here, we show that miR-135a is selectively upregulated within neurons in epileptic brain and report that targeting miR-135a in vivo using antagomirs after onset of spontaneous recurrent seizures can reduce seizure activity at the chronic stage of experimental mTLE in male mice. Further, by using an unbiased approach combining immunoprecipitation and RNA sequencing, we identify several novel neuronal targets of miR-135a, including Mef2a Mef2 proteins are key regulators of excitatory synapse density. Mef2a and miR-135a show reciprocal expression regulation in human (of both sexes) and experimental TLE, and miR-135a regulates dendritic spine number and type through Mef2. Together, our data show that miR-135a is target for reducing seizure activity in chronic epilepsy, and that deregulation of miR-135a in epilepsy may alter Mef2a expression and thereby affect synaptic function and plasticity.SIGNIFICANCE STATEMENT miRNAs are post-transcriptional regulators of gene expression with roles in the pathogenesis of epilepsy. However, the precise mechanism of action and therapeutic potential of most epilepsy-associated miRNAs remain poorly understood. Our study reveals dramatic upregulation of the key neuronal miRNA miR-135a in both experimental and human mesial temporal lobe epilepsy. Silencing miR-135a in experimental temporal lobe epilepsy reduces seizure activity at the spontaneous recurrent seizure stage. These data support the exciting possibility that miRNAs can be targeted to combat seizures after spontaneous seizure activity has been established. Further, by using unbiased approaches novel neuronal targets of miR-135a, including members of the Mef2 protein family, are identified that begin to explain how deregulation of miR-135a may contribute to epilepsy.

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.

A Late Phase of Long-Term Synaptic Depression in Cerebellar Purkinje Cells Requires Activation of MEF2

The MEF2 family of transcription factors restricts excitatory synapse number in an activity-dependent fashion during development, yet MEF2 has not been implicated in long-term synaptic depression (LTD), which is thought to initiate synapse elimination. Mutations in MEF2 pathways are implicated in autism spectrum disorders, which include cerebellar dysfunction. Here, we test the hypothesis that cerebellar LTD requires postsynaptic activation of MEF2. Knockdown of MEF2D produces suppression of the transcription-dependent late phase of LTD in cultured Purkinje cells. The late phase of LTD is also completely blocked in Purkinje cells derived from MEF2A+MEF2D null mice and rescued with plasmids that drive expression of MEF2D but not phosphatase-resistant mutant MEF2D S444D. Wild-type Purkinje cells transfected with a constitutively active form of MEF2 show no alterations of synaptic strength. Thus, postsynaptic activation of MEF2 by S444 dephosphorylation is necessary, but not sufficient, for the late phase of cerebellar LTD.

Synaptopathology Involved in Autism Spectrum Disorder

Autism spectrum disorder (ASD) encompasses a group of multifactorial neurodevelopmental disorders characterized by impaired social communication, social interaction and repetitive behaviors. ASD affects 1 in 59 children, and is about 4 times more common among boys than among girls. Strong genetic components, together with environmental factors in the early stage of development, contribute to the pathogenesis of ASD. Multiple studies have revealed that mutations in genes like NRXN, NLGN, SHANK, TSC1/2, FMR1, and MECP2 converge on common cellular pathways that intersect at synapses. These genes encode cell adhesion molecules, scaffolding proteins and proteins involved in synaptic transcription, protein synthesis and degradation, affecting various aspects of synapses including synapse formation and elimination, synaptic transmission and plasticity. This suggests that the pathogenesis of ASD may, at least in part, be attributed to synaptic dysfunction. In this article, we will review major genes and signaling pathways implicated in synaptic abnormalities underlying ASD, and discuss molecular, cellular and functional studies of ASD experimental models.

Activation of autophagy rescues synaptic and cognitive deficits in fragile X mice

Fragile X syndrome (FXS) is the most frequent form of heritable intellectual disability and autism. Fragile X (Fmr1-KO) mice exhibit aberrant dendritic spine structure, synaptic plasticity, and cognition. Autophagy is a catabolic process of programmed degradation and recycling of proteins and cellular components via the lysosomal pathway. However, a role for autophagy in the pathophysiology of FXS is, as yet, unclear. Here we show that autophagic flux, a functional readout of autophagy, and biochemical markers of autophagy are down-regulated in hippocampal neurons of fragile X mice. We further show that enhanced activity of mammalian target of rapamycin complex 1 (mTORC1) and translocation of Raptor, a defining component of mTORC1, to the lysosome are causally related to reduced autophagy. Activation of autophagy by delivery of shRNA to Raptor directly into the CA1 of living mice via the lentivirus expression system largely corrects aberrant spine structure, synaptic plasticity, and cognition in fragile X mice. Postsynaptic density protein (PSD-95) and activity-regulated cytoskeletal-associated protein (Arc/Arg3.1), proteins implicated in spine structure and synaptic plasticity, respectively, are elevated in neurons lacking fragile X mental retardation protein. Activation of autophagy corrects PSD-95 and Arc abundance, identifying a potential mechanism by which impaired autophagy is causally related to the fragile X phenotype and revealing a previously unappreciated role for autophagy in the synaptic and cognitive deficits associated with fragile X syndrome.

Postsynaptic FMRP Regulates Synaptogenesis In Vivo in the Developing Cochlear Nucleus

A global loss of the fragile X mental retardation protein (FMRP; encoded by the Fmr1 gene) leads to sensory dysfunction and intellectual disabilities. One underlying mechanism of these phenotypes is structural and functional deficits in synapses. Here, we determined the autonomous function of postsynaptic FMRP in circuit formation, synaptogenesis, and synaptic maturation. In normal cochlea nucleus, presynaptic auditory axons form large axosomatic endbulb synapses on cell bodies of postsynaptic bushy neurons. In ovo electroporation of drug-inducible Fmr1-shRNA constructs produced a mosaicism of FMRP expression in chicken (either sex) bushy neurons, leading to reduced FMRP levels in transfected, but not neighboring nontransfected, neurons. Structural analyses revealed that postsynaptic FMRP reduction led to smaller size and abnormal morphology of individual presynaptic endbulbs at both early and later developmental stages. We further examined whether FMRP reduction affects dendritic development, as a potential mechanism underlying defective endbulb formation. Normally, chicken bushy neurons grow extensive dendrites at early stages and retract these dendrites when endbulbs begin to form. Neurons transfected with Fmr1 shRNA exhibited a remarkable delay in branch retraction, failing to provide necessary somatic surface for timely formation and growth of large endbulbs. Patch-clamp recording verified functional consequences of dendritic and synaptic deficits on neurotransmission, showing smaller amplitudes and slower kinetics of spontaneous and evoked EPSCs. Together, these data demonstrate that proper levels of postsynaptic FMRP are required for timely maturation of somatodendritic morphology, a delay of which may affect synaptogenesis and thus contribute to long-lasting deficits of excitatory synapses.SIGNIFICANCE STATEMENT Fragile X mental retardation protein (FMRP) regulates a large variety of neuronal activities. A global loss of FMRP affects neural circuit development and synaptic function, leading to fragile X syndrome (FXS). Using temporally and spatially controlled genetic manipulations, this study provides the first in vivo report that autonomous FMRP regulates multiple stages of dendritic development, and that selective reduction of postsynaptic FMRP leads to abnormal development of excitatory presynaptic terminals and compromised neurotransmission. These observations demonstrate secondary influence of developmentally transient deficits in neuronal morphology and connectivity to the development of long-lasting synaptic pathology in FXS.

The Cadherin Superfamily in Neural Circuit Assembly

The cadherin superfamily comprises a large, diverse collection of cell surface receptors that are expressed in the nervous system throughout development and have been shown to be essential for the proper assembly of the vertebrate nervous system. As our knowledge of each family member has grown, it has become increasingly clear that the functions of various cadherin subfamilies are intertwined: they can be present in the same protein complexes, impinge on the same developmental processes, and influence the same signaling pathways. This interconnectedness may illustrate a central way in which core developmental events are controlled to bring about the robust and precise assembly of neural circuitry.

Dendritic Spine Elimination: Molecular Mechanisms and Implications

Dynamic modification of synaptic connectivity in response to sensory experience is a vital step in the refinement of brain circuits as they are established during development and modified during learning. In addition to the well-established role for new spine growth and stabilization in the experience-dependent plasticity of neural circuits, dendritic spine elimination has been linked to improvements in learning, and dysregulation of spine elimination has been associated with intellectual disability and behavioral impairment. Proper brain function requires a tightly regulated balance between spine formation and spine elimination. Although most studies have focused on the mechanisms of spine formation, considerable progress has been made recently in delineating the neural activity patterns and downstream molecular mechanisms that drive dendritic spine elimination. Here, we review the current state of knowledge concerning the signaling pathways that drive dendritic spine shrinkage and elimination in the cerebral cortex and we discuss their implication in neuropsychiatric and neurodegenerative disease.

The transcription factor MEF2A plays a key role in the differentiation/maturation of rat neural stem cells into neurons

Neural stem cells (NSCs) are self-renewing multipotent stem cells that can be proliferated in vitro and differentiated into neuronal and/or glial lineages, making them an ideal model to study the processes involved in neuronal differentiation. Here we have used NSCs to investigate the role of the transcription factor MEF2A in neuronal differentiation and development in vitro. We show that although MEF2A is present in undifferentiated NSCs, following differentiation it is expressed at significantly higher levels in a subset of neuronal compared to non-neuronal cells. Furthermore, shRNA-mediated knockdown of MEF2A reduces the number of NSC-derived neurons compared to non-neuronal cells after differentiation. Together, these data indicate that MEF2A participates in neuronal differentiation/maturation from NSCs.

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Undifferentiated and differentiated neural stem cells (NSCs) express MEF2A in vitro.

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NSC-derived neurons express more MEF2A than NSC-derived glia.

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Ablating MEF2A reduces NSC differentiation into neurons.

MEF2A regulates mGluR-dependent AMPA receptor trafficking independently of Arc/Arg3.1

Differential trafficking of AMPA receptors (AMPARs) to and from the postsynaptic membrane is a key determinant of the strength of excitatory neurotransmission, and is thought to underlie learning and memory. The transcription factor MEF2 is a negative regulator of memory in vivo, in part by regulating trafficking of the AMPAR subunit GluA2, but the molecular mechanisms behind this have not been established. Here we show, via knockdown of endogenous MEF2A in primary neuronal culture, that MEF2A is specifically required for Group I metabotropic glutamate receptor (mGluR)-mediated GluA2 internalisation, but does not regulate AMPAR expression or trafficking under basal conditions. Furthermore, this process occurs independently of changes in expression of Arc/Arg3.1, a previously characterised MEF2 transcriptional target and mediator of mGluR-dependent long-term depression. These data demonstrate a novel MEF2A-dependent mechanism for the regulation of activity-dependent AMPAR trafficking.

Roles for Arc in metabotropic glutamate receptor-dependent LTD and synapse elimination: Implications in health and disease

The Arc gene is robustly transcribed in specific neural ensembles in response to experience-driven activity. Upon induction, Arc mRNA is transported to dendrites, where it can be rapidly and locally translated by activation of metabotropic glutamate receptors (mGluR1/5). mGluR-induced dendritic synthesis of Arc is implicated in weakening or elimination of excitatory synapses by triggering endocytosis of postsynaptic AMPARs in both hippocampal CA1 and cerebellar Purkinje neurons. Importantly, CA1 neurons with experience-induced Arc mRNA are susceptible, or primed for mGluR-induced long-term synaptic depression (mGluR-LTD). Here we review mechanisms and function of Arc in mGluR-LTD and synapse elimination and propose roles for these forms of plasticity in Arc-dependent formation of sparse neural representations of learned experience. We also discuss accumulating evidence linking dysregulation of Arc and mGluR-LTD in human cognitive disorders such as intellectual disability, autism and Alzheimer's disease.

FMRP-dependent Mdm2 dephosphorylation is required for MEF2-induced synapse elimination

The Myocyte Enhancer Factor 2 (MEF2) transcription factors suppress an excitatory synapse number by promoting degradation of the synaptic scaffold protein, postsynaptic density protein 95 (PSD-95), a process that is deficient in the mouse model of Fragile X Syndrome, Fmr1 KO. How MEF2 activation results in PSD-95 degradation and why this is defective in Fmr1 KO neurons is unknown. Here we report that MEF2 induces a Protein phosphatase 2A (PP2A)-mediated dephosphorylation of murine double minute-2 (Mdm2), the ubiquitin E3 ligase for PSD-95, which results in nuclear export and synaptic accumulation of Mdm2 as well as PSD-95 degradation and synapse elimination. In Fmr1 KO neurons, Mdm2 is hyperphosphorylated, nuclear localized basally, and unaffected by MEF2 activation, which our data suggest due to an enhanced interaction with Eukaryotic Elongation Factor 1α (EF1α), whose protein levels are elevated in Fmr1 KO. Expression of a dephosphomimetic of Mdm2 rescues PSD-95 ubiquitination, degradation and synapse elimination in Fmr1 KO neurons. This work reveals detailed mechanisms of synapse elimination in health and a developmental brain disorder.

Distinct stages of synapse elimination are induced by burst firing of CA1 neurons and differentially require MEF2A/D

Experience and activity refine cortical circuits through synapse elimination, but little is known about the activity patterns and downstream molecular mechanisms that mediate this process. We used optogenetics to drive individual mouse CA1 hippocampal neurons to fire in theta frequency bursts to understand how cell autonomous, postsynaptic activity leads to synapse elimination. Brief (1 hr) periods of postsynaptic bursting selectively depressed AMPA receptor (R) synaptic transmission, or silenced excitatory synapses, whereas more prolonged (24 hr) firing depressed both AMPAR and NMDAR EPSCs and eliminated spines, indicative of a synapse elimination. Both synapse silencing and elimination required de novo transcription, but only silencing required the activity-dependent transcription factors MEF2A/D. Burst firing induced MEF2A/D-dependent induction of the target gene Arc which contributed to synapse silencing and elimination. This work reveals new and distinct forms of activity and transcription-dependent synapse depression and suggests that these processes can occur independently.