Experience-Dependent and Differential Regulation of Local and Long-Range Excitatory Neocortical Circuits by Postsynaptic Mef2c

Developmental Disruption of Mef2c in Medial Ganglionic Eminence-Derived Cortical Inhibitory Interneurons Impairs Cellular and Circuit Function

BACKGROUND

MEF2C is strongly linked to various neurodevelopmental disorders including autism, intellectual disability, schizophrenia, and attention-deficit/hyperactivity disorder. Mice that constitutively lack 1 copy of Mef2c or selectively lack both copies of Mef2c in cortical excitatory neurons display a variety of behavioral phenotypes associated with neurodevelopmental disorders. The MEF2C protein is a transcription factor necessary for cellular development and synaptic modulation of excitatory neurons. MEF2C is also expressed in a subset of cortical GABAergic (gamma-aminobutyric acidergic) inhibitory neurons, but its function in those cell types remains largely unknown.

METHODS

Using conditional deletions of the Mef2c gene in mice, we investigated the role of MEF2C in parvalbumin-expressing interneurons (PV-INs), the largest subpopulation of cortical GABAergic cells, at 2 developmental time points. We performed slice electrophysiology, in vivo recordings, and behavior assays to test how embryonic and late postnatal loss of MEF2C from GABAergic INs impacts their survival and maturation and alters brain function and behavior.

RESULTS

Loss of MEF2C from PV-INs during embryonic, but not late postnatal, development resulted in reduced PV-IN number and failure of PV-INs to molecularly and synaptically mature. In association with these deficits, early loss of MEF2C in GABAergic INs led to abnormal cortical network activity, hyperactive and stereotypic behavior, and impaired cognitive and social behavior.

CONCLUSIONS

MEF2C expression is critical for the development of cortical GABAergic INs, particularly PV-INs. Embryonic loss of function of MEF2C mediates dysfunction of GABAergic INs, leading to altered in vivo patterns of cortical activity and behavioral phenotypes associated with neurodevelopmental disorders.

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.

The function of Mef2c toward the development of excitatory and inhibitory cortical neurons

Neurodevelopmental disorders (NDDs) are caused by abnormal brain development, leading to altered brain function and affecting cognition, learning, self-control, memory, and emotion. NDDs are often demarcated as discrete entities for diagnosis, but empirical evidence indicates that NDDs share a great deal of overlap, including genetics, core symptoms, and biomarkers. Many NDDs also share a primary sensitive period for disease, specifically the last trimester of pregnancy in humans, which corresponds to the neonatal period in mice. This period is notable for cortical circuit assembly, suggesting that deficits in the establishment of brain connectivity are likely a leading cause of brain dysfunction across different NDDs. Regulators of gene programs that underlie neurodevelopment represent a point of convergence for NDDs. Here, we review how the transcription factor MEF2C, a risk factor for various NDDs, impacts cortical development. Cortical activity requires a precise balance of various types of excitatory and inhibitory neuron types. We use MEF2C loss-of-function as a study case to illustrate how brain dysfunction and altered behavior may derive from the dysfunction of specific cortical circuits at specific developmental times.

Local and long-range input balance: A framework for investigating frontal cognitive circuit maturation in health and disease

Frontal cortical circuits undergo prolonged maturation across childhood and adolescence; however, it remains unknown what specific changes are occurring at the circuit level to establish adult cognitive function. With the recent advent of circuit dissection techniques, it is now feasible to examine circuit-specific changes in connectivity, activity, and function in animal models. Here, we propose that the balance of local and long-range inputs onto frontal cognitive circuits is an understudied metric of circuit maturation. This review highlights research on a frontal-sensory attention circuit that undergoes refinement of local/long-range connectivity, regulated by circuit activity and neuromodulatory signaling, and evaluates how this process may occur generally in the frontal cortex to support adult cognitive behavior. Notably, this balance can be bidirectionally disrupted through various mechanisms relevant to psychiatric disorders. Pharmacological or environmental interventions to normalize or reset the local and long-range balance could hold great therapeutic promise to prevent or rescue cognitive deficits.

MEF2C contributes to axonal branching by regulating Kif2c transcription

Neurons are post-mitotic cells, with microtubules playing crucial roles in axonal transport and growth. Kinesin family member 2c (KIF2C), a member of the Kinesin-13 family, possesses the ability to depolymerize microtubules and is involved in remodelling the microtubule lattice. Myocyte enhancer factor 2c (MEF2C) was initially identified as a regulator of muscle differentiation but has recently been associated with neurological abnormalities such as severe cognitive impairment, stereotyping, epilepsy and brain malformations when mutated or deleted. However, further investigation is required to determine which target genes MEF2C acts upon to influence neuronal function as a transcription regulator. Our data demonstrate that knockdown of both Mef2c and Kif2c significantly impacts spinal motor neuron development and behaviour in zebrafish. Luciferase reporter assays and chromosome immunoprecipitation assays, along with down/upregulated expression analysis, revealed that MFE2C functions as a novel transcription regulator for the Kif2c gene. Additionally, the knockdown of either Mef2c or Kif2c expression in E18 cortical neurons substantially reduces the number of primary neurites and axonal branches during neuronal development in vitro without affecting neurite length. Finally, depletion of Kif2c eliminated the effects of overexpression of Mef2c on the neurite branching. Based on these findings, we provided novel evidence demonstrating that MEF2C regulates the transcription of the Kif2c gene thereby influencing the axonal branching.

Early cortical GABAergic interneurons determine the projection patterns of L4 excitatory neurons

During cerebral cortex development, excitatory pyramidal neurons (PNs) establish specific projection patterns while receiving inputs from GABAergic inhibitory interneurons (INs). Whether these inhibitory inputs can shape PNs' projection patterns is, however, unknown. While layer 4 (L4) PNs of the primary somatosensory (S1) cortex are all born as long-range callosal projection neurons (CPNs), most of them acquire local connectivity upon activity-dependent elimination of their interhemispheric axons during postnatal development. Here, we demonstrate that precise developmental regulation of inhibition is key for the retraction of S1L4 PNs' callosal projections. Ablation of somatostatin INs leads to premature inhibition from parvalbumin INs onto S1L4 PNs and prevents them from acquiring their barrel-restricted local connectivity pattern. As a result, adult S1L4 PNs retain interhemispheric projections responding to tactile stimuli, and the mice lose whisker-based texture discrimination. Overall, we show that temporally ordered IN activity during development is key to shaping local ipsilateral S1L4 PNs' projection pattern, which is required for fine somatosensory processing.

FOXP1 regulates the development of excitatory synaptic inputs onto striatal neurons and induces phenotypic reversal with reinstatement

Long-range glutamatergic inputs originating from the cortex and thalamus are indispensable for striatal development, providing the foundation for motor and cognitive functions. Despite their significance, transcriptional regulation governing these inputs remains largely unknown. We investigated the role of a transcription factor encoded by a high-risk autism-associated gene, FOXP1, in sculpting glutamatergic inputs onto spiny projection neurons (SPNs) within the striatum. We find a neuron subtype-specific role of FOXP1 in strengthening and maturing glutamatergic inputs onto dopamine receptor 2-expressing SPNs (D2 SPNs). We also find that FOXP1 promotes synaptically driven excitability in these neurons. Using single-nuclei RNA sequencing, we identify candidate genes that mediate these cell-autonomous processes through postnatal FOXP1 function at the post-synapse. Last, we demonstrate that postnatal FOXP1 reinstatement rescues electrophysiological deficits, cell type-specific gene expression changes, and behavioral phenotypes. Together, this study enhances our understanding of striatal circuit development and provides proof of concept for a therapeutic approach for FOXP1 syndrome and other neurodevelopmental disorders.

Developmental disruption of Mef2c in Medial Ganglionic Eminence-derived cortical inhibitory interneurons impairs cellular and circuit function

MEF2C is strongly linked to various neurodevelopmental disorders (NDDs) including autism, intellectual disability, schizophrenia, and attention-deficit/hyperactivity. Mice constitutively lacking one copy of Mef2c , or selectively lacking both copies of Mef2c in cortical excitatory neurons, display a variety of behavioral phenotypes associated with NDDs. The MEF2C protein is a transcription factor necessary for cellular development and synaptic modulation of excitatory neurons. MEF2C is also expressed in a subset of cortical GABAergic inhibitory neurons, but its function in those cell types remains largely unknown. Using conditional deletions of the Mef2c gene in mice, we investigated the role of MEF2C in Parvalbumin-expressing Interneurons (PV-INs), the largest subpopulation of cortical GABAergic cells, at two developmental timepoints. We performed slice electrophysiology, in vivo recordings, and behavior assays to test how embryonic and late postnatal loss of MEF2C from GABAergic interneurons impacts their survival and maturation, and alters brain function and behavior. We found that loss of MEF2C from PV-INs during embryonic, but not late postnatal, development resulted in reduced PV-IN number and failure of PV-INs to molecularly and synaptically mature. In association with these deficits, early loss of MEF2C in GABAergic interneurons lead to abnormal cortical network activity, hyperactive and stereotypic behavior, and impaired cognitive and social behavior. Our findings indicate that MEF2C expression is critical for the development of cortical GABAergic interneurons, particularly PV-INs. Embryonic loss of function of MEF2C mediates dysfunction of GABAergic interneurons, leading to altered in vivo patterns of cortical activity and behavioral phenotypes associated with neurodevelopmental disorders.

Enhanced Spine Stability and Survival Lead to Increases in Dendritic Spine Density as an Early Response to Local Alpha-Synuclein Overexpression in Mouse Prefrontal Cortex

Lewy Body Dementias (LBD), including Parkinson's disease dementia and Dementia with Lewy Bodies, are characterized by widespread accumulation of intracellular alpha-Synuclein protein deposits in regions beyond the brainstem, including in the cortex. However, the impact of local pathology in the cortex is unknown. To investigate this, we employed viral overexpression of human alpha-Synuclein protein targeting the mouse prefrontal cortex (PFC). We then used in vivo 2-photon microscopy to image awake head-fixed mice via an implanted chronic cranial window to assess the early consequences of alpha-Synuclein overexpression in the weeks following overexpression. We imaged apical tufts of Layer V pyramidal neurons in the PFC of Thy1-YFP transgenic mice at 1-week intervals from 1 to 2 weeks before and 9 weeks following viral overexpression, allowing analysis of dynamic changes in dendritic spines. We found an increase in the relative dendritic spine density following local overexpression of alpha-Synuclein, beginning at 5 weeks post-injection, and persisting for the remainder of the study. We found that alpha-Synuclein overexpression led to an increased percentage and longevity of newly-persistent spines, without significant changes in the total density of newly formed or eliminated spines. A follow-up study utilizing confocal microscopy revealed that the increased spine density is found in cortical cells within the alpha-Synuclein injection site, but negative for alpha-Synuclein phosphorylation at Serine-129, highlighting the potential for effects of dose and local circuits on spine survival. These findings have important implications for the physiological role and early pathological stages of alpha-Synuclein in the cortex.

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.

FOXP1 regulates the development of excitatory synaptic inputs onto striatal neurons and induces phenotypic reversal with reinstatement

Long-range glutamatergic inputs from the cortex and thalamus are critical for motor and cognitive processing in the striatum. Transcription factors that orchestrate the development of these inputs are largely unknown. We investigated the role of a transcription factor and high-risk autism-associated gene, FOXP1, in the development of glutamatergic inputs onto spiny projection neurons (SPNs) in the striatum. We find that FOXP1 robustly drives the strengthening and maturation of glutamatergic input onto dopamine receptor 2-expressing SPNs (D2 SPNs) but has a comparatively milder effect on D1 SPNs. This process is cell-autonomous and is likely mediated through postnatal FOXP1 function at the postsynapse. We identified postsynaptic FOXP1-regulated transcripts as potential candidates for mediating these effects. Postnatal reinstatement of FOXP1 rescues electrophysiological deficits, reverses gene expression alterations resulting from embryonic deletion, and mitigates behavioral phenotypes. These results provide support for a possible therapeutic approach for individuals with FOXP1 syndrome.

Enhanced spine stability and survival lead to increases in dendritic spine density as an early response to local alpha-synuclein overexpression in mouse prefrontal cortex

Lewy Body Dementias (LBD), including Parkinson's disease dementia and Dementia with Lewy Bodies, are characterized by widespread accumulation of intracellular alpha-Synuclein protein deposits in regions beyond the brainstem, including in the cortex. Patients with LBDs develop cognitive changes, including abnormalities in executive function, attention, hallucinations, slowed processing, and cognitive fluctuations. The causes of these non-motor symptoms remain unclear; however, accumulation of alpha-Synuclein aggregates in the cortex and subsequent interference of synaptic and cellular function could contribute to psychiatric and cognitive symptoms. It is unknown how the cortex responds to local pathology in the absence of significant secondary effects of alpha-Synuclein pathology in the brainstem. To investigate this, we employed viral overexpression of human alpha-Synuclein protein targeting the mouse prefrontal cortex (PFC). We then used in vivo 2-photon microscopy to image awake head-fixed mice via an implanted chronic cranial window to assess the early consequences of alpha-Synuclein overexpression in the weeks following overexpression. We imaged apical tufts of Layer V pyramidal neurons in the PFC of Thy1-YFP transgenic mice at 1-week intervals from 1-2 weeks before and 9 weeks following viral overexpression, allowing analysis of dynamic changes in dendritic spines. We found an increase in the relative dendritic spine density following local overexpression of alpha-Synuclein, beginning at 5 weeks post-injection, and persisting for the remainder of the study. We found that alpha-Synuclein overexpression led to an increased percentage and longevity of newly-persistent spines, without significant changes in the total density of newly formed or eliminated spines. A follow up study utilizing confocal microscopy revealed that the increased spine density is found in cortical cells within the alpha-Synuclein injection site, but negative for alpha-Synuclein phosphorylation at Serine-129, highlighting the potential for effects of dose and local circuits on spine survival. These findings have important implications for the physiological role and early pathological stages of alpha-Synuclein in the cortex.

In the developing cerebral cortex: axonogenesis, synapse formation, and synaptic plasticity are regulated by SATB2 target genes

BACKGROUND

Special AT-rich sequence-binding protein 2 is essential for the development of cerebral cortex and key molecular node for the establishment of proper neural circuitry and function. Mutations in the SATB2 gene lead to SATB2-associated syndrome, which is characterized by abnormal development of skeleton and central nervous systems.

METHODS

We generated Satb2 knockout mouse model through CRISPR-Cas9 technology and performed RNA-seq and ChIP-seq of embryonic cerebral cortex. We conducted RT-qPCR, western blot, immunofluorescence staining, luciferase reporter assay and behavioral analysis for experimental verification.

RESULTS

We identified 1363 downstream effector genes of Satb2 and correlation analysis of Satb2-targeted genes and neurological disease genes showed that Satb2 contribute to cognitive and mental disorders from the early developmental stage. We found that Satb2 directly regulate the expression of Ntng1, Cdh13, Kitl, genes important for axon guidance, synaptic formation, neuron migration, and Satb2 directly activates the expression of Mef2c. We also showed that Satb2 heterozygous knockout mice showed impaired spatial learning and memory.

CONCLUSIONS

Taken together, our study supportsroles of Satb2 in the regulation of axonogenesis and synaptic formation at the early developmental stage and provides new insights into the complicated regulatory mechanism of Satb2 and new evidence to elucidate the pathogen of SATB2-associated syndrome.

IMPACT

1363 downstream effector genes of Satb2 were classified into 5 clusters with different temporal expression patterns. We identified Plxnd1, Ntng1, Efnb2, Ephb1, Plxna2, Epha3, Plxna4, Unc5c, and Flrt2 as axon guidance molecules to regulate axonogenesis. 168 targeted genes of Satb2 were found to regulate synaptic formation in the early development of the cerebral cortex. Transcription factor Mef2c is positively regulated by Satb2, and 28 Mef2c-targeted genes can be directly regulated by Satb2. In the Morris water maze test, Satb2+/- mice showed impaired spatial learning and memory, further strengthening that Satb2 can regulate synaptic functions.

The molecular signals that regulate activity-dependent synapse refinement in the brain

The formation of appropriate synaptic connections is critical for the proper functioning of the brain. Early in development, neurons form a surplus of immature synapses. To establish efficient, functional neural networks, neurons selectively stabilize active synapses and eliminate less active ones. This process is known as activity-dependent synapse refinement. Defects in this process have been implicated in neuropsychiatric disorders such as schizophrenia and autism. Here we review the manner and mechanisms by which synapse elimination is regulated through activity-dependent competition. We propose a theoretical framework for the molecular mechanisms of synapse refinement, in which three types of signals regulate the refinement. We then describe the identity of these signals and discuss how multiple molecular signals interact to achieve appropriate synapse refinement in the brain.

The Role of MEF2 Transcription Factor Family in Neuronal Survival and Degeneration

The family of myocyte enhancer factor 2 (MEF2) transcription factors comprises four highly conserved members that play an important role in the nervous system. They appear in precisely defined time frames in the developing brain to turn on and turn off genes affecting growth, pruning and survival of neurons. MEF2s are known to dictate neuronal development, synaptic plasticity and restrict the number of synapses in the hippocampus, thus affecting learning and memory formation. In primary neurons, negative regulation of MEF2 activity by external stimuli or stress conditions is known to induce apoptosis, albeit the pro or antiapoptotic action of MEF2 depends on the neuronal maturation stage. By contrast, enhancement of MEF2 transcriptional activity protects neurons from apoptotic death both in vitro and in preclinical models of neurodegenerative diseases. A growing body of evidence places this transcription factor in the center of many neuropathologies associated with age-dependent neuronal dysfunctions or gradual but irreversible neuron loss. In this work, we discuss how the altered function of MEF2s during development and in adulthood affecting neuronal survival may be linked to neuropsychiatric disorders.

Intelligent Somatosensory Interactive Activities Restore Motor Function to Children with Autism

So far, the biomedical community has not provided clear etiological conclusions and targeted drug treatments. Educational intervention and rehabilitation are the main ways to promote the development of autistic children's ability and change the quality of life. This research mainly explores how intelligent somatosensory interactive activities can restore motor function to children with autism. Case studies are used to investigate the current problems of hand movement training for children with autism and the effect of somatosensory games on rehabilitation training for autism. The experience of autistic children using somatosensory games for hand movement training was analyzed. Through the collection, sorting, and analysis of data, the influence of different factors on users' immersive experience is explored. It designs and implements the system's somatosensory game module, including a detailed introduction to the development platform and key technologies used in the development of the somatosensory game module, and shows the functions, program flow, main features, and implementation effects of the somatosensory game. The development process of the somatosensory interaction system is introduced in detail, including model making, character control, task flow control, collision detection, interactive interface, and natural interaction methods of gesture interaction and voice interaction. This study outlines the concepts related to autism and the characteristics of children with autism. It discusses the feasibility of applying somatosensory games to the hand movement training of children with autism and analyzes the development status and application of somatosensory games in detail to lay the foundation for follow-up research. Moreover, it defines the research content of somatosensory interactive training products and clarifies the design content and direction of the product. The comfort evaluation of the somatosensory game products designed in the study reached 92.9%. This research further proves that somatosensory games have a positive effect.

Progress on the roles of MEF2C in neuropsychiatric diseases

Myocyte Enhancer Factor 2 C (MEF2C), one of the transcription factors of the MADS-BOX family, is involved in embryonic brain development, neuronal formation and differentiation, as well as in the growth and pruning of axons and dendrites. MEF2C is also involved in the development of various neuropsychiatric disorders, such as autism spectrum disorders (ASD), epilepsy, schizophrenia and Alzheimer’s disease (AD). Here, we review the relationship between MEF2C and neuropsychiatric disorders, and provide further insights into the mechanism of these diseases.

MEF2 is a key regulator of cognitive potential and confers resilience to neurodegeneration

[Figure: see text].

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.

Unique properties of dually innervated dendritic spines in pyramidal neurons of the somatosensory cortex uncovered by 3D correlative light and electron microscopy

Pyramidal neurons (PNs) are covered by thousands of dendritic spines receiving excitatory synaptic inputs. The ultrastructure of dendritic spines shapes signal compartmentalization, but ultrastructural diversity is rarely taken into account in computational models of synaptic integration. Here, we developed a 3D correlative light-electron microscopy (3D-CLEM) approach allowing the analysis of specific populations of synapses in genetically defined neuronal types in intact brain circuits. We used it to reconstruct segments of basal dendrites of layer 2/3 PNs of adult mouse somatosensory cortex and quantify spine ultrastructural diversity. We found that 10% of spines were dually innervated and 38% of inhibitory synapses localized to spines. Using our morphometric data to constrain a model of synaptic signal compartmentalization, we assessed the impact of spinous versus dendritic shaft inhibition. Our results indicate that spinous inhibition is locally more efficient than shaft inhibition and that it can decouple voltage and calcium signaling, potentially impacting synaptic plasticity.

Development and plasticity of the corpus callosum

The corpus callosum (CC) connects the cerebral hemispheres and is the major mammalian commissural tract. It facilitates bilateral sensory integration and higher cognitive functions, and is often affected in neurodevelopmental diseases. Here, we review the mechanisms that contribute to the development of CC circuits in animal models and humans. These species comparisons reveal several commonalities. First, there is an early period of massive axonal projection. Second, there is a postnatal temporal window, varying between species, in which early callosal projections are selectively refined. Third, sensory-derived activity influences axonal refinement. We also discuss how defects in CC formation can lead to mild or severe CC congenital malformations.

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.

An essential role for MEF2C in the cortical response to loss of sleep in mice

Neuronal activity and gene expression in response to the loss of sleep can provide a window into the enigma of sleep function. Sleep loss is associated with brain differential gene expression, an increase in pyramidal cell mEPSC frequency and amplitude, and a characteristic rebound and resolution of slow wave sleep-slow wave activity (SWS-SWA). However, the molecular mechanism(s) mediating the sleep-loss response are not well understood. We show that sleep-loss regulates MEF2C phosphorylation, a key mechanism regulating MEF2C transcriptional activity, and that MEF2C function in postnatal excitatory forebrain neurons is required for the biological events in response to sleep loss in C57BL/6J mice. These include altered gene expression, the increase and recovery of synaptic strength, and the rebound and resolution of SWS-SWA, which implicate MEF2C as an essential regulator of sleep function.

Adolescent frontal top-down neurons receive heightened local drive to establish adult attentional behavior in mice

Frontal top-down cortical neurons projecting to sensory cortical regions are well-positioned to integrate long-range inputs with local circuitry in frontal cortex to implement top-down attentional control of sensory regions. How adolescence contributes to the maturation of top-down neurons and associated local/long-range input balance, and the establishment of attentional control is poorly understood. Here we combine projection-specific electrophysiological and rabies-mediated input mapping in mice to uncover adolescence as a developmental stage when frontal top-down neurons projecting from the anterior cingulate to visual cortex are highly functionally integrated into local excitatory circuitry and have heightened activity compared to adulthood. Chemogenetic suppression of top-down neuron activity selectively during adolescence, but not later periods, produces long-lasting visual attentional behavior deficits, and results in excessive loss of local excitatory inputs in adulthood. Our study reveals an adolescent sensitive period when top-down neurons integrate local circuits with long-range connectivity to produce attentional behavior.

Sex-specific impact of prenatal androgens on social brain default mode subsystems

Early-onset neurodevelopmental conditions (e.g., autism) affect males more frequently than females. Androgens may play a role in this male-bias by sex-differentially impacting early prenatal brain development, particularly neural circuits that later develop specialized roles in social cognition. Here, we find that increasing prenatal testosterone in humans is associated with later reduction of functional connectivity between social brain default mode (DMN) subsystems in adolescent males, but has no effect in females. Since testosterone can work directly via the androgen receptor (AR) or indirectly via the estrogen receptor through aromatase conversion to estradiol, we further examined how a potent non-aromatizable androgen, dihydrotestosterone (DHT), acts via the AR to influence gene expression in human neural stem cells (hNSC)-particularly for genes of high-relevance for DMN circuitry. DHT dysregulates a number of genes enriched for syndromic causes of autism and intellectual disability and for genes that in later development are expressed in anatomical patterns that highly correspond to the cortical midline DMN subsystem. DMN-related and DHT-affected genes (e.g., MEF2C) are involved in a number of synaptic processes, many of which impact excitation-inhibition balance. Androgens have male-specific prenatal influence over social brain circuitry in humans and may be relevant towards explaining some component of male-bias in early-onset neurodevelopmental conditions.

Activity-Regulated Transcription: Bridging the Gap between Neural Activity and Behavior

Gene transcription is the process by which the genetic codes of organisms are read and interpreted as a set of instructions for cells to divide, differentiate, migrate, and mature. As cells function in their respective niches, transcription further allows mature cells to interact dynamically with their external environment while reliably retaining fundamental information about past experiences. In this Review, we provide an overview of the field of activity-dependent transcription in the vertebrate brain and highlight contemporary work that ranges from studies of activity-dependent chromatin modifications to plasticity mechanisms underlying adaptive behaviors. We identify key gaps in knowledge and propose integrated approaches toward a deeper understanding of how activity-dependent transcription promotes the refinement and plasticity of neural circuits for cognitive function.

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.

The development of local circuits in the neocortex: recent lessons from the mouse visual cortex

Precise synaptic connections among neurons in the neocortex generate the circuits that underlie a broad repertoire of cortical functions including perception, learning and memory, and complex problem solving. The specific patterns and properties of these synaptic connections are fundamental to the computations cortical neurons perform. How such specificity arises in cortical circuits has remained elusive. Here, we first consider the cell-type, subcellular and synaptic specificity required for generating mature patterns of cortical connectivity and responses. Next, we focus on recent progress in understanding how the synaptic connections among excitatory cortical projection neurons are established during development using the primary visual cortex of the mouse as a model.

Genes regulated by SATB2 during neurodevelopment contribute to schizophrenia and educational attainment

SATB2 is associated with schizophrenia and is an important transcription factor regulating neocortical organization and circuitry. Rare mutations in SATB2 cause a syndrome that includes developmental delay, and mouse studies identify an important role for SATB2 in learning and memory. Interacting partners BCL11B and GATAD2A are also schizophrenia risk genes indicating that other genes interacting with or are regulated by SATB2 are making a contribution to schizophrenia and cognition. We used data from Satb2 mouse models to generate three gene-sets that contain genes either functionally related to SATB2 or targeted by SATB2 at different stages of development. Each was tested for enrichment using the largest available genome-wide association studies (GWAS) datasets for schizophrenia and educational attainment (EA) and enrichment analysis was also performed for schizophrenia and other neurodevelopmental disorders using data from rare variant sequencing studies. These SATB2 gene-sets were enriched for genes containing common variants associated with schizophrenia and EA, and were enriched for genes containing rare variants reported in studies of schizophrenia, autism and intellectual disability. In the developing cortex, genes targeted by SATB2 based on ChIP-seq data, and functionally affected when SATB2 is not expressed based on differential expression analysis using RNA-seq data, show strong enrichment for genes associated with EA. For genes expressed in the hippocampus or at the synapse, those targeted by SATB2 are more strongly enriched for genes associated EA than gene-sets not targeted by SATB2. This study demonstrates that single gene findings from GWAS can provide important insights to pathobiological processes. In this case we find evidence that genes influenced by SATB2 and involved in synaptic transmission, axon guidance and formation of the corpus callosum are contributing to schizophrenia and cognition.

Schizophrenia is a complex disorder caused by many genes. Using new gene discoveries to understand pathobiology is a foundation for development of new treatments. Current drugs for schizophrenia are only partially effective and do not treat cognitive deficits, which are key factors for explaining disability, leading to unemployment, homelessness and social isolation. Genome-wide association studies (GWAS) of schizophrenia have been effective at identifying individual SNPs and genes that contribute to risk but have struggled to immediately uncover the bigger picture of the underlying biology of the disorder. Here we take an individual gene identified in a schizophrenia GWAS called SATB2, which on its own is a very important regulator of brain development. We use functional genomics data from mouse studies to identify sets of others genes that are influenced by SATB2 during development. We show that these gene sets are enriched for common variants associated with schizophrenia and educational attainment (used as a proxy for cognition), and for rare variants that increase risk of various neurodevelopmental disorders. This study provides evidence that the molecular mechanisms that underpin schizophrenia and cognitive function include disruption of biological processes influenced by SATB2 as the brain is being organized and wired during development.

Novel formononetin-7-sal ester ameliorates pulmonary fibrosis via MEF2c signaling pathway

Pulmonary fibrosis is a progressive disorder with poor prognosis and limited treatment options. Therefore, novel therapeutic drugs should be developed in preclinical studies. In this study, we designed and synthesized a novel compound named formononetin-7-sal ester (FS). We also investigated its anti-pulmonary fibrosis ability on transforming growth factor beta 1 (TGF-β1)-stimulated pulmonary epithelial cells and fibroblasts in vitro and on bleomycin (BLM)-induced pulmonary fibrosis in vivo. FS strongly blocked cell proliferation and migration, which were activated by TGF-β1, thereby reducing the expression of lung fibrosis markers, such as vimentin, alpha-smooth muscle actin (α-SMA), Snail, and collagen I and III, and increasing the expression of the epithelial cell marker E-cadherin. FS ameliorated BLM-induced pulmonary fibrosis in mice and decreased histopathologic fibrosis scores and collagen deposition. A low expression of hydroxyproline, vimentin, α-SMA, and Snail and a high expression of E-cadherin were found in FS-treated lungs compared with BLM-instilled lungs. Using the Cignal Finder 45-Pathway Reporter Array, we tested the regulation of FS in pulmonary fibrosis-associated signaling pathways and observed that FS significantly inhibited the myocyte enhancer factor-2c (MEF2c) signaling pathway. Gain- and loss-of-function studies, rescue experiments and promoter activity testing were designed to further confirm this result in vivo and in vitro. Collectively, our results demonstrated that FS prevents pulmonary fibrosis via the MEF2c signaling pathway.

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.

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.

Studies of cortical connectivity using optical circuit mapping methods

An important consideration when probing the function of any neuron is to uncover the source of synaptic input onto the cell, its intrinsic physiology and efferent targets. Over the years, electrophysiological approaches have generated considerable insight into these properties in a variety of cortical neuronal subtypes and circuits. However, as researchers explore neuronal function in greater detail, they are increasingly turning to optical techniques to bridge the gap between local network interactions and behaviour. The application of optical methods has increased dramatically over the past decade, spurred on by the optogenetic revolution. In this review, we provide an account of recent innovations, providing researchers with a primer detailing circuit mapping strategies in the cerebral cortex. We will focus on technical aspects of performing neurotransmitter uncaging and channelrhodopsin-assisted circuit mapping, with the aim of identifying common pitfalls that can negatively influence the collection of reliable data.

Transcribing the connectome: roles for transcription factors and chromatin regulators in activity-dependent synapse development

The wiring of synaptic connections in the developing mammalian brain is shaped by both intrinsic and extrinsic signals. One point where these regulatory pathways converge is via the sensory experience-dependent regulation of new gene transcription. Recent studies have elucidated a number of molecular mechanisms that allow nuclear transcription factors and chromatin regulatory proteins to encode aspects of specificity in experience-dependent synapse development. Here we review the evidence for the transcriptional mechanisms that sculpt activity-dependent aspects of synaptic connectivity during postnatal development and discuss how disruption of these processes is associated with aberrant brain development in autism and intellectual disability.