Convergence of spectrums: neuronal gene network states in autism spectrum disorder.

Autism spectrum disorder (ASD) is a prevalent neurodevelopmental disorder characterized by social deficits and restrictive and/or repetitive behaviors. The breadth of ASD symptoms is paralleled by the multiplicity of genes that have been implicated in its etiology. Initial findings revealed numerous ASD risk genes that contribute to synaptic function. More recently, genomic and gene expression studies point to altered chromatin function and impaired transcriptional control as additional risk factors for ASD. The consequences of impaired transcriptional alterations in ASD involve consistent changes in synaptic gene expression and cortical neuron specification during brain development. The multiplicity of genetic and environmental factors associated with ASD risk and their convergence onto common molecular pathways in neurons point to ASD as a disorder of gene regulatory networks.

Epigenetic regulation of brain region-specific microglia clearance activity.

The rapid elimination of dying neurons and nonfunctional synapses in the brain is carried out by microglia, the resident myeloid cells of the brain. Here we show that microglia clearance activity in the adult brain is regionally regulated and depends on the rate of neuronal attrition. Cerebellar, but not striatal or cortical, microglia exhibited high levels of basal clearance activity, which correlated with an elevated degree of cerebellar neuronal attrition. Exposing forebrain microglia to apoptotic cells activated gene-expression programs supporting clearance activity. We provide evidence that the polycomb repressive complex 2 (PRC2) epigenetically restricts the expression of genes that support clearance activity in striatal and cortical microglia. Loss of PRC2 leads to aberrant activation of a microglia clearance phenotype, which triggers changes in neuronal morphology and behavior. Our data highlight a key role of epigenetic mechanisms in preventing microglia-induced neuronal alterations that are frequently associated with neurodegenerative and psychiatric diseases.

Polycomb repressive complex 2 (PRC2) silences genes responsible for neurodegeneration.

Normal brain function depends on the interaction between highly specialized neurons that operate within anatomically and functionally distinct brain regions. Neuronal specification is driven by transcriptional programs that are established during early neuronal development and remain in place in the adult brain. The fidelity of neuronal specification depends on the robustness of the transcriptional program that supports the neuron type-specific gene expression patterns. Here we show that polycomb repressive complex 2 (PRC2), which supports neuron specification during differentiation, contributes to the suppression of a transcriptional program that is detrimental to adult neuron function and survival. We show that PRC2 deficiency in striatal neurons leads to the de-repression of selected, predominantly bivalent PRC2 target genes that are dominated by self-regulating transcription factors normally suppressed in these neurons. The transcriptional changes in PRC2-deficient neurons lead to progressive and fatal neurodegeneration in mice. Our results point to a key role of PRC2 in protecting neurons against degeneration.

Autism-like syndrome is induced by pharmacological suppression of BET proteins in young mice.

Studies investigating the causes of autism spectrum disorder (ASD) point to genetic, as well as epigenetic, mechanisms of the disease. Identification of epigenetic processes that contribute to ASD development and progression is of major importance and may lead to the development of novel therapeutic strategies. Here, we identify the bromodomain and extraterminal domain-containing proteins (BETs) as epigenetic regulators of genes involved in ASD-like behaviors in mice. We found that the pharmacological suppression of BET proteins in the brain of young mice, by the novel, highly specific, brain-permeable inhibitor I-BET858 leads to selective suppression of neuronal gene expression followed by the development of an autism-like syndrome. Many of the I-BET858-affected genes have been linked to ASD in humans, thus suggesting the key role of the BET-controlled gene network in the disorder. Our studies suggest that environmental factors controlling BET proteins or their target genes may contribute to the epigenetic mechanism of ASD.

Cadherin-8 expression, synaptic localization, and molecular control of neuronal form in prefrontal corticostriatal circuits.

Neocortical interactions with the dorsal striatum support many motor and executive functions, and such underlying functional networks are particularly vulnerable to a variety of developmental, neurological, and psychiatric brain disorders, including autism spectrum disorders, Parkinson's disease, and Huntington's disease. Relatively little is known about the development of functional corticostriatal interactions, and in particular, virtually nothing is known of the molecular mechanisms that control generation of prefrontal cortex-striatal circuits. Here, we used regional and cellular in situ hybridization techniques coupled with neuronal tract tracing to show that Cadherin-8 (Cdh8), a homophilic adhesion protein encoded by a gene associated with autism spectrum disorders and learning disability susceptibility, is enriched within striatal projection neurons in the medial prefrontal cortex and in striatal medium spiny neurons forming the direct or indirect pathways. Developmental analysis of quantitative real-time polymerase chain reaction and western blot data show that Cdh8 expression peaks in the prefrontal cortex and striatum at P10, when cortical projections start to form synapses in the striatum. High-resolution immunoelectron microscopy shows that Cdh8 is concentrated at excitatory synapses in the dorsal striatum, and Cdh8 knockdown in cortical neurons impairs dendritic arborization and dendrite self-avoidance. Taken together, our findings indicate that Cdh8 delineates developing corticostriatal circuits where it is a strong candidate for regulating the generation of normal cortical projections, neuronal morphology, and corticostriatal synapses.

Mint proteins are required for synaptic activity-dependent amyloid precursor protein (APP) trafficking and amyloid ß generation.

Aberrant amyloid ß (Aß) production plays a causal role in Alzheimer disease pathogenesis. A major cellular pathway for Aß generation is the activity-dependent endocytosis and proteolytic cleavage of the amyloid precursor protein (APP). However, the molecules controlling activity-dependent APP trafficking in neurons are less defined. Mints are adaptor proteins that directly interact with the endocytic sorting motif of APP and are functionally important in regulating APP endocytosis and Aß production. We analyzed neuronal cultures from control and Mint knockout neurons that were treated with either glutamate or tetrodotoxin to stimulate an increase or decrease in neuronal activity, respectively. We found that neuronal activation by glutamate increased APP endocytosis, followed by elevated APP insertion into the cell surface, stabilizing APP at the plasma membrane. Conversely, suppression of neuronal activity by tetrodotoxin decreased APP endocytosis and insertion. Interestingly, we found that activity-dependent APP trafficking and Aß generation were blocked in Mint knockout neurons. We showed that wild-type Mint1 can rescue APP internalization and insertion in Mint knockout neurons. In addition, we found that Mint overexpression increased excitatory synaptic activity and that APP was internalized predominantly to endosomes associated with APP processing. We demonstrated that presenilin 1 (PS1) endocytosis requires interaction with the PDZ domains of Mint1 and that this interaction facilitates activity-dependent colocalization of APP and PS1. These findings demonstrate that Mints are necessary for activity-induced APP and PS1 trafficking and provide insight into the cellular fate of APP in endocytic pathways essential for Aß production.

Microtubule plus-end tracking protein CLASP2 regulates neuronal polarity and synaptic function.

Microtubule organization and dynamics are essential during axon and dendrite formation and maintenance in neurons. However, little is known about the regulation of microtubule dynamics during synaptic development and function in mammalian neurons. Here, we present evidence that the microtubule plus-end tracking protein CLASP2 (cytoplasmic linker associated protein 2) is a key regulator of axon and dendrite outgrowth that leads to functional alterations in synaptic activity and formation. We found that CLASP2 protein levels steadily increase throughout neuronal development in the mouse brain and are specifically enriched at the growth cones of extending neurites. The short-hairpin RNA-mediated knockdown of CLASP2 in primary mouse neurons decreased axon and dendritic length, whereas overexpression of human CLASP2 caused the formation of multiple axons, enhanced dendritic branching, and Golgi condensation, implicating CLASP2 in neuronal morphogenesis. In addition, the CLASP2-induced morphological changes led to significant functional alterations in synaptic transmission. CLASP2 overexpression produced a large increase in spontaneous miniature event frequency that was specific to excitatory neurotransmitter release. The changes in presynaptic activity produced by CLASP2 overexpression were accompanied by increases in presynaptic terminal circumference, total synapse number, and a selective increase in presynaptic proteins that are involved in neurotransmitter release. Also, we found a smaller increase in miniature event amplitude that was accompanied by an increase in postsynaptic surface expression of GluA1 receptor localization. Together, these results provide evidence for involvement of the microtubule plus-end tracking protein CLASP2 in cytoskeleton-related mechanisms underlying neuronal polarity and interplay between microtubule stabilization and synapse formation and activity.