MET receptor tyrosine kinase controls dendritic complexity, spine morphogenesis, and glutamatergic synapse maturation in the hippocampus

ARHGAP12 Functions as a Developmental Brake on Excitatory Synapse Function

The molecular mechanisms that promote excitatory synapse development have been extensively studied. However, the molecular events preventing precocious excitatory synapse development so that synapses form at the correct time and place are less well understood. Here, we report the functional characterization of ARHGAP12, a previously uncharacterized Rho GTPase-activating protein (RhoGAP) in the brain. ARHGAP12 is specifically expressed in the CA1 region of the hippocampus, where it localizes to the postsynaptic compartment of excitatory synapses. ARHGAP12 negatively controls spine size via its RhoGAP activity and promotes, by interacting with CIP4, postsynaptic AMPA receptor endocytosis. Arhgap12 knockdown results in precocious maturation of excitatory synapses, as indicated by a reduction in the proportion of silent synapses. Collectively, our data show that ARHGAP12 is a synaptic RhoGAP that regulates excitatory synaptic structure and function during development.

Complete or partial reduction of the Met receptor tyrosine kinase in distinct circuits differentially impacts mouse behavior

BACKGROUND

Our laboratory discovered that the gene encoding the receptor tyrosine kinase, MET, contributes to autism risk. Expression of MET is reduced in human postmortem temporal lobe in autism and Rett Syndrome. Subsequent studies revealed a role for MET in human and mouse functional and structural cortical connectivity. To further understand the contribution of Met to brain development and its impact on behavior, we generated two conditional mouse lines in which Met is deleted from select populations of central nervous system neurons. Mice were then tested to determine the consequences of disrupting Met expression.

METHODS

Mating of Emx1 (cre) and Met (fx/fx) mice eliminates receptor signaling from all cells arising from the dorsal pallium. Met (fx/fx) and Nestin (cre) crosses result in receptor signaling elimination from all neural cells. Behavioral tests were performed to assess cognitive, emotional, and social impairments that are observed in multiple neurodevelopmental disorders and that are in part subserved by circuits that express Met.

RESULTS

Met (fx/fx) /Emx1 (cre) null mice displayed significant hypoactivity in the activity chamber and in the T-maze despite superior performance on the rotarod. Additionally, these animals showed a deficit in spontaneous alternation. Surprisingly, Met (fx/fx; fx/+) /Nestin (cre) null and heterozygous mice exhibited deficits in contextual fear conditioning, and Met (fx/+) /Nestin (cre) heterozygous mice spent less time in the closed arms of the elevated plus maze.

CONCLUSIONS

These data suggest a complex contribution of Met in the development of circuits mediating social, emotional, and cognitive behavior. The impact of disrupting developmental Met expression is dependent upon circuit-specific deletion patterns and levels of receptor activity.

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.

The Multifarious Hippocampal Functions of MicroRNA-137

MicroRNAs (miRs) have emerged as a powerful class of endogenous noncoding RNAs involved in posttranscriptional gene expression regulation. miR-137 has repeatedly been associated with schizophrenia and intellectual disability. Recent studies describe the mechanisms of miR-137 in mediating basic synaptic transmission and plasticity in the hippocampus. A picture is emerging in which miR-137 acts as a potent player in regulating glutamatergic synaptic transmission in the hippocampus by controlling the translation of functionally critical genes at spatially opposite ends of the synapse, contributing to the pathogenesis of cognitive impairments as seen in neurodevelopmental disorders.

Transcriptional maturation of the mouse auditory forebrain

Background

The maturation of the brain involves the coordinated expression of thousands of genes, proteins and regulatory elements over time. In sensory pathways, gene expression profiles are modified by age and sensory experience in a manner that differs between brain regions and cell types. In the auditory system of altricial animals, neuronal activity increases markedly after the opening of the ear canals, initiating events that culminate in the maturation of auditory circuitry in the brain. This window provides a unique opportunity to study how gene expression patterns are modified by the onset of sensory experience through maturity. As a tool for capturing these features, next-generation sequencing of total RNA (RNAseq) has tremendous utility, because the entire transcriptome can be screened to index expression of any gene. To date, whole transcriptome profiles have not been generated for any central auditory structure in any species at any age. In the present study, RNAseq was used to profile two regions of the mouse auditory forebrain (A1, primary auditory cortex; MG, medial geniculate) at key stages of postnatal development (P7, P14, P21, adult) before and after the onset of hearing (~P12). Hierarchical clustering, differential expression, and functional geneset enrichment analyses (GSEA) were used to profile the expression patterns of all genes. Selected genesets related to neurotransmission, developmental plasticity, critical periods and brain structure were highlighted. An accessible repository of the entire dataset was also constructed that permits extraction and screening of all data from the global through single-gene levels. To our knowledge, this is the first whole transcriptome sequencing study of the forebrain of any mammalian sensory system. Although the data are most relevant for the auditory system, they are generally applicable to forebrain structures in the visual and somatosensory systems, as well.

Results

The main findings were: (1) Global gene expression patterns were tightly clustered by postnatal age and brain region; (2) comparing A1 and MG, the total numbers of differentially expressed genes were comparable from P7 to P21, then dropped to nearly half by adulthood; (3) comparing successive age groups, the greatest numbers of differentially expressed genes were found between P7 and P14 in both regions, followed by a steady decline in numbers with age; (4) maturational trajectories in expression levels varied at the single gene level (increasing, decreasing, static, other); (5) between regions, the profiles of single genes were often asymmetric; (6) GSEA revealed that genesets related to neural activity and plasticity were typically upregulated from P7 to adult, while those related to structure tended to be downregulated; (7) GSEA and pathways analysis of selected functional networks were not predictive of expression patterns in the auditory forebrain for all genes, reflecting regional specificity at the single gene level.

Conclusions

Gene expression in the auditory forebrain during postnatal development is in constant flux and becomes increasingly stable with age. Maturational changes are evident at the global through single gene levels. Transcriptome profiles in A1 and MG are distinct at all ages, and differ from other brain regions. The database generated by this study provides a rich foundation for the identification of novel developmental biomarkers, functional gene pathways, and targeted studies of postnatal maturation in the auditory forebrain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1709-8) contains supplementary material, which is available to authorized users.