Transcriptional Mechanisms Regulating Activity Dependent

Child Health And Human Development

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Abstract

Epigenetic and genetic factors contribute to the remodeling of the nervous system and skeletal muscles during development. These tissues have the capacity to modify their functional properties during development, and later in the adult, in response to experience (i.e. activity). The plasticity of neurons and muscles underlie complex cellular processes necessary for learning and memory in the brain, adaptation of muscles to exercise, and when not functioning properly, contribute to numerous neurological and muscle diseases. A commonly held view in Neurobiology has been that presynaptic inputs are instructive and regulate the functional properties of the neurons and muscles they innervate. However, recent work on the development of neuronal circuits in the central and peripheral nervous system indicates that the properties of postsynaptic cells are pre-patterned by lineage cues independently from the nerve. Later, during the maturation of synaptic connections, neuronal growth/differentiation factors and electrical activity) modify the plastic properties of neurons and muscles. The Section on Molecular Neurobiology is utilizing two experimental models to investigate how neural factors and activity regulate neuron and muscle plasticity during development. In the first project, we are investigating how Neuregulins regulate the functional properties of interneuronal synapses in the brain. In the second, we are studying how lineage and motoneuron activity contribute to the emergence and plasticity of different muscle types. A. Neuregulin effects on neuronal plasticity. Recent studies have implicated the NRG family of growth/differentiation factors, and their receptors, in the regulation of synaptic plasticity and behavior. The possible genetic link of NRG-1 to schizophrenia suggests that these factors may regulate cognitive processes. The NRG family is encoded by 3 genes that produce multiple transcripts by alternative splicing, and that signal via a family of receptor tyrosine kinases known as ErbB-1 through ErbB-4. Several years ago we, and others, showed that NRG-1 treatment of neurons promotes the selective expression of subunits comprising neurotransmitter receptors for glutamate (NMDA subtype), acetylcholine and GABA. Using yeast-2-hybrid cloning, biochemical and cell biology techniques, we found that ErbB and NMDA receptors colocalize at postsynaptic sites where they interact with the scaffolding and signaling proteins called PSD-95. More recently, we found that NRG-1 regulates synaptic transmission, consistent with the possible contribution of the NRG/ErbB signaling pathway to behavior and psychological disorders. To understand how this signaling pathway contributes to neuronal plasticity, we have studied the regional and temporal expression of NRGs and ErbB receptors during development. These studies indicate that the NRGs 1-3 and ErbB 1-4 receptors show distinct patterns of expression in the developing brain, suggesting that they serve different functions. Electrophysiological and DNA microarray experiments are being utilized, in combination with knock-out mice, to understand how NRGs may exert different neuronal functions. These genetic approaches, in combination with conventional electrophysiological and cell biological techniques, should help to uncover the role NRG signaling in activity-dependent synaptic plasticity during synaptogenesis and their possible contribution to behavior. B. Developmental and activity-dependent regulation of muscle genes. The developmental and neuronal regulation of slow- and fast-twitch properties of skeletal muscles provides an excellent model to study how patterned electrical activity regulates plasticity of postsynaptic target. The long-term objectives of our laboratory are to identify transcription factors that regulate the emergence of distinct fiber types during development, and that modulate their plasticity in response to motoneuron activity. Our experimental paradigm is the regulation of genes encoding Troponin I slow (TnIs) and fast (TnIf) isoforms, which are specifically expressed in slow- and fast-twitch myofibers, respectively. Initially, these genes are differentially regulated during myoblast differentiation in the fetus, and after birth, distinct patterns of motoneuron impulses (slow: 10 Hertz tonic depolarization; fast: 100 Hertz phasic depolarization) differentially stimulate TnI transcription. Our recent studies on the transcriptional regulation of TnIs indicate that early developmental cues, as well as specific patterns of motoneuron activity, are necessary for the full phenotypic differentiation of muscle fiber types. Our experiments implicate the nuclear factor GTF3 in contributing to the slow-specific transcription of the TnIs gene during early development. We identified 3 novel splice variants of GTF3 (for a total of 6), denoted alpha-2, alpha-3, and gamma-2, and compared their binding properties to the TnIs enhancer using gel shift assays. Interestingly, the full-length GTF3 proteins interacted poorly with their cognate binding site, but a subset of the N-terminally truncated proteins are avid binders. These results suggest that post-translational mechanisms or interactions with other transcription factors regulate the binding capacity of GTF3 to slow enhancers. Experiments are in progress to understand the biochemical and functional properties of this factor in vivo using GTF3 knock-out mice. Persons with Williams Syndrome (WS) have distinctive physical, cognitive and behavior abnormalities that include impaired spatial cognitive skills and myopathies. Recent studies strongly implicate GTF3, and its close homologue GTF2i, as candidate proteins that contribute to the deficiencies observed in patients with WS. Our studies using ectopically transfected GTF3 constructs in adult muscles and GTF3 mutant mice strongly support a role for this factor in regulating muscle contractile properties, which could be related to myopathies observed in WS. The observation that GTF3 and GTF2i are mostly expressed in developing musculature and neurons, raises the possibility that reduction of these factors during embryogenesis could affect the expression of target genes later in development.