One of the greatest current threats to health and lifespan in both the Western world and developing countries is overweight and obesity due to the predisposition for type-2DM, cardiovascular disease, and other manifestations of the metabolic syndrome. Body weight is normally maintained in a narrow range through a complex orchestration of hormonal, neural, and metabolic signals reciprocating between the brain and peripheral tissues. The obesity epidemic has emerged because of recent environmental, sociological, and genetic interactions that have unfavorably tipped the balance in favor of excess caloric storage as body fat. One critical component of the neural circuitry that regulates caloric intake and expenditure is the tiny population of neurons located in the hypothalamus and medulla that express the Pomc gene and synthesize the neuropeptides aMSH and bEndorphin from the precursor protein, proopiomelanocortin (POMC). In the current project period, we made significant progress towards explaining how transcription of Pomc is restricted to these specialized neurons, which are a primary portal to the brain for peripheral signals relating energy flux. We located a modular enhancer locus about 10 kb upstream of exon 1 that directs POMC neuron-specific gene expression and functions independently of pituitary-specific promoter elements. Closer bioinformatic and functional interrogation of this locus revealed two evolutionarily distinct modules within the Mammalian lineage that we named nPE1 and nPE2 for neuronal POMC Enhancer. Mutant strains of mice were generated that carry deletions of individual or both nPE elements (D1, D2, and D1D2). Our preliminary phenotypic analyses of these mice, and parallel strains with insertions of a PGK-neo selection cassette in the distal enhancer locus, led us to the hypotheses that nPE1 and nPE2 possess distinct, nonredundant control over Pomc expression in the brain, and moreover, that at least one additional neuronal enhancer for Pomc has yet to be identified. Core nucleotide sequence motifs within the enhancers and ongoing testing of candidate transcription factors suggest that a unique combination of homeodomain factors is responsible for POMC neuron-specific expression. Therefore, the specific aims for this project renewal are to: 1) Dissect the physiological roles of the Pomc neuronal enhancers nPE1 and nPE2 on the regulation of energy balance;2) Analyze the interrelationships among neuroendocrine signaling pathways, cellular activation, and Pomc enhancer elements that regulate neuronal Pomc expression using the nPE mutant strains;3) Unravel the neuronal Pomc transcriptional code through the identification of the entire set of enhancers and the essential motifs necessary to control Pomc expression in hypothalamic Arc neurons and the NTS;and 4) Identify the transcription factors responsible for cell-specific transactivation of the Pomc gene in the CNS. Completion of these aims will provide fundamental knowledge about an essential neurobiological control point for body mass and may point to new avenues for the prevention or treatment of obesity based on exploitation of the underlying molecular mechanisms.
Among the greatest current threats to public health are the continually increasing rates of overweight, obesity, diabetes, and the metabolic syndrome. A complex set of neural circuits integrates the balance between caloric demand and utilization with the behavioral and psychological processes related to feeding. This project centers on a key molecular component of the brain's feeding circuits, the proopiomelanocortin gene, and how the gene is regulated in specific neurons of the hypothalamus in response to metabolic and hormonal signals including leptin, insulin, and glucose flux.
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