Both behavioral and metabolic consequences of uncontrolled, insulin-deficient diabetes mellitus (uDM) arise in part from the response of key brain areas such as the hypothalamic arcuate nucleus (ARC) and ventromedial hypothalamic nucleus (VMN) to changes in the humoral milieu, including marked decreases in the circulating levels of both insulin and leptin, and elevated levels of ghrelin. Rodent models of uDM therefore constitute a unique and valuable tool with which to study these neuroendocrine control systems. Among ARC neuronal subsets activated in uDM are those that express orexigenic (or 'food intake-stimulatory') peptides such as neuropeptide Y (NPY) and agouti-related peptide (AgRP), whereas adjacent anorexigenic, melanocortin-producing (or 'POMC') neurons are inhibited, a combination of responses implicated in the pronounced increase of food intake characteristic of uDM (termed "diabetic hyperphagia"). At the cellular level, signaling via the insulin receptor substrate-phosphotidylinositol-3 kinase (IRS-PI3K) pathway plays a critical role in insulin action in peripheral tissues and while this pathway is also critical for both leptin and insulin action in the CNS, the specific neuronal subsets involved remain to be determined. Signaling molecules downstream of PI3K include protein kinase B (PKB) and mammalian target of rapamycin (mTOR), both of which are also implicated in hypothalamic control of food intake. Growing evidence also suggests that hypothalamic neurocircuits that sense input from insulin, leptin and ghrelin participate in the control of insulin sensitivity in peripheral tissues, and our recent work implicates dysfunction of these neurocircuits, triggered by reduced PI3K signaling, in the progressive insulin resistance seen in rats with uDM induced by the b-cell toxin, streptozotocin (STZ). Based on these observations, we propose in Specific Aim 1 to employ mouse models of STZ-induced uDM that enable us to identify the specific brain areas and neuronal subsets in which signal transduction via the IRS-PI3K-PKB pathway regulates food intake and glucose metabolism. Specifically, we will use a combination of Cre-loxP genetic and adenoviral gene therapy techniques to increase PKB specifically in NPY/Agrp neurons, POMC neurons, neurons that express leptin receptors, or VMN neurons (that express the transcription factor SF- 1) in mice with STZ-induced uDM. In this way, we will identify neuronal subsets in the ARC and VMN in which reduced PKB signaling contributes to feeding and metabolic consequences of uDM. Similarly, Aim 2 seeks to delineate the role of reduced hypothalamic mTOR signaling in behavioral and metabolic responses to uDM in rats.
In Aim 3, we investigate mechanisms underlying increased plasma ghrelin levels in uDM and determine the contribution made by this increase to hyperphagia and insulin resistance in this setting. Together, this information will shed new light on neuroendocrine mechanisms controlling food intake and insulin sensitivity and help to clarify how dysfunction within these systems contributes to the pathogenesis of disordered feeding behavior and glucose metabolism in obesity and diabetes.
Obesity and diabetes mellitus are closely related metabolic disorders that take an increasing toll on human health. Recent findings suggest that dysfunction of key hypothalamic neurons that regulate food intake, autonomic function and glucose metabolism contributes to the link between these metabolic disorders. The identification of neuronal circuits upon which hormones such as insulin, leptin and ghrelin act to control both food intake and glucose metabolism is therefore an important scientific priority. Towards this end, this proposal employs rat and mouse models of uncontrolled diabetes mellitus to further define neuroendocrine mechanisms controlling food intake, body fat storage and insulin sensitivity in peripheral tissues. By improving our understanding of how mechanisms driving obesity and diabetes are linked to one another within the brain, our ultimate goal is to find new strategies for their prevention and treatment.
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