Role of hypothalamic MC4R in glucose homeostasis via a novel neuroendocrine circuit involving the kidneys and adrenal glands

我们最近发现下丘脑MC4R通过肾上腺素和肾GLUT2影响糖尿来调节血糖水平。此外，我们之前已经证明，糖尿病会降低下丘脑Mc4r的表达，从而导致小鼠对葡萄糖缺乏的缺陷性反调节反应（CRR）。这些临床观察结果背后的分子和综合机制尚不清楚。具体来说，下丘脑MC4R是如何调节血浆肾上腺素水平的？下丘脑MC4R和肾脏GLUT2之间的神经串扰是否通过肾上腺素协调对全身葡萄糖稳态的调节？在我们的初步研究中，我们观察到在促肾上腺皮质激素释放激素（CRH）神经元中选择性地恢复MC4R使MC4R缺失小鼠的血浆肾上腺素水平正常化，表明表达MC4R的CRH神经元在调节血浆肾上腺素水平中的作用。出乎意料的是，肾特异性Glut2敲除小鼠可免受糖尿病介导的CRR缺陷致低血糖的影响，小鼠下丘脑Mc4r和Crh mRNA水平升高，提示肾Glut2对下丘脑黑素皮质素系统的影响。我们的初步数据还表明，肾脏特异性Glut2敲除小鼠低血糖后，肾脏（而非肝脏）糖异生对恢复正常血糖水平的贡献。基于这些初步发现，我们假设下丘脑表达mc4r的CRH神经元在葡萄糖缺乏时被传入肾神经激活，从而增加血浆肾上腺素，从而增加肾脏糖异生并恢复血糖水平。此外，糖尿病患者肾脏GLUT2水平升高会损害表达mc4r的CRH神经元的激活，从而损害对低血糖的反调节反应。我们将通过以下目的来检验这些假设：目的1：确定下丘脑表达mc4r的CRH神经元在整合肾传入神经信号以增加低血糖时血浆肾上腺素的作用。目的2：确定糖尿病损害机体抵抗低血糖能力的分子和综合（中枢与外周）机制——肾GLUT2慢性升高，肾传入神经活性降低，和/或下丘脑表达mc4r的CRH神经元激活钝化。目的3：确立肾糖异生作为葡萄糖来源的作用，防御低血糖，并确定肾上腺素增加葡萄糖重吸收和肾糖异生的肾肾上腺素能受体。总之，这个项目可能会发现一个新的神经内分泌回路，它隐藏在下丘脑、肾脏和肾上腺之间的串扰中，以协调全身葡萄糖稳态。这种整合电路可能会告诉我们糖尿病是如何在葡萄糖缺乏时降低机体增加血浆肾上腺素的能力，从而导致低血糖相关的自主神经衰竭，这是1型或2型糖尿病晚期患者的一种危及生命的疾病。

We recently identified the contribution of hypothalamic MC4R in regulating blood glucose levels by influencing glycosuria through epinephrine and renal GLUT2. Moreover, we have demonstrated previously that diabetes decreases the hypothalamic Mc4r expression that consequently leads to defective counterregulatory response (CRR) to glucose deficits in mice. The molecular and integrative mechanisms underlying these clinically significant observations remain unknown. Specifically, how does hypothalamic MC4R regulates plasma epinephrine levels? Does a neural crosstalk between the hypothalamic MC4R and kidney GLUT2 coordinate the regulation of systemic glucose homeostasis via epinephrine? In our Preliminary Studies, we observed that MC4R restoration selectively in the corticotropin releasing hormone (CRH) neurons normalized plasma epinephrine levels in otherwise MC4R-deficient mice, indicating the role of MC4R- expressing CRH neurons in regulating plasma epinephrine levels. Unexpectedly, kidney-specific Glut2 knockout mice are protected from diabetes-mediated defective CRR to hypoglycemia and the mice show increased Mc4r and Crh mRNA levels in the hypothalamus, suggesting the influence of renal GLUT2 on the hypothalamic melanocortin system. Our Preliminary Data also indicate the contribution of renal, but not liver, gluconeogenesis toward restoration of normal blood glucose levels following hypoglycemia in the kidney-specific Glut2 knockout mice. Based on these preliminary findings, we hypothesize that hypothalamic MC4R-expressing CRH neurons are activated by the afferent renal nerves in response to glucose deficits to enhance plasma epinephrine, which in turn increases renal gluconeogenesis and restores blood glucose levels. Moreover, elevated kidney GLUT2 levels in diabetes compromises this activation of MC4R-expressing CRH neurons to impair counterregulatory response to hypoglycemia. We will test the hypotheses with the following Aims, Aim1: Determine the role of hypothalamic MC4R-expressing CRH neurons in integrating signals from renal afferent nerves to increase plasma epinephrine in response to hypoglycemia. Aim2: Identify the molecular and integrative (central versus peripheral) mechanisms – chronically elevated renal GLUT2, decreased renal afferent nerve activity, and/or blunted activation of hypothalamic MC4R-expressing CRH neurons - through which diabetes compromises the body’s ability to defend against hypoglycemia. Aim3: Establish the contribution of renal gluconeogenesis as a source of glucose to defend against hypoglycemia and identify the renal adrenergic receptors through which epinephrine increases glucose reabsorption and renal gluconeogenesis. Altogether, this project will likely identify a novel neuroendocrine circuit underlying the crosstalk between the hypothalamus, kidney and adrenal gland to coordinate systemic glucose homeostasis. This integrative circuit will probably inform the molecular basis of how diabetes compromises the body’s ability to increase plasma epinephrine in the face of glucose deficits leading to hypoglycemia-associated autonomic failure, a life- threatening condition in patients with type 1 or late-stage type 2 diabetes.