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

我们最近确定了下丘脑MC 4 R通过肾上腺素和肾脏GLUT 2影响糖尿来调节血糖水平的作用。此外，我们以前已经证明，糖尿病降低了下丘脑Mc 4 r的表达，从而导致缺陷的反调节反应（CRR）的葡萄糖赤字小鼠。这些具有临床意义的观察结果背后的分子和整合机制仍然未知。具体来说，下丘脑MC 4 R如何调节血浆肾上腺素水平？下丘脑MC 4 R和肾脏GLUT 2之间的神经串扰是否通过肾上腺素协调全身葡萄糖稳态的调节？在我们的初步研究中，我们观察到，MC 4 R选择性恢复促肾上腺皮质激素释放激素（CRH）神经元正常化，否则MC 4 R缺陷小鼠的血浆肾上腺素水平，表明MC 4 R表达CRH神经元在调节血浆肾上腺素水平的作用。出乎意料的是，肾特异性Glut 2敲除小鼠受到保护，免于糖尿病介导的低血糖的CRR缺陷，并且小鼠显示下丘脑中Mc 4 r和Crh mRNA水平增加，表明肾GLUT 2对下丘脑黑皮质素系统的影响。我们的初步数据还表明，肾脏特异性Glut 2基因敲除小鼠低血糖后，肾脏（而非肝脏）的造血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MC 4 R表达CRH神经元被传入肾神经激活，以响应葡萄糖不足，从而提高血浆肾上腺素，这反过来又增加了肾脏的新生血管和恢复血糖水平。此外，糖尿病患者肾脏GLUT 2水平升高会损害表达MC 4 R的CRH神经元的这种激活，从而损害对低血糖的反调节反应。我们将测试假设与以下目标，目标1：确定下丘脑MC 4 R表达CRH神经元的作用，整合信号从肾传入神经增加血浆肾上腺素响应低血糖。目标2：确定分子和综合（中枢与外周）机制--肾脏GLUT 2慢性升高、肾脏传入神经活动减少和/或下丘脑MC 4 R表达CRH神经元的激活减弱--糖尿病通过这些机制损害身体防御低血糖的能力。目标3：确定肾脏新生血管作为葡萄糖来源对预防低血糖的贡献，并确定肾上腺素增加葡萄糖重吸收和肾脏新生血管的肾脏肾上腺素能受体。总而言之，该项目将可能确定一种新的神经内分泌回路，该回路是下丘脑、肾脏和肾上腺之间协调全身葡萄糖稳态的串扰的基础。这种整合回路可能会告知糖尿病如何在面对葡萄糖缺乏时损害身体增加血浆肾上腺素的能力的分子基础，导致低血糖相关的自主神经功能衰竭，这是1型或晚期2型糖尿病患者的危及生命的病症。

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.