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通过肾上腺素和肾脏GLUT2影响血糖，从而调节血糖水平。此外，我们以前已经证明，糖尿病降低了下丘脑Mc4r的表达，从而导致小鼠对葡萄糖缺乏的缺陷反调节反应(CRR)。这些具有临床意义的观察结果背后的分子和综合机制仍不清楚。具体地说，下丘脑MC4R如何调节血浆肾上腺素水平？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全身葡萄糖稳态的调节？在我们的初步研究中，我们观察到促肾上腺皮质激素释放激素(CRH)神经元中MC4R的选择性恢复使其他MC4R缺陷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，肾脏特异性Glut2基因敲除小鼠受到保护，从糖尿病介导的CRR缺陷到低血糖，并且小鼠下丘脑Mc4r和CRH mRNA水平增加，这表明肾脏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，在肾脏特异性Glut2基因敲除小鼠低血糖后，肾脏而不是肝脏的糖异生对恢复正常血糖水平起到了作用。基于这些初步发现，我们假设下丘脑MC4R表达CRH的神经元被肾传入神经激活，以响应葡萄糖缺乏而增加血浆肾上腺素，进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水平升高损害了表达MC4R的CRH神经元的这种激活，从而削弱了对低血糖的反调节反应。我们将以以下目的验证这些假说：目的1：确定下丘脑表达MC4R的CRH神经元在整合来自肾传入神经的信号以增加血浆肾上腺素对低血糖反应的作用。目的：确定糖尿病损害机体抵抗低血糖的分子和综合机制--肾脏GLUT2慢性升高，肾传入神经活性降低，和/或表达MC4R的CRH神经元激活迟钝。目的：确定肾脏糖异生作为葡萄糖来源对抵抗低血糖的作用，并确定肾上腺素通过何种受体促进葡萄糖重吸收和肾脏糖异生。总而言之，这个项目可能会确定一个新的神经内分泌回路，在下丘脑、肾脏和肾上腺之间的串扰下，以协调全身葡萄糖稳态。这一整合回路可能会为糖尿病的分子基础提供信息，即糖尿病如何在葡萄糖缺乏导致低血糖相关自主神经衰竭的情况下损害人体增加血浆肾上腺素的能力，而低血糖相关自主神经衰竭是1型或晚期2型糖尿病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.