Molecular Basis of Mammalian Manganese Homeostasis

金属对人类健康至关重要，但过量可能有毒。我们对金属稳态的理解主要来自对金属缺乏和过量的遗传性疾病的研究。相对于其他金属，锰（Mn）稳态的分子基础知之甚少，因为遗传性锰相关疾病最近才被发现。2012年，在Mn过量、肝脏和神经系统疾病、红细胞计数增加和促红细胞生成素过量的患者中报告了金属输出蛋白SLC30A10的突变。锰过量是由于胆锰排泄受损，而肝脏，神经系统和血液系统的缺陷归因于锰毒性。利用我们在哺乳动物金属体内平衡方面的专业知识，我们最近证明了鼠Slc30a10不仅对于全身和胆汁Mn排泄是必需的，而且对于Mn通过小肠输出到胃肠道内腔中也是必需的（Mercadante等人，J Clin Invest.，2019年）。这表明SLC 30 A10在Mn稳态中的作用比预期的更复杂，并且肝外器官有助于Mn排泄。在探索SLC30A10缺陷中Mn过量的机制基础时，我们还观察到，尽管存在严重的Mn过量，但Slc30A10缺陷小鼠的Mn吸收却矛盾地增加。我们的初步数据表明，这一令人惊讶的观察结果背后有一个多步骤模型。首先，过量的锰异常地刺激肝脏表达激素促红细胞生成素。其次，促红细胞生成素过量间接抑制了铁调素激素的表达，铁调素是膳食铁吸收的抑制剂。第三，铁调素缺乏导致Mn吸收增加。由于铁调素在Mn体内平衡中的作用还没有很好地建立，我们的模型中的最后一个新步骤是基于我们的观察，即铁调素表达的药理学刺激降低了Slc30a10缺陷小鼠中的Mn水平。我们认为，SLC30A10缺乏症是一个独特的病理生理背景下，铁调素发挥了意想不到的，但关键的作用，锰过量的发展。该提案的目的是建立铁调素和SLC 30A10缺乏症中Mn吸收之间的机制联系，并探索铁调素表达的药理学刺激作为SLC 30A10缺乏症和其他病症中Mn过量的治疗。这将通过遗传、代谢、药理学、放射性同位素、饮食和显微镜技术来实现。拟议的研究将使我们不仅能够进一步建立哺乳动物锰稳态的分子基础，但也设计锰相关疾病的治疗策略的基础上，我们了解锰稳态的潜在机制。

Metals are essential for human health yet potentially toxic in excess. Our understanding of metal homeostasis stems prominently from studies of inherited diseases of metal deficiency and excess. The molecular basis of manganese (Mn) homeostasis is poorly understood relative to other metals, given that inherited Mn-related diseases were only recently identified. In 2012, mutations in a metal export protein SLC30A10 were reported in patients with Mn excess, liver and neurologic disease, increased red blood cell counts, and erythropoietin excess. Mn excess was attributed to impaired biliary Mn excretion, while liver, neurologic, and hematologic defects were ascribed to Mn toxicity. Employing our expertise in mammalian metal homeostasis, we recently demonstrated that murine Slc30a10 is not only essential for systemic and biliary Mn excretion but also for Mn export by the small intestine into the lumen of the gastrointestinal tract (Mercadante et al., J Clin Invest., 2019). This indicates that the role of SLC30A10 in Mn homeostasis is more complicated than anticipated and that extrahepatic organs contribute to Mn excretion. In exploring the mechanistic basis of Mn excess in SLC30A10 deficiency, we also observed that Slc30a10-deficient mice develop paradoxically increased Mn absorption despite severe Mn excess. Our preliminary data suggest a multi-step model underlying this surprising observation. First, excess Mn aberrantly stimulates the liver to express the hormone erythropoietin. Second, erythropoietin excess indirectly suppresses expression of the hormone hepcidin, an inhibitor of dietary iron absorption. Third, hepcidin deficiency leads to increased Mn absorption. As a role for hepcidin in Mn homeostasis is not well-established, this last novel step in our model is based upon our observation that pharmacologic stimulation of hepcidin expression decreases Mn levels in Slc30a10-deficient mice . We propose that SLC30A10 deficiency is a unique pathophysiological context in which hepcidin plays an unexpected but pivotal role in development of Mn excess. The goal of this proposal is to establish the mechanistic link between hepcidin and Mn absorption in SLC30A10 deficiency and to explore pharmacologic stimulation of hepcidin expression as a treatment for Mn excess in SLC30A10 deficiency and other conditions. This will be accomplished using genetic, metabolic, pharmacologic, radioisotopic, dietary, and microscopic techniques. The proposed studies will enable us to not only further establish the molecular basis of mammalian Mn homeostasis but also to design therapeutic strategies for Mn-related diseases based upon our understanding of underlying mechanisms of Mn homeostasis.