Regulation of parathyroid function by the amyloid precursor protein

原发性甲状旁腺功能亢进（PHPT）是一种常见的内分泌肿瘤疾病，主要发生在老年人。在美国，每10万人中就有15到30人患有PHPT，随着美国平均人口老龄化，总体发病率一直在稳步上升。该疾病的生物化学定义是由肿瘤状甲状旁腺组织的甲状旁腺激素（PTH）的组成性分泌升高，导致慢性高钙血症和一系列临床后遗症，包括骨量消耗和骨折风险增加。虽然PHPT的中心生理缺陷——维持钙稳态的失败——已经得到了很好的认识，但驱动这种全身功能障碍的潜在分子机制尚未得到充分的表征。研究正常和肿瘤甲状旁腺组织中钙感知的主要限制是缺乏实验可处理的模型系统，可以忠实地再现完整器官的动态钙响应行为。为了解决这个问题，我们的团队开发了一系列的离体完整组织分析，允许对甲状旁腺功能响应环境钙浓度的动态变化进行疑问性评估。通过对正常和肿瘤人甲状旁腺组织的比较评估，我们发现甲状旁腺瘤的分子和生化功能异质性与临床表现的表型变异相关。与此同时，通过一系列小鼠遗传模型，我们的研究小组最近证明了钙敏感受体（CaSR）和代谢性GABAB1受体（GABAB1R）之间的异络合物的形成可以减弱钙的反应性，使甲状旁腺激素的分泌与环境钙敏感分离。在PHPT患者的甲状旁腺瘤中发现GABAB1R/CaSR复合物的丰度增加，提示GABAB1R/CaSR异源二聚体形成的上调可能有助于肿瘤甲状旁腺组织中钙感知的衰减。事实上，根据最近在神经系统中发表的研究报告，淀粉样蛋白前体蛋白（APP）的各种裂解肽结合并激活GABAB1R，我们发现，通过与APP的肽裂解产物b-淀粉样蛋白（Ab）共孵育，可以刺激小鼠甲状旁腺组织强直分泌PTH。我们进一步证实了APP和参与APP相关肽裂解的所有3个关键分泌酶(sAPPa, sAPPb，和Ab)在人类甲状旁腺腺瘤和小鼠甲状旁腺中的表达，暗示一种新的旁腺机制可能促进甲状旁腺瘤的功能异质性。我们建议通过(1)定量测定各种app衍生肽的表达，并确定它们对正常和PHPT腺瘤人类甲状旁腺组织中钙信号传导和甲状旁腺激素分泌的影响来描述这一机制；(2)研究甲状旁腺特异性APP敲除对体内矿物质、骨骼和激素稳态的影响。这一全新信号轴的发现代表了一个具有挑衅性的新概念途径，可能将年龄依赖性的APP翻译后加工与老年人PHPT发病率增加联系起来。

Primary hyperparathyroidism (PHPT) is a common endocrine neoplastic disorder that predominantly occurs in the elderly. PHPT affects 15 to 30 individuals per 100,000 in the U.S., an overall incidence that has been steadily rising as the nation’s mean population ages. The disease is biochemically defined by constitutively elevated secretion of parathyroid hormone (PTH) from neoplastic parathyroid tissue, leading to chronic hypercalcemia and a spectrum of clinical sequelae including bone mass attrition and increased fracture risk. While the central physiological deficit in PHPT – the failure to maintain calcium homeostasis – is well recognized, the underlying molecular mechanisms that drive this systemic dysfunction have not been fully characterized. A major limitation to studying calcium sensing in normal and neoplastic parathyroid tissue has been the lack of experimentally tractable model systems that can faithfully reproduce the dynamic calcium response behaviors of the intact organ. To address this, our group has developed a series of ex vivo intact tissue assays that allow interrogative assessment of parathyroid gland function in response to dynamic changes in ambient calcium concentration. Through comparative assessment of normal and neoplastic human parathyroid tissue, we found molecular and biochemical functional heterogeneity among parathyroid adenomas that correlated to phenotypic variations in clinical presentation. In parallel, using a series of murine genetic models our group has recently demonstrated that heterocomplex formation between the calcium sensing receptor (CaSR) and the metabotropic GABAB1 receptor (GABAB1R) can attenuate calcium responsiveness, uncoupling PTH secretion from ambient calcium sensing. The abundance of GABAB1R/CaSR complexes was found to be increased in parathyroid adenomas from patients with PHPT, suggesting that upregulation of GABAB1R/CaSR heterodimer formation could contribute to the attenuation of calcium sensing in neoplastic parathyroid tissue. Indeed, based on recently published work in neuronal systems reporting that various cleaved peptides of the amyloid precursor protein (APP) bind and activate GABAB1R, we have found that tonic secretion of PTH by murine parathyroid tissue is stimulated by co-incubation with b-amyloid (Ab), a peptide cleavage product of APP. We further showed robust expression of APP and all 3 key secretases involved in cleavage of APP-related peptides (sAPPa, sAPPb, and Ab) in human parathyroid adenomas and murine parathyroid glands, implicating a novel paracrine mechanism that may foster functional heterogeneity in parathyroid adenomas. We propose to delineate this mechanism by (1) quantitating the expression of the various APP-derived peptides and determining their impact on calcium signaling and PTH secretion in normal and PHPT adenoma human parathyroid tissue; and (2) investigating the effects of parathyroid-specific APP knockout on mineral, skeletal, and hormonal homeostasis in vivo. The discovery of this entirely novel signaling axis represents a provocative new conceptual pathway potentially linking age-dependent post-translational processing of APP to the increased incidence of PHPT in the elderly.