Regulation of parathyroid function by the amyloid precursor protein

原发性甲状旁腺功能亢进症（PHPT）是一种常见的内分泌肿瘤性疾病，主要发生在老年人中。在美国，每 10 万人中有 15 至 30 人患有 PHPT，随着美国平均人口老龄化，这一总体发病率一直在稳步上升。该疾病的生化定义为肿瘤性甲状旁腺组织中甲状旁腺激素 (PTH) 的分泌持续升高，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 的核心生理缺陷（无法维持钙稳态）已被广泛认识，但驱动这种系统功能障碍的潜在分子机制尚未得到充分表征。研究正常和肿瘤性甲状旁腺组织中钙传感的一个主要限制是缺乏能够忠实再现完整器官的动态钙反应行为的实验上易于处理的模型系统。为了解决这个问题，我们的小组开发了一系列离体完整组织测定，可以根据环境钙浓度的动态变化对甲状旁腺功能进行质疑评估。通过对正常和肿瘤性人类甲状旁腺组织的比较评估，我们发现甲状旁腺腺瘤之间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，我们小组最近使用一系列小鼠遗传模型证明，钙传感受体 (CaSR) 和代谢型 GABAB1 受体 (GABAB1R) 之间形成的异质复合物可以减弱钙反应性，从而使 PTH 分泌与环境钙传感脱钩。发现 PHPT 患者的甲状旁腺腺瘤中 GABAB1R/CaSR 复合物的丰度增加，表明 GABAB1R/CaSR 异二聚体形成的上调可能导致肿瘤性甲状旁腺组织中钙感应的减弱。事实上，根据最近发表的神经系统研究报告，淀粉样前体蛋白 (APP) 的各种裂解肽结合并激活 GABAB1R，我们发现，与 APP 的肽裂解产物 b-淀粉样蛋白 (Ab) 共孵育可刺激小鼠甲状旁腺组织分泌 PTH。我们进一步表明，在人甲状旁腺腺瘤和鼠甲状旁腺中，APP 以及参与 APP 相关肽（sAPPa、sAPPb 和 Ab）裂解的所有 3 种关键分泌酶均具有强劲表达，这表明一种新的旁分泌机制可能会促进甲状旁腺腺瘤的功能异质性。我们建议通过以下方式描述这一机制：(1) 定量各种 APP 衍生肽的表达，并确定它们对正常和 PHPT 腺瘤人甲状旁腺组织中钙信号传导和 PTH 分泌的影响； (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条具有启发性的新概念途径，可能将年龄依赖性 APP 翻译后处理与老年人 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.