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性疾病，主要发生在老年人。在美国，每10万人中有15至30人患有PHPT，随着国家平均人口老龄化，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，包括骨量磨损和骨折风险增加。虽然PHPT中的中枢生理缺陷-未能维持钙稳态-已被充分认识，但驱动这种全身功能障碍的潜在分子机制尚未完全表征。研究正常和肿瘤性甲状旁腺组织中钙敏感的一个主要限制是缺乏实验上易于处理的模型系统，该模型系统可以忠实地再现完整器官的动态钙反应行为。为了解决这个问题，我们的小组已经开发了一系列的离体完整组织检测，允许询问评估甲状旁腺功能的动态变化，在周围的钙浓度。通过对正常和肿瘤性人甲状旁腺组织的比较评估，我们发现甲状旁腺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，我们的研究小组最近使用一系列小鼠遗传模型证明，钙敏感受体（CaSR）和代谢型GABAB 1受体（GABAB 1 R）之间的异源复合物形成可以减弱钙的反应性，使PTH分泌与周围钙的敏感性脱钩。GABAB 1 R/CaSR复合物的丰度在PHPT患者的甲状旁腺腺瘤中增加，表明GABAB 1 R/CaSR异二聚体形成的上调可能有助于肿瘤性甲状旁腺组织中钙敏感的衰减。事实上，基于最近发表的神经系统中的工作，报告了淀粉样前体蛋白（APP）的各种裂解肽结合并激活GABAB 1 R，我们发现鼠甲状旁腺组织的PTH的紧张性分泌通过与b-淀粉样蛋白（Ab）共孵育来刺激，我们进一步显示了APP的稳健表达以及参与APP相关肽切割的所有3种关键分泌酶（sAPPa，sAPPb和Ab）在人甲状旁腺腺瘤和小鼠甲状旁腺，暗示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（1）定量各种APP衍生肽的表达并确定其对正常和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来描述这一机制。这一全新的信号轴的发现代表了一个挑衅性的新概念途径，可能将APP的年龄依赖性翻译后加工与老年人PHPT的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.