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疾病，主要发生在老年人。在美国，每10万人中就有15到30人受到PHPT的影响，随着美国平均人口老龄化，总发病率一直在稳步上升。这种疾病的生化定义是肿瘤性甲状旁腺组织分泌甲状旁腺激素(PTH)的结构性增加，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的中枢生理缺陷--未能维持钙稳态--已经得到了很好的认识，但推动这种全身功能障碍的潜在分子机制尚未完全确定。在正常和肿瘤甲状旁腺组织中研究钙敏感的一个主要限制是缺乏实验上容易处理的模型系统来真实地再现完整器官的动态钙反应行为。为了解决这一问题，我们的团队开发了一系列体外完整的组织检测方法，可以根据环境钙浓度的动态变化对甲状旁腺功能进行疑问性评估。通过对正常和肿瘤的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的异质性与临床表现的表型变化有关。同时，我们的团队最近利用一系列小鼠遗传模型证明了钙敏感受体(CaSR)和代谢性GAB1受体(GAB1R)之间形成的异源复合体可以减弱钙的反应性，使甲状旁腺素的分泌与环境钙感知解偶联。在PHPT患者的甲状旁腺腺瘤中发现GAB1R/CaSR复合体的丰度增加，提示GABAB1R/CaSR异二聚体的形成上调可能是肿瘤甲状旁腺组织钙敏感性减弱的原因之一。事实上，基于最近发表的神经系统工作报告，淀粉样前体蛋白(APP)的各种裂解多肽与GABAB1R结合并激活GABAB1R，我们发现小鼠甲状旁腺组织PTH的紧张性分泌通过与APP的多肽裂解产物b-淀粉样蛋白(AB)共同孵育而被刺激。我们进一步显示APP和所有参与APP相关多肽(SappA、SAPPb和Ab3个关键分泌酶)在人甲状旁腺腺瘤和小鼠甲状旁腺腺体中的高表达，暗示了一种新的旁分泌机制，可能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衍生多肽的表达，并确定它们对正常和PHPT腺瘤人类甲状旁腺组织中钙信号和PTH分泌的影响；以及(2)研究甲状旁腺特异性APP基因敲除对体内矿物质、骨骼和激素动态平衡的影响。这一全新的信号轴的发现代表了一条具有挑衅性的新概念路径，可能将APP的年龄依赖的翻译后处理与老年人PHPT发生率的增加联系起来。

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.