Pathophysiologic Regulation of Fgf-23 in Phosphate Homeostasis: Role of Vitamin D

描述（由申请人提供）：本申请是我们拨款的竞争性更新，以确定Fgf23在体内的生理作用。FGF23调节磷酸盐稳态和维生素D代谢。人类血清FGF23水平的变化可导致佝偻病、骨软化症或肿瘤钙质沉着症。我们生成了Fgf23-/-小鼠，以确定Fgf23的表达模式和功能。Fgf23-/-小鼠生长发育严重迟缓，表现为高磷血症、维生素D过多症、多发性软组织钙化和异位骨结节形成、骨质减少、骨质疏松、性腺功能减退、寿命短。我们将这只小鼠与其他转基因系交叉，以证明Fgf23生理学的重要方面，包括1)升高的Fgf23水平导致人类疾病x连锁低磷性佝偻病小鼠模型中的佝偻病，2)Fgf23通过肾脏中磷酸钠共转运体NaPi2a调节血清磷酸盐水平，3)全身Fgf23是磷酸盐稳态的调节剂，但在骨骼中也具有自分泌功能，4)维生素D是Fgf23信号传导的必需中介。我们已经扩展了我们的研究，以更详细地探索Fgf23， PTH和Klotho的相互关系。初步工作已经产生了新颖而令人兴奋的结果，我们将在我们提出的研究中对此进行扩展。首先，我们将使用改变的PTH水平来定义相互独立的Fgf23和Klotho函数。我们已经获得了令人兴奋的数据，表明完全消融PTH可以挽救Klotho-/-小鼠的骨质疏松症和矿化缺陷，但在Fgf23-/-小鼠中却没有这样做。此外，我们发现Fgf23-/-/PTH-/-与Klotho-/-/PTH-/-小鼠甲状旁腺的大小有显著差异，尽管相似的血清生化表明Fgf23与Klotho对甲状旁腺的作用不同。此外，我们将研究断断续续和连续给药PTH（1-34）给Fgf23-/-和Klotho-/-小鼠的结果，以检测PTH的合成代谢和分解代谢作用对骨骼的可能变化。此外，我们将研究骨桥蛋白（OPN）表达增加是否与Fgf23-/-、Klotho-/-和Fgf23-/-/PTH-/-的骨矿化缺陷有关。这种缺陷不会发生在Klotho-/-/PTH-/-小鼠身上，它们的OPN水平相对正常。探讨矿化缺陷的成因机制。其次，我们将使用我们独特的Klotho fl/fl小鼠来研究Klotho在特定组织中的作用。这将使我们能够独立检查Klotho在肾脏和甲状旁腺中的作用，以评估Klotho在这些组织中的表达的相对重要性和独立作用。条件Klotho fl/fl小鼠将通过与Hyp小鼠杂交，注入FGF23蛋白或喂食低钙饮食来挑战它们。我们提出的研究旨在提供关于Fgf23， PTH和Klotho的作用的新数据，其分辨率比迄今为止可能的更高。这项研究将为骨、甲状旁腺和肾脏如何相互沟通以调节矿物质离子稳态和骨代谢提供全新的信息。

DESCRIPTION (provided by applicant): This proposal is a competitive renewal for our grant to determine the physiological role of Fgf23 in vivo. FGF23 regulates phosphate homeostasis and vitamin D metabolism. Changes in serum FGF23 levels in humans result in rickets, osteomalacia, or tumoral calcinosis. We generated Fgf23-/- mice to determine Fgf23 expression patterns and functions. Fgf23-/- mice are severely growth-retarded and displayed hyperphosphatemia, hypervitaminosis D, multiple soft tissue calcifications and ectopic bone nodule formation, osteopenia, osteoidosis, hypogonadism, and a short life span. We crossed this mouse with other transgenic lines to demonstrate significant aspects of Fgf23 physiology, including 1) elevated Fgf23 levels cause rickets in a mouse model of the human disease X-linked hypophosphatemic rickets, 2) Fgf23 regulates serum phosphate levels through sodium-phosphate co-transporter NaPi2a in the kidney, 3) systemic Fgf23 is a regulator of phosphate homeostasis but also has autocrine functions in bone, and 4) vitamin D is a required mediator of Fgf23 signaling. We have expanded our research to explore the interrelationships of Fgf23, PTH and Klotho in more detail. Preliminary work has yielded novel and exciting results, which we will expand upon in our proposed research. First, we will define Fgf23 and Klotho functions that are independent from each other using altered PTH levels. We have obtained exciting data showing that complete ablation of PTH rescues the osteoidosis and mineralization defects in Klotho-/- mice but fails to do so in Fgf23-/- mice. Moreover, we found a marked difference in parathyroid gland size in Fgf23-/-/PTH-/- vs. Klotho-/-/PTH-/- mice despite similar serum biochemistry suggesting that Fgf23 has different effects than Klotho on parathyroid glands. Furthermore, we will examine the outcome of intermittent vs. continuous administration of PTH (1-34) to Fgf23-/- and Klotho-/- mice to detect possible changes in the bone in response to anabolic and catabolic actions of PTH. Moreover, we will investigate whether increased osteopontin (OPN) expression accounts for the bone mineralization defect in Fgf23-/-, Klotho-/-, and Fgf23-/-/PTH-/-. This defect does not occur in Klotho-/-/PTH-/- mice, which exhibit relatively normal OPN levels. We will explore the mechanism responsible for the mineralization defect. Second, we will examine the role of Klotho in specific tissues using our unique Klotho fl/fl mouse. This will allow us to independently examine the effects of Klotho in the kidney and parathyroid glands to assess the relative importance and independent effects of Klotho expression in those tissues. Conditional Klotho fl/fl mice will be challenged either by crossing them with Hyp mice, infusing them with FGF23 protein, or feeding them a low calcium diet. Our proposed research aims to provide novel data regarding the roles of Fgf23, PTH and Klotho in a finer resolution than has been possible so far. This research will provide completely new information about how bone, parathyroid gland, and kidney communicate with each other to regulate mineral ion homeostasis and bone metabolism.