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

描述(由申请人提供)：这项提案是我们为确定FGF23在活体中的生理作用而提供的一项竞争性更新。FGF23调节磷酸盐动态平衡和维生素D代谢。人类血清FGF23水平的变化会导致软骨病、骨软化或肿瘤钙质沉着。我们建立了FGF23-/-小鼠来确定FGF23的表达模式和功能。FGF23-/-小鼠生长严重迟缓，表现为高磷血症、高维生素D、多发性软组织钙化和异位骨结节形成、骨量减少、类骨质症、性腺功能低下和寿命短。我们将这只小鼠与其他转基因系杂交，以展示FGF23生理学的重要方面，包括1)高水平的FGF23导致人类疾病X连锁低磷血症小鼠模型中的软骨病，2)FGF23通过肾脏中的钠磷共转运体NaPi2a调节血磷水平，3)全身性FGF23是磷酸盐稳态的调节者，但在骨骼中也具有自分泌功能，以及4)维生素D是FGF23信号的必需中介。我们扩展了我们的研究，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的和令人兴奋的结果，我们将在我们拟议的研究中扩展这些结果。首先，我们将使用改变的PTH水平定义彼此独立的FGF23和Klotho函数。我们已经获得了令人兴奋的数据，表明完全消融甲状旁腺素可以挽救Klotho-/-小鼠的类骨症和矿化缺陷，但在FGF23-/-小鼠中却无法做到这一点。此外，我们发现FGF23-/-/PTH-/-与Klotho-/-/PTH-/-小鼠的甲状旁腺大小有显著差异，尽管相似的血清生化表明FGF23对甲状旁腺的影响不同于Klotho。此外，我们将研究间歇和连续给药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，这些小鼠的骨桥蛋白水平相对正常。我们将探讨矿化缺陷的机制。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地检查Klotho在肾脏和甲状旁腺中的作用，以评估Klotho在这些组织中表达的相对重要性和独立影响。条件性Klotho fl/fl小鼠将受到挑战，要么将它们与Hyp小鼠杂交，给它们注射FGF23蛋白，要么给它们喂低钙饮食。我们建议的研究旨在提供关于FGF23、PTH和Klotho的作用的新数据，其分辨率比目前可能的更高。这项研究将提供关于骨骼、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的全新信息。

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.