FGF-23 REGULATION OF PHOSPHATE HOMEOSTASIS

描述(由申请人提供)：由于磷酸盐稳态受损而导致的代谢性骨病的病理生理学机制尚未完全了解。研究矿物质代谢紊乱引起的疾病的分子病因学有助于确定新的磷酸盐稳态循环调节因子，统称为“磷酸钙素”。为了分离常染色体显性遗传性低磷血症性软骨病(ADHR)的基因，我们用定位克隆的方法鉴定了磷酸化成纤维细胞生长因子-23(FGF23)，ADHR的特征是继发性低磷血症，继发于肾脏磷酸盐消耗、软骨病和骨折。在当前赠款周期的基础上，我们团队发现了一种与FGF23升高相关的新型隐性磷酸盐消耗障碍的分子基础，并确定ADHR的镜像障碍--家族性肿瘤钙质沉着症(TC)--是由隐性FGF23功能突变丧失引起的。在Klotho(KL)缺失的小鼠中也存在TC表型，我们的数据表明FGF23通过这种受体样分子诱导其生物学作用。然而，介导FGF23依赖的生物活性的KL亚型尚不清楚。我们使用携带人类ADHR FGF23突变的新型敲击小鼠的初步发现表明，存在独特的骨骼变化，为了解FGF23在体内的功能提供了新的见解。尽管已经在确定FGF23在正常情况下和疾病中的作用方面取得了重大进展，但FGF23控制磷酸盐转运的机制仍然不清楚。因此，在这项建议中要检验的中心假设是：FGF23是由骨细胞以一种受调控的方式分泌的，并通过特定的靶分子来控制肾脏中的磷酸盐稳态。我们将通过以下具体目标来解决这一假说：使用小鼠敲打模型测试ADHR的分子机制；确定KL在FGF23在肾脏近端小管和远端小管作用中的作用；以及测试KL亚型在体内指导FGF23生物活性的能力。 从长远来看，这些研究将为与FGF23表达变化相关的综合征的发病机制、更常见的磷酸盐稳态紊乱以及磷酸盐稳态的基础生物学提供关键的洞察力。

DESCRIPTION (provided by applicant): The pathophysiological mechanisms that underlie metabolic bone diseases due to impaired phosphate homeostasis are incompletely understood. Investigating the molecular etiology of disorders caused by disturbed mineral metabolism has been instrumental in identifying new circulating regulators of phosphate homeostasis, collectively referred to as `phosphatonins.' We identified the phosphatonin Fibroblast growth factor-23 (FGF23) in a positional cloning approach to isolate the gene for autosomal dominant hypophosphatemic rickets (ADHR), characterized by hypophosphatemia secondary to renal phosphate wasting, rickets/osteomalacia and fracture. Building upon this work during the current grant cycle, our group discovered the molecular basis for a novel recessive phosphate wasting disorder associated with elevated FGF23, as well as determined that the mirror-image disorder to ADHR, familial tumoral calcinosis (TC), is caused by recessive FGF23 loss of function mutations. A TC phenotype is also present in the Klotho (KL)-null mouse, and our data demonstrate that FGF23 elicits its biological actions through this receptor-like molecule. However, the KL isoform that mediates FGF23-dependent bioactivity is unknown. Our preliminary findings using a novel knock in mouse carrying a human ADHR Fgf23 mutation demonstrate the presence of unique skeletal alterations, providing new insight into FGF23 function in vivo. Although significant progress has been made determining the roles of FGF23 in normal circumstances and in disease, the mechanisms whereby FGF23 controls phosphate handling remain unclear. Thus, the central hypothesis to be tested within this proposal is: FGF23 is secreted from bone cells in a regulated manner and acts through specific target molecules to control phosphate homeostasis in the kidney. We will address this hypothesis by undertaking the following specific aims: To test the molecular mechanisms underlying ADHR using a murine knock in model; to determine the role of KL in the renal proximal tubule and distal tubule actions of FGF23; and to test the KL isoforms for the ability to direct FGF23 bioactivity in vivo. These investigations will, over the long term, provide critical insight into the pathogenesis of syndromes associated with altered FGF23 expression, into more common disorders of disturbed phosphate homeostasis, as well as into the basic biology of phosphate homeostasis.