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

描述（申请人提供）：成纤维细胞生长因子-23（FGF-23）是最近鉴定的分子，并且涉及多种人类疾病的发病机制，包括X连锁低磷酸盐血症（XLH）、致癌性骨软化症（OOM）、常染色体显性低磷酸盐血症佝偻病（ADHR）、家族性肿瘤钙质沉着症（FTC）和慢性肾病。FGF-23是维持磷酸盐稳态和骨骼矿化的最重要和决定性因素之一。这项资助提案的长期目标是确定FGF-23在生理和病理生理条件下的体内功能和调节。作为获得这些目标的初步步骤，我们最近产生了小鼠，其中Fgf-23基因已成功地通过同源重组消融。这些Fgf-23缺失小鼠表现出高磷酸盐血症、维生素D活性增加、骨中过度矿化和软组织中异常钙化。在本申请中，我们建议使用Fgf-23缺失小鼠分析三种必需成分磷酸盐、Fgf-23和维生素D的作用和相互关系。为了确定Fgf-23的体内作用和调节，我们提出通过产生Fgf-23-/- / NaPi 2a-/-双突变小鼠来定义钠-磷酸盐共转运蛋白（NaPi）在Fgf-23缺失小鼠中异常磷酸盐稳态中的作用，如（具体目标1A）中所述。还提出了进一步的研究，以确定烟酰胺在Fgf-23缺失动物中降低血清磷酸盐的作用（特定目的1B）。我们还将调查是否循环FGF-23，完全来自？1（1）胶原蛋白（2.3kb启动子）表达成骨细胞足以挽救Fgf-23缺失动物的异常全身表型。因此，我们建议产生一种小鼠模型，其完全消除内源性FGF-23，但在成骨细胞中表达FGF-23，然后将其释放到循环中（具体目标2）。此外，我们提出通过产生和分子表征Fgf-23-/-/1a羟化酶和Fgf-23/维生素D受体双突变小鼠（Fgf-23-/-/1a（OH）ase-/-; Fgf-23-/-//DR-/-）来研究维生素D在Fgf-23介导的功能中的作用（特异性目的3A，3B）。此外，我们将研究FGF-23在正常钙/正常磷酸盐血症微环境中的体内生物活性，其独立于维生素D信号传导（特异性目的3B）。最后，我们建议使用颅骨成骨细胞和外植体在体外分析FGF-23的自分泌功能，并将研究其作为矿化抑制剂的作用（具体目标4）。成功完成这项拟议的拨款申请将产生数据，这些数据将构成设计策略的基础，以操纵患有各种疾病的患者的异常磷酸盐稳态和有缺陷的骨骼矿化，包括佝偻病，XLH，ADHR，OOM，FTC和慢性肾衰竭，使用FGF-23或其相互作用分子作为潜在的治疗工具。

DESCRIPTION (provided by applicant): Fibroblast growth factor-23 (FGF-23) is a recently identified molecule, and implicated in the pathogenesis of various human diseases, including in X-linked hypophosphatemia (XLH), oncogenic osteomalacia (OOM), autosomal dominant hypophosphatemic rickets (ADHR), familial tumor calcinosis (FTC) and chronic renal diseases. FGF-23 is one of the most important and determinant factors in maintaining phosphate homeostasis, and skeletal mineralization. The long-term objective of this grant proposal is to determine in vivo function, and regulation of FGF-23 in physiological and pathophysiological conditions. As a preliminary step of obtaining such objectives, we have recently generated mice, in which the Fgf-23 gene has been successfully ablated by homologous recombination. These Fgf-23 null mice exhibit hyperphosphatemia, increased vitamin-D activities, excessive mineralization in bone, and abnormal calcifications in the soft tissues. In this grant application, we propose to analyze the effects and interrelationship of three essential components, phosphate, Fgf-23 and vitamin-D, using Fgf-23 null mice. To determine the in vivo roles and regulations of Fgf-23, we propose to define the role of sodium-phosphate co-transporters (NaPi) in abnormal phosphate homeostasis in Fgf-23 null mice by generating Fgf-23-/- / NaPi 2a-/- double mutant mice, as outlined in (Specific Aim 1A). Further studies are also proposed to determine the effects of lowering serum phosphate by nicotinamide in Fgf-23 null animals (Specific Aim 1B). We will also investigate whether circulating FGF-23, exclusively derived from ?1(l) collagen (2.3 kb promoter) expressing osteoblasts, is sufficient to rescue the abnormal systemic phenotype of Fgf-23 null animals. We, therefore, propose to generate a mouse model that is completely ablated for endogenous Fgf-23, but expresses FGF-23 in osteoblasts which is then released into circulation (Specific Aim 2). Furthermore, we proposed to study the role of vitamin D in Fgf-23 mediated functions, by generating and molecular characterization of Fgf-23/1a hydroxylase and Fgf-23/vitamin D receptor double mutant mice (Fgf-23-/-/1a(OH)ase-/-; Fgf-23-/- / \/DR-/-) (Specific Aims 3A, 3B). In addition, we will examine the In vivo bioactivities of Fgf-23 in a normocalcemic/normophosphatemic microenvironment that are independent of vitamin D signaling (Specific Aim 3B). Finally, we propose to analyze the autocrine function of Fgf-23 in vitro using calvarial osteoblasts and explants and will investigate its role as an inhibitor of mineralization (Specific Aim 4). Successful completion of this proposed grant application would generate data that will form the basis to design strategies to manipulate abnormal phosphate homeostasis and defective skeletal mineralization in patients suffering from a wide range of diseases including rickets, XLH, ADHR, OOM, FTC, and chronic renal failure, using FGF-23 or its interacting molecules as a potential therapeutic tool.