Control of FGF23 Bioactivity via Circulating alpha-Klotho

描述(申请人提供)：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剂，统称为磷脂酰肌醇。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。FGF23共同受体Klotho(？KL)与典型的成纤维细胞生长因子受体(FGFR)形成异构体复合体，是正常的磷酸盐代谢所必需的。强调这一点的事实是，KL功能突变的丧失导致终末器官FGF23耐药，并导致ADHR的表型互惠障碍，即家族性高磷血症性肿瘤钙质沉着症(TC)。KL被表达为一种膜结合蛋白(MKL)，它在靶组织中介导依赖于FGF23的信号转导，也是一种主要的循环物种，源于MKL在其邻近的细胞外膜结构域内的蛋白水解性裂解，从而产生“CKL”。CKL物种的生物活性尚不清楚。这里提出的新的初步研究建立了关于FGF23产生的调节的重要的、机械的联系，并显著修改了当前解释磷酸盐稳态的模型。在这一点上，将CKL注射到野生型小鼠体内可以有效地刺激骨FGF23 mRNA的产生，导致血清FGF23的高度升高。实验动物表现出严重的低磷血症、肾脏1，25(OH)2维生素D生成途径的改变和甲状旁腺功能亢进症，并伴有骨矿物质含量减少和骨折。此外，体内的血CKL浓度与磷酸盐代谢的变化相关，CKL表现出FGFR依赖的活性。值得注意的是，使用CKL治疗的小鼠是一名Klotho基因易位(t9：13)患者的生物学表型，这导致血清CKL浓度显著增加。总之，这些新的发现表明，决定FGF23产生的分子机制，以及通过KL亚型控制FGF23生物活性和表达的分子机制仍有待确定。因此，需要检验的中心假设是：KL的CKL形式通过刺激FGF23的产生来控制FGF23靶组织和骨骼之间的内分泌动态轴。我们期望我们的研究将为罕见和常见的FGF23表达改变的综合征，如CKD-MBD，以及磷酸盐代谢的基础生物学提供新的、翻译上的见解。

DESCRIPTION (provided by applicant): 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. The FGF23 co-receptor ¿Klotho (¿KL), acting in a heteromeric complex with a canonical FGF receptor (FGFR), is required for normal phosphate metabolism. This is emphasized by the fact that ¿KL loss of function mutations lead to end organ Fgf23 resistance, and cause the phenotypic reciprocal disorder to ADHR, familial hyperphosphatemic tumoral calcinosis (TC). ¿KL is expressed as a membrane-bound protein (mKL) that mediates Fgf23-dependent signaling in target tissues, as well as a major circulating species that originates from the proteolytic cleavage of mKL within its juxta extracellular membrane domain to derive 'cKL'. The biological activity of the cKL species is unknown. The novel preliminary studies presented herein make important, mechanistic connections regarding the regulation of Fgf23 production, and significantly modify the current models explaining phosphate homeostasis. In this regard, cKL delivery to wild type mice potently stimulates bone Fgf23 mRNA production, leading to highly elevated serum Fgf23. The treated animals manifest severe hypophosphatemia, alterations in renal 1,25(OH)2 vitamin D production pathways and hyperparathyroidism, with reduced bone mineral content and fractures. Further, blood cKL concentrations in vivo are correlated with changes in phosphate metabolism, and cKL demonstrates FGFR-dependent activity. Of significance, the mice treated with cKL are biological phenocopies of a patient with an ¿Klotho gene translocation (t9:13), that resulted in markedly increased serum cKL concentrations. Collectively, these new findings demonstrate that the molecular mechanisms dictating Fgf23 production, as well as the control of Fgf23 bioactivity and expression via the ¿KL isoforms remain to be defined. Thus, the central hypothesis to be tested is: the cKL form of ¿KL controls an endocrine homeostatic axis between FGF23 target tissues and the skeleton by stimulating Fgf23 production. We expect our studies will provide novel, translational insight into rare, and common syndromes of altered FGF23 expression such as CKD-MBD, and into the basic biology of phosphate metabolism.