Changes in phosphate metabolism cause pathologic cardiac remodeling in chronic kidney disease (CKD)

摘要 成纤维细胞生长因子23是一种骨源性激素，通过成纤维细胞生长因子受体(FGFR)靶向肾脏。 和Klotho，一种跨膜蛋白(MKL)，作为FGF23的共同受体，从而增加肾脏 排磷和降低血磷水平。在慢性肾病(CKD)患者中， FGF23-反应性和磷酸盐重吸收受损，导致血清磷酸盐增加 在骨骼中的浓度和FGF23的产生。临床研究表明，血清FGF23水平升高 与慢性肾脏病的负性结局密切相关，如心肌肥厚和心血管疾病 死亡率。我们的翻译工作表明，循环中的FGF23可以直接导致组织损伤，即 与慢性肾脏病有关。通过激活成纤维细胞生长因子受体4和随后的磷脂酶C 磷脂酶C/钙调神经磷酸酶/活化T细胞核因子在心肌细胞中的信号转导 啮齿类动物的心肌肥大和纤维化。这种病理效应独立于Klotho发生，而Klotho不是 表达在内心深处。Klotho也以截断的可溶性形式(‘SKL’)存在，它是由 MKL，并从肾脏释放。实验研究表明，SKL具有组织保护作用，包括 心脏中的抗肥大和抗纤维化作用，类似于活性维生素D(‘1，25D’)，它也起到 心脏保护荷尔蒙。在CKD患者中，血清SKL和1，25D水平显著降低，并且 据认为，SKL和1，25D的保护作用的丧失导致了CKD相关的组织损伤。自.以来 我们已发表的和初步的工作表明SKL和1，25D可以抑制FGF23诱导的信号转导和 在培养的心肌细胞肥大，我们假设SKL和1，25D减弱病理作用。 FGF23在心脏。在目标1中，我们将在高腺嘌呤饮食(AD)的小鼠中进行研究，这是一种高水平CKD的模型。 发生心肌肥厚的FGF23，AAV递送的全身SKL升高是否会减弱心脏 肾脏特异性Klotho缺失小鼠模型中是否存在SKL的肥大，反之亦然 加重了心脏损伤。在分离的心肌细胞中，我们将研究SKL抗心肌损伤的机制。 并确定SKL是否通过与FGF23相互作用而阻断FGF23与FGFR4和/或 FGFR4激活。在目标2中，我们将确定1，25D对小鼠的心脏保护作用 AD，以及具有结构性FGFR4激活和心肌肥厚的转基因小鼠。我们会 还研究了心脏特异性维生素受体(VDR)自发缺失小鼠的IF 心肌肥厚，AD或渗透压小泵输注引起的血清FGF23水平升高 重组FGF23加重心脏损伤。我们推测，通过研究的机制，升高的血清 FGF23和血清SKL和1，25D水平降低是CKD的三个临床特征，协同作用 与慢性肾脏病相关的心脏损伤。药物干预的组合激活VDR，升高 循环SKL和阻断FGF23/FGFR4信号通路可能对CKD患者有心脏保护作用。

ABSTRACT Fibroblast growth factor (FGF) 23 is a bone-derived hormone that targets the kidney via FGF receptors (FGFR) and klotho, a transmembrane protein (‘mKL’) that acts as an FGF23 co-receptor, thereby increasing renal phosphate excretion and lowering serum phosphate levels. In patients with chronic kidney disease (CKD), FGF23-responsiveness and phosphate reabsorption are impaired, leading to increased serum phosphate concentrations and FGF23 production in bone. Clinical studies have shown that elevated serum FGF23 levels are strongly associated with negative outcomes in CKD, such as cardiac hypertrophy and cardiovascular mortality. Our translational work indicates that circulating FGF23 can directly contribute to tissue injury that is associated with CKD. By activating FGF receptor (FGFR) 4 and subsequent phospholipase C (PLC)/calcineurin/nuclear factor of activated T cells (NFAT) signaling in cardiac myocytes, FGF23 induces cardiac hypertrophy and fibrosis in rodents. This pathologic effect occurs independently of klotho that is not expressed in the heart. Klotho also exists in a truncated soluble form (‘sKL’) that is generated by proteolysis of mKL and released from the kidney. Experimental studies indicate that sKL has tissue-protective effects, including anti-hypertrophic and anti-fibrotic actions in the heart, similar to active vitamin D (‘1,25D‘), which also acts as a cardio-protective hormone. In patients with CKD, serum levels of sKL and 1,25D are significantly reduced, and it is thought that a loss of sKL’s and 1,25D’s protective effects contributes to CKD-associated tissue injury. Since our published and preliminary work indicates that sKL and 1,25D can inhibit FGF23-induced signaling and hypertrophy in cultured cardiac myocytes, we hypothesize that sKL and 1,25D attenuate pathologic actions of FGF23 in the heart. In Aim 1, we will study in mice on an adenine-rich diet (AD), a model for CKD with high FGF23 that develops cardiac hypertrophy, whether systemic sKL elevations by AAV delivery attenuate cardiac hypertrophy, and vise-versa, whether the absence of sKL in a mouse model with kidney-specific klotho deletion accelartes cardiac injury. In isolated cardiac myocytes, we will study the mechanism underlying sKL’s anti- hypertrophic effects and determine if by interacting with FGF23, sKL can block FGF23 binding to FGFR4 and/or FGFR4 activation. In Aim 2, we will determine if administration of 1,25D has cardio-protective effects in mice on AD, as well as in genetically modified mice with constitutive FGFR4 activation and cardiac hypertrophy. We will also study if in mice with cardiac-specific deletion of the vitamin receptor (VDR) which spontaneously develop cardiac hypertrophy, the elevation of serum FGF23 levels by AD or by osmotic minipump infusions of recombinant FGF23 aggravates cardiac injury. We postulate that via the studied mechanism, elevated serum FGF23, and reduced serum levels of sKL and 1,25D, three clinical hallmarks of CKD, synergistically contribute to CKD-associated cardiac injury. A combination of pharmacological interventions activating VDR, elevating circulating sKL and blocking FGF23/FGFR4 signaling might have cardio-protective effects in patients with CKD.