A Parathyroid Hormone/Salt Inducible Kinase Signaling Axis Controls Renal Vitamin D Activation and Organismal Calcium Homeostasis [SIK]. A Parathyroid Hormone/Salt Inducible Kinase Signaling Axis Controls Renal Vitamin D Activation and Organismal Calcium Homeostasis [SIK]

The renal actions of parathyroid hormone (PTH) promote 1,25-vitamin D generation; however, the signaling mechanisms in renal epithelial cells downstream of the PTH receptor that control vitamin D metabolism remain unknown. Here we demonstrate that Salt Inducible Kinases (SIKs) control renal 1,25-vit D production downstream of PTH signaling via regulating Cyp27b1 expression. As PTH inhibits the cellular activity of SIKs by cAMP-dependent PKA phosphorylation in bone, we hypothesized that SIKs would also work as an essential mediator of PTH doownstream signaling in kidney, thus regulate vitamin D metabolism. Whole tissue and single cell transcriptomics in kidney demonstrates that both PTH and pharmacologic SIK inhibitors regulate a vitamin D gene module in specific proximal tubule cells. Moreover, small molecule SIK inhibitors directly increase 1,25-vit D production and renal Cyp27b1 mRNA expression in mice and in human embryonic stem cell-derived kidney organoids. Global- and kidney-specific Sik2/Sik3 mutant mice show Cyp27b1 upregulation, elevated serum levels of 1,25-vit D, and PTH-independent hypercalcemia. The SIK substrate CRTC2 shows PTH and SIK inhibitor-inducible binding to key Cyp27b1 regulatory enhancers in the kidney, which are also required for SIK inhibitors to increase Cyp27b1 in vivo. Lastly, in a podocyte injury mouse model of chronic kidney disease (CKD) characterized by low 1,25-vit D levels, SIK inhibitor treatment stimulates both renal Cyp27b1 expression and 1,25-vit D production. Overall design: Kidney mRNA profiles from Ctrl and Six2 Cre-SIK1f/f;SIK2f/f,SIK3f/f mice (n= 4 per genotype) were generated by deep sequencing

甲状旁腺激素（parathyroid hormone, PTH）的肾脏生物学效应可促进1,25-维生素D的生成，但目前尚不明确PTH受体下游肾上皮细胞内的信号机制如何调控维生素D代谢。本研究证实，盐诱导激酶（Salt Inducible Kinases, SIKs）可通过调控Cyp27b1的表达，在PTH信号通路下游介导肾脏1,25-维生素D的生成。
既往研究显示，在骨骼组织中，PTH可通过环磷酸腺苷（cAMP）依赖的蛋白激酶A（PKA）磷酸化作用抑制SIKs的细胞活性；据此我们推测，SIKs同样是肾脏中PTH下游信号通路的关键介导因子，进而参与调控维生素D代谢。
肾脏整体组织与单细胞转录组学分析表明，PTH与药理学SIK抑制剂均可在特定近端肾小管细胞中调控维生素D相关基因模块。此外，小分子SIK抑制剂可在小鼠及人类胚胎干细胞衍生的肾脏类器官中，直接上调1,25-维生素D的生成水平与肾脏Cyp27b1的mRNA表达量。
全身及肾脏组织特异性Sik2/Sik3敲除小鼠模型中，可见Cyp27b1表达上调、血清1,25-维生素D水平升高，且出现不依赖PTH的高钙血症。SIK的底物CRTC2可在PTH及SIK抑制剂的诱导下，结合肾脏内Cyp27b1的关键调控增强子区域；该区域同样是SIK抑制剂在体内上调Cyp27b1表达所必需的。
最后，在以1,25-维生素D水平低下为特征的慢性肾脏病（chronic kidney disease, CKD）足细胞损伤小鼠模型中，SIK抑制剂治疗可同时促进肾脏Cyp27b1的表达与1,25-维生素D的生成。
整体实验设计：通过深度测序获取对照组与Six2 Cre-SIK1f/f;SIK2f/f;SIK3f/f小鼠（每组n=4）的肾脏mRNA转录组图谱。