Department of Physiology, Johns Hopkins University School of Medicine, Baltimore, MD, USA.
Department of Medicine, University of Maryland School of Medicine, Baltimore, MD, USA.
J Physiol. 2019 Jan;597(2):499-519. doi: 10.1113/JP276796. Epub 2018 Oct 17.
Significant and selective up-regulation of the Na /H exchanger NHA2 (SLC9B2) was observed in cysts of patients with autosomal dominant polycystic kidney disease. Using the MDCK cell model of cystogenesis, it was found that NHA2 increases cyst size. Silencing or pharmacological inhibition of NHA2 inhibits cyst formation in vitro. Polycystin-1 represses NHA2 expression via Ca /NFAT signalling whereas the dominant negative membrane-anchored C-terminal fragment (PC1-MAT) increased NHA2 levels. Drugs (caffeine, theophylline) and hormones (vasopressin, aldosterone) known to exacerbate cysts elicit NHA2 expression. Taken together, the findings reveal NHA2 as a potential new player in salt and water homeostasis in the kidney and in the pathogenesis of polycystic kidney disease.
Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in PKD1 and PKD2 encoding polycystin-1 (PC1) and polycystin-2 (PC2), respectively. The molecular pathways linking polycystins to cyst development in ADPKD are still unclear. Intracystic fluid secretion via ion transporters and channels plays a crucial role in cyst expansion in ADPKD. Unexpectedly, we observed significant and selective up-regulation of NHA2, a member of the SLC9B family of Na /H exchangers, that correlated with cyst size and disease severity in ADPKD patients. Using three-dimensional cultures of MDCK cells to model cystogenesis in vitro, we showed that ectopic expression of NHA2 is causal to increased cyst size. Induction of PC1 in MDCK cells inhibited NHA2 expression with concordant inhibition of Ca influx through store-dependent and -independent pathways, whereas reciprocal activation of Ca influx by the dominant negative membrane-anchored C-terminal tail fragment of PC1 elevated NHA2. We showed that NHA2 is a target of Ca /NFAT signalling and is transcriptionally induced by methylxanthine drugs such as caffeine and theophylline, which are contraindicated in ADPKD patients. Finally, we observed robust induction of NHA2 by vasopressin, which is physiologically consistent with increased levels of circulating vasopressin and up-regulation of vasopressin V2 receptors in ADPKD. Our findings have mechanistic implications on the emerging use of vasopressin V2 receptor antagonists such as tolvaptan as safe and effective therapy for polycystic kidney disease and reveal a potential new regulator of transepithelial salt and water transport in the kidney.
常染色体显性多囊肾病（ADPKD）患者的囊肿中观察到钠离子/氢交换器 NHA2（SLC9B2）的显著和选择性上调。在 MDCK 细胞囊肿发生模型中，发现 NHA2 增加了囊肿大小。体外沉默或药理学抑制 NHA2 可抑制囊肿形成。多囊蛋白-1 通过 Ca/NFAT 信号抑制 NHA2 的表达，而膜锚定的 C 端片段（PC1-MAT）的显性负性抑制 NHA2 水平增加。已知加重囊肿的药物（咖啡因、茶碱）和激素（血管加压素、醛固酮）可诱导 NHA2 表达。总之，这些发现揭示了 NHA2 作为肾脏中盐和水稳态以及多囊肾病发病机制的潜在新参与者。
常染色体显性多囊肾病（ADPKD）是由 PKD1 和 PKD2 编码的多囊蛋白-1（PC1）和多囊蛋白-2（PC2）突变引起的。将多囊蛋白与 ADPKD 中囊肿发育联系起来的分子途径仍不清楚。离子转运体和通道介导的囊内液分泌在 ADPKD 中囊肿扩张中起关键作用。出乎意料的是，我们观察到 SLC9B 家族的钠离子/氢交换器 NHA2 的显著和选择性上调，这与 ADPKD 患者的囊肿大小和疾病严重程度相关。使用 MDCK 细胞的三维培养来模拟体外囊肿发生，我们表明 NHA2 的异位表达是导致囊肿增大的原因。在 MDCK 细胞中诱导 PC1 会通过依赖和不依赖储存的途径抑制 Ca 内流，从而抑制 NHA2 的表达，而 PC1 的膜锚定的 C 端尾片段的显性负性激活则会升高 NHA2。我们表明 NHA2 是 Ca/NFAT 信号的靶标，并且可被甲基黄嘌呤类药物（如咖啡因和茶碱）转录诱导，这些药物在 ADPKD 患者中是禁忌的。最后，我们观察到血管加压素强烈诱导 NHA2，这与 ADPKD 中循环血管加压素水平升高和血管加压素 V2 受体上调生理上一致。我们的研究结果对新兴的血管加压素 V2 受体拮抗剂（如托伐普坦）作为多囊肾病安全有效的治疗方法具有机制意义，并揭示了肾脏中跨上皮盐和水转运的潜在新调节剂。
