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The transient receptor potential channel C6 (TRPC6) is a slit diaphragm-associated protein in podocytes involved in regulating glomerular filter function. Gain-of-function mutations in TRPC6 cause hereditary focal segmental glomerulosclerosis (FSGS), and several human acquired proteinuric diseases show increased glomerular TRPC6 expression. Angiotensin II (AngII) is a key contributor to glomerular disease and may regulate TRPC6 expression in nonrenal cells. We demonstrate that AngII regulates TRPC6 mRNA and protein levels in cultured podocytes and that AngII infusion enhances glomerular TRPC6 expression in vivo. In animal models for human FSGS (doxorubicin nephropathy) and increased renin-angiotensin system activity (Ren2 transgenic rats), glomerular TRPC6 expression was increased in an AngII-dependent manner. TRPC6 expression correlated with glomerular damage markers and glomerulosclerosis. We show that the regulation of TRPC6 expression by AngII and doxorubicin requires TRPC6-mediated Ca(2+) influx and the activation of the Ca(2+)-dependent protein phosphatase calcineurin and its substrate nuclear factor of activated T cells (NFAT). Accordingly, calcineurin inhibition by cyclosporine decreased TRPC6 expression and reduced proteinuria in doxorubicin nephropathy, whereas podocyte-specific inducible expression of a constitutively active NFAT mutant increased TRPC6 expression and induced severe proteinuria. Our findings demonstrate that the deleterious effects of AngII on podocytes and its pathogenic role in glomerular disease involve enhanced TRPC6 expression via a calcineurin/NFAT positive feedback signaling pathway.

Abstract. The family of epithelial Ca 2ϩ channels consists of two highly homologues members, TRPV5 and TRPV6, which constitute the apical Ca 2ϩ entry mechanism in active Ca 2ϩ (re)absorption in kidney and small intestine. In kidney, TRPV5 expression has been extensively studied, whereas TRPV6 localization and regulation has been largely confined to the small intestine. The present study investigated the renal distribution of TRPV6 and regulation by 1,25-dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ). In mouse kidney, TRPV6 was detected by immunohistochemistry at the apical domain of the distal convoluted tubules (DCT2), connecting tubules (CNT), and cortical and medullary collecting ducts (CD). Furthermore, several putative vitamin D-responsive elements were detected upstream of the mouse TRPV6 start codon, and 1,25(OH) 2 D 3 treatment significantly increased renal TRPV6 mRNA and protein expression. In DCT2 and CNT, TRPV6 co-localizes with the other known Ca 2ϩ transport proteins, including TRPV5 and calbindin-D 28K . Together, these data suggest a role for TRPV6 in 1,25(OH) 2 D 3 -stimulated Ca 2ϩ reabsorption in these segments. Interestingly, distribution of TRPV6 extended to the CD, where it localized to the apical domain of principal and intercalated cells, which are not generally implicated in active Ca 2ϩ reabsorption. In addition, TRPV6 mRNA levels were quantified in a large set of tissues, and in the order of decreasing expression level were detected: prostate Ͼ stomach, brain Ͼ lung Ͼ duodenum, kidney, bone, cecum, heart Ͼ colon Ͼ skeletal muscle Ͼ pancreas. Therefore, additional physiologic functions for TRPV6 are feasible. In conclusion, TRPV6 is expressed along the apical domain of DCT2, CNT, and CD, where TRPV6 expression is positively regulated by 1,25(OH) 2 D 3 .Active Ca 2ϩ absorption plays a key role in Ca 2ϩ homeostasis and takes place in Ca 2ϩ

Hepatocyte nuclear factor 1b (HNF1b)-associated disease is a recently recognized clinical entity with a variable multisystem phenotype. Early reports described an association between HNF1B mutations and maturity-onset diabetes of the young. These patients often presented with renal cysts and renal function decline that preceded the diabetes, hence it was initially referred to as renal cysts and diabetes syndrome. However, it is now evident that many more symptoms occur, and diabetes and renal cysts are not always present. The multisystem phenotype is probably attributable to functional promiscuity of the HNF1b transcription factor, involved in the development of the kidney, urogenital tract, pancreas, liver, brain, and parathyroid gland. Nephrologists might diagnose HNF1b-associated kidney disease in patients referred with a suspected diagnosis of autosomal dominant polycystic kidney disease, medullary cystic kidney disease, diabetic nephropathy, or CKD of unknown cause. Associated renal or extrarenal symptoms should alert the nephrologist to HNF1b-associated kidney disease. A considerable proportion of these patients display hypomagnesemia, which sometimes mimics Gitelman syndrome. Other signs include early onset diabetes, gout and hyperparathyroidism, elevated liver enzymes, and congenital anomalies of the urogenital tract. Because many cases of this disease are probably undiagnosed, this review emphasizes the clinical manifestations of HNF1b-associated disease for the nephrologist.

Abstract. FK506 (tacrolimus) and dexamethasone are potent immunosuppressants known to induce significant side effects on mineral homeostasis, including hypercalciuria and hypomagnesemia. However, the underlying molecular mechanisms remain unknown. The present study investigated the effects of FK506 and dexamethasone on the expression of proteins involved in active Ca 2ϩ reabsorption: the epithelial Ca 2ϩ channel TRPV5 and the cytosolic Ca 2ϩ -binding protein calbindin-D 28K . In addition, the renal expression of the putative Mg 2ϩ channel TRPM6, suggested to be involved in transcellular Mg 2ϩ reabsorption, was determined. Administration of FK506 to rats by daily oral gavage during 7 d significantly enhanced the urinary excretion of Ca 2ϩ and Mg 2ϩ and induced a significant hypomagnesemia. FK506 significantly decreased the renal mRNA expression of TRPV5 (62 Ϯ 7% relative to controls), calbindin-D 28K (9 Ϯ 1%), and TRPM6 (52 Ϯ 8%), as determined by real-time quantitative PCR analysis. Furthermore, semiquantitative immunohistochemistry showed reduced renal protein abundance of TRPV5 (24 Ϯ 5%) and calbindin-D 28K (29 Ϯ 4%), altogether suggesting that downregulation of these transport proteins is responsible for the FK506-induced Ca 2ϩ and Mg 2ϩ wasting. In contrast, dexamethasone significantly enhanced renal TRPV5 (150 Ϯ 15%), calbindin-D 28K (177 Ϯ 23%), and TRPM6 (156 Ϯ 20%) mRNA levels along with TRPV5 (211 Ϯ 8%) and calbindin-D 28K (176 Ϯ 5%) protein abundance in the presence of significantly increased Ca 2ϩ and Mg 2ϩ excretion. This indicated that these proteins are directly or indirectly regulated by dexamethasone. In conclusion, FK506 and dexamethasone induce renal Ca 2ϩ and Mg 2ϩ wasting, albeit by different mechanisms. Downregulation of specific Ca 2ϩ and Mg 2ϩ transport proteins provides a molecular mechanism for FK506-induced hypercalciuria and hypomagnesemia, whereas dexamethasone positively regulates these proteins.

Chronic metabolic acidosis results in renal Ca2؉ and Mg 2؉ wasting, whereas chronic metabolic alkalosis is known to exert the reverse effects. It was hypothesized that these adaptations are mediated at least in part by the renal Ca 2؉

Reports suggest a role of endothelial dysfunction and loss of endothelial barrier function in COVID-19. It is well established that the endothelial glycocalyx-degrading enzyme heparanase contributes to vascular leakage and inflammation. Low molecular weight heparins (LMWH) serve as an inhibitor of heparanase. We hypothesize that heparanase contributes to the pathogenesis of COVID-19, and that heparanase may be inhibited by LMWH. To test this hypothesis, heparanase activity and heparan sulfate levels were measured in plasma of healthy controls (n = 10) and COVID-19 patients (n = 48). Plasma heparanase activity and heparan sulfate levels were significantly elevated in COVID-19 patients. Heparanase activity was associated with disease severity including the need for intensive care, lactate dehydrogenase levels, and creatinine levels. Use of prophylactic LMWH in non-ICU patients was associated with a reduced heparanase activity. Since there is no other clinically applied heparanase inhibitor currently available, therapeutic treatment of COVID-19 patients with low molecular weight heparins should be explored.

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