TRPV4 Dysfunction Promotes Renal Cystogenesis in Autosomal Recessive Polycystic Kidney Disease

The molecular mechanism of cyst formation and expansion in autosomal recessive polycystic kidney disease (ARPKD) is poorly understood, but impaired mechanosensitivity to tubular flow and dysfunctional calcium signaling are important contributors. The activity of the mechanosensitive Ca²⁺-permeable TRPV4 channel underlies flow-dependent Ca²⁺ signaling in murine collecting duct (CD) cells, suggesting that this channel may contribute to cystogenesis in ARPKD. Here, we developed a method to isolate CD-derived cysts and studied TRPV4 function in these cysts laid open as monolayers and in nondilated split-open CDs in a rat model of ARPKD. In freshly isolated CD-derived cyst monolayers, we observed markedly impaired TRPV4 activity, abnormal subcellular localization of the channel, disrupted TRPV4 glycosylation, decreased basal [Ca²⁺]_i, and loss of flow-mediated [Ca²⁺]_i signaling. In contrast, nondilated CDs of these rats exhibited functional TRPV4 with largely preserved mechanosensitive properties. Long-term systemic augmentation of TRPV4 activity with a selective TRPV4 activator significantly attenuated the renal manifestations of ARPKD in a time-dependent manner. At the cellular level, selective activation of TRPV4 restored mechanosensitive Ca²⁺ signaling as well as the function and subcellular distribution of TRPV4. In conclusion, the functional status of TRPV4, which underlies mechanosensitive Ca²⁺ signaling in CD cells, inversely correlates with renal cystogenesis in ARPKD. Augmenting TRPV4 activity may have therapeutic potential in ARPKD.Polycystic kidney disease (PKD) is a cohort of monogenic disorders that result in development and subsequent growth of renal cysts filled with fluid.^1–5 Cyst enlargement compromises function of surrounding nephrons and progresses to ESRD.^1,6 In the more common form of PKD, autosomal dominant PKD (ADPKD), which is caused by mutations of polycystin 1 (PC1) and polycystin 2 (PC2), renal cysts are formed along the full length of the nephron with prevalence to the collecting duct (CD).^1,7 In the rarer and more severe autosomal recessive PKD (ARPKD), renal cyst formation is virtually restricted to the CD.^1,2,5,8 Mutations of the PKHD1 gene encoding fibrocystin underlie the genetic basis of the disease.^6,8,9 Although the exact function of the protein is unknown, fibrocystin was shown to be expressed in primary cilia where it can interact and form complexes with PC2, possibly participating in mechanotransduction.^10–12It is accepted that the CD cells elevate [Ca²⁺]_i in response to mechanical stress arising from variations in tubular flow or tubular composition.^13–23 Impaired mechanosensitive [Ca²⁺]_i responses, reported for both cultured ADPKD²⁴ and ARPKD^25,26 cells, point to a possible fundamental role of disrupted [Ca²⁺]_i signaling in cystogenesis. The central cilia and cilia-associated PC1 and PC2 were proposed to mediate flow-induced cellular responses.^19,27 However, homomeric PC2 channels are not mechanosensitive and fail to increase [Ca²⁺]_i in response to flow and hypotonicity.^28,29 Furthermore, intercalated cells, which lack primary cilia, respond to flow changes with comparable increases in [Ca²⁺]_i as observed in principal cells, which have primary cilia.^16,30 Therefore, additional mechanisms conferring mechanosensitivity to the CD cells need to be considered.Transient receptor potential (TRP) channels are known to participate in cellular responses to a variety of environmental stimuli, including thermosensation, chemosensation, and mechanical forces (reviewed in Song and Yuan³¹). Several TRP channels, including TRPC3, TRPC6, and TRPV4, can be detected in the native CD cells and CD-originated cultured lines.^19,32–34 Among these channels, TRPV4 has routinely been shown to be activated by mechanical stimuli.^34–38 Indeed, we documented that endogenous TRPV4 in M-1 CD cells is stimulated by increases in flow, a response that is abolished by TRPV4 small interfering RNA knockdown.^34,38 We further demonstrated a lack of flow-mediated [Ca²⁺]_i elevations in CD from TRPV4−/− mice.³⁰ Consistently, flow-mediated Ca²⁺-dependent K⁺ secretion in the CD is disrupted in TRPV4 knockout animals.³⁹ TRPV4 directly interacts with PC2 to form mechanosensitive heteromeric complexes.^28,29 The fact that PC2 interacts with both PC1⁴⁰ and fibrocystin^10–12 suggests that TRPV4 could be an essential part of this mechanotransducing sensory complex.Current PKD management is directed toward pharmacologic interference with abnormal signaling pathways causing exaggerated cell proliferation, dedifferentiation, apoptosis, and cyst growth.⁴¹ Specifically, PKD is associated with elevated circulating vasopressin levels, increased basal cellular cAMP levels, and strong upregulation of cAMP-dependent fluid secretion and proliferation.^42,43 V2 antagonism greatly diminishes disease progression in rodent models of both ADPKD and ARPKD.^42,44 Elevated cAMP levels might be directly related to the reduced [Ca²⁺]_i, possibly due to impaired ability to sense changes in flow.^42,44,45 This raises the possibility that manipulation with the mechanosensitivity in the CD along the TRPV4 axis modulates [Ca²⁺]_i signalization and, in turn, renal cystogenesis.In this study, we developed a new approach to isolate native CD-derived cyst monolayers and nondilated CDs from a rat model of ARPKD to thoroughly investigate how functional TRPV4 status determines the development and growth of renal cysts. We found that the disease leads to disruption of mechanosensitive [Ca²⁺]_i signaling and impaired TRPV4 activity specifically in CD cysts but not in nondilated CDs. Long-term pharmacologic potentiation of TRPV4 activity gradually restores mechanosensitivity in cyst cells and greatly blunts renal ARPKD progression. From a global prospective, this study establishes a temporal link between disruption of TRPV4-based mechanosensitivity in the CD and cystogenesis. This also suggests pharmacologic potential of targeting TRPV4 activity as a treatment strategy in retarding development of ARPKD.     

Morbidity from Congenital Hepatic Fibrosis after Renal Transplantation for Autosomal Recessive Polycystic Kidney Disease

Presentation of autosomal recessive polycystic kidney disease (ARPKD) ranges from severe renal impairment and a high mortality rate in infancy to older children and adolescents with minimal renal disease and complications of congenital hepatic fibrosis (CHF), cholangitis and portal hypertension. Renal transplantation improves prognosis but it is unclear whether CHF in transplanted children follows the same clinical course as in older children with less severe renal disease. The aim of this study was to evaluate morbidity from CHF in ARPKD post renal transplantation. Data were analyzed for six males and eight females, transplanted for ARPKD (mean age 8.3 years, range 1-22.3 years) at the University of Minnesota between 1972 and 1998. Follow-up was for a mean of 14.5 years (range 3.1-33.6 years). One and 5 years patient survival rates were 93% and 86%, respectively. Overall five patients (36%) died; 4/5 deaths were related to CHF. Causes of death were hepatic failure immediately post transplant (n = 1), septicemia related to bile duct dilatation (n = 3) and multiorgan failure (n = 1). One and 5years graft survival rates were 87% and 70%, respectively. One patient had a combined liver-kidney transplant and two were re-transplanted. Initial signs of CHF were splenomegaly (n = 5), hepatosplenomegaly (n = 4) and gastrointestinal bleed (n = 2). Progression of CHF through childhood included hypersplenism (n = 7), esophageal varices with gastrointestinal bleeding (n = 5) and bile duct dilatation (n = 5). Portal hypertension was treated with portosystemic shunt (n = 3), sclerotherapy (n = 2), banding of varices (n = 1) and transjugular intrahepatic portosystemic shunt (n = 1). Of the nine survivors (mean age 12.8 years) 78% have functioning grafts (one liver-kidney transplant), 63% have portal hypertension and 22% have asymptomatic biliary dilatation. Complications of CHF developed in 79% of children who received a renal transplant for ARPKD. Mortality related to CHF occurred in 29% and accounted for 80% (4/5) of the deaths.     

Native Nephrectomy in Renal Transplant Recipients With Autosomal Dominant Polycystic Kidney Disease

Introduction

Patients with autosomal dominant polycystic kidney disease (ADPKD) represent about 10% of kidney transplant recipients (KTR) and have unique needs regarding acceptance for this procedure. Whether native kidney nephrectomy (NKN) affects kidney transplantation (KT) outcomes remains a matter of debate, and more data is needed to establish a standard approach to KTR with ADPKD.

Adrenal Insufficiency Due to Isolated Adrenocorticotropin Deficiency Complicated by Autosomal Recessive Polycystic Kidney Disease

We describe a 29-old-year Japanese man with autosomal recessive polycystic kidney disease who was frequently hypoglycemic. Insulinoma as a cause of hypoglycemia was denied because the ratio of plasma immunoreactive insulin to glucose was low. Adrenal insufficiency was diagnosed because of the low urinary excretion of 17-hydroxycorticosteroids, and both blunted responses of plasma cortisol to an intravenous injection of adrenocorticotropin and of plasma adrenocorticotropin to an intravenous injection of human corticotropin releasing hormone were observed, although basal plasma concentrations of cortisol and adrenocorticotropin were normal. The elusion profile of plasma sample from our patient chromatographed on a Sephadex G-75 column showed two peaks of (1-39)-ACTH and β-lipotropin, with no evidence of high molecular weight form of ACTH. The plasma concentrations of thyroid stimulating hormone and growth hormone were within the normal range. These findings indicated that this patient with autosomal recessive polycystic kidney disease was associated with adrenal insufficiency due to isolated adrenocorticotropin deficiency.     

肾脏体积指标在监测成人型多囊肾病病情进展中的应用

成人型多囊肾病又称常染色体显性遗传性多囊肾病(ADPKD)是最常见的单基因遗传性肾病,发病率约为1/1000～1/500[1,2].     

mTOR抑制剂在ADPKD中的应用

常染色体显性多囊肾病(autosomal dominant polycystic kidney disease,ADPKD)是人类最常见的遗传性肾病,其发病率为0.1%～2.5%[1],就诊多见于成人,其临床表现主要有肾脏体积增大、血尿、蛋白尿、高血压.     

Imaging-Based Diagnosis of Autosomal Dominant Polycystic Kidney Disease

The clinical use of conventional ultrasonography (US) in autosomal dominant polycystic kidney disease (ADPKD) is currently limited by reduced diagnostic sensitivity, especially in at-risk subjects younger than 30 years of age. In this single-center prospective study, we compared the diagnostic performance of MRI with that of high-resolution (HR) US in 126 subjects ages 16–40 years born with a 50% risk of ADPKD who underwent both these renal imaging studies and comprehensive PKD1 and PKD2 mutation screening. Concurrently, 45 healthy control subjects without a family history of ADPKD completed the same imaging protocol. We analyzed 110 at-risk subjects whose disease status was unequivocally defined by molecular testing and 45 unaffected healthy control subjects. Using a total of >10 cysts as a test criterion in subjects younger than 30 years of age, we found that MRI provided both a sensitivity and specificity of 100%. Comparison of our results from HR US with those from a previous study of conventional US using the test criterion of a total of three or more cysts found a higher diagnostic sensitivity (approximately 97% versus approximately 82%) with a slightly decreased specificity (approximately 98% versus 100%) in this study. Similar results were obtained in test subjects between the ages of 30 and 40 years old. These results suggest that MRI is highly sensitive and specific for diagnosis of ADPKD. HR US has the potential to rival the diagnostic performance of MRI but is both center- and operator-dependent.     

Predictors of Autosomal Dominant Polycystic Kidney Disease Progression

Autosomal dominant polycystic kidney disease is a genetic disorder associated with substantial variability in its natural course within and between affected families. Understanding predictors for rapid progression of this disease has become increasingly important with the emergence of potential new treatments. This systematic review of the literature since 1988 evaluates factors that may predict and/or effect autosomal dominant polycystic kidney disease progression. Predicting factors associated with early adverse structural and/or functional outcomes are considered. These factors include PKD1 mutation (particularly truncating mutation), men, early onset of hypertension, early and frequent gross hematuria, and among women, three or more pregnancies. Increases in total kidney volume and decreases in GFR and renal blood flow greater than expected for a given age also signify rapid disease progression. Concerning laboratory markers include overt proteinuria, macroalbuminuria, and perhaps, elevated serum copeptin levels in affected adults. These factors and others may help to identify patients with autosomal dominant polycystic kidney disease who are most likely to benefit from early intervention with novel treatments.