Caspase inhibition reduces tubular apoptosis and proliferation and slows disease progression in polycystic kidney disease

Cleavage fragments of the C-terminal tail of polycystin-1 are regulated by oxidative stress and induce mitochondrial dysfunction

Mutations in the gene encoding polycystin-1 (PC1) are the most common cause of autosomal dominant polycystic kidney disease (ADPKD). Cysts in ADPKD exhibit a Warburg-like metabolism characterized by dysfunctional mitochondria and aerobic glycolysis. PC1 is an integral membrane protein with a large extracellular domain, a short C-terminal cytoplasmic tail and shares structural and functional similarities with G protein-coupled receptors. Its exact function remains unclear. The C-terminal cytoplasmic tail of PC1 undergoes proteolytic cleavage, generating soluble fragments that are overexpressed in ADPKD kidneys. The regulation, localization, and function of these fragments is poorly understood. Here, we show that a ∼30 kDa cleavage fragment (PC1-p30), comprising the entire C-terminal tail, undergoes rapid proteasomal degradation by a mechanism involving the von Hippel-Lindau tumor suppressor protein. PC1-p30 is stabilized by reactive oxygen species, and the subcellular localization is regulated by reactive oxygen species in a dose-dependent manner. We found that a second, ∼15 kDa fragment (PC1-p15), is generated by caspase cleavage at a conserved site (Asp-4195) on the PC1 C-terminal tail. PC1-p15 is not subject to degradation and constitutively localizes to the mitochondrial matrix. Both cleavage fragments induce mitochondrial fragmentation, and PC1-p15 expression causes impaired fatty acid oxidation and increased lactate production, indicative of a Warburg-like phenotype. Endogenous PC1 tail fragments accumulate in renal cyst-lining cells in a mouse model of PKD. Collectively, these results identify novel mechanisms regarding the regulation and function of PC1 and suggest that C-terminal PC1 fragments may be involved in the mitochondrial and metabolic abnormalities observed in ADPKD.

Identification of renal cyst cells of type I Nephronophthisis by single-nucleus RNA sequencing

Background: Nephronophthisis (NPH) is the most common genetic cause of end-stage renal disease (ESRD) in childhood, and NPHP1 is the major pathogenic gene. Cyst formation at the corticomedullary junction is a pathological feature of NPH, but the mechanism underlying cystogenesis is not well understood. The isolation and identification of cystic cell subpopulation could help to identify their origins and provide vital clues to the mechanisms underlying cystogenesis in NPH. Methods: Single-nucleus RNA sequencing (snRNA-seq) was performed to produce an atlas of NPHP1 renal cells. Kidney samples were collected from WT (Nphp1 +/+) mice and NPHP1 (Nphp1 del2-20/del2-20) model mice. Results: A comprehensive atlas of the renal cellular landscape in NPHP1 was generated, consisting of 14 basic renal cell types as well as a subpopulation of DCT cells that was overrepresented in NPHP1 kidneys compared to WT kidneys. GO analysis revealed significant downregulation of genes associated with tubular development and kidney morphogenesis in this subpopulation. Furthermore, the reconstruction of differentiation trajectories of individual cells within this subpopulation confirmed that a specific group of cells in NPHP1 mice become arrested at an early stage of differentiation and proliferate to form cysts. We demonstrate that Niban1 is a specific molecular marker of cystic cells in both mice and human NPHP1. Conclusion: In summary, we report a novel subpopulation of DCT cells, marked by Niban1, that are classified as cystic cells in the NPHP1 mice kidney. These results offer fresh insights into the cellular and molecular basis of cystogenesis in NPH.

Primary cilia and hypoxia-associated signaling in developmental odontogenic cysts in relation to autosomal dominant polycystic kidney disease - A novel insight

Developmental cysts are pathological epithelial-lined cavities arising in various organs as a result of systemic or hereditary diseases. Molecular mechanisms involved in the formation of developmental odontogenic cysts (OCs) are not fully understood yet; the cystogenesis of renal cysts originating from the autosomal dominant polycystic kidney disease (ADPKD) has been, however, explored in much greater detail. This narrative review aimed i) to summarize molecular and cellular processes involved in the formation and growth of developmental OCs, especially dentigerous cysts (DCs) and odontogenic keratocysts (OKCs), ii) to find if there are any similarities in their cystogenesis to ADPKD cysts, and, based on that, iii) to suggest potential factors, candidate molecules, and mechanisms that could be involved in the DC formation, thus proposing further research directions. Here we suggest a possible association of developmental OCs with primary cilia disruption and with hypoxia, which have been previously linked with cyst formation in ADPKD patients. This is illustrated on the imagery of tissues from an ADPKD patient (renal cyst) and from developmental OCs, supporting the similarities in cell proliferation, apoptosis, and primary cilia distribution in DC/OKC/ADPKD tissues. Based on all that, we propose a novel hypothesis of OCs formation suggesting a crucial role of mutations associated with the signaling pathways of primary cilia (in particular, Sonic Hedgehog). These can lead to excessive proliferation and formation of cell agglomerates, which is followed by hypoxia-driven apoptosis in the centers of such agglomerates (controlled by molecules such as Hypoxia-inducible factor-1 alpha), leading to cavity formation and, finally, the OCs development. Based on this, we propose future perspectives in the investigation of OC pathogenesis.

Apoptotic cell death in disease-Current understanding of the NCCD 2023

Apoptosis is a form of regulated cell death (RCD) that involves proteases of the caspase family. Pharmacological and genetic strategies that experimentally inhibit or delay apoptosis in mammalian systems have elucidated the key contribution of this process not only to (post-)embryonic development and adult tissue homeostasis, but also to the etiology of multiple human disorders. Consistent with this notion, while defects in the molecular machinery for apoptotic cell death impair organismal development and promote oncogenesis, the unwarranted activation of apoptosis promotes cell loss and tissue damage in the context of various neurological, cardiovascular, renal, hepatic, infectious, neoplastic and inflammatory conditions. Here, the Nomenclature Committee on Cell Death (NCCD) gathered to critically summarize an abundant pre-clinical literature mechanistically linking the core apoptotic apparatus to organismal homeostasis in the context of disease.

Molecular Mechanisms of Epigenetic Regulation, Inflammation, and Cell Death in ADPKD

Autosomal dominant polycystic kidney disease (ADPKD) is a genetic disorder, which is caused by mutations in the PKD1 and PKD2 genes, characterizing by progressive growth of multiple cysts in the kidneys, eventually leading to end-stage kidney disease (ESKD) and requiring renal replacement therapy. In addition, studies indicate that disease progression is as a result of a combination of factors. Understanding the molecular mechanisms, therefore, should facilitate the development of precise therapeutic strategies for ADPKD treatment. The roles of epigenetic modulation, interstitial inflammation, and regulated cell death have recently become the focuses in ADPKD. Different epigenetic regulators, and the presence of inflammatory markers detectable even before cyst growth, have been linked to cyst progression. Moreover, the infiltration of inflammatory cells, such as macrophages and T cells, have been associated with cyst growth and deteriorating renal function in humans and PKD animal models. There is evidence supporting a direct role of the PKD gene mutations to the regulation of epigenetic mechanisms and inflammatory response in ADPKD. In addition, the role of regulated cell death, including apoptosis, autophagy and ferroptosis, have been investigated in ADPKD. However, there is no consensus whether cell death promotes or delays cyst growth in ADPKD. It is therefore necessary to develop an interactive picture between PKD gene mutations, the epigenome, inflammation, and cell death to understand why inherited PKD gene mutations in patients may result in the dysregulation of these processes that increase the progression of renal cyst formation.

Ferroptosis Promotes Cyst Growth in Autosomal Dominant Polycystic Kidney Disease Mouse Models

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD), the most common inherited kidney disease, is regulated by different forms of cell death, including apoptosis and autophagy. However, the role in ADPKD of ferroptosis, a recently discovered form of cell death mediated by iron and lipid metabolism, remains elusive.

METHODS

To determine a pathophysiologic role of ferroptosis in ADPKD, we investigated whether the absence of Pkd1 (encoding polycystin-1) affected the expression of key factors involved in the process of ferroptosis, using Western blot and qRT-PCR analysis in Pkd1 mutant renal cells and tissues. We also examined whether treatment with erastin, a ferroptosis inducer, and ferrostain-1, a ferroptosis inhibitor, affected cyst growth in Pkd1 mutant mouse models.

RESULTS

We found that kidney cells and tissues lacking Pkd1 exhibit extensive metabolic abnormalities, including reduced expression of the system Xc- amino acid antiporter (critical for import of cystine), of iron exporter (ferroportin), and of GPX4 (a key and negative regulator of ferroptosis). The abnormalities also include increased expression of iron importers (TfR1, DMT1) and HO-1, which in turn result in high iron levels, low GSH and GPX4 activity, increased lipid peroxidation, and propensity to ferroptosis. We further found that erastin increased, and ferrostatin-1 inhibited ferroptotic cell death and proliferation of Pkd1-deficient cells in kidneys from Pkd1 mutant mice. A lipid peroxidation product increased in Pkd1-deficient cells, 4HNE, promoted the proliferation of survived Pkd1 mutant cells via activation of Akt, S6, Stat3, and Rb during the ferroptotic process, contributing to cyst growth.

CONCLUSION

These findings indicate that ferroptosis contributes to ADPKD progression and management of ferroptosis may be a novel strategy for ADPKD treatment.

Urinary Levels of Cathepsin B in Preterm Newborns

Increased investment in perinatal health in developing countries has improved the survival of preterm newborns, but their significant multiorgan immaturity is associated with short and long-term adverse consequences. Cathepsin B, as a protease with angiogenic properties, may be related to the process of nephrogenesis. A total of 88 neonates (60 premature children, 28 healthy term children) were included in this prospective study. We collected urine samples on the first or second day of life. In order to determine the concentration of cathepsin B in the urine, the commercially available enzyme immunoassay was used. The urinary concentrations of cathepsin B normalized with the urinary concentrations of creatinine (cathepsin B/Cr.) in newborns born at 30-34, 35-36, and 37-41 (the control group) weeks of pregnancy were (median, Q1-Q3) 4.00 (2.82-5.12), 3.07 (1.95-3.90), and 2.51 (2.00-3.48) ng/mg Cr, respectively. Statistically significant differences were found between the group of newborns born at 30-34 weeks of pregnancy and the control group (p < 0.01), and between early and late preterm babies (PTB) (p < 0.05). The group of children born at 35-36 weeks of pregnancy and the control group did not differ significantly. This result suggests that the elevated urinary cathepsin B/Cr. level may be the result of the kidneys' immaturity in preterm newborns.

Macrophage migration inhibitory factor is regulated by HIF-1α and cAMP and promotes renal cyst cell proliferation in a macrophage-independent manner

Progressive cyst growth leads to decline of renal function in polycystic kidney disease. Macrophage migration inhibitory factor (MIF) was found to be upregulated in cyst-lining cells in a mouse model of polycystic kidney disease and to promote cyst growth. In addition, MIF can be secreted by tubular cells and may contribute to cyst growth in an autocrine manner. However, the underlying mechanisms leading to induction of MIF in cyst-lining cells remained elusive. Here, we demonstrate that hypoxia-inducible transcription factor (HIF) 1α upregulates MIF in cyst-lining cells in a tubule-specific PKD1 knockout mouse. Pharmacological stabilization of HIF-1α resulted in significant increase of MIF in cyst epithelial cells whereas tubule-specific knockout of HIF-1α prevented MIF upregulation. Identical regulation could be found for ABCA1, which has been shown to act as a transport protein for MIF. Furthermore, we show that MIF and ABCA1 are direct target genes of HIF-1α in human primary tubular cells. Next to HIF-1α and hypoxia, we found MIF being additionally regulated by cAMP which is a strong promotor of cyst growth. In line with these findings, HIF-1α- and cAMP-dependent in vitro cyst growth could be decreased by the MIF-inhibitor ISO-1 which resulted in reduced cyst cell proliferation. In conclusion, HIF-1α and cAMP regulate MIF in primary tubular cells and cyst-lining epithelial cells, and MIF promotes cyst growth in the absence of macrophages. In line with these findings, the MIF inhibitor ISO-1 attenuates HIF-1α- and cAMP-dependent in vitro cyst enlargement. KEY MESSAGES: • MIF is upregulated in cyst-lining cells in a polycystic kidney disease mouse model. • MIF upregulation is mediated by hypoxia-inducible transcription factor (HIF) 1α. • ABCA1, transport protein for MIF, is also regulated by HIF-1α in vitro and in vivo. • MIF is additionally regulated by cAMP, a strong promotor of cyst growth. • MIF-inhibitor ISO-1 reduces HIF-1α- and cAMP-dependent cyst growth.

Apoptosis and autophagy in polycystic kidney disease (PKD)

Apoptosis in the cystic epithelium is observed in most rodent models of polycystic kidney disease (PKD) and in human autosomal dominant PKD (ADPKD). Apoptosis inhibition decreases cyst growth, whereas induction of apoptosis in the kidney of Bcl-2 deficient mice increases proliferation of the tubular epithelium and subsequent cyst formation. However, alternative evidence indicates that both induction of apoptosis as well as increased overall rates of apoptosis are associated with decreased cyst growth. Autophagic flux is suppressed in cell, zebra fish and mouse models of PKD and suppressed autophagy is known to be associated with increased apoptosis. There may be a link between apoptosis and autophagy in PKD. The mammalian target of rapamycin (mTOR), B-cell lymphoma 2 (Bcl-2) and caspase pathways that are known to be dysregulated in PKD, are also known to regulate both autophagy and apoptosis. Induction of autophagy in cell and zebrafish models of PKD results in suppression of apoptosis and reduced cyst growth supporting the hypothesis autophagy induction may have a therapeutic role in decreasing cyst growth, perhaps by decreasing apoptosis and proliferation in PKD. Future research is needed to evaluate the effects of direct autophagy inducers on apoptosis in rodent PKD models, as well as the cause and effect relationship between autophagy, apoptosis and cyst growth in PKD.

Targeting new cellular disease pathways in autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited cause of end-stage renal failure. Understanding the molecular and cellular pathogenesis of ADPKD could help to identify new targets for treatment. The classic cellular cystic phenotype includes changes in proliferation, apoptosis, fluid secretion, extracellular matrix and cilia function. However, recent research, suggests that the cellular cystic phenotype could be broader and that changes, such as altered metabolism, autophagy, inflammation, oxidative stress and epigenetic modification, could play important roles in the processes of cyst initiation, cyst growth or disease progression. Here we review these newer cellular pathways, describe evidence for their possible links to cystic pathogenesis or different stages of disease and discuss the options for developing novel treatments.

Extracellular Nucleotides and P2 Receptors in Renal Function

The understanding of the nucleotide/P2 receptor system in the regulation of renal hemodynamics and transport function has grown exponentially over the last 20 yr. This review attempts to integrate the available data while also identifying areas of missing information. First, the determinants of nucleotide concentrations in the interstitial and tubular fluids of the kidney are described, including mechanisms of cellular release of nucleotides and their extracellular breakdown. Then the renal cell membrane expression of P2X and P2Y receptors is discussed in the context of their effects on renal vascular and tubular functions. Attention is paid to effects on the cortical vasculature and intraglomerular structures, autoregulation of renal blood flow, tubuloglomerular feedback, and the control of medullary blood flow. The role of the nucleotide/P2 receptor system in the autocrine/paracrine regulation of sodium and fluid transport in the tubular and collecting duct system is outlined together with its role in integrative sodium and fluid homeostasis and blood pressure control. The final section summarizes the rapidly growing evidence indicating a prominent role of the extracellular nucleotide/P2 receptor system in the pathophysiology of the kidney and aims to identify potential therapeutic opportunities, including hypertension, lithium-induced nephropathy, polycystic kidney disease, and kidney inflammation. We are only beginning to unravel the distinct physiological and pathophysiological influences of the extracellular nucleotide/P2 receptor system and the associated therapeutic perspectives.

A study of sirolimus and mTOR kinase inhibitor in a hypomorphic Pkd1 mouse model of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (PKD) is characterized by cyst formation and growth, which are partially driven by abnormal proliferation of tubular cells. Proproliferative mechanistic target of rapamycin (mTOR) complexes 1 and 2 (mTORC1 and mTORC2) are activated in the kidneys of mice with PKD. Sirolimus indirectly inhibits mTORC1. Novel mTOR kinase inhibitors directly inhibit mTOR kinase, resulting in the inhibition of mTORC1 and mTORC2. The aim of the present study was to determine the effects of sirolimus versus the mTOR kinase inhibitor torin2 on cyst growth and kidney function in the Pkd1 p.R3277C (Pkd1^RC/RC) mouse model, a hypomorphic Pkd1 model orthologous to the human condition, and to determine the effects of sirolimus versus torin2 on mTORC1 and mTORC2 signaling in PKD1^-/- cells and in the kidneys of Pkd1^RC/RC mice. In vitro, both inhibitors reduced mTORC1 and mTORC2 phosphorylated substrates and negatively impacted cellular metabolic activity, as measured by MTT assay. Pkd1^RC/RC mice were treated with sirolimus or torin2 from 50 to 120 days of age. Torin2 was as effective as sirolimus in decreasing cyst growth and improving loss of kidney function. Both sirolimus and torin2 decreased phosphorylated S6 protein, phosphorylated eukaryotic translation initiation factor 4E-binding protein 1, phosphorylated Akt, and proliferation in Pkd1^RC/RC kidneys. In conclusion, torin2 and sirolimus were equally effective in decreasing cyst burden and improving kidney function and mediated comparable effects on mTORC1 and mTORC2 signaling and proliferation in the Pkd1^RC/RC kidney.

Autosomal dominant polycystic kidney disease: Disrupted pathways and potential therapeutic interventions

Autosomal dominant polycystic kidney disease (ADPKD) is a monogenic inherited renal cystic disease that occurs in different races worldwide. It is characterized by the development of a multitude of renal cysts, which leads to massive enlargement of the kidney and often to renal failure in adulthood. ADPKD is caused by a mutation in PKD1 or PKD2 genes encoding the proteins polycystin-1 and polycystin-2, respectively. Recent studies showed that cyst formation and growth result from deregulation of multiple cellular pathways like proliferation, apoptosis, metabolic processes, cell polarity, and immune defense. In ADPKD, intracellular cyclic adenosine monophosphate (cAMP) promotes cyst enlargement by stimulating cell proliferation and transepithelial fluid secretion. Several interventions affecting many of these defective signaling pathways have been effective in animal models and some are currently being tested in clinical trials. Moreover, the stem cell therapy can improve nephropathies and according to studies were done in this field, can be considered as a hopeful therapeutic approach in future for PKD. This study provides an in-depth review of the relevant molecular pathways associated with the pathogenesis of ADPKD and their implications in development of potential therapeutic strategies.

Inhibition of Mast Cell Degranulation With Cromolyn Sodium Exhibits Organ-Specific Effects in Polycystic Kidney (PCK) Rats

Autosomal recessive polycystic kidney disease (ARPKD) is a monogenic disease characterized by development of hepatorenal cysts, pericystic fibrosis, and inflammation. Previous studies show that mast cell (MC) mediators such as histamine induce proliferation of cholangiocytes. We observed robust MC accumulation around liver cysts, but not kidney cysts, in polycystic kidney (PCK) rats (an animal model of ARPKD). Therefore, we hypothesized that MCs contribute to hepatic cyst growth in ARPKD. To test this hypothesis, we treated PCK rats with 1 of 2 different MC stabilizers, cromolyn sodium (CS) or ketotifen, or saline. The CS treatment decreased MC degranulation in the liver and reduced serum tryptase (an MC granule component). Interestingly, we observed an increase in liver to body weight ratio after CS treatment paralleled by a significant increase in individual cyst size. Hepatic fibrosis was not affected by CS treatment. The CS treatment increased hepatic cyst wall epithelial cell (CWEC) proliferation and decreased cell death. Ketotifen treatment also increased hepatic cyst size. In vitro, CS treatment did not affect proliferation of isolated hepatic CWECs from PCK rats. In contrast, CS decreased kidney to body weight ratio paralleled by a significant decrease in individual cyst size. The percentage of kidney to body weight ratio was strongly correlated with serum renin (an MC granule component). Ketotifen did not affect kidney cyst growth. Collectively, these data suggest that CS affects hepatic and renal cyst growth differently in PCK rats. Moreover, CS may be beneficial to renal cystic disease but may exacerbate hepatic cyst growth in ARPKD.

Effects of colchicine on renal fibrosis and apoptosis in obstructed kidneys

BACKGROUND/AIMS

Colchicine is an established drug for microtubule stabilization that may reduce tissue injury. No data were available that its effects may depend on the dosage of colchicine. We investigated the anti-fibrotic and apoptotic effects of various dose of colchicine in a unilateral ureteral obstruction (UUO) model.

METHODS

Thirty-six Sprague-Dawley rats were randomly assigned into six groups. Two sham groups were divided into a vehicle-treated or colchicine-treated group (100 μg/kg/day). Four UUO groups were treated with either vehicle or three different doses of colchicine for 7 days (30, 60, and 100 μg/kg/day, intraperitoneally). All of the animals were sacrificed on day 7.

RESULTS

Colchicine treatment diminished acetylated α-tubulin and tumor growth factor-β immunoreactivities in the cortical area of the 7-day obstructed kidneys, which was in dose dependent manner. Colchicine attenuated tubulointerstitial damage and apoptosis in both cortical and medullary area, and beneficial effects of colchicine therapy were dramatically shown at the higher dosage of colchicine. The expression levels of cleaved caspase-3, ED-1, and fibronectin were decreased in UUO animals.

CONCLUSIONS

We found that the proper dosage of colchicine may have anti-fibrotic and anti-apoptotic effects in obstructed kidneys. For clinical applications, an optimal dose of colchicine should be evaluated to maximize the prevention of renal disease progression.

Role of Sirolimus in renal tubular apoptosis in response to unilateral ureteral obstruction

Renal tubule cell apoptosis plays a pivotal role in the progression of chronic renal diseases. The previous study indicates that Sirolimus is effective on unilateral ureteral obstruction (UUO)-induced renal fibrosis. However, the role of Sirolimus in renal tubular apoptosis induced by UUO has not yet been addressed. The aim of this study was to determine the role of Sirolimus in renal tubular apoptosis induced by UUO. Male Sprague-Dawley rats were divided into three groups, sham-operated rats, and after which unilateral ureteral obstruction (UUO) was performed: non-treated and sirolimus-treated (1mg/kg). After 4, 7 and 14 d, animals were sacrificed and blood, kidney tissue samples were collected for analyses. Histologic changes and interstitial collagen were determined microscopically following HE and Masson's trichrome staining. The expression of PCNA was investigated using immunohistochemistry and the expression of Bcl-2, Bax, caspase-9, and caspase-3 were investigated using Western blot in each group. Tubular apoptotic cell deaths were assessed by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) assay. Sirolimus administration resulted in a significant reduction in tubulointerstitial fibrosis scores. After UUO, there was an increase in tubular and interstitial apoptosis in untreated controls as compared to Sirolimus treatment rats (P<0.05). In addition, the expression of PCNA, Bcl-2, Bax, caspase-9, and caspase-3 in obstructed kidney was characterized by immunohistochemistry and Western blot analyses demonstrating that sirolimus treatment significantly reduced PCNA, Bax, caspase-9 and cleaved caspase-3 expression compared to those observed in controls (P<0.05), whereas, Bcl-2 in the obstructed kidney were decreased in untreated controls compared to Sirolimus treatment rats subjected to the same time course of obstruction (P<0.05). We demonstrated a marked renoprotective effect of sirolimus by inhibition of UUO-induced renal tubular apoptosis in vivo.

The Multifaceted Role of the Lysosomal Protease Cathepsins in Kidney Disease

Kidney disease is worldwide the 12th leading cause of death affecting 8–16% of the entire population. Kidney disease encompasses acute (short-lasting episode) and chronic (developing over years) pathologies both leading to renal failure. Since specific treatments for acute or chronic kidney disease are limited, more than 2 million people a year require dialysis or kidney transplantation. Several recent evidences identified lysosomal proteases cathepsins as key players in kidney pathophysiology. Cathepsins, originally found in the lysosomes, exert important functions also in the cytosol and nucleus of cells as well as in the extracellular space, thus participating in a wide range of physiological and pathological processes. Based on their catalytic active site residue, the 15 human cathepsins identified up to now are classified in three different families: serine (cathepsins A and G), aspartate (cathepsins D and E), or cysteine (cathepsins B, C, F, H, K, L, O, S, V, X, and W) proteases. Specifically in the kidney, cathepsins B, D, L and S have been shown to regulate extracellular matrix homeostasis, autophagy, apoptosis, glomerular permeability, endothelial function, and inflammation. Dysregulation of their expression/activity has been associated to the onset and progression of kidney disease. This review summarizes most of the recent findings that highlight the critical role of cathepsins in kidney disease development and progression. A better understanding of the signaling pathways governed by cathepsins in kidney physiopathology may yield novel selective biomarkers or therapeutic targets for developing specific treatments against kidney disease.

On the Many Actions of Ouabain: Pro-Cystogenic Effects in Autosomal Dominant Polycystic Kidney Disease

Ouabain and other cardenolides are steroidal compounds originally discovered in plants. Cardenolides were first used as poisons, but after finding their beneficial cardiotonic effects, they were rapidly included in the medical pharmacopeia. The use of cardenolides to treat congestive heart failure remained empirical for centuries and only relatively recently, their mechanisms of action became better understood. A breakthrough came with the discovery that ouabain and other cardenolides exist as endogenous compounds that circulate in the bloodstream of mammals. This elevated these compounds to the category of hormones and opened new lines of investigation directed to further study their biological role. Another important discovery was the finding that the effect of ouabain was mediated not only by inhibition of the activity of the Na,K-ATPase (NKA), but by the unexpected role of NKA as a receptor and a signal transducer, which activates a complex cascade of intracellular second messengers in the cell. This broadened the interest for ouabain and showed that it exerts actions that go beyond its cardiotonic effect. It is now clear that ouabain regulates multiple cell functions, including cell proliferation and hypertrophy, apoptosis, cell adhesion, cell migration, and cell metabolism in a cell and tissue type specific manner. This review article focuses on the cardenolide ouabain and discusses its various in vitro and in vivo effects, its role as an endogenous compound, its mechanisms of action, and its potential use as a therapeutic agent; placing especial emphasis on our findings of ouabain as a pro-cystogenic agent in autosomal dominant polycystic kidney disease (ADPKD).

Role of HIF-1α and CASPASE-3 in cystogenesis of odontogenic cysts and tumors

OBJECTIVES

Odontogenic cysts and tumors are the most relevant lesions that affect the gnathic bones. These lesions have in common the formation of cystic areas and this common feature may suggest involvement of similar mechanisms. The hypoxia inducible factor 1 alpha (HIF-1α), a responsive protein to hypoxia and caspase-3, an irreversible apoptosis marker, may contribute to cyst formation. Thus, this study aimed to investigate the immunoexpression of these proteins in odontogenic cysts and tumors.

MATERIAL AND METHODS

Twenty cases of ameloblastoma, keratocystic odontogenic tumor (KOT) (n = 20), radicular cyst (RC) (n = 18), dentigerous cyst (DC) (n = 11), calcifying cystic odontogenic tumor (n = 8), and dental follicle (DF) (n = 10) were used to investigate HIF-1α and caspase-3 expression in sequential serial cuts by immunohistochemistry.

RESULTS

HIF-1α was overexpressed in RC, DC, and ameloblastoma when compared with DF. The basal and sometimes the lower suprabasal layer showed no or very low expression in DC, KOT, and ameloblastoma, the last also showing strong expression in solid epithelial areas and initial cystic formation regions. Caspase-3 was found to be overexpressed in all lesions, with the highest expression in odontogenic cysts compared to tumors. HIF-1α and caspase-3 were localized in similar areas of the same lesions, especially in the epithelium surrounding cystic formations.

CONCLUSIONS

This study showed distinct immunoexpression of HIF-1α and caspase-3 in odontogenic cyst and tumors, with higher expression observed in odontogenic cysts.

CLINICAL RELEVANCE

These findings suggest a possible correlation between hypoxia, apoptosis, and cystogenesis, leading to understand the mechanisms responsible to cystic formation in odontogenic lesions.

Mitochondrial reactive oxygen species-mediated NLRP3 inflammasome activation contributes to aldosterone-induced renal tubular cells injury

Aldosterone (Aldo) is an independent risk factor for chronic kidney disease (CKD), and although Aldo directly induces renal tubular cell injury, the underlying mechanisms remain unclear. NLRP3 inflammasome and mitochondrial reactive oxygen species (ROS) have recently been implicated in various kinds of CKD. The present study hypothesized that mitochondrial ROS and NLRP3 inflammasome mediated Aldo–induced tubular cell injury. The NLRP3 inflammasome is induced by Aldo in a dose- and time-dependent manner, as evidenced by increased NLRP3, ASC, caspase-1, and downstream cytokines, such as interleukin (IL)-1β and IL-18. The activation of the NLRP3 inflammasome was significantly prevented by the selective mineralocorticoid receptor (MR) antagonist eplerenone (EPL) (P < 0.01). Mice harboring genetic knock-out of NLRP3 (NLRP3^−/−) showed decreased maturation of renal IL-1β and IL-18, reduced renal tubular apoptosis, and improved renal epithelial cell phenotypic alternation, and attenuated renal function in response to Aldo-infusion. In addition, mitochondrial ROS was also increased in Aldo-stimulated HK-2 cells, as assessed by MitoSOX^TM red reagent. Mito-Tempo, the mitochondria-targeted antioxidant, significantly decreased HK-2 cell apoptosis, oxidative stress, and the activation of NLRP3 inflammasome. We conclude that Aldo induces renal tubular cell injury via MR dependent, mitochondrial ROS-mediated NLRP3 inflammasome activation.

Differences in the timing and magnitude of Pkd1 gene deletion determine the severity of polycystic kidney disease in an orthologous mouse model of ADPKD

Development of a disease‐modifying therapy to treat autosomal dominant polycystic kidney disease (ADPKD) requires well‐characterized preclinical models that accurately reflect the pathology and biochemical changes associated with the disease. Using a Pkd1 conditional knockout mouse, we demonstrate that subtly altering the timing and extent of Pkd1 deletion can have a significant impact on the origin and severity of kidney cyst formation. Pkd1 deletion on postnatal day 1 or 2 results in cysts arising from both the cortical and medullary regions, whereas deletion on postnatal days 3–8 results in primarily medullary cyst formation. Altering the extent of Pkd1 deletion by modulating the tamoxifen dose produces dose‐dependent changes in the severity, but not origin, of cystogenesis. Limited Pkd1 deletion produces progressive kidney cystogenesis, accompanied by interstitial fibrosis and loss of kidney function. Cyst growth occurs in two phases: an early, rapid growth phase, followed by a later, slow growth period. Analysis of biochemical pathway changes in cystic kidneys reveals dysregulation of the cell cycle, increased proliferation and apoptosis, activation of Mek‐Erk, Akt‐mTOR, and Wnt‐β‐catenin signaling pathways, and altered glycosphingolipid metabolism that resemble the biochemical changes occurring in human ADPKD kidneys. These pathways are normally active in neonatal mouse kidneys until repressed around 3 weeks of age; however, they remain active following Pkd1 deletion. Together, this work describes the key parameters to accurately model the pathological and biochemical changes associated with ADPKD in a conditional mouse model.

MicroRNA-21 Aggravates Cyst Growth in a Model of Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD), one of the most common monogenetic disorders, is characterized by kidney failure caused by bilateral renal cyst growth. MicroRNAs (miRs) have been implicated in numerous diseases, but the role of these noncoding RNAs in ADPKD pathogenesis is still poorly defined. Here, we investigated the role of miR-21, an oncogenic miR, in kidney cyst growth. We found that transcriptional activation of miR-21 is a common feature of murine PKD. Furthermore, compared with renal tubules from kidney samples of normal controls, cysts in kidney samples from patients with ADPKD had increased levels of miR-21. cAMP signaling, a key pathogenic pathway in PKD, transactivated miR-21 promoter in kidney cells and promoted miR-21 expression in cystic kidneys of mice. Genetic deletion of miR-21 attenuated cyst burden, reduced kidney injury, and improved survival of an orthologous model of ADPKD. RNA sequencing analysis and additional in vivo assays showed that miR-21 inhibits apoptosis of cyst epithelial cells, likely through direct repression of its target gene programmed cell death 4 Thus, miR-21 functions downstream of the cAMP pathway and promotes disease progression in experimental PKD. Our results suggest that inhibiting miR-21 is a potential new therapeutic approach to slow cyst growth in PKD.

Protection From Apoptotic Cell Death During Cold Storage Followed by Rewarming in 13-Lined Ground Squirrel Tubular Cells: The Role of Prosurvival Factors X-Linked Inhibitor of Apoptosis and PhosphoAkt

BACKGROUND

Hibernators, such as the 13-lined ground squirrel, endure severe hypothermia during torpor followed by periodic rewarming (REW) during interbout arousal (IBA), proapoptotic conditions that are lethal to nonhibernating mammals. We have previously shown that 13-lined ground squirrel tubular cells are protected from apoptotic cell death during IBA. To understand the mechanism of protection, we developed an in vitro model of prolonged cold storage (CS) followed by REW, which is akin to the in vivo changes of hypothermia followed by REW observed during IBA. We hypothesized that renal tubular epithelial cells (RTECs) isolated from hibernating ground squirrels would be protected against apoptosis during CS/REW versus nonhibernating mouse RTECs.

METHODS

Isolated hibernating ground squirrel and mouse RTECs were subjected to CS at 4°C for 24 hours followed by REW to 37°C for 24 hours (CS/REW).

RESULTS

Ground squirrel RTECs had significantly less apoptosis compared to mouse RTECs when subjected to CS/REW. Next, we hypothesized that the mechanism of protection was related to the antiapoptotic proteins X-linked inhibitor of apoptosis (XIAP), phospho-Akt (pAkt), and phospho-BAD. There was a significantly increased pAkt and pBAD expression in ground squirrel versus mouse RTECs subjected to CS/REW. The XIAP expression was maintained in ground squirrel RTECs but was significantly decreased in mouse RTECs after CS/REW. Ground squirrel RTECs in which gene expression of Akt1 and XIAP was silenced lost their protection and demonstrated increased apoptosis and cleaved caspase-3 expression after CS/REW.

CONCLUSIONS

Our findings suggest that ground squirrel RTECs are protected against apoptosis during prolonged CS/REW by the "prosurvival" factors XIAP and pAkt.

Ouabain Enhances ADPKD Cell Apoptosis via the Intrinsic Pathway

Progression of autosomal dominant polycystic kidney disease (ADPKD) is highly influenced by factors circulating in blood. We have shown that the hormone ouabain enhances several characteristics of the ADPKD cystic phenotype, including the rate of cell proliferation, fluid secretion and the capacity of the cells to form cysts. In this work, we found that physiological levels of ouabain (3 nM) also promote programmed cell death of renal epithelial cells obtained from kidney cysts of patients with ADPKD (ADPKD cells). This was determined by Alexa Fluor 488 labeled-Annexin-V staining and TUNEL assay, both biochemical markers of apoptosis. Ouabain-induced apoptosis also takes place when ADPKD cell growth is blocked; suggesting that the effect is not secondary to the stimulatory actions of ouabain on cell proliferation. Ouabain alters the expression of BCL family of proteins, reducing BCL-2 and increasing BAX expression levels, anti- and pro-apoptotic mediators respectively. In addition, ouabain caused the release of cytochrome c from mitochondria. Moreover, ouabain activates caspase-3, a key “executioner” caspase in the cell apoptotic pathway, but did not affect caspase-8. This suggests that ouabain triggers ADPKD cell apoptosis by stimulating the intrinsic, but not the extrinsic pathway of programmed cell death. The apoptotic effects of ouabain are specific for ADPKD cells and do not occur in normal human kidney cells (NHK cells). Taken together with our previous observations, these results show that ouabain causes an imbalance in cell growth/death, to favor growth of the cystic cells. This event, characteristic of ADPKD, further suggests the importance of ouabain as a circulating factor that promotes ADPKD progression.

Pathways, perspectives and pursuits in polycystic kidney disease

Polycystic kidney disease (PKD) is characterized by the growth of numerous cysts in the kidneys. When cysts form in the kidneys, they are filled with fluid. PKD cysts can profoundly enlarge the kidneys while replacing much of the normal structure, resulting in reduced kidney function and leading to kidney failure. Autosomal dominant polycystic kidney disease (ADPKD) is a hereditary disease that occurs in one out of 1000 humans. PKD and its causes are being dissected through studies of human populations and through the use of animal models. Mouse models in particular have made a substantial contribution to our understanding of the gene pathways involved in the pathogenesis and the nature of signaling molecules that act in a tissue-specific manner at critical stages of cyst development. PKD has a number of characteristics that make it uniquely challenging for the development of therapies to slowdown disease progression. This review provides current understanding of the etiopathology, pathways involved and therapeutic targets of PKDs.

Anoctamin 6 is localized in the primary cilium of renal tubular cells and is involved in apoptosis-dependent cyst lumen formation

Primary cilia are antenna-like structures projected from the apical surface of various mammalian cells including renal tubular cells. Functional or structural defects of the cilium lead to systemic disorders comprising polycystic kidneys as a key feature. Here we show that anoctamin 6 (ANO6), a member of the anoctamin chloride channel family, is localized in the primary cilium of renal epithelial cells in vitro and in vivo. ANO6 was not essential for cilia formation and had no effect on in vitro cyst expansion. However, knockdown of ANO6 impaired cyst lumen formation of MDCK cells in three-dimensional culture. In the absence of ANO6, apoptosis was reduced and epithelial cells were incompletely removed from the center of cell aggregates, which form in the early phase of cystogenesis. In line with these data, we show that ANO6 is highly expressed in apoptotic cyst epithelial cells of human polycystic kidneys. These data identify ANO6 as a cilium-associated protein and suggest its functional relevance in cyst formation.

The hallmarks of cancer: relevance to the pathogenesis of polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a progressive inherited disorder in which renal tissue is gradually replaced with fluid-filled cysts, giving rise to chronic kidney disease (CKD) and progressive loss of renal function. ADPKD is also associated with liver ductal cysts, hypertension, chronic pain and extra-renal problems such as cerebral aneurysms. Intriguingly, improved understanding of the signalling and pathological derangements characteristic of ADPKD has revealed marked similarities to those of solid tumours, even though the gross presentation of tumours and the greater morbidity and mortality associated with tumour invasion and metastasis would initially suggest entirely different disease processes. The commonalities between ADPKD and cancer are provocative, particularly in the context of recent preclinical and clinical studies of ADPKD that have shown promise with drugs that were originally developed for cancer. The potential therapeutic benefit of such repurposing has led us to review in detail the pathological features of ADPKD through the lens of the defined, classic hallmarks of cancer. In addition, we have evaluated features typical of ADPKD, and determined whether evidence supports the presence of such features in cancer cells. This analysis, which places pathological processes in the context of defined signalling pathways and approved signalling inhibitors, highlights potential avenues for further research and therapeutic exploitation in both diseases.

An mTOR kinase inhibitor slows disease progression in a rat model of polycystic kidney disease

BACKGROUND

The mTOR pathway, which consists of mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), is activated in polycystic kidney disease (PKD) kidneys. Sirolimus and everolimus indirectly bind and inhibit mTORC1. A novel group of drugs, the mTOR kinase inhibitors, directly bind to mTOR kinase, thus inhibiting both mTORC1 and 2. The aim of the study was to determine the therapeutic effect of an mTOR kinase inhibitor, PP242, in the Han:SPRD rat (Cy/+) model of PKD.

METHODS

Male rats were treated with PP242 5 mg/kg/day IP or vehicle for 5 weeks.

RESULTS

PP242 significantly reduced the kidney enlargement, the cyst density and the blood urea nitrogen in Cy/+ rats. On immunoblot of kidneys, PP242 resulted in a decrease in pS6, a marker of mTORC1 signaling and pAkt(Ser473), a marker of mTORC2 signaling. mTORC plays an important role in regulating cytokine production. There was an increase in IL-1, IL-6, CXCL1 and TNF-α in Cy/+ rat kidneys that was unaffected by PP242. Apoptosis or proliferation is known to play a causal role in cyst growth. PP242 had no effect on caspase-3 activity, TUNEL positive or active caspase-3-positive tubular cells in Cy/+ kidneys. PP242 reduced the number of proliferating cells per cyst and per non-cystic tubule in Cy/+ rats.

CONCLUSIONS

In a rat model of autosomal dominant polycystic kidney disease, PP242 treatment (i) decreases proliferation in cystic and non-cystic tubules; (ii) inhibits renal enlargement and cystogenesis and (iii) significantly reduces the loss of kidney function.

Dichloroacetate treatment accelerates the development of pathology in rodent autosomal recessive polycystic kidney disease

Dichloroacetate (DCA) is a toxicant by-product from the chlorination disinfection process for municipal water. The levels would not affect people with normal renal and liver function. However, people with impaired renal or liver function may have an increased susceptibility to DCA toxicity as those are the organs affected by DCA. People (and rodents) with polycystic kidney disease (PKD) are polyuric, drink more fluids, and have both renal and liver pathology. In PKD, renal tubules and biliary epithelial cells proliferate to form cysts, which can eventually cause renal and/or liver dysfunction. Therefore, PKD may be a predisposing condition with an increased sensitivity to DCA toxicity. PCK rats are an orthologous model of human autosomal recessive PKD and were treated with 75 mg/l DCA in their drinking water. Male and female PCK and male Sprague-Dawley rats were treated from 4 to 8 wk of age, after which the severity of the renal and liver pathology induced by DCA were assessed. Only male PCK rats were adversely affected by DCA treatment, with an increase in the severity of renal cystic disease evinced by an increase in cystic enlargement and proteinuria. In conclusion, the chlorination byproduct DCA may adversely affect those with a preexisting renal disease, especially those who are polydipsic, like those with PKD.

Effects of cell-type-specific expression of a pan-caspase inhibitor on renal fibrogenesis

BACKGROUND

The caspase family of enzymes is grouped into two major sub-families, namely apoptotic and inflammatory caspases, which play central roles in the induction of apoptosis, regulation of inflammation and immunity, and cellular differentiation.

METHODS

The role of caspase activation in tubular epithelium and interstitial cells of 3 lines of transgenic mice with obstructed nephropathy was examined: p35 mice bearing the pan-caspase inhibitor protein expressed by the p35 gene separated from the universal CAG promoter by a floxed STOP sequence were crossed with γGT.Cre and FSP1.Cre mice that express Cre recombinase in the cortical tubular epithelium and FSP1(+) interstitial cells, respectively. The γGT.Cre;p35, FSP1.Cre;p35 and p35 control mice were then challenged with unilateral ureter obstruction (UUO).

RESULTS

Proinflammatory parameters such as protein levels of active IL-1β subunit and mRNA levels of TNF-α and NOD-like receptor pyrin domain containing-3, and profibrogenic parameters such as interstitial matrix deposition and mRNA levels of fibronectin EIIIA isoform and α1 chain of procollagen type I in the kidneys were significantly increased at 7 days in the FSP1.Cre;p35- and p35-UUO mice, but not in the γGT.Cre;p35-UUO mice. These changes paralleled the numbers of apoptotic nuclei in tubules, but not in interstitial cells, and the protein levels of active caspase-3 subunit in the kidneys of FSP1.Cre;p35-, p35- and γGT.Cre;p35-UUO mice.

CONCLUSION

This study provides evidence of the critical role of caspase activation in the tubular epithelium, but not in FSP1(+) interstitial cells, in apoptosis and inflammasome induction, leading to proinflammatory and profibrogenic processes in fibrous kidneys with UUO.

An mTOR anti-sense oligonucleotide decreases polycystic kidney disease in mice with a targeted mutation in Pkd2

Autosomal dominant polycystic kidney disease (ADPKD) is the most common life-threatening hereditary disease in the USA. In human ADPKD studies, sirolimus, a mammalian target of rapamycin complex 1 (mTORC1) inhibitor, had little therapeutic effect. While sirolimus robustly inhibits mTORC1, it has a minimal effect on mTOR complex 2 (mTORC2). Polycystic kidneys of Pkd2WS25/- mice, an orthologous model of human ADPKD caused by a mutation in the Pkd2 gene, had an early increase in pS6 (marker of mTORC1) and pAktSer(473) (marker of mTORC2). To investigate the effect of combined mTORC1 and 2 inhibition, Pkd2WS25/- mice were treated with an mTOR anti-sense oligonucleotide (ASO) that blocks mTOR expression thus inhibiting both mTORC1 and 2. The mTOR ASO resulted in a significant decrease in mTOR protein, pS6 and pAktSer(473). Pkd2WS25/- mice treated with the ASO had a normalization of kidney weights and kidney function and a marked decrease in cyst volume. The mTOR ASO resulted in a significant decrease in proliferation and apoptosis of tubular epithelial cells. To demonstrate the role of mTORC2 on cyst growth, Rictor, the functional component of mTORC2, was silenced in Madin-Darby canine kidney cell cysts grown in 3D cultures. Silencing Rictor significantly decreased cyst volume and expression of pAktSer(473). The decreased cyst size in the Rictor silenced cells was reversed by introduction of a constitutively active Akt1. In vitro, combined mTORC1 and 2 inhibition reduced cyst growth more than inhibition of mTORC1 or 2 alone. In conclusion, combined mTORC1 and 2 inhibition has therapeutic potential in ADPKD.

Polycystic kidney disease: a case of suppressed autophagy?

Autosomal-dominant polycystic kidney disease is the most common form of polycystic kidney disease in adults and is caused by a mutation in the polycystic kidney disease 1 or 2 genes, which encode, respectively, polycystin-1 and polycystin-2. Autophagy is present in polycystic kidneys in rat and mouse models of polycystic kidney disease. Autophagy has yet to be shown in human polycystic kidney disease kidneys. The mechanism of cyst growth has been studied extensively in vitro and in vivo. Multiple molecules and signaling pathways have been implicated in cyst growth including mammalian target of rapamycin, the renin-angiotensin-aldosterone system, vasopressin and cyclic adenosine monophosphate, epidermal growth factor and insulin-like growth factor tyrosine kinases, vascular endothelial growth factor, extracellular signal-related kinase, tumor necrosis factor-α, cyclin-dependent kinases, caspases and apoptosis, and cyclic adenosine monophosphate-activated protein kinases. Many of the agents that inhibit these signaling pathways and slow cyst growth are also autophagy inducers such as mammalian target of rapamycin inhibitors, cyclin-dependent kinase inhibitors, caspase inhibitors, tyrosine kinase inhibitors, metformin, curcumin, and triptolide. There are reasons to believe that suppression of autophagy may play a role in cyst formation and growth. This review presents the hypothesis that suppression of autophagy may play a role in cyst formation and growth, based on the following: (1) many of the agents that protect against polycystic kidney disease also induce autophagy, (2) suppression of autophagy in polycystic kidney disease 1 knockout cells, (3) a defect in autophagy in congenital polycystic kidney mice with polycystic kidney disease, (4) how suppressed autophagy may relate to apoptosis in polycystic kidney disease, and (5) conditions with defective cilia, the ciliopathies, are associated with decreased autophagy.

Endothelial dysfunction occurs prior to clinical evidence of polycystic kidney disease

OBJECTIVE

Polycystic kidney disease (PKD), a monogenic disease with an autosomal dominant or an autosomal recessive form of inheritance (ARPKD), is the most common genetic cause of renal dysfunction and end-stage renal failure. In addition to the development of cysts, the autosomal form of PKD is associated with vascular endothelial dysfunction, a marker of vascular disease. Whether vascular endothelial dysfunction is also present in ARPKD, and its relationship with renal dysfunction remain to be determined.

METHODS

ARPKD rats (PCK model) and controls were studied at 6 and 10 weeks of age, and mean arterial pressure and renal function were measured. Aortic endothelial function was assessed using organ chamber techniques. Aortic endothelial cells (ECs) were isolated, characterized and their function studied.

RESULTS

Compared to controls, ARPKD animals had a decrease in the vasorelaxation to endothelium-dependent vasodilators, even prior to changes in mean arterial pressure or renal function. The abnormal vasoreactivity was corrected with L-arginine (a precursor of nitric oxide, NO), while the expression of endothelial NO synthase (eNOS) was unchanged. Furthermore, isolated ECs from 6-week-old ARPKD animals showed increased oxidative stress, with preserved eNOS expression and abnormal patterns of migration and angiogenic capacity (measured by the scratch and tube formation assays, respectively).

CONCLUSION

ARPKD leads to impairments in aortic vascular function and ECs at an early stage, which can have significant functional consequences, potentially representing a novel therapeutic target in this disease.

Mouse models of polycystic kidney disease induced by defects of ciliary proteins

Polycystic kidney disease (PKD) is a common hereditary disorder which is characterized by fluid-filled cysts in the kidney. Mutation in either PKD1, encoding polycystin-1 (PC1), or PKD2, encoding polycystin-2 (PC2), are causative genes of PKD. Recent studies indicate that renal cilia, known as mechanosensors, detecting flow stimulation through renal tubules, have a critical function in maintaining homeostasis of renal epithelial cells. Because most proteins related to PKD are localized to renal cilia or have a function in ciliogenesis. PC1/PC2 heterodimer is localized to the cilia, playing a role in calcium channels. Also, disruptions of ciliary proteins, except for PC1 and PC2, could be involved in the induction of polycystic kidney disease. Based on these findings, various PKD mice models were produced to understand the roles of primary cilia defects in renal cyst formation. In this review, we will describe the general role of cilia in renal epithelial cells, and the relationship between ciliary defects and PKD. We also discuss mouse models of PKD related to ciliary defects based on recent studies. [BMB Reports 2013; 46(2): 73-79]

Comparative proteomic analysis suggests that mitochondria are involved in autosomal recessive polycystic kidney disease

Autosomal recessive polycystic kidney disease (ARPKD), characterized by ectatic collecting duct, is an infantile form of PKD occurring in 1 in 20 000 births. Despite having been studied for many years, little is known about the underlying mechanisms. In the current study, we employed, for the first time, a MS-based comparative proteomics approach to investigate the differently expressed proteins between kidney tissue samples of four ARPKD and five control individuals. Thirty two differently expressed proteins were identified and six of the identified protein encoding genes performed on an independent group (three ARPKD subjects, four control subjects) were verified by semi-quantitative RT-PCR, and part of them were further validated by Western blot and immunohistochemistry. Moreover, similar alteration tendency was detected after downregulation of PKHD1 by small interfering RNA in HEK293T cell. Interestingly, most of the identified proteins are associated with mitochondria. This implies that mitochondria may be implicated in ARPKD. Furthermore, the String software was utilized to investigate the biological association network, which is based on known and predicted protein interactions. In conclusion, our findings depicted a global understanding of ARPKD progression and provided a promising resource of targeting protein, and shed some light further investigation of ARPKD.

Polycystins and cellular Ca2+ signaling

The cystic phenotype in autosomal dominant polycystic kidney disease is characterized by a profound dysfunction of many cellular signaling patterns, ultimately leading to an increase in both cell proliferation and apoptotic cell death. Disturbance of normal cellular Ca²⁺ signaling seems to be a primary event and is clearly involved in many pathways that may lead to both types of cellular responses. In this review, we summarize the current knowledge about the molecular and functional interactions between polycystins and multiple components of the cellular Ca²⁺-signaling machinery. In addition, we discuss the relevant downstream responses of the changed Ca²⁺ signaling that ultimately lead to increased proliferation and increased apoptosis as observed in many cystic cell types.

CDK inhibitors R-roscovitine and S-CR8 effectively block renal and hepatic cystogenesis in an orthologous model of ADPKD

Autosomal dominant polycystic kidney disease (ADPKD) and other forms of PKD are associated with dysregulated cell cycle and proliferation. Although no effective therapy for the treatment of PKD is currently available, possible mechanism-based approaches are beginning to emerge. A therapeutic intervention targeting aberrant cilia-cell cycle connection using CDK-inhibitor R-roscovitine showed effective arrest of PKD in jck and cpk models that are not orthologous to human ADPKD. To evaluate whether CDK inhibition approach will translate into efficacy in an orthologous model of ADPKD, we tested R-roscovitine and its derivative S-CR8 in a model with a conditionally inactivated Pkd1 gene (Pkd1 cKO). Similar to ADPKD, Pkd1 cKO mice developed renal and hepatic cysts. Treatment of Pkd1 cKO mice with R-roscovitine and its more potent and selective analog S-CR8 significantly reduced renal and hepatic cystogenesis and attenuated kidney function decline. Mechanism of action studies demonstrated effective blockade of cell cycle and proliferation and reduction of apoptosis. Together, these data validate CDK inhibition as a novel and effective approach for the treatment of ADPKD.

Targeted expression of a pan-caspase inhibitor in tubular epithelium attenuates interstitial inflammation and fibrogenesis in nephritic but not nephrotic mice

The caspase family of enzymes participates in apoptotic and proinflammatory reactions in any cell. Here we studied the role of caspase activation in the tubular epithelium of diseased kidneys using mice transgenic for the baculovirus pan-caspase inhibitor p35 gene held in a nonexpressed state (control mice) but target-expressed in the renal proximal tubule cells when crossed with mice expressing Cre recombinase under the control of the γ-glutamyltransferase promoter. Proinflammatory and profibrogenic parameters such as the number of monocytes and fibroblasts in the kidneys were significantly increased at 28 days in the control mice, but not in the renal tubule-targeted mice expressing p35 in a nephrotoxic serum nephritis model of disease. These cellular changes paralleled the number of apoptotic tubular cells and protein levels of active caspase-3 in the kidneys at 7 and 28 days of both the control and proximal tubule-targeted mice. Surprisingly, all of these parameters were not significantly affected at 7 and 28 days by targeted p35 expression in tubular epithelium when compared with nontargeted control mice in a model of adriamycin nephrosis. Thus, our study shows the critical role of caspase activation in the tubular epithelium in apoptosis along with proinflammatory and profibrogenic processes in nephrotoxic serum nephritis but not adriamycin nephrosis.

Emerging role of autophagy in kidney function, diseases and aging

Autophagy is a highly conserved process that degrades cellular long-lived proteins and organelles. Accumulating evidence indicates that autophagy plays a critical role in kidney maintenance, diseases and aging. Ischemic, toxic, immunological, and oxidative insults can cause an induction of autophagy in renal epithelial cells modifying the course of various kidney diseases. This review summarizes recent insights on the role of autophagy in kidney physiology and diseases alluding to possible novel intervention strategies for treating specific kidney disorders by modifying autophagy.

Loss of GM3 synthase gene, but not sphingosine kinase 1, is protective against murine nephronophthisis-related polycystic kidney disease

Genetic forms of polycystic kidney diseases (PKDs), including nephronophthisis, are characterized by formation of fluid-filled cysts in the kidneys and progression to end-stage renal disease. No therapies are currently available to treat cystic diseases, making it imperative to dissect molecular mechanisms in search of therapeutic targets. Accumulating evidence suggests a pathogenic role for glucosylceramide (GlcCer) in multiple forms of PKD. It is not known, however, whether other structural glycosphingolipids (GSLs) or bioactive signaling sphingolipids (SLs) modulate cystogenesis. Therefore, we set out to address the role of a specific GSL (ganglioside GM3) and signaling SL (sphingosine-1-phosphate, S1P) in PKD progression, using the jck mouse model of nephronopthisis. To define the role of GM3 accumulation in cystogenesis, we crossed jck mice with mice carrying a targeted mutation in the GM3 synthase (St3gal5) gene. GM3-deficient jck mice displayed milder PKD, revealing a pivotal role for ganglioside GM3. Mechanistic changes in regulation of the cell-cycle machinery and Akt-mTOR signaling were consistent with reduced cystogenesis. Dramatic overexpression of sphingosine kinase 1 (Sphk1) mRNA in jck kidneys suggested a pathogenic role for S1P. Surprisingly, genetic loss of Sphk1 exacerbated cystogenesis and was associated with increased levels of GlcCer and GM3. On the other hand, increasing S1P accumulation through pharmacologic inhibition of S1P lyase had no effect on the progression of cystogenesis or kidney GSL levels. Together, these data suggest that genes involved in the SL metabolism may be modifiers of cystogenesis, and suggest GM3 synthase as a new anti-cystic therapeutic target.

Deletion of growth hormone receptor gene but not visceral fat removal decreases expression of apoptosis-related genes in the kidney-potential mechanism of lifespan extension

Mice homozygous for the targeted disruption of the growth hormone (GH) receptor (Ghr) gene (GH receptor knockout; GHRKO; KO) are hypoinsulinemic, highly insulin sensitive, normoglycemic, and long-lived. Visceral fat removal (VFR) is a surgical intervention which improves insulin signaling in normal (N) mice and rats and extends longevity in rats. We have previously demonstrated decreased expression level of certain pro-apoptotic genes in skeletal muscles and suggested that this may contribute to the regulation of longevity in GHRKO mice. Alterations in apoptosis-related genes expression in the kidneys also may potentially lead to lifespan extension. In this context, we decided to examine the renal expression of the following genes: caspase-3, caspase-9, caspase-8, bax, bad, bcl-2, Smac/DIABLO, Apaf-1, p53, and cytochrome c1 (cyc1) in male GHRKO and N mice subjected to VFR or sham surgery, at approximately 6 months of age. The kidneys were collected 2 months after VFR. As a result, caspase-3, caspase-9, and bax expressions were decreased in KO mice as compared to N animals. Expressions of Smac/DIABLO, caspase-8, bcl-2, bad, and p53 did not differ between KOs and N mice. VFR did not change the expression of the examined genes in KO or N mice. In conclusion, endocrine abnormalities in GHRKO mice result in decreased expression of pro-apoptotic genes and VFR did not alter the examined genes expression in N and KO mice. These data are consistent with a model in which alterations of GH signaling and/or insulin sensitivity lead to increased lifespan mediated by decreased renal expression of pro-apoptotic genes.

Mechanoprotection by polycystins against apoptosis is mediated through the opening of stretch-activated K(2P) channels

How renal epithelial cells respond to increased pressure and the link with kidney disease states remain poorly understood. Pkd1 knockout or expression of a PC2 pathogenic mutant, mimicking the autosomal dominant polycystic kidney disease, dramatically enhances mechanical stress-induced tubular apoptotic cell death. We show the presence of a stretch-activated K(+) channel dependent on the TREK-2 K(2P) subunit in proximal convoluted tubule epithelial cells. Our findings further demonstrate that polycystins protect renal epithelial cells against apoptosis in response to mechanical stress, and this function is mediated through the opening of stretch-activated K(2P) channels. Thus, to our knowledge, we establish for the first time, both in vitro and in vivo, a functional relationship between mechanotransduction and mechanoprotection. We propose that this mechanism is at play in other important pathologies associated with apoptosis and in which pressure or flow stimulation is altered, including heart failure or atherosclerosis.

Mechanism-based therapeutics for autosomal dominant polycystic kidney disease: recent progress and future prospects

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited kidney disease, accounting for up to 10% of patients on renal replacement therapy. There are presently no proven treatments for ADPKD and an effective disease-modifying drug would have significant implications for patients and their families. Since the identification of PKD1 and PKD2, there has been an explosion in knowledge identifying new disease mechanisms and testing new drugs. Currently, the three major treatment strategies are to: (1) reduce cAMP levels; (2) inhibit cell proliferation, and (3) reduce fluid secretion. Several compounds shown to be effective in preclinical models have already undergone clinical trials and more are planned. In addition, a whole raft of other compounds have been developed from preclinical studies. The purpose of this paper is to evaluate the results of recent published trials, review current trials and highlight the most promising compounds in the pipeline. There appears to be no shortage of potential candidates, but several key issues need to be addressed to facilitate clinical translation.

2-Hydroxyestradiol slows progression of experimental polycystic kidney disease

Male gender is a risk factor for progression of polycystic kidney disease (PKD). 17β-Estradiol (E2) protects experimentally, but clinical use is limited by adverse effects. Novel E2 metabolites provide many benefits of E2 without stimulating the estrogen receptor, and thus may be safer. We hypothesized that E2 metabolites are protective in a model of PKD. Studies were performed in male control Han:SPRD rats, and in cystic males treated with orchiectomy, 2-methoxyestradiol, 2-hydroxyestradiol (2-OHE), or vehicle, from age 3 to 12 wk. Cystic rats exhibited renal functional impairment (∼50% decrease in glomerular filtration and renal plasma flow rates, P < 0.05) and substantial cyst development (20.5 ± 2.0% of cortex area). 2-OHE was the most effective in limiting cysts (6.0 ± 0.7% of cortex area, P < 0.05 vs. vehicle-treated cystic rats) and preserving function, in association with suppression of proliferation, apoptosis, and angiogenesis markers. Downregulation of p21 expression and increased expression of Akt, the mammalian target of rapamycin (mTOR), and some of its downstream effectors were significantly reversed by 2-OHE. Thus, 2-OHE limits disease progression in a cystic rodent model. Mechanisms include reduced renal cell proliferation, apoptosis, and angiogenesis. These effects may be mediated, at least in part, by preservation of p21 and suppression of Akt and mTOR. Estradiol metabolites may represent a novel, safe intervention to slow progression of PKD.

The Raf kinase inhibitor PLX5568 slows cyst proliferation in rat polycystic kidney disease but promotes renal and hepatic fibrosis

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is a common cause of renal failure. Aberrant epithelial cell proliferation is a major cause of progressive cyst enlargement in ADPKD. Since activation of the Ras/Raf signaling system has been detected in cyst-lining epithelia, inhibition of Raf kinase has been proposed as an approach to retard the progression of ADPKD. Methods and results. PLX5568, a novel selective small molecule inhibitor of Raf kinases, attenuated proliferation of human ADPKD cyst epithelial cells. It reduced in vitro cyst growth of Madin-Darby Canine Kidney cells and of human ADPKD cells within a collagen gel. In male cy/+ rats with polycystic kidneys, PLX5568 inhibited renal cyst growth along with a significant reduction in the number of proliferating cell nuclear antigen- and phosphorylated extracellular signal-regulated kinase-positive cyst-lining epithelial cells. Furthermore, treated animals showed increased capacity to concentrate urine. However, PLX5568 did not lead to a consistent improvement of renal function. Moreover, although relative cyst volume was decreased, total kidney-to-body weight ratio was not significantly reduced by PLX5568. Further analyses revealed a 2-fold increase of renal and hepatic fibrosis in animals treated with PLX5568.

CONCLUSIONS

PLX5568 attenuated cyst enlargement in vitro and in a rat model of ADPKD without improving kidney function, presumably due to increased renal fibrosis. These data suggest that effective therapies for the treatment of ADPKD will need to target fibrosis as well as the growth of cysts.

Hypoxia-inducible factor-1α (HIF-1α) and autophagy in polycystic kidney disease (PKD)

Cyst expansion in polycystic kidney disease (PKD) results in localized hypoxia in the kidney that may activate hypoxia-inducible factor-1α (HIF-1α). HIF-1α and autophagy, a form of programmed cell repair, are induced by hypoxia. The purposes were to determine HIF-1α expression and autophagy in rat and mouse models of PKD. HIF-1α was detected by electrochemiluminescence. Autophagy was visualized by electron microscopy (EM). LC3 and beclin-1, markers of autophagy, were detected by immunoblotting. Eight-week-old male heterozygous (Cy/+) and 4-wk-old homozygous (Cy/Cy) Han:SPRD rats, 4-wk-old cpk mice, and 112-day-old Pkd2WS25/- mice with a mutation in the Pkd2 gene were studied. HIF-1α was significantly increased in massive Cy/Cy and cpk kidneys and not smaller Cy/+ and Pkd2WS25/- kidneys. On EM, features of autophagy were seen in wild-type (+/+), Cy/+, and cpk kidneys: autophagosomes, mitophagy, and autolysosomes. Specifically, autophagosomes were found on EM in the tubular cells lining the cysts in cpk mice. The increase in LC3-II, a marker of autophagosome production and beclin, a regulator of autophagy, in Cy/Cy and cpk kidneys, followed the same pattern of increase as HIF-1α. To determine the role of HIF-1α in cyst formation and/or growth, Cy/+ rats, Cy/Cy rats, and cpk mice were treated with the HIF-1α inhibitor 2-methoxyestradiol (2ME2). 2ME2 had no significant effect on kidney volume or cyst volume density. In summary, HIF-1α is highly expressed in the late stages of PKD and is associated with an increase in LC3-II and beclin-1. The first demonstration of autophagosomes in PKD kidneys is reported. Inhibition of HIF-1α did not have a therapeutic effect.