Polycystic kidney disease: genes, proteins, animal models, disease mechanisms and therapeutic opportunities

ADPKD variants in the PKD2 pore helix cause structural collapse of the gate and distinct forms of channel dysfunction

PKD2 is a member of the polycystin subfamily of transient receptor potential (TRP) ion channel subunits which traffic and function in primary cilia organelle membranes. Millions of individuals carry pathogenic genetic variants in PKD2 that cause a life-threatening condition called autosomal dominant polycystic kidney disease (ADPKD). Although ADPKD is a common monogenetic disorder, there is no drug cure or available therapeutics which address the underlying channel dysregulation. Furthermore, the structural and mechanistic impact of most disease-causing variants are uncharacterized. Using direct cilia electrophysiology, cryogenic electron microscopy (cryo-EM), and super resolution imaging, we have discovered mechanistic differences in channel dysregulation caused by three germline missense variants located in PKD2's pore helix 1. Variant C632R reduces protein thermal stability, resulting in impaired channel assembly and abolishes primary cilia trafficking. In contrast, variants F629S and R638C retain native cilia trafficking, but exhibit gating defects. Resolved cryo-EM structures (2.7-3.2Å) of the variants indicate loss of critical pore helix interactions and precipitate allosteric collapse of the channels inner gate. Results demonstrate how ADPKD-causing these mutations have divergent and ranging impacts on PKD2 function, despite their shared structural proximity. These unexpected findings underscore the need for mechanistic characterization of polycystin variants, which may guide rational drug development of ADPKD therapeutics.

Regarding polycystin nomenclature

The revised and current IUPHAR/BPS nomenclature creates ambiguity regarding the genetic identity of the polycystin family members of transient receptor potential ion channels (TRPP), especially when cross-referencing manuscripts that describe subunits using the former system 1 . Traditionally, the products of polycystin genes (e.g., PKD2) are referred to as polycystin proteins (e.g., polycystin-2). For simplicity and to prevent confusion, we will refer to the polycystin gene name rather than differentiating gene and protein with separate names- a nomenclature we have recently outlined (Annual Reviews in Physiology, Esarte Palomero et al. 2023) 2.

Calcium and Neural Stem Cell Proliferation

Intracellular calcium plays a pivotal role in central nervous system (CNS) development by regulating various processes such as cell proliferation, migration, differentiation, and maturation. However, understanding the involvement of calcium (Ca2+) in these processes during CNS development is challenging due to the dynamic nature of this cation and the evolving cell populations during development. While Ca2+ transient patterns have been observed in specific cell processes and molecules responsible for Ca2+ homeostasis have been identified in excitable and non-excitable cells, further research into Ca2+ dynamics and the underlying mechanisms in neural stem cells (NSCs) is required. This review focuses on molecules involved in Ca2+ entrance expressed in NSCs in vivo and in vitro, which are crucial for Ca2+ dynamics and signaling. It also discusses how these molecules might play a key role in balancing cell proliferation for self-renewal or promoting differentiation. These processes are finely regulated in a time-dependent manner throughout brain development, influenced by extrinsic and intrinsic factors that directly or indirectly modulate Ca2+ dynamics. Furthermore, this review addresses the potential implications of understanding Ca2+ dynamics in NSCs for treating neurological disorders. Despite significant progress in this field, unraveling the elements contributing to Ca2+ intracellular dynamics in cell proliferation remains a challenging puzzle that requires further investigation.

Experimental Models of Polycystic Kidney Disease: Applications and Therapeutic Testing

Polycystic kidney diseases (PKDs) are genetic disorders characterized by the formation and expansion of numerous fluid-filled renal cysts, damaging normal parenchyma and often leading to kidney failure. Although PKDs comprise a broad range of different diseases, with substantial genetic and phenotypic heterogeneity, an association with primary cilia represents a common theme. Great strides have been made in the identification of causative genes, furthering our understanding of the genetic complexity and disease mechanisms, but only one therapy so far has shown success in clinical trials and advanced to US Food and Drug Administration approval. A key step in understanding disease pathogenesis and testing potential therapeutics is developing orthologous experimental models that accurately recapitulate the human phenotype. This has been particularly important for PKDs because cellular models have been of limited value; however, the advent of organoid usage has expanded capabilities in this area but does not negate the need for whole-organism models where renal function can be assessed. Animal model generation is further complicated in the most common disease type, autosomal dominant PKD, by homozygous lethality and a very limited cystic phenotype in heterozygotes while for autosomal recessive PKD, mouse models have a delayed and modest kidney disease, in contrast to humans. However, for autosomal dominant PKD, the use of conditional/inducible and dosage models have resulted in some of the best disease models in nephrology. These have been used to help understand pathogenesis, to facilitate genetic interaction studies, and to perform preclinical testing. Whereas for autosomal recessive PKD, using alternative species and digenic models has partially overcome these deficiencies. Here, we review the experimental models that are currently available and most valuable for therapeutic testing in PKD, their applications, success in preclinical trials, advantages and limitations, and where further improvements are needed.

Liver transplantation for biliary cysts: perioperative and long-term outcomes

BACKGROUND

Biliary cysts (BC) is a rare indication for orthotopic liver transplantation (OLT).

METHODS

We queried the UNOS dataset to identify patients who underwent OLT for Caroli's disease (CD) and choledochal cysts (CC). All patients with BC (CD + CC) were compared to a cohort of patients transplanted for other indications. Patients with CC were also compared to those with CD. Cox proportional hazard model was performed to assess predictors of graft and patient survival.

RESULTS

261 patients underwent OLT for BC. Patients with BC had better pre-operative liver function compared to those transplanted for other indications. 5-year graft and patient survival were 72% and 81%, respectively, similar to those transplanted for other indications after matching. Patients with CC were younger and had increased preoperative cholestasis compared to those with CD. Donor age, race, and gender were predictors of poor graft and patient survival in patients transplanted for CC.

CONCLUSIONS

Patients with BC have similar outcomes to those transplanted for other indications and more frequently require MELD score exception. In patients transplanted for choledochal cysts, female gender, donor age, and African-American race were independent predictors of poor survival. Pediatric patients transplanted for Caroli's disease had better survival compared to adults.

Polycystin Channel Complexes

Polycystin subunits can form hetero- and homotetrameric ion channels in the membranes of various compartments of the cell. Homotetrameric polycystin channels are voltage- and calcium-modulated, whereas heterotetrameric versions are proposed to be ligand- or autoproteolytically regulated. Their importance is underscored by variants associated with autosomal dominant polycystic kidney disease and by vital roles in fertilization and embryonic development. The diversity in polycystin assembly and subcellular distribution allows for a multitude of sensory functions by this class of channels. In this review, we highlight their recent structural and functional characterization, which has provided a molecular blueprint to investigate the conformational changes required for channel opening in response to unique stimuli. We consider each polycystin channel type individually, discussing how they contribute to sensory cell biology, as well as their impact on the physiology of various tissues.

Expression of active B-Raf proto-oncogene in kidney collecting ducts induces cyst formation in normal mice and accelerates cyst growth in mice with polycystic kidney disease

Polycystic kidney disease (PKD) is characterized by the formation and progressive enlargement of fluid-filled cysts due to abnormal cell proliferation. Cyclic AMP agonists, including arginine vasopressin, stimulate ERK-dependent proliferation of cystic cells, but not normal kidney cells. Previously, B-Raf proto-oncogene (BRAF), a MAPK kinase kinase that activates MEK-ERK signaling, was shown to be a central intermediate in the cAMP mitogenic response. However, the role of BRAF on cyst formation and enlargement in vivo had not been demonstrated. To determine if active BRAF induces kidney cyst formation, we generated transgenic mice that conditionally express BRAFV600E, a common activating mutation, and bred them with Pkhd1-Cre mice to express active BRAF in the collecting ducts, a predominant site for cyst formation. Collecting duct expression of BRAFV600E (BRafCD) caused kidney cyst formation as early as three weeks of age. There were increased levels of phosphorylated ERK (p-ERK) and proliferating cell nuclear antigen, a marker for cell proliferation. BRafCD mice developed extensive kidney fibrosis and elevated blood urea nitrogen, indicating a decline in kidney function, by ten weeks of age. BRAFV600E transgenic mice were also bred to Pkd1RC/RC and pcy/pcy mice, well-characterized slowly progressive PKD models. Collecting duct expression of active BRAF markedly increased kidney weight/body weight, cyst number and size, and total cystic area. There were increased p-ERK levels and proliferating cells, immune cell infiltration, interstitial fibrosis, and a decline in kidney function in both these models. Thus, our findings demonstrate that active BRAF is sufficient to induce kidney cyst formation in normal mice and accelerate cystic disease in PKD mice.

Retarding Progression of Chronic Kidney Disease in Autosomal Dominant Polycystic Kidney Disease with Metformin and Other Therapies: An Update of New Insights

Autosomal dominant polycystic kidney disease (ADPKD) is the most frequent single-gene disorder leading to renal failure. Current therapies are aimed to treat renal and extrarenal complications of ADPKD, but improved knowledge of the pathophysiological mechanisms leading to the generation and growth of cysts has permitted the identification of new drug candidates for clinical trials. Among these, in this review, we will examine above all the role of metformin, hypothesized to be able to activate the AMP-activated protein kinase (AMPK) pathway and potentially modulate some mechanisms implicated in the onset and the growth of the cysts.

Autosomic dominant polycystic kidney disease and metformin: Old knowledge and new insights on retarding progression of chronic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common congenital kidney disorder, generally caused by mutations in the PKD1 and PKD2 genes, coding for polycystins 1 and 2. Its pathogenesis is accompanied by alterations of the cAMP, mTOR, MAPK/ERK, and JAK/STAT pathways. ADPKD is clinically characterized by the formation of many growing cysts with kidney enlargement and a progressive damage to the parenchyma, up to its complete loss of function, and the onset of end-stage renal disease (ESRD). The current aim of ADPKD therapy is the inhibition of cyst development and retardation of chronic kidney disease progression. Several drugs have been recently included as potential therapies for ADPKD including metformin, the drug of choice for the treatment of type 2 diabetes mellitus, according to its potential inhibitory effects on cystogenesis. In this review, we summarize preclinical and clinical evidence endorsing or rejecting metformin administration in ADPKD evolution and pathological mechanisms. We explored the biology of APDKD and the role of metformin in slowing down cystogenesis searching PubMed and Clinical Trials to identify relevant data from the database inception to December 2020. From our research analysis, evidence for metformin as emerging cure for ADPKD mainly arise from preclinical studies. In fact, clinical studies are still scanty and stronger evidence is awaited. Its effects are likely mediated by inhibition of the ERK pathway and increase of AMPK levels, which are both linked to ADPKD pathogenesis.

Genitourinary Interventions in Autosomal Dominant Polycystic Kidney Disease: Clinical Recommendations for Urologic and Transplant Surgeons

Autosomal dominant polycystic kidney disease is the fourth most common single cause of end-stage renal disease worldwide with both renal and extrarenal manifestations, resulting in significant morbidity. Approaches to the management of this disease vary widely, with no broadly accepted practice guidelines. Herein, we reviewed the various surgical and interventional management options that are targeted toward treating the symptoms or addressing the resulting kidney failure. Novel treatment modalities such as celiac plexus blockade and renal denervation appear to be promising in pain relief; however, further studies are lacking. Renal cyst decortication seems to have a higher success rate in targeting cyst-related pain compared with aspiration only. In terms of requiring major surgical intervention, such as need and timing of native nephrectomy, there are several considerations when deciding on transplantation with or without a pretransplant native nephrectomy. Patients who are not candidates for native nephrectomy may consider transcatheter arterial embolization. Based on our review of the contemporary indications for genitourinary interventions in the management of autosomal dominant polycystic kidney disease, we propose an algorithm that depicts the decision-making process on assessing the indications and timing of native nephrectomy in patients with end-stage renal disease awaiting transplant.

Histomorphology and Immunohistochemistry of a Congenital Nephromegaly Demonstrate Concurrent Features of Heritable and Acquired Cystic Nephropathies in a Girgentana Goat (Capra falconeri)

Polycystic kidney diseases (PKD) represent frequent congenital and adult nephropathies in humans and domestic animals. This report illustrates an uncommon state of congenital PKD in a girgentana goat (Capra falconeri). A stillborn female goat kid was submitted for postmortem examination and underwent macroscopic and microscopic examination. The kidneys showed a bilateral nephromegaly and a perpendicular polycystic altered texture of the renal parenchyma. Renal tissue sections were comprehensively investigated by histopathology (overview and special stains), immunohistochemistry (CD10, CD117, pan-cytokeratin, cytokeratin 7, E-cadherin, Pax2, Pax8, and vimentin), and electron microscopy (SEM, TEM). Histopathology of renal tissue sections revealed polycystic alterations of the renal parenchyma as well as conspicuous polypoid proliferates/projections of the renal tubular epithelium, which showed clear cell characteristics. Furthermore, epithelial projections were indicative for epithelio-mesenchymal-transition, cellular depolarization, and strong expression of differentiation markers Pax2, Pax8, and CD10. Ultrastructural morphology of the projections was characterized by numerous diffusely distributed, demarked round cytoplasmic structures and several apico-lateral differentiations. Additionally, hepatic malformations comprising biliary duct proliferation with saccular dilation and bridging fibrosis were observed. Notably, this report describes the first case of a congenital cystic nephropathy with overlapping features of heritable and acquired nephropathies in any species. Epithelio-mesenchymal-transition and altered cadherin expression seem to be crucial components of a suspected pathomechanism during cystogenesis.

Metabolic Changes in Polycystic Kidney Disease as a Potential Target for Systemic Treatment

Autosomal recessive and autosomal dominant polycystic kidney disease (ARPKD, ADPKD) are systemic disorders with pronounced hepatorenal phenotypes. While the main underlying genetic causes of both ARPKD and ADPKD have been well-known for years, the exact molecular mechanisms resulting in the observed clinical phenotypes in the different organs, remain incompletely understood. Recent research has identified cellular metabolic changes in PKD. These findings are of major relevance as there may be an immediate translation into clinical trials and potentially clinical practice. Here, we review important results in the field regarding metabolic changes in PKD and their modulation as a potential target of systemic treatment.

An Overview of In Vivo and In Vitro Models for Autosomal Dominant Polycystic Kidney Disease: A Journey from 3D-Cysts to Mini-Pigs

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inheritable cause of end stage renal disease and, as of today, only a single moderately effective treatment is available for patients. Even though ADPKD research has made huge progress over the last decades, the precise disease mechanisms remain elusive. However, a wide variety of cellular and animal models have been developed to decipher the pathophysiological mechanisms and related pathways underlying the disease. As none of these models perfectly recapitulates the complexity of the human disease, the aim of this review is to give an overview of the main tools currently available to ADPKD researchers, as well as their main advantages and limitations.

Extracellular matrix, integrins, and focal adhesion signaling in polycystic kidney disease

In autosomal dominant polycystic kidney disease (ADPKD), the inexorable growth of numerous fluid-filled cysts leads to massively enlarged kidneys, renal interstitial damage, inflammation, and fibrosis, and progressive decline in kidney function. It has long been recognized that interstitial fibrosis is the most important manifestation associated with end-stage renal disease; however, the role of abnormal extracellular matrix (ECM) production on ADPKD pathogenesis is not fully understood. Early evidence showed that cysts in end-stage human ADPKD kidneys had thickened and extensively laminated cellular basement membranes, and abnormal regulation of gene expression of several basement membrane components, including collagens, laminins, and proteoglycans by cyst epithelial cells. These basement membrane changes were also observed in dilated tubules and small cysts of early ADPKD kidneys, indicating that ECM alterations were early features of cyst development. Renal cystic cells were also found to overexpress several integrins and their ligands, including ECM structural components and soluble matricellular proteins. ECM ligands binding to integrins stimulate focal adhesion formation and can promote cell attachment and migration. Abnormal expression of laminin-332 (laminin-5) and its receptor α₆β₄ stimulated cyst epithelial cell proliferation; and mice that lacked laminin α₅, a component of laminin-511 normally expressed by renal tubules, had an overexpression of laminin-332 that was associated with renal cyst formation. Periostin, a matricellular protein that binds α_Vβ₃- and α_Vβ₅-integrins, was found to be highly overexpressed in the kidneys of ADPKD and autosomal recessive PKD patients, and several rodent models of PKD. α_Vβ₃-integrin is also overexpressed by cystic epithelial cells, and the binding of periostin to α_Vβ₃-integrin activates the integrin-linked kinase and downstream signal transduction pathways involved in tissue repair promoting cyst growth, ECM synthesis, and tissue fibrosis. This chapter reviews the roles of the ECM, integrins, and focal adhesion signaling in cyst growth and fibrosis in PKD.

Premise for Standardized Sepsis Models

Sepsis morbidity and mortality exacts a toll on patients and contributes significantly to healthcare costs. Preclinical models of sepsis have been used to study disease pathogenesis and test new therapies, but divergent outcomes have been observed with the same treatment even when using the same sepsis model. Other disorders such as diabetes, cancer, malaria, obesity, and cardiovascular diseases have used standardized, preclinical models that allow laboratories to compare results. Standardized models accelerate the pace of research and such models have been used to test new therapies or changes in treatment guidelines. The National Institutes of Health mandated that investigators increase data reproducibility and the rigor of scientific experiments and has also issued research funding announcements about the development and refinement of standardized models. Our premise is that refinement and standardization of preclinical sepsis models may accelerate the development and testing of potential therapeutics for human sepsis, as has been the case with preclinical models for other disorders. As a first step toward creating standardized models, we suggest standardizing the technical standards of the widely used cecal ligation and puncture model and creating a list of appropriate organ injury and immune dysfunction parameters. Standardized sepsis models could enhance reproducibility and allow comparison of results between laboratories and may accelerate our understanding of the pathogenesis of sepsis.

Proliferative signaling by ERBB proteins and RAF/MEK/ERK effectors in polycystic kidney disease

A primary pathological feature of polycystic kidney disease (PKD) is the hyperproliferation of epithelial cells in renal tubules, resulting in formation of fluid-filled cysts. The proliferative aspects of the two major forms of PKD-autosomal dominant PKD (ADPKD), which arises from mutations in the polycystins PKD1 and PKD2, and autosomal recessive PKD (ARPKD), which arises from mutations in PKHD1-has encouraged investigation into protein components of the core cell proliferative machinery as potential drivers of PKD pathogenesis. In this review, we examine the role of signaling by ERBB proteins and their effectors, with a primary focus on ADPKD. The ERBB family of receptor tyrosine kinases (EGFR/ERBB1, HER2/ERBB2, ERBB3, and ERBB4) are activated by extracellular ligands, inducing multiple pro-growth signaling cascades; among these, activation of signaling through the RAS GTPase, and the RAF, MEK1/2, and ERK1/2 kinases enhance cell proliferation and restrict apoptosis during renal tubuloepithelial cyst formation. Characteristics of PKD include overexpression and mislocalization of the ERBB receptors and ligands, leading to enhanced activation and increased activity of downstream signaling proteins. The altered regulation of ERBBs and their effectors in PKD is influenced by enhanced activity of SRC kinase, which is promoted by the loss of cytoplasmic Ca²⁺ and an increase in cAMP-dependent PKA kinase activity that stimulates CFTR, driving the secretory phenotype of ADPKD. We discuss the interplay between ERBB/SRC signaling, and polycystins and their depending signaling, with emphasis on thes changes that affect cell proliferation in cyst expansion, as well as the inflammation-associated fibrogenesis, which characterizes progressive disease. We summarize the current progress of preclinical and clinical trials directed at inhibiting this signaling axis, and discuss potential future strategies that may be productive for controlling PKD.

Drug prioritization using the semantic properties of a knowledge graph

Compounds that are candidates for drug repurposing can be ranked by leveraging knowledge available in the biomedical literature and databases. This knowledge, spread across a variety of sources, can be integrated within a knowledge graph, which thereby comprehensively describes known relationships between biomedical concepts, such as drugs, diseases, genes, etc. Our work uses the semantic information between drug and disease concepts as features, which are extracted from an existing knowledge graph that integrates 200 different biological knowledge sources. RepoDB, a standard drug repurposing database which describes drug-disease combinations that were approved or that failed in clinical trials, is used to train a random forest classifier. The 10-times repeated 10-fold cross-validation performance of the classifier achieves a mean area under the receiver operating characteristic curve (AUC) of 92.2%. We apply the classifier to prioritize 21 preclinical drug repurposing candidates that have been suggested for Autosomal Dominant Polycystic Kidney Disease (ADPKD). Mozavaptan, a vasopressin V2 receptor antagonist is predicted to be the drug most likely to be approved after a clinical trial, and belongs to the same drug class as tolvaptan, the only treatment for ADPKD that is currently approved. We conclude that semantic properties of concepts in a knowledge graph can be exploited to prioritize drug repurposing candidates for testing in clinical trials.

Metabolomics Approaches for the Diagnosis and Understanding of Kidney Diseases

Diseases of the kidney are difficult to diagnose and treat. This review summarises the definition, cause, epidemiology and treatment of some of these diseases including chronic kidney disease, diabetic nephropathy, acute kidney injury, kidney cancer, kidney transplantation and polycystic kidney diseases. Numerous studies have adopted a metabolomics approach to uncover new small molecule biomarkers of kidney diseases to improve specificity and sensitivity of diagnosis and to uncover biochemical mechanisms that may elucidate the cause and progression of these diseases. This work includes a description of mass spectrometry-based metabolomics approaches, including some of the currently available tools, and emphasises findings from metabolomics studies of kidney diseases. We have included a varied selection of studies (disease, model, sample number, analytical platform) and focused on metabolites which were commonly reported as discriminating features between kidney disease and a control. These metabolites are likely to be robust indicators of kidney disease processes, and therefore potential biomarkers, warranting further investigation.

Pharmacokinetic Evaluation of Nimotuzumab in Patients With Autosomal Dominant Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD) is a genetic disease characterized by an overexpression and mislocalization of epidermal growth factor receptor (EGFR) to the apical membranes of cystic epithelial cells. Nimotuzumab is a humanized antibody that recognizes an extracellular domain III of human EGFR. The aim of this study was to assess the pharmacokinetic behavior of nimotuzumab in patients with ADPKD given as a single dose. A phase I, single-center, and noncontrolled open clinical study was conducted. Five patients were enrolled at each of the following fixed-dose levels: 50, 100, 200, and 400 mg. Intravenous continuous infusions of nimotuzumab were administered every 14 days during a year, except the first administration, when blood samples were drawn during 28 days for pharmacokinetic assessments. Subjects were closely monitored during the trial and at completion of the administration of nimotuzumab, including the anti-idiotypic response. For the first time, nimotuzumab was used for treating a nononcological disease. The administration of nimotuzumab showed dose-dependent kinetics. Nimotuzumab does not develop anti-idiotypic response against the murine portion present in the hypervariable region of the antibody present in the serum of the patients treated. No significant differences were found in the systemic clearance between the 100- and 400-mg dose, which indicates that the optimal biological dose is in this range of dose.

Role of calcium in adult onset polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in genes encoding the polycystin (PC) 1 and 2 proteins. The goal of this study was to determine the role of calcium in regulating cyst growth. Stromal interaction molecule 1 (STIM1) protein expression was 15-fold higher in PC1-null proximal tubule cells (PN) than in heterozygote (PH) controls and 2-fold higher in an inducible, PC1 knockout, mouse model of ADPKD compared to a non-cystic match control. IP3 receptor protein expression was also higher in the cystic mice. Knocking down STIM1 with siRNA reduced cyst growth and lowered cAMP levels in PN cells. Fura2 measurements of intracellular Ca2+ showed higher levels of intracellular Ca2+, SOCE and thaspigargin-stimulated ER Ca2+ release in PN vs. PH cells. There was a dramatic reduction in thapsigargin-stimulated release of ER Ca2+ following STIM1 silencing or application of 2-APB, consistent with altered ER Ca2+ movement; the protein expression of the Ca2+-dependent adenylyl cyclases (AC) AC3 and AC6 was up- and down-regulated, respectively. Like STIM1 knockdown, application of the calmodulin inhibitor W7 lowered cAMP levels, further indicating that STIM1 regulates AC3 via Ca2+ We conclude that the high levels of STIM1 in ADPKD cells play a role in supporting cyst growth and promoting high cAMP levels and an increased release of Ca2+ from the ER. Thus, our results provide novel therapeutic targets for treating ADPKD.

Effect of Trehalose Supplementation on Autophagy and Cystogenesis in a Mouse Model of Polycystic Kidney Disease

Autophagy impairment has been demonstrated in the pathogenesis of autosomal dominant polycystic kidney disease (ADPKD) and could be a new target of treatment. Trehalose is a natural, nonreducing disaccharide that has been shown to enhance autophagy. Therefore, we investigated whether trehalose treatment reduces renal cyst formation in a Pkd1-hypomorphic mouse model. Pkd1 miRNA transgenic (Pkd1 miR Tg) mice and wild-type littermates were given drinking water supplemented with 2% trehalose from postnatal day 35 to postnatal day 91. The control groups received pure water or 2% sucrose for the control of hyperosmolarity. The effect on kidney weights, cystic indices, renal function, cell proliferation, and autophagic activities was determined. We found that Pkd1 miR Tg mice had a significantly lower renal mRNA expression of autophagy-related genes, including atg5, atg12, ulk1, beclin1, and p62, compared with wild-type control mice. Furthermore, immunohistochemical analysis showed that cystic lining cells had strong positive staining for the p62 protein, indicating impaired degradation of the protein by the autophagy-lysosome pathway. However, trehalose treatment did not improve reduced autophagy activities, nor did it reduce relative kidney weights, plasma blood urea nitrogen levels, or cystatin C levels in Pkd1 miR Tg mice. Histomorphological analysis revealed no significant differences in the renal cyst index, fibrosis score, or proliferative score among trehalose-, sucrose-, and water-treated groups. Our results demonstrate that adding trehalose to drinking water does not modulate autophagy activities and renal cystogenesis in Pkd1-deficient mice, suggesting that an oral supplement of trehalose may not affect the progression of ADPKD.

Impact of miR-192 and miR-194 on cyst enlargement through EMT in autosomal dominant polycystic kidney disease

Altered miRNA (miR) expression occurs in various diseases. However, the therapeutic effect of miRNAs in autosomal dominant polycystic kidney disease (ADPKD) is unclear. Genome-wide analyses of miRNA expression and DNA methylation status were conducted to identify crucial miRNAs in end-stage ADPKD. miR-192 and -194 levels were down-regulated with hypermethylation at these loci, mainly in the intermediate and late stages, not in the early stage, of cystogenesis, suggesting their potential impact on cyst expansion. Cyst expansion has been strongly associated with endothelial-mesenchymal transition (EMT). Zinc finger E-box-binding homeobox-2 and cadherin-2, which are involved in EMT, were directly regulated by miR-192 and -194. The therapeutic effect of miR-192 and -194 in vivo and in vitro were assessed. Restoring these miRs by injection of precursors influenced the reduced size of cysts in Pkd1 conditional knockout mice. miR-192 and -194 may act as potential therapeutic targets to control the expansion and progression of cysts in patients with ADPKD.-Kim, D. Y., Woo, Y. M., Lee, S., Oh, S., Shin, Y., Shin, J.-O., Park, E. Y., Ko, J. Y., Lee, E. J., Bok, J., Yoo, K. H., Park, J. H. Impact of miR-192 and miR-194 on cyst enlargement through EMT in autosomal dominant polycystic kidney disease.

A potential strategy for reducing cysts in autosomal dominant polycystic kidney disease with a CFTR corrector

Autosomal dominant polycystic kidney disease (ADPKD) is associated with progressive enlargement of cysts, leading to a decline in function and renal failure that cannot be prevented by current treatments. Mutations in pkd1 and pkd2, encoding the polycystin 1 and 2 proteins, induce growth-related pathways, including heat shock proteins, as occurs in some cancers, raising the prospect that pharmacological interventions that target these pathways might alleviate or prevent ADPKD. Here, we demonstrate a role for VX-809, a corrector of cystic fibrosis transmembrane conductance regulator (CFTR), conventionally used to manage cystic fibrosis in reducing renal cyst growth. VX-809 reduced cyst growth in Pkd1-knockout mice and in proximal, tubule-derived, cultured Pkd1 knockout cells. VX-809 reduced both basal and forskolin-activated cAMP levels and also decreased the expression of the adenylyl cyclase AC3 but not of AC6. VX-809 also decreased resting levels of intracellular Ca²⁺ but did not affect ATP-stimulated Ca²⁺ release. Notably, VX-809 dramatically decreased thapsigargin-induced release of Ca²⁺ from the endoplasmic reticulum (ER). VX-809 also reduced the levels of heat shock proteins Hsp27, Hsp70, and Hsp90 in mice cystic kidneys, consistent with the restoration of cellular proteostasis. Moreover, VX-809 strongly decreased an ER stress marker, the GADD153 protein, and cell proliferation but had only a small effect on apoptosis. Given that administration of VX-809 is safe, this drug potentially offers a new way to treat patients with ADPKD.

Polycystin 1 loss of function is directly linked to an imbalance in G-protein signaling in the kidney

The development of the kidney relies on the establishment and maintenance of a precise tubular diameter of its functional units, the nephrons. This process is disrupted in polycystic kidney disease (PKD), resulting in dilations of the nephron and renal cyst formation. In the course of exploring G-protein-coupled signaling in the Xenopus pronephric kidney, we discovered that loss of the G-protein α subunit, Gnas, results in a PKD phenotype. Polycystin 1, one of the genes mutated in human PKD, encodes a protein resembling a G-protein-coupled receptor. Furthermore, deletion of the G-protein-binding domain present in the intracellular C terminus of polycystin 1 impacts functionality. A comprehensive analysis of all the G-protein α subunits expressed in the Xenopus pronephric kidney demonstrates that polycystin 1 recruits a select subset of G-protein α subunits and that their knockdown - as in the case of Gnas - results in a PKD phenotype. Mechanistically, the phenotype is caused by increased endogenous G-protein β/γ signaling and can be reversed by pharmacological inhibitors as well as knocking down Gnb1. Together, our data support the hypothesis that G proteins are recruited to the intracellular domain of PKD1 and that this interaction is crucial for its function in the kidney.

Recent advances in the molecular diagnosis of polycystic kidney disease

INTRODUCTION

Polycystic kidney disease (PKD) is clinically and genetically heterogeneous and constitutes the most common heritable kidney disease. Most patients are affected by the autosomal dominant form (ADPKD) which generally is an adult-onset multisystem disorder. By contrast, the rarer recessive form ARPKD usually already manifests perinatally or in childhood. In some patients, however, ADPKD and ARPKD can phenotypically overlap with early manifestation in ADPKD and only late onset in ARPKD. Progressive fibrocystic renal changes are often accompanied by severe hepatobiliary changes or other extrarenal abnormalities. Areas covered: A reduced dosage of disease proteins disturbs cell homeostasis and explains a more severe clinical course in some PKD patients. Cystic kidney disease is also a common feature of other ciliopathies and genetic syndromes. Genetic diagnosis may guide clinical management and helps to avoid invasive measures and to detect renal and extrarenal comorbidities early in the clinical course. Expert Commentary: The broad phenotypic and genetic heterogeneity of cystic and polycystic kidney diseases make NGS a particularly powerful approach. Interpretation of data becomes the challenge and bench and bedside benefit from digitized multidisciplinary interrelationships.

Histone deacetylase 6 inhibition reduces cysts by decreasing cAMP and Ca2+ in knock-out mouse models of polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is associated with progressive enlargement of multiple renal cysts, often leading to renal failure that cannot be prevented by a current treatment. Two proteins encoded by two genes are associated with ADPKD: PC1 (pkd1), primarily a signaling molecule, and PC2 (pkd2), a Ca²⁺ channel. Dysregulation of cAMP signaling is central to ADPKD, but the molecular mechanism is unresolved. Here, we studied the role of histone deacetylase 6 (HDAC6) in regulating cyst growth to test the possibility that inhibiting HDAC6 might help manage ADPKD. Chemical inhibition of HDAC6 reduced cyst growth in PC1-knock-out mice. In proximal tubule-derived, PC1-knock-out cells, adenylyl cyclase 6 and 3 (AC6 and -3) are both expressed. AC6 protein expression was higher in cells lacking PC1, compared with control cells containing PC1. Intracellular Ca²⁺ was higher in PC1-knock-out cells than in control cells. HDAC inhibition caused a drop in intracellular Ca²⁺ and increased ATP-simulated Ca²⁺ release. HDAC6 inhibition reduced the release of Ca²⁺ from the endoplasmic reticulum induced by thapsigargin, an inhibitor of endoplasmic reticulum Ca²⁺-ATPase. HDAC6 inhibition and treatment of cells with the intracellular Ca²⁺ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) reduced cAMP levels in PC1-knock-out cells. Finally, the calmodulin inhibitors W-7 and W-13 reduced cAMP levels, and W-7 reduced cyst growth, suggesting that AC3 is involved in cyst growth regulated by HDAC6. We conclude that HDAC6 inhibition reduces cell growth primarily by reducing intracellular cAMP and Ca²⁺ levels. Our results provide potential therapeutic targets that may be useful as treatments for ADPKD.

A Drug Development Tool for Trial Enrichment in Patients With Autosomal Dominant Polycystic Kidney Disease

Introduction

Total kidney volume (TKV) is a promising imaging biomarker for tracking and predicting the natural history of patients with autosomal dominant polycystic kidney disease.

Methods

A drug development tool was developed by linking longitudinal TKV measurements to the probability of a 30% decline of estimated glomerular filtration rate (eGFR) or end-stage renal disease. Drug development tools were developed based on observational data collected over multiple decades for an eGFR decline and end-stage renal disease in 641 and 866 patients with autosomal dominant polycystic kidney disease, respectively.

Results

The statistical association between predicted TKV at the time of a 30% decline of eGFR and that at the time of end-stage renal disease were both highly significant (P < 0.0001). The drug development tool was applied to demonstrate the utility of trial enrichment according to prespecified baseline TKV, age, and eGFR as enrollment criteria in hypothetical clinical trials. Patients with larger TKV (≥1000 ml) displayed steeper slopes of hazard, which translated into a higher risk of a 30% decline of eGFR within each baseline age (< or ≥40 years) or baseline eGFR (< or ≥50 ml/min per 1.73 m²) subgroups.

Discussion

These results suggest that, when eGFR is preserved, patients with larger TKV are more likely to progress to a 30% decline of eGFR within the course of a clinical trial, whereas eGFR and age displayed limited predictive value of disease progression in early disease. Pharmaceutical sponsors and academic investigators are encouraged to prospectively employ the above drug development tool to optimize trial designs in patients with autosomal dominant polycystic kidney disease.

Demographics of polycystic kidney disease and captive population viability in pygmy hippopotamus (Choeropsis liberiensis)

Polycystic kidney disease (PKD) was previously diagnosed at necropsy in several pygmy hippopotami (Choeropsis liberiensis) from the Smithsonian National Zoo and Zoo Basel, suggesting a threat to the long-term viability of the captive population. We determined the incidence and demographics of PKD in the captive population historically; we tested if the condition is linked to pedigree; we investigated mode of inheritance; we examined effects of PKD on longevity; we conducted survival analysis; and we examined long-term population viability. Thirty-seven percent of 149 necropsied adult pygmy hippos were affected by PKD, and it was more common in females, controlling for the overall female-biased sex-ratio. Prevalence increased significantly with age, but most hippos were beyond their reproductive prime before developing clinical signs; thus fecundity was likely unaffected. PKD was linked to pedigree and may exhibit X-linked dominance, but further research is needed to definitively establish the mode of inheritance. PKD did not affect longevity, overall or within any age class. There was no significant correlation between inbreeding coefficient (F) and PKD, and the prevalence in wild-caught and captive-born animals was similar. Longevity for both captive-born and inbred hippos (F > 0) was significantly shorter than longevity for their wild-caught and non-inbred counterparts. Demographic projections indicated the extant population will likely experience a slow increase over time, provided there are no space constraints. We conclude that although PKD is an important cause of morbidity and mortality in pygmy hippos, the condition is not a primary concern for overall viability of the captive population.

The importance of total kidney volume in evaluating progression of polycystic kidney disease

The rate at which autosomal dominant polycystic kidney disease (ADPKD) progresses to end-stage renal disease varies widely and is determined by genetic and non-genetic factors. The ability to determine the prognosis of children and young adults with ADPKD is important for the effective life-long management of the disease and to enable the efficacy of emerging therapies to be determined. Total kidney volume (TKV) reflects the sum volume of hundreds of individual cysts with potentially devastating effects on renal function. The sequential measurement of TKV has been advanced as a dynamic biomarker of disease progression, yet doubt remains among nephrologists and regulatory agencies as to its usefulness. Here, we review the mechanisms that lead to an increase in TKV in ADPKD, and examine the evidence supporting the conclusion that TKV provides a metric of disease progression that can be used to assess the efficacy of potential therapeutic regimens in children and adults with ADPKD. Moreover, we propose that TKV can be used to monitor treatment efficacy in patients with normal levels of renal function, before the pathologic processes of ADPKD cause extensive fibrosis and irreversible loss of functioning renal tissue.

Inhibition of cyst growth in PCK and Wpk rat models of polycystic kidney disease with low doses of peroxisome proliferator-activated receptor γ agonists

BACKGROUND AND OBJECTIVES

The studies were designed to test the efficacy of two peroxisome proliferator-activated receptor γ (PPARγ) agonists in two rodent models of polycystic kidney disease (PKD).

MATERIALS AND METHODS

The PCK rat is a slowly progressing cystic model while the Wpk^-/- rat is a rapidly progressing model. PCK rats were fed with a pharmacological (0.4 mg/kg body weight [BW]) and a sub-pharmacological (0.04 mg/kg BW) dose of rosiglitazone (week 4-28). Wpk^-/- rats were fed with pharmacological (2.0 mg/kg BW) and sub-pharmacologic (0.2 mg/kg BW) doses of pioglitazone from day 5 to 18. At termination, kidney weights of treated versus untreated cystic animals were used to determine efficacy. The current studies were also compared with previous studies containing higher doses of PPARγ agonists. The concentrations used in the animals were calculated with reference to equivalent human doses for both drugs.

RESULTS

The current studies demonstrate: 1) that low, pharmacologically relevant, doses of the PPARγ agonists effectively inhibit cyst growth; 2) there is a class action of the drugs with both commercially available PPARγ agonists, rosiglitazone, and pioglitazone, inhibiting cyst growth; 3) the drugs showed efficacy in two different preclinical cystic models. In the PCK rat, animals fed with a sub-pharmacological dose of rosiglitazone for 24 weeks had significantly lower kidney weights than untreated animals (3.68 ± 0.13 g vs. 4.17 ± 0. 11 g, respectively, P < 0.01) while treatment with a pharmacologic dose had no significant effect on kidney weight. The rapidly progressing Wpk^-/- rats were fed with pharmacological and sub-pharmacologic doses of pioglitazone from day 5 to 18 and the kidneys were compared with non-treated, cystic animals. Kidney weights on the pharmacologic dose were not statistically lower than the untreated animals while rats fed a sub-pharmacologic dose showed a significant decrease compared with untreated animals (3.35 ± 0.15 g vs. 4.55 ± 0.46 g, respectively, P = 0.045).

CONCLUSION

Concentrations of PPARγ agonists below the human equivalent diabetic doses are effective in slowing cyst growth in two rodent models of PKD.

STIM1fl/fl Ksp-Cre Mouse has Impaired Renal Water Balance

BACKGROUND/AIM

STIM1 is as an essential component in store operated Ca2+ entry. However give the paucity of information on the role of STIM1 in kidney, the aim was to study the function of STIM1 in the medulla of the kidney.

METHODS

we crossed a Ksp-cre mouse with another mouse containing two loxP sites flanking Exon 6 of STIM1. The Ksp-cre mouse is based upon the Ksp-cadherin gene promoter which expresses cre recombinase in developing nephrons, collecting ducts (SD) and thick ascending limbs (TAL) of the loop of Henle.

RESULTS

The offspring of these mice are viable without gross morphological changes, however, we noticed that the STIM1 Ksp-cre knockout mice produced more urine compared to control. To examine this more carefully, we fed mice low (LP) and high protein (HP) diets respectively. When mice were fed HP diet STIM1 ko mice had significantly increased urinary volume and lower specific gravity compared to wt mice. In STIM1 ko mice fed HP diet urine creatinine and urea were significantly lower compared to wt mice fed HP diet, however the fractional excretion was the same.

CONCLUSION

These data support the idea that STIM1 ko mice have impaired urinary concentrating ability when challenged with HP diet is most likely caused by impaired Ca2+-dependent signal transduction through the vasopressin receptor cascade.

CaMKII as a pathological mediator of ER stress, oxidative stress, and mitochondrial dysfunction in a murine model of nephronophthisis

Polycystic kidney diseases (PKDs) are genetic diseases characterized by renal cyst formation with increased cell proliferation, apoptosis, and transition to a secretory phenotype at the expense of terminal differentiation. Despite recent progress in understanding PKD pathogenesis and the emergence of potential therapies, the key molecular mechanisms promoting cystogenesis are not well understood. Here, we demonstrate that mechanisms including endoplasmic reticulum stress, oxidative damage, and compromised mitochondrial function all contribute to nephronophthisis-associated PKD. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is emerging as a critical mediator of these cellular processes. Therefore, we reasoned that pharmacological targeting of CaMKII may translate into effective inhibition of PKD in jck mice. Our data demonstrate that CaMKII is activated within cystic kidney epithelia in jck mice. Blockade of CaMKII with a selective inhibitor results in effective inhibition of PKD in jck mice. Mechanistic experiments in vitro and in vivo demonstrated that CaMKII inhibition relieves endoplasmic reticulum stress and oxidative damage and improves mitochondrial integrity and membrane potential. Taken together, our data support CaMKII inhibition as a new and effective therapeutic avenue for the treatment of cystic diseases.

Ouabain Regulates CFTR-Mediated Anion Secretion and Na,K-ATPase Transport in ADPKD Cells

Cyst enlargement in autosomal dominant polycystic kidney disease (ADPKD) requires the transepithelial secretion of fluid into the cyst lumen. We previously showed that physiological amounts of ouabain enhance cAMP-dependent fluid secretion and cyst growth of human ADPKD cyst epithelial cells in culture and formation of cyst-like dilations in metanephric kidneys from Pkd1 mutant mice. Here, we investigated the mechanisms by which ouabain promotes cAMP-dependent fluid secretion and cystogenesis. Ouabain (3 nM) enhanced cAMP-induced cyst-like dilations in embryonic kidneys from Pkd1 (m1Bei) mice, but had no effect on metanephroi from Pkd1 (m1Bei) mice that lack expression of the cystic fibrosis transmembrane conductance regulator (CFTR). Similarly, ouabain stimulation of cAMP-induced fluid secretion and in vitro cyst growth of ADPKD cells were abrogated by CFTR inhibition, showing that CFTR is required for ouabain effects on ADPKD fluid secretion. Moreover, ouabain directly enhanced the cAMP-dependent Cl(-) efflux mediated by CFTR in ADPKD monolayers. Ouabain increased the trafficking of CFTR to the plasma membrane and up-regulated the expression of the CFTR activator PDZK1. Finally, ouabain decreased plasma membrane expression and activity of the Na,K-ATPase in ADPKD cells. Altogether, these results show that ouabain enhances net fluid secretion and cyst formation by activating apical anion secretion via CFTR and decreasing basolateral Na(+) transport via Na,K-ATPase. These results provide new information on the mechanisms by which ouabain affects ADPKD cells and further highlight the importance of ouabain as a non-genomic stimulator of cystogenesis in ADPKD.

Magnetic resonance microscopy of renal and biliary abnormalities in excised tissues from a mouse model of autosomal recessive polycystic kidney disease

Polycystic kidney disease (PKD) is transmitted as either an autosomal dominant or recessive trait and is a major cause of renal failure and liver fibrosis. The cpk mouse model of autosomal recessive PKD (ARPKD) has been extensively characterized using standard histopathological techniques after euthanasia. In the current study, we sought to validate magnetic resonance microscopy (MRM) as a robust tool for assessing the ARPKD phenotype. We used MRM to evaluate the liver and kidney of wild-type and cpk animals at resolutions <100 μm and generated three-dimensional (3D) renderings for pathological evaluation. Our study demonstrates that MRM is an excellent method for evaluating the complex, 3D structural defects in this ARPKD mouse model. We found that MRM was equivalent to water displacement in assessing kidney volume. Additionally, using MRM we demonstrated for the first time that the cpk liver exhibits less extensive ductal arborization, that it was reduced in volume, and that the ductal volume was disproportionately smaller. Histopathology indicates that this is a consequence of bile duct malformation. With its reduced processing time, volumetric information, and 3D capabilities, MRM will be a useful tool for future in vivo and longitudinal studies of disease progression in ARPKD. In addition, MRM will provide a unique tool to determine whether the human disease shares the newly appreciated features of the murine biliary phenotype.

Use of Ultra-high Field MRI in Small Rodent Models of Polycystic Kidney Disease for In Vivo Phenotyping and Drug Monitoring

Several in vivo pre-clinical studies in Polycystic Kidney Disease (PKD) utilize orthologous rodent models to identify and study the genetic and molecular mechanisms responsible for the disease, and are very convenient for rapid drug screening and testing of promising therapies. A limiting factor in these studies is often the lack of efficient non-invasive methods for sequentially analyzing the anatomical and functional changes in the kidney. Magnetic resonance imaging (MRI) is the current gold standard imaging technique to follow autosomal dominant polycystic kidney disease (ADPKD) patients, providing excellent soft tissue contrast and anatomic detail and allowing Total Kidney Volume (TKV) measurements.A major advantage of MRI in rodent models of PKD is the possibility for in vivo imaging allowing for longitudinal studies that use the same animal and therefore reducing the total number of animals required. In this manuscript, we will focus on using Ultra-high field (UHF) MRI to non-invasively acquire in vivo images of rodent models for PKD. The main goal of this work is to introduce the use of MRI as a tool for in vivo phenotypical characterization and drug monitoring in rodent models for PKD.

Effects of hydration in rats and mice with polycystic kidney disease

Vasopressin and V2 receptor signaling promote polycystic kidney disease (PKD) progression, raising the question whether suppression of vasopressin release through enhanced hydration can delay disease advancement. Enhanced hydration by adding 5% glucose to the drinking water has proven protective in a rat model orthologous to autosomal recessive PKD. We wanted to exclude a glucose effect and explore the influence of enhanced hydration in a mouse model orthologous to autosomal dominant PKD. PCK rats were assigned to normal water intake (NWI) or high water intake (HWI) groups achieved by feeding a hydrated agar diet (HWI-agar) or by adding 5% glucose to the drinking water (HWI-glucose), with the latter group used to recapitulate previously published results. Homozygous Pkd1 R3277C (Pkd1(RC/RC)) mice were assigned to NWI and HWI-agar groups. To evaluate the effectiveness of HWI, kidney weight and histomorphometry were assessed, and urine vasopressin, renal cAMP levels, and phosphodiesterase activities were measured. HWI-agar, like HWI-glucose, reduced urine vasopressin, renal cAMP levels, and PKD severity in PCK rats but not in Pkd1(RC/RC) mice. Compared with rat kidneys, mouse kidneys had higher phosphodiesterase activity and lower cAMP levels and were less sensitive to the cystogenic effect of 1-deamino-8-d-arginine vasopressin, as previously shown for Pkd1(RC/RC) mice and confirmed here in Pkd2(WS25/-) mice. We conclude that the effect of enhanced hydration in rat and mouse models of PKD differs. More powerful suppression of V2 receptor-mediated signaling than achievable by enhanced hydration alone may be necessary to affect the development of PKD in mouse models.

Mesenchymal Stromal Cells Improve Renovascular Function in Polycystic Kidney Disease

Polycystic kidney disease (PKD) is a common cause of end-stage renal failure, for which there is no accepted treatment. Progenitor and stem cells have been shown to restore renal function in a model of renovascular disease, a disease that shares many features with PKD. The objective of this study was to examine the potential of adult stem cells to restore renal structure and function in PKD. Bone marrow-derived mesenchymal stromal cells (MSCs, 2.5 × 10(5)) were intrarenally infused in 6-week-old PCK rats. At 10 weeks of age, PCK rats had an increase in systolic blood pressure (SBP) versus controls (126.22 ± 2.74 vs. 116.45 ± 3.53 mmHg, p < 0.05) and decreased creatinine clearance (3.76 ± 0.31 vs. 6.10 ± 0.48 µl/min/g, p < 0.01), which were improved in PKD animals that received MSCs (SBP: 114.67 ± 1.34 mmHg, and creatinine clearance: 4.82 ± 0.24 µl/min/g, p = 0.001 and p = 0.003 vs. PKD, respectively). MSCs preserved vascular density and glomeruli diameter, measured using microcomputed tomography. PCK animals had increased urine osmolality (843.9 ± 54.95 vs. 605.6 ± 45.34 mOsm, p < 0.01 vs. control), which was improved after MSC infusion and not different from control (723.75 ± 56.6 mOsm, p = 0.13 vs. control). Furthermore, MSCs reduced fibrosis and preserved the expression of proangiogenic molecules, while cyst size and number were unaltered by MSCs. Delivery of exogenous MSCs improved vascular density and renal function in PCK animals, and the benefit was observed up to 4 weeks after a single infusion. Cell-based therapy constitutes a novel approach in PKD.

Translational research in ADPKD: lessons from animal models

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in PKD1 or PKD2, which encode polycystin-1 and polycystin-2, respectively. Rodent models are available to study the pathogenesis of polycystic kidney disease (PKD) and for preclinical testing of potential therapies-either genetically engineered models carrying mutations in Pkd1 or Pkd2 or models of renal cystic disease that do not have mutations in these genes. The models are characterized by age at onset of disease, rate of disease progression, the affected nephron segment, the number of affected nephrons, synchronized or unsynchronized cyst formation and the extent of fibrosis and inflammation. Mouse models have provided valuable mechanistic insights into the pathogenesis of PKD; for example, mutated Pkd1 or Pkd2 cause renal cysts but additional factors are also required, and the rate of cyst formation is increased in the presence of renal injury. Animal studies have also revealed complex genetic and functional interactions among various genes and proteins associated with PKD. Here, we provide an update on the preclinical models commonly used to study the molecular pathogenesis of ADPKD and test potential therapeutic strategies. Progress made in understanding the pathophysiology of human ADPKD through these animal models is also discussed.

Dietary n-3 polyunsaturated fatty acids or soy protein isolate did not attenuate disease progression in a female rat model of autosomal recessive polycystic kidney disease

Polycystic kidney disease (PKD) is an incurable genetic disorder that is characterized by multiple benign cysts. As PKD advances, cyst growth increases kidney volume, decreases renal function, and may lead to end-stage renal disease; however, in a PKD rat model, feeding soy protein isolate (SPI) reduced cyst proliferation and growth. The n-3 polyunsaturated fatty acids (PUFAs) are noted for their anti-inflammatory actions. Therefore, diet therapy could offer a potentially efficacious, safe, and cost-effective strategy for treating PKD. The objective of this study was to investigate the role of soy protein and/or n-3 PUFAs on PKD progression and severity in the rat model of autosomal recessive PKD. We hypothesized that the antiproliferative and anti-inflammatory actions associated with soy protein and n-3 PUFA supplementation will attenuate PKD progression in female PCK rats. For 12 weeks, young (age, 28 days) female PCK rats were randomly assigned (n=12/group) to 4 different diets: casein±corn oil, casein±soybean oil, SPI±soybean oil, or SPI±1:1 soybean/salmon oil (SPI±SB). The feeding of the different protein and lipid sources had no significant effect on relative kidney weight. Histologic evaluation showed no significant differences in cortical or medullary cyst size, interstitial inflammation, and fibrosis among diet groups. However, rats fed SPI±SB diet had cortical cyst obstruction and the highest (P<.01) serum blood urea nitrogen concentration. Rats fed SPI±SB diet had the highest (P<.001) renal docosahexaeonic acid, but there were no significant differences in renal tissue inflammation and proliferation gene expression among the diet groups. Based on these results, dietary soy protein and/or n-3 PUFAs did not attenuate disease progression or severity in the female PCK rat model of autosomal recessive PKD.

The bigger the better: determining nephron size in kidney

The main functions of the kidney are to excrete metabolic waste products and actively reabsorb essential molecules such as amino acids, ions, glucose and water. In humans, a wide range of genetic disorders exist characterized by wasting of metabolically important compounds. At the cellular level, more than 20 highly specialized renal epithelial cell types located in different segments of the nephron contribute to the reabsorption process. In particular, proximal tubular cells play a crucial role and are uniquely adapted to maximize reabsorption efficiency. They accommodate high numbers of transporters and channels by increasing the apical surface area in contact with the primary filtrate by forming a brush border as well as undergoing hypertrophy and hyperplasia. This adaptation is evolutionarily conserved and is detected in the primitive pronephric kidney of fish and amphibians as well as the metanephric kidney of higher vertebrates. Surprisingly, signaling pathways regulating these three processes have remained largely unknown. Here we summarize recent studies that highlight the early phases of kidney development as a critical juncture in establishing proximal tubule size.

Effects of combined administration of rapamycin, tolvaptan, and AEZ-131 on the progression of polycystic disease in PCK rats

Both experimental and clinical studies have suggested that any potential treatment of polycystic kidney disease (PKD) should start early and last for a long time to be effective, with unavoidable side reactions and considerable costs. The aim of the present study was to test how low doses of rapamycin (RAPA; 0.15 mg/kg ip for 4 days/wk), tolvaptan (TOLV; 0.005% in diet), or AEZ-131 (AEZ; a novel ERK inhibitor, 30 mg/kg for 3 days/wk by gavage), alone and in association, affect the progression of polycystic renal disease in PCK rats. Rats were treated for 8 wk starting at 4-6 wk of age. The efficacy of low doses of such drugs in inhibiting their respective targets was confirmed by immunoblot experiments. Compared with rats in the control (CON) group, RAPA treatment caused a significant reduction in cyst volume density (CVD; -19% vs. the CON group) and was numerically similar to that in TOLV-treated rats (-18%, not significiant), whereas AEZ treatment was not effective. RAPA + TOLV treatment resulted in a significantly lower CVD (-49% vs. the CON group) and was associated with a striking decrease in cAMP response element-binding protein phosphorylation, and similar data were detected in RAPA + AEZ-treated rats (-42%), whereas TOLV + AEZ treatment had virtually no effect. RAPA administration significantly lessened body weight gain, whereas TOLV administration resulted a mild increase in diuresis and a significant increase in cAMP urinary excretion. Histological data of tubular proliferation were in full agreement with CVD data. In conclusion, this study demonstrates that the association of low doses of RAPA, TOLV, and AEZ slows the progression of PKD with limited side effects, suggesting the use of combined therapies also in clinical trials.

Tolvaptan delays the onset of end-stage renal disease in a polycystic kidney disease model by suppressing increases in kidney volume and renal injury

Tolvaptan, a selective vasopressin V2 receptor antagonist, slows the increase in total kidney volume and the decline in kidney function in patients with the results of the Tolvaptan Efficacy and Safety in Management of Autosomal Dominant Polycystic Kidney Disease and Outcome (TEMPO) 3:4 trial. However, it was unclear which dose of tolvaptan was optimal or whether tolvaptan was able to delay progression to end-stage renal disease (ESRD). Here we examined the relationship with aquaresis and the inhibitory effect on cyst development in short-term treatment and mortality as an index of ESRD in long-term treatment with tolvaptan using DBA/2FG-pcy mice, an animal model of nephronophthisis. With short-term treatment from 5 to 15 weeks of age, tolvaptan (0.01-0.3% via diet) dose-dependently enhanced aquaresis, prevented increases in kidney weight and cyst volume, and was associated with significant reductions in kidney cAMP levels and extracellular signal-regulated kinase activity. Maximal effects of tolvaptan on aquaresis and the prevention of development of polycystic kidney disease (PKD) were obtained at 0.1%. Interestingly, tolvaptan also dose-dependently reduced urinary neutrophil gelatinase-associated lipocalin levels in correlation with the kidney volume. With long-term treatment from 5 to 29 weeks of age, tolvaptan significantly attenuated the increase in kidney volume by up to 50% and reduced urinary albumin excretion. Furthermore, tolvaptan significantly reduced the mortality rate to 20%, compared with 60% in the control group. These data indicate that tolvaptan may delay the onset of ESRD in PKD by suppressing the increases in kidney volume and renal injury, providing a promising treatment for PKD.

Targeting of receptor for advanced glycation end products suppresses cyst growth in polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited renal disorder. Although a myriad of research groups have attempted to identify a new therapeutic target for ADPKD, no drug has worked well in clinical trials. Our research group has focused on the receptor for advanced glycation end products (RAGE) gene as a novel target for ADPKD. This gene is involved in inflammation and cell proliferation. We have already confirmed that blocking RAGE function attenuates cyst growth in vitro. Based on this previous investigation, our group examined the effect of RAGE on cyst enlargement in vivo. PC2R mice, a severe ADPKD mouse model that we generated, were utilized. An adenovirus containing anti-RAGE shRNA was injected intravenously into this model. We observed that RAGE gene knockdown resulted in loss of kidney weight and volume. Additionally, the cystic area that originated from different nephron segments decreased in size because of down-regulation of the RAGE gene. Blood urea nitrogen and creatinine values tended to be lower after inhibiting RAGE. Based on these results, we confirmed that the RAGE gene could be an effective target for ADPKD treatment.

Adenylyl cyclase 6 deficiency ameliorates polycystic kidney disease

cAMP is an important mediator of cystogenesis in polycystic kidney disease (PKD). Several adenylyl cyclase (AC) isoforms could mediate cAMP accumulation in PKD, and identification of a specific pathogenic AC isoform is of therapeutic interest. We investigated the role of AC6 in a mouse model of PKD that is homozygous for the loxP-flanked PKD1 gene and heterozygous for an aquaporin-2-Cre recombinase transgene to achieve collecting duct-specific gene targeting. Collecting duct-specific knockout of polycystin-1 caused massive renal cyst formation, kidney enlargement, and severe kidney failure, with a mean survival time of 2 months. In contrast, coincident collecting duct-specific knockout of polycystin-1 and AC6 (also homozygous for the floxed ADCY6 gene) markedly decreased kidney size and cystogenesis, improved renal function, reduced activation of the B-Raf/ERK/MEK pathway, and greatly increased survival. Absence of collecting duct AC6 did not alter urinary cAMP excretion or kidney cAMP concentration. In conclusion, AC6 is a key mediator of cyst formation and renal injury in a model of PKD.

Pathophysiology of childhood polycystic kidney diseases: new insights into disease-specific therapy

Autosomal dominant polycystic kidney disease (ADPKD) and autosomal recessive polycystic kidney disease (ARPKD) are significant causes of morbidity and mortality in children and young adults. ADPKD, with an incidence of 1:400 to 1:1,000, affects more than 13 million individuals worldwide and is a major cause of end-stage renal disease in adults. However, symptomatic disease is increasingly recognized in children. ARPKD is a dual-organ hepatorenal disease with an incidence of 1:20,000 to 1:40,000 and a heterozygote carrier rate of 1 in 70. Currently, no clinically significant disease-specific therapy exists for ADPKD or ARPKD. The genetic basis of both ADPKD and ARPKD have been identified, and delineation of the basic molecular and cellular pathophysiology has led to the discovery that abnormal ADPKD and ARPKD gene products interact to create "polycystin complexes" located at multiple sites within affected cells. The extracellular matrix and vessels produce a variety of soluble factors that affect the biology of adjacent cells in many dynamic ways. This review will focus on the molecular and cellular bases of the abnormal cystic phenotype and discuss the clinical translation of such basic data into new therapies that promise to alter the natural history of disease for children with genetic PKDs.

Osteocytes: master orchestrators of bone

Osteocytes comprise the overwhelming majority of cells in bone and are its only true "permanent" resident cell population. In recent years, conceptual and technological advances on many fronts have helped to clarify the role osteocytes play in skeletal metabolism and the mechanisms they use to perform them. The osteocyte is now recognized as a major orchestrator of skeletal activity, capable of sensing and integrating mechanical and chemical signals from their environment to regulate both bone formation and resorption. Recent studies have established that the mechanisms osteocytes use to sense stimuli and regulate effector cells (e.g., osteoblasts and osteoclasts) are directly coupled to the environment they inhabit-entombed within the mineralized matrix of bone and connected to each other in multicellular networks. Communication within these networks is both direct (via cell-cell contacts at gap junctions) and indirect (via paracrine signaling by secreted signals). Moreover, the movement of paracrine signals is dependent on the movement of both solutes and fluid through the space immediately surrounding the osteocytes (i.e., the lacunar-canalicular system). Finally, recent studies have also shown that the regulatory capabilities of osteocytes extend beyond bone to include a role in the endocrine control of systemic phosphate metabolism. This review will discuss how a highly productive combination of experimental and theoretical approaches has managed to unearth these unique features of osteocytes and bring to light novel insights into the regulatory mechanisms operating in bone.