Role of calcium in polycystic kidney disease: From signaling to pathology

Melanin-like nanoparticles slow cyst growth in ADPKD by dual inhibition of oxidative stress and CREB

Melanin-like nanoparticles (MNPs) have recently emerged as valuable agents in antioxidant therapy due to their excellent biocompatibility and potent capacity to scavenge various reactive oxygen species (ROS). However, previous studies have mainly focused on acute ROS-related diseases, leaving a knowledge gap regarding their potential in chronic conditions. Furthermore, apart from their well-established antioxidant effects, it remains unclear whether MNPs target other intracellular molecular pathways. In this study, we synthesized ultra-small polyethylene glycol-incorporated Mn2+-chelated MNP (MMPP). We found that MMPP traversed the glomerular filtration barrier and specifically accumulated in renal tubules. Autosomal dominant polycystic kidney disease (ADPKD) is a chronic genetic disorder closely associated with increased oxidative stress and featured by the progressive enlargement of cysts originating from various segments of the renal tubules. Treatment with MMPP markedly attenuated oxidative stress levels, inhibited cyst growth, thereby improving renal function. Interestingly, we found that MMPP effectively inhibits a cyst-promoting gene program downstream of the cAMP-CREB pathway, a crucial signaling pathway implicated in ADPKD progression. Mechanistically, we observed that MMPP directly binds to the bZIP DNA-binding domain of CREB, leading to competitive inhibition of CREB's DNA binding ability and subsequent reduction in CREB target gene expression. In summary, our findings identify an intracellular target of MMPP and demonstrate its potential for treating ADPKD by simultaneously targeting oxidative stress and CREB transcriptional activity.

Calcium signaling crosstalk between the endoplasmic reticulum and mitochondria, a new drug development strategies of kidney diseases

Calcium (Ca2+) acts as a second messenger and constitutes a complex and large information exchange system between the endoplasmic reticulum (ER) and mitochondria; this process is involved in various life activities, such as energy metabolism, cell proliferation and apoptosis. Increasing evidence has suggested that alterations in Ca2+ crosstalk between the ER and mitochondria, including alterations in ER and mitochondrial Ca2+ channels and related Ca2+ regulatory proteins, such as sarco/endoplasmic reticulum Ca2+-ATPase (SERCA), inositol 1,4,5-trisphosphate receptor (IP3R), and calnexin (CNX), are closely associated with the development of kidney disease. Therapies targeting intracellular Ca2+ signaling have emerged as an emerging field in the treatment of renal diseases. In this review, we focused on recent advances in Ca2+ signaling, ER and mitochondrial Ca2+ monitoring methods and Ca2+ homeostasis in the development of renal diseases and sought to identify new targets and insights for the treatment of renal diseases by targeting Ca2+ channels or related Ca2+ regulatory proteins.

Novel Potential Therapeutic Targets in Autosomal Dominant Polycystic Kidney Disease from the Perspective of Cell Polarity and Fibrosis

Autosomal dominant polycystic kidney disease (ADPKD), a congenital genetic disorder, is a notable contributor to the prevalence of chronic kidney disease worldwide. Despite the absence of a complete cure, ongoing research aims for early diagnosis and treatment. Although agents such as tolvaptan and mTOR inhibitors have been utilized, their effectiveness in managing the disease during its initial phase has certain limitations. This review aimed to explore new targets for the early diagnosis and treatment of ADPKD, considering ongoing developments. We particularly focus on cell polarity, which is a key factor that influences the process and pace of cyst formation. In addition, we aimed to identify agents or treatments that can prevent or impede the progression of renal fibrosis, ultimately slowing its trajectory toward end-stage renal disease. Recent advances in slowing ADPKD progression have been examined, and potential therapeutic approaches targeting multiple pathways have been introduced. This comprehensive review discusses innovative strategies to address the challenges of ADPKD and provides valuable insights into potential avenues for its prevention and treatment.

Investigational agents for autosomal dominant polycystic kidney disease: preclinical and early phase study insights

INTRODUCTION

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is the most common inherited kidney condition caused by a single-gene mutation. It leads patients to kidney failure in more than 50% of cases by the age of 60, and, given the dominant inheritance, this disease is present in the family history in more than 90% of cases.

AREAS COVERED

This review aims to analyze the set of preclinical and early-phase studies to provide a general view of the current progress on ADPKD therapeutic options. Articles from PubMed and the current status of the trials listed in clinicaltrials.gov were examined for the review.

EXPERT OPINION

Many potential therapeutic targets are currently under study for the treatment of ADPKD. A few drugs have reached the clinical phase, while many are currently still in the preclinical phase. Organoids could be a novel approach to the study of drugs in this phase. Other than pharmacological options, very important developing approaches are represented by gene therapy and the use of MiRNA inhibitors.

N-methyl-d-aspartate receptor 1 activation mediates cadmium-induced epithelial-mesenchymal transition in proximal tubular cells

Cadmium (Cd) is a commonly found environmental pollutant and is known to damage multiple organs with kidneys being the most common one. N-methyl-d-aspartate receptor 1 (NMDAR1) is a ligand-gated ion channel that is highly permeable to calcium ion (Ca2+). Because Cd2+ and Ca2+ have structural and physicochemical similarities, whether and how Cd could interfere NMDAR1 function to cause renal epithelial cells dysfunction remains unknown. In this study, we investigated the role of NMDAR1 in Cd-induced renal damage and found that Cd treatment upregulated NMDAR1 expression and promoted epithelial-mesenchymal transition (EMT) in mouse kidneys in vivo and human proximal tubular epithelial HK-2 cells in vitro, which were accompanied with activation of the inositol-requiring enzyme 1 (IRE-1α) / spliced X box binding protein-1 (XBP-1s) pathway, an indicative of endoplasmic reticulum (ER) stress. Mechanistically, NMDAR1 upregulation by Cd promoted Ca2+ channel opening and Ca2+ influx, resulting in ER stress and subsequently EMT in HK-2 cells. Inhibition of NMDAR1 by pharmacological antagonist MK-801 significantly attenuated Cd-induced Ca2+ influx, ER stress, and EMT. Pretreatment with the IRE-1α/XBP-1s pathway inhibitor STF-083010 also restored the epithelial phenotype of Cd-treated HK-2 cells. Therefore, our findings suggest that NMDAR1 activation mediates Cd-induced EMT in proximal epithelial cells likely through the IRE-1α/XBP-1s pathway, supporting the idea that NMDAR1 could be a potential therapeutic target for Cd-induced renal damage.

CFTR and PC2, partners in the primary cilia in autosomal dominant polycystic kidney disease

Defects in the primary cilium are associated with autosomal dominant polycystic kidney disease (ADPKD). We used a combination of animal models, Western blotting, and confocal microscopy and discovered that CFTR and polycystin 2 (PC2) are both colocalized to the cilium in normal kidneys, with the levels of both being decreased in cystic epithelia. Cilia were longer in CFTR-null mice and in cystic cells in our ADPKD animal models. We examined septin 2, known to play a role in cilia length, to act as a diffusion barrier and to serve as an enhancer of proliferation. We found that septin 2 protein levels were upregulated and colocalized strongly with CFTR in cystic cells. Application of VX-809, the CFTR corrector, restored CFTR and PC2 toward normal in the cilia, decreased the protein levels of septin 2, and drastically reduced septin 2 colocalization with CFTR. Our data suggest that CFTR is present in the cilia and plays a role there, perhaps through its conductance of Cl-. We also postulate that septin 2 is important for localizing CFTR to the apical membrane in cystic epithelia.NEW & NOTEWORTHY CFTR is present in the primary cilia together with polycystin 2 (PC2). Ablation of CFTR makes cilia longer suggesting that CFTR plays a role there, perhaps through its conductance of Cl.

Role of PKD2 in the endoplasmic reticulum calcium homeostasis

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in the PKD1 or PKD2 gene which encodes membrane receptor PKD1 and cation channel PKD2, respectively. PKD2, also called transient receptor potential polycystin-2 (TRPP2), is a Ca²⁺-permeable channel located on the membrane of cell surface, primary cilia, and endoplasmic reticulum (ER). Ca²⁺ is closely associated with diverse cellular functions. While ER Ca²⁺ homeostasis depends on different Ca²⁺ receptors, channels and transporters, the role of PKD2 within the ER remains controversial. Whether and how PKD2-mediated ER Ca²⁺ leak relates to ADPKD pathogenesis is not well understood. Here, we reviewed current knowledge about the biophysical and physiological properties of PKD2 and how PKD2 contributes to ER Ca²⁺ homeostasis.

The Mitochondrial Ca2+ import complex is altered in ADPKD

Mutations in either of the polycystic kidney disease genes, PKD1 or PKD2, engender the growth of cysts, altering renal function. Cystic growth is supported by major changes in cellular metabolism, some of which involve the mitochondrion, a major storage site for Ca²⁺ and a key organelle in cellular Ca²⁺ signaling. The goal here was to understand the role of components of the mitochondrial Ca²⁺ uptake complex in PC1-mutant cells in autosomal dominant polycystic kidney disease (ADPKD). We found that the mitochondrial Ca²⁺ uniporter (MCU) and voltage-dependent anion channels 1& 3 (VDAC) were down-regulated in different mouse and cell models of ADPKD along with the Ca²⁺-dependent enzyme, pyruvate dehydrogenase phosphatase (PDHX). The release of Ca²⁺ from the endoplasmic reticulum, and Ca²⁺ uptake by the mitochondria were upregulated in PC1(polycystin)-null cells. We also observed an enhanced staining with MitoTracker Red CMXRos in PC1-null cultured cells than in PC1-containing cells and a substantially higher increase in response to ER Ca²⁺ release. Increased colocalization of the Ca²⁺ sensitive dye, rhodamine2, with MitoTracker Green suggested an increase Ca²⁺ entry into the mitochondria in PC1 null cells subsequent to Ca²⁺ release from the ER or from Ca²⁺ entry from the extracellular solution. These data clearly demonstrate abnormal release of Ca²⁺ by the ER and corresponding alterations in Ca²⁺ uptake by the mitochondria in PC1^-null cells. Importantly, inhibiting mitochondrial Ca²⁺ uptake with the specific inhibitor Ru360 inhibited cyst growth and altered both apoptosis and cell proliferation. We further show that the decrease in mitochondrial proteins and abnormally high Ca²⁺ signaling can be reversed by application of the cystic fibrosis (CFTR) corrector, VX-809. We conclude that enhanced Ca²⁺ signaling and alterations in proteins association with the mitochondrial Ca²⁺ uptake complex are associated with malfunction of PC1. Finally, our results identify novel therapeutic targets for treating ADPKD.

Recent advances in understanding ion transport mechanisms in polycystic kidney disease

This review focuses on the most recent advances in the understanding of the electrolyte transport-related mechanisms important for the development of severe inherited renal disorders, autosomal dominant (AD) and recessive (AR) forms of polycystic kidney disease (PKD). We provide here a basic overview of the origins and clinical aspects of ARPKD and ADPKD and discuss the implications of electrolyte transport in cystogenesis. Special attention is devoted to intracellular calcium handling by the cystic cells, with a focus on polycystins and fibrocystin, as well as other calcium level regulators, such as transient receptor potential vanilloid type 4 (TRPV4) channels, ciliary machinery, and purinergic receptor remodeling. Sodium transport is reviewed with a focus on the epithelial sodium channel (ENaC), and the role of chloride-dependent fluid secretion in cystic fluid accumulation is discussed. In addition, we highlight the emerging promising concepts in the field, such as potassium transport, and suggest some new avenues for research related to electrolyte handling.

Coregulation Analysis of Mechanistic Biomarkers in Autosomal Dominant Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary kidney disorder leading to deterioration of kidney function and end stage kidney disease (ESKD). A number of molecular processes are dysregulated in ADPKD but the exact mechanism of disease progression is not fully understood. We measured protein biomarkers being linked to ADPKD-associated molecular processes via ELISA in urine and serum in a cohort of ADPKD patients as well as age, gender and eGFR matched CKD patients and healthy controls. ANOVA and t-tests were used to determine differences between cohorts. Spearman correlation coefficient analysis was performed to assess coregulation patterns of individual biomarkers and renal function. Urinary epidermal growth factor (EGF) and serum apelin (APLN) levels were significantly downregulated in ADPKD patients. Serum vascular endothelial growth factor alpha (VEGFA) and urinary angiotensinogen (AGT) were significantly upregulated in ADPKD patients as compared with healthy controls. Arginine vasopressin (AVP) was significantly upregulated in ADPKD patients as compared with CKD patients. Serum VEGFA and VIM concentrations were positively correlated and urinary EGF levels were negatively correlated with urinary AGT levels. Urinary EGF and AGT levels were furthermore significantly associated with estimated glomerular filtration rate (eGFR) in ADPKD patients. In summary, altered protein concentrations in body fluids of ADPKD patients were found for the mechanistic markers EGF, APLN, VEGFA, AGT, AVP, and VIM. In particular, the connection between EGF and AGT during progression of ADPKD warrants further investigation.

Inward Ca2+ current through the polycystin-2-dependent channels of renal primary cilia

In 15% of cases, autosomal dominant polycystic kidney disease arises from defects in polycystin-2 (PC2). PC2 is a member of the polycystin transient receptor potential subfamily of cation-conducting channels and is expressed in the endoplasmic reticulum and primary cilium of renal epithelial cells. PC2 opposes a procystogenic influence of the cilium, and it has been proposed that this beneficial effect is mediated in part by a flow of Ca²⁺ through PC2 channels into the primary cilium. However, previous efforts to determine the permeability of PC2 channels to Ca²⁺ have yielded widely varying results. Here, we report the mean macroscopic Ca²⁺ influx through native PC2 channels in the primary cilia of mIMCD-3 cells, which are derived from the murine inner medullary collecting duct. Under conditions designed to isolate inward Ca²⁺ currents, a small inward Ca²⁺ current was detected in cilia with active PC2 channels but not in cilia lacking those channels. The current was activated by the addition of 10 µM internal Ca²⁺, which is known to activate ciliary PC2 channels. It was blocked by 10 µM isosakuranetin, which blocks the same channels. On average, the current amplitude was -1.8 pA at -190 mV; its conductance from -50 to -200 mV averaged 20 pS. Thus, native PC2 channels of renal primary cilia are able to conduct a small but detectable Ca²⁺ influx under the conditions tested. The possible consequences of this influx are discussed.NEW & NOTEWORTHY In autosomal dominant polycystic kidney disease, it is proposed that Ca²⁺ entering the primary cilium through polycystin-2 (PC2) channels may limit the formation of cysts. Recent studies predict that any macroscopic Ca²⁺ influx through these channels should be small. We report that the native PC2 channels in primary cilia of cultured renal epithelial cells can allow a small macroscopic calcium influx. This may allow a significant accumulation of Ca²⁺ in the cilium in vivo.

Modeling Neoplastic Growth in Renal Cell Carcinoma and Polycystic Kidney Disease

Renal cell carcinoma (RCC) and autosomal dominant polycystic kidney disease (ADPKD) share several characteristics, including neoplastic cell growth, kidney cysts, and limited therapeutics. As well, both exhibit impaired vasculature and compensatory VEGF activation of angiogenesis. The PI3K/AKT/mTOR and Ras/Raf/ERK pathways play important roles in regulating cystic and tumor cell proliferation and growth. Both RCC and ADPKD result in hypoxia, where HIF-α signaling is activated in response to oxygen deprivation. Primary cilia and altered cell metabolism may play a role in disease progression. Non-coding RNAs may regulate RCC carcinogenesis and ADPKD through their varied effects. Drosophila exhibits remarkable conservation of the pathways involved in RCC and ADPKD. Here, we review the progress towards understanding disease mechanisms, partially overlapping cellular and molecular dysfunctions in RCC and ADPKD and reflect on the potential for the agile Drosophila genetic model to accelerate discovery science, address unresolved mechanistic aspects of these diseases, and perform rapid pharmacological screens.

Changes of Motile Ciliary Phenotype in Patients with Primary Ciliopathies

Primary ciliopathies are a group of disorders associated with abnormal formation and function of primary cilia. Many cilia-associated proteins found in primary cilia are also present in motile cilia. Such proteins are important for the ciliary base, such as the transition zone or basal bodies, and the intraflagellar transport. Their exact role in the respiratory motile cilia is unsettled. In this prospective clinical single-center study, we investigated the hypothesis that these proteins regulate the function of motile cilia. We addressed the issue by defining the motile cilia beat frequency in the respiratory tract of patients with primary ciliopathies accompanied by chronic kidney disease and comparing it in those without kidney involvement. Ciliary beat frequency in the nasal mucosa samples was evaluated by the ciliary analysis software LabVIEW. Both children and their parents with primary ciliopathies and kidney involvement had significantly lower median airway ciliary beat frequencies than those without kidney involvement who have normal ciliary motility. Further, the ciliary beat frequency is inversely associated with the serum creatinine level. These findings strongly suggest that kidney involvement in patients with primary ciliopathy may underlie the development of motile cilia dysfunction in the respiratory tract, potentially increasing respiratory morbidity.

Autosomal dominant polycystic kidney disease and pioglitazone for its therapy: a comprehensive review with an emphasis on the molecular pathogenesis and pharmacological aspects

Autosomal dominant polycystic kidney disease (ADPKD) is an inherited chronic kidney disorder (CKD) that is characterized by the development of numerous fluid-filled cysts in kidneys. It is caused either due to the mutations in the PKD1 or PKD2 gene that encodes polycystin-1 and polycystin-2, respectively. This condition progresses into end-stage renal disorder if the renal or extra-renal clinical manifestations remain untreated. Several clinical trials with a variety of drugs have failed, and the only Food and Drugs Administration (FDA) approved drug to treat ADPKD to date is tolvaptan that works by antagonizing the vasopressin-2 receptor (V2R). The pathology of ADPKD is complex and involves the malfunction of different signaling pathways like cAMP, Hedgehog, and MAPK/ERK pathway owing to the mutated product that is polycystin-1 or 2. A measured yet substantial number of preclinical studies have found pioglitazone to decrease the cystic burden and improve the renal function in ADPKD. The peroxisome proliferator-activated receptor-gamma is found on the epithelial cells of renal collecting tubule and when it gets agonized by pioglitazone, confers efficacy in ADPKD treatment through multiple mechanisms. There is only one clinical trial (ongoing) wherein it is being assessed for its benefits and risk in patients with ADPKD, and is expected to get approval from the regulatory body owing to its promising therapeutic effects. This article would encompass the updated information on the epidemiology, pathophysiology of ADPKD, different mechanisms of action of pioglitazone in the treatment of ADPKD with preclinical and clinical shreds of evidence, and related safety updates.

Roles of telomeres and telomerase in age‑related renal diseases (Review)

Age‑related renal diseases, which account for various progressive renal disorders associated with cellular and organismal senescence, are becoming a substantial public health burden. However, their aetiologies are complicated and their pathogeneses remain poorly understood. Telomeres and telomerase are known to be essential for maintaining the integrity and stability of eukaryotic genomes and serve crucial roles in numerous related signalling pathways that activate renal functions, such as repair and regeneration. Previous studies have reported that telomere dysfunction served a role in various types of age‑related kidney disease through various different molecular pathways. The present review aimed to summarise the current knowledge of the association between telomeres and ageing‑related kidney diseases and explored the contribution of dysfunctional telomeres to these diseases. The findings may help to provide novel strategies for treating patients with renal disease.

Targeting E3 Ubiquitin Ligases and Deubiquitinases in Ciliopathy and Cancer

Cilia are antenna-like structures present in many vertebrate cells. These organelles detect extracellular cues, transduce signals into the cell, and play an essential role in ensuring correct cell proliferation, migration, and differentiation in a spatiotemporal manner. Not surprisingly, dysregulation of cilia can cause various diseases, including cancer and ciliopathies, which are complex disorders caused by mutations in genes regulating ciliary function. The structure and function of cilia are dynamically regulated through various mechanisms, among which E3 ubiquitin ligases and deubiquitinases play crucial roles. These enzymes regulate the degradation and stabilization of ciliary proteins through the ubiquitin-proteasome system. In this review, we briefly highlight the role of cilia in ciliopathy and cancer; describe the roles of E3 ubiquitin ligases and deubiquitinases in ciliogenesis, ciliopathy, and cancer; and highlight some of the E3 ubiquitin ligases and deubiquitinases that are potential therapeutic targets for these disorders.

Shear stress induced by fluid flow produces improvements in tissue-engineered cartilage

Tissue engineering aims to create implantable biomaterials for the repair and regeneration of damaged tissues. In vitro tissue engineering is generally based on static culture, which limits access to nutrients and lacks mechanical signaling. Using shear stress is controversial because in some cases it can lead to cell death while in others it promotes tissue regeneration. To understand how shear stress works and how it may be used to improve neotissue function, a series of studies were performed. First, a tunable device was designed to determine optimal levels of shear stress for neotissue formation. Then, computational fluid dynamics modeling showed the device applies fluid-induced shear (FIS) stress spanning three orders of magnitude on tissue-engineered cartilage (neocartilage). A beneficial window of FIS stress was subsequently identified, resulting in up to 3.6-fold improvements in mechanical properties of neocartilage in vitro. In vivo, neocartilage matured as evidenced by the doubling of collagen content toward native values. Translation of FIS stress to human derived neocartilage was then demonstrated, yielding analogous improvements in mechanical properties, such as 168% increase in tensile modulus. To gain an understanding of the beneficial roles of FIS stress, a mechanistic study was performed revealing a mechanically gated complex on the primary cilia of chondrocytes that is activated by FIS stress. This series of studies places FIS stress into the arena as a meaningful mechanical stimulation strategy for creating robust and translatable neotissues, and demonstrates the ease of incorporating FIS stress in tissue culture.

Structure and function of polycystin channels in primary cilia

Variants in genes which encode for polycystin-1 and polycystin-2 cause most forms of autosomal dominant polycystic disease (ADPKD). Despite our strong understanding of the genetic determinants of ADPKD, we do not understand the structural features which govern the function of polycystins at the molecular level, nor do we understand the impact of most disease-causing variants on the conformational state of these proteins. These questions have remained elusive because polycystins localize to several organelle membranes, including the primary cilia. Primary cilia are microtubule based organelles which function as cellular antennae. Polycystin-2 and related polycystin-2 L1 are members of the transient receptor potential (TRP) ion channel family, and form distinct ion channels in the primary cilia of disparate cell types which can be directly measured. Polycystin-1 has both ion channel and adhesion G-protein coupled receptor (GPCR) features-but its role in forming a channel complex or as a channel subunit chaperone is undetermined. Nonetheless, recent polycystin structural determination by cryo-EM has provided a molecular template to understand their biophysical regulation and the impact of disease-causing variants. We will review these advances and discuss hypotheses regarding the regulation of polycystin channel opening by their structural domains within the context of the primary cilia.

Mapping of parathyroid neoplasms based on grey scale ultrasound images and histopathological whole slide images

PURPOSE

In this study, echogenicity and histopathological projections of parathyroid neoplasia in grey mode ultrasonography were compared with whole side imaging (WSI). The utility of the data obtained for clinical assessment was evaluated.

METHODS

In 57 patients operated for hyperparathyroidism, the parathyroid gland was sampled in the sagittal plane. The lesion slides were scanned. The WSI was rendered digital. The histopathological slide images scanned with USG images were matched. With the İmage J program, the areas of cell types and morphological changes were measured.

RESULTS

In parathyroid neoplasms, hypoechoic areas were found to be matched with 21% [55.3%] chief cell, 2 [5.0%] oncocytic cell and 8%[20.0%] cystic morphology. Of the 57 patients, 26 [45.61%] had a cystic area. It was seen that hyperechogenic areas match more connective tissue areas than chef cell [p < 0.05]. There was less clear cell in hyperechogenic areas than iso-hyperechogenic areas [p < 0.05]. The ratio of fat tissue echogenicity was lower in hypoechoic areas than hyperechoic [p < 0.05]. There was a positive correlation between PTH and the entire tissue area [r = 0.377, p = 0.004]. A positive directional moderate linear relationship was found between blood calcium level and parathyroid adenoma [rho = 0.530, p = 0.009] and percentage [rho = 0.416, p = 0.048]. When atypical adenomas and adenoma cases were compared, no significant difference was found between the cystic and chief cell areas [p > 0.05].

CONCLUSION

The hypoechogenicity seen in USG was observed to be compatible with chief cell, hyperechogenic areas in USG were compatible with connective tissue and fat tissue. As the cystic area increased, blood calcium levels were higher in adenomas. We think that the results of this study will be guiding to evaluate the reflections of the detailed morphometric studies.

Polycystin 2: A calcium channel, channel partner, and regulator of calcium homeostasis in ADPKD

Polycystin 2 (PC2) is one of two main protein types responsible for the underlying etiology of autosomal dominant polycystic kidney disease (ADPKD), the most prevalent monogenic renal disease in the world. This debilitating and currently incurable condition is caused by loss-of-function mutations in PKD2 and PKD1, the genes encoding for PC2 and Polycystin 1 (PC1), respectively. Two-hit mutation events in these genes lead to renal cyst formation and eventual kidney failure, the main hallmarks of ADPKD. Though much is known concerning the physiological consequences and dysfunctional signaling mechanisms resulting from ADPKD development, to best understand the requirement of PC2 in maintaining organ homeostasis, it is important to recognize how PC2 acts under normal conditions. As such, an array of work has been performed characterizing the endogenous function of PC2, revealing it to be a member of the transient receptor potential (TRP) channel family of proteins. As a TRP protein, PC2 is a nonselective, cation-permeant, calcium-sensitive channel expressed in all tissue types, where it localizes primarily on the endoplasmic reticulum (ER), primary cilia, and plasma membrane. In addition to its channel function, PC2 interacts with and acts as a regulator of a number of other channels, ultimately further affecting intracellular signaling and leading to dysfunction in its absence. In this review, we describe the biophysical and physiological properties of PC2 as a cation channel and modulator of intracellular calcium channels, along with how these properties are altered in ADPKD.

Polycystin-1 Inhibits Cell Proliferation through Phosphatase PP2A/B56α

Autosomal dominant polycystic kidney disease (ADPKD) is associated with a number of cellular defects such as hyperproliferation, apoptosis, and dedifferentiation. Mutations in polycystin-1 (PC1) account for ∼85% of ADPKD. Here, we showed that wild-type (WT) or mutant PC1 composed of the last five transmembrane (TM) domains and the C-terminus (termed PC1-5TMC) inhibits cell proliferation and protein translation, as well as the downstream effectors of mTOR, consistent with previous reports. Knockdown of B56α, a subunit of the protein phosphatase 2A (PP2A) complex, or application of PP2A inhibitor okadaic acid or calyculin A, abolished the inhibitory effect of PC1 and PC1-5TMC on proliferation, indicating that PP2A/B56α mediates the regulation of cell proliferation by PC1. In addition to the phosphorylated S6 and 4EBP1, B56α was also downregulated by PC1 and PC1-5TMC. Furthermore, the downregulation of B56α, which may be mediated by mTOR but not AKT, can account for the dependence of PC1-inhibited proliferation on PP2A.

Ciliary exclusion of Polycystin-2 promotes kidney cystogenesis in an autosomal dominant polycystic kidney disease model

The human PKD2 locus encodes Polycystin-2 (PC2), a TRPP channel that localises to several distinct cellular compartments, including the cilium. PKD2 mutations cause Autosomal Dominant Polycystic Kidney Disease (ADPKD) and affect many cellular pathways. Data underlining the importance of ciliary PC2 localisation in preventing PKD are limited because PC2 function is ablated throughout the cell in existing model systems. Here, we dissect the ciliary role of PC2 by analysing mice carrying a non-ciliary localising, yet channel-functional, PC2 mutation. Mutants develop embryonic renal cysts that appear indistinguishable from mice completely lacking PC2. Despite not entering the cilium in mutant cells, mutant PC2 accumulates at the ciliary base, forming a ring pattern consistent with distal appendage localisation. This suggests a two-step model of ciliary entry; PC2 first traffics to the cilium base before TOP domain dependent entry. Our results suggest that PC2 localisation to the cilium is necessary to prevent PKD.

The molecular role of ciliary Polycystin-2 (PC2) in cyst formation and polycystic kidney disease (ADKPD) is unclear. Here, the authors identify a PC2 mutant lacking ciliary localisation but with active Ca2+ channel function in mice, that is sufficient to generate an ADPKD phenotype.

A Novel Role for Polycystin-2 (Pkd2) in P. tetraurelia as a Probable Mg2+ Channel Necessary for Mg2+-Induced Behavior

A human ciliopathy gene codes for Polycystin-2 (Pkd2), a non-selective cation channel. Here, the Pkd2 channel was explored in the ciliate Paramecium tetraurelia using combinations of RNA interference, over-expression, and epitope-tagging, in a search for function and novel interacting partners. Upon depletion of Pkd2, cells exhibited a phenotype similar to eccentric (XntA1), a Paramecium mutant lacking the inward Ca²⁺-dependent Mg²⁺ conductance. Further investigation showed both Pkd2 and XntA localize to the cilia and cell membrane, but do not require one another for trafficking. The XntA-myc protein co-immunoprecipitates Pkd2-FLAG, but not vice versa, suggesting two populations of Pkd2-FLAG, one of which interacts with XntA. Electrophysiology data showed that depletion and over-expression of Pkd2 led to smaller and larger depolarizations in Mg²⁺ solutions, respectively. Over-expression of Pkd2-FLAG in the XntA1 mutant caused slower swimming, supporting an increase in Mg²⁺ permeability, in agreement with the electrophysiology data. We propose that Pkd2 in P. tetraurelia collaborates with XntA for Mg²⁺-induced behavior. Our data suggest Pkd2 is sufficient and necessary for Mg²⁺ conductance and membrane permeability to Mg²⁺, and that Pkd2 is potentially a Mg²⁺-permeable channel.

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.

p53 mutation regulates PKD genes and results in co-occurrence of PKD and tumorigenesis

Objective

Polycystic kidney disease (PKD) is the major cause of kidney failure and mortality in humans. It has always been suspected that the development of cystic kidney disease shares features with tumorigenesis, although the evidence is unclear.

Methods

We crossed p53 mutant mice (p53N236S, p53S) with Werner syndrome mice and analyzed the pathological phenotypes. The RNA-seq, ssGSEA analysis, and real-time PCR were performed to dissect the gene signatures involved in the development of disease phenotypes.

Results

We found enlarged kidneys with fluid-filled cysts in offspring mice with a genotype of G3mTerc^-/-WRN^-/-p53^S/S (G3TM). Pathology analysis confirmed the occurrence of PKD, and it was highly correlated with the incidence of tumorigenesis. RNA-seq data revealed the gene signatures involved in PKD development, and demonstrated that PKD and tumorigenesis shared common pathways, including complement pathways, lipid metabolism, mitochondria energy homeostasis and others. Interestingly, this G3TM PKD and the classical PKD1/2 deficient PKD shared common pathways, possibly because the mutant p53S could regulate the expression levels of PKD1/2, Pkhd1, and Hnf1b.

Conclusions

We established a dual mouse model for PKD and tumorigenesis derived from abnormal cellular proliferation and telomere dysfunction. The innovative point of our study is to report PKD occurring in conjunction with tumorigenesis. The gene signatures revealed might shed new light on the pathogenesis of PKD, and provide new molecular biomarkers for clinical diagnosis and prognosis.

Olive leaf extract counteracts cell proliferation and cyst growth in an in vitro model of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is characterized by progressive enlargement of kidney cysts, leading to chronic kidney disease. Since the available treatment for ADPKD is limited, there is emerging interest for natural compounds as potential therapeutic candidates. The aim of our study was to investigate whether an olive leaf extract may be able to counteract the cyst growth in an in vitro model of ADPKD. We treated WT9-12 cells with an olive leaf extract (OLE). In monolayer culture we evaluated cell viability by the MTT assay, protein expression by western-blot analysis and apoptosis by DNA laddering and TUNEL assays. For functional studies we used transient transfection and ChIP assays. Intracellular calcium measurement was performed with a spectrofluorimeter using a fluorescent probe. 3D-cell-culture was used for cyst growth studies. OLE reduced the WT9-12 cell growth rate and affected intracellular signaling due to high c-AMP levels, as OLE reduced PKA levels, enhanced p-AKT, restored B-Raf-inactivation and down-regulated p-ERK. We elucidated the molecular mechanism by which OLE, via Sp1, transactivates the p21WAF1/Cip1 promoter, whose levels are down-regulated by mutated PKD1. We demonstrated that p-AKT up-regulation also played a crucial role in the OLE-induced anti-apoptotic effect and that OLE ameliorated intracellular calcium levels, the primary cause of ADPKD. Finally, using a 3D-cell-culture model we observed that OLE reduced the cyst size. Therefore, multifaceted OLE may be considered a new therapeutic approach for ADPKD treatment.

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.

Activation of Calcium-Sensing Receptor increases intracellular calcium and decreases cAMP and mTOR in PKD1 deficient cells

Clinical and fundamental research suggest that altered calcium and cAMP signaling might be the most proximal events in ADPKD pathogenesis. Cells from ADPKD cysts have a reduced resting cytosolic calcium [Ca²⁺]_i and increased cAMP levels. CaSR plays an essential role in regulating calcium homeostasis. Its activation is associated with [Ca²⁺]_i increase and cAMP decrease, making CaSR a possible therapeutic target. Human conditionally immortalized Proximal Tubular Epithelial cells (ciPTEC) with stable knockdown of PKD1 (ciPTEC-PC1KD) and ciPTEC generated from an ADPKD1 patient (ciPTEC-PC1Pt) were used as experimental tools. CaSR functional expression was confirmed by studies showing that the calcimimetic NPS-R568 induced a significant increase in [Ca²⁺]_i in ciPTEC-PC1KD and ciPTEC-PC1Pt. Resting [Ca²⁺]_i were significantly lower in ciPTEC-PC1KD with respect to ciPTECwt, confirming calcium dysregulation. As in native cyst cells, significantly higher cAMP levels and mTOR activity were found in ciPTEC-PC1KD compared to ciPTECwt. Of note, NPS-R568 treatment significantly reduced intracellular cAMP and mTOR activity in ciPTEC-PC1KD and ciPTEC-PC1Pt. To conclude, we demonstrated that selective CaSR activation in human ciPTEC carrying PKD1 mutation increases [Ca²⁺]_i, reduces intracellular cAMP and mTOR activity, reversing the principal dysregulations considered the most proximal events in ADPKD pathogenesis, making CaSR a possible candidate as therapeutic target.

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.

Functional Study of the Primary Cilia in ADPKD

The primary cilium is a microtubule-based organelle that is considered to be a cellular antennae, because proteins related to multiple signaling pathways such as Wnt, PDGFRα, Hh, and mechanosignaling are localized to the membrane of the primary cilium. In the kidney, primary cilia extend from the cell membrane to the lumen of renal tubules to respond to fluidic stress. Recent studies have indicated that the disruption of ciliary proteins including polycystin-1 (PC1), polycystin-2 (PC2), and members of the intraflagellar transport (IFT) family induce the development of polycystic kidney disease (PKD), suggesting that the malformation or absence of primary cilia is a driving force of the onset of PKD. Therefore, in this chapter, the renal cystogenesis mechanism induced by cilia defects and pathogenic ciliary proteins associated with PKD development will be described.

Motile and non-motile cilia in human pathology: from function to phenotypes

Ciliopathies are inherited human disorders caused by both motile and non-motile cilia dysfunction that form an important and rapidly expanding disease category. Ciliopathies are complex conditions to diagnose, being multisystem disorders characterized by extensive genetic heterogeneity and clinical variability with high levels of lethality. There is marked phenotypic overlap among distinct ciliopathy syndromes that presents a major challenge for their recognition, diagnosis, and clinical management, in addition to posing an on-going task to develop the most appropriate family counselling. The impact of next-generation sequencing and high-throughput technologies in the last decade has significantly improved our understanding of the biological basis of ciliopathy disorders, enhancing our ability to determine the possible reasons for the extensive overlap in their symptoms and genetic aetiologies. Here, we review the diverse functions of cilia in human health and disease and discuss a growing shift away from the classical clinical definitions of ciliopathy syndromes to a more functional categorization. This approach arises from our improved understanding of this unique organelle, revealed through new genetic and cell biological insights into the discrete functioning of subcompartments of the cilium (basal body, transition zone, intraflagellar transport, motility). Mutations affecting these distinct ciliary protein modules can confer different genetic diseases and new clinical classifications are possible to define, according to the nature and extent of organ involvement. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.