Polycystin-2 is an intracellular calcium release channel

Cilia-enriched oxysterol 7β,27-DHC is required for polycystin ion channel activation

Polycystin-1 (PC-1) and PC-2 form a heteromeric ion channel complex that is abundantly expressed in primary cilia of renal epithelial cells. This complex functions as a non-selective cation channel, and mutations within the polycystin complex cause autosomal dominant polycystic kidney disease (ADPKD). The spatial and temporal regulation of the polycystin complex within the ciliary membrane remains poorly understood. Using both whole-cell and ciliary patch-clamp recordings, we identify a cilia-enriched oxysterol, 7β,27-dihydroxycholesterol (DHC), that serves as a necessary activator of the polycystin complex. We further identify an oxysterol-binding pocket within PC-2 and showed that mutations within this binding pocket disrupt 7β,27-DHC-dependent polycystin activation. Pharmacologic and genetic inhibition of oxysterol synthesis reduces channel activity in primary cilia. In summary, our findings reveal a regulator of the polycystin complex. This oxysterol-binding pocket in PC-2 may provide a specific target for potential ADPKD therapeutics.

Disease-associated missense mutations in the pore loop of polycystin-2 alter its ion channel function in a heterologous expression system

Polycystin-2 (PC2) is mutated in ∼15% of patients with autosomal dominant polycystic kidney disease (ADPKD). PC2 belongs to the family of transient receptor potential (TRP) channels and can function as a homotetramer. We investigated whether three disease-associated mutations (F629S, C632R, or R638C) localized in the channel's pore loop alter ion channel properties of human PC2 expressed in Xenopus laevis oocytes. Expression of wild-type (WT) PC2 typically resulted in small but measurable Na+ inward currents in the absence of extracellular divalent cations. These currents were no longer observed when individual pore mutations were introduced in WT PC2. Similarly, Na+ inward currents mediated by the F604P gain-of-function (GOF) PC2 construct (PC2 F604P) were abolished by each of the three pore mutations. In contrast, when the mutations were introduced in another GOF construct, PC2 L677A N681A, only C632R had a complete loss-of-function effect, whereas significant residual Na+ inward currents were observed with F629S (∼15%) and R638C (∼30%). Importantly, the R638C mutation also abolished the Ca2+ permeability of PC2 L677A N681A and altered its monovalent cation selectivity. To elucidate the molecular mechanisms by which the R638C mutation affects channel function, molecular dynamics (MD) simulations were used in combination with functional experiments and site-directed mutagenesis. Our findings suggest that R638C stabilizes ionic interactions between Na+ ions and the selectivity filter residue D643. This probably explains the reduced monovalent cation conductance of the mutant channel. In summary, our data support the concept that altered ion channel properties of PC2 contribute to the pathogenesis of ADPKD.

The ion channels of endomembranes

The endomembrane system consists of organellar membranes in the biosynthetic pathway [endoplasmic reticulum (ER), Golgi apparatus, and secretory vesicles] as well as those in the degradative pathway (early endosomes, macropinosomes, phagosomes, autophagosomes, late endosomes, and lysosomes). These endomembrane organelles/vesicles work together to synthesize, modify, package, transport, and degrade proteins, carbohydrates, and lipids, regulating the balance between cellular anabolism and catabolism. Large ion concentration gradients exist across endomembranes: Ca2+ gradients for most endomembrane organelles and H+ gradients for the acidic compartments. Ion (Na+, K+, H+, Ca2+, and Cl-) channels on the organellar membranes control ion flux in response to cellular cues, allowing rapid informational exchange between the cytosol and organelle lumen. Recent advances in organelle proteomics, organellar electrophysiology, and luminal and juxtaorganellar ion imaging have led to molecular identification and functional characterization of about two dozen endomembrane ion channels. For example, whereas IP3R1-3 channels mediate Ca2+ release from the ER in response to neurotransmitter and hormone stimulation, TRPML1-3 and TMEM175 channels mediate lysosomal Ca2+ and H+ release, respectively, in response to nutritional and trafficking cues. This review aims to summarize the current understanding of these endomembrane channels, with a focus on their subcellular localizations, ion permeation properties, gating mechanisms, cell biological functions, and disease relevance.

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.

Clinical manifestation, epidemiology, genetic basis, potential molecular targets, and current treatment of polycystic liver disease

Polycystic liver disease (PLD) is a rare condition observed in three genetic diseases, including autosomal dominant polycystic liver disease (ADPLD), autosomal dominant polycystic kidney disease (ADPKD), and autosomal recessive polycystic kidney disease (ARPKD). PLD usually does not impair liver function, and advanced PLD becomes symptomatic when the enlarged liver compresses adjacent organs or increases intra-abdominal pressure. Currently, the diagnosis of PLD is mainly based on imaging, and genetic testing is not required except for complex cases. Besides, genetic testing may help predict patients' prognosis, classify patients for genetic intervention, and conduct early treatment. Although the underlying genetic causes and mechanisms are not fully understood, previous studies refer to primary ciliopathy or impaired ciliogenesis as the main culprit. Primarily, PLD occurs due to defective ciliogenesis and ineffective endoplasmic reticulum quality control. Specifically, loss of function mutations of genes that are directly involved in ciliogenesis, such as Pkd1, Pkd2, Pkhd1, and Dzip1l, can lead to both hepatic and renal cystogenesis in ADPKD and ARPKD. In addition, loss of function mutations of genes that are involved in endoplasmic reticulum quality control and protein folding, trafficking, and maturation, such as PRKCSH, Sec63, ALG8, ALG9, GANAB, and SEC61B, can impair the production and function of polycystin1 (PC1) and polycystin 2 (PC2) or facilitate their degradation and indirectly promote isolated hepatic cystogenesis or concurrent hepatic and renal cystogenesis. Recently, it was shown that mutations of LRP5, which impairs canonical Wnt signaling, can lead to hepatic cystogenesis. PLD is currently treated by somatostatin analogs, percutaneous intervention, surgical fenestration, resection, and liver transplantation. In addition, based on the underlying molecular mechanisms and signaling pathways, several investigational treatments have been used in preclinical studies, some of which have shown promising results. This review discusses the clinical manifestation, complications, prevalence, genetic basis, and treatment of PLD and explains the investigational methods of treatment and future research direction, which can be beneficial for researchers and clinicians interested in PLD.

Cardiovascular perspectives of the TRP channel polycystin 2

Calcium release from the endoplasmic reticulum (ER) is predominantly driven by two key ion channel receptors, inositol 1, 4, 5-triphosphate receptor (InsP3R) in non-excitable cells and ryanodine receptor (RyR) in excitable and muscle-based cells. These calcium transients can be modified by other less-studied ion channels, including polycystin 2 (PC2), a member of the transient receptor potential (TRP) family. PC2 is found in various cell types and is evolutionarily conserved with paralogues ranging from single-cell organisms to yeasts and mammals. Interest in the mammalian form of PC2 stems from its disease relevance, as mutations in the PKD2 gene, which encodes PC2, result in autosomal dominant polycystic kidney disease (ADPKD). This disease is characterized by renal and liver cysts, and cardiovascular extrarenal manifestations. However, in contrast to the well-defined roles of many TRP channels, the role of PC2 remains unknown, as it has different subcellular locations, and the functional understanding of the channel in each location is still unclear. Recent structural and functional studies have shed light on this channel. Moreover, studies on cardiovascular tissues have demonstrated a diverse role of PC2 in these tissues compared to that in the kidney. We highlight recent advances in understanding the role of this channel in the cardiovascular system and discuss the functional relevance of PC2 in non-renal cells.

Polycystin-2 Mediated Calcium Signalling in the Dictyostelium Model for Autosomal Dominant Polycystic Kidney Disease

Autosomal dominant polycystic kidney disease (ADPKD) occurs when the proteins Polycystin-1 (PC1, PKD1) and Polycystin-2 (PC2, PKD2) contain mutations. PC1 is a large membrane receptor that can interact and form a complex with the calcium-permeable cation channel PC2. This complex localizes to the plasma membrane, primary cilia and ER. Dysregulated calcium signalling and consequential alterations in downstream signalling pathways in ADPKD are linked to cyst formation and expansion; however, it is not completely understood how PC1 and PC2 regulate calcium signalling. We have studied Polycystin-2 mediated calcium signalling in the model organism Dictyostelium discoideum by overexpressing and knocking down the expression of the endogenous Polycystin-2 homologue, Polycystin-2. Chemoattractant-stimulated cytosolic calcium response magnitudes increased and decreased in overexpression and knockdown strains, respectively, and analysis of the response kinetics indicates that Polycystin-2 is a significant contributor to the control of Ca2+ responses. Furthermore, basal cytosolic calcium levels were reduced in Polycystin-2 knockdown transformants. These alterations in Ca2+ signalling also impacted other downstream Ca2+-sensitive processes including growth rates, endocytosis, stalk cell differentiation and spore viability, indicating that Dictyostelium is a useful model to study Polycystin-2 mediated calcium signalling.

Molecular and structural basis of the dual regulation of the polycystin-2 ion channel by small-molecule ligands

Mutations in the PKD2 gene, which encodes the polycystin-2 (PC2, also called TRPP2) protein, lead to autosomal dominant polycystic kidney disease (ADPKD). As a member of the transient receptor potential (TRP) channel superfamily, PC2 functions as a non-selective cation channel. The activation and regulation of the PC2 channel are largely unknown, and direct binding of small-molecule ligands to this channel has not been reported. In this work, we found that most known small-molecule agonists of the mucolipin TRP (TRPML) channels inhibit the activity of the PC2_F604P, a gain-of-function mutant of the PC2 channel. However, two of them, ML-SA1 and SF-51, have dual regulatory effects, with low concentration further activating PC2_F604P, and high concentration leading to inactivation of the channel. With two cryo-electron microscopy (cryo-EM) structures, a molecular docking model, and mutagenesis results, we identified two distinct binding sites of ML-SA1 in PC2_F604P that are responsible for activation and inactivation, respectively. These results provide structural and functional insights into how ligands regulate PC2 channel function through unusual mechanisms and may help design compounds that are more efficient and specific in regulating the PC2 channel and potentially also for ADPKD treatment.

Autosomal Dominant Polycystic Kidney Disease: Extrarenal Involvement

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary kidney disorder, but kidneys are not the only organs involved in this systemic disorder. Individuals with the condition may display additional manifestations beyond the renal system, involving the liver, pancreas, and brain in the context of cystic manifestations, while involving the vascular system, gastrointestinal tract, bones, and cardiac valves in the context of non-cystic manifestations. Despite kidney involvement remaining the main feature of the disease, thanks to longer survival, early diagnosis, and better management of kidney-related problems, a new wave of complications must be faced by clinicians who treated patients with ADPKD. Involvement of the liver represents the most prevalent extrarenal manifestation and has growing importance in the symptom burden and quality of life. Vascular abnormalities are a key factor for patients' life expectancy and there is still debate whether to screen or not to screen all patients. Arterial hypertension is often the earliest onset symptom among ADPKD patients, leading to frequent cardiovascular complications. Although cardiac valvular abnormalities are a frequent complication, they rarely lead to relevant problems in the clinical history of polycystic patients. One of the newest relevant aspects concerns bone disorders that can exert a considerable influence on the clinical course of these patients. This review aims to provide the "state of the art" among the extrarenal manifestation of ADPKD.

The molecular structure and function of fibrocystin, the key gene product implicated in autosomal recessive polycystic kidney disease (ARPKD)

Autosomal recessive polycystic kidney disease is an early onset inherited hepatorenal disorder affecting around 1 in 20,000 births with no approved specific therapies. The disease is almost always caused by variations in the polycystic kidney and hepatic disease 1 gene, which encodes fibrocystin (FC), a very large, single-pass transmembrane glycoprotein found in primary cilia, urine and urinary exosomes. By comparison to proteins involved in autosomal dominant PKD, our structural and molecular understanding of FC has lagged far behind such that there are no published experimentally determined structures of any part of the protein. Bioinformatics analyses predict that the ectodomain contains a long chain of immunoglobulin-like plexin-transcription factor domains, a protective antigen 14 domain, a tandem G8-TMEM2 homology region and a sperm protein, enterokinase and agrin domain. Here we review current knowledge on the molecular function of the protein from a structural perspective.

Polycystin-2-dependent transcriptome reveals early response of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in polycystin genes, Pkd1 and Pkd2, but the underlying pathogenic mechanisms are poorly understood. To identify genes and pathways that operate downstream of polycystin-2 (PC2), a comprehensive gene expression database was created, cataloging changes in the transcriptome immediately following PC2 protein depletion. To explore cyst initiation processes, an immortalized mouse inner medullary collecting duct line was developed with the ability to knock out the Pkd2 gene conditionally. Genome-wide transcriptome profiling was performed using RNA sequencing in the cells immediately after PC2 was depleted and compared with isogenic control cells. Differentially expressed genes were identified, and a bioinformatic analysis pipeline was implemented. Altered expression of candidate cystogenic genes was validated in Pkd2 knockout mice. The expression of nearly 900 genes changed upon PC2 depletion. Differentially expressed genes were enriched for genes encoding components of the primary cilia, the canonical Wnt pathway, and MAPK signaling. Among the PC2-dependent ciliary genes, the transcription factor Glis3 was significantly downregulated. MAPK signaling formed a key node at the epicenter of PC2-dependent signaling networks. Activation of Wnt and MAPK signaling, concomitant with the downregulation of Glis3, was corroborated in Pkd2 knockout mice. The data identify a PC2 cilia-to-nucleus signaling axis and dysregulation of the Gli-similar subfamily of transcription factors as a potential initiator of cyst formation in ADPKD. The catalog of PC2-regulated genes should provide a valuable resource for future ADPKD research and new opportunities for drug development.NEW & NOTEWORTHY Autosomal dominant polycystic kidney disease (ADPKD) is the most common inherited kidney disease. Mutations in polycystin genes cause the disease, but the underlying mechanisms of cystogenesis are unknown. To help fill this knowledge gap, we created an inducible cell model of ADPKD and assembled a catalog of genes that respond in immediate proximity to polycystin-2 depletion using transcriptomic profiling. The catalog unveils a ciliary signaling-to-nucleus axis proximal to polycystin-2 dysfunction, highlighting Glis, Wnt, and MAPK signaling.

Convergence of Calcium Channel Regulation and Mechanotransduction in Skeletal Regenerative Biomaterial Design

Cells are known to perceive their microenvironment through extracellular and intracellular mechanical signals. Upon sensing mechanical stimuli, cells can initiate various downstream signaling pathways that are vital to regulating proliferation, growth, and homeostasis. One such physiologic activity modulated by mechanical stimuli is osteogenic differentiation. The process of osteogenic mechanotransduction is regulated by numerous calcium ion channels-including channels coupled to cilia, mechanosensitive and voltage-sensitive channels, and channels associated with the endoplasmic reticulum. Evidence suggests these channels are implicated in osteogenic pathways such as the YAP/TAZ and canonical Wnt pathways. This review aims to describe the involvement of calcium channels in regulating osteogenic differentiation in response to mechanical loading and characterize the fashion in which those channels directly or indirectly mediate this process. The mechanotransduction pathway is a promising target for the development of regenerative materials for clinical applications due to its independence from exogenous growth factor supplementation. As such, also described are examples of osteogenic biomaterial strategies that involve the discussed calcium ion channels, calcium-dependent cellular structures, or calcium ion-regulating cellular features. Understanding the distinct ways calcium channels and signaling regulate these processes may uncover potential targets for advancing biomaterials with regenerative osteogenic capabilities.

Pharmacological Regulation of Endoplasmic Reticulum Structure and Calcium Dynamics: Importance for Neurodegenerative Diseases

The endoplasmic reticulum (ER) is the largest organelle of the cell, composed of a continuous network of sheets and tubules, and is involved in protein, calcium (Ca2+), and lipid homeostasis. In neurons, the ER extends throughout the cell, both somal and axodendritic compartments, and is highly important for neuronal functions. A third of the proteome of a cell, secreted and membrane-bound proteins, are processed within the ER lumen and most of these proteins are vital for neuronal activity. The brain itself is high in lipid content, and many structural lipids are produced, in part, by the ER. Cholesterol and steroid synthesis are strictly regulated in the ER of the blood-brain barrier protected brain cells. The high Ca2+ level in the ER lumen and low cytosolic concentration is needed for Ca2+-based intracellular signaling, for synaptic signaling and Ca2+ waves, and for preparing proteins for correct folding in the presence of high Ca2+ concentrations to cope with the high concentrations of extracellular milieu. Particularly, ER Ca2+ is controlled in axodendritic areas for proper neurito- and synaptogenesis and synaptic plasticity and remodeling. In this review, we cover the physiologic functions of the neuronal ER and discuss it in context of common neurodegenerative diseases, focusing on pharmacological regulation of ER Ca2+ Furthermore, we postulate that heterogeneity of the ER, its protein folding capacity, and ensuring Ca2+ regulation are crucial factors for the aging and selective vulnerability of neurons in various neurodegenerative diseases. SIGNIFICANCE STATEMENT: Endoplasmic reticulum (ER) Ca2+ regulators are promising therapeutic targets for degenerative diseases for which efficacious drug therapies do not exist. The use of pharmacological probes targeting maintenance and restoration of ER Ca2+ can provide restoration of protein homeostasis (e.g., folding of complex plasma membrane signaling receptors) and slow down the degeneration process of neurons.

Functions of the primary cilium in the kidney and its connection with renal diseases

The nonmotile primary cilium is a sensory structure found on most mammalian cell types that integrates multiple signaling pathways involved in tissue development and postnatal function. As such, mutations disrupting cilia activities cause a group of disorders referred to as ciliopathies. These disorders exhibit a wide spectrum of phenotypes impacting nearly every tissue. In the kidney, primary cilia dysfunction caused by mutations in polycystin 1 (Pkd1), polycystin 2 (Pkd2), or polycystic kidney and hepatic disease 1 (Pkhd1), result in polycystic kidney disease (PKD), a progressive disorder causing renal functional decline and end-stage renal disease. PKD affects nearly 1 in 1000 individuals and as there is no cure for PKD, patients frequently require dialysis or renal transplantation. Pkd1, Pkd2, and Pkhd1 encode membrane proteins that all localize in the cilium. Pkd1 and Pkd2 function as a nonselective cation channel complex while Pkhd1 protein function remains uncertain. Data indicate that the cilium may act as a mechanosensor to detect fluid movement through renal tubules. Other functions proposed for the cilium and PKD proteins in cyst development involve regulation of cell cycle and oriented division, regulation of renal inflammation and repair processes, maintenance of epithelial cell differentiation, and regulation of mitochondrial structure and metabolism. However, how loss of cilia or cilia function leads to cyst development remains elusive. Studies directed at understanding the roles of Pkd1, Pkd2, and Pkhd1 in the cilium and other locations within the cell will be important for developing therapeutic strategies to slow cyst progression.

Co-Inheritance of Pathogenic Variants in PKD1 and PKD2 Genes Determined by Parental Segregation and De Novo Origin: A Case Report

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary renal disease, and it is typically caused by PKD1 and PKD2 heterozygous variants. Nonetheless, the extensive phenotypic variability observed among affected individuals, even within the same family, suggests a more complex pattern of inheritance. We describe an ADPKD family in which the proband presented with an earlier and more severe renal phenotype (clinical diagnosis at the age of 14 and end-stage renal disease aged 24), compared to the other affected family members. Next-generation sequencing (NGS)-based analysis of polycystic kidney disease (PKD)-associated genes in the proband revealed the presence of a pathogenic PKD2 variant and a likely pathogenic variant in PKD1, according to the American College of Medical Genetics and Genomics (ACMG) criteria. The PKD2 nonsense p.Arg872Ter variant was segregated from the proband's father, with a mild phenotype. A similar mild disease presentation was found in the proband's aunts and uncle (the father's siblings). The frameshift p.Asp3832ProfsTer128 novel variant within PKD1 carried by the proband in addition to the pathogenic PKD2 variant was not found in either parent. This report highlights that the co-inheritance of two or more PKD genes or alleles may explain the extensive phenotypic variability among affected family members, thus emphasizing the importance of NGS-based techniques in the definition of the prognostic course.

Identified eleven exon variants in PKD1 and PKD2 genes that altered RNA splicing by minigene assay

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is a common monogenic multisystem disease caused primarily by mutations in the PKD1 gene or PKD2 gene. There is increasing evidence that some of these variants, which are described as missense, synonymous or nonsense mutations in the literature or databases, may be deleterious by affecting the pre-mRNA splicing process.

RESULTS

This study aimed to determine the effect of these PKD1 and PKD2 variants on exon splicing combined with predictive bioinformatics tools and minigene assay. As a result, among the 19 candidate single nucleotide alterations, 11 variants distributed in PKD1 (c.7866C > A, c.7960A > G, c.7979A > T, c.7987C > T, c.11248C > G, c.11251C > T, c.11257C > G, c.11257C > T, c.11346C > T, and c.11393C > G) and PKD2 (c.1480G > T) were identified to result in exon skipping.

CONCLUSIONS

We confirmed that 11 variants in the gene of PKD1 and PKD2 affect normal splicing by interfering the recognition of classical splicing sites or by disrupting exon splicing enhancers and generating exon splicing silencers. This is the most comprehensive study to date on pre-mRNA splicing of exonic variants in ADPKD-associated disease-causing genes in consideration of the increasing number of identified variants in PKD1 and PKD2 gene in recent years. These results emphasize the significance of assessing the effect of exon single nucleotide variants in ADPKD at the mRNA level.

TRP (transient receptor potential) ion channel family: structures, biological functions and therapeutic interventions for diseases

Transient receptor potential (TRP) channels are sensors for a variety of cellular and environmental signals. Mammals express a total of 28 different TRP channel proteins, which can be divided into seven subfamilies based on amino acid sequence homology: TRPA (Ankyrin), TRPC (Canonical), TRPM (Melastatin), TRPML (Mucolipin), TRPN (NO-mechano-potential, NOMP), TRPP (Polycystin), TRPV (Vanilloid). They are a class of ion channels found in numerous tissues and cell types and are permeable to a wide range of cations such as Ca²⁺, Mg²⁺, Na⁺, K⁺, and others. TRP channels are responsible for various sensory responses including heat, cold, pain, stress, vision and taste and can be activated by a number of stimuli. Their predominantly location on the cell surface, their interaction with numerous physiological signaling pathways, and the unique crystal structure of TRP channels make TRPs attractive drug targets and implicate them in the treatment of a wide range of diseases. Here, we review the history of TRP channel discovery, summarize the structures and functions of the TRP ion channel family, and highlight the current understanding of the role of TRP channels in the pathogenesis of human disease. Most importantly, we describe TRP channel-related drug discovery, therapeutic interventions for diseases and the limitations of targeting TRP channels in potential clinical applications.

Calcium signaling in polycystic kidney disease- cell death and survival

Polycystic kidney disease is typified by cysts in the kidney and extra-renal manifestations including hypertension and heart failure. The main genetic underpinning this disease are loss-of function mutations to the two polycystin proteins, polycystin 1 and polycystin 2. Molecularly, the disease is characterized by changes in multiple signaling pathways including down regulation of calcium signaling, which, in part, is contributed by the calcium permeant properties of polycystin 2. These signaling pathways enable the cystic cells to survive and avoid cell death. This review focuses on the studies that have emerged in the past 5 years describing how the structural insights gained from PC-1 and PC-2 inform the calcium dependent molecular pathways of autophagy and the unfolded protein response that are regulated by the polycystin proteins and how it leads to cell survival and/or cell death.

Calcium Homeostasis, Transporters, and Blockers in Health and Diseases of the Cardiovascular System

Calcium is a highly positively charged ionic species. It regulates all cell types' functions and is an important second messenger that controls and triggers several mechanisms, including membrane stabilization, permeability, contraction, secretion, mitosis, intercellular communications, and in the activation of kinases and gene expression. Therefore, controlling calcium transport and its intracellular homeostasis in physiology leads to the healthy functioning of the biological system. However, abnormal extracellular and intracellular calcium homeostasis leads to cardiovascular, skeletal, immune, secretory diseases, and cancer. Therefore, the pharmacological control of calcium influx directly via calcium channels and exchangers and its outflow via calcium pumps and uptake by the ER/SR are crucial in treating calcium transport remodeling in pathology. Here, we mainly focused on selective calcium transporters and blockers in the cardiovascular system.

The diverse effects of pathogenic point mutations on ion channel activity of a gain-of-function polycystin-2

Autosomal dominant polycystic kidney disease is caused by mutations in PKD1 or PKD2 genes. The latter encodes polycystin-2 (PC2, also known as TRPP2), a member of the transient receptor potential ion channel family. Despite most pathogenic mutations in PKD2 being truncation variants, there are also many point mutations, which cause small changes in protein sequences but dramatic changes in the in vivo function of PC2. How these mutations affect PC2 ion channel function is largely unknown. In this study, we systematically tested the effects of 31 point mutations on the ion channel activity of a gain-of-function PC2 mutant, PC2_F604P, expressed in Xenopus oocytes. The results show that all mutations in the transmembrane domains and channel pore region, and most mutations in the extracellular tetragonal opening for polycystins domain, are critical for PC2_F604P channel function. In contrast, the other mutations in the tetragonal opening for polycystins domain and most mutations in the C-terminal tail cause mild or no effects on channel function as assessed in Xenopus oocytes. To understand the mechanism of these effects, we have discussed possible conformational consequences of these mutations based on the cryo-EM structures of PC2. The results help gain insight into the structure and function of the PC2 ion channel and the molecular mechanism of pathogenesis caused by these mutations.

Polycystic kidney disease: novel insights into polycystin function

Autosomal dominant polycystic kidney disease (ADPKD) is a life-threatening monogenic disease caused by mutations in PKD1 and PKD2 that encode polycystin 1 (PC1) and polycystin 2 (PC2). PC1/2 localize to cilia of renal epithelial cells, and their function is believed to embody an inhibitory activity that suppresses the cilia-dependent cyst activation (CDCA) signal. Consequently, PC deficiency results in activation of CDCA and stimulates cyst growth. Recently, re-expression of PCs in established cysts has been shown to reverse PKD. Thus, the mode of action of PCs resembles a 'counterbalance in cruise control' to maintain lumen diameter within a designated range. Herein we review recent studies that point to novel arenas for future PC research with therapeutic potential for ADPKD.

Polycystin-2 Associates With Malignancy in Meningiomas

The involvement of polycystin-2 (PC2) in cell survival pathways raises questions about its role in carcinogenesis. Aberrant expression of PC2 has been associated with malignancy in various tumors. No evidence exists referring to PC2 expression in meningiomas. The aim of this study was to investigate the expression levels of PC2 in meningiomas and compare them with normal brain samples including leptomeninges. PC2 immunohistochemical expression was quantitatively analyzed in archival tissue from 60 patients with benign (WHO grade 1) and 22 patients with high-grade (21: WHO grade 2 and 1: grade 3) meningiomas. Specifically, the labeling index [the percentage of positive (labeled) cells out of the total number of tumor cells counted] was determined. PC2 mRNA levels were evaluated by quantitative real-time polymerase chain reaction. PC2 immunostaining was not detected in the leptomeninges. Gene expression analysis revealed increased levels of PC2 in WHO grade 1 ( P = 0.008) and WHO grade 2 ( P = 0.0007) meningiomas compared with that of normal brains. PC2 expression was significantly associated with an ascending grade of malignancy by both immunohistochemistry and quantitative real-time polymerase chain reaction ( P < 0.05). Recurrent meningiomas displayed higher levels of PC2 compared with primary meningiomas ( P = 0.008). Although no significant association of PC2 with the overall survival of the patients was found ( P > 0.05), it was noticed that the patients with WHO grade 2 meningiomas with low expression of PC2 survived longer compared with the patients with WHO grade 1 meningioma with high expression of PC2 (mean survival 49.5 and 28 months, respectively). The above results indicate a possible association of PC2 with malignancy in meningiomas. However, the mechanisms underlying PC2 implication in meningioma pathogenesis should be further elucidated.

High calcium transport by Polycystin-2 (TRPP2) induces channel clustering and oscillatory currents

The regulation by Ca²⁺ of Ca²⁺-permeable ion channels represents an important mechanism in the control of cell function. Polycystin-2 (PC2, TRPP2), a member of the TRP channel family (Transient Potential Receptor), is a Ca²⁺ permeable non-selective cation channel. Previous studies from our laboratory demonstrated that physiological concentrations of Ca²⁺ do not regulate in vitro translated PC2 (PC2_iv) channel activity. However, the issue as to PC2's Ca²⁺ permeability and regulation remain ill-defined, in particular because Ca²⁺ transport is usually observed in the presence of other ionic gradients. In this study, we assessed Ca²⁺ transport by PC2_iv in a lipid bilayer reconstitution system in a high Ca²⁺ gradient (CaCl₂ 100 mM cis, CaCl₂ 10 mM trans) in the presence of either 3:7 or 7:3 1-palmitoyl-2-oleoyl-choline and ethanolamine lipid mixtures. Reconstituted PC2_iv showed spontaneous Ca²⁺ currents in both lipid mixtures, with a maximum conductance of 63 ± 13 pS (n = 19) and 105 pS ± 9.8 (n = 9), respectively. In both cases, we best fitted the experimental data with the Goldman-Hodgkin-Katz equation, observing a reversal potential (V_rev ∼ -27 mV) consistent with strict Ca²⁺ selectivity. The R742X mutated PC2 (PC2_R742X), lacking the carboxy terminal domain of the channel showed no differences with wild type PC2. Interestingly, we also observed the onset of spontaneous Ca²⁺ current oscillations whenever PC2-containing samples were reconstituted in the 3:7, but not 7:3 POPC:POPE lipid mixture. The amplitude and frequency of the ionic oscillations were highly dependent on the applied voltage, the imposed Ca²⁺ gradient, and the presence of high Ca²⁺, which induced PC2 channel clustering as observed by atomic force microscopy (AFM). We also used the QuB suite to kinetically model the PC2 channel Ca²⁺ oscillations based on the presence of subconductance states in the channel. The encompassed evidence supports a high Ca²⁺ permeability by PC2, and a novel oscillatory mechanism dependent on the presence of Ca²⁺ and phospholipids that provides the first evidence for the relation between stochasticity and deterministic processes mediated by ion channels.

Expression of RSUME is associated with poor prognosis in clear cell Renal Carcinoma: involvement of ROS related metabolism

INTRODUCTION

RSUME (RWD domain-containing protein SUMO Enhancer), RWD domain containing 3 (RWDD3) gene product, is upregulated by hypoxia and expressed in organs prone to develop von Hippel-Lindau (VHL) syndrome tumors.

MATERIALS AND METHODS

We evaluated RSUME prognostic value in clear cell renal cell carcinoma (ccRCC) based mainly on the dataset (KIRC) from patients in The Cancer Genome Atlas (TCGA). Wilcoxon signed-rank test and one-way analysis of variance (ANOVA) followed by Tukey's test were used to evaluate relationships between clinicopathological features and RSUME expression and univariate and multivariate Cox regression analysis methods were used to evaluate prognostic factors. The biological function of RSUME was assessed by gene set enrichment analysis (GSEA). For validation, total amount of ROS was detected in ccRCC cell lines using dichlorofluorescin diacetate.

RESULTS

RSUME is highly expressed in tumor tissues compared with normal tissues (P = .006, P = .039, P = .002, P = .036, P < .001) and associates with tumor T (P = .018) and tumor M (P = .036) advanced stages and higher extent cysts (P = .005). RSUME expression appears to be an independent risk factor for overall survival (OS) (P = .002) and disease-specific survival (DSS) (P = .026) in ccRCC patients. GSEA showed enrichment of relevant glycerophospholipid- and ROS-related pathways in RSUME high-expression phenotype. ROS diminished levels in RSUME-silenced ccRCC cell lines validated RSUME relevance in ROS-related pathways.

CONCLUSION

RSUME high expression may predict poor prognosis in ccRCC and impact through its action on metabolism and ROS related pathways.

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.

Deficit of PKHD1L1 in the dentate gyrus increases seizure susceptibility in mice

Epilepsy is a chronic neurological disorder featuring recurrent, unprovoked seizures, which affect more than 65 million people worldwide. Here, we discover that the PKHD1L1, which is encoded by polycystic kidney and hepatic disease1-like 1 (Pkhd1l1), wildly distributes in neurons in the central nervous system (CNS) of mice. Disruption of PKHD1L1 in the dentate gyrus region of the hippocampus leads to increased susceptibility to pentylenetetrazol-induced seizures in mice. The disturbance of PKHD1L1 leads to the overactivation of the mitogen-activated protein kinase (MAPK)/extracellular regulated kinase (ERK)-Calpain pathway, which is accompanied by remarkable degradation of cytoplasmic potassium chloride co-transporter 2 (KCC2) level together with the impaired expression and function of membrane KCC2. However, the reduction of membrane KCC2 is associated with the damaged inhibitory ability of the vital GABA receptors, which ultimately leads to the significantly increased susceptibility to epileptic seizures. Our data, thus, indicate for the first time that Pkhd1l1, a newly discovered polycystic kidney disease (PKD) association gene, is required in neurons to maintain neuronal excitability by regulation of KCC2 expression in CNS. A new mechanism of the clinical association between genetic PKD and seizures has been built, which could be a potential therapeutic target for treating PKD-related seizures.

SCF-SKP2 E3 ubiquitin ligase links mTORC1/ER stress/ISR with YAP activation in murine renal cystogenesis

The Hippo pathway nuclear effector Yes-associated protein (YAP) potentiates the progression of polycystic kidney disease (PKD) arising from ciliopathies. The mechanisms underlying the increase in YAP expression and transcriptional activity in PKD remain obscure. We observed that in kidneys from mice with juvenile cystic kidney (jck) ciliopathy, the aberrant hyperactivity of mechanistic target of rapamycin complex 1 (mTORC1), driven by ERK1/2 and PI3K/AKT cascades, induced ER proteotoxic stress. To reduce this stress by reprogramming translation, the protein kinase R-like ER kinase-eukaryotic initiation factor 2α (PERK/eIF2α) arm of the integrated stress response (ISR) was activated. PERK-mediated phosphorylation of eIF2α drove the selective translation of activating transcription factor 4 (ATF4), potentiating YAP expression. In parallel, YAP underwent K63-linked polyubiquitination by SCF S-phase kinase-associated protein 2 (SKP2) E3 ubiquitin ligase, a Hippo-independent, nonproteolytic ubiquitination that enhances YAP nuclear trafficking and transcriptional activity in cancer cells. Defective ISR cellular adaptation to ER stress in eIF2α phosphorylation-deficient jck mice further augmented YAP-mediated transcriptional activity and renal cyst growth. Conversely, pharmacological tuning down of ER stress/ISR activity and SKP2 expression in jck mice by administration of tauroursodeoxycholic acid (TUDCA) or tolvaptan impeded these processes. Restoring ER homeostasis and/or interfering with the SKP2-YAP interaction represent potential therapeutic avenues for stemming the progression of renal cystogenesis.

Establishment of transgenic pigs overexpressing human PKD2-D511V mutant

Numerous missense mutations have been reported in autosomal dominant polycystic kidney disease which is one of the most common renal genetic disorders. The underlying mechanism for cystogenesis is still elusive, partly due to the lack of suitable animal models. Currently, we tried to establish a porcine transgenic model overexpressing human PKD2-D511V (hPKD2-D511V), which is a dominant-negative mutation in the vertebrate in vitro models. A total of six cloned pigs were finally obtained using somatic cell nuclear transfer. However, five with functional hPKD2-D511V died shortly after birth, leaving only one with the dysfunctional transgenic event to survive. Compared with the WT pigs, the demised transgenic pigs had elevated levels of hPKD2 expression at the mRNA and protein levels. Additionally, no renal malformation was observed, indicating that hPKD2-D511V did not alter normal kidney development. RNA-seq analysis also revealed that several ADPKD-related pathways were disturbed when overexpressing hPKD2-D511V. Therefore, our study implies that hPKD2-D511V may be lethal due to the dominant-negative effect. Hence, to dissect how PKD2-D511V drives renal cystogenesis, it is better to choose in vitro or invertebrate models.

Polycystin-2 (TRPP2) regulates primary cilium length in LLC-PK1 renal epithelial cells

Polycystin-2 (PC2, TRPP2) is a Ca²⁺ permeable nonselective cation channel whose dysfunction generates autosomal dominant polycystic kidney disease (ADPKD). PC2 is present in different cell locations, including the primary cilium of renal epithelial cells. However, little is known as to whether PC2 contributes to the primary cilium structure. Here, we explored the effect(s) of external Ca²⁺, PC2 channel blockers, and PKD2 gene silencing on the length of primary cilia in wild-type LLC-PK1 renal epithelial cells. Confluent cell monolayers were fixed and immuno-labeled with an anti-acetylated α-tubulin antibody to identify primary cilia and measure their length. Although primary cilia length measurements did not follow a Normal distribution, the data were normalized by Box-Cox transformation rendering statistical differences under all experimental conditions. Cells exposed to high external Ca²⁺ (6.2 mM) decreased a 13.5% (p < 0.001) primary cilia length as compared to controls (1.2 mM Ca²⁺). In contrast, the PC2 inhibitors amiloride (200 μM) and LiCl (10 mM), both increased primary ciliary length by 33.2% (p < 0.001), and 17.4% (p < 0.001), respectively. PKD2 gene silencing by siRNA elicited a statistically significant, 10.3% (p < 0.001) increase in primary cilia length compared to their respective scrambled RNA transfected cells. The data indicate that conditions that regulate PC2 function or gene expression modify the length of primary cilia in renal epithelial cells. Blocking of PC2 mitigates the effects of elevated external Ca²⁺ concentration on primary cilia length. Proper regulation of PC2 function in the primary cilium may be essential in the onset of mechanisms that trigger cyst formation in ADPKD.

Genetics, pathobiology and therapeutic opportunities of polycystic liver disease

Polycystic liver diseases (PLDs) are inherited genetic disorders characterized by progressive development of intrahepatic, fluid-filled biliary cysts (more than ten), which constitute the main cause of morbidity and markedly affect the quality of life. Liver cysts arise in patients with autosomal dominant PLD (ADPLD) or in co-occurrence with renal cysts in patients with autosomal dominant or autosomal recessive polycystic kidney disease (ADPKD and ARPKD, respectively). Hepatic cystogenesis is a heterogeneous process, with several risk factors increasing the odds of developing larger cysts. Depending on the causative gene, PLDs can arise exclusively in the liver or in parallel with renal cysts. Current therapeutic strategies, mainly based on surgical procedures and/or chronic administration of somatostatin analogues, show modest benefits, with liver transplantation as the only potentially curative option. Increasing research has shed light on the genetic landscape of PLDs and consequent cholangiocyte abnormalities, which can pave the way for discovering new targets for therapy and the design of novel potential treatments for patients. Herein, we provide a critical and comprehensive overview of the latest advances in the field of PLDs, mainly focusing on genetics, pathobiology, risk factors and next-generation therapeutic strategies, highlighting future directions in basic, translational and clinical research.

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.

Susceptibility genes of hyperuricemia and gout

Gout is a chronic metabolic disease that seriously affects human health. It is also a major challenge facing the world, which has brought a heavy burden to patients and society. Hyperuricemia (HUA) is the most important risk factor for gout. In recent years, with the improvement of living standards and the change of dietary habits, the incidence of gout in the world has increased dramatically, and gradually tends to be younger. An increasing number of studies have shown that gene mutations may play an important role in the development of HUA and gout. Therefore, we reviewed the existing literature and summarized the susceptibility genes and research status of HUA and gout, in order to provide reference for the early diagnosis, individualized treatment and the development of new targeted drugs of HUA and gout.

Channel Function of Polycystin-2 in the Endoplasmic Reticulum Protects against Autosomal Dominant Polycystic Kidney Disease

BACKGROUND

Mutations of PKD2, which encodes polycystin-2, cause autosomal dominant polycystic kidney disease (ADPKD). The prevailing view is that defects in polycystin-2-mediated calcium ion influx in the primary cilia play a central role in the pathogenesis of cyst growth. However, polycystin-2 is predominantly expressed in the endoplasmic reticulum (ER) and more permeable to potassium ions than to calcium ions.

METHODS

The trimeric intracellular cation (TRIC) channel TRIC-B is an ER-resident potassium channel that mediates potassium-calcium counterion exchange for inositol trisphosphate-mediated calcium ion release. Using TRIC-B as a tool, we examined the function of ER-localized polycystin-2 and its role in ADPKD pathogenesis in cultured cells, zebrafish, and mouse models.

RESULTS

Agonist-induced ER calcium ion release was defective in cells lacking polycystin-2 and reversed by exogenous expression of TRIC-B. Vice versa, exogenous polycystin-2 reversed an ER calcium-release defect in cells lacking TRIC-B. In a zebrafish model, expression of wild-type but not nonfunctional TRIC-B suppressed polycystin-2-deficient phenotypes. Similarly, these phenotypes were suppressed by targeting the ROMK potassium channel (normally expressed on the cell surface) to the ER. In cultured cells and polycystin-2-deficient zebrafish phenotypes, polycystin-2 remained capable of reversing the ER calcium release defect even when it was not present in the cilia. Transgenic expression of Tric-b ameliorated cystogenesis in the kidneys of conditional Pkd2-inactivated mice, whereas Tric-b deletion enhanced cystogenesis in Pkd2-heterozygous kidneys.

CONCLUSIONS

Polycystin-2 in the ER appears to be critical for anticystogenesis and likely functions as a potassium ion channel to facilitate potassium-calcium counterion exchange for inositol trisphosphate-mediated calcium release. The results advance the understanding of ADPKD pathogenesis and provides proof of principle for pharmacotherapy by TRIC-B activators.

Polycystin-2 (PC2) is a key determinant of in vitro myogenesis

The development of skeletal muscle (myogenesis) is a well-orchestrated process where myoblasts withdraw from the cell cycle and differentiate into myotubes. Signaling by fluxes in intracellular calcium (Ca²⁺) is known to contribute to myogenesis, and increased mitochondrial biogenesis is required to meet the metabolic demand of mature myotubes. However, gaps remain in the understanding of how intracellular Ca²⁺ signals can govern myogenesis. Polycystin-2 (PC2 or TRPP1) is a nonselective cation channel permeable to Ca²⁺. It can interact with intracellular calcium channels to control Ca²⁺ release and concurrently modulates mitochondrial function and remodeling. Due to these features, we hypothesized that PC2 is a central protein in mediating both the intracellular Ca²⁺ responses and mitochondrial changes seen in myogenesis. To test this hypothesis, we created CRISPR/Cas9 knockout (KO) C2C12 murine myoblast cell lines. PC2 KO cells were unable to differentiate into myotubes, had impaired spontaneous Ca²⁺ oscillations, and did not develop depolarization-evoked Ca²⁺ transients. The autophagic-associated pathway beclin-1 was downregulated in PC2 KO cells, and direct activation of the autophagic pathway resulted in decreased mitochondrial remodeling. Re-expression of full-length PC2, but not a calcium channel dead pathologic mutant, restored the differentiation phenotype and increased the expression of mitochondrial proteins. Our results establish that PC2 is a novel regulator of in vitro myogenesis by integrating PC2-dependent Ca²⁺ signals and metabolic pathways.

A scalable organoid model of human autosomal dominant polycystic kidney disease for disease mechanism and drug discovery

Human pluripotent stem-cell-derived organoids are models for human development and disease. We report a modified human kidney organoid system that generates thousands of similar organoids, each consisting of 1-2 nephron-like structures. Single-cell transcriptomic profiling and immunofluorescence validation highlighted patterned nephron-like structures utilizing similar pathways, with distinct morphogenesis, to human nephrogenesis. To examine this platform for therapeutic screening, the polycystic kidney disease genes PKD1 and PKD2 were inactivated by gene editing. PKD1 and PKD2 mutant models exhibited efficient and reproducible cyst formation. Cystic outgrowths could be propagated for months to centimeter-sized cysts. To shed new light on cystogenesis, 247 protein kinase inhibitors (PKIs) were screened in a live imaging assay identifying compounds blocking cyst formation but not overall organoid growth. Scaling and further development of the organoid platform will enable a broader capability for kidney disease modeling and high-throughput drug screens.

TRPP2 ion channels: The roles in various subcellular locations

TRPP2 (PC2, PKD2 or Polycytin-2), encoded by PKD2 gene, belongs to the nonselective cation channel TRP family. Recently, the three-dimensional structure of TRPP2 was constructed. TRPP2 mainly functions in three subcellular compartments: endoplasmic reticulum, plasma membrane and primary cilia. TRPP2 can act as a calcium-activated intracellular calcium release channel on the endoplasmic reticulum. TRPP2 also interacts with other Ca²⁺ release channels to regulate calcium release, like IP3R and RyR2. TRPP2 acts as an ion channel regulated by epidermal growth factor through activation of downstream factors in the plasma membrane. TRPP2 binding to TRPC1 in the plasma membrane or endoplasmic reticulum is associated with mechanosensitivity. In cilium, TRPP2 was found to combine with PKD1 and TRPV4 to form a complex related to mechanosensitivity. Because TRPP2 is involved in regulating intracellular ion concentration, TRPP2 mutations often lead to autosomal dominant polycystic kidney disease, which may also be associated with cardiovascular disease. In this paper, we review the molecular structure of TRPP2, the subcellular localization of TRPP2, the related functions and mechanisms of TRPP2 at different sites, and the diseases related to TRPP2.

Cilia-Localized Counterregulatory Signals as Drivers of Renal Cystogenesis

Primary cilia play counterregulatory roles in cystogenesis—they inhibit cyst formation in the normal renal tubule but promote cyst growth when the function of polycystins is impaired. Key upstream cilia-specific signals and components involved in driving cystogenesis have remained elusive. Recent studies of the tubby family protein, Tubby-like protein 3 (TULP3), have provided new insights into the cilia-localized mechanisms that determine cyst growth. TULP3 is a key adapter of the intraflagellar transport complex A (IFT-A) in the trafficking of multiple proteins specifically into the ciliary membrane. Loss of TULP3 results in the selective exclusion of its cargoes from cilia without affecting their extraciliary pools and without disrupting cilia or IFT-A complex integrity. Epistasis analyses have indicated that TULP3 inhibits cystogenesis independently of the polycystins during kidney development but promotes cystogenesis in adults when polycystins are lacking. In this review, we discuss the current model of the cilia-dependent cyst activation (CDCA) mechanism in autosomal dominant polycystic kidney disease (ADPKD) and consider the possible roles of ciliary and extraciliary polycystins in regulating CDCA. We then describe the limitations of this model in not fully accounting for how cilia single knockouts cause significant cystic changes either in the presence or absence of polycystins. Based on available data from TULP3/IFT-A-mediated differential regulation of cystogenesis in kidneys with deletion of polycystins either during development or in adulthood, we hypothesize the existence of cilia-localized components of CDCA (cCDCA) and cilia-localized cyst inhibition (CLCI) signals. We develop the criteria for cCDCA/CLCI signals and discuss potential TULP3 cargoes as possible cilia-localized components that determine cystogenesis in kidneys during development and in adult mice.

Polycystin-2 (TRPP2): Ion channel properties and regulation

Polycystin-2 (TRPP2, PKD2, PC2) is the product of the PKD2 gene, whose mutations cause Autosomal Dominant Polycystic Kidney Disease (ADPKD). PC2 belongs to the superfamily of TRP (Transient Receptor Potential) proteins that generally function as Ca²⁺-permeable nonselective cation channels implicated in Ca²⁺ signaling. PC2 localizes to various cell domains with distinct functions that likely depend on interactions with specific channel partners. Functions include receptor-operated, nonselective cation channel activity in the plasma membrane, intracellular Ca²⁺ release channel activity in the endoplasmic reticulum (ER), and mechanosensitive channel activity in the primary cilium of renal epithelial cells. Here we summarize our current understanding of the properties of PC2 and how other transmembrane and cytosolic proteins modulate this activity, providing functional diversity and selective regulatory mechanisms to its role in the control of cellular Ca²⁺ homeostasis.

Regenerative Calcium Currents in Renal Primary Cilia

Polycystic kidney disease (PKD) is a leading cause of end-stage renal disease. PKD arises from mutations in proteins, one a Ca2+-conducting channel, expressed in the primary cilia of renal epithelial cells. A common hypothesis is that Ca2+ entering through ciliary ion channels may reduce cystogenesis. The cilia have at least two Ca2+-conducting channels: polycystin-2 (PC2) and TRPV4 (transient receptor potential (TRP) cation channel, subfamily V, member 4), but how substantially they can increase intraciliary Ca2+ is unknown. By recording channel activities in isolated cilia, conditions are identified under which the channels can increase free Ca2+ within the cilium by at least 500-fold through regenerative (positive-feedback) signaling. Ca2+ that has entered through a channel can activate the channel internally, which increases the Ca2+ influx, and so on. Regenerative signaling is favored when the concentration of the Ca2+ buffer is reduced or when a slower buffer is used. Under such conditions, the Ca2+ that enters the cilium through a single PC2 channel is sufficient to almost fully activate that same channel. Regenerative signaling is not detectable with reduced external Ca2+. Reduced buffering also allows regenerative signaling through TRPV4 channels, but not through TRPM4 (TRP subfamily M, member 4) channels, which are activated by Ca2+ but do not conduct it. On a larger scale, Ca2+ that enters through TRPV4 channels can cause secondary activation of PC2 channels. I discuss the likelihood of regenerative ciliary Ca2+ signaling in vivo, a possible mechanism for its activation, and how it might relate to cystogenesis.

TRP Channels as Molecular Targets to Relieve Endocrine-Related Diseases

Transient receptor potential (TRP) channels are polymodal channels capable of sensing environmental stimuli, which are widely expressed on the plasma membrane of cells and play an essential role in the physiological or pathological processes of cells as sensors. TRPs often form functional homo- or heterotetramers that act as cation channels to flow Na⁺ and Ca²⁺, change membrane potential and [Ca²⁺]_i (cytosolic [Ca²⁺]), and change protein expression levels, channel attributes, and regulatory factors. Under normal circumstances, various TRP channels respond to intracellular and extracellular stimuli such as temperature, pH, osmotic pressure, chemicals, cytokines, and cell damage and depletion of Ca²⁺ reserves. As cation transport channels and physical and chemical stimulation receptors, TRPs play an important role in regulating secretion, interfering with cell proliferation, and affecting neural activity in these glands and their adenocarcinoma cells. Many studies have proved that TRPs are widely distributed in the pancreas, adrenal gland, and other glands. This article reviews the specific regulatory mechanisms of various TRP channels in some common glands (pancreas, salivary gland, lacrimal gland, adrenal gland, mammary gland, gallbladder, and sweat gland).

Enhanced β-adrenergic response in mice with dominant-negative expression of the PKD2L1 channel

Polycystic kidney disease (PKD) is the most common genetic cause of kidney failure in humans. Among the various PKD-related molecules, PKD2L1 forms cation channels, but its physiological importance is obscure. In the present study, we established a transgenic mouse line by overexpressing the dominant-negative form of the mouse PKD2L1 gene (i.e., lacking the pore-forming domain). The resulting PKD2L1del-Tg mice exhibited supraventricular premature contraction, as well as enhanced sensitivity to β-adrenergic stimulation and unstable R-R intervals in electrocardiography. During spontaneous atrial contraction, PKD2L1del-Tg atria showed enhanced sensitivity to isoproterenol, norepinephrine, and epinephrine. Action potential recording revealed a shortened action potential duration in PKD2L1del-Tg atria in response to isoproterenol. These findings indicated increased adrenergic sensitivity in PKD2L1del-Tg mice, suggesting that PKD2L1 is involved in sympathetic regulation.

Impact of lipid rafts on transient receptor potential channel activities

Members of the transient receptor potential (TRP) superfamily are cation channels that are expressed in nearly every mammalian cell type and respond as cellular sensors to various environmental stimuli. Light, pressure, osmolarity, temperature, and other stimuli can induce TRP calcium conductivity and correspondingly trigger many signaling processes in cells. Disruption of TRP channel activity, as a rule, harms cellular function. Despite numerous studies, the mechanisms of TRP channel regulation are not yet sufficiently clear, in part, because TRP channels are regulated by a broad set of ligands having diverse physical and chemical features. It is now known that some TRP members are located in membrane microdomains termed lipid rafts. Moreover, interaction between specific raft-associated lipids with channels may be a key regulation mechanism. This review examines recent findings related to the roles of lipid rafts in regulation of TRP channel activity. The mechanistic events of channel interactions with the main lipid raft constituent, cholesterol, are being clarified. Better understanding of mechanisms behind such interactions would help establish the key elements of TRP channel regulation and hence allow control of cellular responses to environmental stimuli.

Ion Channels of Cilia: Paramecium as a Model

Holotrichous ciliates, like Paramecium, swim through their aqueous environment by beating their many cilia. They can alter swimming speed and direction, which seems to have mesmerized early microscopists of the 1600's. We know from extensive and elegant physiological studies and generation of mutants that these cells can be considered little swimming neurons because their ciliary beating is under bioelectric control of ion channels in the cilia. This chapter will focus on the ionic control of swimming behavior by ciliary ion channels, primarily in the holotrichous ciliate Paramecium. Voltage gated and calcium activated channels for calcium, magnesium, sodium, and potassium are regulated in a closely orchestrated manner that allows cilia to bend and propel the cell forward or backward. Sensory input that generates receptor potentials feeds into the control of this channel activity and allows the cell to turn or speed up. This in turn helps the cell to avoid predators or toxic conditions. While the focus is on P. tetraurelia and P. caudatum, the principles of ciliary ion channel activity and control are easily extendable to other ciliates and protists. The high conservation of channel and ion pump structures also extends the lessons from Paramecium to higher organisms.

Using Paramecium as a Model for Ciliopathies

Paramecium has served as a model organism for the studies of many aspects of genetics and cell biology: non-Mendelian inheritance, genome duplication, genome rearrangements, and exocytosis, to name a few. However, the large number and patterning of cilia that cover its surface have inspired extraordinary ultrastructural work. Its swimming patterns inspired exquisite electrophysiological studies that led to a description of the bioelectric control of ciliary motion. A genetic dissection of swimming behavior moved the field toward the genes and gene products underlying ciliary function. With the advent of molecular technologies, it became clear that there was not only great conservation of ciliary structure but also of the genes coding for ciliary structure and function. It is this conservation and the legacy of past research that allow us to use Paramecium as a model for cilia and ciliary diseases called ciliopathies. However, there would be no compelling reason to study Paramecium as this model if there were no new insights into cilia and ciliopathies to be gained. In this review, we present studies that we believe will do this. For example, while the literature continues to state that immotile cilia are sensory and motile cilia are not, we will provide evidence that Paramecium cilia are clearly sensory. Other examples show that while a Paramecium protein is highly conserved it takes a different interacting partner or conducts a different ion than expected. Perhaps these exceptions will provoke new ideas about mammalian systems.

Polycystin-1 dependent regulation of polycystin-2 via GRP94, a member of HSP90 family that resides in the endoplasmic reticulum

Autosomal dominant polycystic kidney disease is a common inherited renal disorder that results from mutations in either PKD1 or PKD2, encoding polycystin-1 (PC1) and polycystin-2 (PC2), respectively. Downregulation or overexpression of PKD1 or PKD2 in mouse models results in renal cyst formation, suggesting that the quantity of PC1 and PC2 needs to be maintained within a tight functional window to prevent cystogenesis. Here we show that enhanced PC2 expression is a common feature of PKD1 mutant tissues, in part due to an increase in Pkd2 mRNA. However, our data also suggest that more effective protein folding contributes to the augmented levels of PC2. We demonstrate that the unfolded protein response is activated in Pkd1 knockout kidneys and in Pkd1 mutant cells and that this is coupled with increased levels of GRP94, an endoplasmic reticulum protein that is a member of the HSP90 family of chaperones. GRP94 was found to physically interact with PC2 and depletion or chemical inhibition of GRP94 led to a decrease in PC2, suggesting that GRP94 serves as its chaperone. Moreover, GRP94 is acetylated and binds to histone deacetylase 6 (HDAC6), a known deacetylase and activator of HSP90 proteins. Inhibition of HDAC6 decreased PC2 suggesting that HDAC6 and GRP94 work together to regulate PC2 levels. Lastly, we showed that inhibition of GRP94 prevents cAMP-induced cyst formation in vitro. Taken together our data uncovered a novel HDAC6-GRP94-related axis that likely participates in maintaining elevated PC2 levels in Pkd1 mutant cells.

Calcium, an Emerging Intracellular Messenger for the Hippo Pathway Regulation

The Hippo pathway is a conserved signaling network regulating organ development and tissue homeostasis. Dysfunction of this pathway may lead to various diseases, such as regeneration defect and cancer. Studies over the past decade have found various extracellular and intracellular signals that can regulate this pathway. Among them, calcium (Ca²⁺) is emerging as a potential messenger that can transduce certain signals, such as the mechanical cue, to the main signaling machinery. In this process, rearrangement of the actin cytoskeleton, such as calcium-activated actin reset (CaAR), may construct actin filaments at the cell cortex or other subcellular domains that provide a scaffold to launch Hippo pathway activators. This article will review studies demonstrating Ca²⁺-mediated Hippo pathway modulation and discuss its implication in understanding the role of actin cytoskeleton in regulating the Hippo pathway.

Calcium signaling through a transient receptor channel is important for Toxoplasma gondii growth

Transient receptor potential (TRP) channels participate in calcium ion (Ca²⁺) influx and intracellular Ca²⁺ release. TRP channels have not been studied in Toxoplasma gondii or any other apicomplexan parasite. In this work, we characterize TgGT1_310560, a protein predicted to possess a TRP domain (TgTRPPL-2), and determined its role in Ca²⁺ signaling in T. gondii, the causative agent of toxoplasmosis. TgTRPPL-2 localizes to the plasma membrane and the endoplasmic reticulum (ER) of T. gondii. The ΔTgTRPPL-2 mutant was defective in growth and cytosolic Ca²⁺ influx from both extracellular and intracellular sources. Heterologous expression of TgTRPPL-2 in HEK-3KO cells allowed its functional characterization. Patching of ER-nuclear membranes demonstrates that TgTRPPL-2 is a non-selective cation channel that conducts Ca²⁺. Pharmacological blockers of TgTRPPL-2 inhibit Ca²⁺ influx and parasite growth. This is the first report of an apicomplexan ion channel that conducts Ca²⁺ and may initiate a Ca²⁺ signaling cascade that leads to the stimulation of motility, invasion, and egress. TgTRPPL-2 is a potential target for combating toxoplasmosis.

TRPV1 Hyperfunction Contributes to Renal Inflammation in Oxalate Nephropathy

Inflammation worsens oxalate nephropathy by exacerbating tubular damage. The transient receptor potential vanilloid 1 (TRPV1) channel is present in kidney and has a polymodal sensing ability. Here, we tested whether TRPV1 plays a role in hyperoxaluria-induced renal inflammation. In TRPV1-expressed proximal tubular cells LLC-PK₁, oxalate could induce cell damage in a time- and dose-dependent manner; this was associated with increased arachidonate 12-lipoxygenase (ALOX12) expression and synthesis of endovanilloid 12(S)-hydroxyeicosatetraenoic acid for TRPV1 activation. Inhibition of ALOX12 or TRPV1 attenuated oxalate-mediated cell damage. We further showed that increases in intracellular Ca²⁺ and protein kinase C α activation are downstream of TRPV1 for NADPH oxidase 4 upregulation and reactive oxygen species formation. These trigger tubular cell inflammation via increased NLR family pyrin domain-containing 3 expression, caspase-1 activation, and interleukin (IL)-1β release, and were alleviated by TRPV1 inhibition. Male hyperoxaluric rats demonstrated urinary supersaturation, tubular damage, and oxidative stress in a time-dependent manner. Chronic TRPV1 inhibition did not affect hyperoxaluria and urinary supersaturation, but markedly reduced tubular damage and calcium oxalate crystal deposition by lowering oxidative stress and inflammatory signaling. Taking all these results together, we conclude that TRPV1 hyperfunction contributes to oxalate-induced renal inflammation. Blunting TRPV1 function attenuates hyperoxaluric nephropathy.

Transient receptor potential polycystin-2 (TRPP2) regulates motility and intracellular calcium of porcine sperm

Polycystin-2, also known as transient receptor potential polycystin-2 (TRPP2), is a membrane protein that regulates calcium homeostasis in renal epithelial cells. Mutations in PKD2, the gene encoding human TRPP2, cause enlarged cystic kidneys and contribute to polycystic kidney disease (PKD). Male Drosophila melanogaster with mutations in amo, the homolog of PKD2, display a mild decrease in sperm motility but have a drastic reduction in fertility due to failed sperm migration and storage within the female tract. Although TRPP2 has critical roles for Drosophila sperm function, the protein has not been described in mammalian sperm. Herein, we report the localization of TRPP2 in porcine sperm and identify functions of TRPP2 in regulating intracellular Ca²⁺ and motility. Porcine sperm treated with an antibody to TRPP2 in capacitating medium had reduced average path velocity and curvilinear velocity (p < .05). Blocking TRPP2 also increased sperm tail beat-cross frequency (p < .05). After 90 min of capacitation, sperm incubated with TRPP2 antibody had decreased intracellular Ca²⁺ concentration compared to controls (p < .05), consistent with TRPP2 function as a plasma membrane cation channel. This is the first report that mammalian sperm contain TRPP2, which appears to regulate intracellular Ca²⁺ and motility patterns in porcine sperm.