Vascular polycystin proteins in health and disease

PKD1 (polycystin 1) and PKD2 (polycystin 2) are expressed in a variety of different cell types, including arterial smooth muscle and endothelial cells. PKD1 is a transmembrane domain protein with a large extracellular N-terminus that is proposed to act as a mechanosensor and receptor. PKD2 is a member of the transient receptor potential (TRP) channel superfamily which is also termed TRPP1. Mutations in the genes which encode PKD1 and PKD2 lead to autosomal dominant polycystic kidney disease (ADPKD). ADPKD is one of the most prevalent monogenic disorders in humans and is associated with extrarenal and vascular complications, including hypertension. Recent studies have uncovered mechanisms of activation and physiological functions of PKD1 and PKD2 in arterial smooth muscle and endothelial cells. It has also been found that PKD function is altered in the vasculature during ADPKD and hypertension. We will summarize this work and discuss future possibilities for this area of research.

Structure-guided discovery of protein and glycan components in native mastigonemes

Mastigonemes, the hair-like lateral appendages lining cilia or flagella, participate in mechanosensation and cellular motion, but their constituents and structure have remained unclear. Here, we report the cryo-EM structure of native mastigonemes isolated from Chlamydomonas at 3.0 Å resolution. The long stem assembles as a super spiral, with each helical turn comprising four pairs of anti-parallel mastigoneme-like protein 1 (Mst1). A large array of arabinoglycans, which represents a common class of glycosylation in plants and algae, is resolved surrounding the type II poly-hydroxyproline (Hyp) helix in Mst1. The EM map unveils a mastigoneme axial protein (Mstax) that is rich in heavily glycosylated Hyp and contains a PKD2-like transmembrane domain (TMD). Mstax, with nearly 8,000 residues spanning from the intracellular region to the distal end of the mastigoneme, provides the framework for Mst1 assembly. Our study provides insights into the complexity of protein and glycan interactions in native bio-architectures.

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.

Deletion of Aurora kinase A prevents the development of polycystic kidney disease in mice

Aurora Kinase A (AURKA) promotes cell proliferation and is overexpressed in different types of polycystic kidney disease (PKD). To understand AURKA's role in regulating renal cyst development we conditionally deleted the gene in mouse models of Autosomal Dominant PKD (ADPKD) and Joubert Syndrome, caused by Polycystin 1 (Pkd1) and Inositol polyphosphate-5-phosphatase E (Inpp5e) mutations respectively. We show that while Aurka is dispensable for collecting duct development and homeostasis, its deletion prevents cyst formation in both disease models. Cross-comparison of transcriptional changes implicated AKT signaling in cyst prevention and we show that (i) AURKA and AKT physically interact, (ii) AURKA regulates AKT activity in a kinase-independent manner and (iii) inhibition of AKT can reduce disease severity. AKT activation also regulates Aurka expression, creating a feed-forward loop driving renal cystogenesis. We find that the AURKA kinase inhibitor Alisertib stabilises the AURKA protein, agonizing its cystogenic functions. These studies identify AURKA as a master regulator of renal cyst development in different types of PKD, functioning in-part via AKT.

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.

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.

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.

Role of TRPM2 in brain tumours and potential as a drug target

Ion channels are ubiquitously expressed in almost all living cells, and are the third-largest category of drug targets, following enzymes and receptors. The transient receptor potential melastatin (TRPM) subfamily of ion channels are important to cell function and survival. Studies have shown upregulation of the TRPM family of ion channels in various brain tumours. Gliomas are the most prevalent form of primary malignant brain tumours with no effective treatment; thus, drug development is eagerly needed. TRPM2 is an essential ion channel for cell function and has important roles in oxidative stress and inflammation. In response to oxidative stress, ADP-ribose (ADPR) is produced, and in turn activates TRPM2 by binding to the NUDT9-H domain on the C-terminal. TRPM2 has been implicated in various cancers and is significantly upregulated in brain tumours. This article reviews the current understanding of TRPM2 in the context of brain tumours and overviews the effects of potential drug therapies targeting TRPM2 including hydrogen peroxide (H₂O₂), curcumin, docetaxel and selenium, paclitaxel and resveratrol, and botulinum toxin. It is long withstanding knowledge that gliomas are difficult to treat effectively, therefore investigating TRPM2 as a potential therapeutic target for brain tumours may be of considerable interest in the fields of ion channels and pharmacology.

A plasma membrane-localized polycystin-1/polycystin-2 complex in endothelial cells elicits vasodilation

Polycystin-1 (PC-1, PKD1), a receptor-like protein expressed by the Pkd1 gene, is present in a wide variety of cell types, but its cellular location, signaling mechanisms, and physiological functions are poorly understood. Here, by studying tamoxifen-inducible, endothelial cell (EC)-specific Pkd1 knockout (Pkd1 ecKO) mice, we show that flow activates PC-1-mediated, Ca2+-dependent cation currents in ECs. EC-specific PC-1 knockout attenuates flow-mediated arterial hyperpolarization and vasodilation. PC-1-dependent vasodilation occurs over the entire functional shear stress range and via the activation of endothelial nitric oxide synthase (eNOS) and intermediate (IK)- and small (SK)-conductance Ca2+-activated K+ channels. EC-specific PC-1 knockout increases systemic blood pressure without altering kidney anatomy. PC-1 coimmunoprecipitates with polycystin-2 (PC-2, PKD2), a TRP polycystin channel, and clusters of both proteins locate in nanoscale proximity in the EC plasma membrane. Knockout of either PC-1 or PC-2 (Pkd2 ecKO mice) abolishes surface clusters of both PC-1 and PC-2 in ECs. Single knockout of PC-1 or PC-2 or double knockout of PC-1 and PC-2 (Pkd1/Pkd2 ecKO mice) similarly attenuates flow-mediated vasodilation. Flow stimulates nonselective cation currents in ECs that are similarly inhibited by either PC-1 or PC-2 knockout or by interference peptides corresponding to the C-terminus coiled-coil domains present in PC-1 or PC-2. In summary, we show that PC-1 regulates arterial contractility through the formation of an interdependent signaling complex with PC-2 in ECs. Flow stimulates PC-1/PC-2 clusters in the EC plasma membrane, leading to eNOS, IK channel, and SK channel activation, vasodilation, and a reduction in blood pressure.

Polycystin-1 and hydrostatic pressure are implicated in glioblastoma pathogenesis in vitro

The mechanobiological aspects of glioblastoma (GBM) pathogenesis are largely unknown. Polycystin‐1 (PC1) is a key mechanosensitive protein which perceives extracellular mechanical cues and transforms them into intracellular biochemical signals that elicit a change in cell behaviour. The aim of the present study was to investigate if and how PC1 participates in GBM pathogenesis under a mechanically induced microenvironment. Therefore, we subjected T98G GBM cells to continuous hydrostatic pressure (HP) and/or PC1 blockade and evaluated their effect on cell behaviour, the activity of signalling pathways and the expression of mechano‐induced transcriptional regulators and markers associated with properties of cancer cells. According to our data, PC1 and HP affect GBM cell proliferation, clonogenicity and migration; the diameter of GBM spheroids; the phosphorylation of mechanistic target of rapamycin (mTOR), extracellular signal‐regulated kinase (ERK) and focal adhesion kinase (FAK); the protein expression of transcription cofactors YES‐associated protein (YAP) and transcriptional coactivator with PDZ‐binding motif (TAZ); and the mRNA expression of markers related to anti‐apoptosis, apoptosis, angiogenesis, epithelial to mesenchymal transition (EMT) and proliferation. Together, our in vitro results suggest that PC1 plays an important role in GBM mechanobiology.

Molecular detection of polycystic kidney disease in Persian and Persian-related breeds in Iran

Objectives

This study was aimed at detecting feline autosomal dominant polycystic kidney disease in a population of Persian and Persian-related breeds by a molecular method in Iran.

Methods

Buccal swab samples were collected from 47 cats and examined with a touchdown PCR method. Additionally, partial sequencing was performed in two cats with bilateral renal cysts.

Results

Twenty-two cats (46.8%) were diagnosed as heterozygous for this mutation. Sequence analysis of two cats showed C to A point mutation in the PKD1 gene, as in previous studies.

Conclusions and relevance

Prevalence of this disease is high in Iran, highlighting the need for molecular screening tests before including cats in breeding programmes.

Structural basis for Ca2+ activation of the heteromeric PKD1L3/PKD2L1 channel

The heteromeric complex between PKD1L3, a member of the polycystic kidney disease (PKD) protein family, and PKD2L1, also known as TRPP2 or TRPP3, has been a prototype for mechanistic characterization of heterotetrametric TRP-like channels. Here we show that a truncated PKD1L3/PKD2L1 complex with the C-terminal TRP-fold fragment of PKD1L3 retains both Ca²⁺ and acid-induced channel activities. Cryo-EM structures of this core heterocomplex with or without supplemented Ca²⁺ were determined at resolutions of 3.1 Å and 3.4 Å, respectively. The heterotetramer, with a pseudo-symmetric TRP architecture of 1:3 stoichiometry, has an asymmetric selectivity filter (SF) guarded by Lys2069 from PKD1L3 and Asp523 from the three PKD2L1 subunits. Ca²⁺-entrance to the SF vestibule is accompanied by a swing motion of Lys2069 on PKD1L3. The S6 of PKD1L3 is pushed inward by the S4-S5 linker of the nearby PKD2L1 (PKD2L1-III), resulting in an elongated intracellular gate which seals the pore domain. Comparison of the apo and Ca²⁺-loaded complexes unveils an unprecedented Ca²⁺ binding site in the extracellular cleft of the voltage-sensing domain (VSD) of PKD2L1-III, but not the other three VSDs. Structure-guided mutagenic studies support this unconventional site to be responsible for Ca²⁺-induced channel activation through an allosteric mechanism.

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.

Therapeutic advances in ADPKD: the future awaits

Autosomal dominant polycystic kidney disease (ADPKD) is a heterogeneous genetic disorder included in ciliopathies, representing the fourth cause of end stage renal disease (ESRD), with an estimated prevalence between 1:1000 and 1:2500. It is mainly caused by mutations in the PKD1 and PKD2 genes encoding for polycystin 1 (PC1) and polycystin 2 (PC2), which regulate differentiation, proliferation, survival, apoptosis, and autophagy. The advances in the knowledge of multiple molecular pathways involved in the pathophysiology of ADPKD led to the development of several treatments which are currently under investigation. Recently, the widespread approval of tolvaptan and, in Italy, of long-acting release octreotide (octreotide-LAR), represents but the beginning of the new therapeutic management of ADPKD patients. Encouraging results are expected from ongoing randomized controlled trials (RCTs), which are investigating not only drugs acting on the calcium/cyclic adenosin monoposphate (cAMP) pathway, the most studied target so far, but also molecules targeting specific pathophysiological pathways (e.g. epidermal growth factor (EGF) receptor, AMP-activated protein kinase (AMPK) and KEAP1-Nrf2) and sphingolipids. Moreover, studies on animal models and cultured cells have also provided further promising therapeutic strategies based on the role of intracellular calcium, cell cycle regulation, MAPK pathway, epigenetic DNA, interstitial inflammation, and cell therapy. Thus, in a near future, tailored therapy could be the key to changing the natural history of ADPKD thanks to the vigorous efforts that are being made to implement clinical and preclinical studies in this field. Our review aimed to summarize the spectrum of drugs that are available in the clinical practice and the most promising molecules undergoing clinical, animal, and cultured cell studies.

Wnt3a Stimulation Promotes Primary Ciliogenesis through β-Catenin Phosphorylation-Induced Reorganization of Centriolar Satellites

Primary cilium is an antenna-like microtubule-based cellular sensing structure. Abnormal regulation of the dynamic assembly and disassembly cycle of primary cilia is closely related to ciliopathy and cancer. The Wnt signaling pathway plays a major role in embryonic development and tissue homeostasis, and defects in Wnt signaling are associated with a variety of human diseases, including cancer. In this study, we provide direct evidence of Wnt3a-induced primary ciliogenesis, which includes a continuous pathway showing that the stimulation of Wnt3a, a canonical Wnt ligand, promotes the generation of β-catenin p-S47 epitope by CK1δ, and these events lead to the reorganization of centriolar satellites resulting in primary ciliogenesis. We have also confirmed the application of our findings in MCF-7/ADR cells, a multidrug-resistant tumor cell model. Thus, our data provide a Wnt3a-induced primary ciliogenesis pathway and may provide a clue on how to overcome multidrug resistance in cancer treatment.

PKD1-Dependent Renal Cystogenesis in Human Induced Pluripotent Stem Cell-Derived Ureteric Bud/Collecting Duct Organoids

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary kidney disease leading to renal failure, wherein multiple cysts form in renal tubules and collecting ducts derived from distinct precursors: the nephron progenitor and ureteric bud (UB), respectively. Recent progress in induced pluripotent stem cell (iPSC) biology has enabled cyst formation in nephron progenitor-derived human kidney organoids in which PKD1 or PKD2, the major causative genes for ADPKD, are deleted. However, cysts have not been generated in UB organoids, despite the prevalence of collecting duct cysts in patients with ADPKD.

METHODS

CRISPR-Cas9 technology deleted PKD1 in human iPSCs and the cells induced to differentiate along pathways leading to formation of either nephron progenitor or UB organoids. Cyst formation was investigated in both types of kidney organoid derived from PKD1-deleted iPSCs and in UB organoids generated from iPSCs from a patient with ADPKD who had a missense mutation.

RESULTS

Cysts formed in UB organoids with homozygous PKD1 mutations upon cAMP stimulation and, to a lesser extent, in heterozygous mutant organoids. Furthermore, UB organoids generated from iPSCs from a patient with ADPKD who had a heterozygous missense mutation developed cysts upon cAMP stimulation.

CONCLUSIONS

Cysts form in PKD1 mutant UB organoids as well as in iPSCs derived from a patient with ADPKD. The organoids provide a robust model of the genesis of ADPKD.

Identification of missense and synonymous variants in Iranian patients suffering from autosomal dominant polycystic kidney disease

Background

Autosomal dominant polycystic kidney disease (ADPKD), the predominant type of inherited kidney disorder, occurs due to PKD1 and PKD2 gene mutations. ADPKD diagnosis is made primarily by kidney imaging. However, molecular genetic analysis is required to confirm the diagnosis. It is critical to perform a molecular genetic analysis when the imaging diagnosis is uncertain, particularly in simplex cases (i.e. a single occurrence in a family), in people with remarkably mild symptoms, or in individuals with atypical presentations. The main aim of this study is to determine the frequency of PKD1 gene mutations in Iranian patients with ADPKD diagnosis.

Methods

Genomic DNA was extracted from blood samples from 22 ADPKD patients, who were referred to the Qaem Hospital in Mashhad, Iran. By using appropriate primers, 16 end exons of PKD1 gene that are regional hotspots, were replicated with PCR. Then, PCR products were subjected to DNA directional Sanger sequencing.

Results

The DNA sequencing in the patients has shown that exons 35, 36 and 37 were non- polymorphic, and that most mutations had occurred in exons 44 and 45. In two patients, an exon-intron boundary mutation had occurred in intron 44. Most of the variants were missense and synonymous types.

Conclusion

In the present study, we have shown the occurrence of nine novel missense or synonymous variants in PKD1 gene. These data could contribute to an improved diagnostic and genetic counseling in clinical settings.

Loss of polycystins suppresses deciliation via the activation of the centrosomal integrity pathway

The primary cilium is a microtubule-based, antenna-like organelle housing several signaling pathways. It follows a cyclic pattern of assembly and deciliation (disassembly and/or shedding), as cells exit and re-enter the cell cycle, respectively. In general, primary cilia loss leads to kidney cystogenesis. However, in animal models of autosomal dominant polycystic kidney disease, a major disease caused by mutations in the polycystin genes (Pkd1 or Pkd2), primary cilia ablation or acceleration of deciliation suppresses cystic growth, whereas deceleration of deciliation enhances cystogenesis. Here, we show that deciliation is delayed in the cystic epithelium of a mouse model of postnatal deletion of Pkd1 and in Pkd1- or Pkd2-null cells in culture. Mechanistic experiments show that PKD1 depletion activates the centrosomal integrity/mitotic surveillance pathway involving 53BP1, USP28, and p53 leading to a delay in deciliation. Reduced deciliation rate causes prolonged activation of cilia-based signaling pathways that could promote cystic growth. Our study links polycystins to cilia dynamics, identifies cellular deciliation downstream of the centrosomal integrity pathway, and helps explain pro-cystic effects of primary cilia in autosomal dominant polycystic kidney disease.

Heterotrimeric Gα12/13 proteins in kidney injury and disease

Gα₁₂ and Gα₁₃ are ubiquitous members of the heterotrimeric guanine nucleotide-binding protein (G protein) family that play central and integrative roles in the regulation of signal transduction cascades within various cell types in the kidney. Gα₁₂/Gα₁₃ proteins enable the kidney to adapt to an ever-changing environment by transducing stimuli from cell surface receptors and accessory proteins to effector systems. Therefore, perturbations in Gα₁₂/Gα₁₃ levels or their activity can contribute to the pathogenesis of various renal diseases, including renal cancer. This review will highlight and discuss the complex and expanding roles of Gα₁₂/Gα₁₃ proteins on distinct renal pathologies, with emphasis on more recently reported findings. Deciphering how the different Gα₁₂/Gα₁₃ interaction networks participate in the onset and development of renal diseases may lead to the discovery of new therapeutic strategies.

Analysis from the perspective of cilia: the protective effect of PARP inhibitors on visual function during light-induced damage

PURPOSE

To analyze the protective effect of PARP inhibitors on light-damaged retina and explore its possible mechanism from the perspective of ciliopathy.

METHODS

A systematic review of the literature was performed to investigate the protection of PARP inhibition on light-damaged cilia. PubMed database was retrieved to find the relevant studies and 119 literatures were involved in the review.

RESULTS

In retina, the outer segment of photoreceptor is regarded as a special type of primary cilium, so various retinal diseases actually belong to a type of ciliopathy. The retina is the only central nervous tissue exposed to light, but poly (ADP-ribose) polymerase (PARP), as a nuclear enzyme repairing DNA breaks, is overactivated during the light-induced DNA damage, and is involved in the cell death cascade. Studies show that both ATR and phosphorylated Akt colocalize with cilium and play an important role in regulating ciliary function. PARP may function at ATR or PI3K/Akt signal to exert protective effect on cilia.

CONCLUSION

PARP inhibitors may protect the cilia/OS of photoreceptor during light-induced damage, which the possible mechanism may be involved in the activation of ATR and PI3K/Akt signal.

The ion channel function of polycystin-1 in the polycystin-1/polycystin-2 complex

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in PKD1 or PKD2 gene, encoding the polycystic kidney disease protein polycystin-1 and the transient receptor potential channel polycystin-2 (also known as TRPP2), respectively. Polycystin-1 and polycystin-2 form a receptor-ion channel complex located in primary cilia. The function of this complex, especially the role of polycystin-1, is largely unknown due to the lack of a reliable functional assay. In this study, we dissect the role of polycystin-1 by directly recording currents mediated by a gain-of-function (GOF) polycystin-1/polycystin-2 channel. Our data show that this channel has distinct properties from that of the homomeric polycystin-2 channel. The polycystin-1 subunit directly contributes to the channel pore, and its eleven transmembrane domains are sufficient for its channel function. We also show that the cleavage of polycystin-1 at the N-terminal G protein-coupled receptor proteolysis site is not required for the activity of the GOF polycystin-1/polycystin-2 channel. These results demonstrate the ion channel function of polycystin-1 in the polycystin-1/polycystin-2 complex, enriching our understanding of this channel and its role in ADPKD.

Therapeutic perspectives for structural and functional abnormalities of cilia

Ciliopathies are a group of hereditary disorders that result from structural or functional abnormalities of cilia. Recent intense research efforts have uncovered the genetic bases of ciliopathies, and our understanding of the assembly and functions of cilia has been improved significantly. Although mechanism-specific therapies for ciliopathies have not yet received regulatory approval, the use of innovative therapeutic modalities such as oligonucleotide therapy, gene replacement therapy, and gene editing in addition to symptomatic treatments are expected to provide valid treatment options in the near future. Moreover, candidate chemical compounds for developing small molecule drugs to treat ciliopathies have been identified. This review introduces the key features of cilia and ciliopathies, and summarizes the advances as well as the challenges that remain with the development of therapies for treating ciliopathies.

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.

Pkd1-targeted mutation reveals a role for the Wolffian duct in autosomal dominant polycystic kidney disease

Several life-threatening diseases of the kidney have their origins in mutational events that occur during embryonic development. In this study, we investigate the role of the Wolffian duct (WD), the earliest embryonic epithelial progenitor of renal tubules, in the etiology of autosomal dominant polycystic kidney disease (ADPKD). ADPKD is associated with a germline mutation of one of the two Pkd1 alleles. For the disease to occur, a second event that disrupts the expression of the other inherited Pkd1 allele must occur. We postulated that this secondary event can occur in the pronephric WD. Using Cre-Lox recombination, mice with WD-specific deletion of one or both Pkd1 alleles were generated. Homozygous Pkd1-targeted deletion in WD-derived tissues resulted in mice with large cystic kidneys and serologic evidence of renal failure. In contrast, heterozygous deletion of Pkd1 in the WD led to kidneys that were phenotypically indistinguishable from control in the early postnatal period. High-throughput sequencing, however, revealed underlying gene and microRNA (miRNA) changes in these heterozygous mutant kidneys that suggest a strong predisposition toward developing ADPKD. Bioinformatic analysis of this data demonstrated an upregulation of several miRNAs that have been previously associated with PKD; pathway analysis further demonstrated that the differentially expressed genes in the heterozygous mutant kidneys were overrepresented in signaling pathways associated with maintenance and function of the renal tubular epithelium. These results suggest that the WD may be an early epithelial target for the genetic or molecular signals that can lead to cyst formation in ADPKD.

A role for polycystin-1 and polycystin-2 in neural progenitor cell differentiation

Polycystin-1 (PC1) and polycystin-2 (PC2) are transmembrane proteins encoded by the Pkd1 and Pkd2 genes, respectively. Mutations in these genes are causative for the development of autosomal-dominant polycystic kidney disease. A prominent feature of this disease is an unbalanced cell proliferation. PC1 and PC2 physically interact to form a complex, which localizes to the primary cilia of renal epithelial cells. Recently, PC1 and PC2 have also been described to be present in primary cilia of radial glial cells (RGCs) and to contribute to the planar cell polarity of late RGCs and E1 ependymal cells. As neural progenitor cells (NPCs), early RGCs have to balance proliferation for expansion, or for self-renewal and differentiation to generate neurons. It is not known whether the polycystins play a role in this process. Here, we show that PC1 and PC2 are expressed in RGCs of the developing mouse cerebral cortex during neurogenesis. Loss-of-function analysis and cell-based assays reveal that a reduction of PC1 or PC2 expression leads to increased NPC proliferation, while the differentiation to neurons becomes impaired. The increased NPC proliferation is preceded by enhanced Notch signaling and accompanied by a rise in the number of symmetric cell divisions. The transcription factor STAT3 seems to be mechanistically important for polycystin signaling in NPCs as either STAT3 knockdown or inhibition of STAT3 function abrogates the increased proliferation driven by reduced polycystin expression. Our findings indicate that PC1 and PC2 are critical for maintaining a balance between proliferation and differentiation of NPCs.

Polycystin-1 affects cancer cell behaviour and interacts with mTOR and Jak signalling pathways in cancer cell lines

Polycystic Kidney Disease (PKD), which is attributable to mutations in the PKD1 and PKD2 genes encoding polycystin‐1 (PC1) and polycystin‐2 (PC2) respectively, shares common cellular defects with cancer, such as uncontrolled cell proliferation, abnormal differentiation and increased apoptosis. Interestingly, PC1 regulates many signalling pathways including Jak/STAT, mTOR, Wnt, AP‐1 and calcineurin‐NFAT which are also used by cancer cells for sending signals that will allow them to acquire and maintain malignant phenotypes. Nevertheless, the molecular relationship between polycystins and cancer is unknown. In this study, we investigated the role of PC1 in cancer biology using glioblastoma (GOS3), prostate (PC3), breast (MCF7), lung (A549) and colorectal (HT29) cancer cell lines. Our in vitro results propose that PC1 promotes cell migration in GOS3 cells and suppresses cell migration in A549 cells. In addition, PC1 enhances cell proliferation in GOS3 cells but inhibits it in MCF7, A549 and HT29 cells. We also found that PC1 up‐regulates mTOR signalling and down‐regulates Jak signalling in GOS3 cells, while it up‐regulates mTOR signalling in PC3 and HT29 cells. Together, our study suggests that PC1 modulates cell proliferation and migration and interacts with mTOR and Jak signalling pathways in different cancer cell lines. Understanding the molecular details of how polycystins are associated with cancer may lead to the identification of new players in this devastating disease.

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.

Protein phosphatase 1α interacts with a novel ciliary targeting sequence of polycystin-1 and regulates polycystin-1 trafficking

Autosomal dominant polycystic kidney disease (ADPKD) is the most common genetic disorder causing renal failure. Mutations of polycystic kidney disease 1 (PKD1) account for most ADPKD cases. Defective ciliary localization of polycystin-1 (PC1), a large integral membrane protein encoded by PKD1, underlies the pathogenesis of a subgroup of patients with ADPKD. However, the mechanisms by which PC1 and other ciliary proteins traffic to the primary cilium remain poorly understood. A ciliary targeting sequence (CTS) that resides in ciliary receptors is considered to function in the process. It has been reported that the VxP motif in the intracellular C-terminal tail of PC1 functions as a CTS in an ADP ribosylation factor 4 (Arf4)/ArfGAP with SH3 domain, ankyrin repeat and PH domain 1 (ASAP1)-dependent manner. However, other recent studies have revealed that this motif is dispensable for PC1 trafficking to cilia. In this study, we identified a novel CTS consisting of 8 residues (RHKVRFEG) in the PC1 C tail. We found that this motif is sufficient to bind protein phosphatase 1 (PP1)α, a ubiquitously expressed phosphatase in the phosphoprotein phosphatase (PPP) family. Mutations in this CTS motif disrupt binding with PP1α and impair ciliary localization of PC1. Additionally, short hairpin RNA-mediated knockdown of PP1α results in reduced ciliary localization of PC1 and elongated cilia, suggesting a role for PP1α in the regulation of ciliary structure and function.-Luo, C., Wu, M., Su, X., Yu, F., Brautigan, D. L., Chen, J., Zhou, J. Protein phosphatase 1α interacts with a novel ciliary targeting sequence of polycystin-1 and regulates polycystin-1 trafficking.

Primary Cilia Exhibit Mechanosensitivity to Cyclic Tensile Strain and Lineage-Dependent Expression in Adipose-Derived Stem Cells

Non-motile primary cilia are dynamic cellular sensory structures and are expressed in adipose-derived stem cells (ASCs). We have previously shown that primary cilia are involved in chemically-induced osteogenic differentiation of human ASC (hASCs) in vitro. Further, we have reported that 10% cyclic tensile strain (1 Hz, 4 hours/day) enhances hASC osteogenesis. We hypothesize that primary cilia respond to cyclic tensile strain in a lineage dependent manner and that their mechanosensitivity may regulate the dynamics of signaling pathways localized to the cilium. We found that hASC morphology, cilia length and cilia conformation varied in response to culture in complete growth, osteogenic differentiation, or adipogenic differentiation medium, with the longest cilia expressed in adipogenically differentiating cells. Further, we show that cyclic tensile strain both enhances osteogenic differentiation of hASCs while it suppresses adipogenic differentiation as evidenced by upregulation of RUNX2 gene expression and downregulation of PPARG and IGF-1, respectively. This study demonstrates that hASC primary cilia exhibit mechanosensitivity to cyclic tensile strain and lineage-dependent expression, which may in part regulate signaling pathways localized to the primary cilium during the differentiation process. We highlight the importance of the primary cilium structure in mechanosensing and lineage specification and surmise that this structure may be a novel target in manipulating hASC for in tissue engineering applications.

A PKD1L3 splice variant in taste buds is not cleaved at the G protein-coupled receptor proteolytic site

Mutations in polycystin proteins PKD1 and TRPP2 lead to autosomal dominant polycystic kidney disease. These two proteins form a receptor-ion channel complex on primary cilia. PKD1 undergoes an autoproteolysis at the N terminal G-protein-coupled receptor proteolytic site (GPS), which is essential for the function of PKD1. Whether GPS cleavage happens in other PKD proteins and its functional consequence has remained elusive. Here we studied the GPS cleavage of PKD1L3, a protein that associates with TRPP3 in taste cells and may play a role in sour taste. Our results show that PKD1L3 also undergoes GPS cleavage. Mutation at the GPS abolishes the cleavage, and the non-cleavable mutant does not traffic to the plasma membrane when associated with TRPP3. We also found that a splice variant of PKD1L3, which was originally identified in taste buds, is not cleaved. Amino acids L708 and S709, which are missing in this splice variant, are crucial for the GPS cleavage of PKD1L3 and the trafficking of the PKD1L3/TRPP3 complex. Our results gain insight into the molecular mechanism of the GPS cleavage of PKD1L3. The presence of the non-cleavable variant suggests the potential in vivo function of uncleaved PKD proteins.

Shear stress induced nuclear shrinkage through activation of Piezo1 channels in epithelial cells

The cell nucleus responds to mechanical cues with changes in size, morphology, and motility. Previous work showed that external forces couple to nuclei through the cytoskeleton network, but we show here that changes in nuclear shape can be driven solely by calcium levels. Fluid shear stress applied to MDCK cells caused the nuclei to shrink through a Ca2+ dependent signaling pathway. Inhibiting mechanosensitive Piezo1 channels with GsMTx4 prevented nuclear shrinkage. Piezo1 knockdown also significantly reduced the nuclear shrinkage. Activation of Piezo1 with the agonist Yoda1 caused similar nucleus shrinkage without shear stress. These results demonstrate that Piezo1 channel is a key element for transmitting shear force input to nuclei. To ascertain the relative contributions of Ca2+ to cytoskeleton perturbation, we examined the F-actin reorganization under shear stress and static conditions, and showed that reorganization of the cytoskeleton is not necessary for nuclear shrinkage. These results emphasize the role of the mechanosensitive channels as primary transducers in force transmission to the nucleus.

Guided tissue organization and disease modeling in a kidney tubule array

Three-dimensional (3D) in vitro kidney tubule models have either largely relied on the self-morphogenetic properties of the mammalian cells or used an engineered microfluidic platform with a monolayer of cells cultured on an extracellular matrix (ECM) protein coated porous membrane. These systems are used to understand critical processes during kidney development and transport properties of renal tubules. However, high variability and lack of kidney tubule-relevant geometries among engineered structures limit their utility in disease research and pre-clinical drug testing. Here, we report a novel bioengineered guided kidney tubule (gKT) array system that incorporates in vivo-like physicochemical cues in 3D culture to reproducibly generate homogeneous kidney tubules. The system utilizes a unique 3D micro-molded ECM platform in human physiology-scale dimensions (50-μm diameter) and relevant shapes to guide cells towards formation of mature tubule structures. The guided kidney tubules in our array system display enhanced tubule homogeneity with in vivo-like structural and functional features as evaluated by marker protein localization and epithelial transport analysis. Furthermore, the response of gKT structures to forskolin treatment exhibits characteristic tissue transformations from tubules to expanding cysts. Moreover, acute cisplatin injury causes induction of Kidney Injury Molecule-1 (KIM-1) expression as well as tubular necrosis and apoptosis. Thus the gKT array system offers enhanced structural uniformity with accurate in vivo-like tissue architecture, and will have broad applications in kidney tubule disease pathophysiology (including ciliopathies and drug-induced acute kidney injury), and will enhance pre-clinical drug screening studies.

Structure of the human PKD1-PKD2 complex

Mutations in two genes, PKD1 and PKD2, account for most cases of autosomal dominant polycystic kidney disease, one of the most common monogenetic disorders. Here we report the 3.6-angstrom cryo–electron microscopy structure of truncated human PKD1-PKD2 complex assembled in a 1:3 ratio. PKD1 contains a voltage-gated ion channel (VGIC) fold that interacts with PKD2 to form the domain-swapped, yet noncanonical, transient receptor potential (TRP) channel architecture. The S6 helix in PKD1 is broken in the middle, with the extracellular half, S6a, resembling pore helix 1 in a typical TRP channel. Three positively charged, cavity-facing residues on S6b may block cation permeation. In addition to the VGIC, a five–transmembrane helix domain and a cytosolic PLAT domain were resolved in PKD1. The PKD1-PKD2 complex structure establishes a framework for dissecting the function and disease mechanisms of the PKD proteins.

Retromer associates with the cytoplasmic amino-terminus of polycystin-2

Autosomal dominant polycystic kidney disease (ADPKD) is the most common monogenic human disease, with around 12.5 million people affected worldwide. ADPKD results from mutations in either PKD1 or PKD2, which encode the atypical G-protein coupled receptor polycystin-1 (PC1) and the transient receptor potential channel polycystin-2 (PC2), respectively. Although altered intracellular trafficking of PC1 and PC2 is an underlying feature of ADPKD, the mechanisms which govern vesicular transport of the polycystins through the biosynthetic and endosomal membrane networks remain to be fully elucidated. Here, we describe an interaction between PC2 and retromer, a master controller for the sorting of integral membrane proteins through the endo-lysosomal network. We show that association of PC2 with retromer occurs via a region in the PC2 cytoplasmic amino-terminal domain, independently of the retromer-binding Wiskott-Aldrich syndrome and scar homologue (WASH) complex. Based on observations that retromer preferentially interacts with a trafficking population of PC2, and that ciliary levels of PC1 are reduced upon mutation of key residues required for retromer association in PC2, our data are consistent with the identification of PC2 as a retromer cargo protein.This article has an associated First Person interview with the first author of the paper.

Ciliogenesis is reciprocally regulated by PPARA and NR1H4/FXR through controlling autophagy in vitro and in vivo

The primary cilia are evolutionarily conserved microtubule-based cellular organelles that perceive metabolic status and thus link the sensory system to cellular signaling pathways. Therefore, ciliogenesis is thought to be tightly linked to autophagy, which is also regulated by nutrient-sensing transcription factors, such as PPARA (peroxisome proliferator activated receptor alpha) and NR1H4/FXR (nuclear receptor subfamily 1, group H, member 4). However, the relationship between these factors and ciliogenesis has not been clearly demonstrated. Here, we present direct evidence for the involvement of macroautophagic/autophagic regulators in controlling ciliogenesis. We showed that activation of PPARA facilitated ciliogenesis independently of cellular nutritional states. Importantly, PPARA-induced ciliogenesis was mediated by controlling autophagy, since either pharmacological or genetic inactivation of autophagy significantly repressed ciliogenesis. Moreover, we showed that pharmacological activator of autophagy, rapamycin, recovered repressed ciliogenesis in ppara^-/- cells. Conversely, activation of NR1H4 repressed cilia formation, while knockdown of NR1H4 enhanced ciliogenesis by inducing autophagy. The reciprocal activities of PPARA and NR1H4 in regulating ciliogenesis were highlighted in a condition where de-repressed ciliogenesis by NR1H4 knockdown was further enhanced by PPARA activation. The in vivo roles of PPARA and NR1H4 in regulating ciliogenesis were examined in greater detail in ppara^-/- mice. In response to starvation, ciliogenesis was facilitated in wild-type mice via enhanced autophagy in kidney, while ppara^-/- mice displayed impaired autophagy and kidney damage resembling ciliopathy. Furthermore, an NR1H4 agonist exacerbated kidney damage associated with starvation in ppara^-/- mice. These findings indicate a previously unknown role for PPARA and NR1H4 in regulating the autophagy-ciliogenesis axis in vivo.

Cryo-EM structure of the polycystic kidney disease-like channel PKD2L1

PKD2L1, also termed TRPP3 from the TRPP subfamily (polycystic TRP channels), is involved in the sour sensation and other pH-dependent processes. PKD2L1 is believed to be a nonselective cation channel that can be regulated by voltage, protons, and calcium. Despite its considerable importance, the molecular mechanisms underlying PKD2L1 regulations are largely unknown. Here, we determine the PKD2L1 atomic structure at 3.38 Å resolution by cryo-electron microscopy, whereby side chains of nearly all residues are assigned. Unlike its ortholog PKD2, the pore helix (PH) and transmembrane segment 6 (S6) of PKD2L1, which are involved in upper and lower-gate opening, adopt an open conformation. Structural comparisons of PKD2L1 with a PKD2-based homologous model indicate that the pore domain dilation is coupled to conformational changes of voltage-sensing domains (VSDs) via a series of π-π interactions, suggesting a potential PKD2L1 gating mechanism.

Sclt1 deficiency causes cystic kidney by activating ERK and STAT3 signaling

Ciliopathies form a group of inherited disorders sharing several clinical manifestations because of abnormal cilia formation or function, and few treatments have been successful against these disorders. Here, we report a mouse model with mutated Sclt1 gene, which encodes a centriole distal appendage protein important for ciliogenesis. Sodium channel and clathrin linker 1 (SCLT1) mutations were associated with the oral-facial-digital syndrome (OFD), an autosomal recessive ciliopathy. The Sclt1-/- mice exhibit typical ciliopathy phenotypes, including cystic kidney, cleft palate and polydactyly. Sclt1-loss decreases the number of cilia in kidney; increases proliferation and apoptosis of renal tubule epithelial cells; elevates protein kinase A, extracellular signal-regulated kinases, SMAD and signal transducer and activator of transcription 3 (STAT3) pathways; and enhances pro-inflammation and pro-fibrosis pathways with disease progression. Embryonic kidney cyst formation of Sclt1-/- mice was effectively reduced by an anti-STAT3 treatment using pyrimethamine. Overall, we reported a new mouse model for the OFD; and our data suggest that STAT3 inhibition may be a promising treatment for SCLT1-associated cystic kidney.

Quercetin inhibits renal cyst growth in vitro and via parenteral injection in a polycystic kidney disease mouse model

Autosomal dominant polycystic kidney disease (ADPKD) is a common monogenic disease characterized by massive enlargement of fluid-filled cysts in the kidney.

Recent advances in the molecular diagnosis of polycystic kidney disease

INTRODUCTION

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

Routes and machinery of primary cilium biogenesis

Primary cilia are solitary, microtubule-based protrusions of the cell surface that play fundamental roles as photosensors, mechanosensors and biochemical sensors. Primary cilia dysfunction results in a long list of developmental and degenerative disorders that combine to give rise to a large spectrum of human diseases affecting almost any major body organ. Depending on the cell type, primary ciliogenesis is initiated intracellularly, as in fibroblasts, or at the cell surface, as in renal polarized epithelial cells. In this review, we have focused on the routes of primary ciliogenesis placing particular emphasis on the recently described pathway in renal polarized epithelial cells by which the midbody remnant resulting from a previous cell division event enables the centrosome for initiation of primary cilium assembly. The protein machinery implicated in primary cilium formation in epithelial cells, including the machinery best known for its involvement in establishing cell polarity and polarized membrane trafficking, is also discussed.

Loss of Glis2/NPHP7 causes kidney epithelial cell senescence and suppresses cyst growth in the Kif3a mouse model of cystic kidney disease

Enlargement of kidney tubules is a common feature of multiple cystic kidney diseases in humans and mice. However, while some of these pathologies are characterized by cyst expansion and organ enlargement, in others, progressive interstitial fibrosis and kidney atrophy prevail. The Kif3a knockout mouse is an established non-orthologous mouse model of cystic kidney disease. Conditional inactivation of Kif3a in kidney tubular cells results in loss of primary cilia and rapid cyst growth. Conversely, loss of function of the gene GLIS2/NPHP7 causes progressive kidney atrophy, interstitial inflammatory infiltration, and fibrosis. Kif3a null tubular cells have unrestrained proliferation and reduced stabilization of p53 resulting in a loss of cell cycle arrest in the presence of DNA damage. In contrast, loss of Glis2 is associated with activation of checkpoint kinase 1, stabilization of p53, and induction of cell senescence. Interestingly, the cystic phenotype of Kif3a knockout mice is partially rescued by genetic ablation of Glis2 and pharmacological stabilization of p53. Thus, Kif3a is required for cell cycle regulation and the DNA damage response, whereas cell senescence is significantly enhanced in Glis2 null cells. Hence, cell senescence is a central feature in nephronophthisis type 7 and Kif3a is unexpectedly required for efficient DNA damage response and cell cycle arrest.

Polycystin-1 inhibits eIF2α phosphorylation and cell apoptosis through a PKR-eIF2α pathway

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in PKD1 or PKD2 which encodes polycystin-1 (PC1) and polycystin-2, respectively. PC1 was previously shown to slow cell proliferation and inhibit apoptosis but the underlying mechanisms remain elusive or controversial. Here we showed in cultured mammalian cells and Pkd1 knockout mouse kidney epithelial cells that PC1 and its truncation mutant comprising the last five transmembrane segments and the intracellular C-terminus (PC1-5TMC) down-regulate the phosphorylation of protein kinase R (PKR) and its substrate eukaryotic translation initiation factor 2 alpha (eIF2α). PKR is known to be activated by interferons and dsRNAs, inhibits protein synthesis and induces apoptosis. By co-immunoprecipitation experiments we found that PC1 truncation mutants associate with PKR, or with PKR and its activator PACT. Further experiments showed that PC1 and PC1-5TMC reduce phosphorylation of eIF2α through inhibiting PKR phosphorylation. Our TUNEL experiments using tunicamycin, an apoptosis inducer, and GADD34, an inhibitor of eIF2α phosphorylation, demonstrated that PC1-5TMC inhibits apoptosis of HEK293T cells in a PKR-eIF2α-dependent manner, with concurrent up- and down-regulation of Bcl-2 and Bax, respectively, revealed by Western blotting. Involvement of PC1-regulated eIF2α phosphorylation and a PKR-eIF2α pathway in cell apoptosis may be an important part of the mechanism underlying ADPKD pathogenesis.

Flow induced adherens junction remodeling driven by cytoskeletal forces

Adherens junctions (AJs) are a key structural component for tissue organization and function. Under fluid shear stress, AJs exhibit dynamic assembly/disassembly, but how shear stress couples to AJs is unclear. In MDCK cells we measured simultaneously the forces in cytoskeletal α-actinin and the density and length of AJs using a genetically coded optical force sensor, actinin-sstFRET, and fluorescently labeled E-cadherin (E-cad). We found that shear stress of 0.74dyn/cm² for 3h significantly enhanced E-cad expression at cell-cell contacts and this phenomenon has two phases. The initial formation of segregated AJ plaques coincided with a decrease in cytoskeletal tension, but an increase in tension was necessary for expansion of the plaques and the formation of continuous AJs in the later phase. The changes in cytoskeletal tension and reorganization appear to be an upstream process in response to flow since it occurred in both wild type and dominant negative E-cad cells. Disruption of F-actin with a Rho-ROCK inhibitor eliminated AJ growth under flow. These results delineate the shear stress transduction paths in cultured cells, which helps to understand pathology of a range of diseases that involve dysfunction of E-cadherin.

Disease modeling in genetic kidney diseases: zebrafish

Growing numbers of translational genomics studies are based on the highly efficient and versatile zebrafish (Danio rerio) vertebrate model. The increasing types of zebrafish models have improved our understanding of inherited kidney diseases, since they not only display pathophysiological changes but also give us the opportunity to develop and test novel treatment options in a high-throughput manner. New paradigms in inherited kidney diseases have been developed on the basis of the distinct genome conservation of approximately 70 % between zebrafish and humans in terms of existing gene orthologs. Several options are available to determine the functional role of a specific gene or gene sets. Permanent genome editing can be induced via complete gene knockout by using the CRISPR/Cas-system, among others, or via transient modification by using various morpholino techniques. Cross-species rescues succeeding knockdown techniques are employed to determine the functional significance of a target gene or a specific mutation. This article summarizes the current techniques and discusses their perspectives.

Identification and Expression Analysis of the Complete Family of Zebrafish pkd Genes

Polycystic kidney disease (PKD) proteins are trans-membrane proteins that have crucial roles in many aspects of vertebrate development and physiology, including the development of many organs as well as left–right patterning and taste. They can be divided into structurally-distinct PKD1-like and PKD2-like proteins and usually one PKD1-like protein forms a heteromeric polycystin complex with a PKD2-like protein. For example, PKD1 forms a complex with PKD2 and mutations in either of these proteins cause Autosomal Dominant Polycystic Kidney Disease (ADPKD), which is the most frequent potentially-lethal single-gene disorder in humans. Here, we identify the complete family of pkd genes in zebrafish and other teleosts. We describe the genomic locations and sequences of all seven genes: pkd1, pkd1b, pkd1l1, pkd1l2a, pkd1l2b, pkd2, and pkd2l1. pkd1l2a/pkd1l2b are likely to be ohnologs of pkd1l2, preserved from the whole genome duplication that occurred at the base of the teleosts. However, in contrast to mammals and cartilaginous and holostei fish, teleosts lack pkd2l2, and pkdrej genes, suggesting that these have been lost in the teleost lineage. In addition, teleost, and holostei fish have only a partial pkd1l3 sequence, suggesting that this gene may be in the process of being lost in the ray-finned fish lineage. We also provide the first comprehensive description of the expression of zebrafish pkd genes during development. In most structures we detect expression of one pkd1-like gene and one pkd2-like gene, consistent with these genes encoding a heteromeric protein complex. For example, we found that pkd2 and pkd1l1 are expressed in Kupffer's vesicle and pkd1 and pkd2 are expressed in the developing pronephros. In the spinal cord, we show that pkd1l2a and pkd2l1 are co-expressed in KA cells. We also identify potential co-expression of pkd1b and pkd2 in the floor-plate. Interestingly, and in contrast to mouse, we observe expression of all seven pkd genes in regions that may correspond to taste receptors. Taken together, these results provide a crucial catalog of pkd genes in an important model system for elucidating cell and developmental processes and modeling human diseases and the most comprehensive analysis of embryonic pkd gene expression in any vertebrate.

Extracellular Loops Are Essential for the Assembly and Function of Polycystin Receptor-Ion Channel Complexes

Polycystin complexes, or TRPP-PKD complexes, made of transient receptor potential channel polycystin (TRPP) and polycystic kidney disease (PKD) proteins, play key roles in coupling extracellular stimuli with intracellular Ca²⁺ signals. For example, the TRPP2-PKD1 complex has a crucial function in renal physiology, with mutations in either protein causing autosomal dominant polycystic kidney disease. In contrast, the TRPP3-PKD1L3 complex responds to low pH and was proposed to be a sour taste receptor candidate. It has been shown previously that the protein partners interact via association of the C-terminal or transmembrane segments, with consequences for the assembly, surface expression, and function of the polycystin complexes. However, the roles of extracellular components, especially the loops that connect the transmembrane segments, in the assembly and function of the polycystin complex are largely unknown. Here, with an immunoprecipitation method, we found that extracellular loops between the first and second transmembrane segments of TRPP2 and TRPP3 associate with the extracellular loops between the sixth and seventh transmembrane segments of PKD1 and PKD1L3, respectively. Immunofluorescence and electrophysiology data further confirm that the associations between these loops are essential for the trafficking and function of the complexes. Interestingly, most of the extracellular loops are also found to be involved in homomeric assembly. Furthermore, autosomal dominant polycystic kidney disease-associated TRPP2 mutant T448K significantly weakened TRPP2 homomeric assembly but had no obvious effect on TRPP2-PKD1 heteromeric assembly. Our results demonstrate a crucial role of these functionally underexplored extracellular loops in the assembly and function of the polycystin complexes.

Primary Cilia in Cystic Kidney Disease

Primary cilia are small, antenna-like structures that detect mechanical and chemical cues and transduce extracellular signals. While mammalian primary cilia were first reported in the late 1800s, scientific interest in these sensory organelles has burgeoned since the beginning of the twenty-first century with recognition that primary cilia are essential to human health. Among the most common clinical manifestations of ciliary dysfunction are renal cysts. The molecular mechanisms underlying renal cystogenesis are complex, involving multiple aberrant cellular processes and signaling pathways, while initiating molecular events remain undefined. Autosomal Dominant Polycystic Kidney Disease is the most common renal cystic disease, caused by disruption of polycystin-1 and polycystin-2 transmembrane proteins, which evidence suggests must localize to primary cilia for proper function. To understand how the absence of these proteins in primary cilia may be remediated, we review intracellular trafficking of polycystins to the primary cilium. We also examine the controversial mechanisms by which primary cilia transduce flow-mediated mechanical stress into intracellular calcium. Further, to better understand ciliary function in the kidney, we highlight the LKB1/AMPK, Wnt, and Hedgehog developmental signaling pathways mediated by primary cilia and misregulated in renal cystic disease.

The spectrum of autosomal dominant polycystic kidney disease in children and adolescents

Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary renal disorder. It is characterized by the development of renal cysts and kidney enlargement and ultimately leads to renal failure typically in the sixth decade of life. Although most patients are asymptomatic until well into adulthood, renal cysts develop much earlier, often in utero. Significant renal anatomic and cystic expansion typically occurs before clinical manifestations in children and young adults with AKPKD. The cyst burden detected by imaging represents the minority of cyst burden, and renal and cardiovascular abnormalities are the most common manifestations in children with ADPKD. Here we review the molecular pathogenesis of ADPKD, discuss the screening, diagnosis and clinical manifestations of this renal disorder in childhood and adolescents and review treatment options and potential therapies currently being tested.

A biomimetic bioelectronic tongue: A switch for On- and Off- response of acid sensations

The perception of sour taste in mammals is important for its basic modality properties and avoiding toxic substances. We explore a biomimetic bioelectronic tongue, which integrate MEA (microelectrode array) and taste receptor cell for acid detection as a switch. However, the acid-sensing mechanism and coding of the taste receptor cells in the periphery is not well understood, with long-standing debate. Therefore, we firstly construct a Hodgkin-Huxley type mathematical model of whole-cell acid-sensing taste receptor cells based on the electrophysiologic patch clamp recordings with different acid sensitive receptor expressing and different acidic stimulations. ASICs and PKDL channels are two most promising candidates for acidic sensation. ASICs channels contribute to the On response, and PKDL channels coding the Offset stimulations respectively, which function as a pair for switch. Therefore, with the advantage of effective and noninvasive detection for MEA, a sour taste biosensor based on MEA and taste receptor cells was designed and established to detect sour response from the elementary acid sensitive taste receptor cells during and after stimulus. From simulation and extracelluar potential recordings, we found the biomimetic bioelectronic tongue was acid-sensitive, as acid stimulation pH decrease, the firing frequency significantly increase. Furthermore, this reliable and effective MEA based bioelectronic tongue functioned as a switch for stimulation On and Off. This study provided a powerful platform to recognize sour stimulation and help elucidate the sour taste sensation and coding mechanism.