Characterization of purinergic receptor expression in ARPKD cystic epithelia

Purinergic 2X 4 (P2X4), but not P2X7, receptors increase cytosolic [Ca2+] and stimulate mucin secretion in rat conjunctival goblet cells to maintain ocular surface health

Ionotropic purinergic receptors (P2XRs) are activated by ATP and ATP analogs. ATP can be released through ATP-permeable channels such as the pannexin hemichannels. Upon activation, the P2XRs become permeable to Ca2+, a potent stimulator of mucin secretion in conjunctival goblet cells (CGCs). The purpose of this study was to investigate the presence and function of P2XRs in CGCs. We also examined the presence of pannexin hemichannels. Rat first passage CGCs were stained with the goblet cell marker anti-cytokeratin 7 antibody and specific antibodies to P2X1-7 receptors and pannexin 1-3. mRNA expression was determined by RT-PCR using primers specific to P2XRs and pannexins. Proteins were identified with Western blotting (WB) using the same antibodies as for immunofluorescence (IF) microscopy. To study receptor function, CGCs were incubated with Fura 2-AM, exposed to agonists and antagonists, and intracellular [Ca2+] ([Ca2+]i) measured. [Ca2+]i was also measured after knock down of P2X4 and P2X7 receptor expression, and when exploiting P2XR specific characteristics. Lastly, mucin secretion was measured after the addition of several P2XR agonists. All P2XRs and pannexins were visualized with IF microscopy, and identified with RT-PCR and WB. [Ca2+]i was significantly increased when stimulated with ATP (10-7-10-4 M). Suramin, a non-selective P2XR antagonist at 10-4 M did not reduce ATP-induced peak [Ca2+]i. The potent P2X7 agonist, BzATP (10-7-10-4 M) increased the [Ca2+]i, although to a lesser extent than ATP. When measuring [Ca2+]i the effect of repeated applications of ATP at 10-5 or 10-6 M the response "desensitized" after 30-60 s. The P2X4 specific antagonist 5-BDBD decreased the P2X4 agonist, 2MeSATP,-induced [Ca2+]i increase. Furthermore, siRNA against the P2X4R, but not the P2X7R, decreased agonist-induced peak [Ca2+]i. ATP (10-5 M), BzATP (10-4 M) and 2MeSATP (10-5 M) induced mucin secretion. We conclude that all seven P2XRs are present in cultured rat CGCs. Of the P2XRs, only activation of the homotrimeric P2X4R appears to increase [Ca2+]i and induce mucin secretion. The P2X4R in CGCs offers a new therapeutic target for protective mucin secretion.

Probenecid slows disease progression in a murine model of autosomal dominant polycystic kidney disease

Development of autosomal dominant polycystic kidney disease (ADPKD) involves renal epithelial cell abnormalities. Cystic fluid contains a high level of ATP that, among other effects, leads to a reduced reabsorption of electrolytes in cyst-lining cells, and thus results in cystic fluid accumulation. Earlier, we demonstrated that Pkd1RC/RC mice, a hypomorphic model of ADPKD, exhibit increased expression of pannexin-1, a membrane channel capable of ATP release. In the current study, we found that human ADPKD cystic epithelia have higher pannexin-1 abundance than normal collecting ducts. We hypothesized that inhibition of pannexin-1 function with probenecid can be used to attenuate ADPKD development. Renal function in male and female Pkd1RC/RC and control mice was monitored between 9 and 20 months of age. To test the therapeutic effects of probenecid (a uricosuric agent and a pannexin-1 blocker), osmotic minipumps were implanted in male and female Pkd1RC/RC mice, and probenecid or vehicle was administered for 42 days until 1 year of age. Probenecid treatment improved glomerular filtration rates and slowed renal cyst formation in male mice (as shown in histopathology). The mechanistic effects of probenecid on sodium reabsorption and fluid transport were tested on polarized mpkCCDcl4 cells subjected to short-circuit current measurements, and in 3D cysts grown in Matrigel. In the mpkCCDcl4 epithelial cell line, probenecid elicited higher ENaC currents and attenuated in vitro cyst formation, indicating lower sodium and less fluid retention in the cysts. Our studies open new avenues of research into targeting pannexin-1 in ADPKD pathology.

Ion channels and channelopathies in glomeruli

An essential step in renal function entails the formation of an ultrafiltrate that is delivered to the renal tubules for subsequent processing. This process, known as glomerular filtration, is controlled by intrinsic regulatory systems and by paracrine, neuronal, and endocrine signals that converge onto glomerular cells. In addition, the characteristics of glomerular fluid flow, such as the glomerular filtration rate and the glomerular filtration fraction, play an important role in determining blood flow to the rest of the kidney. Consequently, disease processes that initially affect glomeruli are the most likely to lead to end-stage kidney failure. The cells that comprise the glomerular filter, especially podocytes and mesangial cells, express many different types of ion channels that regulate intrinsic aspects of cell function and cellular responses to the local environment, such as changes in glomerular capillary pressure. Dysregulation of glomerular ion channels, such as changes in TRPC6, can lead to devastating glomerular diseases, and a number of channels, including TRPC6, TRPC5, and various ionotropic receptors, are promising targets for drug development. This review discusses glomerular structure and glomerular disease processes. It also describes the types of plasma membrane ion channels that have been identified in glomerular cells, the physiological and pathophysiological contexts in which they operate, and the pathways by which they are regulated and dysregulated. The contributions of these channels to glomerular disease processes, such as focal segmental glomerulosclerosis (FSGS) and diabetic nephropathy, as well as the development of drugs that target these channels are also discussed.

Caspase-1 and the inflammasome promote polycystic kidney disease progression

We and others have previously shown that the presence of renal innate immune cells can promote polycystic kidney disease (PKD) progression. In this study, we examined the influence of the inflammasome, a key part of the innate immune system, on PKD. The inflammasome is a system of molecular sensors, receptors, and scaffolds that responds to stimuli like cellular damage or microbes by activating Caspase-1, and generating critical mediators of the inflammatory milieu, including IL-1β and IL-18. We provide evidence that the inflammasome is primed in PKD, as multiple inflammasome sensors were upregulated in cystic kidneys from human ADPKD patients, as well as in kidneys from both orthologous (PKD1 RC/RC or RC/RC) and non-orthologous (jck) mouse models of PKD. Further, we demonstrate that the inflammasome is activated in female RC/RC mice kidneys, and this activation occurs in renal leukocytes, primarily in CD11c+ cells. Knock-out of Casp1, the gene encoding Caspase-1, in the RC/RC mice significantly restrained cystic disease progression in female mice, implying sex-specific differences in the renal immune environment. RNAseq analysis implicated the promotion of MYC/YAP pathways as a mechanism underlying the pro-cystic effects of the Caspase-1/inflammasome in females. Finally, treatment of RC/RC mice with hydroxychloroquine, a widely used immunomodulatory drug that has been shown to inhibit the inflammasome, protected renal function specifically in females and restrained cyst enlargement in both male and female RC/RC mice. Collectively, these results provide evidence for the first time that the activated Caspase-1/inflammasome promotes cyst expansion and disease progression in PKD, particularly in females. Moreover, the data suggest that this innate immune pathway may be a relevant target for therapy in PKD.

Modulation of P2X4 receptor activity by ivermectin and 5-BDBD has no effect on the development of ARPKD in PCK rats

Autosomal recessive polycystic kidney disease (ARPKD) is an inherited pathology caused mainly by mutations of the polycystic kidney and hepatic disease 1 (PKHD1) gene, which usually leads to end-stage renal disease. Previous studies suggested that the P2X purinoreceptor 4 (P2X₄ R) may play an important role in the progression of ARPKD. To test this hypothesis, we assessed the chronic effects of ivermectin (P2X₄ R allosteric modulator) and 5-BDBD (P2X₄ R antagonist) on the development of ARPKD in PCK/CrljCrl-Pkhd1pck/CRL (PCK) rats. Our data indicated that activation of ATP-mediated P2X₄ R signaling with ivermectin for 6 weeks in high dose (50 mg/L; water supplementation) decreased the total body weight of PCK rats while the heart and kidney weight remained unaffected. Smaller doses of ivermectin (0.5 or 5 mg/L, 6 weeks) or the inhibition of P2X₄ R signaling with 5-BDBD (18 mg/kg/day, food supplement for 8 weeks) showed no effect on electrolyte balance or the basic physiological parameters. Furthermore, cystic index analysis for kidneys and liver revealed no effect of smaller doses of ivermectin (0.5 or 5 mg/L) and 5-BDBD on the cyst development of PCK rats. We observed a slight increase in the cystic liver index on high ivermectin dose, possibly due to the cytotoxicity of the drug. In conclusion, this study revealed that pharmacological modulation of P2X₄ R by ivermectin or 5-BDBD does not affect the development of ARPKD in PCK rats, which may provide insights for future studies on investigating the therapeutic potential of adenosine triphosphate (ATP)-P2 signaling in PKD diseases.

Modulation of blood pressure regulatory genes in the Agtrap-Plod1 locus associated with a deletion in Clcn6

The AGTRAP-PLOD1 locus is a conserved gene cluster containing several blood pressure regulatory genes, including CLCN6, MTHFR, NPPA, and NPPB. Previous work revealed that knockout of Clcn6 on the Dahl Salt-Sensitive (SS) rat background (SS-Clcn6) resulted in lower diastolic blood pressure compared to SS-WT rats. Additionally, a recent study found sickle cell anemia patients with mutations in CLCN6 had improved survival and reduced stroke risk. We investigated whether loss of Clcn6 would delay the mortality of Dahl SS rats on an 8% NaCl (HS) diet. No significant difference in survival was found. The ability of Clcn6 to affect mRNA expression of nearby Mthfr, Nppa, and Nppb genes was also tested. On normal salt (0.4% NaCl, NS) diets, renal Mthfr mRNA and protein expression were significantly increased in the SS-Clcn6 rats. MTHFR reduces homocysteine to methionine, but no differences in circulating homocysteine levels were detected. Nppa mRNA levels in cardiac tissue from SS-Clcn6 rat in both normotensive and hypertensive conditions were significantly reduced compared to SS-WT. Nppb mRNA expression in SS-Clcn6 rats on a NS diet was also substantially decreased. Heightened Mthfr expression would be predicted to be protective; however, diminished Nppa and Nppb expression could be deleterious and by preventing or blunting vasodilation, natriuresis, and diuresis that ought to normally occur to offset blood pressure increases. The conserved nature of this genetic locus in humans and rats suggests more studies are warranted to understand how mutations in and around these genes may be influencing the expression of their neighbors.

P2 purinergic receptor dysregulation in urologic disease

P2 purinergic receptors are involved in the normal function of the kidney, bladder, and prostate via signaling that occurs in response to extracellular nucleotides. Dysregulation of these receptors is common in pathological states and often associated with disease initiation, progression, or aggressiveness. Indeed, P2 purinergic receptor expression is altered across multiple urologic disorders including chronic kidney disease, polycystic kidney disease, interstitial cystitis, urinary incontinence, overactive bladder syndrome, prostatitis, and benign prostatic hyperplasia. P2 purinergic receptors are likewise indirectly associated with these disorders via receptor-mediated inflammation and pain, a common characteristic across most urologic disorders. Furthermore, select P2 purinergic receptors are overexpressed in urologic cancer including renal cell carcinoma, urothelial carcinoma, and prostate adenocarcinoma, and pre-clinical studies depict P2 purinergic receptors as potential therapeutic targets. Herein, we highlight the compelling evidence for the exploration of P2 purinergic receptors as biomarkers and therapeutic targets in urologic cancers and other urologic disease. Likewise, there is currently optimism for P2 purinergic receptor-targeted therapeutics for the treatment of inflammation and pain associated with urologic diseases. Further exploration of the common pathways linking P2 purinergic receptor dysregulation to urologic disease might ultimately help in gaining new mechanistic insight into disease processes and therapeutic targeting.

SGLT2 inhibition effect on salt-induced hypertension, RAAS, and Na+ transport in Dahl SS rats

Na+-glucose cotransporter-2 (SGLT2) inhibitors are the new mainstay of treatment for diabetes mellitus and cardiovascular diseases. Despite the remarkable benefits, the molecular mechanisms mediating the effects of SGLT2 inhibitors on water and electrolyte balance are incompletely understood. The goal of this study was to determine whether SGLT2 inhibition alters blood pressure and kidney function via affecting the renin-angiotensin-aldosterone system (RAAS) and Na+ channels/transporters along the nephron in Dahl salt-sensitive rats, a model of salt-induced hypertension. Administration of dapagliflozin (Dapa) at 2 mg/kg/day via drinking water for 3 wk blunted the development of salt-induced hypertension as evidenced by lower blood pressure and a left shift of the pressure natriuresis curve. Urinary flow rate, glucose excretion, and Na+- and Cl--to-creatinine ratios increased in Dapa-treated compared with vehicle-treated rats. To define the contribution of the RAAS, we measured various hormones. Despite apparent effects on Na+- and Cl--to-creatinine ratios, Dapa treatment did not affect RAAS metabolites. Subsequently, we assessed the effects of Dapa on renal Na+ channels and transporters using RT-PCR, Western blot analysis, and patch clamp. Neither mRNA nor protein expression levels of renal transporters (SGLT2, Na+/H+ exchanger isoform 3, Na+-K+-2Cl- cotransporter 2, Na+-Cl- cotransporter, and α-, β-, and γ-epithelial Na+ channel subunits) changed significantly between groups. Furthermore, electrophysiological experiments did not reveal any difference in Dapa treatment on the conductance and activity of epithelial Na+ channels. Our data suggest that SGLT2 inhibition in a nondiabetic model of salt-sensitive hypertension blunts the development of salt-induced hypertension by causing glucosuria and natriuresis without changes in the RAAS or the expression or activity of the main Na+ channels and transporters.NEW & NOTEWORTHY The present study indicates that Na+-glucose cotransporter-2 (SGLT2) inhibition in a nondiabetic model of salt-sensitive hypertension blunts the development and magnitude of salt-induced hypertension. Chronic inhibition of SGLT2 increases glucose and Na+ excretion without secondary effects on the expression and function of other Na+ transporters and channels along the nephron and hormone levels in the renin-angiotensin-aldosterone system. These data provide novel insights into the effects of SGLT2 inhibitors and their potential use in hypertension.

Genetic Kidney Diseases (GKDs) Modeling Using Genome Editing Technologies

Genetic kidney diseases (GKDs) are a group of rare diseases, affecting approximately about 60 to 80 per 100,000 individuals, for which there is currently no treatment that can cure them (in many cases). GKDs usually leads to early-onset chronic kidney disease, which results in patients having to undergo dialysis or kidney transplant. Here, we briefly describe genetic causes and phenotypic effects of six GKDs representative of different ranges of prevalence and renal involvement (ciliopathy, glomerulopathy, and tubulopathy). One of the shared characteristics of GKDs is that most of them are monogenic. This characteristic makes it possible to use site-specific nuclease systems to edit the genes that cause GKDs and generate in vitro and in vivo models that reflect the genetic abnormalities of GKDs. We describe and compare these site-specific nuclease systems (zinc finger nucleases (ZFNs), transcription activator-like effect nucleases (TALENs) and regularly clustered short palindromic repeat-associated protein (CRISPR-Cas9)) and review how these systems have allowed the generation of cellular and animal GKDs models and how they have contributed to shed light on many still unknown fields in GKDs. We also indicate the main obstacles limiting the application of these systems in a more efficient way. The information provided here will be useful to gain an accurate understanding of the technological advances in the field of genome editing for GKDs, as well as to serve as a guide for the selection of both the genome editing tool and the gene delivery method most suitable for the successful development of GKDs models.

Involvement of ceramide biosynthesis in increased extracellular vesicle release in Pkd1 knock out cells

Autosomal Dominant Polycystic Kidney Disease (ADPKD) is an inherited disorder characterized by the development of renal cysts, which frequently leads to renal failure. Hypertension and other cardiovascular symptoms contribute to the high morbidity and mortality of the disease. ADPKD is caused by mutations in the PKD1 gene or, less frequently, in the PKD2 gene. The disease onset and progression are highly variable between patients, whereby the underlying mechanisms are not fully elucidated. Recently, a role of extracellular vesicles (EVs) in the progression of ADPKD has been postulated. However, the mechanisms stimulating EV release in ADPKD have not been addressed and the participation of the distal nephron segments is still uninvestigated. Here, we studied the effect of Pkd1 deficiency on EV release in wild type and Pkd1^-/- mDCT15 and mIMCD3 cells as models of the distal convoluted tubule (DCT) and inner medullary collecting duct (IMCD), respectively. By using nanoparticle tracking analysis, we observed a significant increase in EV release in Pkd1^-/- mDCT15 and mIMCD3 cells, with respect to the wild type cells. The molecular mechanisms leading to the changes in EV release were further investigated in mDCT15 cells through RNA sequencing and qPCR studies. Specifically, we assessed the relevance of purinergic signaling and ceramide biosynthesis enzymes. Pkd1^-/- mDCT15 cells showed a clear upregulation of P2rx7 expression compared to wild type cells. Depletion of extracellular ATP by apyrase (ecto-nucleotidase) inhibited EV release only in wild type cells, suggesting an exacerbated signaling of the extracellular ATP/P2X7 pathway in Pkd1^-/- cells. In addition, we identified a significant up-regulation of the ceramide biosynthesis enzymes CerS6 and Smpd3 in Pkd1^-/- cells. Altogether, our findings suggest the involvement of the DCT in the EV-mediated ADPKD progression and points to the induction of ceramide biosynthesis as an underlying molecular mechanism. Further studies should be performed to investigate whether CerS6 and Smpd3 can be used as biomarkers of ADPKD onset, progression or severity.

Recent advances in understanding ion transport mechanisms in polycystic kidney disease

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

Characterization of purinergic receptor 2 signaling in podocytes from diabetic kidneys

Growing evidence suggests that renal purinergic signaling undergoes significant remodeling during pathophysiological conditions such as diabetes. This study examined the renal P2 receptor profile and ATP-mediated calcium response from podocytes in glomeruli from kidneys with type 1 or type 2 diabetic kidney disease (DKD), using type 2 diabetic nephropathy (T2DN) rats and streptozotocin-injected Dahl salt-sensitive (type 1 diabetes) rats. A dramatic increase in the ATP-mediated intracellular calcium flux in podocytes was observed in both models. Pharmacological inhibition established that P2X₄ and P2X₇ are the major receptors contributing to the augmented ATP-mediated intracellular calcium signaling in diabetic podocytes. The transition in purinergic receptor composition from metabotropic to ionotropic may disrupt intracellular calcium homeostasis in podocytes resulting in their dysfunction and potentially further aggravating DKD progression.

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Diabetic podocytes have sustained intracellular Ca²⁺ signaling in response to ATP

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Podocyte purinergic receptor signaling is predominantly ionotropic in diabetes

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Both type 1 and 2 diabetic podocytes have similar purinergic receptor remodeling

Conversion of extracellular ATP into adenosine: a master switch in renal health and disease

ATP and its ultimate degradation product adenosine are potent extracellular signalling molecules that elicit a variety of pathophysiological functions in the kidney through the activation of P2 and P1 purinergic receptors, respectively. Extracellular purines can modulate immune responses, balancing inflammatory processes and immunosuppression; indeed, alterations in extracellular nucleotide and adenosine signalling determine outcomes of inflammation and healing processes. The functional activities of ectonucleotidases such as CD39 and CD73, which hydrolyse pro-inflammatory ATP to generate immunosuppressive adenosine, are therefore pivotal in acute inflammation. Protracted inflammation may result in aberrant adenosinergic signalling, which serves to sustain inflammasome activation and worsen fibrotic reactions. Alterations in the expression of ectonucleotidases on various immune cells, such as regulatory T cells and macrophages, as well as components of the renal vasculature, control purinergic receptor-mediated effects on target tissues within the kidney. The role of CD39 as a rheostat that can have an impact on purinergic signalling in both acute and chronic inflammation is increasingly supported by the literature, as detailed in this Review. Better understanding of these purinergic processes and development of novel drugs targeting these pathways could lead to effective therapies for the management of acute and chronic kidney disease.

Pannexin-1 mediates fluid shear stress-sensitive purinergic signaling and cyst growth in polycystic kidney disease

Tubular ATP release is regulated by mechanosensation of fluid shear stress (FSS). Polycystin-1/polycystin-2 (PC1/PC2) functions as a mechanosensory complex in the kidney. Extracellular ATP is implicated in polycystic kidney disease (PKD), where PC1/PC2 is dysfunctional. This study aims to provide new insights into the ATP signaling under physiological conditions and PKD. Microfluidics, pharmacologic inhibition, and loss-of-function approaches were combined to assess the ATP release in mouse distal convoluted tubule 15 (mDCT15) cells. Kidney-specific Pkd1 knockout mice (iKsp-Pkd1^-/- ) and zebrafish pkd2 morphants (pkd2-MO) were as models for PKD. FSS-exposed mDCT15 cells displayed increased ATP release. Pannexin-1 inhibition and knockout decreased FSS-modulated ATP release. In iKsp-Pkd1^-/- mice, elevated renal pannexin-1 mRNA expression and urinary ATP were observed. In Pkd1^-/- mDCT15 cells, elevated ATP release was observed upon the FSS mechanosensation. In these cells, increased pannexin-1 mRNA expression was observed. Importantly, pannexin-1 inhibition in pkd2-MO decreased the renal cyst growth. Our results demonstrate that pannexin-1 channels mediate ATP release into the tubular lumen due to pro-urinary flow. We present pannexin-1 as novel therapeutic target to prevent the renal cyst growth in PKD.

Knockout of P2rx7 purinergic receptor attenuates cyst growth in a rat model of ARPKD

The severity of polycystic kidney diseases (PKD) depends on the counterbalancing of genetic predisposition and environmental factors exerting permissive or protective influence on cyst development. One poorly characterized phenomenon in the cystic epithelium is abnormal purinergic signaling. Earlier experimental studies revealed the high importance of the ionotropic P2X receptors (particularly, P2X7) in the pathophysiology of the cyst wall. To study mechanisms of P2X7 involvement in cyst growth and aspects of targeting these receptors in PKD treatment we performed a CRISPR/SpCas9-mediated global knockout of the P2rx7 gene in PCK rats, a model of autosomal recessive PKD (ARPKD). A single base insertion in exon 2 of the P2rx7 gene in the renal tissues of homozygous mutant animals leads to lack of P2X7 protein that did not affect their viability or renal excretory function. However, PCK.P2rx7 rats demonstrated slower cyst growth (but not formation of new cysts) compared with heterozygous and PCK.P2rx7⁺ littermates. P2X7 receptors are known to activate pannexin-1, a plasma channel capable of releasing ATP, and we found here that pannexin-1 expression in the cystic epithelium is significantly higher than in nondilated tubules. P2X7 deficiency reduces renal pannexin-1 protein expression and daily urinary ATP excretion. Patch-clamp analysis revealed that lack of P2X7 increases epithelial sodium channel activity in renal tissues and restores impaired channel activity in cysts. Interpretation of our current data in the context of earlier studies strongly suggests that P2X7 contributes to cyst growth by increasing pannexin-1-dependent pathogenic ATP release into the lumen and reduction of sodium reabsorption across the cyst walls.

Vibrodissociation method for isolation of defined nephron segments from human and rodent kidneys

Our current knowledge of the properties of renal ion channels responsible for electrolytes and cell energy homeostasis mainly relies on rodent studies. However, it has not been established yet to what extent their characteristics can be generalized to those of humans. The present study was designed to develop a standardized protocol for the isolation of well-preserved glomeruli and renal tubules from rodent and human kidneys and to assess the functional suitability of the obtained materials for physiological studies. Separation of nephron segments from human and rodent kidneys was achieved using a novel vibrodissociation technique. The integrity of isolated renal tubules and glomeruli was probed via electrophysiological analysis and fluorescence microscopy, and the purity of the collected fractions was confirmed using quantitative RT-PCR with gene markers for specific cell types. The developed approach allows rapid isolation of well-preserved renal tubules and glomeruli from human and rodent kidneys amenable for electrophysiological, Ca²⁺ imaging, and omics studies. Analysis of the basic electrophysiological parameters of major K⁺ and Na⁺ channels expressed in human cortical collecting ducts revealed that they exhibited similar biophysical properties as previously reported in rodent studies. Using vibrodissociation for nephron segment isolation has several advantages over existing techniques: it is less labor intensive, requires little to no enzymatic treatment, and produces large quantities of well-preserved experimental material in pure fractions. Applying this method for the separation of nephron segments from human and rodent kidneys may be a powerful tool for the indepth assessment of kidney function in health and disease.

Extracellular Nucleotides and P2 Receptors in Renal Function

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

Loss of primary cilia increases polycystin-2 and TRPV4 and the appearance of a nonselective cation channel in the mouse cortical collecting duct

Flow-related bending of cilia results in Ca²⁺ influx through a polycystin-1 (Pkd1) and polycystin-2 (Pkd2) complex, both of which are members of the transient receptor potential (TRP) family (TRPP1 and TRPP2, respectively). Deletion of this complex as well as cilia result in polycystic kidney disease. The Ca²⁺ influx pathway has been previously characterized in immortalized collecting duct cells without cilia and found to be a 23-pS channel that was a multimere of TRPP2 and TRPV4. The purpose of the present study was to determine if this TRPP2 and TRPV4 multimere exists in vivo. Apical channel activity was measured using the patch-clamp technique from isolated split-open cortical collecting ducts from adult conditional knockout mice with (Ift88^flox/flox) or without (Ift88^-/-) cilia. Single tubules were isolated for measurements of mRNA for Pkd1, Pkd2, Trpv4, and epithelial Na⁺ channel subunits. The predominant channel activity from Ift88^flox/flox mice was from epithelial Na⁺ channel [5-pS Na⁺-selective channels with long mean open times (475.7 ± 83.26 ms) and open probability > 0.2]. With the loss of cilia, the predominant conductance was a 23-pS nonselective cation channel (reversal potential near 0) with a short mean open time (72 ± 17 ms), open probability < 0.08, and a characteristic flickery opening. Loss of cilia increased mRNA levels for Pkd2 and Trpv4 from single isolated cortical collecting ducts. In conclusion, 23-pS channels exist in vivo, and activity of this channel is elevated with loss of cilia, consistent with previous finding of an elevated-unregulated Ca²⁺-permeable pathway at the apical membrane of collecting duct cells that lack cilia.

Role of TRPC6 in Progression of Diabetic Kidney Disease

PURPOSE OF REVIEW

The underlining goal of this review is to offer a concise, detailed look into current knowledge surrounding transient receptor potential canonical channel 6 (TRPC6) in the progression of diabetic kidney disease (DKD).

RECENT FINDINGS

Mutations and over-activation in TRPC6 channel activity lead to the development of glomeruli injury. Angiotensin II, reactive oxygen species, and other factors in the setting of DKD stimulate drastic increases in calcium influx through the TRPC6 channel, causing podocyte hypertrophy and foot process effacement. Loss of the podocytes further promote deterioration of the glomerular filtration barrier and play a major role in the development of both albuminuria and the renal injury in DKD. Recent genetic manipulation with TRPC6 channels in various rodent models provide additional knowledge about the role of TRPC6 in DKD and are reviewed here. The TRPC6 channel has a pronounced role in the progression of DKD, with deviations in activity yielding detrimental outcomes. The benefits of targeting TRPC6 or its upstream or downstream signaling pathways in DKD are prominent.

ATP release into ADPKD cysts via pannexin-1/P2X7 channels decreases ENaC activity

Genetic predisposition is necessary for polycystic kidney disease (PKD) initiation, although there are other, incompletely identified downstream processes that are required for cyst growth. Their characterization may provide a unique opportunity for clinical interventions. One of the poorly studied phenomena in PKD is high ATP content in cysts. Unfortunately, neither origins of uncontrolled ATP release, nor consequences of abnormal purinergic signaling in relation to epithelial transport are well explored in the polycystic kidney. We tested the distribution of pannexin-1 (Panx1) and P2X7, two proteins potentially involved in ATP release, in the kidneys of the Pkd1^RC/RC mice, a model of autosomal dominant PKD (ADPKD). Abundances of both proteins were abnormally increased in the cyst lining cells compared to non-dilated collecting ducts. To establish if pannexin-1 contributes to ATP release in the collecting ducts (CD), we measured luminal accumulation of ATP in M1 cell renal CD monolayers, and found that treatment with probenecid, a Panx1 blocker, prevents ATP release. Single channel patch clamp analysis of polarized M1 cells revealed that apical stimulation of P2X receptors with αβ-MeATP acutely reduces ENaC activity. We conclude that in ADPKD progression, an abnormal hyperexpression of both PANX1 and P2RX7 occurs in the cyst lining epithelial cells. High abundance of both proteins is not typical for non-dilated CDs but, when it happens in cysts, pannexin1/P2X7 cooperation elevates ATP release into the luminal space. High ATP level is a pathogenic factor facilitating cystogenesis by reducing ENaC-mediated reabsorption from the lumen.