Extracellular ATP through P2 receptors activates AMP-activated protein kinase and suppresses superoxide generation in cultured mouse podocytes

Purinergic P2 receptors: Involvement and therapeutic implications in diabetes-related glomerular injury

The purinergic activation of P2 receptors initiates a powerful and rapid signaling cascade that contributes to the regulation of an array of physiological and pathophysiological processes in many organs, including the kidney. P2 receptors are broadly distributed in both epithelial and vascular renal cells. Disturbances of purinergic signaling can lead to impairments in renal function. A growing body of evidence indicates changes in P2 receptor expression and nucleotide metabolism in chronic renal injury and inflammatory diseases. Increasing attention has focused on purinergic P2X7 receptors, which are not normally expressed in healthy kidney tissue but are highly expressed at sites of tissue damage and inflammation. Under hyperglycemic conditions, several mechanisms that are linked to purinergic signaling and involve nucleotide release and degradation are disrupted, resulting in the accumulation of adenosine 5'-triphosphate in the bloodstream in diabetes. Dysfunction of the purinergic system might be associated with serious vascular complications in diabetes, including diabetic nephropathy. This review summarizes our current knowledge of the role of P2 receptors in diabetes-related glomerular injury and its implications for new therapeutics for diabetic nephropathy.

Extracellular ATP modulates podocyte function through P2Y purinergic receptors and pleiotropic effects on AMPK and cAMP/PKA signaling pathways

Podocytes and their foot processes interlinked by slit diaphragms, constitute a continuous outermost layer of the glomerular capillary and seem to be crucial for maintaining the integrity of the glomerular filtration barrier. Purinergic signaling is involved in a wide range of physiological processes in the renal system, including regulating glomerular filtration. We evaluated the role of nucleotide receptors in cultured rat podocytes using non-selective P2 receptor agonists and agonists specific for the P2Y₁, P2Y₂, and P2Y₄ receptors. The results showed that extracellular ATP evokes cAMP-dependent pathways through P2 receptors and influences remodeling of the podocyte cytoskeleton and podocyte permeability to albumin via coupling with RhoA signaling. Our findings highlight the relevance of the P2Y₄ receptor in protein kinase A-mediated signal transduction to the actin cytoskeleton. We observed increased cAMP concentration and decreased RhoA activity after treatment with a P2Y₄ agonist. Moreover, protein kinase A inhibitors reversed P2Y₄-induced changes in RhoA activity and intracellular F-actin staining. P2Y₄ stimulation resulted in enhanced AMPK phosphorylation and reduced reactive oxygen species generation. Our findings identify P2Y-PKA-RhoA signaling as the regulatory mechanism of the podocyte contractile apparatus and glomerular filtration. We describe a protection mechanism for the glomerular barrier linked to reduced oxidative stress and reestablished energy balance.

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.

Functional and therapeutic importance of purinergic signaling in polycystic kidney disease

Polycystic kidney diseases (PKD) are a group of inherited nephropathies marked with the formation of fluid-filled cysts along the nephron. This renal disorder affects millions of people worldwide, but current treatment strategies are unfortunately limited to supportive therapy, dietary restrictions, and, eventually, renal transplantation. Recent advances in PKD management are aimed at targeting exaggerated cell proliferation and dedifferentiation to interfere with cyst growth. However, not nearly enough is known about the ion transport properties of the cystic cells, or specific signaling pathways modulating channels and transporters in this condition. There is growing evidence that abnormally elevated concentrations of adenosine triphosphate (ATP) in PKD may contribute to cyst enlargement; change in the profile of purinergic receptors may also result in promotion of cystogenesis. The current mini-review is focused on the role of ATP and associated signaling affecting ion transport properties of the renal cystic epithelia.

Chloride secretion by renal collecting ducts

PURPOSE OF REVIEW

Renal collecting ducts maintain NaCl homeostasis by fine-tuning urinary excretion to balance dietary salt intake. This review focuses on recent studies on transcellular Cl secretion by collecting ducts, its regulation and its role in cyst growth in autosomal dominant polycystic kidney disease (ADPKD).

RECENT FINDINGS

Lumens of nonperfused rat medullary collecting ducts collapse in control media but expand with fluid following treatment with cAMP, demonstrating the capacity for both salt absorption and secretion. Recently, inhibition of apical epithelial Na channels (ENaC) unmasked Cl secretion in perfused mouse cortical collecting ducts (CCDs), involving Cl uptake by basolateral NKCC1 and efflux through apical Cl channels. AVP, the key hormone for osmoregulation, promotes cystic fibrosis transmembrane conductance regulator (CFTR)-mediated Cl secretion. In addition, prostaglandin E2 stimulates Cl secretion through both CFTR and Ca-activated Cl channels.

SUMMARY

Renal Cl secretion has been commonly overlooked because of the overwhelming capacity for the nephron to reabsorb NaCl from the glomerular filtrate. In ADPKD, Cl secretion plays a central role in the accumulation of cyst fluid and the remarkable size of the cystic kidneys. Investigation of renal Cl secretion may provide a better understanding of NaCl homeostasis and identify new approaches to reduce cyst growth in PKD.

AMPK Suppresses Connexin43 Expression in the Bladder and Ameliorates Voiding Dysfunction in Cyclophosphamide-induced Mouse Cystitis

Bladder voiding dysfunction is closely related to local oxidation, inflammation, and enhanced channel activities. Given that the AMP-activated protein kinase (AMPK) has anti-oxidative, anti-inflammatory and channel-inhibiting properties, we examined whether and how AMPK affected bladder activity. AMPK activation in rat bladder smooth muscle cells (BSMCs) using three different AMPK agonists resulted in a decrease in connexin43 (Cx43) expression and function, which was associated with reduced CREB phosphorylation, Cx43 promoter activity and mRNA expression, but not Cx43 degradation. Downregulation of CREB with siRNA increased Cx43 expression. A functional analysis revealed that AMPK weakened BSMC contraction and bladder capacity. AMPK also counteracted the IL-1β- and TNFα-induced increase in Cx43 in BSMCs. In vivo administration of the AMPK agonist AICAR attenuated cyclophosphamide-initiated bladder oxidation, inflammation, Cx43 expression and voiding dysfunction. Further analysis comparing the responses of the wild-type (Cx43(+/+)) and heterozygous (Cx43(+/-)) Cx43 mice to cyclophosphamide revealed that the Cx43(+/-) mice retained a relatively normal micturition pattern compared to the Cx43(+/+) mice. Taken together, our results indicate that AMPK inhibits Cx43 in BSMCs and improves bladder activity under pathological conditions. We propose that strategies that target AMPK can be developed as novel therapeutic approaches for treating bladder dysfunction.

Purinergic signalling in the kidney in health and disease

The involvement of purinergic signalling in kidney physiology and pathophysiology is rapidly gaining recognition and this is a comprehensive review of early and recent publications in the field. Purinergic signalling involvement is described in several important intrarenal regulatory mechanisms, including tuboglomerular feedback, the autoregulatory response of the glomerular and extraglomerular microcirculation and the control of renin release. Furthermore, purinergic signalling influences water and electrolyte transport in all segments of the renal tubule. Reports about purine- and pyrimidine-mediated actions in diseases of the kidney, including polycystic kidney disease, nephritis, diabetes, hypertension and nephrotoxicant injury are covered and possible purinergic therapeutic strategies discussed.

5'-AMP-activated protein kinase attenuates adriamycin-induced oxidative podocyte injury through thioredoxin-mediated suppression of the apoptosis signal-regulating kinase 1-P38 signaling pathway

Oxidative stress-induced podocyte injury is one of the major mechanisms underlying the initiation and progression of glomerulosclerosis. 5'-AMP-activated protein kinase (AMPK), a serine/threonine kinase that senses intracellular energy status and maintains energy homeostasis, is reported to have antioxidative effects. However, little is known about its application and mechanism. In this study, we investigated whether and how AMPK affected oxidative podocyte injury induced by Adriamycin (ADR; Wako Pure Chemical, Osaka, Japan). Exposure of podocytes to ADR resulted in cell injury, which was preceded by increased reactive oxygen species (ROS) generation and P38 activation. Prevention of oxidative stress with the antioxidant N-acetyl-cysteine and glutathione or inhibition of P38 with SB203580 [4-(4-fluorophenyl)-2-(4-methylsulfinylphenyl)-5-(4-pyridyl)1H-imidazole] attenuated cell injury. Activation of AMPK with three structurally different AMPK activators also protected podocytes from ADR-elicited cell injury. This effect was associated with strong suppression of oxidative stress-sensitive kinase apoptosis signal-regulating kinase 1 (ASK1) and P38 without obvious influence on ROS level. Further analyses revealed that AMPK promoted thioredoxin (Trx) binding to ASK1. Consistently, AMPK potently suppressed the expression of thioredoxin-interacting protein (TXNIP), a negative regulator of Trx, whereas it significantly enhanced the activity of Trx reductases that convert oxidized Trx to reduced form. In further support of a key role of Trx, downregulation or inhibition of Trx exaggerated but downregulation of TXNIP attenuated the cell injury. These results indicate that AMPK prevents oxidative cell injury through Trx-mediated suppression of ASK1-P38 signaling pathway. Our findings thus provide novel mechanistic insights into the antioxidative actions of AMPK. AMPK could be developed as a novel therapeutic target for treatment of oxidative cell injury.

Pharmacological characterization of the P2 receptors profile in the podocytes of the freshly isolated rat glomeruli

Calcium flux in the podocytes is critical for normal and pathophysiological regulation of these types of cells, and excessive calcium signaling results in podocytes damage and improper glomeruli function. Purinergic activation of P2 receptors is a powerful and rapid signaling process; however, the exact physiological identity of P2 receptors subtypes in podocytes remains essentially unknown. The goal of this study was to determine the P2 receptor profile in podocytes of the intact Sprague-Dawley rat glomeruli using available pharmacological tools. Glomeruli were isolated by differential sieving and loaded with Fluo-4/Fura Red cell permeable calcium indicators, and the purinergic response in the podocytes was analyzed with ratiometric confocal fluorescence measurements. Various P2 receptors activators were tested and compared with the effect of ATP, specifically, UDP, MRS 2365, bzATP, αβ-methylene, 2-meSADP, MRS 4062, and MRS 2768, were analyzed. Antagonists (MRS 2500, 5-BDBD, A438079, and NF 449) were tested when 10 μM ATP was applied as the EC50 for ATP activation of the calcium influx in the podocytes was determined to be 10.7 ± 1.5 μM. Several agonists including MRS 2365 and 2-meSADP caused calcium flux. Importantly, only the P2Y1-specific antagonist MRS 2500 (1 nM) precluded the effects of ATP concentrations of the physiological range. Immunohistochemical analysis confirmed that P2Y1 receptors are highly expressed in the podocytes. We conclude that P2Y1 receptor signaling is the predominant P2Y purinergic pathway in the glomeruli podocytes and P2Y1 might be involved in the pathogenesis of glomerular injury and could be a target for treatment of kidney diseases.

Induction of AMPK activity corrects early pathophysiological alterations in the subtotal nephrectomy model of chronic kidney disease

The rat kidney ablation and infarction (A/I) model of subtotal or 5/6th nephrectomy is the most commonly studied model of nondiabetic chronic kidney disease (CKD). The A/I kidney at 1 wk exhibits reductions in kidney function, as determined by glomerular filtration rate, and diminished metabolic efficiency as determined by oxygen consumption per sodium transport (QO2/TNa). As renoprotective AMPK activity is affected by metabolic changes and cellular stress, we evaluated AMPK activity in this model system. We show that these early pathophysiological changes are accompanied by a paradoxical decrease in AMPK activity. Over time, these kidney parameters progressively worsen with extensive kidney structural, functional, metabolic, and fibrotic changes observed at 4 wk after A/I. We show that induction of AMPK activity with either metformin or 5-aminoimidazole-4-carboxamide ribonucleotide increases AMPK activity in this model and also corrects kidney metabolic inefficiency, improves kidney function, and ameliorates kidney fibrosis and structural alterations. We conclude that AMPK activity is reduced in the subtotal nephrectomy model of nondiabetic CKD, that altered regulation of AMPK is coincident with the progression of disease parameters, and that restoration of AMPK activity can suppress the progressive loss of function characteristic of this model. We propose that induction of AMPK activity may prove an effective therapeutic target for the treatment of nondiabetic CKD.

Expression of membrane-bound NPP-type ecto-phosphodiesterases in rat podocytes cultured at normal and high glucose concentrations

The ecto-nucleotide pyrophosphatase/phosphodiesterase family (E-NPPs) contains two membrane-bound members: E-NPP1 and E-NPP3. These enzymes mediate hydrolysis of extracellular nucleotides and their abnormal expression may affect intracellular signal transduction pathways, leading to cellular dysfunction, e.g., insulin resistance. Podocytes are insulin-dependent glomerular epithelial cells that regulate the glomerular filtration rate. Pathology of podocytes is a hallmark of diabetic nephropathy. Here, we investigated the expressions of E-NPP1 and E-NPP3 and activity of E-NPP enzymes in rat podocytes cultured with 5mM (NG) or 30 mM glucose (HG). Insulin resistance was determined by measuring changes in [1,2-(3)H]-deoxy-D-glucose uptake in response to insulin. mRNAs of E-NPP1 and E-NPP3 were detected within podocytes. The E-NPP expressions were confirmed at the protein level using western blot and immunofluorescence techniques. At NG, insulin (300 nM, 3 min) increased glucose uptake 1.5-fold; however, this effect was abolished at HG. The protein expressions of E-NPP1 and E-NPP3 were not affected at HG. The E-NPP activities were 24.68±0.72 and 26.51±1.55 nmol/min/mg protein at NG and HG, respectively. In conclusion, ecto-nucleotide pyrophosphatase/phosphodiesterase 1 and 3 are expressed on podocytes, but changes in expression of these enzymes are most likely not involved in etiology of insulin resistance in podocytes.