Immunolocalization of ectonucleotidases along the rat nephron

Adenosine stimulates the basolateral 50 pS K+ channel in renal proximal tubule via adenosine-A1 receptor

Background: The basolateral potassium channels play an important role in maintaining the membrane transport in the renal proximal tubules (PT) and adenosine receptors have been shown to regulate the trans-epithelial Na+ absorption in the PT. The aim of the present study is to explore whether adenosine also regulates the basolateral K+ channel of the PT and to determine the adenosine receptor type and the signaling pathway which mediates the effect of adenosine on the K+ channel. Methods: We have used the single channel recording to examine the basolateral K+ channel activity in the proximal tubules of the mouse kidney. All experiments were performed in cell-attached patches. Results: Single channel recording has detected a 50 pS inwardly-rectifying K+ channel with high channel open probability and this 50 pS K+ channel is a predominant type K+ channel in the basolateral membrane of the mouse PT. Adding adenosine increased 50 pS K+ channel activity in cell-attached patches, defined by NPo (a product of channel Numbers and Open Probability). The adenosine-induced stimulation of the 50 pS K+ channel was absent in the PT pretreated with DPCPX, a selective inhibitor of adenosine A1 receptor. In contrast, adenosine was still able to stimulate the 50 pS K+ channel in the PT pretreated with CP-66713, a selective adenosine A2 receptor antagonist. This suggests that the stimulatory effect of adenosine on the 50 pS K+ channel of the PT was mediated by adenosine-A1 receptor. Moreover, the effect of adenosine on the 50 pS K+ channel was blocked in the PT pretreated with U-73122 or Calphostin C, suggesting that adenosine-induced stimulation of the 50 pS K+ channels of the PT was due to the activation of phospholipase C (PLC) and protein kinase C (PKC) pathway. In contrast, the inhibition of phospholipase A2 (PLA2) with AACOCF3 or inhibition of protein kinase A (PKA) with H8 failed to block the adenosine-induced stimulation of the 50 pS K+ channel of the PT. Conclusion: We conclude that adenosine activates the 50 pS K+ channels in the basolateral membrane of PT via adenosine-A1 receptor. Furthermore, the effect of adenosine on the 50 pS K+ channel is mediated by PLC-PKC signaling pathway.

Burnstock oration - purinergic signalling in kidney transplantation

Kidney transplantation is the preferred treatment for individuals with kidney failure offering improved quality and quantity of life. Despite significant advancements in short term graft survival, longer term survival rates have not improved greatly mediated in large by chronic antibody mediated rejection. Strategies to reduce the donor kidney antigenic load may translate to improved transplant survival. CD39 on the vascular endothelium and on circulating cells, in particular regulatory T cells (Treg), is upregulated in response to hypoxic stimuli and plays a critical role in regulating the immune response removing proinflammatory ATP and generating anti-inflammatory adenosine. Herein, the role of CD39 in reducing ischaemia-reperfusion injury (IRI) and on Treg within the context of kidney transplantation is reviewed.

Extracellular vesicles regulate purinergic signaling and epithelial sodium channel expression in renal collecting duct cells

Purinergic signaling regulates several renal physiological and pathophysiological processes. Extracellular vesicles (EVs) are nanoparticles released by most cell types, which, in non-renal tissues, modulate purinergic signaling. The aim of this study was to investigate the effect of EVs from renal proximal tubule (HK2) and collecting duct cells (HCD) on intra- and intersegment modulation of extracellular ATP levels, the underlying molecular mechanisms, and the impact on the expression of the alpha subunit of the epithelial sodium channel (αENaC). HK2 cells were exposed to HK2 EVs, while HCD cells were exposed to HK2 and HCD EVs. Extracellular ATP levels and αENaC expression were measured by chemiluminescence and qRT-PCR, respectively. ATPases in EV populations were identified by mass spectrometry. The effect of aldosterone was assessed using EVs from aldosterone-treated cells and urinary EVs (uEVs) from primary aldosteronism (PA) patients. HK2 EVs downregulated ectonucleoside-triphosphate-diphosphohydrolase-1 (ENTPD1) expression, increased extracellular ATP and downregulated αENaC expression in HCD cells. ENTPD1 downregulation could be attributed to increased miR-205-3p and miR-505 levels. Conversely, HCD EVs decreased extracellular ATP levels and upregulated αENaC expression in HCD cells, probably due to enrichment of 14-3-3 isoforms with ATPase activity. Pretreatment of donor cells with aldosterone or exposure to uEVs from PA patients enhanced the effects on extracellular ATP and αENaC expression. We demonstrated inter- and intrasegment modulation of renal purinergic signaling by EVs. Our findings postulate EVs as carriers of information along the renal tubules, whereby processes affecting EV release and/or cargo may impact on purinergically regulated processes.

Identification of Genomic Regions Influencing N-Metabolism and N-Excretion in Lactating Holstein- Friesians

Excreted nitrogen (N) of dairy cows contribute to environmental eutrophication. The main N-excretory metabolite of dairy cows is urea, which is synthesized as a result of N-metabolization in the liver and is excreted via milk and urine. Genetic variation in milk urea (MU) has been postulated but the complex physiology behind the trait as well as the tremendous diversity of processes regulating the N-metabolism impede the consistent determination of causal regions in the bovine genome. In order to map the genetic determinants affecting N-excretion, MU and eight other N-excretory metabolites in milk and urine were assessed in a genome-wide association study. Therefore phenotypes of 371 Holstein- Friesians were obtained in a trial on a dairy farm under near commercial conditions. Genotype data comprised SNP information of the Bovine 50K MD Genome chip (45,613 SNPs). Significantly associated genomic regions for MU concentration revealed GJA1 (BTA 9), RXFP1, and FRY1 (both BTA 12) as putative candidates. For milk urea yield (MUY) a promising QTL on BTA 17 including SH3D19 emerged, whereas RCAN2, CLIC5, ENPP4, and ENPP5 (BTA 23) are suggested to influence urinary urea concentration. Minor N-fractions in milk (MN) may be regulated by ELF2 and SLC7A11 (BTA 17), whilst ITPR2 and MYBPC1 (BTA 5), STIM2 (BTA 6), SGCD (BTA 7), SLC6A2 (BTA 18), TMCC2 and MFSD4A (BTA 16) are suggested to have an impact on various non-urea-N (NUN) fractions excreted via urine. Our results highlight genomic regions and candidate genes for N-excretory metabolites and provide a deeper insight into the predisposed component to regulate the N-metabolism in dairy cows.

Purinergic signalling in the kidney - A beginning with Geoffrey Burnstock

This not an original publication or a current and up-to-date review of purinergic signalling and kidney function, but rather a tribute to Professor Geoffrey Burnstock, written as a short and personal memoir of our early collaborative work together on this topic: our beginnings and the subsequent journey we took with our many valued collaborators along the way.

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.

Biochemical analysis of ectonucleotidases on primary rat vascular smooth muscle cells and in silico investigation of their role in vascular diseases

Vascular smooth muscle cells (VSMCs) exhibit a high degree of plasticity when they undergo the progression from a normal to a disease condition, which makes them a potential target for evaluating early markers and for the development of new therapies. Purinergic signalling plays a key role in vascular tonus control, ATP being an inductor of vasoconstriction, whereas adenosine mediates a vasodilation effect antagonising the ATP actions. The control of extracellular ATP and adenosine levels is done by ectonucleotidases, which represent a potential target to be evaluated in the progression of cardiovascular diseases. In this study, we analysed the basal activity and expression of the ectonucleotidases in aortic rat VSMCs, and we further performed in silico analysis to determine the expression of those enzymes in conditions that mimicked vascular diseases. Cultured in vitro VSMCs showed a prominent expression of Entpd1 followed by Entpd2 and Nt5e (CD73) and very low levels of Entpd3. Slightly faster AMP hydrolysis was observed when compared to ATP and ADP nucleotides. In silico analysis showed that the ectonucleotidases were modulated after induction of conditions that can lead to vascular diseases such as, hypertensive and hypotensive mice models (Nt5e); exposition to high-fat (Entpd1 and Entpd2) or high-phosphate (Nt5e) diet; mechanical stretch (Entpd1, Entpd2 and Nt5e); and myocardial infarction (Entpd1). Our data show that VSMCs are able to efficiently metabolise the extracellular nucleotides generating adenosine. The modulation of Entpd1, Entdp2 and Nt5e in vascular diseases suggests these ectoenzymes as potential targets or markers to be investigated in future studies.

Cell type- and tissue-specific functions of ecto-5'-nucleotidase (CD73)

Ecto-5'-nucleotidase [cluster of differentiation 73 (CD73)] is a ubiquitously expressed glycosylphosphatidylinositol-anchored glycoprotein that converts extracellular adenosine 5'-monophosphate to adenosine. Anti-CD73 inhibitory antibodies are currently undergoing clinical testing for cancer immunotherapy. However, many protective physiological functions of CD73 need to be taken into account for new targeted therapies. This review examines CD73 functions in multiple organ systems and cell types, with a particular focus on novel findings from the last 5 years. Missense loss-of-function mutations in the CD73-encoding gene NT5E cause the rare disease "arterial calcifications due to deficiency of CD73." Aside from direct human disease involvement, cellular and animal model studies have revealed key functions of CD73 in tissue homeostasis and pathology across multiple organ systems. In the context of the central nervous system, CD73 is antinociceptive and protects against inflammatory damage, while also contributing to age-dependent decline in cortical plasticity. CD73 preserves barrier function in multiple tissues, a role that is most evident in the respiratory system, where it inhibits endothelial permeability in an adenosine-dependent manner. CD73 has important cardioprotective functions during myocardial infarction and heart failure. Under ischemia-reperfusion injury conditions, rapid and sustained induction of CD73 confers protection in the liver and kidney. In some cases, the mechanism by which CD73 mediates tissue injury is less clear. For example, CD73 has a promoting role in liver fibrosis but is protective in lung fibrosis. Future studies that integrate CD73 regulation and function at the cellular level with physiological responses will improve its utility as a disease target.

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.

CD39-adenosinergic axis in renal pathophysiology and therapeutics

Extracellular ATP interacts with purinergic type 2 (P2) receptors and elicits many crucial biological functions. Extracellular ATP is sequentially hydrolyzed to ADP and AMP by the actions of defined nucleotidases, such as CD39, and AMP is converted to adenosine, largely by CD73, an ecto-5'-nucleotidase. Extracellular adenosine interacts with P1 receptors and often opposes the effects of P2 receptor activation. The balance between extracellular ATP and adenosine in the blood and extracellular fluid is regulated chiefly by the activities of CD39 and CD73, which constitute the CD39-adenosinergic axis. In recent years, several studies have shown this axis to play critical roles in transport of water/sodium, tubuloglomerular feedback, renin secretion, ischemia reperfusion injury, renal fibrosis, hypertension, diabetic nephropathy, transplantation, inflammation, and macrophage transformation. Important developments include global and targeted gene knockout and/or transgenic mouse models of CD39 or CD73, biological or small molecule inhibitors, and soluble engineered ectonucleotidases to directly impact the CD39-adenosinergic axis. This review presents a comprehensive picture of the multiple roles of CD39-adenosinergic axis in renal physiology, pathophysiology, and therapeutics. Scientific advances and greater understanding of the role of this axis in the kidney, in both health and illness, will direct development of innovative therapies for renal diseases.

ATP metabolizing enzymes ENPP1, 2 and 3 are localized in sensory neurons of rat dorsal root ganglion

In dorsal root ganglion (DRG) neurons, ATP is an important neurotransmitter in nociceptive signaling through P2 receptors (P2Rs) such as P2X2/3R, and adenosine is also involved in anti-nociceptive signaling through adenosine A1R. Thus, the clearance system for adenine nucleotide/nucleoside plays a critical role in regulation of nociceptive signaling, but there is little information on it, especially ectoenzyme expression profiles in DRG. In this study, we examined expression and localization of ecto-nucleotide pyrophosphatase/phosphodiesterases (ENPPs), by which ATP is metabolized to AMP, in rat DRG. The mRNA expression levels of ENPP2 were greater than those of ENPP1 and ENPP3 in rat DRGs. On immunohistochemical analysis, ENPP1, 2 and 3 were found in soma of DRG neurons. Immunopositive rate of ENPP3 was greater than that of ENPP1 and ENPP2 in all DRG neurons. ENPP3, as compared with ENPP1 and ENPP2, was expressed mainly by isolectin B4-positive cells, and slightly by neurofilament 200-positive ones. In this way, the expression profile of ENPP1, 2 and 3 was different in DRGs, and they were mainly expressed in small/medium-sized DRG neurons. Moreover, ENPP1-, 2- and 3-immunoreactivities were colocalized with P2X2R, P2X3R and prostatic acid phosphatase (PAP), as an ectoenzyme for metabolism from AMP to adenosine. Additionally, PAP-immunoreactivity was colocalized with equilibrative nucleoside transporter (ENT) 1, as an adenosine uptake system. These results suggest that the clearance system consisted of ENPPs, PAP and ENT1 plays an important role in regulation of nociceptive signaling in sensory neurons.

Generation and characterization of polyclonal and monoclonal antibodies to human NTPDase2 including a blocking antibody

Nucleoside triphosphate diphosphohydrolase-2 (NTPDase2) is an ectonucleotidase that modulates P2 receptor activation by hydrolyzing ATP to ADP. In rodents, NTPDase2 is expressed by several specialized cell types such as vascular adventitial cells, neuroglial cells, hepatic portal fibroblasts, gustatory type I cells, and cells within the connective tissues of reproductive and gastrointestinal organs. Much less is known regarding the expression and function of NTPDase2 in humans. Here, we developed specific research tools to study human NTPDase2. We generated mouse monoclonal antibodies and rabbit polyclonal antibodies specific to human NTPDase2 and validated their specificity by western blot, immunocytochemistry, immunohistochemistry, and flow cytometry. In addition, one monoclonal antibody named hN2-D5 _s specifically inhibits human NTPDase2 enzymatic activity but not mouse nor rat NTPDase2. Using these antibodies, NTPDase2 immunoreactivity was detected on glial cells of the human enteric nervous system suggesting a function of the enzyme in intestinal motility. In conclusion, the new antibodies described in our work are novel tools that will enhance future studies of NTPDase2 expression and function in humans.

Generation and Characterization of Specific Antibodies to the Murine and Human Ectonucleotidase NTPDase8

The ectonucleotidase nucleoside triphosphate diphosphohydrolase-8 (NTPDase8) is the last member of the Ecto-NTPDase family to be discovered and characterized. It is a transmembrane protein which regulates the concentration of the agonists of P1 and P2 receptors at the cell surface. The functions of the enzyme are still not known partly due to the lack of specific tools such as antibodies. In this work, guinea pig polyclonal antibodies against mouse NTPDase8 and mouse monoclonal antibodies against human NTPDase8 have been generated and characterized. For the production of antibodies against mouse NTPDase8 several techniques have been tried. Several peptide antigens in several hosts (rabbit, rat, hamster, and guinea pig) failed to give a positive reaction suggesting that NTPDase8 is poorly immunogenic. In this study, we describe the successful process that led to anti-mouse NTPDase8, namely the cDNA immunization technique. Monoclonal antibodies to human NTPDase8 were also obtained by cDNA immunization followed by a final injection with transfected human embryonic kidney (HEK 293T) cells expressing human NTPDase8. The specificity of these antibodies was evaluated by Western blot, immunocytochemistry, immunohistochemistry and flow cytometry. In contrast, all commercial antibodies to NTPDase8 peptides that we have tested failed to give a specific positive signal against the expressed NTPDase8 protein when used to probe Western blots. In addition, immunohistochemistry experiments confirmed the presence of NTPDase8 in mouse liver canaliculi. The tools generated in this work will help characterize NTPDase8 localization and function in future studies and its contribution to the modulation of P1 and P2 receptor activation.

Purinergic signaling in kidney disease

Nucleotides are key subunits for nucleic acids and provide energy for intracellular metabolism. They can also be released from cells to act physiologically as extracellular messengers or pathologically as danger signals. Extracellular nucleotides stimulate membrane receptors in the P2 and P1 family. P2X are ATP-activated cation channels; P2Y and P1 are G-protein coupled receptors activated by ATP, ADP, UTP, and UDP in the case of P2 or adenosine for P1. Renal P2 receptors influence both vascular contractility and tubular function. Renal cells also express ectonucleotidases that rapidly hydrolyze extracellular nucleotides. These enzymes integrate this multireceptor purinergic-signaling complex by determining the nucleotide milieu to titrate receptor activation. Purinergic signaling also regulates immune cell function by modulating the synthesis and release of various cytokines such as IL1-β and IL-18 as part of inflammasome activation. Abnormal or excessive stimulation of this intricate paracrine system can be pro- or anti-inflammatory, and is also linked to necrosis and apoptosis. Kidney tissue injury causes a localized increase in ATP concentration, and sustained activation of P2 receptors can lead to renal glomerular, tubular, and vascular cell damage. Purinergic receptors also regulate the activity and proliferation of fibroblasts, promoting both inflammation and fibrosis in chronic disease. In this short review we summarize some of the recent findings related to purinergic signaling in the kidney. We focus predominantly on the P2X7 receptor, discussing why antagonists have so far disappointed in clinical trials and how advances in our understanding of purinergic signaling might help to reposition these compounds as potential treatments for renal disease.

Inhibition of P2X Receptors Protects Human Monocytes against Damage by Leukotoxin from Aggregatibacter actinomycetemcomitans and α-Hemolysin from Escherichia coli

α-Hemolysin (HlyA) from Escherichia coli and leukotoxin A (LtxA) from Aggregatibacter actinomycetemcomitans are important virulence factors in ascending urinary tract infections and aggressive periodontitis, respectively. The extracellular signaling molecule ATP is released immediately after insertion of the toxins into plasma membranes and, via P2X receptors, is essential for the erythrocyte damage inflicted by these toxins. Moreover, ATP signaling is required for the ensuing recognition and phagocytosis of damaged erythrocytes by the monocytic cell line THP-1. Here, we investigate how these toxins affect THP-1 monocyte function. We demonstrate that both toxins trigger early ATP release and a following increase in the intracellular Ca²⁺ concentration ([Ca²⁺]_i) in THP-1 monocytes. The HlyA- and LtxA-induced [Ca²⁺]_i response is diminished by the P2 receptor antagonist in a pattern that fits the functional P2 receptor expression in these cells. Both toxins are capable of lysing THP-1 cells, with LtxA being more aggressive. Either desensitization or blockage of P2X₁, P2X₄, or P2X₇ receptors markedly reduces toxin-induced cytolysis. This pattern is paralleled in freshly isolated human monocytes from healthy volunteers. Interestingly, only a minor fraction of the toxin-damaged THP-1 monocytes eventually lyse. P2X₇ receptor inhibition generally prevents cell damage, except from a distinct cell shrinkage that prevails in response to the toxins. Moreover, we find that preexposure to HlyA preserves the capacity of THP-1 monocytes to phagocytose damaged erythrocytes and may induce readiness to discriminate between damaged and healthy erythrocytes. These findings suggest a new pharmacological target for protecting monocytes during exposure to pore-forming cytolysins during infection or injury.

Regulation of Vascular and Renal Function by Metabolite Receptors

To maintain metabolic homeostasis, the body must be able to monitor the concentration of a large number of substances, including metabolites, in real time and to use that information to regulate the activities of different metabolic pathways. Such regulation is achieved by the presence of sensors, termed metabolite receptors, in various tissues and cells of the body, which in turn convey the information to appropriate regulatory or positive or negative feedback systems. In this review, we cover the unique roles of metabolite receptors in renal and vascular function. These receptors play a wide variety of important roles in maintaining various aspects of homeostasis-from salt and water balance to metabolism-by sensing metabolites from a wide variety of sources. We discuss the role of metabolite sensors in sensing metabolites generated locally, metabolites generated at distant tissues or organs, or even metabolites generated by resident microbes. Metabolite receptors are also involved in various pathophysiological conditions and are being recognized as potential targets for new drugs. By highlighting three receptor families-(a) citric acid cycle intermediate receptors, (b) purinergic receptors, and

Purinergic receptors in tubulointerstitial inflammatory cells: a pathophysiological mechanism of salt-sensitive hypertension

Recent studies have suggested that both the tubulointerstitial inflammatory cells and the activation of purinergic receptors integrate common mechanisms that result in salt-sensitive hypertension. The basis of this hypothesis is that renal endothelial cells release ATP in response to shear stress in the setting of hypertension. It has been demonstrated that the over-expression and activation of the P2X7, P2Y12 and P2X1 receptors favour the elevation of blood pressure induced by high-salt intake. In addition, the release of interleukins and inflammatory mediators in the tubulointerstitial area appears to be related to the activation of these receptors. Renal vasoconstriction and tubulointerstitial injury develop as a result, which increase sodium reabsorption by epithelial cells. Consistent with these effects, the reduction of tubulointerstitial inflammation caused by immunosuppressants, such as mycophenolate mofetil, prevents the development of salt-sensitive hypertension. Also, P2X7-receptor knockout mice develop minor renal injury when hypertension is induced via the administration of deoxycorticosterone acetate and a high-salt diet. In the setting of angiotensin II-induced hypertension, which is an early stage in the development of salt-sensitive hypertension, an acute blockade with the specific, non-selective P2 antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid prevented the renal vasoconstriction induced by angiotensin II. In addition, it normalized glomerular haemodynamics and restored sodium excretion to control values. These findings suggest that chronic administration of P2 purinergic antagonists may prevent the deleterious effects of purinergic receptors during the development of salt-sensitive hypertension.

Impaired natriuretic response to high-NaCl diet plus aldosterone infusion in mice overexpressing human CD39, an ectonucleotidase (NTPDase1)

Extracellular nucleotides acting through P2 receptors facilitate natriuresis. To define how purinergic mechanisms are involved in sodium homeostasis, we used transgenic (TG) mice that globally overexpress human CD39 (hCD39, NTPDase1), an ectonucleotidase that hydrolyzes extracellular ATP/ADP to AMP, resulting in an altered extracellular purine profile. On a high-sodium diet (HSD, 3.5% Na(+)), urine volume and serum sodium were significantly higher in TG mice but sodium excretion was unaltered. Furthermore, TG mice showed an attenuated fall in urine aldosterone with HSD. Western blot analysis revealed significantly lower densities (∼40%) of the β-subunit of the epithelial sodium channel (ENaC) in medulla, and the major band (85-kDa) of γ-ENaC in TG mice cortex. To evaluate aldosterone-independent differences, in a second experiment, aldosterone was clamped by osmotic minipump at 20 μg/day, and mice were fed either an HSD or a low-sodium diet (LSD, 0.03% Na(+)). Here, no differences in urine volume or osmolality, or serum aldosterone were found, but TG mice showed a modest, yet significant impairment in late natriuresis (days 3 and 4). Several major sodium transporters or channel subunits were differentially expressed between the genotypes. HSD caused a downregulation of Na-Cl cotransporter (NCC) in both genotypes; and had higher cortical levels of NCC, Na-K-ATPase (α-1 subunit), and α- and γ-ENaC. The Na-K-2Cl cotransporter (NKCC2) was downregulated by HSD in wild-type mice, but it increased in TG mice. In summary, our data support the concept that extracellular nucleotides facilitate natriuresis; they also reveal an aldosterone-independent downregulation of major renal sodium transporters and channel subunits by purinergic signaling.

Renal P2 receptors and hypertension

The regulation of extracellular fluid volume is a key component of blood pressure homeostasis. Long-term blood pressure is stabilized by the acute pressure natriuresis response by which changes in renal perfusion pressure evoke corresponding changes in renal sodium excretion. A wealth of experimental evidence suggests that a defect in the pressure natriuresis response contributes to the development and maintenance of hypertension. The mechanisms underlying the relationship between renal perfusion pressure and sodium excretion are incompletely understood. Increased blood flow through the vasa recta increases renal interstitial hydrostatic pressure, thereby reducing the driving force for transepithelial sodium reabsorption. Paracrine signalling also contributes to the overall natriuretic response by inhibiting tubular sodium reabsorption in several nephron segments. In this brief review, we discuss the role of purinergic signalling in the renal control of blood pressure. ATP is released from renal tubule and vascular cells in response to increased flow and can activate P2 receptor subtypes expressed in both epithelial and vascular endothelial/smooth muscle cells. In concert, these effects integrate the vascular and tubular responses to increased perfusion pressure and targeting P2 receptors, particularly P2X7, may prove beneficial for treatment of hypertension.

Rat submandibular glands secrete nanovesicles with NTPDase and 5'-nucleotidase activities

Extracellular nucleotides modulate a wide number of biological processes such as neurotransmission, platelet aggregation, muscle contraction, and epithelial secretion acting by the purinergic pathway. Nucleotidases as NTPDases and ecto-5'-nucleotidase are membrane-anchored proteins that regulate extracellular nucleotide concentrations. In a previous work, we have partially characterized an NTPDase-like activity expressed by rat submandibular gland microsomes, giving rise to the hypothesis that membrane NTPDases could be released into salivary ducts to regulate luminal nucleotide concentrations as was previously proposed for ovarian, prostatic, and pancreatic secretions. Present results show that rat submandibular glands incubated in vitro release membrane-associated NTPDase and ecto-5'-nucleotidase activities. Electron microscopy images show that released membranes presenting nucleotidase activity correspond to exosome-like vesicles which are also present at microsomal fraction. Both exosome release and nucleotidase activities are raised by adrenergic stimulation. Nucleotidase activities present the same kinetic characteristics than microsomal nucleotidase activity, corresponding mainly to the action of NTPDase2 and NTPDase3 isoforms as well as 5'-nucleotidase. This is consistent with Western blot analysis revealing the presence of these enzymes in the microsomal fraction.

8-BuS-ATP derivatives as specific NTPDase1 inhibitors

BACKGROUND AND PURPOSE

Ectonucleotidases control extracellular nucleotide levels and consequently, their (patho)physiological responses. Among these enzymes, nucleoside triphosphate diphosphohydrolase-1 (NTPDase1), -2, -3 and -8 are the major ectonucleotidases responsible for nucleotide hydrolysis at the cell surface under physiological conditions, and NTPDase1 is predominantly located at the surface of vascular endothelial cells and leukocytes. Efficacious inhibitors of NTPDase1 are required to modulate responses induced by nucleotides in a number of pathological situations such as thrombosis, inflammation and cancer.

EXPERIMENTAL APPROACH

Here, we present the synthesis and enzymatic characterization of five 8-BuS-adenine nucleotide derivatives as potent and selective inhibitors of NTPDase1.

KEY RESULTS

The compounds 8-BuS-AMP, 8-BuS-ADP and 8-BuS-ATP inhibit recombinant human and mouse NTPDase1 by mixed type inhibition, predominantly competitive with Ki values <1 μM. In contrast to 8-BuS-ATP which could be hydrolyzed by other NTPDases, the other BuS derivatives were resistant to hydrolysis by either NTPDase1, -2, -3 or -8. 8-BuS-AMP and 8-BuS-ADP were the most potent and selective inhibitors of NTPDase1 expressed in human umbilical vein endothelial cells as well as in situ in human and mouse tissues. As expected, as a result of their inhibition of recombinant human NTPDase1, 8-BuS-AMP and 8-BuS-ADP impaired the ability of this enzyme to block platelet aggregation. Importantly, neither of these two inhibitors triggered platelet aggregation nor prevented ADP-induced platelet aggregation, in support of their inactivity towards P2Y1 and P2Y12 receptors.

CONCLUSIONS AND IMPLICATIONS

The 8-BuS-AMP and 8-BuS-ADP have therefore potential to serve as drugs for the treatment of pathologies regulated by NTPDase1.

Comparison between male and female Sprague-Dawley rats in the response of urinary biomarkers to injury induced by gentamicin

Differences were examined between male and female Sprague-Dawley rats in the response of 16 urinary biomarkers (measured using several assay platforms) to renal injury produced by gentamicin administered subcutaneously for 10 days at a dosage of 75 mg/kg. Urinary biomarkers expressed as fold difference from contemporaneous controls and renal histopathology were assessed after 3 and 10 doses. On day 4, minimal proximal tubular changes were observed microscopically in all males but no females; on day 11, more extensive and more severe injury was observed to a similar extent in all animals of both sexes. Modest increases (maximum 5-fold) in all urinary biomarkers (except epidermal growth factor [EGF], which was decreased) on day 4 and marked elevations (maximum 271-fold) on day 11 were seen consistently in both sexes. However, the magnitude of the increases differed between the sexes. On day 4, despite the lack of tubular injury, many biomarkers were more elevated in females than males but this rarely led to statistically significant sex differences; only 2 biomarkers (β2-microglobulin and total protein) showed a greater increase in males than females in line with the histopathology. On day 11, there were many more biomarkers that showed a statistically significant difference between the sexes in fold change with treatment; in line with the results on day 4, the majority of biomarkers were more increased in females than males. It remains unresolved if sex differences in the magnitude of biomarker response at injury threshold would lead to any difference in diagnostic interpretation between the sexes. These data highlight the need for publication of more studies using animals of both sexes to fully explore the influence of sex on the diagnostic performance of the novel biomarkers.

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.

NTPDase3 and ecto-5'-nucleotidase/CD73 are differentially expressed during mouse bladder cancer progression

According to the World Health Organization, bladder cancer is the seventh most common cancer among men in the world. The current treatments for this malignancy are not efficient to prevent the recurrence and progression of tumors. Then, researches continue looking for better therapeutic targets which can end up in new and more efficient treatments. One of the recent findings was the identification that the purinergic system was involved in bladder tumorigenesis. The ectonucleotidases, mainly ecto-5'-nucleotidase/CD73 have been revealed as new players in cancer progression and malignity. In this work, we investigated the NTPDase3 and ecto-5'-nucleotidase/CD73 expression in cancer progression in vivo. Bladder tumor was induced in mice by the addition of 0.05 % of N-butyl-N-(hydroxybutyl)-nitrosamine (BBN) in the drinking water for 4, 8, 12, 18, and 24 weeks. After this period, mice bladders were removed for histopathology analysis and immunofluorescence assays. The bladder of animals which has received BBN had alterations, mainly inflammation, in initial times of tumor induction. After 18 weeks, mice's bladder has developed histological alterations similar to human transitional cell carcinoma. The cancerous urothelium, from mice that received BBN for 18 and 24 weeks, presented a weak immunostaining to NTPDase3, in contrast to an increased expression of ecto-5'-nucleotidase/CD73. The altered expression of NTPDase3 and ecto-5'-nucleotidase/CD73 presented herein adds further evidence to support the idea that alterations in ectonucleotidases are involved in bladder tumorigenesis and reinforce the ecto-5'-nucleotidase/CD73 as a future biomarker and/or a target for pharmacological therapy of bladder cancer.

Therapeutic potentials of ecto-nucleoside triphosphate diphosphohydrolase, ecto-nucleotide pyrophosphatase/phosphodiesterase, ecto-5'-nucleotidase, and alkaline phosphatase inhibitors

The modulatory role of extracellular nucleotides and adenosine in relevance to purinergic cell signaling mechanisms has long been known and is an object of much research worldwide. These extracellular nucleotides are released by a variety of cell types either innately or as a response to patho-physiological stress or injury. A variety of surface-located ecto-nucleotidases (of four major types; nucleoside triphosphate diphosphohydrolases or NTPDases, nucleotide pyrophosphatase/phosphodiesterases or NPPs, alkaline phosphatases APs or ALPs, and ecto-5'-nucleotidase or e5NT) are responsible for meticulously controlling the availability of these important signaling molecules (at their respective receptors) in extracellular environment and are therefore crucial for maintaining the integrity of normal cell functioning. Overexpression of many of these ubiquitous ecto-enzymes has been implicated in a variety of disorders including cell adhesion, activation, proliferation, apoptosis, and degenerative neurological and immunological responses. Selective inhibition of these ecto-enzymes is an area that is currently being explored with great interest and hopes remain high that development of selective ecto-nucleotidase inhibitors will prove to have many beneficial therapeutic implications. The aim of this review is to emphasize and focus on recent developments made in the field of inhibitors of ecto-nucleotidases and to highlight their structure activity relationships wherever possible. Most recent and significant advances in field of NTPDase, NPP, AP, and e5NT inhibitors is being discussed in detail in anticipation of providing prolific leads and relevant background for research groups interested in synthesis of selective ecto-nucleotidase inhibitors.

Detection of endogenous substances with enzymatic microelectrode biosensors in the kidney

Direct real-time measurements of purinergic agents and reactive oxygen species concentrations have been of great value in understanding the functional roles of these substances in a number of diseases including chronic kidney disease and hypertension. The interstitial concentrations of these intermediate signaling molecules and dynamics of their release are important autocrine and paracrine factors in the kidney, which play a key role in the regulation of oxidative stress, inflammation, and kidney damage. Analysis of signaling mechanisms, especially in vivo and ex vivo, has been slowed by deficiencies of existing methods for direct measurements of the signaling molecules concentrations in whole organs and acute changes in response to endocrine factors. The multienzymatic microelectrode biosensors technique was originally developed and used for the detection of purines release in the brain and in present could be modified to identify the interplay between different substances that could be measured simultaneously in whole organs, such as the kidney. Adaptation of this method for renal and cardiovascular studies represents a unique powerful approach for real-time monitoring of substance level fluctuations in organs or tissues under normal or pathological conditions.

Extracellular pyrophosphate in the kidney: how does it get there and what does it do?

Pyrophosphate (PPi) is well known as a regulator of calcification, and the ANKH (ANK in mouse) protein has a role in the membrane transport of PPi. Earlier work concentrated on bones and joints, but ANKH is also likely to have important roles in the kidney, with newer studies focusing on vascular calcification in renal failure. Renal calcification can occur due to a naturally occurring ANK mouse mutation, yet other ANK mutations do not cause a renal phenotype. Despite evidence over 10 years of ANKH's involvement in PPi transport, efflux of PPi via ANKH has never been demonstrated. Rather than physically moving PPi, the ANKH protein may assist its membrane transport in other ways such as by hydrolysis and compartmentalisation. Protein complexes may account for effects of ANKH that are specific to particular tissues. In the kidney, recent localisation data may be helpful in suggesting physiological roles for ANKH, such as its co-localisation with aquaporin-2 and cilial proteins. Such diverse functions would reflect the ubiquitous nature of ANKH in tissues and its profound evolutionary conservation.

Characterization of ecto-ATPase activity in the surface of LLC-PK1 cells and its modulation by ischemic conditions

BACKGROUND

The concentration of extracellular nucleotides is regulated by enzymes that have their catalytic site facing the extracellular space, the so-called ecto-enzymes.

METHODS

We used LLC-PK1 cells, a well-characterized porcine renal proximal tubule cell line, to biochemically characterize ecto-ATPase activity in the luminal surface. The [γ-(32)P]Pi released after reaction was measured in aliquots of the supernatant by liquid scintillation.

RESULTS

This activity was linear with time up to 20min of reaction and stimulated by divalent metals. The ecto-ATPase activity measured in the presence of 5mM MgCl(2) was (1) optimum at pH 8, (2) insensitive to different inhibitors of intracellular ATPases, (3) inhibited by 1mM suramin, an inhibitor of ecto-ATPases, (4) sensitive to high concentrations of sodium azide (NaN(3)) and (5) also able to hydrolyze ADP in the extracellular medium. The ATP:ADP hydrolysis ratio calculated was 4:1. The ecto-ADPase activity was also inhibited by suramin and NaN(3). The dose-response of ATP revealed a hyperbolic profile with maximal velocity of 25.2±1.2nmol Pixmg(-1)xmin(-1) and K(0.5) of 0.07±0.01mM. When cells were submitted to ischemia, the E-NTPDase activity was reduced with time, achieving 71% inhibition at 60min of ischemia.

CONCLUSION

Our results suggest that the ecto-ATPase activity of LLC-PK1 cells has the characteristics of a type 3 E-NTPDase which is inhibited by ischemia.

GENERAL SIGNIFICANCE

This could represent an important pathophysiologic mechanism that explains the increase in ATP concentration in the extracellular milieu in the proximal tubule during ischemia.

Proliferative and nonproliferative lesions of the rat and mouse urinary system

The INHAND Project (International Harmonization of Nomenclature and Diagnostic Criteria for Lesions in Rats and Mice) is a joint initiative of the Societies of Toxicologic Pathology from Europe (ESTP), Great Britain (BSTP), Japan (JSTP), and North America (STP) to develop an internationally accepted nomenclature for proliferative and nonproliferative lesions in laboratory animals. The purpose of this publication is to provide a standardized nomenclature for classifying lesions observed in the urinary tract of rats and mice. The standardized nomenclature of urinary tract lesions presented in this document is also available electronically on the Internet (http://www.goreni.org/). Sources of material included histopathology databases from government, academia, and industrial laboratories throughout the world. Content includes spontaneous developmental and aging lesions as well as those induced by exposure to test materials. A widely accepted and utilized international harmonization of nomenclature for urinary tract lesions in laboratory animals will decrease confusion among regulatory and scientific research organizations in different countries and provide a common language to increase and enrich international exchanges of information among toxicologists and pathologists.

Cellular function and molecular structure of ecto-nucleotidases

Ecto-nucleotidases play a pivotal role in purinergic signal transmission. They hydrolyze extracellular nucleotides and thus can control their availability at purinergic P2 receptors. They generate extracellular nucleosides for cellular reuptake and salvage via nucleoside transporters of the plasma membrane. The extracellular adenosine formed acts as an agonist of purinergic P1 receptors. They also can produce and hydrolyze extracellular inorganic pyrophosphate that is of major relevance in the control of bone mineralization. This review discusses and compares four major groups of ecto-nucleotidases: the ecto-nucleoside triphosphate diphosphohydrolases, ecto-5'-nucleotidase, ecto-nucleotide pyrophosphatase/phosphodiesterases, and alkaline phosphatases. Only recently and based on crystal structures, detailed information regarding the spatial structures and catalytic mechanisms has become available for members of these four ecto-nucleotidase families. This permits detailed predictions of their catalytic mechanisms and a comparison between the individual enzyme groups. The review focuses on the principal biochemical, cell biological, catalytic, and structural properties of the enzymes and provides brief reference to tissue distribution, and physiological and pathophysiological functions.

P2Y receptors and kidney function

Cellular release of nucleotides is of physiological importance to regulate and maintain cell function and integrity. Also in the tubular and collecting duct system of the kidney, nucleotides are released in response to changes in cell volume or luminal flow rate and act in a paracrine and autocrine way on basolateral and luminal P2Y receptors. Recent studies using gene knockout mice assigned a prominent role to G protein-coupled P2Y(2) receptors, which are activated by both ATP and UTP. The antidiuretic hormone, arginine-vasopressin (AVP), and possibly an increase in collecting duct cell volume induce ATP release. The subsequent activation of P2Y(2) receptors inhibits AVP-induced cAMP formation and water reabsorption, which stabilizes cell volume and facilitates water excretion. An increase in NaCl intake enhances luminal release of ATP and UTP in the aldosterone-sensitive distal nephron which by activating apical P2Y(2) receptors and phospholipase C lowers the open probability of the epithelial sodium channel ENaC, thereby facilitating sodium excretion. Thus, the renal ATP/UTP/P2Y(2) receptor system not only serves to preserve cell volume and integrity but is also regulated by stimuli that derive from body NaCl homeostasis. The system also inhibits ENaC activity during aldosterone escape, i.e. when sodium reabsorption via ENaC is inappropriately high. The P2Y(2) receptor tone inhibits the expression and activity of the Na-K-2Cl cotransporter NKCC2 in the thick ascending limb and mediates vasodilation. While the role of other P2Y receptors in the kidney is less clear, the ATP/UTP/P2Y(2) receptor system regulates NaCl and water homeostasis and blood pressure.

Biomarkers of Collecting Duct Injury in Han-Wistar and Sprague-Dawley Rats Treated with N-Phenylanthranilic Acid

N-phenylanthranilic acid is a chloride channel blocker that causes renal papillary necrosis in rats. Studies were conducted in two strains of male rats to evaluate novel biomarkers of nephrotoxicity. Han-Wistar rats were given daily oral doses of 50, 350, or up to 700 mg/kg/day of NPAA, and Sprague-Dawley rats were given 50 or 400 mg/kg/day of NPAA. Rats were euthanized on days 8 and 15. The candidate kidney injury biomarkers renal papillary antigen-1 (RPA-1, for collecting duct injury), clusterin (for general kidney injury), α-glutathione-S-transferase (a proximal tubular marker), and µ-glutathione-S-transferase (a distal tubular marker) were measured in urine by enzyme immunoassay. Characteristic degeneration and necrosis of the collecting duct and renal papilla were observed in Han-Wistar rats at the high dose on day 8 and at the mid and high doses on day 15, and in Sprague-Dawley rats given the high dose on days 8 and 15. Increases in urinary RPA-1, and to a lesser extent urine clusterin, were generally associated with the presence of collecting duct injury and were more sensitive than BUN and serum creatinine. On the other hand, decreases in α-glutathione-S-transferase without proximal tubule lesions in both strains and decreases in µ-glutathione-S-transferase in Sprague-Dawley rats only were not associated with morphological proximal or distal tubule abnormalities, so both were of less utility. It was concluded that RPA-1 is a new biomarker with utility in the detection of collecting duct injury in papillary necrosis in male rats.

Immunohistochemical identification of kidney nephron segments in the dog, rat, mouse, and cynomolgus monkey

Kidney is a major target organ in preclinical studies. In recent years, intense research has been undertaken to characterize novel renal toxicity biomarkers. In this context, we studied nephron segment specific antibodies against aquaporin-1 (AQP-1), α-glutathione-S-transferase (alpha-GST), Tamm-Horsfall protein (TH), calbindin-D(28K) (CalD), and aquaporin-2 (AQP-2), using an immunoperoxidase method on formalin-fixed paraffin-embedded kidney tissues of dogs, rats, mice, and Cynomolgus monkeys. AQP-1 was specific for proximal tubules and thin descending limbs of Henle's loops and AQP-2 for connecting and collecting ducts in dogs, rats, mice, and Cynomolgus monkeys. Alpha-GST stained the straight part of proximal tubules in dogs and proximal convoluted tubule and straight part of proximal tubules in rats. TH was specific for thick ascending limbs of Henle's loops in mice, rats, dogs, and Cynomolgus monkeys and stained additionally scattered cells in cortical connecting/collecting ducts of dogs. CalD was found in distal convoluted tubules and cortical connecting and collecting ducts of dogs, rats, and mice and in distal convoluted tubules, connecting ducts, and cortical and medullary collecting ducts of Cynomolgus monkey. This panel of antibodies may be a helpful tool to identify renal tubules by light microscopy in preclinical studies and to validate new biomarkers of renal toxicity.

Impact of ectoenzymes on p2 and p1 receptor signaling

P2 receptors that are activated by extracellular nucleotides (e.g., ATP, ADP, UTP, UDP, Ap(n)A) and P1 receptors activated by adenosine control a diversity of biological processes. The activation of these receptors is tightly regulated by ectoenzymes that metabolize their ligands. This review presents these enzymes as well as their roles in the regulation of P2 and P1 receptor activation. We focus specifically on the role of ectoenzymes in processes of our interest, that is, inflammation, vascular tone, and neurotransmission. An update on the development of ectonucleotidase inhibitors is also presented.

The extracellular cAMP-adenosine pathway regulates expression of renal D1 dopamine receptors in diabetic rats

Activation of D1 dopamine receptors expressed in the kidneys promotes the excretion of sodium and regulates sodium levels during increases in dietary sodium intake. A decrease in the expression or function of D1 receptors results in increased sodium retention which can potentially lead to the development of hypertension. Studies have shown that in the absence of functional D1 receptors, in null mice, the systolic, diastolic, and mean arterial pressures are higher. Previous studies have shown that the expression and function of D1 receptors in the kidneys are decreased in animal models of diabetes. The mechanisms that down-regulate the expression of renal D1 receptor gene in diabetes are not well understood. Using primary renal cells and acutely isolated kidneys from the streptozotocin-induced rat diabetic model, we demonstrate that the renal D1 receptor expression is down-regulated by the extracellular cAMP-adenosine pathway in vitro and in vivo. In cultures of primary renal cells, a 3 mm, 60-h cAMP treatment down-regulated the expression of D1 receptors. In vivo, we determined that the plasma and urine cAMP levels as well as the expression of 5'-ectonucleotidase, tissue-nonspecific alkaline phosphatase, and adenosine A2a receptors are significantly increased in diabetic rats. Inhibitors of 5'-ectonucleotidase and tissue-nonspecific alkaline phosphatase, α,β-methyleneadenosine 5'-diphosphate, and levamisole, respectively, blocked the down-regulation of D1 receptors in the primary renal cells and in the kidney of diabetic animals. The results suggest that inhibitors of the extracellular cAMP-adenosine pathway reverse the down-regulation of renal D1 receptor in diabetes.

Distribution of ecto-nucleotidases in mouse sensory circuits suggests roles for nucleoside triphosphate diphosphohydrolase-3 in nociception and mechanoreception

Nucleotide-activated P2X channels and P2Y metabotropic receptors participate in nociceptive signaling. Agonist availability is regulated by nucleoside triphosphate diphosphohydrolase-1 (NTPDase1), -2, -3, and -8, a family of enzymes that hydrolyze extracellular ATP to generate ADP (a P2Y agonist) and AMP. They provide a major source of extracellular AMP, the substrate for adenosine production by ecto-5'-nucleotidase (NT5E), and thereby regulate adenosine (P1) receptor signaling. NTPDases vary in their efficiency of tri- and diphosphate hydrolysis; therefore, which family members are expressed impacts nucleotide availability and half-life. This study employed enzyme activity histochemistry to examine the distribution of ATPase activity and immunohistochemistry for NTPDase1, 2, 3, and 8 in dorsal root ganglion (DRG) and spinal cord. Nucleotidase activity was robust in spinal dorsal horn, confirming that nociceptive pathways are a major site of nucleotide transmission. In DRG, extensive staining revealed ATPase activity in a subset of neurons and in non-neuronal cells. mRNA for NTPDase1-3, but not NTPDase8, was detected in lumbar DRG and spinal cord. Immunoreactivity for NTPDase3 closely matched the distribution of ATPase activity, labeling DRG central projections in the dorsal root and superficial dorsal horn, as well as intrinsic spinal neurons concentrated in lamina II. In DRG, NTPDase3 co-localized with markers of nociceptors and with NT5E. In addition, labeling of a subset of larger-diameter neurons in DRG was consistent with intense staining of Meissner corpuscle afferents in glabrous skin. Merkel cells and terminal Schwann cells of hair follicle afferents were also labeled, but the axons themselves were negative. We propose that NTPDase3 is a key regulator of nociceptive signaling that also makes an unexpected contribution to innocuous tactile sensation.

Regulation of renal NaCl and water transport by the ATP/UTP/P2Y2 receptor system

Extracellular nucleotides (e.g., ATP) activate ionotropic P2X and metabotropic P2Y receptors in the plasma membrane to regulate and maintain cell function and integrity. This includes the renal tubular and collecting duct system, where the locally released nucleotides act in a paracrine and autocrine way to regulate transport of electrolytes and water and maintain cell volume. A prominent role has been assigned to Gq-coupled P2Y(2) receptors, which are typically activated by both ATP and UTP. Studies in gene knockout mice revealed an antihypertensive activity of P2Y(2) receptors that is linked to vasodilation and an inhibitory influence on renal salt reabsorption. Flow induces apical ATP release in the thick ascending limb, and first evidence indicates an inhibitory influence of P2Y(2) receptor tone on the expression and activity of the Na-K-2Cl cotransporter NKCC2 in this segment. The apical ATP/UTP/P2Y(2) receptor system in the connecting tubule/cortical collecting duct mediates the inhibitory effect of dietary salt on the open probability of the epithelial sodium channel ENaC and inhibits ENaC activity during aldosterone escape. Connexin 30 has been implicated in the luminal release of the ATP involved in the regulation of ENaC. An increase in collecting duct cell volume in response to manipulating water homeostasis increases ATP release. The subsequent activation of P2Y(2) receptors inhibits vasopressin-induced cAMP formation and water reabsorption, which facilitates water excretion and stabilizes cell volume. Thus recent studies have established the ATP/UTP/P2Y(2) receptor system as a relevant regulator of renal salt and water homeostasis and blood pressure regulation. The pathophysiological relevance and therapeutic potential remains to be determined, but dual effects of P2Y(2) receptor activation on both the vasculature and renal salt reabsorption implicate these receptors as potential therapeutic targets in hypertension.

Involvement of Runx3 in the basal transcriptional activation of the mouse angiotensin II type 1 receptor-associated protein gene

We previously cloned a molecule that interacts with angiotensin II type 1 (AT1) receptor to exert an inhibitory function on AT1 receptor signaling that we named ATRAP/Agtrap (for AT1 receptor-associated protein). In the present study we examined the regulation of basal ATRAP gene expression using renal distal convoluted tubule cells. We found that serum starvation upregulated basal expression of ATRAP gene, a response that required de novo mRNA and protein synthesis. Luciferase assay revealed that the proximal promoter region directs transcription and that a putative binding site of runt-related transcription factors (RBE) is important for transcriptional activation. The results of RBE-decoy transfection and endogenous knockdown by small interference RNA showed that the runt-related transcription factor Runx3 is involved in ATRAP gene expression. Chromatin immunoprecipitation assay also supported the binding of Runx3 to the ATRAP promoter in renal distal convoluted tubule cells. Immunohistochemistry demonstrated the expression of Runx3 and ATRAP proteins in the distal convoluted and connecting tubules of the kidney in consecutive sections. Furthermore, the Runx3 immunostaining was decreased together with a concomitant suppression of ATRAP expression in the affected kidney after 7 days of unilateral ureteral obstruction. These findings indicate that Runx3 plays a role in ATRAP gene expression in renal distal tubular cells both in vitro and in vivo.

Identification of the ectonucleotidases expressed in mouse, rat, and human Langerhans islets: potential role of NTPDase3 in insulin secretion

Extracellular nucleotides and adenosine regulate endocrine pancreatic functions such as insulin secretion by Langerhans islet β-cells via the activation of specific P2 and P1 receptors. Membrane-bound ectonucleotidases regulate the local concentration of these ligands and consequently control the activation of their receptors. The objective of this study was to identify and localize the major ectonucleotidases, namely NTPDases and ecto-5'-nucleotidase, present in the endocrine pancreas. In addition, the potential implication of ecto-ATPase activity on insulin secretion was investigated in the rat β-cell line INS-1 (832/13). The localization of ectonucleotidase activity and protein was carried out in situ by enzyme histochemistry and immunolocalization in mouse, rat, and human pancreas sections. NTPDase1 was localized in all blood vessels and acini, and NTPDase2 was localized in capillaries of Langerhans islets and in peripheral conjunctive tissue, whereas NTPDase3 was detected in all Langerhans islet cell types. Interestingly, among the mammalian species tested, ecto-5'-nucleotidase was present only in rat Langerhans islet cells, where it was coexpressed with NTPDase3. Notably, the inhibition of NTPDase3 activity by BG0136 and NF279 facilitated insulin release from INS-1 (832/13) cells under conditions of low glycemia, probably by affecting P2 receptor activation. NTPDase3 activity also regulated the inhibitory effect of exogenous ATP in the presence of a high glucose concentration most likely by controlling adenosine production. In conclusion, all pancreatic endocrine cells express NTPDase3 that was shown to modulate insulin secretion in rat INS-1 (832/13) β-cells. Ecto-5'-nucleotidase is expressed in rat Langerhans islet cells but absent in human and mouse endocrine cells.

Evaluation of Novel Biomarkers of Nephrotoxicity in Two Strains of Rat Treated with Cisplatin

Cisplatin is an anticancer agent that induces renal proximal tubule lesions in many species. Studies were conducted in Sprague-Dawley and Han-Wistar rats to evaluate the utility of novel preclinical biomarkers of nephrotoxicity for renal lesions caused by this compound. Groups of 10 males of each strain were given a single intraperitoneal injection of 0.3, 1, or 3 mg/kg cisplatin and were sacrificed on days 2, 3, and 5. The novel biomarkers α-glutathione-S-transferase (α-GST) (for proximal tubular injury), μ-glutathione-S-transferase (μ-GST) (for distal tubular injury), clusterin (for general kidney injury), and renal papillary antigen-1 (RPA-1) (for collecting duct injury) were measured in urine by enzyme immunoassay. Histologically, degeneration and necrosis of the S3 segment of the renal proximal tubule were observed on day 2 (Han-Wistar) and days 3 and 5 (both strains) at 1 and 3 mg/kg. Results showed that in both strains of rats, urinary α-GST and clusterin can be detected in urine soon after injury, are more sensitive than BUN and serum creatinine, and therefore are usable as noninvasive biomarkers of proximal tubule injury. Changes in both μ-GST or RPA-1 were considered to represent secondary minor effects of proximal tubular injury on distal segments of the nephron.

Expression of angiotensin II type 1 receptor-interacting molecule in normal human kidney and IgA nephropathy

The intrarenal renin-angiotensin system plays a crucial role in the regulation of renal circulation and sodium reabsorption through the activation of vascular, glomerular, and tubular angiotensin II type 1 (AT(1)) receptor signaling. We previously cloned a molecule that specifically interacted with the murine AT(1) receptor to inhibit AT(1) receptor signaling, which we named ATRAP (for AT(1) receptor-associated protein). Since murine ATRAP was shown to be highly expressed in the kidney, in the present study we investigated expression and distribution of human ATRAP in normal kidney and renal biopsy specimens from patients with IgA nephropathy. In the normal human kidney, both ATRAP mRNA and protein were widely and abundantly distributed along the renal tubules from Bowman's capsule to the medullary collecting ducts. In all renal tubular epithelial cells, the ATRAP protein colocalized with the AT(1) receptor. In renal biopsy specimens with IgA nephropathy, a significant positive correlation between ATRAP and AT(1) receptor gene expression was observed. There was also a positive relationship between tubulointerstitial ATRAP expression and the estimated glomerular filtration rate in patients with IgA nephropathy. Furthermore, we examined the function of the tubular AT(1) receptor using an immortalized cell line of mouse distal convoluted tubule cells (mDCT) and found that overexpression of ATRAP by adenoviral gene transfer suppressed the angiotensin II-mediated increases in transforming growth factor-β production in mDCT cells. These findings suggest that ATRAP might play a role in balancing the renal renin-angiotensin system synergistically with the AT(1) receptor by counterregulatory effects in IgA nephropathy and propose an antagonistic effect of tubular ATRAP on AT(1) receptor signaling.

L-NAME-treatment alters ectonucleotidase activities in kidney membranes of rats

AIMS

To investigate the effect of N(omega)-Nitro-L-arginine methyl ester (l-NAME) treatment, known to induce a sustained elevation of blood pressure, on ectonucleotidase activities in kidney membranes of rats.

MAIN METHODS

L-NAME (30 mg/kg/day) was administered to Wistar rats for 14 days in the drinking water. Enzyme activities were determined colorimetrically and their gene expression patterns were analyzed by semi-quantitative RT-PCR. The metabolism of ATP and the accumulation of adenosine were evaluated by HPLC in kidney membranes from control and hypertensive rats. PKC phosphorylation state was investigated by Western blot.

KEY FINDINGS

We observed an increase in systolic blood pressure from 115+/-12 mmHg (control group) to 152+/-18 mmHg (l-NAME-treated group). Furthermore, the hydrolysis of ATP, ADP, AMP, and p-Nph-5'TMP was also increased (17%, 35%, 27%, 20%, respectively) as was the gene expression of NTPDase2, NTPDase3 and NPP3 in kidneys of hypertensive animals. Phospho-PKC was increased in hypertensive rats.

SIGNIFICANCE

The general increase in ATP hydrolysis and in ecto-5'-nucleotidase activity suggests a rise in renal adenosine levels and in renal autoregulatory responses in order to protect the kidney against the threat presented by hypertension.

Proteomic analysis of brush-border membrane vesicles isolated from purified proximal convoluted tubules

The renal proximal convoluted tubule is the primary site of water, electrolyte and nutrient reabsorption and of active secretion of selected molecules. Proteins in the apical brush-border membrane facilitate these functions and initiate some of the cellular responses to altered renal physiology. The current study uses two-dimensional liquid chromatography/mass spectrometry to compare brush border membrane vesicles isolated from rat renal cortex (BBMV(CTX)) and from purified proximal convoluted tubules (BBMV(PCT)). Both proteomic data and Western blot analysis indicate that the BBMV(CTX) contain apical membrane proteins from cortical cells other than the proximal tubule. This heterogeneity was greatly reduced in the BBMV(PCT). Proteomic analysis identified 193 proteins common to both samples, 21 proteins unique to BBMV(CTX), and 57 proteins unique to BBMV(PCT). Spectral counts were used to quantify relative differences in protein abundance. This analysis identified 42 and 50 proteins that are significantly enriched (p values <or=0.001) in the BBMV(CTX) and BBMV(PCT), respectively. These data were validated by measurement of gamma-glutamyltranspeptidase activity and by Western blot analysis. The combined results establish that BBMV(PCT) are primarily derived from the proximal convoluted tubule (S1 and S2 segments), whereas BBMV(CTX) include proteins from the proximal straight tubule (S3 segment). Analysis of functional annotations indicated that BBMV(PCT) are enriched in mitochondrial proteins and enzymes involved in glucose and organic acid metabolism. Thus the current study reports a detailed proteomic analysis of the brush-border membrane of the rat renal proximal convoluted tubule and provides a database for future hypothesis-driven research.

ATP, P2 receptors and the renal microcirculation

Purinoceptors are rapidly becoming recognised as important regulators of tissue and organ function. Renal expression of P2 receptors is broad and diverse, as reflected by the fact that P2 receptors have been identified in virtually every major tubular/vascular element. While P2 receptor expression by these renal structures is recognised, the physiological functions that they serve remains to be clarified. Renal vascular P2 receptor expression is complex and poorly understood. Evidence suggests that different complements of P2 receptors are expressed by individual renal vascular segments. This unique distribution has given rise to the postulate that P2 receptors are important for renal vascular function, including regulation of preglomerular resistance and autoregulatory behaviour. More recent studies have also uncovered evidence that hypertension reduces renal vascular reactivity to P2 receptor stimulation in concert with compromised autoregulatory capability. This review will consolidate findings related to the role of P2 receptors in regulating renal microvascular function and will present areas of controversy related to the respective roles of ATP and adenosine in autoregulatory resistance adjustments.

P2 receptors in renal pathophysiology

Our knowledge and understanding of the P2 receptor signalling system in the kidney have increased significantly in the last ten years. The broad range of physiological roles proposed for this receptor system and the variety of P2 receptor subtypes found in the kidney suggest that any disturbance of function may contribute to several pathological processes. So far, most reports of a possible pathophysiological role for this system in the kidney have focussed on polycystic kidney disease, where abnormal P2 receptor signalling might be involved in cyst expansion and disease progression, and on the P2X(7) receptor, a unique P2X subtype, which when activated enhances inflammatory cytokine release and production, and also cell death. Expression of this particular receptor is upregulated in some forms of chronic renal injury and inflammatory diseases. Further studies of adenosine triphosphate signalling and P2 receptor expression in renal disorders could provide us with novel insights into the role of these receptors in both normal and abnormal kidney function.

Ectonucleotidases in the kidney

Members of all four families of ectonucleotidases, namely ectonucleoside triphosphate diphosphohydrolases (NTPDases), ectonucleotide pyrophosphatase/phosphodiesterases (NPPs), ecto-5'-nucleotidase and alkaline phosphatases, have been identified in the renal vasculature and/or tubular structures. In rats and mice, NTPDase1, which hydrolyses ATP through to AMP, is prominent throughout most of the renal vasculature and is also present in the thin ascending limb of Henle and medullary collecting duct. NTPDase2 and NTPDase3, which both prefer ATP over ADP as a substrate, are found in most nephron segments beyond the proximal tubule. NPPs catalyse not only the hydrolysis of ATP and ADP, but also of diadenosine polyphosphates. NPP1 has been identified in proximal and distal tubules of the mouse, while NPP3 is expressed in the rat glomerulus and pars recta, but not in more distal segments. Ecto-5'-nucleotidase, which catalyses the conversion of AMP to adenosine, is found in apical membranes of rat proximal convoluted tubule and intercalated cells of the distal nephron, as well as in the peritubular space. Finally, an alkaline phosphatase, which can theoretically catalyse the entire hydrolysis chain from nucleoside triphosphate to nucleoside, has been identified in apical membranes of rat proximal tubules; however, this enzyme exhibits relatively high K (m) values for adenine nucleotides. Although information on renal ectonucleotidases is still incomplete, the enzymes' varied distribution in the vasculature and along the nephron suggests that they can profoundly influence purinoceptor activity through the hydrolysis, and generation, of agonists of the various purinoceptor subtypes. This review provides an update on renal ectonucleotidases and speculates on the functional significance of these enzymes in terms of glomerular and tubular physiology and pathophysiology.

Attenuated, flow-induced ATP release contributes to absence of flow-sensitive, purinergic Cai2+ signaling in human ADPKD cyst epithelial cells

Flow-induced cytosolic Ca2+ Ca(i)2+ signaling in renal tubular epithelial cells is mediated in part through P2 receptor (P2R) activation by locally released ATP. The ability of P2R to regulate salt and water reabsorption has suggested a possible contribution of ATP release and paracrine P2R activation to cystogenesis and/or enlargement in autosomal dominant polycystic kidney disease (ADPKD). We and others have demonstrated in human ADPKD cyst cells the absence of flow-induced Ca(i)2+ signaling exhibited by normal renal epithelial cells. We now extend these findings to primary and telomerase-immortalized normal and ADPKD epithelial cells of different genotype and of both proximal and distal origins. Flow-induced elevation of Ca(i)2+ concentration ([Ca2+](i)) was absent from ADPKD cyst cells, but in normal cells was mediated by flow-sensitive ATP release and paracrine P2R activation, modulated by ecto-nucleotidase activity, and abrogated by P2R inhibition or extracellular ATP hydrolysis. In contrast to the elevated ATP release from ADPKD cells in static isotonic conditions or in hypotonic conditions, flow-induced ATP release from cyst cells was lower than from normal cells. Extracellular ATP rapidly reduced thapsigargin-elevated [Ca2+](i) in both ADPKD cyst and normal cells, but cyst cells lacked the subsequent, slow, oxidized ATP-sensitive [Ca2+](i) recovery present in normal cells. Telomerase-immortalized cyst cells also exhibited altered CD39 and P2X7 mRNA levels. Thus the loss of flow-induced, P2R-mediated Ca(i)2+ signaling in human ADPKD cyst epithelial cells was accompanied by reduced flow-sensitive ATP release, altered purinergic regulation of store-operated Ca2+ entry, and altered expression of gene products controlling extracellular nucleotide signaling.