Adenosine A2A receptor blocks the A 1 receptor inhibition of renal Na + transport and oxygen consumption.

A high renal oxygen (O2 ) need is primarily associated with the renal tubular O2 consumption (VO2 ) necessary for a high rate of sodium (Na+ ) transport. Limited O2 availability leads to increased levels of adenosine, which regulates the kidney via activation of both A1 and A2A adenosine receptors (A1R and A2AR, respectively). The relative contributions of A1R and A2AR to the regulation of renal Na+ transport and VO2 have not been determined. We demonstrated that A1R activation has a dose-dependent biphasic effect on both renal Na+ /H+ exchanger-3 (NHE3), a major player in Na+ transport, and VO2 . Here, we report concentration-dependent effects of adenosine: less than 5 × 10-7 M adenosine-stimulated NHE3 activity; between 5 × 10-7 M and 10-5 M adenosine-inhibited NHE3 activity; and greater than 10-5 M adenosine reversed the change in NHE3 activity (returned to baseline). A1R activation mediated the activation and inhibition of NHE3 activity, whereas 10-4 M adenosine had no effect on the NHE3 activity due to A2AR activation. The following occurred when A1R and A2AR were activated: (a) Blockade of the A2AR receptor restored the NHE3 inhibition mediated by A1R activation, (b) the NHE-dependent effect on VO2 mediated by A1R activation became NHE independent, and (c) A2AR bound to A1R. In summary, A1R affects VO2 via NHE-dependent mechanisms, whereas A2AR acts via NHE-independent mechanisms. When both A1R and A2AR are activated, the A2AR effect on NHE3 and VO2 predominates, possibly via an A1R-A2AR protein interaction. A2AR-A1R heterodimerization is proposed as the molecular mechanism enabling the NHE-independent control of renal VO2 .

Loss of diacylglycerol kinase epsilon in mice causes endothelial distress and impairs glomerular Cox-2 and PGE2 production.

Thrombotic microangiopathy (TMA) is a disorder characterized by microvascular occlusion that can lead to thrombocytopenia, hemolytic anemia, and glomerular damage. Complement activation is the central event in most cases of TMA. Primary forms of TMA are caused by mutations in genes encoding components of the complement or regulators of the complement cascade. Recently, we and others have described a genetic form of TMA caused by mutations in the gene diacylglycerol kinase-ε (DGKE) that encodes the lipid kinase DGKε (Lemaire M, Fremeaux-Bacchi V, Schaefer F, Choi MR, Tang WH, Le Quintrec M, Fakhouri F, Taque S, Nobili F, Martinez F, Ji WZ, Overton JD, Mane SM, Nurnberg G, Altmuller J, Thiele H, Morin D, Deschenes G, Baudouin V, Llanas B, Collard L, Majid MA, Simkova E, Nurnberg P, Rioux-Leclerc N, Moeckel GW, Gubler MC, Hwa J, Loirat C, Lifton RP. Nat Genet 45: 531-536, 2013; Ozaltin F, Li BH, Rauhauser A, An SW, Soylemezoglu O, Gonul II, Taskiran EZ, Ibsirlioglu T, Korkmaz E, Bilginer Y, Duzova A, Ozen S, Topaloglu R, Besbas N, Ashraf S, Du Y, Liang CY, Chen P, Lu DM, Vadnagara K, Arbuckle S, Lewis D, Wakeland B, Quigg RJ, Ransom RF, Wakeland EK, Topham MK, Bazan NG, Mohan C, Hildebrandt F, Bakkaloglu A, Huang CL, Attanasio M. J Am Soc Nephrol 24: 377-384, 2013). DGKε is unrelated to the complement pathway, which suggests that unidentified pathogenic mechanisms independent of complement dysregulation may result in TMA. Studying Dgke knockout mice may help to understand the pathogenesis of this disease, but no glomerular phenotype has been described in these animals so far. Here we report that Dgke null mice present subclinical microscopic anomalies of the glomerular endothelium and basal membrane that worsen with age and develop glomerular capillary occlusion when exposed to nephrotoxic serum. We found that induction of cyclooxygenase-2 and of the proangiogenic prostaglandin E2 are impaired in Dgke null kidneys and are associated with reduced expression of the antithrombotic cell adhesion molecule platelet endothelial cell adhesion molecule-1/CD31 in the glomerular endothelium. Notably, prostaglandin E2 supplementation was able to rescue motility defects of Dgke knockdown cells in vitro and to restore angiogenesis in a test in vivo. Our results unveil an unexpected role of Dgke in the induction of cyclooxygenase-2 and in the regulation of glomerular prostanoids synthesis under stress.

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

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

Dual Effect of Adenosine A1 Receptor Activation on Renal O2 Consumption.

The high requirement of O2 in the renal proximal tubule stems from a high rate of Na(+) transport. Adenosine A1 receptor (A1R) activation regulates Na(+) transport in this nephron segment. Thus, the effect of the acute activation and the mechanisms of A1R on the rate of O2 consumption were evaluated. The A1R-antagonist, 8-cyclopentyl-1,3-dipropylxanthine (CPX) and adenosine deaminase (ADA), which metabolize endogenous adenosine, reduced O2 consumption (40-50%). Replacing Na(+) in the buffer reversed the ADA- or CPX-mediated reduction of O2 consumption. Blocking the Na/H-exchanger activity, which decreases O2 usage per se, did not enhance the ADA- or CPX-induced inhibition of O2 consumption. These data indicate that endogenous adenosine increases O2 usage via the activation of Na(+) transport. In the presence of endogenous adenosine, A1R was further activated by the A1R-agonist N(6)-cyclopentyladenosine (CPA); CPA inhibited O2 usage (30%) and this effect also depended on Na(+) transport. Moreover, a low concentration of CPA activated O2 usage in tissue pretreated with ADA, whereas a high concentration of CPA inhibited O2 usage; both effects depended on Na(+). Protein kinase C signaling mediated the inhibitory effect of A1R, while adenylyl cyclase mediated its stimulatory effect on O2 consumption. In summary, increasing the local concentrations of adenosine can either activate or inhibit O2 consumption via A1R, and this mechanism depends on Na(+) transport. The inhibition of O2 usage by A1R activation might restore the compromised balance between energy supply and demand under pathophysiological conditions, such as renal ischemia, which results in high adenosine production.

Hedgehog signaling indirectly affects tubular cell survival after obstructive kidney injury.

Hedgehog (Hh) is an evolutionary conserved signaling pathway that has important functions in kidney morphogenesis and adult organ maintenance. Recent work has shown that Hh signaling is reactivated in the kidney after injury and is an important mediator of progressive fibrosis. Pericytes and fibroblasts have been proposed to be the principal cells that respond to Hh ligands, and pharmacological attenuation of Hh signaling has been considered as a possible treatment for fibrosis, but the effect of Hh inhibition on tubular epithelial cells after kidney injury has not been reported. Using genetically modified mice in which tubule-derived hedgehog signaling is increased and mice in which this pathway is conditionally suppressed in pericytes that express the proteoglycan neuron glial protein 2 (NG2), we found that suppression of Hh signaling is associated with decreased macrophage infiltration and tubular proliferation but also increased tubular apoptosis, an effect that correlated with the reduction of tubular ß-catenin activity. Collectively, our data suggest a complex function of hedgehog signaling after kidney injury in initiating both reparative and proproliferative, prosurvival processes.

Mutations in ANKS6 cause a nephronophthisis-like phenotype with ESRD.

Nephronophthisis (NPHP) is one of the most common genetic causes of CKD; however, the underlying genetic abnormalities have been established in <50% of patients. We performed genome-wide analysis followed by targeted resequencing in a Turkish consanguineous multiplex family and identified a canonic splice site mutation in ANKS6 associated with an NPHP-like phenotype. Furthermore, we identified four additional ANKS6 variants in a cohort of 56 unrelated patients diagnosed with CKD due to nephronophthisis, chronic GN, interstitial nephritis, or unknown etiology. Immunohistochemistry in human embryonic kidney tissue demonstrated that the expression patterns of ANKS6 change substantially during development. Furthermore, we detected increased levels of both total and active ß-catenin in precystic tubuli in Han:SPRD Cy/+ rats. Overall, these data indicate the importance of ANKS6 in human kidney development and suggest a mechanism by which mutations in ANKS6 may contribute to an NPHP-like phenotype in humans.

The biophysical and molecular basis of intracellular pH sensing by Na+/H+ exchanger-3.

Epithelial Na(+)/H(+) exchanger-3 (NHE3) transport is fundamental for renal and intestinal sodium reabsorption. Cytoplasmic protons are thought to serve as allosteric modifiers of the exchanger and to trigger its transport through protein conformational change. This effect presupposes an intracellular pH (pHi) dependence of NHE3 activity, although the biophysical and molecular basis of NHE3 pHi sensitivity have not been defined. NHE3, when complexed with the calcineurin homologous protein-1 (CHP1), had a shift in pHi sensitivity (0.4 units) toward the acidic side in comparison with NHE3 alone, as measured by oscillating pH electrodes combined with whole-cell patch clamping. Indeed, CHP1 interaction with NHE3 inhibited NHE3 transport in a pHi -dependent manner. CHP1 binding to NHE3 also affected its acute regulation. Intracellular perfusion of peptide from the CHP1 binding region (or pHi modification to reduce the CHP1 amount bound to NHE3) was permissive and cooperative for dopamine inhibition of NHE3 but reversed that of adenosine. Thus, CHP1 interaction with NHE3 apparently establishes the exchanger set point for pHi, and modification in this set point is effective in the hormonal stimuli-mediated regulation of NHE3. CHP1 may serve as a regulatory cofactor for NHE3 conformational change, dependent on intracellular protonation.

DGKE variants cause a glomerular microangiopathy that mimics membranoproliferative GN.

Renal microangiopathies and membranoproliferative GN (MPGN) can manifest similar clinical presentations and histology, suggesting the possibility of a common underlying mechanism in some cases. Here, we performed homozygosity mapping and whole exome sequencing in a Turkish consanguineous family and identified DGKE gene variants as the cause of a membranoproliferative-like glomerular microangiopathy. Furthermore, we identified two additional DGKE variants in a cohort of 142 unrelated patients diagnosed with membranoproliferative GN. This gene encodes the diacylglycerol kinase DGKε, which is an intracellular lipid kinase that phosphorylates diacylglycerol to phosphatidic acid. Immunofluorescence confocal microscopy demonstrated that mouse and rat Dgkε colocalizes with the podocyte marker WT1 but not with the endothelial marker CD31. Patch-clamp experiments in human embryonic kidney (HEK293) cells showed that DGKε variants affect the intracellular concentration of diacylglycerol. Taken together, these results not only identify a genetic cause of a glomerular microangiopathy but also suggest that the phosphatidylinositol cycle, which requires DGKE, is critical to the normal function of podocytes.

Calcineurin homologous protein: a multifunctional Ca2+-binding protein family.

The calcineurin homologous protein (CHP) belongs to an evolutionarily conserved Ca(2+)-binding protein subfamily. The CHP subfamily is composed of CHP1, CHP2, and CHP3, which in vertebrates share significant homology at the protein level with each other and between other Ca(2+)-binding proteins. The CHP structure consists of two globular domains containing from one to four EF-hand structural motifs (calcium-binding regions composed of two helixes, E and F, joined by a loop), the myristoylation, and nuclear export signals. These structural features are essential for the function of the three members of the CHP subfamily. Indeed, CHP1-CHP3 have multiple and diverse essential functions, ranging from the regulation of the plasma membrane Na(+)/H(+) exchanger protein function, to carrier vesicle trafficking and gene transcription. The diverse functions attributed to the CHP subfamily rendered an understanding of its action highly complex and often controversial. This review provides a comprehensive and organized examination of the properties and physiological roles of the CHP subfamily with a view to revealing a link between CHP diverse functions.

Epithelial-to-mesenchymal transition induces cell cycle arrest and parenchymal damage in renal fibrosis.

Kidney fibrosis is marked by an epithelial-to-mesenchymal transition (EMT) of tubular epithelial cells (TECs). Here we find that, during renal fibrosis, TECs acquire a partial EMT program during which they remain associated with their basement membrane and express markers of both epithelial and mesenchymal cells. The functional consequence of the EMT program during fibrotic injury is an arrest in the G2 phase of the cell cycle and lower expression of several solute and solvent transporters in TECs. We also found that transgenic expression of either Twist1 (encoding twist family bHLH transcription factor 1, known as Twist) or Snai1 (encoding snail family zinc finger 1, known as Snail) expression is sufficient to promote prolonged TGF-ß1-induced G2 arrest of TECs, limiting the cells' potential for repair and regeneration. In mouse models of experimentally induced renal fibrosis, conditional deletion of Twist1 or Snai1 in proximal TECs resulted in inhibition of the EMT program and the maintenance of TEC integrity, while also restoring cell proliferation, dedifferentiation-associated repair and regeneration of the kidney parenchyma and attenuating interstitial fibrosis. Thus, inhibition of the EMT program in TECs during chronic renal injury represents a potential anti-fibrosis therapy.

DINE-1, the highest copy number repeats in Drosophila melanogaster are non-autonomous endonuclease-encoding rolling-circle transposable elements (Helentrons).

BACKGROUND: The Drosophila INterspersed Elements-1 (DINE-1/INE1) transposable elements (TEs) are the most abundant component of the Drosophila melanogaster genome and have been associated with functional gene duplications. DINE-1 TEs do not encode any proteins (non-autonomous) thus are moved by autonomous partners. The identity of the autonomous partners has been a mystery. They have been allied to Helitrons (rolling-circle transposons), MITEs (DNA transposons), and non-LTR retrotransposons by different authors. RESULTS: We report multiple lines of bioinformatic evidence that illustrate the relationship of DINE-1 like TEs to endonuclease-encoding rolling-circle TEs (Helentrons). The structural features of Helentrons are described, which resemble the organization of the non-autonomous partners, but differ significantly from canonical Helitrons. In addition to the presence of an endonuclease domain fused to the Rep/Helicase protein, Helentrons have distinct structural features. Evidence is presented that illustrates that Helentrons are widely distributed in invertebrate, fish, and fungal genomes. We describe an intermediate family from the Phytophthora infestans genome that phylogenetically groups with Helentrons but that displays Helitron structure. In addition, evidence is presented that Helentrons can capture gene fragments in a pattern reminiscent of canonical Helitrons. CONCLUSIONS: We illustrate the relationship of DINE-1 and related TE families to autonomous partners, the Helentrons. These findings will allow their proper classification and enable a more accurate understanding of the contribution of rolling-circle transposition to the birth of new genes, gene networks, and genome composition.