Adhesion-GPCR Gpr116 (ADGRF5) expression inhibits renal acid secretion

The diversity and near universal expression of G protein-coupled receptors (GPCR) reflects their involvement in most physiological processes. The GPCR superfamily is the largest in the human genome, and GPCRs are common pharmaceutical targets. Therefore, uncovering the function of understudied GPCRs provides a wealth of untapped therapeutic potential. We previously identified an adhesion-class GPCR, Gpr116, as one of the most abundant GPCRs in the kidney. Here, we show that Gpr116 is highly expressed in specialized acid-secreting A-intercalated cells (A-ICs) in the kidney using both imaging and functional studies, and we demonstrate in situ receptor activation using a synthetic agonist peptide unique to Gpr116. Kidney-specific knockout (KO) of Gpr116 caused a significant reduction in urine pH (i.e., acidification) accompanied by an increase in blood pH and a decrease in pCO2 compared to WT littermates. Additionally, immunogold electron microscopy shows a greater accumulation of V-ATPase proton pumps at the apical surface of A-ICs in KO mice compared to controls. Furthermore, pretreatment of split-open collecting ducts with the synthetic agonist peptide significantly inhibits proton flux in ICs. These data suggest a tonic inhibitory role for Gpr116 in the regulation of V-ATPase trafficking and urinary acidification. Thus, the absence of Gpr116 results in a primary excretion of acid in KO mouse urine, leading to mild metabolic alkalosis ("renal tubular alkalosis"). In conclusion, we have uncovered a significant role for Gpr116 in kidney physiology, which may further inform studies in other organ systems that express this GPCR, such as the lung, testes, and small intestine.

PF-06869206 is a selective inhibitor of renal Pi transport: evidence from in vitro and in vivo studies

Plasma phosphate (P_i) levels are tightly controlled, and elevated plasma P_i levels are associated with an increased risk of cardiovascular complications and death. Two renal transport proteins mediate the majority of P_i reabsorption: Na⁺-phosphate cotransporters Npt2a and Npt2c, with Npt2a accounting for 70-80% of P_i reabsorption. The aim of the present study was to determine the in vitro effects of a novel Npt2a inhibitor (PF-06869206) in opossum kidney (OK) cells as well as determine its selectivity in vivo in Npt2a knockout (Npt2a^-/-) mice. In OK cells, Npt2a inhibitor caused dose-dependent reductions of Na⁺-dependent P_i uptake (IC₅₀: ~1.4 μmol/L), whereas the unselective Npt2 inhibitor phosphonoformic acid (PFA) resulted in an ~20% stronger inhibition of P_i uptake. The dose-dependent inhibitory effects were present after 24 h of incubation with both low- and high-P_i media. Michaelis-Menten kinetics in OK cells identified an ~2.4-fold higher K_m for P_i in response to Npt2a inhibition with no significant change in apparent V_max. Higher parathyroid hormone concentrations decreased P_i uptake equivalent to the maximal inhibitory effect of Npt2a inhibitor. In vivo, the Npt2a inhibitor induced a dose-dependent increase in urinary P_i excretion in wild-type mice (ED₅₀: ~23 mg/kg), which was completely absent in Npt2a^-/- mice, alongside a lack of decrease in plasma P_i. Of note, the Npt2a inhibitor-induced dose-dependent increase in urinary Na⁺ excretion was still present in Npt2a^-/- mice, a response possibly mediated by an off-target acute inhibitory effect of the Npt2a inhibitor on open probability of the epithelial Na⁺ channel in the cortical collecting duct.

Essential role of Kir5.1 channels in renal salt handling and blood pressure control

Supplementing diets with high potassium helps reduce hypertension in humans. Inwardly rectifying K+ channels Kir4.1 (Kcnj10) and Kir5.1 (Kcnj16) are highly expressed in the basolateral membrane of distal renal tubules and contribute to Na+ reabsorption and K+ secretion through the direct control of transepithelial voltage. To define the importance of Kir5.1 in blood pressure control under conditions of salt-induced hypertension, we generated a Kcnj16 knockout in Dahl salt-sensitive (SS) rats (SSKcnj16-/-). SSKcnj16-/- rats exhibited hypokalemia and reduced blood pressure, and when fed a high-salt diet (4% NaCl), experienced 100% mortality within a few days triggered by salt wasting and severe hypokalemia. Electrophysiological recordings of basolateral K+ channels in the collecting ducts isolated from SSKcnj16-/- rats revealed activity of only homomeric Kir4.1 channels. Kir4.1 expression was upregulated in SSKcnj16-/- rats, but the protein was predominantly localized in the cytosol in SSKcnj16-/- rats. Benzamil, but not hydrochlorothiazide or furosemide, rescued this phenotype from mortality on a high-salt diet. Supplementation of high-salt diet with increased potassium (2% KCl) prevented mortality in SSKcnj16-/- rats and prevented or mitigated hypertension in SSKcnj16-/- or control SS rats, respectively. Our results demonstrate that Kir5.1 channels are key regulators of renal salt handling in SS hypertension.

Collecting duct prorenin receptor knockout reduces renal function, increases sodium excretion, and mitigates renal responses in ANG II-induced hypertensive mice

Augmented intratubular angiotensin (ANG) II is a key determinant of enhanced distal Na⁺ reabsorption via activation of epithelial Na⁺ channels (ENaC) and other transporters, which leads to the development of high blood pressure (BP). In ANG II-induced hypertension, there is increased expression of the prorenin receptor (PRR) in the collecting duct (CD), which has been implicated in the stimulation of the sodium transporters and resultant hypertension. The impact of PRR deletion along the nephron on BP regulation and Na⁺ handling remains controversial. In the present study, we investigate the role of PRR in the regulation of renal function and BP by using a mouse model with specific deletion of PRR in the CD (_CDPRR-KO). At basal conditions, _CDPRR-KO mice had decreased renal function and lower systolic BP associated with higher fractional Na⁺ excretion and lower ANG II levels in urine. After 14 days of ANG II infusion (400 ng·kg^-1·min^-1), the increases in systolic BP and diastolic BP were mitigated in _CDPRR-KO mice. _CDPRR-KO mice had lower abundance of cleaved αENaC and γENaC, as well as lower ANG II and renin content in urine compared with wild-type mice. In isolated CD from _CDPRR-KO mice, patch-clamp studies demonstrated that ANG II-dependent stimulation of ENaC activity was reduced because of fewer active channels and lower open probability. These data indicate that CD PRR contributes to renal function and BP responses during chronic ANG II infusion by enhancing renin activity, increasing ANG II, and activating ENaC in the distal nephron segments.

Sustained Elevated Adenosine via ADORA2B Promotes Chronic Pain through Neuro-immune Interaction

The molecular mechanisms of chronic pain are poorly understood and effective mechanism-based treatments are lacking. Here, we report that mice lacking adenosine deaminase (ADA), an enzyme necessary for the breakdown of adenosine, displayed unexpected chronic mechanical and thermal hypersensitivity due to sustained elevated circulating adenosine. Extending from Ada(-/-) mice, we further discovered that prolonged elevated adenosine contributed to chronic pain behaviors in two additional independent animal models: sickle cell disease mice, a model of severe pain with limited treatment, and complete Freund's adjuvant paw-injected mice, a well-accepted inflammatory model of chronic pain. Mechanistically, we revealed that activation of adenosine A2B receptors on myeloid cells caused nociceptor hyperexcitability and promoted chronic pain via soluble IL-6 receptor trans-signaling, and our findings determined that prolonged accumulated circulating adenosine contributes to chronic pain by promoting immune-neuronal interaction and revealed multiple therapeutic targets.

Abstract 021: Nephron Specific Deletion of the Prorenin Receptor Modulates Blood Pressure and Urinary Na+ Excretion

The nephron prorenin receptor (PRR) may modulate blood pressure (BP) and Na+ balance. Since previous models of PRR knockout (KO) mice had early lethality and/or structural defects, we developed an inducible nephron-wide PRR KO using the Pax8/LC1 transgenes. Disruption of nephron PRR at 1 month of age caused no renal histological abnormalities. On a normal Na+ diet, wild-type (WT) and PRR KO mice had similar BP and Na+ excretion. However, PRR KO mice had elevated PRC (KO- 377 ± 77 vs WT- 127 ± 19 ng Ang-I/ml/hr) and a 50% decrease in renal ENaC-α protein. Protein levels of NHE3, NKCC2, NCC and ENaC-β/γ were similar between the two groups. Treatment with mouse prorenin (10 nM for 30 min) increased ENaC channel number by 2-fold, but not open probability, in isolated split-open cortical collecting ducts (CCD) from WT mice; this was prevented by Akt inhibition (A6730) but unaffected by blockade of AT-1 (losartan), ERK1/2 (U0126) or p38 MAPK (SB203580). Addition of prorenin (10 nM) did not change isolated CCD [Ca2+]i as assessed by Fura-2 loading (10 min exposure with readings every 3 sec). On a low Na+ diet, PRR KO mice had increased Na+ excretion (Day 2: KO - 66 ± 11 vs WT- 42 ± 6 μmol/day; Day 6: KO - 39 ± 4 vs ET- 23 ± 4 μmol/day) however, no differences in BP were observed. PRC was elevated in PRR KO mice on a low Na+ diet (KO- 384 ± 40 vs WT-174 ± 12 ng/ Ang-I/ml/hr). PRR KO mice had an attenuated hypertensive response to Angiotensin-II (Ang-II) infusion at 600 ng/Kg/min for 2 weeks (MAP: KO - 117 ± 4 vs WT - 133 ± 4 mm Hg over the course of Ang-II infusion). Urinary Na+ excretion was elevated in Ang-II treated PRR KO mice as compared to WT mice (KO-344 ± 14 vs WT-268 ±30 μmol/day). Taken together, these data indicate that nephron PRR, likely via direct prorenin/renin stimulation of an Akt-dependent pathway, stimulates CCD ENaC activity. Absence of nephron PRR promotes Na+ wasting and reduces the hypertensive response to Ang-II.

Af17 deficiency increases sodium excretion and decreases blood pressure

The putative transcription factor AF17 upregulates the transcription of the epithelial sodium channel (ENaC) genes, but whether AF17 modulates sodium homeostasis and BP is unknown. Here, we generated Af17-deficient mice to determine whether deletion of Af17 leads to sodium wasting and low BP. Compared with wild-type mice, Af17-deficient mice had lower BP (11 mmHg), higher urine volume, and increased sodium excretion despite mildly increased plasma concentrations of aldosterone. Deletion of Af17 led to increased dimethylation of histone H3 K79 and reduced ENaC function. The attenuated function of ENaC resulted from decreased ENaC mRNA and protein expression, fewer active channels, lower open probability, and reduced effective activity. In contrast, inducing high levels of plasma aldosterone by a variety of methods completely compensated for Af17 deficiency with respect to sodium handling and BP. Taken together, these data identify Af17 as a potential locus for the maintenance of sodium and BP homeostasis and suggest that a particular histone modification is directly linked to these processes. Af17-mediated regulation of BP is largely, but not exclusively, the result of modulating ENaC, suggesting it has potential as a therapeutic target for the control of BP.

Ras couples phosphoinositide 3-OH kinase to the epithelial Na+ channel

Aldosterone induces the expression of the small G protein K-Ras. Both K-Ras and its 1st effector phosphoinositide 3-OH kinase (PI3-K) are necessary and sufficient for the activation of ENaC increasing channel open probability. The cell signaling mechanism by which K-Ras enhances ENaC activity, however, is uncertain. We demonstrate here that K-Ras significantly activates human ENaC reconstituted in Chinese hamster ovary cells approximately 3-fold. Activation in response to K-Ras was sensitive to the irreversible PI3-K inhibitor wortmannin but not the competitive LY294002 inhibitor of this phospholipid kinase. Similarly, a PI3-K 1st effector-specific Ras mutant (G12:C40) enhanced ENaC activity in a wortmannin but not LY294002 sensitive manner. Constitutively active PI3-K also enhanced ENaC activity but in a wortmannin and LY294002 sensitive manner with the effects of PI3-K and K-Ras not being additive. The activation of ENaC by PI3-K was also sensitive to intracellular GDPbetaS. Constitutively active PI3-K that is incapable of interacting with K-Ras (K227E p110alpha) acted as dominant negative with respect to the regulation of ENaC even in the presence of K-Ras. K-Ras is known to directly interact with PI3-K with aldosterone promoting this interaction. Here we demonstrate that K-Ras also interacts with ENaC through an, as yet, undetermined mechanism. We conclude that K-Ras enhances ENaC activity by localizing PI3-K near the channel and stimulating of PI3-K activity.

Acetylcholine-induced calcium signalling in adrenaline- and noradrenaline-containing adrenal chromaffin cells

Adrenal chromaffin cells secrete catecholamines in response to cholinergic receptor activation by acetylcholine (ACh). Characteristics of Ca(2+) transients induced by activation of nicotinic (nAChRs) and muscarinic (mAChRs) receptors were analyzed using Fura-2 fluorescent measurements on rat chromaffin cells. We first found two populations of chromaffin cells, which differently responded on AChR stimulation. In the first group (n-cells), consecutive ACh applications evoked persistent Ca(2+) transients, whereas desensitizing transients were observed in the other group (m-cells). The AChR agonists and antagonists precisely imitated or abolished the ACh action on n- and m-type cells, respectively. Cytochemical staining showed that n-cells contained adrenaline, whereas m-cells-noradrenaline. Thus, for the first time we found that nAChRs and mAChRs are differentially expressed in adrenergic and noradrenergic chromaffin cells, respectively. Our data suppose that chromaffin cells can be differentially regulated by incoming ACh signals and in such way release different substances-adrenaline and noradrenaline.