Receptor Protein Tyrosine Phosphatase γ, CO2 Sensing in Proximal Tubule and Acid Base Homeostasis
Authors: Manoocher Soleimani
Journal: Journal of the American Society of Nephrology (2016)

The proper balance between acids and bases in the circulation is essential for systemic pH homeostasis, which, in turn, plays a critical role in the operation of biologic systems, ranging from cellular enzymes to chemical reactions, and encompassing every possible tissue or organ, including cardiac cells, neurons, myocytes, etc. The kidney plays an essential role in systemic pH homeostasis by completely reabsorbing all of the filtered bicarbonate (HCO3−), with approximately 85%–90% of this function being accomplished in the proximal tubule. How the proximal tubule senses acute changes in systemic carbon dioxide (CO2) or HCO3− and how that affects its HCO3− reabsorption capabilities are poorly characterized. In this issue of the Journal of the American Society of Nephrology, Zhou et al. demonstrate that deletion of the receptor protein tyrosine phosphatase-γ (RPTPγ) abolishes the effect of CO2 and HCO3− alteration on HCO3−–absorbing ability (JHCO3−) in the proximal tubule. They also demonstrate that mice lacking RPTPγ are unable to recover from systemic acidosis after exposure to a systemic acid load. These results suggest that RPTPγ is likely a novel extracellular CO2/HCO3− sensor critical for pH homeostasis.The key player in pH homeostasis is ubiquitous carbonic anhydrase, which produces instantaneous equilibrium of carbon dioxide (CO2) and carbonic acid (H2CO3) in biologic systems, causing rapid dissociation into acid (H+) and bicarbonate (HCO3−) according to the following reactions:Regulation of systemic (blood) pH in mammals is dependent on the balance between the concentration of H2CO3 and HCO3− ions (above reaction) according to the Henderson–Hasselbalch equation: pH=6.1+ log ([HCO3−]/[0.03×pCO2]). As noted, the ratio of HCO3− to H2CO3 determines the systemic pH, which can alter in response to changes in CO2 or HCO3− levels. The body will maintain the systemic pH in a narrow physiologic range by removing CO2 through respiration and reabsorbing the filtered HCO3− and excreting excess H+via the kidney. The kidney filters approximately 4200–4500 meq of HCO3−, but is able to reabsorb all of the filtered HCO3− under baseline conditions by secreting equal amounts of H+ into the tubular fluid.1 The majority of filtered HCO3− is reabsorbed in the kidney proximal tubule by H+ secretion across the luminal membrane via the sodium (Na+)/hydrogen (H+) exchanger 3 (NHE-3; SLC9A3) and H+ ATPase, working in tandem with the basolateral Na+:HCO3− cotransporter, SLC4A4 (NBC-1; NBC-e1), which cotransports three HCO3− ions for each Na+ ion.1–4 A number of mutations in the C- and N-termini of NBC-e1 impair its function, resulting in HCO3− wasting and proximal renal tubular acidosis in humans.5–7 Genetic deletion of NBC-e1 in mice causes a similar phenotype, with very low plasma HCO3− and arterial pH. While there are no known loss-of-function mutations for NHE-3, the inactivation of NHE-3 in mouse proximal tubule causes HCO3− wasting and metabolic acidosis.8,9 Taken together, these studies demonstrate that intact and functional NHE-3 and NBC-e1 are crucial for HCO3− reabsorption in the proximal tubule and systemic acid-base homeostasis.1–9 In order to prevent dramatic changes in intra- or extracellular pH, cells must be capable of sensing and responding to the levels of CO2, HCO3−, and H+ in their surrounding environment. These sensors can trigger appropriate responses in their vicinity, such as the modulation of signaling molecules/enzymes, alteration in membrane potential, or modification of the phosphorylation state of transporters, with the ultimate goal of altering the activity of acid-base transporters and maintaining the ratio of H2CO3 and HCO3−, hence preventing dramatic changes in pH. Sensing pH There are several bio-sensing molecules expressed in various tissues, including kidney cells, that sense changes in pH, CO2, or HCO3−. Sensing the systemic pH (or H+ concentration in the extracellular space) is predominantly mediated via G protein-coupled receptors (GPCRs) or H+-sensitive ion channels. At least three GPCR molecules are activated by acidic pH (increased H+ ion concentration) and may in turn activate certain signaling molecules and transporters.10 These include: (1) ovarian cancer GPCR 1 (OGR1, also known as GPR68), (2) GPR4 (also known as GPRC6.1), and (3) T cell death–associated gene (8TDAG8, GPR65). In addition to the H+-sensing GPCRs, systemic acidity is also sensed via two H+-sensitive ion channels: transient receptor potential channels and H+-sensing ion channels.10 Although some of these molecules are expressed in the kidney, none seems to be a pH sensor in the proximal tubule. Interestingly, in addition to the extracellular (systemic) pH sensors (above), kidney proximal tubule cells express Pyk2, which is located in the cytoplasm, activated by intracellular acidosis, and consequently stimulates the apical Na+/H+ exchanger NHE-3 (SLC9A3) and the basolateral Na+:HCO3− cotransporter 1 NBC1 (SLC4A4), with the net effect of enhancing H+ secretion into the lumen and absorbing HCO3−.11 Pyk2 is a member of the focal adhesion kinase family of tyrosine kinases and its activation is followed by the activation of tyrosine kinase c-Src.11,12 Sensing CO2/HCO3− There are very few CO2/HCO3−-sensing molecules that are also expressed in the kidney. One notable molecule is the soluble adenylyl cyclase (sAC; also known as soluble adenyl cyclase 10, ADCY10, or Sacy), which is primarily expressed in the cytoplasm but is also found in the organelles of cells. sAC is a potential HCO3− sensor and is a source of cAMP, which affects several ion-transporting processes along the length of the nephron.6 In the kidney, sAC is primarily expressed in cells of the thick ascending loop of Henle, the distal tubule, and the collecting duct.10,12 Its localization in the proximal tubule remains controversial. An interaction between sAC and Pyk2 has been proposed, which could indicate a potential role for sAC in regulating NHE-3 and NBC-1 in the proximal tubule. Additional studies are needed to firmly establish the role of sAC as a candidate to integrate changes in systemic pH alterations with ion transporters in the kidney proximal tubule. In mammals, CO2 is sensed in chemoreceptors and kidney, respiratory, and several other tissues. The difficulties associated with differentiating between the direct effects of CO2 and the effects of pH and/or HCO3− may have complicated the design of experiments to test CO2 sensing by kidney cells. The use of out-of-equilibrium CO2/HCO3− solutions, established in the laboratory of the senior author of the current studies, is one of the few ways in which this issue has been examined.13,14 Using this approach, inhibitors of tyrosine kinases were found to promote H+ secretion/HCO3− reabsorption in response to basolateral (blood) CO2 in the kidney proximal tubules.14 These experiments suggest that molecules distinct from sAC or the Pyk2/c-Src pathway are responsible for activating apical H+ secretion and HCO3− reabsorption in response to basolateral CO2. No CO2 chemosensor molecule has been unambiguously identified in kidney proximal tubule cells. In the present studies, Zhou et al. demonstrate the localization of RPTPγ on the blood-facing domain of the basolateral membranes of proximal convoluted tubule cells.15 In isolated proximal tubules from RPTPγ-knockout (KO) mice, the authors found that the dependence of JHCO3− on basolateral CO2 concentration was abolished; however, the effect of basolateral HCO3− on JHCO3− remained comparable in wild-type and mutant mice.15 The authors concluded that RPTPγ appears to be a novel extracellular CO2/HCO3− sensor in proximal tubule cells and may be critical for regulating HCO3− absorption and pH homeostasis.15 In addition to the impaired ability to sense basolateral CO2 alteration in proximal tubule cells, RPTPγ-KO mice displayed an inability to recover from ammonium chloride-induced metabolic acidosis.15 Indeed, blood HCO3− and pH in RPTPγ-KO mice were significantly lower versus baseline conditions after exposure to ammonium chloride acid load, and in comparison to wild-type mice, which completely recovered from systemic acidosis after 7 days of acid load.15 Intriguingly, RPTPγ -KO mice displayed significant downregulation of apical Na+/H+ exchanger NHE-3 expression under baseline conditions.15 This raises the possibility that RPTPγ can function both as a CO2 sensor and as a regulator of NHE-3, the main HCO3−-absorbing transporter in the kidney proximal tubule. Taken together, these results may suggest that RPTPγ has two distinct functions. In acute states, it works as a basolateral CO2 sensor; whereas, in chronic states, RPTPγ may have a more complicated effect on the transcription of several ion transporters on the apical membrane of the proximal tubule, including NHE-3. Whether the CO2-sensing properties of RPTPγ may ultimately impact NHE-3 expression via the regulation of intracellular signals, such as Pyk2 or other molecules, is intriguing and worthy of experimentation. Additional studies examining the effect of inducible deletion of RPTPγ in the proximal tubule on Pyk2/c-Src pathway and/or NHE-3 expression/activity could provide critical information into possible links between the CO2-sensing mechanism of RPTPγ, metabolic acidosis, and acid-base homeostasis. Disclosures None.