The renal proximal tubule (PT) reabsorbs ~80% of the HCO-3 filtered at the glomerulus. Previous work on isolated perfused rabbit and mouse PTs shows that the rate of HCO-3 reabsorption (JHCO3) markedly rises with increases in basolateral [CO2] ([CO2]BL) or decreases in [HCO-3]BL, but is not altered by changes in pHBL. Thus, the PT must have a mechanism for sensing CO2/HCO-3. The CO2-evoked increase in JHCO3 is blocked by antagonists of apical ANG II AT1a receptors and by specific inhibitors of the ErbB family of receptor tyrosine kinases (e.g., ErbB1). We were intrigued to learn of an orphaned receptor protein tyrosine phosphatase (RPTP3) with a putative extracellular ligand binding domains that strongly resembles carbonic anhydrase (CA). Preliminary data suggest that knocking out RPTP3 eliminates the JHCO3 response to both ?[CO2]BL and ?[HCO-3]BL. The three aims are a multidisciplinary approach to address, at three levels of integration, how the PT senses ?[CO2]BL and alters JHCO3: (1) Perfused PTs. What is the role of RPTP3? Do elevations in [CO2]BL enhance the response to added luminal ANG II or ANG-(1-7)? Does chronic acidosis enhance the CO2-evoked increase in JHCO3? And are AT1a-null and RPTP3-null mice more sensitive to chronic acidosis? (2) Biochemistry in PT suspensions. Does CO2/HCO-3 produce a unique RPTP3-dependent phosphotyrosine fingerprint on ErbB1? Can a proteomic approach identify downstream - targets of CO2/RPTP3/ErbB1? (3) The RPTP3 molecule. Is it CO2 or HCO3 that activates the phosphatase activity of RPTP3? Where is RPTP3 located? Can mutations in the CA- like domain of RPTP3 generate CA activity? Can a CA inhibitor bind to the CA-like domain? What is the crystal structure of the CA-like domain? The proposed work will illuminate a highly novel mechanism for sensing CO2 and or HCO-3-independent of changes in pH-and could have important implications for clinical approaches to acidosis, hypertension, and cancer.

Public Health Relevance

The acid-base status of the body is predominantly maintained by secretion of bodily H+ (acid) across the proximal tubule (PT) cells of the kidney into fluid that is eventually excreted as urine. Acid secretion by PT cells is not directly stimulated by increased body acid per se-but by the ratio of body CO2/HCO-3 (the components of soda water), which itself is acid-sensitive. The goals of this project are to identify the CO2-sensing protein in the PT and its downstream targets, and to understand the highly novel mechanism by which increased CO2 increases acid secretion. The proposed work could have important implications for clinical approaches to acidosis, hypertension, and cancer.

Agency
National Institute of Health (NIH)
Institute
National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
Type
Research Project (R01)
Project #
5R01DK081567-04
Application #
8272680
Study Section
Cellular and Molecular Biology of the Kidney Study Section (CMBK)
Program Officer
Ketchum, Christian J
Project Start
2009-05-04
Project End
2014-04-30
Budget Start
2012-05-01
Budget End
2013-04-30
Support Year
4
Fiscal Year
2012
Total Cost
$497,728
Indirect Cost
$180,704
Name
Case Western Reserve University
Department
Physiology
Type
Schools of Medicine
DUNS #
077758407
City
Cleveland
State
OH
Country
United States
Zip Code
44106
Boedtkjer, Ebbe; Hansen, Kristoffer B; Boedtkjer, Donna Mb et al. (2016) Extracellular HCO3- is sensed by mouse cerebral arteries: Regulation of tone by receptor protein tyrosine phosphatase γ. J Cereb Blood Flow Metab 36:965-80
Zhou, Yuehan; Skelton, Lara A; Xu, Lumei et al. (2016) Role of Receptor Protein Tyrosine Phosphatase γ in Sensing Extracellular CO2 and HCO3. J Am Soc Nephrol 27:2616-21
Occhipinti, Rossana; Boron, Walter F (2015) Mathematical modeling of acid-base physiology. Prog Biophys Mol Biol 117:43-58
Skelton, Lara A; Boron, Walter F (2015) Effect of acute acid-base disturbances on the phosphorylation of phospholipase C-γ1 and Erk1/2 in the renal proximal tubule. Physiol Rep 3:
Schmandt, Nicolaus; Velisetty, Phanindra; Chalamalasetti, Sreevatsa V et al. (2015) A chimeric prokaryotic pentameric ligand-gated channel reveals distinct pathways of activation. J Gen Physiol 146:323-40
Salameh, Ahlam Ibrahim; Ruffin, Vernon A; Boron, Walter F (2014) Effects of metabolic acidosis on intracellular pH responses in multiple cell types. Am J Physiol Regul Integr Comp Physiol 307:R1413-27
Occhipinti, Rossana; Musa-Aziz, Raif; Boron, Walter F (2014) Evidence from mathematical modeling that carbonic anhydrase II and IV enhance CO2 fluxes across Xenopus oocyte plasma membranes. Am J Physiol Cell Physiol 307:C841-58
Musa-Aziz, Raif; Occhipinti, Rossana; Boron, Walter F (2014) Evidence from simultaneous intracellular- and surface-pH transients that carbonic anhydrase IV enhances CO2 fluxes across Xenopus oocyte plasma membranes. Am J Physiol Cell Physiol 307:C814-40
Parker, Mark D; Boron, Walter F (2013) The divergence, actions, roles, and relatives of sodium-coupled bicarbonate transporters. Physiol Rev 93:803-959
Geyer, R Ryan; Musa-Aziz, Raif; Enkavi, Giray et al. (2013) Movement of NH₃ through the human urea transporter B: a new gas channel. Am J Physiol Renal Physiol 304:F1447-57

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