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The mechanisms of alkali therapy in targeting renal diseases. Biochem Soc Trans 2023; 51:223-232. [PMID: 36744634 DOI: 10.1042/bst20220690] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/08/2022] [Revised: 12/28/2022] [Accepted: 01/19/2023] [Indexed: 02/07/2023]
Abstract
Chronic kidney disease (CKD) is characterized by progressive reduction in kidney function and treatments aiming at stabilizing or slowing its progression may avoid or delay the necessity of kidney replacement therapy and the increased mortality associated with reduced kidney function. Metabolic acidosis, and less severe stages of the acid stress continuum, are common consequences of CKD and some interventional studies support that its correction slows the progression to end-stage kidney disease. This correction can be achieved with mineral alkali in the form of bicarbonate or citrate salts, ingestion of diets with fewer acid-producing food components or more base-producing food components, or a pharmacological approach. In this mini-review article, we summarize the potential mechanisms involved in the beneficial effects of alkali therapy. We also discuss the perspectives in the field and challenges that must be overcome to advance our understanding of such mechanisms.
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Protons in Gating the Kv1.2 Channel: A Calculated Set of Protonation States in Response to Polarization/Depolarization of the Channel, with the Complete Proposed Proton Path from Voltage Sensing Domain to Gate. MEMBRANES 2022; 12:membranes12070718. [PMID: 35877921 PMCID: PMC9318985 DOI: 10.3390/membranes12070718] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 03/31/2022] [Revised: 07/07/2022] [Accepted: 07/08/2022] [Indexed: 12/04/2022]
Abstract
We have in the past proposed that proton motion constitutes the gating current in the potassium channel Kv1.2 and is responsible for the gating mechanism. For this to happen, there must be a proton path between the voltage-sensing domain (VSD) and the channel gate, and here we present quantum calculations that lead to a specific pair of proton paths, defined at the molecular level, with well-defined water molecule linkages, and with hydrogen bonding between residues; there is also at least one interpath crossover, where protons can switch paths. Quantum calculations on the entire 563-atom system give the complete geometry, the energy, and atomic charges. Calculations show that three specific residues (in the pdb 3Lut numbering, H418, E327, R326), and the T1 intracellular moiety, all of which have been shown experimentally to be involved in gating, would necessarily be protonated or deprotonated in the path between the VSD and the gate. Hydroxyl reorientation of serine and threonine residues are shown to provide a means of adjusting proton directions of motion. In the deprotonated state for K312, a low energy state, our calculations come close to reproducing the X-ray structure. The demonstration of the existence of a double proton path between VSD and gate supports the proposed proton gating mechanism; when combined with our earlier demonstration of proton generation in the VSD, and comparison with other systems that are known to move protons, we are close to achieving the definition of a complete gating mechanism in molecular detail. The coupling of the paths to the VSD, and to the PVPV section that essentially forms the gate, can be easily seen from the results of the calculation. The gate itself remains for further computations.
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Babich V, Vadnagara K, Di Sole F. Adenosine A 2A receptor blocks the A 1 receptor inhibition of renal Na + transport and oxygen consumption. J Cell Physiol 2019; 234:13917-13930. [PMID: 30633335 DOI: 10.1002/jcp.28074] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/19/2018] [Accepted: 12/18/2018] [Indexed: 11/11/2022]
Abstract
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 .
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Affiliation(s)
- Victor Babich
- Department of Physiology and Pharmacology, Des Moines University, Des Moines, Iowa.,School of Liberal Arts and Sciences, Mercy College of Health Sciences, Des Moines, Iowa.,Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas.,Department of Medicine, University of Maryland School of Medicine, Baltimore, Maryland
| | - Komal Vadnagara
- Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas
| | - Francesca Di Sole
- Department of Physiology and Pharmacology, Des Moines University, Des Moines, Iowa.,Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas.,Department of Medicine, University of Maryland School of Medicine, Baltimore, Maryland
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Babich V, Henry MK, Di Sole F. Application of Electrophysiology Measurement to Study the Activity of Electro-Neutral Transporters. J Vis Exp 2018. [PMID: 29443070 DOI: 10.3791/56630] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/31/2022] Open
Abstract
The transport of ions through cell membranes ensures the fine control of ion content within and outside the cell that is indispensable for cell survival. These transport mechanisms are mediated by the activities of specialized transporter proteins. Specifically,pH dynamics are finely controlled by plasma membrane proton (H+) extrusion systems, such as the Na+/H+ exchanger (NHE) protein family. Despite extensive efforts to study the mechanisms underlying NHE regulation, our current understanding of the biophysical and molecular properties of the NHE family is inadequate because of the limited availability of methods to effectively measure NHE activity. In this manuscript, we used H+-selective electrodes during whole-cell patch clamping recording to measure NHE-induced H+ flux. We proposed this approach to overcome some limitations of typically used methods to measure NHE activity, such as radioactive uptake and fluorescent membrane permeants. Measurement of NHE activity using the described method enables high sensitivity and time resolution and more efficient control of intracellular H+ concentrations. H+-selective electrodes are based on the fact that transporter activity creates an ion gradient in close proximity to the cell membrane. An H+-selective electrode moving up to and away from the cell membrane in a repetitive, oscillatory fashion records a voltage difference that is dependent on H+ flux. While H+-selective electrodes are used to detect H+ flux moving out of the cell, the patch clamp method in the whole-cell configuration is used to control the intracellular ion composition. Moreover, application of the giant patch clamp technique allows modification of the intracellular composition of not only ions but also lipids. The transporter activity of NHE isoform 3 (NHE3) was measured using this technical approach to study the molecular basis of NHE3 regulation by phosphoinositides.
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Affiliation(s)
- Victor Babich
- Physiology and Pharmacology Department, Des Moines University; School of Liberal Arts and Sciences, Mercy College of Health Sciences
| | - Matthew K Henry
- Physiology and Pharmacology Department, Des Moines University
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Dominguez Rieg JA, de la Mora Chavez S, Rieg T. Novel developments in differentiating the role of renal and intestinal sodium hydrogen exchanger 3. Am J Physiol Regul Integr Comp Physiol 2016; 311:R1186-R1191. [PMID: 27733387 DOI: 10.1152/ajpregu.00372.2016] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/01/2016] [Revised: 09/22/2016] [Accepted: 10/05/2016] [Indexed: 11/22/2022]
Abstract
The Na+/H+ exchanger isoform 3 (NHE3) facilitates Na+ absorption and H+ secretion and is expressed in the intestine, proximal tubule, and thick ascending limb of the kidney. While the function of NHE3 for Na+ and [Formula: see text](re)absorption has been defined using conventional NHE3 knockout mice (NHE3-/-), the recent generation of conditional NHE3 knockout mice started to give critical new insight into the role of this protein by allowing for temporal and spatial control of NHE3 expression. For example, in contrast to NHE3-/- mice, knockout of NHE3 in the S1 and S2 segments of the proximal tubule or along the entire tubule/collecting duct does not cause any lethality. Nonabsorbable NHE3 inhibitors have been developed, and preclinical as well as clinical trials indicate possible pharmacological use in fluid overload, hypertension, chronic kidney disease, hyperphosphatemia, and constipation. Some of the therapeutic considerations seem to be directly related to the pharmacodynamic properties of these drugs; however, little is known about the effects of these nonabsorbable NHE3 inhibitors on intestinal phosphate transport and the mechanisms so far remain elusive. This review focuses on novel findings of NHE3 in the intestine and the kidney as well as novel drug developments targeting NHE3.
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Affiliation(s)
- Jessica A Dominguez Rieg
- Department of Basic Sciences, Bastyr University California, San Diego, California.,Veterans Affairs San Diego Healthcare System, San Diego, California; and
| | | | - Timo Rieg
- Veterans Affairs San Diego Healthcare System, San Diego, California; and .,Division of Nephrology-Hypertension, Department of Medicine, University of California San Diego, La Jolla, California
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Sodium-Proton (Na+/H+) Antiporters: Properties and Roles in Health and Disease. Met Ions Life Sci 2016; 16:391-458. [DOI: 10.1007/978-3-319-21756-7_12] [Citation(s) in RCA: 56] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/22/2023]
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Allman E, Wang Q, Walker RL, Austen M, Peters MA, Nehrke K. Calcineurin homologous proteins regulate the membrane localization and activity of sodium/proton exchangers in C. elegans. Am J Physiol Cell Physiol 2015; 310:C233-42. [PMID: 26561640 DOI: 10.1152/ajpcell.00291.2015] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2015] [Accepted: 11/09/2015] [Indexed: 11/22/2022]
Abstract
Calcineurin B homologous proteins (CHP) are N-myristoylated, EF-hand Ca(2+)-binding proteins that bind to and regulate Na(+)/H(+) exchangers, which occurs through a variety of mechanisms whose relative significance is incompletely understood. Like mammals, Caenorhabditis elegans has three CHP paralogs, but unlike mammals, worms can survive CHP loss-of-function. However, mutants for the CHP ortholog PBO-1 are unfit, and PBO-1 has been shown to be required for proton signaling by the basolateral Na(+)/H(+) exchanger NHX-7 and for proton-coupled intestinal nutrient uptake by the apical Na(+)/H(+) exchanger NHX-2. Here, we have used this genetic model organism to interrogate PBO-1's mechanism of action. Using fluorescent tags to monitor Na(+)/H(+) exchanger trafficking and localization, we found that loss of either PBO-1 binding or activity caused NHX-7 to accumulate in late endosomes/lysosomes. In contrast, NHX-2 was stabilized at the apical membrane by a nonfunctional PBO-1 protein and was only internalized following its complete loss. Additionally, two pbo-1 paralogs were identified, and their expression patterns were analyzed. One of these contributed to the function of the excretory cell, which acts like a kidney in worms, establishing an alternative model for testing the role of this protein in membrane transporter trafficking and regulation. These results lead us to conclude that the role of CHP in Na(+)/H(+) exchanger regulation differs between apical and basolateral transporters. This further emphasizes the importance of proper targeting of Na(+)/H(+) exchangers and the critical role of CHP family proteins in this process.
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Affiliation(s)
- Erik Allman
- Departments of Pharmacology and Physiology and Medicine, University of Rochester School of Medicine and Dentistry, Rochester, New York; Department of Biochemistry and Molecular Biology, Pennsylvania State University, University Park, Pennsylvania
| | - Qian Wang
- Departments of Pharmacology and Physiology and Medicine, University of Rochester School of Medicine and Dentistry, Rochester, New York
| | - Rachel L Walker
- Departments of Pharmacology and Physiology and Medicine, University of Rochester School of Medicine and Dentistry, Rochester, New York
| | - Molly Austen
- Departments of Pharmacology and Physiology and Medicine, University of Rochester School of Medicine and Dentistry, Rochester, New York
| | | | - Keith Nehrke
- Departments of Pharmacology and Physiology and Medicine, University of Rochester School of Medicine and Dentistry, Rochester, New York;
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Babich V, Vadnagara K, Di Sole F. Dual Effect of Adenosine A1Receptor Activation on Renal O2Consumption. J Cell Physiol 2015; 230:3093-104. [DOI: 10.1002/jcp.25050] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/28/2014] [Accepted: 05/18/2015] [Indexed: 12/22/2022]
Affiliation(s)
- Victor Babich
- Department of Medicine; University of Maryland School of Medicine; Maryland
- Department of Internal Medicine; University of Texas Southwestern Medical Center; Dallas Texas
- Physiology and Pharmacology Department; Des Moines University; Iowa
| | - Komal Vadnagara
- Department of Internal Medicine; University of Texas Southwestern Medical Center; Dallas Texas
| | - Francesca Di Sole
- Department of Medicine; University of Maryland School of Medicine; Maryland
- Department of Internal Medicine; University of Texas Southwestern Medical Center; Dallas Texas
- Physiology and Pharmacology Department; Des Moines University; Iowa
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Babich V, Di Sole F. The Na+/H+ Exchanger-3 (NHE3) Activity Requires Ezrin Binding to Phosphoinositide and Its Phosphorylation. PLoS One 2015; 10:e0129306. [PMID: 26042733 PMCID: PMC4455992 DOI: 10.1371/journal.pone.0129306] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/09/2015] [Accepted: 05/08/2015] [Indexed: 11/19/2022] Open
Abstract
Na+/H+ exchanger-3 (NHE3) plays an essential role in maintaining sodium and fluid homeostasis in the intestine and kidney epithelium. Thus, NHE3 is highly regulated and its function depends on binding to multiple regulatory proteins. Ezrin complexed with NHE3 affects its activity via not well-defined mechanisms. This study investigates mechanisms by which ezrin regulates NHE3 activity in epithelial Opossum Kidney cells. Ezrin is activated sequentially by phosphatidylinositol-4,5-bisphosphate (PIP2) binding and phosphorylation of threonine 567. Expression of ezrin lacking PIP2 binding sites inhibited NHE3 activity (-40%) indicating that ezrin binding to PIP2 is required for preserving NHE3 activity. Expression of a phosphomimetic ezrin mutated at the PIP2 binding region was sufficient not only to reverse NHE3 activity to control levels but also to increase its activity (+80%) similar to that of the expression of ezrin carrying the phosphomimetic mutation alone. Calcineurin Homologous Protein-1 (CHP1) is part, with ezrin, of the NHE3 regulatory complex. CHP1-mediated activation of NHE3 activity was blocked by expression of an ezrin variant that could not be phosphorylated but not by an ezrin variant unable to bind PIP2. Thus, for NHE3 activity under baseline conditions not only ezrin phosphorylation, but also ezrin spatial-temporal targeting on the plasma membrane via PIP2 binding is required; however, phosphorylation of ezrin appears to overcome the control of NHE3 transport. CHP1 action on NHE3 activity is not contingent on ezrin binding to PIP2 but rather on ezrin phosphorylation. These findings are important in understanding the interrelation and dynamics of a CHP1-ezrin-NHE3 regulatory complex.
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Affiliation(s)
- Victor Babich
- Department of Medicine, University of Maryland School of Medicine, Baltimore, Maryland, United States of America
- Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas, United States of America
| | - Francesca Di Sole
- Department of Medicine, University of Maryland School of Medicine, Baltimore, Maryland, United States of America
- Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas, United States of America
- Physiology and Pharmacology Department, Des Moines University, Iowa, United States of America
- * E-mail:
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Ho CY, Choy CH, Wattson CA, Johnson DE, Botelho RJ. The Fab1/PIKfyve phosphoinositide phosphate kinase is not necessary to maintain the pH of lysosomes and of the yeast vacuole. J Biol Chem 2015; 290:9919-28. [PMID: 25713145 DOI: 10.1074/jbc.m114.613984] [Citation(s) in RCA: 42] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/23/2014] [Indexed: 12/30/2022] Open
Abstract
Lysosomes and the yeast vacuole are degradative and acidic organelles. Phosphatidylinositol 3,5-bisphosphate (PtdIns(3,5)P2), a master architect of endolysosome and vacuole identity, is thought to be necessary for vacuolar acidification in yeast. There is also evidence that PtdIns(3,5)P2 may play a role in lysosomal acidification in higher eukaryotes. Nevertheless, these conclusions rely on qualitative assays of lysosome/vacuole pH. For example, quinacrine, an acidotropic fluorescent base, does not accumulate in the vacuoles of fab1Δ yeast. Fab1, along with its mammalian ortholog PIKfyve, is the lipid kinase responsible for synthesizing PtdIns(3,5)P2. In this study, we employed several assays that quantitatively assessed the lysosomal and vacuolar pH in PtdIns(3,5)P2-depleted cells. Using ratiometric imaging, we conclude that lysosomes retain a pH < 5 in PIKfyve-inhibited mammalian cells. In addition, quantitative fluorescence microscopy of vacuole-targeted pHluorin, a pH-sensitive GFP variant, indicates that fab1Δ vacuoles are as acidic as wild-type yeast. Importantly, we also employed fluorimetry of vacuoles loaded with cDCFDA, a pH-sensitive dye, to show that both wild-type and fab1Δ vacuoles have a pH < 5.0. In comparison, the vacuolar pH of the V-ATPase mutant vph1Δ or vph1Δ fab1Δ double mutant was 6.1. Although the steady-state vacuolar pH is not affected by PtdIns(3,5)P2 depletion, it may have a role in stabilizing the vacuolar pH during salt shock. Overall, we propose a model in which PtdIns(3,5)P2 does not govern the steady-state pH of vacuoles or lysosomes.
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Affiliation(s)
- Cheuk Y Ho
- From the Department of Chemistry and Biology and the Molecular Science Program, Ryerson University, Toronto, Ontario M5B2K3, Canada and
| | - Christopher H Choy
- From the Department of Chemistry and Biology and the Molecular Science Program, Ryerson University, Toronto, Ontario M5B2K3, Canada and
| | | | - Danielle E Johnson
- the Program in Cell Biology, Hospital for Sick Children, Toronto, Ontario M5G1X8, Canada
| | - Roberto J Botelho
- From the Department of Chemistry and Biology and the Molecular Science Program, Ryerson University, Toronto, Ontario M5B2K3, Canada and the Program in Cell Biology, Hospital for Sick Children, Toronto, Ontario M5G1X8, Canada
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