The transport of gases across cell membranes is one of the most fundamental of physiological processes-O2 for oxidative metabolism, CO2 for acid-base balance, and NH3 for waste disposal. CO2 retention and hyperammonemia are key components of diseases that are major public health concerns. The traditional dogma had been that all gases cross all cell membranes by diffusing through membrane lipid. However, some membranes are gas impermeable and require protein 'gas channels' such as the aquaporins AQP1 (abundant in red blood cells) and AQP5 (abundant in airway epithelia) to conduct gases such as CO2 and NH3. Movement of these gases through AQPs results in a disturbance of pH in microdomains around the channels that we can measure using a pH microelectrode. However, the mechanism of gas conduction is poorly understood. Molecular dynamic simulations, measurements of the pH beneath an electrode touching the surface (pHS) of a model spherical cell (Xenopus oocytes), as well as a mathematical model addressing these pHS changes have provided the first insights into CO2 and NH3 movement through channels. However, a fundamental understanding of such movements across cell membranes requires more advanced multi-scale mathematical models (microscopic, mesoscopic, sub-macroscopic and macroscopic) in order to elucidate mechanisms of gas permeation in normal and pathological states. The PIs (Drs. Boron, Somersalo, and Tajkhorshid) propose to combine state-of-the-art molecular dynamic simulations and computational modeling with novel experimental studies to develop a predictive mathematical model for permeation of various gases across diverse cell membranes of different protein composition, based on integration of data from complementary methodologies across a range of spatial and temporal scales. We will run molecular dynamic simulations of NH3 and CO2 passage through wild-type, mutant, chemically modified, and metal-bound aquaporins in Aim 1 to predict single channel permeabilities (microscopic scale) that will inform the modeling in Aim 2 and cell physiology in Aim 3.
In Aim 2, we will create new computational models of gas transport through single and multiple aquaporins in a lipid bilayer (mesoscopic scale), beneath the pHS electrode (sub-macroscopic scale) and in the whole cell (macroscopic scale). Finally in Aim 3, informed by Aims 1 and 2, we will validate the simulations and models in oocytes using electrophysiological and optical methods.

Public Health Relevance

Our computational models of the movement of ammonia and carbon dioxide gases across biological membranes will enhance understanding of the causes, consequences, and treatments of major public health concerns, such as COPD, interstitial lung disease, and ventilatory failure (associated with CO2 retention), and liver failure (hyperammonemia).

National Institute of Health (NIH)
National Institute of General Medical Sciences (NIGMS)
Research Project--Cooperative Agreements (U01)
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Special Emphasis Panel (ZEB1)
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Lyster, Peter
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Case Western Reserve University
Schools of Medicine
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Mahinthichaichan, Paween; Morris, Dylan M; Wang, Yi et al. (2018) Selective Permeability of Carboxysome Shell Pores to Anionic Molecules. J Phys Chem B 122:9110-9118
Mahinthichaichan, Paween; Gennis, Robert B; Tajkhorshid, Emad (2018) Cytochrome aa3 Oxygen Reductase Utilizes the Tunnel Observed in the Crystal Structures To Deliver O2 for Catalysis. Biochemistry 57:2150-2161
Mahinthichaichan, Paween; Gennis, Robert B; Tajkhorshid, Emad (2018) Bacterial denitrifying nitric oxide reductases and aerobic respiratory terminal oxidases use similar delivery pathways for their molecular substrates. Biochim Biophys Acta Bioenerg 1859:712-724
Guo, Yi-Min; Liu, Ying; Liu, Mei et al. (2017) Na+/HCO3-Cotransporter NBCn2 Mediates HCO3-Reclamation in the Apical Membrane of Renal Proximal Tubules. J Am Soc Nephrol 28:2409-2419
Mahinthichaichan, Paween; Gennis, Robert B; Tajkhorshid, Emad (2016) All the O2 Consumed by Thermus thermophilus Cytochrome ba3 Is Delivered to the Active Site through a Long, Open Hydrophobic Tunnel with Entrances within the Lipid Bilayer. Biochemistry 55:1265-78
Wang, Cun; Hu, Honghong; Qin, Xue et al. (2016) Reconstitution of CO2 Regulation of SLAC1 Anion Channel and Function of CO2-Permeable PIP2;1 Aquaporin as CARBONIC ANHYDRASE4 Interactor. Plant Cell 28:568-82
Boedtkjer, Ebbe; Hansen, Kristoffer B; Boedtkjer, Donna M B et al. (2016) Extracellular HCO3- is sensed by mouse cerebral arteries: Regulation of tone by receptor protein tyrosine phosphatase ?. J Cereb Blood Flow Metab 36:965-80
Zhou, Yuehan; Skelton, Lara A; Xu, Lumei et al. (2016) Role of Receptor Protein Tyrosine Phosphatase ? in Sensing Extracellular CO2 and HCO3. J Am Soc Nephrol 27:2616-21
Cooper, Gordon J; Occhipinti, Rossana; Boron, Walter F (2015) Rebuttal from Gordon J. Cooper, Rossana Occhipinti and Walter F. Boron. J Physiol 593:5033
Vaughan-Jones, Richard D; Boron, Walter F (2015) Integration of acid-base and electrolyte disorders. N Engl J Med 372:389

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