Chemistry and Mathematics (minor). 1991. Virginia Commonwealth University, Richmond, VA.
Physiology. 1997. Medical College of Virginia, Richmond, VA.
Department of Pharmacology and Cancer Biology. 1998. Duke University Medical Center. Durham, NC.
Cystic Fibrosis/Pulmonary Research & Treatment Center. 1999. University of North Carolina, Chapel Hill, NC.
Research Assistant Professor of Medicine
Laboratory: Rm. 6-021 Thurston-Bowles
The proper hydration of the airway surfaces is critical for lung health. For instance, in cystic fibrosis (CF) lung disease, defects in CFTR-mediated epithelial Cl--transport result in a net excess of epithelial salt (NaCl) and H20 absorption, which deplete the airways surface liquid (ASL) layer resulting in dehydrated mucus, inefficient mucociliary clearance (MCC), and progressive deterioration of lung function that accounts for 95% of the morbidity and mortality of CF patients.
My research interests are focused on ion transport occurring at the cellular and single protein levels. My laboratory uses highly sensitive electrophysiological techniques (patch-clamp) to characterize the biophysical and pharmacological properties of the epithelial Na+-channel (ENaC), a protein that is important in the airways for lung liquid clearance at birth, and later, for setting the ASL height necessary for optimal mucus fluidity and efficient MCC.
CF airways are burdened with excessive neutrophil-derived proteases and in 2004, we discovered a “near-silent” ENaC (NS-ENaC) population whose activity is robustly stimulated by this class of proteases (see figure). From these original observations, we predicted that by targeting NS-ENaCs, these proteases would increase epithelial Na+- absorption. This hypothesis was subsequently confirmed by us and other investigators. Today we are using patch-clamp, two electrode voltage-clamp and site-directed mutagenesis approaches to investigate proteases that activate ENaC as well as the cleavage(s) sites in ENaC subunits responsible for channel activation.
Proteolytic inhibitors designed to prevent proteases from activating ENaCs should limit excessive Na+- absorption and ASL volume depletion in CF airways. However, once ENaC is proteolytically activated, a second important area of my research is focused on ENaC pharmacology and blocker-kinetics, which like proteolytic inhibitors, could provide a useful therapeutic approach to quell excessive Na+-absorption in CF airways.
Serine protease regulation of ENaC activity. Left panel (top trace); single channel current record depicting activity of a near-silent ENaC; note brief and infrequent channel openings. (Middle trace);brief protease exposure causes robust stimulation of channel open-probability (Po) leading to increased Na+-absorption. (Bottom trace); once activated, channel activity is inhibited in the presence of an ENaC blocker. Right panel. Protease model of ENaC regulation. Proteases (e.g., CAPs and furin) cleave ENaC subunits. Channel cleavage is correlated with increased amiloride-sensitive Na+-absorption. Normally airway proteolytic activity is regulated by protease inhibitors (p) present in the surface liquid. CF airways are burdened with proteases (e.g., neutrophil elastase) that overwhelm these inhibitors. Because neutrophil protease-stimulated ENaC activity likely contributes to excessive Na+-absorption in CF airways, therapeutic strategies involve development of potent proteolytic inhibitors and ENaC blockers.
- Hirsh, AJ, J Zhang, A Zamurs, J Fleegle, WR Thelin, RA. Caldwell, JR. Sabater, WM Abraham, M Donowitz, B Cha, KB Johnson, JA St- George, MR Johnson, and RC Boucher. 2008. Pharmacological properties of N-(3,5-diamino-6-chloropyrazine-2-carbonyl)-N'-4-[4-(2,3-dihydroxypropoxy)phenyl]butyl-guanidine methanesulfonate (552-02), a novel epithelial sodium channel blocker with potential clinical efficacy for cystic fibrosis lung disease. J Pharmacol Exp Ther. 325:77-88.
- Moody M, Pennington C, Schultz C, Caldwell R, Dinkel C, Rossi MW, McNamara S, Widdicombe J, Gabriel S, Traynor-Kaplan AE. 2005. An inositol polyphosphate derivative inhibits Na+ flux and improves fluid dynamics in cystic fibrosis airway epithelia. Am J Physiol Cell Physiol. 289: C512-C520.
- Caldwell, RA, RC Boucher, and MJ Stutts. 2005. Neutrophil elastase activates near-silent epithelial Na+-channels and increases airway epithelial Na+-transport. Am J Physiol Lung Cell Mol Physiol 288: L813-L819.
- Caldwell, RA, RC Boucher, and MJ Stutts. 2004. Serine protease activation of near-silent epithelial Na+-channels. Am J Physiol 286: C190-C194.
- Caldwell, RA, BR Grubb, R Tarran, RC Boucher, MR Knowles, and PM Barker. 2002. In vivo airway surface liquid Cl-analysis with solid-state electrodes. J Gen Physiol 119: 3-14.