The proper hydration of the airway surfaces is critical for lung health. In cystic fibrosis (CF) lung disease, defects in CFTR-mediated Cl-transport result in a net excess of epithelial salt (NaCl) and H2O absorption, which deplete the airways surface liquid (ASL) layer resulting in dehydrated mucus, inefficient mucociliary clearance (MCC), progressive deterioration of lung function, and significant morbidity and mortality.
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 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. See full-size image here...
My research focuses on ion transport at the cellular (whole-cell) and single protein levels. I use highly sensitive electrophysiological techniques (e.g. patch-clamp) to characterize the biophysical and pharmacological properties of the epithelial Na+ and Cl- channels (ENaC), a protein that is important in the kidney for Na+ balance and blood pressure regulation, and in 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 stimulate ENaC activity 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 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.
- Drs. Diane Grove and Douglas Cyr, UNC Department of Cell Biology. We have been testing compounds called “correctors” on the surface expression and function of the CFTR Cl- channel. These reagents were developed to promote the maturation and cell surface trafficking of CFTRdF508, the most common mutation associated with severe lung disease in the CF population. Mutations in other aspects of CFTR also cause defective maturation and trafficking to the surface membrane. In particular, the disease-causing mutation CFTRV232D results in diminished surface expression, and reduced epithelial Cl- transport. Preliminary results from single-channel patch-clamp analysis indicate CFTR corrector-4a increases functional surface expression of this mutant Cl- channel and may be of potential therapeutic benefit for this and other CFTR mutations associated with maturation and trafficking defects.
- Peter Bove, PhD, UNC CF Center. Patch-clamp analysis of Cl- channels in alveolar type II (ATII) cells. Briefly, this project aims to identify alternate (non-CFTR) Cl- channels located in the apical membrane of airway epithelia, regulation, and pharmacology. Pharmacologic activation of an alternate Cl- channel may bypass defective airway epithelial Cl- transport caused by CFTR mutations.
Ray A Caldwell, PhD, Assistant Professor of Medicine
BS Chemistry and Mathematics (minor). 1991. Virginia Commonwealth University, Richmond, VA.
PhD 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.
Hirsh, AJ, J Zhang, A Zamurs, J Fleegle, WR Thelin, RA. Caldwell, et al. 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, C Pennington, C Schultz, R Caldwell et al. 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, et al. 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, et al. 2004. Serine protease activation of near-silent epithelial Na+-channels. Am J Physiol 286: C190-C194.
Caldwell, RA, BR Grubb, et al. 2002. In vivo airway surface liquid Cl- analysis with solid-state electrodes. J Gen Physiol 119: 3-14.
Dr. Ray A Caldwell, Assistant Professor of Medicine
Contact Information6021 Thurston-Bowles Bldg.
Campus Box #7248
The University of North Carolina at Chapel Hill
Chapel Hill, NC 27599
Phone: (919) 966-7057
Fax: (919) 966-5178