1. Department of Pharmacology, University of North Carolina School of Medicine, CB 7032, 103 Mason Farm Road, Chapel Hill, NC 27599, USA
2. Center for Gastrointestinal Biology and Disease, University of North Carolina School of Medicine, CB 7032, 103 Mason Farm Road, Chapel Hill, NC 27599, USA
3. Department of Biology, Vertex Pharmaceuticals Inc., Laval, QC, Canada H7V 4A7
4. Department of Medicine, University of North Carolina, Chapel Hill, NC 27599, USA
5. Department of Microbiology and Immunology, University of North Carolina, Chapel Hill, NC 27599, USA