Subcellular localization of CFTR to endosomes in a ductal epithelium

Plasma membrane chloride transport by the cystic fibrosis transmembrane conductance regulator (CFTR) may be regulated by cellular processes that affect the cycling of CFTR with the plasma membrane. Testing this hypothesis requires cytochemical evidence for the presence of a subcellular compartment of CFTR. In this study, the subcellular distribution of CFTR in a normal epithelial cell population was characterized using immunofluorescence and immunoelectron microscopy. Two anti-CFTR antibodies, raised against different epitopes of the CFTR molecule, specifically labeled the apical pole of striated duct epithelial cells in tissue sections of rat submandibular gland. By use of electron microscopy, the CFTR immunoreactivity was associated with the apical plasma membrane and the membranes of many subapical vesicles. In this preparation, some of the CFTR-labeled vesicles were also labeled with antibodies against transferrin receptor and rab4, two markers of early endosomes and receptor-mediated endocytosis. These observations provide direct cytochemical evidence for the existence of peripherally located CFTR-expressing endosomes and support the hypothesis that membrane recycling may contribute to CFTR function.