HETEROLOGOUS EXPRESSION OF DELTA-F508 CFTR RESULTS IN DECREASED SIALYLATION OF MEMBRANE GLYCOCONJUGATES

The cystic fibrosis transmembrane conductance regulator (CFTR) is comm only mutated in cystic fibrosis to the Delta F508 CFTR. CFTR has been shown to function as a adenosine 3',5'-cyclic monophosphate-dependent Cl- channel at the cell surface, and there is evidence to suggest that CFTR may also haw a role in transmembrane Cl- conductance in intracel lular membrane compartments. Studies using cells from cystic fibrosis patients or heterologous expression systems have demonstrated that def ective Cl- conductance at the cell surface and defective acidification of the Golgi compartment are associated with the presence of mutant f orms of CFTR. It is possible that mutation of CFTR could also result i n altered Golgi function, consistent with reports of changes in the gl ycosylation of cell surface and secreted glycoproteins in cystic fibro sis. Glycosylation of cell surface glycoproteins, particularly levels of sialylation, may also be related to the increased binding of Pseudo monas to cystic fibrosis cells. The current study was undertaken to co mpare the sialylation of cell surface glycoconjugates in heterologous cells overexpressing normal CFTR or Delta F508 CFTR. The presence of s ialylated residues on cells was assessed by the surface binding of the specific lectins, wheat germ agglutinin and elderberry bark lectin. A fluorescent cholera toxin B subunit probe was used to measure surface binding to sialylated gangliosides in transfected cells. Our studies show that cells lacking CFTR and cells expressing normal CFTR have una ltered levels of sialylation. In contrast, cells expressing the Delta F508 CFTR have significantly decreased amounts of sialylated glycoprot eins and gangliosides on the cell surface. These results suggest that Delta F508 CFTR may perturb intracellular functions, probably at the l evel of Golgi processing, resulting in the appearance of cell surface asialoglycoconjugates.