GLYCOSYLATION OF CD44 NEGATIVELY REGULATES ITS RECOGNITION OF HYALURONAN

Although CD44 is expressed on a wide variety of cell types, few of the m use it to recognize the ligand hyaluronan (HA). A glycosylation-defe ctive clone of Chinese hamster ovary cells (Lee and) bound HA, demonst rating that complete processing of glycoproteins with addition of a fu ll complement of sialic acid is not required. On the contrary, subsequ ent findings revealed that complex sugars on CD44 can actually inhibit ligand recognition. Two subclones of wild-type Chinese hamster ovary cells with similar amounts of surface CD44 were isolated on the basis of HA binding and found to differ with respect to CD44 size as well as staining with fluorescent lectins. Treatment of the nonbinding clone with tunicamycin reduced the size of the protein and allowed the cells to recognize HA via CD44. This function was also induced by treatment with deglycosylating enzymes (either a mixture of endoglycosidase F a nd N-glycosidase F or neuraminidase alone). A possible role for glycos ylation in regulation of adhesion was then sought with a series of nor mal and transformed murine cells. Disruption of glycosylation or treat ment with deglycosylating enzymes did not induce ligand binding in an interleukin 7-dependent pre-B cell line, and splenic B cells also appe ared to be in an inactive state. Some normal B cells acquired the abil ity to recognize HA after stimulation with lipopolysaccharide or inter leukin 5 and had distinctive surface characteristics (loss of immunogl obulin D and acquisition of CD43). An additional subset of activated c ells might have been in a transitional state, because the cells bound ligand after neuraminidase treatment. The ligand-binding ability of a purified CD44-immunoglobulin fusion protein dramatically increased aft er neuraminidase treatment. Thus, differential glycosylation of this m olecule is sufficient to influence its recognition function. Cell adhe sion involving HA can be regulated by multiple mechanisms, one of whic h involves variable glycosylation of CD44.