Expression and function of the collagen receptor GPVI during megakaryocytematuration

In this report, the expression and function of the platelet collagen recept or glycoprotein VI (GPVI) were studied in human megakaryocytes during diffe rentiation and maturation of mobilized blood and cord blood derived CD34(+) cells. By flow cytometry, using an anti-GPVI monoclonal antibody or convul xin, a GPVI-specific ligand, GPVI was detected only on CD41(+) cells includ ing some CD41(+)/CD34(+) cells, suggesting expression at a stage of differe ntiation similar to CD41, These results were confirmed at the mRNA level us ing reverse transcription-polymerase chain reaction. GPVI expression was lo w during megakaryocytic differentiation but increased in the more mature me gakaryocytes (CD41(high)). As in platelets, megakaryocyte GPVI associates w ith the Fc receptor gamma chain (FcR gamma). The FcR gamma chain was detect ed at the RNA and protein level at all stages of megakaryocyte maturation p receding the expression of GPVI. The other collagen receptor, alpha (2)beta (1) integrin (CD49b/CD29), had a pattern of expression similar to GPVI, Me gakaryocytic GPVI was recognized as a 55-kDa protein by immunoblotting and ligand blotting, and thus it presented a slightly lower apparent molecular mass than platelet GPVI (58 kDa), Megakaryocytes began to adhere to immobil ized convulxin via GPVI after only 8-10 days of culture, at a time when meg akaryocytes were maturing. At this stage of maturation, they also adhered t o immobilized collagen by alpha (2)beta (1) integrin-dependent and -indepen dent mechanisms. Convulxin induced a very similar pattern of protein tyrosi ne phosphorylation in megakaryocytes and platelets including Syk, FcR gamma , and PLCgamma2. Our results showed that GPVI is expressed early during meg akaryocytic differentiation but functionally allows megakaryocyte adherence to collagen only at late stages of differentiation when its expression inc reases.