THE SYNTHESIS AND LOCALIZATION OF ALTERNATIVELY SPLICED FIBRONECTIN EIIIB IN RESTING AND THROMBIN-TREATED MEGAKARYOCYTES

There are several species of alternatively spliced fibronection (FN). One of these, FN EIIIB, is primarily present in embryonic and in proli ferating and migrating cells and is believed to be important for cell maturation. We have studied the synthesis, localization. and secretion of this FN isoform in isolated guinea pig megakaryocytes, nonmegakary ocytic bone marrow cells, and platelets. There was 7.5 times more gene ral FN in megakaryocytes than in nonmegakaryocytic cells based on the analysis of equivalent amounts of protein. FN EIIIB was detected by We stern blotting in megakaryocytes but not in nonmegakaryocytic cells pr esent in bone marrow. Neither megakaryocytes nor platelets secreted FN EIIIB, while megakaryocytes secreted 25.3% +/- 4.6% general FN and pl atelets secreted about 61% general FN in response to thrombin. Analysi s of immunostained cells by confocal microscopy revealed that FN EIIIB had been redistributed to the surface of megakaryocytes in response t o thrombin. Synthesis was studied by metabolic labeling, and megakaryo cytes were shown to synthesize FN and FN EIIIB. Thus, megakaryocytes a nd platelets are among a small number of adult cells and tissues that synthesize and contain FN EIIIB. The expression of FN EIIIB on the meg akaryocyte surface may influence migration and maturation. (C) 1996 by The American Society of Hematology.