Developmental expression of mRNAs encoding platelet proteins in rat megakaryocytes

The committed bone marrow megakaryocyte (MK) progenitor undergoes a series of highly regulated stages of development resulting in a large multi-nucleated platelet-producing cell. We studied the developmental expression of the mRNA for two alpha granule proteins, fibronectin (FN) and fibrinogen gamma chain (gamma-FIB), and a cytoskeletal protein, actin, in MKs from marrow of Sprague-Dawley rats. By the method of in situ RNA:RNA hybridization, we showed that mRNAs for the alpha granule proteins were expressed most abundantly in a population of 15-microns diameter promegakaryocytes and in cells as small as 10 microns whose identity as immature MKs was inferred by positive staining for platelet- and MK-specific markers. gamma-FIB and FN mRNAs were present in reduced abundance in a small proportion of intermediate MKs; however, little or no expression was seen in mature platelet-producing MKs. In contrast, high levels of actin mRNA were expressed predominantly in mature, multi-nucleated MKs, and less abundantly in the immature forms. These results suggest that FN and gamma-FIB are transcribed early in MK development to permit translation and packaging of the protein into alpha granules, after which transcription ceases. On the other hand, transcription of actin occurs continuously throughout development, with highest levels in mature platelet-producing MKs, in which actin is needed for shape changes and intracellular movement of organelles. Our data suggest that in situ RNA:RNA hybridization for platelet-specific markers will provide additional criteria by which to establish MK lineage in immature marrow progenitors.