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 α granule proteins, fibronectin (FN) and fibrinogen y chain (γ-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 α granule proteins were expressed most abundantly in a population of 15-μm diameter promegakaryocytes and in cells as small as 10 μm whose identity as immature MKs was inferred by positive staining for platelet- and MK-specific markers. -γ-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 y-FIB are transcribed early in MK development to permit translation and packaging of the protein into α 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.