Synthesis of transforming growth factor-β1 by megakaryocytes and its localization to megakaryocyte and platelet α-granules

Abstract

We have directly demonstrated that megakaryocytes are a major site of synthesis and storage of transforming growth factor-β1 (TGF/β1) by combined immunohistochemical, immunocytochemical, and in situ hybridization methods. The presence of TGF/β1 messenger RNA (mRNA) in mature megakaryocytes in adult rat spleen and bone marrow (BM) was established by in situ hybridization. Localization of TGF/β1 protein to intact α-granules of megakaryocytes, its putative storage site, was accomplished in glycol-methacrylate embedded porcine BM with an immunoperoxidase technique and light microscopy. The TGF/β1 was sequestered in intracytoplasmic granules in a pattern virtually identical to that of another α-granule marker protein, fibrinogen. This observation strongly suggests packaging of TGF/β1 into this organelle within megakaryocytes. That TGF/β1 mRNA was localized to megakaryocytes suggests that the TGF/β1 found in the α-granules in platelets originates with megakaryocyte synthesis. The α-granule localization of TGF/β1, as well as fibrinogen, was also demonstrated in isolated platelets at the ultrastructural level by electronmicroscopy (EM) and postembedding colloidal-gold immunocytochemistry, thus directly demonstrating that α-granules are the final storage site for TGF/β1 in mature platelets. © 1990 by The American Society of Hematology.