Replacement of m-calpain by mu-calpain during maturation of megakaryocytes and possible involvement in platelet formation.

Localization of calpains in human bone marrow cells was studied immunohistochemically employing monoclonal antibodies against the high-Ca(2+)-requiring form (m-calpain) and the low-Ca(2+)-requiring form (mu-calpain). Most cells were stained with anti-m-calpain more strongly than with anti-mu-calpain, and staining with anti-mu-calpain was prominent only in megakaryocytes. To confirm the result, megakaryoblastic cell line (T-33) cells were subjected to immunoblot analysis. However no immunoreactivity to mu-calpain was seen in T-33 cells. Bone marrow from a patient with idiopathic thrombocytopenic purpura showed immature megakaryocytes (stage II) strongly stained by anti-m-calpain antibody while mature cells (stage III) were strongly stained by anti-mu-calpain antibody. These results suggest that mu-calpain plays a crucial role in mature megakaryocytes, possibly in platelet production.