Thrombopoietin: The primary regulator of platelet production

Although only very recently cloned and expressed, initial studies indicate that Tpo stimulates both the proliferation and differentiation of megakaryocytic progenitor cells, is essential for the full maturation of megakaryocytes, and acts to enhance platelet production and to speed recovery after cytoreductive therapies. Together with the finding that all of the thrombopoietic activity of thrombocytopenic plasma can be removed with immobilized Mpl receptor and that in the presence of Tpo nearly all assayable CFU-Mk can be induced to develop into megakaryocytic colonies, it has become clear that (1) ML is identical with the long sought after hormone, Tpo, and that (2) Tpo is the primary physiologic regulator of platelet production. Despite premature conclusions to the contrary, to paraphrase Descartes, 'it has been cloned, therefore, it exists.' Despite these early gains, much work remains. The mechanisms of regulation of Tpo expression are unclear. Whether end cell destruction modulates Tpo blood levels needs verification; if operative, it is unlikely to be the sole mechanism of regulation. Other systems need to be uncovered and investigated. The role of protein processing in the physiology of Tpo is unclear. Whether the smaller forms obtained from plasma are truncated secondary to proteolysis introduced during purification or represent the physiologically active forms that circulate needs careful study. If physiologic, what is the structural basis and the reason for processing? Such questions should provide an active area of research in the near future. Initial studies of v-mpl clearly indicated that the viral gene induces a pan-myeloid proliferative syndrome. Furthermore, Tpo has effects on both erythroid and granulocyte/macrophage progenitors. In light of these findings, the role of the Mpl/Tpo system on hematopoietic stem cells and on other aspects of hematopoiesis needs to be addressed. The nature of the Tpo receptor is unsettled. It is clear that Mpl forms pan of the signalling complex. However, a growing number of cytokine receptors, previously believed to be composed of a single polypeptide species, have recently been shown to be composed of multiple subunits. Careful kinetic analysis and cross-linking studies and possibly gene knockout strategies will be required to dissect the Tpo receptor. Once engaged, the receptor-initiated pathways through which Tpo signals must be discerned. A growing literature suggests that multiple parallel pathways transmit the complex signals of proliferation and differentiation to the nucleus; whether these signals are similar or identical to those used by other members of the hematopoietic receptor family or are specific to megakaryocytes will require further investigation. Is Tpo the end? Are all of the hematopoietic cytokines now in place? The answer to these two questions will require the demonstration of platelet formation in a single-cell, serum-free CFU-Mk assay system containing only defined, purified cytokine combinations. We are presently at the threshold of a molecular understanding of megakaryopoiesis; we now have the questions and important new tools with which to answer them. It is hoped that a thorough understanding of these and of other aspects of thrombopoiesis will derive from productive studies in the ensuing years.