THE PURIFICATION OF MEGAPOIETIN - A PHYSIOLOGICAL REGULATOR OF MEGAKARYOCYTE GROWTH AND PLATELET PRODUCTION

The circulating blood platelet is produced by the bone marrow megakary ocyte. In response to a decrease in the platelet count, megakaryocytes increase in number and ploidy. Although this feedback loop has long b een thought to be mediated by a circulating hematopoietic Factor, no s uch factor has been purified. Using a model of thrombocytopenia in she ep, we have identified an active substance called megapoietin, which s timulated an increase in the number and ploidy of megakaryocytes in bo ne marrow culture. Circulating levels of this factor could be quantifi ed with this assay and were found to be inversely proportional to the platelet count of the sheep. Levels increased from <0.26 pM in normal sheep to 25-40 pM in thrombocytopenic sheep. From large amounts of thr ombo cytopenic sheep plasma we have purified a 31,200-Da protein and f ound that it retained the ability to stimulate both megakaryocyte numb er and ploidy in vitro. Injection of partially purified megapoietin in to rats stimulated a 24% increase in megakaryocyte number and a 60% in crease in mean ploidy as well as a 77% increase in the platelet count. Sheep platelets bound megapoietin and the amount of platelets require d to eliminate half the activity in vitro was close to the amount asso ciated with this same level of activity in vivo. We believe that megap oietin is the physiologically relevant mediator of megakaryocyte growt h and platelet production. Moreover, our data suggest that the level o f megapoietin is directly determined by the ability of platelets to re move megapoietin from the circulation.