Thrombopoietin (TPO) induces thrombocytic colony formation of kidney cells synergistically with kit ligand A and a non-secretory TPO variant exists in common carp

The development of mammalian megakaryocytes and platelets is regulated by numerous cytokine signals, primarily through the thrombopoietin (TPO)/c-MPL axis. Although non-mammalian vertebrates are known to possess nucleated thrombocytes functionally equivalent to mammalian platelets, the dynamics of the thrombocyte development remains unclear. Here we identified TPO and a splice variant (TPO-v) caused by the intron retention in common carp (Cyprinus carpio). Both the tpo and its variant transcripts were highly expressed in heart and liver. Recombinant carp TPO (rcTPO) was produced and purified in HEK293T cells stably expressing tpo, but TPO-v was shown not to be secreted from the transfectants. rcTPO induced the formation of colony-forming unit-thrombocyte (CFU-T) colonies which were recognized by a monoclonal antibody against carp thrombocytes expressing c-mpl and cd41, in a dose-dependent manner. The combination of rcTPO and recombinant carp Kit ligand A (rcKITLA) exerted a significant synergistic effect on three types of colony formation: thrombocytic colonies, thrombocytic burst colonies and thrombocytic/erythroid colonies. Utilizing this colony assay to examine the distribution of thrombocytic progenitor cells in carp, we demonstrated that carp head and trunk kidney play a primary role in thrombopoiesis, while the spleen does not. Our results indicate that carp possess mechanisms of TPO- and KITLA-dependent thrombopoiesis similar to those in other vertebrates and the sites of thrombopoiesis are restricted to the kidney, the primary hematopoietic organ in the teleost fish.