A marine sponge-derived lectin reveals hidden pathway for thrombopoietin receptor activation.

N-glycan-mediated activation of the thrombopoietin receptor (MPL) under pathological conditions has been implicated in myeloproliferative neoplasms induced by mutant calreticulin, which forms an endogenous receptor-agonist complex that traffics to the cell surface and constitutively activates the receptor. However, the molecular basis for this mechanism is elusive because oncogenic activation occurs only in the cell-intrinsic complex and is thus cannot be replicated with external agonists. Here, we describe the structure and function of a marine sponge-derived MPL agonist, thrombocorticin (ThC), a homodimerized lectin with calcium-dependent fucose-binding properties. In-depth characterization of lectin-induced activation showed that, similar to oncogenic activation, sugar chain-mediated activation persists due to limited receptor internalization. The strong synergy between ThC and thrombopoietin suggests that ThC catalyzes the formation of receptor dimers on the cell surface. Overall, the existence of sugar-mediated MPL activation, in which the mode of activation is different from the original ligand, suggests that receptor activation is unpredictably diverse in living organisms.

A novel sponge-derived protein thrombocorticin is a new agonist for thrombopoietin receptor.

We screened 868 marine extracts in search of hematopoietic molecules resulted in findings of several extracts that proliferated Ba/F3-HuMpl cells but not the cells expressed with other hematopoietic cytokine receptors, EPO and G-CSF. Separation of the most potent extract of a Micronesian sponge Corticium sp., guided by the cell proliferation assay using Ba/F3-HuMpl cells resulted in an isolation of thrombocorticin (ThC), a novel 14 kDa protein as an active principal. ThC displayed concentration-dependent proliferation of Ba/F3-HuMpl cells, and had a stronger activity than that of eltrombopag, a small molecule drug used to treat thrombocytopenia. ThC induced phosphorylation of STAT5, suggesting that it activates Jak/STAT pathway as in the case of TPO. These results together indicated that ThC is a specific agonist for c-Mpl, although the size and shape differs largely from TPO. Here we present isolation, characterization and biological activity of ThC.