CLEC-2 stimulates IGF-1 secretion from podoplanin-positive stromal cells and positively regulates erythropoiesis in mice.

BACKGROUND Erythropoiesis is a complex multistep process by which erythrocytes are produced. C-type lectin-like receptor 2 (CLEC-2) is a podoplanin (PDPN) receptor almost exclusively expressed on the surface of platelets and megakaryocytes. Deletion of megakaryocyte/platelet CLEC-2 was reported to cause anemia along with thrombocytopenia in mice. PDPN-expressing stromal cells in the bone marrow (BM) were also reported to facilitate megakaryocyte expansion and maturation depending on the CLEC-2/PDPN interaction. OBJECTIVES We investigated how specific deletion of CLEC-2 in megakaryocytes/platelets leads to anemia. METHODS We used flow-cytometry to analyze maturation of erythroblasts, apoptotic cell death, and cell cycle distribution. CLEC-2 stimulated PDPN-expressing stromal cell conditioned medium was analyzed by cytokine array and ELISA, and co-cultured with immature erythroblasts. Cytokine levels in serum and BM extracellular fluid were quantified by ELISA. RESULTS We observed increased apoptosis of BM erythroblasts in megakaryocyte/platelet-specific CLEC-2 conditional knockout (Clec1bΔPLT ) mice. Moreover, PDPN-expressing stromal cells in the BM secreted insulin-like growth factor 1 (IGF-1) depending on the CLEC-2/PDPN interaction. Pretreatment with IGF-1 receptor inhibitor increased apoptosis rate and decreased the proliferation of erythroblasts in vitro. Furthermore, in Clec1bΔPLT mice, IGF-1 concentrations in serum and BM extracellular fluid were decreased, and IGF-1 replacement in Clec1bΔPLT mice attenuated anemia. CONCLUSIONS Our findings suggest that IGF-1 secretion from PDPN-expressing stromal cells by CLEC-2 stimulation positively regulates erythroblasts. This novel mechanism of erythropoiesis regulation indicates that a microenvironment consisting of megakaryocytes and PDPN-expressing stromal cells supports erythropoiesis.