The secreted lymphangiogenic factor CCBE1 is essential for fetal liver erythropoiesis

Abstract

• The secreted lymphangiogenic protein CCBE1 is essential for fetal but not postnatal erythropoiesis.• Loss of CCBE1 impairs erythroblastic island formation and function.The secreted protein CCBE1 is required for lymphatic vessel growth in fish and mice, and mutations in the CCBE1 gene cause Hennekam syndrome, a primary human lymphedema.Here we show that loss of CCBE1 also confers severe anemia in midgestation mouse embryos due to defective definitive erythropoiesis. Fetal liver erythroid precursors of Ccbe1 null mice exhibit reduced proliferation and increased apoptosis. Colony-forming assays and hematopoietic reconstitution studies suggest that CCBE1 promotes fetal liver erythropoiesis cell nonautonomously. Consistent with these findings, Ccbe1 lacZ reporter expression is not detected in hematopoietic cells and conditional deletion of Ccbe1 in hematopoietic cells does not confer anemia. The expression of the erythropoietic factors erythropoietin and stem cell factor is preserved in CCBE1 null embryos, but erythroblastic island (EBI) formation is reduced due to abnormal macrophage function. In contrast to the profound effects on fetal liver erythropoiesis, postnatal deletion of Ccbe1 does not confer anemia, even under conditions of erythropoietic stress, and EBI formation is normal in the bone marrow of adult CCBE1 knockout mice. Our findings reveal that CCBE1 plays an essential role in regulating the fetal liver erythropoietic environment and suggest that EBI formation is regulated differently in the fetal liver and bone marrow. (Blood. 2013;121(16):3228-3236)

Proteolytic activation defines distinct lymphangiogenic mechanisms for VEGFC and VEGFD

“…Studies of mutant zebrafish that entirely lack lymphatic vessel development and rare individuals with a primary lymphedema disorder known as Hennekam syndrome have identified collagen-and calcium-binding EGF domains 1 (CCBE1) as a secreted protein that is required for lymphatic vascular development (19)(20)(21). Loss of CCBE1 completely blocks lymphatic vascular development in a manner similar to that induced by loss of VEGFC or VEGFR3 (22)(23)(24), but has no effect on blood vessel growth. Recent studies have implicated CCBE1 and a disintegrin and metalloproteinase with thrombospondin motifs 3 (ADAMTS3) in the regulation of VEGFC processing (25)(26)(27) and generated a model of lymphangiogenesis in which VEGFR3-bound VEGFC is cleaved by CCBE1 and ADAMTS3 during receptor activation (25).…”

Cell Adhesion Mediated by VCAM–ITGα9 Interactions Enables Lymphatic Development

“…Such rapid growth and invasion of pre-existing tissues requires factors that support LEC proliferation (e.g. VEGFC 5, 6 and CCBE1 7–10 ), guide LEC migration (e.g. CXCL12 and CXCR4 11 ), and mediate the physical movement of LECs.…”

“…In the mouse this primary structure is the lymph sac 3 , while in the fish it is a line of parachordal lymphangioblasts 4 . In both fish and mice lymphatic vessels arise in response to conserved molecular cues such as the secreted factors VEGF-C and CCBE1 5–10 . However, the molecular mechanisms by which lymphatic endothelial cells (LECs) rapidly migrate to generate this network remain incompletely understood.…”

Critical requirement of VEGF-C in transition to fetal erythropoiesis

“…So far, there are no data on the in vivo function of VEGF-C in hematopoiesis. However, recent reports indicate that Ccbe1 and Adamts3 are necessary for fetal erythropoiesis, 21,22 suggesting that VEGF-C has a potential function in hematopoiesis.…”