HEMATOPOIESIS AND STEM CELLS Modulation of TGF- signaling by endoglin in murine hemangioblast development and primitive hematopoiesis

Endoglin (Eng), an accessory receptor for the transforming growth factor (TGF-) superfamily, is required for proper heman-gioblast and primitive hematopoietic de-velopment. However the mechanism by which endoglin functions at this early developmental stage is currently un-known. Transcriptional analyses of differ-entiating eng/ and eng/ ES cells re-vealed that lack of endoglin leads to profound reductions in the levels of key hematopoietic regulators, including Scl, Lmo2, and Gata2. We also detected lower levels of phosphorylated Smad1 (pS-mad1), a downstream target signaling molecule associated with the TGF- path-way. Using doxycycline-inducible ES cell lines, we interrogated the TGF- signal-ing pathway by expressing activated forms of ALK-1 and ALK-5, type I recep-tors for TGF-. Our results indicate that ALK-1 signaling promotes hemangio-blast development and hematopoiesis, as evidenced by colony assays, gene expres-sion and FACS analyses, whereas signal-ing by ALK-5 leads to the opposite effect, inhibition of hemangioblast and hemato-poietic development. In Eng/ ES cells, ALK-1 rescued both the defective heman-gioblast development, and primitive eryth-ropoiesis, indicating that ALK-1 signaling can compensate for the absence of en-doglin. We propose that endoglin regu-lates primitive hematopoiesis by modulat-ing the activity of the Smad1/5 signaling pathway in early stages of development. (Blood. 2011;118(1):88-97