No abstract

Human embryonic stem cells (hESCs) self-renew indefinitely and give rise to derivatives of all three primary germ layers, yet little is known about the signaling cascades that govern their pluripotent character. Because it plays a prominent role in the early cell fate decisions of embryonic development, we have examined the role of TGFβ superfamily signaling in hESCs. We found that, in undifferentiated cells, the TGFβ/activin/nodal branch is activated (through the signal transducer SMAD2/3) while the BMP/GDF branch(SMAD1/5) is only active in isolated mitotic cells. Upon early differentiation, SMAD2/3 signaling is decreased while SMAD1/5 signaling is activated. We next tested the functional role of TGFβ/activin/nodal signaling in hESCs and found that it is required for the maintenance of markers of the undifferentiated state. We extend these findings to show that SMAD2/3 activation is required downstream of WNT signaling, which we have previously shown to be sufficient to maintain the undifferentiated state of hESCs. Strikingly, we show that in ex vivo mouse blastocyst cultures, SMAD2/3 signaling is also required to maintain the inner cell mass (from which stem cells are derived). These data reveal a crucial role for TGFβ signaling in the earliest stages of cell fate determination and demonstrate an interconnection between TGFβ and WNT signaling in these contexts.

SUMMARY The phenotypic attributes and molecular determinants for the regeneration of bone marrow (BM) sinusoidal endothelial cells (SECs) and their contribution to hematopoiesis are unknown. We show that after myelosuppression VEGFR2 activation promotes reassembly of regressed SECs, reconstituting hematopoietic stem and progenitor cells (HSPCs). VEGFR2 and VEGFR3 expression are restricted to BM vasculature, demarcating a continuous network of VEGFR2+VEGFR3+Sca1− SECs and VEGFR2+VEGFR3−Sca1+ arterioles. While chemotherapy (5FU) and sublethal irradiation (650 rad) induce minor SEC regression, lethal irradiation (950 rad) induces severe regression of SECs requiring BM transplantation (BMT) for regeneration. Conditional deletion of VEGFR2 in adult mice blocks regeneration of SECs in sublethally irradiated animals, preventing hematopoietic reconstitution. Inhibition of VEGFR2 signaling in lethally irradiated wild type mice rescued with BMT severely impairs SEC reconstruction, preventing engraftment and reconstitution of HSPCs. Therefore, activation of VEGFR2 is critical for regeneration of VEGFR3+Sca1− SECs that are essential for engraftment and restoration of HSPCs and hematopoiesis.