Rap1 Signal Modulators Control the Maintenance of Hematopoietic Progenitors in Bone Marrow and Adult Long-term Hematopoiesis

Adult long-term hematopoiesis depends on sustaining hematopoietic stem/progenitor cells (HSPC) in bone marrow (BM) niches, where their balance of quiescence, self-renewal, and hematopoietic differentiation is tightly regulated. Although various BM stroma cells that produce niche factors have been identified, regulation of the intrinsic responsiveness of HSPC to the niche factors remains elusive. We previously reported that mice deficient for Sipa1, a Rap1 GTPase-activating protein, develop diverse hematopoietic disorders of late onset. Here we showed that transplantation of BM cells expressing membrane-targeted C3G (C3G-F), a Rap1 GTP/GDP exchanger, resulted in the progressive decline of the numbers of HSPC repopulated in BM with time and impaired long-term hematopoiesis of all cell lineages. C3G-F/HSPC were sustained for months in spleen retaining hematopoietic potential, but these cells inefficiently contributed to overall hematopoietic reconstitution. C3G-F/HSPC showed enhanced proliferation and differentiation with accelerated progenitor cell exhaustion in response to stem cell factor (SCF). Using a Ba/F3 cell line, we confirmed that the increased basal Rap1GTP levels with C3G-F expression caused a markedly prolonged activation of c-Kit receptor and downstream signaling through SCF ligation. A minor population of C3G-F/HSPC also showed enhanced proliferation in the presence of thrombopoietin (TPO) compared to Vect/HSPC. Current results suggest an important role of basal Rap1 activation status of HSPC in their maintenance in BM for sustaining long-term adult hematopoiesis.