Rap1 signal modulators control the maintenance of hematopoietic progenitors in bone marrow and adult long‐term hematopoiesis

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