Absence of the Rho GTPase Activating Protein p190-B Enhances Long Term Hematopoietic Stem Cell Engraftment

Hematopoietic stem cell (HSC) engraftment is a multistep process involving HSC homing to bone marrow (BM), self-renewal, proliferation and differentiation to mature blood cells. However, the molecular regulation of HSC engraftment is still poorly defined. Small Rho GTPases are critical regulator of cell migration, proliferation and differentiation in multiple cell types. While their role in HSC functions has begun to be understood, the role of their regulator in vivo has been understudied. P190-B GTPase Activating Protein (GAP), a negative regulator of Rho activity, has been implicated in regulating cell size and adipogenesis-myogenesis cell fate determination during fetal development (Sordella, Dev Cell, 2002; Cell 2003). Here, we investigated the role of p190-B in HSC/P engraftment. Since mice lacking p190-B die before birth, serial competitive repopulation assay was performed using fetal liver (FL) tissues from day E14.5 WT and p190-B^−/− embryos. WT and p190-B^−/− FL cells exhibited similar levels of engraftment in primary recipients. However, the level of contribution of p190-B^−/− cells to peripheral blood and bone marrow was maintained between the primary and secondary recipients and still easily detectable in tertiary recipients, while the level of contribution of FL WT cells dramatically decreased with successive serial transplantion and was barely detectable in tertiary recipients. The contribution to T cell, B cell and myeloid cell reconstitution was similar between the genotypes. A pool of HSC was maintained in serially transplanted p190-B^−/− animals, since Lin^negSca^posKit^pos (LSK) cells were still present in the BM of p190-B^−/− secondary engrafted mice while this population disappeared in WT controls. Importantly, this enhanced long term engraftment was due to a difference in the functional capacity of p190-B^−/− HSC compared to WT HSC since highly enriched p190-B^−/− HSC (LSK) demonstrated similar enhanced serial transplantation potential. Because previous studies have suggested that the loss of long term function of HSC during serial transplantation can depend, at least in part, on the upregulation of the cyclin dependent kinase inhibitor p16^Ink4a (Ito et al, Nat Med 2006), the expression of p16^Ink4a was examined during serial transplantation. While expression of p16^Ink4a increased in WT HSC in primary and secondary recipients, p16^Ink4a remained low in p190-B^−/− HSC, which indicated that p190-B-deficiency represses the upregulation of p16^Ink4a in HSC in primary and secondary transplant recipients. This provides a possible mechanism of p190-B-mediated HSC functions. We next examined whether p190-B-deficiency may preserve the repopulating capacity of HSC/P during ex vivo cytokine-induced culture. While freshly isolated LSK cells from WT and p190-B^−/− mice exhibited comparable intrinsic clonogenic capacity, the frequency of colony-forming unit after 7 days in culture was 2 fold-higher in p190-B^−/− compared with WT cultures, resulting in a net CFU expansion. Furthermore, competitive repopulation assays showed significantly higher repopulating activity in mice that received p190-B^−/− cultured cells compared with WT cells equivalent to a 4.4-fold increase in the estimated frequency of repopulating units. Interestingly, p190-deficiency did not alter cell cycling rate or survival both in vivo and in vitro. Therefore, p190-B-deficiency maintains key HSC functions either in vivo or in ex vivo culture without altering cycling rate and survival of these cells. These findings define p190-B as a critical regulator of HSC functions regulating self renewal activity while maintaining a balance between proliferation and differentiation.