Rb is dispensable for self-renewal and multilineage differentiation of adult hematopoietic stem cells

Abstract

Stem cells have been identified as essential for maintaining multiple organ systems, including the hematopoietic system. The distinct cell fates of self-renewal and differentiation of hematopoietic stem cells (HSCs) depend on cell division. Recently, several negative regulators of the cell cycle, such as the cyclin-dependent kinase inhibitors p21 Cip1 , p27 Kip1 , and p16 INK4a ͞p19 ARF , have been demonstrated to have a role in regulating HSC fate decisions, suggesting that regulation of the G 1-S phase transition can contribute to HSC self-renewal. Because the retinoblastoma protein, Rb, plays a central role in the regulation of the G 1-S phase cell cycle, we sought to determine whether it has an intrinsic role in the regulation of HSC fate. Surprisingly, we found that HSC function was essentially normal in the absence of Rb. Rb ⌬/⌬ HSCs contributed normally to both myeloid and lymphoid lineages in both primary and secondary recipients, and no evidence of transformation was observed. Additionally, we observed a mild myeloid expansion and decrease in mature B cells within the Rb ⌬/⌬ bone marrow but a similar contribution to phenotypic HSC populations compared with nondeleted bone marrow. The Rb family members p107 and p130 were not deregulated in cells in which Rb had been deleted, as determined by quantitative RT-PCR on the highly enriched stem and primitive progenitor cell lin ؊ c-Kit ؉ Sca-1 ؉ population. These studies demonstrate that Rb is not intrinsically required for selfrenewal and multilineage differentiation of adult HSCs.retinoblastoma ͉ conditional mutation ͉ bone marrow transplantation

Cell cycle regulation in hematopoietic stem cells

“…For instance, no hematopoietic phenotype was observed in p130-deficient mice (Cobrinik et al, 1996), and p107 deletion resulted in only a mild myeloid hyperplasia (LeCouter et al, 1998). Removal of pRb had also no effect on HSC self-renewal as assessed by serial transplantation (Walkley and Orkin 2006), and although pRb-deficient mice exhibited myeloid expansion, this was shown to be a non-cell autonomous effect (Walkley and Orkin 2006;Walkley et al, 2007). Strikingly, however, conditional deletion of all three Rb family members in of a constitutively low level of cell cycle activity for proper function of the blood system during adult life.…”

Networks Establishing Hematopoietic Stem Cell Multipotency and Self-Renewal

“…In contrast to the germline Patched +/− mode, the mode of conditional deletion of Smoothened in the adult hematopoietic compartment was used in other two studies. However, the negative effects of the Hh pathway on HSC quiescence were shown in one study (Walkley & Orkin, 2006) and not in another (Stead et al, 2002). The discrepancy is possibly due to a distinct mode of deletion.…”

Validation of signalling pathways: Case study of the p16-mediated pathway

“…As a result p16 binds to CDK4/6 inhibiting its activity thereby preventing Rb phosphorylation. 6,7 This leads to irreversible arrest in G1 phase of cell cycle. When DNA damage is detected, the action of p16 again targets CDK4/6 and results in arrest in G1 phase until DNA damage is repaired.…”