A Cytoplasmic Fusion/Cell Division Model for Renewal of Hematopoietic Stem Cell.

Phenotypic attributes and functional properties of murine and human hematopoietic stem cells (HSCs) and progenitor cells are well characterized. However, the mechanisms underlying the renewal of HSCs and maintenance of stem cell state are poorly understood. In the course of a four-year study using a competitive repopulation assay, we have found a novel stem cell population that gives rise to mature CD45.2⁻CD45.1⁺EGFP⁺ cells in lethally-irradiated recipient mice (CD45.1⁺) transplanted with retrovirally transduced CD45.2⁺EGFP⁺ plus untransduced CD45.2⁺ donor bone marrow cells. The CD45.2⁻CD45.1⁺EGFP⁺ cells were detected from all 4 independent transplantation experiments and they persist in recipients for nearly their lifespan. Under the experimental conditions, the CD45.2⁻CD45.1⁺EGFP⁺ cells ranged from 4 ∼ 25% of the total transduced cells. The proportions of CD45.2⁻CD45.1⁺EGFP⁺ lymphocytes and myeloid cells in peripheral blood were respectively 21.9% and 78.1%, which differs dramatically from those of CD45.2⁺EGFP⁺ (77.5% and 22.6%) and total CD45.2⁺ cells (76% and 23.8%). Thus, the differentiation capacity of this type of stem cell differs substantially from that of known murine c-Kit⁺ Thy1.1^low Lin⁻Sca–1⁺ stem cells in that it is largely restricted to the myeloid lineage. Moreover, these cells are diploid based on DNA content analysis. We designate these novel stem cells as HSC partner cells. Based on these unexpected findings and ruling out technical artifacts, we propose a Cytoplasmic Fusion/Cell Division model for renewal and maintenance of HSCs, which is briefly described as follows. In response to a need for renewal, a canonical cKit⁺ Thy1.1^low Lin⁻Sca–1⁺ stem cell fuses with a HSC partner cell. This cell fusion results in combining cytoplasmic components from both types of cells; however, no nuclear fusion takes place. Immediately after the fusion, the combined intracellular factors induce the fused cell to undergo an asymmetric cell division (ACD) which generates a new HSC and a new HSC partner. Then, each of these two daughter cells undergoes a finite number of cell divisions, which are similar to symmetric mitosis in nature and can be regulated by intrinsic and/or exogenous factors. This limited cell expansion creates transitional stem cell pools for both the canonical HSCs and HSC partner. The transitional stem cell pools can be thought of as short-term stem cell “working pools”. Sizes of the transitional stem cell pools can change on demand. A relatively small fraction of HSCs in the transitional stem cell pool can enter G₀ state and remain quiescent until next fusion; thus, these few newly generated quiescent HSCs replenish the long-term HSC pool. Most transitional HSCs are stimulated to proliferate and differentiate into both lymphoid and myeloid lineages. Similarly, a small portion of HSC partner cells in its transitional pool replenish the original HSC partner pool. However, most transitional HSC partners proliferate and differentiate mainly into myeloid cells. The differentiation potential of HSC partners is largely restricted to the myeloid lineage and its capacity for lymphoid lineage is limited. This Cytoplasmic Fusion/Cell Division theory suggests a mechanism for renewal of HSCs and may also explain some well known and established phenomena about HSCs.