Id proteins belong to the basic helix-loop-helix family of transcription factors and act as dominant negative forms of E-protein transcriptional activators. Id mediated E protein silencing has an essential role in restricting differentiation and maintaining self-renewal in embryonic stem cells. However, the role of Id1 in adult stem cells including HSCs has not been described thus far. Having detected relatively high levels of Id1 mRNA in murine adult HSCs (compared to the committed myeloid progenitor cells) we examined the in vivo HSC function in Id1 deficient mice.

We observed a >2 fold reduction in HSC frequency in the bone marrow in 8-10w old Id1−/− mice compared to Id1+/+ animals, detected by both lin-c-kit+Sca-1+ (LKS) cell surface marker profile and Hoechst 33342 dye efflux - “side population” phenotype, as well as a ~25% decrease in total bone marrow cellularity. Although Id1 deficient HSCs show robust long-term competitive repopulating capacity in primary transplant recipients, they have markedly impaired hematopoietic function upon secondary transplantation. Id1 null HSCs show a higher rate of S-phase entry in vivo as measured by 3 day BrdU incorporation (ko: 79.0±3.9% vs. wt: 49.7±7.4) and faster initial doubling times in response to cytokine stimulation in vitro during the first 2 days of culture. This failure to maintain normal HSC numbers and the diminished repopulating capacity, in the presence of enhanced cell cycling, suggests a defect in the regulation of self-renewal in Id1 deficient HSCs. Considering the general function of Id1 as an inhibitor of differentiation, the observed effect of Id1 loss could be explained by the excessive recruitment of LKS cells into the actively proliferating differentiated progenitor pool, at the expense of their self-renewal capacity. Consistent with this, sorted Id1−/− HSCs show accelerated expression of cell surface lineage markers in vitro and an increased ratio of CFU-S8 /CFU-S12 in the in vivo spleen colony forming assay.

Global gene expression profiling of Id1+/+ vs. Id1−/− hematopoietic cells (using Affymetrix MOE430 Plus chips) revealed insignificant transcriptional deregulation in the committed myeloid progenitor subsets (CMP, GMP, MEP) in the absence of Id1. Meanwhile, Id1−/− HSCs showed a marked change in gene expression pattern (more than 1500 genes with a ≥2 fold difference in expression levels). Differentially regulated transcripts in Id1+/+ vs. Id1−/− HSCs significantly overlap (~30%) with the observed changes in gene expression that accompany the transition of HSCs to the common myeloid progenitor phenotype. Specifically, genes such as c/EBPα and GATA1 are significantly upregulated in Id1 null immunophenotypic HSCs, consistent with an earlier than normal commitment to myelo-erythroid differentiation. In contrast, several known transcriptional regulators of HSC self-renewal (Bmi1, Gfi1, HoxB4) show no significant change in expression pattern. These data clearly indicate the unique role of Id1 in regulating HSC self-renewal by restricting the rate of HSC commitment to the myeloid progenitor cell fate.

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