GABPα is required for maintaining an HSC and progenitor pool. (A) Induction of GABPα-floxed allele and experimental timeline. Mx1Cre-GABPα^FL/+ and Mx1Cre-GABPα^FL/− mice at 5 weeks old were injected intraperitoneally with 25 μg/g body weight of pIpC as indicated. The last day of injection was designated day 0 and BM cells were harvested on indicated days for analysis. (B) Flow cytometric analysis of LSKs and c-Kit⁺ myeloid progenitors in lineage-negative BM cells. BM cells were isolated on indicated days from pIpC-treated Mx1Cre-GABPα^FL/+ or Mx1Cre-GABPα^FL/− mice and surface-stained. Percentages of LSKs and c-Kit⁺ myeloid progenitors in Lin⁻ BM cells are shown. (C) Total BM cellularity, LSK frequency and numbers. LSK frequency was expressed as percentages of BM nucleated cells. Absolute counts of total BM cells and LSKs were obtained from 2 tibias and 2 femurs in each mouse. (D) Numbers of myeloid progenitors, granulocytes, and developing B cells in the BM. BM cells were isolated from Mx1Cre-GABPα^FL/+ or control mice during 6-12 days after pIpC treatment, and stained for Lin⁻c-Kit⁺ myeloid progenitors, Gr.1⁺, and B220⁺ cells. The absolute counts were from 2 hind limbs as in panel C. (E) Analysis of LT-HSC, ST-HSC, and MPP subsets in LSKs. LSK cells were fractionated based on CD34 and Flt3 expression, and the percentage of each subset was shown in the flow cytometric profile (left panel). Absolute count of each subset was shown in the right panel. All flow cytometric data are representative of at least 3 independent experiments with similar results, and bar graphs are means ± standard deviation (SD) of pooled results (n ≥ 6). Statistical significance was calculated using the Student t test, with P values shown in each relevant panel.