p53 regulates hematopoietic stem cell quiescence

Abstract

The importance of the p53 protein in the cellular response to DNA damage is well known, but its function during steady-state hematopoiesis has not been established. We have defined a critical role of p53 in regulating hematopoietic stem cell quiescence, especially in promoting the enhanced quiescence seen in HSCs that lack the MEF/ELF4 transcription factor. Transcription profiling of HSCs isolated from wild-type and p53 null mice identified Gfi-1 and Necdin as p53 target genes, and using lentiviral vectors to upregulate or knockdown the expression of these genes, we show their importance in regulating HSC quiescence. Establishing the role of p53 (and its target genes) in controlling the cell-cycle entry of HSCs may lead to therapeutic strategies capable of eliminating quiescent cancer (stem) cells.

Figure 1. Maintaining HSC quiescence by p53

(A) Multicolor flow cytometry was used to determine the percentage of hematopoietic stem/progenitor cells (Lin⁻ Sca-1⁺ ) in the G0 phase of the cell cycle [defined as cells with low Pyronin Y content that contain 2n DNA (G0/G1)]. Total bone marrow cells from wild type and p53 ^−/− mice were stained with Pyronin Y and Hoechst 33342. One representative experiment is shown on the left. The graph on the right indicates the mean percentage (± SD) of G0 cells present (p < 0.005, n=9). (B). Side population (SP) cells (CD34⁻LSKs) from wild type and p53 ^−/− mice were identified by Hoechst 33342 staining and the use of blue and red filters. The bar graph on the right indicates the mean percentage (± SD) of SP cells present (p < 0.003, n=7). (C) Cell cycle analysis of CD34⁻LSK cells was performed by staining with Hoechst 33342 and Ki67 and analyzed by FACS. Data shown are the mean values ± SD (p < 0.0001, n=9). (D) The proliferation of CD34-LSK cells was measured by in vivo BrdU incorporation over 48 hours. Greater proliferation of p53 ^−/− CD34-LSK cells was seen (60% versus 30% for wild-type CD34-LSK cells; p < 0.009, n= 5).

Figure 2. Increased HSC frequency in p53 ^−/− Mef ^−/− mice

(A) Increased LSK cell frequency in p53 ^−/− Mef ^−/− mice. Lin−Sca-1⁺ c-Kit⁺ cells were qu by flow cytometry. Data shown are the mean percentage (± SD) of LSK cells in the marrow (p <0.0002, n=13). (B) Increased LT-HSC frequency in the marrow of p53 ^−/− Mef ^−/− mice. The frequency of LT-HSCs (CD150⁺Sca-1⁺c-kit⁺CD41-CD48-Lin-) was defined by flow cytometric analysis of SLAM cell surface markers. p53 ^−/−, Mef ^−/− and p53 ^−/− Mef ^−/− mice all showed a significant increase in the frequency of CD150⁺CD41-CD48-LSK cells, which correspond to LT-HSCs. Data shown are the mean percentage (± SD) of LT-HSCs (n=10). (C) Bone marrow cells from wild type, p53 ^−/−, Mef ^−/− and p53 ^−/− Mef ^−/− mice were stained with stem and progenitor cell surface markers, and apoptosis was assessed using PI and Annexin-V staining. Data shown are mean percentage (± SD) of Annexin-V⁺ / PI⁻ LSK cells (P= 0.2 by ANOVA; i.e., statistically not significant, n=5).

Figure 3. The increased HSC self-renewal capacity of Mef null mice does not depend on p53

(A) The steady-state level of bone marrow cobblestone area-forming cells was evaluated by scoring colonies at week 5. Data shown are mean values (± SD) (n=3). (B) LSK cells (1× 10³) from wild type, p53 ^−/−, Mef ^−/− and p53 ^−/− Mef ^−/− mice were cultured on MS5 stromal cells for 4 weeks and test for colony formation in the LTC-IC assay. Data shown are the mean number (± SD) of colonies formed (n=3). (C) Serial replating studies. Myeloid progenitors were quantified by methycellulose culture using BMMCs from wild type, p53 ^−/−, Mef ^−/− and p53 ^−/− Mef ^−/− mice. The methycellulose cultures were serially replated, weekly, for 4 weeks. Mean values (± SD) shown (n=3). (D) Lethally irradiated recipient mice (CD45.1) were transplanted with 1 × 10³ LSK cells from wild type, p53 ^−/−, Mef ^−/− and p53^−/− Mef ^−/− mice (CD45.2) plus 5 × 10⁵ competitor cells (CD45.1) in competitive repopulation assays (described in the Experimental Procedures). The bar graph on the right shows the mean percentage (± SD) of donor derived (CD45.2) cells in the peripheral blood 16 weeks post transplantation (n=4). (E) The LTC-IC assay was used to enumerate primitive hematopoietic stem cells, using limiting dilutions of LSK cells that were first cultured for 5 weeks on MS5 stroma and then cultured on methylcellulose for the readout. The number of wells that lacked LTC-ICs is graphed vs. the number of LSK cells per well.

Figure 4. p53 is essential for maintaining hematopoietic stem cell quiescence in MEF null mice

(A) Multicolor flow cytometry was used to determine the percentage of Lin⁻ Sca-1⁺ cells in the G0 phase of the cell cycle [defined as cells with low Pyronin Y content that contain 2n DNA (G0/G1)]. Data shown are the mean percentage (± SD) (n=6). (B) Analysis of SP cells within the CD34⁻LSK cells. CD34⁻LSK cells from wild type, p53 ^−/−, Mef ^−/− and p53 ^−/− Mef ^−/− mice were examined for the proportion of SP cells. Data shown are the mean percentage (± SD) of SP cells (n=3). (C) Bone marrow cells from wild type, p53 ^−/−, Mef ^−/− and p53 ^−/− Mef ^−/− mice obtained 12 hrs after a dose of total-body irradiation (TBI, 6.5 Gy) were assessed for apoptosis using PI and Annexin-V staining. Data shown are mean percentage (± SD) of Annexin-V⁺ / PI LSK cells (n=3). (D) γ-H2AX foci generation in HSCs following irradiation. LSK cells from wild type, p53 ^−/−, Mef ^−/− and p53 ^−/− Mef ^−/− mice were immunostained for γ-H2AX 3 hrs after irradiation (200 rads)(and DAPI stained to identify the nuclei). The bar graph at the bottom shows the percentage of LSK cells that show 0–2, 3–5 or > 5 foci (200 cells are counted in each genotype).

Figure 5. Enhanced HSC quiescence in MEF null mice does not depend on p21

(A) Increased p21 and p27 expression in Mef ^−/− LSK cells. The relative mRNA expression level of p21, p27, and p16 in LSK cells from wild type and Mef ^−/− mice were evaluated by qPCR and normalized to HPRT expression. Data shown are the mean ratio (± SD) of transcript levels over HPRT (n=2). (B) Multicolor flow cytometry was used to determine the percentage of hematopoietic stem cells in the G0 phase of the cell cycle. Lin⁻ Sca-1⁺ cells in G0 phase are defined as cells with low Pyronin Y content that contain 2n DNA (G0/G1). The bar graph on the right shows the mean percentage (± SD) (n=5). (C) Cell cycle analysis of LSK cells from wild type, p21 ^−/−, Mef ^−/− and p21 ^−/− Mef ^−/− mice. Cells were stained with Hoechst 33342 and Ki67 and analyzed by FACS. Data shown are the mean percentage (± SD) (n=6). (D) γ-H2AX foci generation in HSCs following irradiation. LSKs from wild type, p21 ^−/−, Mef ^−/− and p21 ^−/− Mef ^−/− mice were immunostained for γ-H2AX 3 hrs after irradiation (200 rads). DAPI staining was used to identify the nuclei. The bar graph at the bottom shows the percentage of LSK cells that show 0–2, 3–5 or > 5 foci (200 cells are counted for each genotype).

Figure 6. Gfi-1 and Necdin are direct transcriptional targets and functional mediators of p53 in HSCs

(A) Transcript profiling of LSK cells isolated from wild type, p53 ^−/− , p53 ^−/− Mef ^−/− mice were analyzed by Affymetrix oligonucleotide array. Genes that are differentially expressed in both p53 ^−/− and p53 ^−/− Mef ^−/− HSCs compared to wild type cells are shown. We utilized Ingenuity Pathways Analysis (Ingenuity Systems) to group genes into specific canonical pathways. FC stands for fold change. (B) Increased Gfi-1 and Necdin expression in Mef ^−/− LSK cells. The relative mRNA expression level of p16, p21, Gfi-1, and Necdin in LSK cells from wild type, Mef ^−/−, p53 ^−/− Mef ^−/− mice were evaluated by qPCR and normalized to HPRT expression. Data shown are the mean ratio (± SD) of transcript levels relative to HPRT (n=2). (C) p53 transactivates both the Necdin and Gfi-1 promoters. Hela cells were transfected with Necdin promoter or Gfi-1 promoter driven luciferase reporter plasmids containing either wild-type p53 binding sites or mutant p53 binding sites. Luciferase activity was assayed 24 hours after transfection. Values are means (± SD) (n=3). (D) p53 binds to the Gfi-1 and Ndn promoters in vivo. Chromatin bound DNA from Lin⁻ bone marrow cells was immunoprecipitated with a p53-specific antibody, or with normal mouse IgG. qPCR amplification was performed on corresponding templates using primers for the Gfi-1, Necdin, or p21 genes. (E) Downregulation of Gfi-1 and Necdin expression in Mef ^−/− cells decreases HSC quiescence. Mef ^−/− Lin−Sca-1⁺ cells were nucleofected with control, Gfi-1 or Necdin siRNAs. 24 hours post nucleofection, cells were stained with Pyronin Y and Hoechst 33342 and analyzed by FACS. Values are means (± SD) (p < 0.004, n=5). The effectiveness of the knockdown for each siRNA (vs control siRNA) is shown on the right.

Figure 7. Necdin functions as a rheostat to regulate HSC quiescence and maintenance

(A) Effect of downregulating Necdin expression on HSC quiescence. Wild type Lin⁻ cells were nucleofected with control or Necdin siRNAs. 24 hours post nucleofection, the cells were stained with Pyronin Y and Hoechst 33342 and analyzed by FACS. Values are means (± SD) (p < 0.04, n=2). (B) Effect of overexpressing Necdin on HSC quiescence. Wild type Lin⁻ cells were infected with control or Necdin expressiing lentiviruses. 48 hours post infection, the cells were stained with Pyronin Y and Hoechst 33342 and analyzed by FACS. Values shown are the means (± SD) (p < 0.02, n=3). (C). Effects of downregulating Necdin expression on serial replating of hematopoietic stem/progenitor cells in methycellulose assays. Values shown are means(± SD) (n=3).