Lung stem cell self-renewal relies on BMI1-dependent control of expression at imprinted loci

Abstract

BMI1 is required for the self-renewal of stem cells in many tissues including the lung epithelial stem cells, Bronchioalveolar Stem Cells (BASCs). Imprinted genes, which exhibit expression from only the maternally or paternally inherited allele, are known to regulate developmental processes, but what their role is in adult cells remains a fundamental question. Many imprinted genes were derepressed in Bmi1 knockout mice, and knockdown of Cdkn1c (p57) and other imprinted genes partially rescued the self-renewal defect of Bmi1 mutant lung cells. Expression of p57 and other imprinted genes was required for lung cell self-renewal in culture and correlated with repair of lung epithelial cell injury in vivo. Our data suggest that BMI1-dependent regulation of expressed alleles at imprinted loci, distinct from imprinting per se, is required for control of lung stem cells. We anticipate that the regulation and function of imprinted genes is crucial for self-renewal in diverse adult tissue-specific stem cells.

Figure 1

De-repression of imprinted genes in Bmi1 mutant lung cells. (A) Gene expression differences of homeobox (hox) genes, paternally expressed genes (PEGs), and maternally expressed genes (MEGs) from three samples each of Bmi1 wild-type (WT) and mutant lung cells, as assessed by Affymetrix Mouse 430 2.0 gene expression microarray. Red indicates up-regulated expression, green is down-regulated expression, and differences with FDR (False Discovery Rate) <0.1 are marked on left of heatmap (in red, up-regulated; in green, down-regulated). (B) Validation of differential expression of homeodomain genes and CDK inhibitors in Bmi1 wild-type and mutant lung cells by qPCR. Expression from mutant samples (grey bars) shown as fold change relative to wild-type samples (black bars) set to 1. (C) Table showing the 10 gene sets revealed to be most overrepresented in Bmi1 mutant samples using GSEA. Red, upregulated gene sets; green, downregulated gene sets. (D) qPCR analysis of expression of imprinted gene network members in lung cells from wild-type (black bars) and mutant (grey bars) mice, normalized as in (b). The differential expression of all genes shown was statistically significant. (E) Comparison of p57 expression in wild-type (black bars) and mutant (gray bars) ciliated cells (Epcam-positive, CD24-high), BASCs (Epcam-positive, Sca-1-low, CD24-low) and AT2 cells (Epcam-positive, Sca-1-neg, CD24-pos/neg). Asterisk (*) indicates P≤0.05. Error bars, standard deviation. See also Table S1, Table S2, Fig S1.

Figure 2

Lung cell self-renewal in culture is dependent on precise regulation of imprinted gene expression. Formation of secondary colonies following knockdown of various imprinted genes in Bmi1 mutant (A) or WT (B,C) lung cells by retroviral short hairpin RNA (shRNA) expression is shown. Each data point represents an individual well of primary colony cells plated for secondary colony formation, with 0 plotted for wells that lacked secondary colonies and 1 for wells with 1 or more secondary colonies. Infection with empty retroviral vector, sh-Scrambled (Scram), and sh-Firefly luciferase (FF2) served as negative controls. Additional controls, Snrpn, p21, p27. (A) Secondary colony formation in mutant cells infected with empty, Scram shRNA, p57 (sh2) shRNA, a MEG pool that included p57 (sh3), Grb10, and Ube3a shRNAs, a PEG pool that included Igf2, Snrpn, Peg3, and Dlk1 shRNAs, or a combined pool with all shRNAs (MEG + PEG) is shown. (B,C) Secondary colony formation in wild-type cells after infection with individual shRNA as indicated or 3 different shRNAs directed against p57. Results in A,B,C are data from four, three and three experiments, respectively. Asterisk (*) indicates P<0.05. D. qPCR analysis of knockdown efficiency of p57 by sh3 in wild-type or Bmi1 mutant lung cells; p57 expression was measured relative to empty retrovirus control-infected wild-type cells. Error bars, standard error (A–C) or standard deviation (D). See also Fig S2.

Fig. 3

p57 is dynamically regulated during bronchiolar cell repair in wild-type lung but not Bmi1 mutants. Sections were stained with antisera for p57 (red), SP-C (blue), and CCSP (green) along with DAPI (not shown) and four color imaging was performed to score BASCs (CCSP+ SPC+) and Clara cells (CCSP+ SPC-). Ciliated cells were assessed by acetylated tubulin staining. (A–E) Sections from wild-type lungs, isolated from vehicle-treated control mice (A, mock) or mice 2 (B), 3 (C), 5 (D), or 7 (E) days following treatment with naphthalene. For simplicity, only p57, CCSP and SPC staining are shown. Arrows, p57 positive Clara cells. (*), Background red signal in red blood cells/blood vessels (*). (F,G) Fluctuation in p57 levels in wild-type lungs at different time points after naphthalene treatment in BASCS (F) and Clara cells (G) was determined by four color staining as described above. (H) Quantification of BASCs, Clara cells or ciliated cells positive for p57 in Bmi1 mutant and wild-type lung cells at day 5 (d5, black bars) and day 7 (d7, white bars) following naphthalene-induced lung injury. Wild-type data are same as shown in F,G. Asterisk (*) indicates P<0.05; ns = non-significant difference. Error bars, standard deviation. (I,J) Bmi1 mutant lungs 5 days (I) or 7 days (J) after naphthalene treatment were stained as in A–E. See also Fig S3.

Fig. 4

Bmi1-dependent regulation of imprinted loci. (A) The average DNA methylation ratio was assessed in 4 independent biological replicates each of Bmi1 wild-type and mutant BASCs by RRBS. The DNA methylation ratio was averaged across all wild-type (black bars) or Bmi1 mutant (gray bars) imprinted gene promoter regions and DMRs; error bars represent standard errors for each pair. (B–D) Modulation of Bmi1 levels and high resolution melt curve analysis (HRM) to determine allele-specific expression in F1 Cast/129 MEFs. Levels of Bmi1 (B) and p57 (C) after infection with lentivirus expressing shBmi1 are shown, relative to empty vector virus. HRM results (D) for p57 showing expression from pure Cast cDNA (green, paternal imprinted allele control), pure 129 cDNA (black, maternal expressed allele control), empty virus infected F1 MEFs (blue), and shBmi1 F1 MEFs (red). Values shown are the average of three replicates from a representative experiment. See also Fig S4.