Eneral effect, we compared the AZ876 levels of total H3K9me3 in C2C12 myoblasts transfected with NSsiRNA and HP1asiRNA. HP1a deficient cells have higher levels of H3K9me3 (Fig. 5E), suggesting that HP1a regulates global H3K9me3 in myoblasts. Consistent with the downgregulation of myogenin expression after knockdown of HP1a, H3K9me3 levels at the myogenin promoter were higher in HP1asiRNA transfected myoblasts compared to NSsiRNA transfected cells (Fig. 5F).JHDM3A binding was not due to decreased protein levels in HP1a depleted cells (Fig. 6B). To ascertain if JHDM3A associates with HP1a, we overexpressed Flag-JHDM3A and GFP-HP1a in NIH3T3 cells and immunoprecipitated the resultant lysates with either anti-Flag (Fig. 6C) or anti-GFP antibodies (Fig. 6D). Western blotting of the precipitated complexes with anti-HP1a or anti-JHDM3A antibodies revealed that JHDM3A associates with HP1a. Co-immunoprecipitation analyses on lysates prepared from unmodified C2C12 cells confirmed that endogenous HP1a and JHDM3A can also associate in vivo (Fig. 6E and F). We further confirmed HP1a-JHDM3A PHCCC interaction using in vitro binding assays. In vitro translated 35S-methionine-labeled JHDM3A interacted with a recombinant GST-HP1a fusion protein (Fig. 6G). If HP1a recruitment of JHDM3A to Lbx1 exon 2 were required for efficient gene expression we would expect that depletion of JHDM3A in myoblasts would recapitulate the effects of HP1a deficiency, at least on Lbx1, MyoD and myogenin transcription. C2C12 cells were transfected with JHDM3AsiRNA and knockdown efficiency was confirmed by examining JHDM3A mRNA levels by RT-PCR (Fig. 6H). Consistent with the effect of depleting HP1a on myogenic genes expression, JHDM3A depletion led to a marked reduction in Lbx1, MyoD and myogenin transcript levels in myoblasts (Fig. 6H), which subsequently normalized with differentiation (Fig. S5A). We did not detect significant change of mRNA levels of Pax7 and Myf5 in JHDM3A knockdown cells (Fig. 6H). We further examined the H3K9me3 status on exon 2 of Lbx1 and detected higher levels of H3K9me3 in JHDM3AsiRNA transfected myoblasts compared to NSsiRNA transfected cells (Fig. 6I). Knockdown of JHDM3A did not affect HP1a expression (Fig. 6J). Taken together, these results support a model whereby HP1a facilitates early myogenic gene expression in myoblasts through recruitment of JHDM3A to target genes.DiscussionIn this study, we demonstrated that among the three isoforms of HP1, HP1a was specifically required for myogenic gene expression in myoblasts. Superficially, this data appears at odds with recent reports that demonstrated HP1 proteins negatively regulate the transcriptional activity of MyoD and MEF2C [12,16]; however, HP1’s effect on endogenous myogenic genes was not examined in these studies. Instead, consistent with our results, it was reported that down-regulation of HP1a impaired differentiation. The molecular basis for this paradox was postulated that this effect related to a loss of HP1a-dependent suppression on expression of proliferation-associated genes [16]. We did not detect significant upregulation of E2F-dependent target genes after depletion of HP1a in C2C12 myoblasts, thus failure to downregulate proliferation associated genes could not account for this finding. In contrast, depletion of HP1a blocked expression of myogenic genes even before differentiation induction (Fig. 2C). The defect in skeletal muscle differentiation in HP1a-depletedHP1a Promotes Myogenic Gene Expr.Eneral effect, we compared the levels of total H3K9me3 in C2C12 myoblasts transfected with NSsiRNA and HP1asiRNA. HP1a deficient cells have higher levels of H3K9me3 (Fig. 5E), suggesting that HP1a regulates global H3K9me3 in myoblasts. Consistent with the downgregulation of myogenin expression after knockdown of HP1a, H3K9me3 levels at the myogenin promoter were higher in HP1asiRNA transfected myoblasts compared to NSsiRNA transfected cells (Fig. 5F).JHDM3A binding was not due to decreased protein levels in HP1a depleted cells (Fig. 6B). To ascertain if JHDM3A associates with HP1a, we overexpressed Flag-JHDM3A and GFP-HP1a in NIH3T3 cells and immunoprecipitated the resultant lysates with either anti-Flag (Fig. 6C) or anti-GFP antibodies (Fig. 6D). Western blotting of the precipitated complexes with anti-HP1a or anti-JHDM3A antibodies revealed that JHDM3A associates with HP1a. Co-immunoprecipitation analyses on lysates prepared from unmodified C2C12 cells confirmed that endogenous HP1a and JHDM3A can also associate in vivo (Fig. 6E and F). We further confirmed HP1a-JHDM3A interaction using in vitro binding assays. In vitro translated 35S-methionine-labeled JHDM3A interacted with a recombinant GST-HP1a fusion protein (Fig. 6G). If HP1a recruitment of JHDM3A to Lbx1 exon 2 were required for efficient gene expression we would expect that depletion of JHDM3A in myoblasts would recapitulate the effects of HP1a deficiency, at least on Lbx1, MyoD and myogenin transcription. C2C12 cells were transfected with JHDM3AsiRNA and knockdown efficiency was confirmed by examining JHDM3A mRNA levels by RT-PCR (Fig. 6H). Consistent with the effect of depleting HP1a on myogenic genes expression, JHDM3A depletion led to a marked reduction in Lbx1, MyoD and myogenin transcript levels in myoblasts (Fig. 6H), which subsequently normalized with differentiation (Fig. S5A). We did not detect significant change of mRNA levels of Pax7 and Myf5 in JHDM3A knockdown cells (Fig. 6H). We further examined the H3K9me3 status on exon 2 of Lbx1 and detected higher levels of H3K9me3 in JHDM3AsiRNA transfected myoblasts compared to NSsiRNA transfected cells (Fig. 6I). Knockdown of JHDM3A did not affect HP1a expression (Fig. 6J). Taken together, these results support a model whereby HP1a facilitates early myogenic gene expression in myoblasts through recruitment of JHDM3A to target genes.DiscussionIn this study, we demonstrated that among the three isoforms of HP1, HP1a was specifically required for myogenic gene expression in myoblasts. Superficially, this data appears at odds with recent reports that demonstrated HP1 proteins negatively regulate the transcriptional activity of MyoD and MEF2C [12,16]; however, HP1’s effect on endogenous myogenic genes was not examined in these studies. Instead, consistent with our results, it was reported that down-regulation of HP1a impaired differentiation. The molecular basis for this paradox was postulated that this effect related to a loss of HP1a-dependent suppression on expression of proliferation-associated genes [16]. We did not detect significant upregulation of E2F-dependent target genes after depletion of HP1a in C2C12 myoblasts, thus failure to downregulate proliferation associated genes could not account for this finding. In contrast, depletion of HP1a blocked expression of myogenic genes even before differentiation induction (Fig. 2C). The defect in skeletal muscle differentiation in HP1a-depletedHP1a Promotes Myogenic Gene Expr.
