Unexpected distinct roles of the related histone H3 lysine 9 methyltransferases G9a and G9a-Like Protein in myoblasts

Lysine methyltransferases G9a and GLP (G9a- Like Protein) form functional heterodimeric complexes that establish mono- and dimethylation on histone H3 lysine 9 (H3K9me1, H3K9me2) in euchromatin. Here we describe unexpected opposite individual roles for G9a and GLP during skeletal muscle terminal differentiation. Indeed, gain- or loss-of-function assays in myoblasts showed that in consistency with previous reports that G9a inhibits terminal differentiation. But GLP plays a more complicated role in muscle differentiation since both knockdown and overexpression inhibits terminal differentiation. Unexpectedly, in contrast to G9a, we show that GLP overexpression promotes abnormal expression of muscle differentiationspecific genes in proliferating myoblasts. Transcriptomic analysis indicates that G9a and GLP regulate different sets of genes. GLP, but not G9a, inhibits at the transcriptional level proteasome subunit-encoding genes resulting in an inhibition of the proteasome activities. Subsequently, GLP, but not G9a, overexpression stabilizes MyoD that is likely to be responsible for muscle markers expression in proliferating myoblasts. Microarray has been perform in duplicate or triplicate on total extract RNA from C2C12 cells (siRNA control, siRNA against G9a or Glp)