Posttranscriptional Regulation of Histone Lysine Methyltransferase GLP in Embryonic Male Mouse Germ Cells1

The epigenetic status of germ cells changes dynamically during development. In this study, we analyzed the dynamics of histone H3 lysine 9 dimethylation (H3K9me2), a highly conserved mark of epigenetic silencing, and the expression of two lysine methyltransferases, G9a/Ehmt2/KMT1C and GLP/Ehmt1/KMT1D, in murine male embryonic germ cells after sex determination. Our previous studies established that G9a and GLP are the primary enzymes for H3K9me2 and predominantly exist as a G9a-GLP heteromeric complex that appears to be a functional H3K9 methyltransferase in vivo. During the period from Embryonic Day (E) 13.5 to E18.5 in mice, gonadal H3K9me2 levels were substantially lower in germ cells than in cells of nongerm lineage. Immunohistochemical analysis showed that during this phase in development, GLP level, but not G9a level, was also significantly lower in male germ cells. However, GLP mRNA was present in E13 and E16 male germ cells, with levels similar to those in cells of nongerm lineage. Interestingly, GLP is upregulated in embryonic male germ cells deficient for Nanos2, which encodes a germ cell-specific RNA-binding protein. Our data suggest that GLP protein expression is posttranscriptionally regulated in murine embryonic male germ cells after sex determination and that low H3K9me2 level results from the absence of GLP (severe reduction of the G9a-GLP heteromeric complex).