Successful cleavage development of mammalian embryos requires precise activation and repression of transcription. Covalent modifications to histone proteins, such as methylation and acetylation, play a key role in transcriptional regulation. In particular, dimethylation of the lysine 9 residue of histone protein H3 (H3K9) results in gene silencing and heterochromatin formation. Our lab has previously shown that transcripts encoding the five histone methyltransferases known to methylate H3K9 (Suv39h1, Suv39h2, ESET, G9a, and EHMT1) are present in different amounts during oocyte maturation and cleavage development. Specifically, Suv39h2 is in the greatest abundance in GV and metaphase II stage oocytes and is also present throughout cleavage development. The aim of this study was to determine the localization of Suv39h2 protein in the GV-stage oocyte and pronuclear, 2 cell, and 4 cell stage parthenogenetic porcine embryos. We hypothesized that Suv39h2 protein would localize to the nucleus based on its high transcript abundance throughout cleavage development. To test this hypothesis, we performed a microinjection experiment in which mRNA encoding a porcine Suv39h2-GFP fusion protein was injected into metaphase II porcine oocytes. Porcine oocytes were matured in a defined medium (TCM-199 supplemented with 0.1% PVA, 0.069 mg mL–1 cysteine, 10 ng mL–1 EGF, 0.5 IU mL–1 LH and FSH) for 42 to 44 h at 39°C in 5% CO2, then denuded of cumulus cells just before microinjection. Two separate treatment groups were microinjected intracytoplasmically with 1 μg μL–1 GFP or Suv39h2-GFP mRNA, respectively. Microinjection was performed using a FemtoJet microinjector (Eppendorf, Hamburg, Germany). The treatment groups and non-injected controls were electroactivated independently and cultured in PZM medium supplemented with 3 mg mL–1 BSA for 12 (pronuclear), 24 (2 cell), or 48 (4 cell) hours at 39°C in 5% CO2, depending on desired stage of development. Before visualization under UV light, embryos were stained with Hoechst 33342 for 15 minutes. Oocytes and embryos were analyzed for GFP expression at the GV, pronuclear, 2 and 4 cell stages of development using epifluorescence microscopy. Two to four biological replicates were performed for each stage of embryo development. We found that Suv39h2-GFP protein showed nuclear localization in most GV-stage oocytes (n = 11/14) and pronuclear (n = 17/17), 2-cell (n = 34/36), and 4-cell (n = 9/9) stage embryos. Chi square analysis revealed this pattern to be different from that observed in embryos injected with GFP mRNA, where GFP did not display nuclear localization at any stage of development (n = 12; P < 0.05). These results indicate that Suv39h2 is localized in the nucleus of oocytes and cleaved embryos, which suggests that this histone methyltransferase plays an important role in methylating H3K9.
Regulated nuclear accumulation of a histone methyltransferase times the onset of heterochromatin formation in C. elegans embryos