Regulated Synthesis and Localization of DNA Methyltransferase during Spermatogenesis1

Abstract

Differences in the methylation patterns of male and female gamete DNA are likely to be involved in genomic imprinting. However, little is known of the mechanisms that regulate de novo methylation and demethylation during gametogenesis. We report here that the well-characterized M(r) 190,000 form of DNA methyltransferase (the only known form) is present in isolated mitotic, meiotic, and postmeiotic male germ cells, with the exception of meiotic pachytene spermatocytes, where the protein is undetectable by immunoblot analysis and a novel 6.2-kb DNA methyltransferase transcript is present. Whereas replication and methylation are coupled in somatic cells, the presence of DNA methyltransferase in postreplicative male germ cells is consistent with previously observed de novo methylation events in these cells. Immunofluorescence experiments revealed that DNA methyltransferase is localized to the nuclei of male germ cells, with a subset of spermatogonia and postreplicative leptotene/zygotene spermatocytes displaying prominent nuclear foci that are strongly enriched in DNA methyltransferase. The data suggest that down-regulation of DNA methyltransferase expression during the pachytene stage of meiosis utilizes an mechanism that is associated with the production of a larger mRNA, and that de novo methylation in leptotene/zygotene spermatocytes may take place in spatially restricted nuclear domains that are enriched in DNA methyltransferase.

Expression of DNA Methyltransferase (Dnmt1) in Testicular Germ Cells during Development of Mouse Embryo.

“…6), it strongly suggested that not Dnmt1 but some other DNA methyltransferase(s) such as Dnmt3a or Dnmt3b contributes to the creation of DNA methylation patterns including the imprinted genes. This conclusion, however, does not mean that once the DNA methylation pattern is formed in the arrested gonocytes, it is not maintained by Dnmt1 in spermatogonia when the cells reenter the proliferation stage and Dnmt1 is strongly expressed in those cells (Jue et al, 1995).…”

Gamete imprinting: setting epigenetic patterns for the next generation

“…The absence in prenatal male germ cells of either DNMT3a or DNMT3L results in the failure of spermatogenesis and infertility. In addition, DNMT1, DNMT3a, DNMT3b and DNMT3L are expressed and developmentally regulated in the postnatal testis (Jue et al 1995;Mertineit et al 1998;La Salle et al 2004). The precise roles of each enzyme in the maintenance and de novo methylation during postnatal spermatogenesis have not yet been delineated and will likely require enzyme-and germ cell-specific gene-targeting or ablation experiments.…”

Proliferation Stage-dependent Expression of DNA Methyltransferase (Dnmt1) in Mouse Small Intestine.

“…In pachytene-stage spermatocytes, Dnmt1 utilizes the first exon, which is different from those in oocytes or somatic cells, and the transcript is not translated into functional Dnmt1 (Mertineit et al, 1998). As a consequence, Dnmt1 abruptly decreases in spermatocytes at this stage (Jue et al, 1995). Recently, the transcript, of which 5' sequence is identical to that in pachytene spermatocyte, was also identified in neurons (Inano et al, 2000) and differentiated myotubes (Aguirre-Arteta et al, 2000).…”