Chromatin and Transcription Transitions of Mammalian Adult Germline Stem Cells and Spermatogenesis

Abstract

Adult germline stem cells (AGSCs) self-renew (Thy1(+) enriched) or commit to gametogenesis (Kit(+) enriched). To better understand how chromatin regulates AGSC biology and gametogenesis, we derived stage-specific high-resolution profiles of DNA methylation, 5hmC, histone modifications/variants, and RNA-seq in AGSCs and during spermatogenesis. First, we define striking signaling and transcriptional differences between AGSC types, involving key self-renewal and proliferation pathways. Second, key pluripotency factors (e.g., Nanog) are silent in AGSCs and bear particular chromatin/DNAme attributes that may "poise" them for reactivation after fertilization. Third, AGSCs display chromatin "poising/bivalency" of enhancers and promoters for embryonic transcription factors. Remarkably, gametogenesis occurs without significant changes in DNAme and instead involves transcription of DNA-methylated promoters bearing high RNAPol2, H3K9ac, H3K4me3, low CG content, and (often) 5hmC. Furthermore, key findings were confirmed in human sperm. Here, we reveal AGSC signaling asymmetries and chromatin/DNAme strategies in AGSCs to poise key transcription factors and to activate DNA-methylated promoters during gametogenesis.

Teratozoospermia and asthenozoospermia are associated with specific epigenetic signatures

“…It must be noted that while what we have identified is intriguing, we may simply be observing a downstream change to the epigenome that is not causative of the gross semen abnormalities, but is reflective of a more global causative change. Further, while there is strong data to suggest that methylation marks remain stable from the AGSC to the mature spermatozoa (Hammoud et al, 2014), this does not mean that the currently identified perturbations did not arise in the spermatogenesis cycle, in which case the stem cells would be largely unaffected. Clearly, much more investigation is required to understand the true nature of these marks and their potential impact on fertility.…”

“…Among the most promising marks to accomplish this are residual epigenetic alterations, including DNA methylation patterns, that are used in transcriptional regulation in the AGSC and are largely maintained throughout spermatogenesis to the mature spermatozoa (Hammoud et al, 2014). Because of the regulatory role of DNA methylation, such marks offer a potential snapshot of the events that occur early in spermatogenesis where programmatic problems may contribute to various types of sperm abnormalities.…”

Exposure to the widely used herbicide atrazine results in deregulation of global tissue-specific RNA transcription in the third generation and is associated with a global decrease of histone trimethylation in mice

“…To identify peaks of SG fractions we compared the differential peaks with published dataset (95). The differential regions of SG fraction are enriched in cell cycle and RNA processing genes.…”

Poised chromatin in the mammalian germ line

“…As a result, the poised chromatin state is thought to be functionally associated with, and important for, pluripotency (Azuara et al, 2006;Bernstein et al, 2006;Mikkelsen et al, 2007). Several recent studies (Hammoud et al, 2009(Hammoud et al, , 2014Mochizuki et al, 2012;Erkek et al, 2013;Lesch et al, 2013;Ng et al, 2013;Sachs et al, 2013) have collectively shown that poised chromatin is maintained at developmentally critical gene promoters at multiple stages of male and female germ cell development in the mouse (Table 1). Germ cells represent a unique in vivo cell population: although they undergo extensive cellular differentiation as unipotent cells during gametogenesis, they nevertheless contribute to a totipotent embryo at fertilization ( Fig.…”

“…It is present in adult germline stem cells (Hammoud et al, 2014), which, like early fetal germ cells, can give rise to EG cell lines in culture (Kanatsu-Shinohara et al, 2004). Furthermore, differentiating adult male germ cells that have initiated or completed meiosis ( pachytene spermatocytes or round spermatids, respectively) also retain a poised epigenetic state at a majority of the same genes that are poised in early fetal germ cells, thus highlighting the maintenance of the bivalent state throughout the germline cycle (Lesch et al, 2013).…”