Epigenetic mechanisms are critical for germline development and fertility, and correct epigenetic regulation during development impacts offspring health and disease susceptibility. Meiotic silencing is an epigenetic phenomenon active in many animal species that targets unpaired/unsynapsed chromosomes and chromosomal regions. It is widely conserved and hypothesized to function in several ways to promote fertility and gamete quality. Meiotic silencing appears to have similar functions in Caenorhabditis elegans and mammals where it targets the heterogametic sex chromosomes and other chromosomes or large chromosomal regions that may fail to pair due to mutation or rearrangement. In C. elegans, as in mammals, unsynapsed chromosomes accumulate an elevated level of histone H3 lysine demethylation (H3K9me2) during early first meiotic prophase. This conserved histone modification is characteristic of heterochromatin, therefore its presence suggests a more highly condensed chromatin structure. Although meiotic silencing is a conserved phenomenon, we do not understand the mechanism by which it is accomplished in any organism or how it functions to promote fertility. We previously found that meiotic H3K9me2 distribution in C. elegans depends on the activity of a branch small RNA network whose components include: EGO-1, an RNA-directed RNA polymerase; CSR-1 Argonaute; EKL-1, a Tudor domain protein; and DRH-3, a DEAD-box helicase. Now, we have identified a SWI/SNF protein, HARP-1, that associates physically with EKL-1 and also with MET-2, the histone methyltransferase responsible for germline H3K9me2. SWI/SNF family proteins are ATP-dependent DNA helicases, many of whom function as chromatin remodelers. Our analysis of meiotic silencing in C. elegans provides a model for understanding the function of meiotic silencing in germline development, for tissue-specific regulation of histone modifications, and for chromatin regulation via small RNA-mediated mechanisms. The long-term goal of our research is to understand the mechanisms and developmental importance of meiotic silencing. The central hypothesis of this proposal is that MET-2 activity in meiotic cells is directed by association with the SWI/SNF family member, HARP-1, which mediates its regulation by the small RNA machinery. Here, we propose two specific aims where we will test alternative hypotheses for how MET-2, HARP-1, and the small RNA machinery cooperate to accomplish meiotic silencing.
In Aim 1, we elucidate the biological function HARP-1 during meiosis and describe the functional relationship between HARP-1 and MET-2.
In Aim 2, we will describe the functional relationship between HARP-1 and EKL-1, as well as other components of the EGO-1/CSR-1 small RNA pathway. These two aims will provide complementary information critical for understanding the mechanism of meiotic silencing.
This proposal addresses questions related to fertility and resistance to disease. The project will investigate processes that are required for formation of functional sperm and eggs, and which may also help to reduce the susceptibility of offspring to particular diseases.
