Investigating the mode and mechanism of histone inheritance across species

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 DESCRIPTION (provided by applicant): Epigenetic phenomena refer to changes in gene expression inherited through cell divisions without changing the underlying DNA sequences. It is the epigenetic information marked by DNA methylation, histone modifications, non-coded RNA-mediated processing, etc., that direct cells with identical genomes to become distinct cell types throughout metazoan biology. Failure of this epigenetic regulation leads to abnormalities in stem cell behavior, which underlies diverse diseases including muscular dystrophy, diabetes, infertility, and many types of cancers. A central enigma in asymmetric stem cell division is how the epigenetic memory is retained to govern self-renewal of one daughter cell, while permitting differentiation of the other daughter cell. Recently, our lab has discovered that during the asymmetric cell division of Drosophila male germline stem cells (GSC), the preexisting histone 3 (H3) is selectively segregated to the GSCs, whereas newly synthesized H3 are enriched in the differentiating daughter cell. This asymmetric histone inheritance can provide the means for cells to impart distinct epigenetic information to the two daughter cells before their fates are determined. Employing a combination of molecular genetics and cell biology tools in both Drosophila and C. elegans as dual model systems with distinct advantages, this proposal aims to define (1) how the extrinsic signals emanating from the stem cell niche regulate intrinsic histone asymmetry of GSCs, (2) determine whether asymmetric histone inheritance is conserved, and (3) examine whether it is a broader mechanism used in asymmetric cell divisions in multiple lineages throughout development to maintain epigenetic memory. Applying the Drosophila male GSC model, studies have revealed that both intrinsic factors and extrinsic cues regulate GSC identity and activity. The extrinsic mechanisms include signals emanating from the niche, the extracellular matrix, and membrane bound molecules. Intriguingly, we have demonstrated that at least one of these extrinsic signals emanating from the niche are necessary to regulate asymmetric histone inheritance. We plan to further characterize these extrinsic pathways, which are in a unique position within the niche to integrate with intrinsic regulators of asymmetric histone inheritance. We will also exploit a C. elegans genetic model to address (1) whether asymmetric histone inheritance is a conserved mechanism across different species and (2) if this asymmetric histone inheritance specific for stem cells or a broader mechanism used in asymmetrically dividing cells to specify distinct cell fates. C. elegans is an ideal genetic model organism to study asymmetric histone inheritance with distinct experimental advantages to address both of these questions. The proposed study should uncover a fascinating element of epigenetic regulation during asymmetric cell division throughout development and generate a potentially transformative impact relevant to the fields of stem cell biology, epigenetics, regenerative medicine, genetics, and asymmetric cell division.