PgmNr D1230: Investigating Histone Inheritance Patterns at Specific Genomic Loci.

Authors:
E. Kahney 1 ; M. Wooten 2 ; L. Feng 3 ; X. Chen 4


Institutes
1) Johns Hopkins University, Baltimore, MD; 2) Johns Hopkins University, Baltimore, MD; 3) Johns Hopkins University, Baltimore, MD; 4) Johns Hopkins University, Baltimore, MD.


Keyword: asymmetric cell division

Abstract:

Stem cells rely on epigenetic mechanisms to regulate cell identity as they undergo asymmetric cell division (ACD) to give rise to a daughter that self-renews and another daughter that differentiates, despite each containing identical genomes. Pioneering work from our lab developed a dual-color labeling system to study ACD in Drosophila male germline stem cell (GSCs) and found that preexisting (old) histones are preferentially retained in the self-renewed GSC while newly synthesized (new) histones are preferentially enriched in the differentiating daughter. Histone modifications play important roles in regulating gene expression and thus stem cells may maintain their identity by selectively inheriting preexisting histones. Hypothesis: During ACD, the self-renewing daughter inherits old histones with modifications that maintain active expression at stem cell-promoting genes and repression at differentiation genes. This allows for continuation of the stem cell state by maintaining epigenetic memory. Meanwhile, the differentiating daughter inherits new histones in these regions, erasing the epigenetic stem cell memory and allowing for differentiation to occur.

I will investigate the conservation of asymmetric histone inheritance during ACD in the Drosophila female germline and elucidate the epigenetic histone modifications present at genomic loci that undergo gene expression switching during differentiation. Using previously described methods, I have compared the histone inheritance patterns found in actively dividing female GSCs to those found in male GSCs. We know that during ACD in the male germline, old versus new histones are almost completely segregated between the two daughters. In contrast, my new data show that global histone inheritance patterns in actively dividing female GSCs appear symmetric, but correlation mapping can identify significant regions of non-overlapping old and new histone signal. The difference in gene expression programs for male versus female gametogenesis might underlie the global chromatin change seen during male GSC differentiation but local change during female GSC or other cell lineage differentiation, which undergo far fewer expression changes. To explore this possibility, I have created an array of fluorescent probes targeting well-characterized genes associated with promoting the stem-cell state or the differentiation program. To visualize changes in the epigenome that occur during ACD, I will add these fluorescent probes along with antibodies against old versus new histones as well as specific histone modifications in order to perform a proximity ligation assay. This method will reveal how the epigenome signature changes between the renewed stem cell daughter versus the daughter destined to differentiate in ACD.