Development and Application of a Novel Method to Study Histone Inheritance in Asymmetrically Dividing Cells

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Proper development depends on asymmetric cell division (ACD), a process by which dividing stem cells produce a renewed stem cell and a differentiating cell. Many intrinsic and extrinsic factors guiding ACD have been found and characterized. Yet, the contribution of chromatin to cell-fate determination is poorly understood. Previously, our lab discovered asymmetries in histone and histone post-translational modification (hPTM) inheritance in Drosophila male germ cells. Further dissection of this process revealed it functions and is regulated in three-steps: 1) histone asymmetry is established during S-phase; 2) histone asymmetry is distinguished during M-phase; 3) the readout of the inherited asymmetric histones guides cell cycle progression following mitotic exit. Interestingly, disruption of these asymmetries results in both stem cell loss and overproliferation phenotypes, suggesting that asymmetric histone inheritance is an essential process in tissue health. Further, deterioration of this process may be common among diseases including age-related tissue degeneration and cancer. I hypothesize that asymmetric histone inheritance is a general mechanism that plays a crucial role in stem cell homeostasis and cell-fate determination during development. However, our ability to track histones and hPTMs at specific loci within single cells is severely limited at this time. Thus, we are in urgent need of new tools to study the roles and consequences of asymmetric histone inheritance in development, stem cell homeostasis, and cell-fate determination. In this proposal, I will 1) broaden our understanding of pervasiveness, roles, and patterns of histone inheritance in asymmetrically dividing cells and 2) develop a novel method for labeling non-repetitive loci to track epigenomic features. First, I will express histones tagged with the photoconvertible Dendra2 in Drosophila neuronal stem cell lineages. Neuroblasts (NBs), their progenies ganglion mother cells (GMCs), and transit-amplifying intermediate neuronal progenitors (INPs) are a well-studied stem cell model system. Following 405nm light induced photoconversion, I will observe the inheritance patterns of old (red) versus new (green) histones in type I and II NBs. These studies will 1) expand our knowledge on histone inheritance in ACD, and 2) reveal whether histone inheritance patterns change as asymmetrically dividing cells age and lose potency (e.g. INPs). Tracking epigenomic information at specific genomic locations has traditionally been performed using FISH. However, the heat denaturation required for FISH is either partially or completely incompatible with detection of DNA-associated proteins. To overcome this limitation, I will adapt Oligopaint technologies to design guide RNA libraries for tethering dCas9 to specific, non-repetitive loci. This will allow me to track histone and hPTM inheritance patterns at specific genomic loci with single-cell resolution. These works will both enhance our understanding of the mechanisms underlying ACD and provide new tools facilitating the detection of epigenomic alterations in research and clinical settings.