| Summary: | This thesis explores the unequal partitioning of damaged proteins during mitosis and its implications for cell fate. Initially described in unicellular organisms, it was unclear if this method was used in vivo in multicellular organisms and had functional consequences in mammalian cells. To determine if this asymmetry was conserved in multicellular organisms, I studied three stem/progenitor populations in Drosophila: the larval neuroblast, adult female germline stem cell, and adult intestinal stem cell. Each cell type was found to asymmetrically segregate damaged proteins identified by the 2,4-hydroxynonenal (HNE) modification, which are associated with oxidative stress and age. Both the larval neuroblast and female germline stem cell were found to retain damaged proteins during division, whereas the intestinal stem cell segregated damaged proteins to differentiating progeny. I suggest that functional lifespan, and not cell type, determines the cell that receives the majority of damaged proteins during division. In each cell type, damaged proteins were associated with DE-Cadherin, a common component of the stem cell niche and removal from the niche was associated with reduced damaged protein polarization. Interestingly, when larval neuroblasts were mechanically dissociated from their niche and placed in culture, the internal polarization of damaged proteins was found to increase with progression through the cell-cycle. Therefore, I suggest that both the niche and intrinsic factors play a role in the asymmetric division of damaged proteins. To determine if an asymmetric division of damaged proteins influenced cell fate, I used a mammalian cell line with inducible expression of misfolded Huntingtin, which shares similar properties to damaged proteins. This study also revealed that the conformation of damaged proteins impacts cell fate: cells with diffuse Huntingtin displayed greater proliferation and reduced resistance to stress. Tracking cells containing an aggregate with live-imaging revealed that the cell that inherits the aggregate has a longer cell-cycle and an enhanced capacity to differentiate. Therefore, the asymmetric inheritance of damaged proteins impacts cell fate. In the final chapter of this thesis, I discuss the implications of an asymmetric division of damaged proteins on cell fate and how this information can be applied to cancer treatments.
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