Cell competition, aneuploidy, and aging

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Project Summary/Abstract Aging, the progressive deterioration of tissue and organ function, is the largest risk factor for developing disease. Increasing evidence points to the accumulation of damaged cell cycle-arrested cells with age (cellular senescence) as a cause for the development of certain aging-associated diseases. Clearance of senescent cells results in improvement in age-related phenotypes. Accordingly, limiting the emergence of senescent cells is expected to mitigate age-related phenotypes. Recent studies, including our own, point to aneuploidy, an abnormal chromosome number, as a feature of senescent cells. Our recent studies in Drosophila melanogaster demonstrate that cell competition, the elimination of cells based on their difference from other neighboring cells rather than their intrinsic properties, is a means for removing aneuploid cells because of copy number changes in Ribosomal protein (Rp) genes. In particular, cell competition that eliminates aneuploid cells is anticipated to prevent the accumulation of senescent cells. The 80 eukaryotic Rp genes map across the entire chromosome complement, thus aneuploidy or other large- scale genome changes almost always affect their copy number making cell competition based on Rp gene dosage a strategic mode to recognize and eliminate aneuploid cells. A specific mutation affecting RpS12 in Drosophila prevents the elimination of Rp+/- cells by cell competition, and permits greater survival of aneuploid cells in Drosophila. We will test whether a homologous process affects incidence of aneuploidy and aging phenotypes in a novel mouse model, Rps12 mutant mouse strains, and an aneuploidy induced mouse model whereby aneuploidy is induced by reduced expression of Bub1, an essential kinase for proper chromosome segregation. In Aim 1 we will Determine whether Rps12D90/D90 mice accumulate aneuploid cells, focusing on the cortex. Prior studies establish that aneuploidy is prominent in the embryonic cortex but decreases significantly in frequency by 4-month post-partum, consistent with loss of aneuploid cells during this period. Using custom developed assays for the quantification of aneuploidy in single cells we will measure aneuploid cell frequencies at E13.5 and at 4 months post-partum in RpS12 mutant cortex and in wild type cortex from sibling controls. These assays will establish whether RpS12 plays a conserved role in recognizing and eliminating spontaneous aneuploid cells in mouse cortex. In Aim 2 we will lineage-trace aneuploid cells to determine whether Rps12D90/D90 mice are defective for eliminating aneuploid cells. Conditional reduction in Bub1 expression driven by Homeobox protein 1 -Emx1-Cre will generate clones of Bub1 defective aneuploid cells that will be identified, in the ROSAnT-nG lineage-tracing background, as cells expressing EGFP rather than tdTomato. FACS analysis of isolated nuclei from whole brain will provide EGFP/tdTomato ratios to specifically address roles of RpS12 in aneuploid cell survival or elimination. Lastly, in Aim 3 we will perform a longitudinal study to assess the health of aging Rps12D90/D90 mice. Assays for physical, behavioral, metabolic, and inflammatory deterioration that mark aging will be performed on young (6 months) and aged (22 months old) Rps12D90/D90 mice in comparison with matched controls to assess aging. Globally, these studies represent a first exploration in mammals of the hypothesis that cell competition can prevent the accumulation of senescent cell by eliminating those aneuploid cells that reduce Rp gene copy number, as is the case in Drosophila and will determine if the role of RpS12 in this process is conserved and can be exploited as a potential route to rejuvenation.