Date Published: March 21, 2018
Publisher: Impact Journals
Author(s): Andrew C. Peifer, Patrick H. Maxwell.
Retrotransposons are abundant mobile DNA elements in eukaryotic genomes that are more active with age in diverse species. Details of the regulation and consequences of retrotransposon activity during aging remain to be determined. Ty1 retromobility in Saccharomyces cerevisiae is more frequent in mother cells compared to daughter cells, and we found that Ty1 was more mobile in nonquiescent compared to quiescent subpopulations of stationary phase cells. This retromobility asymmetry was absent in mutant strains lacking BRP1 that have reduced expression of the essential Pma1p plasma membrane proton pump, lacking the mRNA decay gene LSM1, and in cells exposed to a high concentration of calcium. Mother cells had higher levels of Ty1 Gag protein than daughters. The proportion of protease-processed Gag decreased as cells transitioned to stationary phase, processed Gag was the dominant form in nonquiescent cells, but was virtually absent from quiescent cells. Treatment with calcium reduced total Gag levels and the proportion of processed Gag, particularly in mother cells. We also found that Ty1 reduced the fitness of proliferating but not stationary phase cells. These findings may be relevant to understanding regulation and consequences of retrotransposons during aging in other organisms, due to conserved impacts and regulation of retrotransposons.
Retrotransposons copy their RNA transcripts through reverse transcription to produce cDNA molecules that are inserted into genomes, and they have been associated with aging in several model organisms . Retrotransposon expression, integration of new cDNA copies (retrotransposition), or both are increased with age in S. cerevisiae, C. elegans, D. melanogaster, mice, and normal diploid human cells . Correlations between retrotransposon activity and aging-related genomic instability have also been observed [3–6].
We have shown that Ty1 retromobility asymmetry between mother and daughter cells as well as NQ and Q cells depends on mRNA decay factors, pH homeostasis, and can be abolished by exposure to high calcium. Total Ty1 Gag levels were increased in mother cells compared to their daughters, which could account for increased retromobility in mothers, but is in contrast to our previous results from analysis of a Gag-GFP fusion protein by microscopy . The GFP fusion for that construct is at the processing site for Gag, which prevents protease processing of Gag-GFP, and we found that nearly all Gag in cells in our mother-daughter sorting experiments is the processed p45 form. There was a higher level of Gag-GFP in mothers compared to daughters as measured on Western blots, but the asymmetry was not as pronounced as for wild type Gag. Perhaps the lack of processing or some other aspect of the fusion diminishes its asymmetric distribution. There was a steady decrease in mature p45-Gag as cells transitioned from exponential phase to stationary phase without a substantial decrease in total Gag. NQ cells contained p45-Gag, while Q cells contained unprocessed p49 or phosphorylated p47. Accumulation of immature p49-Gag reflects inhibition of VLP formation or maturation , and accumulation of p47 is associated with VLPs that have reduced levels of Pol protein and reduced reverse transcriptase activity . An increased proportion of either of these forms of Gag could account for reduced retromobility during stationary phase and in Q cells. Exposure to high calcium reduced total Gag levels and increased the proportion of p47/49-Gag particularly in mothers and NQ cells, consistent with the absence of retromobility asymmetry in treated cells. Many factors regulating retrotransposition and impacts of retrotransposons are conserved between Ty1 and mammalian retrotransposons [2,8,24], and our results may prove relevant to further understanding the potential impact of mammalian retrotransposons on aging.