April 22, 2022
Replicative lifespan (RLS), determined by the number of daughter cells a mother cell produces before death, has proved to be an effective model for studying aging in budding yeast. The chromatin-associated proteins Sir2p and Fob1p have been shown to modulate ribosomal DNA (rDNA) and impact the formation of extrachromosomal rDNA circles (ERCs), an accumulation of which is linked to a shorter lifespan.
A recent study by Hotz M et al. in PNAS has shown that chromosomal rDNA copy number (CN) positively correlates with RLS in budding yeast. The authors performed whole-genome sequencing (WGS) of 13 wild-type strains and analyzed the lifespan data, which showed an increased rDNA CN along with enhanced RLS. Additionally, the data showed that the rDNA CN explains the majority (~ 70%) of RLS variation observed in almost identical wild-type strains.
To understand this correlation, the authors analyzed ERC levels in aging cells and fob1-Δ strains, in which ERC levels are low. Together, the analysis concluded that ERCs are inversely correlated with rDNA CN. Exploring further, the authors found that cells with lower rDNA CN showed improved accessibility of the upstream activating factor (UAF) complex binding site at the SIR2 locus. This change in chromatin accessibility reduces the expression of SIR2, causing higher ERC levels and thus a shortened lifespan, implicating both Sir2p levels and ERCs as the underlying cause of the CN-RLS correlation.
Additionally, the authors analyzed the CN-RLS relationship in a set of mutant strains (such as hda2-Δ, upb8-Δ, gpa2-Δ, etc.), all known to increase lifespan. The data showed that while some mutants appeared to impact the CN-RLS relationship, rDNA CN strongly influenced the RLS of these mutant strains (except for fob1-Δ).
Thus, the study demonstrates how rDNA CN impacts yeast lifespan by regulating certain aging factors and highlights rDNA copy number as an essential parameter to examine in aging studies.
Categories: Research Spotlight
March 25, 2022
Yeast researchers have long observed asymmetry in cytosolic pH between mother and daughter cells. Likewise, researchers have detailed the accumulation of long-lived asymmetrically retained proteins (LARPs) in mother cells, one of which is the plasma membrane proton pump Pma1p. Pma1p transports protons out of the cytosol, thereby increasing pH, and mother cells show marked increases in vacuolar pH as they age. As mother cells have a shorter replicative life span (RLS) than daughter cells, and aging factors have been linked to pH, it is reasonable to ask whether accumulation of Pma1p itself reduces life span in mother cells.
To address this specific question, Yoon et al. in a recent issue of International Journal of Molecular Sciences described screening for suppressors of asymmetric inheritance of Pma1p. They identified three vacuolar protein sorting genes (VPS8, VPS9, and VPS21) for which mutation resulted in high percentages of abnormally symmetric distribution of Pma1p-GFP. As all three of these genes are involved in endocytosis, they asked whether these mutations affected all proteins that are asymmetrically distributed between mother and daughter cells, or just those that reside in the plasma membrane. They found the latter, that the defect is restricted to PM proteins.
Using these mutants with abnormal asymmetric distribution of the proton pump, the authors asked if defects in asymmetric distribution of Pma1p caused changes in aging. Did mother cells without accumulated Pma1p have a longer life span? The answer was a clear “No.” There was little difference in RLS between mutant and wild type, showing that asymmetric distribution of Pma1p does not correlate with aging.
These results are in fact a bit surprising, because aging in mother cells had been previously correlated with a mutation in PMA1 (the pma1-105 allele, Henderson et al., 2014), which increased lifespan by about 30%. Thus, it appears that Pma1p plays a role in aging, but not via asymmetric distribution between mother and daughter cells.
This study adds to what is known about a complicated system. As usual, the awesome power of yeast genetics (#APOYG) provides an excellent forum in which to ask complicated questions in a simple system. Hopefully the links between pH, aging and asymmetry will be revealed in future experiments.
Categories: Research Spotlight