Excessive Cell Growth Causes Cytoplasm Dilution And Contributes to Senescence

https://www.sciencedirect.com/science/article/pii/S0092867419300510

Gabriel E. Neurohr, Rachel L. Terry, Jette Lengefeld, Megan Bonney, Gregory P. Brittingham, Fabien Moretto, Teemu P. Miettinen, Laura Pontano Vaites, Luis M. Soares, Joao A. Paulo, J. Wade Harper, Stephen Buratowski, Scott Manalis, Folkert J. van Werven, Liam J. Holt, Angelika Amon

• Cells of a particular type tend to display a relatively narrow range of cell sizes (relative to the orders-of-magnitude difference in cell size between cells of different types). 
• When cell cycle is blocked in budding yeast, cells continue to grow. The authors were able to reversibly arrest the cell cycle by perturbing a particular gene (CDC28), providing them with a x12-fold variation in cell volume. Note that denying these cells with glucose, or applying cyclohexamide, prevented the mutants from growing large.
• When allowed to re-enter the cell cycle, larger cells proliferated more slowly, and delays cell cycle progression.
• The authors observed that cell cycle regulators are produced at a lower rate in oversized cells (although the pool-size was comparable to normal cells).
• When cells exceeded ~200 fL, cellular growth shifted from exponential to linear
• In the linear growth regime, cell volume increased faster than total RNA and protein, suggesting dilution of cellular macro-molecules. Direct measurement of cellular density showed a 36% reduction in total cell density, largely explained by reductions in protein and RNA mass.
•  Transcriptome and proteome analysis suggested that general transcription and translation machineries becomes limiting in large cells.
• Using nocodazole to generate diploid cells, large diploid cells grew faster than large haploid cells, and also progressed faster through the cell cycle. The authors therefore suggest that the nDNA:(cytoplasmic volume) ratio is what limits cell growth in oversized cells.
• The authors found that the majority of old yeast cells (>16 cell divisions) were >200 fL and display many of the phenotypes of oversized cells. 
• Excessive increase in cell size was sufficient to reduce lifespan
• The authors tested many of these observations in human fibroblasts.

Thoughts
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• Some of the findings in here, especially relating to cellular growth rates, remind me of this
• Quantifying single-cell mtDNA copy number in this system would be extremely interesting!