March 13, 2017
A square slab can make an excellent door stop. Over time, though, the corners can get chipped, making the slab a bit rounded. This new bit of rock makes a less useful doorstop, but a much better wheel! The chipping and aging of the original stone has made it worse in some situations, but better in others.
A new study by Frenk and coworkers in Aging Cell shows that something similar can happen in yeast. Young yeast are much better at utilizing glucose, but older yeast have them beat with galactose (as well as with raffinose and acetate).
One way to think about this is that age turns yeast from a glucose specialist into a sugar generalist. Aging chips away at a yeast cell’s ability to use glucose, but this loss results in a gain in its ability to use galactose.
So at least in the right environment (i.e., when there’s lots of galactose around), with our old friend Saccharomyces cerevisiae, there can be advantages to getting older.
What makes this particularly fascinating is that at least in yeast, this suggests that there may be a positive selection for aging because of the advantage it can give in certain environments. Those yeast who are ageless would compete less well compared to their aging counterparts when their glucose was taken away. The aging process wins out over immortality!
Frenk and coworkers used a relatively simple experimental set up. Take young cells and old cells, mix them together, and see which outcompetes the other using various sugars.
They used yeast that had been aged for 6, 24, and 48 hours in glucose. This is a nice range as 6-hour “old” yeast are fully viable, 24-hour “old” yeast are starting to suffer a bit in the reproductive viability department, and the 48-hour “old” yeast have passed the median lifetime of a yeast cell. Young adult, middle aged, and elderly yeast.
In the first experiment, they compared these yeast to log-phase yeast which the authors refer to as young (vs. the other three which are referred to as aged).
While the 6 hour yeast could hold its own against the young yeast in glucose, the 24-hour and 48-hour yeast grew much more slowly. This is what you would expect, the younger yeast growing faster than the older yeast. The young guns outdoing the older generations.
The situation was different in galactose. Here, the elderly, 48-hour yeast, ran circles around the young yeast. They blew them out of the water.
And it appears to be an age thing. When they compared 6- and 48-hour yeast that were aged in galactose instead of glucose, the more aged yeast still won. So, it wasn’t the shift in environment that caused the difference, it was in the older yeast cells all along.
The change is also not permanent. The offspring of the older yeast weren’t any better at growing in galactose than the younger yeast were. Only the cells that had lived a longer life could use galactose so well.
A concern here is that yeast as old as 48 hours are pretty rare in the wild. But when they changed assays and looked at colony size as opposed to competition, they saw that even 18-hour yeast had an advantage over the young whippersnappers.
This was such a surprising result that they also looked at cell cycle times of individual aged cells and their daughters. The older mother cells cycled faster in galactose than their daughters. And the opposite was true in glucose.
So it really looks like there are advantages to growing older. Things break down a bit, but that breakdown uncovers new talents that had previously lain dormant.
If you’re a yeast, growing old is not a one-way decline into dotage. You gain new abilities that, under the right conditions, let you outcompete your children! The older cells are selected for in the right environments. #APOYG shows us something good about growing older.
by Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight
Tags: aging , evolution of aging , generalist , specialist