October 21, 2015
In the old days, a car came with the bare minimum of features to get from point A to point B. The windows rolled down with a crank and it usually had a radio. That was about it.
As the car has evolved, it has gained a huge number of bells and whistles. There are power windows and power brakes, a baffling number of computer-based bonus features, personal wifi hotspots, and so on. All of these have undoubtedly made cars more fun and comfortable to drive. But they have come at a cost. Many cars simply do not last as long as their predecessors because these extras break easily.
Turns out life may be like a modern car. It has lots of nice features that help it to do better in the world. But a lot of these features may shorten its life span.
This point was reinforced in a recent study by McCormick and coworkers. They painstakingly searched through a library of 4,698 single gene deletion strains in S. cerevisiae and found that 238 of these strains were able to produce significantly more buds over their lifetime. Many nonessential genes seem to shorten a yeast’s life.
And boy was it painstaking! Believe it or not, they manually dissected over 2.2 million individual yeast daughter cells to generate these results. Luckily it was worth it, as they found so many interesting things.
First off, many of the genes they found fall into a set of five pathways that includes cytosolic and mitochondrial translation, the SAGA complex, protein mannosylation, the TCA cycle, and proteasomal activity. So there are certain pathways we can target to extend the lifespan of our friend yeast. And even better, yeast may not be the only beneficiary of these studies.
Two of the pathways, cytosolic and mitochondrial translation and the TCA cycle, have also been found to be significant in extending the life of the roundworm C. elegans. These pathways are also shared with humans.
And just because the authors found no overlap with the other three pathways in other beasts doesn’t mean they may not be targets for life extension in them too. It could be that previous screens in C. elegans simply missed genes from these pathways.
It could also be that what is found in yeast may turn out to be important in people but not in C. elegans. For example, the authors failed to find any equivalent to the SAGA complex in C. elegans. Either the roundworm lost this complex during evolution, or the homologs between yeast and C. elegans are so different that they’re unrecognizable. In any event, humans at least do have an equivalent to SAGA, called STAGA.
All of this suggests that there may be common ways to make organisms, including people, live longer, healthier lives. Here’s hoping!
And these five pathways are certainly not the whole story. The majority of the genes McCormick and coworkers identified were not in these five, which means there are probably lots of other ways to get at living longer.
One fascinating example that the authors decided to look at in depth was LOS1. Deleting it had one of the biggest effects on a yeast’s reproductive life span.
At first this seems a little weird, as Los1p exports tRNAs out of the nucleus. As expected, deleting LOS1 led to a buildup in tRNAs in the nucleus. The authors confirmed that this buildup is important by showing that overexpressing MTR10, a gene involved in transporting tRNAs from the cytoplasm to the nucleus, led to a longer lived yeast with a buildup of tRNAs in its nucleus.
The next step was to figure out why having a lot of tRNA in the nucleus makes yeast live longer. It was known previously that Los1p is kept out of the nucleus under glucose starvation conditions. The authors confirmed this result.
Most everyone knows that restricting calorie intake (also called dietary restriction or DR) can extend the lives of most every beast tested so far, including yeast. The authors found that growing a los1 deletion strain at low glucose did not increase the lifespan of this strain any further. It thus appears that an important consequence of DR is keeping Los1p out of the nucleus and thereby increasing the amount of tRNA in the nucleus.
While we don’t know yet exactly why keeping tRNAs in the nucleus helps yeast live longer, it is interesting that the increased lifespan associated with the loss of LOS1 is linked to caloric restriction. Finding a way to inhibit Los1p has to be better than starving yourself!
This study has identified 238 genes to follow up on for future studies. And of course there is a whole class of genes that haven’t yet been investigated—the essential genes! Many of these may be important for extending life too.
Stripping life down to its bare essentials may help individuals live longer at the expense of being the most fit in terms of survival in the hurly burly world of nature. After all, those “nonessential” genes undoubtedly have a function in helping yeast outcompete their less well-endowed yeast neighbors. Just like those power sliding doors are way better than the manual ones on a minivan.
But if you want a long-lived minivan, get the one with the manual doors. And if you want a long-lived yeast (or person), get rid of some of those nonessential genes that cause you to break down.
by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight