Thread by @CharlesMBrenner

I'll defend model organisms. However, the proposal that something is conserved for longevity needs to be specifically examined. It's time to explain something about yeast that ppl haven't thought about enough and yes, this concerns SIR2 /1

https://twitter.com/Longevity_Doc/status/1428162860815773702

there are two assays one can use to analyze lifespan in yeast. The 1st one is called chronological lifespan. It's simple. Grow a culture of yeast and see how long the cells are viable to form a colony. Chronological lifespan is measured in days or weeks /2

it might surprise you that sir2 DELETION lives longer in this assay. the work was done by Valter Longo and was published in Cell cell.com/fulltext/S0092… ... let that result sink in a minute /3

The 2nd way to measure aging in yeast is to see how many times a single mother cell can produce daughters. This is called replicative aging. Realize that half of the cells in a dividing culture have never been mother cells (they budded off mothers and are brand new daughters) /4

The other half of the culture have been mothers some number of times. 25% of the culture has been a mother once, 12.5% has been a mother twice, 6.25% has been a mother three times. Get it? /5

There are elaborate tricks to purify old mothers--they'd be mother cells that have 10 or more bud scars. If you look for these cells in a growing culture, you will NOT FIND THEM! /6

Amazingly, a yeast mother can produce daughters about 20 times & if you calorie restrict it, it can do this around 40 times. We know this ONLY BECAUSE we install a graduate student at a microscope for a week, who removes the daughter cells every 90 minutes or so & counts /7

the number of replicative cycles of physically separated mothers. it's true that more SIR2 extends the mother's replicative lifespan (just as lack of SIR2 extends chronological lifespan). also true that yeast needs SIR2 to extend replicative lifespan on calorie restriction /8

but the idea that based on this result, one would expect SIR2 to have been conserved as a longevity gene is silly on its face. there is no selective advantage for this phenotype & there are no observable old mothers in a growing culture /9

Based on the yeast results, if you were making an evolutionary prediction--a gene was conserved to confer a longevity survival advantage--you'd be influenced by the chronological aging experiments: sir2 deletion survives better to form a colony from an old culture /10

In contrast, the replicative longevity phenotype of SIR2 is totally dispensable: >97% of the cells in a culture have been mothers 4 times or less! 20-time mothers have no selective pressure on them. Thus, when ppl did worm & fly experiments in particular strain backgrounds /11

they should have been surprised that there were any effects of SIR2 overexpression & looked at other strains themselves. They would have saved TENS OF BILLIONS OF $ of future investment in sirtuins if they looked critically at their own results /12

Instead of questioning the relevance of a yeast assay for whether incredibly rare 20-time mothers can form a daughter a 21st time & recalling the long-lived phenotype of sir2 yeast in the chronological assay, they were confirmation biased /13

High impact journals published the results claiming that SIR2 genes are anti-aging in animals to great fanfare and the sirtuin cult was formed in earnest. Later, when these results were debunked, others have come along to express a sirtuin in one tissue or another /14

in experiments that are intended to rehabilitate the confirmation biased results. We're told you do get the result if SIR2 is overexpressed in the fat body, etc, etc. Sorry, that's not what the prediction was. The prediction--based on a totally dispensable phenotype of /15

old yeast mothers was that overexpression of SIRT1 homologs should extend lifespan. Not just in some worm or fly strains, in all of them. And not if you only turn it up in one tissue but the idea is that this is a fundamental longevity gene that you want to rev up in ppl /16

I'm explaining all this bc @BelenkyLab & I showed that NR extends yeast lifespan in the old mother yeast assay. I also consulted for Sirtris & know the data as well as anyone. Sirtuins are moderately interesting enzymes & they use NAD /17

They are LESS REGULATED than almost any NAD-dependent enzyme that I can think of: certainly less regulated than members of the PARP superfamily, SARM1, the intracellular form of CD38 or any redox enzyme /18

They have not been shown to mediate the beneficial effects of NAD repletion, though that is what many groups have tried to show. Sirtuins are said to be longevity genes but after 20+ years, it's a fact that the only good data are in yeast! /19

In the chronological aging assay, SIR2 is an antilongevity gene & in the old mother assay, SIR2 is a longevity gene that doesn't confer a survival advantage that would qualify it as a longevity gene. Now, back to figuring out why our metabolism declines at 60 ☮️😏✌️