From Yeasts to Chloroquine

This paper is from Hughes and Gottschling

An early age increase in vacuolar pH limits mitochondrial function and lifespan in yeast

It got a mention in the blog back in 2012 when it was freshly published. The group have gone on to study yeasts, ageing and the lysosome-like vacuole of yeasts. Their core finding is that vacuolar pH controls mitochondrial "health" which controls ageing, at least in their model.

The group has been very busy and earlier this year this paper was published from Hughes' lab:

Cysteine Toxicity Drives Age-Related Mitochondrial Decline by Altering Iron Homeostasis

The paper describes a very long series (way too many to detail here) of experiments aimed at adjusting vacuolar pH upwards and downwards and observing the effect on the survival of mother yeast cells through repeated cell divisions (replicative age rather than chronological age, there are arguments about which matters most).

Bottom line: Acidifying vacuolar pH extends lifespan, reducing its acidity shortens it.

Why should that be?

Their next series of experiments demonstrated that cysteine toxicity was the driver of early mitochondrial functional decline secondary to loss of vacuolar acidity. Cysteine is normally harmless and essential for life. Your cells love it, just so long as it is within the vacuole (or lysosome in humans), not in the cytoplasm. It's kept there by a vacuolar amino acid transporter driven by the vacuole proton gradient. The pH gradient is generated using a vacuolar vATP-ase to pump protons from the cytoplasm in to the vacuole, using ATP. It's related to the mitochondrial ATP synthase but normally runs in reverse.

If, on a long term basis, vacuolar pH rises (ie the vATP-ase fails), cysteine is released from the vacuole in to the cytoplasm where it auto-oxidises, generating much too much hydrogen peroxide. This reacts with the iron-sulphur clusters of complex I and many other crucial enzymes in the mitochondria. In old age cysteine becomes toxic through vacuolar failure.

I've been interested in this for some time because Barja and Sinclair have both intimated that they are tending to avoid animal proteins in favour of low cysteine/methionine plant proteins. Cysteine is the cellular executioner when vacuole pH rises during the old age of yeasts or lysosomal pH rises in ageing mammalian cells. It's interesting because methionine restriction (which reduces cysteine levels) appears to core to the longevity promotion seen with caloric restriction or protein restriction in mice fed on crapinabag.

You have to wonder whether we are looking at this the wrong way round. What if crapinanbag, based on starch and sucrose, causes early onset lysosomal failure which can be ameliorated by removing the cysteine, which is the cellular execution mechanism?

This would make methionine restriction's longevity extension rather specific to glucose based metabolism. My biases would tend to favour this point of view. There are no data, yet.



As an aside:

Now, I have speculated that both influenza and corona viruses need anabolic processes generated by mTOR activation. This requires acute acidification of the lysosome. Blocking acute lysosomal acidification is one technique currently being investigated for treating the life threatening pneumonia which develops in susceptible individuals during the current COVID-19 pandemic. There are suggestions that chloroquine, a suppressor of lysosomal acidification, might be an effective treatment. My guess is because it blocks anabolism.

There is probably a fine line between suppressing anabolism and releasing a mitochondrial-executing concentration of cysteine.

Neither Hughes nor Gottschling were considering therapeutic inhibition of vacuolar acidification as a stratagem for anything. They were more interested in avoiding long term loss of vacuolar acidity to delay mitochondrial function decline. But blunting anabolism without causing catastrophic cysteine release is a current anti-viral/anti-neoplastic therapeutic target.

You can see that the drug chloroquine a) might work and b) might be very toxic in overdose.

It does currently appear that it might work but we should never forget that "clinical experience is no guarantee of therapeutic efficacy".

However it would be great if it really did work.

Peter