Can We Break The Mouse ‘Lifespan Wall’?

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Over the past 100 years or so, human life expectancy has roughly doubled. That’s an incredible achievement, but most of this gain is the result of fewer early deaths, mostly from infectious diseases. We owe our life expectancy primarily to innovations like vaccines, antibiotics and improved sanitation, and less to advances in cancer treatments and heart surgery. While those things have increased our life expectancy, they have not increased our health expectancy – in fact, there’s little evidence we’re getting cancer, heart disease or diabetes any later in life than we did 100 or even 1000 years ago. This means that people are living longer with debilitating age-related diseases.

The main goal of treatments targeting ageing is to slow down the ageing process itself or to repair age-related damage to our tissues and organs so that they continue to function for longer. However, any treatment that truly did slow or reverse ageing would be expected to also extend lifespan. This doesn’t just mean average lifespan or life expectancy – we would expect the longest-living humans to live longer and we would expect the lifespan record of Jeanne Calment (who died at 122) to be surpassed. This isn’t something we should shy away from, but actively pursue – there’s no shame in wanting to live longer.

The ‘Wall’

Many interventions that target the biology of ageing have effects in mice that would seem to suggest a slowing or reversal of the ageing process. Mice not only appear to age more slowly and develop disease later in life, but also achieve greater maximum lifespans. The first intervention shown to extend mouse lifespan was calorie restriction – a sharp reduction in calorie intake without causing malnutrition. Unless started very early in life, calorie restriction generally increases maximum lifespan in mice by around 4 months, a roughly 15% increase.

Since this discovery, many different methods of calorie restriction have been tested. Scientists have discovered drugs that mimic many of the biological effects of calorie restriction, as well as additional treatments that target other mechanisms of ageing such as telomere shortening and the buildup of senescent cells. However, none of these strategies have succeeded in surpassing the effects of calorie restriction when given in later life.

This apparent lifespan ‘wall’ is unlikely to be a hard limit, but rather a result of ageing being comprised of many interlinked processes. Targeting any given process by itself is unlikely to produce radical lifespan extension. However, there’s little research examining the effects of combinations of different strategies. We might expect that targeting multiple different biological processes at the same time would have some additive effects, allowing the record lifespan extension of calorie restriction to be surpassed. If this approach works, it’s good news for humans. If it doesn’t, it suggests that some of our assumptions about ageing are wrong and should point us towards a better understanding.

The Plan

In order to explore these ideas, the Longevity Escape Velocity Foundation (LEV) is running several mouse studies, the first of which began in 2023 and was completed earlier this year. That study, called RMR1 (Robust Mouse Rejuvenation), looked at 10 different combinations of treatments previously shown to extend mouse lifespan to see whether their effects would add up.

The Mice: RMR1 used a total of 1000 mice, half male and half female. This is important because the interventions being tested are known to have different effects in male and female mice. Treatment began at 18 months of age, with an 18 month-old mouse being very roughly equivalent to a human in their 60s. This is also important – interventions can impacted ageing differently depending on whether they are administered in late or early life, and we ideally want interventions that will work for everyone.

The mice used belonged to a strain called C57Bl/6J. These are highly inbred mice with numerous health problems that genetically diverse mice don’t get. They aren’t ideal models for human ageing, but the goal of this study was to determine whether the effects of multiple lifespan extending interventions could be additive. Since the interventions being tested had all been shown to extend lifespan in Bl/6 mice, researchers opted to use this strain.

The interventions: The study tested 4 different interventions in 10 different combinations. The first of those treatments was rapamycin, a drug that mimics some of the effects of calorie restriction. The remaining three focused on repairing age-related damage that has already occurred. The 4 interventions were as follows:

• Rapamycin: Rapamycin is a drug that was initially used to suppress the immune system in organ transplant recipients. It appears to extend lifespan in animals primarily by inhibiting a protein called mTOR, which regulates how cells handle energy from nutrients. mTOR is also suppressed during calorie restriction, and rapamycin and calorie restriction are thought to affect ageing through similar pathways.
• Gal-Nav: Galacto-conjugation of Navitoclax (Gal-Nav) is a type of drug known as a senolytic, which means that it removes senescent cells. Senescent cells are cells that have stopped dividing but refuse to die, and are believed to contribute to ageing largely through the release of harmful signalling molecules.
• mTERT gene therapy: mTERT is the mouse version of the TERT gene, which encodes an essential component of telomerase. Telomerase is an enzyme that repairs telomeres – the protective caps on the ends of our chromosomes. Telomeres become shorter each time a cell divides, eventually preventing it from dividing any further. Longer-lived animals tend to have slower rates of telomere shortening, while immortal organisms like hydra do not undergo telomere shortening at all. Delivering the mTERT gene via gene therapy has been shown to extend mouse lifespan.
• Haematopoietic Stem Cell Transplant (HSCT): Haematopoietic stem cells are cells within the bone marrow that generate new red and white blood cells. In old age, the ability of these stem cells to renew themselves declines, and they also begin to produce too many of certain blood cell types at the expense of others. Research has shown that transplanting haematopoietic stem cells from the bone marrow of young mice into old mice can extend lifespan.

There were 100 mice to a group. One group received all of of the above interventions, 4 groups received one intervention each, 4 groups received all but one intervention each, and the last group received a control treatment. In the groups not receiving all of the interventions, half of the mice received a mock treatment for the intervention(s) they weren’t receiving, while the other half did not.

What Happened?

The results of the study were quite different in male and female mice, with the data for females being the clearest. The bad news is that the group receiving all treatments did not fare any better than those receiving rapamycin alone – the drug that works similarly to calorie restriction. Furthermore, the mice receiving rapamycin together with two other treatments appeared to die off more quickly than those receiving rapamycin alone.

The good news is that aside from this exception, the mice receiving combinations of multiple treatments did appear to fare better than those receiving only one of the three damage repair treatments. This wasn’t just due to the presence of rapamycin in the multi-treatment groups. At the study’s half-way point, the mice receiving all treatments apart from rapamycin were doing just as well as all of the other combination groups and the rapamycin group. By the time 50% of the mice in the no rapamycin group were dead, 70% of the HSCT mice were dead and 75% of the Gal-Nav mice were dead. However, by the end of the study, survival in the no rapamycin group was no better than in mice receiving no treatment at all.

There are a few ways this could be interpreted. One is that the non-rapamycin interventions extended lifespan at the start of the study but their effects wore off towards the end. This could be explained by the fact that these were one-time treatments given to mice at the beginning of the study, while rapamycin was in the mice’s food for the entire duration. It may be that some of the treatments needed to be repeated in order to maintain their benefits until the end of the study.

However, it’s also important to remember that as this kind of study progresses, the sample sizes become smaller and can be more heavily influenced by chance. 20% survival in a given group corresponds to just 10 mice, so just a few mice dying or living unexpectedly long has a larger impact, a consideration that is well illustrated by the results from the male mice.

As with the female mice, the male mice receiving all 4 treatments did well, but after around 10% (5 mice) remain, they all died off very quickly. This could easily be due to ‘bad luck’ and is why focusing on the single longest living organism is not a good way to measure lifespan extension – there will always be outliers.

However, the rest of the data is rather confusing and contrasts starkly with the female data. Firstly, the male mice receiving rapamycin had worse survival than the all-four treatment group for most of the study, but caught up at the end. Secondly, some of the treatment groups fared a lot worse than the mice receiving no treatment at all. This included the no-rapamycin group, which showed the opposite pattern to the female mice – they fared poorly for most of the study, but the last 5 surviving mice then fared exceptionally well. Again, this could be an anomaly, and we’ll need to wait for the statistical analysis to be released before we can make any more detailed interpretations.

What’s Next

After RMR1, the next step will be to gather more data. That doesn’t just mean more mice, but also testing more interventions and collecting data about more than just lifespan. This is what RMR2 is going to attempt.

RMR2 will be conducted in a similar way to RMR1, but with some key differences. It will include 2000 mice and 8 interventions in 20 different combinations. Since rapamycin was consistently effective, it will be given to all of the treatment group mice instead of being included as one of the 8 treatments, which should make it clear whether any of the other treatments have additional lifespan-extending effects.

To address some of the limitations of the first study that we mentioned, some of the treatments will be administered multiple times throughout the study, not just at the start. RMR2 might also use genetically diverse mice instead of BL/6 mice. You can find a full list of the 8 planned treatments here, with detailed descriptions of how they work.