Who can have dysfunctional mitochondria yet live longer?
Every biomedical field has a slightly different definition of aging. There sure is plenty of common ground. One definition stands out because it makes the most sense in the fewest words. That is, you guessed it, the epigenetic definition: aging is the loss of epigenetic information, thanks to Sinclair’s team for the information theory of aging (Lu et al., 2023; Yang et al., 2023).
In this paper
“Aging-associated decline of phosphatidylcholine synthesis is a malleable trigger of natural mitochondrial aging”
Tetiana Polizhaieva from Maria A. Ermolaeva's team at the Leibniz Institute on Aging, assessed aging through mitochondrial function (and dysfunction).
One of the first topics they addressed was whether mitochondrial decline preceded other wear and tear in the organism. For this, they looked at worms.
The model organism was Caenorhabditis elegans: control worms with two mitochondrial mutant strains: clk-1 and isp-1. Clk-1 encode for the protein essential in ubiquinone biosynthesis and isp-1 encodes for an iron sulfur protein of complex III of mitochondria (Dues et al.; Felkai et al.). Strangely, these mutated strains live longer than the control worms.
The researchers followed the worms across adulthood and focused on how their overall proteins change throughout these time points:
young (day 1),
middle-aged (day 5)
old (day 10)
By day 5, control worms had accumulated significant wear and tear, measured by stress response proteins. However, the most significant metabolic changes occurred in the latest stage (day 10).
Molecular aging was already happening before mitochondrial dysfunction and decline, but this could be a part of the wear and tear as worms get old, such as loss of the mitochondrial network that has been identified as a hallmark of aging (Gaffney et al., 2018).
The two mitochondrial mutant worms presented an intriguing puzzle. If they can live longer despite their impaired mitochondria, maybe they have learned to adapt to this “new normal”, but how?
The authors looked more closely at proteins that declined most during normal aging but were maintained in the long-lived mitochondrial mutants. The protein that stood out- you probably didn’t guess this one- was S-adenosylmethionine synthase 1, or SAMS-1.
SAMS-1 produces S-adenosylmethionine (SAM), one of the cell's major methyl donors (Monné et al., 2022). SAM participates in numerous biochemical reactions, including DNA and histone methylation as well as lipid metabolism.
In normal worms, SAMS-1 declined substantially with age. In the two long-lived worms, the decline was a lot less steep. So is this the reason? Could maintaining SAMS-1 levels help these worms adapt to mitochondrial dysfunction?
The not so quick nor dirty way to figure this was to remove SAMS-1.
The researchers knocked down sams-1 with RNA interference. Things got weird fast. In control worms, reducing sams-1 extended lifespan, consistent with previous observations (Ching et al., 2010; Hansen et al., 2005), yet in the long-lived mitochondrial mutants, the same manipulation shortened lifespan.
SAMS-1 impact on longevity, hence depends on the condition of the mitochondria, and the worms with impaired mitochondria need SAMS-1 to live long.
What happens to mitochondria without SAMS-1?
One needs to “see” things to understand, so that's exactly what the researchers did.
They used worms expressing GFP within the mitochondria of their muscle cells, allowing the mitochondrial network to be visualized. Normally, these mitochondria formed beautiful elongated, tubular networks. But guess what happened to the beautiful networks?
This happened:
The long tubular mitochondrial network became round and bubbly. You may think they fused together like oil in water, but that’s not it; this looked more like mitochondria fission, as they became more fragmented (figure above is taken from the paper with permission).
From this observation, the authors concluded that SAMS-1 loss disrupted mitochondrial network integrity. Morphology alone was not all that was changed, of course.
To investigate what could underlie this change, the authors turned to another fluorescent worm, this time carrying hsp-6p::GFP.
HSP-6 is associated with the mitochondrial unfolded protein response (UPRmt), that is a protective stress-response pathway activated when mitochondrial protein homeostasis is disturbed. More GFP means greater activation of this mitochondrial stress response.
Following sams-1 knockdown in worms carrying the hsp GFP mark, this happened:
Figure from the paper, with permission. Note mip-1 is the positive control.
So, loss of SAMS-1 produced two phenotypes:
fragmented mitochondrial networks and activation of the mitochondrial unfolded protein response (UPRmt).
This raised the next question. SAMS-1 is an enzyme that makes SAM. Perhaps the problem wasn't SAMS-1 itself; the mitochondria were suffering because SAM was becoming limiting.
Give the worms SAM back!
The researchers supplemented the worms with 10 mM SAM & this happened:
Focus on the bar charts with red box around it, see how the stress response, measured by the heat shock protein GFP, has reduced. In other words, putting SAM back helped restore mitochondrial homeostasis. How was SAM exerting this influence?
And that trail led to a major phospholipid of cell membrane: phosphatidylcholine, or PC.
Membrane lipid composition is intimately connected to mitochondrial structure and dynamics. SAM contributes to one pathway used to synthesize PC. Two enzymes, PMT-1 and PMT-2, use SAM during the production of phosphocholine, which ultimately feeds into PC synthesis. If this pathway connected SAM to mitochondrial health, disrupting it should reproduce at least some of what happened when sams-1 was knocked down. Did this experimentally conform to the prediction?
When PMT-1 was knocked down, mitochondria fragmented again. More intriguingly, its effects on lifespan echoed the SAMS-1: reducing pmt-1 extended lifespan in control worms, but diminished the longevity of the long-lived mutants. There were a lot of parallels.
SAMS-1 produces SAM, and SAM is involved in PC production, and PC is important in mitochondrial function.
PC was looking suspicious. So researchers asked: can supplementing the media with PC (or choline as a more stable form) rescue the impaired mitochondrial phenotype in control worms?
They supplemented the worms with choline, which showed results similar to PC but was much simpler to administer; in adulthood on day 4 and day 6 (AD4 and AD6), the tubular mitochondrial network was partially restored, as shown in Figure E below, from the paper with permission.
The authors started with an age-associated molecular change, followed through the metabolic pathway, knocked down key proteins within the pathway and reproduced impaired mitochondria. They show that supplementing the worms' diet with key nutrients can ameliorate the mitochondrial abnormalities. As the worms age, significant molecular changes appear before mitochondrial decline. Meanwhile, worms carrying lifelong mitochondrial mutations could live longer as they have adapted throughout their life to cope with the defects.
One lesson here is that mitochondrial aging isn't about accumulating broken mitochondria, but about losing the cellular systems that keep mitochondria functional under stress.
And, at least in a tiny worm, some of that resilience can be influenced by what is available in the diet. Fascinating how diet can rescue genetic defects!
A special thank you to Dr. Maria Ermolaeva for giving us permission to use their figures in this post!
References:
Ching, T. T., Paal, A. B., Mehta, A., Zhong, L., & Hsu, A. L. (2010). drr-2 encodes an eIF4H that acts downstream of TOR in diet-restriction-induced longevity of C. elegans. Aging Cell, 9(4), 545-557. https://doi.org/10.1111/j.1474-9726.2010.00580.x
Dues, D. J., Schaar, C. E., Johnson, B. K., Bowman, M. J., Winn, M. E., Senchuk, M. M., & Van Raamsdonk, J. M. (2017). Uncoupling of oxidative stress resistance and lifespan in long-lived isp-1 mitochondrial mutants in Caenorhabditis elegans. Free radical biology & medicine, 108, 362–373. https://doi.org/10.1016/j.freeradbiomed.2017.04.004
Felkai, S., Ewbank, J. J., Lemieux, J., Labbé, J. C., Brown, G. G., & Hekimi, S. (1999). CLK-1 controls respiration, behavior and aging in the nematode Caenorhabditis elegans. The EMBO journal, 18(7), 1783–1792. https://doi.org/10.1093/emboj/18.7.1783
Gaffney, C. J., Pollard, A., Barratt, T. F., Constantin-Teodosiu, D., Greenhaff, P. L., & Szewczyk, N. J. (2018). Greater loss of mitochondrial function with ageing is associated with earlier onset of sarcopenia in C. elegans. Aging, 10(11), 3382-3396. https://doi.org/10.18632/aging.101654
Hansen, M., Hsu, A. L., Dillin, A., & Kenyon, C. (2005). New genes tied to endocrine, metabolic, and dietary regulation of lifespan from a Caenorhabditis elegans genomic RNAi screen. PLoS Genet, 1(1), 119-128. https://doi.org/10.1371/journal.pgen.0010017
Lu, Y. R., Tian, X., & Sinclair, D. A. (2023). The Information Theory of Aging. Nature Aging, 3(12), 1486-1499. https://doi.org/10.1038/s43587-023-00527-6
Monné, M., Marobbio, C. M. T., Agrimi, G., Palmieri, L., & Palmieri, F. (2022). Mitochondrial transport and metabolism of the major methyl donor and versatile cofactor S-adenosylmethionine, and related diseases: A review†. IUBMB Life, 74(7), 573-591. https://doi.org/https://doi.org/10.1002/iub.2658
Poliezhaieva, T., Li, Y., Chaudhari, P. S., Isildak, U., Alonso-Pernas, P., Valentim, I. S., Su, F., Espada, L., Bayar, M., Fu, L., Koeberle, A., Donertas, H. M., & Ermolaeva, M. A. (2026). Aging-associated decline of phosphatidylcholine synthesis is a malleable trigger of natural mitochondrial aging. Nat. Commun., 17(1). https://doi.org/10.1038/s41467-026-71508-7
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