Scientists have found that in killifish, old age is associated with a dampened response to fasting and identified the protein that might be responsible. A similar mechanism might exist in humans [1].
Fast less furious
Various fasting regimens are thought to confer health and longevity benefits [2], and some potentially geroprotective drugs actually mimic caloric restriction. However, recent research suggests that the response to fasting might be blunted by old age [3], limiting the potential effect of dietary interventions.
In this new study published in Nature Aging, the researchers used turquoise killifish, a popular fast-aging animal model, to investigate age-related changes in response to fasting and refeeding. Following a five-day fast, a transcriptomic analysis revealed almost 2,500 differentially expressed genes (DEGs) in the visceral adipose tissue of young fish. Just like in mammals, fasting negatively affected many genes related to energy expenditure, protein synthesis, and cellular proliferation. However, in older fish, the transcriptomic effect of fasting was much more modest: only 359 DEGs.
Further analysis found that the reason was not a blunted response to fasting, as the researchers initially assumed. Rather, old animals had a more fasting-like transcriptomic profile by default, and subsequent fasting could only nudge it further a bit. For instance, fasting leads to the release of non-esterified fatty acids (NEFAs) as an alternative energy source. However, both fed and fasted older killifish had elevated NEFA levels.
Aging that looks like fasting
Aging is known to affect feeding behavior. In humans, with age, the recommended caloric intake decreases slightly. The fish in this experiment also ate less as they aged but without showing signs of starvation. Post-meal glucose levels were comparable between young and old fed fish.
Further transcriptional analysis showed more similarity between fasting and aging: 23% of genes changed by those two conditions overlapped, and 92% of such genes changed in the same direction. According to the researchers, one outcome of those genetic changes was decreased energy and lipid metabolism.
Among notable proteins whose levels were altered by fasting and aging was AMPK, a fuel-sensing enzyme central to energy metabolism that often pops up on geroscientists’ radar. In young animals, fasting induced upregulation of one AMPK subunit, γ1, and downregulation of another, γ2, but in older animals, this did not happen.
A bump in lifespan
The researchers introduced a mutation that leads to γ1 activation. Old fish carrying this mutation exhibited a much more pronounced transcriptional response to fasting with little resemblance to age-related transcriptional changes. “Sustained AMPKγ1 complex activation counters the age-associated FLTP (fasting-like transcriptional profile) and maintains energy and lipid metabolism responsiveness to feeding late in life,” the researchers commented.
Increased energy metabolism observed in the mutated fish led to improved tissue homeostasis. Aged mutated animals showed notable upregulation of genes related to DNA repair and cellular proliferation. Inflammatory pathways were among the downregulated, as were genes related to obesity, insulin resistance, and type 2 diabetes. The fish also showed reduced fasting blood glucose and triglycerides along with less age-related visceral adiposity. Most importantly, the mutation increased the fish’s lifespan by 20%.
Not just fish
How relevant is this discovery for humans? The researchers cite an earlier study in which fasting resulted in a similar dynamic in AMPK subunits, although that finding did not reach statistical significance [4]. Using a human gene dataset, the researchers found a significant age-related decrease in PRKAG1, the gene that codes for the AMPKγ1 subunit, in several tissues. Finally, the researchers obtained blood samples from 93 donors aged 65-90 with a broad range of age-related diseases and functional levels. The values of the multidimensional prognostic index (MPI), a measurement of general health, were negatively correlated with PRKAG1 expression, suggesting reduced multimorbidity and improved functional skills in people with higher levels of the protein.
The fasting–refeeding cycle triggers an inverse oscillatory expression of genes encoding the AMP-activated protein kinase (AMPK) regulatory subunits Prkag1 (γ1) and Prkag2 (γ2) in young individuals. Aging blunts such regulation, resulting in reduced Prkag1 expression. Transgenic fish with sustained AMPKγ1 countered the fasting-like transcriptional program, exhibiting a more youthful feeding and fasting response in older age, improved metabolic health and longevity. Accordingly, Prkag1 expression declines with age in human tissues and is associated with multimorbidity and multidimensional frailty risk. Thus, selective activation of AMPKγ1 prevents metabolic quiescence and preserves healthy aging in vertebrates, offering potential avenues for intervention.
Literature
[1] Ripa, R., Ballhysa, E., Steiner, J. D., Laboy, R., Annibal, A., Hochhard, N., … & Antebi, A. (2023). Refeeding-associated AMPK?1 complex activity is a hallmark of health and longevity. Nature Aging, 1-17.
[2] Anton, S. D., Moehl, K., Donahoo, W. T., Marosi, K., Lee, S. A., Mainous III, A. G., … & Mattson, M. P. (2018). Flipping the metabolic switch: understanding and applying the health benefits of fasting. Obesity, 26(2), 254-268.
[3] Hahn, O., Drews, L. F., Nguyen, A., Tatsuta, T., Gkioni, L., Hendrich, O., … & Partridge, L. (2019). A nutritional memory effect counteracts the benefits of dietary restriction in old mice. Nature metabolism, 1(11), 1059-1073.
[4] Defour, M., Michielsen, C. C., O’donovan, S. D., Afman, L. A., & Kersten, S. (2020). Transcriptomic signature of fasting in human adipose tissue. Physiological genomics, 52(10), 451-467.
