Adding ATP to the growth medium extended yeast lifespan by 41% and by 60% in yeast carrying a parasite’s ATP transporter gene
Adding ATP to the growth medium extended yeast lifespan by 41% and by 60% in yeast carrying a parasite’s ATP transporter gene
Alaattin Kaya’s group at the University of Virginia posted the second version of their preprint in eLife on October 14. A yeast cell’s lifespan is measured by the number of daughter cells it produces by budding (cell divisions); 41% and 60% are increases in the median number of divisions.
Mitochondria often produce less ATP as cells age. Testing whether this causes aging or results from it is difficult because cells die without ATP synthesis. The authors of the study worked around this problem by supplying ATP from outside the cell, using a gene encoding an ATP transporter from the parasite Encephalitozoon cuniculi. This parasite lacks the Krebs cycle, a component of cellular respiration, and uses these transporters to take ATP from its host cell. The researchers introduced one of these transporters, NTT1, into the yeast cell membrane, allowing the cells to take up ATP from the medium.
Without supplementation, cells carrying NTT1 had shorter lifespans than ordinary yeast cells, and measurements showed that they contained less ATP. Half an hour in ATP-supplemented medium increased ATP levels tenfold in these cells; no increase was detected in ordinary yeast. At 10 micromoles of ATP per liter, the median number of divisions increased by 41% in ordinary yeast and by 60% in yeast carrying NTT1, each compared with the corresponding group without supplementation.
Ordinary yeast also lived longer, although the authors’ measurements showed no ATP uptake. Added ATP altered the expression of 500 genes in these cells, leading the authors to suggest that yeast have a sensor that detects external ATP. In yeast carrying NTT1, ATP uptake suppressed the expression of genes involved in glucose uptake and breakdown, respiration, and the removal of damaged cellular components. The authors propose that cells break down nutrients to produce ATP, generating damaging byproducts in the process. An external supply of ATP could reduce the need to break down nutrients, resulting in less damage and less need to remove damaged components.
To test the role of mitochondria, the researchers removed mitochondrial DNA from some yeast cells, preventing them from carrying out respiration. These experiments used 10 millimoles of ATP per liter, a thousand times the main experimental dose. This dose extended lifespan in ordinary yeast but provided a much smaller benefit to cells carrying NTT1, an effect the authors describe as toxicity. Without respiration, both this toxicity and the shorter lifespan of NTT1 cells without supplementation disappeared, while ordinary yeast still benefited from supplementation. The authors distinguish two mechanisms: external ATP extends lifespan independently of mitochondria, while the effects of imported ATP depend on them. Excess ATP inside the cell is harmful only when respiration is active.
The cells were classified according to two aging pathways. Without supplementation, about 60% of ordinary yeast cells follow the first pathway: the DNA region containing ribosomal RNA genes becomes unstable, mitochondria remain functional, and cells undergo an average of 18 divisions. In the remaining 40%, mitochondria fail, ATP levels fall, and cells undergo an average of 12 divisions. ATP supplementation reduced the proportion of cells following the second pathway in both groups.
The eLife editors consider the lifespan extension well supported, while its cause remains unresolved: intracellular ATP or a stress response to external ATP. Reviewer №2 argues that the extension depends on neither the transporter nor intracellular ATP levels:
“You exposed the cells to what looks like osmotic stress, and it did not even matter whether they had mitochondria to respond.”
Reviewer №2 cites Kaeberlein’s 2002 study, in which a high concentration of dissolved substances in the medium (osmotic stress) extended yeast lifespan by activating the stress-response protein Hog1. In the Kaya group’s study, however, ATP reduced levels of the active form of Hog1.
Reviewer №1 suggests that NTT1 may also transport ATP out of the cell, which would explain the low ATP levels in NTT1 cells without supplementation. The reviewer proposes testing this with labeled ATP so that the movement of the molecules can be tracked.