Peer review of: Independent regulation of age associated fat accumulation and longevity (2020)

Peer Review File  

 

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
The authors made a very interesting observation that there is lipid droplet accumulation in ageing 
cells (chronologically aged yeast cells) and that this is associated with expression level of a gene in 
the kynurenine pathway, i.e. BNA2. Through over-expression of this gene accumulation of lipid 
droplets does not happen, and the cells have increased life-span. This is a very interesting 
observation, but unfortunately as the paper stands now there is no real mechanistic explanation 
for the observation. The authors do evaluate the expression of other genes in the pathway, and 
show that deletion of these genes does not affect the observations. They also measured the 
intracellular levels of kynurenine and find that this is highly elevated in the BNA2 over-expressed 
cells. This led to an argument that the coupling between the kynurenine pathway and lipid droplet 
accumulation is due to an increased flux through the first pathway, and this then drains 
metabolites otherwise needed for lipid metabolism. This argument is questionable as the flux 
towards lipid metabolism is orders of magnitude larger than the flux through the kynurenine 
pathway, even when there is a large accumulation of kynurenine in the cells. 
 
The findings are, however, still very interesting, and I think the authors should just study more 
about the links between lipid metabolism and kynurenine metabolism, and in particular as yeast is 
an excellent model organism for doing this. Kynurenine is known to be a regulator of lipid 
metabolism and is down-regulating phospholipid metabolism, which may be the cause of the 
decrease LD formation. There are a large number of studies on kynurenine metabolism in human 
cells as this metabolite is a key regulator of the immune system, and the study is therefore highly 
relevant and interesting, but the authors do not make any of these connections. A good review for 
the authors to start would be Sas et al. (2018) from Molecules 23:E191 (also discussing 
interactions of mitochondria metabolism and kynurenine metabolism). If the authors can make a 
solid link between lipid metabolism and kynurenine metabolism, and then further to ageing then 
they have a very solid story that would be a much interest to the community. But as the story 
stands now it is not suitable for Nature Communication. 
 
 
Reviewer #3 (Remarks to the Author): 
 
Authors present interesting data and discussion of age associated changes of lipid metabolism to 
be associated with longevity. The study as well as the presentation of results is convincingly 
performed as are the conclusions drawn. However, there are a few shortcomings which should be 
improved. 
Number of replicates is sometimes only 3 or even 2 only (e.g. see figs., 3 and 4), which is not 
really acceptable for drawing any conclusions. 
Discussion should include the fact that not all human cell types regulate tryptophan metabolism in 
a uniform way. Especially the immune system is prone for distinct considerations, since nitric oxide 
radical formation differs between human and murine monocytes/macrophages., and nitric oxide is 
crucial in the regulation of tryptophan catabolism when it inhibits expression an d function of the 
initial enzyme of tryptophan metabolism namely indoleamine 2,3-dioxygenase. So results cannot 
be generalized easily. 
Minor: Denominations of compounds in the paper switches between acids derived from tryptophan 
conversion and their corresponding anions. These denominations should be uniform as long as not 
explained by different pH. 
 
 
 
Reviewer #4 (Remarks to the Author): 
 
Beas et al. demonstrate that replicative aging of budding yeast coincides with an accumulation of 

lipid droplets in mother cells. They show that such replicative age-related build-up of lipid droplets 
can be suppressed, and replicative lifespan can be prolonged in response to an overexpression of 
the gene BNA2 known to code an enzyme catalyzing the first step of NAD+ synthesis. The authors 
provide evidence that the overexpression of BNA2 extends yeast replicative lifespan not because it 
suppresses the observed aging-associated accumulation of lipid droplets and neutral lipid 
deposition but because it redirects the flow of metabolites from lipid synthesis to tryptophan 
synthesis. They also establish that an excessive deposition of neutral lipid in lipid droplets is an 
essential contributor to longevity assurance in replicatively aging yeast exposed to low 
temperatures. 
 
The manuscript is clearly written and of high technical quality. The claims are convincing, fully 
supported by the experimental data and appropriately discussed in the context of previous 
literature. The authors have been fair in their treatment of previous literature. They provided 
enough methodological details for the experiments to be reproduced by others. The manuscript 
provides a critical step towards understanding mechanisms linking replicative cellular aging to age-
related and environmentally controlled changes in cellular metabolism. Therefore, the manuscript 
is suitable for Nature Communications, and I recommend accepting it for publication.