Folate status of gut microbiome affects Caenorhabditis eleganslifespan
© Nguyen and Clarke; licensee BioMed Central Ltd. 2012
Received: 26 July 2012
Accepted: 30 July 2012
Published: 31 July 2012
In a paper in BMC Biology Virk et al. show that Caenorhabditis elegans lifespan is extended in response to a diet of folate-deficient Escherichia coli. The deficiencies in folate biosynthesis were due to an aroD mutation, or treatment of E. coli with sulfa drugs, which are mimics of the folate precursor para-aminobenzoic acid. This study suggests that pharmacological manipulation of the gut microbiome folate status may be a viable approach to slow animal aging, and raises questions about folate supplementation.
See research article http://www.http://www.biomedcentral.com/1741-7007/10/67
Aging in Caenorhabditis elegans - nature versus nuture (or a diet of E. coli)?
The first genes extending lifespan were identified in the nematode Caenorhabditis elegans , and several of the metabolic pathways they are involved in are conserved in other species, including flies, mice and humans. Some of the pioneering studies on aging in C. elegans made use of RNA interference (RNAi) to manipulate gene expression, a strategy that is easily applied in C. elegans by feeding the worms on Escherichia coli strains expressing the relevant RNA sequence.
pABA and sulfa drugs - do they impact more than just folate metabolism?
The results show that inhibition of folate synthesis in the microbe extends C. elegans lifespan. Is a decreased supply of folate per se mediating lifespan extension? Virk et al. tested whether the direct addition of folate to the aroD E. coli diet reversed C. elegans lifespan extension. At the outset, this seemed like a straightforward experiment: E. coli (and other microbes) do not assimilate exogenous folates (this is why the sulfa drugs are so effective). However, the results obtained showed intermediate effects. Folate supplementation only partially reversed the lifespan extension. In fact, the authors showed that this folate supplementation experiment is not so simple. When the E. coli aroD mutant was grown in the presence of exogenous folate the formyl-THF-Glu3 content of the bacteria increased! While microbial folate catabolism pathways are not completely understood, it is clear that E. coli has a transporter that allows for uptake of pABA-glutamate, a folate breakdown product (Figure 2), and a hydrolase that can process pABA-glu into free pABA and glutamate . Hence, the catabolism of exogenously added folate by E. coli (and by the gut microbiome) leads to de novo synthesis of more folate. Finally, the extent to which C. elegans takes up exogenously added folate was not determined, although this may have important implications for whether folate supplementation is an effective way to boost folate stores in C. elegans.
So the jury is still out - clearly the microbe's folate status affects C. elegans lifespan. But whether this is due to the content of folate in the diet, or to indirect effects of the folate status on E. coli or C. elegans is still an open question. It would be interesting to test folate supplementation with axenic medium, where E. coli is eliminated from the C. elegans diet and the folate content could be precisely defined . It would also be important to determine whether the enhanced lifespan of C. elegans fed either the E. coli aroD mutant, or OP50 treated with SMX, is related to the extent of bacterial colonization of the worm intestine .
It is intriguing to consider other possible fates of pABA in this model, as addition of pABA did completely reverse the lifespan extension in C. elegans fed the aroD- diet. In the yeast Saccharomyces cerevisiae, pABA serves as an alternative aromatic ring precursor in the biosynthesis of coenzyme Q . The fate of pABA as a ring precursor of coenzyme Q in E. coli or C. elegans is still uncertain. Hence, it would be of interest to monitor the effect of 4-HB or pABA supplementation on the coenzyme Q content in E. coli aroD mutants. Virk et al. showed that addition of 4-HB, an established ring precursor of coenzyme Q in E. coli, did not reverse the lifespan extension. Nonetheless, since C. elegans fed respiratory defective mutant strains of E. coli diet show an extended lifespan , the possible impact of SMX and pABA on coenzyme Q content and respiratory metabolism in E. coli remains an intriguing avenue of future investigation.
Not only do sulfa drugs directly inhibit folate biosynthesis through competition of SMX with pABA at the dihydropteroate synthase step, but they also produce sulfa-dihydropteroate in the process, and this in turn acts as an inhibitor of dihydrofolate reductase (DHFR), the final step in the production of THF . Although sulfa-dihydropteroate is excreted by E. coli, it is likely to be present in the worm gut. This raises the question of whether other drugs (such as methotrexate, an inhibitor of dihydrofolate reductase and widely used in chemotherapy) might also impart lifespan extension effects when added to the worm diet.
Folate supplementation, sulfa drugs, and human aging
Like C. elegans, mammals are unable to synthesize folate and acquire the metabolite through diet and gut microflora production. Since 1998, the US Food and Drug Administration has required folate supplementation in all cereal grains, which has resulted in higher blood folate content of the adult, non-supplement using population . Recently, a study on the gut microflora of 531 human subjects across a wide range of ages, ethnicities, and geography showed that microbes residing in babies are enriched in genes involved in de novo folate biosynthesis, whereas the microbes residing in adult subjects were enriched in genes that metabolize dietary folate and THF . However, because folate supplementation regulations and diet differ in the sampling population, there is insufficient data to assess whether the changes in microbial folate biosynthesis gene expression are linked to dietary folate. Interestingly, Virk et al. note that sulfa drugs have been reported to inhibit microbiome folate synthesis and extend lifespan in rats . While the mechanism remains to be determined regarding how genetic or pharmacological knockdown of folate in E. coli can enhance C. elegans lifespan, Virk et al. have raised the intriguing possibility that manipulation of the folate status of gut microflora may impact lifespan in other species.
This work was supported by the National Science Foundation Grant 0919609 (to CFC).
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