Vision Influences Circadian Rhythms to Interact with Calorie Restriction and Aging

Circadian mechanisms have been found to influence aging in short-lived species, though the degree to which this is relevant to treating aging in longer-lived species such as our own is up for debate. All too much of the metabolic response to stress, and items such as circadian rhythms that have an impact on that response, have far larger effects on the pace of aging in short-lived species than in long-lived ones. The work here, in which vision is found to alter circadian mechanisms and thus also the beneficial calorie restriction response, is interesting in the academic sense, in the same way that it is interesting that scent can disrupt the calorie restriction response, but most likely of little to no practical use.

Researchers conducted a broad survey to see what genes oscillate in a circadian fashion when flies on an unrestricted diet were compared with those fed just 10 percent of the protein of the unrestricted diet. Immediately, they noticed numerous genes that were both diet-responsive and also exhibiting ups and downs at different time points, or "rhythmic." They then discovered that the rhythmic genes that were activated the most with dietary restriction all seemed to be coming from the eye, specifically from photoreceptors, the specialized neurons in the retina of the eye that respond to light.

This finding led to a series of experiments designed to understand how eye function fit into the story of how dietary restriction can extend lifespan. For example, researchers set up experiments showing that keeping flies in constant darkness extended their lifespan. "That seemed very strange to us. We had thought flies needed the lighting cues to be rhythmic, or circadian." They then used bioinformatics to ask: Do the genes in the eye that are also rhythmic and responsive to dietary restriction influence lifespan? The answer was yes they do.

The biggest question raised by this work as it might apply to humans is, simply, do photoreceptors in mammals affect longevity? Probably not as much as in fruit flies, said Hodge, noting that the majority of energy in a fruit fly is devoted to the eye. But since photoreceptors are just specialized neurons, "the stronger link I would argue is the role that circadian function plays in neurons in general, especially with dietary restrictions, and how these can be harnessed to maintain neuronal function throughout aging."


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