Optimism is a trait often associated with youth, with tough life experiences jading us towards pessimism as we age. The link between age and pessimism may originate from some of our smallest components: our chromosomes, which are capped by protective structures called telomeres. In humans and other animals, telomeres degrade as we age, increasing the risk of damage to the important genetic information that these caps defend, affecting an individual's health and lifespan. Anecdotal evidence also links age-related changes in behavior with telomere shortening, particularly greater pessimism. But whether this change is directly linked to activity down at the level of the telomere remains unclear. To better understand the link between telomeres and behavior, Felipe Espigares (Instituto Gulbenkian de Ciência, Portugal) and colleagues from institutions in Portugal, Spain and France dug into these ideas with genetically altered zebrafish.

Espigares and his collaborators focused on the enzyme telomerase, which is responsible for delaying, or even reversing, the process of cellular aging in telomeres. Telomerase is often thought of as an ‘anti-aging potion’, as its main purpose is to protect and repair our telomeres from the gradual shrinkage that comes with age. Yet, production of this enzyme declines just when we need it most – as we get older. The researchers took advantage of the powerful action of this ‘immortality’ enzyme to test how telomere shortening alters our behavior, by breeding zebrafish that lacked telomerase altogether and hence fast-forwarded individuals to have the telomere length typically experienced by much older fish. They then tested where these telomerase-deficient individuals sat on the optimism–pessimism spectrum compared with normal fish.

Using red and green cards, the team trained the fish to associate one color with a tasty reward and the other with a less pleasant experience (capture in a net). Then, the researchers tested how optimistic or pessimistic each fish was by recording how long it took them to approach an ambiguous half-red/half-green card. They reasoned that the optimists would approach the card more quickly, keen to pick up the reward that they interpreted the card to signal. The pessimists would err on the side of caution, believing that the confusing card would accompany a nasty net surprise. If their hypothesis rang true, telomerase would encourage optimism, while the lack of teleromase would generate pessimism.

When the fish were young (4 months old), telomerase absolutely encouraged optimism, as individuals with abundant telomerase (and the hefty telomeres that this enzyme supports) made a beeline towards the mixed-signal card, with an optimistic expectation of the reward that they would reap. Telomerase deficiency, in contrast, made the fish do the opposite – drag their heels in approaching the card, wary of what they might encounter. While the researchers expected to see this telomerase-generated optimism even at an older age (9 months), the magic of telomerase had seemingly worn off, with fish equally pessimistic whether or not their telomeres benefited from the protection and repair of telomerase.

These results imply that telomere shortening encourages premature pessimism in young individuals. While telomerase helps to prevent some of the effects of aging, it isn't the magic, fountain-of-youth potion for long-term physical and behavioral health. So, maybe a touch of pessimism is inevitable with age, but growing evidence proposes that we should still help to protect our telomeres (and potentially our optimism) through a generally healthy lifestyle, including a balanced diet and exercise.

S. A. M.
M. G.
R. F.
Short telomeres drive pessimistic judgement bias in zebrafish
Biol. Lett.
. doi:10.1098/rsbl.2020.0745