Short-term elevations in glucocorticoids do not alter telomere lengths: A systematic review and meta-analysis of non-primate vertebrate studies

BackgroundThe neuroendocrine stress response allows vertebrates to cope with stressors via the activation of the Hypothalamic-Pituitary-Adrenal (HPA) axis, which ultimately results in the secretion of glucocorticoids (CORT). Glucocorticoids have pleiotropic effects on behavior and physiology, and might influence telomere length dynamics. During a stress event, CORT mobilizes energy towards survival mechanisms rather than to telomere maintenance. Additionally, reactive oxidative species produced in response to increased CORT levels can damage telomeres, also leading to telomere shortening. In our systematic review and meta-analysis, we tested whether CORT levels impact telomere length and if this relationship differs among time frame, life history stage, or stressor type. We hypothesized that elevated CORT levels are linked to a decrease in telomere length. MethodsTo test this hypothesis, we conducted a literature search for studies investigating the relationship between telomere length and CORT levels in non-human vertebrates using four search engines: Web of Science, Google Scholar, Pubmed and Scopus, last searched on September 27th, 2020. This review identified 31 studies examining the relationship between CORT and telomere length. We pooled the data using Fishers Z for 15 of these studies. All quantitative studies underwent a risk of bias assessment. This systematic review study was registered in the Open Science Framework Registry (https://osf.io/rqve6). ResultsThe pooled effect size from fifteen studies and 1066 study organisms shows no relationship between CORT and telomere length ((Fishers Z= 0.1042, 95% CI = 0.0235; 0.1836). While these results support some previous findings, other studies have found a direct relationship between CORT and telomere dynamics, suggesting underlying mechanisms or concepts that are not currently taken into account in our analysis. The risk of bias assessment revealed an overall low risk of bias with occasional instances of bias from missing outcome data or bias in the reported result. ConclusionWe highlight the need for more targeted experiments to understand how conditions, such as experimental timeframes, stressor(s), and stressor magnitudes can drive a relationship between the neuroendocrine stress response and telomere length.