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Horses (Equus ferus caballus) have evolved over the past 300 years in response to man‐made selection for particular athletic traits. Some of the selected traits were selected based on the size and horses’ muscular power (eg Clydesdales), whereas other breeds were bred for peak running performance (eg Thoroughbred and Arabian). Although the physiological changes and some of the cellular adaptations responsible for athletic potential of horses have been identified, the molecular mechanisms are only just beginning to be comprehensively investigated. The purpose of this review was to outline and discuss the current understanding of the molecular mechanisms underpinning the athletic performance and cardiorespiratory fitness in athletic breeds of horses. A brief review of the biology of epigenetics is provided, including discussion on DNA methylation, histone modifications and small RNAs, followed by a summary and critical review of the current work on the exercise‐induced epigenetic and transcriptional changes in horses. Important unanswered questions and currently unexplored areas that deserve attention are highlighted. Finally, a rationale for the analysis of epigenetic modifications in the context with exercise‐related traits and ailments associated with athletic breeds of horses is outlined in order to help guide future research.

Ageing causes a gradual deterioration of bodily functions and telomere degradation. Excessive telomere shortening leads to cellular senescence and decreases tissue vitality. Six proteins, called shelterin, protect telomere integrity and control telomere length through telomerase-dependent mechanisms. Exercise training appears to maintain telomeres in certain somatic cells, although the underlying molecular mechanisms are incompletely understood. Here, we examined the influence of a single bout of vigorous exercise training on leukocyte telomerase reverse transcriptase (TERT) and shelterin gene expression, and the abundance of three microRNAs (miRNAs) implicated in biological ageing (miRNA-143, -223 and -486-5p) in an elite athlete and large animal model, Thoroughbred horses. Gene and miRNA expression were analysed using primer-based and TaqMan Assay qPCR. Leukocyte TRF1, TRF2 and POT1 expression were all significantly increased whilst miR-223 and miR-486-5p were decreased immediately after vigorous exercise (all p < 0.05), and tended to return to baseline levels 24 h after training. Relative to the young horses (~ 3.9 years old), middle-aged horses (~ 14.8 years old) exhibited reduced leukocyte TERT gene expression, and increased POT1 and miR-223 abundance (all p < 0.05). These data demonstrate that genes transcribing key components of the shelterin-telomere complex are influenced by ageing and dynamically regulated by a single bout of vigorous exercise in a large, athletic mammal — Thoroughbred horses. Our findings also implicate TERT and shelterin gene transcripts as potential targets of miR-223 and miR-486-5p, which are modulated by exercise and may have a role in the telomere maintenance and genomic stability associated with long-term aerobic training.

Thoroughbred racehorses possess superior cardiorespiratory fitness levels and are at the pinnacle of athletic performance compared to other breeds of horses. Although equine athletes have undergone years of artificial selection for racing performance, musculoskeletal injuries and illnesses are common and concerns relating to animal welfare have been proposed. Leukocyte telomere length is indicative of biological age, and accelerated telomere shortening occurs with excess physical and psychological stress. This study was designed to explore the association between leukocyte telomere length, biological factors (age, sex and coat colour), training status, winnings and race history parameters. Blood was collected from 146 Thoroughbred racehorses from around Geelong, Victoria, Australia. DNA was extracted from leukocytes; telomere length was measured using qPCR and analysed in context with traits obtained from the Racing Australia website. Age was inversely correlated with telomere length (r = -0.194, P = 0.019). The oldest horses (≥11 years) in the highest age quartile possessed shorter telomeres compared to younger horses in the first, second and third quartiles (≤2, 3-5 and 6-10 years respectively; P < 0.05). No statistically significant associations were observed between telomere length and biological factors, training status, winnings or race history parameters in age-adjusted analyses. The study findings suggest that Thoroughbred horses may undergo age-related telomere shortening similar to other mixed breeds and humans. Despite concerns from some quarters regarding the welfare of racehorses, there was a lack of accelerated biological ageing observed in the present study, as indicated by leukocyte telomere length.

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