Researchers chart age-related changes in gene expression in equine tendons

Changes in gene expression in horse tendon tissue at different ages have been identified by researchers, providing clues to how they might respond to therapeutics as the animals get older.

Equine tendon problems are an ongoing challenge due to their inadequate healing capacity, potential for incomplete repair, and subsequent increased re-injury rates.

Monica Pechanec and Michael Mienaltowski, writing in the journal BMC Research Notes, said tendon injuries represent almost 50% of musculoskeletal injuries in horses, and re-injury of recovered tendons such as the superficial digital flexor tendon recur at least 50% of the time within three years.

Moreover, aging significantly affects the overall outcome due to decreased tenocyte proliferation, increased matrix degeneration, and impaired structure at the extracellular, collagen, and gene expression levels.

“Although it is well-established that tendon healing decreases with age, as seen in mechanical and age-related injury model studies, the biological processes and molecular functions underlying aging in the equine tendon are not well understood,” they said.

Additionally, epigenetic changes associated with aging could contribute to impaired tendon healing, but limited studies have considered this process in aging tendons.

Moreover, it is understood that differences exist in the contribution of the tendon and the connective tissue sheath surrounding the tendon (the peritenon), particularly in immature versus mature tendons.

Pechanec and Mienaltowski, both with the Department of Animal Science at the University of California, Davis, said transcriptomic and epigenetic assessments of tendon aging within both the tendon and the sheath could provide essential biological context for predicting how tendon cells might respond to therapeutics for injuries.

The pair set out to learn more about gene expression markers that define each region and the markers associated with maturation and aging. They also sought to determine associations between DNA methylation and marker expression changes by age and location in the tendon.

DNA methylation is the process in which methyl groups are added to the DNA molecule. It can change the activity of a DNA segment without changing the sequence.

Tendon tissue samples from 14 Thoroughbred horses aged from 3 to 23 were collected for analysis. All had been euthanized for reasons unrelated to the study.

The tissue samples were analyzed using molecular-based techniques.

The researchers found that regional distinctions existed regardless of age. Genes such as DCN, COMP and LOX maintained elevated expression in the tendon itself while genes such as GSN (a cell migration, proliferation, and inflammatory gene) and AHNAK (a cell proliferation and differentiation gene) were more abundant in the peritenon region.

The abundance of these genes in the peritenon region could contribute to the cells’ role in early migration, proliferation, and inflammatory response that accompanies injuries.

They found evidence of increased gene activity in adolescent and geriatric populations, but this was lower during midlife. It was, they said, a notable finding.

In adolescence, the differentially expressed genes between the tendon and peritenon were similar in amount.

However, in the geriatric tendon, expression shifted downward for the tendon and upward for the peritenon region.

Given the decreased capacity of the tendon proper to maintain extracellular matrix homeostasis with age – which was even seen with increased biomineralization activity in geriatric tendons – these expression findings could suggest the increased activity of peritenon cells to compensate.

Indeed, the changes in gene expression identified provide support for the view that, even in uninjured tendons, at a geriatric age, the tendon is already undergoing some repair process as a result of compositional changes.

“Growth, development, and maturation occur with adolescence; responses to degenerating structure ensue in the geriatric age,” they said.

Contrastingly, during midlife, differentially expressed genes were minimal between the tendon and peritenon regions, indicating that there was a level of homeostasis occurring with the two tissue types at that stage of life.

The authors also found regional differences in DNA methylation in the study samples.

“Notably, when evaluating all ages of tendon proper against peritenon, five genes (HAND2, CHD9, RASL11B, ADGRD1, and COL14A1) had regions of differential methylation as well as differential gene expression.”

Pechanec, M.Y., Mienaltowski, M.J. Decoding the transcriptomic expression and genomic methylation patterns in the tendon proper and its peritenon region in the aging horse. BMC Res Notes 16, 267 (2023). https://doi.org/10.1186/s13104-023-06562-1

The study, published under a Creative Commons License, can be read here.