In Aging Cell, researchers have published new data on the relationship between senescence and the extracellular matrix in the tendons of older people.
Easy to injure, hard to heal
The researchers begin this paper by pointing out that injuries to the musculoskeletal system are responsible for over a quarter of the years that elderly people spend living with disability instead of good health [1]. A significant portion of these injuries are to the tendons; for example, over half of people over 80 have sustained injuries to the tendons of a shoulder’s rotator cuff [2]. Compounding the problem, older people have a much harder time healing from these injuries than younger people [3].
The tendons are largely composed of the extracellular matrix (ECM), which is primarily made of collagen, along with an interfascicular matrix composed of collagens and proteins [4]. Exactly how these tissues age is unclear, as data often depends on the exact age of the person and the specific tendon measured. Cross-linking of this collagen plays a significant role [5], as do age-related cellular changes: with aging, the cells in the tendons do not proliferate as well [4], which is likely to be a reason for the slower healing.
The cells responsible for tendon maintenance (tenocytes) engage in a balancing act, producing matrix metalloproteinases (MMPs) to destroy damaged matrix tissues [6] while also synthesizing proteins with which to rebuild these tissues [7]. Exercise and a lack of exercise can affect this process as well.
However, cellular senescence might put its thumb on the scale. Senescent cells secrete multiple compounds, and MMPs are among them [8]. These researchers note that because 2D models do not accurately replicate how cells interact with each other and with the matrices surrounding them, the effects of senescence on tendons have not been properly examined. For this purpose, they used a system of live tendon explants that they had used in a previous study [9].
Why young cells heal better
There were four groups of cells used in total: three of them were taken from young mice, with one group being exposed to radiation and another group being exposed to doxorubicin, both of which induce senescence. One group of young cells was not exposed to any senescence inducers, and the fourth group was from naturally aged mice.
After confirming that doxorubicin and radiation induce senescence both in tendons and in tenocytes, the researchers examined how these cells functioned by placing them in a cellular culture. This environment is devoid of stresses, so it replicates a mechanical-unloading injury. Interestingly, natural aging was less harmful in some respects than induced senescence: the cells exposed to both of these inducers lost the ability to produce common Type 1 collagen over time in this culture, and critical proteoglycans used for tissue creation were less present as well. The researchers hypothesized that this reflects senescent tendons’ inability to heal properly.
Similarly, amino acids that are part of the collagen formation process were significantly more present in the healthy young tendons than in the other three. The researchers also discovered metabolic changes in the cells, although that was outside the scope of this experiment.
Not all MMPs are the same
Some of the MMPs that the researchers measured had completely different trajectories than others. After 14 days in culture, MMP-1 was significantly less expressed in the three senescent groups and maintained in the young group. MMP-3, curiously, was increased the most in the induced-senescence groups, increasing nearly as much in the young group and only somewhat increasing in the aged roup. MMP-13, on the other hand, also increased in all groups, but by far increasing the most in the naturally aged group. These findings, in addition to findings showing that inflammatory chemicals were broadly increased in all groups, surprised the researchers, who had expected more evidence of tissue breakdown due to senescence.
These findings are explained by a lack of stress being interpreted as injury, which causes the cells to behave in a way that is similar to senescence. However, not all of these changes are the same; for example, cells responding to injury do not stop dividing as senescent cells do, and it is unclear whether or not these changes are permanent [10].
The researchers then performed a preliminary investigation with tissues under stress, comparing doxorubicin-treated young cells to untreated young cells. Amazingly, they found few differences in the performance of these tissues. Therefore, they had to conclude that the senescence-associated secretory phenotype (SASP) is either not a major part of failing tendon function under normal circumstances or affects it in a way that this study was unable to detect. While these are largely negative results, they clear an important space and encourage investigation into other areas.
Literature
[1] Briggs, A. M., Cross, M. J., Hoy, D. G., Sànchez-Riera, L., Blyth, F. M., Woolf, A. D., & March, L. (2016). Musculoskeletal health conditions represent a global threat to healthy aging: a report for the 2015 World Health Organization world report on ageing and health. The Gerontologist, 56(suppl_2), S243-S255.
[2] Teunis, T., Lubberts, B., Reilly, B. T., & Ring, D. (2014). A systematic review and pooled analysis of the prevalence of rotator cuff disease with increasing age. Journal of shoulder and elbow surgery, 23(12), 1913-1921.
[3] Ackerman, J. E., Bah, I., Jonason, J. H., Buckley, M. R., & Loiselle, A. E. (2017). Aging does not alter tendon mechanical properties during homeostasis, but does impair flexor tendon healing. Journal of Orthopaedic Research, 35(12), 2716-2724.
[4] Siadat, S. M., Zamboulis, D. E., Thorpe, C. T., Ruberti, J. W., & Connizzo, B. K. (2021). Tendon extracellular matrix assembly, maintenance and dysregulation throughout life. Progress in Heritable Soft Connective Tissue Diseases, 45-103.
[5] Couppe, C., Hansen, P., Kongsgaard, M., Kovanen, V., Suetta, C., Aagaard, P., … & Magnusson, S. P. (2009). Mechanical properties and collagen cross-linking of the patellar tendon in old and young men. Journal of applied physiology, 107(3), 880-886.
[6] Sbardella, D., R Tundo, G., Francesco Fasciglione, G., Gioia, M., Bisicchia, S., Gasbarra, E., … & Marini, S. (2014). Role of metalloproteinases in tendon pathophysiology. Mini Reviews in Medicinal Chemistry, 14(12), 978-987.
[7] Aggouras, A. N., Stowe, E. J., Mlawer, S. J., & Connizzo, B. (2024). Aged Tendons Exhibit Altered Mechanisms of Strain-Dependent Extracellular Matrix Remodeling. Journal of Biomechanical Engineering, 1-41.
[8] Coppé, J. P., Desprez, P. Y., Krtolica, A., & Campisi, J. (2010). The senescence-associated secretory phenotype: the dark side of tumor suppression. Annual review of pathology: mechanisms of disease, 5(1), 99-118.
[9] Connizzo, B. K., Piet, J. M., Shefelbine, S. J., & Grodzinsky, A. J. (2020). Age-associated changes in the response of tendon explants to stress deprivation is sex-dependent. Connective tissue research, 61(1), 48-62.
[10] Chu, X., Wen, J., & Raju, R. P. (2020). Rapid senescence‐like response after acute injury. Aging Cell, 19(9), e13201.
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