(L-479) Age-Dependency of Regenerative Engineering and Rehabilitation in Musculoskeletal Injury

Introduction:: Volumetric muscle loss (VML) occurs when greater than 20% of muscle mass is injured through trauma or surgery, irreparably damaging the intrinsic regenerative ability of skeletal muscle and leading to impairment of muscle function. Previous preclinical models, motivated by the current lack of effective clinical treatments, have demonstrated that VML can be significantly repaired using regenerative rehabilitation strategies such as biomaterial constructs and exercise. The addition of rehabilitative exercise not only recapitulates the clinical practice of prescribing physical therapy post-injury but also enhances the therapeutic effects of transplanted constructs by improving muscle force production, innervation, and vascularization (1,2). The population of interest for VML has historically be active military service members, thus these preclinical models have primarily utilized young mice. However, given that a predicted 1 in 6 individuals will be over the age of 65 by 2050 (3), a growing number of elderly patients will present with VML injuries which result from situations such as vehicular trauma or surgical resection of soft tissue sarcomas. Age-related changes in muscle physiology, such as reduced satellite cell function (4), can obstruct muscle regeneration and may also influence the healing response following VML. Regenerative strategies must be evaluated in a broader patient population to design musculoskeletal therapeutics which are effective across all ages. The objective of this study is to investigate how aging impacts the therapeutic effectiveness of engineered skeletal muscle and rehabilitative exercise following VML.

Materials and Methods:: Young (~ 8-week-old) and old (~80-week-old) C57BL/6 mice, which roughly equate to 20- and 70-year-old humans respectively (5), underwent a 20-30% surgical VML injury of the tibialis anterior (TA) muscle. The VML was left untreated or was treated using engineered skeletal muscle. The engineered muscle consisted of aligned collagen nanofibers which were fabricated using a shear-based extrusion method and then seeded with GFP+ primary mouse myoblasts which were pre-differentiated into myotubes prior to transplant. Mice were further subdivided into cohorts that received voluntary running exercise and those that did not. For the exercised cohort, mice were placed into cage wheels 7 days post-surgery and allowed to free run for 21 days while the non-exercised cohort received normal cage activity (Fig. 1a). Muscle regeneration was assessed using histology and in situ muscle physiology.

Results, Conclusions, and Discussions:: Young mice which received the combined treatment of engineered muscle and exercise demonstrated significantly larger TA muscle mass (N>4, p< 0.05) (Fig. 1b) and improved muscle function with a 150% increase in maximum contractile force (N>4, p< 0.001) compared to young no-treatment controls 28 days post-injury (Fig. 1c). Preliminary data also suggests that the combination treatment improved muscle regeneration in young mice by doubling the density of regenerating myofibers (Fig. 1d). Aging diminished the efficacy of the combined regenerative treatment as there were no significant differences in TA muscle mass or force production among the old experimental groups and the density of regenerating myofibers only increased by 60% compared to old no-treatment controls (Fig. 1b-d). This study suggests that age-related differences in muscle regenerative capacity decrease the effectiveness of regenerative rehabilitation strategies following VML and highlights the importance of considering age when designing musculoskeletal therapeutics.

Acknowledgements (Optional): :

References (Optional): : 1. Nakayama et al. Rehabilitative exercise and spatially patterned nanofibrillar scaffolds enhance vascularization and innervation following volumetric muscle loss. NPJ Regen Med 3, 16 (2018).
2. Quarta et al. Bioengineered constructs combined with exercise enhance stem cell-mediated treatment of volumetric muscle loss. Nat Commun 8, 15613 (2017).
3. United Nations World Population Prospects: the 2019 Revision. https://population.un.org/wpp/
4. Distefano and Goodpaster. Effects of exercise and aging on skeletal muscle. CSH Perspect Med 8, 3 (2018).
5. The Jackson Laboratory. https://www.jax.org/jax-mice-and-services/find-and-order-jax-mice/most-popular-jax-mice-strains/aged-b6