Mechano-immuno-therapy for Skeletal Muscle Repair

Abstract

Traumatic skeletal muscle injuries account for a significant fraction of all healthcare-related costs and compromise quality of life for thousands of patients each year. Severe muscle injuries involve damage that is beyond the ability of the tissue to self-repair, thus requiring significant therapeutic intervention. Current treatment strategies are limited in their ability to prevent fibrosis and direct functional healing, and are often invasive or involve the risk of serious adverse effects. As a safer, more conservative treatment approach, mechanical loading-based therapies (e.g., massage) offer the opportunity to non-invasively modulate tissue repair by leveraging the mechanosensitivity of muscle. This thesis investigates the use of “mechano-immuno-therapy” for skeletal muscle regeneration with the guiding hypothesis that mechanical loading can promote myogenesis and attenuate inflammation after injury.

In this work, a soft robotic actuator was designed to provide force feedback-controlled compressive loading to severely injured skeletal muscle in a young adult murine model. Two weeks of mechanical loading treatment led to significant improvements in muscle function, structural repair, and the restoration of a mature muscle fiber type distribution. Using this device, a systematic approach was also taken to evaluate the effects of mechanical loading on the immune system, a key regulator of injury repair. Mechanical loading led to the broad downregulation of several key pro-inflammatory markers within the tissue as well as a specific reduction in the number of intramuscular neutrophils within the first three days of treatment. The functional improvements observed with mechanical loading were linked to this reduction in both neutrophil numbers and neutrophil-derived cytokines, which otherwise were shown to impede myogenesis.

Since aging leads to a substantial decline in the regenerative capacity of skeletal muscle, the second part of this work evaluated whether the benefits of mechanical loading observed in young muscle were preserved with age. Applying the same loading conditions used in young muscle to aged tissue did not improve muscle recovery and, instead, exacerbated the aged inflammatory response and promoted an abnormal muscle fiber type distribution. Mechanical loading further disrupted the regenerative behavior of aged muscle stem cells, suggesting that the loading treatment was hindering the healing process. Importantly, the introduction of anti-inflammatory therapy alongside mechanical loading reversed these negative effects on the stem cell niche, thereby enabling the response of aged muscle to the loading treatment regimen.

In summary, this thesis demonstrates the utility of mechanical loading-based therapies to aid in the regeneration of skeletal muscle after traumatic injury in young animals but uncovers negative impacts of the same treatment parameters on aged tissue, thus emphasizing the importance of developing therapies using a patient-specific approach. Critically, this work discovers a mechanism to enable functional response of muscle to loading even in the context of age using inflammatory control, which is a therapeutic approach that could offer immediate clinical translation.