Cellular Mechanisms Involved in Pathology Following Musculo-tendinous Injuries

Skeletal muscle strain injury is one of the most common sports injuries, which is caused by excessive tensile strain of the muscle and affects the region where muscle fascicles insert into the connective tissue (aponeurosis). It is therefore not an injury solely affecting skeletal muscle, but the skeletal muscle-aponeurosis interface. Having sustained one strain injury increases the risk of re-injury substantially, changes the biomechanics of the involved muscle and leads to a significantly enlarged aponeurosis. Recently, the investigators have discovered the accumulation of ectopic fat in the intra-fascicular space and within myofibrils. The built-up of ectopic fat in skeletal muscle is a degenerative state and has mostly been described for rotator cuff injuries where it is associated with worse shoulder functional scores. At the same time, the investigators have observed a high cellularity in the pathologic muscle-aponeurosis following strain injuries, but it is not known what cell types these cells are. It seems therefore likely that at least some of the cells are Fibro/adipogenic progenitors (FAPs), which contribute to the fat accumulation in strain injured muscles and that fat accumulation might be a factor contributing to the high risk of recurrent injuries, as well as other medical problems such as pain and decreased function often described long time after a strain injury.

Another common injury of the lower leg is the full Achilles tendon rupture. Recent data show that patients have long-term impairments in the injured leg when compared to the non-injured side. Also after full Achilles tendon ruptures, fatty degeneration of the muscles in the calf has been reported and has been linked to poor patient outcome. Recently, the investigators have observed fatty infiltration of calf muscles in patients following Achilles tendon rupture, which might lead to a decrease in muscle function and thereby to a poor patient outcome.

Despite the high prevalence of muscle strain injuries, very little is known about the cellular mechanisms following a strain injury. In addition, the literature on the cellular composition and alterations following Achilles tendon ruptures is scarce. A decade ago, a group of cells named fibro/adipogenic progenitors (FAPs) have been identified in the interstitial space of skeletal muscle based on the presence of stem cell antigen 1+, CD34+, Platelet Derived Growth Factor Receptor α (PDGFR α)+ . Aside from their supporting activity in muscle regeneration, FAPs have the potential to adopt an adipogenic or profibrotic phenotype. Despite the high incidence, it has not been studied whether the traumatic injury and the subsequent biomechanical changes in the skeletal muscle and associated connective tissue following strain injuries or Achilles tendon rupture activate FAPs, and how these cells contribute to the fatty accumulation in the skeletal muscle.

Hypotheses:

1. There is a high amount of fibro-adipogenic progenitors (FAPs) with an adipogenic pattern in pathologic skeletal muscle following a muscle strain injury and a full Achilles tendon rupture. The investigators hypothesize that there is an increased number of FAPs with an adipogenic signature already in the acute phase after a strain injury, but a significantly higher number in the chronic stage as well as in the muscle following an Achilles tendon rupture.
2. Mechanical cues are a major driver of the phenotypic drift of FAPs. The lack of mechanical stimuli initiates the adoption of an adipogenic pathway in naïve FAPs, whereas naïve FAPs exposed to mechanical stimuli will maintain their undifferentiated phenotype.
3. The adherence of FAPs to a soft material will activate the adoption of an adipogenic phenotype, whereas a stiff material will favor a more fibrotic phenotype in naïve FAP's isolated from healthy skeletal muscle.