ACL injury reduces satellite cell abundance and promotes fibrogenic cell expansion within skeletal muscle

Abstract

Anterior cruciate ligament (ACL) injuries are associated with significant loss of strength in knee extensor muscles that persists despite physical therapy. The underlying mechanisms responsible for this protracted muscle weakness are poorly understood; however, we recently showed significant myofiber atrophy and altered muscle phenotype following ACL injury. We sought to further explore perturbations in skeletal muscle morphology and progenitor cell activity following an ACL injury. Muscle biopsies were obtained from the injured and non-injured vastus lateralis of young adults (n=10) following ACL injury, and histochemical/immunohistochemical analyses were undertaken to determine collagen content, abundance of connective tissue fibroblasts, fibrogenic/adipogenic progenitor (FAP) cells, satellite cells, in addition to indices of muscle fiber denervation and myonuclear apoptosis. The injured limb showed elevated collagen content (p<0.05), in addition to a greater abundance of fibroblasts and FAPs (p<0.05) in the injured limb. Fibroblast content was correlated with increased accumulation of extracellular matrix in the injured limb as well. A higher frequency of interstitial nuclei were positive for phospho-SMAD3 in the injured limb (p<0.05), providing some evidence for activation of a fibrogenic program through transforming growth factor β following an ACL injury. The injured limb also displayed reduced satellite cell abundance, increased fiber denervation and DNA damage associated with apoptosis (p<0.05), indicating alterations within the muscle itself after the ligament injury. Injury of the ACL induces a myriad of negative outcomes within knee extensor muscles, which likely compromise the restorative capacity and plasticity of skeletal muscle, impeding rehabilitative efforts.

Anterior cruciate ligament tear induces a sustained loss of muscle fiber force production

“…However, recent studies have pointed to clear evidence of maladaptive muscle responses, with ACL-R patients demonstrating increased circulating levels of the atrophy-inducing signaling molecules myostatin and transforming growth factor-b up to 5 weeks after surgical repair, 3 and decreased quadriceps muscle stem cell density and muscle fiber CSA. 16,17 This is further supported by animal studies that reported reduced in vitro whole muscle force production after ACL tear, with increased expression of atrophy-associated genes such as the E3 ubiquitin ligases atrogin-1, MuRF-1, and MUSA-1 that can target proteins for enzymatic breakdown. 18,19 In the current study, the lack of appreciable changes in fiber CSA along with substantial changes in F o , suggest that there is a marked reduction in the density of myofibrils, which are the force generating organelles in muscle fibers.…”

“…Much of the literature on neuromuscular changes in ACL‐R patients has not considered intrinsic changes to the muscle fibers themselves. However, recent studies have pointed to clear evidence of maladaptive muscle responses, with ACL‐R patients demonstrating increased circulating levels of the atrophy‐inducing signaling molecules myostatin and transforming growth factor‐β up to 5 weeks after surgical repair, and decreased quadriceps muscle stem cell density and muscle fiber CSA …”

Reduced mitochondrial lipid oxidation leads to fat accumulation in myosteatosis

“…Skeletal muscle often displays chronic degenerative changes following injury, including atrophy and weakness of muscle fibers, and fibrotic changes to the extracellular matrix (1)(2)(3)(4). Some muscles also display a pathological accumulation of lipid in response to chronic injury or disease, which is referred to as myosteatosis or fatty degeneration (5,6).…”

Reduced mitochondrial lipid oxidation leads to fat accumulation in myosteatosis

“…Skeletal muscle often displays chronic degenerative changes following injury, including atrophy and weakness of muscle fibers and fibrotic changes to the extracellular matrix (ECM) (1)(2)(3)(4). Some muscles also display a pathologic accumulation of lipid in response to chronic injury or disease, which is referred to as myosteatosis or fatty degeneration (5,6).…”