Impaired ECM Remodeling and Macrophage Activity Define Necrosis and Regeneration Following Damage in Aged Skeletal Muscle

Abstract

<jats:p>Regenerative capacity of skeletal muscle declines with age, the cause of which remains largely unknown. We investigated extracellular matrix (ECM) proteins and their regulators during early regeneration timepoints to define a link between aberrant ECM remodeling, and impaired aged muscle regeneration. The regeneration process was compared in young (three month old) and aged (18 month old) C56BL/6J mice at 3, 5, and 7 days following cardiotoxin-induced damage to the tibialis anterior muscle. Immunohistochemical analyses were performed to assess regenerative capacity, ECM remodeling, and the macrophage response in relation to plasminogen activator inhibitor-1 (PAI-1), matrix metalloproteinase-9 (MMP-9), and ECM protein expression. The regeneration process was impaired in aged muscle. Greater intracellular and extramyocellular PAI-1 expression was found in aged muscle. Collagen I was found to accumulate in necrotic regions, while macrophage infiltration was delayed in regenerating regions of aged muscle. Young muscle expressed higher levels of MMP-9 early in the regeneration process that primarily colocalized with macrophages, but this expression was reduced in aged muscle. Our results indicate that ECM remodeling is impaired at early time points following muscle damage, likely a result of elevated expression of the major inhibitor of ECM breakdown, PAI-1, and consequent suppression of the macrophage, MMP-9, and myogenic responses.</jats:p>

PAI-1, the Plasminogen System, and Skeletal Muscle

“…PAI-1 is expressed in vasculature (endothelial and smooth muscle cells), immune cells, heart, liver, kidney, adipose tissue, as well as some cancer cell types [ 18 , 20 , 103 ]. Skeletal muscle also appears to express PAI-1, at least during regeneration, suggesting that PAI-1 plays a role in modulating skeletal muscle ECM [ 29 , 41 , 42 ] (GEO dataset: GDS234; Reference series GSE469). Regardless of tissue origin, the transduction of Serpine1 (i.e., gene encoding for PAI-1) remains similar across most if not all tissue types.…”

“…Together these data suggest that unbound/soluble uPA activity is needed for muscle regeneration, and PAI-1 can negatively impact its role in regeneration. In fact, in diabetic and aged muscle, where PAI-1 levels are elevated, aberrant macrophage infiltration has also been observed [ 27 , 29 ]. Interestingly, however, the chemical inhibition of PAI-1 by tiplaxtinin (PAI-039) was found to rescue early impairments in regeneration in diabetic muscle, particularly in the necrotic regions, and likely by facilitating macrophage infiltration [ 27 , 128 ].…”

“…Given that PAI-1 is present within the circulation under normal and pathological conditions, it is unsurprising that this protein is capable of affecting virtually all tissues/organs in the body. For instance, high levels of PAI-1 have been implicated in cancer [ 16 , 17 , 18 , 19 , 20 ], cardiovascular disease [ 21 , 22 , 23 ], diabetic complications [ 23 , 24 ], renal disease [ 25 , 26 ], musculoskeletal disorders [ 27 , 28 , 29 , 30 ], and tissue fibrosis [ 31 , 32 , 33 ]. In skeletal muscle, PAI-1 plays critical roles in response to skeletal muscle injury and in myopathic conditions [ 27 , 28 , 29 , 34 , 35 ], further highlighting the importance of the plasminogen system and ECM remodeling to skeletal muscle.…”

“…For instance, high levels of PAI-1 have been implicated in cancer [ 16 , 17 , 18 , 19 , 20 ], cardiovascular disease [ 21 , 22 , 23 ], diabetic complications [ 23 , 24 ], renal disease [ 25 , 26 ], musculoskeletal disorders [ 27 , 28 , 29 , 30 ], and tissue fibrosis [ 31 , 32 , 33 ]. In skeletal muscle, PAI-1 plays critical roles in response to skeletal muscle injury and in myopathic conditions [ 27 , 28 , 29 , 34 , 35 ], further highlighting the importance of the plasminogen system and ECM remodeling to skeletal muscle. Indeed, several studies have identified the importance of plasminogen system function but have yet to fully elucidate a role for PAI-1 in skeletal muscle [ 35 , 36 , 37 , 38 , 39 , 40 ], although recent evidence suggests that skeletal muscle is also a site of PAI-1 synthesis [ 29 , 41 , 42 ].…”

“…In skeletal muscle, PAI-1 plays critical roles in response to skeletal muscle injury and in myopathic conditions [ 27 , 28 , 29 , 34 , 35 ], further highlighting the importance of the plasminogen system and ECM remodeling to skeletal muscle. Indeed, several studies have identified the importance of plasminogen system function but have yet to fully elucidate a role for PAI-1 in skeletal muscle [ 35 , 36 , 37 , 38 , 39 , 40 ], although recent evidence suggests that skeletal muscle is also a site of PAI-1 synthesis [ 29 , 41 , 42 ]. Control of PAI-1 levels in the circulation is of clinical importance and while studies generally indicate that circulating PAI-1 can be controlled with exercise [ 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 ] or through the administration of pharmacological PAI-1 inhibitors [ 17 , 22 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 ].…”