The plasminogen system is a critical proteolytic system responsible for the remodeling of the extracellular matrix (ECM). The master regulator of the plasminogen system, plasminogen activator inhibitor-1 (PAI-1), has been implicated for its role in exacerbating various disease states not only through the accumulation of ECM (i.e., fibrosis) but also its role in altering cell fate/behaviour. Examination of PAI-1 has extended through various tissues and cell-types with recent investigations showing its presence in skeletal muscle. In skeletal muscle, the role of this protein has been implicated throughout the regeneration process, and in skeletal muscle pathologies (muscular dystrophy, diabetes, and aging-driven pathology). Needless to say, the complete function of this protein in skeletal muscle has yet to be fully elucidated. Given the importance of skeletal muscle in maintaining overall health and quality of life, it is critical to understand the alterations—particularly in PAI-1—that occur to negatively impact this organ. Thus, we provide a comprehensive review of the importance of PAI-1 in skeletal muscle health and function. We aim to shed light on the relevance of this protein in skeletal muscle and propose potential therapeutic approaches to aid in the maintenance of skeletal muscle health.

Muscle weakness and wasting are defining features of cancer-induced cachexia. Mitochondrial stress occurs before atrophy in certain muscles, but the possibility of heterogeneous responses between muscles and across time remains unclear. Using mice inoculated with Colon-26 cancer, we demonstrate that specific force production was reduced in quadriceps and diaphragm at 2 weeks in the absence of atrophy. At this time, pyruvate-supported mitochondrial respiration was lower in quadriceps while mitochondrial H ₂ O ₂ emission was elevated in diaphragm. By 4 weeks, atrophy occurred in both muscles, but specific force production increased to control levels in quadriceps such that reductions in absolute force were due entirely to atrophy. Specific force production remained reduced in diaphragm. Mitochondrial respiration increased and H ₂ O ₂ emission was unchanged in both muscles versus control while mitochondrial creatine sensitivity was reduced in quadriceps. These findings indicate muscle weakness precedes atrophy and is linked to heterogeneous mitochondrial alterations that could involve adaptive responses to metabolic stress. Eventual muscle-specific restorations in specific force and bioenergetics highlight how the effects of cancer on one muscle do not predict the response in another muscle. Exploring heterogeneous responses of muscle to cancer may reveal new mechanisms underlying distinct sensitivities, or resistance, to cancer cachexia.

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.

Skeletal muscle is a complex tissue comprising multinucleated and post-mitotic cells (i.e., myofibers). Given this, skeletal muscle must maintain a fine balance between growth and degradative signals. A major system regulating the remodeling of skeletal muscle is autophagy, where cellular quality control is mediated by the degradation of damaged cellular components. The accumulation of damaged cellular material can result in elevated apoptotic signaling, which is particularly relevant in skeletal muscle given its post-mitotic nature. Luckily, skeletal muscle possesses the unique ability to regenerate in response to injury. It is unknown whether a relationship between autophagy and apoptotic signaling exists in injured skeletal muscle and how autophagy deficiency influences myofiber apoptosis and regeneration. In the present study, we demonstrate that an initial inducible muscle-specific autophagy deficiency does not alter apoptotic signaling following cardiotoxin injury. This finding is presumably due to the re-establishment of ATG7 levels following injury, which may be attributed to the contribution of a functional Atg7 gene from satellite cells. Furthermore, the re-expression of ATG7 resulted in virtually identical regenerative potential. Overall, our data demonstrate that catastrophic injury may “reset” muscle gene expression via the incorporation of nuclei from satellite cells.

The degree to which mitochondrial‐linked programmed cell death pathways contribute to skeletal muscle atrophy during cancer remains unknown. Here we combined a novel and robust mouse model of metastatic ovarian cancer with chronic administration of the mitochondrial‐targeted antioxidant SkQ1 to determine the time‐dependent and muscle‐specific relationships of mitochondrial‐linked apoptotic and necroptotic signalling to the development of muscle atrophy in the type IIB‐rich gastrocnemius. Early‐stage ovarian cancer reduced type IIB fibre cross‐sectional area in the gastrocnemius but did not alter mitochondrial H ₂ O ₂ emission despite increased activities of mitochondrial‐linked caspase‐9 and ‐3 regulators of apoptosis. During late‐stage ovarian cancer sustained atrophy was associated with increased mitochondrial H ₂ O ₂ emission potential in vitro , a greater probability of calcium‐triggered mitochondrial permeability transition and increases in downstream caspase‐9 and ‐3 activities. SkQ1 attenuated mitochondrial H ₂ O ₂ emission and caspase‐9 and ‐3 activities in late‐stage ovarian cancer but did not prevent atrophy. Necroptosis markers were heterogeneous across time with total RIPK1 increasing during early‐stage cancer, which reverted to normal levels by late stages, whereas phosphorylated RIPK3 decreased below control levels. These discoveries indicate that preventing increases in mitochondrial‐linked apoptotic caspase‐9 and ‐3 activities during late‐stage ovarian cancer with SkQ1 does not prevent atrophy of type II B fibres. Furthermore necroptotic markers are inconclusive during cancer in this muscle type but are not modified by SkQ1. These results do not support a causal relationship between mitochondrial H ₂ O ₂ ‐linked apoptotic or necroptotic signalling and atrophy in type IIB fibres during ovarian cancer but do not rule out potential relationships in other muscle types. image Key points: Cancer increases mitochondrial reactive oxygen species (ROS) in skeletal muscle during atrophy, but the role of ROS in regulating cell death remains unknown. We show that attenuating gastrocnemius mitochondrial ROS with the mitochondrial‐targeted antioxidant SkQ1 prevented mitochondrial‐linked pro‐apoptotic caspase 9‐ and ‐3 activities but did not affect markers of necroptosis in a mouse model of ovarian cancer. Reductions in gastrocnemius muscle fibre cross‐sectional areas and the wet weights of several muscles were not prevented by SkQ1. These findings demonstrate that mitochondrial ROS regulate apoptotic caspases but not necroptotic proteins, and neither pathway is linked to gastrocnemius atrophy in mice with ovarian cancer. The degree to which mitochondrial ROS‐linked cell death pathways regulate muscle mass in other muscle types and cancer models requires further investigation.

Skeletal muscle is a heterogeneous tissue composed of fibers with distinct contractile, metabolic, and molecular characteristics. This intrinsic heterogeneity influences how individual fibers respond to physiological stimuli, pathological stress, and cellular remodeling processes such as autophagy. Skeletal muscle autophagy is essential for maintaining proteostasis and organelle quality, particularly in high-demand tissues like skeletal muscle. However, emerging evidence indicates that autophagy is not uniformly regulated across all muscles and fibers within a skeletal muscle. Fast/glycolytic fibers, characterized by faster contractile speed and high glycolytic capacity, exhibit greater autophagic flux potentially driven by activation of energy signals, calcium, and redox-sensitive pathways. In contrast, slow/oxidative fibers, characterized by slow contractile speed and higher oxidative metabolism, show lower inducible autophagy despite elevated basal expression of autophagy-related proteins. These differences are compounded by fiber type – specific organelle architecture, recruitment patterns during activity and disuse, and substrate availability and utilization. Further, pathological conditions such as disuse, chronic disease, and myopathies often induce fiber type alterations as well as changes to organelle content and function that are closely associated with changes in autophagy signaling. Additionally, species and strain variability add another layer of complexity, complicating both the interpretation and translational relevance of autophagy studies in skeletal muscle. This review synthesizes current evidence linking skeletal muscle phenotype to autophagy regulation and highlights the need to consider skeletal muscle heterogeneity as a central variable in skeletal muscle autophagy research. A deeper understanding of skeletal muscle type/fiber-specific autophagy will improve our ability to interpret experimental findings and develop targeted interventions for skeletal muscle dysfunction.