Urokinase-type plasminogen activator and macrophages are required for skeletal muscle hypertrophy in mice

Abstract

Adult skeletal muscle possesses remarkable potential for growth in response to mechanical loading; however, many of the cellular and molecular mechanisms involved remain undefined. The hypothesis of this study was that the extracellular serine protease, urokinase-type plasminogen activator (uPA), is required for muscle hypertrophy, in part by promoting macrophage accumulation in muscle subjected to increased mechanical loading. Compensatory muscle hypertrophy was induced in mouse plantaris (PLT) muscles by surgical ablation of synergist muscles. Following synergist ablation, PLT muscles in wild-type mice demonstrated edema and infiltration of neutrophils and macrophages but an absence of overt muscle fiber damage. Sham procedures resulted in no edema or accumulation of inflammatory cells. In addition, synergist ablation was associated with a large increase in activity of uPA in the PLT muscle. uPA-null mice demonstrated complete abrogation of compensatory hypertrophy associated with reduced macrophage accumulation, indicating that uPA is required for hypertrophy. Macrophages isolated from wild-type PLT muscle during compensatory hypertrophy expressed uPA and IGF-I, both of which may contribute to hypertrophy. To determine whether macrophages are required for muscle hypertrophy, clodronate liposomes were administered to deplete macrophages in wild-type mice; this resulted in reduced muscle hypertrophy. Decreased macrophage accumulation was associated with reduced cell proliferation but did not alter signaling through the mammalian target of rapamycin pathway. These data indicate that uPA and macrophages are required for muscle hypertrophy following synergist ablation.

Systematic review of the synergist muscle ablation model for compensatory hypertrophy

“…The increase in the cross-sectional area of the muscle and muscle fibers was studied using histological techniques. 1,[7][8][9]60 The mean increase in cross-sectional area in comparison to the control was 66 ± 4% at day 14, demonstrating that compensatory hypertrophy is an effective model for increasing muscle mass.…”

“…[1][2][3] Hypertrophy is an example of this plasticity and refers to the increase in muscle mass necessary to enable the muscle to optimize its response to the demands of sustaining and generating force. 1,2,[4][5][6] Skeletal muscle mass is regulated by a variety of stimuli, the best known of which is mechanical overload. The muscle adaptation process can be induced by stretching/ immobilization, 44,46 compensatory mechanisms (chronic).…”

“…The muscle adaptation process can be induced by stretching/ immobilization, 44,46 compensatory mechanisms (chronic). 1,[2][3][4][5][6]8,9,12,14,[18][19][20][61][62][63][64][65] and exercise/training. 33,46 Evidence of this is derived from a large number of studies demonstrating that overload leads to an increase in muscle mass and cross-sectional area of the muscle fibers and induces chronic changes in the balance between the synthesis and degradation of proteins.…”

“…3,7,12,13,20,59 The ablation of synergists for compensatory hypertrophy consists of the surgical removal of all or part of synergistic muscles, which can be either unilateral or bilateral, to generate chronic functional overload that causes hypertrophy. 3,7,12,13,20,59 According to Parvaresh et al, 1 complete muscle removal can compromise the neurovascular supply, which increases edema and the recovery of the animal in the postoperative period. Thus, the removal of only the distal portion of synergist muscle is recommended (Figure 1).…”

Regulatory interactions between muscle and the immune system during muscle regeneration

“…Recent findings show that the genetic ablation of IL-6 greatly slows muscle growth in a model of compensatory muscle hypertrophy in mice where the gastrocnemius tendon was sectioned to increase loading and growth in the synergistic plantaris muscle (99). Although this treatment causes muscle inflammation (21), which could be a source of IL-6 in the model, the investigators showed that Pax7-expressing cells in the overloaded muscle also expressed IL-6, suggesting that both muscle cells and Th1 myeloid cells could provide IL-6 that is important for muscle growth. Apparently, slowed growth in the IL-6 null mutant muscle reflected loss of normal muscle cell differentiation, at least in part.…”