Abstract
In muscle regeneration, infiltrating myeloid cells, such as macrophages mediate muscle inflammation by releasing key soluble factors. One such factor, insulin-like growth factor 1 (IGF-1), suppresses inflammatory cytokine expression and mediates macrophage polarization to anti-inflammatory phenotype during muscle injury. Previously the IGF-1Ea isoform was shown to be anti-inflammatory. Another isoform of IGF-1, mechano-growth factor (MGF), is structurally and functionally distinct from IGF-1Ea, but its role in muscle inflammation has not yet been characterized. In this study, we hypothesized that MGF expression in muscle injury modulates muscle inflammation. We first investigated changes of transcription and expression of MGF in response to skeletal muscle injury induced by cardiotoxin (CTX) in vivo. At 1–2 days post-injury, Mgf expression was significantly upregulated and positively correlated with that of inflammatory cytokines. Immunostaining revealed that infiltration of neutrophils and macrophages coincided with Mgf upregulation. Furthermore, infiltrating neutrophils and macrophages expressed Mgf, suggesting their contribution to MGF upregulation in muscle injury. Macrophages seem to be the predominant source of MGF in muscle injury, whereas neutrophil depletion did not affect muscle Mgf expression. Given the association of MGF and macrophages, we then studied whether MGF could affect macrophage infiltration and polarization. To test this, we overexpressed MGF in CTX-injured muscles and evaluated inflammatory marker expression, macrophage populations, and muscle regeneration outcomes. MGF overexpression delayed the resolution of macrophages, particularly the pro-inflammatory phenotype. This coincided with upregulation of inflammatory markers. Annexin V-based flow cytometry revealed that MGF overexpression likely delays macrophage resolution by limiting macrophage apoptosis. Although MGF overexpression did not obviously affect muscle regeneration outcomes, the findings are novel and provide insights on the physiological roles of MGF in muscle regeneration.
Highlights
Skeletal muscle repair following injury involves a well-orchestrated series of events
It is known that expression of pro-inflammatory cytokines, such as tumor necrosis factor-α (TNF-α) and C–C motif ligand 2 (CCL2) elevated after injury (Tidball et al, 2014; Tidball, 2017)
We examined the expression of genes associated with muscle regeneration, including paired-box protein 7 (Pax7), myogenic differentiation 1 (Myod), Myogenin, and embryonic (Myh3) and neonatal (Myh8) myosin heavy chain in mechano-growth factor (MGF)-overexpressing CTX-injured muscles (n = 6/treatment/timepoint; Figure 9C)
Summary
Skeletal muscle repair following injury involves a well-orchestrated series of events. Numerous studies investigated the role of inflammatory cytokines, chemokines, and growth factors in mediating muscle inflammatory response (Pelosi et al, 2007; Tidball and Welc, 2015; Tonkin et al, 2015; Tidball, 2017). One of these factors, insulin-like growth factor 1 (IGF-1), is important for both resolving muscle inflammation and promoting myogenesis. It has been shown in vivo that overexpression of MGF exerts a more potent hypertrophic effect than IGF-1Ea (Brisson et al, 2014)
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