Muscle injuries and strategies for improving their repair

Muscle degeneration and inflammation

Active muscle degeneration and inflammation occur within the first few days after injury. The initial event is necrosis of the muscle fibers, which is triggered by disruption of local homeostasis and particularly by unregulated influx of calcium through sarcolemma lesions (Tidball 2011). Excess in cytoplasmic calcium causes proteases and hydrolases activation that contribute to muscle damage and also causes activation of enzymes that drive the production of mitogenic substances for muscle and immune cells (Tidball 2005). After muscle degeneration, neutrophils are the first inflammatory cells infiltrating the lesion. A large number of pro-inflammatory molecules such as cytokines (TNF-α, IL-6), chemokine (CCL17, CCL2) and growth factors (FGF, HGF, IGF-I, VEGF; TGF-β1) are secreted by neutrophils in order to create a chemoattractive microenvironment for other inflammatory cells such as monocytes and macrophages (Tidball 1995; Toumi and Best 2003). Two types of macrophages are identified during muscle regeneration (McLennan 1996), which appear sequentially during muscle repair (Arnold et al. 2007). M1 macrophages, defined as pro-inflammatory macrophages, act during the first few days after injury,. contribute to cell lysis, removal of cellular debris and stimulate myoblast proliferation. Conversely, M2 macrophages, defined as anti-inflammatory macrophages, act 2 to 4 days after injury, attenuate the inflammatory response and favor muscle repair by promoting myotubes formation (Tidball and Wehling-Henricks 2007; Chazaud 2014; Chazaud et al. 2003). Macrophages, infiltrating injured muscle, are key players of the healing process (Zhao et al. 2016), able to participate in the muscle regeneration process or to favor fibrosis (Munoz-Canoves and Serrano 2015; Lemos et al. 2015).

Connective tissue/fibrosis

Connective tissue remodeling is an important step of the regenerative muscle process. Rapidly after muscle injury, a gap is formed between damaged muscle fibers and filled with a hematoma. Muscle injuries can be clinically classified depending of the nature of the hematoma (size, location). Late elimination of the hematoma is known to delay skeletal muscle regeneration, to improve fibrosis and to reduce biomechanical properties of the healing muscle (Beiner et al. 1999). In rare complication, major muscle injuries may lead to the development of myositis ossificans that will impair muscle regeneration and repair (Beiner and Jokl 2002) (Walczak et al. 2015).

The presence of fibrin and fibronectin at the injury site, initiate the formation of an extracellular matrix that is rapidly invaded by fibroblasts (Darby et al. 2016; Desmouliere and Gabbiani 1995). Fibrogenic cytokines such as transforming growth factor β1 (TGF-β1) participate to excessive fibroblasts/myofibroblasts proliferation and to an increase in type I/III collagens, laminin and fibronectin production (Lehto et al. 1985). In its initial phase, the fibrotic response is beneficial, stabilizing the tissue and acting as a scaffold for myofibers regeneration. Nevertheless, an excessive collagen synthesis post injury, often result in an increase of scar tissue size over time that can prevent normal muscle function (Mann et al. 2011). Many growth factors are involved in the development of fibrosis, such as Connective Tissue Growth Factor (CTGF), Platelet-Derived Growth Factor (PDGF) or myostatin. TGF-β1, by stimulating fibroblasts/myofibroblasts to produce extracellular proteins such as fibronectin and type I/III collagen, has been identified as the key element in this process (Mann et al. 2011),. Although fibroblasts are the major collagen-producing cells in skeletal muscle, TGF-β1 have also an effect directly on myoblasts causing their conversion to myofibroblasts. Thus myoblasts initially acting to repair damaged myofibers, will produce significant level of collagen and will contribute to muscle fibrosis (Li and Huard 2002).

R evascularization

The restoration of the blood supply in the injured skeletal muscle is one of the first signs of muscle regeneration and is essential to its success. Without revascularization, muscle regeneration is incomplete and a significant fibrosis occurs (Best et al. 2012; Ota et al. 2011). After muscle trauma, blood vessels rupture induces tissue hypoxia at the injury site (Jarvinen et al. 2005). New capillaries formation quickly after injury is therefore necessary (Scholz et al. 2003) for a functional muscle recovery. Secretion of angiogenic factors such as vascular endothelial growth factor (VEGF) at the lesion site is important and several studies have shown that VEGF, by favoring angiogenesis, improve skeletal muscle repair (Deasy et al. 2009; Frey et al. 2012).

Innervation

Muscle repair is complete when injured myofibers are fully regenerated and become innervated. The synaptic contact between a motor neuron and its target muscle fiber, often take place at a specific site in the central region of myofibers, the neuromuscular junction (NMJ) (Wu et al. 2010). NMJ are essential for maturation and functional activity of regenerating muscles. Within 2–3 weeks after muscle damage, the presence of newly formed NMJ is observed in regenerative muscle (Rantanen et al. 1995; Vaittinen et al. 2001).

Growth factors

Stem cells

Transplantation of satellite cell-derived myoblasts has long been explored as a promising approach for treatment of skeletal muscle disorders. After an initial demonstration that normal myoblasts can restore dystrophin expression in mdx mice (Partridge et al. 1989), clinical trials, in which allogeneic normal human myoblasts were injected intramuscularly several times in dystrophic young boys muscles, have not been successful (Law et al. 1990; Mendell et al. 1995). Even recently, despite clear improvement in methodologies that enhance the success of myoblast transplantation in Duchenne patients (Skuk et al. 2007), outcomes of clinical trials are still disappointing. These experiments have raised concerns about the limited migratory and proliferative capacities of human myoblasts, as well as their limited life span in vivo. It led to the investigations of other muscle stem cells sources that could overcome these limitations and outperform the success of muscle cell transplantation. Among all these non-satellite myogenic stem cells, human mesoangioblasts, human myogenic-endothelial cells and human muscle–derived CD133+ have shown myogenic potentials in vitro and in vivo (Sampaolesi et al. 2006; Zheng et al. 2007; Meng et al. 2014). The use of such myogenic progenitors cells for improving muscle healing may become an interesting therapeutic alternative (Tedesco and Cossu 2012; Tedesco et al. 2010; Chen et al. 2012). A first phase I/IIa clinical trial has recently demonstrated that intra arterial injections of human mesoangioblasts are safe but display only very limited clinical efficacy in Duchenne patients (Cossu et al. 2015).

Scaffolds

Myogenic precursor cell survival and migration is greatly increased by using appropriate scaffold composition and growth factor delivery (Hill et al. 2006) (Boldrin et al. 2007). Controlling the microenvironment of injected myogenic cells using biological scaffolds enhance muscle regeneration (Borselli et al. 2011). Ideally, using an appropriate extracellular matrix (ECM) composition and stiffness, scaffolds should best replicate the in vivo milieu and mechanical microenvironment (Gilbert et al. 2010) (Engler et al. 2006). A combination of stem cells, biomaterial-based scaffolds and growth factors may provide a therapeutic option to improve regeneration of injured skeletal muscles (Jeon and Elisseeff 2016).

Anti-fibrotic therapy

TGF-β1 is expressed at high levels and plays an important role in the fibrotic cascade that occurs after the onset of muscle injury (Bernasconi et al. 1995; Li et al. 2004). Therefore, neutralization of TGF-β1 expression in injured skeletal muscle should inhibit the formation of scar tissue. Indeed, the use of anti-fibrotic agents (ie decorin, relaxin, antibody against TGF-β1…) that inactivate TGF-β1 signaling pathways reduces muscle fibrosis and, consequently, improve muscle healing, leading to a near complete recovery of lacerated muscle (Fukushima et al. 2001; Li et al. 2007). Losartan, an angiotensin II receptor antagonist, neutralize the effect of TGF-β1 and reduce fibrosis, making it the treatment of choice, since it already has FDA approval to be used clinically (Bedair et al. 2008; Park et al. 2012; Terada et al. 2013). Suramin, also approved by the FDA, blocks TGF-β1 pathway and reduces muscle fibrosis in experimental model (Chan et al. 2003; Taniguti et al. 2011).

Mechanical stimulation

Mechanical stimulation may offer a simple and effective approach to enhance skeletal muscle regeneration. Stretch activation, mechanical conditioning but also massage therapy or physical manipulation of injured skeletal muscles have shown multiple benefit effects on muscle biology and function in vitro and in vivo (Tatsumi et al. 2001);(Best et al. 2012) (Crane et al. 2012; Kumar et al. 2002; Gilbert et al. 2010; Powell et al. 2002). Recently, Cezar and colleagues demonstrates that mechanical forces are as important biological regulators as chemicals and genes, and underlines the immense potential of developing mechano-therapies to treat muscle damage (Cezar et al. 2016). A recent study also demonstrated that a treatment based on ultrasound-guided intra-tissue percutaneous electrolysis (EPI technique) enhances the treatment of muscle injuries (Abat et al. 2015). Altogether, these results suggest that mechanical stimulation should be considered as a possible therapy to improve muscle regeneration and repair.