Skeletal muscle fibrosis: the effect of stromal-derived factor-1a-loaded collagen scaffolds

Skeletal muscle tissue repairs itself by the activation of satellite cells, which are associated with the myofibers [1,2]. Activated satellite cells migrate to the site of injury and generate myoblasts, which eventually differentiate and fuse to each other or to damaged myofibers to restore muscle structure and function [3–5]. The satellite cells also have the capacity to replenish their numbers by self-renewal for future regeneration cycles [6–8]. However, fibrosis can also occur, which prevents full functional recovery of the muscle [3,4,9,10]. Many approaches have been developed to improve muscle regeneration. The application of growth factors, such as IGF-I, FGF-II, HGF, NGF or GCSF, has been demonstrated to improve muscle regeneration by inducing satellite cell proliferation and differentiation [11–15]. Inhibition of TGF-b can reduce the extent of fibrosis, and promotes muscle regeneration [11,16]. The injection of cell types such as satellite cell-derived myoblasts, side population cells, muscle-derived stem cells, mesoangioblasts, pericytes and CD133 stem cells improves muscle regeneration after muscle injury, but also in muscle diseases, such as Duchenne muscular dystrophy [8,17–22]. Many muscle injury models exist, such as crush injury, freeze injury, toxin-induced injury, strains, contusions, lacerations and muscle disease models, which can induce minor muscle fibrosis [3,4,23]. However, full-thickness defects, which result in the loss of muscle tissue and the formation of large fibrotic lesions, are not widely studied. Such a model represents muscle resection after tumor ablation or other surgical muscle traumas. It can be used to develop methods to (re)generate skeletal muscle tissue and inhibit the formation of fibrotic tissue by implantation of a regenerative scaffold. To achieve this, the addition of growth factors or cells alone is not sufficient. Tissue-engineered constructs are required to fill up the defect and provide the necessary structural cues for the satellite cells. Several studies using such scaffolds have been performed with varying results, but loading of the scaffolds with growth factors and/or cells generally improves muscle regeneration [24–27]. However, the myogenic potential of satellite cells is readily lost during culture [8,28]. Furthermore, transplanted satellite cells and myoblasts rarely survive and their migration into the muscle tissue is limited [17,29]. Since satellite cells are the primary cells for muscle regeneration, we loaded collagen scaffolds with stromal-derived factor (SDF)-1a to attract resident satellite cells into the defect. SDF-1a is a CXC chemokine that controls processes such as trafficking and transendothelial migration of hematopoietic cells [30]. SDF-1a binds to CXCR4, which is also present on satellite cells [31]. During embryo genesis, SDF-1a regulates the migration of muscle precursor cells [32,33]. Moreover, in adulthood, SDF-1a is expressed by myofibers and induces migration of satellite cells [34,35]. Therefore, the aim Aim: To develop a model for muscle fibrosis based on full-thickness muscle defects, and to evaluate the effects of implanted stromal-derived factor (SDF)-1a-loaded collagen scaffolds. Methods: Full-thickness defects 2 mm in diameter were made in the musculus soleus of 48 rats and either left alone or filled with SDF-1a-loaded collagen scaffolds. At 3, 10, 28 and 56 days postsurgery, muscles were analyzed for collagen deposition, satellite cells, myofibroblasts and macrophages. Results: A significant amount of collagen-rich fibrotic tissue was formed, which persisted over time. Increased numbers of satellite cells were present around, but not within, the wounds. Satellite cells were further upregulated in regenerating tissue when SDF-1a-loaded collagen scaffolds were implanted. The scaffolds also attracted macrophages, but collagen deposition and myofibroblast numbers were not affected. Conclusion: Persistent muscle fibrosis is induced by full-thickness defects 2 mm in diameter. SDF-1a-loaded collagen scaffolds accelerated muscle regeneration around the wounds, but did not reduce muscle fibrosis.