Role of matrix metalloproteinases in skeletal muscle: migration, differentiation, regeneration and fibrosis

Abstract

Matrix metalloproteases (MMPs) are key regulatory molecules in the formation, remodeling and degradation of extracellular matrix (ECM) components in both physiological and pathological processes in many tissues. In skeletal muscle, MMPs play an important role in the homeostasis and maintenance of myofiber functional integrity by breaking down ECM and regulating skeletal muscle cell migration, differentiation and regeneration. Skeletal muscle satellite cells, a group of quiescent stem cells located between the basement membrane and the plasmalemma of myofibers, are responsible for lifelong maintenance and repairing, which can be activated and as a result migrate underneath the basement membrane to promote regeneration at the injured site. MMPs are able to degrade ECM components, thereby facilitating satellite cell migration and differentiation. This current review will focus on the critical roles of MMPs in skeletal muscle injury and repair, which include satellite cell activation with migration and differentiation. The effect of MMPs on muscle regeneration and fibrous scar tissue formation, as well as therapeutic insights for the future will be explored.

Figure 1

MMP-1 activates myoblast migration at various conditions in vitro. The culturing myoblasts (C2C12) were artificially wounded by disrupting the monolayer with a sterile pipette, and were then placed in serum-free DMEM supplemented with 1% penicillin/streptomycin and treated with 0, 1.0, 10 or 100 ng/mL of MMP-1 (M1802, Sigma, St. Louis, MO). Cells were incubated for 1, 4, 6 and 24 hours to allow for migration back into the wound area. Cells were then fixed in cold methanol, washed with phosphate buffered saline (PBS), and then stained with 4′,6-Diamidino-2-phenylindole (DAPI, Sigma, St. Louis, MO) to help visualize cell migration. Northern Eclipse Software (Empix Imaging Inc., Mississauga, ON, Canada) was used to quantify the average migration distance of C2C12 cells that traveled past the original wound demarcation. C2C12 show an enhanced ability to migrate into the wound area after of treatment with MMP1 (10 ng/mL) in non-coated dishes (A). In conditions more accurately representing in vivo ECM content, e.g., type I collagen (B) or fibronectin (C)-coated plates, C2C12 present more aggressive movement compared with control non-treated conditions, especially at 24 hours observation (A–C). (*p < 0.05, **p < 0.01).

Figure 2

MMP enhances myoblast transplantation in the skeletal muscle of mdx mice. 1 × 10⁵ LacZ positive C2C12 cells and fluorescent beads were injected into the gastrocnemius muscles (GMs) of mdx/SCID mice along with 200 ng of MMP-1 (Sigma) in the left GMs and PBS in the right GMs. GMs were harvested at different time points. LacZ staining with Eosin, along with immunohistochemistry for dystrophin was performed. Results showed that MMP-1 treated cells fused into muscle grafts to a greater degree (A) compared to control cells (C) at two weeks post transplantation. Using immunohistochemistry, higher numbers of dystrophin positive myofibers with larger size were detected within MMP-1 treated muscle (B) compared to control (D) at two weeks. It shows that MMP-1 treated C2C12 cells migrated at increased distances from the original injection site and showed a greater degree of engraftment. Red, dystrophin; green, beads; blue, nuclei; white asterisks, dystrophin positive myofibers.