The Chemokine Cxcl10 is Not Needed For Normal Skeletal Muscle Regeneration Following a Toxin-induced Injury

Abstract

PURPOSE: The CCL2/CCR2 chemokine receptor axis is necessary for successful muscle regeneration following injury. Recently, another chemokine known as CXCL10 (or IP-10) was found to be elevated in human skeletal muscle following muscle-damaging eccentric contractions. This finding may indicate that CXCL10, like CCL2, is important for muscle regeneration. METHODS:To test this, we measured functional and histological markers of muscle regeneration in mice out to 14 days post muscle injury (DPI) in wild type (WT) mice and CXCL10 knockout (KO) mice (n = 13 and 12, respectively). The muscle injury was induced by cardiotoxin (CTX) injection into the tibialis anterior (TA) muscle of a hind limb of each mouse. The opposite limb was injected with saline to serve as a within-animal sham control. At 2, 7 and 14 DPI muscle function of both the CTX- and saline-injected TA muscles was assessed using an in situ contraction preparation. After the functional testing, the TA muscles were excised, weighed, frozen, cut by cross section and mounted on glass slides for histological studies. Mounted muscle samples were stained using hematoxylin and eosin to identify central nucleated fibers and quantify cross sectional area. Samples were also stained immunohistochemically for embryonic myosin heavy chain (eMyHC) to identify the number of fibers undergoing regeneration. RESULTS: Relative to the saline-injected TA, strength significantly decreased (p<0.05) at 2 (WT=46.7±15.9%, KO=44±9.4%) and 7 (WT=65±8.3%, KO= 66±15.9%) DPI, and was recovered by 14 DPI. No significant differences in muscle strength were found between genotypes. Compared to the control TA, the CTX-injected TA muscle mass was significantly reduced (p<0.05) at 7 DPI (WT=86±3%, KO=91±9%) compared to 14 DPI (WT=105±8.3%, KO= 105±11%). No significant differences in muscle mass were found between genotypes. Cross-sectional area of regenerating myofibers was greater at 14 DPI compared to 7 (p<0.05), yet again no significant differences were found between genotypes. The number of myofibers expressing eMyHC was greater at 7 DPI than 14 (p<0.05), with no significant difference between genotypes. CONCLUSIONS: We conclude that CXCL10 is not necessary for normal muscle regeneration from a toxin-induced muscle injury in mice.