The study evaluates the influence of monocytes/macrophages in the mechanisms of skeletal muscle injury using a mouse model and selective depletion of peripheral monocyte with systemic injections of liposomal clodronate (dichloromethylene bisphosphonate). This pharmacological treatment has been demonstrated to induce specific apoptotic death in monocytes and phagocytic macrophages. In the current studies, the liposomal clodronate injections resulted in a marked attenuation of the peak inflammatory response in the freeze-injured muscle in the first three days after injury. The effect was accompanied by a transient reduction (at day 1 or 3 postinjury) of the expression of several genes coding for inflammatory, as well as growth-related mediators, including TNF, monocyte chemoattractant protein (MCP)-1, thioredoxin, high-mobility group AT-hook 1, insulin-like growth factor-binding protein (IGFBP), and IGF-1. In contrast, the expression of major myogenic factors (i.e., MyoD and myogenin) directly involved in the activation/proliferation and differentiation of muscle precursor cells was not altered by the clodronate liposome treatment. The repair process in the injured muscle of clodronate liposome-treated mice was characterized by prolonged clearance of necrotic myofibers and a tendency for increased muscle fat accumulation at day 9 and 14 postinjury, respectively. In conclusion, a significant reduction of the initial monocyte/macrophage influx into the injured muscle is associated with not improved, but moderately impaired, repair processes after skeletal muscle injury.

The purpose of this study was to determine whether certain chemokines, which are highly expressed in injured skeletal muscle, are involved in the repair and functional recovery of the muscle after traumatic injury. In wild-type control mice, mRNA transcripts of macrophage inflammatory protein (MIP)-1alpha, MIP-1beta, and monocyte chemoattractant protein (MCP)-1 as well as their major receptors, CCR5 and CCR2, increased after freeze injury and gradually returned to control (uninjured) levels by 14 days. Muscle function and histological characteristics were monitored in injured mice that were genetically deficient for the CCR5 receptor (a major receptor for MIP-1alpha and MIP-1beta) and also rendered MCP-1 deficient with neutralizing antibodies. To dissect the role of these chemokines, additional studies were conducted in CCR5- and CCR2-deficient mice. CCR5-/- mice injected with MCP-1 antiserum for the first 3 days after injury exhibited a twofold greater maximal isometric tetanic torque deficit at 14 days after injury than did controls (i.e., 33% vs. 17%; P = 0.002). The impaired functional recovery was accompanied with an increased fat infiltration within the regenerating muscle without a significant difference in the influx of inflammatory cells, including macrophages. Strength recovery was also impaired in mice deficient for the receptor of MCP-1, CCR2, but not in CCR5-/- mice that were not injected with MCP-1 antiserum. The data suggest that MCP-1/CCR2 plays a role in the regeneration and recovery of function after traumatic muscle injury.

Common acute injuries to skeletal muscle can lead to significant pain and disability. The current therapeutic approaches for treating muscle injuries are dependent on the clinical severity but not on the type of injury. In the present studies, the pathophysiology and molecular pathways associated with two different types of skeletal muscle injury, one induced by direct destruction of muscle tissue (i.e. FI) and the other induced by a contractile overload (more specifically high-force eccentric contractions, i.e. CI) were compared side by side. Histopathological evaluation and measurements of muscle strength were accompanied by analyses of expression for 12 488 known genes at four time points ranging from 6 h to 7 days after injury. Real-time RT-PCR was used to confirm some of the injury type differences in the temporal profiles of gene expression. Our data revealed several pools of genes, including early induction of transcription, myogenic and stress-responsive factors, common for both types of injury as well as pools of genes expressed specifically with one of the injury types. Only CI activated a set of genes associated with the repair of impaired proteins and structures including genes related to apoptosis, whereas FI uniquely activated gene sets involved in extensive inflammatory responses, tissue remodelling, angiogenesis and myofibre/extracellular matrix synthesis. In conclusion, knowledge of the sets of genes associated specifically with the nature of the injury may have application for development of new strategies for acceleration of the recovery process in injured skeletal muscle.

Traumatic skeletal muscle injury causes a specific sequence of cellular events consisting of degeneration, inflammation, regeneration, and fibrosis. The role of early posttraumatic mechanisms, including acute inflammatory response, in muscle repair is not well understood. In the present study, oligonucleotide microarray analyses were used to examine the candidate genes that are involved in these early events of the muscle injury/repair process. cDNA was prepared from the injured and control tibialis anterior (TA) muscle of mice 24 h postinjury and labeled with the fluorescent dye Cy5 or Cy3 prior to hybridization to a DNA microarray. The microarray analysis, including 732 genes, was conducted in triplicate, and we describe only genes modulated by the injury showing a differential expression (both increased and decreased) 1.7-fold or greater (p < 0.05) from control uninjured TA muscle. Selected expression patterns were confirmed by other gene expression detection methods, including real-time reverse transcription-polymerase chain reaction (RT-PCR) and RNase protection assay (RPA) or immunohistochemistry detection methods. The upregulated genes (2.8%) were mainly associated with inflammation, oxidative stress, and cell proliferation, whereas the downregulated genes (3.2%) were related to metabolic and cell signaling pathways. In addition, the study suggested that chemokines, such as monocyte chemoattractant protein-1 (MCP-1), associated with monocyte/macrophage influx and activation, are abundantly expressed in postinjured muscle, and they might play a role in traumatic muscle injury/recovery processes.

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