The skeletal muscles in ob/ob mice had a smaller cross-sectional area in type II fibers, with excessive deposition of lipid and collagen in the area of the intermyofibers, when compared to the control group. Despite this, the recovery of motor function was equal between ob/ob and control mice following the nerve injury. Collagen deposition was increased both groups due to nerve injury, especially in ob/ob mice.
Obesity is a causative factor for the disorders of various organs including cardiovascular tissues and the liver, which is a target of ectopic lipid accumulation. The excessive lipid accumulation to the non-adipose tissues can cause cell dysfunction or cell death via pro-inflammation, and these processes have been defined as lipotoxicity [18]. As a result, inflammation and fibrosis are caused to the heart and the liver, leading to diastolic dysfunction of the heart and non-alcoholic steatohepatitis (NASH), respectively [8, 9]. Consequently, these conditions can progress to serious conditions including heart failure and liver cirrhosis. The lipotoxicity is a concern for the skeletal muscles, since skeletal muscles are also a target for ectopic lipid accumulation in individuals with obesity. In fact, we found a number of studies regarding the pathophysiology of the lipotoxicity to skeletal muscle cells based on in vitro study [19, 20], while the adverse effect of lipotoxicity in vivo and the functional consequences are unresolved.
In the present study, we used ob/ob mice as the animal model of obesity, in which lipid accumulation and increases of adipocytes were clearly shown in the gastrocnemius muscles. Therefore, the ob/ob mice are an appropriate model of in vivo exploration into the adverse effect of lipotoxicity to the skeletal muscles. Furthermore, the skeletal muscles were observed to be atrophic in the ob/ob mice, in which reduction of cross-sectional areas of type II fibers was predominant. In addition, the skeletal muscles had more collagen deposition between the myofibers in the ob/ob mice than in the control mice. These findings are similar to those in liver and heart patients with obesity [21, 22], suggesting that the skeletal muscles in the obese subjects could be atrophic through chronic inflammation owing to lipotoxicity. In fact, a previous study showed that obese adults had a reduction of quadriceps muscle strength relative to body mass compared to non-obese adults [23].
Furthermore, previous studies reported that patients with obesity had inferior function in comparison to non-obese patients after surgery for peripheral nerve disorders, even though the surgical treatments had good clinical results in comparison to the preoperative condition. Roh et al. stated that patients with metabolic syndrome, which is strongly related to obesity, had decreased pinch strength and delay of functional recovery after the surgical treatment for carpal tunnel syndrome [24]. Burgstaller et al. described fewer obese patients with meaningful improvement than non-obese individuals in the surgical treatment for lumbar canal stenosis [25]. These studies suggest obesity is associated with insufficient improvement due to impairment during the denervation and re-innervation process in the surgical treatment of peripheral disorders.
Nonetheless, the improvements in the functional status were equal between ob/ob and control mice after the nerve injury, while histological analysis showed that recovery of the cross-sectional area in type II fibers was delayed and poor in the ob/ob mice than in the control mice. In general, type I fibers are associated with endurance, while type II fibers are characterized as ‘fast fibers’, and associated with strength of skeletal muscles [26]. The results might indicate inferior recovery of muscle strength after the nerve injury in patients with obesity, even though the muscle strength was not measured in this study. By contrast, the recovery of cross-sectional area of type I fibers was not different between the two groups, which is likely to explain the reason for no difference in functional status.
Likewise, an increase of type I fiber grouping was observed in the ob/ob mice, especially after the nerve injury. Previous studies have demonstrated that re-innervation of type I fibers is faster than that of type II fibers, in which preferential re-innervation of type I fibers in recovery following nerve injury leads to an increase in type I grouping in reinnervated muscles [11, 27]. Furthermore, shifts of fiber groupings are associated with aging in the denervation and re-innervation processes, suggesting that an increase of type I grouping might influence the function of reinnervated muscles, such as contraction of the myofibers [11]. This study suggests obesity could not only cause a shift of fiber type in the skeletal muscles which result in decreased muscular strength, but also cause insufficient recovery after the nerve injury.
Furthermore, the collagen deposition on the intermyofibers was increased in the skeletal muscles following the nerve injury [28]. The fibrotic formation is characterized by an anomalous accumulation of the extracellular matrix, such as collagen around inflamed tissues, and leads to the dysfunction in various tissues including the heart and liver [29]. In myocardial tissue, this reactive and progressive interstitial fibrosis contributes to myocardial stiffness with progression to ventricular dysfunction. Even though the pathological significance of the fibrosis is less elucidated in the skeletal muscles than in other organs, fibrosis of the skeletal muscle is a characteristic feature of muscular dystrophin, myopathies, and traumatic injuries [30]. In addition, previous studies show that fibrosis caused a decrease in skeletal muscular strength [31–33]. Therefore, abnormal accumulation of fibrotic tissues, which were observed in the denervated and reinnervated muscles of ob/ob mice, also may lead to stiffness of the skeletal muscles and result in a reduction of contractile force. In the future, reduction of body weight, anti-fibrotic drugs, and inhibition of fatty acid for the recovery of motor function in peripheral nerve disorders need to be evaluated.
In conclusion, this study showed that the skeletal muscles of ob/ob mice were smaller in cross-sectional area of type II fibers with excessive deposition of lipid and collagen at the interstitial area at myofibers. Following injury of the sciatic nerve, recovery of motor function was equal between ob/ob and control mice following nerve injury, whereas the skeletal muscles of ob/ob mice not only had a significantly smaller sectional area of type II fibers than the control mice, but also comprised an increase of type I fiber grouping during denervation and re-innervation. In addition, collagen deposition was significantly increased in the skeletal muscles of the ob/ob mice after the nerve injury. We suggest that lipotoxicity to type II fibers, grouping of type I fibers, and interstitial fibrosis due to collagen deposition could be the causes of insufficient improvement after surgery for peripheral nerve injury in obese individuals.