Joint contracture is a common disease in rehabilitation medicine, which can have a great impact on patients’ quality of life [1]. After the onset of joint contracture, muscle atrophy occurs, intra-articular tissue adhesion happens, and bone changes take place around the joint [2, 3]. Furthermore, the function of impaired joints may be limited, which can affect the patients' abilities in daily living. Prompt and effective treatment is important for the prognosis of joint contracture. however, the timing of rehabilitation treatment and the choice of rehabilitation intervention methods are still controversial.
Joint contracture is often accompanied by morphological changes of various tissues around the joint. According to the anatomical factors limiting joint range of motion, it can be divided into myogenic factors (muscle, tendon, fascia, etc.) and arthrogenic factors (including bone, cartilage, joint capsule and ligament, etc.). The limiting factors of joint motion are mainly muscular factors in the early stage of joint contracture, while articular factors plays the leading role in the middle and late stages [4, 5]. To our knowledge, the treatment of joint contracture is very difficult after the advanced stage. Therefore, the suppression of myogenic factors in the early stage of joint contracture is of great importance to the prognosis of these patients.
Myogenic contracture is primarily manifested as skeletal muscle atrophy, which is characterized by the reduction of muscle mass, cross-sectional area and muscle length in muscles around the affected joint [5, 6]. After the formation of joint contracture caused by joint immobilization, the atrophied muscle tissue around the joint showed a light cytoplasm staining and reticular pattern, with a relative increase in stroma and nucleus, as well as innervation and aggregation of the nucleus [7, 8]. This phenomenon indicates that the catabolism of muscle protein in skeletal muscle tissue was enhanced. Our previous studies suggest that the occurrence of disuse muscular atrophy is related to the happening of joint contracture [5, 9]. The decrease in muscle wet weights is one of the most direct reflections of muscle atrophy [10]. Previous studies have suggested that atrogin-1 is one of the important muscle atrophy-related genes [11], so the elevated expression of atrogin-1 can be used to reflect muscle atrophy. However, the mechanism of periarticular muscle disuse atrophy following joint immobilization is still not thoroughly understood.
Disuse muscular atrophy is regulated by a variety of mechanisms, of which autophagy - lysosomal pathway may be one of the most important signaling pathways [12]. Autophagy is divided into selective autophagy and non-selective autophagy. Most of the previous studies focused on non-selective autophagy, with only a few on selective autophagy. Parkin-mediated mitophagy signaling pathway is one of the classical signaling pathways that regulate mitophagy at present [13]. Under cellular stress conditions, the reduction of circumstances of mitochondrial membrane potential may cause the mutation of mitochondrial DNA mutation and increase in unfolded proteins [14]. Intracellular protein kinase Pink1 aggregates in the outer membrane of the damaged mitochondria and is self-phosphorylated. Pink1 is phosphorylated by the ubiquitin ligase Parkin and is recruited to mitochondria to activate Parkin signal. Concurrently, the process also promotes the phosphorylation of ubiquitin on the outer membrane of mitochondria, which further activates Parkin. Ubiquitin binds to LC3-II through autophagy receptor proteins (p62, etc.) to form autophagosomes. The autophagosome then fuses with the lysosome to form the autophagosome, and the mitochondria and related proteins are finally degraded by lysosomal hydroxylase [15]. Previous studies have suggested that Parkin-mediated mitophagy may be activated after atrophic muscle changes happened following joint immobilization [16]. Recently, scientists have recognized that mitophagy signaling pathways may be related to skeletal muscle atrophy. However, the exact mechanism that may regulate skeletal muscle mitophagy is still unclear.
The endoplasmic reticulum (ER) is an intracellular organelle in which proteins are modified. Previous studies have shown that mitophagy can be regulated by endoplasmic reticulum stress [17]. The endoplasmic reticulum stress responds to the load of unfolded proteins by activating intracellular signal transduction pathways, which is known as unfolded protein response (UPR) [18]. Previous study has suggested that UPR pathways play pivotal roles in muscle stem cell homeostasis, myogenic differentiation and regeneration of injured skeletal muscle [19]. In general, at least three branches of mechanisms can regulate the expressions of a large number of genes that can maintain homeostasis or induction in the endoplasmic reticulum apoptosis. These are protein kinase RNA-like ER kinase (PERK), activated transcription factor-6 (ATF6) and inositol-demanding enzyme-1 (IRE1) [20]. Among these three pathways, PERK signaling pathway can participate in the formation of myotubules and have a regulatory effect on the synthesis of myocytes [21, 22]. PERK is responsible for reducing the overload of misfolded proteins, thereby alleviating ER stress. Kang C et al. showed that mitophagy mediated by the Parkin signaling pathway is elevated in the atrophic muscle after immobilization of unilateral lower limb [23]. The study of Deval C et al. also suggested that mitophagy levels in gastrocnemius and tibialis anterior muscles were increased after lower limb immobilization [24]. These results indicated that the level of mitophagy may increase when skeletal muscle atrophy occurs. Previous studies have suggested that Parkin-mediated mitophagy can be regulated by the PERK signal in placental tissues [25]. However, few studies have concentrated on the regulatory role of PERK on Parkin in atrophic skeletal muscles.
Low frequency electrical stimulation (LFES) is a safe and effective physical agents therapy method in rehabilitative medicine, which can increase contractile function of muscle fiber which can then be used for the treatment of skeletal muscle atrophy [26]. Rectus femoris muscle is a commonly used treatment site of electrical stimulation for patients with quadriceps muscle atrophy in clinical practice. The current study involves an investigation of the effects of electrical stimulation on disuse muscular atrophy in a rabbit model of knee joint contracture and an exploration of the molecular mechanisms that underlie the initiation and progression of this pathology. We hypothesized that PERK-regulated mitophagy may play an important role in the pathology of joint contracture and that electrical stimulation may inhibit disuse muscular atrophy through PERK-regulated mitophagy.