Emerging roles of ER stress and unfolded protein response pathways in skeletal muscle health and disease

Abstract

Skeletal muscle is the most abundant tissue in the human body and can adapt its mass as a consequence of physical activity, metabolism, growth factors, and disease conditions. Skeletal muscle contains an extensive network of endoplasmic reticulum (ER), called sarcoplasmic reticulum, which plays an important role in the regulation of proteostasis and calcium homeostasis. In many cell types, environmental and genetic factors that disrupt ER function cause an accumulation of misfolded and unfolded proteins in the ER lumen that ultimately leads to ER stress. To alleviate the stress and restore homeostasis, the ER activates a signaling network called the unfolded protein response (UPR). The UPR has three arms, which regulate protein synthesis and expression of many ER chaperone and regulatory proteins. However, the role of individual UPR pathways in skeletal muscle has just begun to be investigated. Recent studies suggest that UPR pathways play pivotal roles in muscle stem cell homeostasis, myogenic differentiation, and regeneration of injured skeletal muscle. Moreover, markers of ER stress and the UPR are activated in skeletal muscle in diverse conditions such as exercise, denervation, starvation, high fat diet, cancer cachexia, and aging. Accumulating evidence also suggests that ER stress may have important roles in the pathogenesis of inflammatory myopathies and genetic muscle disorders. The purpose of this review article is to discuss the role and potential mechanisms by which ER stress and the individual arms of the UPR regulate skeletal muscle formation, plasticity, and function in various physiological and pathophysiological conditions.

Keywords: ER stress; UPR; atrophy; myogenesis; skeletal muscle.

Figure 1. Schematic representation of the unfolded protein response pathways in skeletal muscle

Under normal physiological conditions, the PERK, IRE1, and ATF6 proteins are maintained in an inactive state by binding to BiP. Stress in skeletal muscle caused by aging, disease, exercise, starvation, cachexia, denervation, or high fat diet causes BiP to disassociate from these proteins and preferentially binds to the misfolded proteins in the ER lumen. Upon release from BiP, PERK is auto-phosphorylated leading to a cascade of signals including phosphorylation of eIF2α and translation of ATF4, a potent transcription factor. Another ER transmembrane sensor, IRE1, also becomes activated by autophosphorylation during ER stress. Through its endonuclease activity, IRE1 promotes splicing of a 26-base intron from XBP1 mRNA as well as phosphorylation of JNK. Lastly, once activated, ATF6 moves from the ER to the Golgi apparatus to be cleaved by site-1 proteases. The cleaved N-terminal fragment of ATF6 is then transported to the nucleus where it acts in combination with sXBP1 and ATF4 to alleviate ER stress by regulating gene expression and protein synthesis. In addition to regulation of ER related genes, activation of ER stress has been shown to increase skeletal muscle atrophy by inhibition of protein synthesis through eIF2α, activation of JNK through IRE1, and potentially through augmenting the expression of components of UPS and ALS. Activation of ATF6 increases skeletal muscle adaptation by increasing expression of metabolic genes through interaction with PGC1α. ER stress has also been linked to insulin resistance through activation of ERK signaling and inhibition of AMPK. However, specific activation of PERK signaling leads to secretion of FGF21, which is shown to alleviate insulin resistance.

Figure 2. Schematic diagram illustrating the effect of ER stress pathways on myogenesis

Adult muscle stem cells or satellite cells reside between the sarcolemma and the basal lamina of mature muscle in a quiescent state and are maintained by constitutively phosphorylated PERK and phosphorylated eIF2α. Dephosphorylation of PERK and eIF2α promotes progression and commitment of satellite cells to the myogenic lineage. Prior to differentiation, myoblasts undergo an apoptotic pruning process facilitated by ATF6, ensuring that only differentiation-competent myoblasts proceed. Competent myoblasts differentiate into myocytes; however, CHOP has been found to inhibit this transition. Myocytes fuse with one another to form multi-nucleated myotubes. IRE1 and sXBP1 have been shown to disrupt the fusion process, resulting in thin, unhealthy cells through inhibition of MyoD and Myogenin. Myotubes undergo a maturation process to form adult myofibers with their own supply of quiescent satellite cells available for repair of damaged tissue due to injury or disease.