Calsequestrin, a new modulator of unfolded protein response in skeletal and cardiac muscle

Abstract

Cellular responses to stress are an integral part of muscle physiology and pathology. To deal with stress, cardiac and skeletal muscle cells utilize various mechanisms, engaging intercellular organelles including the endoplasmic reticulum/sarcoplasmic reticulum (ER/SR) [1]. The disruptions of ER functions are often observed in ischemic, hypertrophic and failing hearts [1]. However, the molecular mechanisms governing muscle ER stress regulation in the muscle are still unclear. Disruption of ER/SR hemostasis with physiological and pathological stimuli triggers a coping response, so called unfolded protein response (UPR). One of the UPR signaling pathways involves stress sensor inositol‐requiring enzyme 1α (IRE1α), an integral ER/SR membrane protein kinase and mRNA splicing enzyme. Here, we discovered that calsequestrin, a muscle Ca2+ binding protein localized to junctional SR, interacts with UPR stress sensor IRE1α, and contribute to the modulation of stress responses in the muscle. Both skeletal and cardiac isoforms of calsequestrin bind directly to IRE1α as measured by Microscale Thermophoresis (MST), Surface Plasmon Resonance (SPR), and His‐tag Ni‐NTA‐agarose pull down techniques. Although calsequestrin contains three thioredoxin‐like domains its interaction with IRE1α did not involve cysteine residues in IRE1α. Iodixanol gradient fractionation and immunostaining of skeletal muscle sections or isolated cardiomyocytes showed enrichment of IRE1α protein in junctional SR where it is colocalized with calsequestrin. MST analysis revealed that calsequestrin binding to IRE1α has inhibitory effect on IRE1α by preventing dimerization/oligomerization IRE1α required for X‐box binding protein 1 (XBP1) splicing activity. Consequently, cardiac calsequestrin‐deficient cardiomyocytes have increased activity of IRE1α. We concluded that calsequestrin is a new IRE1α binding partner and the protein plays a role in modulation of stress responses in skeletal and cardiac muscle.Support or Funding InformationCanada Institutes of Health Research (CIHR)This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.