Two pools of IRE1α in cardiac and skeletal muscle cells

Abstract

The endoplasmic reticulum (ER) plays a central role in cellular stress responses via mobilization of ER stress coping responses, such as the unfolded protein response (UPR). The inositol‐requiring 1α (IRE1α) is an ER stress sensor and component of the UPR. Muscle cells also have a well‐developed and highly subspecialized membrane network of smooth ER called the sarcoplasmic reticulum (SR) surrounding myofibrils and specialized for Ca2+ storage, release, and uptake to control muscle excitation‐contraction coupling. Here, we describe 2 distinct pools of IRE1α in cardiac and skeletal muscle cells, one localized at the perinuclear ER and the other at the junctional SR. We discovered that, at the junctional SR, calsequestrin binds to the ER luminal domain of IRE1α, inhibiting its dimerization. This novel interaction of IRE1α with calsequestrin, one of the highly abundant Ca2+ handling proteins at the junctional SR, provides new insights into the regulation of stress coping responses in muscle cells.—Wang, Q., Groenendyk, J., Paskevicius, T., Qin, W., Kor, K. C., Liu, Y., Hiess, F., Knollmann, B. C., Chen, S. R. W., Tang, J., Chen, X.‐Z., Agellon, L. B., Michalak, M. Two pools of IRE1α in cardiac and skeletal muscle cells. FASEB J. 33, 8892–8904 (2019). http://www.fasebj.org

Recent advances in signal integration mechanisms in the unfolded protein response

“…By contrast, the ER lumenal protein canopy homolog 2 (CNPY2) binds only to PERK during ER stress to regulate downstream signaling 45 . Moreover, in skeletal muscle cells, the ER oxidoreductase calsequestrin binds IRE1 to inhibit its activity 46 . These findings support a view wherein the signaling capacity of UPR sensors can be refined to accommodate specific outputs according to the specific needs of cells and tissues.…”

Phylogenetic and biochemical analysis of calsequestrin structure and association of its variants with cardiac disorders

“…Casq2 binds to IRE1α, an ER/SR stress sensor and squelches IRE1α activity 55 . We used MST thermophoresis to test whether Casq2 mutations affected Casq2 interaction with the luminal domain of IRE1α.…”

“…Purification was performed using binding buffer containing 50 mM Tris–HCl, pH 8.0, 300 mM NaCl, and protein eluted with buffer containing 50 mM Tris–HCl, pH 8.0, 300 mM NaCl, and 250 mM imidazole. The 6xHis tagged ER luminal domain of IRE1α (IRE1-NLD) was expressed in COS-1 cells and purified by Ni-NTA agarose chromatography 55 . Protein concentration was determined using Bio-Rad DC protein assay (Bio-Rad, 5000111) as recommended by the manufacturer.…”

“…Recently, crystal structure studies of the Casq2 K180R maps the mutation to the filament-forming interface 67 , and it was proposed that disrupted Casq2 polymer formation may be responsible for Casq2 mutant-associated CPVT. Casq2 binds directly to the luminal domain of ER stress sensor IRE1α at the junctional SR to prevent the activation of IRE1α 55 . Interestingly, of all mutants tested in this www.nature.com/scientificreports/ study, only Casq2 K180R showed altered binding to the luminal domain of IRE1α.…”

Calsequestrins New Calcium Store Markers of Adult Zebrafish Cerebellum and Optic Tectum

“…Such a condensation property is missing in other lumenal proteins, widely distributed throughout the ER lumen. Very recently, a role for cardiac Casq in stress response has been proposed (Wang et al, 2019). By and large, these biophysical and physiological properties define and control Ca 2+ store capacity in the ER of neuronal cells: nonetheless, Casq expression in the brain of fish and of mammals has been poorly investigated.…”