Hrd1 and ER-Associated Protein Degradation, ERAD, Are Critical Elements of the Adaptive ER Stress Response in Cardiac Myocytes

Abstract

Rationale Hrd1 is an endoplasmic reticulum (ER)-transmembrane E3 ubiquitin ligase that has been studied in yeast, where it contributes to ER protein quality control by ER-associated degradation (ERAD) of misfolded proteins that accumulate during ER stress. Neither Hrd1 nor ERAD have been studied in the heart, or in cardiac myocytes, where protein quality control is critical for proper heart function. Objective The objectives of this study were to elucidate roles for Hrd1 in ER stress, ERAD, and viability in cultured cardiac myocytes and in the mouse heart, in vivo. Methods and Results The effects of siRNA-mediated Hrd1 knockdown were examined in cultured neonatal rat ventricular myocytes. The effects of adeno-associated virus (AAV)-mediated Hrd1 knockdown and overexpression were examined in the hearts of mice subjected to pressure-overload induced pathological cardiac hypertrophy, which challenges protein-folding capacity. In cardiac myocytes, the ER stressors, thapsigargin (TG) and tunicamycin (TM) increased ERAD, as well as adaptive ER stress proteins, and minimally affected cell death. However, when Hrd1 was knocked down, TG and TM dramatically decreased ERAD, while increasing maladaptive ER stress proteins and cell death. In vivo, Hrd1 knockdown exacerbated cardiac dysfunction, and increased apoptosis and cardiac hypertrophy, while Hrd1 overexpression preserved cardiac function, and decreased apoptosis and attenuated cardiac hypertrophy in the hearts of mice subjected to pressure-overload. Conclusions Hrd1 and ERAD are essential components of the adaptive ER stress response in cardiac myocytes. Hrd1 contributes to preserving heart structure and function in a mouse model of pathological cardiac hypertrophy.

Proteostasis in cardiac health and disease

“…Increasing proteo toxic stress in experimental animal models by inhibition or knock out of important enzymes involved in PQCoften in mice with transverse aortic binding -implicate the cardiac activation of various signalling pathways associated with progression or attenu ation of heart dis ease. Signalling includes detrimental pathways, such as the calcineurin-NFAT pathway 150 , phosphorylation of eukaryotic translation initiation factor (eIF) 2α 151 , and induction of apoptosis [151][152][153] , but also favourable path ways, such as protection against oxidative stress through upregu lation of catalase 154 , or transcriptional activation of the hexo samine biosynthetic pathway supplying the sub strate for protein glycosylation 150 . Nevertheless, despite disclosure of these pathways, the dominant pathways linking cardio myocyte proteotoxic stress to subsequent cell death and cardiac remodelling in various cardiac conditions are still not well defined.…”

Unfolded protein response in brain ischemia: A timely update

“…75 Derl3 is upregulated in the myocardial infarct border zone and protects cardiomyocytes from ischemic stress-induced damage, while Hrd1 decreases cardiac pathology associated with pressure overload. 22,76 Thus, cell-specific genetic modulation of UPR branches provides important information about the role of individual UPR pathways in the disease under investigation, and helps to identify novel targets for therapeutic intervention.…”

Selenoprotein S-dependent Selenoprotein K Binding to p97(VCP) Protein Is Essential for Endoplasmic Reticulum-associated Degradation

“…Therefore, ERAD can directly regulate physiological processes that occur in the ER, and, furthermore, ERAD is essential for relieving ER stress by degrading misfolded proteins. Abnormal ERAD is involved in the pathogenesis of diseases that are related to ER stress-mediated apoptosis, such as neurodegenerative diseases, cardiac hypertrophy, and diabetes (5)(6)(7). p97(VCP) is a highly abundant protein with many roles in diverse biological processes that include ERAD, mitochondrialassociated degradation, and DNA repair (8 -10).…”

“…The interaction of p97(VCP) with its binding partners and co-factors plays a key role in its activity and subcellular location (10). When p97(VCP) is relocalized to the ER membrane via its interaction with its binding partners, such as SelS, gp78, and Hrd1 (7,(12)(13)(14), it has an essential function in the maintenance of ER homeostasis and the regulation of ER stress through ERAD (15).…”