UC San Diego

Optimal conditions are required for proper protein folding in the ER. Cellular stresses can compromise these conditions, leading to an increase in misfolded proteins within the ER lumen, a condition known as ER stress. One such stress experienced by cardiac myocytes is myocardial infarction (MI), in which nutrients and oxygen are deprived to a particular region of the heart due to a coronary blockage. Upon ER stress, a protective signaling pathway known as the unfolded protein response (UPR) is triggered by three major ER membrane-bound stress sensors, PERK, IRE1, and ATF6. Pre-activating ATF6 prior to stress has been shown to decrease cell death, and increase cardiac performance following the stress; however, the consequence of ATF6 activation on overall gene expression, and the specific mechanisms of ATF6-mediated protection, have not been investigated. This work focused on identifying ATF6-activated genes in the hearts of transgenic mice. In addition, this work identified two novel mechanisms of ATF6-mediated protection. The first involves the activation of RCAN1, a protein whose role in calcineurin-NFAT hypertrophic signaling is well known, but whose role in ER stress signaling had not been studied. RCAN1 was shown to be ER stress- and ATF6-inducible. This represents the first known study identifying a potential interface between ER stress and hypertrophic signaling. In addition, this work identified Derlin-3 (Derl3), a component of ER-associated degradation (ERAD), as another novel target of ATF6, as it displayed among the most dynamic up-regulation by ATF6 ever seen in the heart. Derl3 was also up-regulated in the heart following MI and in cultured myocytes following simulated ischemia (sI). Derl3 overexpression increased clearance of known misfolded proteins and attenuated apoptotic signaling and death in response to sI, while knock-down exacerbated death. These results indicated that enhancing components of ERAD can protect against cardiac stress, solidifying the critical importance of ERAD and protein quality control in the heart. Finally, this work was the first to investigate the role of ATF6 in microRNA gene regulation in the heart, uncovering potential novel connections between microRNAs and ER stress signaling