ATF6 as a Regulator of Atrial Myocyte ER Proteostasis - The endoplasmic reticulum (ER) is the synthesis site of many membrane and secreted proteins, amounting to as much as 40% of all protein synthesis. Robust heart function depends on ER proteins, such as ion channels and hormones, emphasizing the importance of ER protein homeostasis, i.e. ER proteostasis in the heart. ER proteostasis comprises the balance of ER protein synthesis, folding, trafficking and ER associated degradation (ERAD) of damaged and misfolded proteins. Little is known about ER proteostasis in atrial myocytes, where we posit it to be essential for production of important peptide hormones in the classical ER secretory pathway. ER proteostasis is maintained mainly by the ER unfolded protein response (UPR). We previously showed that the aspect of the UPR mediated by the ER-localized transcription factor, ATF6, which detects and is activated by increased demand for ER protein folding, is required for adaptive growth of ventricular myocytes during exercise and pressure overload. In contrast to ventricular myocytes, atrial myocytes are endocrine cells that use the classical ER secretory pathway, comprised of the ER, Golgi and secretory granules, to synthesize, process, store and, upon the appropriate stimulus, secrete hormones, such as the blood pressure (BP) lowering peptide, atrial natriuretic peptide (ANP). Consistent with the potential importance of ATF6 in atrial myocytes, we found that atria express considerably more ATF6 than ventricles, perhaps to maintain robust hormone production. This concept is supported by our finding that atrial myocyte ATF6 was upregulated by conditions that stimulate ANP synthesis and secretion and that ATF6 deletion decreased ANP most likely due to the loss of ATF6-fortified ER secretory pathway vitality. These preliminary results support the hypothesis that ATF6 is required for expression of “service” proteins that reside in the ER secretory pathway, such as chaperones, disulfide isomerases, ERAD components, that maintain a robust ER secretory pathway proteostasis needed for efficient production of “client” proteins that are made in, and routed through the ER secretory pathway. Our specific aims will examine the effects of atrial myocyte specific ATF6 loss- and gain-of-function in mice on 1- atrial myocyte ANP expression, folding, secretion and ANP-based cardiovascular physiology, in vivo; 2- the atrial ER proteome and ER secretome; and 3- renewal of the atrial proteome. We believe these studies are significant because they will reveal new information about the nature of ATF6 and the UPR in atrial myocyte ER secretory pathway proteostasis in a mouse model that mimics the dietary sodium intake that contributes to hypertension and cardiovascular disease in humans. We believe that the innovation of the studies lies in the novelty of the techniques as applied to the problem, including atrial myocyte specific gene targeting, a first-of-its-kine, in vivo ERAD assay, and complementary proteomics approaches. Taken together these novel technologies place us in a strong position to examine what we believe to be critical functions for ATF6 in the atrial myocyte ER secretory pathway which may impact a wide range of atrial functions.
