Abstract P134: Synoviolin Is a Stress-Inducible Endoplasmic/Sarcoplasmic Reticulum E3 Ubiquitin Ligase that Preserves Cardiac Function

Abstract

We recently reported that synoviolin1 (syvn1) is a novel endoplasmic reticulum stress response (ERSR) protein that is up-regulated in the mouse heart by ATF6, a cardioprotective, nodal transcription factor of the ERSR. Syvn1 is a unique E3 ubiquitin ligase that retrotranslocates misfolded proteins from endoplasmic/sarcoplasmic reticulum (ER/SR) to the cytosol and subsequently polyubiquitinates them, targeting them for degradation. We now report that syvn1 expression is induced with tunicamycin, thapsigargin, and dithiothreitol, ER stressors that activate ATF6. Moreover, adenovirus-mediated syvn1 overexpression in neonatal rat ventricular cardiac myocytes (NRVCMs) increases contractility. Consistent with this finding, syvn1 overexpression increases calcium transient amplitude as well as diastolic calcium. We also find that syvn1 overexpression decreases secretion of MANF, a protective, anti-hypertrophic, ER/SR protein which we find is conditionally secreted when ER/SR calcium is depleted. We also report MANF as the first example of a is protein whose secretion from ventricular myocytes is conditionally dependent on ER/SR calcium. Furthermore, syvn1 reduces the growth of NRVCMs treated with the α-adrenergic agonist, phenylephrine. Moreover, while knockdown of syvn1 in NRVCMs using syvn1-targeted siRNA increases cell death, overexpression of syvn1 promotes cell survival. To investigate the roles of syvn1, in vivo , we examined the effects of syvn1 overexpression in cardiac myocytes in a mouse model of trans-aortic banding. Syvn1 overexpression, in vivo, was achieved by intravenous delivery of recombinant AAV serotype 9 (AAV9; cardiac specific serotype) with MLC2v promoter driving syvn1 expression. Baseline cardiac function, as measured by echocardiography six weeks after gene delivery, shows no difference between AAV9-control and AAV9-syvn1 treated mice. Trans-aortic banding decreases cardiac function in mice injected with AAV9-control. In contrast, cardiac function is preserved in mice injected with AAV9-syvn1. The findings in this study suggest that syvn1 is a novel stress-inducible cardiac E3 ligase with unique functions in regulating protein secretion, maintaining cell viability, and preserving cardiac function.