The ubiquitin proteasome system (UPS) plays a crucial role in biological processes integral to the development of the cardiovascular system and cardiovascular diseases. The UPS prototypically recognizes specific protein substrates and places polyubiquitin chains on them for subsequent destruction by the proteasome. This system is in place to degrade not only misfolded and damaged proteins, but is essential also in regulating a host of cell signaling pathways involved in proliferation, adaptation to stress, regulation of cell size, and cell death. During the development of the cardiovascular system, the UPS regulates cell signaling by modifying transcription factors, receptors, and structural proteins. Later, in the event of cardiovascular diseases as diverse as atherosclerosis, cardiac hypertrophy, and ischemia reperfusion injury, ubiquitin ligases and the proteasome are implicated in protecting and exacerbating clinical outcomes. However, when misfolded and damaged proteins are ubiquitinated by the UPS, their destruction by the proteasome is not always possible due to their aggregated confirmations. Recent studies have discovered how these ubiquitinated misfolded proteins can be destroyed by alternative “specific” mechanisms. The cytosolic receptors p62, NBR, and HDAC6 recognize aggregated ubiquitinated proteins and target them for autophagy in the process of “selective autophagy”. Even the ubiquitination of multiple proteins within whole organelles that drive the more general macro-autophagy may be due, in part, to similar ubiquitin-driven mechanisms. In summary, the cross-talk between the UPS and autophagy highlight the pivotal and diverse roles the UPS plays in maintaining protein quality control and regulating cardiovascular development and disease.

Despite improvements in interventions of acute coronary syndromes, primary reperfusion therapies restoring blood flow to ischemic myocardium leads to the activation of signaling cascades that induce cardiomyocyte cell death. These signaling cascades, including the mitogen-activated protein kinase signaling pathways, activate cardiomyocyte death in response to both ischemia and reperfusion. We have previously identified muscle ring finger-1 (MuRF1) as a cardiac-specific protein that regulates cardiomyocyte mass through its ubiquitin ligase activity, acting to degrade sarcomeric proteins and inhibit transcription factors involved in cardiac hypertrophy signaling. To determine MuRF1's role in cardiac ischemia/reperfusion (I/R) injury, cardiomyocytes in culture and intact hearts were challenged with I/R injury in the presence and absence of MuRF1. We found that MuRF1 is cardioprotective, in part, by its ability to prevent cell death by inhibiting Jun N-terminal kinase (JNK) signaling. MuRF1 specifically targets JNK's proximal downstream target, activated phospho-c-Jun, for degradation by the proteasome, effectively inhibiting downstream signaling and the induction of cell death. MuRF1's inhibitory affects on JNK signaling through its ubiquitin proteasome-dependent degradation of activated c-Jun is the first description of a cardiac ubiquitin ligase inhibiting mitogen-activated protein kinase signaling. MuRF1's cardioprotection in I/R injury is attenuated in the presence of pharmacologic JNK inhibition in vivo, suggesting a prominent role of MuRF1's regulation of c-Jun in the intact heart.

BackgroundCardiac insults such as ischemia, infarction, hypertrophy and dilatation are often accompanied by altered abundance and/or localization of the connexin43 gap junction protein, which may predispose towards arrhythmic complications. Models of chronic dyssynchronous cardiac activation have also been shown to result in redistribution of connexin43 in cardiomyocytes. We hypothesized that alterations in connexin43 expression and localization in the mouse heart might be induced by ventricular pacing over a short period of time.ResultsThe subdiaphragmatic approach was used to pace a series of wild type mice for six hours before the hearts were removed for analysis. Mice were paced at 10–15% above their average anesthetized sinus rate and monitored to ensure 1:1 capture. Short-term pacing resulted in a significant reduction in connexin43 mRNA abundance, a partial redistribution of connexin43 from the sarcolemma to a non-sarcolemmal fraction, and accumulation of ubiquitinated connexin43 without a significant change in overall connexin43 protein levels. These early pacing-induced changes in connexin43 expression were not accompanied by decreased cardiac function, prolonged refractoriness or increased inducibility into sustained arrhythmias.ConclusionOur data suggest that short-term pacing is associated with incipient changes in the expression of the connexin43 gap junction, possibly including decreased production and a slowed rate of degradation. This murine model may facilitate the study of early molecular changes induced by pacing and may ultimately assist in the development of strategies to prevent gap junction remodeling and the associated arrhythmic complications of cardiac disease.

Connexin43 (Cx43), a component of gap junctions, has a relatively large carboxyterminal region with multiple proteomic interactions. Proteomic interactions with its cytoplasmic loop, however, are poorly defined. The goal of this study is to examine proteomic interactions involving the cytoplasmic loop (CL) of Cx43. The authors utilized various techniques, including glutathione-S-transferase (GST) pull-down, immunoblot analysis, two-dimensional (2D) gel electrophoresis, and mass spectrometry, to elucidate binding partners for Cx43-CL. The authors identified novel interactions with Cx43-CL involving a-and b-tubulin, myelin basic protein, and Pura. Because tubulin interacts with the C-terminus of Cx43 (Cx43-CT), the authors further investigated the nature of the interaction between b-tubulin and Cx43-CL. b-Tubulin binds with the full length of Cx43-CL with approximately one-fifth the affinity of the interaction between Cx43-CT and b-tubulin. This study demonstrates novel proteomic interactions involving Cx43-CL that may lead to a more complete understanding of trafficking and gating of gap junction channels.

. Decreased connexin43 expression in the mouse heart potentiates pacing-induced remodeling of repolarizing currents.