-Atrial fibrillation (AF) is frequently associated with enhanced inflammatory response. The "NACHT, LRR and PYD domain containing protein 3" (NLRP3)-inflammasome mediates caspase-1 activation and interleukin-1β release in immune cells, but is not known to play a role in cardiomyocytes (CMs). Here, we assessed the role of CM NLRP3-inflammasome in AF. -NLRP3-inflammasome activation was assessed by immunoblot in atrial whole-tissue lysates and CMs from patients with paroxysmal (pAF) or long-standing persistent (chronic) AF (cAF). To determine whether CM-specific activation of NLPR3 is sufficient to promote AF, a CM-specific knock-in mouse model expressing constitutively active NLRP3 (CM-KI) was established. In vivo electrophysiology was used to assess atrial arrhythmia vulnerability. To evaluate the mechanism of AF, electrical activation pattern, Ca spark frequency (CaSF), atrial effective refractory period (AERP), and morphology of atria were evaluated in CM-KI mice and WT littermates. -NLRP3-inflammasome activity was increased in atrial CMs of pAF and cAF patients. CM-KI mice developed spontaneous premature atrial contractions and inducible AF, which was attenuated by a specific NLRP3-inflammasome inhibitor, MCC950. CM-KI mice exhibited ectopic activity, abnormal sarcoplasmic-reticulum Ca-release, AERP shortening and atrial hypertrophy. Adeno-associated virus subtype-9 mediated CM-specific knockdown of suppressed AF development in CM-KI mice. Finally, genetic inhibition of prevented AF development in CREM transgenic mice, a well-characterized mouse model of spontaneous AF. -Our study establishes a novel pathophysiological role for CM NLRP3-inflammasome signaling with a mechanistic link to the pathogenesis of AF, and establishes inhibition of NLRP3 as a potential novel AF-therapy approach.

Muscle ring finger (MuRF) proteins have been implicated in transmitting mechanical forces to cell signaling pathways through their interactions with the giant protein titin. Recent evidence has linked mechanicallyinduced stimuli with the control of serum response factor activity and localization through MuRF2. This observation is particularly intriguing in the context of cardiac hypertrophy, where serum response factor transactivation is a key event necessary for the induction of cardiac hypertrophy in response to increased afterload. We have previously reported that MuRF1, which is also a titinassociated protein, exerts antihypertrophic activity in vitro. In the present study, we induced cardiac hypertrophy in mice lacking MuRF1 and MuRF2 to distinguish the physiologic role of these divergent proteins in vivo. We identified for the first time that MuRF1, but not MuRF2, plays a key role in regulating the induction of cardiac hypertrophy, likely by its direct interactions with serum response factor. These studies describe for the first time distinct and nonoverlapping functional characteristics of MuRF1 and MuRF2 in response to cardiac stress in vivo.T he muscle ring finger (MuRF) proteins are striated muscle-specific proteins that have been implicated in various aspects of contractile regulation and myogenic responses. 1 Whereas MuRF3 is primarily microtubuleassociated, both MuRF1 and MuRF2 are associated with the giant sarcomeric protein titin. Titin spans half sarcomeres and is ideally positioned to sense mechanical loading. It has been speculated that MuRF2 participates in a circuit that links mechanical force generation to transcriptional responses in cardiac myocytes by modulating the activity and localization of the transcription factor serum response factor (SRF) in response to changes in mechanical stimuli. 2 In this model, titin undergoes conformational changes in response to stretch, allowing its titin kinase domain to interact with scaffolding proteins (nbr1 and p62), which in turn interact with MuRF2. 2 MuRF2 subsequently associates directly with the transactivation domain of SRF, and is able to inhibit its nuclear localization and transcriptional activity. 2 If the model whereby MuRF2 links titin dynamics with SRF activity is correct, then MuRF2 should play a necessary role in suppressing hypertrophic responses elicited by mechanical forces.MuRF1 also associates with titin, although its regulation by mechanical stress has not been directly tested. Instead, MuRF1 is a well-characterized RING-finger-dependent ubiquitin ligase that is active toward the sarcomeric protein troponin I. 3 In addition, MuRF1 inhibits PKC activity through interactions with RACK1, the receptor for activated protein kinase C protein, which in turn suppresses focal adhesion kinase and ERK1/2 in cardiomyocytes. 4 The inhibitory activity of MuRF1 in the setting of cardiomyocyte hypertrophy has been demonstrated in cultured cells, but cardiac phenotypes of mice deficient in MuRF1 have not been tested. 4 Similarly, the role for MuR...

Abstract-Muscle ring finger (MuRF)1 is a muscle-specific protein implicated in the regulation of cardiac myocyte size and contractility. MuRF2, a closely related family member, redundantly interacts with protein substrates and heterodimerizes with MuRF1. Mice lacking either MuRF1 or MuRF2 are phenotypically normal, whereas mice lacking both proteins develop a spontaneous cardiac and skeletal muscle hypertrophy, indicating cooperative control of muscle mass by MuRF1 and MuRF2. To identify the unique role that MuRF1 plays in regulating cardiac hypertrophy in vivo, we created transgenic mice expressing increased amounts of cardiac MuRF1. Adult MuRF1 transgenic (Tg ϩ ) hearts exhibited a nonprogressive thinning of the left ventricular wall and a concomitant decrease in cardiac function. Experimental induction of cardiac hypertrophy by transaortic constriction (TAC) induced rapid failure of MuRF1 Tg ϩ hearts. Microarray analysis identified that the levels of genes associated with metabolism (and in particular mitochondrial processes) were significantly altered in MuRF1 Tg ϩ hearts, both at baseline and during the development of cardiac hypertrophy. Surprisingly, ATP levels in MuRF1 Tg ϩ mice did not differ from wild-type mice despite the depressed contractility following TAC. In comparing the level and activity of creatine kinase (CK) between wild-type and MuRF1 Tg ϩ hearts, we found that mCK and CK-M/B protein levels were unaffected in MuRF1 Tg ϩ hearts; however, total CK activity was significantly inhibited. We conclude that increased expression of cardiac MuRF1 results in a broad disruption of primary metabolic functions, including alterations in CK activity that leads to increased susceptibility to heart failure following TAC. This study demonstrates for the first time a role for MuRF1 in the regulation of cardiac energetics in vivo. Key Words: muscle ring finger-1 Ⅲ MuRF1 Ⅲ ubiquitin ligase Ⅲ cardiac hypertrophy Ⅲ heart failure Ⅲ creatine kinase T he muscle ring finger (MuRF) proteins are striated muscle-specific proteins that have been implicated in various aspects of contractile regulation and myogenic responses. 1 MuRF1 is a well-characterized RING finger-dependent ubiquitin ligase that targets sarcomere proteins, such as cardiac troponin (cTn)I, during the process of skeletal muscle atrophy. 2,3 MuRF1 has also been implicated in the regulation of cardiac myocyte size and contractility [3][4][5] and inhibits the development of cardiac hypertrophy, 6,7 a dynamic process commonly thought of as a precursor to heart failure. 8 To date, the study of the regulation of cardiac muscle mass by MuRF1 has centered around its involvement in the regulation of sarcomere protein degradation. Although this is certainly an important function, in this report, we propose that MuRF1 operates in a broader capacity that encompasses both protein turnover as well as control of cardiac metabolism.Soon after the discovery of MuRF1, the related proteins MuRF2 and MuRF3 were identified as interacting proteins capable of forming hetero...

Pathological cardiac hypertrophy, induced by various etiologies such as high blood pressure and aortic stenosis, develops in response to increased afterload and represents a common intermediary in the development of heart failure. Understandably then, the reversal of pathological cardiac hypertrophy is associated with a significant reduction in cardiovascular event risk and represents an important, yet underdeveloped, target of therapeutic research. Recently, we determined that muscle ring finger-1 (MuRF1), a muscle-specific protein, inhibits the development of experimentally induced pathological; cardiac hypertrophy. We now demonstrate that therapeutic cardiac atrophy induced in patients after left ventricular assist device placement is associated with an increase in cardiac MuRF1 expression. This prompted us to investigate the role of MuRF1 in two independent mouse models of cardiac atrophy: 1) cardiac hypertrophy regression after reversal of transaortic constriction (TAC) reversal and 2) dexamethasone-induced atrophy. Using echocardiographic, histological, and gene expression analyses, we found that upon TAC release, cardiac mass and cardiomyocyte cross-sectional areas in MuRF1(-/-) mice decreased approximately 70% less than in wild type mice in the 4 wk after release. This was in striking contrast to wild-type mice, who returned to baseline cardiac mass and cardiomyocyte size within 4 days of TAC release. Despite these differences in atrophic remodeling, the transcriptional activation of cardiac hypertrophy measured by beta-myosin heavy chain, smooth muscle actin, and brain natriuretic peptide was attenuated similarly in both MuRF1(-/-) and wild-type hearts after TAC release. In the second model, MuRF1(-/-) mice also displayed resistance to dexamethasone-induced cardiac atrophy, as determined by echocardiographic analysis. This study demonstrates, for the first time, that MuRF1 is essential for cardiac atrophy in vivo, both in the setting of therapeutic regression of cardiac hypertrophy and dexamethasone-induced atrophy.

Low-density lipoprotein receptor-related protein 1 (LRP1) regulates lipid and glucose metabolism in liver and adipose tissue. It is also involved in central nervous system regulation of food intake and leptin signalling. Here we demonstrate that endothelial Lrp1 regulates systemic energy homeostasis. Mice with endothelial-specific Lrp1 deletion display improved glucose sensitivity and lipid profiles combined with increased oxygen consumption during high-fat-diet-induced obesity. We show that the intracellular domain of Lrp1 interacts with the nuclear receptor Pparγ, a central regulator of lipid and glucose metabolism, acting as its transcriptional co-activator in endothelial cells. Therefore, Lrp1 not only acts as an endocytic receptor but also directly participates in gene transcription. Our findings indicate an underappreciated functional role of endothelium in maintaining systemic energy homeostasis.