Canopy 2 attenuates the transition from compensatory hypertrophy to dilated heart failure in hypertrophic cardiomyopathy.

AIMS: A mismatch between adequate angiogenesis and overgrowth of myocytes may be a critical mechanism controlling the transition from adaptive hypertrophy to heart failure. Canopy 2 (CNPY2) was recently identified as a secreted, HIF-1α-regulated angiogenic growth factor. As angiogenic factors play important roles in the development of myocardial hypertrophy, we investigated the role of CNPY2 in molecular and functional changes during development of chronic heart failure using cardiac-specific transgenic (TG) mice that overexpress human CNPY2. METHODS AND RESULTS: We generated TG mice that constitutively express CNPY2 in the myocardium. Cardiomyopathy was induced in TG and wild-type (WT) mice by transverse aortic constriction (TAC). WT mice developed significant ventricular hypertrophy at 4 weeks and severe dilatation and heart failure at 12 weeks after TAC. However, TG mice preserved much better cardiac structure and function, with less severe ventricular dilatation and markedly reduced cardiac apoptosis and fibrosis following TAC. Excess CNPY2 in TG mice prevented significant loss of vasculature up to 12 weeks after TAC injury, resulting in a better local myocardial environment that facilitated myocyte survival and prevented excessive matrix remodelling compared with WT mice. TG mice had less accumulation of endogenous tumor suppressor p53 after TAC, indicating intrinsic activation of the p53-mediated repression of HIF-1α, and Cnpy2 was diminished in TG mice compared with WT controls. CONCLUSION: Our study showed a correlation between downregulation of endogenous mouse Cnpy2 and p53-mediated HIF-1α inhibition during late-stage hypertrophic development. Additional CNPY2 attenuated the transition from compensatory hypertrophic response to maladaptive ventricular dilatation and heart failure.

Expression of CNPY2 in mouse tissues: quantification and localization.

Canopy FGF signaling regulator 2 (CNPY2) is a FGF21-modulated protein containing a saposin B-type domain. In vitro studies have shown CNPY2 is able to enhance neurite outgrowth in neurons and stabilize the expression of low density lipoprotein receptor in macrophages and hepatocytes. However, no in vivo data are available on the normal expression of CNPY2 and information is lacking on which cell types express this protein in tissues. To address this, the present study examined CNPY2 expression at the mRNA and protein levels. Quantitative PCR and ELISA examination of mouse tissues showed that CNPY2 varies between organs, with the highest expression in the heart, lung and liver. Immunohistochemistry detected CNPY2 in a variety of cell types including skeletal, cardiac and smooth muscle myocytes, endothelial cells and epithelial cells. CNPY2 was also detectable in mouse blood and human and mouse uteri. These data demonstrate CNPY2 is widely distributed in tissues and suggest the protein has biological functions that have yet to be identified. Using these new observations we discuss possible functions of the protein.

Multiple signaling pathways regulate contractile activity-mediated PGC-1α gene expression and activity in skeletal muscle cells.

PGC-1α is an important transcriptional coactivator that plays a key role in mediating mitochondrial biogenesis. Within seconds of the onset of contractile activity, a number of rapid cellular events occur that form part of the initial signaling processes involved in PGC-1α gene regulation, such as elevations in cytoplasmic calcium, AMPK and p38 activation, and elevated ROS production. We observed that basal levels of PGC-1α promoter activity were more sensitive to resting Ca(2+) levels, compared to ROS, p38 or, AMPK signaling. Moreover, enhanced PGC-1α transcription and post-translational activity on DNA were a result of the activation of multiple signal transduction pathways during contractile activity of myotubes. AMPK, ROS, and Ca(2+) appear to be necessary for the regulation of contractile activity-induced PGC-1α gene expression, governed partly through p38 MAPK and CaMKII activity. Whether these signaling pathways are arranged as a linear sequence of events, or as largely independent pathways during contractile activity, remains to be determined.

Expression of mitochondrial fission and fusion regulatory proteins in skeletal muscle during chronic use and disuse.

INTRODUCTION: The mitochondrial network within cells is mediated by fission and fusion processes. METHODS: We investigated the expression of the proteins responsible for these events during conditions of altered oxidative capacity. RESULTS: With chronic contractile activity, the mitochondrial reticulum increased in size, along with concomitant increases in the fusion proteins Opa1 and Mfn2 (by 36% and 53%; P < 0.05). When we induced muscle disuse through denervation for 7 days, fragmented mitochondria were observed, along with significant decreases in the expression of Mfn2 and Opa1 (by 84% and 70%). To assess the effects of aging on mitochondrial morphology, young (5 month) and aged (35 month) Fisher 344 Brown Norway rats were used. Aged animals also possessed smaller mitochondria and displayed increased levels of fission proteins. CONCLUSIONS: Chronic muscle use increases the ratio of fusion:fission proteins, leading to reticular mitochondria, whereas muscle disuse and aging result in a decrease in this ratio, culminating in fragmented organelles.

Sirtuin 1-mediated effects of exercise and resveratrol on mitochondrial biogenesis.

The purpose of this study was to evaluate the role of sirtuin 1 (SirT1) in exercise- and resveratrol (RSV)-induced skeletal muscle mitochondrial biogenesis. Using muscle-specific SirT1-deficient (KO) mice and a cell culture model of differentiated myotubes, we compared the treatment of resveratrol, an activator of SirT1, with that of exercise in inducing mitochondrial biogenesis. These experiments demonstrated that SirT1 plays a modest role in maintaining basal mitochondrial content and a larger role in preserving mitochondrial function. Furthermore, voluntary exercise and RSV treatment induced mitochondrial biogenesis in a SirT1-independent manner. However, when RSV and exercise were combined, a SirT1-dependent synergistic effect was evident, leading to enhanced translocation of PGC-1α and SirT1 to the nucleus and stimulation of mitochondrial biogenesis. Thus, the magnitude of the effect of RSV on muscle mitochondrial biogenesis is reliant on SirT1, as well as the cellular environment, such as that produced by repeated bouts of exercise.

The effects of chronic muscle use and disuse on cardiolipin metabolism.

Cardiolipin (CL) is a phospholipid that maintains the integrity of mitochondrial membranes. We previously demonstrated that CL content increases with chronic muscle use, and decreases with denervation-induced disuse. To investigate the underlying mechanisms, we measured the mRNA expression of 1) CL synthesis enzymes cardiolipin synthase (CLS) and CTP:PA-cytidylyltransferase-1 (CDS-1); 2) remodeling enzymes tafazzin and acyl-CoA:lysocardiolipin acyltransferase-1 (ALCAT1); and 3) outer membrane CL enzymes, mitochondrial phospholipase D and phospholipid scramblase 3 (Plscr3), during chronic contractile activity (CCA)-induced mitochondrial biogenesis and denervation. With CCA, CDS-1 expression increased by 128%, parelleling CL levels. Surprisingly, denervation also led to large increases in CDS-1 and CLS, despite a decrease in mitochondria, possibly due to a compensatory mechanism to restore lost CL. ALCAT1 decreased by 32% with CCA, but increased by 290% following denervation, indicating that both CCA and denervation alter CL remodeling. CCA and denervation also elicited 60-90% increases in Plscr3, likely to facilitate CL movement to the outer membrane. The expression of these genes was not affected by aging, but changes due to CCA and denervation were attenuated compared with young animals. The absence of PPARγ coactivator-1α in knockout animals led to a decrease in CDS-1 and an increase in ALCAT1 mRNA levels, implicating PGC-1α in regulating both CL synthesis and remodeling. These data suggest that chronic muscle use and disuse modify the expression of mRNAs encoding CL metabolism enzymes. Our data also illustrate, for the first time, that PPARγ coactivator-1α regulates the CL metabolism pathway in muscle.

Effect of denervation-induced muscle disuse on mitochondrial protein import.

This study determined whether muscle disuse affects mitochondrial protein import and whether changes in protein import are related to mitochondrial content and function. Protein import was measured using a model of unilateral peroneal nerve denervation in rats for 3 (n = 10), 7 (n = 12), or 14 (n = 14) days. We compared the import of preproteins into the matrix of subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria isolated from the denervated and the contralateral control tibialis anterior muscles. Denervation led to 50% and 29% reductions in protein import after 14 days of disuse in SS and IMF mitochondria, respectively. This was accompanied by significant decreases in mitochondrial state 3 respiration, muscle mass, and whole muscle cytochrome c oxidase activity. To investigate the mechanisms involved, we assessed disuse-related changes in 1) protein import machinery components and 2) mitochondrial function, reflected by respiration and reactive oxygen species (ROS) production. Denervation significantly reduced the expression of translocases localized in the inner membrane (Tim23), outer membrane (Tom20), and mitochondrial heat shock protein 70 (mtHsp70), especially in the SS subfraction. Denervation also resulted in elevated ROS generation, and exogenous ROS was found to markedly reduce protein import. Thus our data indicate that protein import kinetics are closely related to alterations in mitochondrial respiratory capacity (r = 0.95) and are negatively impacted by ROS. Deleterious changes in the protein import system likely facilitate the reduction in mitochondrial content and the increase in organelle dysfunction (i.e., increased ROS production and decreased respiration) during chronic disuse, which likely contribute to the activation of degradative pathways leading to muscle atrophy.