Compliant Titin Isoform Content Is Reduced in Left Ventricles of Sedentary Versus Active Rats

RNA binding proteins as mediators of pathological cardiac remodeling

RNA binding proteins (RBPs) play a central in the post-transcriptional regulation of gene expression, which can account for up to 50% of all variations in protein expression within a cell. Following their binding to target RNAs, RBPs most typically confer changes in gene expression through modulation of alternative spicing, RNA stabilization/degradation, or ribosome loading/translation rate. All of these post-transcriptional regulatory processes have been shown to play a functional role in pathological cardiac remodeling, and a growing body of evidence is beginning to identify the mechanistic contribution of individual RBPs and their cardiac RNA targets. This review highlights the mechanisms of RBP-dependent post-transcriptional gene regulation in cardiomyocytes and fibroblasts and our current understanding of how RNA binding proteins functionally contribute to pathological cardiac remodeling.

A new insight on copper: Promotion of collagen synthesis and myofiber growth and development in juvenile grass carp (Ctenopharyngodon idella)

Copper (Cu) is a trace element, essential for fish growth. In the current study, in addition to growth performance, we first explored the effects of Cu on collagen synthesis and myofiber growth and development in juvenile grass carp (Ctenopharyngodon idella). A total of 1080 fish (11.16 ± 0.01 g) were randomly divided into 6 treatments (3 replicates per treatment) to receive five doses of organic Cu, which were Cu citrate (CuCit) at 0.99 (basal diet), 2.19, 4.06, 6.15, and 8.07 mg/kg, and one dose of inorganic Cu (CuSO4·5H2O at 3.15 mg/kg), for 9 weeks. The results showed appropriate Cu level (4.06 mg/kg) enhanced growth performance, improved nutritional Cu status, and downregulated Cu-transporting ATPase 1 mRNA levels in the hepatopancreas, intestine, and muscle of juvenile grass carp. Meanwhile, collagen content in fish muscle was increased after Cu intake, which was probably due to the following pathways: (1) activating CTGF/TGF-β1/Smads signaling pathway to regulate collagen transcription; (2) upregulating of La ribonucleoprotein domain family 6 (LARP6) mRNA levels to regulate translation initiation; (3) increasing proline hydroxylase, lysine hydroxylase, and lysine oxidase activities to regulate posttranslational modifications. In addition, optimal Cu group increased myofiber diameters and the frequency of myofibers with diameter >50 μm, which might be associated with upregulation of cyclin B, cyclin D, cyclin E, proliferating cell nuclear antigen, myogenic determining factor (MyoD), myogenic factor 5, myogenin (MyoG), myogenic regulatory factor 4 and myosin heavy chain (MyHC) and downregulation of myostatin mRNA levels, increasing protein levels of MyoD, MyoG and MyHC in fish muscle. Finally, based on percentage weight gain (PWG), serum ceruloplasmin (Cp) activity and collagen content in fish muscle, Cu requirements were determined as 4.74, 4.37 and 4.62 mg/kg diet (CuCit as Cu source) of juvenile grass carp, respectively. Based on PWG and Cp activity, compared to CuSO4·5H2O, the efficacy of CuCit were 131.80% and 115.38%, respectively. Our findings provide new insights into Cu supplementation to promote muscle growth in fish, and help improve the overall productivity of aquaculture.

Increased Expression of N2BA Titin Corresponds to More Compliant Myofibrils in Athlete's Heart

Long-term exercise induces physiological cardiac adaptation, a condition referred to as athlete’s heart. Exercise tolerance is known to be associated with decreased cardiac passive stiffness. Passive stiffness of the heart muscle is determined by the giant elastic protein titin. The adult cardiac muscle contains two titin isoforms: the more compliant N2BA and the stiffer N2B. Titin-based passive stiffness may be controlled by altering the expression of the different isoforms or via post-translational modifications such as phosphorylation. Currently, there is very limited knowledge about titin’s role in cardiac adaptation during long-term exercise. Our aim was to determine the N2BA/N2B ratio and post-translational phosphorylation of titin in the left ventricle and to correlate the changes with the structure and transverse stiffness of cardiac sarcomeres in a rat model of an athlete’s heart. The athlete’s heart was induced by a 12-week-long swim-based training. In the exercised myocardium the N2BA/N2B ratio was significantly increased, Ser11878 of the PEVK domain was hypophosphorlyated, and the sarcomeric transverse elastic modulus was reduced. Thus, the reduced passive stiffness in the athlete’s heart is likely caused by a shift towards the expression of the longer cardiac titin isoform and a phosphorylation-induced softening of the PEVK domain which is manifested in a mechanical rearrangement locally, within the cardiac sarcomere.

Mechanical and Structural Remodeling of Cardiac Muscle after Aerobic and Resistance Exercise Training in Rats

INTRODUCTION

Aerobic and resistance exercise training results in distinct structural changes of the heart. The mechanics of how cardiac cells adapt to resistance training and the benefits to cells when combining aerobic and resistance exercise remains largely unknown. The purpose of this study was to compare mechanical adaptations of skinned cardiac fiber bundles after chronic resistance, aerobic and combined exercise training in rats. We hypothesized that differences in mechanical function on the fiber bundle level coincide with differences previously reported in the structure of the heart.

METHOD

Twelve-week-old rats were assigned to (i) an aerobic running group (n = 6), (ii) a ladder climbing resistance group (n = 6), (iii) a combination group subjected to aerobic and resistance training (n = 6), or (iv) a sedentary (control) group (n = 5). Echocardiography was used to measure cardiac structural remodeling. Skinned cardiac fiber bundles were used to determine active and passive force properties, maximal shortening velocity, and calcium sensitivity.

RESULTS

Aerobically trained animals had 43%-49% greater ventricular volume and myocardial thickness, and a 4%-17% greater shortening velocity and calcium sensitivity compared with control group rats. Resistance-trained rats had 37%-71% thicker ventricular walls, a 56% greater isometric force production, a 9% greater shortening velocity, and a 4% greater calcium sensitivity compared with control group rats. The combination exercise-trained rats had 25%-43% greater ventricular volume and myocardial wall thickness, a 55% greater active force production, a 7% greater shortening velocity, and a 60% greater cross-bridge cooperativity compared with control group rats.

CONCLUSIONS

The heart adapts differently to each exercise modality, and a combination of aerobic and resistance training may have the greatest benefit for cardiac health and performance.