Isomyosins, microtubules and desmin during the onset of cardiac hypertrophy in the rat

High molecular mass proteomics analyses of left ventricle from rats subjected to differential swimming training

BACKGROUND

Regular exercises are commonly described as an important factor in health improvement, being directly related to contractile force development in cardiac cells.In order to evaluate the links between swimming exercise intensity and cardiac adaptation by using high molecular mass proteomics, isogenic Wistar rats were divided into four groups: one control (CG) and three training groups (TG's), with low, moderate and high intensity of exercises.In order to evaluate the links between swimming exercise intensity and cardiac adaptation by using high molecular mass proteomics, isogenic Wistar rats were divided into four groups: one control (CG) and three training groups (TG's), with low, moderate and high intensity of exercises.

RESULTS

Findings here reported demonstrated clear morphologic alterations, significant cellular injury and increased energy supplies at high exercise intensities. α-MyHC, as well proteins associated with mitochondrial oxidative metabolism were shown to be improved. α-MyHC expression increase 1.2 fold in high intensity training group when compared with control group. α-MyHC was also evaluated by real-time PCR showing a clear expression correlation with protein synthesis data increase in 8.48 fold in high intensity training group. Other myofibrillar protein, troponin , appear only in high intensity group, corroborating the cellular injury data. High molecular masses proteins such as MRS2 and NADH dehydrogenase, involved in metabolic pathways also demonstrate increase expression, respectily 1.5 and 1.3 fold, in response to high intensity exercise.

CONCLUSIONS

High intensity exercise demonstrated an increase expression in some high molecular masses myofibrilar proteins, α-MyHC and troponin. Furthermore this intensity also lead a significant increase of other high molecular masses proteins such as MRS2 and NADH dehydrogenase in comparison to low and moderate intensities. However, high intensity exercise also represented a significant degree of cellular injury, when compared with the individuals submitted to low and moderate intensities.

Redox proteomics identification of oxidatively modified myocardial proteins in human heart failure: implications for protein function

Increased oxidative stress in a failing heart may contribute to the pathogenesis of heart failure (HF). The aim of this study was to identify the oxidised proteins in the myocardium of HF patients and analyse the consequences of oxidation on protein function. The carbonylated proteins in left ventricular tissue from failing (n = 14) and non-failing human hearts (n = 13) were measured by immunoassay and identified by proteomics. HL-1 cardiomyocytes were incubated in the presence of stimuli relevant for HF in order to assess the generation of reactive oxygen species (ROS), the induction of protein carbonylation, and its consequences on protein function. The levels of carbonylated proteins were significantly higher in the HF patients than in the controls (p<0.01). We identified two proteins that mainly underwent carbonylation: M-type creatine kinase (M-CK), whose activity is impaired, and, to a lesser extent, α-cardiac actin. Exposure of cardiomyocytes to angiotensin II and norepinephrine led to ROS generation and M-CK carbonylation with loss of its enzymatic activity. Our findings indicate that protein carbonylation is increased in the myocardium during HF and that these oxidative changes may help to explain the decreased CK activity and consequent defects in energy metabolism observed in HF.

β-myosin heavy chain is induced by pressure overload in a minor subpopulation of smaller mouse cardiac myocytes

RATIONALE

Induction of the fetal hypertrophic marker gene β-myosin heavy chain (β-MyHC) is a signature feature of pressure overload hypertrophy in rodents. β-MyHC is assumed present in all or most enlarged myocytes.

OBJECTIVE

To quantify the number and size of myocytes expressing endogenous β-MyHC by a flow cytometry approach.

METHODS AND RESULTS

Myocytes were isolated from the left ventricle of male C57BL/6J mice after transverse aortic constriction (TAC), and the fraction of cells expressing endogenous β-MyHC was quantified by flow cytometry on 10,000 to 20,000 myocytes with use of a validated β-MyHC antibody. Side scatter by flow cytometry in the same cells was validated as an index of myocyte size. β-MyHC-positive myocytes constituted 3 ± 1% of myocytes in control hearts (n=12), increasing to 25 ± 10% at 3 days to 6 weeks after TAC (n=24, P<0.01). β-MyHC-positive myocytes did not enlarge with TAC and were smaller at all times than myocytes without β-MyHC (≈70% as large, P<0.001). β-MyHC-positive myocytes arose by addition of β-MyHC to α-MyHC and had more total MyHC after TAC than did the hypertrophied myocytes that had α-MyHC only. Myocytes positive for β-MyHC were found in discrete regions of the left ventricle in 3 patterns: perivascular, in areas with fibrosis, and in apparently normal myocardium.

CONCLUSIONS

β-MyHC protein is induced by pressure overload in a minor subpopulation of smaller cardiac myocytes. The hypertrophied myocytes after TAC have α-MyHC only. These data challenge the current paradigm of the fetal hypertrophic gene program and identify a new subpopulation of smaller working ventricular myocytes with more myosin.

The sarcomere and sarcomerogenesis

Striated muscle owes its name to the microscopic appearance, caused by the longitudinal alignment of thousands of highly ordered contractile units, the sarcomeres. The assembly (and disassembly) of these multiprotein complexes (sarcomere assembly or sarcomerogenesis) follows ordered pathways, which are regulated on the transcriptional, translational and posttranslational level. Furthermore, myofibril assembly involves the participation of transient scaffolds and adaptors, notably the microtubule network. Studies in cell culture and developing embryos have revealed common pathways of sarcomere assembly in heart and skeletal muscle. Disruptions in these pathways are implicated in muscle diseases.

Microtubules Modulate the Stiffness of Cardiomyocytes Against Shear Stress

Although microtubules are involved in various pathological conditions of the heart including hypertrophy and congestive heart failure, the mechanical role of microtubules in cardiomyocytes under such conditions is not well understood. In the present study, we measured multiple aspects of the mechanical properties of single cardiomyocytes, including tensile stiffness, transverse (indentation) stiffness, and shear stiffness in both transverse and longitudinal planes using carbon fiber–based systems and compared these parameters under control, microtubule depolymerized (colchicine treated), and microtubule hyperpolymerized (paclitaxel treated) conditions. From all of these measurements, we found that only the stiffness against shear in the longitudinal plane was modulated by the microtubule cytoskeleton. A simulation model of the myocyte in which microtubules serve as compression-resistant elements successfully reproduced the experimental results. In the complex strain field that living myocytes experience in the body, observed changes in shear stiffness may have a significant influence on the diastolic property of the diseased heart.

Transient association of titin and myosin with microtubules in nascent myofibrils directed by the MURF2 RING-finger protein

Assembly of muscle sarcomeres is a complex dynamic process and involves a large number of proteins. A growing number of these have regulatory functions and are transiently present in the myofibril. We show here that the novel tubulin-associated RING/B-box protein MURF2 associates transiently with microtubules, myosin and titin during sarcomere assembly. During sarcomere assembly, MURF2 first associates with microtubules at the exclusion of tyrosinated tubulin. Then, MURF2-labelled microtubules associate transiently with sarcomeric myosin and later with A-band titin when non-striated myofibrils differentiate into mature sarcomeres. Finally, MURF2 labelled microtubules disappear from the sarcomere after the incorporation of myosin filaments and the elongation of titin. This suggests that the incorporation of myosin into nascent sarcomeres and the elongation of titin require an active, microtubule-dependent transport process and that MURF2-associated microtubules play a role in the alignment and extension of nascent sarcomeres. MURF2 is expressed in at least four isoforms, of which a 27 kDa isoform is cardiac specific. A C-terminal isoform is generated by alternative reading frame use, a novelty in muscle proteins. In mature cardiac sarcomeres, endogenous MURF2 can associate with the M-band, and is translocated to the nucleus. MURF2 can therefore act as a transient adaptor between microtubules, titin and nascent myosin filaments, as well as being involved in signalling from the sarcomere to the nucleus.

Microtubule involvement in the adaptation to altered mechanical load in developing chick myocardium

Mechanical load regulates ventricular growth, function, and structure from the earliest stages of cardiac morphogenesis through senescence. Dramatic changes in cardiac form and function have been defined for developing cardiovascular systems, and changes in mechanical loading conditions can produce structural malformations such as left heart hypoplasia. To date, relatively little is known regarding the interactions between changes in mechanical load, morphogenesis, and the material properties of the embryonic heart. We tested the hypothesis that passive material properties in the embryonic heart change in response to altered mechanical load and that microtubules play an important role in this adaptive response. We measured biaxial passive stress-strain relations in left ventricular (LV) myocardial strips in chick embryos at Hamburger-Hamilton stage 27 following left atrial ligation (LAL) at stage 21 to reduce LV volume load and create left heart hypoplasia. Following LAL, myocardial stresses at given strains and circumferential stiffness increased versus control strips. Western blot analysis of LAL embryos showed an increase in both total and polymerized beta-tubulin and confocal microscopy confirmed an increase in microtubule density in the LV compact layer versus control. Following colchicine treatment, LV stresses and stiffness normalized in LAL specimens and microtubule density following colchicine was similar in LAL to control. In contrast, Taxol treatment increased myocardial stresses and stiffness in control strips to levels beyond LAL specimens. Thus, the material properties of the developing myocardium are regulated by mechanical load and microtubules play a role in this adaptive response during cardiac morphogenesis.

Nonsynchronous accumulation of alpha-skeletal actin and beta-myosin heavy chain mRNAs during early stages of pressure-overload--induced cardiac hypertrophy demonstrated by in situ hybridization

The development of cardiac hypertrophy secondary to pressure overload is accompanied by isoformic changes of contractile proteins such as myosin and actin. 35S-Labeled complementary RNA (cRNA) probes and in situ hybridization procedures were used for analysis of the regional distribution of newly formed transcripts from alpha-skeletal actin (alpha-sk-actin) and beta-myosin heavy chain (beta-MHC) genes during the early stages of pressure overload. The study was performed in 25-day-old rats submitted to a thoracic aortic stenosis and killed after surgery at times ranging from 4 hours to 3 days. Neither alpha-sk-actin nor beta-MHC messenger RNA (mRNA) was detected in the hearts of normal and sham-operated animals. However, alpha-sk-actin mRNA accumulated throughout the entire left ventricle as early as 4 hours after aortic stenosis, and by 12 hours was also detected in the left atrium. In contrast, beta-MHC mRNA was hardly detectable before day 1, and by days 2-3 was mainly restricted to the inner part of the left ventricle and around the coronary arteries. The absence of spatial and temporal coordination in the accumulation of alpha-sk-actin and beta-MHC mRNAs indicates that different signals and/or regulatory mechanisms are implicated in the induction of the two genes in response to hemodynamic overload.

Microtubules and desmin filaments during onset of heart hypertrophy in rat: a double immunoelectron microscope study

The distribution of tubulin and desmin, the constituent proteins of microtubules and intermediate filaments, respectively, were studied in normal and hypertrophied rat myocardium by high-resolution immunofluorescence and immunoelectron microscopy. Cardiac hypertrophy was induced in 25-day-old rats by aortic stenosis. In the normal heart, double immunolabelling of ultrathin frozen sections of papillary muscle using gold-labelled probes for tubulin and desmin showed that microtubules ran primarily in a longitudinal direction through the intermyofibrillar spaces, perpendicularly to the desmin filaments. Microtubules were present near nuclei, mitochondria, and plasma membranes, while desmin filaments formed transverse connections between adjacent Z disks. No tubulin was observed near the intercalated disks, which were rich in desmin filaments. In hypertrophied hearts, myocytes exhibited the typical morphological features of developing hypertrophy. While there was little difference in the distribution of the microtubules around mitochondria and at the plasma membrane, considerable increases were seen near the nuclei and along the myofibrils. Desmin labelling was distributed transversely as in the controls; however, sometimes it was longitudinally oriented either in the intermyofibrillar space linking 2 Z disks out of register or along digitations of the intercalated disks connecting neighboring desmosomes. The unique rearrangement of desmin and tubulin filaments in hypertrophied cardiac myocytes emphasizes their distinct role in myocyte organization. We suggest that, during the development of cardiac hypertrophy, desmin filaments are mainly involved in maintaining the myofibrils in register, whereas the degree of assembly of microtubules is correlated with the rate of protein synthesis and with myofibrillogenesis.