Generation of new intercellular junctions between cardiocytes. A possible mechanism compensating for mechanical overload in the hypertrophied human adult myocardium

The Primary Alteration of Ventricular Myocardium Conduction: The Significant Determinant of Left Bundle Branch Block Pattern

Intraventricular conduction disturbances (IVCD) are currently generally accepted as ECG diagnostic categories. They are characterized by defined QRS complex patterns that reflect the abnormalities in the intraventricular sequence of activation that can be caused by pathology in the His-Purkinje conduction system (HP) or ventricular myocardium. However, the current understanding of the IVCD's underlying mechanism is mostly attributed to HP structural or functional alterations. The involvement of the working ventricular myocardium is only marginally mentioned or not considered. This opinion paper is focused on the alterations of the ventricular working myocardium leading to the most frequent IVCD pattern-the left bundle branch block pattern (LBBB). Recognizing the underlying mechanisms of the LBBB patterns and the involvement of the ventricular working myocardium is of utmost clinical importance, considering a patient's prognosis and indication for cardiac resynchronization therapy.

Missing Link between Molecular Aspects of Ventricular Arrhythmias and QRS Complex Morphology in Left Ventricular Hypertrophy

The aim of this opinion paper is to point out the knowledge gap between evidence on the molecular level and clinical diagnostic possibilities in left ventricular hypertrophy (LVH) regarding the prediction of ventricular arrhythmias and monitoring the effect of therapy. LVH is defined as an increase in left ventricular size and is associated with increased occurrence of ventricular arrhythmia. Hypertrophic rebuilding of myocardium comprises interrelated processes on molecular, subcellular, cellular, tissue, and organ levels affecting electrogenesis, creating a substrate for triggering and maintaining arrhythmias. The knowledge of these processes serves as a basis for developing targeted therapy to prevent and treat arrhythmias. In the clinical practice, the method for recording electrical phenomena of the heart is electrocardiography. The recognized clinical electrocardiogram (ECG) predictors of ventricular arrhythmias are related to alterations in electrical impulse propagation, such as QRS complex duration, QT interval, early repolarization, late potentials, and fragmented QRS, and they are not specific for LVH. However, the simulation studies have shown that the QRS complex patterns documented in patients with LVH are also conditioned remarkably by the alterations in impulse propagation. These QRS complex patterns in LVH could be potentially recognized for predicting ventricular arrhythmia and for monitoring the effect of therapy.

The extracellular matrix proteoglycan lumican improves survival and counteracts cardiac dilatation and failure in mice subjected to pressure overload

Left ventricular (LV) dilatation is a key step in transition to heart failure (HF) in response to pressure overload. Cardiac extracellular matrix (ECM) contains fibrillar collagens and proteoglycans, important for maintaining tissue integrity. Alterations in collagen production and cross-linking are associated with cardiac LV dilatation and HF. Lumican (LUM) is a collagen binding proteoglycan with increased expression in hearts of patients and mice with HF, however, its role in cardiac function remains poorly understood. To examine the role of LUM in pressure overload induced cardiac remodeling, we subjected LUM knock-out (LUMKO) mice to aortic banding (AB) and treated cultured cardiac fibroblasts (CFB) with LUM. LUMKO mice exhibited increased mortality 1-14 days post-AB. Echocardiography revealed increased LV dilatation, altered hypertrophic remodeling and exacerbated contractile dysfunction in surviving LUMKO 1-10w post-AB. LUMKO hearts showed reduced collagen expression and cross-linking post-AB. Transcriptional profiling of LUMKO hearts by RNA sequencing revealed 714 differentially expressed transcripts, with enrichment of cardiotoxicity, ECM and inflammatory pathways. CFB treated with LUM showed increased mRNAs for markers of myofibroblast differentiation, proliferation and expression of ECM molecules important for fibrosis, including collagens and collagen cross-linking enzyme lysyl oxidase. In conclusion, we report the novel finding that lack of LUM attenuates collagen cross-linking in the pressure-overloaded heart, leading to increased mortality, dilatation and contractile dysfunction in mice.

Electrical and structural remodeling in left ventricular hypertrophy-a substrate for a decrease in QRS voltage?

Electrical remodeling in advanced stages of cardiovascular diseases creates a substrate for triggering and maintenance of arrhythmias. The electrical remodeling is a continuous process initiated already in the early stages of cardiological pathology. The aim of this opinion article was to discuss the changes in electrical properties of myocardium in left ventricular hypertrophy (LVH), with special focus on its early stage, as well as their possible reflection in the QRS amplitude of the electrocardiogram. It critically appraises the classical hypothesis related to the QRS voltage changes in LVH. The hypothesis of the relative voltage deficit is discussed in the context of supporting evidence from clinical studies, animal experiments, and simulation studies. The underlying determinants of electrical impulse propagation which may explain discrepancies between "normal" ECG findings and increased left ventricular size/mass in LVH are reviewed.

Left ventricular mass by 12-lead electrocardiogram in healthy subjects: comparison to cardiac magnetic resonance imaging

The ability to estimate left ventricular mass (LVM) from the standard 12-lead electrocardiogram (ECG) has been shown to be limited because there is a considerable variability of the normal 12-lead ECG due to demographic and anthropometric variables. We sought to study LVM in healthy subjects and its relationship with QRS duration, and established electrocardiographic criteria for left ventricular hypertrophy. Cardiac magnetic resonance imaging was used to measure LVM. Seventy-one healthy volunteers (36 men; age range, 21-82 years) were studied. All ECG criteria tested showed a statistically significant relationship with LVM. The highest R value was found between LVM and QRS duration, as well as the 12-lead voltage-duration product (R = 0.59, P < .001 for both). The lowest R value was found for the Sokolow-Lyon voltage criterion (R = 0.25, P = .033). Left ventricular mass differed significantly between sexes, as did all ECG criteria except the Sokolow-Lyon criterion. Thus, in healthy subjects, QRS duration alone is equally or more strongly correlated to LVM than are established electrocardiographic left ventricular hypertrophy criteria.

Induction of oxidative stress and disintegrin metalloproteinase in human heart end-stage failure

Collagen degradation is required for the creation of new integrin binding sites necessary for cell survival. However, a complete separation between the matrix and the cell leads to apoptosis, dilatation, and failure. Previous studies have demonstrated increased metalloproteinase activity in the failing myocardium. To test the hypothesis that disintegrin metalloproteinase (DMP) is induced in human heart end-stage failure, left ventricle tissue from ischemic cardiomyopathic (ICM, n = 10) and dilated cardiomyopathic (DCM, n = 10) human hearts were obtained at the time of orthotopic cardiac transplant. Normal (n = 5) tissue specimens were obtained from unused hearts. The levels of reduced oxygen species (ROS) were 12 +/- 2, 25 +/- 3, and 16 +/- 2 nmol (means +/- SE, P < 0.005) in normal, ICM, and DCM, respectively, by spectrofluorometry. The percent levels of endothelial cells were 100 +/- 15, 35 +/- 19, and 55 +/- 11 in normal, ICM, and DCM, respectively, by CD31 labeling. The levels of nitrotyrosine by Western analysis were significantly increased, and endothelial nitric oxide (NO) by the Griess method was decreased in ICM and DCM compared with normal tissue. The synthesis and degradation of beta(1)-integrin and connexin 43 were significantly increased in ICM and DCM compared with normal hearts by Western analysis. Levels of DMP were increased, and levels of cardiac inhibitor of metalloproteinase (CIMP) were decreased. Aggrecanase activity of DMP was significantly increased in ICM and DCM hearts compared with normal. These results suggest that the occurrence of cardiomyopathy is significantly confounded by the increase in ROS, nitrotyrosine, and DMP activity. This increase is associated with decreased NO, endothelial cell density, and CIMP. In vitro, treatment of CIMP abrogated the DMP activity. The treatment with CIMP may prevent degradation of integrin and connexin and ameliorate heart failure.

Dynamics of early contact formation in cultured adult rat cardiomyocytes studied by N-cadherin fused to green fluorescent protein

We investigated dynamic events during the formation of intercalated disc-like structures of adult rat cardiomyocytes (ARC) in long-term culture. Given the complexity of ARC cytoIarchitecture after de- and re-differentiation, and the non-uniform morphological development of individual cells, green fluorescent protein (GFP) technology was used to track N-cadherin in living cells. Sorting and functionality of the GFP fusion protein was tested in ARC. Isolated ARC were micro-injected with the expression construct at the onset of spreading in culture, and the fluorescence signals were tracked during contact formation and in fully redifferentiated living cells. The first contact sites were found to be established by cellular protrusions, which were marked by an ultrastructure similar to microspikes and probably have a role as exploratory units in the spreading phase. Subsequently, initial contact sites served as anchorage for the most prominent stress fibre-like structures. The fusion protein appeared before connexin-43 at newly established cell-cell contacts. Membrane invaginations at the sarcolemma facing the substratum of cultured ARC may be responsible for the appearance of a striped pattern of N-cadherin and other adherens junction proteins away from intercalated disc-like structures. The stripes were immobile in redifferentiated cells, while the distinct small fluorescent particles in the cell body were found to move directionally at speeds around 10 micro m/min. These results contribute to the understanding of the mechanisms of cell-cell contact formation of adult cardiomyocytes, which is a prerequisite for any future implantation technology.

Backscattered electron imaging: A new method for the study of cardiomyocyte architecture using scanning electron microscopy

Scanning electron microscopy (SEM) with secondary electron emissions is useful for the study of cardiomyocyte architecture, however, the information is limited from the cell surface. Whereas backscattered electron (BSE) emission can give a high-resolution image of the specimen's intracellular structure after heavy metal staining. In this study, we applied BSE imaging analysis to the study of the arrangement of cardiomyocytes in the myocardium. The tissue specimens from a normal fresh monkey heart, normal human heart obtained at autopsy, and surgically resected tissue from a patient with old myocardial infarction in the left ventricular aneurysmectomy were used. The tissue specimens were fixed in neutral formalin, treated with NaOH and then stained with Gomori's silver methenamine reagent followed by tannic acid and osmium tetroxide. After dehydration and drying, the specimens were coated with carbon and examined by SEM with a BSE detector. In the tissue preparations, the A bands of sarcomeres were selectively stained with silver so that the arrangements of subsarcolemmal myofibrils and the intercalated discs were clearly seen in the BSE images. In the left ventricular aneurysmal walls of old myocardial infarction, atrophied cardiomyocytes with disarray of subsarcolemmal myofibrils were observed. The results strongly suggest that BSE images are further applicable to the study of the architecture of cardiac myocytes and their branches, and the arrangement of intracellular myofibrils in various diseased myocardium.

On the nature of cell death during remodeling of hypertrophied human myocardium

Cardiocyte loss during myocardial hypertrophy leads to progressive dysfunction in human hearts with chronic hemodynamic overload. The mechanism for such cell elimination is unknown. We examined lysosomal participation in cardiocytic degradation present in human cardiac biopsies, utilizing electron microscopic cytochemistry (acid phosphatase). Lysosomes were significantly increased in number (t-test, P<0.001) in 50 hemodynamically overloaded hearts (375+/-69, mean+/-s.e.m., per 5,000 microm(2) cardiocytic area; eight controls, 38+/-11). Secondary lysosomes were prominent near degenerative intracellular organelles in both hypertrophic and atrophic cardiocytes. Increased lysosomal and phagocytic activity in the cytoplasm without typical nuclear apoptosis resembled cytoplasmic degradation in developmental programmed cell death described in different tissues. We also demonstrated cardiocytic DNA degradation (in situ nick-end labeling) in autopsy hearts, including 299 nuclei normalized per 10(6) observed nuclei from five concentrically hypertrophied hearts, 1961 nuclei from five eccentrically hypertrophied hearts, and no positive nuclei in five controls. We postulate a chronic self-controlled cytoplasmic proteolysis in cardiocytes, not initially associated with either nuclear degradation or intercellular dehiscence but later possibly accompanied by apoptotic nuclear elimination, and leading to apoptotic cell death.

Remodeling of cardiomyocytes and their branches in juvenile, adult, and senescent spontaneously hypertensive rats and Wistar Kyoto rats: comparative morphometric analyses by scanning electron microscopy

Scanning electron microscopy was used to compare the shape, size, and connection of left ventricular (LV) myocytes between spontaneously hypertensive rats (SHR) and Wistar Kyoto rats (WKY) at 3, 8, 15, 35, and 63 weeks of age. For either strain at each age, five rats were studied, in which LV myocytes consisted of a cylindrical trunk with series (SB) and/ or lateral branch(es) (LB) and step formations; cell junctions had 12 common basic patterns. The length (L), width (W), and L/W ratio of the myocytes, and various indices for SB, LB, and three selected types of cell junctions were measured in 100 cells from each heart and averaged for comparison studies. In the growing period (3-8 weeks of age), the LV myocytes were similar in shape and width in the two age-matched strains and grew similarly with the same L/W ratio. In adolescent (15-week-old) WKY, LV cells grew with the same L/W ratio as in the younger rats, whereas in adolescent SHR, the cells showed a much greater increase in width than in length (disproportionate hypertrophy), the LB proliferated significantly, and the numbers of step-to-step and side-to-side junctions were diminished. In adult (15-35-week-old) WKY, LV cells continued to grow without much change in SB, LB, and the cell junctions, whereas in adult SHR, LV hypertrophy progressed with enhanced cardiomyocyte hypertrophy, increased number of SB, LB, and step-to-end junctions, and reduction in the number of step-to-step and side-to-side junctions per cell. In aged (63-week-old) WKY and SHR, the indices of LV myocytes, SB, LB, and cell junctions did not differ from those in adult WKY and SHR, except for LB thinning in the WKY and significant LB loss in the SHR. Age-related reductions in side-to-side- and step-to-step junctions, and LB loss with myocardial fibrosis in adult and aged SHR may indicate increased loss of gap junctions which couple the cells for transverse conduction, and contribute to anisotropic discontinuous propagation and potential reentrant LV arrhythmias.