Contractile responses of myocytes isolated from patients with cardiomyopathy

Enhanced NCLX-dependent mitochondrial Ca2+ efflux attenuates pathological remodeling in heart failure

Mitochondrial calcium (_mCa²⁺) uptake couples changes in cardiomyocyte energetic demand to mitochondrial ATP production. However, excessive _mCa²⁺ uptake triggers permeability transition and necrosis. Despite these established roles during acute stress, the involvement of _mCa²⁺ signaling in cardiac adaptations to chronic stress remains poorly defined. Changes in NCLX expression are reported in heart failure (HF) patients and models of cardiac hypertrophy. Therefore, we hypothesized that altered _mCa²⁺ homeostasis contributes to the hypertrophic remodeling of the myocardium that occurs upon a sustained increase in cardiac workload. The impact of _mCa²⁺ flux on cardiac function and remodeling was examined by subjecting mice with cardiomyocyte-specific overexpression (OE) of the mitochondrial Na⁺/Ca²⁺ exchanger (NCLX), the primary mediator of _mCa²⁺ efflux, to several well-established models of hypertrophic and non-ischemic HF. Cardiomyocyte NCLX-OE preserved contractile function, prevented hypertrophy and fibrosis, and attenuated maladaptive gene programs in mice subjected to chronic pressure overload. Hypertrophy was attenuated in NCLX-OE mice, prior to any decline in cardiac contractility. NCLX-OE similarly attenuated deleterious cardiac remodeling in mice subjected to chronic neurohormonal stimulation. However, cardiomyocyte NCLX-OE unexpectedly reduced overall survival in mice subjected to severe neurohormonal stress with angiotensin II + phenylephrine. Adenoviral NCLX expression limited _mCa²⁺ accumulation, oxidative metabolism, and de novo protein synthesis during hypertrophic stimulation of cardiomyocytes in vitro. Our findings provide genetic evidence for the contribution of _mCa²⁺ to early pathological remodeling in non-ischemic heart disease, but also highlight a deleterious consequence of increasing _mCa²⁺ efflux when the heart is subjected to extreme, sustained neurohormonal stress.

SLMAP-3 is downregulated in human dilated ventricles and its overexpression promotes cardiomyocyte response to adrenergic stimuli by increasing intracellular calcium

Structural dilation of cardiomyocytes (CMs) imposes a decline in cardiac performance that precipitates cardiac failure and sudden death. Since membrane proteins are implicated in dilated cardiomyopathy and heart failure, we evaluated the expression of the sarcolemmal membrane-associated protein (SLMAP) in dilated cardiomyopathy and its effect on CM contraction. We found that all 3 SLMAP isoforms (SLMAP-1, -2, and -3) are expressed in CMs and are downregulated in human dilated ventricles. Knockdown of SLMAPs in cultured CMs transduced with recombinant adeno-associated viral particles releasing SLMAP-shRNA precipitated reduced spontaneous contractile rate that was not fully recovered in SLMAP-depleted CMs challenged with isoproterenol (ISO), thus phenotypically mimicking heart failure performance. Interestingly, the overexpression of the SLMAP-3 full-length isoform induced a positive chronotropic effect in CMs that was more pronounced in response to ISO insult (vs. ISO-treated naïve CMs). Confocal live imaging showed that H9c2 cardiac myoblasts overexpressing SLMAP-3 exhibit a higher intracellular calcium transient peak when treated with ISO (vs. ISO-treated cells carrying a control adeno-associated viral particle). Proteomics revealed that SLMAP-3 interacts with the regulator of CM contraction, striatin. Collectively, our data demonstrate that SLMAP-3 is a novel regulator of CM contraction rate and their response to adrenergic stimuli. Loss of SLMAPs phenotypically mimics cardiac failure and crystallizes SLMAPs as predictive of dilated cardiomyopathy and heart failure.

Role of titin in cardiomyopathy: from DNA variants to patient stratification

Dilated cardiomyopathy (DCM) affects approximately 1 in 250 individuals and is the leading indication for heart transplantation. DCM is often familial, and the most common genetic predisposition is a truncating variation in the giant sarcomeric protein, titin, which occurs in up to 15% of ambulant patients with DCM and 25% of end-stage or familial cases. In this article, we review the evidence for the role of titin truncation in the pathogenesis of DCM and our understanding of the molecular mechanisms and pathophysiological consequences of variation in the gene encoding titin (TTN). Such variation is common in the general population (up to 1% of individuals), and we consider key features that discriminate variants with disease-causing potential from those that are benign. We summarize strategies for clinical interpretation of genetic variants for use in the diagnosis of patients and the evaluation of their relatives. Finally, we consider the contemporary and potential future role for genetic stratification in cardiomyopathy and in the general population, evaluating titin variation as a predictor of outcome and treatment response for precision medicine.

Aβ Amyloid Pathology Affects the Hearts of Patients With Alzheimer's Disease: Mind the Heart

BACKGROUND

Individually, heart failure (HF) and Alzheimer's disease (AD) are severe threats to population health, and their potential coexistence is an alarming prospect. In addition to sharing analogous epidemiological and genetic profiles, biochemical characteristics, and common triggers, the authors recently recognized common molecular and pathological features between the 2 conditions. Whereas cognitive impairment has been linked to HF through perfusion defects, angiopathy, and inflammation, whether patients with AD present with myocardial dysfunction, and if the 2 conditions bear a common pathogenesis as neglected siblings are unknown.

OBJECTIVES

Here, the authors investigated whether amyloid beta (Aβ) protein aggregates are present in the hearts of patients with a primary diagnosis of AD, affecting myocardial function.

METHODS

The authors examined myocardial function in a retrospective cross-sectional study from a cohort of AD patients and age-matched controls. Imaging and proteomics approaches were used to identify and quantify Aβ deposits in AD heart and brain specimens compared with controls. Cell shortening and calcium transients were measured on isolated adult cardiomyocytes.

RESULTS

Echocardiographic measurements of myocardial function suggest that patients with AD present with an anticipated diastolic dysfunction. As in the brain, Aβ₄₀ and Aβ₄₂ are present in the heart, and their expression is increased in AD.

CONCLUSIONS

Here, the authors provide the first report of the presence of compromised myocardial function and intramyocardial deposits of Aβ in AD patients. The findings depict a novel biological framework in which AD may be viewed either as a systemic disease or as a metastatic disorder leading to heart, and possibly multiorgan failure. AD and HF are both debilitating and life-threatening conditions, affecting enormous patient populations. Our findings underline a previously dismissed problem of a magnitude that will require new diagnostic approaches and treatments for brain and heart disease, and their combination.

Modelling sarcomeric cardiomyopathies in the dish: from human heart samples to iPSC cardiomyocytes

One of the obstacles to a better understanding of the pathogenesis of human cardiomyopathies has been poor availability of heart-tissue samples at early stages of disease development. This has possibly changed by the advent of patient-derived induced pluripotent stem cell (hiPSC) from which cardiomyocytes can be derived in vitro. The main promise of hiPSC technology is that by capturing the effects of thousands of individual gene variants, the phenotype of differentiated derivatives of these cells will provide more information on a particular disease than simple genotyping. This article summarizes what is known about the ‘human cardiomyopathy or heart failure phenotype in vitro’, which constitutes the reference for modelling sarcomeric cardiomyopathies in hiPSC-derived cardiomyocytes. The current techniques for hiPSC generation and cardiac myocyte differentiation are briefly reviewed and the few published reports of hiPSC models of sarcomeric cardiomyopathies described. A discussion of promises and challenges of hiPSC-modelling of sarcomeric cardiomyopathies and individualized approaches is followed by a number of questions that, in the view of the authors, need to be answered before the true potential of this technology can be evaluated.

Cardiac protein abnormalities in dilated cardiomyopathy detected by two-dimensional polyacrylamide gel electrophoresis

The aim of the investigation was to determine whether there are specific global quantitative and qualitative changes in protein expression in heart tissue from patients with dilated cardiomyopathy (DCM) compared with ischaemic heart disease and undiseased tissue. Two-dimensional (2-D) polyacrylamide gel electrophoresis and computer analysis was used to study protein alteration in DCM biopsy material (n=28) compared with donor heart biopsy samples (n=9) and explanted hearts from individuals suffering from ischaemic heart disease (IHD; n = 21). A total of 88 proteins displayed decreased abundance in DCM versus IHD material while five proteins had elevated levels in the DCM group (p<0.01). The most prominent changes occurred in the contractile protein myosin light chain 2 and in a group of proteins identified as desmin. These changes do not appear to be artefactual degradation events occurring during sample processing. These proteins are not apparent in electrophoretic separations of vascular tissue or cultured endothelial cells, mesothelial cells or cardiac fibroblasts, which are clearly distinguishable from the 2-D protein patterns of whole heart and of isolated cardiac myocytes and do not appear to reflect variations in the cellular composition of biopsy samples. The different protein patterns observed in cardiomyopathy showed no obvious relationship with New York Heart Association (NYHA) functional class or haemodynamic parameters. The study has demonstrated significant alterations in quantitative protein expression in the DCM heart which would have serious implications for myocyte function. These changes might be explained by altered protease activity in DCM which could exacerbate contractile dysfunction in the failing heart.

Electrical restitution in diseased human ventricular myocardium

Action potential configuration and electrical restitution were studied in diseased human ventricular muscle by comparing the characteristics of hypertrophic (HYP) and dilated (DIL) human ventricular preparations. Conventional microelectrode techniques were used to evaluate action potentials evoked at increasingly longer diastolic intervals. The steady-state action potential duration (APD90) was significantly longer in DIL than in HYP preparations (393 +/- 5 ms, n = 4 and 296 +/- 11 ms, n = 4, respectively; P < 0.001, mean +/- SEM). In the dilated preparations studied at long diastolic intervals, the initial period of rapid repolarization (phase 1) was absent, and the rate of final repolarization (phase 3) was reduced. Electrical restitution relations in these preparations were fitted as the sum of two exponentials. The time constant of the fast component was significantly longer in DIL than in HYP preparations (242 +/- 9 ms and 121 +/- 4 ms, respectively; P < 0.001). No difference was observed in the time constants for the slow component of restitution in the two groups. Electrical restitution was also studied in single human ventricular myocytes by using patch clamp techniques. The initial 600 ms period of restitution was fitted in these cells to a monoexponential function. The time constant for this period of the restitution relation was significantly longer, while the estimated amplitude of this early rising phase was significantly lower in human cells obtained from DIL hearts than the respective parameters obtained in the healthy canine and guinea pig cells also examined. The observed changes in the restitution kinetics of the dilated human heart are, likely, the consequence of alterations in the ionic currents that underlie the cardiac action potential.

Reduced contraction and altered frequency response of isolated ventricular myocytes from patients with heart failure

BACKGROUND

Previous work has failed to demonstrate reduced maximal contraction of isolated ventricular myocytes from failing human hearts compared with nonfailing control hearts. The effect of alterations in stimulation frequency and temperature on the contraction of isolated ventricular myocytes has been investigated. Left ventricular myocytes were isolated from the hearts of patients with severe heart failure undergoing heart transplantation and compared with myocytes isolated from myocardial biopsies from patients with coronary disease but preserved left ventricular systolic function or from myocytes from rejected donor hearts.

METHODS AND RESULTS

Myocytes were exposed to either a maximally activating level of extracellular calcium at 37 degrees C or to 2 mmol/L calcium at 32 degrees C. There was no significant difference in the contraction amplitude between myocytes from failing and nonfailing hearts at 0.2 Hz. With increasing stimulation frequency, there was a reduction in contraction amplitude in cells from failing hearts relative to control hearts in both maximal calcium from 0.33 Hz (4.5% versus 6.6%) to 1.4 Hz (3.9% versus 8.8%) (ANCOVA, P < .001) and at 2 mmol/L calcium from 0.50 Hz (2.3% versus 3.5%) to 1.4 Hz (1.8% versus 3.9%) (ANCOVA, P < .001). The time to peak contraction and the times to 50% and 90% relaxation were prolonged in myocytes from failing hearts at stimulation rate of 0.2 Hz (P < .01), but only the time to 50% relaxation was prolonged at 1.0 Hz (P < .05).

CONCLUSIONS

Reduced contraction, slowed relaxation, and impaired frequency response occurring at the level of the individual ventricular myocyte can be demonstrated in human heart failure. This demonstrates that disruption of myocyte function can contribute to both the systolic and the diastolic abnormalities that occur in the failing human heart.