Michael Ibrahim

Prolonged mechanical unloading affects cardiomyocyte excitation-contraction coupling, transverse-tubule structure, and the cell surface

Prolonged mechanical unloading (UN) of the heart is associated with detrimental changes to the structure and function of cardiomyocytes. The mechanisms underlying these changes are unknown. In this study, we report the influence of UN on excitation‐contraction coupling, Ca2+‐induced Ca2+ release (CICR) in particular, and transverse (t)‐tubule structure. UN was induced in male Lewis rat hearts by heterotopic abdominal heart transplantation. Left ventricular cardiomyocytes were isolated from the transplanted hearts after 4 wk and studied using whole‐cell patch clamping, confocal microscopy, and scanning ion conductance microscopy (SICM). Recipient hearts were used as control (C). UN reduced the volume of cardiomyocytes by 56.5% compared with C (UN, n=90; C, n=59; P<0.001). The variance of time‐to‐peak of the Ca2+ transients was significantly increased in unloaded cardiomyocytes (UN 227.4±24.9 ms2, n=42 vs. C 157.8±18.0 ms2, n=40; P<0.05). UN did not alter the action potential morphology or whole‐cell L‐type Ca2+ current compared with C, but caused a significantly higher Ca2+ spark frequency (UN 3.718±0.85 events/ 100 µm/s, n=47 vs. C 0.908±0.186 events/100 µm/s, n=45; P<0.05). Confocal studies showed irregular distribution of the t tubules (power of the normal t‐tubule frequency: UN 8.13±1.12×105, n=57 vs. C 20.60±3.174 × 105, n=56; P< 0.001) and SICM studies revealed a profound disruption to the openings of the t tubules and the cell surface in unloaded cardiomyocytes. We show that UN leads to a functional uncoupling of the CICR process and identify disruption of the t‐tubule‐sarcoplasmic reticulum interaction as a possible mechanism.—Ibrahim, M., Al Masri, A., Navaratnarajah, M., Siedlecka, U., Soppa, G. K., Moshkov, A., Abou AlSaud, S., Gorelik, J., Yacoub, M. H., Terracciano, C. M. N. Prolonged mechanical unloading affects cardiomyocyte excitation‐contraction coupling, transverse‐tubule structure, and the cell surface. FASEB J. 24, 3321–3329 (2010). www.fasebj.org

Mechanical unloading reverses transverse tubule remodelling and normalizes local Ca2+-induced Ca2+release in a rodent model of heart failure

Ca2+‐induced Ca2+ release (CICR) is critical for contraction in cardiomyocytes. The transverse (t)‐tubule system guarantees the proximity of the triggers for Ca2+ release [L‐type Ca2+ channel, dihydropyridine receptors (DHPRs)] and the sarcoplasmic reticulum Ca2+ release channels [ryanodine receptors (RyRs)]. Transverse tubule disruption occurs early in heart failure (HF). Clinical studies of left ventricular assist devices in HF indicate that mechanical unloading induces reverse remodelling. We hypothesize that unloading of failing hearts normalizes t‐tubule structure and improves CICR.

A critical role for Telethonin in regulating t-tubule structure and function in the mammalian heart

The transverse (t)-tubule system plays an essential role in healthy and diseased heart muscle, particularly in Ca(2+)-induced Ca(2+) release (CICR), and its structural disruption is an early event in heart failure. Both mechanical overload and unloading alter t-tubule structure, but the mechanisms mediating the normally tight regulation of the t-tubules in response to load variation are poorly understood. Telethonin (Tcap) is a stretch-sensitive Z-disc protein that binds to proteins in the t-tubule membrane. To assess its role in regulating t-tubule structure and function, we used Tcap knockout (KO) mice and investigated cardiomyocyte t-tubule and cell structure and CICR over time and following mechanical overload. In cardiomyocytes from 3-month-old KO (3mKO), there were isolated t-tubule defects and Ca(2+) transient dysynchrony without whole heart and cellular dysfunction. Ca(2+) spark frequency more than doubled in 3mKO. At 8 months of age (8mKO), cardiomyocytes showed progressive loss of t-tubules and remodelling of the cell surface, with prolonged and dysynchronous Ca(2+) transients. Ca(2+) spark frequency was elevated and the L-type Ca(2+) channel was depressed at 8 months only. After mechanical overload obtained by aortic banding constriction, the Ca(2+) transient was prolonged in both wild type and KO. Mechanical overload increased the Ca(2+) spark frequency in KO alone, where there was also significantly more t-tubule loss, with a greater deterioration in t-tubule regularity. In conjunction, Tcap KO showed severe loss of cell surface ultrastructure. These data suggest that Tcap is a critical, load-sensitive regulator of t-tubule structure and function.

Reversibility of T-tubule remodelling in heart failure: mechanical load as a dynamic regulator of the T-tubules

The T-tubule system in ventricular cardiomyocytes is essential for synchronous Ca(2+) handling, and, therefore, efficient contraction. T-tubular remodelling is a common feature of heart disease. In this review, we discuss whether t-tubular remodelling can be reversed and which factors may be implicated in this process. In particular, we focus on the interaction between mechanical load variation and T-tubule structure and function. What is the evidence of this relationship? What is the role of different degrees and durations of mechanical load variation? In what settings might mechanical load variation have detrimental or beneficial effects on T-tubule structure and function? What are the molecular determinants of this interaction? Ultimately this discussion is used to address the question of whether mechanical load variation can provide an understanding to underpin attempts to induce recovery of the T-tubule system. In reviewing these questions, we define what remains to be discovered in understanding T-tubule recovery.

Modern management of systolic anterior motion of the mitral valve

Systolic anterior motion (SAM) of the mitral valve (MV) can be a life-threatening condition. The SAM can result in severe left ventricular outflow tract obstruction and/or mitral regurgitation and is associated with an up to 20% risk of sudden death (which is substantially lower in hypertrophic cardiomyopathy (HCM)). The mechanisms of SAM are complex and depend on the functional status of the ventricle. The SAM can occur in the normal population, but is typically observed in patients with HCM or following MV repair. Echocardiography (2D, 3D and stress) has a central diagnostic role as the application of echocardiographic SAM predictors allows the incorporation of prevention techniques during surgery and post-operative SAM assessment. Cardiac magnetic resonance imaging has a special role in understanding the dynamic nature of SAM, especially in anatomically atypical hearts (including HCM). This article describes what the clinician needs to know about SAM ranging from pathophysiological mechanisms and imaging modalities to conservative (medical) and surgical approaches and their respective outcomes. A stepwise approach is advocated consisting of medical therapy, followed by aggressive volume loading and beta-adrenoceptor blockade. Surgery is the final option. The correct choice of surgical technique requires an understanding of the anatomical substrate of SAM.

Artificial chordae for degenerative mitral valve disease: critical analysis of current techniques

The surgical repair of degenerative mitral valve disease involves a number of technical points of importance. The use of artificial chordae for the repair of degenerative disease has increased as a part of the move from mitral valve replacement to repair of the mitral valve. The use of artificial chordae provides an alternative to the techniques pioneered by Carpentier (including the quadrangular resection, transfer of native chordae and papillary muscle shortening/plasty), which can be more technically difficult. Despite a growth in their uptake and the indications for their use, a number of challenges remain for the use of artificial chordae in mitral valve repair, particularly in the determination of the correct length to ensure optimal leaflet coaptation. Here, we analyse over 40 techniques described for artificial chordae mitral valve repair in the setting of degenerative disease.

Phosphoregulation of the Titin-cap Protein Telethonin in Cardiac Myocytes

Background: Telethonin mutations are associated with cardiomyopathy through unknown mechanisms. Results: Telethonin is a substrate for CaMK family kinases and exists in a bis-phosphorylated state in cardiomyocytes, in which non-phosphorylated telethonin disrupts transverse tubule organization and intracellular calcium transients. Conclusion: Telethonin phosphorylation is critical for the maintenance of normal cardiomyocyte morphology and calcium handling. Significance: Disruption of phospho-telethonin functions may contribute to pathogenesis in cardiomyopathy. Telethonin (also known as titin-cap or t-cap) is a muscle-specific protein whose mutation is associated with cardiac and skeletal myopathies through unknown mechanisms. Our previous work identified cardiac telethonin as an interaction partner for the protein kinase D catalytic domain. In this study, kinase assays used in conjunction with MS and site-directed mutagenesis confirmed telethonin as a substrate for protein kinase D and Ca2+/calmodulin-dependent kinase II in vitro and identified Ser-157 and Ser-161 as the phosphorylation sites. Phosphate affinity electrophoresis and MS revealed endogenous telethonin to exist in a constitutively bis-phosphorylated form in isolated adult rat ventricular myocytes and in mouse and rat ventricular myocardium. Following heterologous expression in myocytes by adenoviral gene transfer, wild-type telethonin became bis-phosphorylated, whereas S157A/S161A telethonin remained non-phosphorylated. Nevertheless, both proteins localized predominantly to the sarcomeric Z-disc, where they partially replaced endogenous telethonin. Such partial replacement with S157A/S161A telethonin disrupted transverse tubule organization and prolonged the time to peak of the intracellular Ca2+ transient and increased its variance. These data reveal, for the first time, that cardiac telethonin is constitutively bis-phosphorylated and suggest that such phosphorylation is critical for normal telethonin function, which may include maintenance of transverse tubule organization and intracellular Ca2+ transients.

Cardiomyocyte Ca2+ handling and structure is regulated by degree and duration of mechanical load variation

Cardiac transverse (t)‐tubules are altered during disease and may be regulated by stretch‐sensitive molecules. The relationship between variations in the degree and duration of load and t‐tubule structure remains unknown, as well as its implications for local Ca2+‐induced Ca2+ release (CICR). Rat hearts were studied after 4 or 8 weeks of moderate mechanical unloading [using heterotopic abdominal heart–lung transplantation (HAHLT)] and 6 or 10 weeks of pressure overloading using thoracic aortic constriction. CICR, cell and t‐tubule structure were assessed using confocal‐microscopy, patch‐clamping and scanning ion conductance microscopy. Moderate unloading was compared with severe unloading [using heart‐only transplantation (HAHT)]. Mechanical unloading reduced cardiomyocyte volume in a time‐dependent manner. Ca2+ release synchronicity was reduced at 8 weeks moderate unloading only. Ca2+ sparks increased in frequency and duration at 8 weeks of moderate unloading, which also induced t‐tubule disorganization. Overloading increased cardiomyocyte volume and disrupted t‐tubule morphology at 10 weeks but not 6 weeks. Moderate mechanical unloading for 4 weeks had milder effects compared with severe mechanical unloading (37% reduction in cell volume at 4 weeks compared to 56% reduction after severe mechanical unloading) and did not cause depression and delay of the Ca2+ transient, increased Ca2+ spark frequency or impaired t‐tubule and cell surface structure. These data suggest that variations in chronic mechanical load influence local CICR and t‐tubule structure in a time‐ and degree‐dependent manner, and that physiological states of increased and reduced cell size, without pathological changes are possible.

What factors influence British medical students' career intentions?

Abstract Aim: The aim of this study was to identify factors that influence career choice in UK medical students. Methods: Students at seven institutions were invited to rate how important various factors were on influencing their career choices and how interested they were in pursuing different specialties. The influence of interpersonal relationship networks on career choice was also evaluated. Results: 641 responses were collected. 44% (283) were male, 16% (105) were graduates and 41% (263) were final-year students. For Dermatology (p = 0.009), Paediatrics (p = 0.000), Radiology (p = 0.000), Emergency Medicine (p = 0.018) and Cardiothoracic Surgery (p = 0.000), there was a clear correlation between completing a clinical attachment and an interest in pursuing the specialty. Perceived characteristics of the speciality, individually and in clusters were considered important by specific subgroups of students, such as those interested in surgery. These students considered prestige (p = 0.0003), role models (p = 0.014), financial rewards after training (p = 0.0196) and technical challenge (p = 0.0011) as important factors. Demographics such as sex and age played a significant role in career choice. Interpersonal relationship networks do not have a significant influence on career intentions. Conclusions: This study shows that the career intentions of British medical students are influenced by their undergraduate experience and by the weight they place on different specialty-related factors.

Off-Pump Versus On-Pump Coronary Revascularization: Meta-Analysis of Mid- and Long-Term Outcomes

BACKGROUND Early outcomes for off-pump coronary artery bypass grafting (OPCAB) have been extensively compared with on-pump coronary revascularization (ONCAB); however, the long-term effects of OPCAB continue to be debated. This study aims to compare the mid-term (>1year; ≤5 years) and long-term (>5 years) survival and major adverse cardiovascular and cerebrovascular events of OPCAB versus ONCAB. METHODS A systematic search identified 32 studies meeting our inclusion criteria. These were analyzed using random effects modeling, with subgroup evaluation according to study type. Primary outcomes were mid- and long-term survival over a follow-up period greater than 1 year. Secondary outcomes were mid- and long-term events including repeat revascularization, myocardial infarction, angina, heart failure, and cerebrovascular accidents. RESULTS Off-pump coronary artery bypass grafting confers similar overall mid-term survival when compared with ONCAB (hazard ratio, 1.06; 95% confidence interval, 0.95 to 1.19; p=0.31). On-pump coronary artery bypass grafting was associated with a significant trend towards a long-term survival advantage (hazard ratio, 1.06; 95% confidence interval, 1.00 to 1.13; p=0.05); however, this was no longer present when subgroup analysis of only randomized controlled trials, registry-based studies, and propensity-matched studies was performed. There was an increase in angina recurrence among two studies after OPCAB, but no difference was seen in 11 other studies reporting data as odds ratio. No significant differences were observed in other secondary outcomes. CONCLUSIONS This analysis demonstrates comparable mid-term mortality and mid- to long-term morbidity between OPCAB and ONCAB. On-pump coronary artery bypass grafting may be associated with improved long-term survival when all study types are analyzed; however, analysis of only randomized controlled trials and propensity-matched studies demonstrates comparable long-term mortality between OPCAB and ONCAB.