Transmembrane electrical characteristics of cultured human skeletal muscle cells

Spatial profiles of electrical mismatch determine vulnerability to conduction failure across a host-donor cell interface

BACKGROUND

Electrophysiological mismatch between host cardiomyocytes and donor cells can directly affect the electrical safety of cardiac cell therapies; however, the ability to study host-donor interactions at the microscopic scale in situ is severely limited. We systematically explored how action potential (AP) differences between cardiomyocytes and other excitable cells modulate vulnerability to conduction failure in vitro.

METHODS AND RESULTS

AP propagation was optically mapped at 75 μm resolution in micropatterned strands (n=152) in which host neonatal rat ventricular myocytes (AP duration=153.2±2.3 ms, conduction velocity=22.3±0.3 cm/s) seamlessly interfaced with genetically engineered excitable donor cells expressing inward rectifier potassium (Kir2.1) and cardiac sodium (Na(v)1.5) channels with either weak (conduction velocity=3.1±0.1 cm/s) or strong (conduction velocity=22.1±0.4 cm/s) electrical coupling. Selective prolongation of engineered donor cell AP duration (31.9-139.1 ms) by low-dose BaCl2 generated a wide range of host-donor repolarization time (RT) profiles with maximum gradients (∇RT(max)) of 5.5 to 257 ms/mm. During programmed stimulation of donor cells, the vulnerable time window for conduction block across the host-donor interface most strongly correlated with ∇RT(max). Compared with well-coupled donor cells, the interface composed of poorly coupled cells significantly shortened the RT profile width by 19.7% and increased ∇RT(max) and vulnerable time window by 22.2% and 19%, respectively. Flattening the RT profile by perfusion of 50 μmol/L BaCl2 eliminated coupling-induced differences in vulnerability to block.

CONCLUSIONS

Our results quantify how the degree of electrical mismatch across a cardiomyocyte-donor cell interface affects vulnerability to conduction block, with important implications for the design of safe cardiac cell and gene therapies.

Transcription factors MYOCD, SRF, Mesp1 and SMARCD3 enhance the cardio-inducing effect of GATA4, TBX5, and MEF2C during direct cellular reprogramming

Transient overexpression of defined combinations of master regulator genes can effectively induce cellular reprogramming: the acquisition of an alternative predicted phenotype from a differentiated cell lineage. This can be of particular importance in cardiac regenerative medicine wherein the heart lacks the capacity to heal itself, but simultaneously contains a large pool of fibroblasts. In this study we determined the cardio-inducing capacity of ten transcription factors to actuate cellular reprogramming of mouse embryonic fibroblasts into cardiomyocyte-like cells. Overexpression of transcription factors MYOCD and SRF alone or in conjunction with Mesp1 and SMARCD3 enhanced the basal but necessary cardio-inducing effect of the previously reported GATA4, TBX5, and MEF2C. In particular, combinations of five or seven transcription factors enhanced the activation of cardiac reporter vectors, and induced an upregulation of cardiac-specific genes. Global gene expression analysis also demonstrated a significantly greater cardio-inducing effect when the transcription factors MYOCD and SRF were used. Detection of cross-striated cells was highly dependent on the cell culture conditions and was enhanced by the addition of valproic acid and JAK inhibitor. Although we detected Ca²⁺ transient oscillations in the reprogrammed cells, we did not detect significant changes in resting membrane potential or spontaneously contracting cells. This study further elucidates the cardio-inducing effect of the transcriptional networks involved in cardiac cellular reprogramming, contributing to the ongoing rational design of a robust protocol required for cardiac regenerative therapies.

Cardiac stem cell therapy and arrhythmogenicity: prometheus and the arrows of Apollo and Artemis

Cardiac cell therapy is an expanding scientific field which is yielding new insights into the pathogenesis of cardiac disease and offers new therapeutic strategies. Inherent to both these areas of research are the electrical properties of individual cells, the electrical interplay between cardiomyocytes, and their roles in arrhythmogenesis. This review discusses the potential mechanisms by which various candidate cells for cardiac therapy may modulate the ventricular arrhythmic substrate and highlights the data and lessons learnt from the clinical cardiac cell therapy trials published to date. Pro- and antiarrhythmic mechanistic factors are discussed, and the importance of their consideration in the design of any future clinical cell therapy trials.

Calcium dyshomeostasis in beta-amyloid and tau-bearing skeletal myotubes

The relative scarcity of inclusion-affected muscle cells or markers of cell death in inclusion body myositis (IBM) is in distinction to the specific and early intracellular deposition of several Alzheimer's Disease (AD)-related proteins. The current study examined the possible correlation between myotube beta-amyloid and/or Tau accumulations and a widespread mishandling of intracellular muscle calcium concentration that could potentially account for the unrelenting weakness in affected patients. Cultured myogenic cells (C(2)C(12)) expressed beta-amyloid-42 (Abeta(42)) and fetal Tau peptides, as human transgenes encoded by herpes simplex virus, either individually or concurrently. Co-expression of Abeta(42) in C(2)C(12) myotubes resulted in hyperphosphorylation of Tau protein that was not observed when Tau was expressed alone. Resting calcium concentration and agonist-induced RyR-mediated Ca(2+) release were examined using calcium-specific microelectrodes and Fluo-4 epifluorescence, respectively. Co-expression of Abeta(42) and Tau cooperatively elevated basal levels of myoplasmic-free calcium, an effect that was accompanied by depolarization of the plasma membrane. Sarcoplasmic reticulum (SR) calcium release, induced by KCl depolarization, was not affected by Abeta(42) or Tau. In contrast, expression of Abeta(42), Tau, or Abeta(42) together with Tau resulted in enhanced sensitivity of ryanodine receptors to activation by caffeine. Notably, expression of beta-amyloid, alone, was sufficient to result in an increased sensitivity to direct activation by caffeine. Current results indicate that amyloid proteins cooperate to raise resting calcium levels and that these effects are associated with a passive SR Ca(2+) leak and Tau hyperphosphorylation in skeletal muscle.

Ca2+ influx through alpha1S DHPR may play a role in regulating Ca2+ release from RyR1 in skeletal muscle

Differentiated primary myotubes isolated from wild-type mice exhibit ryanodine-sensitive, spontaneous global Ca2+ oscillations as well as spontaneous depolarizations in the plasma membrane. Immunolabeling of these myotubes showed expression of both alpha1S dihydropyridine receptors (DHPRs) and ryanodine-sensitive Ca2+-release channel 1 (RyR1), the two key proteins in skeletal excitation-contraction (E-C) coupling. Spontaneous global Ca2+ oscillations could be inhibited by addition of 0.1 mM CdCl2/0.5 mM LaCl3 or 5 microM nifedipine to the extracellular bathing solution. After either treatment, Ca2+ oscillations could be restored upon extensive washing. Although exposure to DHPR antagonists completely blocked Ca2+ oscillations, normal orthograde signaling between DHPRs and RyRs, such as that elicited by 80 mM KCl depolarization, was still observed. In addition, we showed that spontaneous Ca2+ oscillations were never present in cultured mdg myotubes, which lack the expression of alpha1SDHPRs. These results suggest that under physiological conditions in conjunction with the mechanical coupling between the alpha1SDHPRs and RyR1, the initiation of Ca2+ oscillations in myotubes may be facilitated, in part, by the Ca2+ influx through the alpha1s-subunit of the DHPR.

Human embryonic stem cells can differentiate into myocytes with structural and functional properties of cardiomyocytes

The study of human cardiac tissue development is hampered by the lack of a suitable in vitro model. We describe the phenotypic properties of cardiomyocytes derived from human embryonic stem (ES) cells. Human ES cells were cultivated in suspension and plated to form aggregates termed embryoid bodies (EBs). Spontaneously contracting areas appeared in 8.1% of the EBs. Cells from the spontaneously contracting areas within EBs were stained positively with anti-cardiac myosin heavy chain, anti--alpha-actinin, anti-desmin, anti--cardiac troponin I (anti-cTnI), and anti-ANP antibodies. Electron microscopy revealed varying degrees of myofibrillar organization, consistent with early-stage cardiomyocytes. RT-PCR studies demonstrated the expression of several cardiac-specific genes and transcription factors. Extracellular electrograms were characterized by a sharp component lasting 30 +/- 25 milliseconds, followed by a slow component of 347 +/- 120 milliseconds. Intracellular Ca(2+) transients displayed a sharp rise lasting 130 +/- 27 milliseconds and a relaxation component lasting 200--300 milliseconds. Positive and negative chronotropic effects were induced by application of isoproterenol and carbamylcholine, respectively. In conclusion, the human ES cell--derived cardiomyocytes displayed structural and functional properties of early-stage cardiomyocytes. Establishment of this unique differentiation system may have significant impact on the study of early human cardiac differentiation, functional genomics, pharmacological testing, cell therapy, and tissue engineering.