Alonso P. Moreno

A Model of Electrical Conduction in Cardiac Tissue Including Fibroblasts

Fibroblasts are abundant in cardiac tissue. Experimental studies suggested that fibroblasts are electrically coupled to myocytes and this coupling can impact cardiac electrophysiology. In this work, we present a novel approach for mathematical modeling of electrical conduction in cardiac tissue composed of myocytes, fibroblasts, and the extracellular space. The model is an extension of established cardiac bidomain models, which include a description of intra-myocyte and extracellular conductivities, currents and potentials in addition to transmembrane voltages of myocytes. Our extension added a description of fibroblasts, which are electrically coupled with each other and with myocytes. We applied the extended model in exemplary computational simulations of plane waves and conduction in a thin tissue slice assuming an isotropic conductivity of the intra-fibroblast domain. In simulations of plane waves, increased myocyte–fibroblast coupling and fibroblast–myocyte ratio reduced peak voltage and maximal upstroke velocity of myocytes as well as amplitudes and maximal downstroke velocity of extracellular potentials. Simulations with the thin tissue slice showed that inter-fibroblast coupling affected rather transversal than longitudinal conduction velocity. Our results suggest that fibroblast coupling becomes relevant for small intra-myocyte and/or large intra-fibroblast conductivity. In summary, the study demonstrated the feasibility of the extended bidomain model and supports the hypothesis that fibroblasts contribute to cardiac electrophysiology in various manners.

Src Utilizes Cas to Block Gap Junctional Communication Mediated by Connexin43

The Src tyrosine kinase phosphorylates Cas (Crk-associated substrate) to confer anchorage independence and invasive growth potential to transformed cells. Gap junctional communication is often lower between aggressive tumor cells compared with normal or benign precursors. The gap junction protein connexin43 (Cx43) is a tumor suppressor that can inhibit tumor cell growth. Src can phosphorylate Cx43 to block gap junctional communication between transformed cells. However, mechanisms by which this event actually closes intercellular channels have not been clearly defined. Here, we report that Src and Cas associate with each other at intercellular junctions. In addition, Cas is required for Src to reduce dye transfer and electrical coupling between cells expressing Cx43. Thus, Src utilizes Cas to inhibit gap junctional communication mediated by Cx43. This finding introduces a novel role of the Cas focal adhesion linker protein in the gap junction complex. This observation may help explain how gap junctional communication can be suppressed between malignant and metastatic tumor cells.

The voltage-sensitive dye di-4-ANEPPS slows conduction velocity in isolated guinea pig hearts

BACKGROUND Voltage-sensitive dyes are important tools for mapping electrical activity in the heart. However, little is known about the effects of voltage-sensitive dyes on cardiac electrophysiology. OBJECTIVE To test the hypothesis that the voltage-sensitive dye di-4-ANEPPS modulates cardiac impulse propagation. METHODS Electrical and optical mapping experiments were performed in isolated Langendorff perfused guinea pig hearts. The effect of di-4-ANEPPS on conduction velocity and anisotropy of propagation was quantified. HeLa cells expressing connexin 43 were used to evaluate the effect of di-4-ANEPPS on gap junctional conductance. RESULTS In electrical mapping experiments, di-4-ANEPPS (7.5 μM) was found to decrease both longitudinal and transverse conduction velocities significantly compared with control. No change in the anisotropy of propagation was observed. Similar results were obtained in optical mapping experiments. In these experiments, the effect of di-4-ANEPPS was dose dependent. di-4-ANEPPS had no detectable effect on connexin 43-mediated gap junctional conductance in transfected HeLa cells. CONCLUSION Our results demonstrate that the voltage-sensitive dye di-4-ANEPPS directly and dose-dependently modulates cardiac impulse propagation. The effect is not likely mediated by connexin 43 inhibition. Our results highlight an important caveat that should be taken into account when interpreting data obtained using di-4-ANEPPS in cardiac preparations.

Functional and Pharmacological Analysis of Cardiomyocytes Differentiated from Human Peripheral Blood Mononuclear-Derived Pluripotent Stem Cells

Summary Advances in induced pluripotent stem cell (iPSC) technology have set the stage for routine derivation of patient- and disease-specific human iPSC-cardiomyocyte (CM) models for preclinical drug screening and personalized medicine approaches. Peripheral blood mononuclear cells (PBMCs) are an advantageous source of somatic cells because they are easily obtained and readily amenable to transduction. Here, we report that the electrophysiological properties and pharmacological responses of PBMC-derived iPSC CM are generally similar to those of iPSC CM derived from other somatic cells, using patch-clamp, calcium transient, and multielectrode array (MEA) analyses. Distinct iPSC lines derived from a single patient display similar electrophysiological features and pharmacological responses. Finally, we demonstrate that human iPSC CMs undergo acute changes in calcium-handling properties and gene expression in response to rapid electrical stimulation, laying the foundation for an in-vitro-tachypacing model system for the study of human tachyarrhythmias.

Absence of glucose transporter 4 diminishes electrical activity of mouse hearts during hypoxia

•  What is the central question of this study? The aim of this study was to examine quantitatively whether glucose transporter 4 deficiency leads to more severe alterations in cardiac electrical activity during cardiac stress. •  What is the main finding and what is its importance? When compared with hearts from corresponding control littermates, the measured epicardial potentials from the surface of cardiac‐selective glucose transporter 4‐ablated mouse hearts during hypoxia showed the following differences: (i) significant decreases in the maximal downstroke of the potentials; (ii) increased activation time; and (iii) greater alterations in the activation sequence.

Computational simulations of asymmetric fluxes of large molecules through gap junction channel pores

Gap junction channels are formed out of connexin isoforms, which enable molecule and ion selective diffusion amongst neighboring cells. HeLa cells expressing distinct connexins (Cx) allow the formation of heterotypic channels, where we observed a molecular charge-independent preferential flux of large fluorescent molecules in the Cx45 to Cx43 direction. We hypothesize that the pores shape is a significant factor along-side charge and transjunctional voltages for this asymmetric flux. To test this hypothesis, we developed a 3D computational model simulating Brownian diffusion of large molecules in a gap junction channel pore. The basic pore contour was derived from x-ray crystallographic structures of Cx43 and Cx26 and approximated using basic geometric shapes. Lucifer yellow dye molecules and cesium counter-ions were modeled as spheres using their respective Stokes radii. Our simulation results from simple diffusion and constant concentration gradient experiments showed that only charged particles yield asymmetric fluxes in heterotypic pores. While increasing the inner mouth size resulted in a near-quadratic rise in flux, the rise was asymptotic for outer mouth radii increase. Probability maps and average force per particle per pore section explain the asymmetric flux with variation in pore shape. Furthermore, the simulation results are in agreement with our in vitro experimental results with HeLa cells in Cx43-Cx45 heterotypic configurations. The presence of asymmetric fluxes can help us to understand effects of the molecular structure of the pore and predict potential differences in vivo.

Novel xeno‑free human heart matrix‑derived three‑dimensional scaffolds

RationaleMyocardial infarction (MI) results in damaged heart tissue which can progress to severely reduce cardiac function, leading to death. Recent studies have injected dissociated, suspended cardiac cells into coronary arteries to restore function with limited results attributed to poor cell retention and cell death. Extracellular matrix (ECM) injected into damaged cardiac tissue sites show some promising effects. However, combined use of human cardiac ECM and cardiac cells may produce superior benefits to restore cardiac function.ObjectiveThis study was designed to assess use of new three-dimensional human heart ECM-derived scaffolds to serve as vehicles to deliver cardiac-derived cells directly to damaged heart tissue and improve cell retention at these sites while also providing biomechanical support and attracting host cell recruitment.Methods and ResultsECM-derived porous protein scaffolds were fabricated from human heart tissues. These scaffolds were designed to carry, actively promote and preserve cardiac cell phenotype, viability and functional retention in tissue sites. ECM scaffolds were optimized and were seeded with human cardiomyocytes, cultured and subsequently implanted ex vivo onto infarcted murine epicardium. Seeded human cardiomyocytes readily adhered to human cardiac-derived ECM scaffolds and maintained representative phenotypes including expression of cardiomyocyte-specific markers, and remained electrically synchronous within the scaffold in vitro. Ex vivo, cardiomyocyte-seeded ECM scaffolds spontaneously adhered and incorporated into murine ventricle.ConclusionsDecellularized human cardiac tissue-derived 3D ECM scaffolds are effective delivery vehicles for human cardiac cells to directly target ischemic heart tissue and warrant further studies to assess their therapeutic potential in restoring essential cardiac functions.

Mono-Heteromeric Configurations of Gap Junction Channels Formed by Connexin43 and Connexin45 Reduce Unitary Conductance and Determine both Voltage Gating and Metabolic Flux Asymmetry

In cardiac tissues, the expression of multiple connexins (Cx40, Cx43, Cx45, and Cx30.2) is a requirement for proper development and function. Gap junctions formed by these connexins have distinct permeability and gating mechanisms. Since a single cell can express more than one connexin isoform, the formation of hetero-multimeric gap junction channels provides a tissue with an enormous repertoire of combinations to modulate intercellular communication. To study further the perm-selectivity and gating properties of channels containing Cx43 and Cx45, we studied two monoheteromeric combinations in which a HeLa cell co-transfected with Cx43 and Cx45 was paired with a cell expressing only one of these connexins. Macroscopic measurements of total conductance between cell pairs indicated a drastic reduction in total conductance for mono-heteromeric channels. In terms of Vj dependent gating, Cx43 homomeric connexons facing heteromeric connexons only responded weakly to voltage negativity. Cx45 homomeric connexons exhibited no change in Vj gating when facing heteromeric connexons. The distributions of unitary conductances (γj) for both mono-heteromeric channels were smaller than predicted, and both showed low permeability to the fluorescent dyes Lucifer yellow and Rhodamine123. For both mono-heteromeric channels, we observed flux asymmetry regardless of dye charge: flux was higher in the direction of the heteromeric connexon for MhetCx45 and in the direction of the homomeric Cx43 connexon for MhetCx43. Thus, our data suggest that co-expression of Cx45 and Cx43 induces the formation of heteromeric connexons with greatly reduced permeability and unitary conductance. Furthermore, it increases the asymmetry for voltage gating for opposing connexons, and it favors asymmetric flux of molecules across the junction that depends primarily on the size (not the charge) of the crossing molecules.

Evaluation of multiple biological therapies for ischemic cardiac disease

The development of cell- and gene-based strategies for regenerative medicine offers a therapeutic option for the repair and potential regeneration of damaged cardiac tissue post-myocardial infarction (MI). Human umbilical cord subepithelial cell-derived stem cells (hUC-SECs), human bone marrow-derived mesenchymal stem cells (hBM-MSCs), and human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), all derived from human tissue, have been shown to have in vitro and in vivo therapeutic potential. Additionally, S100a1, VEGF165, and stromal-derived factor-1α (SDF-1α) genes all have the potential to improve cardiac function and/or effect adverse remodeling. In this study, we compared the therapeutic potential of hBM-MSCs, hUC-SECs, and hiPSC-CMs along with plasmid-based genes to evaluate the in vivo potential of intramyocardially injected biologics to enhance cardiac function in a mouse MI model. Human cells derived from various tissue types were expanded under hypoxic conditions and injected intramyocardially into mice that had undergone left anterior descending (LAD) artery ligation. Similarly, plasmids were also injected into three groups of mice after LAD ligation. Seven experimental groups were studied in total: (1) control (saline), (2) hBM-MSCs, (3) hiPSC-CMs, (4) hUC-SECs, (5) S100a1 plasmid, (6) VEGF165 plasmid, and (7) SDF-1α plasmid. We evaluated echocardiography, hemodynamic catheterization measurements, and histology at 4 and 12 weeks post-biologic injection. Significant improvement was observed in cardiac function and contractility in hiPSC-CM and S100a1 groups and a significant reduction in left ventricle scar within the hUC-SEC group and a slight improvement in the SDF-1α and VEGF165 groups compared to the control group. These results demonstrate the potential for new biologic therapies to reduce scar burden and improve contractile function.

The Maximal Downstroke of Epicardial Potentials as an Index of Electrical Activity in Mouse Hearts

The maximal upstroke of transmembrane voltage (dV<i>m</i>/dt_max) has been used as an indirect measure of sodium current I_Na upon activation in cardiac myocytes. However, sodium influx generates not only the upstroke of V<i>m</i>, but also the downstroke of the extracellular potentials V including epicardial surface potentials V_es. The purpose of this study was to evaluate the magnitude of the maximal downstroke of V_es (|dV_es/dt_min|) as a global index of electrical activation, based on the relationship of dV<i>m</i>/dt_max to I_Na. To fulfill this purpose, we examined |dV_es/dt_min| experimentally using isolated perfused mouse hearts and computationally using a 3-D cardiac tissue bidomain model. In experimental studies, a custom-made cylindrical “cage” array with 64 electrodes was slipped over mouse hearts to measure V_es during hyperkalemia, ischemia, and hypoxia, which are conditions that decrease I_Na. Values of |dV_es/dt_min| from each electrode were normalized (|dV_es/dt_min|<i>n</i>) and averaged (|dV_es/dt_min|_na). Results showed that |dV_es/dt_min|_na decreased during hyperkalemia by 28, 59, and 79% at 8, 10, and 12 mM [K⁺]<i>o</i>, respectively. |dV_es/dt_min| also decreased by 54 and 84% 20 min after the onset of ischemia and hypoxia, respectively. In computational studies, |dV_es/dt_min| was compared to dV<i>m</i>/dt_max at different levels of the maximum sodium conductance G_Na, extracellular potassium ion concentration [K⁺]<i>o</i>, and intracellular sodium ion concentration [Na⁺]<i>i</i>, which all influence levels of I_Na. Changes in |dV_es/dt_min|<i>n</i> were similar to dV<i>m</i>/dt_max during alterations of G_Na , [K⁺]<i>o</i>, and [Na⁺]<i>i</i>. Our results demonstrate that |dV_es/dt_min|_na is a robust global index of electrical activation for use in mouse hearts and, similar to dV<i>m</i>/dt_max, can be used to probe electrophysiological alterations reliably. The index can be readily measured and evaluated, which makes it attractive for characterization of, for instance, genetically modified mouse hearts and drug effects on cardiac tissue.