Engineering prokaryotic channels for control of mammalian tissue excitability

Prokaryotic voltage-gated sodium channels are more effective than endogenous Nav1.5 channels in rescuing cardiac action potential conduction: an in silico study

Methods to augment Na+ current in cardiomyocytes hold potential for the treatment of various cardiac arrhythmias involving conduction slowing. Because the gene coding cardiac Na+ channel (Nav1.5) is too large to fit in a single adeno-associated virus (AAV) vector, new gene therapies are being developed to enhance endogenous Nav1.5 current (by overexpression of chaperon molecules or use of multiple AAV vectors) or to exogenously introduce prokaryotic voltage-gated Na+ channels (BacNav) whose gene size is significantly smaller than that of the Nav1.5. In this study, based on experimental measurements in heterologous expression systems, we developed an improved computational model of the BacNav channel, NavSheP D60A. We then compared in silico how NavSheP D60A expression vs. Nav1.5 augmentation affects the electrophysiology of cardiac tissue. We found that the incorporation of BacNav channels in both adult guinea pig and human cardiomyocyte models increased their excitability and reduced action potential duration. When compared with equivalent augmentation of Nav1.5 current in simulated settings of reduced tissue excitability, the addition of the BacNav current was superior in improving the safety of conduction under conditions of current source-load mismatch, reducing the vulnerability to unidirectional conduction block during premature pacing, preventing the instability and breakup of spiral waves, and normalizing the conduction and ECG in Brugada syndrome tissues with mutated Nav1.5. Overall, our studies show that compared with a potential enhancement of the endogenous Nav1.5 current, expression of the BacNav channels with their slower inactivation kinetics can provide greater anti-arrhythmic benefits in hearts with compromised action potential conduction.NEW & NOTEWORTHY Slow action potential conduction is a common cause of various cardiac arrhythmias; yet, current pharmacotherapies cannot augment cardiac conduction. This in silico study compared the efficacy of recently proposed antiarrhythmic gene therapy approaches that increase peak sodium current in cardiomyocytes. When compared with the augmentation of endogenous sodium current, expression of slower-inactivating bacterial sodium channels was superior in preventing conduction block and arrhythmia induction. These results further the promise of antiarrhythmic gene therapies targeting sodium channels.

Engineering Functional Membrane-Membrane Interfaces by InterSpy

Engineering synthetic interfaces between membranes has potential applications in designing non-native cellular communication pathways and creating synthetic tissues. Here, InterSpy is introduced as a synthetic biology tool consisting of a heterodimeric protein engineered to form and maintain membrane-membrane interfaces between apposing synthetic as well as cell membranes through the SpyTag/SpyCatcher interaction. The inclusion of split fluorescent protein fragments in InterSpy allows tracking of the formation of a membrane-membrane interface and reconstitution of functional fluorescent protein in the space between apposing membranes. First, InterSpy is demonstrated by testing split protein designs using a mammalian cell-free expression (CFE) system. By utilizing co-translational helix insertion, cell-free synthesized InterSpy fragments are incorporated into the membrane of liposomes and supported lipid bilayers with the desired topology. Functional reconstitution of split fluorescent protein between the membranes is strictly dependent on SpyTag/SpyCatcher. Finally, InterSpy is demonstrated in mammalian cells by detecting fluorescence reconstitution of split protein at the membrane-membrane interface between two cells each expressing a component of InterSpy. InterSpy demonstrates the power of CFE systems in the functional reconstitution of synthetic membrane interfaces via proximity-inducing proteins. This technology may also prove useful where cell-cell contacts and communication are recreated in a controlled manner using minimal components.

Computational design of custom therapeutic cells to correct failing human cardiomyocytes

Background

Myocardial delivery of non-excitable cells-namely human mesenchymal stem cells (hMSCs) and c-kit⁺ cardiac interstitial cells (hCICs)-remains a promising approach for treating the failing heart. Recent empirical studies attempt to improve such therapies by genetically engineering cells to express specific ion channels, or by creating hybrid cells with combined channel expression. This study uses a computational modeling approach to test the hypothesis that custom hypothetical cells can be rationally designed to restore a healthy phenotype when coupled to human heart failure (HF) cardiomyocytes.

Methods

Candidate custom cells were simulated with a combination of ion channels from non-excitable cells and healthy human cardiomyocytes (hCMs). Using a genetic algorithm-based optimization approach, candidate cells were accepted if a root mean square error (RMSE) of less than 50% relative to healthy hCM was achieved for both action potential and calcium transient waveforms for the cell-treated HF cardiomyocyte, normalized to the untreated HF cardiomyocyte.

Results

Custom cells expressing only non-excitable ion channels were inadequate to restore a healthy cardiac phenotype when coupled to either fibrotic or non-fibrotic HF cardiomyocytes. In contrast, custom cells also expressing cardiac ion channels led to acceptable restoration of a healthy cardiomyocyte phenotype when coupled to fibrotic, but not non-fibrotic, HF cardiomyocytes. Incorporating the cardiomyocyte inward rectifier K⁺ channel was critical to accomplishing this phenotypic rescue while also improving single-cell action potential metrics associated with arrhythmias, namely resting membrane potential and action potential duration. The computational approach also provided insight into the rescue mechanisms, whereby heterocellular coupling enhanced cardiomyocyte L-type calcium current and promoted calcium-induced calcium release. Finally, as a therapeutically translatable strategy, we simulated delivery of hMSCs and hCICs genetically engineered to express the cardiomyocyte inward rectifier K⁺ channel, which decreased action potential and calcium transient RMSEs by at least 24% relative to control hMSCs and hCICs, with more favorable single-cell arrhythmia metrics.

Conclusion

Computational modeling facilitates exploration of customizable engineered cell therapies. Optimized cells expressing cardiac ion channels restored healthy action potential and calcium handling phenotypes in fibrotic HF cardiomyocytes and improved single-cell arrhythmia metrics, warranting further experimental validation studies of the proposed custom therapeutic cells.

Engineered bacterial voltage-gated sodium channel platform for cardiac gene therapy

Therapies for cardiac arrhythmias could greatly benefit from approaches to enhance electrical excitability and action potential conduction in the heart by stably overexpressing mammalian voltage-gated sodium channels. However, the large size of these channels precludes their incorporation into therapeutic viral vectors. Here, we report a platform utilizing small-size, codon-optimized engineered prokaryotic sodium channels (BacNav) driven by muscle-specific promoters that significantly enhance excitability and conduction in rat and human cardiomyocytes in vitro and adult cardiac tissues from multiple species in silico. We also show that the expression of BacNav significantly reduces occurrence of conduction block and reentrant arrhythmias in fibrotic cardiac cultures. Moreover, functional BacNav channels are stably expressed in healthy mouse hearts six weeks following intravenous injection of self-complementary adeno-associated virus (scAAV) without causing any adverse effects on cardiac electrophysiology. The large diversity of prokaryotic sodium channels and experimental-computational platform reported in this study should facilitate the development and evaluation of BacNav-based gene therapies for cardiac conduction disorders.

In vitro discovery of novel prokaryotic ion channel candidates for antiarrhythmic gene therapy

Sudden cardiac death continues to have a devastating impact on public health prompting the continued efforts to develop more effective therapies for cardiac arrhythmias. Among different approaches to normalize function of ion channels and prevent arrhythmogenic remodeling of tissue substrate, cardiac cell and gene therapies are emerging as promising strategies to restore and maintain normal heart rhythm. Specifically, the ability to genetically enhance electrical excitability of diseased hearts through voltage-gated sodium channel (VGSC) gene transfer could improve velocity of action potential conduction and act to stop reentrant circuits underlying sustained arrhythmias. For this purpose, prokaryotic VGSC genes are promising therapeutic candidates due to their small size (<1kb) and potential to be effectively packaged in adeno-associated viral (AAV) vectors and delivered to cardiomyocytes for stable, long-term expression. This article describes a versatile method to discover and characterize novel prokaryotic ion channels for use in gene and cell therapies for heart disease including cardiac arrhythmias. Detailed protocols are provided for: (1) identification of potential ion channel candidates from large genomic databases, (2) candidate screening and characterization using site-directed mutagenesis and engineered human excitable cell system and, (3) candidate validation using electrophysiological techniques and an in vitro model of impaired cardiac impulse conduction.

Optically activated, customizable, excitable cells

Genetically encoded fluorescent biosensors are powerful tools for studying complex signaling in the nervous system, and now both Ca2+ and voltage sensors are available to study the signaling behavior of entire neural circuits. There is a pressing need for improved sensors, but improving them is challenging because testing them involves a low throughput, labor-intensive processes. Our goal was to create synthetic, excitable cells that can be activated with brief pulses of blue light and serve as a medium throughput platform for screening the next generation of sensors. In this live cell system, blue light activates an adenylyl cyclase enzyme (bPAC) that increases intracellular cAMP (Stierl M et al. 2011). In turn, the cAMP opens a cAMP-gated ion channel. This produces slow, whole-cell Ca2+ transients and voltage changes. To increase the speed of these transients, we add the inwardly rectifying potassium channel Kir2.1, the bacterial voltage-gated sodium channel NAVROSD, and Connexin-43. The result is a highly reproducible, medium-throughput, live cell system that can be used to screen voltage and Ca2+ sensors.

Altering integrin engagement regulates membrane localization of Kir2.1 channels

How ion channels localize and distribute on the cell membrane remains incompletely understood. We show that interventions that vary cell adhesion proteins and cell size also affect the membrane current density of inward-rectifier K+ channels (Kir2.1; encoded by KCNJ2) and profoundly alter the action potential shape of excitable cells. By using micropatterning to manipulate the localization and size of focal adhesions (FAs) in single HEK293 cells engineered to stably express Kir2.1 channels or in neonatal rat cardiomyocytes, we establish a robust linear correlation between FA coverage and the amplitude of Kir2.1 current at both the local and whole-cell levels. Confocal microscopy showed that Kir2.1 channels accumulate in membrane proximal to FAs. Selective pharmacological inhibition of key mediators of protein trafficking and the spatially dependent alterations in the dynamics of Kir2.1 fluorescent recovery after photobleaching revealed that the Kir2.1 channels are transported to the cell membrane uniformly, but are preferentially internalized by endocytosis at sites that are distal from FAs. Based on these results, we propose adhesion-regulated membrane localization of ion channels as a fundamental mechanism of controlling cellular electrophysiology via mechanochemical signals, independent of the direct ion channel mechanogating.

Physiologic, Pathologic, and Therapeutic Paracrine Modulation of Cardiac Excitation-Contraction Coupling

Cardiac excitation-contraction coupling (ECC) is the orchestrated process of initial myocyte electrical excitation, which leads to calcium entry, intracellular trafficking, and subsequent sarcomere shortening and myofibrillar contraction. Neurohumoral β-adrenergic signaling is a well-established mediator of ECC; other signaling mechanisms, such as paracrine signaling, have also demonstrated significant impact on ECC but are less well understood. For example, resident heart endothelial cells are well-known physiological paracrine modulators of cardiac myocyte ECC mainly via NO and endothelin-1. Moreover, recent studies have demonstrated other resident noncardiomyocyte heart cells (eg, physiological fibroblasts and pathological myofibroblasts), and even experimental cardiotherapeutic cells (eg, mesenchymal stem cells) are also capable of altering cardiomyocyte ECC through paracrine mechanisms. In this review, we first focus on the paracrine-mediated effects of resident and therapeutic noncardiomyocytes on cardiomyocyte hypertrophy, electrophysiology, and calcium handling, each of which can modulate ECC, and then discuss the current knowledge about key paracrine factors and their underlying mechanisms of action. Next, we provide a case example demonstrating the promise of tissue-engineering approaches to study paracrine effects on tissue-level contractility. More specifically, we present new functional and molecular data on the effects of human adult cardiac fibroblast conditioned media on human engineered cardiac tissue contractility and ion channel gene expression that generally agrees with previous murine studies but also suggests possible species-specific differences. By contrast, paracrine secretions by human dermal fibroblasts had no discernible effect on human engineered cardiac tissue contractile function and gene expression. Finally, we discuss systems biology approaches to help identify key stem cell paracrine mediators of ECC and their associated mechanistic pathways. Such integration of tissue-engineering and systems biology methods shows promise to reveal novel insights into paracrine mediators of ECC and their underlying mechanisms of action, ultimately leading to improved cell-based therapies for patients with heart disease.

Biomaterializing the promise of cardiac tissue engineering

During an average individual's lifespan, the human heart pumps nearly 200 million liters of blood delivered by approximately 3 billion heartbeats. Therefore, it is not surprising that native myocardium under this incredible demand is extraordinarily complex, both structurally and functionally. As a result, successful engineering of adult-mimetic functional cardiac tissues is likely to require utilization of highly specialized biomaterials representative of the native extracellular microenvironment. There is currently no single biomaterial that fully recapitulates the architecture or the biochemical and biomechanical properties of adult myocardium. However, significant effort has gone toward designing highly functional materials and tissue constructs that may one day provide a ready source of cardiac tissue grafts to address the overwhelming burden of cardiomyopathic disease. In the near term, biomaterial-based scaffolds are helping to generate in vitro systems for querying the mechanisms underlying human heart homeostasis and disease and discovering new, patient-specific therapeutics. When combined with advances in minimally-invasive cardiac delivery, ongoing efforts will likely lead to scalable cell and biomaterial technologies for use in clinical practice. In this review, we describe recent progress in the field of cardiac tissue engineering with particular emphasis on use of biomaterials for therapeutic tissue design and delivery.

Ion channel engineering for modulation and de novo generation of electrical excitability

Ion channels play essential roles in regulating electrical properties of excitable tissues. By leveraging various ion channel gating mechanisms, scientists have developed a versatile set of genetically encoded tools to modulate intrinsic tissue excitability under different experimental settings. In this article, we will review how ion channels activated by voltage, light, small chemicals, stretch, and temperature have been customized to enable control of tissue excitability both in vitro and in vivo. Advantages and limitations of each of these ion channel-engineering platforms will be discussed and notable applications will be highlighted. Furthermore, we will describe recent progress on de novo generation of excitable tissues via expression of appropriate sets of engineered voltage-gated ion channels and discuss potential therapeutic implications.

Generation and customization of biosynthetic excitable tissues for electrophysiological studies and cell-based therapies

We describe a two-stage protocol to generate electrically excitable and actively conducting cell networks with stable and customizable electrophysiological phenotypes. Using this method, we have engineered monoclonally derived excitable tissues as a robust and reproducible platform to investigate how specific ion channels and mutations affect action potential (AP) shape and conduction. In the first stage of the protocol, we combine computational modeling, site-directed mutagenesis, and electrophysiological techniques to derive optimal sets of mammalian and/or prokaryotic ion channels that produce specific AP shape and conduction characteristics. In the second stage of the protocol, selected ion channels are stably expressed in unexcitable human cells by means of viral or nonviral delivery, followed by flow cytometry or antibiotic selection to purify the desired phenotype. This protocol can be used with traditional heterologous expression systems or primary excitable cells, and application of this method to primary fibroblasts may enable an alternative approach to cardiac cell therapy. Compared with existing methods, this protocol generates a well-defined, relatively homogeneous electrophysiological phenotype of excitable cells that facilitates experimental and computational studies of AP conduction and can decrease arrhythmogenic risk upon cell transplantation. Although basic cell culture and molecular biology techniques are sufficient to generate excitable tissues using the described protocol, experience with patch-clamp techniques is required to characterize and optimize derived cell populations.

Engineering human pluripotent stem cells into a functional skeletal muscle tissue

The generation of functional skeletal muscle tissues from human pluripotent stem cells (hPSCs) has not been reported. Here, we derive induced myogenic progenitor cells (iMPCs) via transient overexpression of Pax7 in paraxial mesoderm cells differentiated from hPSCs. In 2D culture, iMPCs readily differentiate into spontaneously contracting multinucleated myotubes and a pool of satellite-like cells endogenously expressing Pax7. Under optimized 3D culture conditions, iMPCs derived from multiple hPSC lines reproducibly form functional skeletal muscle tissues (iSKM bundles) containing aligned multi-nucleated myotubes that exhibit positive force-frequency relationship and robust calcium transients in response to electrical or acetylcholine stimulation. During 1-month culture, the iSKM bundles undergo increased structural and molecular maturation, hypertrophy, and force generation. When implanted into dorsal window chamber or hindlimb muscle in immunocompromised mice, the iSKM bundles survive, progressively vascularize, and maintain functionality. iSKM bundles hold promise as a microphysiological platform for human muscle disease modeling and drug development.

Engineered cardiac tissue patch maintains structural and electrical properties after epicardial implantation

Functional cardiac tissue engineering holds promise as a candidate therapy for myocardial infarction and heart failure. Generation of "strong-contracting and fast-conducting" cardiac tissue patches capable of electromechanical coupling with host myocardium could allow efficient improvement of heart function without increased arrhythmogenic risks. Towards that goal, we engineered highly functional 1 cm × 1 cm cardiac tissue patches made of neonatal rat ventricular cells which after 2 weeks of culture exhibited force of contraction of 18.0 ± 1.4 mN, conduction velocity (CV) of 32.3 ± 1.8 cm/s, and sustained chronic activation when paced at rates as high as 8.7 ± 0.8 Hz. Patches transduced with genetically-encoded calcium indicator (GCaMP6) were implanted onto adult rat ventricles and after 4-6 weeks assessed for action potential conduction and electrical integration by two-camera optical mapping of GCaMP6-reported Ca²⁺ transients in the patch and RH237-reported action potentials in the recipient heart. Of the 13 implanted patches, 11 (85%) engrafted, maintained structural integrity, and conducted action potentials with average CVs and Ca²⁺ transient durations comparable to those before implantation. Despite preserved graft electrical properties, no anterograde or retrograde conduction could be induced between the patch and host cardiomyocytes, indicating lack of electrical integration. Electrical properties of the underlying myocardium were not changed by the engrafted patch. From immunostaining analyses, implanted patches were highly vascularized and expressed abundant electromechanical junctions, but remained separated from the epicardium by a non-myocyte layer. In summary, our studies demonstrate generation of highly functional cardiac tissue patches that can robustly engraft on the epicardial surface, vascularize, and maintain electrical function, but do not couple with host tissue. The lack of graft-host electrical integration is therefore a critical obstacle to development of efficient tissue engineering therapies for heart repair.

Virtual cardiac monolayers for electrical wave propagation

The complex structure of cardiac tissue is considered to be one of the main determinants of an arrhythmogenic substrate. This study is aimed at developing the first mathematical model to describe the formation of cardiac tissue, using a joint in silico-in vitro approach. First, we performed experiments under various conditions to carefully characterise the morphology of cardiac tissue in a culture of neonatal rat ventricular cells. We considered two cell types, namely, cardiomyocytes and fibroblasts. Next, we proposed a mathematical model, based on the Glazier-Graner-Hogeweg model, which is widely used in tissue growth studies. The resultant tissue morphology was coupled to the detailed electrophysiological Korhonen-Majumder model for neonatal rat ventricular cardiomyocytes, in order to study wave propagation. The simulated waves had the same anisotropy ratio and wavefront complexity as those in the experiment. Thus, we conclude that our approach allows us to reproduce the morphological and physiological properties of cardiac tissue.

Age-dependent functional crosstalk between cardiac fibroblasts and cardiomyocytes in a 3D engineered cardiac tissue

Complex heterocellular interactions between cardiomyocytes and fibroblasts in the heart involve their bidirectional signaling via cell-cell contacts, paracrine factors, and extracellular matrix (ECM). These interactions vary with heart development and pathology leading to changes in cardiac structure and function. Whether cardiac fibroblasts of different ages interact differentially with cardiomyocytes to distinctly impact their function remains unknown. Here, we explored the direct structural and functional effects of fetal and adult cardiac fibroblasts on cardiomyocytes using a tissue-engineered 3D co-culture system. We show that the age of cardiac fibroblasts is a strong determinant of the structure, function, and molecular properties of co-cultured tissues. In particular, in vitro expanded adult, but not fetal, cardiac fibroblasts significantly deteriorated electrical and mechanical function of the co-cultured cardiomyocytes, as evidenced by slower action potential conduction, prolonged action potential duration, weaker contractions, higher tissue stiffness, and reduced calcium transient amplitude. This functional deficit was associated with structural and molecular signatures of pathological remodeling including fibroblast proliferation, interstitial collagen deposition, and upregulation of pro-fibrotic markers. Our studies imply critical roles of the age of supporting cells in engineering functional cardiac tissues and provide a new physiologically relevant in vitro platform to investigate influence of heterocellular interactions on cardiomyocyte function, development, and disease.

STATEMENT OF SIGNIFICANCE

Previous studies have shown that cardiomyocytes and fibroblasts in the heart interact through direct contacts, paracrine factors, and matrix-mediated crosstalk. However, whether cardiac fibroblasts of different ages distinctly impact cardiomyocyte function remains elusive. We employed a tissue-engineered hydrogel-based co-culture system to study interactions of cardiomyocytes with fetal or adult cardiac fibroblasts. We show that the age of cardiac fibroblasts is a strong determinant of the structure, function, and molecular properties of engineered cardiac tissues and that key features of fibrotic myocardium are replicated by supplementing cardiomyocytes with expanded adult but not fetal fibroblasts. These findings relate to implantation of stem cell-derived cardiomyocytes in adult myocardium and warrant further studies of how age and source of non-myocytes impact cardiac function and maturation.

Modeling an Excitable Biosynthetic Tissue with Inherent Variability for Paired Computational-Experimental Studies

To understand how excitable tissues give rise to arrhythmias, it is crucially necessary to understand the electrical dynamics of cells in the context of their environment. Multicellular monolayer cultures have proven useful for investigating arrhythmias and other conduction anomalies, and because of their relatively simple structure, these constructs lend themselves to paired computational studies that often help elucidate mechanisms of the observed behavior. However, tissue cultures of cardiomyocyte monolayers currently require the use of neonatal cells with ionic properties that change rapidly during development and have thus been poorly characterized and modeled to date. Recently, Kirkton and Bursac demonstrated the ability to create biosynthetic excitable tissues from genetically engineered and immortalized HEK293 cells with well-characterized electrical properties and the ability to propagate action potentials. In this study, we developed and validated a computational model of these excitable HEK293 cells (called “Ex293” cells) using existing electrophysiological data and a genetic search algorithm. In order to reproduce not only the mean but also the variability of experimental observations, we examined what sources of variation were required in the computational model. Random cell-to-cell and inter-monolayer variation in both ionic conductances and tissue conductivity was necessary to explain the experimentally observed variability in action potential shape and macroscopic conduction, and the spatial organization of cell-to-cell conductance variation was found to not impact macroscopic behavior; the resulting model accurately reproduces both normal and drug-modified conduction behavior. The development of a computational Ex293 cell and tissue model provides a novel framework to perform paired computational-experimental studies to study normal and abnormal conduction in multidimensional excitable tissue, and the methodology of modeling variation can be applied to models of any excitable cell.

One of the major challenges in trying to understand how arrhythmias can form in cardiac tissue is studying how the electrical activity of cardiac cells is affected by their surroundings. Current approaches have focused on studying cardiac cells in vitro and using computational models to elucidate the mechanisms behind experimental findings. However, tissue culture techniques are limited to working with neonatal, rather than adult, cells, and computational modeling of these cells has proven challenging. In this work, we have a developed a new approach for conducting paired experimental and computational studies by using a cell line engineered with the minimum machinery for excitability, and a computational model derived and validated directly from this cell line. In order to create a model that reproduces the diversity, rather than simply the average behavior, of experimental studies, we have incorporated a simple yet novel method of inherent variability, and explored what types of experimental variation must be incorporated into the model to recapitulate experimental findings. Using this new platform for paired experimental-computational studies with inherent variability, we will be able to study and better understand how changes in cardiac structure such as fibrosis and heterogeneity lead to conduction slowing, conduction failure, and arrhythmogenesis.