Dynamic loading of human engineered heart tissue enhances contractile function and drives a desmosome-linked disease phenotype

The role that mechanical forces play in shaping the structure and function of the heart is critical to understanding heart formation and the etiology of disease but is challenging to study in patients. Engineered heart tissues (EHTs) incorporating human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes have the potential to provide insight into these adaptive and maladaptive changes. However, most EHT systems cannot model both preload (stretch during chamber filling) and afterload (pressure the heart must work against to eject blood). Here, we have developed a new dynamic EHT (dyn-EHT) model that enables us to tune preload and have unconstrained contractile shortening of >10%. To do this, three-dimensional (3D) EHTs were integrated with an elastic polydimethylsiloxane strip providing mechanical preload and afterload in addition to enabling contractile force measurements based on strip bending. Our results demonstrated that dynamic loading improves the function of wild-type EHTs on the basis of the magnitude of the applied force, leading to improved alignment, conduction velocity, and contractility. For disease modeling, we used hiPSC-derived cardiomyocytes from a patient with arrhythmogenic cardiomyopathy due to mutations in the desmoplakin gene. We demonstrated that manifestation of this desmosome-linked disease state required dyn-EHT conditioning and that it could not be induced using 2D or standard 3D EHT approaches. Thus, a dynamic loading strategy is necessary to provoke the disease phenotype of diastolic lengthening, reduction of desmosome counts, and reduced contractility, which are related to primary end points of clinical disease, such as chamber thinning and reduced cardiac output.

NaV1.1 and NaV1.6 selective compounds reduce the behavior phenotype and epileptiform activity in a novel zebrafish model for Dravet Syndrome

Dravet syndrome is caused by dominant loss-of-function mutations in SCN1A which cause reduced activity of Nav1.1 leading to lack of neuronal inhibition. On the other hand, gain-of-function mutations in SCN8A can lead to a severe epileptic encephalopathy subtype by over activating NaV1.6 channels. These observations suggest that Nav1.1 and Nav1.6 represent two opposing sides of the neuronal balance between inhibition and activation. Here, we hypothesize that Dravet syndrome may be treated by either enhancing Nav1.1 or reducing Nav1.6 activity. To test this hypothesis we generated and characterized a novel DS zebrafish model and tested new compounds that selectively activate or inhibit the human NaV1.1 or NaV1.6 channel respectively. We used CRISPR/Cas9 to generate two separate Scn1Lab knockout lines as an alternative to previous zebrafish models generated by random mutagenesis or morpholino oligomers. Using an optimized locomotor assay, spontaneous burst movements were detected that were unique to Scn1Lab knockouts and disappear when introducing human SCN1A mRNA. Besides the behavioral phenotype, Scn1Lab knockouts show sudden, electrical discharges in the brain that indicate epileptic seizures in zebrafish. Scn1Lab knockouts showed increased sensitivity to the GABA antagonist pentylenetetrazole and a reduction in whole organism GABA levels. Drug screenings further validated a Dravet syndrome phenotype. We tested the NaV1.1 activator AA43279 and two novel NaV1.6 inhibitors MV1369 and MV1312 in the Scn1Lab knockouts. Both type of compounds significantly reduced the number of spontaneous burst movements and seizure activity. Our results show that selective inhibition of NaV1.6 could be just as efficient as selective activation of NaV1.1 and these approaches could prove to be novel potential treatment strategies for Dravet syndrome and other (genetic) epilepsies. Compounds tested in zebrafish however, should always be further validated in other model systems for efficacy in mammals and to screen for potential side effects.

An automated hybrid bioelectronic system for autogenous restoration of sinus rhythm in atrial fibrillation

Because of suboptimal therapeutic strategies, restoration of sinus rhythm in symptomatic atrial fibrillation (AF) often requires in-hospital delivery of high-voltage shocks, thereby precluding ambulatory AF termination. Continuous, rapid restoration of sinus rhythm is desired given the recurring and progressive nature of AF. Here, we present an automated hybrid bioelectronic system for shock-free termination of AF that enables the heart to act as an electric current generator for autogenous restoration of sinus rhythm. We show that local, right atrial delivery of adenoassociated virus vectors encoding a light-gated depolarizing ion channel results in efficient and spatially confined transgene expression. Activation of an implanted intrathoracic light-emitting diode device allows for termination of AF by illuminating part of the atria. Combining this newly obtained antiarrhythmic effector function of the heart with the arrhythmia detector function of a machine-based cardiac rhythm monitor in the closed chest of adult rats allowed automated and rapid arrhythmia detection and termination in a safe, effective, repetitive, yet shock-free manner. These findings hold translational potential for the development of shock-free antiarrhythmic device therapy for ambulatory treatment of AF.

Generation and primary characterization of iAM-1, a versatile new line of conditionally immortalized atrial myocytes with preserved cardiomyogenic differentiation capacity

Aims

The generation of homogeneous cardiomyocyte populations from fresh tissue or stem cells is laborious and costly. A potential solution to this problem would be to establish lines of immortalized cardiomyocytes. However, as proliferation and (terminal) differentiation of cardiomyocytes are mutually exclusive processes, their permanent immortalization causes loss of electrical and mechanical functions. We therefore aimed at developing conditionally immortalized atrial myocyte (iAM) lines allowing toggling between proliferative and contractile phenotypes by a single-component change in culture medium composition.

Methods and results

Freshly isolated neonatal rat atrial cardiomyocytes (AMs) were transduced with a lentiviral vector conferring doxycycline (dox)-controlled expression of simian virus 40 large T antigen. Under proliferative conditions (i.e. in the presence of dox), the resulting cells lost most cardiomyocyte traits and doubled every 38 h. Under differentiation conditions (i.e. in the absence of dox), the cells stopped dividing and spontaneously reacquired a phenotype very similar to that of primary AMs (pAMs) in gene expression profile, sarcomeric organization, contractile behaviour, electrical properties, and response to ion channel-modulating compounds (as assessed by patch-clamp and optical voltage mapping). Moreover, differentiated iAMs had much narrower action potentials and propagated them at >10-fold higher speeds than the widely used murine atrial HL-1 cells. High-frequency electrical stimulation of confluent monolayers of differentiated iAMs resulted in re-entrant conduction resembling atrial fibrillation, which could be prevented by tertiapin treatment, just like in monolayers of pAMs.

Conclusion

Through controlled expansion and differentiation of AMs, large numbers of functional cardiomyocytes were generated with properties superior to the differentiated progeny of existing cardiomyocyte lines. iAMs provide an attractive new model system for studying cardiomyocyte proliferation, differentiation, metabolism, and (electro)physiology as well as to investigate cardiac diseases and drug responses, without using animals.

2160Continuous shock-free termination of atrial fibrillation by local optogenetic therapy and arrhythmia-triggered activation of an implanted light source

Background

Maintenance of sinus rhythm is the primary therapeutic goal for symptomatic atrial fibrillation (AF) patients but remains difficult to achieve because of suboptimal treatment options. While being effective in detecting and terminating AF, the widespread use of implantable atrial defibrillators is limited due to patients intolerance to repeated shocks. The negative adverse effects of electroshock therapy can hypothetically be overcome by allowing the heart itself to produce the electric current required for arrhythmia termination. As a result, the effector function of an electrical defibrillator would be provided by the heart itself, and therefore no longer rely on electronics, but on bioelectricity instead.

Purpose

To develop a hybrid bio-electronic system for automated and acute shock-free AF treatment.

Methods

To equip the heart with the effector function of the envisioned AF termination system, adeno-associated virus (AAV) vectors encoding red-activatable channelrhodopsin (ReaChR) (n=12) or citrine (n=4) were delivered locally to the right atrium (RA) of adult Wistar rats by gene painting. Four to 8 weeks later, AF was induced in vivo by atrial burst pacing after carbachol administration, followed by programmed local illumination of the RA by an implanted intrathoracic LED device whose activation was automatically regulated by an electrocardiogram (ECG)-based cardiac rhythm monitor.

Results

Gene painting of the RA resulted in transmural transduction of right atrial myocytes (78±6%) with minimum transgene expression of the left atrium and ventricles (6±2% and <0.5%, respectively). Electrophysiological assessments revealed no significant differences in ECG characteristics, atrial action potential duration and conduction velocity when compared to baseline or citrine control animals. Feasibility of optical AF termination was first assessed in an open-chest rat model, showing that a single 470-nm light pulse (3.5 mW/mm2, 1000 ms) efficiently terminated AF in all ReaChR-expressing rats with an average termination efficacy of 94±3% (n=12) vs. 3±3% (n=4) in citrine-expressing control animals (p<0.01). AF termination efficacy remained superb following automated detection and termination of AF by ECG-triggered activation of the implanted intra-thoracic LED in closed-chest ReaChR-expressing rats (96±4%), n=4), whereas none of the AF episodes were terminated in control rats (0%, n=4) (p<0.01). No bradycardias or other arrhythmias were observed following optical AF termination.

Conclusions

By using a hybrid bio-electronic approach to modulate cardiac excitability, our study delivers proof that AF can be detected and terminated automatically in a safe, effective and repetitive, yet shock-free manner. These findings may create the basis for the development of pain-free device therapy for cardiac arrhythmias, thereby paving the way for ambulatory AF treatment with the perspective to improve patients' prognosis and quality of life.

Acknowledgement/Funding

NWO Vidi grant (1714336) and ERC Starting Grant (716509) both to D.A.P.

P1229Massive expansion of native human atrial cardiomyocytes through immortogenetics: generation of the hiAM cell lines

Background

Preclinical cardiac research greatly depends on animal-derived cellular models, thereby hampering clinical translation. While upcoming human pluripotent stem cell technology seems to decrease this gap between bench and bedside, its complex/multi-step protocol to produce cardiac muscle cells, its required expertise, and its trouble to produce large numbers of phenotypically homogeneous cardiomyocytes so far has limited broad application.

Purpose

We aimed to conditionally immortalize native human atrial cardiomyocytes to produce natural and standardized lines of these cells by gaining full control over their proliferation and differentiation.

Methods

Human fetal atria (gestational age 18 weeks) were dissociated and transduced with a lentiviral vector directing myocyte-specific and doxycycline-inducible expression of simian virus 40 large T antigen (here defined as immortogenetics). Addition of doxycycline to the culture medium pushed cardiomyocytes towards a proliferative phenotype. In total, 125 proliferating monoclones were isolated, expanded and screened for their cardiomyogenic differentiation capacity upon doxycycline removal. Selected clones were characterised using various molecular biological and electrophysiological assays.

Results

Upon doxycycline removal (i.e. under differentiation conditions), cells spontaneously reacquired a cardiomyocyte-like appearance as judged by phase-contrast microscopy and were observed contracting. Simultaneously, these cells stopped proliferating, which was accompanied by a drop in large T level, loss of Ki67 expression and the development of sarcomeres with striated α-actinin and troponin T staining patterns. These cells were tagged conditionally immortalized human atrial cardiomyocytes (hereinafter called hiAMs). Optical voltage mapping of hiAM monolayers revealed excitable cells showing homogeneous spreading of action potentials at 22,5±3,1 cm/s following 1-Hz point stimulation, with a mean APD80 of 139±22 ms. Monolayers of hiAMs could easily be created as big as 10cm2 while continuing to display homogenous conduction throughout the culture. Single-cell patch clamp recordings of a hiAM clone in current-clamp mode confirmed excitability with a resting membrane potential of −62,2±4,3 mV, peak potential of 39,4±3,9 mV and APD80 of 339±9 ms.

Excitable monolayer of hiAMs

Conclusion

We have generated first-of-a-kind lines of natural human atrial cardiomyocytes through immortogenetics, allowing massive cell expansion under proliferation conditions and robust formation of cross-striated, contractile and excitable cardiomyocytes after differentiation. Thereby, a user-friendly, clinically-relevant and much-anticipated research model has been produced, which application could range from multi-scale electrophysiological studies and drug response studies to disease modelling and myocardial regeneration.

Localized Optogenetic Targeting of Rotors in Atrial Cardiomyocyte Monolayers

BACKGROUND

Recently, a new ablation strategy for atrial fibrillation has emerged, which involves the identification of rotors (ie, local drivers) followed by the localized targeting of their core region by ablation. However, this concept has been subject to debate because the mode of arrhythmia termination remains poorly understood, as dedicated models and research tools are lacking. We took a unique optogenetic approach to induce and locally target a rotor in atrial monolayers.

METHODS AND RESULTS

Neonatal rat atrial cardiomyocyte monolayers expressing a depolarizing light-gated ion channel (Ca²⁺-translocating channelrhodopsin) were subjected to patterned illumination to induce single, stable, and centralized rotors by optical S1-S2 cross-field stimulation. Next, the core region of these rotors was specifically and precisely targeted by light to induce local conduction blocks of circular or linear shapes. Conduction blocks crossing the core region, but not reaching any unexcitable boundary, did not lead to termination. Instead, electric waves started to propagate along the circumference of block, thereby maintaining reentrant activity, although of lower frequency. If, however, core-spanning lines of block reached at least 1 unexcitable boundary, reentrant activity was consistently terminated by wave collision. Lines of block away from the core region resulted merely in rotor destabilization (ie, drifting).

CONCLUSIONS

Localized optogenetic targeting of rotors in atrial monolayers could lead to both stabilization and destabilization of reentrant activity. For termination, however, a line of block is required reaching from the core region to at least 1 unexcitable boundary. These findings may improve our understanding of the mechanisms involved in rotor-guided ablation.

Clinical and genetic analysis of a family with two rare reflex epilepsies

PURPOSE

To determine clinical phenotypes, evolution and genetic background of a large family with a combination of two unusual forms of reflex epilepsies.

METHOD

Phenotyping was performed in eighteen family members (10 F, 8 M) including standardized EEG recordings with intermittent photic stimulation (IPS). Genetic analyses (linkage scans, Whole Exome Sequencing (WES) and Functional studies) were performed using photoparoxysmal EEG responses (PPRs) as affection status.

RESULTS

The proband suffered from speaking induced jaw-jerks and increasing limb jerks evoked by flickering sunlight since about 50 years of age. Three of her family members had the same phenotype. Generalized PPRs were found in seven members (six above 50 years of age) with myoclonus during the PPR. Evolution was typical: Sensitivity to lights with migraine-like complaints around adolescence, followed by jerks evoked by lights and spontaneously with dropping of objects, and strong increase of light sensitivity and onset of talking induced jaw jerks around 50 years. Linkage analysis showed suggestive evidence for linkage to four genomic regions. All photosensitive family members shared a heterozygous R129C mutation in the SCNM1 gene that regulates splicing of voltage gated ion channels. Mutation screening of 134 unrelated PPR patients and 95 healthy controls, did not replicate these findings.

CONCLUSION

This family presents a combination of two rare reflex epilepsies. Genetic analysis favors four genomic regions and points to a shared SCNM1 mutation that was not replicated in a general cohort of photosensitive subjects. Further genetic studies in families with similar combination of features are warranted.

Febrile temperatures unmask biophysical defects in Nav1.1 epilepsy mutations supportive of seizure initiation

Generalized epilepsy with febrile seizures plus (GEFS+) is an early onset febrile epileptic syndrome with therapeutic responsive (a)febrile seizures continuing later in life. Dravet syndrome (DS) or severe myoclonic epilepsy of infancy has a complex phenotype including febrile generalized or hemiclonic convulsions before the age of 1, followed by intractable myoclonic, complex partial, or absence seizures. Both diseases can result from mutations in the Na_v1.1 sodium channel, and initially, seizures are typically triggered by fever. We previously characterized two Na_v1.1 mutants—R859H (GEFS+) and R865G (DS)—at room temperature and reported a mixture of biophysical gating defects that could not easily predict the phenotype presentation as either GEFS+ or DS. In this study, we extend the characterization of Na_v1.1 wild-type, R859H, and R865G channels to physiological (37°C) and febrile (40°C) temperatures. At physiological temperature, a variety of biophysical defects were detected in both mutants, including a hyperpolarized shift in the voltage dependence of activation and a delayed recovery from fast and slow inactivation. Interestingly, at 40°C we also detected additional gating defects for both R859H and R865G mutants. The GEFS+ mutant R859H showed a loss of function in the voltage dependence of inactivation and an increased channel use-dependency at 40°C with no reduction in peak current density. The DS mutant R865G exhibited reduced peak sodium currents, enhanced entry into slow inactivation, and increased use-dependency at 40°C. Our results suggest that fever-induced temperatures exacerbate the gating defects of R859H or R865G mutants and may predispose mutation carriers to febrile seizures.

Nav 1.1 dysfunction in genetic epilepsy with febrile seizures-plus or Dravet syndrome

Relatively few SCN1A mutations associated with genetic epilepsy with febrile seizures-plus (GEFS+) and Dravet syndrome (DS) have been functionally characterized. In contrast to GEFS+, many mutations detected in DS patients are predicted to have complete loss of function. However, functional consequences are not immediately apparent for DS missense mutations. Therefore, we performed a biophysical analysis of three SCN1A missense mutations (R865G, R946C and R946H) we detected in six patients with DS. Furthermore, we compared the functionality of the R865G DS mutation with that of a R859H mutation detected in a GEFS+ patient; the two mutations reside in the same voltage sensor domain of Na(v) 1.1. The four mutations were co-expressed with β1 and β2 subunits in tsA201 cells, and characterized using the whole-cell patch clamp technique. The two DS mutations, R946C and R946H, were nonfunctional. However, the novel voltage sensor mutants R859H (GEFS+) and R865G (DS) produced sodium current densities similar to those in wild-type channels. Both mutants had negative shifts in the voltage dependence of activation, slower recovery from inactivation, and increased persistent current. Only the GEFS+ mutant exhibited a loss of function in voltage-dependent channel availability. Our results suggest that the R859H mutation causes GEFS+ by a mixture of biophysical defects in Na(v) 1.1 gating. Interestingly, while loss of Na(v) 1.1 function is common in DS, the R865G mutation may cause DS by overall gain-of-function defects.

Heterogeneity at the JME 6p11?12 Locus: Absence of Mutations in the EFHC1 Gene in Linked Dutch Families

PURPOSE

The EFHC1 gene, encoding a protein with a Ca(2+)-sensing EF-hand motif, is localized at 6p12 and was recently reported as mutated in six Mexican juvenile myoclonic epilepsy (JME) families linked to this region. We had previously confirmed linkage between JME and 6p11-12 in 18 Dutch families, and shown exclusionary lod scores at 6p21.3. We therefore evaluated the relevance of EFHC1 in our set of 6p11-12-linked families.

METHODS

We screened all coding and regulatory regions of EFHC1 by direct sequencing, and the detected variants were tested in a case-control association study.

RESULTS

We found none of the five mutations previously reported in the Mexican families, but identified nine variants, three of which are novel: 5' upstream region (c.-146_147delGC), nonsynonymous (R159W, R182H, M448T, I619L), intronic (IVS3 + 10A>G, IVS8 + 175_176delTT, IVS10 + 59C>T), and 3' UTR (c.+121C>A). These variants did not cosegregate with JME and did not account for the observed linkage at the 6p11-12 locus. Furthermore, no significant association was detected between JME and these variants in 112 unrelated patients and 180 controls. Finally, none of the mutations reported in Mexican families was found in 100 unrelated patients.

CONCLUSIONS

We found no evidence that EFHC1 is a major genetic risk factor for JME susceptibility in Dutch patients. The EFHC1 variants reported in Mexican families may be mendelian variants specific for those families, suggesting that for Dutch patients and possibly many other populations, the main disease variant at the 6p11-12 is yet to be identified.