A Closed-Loop Electrical Stimulation System for Cardiac Cell Cultures

DC electrical stimulation enhances proliferation and differentiation on N2a and MC3T3 cell lines

Background

Electrical stimulation is a novel tool to promote the differentiation and proliferation of precursor cells. In this work we have studied the effects of direct current (DC) electrical stimulation on neuroblastoma (N2a) and osteoblast (MC3T3) cell lines as a model for nervous and bone tissue regeneration, respectively. We have developed the electronics and encapsulation of a proposed stimulation system and designed a setup and protocol to stimulate cell cultures.

Methods

Cell cultures were subjected to several assays to assess the effects of electrical stimulation on them. N2a cells were analyzed using microscope images and an inmunofluorescence assay, differentiated cells were counted and neurites were measured. MC3T3 cells were subjected to an AlamarBlue assay for viability, ALP activity was measured, and a real time PCR was carried out.

Results

Our results show that electrically stimulated cells had more tendency to differentiate in both cell lines when compared to non-stimulated cultures, paired with a promotion of neurite growth and polarization in N2a cells and an increase in proliferation in MC3T3 cell line.

Conclusions

These results prove the effectiveness of electrical stimulation as a tool for tissue engineering and regenerative medicine, both for neural and bone injuries. Bone progenitor cells submitted to electrical stimulation have a higher tendency to differentiate and proliferate, filling the gaps present in injuries. On the other hand, neuronal progenitor cells differentiate, and their neurites can be polarized to follow the electric field applied.

A Biohybrid Setup for Coupling Biological and Neuromorphic Neural Networks

Developing technologies for coupling neural activity and artificial neural components, is key for advancing neural interfaces and neuroprosthetics. We present a biohybrid experimental setting, where the activity of a biological neural network is coupled to a biomimetic hardware network. The implementation of the hardware network (denoted NeuroSoC) exhibits complex dynamics with a multiplicity of time-scales, emulating 2880 neurons and 12.7 M synapses, designed on a VLSI chip. This network is coupled to a neural network in vitro, where the activities of both the biological and the hardware networks can be recorded, processed, and integrated bidirectionally in real-time. This experimental setup enables an adjustable and well-monitored coupling, while providing access to key functional features of neural networks. We demonstrate the feasibility to functionally couple the two networks and to implement control circuits to modify the biohybrid activity. Overall, we provide an experimental model for neuromorphic-neural interfaces, hopefully to advance the capability to interface with neural activity, and with its irregularities in pathology.

When the Ostrich-Algorithm Fails: Blanking Method Affects Spike Train Statistics

Modern electroceuticals are bound to employ the usage of electrical high frequency (130-180 Hz) stimulation carried out under closed loop control, most prominent in the case of movement disorders. However, particular challenges are faced when electrical recordings of neuronal tissue are carried out during high frequency electrical stimulation, both in-vivo and in-vitro. This stimulation produces undesired artifacts and can render the recorded signal only partially useful. The extent of these artifacts is often reduced by temporarily grounding the recording input during stimulation pulses. In the following study, we quantify the effects of this method, "blanking," on the spike count and spike train statistics. Starting from a theoretical standpoint, we calculate a loss in the absolute number of action potentials, depending on: width of the blanking window, frequency of stimulation, and intrinsic neuronal activity. These calculations were then corroborated by actual high signal to noise ratio (SNR) single cell recordings. We state that, for clinically relevant frequencies of 130 Hz (used for movement disorders) and realistic blanking windows of 2 ms, up to 27% of actual existing spikes are lost. We strongly advice cautioned use of the blanking method when spike rate quantification is attempted.

Impact statement

Blanking (artifact removal by temporarily grounding input), depending on recording parameters, can lead to significant spike loss. Very careful use of blanking circuits is advised.

Simultaneous stimulation and recording of cardiac depolarization enabled by high-frequency stimulation

High-frequency stimulation techniques have been recently proposed for the pacing and control of excitability of cardiac tissues. This paper introduces a system designed specifically for such stimulation, and demonstrates the unique ability to record depolarization events on the same electrode used for stimulation, during the stimulus. Experimental results with HL-1 cardiomyocytes are presented, highlighting key concepts enabled by this system, such as direct strength-duration relationship measurement and beat-to-beat stimulation threshold monitoring following pacing onset or pharmacological modulation.

Characterisation of electrophysiological conduction in cardiomyocyte co-cultures using co-occurrence analysis

Cardiac arrhythmias are disturbances of the electrical conduction pattern in the heart with severe clinical implications. The damage of existing cells or the transplantation of foreign cells may disturb functional conduction pathways and may increase the risk of arrhythmias. Although these conduction disturbances are easily accessible with the human eye, there is no algorithmic method to extract quantitative features that quickly portray the conduction pattern. Here, we show that co-occurrence analysis, a well-established method for feature recognition in texture analysis, provides insightful quantitative information about the uniformity and the homogeneity of an excitation wave. As a first proof-of-principle, we illustrate the potential of co-occurrence analysis by means of conduction patterns of cardiomyocyte-fibroblast co-cultures, generated both in vitro and in silico. To characterise signal propagation in vitro, we perform a conduction analysis of co-cultured murine HL-1 cardiomyocytes and murine 3T3 fibroblasts using microelectrode arrays. To characterise signal propagation in silico, we establish a conduction analysis of co-cultured electrically active, conductive cardiomyocytes and non-conductive fibroblasts using the finite element method. Our results demonstrate that co-occurrence analysis is a powerful tool to create purity-conduction relationships and to quickly quantify conduction patterns in terms of co-occurrence energy and contrast. We anticipate this first preliminary study to be a starting point for more sophisticated analyses of different co-culture systems. In particular, in view of stem cell therapies, we expect co-occurrence analysis to provide valuable quantitative insight into the integration of foreign cells into a functional host system.

A device for separated and reversible co-culture of cardiomyocytes

A novel technique is introduced for patterning and controllably merging two cultures of adherent cells on a microelectrode array (MEA) by separation with a removable physical barrier. The device was first demonstrated by separating two cardiomyocyte populations, which upon merging synchronized electrical activity. Next, two applications of this co-culture device are presented that demonstrate its flexibility as well as outline different metrics to analyze co-cultures. In a differential assay, the device contained two distinct cell cultures of neonatal wild-type and beta-adrenergic receptor (beta-AR) knockout cardiomyocytes and simultaneously exposed them with the beta-AR agonist isoproterenol. The beat rate and action potential amplitude from each cell type displayed different characteristic responses in both unmerged and merged states. This technique can be used to study the role of beta-receptor signaling and how the corresponding cellular response can be modulated by neighboring cells. In the second application, action potential propagation between modeled host and graft cell cultures was shown through the analysis of conduction velocity across the MEA. A co-culture of murine cardiomyocytes (host) and murine skeletal myoblasts (graft) demonstrated functional integration at the boundary, as shown by the progression of synchronous electrical activity propagating from the host into the graft cell populations. However, conduction velocity significantly decreased as the depolarization waves reached the graft region due to a mismatch of inherent cell properties that influence conduction.

Electrically evoked auditory steady state responses in cochlear implant users

Auditory steady state responses are neural potentials in response to repeated auditory stimuli. This study shows that electrically evoked auditory steady state responses (EASSRs) to low-rate pulse trains can be reliably recorded by electrodes placed on the scalp of a cochlear implant (CI) user and separated from the artifacts generated by the electrical stimulation. Response properties are described, and the predictive value of EASSRs for behaviorally hearing thresholds is analyzed. For six users of a Cochlear Nucleus CI, EASSRs to symmetric biphasic pulse trains with rates between 35 and 47 Hz were recorded with seven scalp electrodes. The influence of various stimulus parameters was assessed: pulse rate, stimulus intensity, monopolar or bipolar stimulation mode, and presentation of either one pulse train on one electrode or interleaved pulse trains with different pulse rates on multiple electrodes. To compensate for the electrical artifacts caused by the stimulus pulses and radio frequency transmission, different methods of artifact reduction were employed. The validity of the recorded responses was confirmed by recording on-off responses, determination of response latency across the measured pulse rates, and comparison of amplitude growth of stimulus artifact and response amplitude. For stimulation in the 40 Hz range, response latencies of 35.6 ms (SD = 5.3 ms) were obtained. Responses to multiple simultaneous stimuli on different electrodes can be evoked, and the electrophysiological thresholds determined from EASSR amplitude growth in the 40 Hz range correlate well with behaviorally determined threshold levels for pulse rates of 41 Hz.

Current-Controlled Electrical Point-Source Stimulation of Embryonic Stem Cells

Stem cell therapy is emerging as a promising clinical approach for myocardial repair. However, the interactions between the graft and host, resulting in inconsistent levels of integration, remain largely unknown. In particular, the influence of electrical activity of the surrounding host tissue on graft differentiation and integration is poorly understood. In order to study this influence under controlled conditions, an in vitro system was developed. Electrical pacing of differentiating murine embryonic stem (ES) cells was performed at physiologically relevant levels through direct contact with microelectrodes, simulating the local activation resulting from contact with surrounding electroactive tissue. Cells stimulated with a charged balanced voltage-controlled current source for up to 4 days were analyzed for cardiac and ES cell gene expression using real-time PCR, immunofluorescent imaging, and genome microarray analysis. Results varied between ES cells from three progressive differentiation stages and stimulation amplitudes (nine conditions), indicating a high sensitivity to electrical pacing. Conditions that maximally encouraged cardiomyocyte differentiation were found with Day 7 EBs stimulated at 30 microA. The resulting gene expression included a sixfold increase in troponin-T and a twofold increase in beta-MHCwithout increasing ES cell proliferation marker Nanog. Subsequent genome microarray analysis revealed broad transcriptome changes after pacing. Concurrent to upregulation of mature gene programs including cardiovascular, neurological, and musculoskeletal systems is the apparent downregulation of important self-renewal and pluripotency genes. Overall, a robust system capable of long-term stimulation of ES cells is demonstrated, and specific conditions are outlined that most encourage cardiomyocyte differentiation.

Peptide-functionalized microfabricated structures for improved on-chip neuronal adhesion

Extracellular, high signal-to-noise ratio recordings from electrogenic cells require a tight coupling between the cellular membrane and the recording electrode. Self assembled monolayers (SAMs) of alkanethiols functionalized with peptides were used in combination with micro- and nano-structured features on the sensor surface. This combination of surface chemistry and topography triggers a phagocytosis-like engulfment and ensures tight coupling. In this paper we report the results concerning usage of different SAMs and the influence of the peptide concentration towards cell adhesion and outgrowth. Later on, the optimized peptide functionalized SAMs were applied on micro- and nano-structured sensor surfaces. As a result, phagocytosis-like events could be shown using focused ion beam SEM and confocal fluorescence imaging.

Local electrical stimulation of cultured embryonic cardiomyocytes with sub-micrometer nail structures

In this paper, we demonstrate the feasibility of selective extracellular electrical stimulation at the (sub)cellular level in dissociated cultured cells. Using a CMOS-compatible process, we have fabricated an electrode array with sub-micrometer nail probes. Due to their particular configuration, the nails are strongly engulfed by the cellular membrane. By measuring the calcium signals, we found that electrical stimulation via the micronails activates the cell locally, in a dose-dependent manner, with very low applied currents. The results suggest the applicability of the device in pharmacological or signal propagation studies.

A retrofitted neural recording system with a novel stimulation IC to monitor early neural responses from a stimulating electrode

Extracellular electrical stimulation is increasingly used for in vitro neural experimentation, including brain slices and cultured cells. Although it is desirable to record directly from the stimulating electrode, relatively high stimulation levels make it extremely difficult to record immediately after the stimulation. We have shown that this is feasible by a stimulation system (analog IC) that includes the feature of active electrode discharge. Here, we piggybacked the new IC onto an existing recording amplifier system, making it possible to record neural responses directly from the stimulating channel as early as 3 ms after the stimulation. We used the retrofitted recording system to stimulate and record from dissociated hippocampal neurons in culture. This new strategy of retrofitting an existing system is a simple but attractive approach for instrumentation designers interested in adding a new feature for extracellular recording without replacing already existing recording systems.

Temporal resolution of stimulation threshold: a tool for electrophysiologic analysis

Electrical stimulation of cardiac cultures with closed-loop control permits the determination of threshold in real time. The temporal response of stimulation threshold and underlying cell membrane excitability is valuable information for understanding the complex electrophysiologic processes within cardiac cells and can aid in understanding the mechanisms and effects of pharmaceuticals or other stimuli. This work presents the temporal response of stimulation threshold measured using HL-1 cardiac myocytes when exposed to changes in temperature and extracellular potassium concentration. These changes mimic systemic alteration of excitability and conditions that can result from ischemia in the heart. The results demonstrate the efficacy of stimulation threshold as a physiologic indicator and illustrate transient effects with both fast and slow time constants that can be resolved using a system that determines stimulation threshold in real time.

Cardiac differentiation of embryonic stem cells with point-source electrical stimulation

The use of pluripotent stem cells as a means to repair damaged heart tissue has recently emerged as a promising, yet controversial therapy. Despite the different approaches and the variety of cell types used, many of these procedures have been met with mixed success. The lack of understanding of the differentiation and integration process, notably with respect to electrical signaling, significantly hampers the development of these therapies. A system was thus developed allowing the use of point source electrical stimulation on embryonic stem (ES) cells to study the effect of physiologically-relevant electrical stimulus. When modulating the amplitude of the stimulus over various differentiation stages of embryonic stem cells, differences in the proportions of cardiomyocytes to embryonic stem cells were observed through quantitative PCR. The use of this technique might have larger applications in understanding molecular pathways towards the regeneration process.

Microfluidic patterning for fabrication of contractile cardiac organoids

The development of in vitro methods of engineering three-dimensional cardiac tissues can be useful for tissue replacement, diagnostics and drug discovery. Here, we introduce the use of patterned hyaluronic acid (HA) substrates generated using microfluidic patterning as a method of fabricating 3D cardiac organoids. HA micropatterns served as inductive templates for organoid assembly. Upon seeding, cardiomyocytes elongated and aligned along the pattern direction attaching preferentially to the glass substrate and the interface between HA patterns and glass substrate. After 3 days in culture, the linearly aligned myocytes detached from the surface and formed contractile cardiac organoids. The procedure can be utilized to simply, rapidly and inexpensively create in vitro cardiac tissue models.