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What is a microelectrode array (MEA)?

Microelectrode arrays (MEA), also known as multielectrode arrays, contain a grid of tightly spaced electrodes embedded in the culture surface of the well. Electrically active cells, such as cardiomyocytes, are cultured on top of the electrodes. When neurons fire action potentials, the electrodes measure the extracellular voltage on a microsecond timescale. As the cells attach and connect with one another, an MEA can simultaneously sample from many locations across the culture to detect propagation and synchronization of cardiac activity across the syncytium.

That’s it, an electrode and your cells. No dyes, no incubation steps, no perfusion, no positioning things just-so; just your cells in a well. Because the electrodes are extracellular (they do not poke into the cells), the recording is noninvasive and does not alter the behavior of the cells, you can measure the activity of your culture for seconds or even months!

An MEA of 64 electrodes embedded in the substate at the bottom of a well.

Cardiomyocytes attach to the array and form a network. The microelectrodes detect the action potentials fired as well as their propagation across the network.

 

Heartbeats in a dish

When cardiomyocytes are cultured on top of an MEA, they attach and connect to form a spontaneously beating sheet of cells, called a syncytium. When one cardiomyocyte fires an action potential, the electrical activity propagates across the syncytium causing each cell to fire and then contract. The electrodes detect each individual action potential and contraction, as well as the propagation of this activity across the array.

The propagating electrical signal is detected by the electrodes as an extracellular field potential. The field potential derives from the underlying cardiac action potential, but more closely resembles a clinical electrocardiogram (ECG) signal. The initial depolarization phase is seen as a sharp spike, similar to the QRS complex, and the slow repolarization is seen as a small slow spike, like a T-wave. The time from the depolarization to repolarization is termed the field potential duration (FPD) and is a key metric in predictive cardiotoxicity screening assays.

While most record the cardiac field potential, the Maestro Pro and Edge MEA systems can also measure local extracellular action potentials, or LEAP. LEAP induction increases the coupling between the microelectrodes and the cardiomyocytes, transforming the extracellular signal from a field potential to an action potential. LEAP provides additional and complementary metrics such as rise time, action potential duration (APD), triangulation, and automated early after depolarization (EAD) detection.

The cardiac action potential propagates from cell to cell across the syncytium. The MEA detects this activity as an extracellular field potential, which closely resembles the clinical ECG.

Do more with multiwell  

Axion BioSystems offers multiwell plates at many throughputs, from 6-wells to 96-wells, with an MEA embedded in the bottom of each well. Each well represents its own unique cell culture and conditions, creating up to 96 experiments on one plate. Multiwell MEA allows you to study complex human biology in a dish, from a single cell firing to network activity, across many conditions and cell types at once.

Contractility: Using array-based impedance

Every beat of the heart is characterized by a contraction of the heart that pumps the blood out to the body. When cardiomycoytes are cultured on top of electrodes, they form a spontaneously beating syncytium. With each beat, the cells contract and relax, changing their shape and coverage over the electrodes. These changes can be measured as a change in impedance, or contractility.

Contractility is often used to characterize the mechanical properties of induced pluripotent stem cell-derived cardiomyocytes and to detect the effects of compounds on cardiac contractile function (e.g. inotropes). Measures such as beat amplitude, beat period, and excitation-contraction delay reveal changes in contractile function due to cardiomyocyte maturation or compound addition.

The Maestro Pro and Edge provide key parameters of cardiomyocyte contractile function, including beat amplitude, beat period, and excitation-contraction delay. The Maestro systems are the only platforms that can measure the coupling between the electrical (field potential) and contractile signals from the same cells over the same electrode at the same time.

Most contractility platforms measure contractility from one or two large electrodes. The Maestro Pro and Edge use an array of microelectrodes to measure high-resolution contractility from multiple locations across the syncytium. Array-based contractility can be used to track propagation and characterize variability. In addition, by simultaneously monitoring both the field potential and the contraction from the same electrode, the Maestro provides highly precise measurements of beat timing and excitation-contraction delays.

(A) Contraction and relaxation are detected as an increase and decrease in contractility (gray arrows). (B) The microelectrode array detects regions that are contracting while others are being stretched. (C) This pattern is represented as a contractility map, where the relative size of each circle indicates whether local cells are contracting or being stretched.

Contractility in 3D

Using an array of microelectrodes for contractility offers many advantages, including assessing variability across the syncytium and variations in cell culture coverage. Recording from microelectrodes enables advanced applications, such as measuring contractility from multiple 3D cardiac spheroids in the same well. A larger electrode would either smear these signals or fail to detect them at all. The microelectrodes provide high spatial resolution recordings to detect the distinct beating patterns in each area of the well.

Seven spheroids of human induced pluripotent stem cell-derived cardiomyocytes were deposited in a CytoView MEA 6-well plate. The spheroids beat independently, exhibiting unique contractile patterns that were detected and distinguished using array-based contractility on the Maestro.

Optogenetics: Using light to control cells

Optogenetics is a technique that involves the use of light to control cell function. Cells are first genetically modified to express light-sensitive ion channels, called opsins. Then, light can be used to activate the opsin. The most well-known opsins are light-gated ion channels that can control the excitability of the cell membrane. When activated by the opsin-specific wavelength of light, the channels open allowing ions to flow across the cell membrane to either excite or inhibit the cell. Optogenetics enables precise control over a targeted cell population.

Many opsins, many options

Since the first microbial opsin was introduced into mammalian neurons in 2005, many different opsins have been used to control the excitability of electroactive cells such as neurons and cardiomyocytes. Each opsin is sensitive to a specific wavelength range, or color of light and induces a precise biological event.

Channelrhodopsin (ChR2), for example, is activated by blue light. When ChR2 opens, positive cations (like sodium and calcium), flow into the cell, depolarizing or exciting the cell. In contrast, halorhodopsin and archaerhodopsin both inhibit cell excitability by hyperpolarizing the cell in response to yellow or green light, respectively. With optogenetics, you can turn on and off cells like a light switch.

The timing of these light-activated events is fast, facilitating highly precise control. First generation opsins, such as channelrhodopsin, open and close in milliseconds, ideal for kicking off an action potential. Second generation opsins have fined tuned kinetics for even faster, more precise control or slower, longer-lasting inhibition. For example, step-function opsins stay open until another pulse of light switches them off.

Optogenetics can precisely control which cells are turned on or off by employing different gene promoters for opsin expression. Opsins can be expressed in all neurons or used to control specific subpopulations. Unlike electrical stimulation, which excites all nearby cells, optical stimulation can be finely targeted to the cells expressing the opsins responsive to a narrow band of light wavelengths.

In summary, optogenetics is a powerful toolbox for precise control over targeted cell populations at fast time scales. Superior spatial and temporal control, reversibility, and easy stimulus delivery make exploring complex biology simpler than ever before.

More than just ion channels

As the field has advanced, opsins have been used to control more than just ion flow. Light-activated gene expression with light-inducible transcription factors can control the proteins made by cells. The combination of optogenetics with CRISPR provides even greater control over CRISPR/Cas9 gene editing.

Opsins have also been incorporated into many biochemical and intracellular signaling pathways to control key protein functions. MAPK and PI3K pathways, Rho family GTPase activation, apoptosis, and protein trafficking can now all be precisely controlled by light.

Shining light in vitro

Sophisticated biology like optogenetics demands sophisticated technology to explore it. In vitro technology relied on single wavelength lasers and custom lab-specific tools while many in vivo technologies were quickly developed for optical stimulation. The Lumos Optical Stimulation system is the first-of-its-kind multiwell optical stimulator with the ability to deliver up to four wavelengths of light per well with microsecond precision. From controlling the excitability of your neurons to pacing the beating of your cardiomyocytes, discover how the Lumos and optogenetics can revolutionize your assay.