What is TEER? – Trans-Epithelial Electrical Resistance Assay

TEER stands for Trans-Epithelial Electrical Resistance assay, a non-invasive method to measure the electrical resistance of cell and tissue barriers in vitro.

Tissue barriers support the homeostasis of vital tissue and organ functions by precisely regulating the transport of ions, biomolecules, drugs, cells, and microbial organisms (Nazari 2023 et al.). Epithelial cells are the major building block for many organs such as skin, lung, kidney, liver and intestine. The epithelium and endothelium form a protective barrier, regulating transport in and out of the cells, amongst other important biological functions.

TEER assays are an excellent method to determine barrier, and thereby epithelial and endothelial tissue, integrity. There is plenty to learn on this topic, however, in this article we will focus on:

What is a TEER assay?

How is a TEER measurement performed?

How to calculate TEER?

Which variables can affect TEER measurements?

But most importantly:

How can TEER measurements help you with your research?

What is a TEER assay?

A Trans-Epithelial Electrical Resistance (TEER) assay, is a method to determine the electrical resistance of a cellular tissue. Hereby, a current flow is passed through the cell layer to determine the electrical resistance.

“[Electrical resistance is measured in ohm and]… is equal to the resistance of a circuit in which a potential difference of one volt produces a current of one ampere (1Ω = 1 V/A); or, the resistance in which one watt of power is dissipated when one ampere flows through it” (Britannica).

A trademark of TEER assays is that they can measure the electrical resistance of living cells in real-time, non-invasive and label-free which makes it an excellent technique to monitor the cellular barrier integrity. TEER values increase upon rising cell proliferation rates and formation of functional cell-cell junctional complexes resulting in maximum confluency of the cell layer (almost gap free coverage of cultivation surface with adherent cells). Similarly, if TEER values decrease it can indicate a disturbance or damage of the barrier through injury, chemical exposure, or disease development.

How is a TEER measurement performed?

There are two approaches to determine TEER. They are referred to as Ohm’s law method and impedance spectroscopy (Holzreuter 2024 et.al.). Common for both is the application of a current and measurement of the resulting electrical resistance across a single or multiple tissue layers.

The chopstick method is a traditional TEER assay by which electrodes are places in the cell culture medium on each side of the confluent cell layer as shown in the figure below.

An alternating current (AC) between the two electrodes is induced, and the electrical resistance is measured by using a Voltohmmeter. The alternating current is used instead of a direct current (DC) to avoid damaging of cells and electrodes due to charging effects (Scrinivasan 2015 et al.). The lower the current flows, the lower, the TEER value and thereby the thickness and/or confluence of the cell layer.

To ensure full functionality of the TEER assay, cells need to be cultured in inserts containing semi-permeable membranes allowing for transport of cell culture media and ions. TEER measurements are then performed between the electrodes in the upper and the lower compartment.

Despite the easy operability, the traditional chopstick method is associated with low throughput, high variability and is not suitable for integration into organ-on-chip platforms due to electrode size and instability leading to inconsistent and erratic data.

Chip platforms provided by Dynamic42 typically consist of two perfused compartments (top and bottom) separated by a porous membrane on which different cell types depending on the organ model can be seeded and cultured independently. The base body of the chip is sealed by a bonding foil on the top and bottom side. Within this foil the gold electrodes are spotted on the inside in the upper and lower chip chambers as shown in the Figure below.

The direct integration of immobilized electrodes within the chip and in close proximity to the tissue reduces signal noise caused by electrode motion as observed in the chopstick method. This also decreases the risk for technical errors that could lead to cell damage, as electrodes are already immobilized and do not need to be placed in the culture system.

How to calculate TEER?

TEER is typically expressed in Ω × cm². To calculate TEER, the surface area of the culture chamber (in cm²) is multiplied by the tissue resistance (R_Tissue) (Michigan Medicine). The reason for doing so is that the resistance of the measured tissue is inversely proportional to the culture area. Therefore, one multiplies area with resistance to produce values that are comparable between different culture systems (Holzreuter 2024 et al.).

The tissue resistance is the total resistance measured with cultured cells (R_Total) subtracted by the resistance of a blank culture chamber (R_Blank) containing cell culture media.

See an example below for better illustration:

Vessel – 6-well Transwell

*Surface area (M_Area) = 5.76 cm²

R_Total = 2200 Ω

R_Blank = 200 Ω

R_Tissue = R_Total – R_Blank = 2200 Ω – 200 Ω = 2000 Ω

TEER = R_Tissue x M_Area = 2000 Ω * 5.76 cm² = 11,520 Ω*cm²

If you need reference values for your TEER measurements, please take a look at table 1, 2 and 3 in Scrinivasan 2015 et al., which contain a comprehensive summary of TEER values in blood-brain-barrier (BBB), gastrointestinal tract and pulmonary models, respectively.

*Please note values given are for demonstration purposes only and might vary depending on the supplier.

Which variables can affect TEER measurements?

Variations in TEER values can arise due to factors such as changing environmental factors like temperature, medium formulation, vibrations, position of electrodes and passage number of cells (cellqart).

Temperature deviations can drastically alter TEER measurements, as values increase at lower temperatures and decrease at higher ones. Therefore, it’s highly recommended to perform measurements inside an incubator maintaining constant temperature of 37°C. If the TEER assay needs to be done at room temperature, equilibration of temperature prior to TEER measurements is mandatory. Interestingly, Blume 2009 et al. has devised a mathematical method to temperature correct TEER values, in an experiment using Cancer coli-2 (Caco-2)and Immortal Human Pancreatic Duct Epithelial (HPDE)  cells, to solve this issue.

Specifically, the chop stick method is very sensitive to accurate placement of the electrodes as different locations can lead to high variability of the values measured.

Besides those environmental factors, the properties of the model itself can influence the resulting TEER values. Specifically, shear and mechanical stress as well as interactions with other cells types can drastically alter results (Holzreuter 2024 et al.). Which is why the choice of tissue or organ model is just as important as choice of measurement method to receive values replicating in vivo conditions.

How TEER measurement can help your research

The primary advantage of TEER assays is the non-invasive monitoring of living cells in real-time at different developmental stages over a desired time frame. Furthermore, TEER values are excellent indicators of barrier integrity, which is important for transport of drugs or chemicals. And most importantly, TEER is much more sensitive than other methods such as permeable, fluorescent dyes (e.g. FITC-Dextrane).

TEER assays in drug development

TEER measurements of epithelial and endothelial layers are of special interest in drug development to predict mechanisms of drug-induced disruption of cell-cell contacts or cell death leading to enhanced permeability and as a quality parameter to monitor stable cell growth and confluency. It furthermore aids to understand disease, injuries and drugs that are affecting the barrier tissue (Scrinivasan 2015 et al.). TEER assays are also of great interest for infection processes and translocation of pathogens in the lung and intestine.

TEER assays in BBB models

Whereas epithelial and endothelial layers can be found in a lot of tissue and organ models, barrier measurements play a special role for blood-brain barrier (BBB) models. The most crucial function of the BBB is to protect the brain from toxic or otherwise harmful substances circulating in the blood. It forms a semi-permeable membrane that regulates molecule and ion movement (Dotiwala 2023 et al.). Indented to protect the brain, this barrier function poses a challenge to drug development as it blocks the transport of large therapeutics (> 1kDa). Specifically, neurotherapeutics comprised of proteins and peptides (500-1000 Da) are affected by this (Scrinivasan 2015 et al.). This demonstrates why the development of BBB models that allow for the measurement of barrier function with TEER is of great importance.

TEER assays in intestinal models

The intestine forms the largest barrier in the human body towards its external environment. On one hand, it must selectively take up nutrients from the intestinal lumen and on the other impede the invasion of potentially harmful organisms or molecules (Assimakopoulos 2018 et al.). TEER assays are widely used to understand the intestinal barrier function for research topics such as:

Mechanisms of gastrointestinal protection by probiotic bacteria (Yuan 2020 et al.)

Disruption of the intestinal epithelial barrier by mucosal inflammation (Lechuga 2023 et al.)

The effects of aging on intestinal barrier function in humans (Wilms 2020 et al.)

TEER assays in lung models

Similarly to the intestine, the lung forms an interface to the external environment. It thereby serves as a barrier but also forms a potential entry side for pathogens like viruses and bacteria as well as environmental factors (toxins, air pollutants, allergens) (Herminghaus 2022 et al.).

Differences in the alveolar epithelial barrier integrity in vitro can be detected using TEER assays in lung models and thereby aid in understanding infection and disease mechanisms leading to susceptibility of the lung. (Frank 2012 et al.).

Summary

Tissue barriers are a vital part of many organ structures such as skin, lung, kidney, liver and intestine. Cellular compartmentalization and zonation support the maintenance of homeostasis by separating organs structures from their surrounding environment and enabling selective transport of molecules across the barrier. Characterization of this barrier is best achieved with TEER assays integrated in organ-on-chip models posing a non-invasive and fast way of determining barrier integrity. This in vitro method shows particular importance for the characterization of barrier models for use in drug development, disease modeling or infection studies.  Such models enable perfusion of the organ model for a physiological development of the barrier, and they offer a range of technical advantages such as continues, real-time, non-invasive measurement of TEER and minimal signal noise due to electrodes being immobilized in the chip.

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