The Trans-Epithelial/Endothelial Electrical Resistance (TEER) assay is a technique used in cell biology to quantitatively assess the integrity of biological barriers. TEER measures the electrical resistance across a layer of cells, which serves as a direct indicator of the barrier’s tightness or “leakiness.” This non-invasive method allows researchers to monitor the health and function of cell layers in real-time within a controlled laboratory setting.
A high TEER value signifies a tight, well-formed barrier with minimal leakage, while a low value suggests a compromised barrier. The resistance value is directly proportional to how well the cell layer is sealed against the movement of ions and molecules. This makes the assay a standard tool for verifying the formation of a functional barrier before proceeding with complex experiments.
Biological Barriers and Tight Junctions
Biological barriers are composed of continuous sheets of cells, known as epithelia or endothelia, that line surfaces throughout the body, separating compartments. Epithelial cells cover external surfaces and internal cavities, such as the gut lining and the lungs, while endothelial cells form the inner lining of blood vessels. These cellular sheets are selectively permeable, allowing necessary nutrients to pass while blocking pathogens and toxins.
The structural integrity of these barriers is maintained by a network of protein structures that physically connect adjacent cells. Tight junctions, also known as zonulae occludentes, function as the primary gatekeepers. They form a continuous, belt-like seal around the cell’s circumference, effectively closing the space between neighboring cells.
The tight junction seal is a dynamic, regulated barrier that controls the movement of substances through the paracellular pathway. Proteins such as claudins and occludin are the main components of these junctions, forming strands that act like a molecular zipper. The specific combination of these proteins determines the selectivity and overall tightness of the barrier.
Measuring Barrier Integrity
The TEER assay is based on the principle of electrical resistance, which is the opposition to the flow of an electrical current. In the laboratory, barrier-forming cells are grown as a single layer on a permeable membrane, typically a Transwell insert. This setup creates two distinct compartments separated by the cell monolayer, mimicking physiological separation, such as the apical side and the basolateral side.
To measure resistance, a weak alternating electrical current is applied across the cell monolayer using specialized electrodes placed in the fluid on both sides. The instrument measures the voltage drop and uses Ohm’s Law to calculate the resistance of the cell layer. The resistance contributed by the underlying fluid and the porous membrane is subtracted to isolate the resistance of the cell monolayer alone.
The resulting TEER value is expressed in the standardized unit of Ohm multiplied by square centimeter ($\Omega \cdot \text{cm}^2$). This unit normalizes the measurement to account for the total area of the cell layer, allowing for comparisons between different experiments and cell types. A higher calculated value indicates that the cell layer is strongly resisting the flow of ions, confirming an intact tight junction barrier.
The non-destructive nature of the electrical measurement is a major advantage of the TEER assay, permitting continuous, real-time monitoring of the same cell layer over hours or days. Researchers can track the development of the barrier as cells grow and mature, seeing the resistance value rise until it reaches a stable plateau, indicating confluence. Monitoring dynamic changes is important when testing the effects of a compound, where a sudden drop in TEER flags a negative impact on the barrier’s function.
Key Applications in Research
The ability to accurately measure barrier integrity makes the TEER assay an indispensable tool across several fields of biomedical research.
Drug Screening and Absorption
In pharmaceutical research, TEER is used extensively in drug screening to predict how well a potential medication will be absorbed by the body. Researchers assess drug transport across models of the intestinal barrier, determining if a compound can effectively pass through the gut lining to reach the bloodstream. The assay is also applied to models of the blood-brain barrier (BBB) to evaluate a drug’s ability to enter the central nervous system. A high TEER value in a BBB model suggests low permeability, indicating a poor candidate drug for neurological conditions.
Toxicology Studies
The assay is a standard method in toxicology studies, where scientists test how various environmental toxins or chemicals compromise epithelial layers, such as those of the lungs, skin, or intestines. A substance that causes a significant drop in TEER is identified as an agent that disrupts cell-cell adhesion and barrier function.
Disease Modeling
The TEER assay is central to modeling human diseases where barrier dysfunction is a defining feature. Conditions such as inflammatory bowel diseases are characterized by a compromised intestinal epithelial barrier. Researchers use TEER to track how inflammatory molecules or pathogenic bacteria decrease the resistance of the cell monolayer, helping to understand disease progression and mechanisms. This allows for the testing of new therapeutic strategies, such as probiotics or anti-inflammatory drugs, by observing if they can restore the TEER value to a healthy range.