Background

The intestinal epithelium is represented as a cellular monolayer that separates the luminal content from the rest of the body [1]. Apart from enabling digestion and absorption of food, it serves as a crucial barrier for warding off potentially pathogenic microbes [1]. Damage and impairment of the gut barrier are observed through the course of various intestinal diseases such as necrotizing enterocolitis and inflammatory bowel disease [2,3].

A major boost to the in vitro modeling of the intestinal epithelial barrier came with the emergence of organoid technology. Organoids are self-organizing, three-dimensional (3D) structures that are grown in vitro from stem cells [4]. They recapitulate many structural and functional aspects of their parent organ more accurately than the commonly used two-dimensional (2D) cell lines [4]. Therefore, organoids have become a frequently used model among researchers. Since it remains challenging to study epithelial barrier formation in 3D structures, several studies cultured the intestinal organoids as a monolayer in a Transwell system [1].

The Transwell system is the most commonly used in vitro model for intestinal barrier research [5,6]. It consists of a porous cell culture insert that can be placed in a traditional cell culture well plate [5]. The gut epithelial cells are grown on the permeable membrane of the insert to create a cell monolayer with a luminal and basolateral compartment. Evaluation of barrier formation is assessed by measuring the transepithelial electrical resistance (TEER) with electrodes placed on either side of the membrane [5,6]. While measuring TEER with the typical handheld chopstick electrode epithelial voltohmmeter (EVOM) device remains the gold standard for assessing the integrity of barrier models, this technique requires removal of the cultures from the incubator to test each well individually. Such approach might disrupt the cell layers and, in addition, variations in electrode positioning could hinder reproducibility of measurement because of non-uniform electric field created by the chopstick electrodes [6].

An alternative to EVOM is electrical cell–substrate impedance sensing (ECIS). ECIS uses the same principle as the EVOM, except that the electrodes are integrated into the bottom of the well on which the cells are grown [6]. The presence of the electrodes in the wells allows cells to attach and proliferate directly on top of the electrodes, resulting in more localized and sensitive impedance measurements of the cell barrier. The ECIS enables impedance measurements at a broad scale of electrical frequencies, ranging from 62.5 Hz to 64 kHz. In addition, depending on the electrode placement and size, the ECIS can be used to determine additional properties of the epithelial cell layer such as cell attachment, migration, and proliferation [6]. Studying cell attachment, proliferation, and spreading yields insights into fundamental cellular behaviors, enhancing our understanding of basic biological processes in health and disease. Apart from monitoring cell barrier formation, the cells can be subjected to (lethal) electrical currents to inflict cellular damage [7]. The ECIS is therefore a potentially interesting cell-based system to study gut barrier formation and to screen for drugs capable of resolving cell damage and achieving higher mucosal healing rates. Here, we show that the ECIS technology can be successfully applied on gut organoid 2D monolayers.