Epithelial cells line many of the hollow organs in our bodies. A single layer of epithelium is composed of tightly packed individual epithelial cells that are connected to each other by protein complexes called tight junctions. Epithelia act as barriers between the external environment and the internal milieu. This may be the airway epithelium, guarding the body against airborne pathogens, or the intestinal epithelium, protecting the body against opportunistic microbes in the gut. Epithelial barrier function is achieved by limiting the transport and flow of molecules between and through epithelial cells, from the lumen of organs into the bloodstream. Transport through the epithelial layer can occur by one of two processes: transcellular transport (transit through the cell through the membrane) or paracellular transport (transit between adjacent epithelial cells through tight junctions).
Epithelial cells are often referred to as “polarized,” which means they have distinct membrane compartments, apical and basolateral. The apical membrane faces the lumen and is the “top” of the cell, while the basolateral membrane faces the internal environment and encompasses the sides and “bottom” of the cell. The basolateral and apical membranes of epithelial cells are divided and maintained by the presence of tight junctions, and often express different transmembrane proteins and transporters.
Air-Liquid Interface Culture Systems Mimic the Airway Environment
Epithelial cells typically do not polarize when grown on plastic, so researchers use special permeable culture inserts that allow media to contact cells on both apical and basolateral sides. This mimics the environment in the body and stimulates cells to differentiate and polarize. Polarization and epithelial “tightness” is assessed by measuring transepithelial electrical resistance, which measures electrical resistance across the epithelial monolayer; a tighter epithelium will have greater resistance.
For studies of airway epithelium, scientists have developed a specialized polarized culture system in which cells are grown in an air-liquid interface to mimic the airway. To use an air-liquid interface, airway epithelial cells are grown on permeable inserts and medium is only added to the basolateral side of the dish, leaving the apical surface of cells to contact air.Multiple studies have reported the use of Lifeline® airway epithelial cells in air-liquid interfaces to study respiratory virus infection. Lifeline® human bronchial/tracheal epithelial cells grown in an air-liquid interface were used in a 2013 study by Hauser et al. to demonstrate that compared to human respiratory epithelium, swine respiratory epithelial cells have a reduced antiviral response. They concluded this might help explain the increased ability of influenza viruses from multiple origins (i.e. – avian or human) to recombine in swine.
Additionally, Dominguez et al. published a 2014 study that used Lifeline® human tracheal/bronchial epithelial cells grown in an air-liquid interface to isolate, propagate, sequence, and classify patient-derived samples of human coronavirus. Finally, in a 2015 study, Qian and colleagues used Lifeline® human tracheal/bronchial epithelial cells grown in an air-liquid interface to identify the receptor-binding domain in the spike (S) protein of the human beta-coronavirus HKU1, which regulates binding of virus to its host cell, initiating infection. These reports clearly demonstrate that Lifeline® bronchial/tracheal cells cultured in an air-liquid interface provide an excellent in vitro model of the human airway.
Lifeline® provides multiple human lung-airway epithelial cells, including:
Lifeline® Human Bronchial Epithelial Cells in Intestinal Research
The cystic fibrosis transmembrane conductance regulator (CFTR) is an ion transporter found on the apical membrane of epithelial cells that controls the flow of chloride ions into and out of the cell. The role of the CFTR is tightly linked to water transport, which is particularly important in the airway. In fact, cystic fibrosis is a disease associated with CFTR mutations that render the protein dysfunctional. This impedes the regular flow of ions and water across cell membranes, leading to a loss of water from the airway lumen, and a buildup of thick mucus. The airway epithelium is similar to the intestinal epithelium, which also secretes mucus to facilitate the transit of feces through the tract. Constipation is a common condition that often results from a lack of water, causing dry, hard stools that are difficult to pass. Cancer patients and patients needing opioid-based pain management often suffer constipation as a side effect, which can lead to cessation of opioid-based therapies.
In a paper from 2017, Harada and colleagues set out to investigate the mechanism of mashiningan (MNG), which is used to treat constipation. The researchers tested whether MNG could alleviate constipation in normal rats and rats treated with codeine phosphate (CPH), an opioid drug. They first found that MNG treatment increased fecal weight in normal rats. Next, they demonstrated that MNG increased fecal count and weight in rats treated with CPH that suffered from decreased fecal counts and weight. Additionally, MNG softened feces from rats treated with CPH, who rarely passed soft feces. To determine the mechanism by which CPH-induced constipation is relieved by MNG, the group measured small intestinal fluid secretion in fasted rats, which was significantly increased in a MNG dose-dependent manner.
Interestingly, addition of a CFTR inhibitor blocked MNG-induced fluid secretion, suggesting MNG activates CFTR. To dissect the mechanism further, the researchers grew Lifeline® human bronchial/tracheal epithelial cells in an air-liquid interface and treated them with MNG with or without a CFTR inhibitor. They evaluated CFTR activity using an Ussing chamber assay, which measures ion flux through electrical current. They found that treatment with MNG resulted in a negative current, indicating activation of CFTR and release of chloride (which is negatively charged) into the medium; addition of a CFTR inhibitor reversed this effect and resulted in a positive current. Together, these results suggest that MNG alleviates opioid-induced constipation by increasing fluid secretion through activation of CFTR.
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