There are many varieties of intercellular junctions, which are expressed to a greater or lesser extent depending on cell type. Three of the main categories of these, present on most epithelial and endothelial cells, are tight junctions, adherens junctions, and gap junctions. They are differentiated by both structure and function.

act as gates between cell surfaces, and are extremely dependent on cell polarity. Depending on subtype, they can be composed of varying proteins, such as claudin or occludin. Anchored within and through the plasma membrane, tight junctions do not directly transfer material between cells, but they are essential for protein signaling and intercellular crosstalk via protein-protein interactions. They also help regulate ion and other small molecular transfer between cells that provide a basis for cellular signaling. These signals control not only standard cell behavior such as migration, but also lifecycle actions like cell proliferation, differentiation, and death.

are similar to tight junctions, as they are both anchored within the plasma membrane and transmit signals but not materials between cells. Adherens junctions are made primarily of E-cadherin transmembrane protein, and provide not only adhesive contacts for epithelial cells, but can bind to intracellular catenins. This facilitates the transfer of signals to organize the cytoskeletal structure as well as intracellular regulation of activities as important as gene transcription.

in particular are unique in that they are made of two interdocked cylinders made of connexin. Six of these molecules individually make up what is known as a hemichannel, which span the plasma membrane of each cell, facilitating easy transfer of small molecules when they meet and interlock to form a complete intercellular channel.

Gap junctions are specialized intercellular membrane channels constituted with connexin that selectively facilitate the passage of small molecules of <1.2 kD across cells. They are tightly regulated by voltage, growth factors, cAMP, and retinoids, and they are modulated by phosphorylation. During cellular membrane depolarization (>+20mV) the channels will be open, but closed otherwise. Gap junctions have been implicated in inflammatory diseases and tumor behavior, and severe dysfunction results in cell death and multiple problems with the organism as a whole.

Since gap junctions transfer so many important molecular types (including second messengers as well as more typical yet essential ions) they are central in multiple regulatory cellular communication pathways that regulate cellular and tissue homeostasis. This importance is broadly termed gap junctional intercellular communication (GJIC) and has implications up to and including organ health as well as cell behavior and maintenance.

Simplicity is the key to experimental setup. The cell population is divided evenly, with one population incubated with a low-toxicity cytoplasmic dye. This dye will be contained within those living cells. The other population will be stained with a plasma membrane dye in an alternative color. A wide spectral separation between the colors is ideal, to minimize signal overlap. After washing and mixing, any of a variety of experimental stimuli can be applied (or simply a period of time will be permitted to pass) and the mixed population of cells can be either analyzed via flow cytometry or simply visualized with a fluorescence microscope.

The possible results can be grouped into four possibilities: double-positive cells showing both colors of dye are those that have exchanged cytoplasmic material via gap junction. Double-negative cells (no staining at all) are dead, and are a useful control group as well as a measure of the overall health of the population. Other populations will be stained only with the colors of the dyes they were incubated with, and these single-signal cells are then used to compare to the double-positive cells, with that proportion being a determination of gap junction activity.

Exemplifying this type of noninvasive gap junction investigation, the Cell Meter™ Fluorescence Gap Junction Tracing Kit provides a reliable and robust assay for the in vitro determination of gap junction function. The cell population under study is divided such that one fraction is loaded with a lipophilic cell plasma membrane permeable dye, Calcein UltraGreen™ AM, that is hydrolyzed upon cellular uptake by cytoplasmic esterases to yield Calcein UltraGreen, a highly fluorescent and well-retained and membrane-impermeable molecule.

Calceins are a fairly large family of dyes commonly used to determine cell viability and plasma membrane integrity. As a nearly neutral substrate, the am esters of these dyes can diffuse freely and noninvasively across the plasma membrane and permeate the cytosol of live cells. Once inside, cytosolic esterases in metabolically active cells rapidly hydrolyze the non-fluorescent calcein AM substrates into calcein products retained by cells with intact plasma membranes. Since dead cells lack esterase activity, only viable cells are labeled. Compared to other live-cell labeling reagents, such as BCECF AM or carboxyfluorescein diacetate, the brighter fluorescence, photostability, and low cytotoxicity of calcein AM is advantageous for a variety of studies, including cell tracking, cell adhesion, chemotaxis,multidrug resistance, cell viability, apoptosis, and cytotoxicity. Many colors are available, although the green spectra is the most commonly employed, for this kit as well as for other similar ones.

The other fraction is loaded DiD, which is a lipophilic membrane dye that diffuses laterally to stain the entire cell membrane in deep red fluorescence upon incorporation into membranes. DiI, DiO, DiD and DiR dyes are a family of lipophilic fluorescent stains for labeling membranes and other hydrophobic structures. The fluorescence of these environment-sensitive dyes is greatly enhanced when incorporated into membranes or bound to lipophilic biomolecules such as proteins although they are weakly fluorescent in water. They have high extinction coefficients, polarity-dependent fluorescence and short excited-state lifetimes. Once applied to cells, these dyes diffuse laterally within the cellular plasma membranes, resulting in even staining of the entire cell at their optimal concentrations. Among them DiD is well excited by the 633 nm He-Ne laser, and has much longer excitation and emission wavelengths than those of DiI, providing a valuable alternative for labeling cells and tissues that have significant intrinsic fluorescence.

Once the fractions have been loaded sufficiently according to the experimental cell type, the two fractions are mixed and incubated under coculture conditions. Calcein UltraGreen is transferred to the DiD-stained cells through gap junctions. The assessment of this uptake can be monitored by fluorescence imaging or flow cytometry.

The following example protocols can be used as a guideline and should be optimized according to experimental needs.

For flow cytometric analysis, ensure that the machine is fully cleaned and calibrated prior to beginning the procedure.

Once cells are in suspension or for cells in suspension, wash cells twice with DPBS or buffer of your choice. Resuspend cells in HHBS or DPBS or buffer of your choice and measure the response with 530/30 nm filter (FITC channel) and 660/20 nm filter (APC channel).

The success of the procedure will dictate how the data should be gated. Clear groupings indicate an ideal experiment, but a lack of clearly separated populations strongly implies one or more issues. As is typical for most cell activity experiments, use negative and positive controls to calibrate and discern meaningful data.

Monitor the cells with a fluorescence microscope using the FITC and Cy5 filter sets. In most cases, standard settings will be sufficient, but optimize as needed.

Most gap junction assays, including the one used here to demonstrate, have an incubation timeframe that has been tested and optimized to provide enough time for cells to begin displaying double positive signal (cells that will generate both green and red fluorescence) due to cytoplasmic interchange via gap junctions. Cell populations are likely to continue to increasingly show this double positive trend as time goes on, although cell type and experimental design will affect when this tendency will plateau and/or begin to decrease. The image above shows images at both 3 and 5 hours after initial cocultured incubation, as an example.

Based on experimental need, either prepared kits such as demonstrated earlier in this article can be used, or the same principles can be applied on a case-by-case basis. When building a procedure in-house, each aspect must be tested and optimized. For either flow cytometry or fluorescence microscopy platforms, the incubation time and dye concentrations must be assessed for best results, and other platform-specific considerations may come into play as well as variation in behavior based on cell lines.

Although upgraded reagents such as Calcein Ultragreen AM give better performance, cheaper classic options such as Calcein AM are still used frequently. In either case, cellular incubation time should be balanced between functionality and laboratory feasibility. For the image above, testing showed that any incubation longer than 5 hours provided little more data, and in fact less as time went on. This knowledge, despite the relatively low toxicity of the dyes, can inform not only procedure starting times (to permit adequate but not over-incubation) to achieve the best possible experimental results.

For determining ideal dye concentrations, the cells should be incubated with a range of molarities. Many kits and protocols will have recommended ranges, which are usually the best starting point. Even for dyes that have minimal cytotoxicity, high concentrations will frequently have deleterious effects to the health and proliferation rates of the cell population.

Whether using a flow cytometer or a fluorescence microscope, visualizing and assessing gap junction activity has multiple possible applications. By using fluorescent labels, suitably optimized for a given procedure, cellular functions can be investigated on any platform. Gap junction analysis provides insight on many other aspects of cell health and behavior, and so tracking it is helpful in multiple fields of study, for a holistic understanding of intercellular communication.