Carcinoembryonic antigen-related cellular adhesion molecules (CEACAM) family proteins are cell surface glycoproteins involved in various cellular activities, including cellular growth, differentiation, tumorigenesis, inflammation, and infection. It is believed that these different and sometimes controversial signaling pathways are regulated through the homo- and heterophilic interactions of CEACAMs, which is still poorly understood. For example, during Neisseria gonorrhoeae infection, CEACAM1 and CEACAM5 both respond to bacterial attachment and trigger bacterial uptake; however, the mechanisms of engulfment differ. It is known that CEACAM1 and 5 can both form cis and trans homophilic dimers, as well as heterophilic dimers with each other. We are interested in how these different combinations of inter- /intracellular interactions occur and regulate different cell activities. We achieve this goal by visualizing fluorescently tagged EYFP-CEACAM1 and mCherry-CEACAM5 expressed in HeLa cells using total internal reflection (TIRF) microscopy coupled with homoFRET, which gives us information on the spatial-temporal distribution and association dynamics. With 3-dimensional fluorescence imaging, we also visualized CEACAM1 and CEACAM5 responding to N. gonorrhoeae infection, including the attachment and engulfment of the bacteria. Moreover, with the additional label-free digital holographic microscopy (DHM) method, we can get the volumetric information of the cell and bacteria alongside fluorescence imaging of CEACAMs, as well as real-time N. gonorrhoeae tracking during live imaging. These approaches give us a better understanding of the CEACAM associations and their role in infection mechanisms.
Liang et al. (Sun,) studied this question.