The “In Situ” Proximity Ligation Assay to Probe Protein–Protein Interactions in Intact Tissues

The proximity ligation assay (PLA) is a sensitive and specific technique to visualize proteins, their posttranslational modifications and activation state as well as protein–protein interactions. The assay is based on the employment of proximity probes, composed by oligonucleotide-conjugated antibodies, to recognize a couple of specific targets. The binding of probes in close proximity allows for their hybridization by connector oligonucleotides, that can form a circular DNA strand. These DNA circles can then be amplified by polymerase chain reaction. Finally, the conjugation of fluorescence-labelled oligonucleotides with the amplification product allows for the localized detection of individual or interacting proteins in cells and tissues. Here, we describe the use of “in situ” PLA to visualize the localization of protein–protein interactions in intact tissues.