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Cellular functions are regulated and executed by complex protein interaction networks. Accordingly, it is essential to understand the interplay between proteins in determining the activity status of signaling cascades. New methods are therefore required to provide information on different protein interaction events at the single cell level in heterogeneous cell populations such as in tissue sections. Here, we describe a multiplex proximity ligation assay for simultaneous visualization of multiple protein complexes in situ. The assay is an enhancement of the original proximity ligation assay, and it is based on using proximity probes labeled with unique tag sequences that can be used to read out which probes, from a pool of probes, have bound a certain protein complex. Using this approach, it is possible to gain information on the constituents of different protein complexes, the subcellular location of the complexes, and how the balance between different complex constituents can change between normal and malignant cells, for example. As a proof of concept, we used the assay to simultaneously visualize multiple protein complexes involving EGFR, HER2, and HER3 homo- and heterodimers on a single-cell level in breast cancer tissue sections. The ability to study several protein complex formations concurrently at single cell resolution could be of great potential for a systems understanding, paving the way for improved disease diagnostics and possibilities for drug development.

Background: Sensitive detection of protein interactions and post-translational modifications of native proteins is a challenge for research and diagnostic purposes. A method for this, which could be used in point of care devices should be cheap and robust.

Results: Building on hybridization chain reaction, we designed a four hairpin system which is metastable in solution at 37°C for several hours and undergoes rapid signal amplification upon introduction of an initiator oligonucleotide. When the proximity hairpins are conjugated to antibodies these proximity probes in combination with the HCR hairpins and the initiator oligonucleotide provide a specific, enzyme free method to detect HIF-1α/HIF-1β and potentially other protein interactions and PTMs in situ. Furthermore it was possible to detect single proteins in the different compartments of the cell, further proving the specificity of this technique.

Conclusion: In this study we present proximity dependent HCR, which is a cheap and robust method to detect protein interactions and post-translational modifications. Because of its independence from enzymes the technique has only low demands on storage and handling which makes it interesting for point of care devices.