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We have redesigned probes for in situ proximity ligation assay (PLA), resulting in more efficient localized detection of target proteins. In situ PLA depends on recognition of target proteins by pairs of antibody-oligonucleotide conjugates (PLA probes), which jointly give rise to DNA circles that template localized rolling circle amplification reactions. The requirement for dual recognition of the target proteins improves selectivity by ignoring any cross-reactivity not shared by the antibodies, and it allows detection of protein-protein interactions and post-translational modifications. We herein describe an improved design of the PLA probes -UnFold probes - where all elements required for formation of circular DNA strands are incorporated in the probes. Premature interactions between the UnFold probes are prevented by including an enzymatic "unfolding" step in the detection reactions. This allows DNA circles to form by pairs of reagents only after excess reagents have been removed. We demonstrate the performance of UnFold probes for detection of protein-protein interactions and post-translational modifications in fixed cells and tissues, revealing considerably more efficient signal generation. We also apply the UnFold probes to detect IL-6 in solution phase after capture on solid supports, demonstrating increased sensitivity over both normal sandwich enzyme-linked immunosorbent assays and conventional PLA assays.

Proximity based detection methods are invaluable tools in the field of molecular biology, increasing selectivity and allowing for analysis of protein interactions. ProxHCR utilizes pairs of antibodies labelled with oligonucleotides to probe for proximal binding and to initiate a hybridization chain reaction (HCR) to generate an amplified detection signal. As HCR is based upon hybridization of DNA hairpins, the performance is dependent on salt concentrations and temperature. Herein we have redesigned the proxHCR system to increase the performance and to reduce dependency on temperature and salt concentrations. The new oligonucleotides provide an increased signal when performed at physiological salt concentrations and in room temperature.

Hybridization chain-reaction (HCR) is technique to generate a linear polymerization of oligonucleotide hairpins, used in multiple molecular biology methods. The HCR reaction is dependent on that every hairpin is metastable in the absence of a triggering oligonucleotide and that every hairpin can continue the polymerization, which places a strong demand on oligonucleotide quality. In this paper we show how further purification can greatly increase polymerization potential. We found that a single extra PAGE-purification could greatly enhance hairpin polymerization both in solution and in situ. Purification using a ligation-based method further improved polymerization, yielding in situ immunoHCR stains at least 3.4-times stronger than non-purified control. This demonstrates the importance of not only good sequence design of the oligonucleotide hairpins, but also the demand for high quality oligonucleotides to accomplish a potent and specific HCR.

To accurately diagnose cancer and other aberrant cellular conditions, investigations of several parameters are required. Not only morphology, protein and gene expression but also more complex investigations, such as protein modifications, translocations and interactions can provide essential data. In order to investigate such cellular events, several molecular tools have been developed. Among them is proximity-dependent initiation of hybridization chain reaction (ProxHCR), an inexpensive tool developed to provide accurate analysis of proximity events within cells. While previous versions of ProxHCR has been capable of detecting protein-proximity, lacking signal strength has resulted in poor ability to properly quantify imaged results. Here we show how further purification of the HCR detection hairpins considerably increase signal strength. The increased signal strength allow for accurate quantification of proximity events in line with the established PLA method. Furthermore we show how ProxHCR can be used to track PDGFR-β phosphorylation and successive recruitment of GRB2 and PI3K over time. These results show how ProxHCR can be a valuable, enzyme free alternative to other proximity analysis tools.