Irene Weibrecht

Characterizing proteins and their interactions in cells and tissues using the in situ proximity ligation assay

The activity of proteins is typically regulated by secondary modifications and by interactions with other partners, resulting in the formation of protein complexes whose functions depend on the participating proteins. Accordingly, it is of central importance to monitor the presence of interaction complexes as well as their localization, thus providing information about the types of cells where the proteins are located and in what sub-cellular compartment these interactions occur. Several methods for visualizing protein interactions in situ have been developed during the last decade. These methods in most cases involve genetic constructs, and they have been successfully used in assays of living cell maintained in tissue culture, but they cannot easily be implemented in studies of clinical specimens. For such samples, affinity reagents like antibodies can be used to target the interacting proteins. In this review we will describe the in situ proximity ligation assays (in situ PLA), a method that is suitable for visualizing protein interactions in both tissue sections and in vitro cell lines, and we discuss research tasks when this or other method may be selected.

In situ detection of phosphorylated platelet-derived growth factor receptor beta using a generalized proximity ligation method.

Improved methods are needed for in situ characterization of post-translational modifications in cell lines and tissues. For example, it is desirable to monitor the phosphorylation status of individual receptor tyrosine kinases in samples from human tumors treated with inhibitors to evaluate therapeutic responses. Unfortunately the leading methods for observing the dynamics of tissue post-translational modifications in situ, immunohistochemistry and immunofluorescence, exhibit limited sensitivity and selectivity. Proximity ligation assay is a novel method that offers improved selectivity through the requirement of dual recognition and increased sensitivity by including DNA amplification as a component of detection of the target molecule. Here we therefore established a generalized in situ proximity ligation assay to investigate phosphorylation of platelet-derived growth factor receptor β (PDGFRβ) in cells stimulated with platelet-derived growth factor BB. Antibodies specific for immunoglobulins from different species, modified by attachment of DNA strands, were used as secondary proximity probes together with a pair of primary antibodies from the corresponding species. Dual recognition of receptors and phosphorylated sites by the primary antibodies in combination with the secondary proximity probes was used to generate circular DNA strands; this was followed by signal amplification by replicating the DNA circles via rolling circle amplification. We detected tyrosine phosphorylated PDGFRβ in human embryonic kidney cells stably overexpressing human influenza hemagglutinin-tagged human PDGFRβ in porcine aortic endothelial cells transfected with the β-receptor, but not in cells transfected with the α-receptor, and also in immortalized human foreskin fibroblasts, BJ hTert, endogenously expressing the PDGFRβ. We furthermore visualized tyrosine phosphorylated PDGFRβ in tissue sections from fresh frozen human scar tissue undergoing wound healing. The method should be of great value to study signal transduction, screen for effects of pharmacological agents, and enhance the diagnostic potential in histopathology.

WRAP53 Is Essential for Cajal Body Formation and for Targeting the Survival of Motor Neuron Complex to Cajal Bodies

The WRAP53 protein regulates the formation and maintenance of Cajal bodies (nuclear sub-organelles), as well as directs the recruitment of nuclear factors to Cajal bodies.

Flow cytometric in situ proximity ligation analyses of protein interactions and post-translational modification of the epidermal growth factor receptor family

Interactions between members of the epidermal growth factor receptor (EGFR) family mediates cellular responses to ligand stimulation. Measurement of these interactions could provide important information and may prove useful as prognostic markers in malignancy. Therefore, to develop methods to study these interactions in genetically unmodified cells, such as clinical samples, in a sensitive and selective way, with good statistical accuracy, is important. The in situ proximity ligation assay (in situ PLA) was used to quantify homo‐ and heteromeric interactions between EGFR and HER2 in cultured cells, using flow cytometry as the readout method. Cells were monitored for changes in dimerization patterns and phosphorylation status upon stimulation. The different cell lines displayed varying amounts of interactions between EGFR and HER2, but the amount of dimerization was not found to be affected significantly upon stimulation by EGF. Activation of EGFR could be visualized by in situ PLA, but not by immunofluorescence staining. In situ PLA was successfully used to study receptor dimerization and activation of the EGF‐receptor family with high selectivity and sensitivity. The combination of in situ PLA and flow cytometry provided a statistically powerful way of analyzing protein‐protein interactions and post‐translational modifications on a single‐cell basis.

In situ detection of individual mRNA molecules and protein complexes or post-translational modifications using padlock probes combined with the in situ proximity ligation assay

Analysis at the single-cell level is essential for the understanding of cellular responses in heterogeneous cell populations, but it has been difficult to perform because of the strict requirements put on detection methods with regard to selectivity and sensitivity (i.e., owing to the cross-reactivity of probes and limited signal amplification). Here we describe a 1.5-d protocol for enumerating and genotyping mRNA molecules in situ while simultaneously obtaining information on protein interactions or post-translational modifications; this is achieved by combining padlock probes with in situ proximity ligation assays (in situ PLA). In addition, we provide an example of how to design padlock probes and how to optimize staining conditions for fixed cells and tissue sections. Both padlock probes and in situ PLA provide the ability to directly visualize single molecules by standard microscopy in fixed cells or tissue sections, and these methods may thus be valuable for both research and diagnostic purposes.

Increasing the dynamic range of in situ PLA

Genomic DNA is the template of life - the entity which is characterized by a self-sustaining anatomical development, regulated signaling processes, the ability to reproduce and to respond to stimuli. Through what is classically known as the central dogma, the genome is transcribed into mRNA, which in turn is translated into proteins. The proteins take part in most, if not all, cellular processes, and it is by unraveling these processes that we can begin to understand life and disease-causing mechanisms.In vitro and in vivo assays are two levels at which protein communication may be studied, and which permit manipulation and control over the proteins under investigation. But in order to retrieve a representation of the processes as close to reality as possible, in situ analysis may instead be applied as a complement to the other two levels of study. In situ PLA offers the ability to survey protein activity in tissue samples and primary cell lines, at a single cell level, detecting single targets in their natural unperturbed environment. In this thesis new developments of the in situ PLA are described, along with a new technique offering in situ enzyme-free detection of proximity between biomolecules.The dynamic range of in situ PLA has now been increased by several orders of magnitude to cover analogous ranges of protein expression; the output signals have been modified to offer a greater signal-to-noise ratio and to limit false-positive-rates while also extending the dynamic range further; simultaneous detection of multiple protein complexes is now possible; proximity-HCR is presented as a robust and inexpensive enzyme-free assay for protein complex detection.The thesis also covers descriptions on how the techniques may be simultaneously applied, also together with other techniques, for the multiple data-point acquisition required by the emerging realm of systems biology. A future perspective is presented for how much more information may be simultaneously acquired from tissue samples to describe biomolecular interactions in a new manner. This will allow new types of biomarkers and drugs to be discovered, and a new holistic understanding of life.

Functional Overlap Between Chondroitin and Heparan Sulfate Proteoglycans During VEGF-Induced Sprouting Angiogenesis

Objective—Heparan sulfate proteoglycans regulate key steps of blood vessel formation. The present study was undertaken to investigate if there is a functional overlap between heparan sulfate proteoglycans and chondroitin sulfate proteoglycans during sprouting angiogenesis. Methods and Results—Using cultures of genetically engineered mouse embryonic stem cells, we show that angiogenic sprouting occurs also in the absence of heparan sulfate biosynthesis. Cells unable to produce heparan sulfate instead increase their production of chondroitin sulfate that binds key angiogenic growth factors such as vascular endothelial growth factor A, transforming growth factor &bgr;, and platelet-derived growth factor B. Lack of heparan sulfate proteoglycan production however leads to increased pericyte numbers and reduced adhesion of pericytes to nascent sprouts, likely due to dysregulation of transforming growth factor &bgr; and platelet-derived growth factor B signal transduction. Conclusion—The present study provides direct evidence for a previously undefined functional overlap between chondroitin sulfate proteoglycans and heparan sulfate proteoglycans during sprouting angiogenesis. Our findings provide information relevant for potential future drug design efforts that involve targeting of proteoglycans in the vasculature.

Visualising individual sequence-specific protein-DNA interactions in situ

Gene expression - a key feature for modulating cell fate-is regulated in part by histone modifications, which modulate accessibility of the chromatin to transcription factors. Until now, protein-DNA interactions (PDIs) have mostly been studied in bulk without retrieving spatial information from the sample or with poor sequence resolution. New tools are needed to reveal proteins interacting with specific DNA sequences in situ for further understanding of the orchestration of transcriptional control within the nucleus. We present herein an approach to visualise individual PDIs within cells, based on the in situ proximity ligation assay (PLA). This assay, previously used for the detection of protein-protein interactions in situ, was adapted for analysis of target PDIs, using padlock probes to identify unique DNA sequences in complex genomes. As a proof-of-principle we detected histone H3 interacting with a 26 bp consensus sequence of the Alu-repeat abundantly expressed in the human genome, but absent in mice. However, the mouse genome contains a highly similar sequence, providing a model system to analyse the selectivity of the developed methods. Although efficiency of detection currently is limiting, we conclude that in situ PLA can be used to achieve a highly selective analysis of PDIs in single cells.

In Situ Proximity Ligation Assay for Microscopy and Flow Cytometry

The ability to study proteins and protein interactions inside cells and tissues is important for elucidating how cells function in health and disease. The in situ proximity ligation assay (in situ PLA) presented here can be used to visualize proteins, protein‐protein interactions, and post‐translational modifications in cells and tissues. The method is based upon the use of antibodies that target the proteins involved in an interaction; hence, the method has the advantage that it can be used in clinical specimens, providing localized, quantifiable single molecule detection in single cells. This unit describes how in situ PLA can be used with fluorescence microscopy and flow cytometry to study proteins (obtaining high sensitivity and specificity of detection) and protein interactions. It also includes information on expected results and information on how to troubleshoot the assay. Curr. Protoc. Cytom. 56:9.36.1‐9.36.15.

Simultaneous Visualization of Both Signaling Cascade Activity and End-Point Gene Expression in Single Cells

We have developed an approach for simultaneous detection of individual endogenous protein modifications and mRNA molecules in single cells in situ. For this purpose we combined two methods previously developed in our lab: in situ proximity ligation assay for the detection of individual protein interactions and -modifications and in situ detection of single mRNA molecules using padlock probes. As proof-of-principle, we demonstrated the utility of the method for simultaneous detection of phosphorylated PDGFRβ and DUSP6/MKP-3 mRNA molecules in individual human fibroblasts upon PDGF-BB stimulation. Further we applied drugs disrupting the PDGFRβ signaling pathway at various sites to show that this combined method can concurrently monitor the molecular effect of the drugs, i.e. inhibition of downstream signaling from the targeted node in the signaling pathway. Due to its ability to detect different types of molecules in single cells in situ the method presented here can contribute to a deeper understanding of cell-to-cell variations and can be applied to e.g. pinpoint effector sites of drugs in a signaling pathway.