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Super-resolution optical fluctuation imaging (SOFI) is a super-resolution microscopy technique that overcomes the diffraction limit by analyzing intensity fluctuations of statistically independent emitters in a time series of images. The final images are background-free and show confocality and enhanced spatial resolution (super-resolution). Fluorophore photobleaching, however, is a key limitation for recording long time series of images that will allow for the calculation of higher order SOFI results with correspondingly increased resolution. Here, we demonstrate that photobleaching can be circumvented by using fluorophore labels that reversibly and transiently bind to a target, and which are being replenished from a buffer which serves as a reservoir. Using fluorophore-labeled short DNA oligonucleotides, we labeled cellular structures with target-specific antibodies that contain complementary DNA sequences and record the fluctuation events caused by transient emitter binding. We show that this concept bypasses extensive photobleaching and facilitates two-color imaging of cellular structures with SOFI.
Receptor tyrosine kinases (RTKs) orchestrate cell motility and differentiation. Deregulated RTKs may promote cancer and are prime targets for specific inhibitors. Increasing evidence indicates that resistance to inhibitor treatment involves receptor cross-interactions circumventing inhibition of one RTK by activating alternative signaling pathways. Here, we used single-molecule super-resolution microscopy to simultaneously visualize single MET and epidermal growth factor receptor (EGFR) clusters in two cancer cell lines, HeLa and BT-20, in fixed and living cells. We found heteromeric receptor clusters of EGFR and MET in both cell types, promoted by ligand activation. Single-protein tracking experiments in living cells revealed that both MET and EGFR respond to their cognate as well as non-cognate ligands by slower diffusion. In summary, for the first time, we present static as well as dynamic evidence of the presence of heteromeric clusters of MET and EGFR on the cell membrane that correlates with the relative surface expression levels of the two receptors
TNFR1 is a crucial regulator of NF‐ĸB‐mediated proinflammatory cell survival responses and programmed cell death (PCD). Deregulation of TNFα‐ and TNFR1‐controlled NF‐ĸB signaling underlies major diseases, like cancer, inflammation, and autoimmune diseases. Therefore, although being routinely used, antagonists of TNFα might also affect TNFR2‐mediated processes, so that alternative approaches to directly antagonize TNFR1 are beneficial. Here, we apply quantitative single‐molecule localization microscopy (SMLM) of TNFR1 in physiologic cellular settings to validate and characterize TNFR1 inhibitory substances, exemplified by the recently described TNFR1 antagonist zafirlukast. Treatment of TNFR1‐mEos2 reconstituted TNFR1/2 knockout mouse embryonic fibroblasts (MEFs) with zafirlukast inhibited both ligand‐independent preligand assembly domain (PLAD)‐mediated TNFR1 dimerization as well as TNFα‐induced TNFR1 oligomerization. In addition, zafirlukast‐mediated inhibition of TNFR1 clustering was accompanied by deregulation of acute and prolonged NF‐ĸB signaling in reconstituted TNFR1‐mEos2 MEFs and human cervical carcinoma cells. These findings reveal the necessity of PLAD‐mediated, ligand‐independent TNFR1 dimerization for NF‐ĸB activation, highlight the PLAD as central regulator of TNFα‐induced TNFR1 oligomerization, and demonstrate that TNFR1‐mEos2 MEFs can be used to investigate TNFR1‐antagonizing compounds employing single‐molecule quantification and functional NF‐ĸB assays at physiologic conditions.