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Fluorescence microscopy, with its molecular specificity, is one of the major characterization methods used in the life sciences to understand complex biological systems. Super-resolution approaches1–6 can achieve resolution in cells in the range of 15 to 20 nm, but interactions between individual biomolecules occur at length scales below 10 nm and characterization of intramolecular structure requires Ångström resolution. State-of-the-art super-resolution implementations7–14 have demonstrated spatial resolutions down to 5 nm and localization precisions of 1 nm under certain in vitro conditions. However, such resolutions do not directly translate to experiments in cells, and Ångström resolution has not been demonstrated to date. Here we introdue a DNA-barcoding method, resolution enhancement by sequential imaging (RESI), that improves the resolution of fluorescence microscopy down to the Ångström scale using off-the-shelf fluorescence microscopy hardware and reagents. By sequentially imaging sparse target subsets at moderate spatial resolutions of >15 nm, we demonstrate that single-protein resolution can be achieved for biomolecules in whole intact cells. Furthermore, we experimentally resolve the DNA backbone distance of single bases in DNA origami with Ångström resolution. We use our method in a proof-of-principle demonstration to map the molecular arrangement of the immunotherapy target CD20 in situ in untreated and drug-treated cells, which opens possibilities for assessing the molecular mechanisms of targeted immunotherapy. These observations demonstrate that, by enabling intramolecular imaging under ambient conditions in whole intact cells, RESI closes the gap between super-resolution microscopy and structural biology studies and thus delivers information key to understanding complex biological systems.

SUMMARYTo fully understand biological processes and functions, it is necessary to reveal the molecular heterogeneity of cells and even subcellular assemblies by gaining access to the location and interaction of all biomolecules. The study of protein arrangements has seen significant advancements through super-resolution microscopy, but such methods are still far from reaching the multiplexing capacity of spatial proteomics. Here, we introduce Secondary label-based Unlimited Multiplexed DNA-PAINT (SUM-PAINT), a high-throughput imaging method capable of achieving virtually unlimited multiplexing at better than 15 nm spatial resolution. Using SUM-PAINT, we generated the most extensive multiprotein dataset to date at single-protein spatial resolution, comprising up to 30 distinct protein targets in parallel and adapted omics-inspired analysis workflows to explore these feature-rich datasets. Remarkably, by examining the multiplexed protein content of almost 900 individual synapses at single-protein resolution, we revealed the complexity of synaptic heterogeneity, ultimately leading to the discovery of a new synapse type. This work provides not only a feature-rich resource for researchers, but also an integrated data acquisition and analysis workflow for comprehensive spatial proteomics at single-protein resolution, paving the way for ‘Localizomics’.

Aptamers are oligonucleotides with antibody-like binding function, selected from large combinatorial libraries. In this study, we modified a DNA aptamer library with N-hydroxysuccinimide esters, enabling covalent reactivity with cognate proteins. We selected for the ability to bind to mouse monoclonal antibodies, resulting in the isolation of two distinct covalent binding motifs. The covalent aptamers are specific for the Fc region of mouse monoclonal IgG1 and are cross-reactive with mouse IgG2a and other IgGs. Investigation into the covalent reactivity of the aptamers revealed a dependence on micromolar concentrations of Cu2+ions which can be explained by residual catalyst remaining after modification of the aptamer library. The aptamers were successfully used as adapters in the formation of antibody-oligonucleotide conjugates (AOCs) for use in detection of HIV protein p24 and super-resolution imaging of actin. This work introduces a new method for the site-specific modification of native monoclonal antibodies and may be useful in applications requiring AOCs or other antibody conjugates.

We present a novel way to enhance the information content of fluorescence imaging by encoding information in the microscope point-spread function (PSF). In contrast to methods that modify the shape of the PSF itself, our approach encodes the additional information via the creation of faithful copies of the original PSF. Our method is compatible with operation over a broad wavelength range, other PSF engineering modalities, and existing analysis tools. We demonstrate this approach by developing the ‘Circulator’, an optical add-on made of off-the-shelf components that encodes the emission band of the fluorophore into the PSF. The device creates four copies of the original PSF at well-defined relative orientations and positions, where the presence or intensity of each spot indicates the emission color of the fluorophore. Using this device, simultaneous multicolor super-resolution and single-molecule microscopy can be performed using the full field of view of a single camera, resulting in a pronounced increase in experimental throughput. We demonstrate our approach in simultaneous three-color super-resolution imaging with single-molecule localization microscopy (SMLM) and super-resolution optical fluctuation imaging (SOFI).

Super-resolution microscopy methods enable the visualization of biological processes on the level of a few nanometers. However, the application of these techniques in biological systems is limited by the availability of small affinity reagents. Slow off-rate-modified aptamers as nucleic acid analogues to antibodies have been successfully applied to improve the resolution and quantification of DNA-PAINT. In this chapter, we describe a protocol for using SOMAmers as labeling reagents for super-resolution microscopy.

DNA-PAINT based single-particle tracking (DNA-PAINT-SPT) has recently significantly enhanced observation times in in vitro SPT experiments by overcoming the constraints of fluorophore photobleaching. However, with the reported implementation, only a single target can be imaged and the technique cannot be applied straight to live cell imaging. Here we report on leveraging this technique from a proof-of-principle implementation to a useful tool for the SPT community by introducing simultaneous live cell dual-colour DNA-PAINT-SPT for quantifying protein dimerisation and tracking proteins in living cell membranes, demonstrating its improved performance over single-dye SPT.

Cell-extracellular matrix sensing plays a crucial role in cellular behavior and leads to the formation of a macromolecular protein complex called the focal adhesion. Despite their importance in cellular decision making, relatively little is known about cell-matrix interactions and the intracellular transduction of an initial ligand-receptor binding event on the single-molecule level. Here, we combine cRGD-ligand-decorated DNA tension sensors with DNA-PAINT super-resolution microscopy to study the mechanical engagement of single integrin receptors and the downstream influence on actin bundling. We uncover that integrin receptor clustering is governed by a non-random organization with complexes spaced at 20–30 nm distances. The DNA-based tension sensor and analysis framework provide powerful tools to study a multitude of receptor-ligand interactions where forces are involved in ligand-receptor binding., Relatively little is known about cell-matrix interactions and the intracellular transduction of an initial ligand-receptor binding event on the single-molecule level. Here authors combine ligand-decorated DNA tension sensors with DNA-PAINT super-resolution microscopy to study the mechanical engagement of single integrin receptors and the downstream influence on actin bundling.

The centrosome provides an intracellular anchor for the cytoskeleton, regulating cell division, cell migration, and cilia formation. We used spatial proteomics to elucidate protein interaction networks at the centrosome of human induced pluripotent stem cell–derived neural stem cells (NSCs) and neurons. Centrosome-associated proteins were largely cell type–specific, with protein hubs involved in RNA dynamics. Analysis of neurodevelopmental disease cohorts identified a significant overrepresentation of NSC centrosome proteins with variants in patients with periventricular heterotopia (PH). Expressing the PH-associated mutant pre-mRNA-processing factor 6 (PRPF6) reproduced the periventricular misplacement in the developing mouse brain, highlighting missplicing of transcripts of a microtubule-associated kinase with centrosomal location as essential for the phenotype. Collectively, cell type–specific centrosome interactomes explain how genetic variants in ubiquitous proteins may convey brain-specific phenotypes.

A standard procedure to study cellular elements is via immunostaining followed by optical imaging. This methodology typically requires target-specific primary antibodies (1.Abs), which are revealed by secondary antibodies (2.Abs). Unfortunately, the antibody bivalency, polyclonality, and large size can result in a series of artifacts. Alternatively, small, monovalent probes, such as single-domain antibodies (nanobodies) have been suggested to minimize these limitations. The discovery and validation of nanobodies against specific targets are challenging, thus only a minimal amount of them are currently available. Here, we used STED, DNA-PAINT, and light-sheet microscopy, to demonstrate that secondary nanobodies (1) increase localization accuracy compared to 2.Abs; (2) allow direct pre-mixing with 1.Abs before staining, reducing experimental time, and enabling the use of multiple 1.Abs from the same species; (3) penetrate thick tissues more efficiently; and (4) avoid probe-induced clustering of target molecules observed with conventional 2.Abs in living or poorly fixed samples. Altogether, we show how secondary nanobodies are a valuable alternative to 2.Abs. Associated articles - Correction: Circumvention of common labelling artefacts using secondary nanobodi…

Antigen binding by B cell receptors (BCRs) on cognate B cells elicits a response that eventually leads to production of antibodies. However, it is unclear what the distribution of BCRs is on the naïve B cell and how antigen binding triggers the first step in BCR signaling. Using DNA-PAINT super-resolution microscopy, we find that most BCRs are present as monomers, dimers, or lower-order oligomers on resting B cells, with a nearest-neighbor inter-Fab distance of 20-30 nm. We leverage a Holliday junction nanoscaffold to engineer monodisperse model antigens with precision-controlled affinity and valency, and find that the antigen exerts agonistic effects on the BCR as a function of increasing affinity and avidity. Monovalent macromolecular antigens can activate the BCR at high concentrations, whereas micromolecular antigens cannot, demonstrating that antigen binding does not directly drive activation. Based on this, we propose a novel BCR activation model determined by the antigen footprint.

Super-resolution (SR) microscopy techniques have been advancing the understanding of neuronal protein networks and interactions. Unraveling the arrangement of proteins with molecular resolution provided novel insights into neuron cytoskeleton structure and actin polymerization dynamics in synaptic spines. Recent improvements in quantitative SR imaging have been applied to synaptic protein clusters and with improved multiplexing technology, the interplay of multiple protein partners in synaptic active zones has been elucidated. While all SR techniques come with benefits and drawbacks, true molecular quantification is a major challenge with the most complex requirements for labeling reagents and careful experimental design. In this perspective, we provide an overview of quantitative SR multiplexing and discuss in greater detail the quantification and multiplexing capabilities of the SR technique DNA-PAINT. Using predictable binding kinetics of short oligonucleotides, DNA-PAINT provides two unique approaches to address multiplexed molecular quantification: qPAINT and Exchange-PAINT. With precise and accurate quantification and spectrally unlimited multiplexing, DNA-PAINT offers an attractive route to unravel complex protein interaction networks in neurons. Finally, while the SR community has been pushing technological advances from an imaging technique perspective, the development of universally available, small, efficient, and quantitative labels remains a major challenge in the field.

The NLRP3 inflammasome is a central component of the innate immune system. Its activation leads to the formation of a supramolecular assembly of the inflammasome adaptor ASC, denoted as 'ASC speck'. Different models of the overall structure of the ASC speck, as well as the entire NLRP3 inflammasome, have been reported in the literature. While many experiments involve overexpression or in vitro reconstitution of recombinant ASC, the cytoplasmic endogenous ASC speck remains difficult to study due to its relatively small size and structural variability. Here, we use a combination of fluorescence imaging techniques including dual-color 3D super-resolution imaging (dSTORM and DNA-PAINT) to visualize the endogenous ASC speck following NLRP3 inflammasome activation. We observe that the complex varies in diameter between ~800 and 1000 nm and is composed of a dense core from which filaments reach out into the periphery. We used a combination of anti-ASC antibodies as well as a much smaller nanobody for labeling and show that the larger complexes do not reliably label the dense core whereas the nanobody, which has a lower binding affinity, is less efficient in labeling the lower-density periphery. Imaging whole cells using dSTORM, furthermore, allowed us to sort the imaged structures into a quasi-temporal sequence suggesting that the endogenous ASC speck becomes mainly denser but not much larger during its formation.

The design of dynamic, reconfigurable devices is crucial for the bottom-up construction of artificial biological systems. DNA can be used as an engineering material for the de-novo design of such dynamic devices. A self-assembled DNA origami switch is presented that uses the transition from double- to single-stranded DNA and vice versa to create and annihilate an entropic force that drives a reversible conformational change inside the switch. It is distinctively demonstrated that a DNA single-strand that is extended with 0.34 nm per nucleotide - the extension this very strand has in the double-stranded configuration - exerts a contractive force on its ends leading to large-scale motion. The operation of this type of switch is demonstrated via transmission electron microscopy, DNA-PAINT super-resolution microscopy and darkfield microscopy. The work illustrates the intricate and sometimes counter-intuitive forces that act in nanoscale physical systems that operate in fluids.

Single-Molecule Localization Microscopy (SMLM) has revolutionized light microscopy by enabling optical resolution down to a few nanometer. Yet, localization precision commonly doesn’t suffice to visually resolve single subunits in molecular assemblies or multimeric complexes. Since each targeted molecule contributes localizations during image acquisition, molecular counting approaches to reveal the target copy numbers within localization clusters have been persistently proposed since the early days of SMLM, most of which rely on preliminary knowledge of the dye photo-physics or on a calibration to a reference. Previously, we developed localization-based Fluorescence Correlation Spectroscopy (lbFCS) as an absolute ensemble counting approach for the SMLM-variant DNA-PAINT (Points Accumulation for Imaging in Nanoscale Topography), for the first time circumventing the necessity for reference calibrations. Here, we present an extended concept termed lbFCS+ which allows absolute counting of copy numbers for individual localization clusters in a single DNA-PAINT image. In lbFCS+, absolute counting of fluorescent loci contained in individual nanoscopic volumes is achieved via precise measurement of the local hybridization rates of the fluorescently-labeled oligonucleotides (’imagers’) employed in DNA-PAINT imaging. In proof-of-principle experiments on DNA origami nanostructures, we demonstrate the ability of lbFCS+ to truthfully determine molecular copy numbers and imager association and dissociation rates in well-separated localization clusters containing up to ten docking strands. For N ≤ 4 target molecules, lbFCS+ is even able to resolve integers, providing the potential to study the composition of up to tetrameric molecular complexes. Further, we show that lbFCS+ allows to resolve heterogeneous binding dynamics enabling the distinction of stochastically generated and a priori indistinguishable DNA assemblies. Beyond advancing quantitative DNA-PAINT imaging, we believe that lbFCS+ could find promising applications ranging from bio-sensing to DNA computing.

The precise spatial localization of proteins in situ by super-resolution microscopy (SRM) demands their targeted labeling. Positioning reporter molecules as close as possible to the target remains a challenge in primary cells or tissues from patients that cannot be easily genetically modified. Indirect immunolabeling introduces relatively large linkage errors, whereas site-specific and stoichiometric labeling of primary antibodies relies on elaborate chemistries. In this study, we developed a simple two-step protocol to site-specifically attach reporters such as fluorophores or DNA handles to several immunoglobulin G (IgG) antibodies from different animal species and benchmarked the performance of these conjugates for 3D STORM (stochastic optical reconstruction microscopy) and DNA-PAINT (point accumulation in nanoscale topography). Glutamine labeling was restricted to two sites per IgG and saturable by exploiting microbial transglutaminase after removal of N-linked glycans. Precision measurements of 3D microtubule labeling shell dimensions in cell lines and human platelets showed that linkage errors from primary and secondary antibodies did not add up. Monte Carlo simulations of a geometric microtubule-IgG model were in quantitative agreement with STORM results. The simulations revealed that the flexible hinge between Fab and Fc segments effectively randomized the direction of the secondary antibody, while the restricted binding orientation of the primary antibody’s Fab fragment accounted for most of the systematic offset between the reporter and α-tubulin. DNA-PAINT surprisingly yielded larger linkage errors than STORM, indicating unphysiological conformations of DNA-labeled IgGs. In summary, our cost-effective protocol for generating well-characterized primary IgG conjugates offers an easy route to precise SRM measurements in arbitrary fixed samples. ISSN:1936-0851 ISSN:1936-086X

DNA-PAINT’s imaging speed has recently been significantly enhanced by optimized sequence design and buffer conditions. However, this implementation has not reached an ultimate speed-limit and importantly is only applicable to imaging single targets. To further improve acquisition speed, we here introduce concatenated, periodic DNA sequence motifs, yielding up to 100-fold faster sampling compared to traditional DNA-PAINT. We extend this approach to six orthogonal sequence motifs, now enabling speed-optimized multiplexed imaging.

DNA points accumulation in nanoscale topography (DNA-PAINT) is a relatively easy-to-implement super-resolution technique. However, image acquisition is slow compared to most other approaches. Here, we overcome this limitation by designing optimized DNA sequences and buffer conditions. We demonstrate our approach in vitro with DNA origami and in situ using cell samples, and achieve an order of magnitude faster imaging speeds without compromising image quality or spatial resolution. This improvement now makes DNA-PAINT applicable to high-throughput studies.

ZUSAMMENFASSUNGJeder Profi muss sein Handwerkszeug kennen, um es optimal einzusetzen. Dies gilt auch für Kanülen, die aufgrund ihres Designs und ihrer technischen Ausführung speziell auf den Gefäßanschluss für die Hämodialyse ausgelegt sind. Der scharfe Facettenschliff erlaubt eine schmerzarme Punktion trotz des großen Durchmessers, welcher für einen ausreichenden Blutfluss erforderlich ist. Mögliche Gewebeschädigungen können durch die Wahl des Punktionswinkels und der Schlifforientierung vermindert werden. Das Back Eye bei arteriellen Kanülen, ein sorgfältiges Anticoring und Farbmarken, die auch bei liegender Kanüle die Schlifforientierung anzeigen, sind besondere Sicherheitsmerkmale. Die Silikonisierung des Kanülenrohres verhindert die Aktivierung des Gerinnungssystems und lässt die Kanüle schmerzarm in das Gewebe gleiten. Moderne Dialysekanülen in Ultra-Dünnwand-Ausführung ermöglichen hohe Blutflüsse bei minimalem Außendurchmesser.

Current optical super-resolution implementations are capable of resolving features spaced just a few nanometers apart. However, translating this spatial resolution to cellular targets is limited by the large size of traditionally employed primary and secondary antibody reagents. Recent advancements in small and efficient protein binders for super-resolution microscopy such as nanobodies or aptamers provide an exciting avenue for the future, however their widespread availability is still limited. To address this issue, we here report the combination of bacterial-derived binders commonly used in antibody purification with DNA-PAINT microscopy. The small size of these protein binders compared to secondary antibodies make them an attractive labeling alternative for emerging super-resolution techniques. We here present a labeling protocol for DNA conjugation of bacterial-derived protein A and G for DNA-PAINT imaging and assay their performance intracellularly by targeting primary antibodies against Tubulin, TOM20, and EGFR and quantify the increase in obtainable resolution. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. We also acknowledge the support of the imaging facility at the MPI of Biochemistry.

Synthetic host-guest complexes are inherently dynamic as they employ weak and reversible noncovalent interactions for their recognition processes. We strategically exploited dynamic supramolecular recognition between fluorescently labeled guest molecules to complementary cucurbit[7]uril hosts to obtain stochastic switching between fluorescence ON- and OFF-states, enabling PAINT-based nanoscopic imaging in cells and tissues.