We demonstrate a method to image multiple molecules within heterogeneous nano-structures at single molecule accuracy using sequential binding and elution of fluorescently labeled antibodies.
Imaging heterogeneous cellular structures using single molecule localization microscopy has been hindered by inadequate localization precision and multiplexing ability. Using fluorescent nano-diamond fiducial markers, we describe the drift correction and alignment procedures required to obtain high precision in single molecule localization microscopy. In addition, a new multiplexing strategy, madSTORM, is described in which multiple molecules are targeted in the same cell using sequential binding and elution of fluorescent antibodies. madSTORM is demonstrated on an activated T cell to visualize the locations of different components within a membrane-bound, multi-protein structure called the T cell receptor microcluster. In addition, application of madSTORM as a general tool for visualization of multi-protein structures is discussed.
A variety of super-resolution microscopy techniques have been developed to overcome the diffraction limit of light microscopy (~200 nm). Among these is a category of techniques called single molecule localization microscopy (SMLM) which includes photo-activation localization microscopy (PALM) and stochastic optical reconstruction microscopy (STORM). SMLM techniques share in the use of fluorophores that can be switched between on (fluorescent) and off (dark/photo-switched) states, allowing sequential localization of fluorescence from single molecules1,2,3.
Due to its compatibility with commercially available dyes and microscopes, direct STORM (dSTORM) has become a widely adopted SMLM technique4. dSTORM can routinely achieve ~10 nm localization precision, defined as the uncertainty in calculating the center of a diffraction-limited point spread function (PSF). However, despite the high precision estimated using localization algorithms5,6,7, accurate determination of the actual location of single molecules has been hampered by a number of issues. First, mechanical movement of the microscope stage during image acquisition adds significant uncertainty to localization precision. As SMLM images are obtained over thousands of time-lapse frames, nano-scale movements of the microscope stage can significantly compromise the precision of the final super-resolution image8. To compensate for stage movement during image acquisition, stage drift is commonly estimated from regression-based fitting of binned localizations from the image itself (cross correlation) or sequential localizations from fiducial markers (fiducial correction)1,9. However, these methods require optimization of multiple parameters for each image stack, and cannot account for stage movements at short time scales such as mechanical vibration. Gold nano-particles and multi-color fluorescent beads have been used as fiducial markers in SMLM, but they are not photo-stable, and result in significantly lower precision after drift correction than the nitrogen vacancy-center fluorescent nano-diamonds (FNDs) used in madSTORM10.
In addition to the diffraction limit, light microscopy is further restricted by spectral limits. Simultaneous visualization of multiple targets requires fluorescent probes with non-overlapping spectral profiles, generally restricting fluorescence-based light microscopy to 6 colors and SMLM to 2-3 colors4,11,12. Moreover, non-linear chromatic aberration causes misalignment of multicolor images, which require extensive alignment procedures using multi-colored fiducial markers8,13. To overcome these limits, previous studies have imaged multiple targets using repetitive photobleaching or chemical quenching of sequentially bound fluorophores14,15,16,17,18,19. While these methods can overcome the spectral limits of microscopy, fluorescence bleaching is known to be a toxic process20, and prolonged bleaching or quenching may cause unwanted side effects such as loss of crosslinking. Furthermore, the accumulation of fluorescent probes could lead to steric blocking of binding sites in the sample, preventing large-scale multiplexing and robust targeting of epitopes. To avoid such steric interference, a recent study achieved multiplexing using stochastic exchange of freely diffusing protein fragments21. Whereas this method allows dense labeling of cellular structures, it requires extensive biochemical preparation to isolate peptide fragments, cannot locate single molecule positions, and does not readily facilitate large-scale multiplexing using commercially available probes. We present a detailed video protocol describing the sequential binding and elution of fluorescently antibodies for multiplexed, antibody size-limited dSTORM (madSTORM) imaging, and use of fluorescent nano-diamonds to achieve precise drift correction and alignment.
Caution: Please consult all relevant material safety data sheets (MSDS) before use. Several of the chemicals used in this protocol are toxic and carcinogenic. Please use all appropriate safety practices when performing the protocol including the use of engineering controls (fume hood, glovebox) and personal protective equipment (safety glasses, gloves, lab coat, full length pants, closed-toe shoes).
1. Multiplexed Imaging of Activated T Cells
2. Drift Correction and Alignment of Multiplexed Image Stacks
The sequential elution and staining method was used to produce the multiplexed madSTORM image of microclusters and other structures in an activated Jurkat T cell (Figure 1, check figure alignments). Each pseudo-colored image represents one round of madSTORM imaging acquired in steps 1.1.1 to 1.3.13. As described previously10, the field of view should be monitored for residual signal from previous, non-eluted antibody to avoid crosstalk between multiplexed targets. Moreover, as the elution efficiency can vary among antibodies, the order of sequential multiplexing should be arranged from the best to worst eluting antibody to minimize steric blocking of epitopes. The final madSTORM image in Figure 1 has been corrected for drift and alignment using AFC and FND fiducial markers as described in section 2.
FNDs were used to perform AFC on localizations from a single, independent FND (Figure 2A, 2B) and an activated Jurkat T cell (Figure 2C, 2D). The AFC drift correction was performed as described in steps 2.1.1-2.1.8 using the algorithms provided. First, visual inspection of the image after drift correction (Fig. 2C) is recommended to ensure proper application of the drift correction algorithm. Second, measuring the standard deviation of localizations in both the X and Y axis is recommended to confirm the precision achieved by AFC. As reported previously10, AFC typically yields better precision than expected by various localization algorithms.
Figure 1: (A) An activated Jurkat T cell, sequentially imaged using multiple rounds of madSTORM imaging. LAT (red), pLAT-Y171 (magenta), pLAT-Y191 (pink), pLAT-Y226(purple), PLCγ1 (light blue), pSLP76-Y128 (green), pSRC-Y416 (aqua), pTCRζ-Y142 (blue), TOM20 (yellow), αTubulin (orange), ZAP70 (brown). Scale bar = 2.5 μm. (B) Expanded image of boxed region in A. Scale bar = 500 nm. Please click here to view a larger version of this figure.
Figure 2: SMLM image of a single FND localized in 30,000 image frames (A) before and (B) after drift correction with averaged fiducial correction. Scale bars = 20 nm (A and B). SMLM image of an activated Jurkat T cell stained with anti-phosphorylated SLP76 (Y128) antibody and FND fiducial markers (C) before and (D) after drift correction with averaged fiducial correction. Scale bar = 2 μm. Please click here to view a larger version of this figure.
The sequential multiplexing, drift correction, and alignment procedures in madSTORM allow precise, highly multiplexed visualization of heterogenous structures in cells.10 In addition, madSTORM avoids the limitations of multi-color STORM such as chromatic aberration and sub-optimal photoswitching/emission properties of non-far red dyes9,12. As the elution step significantly reduces steric interference from sequentially bound antibodies, madSTORM can be used to perform repeated imaging of cell samples beyond the 6-10 rounds of multiplexed imaging achieved by previous techniques9,15,21. Moreover, the sequential elution and staining steps are not restricted to SMLM and can be used for other imaging techniques such as confocal microscopy, SIM and two-photon microscopy. However, given that madSTORM is a sequential immunostaining technique, this technique is limited by the availability of antibodies for each targeted molecule. Each antibody should be carefully tested to ensure specificity, labeling efficiency, and elution ability.
The acquisition settings for madSTORM were chosen to maximize detection of individual photoswitching events, allowing us to achieve 2.5 nm average localization precision. However, the high precision comes at the expense of time, requiring ~3 h for each round of madSTORM imaging as discussed previously10. For faster image acquisition at lower localization precision (5-10 nm), a shorter exposure time and higher EM gain are recommended (e.g. 20 ms exposure, 300X EM gain, 17 MHz at 16 bit, conversion gain 3). Furthermore, multi-color imaging (e.g. Atto-488, Alexa-568, and Alexa-647 nm) can be performed for each round of madSTORM imaging to increase the scale of multiplexing, albeit with a decrease in alignment precision due to chromatic aberration.
We have successfully performed up to 25 rounds of madSTORM imaging using this protocol. As the entire procedure is performed with the coverslip chamber mounted on the microscope stage, it is important to secure the coverslip chamber in the same position. To do this utilize auto focus mode and mount the chamber on the microscope stage using metal clamps. If significant stage drift has occurred, use known positions of FND fiducial markers to re-center the cell sample.
Simultaneous visualization of aligned madSTORM images can be challenging. To simultaneously view 7 or fewer madSTORM images, use Color/merge channels in ImageJ to create a pseudo-colored composite image. For 8 or more images, it is advisable to catagorize the madSTORM images into different groups, merge each group into a composite image, and apply a single LUT color to each composite image.
We used 100 nm nitrogen vacancy-center nano-diamonds due to their brightness and adherence to PLL-coated coverslips. FNDs smaller than 80 nm did not adhere well to PLL-coated coverslips, and FNDs larger than 100 nm were significantly brighter than Alexa-647 dye when excited using 647 nm laser, limiting optimization of camera acquisition settings for Alexa-647 emission. We recommend experimenting with different FND sizes or colors if the 100 nm nitrogen vacancy-center FNDs are not optimal for your imaging setup. Alternatively, nano-gold particles or fluorescent beads can be used as fiducial markers, but are not optimal for the averaged point correction procedure used below in section 2.110. Finally, for NV– vacancy FNDs, the emission intensity can be decreased by up to ~50% in situ by the application of a static magnetic field23. The decrease in intensity is roughly linear up to ~500 Gauss at which point the effect saturates. In practice, the intensity of diamonds in the field of view can be decreased to the desired degree by bringing a permanent magnet closer to the microscope slide from the top.
Lastly, we optimized the elution buffer for removal of antibodies targeting the molecular components of the T cell microcluster. madSTORM is not limited to TCR microclusters, however, as microtubules, mitochondria, F-actin, Vimentin and other molecular structures have been imaged using this technique10. For antibodies targeting other cellular compartments, additional steps may be required to disrupt epitope binding such as higher temperature, lower pH and longer incubation of elution buffer. Conversely, the elution buffer can be used without Tween-20 to target sensitive membrane structures, preventing disruption of non-ionic interactions. We foresee in future experiments that the elution buffer composition will be tailored for individual antibodies to optimize elution efficiency and sample preservation. Furthermore, beyond the application in activated T cells, madSTORM may be feasible with other systems including in vitro molecular complexes, different cell types, and tissue sections. Various parameters of the madSTORM protocol, such as fixation, permeabilization, antibody staining, and elution buffer composition will need to be adjusted for each targeted system.
The authors have nothing to disclose.
We thank Xufeng Wu for access to the STORM microscope. This research was supported by the Intramural Research Program of the National Cancer Institute (NCI) Center for Cancer Research and the National Heart Lung and Blood Institute (NHLBI).
8 well coverslip chamber | Lab-tek | 155409 | |
0.1% Poly-L-Lysine solution | Sigma-Aldrich | P8920 | |
NV-100nm Fluorescent Nano-diamond | Adamas | Red FND | |
anti-CD3ε antibody | BD Biosciences | 555329 | |
Bovine serum albumin, Fraction V | KSE Scientific | 98-100P | |
Triton-X | Sigma-Aldrich | T9284 | |
10% Paraformaldehyde | EMS | 15712 | Carcinogen |
Gelatin from fresh water fish skin | Sigma-Aldrich | G7041 | |
Alexa 647 antibody labeling kit | Thermo Fisher | A20186 | |
Magnesium Chloride hexahydrate | Sigma-Aldrich | 138924 | |
PIPES | Sigma-Aldrich | P6757 | |
Tween-20 | Fisher Scientific | BP337-500 | |
2-Mercaptoethanol | Sigma-Aldrich | M7154 | Highly toxic, air sensitive |
Cysteamine | Sigma-Aldrich | 30070 | Highly toxic |
Cyclooctatetraene 98% | Sigma-Aldrich | 138924 | Highly toxic, air sensitive |
10x PBS | KD Medical | RGF-3210 | |
10x TBS | KD Medical | RGF-3385 |