Quick Guide to STED Sample Preparation

November 14, 2012

This guide is intended as a quick reference guide for the most common questions regarding the preparation of samples for the Leica SP8 TCS STED, by briefly explaining the theoretical basics required from samples for stimulated emission depletion. It provides information about the most common immunofluorescence labeling techniques, working mounting media, basic quality optimization procedures and experiment designs. It also contains a detailed list of reagents, antibodies and disposables frequently and successfully used in super-resolution STED microscopy. In summary, this guide is meant to present the necessary knowledge, including some tips and tricks, to users that are preparing their first super-resolution microscopy sample.

The guide’s focus is set on the Leica TCS SP8 STED and Leica TCS SP8 gated STED microscopes.

Stimulated emission depletion (STED) microscopy is a super-resolution method based on fluorescence confocal imaging, in which images are acquired by scanning a focused light spot over a region of interest and collecting the fluorescence sequentially pixel by pixel. The main strengths of this technology are:

  1. Pure physical resolution improvement, without any additional post-processing below 50 nm.
  2. Intrinsic optical sectioning, enabling the acquisition of planes of roughly 500 nm, 3-dimensional structures, even several tens of microns deep inside the tissue.
  3. Fast image acquisition of several images per second.
  4. Live-imaging capabilities by using either fluorescent proteins or other fluorescent tags.

Step 1: Choice of samples

  • STED can be applied on a big variety of samples, ranging from single cultured flat cells, tissue slices to whole animals, e.g. nematodes (C. elegans) and insects (D. melanogaster). Nonetheless some points should be considered. STED applies a specially developed STED 100x/1.4 oil objective, which has a working distance of 90 μm. Thus, the observed structure should be at most 80 μm away from the coverglass, but preferably within 20 μm range for optimal performance. Additionally, in order to achieve the best results, the refractive index of the mounting medium should match the index of the immersion used (immersion liquid = 1.518, also see Step 5). There are currently no dip-in objectives in our portfolio.
  • The structure has to be optically accessible. Auto-fluorescence, sudden and unpredicted changes of the refractive index (e.g. tissues containing air, myelin and fat) may influence the shape of the focal spot and consequently the performance of the microscope. If experience with clearing solutions is available, it might be worth testing here.
  • During STED imaging, samples are irradiated with strong light at a wavelength of 592 nm. It is of crucial importance that the sample is not absorbing light at this wavelength.

Step 2: Choice of fluorescent dyes

There is a wide range of fluorescent dyes performing well with Leica STED microscopes (see Appendix A). In order to achieve satisfying results within the few hours, in which a system demonstration is normally realized, it is advisable to stay with the repertoire of fluorophores suggested by Leica. If this is not possible, fluorophores should be chosen, which have similar excitation and emission spectra as the suggested dyes. It is also beneficial to start with single color stainings and, as soon as these are approved, move on to more demanding dual color experiments. Please refer to Appendix B for the recommended dyes for dual color experiments, which do not require additional spectral unmixing. For STED to work efficiently, the emission spectra of both dyes need to show significant emission at the STED wavelength (see example of STAR 440SX and Oregon Green 488 in Figure below).

For the spectral separation of two dyes with similar emissions different excitations are required. This is realized by using large Stoke’s shift dyes (BD V500 and STAR 440SX) with absorption spectra located in the violet/blue range.
It is principally possible to include additional dyes for more colors. The emission of these dyes, however, should reside outside the range of the STED detection that could otherwise interfere with STED image quality. It is most likely, if dyes in the orange/red range are selected (e.g. Cy5), that they will absorb the strong STED depletion light and get bleached. Thus all reference images need to be acquired before the STED images.
Note that the use of DAPI and Hoechst should be avoided at all times. Both markers are excited by the STED laser and can lead to strong background in the STED images.

Step 3: Choice of primary antibodies/labeled structures

In order to analyze the performance of STED in a given sample, the experiment is optimally divided into two parts Part A and B might be performed in parallel, with A being the control experiment for B.

A) Evaluation of the STED performance in the according environment
Fluorescent dyes and antibodies are sensible to the surrounding environment (pH, salt concentrations, redox agents). In order to check the performance of dyes from the given sample, in a first step, a well-established primary antibody should be chosen that gives a pecific, bright staining. Even with no real biological relevance, this step helps to ensure that the dye is behaving as expected in the surrounding environment.

B) Working with the structure of interest
In a second step, technically more demanding stainings can be imaged that now address the actual structures of interest.

The labeling density plays an important role in super-resolution, but it cannot be properly checked with conventional microscopes. Often, increasing the antibody concentration during the staining procedure already helps in enhancing the sample quality. It is therefore advisable to work with higher antibody concentrations (2 to 5-fold) for STED imaging to ensure optimal labeling density. It is worth to test different primary and secondary dye concentrations in step A and B. The quality of the stain (brightness/background) should be checked with conventional microscopy prior to the STED imaging.

Step 4: E.g. Antibody staining

Each action performed in an antibody staining procedure has a distinct influence on the sample quality. Each step will be briefly explained by means of a standard immuno-fluorescence protocol for cell culture, as an example to assist the user during the optimization steps for his/her own protocol:

Reagents
• Phosphate buffer saline (PBS), pH 7,4
• 2 % Paraformaldehyde (PFA) in PBS
• 0.1 % Triton in PBS
• Bovine Serum Albumin (BSA)

Procedure
All steps are performed at room temperature, comments are in brackets.

  1. Rinse 3 x with PBS
    (Cells should be washed, culture medium removed by rinsing the culture several times. Tissues should be dissected and cleaned from parts that could hinder image acquisition. Use established lab protocols, if they are known to work. Samples must be treated gently and quickly, which could otherwise lead to premature death and decomposition.)
  2. Fix with 2 % PFA in PBS for 15 min
    (Fixation of samples is a critical step in the sample preparation, as it defines how well the structure will be preserved. With increasing resolution this step becomes more critical and should be addressed with care. PFA is a common fixative, but it is not always the best performing one. Some research in literature and optimization might be required here. Alternatively, a 5 min incubation with ice-cold (–20 °C) 100 % methanol can be used. The methanol fixation does not require additional permeabilization steps. Thus, steps 5 and 6 can be ignored with methanol fixation, also see below.)
  3. Rinse 3 x with PBS
    (Remove higher concentrations of fixatives for following steps.)
  4. Wash 3x with PBS for 5 min
    (Remove the rest of fixatives for following steps.)
  5. Permeabilize with 0.1 % Triton in PBS for 10 min
    (Crucial step to reveal epitopes to primary antibodies. Lower concentrations / shorter incubation times may better preserve the structure, but compromise labeling density. Higher concentrations/longer incubation times may make the epitope more accessible to antibodies but also deteriorate the structure. Some fixatives (e.g. methanol) do not need extra permeabilizing steps.)
  6. Rinse 3 x with PBS
    (Remove permeabilizing agents.)
  7. Block with 2 % BSA in PBS for 1 h
    (Blocking can be performed with different agents, normally consisting of inert proteins that bind to non-specific binding partners, which would otherwise bind to antibodies and increase the unspecific labeling of fluorescent dyes. It is also advisable to use blocking agents while incubating with antibodies, as the serum helps in preserving the cellular structure. Thicker tissues might require longer incubation times.)
  8. Incubate with primary antibody for 1 h
    (Use of higher antibody concentrations might be helpful for STED experiments, longer incubation times frequently give better results, but be aware of potentially increased background. In thicker samples (e.g. whole mounts) incubation may take up to days. Alternatively the incubation can be done at 4 °C over night.)
  9. Wash 3 x with PBS for 5 min
    (Washing steps are important, especially when using high concentration of antibodies. 5 minutes is the absolute minimum for washing steps here. Otherwise move to 10 or 20 minutes incubations and more times for better results. Previous rinsing steps might speed up the process.)
  10. Incubate with secondary antibody for 1 h
    (You might need to adopt/optimize the antibody concentration for your application. Incubation with secondary antibodies should be performed similar to primary antibody incubation. For Oregon Green 488, Chromeo 505, Chromeo 488, Alexa Fluor 488, ATTO 488, STAR 488, STAR 440SX a good starting point are dilutions of 1:100, when bought from commercially available sources, otherwise 5 x higher than the recommended dilution. For Becton and Dickinson V500 stainings use the biotinylated antibody from Jackson Immunoreserach Laboratories at dilutions of 1:100 at this step. Incubations can also be done overnight. Thicker tissues need longer incubation times.)
  11. Wash 3 x with PBS for 5 min
    (Remove unbound antibodies from sample. Longer and more washing steps will increase the quality and specificity of fluorescent label. Previous rinsing steps might speed up the process.)
  12. Additional steps only needed when staining with BD V500:
    • Incubate with Streptavidin-V500 for 30 min
    (Additional step when BD V500 is used for fluorescent labeling. Dilutions of V500 should be of 1:50. Longer incubations might be required for ticker tissues.)
    • Wash 3 x with PBS for 5 min
    (Remove unbound Streptavidin-V500 from sample. Longer and more washing steps will increase the quality and specificity of fluorescent label. Previous rinsing steps might quicken the process.)
  13. Mount
    (See Step 5 below)
  14. Store at 4 °C

Finally, the staining should look crisp and bright, when observed through the ocular (GFP settings for single color, or CFP/YFP settings for dual color) and yield good signal to noise in confocal or widefield fluorescence microscopes.

Step 5: Mounting

The mounting medium should have a refractive index matching the immersion required by the objective to enable highest penetration depth without unwanted aberrations. Furthermore, no auto-fluorescence should be observed when irradiating it with 592 nm laser light, nor should it contain DAPI or Hoechst. In some cases mounting media affected the fluorescence yield of large Stoke’s shift dyes (e.g. VectaShield), or of fluorescent proteins and some green dyes (e.g. TDE – For a list of dyes working in TDE please refer to Staudt et al., 2007). Thus, we do not recommend the utilization of VectaShield together with large Stoke´s shift dyes. Prolong Gold (Invitrogen) has performed well in our hands and is recommended by Leica.

Very good results were also obtained with rather simple self-made glycerol based mounting as described below:

  1. Glycerol
    By combining different amounts of water (or PBS) and glycerol (or even just pure glycerol) the refractive index (RI) can be precisely adjusted between 1.33 and 1.47. Additionally, it is easy to prepare and suitable for longer sample storage at –20  °C.
  2. Mowiol
    Take 6 g of glycerol (analytical grade) and add 2,4 g of Mowiol powder (Calbiochem # 475904), 6 ml of Aqua dest., 12 ml of 0.2 M TRIS buffer with pH 8 and stir the solution for approximately 4 hours. Subsequently let the solution rest for additional 2 hours. Incubate the Mowiol for 10 min at 50 °C (water bath) and centrifuge the solution for 15 min at 5,000 g. Finally, take the supernatant and freeze the medium at –20 °C for storage. Mowiol is so far the best suited medium for STED images and can be used for –20 °C storage of samples.

All Leica objectives with coverglass correction are corrected for #1.5 coverslips (optimal: 0.170 ± 0.01 mm thick, Hecht-Assistent, cat. number 1014/2424), which should be used for mounting and drastically enhances image quality compared to #1 coverslips not only for STED but also for confocal imaging.

Common antifades, e.g. DABCO (2.5 %) or NPG (4 %), also may cause significant changes to the photo-physical properties of dyes and are often used with STED.

Step 6: Live-imaging

Leica TCS SP8 STED module is currently only supported for inverted microscope stands and live-imaging procedures have to be adapted accordingly. Good results have been reported with a series of fluorescent proteins. Best results so far have been obtained with FPs similar to eYFP.

Fluorescent proteins

Excitation (nm)

Depletion

eGFP

488

592

EmGFP (Emerald)

488

592

mCitrine

514

592

eYFP

514

592

Venus

514

592

But also organic dyes used for labeling living cells (e.g. Tubulin Tracker Green, Oregon Green BABTA) were convincing.

If live-imaging experiments are desired, it is advisable to contact the Leica personnel in order to clarify the experimental procedure and if the necessary equipment is present at site.

Appendix A: List of conjugates for staining

Fluorophore

Excitation (nm)

Depletion (nm)

Provider

Cat. number

Biotinylated Antibody

Jackson Immunoresearch

115-065-003 (mouse)
111-065-003 (rabbit)

BD Horizon V500**

458/470

592

Beckton & Dickinson

561419 (Streptavidin)

Abberior STAR 440SX**

458/470

592

Abberior

2-0002-003-7 (mouse)

ATTO 488*

488

592

Sigma-Aldrich

62197 (mouse)
18772 (rabbit)

Abberior STAR 488*

488

592

Abberior

2-0002-006-8 (mouse)
2-0012-006-5 (rabbit)

Alexa Fluor 488*

488

592

life technologies

A11001 (mouse)
A11008 (rabbit)

Chromeo

488

592

Active Motif

15051 (mouse)
15061 (rabbit)

FITC

488

592

Sigma-Aldrich

F0257 (mouse)
F0382 (rabbit)

DyLight 488*

488

592

Thermo Scientific

35502 (mouse)
35552 (rabbit)

Chromeo 505**

488/514

592

Active Motif

15050 (mouse)
15060 (rabbit)

Oregon Green 488**

488/514

592

life technologies

06380 (mouse)
06381 (rabbit)


Appendix A: List of some conjugates reported to work with Leica STED microscopes.
* recommended dyes for single color experiments;
** recommended dyes for single and dual color experiments.

Appendix B: Recommended dual color dye combinations

Dye 1

Dye 2

Name

Excitation

Emission: e.g.

Name

Excitation

Emission: e.g.

BD Horizon V500

485/470

475–510

Oregon Green 488/
Chromeo 505

514/520

523–580

Abberior STAR 440SX

458/470

475–515

Oregon Green 488/
Chromeo 505

514/520

523–580


Excitation: 458 nm/470 nm and 514 nm/520 nm; STED: 592 nm

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