transcript live cell imaging webinar on 18 july 2012 · 1 techniques and methods in live cell...
TRANSCRIPT
1
Techniques and Methods in Live Cell Imaging: Practical Advice for Miscroscopy‐based Research
Webinar 18 July 2012
[0:00:14] Slide 1 Sean Sanders: Hello and a welcome to this Science/AAAS audio webinar. My name
is Sean Sanders and I'm the editor for custom publishing at Science. Imaging technologies can be found today in just about every life
science laboratory. The imaging of live cells in particular, either in culture or in vivo, has enabled many biological questions to be addressed that were not possible with fixed samples. This webinar will examine some of the cutting‐edge technologies available today, and provide you, our audience, with a practical look at the challenges encountered and possible solutions when applying imaging modalities in a basic research setting.
We have a wonderful panel of experts on the line with me today.
They are Dr. Vytas Bindokas from the University of Chicago, Dr. Simon Watkins from the University of Pittsburgh School of Medicine, and Dr. Tomasz Zal from MD Anderson Cancer Center in Houston, Texas. It’s a pleasure to have you all with us today.
Slide 2 Before we get started, I have some information that our audience
might find helpful. Note that you can resize or hide any of the windows in your viewing console. The widgets at the bottom of the console control what you see. Click on these to see the speaker bios, additional information about technologies related to today's discussion, or to download a PDF of the slides.
Each of our speakers will talk briefly about their work. After which
we will have a Q&A session during which our guests will address the questions submitted by our live online viewers. So if you're joining us live, start thinking about some questions now and submit them at any time by typing them into the box on the bottom left of your viewing console and clicking the submit button. If you can't see this box, just click the red Q&A widget at the bottom of the screen.
2
Please remember to keep your questions short and concise, as this will give them the best chance of being put to our panel.
You can also log in to your Facebook, Twitter, or LinkedIn accounts
during the webinar to post updates or send tweets about the event, just click the relevant widgets at the bottom of the screen. For tweets, you can add the hash tag, #sciencewebinar.
Finally, thank you to Leica Microsystems for their sponsorship of
today's webinar. Slide 3 Now, I’d like to introduce our first speaker for this webinar, Dr. Vytas
Bindokas. Dr. Bindokas earned his PhD degree from the University of Illinois at Chicago and trained as a postdoctoral fellow at both the Davis and Riverside campuses of the University of California. He has been a research associate in the University of Chicago’s Department of Neurobiology, Pharmacology and Physiology since 1997, and is currently also the core director for the Biological Sciences Division/Cancer Center/Digestive Diseases Integrated Light Microscopy Core Facility. Dr. Bindokas has extensive experience in many different forms of fluorescence microscopy, including physiologic probes, FRET, FLIM, and superresolution microscopy as well as the associated hardware and software. A very warm welcome to you, Dr. Bindokas.
Dr. Vytas Bindokas: Hello all. Biology is the study of life and its processes. Because
biology is dynamic, it makes sense to observe the processes in context. While there’s great power in seeing biology happen, one must be cautious that the observations are indeed accurate and valid.
Slide 4 One problem is illustrated here. It’s the dreaded observer effect.
While the hazard of using a magnifying lens on a sunny day is really appreciated, the microscope can be just as harsh and worse. The problem increases when going from whole organism down to subcellular detail.
Slide 5 Two major types of live cell microscopy are transmitted in
fluorescence techniques. Bright field or transmitted light is suitable for colored materials like plant cells and even colorless samples can be observed in fine detail with technologies like phase contrast, DIC,
3
Hoffman modulation, etc. This technique is useful for tracking cells and even fine structural detail can be observed. A tip for all modes of imaging transmitted or fluorescence is to limit light exposure to the actual data capture by using automated shutters; don’t illuminate when you don’t need to.
Fluorescence is well suited for finer details since we can more readily
see a bright signal against the dark background. Very many types of fluorescent assay are possible and multiple targets can be observed using vital dyes, natural materials, fluorescent proteins, etc.
[0:05:03] Slide 6 General rules to get the best results are to use the least light, the
brightest probes, and the best detectors. The top panel shows a happy cell with ruffling edges and a broad attachment to the substrate. In panel 4 in lower left well, there was a brief UV flash applied to the cell and now we could see membrane blebbing or having these blister‐like structures form. The cell begins to round and eventually dies. You can get similar effects with any illumination that is too strong not just UV. So what should you avoid? You should also avoid over magnification since magnification concentrates the light. Also, avoid prolonged illumination because this creates greater photo damage and bleaching.
Slide 7 So what are the best detectors? It can be argued that a back‐thinned
EM‐CCD camera has the best efficiency. It has the ability to record a wide range of brightness data, it has good speed, it has very good signal to noise, yet they are very expensive and they tend to have too few and too large pixels. Next, we can get into all sorts of marketing wars. Are more pixels better? Do I need really high speed or can I really get by with a 5‐second exposure? How much bit depth do I need, in other words how many brightness values do I need to record?
The answers all depend on the questions being asked. One key point
is signal to noise in your images. New technologies are available and they are opening new possibilities. This example here shows mitochondria clearly distinguishable from background in just 3 intensity values above the background. That’s what the plot shows.
One more battle, what resolution do you really need for your study,
standard microscopy, confocal, or superresolution? Each technique
4
requires very different light requirements and more than likely, your prep’s light tolerance is going to set this choice for you.
Slide 8 So live cell microscopy has many more problems compared to fixed
sample microscopy. Beware that UV, blue light and other short wavelengths are potentially directly toxic as is any wavelength at very high intensity. You need to use but a few percent of full illumination brightness, in other words microwatts not milliwatts. If you must use UV, you can use longer exposures with less light to build up the image details. You can forestall damage by collecting fewer images over time or in Z, in different focal planes. Try to use redder probes since long wavelengths pose fewer problems.
Next, many materials when illuminated can generate reactive
oxygen species that damage cells and cause bleaching. Riboflavin in cell media and certain dyes are prime examples of this. Workarounds would be to exclude the B vitamins from all imaging media or you use fully defined saline for example hence balance salt solution. You can also radical quenchers or scavengers.
Next, focal drift can be a problem. Try to equilibrate temperature of
the microscope in all solutions. Try keeping the dish volume constant. You can, if you can afford it, use automatic focusing hardware devices. All of those can work well or you can accept that there will be drift and try to collect the 3D image stack and hope somewhere in that volume, you’re going to have that object that you were trying to observe.
Mammalian cells need both elevated temperature and their media
requires 5% CO2. For the short term, one can use alternate buffers or, you know, add HEPES to your normal media. Don’t use normal media, but you can try this for short term. But the best results require incubators.
Slide 9 So here, short‐term imaging can be made using just plain heated
dishes in the upper left as an example of an electronically heated dish. It’s better to heat the dish and the objective, objective heater is seen in the upper right. It’s even better to heat the entire microscope system. Maintaining gas is just not possible in open dishes without some kind of a mini chamber setup and in fact, the mini chamber is a good approach even in these large jacketed schemes because there’s a lot of airflow going through them.
5
Slide 10 Long‐term imaging is now possible using devices such as the
VivaView system. Here, the microscope is actually built into a regular cell incubator and this provides for ideal temperature, gas, and humidity control. This system even has motorized dish covers and you can see an example of that in the lower panel. It actually opens to allow you to feed the cells, to add drugs with the least perturbation of the environment within the microscope system.
[0:10:28] When considering doing this kind of multi‐day experiments, think
about collecting multiple fields of view per dish and multiple dishes/treatments so as you can concurrently collect as much data as possible in the least amount of time.
Slide 11 Next, moving targets are really difficult to deal with. One approach
would be to try to trap the cell into a thick solution. A number of possibilities exist. Thermogels are interesting because they’re liquid when they’re cold and they reversibly gel on warming allowing you to recover the cells fairly easily. The targets can also be trapped in shallow spaces, although this tends to be a short‐term fix. As evaporation happens, you tend to crush the cells in fairly short periods of time. Single targets can also be tracked automatically using certain software and motorized computerized XY stages. You can also try to keep more in view by using a thicker optical section and lower magnification to look at a larger area.
Another issue is the relative motion of multiple targets. Here, using
sequential image captures on one camera or one detector, the objects can move before all of the colored channels are collected giving the appearance that the signals are chasing each other, kind of like head and tail like. They may indeed be chasing each other, but the only way to know for sure is to collect the signals simultaneously.
Slide 12 Finally, many imaging needs are still beyond the current technology.
There are cases where faint signals, high magnification, multiple illuminations must be used. Photo damage still limits these types of experiments. Some fixes might be to some images to increase the signal, to use averaging to deal with detect the noise, to open up the confocality to view more molecules per image, etc. Importantly, you should be watching the technology because new approaches and new hardware are continuously becoming available. For example,
6
the major bottleneck like the diffraction limit has been recently bypassed, probes and dyes continue to be improved so really there’s much more biology, which is now directly observable and in ever greater clarity.
Sean Sanders: Great. Dr. Vytas Bindokas: Thank you for your attention. Sean Sanders: Thank you so much, Dr. Bindokas. Fantastic introduction and we did
have a few questions coming in so I have saved those and if you out there in the audience have additional questions, please do send them in using the question box on the left of your viewing console.
Slide 13 So we’re going to move right on to our second speaker for today and
that is Dr. Simon Watkins. Dr. Watkins was awarded his PhD from the University of Newcastle upon Tyne in England and received postdoctoral training at the Pasteur Institute in Paris, France and at the Dana Farber Cancer Institute in the U.S.
Slide 14 He was recruited to the faculty of the University of Pittsburgh as a
tenure stream assistant professor and was promoted to full professor with tenure in 2001. Currently, Dr. Watkins is a professor of Cell Biology and Physiology, and Immunology, and vice chair of the department of Cell Biology and Physiology. He is the founder of the Center for Biologic Imaging at the University of Pittsburgh, an internationally renowned intellectual center for the application of all aspects of microscopic imaging. His current research focus is understanding the mechanisms of communication between cells of the immune system using molecular and optical imaging tools. Welcome and thanks for being with us, Dr. Watkins.
Dr. Simon Watkins: Thank you. What I’d like to do is grow from the last presentation to
talk about one area of live cell imaging, which seems to be ever present but has had massive improvements over the last short period and this is really how to increase speed.
Slide 15 So why would we want to go fast? Well one can – on this slide is just
a list of the obvious candidates, endocytosis, calcium measurements and obviously as we move more towards organismal biology, we not only need to image the organs into cells, but also motions of things
7
within those organisms, for example, called heartbeat, blood flow, sperm motion, etc.
Slide 16 What I’d like to do now is to look at the microscope and its
components and try and imagine the limiting factors and whether they’ve been solved or not and then I’m going to give you an example of how each one of them is solved. The slowest component is the port changer, that is now fast switching. Another one of the slowest components is the electronic changer objective. This has not been solved but honestly, I don’t think it ever really is a problem, one uses the same objective for nearly all the experiment. Changing colors with cubes has also been solved. Opening and closing shutters at speeds has been solved. Changing the light color has been solved and also we are now using very fast cameras.
[0:15:45] Slide 17 Why would you want to change camera ports for example? Well for
us, the most common reason is when we want to combine modalities. For example, examining cell surface with TIRF and then wanting to find out what happened to the endocytosed or transcytosed molecules where we want to change to a confocal. To do this, we have to have the ports on the same side of the microscope as well, otherwise most microscopes as we all know have ports on opposites sides of the microscope or including the bottom port because you want to register the images easily and one really needs them on the same side. So this has been solved using Galvanometer‐driven port switches. They are available in up to three colors. The obvious one might be one from Intelligent Imaging, which you can see here has multiple ports and uses a Galvo to switch between colors. The Galvanometer runs at the same speed, it changes port in millisecond timescales.
Slide 18 Traditionally, to change the light color in a microscope, one might
use a halogen or Xenon or metal halide source and then combine that with filter wheels called cubes to change colors. These have defined change times, 15 milliseconds perhaps to change colors and also another 15 milliseconds to open and close the camera port, or open and close the shutter. This is a significant issue and probably one of the currently well solved problems but is traditionally one of the hardest things to do when you are doing fast imaging such that
8
we often had two cameras rather than one rather than change colors.
Slide 19 Also, traditionally, the light sources we use, for example here a
mercury or argon source, have spectrally very different signal strength so it’s hard to be quantitative.
Slide 20 I would argue nowadays that the diode illuminators that are
available, which are either diodes or dio pumps, and phosphors are incredibly impressive. The amount of power one can get out of these light sources as you can see from this table here from a seven‐color source measured in milliwatts. This is the filtered power, which is after it’s been through a specific filter are very, very high, in the hundreds of milliwatts range, which as you heard from the last talk is far, far more than you need to do basic fluorescence or any kind of fluorescence microscopy. These are available now from multiple vendors, Lumencor, Lumen, 89North, and also for Zeiss. So if you want to go fast, you need to use these.
Slide 21 The reason is that they have many colors. Unfortunately, they do not
have – you still can’t do UV so Fura imaging is not possible with a diode light source, but they are very fast. Instead of using a shutter, you turn them on and off and they have a rise time of about 0.25 milliseconds, which is exquisitely fast so you can move the shutter. They are continually attenuatable. In other words, you can choose any brightness so you remove the need for density filters. Also, if you combine them with multiband, match to multiband filter cubes, you can really have no moving parts in the microscopes when you change colors. You might need an emission filter wheel if you have bleedthrough problems, but routinely as I’m going to show you in a few images, you can have four colors running very, very fast.
Slide 22 Of course, to do this, you need to combine this with a fast camera.
Now traditionally, one uses cool CCD cameras. This is a slide, which shows how an interlaying camera chip would work where the light is received undetected in one area, one pixel then it moves across to another line and then it’s read out to the side.
Slide 23
9
Also, we also nowadays use EM‐CCDs or electron multiplying CCD cameras a lot. These are generally frame transfer chips and also have speed limitations.
Slide 24 In reality, they are relatively slow. Frame rates are about generally
around 20 million pixels per second. So, if you have a 1024 x 1024 chip, then you’ve got 20 frames per second as your maximum speed. Obviously, you can bend the image, which is where you put the data from many pixels into one pixel, that does increase frame rate because you have less pixels per frame, however, that does decrease resolution.
[0:20:32] Ultimately though, with these camera technologies, one speed is
limited by noise and the noise is limited by heat so the faster you make the camera go, the more heat is generated and therefore the more difficult to use at speed.
Slide 25 This has really been solved now using complementary‐metal‐oxide
semiconductors in which instead of having the pixels reading out serially, one reads out in columns, each pixel essentially is its own camera.
Slide 27 Nowadays and you can find the cameras generally have four times
the real estate of a traditional cool CCD camera and at that full frame which is 2000 x 2000 pixels, the readout is generally about 100 frames per second. As the pixels are read out, entire columns not one in sub arrays or one can read out individual columns with a full line of about 2000 pixels in about 5 microseconds, which essentially means you can do a field with 2000 x 200 pixels at about 1000 frames per second. This is striking and a dramatic improvement of what one could do traditionally.
All the camera manufacturers who make these devices are
improving noise. Right now, the noise is actually very, very low when one compares it with traditional cool CCD devices. There’s still some variance between the pixels because each pixel is its own camera, but to be honest, the technology is improving rapidly and daily. There’s three main manufacturers now Andor, PCO Edge, and Hamamatsu. They all make fine devices.
Slide 28
10
So what happens when we combine the high‐speed light sources with these new cameras? If you trigger the camera, in other words if you use the camera as a trigger source, in other words, it controls other devices, you can then use this to trigger changes in color from the light source. So instead of taking 50 milliseconds, which you would with a filter wheel, you’re actually down to the millisecond range, sub millisecond range. Also, actually one more thing, this can now be combined using a diode light source and the transmitted light illuminator so you can actually change from fluorescence to transmitted light using a combination of fluorescence LEDs and transmitted light, white LEDs.
Slide 29 So here’s an example. What you’re looking at here is and I’ll show
you a movie on the next slide, you’re looking at C. elegans. This slide, these worms came from Cliff Luke and Gary Silverman at Pitt and there are two‐color fluorescence they have, red and green colors. We can image, and this was tested using three colors, if we use the full frame, we get about 23 3‐color images per second. If we reduce the size down to 256 x 2048 using the LED and CMOS camera, we can get about 111 3‐color images per second.
So the next frame shows you a movie of what this looks like.
Remember, this was collected as serial black and white images for each color, not as a 3D, the two‐color image. So right now in the frame, you should be seeing the worms swimming around in perfect red and orange. You’ll notice there’s no delay between the colors and you’ll notice that the colors overlay perfectly. With a slower camera, there’s a sort of jagged, jumpy sort of look to it. Note this image here, this movie here has been slowed down threefold. It’s much, much slower than it’s actually happening in the real life. Okay.
Slide 30 So I do need to comment briefly on the use of confocal microscopes
and speed. This has been made significantly better using resonance scanners, but this is still a single point detector and it’s better if you can use array detector because of this increased sensitivity. Obviously, we combine these with multipinhole confocals and there’s many, many commercial solutions now available, which allow you to go at speed.
[0:25:05] Slide 31 Honestly, if you want to do live cell imaging at speed, the
multipinhole solution has always been better for us in our hands. An
11
example here is the Prairie device, which actually has multiple pinholes as well as being very, very fast. When you use this combined with the triggered devices for cameras, you can go very, very fast.
Slide 32 Unfortunately, you still are using the non‐CMOS cameras, in our
hands, we’re still using EM‐CCD so there is a speed limit because the EM‐CCDs cannot run as quickly as a CMOS camera. Generally, we feel that if you are doing this kind of microscopy, you do need to combine this with a Piezo Z stage to get high‐speed 3D stacks. There are now made by Mad City Labs and PI. But right now, high speed 3D confocal can do two or three 30 Z positions per second. If you do decide you need confocal or you do want to go fast, it’s important that you compare all the various technologies to come up with a solution that best fits your experimental needs be that resonant detector or a multipinhole detector with a multipinhole solution with an array detector.
Slide 33 So in summary, what I’ve tried to show you in the last few minutes is
that speed issues are the new frontier in live cell imaging. Honestly when you go out and you select the system you wish to use, most of these speed problems are solved and widefield can go truly fast without any moving parts and the confocal could go relatively fast with no moving parts.
Also what’s interesting and new is that the new microscope stands
that we’re seeing coming out now have many of these components implemented at their core so the microscope stands themselves have become quicker. With that, I’m going to close and hand off to the next presenter. Thank you.
Sean Sanders: Wonderful. Many thanks, Dr. Watkins. Slide 34 Our final speaker for this webinar is Dr. Tomasz Zal. Dr. Zal
completed his Master’s and PhD degrees in Poland at the Wroclaw University of Technology and the Institute of Immunology and Experimental Therapy of the Polish Academy of Sciences, respectfully. He underwent postdoctoral training at the National Institute for Medical Research in London and at The Scripps Research Institute in California. Dr. Zal then moved to the University of Texas and MD Anderson Cancer Center in Houston, Texas, where he is
12
currently an assistant professor and director of the Immune Imaging Core at the Department of Immunology. The goal of his current research is to understand the spatiotemporal regulation of immune interactions and the role of tissue microenvironment in immunological regulation, for which he employs various dynamic imaging techniques. Dr. Zal has developed new dynamic imaging modalities, such as FRET and intravital dynamics‐immunosignal correlative or iDISC microscopy, and is experienced in numerous imaging technologies, ranging from intravital multiphoton microscopy to FRET to superresolution microscopy. Many thanks for joining us, Dr. Zal.
Slide 35 Dr. Tomasz Zal: Welcome everyone. It is quite clear from the previous presentations
that live cell imaging offers a key advantage as an experimental method. Basically, it allows us to follow the cellular and structural biology simultaneously into space and time. The temporal dimension was nicely covered by Simon so we now know how to set up live microscope, some imaging microscope that use a very high temporal resolution.
In my presentation today, I’d like to talk about the spatial dimension.
Basically, what I will talk about is how to optimize the microscope depending on the situation that we have and the specimen that we handle. For example, how to optimize the optics for aqueous samples for cells in chambers then how to observe single living cells best in vivo. Then I will change gears and change the spatial dimension to the molecular level. We’ll talk briefly about imaging molecular interactions focusing on FRET. Then I will talk about a common problem that we have, which is how to relate the dynamics of cells that we observe during an experiment to the molecular information that cannot be easily visualized in living cells. In that part, I will talk about an imaging technology that we named Dynamics‐Immunosignal Correlative microscopy.
Slide 36 So it’s quite clear that live cells exist on a water environment and for
optics this really presents a challenge. To explain this, let’s compare an ideal fixed specimen on the left with a live cell chamber on the right. In the fixed cell specimen, these cells are mounted in a mounting medium whose refractive index is in an ideal situation very similar to the refractive index of the cover glass. Since the indices of the immersion oil is ‐‐ active index of the immersion oil is almost the same as that of the glass, the light travels from the fluorescent
13
object to the lens in straight lines. An important implication is that it is now possible to focus up and down through this cell without experiencing any spherical aberration.
[0:30:47] Now, the same oil immersion objective if it’s used for a live cell in a
chamber filled with water medium, which has a lower refractive index, if we did that we would experience rapidly increasing spherical aberration and loss of detail if you wanted to focus away from the bottom of the dish.
If we look at the right, to overcome this problem, live cells are best
imaged with special objectives that are designed to maintain the optical contact with the cover glass using water rather oil. So in this situation, the light travels from the fluorescent object and at the most we’re talking about fluorescence today. The light travels from the fluorescent object to the objective and through the bottom of the dish and on that path, it gets bent twice in equally opposite angles. So basically, the ray that leaves the sample and the ray that reaches the objective, these two rays are now in parallel. So again, now it becomes possible to focus deep within the sample without spherical aberration so cells can be imaged freely.
Now, there is an important point. So in this arrangement now unlike
in fixed cell situation, the thickness of the cover slip is critical. It is important to compensate for the thickness of the cover glass with the variations that you may have using the objective, the collar that should be on a good water immersion objective. You can also use the same collar to compensate for the temperature that we have in the chamber.
So, it would seem that all problems are solved except that the water
that couples the objective to the cover glass bottom evaporates during prolonged kind of experiments. This problem has been now solved recently by using objectives that are optimized for silicone oil as the immersion medium of the water. So basically, by using silicone instead of water, we can avoid the evaporation and also silicone doesn’t show moisture as for example glass or wood. Another bonus of using silicone is the refractive index is slightly higher than that of water and for certain types of cells and organelles, this may be a better match because the refractive index of certain structures is a little higher than that of water. Of course, an old‐fashioned way to solve the above‐mentioned problem would be also to replenish the water using a small pump.
14
Slide 37 Yet another arrangement is available for live cell imaging on upright
microscopes. But if we go back to the previous slide, I’d like to mention briefly about one additional topic that was already mentioned before, which is how to maintain the focus. Actually let’s stay on this slide. By setting a lower refractive index in the imaging chamber, we can now realize correction of the focal drift by introducing a longer wavelength light through the objective that will now bounce from the boundary between the cover glass and the water in medium and go back into the objective, it can be now picked up by a sensor so that a negative feedback loop can now compensate for any drift. So this is known as autofocus and this technology is now available from various manufacturers.
Slide 38 So, water or silicone immersion works well with live cell chambers
using cover glass bottom. But for upright microscope, it’s possible to directly dip the objective in the open dish. Water immersion, water‐dipping objectives are designed to provide sharp images without a cover slip so we need a different type of objective which is also very expensive I should mention.
[0:35:25] The direct dipping method is actually associated with its own
problems. So, it’s difficult to keep the cells sterile, media tend to evaporate. However, it is easy now to introduce tools such as micropipettes and electrodes into direct contact with these cells. Importantly, the direct dipping configuration has been widely adopted for in vivo microscopy in particular for multiphoton laser scanning microscopy, which is my second topic today.
Without going into details, in 2‐photon microscopy, it is now possible
to excite fluorophores using a lower energy light, near‐infrared light and fluorophore is basically absorbed two or more lower energy photons almost simultaneously and this process is very efficient only in the focus of the objective. Sorry about that ringing, I couldn’t switch my primary line. The 2‐photon excitation allows us to remove the pinhole so that we can now detect all photons, which contribute to the image increased sensitivity.
Now, what I’m getting to is a problem with keeping the tissue steady
when we have the dipping configuration. Basically, the top of the tissue surface can now move unless the tissue is intrinsically not moving at all such as the bone for example. But if we have the lungs
15
or the intestine or some other soft tissue that is motile, the direct dipping configuration is problematic.
To overcome this instability, what we do in our lab is to use a holder,
which basically it consists of a cover glass and a suction. You can see it in the lower right. In this device, soft tissue is gently sucked against the cover glass and the exposed surface now becomes nicely stabilized against the glass. So similar devices were described in the literature. We adapted it for our needs for lung imaging.
So now, we are going back with intravital imaging to the optics that
requires water immersion objectives again. As of today, several manufacturers offer excellent water immersion objectives that can be used for intravital imaging. Basically, those are low magnification objectives, but has high numerical apertures and they’re corrected for cover glass.
Slide 39 So since we are talking about intravital multiphoton imaging, to
briefly discuss another problem, which is basically caused by having to do all the imaging with just a single laser. So, current multiphoton microscopes remain limited by allowing the excitation with only one wavelength. This presents a problem because it might be difficult to excite two or more distinct fluorophores with just one wavelength in an optimal way. As you can see on the spectrum on the left, such a situation occurs if we have for example cyano and yellow and red fluorescent proteins.
What we do in our lab is to use dual laser multiphoton excitation. On
the right, you can see the results, which comes from a time‐lapse movie that I will not show today recorded in vivo, in the lungs. So, we are basically combining the soft tissue holder here with dual 2‐photon excitation. We can now see four signals. We can see the CFP, YFP, DsRed as well as second harmonic generation and all this is picked up by only two detectors by multiplexing the excitation.
Slide 40 So what if we don’t have a 2‐photon microscope, can we use our
regular confocal for in vivo work? So, the multiphoton excitation clearly is the preferred method, but in fact we can use confocal scanning for certain in vivo situations. So let’s compare briefly the advantages of multiphoton and confocal imaging as it stands today.
[0:40:18]
16
So, it’s granted that the depth of useful imaging is better in 2‐photon microscopy. We can go typically about 300 microns deep into the tissue whereas in confocal we can go 100 microns, but often this is quite sufficient. It is said that photobleaching and phototoxicity is lower in multiphoton microscopes, but what we actually see is quite opposite. Confocal can be quite mild on cells if we use the low laser powers of course. It’s also important to use resonant scannings to decrease the dwell time and accelerate the speed of imaging.
If we have a single multiphoton laser, of course the number of
channels is low 2 to 4 whereas in confocal we can increase the number of useful colors. The resolution on multiphoton is a little bit worse. When using confocal, we can also use additional sources of contrast such as autofluorescence and reflection. Of course, the price is much, much lower as well, which is important.
Slide 41 So, to move on to the dimension of molecular interactions, there are
of course numerous methods that are available to detect molecular interactions in living cells. Here, I have arranged these three major approaches on a triangle. This arrangement is to convey that there is no single method for imaging molecular interactions that is better than other.
Let’s take FRET. In this method, the donor fluorophore is excited and
the energy is transferred to the acceptor depending on the distance. The key measure of FRET is the efficiency, which basically is a value from 0% to 100% of the photons that are picked up by the donor and then transferred to the acceptor. If the efficiency, the E value, is known, then we can calculate the distance between the fluorophores, which of course is of great importance for understanding the biology. FRETiness can be measured in several ways as shown here on here sensitized emission, donor lifetime FLIM, polarization. The key advantage of all those FRET methods is that they can be performed in real time with a higher resolution.
In contrast, bimolecular fluorescence complementation doesn’t
allow to follow interactions in real time because the principle of the method is based on the recombination of two pieces of a fluorescent protein into fluorescent for a molecule and that process is irreversible. However, fluorescent complementation is very sensitive in contrast to FRET, which is actually a method that is not very sensitive. There’s also fluorescence cross‐correlation available, but it’s not often used for imaging so I will not go into it today. So this is
17
the case with FRET as the non‐perfect, but preferred method, to image molecular interactions in living cells.
Slide 42 In this slide, I briefly summarized basically a recipe how to calculate
FRET efficiency from desensitized emission, which is still a topic of certain confusion in the community. Just to make the long story short, we basically need three images of which the first two images where we excite the donor and read the emission of the donor and the acceptor, which contains the sensitized emission. These two images are collected simultaneously preferably with dual cameral or an image splitter and then we quickly change the excitation and collect the image of acceptor and we need that said image to eliminate the bleedthrough, which is present in all these three images at certain levels. Then we can calculate FRET efficiency based on the formula that you can see here after we collect the bleedthrough or the SE, the sensitized emission. Important feature of this method is that we can now correct for fluorophore photobleaching so that we can easily follow FRET in time or in 3D as shown on the right and I will not go into this detail today.
Slide 43 So now, we have the data, we have the dynamics of our cells that we
observed. The question now is how do we now relate this dynamics to the biology. Using various type of software, we can observe and quantify very nicely motility parameters. For example for translational motility, we can answer the question is the motility now faster or slower due to treatment or genetic mutation, does it become random or directional or is it steady or with arrests? We can follow shape dynamics in various ways. We can follow calcium fluxes.
[0:45:40] But what we would like to do really is to relate this dynamics to
molecular information that may not be readily available for imaging in a live cell. Of course, we can quantify all these parameters using various software, which tends to be expensive so let me just mention again that ImageJ is a free package that is very powerful if you download the correct plugins.
Slide 44 So, the method that we used to now relate the dynamics of cells to
molecular information of almost any kind is the Dynamics‐ImmnunoSignal Correlative microscopy or DISC microscopy. But I think it’s very simple. What we do is to visualize live cells in the dish
18
or in vivo and then we fix those cells in a rapid manner using formaldehyde. Then we can take the specimen and stain it for almost any molecule including signaling proteins using typical immunofluorescence technologies. What we can do now is to align the resulting images with the prior intravital or dynamic recordings using landmarks and then we can do that on a subcellular basis.
Slide 45 So to explain this, let’s quickly go over this last example. Here we can
see a movie, which I hope you can see on your screen, where you can see a red tumor, which is surrounded by green T cells. These T cells are very heterogeneous. Some of them are moving in a translational manner. Some of them are interacting immobilized. We would like to now understand how these cells are different, but we cannot do that in vivo because our ways to label the cells in vivo might be very limited.
Slide 46 So, we are now fixing this movie at the last frame and in this slide
you should see now the last frame of the movie on the left and after fixation you see the same area in the specimen observed on a slide.
Slide 47 We can now identify every single cell by numbers. You can recognize
that the shapes remain unchanged. Slide 48 So, what we can now do is to basically stain our cells for almost any
molecule that we choose. Here, just a proof of principle, we are staining for CD4 and CD8, which label the helper and cytotoxic T‐cells. So we can now give names to our cells. Here is a CD4 cell, here’s a CD 8 cell.
Slide 49 We can feed this information back into the intravital movie and we
can now identify the motility of these two populations. Of course, we can repeat this process for almost any fluorophore and any molecule using antibodies that are available. In this particular example, we are finding that CD8 cells are slower and they are rested for longer times when interacting with the tumor.
Slide 50 So to pull it all together, what I talked about today is basically how to
adjust the microscope spatial resolution depending on the specimen.
19
The conclusion is that special objectives are required. They are expensive, but they are very critical. For in vivo imaging, I didn’t mention it, but I like to emphasize that the upright configuration is probably more versatile in particular when you use a suction‐based tissue holder than the inverted configuration for example. It’s important to use adjustable objectives to correct for refractive index changes and that’s also important for in vivo work. You can use confocal for intravital imaging quite efficiently if you don’t have 2‐photon.
[0:50:03] As far as imaging of molecular interactions, we very much like FRET
by sensitized emission and we can use it in a quantitative manner to calculate FRET efficiency. We like this method because it’s gentle on live cells and fast and quantitative.
Dealing with the data of course is the weakest link in the chain, it’s
most time consuming. But one thing that we are doing now is to correlate the dynamics of cells with the molecular information that we can obtain by regular immunofluorescence microscopy.
If you’d like to see more on the topic, here’s a reference to our
recent publication. So with that, I’d like to finish and thank you for listening.
Sean Sanders: Wonderful. Thank you so much, Dr. Zal, and thank you to all of our
speakers for the fascinating presentations. Slide 51 We’re going to move right on now to the questions submitted by our
online viewers. We have about 10 minutes so we’ll get through as many of those questions as possible. A quick reminder to those watching us live that you can still submit questions by typing them into the textbox and clicking the submit button.
So I’m going to start with a question for the entire panel that asks,
and maybe we’ll start with you, Dr. Bindokas, is there a recommended approach for assessing the viability of cells in culture when performing fluorescent imaging?
Dr. Vytas Bindokas: Primarily, you can use the morphology of the cells if you’re using
transmitted light modes. From experience, you can get an impression of what the cell will look like in a normal fashion.
Slide 6
20
For example, this slide I showed, these cells have protruding lamelopodia the sheet‐like extensions and the fingerlike filopodia, which are observable over time and in the bottom row, you can see what happens to the cell when it’s unhealthy. The membrane is being disrupted, vacuoles will form in the cell, the cell will round up and basically detach from the media. You can also use fluorescent techniques and I’ll pass that to my colleagues.
Sean Sanders: Dr. Watkins? Slide 51 Dr. Simon Watkins: So absolute health there are dyes one can use, for example live cell,
dead cell, live/dead kits. One we use a lot is ‐‐‐ one of the first components of the cells become perturbed and distressed is the mitochondrion so cells MitoTracker dyes work very well. Of course, most cells are generating some RO species, reactive oxygen species when they’re under stress so dyes like DHE or if you think it’s mitochondrial MitoSox all give you some indication of cell stress. But I would also agree that cell morphology is a very powerful component when you’re looking at the cells in a dish.
Sean Sanders: And, Dr. Zal, anything to add? Dr. Tomasz Zal: Yes. So what we’d like to do to look at the viability, the other way is
to how cells move. If the motility remains consistent throughout the whole imaging sequence and if there were no intentional perturbations, we like that. Another thing that we do is to take the cells after imaging and culture them and see if they keep on growing and dividing.
Sean Sanders: So I also had a question and that is maybe related to this asking if
there’s any reagents that are available that work as radical scavengers to keep cells healthy maybe something like vitamin C and would this help? Dr. Zal, maybe you could talk to that?
Dr. Tomasz Zal: Trolox is one reagent that’s been reported to decrease
photobleaching and increase viability vitamin E as well. Sean Sanders: Excellent. Dr. Simon Watkins: We use Oxyrase. Oxyrase is another product, which is very good. So
it scavenges oxygen so you’re working in a slightly low oxygen environment, but it improves the scavenging of – decreases free radical generation.
21
Dr. Vytas Bindokas: You can also use propyl gallate, which is a nontoxic fairly general
scavenger. There are scavenging enzyme systems for instance B27 supplements that can be added to the solution. These work outside the cell, not inside the cell. Beta‐carotene can be a scavenger for singlet oxygen. EzyTe is great, but it’s going to kill your cells.
Sean Sanders: Great. So a broad question now that maybe I’ll send to you, Dr.
Watkins, this person is interested in live imaging of nanometer scale bacteria on an outer surface membrane and they were asking is this possible? So I guess the broader question is what is the current resolution limit of the microscopy techniques that are available?
Dr. Simon Watkins: So most live cell techniques rely on minimizing and this was alluded
to at the beginning of this discussion. So the superresolution techniques be they STRM, STED, PALM, FLIM, whatever four‐letter acronym you wish to use, often rely on collecting many, many images. For example to do a good PALM or STRM image stochastic optical reconstruction microscopy or photo‐activated localization microscopy demand tens of thousands of images. So while some people are using them for live cell imaging, they’re really not ready for primetime. Structured illumination microscopy, which demands many, many less images, but also does not give you quite the same resolution improvement is probably currently a viable option.
[0:55:51] So to give you an idea, normally the ultimate limit of resolution in XY
on a microscope is about 200 nanometers. Structured illumination microscopy will bring that down to about 125 nanometers or maybe a little less. STRM and PALM go down into the 20s to 30 nanometers. You could also use STED, stimulated emission depletion microscopy, which is a confocal technique, but that brings all the dangers of phototoxicity that come with a point scanning system.
So my own personal bias is towards FLIM imaging in that it gives you
a definite improvement in X, Y, and Z resolution. But more importantly, it doesn’t demand use of different fluorophores than the one we’re using at the moment. It works with any fluorophore, as it’s an optical trick rather than depending on a dye or a probe. However, we still have problems resolving the range that this question was asked at, you know, and I think in this case like many of us, we go back to electron microscopy, which is obviously a non‐live cell approach.
22
Sean Sanders: Fantastic. A question for you, Dr. Bindokas, about the CyGEL. This viewer asks when to use a CyGEL slow tracking of fluorescently labeled compounds that are being released from the cells and monitored in real time?
Dr. Vytas Bindokas: I suppose that’s possible. It depends on what the particular material
is. I’m not convinced I know what the pore size of that CyGEL gel. So it may slow down the diffusion, it probably does, but I would say that’s a better question for their product support people.
Sean Sanders: Great. Dr. Simon Watkins: Can I speak to the CyGEL question for one second? Sean Sanders: Sure, absolutely. Dr. Simon Watkins: This is Simon Watkins. CyGELs are not very gas permeable so you
can’t use them for longer term imaging. They really are already used for short term imaging. But they do allow you to image non‐adherent cells in a live situation but not for long term imaging.
Sean Sanders: Well that’s actually a perfect lead‐in to my next question, which was
asking about imaging of non‐adherent cells so perhaps you could talk a little bit more about that then we can take comments from the other speakers too.
Dr. Simon Watkins: Actually, that was my answer. Sean Sanders: Okay. Fantastic. Dr. Zal, any other— Dr. Simon Watkins: Using CyGEL is a good way of doing it. There are certain situations for
example we’ve done a fair amount of imaging of T‐cells where we’re looking at calcium flux and we actually put the T‐cell receptor on the dish and then dropped T‐cells in. They come down and adhere to the dish because of the receptor on the dish surface, but those are generally very specific products. Loosely adherent cells, you can either use Matrigel to embed the cells in or you can use poly‐lysine or collagen‐coated dishes, they all help, but they do not specifically solve the problem of totally non‐adherent cells.
Sean Sanders: Dr. Zal, some more comments? Dr. Tomasz Zal: I think it’s been answered. It’s important to remember that non‐
adherent cells in real life they actually are surrounded by some kind
23
of matrix unless they are in the blood so it’s really important to put those non‐adherent cells in some kind of matrix like Matrigel or collagen matrix and to really look at those interactions in such an environment.
Sean Sanders: Excellent. So we’re at the top of the hour, but I’m going to try
squeeze in just a few more questions before we finish and I’m going to stay with you, Dr. Zal. It’s a question about your iDISC technique. The viewer is asking if it’s possible to extend DISC microscopy to look at intracellular events such as pathogen entry into cells?
Dr. Tomasz Zal: And this is actually exactly what we did and I didn’t cover it today,
not pathogen entry, but we looked at intracellular protein phosphorylation during T‐cell receptor activation. So for further reference I would refer to the publication that I showed on the last slide.
[1:00:03] Basically, we looked at dendritic epidermal T‐cells and then we
followed their motility in vivo and then we fixed the tissue and looked at proteins and kinase phosphorylation CD3 receptor phospho forms and so on, zeta chains and we could exactly where T‐cell receptor was activated in vivo and at the sub cellular resolution.
Sean Sanders: Perfect. So, Dr. Bindokas, a good question for you on temperature.
How important is the temperature for living cell imaging experiments and how long would one need to maintain the temperature for an experiment? For instance, is a 30‐minute imaging experiment considered a long time and would that timespan be enough to affect the cells?
Dr. Vytas Bindokas: It’s quite possible that even a short departure from 37 degrees can
affect the biology you’re trying to study. It depends really on the enzymes and how quickly they double their speed of action, the Q10 factor. You know, a couple degrees could make a difference. In general, you want mammalian cell physiology to be above absolutely 32 degrees Centigrade. Below that, most systems are affected. If they’re affected at 36 versus 37, it may depend on the particular system. You know, having good temperature control is critical for long‐term imaging, but even I the short term it could be a significant factor.
Sean Sanders: Any other comments from the other speakers? If not, I’m not going
to ask you all a final question and we’ll start with Dr. Bindokas.
24
Where do you think the biggest advances in microscopy are going to be seen in the next say two to five years and what are you looking for in order to really drive your research?
Dr. Vytas Bindokas: The push is still smaller and smaller. The diffraction limit is currently
broken. We’re down to a few tenths of nanometers, but that’s still large compared to many of the individual proteins and molecules that we’re interested in studying.
Sean Sanders: Great. Dr. Watkins? Dr. Simon Watkins: I think a combination of scale and speed. Many of the high
superresolution techniques are speed limited. By integrating the faster, ever faster cameras and detectors, cameras and light sources we’ll be able to improve the speed at which we can use these superresolution methods such that we can use them practically within a live cell situation.
Sean Sanders: And final word for you, Dr. Zal. Dr. Tomasz Zal: Oh, well what I am very excited about is the optogenetics basically
not only to image cells live, but to use light to manipulate cell behavior and to interrogate the processes with subcellular resolution and this is now possible with channelrhodopsin molecules that can be activated to induce ion fluxes in a particular sub region of a cell. So this combined with imaging already is very informative and I’d like to see this employed more often actually.
Sean Sanders: Fantastic. Well unfortunately, we are out of time for this webinar so
many thanks to our speakers for providing such interesting talks and discussion and for being here to answer questions for our audience. They were Dr. Vytas Bindokas from the University of Chicago, Dr. Simon Watkins from the University of Pittsburgh School of Medicine, and Dr. Tomasz Zal from the MD Anderson Cancer Center.
Many thanks to our live online audience for all the questions you
submitted to the panel. I’m sorry that we didn't manage to get through all of them.
Slide 52 Please go to the URL now at the bottom of your slide viewer to learn
more about resources related to today’s discussion and look out for more webinars from Science available at webinar.sciencemag.org.
25
This particular webinar will be made available to view again as an on‐demand presentation within approximately 48 hours from now.
We'd love to hear what you thought of the webinar, send us an
email at the address now up in the slide viewer, [email protected]. Again, thank you to our wonderful panel and to Leica Microsystems
for their kind sponsorship of today's educational seminar. Goodbye. [1:04:35] End of Audio