transcript stem cell imaging tips, tricks and best practices cell... · 1 stem cell imaging: tips,...
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Stem Cell Imaging: Tips, Tricks & Best Practices [0:00:20] Slide 1 Sean Sanders: Hello and a warm welcome to this Science/AAAS webinar. My name
is Sean Sanders and I'm the editor for custom publishing at Science. In today's webinar, we'll be talking about the preparation and
imaging of stem cells. The unique pluripotent nature of stem cells makes them of particular interest to researchers looking to treat and cure a number of disorders, including neurodegenerative diseases, heart disease, and spinal cord injuries. In order to investigate stem cell function in vivo and in vitro, researchers need robust methodologies and accurate imaging capabilities.
In this webinar, our expert panel will focus on best practices for the
manipulation and imaging of stem cells in the laboratory, and explain how they have successfully applied the latest imaging solutions to advance our understanding of stem cells and their application in disease treatment.
Slide 2 I'm very pleased to welcome three very knowledgeable speakers for
our webinar today. They are Dr. Weibo Cai from the University of Wisconsin‐Madison in Madison, Wisconsin, Dr. David Schaffer from the University of California at Berkeley, and finally Dr. Clemens Cabernard from the Biozentrum at the University of Basel in Switzerland. I very much appreciate all of you taking the time to be with us today.
Before we get started, I have some important information for our
audience. Please 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. If you are watching 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 the
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box, click the red Q&A widget at the bottom of the screen. Please remember to keep your questions short and to the point as this will give them the best chance of being put to our panel.
You can also log in to Facebook, Twitter, or LinkedIn 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. Now, I'd like to introduce our first speaker for today and that is Dr.
Weibo Cai. Dr. Cai is currently an Assistant Professor of Radiology and Medical Physics at the University of Wisconsin‐Madison School of Medicine and Public Health. He received his Ph.D. in Chemistry from the University of California, San Diego and did his postdoctoral research at the Molecular Imaging Program at Stanford University. In 2008, Dr. Cai joined the University of Wisconsin‐Madison where his research has focused on molecular imaging and nanotechnology. Dr. Cai is a prolific author and has won many prestigious awards, including the Society of Nuclear Medicine Young Professionals Committee Best Basic Science Award in 2007 and the European Association of Nuclear Medicine Springer Prize in 2011. Dr. Cai, a warm welcome to you.
Slide 3 Dr. Weibo Cai: Thank you, Sean, for the very kind introduction. Welcome everyone
to this Science webinar series. Today, I'm going to give a brief overview about the techniques that we use to track stem cells in vivo while Dr. Schaffer and Dr. Cabernard will be more focusing on the cell‐based imaging studies.
Slide 4 So, the field of molecular imaging has really expanded tremendously
over the first decade of this century and on this slide, you'll see some of the major techniques that people use for imaging applications, many of these can be applied to stem cells. For example, the reporter gene techniques or the imaging of the metabolism where you can use this to label cells and then track them in vivo and I will give you a few examples about this a little bit later in the talk.
Slide 5
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In terms of imaging techniques, there are several different imaging modalities and here are the six major imaging modalities that people use for in vivo imaging. Many of these have the corresponding clinical imaging equipment and the last decade has really witnessed very tremendous advancement in terms of making these scanners for many more imaging studies. So, the six techniques include optical imaging, which includes fluorescence and bioluminescence, also MRI (Magnetic Resonance Imaging), and then PET (Positron Emission Tomography), SPECT which is Single Photon Imaging Computed Tomography, and ultrasound, and the CT. For ultrasound and the CT, they are not very commonly used for stem cell tracking in vivo so most of it is focused on the other four techniques namely optical, MRI, PET, and SPECT.
[0:05:09] Slide 6 So, this next slide shows some of the agents that have been used for
tracking cells in vivo. For fluorescence, there are three major categories of fluorophores. One is the organic dye, the other one is fluorescent protein, the most famous being the green fluorescent protein or red fluorescent protein, recently, they are also [indiscernible] fluorescent protein, and nanoparticles such as quantum dots, which also are fluorescent.
For bioluminescence imaging, they are typically focused on
luciferases like firefly luciferase, renilla luciferase. For MRI, the major agents used include iron oxide nanoparticles or some gadolinium‐based chelates.
Slide 7 Those three techniques do not use any radioactivity at all and there
are also a few agents that are radio‐labeled where you can use this for PET imaging or SPECT imaging. Here, I gave a few examples about some of the commonly used agents for labeling cells so that you can track it with SPECT or PET. For example, indium‐111‐labeled oxine and technetium‐99m‐labeled HMPAO, which can be used for SPECT imaging. While for PET, FDG is one of the most commonly used agent for clinical use in oncology and it has also been used for stem cell imaging.
Slide 8 So, I'll give you just a few examples of each of these techniques, and
this slide shows one of the earliest clinical studies on tracking cells in vivo using 18F‐FDG labeled bone marrow cells. In A and B, they
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injected unselected bone marrow cells, which means the stem cell population is very low and it does not home to the heart as much as in C and D where they preselect some of the cells through FACS analysis using CD34 as a stem cell marker, and they found that it indeed goes to the heart at much higher frequencies than the unselected cells. So this is one of the earliest studies in terms of in vivo imaging in patients of labeled stem cells and this used positron emission tomography as the imaging technique labeled with 18F‐FDG, which is a glucose analogue.
Slide 9 This slide shows a reporter‐gene based technique for tracking cells in
vivo using a multimodality imaging and this is done in animal models. In this case, the reporter gene includes three different proteins. One is firefly luciferase, which can be used for bioluminescence imaging. The other one is a red fluorescent protein, which can be used for FACS analysis and microscopy studies to isolate the cells. And then the TTK, which is a truncated thymidine kinase, which is a reporter gene for PET, for positron emission tomography, which can be used for long‐term monitoring of the stably transfected cells. In this case, they used RFP to select the best transfection method. They found that lentiviral method gives the highest transfection efficiency and also is confirmed by microscopy studies.
Slide 10 So, for in vivo tracking, the top shows bioluminescence imaging
where they found that after injection of these cells into the heart of the rat, the signal goes up in for about four weeks showing that the cells are proliferating and this was also confirmed via PET, which is shown on the lower panel. The two imaging modalities match pretty well.
Slide 11 One extra feature about this reporter gene is that this TK reporter
gene is also a suicide gene. So if something goes wrong,… as we all know one of the major fears of pluripotent stem cells is that they form teratomas, and in this case, the reporter is also a suicide gene. So we can administer a certain drug, which will eliminate the teratoma as shown here in the left where most of the signals are gone for both bioluminescence and PET on the left panel while in the right panel saline control shows that the teratoma keeps growing.
Slide 12
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So which is the best method to use for in vivo imaging? Here is a kind of very short summary about the different imaging techniques and radiolabeling includes both PET and SPECT or MRI typically includes gadolinium chelates or iron oxide nanoparticles. Reporter genes typically include either single modality reporter gene or multimodality reporter gene.
In terms of biocompatibility, which is also kind of the least toxicity,
radiolabeling and MRI has both been shown to be pretty safe. Reporter gene has not been extensively validated.
[0:10:03] In terms of least perturbation to the biological function,
radiolabeling and MRI has also been shown to be very safe while reporter gene techniques still need a little bit more validation, but many studies have shown that indeed reporter genes did not affect the biocompatibility of all the biological functions.
In terms of anatomical location, this depends on the imaging
techniques we are using. MRI can give a very high resolution while the radiolabeling with PET or SPECT, resolution is a little bit lower and reporter genes depending on which techniques they are using, you can get either high resolution or low resolution.
In terms of cell viability, if you label the cells with radiolabeling or
MRI, you don’t really know whether the cells are alive or dead because all you are detecting is the actual signal of the label. For the reporter gene techniques, you'll know for sure the cells are alive because if the cells are not alive, they're not expressing the protein then you cannot detect it.
In terms of dilution in signal, when the cells divide, those that are
radio‐labeled or MRI labeled will be diluted into daughter cells while the reporter genes if they are stably transfected, they should be quite stable for long term tracking.
Slide 13 to Slide 14 So, these techniques can apply to not only stem cells but also
tracking on T cells if you're interested. It can also be used to track stem cells so that we can better understand stem cell biology.
Slide 15 Of course, one of the hottest topics on stem cells is induced
pluripotent stem cells, which were developed by a group in Kyoto University and also another group in the University of Wisconsin‐
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Madison. These induced pluripotent stem cells can also be tracked long term and there's a recent publication on stem cell earlier this year. They showed that you can use bioluminescence imaging where the cells were stably transfected with firefly luciferase and they were able to monitor the long term proliferation in vivo as shown in the bottom panel of this slide.
Slide 16 So, to quickly summarize, the wide availability of these small animal
imaging systems has really made molecular imaging a very useful tool for stem cell research as well as several other biological disciplines. In terms of the techniques used for stem cell tracking, there are generally speaking four major techniques: Optical imaging, which includes fluorescence and bioluminescence, MRI where the cells are labeled with magnetic nanoparticles or magnetic compounds, and nuclear imaging where the cells are labeled with radio‐labeled agents, and reporter gene approach which can be either single modality of one of the abovementioned three or combinations of multiple imaging techniques.
Nuclear imaging basically means PET and SPECT, and MRI is useful
for measuring acute deliveries. You can see exactly where it goes right after injection, but it cannot give you too much information in terms of the long‐term survival, proliferation, or differentiation.
The reporter gene approach is most suitable for long‐term tracking,
but it will need a lot more validation before it can eventually go to human studies. But nonetheless, some of the reporter gene‐based imaging studies have been in patients, probably not in stem cells, but in some other disciplines so there is definitely a future for this.
In terms of imaging techniques, the different imaging techniques
they are complementary. They can allow for cross validation. They are more complementary rather than competitive.
Slide 17 So, if you're interested in more details, I'll give you a few examples of
review articles. The first three were written by us over the last couple of years and the last one was a short review comparing the imaging techniques for tracking of stem cell therapy in cardiology, which was written by a group at Stanford University.
So with that, I'd like to stop here and pass it on to Dr. David Schaffer
who is the next speaker.
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Sean Sanders: Great. Thank you very much, Dr. Cai, excellent presentation. Just a
reminder to the audience, if you'd like to download the slides, you can just click on the green resource web page icon and there's a link there to download the slides if you want to take a look at those resources once again.
Slide 18 Now, we're going to move on to our second speaker for today, that's
Dr. David Schaffer. Dr. Schaffer is a professor of Chemical and Biomolecular Engineering, Bioengineering, and Neuroscience at University of California Berkeley, where he also serves as the director of the Berkeley Stem Cell Center. He graduated from Stanford University with a Bachelor's degree in chemical engineering in 1993. Afterwards, he attended the Massachusetts Institute of Technology and earned his Ph.D. also in chemical engineering in 1998. Finally, he did a postdoctoral fellowship at the Salk Institute for Biological Studies in La Jolla, California before moving to UC Berkeley in 1999. At Berkeley, Dr. Schaffer applies engineering principles to enhance stem cell and gene therapy approaches for neuroregeneration, work that includes developing new technologies to enable mechanistic investigation of stem cell control. Dr. Schaffer has received numerous awards during his career including a National Science Foundation CAREER Award, a Whitaker Foundation Young Investigator Award, and he was also named Technology Review Top 100 Innovator. A very warm welcome to you, Dr. Schaffer, thanks for being here.
[0:15:32] Dr. David Schaffer: Thank you very much. I'm very happy to be a part of this panel. So,
our group is very interested in developing and utilizing technologies to image cells and specifically stem cells at the single cell level.
Slide 19 to slide 20 As stem cell researchers know, a major motivation for doing this is
that stem cells exhibit sometimes random or stochastic behavior. So specifically, you can take a field of isogenic cells, apply what you think is the uniform signal to that population of cells, and end up getting a very broad distribution of outcomes from the cell behaviors. So, we need to or are interested in imaging or getting information at the single cell level.
In addition, the stem cell niche present cells with a lot of rich
information both biochemical in nature as well as biophysical or
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mechanical sometimes in nature and we're interested therefore in gathering a broad range of information from cells, in other words using different imaging modalities to gather information.
So, we work with three different technologies and I'll mirror the
comments of Weibo in saying that these are very complementary approaches to gain or interrogate different types of information from the cell. So specifically, single cell fluorescent imaging in real time, single cell biophysical measurements in real time, and then moving at least towards single cell measurements in vivo.
Slide 21 So, in order to be able to image a cell in a fluorescent mode, you
obviously need to introduce a sensor such as a genetic fluorescent reporter into the cell. Often, the way that this is done is that one takes out regulatory information from the inside of a cell, let's say a promoter that one whose activity one would like to monitor, and then puts on top of that a fluorescent protein, for example or luciferase, and then in some cases, we'll truncate that promoter in order to be able to make it fit into a delivery vehicle then reintroduce this in the cells. As a result of this reintroduction, you lose some control over the localization inside the genome and furthermore they are often many copies that integrate into the genome, sometimes concatemeric, sometimes a different loci.
So, this technology provides somewhat poor control over copy
number, therefore, expression level. If one is actually tagging fluorescent protein, losing control over the expression level can have some deleterious effect on the outcome of the experiment. Furthermore, these randomly integrated constructs don’t really report on the chromatin environment of the endogenous gene.
So, another approach that's been utilized over the past several
decades is to take a piece of genetic information such as a fluorescence or reporter and knock it into the genome and tag it on to a gene whose activity one would like to monitor. This provides higher fidelity reporting of endogenous protein levels, perhaps less disruptive to cell as a result, and furthermore maintains information about the chromatin environment of the gene that one would like to study. But one problem of course encountered within this field is that mammalian gene targeting and for example, stem cell targeting is still quite inefficient compared to other organisms such as yeast.
Slide 22
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So it was discovered a number of years ago or several years ago that one can utilize a virus to mediate homologous recombination or editing of an endogenous locus with very high efficiency. So, specifically adeno‐associated virus because of the form of its genome mediates gene targeting or homologous recombination at rates that are of the order 103 to 104 more efficiently than the corresponding plasmid. This can be utilized to introduce sensors, to knock out genes, introduce mutations or in the long term, one could think about this even therapeutically for correcting a patient's genome at the stem cell.
Slide 23 However, AAV is notoriously bad at mediating gene delivery to many
different cell types that's shown here on this slide where regardless of whether you're talking about an ES, a neural stem cells, or a murine ES cell. Unfortunately, this data indicate that AAV is very poor at carrying its genome inside the cells and therefore cannot mediate homologous recombination very efficiently.
Slide 24 So, our group over the past eight years or so has been utilizing a high
throughput protein engineering technique or technology, directed evolution technology, to adapt AAV and engineer it for high efficiency gene delivery to a number of different stem cell types. As a result of getting more copies of the genome into the nucleus of the stem cell, you can then mediate homologous recombination and knock in the fluorescent proteins at a much higher rate. So shown here with using this gene targeting construct, we're now targeting neural stem cells at a rate of about 0.15% whereas I believe there was no previous published report of any gene targeting within the neural stem cell using a plasmid.
[0:20:02] Slide 25 This can also be applied more recently to human embryonic stem
cells and human IPS cells where a plasmid construct typically gets gene targeting efficiency of 1 in 1M to 1 in 10M cells and we're now achieving gene targeting rates on the order of 0.1%.
In addition, it's been reported that you can use a nuclease to cut
specific sites within a genome such as in hESC and achieve higher efficiencies. However, this AAV approach doesn't require creating a brand new genome or creating a brand new nucleus. As a result of this, we're not knocking in fluorescent proteins into a number of
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genes inside a stem cell so that we can study the process of neural differentiation across a colony for example in real time and shown here is an image for OC4.
Slide 26 So in addition to using these sensors as ways to probe the
biochemical cues that the stem cell is receiving, we're also interested in gathering information about biophysical cues.
Slide 27 So, cells that's known in tissues inside the body exhibit a broad range
of mechanical properties. So for example, our bodies reside typically in mechanical ranges on the order of 102 to 106 units of stiffness, this Pascal unit. However, when we culture cells on glass or plastic, you're often taking them out of this soft gel tissue environment and placing them on a rock‐hard surface that can be millions of folds stiffer in its properties.
Slide 28 So, we've been studying the effects of mechanical properties on
stem cell differentiation and in this case, we're studying neural stem cells, which are shown here in this tissue slice in blue.
Slide 29 to slide 30 We can place these cells in culture on top of surfaces of different
stiffness and if you quantify these effects, you see that stem cells on a stiff gel, on a stiff surface are predisposed or biased strongly towards differentiating into a glial lineage whereas on a very soft surface, they're very strongly biased to differentiating into a neuron. So all the biochemistry in this case is identical and all that's changing is the mechanical properties of the environments in which the cell is living.
Slide 31 So we're extremely interested therefore in gathering information
that's not just biochemical in nature but also biophysical or mechanical in nature. So, we've been using an imaging modality referred to as atomic force microscopy. This is an outdated analogy, but you can kind of think of it as a record player where you take the needle and tap into the cell and by getting information about the force and the displacement of that tapping, you can image or raster across the cell and you can also get mechanical information about the cells.
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Slide 32 So for example, and this is a real‐time dataset that can be gathered
on live cells in culture in a nondestructive way, you can both image the cells using AFM as well get information about how their mechanical properties or stiffness adapts to that of their environment.
Slide 33 In addition, you can use again fluorescence microscopy to gather
information about the biophysical properties of a cell. This is work done by Sanjay Kumar a collaborator at UC Berkeley where he has used laser ablation of cytoskeletal elements inside of a cell where you can actually use an infrared laser to punch a hole out into the middle of a cytoskeletal element and then image in real time using visible fluorescence microscopy, the retraction of an actin microfilament. This data can be used to gather high resolution information about the mechanical properties of a cytoskeletal network as well as the cytosol inside of a cell.
Slide 34 to Slide 35 So finally, I'm going to talk about another imaging modality that's
been developed at Berkeley and we're collaborating with the developer of this, Professor Steve Conolly in our bioengineering department. This is a modality called magnetic particle imaging. This is a very different technology from MRI. The way that it works is that a magnetic field is applied to a sample in such a way that a small magnetic particle such as a similar type of particle that's sometimes used in MRI is scanned across the magnetic field. It reaches a zero field point where at that stage, the properties or the magnetization of that small particle flips from one direction to another. That can be sensed in an inductive way and as a result of that flipping of the magnetic field of a sensor or a particle; you get a signal that can be imaged.
The advantages of this modality are that unlike sometimes optical
imaging, the body is completely transparent to magnetic fields so you can image very, very deep into tissue. In addition, it has the potential to be up to three orders of magnitude more sensitive compared to MRI and therefore you could begin to get down to very small numbers of cells.
Slide 36 So this can be used initially for example angiography applications
where you can image an entire organism soon or very shortly after
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injection or introduction of these particles into the organism. Shown here is a 3D reconstruction of a mouse. Finally, we've been collaborating with Steve Conolly to be able to apply this towards, in this case, human embryonic stem cells.
Slide 37 We're now down to the stage where we can detect 5000 cells within
a tissue sample and both for technical as well as theoretical reasons; we think we can push that down by several orders of magnitude further so that we can get towards single cell imaging in vivo.
[0:25:14] Slide 38 So with that, I'd like to thank the people who really did this work,
both our collaborators Steve Conolly and Sanjay Kumar as well as other investigators within our lab. So thank you very much.
Slide 39 Sean Sanders: Great. Thank you so much, Dr. Schaffer. Our final speaker for this
webinar is Dr. Clemens Cabernard. Dr. Cabernard completed his undergraduate and graduate degrees at the Biozentrum at the University of Basel in Switzerland, after which he undertook postdoctoral training at the Institutes of Molecular Biology and Neuroscience in Eugene, Oregon in the US. He then returned to the Biozentrum and he currently holds a Swiss National Science Foundation professorship in Growth and Development and Neurobiology. Dr. Cabernard’s research uses advanced imaging techniques to visualize asymmetric stem cell division, particularly in Drosophila neuroblasts. Welcome to you, Dr. Cabernard.
Dr. Clemens Cabernard: Thank you very much, Sean. It's a pleasure to be here. Slide 40 So, during the remainder of this webinar, I will be talking about live
imaging in Drosophila melanogaster brains and particularly, we are interested in asymmetric cell division of neural stem cells of the fly also called neuroblasts.
Slide 41 This image sequence nicely illustrates we can see that these
neuroblasts are highly polarized cells along the internal apical‐basal axis and this then results in the complete segregation of these green cell fate determinants into the small differentiating ganglion mother cell, but at the same time also forming a self‐renewed apical neuroblast, which is also different in size.
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Slide 42 So, we are usually imaging these neuroblasts in third instar larval
brain depicted with these dark blue spheres in one or the other brain lobe. These neuroblasts are really closely located to the brain surface, which makes them ideally suited for imaging in general and live imaging in particular.
Here, this overview movie shows four pseudo colored mitotic
neuroblasts and since these are fly cells, not only can we easily label subcellular structures such as the mitotic spindles, but can also of course employ all the genetic tools to manipulate these cells at our will.
Slide 43 So in order to live image these neuroblasts, we are using a special
setup depicted in this slide here. Essentially, it's consisting of this metal base, which was originally published in '94 by Dan Kiehart. This metal base is also taking advantage of a metal split ring, which can then be adapted or assembled in a particular way and that's shown on the next slide here.
Slide 44 Essentially what we're using is a gas permeable membrane on which
our specimens are sitting. We then attach this membrane to this metal base and cover the entire setup with a coverslip and seal it off with Vaseline.
Slide 45 In terms of imaging mode, if this setup is used to image with an
upright microscope as most of us probably have at our disposal, the specimen is sitting in a small chamber with the gas permeable membrane on the bottom and a coverslip on the top. This really enables the neuroblast to be very close to the objective and therefore provides very high optical resolution.
Slide 46 However, we can also use the same imaging setup to image on an
inverted microscope by just flipping this imaging chamber around. Slide 47 In terms of instrumentation, we have originally started using single
point confocal microscopes, which as you probably all know, provide good optical resolution. They have as an advantage an electronic
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zoom option as well as adjustable emission filters. In addition, we can have several laser lines, which allow us to image several different fluorophores. The only real disadvantage to single point confocal microscopes is probably the fact that acquisition mode is relatively slow.
Slide 48 In the last couple of years, we have therefore been using spinning‐
disk microscopes, which similar to single point confocals also have good optic resolution, provide several laser lines, have in contrast to single point confocal very fast acquisition mode, and also deploy low phototoxicity on the specimen. The only disadvantage in this particular case with this microscope is probably no electronic zoom can be applied and emission filters are fixed, which somehow takes away some of the flexibility.
Slide 49 So in this slide, we see some of the probes we can use in Drosophila
in general and neuroblast in particular to image and label subcellular structures such as the DNA, the mitotic spindle. We can label microtubule + ends, centrosomes, but also furrow markers such as myosin is used, and then last but not least, we also have polarity markers to distinguish between apical and basal cortex.
Traditionally, most of these constructs were tagged with GFP based
on its inherent stability and also relative brightness, but nowadays with the advantage of new fluorescent proteins of course novel tags also employed and Cherry is just one example of this whole cascade.
[0:30:36] So, I also would like to point out, as Dave already mentioned, it's of
course important how do you deliver these probes into the cell or at least how you express those. In Drosophila, we have several possibilities to do this. On one hand, we can use endogenous promoter and enhancer elements to drive expression of these reporter constructs and this can be done by BAC‐mediated recombineering or even homologous recombination.
In addition, we also have binary expression system called the GAL4‐
UAS system, which allows us to express these constructs in a tissue‐specific manner also in a cell type specific manner; however, proteins might not be expressed at physiological levels.
Slide 50
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So here in this slide, I'd like to show you two examples of asymmetrically dividing neuroblasts. In this particular example, we see a third instar larval brain neuroblast image with a spinning‐disk microscope. In green, we see Par3 tagged with GFP localizing to the apical cortex and the mitotic spindle is also outlined in white. I hope that we can appreciate from these images here that using spinning‐disk microscopy; we can achieve good optic resolution as well as high temporal and spatial resolution.
Slide 51 On my next slide, we will see another example of an asymmetrically
dividing third instar larval neuroblast in this case labeled with myosin in green as well as microtubule binding protein in white to outline the mitotic spindle. Also here, we can appreciate how dynamic this marker as we localize and therefore live imaging is really an important and very precise tool to visualize the very dynamic process asymmetric cell division in stem cells.
Slide 52 So, to wrap this up, I'd like to provide three conclusions. As a general
statement, I would like to say that live imaging really provides an ideal tool to elucidate the basic cellular and molecular mechanism of asymmetric cell division. One of the tricks I learned or one of the tips I learned over the course of my imaging studies was that minimizing laser power and exposure time to laser is really key for successful imaging especially for long term stem cell imaging.
I would also like to point out that in our case at least using a
mounting setup, which allows exchange of gases, is really a great advantage and maintains the viability of these cells over a long, long time.
Again, as mentioned earlier, one of the best practices I can mention
is that whenever possible, the use of endogenous regulatory elements to drive reporter construct should be considered. This really will allow it to express these proteins on their physiological levels.
Slide 53 With this last slide, I'd like to acknowledge some of my former lab
mates, Karsten as well as Sarah, which really pioneered some of these live imaging studies and also taught me a great deal about it. I'd like to acknowledge my financial support. Currently, my lab is mostly funded by the Swiss National Science Foundation. I'd like to thank my host the Biozentrum from the University of Basel for
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providing a great work environment. Thank you very much and I'm looking forward to questions.
Sean Sanders: Great. Thanks so much, Dr. Cabernard, and many thanks to all of our
speakers for their fantastic and very informative presentations. We're going to move now to questions submitted by our online viewers.
Slide 54 Just a quick reminder to those watching us live that you can still
submit your questions by typing them into the text box and clicking the submit button. If you don't see this box on your screen, just click the red Q&A icon and it should appear.
Also a reminder, you can download the slides, just click on the green
widgets with the folder icon at the bottom of your screen and that should give you a link to download the slides. Also, if you missed some of those movies, this webinar will be made available to view again as an on‐demand video usually within about 48 hours from now so you can re‐watch those excellent movies, very interesting.
So, I'm going to move on to our first question for today and I think
this is probably going to be best suited to Dr. Cabernard and Dr. Schaffer. This person asks, could the speakers briefly discuss one of the most difficult sample preparations that they've had to deal with in their research and describe the solution that they've found for that issue? So let's start with Dr. Schaffer.
[0:35:24] Dr. David Schaffer: Sure. I'll describe it not necessarily from the imaging itself but
actually the preparation of the cells beforehand, before they were imaged. I think the field recognizes probably as a whole that the best way to prepare a cell for fluorescent imaging with a genetic sensor is to actually have that genetic sensor reporting on the activity of the endogenous protein specifically knocking that sensor into different genes within the cell that one would like to study. So, that's very difficult to do and that's probably the reason that we invested a significant amount of effort to develop technologies that can mediate very high efficiency gene targeting. So I described some of that within our talk and we're now enjoying the fruits of that initial investment of effort.
Sean Sanders: Dr. Cabernard?
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Slide 45 Dr. Clemens Cabernard: Well, from my point of view, I would like to point out that I guess in
order to get high quality images, it's really important that the setup is optimized. I think I can speak from experience, I had to learn the hard way that in our case, there's really involves a lot of dissecting. So in order to get from the actual specimen to the image, a number of dissecting steps have to be performed. Once these brains are actually dissected and mounted on an imaging slide, our next difficulty is then to really provide or at least ensure that these neuroblasts are in very close proximity to the objective. This really requires a lot of practice and manually requires that some of the imaging solution where these specimens are sitting in is being removed. So again, that really is something, which has to be practiced and cannot just be learned from one day to another.
In addition, I'd like to point out another difficulty we encountered
during live imaging is that for every probe or every reporter we use, usually the settings have to be adjusted on the microscope. This is again a trial and error phase, which can lead to some frustration, but of course it has to be practiced and it has to be done for every specimen, which takes some time.
Sean Sanders: A question for you, Dr. Cai. In terms of researching stem cells in
neurological imaging studies, would the MRI be more efficient than radiolabeling methods to show the different receptors?
Dr. Weibo Cai: Well, this I guess it depends on what question you are asking
because MRI does give you a much higher resolution and typically you can go to probably 100‐micron resolution while labeling with radioactive agents might typically will give you a resolution of a few millimeters. So, I think in that case, nuclear radioactive agent based imaging probably will not give you a very good biological information. It can probably only tell you where it's going.
I'm not sure whether I got the question about the receptors because
in this case, direct labeling is probably not the way to go. I think it's probably going to be some kind of conditional promoters type of reporter gene based approach. So, I think MRI is probably more accessible and while PET or SPECT does require a lot of infrastructure. Like you'll need to have the tracer, you need to have people, you need to collaborate with a radio chemist to synthesize the tracer, you also need to have the kind of license to handle radioactive materials so it's a lot more complicated than imaging with MRI or with optical techniques.
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Sean Sanders: Okay. Excellent. So a question for Dr. Schaffer now that just came in.
Could you talk a little bit more about you mentioned mixed differentiation conditions and the viewer is asking for a little bit more detail on that.
Dr. David Schaffer: Sure. In this case, we were really interested in seeing how
mechanical properties or biophysical properties at the microenvironment could tip the balance one way or another when the cell is faced with the choice of a 50/50 decision about whether to go down one lineage or another. So we've studied this in the context of human embryonic stem cells more recently, but the data I showed there were for adult neural stem cells, stem cells isolated from the adult hippocampus of the mammalian brain. The cells were given biochemical cues that led them to flip a coin and decide whether they wanted to go down a neuronal lineage or a glial lineage. Under those identical biochemical conditions, it turns out the mechanical properties the environments could almost completely tip the balance one way versus the other.
[0:40:10] Sean Sanders: Another quick question for you, Dr. Schaffer, a technical question,
would you recommend coating glass bottom dishes for stem cell imaging?
Dr. David Schaffer: Yes. The stem cells that we utilized, we primarily utilized neural stem
cells, human induced pluripotents, and human embryonic, all of those require a coating on the surface on the substrate in order to attach to the surfaces and be grown either as colonies or as monolayers. So for the neural stem cells, we typically used laminin and something like poly‐ornithine or poly‐lysine and for the human embryonic stem cells, the field is probably converged on the standard of using, if you're not using a feeder layer, using a Matrigel coating on the surface. There are an increasing number of fully synthetic surfaces that are composed either of a polymer with good properties or with one or more defined peptides that can engage with specific adhesion receptors on the cell surface. At this stage, these are more at the research level and had not been mass produced to the point where they could be adapted as a standard across the field yet though.
Sean Sanders: Now, do you find that in dishes that stem cells are motile or mobile
or not?
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Dr. David Schaffer: So neural stem cells, the ones that we work with are highly motile. So on the stiff surfaces, the mechanically stiff substrates that we utilize, the cells are highly migratory. As you move towards softer substrates, the cells tend to be a bit more stationary. For the human embryonic stem cells, I kind of liken them to a rugby scrum almost where there's a large mass of cells aggregating together in a colony and we haven't really seen too much motility of the colony as a whole, but certainly when cells on the periphery of that colony begins to differentiate, they migrate away.
Sean Sanders: Dr. Cabernard, do you have any input on that? Dr. Clemens Cabernard: Well in terms of motility? Sean Sanders: Yeah. Dr. Clemens Cabernard: We have not seen neuroblasts to move around either in an intact
organ or in culture. What I have to say though of course is these neuroblasts are non‐adherent cells, if you just put them in a Petri dish, they will move around.
Sean Sanders: Uh‐hum. Dr. Clemens Cabernard: So if somebody wants to image in a different setup, it's advisable to
use a certain coating such as a laminin surface to have them stick to the slide to a certain degree.
Sean Sanders: Great. Dr. Cai, you mentioned that magnetic resonance imaging is
useful for measuring the efficiency of acute delivery. Can you talk about the temporal limitations of these methods, in other words how fast images can be obtained?
Dr. Weibo Cai: Well, in this case for MRI, typically you could get a scan done within
a few minutes, for nuclear medicine techniques it's also a few minutes so on order of minutes.
Sean Sanders: Okay. Great. Another question for you, sorry, hold on one second, I
have it right here. How do you test the biocompatibility of the chemical cell tags that you use?
Dr. Weibo Cai: Well in this case then, typically people can just look at the expression
of the stem cell markers, you know, the various markers. Some people also go even further and look at, you know, microarray and also look at the gene expression patterns and compare whether you
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see any differences. Many studies have shown that there's really not a big difference in terms of, you know, before and after labeling and of course that has to be studied very rigorously on case‐by‐case basis.
Sean Sanders: Dr. Cabernard, for your experiments, what is the maximum time
duration of time‐lapse imaging? Dr. Clemens Cabernard: Well, that's a good question. I think the longest we ever imaged was
about 20 to 24 hours with of course lower time resolution, but technically it's possible.
Sean Sanders: Okay. Back to you, Dr. Schaffer. Have you considered using ethidium
bromide histology microscope slides and UV light with UV blocking lenses?
Dr. David Schaffer: Ethidium bromide to be able to visualize the nucleus in real time? Sean Sanders: I believe so, yeah. Dr. David Schaffer: Well ethidium bromide is of course a DNA intercalator and things like
Hoechst and ethidium bromide. Hoechst is a permeate to live cells so you can visualize live cells with Hoechst. But even so, since it interacts with the DNA, it can perturb the function of the cell and chromatin mediated biology. So rather than imaging the nucleus using a chemical dye, the field as a whole and we've done this to a small extent, has been genetically labeling nuclei and DNA by for example fusing a histone to a fluorescent protein. So that is probably the approach I would tend to adapt more so than a chemical dye that could interact with DNA.
[0:45:13] Sean Sanders: I'm going to stay with you for a second. A question that just came in
that comes back to something you were talking about just a few minutes ago, do you think your findings on the influence of stress on these cells could possibly explain or help explain differences in stem cell adaption to suspension versus surface growth?
Dr. David Schaffer: Yes, I do. I think it can. So in the case of a neural stem cell for
example the brain is a couple of hundred of these Pascal units in stiffness, although there is heterogeneity, there's stiffness gradients within the brain. The brain it turns out changes stiffness within age as well. So, when you take a stem cell out of the brain and put it on top of a dish, it's about a hundred million‐fold increase in the
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stiffness of the environment that the cell is experiencing, and that pretty dramatic change in the stiffness does affect a variety of cell behaviors, anything from adhesion and motility through, our results are indicating, a differentiation. So if you have a cell in a neurosphere or an embryoid body compared to being adherent on to a dish, it's going to experience a very different mechanical property.
So there are certainly other biochemical differences between
adhered cells versus a ball of cells since cells of course in the interior of an EB or a neurosphere would experience a very different environment for those on the exterior. But certainly mechanical differences are very significant between adherent versus suspension cultures.
Sean Sanders: So, Dr. Cai, I think this question might be best directed at you, but
I'm happy for the others to answer as well. This viewer says that they're trying to monitor the dynamic protein modification state of a specific transcription factor during stem cell differentiation. Do you have some suggestions or tips on some things they might need to consider when they're doing this?
Dr. Weibo Cai: Yeah. I was looking at this question and I think probably David and
Clemens will have a better answer to this. I think this one is definitely in need of some kind of reporter gene techniques in a way of specifically putting that gene, you know, on the specific transcription factor to do longitudinal imaging?
Sean Sanders: Uh‐hum. Dr. Weibo Cai: Dave, do you have any comments on this? Dr. David Schaffer: Sure. I mean we're sort of the proponents again of knocking‐in into
endogenous loci so that you ‐‐ it's been shown of course that some promoter enhancer elements that regulate the expression of a given gene can be tens of KB away from the transcriptional start site for that gene. So the only way that one can really replicate and reproduce that regulation is by inserting a sensor into the actual endogenous site of the gene. So that's the approach that we would like to take.
Sean Sanders: Dr. Cabernard, anything to add?
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Dr. Clemens Cabernard: Yes. So, I was thinking about this question too and I thought if somebody is known, you know, if something is known about the changes of modification state of that transcription factor, one could of course try to design a FRET probe, which would take into account any confirmation changes this transcription factor undergoes. So that would then, I guess, be the best way to visualize and even to image the dynamics of these transcriptional or the changes within this transcription factor.
Sean Sanders: I have another question for you, Dr. Cabernard. I know you've been
working mostly on the Drosophila brain, but maybe you can give this viewer some tips. They're asking about releasing, finding a method to release more insect stem cells from mid gut tissue during isolation. So maybe you can talk about how you do your isolations and what might work for them?
Dr. Clemens Cabernard: Right. Well that's kind of a difficult question. In our case, I guess we
never faced the problem that we don’t have enough cells because the brains we usually use; they each contain 50 neuroblasts per brain lobe. So the neuroblasts can theoretically be imaged, although we usually only image the one on the very top so closest to the cover slide. However, in the case of the mid gut, I don’t really know how one could achieve a higher number except for dissecting more mid gut tissue, I guess that will be my approach I would be choosing.
Sean Sanders: Dr. Schaffer, one for you. The dyes that you use as cell tags, how do
they enter the cell membrane? Dr. David Schaffer: This is probably the super paramagnetic iron oxide for the magnetic
particle imaging. Those are small particles that are ‐‐ we actually or Steve Conolly I should mention actually uses these particles that are of the order 10, 15 nanometers in size they originally developed for MRI as contrast agents for MRI and then uses it in this different imaging technology, MPI (Magnetic Particle Imaging). The way that we end up introducing them to cells is either to do it very nonspecifically where you coat the outside surface of that particle with something cationic like protamine sulfate or poly‐lysine such that it undergoes nonspecific endocytosis within the cells. Or you could alternatively tag it with something that's specific, a ligand that would direct it to particular cell types and that would have for example utility for in vivo imaging and diagnostic purposes for example if you could generate a particle that localized to a tumor.
[0:50:31]
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Sean Sanders: Great. Dr. Cai, I think you might be able to take this one. What do you feel is the optimum ratio of the number of cells to the amount of chemical tag that you use?
Dr. Weibo Cai: We typically do not do too much of chemical labeling. Sean Sanders: Uh‐hum. Dr. Weibo Cai: We are kind of more into the reporter gene based approach using
other firefly luciferase, sometimes we also do iron oxide labeling. Sean Sanders: Uh‐hum. Dr. Weibo Cai: In terms of the actual amount, I don't really have the exact numbers,
but I think many of these details are kind of in the methods section in some of the actual research articles.
Sean Sanders: Uh‐hum. Dr. Weibo Cai: At this point, I just don’t have an exact number in mind. Sean Sanders: Right. Anyone else want to add anything on that? Not for now?
Okay. I wanted to ask maybe a more general question to Dr. Cabernard and Dr. Schaffer about what they feel are the most useful and flexible imaging tools for live cell imaging that they've had experience with. So maybe, Dr. Cabernard, you can start first.
Dr. Clemens Cabernard: Okay. Well, I guess one of the most useful setups I would think is
probably a regular single point scanning confocal system especially for labs which usually do immunohistochemistry assays anyways and usually these scopes are ‐‐ I mean they are fairly distributed all over these institutes. So I think this setup can at least to start with definitely be used to live image stem cells and then depending on the needs of course, you know, if one really wants to image at a high temporal resolution, it would be advisable to move on to a spinning‐disk system.
Sean Sanders: And, Dr. Schaffer? Dr. David Schaffer: Yes. One way of answering that maybe is that I run a stem cell core
at Berkeley that has a lot of imaging equipment within it and certainly our confocal and we have a 2P as well on the same rig, gets a good amount of business. The thing that probably surprised me the most is that the instrument that gets booked 24/7 is one of these
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high content imagers, Cellomics makes one of these, General Electric makes one of these, we got one from Molecular Devices or MDS. These are imagers that I'm sure many people are familiar with where you can load samples into it.
It's just a regular epifluorescence microscope, but it has robotics and
software associated with it that will enable you to for example scan every well at the bottom of a 96‐well plate or a 384‐well plate every 15 minutes for 24 hours in a row, and then process the images to be able to gain quantitative information from them such as the number of cells of different identities or the amounts of label that's in the nucleus versus the cytosol, the number of branches that a neurite has. So this high content information, although you can fill up hard drives pretty quickly is able to get a huge amount of data that enables us to consider experiments we simply would not have considered three years ago.
Sean Sanders: So, Dr. Schaffer, a question came in about your opinion on whether
stem cell fate based on changing the substrate elasticity may have real functional implications for stem cell differentiation in vivo or do you believe that this might just be an artifact that you're seeing in the laboratory.
Dr. David Schaffer: Sure. So I'll start out heuristically by saying that of course the
stiffness of tissue inside the body ranging all the way from adipose which is very soft, brain which is not that much harder, all the way through bone, it ranges over about four orders of magnitude in stiffness. The brain itself and the hippocampus that we study has around a five‐fold variation in stiffness and that variation occurs across the exact same range that we're observing flips in cell fate as a function of stiffness.
So it's difficult to directly change the stiffness of a tissue like the
brain without changing other biochemical properties, but we've done a lot of work in analyzing the mechanism by which a cell responds to these mechanical properties and have gone a lot into the biochemistry and the signal transduction involved. So what we can say is that if we go in vivo and use viral vectors to manipulate signaling pathways that respond to mechanical differences, we see the analogous flips in cell fate in vivo that we observe in vitro in response to stiffness changes.
[0:55:06]
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Sean Sanders: Dr. Cabernard, I'm coming to you for a fairly specific question on fly neurons. This viewer is trying to culture them on poly‐lysine‐coated dishes and they said they're finding quite a lot of debris and floating cells and not all the cells are attaching. Maybe you can advise them on something they should be looking at?
Dr. Clemens Cabernard: Well, that's pretty specific. I'm not sure how I could provide any
advice to this question. What I would say is probably obviously what happens here is that if a lot of debris is being observed that cells are not happy with the substrate so they're probably undergo apoptosis or retract or whatever might happen with these cells. So I would probably try to use different coatings, different slides, maybe even different glasses or different manufacturers for glass slides and then of course use different substrates as their coating surface as well.
Sean Sanders: Excellent. Thank you. So we're coming up to the end of our hour, but
I'm going to try to sneak a couple more questions in. One for you Dr. Cai.
Dr. Weibo Cai: Yeah. Sean Sanders: This asks, is bioluminescence amenable to the recently developed
family of super resolution microscopy techniques? Dr. Weibo Cai: Probably not at this point. I think most of the super resolution
imaging are using fluorescent proteins rather than bioluminescence and also bioluminescence the signal is kind of relative to ‐‐ it's a lot weaker, but there's not much background. So at this point I don’t think is bioluminescence is amenable to those super resolution imaging.
Sean Sanders: Another question for you, which imaging methods are being used to
complement one another in multimodal or correlative manner to facilitate stem cell research on different scales, in other whole animal to organs, organs to tissues, etc?
Dr. Weibo Cai: I think fluorescence imaging it can be used to reach different scales,
but typically the fluorescence signal does not penetrate very well so you can only at superficial tissue. Let's say for mice, typically you can only image to maybe a few millimeters deep. The typical correlation people use are probably bioluminescence and say the multimodality reporter gene techniques that typically use bioluminescence and the PET. Those are the two that's most widely used for in vivo imaging while the fluorescence is mostly for a selection of cells like, you
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know, FACS analysis, not very widely used for in vivo imaging at this point.
Sean Sanders: Dr. Schaffer, a question about AAV that you talked about a little bit
in your presentation, what do you foresee might be the future for AAV mutants specifically for the improved efficiencies in various stem cell types. Do you think there's going to be improvement there?
Dr. David Schaffer: Well, that's one of the goals of our research, to be able to engineer
AAV for improved efficiency on the stem cells that are of interest to ourselves and hopefully of some interest to others within the field. So in terms of efficiencies, we have created mutants that are really good so far on neural stem cells and different mutants that are good on human embryonic stem cells and human induced pluripotent stem cells. For the NCSs, we're getting efficiencies on the order of, you know, 80% cells are transduced by the vector and of those 80%, around 0.15% ‐‐ if your goal is to do a homologous recombination or a gene targeting then it results in about a 0.15% gene targeting rate, which is quite good within the field. For the human embryonic stem cells, we're getting around 60% gene expression and that's compared to for example a nucleofection, a plasmid transfection method that perhaps gets around 30% or so. So then I think that at this stage we're not efficiency limited as far as delivery goes anymore.
Sean Sanders: Fantastic. Well unfortunately, that's all the time we have for
questions today. So on behalf of myself and our viewing audience, I wanted to thank our excellent speakers for providing such interesting talks and a very engaging discussion: Dr. Weibo Cai from the University of Wisconsin‐Madison, Dr. David Schaffer from the University of California at Berkeley, and Dr. Clemens Cabernard from the University of Basel.
Many thanks to our online audience for the questions you submitted
to the panel. I'm sorry that we didn't manage to get to all of them. Slide 55 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 the more webinars from Science available at www.sciencemag.org/webinar. As I mentioned before, this webinar will be made available to view again as an on‐demand presentation within approximately 48 hours from now.
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We'd love to hear what you thought of the webinar, just send us an
email at the address now up in your slide viewer; [email protected]. Again, thank you to our panel and to Leica Microsystems for their
kind sponsorship of today’s educational seminar. Goodbye. [1:00:23] End of Audio