Prof. Dr. Yoshiki Sasai, Riken Center for Developmental Biology, Kobe, Japan
An Interview with the 2013 Hans Sigrist Prize Winner
While Prof. Dr. Yoshiki Sasai, our 2013 Hans Sigrist Prize winner, was in Bern to receive the prize at Dies Academicus in December, our Foundation Manager from the Hans Sigrist Foundation (HSF) sat down with him to ask him a few questions about his research and future plans.
HSF: Can you tell us a little about your recent work, aimed at our readers who are not stem cell scientists?
Sasai: I am particularly interested in the field of embryo origin, how complex organs such as brains or eyes can form, from relatively simple cells called stem cells. It is a mysterious process. Although these complex organs form in the mother's body, it is not like the mother put her hands into her body and made a nice shape. Actually, what happens is that after conception, the fertilized egg keeps on dividing, and somehow, these cells spontaneously develop the structure including organs before the embryo is formed. This is not simple, spontaneous generation, but it is programmed. Because if you have a brother who is a homozygous zygote (identical twin) with you, he should be very similar to you in many senses, including the shape of body and shape of organs, but homozygous twins are not totally genetically encoded. But most of organ development is genetically encoded. A mouse makes a mouse eye, a human makes a human eye. I found that this is a very interesting and mysterious area of biology, so over the last 12 years, my laboratory has been challenging this question, by using stem cell cultures to recapitulate organ development.
HSF: What made you use the specific approach that you did to working with stem cells. What inspired you to make the changes you did to stem cell research?
Sasai: What we really wanted to do was recapitulate the baby's development that usually happens in the body. We wanted to bring that into the in vitro context. In mammalian development, including human development, part of the body comes from only 30 cells, it is a ball of 30 cells, present in the conceived embryo, that is called the inner cell mass. These cells can become anything and everything in the body - brain, heart, bone, or gonadal tissue, the sperm and egg. They are the almighty kind of cells, the totipotent stem cells; they can develop into any kind of cells. In this field, we call that differentiation. In the end, a neuron is a neuron, and a heart is a heart, and a heart will not become a neuron anymore. There are two things we improved to recapitulate organ development from stem cells in culture. The first one is substantial improvement of the differentiation protocol. The stem cells can be steered to differentiate into a specific lineage of cells, such as cerebral cortex tissue or retinal tissue in the eye. Initially, over the last 15 years or so, many people in the field are working on this kind of differentiation to get 10 or 20 percent efficiency. In our case, we improved our culture in such a way that the cells of interest occupy more than 40 to 50 percent of the entire culture.
Once the cells of interest occupy 1/3 of the culture, we found something special happening. When the cells of interest are mixed with other kinds of cells, for example, if the cortex cells are mixed with other brain tissues in the culture, they do not do much, they just become cortical neurons. So, that is O.K., and that is probably O.K. to be used for regenerative medicine, or to be used for drug discovery for Alzheimers disease.
HSF: So the researchers would test the drugs on the cells?
Sasai: Yes, but we reached the condition where the differentiation efficiency is more than a third, like 40 or 50 percent, then the majority of the population starts to do their own job, by talking to each other. So, they started very complex intercellular interactions, cell-cell interactions. And without external instruction, without my instruction, they started their own program that is internally programmed in their genome. So, it is like, if you are a cell, let's say you are a cortical neuron, when you see at a party that there are lots of different kinds of people, cortical neurons, heart muscles, and eye cells, and so on, then probably you do not feel like, "I should be just one of them." But when you see that the guys and girls all around are cortical neurons, you start to talk to each other and say "Let's make our structure, let's make cortical tissue".
HSF: So, it's sort of like peer pressure?
Sasai: Yes, and this community effect apparently has a threshold, it only happens when it reaches a critical level of cells of the same kind. Then, their intercellular interaction becomes really strong, and they start some kind of structurization - they start to make structures, even in vitro.
I do not know about the people in Switzerland, but perhaps I can best explain it by saying if you go to somewhere like Munich, at Oktoberfest, and you have 40 or 50 people, and they start to drink, and wrap their arms around each other's shoulders and sway back and forth and sing. This would not happen if these guys are just in the middle of New York. So, this is the community effect. And the way they do it is Bavarian type, because it is programmed from their childhood. So, my finding was first that when the number of cell progenitors accumulates and reaches the critical mass, then they start their own program. That is one thing.
The second thing is that we invented a 3D culture. Most of the previous stem cell work used a 2D culture, so that the cells are attached to the culture plate as a monolayer. In the body, there is no cell that actually behaves that way, especially in the brain or eyes, which are my interest. They are not one sheet of cells, they have a 3D structure. Being pasted to the bottom of the culture plate is like crucifying them - they are under very strong stress and they do not have freedom. So, we decided to make 3D cultures of several thousand cells, and culture them in a homogenous medium, which has been optimized to induce a retina, for instance, or cortical tissue. So, when you culture that, it becomes retinal or cortical cells, depending upon the medium you choose. Then, I just let them do what they want to do. It is like a boy meets a girl, but in front of the parents, they will not do anything at all.
Anyway, they start making a single tissue, but in the case of retinal cells, they really make a fetal eye structure, in the correct size and the correct shape, in other words, they know what to make.
HSF: That's fascinating.
Sasai: It is fascinating. Each cell probably does not know what to make, but as a mass of cells, several thousand cells, if they are already committed to becoming a retina, then they make retinas. You can easily imagine what would happen with Munich people, if you have 3,000 of them, with beer in the medium, well, that is what is programmed. It was really difficult to analyze these things in vivo, in the embryo, because there are so many different tissues around, so you cannot analyze cell-cell communications, but when you take it out into a culture tube, with minimal components, you can observe it, because it is not surrounded by all the other tissues. This size of such a structure is about 0.5 mm to 1 mm. This size is called mesoscopic, not macroscopic or microscopic, but somewhere in the middle.
It is not New York, but it is not a very small old town, but the size of .... I do not know.
HSF: How many cells did you say it is?
Sasai: Around 3,000 or 4,000.
HSF: So, it is a thriving Swiss village.
Sasai: Right. Probably there, you do not need parliament or say, local laws, but they know what they have to do, how to make their village, just by talking to each other.
HSF: So, you let a certain type of cells rule themselves because they know what to do?
Sasai: My finding, what it means, is that there are lots of local rules that determine the fine structure, that are working, not just through dictated instructive signals, say control towers. There are some, but most of the fine structures are locally determined. For instance, the eye, is a local autonomous kind of development. But where to make the eye, is dictated by organizer signals. We found a new concept at the mesoscopic level for cell autonomy over cell society.
HSF: Can you tell us about the implications of these 3D cultures?
Sasai: Conventionally, when you do tissue analysis, you make a slice of the tissue and stain it with a certain colored medium, and then look at it under the microscope. Each slice gives 2D information. These days, at the mesocopic level (1 mm or less), you do not have to make slides. We have optic slicing, providing 3D images, similar to a CAT Scan. In cooperation with Olympus, we have optimized that for 3D. We have also further improved the incubator, we make it fit with this 3D microscope, so you can keep on recording all the cells in the culture for more than 2 weeks, the entire process, from 3,000 cells, watch them growing, growing, making an optic cup, etc., capturing everything at the single cell level - that is amazing. This is called in toto imaging.
So from that kind of information, we could extract some mechanisms, on how the optic cup forms spontaneously. The spontaneous formation of optic cups, is very, in a sense, strange, because no one pushes it to make this kind of structure. We could learn that only three local tissue mechanics, when they are combined in the right order, can make this optic cup shape.
HSF: Where do you hope to go next in your work, and how will the Hans Sigrist Prize money help you reach that next step?
Sasai: I have two directions in mind for my studies.
The first thing is the basic question, the main concept I would like to pursue, is how multiple cells, like a thousand cells, can interact in a complex way, to make all the structures, such as organs. This is fundamentally important, because the cells are not like just a block in form, the cell itself divides and changes shape and moves in a very flexible way. Cell-cell interactions are very flexible as well, and also dynamic, involving many cells. Common sense will tell you that what ends up, should be chaotic, there is no order. Each element is so flexible, their relationships are very flexible. What happens should be just a mess.
HSF: Anarchy, right?
Sasai: Yes, but during development and also in the stem cell culture system, the opposite is reality, at least under certain conditions. From just a simple aggregate of cells, you can make an eye. This is a very basic biological question - this is how our body is made. God somehow made an amazing program that self-develops, so that the cells are dividing and dividing, talking to each other, and make a very ordered thing. I want to understand the principles of that.
The second thing is to transfer our technology to translational researchers and pharmaceutical companies, so that they can use real tissue, not cells, for their therapeutics or drug discovery. For instance, our method of self-organization of the retina, can make a human retina structure, with all the components in it, and they are very, very similar, even in their fine structure. So, now you have a very young retina in your reach, which you have never been able to get previously. We are transferring this technique to biologists in such a way that they can develop the method of transplantation for diseases of the eye, especially retinitis pigmentosa, in which photo receptors are gradually dying, and over time, ends up in an almost total loss of vision. The treatment of this is very difficult, because photo receptors are not easily obtained, and also the number and the density of photo receptors in the eye are under an amazing level of attack. You cannot really rescue these patients by injecting a few cells into their eyes. So, now we have a real retina, having a lot of photo receptors, as a seed. It gives a totally different chance for translational researchers to make a new transplantation method. That is what I really hope. I am helping them to improve. A deeper understanding of the basic principles is also very helpful here.
I really appreciate the Hans Sigrist Prize, as it will allow me to do new principle-based studies, for which it is harder to find funding. The government or companies are more interested in funding translational research.
HSF: The foundational work is where you think the Hans Sigrist Prize will be the most helpful?
Sasai: Yes. Also, I really need to have wider contact with non-biologists, because this kind of multicellular interaction that orders the structure, that is called the self-organization, this field was previously a part of complex physics. Also, we need to develop more fine measurement devices, like new microscopes, so I need to discuss and collaborate with many people around the world, with different specialties, so this award will also help me travel to meet these people. Also, probably, I will use this prize money to help make a sort of prototype machine for some kind of measurement. That would make me happy.
HSF: Could you tell me a bit about your experience so far on this, your first trip to Bern and to the University of Bern.
Sasai: Yes, I had the chance to spend the day yesterday with Prof. Eliane Müller at the Vetsuisse Faculty and meet her staff. I discovered that the culture medium I am using, actually two of them, are from her spin-off university venture, CELLnTEC advanced systems AG. We discussed which medium is good for which kind of cultures. They told me they are further improving some of their culture mediums, and I got a couple of new beta testing bottles.
HSF: Great, so we are sending you back with souvenirs from the University of Bern that may be useful in your research too!