SOLS Podcasts

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Peggy Coulombe: Hi! This is Peggy Coulombe with School of Life Sciences, and this is Science Studio.

There isn't a person on the planet, besides maybe the Pope, who wouldn't like to know the dark and seamy secret underlying sexual attraction. With us today is Doug Chandler, a professor in School of Life Sciences. Doug's discovered the first chemoattractant in vertebrates, actually frogs. Thanks for coming to speak with us, Doug.

Doug Chandler: Thanks, Peggy.

Peggy: Tell me a little bit about why you work with frogs.

Doug: Well, like most scientists, we generally use animals as models, and we try to pick the most interesting, or the best, for the problem we're interested in. Likewise, you can have a problem which types of animals will help answer.

So I started off being interested in cells that secrete hormones, and initially I worked on the exocrine pancreas, which secretes digestive enzymes. From there I went on to working with mast cells, which secrete histamine, and they're involved in tissue inflammation. And from there I finally went on to studying reproductive cells, but that was only because eggs, actually, are secretory cells.

Peggy: Mmm.

Doug: And right at fertilization, they secrete proteins that are enzymes, and structural proteins that modify the coating around the egg to change it from being a sperm-receptive coat that sperm actually bind to, to being a coat that actually acts as a barrier to further sperm getting to the egg and thereby contributing too many chromosomes to the new embryo, which ultimately leads to death, so that's something to be avoided.

From there, in that I had an interest in secretory cells, I jumped on to seeing how beautiful the egg was. In those days, I was using sea urchin eggs, which are a common model, because so many of them are produced by the female sea urchin.

Peggy: Millions.

Doug: Millions. Almost billions, like the galaxies. [laughs] But millions. So it's a biochemist's dream and a cell biologist's dream.

But it happened that I was doing some electron microscopy, and looking at the egg's surface. And on the surface, I saw an aerial view of an extracellular matrix, a coating on the egg's surface, that had never been seen like that before. And it was just so interesting, and so fundamental in terms of being a sperm-binding network of proteins, that I started studying that instead of secretion. I gradually changed to studying the extracellular matrix.

So we continued doing that work on the extracellular matrix of sea urchin eggs, and it became natural for students entering my lab to want to see what other eggs look like. And there was one graduate student who wanted to do frog eggs, and that was easy, because one of my colleagues was studying Xenopus laevis, and routinely harvested eggs.

So she started on that, and, indeed, the frog egg turned out to be a beautiful subject, and then finally we got to the point where we said, OK, this structure that lies on the surface of the frog egg binds sperm. From there, the sperm will wiggle through that layer and get to the egg plasma membrane. The sperm then fuses its membrane with the egg membrane, and then it can donate its chromosomes to the egg cytoplasm.

A lot of people had studied that, and similar coats in mammalian eggs, and, likewise, sea urchin eggs and other invertebrates. But we realized that we were also interested in this outer coating around the frog egg. It's these translucent jelly layers that just sort of pop right out when you look at a bunch of frog eggs in a pond. You see they're all glistening and transparent.

Peggy: Mm-hmm. Absolutely, yeah.

Doug: So we thought, well, does anybody know anything about that? We've certainly visualized them by electron microscopy, but what do they do?

And, as usual, you go to the literature and see what people have done on a particular subject, and, surprisingly, nobody knew what the jelly layers did in regards to helping fertilization. And yet, on the other hand, people had taken the jelly layers away, and fertilization stopped, basically. So, clearly, they were important.

Peggy: Yeah.

Doug: So that led us to saying, "Here's a problem that needs to be solved." So that's how we got into studying the jelly layers, and out of that we found out that this protein, allurin, that resides in the jelly layers, actually is a chemoattractant for sperm.

Peggy: And how does this chemoattractant work? Because you've got all these layers of jelly. How does it actually attract sperm?

Doug: OK. Well, I'll tell you parts that we know about. In terms of the global picture, we would like to know how it works. [laughter] But there's still a lot left to figure out.

Well, what we do know is that when the egg grows in the ovary, it doesn't yet have a jelly coat. But when it is spawned, it passes through this long tube called an oviduct, which essentially is equivalent to the oviduct, or Fallopian tube, in mammals. And it's the cells in that oviduct that actually make the jelly. So, as the egg goes down the oviduct, it accumulates layer after layer of jelly, and then, finally, when it's spawned out into the pond water, has all these jelly layers.

Well, as you would predict, because this protein, allurin, is found in the jelly, it is actually synthesized by these oviduct cells.

Peggy: Ah, interesting.

Doug: It's kind of funny. You know, the oviduct works so efficiently that if you put a little ball bearing at the beginning, about the size of the egg, the oviduct will actually pull it through and coat it with jelly, even though it isn't an egg.

Peggy: A ball bearing. [laughs]

Doug: Yeah, right. [laughs] So anything that goes through there gets coated with jelly. But we have seen, microscopically, that allurin is within that coat, and also that it leaves that coat very quickly once the egg hits the pond water. What seems to happen is that the jelly layers expand because they suck up--

Peggy: Water?

Doug: --pond water, kind of like a sponge, and then, in doing that, the structure expands, and that seems to allow these proteins like allurin, that are smaller proteins, out into the pond water. And that's where they meet the sperm.

Peggy: So do the sperm essentially swim up a concentration gradient?

Doug: That's the theory. Now, why I say "theory" is because it's actually very difficult to measure a concentration gradient in practice. And while people studying chemotaxis essentially assume there's a gradient in the way they set up their stage for chemotactic chemicals interacting with a cell, it's seldom that they actually measure it.

And, indeed, it's hard to measure something that's floating around and not staying still as to "What's it's chemical concentration gradient?" Nevertheless, what we have done is showed that the amount of allurin in the jelly exits the jelly within just a few minutes.

We've shown that by what's called immunocytochemistry, where you take an antibody, in this case it'll react specifically with our particular allurin protein. You use that to detect where the allurin is. It can be done quantitatively. So what we find is that the jelly layers initially have a lot of allurin, and within four minutes, half of it's gone.

Peggy: Hmm.

Doug: And, obviously, subsequently, probably about 90% of it leaves the jelly. Now, the question, though, is, when it finds a sperm, what happens? And that's the part that we still don't know about very well.

Peggy: Don't know exactly.

Doug: We know, number one, that if we put sperm in a chamber, where we've set it up so that there is a concentration gradient, the sperm will swim towards the higher concentration of the protein. And that's why we know it's a chemoattractant.

Peggy: So, in your setup, you have then the eggs or the jelly on one side, or...?

Doug: In some cases we have jellied eggs, and that'd be close to the biological system. In other cases we essentially have chemically solubulized the jelly from eggs, and we put a small drop of that in our chamber.

Peggy: OK.

Doug: And, likewise, we've gotten to the point where we can actually purify this particular protein from all of the proteins that are in the jelly extract. So the purified protein does it, too.

Peggy: Since this is the first vertebrate chemoattractant that's been isolated, how do you know--since it's in frogs--how do you know it has any application to, maybe, people, for example?

Doug: Well, we don't know yet, but there are some interesting leads. Those leads include--I should say this is kind of exploratory data that is not yet published--but our clues include, one, that if you set up a chemotaxis system using a mouse, that is, a mammal, mouse sperm, that allurin actually appears to be a chemoattractant for mouse sperm.

Peggy: So something about allurin, for frogs, also attracts sperm in mice.

Doug: Right. And we really don't know why that might be, but there are a couple of other clues. One can determine the amino acid sequence of a protein, and ask, "How does that sequence compare to other proteins?" And, of course, there's a huge database now available to do that.

And when we did that, we found that allurin is related by sequence to some mammalian sperm-binding proteins. And, in fact, one of them is thought to bind the sperm in the male reproductive tract, but then be carried with the sperm into the female tract and actually be present at the time of sperm-egg fusion. So some people predict that it might have a role in that process.

So these kinds of proteins clearly are not unknown to mammalian systems. It's just that no one's found one in the female reproductive tract of a mammal yet. Nevertheless, we have another really interesting clue, and that is that if you look for similar proteins in the female mouse, you can actually find one that seems to be related to allurin, because the antibody for allurin will bind to it, meaning it's probably a similar protein. We don't know what that is yet, but we're going to find out.

Peggy: This kind of research is potentially very exciting on a lot of different levels, but one in particular is that if you can identify how it works in a mammalian system, that potentially you can help people, couples, with issues like fertility.

Doug: It's possible. We would like to think that. Anytime you have a natural, whether it be a protein, or a drug, or a compound that binds to a cell and initiates a certain behavior, and usually that has to be carried out through what's called a signaling pathway--there is a binding site on the cell surface that leads to interactions of proteins inside the cell, and finally it regulates some response on the part of the cell.

Well, presumably, that's what's happening when allurin binds to sperm. We know that it binds. We do have data on that. But we don't know the rest of the story inside the cell that really guides its swimming.

Now, really, what I was getting to is that anytime you have such a molecule binding to a cell to make it behave in some certain way, you have the possibility for manipulating that pathway. We see it all the time in drugs that pharmaceutical companies make. Often they're drugs that interact with the receptors for some natural hormone or neurotransmitter, and therefore makes our body behave in a different way.

Well, you can apply that to allurin. We might find in the future that allurin or its analogues might act as, say, artificial activators of sperm, or maybe some might act to block sperm's action. And in this way, one could anticipate that these might be compounds useful for stimulating fertilization or blocking fertilization, in the manner of contraceptives.

Peggy: When I first came to your office, one of the first things I noticed were your oil paintings. And on the wall, for those of you who can't see the publication in front of me, there's a photograph of one of Doug's paintings that looks like a galaxy, and there's a planet with an outer orbit, and there are two large sperm spiraling their way in.

Many other of Doug's paintings are landscapes and snowscapes and all sorts of beautiful pieces of work. Can you tell me something about how you came to paint?

Doug: Oh, well, it's a hobby of mine. We all enjoy our hobbies. I think, often, we have to thank our parents for getting us involved in things that we then carry with us, like the little kid who grumbles over having to learn piano but then later in life decides it's really a lot of fun.

Well, I was sort of like that with painting or drawing. I had a couple of friends in junior high and high school who also had the same interest, and, not my mom, but my friend's mom saw to it that my friend Dana got a chance to take art lessons from a professional artist. She needed some help with that in the sense of, "Well, it's not going to cost as much if you get two of your buddies to join in with you."

Well, I was one of those buddies. And we actually had a lot of fun. We would go every Saturday afternoon to an artist's studio to draw and paint, and, well, this shows a little prejudice, but it was a lot more fun than going to the high school football game where everybody else was. [laughter] We had a blast. And you look back on that, and you realize how lucky you were that you were in that situation. So out of that, I really just gained a lifelong enjoyment of painting that I do every so often.

Peggy: And your friends still paint, too, as well.

Doug: Yes. The two other fellows, one of them is a professional artist, Ernie Norcia, he's in the Philadelphia area. He's a portrait painter, and I think is highly sought after. And the other is Dana Everett. His mom was the one who saw to it that we got a chance to do this. And he has been in design his whole life, but he is now in retirement and going back to actual oil painting, so I've seen a few of those on the web.

My interests were strong, but I always had this stronger interest in science, so it took the first priority, I would say--

Peggy: In your life?

Doug: I saved art for fun.

Peggy: [laughs] But do you see art and science as having things in common?

Doug: Oh, yes. Very much so. You know, behind each is a creative individual, or the enjoyment of creating. In art, you're creating something rather obvious that other people can enjoy as well, we hope, but the actual enjoyment of the artist in making it is what he or she is, therefore.

And, likewise, in science, really you're trying to put together what amounts to clues in a mystery story, and in doing so, you're creating a story that in a sense didn't exist before. So you get the joy of creating that, and hopefully other people will see it, some will understand it. Science isn't as immediate, though, as art is.

Peggy: Well, it seems like they've both been long-term projects.

Doug: Yes, yes. Certainly science has stuck with me every year of my life, I would say. Some years better than others. And art has sort of come and gone, and come and gone, but it always comes back again.

Peggy: For a student coming into your lab, what would you hope that they came with and left with?

Doug: Hmm. I think most people in education really want to impart--well, in a way it's a gift, but it's something that works both ways. You want to have your student be successful, enjoy being creative, in the same way you have. You want them to gain abilities, and you take pride in that.

People often talk about their students being their academic family, and that, as a mentor, you're sort of the academic parent of your students, and there definitely is some truth in the way those words are used. You feel responsibility, and you feel pride in what your students do, and you don't want to let them get out there on their own without having a proper foundation.

And, really, it isn't just the most obvious things, like, "Well, you've got to know how to do X, Y, and Z techniques. You have to know how to write a paper, " or something like that. You want them to be a thinking person that can live in the adult world of science, and it takes a lot of skills to do that.

Peggy: Well, Doug, I want to thank you so much for coming today. And for those of you who don't know it, the person I was supposed to speak with originally had to cancel, and so Doug was very kind to let me drag him out of his office and join us today. And I want to wish you all the best in your research.

Doug: Well, thank you, Peggy. It's been a lot of fun.

Peggy: Thank you for joining us today. This is Peggy Coulombe, with Science Studio at Arizona State University, in the College of Liberal Arts and Sciences, and have a good day.

Froggy goes a-courting

Transcript - [Printable PDF format]

SOLS Podcasts 2007