Misty Riddle, PhD, Post-Doctoral Fellow and Evolutionary Developmental Biologist talks about discoveries found in cave fish and how it relates to diabetes in humans.
Interview conducted by Ivanhoe Broadcast News in May 2018.
Tell me about cave fish, how did you get to this point?
Misty: Cave fish are a really exciting model system to work with. For one thing they’re just really cool to look at right? So we have these two different morphotypes: one that’s found living in rivers in Northeastern Mexico and one that thrives in these underground limestone caves. And they look very different from one another but they’re the same species so the river fish has pigment, has these big eyes and the cave fish has no pigment they are like these white pinkish fish with no eyes at all. They were first discovered in about the nineteen thirties by some cavers who said, oh there’s some fish here, but they weren’t brought in to the laboratory until much later than that. And the first thing people started looking at was eye loss because that’s sort of the most obvious thing to ask about how they lost their eyes and pigment loss. Then also they have some differences in their behavior but nobody had really asked how do they survive in a cave with so little food. So how has their metabolism changed to allow them to go much of the year without any food at all. Caves are completely dark, they don’t have any light and they don’t have any plants so photosynthesis can’t take place. So they’re really dependent on things coming in to the cave from the outside and that can happen through floods or from bats. But we were interested in not necessarily how they look or how they behave but how do they actually survive with so little food.
What is it about their systems that is unique that you and your fellow researchers are learning from?
Misty: When we started to look at their metabolism, one thing that we already knew is that they were really fat. If you dissect these fish or even just looking at them they seem to store fat everywhere. We wondered what other type of metabolic changes would there be. We know in humans sometimes obesity is associated with dysregulation of glucose homeostasis, and so that’s one thing we decided to study, what does their blood glucose regulation look like. Luckily we don’t have to go in the field and do this work, we don’t have to travel to Mexico to do this we have the fish in the lab and we can feed them whatever diet that we want to feed them on and see how they respond to that diet. And what we found is the fish that live in the rivers they have a normal blood glucose level whereas the fish that thrive in the caves have a highly elevated blood glucose level. That sort of told us something was different in the way that they regulate their glucose levels. The next thing we did was challenge them to clear glucose from their blood and we did that by giving them a lot of glucose, you can actually just inject the fish. You see this dramatic spike in the blood glucose level and after a certain period of time the river fish they bring that level down to normal, the cave fish can’t do that as well it seems to be a delay in clearing the glucose from their blood. And so this told us that something was happening with this insulin signaling pathway that might be changed in these fish.
From there where do you go?
Misty: There’s two sorts of avenues that we went down. For example, for people with diabetes there’s two types of diabetes Type I and Type II. Type I you don’t make enough insulin and insulin is what tells your cells to absorb glucose from the blood after you’ve eaten a big meal, insulin levels go up your cells absorb that glucose from the blood. So Type I diabetes just doesn’t make enough insulin. Type II is that your cells don’t respond to the signal, they can’t hear that signal to absorb glucose from the blood. So we first measured the cave fish insulin level maybe they don’t make enough insulin. It turns out that they do they had no change they seemed to be making plenty of insulin. Their pancreas development looked normal, so that told us that it must be the flip side maybe their cells aren’t responding to that signal as well. And we found that actually was the case through some in vitro methods. And we then asked is there a genetic change that could be linked to those difference in insulin resistance. And what’s really cool about this fish is that we have the entire genome sequenced. We can ask about what are genetic changes that might be associated with this insulin resistance. And because of work in humans and other model systems we know the genetic sequences of those genes so insulin receptor for example. And we actually found that there was a mutation in insulin receptor in these fish. And what was really surprising was they had a mutation that was the same mutation found in some humans that have this really rare form of insulin resistance that’s really deadly. The cave fish have this mutation and this river fish don’t. And the mutation seems to be preventing insulin from binding well to the receptor. So that’s sort of the cells not receiving that signal.
When you have that kind of information and it’s a long way down the line, but what kind of path does that give researchers, yourself and others looking in to this to make the leap to humans?
Misty: Now here we have this fish that’s thriving with a mutation that’s deadly in humans, how is it doing that? What are the mechanisms that fish has that allows it to have this mutation and survive. If we can learn about how that’s different maybe we could apply that to humans or understanding why humans with the mutation have these detrimental effects that the fish avoid. And in fact it’s even advantageous to this fish to have it. So that’s the surprising thing. We asked why would they even have this mutation, why would they even have dysregulated glucose homeostasis this seems like a bad thing to have. At least in humans it’s a really bad thing to have. And we were able to take advantage of the fact that you can actually interbreed these fish. You can take the surface fish and cave fish produce hybrids and then look at their offspring. And that allowed us to look at fish that carry the mutation and fish that don’t and ask what kind of characteristic is having the mutation associated with. And the fish that have the mutation grew bigger, they had higher weight. They were able to gain weight more. And so what we think is it gives them an advantage to gain weight and if you’re a cave fish that’s a good thing because there’s going to be a time when food is really scarce. It’s not detrimental and it’s actually giving them an advantage. One really cool thing that was part of the paper is it could be a mutation near that gene that was affected and to eliminate that possibility we used this DNA editing technology called CRISPR. And we were able to insert the cave fish mutation into zebrafish. And those zebrafish grew bigger so that really told us that this mutation was associated with the increased weight gain.
So you knew you were in exact right place when you saw the impact on the zebra fish?
Misty: Exactly, yeah.
With the cave fish you said that they aren’t impacted by this deadly form that would take humans out by age three?
Misty: Yeah.
They weren’t impacted. Are there other surprising factors about the fish, do they show any other bad signs, bad side effects?
Misty: They don’t seem to have the negative health consequences. And we have limited number of ways to test that. One way is lifespan, so if you’re dead you’re unhealthy if you’re alive you’re at least doing okay. And these fish are incredibly long lived so the oldest fish that are in a lab setting are fifteen years old. And those fish were collected from the wild. So who knows how old they were when they were collected, they’re at least fifteen years old. And the surface fish have started to show some signs of senescence which in fish it’s kind of weird that it’s like sinking skin and like an arched back. But in the cave fish they don’t seem to show those same signs of aging. And we have really limited numbers of those really old fish but it seems like the cave fish don’t have advanced signs of aging. The other thing that we measured was something called advanced glycation end-products. In humans when you have this chronic elevation in blood sugar levels really simply put, your proteins in your blood start to get kind of sugar coated and that’s bad because they can’t work properly if they have all this extra sugar on them. We know that cave fish have this elevated glucose in their blood so we asked do they actually have sugar coated proteins in their blood and it turns out that they don’t. Those advanced glycation end-products are a really promising target for diabetes associated pathologies. The cave fish don’t accumulate them. That’s what I see as a potential really interesting avenue to pursue, is how are they not getting those end products.
You touched on this but I want to ask again. Could you help take our viewers from being able to pinpoint these specific genes for drug development down the road? If you know exactly what’s going on, how will that help potentially down the road in humans?
Misty: We know that the fish have these characteristics like elevated glucose and this mutation and the fish don’t have the negative health consequences of that there’s certainly a lot of work to figure out why they don’t, but if we figure out why potentially it’s something that’s relevant to treating humans.
Is there anything I didn’t ask you that you would want people to know about this research?
Misty: I don’t think so.
Can you give a statement about what basic scienc can lead to?
Misty: That’s a great idea. As research scientists there’s a lot of interesting model organisms. Like research going on in humans is very important, research in mice and we have C. elegans. But in some ways we’re ignoring the whole world of animals so there’s such diversity in animals and they’re thriving in so many different environments that it’s really a great opportunity to do more basic science of just going out in the world and seeing how animals survive in different conditions. And we might learn something quite interesting like we have in the cave fish.
END OF INTERVIEW
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