Michael W. Young, PhD, is an American biologist, geneticist, investigator, and professor and vice president for academic affairs at Rockefeller University in New York City. In October, he became a recipient of the Nobel Prize in Physiology or Medicine. Along with co-recipients Michael Rosbash and Jeffrey Hall, Young is credited with identifying the molecular basis of circadian rhythms.
Dr Young has devoted his life's work to exploring the ways in which circadian rhythms arise from interactions among certain genes and their proteins, which set up molecular oscillations. A member of the National Academy of Sciences and a fellow of the American Academy of Microbiology, he has been studying circadian clocks for nearly three decades.
Among his many accomplishments, Dr Young identified genes that affect the regulation of sleep in Drosophila, uncovering specific neurons whose activity promotes sleep. He is a recipient of the 2013 Shaw Prize in Life Science and Medicine, the 2013 Wiley Prize in Biomedical Sciences, the 2012 Canada Gairdner Foundation International Award, the 2012 Massry Prize, the 2011 Louisa Gross Horwitz Prize for Biology or Biochemistry, and the 2009 Neuroscience Prize of the Peter and Patricia Gruber Foundation.
Medical journalist Marc Gozlan, MD, on behalf of Medscape, spoke with Dr Young to discuss his work and learn more about the man behind the Nobel Prize.
Early Days in the Backyard
Medscape: When and how did you first develop an interest in science and biology?
Dr Young: I grew up in Miami. It is a very tropical climate. We kids used to go through the backyards of the whole neighborhood, looking for birds, lizards, and snakes—things of that sort. Miami is a tourist destination, and it's full of parks. There is the Parrot Jungle, the Monkey Jungle, the Serpentarium. These were privately owned, but they were operated with an eye toward tourism.
This park called the Parrot Jungle was just a few miles away from my house. It probably occupied 50-100 acres, and it was an open park. It had all these exotic birds. There was no top, no cage.
Medscape: Did animals often escape from the private parks in the area?
Dr Young: Yes. Our backyard was my jungle. Animals were always escaping. On many occasions, I would go out in the backyard and there'd be some exotic bird from the Parrot Jungle.
The same was true for [other animals]. You'd have an alligator wander into the backyard (a small one—probably escaped from one of these parks, because we were far enough from the Everglades). And we were just hoping that none of the snakes would get out of the Serpentarium. Also, I would find iguanas, parrots, and toucans in our trees.
But the other feature of the neighborhood was there were many botanical gardens. There were also nurseries that sold tropical plants, and everyone around us had these exotic plants.
Medscape: Exotic plants were a regular part of your environment?
Dr Young: Yes. Right next door, there was a plant that produced white flowers that would have a dried-up appearance during the day. But if you came over around sunset, around early evening, they would be in huge blooms. So, they would be wide open. At that time, it just shocked me that you could get these rapid changes and that this was something a plant could do. I was probably 8 or 9 years old.
Medscape: What did your parents do?
Dr Young: Both of my parents were born and grew up in Knoxville, Tennessee. My father was in the Army Air Force in the 1940s. After World War II, he came back to Knoxville and met my mother. They decided pretty quickly to get married and went to Miami, Florida, for their honeymoon in 1946. I think they liked the warm weather, and they decided to stay.
My father managed aluminum ingot sales throughout the southeastern United States for Olin Mathieson Chemical Corporation. My mother worked as a secretary in a legal office in Miami. I was born in March 1949 in Miami.
Medscape: Do you think this fascination with biology was fostered from reading books?
Dr Young: I was probably 11 or 12 years old when my parents gave me a book on Darwin, evolution, and some biological mysteries. There was a description of biological clocks that control the movements of flowers and help birds or insects to navigate, but it was clear that the mechanisms of these clocks were unknown.
Medscape: Did your parents encourage your interest in science and biology?
Dr Young: Yes, they did. I was always getting them to buy me books that had something to do with nature. Very often, they were books that were most impressive for their photographs of wild animals. But I do remember I had noticed a book in a bookstore. It was a large book, and the title was The Wonders of Life on Earth. It was produced by Time Life. It was full of colorful illustrations of different forms of sea life and terrestrial life.
A theme that ran through the whole book was Darwin and the theory of evolution, and how all of these forms of life had propagated. There was a section on heredity and genetics, and another section on molecular biology.
I was probably 11 at the time, and this was when I first encountered DNA as the hereditary material. I still have this book. I think the publication date of this book is around 1962, maybe 1963, and it already had these stick models of DNA—photographs of them, and an explanation what DNA was. It even went into a little bit of DNA hybridization—the fact that you can melt DNA and allow it to reanneal.
There were a couple of pages that talked about the experiments of Marmur and Doty on denaturation of DNA. I remember being fascinated with the chapter about his experiments on taking DNA apart and putting back together the complementary strands. Years later, I met Julius Marmur. He was in the department where my wife worked, and I had a great time showing him this book.
Medscape: Did your parents offer you scientific equipment, such as a microscope?
Dr Young: Yes. Microscopes often came with a few prepared slides and such things as onion skin and similar items where you could see some cell structure. And of course, I immediately started sticking in things like blood cells. I remember pricking my finger and putting blood cells on a slide and being really amazed at these wonderful little bubble-like images of my own blood that I could get on this simple light microscope. I also had dissecting sets, where you would get all these instruments for slicing up a dead frog.
I also got a chemistry set. I used to have compounds such that I could easily generate hydrogen gas from reactions and cause small explosions with a match. I can remember begging my father to bring me some potassium chlorate, because I knew I could make the oxygen from potassium chlorate. Of course, you could not get that in the chemistry set or in the hobby shops where you would buy extra chemicals, because it had explosive potential. My father did finally bring me home a bottle of potassium chlorate from one of his colleagues in the company, and I proceeded to make some oxygen using it. Again, it's something that would make a match glow—things of that sort.
So came the destruction of a terracotta floor with my chemistry set! I can't even remember what the compounds were that I was putting together, but something started bubbling and sending shooting beads of liquid out onto the terracotta floor. My mother was very disappointed in my early experimental abilities, my early research. After that, I got moved to the garage.
Medscape: Were you also interested in building things?
Dr Young: When I was 11 or 12, I had an interest in machinery and built a go-kart from a frame I found at a used bike store near my house and a gasoline engine I unbolted from the family vertical-drive lawnmower. It was very crude. I ran a chain from a sprocket that I put on the crankshaft directly to another on a rear half-axle. It looked like a Formula 1 go-kart.
You pushed it to start it. It would take off, and you would have to run alongside while you were holding the steering wheel and jump into the seat and, to keep it going, mash on the gas. It would do about 30 mph. There were drum brakes. You would put your foot on the brake and slow down a little bit at the stop sign, and you would look around for cars that might hit you, then do a quick corner or U-turn or something instead of stopping. It was a dangerous device, but it was great fun.
Medscape: Your family moved to Dallas, Texas, when you were in high school, and then you graduated from the University of Texas at Austin in 1971.
Dr Young: That's right. During my last year of undergraduate studies, I took a course in genetics. It seemed to me not much in the way of the mechanism was understood. There were courses with taxonomy or phylogeny, and there would be a lot of descriptive information, but genetics seemed to be a subject where you could dig in and learn something about the mechanism. That became apparent from the way the course was taught by Burke Judd, who was a professor in the Department of Zoology. I asked him if I could do a summer project, because the University of Texas had these courses that were called research courses, where you could work for summer under a faculty member.
A Summer Research Project
Medscape: How did your research with Burke Judd go?
Dr Young: That summer research project turned out to be much more interesting than I had even imagined. I became exposed to the way in which research questions are posed and the way to move forward. I had the feeling that I was actively engaged in new research—research that was really moving, not just repeating things that had been seen in a textbook. In fact, Burke Judd let me ask truly new questions, I became very interested, and he and his postdoc encouraged me to stay on. I had some luck with the project during the summer. I was making mutations to see whether a certain class of genes could be found.
Medscape: How did you make the decision to focus on molecular genetics of a biological clock in Drosophila?
Dr Young: The course that focused on biological clocks came a little bit later, but certainly my decision to use Drosophila to look at contemporary problems in genetics developed that summer. What I became interested in was the surprising amount of DNA found in eukaryotic chromosomes, much more than is in bacteria. Then the fascinating questions became "What's the structure of a eukaryotic gene? What does the eukaryotic gene look like, that might make it so different from a bacterial gene?"
Medscape: You went as a postdoc at Stanford in 1975 to learn molecular biology, correct?
Dr Young: That's right. I began my PhD program in 1971, and I graduated from the University of Texas in 1975. In August of that year, I moved to Stanford to work as postdoctoral fellow. None of the experiments I was performing when I was in Texas involved molecular biology. They were all classical genetics. I was able to learn a lot about the period gene by doing those classical genetic experiments without touching a molecule—just by doing crosses between flies and following chromosomes, doing genetic complementation mapping.
I only applied to one place to do postdoctoral work; it was Dave Hogness's lab. I didn't even bother to talk to anyone else, because I was so sure that the only way to get meaningful information was with the new methods that were being developed at Stanford. I was with Dave for 2 years. We were busy just trying to make sense of some of the first Drosophila DNA clones that were coming out of these libraries.
Medscape: Then you went to Rockefeller University in January 1978?
Dr Young: Exactly. After a couple of years at Rockefeller, I realized that I could combine some of the techniques from the old genetics, classical genetics explored in Texas, with some of the molecular cloning—molecular biology knowledge—that I had gotten from Stanford. I could bring these two things together and go after specific genes in well-studied regions, genetically studied regions of the X chromosome.
Medscape: So you decided to try to work on the period gene?
Dr Young: In fact, we worked on two genes, period (per) and notch, that were very close to the white region where I worked as a graduate student. I realized that I had ways of locating the per gene using those chromosomes rearrangements. And we did wind up isolating both of those genes, although we eventually became much more intrigued by the circadian rhythm problem and put more of our forces into that. I made that decision to really locate and identify the per gene, probably in 1981.
Between zeste and white
Medscape: How did you become interested in per, which turned out many years later to be the first clock gene to be recognized in Drosophila melanogaster? Did it all begin when you reported that mutations of the per locus were associated with breakpoints of several chromosomal rearrangements and characterized with respect to their effects on eclosion rhythms?
Dr Young: My interest in period came in about 1972, and it closely followed the report that Ron Konopka and Seymour Benzer published in Proceedings of the National Academy of Sciences (PNAS). The preliminary chromosome locations that they had performed in that paper suggested that their gene that was controlling these biological rhythms might map close to an area that I was trying to saturate with regard to mutations and genetic functions. It was between two genes, called white and zeste. I was trying to find out whether there might be other genes in that interval that had been missed because they could not be mutated to give embryonic inviability.
Medscape: You must have been intrigued that the group at the California Institute of Technology—Konopka and Benzer—had found these mutations that affected the behavior of the fruit fly.
Dr Young: I wondered whether they might first map within this region that I was working on and whether they would be alternatively mutated forms of genes that I already recognized, or whether they would be something completely new. So I asked Konopka and Benzer to send the mutants. Konopka encouraged my interest and sent them along. I found that I could locate them using a series of partial deficiencies and partial duplications for that region. Using these rearrangement chromosomaI sequences, I found that I could locate the per gene—that in fact, the gene was going to map somewhere between the zeste and the white loci. So it was falling in this interval.
Medscape: And so you succeeded in mapping the period gene more precisely.
Dr Young: Yes. I found one chromosome translocation, which placed the tip of the X chromosome onto the fourth chromosome. In doing that, it had produced a clock mutation: In other words, when I took that translocation and tested it against the rhythmic mutation as a heterozygote with the rhythmic mutation that Konopka had isolated, I found that I got defective flies—flies that had abnormal circadian behavior.
Medscape: That gave you an even more precise location for the per gene.
Dr Young: Exactly. I had been mapping the period gene effectively between two previously recognized genes in that interval. And so I tested several of the lethal mutations from those neighboring genes, and none of them had an interaction with the period mutation. That was important, because it indicated that period could not be mutated to give a lethal phenotype. So, I had a breakpoint mutation.
Ron Konopka had three mutations: per0 , pers , and perl . per0 mutants caused arrhythmia with respect to both locomotor activity and eclosion. perl mutants were rhythmic, but the period of locomotor activity rhythms and eclosion rhythms were lengthened to 29 hours, and pers mutants has short period rhythms of 19 hours.
Medscape: All of these mutations gave a behavioral phenotype?
Dr Young: Absolutely; it was as if the gene were dedicated to this behavior. So that changed my thinking quite a lot. By using overlapping deficiencies, defined by a breakpoint in each chromosomal rearrangement, we got an arrhythmic phenotype. That proved that loss of the gene produces this arrhythmicity.
Also, a particular translocation and two deficiencies allowed us to know what region of the DNA would have to contain the period locus. So, the chromosomal interval lying between these two breakpoints, called TEM and JC43, included the per locus.
Medscape: So per could correspond to any of several transcription units that could be mapped to that deficient region of DNA.
Dr Young: Exactly. The real help was the JC43 translocation because, instead of eliminating a portion of the region, it had taken the left portion of the entire region and joined it to the fourth chromosome. And what we realized was that these flies had some clock activity. This translocation gave very long period behavior rhythms. That indicated that there was still some function remaining for the period gene.
The break on the translocation must have affected the way the period gene could function. So when we looked for changes in the way in which the transcription units in that area behaved, we found that one of the genes had been broken by this translocation at the 3' end of the transcription unit. There was a 7000–base-pair transcription that had the JC43 break right at one end of it.
Medscape: That was the article you published in April 1984 with your postdoctoral student Ted Bargiello in PNAS.
Dr Young: That's correct. It was predicted that the PER could still be produced, but its control could be altered by virtue of the change in the sequences near one end of the messenger RNA. So that was in fact the first period mutation that was physically mapped and understood to affect a product of the per locus.
A Device the Size of a Shoebox
Medscape: Could tell us about your screen procedure that included the analysis of the time of eclosion, but later also the analysis of the locomotor activity of the flies?
Dr Young: In 1984, we had to hand-collect flies that were emerging from the pupal stage. The eclosion of wild-type flies occurs with a circadian rhythm in populations of Drosophila. They emerge early in the morning, and so the period mutants did that incorrectly. That was one of our assays that we used to follow the mutant flies.
The other assay that we used was on monitoring up to five flies at a time. Here at Rockefeller, we were able to build a device that has a place where a small piece of glass tubing, resembling a pipette about 3 inches long, could be placed in the device. On one side of that glass pipette, you have a light-emitting diode that produced far-red light. On the other side of the pipette is a phototransistor. It is set up to produce an electrical impulse if a moving fly breaks the light beam as it travels along the length of this 5 mm × 50 mm transparent glass pipette.
Medscape: What about the recording machine?
Dr Young: The device was wired to a large machine, called an Esterline Angus event recorder; this was the machine that produced a chart paper. Over the course of 24 hours, we would have a few feet of chart paper with a red line extending from each of five pens working in parallel, because we could monitor five flies at a time with this device. The pen would move when there was an electrical impulse coming from the device. We would fill it every day with red ink.
So we would have control flies and experimental flies; we would have our arrhythmic flies. At the end of 1984, what we had were flies that had received injections of the transcription unit, and the one that we saw had to be the period gene from the JC43 translocation.
Medscape: I imagine you had a look at the recordings each morning.
Dr Young: Oh, yes! Each morning, we gathered the paper that had been pumped out onto the floor overnight, and we would look at the lines of red ink for places where the pen was wiggling and places where the pen was quiet. Wiggles indicated times when the fly was moving around, and the straight lines were times when the fly was quiet, resting. Then we would cut the paper up and stack it in 24-hour intervals, so that we could see what the patterns of behavior were for each fly.
The wild-type flies would be active for 12 hours and inactive for 12 hours. Over and over again, we'd see that pattern in these records. The arrhythmic control flies would not show any of that; they would just be erratically active and erratically resting. In contrast, the flies that had received the clone DNA from the period gene had gotten their patterns back, so they were now showing alternating days of activity and nights of inactivity.
Medscape: What was the size of this device?
Dr Young: It was about the size of a shoebox. All of this was done with the flies in the dark incubator; the flies could not see light at any time, and they could not see light cycles or experience temperature cycles. The environment was kept stable. So we knew that what we were measuring was some endogenous ability of the fly, and moreover that we could use this clone DNA to transfer that endogenous ability from one fly to another.
This device was used in the early 1980s. It is the work that we published in 1984 in two papers, chiefly the Nature article, where we showed the restoration of circadian behavioral rhythms by gene transfer from a rhythmic fly to an arrhythmic fly.
Medscape: Drosophila melanogaster born and reared in constant darkness exhibit circadian locomotor activity rhythms as adults, correct?
Dr Young: Yes, this is correct. We actually tested it ourselves in a paper we published in February 1992 in PNAS. We found that flies that have never seen light in their lives will give locomotor monitor rhythms that look just like those from the wild-type flies. It is just that you cannot predict the phase of the oscillation of the rhythms. Therefore, every individual fly that has never seen light starts off with its own unique phase, until you get population arrhythmicity. But that is coming from individual rhythms that are just uncoupled from one another.
Finding the Molecules That Make Clocks Tick
Medscape: Ten years after the isolation of the per gene, in March 1994, your team published two landmark articles in Science[6,7] You reported the discovery of an additional period mutant in fly, called timeless (TIM). Positional cloning was used to isolate TIM on chromosome 2.
Dr Young: In these first two papers in 1994, what we showed is that the cycling of the period gene stopped in the flies with a timeless mutation. In other words, the timeless mutation caused behavioral and molecular arrhythmicity. We also showed that lots of TIM caused the PER to disappear. The PER was no longer produced even though period RNA was being made. So this also meant that timeless was somehow coupled in its function to period.
Medscape: So you showed an elevated level of cytoplasmic PER protein in this new arrhythmic timeless allele.
Dr Young: Yes. That was also satisfying because of what it meant when we went out looking for another gene that affected circadian rhythms. Instead of finding a new pathway, we came back to the same mechanism that PER was involved with. We came back to the same single clock, so that period and timeless somehow worked together to apply circadian rhythms.
Medscape: You showed 1 year later, in 1995, that the molecular rhythms of PER and TIM are interdependent, with mutation at either locus eliciting corresponding changes in the cycles produced by both locus.[8,9,10]
Dr Young: That's correct. There were three more papers in 1995 that showed that per mutations would cause arrhythmicity of molecular cycles of timeless.[8,9,10] We had previously shown that timeless mutations would exhibit molecular arrhythmicity in period.
Medscape: Could you offer a word on PER as a member of a family of proteins that contain a protein interaction domain called PAS?
Dr Young: It is an important aspect of the PER connection with other proteins involved in the molecular circadian rhythm in Drosophila. This acronym, PAS, is based on the first letters of the seminal trio of proteins found to contain this domain (Per-Arnt-Sim).
The PAS domain turns out to be a protein interaction domain that is very common in transcription factors. PER is a protein that does not bind to DNA, but it always works with another protein that has a DNA binding sequence. So PER binds to the clock-cycle protein complex. Those proteins have DNA binding domains associated with them, and PER uses its PAS domain for the interactions with clock and cycle. So PER modifies the action of the transcription factors clock and cycle by acting as a repressor in order to inhibit clock-cycle–mediated gene expression.
Medscape: Another important piece of the puzzle was discovered when you reported that the association of the protein PER with TIM is a prerequisite for nuclear co-entry of this heterodimer, correct?
Dr Young: Yes. It started in 1994 with the paper by Leslie Vosshall showing a block in nuclear localization of the period protein by a second clock mutation, called timeless. So we saw that the period proteins could not get to nuclei if TIM proteins were not present. And we reported in Science, in 1996, that the co-expression of PER and TIM in cultured Drosophila cells results in physical association and nuclear localization of both proteins, but expression of PER in the absence of TIM leads to cytoplasmic accumulation in cells. So we assumed that this physical association is a prerequisite for their moving to nuclei.
We also reported that just as TIM required for PER nuclear localization, PER must be present for nuclear localization of TIM, because TIM accumulates cytoplasmically in per0 mutants.
Medscape: In March 1996, you reported in Science that TIM levels are suppressed within minutes of exposure of the organism to light. TIM may therefore function to link transcriptional repression to external temporal cues.
Dr Young: The experiments showing that TIM is a trigger for response to the environmental cues of light came from showing that any time you shine light on a fly that is making TIM, TIM will go away within a matter of a minute or two, in response to that light. It is due to its interaction with a photoreceptor, called cryptochrome, which binds to TIM when activated by light and cause it to be modified and degraded. That will happen whether or not PER is around. So even in a per0 fly that is making TIM, you can see that light response.
A Delayed Mechanism That Produces Oscillations
Medscape: I imagine the discovery of the timeless gene was a moment of great excitement.
Dr Young: Yes. It was really exciting when we found the effects of timeless. It was interesting that we now had another gene to study and that it had effects that were as strong as period.
But when we found that TIM affects the way PER was produced, whether PER could accumulate, and whether PER could go to the nucleus, it seemed like we were beginning to see a mechanism. It was not just going to be an abstract thing; there was a tangible machine. We were going to pick apart the machinery and see how this machine worked.
It was terribly exciting at that stage, because so much came into understanding. We could see, for example, that if period and timeless were oscillating and if they controlled their own synthesis and depended on each other, that gave us a way of turning a feedback loop into something that would oscillate. So we knew there had to be something that would turn a feedback loop into an oscillator.
Then, seeing this required interaction between PER and TIM—seeing that they were delayed in coming back into the nucleus—that sort of opened the door into understanding. "Aha! We've got a delay; these are delayed feedback loops." These are proteins whose action is deliberately delayed in order for us to go from swings of synthesis to swings of repression. So that was a great thing.
Medscape: Your team reported in Cell in 1998 one additional circadian component: doubletime (dbt). This novel Drosophila clock gene regulates PER protein accumulation. The identification of DOUBLETIME, a kinase involved in circadian rhythms, was a major step forward in the understanding of the mechanistic basis of circadian phosphorylation.
Dr Young: The dbt gene was originally identified in genetic screens for mutants with altered circadian locomotor activity rhythms. The mutation originally defining the locus, dbtS , produced flies with short period rhythms. In fact, doubletime was the first gene in the molecular control of the circadian behavioral rhythm in Drosophila that was not a transcription factor that was discovered.
The fact that doubletime was an enzyme, a kinase, explained why PER goes away when TIM is not present. In fact, when PER is not bound to TIM—when it is by itself—it is constantly being broken down by its interaction with doubletime. So PER proteins that are not bound to TIM will interact with doubletime. Doubletime will phosphorylate PER providing a signal for degradation. So the response to environmental cues critically depends on cryptochrome and TIM interactions on the one hand, and then secondarily on the response of PER proteins that have lost their TIM partners.
The other way that doubletime plays a crucial role is in controlling the rate of accumulation of PER and by providing a delay that is crucial for producing oscillations in constant darkness. PER proteins are removed by the action of doubletime only until quite high levels of TIM are available begin to make heterodimers. In other words, the balance has shifted from making PER/DOUBLETIME complexes to making PER/TIM complexes. So that will presumably delay the appearance of the first PER/TIM complexes, because DOUBLETIME is keeping the level of PER proteins so low.
Three Decades Later
Medscape: What does circadian research tell us in terms of systems-level biology, where a molecular oscillator ticking in key neurons in the brain orchestrates a large number of molecules in multiple tissues to generate overt behavioral rhythms?
Dr Young: It indicates to me that we are organisms defined by time. Every hour of the day, so many tissues are changing with respect to their activities, and all under the umbrella orchestration of these circadian clocks.
It struck me that there is interesting new research conducted in France by Pierre Chambon. These researchers were looking at commensal bacteria in the control of intestinal epithelial cell homeostasis. Their work was published in Cell in May 2013.
It turns out that there are circadian clocks that are switched on under the influence of the microbiota in the gut, but that the clocks are not running in germ-free organisms. For me, this suggests that when microbes are present, the intestinal epithelial cells are called upon to deal with this new environmental situation. To deal with this new microbial environment, you have to turn on genes; you have to first turn on a whole circadian program that says when those genes are going to be turned on.
Perhaps some specialized functions require the orchestration of many genes that not only have to work together in a sort of agnostic sense, but also in a sense of temporal control—that it is important that they come online in a given sequence, either together or A following B following C. So the temporal order of gene expression to the activity of a cell is at the basic root of gene regulation. And the way that biological systems get that done is with their circadian clock. The symbiotic dialogue between the gut and the microbiota not only involves external bacterial cues interacting with epithelial cell receptors, but also requires the integrity of their circadian clock that opens a temporal window during which bacterial component signals can be transduced by the epithelial cell receptors.
Medscape: More than 30 years have passed since the beginning of your work on biological clocks. Today, does the impact of your work surprise you? It spans so many different fields: sleep disorders, cancer, immunity, metabolism.
Dr Young: Yes, it does. When I first became interested in this problem myself, it was just a problem of behavior. We had mammalian studies indicating that we had identified a small region in the brain, the suprachiasmatic nucleus of the hypothalamus, that controlled locomotor activity rhythms. I think that many could argue that all circadian clock functions map to that small region in the brain and nowhere else—that this was the master clock.
We began to see that things could be more complicated with the isolation of the period gene, because we found sites of expression outside the brain, right from the beginning. Indeed, per RNA was found all over the place, and it was quite confusing. Of course, now we know that in the majority of these cases, we got per expression that is associated with autonomous clock activity.
So, it was a surprise. There was so much in the field that was focused on the brain. In the 1970s, the suprachiasmatic nucleus was seen as so central for so many years, and then things broke out. There was the study in 1997 published in Science by Jeff Hall and Steve Kay that showed that there are circadian clocks all over the fly body. Shortly after, in 1998, Ueli Schibler in Geneva found that cultured rat fibroblasts had their own clocks in a dish. Daily oscillations with an average period length of 22.5 hours were recorded in these rodent cells. And then liver, pancreatic, lung, and muscular clocks were reported. Sometimes they work together, and sometimes they fight each other.
Medscape: Which parts of the work on the genetics and molecular biology of rhythms in Drosophila could be the most relevant for human medicine?
Dr Young: We are doing some work with human cultured cells, working with patients and culturing cells, to look at sleep disorders. It is very clear that a lot is going to be learned about sleep disorders and the inheritability of sleep disorders from these kinds of studies.
Medscape: Indeed, there are two well-delineated circadian sleep pathologies: advanced sleep phase syndrome and delayed sleep phase syndrome. Familial advanced sleep phase syndrome was linked to a single gene mutation on chromosome 2 in one large kindred. The altered gene is period 2, which was originally identified in studies of Drosophila circadian rhythms. In the human disorder, affected family members begin sleep about an hour after sunset and are wide awake by 4 AM. So these individuals routinely fall asleep in the late afternoon or early evening, only to wake up in the middle of the night.
Dr Young: That's correct. But the field of clock research is branching out so quickly into very different areas of human or mammalian biology. The kinds of things that I find striking are experiments in which some of these genes are ablated. We find very strong metabolic consequences of some of these.
Some genetically engineered mouse models of circadian rhythmicity are obese and show signs of a metabolic syndrome. Loss of function of the autonomous clocks in the liver can produce hypoglycemia. Some pancreatic islet cell-specific clock knockout mice have impaired glucose tolerance and reduced insulin secretion. Studies of these mutants have shown that they have a reduced insulin response to glucose, indicating that the pancreatic clock could be a direct contributor to normal insulin sensitivity and protect against diabetes mellitus.
Interestingly, in response to wounds, there is circadian regulation in the skin regenerative process. Recent research suggests that the loss of one protein partner of the circadian PER proteins resulted in defective wound repair in mice. It has also been shown that mice lacking the genes Per1, Per2, Cry1, and Cry2 show increased spontaneous and radiation-induced tumor development. So there is a very broad set of responses to damaged clocks that we are now seeing in mammals that makes it clear that circadian rhythmicity is much more than just behavior.
Medscape: Why might circadian rhythms be important for understanding cancer?
Dr Young: It is very unclear so far why changes in circadian rhythms affect the frequency of cancer. Cancer has been associated with shift work. As I said, in mice, it has been associated with a loss of clock genes. It is possible that there are protective elements of circadian rhythmicity with regard to DNA damage response mechanisms. What we are seeing in some of these cases is a DNA damage response aberration that is downstream from the loss of circadian rhythmicity. I think we will understand more about the pathways to cancer in mutant mice that have a higher rate of tumor formation.
Looking Back and to the Future
Medscape: Could you provide a brief historical perspective on the first observations of a circadian clock?
Dr Young: Androsthenes, a scribe of Alexander the Great in the fourth century BC, was one of the first to record observations of a circadian clock. There was also the French astronomer Jean-Jacques d'Ortous de Marain, who observed that the leaves of the heliotrope, which open in the morning and close in the evening, continued to open and close even in the absence of a daily light/dark cycle. He wanted to see how much the environment influences the activities of plants and found that when he took the plants into his cellar, the activities persisted for a few days. Plants continued to rhythmically unfold their leaves in the morning and fold them in the evening, even when they were maintained in constant darkness. In 1729, d'Ortous de Marain had discovered an innate regulatory mechanism that allowed the plants to anticipate when daylight would be expected to begin and to wane.
Medscape: Readers are always fascinated by the personal lives of Nobel Prize winners. Could you tell us a little bit about your wife Laurel Eckhardt, who is a scientist?
Dr Young: I met Laurel in Austin at the University of Texas when we were both undergraduates. She and I took a genetics course together. She was both bright and attractive. A year later, I was in graduate school and she had Burke Judd, who was my advisor, as her undergraduate advisor. The following summer, Burke introduced me to Laurel properly, and then suddenly she paid attention to me.
Later, we both went to Stanford. She was in the genetics department, and I was in biochemistry. We were on the same floor, but right around the corner from each other. She worked with Len Herzenberg in immunogenetics.
Then we decided to go to New York. I took a job at Rockefeller, and 2 years later, Laurel came on as a postdoc with an immunologist at Einstein College of Medicine. Right now, she is head of the PhD program for the biological sciences for all campuses of the City University of New York, in addition to having a lab that does research in immunogenetics. So we still have plenty of conversations about genetics and biology.
Medscape: What about your children?
Dr Young: Our daughters, Natalie and Arissa, now in their 30s, grew up in this household where there was lots of science around them all the time. I used to bring home fruit flies with different mutant characteristics. In the mornings, I would leave and I would say, "Okay, I'm going to bring you home some flies at the end of the day, and I want you to tell me what you want me to build." They would say, "Oh, I want one with red eyes," or "I want one with white eyes," or "I want a yellow one"—things like this. So I guided them a little bit on their requests. I would bring home things that we had in our stock collection.
Medscape: Can you tell us a little bit about your hobbies?
Dr Young: We spend a lot of time in New Mexico. We have a place out in the canyon lands. It is in the Rocky Mountains where they extend down into New Mexico. This is just northwest of Santa Fe.
We do a lot of hiking and fossil collecting. The geology out there is really pretty spectacular. It has been deposited in layers over huge spans of time. You can walk through these very different time zones; down at the bottom of one of our canyons, the rocks are about 250 million years old. They are Permian rocks. You find one set of fossils down there that are really very primitive, mostly plant fossils. And as you go up the walls of the canyon, you get into areas that are Triassic. Some areas are about 200-220 million years old and are full of amphibian bones. There are these terrific maps that are produced by the New Mexico Geological Survey of the ages of the rocks in that area, so you always know exactly where you are with respect to the evolutionary history of the place.
Medscape: Did you ever imagine, in the early 1980s, that you would spend the next 30 years working on circadian behavioral rhythms? How do you explain that it could occupy your interests for so long? Is there something other than pure intellectual curiosity, and how did you stay interested?
Dr Young: That's a great question. You know what happened was that originally it was a small problem. We had hoped that we would find something quickly, some hints quickly about how the period gene caused rhythmicity. And of course, it took much more work.
At the beginning of the 1990s, we started getting multiple genes that had many very interesting properties. The kinds of questions, the number of approaches, kept broadening as we entered this. We found ourselves going from genetics to molecular biology to cell biology to protein biochemistry. There was always plenty to work on, and new things seemed just as fresh and open. Big questions seemed to be there as we turned each stone.
During the time that we were working on circadian rhythms, we were working on other things as well. We worked for a while on transposable elements, on the gene Notch in early development. But the questions associated with the clock, the answers that we got, pointed toward growth instead of constriction. I think that is what kept me devoting more and more of the laboratory's efforts to the problem of circadian clocks and away from the other topics that seemed to grow at a less spectacular rate.
Medscape: With distance and hindsight, what do you consider to be your main talents and character traits as a scientist?
Dr Young: Persistence is helpful. And you know, there have been many times when we didn't see a way forward—where we just kept pushing and finally something would pop up, so we could say, "Aha, here's a clue." So persistence is one of the most important.
I have always had an interest in evolution: the diversity of organisms and the many different kinds of adaptations that organisms are able to do when you look from species to species, genera to genera, phyla to phyla. So these are all mysteries that are being uncovered, and to be a part of that is something that keeps you going.
I find that I'm interested in many areas of biology, and that has helped me to be to get a view of new results, and see that new results can be pushed from different directions.
I don't think everybody knew about circadian rhythms when I was growing up—when I was a graduate student, for example. I think this was off the radar of many, but because I'd had a childhood where I had been exposed to a lot of very general biological phenomena, I was sort of prepared to take on such a problem as this.
Medscape: What kind of advice do you give to young scientists working with you?
Dr Young: They should pursue things that they are interested in, and not be pulled in too quickly to things that seem utterly practical. I think there is a tendency now to be pushed in that direction. There are safe and practical things that scientists can do, and then there are things that are not necessarily full of risk but have inherently more potential for surprise and excitement. I think my advice would be to give yourself the opportunity to find something new and truly exciting.
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Cite this: 2017 Nobel Prize Winner Michael W. Young: An Interview - Medscape - Nov 17, 2017.