Steven M. Reppert

Last updated
Steven M. Reppert
Born (1946-09-04) September 4, 1946 (age 77)
Sioux City, Iowa
CitizenshipUnited States
Alma mater
Known for
  • Fetal circadian clock
  • Melatonin receptors
  • Circadian clock mechanism in mammals
  • Monarch butterfly sun compass
Scientific career
Fields
Institutions
  • Chair of the Department of Neurobiology at UMass Medical School (2001–2013)
  • Higgins Family Professor of Neuroscience UMass Medical School (2001–2017)
  • Distinguished Professor of Neurobiology at UMass Medical School (2014–2017)
  • Distinguished Professor Emeritus of Neurobiology at UMass Medical School (2017–)

Steven M. Reppert (born September 4, 1946) is an American neuroscientist known for his contributions to the fields of chronobiology and neuroethology. His research has focused primarily on the physiological, cellular, and molecular basis of circadian rhythms in mammals and more recently on the navigational mechanisms of migratory monarch butterflies. He was the Higgins Family Professor of Neuroscience at the University of Massachusetts Medical School from 2001 to 2017, and from 2001 to 2013 was the founding chair of the Department of Neurobiology. Reppert stepped down as chair in 2014. He is currently distinguished professor emeritus of neurobiology.

Contents

Biography

Early life

Steven Reppert grew up in the village of Pender, Nebraska, and graduated from Pender Public High School in 1964. His interest in science began in childhood with the cecropia moth—an insect made famous by Harvard biologist Carroll M. Williams, who used the moth in his pioneering work on the role of juvenile hormone in molting and metamorphosis. [1] Reppert continues to rear cecropia from egg to adult each summer.

Education and career

Reppert received his BS and MD in 1973 (with distinction) from the University of Nebraska College of Medicine and was elected as a medical student to the Alpha Omega Alpha Honor Medical Society. From 1973 to 1976 he did an internship and residency in pediatrics at the Massachusetts General Hospital. From 1976 to 1979 Reppert was a postdoctoral fellow in neuroendocrinology at the National Institute of Child Health and Human Development in Bethesda, Maryland, in David C. Klein's laboratory, which focuses on the pineal gland and circadian biology. [2] Reppert was on the faculty at the Massachusetts General Hospital and Harvard Medical School beginning in 1979 and was promoted to professor in 1993; he directed the Laboratory of Developmental Chronobiology at the Massachusetts General Hospital from 1983 to 2001, when he moved to the University of Massachusetts Medical School. [3]

Research

Reppert has published more than 180 papers. He is the principal inventor on seven patents derived from his research. [4]

Fetal circadian clocks

Rodent studies have shown that the master brain clock in the suprachiasmatic nucleus (SCN) is functional in the fetus before the fetal brain is capable of registering the presence of light. Reppert and colleagues reported that the fetal SCN is entrained to the light-dark cycle before the retinohypothalamic pathway innervates the SCN from the eye. [5] This finding indicates that the mother, and her entrainment to ambient light-dark cycles, provides the necessary information to the fetus for synchronization. As Reppert states, “Mom is functioning as the transducer for the fetal circadian system. She takes in light information to her circadian system, and then that is communicated to the fetal circadian system.” [6] This fetal entrainment persists into the postnatal period and ensures that neonatal behavioral patterns are properly tuned with the environment. Dopamine and melatonin can both act as perinatal maternal entraining signals. [7]

Mammalian circadian clocks

Steven Reppert and colleagues have made seminal contributions that provide insight into the mammalian circadian clock mechanism.

Cell autonomy in the SCN

Reppert and colleagues discovered that the SCN contains a large population of autonomous, single-cell circadian oscillators. [8] They cultured cells from neonatal rat SCN on fixed microelectrode array that allowed them to monitor individual SCN neuron activity in culture. Circadian rhythms expressed by neurons in the same culture were not synchronized, indicating that they functioned independently of one another.

Functions of mouse clock genes: PERIOD2 and PERIOD3

Reppert and coworkers also discovered the mouse clock genes mPer2 and mPer3 and defined their functions. They found that the mPER2 and mPER3 proteins, as well as the previously discovered mPER1, share several regions of homology with one another and with Drosophila PER. [9] [10] Reppert and coworkers found different light responses among the three Per genes. [10] Unlike mPer1 and mPer2 mRNA levels, mPer3 mRNA levels are not acutely altered by light exposure during the subjective night. They also found that mPer1–3 are widely expressed in tissues outside the brain, including the liver, skeletal muscles, and testis. To determine the function of mPER1–3, Reppert and colleagues disrupted the three genes encoding them. [11] Using double-mutant mice, they showed that mPER3 functions outside the core circadian clockwork, whereas both mPER1 and mPER2 are necessary for rhythmicity.

Negative transcriptional feedback loop

Reppert and colleagues discovered that the two mouse cryptochromes, mCRY1 and mCRY2, function as the primary transcriptional repressors of clock gene expression, and the mPER proteins are necessary for CRY nuclear translocation. [12] This work provided the first portrayal of a negative transcriptional feedback loop as the major gear driving the mouse molecular clock. [13]

Interlocking transcriptional feedback loops

Reppert and colleagues found that the core mechanisms for the SCN in mammals consist of interacting positive and negative transcriptional feedback loops. [14] The first loop is an autoregulatory negative transcriptional feedback loop in which the mCRY proteins negatively regulate mCry and mPer gene transcription. The second interlocking feedback loop involves the rhythmic regulation of Bmal1. Rhythmicity of Bmal1 is not necessary for clockwork function, but it helps modulate the robustness of rhythmicity.

CLOCK and NPAS2

Reppert and colleagues discovered that the transcription factors CLOCK and NPAS2 have overlapping roles in the SCN, revealing a new and unexpected role for NPAS2. [15] His lab observed that CLOCK-deficient mice continue to have behavioral and molecular rhythms, which showed that CLOCK is not essential for circadian rhythm in locomotor activity in mice. They then determined, by investigating CLOCK-deficient mice, that NPAS2 is a paralog of CLOCK and can functionally substitute CLOCK by dimerizing with BMAL1. Finally, they found—by investigating CLOCK-deficient, NPAS2-deficient, and double-mutant mice—that circadian rhythms in peripheral oscillators require CLOCK. [15] Thus, there is a fundamental difference between CLOCK and NPAS2 that is tissue dependent.

Mammalian melatonin receptors

In 1994, Reppert cloned human and sheep Mel1a melatonin receptor, the first in a family of GPCRs that bind the pineal hormone melatonin, and localized its expression in the mammalian brain to the SCN and the hypophyseal pars tuberalis. [16] Mel1a is believed to be responsible for the circadian effects of melatonin and the reproductive actions in seasonal breeding mammals. [16]

In 1995, Reppert cloned and characterized the Mel1b melatonin receptor. He and colleagues found that the receptor was predominantly expressed in the retina, where it is believed to modify light-dependent retinal functions. [17] They identified outbred populations of Siberian hamsters that lacked functional Mel1b but maintained circadian and reproductive responses to melatonin; [18] these data indicate that Mel1b is not necessary for the circadian and reproductive actions of melatonin, which instead depend on Mel1a.

Elucidation of the molecular nature of the melatonin receptors has facilitated definition of their ligand-binding characteristics and aided the development of melatonin analogs that are now used to treat sleep disorders and depression. [16]

Insect cryptochromes

In 2003, Reppert began investigating the functional and evolutionary properties of the CRY protein in the monarch butterfly. He identified two Cry genes in the monarch, Cry1 and Cry2. [19] His work demonstrated that the monarch CRY1 protein is functionally analogous to Drosophila CRY, the blue-light photoreceptor necessary for photoentrainment in the fly. He also demonstrated that monarch CRY2 is functionally analogous to vertebrate CRYs and that monarch CRY2 acts as a potent transcriptional repressor in the circadian clock transcriptional translation feedback loop of the butterfly, as his group previously showed for the two mouse CRYs. [12] These data propose the existence of a novel circadian clock unique to some non-drosophilid insects that possesses mechanisms characteristic of both the Drosophila and the mammalian clocks. [20] Other insects, such as bees and ants, possess only a vertebrate-like CRY, and their circadian clocks are even more vertebrate like. [21] Drosophila is the only known insect that does not possess a vertebrate-like CRY.

In 2008, Reppert and colleagues discovered the necessity of CRY for light-dependent magnetoreception responses in Drosophila. They also showed that magnetoreception requires UVA/blue light, the spectrum corresponding with the action spectrum of Drosophila CRY. [22] These data were the first to genetically implicate CRY as a component of the input pathway or the chemical-based pathway of magnetoreception. Applying these findings to his work with the monarch, Reppert’s group showed that both monarch CRY1 and CRY2 proteins, when expressed as a transgene in CRY-deficient flies, successfully restore light-dependent magnetosensitivity function. These results propose the presence of a CRY-mediated magnetosensitivity system in monarchs that may act in concordance with the sun compass to aid navigation. In 2011, Reppert's lab also discovered that human CRY2 can substitute as a functional magnetoreceptor in CRY-deficient flies, a discovery that warrants additional research into magnetosensitivity in humans. [23] [24] However, interpretation of the above CRY-dependent magnet work needs to be viewed in the context of a paper by Bassetto et al. 2023 that suggests that there is no evidence for magnetic field effects on behaviour in Drosophila. [25] Moreover, the authors could not reproduce magnetosensitivity in Drosophila using the binary T-maze apparatus developed in Reppert’s lab. [22] Reppert defends the work from his lab showing fruit fly magnetosensitivity and questions the conclusions reported in Bassetto et al., 2023. [26] Further work is needed to refute or verify the proposed lack of magnetoreception in Drosophila.

Monarch butterfly migration

Since 2002, Reppert and coworkers have pioneered the study of the biological basis of monarch butterfly migration. [27] [28] Each fall, millions of monarchs from the eastern United States and southeastern Canada migrate as much as 4,000 km to overwinter in roosts in Central Mexico. [29] Monarch migration is not a learned activity, given that migrants flying south are at least two generations removed from the previous year's migrants. [30] Thus, migrating monarchs must have some genetically based navigational mechanism.

Reppert and colleagues have focused on a novel circadian clock mechanism and its role in time-compensated sun compass orientation, a major navigational strategy that butterflies use during their fall migration. [29] Using clock-shift experiments, they showed that the circadian clock must interact with the sun compass to enable migrants to maintain a southerly flight direction as the sun moves daily across the sky. [31] Reppert collaborated with Eli Shlizerman at the University of Washington and Daniel Forger at the University of Michigan to propose a working mathematical model of the time-compensated sun compass. [32]

Clockwork mechanism

The monarch clockwork model, which has both Drosophila-like and mammal-like aspects, is unique because it employs two distinct CRY proteins. As presented in a review article, [28] the clock mechanism, on a gene/protein level, operates as follows:

  • In an autoregulatory transcriptional feedback loop, heterodimers of CLOCK (CLK) and CYCLE (CYC) form and drive the transcription of the Per, Tim, and Cry2 genes.
  • TIM, PER, and CRY2 proteins are translated and form complexes in the cytoplasm.
  • 24 hours later, CRY2 returns to the nucleus and inhibits CLK:CYC transcription.
  • Meanwhile, PER is progressively phosphorylated, which may aid CRY2 translocation into the nucleus.
  • CRY1 protein is a circadian photoreceptor that, when exposed to light, causes TIM degradation, allowing light to gain access to the central clock mechanism for photic entrainment.

Antennal clocks

Reppert’s lab expanded upon Fred Urquhart's postulation that antennae play a role in monarch migration. In 2009 Reppert and coworkers Christine Merlin and Robert Gegear reported that, despite previous assumptions that the time-compensation clocks are located exclusively in the brain, there are also clocks located in the antennae, which "are necessary for proper time-compensated sun compass orientation in migratory monarch butterflies.” [33] They concluded this by comparing the sun compass orientation of monarch migrants with intact antennae and those whose antennae had been removed. [33] Reppert's lab also studied antennae in vitro and found that antennal clocks can be directly entrained by light and can function independently from the brain. [33] Further research is needed, however, on the interaction between the circadian clocks in monarch butterfly's antennae and the sun compass in the brain.

In 2012, Reppert and colleagues determined that only a single antenna is sufficient for sun compass orientation. They did so by painting one antenna black to cause discordant light exposure between the two antennae; the single not-painted antenna was sufficient for orientation. All four clock genes (per, tim, cry1, and cry2) were expressed in the various studied areas of the antenna, suggesting that "light entrained circadian clocks are distributed throughout the length of the monarch butterfly antenna." [34]

In 2013, Reppert and Patrick Guerra showed that spring remigrants also use an antenna-dependent time-compensated sun compass to direct their northward flight from Mexico to the southern United States. [35]

Sun compass

Using anatomical and electrophysiological studies of the monarch butterfly brain, Stanley Heinze working in Reppert’s lab provided evidence that the central complex, a midline structure in the central brain, is likely the site of the sun compass. [36]

Magnetic compass

Reppert and colleagues Patrick Guerra and Robert Gegear showed that migratory monarchs can use a light-dependent, inclination-based, magnetic compass for navigation on overcast days. [37] Genetic studies from Christine Merlin’s laboratory show that the photoreceptive CRY1 protein is essential for the monarch’s light-sensitive magnetic compass. [38] The successful use of reverse genetics in monarchs would indicate that the butterfly is an excellent choice for helping to delineate the molecular mechanism underlying light-dependent magnetosensing in the context of compass navigation.

Temperature

Reppert and Patrick Guerra showed that fall migrants prematurely exposed to overwintering-like coldness reverse their flight orientation to the north. The temperature microenvironment at the overwintering site is essential for successful completion of the migration cycle: without cold exposure, aged migrants continue to orient to the south. The discovery that coldness triggers the northward flight direction in spring remigrants underscores how vulnerable the migration may be to climate change. [39] [40]

Monarch butterfly genome

In 2011, Reppert and colleagues presented the draft sequence of the monarch butterfly genome and a set 16,866 protein-coding genes. This is the first characterized genome of a butterfly and of a long-distance migratory species. [41] [42] [43]

In 2012, Reppert and colleagues established MonarchBase, an integrated database for the genome of Danaus plexippus. The goal of the project was to make genomic and proteomic information about monarch butterflies accessible to biological and lepidopteran communities. [44]

In 2013, Christine Merlin and Scot Wolfe developed in Reppert’s lab a novel gene-targeting approach in monarchs that uses a zinc finger nuclease strategy to define the essential nature of CRY2 for clockwork function in lepidopterans. [45] Targeted mutagenesis of Cry2 indeed resulted in the in vivo disruption of circadian behavior and the molecular clock mechanism. Further work in Merlin’s lab has shown that nuclease strategies are powerful tools for targeting additional clock genes in monarchs and altering gene function. [46]

In 2016, Reppert collaborated with Marcus Kronforst at the University of Chicago and others to use population genetic studies to define the evolutionary history of the monarch migration. [47]

Awards and honors

Related Research Articles

<span class="mw-page-title-main">Circadian rhythm</span> Natural internal process that regulates the sleep-wake cycle

A circadian rhythm, or circadian cycle, is a natural oscillation that repeats roughly every 24 hours. Circadian rhythms can refer to any process that originates within an organism and responds to the environment. Circadian rhythms are regulated by a circadian clock whose primary function is to rhythmically co-ordinate biological processes so they occur at the correct time to maximize the fitness of an individual. Circadian rhythms have been widely observed in animals, plants, fungi and cyanobacteria and there is evidence that they evolved independently in each of these kingdoms of life.

<span class="mw-page-title-main">Suprachiasmatic nucleus</span> Part of the brains hypothalamus

The suprachiasmatic nucleus or nuclei (SCN) is a small region of the brain in the hypothalamus, situated directly above the optic chiasm. It is the principal circadian pacemaker in mammals, responsible for generating circadian rhythms. Reception of light inputs from photosensitive retinal ganglion cells allow it to coordinate the subordinate cellular clocks of the body and entrain to the environment. The neuronal and hormonal activities it generates regulate many different body functions in an approximately 24-hour cycle.

<span class="mw-page-title-main">Cryptochrome</span> Class of photoreceptors in plants and animals

Cryptochromes are a class of flavoproteins found in plants and animals that are sensitive to blue light. They are involved in the circadian rhythms and the sensing of magnetic fields in a number of species. The name cryptochrome was proposed as a portmanteau combining the chromatic nature of the photoreceptor, and the cryptogamic organisms on which many blue-light studies were carried out.

Period (per) is a gene located on the X chromosome of Drosophila melanogaster. Oscillations in levels of both per transcript and its corresponding protein PER have a period of approximately 24 hours and together play a central role in the molecular mechanism of the Drosophila biological clock driving circadian rhythms in eclosion and locomotor activity. Mutations in the per gene can shorten (perS), lengthen (perL), and even abolish (per0) the period of the circadian rhythm.

<span class="mw-page-title-main">PER2</span> Protein-coding gene in the species Homo sapiens

PER2 is a protein in mammals encoded by the PER2 gene. PER2 is noted for its major role in circadian rhythms.

<span class="mw-page-title-main">FBXL3</span> Protein-coding gene in the species Homo sapiens

FBXL3 is a gene in humans and mice that encodes the F-box/LRR-repeat protein 3 (FBXL3). FBXL3 is a member of the F-box protein family, which constitutes one of the four subunits in the SCF ubiquitin ligase complex.

<span class="mw-page-title-main">Period circadian protein homolog 1</span> Protein-coding gene in the species Homo sapiens

Period circadian protein homolog 1 is a protein in humans that is encoded by the PER1 gene.

<span class="mw-page-title-main">PAS domain</span> Protein domain

A Per-Arnt-Sim (PAS) domain is a protein domain found in all kingdoms of life. Generally, the PAS domain acts as a molecular sensor, whereby small molecules and other proteins associate via binding of the PAS domain. Due to this sensing capability, the PAS domain has been shown as the key structural motif involved in protein-protein interactions of the circadian clock, and it is also a common motif found in signaling proteins, where it functions as a signaling sensor.

In molecular biology, an oscillating gene is a gene that is expressed in a rhythmic pattern or in periodic cycles. Oscillating genes are usually circadian and can be identified by periodic changes in the state of an organism. Circadian rhythms, controlled by oscillating genes, have a period of approximately 24 hours. For example, plant leaves opening and closing at different times of the day or the sleep-wake schedule of animals can all include circadian rhythms. Other periods are also possible, such as 29.5 days resulting from circalunar rhythms or 12.4 hours resulting from circatidal rhythms. Oscillating genes include both core clock component genes and output genes. A core clock component gene is a gene necessary for to the pacemaker. However, an output oscillating gene, such as the AVP gene, is rhythmic but not necessary to the pacemaker.

<span class="mw-page-title-main">Monarch butterfly migration</span> Migrations, mainly across North America

Monarch butterfly migration is the phenomenon, mainly across North America, where the subspecies Danaus plexippus plexippus migrates each autumn to overwintering sites on the West Coast of California or mountainous sites in Central Mexico. Other populations from around the world perform minor migrations or none at all. This massive movement of butterflies has been recognized as "one of the most spectacular natural phenomena in the world".

Hitoshi Okamura is a Japanese scientist who specializes in chronobiology. He is currently a professor of Systems Biology at Kyoto University Graduate School of Pharmaceutical Sciences and the Research Director of the Japan Science Technology Institute, CREST. Okamura's research group cloned mammalian Period genes, visualized clock oscillation at the single cell level in the central clock of the SCN, and proposed a time-signal neuronal pathway to the adrenal gland. He received a Medal of Honor with Purple Ribbon in 2007 for his research and was awarded Aschoff's Ruler for his work on circadian rhythms in rodents. His lab recently revealed the effects of m6A mRNA methylation on the circadian clock, neuronal communications in jet lag, and the role of dysregulated clocks in salt-induced hypertension.

Steve A. Kay is a British-born chronobiologist who mainly works in the United States. Dr. Kay has pioneered methods to monitor daily gene expression in real time and characterized circadian gene expression in plants, flies and mammals. In 2014, Steve Kay celebrated 25 years of successful chronobiology research at the Kaylab 25 Symposium, joined by over one hundred researchers with whom he had collaborated with or mentored. Dr. Kay, a member of the National Academy of Sciences, U.S.A., briefly served as president of The Scripps Research Institute. and is currently a professor at the University of Southern California. He also served on the Life Sciences jury for the Infosys Prize in 2011.

Transcription-translation feedback loop (TTFL) is a cellular model for explaining circadian rhythms in behavior and physiology. Widely conserved across species, the TTFL is auto-regulatory, in which transcription of clock genes is regulated by their own protein products.

In the field of chronobiology, the dual circadian oscillator model refers to a model of entrainment initially proposed by Colin Pittendrigh and Serge Daan. The dual oscillator model suggests the presence of two coupled circadian oscillators: E (evening) and M (morning). The E oscillator is responsible for entraining the organism’s evening activity to dusk cues when the daylight fades, while the M oscillator is responsible for entraining the organism’s morning activity to dawn cues, when daylight increases. The E and M oscillators operate in an antiphase relationship. As the timing of the sun's position fluctuates over the course of the year, the oscillators' periods adjust accordingly. Other oscillators, including seasonal oscillators, have been found to work in conjunction with circadian oscillators in order to time different behaviors in organisms such as fruit flies.

<span class="mw-page-title-main">Sato Honma</span>

Sato Honma is a Japanese chronobiologist who researches the biological mechanisms of circadian rhythms. She mainly collaborates with Ken-Ichi Honma on publications, and both of their primary research focuses are the human circadian clock under temporal isolation and the mammalian suprachiasmatic nucleus (SCN), its components, and associates. Honma is a retired professor at the Hokkaido University School of Medicine in Sapporo, Japan. She received her Ph.D. in physiology from Hokkaido University. She taught physiology at the School of Medicine and then at the Research and Education Center for Brain Science at Hokkaido University. She is currently the director at the Center for Sleep and Circadian Rhythm Disorders at Sapporo Hanazono Hospital and works as a somnologist.

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Martha Ulbrick Gillette is a chronobiologist and neurobiologist with research focusing on the effects of circadian clocks on integrative brain functions metabolism and the molecular mechanisms involved in signaling pathways. She is a fellow of the American Association for the Advancement of Science.

Ken-Ichi Honma is a Japanese chronobiologist who researches the biological mechanisms underlying circadian rhythms. After graduating from Hokkaido University School of Medicine, he practiced clinical psychiatry before beginning his research. His recent research efforts are centered around photic and non-photic entrainment, the structure of circadian clocks, and the ontogeny of circadian clocks. He often collaborates with his wife, Sato Honma, on work involving the mammalian suprachiasmatic nucleus (SCN).

<span class="mw-page-title-main">Christine Merlin</span>

Christine Merlin is a French chronobiologist and an associate professor of biology at Texas A&M University. Merlin's research focuses on the underlying genetics of the monarch butterfly circadian clock and explores how circadian rhythms modulate monarch behavior and navigation.

Raleighplots, or Rayleigh plots, are statistical graphics that serve as graphical representations for a Raleigh test that map a mean vector to a circular plot. Raleigh plots have many applications in the field of chronobiology, such as in studying butterfly migration patterns or protein and gene expression, and in other fields such as geology, cognitive psychology, and physics.

References

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