Paul Hardin (chronobiologist)
Paul Hardin (born September 14, 1960) is a prominent scientist in the field of chronobiology and a pioneering researcher in the understanding of circadian clocks in flies and mammals. Hardin currently serves as a distinguished professor in the biology department at Texas A&M University. He is best known for his discovery of circadian oscillations in the mRNA of the clock gene Period ("per"), the importance of the E-Box in "per" activation, the interlocked feedback loops that control rhythms in activator gene transcription, and the circadian regulation of olfaction in "Drosophila melanogaster". Born in a suburb of  Chicago, Matteson, Illinois, Hardin currently resides in College Station, Texas, with his wife and three children. Academic career. Hardin earned his B.S. in biology at Southern Methodist University (SMU) in 1982. He then continued to pursue a Ph.D in genetics from Indiana University in 1987 with William H. Klein. He went on to conduct his postdoctoral research at Brandeis University under the supervision of chronobiologist Michael Rosbash. From 1991 to 1995, Hardin worked as a professor at Texas A&M University, and from 1995 to 2005 at the University of Houston. Since 2005, Hardin has worked as a professor and  researcher in the biology department at Texas A&M University. He teaches courses on introductory biology, molecular cell biology, and a graduate level class on biological clocks. He also serves as the director of the Texas A&M's Center for Biological Clocks Research and as faculty for the Texas A&M Institute for Neuroscience and PhD program in genetics. In addition, Hardin was also actively involved in the Society for Research on Biological Rhythms; he served as the secretary in 2006, treasurer in 2010, and president in 2016. Research. Discovery of "per" mRNA cycling. In 1971, Ron Konopka, a geneticist at the  California Institute of Technology, discovered the Period gene, which he found to be involved in the circadian clock of "Drosophila". In 1999, Paul Hardin discovered that "per" mRNA underwent strong circadian oscillations by exposing isolated wild-type "per" mRNA to a series of light-dark (LD) cycles followed by cycles of constant darkness (DD). As a post-doctorate in the lab of chronobiologist Dr. Michael Rosbash, Hardin specifically noted that "per" mRNA levels in "Drosophila" brains fluctuate about 10-fold in a typical 24-hour light-dark cycle. Hardin further demonstrated that wild-type protein, PER, can rescue rhythmicity in  the mRNA of an arrhythmic mutant of the "per" gene. His findings suggested that feedback of the PER protein regulates levels of "per" mRNA. Hardin ultimately published his seminal work on the rhythmic nature of "per" mRNA in "Drosophila" in the journal "Nature". This discovery led Hardin and other prominent members in the field of chronobiology to develop a model that describes the clock mechanism in "Drosophila". This model is referred to as the Transcription Feedback Loop, which suggests that the translated protein provides negative feedback on the mRNA transcription of itself. Role of the E-box in "per" activation . In 1997, Hardin, with Haiping Hao and David Allen, analyzed the sequence of the "per" gene in "Drosophila" and found a 69-bp enhancer upstream of the gene. This enhancer sequence contained an E-box (CACGTG), which was determined to be necessary for high-level "per" transcription. As E-boxes are typically bound by proteins containing a basic helix-loop-helix (bHLH) protein structural motif, the presence of an E-box in "per" led to the hypothesis that the proteins involved in circadian rhythms may contain a bHLH domain. This proved to be vital in establishing the function of the  previously discovered CLOCK protein, which was known to play a role in circadian rhythms and contained a bHLH domain as well. This discovery also aided in the identification of the BMAL1 and CYCLE proteins as critical players in the circadian rhythms of mammalian and "Drosophila" circadian systems respectively. Circadian rhythms in olfaction. While teaching at the University of Houston, Hardin, along with fellow scientists Balaji Krishnan and Stuart Dryer, investigated circadian rhythms of olfaction in "Drosophila". Previous experiments had shown that "Drosophila" antennae demonstrate circadian rhythms. However, the mechanism for circadian rhythms in the antennae was unknown. To determine the  mechanism of rhythms in antennae, Hardin and his team kept wild-type and mutant flies, "per" and "tim", in 12:12 light-dark (LD) cycles and measured olfaction in the antennae with an electroantennogram (EAG), that measures the average output of an insect antenna to its brain for a given odor, over a 24-hour period. Only the wild-type flies demonstrated rhythmicity in the electrical activity, which indicated that circadian rhythms were present in the olfactory response. In contrast, the mutants showed no cyclic activity. Therefore, Hardin's team discovered that circadian rhythms control the olfactory response in "Drosophila" antennae  and his results were eventually published in "Nature". Discovery of two interlocked feedback loops in circadian clock. In 1999, Hardin along with Nick Glossop and Lisa Lyons, conducted research on the specific role of "Clk" in the interlocked feedback loops present in "Drosophila" circadian oscillators. It was previously known that five genes ("per", "tim", "dbt", "Clk", and "cyc") controlled circadian rhythms in "Drosophila". The "per"-"tim" regulation mechanism was known at this time, though "Clk" regulation was not yet known. Hardin and his team conducted a series of experiments to identify the two interlocked feedback loops in the circadian mechanism of  Drosophila. This means that the "per"-"tim" feedback loop connects to the "Clk"-"cyc" feedback loop, so that one loop has an effect on the other, and vice versa. They measured wild-type and mutant "Clk" mRNA levels to identify any changes in transcription levels. They observed that the PER-TIM complex suppresses transcription. They hypothesized that the "Clk" repressor was either the CLK-CYC complex itself or a repressor that was activated by CLK-CYC. They observed that the presence of active CLK and CYC resulted in the repression of "Clk", while arrhythmic "per" mutants exhibited low levels of "Clk". This  evidence led them to propose the following model regarding two interlocked feedback loops: In 2003, Hardin's team uncovered the second feedback loop associated with the circadian clock. "vrille (vri)" and "Par Domain Protein 1" ("Pdp1") encode related transcription factors whose expression is directly activated by dCLOCK/CYCLE. They show that VRI and PDP1 proteins feed back and directly regulate "dClock" expression. Thus, VRI and PDP1, together with "dClock" itself, comprise a second feedback loop in the "Drosophila" clock that gives rhythmic expression of "dClock", and probably of other genes, to generate accurate circadian rhythms. Current research. Hardin's current  research centers on the function of the circadian clock in "Drosophila melanogaster". One of Hardin's main research topics is understanding the mechanism behind the circadian rhythms in olfaction and gustatory physiology. His research also focuses on understanding the role of post-translational regulatory mechanisms in the feedback loop that set a 24-hour rhythm. Lastly, his lab has been working on identifying if the interlocked loops in the feedback mechanism function as a circadian oscillator or a clock output. His most recent article discusses the conservation of the transcription feedback loop in not only "Drosophila," but also in other  animal species as well.