Circadian Clocks: Running on Redox

Abstract

. Further experiments demonstrateacquainted, my morning’s self would not recognize my that, like in the feedback loop where CLK and BMAL1evening’s.” Gide’s words describe a phenomenon that are the positive activators, CRY negatively feeds backis built upon literally thousands of physiological and onNPAS2:BMAL1-dependenttranscription.Thismolec-biochemical parameters which systematically change ular loop is evidence of the circadian clock in theseover the course of a day. The changes, which even neurons, located in the forebrain, distinct from the mas-ter pacemaker in the SCN. Given the many clock-con-continue with a period of about a day (circadian) introlled neurobehavioral parameters, this molecular de-constant conditions, are controlled by a biological clockscription may help to understand how the circadianwhich evolved as an adaptation to a cycling, 24 hr envi-clock modulates finger-tapping speed, grip strength,ronment. In mammals, the pacemaker of circadiantime estimation, or even mood in humans (Aschoff andrhythms resides in the hypothalamic suprachiasmaticWever, 1981), behaviors processed in this region of thenucleus (SCN), and therein, by the coupled oscillationbrain. The physiology of mouse rhythms needs to catchgenerated by its individual cells. It is generally acceptedup to the advanced mouse genetics, however, as therethat most cells (not just SCN neurons) have a circadianis relatively little known in mice at this time regardingmachinery; thus, there are numerous peripheral oscilla-circadian physiology, aside from motor activity. We dotors (liver, muscle, lung, and even other parts of theknow that NPAS2 knockout mice are deficient in somebrain). The differences people experience between theirmemory tasks (Garcia et al., 2000), and learning andmorning and evening self is based on a circadian physi-memory are modulated by the circadian system (Wever,ology that is a hierarchical amalgam of neuronal pace-1979).makers and peripheral oscillators.In a companion paper, Rutter et al. (2001) focused onTheissueathandistheintracellularmolecularmecha-one of the genes regulated by the NPAS2 transcriptionnism of circadian rhythmicity. In all molecular geneticfactor complex: a robustly expressed lactate dehydro-systems that have been studied to date, a transcription/genase. Note the logic here: lactate dehydrogenase hastranslation feedback loop is integral to circadian rhyth- the potential to generate either NAD or NADH in themicity. The composition of the loop is almost formulaic reversible conversion between pyruvate and lactate, re-from the bottom to the top of the phylogenetic tree: at dox-regulatedtranscriptionfactorsregulateanumberofits core is transcription, translation and then negative signaling or developmental pathways, and biochemicalfeedback on the initial transcriptive process (Figure 1, feedbacks are common throughout nature. Hence, theas originally proposed in Hardin et al., 1990). Without it, impact of redox state on transcription factor function ofmost circadian properties are lost.Over the last decade, the NPAS2:BMAL1 complex was investigated for feed-manyparticipantsinthefeedbackloophavebeenidenti- back from its hypothetical output (modulated redoxfied. The current cartoon version looks like a figure eight state). In experiments that are a tutorial for molecularskated by a two-year-old on fresh ice, and includes biochemistry, it was determined that the presence ofpositive activators of transcription, negative effectors, NADH or NADPH promotes binding of the heterodimericmodulators (e.g., a kinase), and their interdependent transcription factor complex to DNA, while the oxidizedregulation with appropriate time delays to generate a forms NAD or NADP fail to promote, or even inhibitcirca 24 hr cycle (a simplified version is seen in Figure binding. The concentration of cofactor that is effective2; Shearman et al., 2000; Zheng et al., 2001). is within the physiological range. The suggestion isA recent theme in this area of research is plasticity that DNA binding activity of both NPAS2:BMAL1 andin the various molecular clock components. In mice, CLK:BMAL1, the components utilized by the cellular