Long-term monitoring of circadian rhythms in c-fos gene expression from suprachiasmatic nucleus cultures

Reflections on Several Landmark Advances in Circadian Biology

Circadian Biology intersects with diverse scientific domains, intricately woven into the fabric of organismal physiology and behavior. The rhythmic orchestration of life by the circadian clock serves as a focal point for researchers across disciplines. This retrospective examination delves into several of the scientific milestones that have fundamentally shaped our contemporary understanding of circadian rhythms. From deciphering the complexities of clock genes at a cellular level to exploring the nuances of coupled oscillators in whole organism responses to stimuli. The field has undergone significant evolution lately guided by genetics approaches. Our exploration here considers key moments in the circadian-research landscape, elucidating the trajectory of this discipline with a keen eye on scientific advancements and paradigm shifts.

Using bioluminescence to image gene expression and spontaneous behavior in freely moving mice

Bioluminescence imaging (BLI) of gene expression in live animals is a powerful method for monitoring development, tumor growth, infections, healing, and other progressive, long-term biological processes. BLI remains an effective approach for reducing the number of animals needed to monitor dynamic changes in gene activity because images can be captured repeatedly from the same animals. When examining these ongoing changes, it is sometimes necessary to remove rhythmic effects on the bioluminescence signal caused by the circadian clock's daily modulation of gene expression. Furthermore, BLI using freely moving animals remains limited because the standard procedures can alter normal behaviors. Another obstacle with conventional BLI of animals is that luciferin, the firefly luciferase substrate, is usually injected into mice that are then imaged while anesthetized. Unfortunately, the luciferase signal declines rapidly during imaging as luciferin is cleared from the body. Alternatively, mice are imaged after they are surgically implanted with a pump or connected to a tether to deliver luciferin, but stressors such as this surgery and anesthesia can alter physiology, behavior, and the actual gene expression being imaged. Consequently, we developed a strategy that minimizes animal exposure to stressors before and during sustained BLI of freely moving unanesthetized mice. This technique was effective when monitoring expression of the Per1 gene that serves in the circadian clock timing mechanism and was previously shown to produce circadian bioluminescence rhythms in live mice. We used hairless albino mice expressing luciferase that were allowed to drink luciferin and engage in normal behaviors during imaging with cooled electron-multiplying-CCD cameras. Computer-aided image selection was developed to measure signal intensity of individual mice each time they were in the same posture, thereby providing comparable measurements over long intervals. This imaging procedure, performed primarily during the animal's night, is compatible with entrainment of the mouse circadian timing system to the light cycle while allowing sampling at multi-day intervals to monitor long-term changes. When the circadian expression of a gene is known, this approach provides an effective alternative to imaging immobile anesthetized animals and can removing noise caused by circadian oscillations and body movements that can degrade data collected during long-term imaging studies.

Generation of c-Fos knockout rats, and observation of their phenotype

c-Fos is a useful marker gene of neuron activation for neuroscience and physiology research. The mechanism and function of neural networks have been elucidated using c-Fos reporter knock-in (KI) mice, but the small size of the mice makes it difficult to perform surgical procedures on specific brain regions. On the other hand, there is a large amount of accumulated data on behavioral studies using rats. Thus, the generation of c-Fos reporter rat is expected, but it is difficult to generate gene-modified rats. Furthermore, c-Fos gene abnormality is expected to be severe in rats, as shown in homozygous of c-Fos knockout (KO) mouse, but such analysis has rarely been performed and is not certain. This study generated c-Fos-deficient rats using CRISPR/Cas, with 1067 bp deletion including exon 1 of the c-Fos gene. Homozygous c-Fos KO rats had growth latency and the same tooth and bone abnormality as homozygous c-Fos KO mice but not heterozygous c-Fos KO rats. Therefore, the c-Fos gene in rats is expected to have the same function as that in mice, and the generation of c-Fos reporter KI rats is further anticipated.

Application of Single-Cell Bioluminescent Imaging to Monitor Circadian Rhythms of Individual Plant Cells

The bioluminescent reporter system is a powerful tool for the long-term monitoring of gene expression because of its noninvasive nature. Furthermore, in combination with high-sensitive imaging technology, spatiotemporal analysis on regulation and heterogeneity in gene expression is possible. We developed a single-cell bioluminescent imaging system for plants through a transient gene transfection by particle bombardment. By applying this system to a duckweed species, we succeeded in monitoring circadian rhythms of individual cells in an intact plant for over a week. Here we describe methods for gene transfection by particle bombardment and single-cell bioluminescence monitoring by a high-sensitive camera. This technique provides a platform for characterizing gene expression patterns of individual cells in the same tissue.

Light Has Diverse Spatiotemporal Molecular Changes in the Mouse Suprachiasmatic Nucleus

To be physiologically relevant, the period of the central circadian pacemaker, located in the suprachiasmatic nucleus (SCN), has to match the solar day in a process known as circadian photoentrainment. However, little is known about the spatiotemporal molecular changes that occur in the SCN in response to light. In this study, we sought to systematically characterize the circadian and light effects on activity-dependent markers of transcriptional (cFos), translational (pS6), and epigenetic (pH3) activities in the mouse SCN. To investigate circadian versus light influences on these molecular responses, we harvested brains from adult wild-type mice in darkness at different circadian times (CT) or from mice exposed to a 15-min light pulse at the middle of the subjective day (CT6, no phase shifts), early subjective night (CT14, large phase delays), or late subjective night (CT22, small phase advances). We found that cFos and pS6 exhibited rhythmic circadian expression in the SCN with distinct spatial rhythms, whereas pH3 expression was undetectable at all circadian phases. cFos rhythms were largely limited to the SCN shell, whereas pS6 rhythms encompassed the entire SCN. pH3, pS6, and cFos showed gating in response to light; however, we were surprised to find that the expression levels of these markers were not higher at phases when larger phase shifts are observed behaviorally (CT14 versus CT22). We then used animals lacking melanopsin (melanopsin knockout [MKO]), which show deficits in phase delays, to further investigate whether changes in these molecular markers correspond to behavioral phase shifts. Surprisingly, only pS6 showed deficits in MKOs at CT14. Therefore, our previous understanding of the molecular pathways that lead to circadian photoentrainment needs to be revised.

Illuminating the Activated Brain: Emerging Activity-Dependent Tools to Capture and Control Functional Neural Circuits

Immediate-early genes (IEGs) have long been used to visualize neural activations induced by sensory and behavioral stimuli. Recent advances in imaging techniques have made it possible to use endogenous IEG signals to visualize and discriminate neural ensembles activated by multiple stimuli, and to map whole-brain-scale neural activation at single-neuron resolution. In addition, a collection of IEG-dependent molecular tools has been developed that can be used to complement the labeling of endogenous IEG genes and, especially, to manipulate activated neural ensembles in order to reveal the circuits and mechanisms underlying different behaviors. Here, we review these techniques and tools in terms of their utility in studying functional neural circuits. In addition, we provide an experimental strategy to measure the signal-to-noise ratio of IEG-dependent molecular tools, for evaluating their suitability for investigating relevant circuits and behaviors.

Circadian Clock Regulates Bone Resorption in Mice

The circadian clock controls many behavioral and physiological processes beyond daily rhythms. Circadian dysfunction increases the risk of cancer, obesity, and cardiovascular and metabolic diseases. Although clinical studies have shown that bone resorption is controlled by circadian rhythm, as indicated by diurnal variations in bone resorption, the molecular mechanism of circadian clock-dependent bone resorption remains unknown. To clarify the role of circadian rhythm in bone resorption, aryl hydrocarbon receptor nuclear translocator-like (Bmal1), a prototype circadian gene, was knocked out specifically in osteoclasts. Osteoclast-specific Bmal1-knockout mice showed a high bone mass phenotype due to reduced osteoclast differentiation. A cell-based assay revealed that BMAL1 upregulated nuclear factor of activated T cells, cytoplasmic, calcineurin-dependent 1 (Nfatc1) transcription through its binding to an E-box element located on the Nfatc1 promoter in cooperation with circadian locomotor output cycles kaput (CLOCK), a heterodimer partner of BMAL1. Moreover, steroid receptor coactivator (SRC) family members were shown to interact with and upregulate BMAL1:CLOCK transcriptional activity. Collectively, these data suggest that bone resorption is controlled by osteoclastic BMAL1 through interactions with the SRC family and binding to the Nfatc1 promoter. © 2016 American Society for Bone and Mineral Research.

Transcriptional control of circadian metabolic rhythms in the liver

Diurnal metabolic rhythms add an important temporal dimension to metabolic homeostasis in mammals. Although it remains a challenge to untangle the intricate networks of crosstalk among the body clock, nutrient signalling and tissue metabolism, there is little doubt that the rhythmic nature of nutrient and energy metabolism is a central aspect of metabolic physiology. Disruption of the synchrony between clock and metabolism has been causally linked to diverse pathophysiological states. As such, restoring the rhythmicity of body physiology and therapeutic targeting directed at specific time windows during the day may have important implications in human health and medicine. In this review, we summarize recent findings on the integration of hepatic glucose metabolism and the body clock through a regulatory network centred on the PPARγ coactivator 1 (PGC-1) transcriptional coactivators. In addition, we discuss the transcriptional mechanisms underlying circadian control of the autophagy gene programme and autophagy in the liver.

Acute effects of light on the brain and behavior of diurnal Arvicanthis niloticus and nocturnal Mus musculus

Photic cues influence daily patterns of activity via two complementary mechanisms: (1) entraining the internal circadian clock and (2) directly increasing or decreasing activity, a phenomenon referred to as "masking". The direction of this masking response is dependent on the temporal niche an organism occupies, as nocturnal animals often decrease activity when exposed to light, while the opposite response is more likely to be seen in diurnal animals. Little is known about the neural mechanisms underlying these differences. Here, we examined the masking effects of light on behavior and the activation of several brain regions by that light, in diurnal Arvicanthis niloticus (Nile grass rats) and nocturnal Mus musculus (mice). Each species displayed the expected behavioral response to a 1h pulse of light presented 2h after lights-off, with the diurnal grass rats and nocturnal mice increasing and decreasing their activity, respectively. In grass rats light induced an increase in cFOS in all retinorecipient areas examined, which included the suprachiasmatic nucleus (SCN), the ventral subparaventricular zone (vSPZ), intergeniculate leaflet (IGL), lateral habenula (LH), olivary pretectal nucleus (OPT) and the dorsal lateral geniculate (DLG). In mice, light led to an increase in cFOS in one of these regions (SCN), no change in others (vSPZ, IGL and LH) and a decrease in two (OPT and DLG). In addition, light increased cFOS expression in three arousal-related brain regions (the lateral hypothalamus, dorsal raphe, and locus coeruleus) and in one sleep-promoting region (the ventrolateral preoptic area) in grass rats. In mice, light had no effect on cFOS in these four regions. Taken together, these results highlight several brain regions whose responses to light suggest that they may play a role in masking, and that the possibility that they contribute to species-specific patterns of behavioral responses to light should be explored in future.

A new era for functional labeling of neurons: activity-dependent promoters have come of age

Genetic labeling of neurons with a specific response feature is an emerging technology for precise dissection of brain circuits that are functionally heterogeneous at the single-cell level. While immediate early gene mapping has been widely used for decades to identify brain regions which are activated by external stimuli, recent characterization of the promoter and enhancer elements responsible for neuronal activity-dependent transcription have opened new avenues for live imaging of active neurons. Indeed, these advancements provided the basis for a growing repertoire of novel experiments to address the role of active neuronal networks in cognitive behaviors. In this review, we summarize the current literature on the usage and development of activity-dependent promoters and discuss the future directions of this expanding new field.

Light-dependent and circadian transcription dynamics in vivo recorded with a destabilized luciferase reporter in Neurospora

We show that firefly luciferase is a stable protein when expressed at 25 °C in Neurospora, which limits its use as transcription reporter. We created a short-lived luciferase by fusing a PEST signal to its C-terminus (LUC-PEST) and applied the LUC-PEST reporter system to record in vivo transcription dynamics associated with the Neurospora circadian clock and its blue-light photosensory system over the course of several days. We show that the tool is suitable to faithfully monitor rapid, but also subtle changes in transcription in a medium to high throughput format.

In vitro circadian rhythms: imaging and electrophysiology

In vitro assays have localized circadian pacemakers to individual cells, revealed genetic determinants of rhythm generation, identified molecular players in cell-cell synchronization and determined physiological events regulated by circadian clocks. Although they allow strict control of experimental conditions and reduce the number of variables compared with in vivo studies, they also lack many of the conditions in which cellular circadian oscillators normally function. The present review highlights methods to study circadian timing in cultured mammalian cells and how they have shaped the hypothesis that all cells are capable of circadian rhythmicity.

Slice preparation, organotypic tissue culturing and luciferase recording of clock gene activity in the suprachiasmatic nucleus

A central circadian (~24 hr) clock coordinating daily rhythms in physiology and behavior resides in the suprachiasmatic nucleus (SCN) located in the anterior hypothalamus. The clock is directly synchronized by light via the retina and optic nerve. Circadian oscillations are generated by interacting negative feedback loops of a number of so called "clock genes" and their protein products, including the Period (Per) genes. The core clock is also dependent on membrane depolarization, calcium and cAMP ¹. The SCN shows daily oscillations in clock gene expression, metabolic activity and spontaneous electrical activity. Remarkably, this endogenous cyclic activity persists in adult tissue slices of the SCN ^2-4. In this way, the biological clock can easily be studied in vitro, allowing molecular, electrophysiological and metabolic investigations of the pacemaker function.

The SCN is a small, well-defined bilateral structure located right above the optic chiasm ⁵. In the rat it contains ~8.000 neurons in each nucleus and has dimensions of approximately 947 μm (length, rostrocaudal axis) x 424 μm (width) x 390 μm (height) ⁶. To dissect out the SCN it is necessary to cut a brain slice at the specific level of the brain where the SCN can be identified. Here, we describe the dissecting and slicing procedure of the SCN, which is similar for mouse and rat brains. Further, we show how to culture the dissected tissue organotypically on a membrane ⁷, a technique developed for SCN tissue culture by Yamazaki et al.⁸. Finally, we demonstrate how transgenic tissue can be used for measuring expression of clock genes/proteins using dynamic luciferase reporter technology, a method that originally was used for circadian measurements by Geusz et al.⁹. We here use SCN tissues from the transgenic knock-in PERIOD2::LUCIFERASE mice produced by Yoo et al.¹⁰. The mice contain a fusion protein of PERIOD (PER) 2 and the firefly enzyme LUCIFERASE. When PER2 is translated in the presence of the substrate for luciferase, i.e. luciferin, the PER2 expression can be monitored as bioluminescence when luciferase catalyzes the oxidation of luciferin. The number of emitted photons positively correlates to the amount of produced PER2 protein, and the bioluminescence rhythms match the PER2 protein rhythm in vivo¹⁰. In this way the cyclic variation in PER2 expression can be continuously monitored real time during many days. The protocol we follow for tissue culturing and real-time bioluminescence recording has been thoroughly described by Yamazaki and Takahashi ¹¹.

Noninvasive bioluminescence imaging of c-fos expression in the mouse barrel cortex

Expression of immediate early genes, such as c-fos, has been extensively used as a marker of neural activity. However, their expression in the brain has so far been examined by using invasive procedures. In this study, we tried to image c-fos expression in the mouse barrel cortex noninvasively by detecting bioluminescence produced by the reporter luciferase. To detect asymmetry in c-fos expression in the bilateral barrel cortices, we used ten Fos-Luc mice and removed long whiskers on one side. After 1h of exploration in a novel cage, luciferin was intraperitoneally administrated under gas anesthesia and bioluminescence was measured with a cooled CCD camera. We observed moderate but clear emission over the head that was significantly stronger on the side of removal. After regrowth of the whiskers, the same mice had the vibrissae clipped on the other side. Bioluminescence was again dominant on the side of removal. In three of the mice, c-fos expression was examined immunohistochemically. The distribution of bioluminescence generally agreed with that of the c-fos positive cells though the bioluminescence tended to distribute wider, by around 0.5mm, probably due to scattering of light through the tissues. The results show that expression of c-fos in the mouse barrel cortex can be imaged repeatedly and noninvasively in the living animal.

Fully codon-optimized luciferase uncovers novel temperature characteristics of the Neurospora clock

We report the complete reconstruction of the firefly luciferase gene, fully codon optimized for expression in Neurospora crassa. This reporter enhances light output by approximately 4 log orders over that with previously available versions, now producing light that is visible to the naked eye and sufficient for monitoring the activities of many poorly expressed genes. Time lapse photography of strains growing in race tubes, in which the frq or eas/ccg-2 promoter is used to drive luciferase, shows the highest levels of luciferase activity near the growth front and newly formed conidial bands. Further, we have established a sorbose medium colony assay that will facilitate luciferase-based screens. The signals from sorbose-grown colonies of strains in which the frq promoter drives luciferase exhibit the properties of circadian rhythms and can be tracked for many days to weeks. This reporter now makes it possible to follow the clock in real time, even in strains or under conditions in which the circadian rhythm in conidial banding is not expressed. This property has been used to discover short, ca. 15-h period rhythms at high temperatures, at which banding becomes difficult to observe in race tubes, and to generate a high-resolution temperature phase-response curve.

Imaging gene expression in live transgenic mice after providing luciferin in drinking water

Mice expressing the firefly luciferase gene luc under the control of various gene promoters are used to image long-term changes in tumor growth, infection, development, and circadian rhythms. This novel approach enables ongoing regulation of gene expression to be visualized through repeated imaging of luciferase bioluminescence. Typically, luciferin, the luciferase substrate, is injected into mice before they are anaesthetized for imaging. To avoid the effects of handling and stress from injection on expression of the transgene, oral luciferin delivery methods were tested as an alternative to current methods. For unobscured imaging, a transgenic mouse line containing luc controlled by the enhancer and promoter for the major immediate-early gene of human cytomegalovirus (CMV) was crossed with a hairless albino mouse stock (HRS/J), resulting in the Hr-CMV line. Mice given food and water ad libitum readily drank 1-5 mM luciferin in water or apple juice and could be imaged repeatedly on subsequent days without any apparent adverse effects. Oral and injected luciferin produced similar patterns of luminescence in the body areas examined: abdomen, tail vertebrae, gonads, hind leg, foreleg and others, although the tail showed a slightly brighter relative luminescence after oral luciferin. These results show that luciferin is not appreciably degraded in the digestive tract and can be easily administered orally to avoid injection and any concomitant effects on behavior that could alter gene expression.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

The in vitro real-time oscillation monitoring system identifies potential entrainment factors for circadian clocks

BACKGROUND

Circadian rhythms are endogenous, self-sustained oscillations with approximately 24-hr rhythmicity that are manifested in various physiological and metabolic processes. The circadian organization of these processes in mammals is governed by the master oscillator within the suprachiasmatic nuclei (SCN) of the hypothalamus. Recent findings revealed that circadian oscillators exist in most organs, tissues, and even in immortalized cells, and that the oscillators in peripheral tissues are likely to be coordinated by SCN, the master oscillator. Some candidates for endogenous entrainment factors have sporadically been reported, however, their details remain mainly obscure.

RESULTS

We developed the in vitro real-time oscillation monitoring system (IV-ROMS) by measuring the activity of luciferase coupled to the oscillatory gene promoter using photomultiplier tubes and applied this system to screen and identify factors able to influence circadian rhythmicity. Using this IV-ROMS as the primary screening of entrainment factors for circadian clocks, we identified 12 candidates as the potential entrainment factor in a total of 299 peptides and bioactive lipids. Among them, four candidates (endothelin-1, all-trans retinoic acid, 9-cis retinoic acid, and 13-cis retinoic acid) have already been reported as the entrainment factors in vivo and in vitro. We demonstrated that one of the novel candidates, 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2), a natural ligand of the peroxisome proliferator-activated receptor-gamma (PPAR-gamma), triggers the rhythmic expression of endogenous clock genes in NIH3T3 cells. Furthermore, we showed that 15d-PGJ2 transiently induces Cry1, Cry2, and Roralpha mRNA expressions and that 15d-PGJ2-induced entrainment signaling pathway is PPAR-gamma--and MAPKs (ERK, JNK, p38MAPK)-independent.

CONCLUSION

Here, we identified 15d-PGJ2 as an entrainment factor in vitro. Using our developed IV-ROMS to screen 299 compounds, we found eight novel and four known molecules to be potential entrainment factors for circadian clocks, indicating that this assay system is a powerful and useful tool in initial screenings.

Circadian Regulation of a Viral Gene Promoter in Live Transgenic Mice Expressing Firefly Luciferase

PURPOSE

This study was conducted to test for possible circadian control of viral infection in live animals using bioluminescence imaging of a firefly luciferase transgene.

METHODS

Transgenic mice expressing the firefly luciferase gene under the control of the promoter and enhancer of the human cytomegalovirus major immediate-early gene (CMV::luc) were examined through whole-animal imaging. Mice were crossed with HRS/J hairless albino mice to improve imaging of deep structures.

RESULTS

Transgene expression in the extremities and head was elevated around dusk in mice maintained in cycles of light and dark. Signal was also elevated during the animal's night in mice maintained in extended darkness. The viral promoter was induced during the active phase of the circadian locomotor rhythm in several tissues. Both the acinar cells and islets expressed the transgene in dissociated pancreas cultures.

CONCLUSIONS

These results suggest that viruses may exploit the circadian system for optimal timing of infection at particular phases in several tissue types.

Real-time luminescence reporting of circadian gene expression in mammals

Luminescence reporters have been used successfully in studies of circadian rhythms. Real-time measurements of circadian variations in gene expression were made in living cells, cultured tissues, and whole organisms. Because this technique is relatively easy and continuous noninvasive measurement from tissue cultures allows for a drastic reduction in the number of experimental animals, we believe this method will become a common technique for studying circadian rhythms. Using a multichannel recording apparatus, it may also become a powerful tool for the discovery of new drugs. In the past, measurements were done using hand-made apparatuses or by modifying commercially available equipment. We, along with other investigators, have developed user-friendly equipment for performing circadian rhythms experiments, and these systems are now available commercially. This article describes the use of luminescence reporters in circadian research and provides detailed methods used in these experiments. One of our goals in this article is to reduce experimental variability in different laboratories by proposing standard protocols.

Circadian gene expression in mammalian fibroblasts revealed by real-time luminescence reporting: temperature compensation and damping

Mammalian cells such as rat-1 fibroblasts have been shown to exhibit daily oscillations in the expression of several gene transcripts in culture. After induction, these oscillations persist with a period of approximately 24 h for several days. This characteristic suggests that the oscillations are controlled by a circadian clock, but the crucial criterion of temperature compensation has not been demonstrated for rat-1 fibroblasts. We have developed an automated assay of circadian expression of the mPer1 promoter in rat-1 fibroblasts that have been stably transfected with a luciferase reporter. Using this cell culture-based in vitro luminescent reporter assay, we found that the daily oscillation of mPer1 promoter activity in rat-1 cells is temperature compensated over the range of 28.5-36.5 degrees C. This finding means that these oscillations are bona fide circadian rhythms. Moreover, the circadian clock of these homeothermic mammalian cells not only is temperature compensated but also is overcompensated such that it runs faster at cooler temperatures (Q10 of 0.85-0.88). The oscillations in rat-1 fibroblasts damp more rapidly at cooler temperatures, and damping is not due to cells becoming unhealthy because a second stimulus will reinitiate a robust rhythm. These data show that rat-1 cell cultures that are stably transfected with luminescence reporters are an excellent model system for studying circadian clocks at the cellular level in mammals.

Monitoring immediate-early gene expression through firefly luciferase imaging of HRS/J hairless mice

BACKGROUND

Gene promoters fused to the firefly luciferase gene (luc) are useful for examining gene regulation in live transgenic mice and they provide unique views of functioning organs. The dynamics of gene expression in cells and tissues expressing luciferase can be observed by imaging this enzyme's bioluminescent oxidation of luciferin. Neural pathways involved in specific behaviors have been identified by localizing expression of immediate-early genes such as c-fos. A transgenic mouse line with luc controlled by the human c-fos promoter (fos::luc) has enabled gene expression imaging in brain slice cultures. To optimize imaging of immediate-early gene expression throughout intact mice, the present study examined fos::luc mice and a second transgenic mouse containing luc controlled by the human cytomegalovirus immediate-early gene 1 promoter and enhancer (CMV::luc). Because skin pigments and hair can significantly scatter light from underlying structures, the two transgenic lines were crossed with a hairless albino mouse (HRS/J) to explore which deep structures could be imaged. Furthermore, live anesthetized mice were compared with overdosed mice.

RESULTS

Bioluminescence imaging of anesthetized mice over several weeks corresponded with expression patterns in mice imaged rapidly after a lethal overdose. Both fos::luc and CMV::luc mice showed quantifiable bright bioluminescence in ear, nose, paws, and tail whether they were anesthetized or overdosed. CMV::luc and fos::luc neonates had bioluminescence patterns similar to those of adults, although intensity was significantly higher in neonates. CMV::luc mice crossed with HRS/J mice had high expression in bone, claws, head, pancreas, and skeletal muscle, but less in extremities than haired CMV::luc mice. Imaging of brain bioluminescence through the neonatal skull was also practical. By imaging luciferin autofluorescence it was clear that substrate distribution did not restrict bioluminescence imaging to capillaries after injection. Luciferin treatment and anesthesia during imaging did not adversely affect circadian rhythms in locomotor activity.

CONCLUSIONS

Imaging of gene expression patterns with luciferase can be extended from studies of live animals to rapid imaging of mice following a pentobarbital overdose before significant effects from postmortem changes occurs. Bioluminescent transgenic mice crossed with HRS/J mice are valuable for examining gene expression in deep tissues.

Circadian entrainment to temperature, but not light, in the isolated suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) is the master pacemaker that drives circadian rhythms in mammalian physiology and behavior. The abilities to synchronize to daily cycles in the environment and to keep accurate time over a range of physiologic temperatures are two fundamental properties of circadian pacemakers. Recordings from a bioluminescent reporter (Per1-luc) of Period1 gene activity in rats showed that the cultured SCN entrained to daily, 1.5 degrees C cycles of temperature, but did not synchronize to daily light cycles. Temperature entrainment developed by 1 day after birth. Light cycles failed to affect the isolated SCN of rats aged 2 to 339 days. Entrainment to a 3-h shift in the warm-cool cycle was possible in <3 days with 3 degrees C cycles. Importantly, Per1-luc expression in vitro was similar to that seen in vivo where peak expression occurs approximately 1 h prior to the daily increase in temperature. In addition, the firing rate of individual mouse SCN neurons continued to express near 24-h rhythms from 24-37 degrees C. At lower temperatures, the percentage of rhythmic cells was reduced, but periodicity was temperature compensated. The results indicate that normal rhythms in brain temperature may serve to stabilize rhythmicity of the circadian system in vivo and that temperature compensation of this period is determined at the level of individual SCN cells.

Serotonin phase-shifts the mouse suprachiasmatic circadian clock in vitro

The mammalian circadian clock in the suprachiasmatic nucleus (SCN) receives multiple afferent signals that could potentially modulate its phase. One input, the serotonin (5-HT) projection from the raphe nuclei, has been extensively investigated in rats and hamsters, yet its role(s) in modulating circadian clock phase remains controversial. To expand our investigation of 5-HT modulation of the SCN clock, we investigated the phase-shifting effects of 5-HT and its agonist, (+)8-hydroxy-2-(di-n-propylamino)tetralin (DPAT), when applied to mouse SCN brain slices. 5-HT induced 2-3 h phase advances when applied during subjective day, while non-significant phase shifts were seen after 5-HT application at other times. These phase shifts were completely blocked by the 5-HT antagonist, metergoline. DPAT also induced phase shifts when applied during mid-subjective day, and this effect appeared dose-dependent. Together, these results demonstrate that the mouse SCN, like that of the rat, is directly sensitive to in vitro phase-resetting by 5-HT.

Real-time analysis of rhythmic gene expression in immortalized suprachiasmatic nucleus cells

Immortalized cells derived from the suprachiasmatic nucleus (SCN) retain many properties of the SCN including the capacity to generate circadian rhythms. Stably transfected SCN2.2 cells expressing the human c- promoter linked to a luciferase reporter gene ( /luc) were examined for evidence of transgene responses to stimuli known to induce c- expression and of endogenous rhythmic variation. Bioluminescence-reported transgene expression was induced in SCN2.2 /luc cells following stimulation with fetal bovine serum or KCl. SCN2.2 /luc cells showed 24 h rhythms of bioluminescence with a 9- to 19-fold difference between peak and minimum levels. These results demonstrate that the regulation of /luc transgene expression in SCN2.2 cells is similar to that of the endogenous c- gene in the SCN.

Circadian rhythms in isolated brain regions

The suprachiasmatic nucleus (SCN) of the mammalian hypothalamus has been referred to as the master circadian pacemaker that drives daily rhythms in behavior and physiology. There is, however, evidence for extra-SCN circadian oscillators. Neural tissues cultured from rats carrying the Per-luciferase transgene were used to monitor the intrinsic Per1 expression patterns in different brain areas and their response to changes in the light cycle. Although many Per-expressing brain areas were arrhythmic in culture, 14 of the 27 areas examined were rhythmic. The pineal and pituitary glands both expressed rhythms that persisted for >3 d in vitro, with peak expression during the subjective night. Nuclei in the olfactory bulb and the ventral hypothalamus expressed rhythmicity with peak expression at night, whereas other brain areas were either weakly rhythmic and peaked at night, or arrhythmic. After a 6 hr advance or delay in the light cycle, the pineal, paraventricular nucleus of the hypothalamus, and arcuate nucleus each adjusted the phase of their rhythmicity with different kinetics. Together, these results indicate that the brain contains multiple, damped circadian oscillators outside the SCN. The phasing of these oscillators to one another may play a critical role in coordinating brain activity and its adjustment to changes in the light cycle.

Genetic engineering of neural function in transgenic rodents: towards a comprehensive strategy?

As mammalian genome projects move towards completion, the attention of molecular neuroscientists is currently moving away from gene identification towards both cell-specific gene expression patterns (neuronal transcriptions) and protein expression/interactions (neuronal proteomics). In the long term, attention will increasingly be directed towards experimental interventions which are able to question neuronal function in a sophisticated manner that is cognisant of both transcriptomic and proteomic organization. Central to this effort will be the application of a new generation of transgenic approaches which are now evolving towards an appropriate level of molecular, temporal and spatial resolution. In this review, we summarize recent developments in transgenesis, and show how they have been applied in the principal model species for neuroscience, namely rats and mice. Current concepts of transgene design are also considered together with an overview of new genetically-encoded tools including both cellular indicators such as fluorescent activity reporters, and cellular regulators such as dominant negative signalling factors. Application of these tools in a whole animal context can be used to question both basic concepts of brain function, and also current concepts of underlying dysfuction in neurological diseases.

Phase response curves of a molecular model oscillator: implications for mutual coupling of paired oscillators

Increasing evidence indicates that the accessory medulla is the circadian pacemaker controlling locomotor activity rhythms in insects. A prominent group of neurons of this neuropil shows immunoreactivity to the peptide pigment-dispersing hormone (PDH). In Drosophila melanogaster, the PDH-immunoreactive (PDH-ir) lateral neurons, which also express the clock genes period and timeless, are assumed to be circadian pacemaker cells themselves. In other insects, such as Leucophaea maderae, a subset of apparently homologue PDH-ir cells is a candidate for the circadian coupling pathway of the bilaterally symmetric clocks. Although knowledge about molecular mechanisms of the circadian clockwork is increasing rapidly, very little is known about mechanisms of circadian coupling. The authors used a computer model, based on the molecular feedback loop of the clock genes in D. melanogaster, to test the hypothesis that release of PDH is involved in the coupling between bilaterally paired oscillators. They can show that a combination of all-delay- and all-advance-type interactions between two model oscillators matches best the experimental findings on mutual pacemaker coupling in L. maderae. The model predicts that PDH affects the phosphorylation rate of clock genes and that in addition to PDH, another neuroactive substance is involved in the coupling pathway, via an all-advance type of interaction. The model suggests that PDH and light pulses, represented by two distinct classes of phase response curves, have different targets in the oscillatory feedback loop and are, therefore, likely to act in separate input pathways to the clock.

Differential regulation of fos family genes in the ventrolateral and dorsomedial subdivisions of the rat suprachiasmatic nucleus

Extensive studies have established that light regulates c-fos gene expression in the suprachiasmatic nucleus, the site of an endogenous circadian clock, but relatively little is known about the expression of genes structurally related to c-fos, including fra-1, fra-2 and fosB. We analysed the photic and temporal regulation of these genes at the messenger RNA and immunoreactive protein levels in rat suprachiasmatic nucleus, and we found different expression patterns after photic stimulation and depending on location in the ventrolateral or dorsomedial subdivisions. In the ventrolateral suprachiasmatic nucleus, c-fos, fra-2 and fosB expression was stimulated after a subjective-night (but not subjective-day) light pulse. Expression of the fra-2 gene was prolonged following photic stimulation, with elevated messenger RNA and protein levels that appeared unchanged for at least a few hours beyond the c-fos peak. Unlike c-fos and fra-2, the fosB gene appeared to be expressed constitutively in the ventrolateral suprachiasmatic nucleus throughout the circadian cycle; immunohistochemical analysis suggested that delta FosB was the protein product accounting for this constitutive expression, while FosB was induced by the subjective-night light pulse. In the dorsomedial suprachiasmatic nucleus, c-fos and fra-2 expression exhibited an endogenous circadian rhythm, with higher levels during the early subjective day, although the relative abundance was much lower than that measured after light pulses in the ventrolateral suprachiasmatic nucleus. Double-label immunohistochemistry suggested that some of the dorsomedial cells responsible for the circadian expression of c-Fos also synthesized arginine vasopressin. No evidence of suprachiasmatic nucleus fra-1 expression was found. In summary, fos family genes exhibit differences in their specific expression patterns in the suprachiasmatic nucleus, including their photic and circadian regulation in separate cell populations in the ventrolateral and dorsomedial subdivisions. The data, in combination with our previous results [Takeuchi J. et al. (1993) Neuron 11, 825-836], suggest that activator protein-1 binding sites on ventrolateral suprachiasmatic nucleus target genes are constitutively occupied by DeltaFosB/JunD complexes, and that c-Fos, Fra-2, FosB and JunB compete for binding after photic stimulation. The differential regulation of fos family genes in the ventrolateral and dorsomedial suprachiasmatic nucleus suggests that their circadian function(s) and downstream target(s) are likely to be cell specific.

Resetting central and peripheral circadian oscillators in transgenic rats

In multicellular organisms, circadian oscillators are organized into multitissue systems which function as biological clocks that regulate the activities of the organism in relation to environmental cycles and provide an internal temporal framework. To investigate the organization of a mammalian circadian system, we constructed a transgenic rat line in which luciferase is rhythmically expressed under the control of the mouse Per1 promoter. Light emission from cultured suprachiasmatic nuclei (SCN) of these rats was invariably and robustly rhythmic and persisted for up to 32 days in vitro. Liver, lung, and skeletal muscle also expressed circadian rhythms, which damped after two to seven cycles in vitro. In response to advances and delays of the environmental light cycle, the circadian rhythm of light emission from the SCN shifted more rapidly than did the rhythm of locomotor behavior or the rhythms in peripheral tissues. We hypothesize that a self-sustained circadian pacemaker in the SCN entrains circadian oscillators in the periphery to maintain adaptive phase control, which is temporarily lost following large, abrupt shifts in the environmental light cycle.

Immediate early gene expression within the visual system: light and circadian regulation in the retina and the suprachiasmatic nucleus

Immediate early genes are a family of genes that share the characteristic of having their expression rapidly and transiently induced upon stimulation of neuronal and non-neuronal cells. In this review, first a short description of the IEGs is given, then it is discussed the stimulus-induced and circadian-induced variations in the expression of IEGs in the visual system, mainly in the retina and the suprachiasmatic nucleus. The possible physiological consequences of these variations in IEG expression are also considered. Finally, we refer to two aspects of our recent studies and those of other laboratories involving light-driven IEG expression. The first is the finding that in the chick retina, the expression of c-fos is differentially modulated in the different cell types and that c-fos regulates the synthesis of the quantitatively most important lipids of all cells, the phospholipids, by a non-genomic mechanism. The second is the occurrence of differential waves of IEG expression in the mammalian suprachiasmatic nucleus regarding light induction or spontaneous oscillations.

The Drosophila dCREB2 gene affects the circadian clock

We report the role of dCREB2, the Drosophila homolog of CREB/CREM, in circadian rhythms. dCREB2 activity cycles with a 24 hr rhythm in flies, both in a light:dark cycle and in constant darkness. A mutation in dCREB2 shortens circadian locomotor rhythm in flies and dampens the oscillation of period, a known clock gene. Cycling dCREB2 activity is abolished in a period mutant, indicating that dCREB2 and Period affect each other and suggesting that the two genes participate in the same regulatory feedback loop. We propose that dCREB2 supports cycling of the Period/Timeless oscillator. These findings support CREB's role in mediating adaptive behavioral responses to a variey of environmental stimuli (stress, growth factors, drug addiction, circadian rhythms, and memory formation) in mammals and long-term memory formation and circadian rhythms in Drosophila.