Drosophila Free-Running Rhythms Require Intercellular Communication

Robust self-sustained oscillations are a ubiquitous characteristic of circadian rhythms. These include Drosophila locomotor activity rhythms, which persist for weeks in constant darkness (DD). Yet the molecular oscillations that underlie circadian rhythms damp rapidly in many Drosophila tissues. Although much progress has been made in understanding the biochemical and cellular basis of circadian rhythms, the mechanisms that underlie the differences between damped and self-sustaining oscillations remain largely unknown. A small cluster of neurons in adult Drosophila brain, the ventral lateral neurons (LN(v)s), is essential for self-sustained behavioral rhythms and has been proposed to be the primary pacemaker for locomotor activity rhythms. With an LN(v)-specific driver, we restricted functional clocks to these neurons and showed that they are not sufficient to drive circadian locomotor activity rhythms. Also contrary to expectation, we found that all brain clock neurons manifest robust circadian oscillations of timeless and cryptochrome RNA for many days in DD. This persistent molecular rhythm requires pigment-dispersing factor (PDF), an LN(v)-specific neuropeptide, because the molecular oscillations are gradually lost when Pdf(01) mutant flies are exposed to free-running conditions. This observation precisely parallels the previously reported effect on behavioral rhythms of the Pdf(01) mutant. PDF is likely to affect some clock neurons directly, since the peptide appears to bind to the surface of many clock neurons, including the LN(v)s themselves. We showed that the brain circadian clock in Drosophila is clearly distinguishable from the eyes and other rapidly damping peripheral tissues, as it sustains robust molecular oscillations in DD. At the same time, different clock neurons are likely to work cooperatively within the brain, because the LN(v)s alone are insufficient to support the circadian program. Based on the damping results with Pdf(01) mutant flies, we propose that LN(v)s, and specifically the PDF neuropeptide that it synthesizes, are important in coordinating a circadian cellular network within the brain. The cooperative function of this network appears to be necessary for maintaining robust molecular oscillations in DD and is the basis of sustained circadian locomotor activity rhythms

In Synch but Not in Step: Circadian Clock Circuits Regulating Plasticity in Daily Rhythms

The suprachiasmatic nucleus (SCN) is a network of neural oscillators that program daily rhythms in mammalian behavior and physiology. Over the last decade much has been learned about how SCN clock neurons coordinate together in time and space to form a cohesive population. Despite this insight, much remains unknown about how SCN neurons communicate with one another to produce emergent properties of the network. Here we review the current understanding of communication among SCN clock cells and highlight a collection of formal assays where changes in SCN interactions provide for plasticity in the waveform of circadian rhythms in behavior. Future studies that pair analytical behavioral assays with modern neuroscience techniques have the potential to provide deeper insight into SCN circuit mechanisms

Oscillatory Bursting as a Mechanism for Temporal Coupling and Information Coding

© Copyright © 2020 Tal, Neymotin, Bickel, Lakatos and Schroeder. Even the simplest cognitive processes involve interactions between cortical regions. To study these processes, we usually rely on averaging across several repetitions of a task or across long segments of data to reach a statistically valid conclusion. Neuronal oscillations reflect synchronized excitability fluctuations in ensembles of neurons and can be observed in electrophysiological recordings in the presence or absence of an external stimulus. Oscillatory brain activity has been viewed as sustained increase in power at specific frequency bands. However, this perspective has been challenged in recent years by the notion that oscillations may occur as transient burst-like events that occur in individual trials and may only appear as sustained activity when multiple trials are averaged together. In this review, we examine the idea that oscillatory activity can manifest as a transient burst as well as a sustained increase in power. We discuss the technical challenges involved in the detection and characterization of transient events at the single trial level, the mechanisms that might generate them and the features that can be extracted from these events to study single-trial dynamics of neuronal ensemble activity

Reflections On Contributing To “Big Discoveries” About The Fly Clock: Our Fortunate Paths As Post-Docs With 2017 Nobel Laureates Jeff Hall, Michael Rosbash, And Mike Young

In the early 1980s Jeff Hall and Michael Rosbash at Brandeis University and Mike Young at Rockefeller University set out to isolate the period (per) gene, which was recovered in a revolutionary genetic screen by Ron Konopka and Seymour Benzer for mutants that altered circadian behavioral rhythms. Over the next 15 years the Hall, Rosbash and Young labs made a series of groundbreaking discoveries that defined the molecular timekeeping mechanism and formed the basis for them being awarded the 2017 Nobel Prize in Physiology or Medicine. Here the authors recount their experiences as post-docs in the Hall, Rosbash and Young labs from the mid-1980s to the mid-1990s, and provide a perspective of how basic research conducted on a simple model system during that era profoundly influenced the direction of the clocks field and established novel approaches that are now standard operating procedure for studying complex behavior

Circadian rhythmicity and light sensitivity of the zebrafish brain

Light is important for entraining circadian rhythms, which regulate a wide range of biological processes. Zebrafish have directly light responsive tissues (Whitmore et al 2000) and are thus a useful vertebrate model for circadian rhythmicity and light sensitivity. Recent studies show the pineal regulates locomotor rhythms (Li et al 2012). However, there are many unresolved questions concerning the neurobiological basis of the zebrafish clock, such as whether neuronal pacemakers, which drive rhythms in other tissues, are present throughout the brain. In this study, per3-luc zebrafish confirm that both central and peripheral tissues are directly light sensitive and have endogenous circadian rhythmicity. Chromogenic in situ hybridization reveals localised expression of several core zebrafish clock genes, a rhythmic gene, per3, and two light responsive genes, cry1a and per2. Adult brain nuclei with expression include the suprachiasmatic nucleus, periventricular grey zone of the optic tectum, and granular cells of the rhombencephalon. Pilot experiments using high-resolution spatial recording of per3-luc brain slices show some of these regions can display robust rhythmicity in DD. Some of the cells expressing clock genes are neurons, and therefore neurons were further investigated. C-fos, a marker for neuronal activity in mammalian photoreceptors, is upregulated in at least four different responses to light in zebrafish, in different brain nuclei. This suggests the brain contains several types of photosensitive cells, which respond to different lighting conditions. Zebrafish larvae exhibit developmental changes in spatial circadian gene expression of per3 and light induction of c-fos. Finally, the photopigment group of opsins were investigated for their potential role in light entrainment. Exorh was prominent solely in the pineal. Rgr1 was found in numerous nuclei, many of which had shown expression of cry1a, per2 and per3. Overall, this thesis shows that the zebrafish brain is not uniformly light sensitive. Localised regions in the zebrafish brain with strong rhythmicity and light sensitivity are neuronal pacemaker candidates

Novel mechanisms operating in the central pacemaker and in the light-synchronization pathway of Drosophila’s circadian clock

PhDMost organisms display circadian rhythms of approximately 24 hours in many aspects of their physiology and behaviour. The synchronization between their internal rhythm and the environmental light-dark cycles is essential for an organism’s survival and fitness. In the fruit fly, Drosophila melanogaster, circadian locomotor activity is controlled by central pacemaker neurons, in which the circadian oscillation of the molecular clock is built on the negative feedback regulation of period (per) and timeless (tim) gene expression. After transcription and translation, PER and TIM proteins form stable heterodimers in the cytoplasm and transfer into the nucleus to suppress their own transcription. Whether other processes including PER homodimerisation and nuclear trafficking are involved in circadian feedback control remains largely unknown. To study the functions of these processes, I attempted to specifically disrupt PER homodimers and nuclear export sequences (NES). I found that PER homodimers are required for PER nuclear translocation, period transcriptional repression, and normal circadian behaviour in flies. I also demonstrated that the potential NES of PER contributes to the repressor activity of PER and temperature compensation of the circadian clock. Light can phase-shift and even disrupt the molecular clock by degrading TIM. Light-dependent TIM degradation in Drosophila is mainly mediated by the photoreceptor CRYPTOCHROME (CRY). However, CRY-independent light-input pathways are also utilized by the Drosophila circadian clock. To explore these pathways, I investigated the function of a novel gene quasimodo (qsm) and found that QSM is a light-responsive protein. In constant light, qsm mutants maintain oscillation of clock proteins and show abnormal behavioral rhythms, indicating impaired photoreception. 6 Functional analysis suggests that qsm may mediate TIM degradation in the absence of CRY, and constitutes part of a novel light-input pathway to the clock. In addition, I found that in conjunction with a functional circadian clock QSM may suppress light induced ultradian rhythms

Circadian deep sequencing reveals stress-response genes that adopt robust rhythmic expression during aging

Disruption of the circadian clock, which directs rhythmic expression of numerous output genes, accelerates aging. To enquire how the circadian system protects aging organisms, here we compare circadian transcriptomes in heads of young and old Drosophila melanogaster. The core clock and most output genes remained robustly rhythmic in old flies, while others lost rhythmicity with age, resulting in constitutive over- or under-expression. Unexpectedly, we identify a subset of genes that adopted increased or de novo rhythmicity during aging, enriched for stress-response functions. These genes, termed late-life cyclers, were also rhythmically induced in young flies by constant exposure to exogenous oxidative stress, and this upregulation is CLOCK-dependent. We also identify age-onset rhythmicity in several putative primary piRNA transcripts overlapping antisense transposons. Our results suggest that, as organisms age, the circadian system shifts greater regulatory priority to the mitigation of accumulating cellular stress

Dynamic Decomposition of Spatiotemporal Neural Signals

Neural signals are characterized by rich temporal and spatiotemporal dynamics that reflect the organization of cortical networks. Theoretical research has shown how neural networks can operate at different dynamic ranges that correspond to specific types of information processing. Here we present a data analysis framework that uses a linearized model of these dynamic states in order to decompose the measured neural signal into a series of components that capture both rhythmic and non-rhythmic neural activity. The method is based on stochastic differential equations and Gaussian process regression. Through computer simulations and analysis of magnetoencephalographic data, we demonstrate the efficacy of the method in identifying meaningful modulations of oscillatory signals corrupted by structured temporal and spatiotemporal noise. These results suggest that the method is particularly suitable for the analysis and interpretation of complex temporal and spatiotemporal neural signals