Endogenous Circadian Oscillator Research Articles

The Free-Running Circasemilunar Period Is Determined by Counting Circadian Clock Cycles in the Marine Midge Clunio Marinus.

Semilunar rhythms are found in numerous marine organisms, but the molecular mechanism and functional principles of endogenous circasemilunar clocks remain elusive. Here, we explore the connection between the free-running circasemilunar clock and the circadian clock in the marine midge Clunio marinus with three different chronobiological assays. First, we found that the free-running circasemilunar period of the adult emergence rhythm in C. marinus changes linearly with diel T-cycle length, supporting a day-counting mechanism. Second, under LD 6:6, periods of circasemilunar and circadian emergence were comparable to those under LD 12:12, indicating that the circasemilunar counter in C. marinus relies on endogenous circadian oscillations rather than external T-cycles. Finally, when desynchronizing the circadian clock with constant light, the free-running circasemilunar emergence rhythm disappeared as well, suggesting that it requires a synchronized circadian clock. These results oppose the long-held view that C. marinus' free-running circasemilunar clock operates independently of the circadian clock. In a broader evolutionary context, our results strengthen the idea that the circasemilunar clocks of dipterous insects are based on different functional principles compared to the circasemilunar or circalunar clocks of marine annelids and algae. These divergent clock principles may indicate multiple evolutionary origins of circasemilunar and circalunar clocks.

Melatonin improves cardiac remodeling and brain-heart sympathetic hyperactivation aggravated by light disruption after myocardial infarction.

Light in the external environment might affect cardiovascular function. The light disruption seems to be related to changes in cardiovascular physiological functions, and disturbing light may be a risk factor for cardiovascular diseases. Prior studies have found that light disruption after myocardial infarction (MI) exacerbates cardiac remodeling, and the brain-heart sympathetic nervous system may be one of the key mechanisms. However, how to improve light-disrupted cardiac remodeling remains unclear. Melatonin is an indoleamine secreted by the pineal gland and controlled by endogenous circadian oscillators within the suprachiasmatic nucleus, which is closely associated with light/dark cycle. This study aimed to explore whether melatonin could improve light-disrupted cardiac remodeling and modulate the brain-heart sympathetic nervous system. Our study revealed that light disruption reduced serum melatonin levels, aggravated cardiac sympathetic remodeling, caused overactivation of the brain-heart sympathetic nervous system, exacerbated cardiac dysfunction, and increased cardiac fibrosis after MI, while melatonin treatment improved light disruption-exacerbated cardiac remodeling and brain-heart sympathetic hyperactivation after MI. Furthermore, RNA-Seq results revealed the significant changes at the cardiac transcription level. In conclusion, melatonin may be a potential therapy for light-disrupted cardiac remodeling.

Transcriptome analysis uncovering regulatory networks and hub genes of Populus photosynthesis and chlorophyll content

Photosynthesis plays vital role in plant growth and development throughout its life, and it is influenced by environmental signals and circadian rhythms. We analyzed the transcriptome landscape of the two poplars progeny with contrasting photosynthesis rates at three times point (ZT4, ZT16, ZT22), constructed gene regulatory network that related to circadian rhythm and photosynthesis. We suggest that the differences in photosynthetic rate between the progenies may originate from plant endogenous circadian oscillators prepare poplar plants for photosynthesis by regulating photosynthesis-associated nuclear genes and carotenoid metabolism genes before dawn, genes associated with plant hormone signal transduction and transcription factor increase leaf size and stomatal movement, the influence of other core regulatory factors on chlorophyll accumulation. Furthermore, overexpression of candidate regulatory gene, AP3 (Potri.007G017000), induced leaf senescence and reduced the content of chlorophyll. These results demonstrated that many potential key regulators are integrated closely with chlorophyll content and photosynthesis.

Statistical mechanics of clock gene networks underlying circadian rhythms

All multicellular organisms embed endogenous circadian oscillators or clocks that rhythmically regulate a wide variety of processes in response to daily environmental cycles. Previous molecular studies using rhythmic mutants for several model systems have identified a set of genes responsible for rhythmic activities and illustrated the molecular mechanisms underlying how disruptions in circadian rhythms are associated with the sort of aberrant cell cycling. However, the wide use of these forward genetic studies is impaired by a limited number of mutations that can be identified or induced only in a single genome, limiting the identification of many other conserved or non-conserved clock genes. Genetic linkage or association mapping provides an unprecedented glimpse into the genome-wide scanning and characterization of genes underlying circadian rhythms. The implementation of sophisticated statistical models into genetic mapping studies can not only identify key clock genes or clock quantitative trait loci (cQTL) but also, more importantly, reveal a complete atlas of the genetic control mechanisms constituted by gene interactomes. Here, we introduce and review an advanced statistical mechanics framework for coalescing all possible clock genes into intricate but well-organized interaction networks that regulate rhythmic cycles. The application of this framework to widely available mapping populations will reshape and further our understanding of the genetic signatures behind circadian rhythms for an enlarged range of species including microbes, plants, and humans.

Aging and the clock: Perspective from flies to humans.

Endogenous circadian oscillators regulate molecular, cellular and physiological rhythms, synchronizing tissues and organ function to coordinate activity and metabolism with environmental cycles. The technological nature of modern society with round-the-clock work schedules and heavy reliance on personal electronics has precipitated a striking increase in the incidence of circadian and sleep disorders. Circadian dysfunction contributes to an increased risk for many diseases and appears to have adverse effects on aging and longevity in animal models. From invertebrate organisms to humans, the function and synchronization of the circadian system weakens with age aggravating the age-related disorders and pathologies. In this review, we highlight the impacts of circadian dysfunction on aging and longevity and the reciprocal effects of aging on circadian function with examples from Drosophila to humans underscoring the highly conserved nature of these interactions. Additionally, we review the potential for using reinforcement of the circadian system to promote healthy aging and mitigate age-related pathologies. Advancements in medicine and public health have significantly increased human life span in the past century. With the demographics of countries worldwide shifting to an older population, there is a critical need to understand the factors that shape healthy aging. Drosophila melanogaster, as a model for aging and circadian interactions, has the capacity to facilitate the rapid advancement of research in this area and provide mechanistic insights for targeted investigations in mammals.

Locomotor activity patterns of domestic cat (Felis silvestris catus) modulated by different light/dark cycles

ABSTRACTThe aim of this study was to investigate the influence of different photoperiod schedules on daily total locomotor activity in cat. Four male cats (4 ± 0.3 kg) were housed individually under controlled temperature during the study and were subjected to four different artificial lighting regimes including 12L:12D (light on at 08:00, light off at 20:00), 12D:12L (light on at 20:00, light off at 08:00), 24L:0D (light on for all experimental period), 0L:24D (constant darkness). The application of the statistical analysis of cosinor indicated the existence of a significant daily rhythm (P < 0.05). In particular, in 12L:12D conditions, the cats were more active during the dark phase compared to the light phase, with a range of variation between 23:30 and 24:30 h. During the 12D:12L cycle, all cats changed their locomotor activity rhythm, so that it coincided with the new phase of the photoperiod. The resynchronization to a new LD cycle was quick. Endogenous rhythms were manifested in all cats with different clarity when subjected to 24L:0D cycles. When the cats were exposed to constant darkness, the endogenous periodicity was not observed. These results suggest the existence of an endogenous circadian oscillator that controls the expression of locomotor activity rhythms in the cat.

Circadian period of luciferase expression shortens with age in human mature adipocytes from obese patients

Daily rhythms in physiology and behavior change with age. An unresolved question is to what extent such age-related alterations in circadian organization are driven by the central clock in the suprachiasmatic nucleus (SCN), modifying timing signals to contributing peripheral tissue oscillators, and are mediated by underlying changes in the local cellular oscillators themselves. Using a bioluminescence reporter approach, we sought to determine whether circadian clock function in human adipocytes from subcutaneous (SAT) and visceral (VAT) adipose tissues changes with age. SAT and VAT biopsies were obtained from obese individuals during gastric bypass surgeries [ n = 16; body mass index: 44.8 ± 11.4 kg/m2; age: 44 ± 9 yr (range: 30-58)]. Cells were isolated and transduced with a lentiviral circadian reporter construct [brain and muscle aryl hydrocarbon receptor nuclear translocator-like:luciferase ( BMAL:LUC)], and bioluminescence was recorded over a period of 3 d. Human BMAL1:LUC adipocytes displayed a robust luminescence rhythm with comparable within-individual periods in mature and preadipocytes ( P > 0.05). With increasing age, the circadian period decreased in mature adipocytes ( P = 0.005) (β = 4 min/yr; P < 0.05). Our ex vivo approach indicated that ageing changes the organization of endogenous circadian oscillators in human adipocytes, independent of SCN signaling.-Kolbe, I., Carrasco-Benso, M. P., López-Mínguez, J., Luján, J., Scheer, F. A. J. L., Oster, H., Garaulet, M. Circadian period of luciferase expression shortens with age in human mature adipocytes from obese patients.

Circadian rhythms of micturition during jet lag.

Micturition behavior follows regular day/night fluctuations, and unwanted increase in micturition could occur during night in jet lag condition. To clarify the effect of jet lag on micturition behavior, we simultaneously detected circadian micturition patterns and locomotor activity rhythms of mice under experimental jet lag conditions, by applying the improved automated Voided Stain on Paper (aVSOP) method. When wild-type (WT) mice were phase-advanced for 8 hours, day-night variation of micturition was disrupted suddenly, and this irregular daily micturition continued until 8 days, although their activity rhythms entrained gradually day by day until 8 days. We also examined how jet lag induced changes of micturition in Per-null mice lacking Per1, Per2 and Per3 genes, whose endogenous clock is completely disrupted. We found both micturition and locomotor activity of Per-null mice promptly entrained to the new LD cycle. These findings suggest that the irregular micturition during jet lag is caused along with the gradual shift of the endogenous clock, and paradoxically, jet lag-associated abnormality was absent when endogenous circadian oscillations were genetically disrupted.

Characterization and comparison of activity profiles exhibited by the cave and surface morphotypes of the blind Mexican tetra, Astyanax mexicanus

Departure from normal circadian rhythmicity and exposure to atypical lighting cues has been shown to adversely affect human health and wellness in a variety of ways. In contrast, adaptation to extreme environments has led many species to alter or even entirely abandon their reliance upon cyclic environmental inputs, principally daily cycles of light and darkness. The extreme darkness, stability and isolation of cave ecosystems has made cave-adapted species particularly attractive systems in which to study the consequences of life without light and the strategies that allow species to survive and even thrive in such environments. In order to further explore these questions, we have assessed the rhythmicity of locomotion in the blind Mexican tetra, Astyanax mexicanus, under controlled laboratory conditions. Using high-resolution video tracking assays, we characterized patterns in locomotor activity and spatial tank usage for members of the surface and Pachón cave populations. Here we demonstrate that cavefish have a higher overall level of activity and use the space within the trial tank differently than surface fish. Further, Pachón cavefish show circadian rhythmicity in both activity and spatial tank usage under a 12:12 light/dark cycle. We provide further evidence that these cavefish retain a weakly light-entrainable, endogenous circadian oscillator with limited capability to sustain rhythms in activity, but not spatial tank usage, in the absence of photic cues. Finally, we demonstrate a putative behavioral “masking effect” contributing to behavioral rhythms and provide evidence that exposure to constant darkness during development may alter behavioral patterns later in life.

Abstract 858: The circadian clock of glioma cells undergoing epithelial-mesenchymal transition

Abstract Highly invasive gliomas contain many cells that have changed to an enhanced migratory state through an epithelial-mesenchymal transition (EMT). These cells have the phenotype of glioma stem cells (GSCs). Similar to GSCs, glioma cells undergoing EMT show phenotypic heterogeneity, altered gene expression, and resistance to anticancer drugs along with increased invasiveness. Circadian rhythms in tumor cells influence the progression and severity of cancer and appear to regulate cell division cycles. Increased cancer incidence and progression have often been linked to disruption or deregulation of the molecular mechanism of the circadian clock. One of the major clock proteins, PER2, has been shown to play a regulatory role as a tumor and EMT suppressor in metastatic breast cancer cell lines. GSCs in tumorsphere cultures contain circadian clocks that may regulate their cancer properties. The possibility of a regulatory role for circadian rhythms in EMT was examined here. We used a standard method to induce EMT in the C6 rat glioma cell line that has known circadian rhythms in gene expression generated by the circadian clocks within its cells. EMT was induced by exchanging the cell culture medium with a serum-free stem cell medium (SCM) containing growth factors (EGF, FGF2, PDGF-alpha-beta). Cell cultures were imaged continuously with a microscope and digital camera in a cell incubator to monitor cell shape, cell death, migration, and apoptosis. During EMT, cells changed from an extended flat state to rounded and spindle shapes, ceased proliferating, and expressed EMT markers ZEB1 and vimentin. At the end of two days in SCM, 33.4% of the cells were ZEB1-positive and only 1.26% were GFAP-positive (n=3 cultures). Cell diameters after EMT were within the size range of C6 GSCs described as Hoechst-negative cells positive for stem cell markers nestin and CD133. Following EMT, small tumorspheres began to form. After initiating EMT, the rounded cells were counted at hourly intervals for up to four days after the medium exchange. As the number of post-EMT cells increased, the population size oscillated, and when examined by Lomb-Scargle periodogram analysis, four cultures had a significant period within the circadian range, 19-29 hours, (average 22.20 ±2.45 SD, p&amp;lt;0.05). One had a 16-hour period and one lacked a significant rhythm. Rhythms were present even when circadian clocks in the cultures were not synchronized with an initial forskolin treatment. We conclude that EMT may be timed by endogenous circadian oscillators in gliomas that favor larger numbers of post-EMT cells at a particular time of day. These results suggest that pharmacological treatments that suppress EMT or target GSCs would be more effective when delivered at particular times during the circadian cycle of the tumor. Citation Format: Arpan De, Dilshan Harshajith Beligala, Vishal Premdev Sharma, Benjamin Ryan Fry, Michael Eric Geusz. The circadian clock of glioma cells undergoing epithelial-mesenchymal transition [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 858. doi:10.1158/1538-7445.AM2017-858

Season-dependent effects of photoperiod and temperature on circadian rhythm of arylalkylamine N-acetyltransferase2 gene expression in pineal organ of an air-breathing catfish, Clarias gariepinus

Arylalkylamine N-acetyltransferase (AANAT) activity, aanat gene expression and melatonin production have been reported to exhibit prominent circadian rhythm in the pineal organ of most species of fish. Three types of aanat genes are expressed in fish, but the fish pineal organ predominantly expresses aanat2 gene. Increase and decrease in daylength is invariably associated with increase and decrease in temperature, respectively. But so far no attempt has been made to delineate the role of photoperiod and temperature in regulation of the circadian rhythm of aanat2 gene expression in the pineal organ of any fish with special reference to seasons. Therefore, we studied effects of various lighting regimes (12L-12D, 16L-8D, 8L-16D, LL and DD) at a constant temperature (25°C) and effects of different temperatures (15°, 25° and 35°C) under a common photoperiod 12L-12D on circadian rhythm of aanat2 gene expression in the pineal organ of Clarias gariepinus during summer and winter seasons. Aanat2 gene expression in fish pineal organ was studied by measuring aanat2 mRNA levels using Real-Time PCR. Our findings indicate that the pineal organ of C. gariepinus exhibits a prominent circadian rhythm of aanat2 gene expression irrespective of photoperiods, temperatures and seasons, and the circadian rhythm of aanat2 gene expression responds differently to different photoperiods and temperatures in a season-dependent manner. Existence of circadian rhythm of aanat2 gene expression in pineal organs maintained in vitro under 12L-12D and DD conditions as well as a free running rhythm of the gene expression in pineal organ of the fish maintained under LL and DD conditions suggest that the fish pineal organ possesses an endogenous circadian oscillator, which is entrained by light-dark cycle.

Drosophila: An Emergent Model for Delineating Interactions between the Circadian Clock and Drugs of Abuse.

Endogenous circadian oscillators orchestrate rhythms at the cellular, physiological, and behavioral levels across species to coordinate activity, for example, sleep/wake cycles, metabolism, and learning and memory, with predictable environmental cycles. The 21st century has seen a dramatic rise in the incidence of circadian and sleep disorders with globalization, technological advances, and the use of personal electronics. The circadian clock modulates alcohol- and drug-induced behaviors with circadian misalignment contributing to increased substance use and abuse. Invertebrate models, such as Drosophila melanogaster, have proven invaluable for the identification of genetic and molecular mechanisms underlying highly conserved processes including the circadian clock, drug tolerance, and reward systems. In this review, we highlight the contributions of Drosophila as a model system for understanding the bidirectional interactions between the circadian system and the drugs of abuse, alcohol and cocaine, and illustrate the highly conserved nature of these interactions between Drosophila and mammalian systems. Research in Drosophila provides mechanistic insights into the corresponding behaviors in higher organisms and can be used as a guide for targeted inquiries in mammals.

Exceptionally short-period circadian clock inCyclosa turbinata: regulation of locomotor and web-building behavior in an orb-weaving spider

A major advantage of having behavior controlled by a circadian clock is that the organism may be able to anticipate, rather than respond to, important daily events in its environment. Here, we describe the behavioral rhythms of locomotor activity and web building in the orb-weaving spider Cyclosa turbinata (Walckenaer, 1841). Web building occurs late in the scotophase, in absolute darkness, and is initiated and completed before lights-on under light:dark cycles in the laboratory. This scheduling presumably enables web-building to occur under the cover of darkness, thereby avoiding visual predators. Locomotor activity occurs predominantly in the dark with a sharp peak within one hour after lights-off and a broader peak occurring before lights-on. The locomotor activity rhythm free runs under constant dark and constant temperature conditions, thus indicating endogenous circadian control. Evidence from the free running rhythm suggests that the first peak under light:dark cycles is a result of masking but that the second peak is attributable to the endogenous circadian oscillator. The period of the free run is exceptionally short, about 19 hours. In comparison with locomotor activity, web building is quite sporadic under constant dark conditions, making detection of periodicities difficult and, therefore, whether web-building is under endogenous circadian control or is driven by exogenous factors remains unresolved.

Time Matters in Ecotoxicological Sampling Due to Circadian Rhythm.

As in general, technological inventions also drive the development in the field of toxicology and ecotoxicology. In the past decade, gene expression analysis has become a universally applied technology allowing many insights into toxicological pathways of environmental contaminants. Due to the novel technologies, including quantitative determination of mRNA by quantitative reverse transcription analysis (qRT-PCR), and semiquantitative methods, such as microarrays and RNA-sequencing technologies, toxicological profiles of contaminants could be identified. For instance, gene expression analysis of genes associated with the hypothalamic-pituitary-gonadal axis (HPG axis) in fish had become a conventional end point for endocrine disrupting chemicals. While these gene expression data provide novel insights into identifying potential toxicological end points and molecular mechanisms, often not enough attention is given to the question of mRNA stabilities and reliabilities of transcriptional data, in particular when links to physiological effects are difficult to make. A crucial factor in this issue is the endogenous circadian oscillations of genes during sampling.

Circadian Clock Genes Are Essential for Normal Adult Neurogenesis, Differentiation, and Fate Determination

Adult neurogenesis creates new neurons and glia from stem cells in the human brain throughout life. It is best understood in the dentate gyrus (DG) of the hippocampus and the subventricular zone (SVZ). Circadian rhythms have been identified in the hippocampus, but the role of any endogenous circadian oscillator cells in hippocampal neurogenesis and their importance in learning or memory remains unclear. Any study of stem cell regulation by intrinsic circadian timing within the DG is complicated by modulation from circadian clocks elsewhere in the brain. To examine circadian oscillators in greater isolation, neurosphere cultures were prepared from the DG of two knockout mouse lines that lack a functional circadian clock and from mPer1::luc mice to identify circadian oscillations in gene expression. Circadian mPer1 gene activity rhythms were recorded in neurospheres maintained in a culture medium that induces neurogenesis but not in one that maintains the stem cell state. Although the differentiating neural stem progenitor cells of spheres were rhythmic, evidence of any mature neurons was extremely sparse. The circadian timing signal originated in undifferentiated cells within the neurosphere. This conclusion was supported by immunocytochemistry for mPER1 protein that was localized to the inner, more stem cell-like neurosphere core. To test for effects of the circadian clock on neurogenesis, media conditions were altered to induce neurospheres from BMAL1 knockout mice to differentiate. These cultures displayed unusually high differentiation into glia rather than neurons according to GFAP and NeuN expression, respectively, and very few BetaIII tubulin-positive, immature neurons were observed. The knockout neurospheres also displayed areas visibly devoid of cells and had overall higher cell death. Neurospheres from arrhythmic mice lacking two other core clock genes, Cry1 and Cry2, showed significantly reduced growth and increased astrocyte proliferation during differentiation, but they generated normal percentages of neuronal cells. Neuronal fate commitment therefore appears to be controlled through a non-clock function of BMAL1. This study provides insight into how cell autonomous circadian clocks and clock genes regulate adult neural stem cells with implications for treating neurodegenerative disorders and impaired brain functions by manipulating neurogenesis.

Synchrony and desynchrony in circadian clocks: impacts on learning and memory.

Circadian clocks evolved under conditions of environmental variation, primarily alternating light dark cycles, to enable organisms to anticipate daily environmental events and coordinate metabolic, physiological, and behavioral activities. However, modern lifestyle and advances in technology have increased the percentage of individuals working in phases misaligned with natural circadian activity rhythms. Endogenous circadian oscillators modulate alertness, the acquisition of learning, memory formation, and the recall of memory with examples of circadian modulation of memory observed across phyla from invertebrates to humans. Cognitive performance and memory are significantly diminished when occurring out of phase with natural circadian rhythms. Disruptions in circadian regulation can lead to impairment in the formation of memories and manifestation of other cognitive deficits. This review explores the types of interactions through which the circadian clock modulates cognition, highlights recent progress in identifying mechanistic interactions between the circadian system and the processes involved in memory formation, and outlines methods used to remediate circadian perturbations and reinforce circadian adaptation.

Feedback Control of Biological Rhythm in Crassulacean Acid Metabolism by CO2-Uptake Signal

The mechanism of endogenous circadian photosynthesis oscillations of plants performing crassulacean acid metabolism (CAM) is investigated in terms of a nonlinear theoretical model. Blasius et al. used throughout continuous time differential equations which adequately reflect the CAM dynamics. The model shows regular endogenous limit cycle oscillations that are stable for a wide range of temperatures in a manner that complies well with experimental data. We have recently presented four types of the feedback controllers for the phase shift of the CAM model of the single cell using the light intensity as an input. In this paper, we deal with the synchronization issues in the coupled cells with light intensity input. We show that the strong coupling causes the unsynchronization of the cells.

Circadian Rhythmicity and Light Sensitivity of the Zebrafish Brain

Traditionally, circadian clocks have been thought of as a neurobiological phenomenon. This view changed somewhat over recent years with the discovery of peripheral tissue circadian oscillators. In mammals, however, the suprachiasmatic nucleus (SCN) in the hypothalamus still retains the critical role of a central synchronizer of biological timing. Zebrafish, in contrast, have always reflected a more highly decentralized level of clock organization, as individual cells and tissues contain directly light responsive circadian pacemakers. As a consequence, clock function in the zebrafish brain has remained largely unexplored, and the precise organization of rhythmic and light-sensitive neurons within the brain is unknown. To address this issue, we used the period3 (per3)-luciferase transgenic zebrafish to confirm that multiple brain regions contain endogenous circadian oscillators that are directly light responsive. In addition, in situ hybridization revealed localised neural expression of several rhythmic and light responsive clock genes, including per3, cryptochrome1a (cry1a) and per2. Adult brain nuclei showing significant clock gene expression include the teleost equivalent of the SCN, as well as numerous hypothalamic nuclei, the periventricular grey zone (PGZ) of the optic tectum, and granular cells of the rhombencephalon. To further investigate the light sensitive properties of neurons, expression of c-fos, a marker for neuronal activity, was examined. c-fos mRNA was upregulated in response to changing light conditions in different nuclei within the zebrafish brain. Furthermore, under constant dark (DD) conditions, c-fos shows a significant circadian oscillation. Taken together, these results show that there are numerous areas of the zebrafish central nervous system, which contain deep brain photoreceptors and directly light-entrainable circadian pacemakers. However, there are also multiple brain nuclei, which possess neither, demonstrating a degree of pacemaker complexity that was not previously appreciated.

Circadian phase‐dependent effect of nitric oxide on L‐type voltage‐gated calcium channels in avian cone photoreceptors

Nitric oxide (NO) plays an important role in phase-shifting of circadian neuronal activities in the suprachiasmatic nucleus and circadian behavior activity rhythms. In the retina, NO production is increased in a light-dependent manner. While endogenous circadian oscillators in retinal photoreceptors regulate their physiological states, it is not clear whether NO also participates in the circadian regulation of photoreceptors. In this study, we demonstrate that NO is involved in the circadian phase-dependent regulation of L-type voltage-gated calcium channels (L-VGCCs). In chick cone photoreceptors, the L-VGCCα1 subunit expression and the maximal L-VGCC currents are higher at night, and both Ras-mitogen-activated protein kinase (MAPK)-extracellular signal-regulated kinase (Erk) and Ras-phosphatidylinositol 3 kinase (PI3K)-protein kinase B (Akt) are part of the circadian output pathways regulating L-VGCCs. The NO-cGMP-protein kinase G (PKG) pathway decreases L-VGCCα1 subunit expression and L-VGCC currents at night, but not during the day, and exogenous NO donor or cGMP decreases the phosphorylation of Erk and Akt at night. The protein expression of neural NO synthase (nNOS) is also under circadian control, with both nNOS and NO production being higher during the day. Taken together, NO/cGMP/PKG signaling is involved as part of the circadian output pathway to regulate L-VGCCs in cone photoreceptors. In cone photoreceptors, the protein expression of neural nitric oxide synthase (nNOS) and NO production are under circadian control. NO-cGMP-protein kinase G (PKG) signaling serves in the circadian output pathway to regulate the circadian rhythms of L-type voltage-gated calcium channels (L-VGCCs) in part through regulating the phosphorylation states of extracellular-signal-regulated kinase (Erk) and protein kinase B (Akt).

Adaptive Estimation of Biological Rhythm in Crassulacean Acid Metabolism with Critical Manifold

The mechanism of endogenous circadian photosynthesis oscillations of plants performing crassulacean acid metabolism (CAM) is investigated in terms of a nonlinear theoretical model. Blasius et al. used throughout continuous time differential equations which adequately reflect the CAM dynamics. The model shows regular endogenous limit cycle oscillations that are stable for a wide range of temperatures in a manner that complies well with experimental data. In this paper, we pay attention to the approximation of the fast modes of the CAM dynamics. Using the zero-epsilon approximation of the slow manifold, we derive the critical manifold that is defined by two algebraic nonlinear equations. The critical manifold allows us to give the algebraic estimate of the order of the tonoplast membrane. The dynamic equation of the order of the tonoplast membrane includes the nonlinear function that gives the equilibrium value of the lipid order of tonoplast functions as a hysteresis switch. We identify the nonlinear function with the measurement signals. Using the basis function expansion of the nonlinear and the critical manifold, we propose an adaptive observer to estimate the tonoplast order and the nonlinear function.