Cell division cycles and circadian oscillators in Euglena

Environment-mediated mutagenetic interference on genetic stabilization and circadian rhythm in plants

Many mortal organisms on this planet have developed the potential to merge all internal as well as external environmental cues to regulate various processes running inside organisms and in turn make them adaptive to the environment through the circadian clock. This moving rotator controls processes like activation of hormonal, metabolic, or defense pathways, initiation of flowering at an accurate period, and developmental processes in plants to ensure their stability in the environment. All these processes that are under the control of this rotating wheel can be changed either by external environmental factors or by an unpredictable phenomenon called mutation that can be generated by either physical mutagens, chemical mutagens, or by internal genetic interruption during metabolic processes, which alters normal functionality of organisms like innate immune responses, entrainment of the clock, biomass reduction, chlorophyll formation, and hormonal signaling, despite its fewer positive roles in plants like changing plant type, loss of vernalization treatment to make them survivable in different latitudes, and defense responses during stress. In addition, with mutation, overexpression of gene components sometimes supresses mutation effect and promote normal circadian genes abundance in the cell, while sometimes it affects circadian functionality by generating arrhythmicity and shows that not only mutation but overexpression also effects normal functional activities of plant. Therefore, this review mainly summarizes the role of each circadian clock genes in regulating rhythmicity, and shows that how circadian outputs are controlled by mutations as well as overexpression phenomenon.

Evolutionary conservations, changes of circadian rhythms and their effect on circadian disturbances and therapeutic approaches

The circadian rhythm is essential for the interaction of all living organisms with their environments. Several processes, such as thermoregulation, metabolism, cognition and memory, are regulated by the internal clock. Disturbances in the circadian rhythm have been shown to lead to the development of neuropsychiatric disorders, including attention-deficit hyperactivity disorder (ADHD). Interestingly, the mechanism of the circadian rhythms has been conserved in many different species, and misalignment between circadian rhythms and the environment results in evolutionary regression and lifespan reduction. This review summarises the conserved mechanism of the internal clock and its major interspecies differences. In addition, it focuses on effects the circadian rhythm disturbances, especially in cases of ADHD, and describes the possibility of recombinant proteins generated by eukaryotic expression systems as therapeutic agents as well as CRISPR/Cas9 technology as a potential tool for research and therapy. The aim is to give an overview about the evolutionary conserved mechanism as well as the changes of the circadian clock. Furthermore, current knowledge about circadian rhythm disturbances and therapeutic approaches is discussed.

Clock Gene Evolution and Functional Divergence

In considering the impact of the earth’s changing geophysical conditions during the history of life, it is surprising to learn that the earth’s rotational period may have been as short as 4 h, as recently as 1900 million years ago (or 1.9 billion years ago). The implications of such figures for the origin and evolution of clocks are considerable, and the authors speculate on how this short rotational period might have influenced the development of the “protoclock” in early microorganisms, such as the Cyanobacteria, during the geological periodsin which they arose and flourished. They then discuss the subsequent duplication of clock genes that took place around and after the Cambrian period, 543 million years ago, and its consequences. They compare the relative divergences of the canonical clock genes, which reveal the Per family to be the most rapidly evolving. In addition, the authors use a statistical test to predict which residues within the PER and CRY families may have undergone functional specialization.

Isolation, expression, and characterization of blue light receptor AUREOCHROME gene from Saccharina japonica (Laminariales, Phaeophyceae)

Photosynthetic stramenopile have chloroplasts of secondary endosymbiotic origin and are significant as aquatic primary productivity and biomass production. In marine environments, many photosynthetic stramenopiles utilize blue light to regulate growth, development, and organelle movement. Aureochrome (AUREO) is a new type blue light photoreceptor specific in photosynthetic stramenopiles. Previously, several AUREO orthologs were reported in genomes of stramenopile members, but the full-length cDNA sequences were completed only in Vaucheria frigida (Xanthophyceae), Fucus distichus (Phaeophyceae), and Ochromonas danica (Chrysophyceae). In this study, the full-length cDNA of AUREO from Saccharina japonica (designated as SjAUREO) was isolated based on homologous cloning and the rapid amplification of cDNA ends (RACE). It characterized by the full length of 1,013 bp with an open reading frame of 612 bp, which encoded a polypeptide of 203 amino acids with predicted molecular weight of 23.08 kDa and theoretical isoelectric point of 7.63. The deduced amino acid sequence of SjAUREO contained one N-terminal basic region/leucine zipper (bZIP) transcription regulation domain and a single light-, oxygen-, or voltage-sensitive (LOV) domain near the C-terminus. Homologous analysis showed that SjAUREO shared 40-92 % similarities with those of other photosynthetic stramenopiles. Phylogenetic analysis revealed close phylogenetic affinity between SjAUREO and AUREO4 of brown alga Ectocarpus siliculosus. Real-time PCR detection revealed that the SjAUREO transcription was markedly increased under BL exposure and dramatically upregulated in the 1-month juvenile sporophyte than those in the 2 and 3-month materials, which indirectly reflected the SjAUREO associated with the BL-mediated photomorphogenesis during the growth and early development of juvenile sporophytes. In vitro expression showed one distinct band existed at ∼27 kDa, and western blot detection proved that it was positive to the anti-His antibody with high specificity. Our results enriched the knowledge of AUREO properties in S. japonica and provided clues to explore the mechanisms underlying diverse physiological responses mediated by BL photoreceptors AUREO in the photosynthetic stramenopiles.

The regulation of plant growth by the circadian clock

Circadian regulated changes in growth rates have been observed in numerous plants as well as in unicellular and multicellular algae. The circadian clock regulates a multitude of factors that affect growth in plants, such as water and carbon availability and light and hormone signalling pathways. The combination of high-resolution growth rate analyses with mutant and biochemical analysis is helping us elucidate the time-dependent interactions between these factors and discover the molecular mechanisms involved. At the molecular level, growth in plants is modulated through a complex regulatory network, in which the circadian clock acts at multiple levels.

Light-dependent regulation of cell division in Ostreococcus: evidence for a major transcriptional input

Cell division often occurs at specific times of the day in animal and photosynthetic organisms. Studies in unicellular photosynthetic algae, such as Chlamydomonas or Euglena, have shown that the photoperiodic control of cell division is mediated through the circadian clock. However, the underlying mechanisms remain unknown. We have studied the molecular basis of light-dependent control of cell division in the unicellular green alga Ostreococcus. We found that cell division obeys a circadian oscillator in Ostreococcus. We provide evidence suggesting that the clock may, at least in part, regulate directly cell division independently of the metabolism. Combined microarray and quantitative real-time reverse transcription-polymerase chain reaction analysis of the main core cell cycle gene expression revealed an extensive transcriptional regulation of cell division by the photoperiod in Ostreococcus. Finally, transcription of the main core cell cycle genes, including cyclins and cyclin-dependent kinases, was shown to be under circadian control in Ostreococcus, suggesting that these genes are potential targets of the circadian clock in the control of cell division.

The cellular mechanism of circadian rhythms--a view on evidence, hypotheses and problems

A stable period length is a characteristic property of circadian oscillations. The question about whether higher frequency oscillators (0.5-8 hr) contribute to or establish the stable circadian periodicity cannot be answered at present. A sequential coupling of quantal subcycles appears possible on the basis of known "ultradian" oscillations. There is, however, no supporting evidence for such a concept. Phase response curves of the circadian clock derived from various perturbing pulses allow qualitative conclusions concerning the perturbed clock process. Deductions from computer simulations also allow conclusions about the phase of this oscillatory process. The distinction between processes (a) essential to the clock mechanism, (b) maintaining and controlling the clock (inputs) and (c) depending on the clock (outputs) on the basis of "oscillatory" and "change of psi or tau after perturbation" seems to be useful but not stringent. Protein synthesis may be an essential or input process. Oscillatory changes of this process may be due to periodic translational control or RNA-supply. Circadian changes in protein concentration and/or activity may depend on periodic synthesis, proteolysis, covalent modifications or aggregations. Specific essential proteins have not been identified conclusively. The large overlap between the group of agents and treatments that phase shift the clock and the group that induces stress proteins suggest that the latter may play a role in the controlling (input) or essential domain. The role of membranes in the clock mechanism is not clear: concepts assuming an essential function are based on circumstantial evidence. The membrane potential as well as Ca2+ may be involved in either input or essential function. Ca(2+)-calmodulin may also be important as concluded from inhibitor experiments. It is tempting to assume that a calmodulin-dependent kinase is part of a periodic protein phosphorylation process, yet it is not clear whether the periodic protein phosphorylation that has been observed is essential or is just another output process.

Ultradian photosynchronization in Tetrahymena pyriformis GLC is related to modal cell generation time: further evidence for a common timer model

This study contains the first report of the photosynchronization of Tetrahymena in the ultradian mode of cell division. Ultradian mode cultures of T. pyriformis GLC were grown at low cell titers in a nephelostat under five different ultradian photocycles and also under constant conditions of illumination. Entrainment was achieved only when the period of the synchronizer did not exceed the nearest modal generation time observed in free-running single cells. Thus, the discrete ranges for photentrainment of ultradian rhythms in Tetrahymena were restricted to modal windows for the generation times in free-run. Cell division was found to be a function of the phase of the ultradian zeitgeber cycle. The cells did not behave as if they had been forced into synchrony by physiological shock; the synchronous populations obtained by this technique behaved like the populations commonly used in circadian studies which had been phased by a cyclic variation within the tolerance range of the organism.