Circadian Rhythms in Three Unicellular Organisms. The Pecularities of the Organisms, the Evidence Brought on Rhythms and their Specific Practical Problems. Outline of Recent Hypotheses

The interplay between circadian clock and viral infections: A molecular perspective

The circadian clock influences almost every aspect of mammalian behavioral, physiological and metabolic processes. Being a hierarchical network, the circadian clock is driven by the central clock in the brain and is composed of several peripheral tissue-specific clocks. It orchestrates and synchronizes the daily oscillations of biological processes to the environment. Several pathological events are influenced by time and seasonal variations and as such implicate the clock in pathogenesis mechanisms. In context with viral infections, circadian rhythmicity is closely associated with host susceptibility, disease severity, and pharmacokinetics and efficacies of antivirals and vaccines. Leveraging the circadian molecular mechanism insights has increased our understanding of clock infection biology and proposes new avenues for viral diagnostics and therapeutics. In this chapter, we address the molecular interplay between the circadian clock and viral infections and discuss the importance of chronotherapy as a complementary approach to conventional medicines, emphasizing the significance of virus-clock studies.

Circadian and ultradian clock-controlled rhythms in unicellular microorganisms

The time structure of a biological system is at least as intricate as its spatial structure. Whereas we have detailed information about the latter, our understanding of the former is still rudimentary. As techniques for monitoring intracellular processes continuously in single cells become more refined, it becomes increasingly evident that periodic behaviour abounds in all time domains. Circadian timekeeping dominates in natural environments. Here the free-running period is about 24 h. Circadian rhythms in eukaryotes and prokaryotes allow predictive matching of intracellular states with environmental changes during the daily cycles. Unicellular organisms provide excellent systems for the study of these phenomena, which pervade all higher life forms. Intracellular timekeeping is essential. The presence of a temperature-compensated oscillator provides such a timer. The coupled outputs (epigenetic oscillations) of this ultradian clock constitute a special class of ultradian rhythm. These are undamped and endogenously driven by a device which shows biochemical properties characteristic of transcriptional and translational elements. Energy-yielding processes, protein turnover, motility and the timing of the cell-division cycle processes are all controlled by the ultradian clock. Different periods characterize different species, and this indicates a genetic determinant. Periods range from 30 min to 4 h. Mechanisms of clock control are being elucidated; it is becoming evident that many different control circuits can provide these functions.

Differentiation, growth and morphogenesis: Acetabularia as a model system

The aim of this paper is to review the present knowledge of the main aspects of differentiation of Acetabularia, a unicellular, eukaryotic organism, and to underline the multiple control pathways modulated by circadian rhythmicity. Growth and morphogenesis are sequentially programmed. Timing of cap differentiation is highly dependent on external conditions. The importance of the sequence of processes is shown by experimental disregulation. The alga is a highly polarized cell, both in morphology and in the relative concentrations of a number of the molecules it contains. Apical cap differentiation is regulated at the post-transcriptional level and could also depend in part on polyamines and on proteolytic activity. Acetabularia displays a number of circadian rhythms (CR). These rhythms form an elaborate biological time structure (also called temporal morphology, or morphology in time as opposed to morphology in space): the distribution in the 24 h cycle of the peaks and troughs of the oscillating functions. The oscillations display fixed relations both with the other functions and with external conditions (such as the transition from dark to light). Interestingly, the CR modulate Acetabularia's development, which is influenced by photoperiod; we present preliminary experiments suggesting that disruption of temporal morphology is deleterious to morphogenesis. Induction of growth and of morphogenesis are totally dependent on blue light. However, blue light receptors in plants arc probably multiple, but we present arguments suggesting that flavin-cytochrome b and the associated KHAM-sensitive molecule are present in Acetabularia plasma membrane and are involved in blue light perception. Agents interfering with different steps of signal perception and transduction show that at least some of these steps are temporally regulated. According to recent experiments from our laboratory, the existence of a redox signalling mechanism appears to be highly probable. The phytohormones (or plant regulators), auxin (indole acetic acid), abscisic acid and ethylene, exert cell-regulatory functions and are involved in Acetabularia differentiation. They also modulate at least some circadian rhythms. Finally, circadian rhythms intervene in differentiation and are proposed to have an integrative function. CONTENTS Summary 1 I. Introduction: the cell cycle and morphology of Acetabularia 2 II. Growth and cap morphogenesis: the developmental programme 3 III. Polarity 5 IV. Temporal morphology 6 V. Induction of growth and cap morphogenesis 9 VI. The plasma membrane 12 VII. Hormones: development and metabolic activity in Acetabularia 12 VIII. Phytohormones receptors and insulin receptors 15 IX. Other possible hormones 16 X. Fundamental role of CR: their intervention in modulating multiple steps in differentiation 16 XI. Conclusions and perspectives 17 Acknowledgements 17 References 17.

Tripping along the trail to the molecular mechanisms of biological clocks

Solving the mechanism of circadian clocks has become an important goal, in part because daily rhythms are running in such a wide variety of organisms, and contribute to many aspects of their well being. Systematic genetic approaches to studying 'the clock' were initiated in fruitflies more than 20 years ago as a novel means by which neural-pacemaking mysteries might be solved. Such chronogenetic investigations gained momentum when they spread to other species, and became molecular. However, the molecular studies were misleading, that is, until some elementary neuro-anatomical observations, involving the expression of a 'clock gene' in Drosophila, gave the experiments in this molecular-neurogenetic area of chronobiology a new direction. The initially neuro-descriptive studies led to the current investigations that involve negatively acting transcription factors and other clock molecules that are presumed to interact with them. In addition, new mutants and clones have been isolated in a timely manner. These mutations and molecules should permit chronogeneticists, working on a wide variety of organisms, to unravel further details of how the clock works, how environmental information finds its way to it, and how it sends information out into the organism's physiology, biochemistry and behavior.