How Light Resets Circadian Clocks

Abstract

The daily revolutions of the earth around its axis are responsible for day and night and its annual orbit around the sun for the seasons with their fluctuations in day length. Most organisms have adapted to these diurnal and annual cycles. The strategies and mechanisms used are quite delicate and complicated.It came as a surprise that photosynthesis and many other processes are, however, additionally controlled by internal clocks. Thus, photosynthesis fluctuates not only during the daily light-dark cycle (=LD; see the List of Abbreviations and http://www.circadian.org/dictionary.html) but also when the plants are kept under LL and constant temperature (Hennessey and Field 1991). However, the period length (period for short) of this rhythmic event then deviates from exactly 24 h and is therefore called circadian (from Latin circa, about, and dies, day). If in the absence of LD and temperature cycles other 24 h time cues (also called zeitgeber, German for time giver) would control the rhythm, it should show an exact 24 h rhythm. This is not the case, demonstrating the endogenous nature of a clock that is locked to light signals (see Fig. 18.1). Spectrum of RhythmsEndogenous rhythms of organisms are not only tuned to the daily cycle of 24 h. The range of rhythms found in organisms covers ultradian (with periods of several hours to very short ones), circadian, and annual (with periods of about a year) rhythms. Other rhythms such as tidal, 14-day, and monthly ones cope with influences of the moon on the earth, mainly on the water movements of the oceans, and they are therefore found in organisms at the coasts and in the sea. Annual rhythms interact with the day-length changes during the year (see below). There are furthermore rhythms with periods covering several years. The following discussion of a "biological clock" is restricted to circadian rhythms. Even they are often not just composed of one clock type but form a "circadian system" consisting of two or more clocks with different prop- Function of Circadian ClocksThe term "clock" usually implies a time-measuring device or function. For instance, the day length (or night length) can be determined by an organism. Since day length is a function of the time of the year (long days in summer, short days in winter), it can be used to time certain events such as flowering or tuber formation of a plant or breeding of birds and mammals during the most appropriate season. These processes are denoted photoperiodism (see Chap. 19).However, a clock can also be used to set a certain temporal order. For instance, the circadian control of our sleepwake cycle ensures that we rise in the morning and fall asleep in the evening at a preferred time. Food intake and digestion are likewise controlled by this clock and gated to certain times of the day (Silver et al. 2011;Duguay and Cermakian 2009;Forsgren 1935). The circadian clock will time these events also under constant conditions. Furthermore, circadian clocks can serve as alarm clocks.

Normal vision can compensate for the loss of the circadian clock

“…Next we aimed to unravel the light-input pathways that are responsible for WT-like activity pattern of the mutants. Drosophila melanogaster has several photoreceptors: the compound eyes, the ocelli, the extraretinal eyelets and Cryptochrome (CRY; reviewed in [ 12 ]). Among these, CRY and the compound eyes have the greatest impact on locomotor activity rhythms.…”

“…Drosophila melanogaster has several & 2015 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution photoreceptors: the compound eyes, the ocelli, the extraretinal eyelets and Cryptochrome (CRY; reviewed in [12]). Among these, CRY and the compound eyes have the greatest impact on locomotor activity rhythms.…”

Differential effects of environmental and endogenous 24h rhythms within a deep-coverage spatiotemporal proteome

“…The abundance of each protein was normalised by the time course mean of the protein, and values were centred around 0 using the scale function in R before applying the heatmap.2 function from the pvclust R package 71 . Relevant proteins from different biological processes were depicted in diagrams based on the following cell categories: cell cycle 28, 29 , light signalling and clock 25, 72, 73 , photosynthesis and chloroplast biosynthesis 74 , and organelle-encoded proteins 41 . Bioinformatic analyses to identify potential candidate proteins were conducted using Standard Protein Basic Local Alignment Search Tool (BLAST) of known protein sequences from other model systems as Arabidopsis thaliana and Homo sapiens using the Ostreococcus genome ORCAE V2 57 .…”

Endogenous ceramide phosphoethanolamine modulates circadian rhythm via neural–glial coupling in Drosophila

“…Morning (M) and evening (E) cells generate M and E activity bouts during the light–dark cycles [ 22 , 23 ]. The clock genes in these neurons, including period ( per ), timeless ( tim ), Clock ( Clk ), Par domain protein 1ϵ ( pdp1ϵ ), vrille ( vri ) and cycle ( cyc ), constitute positive and negative feedback loops through transcriptional/translational activation and inhibition to govern rhythmic output, and form the molecular mechanisms underlying Drosophila circadian oscillators [ 24–26 ].…”