Metabolic oscillations on the circadian time scale in Drosophila cells lacking clock genes

Circadian rhythms are cell‐autonomous biological oscillations with a period of about 24 h. Current models propose that transcriptional feedback loops are the primary mechanism for the generation of circadian oscillations. Within this framework, Drosophila S2 cells are regarded as “non‐rhythmic” cells, as they do not express several canonical circadian components. Using an unbiased multi‐omics approach, we made the surprising discovery that Drosophila S2 cells do in fact display widespread daily rhythms. Transcriptomics and proteomics analyses revealed that hundreds of genes and their products, and in particular metabolic enzymes, are rhythmically expressed in a 24‐h cycle. Metabolomics analyses extended these findings and demonstrate that central carbon metabolism and amino acid metabolism are core metabolic pathways driven by protein rhythms. We thus demonstrate that 24‐h metabolic oscillations, coupled to gene and protein cycles, take place in nucleated cells without the contribution of any known circadian regulators. These results therefore suggest a reconsideration of existing models of the clockwork in Drosophila and other eukaryotic systems.Mol Syst Biol. (2018) 14: e837

Mechanisms and physiological function of daily haemoglobin oxidation rhythms in red blood cells

Cellular circadian rhythms confer temporal organisation upon physiology that is fundamental to human health. Rhythms are present in red blood cells (RBCs), the most abundant cell type in the body, but their physiological function is poorly understood. Here, we present a novel biochemical assay for haemoglobin (Hb) oxidation status which relies on a redox-sensitive covalent haem-Hb linkage that forms during SDS-mediated cell lysis. Formation of this linkage is lowest when ferrous Hb is oxidised, in the form of ferric metHb. Daily haemoglobin oxidation rhythms are observed in mouse and human RBCs cultured in vitro, or taken from humans in vivo, and are unaffected by mutations that affect circadian rhythms in nucleated cells. These rhythms correlate with daily rhythms in core body temperature, with temperature lowest when metHb levels are highest. Raising metHb levels with dietary sodium nitrite can further decrease daytime core body temperature in mice via nitric oxide (NO) signalling. These results extend our molecular understanding of RBC circadian rhythms and suggest they contribute to the regulation of body temperature

Daily magnesium fluxes regulate cellular timekeeping and energy balance

Circadian clocks are fundamental to the biology of most eukaryotes, coordinating behaviour and physiology to resonate with the environmental cycle of day and night through complex networks of clock-controlled genes1, 2, 3. A fundamental knowledge gap exists, however, between circadian gene expression cycles and the biochemical mechanisms that ultimately facilitate circadian regulation of cell biology4, 5. Here we report circadian rhythms in the intracellular concentration of magnesium ions, [Mg2+]i, which act as a cell-autonomous timekeeping component to determine key clock properties both in a human cell line and in a unicellular alga that diverged from each other more than 1 billion years ago6. Given the essential role of Mg2+ as a cofactor for ATP, a functional consequence of [Mg2+]i oscillations is dynamic regulation of cellular energy expenditure over the daily cycle. Mechanistically, we find that these rhythms provide bilateral feedback linking rhythmic metabolism to clock-controlled gene expression. The global regulation of nucleotide triphosphate turnover by intracellular Mg2+ availability has potential to impact upon many of the cell’s more than 600 MgATP-dependent enzymes7 and every cellular system where MgNTP hydrolysis becomes rate limiting. Indeed, we find that circadian control of translation by mTOR8 is regulated through [Mg2+]i oscillations. It will now be important to identify which additional biological processes are subject to this form of regulation in tissues of multicellular organisms such as plants and humans, in the context of health and disease

The biological clock and the molecular basis of lysosomal storage diseases

The lysosomal storage disorders encompass nearly fifty diseases provoked by lack or deficiency of enzymes essential for the breakdown of complex molecules and hallmarked by accumulation in the lysosomes of metabolic residues. Histochemistry and cytochemistry studies evidenced patterns of circadian variation of the lysosomal marker enzymes, suggesting that lysosomal function oscillates rhythmically during the 24-h day. The circadian rhythmicity of cellular processes is driven by the biological clock ticking through transcriptional/translational feedback loops hardwired by circadian genes and proteins. Malfunction of the molecular clockwork may provoke severe deregulation of downstream gene expression regulating a complex array of cellular functions leading to anatomical and functional changes. In this review we highlight that all the genes mutated in lysosomal storage disorders encode circadian transcripts suggesting a direct participation of the biological clock in the pathophysiological mechanisms underlying cellular and tissue derangements hallmarking these hereditary diseases. The 24-h periodicity of oscillation of gene transcription and translation could lead in physiological conditions to circadian rhythmicity of fluctuation of enzyme levels and activity, so that gene transfer could be envisaged to reproduce 24-h periodicity of variation of enzymatic dynamics and circadian rhythmicity could have an impact on the schedule of enzyme replacement therapy. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this chapter (doi:10.1007/8904_2014_354) contains supplementary material, which is available to authorized users