Lipidomics Analyses Reveal Temporal and Spatial Lipid Organization and Uncover Daily Oscillations in Intracellular Organelles.

Cells have evolved mechanisms to handle incompatible processes through temporal organization by circadian clocks and by spatial compartmentalization within organelles defined by lipid bilayers. Recent advances in lipidomics have led to identification of plentiful lipid species, yet our knowledge regarding their spatiotemporal organization is lagging behind. In this study, we quantitatively characterized the nuclear and mitochondrial lipidome in mouse liver throughout the day, upon different feeding regimens, and in clock-disrupted mice. Our analyses revealed potential connections between lipid species within and between lipid classes. Remarkably, we uncovered diurnal oscillations in lipid accumulation in the nucleus and mitochondria. These oscillations exhibited opposite phases and readily responded to feeding time. Furthermore, we found that the circadian clock coordinates the phase relation between the organelles. In summary, our study provides temporal and spatial depiction of lipid organization and reveals the presence and coordination of diurnal rhythmicity in intracellular organelles.

Clock proteins and training modify exercise capacity in a daytime-dependent manner.

Exercise and circadian biology are closely intertwined with physiology and metabolism, yet the functional interaction between circadian clocks and exercise capacity is only partially characterized. Here, we tested different clock mutant mouse models to examine the effect of the circadian clock and clock proteins, namely PERIODs and BMAL1, on exercise capacity. We found that daytime variance in endurance exercise capacity is circadian clock controlled. Unlike wild-type mice, which outperform in the late compared with the early part of their active phase, PERIODs- and BMAL1-null mice do not show daytime variance in exercise capacity. It appears that BMAL1 impairs and PERIODs enhance exercise capacity in a daytime-dependent manner. An analysis of liver and muscle glycogen stores as well as muscle lipid utilization suggested that these daytime effects mostly relate to liver glycogen levels and correspond to the animals' feeding behavior. Furthermore, given that exercise capacity responds to training, we tested the effect of training at different times of the day and found that training in the late compared with the early part of the active phase improves exercise performance. Overall, our findings suggest that clock proteins shape exercise capacity in a daytime-dependent manner through changes in liver glycogen levels, likely due to their effect on animals' feeding behavior.

Physiological and Molecular Dissection of Daily Variance in Exercise Capacity.

Physical performance relies on the concerted action of myriad responses, many of which are under circadian clock control. Little is known, however, regarding the time-dependent effect on exercise performance at the molecular level. We found that both mice and humans exhibit daytime variance in exercise capacity between the early and late part of their active phase. The daytime variance in mice was dependent on exercise intensity and relied on the circadian clock proteins PER1/2. High-throughput gene expression and metabolic profiling of skeletal muscle revealed metabolic pathways that are differently activated upon exercise in a daytime-dependent manner. Remarkably, we discovered that ZMP, an endogenous AMPK activator, is induced by exercise in a time-dependent manner to regulate key steps in glycolytic and fatty acid oxidation pathways and potentially enhance exercise capacity. Overall, we propose that time of day is a major modifier of exercise capacity and associated metabolic pathways.

The Liver in the Eyes of a Chronobiologist.

Our understanding of the molecular underpinnings of the mammalian circadian oscillator and its tight connection to physiology has progressed tremendously during the past decades. The liver is considered the prototypic experimental model tissue for circadian research in peripheral organs. Studies on liver clocks have been highly productive and yielded information about widely different aspects of circadian biology. The liver, as one of the largest organs in the body, has often been used for the identification of core clock and auxiliary clock components, for example, by biochemical purifications. Because the liver is also a major metabolic hub, studies addressing the interplay between circadian clocks and metabolism have been insightful. In addition, the use of liver-specific loss-of-function models for clock components highlighted not only specific physiological roles of the hepatic clock but also its interplay with systemic cues and oscillators in other organs. Recently, technological advances in omics approaches have been successfully applied on the liver, providing a comprehensive depiction of pervasive circadian control of gene expression and protein and metabolite accumulation. In this review, we chose to illuminate specific examples that demonstrate how different experimental approaches--namely, biochemical, metabolic, genetic, and omics methodologies--have advanced our knowledge regarding circadian liver biology and chronobiology in general.

Circadian Clock Control by Polyamine Levels through a Mechanism that Declines with Age.

Polyamines are essential polycations present in all living cells. Polyamine levels are maintained from the diet and de novo synthesis, and their decline with age is associated with various pathologies. Here we show that polyamine levels oscillate in a daily manner. Both clock- and feeding-dependent mechanisms regulate the daily accumulation of key enzymes in polyamine biosynthesis through rhythmic binding of BMAL1:CLOCK to conserved DNA elements. In turn, polyamines control the circadian period in cultured cells and animals by regulating the interaction between the core clock repressors PER2 and CRY1. Importantly, we found that the decline in polyamine levels with age in mice is associated with a longer circadian period that can be reversed upon polyamine supplementation in the diet. Our findings suggest a crosstalk between circadian clocks and polyamine biosynthesis and open new possibilities for nutritional interventions against the decay in clock's function with age.

Circadian clocks and feeding time regulate the oscillations and levels of hepatic triglycerides.

Circadian clocks play a major role in orchestrating daily physiology, and their disruption can evoke metabolic diseases such as fatty liver and obesity. To study the role of circadian clocks in lipid homeostasis, we performed an extensive lipidomic analysis of liver tissues from wild-type and clock-disrupted mice either fed ad libitum or night fed. To our surprise, a similar fraction of lipids (∼17%) oscillated in both mouse strains, most notably triglycerides, but with completely different phases. Moreover, several master lipid regulators (e.g., PPARα) and enzymes involved in triglyceride metabolism retained their circadian expression in clock-disrupted mice. Nighttime restricted feeding shifted the phase of triglyceride accumulation and resulted in ∼50% decrease in hepatic triglyceride levels in wild-type mice. Our findings suggest that circadian clocks and feeding time dictate the phase and levels of hepatic triglyceride accumulation; however, oscillations in triglycerides can persist in the absence of a functional clock.