Final Report Summary - METACYCLES (Uncovering metabolic cycles in mammals and dissecting their interplay with circadian clocks)
Circadian clocks play a major role in orchestrating our daily metabolism, and their disruption can lead to metabolic diseases such as diabetes and obesity. Concomitantly, clocks are tightly coupled to cellular metabolism and respond to nutritional cues (e.g. feeding times), (reviewed in Asher & Schibler, Cell Metab. 2011; Asher & Corsi, Cell 2015; Reinke & Asher, Gastroenterology 2016). Thus, circadian clocks are positioned at the intersection of nutritional inputs and temporal organization of metabolism, however the underlying molecular mechanisms and the potential medical implications are only now beginning to emerge.
Over the past 5 years my lab has been investigating the intricate interplay between circadian clocks, nutrition, and metabolism. We have focused on two principal aspects: (i) the daily control of cellular metabolism by circadian clocks and by nutrition, and (ii) the effects of nutrition and metabolism on circadian clocks. Our central objective is to understand how these processes are reciprocally interrelated at the molecular level. To this end, we centered our efforts on several distinct metabolites (e.g. lipids, polyamines) and specific metabolic pathways (e.g. those of mitochondrial function, oxygen sensing) at various levels ranging from whole animals, through organs and cells to intracellular organelles. Our unique approach has led to significant findings, as summarized below.
By applying lipidomics methodologies to study the roles of circadian clocks and feeding times in lipid homeostasis (Adamovich et al., Cell Metab. 2014), we uncovered daily oscillations in accumulation of lipids, and specifically of triglycerides, in mouse liver. Night-time feeding led to a dramatic decrease (~50%) in hepatic triglyceride levels. Remarkably, we found that a similar fraction of lipids oscillates in wild-type and clock-disrupted mice, but with completely opposite phases. We concluded that circadian clocks and feeding times dictate the phase and level of hepatic lipid accumulation, however lipid oscillations persist in the absence of a functional clock.
The mammalian circadian system consists of a master clock in the brain that synchronizes subsidiary oscillators in peripheral tissues. The presence of cell autonomous oscillations in almost every cell raised the question of whether circadian oscillations occur also in intracellular organelles. Using lipids as ‘time tellers’, i.e. indicators for endogenous time, we identified daily oscillations in nuclei and mitochondria. These oscillations exhibited opposite phases and readily responded to feeding time. We further found that the circadian clock coordinates the phase relationship between the organelles. Collectively, this is the first comprehensive depiction of diurnal oscillations in intracellular organelles and their interrelationships (Aviram et al., Mol Cell 2016).
We recently identified a novel metabolic feedback loop that couples circadian clocks with polyamine biosynthesis (Zwighaft et al., Cell Metab. 2015). We found that clock- and feeding-dependent mechanisms in mice regulate the rhythmic accumulation of key enzymes in polyamine biosynthesis and polyamine levels. In turn, polyamines control the circadian period in cultured cells and in vivo by modulating protein interactions within the core clock circuitry. Importantly, we showed that the decline in polyamine levels with age in mice is associated with a longer circadian period, which can be reversed by dietary supplementation of polyamines. To the best of my knowledge, this was the first demonstration that the circadian clock can be ‘rejuvenated’ merely by nutritional intervention.
Moreover, we examined the hypothesis that the functions of intracellular organelles in general, and of mitochondria in particular, are under circadian control. Using proteomics methodologies, we identified extensive oscillations in mitochondrial protein levels, which peaked predominantly during the early light phase. Several rate-limiting mitochondrial enzymes accumulated daily and were circadian clock dependent. Concurrently, we uncovered daily oscillations in mitochondrial respiration that were substrate specific and peaked at different times of the day. We proposed that circadian clocks regulate the daily utilization of different nutrients by mitochondria, and thus optimize mitochondrial function to accommodate daily changes in energy supply and demand (Neufeld et al., P.N.A.S. 2016).
Lastly, we investigated the role of oxygen as a resetting cue for circadian clocks. We detected daily rhythms in tissue oxygenation in freely moving animals. Oxygen cycles, within the physiological range, were sufficient to synchronize cellular clocks in HIF1-dependent manner. Furthermore, oxygen modulation accelerated the adaptation of mice to the new time in a jet lag protocol in HIF1-dependent fashion. We concluded that oxygen, via HIF1 activation, is a central resetting cue for circadian clocks and proposed oxygen modulation as therapy for jet lag (Adamovich et al., Cell Metab. 2016).
In summary, our studies have delineated different facets of circadian metabolism and dissected their circadian clock and nutritional control at levels ranging from the whole organism to intracellular organelles. Our future goals are to extend and deepen our understanding of the molecular mechanisms and metabolites that connect circadian clocks to nutrition and metabolism.
Over the past 5 years my lab has been investigating the intricate interplay between circadian clocks, nutrition, and metabolism. We have focused on two principal aspects: (i) the daily control of cellular metabolism by circadian clocks and by nutrition, and (ii) the effects of nutrition and metabolism on circadian clocks. Our central objective is to understand how these processes are reciprocally interrelated at the molecular level. To this end, we centered our efforts on several distinct metabolites (e.g. lipids, polyamines) and specific metabolic pathways (e.g. those of mitochondrial function, oxygen sensing) at various levels ranging from whole animals, through organs and cells to intracellular organelles. Our unique approach has led to significant findings, as summarized below.
By applying lipidomics methodologies to study the roles of circadian clocks and feeding times in lipid homeostasis (Adamovich et al., Cell Metab. 2014), we uncovered daily oscillations in accumulation of lipids, and specifically of triglycerides, in mouse liver. Night-time feeding led to a dramatic decrease (~50%) in hepatic triglyceride levels. Remarkably, we found that a similar fraction of lipids oscillates in wild-type and clock-disrupted mice, but with completely opposite phases. We concluded that circadian clocks and feeding times dictate the phase and level of hepatic lipid accumulation, however lipid oscillations persist in the absence of a functional clock.
The mammalian circadian system consists of a master clock in the brain that synchronizes subsidiary oscillators in peripheral tissues. The presence of cell autonomous oscillations in almost every cell raised the question of whether circadian oscillations occur also in intracellular organelles. Using lipids as ‘time tellers’, i.e. indicators for endogenous time, we identified daily oscillations in nuclei and mitochondria. These oscillations exhibited opposite phases and readily responded to feeding time. We further found that the circadian clock coordinates the phase relationship between the organelles. Collectively, this is the first comprehensive depiction of diurnal oscillations in intracellular organelles and their interrelationships (Aviram et al., Mol Cell 2016).
We recently identified a novel metabolic feedback loop that couples circadian clocks with polyamine biosynthesis (Zwighaft et al., Cell Metab. 2015). We found that clock- and feeding-dependent mechanisms in mice regulate the rhythmic accumulation of key enzymes in polyamine biosynthesis and polyamine levels. In turn, polyamines control the circadian period in cultured cells and in vivo by modulating protein interactions within the core clock circuitry. Importantly, we showed that the decline in polyamine levels with age in mice is associated with a longer circadian period, which can be reversed by dietary supplementation of polyamines. To the best of my knowledge, this was the first demonstration that the circadian clock can be ‘rejuvenated’ merely by nutritional intervention.
Moreover, we examined the hypothesis that the functions of intracellular organelles in general, and of mitochondria in particular, are under circadian control. Using proteomics methodologies, we identified extensive oscillations in mitochondrial protein levels, which peaked predominantly during the early light phase. Several rate-limiting mitochondrial enzymes accumulated daily and were circadian clock dependent. Concurrently, we uncovered daily oscillations in mitochondrial respiration that were substrate specific and peaked at different times of the day. We proposed that circadian clocks regulate the daily utilization of different nutrients by mitochondria, and thus optimize mitochondrial function to accommodate daily changes in energy supply and demand (Neufeld et al., P.N.A.S. 2016).
Lastly, we investigated the role of oxygen as a resetting cue for circadian clocks. We detected daily rhythms in tissue oxygenation in freely moving animals. Oxygen cycles, within the physiological range, were sufficient to synchronize cellular clocks in HIF1-dependent manner. Furthermore, oxygen modulation accelerated the adaptation of mice to the new time in a jet lag protocol in HIF1-dependent fashion. We concluded that oxygen, via HIF1 activation, is a central resetting cue for circadian clocks and proposed oxygen modulation as therapy for jet lag (Adamovich et al., Cell Metab. 2016).
In summary, our studies have delineated different facets of circadian metabolism and dissected their circadian clock and nutritional control at levels ranging from the whole organism to intracellular organelles. Our future goals are to extend and deepen our understanding of the molecular mechanisms and metabolites that connect circadian clocks to nutrition and metabolism.