Objective: Brown adipose tissue (BAT) burns fatty acids (FAs) to produce heat, and shows diurnal oscillation in glucose and triglyceride (TG)derived FA-uptake, peaking around wakening. Here we... Show moreObjective: Brown adipose tissue (BAT) burns fatty acids (FAs) to produce heat, and shows diurnal oscillation in glucose and triglyceride (TG)derived FA-uptake, peaking around wakening. Here we aimed to gain insight in the diurnal regulation of metabolic BAT activity. Methods: RNA-sequencing, chromatin immunoprecipitation (ChIP)-sequencing, and lipidomics analyses were performed on BAT samples of wild type C57BL/6J mice collected at 3-hour intervals throughout the day. Knockout and overexpression models were used to study causal relationships in diurnal lipid handling by BAT. Results: We identified pronounced enrichment of oscillating genes involved in extracellular lipolysis in BAT, accompanied by oscillations of FA and monoacylglycerol content. This coincided with peak lipoprotein lipase (Lpl) expression, and was predicted to be driven by peroxisome proliferator-activated receptor gamma (PPARg) activity. ChIP-sequencing for PPARg confirmed oscillation in binding of PPARg to Lpl. Of the known LPL-modulators, angiopoietin-like 4 (Angptl4) showed the largest diurnal amplitude opposite to Lpl, and both Angptl4 knockout and overexpression attenuated oscillations of LPL activity and TG-derived FA-uptake by BAT. Conclusions: Our findings highlight involvement of PPARg and a crucial role of ANGPTL4 in mediating the diurnal oscillation of TG-derived FAuptake by BAT, and imply that time of day is essential when targeting LPL activity in BAT to improve metabolic health. (c) 2022 The Author(s). Published by Elsevier GmbH. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Show less
The noradrenergic Locus Cœruleus is one of the major arousal structures involved in inducing wakefulness. While brain noradrenaline (NA) amounts display 24-h variations, the origin of NA rhythm is... Show moreThe noradrenergic Locus Cœruleus is one of the major arousal structures involved in inducing wakefulness. While brain noradrenaline (NA) amounts display 24-h variations, the origin of NA rhythm is currently unknown. In this study, we tested the hypothesis that NA rhythm could result from its rhythmic synthesis. Therefore, we investigated the 24-h expression profile of NA rate-limiting enzyme, tyrosine hydroxylase (th), in the Locus Cœruleus (LC) of the nocturnal rat and the diurnal rodent Arvicanthis, under 12 h:12 h light/dark (LD) and constant darkness (DD) conditions. In both species, th mRNA levels vary significantly over 24-h. In nocturnal rats, th mRNA profiles show a unimodal rhythm, with peak values in late day in LD, and in the middle of the subjective day in DD. In contrast, th mRNA rhythm in Arvicanthis is characterized by a bimodal profile, with higher levels at the beginning of the day and of the night in LD, and in the middle of the subjective day and night in DD. The rhythmic pattern of th expression may be dependent on a LC clock machinery. Therefore, we investigated the expression of three clock genes, namely bmal1, per1, and per2, and found that their mRNAs display significant variations between day and nighttime points in both species, but in opposite directions. These data show that NA rhythm may be related to circadian expression of th gene in both species, but differs between nocturnal and diurnal rodents. Furthermore, the phase opposition of clock gene expression in the rat compared to Arvicanthis suggests that the clock machinery might be one of the mechanisms involved in th rhythmic expression. Show less
In modern society, circadian rhythms and sleep are often disturbed, which may negatively affect health. This thesis examines these associations and focuses on the basic functioning of sleep and the... Show moreIn modern society, circadian rhythms and sleep are often disturbed, which may negatively affect health. This thesis examines these associations and focuses on the basic functioning of sleep and the circadian system in mice and in humans. Circadian rhythms are orchestrated by ~20,000 neurons in the central clock in the suprachiasmatic nuclei (SCN) in the brain. In mice, a complete abolishment of central clock-driven rhythms resulted in obesity and severe hepatic insulin resistance. An attenuation of rhythms resulted in decreased muscle strength, osteoporosis-like bone changes and transient changes in the immune system. In humans, short sleeping obese individuals with a preference for evening activities ("evening chronotypes") had increased cardiovascular risk factors. Their neurocognitive function was often impaired and could be improved with sleep extension. Insufficient sleep was also associated with an increased risk for osteopenia and sarcopenia. Taken together, disrupted circadian rhythms and insufficient sleep associate with a spectrum of unfavorable health outcomes. Studies described in the thesis provide insight in potential strategies to improve rhythms and sleep: by appropriately timed behavior (active behavior during the active phase; rest during the rest phase), light exposure (light during the subjective day; darkness at night) as well as caffeine intake. Show less
Cardiovascular diseases (CVD) are the leading cause of death worldwide, and disturbances in day-night rhythms have recently been implicated as a novel risk factor for CVD. We investigated the... Show moreCardiovascular diseases (CVD) are the leading cause of death worldwide, and disturbances in day-night rhythms have recently been implicated as a novel risk factor for CVD. We investigated the effects of modulating circadian rhythms on energy metabolism using animal models and by studying plasma metaoblites and lipids in humans. Using animal studies we observed that brown adipose tissue (BAT) is strongly regulated by the biological clock, possibly via circadian glucocorticoid rhythms, and attenuated BAT activity through prolonged light exposure increases adiposity. Research focusing on the rhythm in human BAT, and regulation thereof, is necessary to confirm the translational value of our findings. We also observed that mistimed light exposure enhances atherosclerosis development, which may provide a mechanistic link between the known association between shift work and CVD. We anticipate that living according to the natural circadian rhythms presumably contributes to cardiometabolic health. Since disturbances in day-night rhythms are inevitable in modern society, in the future we may advise individuals at risk for development of CVD refrain from shift work and short sleep duration. In addition, data in this thesis may be useful to design strategies to avoid the disadvantageous metabolic effects of shift work. Show less
Many organisms have developed an internal clock to cope with the daily and seasonal cycles in the environment. In mammals, suprachiasmatic nuclei (SCN) of the hypothalamus control circadian rhythms... Show moreMany organisms have developed an internal clock to cope with the daily and seasonal cycles in the environment. In mammals, suprachiasmatic nuclei (SCN) of the hypothalamus control circadian rhythms in behavior and physiology. Evidence links the proper function of circadian clock to mental and physical health. Aging disturbs the accurate function of the SCN and impairs many rhythms such as sleep-wake cycle. Hence improvement of clock function can aid healthy aging. In chapters 3 and 4 I show the ensemble output of the SCN neuronal network is more robust than individual cells__ output suggesting a compensatory role of the network in aging. Seasonal changes affect the physiology and reproduction success of many organisms. The SCN encodes for day-length by adjusting the pattern of its electrical activity rhythm.. In chapters 5 and 6 I reveal that plasticity in interneuronal and cell-intrinsic functions in the SCN helps the organism to adjust to yearly natural changes in photoperiod. These results imply that extensive artificial light in modern society may alter neurotransmitters action in the SCN. A better understanding of SCN network function and cellular properties facilitate alleviation of modern life-related diseases caused by circadian disturbances and aging. Show less
Throughout the animal kingdom, species have evolved an internal time-keeping system, referred to as a 'biological clock'. This internal clock allows anticipation to profound, but largely... Show moreThroughout the animal kingdom, species have evolved an internal time-keeping system, referred to as a 'biological clock'. This internal clock allows anticipation to profound, but largely predictable, environmental day-night changes on earth. The biological clock drives 24h-rhythms in physiology and behaviour, and aligns the endogenous rhythms to the external solar day in a close temporal relationship. Being in synchrony with the environmental light-dark cycle allows the organism to cope adequately with daily changes in food availability, ambient temperature, the presence of predators, mating opportunities and/or social interactions. Additionally, the biological timing system has a major function in the regulation of seasonal rhythms, for instance in reproduction, animal migration and fur change. In order to be of functional use, the biological clock needs to be adjusted to the 24h cycle of the environment on a daily basis. The most important stimulus to regulate the synchronisation is light, which is detected via specialized eye pigments. Apart from light (photic stimulus), the biological clock is also responsive to non-photic stimuli, such as behavioural activity and pharmacological agents. The research described in this thesis examines how photic and non-photic cues modulate the activity of the biological clock. Show less
The biological clock in mammals is located in the suprachiasmatic nucleus (SCN) and regulates daily and seasonal rhythms. The research presented in this thesis consists of behavioral and... Show moreThe biological clock in mammals is located in the suprachiasmatic nucleus (SCN) and regulates daily and seasonal rhythms. The research presented in this thesis consists of behavioral and electrophysiological experiments to investigate the SCN as a pacemaker of daily and seasonal rhythms. We investigated the activity of single neurons and small groups of neurons in the SCN and their role in photoperiodic adaptations. We found that single cells in the SCN do not code for the length of day, but that photoperiodic encoding is a property of the SCN neuronal network. Show less
De draaiing van de aarde zorgt voor dagelijkse veranderingen in licht en donker. Om op deze dagelijkse veranderingen te kunnen anticiperen hebben alle organismen een ingebouwd mechanisme, namelijk... Show moreDe draaiing van de aarde zorgt voor dagelijkse veranderingen in licht en donker. Om op deze dagelijkse veranderingen te kunnen anticiperen hebben alle organismen een ingebouwd mechanisme, namelijk de biologische klok. De biologische klok bevindt zich in de suprachiasmatische nucleus (SCN) en bestaat uit ongeveer 20 000 neuronen. De SCN ontvangt lichtinformatie uit de omgeving via lichtgevoelige cellen in het netvlies van het oog. Deze cellen geven de informatie door aan de SCN. In het netvlies zijn drie typen lichtgevoelige cellen aanwezig, de staafjes, de kegeltjes en retinale ganglion cellen met melanopsine. In dit proefschrift is de relatieve bijdrage van de verschillende lichtgevoelige cellen op de verwerking van lichtinformatie door de SCN onderzocht. Het onderzoek heeft zich met name gericht op de specifieke bijdragen van de kegeltjes. Ook is onderzocht welke neurotransmitters van invloed zijn op de lichtgevoeligheid van de SCN. Samenvattend laten de resultaten zien dat de hoeveelheid lichtinformatie die de SCN bereikt kan worden bepaald door zowel staafjes, kegeltjes als melanopsine en dat dit afhankelijk is van golflengte, duur en intensiteit van het licht. Daarnaast kan de hoeveelheid lichtinformatie die de SCN bereikt ook afhankelijk zijn van de neurotransmitters adenosine en VIP. Show less
