Mouse genotypes drive the liver and adrenal gland clocks

Circadian characteristics of term and preterm labors

The labor is a physiological event considered to have its own circadian (diurnal) rhythm, but some of the data remain conflicting, especially for preterm births. In this retrospective study, we analyzed the circadian trends of labor onset times in the Slovenian birth cohort from 1990 to 2018 with over 550,000 cases of singleton births. The number of term and preterm labor onsets was calculated for each hour in a day and circadian trends were evaluated for each of the study groups by modeling with a generalized Poisson distribution linked with the cosinor regression model using logarithmic link function. The induced labors were taken as the control group since the timing of labor depends mostly on the working schedule of personnel and not on the intrinsic rhythmic characteristics. For induced labors, the main peak in the number of labor cases was observed in the late morning hours (around 10 AM) for all gestational ages. The prominence of this peak becomes smaller in spontaneous premature labors with gradually disrupting rhythmicity in very preterm and extremely preterm cases. Labors starting with spontaneous contractions peak between 6 and 7 AM and lose the rhythmicity at 35 weeks of gestation while labors starting with a spontaneous rupture of membranes peak at 1 AM and lose the rhythmicity at 31 weeks of gestation, suggesting differences in underlying mechanisms. According to our knowledge, this is the first study that shows differences of circadian trends between different types of spontaneous labors, i.e., labors initiated with contraction and labors initiated with a spontaneous rupture of membranes. Moreover, the obtained results represent evidence of gradual disruption of rhythmicity from mild to extreme prematurity.

Is obstructive sleep apnea a circadian rhythm disorder?

Obstructive sleep apnea is the most common sleep-related breathing disorder worldwide and remains underdiagnosed. Its multiple associated comorbidities contribute to a decreased quality of life and work performance as well as an increased risk of death. Standard treatment seems to have limited effects on cardiovascular and metabolic aspects of the disease, emphasising the need for early diagnosis and additional therapeutic approaches. Recent evidence suggests that the dysregulation of circadian rhythms, processes with endogenous rhythmicity that are adjusted to the environment through various cues, is involved in the pathogenesis of comorbidities. In patients with obstructive sleep apnea, altered circadian gene expression patterns have been demonstrated. Obstructive respiratory events may promote circadian dysregulation through the effects of sleep disturbance and intermittent hypoxia, with subsequent inflammation and disruption of neural and hormonal homeostasis. In this review, current knowledge on obstructive sleep apnea, circadian rhythm regulation, and circadian rhythm sleep disorders is summarised. Studies that connect obstructive sleep apnea to circadian rhythm abnormalities are critically evaluated. Furthermore, pathogenetic mechanisms that may underlie this association, most notably hypoxia signalling, are presented. A bidirectional relationship between obstructive sleep apnea and circadian rhythm dysregulation is proposed. Approaching obstructive sleep apnea as a circadian rhythm disorder may prove beneficial for the development of new, personalised diagnostic, therapeutic and prognostic tools. However, further studies are needed before the clinical approach to obstructive sleep apnea includes targeting the circadian system.

Differences in Diurnal Rhythm of Rod Outer Segment Renewal between 129T2/SvEmsJ and C57BL/6J Mice

In all mammalian species tested to date, rod photoreceptor outer segment renewal is a circadian process synchronized by light with a burst of outer segment fragment (POS) shedding and POS phagocytosis by the adjacent retinal pigment epithelium (RPE) every morning at light onset. Recent reports show that RPE phagocytosis also increases shortly after dark onset in C57BL/6 (C57) mice. Genetic differences between C57 mice and 129T2/SvEmsJ (129) mice may affect regulation of outer segment renewal. Here, we used quantitative methods to directly compare outer segment renewal in C57 and 129 mouse retina. Quantification of rhodopsin-positive phagosomes in the RPE showed that in 129 mice, rod POS phagocytosis after light onset was significantly increased compared to C57 mice, but that 129 mice did not show a second peak after dark onset. Cone POS phagosome content of RPE cells did not differ by mouse strain with higher phagosome numbers after light than after dark. We further quantified externalization of the "eat me" signal phosphatidylserine by outer segment tips, which precedes POS phagocytosis. Live imaging of retina ex vivo showed that rod outer segments extended PS exposure in both strains but that frequency of outer segments with exposed PS after light onset was lower in C57 than in 129 retina. Taken together, 129 mice lacked a burst of rod outer segment renewal after dark onset. The increases in rod outer segment renewal after light and after dark onset in C57 mice were attenuated compared to the peak after light onset in 129 mice, suggesting an impairment in rhythmicity in C57 mice.

Coupling the Circadian Clock to Homeostasis: The Role of Period in Timing Physiology

A plethora of physiological processes show stable and synchronized daily oscillations that are either driven or modulated by biological clocks. A circadian pacemaker located in the suprachiasmatic nucleus of the ventral hypothalamus coordinates 24-hour oscillations of central and peripheral physiology with the environment. The circadian clockwork involved in driving rhythmic physiology is composed of various clock genes that are interlocked via a complex feedback loop to generate precise yet plastic oscillations of ∼24 hours. This review focuses on the specific role of the core clockwork gene Period1 and its paralogs on intra-oscillator and extra-oscillator functions, including, but not limited to, hippocampus-dependent processes, cardiovascular function, appetite control, as well as glucose and lipid homeostasis. Alterations in Period gene function have been implicated in a wide range of physical and mental disorders. At the same time, a variety of conditions including metabolic disorders also impact clock gene expression, resulting in circadian disruptions, which in turn often exacerbates the disease state.