As a key physiological process of the central nervous system, sleep plays a critical role in regulating metabolic homeostasis, immune function, memory consolidation, and so on. Disruption of circadian rhythm and sleep induced by changed light-dark cycles, such as shift work and light pollution, has been linked to a variety of adverse outcomes [33–35]. Therefore, exploring the mechanism of sleep disorders under changed light exposure and identifying potential targets for clinical intervention are of great scientific importance and practical value. Our research took advantage of the unique light-dark cycles in Antarctica to investigate how changes in the photoperiod affect the molecular pathways that regulate the circadian rhythm and sleep among expeditioners. In addition to supporting previous findings, we have, for the first time, discovered that changed light-dark cycle induced alteration of factors regulating neural function, extracellular matrix (ECM) homeostasis, and glucose metabolism, which may contribute to delayed circadian rhythms and sleep disorders. Importantly, we proposed a new candidate gene, SHISA8, as a key regulator of both sleep-wake rhythm and sleep quality (Fig. 6). Hopefully, this study may provide novel targets for the prevention and treatment of sleep disorders under extreme environments with similar stress factors to Antarctica, for example, space travel.
The amount of light required for circadian rhythm entrainment depends on multiple factors. In a controlled environment without scheduled sleep and activity, a 12:12 light-dark cycle with light intensity between 200 and 1000 lux was found to be necessary to maintain circadian synchrony [36]. However, during the austral winter, the artificial illumination in the Zhongshan Station was usually below 150 lux, which may in turn induce circadian misalignment and sleep problems. In this study, we observed a simultaneous delay in bedtime, get up time, and acrophase of activity by 1.24 hours, 1.98 hours, and 1.95 hours, respectively, during the austral winter. Our previous study at the Zhongshan Station also reported that during polar nights, the acrophase of urine 6-sulphatoxymelatonin rhythm was delayed by 2.52 hours [8]. Meanwhile, compared with departure, the sleep onset and offset time were 1.46h and 1.80h later, respectively [8], which is consistent with this study. In addition to the delayed circadian phase, we also observed a decrease in sleep efficiency by 4.49% on average, primarily due to an increase in awakening time after sleep onset. In a longitude study among winter-over expeditioners at German Stations Neumayer II and III from 2008 to 2014, the sleep efficiency of men dropped by 5.2%, similar to our findings [14]. Another research explored the sleep pattern of the Indian expedition team at Maitri Station and reported a greater decrease in sleep efficiency during the mid-winter compared to pre-departure, from 97.7–88.0% [12]. Therefore, the lack of light exposure during the austral winter will impair the sleep quality of expeditioners regardless of the social-cultural background, albeit to different extents.
Multiple interacting neurotransmitters in the central nervous system (CNS) contribute to the transition between human wakefulness and sleep. Specifically, we found both the glutamatergic and dopaminergic systems with wake-promoting effects might be associated with the delayed circadian phase and impaired sleep of expeditioners.
Glutamate is one of the most important and abundant excitatory neurotransmitters in the mammalian CNS [37, 38]. Abnormal increase of the glutamate and glutamine levels in the thalamus may lead to shortened sleep and wakefulness [39], indicating the crucial role of the glutamatergic system in regulating the sleep-wake cycle. In this study, SHISA8, a member of the Shisa family, has been identified as a novel key regulator inducing awakenings after sleep onset. According to previous reports, SHISAs modulate glutaminergic neurotransmission through regulating the surface trafficking of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR), a synaptic glutamate receptor, as well as influence the AMPAR-mediated current amplitude [27]. This may partially explain the arousal effect of SHISA8. Our study suggested that SLC2A1 might be an upstream regulator of SHISA8. SLC2A1 is a glucose transporter primarily located in the blood-brain barrier [40], whose nonsense or missense mutations induced daytime sleepiness and insomnia [41, 42]. Although a recent study in a small cohort suggested increased glutamine level in cerebrospinal fluid of SLC2A1 deficiency patients [43], whether SLC2A1 affects sleep-wake activity through the glutaminergic system or SHISA8 remains largely unexplored.
Dopamine is another major neurotransmitter in the CNS with wake-promoting effects. Medications such as amphetamines, which boost the dopaminergic system, have been widely used for the treatment of narcolepsy [44]. Conversely, dopamine D2 receptor knockout resulted in reduced arousal levels in mice [45]. DOPAC is a neuronal metabolite of dopamine, and we found serum levels of DOPAC among winter-over expeditioners significantly increased during the austral winter and were significantly correlated with sleep disorders. In support of our findings, early studies on patients with chronic insomnia found a positive correlation between urinary DOPAC levels and awakening time during sleep [46]. Other studies also suggested sleep deprivation induced an increase of DOPAC in the basal forebrain and striatum of rats, indicating a correlation between DOPAC levels and the degree of arousal [47, 48]. In addition to neuronal metabolite, we found that EHD3, a cell transporter mediating the recycling of D1-type dopamine receptors after endocytosis [49], may affect circadian rhythm by promoting the expression of the lncRNA ENSG000000273447. However, the specific regulatory mechanism of this pathway remains to be further explored. In summary, the disturbance of the dopamine-dopamine receptor system may be involved in the development of sleep disorders in winter-over expeditioners.
Recent studies revealed the functions of ECM in modulating circadian rhythm and sleep, through interactions with cell surface molecules or extracellular proteases [50]. According to our results, lncRNA ENSG00000268240 may advance the circadian rhythm through inhibiting extracellular protease MMP9 expression. Recent evidence uncovered the role of MMP9 in sleep-wake regulation. Hurtado Alvarado et al. [51] reported a significant increase in MMP9 levels in the hippocampus after sleep restriction. Breast cancer patients with circadian disruption were characterized by higher levels of serum MMP9 compared to controls with stable circadian rhythm [52]. Mechanism studies suggested that MMP9 cleaves pro-BDNF into activated mature BDNF, which in turn binds to and activates TrkB receptor and enhances the signal transduction activity of postsynaptic N-methyl-D-aspartate receptors (NMDAR), members of the glutamate receptor channel superfamily [53]. Therefore, the advance of circadian phase after MMP9 inhibition could be related to the repressed glutamate pathway activity in the CNS. However, the mechanism by which ENSG00000268240 regulates MMP9 expression remains to be further elucidated. LTBP2 is another protein that binds to microfibers in ECM and interacts with transforming growth factor β (TGFβ) family members [54]. In this study, we proposed a novel potential function of LTBP2 that it might mediate the effects of neurodevelopmental regulator SOX5 on sleep, inducing increased WASO and delayed get up time. Further studies are required to verify our findings and clarify their mechanisms.
It is well established that both the central and peripheral circadian clocks can affect glucose metabolism homeostasis through various pathways, such as regulating food intake, hormone secretion, insulin sensitivity, and energy metabolism [55]. Vice versa, glucose levels may also influence sleep-wake activity. Evidence from in vivo and in vitro research demonstrated that the elevation of glucose levels repressed the expression of arousal-regulating orexin and the electrical activity of orexin neurons [56, 57]. In support of this, we found both the glucose transporter SLC2A1 and the pentose phosphate pathway catalytic enzyme PGD may be involved in the development of sleep disorders. The underlying mechanisms could be partially due to the fluctuations of glucose levels in the CNS.
There are still some limitations in this study. Firstly, although we have identified several potential pathways and targets that may regulate sleep status, the results are speculative, and further functional experiments are required to confirm these findings. In addition, how the proposed genes and proteins interact with each other to alter the circadian rhythm and sleep remains largely unexplored.
To summarize, this study, for the first time, presented the neurotransmitter metabolites-genes-proteins interaction network which modulates circadian and sleep disorders in Antarctica and proposed several possible signaling pathways. Importantly, we found SHISA8 as a key candidate gene in sleep regulation, which may provide new insights into the prevention and treatment of sleep disorders under extreme light conditions.