Regulation of the Neuroligin-1 Gene by Clock Transcription Factors

NEUROLIGIN-1 (NLGN1) is a postsynaptic adhesion molecule involved in the regulation of glutamatergic transmission. It has been associated with several features of sleep and psychiatric disorders. Our previous work suggested that transcription of the Nlgn1 gene could be regulated by the transcription factors CLOCK and BMAL1 because they bind to the Nlgn1 gene promoter in vivo. However, whether CLOCK/BMAL1 can directly activate Nlgn1 transcription is not yet known. We thus aimed to verify whether CLOCK/BMAL1, as well as their homologs NPAS2 and BMAL2, can activate transcription via the Nlgn1 promoter by using luciferase assays in COS-7 cells. We also investigated how Nlgn1 expression was affected in Clock mutant mice. Our results show transcriptional activation in vitro mediated by CLOCK/BMAL1 and by combinations with their homologs NPAS2 and BMAL2. Moreover, CLOCK/BMAL1 activation via the Nlgn1 gene fragment was repressed by GSK3β. In vivo, Nlgn1 mRNA expression was significantly modified in the forebrain of Clock mutant mice in a transcript variant-dependent manner. However, no significant change in NLGN1 protein level was observed in Clock mutant mice. These findings will increase knowledge about the transcriptional regulation of Nlgn1 and the relationship between circadian rhythms, mental health, and sleep.

Omics Approaches in Sleep-Wake Regulation

Although sleep seems an obvious and simple behaviour, it is extremely complex involving numerous interactions both at the neuronal and the molecular levels. While we have gained detailed insight into the molecules and neuronal networks responsible for the circadian organization of sleep and wakefulness, the molecular underpinnings of the homeostatic aspect of sleep regulation are still unknown and the focus of a considerable research effort. In the last 20 years, the development of techniques allowing the simultaneous measurement of hundreds to thousands of molecular targets (i.e. 'omics' approaches) has enabled the unbiased study of the molecular pathways regulated by and regulating sleep. In this chapter, we will review how the different omics approaches, including transcriptomics, epigenomics, proteomics, and metabolomics, have advanced sleep research. We present relevant data in the framework of the two-process model in which circadian and homeostatic processes interact to regulate sleep. The integration of the different omics levels, known as 'systems genetics', will eventually lead to a better understanding of how information flows from the genome, to molecules, to networks, and finally to sleep both in health and disease.

The cell adhesion molecule EphA4 is involved in circadian clock functions

Circadian (∼24 h) rhythms of cellular network plasticity in the central circadian clock, the suprachiasmatic nucleus (SCN), have been described. The neuronal network in the SCN regulates photic resetting of the circadian clock as well as stability of the circadian system during both entrained and constant conditions. EphA4, a cell adhesion molecule regulating synaptic plasticity by controlling connections of neurons and astrocytes, is expressed in the SCN. To address whether EphA4 plays a role in circadian photoreception and influences the neuronal network of the SCN, we have analyzed circadian wheel-running behavior of EphA4 knockout (EphA4^-/- ) mice under different light conditions and upon photic resetting, as well as their light-induced protein response in the SCN. EphA4^-/- mice exhibited reduced wheel-running activity, longer endogenous periods under constant darkness and shorter periods under constant light conditions, suggesting an effect of EphA4 on SCN function. Moreover, EphA4^-/- mice exhibited suppressed phase delays of their wheel-running activity following a light pulse during the beginning of the subjective night (CT15). Accordingly, light-induced c-FOS (FBJ murine osteosarcoma viral oncogene homolog) expression was diminished. Our results suggest a circadian role for EphA4 in the SCN neuronal network, affecting the circadian system and contributing to the circadian response to light.

Genetic analysis of Drosophila circadian behavior in seminatural conditions

The study of circadian behavior in model organisms is almost exclusively confined to the laboratory, where rhythmic phenotypes are studied under highly simplified conditions such as constant darkness or rectangular light-dark cycles. Environmental cycles in nature are far more complex, and recent work in rodents and flies has revealed that when placed in natural/seminatural situations, circadian behavior shows unexpected features that are not consistent with laboratory observations. In addition, the recent observations of clockless mutants, both in terms of their circadian behavior and their Darwinian fitness, challenge some of the traditional beliefs derived from laboratory studies about what constitutes an adaptive circadian phenotype. Here, we briefly summarize the results of these newer studies and then describe how Drosophila behavior can be studied in the wild, pointing out solutions to some of the technical problems associated with extending locomotor monitoring to this unpredictable environment. We also briefly describe how to generate sophisticated simulations of natural light and temperature cycles that can be used to successfully mimic the fly's natural circadian behavior. We further clarify some misconceptions that have been raised in recent studies of natural fly behavior and show how these can be overcome with appropriate methodology. Finally, we describe some recent technical developments that will enhance the naturalistic study of fly circadian behavior.

Does prior sepsis alter subsequent circadian and sickness behaviour response to lipopolysaccharide treatment in mice?

Previous data has shown that prior history of immune challenge may affect central and behavioural responses to subsequent immune challenge, either leading to exaggerated responses via priming mechanisms or lessened responses via endotoxin tolerance. In this set of experiments we have examined how previously lipopolysaccharide (LPS)-induced sepsis shapes the response to subsequent treatment with lower dose LPS. After treatment with LPS (5 mg/kg) or saline mice were allowed to recover for 3-4 months before being challenged with a lower dose of LPS (100 μg/kg) for assessment of sickness behaviours. Performance on the open field test and the tail suspension test was assessed, and no evidence was found that prior sepsis altered sickness or depressive-like behaviour following LPS treatment. We then examined the responsiveness of the circadian system of mice to LPS. We found that in control animals, LPS induced a significant phase delay of the behavioural rhythm and that this was not the case in post-septic animals (4-6 weeks after sepsis), indicating that prior sepsis alters the responsivity of the circadian system to subsequent immune challenge. We further assessed the induction of the immediate early genes c-Fos and EGR1 in the hippocampus and the suprachiasmatic nucleus (SCN; the master circadian pacemaker) by LPS in control or post-septic animals, and found that post-septic animals show elevated expression in the hippocampus but not the SCN. These data suggest that previous sepsis has some effect on behavioural and molecular responses to subsequent immune challenge in mice.

Long-lasting effects of sepsis on circadian rhythms in the mouse

Daily patterns of activity and physiology are termed circadian rhythms and are driven primarily by an endogenous biological timekeeping system, with the master clock located in the suprachiasmatic nucleus. Previous studies have indicated reciprocal relationships between the circadian and the immune systems, although to date there have been only limited explorations of the long-term modulation of the circadian system by immune challenge, and it is to this question that we addressed ourselves in the current study. Sepsis was induced by peripheral treatment with lipopolysaccharide (5 mg/kg) and circadian rhythms were monitored following recovery. The basic parameters of circadian rhythmicity (free-running period and rhythm amplitude, entrainment to a light/dark cycle) were unaltered in post-septic animals compared to controls. Animals previously treated with LPS showed accelerated re-entrainment to a 6 hour advance of the light/dark cycle, and showed larger phase advances induced by photic stimulation in the late night phase. Photic induction of the immediate early genes c-FOS, EGR-1 and ARC was not altered, and neither was phase-shifting in response to treatment with the 5-HT-1a/7 agonist 8-OH-DPAT. Circadian expression of the clock gene product PER2 was altered in the suprachiasmatic nucleus of post-septic animals, and PER1 and PER2 expression patterns were altered also in the hippocampus. Examination of the suprachiasmatic nucleus 3 months after treatment with LPS showed persistent upregulation of the microglial markers CD-11b and F4/80, but no changes in the expression of various neuropeptides, cytokines, and intracellular signallers. The effects of sepsis on circadian rhythms does not seem to be driven by cell death, as 24 hours after LPS treatment there was no evidence for apoptosis in the suprachiasmatic nucleus as judged by TUNEL and cleaved-caspase 3 staining. Overall these data provide novel insight into how septic shock exerts chronic effects on the mammalian circadian system.