Regulation and integration in the mammalian circadian system

Insulin-FOXO3 signaling modulates circadian rhythms via regulation of clock transcription

Circadian rhythms are responsive to external and internal cues, light and metabolism being among the most important. In mammals, the light signal is sensed by the retina and transmitted to the suprachiasmatic nucleus (SCN) master clock [1], where it is integrated into the molecular oscillator via regulation of clock gene transcription. The SCN synchronizes peripheral oscillators, an effect that can be overruled by incoming metabolic signals [2]. As a consequence, peripheral oscillators can be uncoupled from the master clock when light and metabolic signals are not in phase. The signaling pathways responsible for coupling metabolic cues to the molecular clock are being rapidly uncovered [3-5]. Here we show that insulin-phosphatidylinositol 3-kinase (PI3K)-Forkhead box class O3 (FOXO3) signaling is required for circadian rhythmicity in the liver via regulation of Clock. Knockdown of FoxO3 dampens circadian amplitude, an effect that is rescued by overexpression of Clock. Subsequently, we show binding of FOXO3 to two Daf-binding elements (DBEs) located in the Clock promoter area, implicating Clock as a transcriptional target of FOXO3. Transcriptional oscillation of both core clock and output genes in the liver of FOXO3-deficient mice is affected, indicating a disrupted hepatic circadian rhythmicity. Finally, we show that insulin, a major regulator of FOXO activity [6-9], regulates Clock levels in a PI3K- and FOXO3-dependent manner. Our data point to a key role of the insulin-FOXO3-Clock signaling pathway in the modulation of circadian rhythms.

Physiology of circadian entrainment

Mammalian circadian rhythms are controlled by endogenous biological oscillators, including a master clock located in the hypothalamic suprachiasmatic nuclei (SCN). Since the period of this oscillation is of approximately 24 h, to keep synchrony with the environment, circadian rhythms need to be entrained daily by means of Zeitgeber ("time giver") signals, such as the light-dark cycle. Recent advances in the neurophysiology and molecular biology of circadian rhythmicity allow a better understanding of synchronization. In this review we cover several aspects of the mechanisms for photic entrainment of mammalian circadian rhythms, including retinal sensitivity to light by means of novel photopigments as well as circadian variations in the retina that contribute to the regulation of retinal physiology. Downstream from the retina, we examine retinohypothalamic communication through neurotransmitter (glutamate, aspartate, pituitary adenylate cyclase-activating polypeptide) interaction with SCN receptors and the resulting signal transduction pathways in suprachiasmatic neurons, as well as putative neuron-glia interactions. Finally, we describe and analyze clock gene expression and its importance in entrainment mechanisms, as well as circadian disorders or retinal diseases related to entrainment deficits, including experimental and clinical treatments.

Casein kinase I epsilon gene transfer into the suprachiasmatic nucleus via electroporation lengthens circadian periods of tau mutant hamsters

Circadian activity rhythms in mammals are controlled by the expression and transcriptional regulation of clock genes in the suprachiasmatic nucleus (SCN). The circadian cycle length in hamsters is regulated in part by casein kinase I epsilon (CKIepsilon). A semidominant mutation (C-->T, R178C, CKIepsilon(tau)) appears to act as a dominant-negative allele to shorten the period of circadian rhythms. We tested this hypothesis in vivo by expressing wild-type CKIepsilon gene in homozygous tau mutant hamsters. High-level CKIepsilon(+/+) gene transfer and expression (as indicated by green fluorescent protein) were obtained by injecting CKIepsilon-containing plasmids bilaterally near the SCN, followed by in vivo electroporation. Rhythmicity reappeared 5-7 days after electroporation, with a gradual increase in circadian period over the next 10 days. The circadian period returned to the baseline over the next 20 days. For the five hamsters with clearest gene expression in the SCN, the mean lengthening time was 39.6 min. Period change was not observed in either control tau mutant hamsters electroporated with plasmids lacking the CKIepsilon gene or in wild-type hamsters with plasmids containing the wild-type CKIepsilon gene. Therefore, normal periodicity in homozygous CKIepsilon(tau) hamsters was partially rescued by expression of the wild-type CKIepsilon gene in the SCN, supporting a competitive and dominant-negative action of the mutant allele. This study shows that electroporation of wild-type CKIepsilon gene into the SCN is sufficient for lengthening the shorter circadian period of tau mutant hamsters in a time-dependent way and supports the conclusion that CKIepsilon(tau) is the cause of the shorter period.

Signaling in the mammalian circadian clock: the NO/cGMP pathway

Mammalian circadian rhythms are generated by a hypothalamic suprachiasmatic nuclei (SCN) clock. Light pulses synchronize body rhythms by inducing phase delays during the early night and phase advances during the late night. Phosphorylation events are known to be involved in circadian phase shifting, both for delays and advances. Pharmacological inhibition of the cGMP-dependent kinase (cGK) or Ca2+/calmodulin-dependent kinase (CaMK), or of neuronal nitric oxide synthase (nNOS) blocks the circadian responses to light in vivo. Light pulses administered during the subjective night, but not during the day, induce rapid phosphorylation of both p-CAMKII and p-nNOS (specifically phosphorylated by CaMKII). CaMKII inhibitors block light-induced nNOS activity and phosphorylation, suggesting a direct pathway between both enzymes. Furthermore, SCN cGMP exhibits diurnal and circadian rhythms with maximal values during the day or subjective day. This variation of cGMP levels appears to be related to temporal changes in phosphodiesterase (PDE) activity and not to guanylyl cyclase (GC) activity. Light pulses increase SCN cGMP levels at circadian time (CT) 18 (when light causes phase advances of rhythms) but not at CT 14 (the time for light-induced phase delays). cGK II is expressed in the hamster SCN and also exhibits circadian changes in its levels, peaking during the day. Light pulses increase cGK activity at CT 18 but not at CT 14. In addition, cGK and GC inhibition by KT-5823 and ODQ significantly attenuated light-induced phase shifts at CT 18. This inhibition did not change c-Fos expression SCN but affected the expression of the clock gene per in the SCN. These results suggest a signal transduction pathway responsible for light-induced phase advances of the circadian clock which could be summarized as follows: Glu-Ca2+-CaMKII-nNOS-GC-cGMP-cGK-->-->clock genes. This pathway offers a signaling window that allows peering into the circadian clock machinery in order to decipher its temporal cogs and wheels.

Cyclic AMP and protein kinase A rhythmicity in the mammalian suprachiasmatic nuclei

The levels of cyclic AMP and protein kinase A, as well as the activity of this enzyme, were measured in the hamster suprachiasmatic nuclei at different time points throughout the daily or circadian cycle. Significant diurnal variations for levels of AMPc and the catalytic subunit of protein kinase A and the activity of this enzyme were found. All of these parameters tended to increase throughout the nocturnal phase, reaching higher values at the end of the night and the beginning of the day and minimal values around the time of lights off. This rhythmicity appears to be under exogenous control, since constant darkness abolished fluctuations throughout the circadian cycle. In vitro incubation in the presence of melatonin during the day significantly decreased cyclic AMP levels and basal protein kinase A activity in the SCN, while neither neuropeptide Y nor light pulses affected these parameters. These results suggest a significant diurnal regulation of the cyclic AMP-dependent system in the hamster circadian clock.

Differential expression of c-fos mRNA in the rat neocortex by in situ hybridization

The c-fos mRNA expression pattern in rat neocortex, was determined in the rat kept in a 12:12 light/dark cycle, in constant dark, or in constant light by in situ hybridization. At the beginning of the light period, c-fos mRNA was induced both in the neocortex and suprachiasmatic nucleus (SCN). Transiently increased c-fos mRNA expression was detected from 0830 to 0900 and soon declined to basal levels. Immediately prior to the beginning of the dark period, c-fos mRNA expression also increased and remained elevated in the neocortex following the dark period. In the constant dark group, c-fos mRNA expression showed no transient elevation at the beginning of the light period. On the other hand, c-fos mRNA expression in the constant light group increased during their subjective dark period as well as normal light/dark cycle. These results demonstrate a circadian pattern of c-fos mRNA expression in the neocortex which is similar to that observed previously in the inner and outer nuclear layers of the retina.

The rhythmic GABAergic system

GABA is the major inhibitory neurotransmitter in the mammalian brain, and has been implicated in the regulation of a variety of behavioral functions, including biological rhythms. The focus of this minireview is the rhythmic variation of the central GABAergic system, comprising fluctuations of GABA levels and turnover, GABA receptor affinity and postsynaptic activity on the chloride ionophore in rodent's brain. Neurochemical rhythms correlated with diurnal and circadian changes in several behaviors associated with the GABA(A) receptor, e.g., anxiolysis-related behavior. GABA is considered to be the principal neurotransmitter of the mammalian circadian system, being present in the suprachiasmatic nuclei and the intergeniculate leaflet. Pharmacological manipulations of GABA(A) receptors phase shift circadian rhythms and alter circadian responses to light. Administration of putative modulators of GABA function, like melatonin or neuroactive steroids, affects the timing of biological rhythms. Therefore, not only does the GABAergic system exhibit strong diurnal and circadian variations, but it also serves as one of the key modulators of the circadian apparatus.