Kazumasa Horikawa

Reduced food anticipatory activity in genetically orexin (hypocretin) neuron-ablated mice.

Daily restricted feeding (RF) produces an anticipatory locomotor activity rhythm and entrains the peripheral molecular oscillator independently of the central pacemaker located in the suprachiasmatic nucleus (SCN). As orexins (hypocretins) are neuropeptides that coordinate sleep/wake patterns and motivated behaviours, such as food seeking, we studied the involvement of orexin in the food anticipatory activity (FAA) induced by RF. Daily RF shifted the mRNA rhythm of a clock‐controlled gene mDbp in the cerebral cortex and caudate putamen but not in the SCN. Under these experimental conditions, prepro‐orexin mRNA and orexin A immunoreactivity in the lateral hypothalamic area (LHA) did not show daily variation. Fasting increased the number of orexin A‐ir cells, while RF did not. However, RF shifted the peak of Fos expression of the orexin neurons from night to day. Genetic ablation of orexin neurons in orexin/ataxin‐3 transgenic mice severely reduced the formation of FAA under RF conditions. The expression of mNpas2 mRNA, a transcription factor thought to be involved in regulation of the food entrainable oscillator as well as mPer1 and mBmal1 mRNA, was reduced in the forebrain of orexin/ataxin‐3 mice. Based on these results, we suggest that activity of the orexin neuron in the LHA contributes to the promotion and maintenance of FAA.

In Vivo Monitoring of Peripheral Circadian Clocks in the Mouse

The mammalian circadian system is comprised of a central clock in the suprachiasmatic nucleus (SCN) and a network of peripheral oscillators located in all of the major organ systems. The SCN is traditionally thought to be positioned at the top of the hierarchy, with SCN lesions resulting in an arrhythmic organism. However, recent work has demonstrated that the SCN and peripheral tissues generate independent circadian oscillations in Per1 clock gene expression in vitro. In the present study, we sought to clarify the role of the SCN in the intact system by recording rhythms in clock gene expression in vivo. A practical imaging protocol was developed that enables us to measure circadian rhythms easily, noninvasively, and longitudinally in individual mice. Circadian oscillations were detected in the kidney, liver, and submandibular gland studied in about half of the SCN-lesioned, behaviorally arrhythmic mice. However, their amplitude was decreased in these organs. Free-running periods of peripheral clocks were identical to those of activity rhythms recorded before the SCN lesion. Thus, we can report for the first time that many of the fundamental properties of circadian oscillations in peripheral clocks in vivo are maintained in the absence of SCN control.

Ca2+/calmodulin-dependent protein kinase II-dependent long-term potentiation in the rat suprachiasmatic nucleus and its inhibition by melatonin

We recently reported that Ca2+/calmodulin‐dependent protein (CaM) kinase II is involved in light‐induced phase delays and Per gene induction in the suprachiasmatic nucleus (SCN). To clarify the activation mechanisms of CaM kinase II by glutamate receptor stimulation in the SCN, we documented CaM kinase II activation following induction of long‐term potentiation (LTP) in the rat SCN. High‐frequency stimulation (100 Hz, 1 sec) applied to the optic nerve resulted in LTP of a postsynaptic field potential in the rat SCN. Unlike LTP in the hippocampal CA1 region, LTP onset in the SCN was slow and partly dependent on N‐methyl‐D‐aspartate receptor activation. LTP induction in the SCN was completely inhibited by treatment with a nitric oxide synthase inhibitor or with a specific CaM kinase II inhibitor. Immunoblotting analysis using phosphospecific antibodies against autophosphorylated CaM kinase II revealed that LTP induction was accompanied by an increase in autophosphorylation. After high‐frequency stimulation, we could visualize activation of CaM kinase II in vasoactive intestinal polypeptide‐positive neurons in the SCN by immunohistochemistry. Treatment with cyclosporin A, a calcineurin inhibitor, potentiated LTP induction in the rat SCN. Interestingly, treatment with melatonin totally prevented LTP induction, without changes in basal synaptic transmission. Analyses of phosphorylation of CaM kinase II, mitogen‐activated protein kinase, and cAMP‐responsive element binding protein revealed that stimulatory and inhibitory effects on CaM kinase II autophosphorylation underlie the effects of cyclosporin A and melatonin, respectively. These results suggest that CaM kinase II plays critical roles in LTP induction in the SCN and that melatonin has inhibitory effects on synaptic plasticity through CaM kinase II.

Restricted feeding induces daily expression of clock genes and Pai-1 mRNA in the heart of Clock mutant mice

Plasminogen activator inhibitor‐1 (PAI‐1) is a key factor of fibrinolytic activity. The activity and mRNA abundance show a daily rhythm. To elucidate the mechanism of daily Pai‐1 gene expression, the expression of Pai‐1 and several clock genes was examined in the heart of homozygous Clock mutant (Clock/Clock) mice. Damping of the daily oscillation of Pai‐1 gene expression in Clock/Clock mice was accompanied with damped or attenuated oscillations of mPer1, mPer2, mBmal1, and mNpas2 mRNA. Daily restricted feeding induced a daily mRNA rhythm of all clock genes and Pai‐1 mRNA in Clock/Clock mice as well as wild‐type mice. The peaks of clock genes and Pai‐1 mRNA were phase‐advanced in the heart of both genotypes after 6 days of restricted feeding. The present results demonstrate that daily Pai‐1 gene expression depends on clock gene expression in the heart and that a functional Clock gene is not required for restricted feeding‐induced resetting of the peripheral clock.

Inhibitory action of brotizolam on circadian and light-induced Per1 and Per2 expression in the hamster suprachiasmatic nucleus

Triazolam reportedly causes phase advances in hamster wheel‐running rhythm after injection during subjective daytime. However, it is unclear whether benzodiazepine affects the Per gene expression accompanying a behavioural phase shift. Brotizolam (0.5–10 mg kg−1) induced large phase advances in hamster rhythm when injected during mid‐subjective daytime (circadian time 6 or 9), but not at circadian time 0, 3 or 15. Brotizolam (5 mg kg−1) significantly reduced the expression of Per1 and Per2 in the suprachiasmatic nucleus 1 and 2 h after injection at circadian time 6, and slightly reduced them at circadian time 20. Injection of 8‐OH‐DPAT (5 mg kg−1) at subjective daytime induced similar phase advances with a reduction of Per1 and Per2 expression. Co‐administration of brotizolam with 8‐OH DPAT failed to potentiate the 8‐OH DPAT‐induced phase advances and reduced Per expression. Both phase advance and rapid induction of Per1 and Per2 in the suprachiasmatic nucleus after light exposure (5 lux, 15 min) at circadian time 20 was strongly attenuated by co‐treatment with brotizolam 5 mg kg−1. The present results strongly suggest that reduction of Per1 and/or Per2 expression during subjective daytime by brotizolam may be an important step in causing a behavioural phase advance. The co‐administration experiment suggests that common mechanism(s) are involved in brotizolam‐ or 8‐OH DPAT‐induced phase advances and the reduction of Per gene expression. These results suggest that brotizolam is not only a good drug for insomnia but also a drug capable of facilitating re‐entrainment like melatonin.

Phase-resetting response to (+)8-OH-DPAT, a serotonin 1A/7 receptor agonist, in the mouse in vivo

Several lines of evidence indicate that 5-HT neuronal systems may play a critical role for the non-photic entrainment of the rodent circadian clock. Although it is well established that (+)8-hydroxy-2-(di-n-propylamino)tetralin [(+)8-OH-DPAT], a 5-HT(1A/7) receptor agonist, causes a phase-advance of behavioral rhythm in hamsters, little is known whether this agent produces phase shifts of activity rhythm in mice. Therefore, we examined the effect of (+)8-OH-DPAT on the mouse locomotor activity rhythm. Systemic administration of this chemical at mid-subjective daytime induced a clear and dose-dependent phase advance, while there were no significant phase shifts at other times (early-subjective day, late-subjective day, or subjective night). Additionally, (+)8-OH-DPAT accelerated the re-entrainment of mouse behavioral rhythm to a 6-h advanced light-dark cycle. These results suggest that we can use mice for understanding the molecular mechanism of (+)8-OH-DPAT-induced phase shift because of availability of clock gene targeted mice.

Rapid damping of food-entrained circadian rhythm of clock gene expression in clock-defective peripheral tissues under fasting conditions

Restricted feeding-induced free-running oscillation of clock genes in the liver was studied in homozygous Clock-mutant (Clock/Clock) mice. Similar to wild-type mice, Clock/Clock mice showed robust food-anticipatory behavioral activity in accordance with a restricted feeding schedule. Also, the peak of all clock gene mRNAs tested was phase-advanced in the liver of Clock/Clock mice as well as wild-type mice, although the amplitude of clock gene expression was low in Clock/Clock mice. The food-anticipatory behavioral rhythm in Clock/Clock mice maintained a period similar to wild-type mice during 2-day fasting after the cessation of restricted feeding. However, during the fasting days after temporal feeding cues were removed, the oscillation of clock genes in the liver and heart, excluding the suprachiasmatic nuclei, appeared to result in arrhythmicity in Clock/Clock mice. Thus, although the CLOCK-based molecular mechanism is not required for the expression of food-anticipatory activity, intact CLOCK protein might be involved in sustaining several cycles of peripheral circadian oscillations after restricted feeding-induced resetting.

Attenuated Food Anticipatory Activity and Abnormal Circadian Locomotor Rhythms in Rgs16 Knockdown Mice

Regulators of G protein signaling (RGS) are a multi-functional protein family, which functions in part as GTPase-activating proteins (GAPs) of G protein α-subunits to terminate G protein signaling. Previous studies have demonstrated that the Rgs16 transcripts exhibit robust circadian rhythms both in the suprachiasmatic nucleus (SCN), the master circadian light-entrainable oscillator (LEO) of the hypothalamus, and in the liver. To investigate the role of RGS16 in the circadian clock in vivo, we generated two independent transgenic mouse lines using lentiviral vectors expressing short hairpin RNA (shRNA) targeting the Rgs16 mRNA. The knockdown mice demonstrated significantly shorter free-running period of locomotor activity rhythms and reduced total activity as compared to the wild-type siblings. In addition, when feeding was restricted during the daytime, food-entrainable oscillator (FEO)-driven elevated food-anticipatory activity (FAA) observed prior to the scheduled feeding time was significantly attenuated in the knockdown mice. Whereas the restricted feeding phase-advanced the rhythmic expression of the Per2 clock gene in liver and thalamus in the wild-type animals, the above phase shift was not observed in the knockdown mice. This is the first in vivo demonstration that a common regulator of G protein signaling is involved in the two separate, but interactive circadian timing systems, LEO and FEO. The present study also suggests that liver and/or thalamus regulate the food-entrained circadian behavior through G protein-mediated signal transduction pathway(s).

Involvement of Stress Kinase Mitogen-activated Protein Kinase Kinase 7 in Regulation of Mammalian Circadian Clock

Background: MKK7 is a kinase involved in the cellular stress response. Results: MKK7 regulates circadian gene expression and the stability of an essential circadian component in unstressed mammalian cells. Conclusion: MKK7 functions as a circadian clock regulator. Significance: Our identification of role of MKK7 in the circadian clock provides insight into the importance of stress-responsive molecules in the maintenance of cellular homeostasis. The stress kinase mitogen-activated protein kinase kinase 7 (MKK7) is a specific activator of c-Jun N-terminal kinase (JNK), which controls various physiological processes, such as cell proliferation, apoptosis, differentiation, and migration. Here we show that genetic inactivation of MKK7 resulted in an extended period of oscillation in circadian gene expression in mouse embryonic fibroblasts. Exogenous expression in cultured mammalian cells of an MKK7-JNK fusion protein that functions as a constitutively active form of JNK induced phosphorylation of PER2, an essential circadian component. Furthermore, JNK interacted with PER2 at both the exogenous and endogenous levels, and MKK7-mediated JNK activation increased the half-life of PER2 protein by inhibiting its ubiquitination. Notably, the PER2 protein stabilization induced by MKK7-JNK fusion protein reduced the degradation of PER2 induced by casein kinase 1ϵ. Taken together, our results support a novel function for the stress kinase MKK7 as a regulator of the circadian clock in mammalian cells at steady state.

Multifactorial Regulation of Daily Rhythms in Expression of the Metabolically Responsive Gene Spot14 in the Mouse Liver

Spot14 is a putative transcriptional regulator for genes involved in fatty acid synthesis. The Spot14 gene is activated in response to lipogenic stimuli such as dietary carbohydrate and is also under circadian regulation. The authors investigated factors responsible for daily oscillation of Spot14 expression. If mice were kept under a 12-h light/12-h dark cycle with ad libitum feeding, Spot14 mRNA levels in the liver reached a peak at an early dark period when mice, as nocturnal animals, start feeding. Under fasting, while Spot14 mRNA levels were generally decreased, the rhythmicity was still maintained, suggesting contribution of both nutritional elements and circadian clock factors on robust rhythmicity of Spot14 expression. Effects of circadian clock factors were confirmed by the observations that the circadian rhythm of Spot14 expression was seen also under the constant darkness and that the rhythmicity was lost in Clock mutant mice. When mice were housed in short-photoperiod (6-h light/18-h dark) and long-photoperiod (18-h light/6-h dark) cycles, rhythms of Spot14 mRNA levels were phase advanced and phase delayed, respectively, being concordant with the notion that Spot14 expression is under the control of the light-entrainable oscillator. As for nutritional mediators, in the liver of db/db mice exhibiting hyperinsulinemia-accompanied hyperglycemia, Spot14 mRNA levels were constantly high without apparent rhythmicity, consistent with previous observations for strong activation of the Spot14 gene by glucose and insulin. Restricted feeding during the 4-h mid-light period caused a phase advance of the Spot14 expression rhythm. On the other hand, restricted feeding during the 4-h mid-dark period led to damping of the rhythmicity, apparently resulting from the separation of phases between effects of the light/dark cycle and feeding on Spot14 expression. Thus, the daily rhythm of Spot14 expression in the liver is under the control of the light-entrainable oscillator, food-entrainable oscillator, and food-derived nutrients, in a separate or cooperative manner.