Role of circadian activation of mitogen-activated protein kinase in chick pineal clock oscillation

Involvement of calcium-calmodulin protein kinase but not mitogen-activated protein kinase in light-induced phase delays and Per gene expression in the suprachiasmatic nucleus of the hamster

It is known that Ca(2+)-dependent phosphorylation of cAMP response element binding protein (CREB) and the rapid induction of mPer1 and mPer2, mouse period genes in the suprachiasmatic nucleus (SCN) are associated with light-induced phase shifting. The CREB/CRE transcriptional pathway has been shown to be activated by calcium/calmodulin dependent kinase II (CaMKII) and mitogen-activated protein kinase (MAPK); however, there is a lack of evidence concerning whether the activation of CaMKII and/or MAPK elicited by photic stimuli are associated with the change in Per gene expression and behavioral phase shifting. In this experiment, we found there was an inhibitory effect by KN93, CaMKII inhibitor, on hamster Per1 and Per2 expression in the SCN and on phase delays in wheel running rhythm induced by light pulses. PD98059 and U0126, MAPK kinase inhibitors, however, affected neither light-induced Per1 and Per2 expression nor behavioral phase delays, even though PD98059 attenuated the light-induced phosphorylation of MAPK in the SCN. The present findings demonstrate that the light-induced activation of CaMKII plays an important role in the induction of Per1 and Per2 mRNA in the hamster SCN as well as phase shifting. These results suggest that gated induction of Per1 and/or Per2 genes through CaMKII-CREB/CRE accompanied with photic stimuli may be a critical step in phase shifting.

Circadian and photic regulation of MAP kinase by Ras- and protein phosphatase-dependent pathways in the chick pineal gland

Chick pineal mitogen-activated protein kinase (MAPK) exhibits circadian activation and light-dependent deactivation at nighttime. Here we report that, in the chick pineal gland, levels of active forms of MAPK, MEK, Raf-1 and Ras exhibited synchronous circadian rhythms with peaks during the subjective night, suggesting a sequential activation of components in the classical Ras-MAPK pathway in a circadian manner. In contrast, the light-dependent deactivation of MAPK was not accompanied by any change of MEK activity, but it was attributed to the light-dependent activation of protein phosphatase dephosphorylating MAPK. These results indicate that the photic and clock signals regulate MAPK activity via independent pathways, and suggest a pivotal role of MAPK in photic entrainment and maintenance of the circadian oscillation.

Circadian regulation of cGMP-gated cationic channels of chick retinal cones. Erk MAP Kinase and Ca2+/calmodulin-dependent protein kinase II

cGMP-gated channels are essential for phototransduction in the vertebrate retina. Here we show that the affinity of these channels for cGMP in chick cones is substantially higher during the subjective night than during the subjective day. This effect persists in constant environmental conditions after entrainment to 12:12 hr light-dark cycles in vitro or in ovo. Circadian modulation of ligand affinity is a posttranslational effect and is driven by rhythms in the activities of two protein kinases: Erk and Ca2+/calmodulin-dependent protein kinase II (CaMKII). Erk is maximally active during the subjective night, whereas CaMKII is maximally active during the subjective day. Acute inhibition of these signaling pathways causes phase-dependent changes in the affinity of the channels for cGMP.

The human PER1 gene is transcriptionally regulated by multiple signaling pathways

The mammalian period (Per) genes are components of the circadian clock and appear to be regulated via an autoregulatory feedback loop. Here we show that the human PER1 (hPER1) gene is synergistically activated by protein kinases A and C (PKA, PKC) and cAMP responsive element binding protein. Activators and inhibitors of PKA as well as PKC modulate endogenous hPER1 expression and hPER1 promoter-driven reporter gene activity in a dose-dependent manner. Our results suggest that the hPER1 promoter acts as a sensor for multiple signaling molecules thereby integrating different physiological parameters. This regulation of hPER1 appears to be significant for rapid adaptation to changing environmental conditions.

Adrenergic regulation of mitogen-activated protein kinase in rat pinealocytes: opposing effects of protein kinase A and protein kinase G

The role of adrenergic stimulation in the regulation of mitogen-activated protein kinase (MAPK) in rat pinealocytes was investigated by measuring phosphorylated MAPK using Western blot analysis and a MAPK enzymatic assay. Stimulation with the endogenous neurotransmitter, norepinephrine (NE; a mixed alpha- and beta-adrenergic agonist), concentration dependently increased the phosphorylation of both p44 and p42 isoforms of MAPK. This effect of NE was blocked by PD98059 and U0126 (two inhibitors of MEK). Treatment with prazosin or propranolol significantly reduced the effect of NE on MAPK phosphorylation, suggesting the involvement of both alpha- and beta-adrenergic receptors. Investigation into the intracellular mechanisms of NE action revealed that the increase in MAPK phosphorylation was blocked by KT5823 (a protein kinase G inhibitor), but was enhanced by H89 (a protein kinase A inhibitor). Calphostin C (a protein kinase C inhibitor) and KN93 (a Ca2+/calmodulin-dependent protein kinase inhibitor) also attenuated NE-mediated MAPK activation, but to a lesser degree. Furthermore, inhibition of MAPK phosphorylation by (Bu)2cAMP was effective in reducing MAPK activation by (Bu)2cGMP, an active phorbol ester or ionomycin. These results indicate that the effect of NE on MAPK phosphorylation represents mainly the integration of two signaling mechanisms, protein kinase A and protein kinase G, each having an opposite effect on MAPK phosphorylation.

Circadian activation of bullfrog retinal mitogen-activated protein kinase associates with oscillator function

The vertebrate retina retains a circadian oscillator, and its oscillation is self-sustained with a period close to 24 h under constant environmental conditions. Here we show that bullfrog retinal mitogen-activated protein kinase (MAPK) exhibits an in vivo circadian rhythm in phosphorylation with a peak at night in a light/dark cycle. The phosphorylation rhythm of MAPK persists in constant darkness with a peak at subjective night, and this self-sustained rhythm is also observed in cultured retinas, indicating its close interaction with the retinal oscillator. The rhythmically phosphorylated MAPK is detected only in a discrete subset of amacrine cells despite ubiquitous distribution of MAPK throughout the retinal layers. Treatment of the cultured retinas with MAPK kinase (MEK) inhibitor PD98059 suppresses MAPK phosphorylation during the subjective night, and this pulse perturbation of MEK activity induces a significant phase delay (4-8 h) of the retinal circadian rhythm in MAPK and MEK phosphorylation. These observations strongly suggest that the site-specific and time-of-day-specific activation of MAPK contributes to the circadian time-keeping mechanism of the retinal clock system.

p38MAPK inhibition enhances basal and norepinephrine-stimulated p42/44MAPK phosphorylation in rat pinealocytes

Interaction between p38MAPK and p42/44MAPK in rat pinealocytes was examined by determining the effects of p38MAPK inhibitors on the phosphorylation of p42/44MAPK using Western blot analysis. Treatment with SB202190, a specific inhibitor of p38MAPK, increased p42/44MAPK phosphorylation in a concentration-dependent manner. SB202190 also enhanced the magnitude and the duration of norepinephrine-activated p42/44MAPK phosphorylation. The effect of SB202190 on p42/44MAPK phosphorylation was abolished by PD98059 or UO126, inhibitors of MEK, suggesting that SB202190 is acting upstream of MEK in activating p42/44MAPK. The SB202190-induced phosphorylation of p42/44MAPK was not blocked by inhibitors of cGMP-dependent kinase (KT5823), protein kinase C (calphostin C) or Ca2+/calmodulin dependent kinase (KN93) suggesting that these pathways may not be involved in the effect of SB202190. SB202190 further increased p42/44MAPK phosphorylation that was stimulated by 8-bromo-cGMP, 4beta phorbol 12-myristate 13-acetate, or ionomycin. In contrast, inhibition of p42/44MAPK phosphorylation by dibutyryl-cAMP persisted when p42/44MAPK phosphorylation was increased by SB202190. Furthermore, inhibition of p42/44MAPK phosphorylation had no effect on p38MAPK activation. These results suggest that inhibition of p38MAPK causes activation of p42/44MAPK and acts synergistically with norepinephrine in the regulation of p42/44MAPK activation in rat pinealocytes.

Developmental regulation of neuronal KCa channels by TGFbeta 1: transcriptional and posttranscriptional effects mediated by Erk MAP kinase

An avian ortholog of transforming growth factor beta1 (TGFbeta1) is the target-derived factor responsible for the developmental expression of large-conductance Ca(2+)-activated K(+) (K(Ca)) channels in chick ciliary ganglion (CG) neurons developing in vivo and in vitro. Application of TGFbeta1 evokes an acute stimulation of K(Ca) that can be observed immediately after cessation of a 12 hr exposure to this factor, that persists in the presence of protein synthesis inhibitors, and that is therefore mediated by posttranslational events. Here we show that a single 3 hr exposure to TGFbeta1 can also induce long-lasting stimulation of macroscopic K(Ca) that persists for at least 3.5 d after the end of the treatment. In contrast to the acute stimulation, this sustained effect is dependent on the transcription and synthesis of new proteins at approximately the time of TGFbeta1 treatment. However TGFbeta1 does not cause increases in the levels of slowpoke alpha subunit transcripts in CG neurons, suggesting that induction of some other protein or proteins is required for sustained enhancement of macroscopic K(Ca). In addition, application of TGFbeta1 evoked an almost immediate but transient phosphorylation of the mitogen-activated protein kinase Erk in CG neurons. TGFbeta1-evoked Erk activation was blocked by the specific MEK1 inhibitor 2- (2'-amino-3'-methoxyphenyl)-oxanaphthalen-4-one (PD98059). Moreover, application of PD98059 blocked both acute and sustained K(Ca) stimulation evoked by TGFbeta1. These results indicate that TGFbeta1 elicits a biphasic stimulation of K(Ca) via activation of an MEK1-Erk pathway and raise the possibility that other neuronal effects of TGFbeta superfamily members entail Erk activation.