Circadian Activation of Bullfrog Retinal Mitogen-activated Protein Kinase Associates with Oscillator Function

Abstract

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-dayspecific activation of MAPK contributes to the circadian time-keeping mechanism of the retinal clock system.The physiology and behavior of living organisms from bacteria to humans show daily fluctuations with a period of approximately 24 h, and the oscillations controlled by autonomous circadian oscillators are termed circadian rhythms (1, 2). These rhythms can be synchronized (entrained) to environmental time cues such as light or temperature but are sustained under constant environmental conditions in the absence of time cues (1). A lot of studies on the Drosophila melanogaster clock system have demonstrated that a transcription/translation-based feedback loop is involved in generating the circadian rhythmicity (2, 3). In Drosophila, a complex of two transcription factors, dCLOCK and CYCLE (dBMAL1), positively regulates both period (per) and timeless (tim) transcription through CACGTG E-box elements found in their promoters (4 -6), leading to an increase in protein levels of both PER and TIM. PER and TIM proteins form a heterodimer and translocate to the nucleus where PER/TIM negatively regulate dCLOCK/CYCLE-induced transcription of their own promoters (5, 7-11). Thus, these positive and negative elements constitute an autoregulatory feedback loop.In vertebrates, autonomous circadian oscillators are located in several neuronal tissues such as the retina, pineal gland, and suprachiasmatic nucleus (SCN) 1 (1, 12-17). Homologs of per, clock, and bmal1 have been identified in vertebrates, and they are expressed in SCN of mammals (reviewed in Refs. 2 and 18). It is now postulated that the molecular framework of the circadian oscillator is conserved from flies to mammals, although each clock component appears to function in a slightly different manner (reviewed in Refs. 19 and 20).In addition to the transcriptional and translational regulation, post-translational modifications of clock gene products play important roles in the clock system. Droso...

Circadian rhythmicity mediated by temporal regulation of the activity of p38 MAPK

“…Additionally, the oscillator components BMAL1 (in mice) and CLK (in Drosophila) can be directly phosphorylated by MAPKs (46,47). Although our work demonstrates circadian clock control of activity of a homolog of the p38 family of MAPKs involved in stress responses, circadian rhythms in the activity of the ERK family of MAPKs involved in growth control are also known in several systems (44,(47)(48)(49)(50)(51)(52)(53)(54)(55). These data suggest that control of MAPK pathways by the circadian clock may be a conserved feature of the output pathways.…”

Circadian Regulation of nocturnin Transcription by Phosphorylated CREB in Xenopus Retinal Photoreceptor Cells

“…Although there is no data on the rhythmicity of these kinases in Xenopus retina, there is a recent report that the activities of Erk MAP kinase and CamKII␣ are rhythmic in chick photoreceptor cells (36), the chick pineal (55), and the mouse SCN (50). In addition, Erk MAP kinase activity also exhibits clear circadian rhythms in bullfrog retina, but this rhythm is confined to a population of amacrine cells in the inner nuclear layer (26).…”

Regulation of the Circadian Oscillator in Xenopus Retinal Photoreceptors by Protein Kinases Sensitive to the Stress-activated Protein Kinase Inhibitor, SB 203580

“…ERK has been suggested to regulate the circadian system in mouse suprachiasmatic nucleus (SCN) (25), chick pineal gland (26, but also see 27), and Bullfrog retina (28) as well as several cell lines (29,30). However, a 6-h pulse of PD 98059 (30 M) given either during the midday or during the midnight had no effect on the phase of melatonin rhythms ( Fig.…”