A role for O-GlcNAcylation in setting circadian clock speed

Abstract

Post-translational modifications of one or more central ''clock'' proteins, most notably time-of-day-dependent changes in phosphorylation, are critical for setting the pace of circadian (y24 h) clocks. In animals, PERIOD (PER) proteins are the key state variable regulating circadian clock speed and undergo daily changes in abundance and cytoplasmic-nuclear distribution that are partly driven by a complex phosphorylation program. Here, we identify O-GlcNAcylation (O-GlcNAc) as a critical post-translational modification in circadian regulation that also contributes to setting clock speed. Knockdown or overexpression of Drosophila O-GlcNAc transferase (ogt) in clock cells either shortens or lengthens circadian behavioral rhythms, respectively. The Drosophila PERIOD protein (dPER) is a direct target of OGT and undergoes daily changes in O-GlcNAcylation, a modification that is mainly observed during the first half of the night, when dPER is predominantly located in the cytoplasm. Intriguingly, the timing of when dPER translocates from the cytoplasm to the nucleus is advanced or delayed in flies, wherein ogt expression is reduced or increased, respectively. Our results suggest that O-GlcNAcylation of dPER contributes to setting the correct pace of the clock by delaying the timing of dPER nuclear entry. In addition, OGT stabilizes dPER, suggesting that O-GlcNAcylation has multiple roles in circadian timing systems.

O- GlcNAc reports ambient temperature and confers heat resistance on ectotherm development

“…More generally, we would like to point out that regulation of O-GlcNAc in D. melanogaster is not governed exclusively by temperature. Indeed, other regulatory inputs are known (24)(25)(26)(27)(28)(29). We would also like to point out that in our analysis of larval extracts (Fig.…”

O-GlcNAcylation of a circadian clock protein: dPER taking its sweet time: Figure 1.

“…Kim et al (2012) show that down regulation as well as overexpression of OGT in Drosophila significantly affect period length but do not abolish rhythmicity. Hence, O-GlcNAcylation may not be essential for the clock mechanism per se, but rather may fulfill a regulatory function.…”

“…The findings by Kim et al (2012) add a new puzzle piece that might fit snugly into this mechanism of progressive phosphorylation. O-GlcNAcylation may directly block phosphorylation sites or slow down the kinetics of phosphorylation of nearby sites and thereby contribute to the characteristic kinetics of progressive phosphorylation of dPER.…”

“…The Drosophila circadian clock is established via a transcriptional feedback loop, in which negatively acting clock proteins (dPER-TIM) rhythmically repress the activity of their positively acting transcription factors (dCLK-CYC) and thereby generate self-sustained oscillations with a period length of ;24 h. Circadian timekeeping is dependent on post-transcriptional modifications of clock proteins, including phosphorylation, sumoylation, acetylation, ADP-ribosylation, and ubiquitination, which regulate their function, localization, and turnover (Mehra et al 2009;Asher et al 2010). Kim et al (2012) show that the circadian negative regulator dPER in Drosophila is modified by O-GlucNAcylation, which impacts on subcellular localization and turnover of the protein and affects the circadian period length. O-GlcNAcylation also plays a role in the circadian clocks of other organisms.…”

“…In light of the finding by Kim et al (2012), the question arises as to whether O-GlcNAcylation is intrinsically required in circadian clocks (e.g., to slow down progressive phosphorylation in order to generate an ;24-h period), or whether GlcNAcylation is a mechanism superimposed onto the clock system to allow an additional layer of regulation. Kim et al (2012) show that down regulation as well as overexpression of OGT in Drosophila significantly affect period length but do not abolish rhythmicity.…”