Regulation of BMAL1 Protein Stability and Circadian Function by GSK3β-Mediated Phosphorylation

Abstract

BackgroundCircadian rhythms govern a large array of physiological and metabolic functions. To achieve plasticity in circadian regulation, proteins constituting the molecular clock machinery undergo various post-translational modifications (PTMs), which influence their activity and intracellular localization. The core clock protein BMAL1 undergoes several PTMs. Here we report that the Akt-GSK3β signaling pathway regulates BMAL1 protein stability and activity.Principal FindingsGSK3β phosphorylates BMAL1 specifically on Ser 17 and Thr 21 and primes it for ubiquitylation. In the absence of GSK3β-mediated phosphorylation, BMAL1 becomes stabilized and BMAL1 dependent circadian gene expression is dampened. Dopamine D2 receptor mediated signaling, known to control the Akt-GSK3β pathway, influences BMAL1 stability and in vivo circadian gene expression in striatal neurons.ConclusionsThese findings uncover a previously unknown mechanism of circadian clock control. The GSK3β kinase phosphorylates BMAL1, an event that controls the stability of the protein and the amplitude of circadian oscillation. BMAL1 phosphorylation appears to be an important regulatory step in maintaining the robustness of the circadian clock.

The intricate dance of post-translational modifications in the rhythm of life

“…Thr21 of BMAL1 is probably the priming site for GSK-3β-catalyzed phosphorylation at Ser17 (ref. 88). Phosphorylation-deficient forms of CLOCK and BMAL1 exhibit less transcriptional activity and greater stability than the wild-type proteins, in agreement with a model in which unstable CLOCK and BMAL1 have higher activity 87,88 .…”

“…88). Phosphorylation-deficient forms of CLOCK and BMAL1 exhibit less transcriptional activity and greater stability than the wild-type proteins, in agreement with a model in which unstable CLOCK and BMAL1 have higher activity 87,88 . Similarly, Thr451 and Thr461 in CLOCK are phosphorylated by cyclin-dependent kinase 5 (Cdk5) and consequently degraded 89 , thus resulting in increased CLOCK-mediated transactivation.…”

“…4). O-GlcNAcylation may compete with the phosphorylation of CLOCK and BMAL1 and indirectly inhibit ubiquitination because phosphorylation of these proteins promotes proteasomal degradation 87,88 . In spite of the central role of protein degradation in regulation of CLOCK and BMAL1, little is known regarding the E3 ligases promoting ubiquitination of CLOCK and BMAL1.…”

Endogenous Dopamine Regulates the Rhythm of Expression of the Clock Protein PER2 in the Rat Dorsal Striatum via Daily Activation of D2Dopamine Receptors

“…The Per2 promoter region contains a CRE-responsive site (TravnickovaBendova et al, 2002) and D 2 receptor stimulation upregulates Per expression in the retina through a CREB-dependent mechanism (Besharse et al, 2004;Yujnovsky et al, 2006). It is possible, however, that DA regulates PER2 expression directly through changes in posttranslational phosphorylation mechanisms (Iitaka et al, 2005) or indirectly by modulating other components of the clock (Beaulieu et al, 2004;Sahar et al, 2010).…”

“…In particular, activation of D 2 receptors has been demonstrated to modulate the circadian effects of light on locomotor activity in mice and to regulate expression of clock genes in the retina (Besharse et al, 2004;Yujnovsky et al, 2006) and striatum (Imbesi et al, 2009;Sahar et al, 2010), a region important for motor control, learning, cognition, and reward. There is no direct evidence, however, that endogenous DA regulates rhythmic expression of clock genes.…”