The sweet tooth of the circadian clock

Abstract

The endogenous circadian clock is a key regulator of daily metabolic processes. On the other hand, circadian clocks in a broad range of tissues can be tuned by extrinsic and intrinsic metabolic cues. The bidirectional interaction between circadian clocks and metabolism involves both transcriptional and post-translational mechanisms. Nuclear receptors exemplify the transcriptional programs that couple molecular clocks to metabolism. The post-translational modifications of the core clock machinery are known to play a key role in metabolic entrainment of circadian clocks. O-linked N-acetylglucosamine modification (O-GlcNAcylation) of intracellular proteins is a key mediator of metabolic response to nutrient availability. This review highlights our current understanding of the role of protein O-GlcNAcylation in mediating metabolic input and output of the circadian clock.

Keywords: O-GlcNAc; circadian clock; metabolism; post-translational modification.

Figure 1 |. Underpinnings of the Self-Sustained Biological Clock |

BMAL1 and CLOCK proteins form a heterodimer that drives transcription of PER and CRY whose heterodimer in turn suppresses the BMAL1/CLOCK complex. As a second layer of regulation, ROR and REV-ERB promote and suppress, respectively, the transcription of BMAL1.

Figure 2 |. PTMs of Clock Components |

Key enzymes responsible for post-translational modifications of clock components relay important external and metabolic signals. The effect of the different PTMs vary depending on the mediating enzyme and target protein. Hexosamine biosynthesis pathway (HBP) flux leads to increased UDP-GlcNAc, the substrate for O-GlcNAcylation by OGT. OGT has been shown to add the O-GlcNAc modification on BMAL1, CLOCK, and PER. AMPK responds to an increase in AMP to ATP ratio leading to phosphorylation of CRY and PER. SIRT1 activity is dependent on increased NAD+ levels in order to deacetylase BMAL1 and PER2. GRK2 responds to increased light signaling through glutamate levels to phosphorylate PER and entrain the central circadian clock in the SCN. Lastly, GSK3-β is reactivated upon decreased insulin signaling inhibition to phosphorylate all four clock components. This figure illustrates the complexity of PTM regulation in clock entrainment.

Figure 3 |. Nutrient Sensing Pathway for O-GlcNAcylation |

Glucose influx into the cell can enter the glycogen biosynthesis pathway (GBP), pentose phosphate pathway (PPP), glycolysis, and hexosamine biosynthesis pathway (HBP), among others. Glutamine fructose-6-phosphate amidotransferase (GFAT) is responsible for controlling the flux of glucose into the HBP. The product of the HBP is UDP-GlcNAc, the substrate for O-GlcNAcylation by OGT.

Figure 4 |. Competing Roles of OGT and GSK3β in Clock Entrainment |

Acting in opposition, OGT promotes the stability of CLOCK and BMAL1 allowing for the transcription of clock controlled genes (CCG) facilitated by the heterodimer while GSK3β primes the two proteins for ubiquitination and subsequent degradation. Additionally, PER protein O-GlcNAcylation delays nuclear translocation while phosphorylation by GSK3β promotes nuclear translocation leading to inhibition of BMAL1/CLOCK function.

Figure 5 |. Clock Entrainment by Nutrient Sensing through O-GlcNAc and GSK3β |

Nutrition increase leads to upregulation in the hexosamine biosynthesis pathway (HBP) leading to increased O-GlcNAcylation and OGT activity while upregulation of the insulin signaling pathway (ISP) leads to downregulation of GSK3β. They work in competition to modulate clock and subsequently, clock controlled genes (CCG). Additionally, these two proteins are believed to regulate one another though the effect of their modification on either’s function remains to be elucidated.