Fig. 2 |. The cell-autonomous core components of the circadian oscillator govern the ~24-hour cycle of gene expression.

The oscillator consists of positive-feedback and negative-feedback loops that intersect via transcriptional and post-transcriptional regulatory mechanisms⁴. In the core loop, circadian locomotor output cycles kaput (CLOCK) and aryl hydrocarbon receptor nuclear translocator-like protein 1 (BMAL1, encoded by ARNTL) heterodimerize to drive expression of period circadian protein homologue (PER) and cryptochrome (CRY) genes via the E-box elements in the morning. PER and CRY proteins subsequently form a repressive complex to inhibit CLOCK and BMAL1 transactivation. The stability of the PER and CRY proteins is regulated by parallel E3 ubiquitin ligase pathways^21–23. In the secondary, or stabilization, loop two subfamilies of nuclear receptors, REV-ERBα and REV-ERBβ (encoded by NR1D1 and NR1D2, respectively) as repressors and RAR-related orphan receptors (RORs) α, β and γ as activators, antagonistically regulate BMAL1 and other target genes at the ROR/REV-ERB-response element (RORE) promoter elements at night-time²⁴. Another key circadian promoter element is the D-box, activated by the proline and acidic amino acid-rich basic leucine zipper (PAR-bZIP) proteins (such as D-box binding PAR bZIP transcription factor; DBP) and repressed by E4 promoter-binding protein 4 (E4BP4; also known as NFIL3)¹⁵. Together, clock-controlled genes (CCGs) are transcriptionally regulated by the three loops via activation of the E-box, RORE and D-box elements in their gene promoter regions. Various ligands (PER/CRY, ROR and REV-ERB ligands) and small molecules (casein kinase 1 (CK1) inhibitors) have been found to regulate core clock components and circadian functions.