CLOCK

CLOCK is a transcription factor that, as a heterodimer with BMAL1, binds E-box elements to drive circadian and clock-controlled gene expression across multiple tissues (PMID:27883226, PMID:25448847). Genome-wide, rhythmic CLOCK:BMAL1 binding promotes nucleosome removal and a transcriptionally permissive chromatin state but is not sufficient on its own to activate transcription, which additionally requires ubiquitously expressed co-binding transcription factors at nearby sites (PMID:29300726). Through direct E-box binding, CLOCK activates specific targets including Pdia3, where CLOCK-driven Pdia3 expression maintains osteoblast viability and bone density (PMID:27883226) and suppresses ER stress and ROS in liver to limit a senescence/aging phenotype (PMID:29283886). CLOCK also controls adiponectin expression in adipocytes both directly and via circadian regulation of PPARgamma and PGC1alpha (PMID:25448847), and in glioma stem cells drives an OLFML3-HIF1alpha-LGMN-CD162 axis promoting immunosuppressive microglial infiltration (PMID:35413115). Beyond transcriptional activation, CLOCK positively regulates NFkappaB: loss of functional CLOCK reduces NFkappaB Lys310 acetylation and increases Ser276 phosphorylation, enhancing NFkappaB nuclear translocation and pro-inflammatory output, while restored CLOCK suppresses this and reduces tissue inflammation (PMID:30716535). CLOCK is functionally redundant in vivo, as Clock knockout mice remain rhythmic, likely through compensation by NPAS2 (PMID:16996737), and in Drosophila clock neurons CLOCK localizes to nuclear-envelope foci during the repression phase and participates in subnuclear repositioning of core clock genes to the periphery in a Lamin B receptor-dependent manner (PMID:34234015).

1. 2006 Medium

   Testing whether CLOCK is an essential core oscillator component, knockout revealed it is dispensable for rhythmicity, redefining it as a redundant rather than indispensable clock factor.

   Evidence Clock knockout mouse with analysis of rhythmic gene expression and locomotor behavior

   PMID:16996737

   Open questions at the time

   • Did not directly demonstrate NPAS2 as the compensating factor
   • Does not address tissue-specific or non-circadian CLOCK functions
2. 2014 Low

   To connect the clock to metabolism, CLOCK:BMAL1 was shown to regulate adiponectin both directly at its promoter and indirectly through PPARgamma/PGC1alpha, establishing a metabolic output of the heterodimer.

   Evidence siRNA knockdown, core clock reconstitution in cell culture, promoter reporter assay, and circadian expression analysis in mouse white adipose tissue

   PMID:25448847

   Open questions at the time

   • No ChIP or E-box mutagenesis to confirm direct binding to the adiponectin promoter
   • Relative contributions of direct vs indirect regulation not quantified
3. 2016 Medium

   Identifying a non-clock target, CLOCK was shown to directly activate Pdia3 via E-box binding, linking CLOCK to osteoblast survival and bone density through in vivo rescue.

   Evidence Luciferase reporter, ChIP, histology, real-time PCR, and forced-expression rescue plus siRNA in Clock-delta19 mutant mice

   PMID:27883226

   Open questions at the time

   • Single tissue context
   • Mechanism of PDIA3 protection of osteoblasts not detailed here
4. 2017 Medium

   Extending the Pdia3 axis, CLOCK-driven Pdia3 was shown to suppress ER stress and ROS in liver, establishing how loss of CLOCK target output produces a senescence/aging phenotype.

   Evidence Immunoblot, immunoprecipitation, real-time PCR, siRNA, and ChIP in Clock-delta19 mutant mice liver

   PMID:29283886

   Open questions at the time

   • Does not establish whether low E-box affinity is the sole cause of reduced Pdia3 in vivo
   • Causality between ER stress relief and aging phenotype is correlative
5. 2018 Medium

   Addressing whether CLOCK:BMAL1 binding is sufficient for activation, genome-wide analysis showed it promotes rhythmic nucleosome removal and permissive chromatin but requires co-binding transcription factors for actual transcription.

   Evidence Meta-analysis of ChIP-seq, H3K27ac, RNA Pol II ChIP-seq, eRNA, and ATAC-seq datasets with fasting perturbation

   PMID:29300726

   Open questions at the time

   • Identity of required co-binding factors at each enhancer not resolved
   • No in vitro reconstitution or mutagenesis to test sufficiency directly
6. 2019 Medium

   Beyond transcriptional activation, CLOCK was shown to control NFkappaB post-translationally, with functional CLOCK promoting Lys310 acetylation and suppressing Ser276 phosphorylation to limit inflammatory signaling in cartilage.

   Evidence ChIP, luciferase, immunoblot, immunoprecipitation, histology, and nanoparticle-mediated in vivo CLOCK expression in Clock-delta19 mice

   PMID:30716535

   Open questions at the time

   • Whether CLOCK directly modifies NFkappaB or acts via an intermediary acetyltransferase is unresolved
   • Single tissue (cartilage/osteoarthritis) context
7. 2021 Medium

   Probing a spatial dimension of clock repression, the CLOCK ortholog was shown to form nuclear-envelope foci and reposition core clock genes to the periphery during the repression phase via Lamin B receptor.

   Evidence Live imaging, DNA-FISH, immunofluorescence, and Lamin B receptor loss-of-function in Drosophila clock neurons

   PMID:34234015

   Open questions at the time

   • Demonstrated in Drosophila ortholog, not confirmed for mammalian CLOCK
   • Molecular mechanism tethering CLOCK to the nuclear envelope unknown
8. 2022 Medium

   Extending CLOCK to disease, the heterodimer was shown to drive an OLFML3-HIF1alpha-LGMN-CD162 axis in glioma stem cells that promotes immunosuppressive microglial infiltration.

   Evidence ChIP, transcriptome and scRNA-seq, TCGA analysis, knockdown/overexpression, and GBM mouse models with anti-PD-1

   PMID:35413115

   Open questions at the time

   • Whether this axis operates rhythmically or independently of circadian timing is unclear
   • Direct E-box binding to OLFML3 vs LGMN not fully delineated

• How CLOCK selects between permissive chromatin priming, direct target activation, and post-translational regulation of partners such as NFkappaB across tissues, and whether nuclear-envelope repositioning operates in mammals, remains open.
• No unifying mechanism for context-dependent transcriptional output
• Mammalian relevance of subnuclear repositioning untested
• Direct vs indirect basis of NFkappaB modification unresolved