CLOCK is the abbreviation of ‘Circadian Locomotor Output Cycles Kaput’ (R).

CLOCK is a protein coding gene. The protein that this gene encoded plays a central role in the regulation of circadian rhythms (R).

CLOCK is a component of the circadian clock oscillator which includes the other proteins (R).

The mammalian circadian clock system is organized in a hierarchy of oscillators (R).

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus is at the top of this hierarchy. SCN is responsible for coordinating independent peripheral oscillators so that a coherent rhythm is regulated at the organismal level (R).

Lesions to the SCN disrupt circadian oscillations producing arrhythmicity (R).

Most of the core components of molecular clock maintain their rhythmicity in the SCN and in peripheral tissues, but some components vary in their intrinsic rhythmic properties across the tissues (R).

The circadian clock that is an internal time-keeping system regulates a wide variety of  physiological processes through the generation of approximately 24 hour circadian rhythms in gene expression, which are translated into rhythms in metabolism and behavior (R).

The circadian clock acts as an important regulator of a wide array of physiological functions including metabolism, sleep, body temperature, blood pressure, endocrine, immune, cardiovascular, and renal function (R).

Clock genes maintain circadian rhythmicity in constant darkness and they can be entrained to a new light/dark cycle (R).

The successive gene activation in the form of a cycle is the underlying principle of a circadian clock . The initial activation of a gene is regulated by the last one in the sequence, making up an auto-regulatory feedback loop for which one cycle takes about 24 hours (R).

Disorders of Clock

In humans, a polymorphism in Clock (rs6832769) is related to personality trait agreeableness (R).

Another single nucleotide polymorphism in Clock (3111C) is associated with diurnal preference and also with increased insomnia, difficulty in losing weight (R).

Variations in Clock gene may lead to an increased risk of breast cancer . Significantly less methylation of Clock promoter region was found in women with breast cancer (R).

Mutations in human circadian genes Per2 and CK1 are associated with the familial sleep disorders (R). In this disorder, patients exhibit early sleep onset followed by early-morning awakening (R).

In mice, Clock can be associated with a sleep disorder, metabolism, pregnancy, and mood disorders (R).

Clock mutant mice sleep less than normal mice each day and display altered levels of plasma glucose and rhythms in food intake (R).

These mutant mice have metabolic syndrome symptoms including hepatic steatosis, hyperleptinemia, hyperglycemia, and hypoinsulinemia (R) and disrupted estrous cycle and increased rates of full-term pregnancy failure (R).

Clock-knockout mice show increased baseline activity, increased locomotor sensitization to cocaine and increased drug reward compared to wild type animals (R).

Clock-null mice show altered response the light, but they continue to express robust circadian locomotor (movement) rhythms even under constant darkness (R).

Bmal1 mutant mice show increases in total sleep time (R).

Cry mutant mice show increases in baseline amount of non-REM sleep (R).

Based these results, the clock genes are involved in the susceptibility to sleep disorders (R).

The Mechanism of  Clock

In the nucleus the protein that CLOCK gene encodes forms a heterodimer with Bmal1  that binds E-box enhancer elements upstream of Period and Cryptochrome genes, and activates the transcription of these genes (R).

This figure shows the principle of circadian clock (R).

Very Advanced:

Positive elements activate the expression of negative elements, which in turn stop the activity of positive elements (R).

In mammalian, the circadian clock mechanism consists of two interlocking, regulatory feedback loops (R).

In the first loop, two transcriptional activators (Bmal1 and Clock) form heterodimers in the cytoplasm and enter the nucleus where the bind to E-box enhancer elements in the promoters of Period (Per1,2) and Cryptochrome (Cry1,2)  genes, and activates the transcription of these genes (R).

Also, various combinations of Per and Cry proteins in the cytoplasm interact with each other and enter the nucleus so that they can inhibit the activity of Bmal1/Clock complex (R).

CRY acts as light-independent inhibitors of Clock/Bmal1 (R).

A second loop regulates the expression of Bmal1 gene. In the nucleus, Bmal1/Clock heterodimers bind to E-boxes present int he promoters of genes that encode the retionic acid-related orphan nuclear receptors (Rev-erba and Rora) , which compete for the ROR relement (RORE) in the Bmal1 promoter. Rora activates Bmal1 expression, while Rev-erba represses it (R). So, the members of ROR and REV-ERB families participate in the control of Clock and Bmal1 expression (R).

Some researchers showed that in the mouse, the core mechanism of circadian clock consist of interacting positive and negative transcription and translation feedback loops (R).

In the Clock/Clock mutant mice, homozygous Per2 mutants and Cry-deficient mice, Bmal1 rhythms were altered. Per2 has a dominant role in the positive regulation of the Bmal1 loop (R). Cry1 and Cry2 are the negative regulators if the Period and Cryptochrome cycles (R).

Transcriptional activation of  the clock genes Per and Cry has required the histone acetyltransferase (HAT) activity of Clock. Histone acetylation promotes transcription through the modification of histones and allows opening of the condensed chromatin (R).

Transcriptional repression is mediated by loss of HAT activity. Per and Cry bind to Bmal1/ clock complex and results in loss of Hat activity by promoting Clock phosphorylation and/or inducing a conformational change of Bmal1/Clock (R).

The following transcription factors affect gene expression:

• ER-alpha
• CUTL1
• CREB
• Arnt
• RORalpha2
• deltaCREB
• PPAR-alpha
• Hlf

Tissue specificity:

Hair follicles (at protein level). Expressed in all tissues examined including spleen, thymus, prostate, testis, ovary, small intestine, colon, leukocytes, heart, brain, placenta, lung, liver, skeletal muscle, kidney and pancreas. Highest levels in testis and skeletal muscle. Low levels in thymus, lung and liver. Expressed in all brain regions with highest levels in cerebellum. Highly expressed in the suprachiasmatic nucleus (SCN).

Enzyme Regulation:

The redox state of the cell can modulate the transcriptional activity of the CLOCK-ARNTL/BMAL1 heterodimer; NADH and NADPH enhance the DNA-binding activity of the heterodimer.

Molecular Function:

• Chromatin Dna Binding
• Core Promoter Binding
• Core Promoter Sequence-Specific Dna Binding
• Dna Binding
• E-Box Binding
• Histone Acetyltransferase Activity
• Rna Polymerase Ii Core Promoter Proximal Region Sequence-Specific Dna Binding
• Sequence-Specific Dna Binding
• Transcriptional Activator Activity, Rna Polymerase Ii Transcription Factor Binding
• Transcription Factor Activity, Rna Polymerase Ii Core Promoter Proximal Region Sequence-Specific Binding
• Transcription Factor Activity, Sequence-Specific Dna Binding

Biological Processes:

• Cellular Response To Ionizing Radiation
• Circadian Regulation Of Gene Expression
• Circadian Rhythm
• Dna Damage Checkpoint
• Negative Regulation Of Glucocorticoid Receptor Signaling Pathway
• Negative Regulation Of Transcription, Dna-Templated
• Photoperiodism
• Positive Regulation Of Inflammatory Response
• Positive Regulation Of Nf-Kappab Transcription Factor Activity
• Positive Regulation Of Transcription, Dna-Templated
• Positive Regulation Of Transcription From Rna Polymerase Ii Promoter
• Proteasome-Mediated Ubiquitin-Dependent Protein Catabolic Process
• Regulation Of Hair Cycle
• Regulation Of Insulin Secretion
• Regulation Of Transcription, Dna-Templated
• Regulation Of Transcription From Rna Polymerase Ii Promoter
• Regulation Of Type B Pancreatic Cell Development
• Response To Redox State
• Signal Transduction
• Spermatogenesis
• Entrainment Of Circadian Clock