Basic helix-loop-helix ARNT-like protein 1
Basic helix-loop-helix ARNT-like protein 1 or aryl hydrocarbon receptor nuclear translocator-like protein 1, or brain and muscle ARNT-like 1 is a protein that in humans is encoded by the BMAL1 gene on chromosome 11, region p15.3. It's also known as MOP3, and, less commonly, bHLHe5, BMAL, BMAL1C, JAP3, PASD3, and TIC.
BMAL1 encodes a transcription factor with a basic helix-loop-helix and two PAS domains. The human BMAL1 gene has a predicted 24 exons, located on the p15 band of the 11th chromosome. The BMAL1 protein is 626 amino acids long and plays a key role as one of the positive elements in the mammalian auto-regulatory transcription-translation negative feedback loop, which is responsible for generating molecular circadian rhythms. Research has revealed that BMAL1 is the only clock gene without which the circadian clock fails to function in humans. BMAL1 has also been identified as a candidate gene for susceptibility to hypertension, diabetes, and obesity, and mutations in BMAL1 have been linked to infertility, gluconeogenesis and lipogenesis problems, and altered sleep patterns. BMAL1, according to genome-wide profiling, is estimated to target more than 150 sites in the human genome, including all of the clock genes and genes encoding for proteins that regulate metabolism.
History
The BMAL1 gene was originally discovered in 1997 by two groups of researchers, John B. Hogenesch et al. in March under the name MOP3 and Ikeda and Nomura in April as part of a superfamily of PAS domain transcription factors. In 1998, Hogenesch's additional characterization of MOP3 revealed that its role as the partner of bHLH-PAS transcription factor CLOCK was essential to mammalian circadian clock function. The MOP3 protein, as it was originally known by the Hogenesch group, was found to dimerize with MOP4, CLOCK, and hypoxia-inducible factors. The names BMAL1 and ARNTL were adopted in later papers. One of BMAL1 protein's earliest discovered functions in circadian regulation was related to the CLOCK-BMAL1 heterodimer, which would bind through an E-box enhancer to activate the transcription of the AVP gene which encodes for vasopressin. However, the gene's importance in circadian rhythms was not fully realized until the knockout of the gene in mice showed complete loss of circadian rhythms in locomotion and other behaviors.Genetics
Regulation of ''Bmal1'' activity
regulates PER protein degradation by inhibiting transcriptional activity of the BMAL1:CLOCK heterodimer in a circadian manner through deacetylation. The degradation of PER proteins prevents the formation of the large protein complex, and thus disinhibits the transcriptional activity of the BMAL1:CLOCK heterodimer. The CRY protein is also signaled for degradation by poly-ubiquitination from the FBXL3 protein resulting in the disinhibition of BMAL1:CLOCK heterodimer activity.In addition to the circadian regulatory TTFL loop, Bmal1 transcription is regulated by competitive binding to the retinoic acid-related orphan receptor response element-binding site within the promoter of Bmal1. The CLOCK/BMAL1 heterodimer also binds to E-box elements in promoter regions of Rev-Erbα and RORα/ß genes, upregulating transcription and translation of REV-ERB and ROR proteins. REV-ERBα and ROR proteins regulate BMAL1 expression through a secondary feedback loop and compete to bind to Rev-Erb/ROR response elements in the Bmal1 promoter, resulting in BMAL1 expression repressed by REV-ERBα and activated by ROR proteins. Other nuclear receptors of the same families ; NR1F2 ; and NR1F3 ) have also been shown to act on Bmal1 transcriptional activity in a similar manner.
Several posttranslational modifications of BMAL1 dictate the timing of the CLOCK/BMAL1 feedback loops. Phosphorylation of BMAL1 targets it for ubiquitination and degradation, as well as deubiquitination and stabilization. Acetylation of BMAL1 recruits CRY1 to suppress the transactivation of CLOCK/BMAL1. The sumoylation of BMAL1 by small ubiquitin-related modifier 3 signals its ubiquitination in the nucleus, leading to transactivation of the CLOCK/BMAL1 heterodimer. CLOCK/BMAL1 transactivation, is activated by phosphorylation by casein kinase 1ε and inhibited by phosphorylation by MAPK. Phosphorylation by CK2α regulates BMAL1 intracellular localization and phosphorylation by GSK3B controls BMAL1 stability and primes it for ubiquitination.
In 2004, Rora was discovered to be an activator of Bmal1 transcription within the suprachiasmatic nucleus, regulated by its core clock. Rora was found to be required for normal Bmal1 expression as well as consolidation of daily locomotor activity. This suggests that the opposing activities of the orphan nuclear receptors RORA and REV-ERBα, the latter of which represses Bmal1 expression, are important in the maintenance of circadian clock function. Currently, Rora is under investigation for its link to autism, which may be a consequence of its function as a circadian regulator.
| Bmal1 Regulator/Modifier | Positive Or Negative Regulator | Direct or Indirect | Mechanism | Source |
| SIRT1 | Negative | Direct | BMAL1:CLOCK heterodimer deacetylation | |
| FBLX3 | Positive | Indirect | Poly-ubiquitination of PER promotes PER degradation | |
| REV-ERBα/β | Negative | Direct | Repression by binding Bmal1 promoter | |
| ROR-α/β/γ | Positive | Direct | Activation by binding Bmal1 promoter | |
| Acetylation | Negative | Direct | Recruits CRY1 to inhibit the BMAL1:CLOCK heterodimer | |
| Small ubiquitin-related modifier 3 | Positive | Direct | Sumoylation of BMAL1 | |
| Casein kinase 1ε | Positive | Direct | Phosphorylation of the CLOCK/BMAL1 heterodimer | |
| MAPK | Negative | Direct | Phosphorylation of the CLOCK/BMAL1 heterodimer | |
| CK2α | Unclear | Direct | Phosphorylation of BMAL1 | |
| GSK3B | Positive | Direct | Phosphorylation of BMAL1 |
Species distribution
Along with mammals such as humans and mice, orthologs of the Arntl gene are also found in fish, birds, reptiles, amphibians, and Drosophila. Unlike mammalian Arntl, circadian regulated, the Drosophila ''Cycle'' is constitutively expressed. In humans, three transcript variants encoding two different isoforms have been found for this gene. The importance of these transcript variants is unknown.Mutations and disease
The Arntl gene is located within the hypertension susceptibility loci of chromosome 1 in rats. A study of single nucleotide polymorphisms within this loci found two polymorphisms that occurred in the sequence encoding for Arntl and were associated with type II diabetes and hypertension. When translated from a rat model to a human model, this research suggests a causative role of Arntl gene variation in the pathology of type II diabetes. Recent phenotype data also suggest this gene and its partner Clock play a role in the regulation of glucose homeostasis and metabolism, which can lead to hypoinsulinaemia or diabetes when disrupted.In regards to other functions, another study shows that the CLOCK/BMAL1 complex upregulates human LDLR promoter activity, suggesting the Arntl gene also plays a role in cholesterol homeostasis. Furthermore, BMAL1 has been shown to influence excitability and seizure threshold. In addition, BMAL1 gene expression, along with that of other core clock genes, were discovered to be lower in patients with bipolar disorder, suggesting a problem with circadian function in these patients. An SNP in Bmal1 was identified as having a link with bipolar disorder. Arntl, Npas2, and Per2 have also been associated with seasonal affective disorder in humans. Alzheimer's patients have different rhythms in BMAL1 methylation suggesting that its misregulation contributes to cognitive deficits. Research has also shown that BMAL1 and other clock genes drive the expression of clock-controlled genes that are associated with Autism Spectrum Disorder. Lastly, BMAL1 has been identified through functional genetic screening as a putative regulator of the p53 tumor suppressor pathway suggesting potential involvement in the circadian rhythms exhibited by cancer cells.
In animal models of multiple sclerosis, namely the experimental autoimmune encephalomyelitis model, it has been shown that daily circadian rhythms can play an important role in disease pathology. Inducing EAE through the active immunization of mice with myelin oligodendrocyte glycoprotein peptide during the rest phase is more efficient in comparison to that during the active phase. Disparity in EAE induction is critically dependent on BMAL1 expression in T cells and myeloid cells. T cell or myeloid-specific deletion of Bmal1 has been shown to cause more severe pathology and is sufficient to abolish the rest vs. active induction effect.