Neuronal PAS domain protein 2 (NPAS2) also known asmember of PAS protein 4 (MOP4) is atranscription factorprotein that in humans is encoded by theNPAS2gene.[5][6] NPAS2 is paralogous toCLOCK, and both are key proteins involved in the maintenance of circadian rhythms in mammals.[7] In thebrain, NPAS2 functions as a generator and maintainer of mammaliancircadian rhythms. More specifically, NPAS2 is an activator oftranscription andtranslation of core clock and clock-controlled genes through its role in anegative feedback loop in thesuprachiasmatic nucleus (SCN), the brain region responsible for the control of circadian rhythms.[8]
The mammalian and mouseNpas2 gene was firstsequenced and characterized in 1997Dr. Steven McKnight's lab and published by Yu-Dong Zhou et al.[9][10] The gene’scDNAs encoding mouse and human forms of NPAS2 were isolated and sequenced. RNA blotting assays were used to demonstrate the selective presence of the gene in brain andspinal cord tissues of mice.In situ hybridization indicated that the pattern of Npas2 mRNA distribution inmouse brain is broad and complex, and is largely non-overlapping with that ofNpas1.[10]
UsingImmunohistochemistry of human testis, Ramasamy et al. (2015) found the presence of NPAS2 protein in bothgerm cells within the tubules of the testes and in theinterstitial space ofLeydig cells.[10]
TheNpas2 gene resides onchromosome 2 at the band q13.[10] The gene is 176,679 bases long and contains 25exons.[11] The predicted 824-amino acid human NPAS2 protein shares 87% sequence identity with mouseNpas2.[10]
TheNpas2 gene has been found to reside on chromosome 1 at 17.98 centimorgans and is 169,505 bases long.[12]
The NPAS2 protein is a member of thebasic helix-loop-helix (bHLH)-PAStranscription factor family and is expressed in theSCN. NPAS2 is aPAS domain-containing protein, which binds other proteins via their own protein-protein (PAS) binding domains. Like its paralogue,CLOCK (another PAS domain-containing protein), the NPAS2 protein can dimerize with theBMAL1 protein and engage in atranscription/translation negative feedback loop (TTFL) to activate transcription of the mammalianPer andCry core clock genes.[8] NPAS2 has been shown to form aheterodimer with BMAL1 in both the brain and in cell lines, suggesting its similarity in function to the CLOCK protein in this TTFL.
Compensation is a key feature of TTFLs that regulate circadian rhythms. BMAL1 compensates for CLOCK in that if CLOCK is absent, BMAL1 willupregulate to maintain the mammalian circadian rhythms. NPAS2 has been shown to be analogous to the function of CLOCK in CLOCK-deficient mice.[8] InClock knockout mice, NPAS2 is upregulated to keep the rhythms intact.[8]Npas2-mutant mice, which do not express functional NPAS2 protein, still maintain robust circadian rhythms in locomotion. However, like CLOCK-deficient mice in the CLOCK/BMAL1 TTFL,Npas2-mutant mice (in the NPAS2/BMAL1 TTFL) still have small defects in their circadian rhythms such as a shortened circadian period and an altered response to changes in the typical light-dark cycle.[8] In addition,Npas2knockout mice show sleep disturbances and have decreased expression ofmPer2 in theirforebrains.[13] Mice without functional alleles of bothClock andNpas2 became arrhythmic once placed in constant darkness, suggesting that both genes have overlapping roles in maintaining circadian rhythms. In bothwild-type andClock knockout mice,Npas2 expression is observed at the same levels, confirming thatNpas2 plays a role in maintaining these rhythms in the absence ofClock.[8]
Npas2 is expressed everywhere in the periphery of the body. Special focus has been given to its function inliver tissues, and itsmRNA isupregulated inClock-mutant mice. However, studies have shown thatNpas2 alone is unable to maintain circadian rhythms in peripheral tissues in the absence of CLOCK protein, unlike in the SCN.[8] One theory to explain this observation is thatneurons in the brain are characterized by intercellular coupling and can thus respond to deficiencies in key clock proteins in nearby neurons to maintain rhythms. In peripheral tissues such as the liver andlung, however, the lack of intercellular coupling does not allow for this compensatory mechanism to occur. A second theory as to why NPAS2 can maintain rhythms in CLOCK-deficient SCNs but not in CLOCK-deficient peripheral tissues, is that there exists an additional unknown factor in the SCN that is not present in peripheral tissues.[8]
NPAS2-deficient mice have been shown to havelong-term memory deficits, suggesting that the protein may play a key role in the acquisition of such memories. This theory was tested by inserting a reporter gene (lacZ) that resulted in the production of an NPAS2 protein lacking the bHLH domain. These mice were then given several tests, including the cued and contextualfear task, and showed long-term memory deficits in both tasks.[14]
NPAS2 has been shown tointeract with:
Npas2genotypes can be determined through tissue samples from which genomicDNA is extracted and assayed. The assay is performed underPCR conditions and can be used to determine specific mutations andpolymorphisms.[19]
Mounting evidence suggests that the NPAS2 protein and other circadian genes are involved intumorigenesis and tumor growth, possibly through their control of cancer-related biologic pathways. Amissense polymorphism in NPAS2 (Ala394Thr) has been shown to be associated with risk of human tumors including breast cancer.[19] These findings provide evidence suggesting a possible role for the circadianNpas2 gene in cancer prognosis. These results have been confirmed in both breast and colorectal cancers.[20]
Current research has revealed an association betweenseasonal affective disorder (SAD) and general mood disorder related to NPAS2,ARNTL, andCLOCK polymorphisms. These genes may influence seasonal variations through metabolic factors such as body weight and appetite.[21][22]
Associated with a connection to mood disorders, NPAS2 has been found to be involved with dopamine degradation. This was first suggested by the observation that the clock components BMAL1 and NPAS2 transcriptionally activated aluciferase reporter driven by themurine monoamine oxidase A (MAOA) promoter in a circadian fashion.[23] This suggested that these two clock components (BMAL1 and NPAS2) directly regulatedMAOAtranscription.[23] Subsequent findings discovered positive transcriptional regulation of BMAL1/NPAS2 by PER2. In mice lacking PER2, bothMAOA mRNA and MAOA protein levels were decreased. Therefore, dopamine degradation was reduced, and dopamine levels in thenucleus accumbens were increased. These findings indicate that degradation ofmonoamines is regulated by the circadian clock. It is very likely that the described clock-mediated regulation of monoamines is relevant for humans, because single-nucleotide polymorphisms inPer2,Bmal1, andNpas2 are associated in an additive fashion with seasonal affective disorder or winter depression.[24]
This article incorporates text from theUnited States National Library of Medicine, which is in thepublic domain.
