NPAS2

The mammalian and mouse Npas2 gene was first sequenced and characterized in 1997 Dr. Steven McKnight's lab and published by Yu-Dong Zhou et al. The gene’s cDNAs 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 and spinal cord tissues of mice. In situ hybridization indicated that the pattern of Npas2 mRNA distribution in mouse brain is broad and complex, and is largely non-overlapping with that of Npas1. Using Immunohistochemistry of human testis, Ramasamy et al. (2015) found the presence of NPAS2 protein in both germ cells within the tubules of the testes and in the interstitial space of Leydig cells. == Structure ==

In humans The Npas2 gene resides on chromosome 2 at the band q13. The predicted 824-amino acid human NPAS2 protein shares 87% sequence identity with mouse Npas2. == Function ==

In the brain The NPAS2 protein is a member of the basic helix-loop-helix (bHLH)-PAS transcription factor family and is expressed in the SCN. NPAS2 is a PAS 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 the BMAL1 protein and engage in a transcription/translation negative feedback loop (TTFL) to activate transcription of the mammalian Per and Cry core clock genes. NPAS2 has been shown to form a heterodimer 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 will upregulate to maintain the mammalian circadian rhythms. NPAS2 has been shown to be analogous to the function of CLOCK in CLOCK-deficient mice. Mice without functional alleles of both Clock and Npas2 became arrhythmic once placed in constant darkness, suggesting that both genes have overlapping roles in maintaining circadian rhythms. In both wild-type and Clock knockout mice, Npas2 expression is observed at the same levels, confirming that Npas2 plays a role in maintaining these rhythms in the absence of Clock. == Interactions ==

NPAS2 has been shown to interact with: • ARNTL (also known as BMAL1). Like Clock, Npas2 mRNA cycles with a similar phase to that of Bmal1, with both peaking 8 hours before the peak of Per2 mRNA expression. This is consistent with the observation that NPAS2 forms a heterodimer with BMAL1 to drive Per2 expression. • EP300. NPAS2 and EP300 interact in a time-dependent, synchronized manner. EP300 is recruited to NPAS2 as a coactivator of clock gene expression. • Retinoic acid receptor alpha (RARα) and retinoid X receptor alpha (RXRα). In peripheral clocks, RARα and RXRα interact with NPAS2 by inhibiting the NPAS2:BMAL1 heterodimer-mediated expression of clock genes. This interaction depends upon humoral signaling by retinoic acid and serves to phase-shift the clock. • Small heterodimer partner (SHP). In the liver circadian clock, NPAS2 and SHP engage in a TTFL: NPAS2 controls the circadian rhythms of SHP by rhythmically binding to its promoter, while SHP inhibits transcription of Npas2 when present. == Clinical significance ==

Npas2 genotypes can be determined through tissue samples from which genomic DNA is extracted and assayed. The assay is performed under PCR conditions and can be used to determine specific mutations and polymorphisms. Polymorphisms and tumorigenesis Mounting evidence suggests that the NPAS2 protein and other circadian genes are involved in tumorigenesis and tumor growth, possibly through their control of cancer-related biologic pathways. A missense polymorphism in NPAS2 (Ala394Thr) has been shown to be associated with risk of human tumors including breast cancer. NPAS2 and mood disorders Current research has revealed an association between seasonal affective disorder (SAD) and general mood disorder related to NPAS2, ARNTL, and CLOCK polymorphisms. These genes may influence seasonal variations through metabolic factors such as body weight and appetite. 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 a luciferase reporter driven by the murine monoamine oxidase A (MAOA) promoter in a circadian fashion. This suggested that these two clock components (BMAL1 and NPAS2) directly regulated MAOA transcription. == See also ==