NPAS2: An analog of clock operative in the mammalian forebrain

Neuronal PAS domain protein 2 (NPAS2) is a transcription factor expressed p rimarily in the mammalian forebrain. NPAS2 is highly related in primary ami no acid sequence to Clock, a transcription factor expressed in the suprachi asmatic nucleus that heterodimerizes with BMAL1 and regulates circadian rhy thm. To investigate the biological role of NPAS2, we prepared a neuroblasto ma cell Line capable of conditional induction of the NPAS2:BMAL1 heterodime r and identified putative target genes by representational difference analy sis, DNA microarrays, and Northern blotting. Coinduction of NPAS2 and BMAL1 activated transcription of the endogenous Per1, Per2, and Cry1 genes, whic h encode negatively activating components of the circadian regulatory appar atus, and repressed transcription of the endogenous BMAL1 gene. Analysis of the frontal cortex of wild-type mice kept in a 24-hour Light-dark cycle re vealed that Per1, Per2, and Cry1 mRNA Levels were elevated during darkness and reduced during Light, whereas BMAL1 mRNA displayed the opposite pattern . In situ hybridization assays of mice kept in constant darkness revealed t hat Per2 mRNA abundance did not oscillate as a function of the circadian cy cle in NPAS2-deficient mice. Thus, NPAS2 likely functions as part of a mole cular clock operative in the mammalian forebrain.