Homeobox-clock protein interaction in zebrafish: A shared mechanism for pineal-specific and circadian gene expression

In non-mammalian vertebrates, the pineal gland is photoreceptive and contains an intrinsic circadian oscillator that drives rhythmic production and secretion of melatonin. These features require an accurate spatio-temporal expression of an array of specific genes in the pineal gland. Among these is the arylalkylamine-N-acetyltransferase (AANAT), a key enzyme in the melatonin production pathway. In zebrafish, pineal-specificity of zfaanat2 is determined by a region designated the pineal-restrictive downstream module (PRDM), which contains three photoreceptor conserved elements (PCEs) and an E-box, elements that are generally associated with photoreceptor-specific and rhythmic expression, respectively. Here, using in vivo and in vitro approaches, it was found that the PRDM PCEs and E-box mediate a synergistic effect of the photoreceptor-specific homeobox OTX5 and rhythmically-expressed clock protein heterodimer, BMAL/CLOCK, on zfaanat2 expression. Furthermore, the distance between the PCEs and E-box was found to be critical for PRDM function, suggesting a possible physical feature of this synergistic interaction. OTX5 - BMAL/CLOCK may act through this mechanism to simultaneously control pineal-specific and rhythmic expression of zfaanat2 and possibly also other pineal and retinal genes.