Melatonin is synthesized in the pineal gland during nighttime in response to nocturnal increase in the activity of the enzyme aralkylamine N-acetyltransferase (AANAT), the transcription of which is modulated by several homeodomain transcription factors. Recent work suggests that the homeodomain transcription factor ISL LIM homeobox 2 (ISL2) is expressed in the pineal gland, but its role is currently unknown. With the purpose of identifying the mechanisms that control pineal expression of Isl2 and the possible function of Isl2 in circadian pineal biology, we report that Isl2 is specifically expressed in the pinealocytes of the rat pineal gland. Its expression exhibits a 24 h rhythm with high transcript and protein levels during the day and a trough in the second half of the night. This rhythm persists in darkness, and lesion studies reveal that it requires intact function of the suprachiasmatic nuclei, suggesting intrinsic circadian regulation. In vivo and in vitro experiments show that pineal Isl2 expression is repressed by adrenergic signaling acting via cyclic AMP; further, Isl2 is negatively regulated by the nocturnal transcription factor cone-rod homeobox. During development, pineal Isl2 expression is detectable from embryonic day 19, preceding Aanat by several days. In vitro knockdown of Isl2 is accompanied by an increase in Aanat transcript levels suggesting that ISL2 represses its daytime expression. Thus, rhythmic expression of ISL2 in pinealocytes is under the control of the suprachiasmatic nucleus acting via adrenergic signaling in the gland to repress nocturnal expression, while ISL2 itself negatively regulates daytime pineal expression of Aanat and thereby suggestively enhances the circadian rhythm in melatonin synthesis.

中文翻译：

---

ISL LIM-同源盒 2 转录因子受昼夜节律肾上腺素信号传导负调节，抑制大鼠松果体松果体细胞中 Aanat 的表达

松果体在夜间合成褪黑激素，以响应夜间芳烷基胺N-乙酰转移酶 (AANAT) 活性的增加，该酶的转录受到多种同源域转录因子的调节。最近的研究表明，同源域转录因子 ISL LIM 同源盒 2 (ISL2) 在松果体中表达，但其作用目前尚不清楚。为了确定控制Isl2表达的机制以及Isl2在昼夜节律松果体生物学中的可能功能，我们报告Isl2在大鼠松果体的松果体细胞中特异性表达。其表达呈现 24 小时节律，白天转录物和蛋白质水平较高，后半夜达到低谷。这种节律在黑暗中持续存在，病变研究表明，它需要视交叉上核的完整功能，这表明内在的昼夜节律调节。体内和体外实验表明，松果体Isl2表达受到环 AMP 作用的肾上腺素信号传导的抑制；此外，Isl2受到夜间转录因子锥杆同源盒的负调控。在发育过程中，松果体Isl2表达从胚胎第 19 天开始即可检测到，比Aanat早几天。Isl2的体外敲低伴随着Aanat转录水平的增加，表明 ISL2 抑制其白天表达。因此，松果体细胞中ISL2的节律性表达受到视交叉上核的控制，通过腺体中的肾上腺素信号传导抑制夜间表达，而ISL2本身负向调节白天松果体Aanat的表达，从而增强褪黑激素合成的昼夜节律。