Takanaka Y, Okano T, Iigo M, Fukada Y
Department of Biophysics and Biochemistry, Graduate School of Science, University of Tokyo, Japan.
J Neurochem. 1998 Mar;70(3):908-13. doi: 10.1046/j.1471-4159.1998.70030908.x.
The phase of a circadian clock in the chicken pineal gland is reset by an environmental light signal, which is captured by the pineal photoreceptive molecule(s). Here we show that the mRNA level of pinopsin, a predominant photoreceptive molecule in the pineal gland, undergoes a diurnal fluctuation in chickens maintained on a light/dark cycle. The mRNA levels in the light were approximately six times higher than those in the dark. This fluctuation was not observed in constant darkness, where the mRNA levels remained low. Subsequent light exposure of chickens increased the amount of pinopsin mRNA regardless of the circadian time. Clearly, the expression of pinopsin gene is controlled by a light signal, independent of the circadian clock. In vitro experiments using cultured pineal glands isolated from the visual system also revealed the light-dependent increase in pinopsin mRNA level, indicating that the pineal photoreceptive molecule(s) is responsible for the induction. These results demonstrate the presence of a feedback loop through which the light signal captured by pinopsin stimulates the transcription of its own gene in the chicken pineal gland. In contrast, pinopsin protein remained at an almost constant level in chickens maintained under the same light/dark cycles. The protein level, however, decreased to approximately 50% of the light/dark level under constant darkness and subsequently increased upon exposure to light after the dark period. It is suggested that, under the light/dark cycles, the pinopsin protein level is kept constant by the light-dependent synthesis, which would compensate for a possible degradation of pinopsin in the daytime.
鸡松果体生物钟的相位会被环境光信号重置，该信号由松果体光感受分子捕获。在此我们表明，在明暗循环条件下饲养的鸡中，松果体中主要的光感受分子视蛋白的mRNA水平呈现昼夜波动。光照下的mRNA水平比黑暗中高出约六倍。在持续黑暗中未观察到这种波动，此时mRNA水平保持较低。随后对鸡进行光照，无论昼夜时间如何，视蛋白mRNA的量都会增加。显然，视蛋白基因的表达受光信号控制，与生物钟无关。使用从视觉系统分离的培养松果体进行的体外实验也揭示了视蛋白mRNA水平的光依赖性增加，表明松果体光感受分子负责这种诱导。这些结果证明存在一个反馈回路，通过该回路视蛋白捕获的光信号刺激其自身基因在鸡松果体中的转录。相比之下，在相同明暗循环条件下饲养的鸡中，视蛋白水平几乎保持恒定。然而，在持续黑暗条件下，蛋白质水平降至明暗循环水平的约50%，随后在黑暗期后光照时增加。这表明，在明暗循环条件下，视蛋白水平通过光依赖性合成保持恒定，这可能会补偿白天视蛋白可能的降解。
