Okano T, Yamazaki K, Kasahara T, Fukada Y
Department of Biophysics and Biochemistry, Graduate School of Science, University of Tokyo, Japan.
J Mol Evol. 1997;44 Suppl 1:S91-7. doi: 10.1007/pl00000057.
The avian pinealocytes have an intrinsic circadian clock function that controls rhythmic synthesis of melatonin, and an environmental light signal can reset the phase of the clock. In addition to the photoendocrine function, the melatonin synthesis of the pinealocytes is regulated by neural signals from sympathetic nerves. Thus the avian pinealocytes show diagnostic characters which seem to represent an evolutionary transition from photosensory cells of lower vertebrates to the neuroendocrinal cells of mammals. To understand the evolutionary background of the regulatory mechanism for the melatonin synthesis in this organ, we screened the chicken pineal cDNA library to find alpha-subunits of heterotrimeric G-proteins involved in the photic and neural regulations. In addition to the transducin-like alpha-subunit (Gt alpha) supposed to mediate the photic pathway, we isolated cDNA clones encoding Gi2 alpha, Gi3 alpha, and Go1 alpha and its splicing variant Go2 alpha. The deduced amino acid sequence of each G alpha had a potential site for pertussis toxin-catalyzed ADP-ribosylation. As it is known that adrenergic receptor-mediated inhibition of melatonin synthesis is blocked by pertussis toxin, the G-proteins identified in the present study are likely to contribute to this neuroendocrine function of the chicken pineal cells.
禽类松果体细胞具有内在的昼夜节律时钟功能，可控制褪黑素的节律性合成，环境光信号能够重置生物钟的相位。除了光内分泌功能外，松果体细胞的褪黑素合成还受交感神经的神经信号调节。因此，禽类松果体细胞呈现出一些特征，似乎代表了从低等脊椎动物的光感受细胞到哺乳动物神经内分泌细胞的进化过渡。为了了解该器官中褪黑素合成调节机制的进化背景，我们筛选了鸡松果体cDNA文库，以寻找参与光调节和神经调节的异源三聚体G蛋白的α亚基。除了推测介导光信号通路的转导素样α亚基（Gtα）外，我们还分离出了编码Gi2α、Gi3α、Go1α及其剪接变体Go2α的cDNA克隆。每个Gα的推导氨基酸序列都有一个百日咳毒素催化的ADP核糖基化潜在位点。由于已知肾上腺素能受体介导的褪黑素合成抑制被百日咳毒素阻断，本研究中鉴定出的G蛋白可能有助于鸡松果体细胞的这种神经内分泌功能。
