Faculty of Life Sciences, University of Manchester, Manchester, UK.
J Neuroendocrinol. 2011 Aug;23(8):725-32. doi: 10.1111/j.1365-2826.2011.02168.x.
In mammals, the pineal hormone melatonin is secreted nocturnally and acts in the pars tuberalis (PT) of the anterior pituitary to control seasonal neuroendocrine function. Melatonin signals through the type 1 Gi-protein coupled melatonin receptor (MT1), inhibiting adenylate cyclase (AC) activity and thereby reducing intracellular concentrations of the second messenger, cAMP. Because melatonin action ceases by the end of the night, this allows a daily rise in cAMP levels, which plays a key part in the photoperiodic response mechanism in the PT. In addition, melatonin receptor desensitisation and sensitisation of AC by melatonin itself appear to fine-tune this process. Opposing the actions of melatonin, thyroid-stimulating hormone (TSH), produced by PT cells, signals through its cognate Gs-protein coupled receptor (TSH-R), leading to increased cAMP production. This effect may contribute to increased TSH production by the PT during spring and summer, and is of considerable interest because TSH plays a pivotal role in seasonal neuroendocrine function. Because cAMP stands at the crossroads between melatonin and TSH signalling pathways, any protein modulating cAMP production has the potential to impact on photoperiodic readout. In the present study, we show that the regulator of G-protein signalling RGS4 is a melatonin-responsive gene, whose expression in the PT increases some 2.5-fold after melatonin treatment. Correspondingly, RGS4 expression is acutely sensitive to changing day length. In sheep acclimated to short days (SP, 8 h light/day), RGS4 expression increases sharply following dark onset, peaking in the middle of the night before declining to basal levels by dawn. Extending the day length to 16 h (LP) by an acute 8-h delay in lights off causes a corresponding delay in the evening rise of RGS4 expression, and the return to basal levels is delayed some 4 h into the next morning. To test the hypothesis that RGS4 expression modulates interactions between melatonin- and TSH-dependent cAMP signalling pathways, we used transient transfections of MT1, TSH-R and RGS4 in COS7 cells along with a cAMP-response element luciferase reporter (CRE-luc). RGS4 attenuated MT1-mediated inhibition of TSH-stimulated CRE-luc activation. We propose that RGS4 contributes to photoperiodic sensitivity in the morning induction of cAMP-dependent gene expression in the PT.
在哺乳动物中，松果体激素褪黑素在夜间分泌，并在前垂体的结节部（PT）发挥作用，以控制季节性神经内分泌功能。褪黑素通过 1 型 Gi 蛋白偶联的褪黑素受体（MT1）发出信号，抑制腺苷酸环化酶（AC）的活性，从而降低第二信使 cAMP 的细胞内浓度。由于褪黑素的作用在夜间结束时停止，这使得 cAMP 水平在一天中上升，这在 PT 中的光周期反应机制中起着关键作用。此外，褪黑素受体脱敏和褪黑素自身对 AC 的敏化似乎使这一过程更加精细。与褪黑素的作用相反，由 PT 细胞产生的促甲状腺激素（TSH）通过其同源 Gs 蛋白偶联受体（TSH-R）发出信号，导致 cAMP 产生增加。这种效应可能有助于 PT 在春季和夏季增加 TSH 的产生，并且由于 TSH 在季节性神经内分泌功能中起着关键作用，因此具有相当大的意义。由于 cAMP 处于褪黑素和 TSH 信号通路的十字路口，任何调节 cAMP 产生的蛋白质都有可能影响光周期的读出。在本研究中，我们表明 G 蛋白信号调节蛋白 RGS4 是一种对褪黑素有反应的基因，其在 PT 中的表达在褪黑素处理后增加约 2.5 倍。相应地，RGS4 的表达对不断变化的日长非常敏感。在适应短日照（SP，每天 8 小时光照）的绵羊中，RGS4 的表达在黑暗开始后急剧增加，在午夜达到峰值，然后在黎明前下降到基础水平。通过将日长延长 8 小时来急性延迟熄灯，会导致傍晚 RGS4 表达的上升相应延迟，并且恢复到基础水平会延迟大约 4 小时进入第二天早上。为了测试 RGS4 表达是否调节褪黑素和 TSH 依赖的 cAMP 信号通路之间的相互作用的假设，我们在 COS7 细胞中转染 MT1、TSH-R 和 RGS4，并使用 cAMP 反应元件荧光素酶报告基因（CRE-luc）。RGS4 减弱了 MT1 介导的 TSH 刺激的 CRE-luc 激活的抑制作用。我们提出，RGS4 有助于 PT 中与光周期相关的 cAMP 依赖性基因表达在早晨的诱导中对光周期的敏感性。
