Ko Gladys Y-P, Ko Michael L, Dryer Stuart E
Department of Biology and Biochemistry and Biological Clocks Program, University of Houston, Houston, Texas 77204-5001, USA.
J Neurosci. 2004 Feb 11;24(6):1296-304. doi: 10.1523/JNEUROSCI.3560-03.2004.
Circadian oscillators in chicken cone photoreceptors regulate the gating properties of cGMP-gated cationic channels (CNGCs) such that they have a higher apparent affinity for cGMP during the subjective night. Here we show that cAMP, acting through protein kinase A (PKA), Ras, and Erk, is part of the circadian output pathway controlling CNGCs. Endogenous and exogenous cAMP cause activation of Erk and Ras, which are more active at night in cones, and increase the apparent affinity of CNGCs for cGMP. The Ras farnesyl transferase inhibitor manumycin-A, and a dominant-negative form of Ras (RasN17) block the circadian rhythms in CNGC gating, as well as the effects of cAMP. A dominant-negative form of the MEK kinase B-Raf also blocks circadian and cAMP modulation of CNGCs. The circadian output pathway modulating CNGC channels is comprised in part of cAMP --> PKA --> Ras --> B-Raf --> MEK --> Erk --> --> CNGCs. cAMP activation of Ras and Erk occur within minutes, whereas modulation of CNGCs requires >1 hr. However, cAMP protagonists do not alter rhythms in cPer2 mRNA, and their effects on CNGCs cannot be attributed to clock phase-shifting.
鸡视锥光感受器中的昼夜节律振荡器调节环磷酸鸟苷门控阳离子通道（CNGCs）的门控特性，使其在主观夜间对环磷酸鸟苷具有更高的表观亲和力。在此我们表明，通过蛋白激酶A（PKA）、Ras和细胞外信号调节激酶（Erk）起作用的环磷酸腺苷（cAMP）是控制CNGCs的昼夜节律输出途径的一部分。内源性和外源性cAMP会导致Erk和Ras的激活，它们在视锥细胞夜间更为活跃，并增加CNGCs对环磷酸鸟苷的表观亲和力。Ras法尼基转移酶抑制剂马马霉素-A以及一种显性负性形式的Ras（RasN17）可阻断CNGC门控的昼夜节律以及cAMP的作用。MEK激酶B-Raf的显性负性形式也可阻断CNGCs的昼夜节律和cAMP调节。调节CNGC通道的昼夜节律输出途径部分由cAMP→PKA→Ras→B-Raf→MEK→Erk→→CNGCs组成。cAMP对Ras和Erk的激活在数分钟内发生，而对CNGCs的调节则需要超过1小时。然而，cAMP激动剂不会改变cPer2 mRNA的节律，并且它们对CNGCs的作用不能归因于时钟相移。
