Daniels Richard L, Takashima Yoshio, McKemy David D
Neuroscience Graduate Program, Neurobiology Section, Department of Biological Sciences, University of Southern California, Los Angeles, California 90089, USA.
J Biol Chem. 2009 Jan 16;284(3):1570-82. doi: 10.1074/jbc.M807270200. Epub 2008 Nov 18.
Cold temperatures robustly activate a small cohort of somatosensory nerves, yet during a prolonged cold stimulus their activity will decrease, or adapt, over time. This process allows for the discrimination of subtle changes in temperature. At the molecular level, cold is detected by transient receptor potential melastatin 8 (TRPM8), a nonselective cation channel expressed on a subset of peripheral afferent fibers. We and others have reported that TRPM8 channels also adapt in a calcium-dependent manner when activated by the cooling compound menthol. Additionally, TRPM8 activity is sensitive to the phospholipid phosphoinositol 4,5-bisphosphate (PIP2), a substrate for the enzyme phospholipase C (PLC). These results suggest an adaptation model whereby TRPM8-mediated Ca2+ influx activates PLC, thereby decreasing PIP2 levels and resulting in reduced TRPM8 activity. Here we tested this model using pharmacological activation of PLC and by manipulating PIP2 levels independent of both PLC and Ca2+. PLC activation leads to adaptation-like reductions in cold- or menthol-evoked TRPM8 currents in both heterologous and native cells. Moreover, PLC-independent reductions in PIP2 had a similar effect on cold- and menthol-evoked currents. Mechanistically, either form of adaptation does not alter temperature sensitivity of TRPM8 but does lead to a change in channel gating. Our results show that adaptation is a shift in voltage dependence toward more positive potentials, reversing the trend toward negative potentials caused by agonist. These data suggest that PLC activity not only mediates adaptation to thermal stimuli, but likely underlies a more general mechanism that establishes the temperature sensitivity of somatosensory neurons.
低温能强烈激活一小群体感神经，但在长时间的冷刺激过程中，它们的活性会随着时间的推移而降低或适应。这一过程有助于辨别温度的细微变化。在分子水平上，冷觉是由瞬时受体电位香草酸亚型8（TRPM8）检测到的，TRPM8是一种在外周传入纤维的一个子集上表达的非选择性阳离子通道。我们和其他人已经报道，当被冷却化合物薄荷醇激活时，TRPM8通道也会以钙依赖的方式适应。此外，TRPM8的活性对磷脂酰肌醇4,5-二磷酸（PIP2）敏感，PIP2是磷脂酶C（PLC）的底物。这些结果提示了一种适应模型，即TRPM8介导的Ca2+内流激活PLC，从而降低PIP2水平，导致TRPM8活性降低。在这里，我们通过PLC的药理学激活以及独立于PLC和Ca2+来操纵PIP2水平来测试这个模型。PLC激活导致异源细胞和天然细胞中冷刺激或薄荷醇诱发的TRPM8电流出现类似适应的降低。此外，独立于PLC降低PIP2对冷刺激和薄荷醇诱发的电流有类似的影响。从机制上讲，任何一种适应形式都不会改变TRPM8的温度敏感性，但确实会导致通道门控的变化。我们的结果表明，适应是电压依赖性向更正电位的转变，扭转了激动剂引起的向负电位的趋势。这些数据表明，PLC活性不仅介导对热刺激的适应，而且可能是建立体感神经元温度敏感性的更普遍机制的基础。
