Johnson Wayne D, Parandaman Vijaya, Onaivi Emmanuel S, Taylor Robert E, Akinshola B Emmanuel
Department of Pharmacology, Howard University College of Medicine, 520 W Street N.W., Washington, DC 20059, USA.
Brain Res. 2008 Jul 30;1222:18-30. doi: 10.1016/j.brainres.2008.05.035. Epub 2008 May 23.
The mechanisms by which agonists and other ligands bind ligand-gated ion channels are important determinants of function in neurotransmitter receptors. The partial agonist, kainic acid (KA) activates a less desensitized, and more robust AMPA receptor (AMPAR) current than full agonists, glutamate or AMPA. Cyclothiazide (CTZ), the allosteric modulator of AMPARs, potentiates receptor currents by inhibiting receptor desensitization resulting from agonist activation. We have constructed an AMPAR GluR1 subunit deletion mutant GluR1L3T(Delta739-784) by deleting the splice-variable "flip/flop" region of the L3 domain in the wild-type receptor and compared its function to that of the wild-type GluR1 receptor and an AMPAR substitution mutant GluR1A782N. When compared to GluR1, the potency of glutamate activation of GluR1L3T was increased, in contrast to a decrease in potency of activation and reduced sensitivity to optimal concentrations of KA. Furthermore, GluR1L3T was totally insensitive to CTZ potentiation of KA and glutamate-activated currents in Xenopus laevis oocytes. The potency of glutamate and KA activation of GluR1A782N was not significantly different from that of the wild-type GluR1 receptor although the mutant receptor currents were more sensitive to CTZ potentiation than the wild-type receptor current. This result is an indication that glutamate and KA binding to the agonist (S1/S2) domain on AMPAR can be modulated by an expendable splice-variable region of the receptor. Moreover, the effect of the allosteric modulator, CTZ on agonist activation of AMPAR can also be modified by a non-conserved amino acid residue substitution within the splice-variable "flip/flop" region.
激动剂和其他配体与配体门控离子通道结合的机制是神经递质受体功能的重要决定因素。部分激动剂 kainic 酸（KA）比完全激动剂谷氨酸或 AMPA 激活的脱敏程度更低、更强劲的 AMPA 受体（AMPAR）电流。环噻嗪（CTZ）是 AMPAR 的变构调节剂，通过抑制激动剂激活导致的受体脱敏来增强受体电流。我们通过删除野生型受体 L3 结构域的可变剪接“翻转/摆动”区域构建了一个 AMPAR GluR1 亚基缺失突变体 GluR1L3T(Delta739 - 784)，并将其功能与野生型 GluR1 受体和 AMPAR 替代突变体 GluR1A782N 的功能进行了比较。与 GluR1 相比，GluR1L3T 的谷氨酸激活效力增加，而激活效力降低以及对最佳浓度 KA 的敏感性降低则相反。此外，在非洲爪蟾卵母细胞中，GluR1L3T 对 KA 和谷氨酸激活电流的 CTZ 增强作用完全不敏感。尽管突变体受体电流比野生型受体电流对 CTZ 增强作用更敏感，但 GluR1A782N 的谷氨酸和 KA 激活效力与野生型 GluR1 受体的效力没有显著差异。这一结果表明，谷氨酸和 KA 与 AMPAR 上激动剂（S1/S2）结构域的结合可被受体的一个可缺失可变剪接区域调节。此外，变构调节剂 CTZ 对 AMPAR 激动剂激活的作用也可通过可变剪接“翻转/摆动”区域内的一个非保守氨基酸残基替代来改变。
