Makino Clint L, Peshenko Igor V, Wen Xiao-Hong, Olshevskaya Elena V, Barrett Ronald, Dizhoor Alexander M
Department of Ophthalmology, Massachusetts Eye and Ear Infirmary and Harvard Medical School, Boston, Massachusetts 02114, USA.
J Biol Chem. 2008 Oct 24;283(43):29135-43. doi: 10.1074/jbc.M804445200. Epub 2008 Aug 22.
Cyclic GMP serves as the second messenger in visual transduction, linking photon absorption by rhodopsin to the activity of ion channels. Synthesis of cGMP in photoreceptors is supported by a pair of retina-specific guanylyl cyclases, retGC1 and -2. Two neuronal calcium sensors, GCAP1 and GCAP2, confer Ca(2+) sensitivity to guanylyl cyclase activity, but the importance and the contribution of each GCAP is controversial. To explore this issue, the gene GUCA1B, coding for GCAP2, was disrupted in mice, and the capacity for knock-out rods to regulate retGC and generate photoresponses was tested. The knock-out did not compromise rod viability or alter outer segment ultrastructure. Levels of retGC1, retGC2, and GCAP-1 expression did not undergo compensatory changes, but the absence of GCAP2 affected guanylyl cyclase activity in two ways; (a) the maximal rate of cGMP synthesis at low [Ca(2+)] dropped 2-fold and (b) the half-maximal rate of cGMP synthesis was attained at a higher than normal [Ca(2+)]. The addition of an antibody raised against mouse GCAP2 produced similar effects on the guanylyl cyclase activity in wild type retinas. Flash responses of GCAP2 knock-out rods recovered more slowly than normal. Knock-out rods became more sensitive to flashes and to steps of illumination but tended to saturate at lower intensities, as compared with wild type rods. Therefore, GCAP2 regulation of guanylyl cyclase activity quickens the recovery of flash and step responses and adjusts the operating range of rods to higher intensities of ambient illumination.
环鸟苷酸（cGMP）作为视觉转导中的第二信使，将视紫红质吸收光子与离子通道的活性联系起来。光感受器中cGMP的合成由一对视网膜特异性鸟苷酸环化酶retGC1和retGC2支持。两种神经元钙传感器GCAP1和GCAP2赋予鸟苷酸环化酶活性对钙离子（Ca²⁺）的敏感性，但每种GCAP的重要性和贡献存在争议。为了探讨这个问题，编码GCAP2的基因GUCA1B在小鼠中被破坏，并测试了敲除视杆细胞调节retGC和产生光反应的能力。敲除并未损害视杆细胞的活力或改变其外段超微结构。retGC1、retGC2和GCAP-1的表达水平未发生代偿性变化，但GCAP2的缺失以两种方式影响鸟苷酸环化酶活性：（a）低钙离子浓度下cGMP合成的最大速率下降了两倍；（b）cGMP合成的半最大速率在高于正常的钙离子浓度下达到。添加针对小鼠GCAP2产生的抗体对野生型视网膜中的鸟苷酸环化酶活性产生了类似的影响。GCAP2敲除视杆细胞的闪光反应恢复比正常情况更慢。与野生型视杆细胞相比，敲除视杆细胞对闪光和光照阶跃变得更加敏感，但在较低强度下趋于饱和。因此，GCAP2对鸟苷酸环化酶活性的调节加快了闪光和阶跃反应的恢复，并将视杆细胞的工作范围调整到更高强度的环境光照。
