南京农业大学学报  2017, Vol. 40 Issue (5): 769-779   PDF    
http://dx.doi.org/10.7685/jnau.201702007
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文章信息

高聪芬, 牛春东, 王利祥, 魏琪, 吴顺凡
GAO Congfen, NIU Chundong, WANG Lixiang, WEI Qi, WU Shunfan
昆虫瞬时感受器电位(TRP)通道研究进展
Advances in insect transient receptor potential(TRP)channels
南京农业大学学报, 2017, 40(5): 769-779
Journal of Nanjing Agricultural University, 2017, 40(5): 769-779.
http://dx.doi.org/10.7685/jnau.201702007

文章历史

收稿日期: 2017-02-09
昆虫瞬时感受器电位(TRP)通道研究进展
高聪芬 , 牛春东, 王利祥, 魏琪, 吴顺凡    
南京农业大学植物保护学院/绿色农药创制与应用技术国家地方联合工程研究中心, 江苏 南京 210095
摘要:瞬时感受器电位(transient receptor potential,TRP)通道是一类位于细胞膜上的阳离子通道。TRP通道基因最先在研究视觉缺陷的突变体黑腹果蝇中被发现,因其受光刺激仅产生瞬时电生理信号而得名。此后基于序列同源性,在不同动物、酵母和藻类中均有发现,在果蝇中共鉴定得到13个基因,并划分为7个亚家族(TRPC、TRPA、TRPM、TRPV、TRPN、TRPP和TRPML)。TRP通道家族基因在昆虫体内扮演着调控各种生理与行为的重要角色,参与调控昆虫各种感觉的发生,例如视觉、嗅觉、听觉、温度感知以及机械刺激感觉等。近年来的研究表明,TRP家族中TRPV亚家族成员nanchunginactive基因所编码的蛋白复合物是杀虫剂吡蚜酮的分子靶标。本文分别介绍了TRP通道家族的命名、分类、结构、进化与激活机制等,并结合最新的文献进展对其参与昆虫各种生理与行为功能的研究进行了详细评述。
关键词TRP通道   生理学   行为学   杀虫剂   分子靶标   
Advances in insect transient receptor potential(TRP)channels
GAO Congfen , NIU Chundong, WANG Lixiang, WEI Qi, WU Shunfan    
College of Plant Protection, Nanjing Agricultural University/State & Local Joint Engineering Research Center of Green Pesticide Invention and Application, Nanjing 210095, China
Abstract: Transient receptor potential(TRP)channels superfamily of ion channels were first discovered in a mutant of Drosophila melanogaster which were visual defected, it was named by the transient electrophysiological signals when we used the light to stimuli this mutant. Thirteen genes were identified in Drosophila based on the sequence homology, and it was divided into seven subfamilies, TRPC, TRPA, TRPM, TRPV, TRPN, TRPP and TRPML. TRP channels also found in other animals, yeast, alage and so on. TRP superfamily involved in all physiological and sensory modalities in insects, such as vision, olfaction, hearing, thermosensation, mechanosensation, etc. Latest research shows that the protein complex of nanchung and inactive which belongs to TRPV subfamily of insect was the target of pymetrozine. In this paper, the nomenclature, classification, structure, evolution and activation mechanism of TRP channel family are introduced respectively, the TRP channels functions of physiological and behavioral in insects are reviewed in detail based on the latest literature.
Key words: TRP channels    physiology    behavior    pesticides    molecular targets   

瞬时感受器电位(transient receptor potential, TRP)通道是一类位于细胞膜上的重要阳离子通道。该离子通道家族存在于酵母、藻类和各种动物体内, 包括蠕虫、昆虫以及哺乳动物等[1], 涉及多种激活机制, 并参与到多种感觉的形成过程中, 如视觉、嗅觉、听觉、温度感觉、机械感觉等[2]。最新研究表明, 昆虫TRP通道家族中的Nanchung(Nan)与Inactive(Iav)蛋白复合物是杀虫剂吡蚜酮(pymetrozine)的分子靶标[3], 这也是首次报道TRP通道家族基因为杀虫剂的分子靶标。目前对于昆虫该家族基因的研究多集中于果蝇, 并取得了诸多进展, 本文在结合最新的文献报道的基础上对TRP通道的命名、分类、结构、进化、激活机制、功能及其应用进行系统阐述, 以期为昆虫学研究提供参考。

1 命名与分类

TRP通道家族首次发现于视觉传导受损的突变体黑腹果蝇(Drosophila melanogaster), 该突变体与野生型的光诱导反应不同, 它们对于强光刺激表现出瞬时的而非持续性的反应, Minke等[4]基于电生理的表现型于1975年首次将其命名为TRP(transient receptor potential)。1989年, Montell等[5]克隆并鉴定了果蝇的Dmtrp基因, 发现将其表达于突变体中可以挽救突变体果蝇对光反应的缺陷。

根据序列同源性及基因结构的相似性, Venkatachalam等[1]将TRP通道共划分为7个亚家族(图 1):TRPC、TRPA、TRPM、TRPV、TRPN、TRPP和TRPML。TRPC亚家族的C代表标准(canonical)或者经典(classical), 该亚家族包含最先在果蝇中发现的Dmtrp基因, 并且该家族的基因与Dmtrp具有极高的同源性, 其他各个亚家族的命名是以该亚家族中首次发现的成员的首字母来进行的[1,5]。TRPV亚家族的V代表辣椒素(vanilloid), 以纪念该家族第一个成员TRPV1(vanilloid receptor 1, VR1) 被辣椒素类化学物质所激活[6]。TRPM亚家族中的M代表黑素瘤(melastatin), 因该家族第一个成员TRPM1(tumor suppressor melastatin 1) 在黑色素瘤细胞中被克隆而得名[7]。TRPA亚家族中的A代表锚蛋白(ankyrin), 源于TRPA1(ankyrin-like with transmembrane domain 1, ANKTM1) 基因[8]。TRPN则是该亚家族唯一成员NompC(No Mechanoreceptor Potential C)的首字母[9]。TRPP中的P代表多囊(polycystic), 源于首次发现的TRPP2(polycystic kidney disease-related protein 2, PKD2)[10]。TRPML中的ML代表黏液素(mucolipin), 源于首次发现的TRPML1(mucolipin 1)[11-12]。同时基于TRP通道家族序列和拓扑结构的差异, 又可将其划分为2组(图 1)[1]:组1包含前5个亚家族, 且与果蝇Dmtrp基因具有较高的同源性; 组2与组1的同源关系较远, 包含其余的2个亚家族(TRPP和TRPML), 这2个亚家族在跨膜序列上存在较高的同源性。

图 1 果蝇瞬时感受器电位(TRP)通道系统进化树及结构示意图 Figure 1 Phylogenetic tree and structures of transient receptor potential(TRP)channels from Drosophila melanogaster(Dm) 选用果蝇TRP通道家族基因的跨膜区氨基酸序列构建进化树, 以蓝色字体的基因为例绘制各亚家族的结构示意图。蓝色圆柱代表跨膜结构域; 红色方块代表锚蛋白重复序列(AR); 黑色方块代表卷曲螺旋区域(CC); 其他结构域名称均在对应图形处标注。浅灰色和黄色背景分别代表TRP通道的2个分组(修改自Venkatachalam等[1])。 The amino acid sequences of Drosophila TRP channels transmembrane domain were used for the phylogenetic tree construction, the gene names of blue font were used for the structure diagram drawing. Blue cylinders represent the transmembrane domain, red squares represent ankyrin repeat(AR), black squares represent coiled-coil(CC), other domain name were annotated in the corresponding graphics. Light gray and yellow background represent two groups of TRP channels(Adapted from Venkatachalam, et al. [1]).
2 结构

TRP通道具有6次跨膜结构域(TM1~TM6), 在TM5与TM6之间形成孔道区域[13], 其N端和C端均位于细胞内(图 1)。控制通道开放的调控元件可能位于TM1~TM4以及胞内的N端和C端上。TRP通道可形成同聚体或异聚体的功能结构域, 在信号转导中发挥重要作用[14]

在组1成员中, 除TRPM亚家族外, 其他亚家族的N端都具有多个锚蛋白重复序列(ankyrin repeats, AR), 但不同通道包含的AR数目不同。TRPC、TRPM和TRPN亚家族在第6跨膜结构域后还具有TRP结构域(TRP domain), 最保守的部分称为TRP box 1(氨基酸序列为EWKFAR)和TRP box 2(氨基酸序列为LPPPFN)[1]。此外, 有些通道的C末端还具有酶结构域, 因此又称为通道酶, 如哺乳动物的TRPM2具有1个Nudix水解酶结构域[15]。组2成员TRPP和TRPML在第1和第2跨膜结构域(TM1、TM2) 存在着1个较大的胞外环, 而组1成员却不存在(图 1)。还有其他的结构域和基序也都会影响TRP通道的功能, 例如卷曲螺旋、钙调素结合位点、EF手形或者氧化磷酸化位点等, 它们的种类和数量多种多样, 即使同一个亚家族中的不同成员间, 也可能会有所不同[16]

3 进化

TRP通道广泛存在于昆虫、蠕虫以及哺乳动物等生物体内(表 1)。此外, 在酵母中也存在TRP通道, 如YVC1[17-19]、PKD2[20]、TRPV[21]等, 这表明TRP通道的出现早于多细胞生物。尽管TRP通道分布广泛, 但大量基因组数据研究表明, 植物中除绿藻(Ostreococcus tauri)、莱茵衣藻(Chlamydomonas reinhardtii)等藻类植物外, 陆地植物并不存在类似于TRP通道的基因, 但是将莱茵衣藻中的TRP通道基因CrTRP1 表达于HEK293细胞后能够表现出与哺乳动物同源TRP通道相关的功能[22], 这可能是在从藻类向陆地植物分化的过程中缺失了TRP通道。值得注意的是, TRPN亚家族广泛存在于无脊椎动物, 一些脊椎动物, 如斑马鱼(Danio rerio)和非洲爪蟾(Xenopus laevis)[23-24]体内也有存在, 但不存在于哺乳动物体内。TRPN亚家族成员与机械感受有关, 这可能是在动物的进化过程中, 该亚家族成员因不适于哺乳动物的机械感受机制而发生了进一步的进化。

表 1 6种昆虫、线虫、老鼠及人类TRP家族比较[1,25] Table 1 Composition of TRP members in six species of insects, worms, mice and humans[1,25]
物种Species TRPC TRPA TRPM TRPV TRPML TRPP TRPN
黑腹果蝇 Drosophila melanogaster 3 4 1 2 1 1 1
家蚕 Bombyx mori 3 6 1 2 1 0 1
意大利蜜蜂 Apis mellifera 3 5 1 2 1 0 1
赤拟谷盗 Tribolium castaneum 3 5 1 2 1 1 1
人体虱 Pediculus humanus 3 4 1 2 1 1 1
豌豆长管蚜 Acyrthosiphon pisum * 2(3) 4 1 2 1 1 1
秀丽隐杆线虫 Caenorhabditis elegans 3 2 4 5 1 1 1
老鼠 Mice 7 1 8 6 3 3 0
人类 Human 6 1 8 6 3 3 0
注:*豌豆长管蚜可能含有更多的TRP亚家族成员。*A. pisum may contain additional TRP subfamily members.

昆虫TRP通道家族的进化也是多种多样的。在已报道开展TRP通道研究的昆虫中, 大部分昆虫具有7个亚家族, 但意大利蜜蜂(Apis mellifera)、丽蝇蛹集金小蜂(Nasonia vitripennis)、红火蚁(Solenopsis invicta)等膜翅目昆虫以及家蚕(Bombyx mori)均缺失TRPP亚家族[25]。TRPA亚家族在不同昆虫中的数目各不相同, 与哺乳动物(1个)和线虫(2个)相比, TRPA亚家族在昆虫中的数目明显偏多(表 1), 可能是昆虫在与其他多细胞动物分化后的进化过程中特异性地扩增了TRPA亚家族成员的数量[26]; 除了数量不同外, 该亚家族在不同昆虫中基因的种类也不相同(表 2), 膜翅目昆虫体内无TRPA1, 但是存在一种特有的TRP通道HsTRPA(Hymenoptera specific TRPA)[27]; 家蚕、豌豆长管蚜(Acyrthosiphon pisum)体内含有2个Wtrw(Waterwitch), 而膜翅目昆虫、家蚕、赤拟谷盗(Tribolium castaneum)体内存在的TRPA5却不存在于果蝇、人体虱、豌豆长管蚜体内[25]。TRPC、TRPV、TRPN、TRPM和TRPML这5个亚家族在昆虫中相对保守, 不存在基因的增加或缺失[26]

表 2 TRPA亚家族在6种昆虫中的分布[25] Table 2 The number of TRPA subfamily members in six species of insects[25]
昆虫Insects TRPA1 Pain Pyx Wtrw HsTRPA TRPA5
黑腹果蝇 Drosophila melanogaster 1 1 1 1 0 0
家蚕 Bombyx mori 1 1 1 2 0 1
意大利蜜蜂 Apis mellifera 0 1 1 1 1 1
赤拟谷盗 Tribolium castaneum 1 1 1 1 0 1
人体虱 Pediculus humanus 1 1 1 1 0 0
豌豆长管蚜 Acyrthosiphon pisum 1 1 0 2 0 0
4 激活机制

TRP通道作为细胞重要的感受器, 会受到细胞内外的化合物、信使分子、渗透压、温度等变化的影响而被激活, 不同TRP通道的激活需要不同类型的刺激, 甚至可能会受到多种类型的刺激同时被激活。例如哺乳动物的TRPV1, 既可以被辣椒素、异硫氰酸烯丙酯(AITC)等化合物激活, 又可被酸碱度、高温等激活[6,28-30]。2015年, Liu等[31]提出一个全新的观点, 他们认为TRP通道家族中绝大多数都属于机械敏感性离子通道(mechanosensitive channels, MSC), TRP通道可以由机械刺激直接激活, 也可通过信号级联等由机械刺激间接激活。根据机械力的传递途径激活机制可分为以下2类:

4.1 机械刺激直接控制开放

几乎所有的TRP通道都位于细胞膜上, 因此胞外来源的力可能在没有信号级联的情况下直接调控通道的开放。Liu等[31]认为, 机械刺激既可以直接导致TRP通道的开放, 又可以通过改变磷脂双分子层的曲率而产生机械张力, 进而导致通道的开放, 并且激活时间少于2 ms的通道就属于机械刺激直接控制开放的通道, 例如果蝇TRPN亚家族的DmNompC, 将其异源表达于非机械敏感型的细胞中, 可使该细胞产生机械响应, 并且能够在受到机械刺激后的1~2 ms之内使通道激活[32]。有些TRP通道的激活可能是因为蛋白质本身具有热敏感性, 温度变化可能会改变这些蛋白的构象从而导致膜脂结构的变化, 也可能是因为温度诱导膜曲率发生改变从而产生机械力, 进而导致温度敏感型TRP通道的开放, 例如DmPain(painless)在高于38 ℃时激活[33], DmPyx(pyrexia)在高于40 ℃时激活[34]

多数TRP通道的N端都具有串联的AR, 其机械直接激活可能需要AR的参与。TRPN亚家族的AR最多, 共29个[9], 这一串联的AR形成了类似于“门控弹簧子”的功能结构, 外力能够引起细胞膜曲率发生改变, 进而导致AR构象的变化, 从而将机械力传递到通道的孔道区域引起通道的开放。Zhang等[35]通过删减或增添果蝇DmNompC的AR后, 发现这样会影响DmNompC蛋白在细胞中的定位, 同时影响果蝇的轻触反应, 进一步证实了该门控模型。

4.2 信号级联引起的机械刺激间接控制开放

在果蝇的光感受细胞中, DmTRP和DmTRPL(transient receptor potential-like)属于典型的具有信号级联反应的通道, 它们在受光照后的几毫秒内就会开放, 并且在10~20 ms内达到响应的最大值[36-37]。光子被成年果蝇视觉器官中的视紫红质捕获后, 异三聚体G蛋白会将信号放大, 激活DmnorpA编码的磷脂酶C(phospholipase C, PLC), 催化4, 5-二磷酸磷脂酰肌醇(phosphatidylinositol 4, 5-bisphosphate, PIP2)水解成为1, 4, 5-三磷酸肌醇(inositol 1, 4, 5-trisphosphate, IP3)、甘油二酯(diacylglycerol, DAG)和质子(H+)[38-39]。而在哺乳动物的视杆细胞和视锥细胞中, 这个信号级联反应与果蝇完全不同, 它们依赖于环磷酸鸟苷(cGMP)作为第二信使, 光诱导可导致最终cGMP水平的下降并引起cGMP门控离子通道的关闭[40]。DAG新陈代谢产生的多不饱和脂肪酸(polyunsaturated fatty acids, PUFA)能够影响DmTRP和DmTRPL通道的激活[41], PUFA可能会影响DmTRP和DmTRPL通道周围的膜结构而产生机械力, 从而导致通道的激活。

TRPC[42]、TRPV[43]、TRPM[43-44]、TRPA[45]、TRPP[46]和TRPML[47]亚家族通道的激活过程多数会有PIP2和其他膜脂类物质的水解发生, 它们通过信号级联进一步改变膜脂的浓度而产生机械力, 这些都有可能是机械敏感型离子通道。温度敏感型TRP通道的激活原因除机械刺激直接控制开放外, 也可能受到PIP2水解的影响, 例如果蝇DmTRPA1在25 ℃以下的激活依赖于G蛋白/PLC信号级联[45], 该信号级联同样依赖于视紫红质, 这属于不依赖于光作用的G蛋白偶联受体类型[48]

5 功能

TRP通道的功能可分为以下5类:光感受、温度感受、机械感受、化学感受、新陈代谢与疾病。这些功能对于生物在自然界中的生存至关重要, 尤其是昆虫, 它们体积较小, 属于变温动物, 同时容易受到天敌的威胁, 因此需要相适应的感觉机制来应对外界环境的变化。目前对于昆虫TRP通道的研究仅局限于较少的物种, 而果蝇作为模式生物, 由于其简单的神经元与方便的遗传学操作手段, 被用作研究TRP通道功能的最佳模型, 并且也研究的最透彻(表 3)。随着RNAi技术的不断成熟及大家对TRP通道的逐渐重视, 其他昆虫TRP通道的研究也相继展开, 但是研究对象仍然局限于较少的物种, 如家蚕、棉铃虫(Helicoverpa armigera)、意大利蜜蜂、冈比亚按蚊(Anopheles gambiae)等, 研究的基因也主要集中于TRPA、TRPC和TRPV这3个亚家族。

表 3 果蝇TRP通道的特征 Table 3 Properties of Drosophila TRP channels
通道
Channels
激活
Activation
功能
Function
参考文献
References
DmTRP Gq/PLC信号通路Gq/PLC signal pathway 光感受Phototransduction [41,49]
躲避寒冷(10 ℃) Cool avoidance(10 ℃) [50]
DmTRPL Gq/PLC信号通路Gq/PLC signal pathway 光感受Phototransduction [41,49]
躲避寒冷(10 ℃) Cool avoidance(10 ℃) [50]
樟脑油Camphor 味觉Gustation [51]
DmTRPγ Gq/PLC信号通路Gq/PLC signal pathway [52]
膜张力Membrane stretch 协调运动Coordinated movement [53]
DmTRPA1 > 26 ℃ 躲避温暖温度(28~35 ℃) Avoidance of warm temperature(28-35 ℃) [50,54-55]
躲避高温伤害(46 ℃) Avoidance of noxious heat(46 ℃) [56,57]
18~24 ℃, Rh1/PLC信号通路Rh1/PLC signal pathway 舒适温度感受(18~24 ℃) Comfortable temperature sensation(18-24 ℃) [45,48]
AITC, N-methylmaleimide, CA 躲避厌恶型非易失性刺激物Avoidance of aversive non-volatileirritants [58]
马兜铃酸(Gq/PLC信号通路) Aristolochic acid(Gq/PLC sig.) 躲避厌恶型促味剂Avoidance of aversive tastants [59]
香茅醛(Gq/PLC信号通路) Citronellal(Gq/PLC sig.) 躲避厌恶型气味Avoidance of aversive odorants [60]
光照Light 躲避强光Avoidance of bright light [61]
躲避机械刺激Avoidance of mechanical stimulation [56]
DmPain 高温( > 38 ℃) Heat( > 38 ℃) 躲避高温伤害( > 38 ℃) Avoidance of noxious heat( > 38 ℃) [33]
躲避机械刺激Avoidance of mechanical stimulation [33]
躲避异硫氰酸酯Avoidance of isothiocyanate [62]
躲避干燥环境Avoidance of dry environments [63]
重力感受Gravity sensation [64]
DmPyx 高温( > 40 ℃) Heat( > 40 ℃) 忍受高温伤害(40 ℃) Noxious heat resistance(40 ℃) [34]
重力感受Gravity sensation [64]
DmWtrw 检测潮湿空气Humid air detection [65]
DmNan 检测干燥空气Dry air detection [65]
运动Locomotion [53]
低渗溶液Hypo-osmotic solution 听觉Hearing [66-69]
重力感受Gravity sensation [64]
DmIav 低渗溶液Hypo-osmotic solution 听觉Hearing [66,69]
重力感受Gravity sensation [64,68]
运动Locomotion [53]
躲避寒冷(14~16 ℃) Cool avoidance(14-16 ℃) [70]
DmNompC 机械刺激Mechanical stimulation 轻触Gentle touch [32]
运动Locomotion [71]
听觉Hearing [68]
DmTRPM Mg2+和Zn2+体内平衡Mg2+ and Zn2+ homeostasis [72-73]
DmPKD2 精子储存Sperm storage [74]
DmTRPML 运动Locomotion [75]
自噬Autophagy [75-76]
清除凋亡细胞Clearance of apoptotic cells [75]
注:—:激活机制或功能未知。—:Activate mechanism or function is unkown.
5.1 光感受

对于昆虫来说, 光感受与其求偶行为、昼夜节律、运动导向等有关。TRP通道中最先发现的DmTRP便与光感受有关, 它是1个Ca2+通透性的具有孔道的通道亚基[77], 并且其突变体对光反应是瞬时的; DmTRPL作为与DmTRP相关的通道, 能够在Dmtrp突变体中感知光, 使果蝇产生光响应, 并且形成双突变体Dmtrpl; Dmtrp可使果蝇致盲[49]。电生理结果证明, Dmtrpl突变体受光刺激后的振幅较小, 而Dmtrp突变体的振幅较大[78]

果蝇幼虫视觉系统相较于成虫而言较为简单, Bolwig器官(Bolwig organ, BO)是幼虫主要的视觉器官, 但是幼虫BO中的视觉传导只有视紫红质与DmTRPL的信号级联, 而无DmTRP的参与[79], 并且对此研究内容较少。美洲大蠊(Periplaneta americana)的Patrpl基因被沉默后发现其视网膜电流受到严重影响, 而沉默Patrp基因则无太大影响, 说明PaTRPL可能对于夜视动物的视觉传导起重要作用[80]。最近研究表明, 果蝇幼虫对强光的反应并非由于BO, 而是依赖于体壁上的第4级多树突神经元(class Ⅳ multidendritic, mdⅣ), 这些神经元受损会影响果蝇对强光、高温、机械刺激以及干燥环境的躲避行为[56,61,63]。果蝇幼虫躲避高强度的光照需要mdⅣ神经元中的DmTRPA1以及味觉受体DmGR28b的参与, 但是DmTRPA1与DmGR28b之间的联系并不清楚[61]

5.2 温度感受

与温度感受相关的TRP通道称为“thermoTRP”。在昆虫中, 温度的变化可以模拟昼夜周期来调控它们的生理节奏[81]。DmTRPA1作为一个温度敏感元件既能感知外界温度的变化速率[82], 也能调控果蝇的生理节奏[83], DmPyx同样可以通过感受温度来调节果蝇的生物钟[84]。BmTRPA1不仅能够使家蚕感知温度变化并迅速做出即时反应, 而且能够调控与温度有关的长期的适应性滞育反应, 影响后代的滞育行为[85]。因此对作为变温动物的昆虫而言, thermoTRP对于昆虫感知外界温度的变化至关重要。

每种昆虫都有其偏爱的温度范围, 作为模式昆虫的果蝇, 它的舒适温度范围是18~24 ℃, DmTRPA1有助于果蝇幼虫更多地选择18 ℃[45]。冈比亚按蚊的幼虫在27 ℃饲养环境中偏好温度为27 ℃及33 ℃, 当饲养温度提升至30 ℃时, 其偏好温度也相应升高了3 ℃。干扰试验表明AgTRPA1能调控幼虫对较高温度的偏好选择[86]。虽然膜翅目昆虫并不具有TRPA1通道[26], 如意大利蜜蜂, 其振翅会产生大量的热, 但仍可维持35 ℃的生存环境, 这是由于膜翅目中特有的HsTRPA来感受周围温度以维持蜂巢温度的稳定[27]

当外界温度超过偏爱范围时, 无论过低或过高均会使昆虫不舒适而产生逃避行为。哺乳动物的TRPM8是一个冷觉感受器[87], 但是并没有研究说明果蝇TRPM也与温度感觉有关[72-73], 不过DmIav、DmTRP和DmTRPL却有助于感觉低于17.5~18 ℃的冷温[50,70]。果蝇DmTRPA1具有4种蛋白质亚型(DmTRPA1-A~D), 并且具有不同的温度激活机制[56,60]。DmTRPA1-A激活的温度阈值约24~29 ℃[56], 果蝇的温度感受器驱使其逃避超过该阈值的温度(非伤害性的不舒适温度)[45,54-55]。虽然DmTRPA1-C不是一个直接的温度感受器, 但是将其过表达于Dmtrpa1 突变体的mdⅣ神经元后仍可挽救幼虫的热逃避缺陷[56]。绿盲蝽(Apolygus lucorum)TRPA1同样存在4种蛋白质亚型, 但只有3种能够被温度激活, 并且激活温度并不相同[88]。果蝇成虫对伤害性高温的响应至少需要DmTRPA1、DmPain、DmPyx这3个TRP通道[34,57]。果蝇幼虫在受到高温刺激( > 38 ℃)时, 身体迅速卷缩, 表现出扭转翻滚的行为, 这种行为依赖于表达在幼虫体壁上mdⅣ神经元中的DmPain[33]。DmPyx通道可被热激活(≥40 ℃), 增强果蝇的热忍耐性, Dmpyx突变体果蝇在40 ℃时便会更快昏迷[34]。赤拟谷盗的TcTRPA1、TcPain和TcPyx同样在其热忍耐过程中发挥重要作用, TcTRPA1 沉默后会影响其对高温(39和42 ℃)的逃避行为[89]。使用辣椒素(TRPV激动剂)处理后的长红锥蝽(Rhodnius prolixus)对热刺激的行为反应减少并且更喜欢低温, 而用辣椒平(TRPV拮抗剂)处理后则结果相反[90]

5.3 机械感受

对于外界的机械刺激做出及时性的响应, 有助于昆虫躲避伤害并适应环境变化。感觉机械刺激后可以是直接由机械力迫使通道开放, 如DmNompC[32]; 也可通过由G蛋白偶联受体参与的信号级联间接开放[91]。其中机械感受又可分为触觉、听觉、本体感受、重力感受和湿度感受。

触觉是生物感受机械接触其本身(特别是体表)的感觉。DmNompC位于果蝇成虫感觉刚毛的顶端, 缺失DmNompC会导致机械刺激成虫感觉刚毛时电生理响应的缺陷[9]。与成虫相比, 果蝇幼虫并不具有机械感觉刚毛, 其主要是通过体壁上的md神经元来感觉轻触和有害碰触[32]。表达于幼虫mdⅢ神经元中的DmNompC缺失同样会引起果蝇幼虫轻触反应的丧失[32]。表达于mdⅣ神经元中的DmPain和DmTRPA1除了感受热刺激外, 也同样能够响应强的机械刺激[33,56]

本体感受对于生物感知身体及其在空间的相对位置具有重要的作用, 这是协调运动所必需的。在果蝇成虫中, 本体感受神经元分布在关节连接处, 例如足、翅、平衡棒等[71]。幼虫中, 本体感受需要多树突神经元中的bipolar dendrite(bd)神经元和第一级dendritic arborization(da)神经元的参与, DmNompC表达于足部的弦音器神经元和bd及da神经元中, 缺失DmNompC会引起果蝇成虫及幼虫的运动失调[71]。DmTRPγ同样对果蝇的协调运动有重要影响, 缺失DmTRPγ的果蝇无法正常协调运动, 难以协调足部运动的精确性, 因此不能够穿越较大的障碍[53]

果蝇成虫的重力感应会引起其负趋地性的行为, 而听觉则有助于它们求偶、识别天敌和感知环境刺激等。果蝇对重力和听觉的刺激可通过位于触角第2节上的弦音感受器来完成, 该器官又被称为江氏器(Johnston′s organ)[64]。TRP通道家族中至少有5个基因参与果蝇的重力或声音的感受。DmNan和DmIav表达于江氏器中2种不同的神经元, 对听觉和重力感应都有作用[64,66-68]。DmNompC表达于幼虫的弦音器神经元中只对听觉起作用[68]。其中DmNan和DmIav通过机械振动检测声音, 后经DmNompC放大声音信号[69]。DmPain和DmPyx只参与重力感应, 并且DmPyx表达于一个跨越触角第2和第3节的非神经元帽细胞中, 能够连接弦音器神经元和运动关节[64]

昆虫检测环境湿度不仅可以防止身体脱水, 更有助于它们选择合适的环境进行产卵。果蝇的湿度感觉受体位于触角末端的刚毛器官中。果蝇DmNan可检测潮湿的空气, DmWtrw可检测干燥的空气[65]。果蝇幼虫体壁上的mdⅣ神经元同样可以通过感受体壁摩擦产生的轻触来逃避干燥的环境, 在幼虫化蛹时生成的摩擦能够使它们逃离潮湿的食物环境, 并需要DmPain和DEG/EnaC通道家族的DmPPK(Pickpocket)的参与[63], 缺失这些基因会影响果蝇化蛹的比例。

5.4 化学感受

化学感受在昆虫的求偶、交配、取食、避害、寻找寄主以及“社会”交往等活动中发挥重要功能, 按照是否与化学物质直接接触可划分为嗅觉和味觉。昆虫的嗅觉和味觉由特定的器官进行感受, 果蝇主要通过触角第3节和下颚须来感知挥发性的化学物质, 这些器官上的嗅觉受体神经元(olfactory receptor neurons, ORN)延伸至触角神经叶, 然后再连接到脑部神经中枢[92]。与果蝇嗅觉有关的主要是2种具有7个跨膜结构域的离子型受体olfactory receptors(OR)[93]和ionotropic receptors(IR)[94], 果蝇TRP通道虽然不能最先检测到气味, 但是它们仍然可以参与嗅觉传导[95]。果蝇的味觉受体神经元(gustatory receptor neurons, GRN)分布于身体各个部位的感受器中[92], 例如喙部顶端的唇瓣、翅、足和雌虫的产卵器等[96], 喙部的GRN延伸到果蝇脑部的食道下神经节, 并且驱动对食物的喜爱或厌恶行为[97-98]

无脊椎动物中第1个克隆得到的与化学感受相关的TRP通道是秀丽隐杆线虫的CeOSM-9[99], Ceosm-9突变体线虫对苯甲醛的气味不再趋避。缺失DmTRPA1的果蝇也不再躲避香茅醛的气味[60], 冈比亚按蚊触角上的AgTRPA1[100]可以被香茅醛直接高效地激活[60]。事实证明, Gq/PLC信号级联反应不仅适用于视觉、响应舒适温度的细小差异, 同样也参与果蝇ORN中的DmTRPA1响应香茅醛的趋避行为的功能[60]。植物产生的马兜铃酸, 也能够激活果蝇喙部GRN中的DmTRPA1[59]。烟草天蛾(Manduca sexta)对马兜铃酸响应具有温度依赖性, 同样需要GRN中的MsTRPA1参与[101]。果蝇DmTRPA1[58]、棉铃虫HaTRPA1[102]、绿盲蝽AlTRPA1[88]还可被AITC、肉桂醛等化合物激活。昆虫TRPA1具有多种蛋白质亚型, 并且不同亚型在不同昆虫中的化学感受功能不同, 例如果蝇DmTRPA1中的4个蛋白质亚型均可被AITC激活[56], 而绿盲蝽AlTRPA1中只有A、C型可被AITC激活[88]

除TRPA1外, 其他TRP通道也有报道可能参与化学感受功能。棉贪夜蛾(Spodoptera littoralis)的SlTRPγ表达于雄性成虫触角的气味感受器中, 可能也与昆虫的嗅觉传导有关[103]。果蝇TRP和TRPL在CO2敏感的触角ORN中共表达, 并且对CO2的趋避作用明显, 可能也参与到Gq/PLC信号级联中[104]。表达于GRN中的DmPain和DmTRPL分别能够对AITC和樟脑的趋避产生作用[51,62]。意大利蜜蜂的AmHsTRPA同样能够被AITC、肉桂醛、樟脑等化合物激活, 并且可被钌红和薄荷醇抑制[27]。蜜蜂在受到天敌或其他威胁的时候会出现螯针伸缩反应(SER), 喂食高浓度的AmHsTRPA激动剂也会出现SER反应, 而AmHsTRPA抑制剂会抑制高温刺激所引起的SER反应[105]

5.5 新陈代谢与疾病

果蝇马氏管中的DmTRPM能将血淋巴中的Mg2+过滤移除, 维持体内稳态[73], 而DmTRPM缺失的果蝇会患有高镁症, 并且还会影响胞内的Zn2+稳态[72]

果蝇TRPP亚家族的同源蛋白DmPKD2位于精子的鞭毛处, 对精子储存有重要作用[74]。果蝇缺失TRPML后, 其细胞自噬能力降低, 然后导致自噬产生的氨基酸数量减少, 进而引起丝氨酸/苏氨酸激酶TORC1活性的降低, 表现出神经退化、运动失调、溶酶体囊泡累积的现象[75-76]

6 总结与展望

TRP通道在昆虫研究中的应用前景非常广阔, 例如家蚕BmTRPA1能够影响其滞育, 这可作为一种调控家蚕季节性适应的手段, 使蚕丝的生产效率最大化[85]。家蚕BmTRPA1、棉铃虫HaTRPA1、意大利蜜蜂AmHsTRPA等均能够被化合物激活, 这为化学控制害虫提供了新的分子靶标。虽然蜜蜂体内不存在TRPA1, 但是蜜蜂的体外寄生螨——狄斯瓦螨(Varroa destructor)具有2种TRPA1的蛋白质亚型(VdTRPA1L与VdTRPA1S)。α-松油醇和香芹酚能够激活VdTRPA1L, 但是不能够激活蜜蜂的AmHsTRPA和果蝇的DmTRPA1;将VdTRPA1L表达于果蝇的味觉神经元后能够改变果蝇的味觉行为, 同时由于α-松油醇能抑制狄斯瓦螨的产卵, 因此可以作为控制蜜蜂寄生螨的一种手段[106]

TRP通道在杀虫剂研究中的应用发现是在2015年, Nesterov等[3]研究发现果蝇TRPV亚家族中的DmNan和DmIav所形成的蛋白复合物是吡蚜酮的分子靶标。而之前吡蚜酮作为一种对刺吸式口器害虫高效的杀虫剂, 多年来一直认为其阻塞昆虫的口针, 影响昆虫进食, 进而导致其死亡。这也是首次报道TRP通道家族基因与杀虫剂有关, 为今后新药剂的研发及杀虫剂毒理学研究提供了新的思路。

昆虫作为一种变温动物, 能够在自然界中存活且种类最多, 这与其感觉机制密不可分。TRP通道家族参与多种感觉的感知, 并且存在于所有的动物体内, 可被多种作用机制激活但归根结底可能均为机械激活。同时, 由于TRP通道家族参与功能的多样性, 在昆虫中的应用非常广泛, 并且其可作为分子靶标来防控农业害虫。但是目前对于该通道家族的研究仅局限于果蝇等少数昆虫, 我们需要继续深入研究昆虫的TRP通道, 为经济昆虫的应用以及新靶标杀虫剂的研发提供可能。

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