昆虫感觉气味的细胞与分子机制研究进展

Advances in cellular and molecular mechanism of insect olfaction

昆虫作为地球上最为成功的类群,已经成功地进化了精细的化学感受系统,通过化学感受系统适应各种复杂的环境,保持种群的繁荣。自1991年在动物中发现嗅觉受体基因以来,关于昆虫感受化学信息的周缘神经系统的分子和细胞机制方面的进展十分迅速。文章主要就昆虫周缘神经系统的感受化学信息的分子和细胞机制进行综述。首先对昆虫感觉气味的细胞机制的研究进展进行简要介绍。昆虫嗅觉神经元在感受化学信息过程中起着极为重要的作用,昆虫嗅觉神经元上表达的嗅觉受体不同而执行着各异的功能。各种嗅觉神经元对于化学信息的感受谱有较大的区别;嗅觉神经元对化学信息类型、浓度、流动动态等产生相应的电生理特征反应。研究表明同一种神经原可以感受多种化学信息,而一种化学信息也可以被多种神经原所感受。由神经原对化学信息感受所形成的特征组合就是感受化学信息的编码。其次较为详细地论述与昆虫感受气味分子相关的一些蛋白质的研究进展。气味分子结合蛋白是一类分子量较小、水溶性的蛋白,主要位于化学感受器神经原树突周围的淋巴液中。在结构上的主要特征是具有6个保守的半光氨酸和由6个α螺旋组成的结合腔。自1981年发现以来,已经在40余种昆虫中发现上百种。由于研究手段的不断进步,已经对该类蛋白的表达特征、结合特性以及三维结构和结合位点进行了大量的研究,提出了多个可能的功能假说,在诸多的假说中,较为广泛接受的是气味分子结合蛋白在昆虫感觉气味的过程中,是与疏水性的气味分子相结合,并将气味分子运输到嗅觉神经原树突膜上的嗅觉受体上。这些处于树突膜上的嗅觉受体则是昆虫感觉气味过程中的另一个十分重要的蛋白质。目前,已经在果蝇、按蚊、蜜蜂和家蚕等10余个昆虫种类中发现上百个嗅觉受体蛋白基因。这类蛋白是跨膜蛋白,一般具有7个跨膜区,整个蛋白的氨基酸残基在400～600个。昆虫的嗅觉受体蛋白的N-端在胞内,而C-端在胞外,这与G耦联蛋白不同。而且,昆虫的一个嗅觉神经元可以表达1～3个嗅觉受体蛋白,也与哺乳动物的一个神经元只表达一种受体蛋白有所不同。每种嗅觉受体可以感受多种气味分子,而一种气味分子可以被多个嗅觉受体所感知,这样组成了感受化学信息的编码谱。最近采用基因敲除技术和膜片钳技术研究发现,昆虫的嗅觉受体蛋白在信号传导中也有特殊性,即嗅觉受体可以直接作为离子通道,而引起动作电位。还有近来的研究表明,神经膜蛋白对于果蝇的性信息素感受神经元感受性信息素cVA是必要的。实际上,昆虫对于化学信息的感受和信号的转导,并不是上述蛋白单独起作用完成的,而是多种蛋白相互作用的结果。论文最后对该领域研究内容进行了展望。

As the most successful animal in the world, insects have evaluated their sophisticated chemoreception systems for adapting to environments and keeping their populations prosperous. Since the olfactory receptor genes identified in 1991, great achievements have been obtained on the molecular and cellular olfaction mechanism of insect. This paper mainly summarized on the molecular and cellular mechanisms advances in peri-neurosystem of insects. Firstly, the olfactory neurons are playing very important role in chemoreception. The neurons on which various olfactory receptors are expressed act their own specific functions. They are characterized as their responsive spectra, strength, and dynamics to various odors. It is demonstrated that one kind of neuron can be evoked by several odorants, in turn one kind of odorant can be accepted by some neurons. And the composition of the response patterns to the odor is the code for chemoreception. Secondly, we give a detail description on advances in molecular aspect. Odorant-binding proteins are small and soluble proteins in sensiUum lymph surrounding the dendrites of olfactory neurons. The features of the proteins are that they have 6 conserved cysteines and 6-a helices which compose a binding pocket. It is reported that more than 100 odorant-binding proteins in more than 40 species of insects have been identified. And few of the proteins have been studied on their expression, binding proporties, 3-deminsional structures and binding sites. Several hypotheses on the physiological function of this kind of proteins have been proposed since they have been found. The proteins as the carriers for transferring hydrophobic odorants to the olfactory receptors on dendrites are most widely suggested. The olfactory receptors with 7 trans-membrane domains, an important family of proteins in olfaction are identified at least more than 100 genes in some 10 species of insects. Further studies indicate that the insect olfactory receptors are not G-coupled proteins. Unlike mammals, insect olfactory neuron can express 1 - 3 olfactory receptors. With the combination of genes knock down and electrophysiological techniques recently the olfactory receptors are found that they may perform as ion channel directly for signal transdnction. Beside the above two families of proteins, sensory neuron membrane protein is demonstrated to play important role in olfaction in insects. In fact, several evidences showed that these three families of proteins interact and conduct the signal transduction, rather than singly. Finally, this paper gives some prospects in the field.