Analysis of the molecular mechanism underlying the maintenance of endosymbiotic system in aphid

蚜虫的内共生系统是通过宿主与共生细菌Buchnera之间的互惠关系来维持的，Buchnera在特定的宿主细胞中被称为细菌细胞。本研究对布赫内拉菌与蚜虫共生的动态进行了研究，得到以下结果：1）布赫内拉菌在细菌细胞中的密度随寄主的生活史而波动，并具有年龄和形态依赖性。Buchnera密度急剧下降，就在最后的蜕皮在有翼的形态alphanes，当间接飞行肌肉的发展，而它在早期幼虫阶段在无翼形态apterae下降。2）蛋白质组学分析的细菌细胞蛋白质的最后蜕皮后，发现增强的羧肽酶-卵黄发生样（CPVL）的表达，在alphanes细菌细胞相比，apterae细菌细胞。用抗CPVL抗血清进行的免疫印迹分析表明，Aphid-CPVL在蜕皮过程中被激活，并作为溶酶体酶降解Buchnera。3）LysoTracker组织化学分析表明，在蜕皮结束后不久，溶酶体系统在蜕皮细菌细胞中被急剧激活。电镜观察表明，在蚜虫共生系统中，寄主溶酶体系统参与了布赫内拉密度的年龄和形态依赖性调节，降解的布赫内拉可能作为营养物质提供给寄主。我们认为，本研究的结果将有助于阐明蚜虫内共生系统维持的分子机制。

Endosymbiotic system in the aphid is maintained through a mutualistic association between the host and a symbiotic bacterium, Buchnera, which is harbored in particular host cell called bacteriocyte. In the present study, we demonstrated the dynamics of Buchnera-aphid symbiosis, and following results were obtained.1) Buchnera density in bacteriocytes fluctuated according to the life cycle of the host in an age- and morph-dependent manner. Buchnera density decreased drastically just around the final ecdysis in the winged morph alatae, when the indirect flight muscles develop, whereas it decreased at the early larviposition stage in the wingless morph apterae.2) Proteomic analysis of the bacteriocyte p roteins just after the final ecdysis revealed the enhanced expression of carboxypeptidase-vitellogenic like (CPVL), in the alatae bacteriocytes compared to the apterae bacteriocytes. Immunoblotting analysis using anti-CPVL antiserum revealed that Aphid-CPVL is activated in the alatae bacteriocytes accompanying with the processing, and functions as a lysosomal enzyme to degrade the Buchnera.3) Histochemical analysis using LysoTracker indicated that the lysosomal system is drastically activated in the alatae bacteriocytes just after the final ecdysis. Further, electron microscope observations revealed many degrading Buchnera that contained irregularly spaced electron-dense areas and were enclosed by a distended symbiosome membrane in the alatae bacteriocytes.These findings suggest that the host lysosomal system is involved in age-and morph-dependent regulation of Buchnera density in the aphid symbiotic system and the degraded Buchnera may be supplied as nutrients to the host. We considered that the result obtained in the present study would contribute in elucidating the molecular mechanism underlying the maintenance of endosymbiotic system in aphid.