Key Laboratory of Insect Developmental and Evolutionary Biology, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai 200032, People's Republic of China.
J Biol Chem. 2012 Apr 20;287(17):14270-9. doi: 10.1074/jbc.M112.354548. Epub 2012 Feb 28.
Many insects eat the green leaves of plants but excrete black feces in an as yet unknown mechanism. Insects cannot avoid ingesting pathogens with food that will be specifically detected by the midgut immune system. However, just as in mammals, many pathogens can still escape the insect midgut immune system and arrive in the hindgut, where they are excreted out with the feces. Here we show that the melanization of hindgut content induced by prophenoloxidase, a key enzyme that induces the production of melanin around invaders and at wound sites, is the last line of immune defense to clear bacteria before feces excretion. We used the silkworm Bombyx mori as a model and found that prophenoloxidase produced by hindgut cells is secreted into the hindgut contents. Several experiments were done to clearly demonstrate that the blackening of the insect feces was due to activated phenoloxidase, which served to regulate the number of bacteria in the hindgut. Our analysis of the silkworm hindgut prophenoloxidase discloses the natural secret of why the phytophagous insect feces is black and provides insight into hindgut innate immunity, which is still rather unclear in mammals.
许多昆虫以植物的绿叶为食，但它们以一种未知的机制排出黑色粪便。昆虫在进食时无法避免摄入食物中的病原体，而这些病原体将被中肠免疫系统特异性检测到。然而，就像哺乳动物一样，许多病原体仍然可以逃避昆虫的中肠免疫系统，并到达后肠，在后肠中与粪便一起排出体外。在这里，我们表明，由酚氧化酶原（一种在入侵者周围和伤口部位诱导黑色素生成的关键酶）诱导的后肠内容物的黑化是在粪便排泄前清除细菌的最后一道免疫防御线。我们使用家蚕（Bombyx mori）作为模型，发现后肠细胞产生的酚氧化酶原被分泌到后肠内容物中。我们进行了几项实验，清楚地证明了昆虫粪便的黑化是由于被激活的酚氧化酶原而起作用的，它可以调节后肠中细菌的数量。我们对家蚕后肠酚氧化酶原的分析揭示了植食性昆虫粪便为何呈黑色的自然秘密，并深入了解了哺乳动物中仍不太清楚的后肠先天免疫。
