Drosophila Sperm Swim Backwards in the Female Reproductive Tract and Are Activated via TRPP2 Ion Channels

BackgroundSperm have but one purpose, to fertilize an egg. In various species including Drosophila melanogaster female sperm storage is a necessary step in the reproductive process. Amo is a homolog of the human transient receptor potential channel TRPP2 (also known as PKD2), which is mutated in autosomal dominant polycystic kidney disease. In flies Amo is required for sperm storage. Drosophila males with Amo mutations produce motile sperm that are transferred to the uterus but they do not reach the female storage organs. Therefore Amo appears to be a mediator of directed sperm motility in the female reproductive tract but the underlying mechanism is unknown.
Methodology/Principal FindingsAmo exhibits a unique expression pattern during spermatogenesis. In spermatocytes, Amo is restricted to the endoplasmic reticulum (ER) whereas in mature sperm, Amo clusters at the distal tip of the sperm tail. Here we show that flagellar localization of Amo is required for sperm storage. This raised the question of how Amo at the rear end of sperm regulates forward movement into the storage organs. In order to address this question, we used in vivo imaging of dual labelled sperm to demonstrate that Drosophila sperm navigate backwards in the female reproductive tract. In addition, we show that sperm exhibit hyperactivation upon transfer to the uterus. Amo mutant sperm remain capable of reverse motility but fail to display hyperactivation and directed movement, suggesting that these functions are required for sperm storage in flies.
Conclusions/SignificanceAmo is part of a signalling complex at the leading edge of the sperm tail that modulates flagellar beating and that guides a backwards path into the storage organs. Our data support an evolutionarily conserved role for TRPP2 channels in cilia.

背景
精子的唯一功能是使卵子受精。在包括黑腹果蝇（Drosophila melanogaster）在内的诸多物种中，雌性的精子储存是生殖过程中的必要环节。Amo是人类瞬时受体电位通道TRPP2（亦称PKD2）的同源蛋白，该通道的突变会引发常染色体显性遗传性多囊肾病。在果蝇中，Amo是精子储存所必需的蛋白。携带Amo突变的黑腹果蝇雄性可产生可运动的精子，这些精子能够被转移至雌性子宫，但无法抵达雌性的精子储存器官。因此，Amo似乎是雌性生殖道内精子定向运动的介导因子，但其背后的分子机制尚不明确。

方法与主要发现
Amo在精子发生过程中呈现独特的表达模式。精母细胞中，Amo仅定位于内质网（endoplasmic reticulum, ER）；而在成熟精子中，Amo聚集于精子尾部的远端尖端。本研究证实，Amo的鞭毛定位是精子储存所必需的。这引出了一个关键问题：位于精子尾部后端的Amo如何调控精子向储存器官的正向运动？为解答该问题，本研究通过对双标记精子进行活体成像，证实黑腹果蝇的精子会在雌性生殖道内反向行进。此外，本研究发现精子被转移至子宫后会发生超激活现象。携带Amo突变的精子仍具备反向运动能力，但无法发生超激活以及定向运动，这表明上述功能是果蝇精子储存所必需的。

结论与意义
Amo是精子尾部前沿信号复合体的组成部分，该复合体可调控鞭毛摆动，并引导精子反向行进至储存器官。本研究数据证实了TRPP2通道在纤毛中发挥的进化保守功能。