Center for Marine Environmental Studies (CMES), Ehime University, Matsuyama, Ehime, 790-8577, Japan.
Graduate School of Bioresources, Mie University, 1577 Kurimamachiya-cho, Tsu, Mie, 514-8507, Japan.
Dis Aquat Organ. 2024 Aug 1;159:37-48. doi: 10.3354/dao03801.
Soft tunic syndrome is an infectious disease caused by the flagellate Azumiobodo hoyamushi, which severely damages the aquaculture of the edible ascidian Halocynthia roretzi. Tunic is a cellulosic extracellular matrix entirely covering the body in ascidians and other tunicates, and its dense cuticle layer covers the tunic surface as a physical barrier against microorganisms. When the tunic of intact H. roretzi individuals was cut into strips, electron-dense fibers (DFs) appeared on the cut surface of the tunic matrix and aggregated to regenerate a new cuticular layer in seawater within a few days. DF formation was partially or completely inhibited in individuals with soft tunic syndrome, and DF formation was also inhibited by the presence of some proteases, indicating the involvement of proteolysis in the process of tunic softening as well as cuticle regeneration. Using pure cultures of the causative flagellate A. hoyamushi, the expression of protease genes and secretion of some proteases were confirmed by RNA-seq analysis and a 4-methylcoumaryl-7-amide substrate assay. Some of these proteases may degrade proteins in the tunic matrix. These findings suggest that the proteases of A. hoyamushi is the key to understanding the mechanisms of cuticular regeneration inhibition and tunic softening.
软皮综合征是一种由尾滴虫 Azumiobodo hoyamushi 引起的传染病，严重破坏了可食用海鞘 Halocynthia roretzi 的水产养殖。海鞘的外骨骼由纤维素组成，完全覆盖其身体，其密集的角质层覆盖在外骨骼表面，形成抵御微生物的物理屏障。当完整的 H. roretzi 个体的外骨骼被切成条状时，在外骨骼基质的切割表面会出现电子致密纤维 (DF)，并在几天后的海水中聚集形成新的角质层。患有软皮综合征的个体中 DF 的形成部分或完全受到抑制，而某些蛋白酶的存在也抑制了 DF 的形成，表明在软皮综合征的过程中，蛋白水解参与了外骨骼软化以及角质层再生。使用致病鞭毛虫 A. hoyamushi 的纯培养物，通过 RNA-seq 分析和 4-甲基香豆素-7-酰胺底物测定证实了蛋白酶基因的表达和一些蛋白酶的分泌。其中一些蛋白酶可能会降解外骨骼基质中的蛋白质。这些发现表明，A. hoyamushi 的蛋白酶是理解角质层再生抑制和外骨骼软化机制的关键。
