A pore-forming protein drives macropinocytosis to facilitate toad water maintaining

Maintaining water balance is a real challenge for amphibians in terrestrial environments. Our previous studies with toad Bombina maxima discovered a pore-forming protein and trefoil factor complex βγ-CAT, which is assembled under tight regulation depending on environmental cues. Here we report an unexpected role for βγ-CAT in toad water maintaining. Deletion of toad skin secretions, in which βγ-CAT is a major component, increased animal mortality under hypertonic stress. βγ-CAT was constitutively expressed in toad osmoregulatory organs, which was inducible under the variation of osmotic conditions. The protein induced and participated in macropinocytosis in vivo and in vitro. During extracellular hyperosmosis, βγ-CAT stimulated macropinocytosis to facilitate water import and enhanced exosomes release, which simultaneously regulated aquaporins distribution. Collectively, these findings uncovered that besides membrane-integrated aquaporin, a secretory pore-forming protein can facilitate toad water maintaining via macropinocytosis induction and exocytosis modulation, especially in responses to osmotic stress.

对于陆生环境中的两栖动物而言，维持体内水平衡是一项切实的挑战。我们此前针对大蹼铃蟾（Bombina maxima）开展的研究发现了一种成孔蛋白与三叶因子复合物βγ-CAT，该复合物会根据环境信号受到严格调控后组装形成。本研究揭示了βγ-CAT在铃蟾维持水平衡中此前未被报道的作用。当去除以βγ-CAT为主要成分的铃蟾皮肤分泌物后，高渗胁迫下的实验动物死亡率显著升高。βγ-CAT在铃蟾的渗透压调节器官中呈组成型表达，且可随渗透环境的变化被诱导上调。该蛋白可在体内及体外诱导并参与巨胞饮作用。当细胞外处于高渗状态时，βγ-CAT可通过刺激巨胞饮以促进水分摄入，同时增强外泌体释放并调控水通道蛋白的分布。综上，本研究发现除膜整合型水通道蛋白外，一种分泌性成孔蛋白可通过诱导巨胞饮与调控胞吐过程，助力铃蟾维持水平衡，尤其是在应对渗透胁迫时发挥关键作用。