University of Massachusetts, Department of Biology, 611 N. Pleasant Street, Amherst, MA 01003, USA.
United States Geological Survey, Patuxent Wildlife Research Center, 1 Migratory Way, Turners Falls, MA 01376, USA.
Curr Biol. 2022 Jun 20;32(12):2765-2771.e4. doi: 10.1016/j.cub.2022.04.006. Epub 2022 Apr 25.
The frog-killing chytrid fungus Batrachochytrium dendrobatidis (Bd) is decimating amphibian populations around the world.Bd has a biphasic life cycle, alternating between motile zoospores that disperse within aquatic environments and sessile sporangia that grow within the mucus-coated skin of amphibians. Zoospores lack cell walls and swim rapidly through aquatic environments using a posterior flagellum and crawl across solid surfaces using actin structures similar to those of human cells.Bd transitions from this motile dispersal form to its reproductive form by absorbing its flagellum, rearranging its actin cytoskeleton, and rapidly building a chitin-based cell wall-a process called "encystation." The resulting sporangium increases in volume by two or three orders of magnitude while undergoing rounds of mitosis without cytokinesis to form a large ceonocyte. The sporangium then cellurizes by dividing its cytoplasm into dozens of new zoospores. After exiting the sporangium through a discharge tube onto the amphibian skin, daughter zoospores can then reinfect the same individual or find a new host. Although encystation is critical to Bd growth, whether and how this developmental transition is triggered by external signals was previously unknown. We discovered that exposure to amphibian mucus triggers rapid and reproducible encystation within minutes. This response can be recapitulated with purified mucin, the bulk component of mucus, but not by similarly viscous methylcellulose or simple sugars. Mucin-induced encystation does not require gene expression but does require surface adhesion, calcium signaling, and modulation of the actin cytoskeleton. Mucus-induced encystation may represent a key mechanism for synchronizing Bd development with the arrival at the host.
蛙类杀伤性壶菌（Batrachochytrium dendrobatidis，Bd）正在使世界各地的两栖动物种群减少。Bd 具有双相生命周期，在运动的游动孢子和生长在两栖动物黏液覆盖的皮肤内的不动孢子囊之间交替。游动孢子缺乏细胞壁，通过后鞭毛在水生环境中快速游动，并使用类似于人类细胞的肌动蛋白结构在固体表面爬行。Bd 通过吸收其鞭毛、重新排列其肌动蛋白细胞骨架并快速构建基于几丁质的细胞壁来从这种运动性扩散形式转变为其生殖形式——这个过程称为“囊化”。形成的孢子囊在经历了几轮没有胞质分裂的有丝分裂后，体积增加了两到三个数量级，从而形成一个大的 ceonocyte。然后，孢子囊通过将其细胞质分裂成几十个新的游动孢子来细胞化。当游动孢子通过放电管离开孢子囊进入两栖动物皮肤后，子代游动孢子可以再次感染同一个体或找到新的宿主。尽管囊化对 Bd 的生长至关重要，但以前不知道这种发育转变是否以及如何被外部信号触发。我们发现，暴露于两栖动物黏液会在数分钟内引发快速且可重复的囊化。这种反应可以用纯化的粘蛋白（黏液的主要成分）重现，但不能用同样粘稠的甲基纤维素或简单糖重现。粘蛋白诱导的囊化不需要基因表达，但需要表面粘附、钙信号和肌动蛋白细胞骨架的调节。粘蛋白诱导的囊化可能代表一种将 Bd 发育与到达宿主同步的关键机制。
