Encystment and Excystment Processes in Acanthamoeba castellanii: An Emphasis on Cellulose Involvement

Abstract

The free-living amoeba Acanthamoeba castellanii is a unicellular eukaryote distributed in a wide range of soil or aquatic environments, either natural or human-made, such as rivers, lakes, drinking water, or swimming pools. Besides its capacity to transport potential pathogens, such as bacteria or viruses, Acanthamoeba spp. can have intrinsic pathogenic properties by causing severe infections at the ocular and cerebral level, named granulomatous amoebic encephalitis and amoebic keratitis, respectively. During its life cycle, A. castellanii alternates between a vegetative and mobile form, named the trophozoite, and a resistant, latent, and non-mobile form, named the cyst. The cyst wall of Acanthamoeba is double-layered, with an inner endocyst and an outer ectocyst, and is mainly composed of cellulose and proteins. The resistance of cysts to many environmental stresses and disinfection treatments has been assigned to the presence of cellulose. The current review aims to present the importance of this glycopolymer in Acanthamoeba cysts and to further report the pathways involved in encystment and excystment.

Keywords: Acanthamoeba castellanii; cellulose; cyst wall; encystment; excystment.

Figure 4

Main cellular steps and endogenous proteins involved in the encystment and excystment processes in Acanthamoeba castellanii. Encystment starts with the formation of the outer layer, the ectocyst, followed by the inner layer, the endocyst. Excystment is characterized by the emergence stage, when the operculum is removed and a cytoplasmic bud appears at the ostiole, and the post-emergence stage when the trophozoite is released and the remaining cyst wall disappears. The operculum and the endocyst are represented in red and green, respectively. NNMT = nicotinamide N-methyltransferase; CSP21 = Cyst-Specific Protein of 21 kDa; Sir2-like = Deacetylase; Arg-methyltransferase 5 = Arginine methyltransferase 5; RegA = AMP phosphodiesterase; PKC = Protein kinase C (from [10,43,44,54,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,85,86,87,88]).

Figure 1

Phase contrast images of Acanthamoeba castellanii. (A) Trophozoite presenting acanthopodia (a) allowing mobility, with several vacuoles (V) around the nucleus (N) presenting a nucleolus (n) in its center. (B) Two cysts characterized by a double layer with the outermost ectocyst (Ec) and the innermost polygonal endocyst (En). These two layers are connected at the ostiole (o). Scale bar = 10 µm.

Figure 2

Transmission electron microscopy image of a cyst of Acanthamoeba castellanii. The cyst wall of A. castellanii is composed of two layers, the inner endocyst (En) and the outer ectocyst (Ec), separated by the intercystic space (IS). Scale bar = 2 µm. (Adapted from [10]).

Figure 3

Illustration depicting the different layers of the Acanthamoeba castellanii cell wall. (A) A. castellanii cyst shows ectocyst and endocyst (in green) separated by an intercystic space. The two layers are closely apposed at the ostiole which is covered by a cap, named the operculum (in red). (B) The cyst wall contains structural sugar polymers (cellulose, glucans, putative chitin) associated with proteins such as lectins.