Acanthamoeba programmed cell death

Abstract

Acanthamoeba is a free-living amoeba, ubiquitously distributed in the natural environment including soil and a plethora of water habitats. It is characterised as an opportunistic parasite that is able to cause several diseases, including life threating granulomatous Acanthamoeba encephalitis and a painful vision-threating keratitis. There is a clear and emerging need to understand how to treat infections caused by this dangerous pathogen, since pharmaceutical approaches have been considered insufficient. The presence and the importance of cell death pathways in unicellular organisms including Acanthamoeba is not yet fully understood and its existence is still debated. This research study presents a set of key characteristics and findings, comprising morphological, biochemical and molecular evidence of Acanthamoeba programmed cell death. Distinctive apoptotic features comprising cell shrinkage, membrane vesiculation and granules appearance which could be easily described as apoptotic like ‘bodies’ formation and nuclear shrinkage, accompanied by large scale chromatin condensation in dense clusters, have been primarily observed. Additionally, mitochondrial dysfunction, characterized by extended mitochondrial outer membrane permeabilization and release of apoptotic factors including cytochrome c, was also noted, indicating a mitochondrially mediated cell death pathway. During the expansion of the aforementioned apoptotic characteristics Acanthamoeba trophozoites were found to maintain their membrane integrity and homeostasis, at least at the early stages of the process. In-depth transcriptomic analysis based on RNA-sequence analysis revealed a plethora of differentially expressed genes between Acanthamoeba undergoing cell death and control trophozoites, indicating the correlation of a more defined and conserved signalling self-destruct program. These discoveries suggest that Acanthamoeba could undergo programmed cell death, which morphologically resembles apoptosis-like cell death, under specific stress conditions. Furthermore, similar characteristics are also found in cell death processes in higher eukaryotes and other unicellular organisms, indicating that biological principles behind this behaviour are widespread and well conserved among species. Identification of Acanthamoeba’s cell death signalling pathways might provide alternatives not only to microorganism’s refractory infection treatment, but also a more extended manipulation might become feasible across other species and systems.