Nucleus-associated actin in Amoeba proteus

The apical cell - An enigmatic somatic cell in leech ovaries - Structure and putative functions

Although somatic cells play an integral role in animal gametogenesis, their organization and function are usually poorly characterized, especially in non-model systems. One such example is a peculiar cell found in leech ovaries - the apical cell (AC). A single AC can be found at the apical tip of each ovary cord, the functional unit of leech ovaries, where it is surrounded by other somatic and germline cells. The AC is easily distinguished due to its enormous size and its numerous long cytoplasmic projections that penetrate the space between neighboring cells. It is also characterized by a prominent accumulation of mitochondria, Golgi complexes and electron-dense vesicles. ACs are also enriched in cytoskeleton, mainly in form of intermediate filaments. Additionally, the AC is connected to neighboring cells via junctions that structurally resemble hemidesmosomes. In spite of numerous descriptive data about the AC, its functions remain poorly understood. Its suggested functions include a role in forming skeleton for the germline cells, and a role in defining a niche for germline stem cells. The latter is more speculative, since germline stem cells have not been identified in leech ovaries. Somatic cells with similar morphological properties to those of the AC have been found in gonads of nematodes - the distal tip cell - and in insects - Verson's cell, hub cells and cap cells. In the present article we summarize information about the AC structure and its putative functions. AC is compared with other well-described somatic cells with potentially similar roles in gametogenesis.

Experimental Listeria-Tetrahymena-Amoeba food chain functioning depends on bacterial virulence traits

Background

Some pathogenic bacteria have been developing as a part of terrestrial and aquatic microbial ecosystems. Bacteria are consumed by bacteriovorous protists which are readily consumed by larger organisms. Being natural predators, protozoa are also an instrument for selection of virulence traits in bacteria. Moreover, protozoa serve as a “Trojan horse” that deliver pathogens to the human body. Here, we suggested that carnivorous amoebas feeding on smaller bacteriovorous protists might serve as “Troy” themselves when pathogens are delivered to them with their preys. A dual role might be suggested for protozoa in the development of traits required for bacterial passage along the food chain.

Results

A model food chain was developed. Pathogenic bacteria L. monocytogenes or related saprophytic bacteria L. innocua constituted the base of the food chain, bacteriovorous ciliate Tetrahymena pyriformis was an intermediate consumer, and carnivorous amoeba Amoeba proteus was a consumer of the highest order. The population of A. proteus demonstrated variations in behaviour depending on whether saprophytic or virulent Listeria was used to feed the intermediate consumer, T. pyriformis. Feeding of A. proteus with T. pyriformis that grazed on saprophytic bacteria caused prevalence of pseudopodia-possessing hungry amoebas. Statistically significant prevalence of amoebas with spherical morphology typical for fed amoebas was observed when pathogenic L. monocytogenes were included in the food chain. Moreover, consumption of tetrahymenas fed with saprophytic L. innocua improved growth of A. proteus population while L. monocytogenes-filled tetrahymenas provided negative effect. Both pathogenic and saprophytic bacteria were delivered to A. proteus alive but only L. monocytogenes multiplied within amoebas. Observed differences in A. proteus population behaviour suggested that virulent L. monocytogenes might slow down restoration of A. proteus ability to hunt again and thus restrict the size of A. proteus population. Comparison of isogenic bacterial pairs that did or did not produce the haemolysin listeriolysin O (LLO) suggested a role for LLO in passing L. monocytogenes along the food chain.

Conclusions

Our results support the idea of protozoa as a means of pathogen delivery to consumers of a higher order and demonstrated a dual role of protozoa as both a “Trojan horse” and “Troy.”

The Chromatin Extrusion Phenomenon in Amoeba proteus Cell Cycle

Amoeba proteus is possibly the best known of all unicellular eukaryotes. At the same time, several quintessential issues of its biology, including some aspects of the cell cycle, remain unsolved. Here, we show that this obligate agamic amoebae and related species have a special type of cyclic polyploidy. Their nucleus has an euploid status only for a small fraction of the cell cycle, during metaphase and telophase. The rest of the time it has an aneuploid status, which is a consequence of polyploidization. Extrusion of "excess" chromatin from the nucleus in late interphase and during prophase results in depolyploidization. Such a strategy of life cycle in unicellular eukaryotes is thought to be the main mechanism of "resetting" the Muller's ratchet and is a satisfactory alternative to the meiotic recombination for agamic protists.

Evidence of conditioned behavior in amoebae

Associative memory is the main type of learning by which complex organisms endowed with evolved nervous systems respond efficiently to certain environmental stimuli. It has been found in different multicellular species, from cephalopods to humans, but never in individual cells. Here we describe a motility pattern consistent with associative conditioned behavior in the microorganism Amoeba proteus. We use a controlled direct-current electric field as the conditioned stimulus, and a specific chemotactic peptide as the unconditioned stimulus. The amoebae are capable of linking two independent past events, generating persistent locomotion movements that can prevail for 44 min on average. We confirm a similar behavior in a related species, Metamoeba leningradensis. Thus, our results indicate that unicellular organisms can modify their behavior during migration by associative conditioning.