Yuuki Kodama

Endosymbionts in Paramecium

Paramecium species are extremely valuable organisms to enable experiments for the reestablishment of endosymbiosis. This is investigated in two different systems, the first with Paramecium caudatum and the endonuclear symbiotic bacterium Holospora species. Although most endosymbiotic bacteria cannot grow outside the host cell as a result of their reduced genome size, Holospora species can maintain their infectivity for a limited time. We found that an 89-kDa periplasmic protein has an important function for Holosporas invasion into the target nucleus, and that Holospora alters the host gene expression; the host thereby acquires resistance against various stresses. The second system is the symbiosis between P. bursaria and symbiotic Chlorella. Alga-free P. bursaria and the algae retain the ability to grow without a partner. Consequently, endosymbiosis between the aposymbiotic host cells and the symbiotic algae can be reestablished easily by mixing them. We now found four checkpoints for the reestablishment of the endosymbiosis between P. bursaria and the algae. The findings in the two systems provide excellent opportunities for us to elucidate not only infection processes but also to assess the associations leading to eukaryotic cell evolution. This paper summarizes recent progresses on reestablishment of the primary and the secondary endosymbiosis in Paramecium.

Symbiotic Chlorella sp. of the ciliate Paramecium bursaria do not prevent acidification and lysosomal fusion of host digestive vacuoles during infection

Summary.Each symbiotic Chlorella sp. of the ciliate Paramecium bursaria is enclosed in a perialgal vacuole derived from the host digestive vacuole, and thereby the alga is protected from digestion by lysosomal fusion. Algae-free cells can be reinfected with algae isolated from algae-bearing cells by ingestion into digestive vacuoles. To examine the timing of acidification and lysosomal fusion of the digestive vacuoles and of algal escape from the digestive vacuole, algae-free cells were mixed with isolated algae or yeast cells stained with pH indicator dyes at 25 ± 1 °C for 1.5 min, washed, chased, and fixed at various time points. Acidification of the vacuoles and digestion of Chlorella sp. began at 0.5 and 2 min after mixing, respectively. All single green Chlorella sp. that had been present in the host cytoplasm before 0.5 h after mixing were digested by 0.5 h. At 1 h after mixing, however, single green algae reappeared in the host cytoplasm, arising from those digestive vacuoles containing both nondigested and partially digested algae, and the percentage of such cells increased to about 40% at 3 h. At 48 h, the single green algae began to multiply by cell division, indicating that these algae had succeeded in establishing endosymbiosis. In contrast to previously published studies, our data show that an alga can successfully escape from the host’s digestive vacuole after acidosomal and lysosomal fusion with the vacuole has occurred, in order to produce endosymbiosis.

Symbiotic alga Chlorella vulgaris of the ciliate Paramecium bursaria shows temporary resistance to host lysosomal enzymes during the early infection process

Summary.Paramecium bursaria free of symbiotic Chlorella species can be experimentally reinfected with algae isolated from algae-bearing cells by ingestion into digestive vacuoles. Isolated symbiotic algae were cloned, mixed with the algae-free P. bursaria at 25 ± 1 °C for 1.5 min, washed and chased, with or without fixation 3 h after mixing. Though genetically identical, a few of the algae were not digested but coexisted with the digested ones in the same vacuole after lysosomal fusion. Light microscopy showed that algal fate did not depend on cell cycle stage or location in the vacuole. Electron microscopy showed that the nondigested algae were not protected by a perialgal vacuole membrane in the digestive vacuole. Moreover, this phenomenon was also observed in the presence of cycloheximide and puromycin, which are known to inhibit algal and host protein synthesis, respectively. These observations suggest that a few algae can acquire temporary resistance to host lysosomal enzymes in order to establish endosymbiosis without algal protein synthesis.

Cycloheximide Induces Synchronous Swelling of Perialgal Vacuoles Enclosing Symbiotic Chlorella vulgaris and Digestion of the Algae in the Ciliate Paramecium bursaria

Cycloheximide is known to inhibit preferentially protein synthesis of symbiotic Chlorella of the ciliate Paramecium bursaria, but to hardly host protein synthesis. Treatment of algae-bearing Paramecium cells with cycloheximide induces synchronous swelling of all perialgal vacuoles that are localized immediately beneath the hosts cell membrane. In this study, the space between the symbiotic algal cell wall and the perialgal vacuole membrane widened to about 25 times its normal width 24 h after treatment with cycloheximide. Then, the vacuoles detached from beneath the hosts cell membrane, were condensed and stained with Gomoris solution, and the algae in the vacuoles were digested. Although this phenomenon is induced only under a fluorescent light condition, and not under a constant dark condition, this phenomenon was not induced in paramecia treated with cycloheximide in the light in the presence of the photosynthesis inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea. These results indicate that algal proteins synthesized in the presence of algal photosynthesis serve some important function to prevent expansion of the perialgal vacuole and to maintain the ability of the perialgal vacuole membrane to protect itself from host lysosomal fusion.

Timing of perialgal vacuole membrane differentiation from digestive vacuole membrane in infection of symbiotic algae Chlorella vulgaris of the ciliate Paramecium bursaria.

Each symbiotic Chlorella of the ciliate Paramecium bursaria is enclosed in a perialgal vacuole derived from the host digestive vacuole to protect from lysosomal fusion. To understand the timing of differentiation of the perialgal vacuole from the host digestive vacuole, algae-free P. bursaria cells were fed symbiotic C. vulgaris cells for 1.5min, washed, chased and fixed at various times after mixing. Acid phosphatase activity in the vacuoles enclosing the algae was detected by Gomoris staining. This activity appeared in 3-min-old vacuoles, and all algae-containing vacuoles demonstrated activity at 30min. Algal escape from these digestive vacuoles began at 30min by budding of the digestive vacuole membrane into the cytoplasm. In the budded membrane, each alga was surrounded by a Gomoris thin positive staining layer. The vacuoles containing a single algal cell moved quickly to and attached just beneath the host cell surface. Such vacuoles were Gomoris staining negative, indicating that the perialgal vacuole membrane differentiates soon after the algal escape from the host digestive vacuole. This is the first report demonstrating the timing of differentiation of the perialgal vacuole membrane during infection of P. bursaria with symbiotic Chlorella.

Secondary symbiosis between Paramecium and Chlorella cells.

Each symbiotic Chlorella species of Paramecium bursaria is enclosed in a perialgal vacuole (PV) membrane derived from the host digestive vacuole (DV) membrane. Algae-free paramecia and symbiotic algae are capable of growing independently and paramecia can be reinfected experimentally by mixing them. This phenomenon provides an excellent model for studying cell-to-cell interaction and the evolution of eukaryotic cells through secondary endosymbiosis between different protists. However, the detailed algal infection process remains unclear. Using pulse labeling of the algae-free paramecia with the isolated symbiotic algae and chase method, we found four necessary cytological events for establishing endosymbiosis. (1) At about 3 min after mixing, some algae show resistance to the host lysosomal enzymes in the DVs, even if the digested ones are present. (2) At about 30 min after mixing, the alga starts to escape from the DVs as the result of the budding of the DV membrane into the cytoplasm. (3) Within 15 min after the escape, the DV membrane enclosing a single green alga differentiates to the PV membrane, which provides protection from lysosomal fusion. (4) The alga localizes at the primary lysosome-less host cell surface by affinity of the PV to unknown structures of the host. At about 24 h after mixing, the alga multiplies by cell division and establishes endosymbiosis. Infection experiments with infection-capable and infection-incapable algae indicate that the infectivity of algae is based on their ability to localize beneath the host surface after escaping from the DVs. This algal infection process differs from known infection processes of other symbiotic or parasitic organisms to their hosts.

Localization of perialgal vacuoles beneath the host cell surface is not a prerequisite phenomenon for protection from the host's lysosomal fusion in the ciliate Paramecium bursaria.

In a ciliate Paramecium bursaria cell, each symbiotic 3-4-mum-diameter Chlorella cell is enclosed within a perialgal vacuole membrane. It localizes near trichocysts beneath the host cell surface. Gomoris staining of this surface shows that it is an acid phosphatase activity-negative area to 5-10mum depth. Trichocysts were removed by treatment with 1mg/ml lysozyme to elucidate whether algal protection from the host lysosomal fusion is controlled by localization of the perialgal vacuole membrane to the acid phosphatase activity-negative area or by the capability of the perialgal vacuole membrane to give protection from lysosomal fusion. The trichocyst-free cell reduced the acid phosphatase activity-negative area to less than 3mum depth at the dorsal surface. However, even though a part of the algal cell had been exposed in the acid phosphatase activity-positive area, the algae were able to attach beneath the host surface and to protect it from lysosomal fusion. Results of this study show that the perialgal vacuole membrane can give protection from host lysosomal fusion, and that the membrane does not require trichocysts for intracellular localization.

Infection of Paramecium bursaria by Symbiotic Chlorella Species

Paramecium bursaria and endosymbiotic Chlorella species retain their ability to grow independently, but can reestablish endosymbiosis by mixing. Infection is induced through the hosts digestive vacuoles (DVs). Acidosomal and lysosomal fusions to the DVs begin at 0.5 and 2-3 min after mixing, respectively.

Cell division and density of symbiotic Chlorella variabilis of the ciliate Paramecium bursaria is controlled by the host's nutritional conditions during early infection process

The association of ciliate Paramecium bursaria with symbiotic Chlorella sp. is a mutualistic symbiosis. However, both the alga-free paramecia and symbiotic algae can still grow independently and can be reinfected experimentally by mixing them. Effects of the hosts nutritional conditions against the symbiotic algal cell division and density were examined during early reinfection. Transmission electron microscopy revealed that algal cell division starts 24 h after mixing with alga-free P. bursaria, and that the algal mother cell wall is discarded from the perialgal vacuole membrane, which encloses symbiotic alga. Labelling of the mother cell wall with Calcofluor White Stain, a cell-wall-specific fluorochrome, was used to show whether alga had divided or not. Pulse labelling of alga-free P. bursaria cells with Calcofluor White Stain-stained algae with or without food bacteria for P. bursaria revealed that the fluorescence of Calcofluor White Stain in P. bursaria with bacteria disappeared within 3 days after mixing, significantly faster than without bacteria. Similar results were obtained both under constant light and dark conditions. This report is the first describing that the cell division and density of symbiotic algae of P. bursaria are controlled by the hosts nutritional conditions during early infection.

Endosymbiosis of Chlorella species to the ciliate Paramecium bursaria alters the distribution of the host’s trichocysts beneath the host cell cortex

Each symbiotic Chlorella of the ciliate Paramecium bursaria is enclosed in a perialgal vacuole membrane derived from the host digestive vacuole membrane. Alga-free paramecia and symbiotic algae can grow independently. Mixing them experimentally can cause reinfection. Earlier, we reported that the symbiotic algae appear to push the host trichocysts aside to become fixed beneath the host cell cortex during the algal reinfection process. Indirect immunofluorescence microscopy with a monoclonal antibody against the trichocysts demonstrates that the trichocysts change their locality to form algal attachment sites and decrease their density beneath the host cell cortex through algal reinfection. Transmission electron microscopy to detect acid phosphatase activity showed that some trichocysts near the host cell cortex are digested by the host lysosomal fusion during algal reinfection. Removal of algae from the host cell using cycloheximide recovers the trichocysts arrangement and number near the host cell cortex. These results indicate that symbiotic algae compete for their attachment sites with preexisting trichocysts and that the algae have the ability to ensure algal attachment sites beneath the host cell cortex.