Structure and function of the cytoskeleton in heliozoa

Does Formation of Multicellular Colonies by Choanoflagellates Affect Their Susceptibility to Capture by Passive Protozoan Predators?

Microbial eukaryotes, critical links in aquatic food webs, are unicellular, but some, such as choanoflagellates, form multicellular colonies. Are there consequences to predator avoidance of being unicellular vs. forming larger colonies? Choanoflagellates share a common ancestor with animals and are used as model organisms to study the evolution of multicellularity. Escape in size from protozoan predators is suggested as a selective factor favoring evolution of multicellularity. Heterotrophic protozoans are categorized as suspension feeders, motile raptors, or passive predators that eat swimming prey which bump into them. We focused on passive predation and measured the mechanisms responsible for the susceptibility of unicellular vs. multicellular choanoflagellates, Salpingoeca helianthica, to capture by passive heliozoan predators, Actinosphaerium nucleofilum, which trap prey on axopodia radiating from the cell body. Microvideography showed that unicellular and colonial choanoflagellates entered the predator's capture zone at similar frequencies, but a greater proportion of colonies contacted axopodia. However, more colonies than single cells were lost during transport by axopodia to the cell body. Thus, feeding efficiency (proportion of prey entering the capture zone that were engulfed in phagosomes) was the same for unicellular and multicellular prey, suggesting that colony formation is not an effective defense against such passive predators.

Immunocytochemical Analysis of α-Tubulin Distribution Before and After Rapid Axopodial Contraction in the Centrohelid Raphidocystis contractilis

The centrohelid Raphidocystis contractilis is a heliozoan that has many radiating axopodia, each containing a bundle of microtubules. Although the rapid contraction of the axopodia at nearly a video rate (30 frames/s) is induced by mechanical stimuli, the mechanism underlying this phenomenon in R. contractilis has not yet been elucidated. In the present study, we described for the first time an adequate immunocytochemical fixation procedure for R. contractilis and the cellular distribution of α-tubulin before and after rapid axopodial contraction. We developed a flow-through chamber equipped with a micro-syringe pump that allowed the test solution to be injected at a flow rate below the threshold required to induce rapid axopodial contraction. Next, we used this injection method for evaluating the effects of different combinations of two fixatives (paraformaldehyde or glutaraldehyde) and two buffers (phosphate buffer or PHEM) on the morphological structure of the axopodia. A low concentration of glutaraldehyde in PHEM was identified as an adequate fixative for immunocytochemistry. The distribution of α-tubulin before and after rapid axopodial contraction was examined using immunocytochemistry and confocal laser scanning fluorescence microscopy. Positive signals were initially detected along the extended axopodia from the tips to the bases and were distributed in a non-uniform manner within the axopodia. Conversely, after the induction of a rapid axopodial contraction, these positive signals accumulated in the peripheral region of the cell. These results indicated that axopodial microtubules disassemble into fragments and/ or tubulin subunits during rapid axopodial contraction. Therefore, we hypothesize that the mechanism of extremely rapid axopodial contraction accompanied by cytoskeletal microtubule degradati

Ca2+-dependent in vitro contractility of a precipitate isolated from an extract of the heliozoonActinophrys sol

Contraction of axopodia in actinophrid heliozoons (protozoa) is induced by a unique contractile structure, the "contractile tubules structure (CTS)". We have previously shown that a cell homogenate of the heliozoon Actinophrys sol yields a precipitate on addition of Ca2+ that is mainly composed of filamentous structures morphologically identical to the CTS. In this study, to further characterize the nature of the CTS in vitro, biochemical and physiological properties of the precipitate were examined. SDS-PAGE analysis showed that the Ca2+-induced precipitate was composed of many proteins, and that no proteins in the precipitate showed any detectable changes in electrophoretic mobility on addition of Ca2+. Addition of extraneous proteins such as bovine serum albumin to the cell homogenate resulted in cosedimentation of the proteins with the Ca2+-induced precipitate, suggesting that the CTS has a high affinity for other proteins that are not related to precipitate formation. Appearance and disappearance of the precipitate were repeatedly induced by alternating addition of Ca2+ and EGTA, and its protein composition remained unchanged even after repeated cycles. When adhered to a glass surface, the precipitate showed Ca2+-dependent contractility with a threshold of 10-100 nM, and this contractility was not inhibited by colchicine or cytochalasin B. The precipitate repeatedly contracted and relaxed with successive addition and removal of Ca2+, indicating that the contraction was controlled by Ca2+ alone with no need for any other energy supply. From our characterization of the precipitate, we concluded that its Ca2+-dependent formation and contraction are associated with the unique contractile organelle, the "contractile tubules structure".

Ca2+-dependent nuclear contraction in the heliozoon Actinophrys sol

Ca2+-dependent contractility was found to exist in the nucleus of the heliozoon protozoan Actinophrys sol. Upon addition of Ca2+ ([Ca2+]free = 2.0 x 10(-3) M), diameters of isolated and detergent-extracted nuclei became reduced from 16.5+/-1.7 microm to 11.0+/-1.3 microm. The threshold level of [Ca2+]free for the nuclear contraction was 2.9 x 10(-7) M. The nuclear contraction was not induced by Mg2+, and was not inhibited by colchicine or cytochalasin B. Contracted nuclei became expanded when Ca2+ was removed by EGTA; thus cycles of contraction and expansion could be repeated many times by alternating addition of Ca2+ and EGTA. The Ca2+-dependent nuclear contractility remained even after high salt treatment, suggesting a possible involvement of nucleoskeletal components in the nuclear contraction. Electron microscopy showed that, in the relaxed state, filamentous structures were observed to spread in the nucleus to form a network. After addition of Ca2+, they became aggregated and constructed a mass of thicker filaments, followed by re-distribution of the filaments spread around inside of the nucleus when Ca2+ was removed. These results suggest that the nuclear contraction is induced by Ca2+-dependent transformation of the filamentous structures in the nucleus.

Axopodial Contraction in the Heliozoon Raphidiophrys contractilis Requires Extracellular Ca2+

Axopodial contraction of the centrohelid heliozoon Raphidiophrys contractilis was induced by mechanical or electrical stimulation. For inducing contraction, extracellular Ca(2+) was required. The threshold level of extracellular Ca(2+) was between 10(-6)-10(-7) M. The speed of axopodial contraction was faster than 3.0 mm/sec. Re-elongation of axopodia started just after contraction, and its initial velocity was approximately 0.30 microm/sec. Electron microscopic observations were carried out using an improved fixative that contained 1 mg/ml ruthenium red and 15 microM Taxol. This fixative prevented artificial retraction of axopodia and resulted in better fixation. A bundle of hexagonally-arranged microtubules was observed in each axopodium, but no other filamentous structures were detected, suggesting that the contractile machinery of axopodia in R. contractilis may be different from that in actinophryid heliozoons in which Ca(2+)-dependent contractile filaments are employed for contraction.

Reactivation of Ca2+-dependent cytoplasmic contraction in permeabilized cell models of the heliozoon Echinosphaerium akamae

Permeabilized cell models of the large heliozoon Echinosphaerium akamae were prepared by treatment with 100 mM EGTA or 1% Triton X-100. When > 10(-6) M Ca(2+) was added to the EGTA-permeabilized cells, axopodial cytoplasm became contracted and several swellings were formed along the axopodial length. Axonemal microtubules remained intact, while higher concentration of Ca(2+) (> 10(-4) M) induced microtubule disassembly and complete breakdown of the axopodia. In Triton-permeabilized cells, cytoplasmic contraction and relaxation of the cell body were induced repeatedly by successive addition and removal of Ca(2+). The contraction did not require ATP, and was not inhibited by cytochalasin B. Electron microscopy showed, in EGTA-permeabilized axopodia, contractile tubules became granulated by the addition of Ca(2+). From these observations, it is strongly suggested that Ca(2+)-dependent granulation of the contractile tubules is responsible for the axopodial contraction.