Secretory organelle docking at the cell membrane of Paramecium cells: dedocking and synchronized redocking of trichocysts

Membrane Traffic and Ca2+ -Signals in Ciliates

A Paramecium cell has as many types of membrane interactions as mammalian cells, as established with monoclonal antibodies by R. Allen and A. Fok. Since then, we have identified key-players, such as SNARE-proteins, Ca²⁺ -regulating proteins, including Ca²⁺ -channels, Ca²⁺ -pumps, Ca²⁺ -binding proteins of different affinity etc. at the molecular level, probed their function and localized them at the light and electron microscopy level. SNARE-proteins, in conjunction with a synaptotagmin-like Ca²⁺ -sensor protein, mediate membrane fusion. This interaction is additionally regulated by monomeric GTPases whose spectrum in Tetrahymena and Paramecium has been established by A. Turkewitz. As known from mammalian cells, GTPases are activated on membranes in conjunction with lumenal acidification by an H⁺ -ATPase. For these complex molecules we found in Paramecium an unsurpassed number of 17 a-subunit paralogs which connect the polymeric head and basis part, V1 and V0. (This multitude may reflect different local functional requirements.) Together with plasmalemmal Ca²⁺ -influx-channels, locally enriched intracellular InsP₃ -type (InsP₃ R, mainly in osmoregulatory system) and ryanodine receptor-like Ca²⁺ -release channels (ryanodine receptor-like proteins, RyR-LP), this complexity mediates Ca²⁺ signals for most flexible local membrane-to-membrane interactions. As we found, the latter channel types miss a substantial portion of the N-terminal part. Caffeine and 4-chloro-meta-cresol (the agent used to probe mutations of RyRs in man during surgery in malignant insomnia patients) initiate trichocyst exocytosis by activating Ca²⁺ -release channels type CRC-IV in the peripheral part of alveolar sacs. This is superimposed by Ca²⁺ -influx, i.e. a mechanism called "store-operated Ca²⁺ -entry" (SOCE). For the majority of key players, we have mapped paralogs throughout the Paramecium cell, with features in common or at variance in the different organelles participating in vesicle trafficking. Local values of free Ca²⁺ -concentration, [Ca²⁺ ]_i , and their change, e.g. upon exocytosis stimulation, have been registered by flurochromes and chelator effects. In parallel we have registered release of Ca²⁺ from alveolar sacs by quenched-flow analysis combined with cryofixation and x-ray microanalysis.

Immunolocalization of Actin in Paramecium Cells

We have selected a conserved immunogenic region from several actin genes of Paramecium, recently cloned in our laboratory, to prepare antibodies for Western blots and immunolocalization. According to cell fractionation analysis, most actin is structure-bound. Immunofluorescence shows signal enriched in the cell cortex, notably around ciliary basal bodies (identified by anti-centrin antibodies), as well as around the oral cavity, at the cytoproct and in association with vacuoles (phagosomes) up to several μm in size. Subtle strands run throughout the cell body. Postembedding immunogold labeling/EM analysis shows that actin in the cell cortex emanates, together with the infraciliary lattice, from basal bodies to around trichocyst tips. Label was also enriched around vacuoles and vesicles of different size including “discoidal” vesicles that serve the formation of new phagosomes. By all methods used, we show actin in cilia. Although none of the structurally well-defined filament systems in Paramecium are exclusively formed by actin, actin does display some ordered, though not very conspicuous, arrays throughout the cell. F-actin may somehow serve vesicle trafficking and as a cytoplasmic scaffold. This is particularly supported by the postembedding/EM labeling analysis we used, which would hardly allow for any large-scale redistribution during preparation.

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 trichocyst's 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.

Molecular identification of a SNAP-25-like SNARE protein in Paramecium

Using database searches of the completed Paramecium tetraurelia macronuclear genome with the metazoan SNAP-25 homologues, we identified a single 21-kDa Qb/c-SNARE in this ciliated protozoan, named P. tetraurelia SNAP (PtSNAP), containing the characteristic dual heptad repeat SNARE motifs of SNAP-25. The presence of only a single Qb/c class SNARE in P. tetraurelia is surprising in view of the multiple genome duplications and the high number of SNAREs found in other classes of this organism. As inferred from the subcellular localization of a green fluorescent protein (GFP) fusion construct, the protein is localized on a variety of intracellular membranes, and there is a large soluble pool of PtSNAP. Similarly, the PtSNAP that is detected with a specific antibody in fixed cells is associated with a number of intracellular membrane structures, including food vacuoles, the contractile vacuole system, and the sites of constitutive endo- and exocytosis. Surprisingly, using gene silencing, we could not assign a role to PtSNAP in the stimulated exocytosis of dense core vesicles (trichocysts), but we found an increased number of food vacuoles in PtSNAP-silenced cells. In conclusion, we identify PtSNAP as a Paramecium homologue of metazoan SNAP-25 that shows several divergent features, like resistance to cleavage by botulinum neurotoxins.

Dense-core secretory vesicle docking and exocytotic membrane fusion in Paramecium cells

Work with Paramecium has contributed to the actual understanding of certain aspects of exocytosis regulation, including membrane fusion. The system is faster and more synchronous than any other dense-core vesicle system described and its highly regular design facilitates correlation of functional and ultrastructural (freeze-fracture) features. From early times on, several crucial aspects of exocytosis regulation have been found in Paramecium cells, e.g. genetically controlled microdomains (with distinct ultrastructure) for organelle docking and membrane fusion, involvement of calmodulin in establishing such microdomains, priming by ATP, occurrence of focal fusion with active participation of integral and peripheral proteins, decay of a population of integral proteins ("rosettes", mandatory for fusion capacity) into subunits and their lateral dispersal during fusion, etc. The size of rosette particles and their dispersal upon focal fusion would be directly compatible with proteolipid V(0) subunits of a V-ATPase, much better than the size predicted for oligomeric SNARE pins (SCAMPs are unknown from Paramecium at this time). However, there are some restrictions for a straightforward interpretation of ultrastructural results. The rather pointed, nipple-like tip of the trichocyst membrane could accommodate only one (or very few) potential V(0) counterpart(s), while the overlaying domain of the cell membrane contains numerous rosette particles. Particle size is compatible with V(0), but larger than that assumed for the SNARE complexes. When membrane fusion is induced in the presence of antibodies against cell surface components, focal fusion is seen to occur with dispersing rosette particles but without dispersal of their subunits and without pore expansion. Clearly, this is required for completing fusion and pore expansion. After cloning SNARE and V(0) components in Paramecium (with increasing details becoming rapidly available), we may soon be able to address the question more directly, whether any of these components or some new ones to be detected, serve exocytotic and/or any other membrane fusions in Paramecium.

Molecular Aspects of Membrane Trafficking in Paramecium

Results achieved in the molecular biology of Paramecium have shed new light on its elaborate membrane trafficking system. Paramecium disposes not only of the standard routes (endoplasmic reticulum --> Golgi --> lysosomes or secretory vesicles; endo- and phagosomes --> lysosomes/digesting vacuoles), but also of some unique features, e.g. and elaborate phagocytic route with the cytoproct and membrane recycling to the cytopharynx, as well as the osmoregulatory system with multiple membrane fusion sites. Exocytosis sites for trichocysts (dense-core secretory vesicles), parasomal sacs (coated pits), and terminal cisternae (early endosomes) display additional regularly arranged predetermined fusion/fission sites, which now can be discussed on a molecular basis. Considering the regular, repetitive arrangements of membrane components, availability of mutants for complementation studies, sensitivity to gene silencing, and so on, Paramecium continues to be a valuable model system for analyzing membrane interactions. This review intends to set a new baseline for ongoing work along these lines.

My favorite cell--Paramecium

A Paramecium cell has a stereotypically patterned surface, with regularly arranged cilia, dense-core secretory vesicles and subplasmalemmal calcium stores. Less strikingly, there is also a patterning of molecules; for instance, some ion channels are restricted to certain regions of the cell surface. This design may explain very effective and selective responses, such as that to Ca(2+) upon stimulation. It enables the cell to respond to a Ca(2+) signal precisely secretion (exocytosis) or by changing its ciliary activity. These responses depend on the location and/or type of signal, even though these two target structures co-exist side-by-side, and normally only limited overlap occurs between the different functions. Furthermore, the patterning of exocytotic sites and the possibility of synchronous exocytosis induction in the sub-second time range have considerably facilitated analyses, and thus led to new concepts of exocytotic membrane fusion. It has been possible to dissect complicated events like overlapping Ca(2+) fluxes produced from external sources and from internal stores. Since molecular genetic approaches have become available for Paramecium, many different gene products have been identified only some of which are known from "higher" eukaryotes. Although a variety of basic cellular functions are briefly addressed to demonstrate the uniqueness of this unicellular organism, this article focuses on exocytosis regulation.

Exocytosis in chromaffin cells of the adrenal medulla

The chromaffin cell has been used as a model to characterize releasable components present in secretory granules and to understand the cellular mechanisms involved in catecholamine release. Recent physiological and biochemical developments have revealed that molecular mechanisms implicated in granule trafficking are conserved in all eukaryotic species: a rise in intracellular calcium triggers regulated exocytosis, and highly conserved proteins are essential elements which interact with each other to form a molecular scaffolding, ensuring the docking of granules at the plasma membrane, and perhaps membrane fusion. However, the mechanisms regulating secretion are multiple and cell specific. They operate at different steps along the life of a granule, from the time of granule biosynthesis up to the last step of exocytosis. With regard to cell specificity, noradrenaline and adrenaline chromaffin cells display different receptor and signaling characteristics that may be important to exocytosis. Characterization of regulated exocytosis in chromaffin cells provides not only fundamental knowledge of neurosecretion but is of additional importance as these cells are used for therapeutic purposes.

Annexins in Paramecium cells. Involvement in site-specific positioning of secretory organelles

Annexins were isolated from Paramecium cell homogenates by standard ethylene glycol tetraacetic acid (EGTA) extraction and 100 000-g centrifugation. Two different antibodies (Abs) against synthetic peptides were used, Call-15 and B15, which in mammalian cells recognize a sequence of annexin II or a common sequence occurring in several annexins (except for annexin II), respectively. With anti-Call-15 Abs, western blots from EGTA extracts showed strongly reactive bands of 44.5 and 46 kDa and of higher values. Some of these bands bound to the 100 000-g pellet fraction when Ca(2+) was added. Immuno- and affinity labelling revealed selective, Ca(2+)-dependent labelling of the cell cortex, with enrichment around trichocyst docking sites (facing subplasmalemmal Ca(2+) stores). Cortical fluorescence labelling decreased in wild-type (7S) cells when trichocyst ghosts were detached after synchronous exocytosis. Similarly, cortical labelling was reduced when intact trichocysts were detached from the cell surface of non-discharge mutant cells (nd9-28 degrees C, showing identical bands on blots), which then contained numerous heavily labelled phagolysosomes. This strongly suggests annexin downregulation. All together, the dynamic labelling of cortical structures we observed strongly supports involvement of calpactin-like annexins in trichocyst docking. Anti-B15 Abs recognized a band of 51 kDa and some of higher values. These Abs selectively labelled the outlines of the cytoproct, the site of spent phagolysosome exocytosis. In conclusion, our data indicate involvement of specific sets of annexins in site-specific positioning and attachment of widely different secretory organelles at the cell surface in Paramecium cells.

Isolation of the membranes from secretory organelles (trichocysts) of Paramecium tetraurelia

We present for the first time a method for isolation of the membranes of extrusive organelles (trichocysts) from sterile culture of different strains of Paramecium tetraurelia. First, trichocysts are isolated according to a new method (Glas-Albrecht, R. and Plattner, H. (1990) Eur. J. Cell Biol. 53, 164-172) with high purity and yield. Then the organelles are subjected to osmotic swelling. Since trichocysts then easily 'decondense' and entangle membranes, these cannot be isolated directly by centrifugation, but only by passage through a filter and subsequent centrifugation. Purity of membrane fractions is analysed by electron microscopy and SDS-PAGE, combined with silver staining or, after biotinylation, by avidin-peroxidase labelling. Molecular masses resolved in our gels are in a range from less than or equal to 15 to greater than or equal to 105 kDa. Main bands obtained with nd9-28 degrees C trichocyst membranes (most bands also being common to wild type trichocysts) are of about 16.5, 19-21, 27-29, 33-34, 44-45 (strong), 47-48 (strong), 57, 61, 65 (strong), 68-71, 75, 81, 94-95 (strong), 104 and greater than or equal to 110 kDa, from a total of approx. 23 bands resolved. There is no remarkable occurrence of dominant protein bands from trichocyst contents ('trichynins'), though these might represent up to 10(3)-times more of the total trichocyst proteins. The ratio of phospholipid/protein is approx. 0.2 mg/mg. The methodology developed might also be valuable for the isolation of extrusome membranes from some other protozoan species.

Secretory granule and synaptic vesicle formation

Sorting of secreted proteins into dense-core secretory granules may involve selective aggregation of regulated secretory proteins, rather than a conventional sortase. Synaptic vesicles, which mediate paracrine communication between adjacent cells, appear to arise by a modification of the early endosome pathway. Targeting to the cell surface involves the actin-based cytoskeleton and small GTP-binding proteins.