Ultrastructural and antigenic preservation of a delicate structure by cryopreparation: identification and immunogold localization during biogenesis of a secretory component (membrane-matrix connection) in Paramecium trichocysts

Signalling in ciliates: long- and short-range signals and molecular determinants for cellular dynamics

In ciliates, unicellular representatives of the bikont branch of evolution, inter- and intracellular signalling pathways have been analysed mainly in Paramecium tetraurelia, Paramecium multimicronucleatum and Tetrahymena thermophila and in part also in Euplotes raikovi. Electrophysiology of ciliary activity in Paramecium spp. is a most successful example. Established signalling mechanisms include plasmalemmal ion channels, recently established intracellular Ca²⁺ -release channels, as well as signalling by cyclic nucleotides and Ca²⁺ . Ca²⁺ -binding proteins (calmodulin, centrin) and Ca²⁺ -activated enzymes (kinases, phosphatases) are involved. Many organelles are endowed with specific molecules cooperating in signalling for intracellular transport and targeted delivery. Among them are recently specified soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs), monomeric GTPases, H⁺ -ATPase/pump, actin, etc. Little specification is available for some key signal transducers including mechanosensitive Ca²⁺ -channels, exocyst complexes and Ca²⁺ -sensor proteins for vesicle-vesicle/membrane interactions. The existence of heterotrimeric G-proteins and of G-protein-coupled receptors is still under considerable debate. Serine/threonine kinases dominate by far over tyrosine kinases (some predicted by phosphoproteomic analyses). Besides short-range signalling, long-range signalling also exists, e.g. as firmly installed microtubular transport rails within epigenetically determined patterns, thus facilitating targeted vesicle delivery. By envisaging widely different phenomena of signalling and subcellular dynamics, it will be shown (i) that important pathways of signalling and cellular dynamics are established already in ciliates, (ii) that some mechanisms diverge from higher eukaryotes and (iii) that considerable uncertainties still exist about some essential aspects of signalling.

A Multigene Family Encoding R-SNAREs in the Ciliate Paramecium tetraurelia

SNARE proteins (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) mediate membrane interactions and are conventionally divided into Q-SNAREs and R-SNAREs according to the possession of a glutamine or arginine residue at the core of their SNARE domain. Here, we describe a set of R-SNAREs from the ciliate Paramecium tetraurelia consisting of seven families encoded by 12 genes that are expressed simultaneously. The complexity of the endomembrane system in Paramecium can explain this high number of genes. All P. tetraurelia synaptobrevins (PtSybs) possess a SNARE domain and show homology to the Longin family of R-SNAREs such as Ykt6, Sec22 and tetanus toxin-insensitive VAMP (TI-VAMP). We localized four exemplary PtSyb subfamilies with GFP constructs and antibodies on the light and electron microscopic level. PtSyb1-1, PtSyb1-2 and PtSyb3-1 were found in the endoplasmic reticulum, whereas PtSyb2 is localized exclusively in the contractile vacuole complex. PtSyb6 was found cytosolic but also resides in regularly arranged structures at the cell cortex (parasomal sacs), the cytoproct and oral apparatus, probably representing endocytotic compartments. With gene silencing, we showed that the R-SNARE of the contractile vacuole complex, PtSyb2, functions to maintain structural integrity as well as functionality of the osmoregulatory system but also affects cell division.

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.

NSF regulates membrane traffic along multiple pathways inParamecium

N-ethylmaleimide (NEM)-sensitive factor (NSF), a regulator of soluble NSF attachment protein receptors (SNAREs), is required for vesicular transport in many eukaryotic cells. In the ciliated protozoon Paramecium, complex but well-defined transport routes exist, constitutive and regulated exocytosis, endocytosis, phagocytosis and a fluid excretory pathway through contractile vacuoles, that can all be studied independently at the whole cell level. To unravel the role of NSF and of the SNARE machinery in this complex traffic, we looked for NSF genes in Paramecium, starting from a partial sequence found in a pilot random sequencing project. We found two very similar genes, PtNSF1 and PtNSF2, which both seem to be expressed. Peptide-specific antibodies (Abs) recognize PtNSF as a 84 kDa band. PtNSF gene silencing results in decreasing phagocytotic activity,while stimulated exocytosis of dense core-vesicles (trichocysts), once firmly attached at the cell membrane, persists. Ultrastructural analysis of silenced cells shows deformation or disappearance of structures involved in membrane traffic. Aggregates of numerous small, smooth vesicles intermingled with branches of ER occur in the cytoplasm and are most intensely labeled with anti-NSF Ab-gold. Furthermore, elongated vesicles of ∼30 nm diameter can be seen attached at cortical calcium storage compartments, the alveolar sacs,whose unknown biogenesis may thus be revealed. Involvement of PtNSF in some low frequency fusion events was visualized in non-silenced cells by immuno-fluorescence, after cautious permeabilization in the presence of ATP-γ-S and NEM. Our data document that PtNSF is involved in distinct pathways of vesicle traffic in Paramecium and that actual sensitivity to silencing is widely different, apparently dependent on the turnover of membrane-to-membrane attachment formation.

Growth and form of secretory granules involves stepwise assembly but not differential sorting of a family of secretory proteins in Paramecium

Paramecium trichocysts are voluminous secretory vesicles consisting of a spindle-shaped body surmounted by a tip that serves to anchor them at exocytotic sites in the plasma membrane. This constrained shape is conferred by the proteins stored in the vesicles, which form an insoluble three-dimensional crystalline array. The constituent polypeptides (Trichocyst Matrix Proteins, TMPs), which assemble during trichocyst biogenesis, are produced by proteolytic processing of soluble proproteins encoded by a large multigene family. In order to investigate the functional significance of the TMP multigene family, which assures the synthesis of a mixture of related polypeptides, we have designed synthetic genes for heterologous expression of three different mature polypeptides, which were used to obtain sequence-specific rabbit antisera. We used these antisera to carry out immunolocalization experiments with wild-type trichocysts at different stages of development and found that the trichocyst matrix consists of two concentric layers containing different TMPs, and that the assembly of each layer corresponds to a distinct phase of trichocyst growth. Examination of mutant trichocysts created by targeted gene silencing of different TMP genes showed that the layer containing the products of the silenced genes is specifically affected, as are all subsequently assembled parts of the structure, consistent with an ordered assembly pathway. This stepwise assembly is not controlled by differential sorting of the TMPs, as single and double label experiments provided evidence that the different TMPs are delivered together to post-Golgi vesicles and developing trichocysts. We present a model for trichocyst biogenesis in which TMP assembly is controlled by protein processing.

Quantitative immunogold localization of protein phosphatase 2B (calcineurin) in Paramecium cells

For immunogold EM labeling analysis, we fixed Paramecium cells in 4% formaldehyde and 0.125% glutaraldehyde, followed by low-temperature embedding in unicryl and UV polymerization. We first quantified some obvious but thus far neglected side effects of section staining on immunogold labeling, using mono- or polyclonal antibodies (Abs) against defined secretory and cell surface components, followed by F(ab)(2)- or protein A-gold conjugates. Use of alkaline lead staining resulted in considerable rearrangement and loss of label unless sections were postfixed by glutaraldehyde after gold labeling. This artifact is specific for section staining with lead. It can be avoided by staining sections with aqueous uranyl acetate only to achieve high-resolution immunogold localization of a protein phosphatase on unicryl sections. In general, phosphatases are assumed to be closely, although loosely, associated with their targets. Because the occurrence of protein phosphatase 2B (calcineurin) in Paramecium has been previously established by biochemical and immunological work, as well as by molecular biology, we have used Abs against mammalian CaN or its subunits, CaN-A and CaN-B, for antigen mapping in these cells by quantitative immunogold labeling analysis. Using ABs against whole CaN, four structures are selectively labeled (with slightly decreasing intensity), i.e., infraciliary lattice (centrin-containing contractile cortical filament network), parasomal sacs (coated pits), and outlines of alveolar sacs (subplasmalemmal calcium stores, tightly attached to the cell membrane), as well as rims of chromatin-containing nuclear domains. In other subcellular regions, gold granules reached densities three to four times above background outside the cell but there was no selective enrichment, e.g., in cilia, ciliary basal bodies, cytosol, mitochondria, trichocysts (dense-core secretory organelles), and non-chromatin nuclear domains. Their labeling density was 4- to 8.5-fold (average 6.5-fold) less than that on selectively labeled structures. Labeling tendency was about the same with Abs against either subunit. Our findings may facilitate the examination of molecular targets contained in the selectively labeled structures. (J Histochem Cytochem 48:1269-1281, 2000)

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.