Trichocysts—Paramecium's Projectile‐like Secretory Organelles

Abstract

This review summarizes biogenesis, composition, intracellular transport, and possible functions of trichocysts. Trichocyst release by Paramecium is the fastest dense core-secretory vesicle exocytosis known. This is enabled by the crystalline nature of the trichocyst "body" whose matrix proteins (tmp), upon contact with extracellular Ca , undergo explosive recrystallization that propagates cooperatively throughout the organelle. Membrane fusion during stimulated trichocyst exocytosis involves Ca mobilization from alveolar sacs and tightly coupled store-operated Ca -influx, initiated by activation of ryanodine receptor-like Ca -release channels. Particularly, aminoethyldextran perfectly mimics a physiological function of trichocysts, i.e. defense against predators, by vigorous, local trichocyst discharge. The tmp's contained in the main "body" of a trichocyst are arranged in a defined pattern, resulting in crossstriation, whose period expands upon expulsion. The second part of a trichocyst, the "tip", contains secretory lectins which diffuse upon discharge. Repulsion from predators may not be the only function of trichocysts. We consider ciliary reversal accompanying stimulated trichocyst exocytosis (also in mutants devoid of depolarization-activated Ca channels) a second, automatically superimposed defense mechanism. A third defensive mechanism may be effectuated by the secretory lectins of the trichocyst tip; they may inhibit toxicyst exocytosis in Dileptus by crosslinking surface proteins (an effect mimicked in Paramecium by antibodies against cell surface components). Some of the proteins, body and tip, are glycosylated as visualized by binding of exogenous lectins. This reflects the biogenetic pathway, from the endoplasmic reticulum via the Golgi apparatus, which is also supported by details from molecular biology. There are fragile links connecting the matrix of a trichocyst with its membrane; these may signal the filling state, full or empty, before and after tmp release upon exocytosis, respectively. This is supported by experimentally produced "frustrated exocytosis", i.e. membrane fusion without contents release, followed by membrane resealing and entry in a new cycle of reattachment for stimulated exocytosis. There are some more puzzles to be solved: Considering the absence of any detectable Ca and of acidity in the organelle, what causes the striking effects of silencing the genes of some specific Ca -release channels and of subunits of the H -ATPase? What determines the inherent polarity of a trichocyst? What precisely causes the inability of trichocyst mutants to dock at the cell membrane? Many details now call for further experimental work to unravel more secrets about these fascinating organelles.

Intracellular symbiosis of algae with possible involvement of mitochondrial dynamics

“…Each zoochlorella cell was observed to be surrounded by multiple numbers of trichocysts, thus partially restricting its free horizontal transfer along the inner surface of the plasma membrane (Supplementary Movie 1). Previous reports on the ultrastructure of other non-symbiotic species of Paramecium showed that subcortical areas are mainly filled with trichocysts and mitochondria without zoochlorella 37, 38 .

Figure 2TEM analyses of the associations between P . bursaria mitochondria and symbiotic zoochlorella.

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Evolutionary Cell Biology of Proteins from Protists to Humans and Plants

“…). In Paramecium , these pumps would be too sluggish to mediate rapid Ca 2+ downregulation, for example, after massive exocytosis stimulation, thus stressing the importance of Ca 2+ binding to high‐capacity CaBPs (Plattner ,b). …”

“…Since PMCA's working kinetics is rather sluggish, as found also in mammalian cells, PMCA was considered to serve mainly for household activity (Brini and Carafoli ). In Paramecium , balance calculations suggest that both P‐types pumps can regulate [Ca 2+ ] at best on a ~60‐min time scale (Plattner ,b). This makes both CaBPs and antiporter systems important for rapid [Ca 2+ ] i downregulation after stimulation.…”

“…This complexity includes the exchange of variable isoforms on the way through the cell and, thus, looks as intriguing as comparable pathways in mammalian cells. For details, see Plattner (,b). …”

“…TRP‐type channels are not known in any detail from ciliates (Plattner and Verkhratsky ; yet see Plattner [,b] for a possible equivalent), whereas they are known from unikonts, such as choanoflagellates (Cai et al. ).…”