My favorite cell—Paramecium

Abstract

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 exocy-tosis regulation. BioEssays 24:649-658, 2002.

Novel Types of Ca2+ Release Channels Participate in the Secretory Cycle of Paramecium Cells

“…Besides [Ca 2ϩ ] homeostasis, the Paramecium cell has to regulate a variety of well-characterized processes (75). This includes exocytosis of dense-core secretory vesicles (trichocysts) (71,74,99). Each cell possesses up to 1,000 trichocysts attached to the cell membrane.…”

One-way calcium spill-over during signal transduction in Paramecium cells: from the cell cortex into cilia, but not in the reverse direction

“…In ciliates dense-core exocytotic vesicles and cilia exist side-by-side in regular intervals [15]. In Paramecium, the distance of these organelles is ∼1 m, thus aggravating differential signal transduction [16]. In Paramecium, depolarisation-induced activation of ciliary Ca 2+ -channels causes ciliary beat reversal and backward swimming, respectively [10,11,13,[17][18][19][20], with no exocytotic response [21].…”

Refilling of cortical calcium stores in Paramecium cells: in situ analysis in correlation with store-operated calcium influx

“…The regular design of the cortex of a Paramecium cell contains not only regularly arranged trichocyst docking sites for immediate release, but also intemlittent "alveolar sacs" (ASs) which encircle each trichocyst (see [14,IS]). ASs are flat sacs,~I 00 nm wide, which represent cortical Ca2+ -stores [27,28] attached at the cell membrane at a distance of~IS nm.…”