Facilitation of Membrane Fusion During Exocytosis and Exocytosis-Coupled Endocytosis and Acceleration of ``Ghost' Detachment in Paramecium by Extracellular Calcium. A Quenched-Flow/Freeze-Fracture Analysis

Abstract

e . Our data are compatible with the assumption that normally exocytotic membrane fusion will include a step of rapid Ca 2+ -mobilization from subplasmalemmal pools (''alveolar sacs'') and, as a superimposed step, a Ca 2+ -influx, since exocytotic membrane fusion can occur at [Ca 2+ ] e even slightly below resting [Ca 2+ ] i . The other important conclusion is that increasing [Ca 2+ ] e facilitates exocytotic and endocytotic membrane fusion, i.e., membrane resealing. In addition, we show for the first time that increasing [Ca 2+ ] e also drives detachment of ''ghosts'' -a novel aspect not analyzed so far in any other system. According to our pilot calculations, a flush of Ca 2+ , orders of magnitude larger than stationary values assumed to drive membrane dynamics, from internal and external sources, drives the different steps of the exo-endocytosis cycle.

Veratridine-Mediated Ca 2+ Dynamics and Exocytosis in Paramecium Cells

“…Only with veratridine, the stimulation of trichocyst exocytosis (analyzed in detail previously by Plattner et al [36]) and the effect on [Ca 2+ ] i increase (this paper) is reduced with [Ca 2+ ] o ‫ס‬ 10 mM (a concentration well tolerated by these cells). This effect remained unexplained until now, particularly since we had shown the opposite effect with AED [35] Fig. 2) and exocytosis is inhibited (Tables 1, 2).…”

“…In Paramecium, veratridine induces trichocyst exocytosis [15,29,36], though less efficiently than the polyamine secretagogue, aminoethyldextrane (AED) [35,38] or caffeine [15,27]. For both these agents we demonstrated, also by fluorochrome analysis, the occurrence of a cortical [Ca 2+ ] i transient orginating primarily from cortical pools ("alveolar sacs" [49]) and reinforced by superimposed Ca 2+ influx from the medium [14,15,26,27].…”

An exocytotic mutant of Paramecium caudatum: membrane fusion without secretory contents release

“…During AED stimulation, Ca z + for trichocyst decondensation would have to come from the outside medium since mobilisation from internal sources in P. tetraurelia wt cells produces exocytotic openings with trichocysts retained in condensed form [22], just as we now find with P. caudatum strain tndl. Even increasing [Ca 2 +]e to 10m M (-100x above normal) did not cause decondensation of trichocysts in tnd 1, while this greatly accelerates all steps of the exo-endocytotic cycle in P. tetraurelia wt :-cells [59]. Trichocysts from tnd1 do not decondense even when exposed to A23187 + Ca Z + (up to 50 mM tested).…”

“…However, for maximal response, AED concentrations had to be increased about lO-fold (to 10 !1M) over those reqUired with P tetraurelia wt cells [14]. Normally mobilisation from subplasmalemmal Ca 2 + pools (alveolar sacs) and Ca 2 + influx cooperate during AED [16][17][18][57][58][59] or caffeine stimulation [15].…”

Inseparable tandem: evolution chooses ATP and Ca2+to control life, death and cellular signalling

“…For membrane-vesicle interaction, various SNARE proteins and synaptotagmin-type Ca 2þ -sensors are present in spatially separated cellular compartments [146,147], in contrast to non-stimulated (constitutive) exocytosis that may not depend on Ca 2þ signals, if one makes painstaking scrutiny [148]. Endocytosis via clathrin-coated pits or uncoated vesicles is synchronized with exocytosis by the pulse of Ca 2þ required for both processes in mammalian [149,150] as well as in protozoan cells [151]. Endocytosis requires rstb.royalsocietypublishing.org Phil.…”