Polar effects of concanavalin A on the cortical cytoskeleton of a molluscan egg (Nassarius reticulatus,Gastropoda) |
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Authors: | Johanna E Speksnijder Katja J Teerds Willem J Hage M René Dohmen |
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Institution: | (1) Department of Experimental Zoology, University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands;(2) Hubrecht Laboratory, Netherlands Institute for Developmental Biology, Uppsalalaan 8, 3584 CT Utrecht, The Netherlands |
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Abstract: | Summary The organization of the surface of fertilizedNassarius reticulatus eggs was probed by investigating the effects of treatment with concanavalin A (Con A). This lectin causes abnormal polar
lobe formation as well as inhibition of cleavage. At low concentrations of Con A (0.3–1.0 μg/ml) the polar lobe constriction
becomes considerably elongated, whereas at higher concentrations (2.5–50 μ/ml) the position of the constriction undergoes
an extreme shift towards the animal pole. In the latter case, the surface of the animal part of the egg forms large protrusions
and folds. Con A also causes resorption of microvilli and disappearance of the extracellular layer covering these villi; this
process starts at the vegetal pole and propagates towards the animal pole. These changes in surface architecture are associated
with profound changes in the organization of filamentous (F-) actin as assessed by confocal laser scanning microscopy of NBD-phallacidin-labelled
eggs. Divalent succinyl-Con A has the same effects on polar lobe formation and surface architecture as tetravalent Con A,
but only at very high concentrations (100–200 μg/ml), indicating that Con A exerts its effects by cross-linkage of its binding
sites. Experiments with cytoskeleton inhibitors (cytochalasin D, colchicine, and nocodazole) reveal that in Con A-treated
eggs — as in untreated eggs — microfilaments, but not microtubules, are involved in the formation of the polar lobe constriction.
The calcium ion channel blocker D600 affects neither normal nor Con A-induced abnormal polar lobe formation, which suggests
that influx of external calcium is not required. In contrast, treatment with TMB-8, an antagonist of internal calcium release,
prevents the formation of a polar lobe in both normal and Con A-treated eggs. Finally, eggs from which the polar lobe has
been removed prior to Con A treatment show none of the effects described, whereas isolated polar lobes react similarly to
intact eggs. These results suggest that binding of Con A to sites present at the vegetal pole of the egg is responsible for
the observed effects of the lectin. |
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Keywords: | Polarity Molluscan development Polar lobe Concanavalin A F-actin |
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