Cell motility during wound healing in giant algal cells: contraction in detergent-permeabilized cell models of Ernodesmis.
Triton-treated cells of the coenocytic green alga, Ernodesmis verticillata, manifest longitudinal contraction and centripetal closure of the cytoplasm in the presence of exogenous free calcium ions. Complete closure of the cytoplasm around the vacuolar remnants is obtained with 1 mM ATP present in the activation medium. The cytoplasmic contractions in these cell models closely resemble normal wound healing motility phenomena previously reported for this organism, thereby providing a system for studying the contractions associated with cellular wound healing. Dye-exclusion experiments with eosin B reveal that the tritonized models are freely permeable, whereas untreated cells do not take up the stain. Ultrastructurally, activated (contracting) models possess a network of microfilaments beneath the plasma membrane remnants in regions of active contraction. Microtubules are associated with the microfilaments, but they also occur throughout the periphery of the entire cells, even in unactivated models. This suggests that microfilaments may be involved in localized contractions leading to wound healing, whereas microtubules are probably providing a cytoskeletal framework upon which the microfilaments act. Colchicine (5 mM) has no effect on contraction in the models; extensive experiments with cytochalasins B and D (up to 100 micrograms/ml) have failed to show regular, reproducible inhibition of contraction also. Trifluoperazine (10 microM) or La3+ (1 mM) decreases the rate and magnitude of contraction to some extent, perhaps by preventing Ca2+ interaction with the contractile apparatus. The threshold concentration of free Ca2+ necessary to activate contraction in the models appears to be 10(-6) M, an order of magnitude higher than the preliminary value reported for non-permeabilized, wounded cells. Free Mg2+ also must be present for activation of models, with a minimum concentration of ca. 5 mM. The requirement of ATP for contraction can be demonstrated by the fact that either excessive washing of cell models prior to activation or inclusion of uncoupling agents in the solutions largely prevents activation of the models unless ATP is added to the Ca2+ -containing activation medium. These results provide further evidence that cellular wound healing in Ernodesmis is similar to the majority of other non-muscle cell motility phenomena in requiring ATP and micromolar amounts of free Ca2+ and that microfilaments are implicated in the contractile process. Moreover, the Ernodesmis cell models should be excellent systems for the further probing of cytoplasmic contractions associated with wound healing at the cellular level of organization.