From Beaumont to poison ivy: marine sponge cell aggregation and the secretory basis of inflammation

Abstract

We have studied Microciona prolifera cells as a model for inflammation and secretion. Dissociated in Ca-, Mg-free seawater with 2.5 mM EDTA, the cells aggregate when exposed to Ca (greater than 5 mM) and Ca ionophores. Extracellular Ca is not required over the course of aggregation; brief pulses of Ca suffice. Aggregation was induced by A23187 in excess EDTA after cells were prepared by pulse Ca. It appeared that Ca ionophore stimulated the secretion of Microciona aggregation factor (MAF) to a locus or in a form inaccessible to external EDTA. Pulse-induced aggregation depended on MAF because it was inhibited by MAF fragments, which are ligands for MAF-binding sites. Sponge cells were preloaded with three fluorescent dyes that monitor aspects of stimulus-secretion coupling: 1) 3,3'-dipropylthiadicarbocyanine iodide (dis-C3-(5)), a carbocyanine dye presumed to report changes in membrane potential; 2) 9-aminoacridine (9AA), which presumably reports secretion from acid vesicles; and 3) chlortetracycline (CTC), presumed to report mobilization of membrane-associated Ca. Exposure of cells either to constant Ca or to pulse Ca stimuli caused prompt decreases in the fluorescence of cells with diS-C3-(5) and increases in fluorescence of cells with 9AA. In contrast, although constant Ca provoked decreases in fluorescence of cells with CTC, a pulse Ca was without effect. Moreover, inhibitors of stimulus-response coupling (e.g., aspirin, sodium salicylate, 5 mM; diclofenac, 100 microM) inhibited sponge aggregation induced by either constant or pulse stimuli. In contrast, like the endogenous mediator of inflammation, leukotriene B4, trienoic alkyl catechols (urushiol) from poison ivy provoked aggregation. These studies suggest the utility of this marine model for analysis of stimulus-response coupling in cells of higher species that also respond to secretagogues in the absence of external Ca.