Aggregation of sponge cells. XIV. Possible substitution of calcium ions by polycations

Identification and further characterization of the specific cell binding fragment from sponge aggregation factor

Monoclonal antibodies (McAbs) were raised against the aggregation factor (AF) from the marine sponge Geodia cydonium. Two clones were identified that secrete McAbs against the cell binding protein of the AF complex. Fab fragments of McAbs: 5D2-D11 completely abolished the activity of the AF to form secondary aggregates from single cells. The McAbs were determined to react with the AF in vitro; this interaction was prevented by addition of the aggregation receptor, isolated and purified from the same species. After dissociation of the AF by sodium dodecyl sulfate and 2-mercaptoethanol, followed by electrophoretical fractionation, a 47-kD protein was identified by immunoblotting which interacted with the McAbs: 5D2-D11. During this dissociation procedure, the sunburst structure of the AF was destroyed. In a second approach, the 47-kD protein was isolated by immunoprecipitation; 12 molecules of this protein species were calculated to be associated with the intact AF particle. The 47-kD AF fragment bound to dissociated Geodia cells with a high affinity (Ka of 7 X 10(8) M-1) even in the absence of Ca++ ions; the number of binding sites was approximately 4 X 10(6)/cell. This interaction was prevented by addition of the aggregation receptor to the 47-kD protein in the homologous cell system. Moreover, it was established that this binding occurs species-specifically. The 47-kD fragment of the AF was localized only extracellularly by indirect immunofluorescence staining in cryostat slices. These data suggest that the 47-kD protein is the cell binding molecule of the AF from Geodia.

Physicochemical and functional characterization of the polymerization process of the Geodia cydonium lectin

The extracellularly localized, galactose-specific lectin from the sponge Geodia cydonium binds at one class of sites, 40 mol Ca2+/mol lectin with an association constant (Ka) of 0.3 X 10(6)M-1. Stoichiometric calculations reveal that in the extracellular milieu 22 mol Ca2+ (maximum) are complexed per mol lectin. Binding of Ca2+ to the lectin increases its apparent Mr from 44000 to 56000 (electrophoretic determination) or from 36500 to 53500 (high-pressure liquid gel chromatographical determination); the s20, w increases from 4.3 S to 4.5 S if Ca2+ is added to the lectin. In the presence of Ca2+ the lectin undergoes a conformational change perhaps by expanding the carbohydrate side chains which are terminated by galactose. Subsequently the lectin molecules polymerize to large three-dimensional clumps (diameter up to 8 micron). Turbidimetric studies reveal an inhibition of the lectin polymerization by lactose. The Ka of the lectin-lectin polymerization rises from 0.9 X 10(6)M-1 to 14.0 X 10(6)M-1 after increasing the Ca2+ concentration (from 1 microM to 100 microM). Parallel with this increase in affinity, the Ka value of the lectin-aggregation factor binding drops from 41.2 X 10(6)M-1 (1 microM Ca2+) to 1.3 X 10(6)M-1 (100 microM Ca2+). In the absence of Ca2+, the Geodia lectin forms 1-10-micron two-dimensional sheets in the presence of homologous glycoconjugates. Cell binding experiments with polyacrylamide gels, containing covalently bound galactose, show that both homologous (Geodia cydonium) and heterologous cells (L5178y) bind with a higher affinity to the lectin-polymer matrix than to the lectin-monomer one. These data suggest that lectin-polymer structures, together with lectin-glycoconjugate associates, are components of the cell-substrate adhesion system(s) of sponges in vivo.

The contribution of the calcium-dependent interaction of aggregation factor molecules to recognition: a system providing additional specificity forces?

Cells from the sponge Microciona prolifera display on their surfaces large but defined proteoglycan complexes (Microciona aggregation factor = MAF) that mediate species-specific cell aggregation by a process requiring high calcium ion concentrations. An analysis of MAF-MAF interactions based on binding studies of MAF to glutaraldehyde-fixed sponge cells and MAF-derivatized beads demonstrates that the requirement for high calcium concentrations can be overcome by extremely small amounts of certain polycations such as polybrene, polylysine, or histones. For measurements of the affinity of these substances of MAF, a method was adopted that partitions 125I-labeled MAF between dextran and polyethyleneglycol in an aqueous two-phase polymer system depending on the net charge of the complex formed. Since only polymers of positive charges affect binding and partitioning at low concentrations, large areas of interaction similar to those found in glycosaminoglycans are proposed for MAF. Through a multitude of appropriately spaced interaction sites, the rather weak selectivity of single charged sites could in such a system still provide strong enough specificities to explain species-specific cell sorting. The biological significance of naturally occurring polycations as well as extracellular calcium includes their role in cell recognition, sorting out as well as the ordered and continual streaming movements of groups of cells seen in the mesohyl of live sponges.

Sponge cell aggregation

Dissociated sponge cell system has proved to be a useful model to study the process of cell aggregation both on cellular and subcellular level. The purpose of this review is to discuss recent results obtained from experiments with the marine sponge Geodia cydonium. Dissociated cells form functional aggregates during a process which can be sub-divided into three phases: first, formation of small primary aggregates in the presence of Ca2+; second, formation of secondary aggregates in the presence of an aggregation factor and third, reconstitution of a functional system of water-containing channels by rearrangement in the secondary aggregates. On subcellular level a series of macromolecules are known which are involved in the control of aggregation and separation of sponge cells: Aggregation factor, aggregation receptor, anti-aggregation receptor, beta-glucuronidase, beta-glucuronosyltransferase, beta-galactosyltransferase, beta-galactosidase and a lectin. These components might be linked in the following sequence: (a) Activation of the aggregation receptor by its enzymic glucuronylation; (b) Adhesive recognition of the cells, mediated by the aggregation factor and the glucuronylated aggregation receptor; (c) Inactivation of the aggregation receptor by its deglucuronylation with the membrane-associated beta-glucuronidase; (d) Cell separation due to either the loss of the recognition site (glucuronic acid) of the aggregation receptor for the aggregation factor or to an inactivation of the aggregation factor by the anti-aggregation receptor. The activity of the anti-aggregation receptor is most likely controlled by the Geodia lectin. The events leading to cell-cell recognition cause a change in the following metabolic events: Increase of oxygen uptake, decrease of cyclic AMP level, increase of cyclic GMP level and stimulation of programmed syntheses.