Self-Recognition and Ca2+-Dependent Carbohydrate-Carbohydrate Cell Adhesion Provide Clues to the Cambrian Explosion

Abstract

The Cambrian explosion of life was a relatively short period approximately 540 Ma that marked a generalized acceleration in the evolution of most animal phyla, but the trigger of this key biological event remains elusive. Sponges are the oldest extant Precambrian metazoan phylum and thus a valid model to study factors that could have unleashed the rise of multicellular animals. One such factor is the advent of self-/non-self-recognition systems, which would be evolutionarily beneficial to organisms to prevent germ-cell parasitism or the introduction of deleterious mutations resulting from fusion with genetically different individuals. However, the molecules responsible for allorecognition probably evolved gradually before the Cambrian period, and some other (external) factor remains to be identified as the missing triggering event. Sponge cells associate through calcium-dependent, multivalent carbohydrate-carbohydrate interactions of the g200 glycan found on extracellular proteoglycans. Single molecule force spectroscopy analysis of g200-g200 binding indicates that calcium affects the lifetime (+Ca/-Ca: 680 s/3 s) and bond reaction length (+Ca/-Ca: 3.47 A/2.27 A). Calculation of mean g200 dissociation times in low and high calcium within the theoretical framework of a cooperative binding model indicates the nonlinear and divergent characteristics leading to either disaggregated cells or stable multicellular assemblies, respectively. This fundamental phenomenon can explain a switch from weak to strong adhesion between primitive metazoan cells caused by the well-documented rise in ocean calcium levels at the end of Precambrian time. We propose that stronger cell adhesion allowed the integrity of genetically uniform animals composed only of "self" cells, facilitating genetic constitutions to remain within the metazoan individual and be passed down inheritance lines. The Cambrian explosion might have been triggered by the coincidence in time of primitive animals endowed with self-/non-self-recognition and of a surge in seawater calcium that increased the binding forces between their calcium-dependent cell adhesion molecules.

Carbohydrate-Carbohydrate Interactions Mediated by Sulfate Esters and Calcium Provide the Cell Adhesion Required for the Emergence of Early Metazoans

“…These calcium-and sulfate-mediated interactions provide the strength and stability (binding lifetime Ͼ10 2 s) necessary to sustain the aggregative properties of AFs and hence the physiological maintenance of cell-cell adhesion. Similar self-interactions mediated by calcium have been observed for the sulfated polysaccharide g200 from the AF of M. prolifera (5), indicating that such a mechanism of intermolecular interaction could be conspicuous in sponges.…”

“…However, the binding forces of these interactions are weaker and markedly less stable (binding lifetime Ͻ10 s), and their reaction lengths fall below the hydrogen bond distance, which is ϳ2.9 Å (51). Similar characteristics were observed for the calcium-free self-interactions of g200 from M. prolifera (5). These calciumand/or sulfate-free interactions likely operate through hydrogen bonds between hydroxyl groups on the polysaccharides, whereas the excess of Na ϩ ions in seawater neutralizes the repulsive forces expected from the highly anionic sulfate and carboxyl groups (10).…”

“…8D, supplemental Table S6). The lifetime of the binding complex dramatically affects its stability because a rapid on-off switching between bound and unbound states is more susceptible to dissociation than longer interactions (5). Therefore, our AFM-SMFS and -DFS results unequivocally show that the calcium-mediated self-interactions of sulfated DaSPs are stronger and last longer than desulfated DaSP despite slightly differing average forces.…”

“…These interaction forces have been attributed to hydrogen bonds between the hydroxyl groups of sugar moieties of polysaccharides (10). In this model calcium ions are responsible only for the approach and organization of the sugar moieties and provide adequate surfaces for further interactions (5). However, the participation of the highly reactive sulfate groups of these polysaccharides was formerly speculated (20).…”

“…Such ancestry makes modern sponges suitable organisms for studying the cellular and molecular aspects of the evolution of multicellularity (5). A pivotal feature of metazoans is their cell adhesion molecular machinery, which directs the formation and maintenance of a distinctive multicellular morphology (6).…”