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

Abstract

Early metazoans had to evolve the first cell adhesion mechanism addressed to maintain a distinctive multicellular morphology. As the oldest extant animals, sponges are good candidates for possessing remnants of the molecules responsible for this crucial evolutionary innovation. Cell adhesion in sponges is mediated by the calcium-dependent multivalent self-interactions of sulfated polysaccharides components of extracellular membrane-bound proteoglycans, namely aggregation factors. Here, we used atomic force microscopy to demonstrate that the aggregation factor of the sponge Desmapsamma anchorata has a circular supramolecular structure and that it thus belongs to the spongican family. Its sulfated polysaccharide units, which were characterized via nuclear magnetic resonance analysis, consist preponderantly of a central backbone composed of 3-␣-Glc1 units partially sulfated at 2-and 4-positions and branches of Pyr(4,6)␣-Gal133-␣-Fuc2(SO 3 )133-␣-Glc4(SO 3 )133-␣-Glc34-linked to the central ␣-Glc units. Single-molecule force measurements of self-binding forces of this sulfated polysaccharide and their chemically desulfated and carboxyl-reduced derivatives revealed that the sulfate epitopes and extracellular calcium are essential for providing the strength and stability necessary to sustain cell adhesion in sponges. We further discuss these findings within the framework of the role of molecular structures in the early evolution of metazoans.Molecular clock estimates, paleobiogeochemical evidence, and the most widely accepted phylogenetic inferences place sponges (phylum Porifera) at the root of the metazoan tree as the oldest extant multicellular animals (1-4). 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). In most animals, protein interactions, such as those in the cell junctions of epithelia, mediate cell adhesion (7); however, sponge multicellularity is sustained by a characteristic cell adhesion and recognition mechanism based on specific carbohydrate-carbohydrate interactions (8).Carbohydrates have vast structural plasticity, ubiquitous distribution in vertebrate and invertebrate tissues, and are commonly associated with the cell surface (9). Carbohydrate selfadhesion is maintained by relatively weak forces that can increase in orders of magnitude upon the multimerization of individual epitopes into polysaccharides (10). Some examples of specific carbohydrate self-interactions include the multivalent binding of Lewis x epitopes involved in the first steps of embryogenesis (11), glycolipid-glycolipid interactions controlling cell adhesion, spreading, and motility (12), and self-interactions of sulfated polysaccharides leading to species-specific cell adhesion in sponges (13).Proteoglycan-like molecules termed aggregation factors (AFs) 4 mediate intercellu...

Dynamic cell–cell adhesion mediated by pericellular matrix interaction – a hypothesis

“…PCMs can also promote dynamic cell-cell adhesion, as documented in sponges. Here, cell sorting-compatible adhesion uses multimeric proteoglycan-like molecules that attach to lectin membrane receptors and bind to identical complexes on adjacent cells through carbohydrate-carbohydrate interactions (Fernandez-Busquets and Burger, 2003;Vilanova et al, 2016). In the closest relatives to the metazoans, the choanoflagellates, a type of colony formation depends on a secreted lectin in a cell surface matrix that is compatible with cell rearrangement (Brunet and King, 2017).…”

“…In the closest relatives to the metazoans, the choanoflagellates, a type of colony formation depends on a secreted lectin in a cell surface matrix that is compatible with cell rearrangement (Brunet and King, 2017). Thus, dynamic adhesion through a cell surface matrix is an ancient mechanism in metazoans (Vilanova et al, 2016).…”

Diverse cell junctions with unique molecular composition in tissues of a sponge (Porifera)

“…They are one of the most phylogenetically divergent groups of animals (King and Rokas, 2017;Simion et al, 2017), their anatomy is fundamentally different from other animals (Leys and Hill, 2012), and there are long-standing questions about the structure and homology of their tissues compared to epithelia in other animals (Leys et al, 2009;Tyler, 2003). It has been argued that sponge cell adhesion relies primarily upon an extracellular proteoglycan complex termed the Aggregation Factor (Bucior and Burger, 2004;Cauldwell et al, 1973;Grice et al, 2017;Haseley et al, 2001;Henkart et al, 1973;Humphreys, 1963;Vilanova et al, 2016). Antibodies raised against the Aggregation Factor have been reported to block reaggregation of dissociated cells (Schütze et al, 2001), and purified Aggregation Factor can mediate adhesion between beads in cell-free assays (Jarchow and Burger, 1998).…”

Microarray Glycan Profiling Reveals Algal Fucoidan Epitopes in Diverse Marine Metazoans

“…Glycan chains which contain fucose are also a part of the sponges' aggregation factors-a highly complex and diverse class of circular proteoglycans also called spongicans. These glycan structures of aggregation factors are responsible for mediating the cell-to-cell adhesion via calcium bridges (Misevic and Burger, 1993;Fernandez-Busquets and Burger, 2003;Bucior et al, 2004;Vilanova et al, 2009Vilanova et al, , 2016.…”