Deciphering a novel mechanism regulating galectin-glycoprotein lattice assembly/disassembly during cell-cell interactions – sweetGalLat

All cells in nature are covered with a dense coating of glycans occuring as modifications of proteins and lipids. This glycome is decoded by specific glycan-binding proteins such as lectins. Lectin-glycan interactions are essential to biological systems, not simply as the “glue” between cells, but as the initiators of a functional cross-talk that modulates their physiology and homeostatic balance. The multivalent nature of both lectins and their saccharide ligands allows the formation of lectin–glycan lattices at the cell surface which help to organize glycoprotein assemblies on the surface of the cell. These ordered arrays of lectins and saccharides on the cell surface are playing leading roles in signal transduction in different biological processes as well as in cell–cell interaction and cell–pathogen interaction. Consequently, cells “fine-tune” the regulation of these processes by modulating the assembly/disassembly of protein-glycan lattices.
One family of multivalent lectins that can organize cell surface lattices is the galectins. Galectins preferentially bind ß-galactoside-containing glycans. They have a variety of functions in specific tissues and mediate developmental processes. Galectin-glycoprotein interactions at the cell surfaces regulate critical physiological and pathological processes. These organized interacting structures are not fixed and altering protein glycosylation or lectin expression are two well-described ways for a cell to dynamically modulate the strength of the lectin-glycoprotein lattice interactions. Very recently, we suggested a third new mechanism of modulation of such crucial interactions in the case of early B cell development. During B-cell development, bone marrow stromal cells secreting galectin-1 (Gal1) constitute a specific niche for pre-BII cells. Besides binding glycans, Gal1 is also a pre-B cell receptor (pre-BCR) ligand that induces receptor clustering, the first checkpoint of B cell differentiation. The Gal1/pre-BCR interaction is the first example of a Gal1/unglycosylated protein interaction in the extracellular compartment. We showed that Gal1/pre-BCR interaction modifies Gal1/glycan affinity towards specific glycan epitopes. This fine tuning of GAL1/glycan interactions may be a strategic mechanism for allowing pre-BCR activation and pre-BII cells departure from their niche.
The concept of galectin binding affinity being affected by a protein-protein interaction is original and novel. This additional level of molecular regulation at the cell surface needs to be fully characterized in vitro and in vivo in order to understand the structural features and specificity regulating this lattice assembly/disassembly. The aims of this project are the structural deciphering of this new mechanism, the visualization of the modulation of galectin lattice interactions in the cellular context, and the exploration of new galectin-dependent lattice targets for such type of regulation. In order to achieve these objectives we will use an approach combining liquid state and on-cell solid-state NMR structural studies, ITC, glycomic technologies and cell biology. This multidisciplinary project will provide in vitro characterization but also in vivo understanding of the fine-tuning of Gal1/glycan interactions. Deciphering this new mechanism and highlighting new lattice modulator proteins will settle a new idea about cell surface recognition phenomena played by galectins. Moreover, achievement of this project will provide a new road map for developing inhibitors targeting galectin-dependent lattices in pathological situations such as cancers.