Eukaryotic cells employ vast and intricate molecular mechanisms to carry out biological functions that help them thrive. One such mechanism is the dynamic movement and reorganizing of the plasma membrane in order to facilitate inter- and intra-cellular events via receptor and/or ligand engagement. The plasma membrane is surrounded by a very dense layer of sugars, or glycans, which extend from glycolipids and glycoproteins, and can interact with galectins, a family of sugar-binding proteins. These interactions form molecular networks called galectin-glycoprotein lattices that function to control the localization, clustering, and retention of glycoproteins to globally regulate cell activation, growth, arrest, and differentiation.
Glycoproteins are built in the endoplasmic reticulum and the Golgi apparatus, in which proteins are folded and then extensively modified with glycans before transport to the cell surface. In the Golgi, the N-acetylglucosaminyl transferases (Mgat1, 2, 4, and 5) mediate the branching of Asparagine (N)-linked glycans in an “assembly line” manner to progressively increase the production of ligands for galectins. More N-glycan branching increases galectin avidity to maintain lattice integrity, cellular homeostasis, and appropriate functional outcomes. Less N-glycan branching reduces galectin avidity, weakens the lattice, and promotes cellular dysfunction and disease states.
We have shown the galectin-glycoprotein lattice and the N-glycan branching pathway are critical regulators of T cells. An intact lattice on T cells promotes basal TCR signaling and appropriate activation states. Furthermore, the lattice inhibits pro-inflammatory TH1/TH17 differentiation, and promotes humoral/immunomodulatory TH2 and anti-inflammatory iTREG differentiation. When the lattice is weakened due to reduced N-glycan branching, T cells become hyperactive and more pro-inflammatory, exacerbating autoimmunity (i.e., EAE and MS).
How the galectin-glycoprotein lattice and N-glycan branching regulates B cells has not been shown before. We provide evidence N-glycan branching promotes B cell generation, differentially regulates CD19/BCR and TLR surface expression and signaling, and affects antigen presentation to CD4+ T cells to influence TH1, TH17, and iTREG differentiation. Furthermore, reduced N-glycan branching in B cells resulted in more severe EAE progression. These studies have begun to delineate how the galectin-glycoprotein lattice regulates B cell homeostasis, and may have implications in B cell targeting therapies.