Glycopolymer probes of signal transduction

Abstract

Glycans are key participants in biological processes ranging from reproduction to cellular communication to infection. Revealing glycan roles and the underlying molecular mechanisms by which glycans manifest their function requires access to glycan derivatives that vary systematically. To this end, glycopolymers (polymers bearing pendant carbohydrates) have emerged as valuable glycan analogs. Because glycopolymers can readily be synthesized, their overall shape can be varied, and they can be altered systematically to dissect the structural features that underpin their activities. This review provides examples in which glycopolymers have been used to effect carbohydrate-mediated signal transduction. Our objective is to illustrate how these powerful tools can reveal the molecular mechanisms that underlie carbohydrate-mediated signal transduction.

Figure 1

Carbohydrate-binding proteins exist as oligomers and can interact with glycans through a variety of mechanisms. An oligomeric protein can interact with an individual cell-surface glycan (A) or with multiple different cell-surface glycans simultaneously (B). Oligomeric proteins can also interact with soluble glycans or soluble oligomeric lectins can engage cell surface glycans (C & D). Soluble proteins can cluster cell-surface glycoproteins to mediate signal transduction (E). Likewise, soluble glycans can cluster cell-surface receptors to mediate signal transduction (F).

Figure 2

(Left) Polymers can be assembled from a wide variety of monomers in a controlled manner to generate polymers of a defined length and valency. (Center) The polymers can be generated with many different topologies. In addition, it is possible to generate polymers bearing multiple functionalities, such as biological ligands or fluorophores. (Right) Finally, a key step in eliciting signal transduction is clustering of cell-surface receptors. Polymers bearing carbohydrates can cluster cell-surface proteins to elicit a signaling output. Figure adapted from reference .

Figure 3

Example polymer backbones generated from common polymerization strategies used in synthesizing glycopolymers. The R substituent represents a linker bearing a carbohydrate ligand, but for many glycopolymers not every monomer unit bears a carbohydrate ligand.

Figure 4

Increasing polymer length (and valency) allows polymers to span multiple binding sites in oligomeric proteins, thereby increasing their functional affinity (avidity).

Figure 5

Peptide nucleic acids (PNAs) were generated to control the spacing of carbohydrates. The library of PNAs was screened against the dimeric antibody 2GI2, which binds to HIV gp120. As the distance between carbohydrates approached the distance between carbohydrate binding sites on 2GI2, the functional affinity increased.

Figure 6

Polymers of sufficient length are capable of bridging multiple surface receptors, clustering them, and initiating signal transduction.

Figure 7

Stenzel and coworkers synthesized linear glycopolymers and diblock glycopolymers, which self assembled to form glycopolymeric micelles. The glycopolymeric micelle was more efficient at clustering Con A than the linear glycopolymer.

Figure 8

An investigation of the role of ligand clustering. Glycopolymers displaying carbohydrate clusters were much more effective at clustering the lectin Con A.

Figure 9

Glycopolymers were used to promote L-selectin signal transduction. Upon clustering of the surface L-selectin by the glycopolymer, signal transduction occurs that leads to the proteolytic cleavage of L-selectin.

Figure 10

Various inhibitors of DC-SIGN. For compound 4, R=H for a monovalent inhibitor; alternatively, R can be a linker appended to synthetic polymer or protein backbone.

Figure 11

The structure of mucins closely resembles that of glycopolymers. Proteoglycans, however, have a different binding epitope arrangement than that found in glycopolymers. It is a testament to glycopolymer utility that they can function as glycosaminoglycan mimics.

Figure 12

Glycopolymers were synthesized by Hsieh-Wilson and coworkers bearing carbohydrate epitopes for CS-A, CS-C, and CS-E. The CS-E glycopolymers were found to inhibit axon regrowth through siganling.

Figure 13

Polymers were designed to contain a nitrophenyl or dinitrophenyl hapten, a carbohydrate ligand for CD22, or both. The hapent-substituted homopolymer only interacts with the B cell receptor complex (BCR), which activates B cell signaling. A copolymer bearing a hapten and a ligand for the lectin CD22 can interact with both the BCR and CD22, which attenuates B cell activation and suppresses immunity.