Glycomimetic Peptides as Therapeutic Tools

Abstract

The entry of peptides into glycobiology has led to the development of a unique class of therapeutic tools. Although numerous and well-known peptides are active as endocrine regulatory factors that bind to specific receptors, and peptides have been used extensively as epitopes for vaccine production, the use of peptides that mimic sugars as ligands of lectin-type receptors has opened a unique approach to modulate activity of immune cells. Ground-breaking work that initiated the use of peptides as tools for therapy identified sugar mimetics by screening phage display libraries. The peptides that have been discovered show significant potential as high-avidity, therapeutic tools when synthesized as multivalent structures. Advantages of peptides over sugars as drugs for immune modulation will be illustrated in this review.

Keywords: CLEC10A; dose response; eczema; glycomimetic peptides; transglutaminase 2.

Figure 1

Structure of the complex of α-amylase with tendamistat. (A) The crystal structure of tendamistat (PDB accession no. 1OK0). The loop structure that binds to the glycan binding site of α-amylase is highlighted. (B) The crystal structure of the complex with tendamistat bound in the glycan binding site of porcine α-amylase (accession no. 1BVN). (C) Model of the 6-mer loop structure of tendamistat (shaded in red) bound to porcine α-amylase (accession no. 1PIF) generated in silico with CABS-Dock (RMSD = 3.561 Å). The predicted binding energy, ΔG′ = −56 to −60 kJ/mol, is nearly the value calculated from K_eq.

Figure 2

Binding of the peptide MYWYPY to ConA (accession no. 5YGM) as predicted by CABS-Dock. (A) A model of the peptide bound to the left side of the Man binding site (Model 2: RMSD = 5.621 Å; ΔG′ = −43 kJ/mol). (B) A model of the peptide bound to the right side of the Man binding site (Model 4: RMSD = 5.997 Å; ΔG′ = −43 kJ/mol). The peptide is shaded in red.

Figure 3

In silico comparison of predicted binding of peptides (A) ARLPR (RMSD = 2.331 Å, ΔG′ = −46.4 kJ/mol) and (B) NPSHPLSG (RMSD = 2.848, ΔG′ = −45.1 kJ/mol) to Siglec-7 (accession no. 1O7V). Modeling was performed with CABS-Dock. Peptides are shaded in red.

Figure 4

In silico comparison of binding of (A) ARLPR and (B) NPSHPLSG to Siglec-5 (accession no. 2ZG1) as predicted by CABS-Dock. The figure shows model 6 for ARLPR (RMSD = 14.47 Å, ΔG′ = −46.4 kJ/mol) and model 1 for NPSHPLSG (RMSD = 4.518 Å, ΔG′ = −44.3 kJ/mol). Peptides are shaded in red.

Figure 5

(A) In silico docking of an arm of sv6D (NQHTPR) to human CLEC10A (accession no. 6PY1, RMSD = 1.343 Å, ΔG′ = −37 kJ/mol) with CABS-Dock. The peptide is enclosed in red shading. (B) The structure in (A) was downloaded into ArgusLab. The position of sv6D in the binding pocket is shown after additional molecular dynamics. The peptide is colored (carbon, grey; nitrogen, blue; oxygen, red) while the binding site is yellow. The positions of His²⁷⁴, His²⁸⁴, and His²⁸⁶ of the binding site are indicated. (C) A lysate of human monocyte-derived DCs was incubated with (1) mouse anti-human CLEC10A, which was recovered with magnetic beads coated with protein A; (2) biotinylated sv6D; or (3) biotinylated svL4, which were recovered with magnetic beads coated with streptavidin. Proteins were eluted from the beads and subjected to electrophoresis. Molecular mass markers are indicated for IgG heavy chain (50 kDa), IgG light chain (25 kDa), and a streptavidin C1 subunit (13.6 kDa). The top band is an instrument marker.

Figure 6

Effect of dose on response in C57BL/6 mice. The peptides were injected subcutaneously, and measured endpoints included proliferation of progenitor peritoneal cells in healthy mice (red circles), inhibition of growth of glioma tumors with cells implanted into the brain (blue circles), and survival of mice with implanted ID8 ovarian cells treated with svL4 (green squares) or sv6D (yellow squares). The response curve reflects the extent of receptor occupancy, with greater than 50% leading to inhibition or tolerance.

Figure 7

Growth of glioma tumor after implantation of GL261 glioma cell line into brain of C57BL/6 mice. svL4 was injected subcutaneously on alternate days, starting at day 7 after implantation, at the doses indicated. Tumor size was measured by NMR [77].

Figure 8

Structures of transglutaminase 2. (A) The inactive, closed conformation (accession no. 3LY6) and (B) the fully open, active conformation (accession no. 2Q3Z). The catalytic site of the enzyme is circled and the position of the cysteine residue that forms the thioester linkage to the substrate in the first step in the reaction is highlighted. The closed conformation is stabilized by Mg²⁺ and a GTP molecule, shown in gray, that binds to β-barrel1 and covers the catalytic site. Calcium displaces Mg²⁺ and GTP and generates the open conformation, which is stabilized by an inactive derivative of the peptide substrate. As shown within the box at the right, the remainder of the tetravalent structure of svL4 and sv6D may restrict entry of an arm into the catalytic site of the closed or partially open conformation.