Heparin antagonism by polyvalent display of cationic motifs on virus-like particles

Abstract

Particles to the rescue! The construction of cationic amino acid motifs on the surface of bacteriophage Qbeta by genetic engineering or chemical conjugation gives particles that are potent inhibitors of the anticoagulant action of heparin, which is a common anticlotting agent subject to clinical overdose.Polyvalent interactions allow biological structures to exploit low-affinity ligand-receptor binding events to affect physiological responses. We describe here the use of bacteriophage Qbeta as a multivalent platform for the display of polycationic motifs that act as heparin antagonists. Point mutations to the coat protein allowed us to generate capsids bearing the K16M, T18R, N10R, or D14R mutations; because 180 coat proteins form the capsid, the mutants provide a spectrum of particles differing in surface charge by as much as +540 units (K16M vs. D14R). Whereas larger poly-Arg insertions (for example, C-terminal Arg(8)) did not yield intact virions, it was possible to append chemically synthesized oligo-Arg peptides to stable wild-type (WT) and K16M platforms. Heparin antagonism by the particles was evaluated by using the activated partial thrombin time (aPTT) clotting assay; this revealed that T18R, D14R, and WT-(R(8)G(2))(95) were the most effective at disrupting heparin-mediated anticoagulation (>95 % inhibition). This activity agreed with measurements of zeta potential (ZP) and retention time on cation exchange chromatography for the genetic constructs, which distribute their added positive charge over the capsid surface (+180 and +360 for T18R and D14R relative to WT). The potent activity of WT-(R(8)G(2))(95), despite its relatively diminished overall surface charge is likely a consequence of the particle's presentation of locally concentrated regions with high positive charge density that interact with heparin's extensively sulfated domains. The engineered cationic capsids retained their ability to inhibit heparin at high concentrations and showed no anticlotting activity of the kind that limits the utility of antiheparin polycationic agents that are currently in clinical use.

Figure 1

A) Crystal structure of wild-type bacteriophage Qβ, highlighting the peptide loops that are composed of coat protein residues 10–18 (red). This region is further magnified in B) dull green N10; bright green I11; white G12, G15; blue K13, K16; red D14; orange Q17; purple T18.

Figure 2

The azide oligo-Arg peptide R₈G₂ 1 and alkyne oligo-Arg peptide R₅ 2 were synthesized by using standard Fmoc chemistry

Figure 3

Attachment of oligo-Arg peptides to WT and K16M Qβ.

Figure 4

Size-exclusion FPLC of 100 μL of 0.6 mg mL⁻¹ A) K16M-(R₅)₅₀ (8) and B) WT-(R₈G₂)₉₅ (9) in 0.1 M KPi pH 7 buffer, showing intact particles in each case. The elution volumes of the dispersed particles are different due to different column packing conditions.

Figure 5

MALDI-MS of coat proteins from A) WT, B) K16M-(R₅)₅₀, and C) WT-(R₈G₂)₉₅, following disassembly and denaturation of the capsids with 5.5 M urea, DTT reduction, and cysteine capping with iodoacetamide.

Figure 6

Concentration dependence of added virus and peptide on coagulation time in the aPTT assay. The data points for protamine and 1 at >400 s represent experiments in which clotting was not observed within that time.