Diving through Membranes: Molecular Cunning to Enforce the Endosomal Escape of Antibody-Targeted Anti-Tumor Toxins

Hemagglutinin (HA) is the receptor-binding and membrane fusion glycoprotein of influenza virus and the target for infectivity-neutralizing antibodies. The structures of three conformations of the ectodomain of the 1968 Hong Kong influenza virus HA have been determined by X-ray crystallography: the single-chain precursor, HA0; the metastable neutral-pH conformation found on virus, and the fusion pH-induced conformation. These structures provide a framework for designing and interpreting the results of experiments on the activity of HA in receptor binding, the generation of emerging and reemerging epidemics, and membrane fusion during viral entry. Structures of HA in complex with sialic acid receptor analogs, together with binding experiments, provide details of these low-affinity interactions in terms of the sialic acid substituents recognized and the HA residues involved in recognition. Neutralizing antibody-binding sites surround the receptor-binding pocket on the membrane-distal surface of HA, and the structures of the complexes between neutralizing monoclonal Fabs and HA indicate possible neutralization mechanisms. Cleavage of the biosynthetic precursor HA0 at a prominent loop in its structure primes HA for subsequent activation of membrane fusion at endosomal pH (Figure 1). Priming involves insertion of the fusion peptide into a charged pocket in the precursor; activation requires its extrusion towards the fusion target membrane, as the N terminus of a newly formed trimeric coiled coil, and repositioning of the C-terminal membrane anchor near the fusion peptide at the same end of a rod-shaped molecule. Comparison of this new HA conformation, which has been formed for membrane fusion, with the structures determined for other virus fusion glycoproteins suggests that these molecules are all in the fusion-activated conformation and that the juxtaposition of the membrane anchor and fusion peptide, a recurring feature, is involved in the fusion mechanism. Extension of these comparisons to the soluble N-ethyl-maleimide-sensitive factor attachment protein receptor (SNARE) protein complex of vesicle fusion allows a similar conclusion.

Since the discovery of SNARE proteins in the late 1980s, SNAREs have been recognized as key components of protein complexes that drive membrane fusion. Despite considerable sequence divergence among SNARE proteins, their mechanism seems to be conserved and is adaptable for fusion reactions as diverse as those involved in cell growth, membrane repair, cytokinesis and synaptic transmission. A fascinating picture of these robust nanomachines is emerging.

Despite continuous improvements in delivery systems, the development of methods for efficient and specific delivery of targeted therapeutic agents still remains an issue in biological treatments such as protein and gene therapy. The endocytic pathway is the major uptake mechanism of cells and any biological agents, such as DNA, siRNA and proteins. These agents become entrapped in endosomes and are degraded by specific enzymes in the lysosome. Thus, a limiting step in achieving an effective biological based therapy is to facilitate the endosomal escape and ensure cytosolic delivery of the therapeutics. Bacteria and viruses are pathogens which use different mechanisms to penetrate the membranes of their target cells and escape the endosomal pathway. Different mechanisms such as pore formation in the endosomal membrane, pH-buffering effect of protonable groups and fusion into the lipid bilayer of endosomes have been proposed to facilitate the endosomal escape. Several viral and bacterial proteins have been identified that are involved in this process. In addition, chemical agents and photochemical methods to rupture the endosomal membrane have been described. New synthetic biomimetic peptides and polymers with high efficacy in facilitating the endosomal escape, low pathogenicity and toxicity have been developed. Each strategy has different characteristics and challenges for designing the best agents and techniques to facilitate the endosomal escape are ongoing. In this review, several mechanisms and agents which are involved in endosomal escape are introduced. Copyright © 2010 Elsevier B.V. All rights reserved.