Rational Design of General Amyloid Interaction Motif (GAIM) Variants that Potently Engage, Remodel and Block the Spread of Amyloid Aggregates Rajaraman Krishnan, Eva Asp, Ming Proschitsky, Charlotte Chung, Michal Lulu, Cassandra Rockwell-Postel, Jason Wright, Richard Fisher
The general amyloid interaction motif (GAIM), derived from a minor capsid protein of the bacteriophage M13, binds a wide variety of amyloid aggregates assembled from amyloid (A) beta, tau, α-synuclein, IAPP and transthyretin in a conformation-dependent manner. GAIM adopts a horse-shoe conformation and consists of two beta strand rich domains connected by a flexible glycine-serine rich linker. A bivalent GAIM Ig-fusion shows improved ability to engage amyloids compared to the GAIM monomer in vitro (Krishnan R et.al, 2014; 2017) and systemic treatment with the Ig fusion effectively lowers both Abeta and tau aggregates in transgenic animal models of Alzheimer disease (Levenson et.al, 2016). NPT088, a first-generation Ig-fusion developed at Proclara is currently in clinical testing for treating Alzheimer's disease. To gain insight into the structural elements that facilitate the GAIM-amyloid-specific interaction, we carried out site-directed mutagenesis to modulate GAIM's amyloid binding surface. Over 100 variants were characterized for: (1) binding to different amyloid conformers of Abeta and tau; (2) amyloid assembly inhibition and remodeling activities; and (3) blocking of cell-to-cell propagation of aggregate seeds. Our studies show that potent binding and remodeling are governed by the progressive interaction and rearrangement of the N1 and the N2 domains of GAIM in relation to the cross-β sheet scaffold of the amyloid target. Selective binding to a subset of amyloid conformers (e.g., towards Abeta or tau aggregates) is driven by polar residues along the N1 domain. Promiscuous interactions with non-amyloid substrates like soluble proteins, sticky molten-globule structures and non-amyloid fibrillar polymers, such as collagen and elastin are driven by the instability of the N2 domain. Finally, the mutagenesis data also shows that surprisingly minor changes along the inner surfaces of the horse shoe conformation, formed by the juxtaposition of N1 and N2 domains, modulate inter-domain stability. These studies further support the mechanism of a novel class of therapeutic candidates based on the GAIM platform that can engage and remove amyloids in brain and peripheral organs.