Aromatic amino acids strongly promote cross-β amyloid formation; whether the amyloidogenicity of aromatic residues is due to high hydrophobicity and β-sheet propensity or formation of stabilizing π-π interactions has been debated. To clarify the role of aromatic residues on amyloid formation, the islet amyloid polypeptide 20-29 fragment [IAPP(20-29)], which contains a single aromatic residue (Phe 23), was adopted as a model. The side chain of residue 23 does not self-associate in cross-β fibrils of IAPP(20-29) (Nielsen et al., Angew Chem Int Ed 2009;48:2118-2121), allowing investigation of the amyloidogenicity of aromatic amino acids in a context where direct π-π interactions do not occur. We prepared variants of IAPP(20-29) in which Tyr, Leu, Phe, pentafluorophenylalanine (F5-Phe), Trp, cyclohexylalanine (Cha), α-naphthylalanine (1-Nap), or β-naphthylalanine (2-Nap) (in order of increasing peptide hydrophobicity) were incorporated at position 23 (SNNXGAILSS-NH2), and the kinetic and thermodynamic effects of these mutations on cross-β self-assembly were assessed. The Tyr, Leu, and Trp 23 variants failed to readily self-assemble at concentrations up to 1.5 mM, while the Cha 23 mutant fibrillized with attenuated kinetics and similar thermodynamic stability relative to the wild-type Phe 23 peptide. Conversely, the F5-Phe, 1-Nap, and 2-Nap 23 variants self-assembled at enhanced rates, forming fibrils with greater thermodynamic stability than the wild-type peptide. These results indicate that the high amyloidogenicity of aromatic amino acids is a function of hydrophobicity, β-sheet propensity, and planar geometry and not the ability to form stabilizing or directing π-π bonds.

EHD1 mediates long-loop recycling of many receptors by forming signaling complexes using its EH domain. We report the design and optimization of cyclic peptides as ligands for the EH domain of EHD1. We demonstrate that the improved affinity from cyclization allows fluorescence-based screening applications for EH domain inhibitors. The cyclic peptide is also unusually well-structured in aqueous solution, as demonstrated using nuclear magnetic resonance-based structural models. Because few EH domain inhibitors have been described, these more potent inhibitors will improve our understanding of the roles of EHD1 in the context of cancer invasion and metastasis.

Recycling of receptors from the endosomal recycling compartment to the plasma membrane is a critical cellular process, and recycling is particularly important for maintaining invasiveness in solid tumors. In this work, we continue our efforts to inhibit EHD1, a critical adaptor protein involved in receptor recycling. We applied a diversity-oriented macrocyclization approach to produce cyclic peptides with varied conformations, but that each contain a motif that binds to the EH domain of EHD1. Screening these uncovered several new inhibitors for EHD1's EH domain, the most potent of which bound with a K of 3.1μM. Several of the most potent inhibitors were tested in a cellular assay that measures extent of vesicle recycling. Inhibiting EHD1 could potentially slow cancer invasiveness and metastasis, and these cyclic peptides represent the most potent inhibitors of EHD1 to date.

The EHD1 protein coordinates the expression of surface proteins, such as β1 integrins, that are highly expressed in our human squamous cell carcinoma (SCC) cell line. EHD1 mediates its effects via its Eps15‐homology (EH) domain that binds proteins containing an NPF sequence motif. This motif forms a β‐turn when bound to the EH domain. Since cyclization promotes turn formation, we asked to what extent constraining a linear NPF peptide promotes turn formation and high affinity. We designed a cyclized NPF peptide using a model of an EH domain‐NPF complex. Linear and cyclized peptides were synthesized, affinities were measured and NMR structures were determined. Head‐to‐tail cyclization increased binding affinity about 5‐fold (Kd, linear=4.7 μM vs. Kd, cyclic=1.0±0.2 μM). Although the linear peptide showed cis‐trans isomerization and poor chemical shift dispersion in NMR spectra, the cyclized peptide adopted a single conformation and higher chemical shift dispersion. Cyclization clearly improves the structure and binding affinities of peptide‐based inhibitors of EHD1 EH domains. We are currently testing whether cyclization improves stability and cell penetration, which would permit the use of EH domain ligands as a novel approach to reducing β1 integrin levels on SCC tumor cells and inhibit disease progression. Supported by a “Tufts Collaborates!” grant (A.A‐H. and J.D.B.) and DP2OD007303 (J.A.K.)