Cyclic Peptides Target CAPON and Modulate Cellular Responses under Alzheimer's Disease-Relevant Stress

CAPON (NOS1AP) is an adaptor protein involved in neuronal nitric oxide synthase (nNOS) signaling and has been implicated in Alzheimer's disease (AD), excitotoxicity, and tau-associated neurodegeneration. Here, we report the identification of cyclic peptide ligands targeting CAPON using phage display screening of a disulfide-constrained peptide library. Phage enrichment, ELISA validation, microscale thermophoresis (MST), and biolayer interferometry (BLI) identified CAP1 as the lead peptide, exhibiting low micromolar binding affinity toward CAPON. Computational studies further supported stable CAPON-CAP1 interactions through complementary hydrophobic and electrostatic contacts. Functionally, CAP1 attenuated Aβ42-induced neuronal toxicity, suppressed NMDA-driven nitric oxide production, and reduced AT8 immunoreactivity under tau-stress conditions in neuronal models. In addition, CAP1 demonstrated favorable preliminary pharmacokinetic properties, including good aqueous solubility, plasma stability, and measurable membrane permeability. Collectively, these findings establish the first cyclic peptide ligands targeting CAPON and identify CAP1 as a promising scaffold for the modulation of CAPON-dependent neurodegenerative signaling.