Probing How Anti‐huntingtin Antibodies Bind the Fibrillar Fuzzy Coat Using Solid‐State NMR

Antibodies are critical for the immune response and serve as important tools due to their ability to recognize specific amino acid sequences, or epitopes. Based on the latter, they are utilized as diagnostic tools in biological and biomedical research. Huntington's disease (HD) is a neurodegenerative condition caused by CAG repeat expansions in the huntingtin (HTT) gene and characterized by amyloid-like protein deposits in patients. Multiple anti-HTT antibodies are used in HD research for their ability to recognize specific HTT inclusions in both post-mortem tissue and in laboratory conditions. Some of the antibodies are seen as detectors of distinct structural motifs. However, most knowledge of their binding mechanism stems from studies of soluble monomers or short fragments of the epitopes, rather than the aggregated, misfolded target protein. Here, we investigate how MW8 antibodies interact with HTT exon 1 (HTTex1) fibrils, using solid-state NMR, electron microscopy, and complementary techniques. Magic angle spinning (MAS) NMR revealed localized impacts of the antibody on exposed parts of the HTTex1 fibrils: the flanking segments that form its "fuzzy coat". Antibody binding affected the structure and dynamics of the fuzzy coat, but also modulated the propensity for forming supramolecular fibril clusters, which has important implications for (reducing) cytotoxicity.