Determining the Spatial Relationship of Membrane-Bound Aquaporin-4 Autoantibodies by STED Nanoscopy.

Determining the spatial relationship of individual proteins in dense assemblies remains a challenge for superresolution nanoscopy. The organization of aquaporin-4 (AQP4) into large plasma membrane assemblies provides an opportunity to image membrane-bound AQP4 antibodies (AQP4-IgG) and evaluate changes in their spatial distribution due to alterations in AQP4 isoform expression and AQP4-IgG epitope specificity. Using stimulated emission depletion nanoscopy, we imaged secondary antibody labeling of monoclonal AQP4-IgGs with differing epitope specificity bound to isolated tetramers (M1-AQP4) and large orthogonal arrays of AQP4 (M23-AQP4). Imaging secondary antibodies bound to M1-AQP4 allowed us to infer the size of individual AQP4-IgG binding events. This information was used to model the assembly of larger AQP4-IgG complexes on M23-AQP4 arrays. A scoring algorithm was generated from these models to characterize the spatial arrangement of bound AQP4-IgG antibodies, yielding multiple epitope-specific patterns of bound antibodies on M23-AQP4 arrays. Our results delineate an approach to infer spatial relationships within protein arrays using stimulated emission depletion nanoscopy, offering insight into how information on single antibody fluorescence events can be used to extract information from dense protein assemblies under a biologic context.