Super-resolution imaging pinpoints the periodic ultrastructure at the human node of Ranvier and its disruption in patients with polyneuropathy

The node of Ranvier is the key element in saltatory conduction along myelinated axons, but its specific protein organization remains elusive in the human species. To shed light on nanoscale anatomy of the human node of Ranvier in health and disease, we assessed human nerve biopsies of patients with polyneuropathy by super-resolution fluorescence microscopy. We applied direct stochastic optical reconstruction microscopy (dSTORM) and supported our data by high-content confocal imaging combined with deep learning-based analysis. As a result, we revealed a similar to 190 nm periodic protein arrangement of cytoskeletal proteins and axoglial cell adhesion molecules in human peripheral nerves. In patients with polyneuropathy, periodic distances increased at the paranodal region of the node of Ranvier, both at the axonal cytoskeleton and at the axoglial junction. In-depth image analysis revealed a partial loss of proteins of the axoglial complex (Caspr-1, neurofascin-155) in combi-nation with detachment from the cytoskeletal anchor protein ss2-spectrin. High content analysis showed that such paranodal disorganization occurred especially in acute and severe axonal neuropathy with ongoing Wallerian degeneration and related cytoskeletal damage. We provide nanoscale and protein-specific evidence for the prominent, but vulnerable role of the node of Ranvier for axonal integrity. Furthermore, we show that super -resolution imaging can identify, quantify and map elongated periodic protein distances and protein interac-tion in histopathological tissue samples. We thus introduce a promising tool for further translational applications of super resolution microscopy.

Automated summary

This study used super-resolution fluorescence microscopy and deep learning analysis to study human nerve biopsies. The results revealed periodic protein arrangement in the node of Ranvier in healthy and diseased human peripheral nerves, with a weakened periodicity in disease conditions associated with cytoskeletal damage. The study demonstrates the potential of super-resolution imaging for clinical applications.