Super-resolution imaging pinpoints ultrastructural changes at the node of Ranvier in patients with polyneuropathy

Abstract

To shed light on nanoscale pathologies in patients with polyneuropathy, we assessed human nerve biopsies by super-resolution fluorescence microscopy. We focused on both physiological protein arrangement and pathological ultrastructural changes at the node of Ranvier, a crucial region of the peripheral myelinated axon. Direct stochastic optical reconstruction microscopy (dSTORM) revealed a ∼190 nm periodic protein arrangement of cytoskeletal proteins and axoglial cell adhesion molecules. Periodic distances increased at the paranodal region of the node of Ranvier in patients with polyneuropathy, both at the axonal cytoskeleton and at the axoglial junction. In-depth image analysis of human nerve biopsies revealed a partial loss of proteins of the axoglial complex (Caspr-1, neurofascin-155) in combination with detachment from the cytoskeletal anchor protein ß2-spectrin. Super-resolution dual-color colocalization data was supported by high-content confocal imaging combined with deep learning-based analysis, indicating that paranodal elongation occurs especially in acute and severe axonal neuropathy, as a possible correlate of Wallerian degeneration and related cytoskeletal damage. Our findings show that super-resolution imaging can identify, quantify and map elongated periodic protein distances in peripheral nerve biopsies for pathophysiological studies and direct implications for diagnostic assessment.Graphical abstractPathological alterations of ultrastructural protein arrangement at the paranodal region of the node of Ranvier in polyneuropathy.Super-resolution microscopy allows for assessing ultrastructural protein arrangement and pathological alterations in patients with polyneuropathy. In human healthy nodes, axoglial and axoskeletal proteins follow a 190 nm periodic arrangement (left). In pathologically altered nodes (right), periodic protein distances of axonal ß2-spectrin elongate, in combination with elongation and partial loss of the axoglial complex of Caspr-1 and neurofascin-155. The axoglial complex itself colocalizes closely even in pathologically altered nodes. A complete loss of the axoglial complex could be the ultrastructural correlate of the detachment of paranodal myelin loops. Scale bar 2 μm.