Interferon-induced GTP-binding protein MX1 drives hyperexcitability in peripheral nerves: a novel mechanism in small fiber neuropathy.

Small fiber neuropathy (SFN) affects small-diameter sensory and autonomic nerve fibers, leading to chronic pain and autonomic dysfunction. While SFN can be associated with diabetes and autoimmune diseases, a significant proportion of cases are idiopathic. Although immune-mediated mechanisms are being recognized increasingly in SFN, their precise role remains unclear. This study investigates the presence of autoantibodies against interferon-induced GTP-binding protein MX (MX1) in SFN patients and explores their potential pathogenic role. A total of 59 patients with skin biopsy-confirmed SFN and 20 healthy controls were recruited. Serum samples were analyzed for the presence of anti-MX1 autoantibodies using enzyme-linked immunosorbent assay (ELISA). Immunohistochemistry was performed on rat sciatic nerves to assess the localization of patient IgG to unmyelinated nerve fibers, and immunocytochemistry and flow cytometry confirmed specific binding to MX1. Functional characterization of MX1 was conducted using whole-cell patch-clamp recordings in dorsal root ganglion (DRG) neurons overexpressing MX1. Additionally, protein interactions between MX1 and transient receptor potential cation channel subfamily C member 6 (TRPC6) were assessed using co-immunoprecipitation and surface biotinylation assays. Anti-MX1 autoantibody levels were significantly elevated in SFN patients compared to controls (p = 0.0278), particularly in the autoimmune SFN subgroup. Patient sera exhibited IgG binding to unmyelinated nerve fibers, with idiopathic and autoimmune SFN cases showing similar staining patterns, suggesting a similar immune-mediated mechanism. Immunocytochemistry showed binding to HEK293-MX1 cells and flow cytometry revealed higher MX1/WT fluorescence intensity ratios in patient sera, further confirming specific immune recognition of MX1. Patch-clamp recordings demonstrated that MX1 overexpression in DRG neurons led to significant membrane depolarization and increased action potential firing frequency (p < 0.0001), indicating heightened neuronal excitability. However, MX1 did not directly interact with TRPC6 or alter its function, suggesting an alternative pathway for its effects. The addition of anti-MX1 IgG did not further modify DRG electrophysiology, implying that the autoimmune component may contribute to SFN pathogenesis through indirect mechanisms. Our findings support the hypothesis that MX1 influences neuronal excitability and plays a role in SFN pathophysiology. Future studies should validate these findings in larger cohorts and explore potential therapeutic strategies targeting MX1-associated pathways in SFN.