Exploring peripheral nerves for pathogenic clues and disease progression biomarkers in ALS mouse models

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the progressive degeneration of upper and lower motor neurons. Increasing evidence suggests that pathological processes affecting motor axons and neuromuscular junctions (NMJ) occur early in the disease course and may contribute significantly to progression. However, the molecular mechanisms underlying early axonal pathology remain incompletely understood. In this study, we investigated early peripheral pathological events in two SOD1G93A mouse models with distinct genetic backgrounds and disease progression rates: the fast-progressing 129Sv strain and the slow-progressing C57BL/6Ola strain. While SOD1-linked cases are generally considered negative for TDP-43 inclusions, emerging evidence suggests that TDP-43 plays an important role in the peripheral nervous system and at the NMJ, and its alterations may potentially influence disease progression even in SOD1-linked cases. We therefore investigated whether TDP-43 alterations are present in the peripheral nervous system of these two mouse models. Plasma neurofilament light chain (NfL) levels were measured to determine the onset of axonal degeneration, and sciatic nerves were collected at multiple pre-symptomatic and at symptomatic stages. We analysed axonal stability and assessed the accumulation of ALS-related proteins, including TDP-43, phosphorylated TDP-43 (pTDP-43), the molecular chaperone PPIA, and mutant human SOD1. Our results indicate that axonal degeneration occurs early in both models, with an earlier and greater increase in plasma NfL levels in fastSOD1G93A mice compared with slowSOD1G93A animals, occurring largely before the onset of motor symptoms. In slowSOD1G93A mice, pTDP-43, the key pathologicalform of TDP-43, and PPIA display opposite distributions across proximal and distal compartments of the sciatic nerve, consistent with a compensatory mechanism aimed at preserving local proteostasis and limiting protein aggregation. In contrast, in fastSOD1G93A mice pTDP-43 accumulates predominantly in the proximal region of the sciatic nerve; however, PPIA does not exhibit the distal enrichment observed in slowSOD1G93A mice and instead increases mainly in the proximal compartment, suggesting that this protective redistribution may be compromised. This alteration may reflect impaired anterograde axonal transport during rapid disease progression, leaving distal axonal regions more vulnerable to proteostatic stress and TDP-43 aggregation. Notably, the peripheral accumulation of TDP-43 and pTDP-43 in fastSOD1G93A mice highlights a compartment-specific aspect of ALS pathology. Overall, these findings support the concept that peripheral axonal proteostasis plays a critical role in ALS progression and that genetic background influences the efficiency of protective molecular responses. Understanding these early peripheral mechanisms may provide new insights into disease pathogenesis and inform therapeutic strategies aimed at preserving axonal integrity.