Neurofilament accumulations in Amyotrophic Lateral Sclerosis patients’ motor neurons impair structural axonal initial segment integrity

Neurofilament (NF) accumulation is a pathological hallmark observed in motor neurons (MNs) of patients with Amyotrophic Lateral Sclerosis (ALS) and levels of NF in fluids is becoming the prime biomarker of ALS progression. However, the underlying mechanisms linking NF accumulations and MN degeneration are not elucidated. Here, we show that integrity of the axonal initial segment (AIS), the region of action potential initiation and of polarized trafficking, is impaired by NF accumulations in human MNs carrying mutations in the two main causal ALS genes: C9ORF72 and SOD1. We show that in mutant MNs derived from induced pluripotent stem cells, both light (NF-L) and phosphorylated heavy chains (pNF-H) accumulated progressively with MN aging and that pNF-H accumulation in the AIS region led to significant structural and molecular alterations. In parallel, ALS MNs acquired altered excitability with aging. Focusing on axonal organization appears as an effective readout for MN cell death and preserving AIS integrity could have important therapeutic implications.