Neurofilament Proteoforms in Amyotrophic Lateral Sclerosis Are Different in Cerebrospinal Fluid and Blood

We used targeted immunopurification-mass spectrometry (IP-MS) to characterize human neurofilament light chain (NfL) proteoforms across various compartments to assess their alterations in amyotrophic lateral sclerosis (ALS). NfL is truncated in cerebrospinal fluid (CSF) and blood in patients with sporadic ALS (sALS) and these proteoforms differ between compartments. Mid-domain species were elevated in CSF whereas plasma NfL proteoforms were mostly comprised of the tail subdomain region. Our results suggest NfL isoforms are proteolyzed and differentially distributed between ALS biofluid compartments and that analyzing by these specific regions or in ratios between regions can provide improvements in biomarker utility. These insights enhance the understanding of NfL and its potential for disease monitoring and therapeutic targeting in ALS. ANN NEUROL 2026;99:857-862.