Stable isotope labeling kinetics of neurofilament light in vitro and in vivo

ImportanceNeurofilament light (NfL) is elevated in CSF and blood across a range of traumatic, inflammatory and neurodegenerative diseases of the central nervous system, and has been increasingly included in clinical trials as an outcome measure of target engagement. Interpreting trajectories of NfL post-treatment has been challenging, prompting a greater need and focus on understanding its pathophysiology. ObjectiveWe measured NfL kinetics in the human central nervous system using stable isotope labeling kinetics (SILK). DesignObservational study. Participants underwent SILK protocol. Infusion of 16 hours with 4mg/kg/h and follow-up lumbar punctures scheduled at 7, 14, 60 and 120 days post-labeling. SettingReferral center - specialist neurology clinic. ParticipantsParticipants with diagnosed primary tauopathies (n=10) were recruited to the Human CNS Tau Kinetics in Tauopathies (TANGLES) study. A control case was examined post-mortem to assess the technical background of the SILK method. ExposureIntravenous infusion of 13C6-leucine, with rates of label incorporation representative of fractional synthesis and fractional clearance rates in vivo and in vitro. Main outcome and MeasureLevel of incorporation of 13C6-leucine tracer into newly-translated NfL divided by the pool of NfL with previously incorporated 12C6-leucine, expressed as a percentage tracer-to-tracee (TTR) ratio. ResultsNfL is rapidly translated in human brain within hours but takes 53 - 162 days to appear in cerebrospinal fluid (CSF). Labeled NfL remains detectable in post-mortem brain tissue 1.5 years post-labeling, indicating an extremely slow turnover in the human CNS. Together, these data suggest the greatest contribution of CSF NfL in neurodegeneration is from slow release of a large pool of previously translated NfL. However, release of newly translated NfL makes a significant contribution. Conclusion and relevanceRapid increases in CSF NfL seen within weeks of disease processes or interventions are likely to reflect release of pre-existing NfL from damaged neurons, but later increases in NfL (>3 months) may also reflect new NfL translation and release. Clinical trials using NfL as an outcome measure to track neurodegeneration would particularly benefit from substantially longer follow-up periods due to the slow turnover of the protein in the central nervous system.