The measurement of biological rather than chronological age is a goal for many research groups. Numerous approaches are under development, and the levels of a wide variety of compounds in the blood have been found to vary with advancing age. The example here, neurofilament light chain, is just one of many. A robust biomarker of biological age, measuring the burden of many different forms of cell and tissue damage, as well as their downstream consequences, will likely be a combination of numerous different measures.
Neurofilament light chain (NfL) is a structural protein found in nerve cells. The nervous system has been implicated in aging and longevity, so the fact that NfL can be detected in human bodily fluids makes it potentially useful as a biomarker for aging. NfL levels are known to increase with age and in response to neurodegenerative diseases, strengthening the case for its use as a biomarker.
To test the idea, an international team of scientists measured NfL levels in blood plasma from a cohort of people aged 21 to 107. They found a non-linear increase and greater variability with age. Plasma proteome data had already been generated from the same cohort, and NfL levels correlated with 53 of the proteins (out of roughly 1300). The proteins correlated with NfL levels are involved in apoptosis as well as synapse formation and plasticity, supporting the notion that plasma NfL levels reflect the activity of pathways associated with neuronal function.
The researchers then evaluated NfL as a predictor of mortality. They collected blood from separate cohorts of centenarians and nonagenarians, measured NfL levels, and tracked the cohorts over the next few years (or until death). They used activities of daily living (ADL) and Mini-Mental State Examination (MMSE) measures to assess the health of the participants. Overall, individuals with lower NfL levels lived longer than those with higher levels and did better on MMSE and ADL measures, though the difference was smaller for ADL. Finally, the team also showed that NfL levels increase with age in mice and that dietary restriction, which is known to extend the lifespan of mice, brings down NfL levels.