OUP accepted manuscript

High precision and cost-effective multiplex quantification of amyloid-β40, amyloid-β42, p181Tau, p217Tau, neurofilament light chain, and glial fibrillary acidic protein from plasma and serum

BackgroundCurrent methods to quantify blood biomarkers for Alzheimer's disease (AD) are expensive and are not widely available.ObjectiveTo develop a low-cost, sensitive, and accurate multiplex assay to quantify Aβ40, Aβ42, p181Tau, p217Tau, NfL, and GFAP biomarkers in plasma and serum based on a widely available technology.MethodsWe used commercial antibodies to Aβ40, Aβ42, p181Tau, p217Tau, NfL, and GFAP, and xMAP Luminex technology, and developed the multiplex 5ADCSI to quantify these biomarkers from plasma and serum. The utility of 5ADCSI was tested in matched cerebrospinal fluid (CSF) and plasma or serum of a cohort of cognitively normal (CN: n = 35), with mild cognitive impairment (MCI: n = 17), and with AD (n = 11) individuals.ResultsThe 5ADCSI demonstrated high specificity and sensitivity, with excellent precision. In clinical samples, moderate to strong correlation is observed between CSF and plasma or serum for Aβ42/40 (r = 0.78), p181Tau/Aβ42 (r = 0.57), p217Tau/Aβ42 (r = 0.72), p181Tau (r = 0.59), p217Tau (r = 0.75), and GFAP (r = 0.59). The AUC of receiver-operator characteristic curve for differentiating CN from AD for plasma/serum and CSF are 0.75, and 0.80 for Aβ42/40, 0.95, 0.91 for p217Tau, 0.76, 0.81 for p181Tau, and 0.73 and 0.78for GFAP, respectively.ConclusionsThe 5ADCSI assay is highly specific, sensitive, and accurate. The wide availability of the base technology of 5ADCSI is an advantage over other similar methods and would allow cost-effective large-scale studies for validation of blood biomarkers for early diagnosis of AD.

Novel insights into the molecular nature of neurofilament light polypeptide species in cerebrospinal fluid

The quantification of neurofilament light polypeptide (NFL) in biofluids is being clinically used to detect and grade general neuronal damage in neurodegenerative diseases and quantify neuronal injury during acute events like traumatic brain injury. Specific assays that target only particular molecular breakdown products of neurofilaments have the potential to distinguish between various pathologies. Nevertheless, the molecular structure of neurofilament light polypeptide in cerebrospinal fluid remains to be elucidated. We characterized neurofilament light polypeptide in cerebrospinal fluid by size-exclusion chromatography, Western blotting and mass spectrometry. Neurofilament light polypeptide in cerebrospinal fluid was found to be composed of aggregates of fragments of the full-length molecule. These aggregates were sensitive to reduction by dithiothreitol and dissociated to monomeric fragments of 6-12 kDa (Western blot), covering most of the coiled-coil domains of neurofilament light polypeptide. Since only cysteine residues can form disulfide bonds, this points to a role of the single cysteine 322 for maintaining the stability of the aggregates. The sequence region covered by the identified fragments ended just a few amino acids C-terminally of the coiled-coil region at a site which had been previously mapped to a calpain cleavage site in the glial fibrillary acidic protein, which is highly homologous to neurofilament light polypeptide in the coiled-coil region. This cleavage site was also confirmed to be present in bovine neurofilament light polypeptide by in vitro digestion of purified neurofilament light polypeptide with calpain-1. The difference of the molecular weights of the reduced and non-reduced forms of neurofilament light polypeptide suggests that neurofilament light polypeptide in CSF consists of disulfide-linked aggregated fragments, most likely tetramers, or alternately dimers in a complex with another binding partner.

Secreted neurofilament light chain after neuronal damage induces myeloid cell activation and neuroinflammation

Neurofilament light chain (NfL) is a neuron-specific cytoskeletal protein that provides structural support for axons and is released into the extracellular space following neuronal injury. While NfL has been extensively studied as a disease biomarker, the underlying release mechanisms and role in neurodegeneration remain poorly understood. Here, we find that neurons secrete low baseline levels of NfL, while neuronal damage triggers calpain-driven proteolysis and release of fragmented NfL. Secreted NfL activates microglial cells, which can be blocked with anti-NfL antibodies. We utilize in vivo single-cell RNA sequencing to profile brain cells after injection of recombinant NfL into the mouse hippocampus and find robust macrophage and microglial responses. Consistently, NfL knockout mice ameliorate microgliosis and delay symptom onset in the SOD1 mouse model of amyotrophic lateral sclerosis (ALS). Our results show that released NfL can activate myeloid cells in the brain and is, thus, a potential therapeutic target for neurodegenerative diseases.

Advances in neurofilament light chain analysis

This chapter provides a comprehensive summary of clinical laboratory testing for neurofilament light chain (NfL) in neurologic disease. A primer on the NfL structure and function is presented with its potential use as a biomarker. The most widely utilized methods for NfL in biologic samples are highlighted and examined. Limitations of current knowledge are considered, as are outstanding questions related to dissemination and standardization of testing. Herein we focus on methods available today and those in development for clinical use. In the final section, a broad vision is presented of how NfL may be utilized in the future to improve diagnosis and treatment of neurologic diseases as well as for maintaining health.

Proteolysis-Based Biomarker Repertoire of the Neurofilament Proteome

Neurodegeneration presents a significant challenge in ageing populations, often being detected too late for effective intervention. Biomarkers have shown great potential in addressing this issue, with neurofilament (Nf) proteins emerging as validated biomarkers presently transitioning from research to routine laboratory use. Whilst advances in large-scale quantitative analyses have enabled the targeted study of proteolytic Nf fragments in blood, the complete landscape of the Nf proteolytic breakdown remains unknown. This study presents a comprehensive atlas of the human Nf isoform (Z) degradome, based on the number of known cleavage sites (x). The full scale of the Nf degradome is described by the formula: Z = ((x + 1) × (x + 2)/2) - 1. The resulting neurofilament degradome atlas (NDA) was validated through a triple-layer approach using in vitro data (open access at: https://doi.org/10.5522/04/25689378.v1). The NDA offers valuable applications in biomarker detection, targeted antibody development, exploration of autoimmunity and understanding Nf aggregate formation. Analysis of the Nf degradome reveals novel insights into neurodegenerative diseases by investigating peptide pools affected by genetic mutations in the Nf genome and alterations in proteolytic pathways. The annotated NDA is publicly available as a database resource, supporting advancements in affinity-based biomarker tests through informed peptide selection and minimising biases in label-free approaches. In conclusion, this study highlights the biological significance of a dynamic pool of coexisting proteolytic Nf peptides, providing a framework that can be applied to other proteins.

Effects of early antihypertensive treatment on cognitive function in patients with acute ischemic stroke with different neurofilament light chain levels

BACKGROUND

It is unclear whether the extent of neuroaxonal damage, as measured by circulating levels of neurofilament light chain (NfL), would modify the effects of early antihypertensive therapy on cognitive performance following stroke. This study aimed to investigate the effects of early blood pressure reduction on the risk of post-stroke cognitive impairment (PSCI) among patients with different plasma NfL levels.

METHODS

A total of 622 eligible patients from a pre-planned ancillary study of CATIS (China Antihypertensive Trial in Acute Ischemic Stroke) were included in this study. The electrochemiluminescence immunoassay technique was used to evaluate Plasma NfL levels at baseline, and the Mini-Mental State Examination (MMSE) in Chinese was used to assess cognition at the 3-month follow-up. An MMSE score of less than 27 was considered as PSCI.

RESULTS

The effect of antihypertensive therapy on PSCI differed according to NfL levels at the 3-month follow-up. In the low NfL group, compared with the control group, antihypertensive treatment reduced the risk of PSCI [adjusted odds ratio (OR), 95 % confidence interval (CI): 0.50 (0.31-0.81)]. However, in the high NfL group, antihypertensive treatment increased the risk of PSCI compared with the control group [adjusted OR, 95 % CI: 1.93 (1.16-3.20)].

CONCLUSIONS

Antihypertensive therapy in the acute phase reduced the risk of PSCI in patients with low plasma NfL levels, but increased the risk in patients with high NfL levels.

Comparisons of neurodegenerative disease biomarkers across different biological fluids from patients with Huntington's disease

Fluid biomarkers play important roles in many aspects of neurodegenerative diseases, such as Huntington's disease (HD). However, a main question relates to how well levels of biomarkers measured in CSF are correlated with those measured in peripheral fluids, such as blood or saliva. In this study, we quantified levels of four neurodegenerative disease-related proteins, neurofilament light (NfL), total tau (t-tau), glial fibrillary acidic protein (GFAP) and YKL-40 in matched CSF, plasma and saliva samples from Huntingtin (HTT) gene-positive individuals (n = 21) using electrochemiluminescence assays. In addition, salivary levels of NfL, t-tau, and GFAP were quantified from a larger cohort (n = 95). We found both positive and negative correlations in the levels of these biomarkers among different biofluids. Most notably, in contrast to the significant positive correlations observed between CSF and plasma levels for NfL and GFAP, we detected significant negative correlations between the CSF and saliva levels of NfL and GFAP. With regard to clinical measures, both plasma and CSF levels of NfL were significantly positively correlated with Total Motor Score and chorea, whereas saliva levels of NfL showed significant correlations in the opposite direction. Additional correlations between salivary biomarkers with clinical data, adjusting for age, sex and CAG repeat length, confirmed that salivary NfL was significantly negatively associated with chorea scores in manifest HD, but not premanifest (PM), individuals. In contrast, salivary t-tau was positively associated with measures of cognition in PM participants. These findings suggest that salivary levels of NfL and t-tau proteins may exemplify non-invasive biomarkers for disease symptoms at different stages of illness. Further, these findings highlight the notion that different forms of disease proteins exist in different biological fluids.

A French multicenter analytical evaluation of the automated Lumipulse G sNfL blood assay (Fujirebio®) and its comparison to four other immunoassays for serum neurofilament light chain assessment in clinical settings

OBJECTIVES

Measurement of serum neurofilament light chain (sNfL) protein is becoming a key biomarker for many neurological diseases. Several immunoassays have been developed to meet these clinical needs, revealing significant differences in terms of variability and results. Here, we propose a French multicenter comparison of 5 sNfL assays.

METHODS

6 replicates of 3 pools with low (10 pg/mL), medium (30 pg/mL) and high (100 pg/mL) sNfL values and one replicate of 12 samples with growing sNfL values were analyzed by six independent French clinical laboratories. The analytical performances of the sNfL blood assay (Fujirebio®) on Lumipulse G were first evaluated then compared to four other immunoassays: NF-light V2 (Quanterix®) on SiMOA HD-X, Human NF-L (Biotechne®) on Ella, R-Plex Human Neurofilament L (MSD®) on Sector 2400; manual ELISA test using Uman Diagnostic/Quanterix®.

RESULTS

Inter-center comparison of the Lumipulse blood assay revealed limited but significant differences in the mean sNfL values across low, medium, and high pools between each city (p < 0.001) and between the two different batches used. Coefficients of variation of pools ranged from 2.0 to 16.9 %. Z-score of sNfL results of the 12 samples ranged from -1.70 to +1.71. Inter-technique comparison showed a systematic difference of sNfL values, with a overestimation of MSD and Ella over other tests. Nonetheless, results were all significantly correlated (p < 0.001).

CONCLUSION

The automated Lumipulse assay produced comparable sNfL values across laboratories; but further adjustments are needed to harmonize sNfL results. Biologists and physicians should be aware of the variability in results between different immunoassay suppliers.

Neurofilament Light Chain under the Lens of Structural Mass Spectrometry

Neurofilament light chain (NfL) is an early nonspecific biomarker in neurodegenerative diseases and traumatic brain injury, indicating axonal damage. This work describes the detailed structural characterization of a selected primary calibrator with the potential to be used in future reference measurement procedure (RMP) development for the accurate quantification of NfL. As a part of the described workflow, the sequence, higher-order structure as well as solvent accessibility, and hydrogen-bonding profile were assessed under three different conditions in KPBS, artificial cerebrospinal fluid, and artificial cerebrospinal fluid in the presence of human serum albumin. The results revealed that NfL is a structurally heterogeneous protein, eliciting a large conformational flexibility. Its structural ensemble changed when it was diluted with an aqueous buffer versus a surrogate matrix, artificial cerebrospinal fluid (aCSF), and/or aCSF with human serum albumin. Various regions of protection and deprotection in the protein head, central helical, and tail domains that experienced altered solvent accessibility and conformational changes caused by different solvent conditions were identified. Moreover, interfacial residues, which may play a role in a potential direct interaction between NfL and human serum albumin, emerged from hydrogen-deuterium exchange mass spectrometry (HDX-MS). These data pinpointed distinct regions of the protein that may participate in such an interaction. Overall, critical quality attributes of a potential primary calibrator for NfL measurements are provided. These findings will ultimately inform ongoing biochemical and clinical assay development procedures and manufacturing practices, giving careful consideration during sample handling and method development.

Serum proteomics reveal APOE-ε4-dependent and APOE-ε4-independent protein signatures in Alzheimer's disease

A deeper understanding of the molecular processes underlying late-onset Alzheimer's disease (LOAD) could aid in biomarker and drug target discovery. Using high-throughput serum proteomics in the prospective population-based Age, Gene/Environment Susceptibility-Reykjavik Study (AGES) cohort of 5,127 older Icelandic adults (mean age, 76.6 ± 5.6 years), we identified 303 proteins associated with incident LOAD over a median follow-up of 12.8 years. Over 40% of these proteins were associated with LOAD independently of APOE-ε4 carrier status, were implicated in neuronal processes and overlapped with LOAD protein signatures in brain and cerebrospinal fluid. We identified 17 proteins whose associations with LOAD were strongly dependent on APOE-ε4 carrier status, with mostly consistent associations in cerebrospinal fluid. Remarkably, four of these proteins (TBCA, ARL2, S100A13 and IRF6) were downregulated by APOE-ε4 yet upregulated due to LOAD, a finding replicated in external cohorts and possibly reflecting a response to disease onset. These findings highlight dysregulated pathways at the preclinical stages of LOAD, including those both independent of and dependent on APOE-ε4 status.

Unlocking early detection of Alzheimer's disease: The emerging role of nanomaterial-based optical sensors

Alzheimer's disease (AD) is a chronic and progressive neurodegenerative disorder that affects millions of individuals worldwide. Researchers have conducted numerous studies to find accurate biomarkers for early AD diagnosis and develop more effective treatments. The main pathological hallmarks of AD are amyloid beta and Tau proteins. Other biomarkers, such as DNA, RNA, and proteins, can also be helpful in early AD diagnosis. To diagnose and treat AD promptly, it is essential to accurately measure the concentration of biomarkers in the cerebrospinal fluid or blood. However, due to the low concentrations of these biomarkers in the body, highly sensitive analytical techniques are required. To date, sensors have become increasingly important due to their high sensitivity, swift detection, and adaptable manipulation features. These qualities make them an excellent substitute for conventional instruments. Nanomaterials are commonly employed in sensors to amplify signals and improve sensitivity. This review paper summarized the integration of nanomaterials in optical sensor systems, including colorimetric, fluorescent, and surface-enhanced Raman scattering sensors for AD biomarkers detection.

An interpretable machine learning-based cerebrospinal fluid proteomics clock for predicting age reveals novel insights into brain aging

Machine learning can be used to create "biologic clocks" that predict age. However, organs, tissues, and biofluids may age at different rates from the organism as a whole. We sought to understand how cerebrospinal fluid (CSF) changes with age to inform the development of brain aging-related disease mechanisms and identify potential anti-aging therapeutic targets. Several epigenetic clocks exist based on plasma and neuronal tissues; however, plasma may not reflect brain aging specifically and tissue-based clocks require samples that are difficult to obtain from living participants. To address these problems, we developed a machine learning clock that uses CSF proteomics to predict the chronological age of individuals with a 0.79 Pearson correlation and mean estimated error (MAE) of 4.30 years in our validation cohort. Additionally, we analyzed proteins highly weighted by the algorithm to gain insights into changes in CSF and uncover novel insights into brain aging. We also demonstrate a novel method to create a minimal protein clock that uses just 109 protein features from the original clock to achieve a similar accuracy (0.75 correlation, MAE 5.41). Finally, we demonstrate that our clock identifies novel proteins that are highly predictive of age in interactions with other proteins, but do not directly correlate with chronological age themselves. In conclusion, we propose that our CSF protein aging clock can identify novel proteins that influence the rate of aging of the central nervous system (CNS), in a manner that would not be identifiable by examining their individual relationships with age.

Multi-peptide characterization of plasma neurofilament light chain in preclinical and mild Alzheimer's disease

Although neurofilament light chain is a well-known marker of neuronal damage, its characterization at the proteoform level is underdeveloped. Here, we describe a new method to profile and quantify neurofilament light chain in plasma at the peptide level, using three in-house monoclonal antibodies targeting distinct protein domains and nano-liquid chromatography coupled to high-resolution tandem mass spectrometry. This study profiled and compared plasma neurofilament light chain to CSF in 102 older individuals (73.9 ± 6.3 years old), 37 of which had a clinical dementia rating greater than 0. We observed elevated neurofilament light chain in preclinical Alzheimer's disease plasma for two measures (NfL101 and NfL324) and CSF for seven measures (NfL92, NfL101, NfL117, NfL137, NfL148, NfL165 and NfL530). We found five plasma peptides (NfL92, NfL101, NfL117, NfL324 and NfL530) significantly associated with age and two (NfL148 and NfL324) with body mass index.

Plasma Neurofilament Light Chain: A Potential Biomarker for Neurological Dysfunction in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a complex disorder characterized by heterogeneous symptoms, which lack specific biomarkers for its diagnosis. This study aimed to investigate plasma neurofilament light chain (NfL) levels as a potential biomarker for ME/CFS and explore associations with cognitive, autonomic, and neuropathic symptoms. Here, 67 ME/CFS patients and 43 healthy controls (HCs) underwent comprehensive assessments, including neuropsychological evaluation, autonomic nervous system (ANS) testing, and plasma NfL level analysis. ME/CFS patients exhibited significantly higher plasma NfL levels compared to HC (F = 4.30, p < 0.05). Correlations were observed between NfL levels and cognitive impairment, particularly in visuospatial perception (r = -0.42; p ≤ 0.001), verbal memory (r = -0.35, p ≤ 0.005), and visual memory (r = -0.26; p < 0.05) in ME/CFS. Additionally, higher NfL levels were associated with worsened autonomic dysfunction in these patients, specifically in parasympathetic function (F = 9.48, p ≤ 0.003). In ME/CFS patients, NfL levels explained up to 17.2% of the results in cognitive tests. Unlike ME/CFS, in HC, NfL levels did not predict cognitive performance. Elevated plasma NfL levels in ME/CFS patients reflect neuroaxonal damage, contributing to cognitive dysfunction and autonomic impairment. These findings support the potential role of NfL as a biomarker for neurological dysfunction in ME/CFS. Further research is warranted to elucidate underlying mechanisms and clinical implications.

Neurofilaments in neurologic disease

Neurofilaments (NFs), major cytoskeletal constituents of neurons, have emerged as universal biomarkers of neuronal injury. Neuroaxonal damage underlies permanent disability in various neurological conditions. It is crucial to accurately quantify and longitudinally monitor this damage to evaluate disease progression, evaluate treatment effectiveness, contribute to novel treatment development, and offer prognostic insights. Neurofilaments show promise for this purpose, as their levels increase with neuroaxonal damage in both cerebrospinal fluid and blood, independent of specific causal pathways. New assays with high sensitivity allow reliable measurement of neurofilaments in body fluids and open avenues to investigate their role in neurological disorders. This book chapter will delve into the evolving landscape of neurofilaments, starting with their structure and cellular functions within neurons. It will then provide a comprehensive overview of their broad clinical value as biomarkers in diseases affecting the central or peripheral nervous system.

NfL concentration in CSF is a quantitative marker of the rate of neurodegeneration in aging and Huntington's disease: a semi-mechanistic model-based analysis

The concentrations of neurofilament light chain (NfL) in cerebrospinal fluid (CSF) and plasma have become key biomarkers of many neurodegenerative diseases, including Huntington's Disease (HD). However, the relationship between the dynamics of NfL concentrations in CSF and the time-course of neurodegeneration (whole brain atrophy) has not yet been described in a quantitative and mechanistic manner. Here, we present a novel semi-mechanistic model, which postulates that the amount of NfL entering the CSF corresponds to the amount of NfL released from damaged neurons, whose degeneration results in a decrease in brain volume. In mathematical terms, the model expresses the NfL concentration in CSF in terms of the NfL concentration in brain tissue, the rate of change of whole brain volume and the CSF flow rate. To test our model, we used a non-linear mixed effects approach to analyze NfL and brain volume data from the HD-CSF study, a 24-month prospective study of individuals with premanifest HD, manifest HD and healthy controls. The time-course of whole brain volume, obtained from MRI, was represented empirically by a 2nd order polynomial, from which its rate of change was computed. CSF flow rates in healthy and HD populations were taken from recent literature data. By estimating the NfL concentration in brain tissue, the model successfully described the time-course of the NfL concentration in CSF in both HD subjects and healthy controls. Furthermore, the model-derived estimate of NfL concentration in brain agreed well with recent direct experimental measurements. The consistency of our model with the NfL and brain volume data suggests that the NfL concentration in CSF reflects the rate, rather than the extent, of neurodegeneration and that the increase in NfL concentration over time is a measure of the accelerating rate of neurodegeneration associated with aging and HD. For HD subjects, the degree of acceleration was found to increase markedly with the number of CAG repeats on their HTT gene. The application of our semi-mechanistic NfL model to other neurodegenerative diseases is discussed.

Neurofilament light protein as a biomarker for spinal muscular atrophy: a review and reference ranges

Spinal muscular atrophy (SMA) is the leading genetic cause of infant mortality, characterized by progressive neuromuscular degeneration resulting from mutations in the survival motor neuron (SMN1) gene. The availability of disease-modifying therapies for SMA therapies highlights the pressing need for easily accessible and cost-effective blood biomarkers to monitor treatment response and for better disease management. Additionally, the wide implementation of newborn genetic screening programs in Western countries enables presymptomatic diagnosis of SMA and immediate treatment administration. However, the absence of monitoring and prognostic blood biomarkers for neurodegeneration in SMA hinders effective disease management. Neurofilament light protein (NfL) is a promising biomarker of neuroaxonal damage in SMA and reflects disease progression in children with SMA undergoing treatment. Recently, the European Medicines Agency issued a letter of support endorsing the potential utilization of NfL as a biomarker of pediatric neurological diseases, including SMA. Within this review, we comprehensively assess the potential applications of NfL as a monitoring biomarker for disease severity and treatment response in pediatric-onset SMA. We provide reference ranges for normal levels of serum based NfL in neurologically healthy children aged 0-18 years. These reference ranges enable accurate interpretation of NfL levels in children and can accelerate the implementation of NfL into clinical practice.

Longitudinal Neuropathological Consequences of Extracranial Radiation Therapy in Mice

Cancer-related cognitive impairment (CRCI) is a consequence of chemotherapy and extracranial radiation therapy (ECRT). Our prior work demonstrated gliosis in the brain following ECRT in SKH1 mice. The signals that induce gliosis were unclear. Right hindlimb skin from SKH1 mice was treated with 20 Gy or 30 Gy to induce subclinical or clinical dermatitis, respectively. Mice were euthanized at 6 h, 24 h, 5 days, 12 days, and 25 days post irradiation, and the brain, thoracic spinal cord, and skin were collected. The brains were harvested for spatial proteomics, immunohistochemistry, Nanostring nCounter® glial profiling, and neuroinflammation gene panels. The thoracic spinal cords were evaluated by immunohistochemistry. Radiation injury to the skin was evaluated by histology. The genes associated with neurotransmission, glial cell activation, innate immune signaling, cell signal transduction, and cancer were differentially expressed in the brains from mice treated with ECRT compared to the controls. Dose-dependent increases in neuroinflammatory-associated and neurodegenerative-disease-associated proteins were measured in the brains from ECRT-treated mice. Histologic changes in the ECRT-treated mice included acute dermatitis within the irradiated skin of the hindlimb and astrocyte activation within the thoracic spinal cord. Collectively, these findings highlight indirect neuronal transmission and glial cell activation in the pathogenesis of ECRT-related CRCI, providing possible signaling pathways for mitigation strategies.

Neurofilaments as biomarkers in neurological disorders - towards clinical application

Neurofilament proteins have been validated as specific body fluid biomarkers of neuro-axonal injury. The advent of highly sensitive analytical platforms that enable reliable quantification of neurofilaments in blood samples and simplify longitudinal follow-up has paved the way for the development of neurofilaments as a biomarker in clinical practice. Potential applications include assessment of disease activity, monitoring of treatment responses, and determining prognosis in many acute and chronic neurological disorders as well as their use as an outcome measure in trials of novel therapies. Progress has now moved the measurement of neurofilaments to the doorstep of routine clinical practice for the evaluation of individuals. In this Review, we first outline current knowledge on the structure and function of neurofilaments. We then discuss analytical and statistical approaches and challenges in determining neurofilament levels in different clinical contexts and assess the implications of neurofilament light chain (NfL) levels in normal ageing and the confounding factors that need to be considered when interpreting NfL measures. In addition, we summarize the current value and potential clinical applications of neurofilaments as a biomarker of neuro-axonal damage in a range of neurological disorders, including multiple sclerosis, Alzheimer disease, frontotemporal dementia, amyotrophic lateral sclerosis, stroke and cerebrovascular disease, traumatic brain injury, and Parkinson disease. We also consider the steps needed to complete the translation of neurofilaments from the laboratory to the management of neurological diseases in clinical practice.

Neurofilament Biophysics: From Structure to Biomechanics

Neurofilaments (NFs) are multisubunit, neuron-specific intermediate filaments consisting of a 10-nm diameter filament "core" surrounded by a layer of long intrinsically disordered protein (IDP) "tails." NFs are thought to regulate axonal caliber during development and then stabilize the mature axon, with NF subunit misregulation, mutation, and aggregation featuring prominently in multiple neurological diseases. The field's understanding of NF structure, mechanics, and function has been deeply informed by a rich variety of biochemical, cell biological, and mouse genetic studies spanning more than four decades. These studies have contributed much to our collective understanding of NF function in axonal physiology and disease. In recent years, however, there has been a resurgence of interest in NF subunit proteins in two new contexts: as potential blood- and cerebrospinal fluid-based biomarkers of neuronal damage, and as model IDPs with intriguing properties. Here, we review established principles and more recent discoveries in NF structure and function. Where possible, we place these findings in the context of biophysics of NF assembly, interaction, and contributions to axonal mechanics.

CSF neurofilament light chain profiling and quantitation in neurological diseases

Neurofilament light chain is an established marker of neuroaxonal injury that is elevated in CSF and blood across various neurological diseases. It is increasingly used in clinical practice to aid diagnosis and monitor progression and as an outcome measure to assess safety and efficacy of disease-modifying therapies across the clinical translational neuroscience field. Quantitative methods for neurofilament light chain in human biofluids have relied on immunoassays, which have limited capacity to describe the structure of the protein in CSF and how this might vary in different neurodegenerative diseases. In this study, we characterized and quantified neurofilament light chain species in CSF across neurodegenerative and neuroinflammatory diseases and healthy controls using targeted mass spectrometry. We show that the quantitative immunoprecipitation-tandem mass spectrometry method developed in this study strongly correlates to single-molecule array measurements in CSF across the broad spectrum of neurodegenerative diseases and was replicable across mass spectrometry methods and centres. In summary, we have created an accurate and cost-effective assay for measuring a key biomarker in translational neuroscience research and clinical practice, which can be easily multiplexed and translated into clinical laboratories for the screening and monitoring of neurodegenerative disease or acute brain injury.

Alzheimer's Disease Biomarker Analysis Using Targeted Mass Spectrometry

Alzheimer's disease (AD) is characterized by several neuropathological changes, mainly extracellular amyloid aggregates (plaques), intraneuronal inclusions of phosphorylated tau (tangles), as well as neuronal and synaptic degeneration, accompanied by tissue reactions to these processes (astrocytosis and microglial activation) that precede neuronal network disturbances in the symptomatic phase of the disease. A number of biomarkers for these brain tissue changes have been developed, mainly using immunoassays. In this review, we discuss how targeted mass spectrometry (TMS) can be used to validate and further characterize classes of biomarkers reflecting different AD pathologies, such as tau- and amyloid-beta pathologies, synaptic dysfunction, lysosomal dysregulation, and axonal damage, and the prospect of using TMS to measure these proteins in clinical research and diagnosis. TMS advantages and disadvantages in relation to immunoassays are discussed, and complementary aspects of the technologies are discussed.

Stability dynamics of neurofilament and GFAP networks and protein fragments

Neurofilaments (NFs) and GFAP are cytoskeletal intermediate filaments (IFs) that support cellular processes unfolding within the uniquely complex environments of neurons and astrocytes, respectively. This review highlights emerging concepts on the transitions between stable and destabilized IF networks in the nervous system. While self-association between transiently structured low-complexity IF domains promotes filament assembly, the opposing destabilizing actions of phosphorylation-mediated filament severing facilitate faster intracellular transport. Cellular proteases, including caspases and calpains, produce a variety of IF fragments, which may interact with N-degron and C-degron pathways of the protein degradation machinery. The rapid adoption of NF and GFAP-based clinical biomarker tests is contrasted with the lagging understanding of the dynamics between the native IF proteins and their fragments.

Urinary biomarkers for amyotrophic lateral sclerosis: candidates, opportunities and considerations

Amyotrophic lateral sclerosis is a relentless neurodegenerative disease that is mostly fatal within 3-5 years and is diagnosed on evidence of progressive upper and lower motor neuron degeneration. Around 15% of those with amyotrophic lateral sclerosis also have frontotemporal degeneration, and gene mutations account for ∼10%. Amyotrophic lateral sclerosis is a variable heterogeneous disease, and it is becoming increasingly clear that numerous different disease processes culminate in the final degeneration of motor neurons. There is a profound need to clearly articulate and measure pathological process that occurs. Such information is needed to tailor treatments to individuals with amyotrophic lateral sclerosis according to an individual's pathological fingerprint. For new candidate therapies, there is also a need for methods to select patients according to expected treatment outcomes and measure the success, or not, of treatments. Biomarkers are essential tools to fulfil these needs, and urine is a rich source for candidate biofluid biomarkers. This review will describe promising candidate urinary biomarkers of amyotrophic lateral sclerosis and other possible urinary candidates in future areas of investigation as well as the limitations of urinary biomarkers.

Concise review on the combined use of immunocapture, mass spectrometry and liquid chromatography for clinical applications

Immunocapture is now a well-established method for sample preparation prior to quantitation of peptides and proteins in complex matrices. This short review will give an overview of some clinical applications of immunocapture methods, as well as protocols with and without enzymatic digestion in a clinical context. The advantages and limitations of both approaches are discussed in detail. Challenges related to the choice of mass spectrometer are also discussed. Top-down, middle-down, and bottom-up approaches are discussed. Even though immunocapture has its limitations, its main advantage is that it provides an additional dimension of separation and/or isolation when working with peptides and proteins. Overall, this short review demonstrates the potential of such techniques in the field of proteomics-based clinical medicine and paves the way for better personalized medicine.

Neurofilament-Light, a Promising Biomarker: Analytical, Metrological and Clinical Challenges

Neurofilament-light chain (Nf-L) is a non-specific early-stage biomarker widely studied in the context of neurodegenerative diseases (NDD) and traumatic brain injuries (TBI), which can be measured in biofluids after axonal damage. Originally measured by enzyme-linked immunosorbent assay (ELISA) in cerebrospinal fluid (CSF), Nf-L can now be quantified in blood with the emergence of ultrasensitive assays. However, to ensure successful clinical implementation, reliable clinical thresholds and reference measurement procedures (RMP) should be developed. This includes establishing and distributing certified reference materials (CRM). As a result of the complexity of Nf-L and the number of circulating forms, a clear definition of what is measured when immunoassays are used is also critical to achieving standardization to ensure the long-term success of those assays. The use of powerful tools such as mass spectrometry for developing RMP and defining the measurand is ongoing. Here, we summarize the current methods in use for quantification of Nf-L in biofluid showing potential for clinical implementation. The progress and challenges in developing RMP and defining the measurand for Nf-L standardization of diagnostic tests are addressed. Finally, we discuss the impact of pathophysiological factors on Nf-L levels and the establishment of a clinical cut-off.

Validity of serum neurofilament light chain as a prognostic biomarker of disease activity in multiple sclerosis

Multiple sclerosis (MS) is a chronic demyelinating and neuroinflammatory disease of the human central nervous system with complex pathoetiology, heterogeneous presentations and an unpredictable course of disease progression. There remains an urgent need to identify and validate a biomarker that can reliably predict the initiation and progression of MS as well as identify patient responses to disease-modifying treatments/therapies (DMTs). Studies exploring biomarkers in MS and other neurodegenerative diseases currently focus mainly on cerebrospinal fluid (CSF) analyses, which are invasive and impractical to perform on a repeated basis. Recent studies, replacing CSF with peripheral blood samples, have revealed that the elevation of serum neurofilament light chain (sNfL) in the clinical stages of MS is, potentially, an ideal prognostic biomarker for predicting disease progression and for possibly guiding treatment decisions. However, there are unresolved factors (the definition of abnormal values of sNfL concentration, the standardisation of measurement and the amount of change in sNfL concentration that is significant) that are preventing its use as a biomarker in routine clinical practice for MS. This updated review critiques these recent findings and highlights areas for focussed work to facilitate the use of sNfL as a prognostic biomarker in MS management.

Neurofilament light chain as neuronal injury marker - what is needed to facilitate implementation in clinical laboratory practice?

Neurobiomarkers have attracted significant attention over the last ten years. One promising biomarker is the neurofilament light chain protein (NfL). Since the introduction of ultrasensitive assays, NfL has been developed into a widely used axonal damage marker of relevance to the diagnosis, prognostication, follow-up, and treatment monitoring of a range of neurological disorders, including multiple sclerosis, amyotrophic lateral sclerosis, and Alzheimer's disease. The marker is increasingly used clinically, as well as in clinical trials. Even if we have validated precise, sensitive, and specific assays for NfL quantification in both cerebrospinal fluid and blood, there are analytical, as well as pre- and post-analytical aspects of the total NfL testing process, including biomarker interpretation, to consider. Although the biomarker is already in use in specialised clinical laboratory settings, a more general use requires some further work. In this review, we provide brief basic information and opinions on NfL as a biomarker of axonal injury in neurological diseases and pinpoint additional work needed to facilitate biomarker implementation in clinical practice.

Elevated neurofilament light chain CSF/serum ratio indicates impaired CSF outflow in idiopathic intracranial hypertension

BACKGROUND

Impaired cerebrospinal fluid (CSF) homeostasis is central to the pathogenesis of idiopathic intracranial hypertension (IIH), although the precise mechanisms involved are still not completely understood. The aim of the current study was to assess the CSF/serum ratio of neurofilament light chain levels (QNfL) as a potential indicator of functional CSF outflow obstruction in IIH patients.

METHODS

NfL levels were measured by single molecule array in CSF and serum samples of 87 IIH patients and in three control groups, consisting of 52 multiple sclerosis (MS) patients with an acute relapse, 21 patients with an axonal polyneuropathy (PNP), and 41 neurologically healthy controls (HC). QNfL was calculated as the ratio of CSF and serum NfL levels. Similarly, we also assessed the CSF/serum ratio of glial fibrillary acidic protein (QGFAP) levels to validate the QNfL data. Routine CSF parameters including the CSF/serum albumin ratio (QAlb) were determined in all groups. Lumbar puncture opening pressure of IIH patients was measured by manometry.

RESULTS

CSF-NfL levels (r = 0.29, p = 0.008) and QNfL (0.40, p = 0.0009), but not serum NfL (S-NfL) levels, were associated with lumbar puncture opening pressure in IIH patients. CSF-NfL levels were increased in IIH patients, MS patients, and PNP patients, whereas sNfL levels were normal in IIH, but elevated in MS and PNP. Remarkably, QNfL (p < 0.0001) as well as QGFAP (p < 0.01) were only increased in IIH patients. QNfL was positively correlated with CSF-NfL levels (r = 0.51, p = 0.0012) and negatively correlated with S-NfL levels (r = - 0.51, p = 0.0012) in HC, while it was only positively associated with CSF-NfL levels in IIH patients (r = 0.71, p < 0.0001). An increase in blood-CSF barrier permeability assessed by QAlb did not lead to a decrease in QNfL in any cohort.

CONCLUSIONS

The observed elevation of QNfL in IIH patients, which was associated with lumbar puncture opening pressure, indicates a reduced NfL transition from the CSF to serum compartment. This supports the hypothesis of a pressure-dependent CSF outflow obstruction to be critically involved in IIH pathogenesis.

Neurofilament light oligomers in neurodegenerative diseases: quantification by homogeneous immunoassay in cerebrospinal fluid

Background

Neurofilament light (NfL) is a widely used biomarker for neurodegeneration. NfL is prone to oligomerisation, but available assays do not reveal the exact molecular nature of the protein variant measured. The objective of this study was to develop a homogeneous ELISA capable of quantifying oligomeric NfL (oNfL) in cerebrospinal fluid (CSF).

Methods

A homogeneous ELISA, based on the same capture and detection antibody (NfL21), was developed and used to quantify oNfL in samples from patients with behavioural variant frontotemporal dementia (bvFTD, n=28), non-fluent variant primary progressive aphasia (nfvPPA, n=23), semantic variant PPA (svPPA, n=10), Alzheimer's disease (AD, n=20) and healthy controls (n=20). The nature of NfL in CSF, and the recombinant protein calibrator, was also characterised by size exclusion chromatography (SEC).

Results

CSF concentration of oNfL was significantly higher in nfvPPA (p<0.0001) and svPPA patients (p<0.05) compared with controls. CSF oNfL concentration was also significantly higher in nfvPPA compared with bvFTD (p<0.001) and AD (p<0.01) patients. SEC data showed a peak fraction compatible with a full-length dimer (~135 kDa) in the in-house calibrator. For CSF, the peak was found in a fraction of lower molecular weight (~53 kDa), suggesting dimerisation of NfL fragments.

Conclusions

The homogeneous ELISA and SEC data suggest that most of the NfL in both the calibrator and human CSF is present as a dimer. In CSF, the dimer appears to be truncated. Further studies are needed to determine its precise molecular composition.

The 2022 Lady Estelle Wolfson lectureship on neurofilaments

Neurofilament proteins (Nf) have been validated and established as a reliable body fluid biomarker for neurodegenerative pathology. This review covers seven Nf isoforms, Nf light (NfL), two splicing variants of Nf medium (NfM), two splicing variants of Nf heavy (NfH),α -internexin (INA) and peripherin (PRPH). The genetic and epigenetic aspects of Nf are discussed as relevant for neurodegenerative diseases and oncology. The comprehensive list of mutations for all Nf isoforms covers Amyotrophic Lateral Sclerosis, Charcot-Marie Tooth disease, Spinal muscular atrophy, Parkinson Disease and Lewy Body Dementia. Next, emphasis is given to the expanding field of post-translational modifications (PTM) of the Nf amino acid residues. Protein structural aspects are reviewed alongside PTMs causing neurodegenerative pathology and human autoimmunity. Molecular visualisations of NF PTMs, assembly and stoichiometry make use of Alphafold2 modelling. The implications for Nf function on the cellular level and axonal transport are discussed. Neurofilament aggregate formation and proteolytic breakdown are reviewed as relevant for biomarker tests and disease. Likewise, Nf stoichiometry is reviewed with regard to in vitro experiments and as a compensatory mechanism in neurodegeneration. The review of Nf across a spectrum of 87 diseases from all parts of medicine is followed by a critical appraisal of 33 meta-analyses on Nf body fluid levels. The review concludes with considerations for clinical trial design and an outlook for future research.

Plasma neurofilament light chain (NfL) reference interval determination in an Age-stratified cognitively unimpaired cohort

BACKGROUND AND AIMS

Neurofilament light chain (NfL) is an emerging biomarker of neurodegenerative disease progression. As plasma NfL increases with age, characterization of NfL concentrations in an age-stratified cognitively unimpaired population was assessed.

MATERIALS AND METHODS

EDTA-plasma samples were measured using the Simoa® NF-light™ Advantage Kit assay. One-sided reference intervals were established from 1100 cognitive normal individuals (588 male, 512 female) aged 20 to 95 years. Of those, 927 samples were obtained from the Mayo Clinic Study of Aging cohort (age > 50 years), and the remainder (age < 50 years) were obtained from individuals without known neurological conditions. All samples were from individuals without known chronic kidney disease, stroke or myocardial infarction, and a body mass index < 30 kg/m2.

RESULTS

The 97.5th percentile limits for the following age ranges (in years) were (pg/mL): 20 s: ≤8.4, 30 s: ≤11.4, 40 s: ≤15.4, 50 s: ≤20.8, 60 s: ≤28.0, 70 s: ≤37.9, 80+: ≤51.2. Sex had no significant effect on reference intervals. Observed NfL concentrations increased at a rate of 3.1 % per year of age.

CONCLUSIONS

Characterization of the rate of NfL concentration increase and decade-wide reference intervals from a neurologically well-characterized patient population will aid in interpretation of NfL during the clinical evaluation of a potential neurodegenerative disease.

A Novel Neurofilament Light Chain ELISA Validated in Patients with Alzheimer's Disease, Frontotemporal Dementia, and Subjective Cognitive Decline, and the Evaluation of Candidate Proteins for Immunoassay Calibration

Neurofilament light chain (Nf-L) is a well-known biomarker for axonal damage; however, the corresponding circulating Nf-L analyte in cerebrospinal fluid (CSF) is poorly characterized. We therefore isolated new monoclonal antibodies against synthetic peptides, and these monoclonals were characterized for their specificity on brain-specific intermediate filament proteins. Two highly specific antibodies, ADx206 and ADx209, were analytically validated for CSF applications according to well-established criteria. Interestingly, using three different sources of purified Nf-L proteins, a significant impact on interpolated concentrations was observed. With a lower limit of analytical sensitivity of 100 pg/mL using bovine Nf-L as the calibrator, we were able to quantify the Nf-L analyte in each sample, and these Nf-L concentrations were highly correlated to the Uman diagnostics assay (Spearman rho = 0.97, p < 0.001). In the clinical diagnostic groups, the new Nf-L ELISA could discriminate patients with Alzheimer’s disease (AD, n = 20) from those with frontotemporal lobe dementia (FTD, n = 20) and control samples with subjective cognitive decline (SCD, n = 20). Henceforth, this novel Nf-L ELISA with well-defined specificity and epitopes can be used to enhance our understanding of harmonizing the use of Nf-L as a clinically relevant marker for neurodegeneration in CSF.

Serum-Based Biomarkers in Neurodegeneration and Multiple Sclerosis

Multiple Sclerosis (MS) is a debilitating disease with typical onset between 20 and 40 years of age, so the disability associated with this disease, unfortunately, occurs in the prime of life. At a very early stage of MS, the relapsing-remitting mobility impairment occurs in parallel with a progressive decline in cognition, which is subclinical. This stage of the disease is considered the beginning of progressive MS. Understanding where a patient is along such a subclinical phase could be critical for therapeutic efficacy and enrollment in clinical trials to test drugs targeted at neurodegeneration. Since the disease course is uneven among patients, biomarkers are needed to provide insights into pathogenesis, diagnosis, and prognosis of events that affect neurons during this subclinical phase that shapes neurodegeneration and disability. Thus, subclinical cognitive decline must be better understood. One approach to this problem is to follow known biomarkers of neurodegeneration over time. These biomarkers include Neurofilament, Tau and phosphotau protein, amyloid-peptide-β, Brl2 and Brl2-23, N-Acetylaspartate, and 14-3-3 family proteins. A composite set of these serum-based biomarkers of neurodegeneration might provide a distinct signature in early vs. late subclinical cognitive decline, thus offering additional diagnostic criteria for progressive neurodegeneration and response to treatment. Studies on serum-based biomarkers are described together with selective studies on CSF-based biomarkers and MRI-based biomarkers.