The dazzling rise of neurofilaments: Physiological functions and roles as biomarkers

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.

Gas6/TAM system as potential biomarker for multiple sclerosis prognosis

Introduction

The protein growth arrest-specific 6 (Gas6) and its tyrosine kinase receptors Tyro-3, Axl, and Mer (TAM) are ubiquitous proteins involved in regulating inflammation and apoptotic body clearance. Multiple sclerosis (MS) is the most common inflammatory demyelinating disease of the central nervous system leading to progressive and irreversible disability if not diagnosed and treated promptly. Gas6 and TAM receptors have been associated with neuronal remyelination and stimulation of oligodendrocyte survival. However, few data are available regarding clinical correlation in MS patients. We aimed to evaluate soluble levels of these molecules in the cerebrospinal fluid (CSF) and serum at MS diagnosis and correlate them with short-term disease severity.

Methods

In a prospective cohort study, we enrolled 64 patients with a diagnosis of clinical isolated syndrome (CIS), radiological isolated syndrome (RIS) and relapsing-remitting (RR) MS according to the McDonald 2017 Criteria. Before any treatment initiation, we sampled the serum and CSF, and collected clinical data: disease course, presence of gadolinium-enhancing lesions, and expanded disability status score (EDSS). At the last clinical follow-up, we assessed EDSS and calculated MS severity score (MSSS) and age-related MS severity (ARMSS). Gas6 and TAM receptors were determined using an ELISA kit (R&D Systems) and compared to neurofilament (NFLs) levels evaluated with SimplePlex™ fluorescence-based immunoassay.

Results

At diagnosis, serum sAxl was higher in patients receiving none or low-efficacy disease-modifying treatments (DMTs) versus patients with high-efficacy DMTs (p = 0.04). Higher CSF Gas6 and serum sAXL were associated with an EDSS <3 at diagnosis (p = 0.04; p = 0.037). Serum Gas6 correlates to a lower MSSS (r2 = -0.32, p = 0.01). Serum and CSF NFLs were confirmed as disability biomarkers in our cohort according to EDSS (p = 0.005; p = 0.002) and MSSS (r2 = 0.27, p = 0.03; r2 = 0.39, p = 0.001). Results were corroborated using multivariate analysis.

Conclusions

Our data suggest a protective role of Gas6 and its receptors in patients with MS and suitable severity disease biomarkers.

Neurofilaments: Novel findings and future challenges

Neurofilaments (NFs) are abundant cytoskeletal proteins that emerge as a critical hub for cell signalling within neurons. As we start to uncover essential roles of NFs in regulating microtubule and organelle dynamics, nerve conduction and neurotransmission, novel discoveries are expected to arise in genetics, with NFs identified as causal genes for various neurodegenerative diseases. This review will discuss how the latest advances in fundamental and translational research illuminate our understanding of NF biology, particularly their assembly, organisation, transport and degradation. We will emphasise the notion that filaments are not one entity and that future challenges will be to apprehend their diverse composition and structural heterogeneity and to scrutinize how this regulates signalling, sustains neuronal physiology and drives pathophysiology in disease.

Loss of TMEM106B exacerbates Tau pathology and neurodegeneration in PS19 mice

TMEM106B, a gene encoding a lysosome membrane protein, is tightly associated with brain aging, hypomyelinating leukodystrophy, and multiple neurodegenerative diseases, including frontotemporal lobar degeneration with TDP-43 aggregates (FTLD-TDP). Recently, TMEM106B polymorphisms have been associated with tauopathy in chronic traumatic encephalopathy (CTE) and FTLD-TDP patients. However, how TMEM106B influences Tau pathology and its associated neurodegeneration, is unclear. Here we show that loss of TMEM106B enhances the accumulation of pathological Tau, especially in the neuronal soma in the hippocampus, resulting in severe neuronal loss in the PS19 Tau transgenic mice. Moreover, Tmem106b-/- PS19 mice develop significantly increased abnormalities in the neuronal cytoskeleton, autophagy-lysosome activities, as well as glial activation, compared with PS19 and Tmem106b-/- mice. Together, our findings demonstrate that loss of TMEM106B drastically exacerbates Tau pathology and its associated disease phenotypes, and provide new insights into the roles of TMEM106B in neurodegenerative diseases.

Gigaxonin Suppresses Epithelial-to-Mesenchymal Transition of Human Cancer Through Downregulation of Snail

Gigaxonin is an E3 ubiquitin ligase that plays a role in cytoskeletal stability. Its role in cancer is not yet clearly understood. Our previous studies of head and neck cancer had identified gigaxonin interacting with p16 for NFκB ubiquitination. To explore its role in cancer cell growth suppression, we analyzed normal and tumor DNA from cervical and head and neck cancers. There was a higher frequency of exon 8 SNP (c.1293 C>T, rs2608555) in the tumor (46% vs. 25% normal, P = 0.011) pointing to a relationship to cancer. Comparison of primary tumor with recurrence and metastasis did not reveal a statistical significance. Two cervical cancer cell lines, ME180 and HT3 harboring exon 8 SNP and showing T allele expression correlated with higher gigaxonin expression, reduced in vitro cell growth and enhanced cisplatin sensitivity in comparison with C allele expressing cancer cell lines. Loss of gigaxonin expression in ME180 cells through CRISPR-Cas9 or siRNA led to aggressive cancer cell growth including increased migration and Matrigel invasion. The in vitro cell growth phenotypes were reversed with re-expression of gigaxonin. Suppression of cell growth correlated with reduced Snail and increased e-cadherin expression. Mouse tail vein injection studies showed increased lung metastasis of cells with low gigaxonin expression and reduced metastasis with reexpression of gigaxonin. We have found an association between C allele expression and RNA instability and absence of multimeric protein formation. From our results, we conclude that gigaxonin expression is associated with suppression of epithelial-mesenchymal transition through inhibition of Snail.

SIGNIFICANCE

Our results suggest that GAN gene exon 8 SNP T allele expression correlates with higher gigaxonin expression and suppression of aggressive cancer cell growth. There is downregulation of Snail and upregulation of e-cadherin through NFκB ubiquitination. We hypothesize that exon 8 T allele and gigaxonin expression could serve as diagnostic markers of suppression of aggressive growth of head and neck cancer.

Comparison of CSF and plasma NfL and pNfH for Alzheimer's disease diagnosis: a memory clinic study

Plasma neurofilament light chain (NfL) is a promising biomarker of axonal damage for the diagnosis of neurodegenerative diseases. Phosphorylated neurofilament heavy chain (pNfH) has demonstrated its value in motor neuron diseases diagnosis, but has less been explored for dementia diagnosis. In a cross-sectional study, we compared cerebrospinal fluid (CSF) and plasma NfL and pNfH levels in n = 188 patients from Lariboisière Hospital, Paris, France, including AD patients at mild cognitive impairment stage (AD-MCI, n = 36) and dementia stage (n = 64), non-AD MCI (n = 38), non-AD dementia (n = 28) patients and control subjects (n = 22). Plasma NfL, plasma and CSF pNfH levels were measured using Simoa and CSF NfL using ELISA. The correlation between CSF and plasma levels was stronger for NfL than pNfH (rho = 0.77 and rho = 0.52, respectively). All neurofilament markers were increased in AD-MCI, AD dementia and non-AD dementia groups compared with controls. CSF NfL, CSF pNfH and plasma NfL showed high performance to discriminate AD at both MCI and dementia stages from control subjects [AUC (area under the curve) = 0.82-0.91]. Plasma pNfH displayed overall lower AUCs for discrimination between groups compared with CSF pNfH. Neurofilament markers showed similar moderate association with cognition. NfL levels displayed significant association with mediotemporal lobe atrophy and white matter lesions in the AD group. Our results suggest that CSF NfL and pNfH as well as plasma NfL levels display equivalent performance in both positive and differential AD diagnosis in memory clinic settings. In contrast to motoneuron disorders, plasma pNfH did not demonstrate added value as compared with plasma NfL.

Relationship between increased serum neurofilament light chain and glial fibrillary acidic protein levels with non-motor symptoms in patients with Parkinson's disease

BACKGROUND

This study set out to investigate the relationship between serum neurofilament light chain (NFL), glial fibrillary acidic protein (GFAP), and various non-motor symptoms (NMSs) in patients with Parkinson's disease (PD).

METHODS

The study included 37 healthy controls (HCs) and 51 PD patients. Clinical assessments of PD symptoms were conducted for all PD patients. The NMSS was utilised to evaluate the NMS burden (NMSB) in individuals. Based on the severity of NMSB, we further categorised the PD group into two subgroups: mild-moderate NMSB group and severe-very severe NMSB group. The amounts of NFL and GFAP in the serum were measured using an extremely sensitive single molecule array (Simoa) method. Statistical analyses were performed on the collected data using SPSS 26.0 and R (version 3.6.3).

RESULTS

Serum GFAP and NFL levels in the PD group with severe-very severe NMSB were significantly higher than those in the mild-moderate NMSB group (GFAP: P < 0.007; NFL: P < 0.009). Serum NFL and GFAP levels had positive correlations with NMSS total scores (GFAP: r = 0.326, P = 0.020; NFL: r = 0.318, P = 0.023) and multiple subdomains. The relationship between the attention/memory domains of NMSS and NFL levels is significantly positive (r = 0.283, P = 0.044). Similarly, the mood/apathy domains of NMSS are also significantly positively correlated with GFAP levels (r = 0.441, P = 0.001). Patients with emotional problems or cognitive impairment had higher GFAP or NFL levels, respectively. Furthermore, it has been demonstrated that NMSs play a mediating role in the quality of life of patients with PD. Moreover, the combination of NFL and GFAP has proven to be more effective than using a single component in identifying PD patients with severe-very severe NMSB.

CONCLUSIONS

The severity of NMSs in PD patients, particularly cognitive and emotional symptoms, was found to be associated with the levels of serum NFL and GFAP. This study marks the first attempt to examine the connection between NMSs of PD and the simultaneous identification of NFL and GFAP levels.

Delirium pathophysiology in cancer: neurofilament light chain biomarker - narrative review

Background Delirium is a debilitating disorder with high prevalence near the end of life, impacting quality of life of patients and their relatives. Timely recognition of delirium can lead to prevention and/or better treatment of delirium. According to current hypotheses delirium is thought to result from aberrant inflammation and neurotransmission, with a possible role for neuronal damage. Neurofilament light chain (NfL) is a protein biomarker in body fluids that is unique to neurons, with elevated levels when neurons are damaged, making NfL a viable biomarker for early detection of delirium. This narrative review summarises current research regarding the pathophysiology of delirium and the potential of NfL as a susceptibility biomarker for delirium and places this in the context of care for patients with advanced cancer.Results Six studies were conducted exclusively on NfL in patients with delirium. Three of these studies demonstrated that high plasma NfL levels preoperatively predict delirium in older adult patients postoperatively. Two studies demonstrated that high levels of NfL in intensive care unit (ICU) patients are correlated with delirium duration and severity. One study found that incident delirium in older adult patients was associated with increased median NfL levels during hospitalisation.Conclusions Targeted studies are required to understand if NfL is a susceptibility biomarker for delirium in patients with advanced cancer. In this palliative care context, better accessible matrices, such as saliva or urine, would be helpful for repetitive testing. Improvement of biological measures for delirium can lead to improved early recognition and lay the groundwork for novel therapeutic strategies.

Serum neurofilament light chain and cognition decline in US elderly: A cross-sectional study

OBJECTIVE

Early identification of cognitive impairment in neurodegenerative diseases like Alzheimer's disease (AD) is crucial. Neurofilament, a potential biomarker for neurological disorders, has gained attention. Our study aims to investigate the relationship between serum neurofilament light (sNfL) levels and cognitive function in elderly individuals in the United States.

METHODS

This cross-sectional study analyzed data from participants aged 60 and above in the National Health and Nutrition Examination Survey (2013-2014). We collected sNfL levels, cognitive function tests, sociodemographic characteristics, comorbidities, and other variables. Weighted multiple linear regression models examined the relationship between ln(sNfL) and cognitive scores. Restricted cubic spline (RCS) visualization explored nonlinear relationships. The stratified analysis examined subgroups' ln(sNfL) and cognitive function association.

RESULTS

The study included 446 participants (47.73% male). Participants with ln(sNfL) levels between 2.58 and 2.81 pg/mL (second quintile) performed relatively well in cognitive tests. After adjusting for multiple factors, ln(sNfL) levels were negatively correlated with cognitive function, with adjusted β (95% CI) as follows: immediate recall test (IRT): -0.763 (-1.301 to -0.224), delayed recall test (DRT): -0.308 (-0.576 to -0.04), animal fluency test (AFT): -1.616 (-2.639 to -0.594), and digit symbol substitution test (DSST): -2.790 (-4.369 to -1.21). RCS curves showed nonlinear relationships between ln(sNfL) and DRT, AFT, with inflection points around 2.7 pg/mL. The stratified analysis revealed a negative correlation between ln(sNfL) and cognition in specific subgroups with distinct features, with an interaction between diabetes and ln(sNfL).

INTERPRETATION

Higher sNfL levels are associated with poorer cognitive function in the elderly population of the United States. sNfL shows promise as a potential biomarker for early identification of cognitive decline.

Neurofilaments in health and Charcot-Marie-Tooth disease

Neurofilaments (NFs) are the most abundant component of mature neurons, that interconnect with actin and microtubules to form the cytoskeleton. Specifically expressed in the nervous system, NFs present the particularity within the Intermediate Filament family of being formed by four subunits, the neurofilament light (NF-L), medium (NF-M), heavy (NF-H) proteins and α-internexin or peripherin. Here, we review the current knowledge on NF proteins and neurofilaments, from their domain structures and their model of assembly to the dynamics of their transport and degradation along the axon. The formation of the filament and its behaviour are regulated by various determinants, including post-transcriptional (miRNA and RBP proteins) and post-translational (phosphorylation and ubiquitination) modifiers. Altogether, the complex set of modifications enable the neuron to establish a stable but elastic NF array constituting the structural scaffold of the axon, while permitting the local expression of NF proteins and providing the dynamics necessary to fulfil local demands and respond to stimuli and injury. Thus, in addition to their roles in mechano-resistance, radial axonal outgrowth and nerve conduction, NFs control microtubule dynamics, organelle distribution and neurotransmission at the synapse. We discuss how the studies of neurodegenerative diseases with NF aggregation shed light on the biology of NFs. In particular, the NEFL and NEFH genes are mutated in Charcot-Marie-Tooth (CMT) disease, the most common inherited neurological disorder of the peripheral nervous system. The clinical features of the CMT forms (axonal CMT2E, CMT2CC; demyelinating CMT1F; intermediate I-CMT) with symptoms affecting the central nervous system (CNS) will allow us to further investigate the physiological roles of NFs in the brain. Thus, NF-CMT mouse models exhibit various degrees of sensory-motor deficits associated with CNS symptoms. Cellular systems brought findings regarding the dominant effect of NF-L mutants on NF aggregation and transport, although these have been recently challenged. Neurofilament detection without NF-L in recessive CMT is puzzling, calling for a re-examination of the current model in which NF-L is indispensable for NF assembly. Overall, we discuss how the fundamental and translational fields are feeding each-other to increase but also challenge our knowledge of NF biology, and to develop therapeutic avenues for CMT and neurodegenerative diseases with NF aggregation.

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.

Loss of TMEM106B exacerbates Tau pathology and neurodegeneration in PS19 mice

TMEM106B, a gene encoding a lysosome membrane protein, is tightly associated with brain aging, hypomyelinating leukodystrophy, and multiple neurodegenerative diseases, including frontotemporal lobar degeneration with TDP-43 aggregates (FTLD-TDP). Recently, TMEM106B polymorphisms have been associated with tauopathy in chronic traumatic encephalopathy (CTE) and FTLD-TDP patients. However, how TMEM106B influences Tau pathology and its associated neurodegeneration, is unclear. Here we show that loss of TMEM106B enhances the accumulation of pathological Tau, especially in the neuronal soma in the hippocampus, resulting in severe neuronal loss in the PS19 Tau transgenic mice. Moreover, Tmem106b-/- PS19 mice develop significantly increased disruption of the neuronal cytoskeleton, autophagy-lysosomal function, and lysosomal trafficking along the axon as well as enhanced gliosis compared with PS19 and Tmem106b-/- mice. Together, our findings demonstrate that loss of TMEM106B drastically exacerbates Tau pathology and its associated disease phenotypes, and provide new insights into the roles of TMEM106B in neurodegenerative diseases.

Serum neurofilament light chain levels in migraine patients: a monocentric case-control study in China

PURPOSE

Serum neurofilament light chain (sNfL) can reflect nerve damage. Whether migraine can cause neurological damage remain unclear. This study assesses sNfL levels in migraine patients and explores whether there is nerve damage in migraine.

METHODS

A case-control study was conducted in Xiamen, China. A total of 138 migraine patients and 70 healthy controls were recruited. sNfL (pg/mL) was measured on the single-molecule array platform. Univariate, Pearson correlation and linear regression analysis were used to assess the relationship between migraine and sNfL levels, with further subgroup analysis by migraine characteristics.

RESULTS

Overall, 85.10% of the 208 subjects were female, with a median age of 36 years. sNfL levels were higher in the migraine group than in the control group (4.85 (3.49, 6.62) vs. 4.11 (3.22, 5.59)), but the difference was not significant (P = 0.133). The two groups showed an almost consistent trend in which sNfL levels increased significantly with age. Subgroup analysis showed a significant increase in sNfL levels in patients with a migraine course ≥ 10 years (β = 0.693 (0.168, 1.220), P = 0.010). Regression analysis results show that age and migraine course are independent risk factors for elevated sNfL levels, and there is an interaction between the two factors. Patients aged < 45 years and with a migraine course ≥ 10 years have significantly increased sNfL levels.

CONCLUSIONS

This is the first study to evaluate sNfL levels in migraine patients. The sNfL levels significantly increased in patients with a migraine course ≥ 10 years. More attention to nerve damage in young patients with a long course of migraine is required.

Intermediate filament proteins are reliable immunohistological biomarkers to help diagnose multiple tissue-specific diseases

Cytoskeletal networks are proteins that effectively maintain cell integrity and provide mechanical support to cells by actively transmitting mechanical signals. Intermediate filaments, which are from the cytoskeleton family and are 10 nanometres in diameter, are unlike actin and microtubules, which are highly dynamic cytoskeletal elements. Intermediate filaments are flexible at low strain, harden at high strain and resist breaking. For this reason, these filaments fulfil structural functions by providing mechanical support to the cells through their different strain-hardening properties. Intermediate filaments are suitable in that cells both cope with mechanical forces and modulate signal transmission. These filaments are composed of fibrous proteins that exhibit a central α-helical rod domain with a conserved substructure. Intermediate filament proteins are divided into six groups. Type I and type II include acidic and basic keratins, type III, vimentin, desmin, peripheralin and glial fibrillary acidic protein (GFAP), respectively. Type IV intermediate filament group includes neurofilament proteins and a fourth neurofilament subunit, α-internexin proteins. Type V consists of lamins located in the nucleus, and the type VI group consists of lens-specific intermediate filaments, CP49/phakinin and filen. Intermediate filament proteins show specific immunoreactivity in differentiating cells and mature cells of various types. Various carcinomas such as colorectal, urothelial and ovarian, diseases such as chronic pancreatitis, cirrhosis, hepatitis and cataract have been associated with intermediate filaments. Accordingly, this section reviews available immunohistochemical antibodies to intermediate filament proteins. Identification of intermediate filament proteins by methodological methods may contribute to the understanding of complex diseases.

A Comparison of Two Analytical Approaches for the Quantification of Neurofilament Light Chain, a Biomarker of Axonal Damage in Multiple Sclerosis

Neurofilament light chain (NfL), is a neuron-specific cytoskeletal protein detected in extracellular fluid following axonal damage. Extensive research has focused on NfL quantification in CSF, establishing it as a prognostic biomarker of disability progression in Multiple Sclerosis (MS). Our study used a new commercially available Enzyme-Linked Immunosorbent Assay (ELISA) kit and Single Molecular Array (Simoa) advanced technology to assess serum NfL levels in MS patients and Healthy Controls (HC). Verifying the most accurate, cost-effective methodology will benefit its application in clinical settings. Blood samples were collected from 54 MS patients and 30 HC. Protocols accompanying the kits were followed. The ELISA thershold was set as 3 S.D. above the mean of the HC. For Simoa, the Z-score calculation created by Jens Kuhle's group was applied (with permission). Samples exceeding the threshold or z-score ≥1.5 indicated subclinical disease activity. To our knowledge, this is the first study to find strong-positive correlation between ELISA and Simoa for the quantification of NfL in serum (r = 0.919). Despite the strong correlation, Simoa has better analytical sensitivity and can detect small changes in samples making it valuable in clinical settings. Further research is required to evaluate whether serum NfL quantification using ELISA could be utilized to predict disability progression.

Transient expression of NF200 by fibers in the nasal septum and rostral telencephalon of developing opossums (Monodelphis domestica)

Marsupials are born very immature and crawl on their mother's belly to attach to teats. Sensory information is required to guide the newborn and to induce attachment to the teat. Olfaction has been classically proposed to influence neonatal behaviors, but recent studies suggest that the central olfactory structures are too immature to account for them. In the newborn opossum, we previously described a fascicle of nerve fibers expressing neurofilament-200 (NF200, a marker of fiber maturity) from the olfactory bulbs to the rostral telencephalon. The course of these fibers is compatible with that of the terminal nerve that, during development, is characterized by the presence of neurons synthetizing gonadotropin hormones (GnRH). To evaluate if these fibers are related to the terminal nerve and if they play a role in precocious behaviors in opossums, we used immunohistochemistry against NF200 and GnRH. The results show that NF200-labeled fibers are present between P0 and P11, but do not reach much further caudally than the septal region. Only a few NF200-labeled fibers were found near the olfactory and vomeronasal epitheliums and they did not penetrate the olfactory bulbs. NF200-labeled fibers follow the same path as fibers labeled for GnRH. In contrast to the latter, NF200-labeled fibers are no longer visible at P15. These results suggest that these fibers are neither from the olfactory nor from the vomeronasal nerves but may be part of the terminal nerve. Their limited caudal extension does not support a role in the sensorimotor behaviors of the newborn opossum.

Neurofilament Levels in Dendritic Spines Associate with Synaptic Status

Neurofilaments are one of the main cytoskeletal components in neurons; they can be found in the form of oligomers at pre- and postsynapses. How their presence is regulated at the postsynapse remains largely unclear. Here we systematically quantified, by immunolabeling, the occurrence of the neurofilament isoform triplet neurofilament light (NFL), medium (NFM), and heavy (NFH) at the postsynapse using STED nanoscopy together with markers of synaptic strength and activity. Our data show that, within dendritic spines, neurofilament isoforms rarely colocalize with each other and that they are present to different extents, with NFL being the most abundant isoform. The amount of the three isoforms correlates with markers of postsynaptic strength and presynaptic activity to varying degrees: NFL shows the highest correlation to both synaptic traits, suggesting its involvement in synaptic response, while NFM exhibits the lowest correlations. By quantifying the presence of neurofilaments at the postsynapse within the context of the synaptic status, this work sheds new light on the regulation of synaptic neurofilaments and their possible contribution to synaptopathies.

Serum neurofilament light chain: a novel biomarker for early diabetic sensorimotor polyneuropathy

AIMS/HYPOTHESIS

No established blood-based biomarker exists to monitor diabetic sensorimotor polyneuropathy (DSPN) and evaluate treatment response. The neurofilament light chain (NFL), a blood biomarker of neuroaxonal damage in several neurodegenerative diseases, represents a potential biomarker for DSPN. We hypothesised that higher serum NFL levels are associated with prevalent DSPN and nerve dysfunction in individuals recently diagnosed with diabetes.

METHODS

This cross-sectional study included 423 adults with type 1 and type 2 diabetes and known diabetes duration of less than 1 year from the prospective observational German Diabetes Study cohort. NFL was measured in serum samples of fasting participants in a multiplex approach using proximity extension assay technology. DSPN was assessed by neurological examination, nerve conduction studies and quantitative sensory testing. Associations of serum NFL with DSPN (defined according to the Toronto Consensus criteria) were estimated using Poisson regression, while multivariable linear and quantile regression models were used to assess associations with nerve function measures. In exploratory analyses, other biomarkers in the multiplex panel were also analysed similarly to NFL.

RESULTS

DSPN was found in 16% of the study sample. Serum NFL levels increased with age. After adjustment for age, sex, waist circumference, height, HbA_1c, known diabetes duration, diabetes type, cholesterol, eGFR, hypertension, CVD, use of lipid-lowering drugs and use of non-steroidal anti-inflammatory drugs, higher serum NFL levels were associated with DSPN (RR [95% CI] per 1-normalised protein expression increase, 1.92 [1.50, 2.45], p<0.0001), slower motor (all p<0.0001) and sensory (all p≤0.03) nerve conduction velocities, lower sural sensory nerve action potential (p=0.0004) and higher thermal detection threshold to warm stimuli (p=0.023 and p=0.004 for hand and foot, respectively). There was no evidence for associations between other neurological biomarkers and DSPN or nerve function measures.

CONCLUSIONS/INTERPRETATION

Our findings in individuals recently diagnosed with diabetes provide new evidence associating higher serum NFL levels with DSPN and peripheral nerve dysfunction. The present study advocates NFL as a potential biomarker for DSPN.

The multifaceted role of neurofilament light chain protein in non-primary neurological diseases

The advancing validation and exploitation of CSF and blood neurofilament light chain protein as a biomarker of neuroaxonal damage has deeply changed the current diagnostic and prognostic approach to neurological diseases. Further, recent studies have provided evidence of potential new applications of this biomarker also in non-primary neurological diseases. In the present review we summarize the state of the art, future perspectives, but also limitations, of neurofilament light chain protein as a CSF and blood biomarker in several medical fields, including intensive care medicine, surgery, internal medicine and psychiatry. In particular, neurofilament light chain protein is associated with the degree of neurological impairment and outcome in patients admitted to intensive care units or in the perioperative phase and it seems to be highly interconnected with cardiovascular risk factors. Beyond that, interesting diagnostic and prognostic insights have been provided by the investigation of neurofilament light chain protein in psychiatric disorders as well as in the current coronavirus disease-19 pandemic and in normal ageing. Altogether, current data outline a multifaceted applicability of CSF and blood neurofilament light chain protein ranging from the critical clinical setting to the development of precision medicine models suggesting a strict interplay between the nervous system pathophysiology and the health-illness continuum.

Proprioceptors-enriched neuronal cultures from induced pluripotent stem cells from Friedreich ataxia patients show altered transcriptomic and proteomic profiles, abnormal neurite extension, and impaired electrophysiological properties

Friedreich ataxia is an autosomal recessive multisystem disorder with prominent neurological manifestations and cardiac involvement. The disease is caused by large GAA expansions in the first intron of the FXN gene, encoding the mitochondrial protein frataxin, resulting in downregulation of gene expression and reduced synthesis of frataxin. The selective loss of proprioceptive neurons is a hallmark of Friedreich ataxia, but the cause of the specific vulnerability of these cells is still unknown. We herein perform an in vitro characterization of human induced pluripotent stem cell-derived sensory neuronal cultures highly enriched for primary proprioceptive neurons. We employ neurons differentiated from healthy donors, Friedreich ataxia patients and Friedreich ataxia sibling isogenic control lines. The analysis of the transcriptomic and proteomic profile suggests an impairment of cytoskeleton organization at the growth cone, neurite extension and, at later stages of maturation, synaptic plasticity. Alterations in the spiking profile of tonic neurons are also observed at the electrophysiological analysis of mature neurons. Despite the reversal of the repressive epigenetic state at the FXN locus and the restoration of FXN expression, isogenic control neurons retain many features of Friedreich ataxia neurons. Our study suggests the existence of abnormalities affecting proprioceptors in Friedreich ataxia, particularly their ability to extend towards their targets and transmit proper synaptic signals. It also highlights the need for further investigations to better understand the mechanistic link between FXN silencing and proprioceptive degeneration in Friedreich ataxia.

Diagnostic Performance of Cerebrospinal Fluid Neurofilament Light Chain and Soluble Amyloid-β Protein Precursor β in the Subcortical Small Vessel Type of Dementia

BACKGROUND

The subcortical small vessel type of dementia (SSVD) is a common subtype of vascular dementia, but there is a lack of disease-specific cerebrospinal fluid (CSF) biomarkers.

OBJECTIVE

We investigated whether CSF concentrations of neurofilament light chain (NFL), soluble amyloid-β protein precursor α (sAβPPα), sAβPPβ, and CSF/serum albumin ratio could separate SSVD from healthy controls, Alzheimer's disease (AD), and mixed dementia (combined AD and SSVD).

METHODS

This was a mono-center study of patients with SSVD (n = 38), AD (n = 121), mixed dementia (n = 62), and controls (n = 96). The CSF biomarkers were measured using immunoassays, and their independent contribution to the separation between groups were evaluated using the Wald test. Then, the area under the receiver operating characteristics curve (AUROC) and 95% confidence intervals (CIs) were calculated.

RESULTS

Elevated neurofilament light chain (NFL) and decreased sAβPPβ independently separated SSVD from controls, and sAβPPβ also distinguished SSVD from AD and mixed dementia. The combination of NFL and sAβPPβ discriminated SSVD from controls with high accuracy (AUROC 0.903, 95% CI: 0.834-0.972). Additionally, sAβPPβ combined with the core AD biomarkers (amyloid-β42, total tau, and phosphorylated tau181) had a high ability to separate SSVD from AD (AUROC 0.886, 95% CI: 0.830-0.942) and mixed dementia (AUROC 0.903, 95% CI: 0.838-0.968).

CONCLUSIONS

The high accuracy of NFL and sAβPPβ to separate SSVD from controls supports that SSVD is a specific diagnostic entity. Moreover, SSVD was distinguished from AD and mixed dementia using sAβPPβ in combination with the core AD biomarkers.

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.

Neurofilament light chain and total tau in the differential diagnosis and prognostic evaluation of acute and chronic inflammatory polyneuropathies

Background and Purpose

To investigate the diagnostic and prognostic value of axonal injury biomarkers in patients with inflammatory polyneuropathies.

Methods

Neurofilament light chain (NfL) and total tau (T‐tau) were measured in the cerebrospinal fluid (CSF) and plasma in 41 patients with Guillain–Barré syndrome (GBS), 32 patients with chronic inflammatory demyelinating polyneuropathy (CIDP), 10 with paraproteinemia‐related demyelinating polyneuropathy (PDN), and 8 with multifocal motor neuropathy (MMN), in comparison with 39 disease‐free controls and 59 other controls. Outcome was measured with the GBS‐disability score (GBS‐ds) or Inflammatory Neuropathy Cause and Treatment (INCAT) disability score.

Results

Neurofilament light chain levels in CSF and plasma were higher in GBS, CIDP, and PDN vs. disease‐free controls. Patients with MMN had higher NfL levels in plasma vs. disease‐free controls, but lower levels in CSF and plasma vs. patients with amyotrophic lateral sclerosis (ALS).

T‐tau levels in plasma were higher in GBS, CIDP, PDN, and MMN vs. all control groups.

Neurofilament light chain levels in CSF and plasma in patients with GBS correlated with GBS‐ds, as higher levels were associated with inability to run after 6 and 12 months. NfL levels in CSF and plasma in CIDP did not correlate significantly with outcome.

Conclusions

Acute and chronic inflammatory neuropathies are associated with an increase in levels of NfL in CSF and plasma, but NfL is validated as a prognostic biomarker only in GBS. NfL could be used in differentiating patients with MMN from ALS. T‐tau in plasma is a novel biomarker that could be used in a diagnostic assessment of patients with acute and chronic inflammatory polyneuropathies.

An age‐dependent mathematical model of neurofilament trafficking in healthy conditions

Neurofilaments (Nfs) are the major structural component of neurons. Their role as a potential biomarker of several neurodegenerative diseases has been investigated in past years with promising results. However, even under physiological conditions, little is known about the leaking of Nfs from the neuronal system and their detection in the cerebrospinal fluid (CSF) and blood. This study aimed at developing a mathematical model of Nf transport in healthy subjects in the 20–90 age range. The model was implemented as a set of ordinary differential equations describing the trafficking of Nfs from the nervous system to the periphery. Model parameters were calibrated on typical Nf levels obtained from the literature. An age‐dependent function modeled on CSF data was also included and validated on data measured in serum. We computed a global sensitivity analysis of model rates and volumes to identify the most sensitive parameters affecting the model’s steady state. Age, Nf synthesis, and degradation rates proved to be relevant for all model variables. Nf levels in the CSF and in blood were observed to be sensitive to the Nf leakage rates from neurons and to the blood clearance rate, and CSF levels were also sensitive to rates representing CSF turnover. An additional parameter perturbation analysis was also performed to investigate possible transient effects on the model variables not captured by the sensitivity analysis. The model provides useful insights into Nf transport and constitutes the basis for implementing quantitative system pharmacology extensions to investigate Nf trafficking in neurodegenerative diseases.

Superresolution Imaging of Cytoskeletal Networks in Fixed Brain Tissue

Emerging evidence suggests that neurodegeneration is directly linked to dysfunction of cytoskeleton; however, visualizing the organization of cytoskeletal structures in brain tissues remains challenging due to the limitation of resolution of light microscopy. Superresolution imaging overcomes this limitation and resolves subcellular structures below the diffraction barrier of light (20-200 nm), while retaining the advantages of fluorescent microscopy such as simultaneous visualization of multiple proteins and increased signal sensitivity and contrast. However, superresolution imaging approaches have been largely limited to very thin samples such as cultured cells growing as a single monolayer. Analysis of thicker tissue sections represents a technical challenge due to high background fluorescence and quality of the tissue preservation methods. Among superresolution microscopy approaches, structured illumination microscopy is one of the most compatible methods for analyzing thicker native tissue samples. We have developed a methodology that allows maximal preservation and quantitative analyses of cytoskeletal networks in tissue sections from a rodent brain. This methodology includes a specialized fixation protocol, tissue preparation, and image acquisition procedures optimized for the characterization of subcellular cytoskeletal structures using superresolution with structured illumination microscopy.

Neurofilament light protein as a biomarker in depression and cognitive function

PURPOSE OF REVIEW

Converging evidence suggest axonal damage is implicated in depression and cognitive function. Neurofilament light protein, measured within serum and cerebrospinal fluid, may be a biomarker of axonal damage. This article examines the emerging evidence implicating neurofilament light protein in depression and cognitive function.

RECENT FINDINGS

Preliminary cross-sectional and case-control studies in cohorts with depression have yielded inconsistent results regarding the association between neurofilament light protein and symptomatology. However, these studies had methodological limitations, requiring further investigation. Importantly, neurofilament light protein concentrations may be a marker of progression of cognitive decline and may be associated with cognitive performance within cognitively intact cohorts.

SUMMARY

Axonal damage is implicated in the neuropathology of depression and cognitive dysfunction. Consequently, neurofilament light protein is an emerging biomarker with potential in depression and cognitive function. Results are more consistent for cognition, requiring more research to assess neurofilament light protein in depression as well as other psychiatric disorders. Future longitudinal studies are necessary to determine whether neurofilament light protein can predict the onset and progression of depression and measure the effectiveness of potential psychiatric interventions and medications.

Neurofilament Light (NF-L) Chain Protein from a Highly Polymerized Structural Component of the Neuronal Cytoskeleton to a Neurodegenerative Disease Biomarker in the Periphery

Neurofilaments (NFs) are critical scaffolding components of the axoskeleton of healthy neurons interacting directly with multiple synaptic-phosphoproteins to support and coordinate neuronal cell shape, cytoarchitecture, synaptogenesis and neurotransmission. While neuronal presynaptic proteins such as synapsin-2 (SYN II) degrade rapidly via the ubiquitin-proteasome pathway, a considerably more stable neurofilament light (NF-L) chain protein turns over much more slowly, and in several neurological diseases is accompanied by a pathological shift from an intracellular neuronal cytoplasmic location into various biofluid compartments. NF-L has been found to be significantly elevated in peripheral biofluids in multiple neurodegenerative disorders, however it is not as widely appreciated that NF-L expression within neurons undergoing inflammatory neurodegeneration exhibit a significant down-regulation in these neuron-specific intermediate-filament components. Down-regulated NF-L in neurons correlates well with the observed axonal and neuronal atrophy, neurite deterioration and synaptic disorganization in tissues affected by Alzheimer's disease (AD) and other progressive, age-related neurological diseases. This Review paper: (i) will briefly assess the remarkably high number of neurological disorders that exhibit NF-L depolymerization, liberation from neuron-specific compartments, mobilization and enrichment into pathological biofluids; (ii) will evaluate how NF-L exhibits compartmentalization effects in age-related neurological disorders; (iii) will review how the shift of NF-L compartmentalization from within the neuronal cytoskeleton into peripheral biofluids may be a diagnostic biomarker for neuronal-decline in all cause dementia most useful in distinguishing between closely related neurological disorders; and (iv) will review emerging evidence that deficits in plasma membrane barrier integrity, pathological transport and/or vesicle-mediated trafficking dysfunction of NF-L may contribute to neuronal decline, with specific reference to AD wherever possible.

Plectin in the Central Nervous System and a Putative Role in Brain Astrocytes

Plectin, a high-molecular-mass cytolinker, is abundantly expressed in the central nervous system (CNS). Currently, a limited amount of data about plectin in the CNS prevents us from seeing the complete picture of how plectin affects the functioning of the CNS as a whole. Yet, by analogy to its role in other tissues, it is anticipated that, in the CNS, plectin also functions as the key cytoskeleton interlinking molecule. Thus, it is likely involved in signalling processes, thereby affecting numerous fundamental functions in the brain and spinal cord. Versatile direct and indirect interactions of plectin with cytoskeletal filaments and enzymes in the cells of the CNS in normal physiological and in pathologic conditions remain to be fully addressed. Several pathologies of the CNS related to plectin have been discovered in patients with plectinopathies. However, in view of plectin as an integrator of a cohesive mesh of cellular proteins, it is important that the role of plectin is also considered in other CNS pathologies. This review summarizes the current knowledge of plectin in the CNS, focusing on plectin isoforms that have been detected in the CNS, along with its expression profile and distribution alongside diverse cytoskeleton filaments in CNS cell types. Considering that the bidirectional communication between neurons and glial cells, especially astrocytes, is crucial for proper functioning of the CNS, we place particular emphasis on the known roles of plectin in neurons, and we propose possible roles of plectin in astrocytes.

Proximal weakness involvement in the first Italian case of Charcot-Marie-Tooth 2CC harboring a novel frameshift variant in NEFH

Charcot-Marie-Tooth (CMT) diseases are a clinically and genetically heterogeneous group of disorders. Different variants in the neurofilament heavy chain (NEFH) gene have been described to cause the CMT2CC subtype. Here we report the first Italian patient affected by CMT2CC, harboring a novel variant in NEFH. In describing our patient, we also reviewed previously CMT2CC individuals, and suggested to consider NEFH variant if patients have an axonal sensory-motor neuropathy with a prominent proximal muscles involvement with early requirement of walking aids or wheelchair, remembering a motor neuron disorder.

Intermediate filaments as effectors of differentiation

After the initial discovery of intermediate filament (IF)-forming proteins in 1968, a decade would elapse before they were revealed to comprise a diverse group of proteins which undergo tissue-, developmental stage-, differentiation-, and context-dependent regulation. Our appreciation for just how large (n = 70), conserved, complex, and dynamic IF genes and proteins are became even sharper upon completion of the human genome project. While there has been extraordinary progress in understanding the multimodal roles of IFs in cells and tissues, even revealing them as direct causative agents in a broad array of human genetic disorders, the link between individual IFs and cell differentiation has remained elusive. Here, we review evidence that demonstrates a role for IFs in lineage determination, cell differentiation, and tissue homeostasis. A major theme in this review is the function of IFs as sensors and transducers of mechanical forces, intersecting microenvironmental cues and fundamental processes through cellular redox balance.