Current application of neurofilaments in amyotrophic lateral sclerosis and future perspectives

Abstract

Motor neuron disease includes a heterogeneous group of relentless progressive neurological disorders defined and characterized by the degeneration of motor neurons. Amyotrophic lateral sclerosis is the most common and aggressive form of motor neuron disease with no effective treatment so far. Unfortunately, diagnostic and prognostic biomarkers are lacking in clinical practice. Neurofilaments are fundamental structural components of the axons and neurofilament light chain and phosphorylated neurofilament heavy chain can be measured in both cerebrospinal fluid and serum. Neurofilament light chain and phosphorylated neurofilament heavy chain levels are elevated in amyotrophic lateral sclerosis, reflecting the extensive damage of motor neurons and axons. Hence, neurofilaments are now increasingly recognized as the most promising candidate biomarker in amyotrophic lateral sclerosis. The potential usefulness of neurofilaments regards various aspects, including diagnosis, prognosis, patient stratification in clinical trials and evaluation of treatment response. In this review paper, we review the body of literature about neurofilaments measurement in amyotrophic lateral sclerosis. We also discuss the open issues concerning the use of neurofilaments clinical practice, as no overall guideline exists to date; finally, we address the most recent evidence and future perspectives.

Keywords: amyotrophic lateral sclerosis; biomarkers; motor neuron disease; neurofilament light chain; phosphorylated neurofilament heavy chain.

Figure 1

This transmission electronic microscope image shows a transverse section of a human motor axon. The image highlights the cytoskeletal scaffold, which is pivotal for both the structure and the function of the axon. The cytoskeleton of every eukaryotic cell is formed by three different structural components, from the larger to the smaller: microtubules, intermediate filaments and microfilaments (not visible in this image). Neurofilaments, the intermediate filaments of neurons, are visible as small dark dots, pointed out by arrows. Microtubules, the largest caliber constituents of the cytoskeleton, are visible as ring-like structures, indicated by arrowheads. Mitochondria, which generate the energy that allows the axonal transport along the cytoskeleton, are labeled with the letter “m”. The myelin sheath (darker and thicker) and the axolemma (thinner, in the inner side) are visible on the left. Sourced from the authors’ laboratory.

Figure 2

Overview of neurofilaments structure. (A) Neurofilament (NF) structure: Nfs are classified in neurofilament light chain (NF-L), neurofilament medium chain (NF-M), neurofilament heavy chain (NF-H) and α-internexin (α-int) according to the molecular mass of their subunits. All NF proteins have an N-terminal head domain, a central α-helical rod domain and a C-terminal tail domain. (B) Assembly of NFs: Monomer subunits form parallel dimers between subunit core domains. Two staggered, antiparallel dimers form tetramers and the lateral association of eight tetramers form cylindrical structures known as unit length filaments. The annealing of the unit length filaments forms long filaments which are further compacted to form mature neurofilaments. (C) NF kinetics in amyotrophic lateral sclerosis (ALS): During the pre-symptomatic stage, axonal damage takes place and NFs are released into the extracellular space and subsequently into cerebrospinal fluid (CSF) and blood. When ALS symptoms appear, large cell death causes extensive release of NF resulting in higher concentration of this biomarker in CSF and serum. NFs levels in the pre-symptomatic and disease phase are influenced by different factors as genetic and environmental factors, disease progression rate, age and ALS phenotype. Sourced from the authors’ laboratory.