Neurofilament phosphoforms: Surrogate markers for axonal injury, degeneration and loss
Introduction
The cytoskeleton of the axon is made of a scaffolding of proteins. Out of these, the most important are the neurofilaments. Neurofilaments are an architectonic amazingly stable tube system of about 10 nm diameter. This size places them intermediate between the smaller microfilaments (7 nm) and the larger microfilaments (≈ 25 nm). Nf are classified as type IV intermediate filaments [1] (Table 1).
To appreciate fully the structural achievement of this protein in cellular architecture, one must put the size of neurons and axons into relation. The neuronal cell-bodies (0.01 to 0.05 mm size) are connected by axons of over 1 m length (the axonal diameter is 1 to 25 μm). This is a factor of 100 000 difference. How does the neuron maintain this enormous structure, more importantly, how can the breakdown of the axon be detected and related to disease?
This review focuses on 4 parts; firstly the neurofilament (Nf) subunit synthesis, its assembly into a tubular structure and the post-translational modifications with particular reference to phosphorylation; secondly on methodological quantitative techniques; thirdly on clinico–pathological relationships related to Nf disassembly including accumulation, phosphorylation and disease; and finally the potential of Nf to serve as surrogate markers for axonal injury, disintegration and loss with particular reference to multiple sclerosis.
Section snippets
The neurofilament triplet protein
The dominant proteins of the axonal cytoskeleton are Nf. Nf are obligate heteropolymers that are composed of 3 subunits, a light (NfL), a medium (NfM) and a heavy (NfH) chain (these are reviewed in detail by Lee and Cleveland [3]). Nf are CNS cell-type specific proteins (Table 1) and qualify Nf as potential surrogate markers of damage to neuron and axon.
Methodological approaches
At present, three methodological approaches have been undertaken in an attempt to quantify Nf: (1) Western/immunoblotting techniques [66], [68], (2) a dot-blot immunoassay [69] and (3) ELISA techniques [31], [70], [71], [72].
Axonal pathology
Nf subunits are actively involved in the pathogenesis of axonal dysfunction and degeneration both as causative agents for disease and as markers for disease progression.
Nf as a surrogate marker for axonal injury in MS
The hypothesis underlying the measurement of Nf as a surrogate marker for axonal damage is outlined in Fig. 7. The following headings are arbitrary and only intended for didactic purposes.
Acknowledgements
I am grateful to Dr Geoffrey Keir and Professor Edward Thompson for comments on the manuscript.
Post-scriptum
Since submission of this review, another study on CSF NfH has been published: Lim ET et al. “Cerebrospinal fluid levels of brain specific proteins in optic neuritis”, Mult Scler 2004; 10: 261–265.
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