Neurofilament

Neurofilaments (NF) are classed as

Any proteinaceous filament that extends in the cytoplasm of a nerve cell is also termed a neurofibril.^[2] This name is used in the neurofibrillary tangles of some neurodegenerative diseases.

Neurofilament proteins

The protein composition of neurofilaments varies widely across different animal phyla. Most is known about mammalian neurofilaments. Historically, mammalian neurofilaments were originally thought to be composed of just three proteins called neurofilament protein NF-L (low molecular weight;

The triplet proteins are named based upon their relative size (low, medium, high). The apparent molecular mass of each protein determined by SDS-PAGE is greater than the mass predicted from the amino sequence. This is due to the anomalous electrophoretic migration of these proteins and is particularly extreme for neurofilament proteins NF-M and NF-H due to their high content of charged amino acids and extensive phosphorylation. All three neurofilament triplet proteins contain long stretches of polypeptide sequence rich in glutamic acid and lysine residues, and NF-M and especially NF-H also contain multiple tandemly repeated serine phosphorylation sites. These sites almost all contain the peptide lysine-serine-proline (KSP), and phosphorylation is normally found on axonal and not dendritic neurofilaments. Human NF-M has 13 of these KSP sites, while human NF-H is expressed from two alleles one of which produces 44 and the other 45 KSP repeats.

Neurofilament assembly and structure

Like other intermediate filament proteins, the neurofilament proteins all share a common central alpha helical region, known as the rod domain because of its rod-like tertiary structure, flanked by amino terminal and carboxy terminal domains that are largely unstructured. The rod domains of two neurofilament proteins dimerize to form an alpha-helical coiled coil. Two dimers associate in a staggered antiparallel manner to form a tetramer. This tetramer is believed to be the basic subunit (i.e. building block) of the neurofilament. Tetramer subunits associate side-to-side to form unit-length filaments, which then anneal end-to-end to form the mature neurofilament polymer, but the precise organization of these subunits within the polymer is not known, largely because of the heterogeneous protein composition and the inability to crystallize neurofilaments or neurofilament proteins. Structural models generally assume eight tetramers (32 neurofilament polypeptides) in a filament cross-section, but measurements of linear mass density suggest that this can vary.

The amino terminal domains of the neurofilament proteins contain numerous phosphorylation sites and appear to be important for subunit interactions during filament assembly. The carboxy terminal domains appear to be intrinsically disordered domains that lack alpha helix or beta sheet. The different sizes of the neurofilament proteins are largely due to differences in the length of the carboxy terminal domains. These domains are rich in acidic and basic amino acid residues. The carboxy terminal domains of NF-M and NF-H are the longest and are modified extensively by post-translational modifications such as

Neurofilaments are found in vertebrate neurons in especially high concentrations in axons, where they are all aligned in parallel along the long axis of the axon forming a continuously overlapping array. They have been proposed to function as space-filling structures that increase axonal diameter. Their contribution to axon diameter is determined by the number of neurofilaments in the axon and their packing density. The number of neurofilaments in the axon is thought to be determined by neurofilament gene expression^[7] and axonal transport. The packing density of the filaments is determined by their side-arms which define the spacing between neighboring filaments. Phosphorylation of the sidearms is thought to increase their extensibility, increasing the spacing between neighboring filaments^[8] by the binding of divalent cations between the sidearms of adjacent filaments^[9][10]

Early in development, axons are narrow processes that contain relatively few neurofilaments. Those axons that become myelinated accumulate more neurofilaments, which drives the expansion of their caliber. After an axon has grown and connected with its

In addition to their structural role in axons, neurofilaments are also cargoes of axonal transport.^[3] Most of the neurofilament proteins in axons are synthesized in the nerve cell body, where they rapidly assemble into neurofilament polymers within about 30 minutes.^[12] These assembled neurofilament polymers are transported along the axon on microtubule tracks powered by microtubule motor proteins.^[13] The filaments move bidirectionally, i.e. both towards the axon tip (anterograde) and towards the cell body (retrograde), but the net direction is anterograde. The filaments move at velocities of up to 8 μm/s on short time scales (seconds or minutes), with average velocities of approximately 1 μm/s.^[14] However, the average velocity on longer time scales (hours or days) is slow because the movements are very infrequent, consisting of brief sprints interrupted by long pauses.^[15][16] Thus on long time scales neurofilaments move in the slow component of axonal transport.

Clinical and research applications

Numerous specific

Neurofilament antibodies are also commonly used in diagnostic neuropathology. Staining with these antibodies can distinguish neurons (positive for neurofilament proteins) from glia (negative for neurofilament proteins).

There is also considerable clinical interest in the use of neurofilament proteins as

• Bielschowsky stain

References