Neurofilament

Neurofilaments (NF) are classed as type IV intermediate filaments found in the cytoplasm of neurons.

Thus mammalian neurofilaments are heteropolymers of up to five different proteins: NF-L, NF-M, NF-H, α-internexin and peripherin.

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.

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.

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 of NF-M and NF-H are the longest and are modified extensively by post-translational modifications such as phosphorylation and glycosylation in vivo.

By electron microscopy, these domains appear as projections called sidearms that appear to contact neighboring filaments.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.

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.

[11] This is caused by an increase in the number of neurofilaments exported from the nerve cell body as well as a slowing of their rate of transport.

[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.

The type VI intermediate filament protein Nestin is expressed in developing neurons and glia.

Nestin is considered a marker of neuronal stem cells, and the presence of this protein is widely used to define neurogenesis.

There is also considerable clinical interest in the use of neurofilament proteins as biomarkers of axonal damage in diseases affecting the central nervous system.

Immunoassays of neurofilament proteins in cerebrospinal fluid and plasma can thus serve as indicators of axonal damage in neurological disorders.

Rat brain cells grown in tissue culture and stained, in green, with an antibody to neurofilament subunit NF-L, which reveals a large neuron. The culture was stained in red for α-internexin, which in this culture is found in neuronal stem cells surrounding the large neuron.
A formalin fixed and paraffin embedded section of human cerebellum stained with an antibody to neurofilament light, NF-L revealed with a brown dye, cell nuclei are revealed with a blue dye. Nuclear rich region at left is granular layer, region at right is molecular layer. The antibody binds processes of basket cells, parallel fiber axons, the perikarya of Purkinje cells and various other axons.
Antibody stain against neurofilament (green) and Ki 67 (red) in a mouse embryo 12.5 days after fertilization . The cells expressing neurofilaments are in the dorsal root ganglia shown in green while proliferating cells are in the ventricular zone in the neural tube and colored red.
Micrograph of white matter (bottom of image) and the anterior horn of the spinal cord showing motor neurons with central chromatolysis . Neurofilament immunostain .