Neurofilament

Neurofilaments are the 10 nanometer (10nm) intermediate filaments found specifically in neurons. They are a major component of the cell's cytoskeleton, and provide support for normal axonal radial growth (i.e. increases in axon's diameter). Neurofilaments are composed of polypeptide chains or subunits that are related structurally to the intermediate filaments of other tissues such as keratin subunits, which make 10nm filaments expressed specifically in epithelia. The family of proteins making intermediate filaments is divided into 5 major classes, the keratins forming the classes I and II. Class III contains the proteins Vimentin, Desmin, Peripherin and Glial Fibrillary Acidic Protein (GFAP). The major neurofilament subunits occupy the class IV family of intermediate filaments. Finally class V contains the nuclear lamins.

Subunit Proteins of Neurofilaments
The three major neurofilament subunits were discovered from studies of proteins transported down axons. Proteins are synthesized within the cell body, and hence they must travel along an axon to reach their final destination. The names given to the three major neurofilament subunits are based upon the apparent molecular weight of the mammalian subunits on SDS-PAGE:


 * the light or lowest (NF-L) runs at 68-70kDa


 * the medium or middle (NF-M) runs at about 145-160kDa


 * the heavy or highest (NF-H) runs at 200-220kDa

The SDS-PAGE molecular weights vary between mammalian species, with larger species usually having larger proteins. The real molecular weights of these proteins are considerably lower than the SDS-PAGE estimates, particularly in the case of NF-H and NF-M. This is due to the highly charged C-terminal regions of the molecules. Neurofilaments are found in vertebrate neurons in especially high concentrations along the axons, where they appear to regulate axonal diameter.

A fourth class IV subunit alpha-internexin, a.k.a. NF66, is found in association with NF-L, NF-M and NF-H in many situations and is expressed earlier in development than the other neurofilament proteins.

A fifth protein belonging to class IV, Nestin, is found in developing neurons and glia, and the presence of this protein is widely used to define neural stem cells. This protein is lost as development proceeds.

The class III intermediate filament protein subunit peripherin is found in neurofilaments along with the class IV subunits in a few neurons, mostly in the peripheral nervous system. Finally another class III intermediate filament subunit, vimentin, is found in developing neurons and a few very unusual neurons in the adult in association with class IV proteins, such as the horizontal neurons of the retina.

In the adult mammal neurofilament subunit proteins coassemble in vivo, forming a heteropolymer that contain NF-L or alpha-internexin plus NF-M or NF-H. Peripherin and vimentin may incorporate into neurofilaments along with these proteins. The NF-H and NF-M proteins have lengthy C-terminal tail domains that appear to control the spacing between neighboring filaments, generating aligned arrays with a fairly uniform interfilament spacing as seen in axons.

Growth
During axonal growth, new neurofilament subunits are incorporated all along the axon in a dynamic process that involves the addition of subunits along the filament length, as well as the addition of subunits at the filament ends.

After an axon has grown and connected with its target cell, the diameter of the axon may increase as much as fivefold.

Neurofilaments are able to determine the diameter of dendrites and axons because their polarity causes them to repel each other.

The level of neurofilament gene expression seems to directly control axonal diameter, which in turn controls how fast electrical signals travel down the axon.

Mutant mice with neurofilament abnormalies have phenotypes resembling amyotrophic lateral sclerosis.

Use in diagnostic pathology
Neurofilament, NF, immunostaining is common in diagnostic neuropathology. It is useful for differentiating neurons (positive for NF) from glia (negative for NF).