Glial fibrillary acidic protein

Glial fibrillary acidic protein (GFAP) is an intermediate filament protein that is expressed by astrocytes- a major population of glial cells in the central nervous system. GFAP is also expressed by ependymal cells, and is present in many peripheral tissues. ref> GFAP is a type III intermediate filament protein that maps, in humans, to 17q21. It is closely related to vimentin, desmin, and peripherin, all of which are involved in the structure and function of the cell’s cytoskeleton.

Structure
Type III intermediate filaments have three domains, the most conserved of which is the rod domain. The DNA sequence for this region may differ between genes for different type III intermediate filament proteins, but the structure of the protein is highly conserved. The rod domain coils around that of another filament to form a dimer, with the N-terminal and C-terminal of each filament aligned. Type III filaments such as GFAP can form both homodimers and heterodimers; GFAP can polymerize with other type III proteins or with neurofilament protein (NF-L). GFAP and other type III IF proteins cannot assemble with keratins, the type I and II intermediate filaments. In cells that express both proteins, two separate intermediate filament networks form.

To form networks, the dimers combine to make staggered tetramers, which are the basic subunits of an intermediate filament. Since rods alone do not form filaments in vitro, it seems that the non-helical domains are needed for filament formation. The head and tail regions are more variable of sequence and structure. The head of GFAP contains two arginines and an aromatic residue that are needed for proper assembly. The sizes of the head and tail regions differ between GFAP and vimentin, which suggests that, when coassembled, they would align head-to-head rather than head-to-tail. This may enable more plastic functionality of the intermediate filament network.

Disease states
Glial scarring occurs in several neurodegenerative conditions, as well as after neural injury. The scar is formed by astrocytes interacting with fibrous tissue to re-establish the glia margins around the central tissue core and is associated with up-regulation of GFAP expression. The scar acts as a barrier to neuronal growth, and thus suppresses neural regeneration.