Glial fibrillary acidic protein

Glial fibrillary acidic protein (GFAP) is a is_associated_with::protein that is encoded by the GFAP is_associated_with::gene in humans.

Glial fibrillary acidic protein is an is_associated_with::intermediate filament (IF) protein that is expressed by numerous cell types of the is_associated_with::central nervous system (CNS) including is_associated_with::astrocytes, and is_associated_with::ependymal cells. GFAP has also been found to be expressed in is_associated_with::glomeruli and peritubular fibroblasts taken from rat kidneys is_associated_with::Leydig cells of the testis in both hamsters and humans, human is_associated_with::keratinocytes, human is_associated_with::osteocytes and is_associated_with::chondrocytes and stellate cells of the pancreas and liver in rats. First described in 1971, GFAP is a type III IF protein that maps, in humans, to 17q21. It is closely related to its non-epithelial family members, is_associated_with::vimentin, is_associated_with::desmin, and is_associated_with::peripherin, which are all involved in the structure and function of the cell’s is_associated_with::cytoskeleton. GFAP is thought to help to maintain is_associated_with::astrocyte is_associated_with::mechanical strength, as well as the shape of cells but its exact function remains poorly understood, despite the number of studies using it as a cell marker. Glial fibrillary acidic protein (GFAP) was named and first isolated and characterized by Lawrence F. Eng in 1969.

Structure
Type III intermediate filaments contain three domains, named the head, rod and tail domains. The specific DNA sequence for the rod domain may differ between different type III intermediate filaments, but the structure of the protein is highly conserved. This rod domain coils around that of another filament to form a dimer, with the is_associated_with::N-terminal and is_associated_with::C-terminal of each filament aligned. Type III filaments such as GFAP are capable of forming both homodimers and heterodimers; GFAP can polymerize with other type III proteins or with is_associated_with::neurofilament protein (NF-L). Interestingly, GFAP and other type III IF proteins cannot assemble with is_associated_with::keratins, the type I and II is_associated_with::intermediate filaments: in cells that express both proteins, two separate intermediate filament networks form, which can allow for specialization and increased variability.

To form networks, the initial GFAP dimers combine to make staggered tetramers, which are the basic subunits of an is_associated_with::intermediate filament. Since rod domains alone in vitro do not form filaments, the non-helical head and tail domains are necessary for filament formation. The head and tail regions have greater variability of sequence and structure. In spite of this increased variability, the head of GFAP contains two conserved is_associated_with::arginines and an is_associated_with::aromatic residue that have been shown to be required for proper assembly.

Function in the central nervous system
GFAP is expressed in the is_associated_with::central nervous system in astrocyte cells. It is involved in many important CNS processes, including cell communication and the functioning of the is_associated_with::blood brain barrier.

GFAP has been shown to play a role in is_associated_with::mitosis by adjusting the filament network present in the cell. During mitosis, there is an increase in the amount of phosphorylated GFAP, and a movement of this modified protein to the cleavage furrow. There are different sets of kinases at work; is_associated_with::cdc2 is_associated_with::kinase acts only at the is_associated_with::G2 phase transition, while other GFAP kinases are active at the cleavage furrow alone. This specificity of location allows for precise regulation of GFAP distribution to the daughter cells. Studies have also shown that GFAP knockout mice undergo multiple degenerative processes including abnormal myelination, white matter structure deterioration, and functional/structural impairment of the is_associated_with::blood–brain barrier. These data suggest that GFAP is necessary for many critical roles in the CNS.

GFAP is proposed to play a role in astrocyte-neuron interactions as well as cell-cell communication. In vitro, using is_associated_with::antisense RNA, astrocytes lacking GFAP do not form the extensions usually present with neurons. Studies have also shown that is_associated_with::Purkinje cells in GFAP knockout mice do not exhibit normal structure, and these mice demonstrate deficits in conditioning experiments such as the eye-blink task. Biochemical studies of GFAP have shown MgCl2 and/or calcium/calmodulin dependent phosphorylation at various serine or threonine residues by PKC and PKA which are two kinases that are important for the cytoplasmic transduction of signals. These data highlight the importance of GFAP for cell-cell communication.

GFAP has also been shown to be important in repair after CNS injury. More specifically for its role in the formation of is_associated_with::glial scars in a multitude of locations throughout the CNS including the eye and brain.

Disease states
There are multiple disorders associated with improper GFAP regulation, and injury can cause glial cells to react in detrimental ways. is_associated_with::Glial scarring is a consequence of several neurodegenerative conditions, as well as injury that severs neural material. The scar is formed by astrocytes interacting with fibrous tissue to re-establish the glial margins around the central injury core and is partially caused by up-regulation of GFAP.

Another condition directly related to GFAP is is_associated_with::Alexander disease, a rare genetic disorder. Its symptoms include mental and physical retardation, dementia, enlargement of the brain and head, spasticity (stiffness of arms and/or legs), and seizures. The cellular mechanism of the disease is the presence of cytoplasmic accumulations containing GFAP and heat shock proteins, known as is_associated_with::Rosenthal fibers. Mutations in the coding region of GFAP have been shown to contribute to the accumulation of Rosenthal fibers. Some of these mutations have been proposed to be detrimental to cytoskeleton formation as well as an increase in is_associated_with::caspase 3 activity, which would lead to increased is_associated_with::apoptosis of cells with these mutations. GFAP therefore plays an important role in the pathogenesis of Alexander disease.

Notably, the expression of some GFAP isoforms have been reported to decrease in response to acute infection or is_associated_with::neurodegeneration. Additionally, reduction in GFAP expression has also been reported in is_associated_with::Wernicke's encephalopathy. The is_associated_with::HIV-1 viral envelope glycoprotein is_associated_with::gp120 can directly inhibit the phosphorylation of GFAP and GFAP levels can be decreased in response to chronic infection with HIV-1, is_associated_with::varicella zoster, and is_associated_with::pseudorabies. Decreases in GFAP expression have been reported in is_associated_with::Down's syndrome, is_associated_with::schizophrenia, is_associated_with::bipolar disorder and depression.

In a study of 22 child patients undergoing extra-corporeal membrane oxygenation (is_associated_with::ECMO), children with abnormally high levels of GFAP were 13 times more likely to die and 11 times more likely to suffer brain injury than children with normal GFAP levels. GFAP levels are already used as a marker of neurologic damage in adults who suffer is_associated_with::strokes and traumatic brain injuries.

Interactions
Glial fibrillary acidic protein has been shown to interact with is_associated_with::MEN1 and is_associated_with::PSEN1.

Isoforms
Although GFAP alpha is the only isoform which is able to assemble homomerically, GFAP has 8 different isoforms which label distinct subpopulations of astrocytes in the human and rodent brain. These isoforms include GFAP kappa, GFAP +1 and the currently best researched GFAP delta. GFAP delta appears to be linked with neural stem cells (NSCs) and may be involved in migration. GFAP+1 is an antibody which labels two isoforms. Although GFAP+1 positive astrocytes are supposedly not reactive astrocytes, they have a wide variety of morphologies including processes of up to 0.95mm (seen in the human brain). The expression of GFAP+1 positive astrocytes is linked with old age and the onset of AD pathology.