Glial fibrillary acidic protein (GFAP) is a protein that is encoded by the GFAP gene in humans. [5] It is a type III intermediate filament (IF) protein that is expressed by numerous cell types of the central nervous system (CNS), including astrocytes [6] and ependymal cells during development. [7] GFAP has also been found to be expressed in glomeruli and peritubular fibroblasts taken from rat kidneys, [8] Leydig cells of the testis in both hamsters [9] and humans, [10] human keratinocytes, [11] human osteocytes and chondrocytes [12] and stellate cells of the pancreas and liver in rats. [13]
GFAP is closely related to the other three non-epithelial type III IF family members, vimentin, desmin and peripherin, which are all involved in the structure and function of the cell's cytoskeleton. GFAP is thought to help to maintain astrocyte mechanical strength [14] 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. The protein was named and first isolated and characterized by Lawrence F. Eng in 1969. [15] In humans, it is located on the long arm of chromosome 17. [16]
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 N-terminal and 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. [17] 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, [18] which can allow for specialization and increased variability.
To form networks, the initial GFAP dimers combine to make staggered tetramers, [19] which are the basic subunits of an intermediate filament. Since rod domains alone in vitro do not form filaments, the non-helical head and tail domains are necessary for filament formation. [17] 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 arginines and an aromatic residue that have been shown to be required for proper assembly. [20]
GFAP is expressed in the central nervous system in astrocyte cells, and the concentration of GFAP differs between different regions in the CNS, where the highest levels are found in medulla oblongata, cervical spinal cord and hippocampus. [6] [21] [22] It is involved in many important CNS processes, including cell communication and the functioning of the blood brain barrier.
GFAP has been shown to play a role in 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. [23] There are different sets of kinases at work; cdc2 kinase acts only at the 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 blood–brain barrier. [24] 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 antisense RNA, astrocytes lacking GFAP do not form the extensions usually present with neurons. [25] Studies have also shown that 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. [26] Biochemical studies of GFAP have shown MgCl2 and/or calcium/calmodulin dependent phosphorylation at various serine or threonine residues by PKC and PKA [27] 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 glial scars in a multitude of locations throughout the CNS including the eye [28] and brain. [29]
In 2016 a CNS inflammatory disorder associated with anti-GFAP antibodies was described. Patients with autoimmune GFAP astrocytopathy developed meningoencephalomyelitis with inflammation of the meninges, the brain parenchyma, and the spinal cord. About one third of cases were associated with various cancers and many also expressed other CNS autoantibodies.
Meningoencephalitis is the predominant clinical presentation of autoimmune GFAP astrocytopathy in published case series. [30] It also can appear associated with encephalomyelitis and parkinsonism. [31]
There are multiple disorders associated with improper GFAP regulation, and injury can cause glial cells to react in detrimental ways. 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 [32] and is partially caused by up-regulation of GFAP. [33]
Another condition directly related to GFAP is 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. [34] The cellular mechanism of the disease is the presence of cytoplasmic accumulations containing GFAP and heat shock proteins, known as Rosenthal fibers. [35] Mutations in the coding region of GFAP have been shown to contribute to the accumulation of Rosenthal fibers. [36] Some of these mutations have been proposed to be detrimental to cytoskeleton formation as well as an increase in caspase 3 activity, [37] which would lead to increased 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 neurodegeneration. [38] Additionally, reduction in GFAP expression has also been reported in Wernicke's encephalopathy. [39] The HIV-1 viral envelope glycoprotein gp120 can directly inhibit the phosphorylation of GFAP and GFAP levels can be decreased in response to chronic infection with HIV-1, [40] varicella zoster, [41] and pseudorabies. [42] Decreases in GFAP expression have been reported in Down's syndrome, schizophrenia, bipolar disorder and depression. [38]
The generally high abundance of GFAP in the CNS has led to a great interest in GFAP as a blood biomarker of acute injury to the brain and spinal cord in different types of disease mechanisms, such as traumatic brain injury and cerebrovascular disease. [43] Elevated blood levels of GFAP are also found in neuroinflammatory diseases, such as multiple sclerosis and neuromyelitis optica, a disease targeting astrocytes. [43] In a study of 22 child patients undergoing extracorporeal membrane oxygenation (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. [44]
Glial fibrillary acidic protein has been shown to interact with MEN1 [45] and PSEN1. [46]
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 0.95 mm (seen in the human brain). The expression of GFAP+1 positive astrocytes is linked with old age and the onset of AD pathology. [47]
Intermediate filaments (IFs) are cytoskeletal structural components found in the cells of vertebrates, and many invertebrates. Homologues of the IF protein have been noted in an invertebrate, the cephalochordate Branchiostoma.
Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system and the peripheral nervous system that do not produce electrical impulses. The neuroglia make up more than one half the volume of neural tissue in the human body. They maintain homeostasis, form myelin in the peripheral nervous system, and provide support and protection for neurons. In the central nervous system, glial cells include oligodendrocytes, astrocytes, ependymal cells and microglia, and in the peripheral nervous system they include Schwann cells and satellite cells.
Alexander disease is a very rare autosomal dominant leukodystrophy, which are neurological conditions caused by anomalies in the myelin which protects nerve fibers in the brain. The most common type is the infantile form that usually begins during the first two years of life. Symptoms include mental and physical developmental delays, followed by the loss of developmental milestones, an abnormal increase in head size and seizures. The juvenile form of Alexander disease has an onset between the ages of 2 and 13 years. These children may have excessive vomiting, difficulty swallowing and speaking, poor coordination, and loss of motor control. Adult-onset forms of Alexander disease are less common. The symptoms sometimes mimic those of Parkinson's disease or multiple sclerosis, or may present primarily as a psychiatric disorder.
Astrocytes, also known collectively as astroglia, are characteristic star-shaped glial cells in the brain and spinal cord. They perform many functions, including biochemical control of endothelial cells that form the blood–brain barrier, provision of nutrients to the nervous tissue, maintenance of extracellular ion balance, regulation of cerebral blood flow, and a role in the repair and scarring process of the brain and spinal cord following infection and traumatic injuries. The proportion of astrocytes in the brain is not well defined; depending on the counting technique used, studies have found that the astrocyte proportion varies by region and ranges from 20% to around 40% of all glia. Another study reports that astrocytes are the most numerous cell type in the brain. Astrocytes are the major source of cholesterol in the central nervous system. Apolipoprotein E transports cholesterol from astrocytes to neurons and other glial cells, regulating cell signaling in the brain. Astrocytes in humans are more than twenty times larger than in rodent brains, and make contact with more than ten times the number of synapses.
Astrogliosis is an abnormal increase in the number of astrocytes due to the destruction of nearby neurons from central nervous system (CNS) trauma, infection, ischemia, stroke, autoimmune responses or neurodegenerative disease. In healthy neural tissue, astrocytes play critical roles in energy provision, regulation of blood flow, homeostasis of extracellular fluid, homeostasis of ions and transmitters, regulation of synapse function and synaptic remodeling. Astrogliosis changes the molecular expression and morphology of astrocytes, in response to infection for example, in severe cases causing glial scar formation that may inhibit axon regeneration.
The tau proteins form a group of six highly soluble protein isoforms produced by alternative splicing from the gene MAPT. They have roles primarily in maintaining the stability of microtubules in axons and are abundant in the neurons of the central nervous system (CNS), where the cerebral cortex has the highest abundance. They are less common elsewhere but are also expressed at very low levels in CNS astrocytes and oligodendrocytes.
Leukodystrophies are a group of, usually, inherited disorders, characterized by degeneration of the white matter in the brain. The word leukodystrophy comes from the Greek roots leuko, "white", dys, "abnormal" and troph, "growth". The leukodystrophies are caused by imperfect growth or development of the glial cells which produce the myelin sheath, the fatty insulating covering around nerve fibers. Leukodystrophies may be classified as hypomyelinating or demyelinating diseases, respectively, depending on whether the damage is present before birth or occurs after. While all leukodystrophies are the result of genetic mutations, other demyelinating disorders have an autoimmune, infectious, or metabolic etiology.
The rostral migratory stream (RMS) is a specialized migratory route found in the brain of some animals along which neuronal precursors that originated in the subventricular zone (SVZ) of the brain migrate to reach the main olfactory bulb (OB). The importance of the RMS lies in its ability to refine and even change an animal's sensitivity to smells, which explains its importance and larger size in the rodent brain as compared to the human brain, as our olfactory sense is not as developed. This pathway has been studied in the rodent, rabbit, and both the squirrel monkey and rhesus monkey. When the neurons reach the OB they differentiate into GABAergic interneurons as they are integrated into either the granule cell layer or periglomerular layer.
Gliosis is a nonspecific reactive change of glial cells in response to damage to the central nervous system (CNS). In most cases, gliosis involves the proliferation or hypertrophy of several different types of glial cells, including astrocytes, microglia, and oligodendrocytes. In its most extreme form, the proliferation associated with gliosis leads to the formation of a glial scar.
Glial cell line-derived neurotrophic factor (GDNF) is a protein that, in humans, is encoded by the GDNF gene. GDNF is a small protein that potently promotes the survival of many types of neurons. It signals through GFRα receptors, particularly GFRα1. It is also responsible for the determination of spermatogonia into primary spermatocytes, i.e. it is received by RET proto-oncogene (RET) and by forming gradient with SCF it divides the spermatogonia into two cells. As the result there is retention of spermatogonia and formation of spermatocyte.
Radial glial cells, or radial glial progenitor cells (RGPs), are bipolar-shaped progenitor cells that are responsible for producing all of the neurons in the cerebral cortex. RGPs also produce certain lineages of glia, including astrocytes and oligodendrocytes. Their cell bodies (somata) reside in the embryonic ventricular zone, which lies next to the developing ventricular system.
Nestin is a protein that in humans is encoded by the NES gene.
Pituicytes are glial cells of the posterior pituitary. Their main role is to assist in the storage and release of neurohypophysial hormones.
Satellite glial cells, formerly called amphicytes, are glial cells that cover the surface of neuron cell bodies in ganglia of the peripheral nervous system. Thus, they are found in sensory, sympathetic, and parasympathetic ganglia. Both satellite glial cells (SGCs) and Schwann cells are derived from the neural crest of the embryo during development. SGCs have been found to play a variety of roles, including control over the microenvironment of sympathetic ganglia. They are thought to have a similar role to astrocytes in the central nervous system (CNS). They supply nutrients to the surrounding neurons and also have some structural function. Satellite cells also act as protective, cushioning cells. Additionally, they express a variety of receptors that allow for a range of interactions with neuroactive chemicals. Many of these receptors and other ion channels have recently been implicated in health issues including chronic pain and herpes simplex. There is much more to be learned about these cells, and research surrounding additional properties and roles of the SGCs is ongoing.
A glial scar formation (gliosis) is a reactive cellular process involving astrogliosis that occurs after injury to the central nervous system. As with scarring in other organs and tissues, the glial scar is the body's mechanism to protect and begin the healing process in the nervous system.
S100 calcium-binding protein B (S100B) is a protein of the S100 protein family.
Excitatory amino acid transporter 2 (EAAT2) also known as solute carrier family 1 member 2 (SLC1A2) and glutamate transporter 1 (GLT-1) is a protein that in humans is encoded by the SLC1A2 gene. Alternatively spliced transcript variants of this gene have been described, but their full-length nature is not known.
Nuclear factor 1 X-type is a protein that in humans is encoded by the NFIX gene. NFI-X3, a splice variant of NFIX, regulates Glial fibrillary acidic protein and YKL-40 in astrocytes.
The Alzheimer type II astrocyte is thought to be a pathological type of cell in the brain; however, its exact pathology remains unknown. Like other astrocytes, it is a non-neuronal glial cell. It's mainly seen in diseases that cause increased levels of ammonia (hyperammonemia), such as chronic liver disease and Wilson's disease.
Autoimmune GFAP Astrocytopathy is an autoimmune disease in which the immune system of the patient attacks a protein of the nervous system called glial fibrillary acidic protein (GFAP). It was described in 2016 by researchers of the Mayo Clinic in the United States.