Agrin
Encyclopedia
Agrin is a large proteoglycan
whose best characterised role is in the development of the neuromuscular junction
during embryogenesis
. Agrin is named based on its involvement in the aggregation of acetylcholine
receptors during synaptogenesis
. In humans, this protein is encoded by the AGRN gene.
This protein has nine domains homologous to protease inhibitors. It may also have functions in other tissues and during other stages of development. It is a major proteoglycan
component in the glomerular basement membrane and may play a role in the renal filtration and cell-matrix interactions.
(NMJ) is called the MuSK receptor (Muscle specific kinase). MuSK is a receptor tyrosine kinase - meaning that it induces cellular signaling by causing the addition of phosphate molecules to particular tyrosines on itself and on proteins which bind the cytoplasmic domain of the receptor.
In addition to MuSK, agrin binds several other proteins on the surface of muscle, including dystroglycan
and laminin
. Apparently these additional binding steps are required to stabilize the NMJ.
The requirement for Agrin & MuSK in the formation of the NMJ was primarily demonstrated by "knockout" mouse studies. In mice which are deficient for either protein, the neuromuscular junction does not form. Many other proteins also comprise the NMJ, and are required to maintain its integrity. For example,
MuSK also binds a protein called "dishevelled" (Dvl), which is in the Wnt signalling pathway. Dvl is additionally required for MuSK-mediated clustering of AChRs, since inhibition of Dvl blocks clustering.
The MuSK receptor apparently recruits casein kinase
2, which is required for clustering.
A protein called rapsyn is then recruited to the primary MuSK scaffold, to induce the additional clustering of acetylcholine receptors
(AChR). This is thought of as the secondary scaffold. A protein called Dok-7
has shown to be additionally required for the formation of the secondary scaffold; it is apparently recruited after MuSK phosphorylation and before acetylcholine receptors are clustered.
(HS) attachment sites within the primary structure of agrin but it is thought that only two of these actually carry HS chains when the protein is expressed.
In fact, one study concluded that at least two attachment sites were necessary by inducing synthetic agents. Since agrin fragments induce acetylcholine receptor aggregation as well as phosphorylation of the MuSK receptor, they spliced them and found that the variant did not trigger phosphorylation. It has also been shown that the G3 domain of agrin is very plastic, meaning it can discriminate between binding partners for a better fit.
Heparan sulfate glycosaminoglycans covalently linked to the agrin protein have been shown to play a role in the clustering of AChR. Interference in the correct formation of heparan sulfate through the addition of chlorate to skeletal muscle cell culture results in a decrease in the frequency of spontaneous acetylcholine receptor (AChR) clustering. It may be that rather than solely binding directly to the agrin protein core a number of components of the secondary scaffold may also interact with its heparan sulfate side chains.
A role in the retention of anionic macromolecules within the vasculature has also been suggested for agrin linked HS at the glomerular or alveolar
basement membrane
.
Proteoglycan
Proteoglycans are proteins that are heavily glycosylated. The basic proteoglycan unit consists of a "core protein" with one or more covalently attached glycosaminoglycan chain. The point of attachment is a Ser residue to which the glycosaminoglycan is joined through a tetrasaccharide bridge...
whose best characterised role is in the development of the neuromuscular junction
Neuromuscular junction
A neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract...
during embryogenesis
Embryogenesis
Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.Embryogenesis starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote...
. Agrin is named based on its involvement in the aggregation of acetylcholine
Acetylcholine
The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...
receptors during synaptogenesis
Synaptogenesis
Synaptogenesis is the formation of synapses. Although it occurs throughout a healthy person's lifespan, an explosion of synapse formation occurs during early brain development...
. In humans, this protein is encoded by the AGRN gene.
This protein has nine domains homologous to protease inhibitors. It may also have functions in other tissues and during other stages of development. It is a major proteoglycan
Proteoglycan
Proteoglycans are proteins that are heavily glycosylated. The basic proteoglycan unit consists of a "core protein" with one or more covalently attached glycosaminoglycan chain. The point of attachment is a Ser residue to which the glycosaminoglycan is joined through a tetrasaccharide bridge...
component in the glomerular basement membrane and may play a role in the renal filtration and cell-matrix interactions.
Mechanism of action
During development, the growing end of motor neuron axons secrete a protein called agrin. This protein binds to several receptors on the surface of skeletal muscle. The receptor which seems to be required for formation of the neuromuscular junctionNeuromuscular junction
A neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract...
(NMJ) is called the MuSK receptor (Muscle specific kinase). MuSK is a receptor tyrosine kinase - meaning that it induces cellular signaling by causing the addition of phosphate molecules to particular tyrosines on itself and on proteins which bind the cytoplasmic domain of the receptor.
In addition to MuSK, agrin binds several other proteins on the surface of muscle, including dystroglycan
Dystroglycan
Dystroglycan is a protein that in humans is encoded by the DAG1 gene.Dystroglycan is one of the dystrophin-associated glycoproteins, which is encoded by a 5.5 kb transcript in Homo sapiens by chromosome 3. There are two exons that are separated by a large intron...
and laminin
Laminin
Laminins are major proteins in the basal lamina , a protein network foundation for most cells and organs...
. Apparently these additional binding steps are required to stabilize the NMJ.
The requirement for Agrin & MuSK in the formation of the NMJ was primarily demonstrated by "knockout" mouse studies. In mice which are deficient for either protein, the neuromuscular junction does not form. Many other proteins also comprise the NMJ, and are required to maintain its integrity. For example,
MuSK also binds a protein called "dishevelled" (Dvl), which is in the Wnt signalling pathway. Dvl is additionally required for MuSK-mediated clustering of AChRs, since inhibition of Dvl blocks clustering.
Signaling
The nerve secretes agrin, resulting in phosphorylation of the MuSK receptor.The MuSK receptor apparently recruits casein kinase
Casein kinase
Casein kinase is a kinase enzyme.* Casein kinase 1* Casein kinase 2...
2, which is required for clustering.
A protein called rapsyn is then recruited to the primary MuSK scaffold, to induce the additional clustering of acetylcholine receptors
ACHR
ACHR may refer to:*American Convention on Human Rights*Asian Centre for Human Rights*Acetylcholine receptor...
(AChR). This is thought of as the secondary scaffold. A protein called Dok-7
Dok-7
Dok-7 is a non-catalytic cytoplasmic adaptor protein that is expressed specifically in muscle and is essential for the formation of neuromuscular synapses. Further, Dok-7 contains pleckstrin homology and phosphotyrosine-binding domains that are critical for Dok-7 function...
has shown to be additionally required for the formation of the secondary scaffold; it is apparently recruited after MuSK phosphorylation and before acetylcholine receptors are clustered.
Structure
There are three potential heparan sulfateHeparan sulfate
Heparan sulfate is a linear polysaccharide found in all animal tissues. It occurs as a proteoglycan in which two or three HS chains are attached in close proximity to cell surface or extracellular matrix proteins...
(HS) attachment sites within the primary structure of agrin but it is thought that only two of these actually carry HS chains when the protein is expressed.
In fact, one study concluded that at least two attachment sites were necessary by inducing synthetic agents. Since agrin fragments induce acetylcholine receptor aggregation as well as phosphorylation of the MuSK receptor, they spliced them and found that the variant did not trigger phosphorylation. It has also been shown that the G3 domain of agrin is very plastic, meaning it can discriminate between binding partners for a better fit.
Heparan sulfate glycosaminoglycans covalently linked to the agrin protein have been shown to play a role in the clustering of AChR. Interference in the correct formation of heparan sulfate through the addition of chlorate to skeletal muscle cell culture results in a decrease in the frequency of spontaneous acetylcholine receptor (AChR) clustering. It may be that rather than solely binding directly to the agrin protein core a number of components of the secondary scaffold may also interact with its heparan sulfate side chains.
A role in the retention of anionic macromolecules within the vasculature has also been suggested for agrin linked HS at the glomerular or alveolar
Pulmonary alveolus
An alveolus is an anatomical structure that has the form of a hollow cavity. Found in the lung parenchyma, the pulmonary alveoli are the dead ends of the respiratory tree, which outcrop from either alveolar sacs or alveolar ducts, which are both sites of gas exchange with the blood as well...
basement membrane
Basement membrane
The basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...
.