Crystallographic structure of chicken agrin.
|Agrin NtA domain|
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.
Agrin was first identified by the U.J. McMahan laboratory, Stanford University.
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 that seems to be required for formation of the neuromuscular junction (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 that bind the cytoplasmic domain of the receptor.
The requirement for Agrin and MuSK in the formation of the NMJ was demonstrated primarily by knockout mouse studies. In mice that 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 nerve secretes agrin, resulting in phosphorylation of the MuSK receptor.
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.
There are three potential heparan sulfate (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 are necessary by inducing synthetic agents. Since agrin fragments induce acetylcholine receptor aggregation as well as phosphorylation of the MuSK receptor, researchers 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.
- PDB 1PZ7; Stetefeld, J., Alexandrescu, A.T., Maciejewski, M.W., Jenny, M., Rathgeb-Szabo, K., Schulthess, T., Landwehr, R., Frank, S., Ruegg, M.A., Kammerer, R.A. (2004). "Modulation of agrin function by alternative splicing and Ca2+ binding". Structure 12: 503–515. doi:10.2210/pdb1pz7/pdb. PMID 15016366.
- Rupp F, Payan DG, Magill-Solc C, Cowan DM, Scheller RH (May 1991). "Structure and expression of a rat agrin". Neuron 6 (5): 811–23. doi:10.1016/0896-6273(91)90177-2. PMID 1851019.
- Kröger S, Schröder JE (October 2002). "Agrin in the developing CNS: new roles for a synapse organizer". News Physiol. Sci. 17 (5): 207–12. doi:10.1152/nips.01390.2002 (inactive 2015-01-01). PMID 12270958. Vancouver style error (help)
- Groffen AJ, Buskens CA, van Kuppevelt TH, Veerkamp JH, Monnens LA, van den Heuvel LP (May 1998). "Primary structure and high expression of human agrin in basement membranes of adult lung and kidney". Eur. J. Biochem. 254 (1): 123–8. doi:10.1046/j.1432-1327.1998.2540123.x. PMID 9652404.
- Tsen G, Halfter W, Kröger S, Cole GJ (1995). "Agrin is a heparan sulfate proteoglycan". J Biol Chem 270 (7): 3392–3399. doi:10.1074/jbc.270.7.3392. PMID 7852425. Vancouver style error (help)
- Groffen AJ, Ruegg MA, Dijkman H, van de Velden TJ, Buskens CA, van den Born J et al. (1998). "Agrin is a major heparan sulfate proteoglycan in the human glomerular basement membrane". J Histochem Cytochem 46 (1): 19–27. doi:10.1177/002215549804600104. PMID 9405491.
- Magill C, Reist NE, Fallon JR, Nitkin RM, Wallace BG, McMahan UJ (1987). "Agrin". Prog. Brain Res. Progress in Brain Research 71: 391–6. doi:10.1016/S0079-6123(08)61840-3. ISBN 978-0-444-80814-1. PMID 3035610.
- Sanes JR, Lichtman JW (November 2001). "Induction, assembly, maturation and maintenance of a postsynaptic apparatus". Nat. Rev. Neurosci. 2 (11): 791–805. doi:10.1038/35097557. PMID 11715056.
- Glass DJ, Bowen DC, Stitt TN, Radziejewski C, Bruno J, Ryan TE et al. (May 1996). "Agrin acts via a MuSK receptor complex". Cell 85 (4): 513–23. doi:10.1016/S0092-8674(00)81252-0. PMID 8653787.
- Sanes JR, Apel ED, Gautam M, Glass D, Grady RM, Martin PT et al. (May 1998). "Agrin receptors at the skeletal neuromuscular junction". Ann. N. Y. Acad. Sci. 841: 1–13. doi:10.1111/j.1749-6632.1998.tb10905.x. PMID 9668217.
- Gautam M, Noakes PG, Moscoso L, Rupp F, Scheller RH, Merlie JP et al. (May 1996). "Defective neuromuscular synaptogenesis in agrin-deficient mutant mice". Cell 85 (4): 525–35. doi:10.1016/S0092-8674(00)81253-2. PMID 8653788.
- Cheusova T, Khan MA, Schubert SW, Gavin AC, Buchou T, Jacob G et al. (July 2006). "Casein kinase 2-dependent serine phosphorylation of MuSK regulates acetylcholine receptor aggregation at the neuromuscular junction". Genes Dev. 20 (13): 1800–16. doi:10.1101/gad.375206. PMC 1522076. PMID 16818610.
- McDonnell KM, Grow WA (2004). "Reduced glycosaminoglycan sulfation diminishes the agrin signal transduction pathway". Dev. Neurosci. 26 (1): 1–10. doi:10.1159/000080706. PMID 15509893.