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 functions by activating the MuSK protein (for Muscle-Specific Kinase), which is a receptor tyrosine kinase required for the formation and maintenance of the neuromuscular junction. Agrin is required to activate MuSK, which is similarly also required for neuromuscular junction formation. Discovery Agrin was first identified by the U.J. McMahan laboratory, Stanford University. Mechanism of action During development in humans, the growing end of motor neuron axons secrete a protein called agrin. When secreted, agrin binds to several receptors on the surface of skeletal muscle. The receptor which appears to be required for the 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. In addition to MuSK, agrin binds several other proteins on the surface of muscle, including dystroglycan and laminin. It is seen that these additional binding steps are required to stabilize the NMJ. 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. Signaling The nerve secretes agrin, resulting in phosphorylation of the MuSK receptor. It seems that the MuSK receptor 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.