Heparan sulfate basal lamina glycoprotein that plays a central role in the formation and the maintenance of the neuromuscular junction (NMJ) and directs key events in postsynaptic differentiation. This neuron-specific (z+) isoform is a component of the AGRN-LRP4 receptor complex that induces the phosphorylation and activation of MUSK. The activation of MUSK in myotubes induces the formation of NMJ by regulating different processes including the transcription of specific genes and the clustering of AChR in the postsynaptic membrane. Calcium ions are required for maximal AChR clustering. AGRN function in neurons is highly regulated by alternative splicing, glycan binding and proteolytic processing. Modulates calcium ion homeostasis in neurons, specifically by inducing an increase in cytoplasmic calcium ions. Functions differentially in the central nervous system (CNS) by inhibiting the alpha(3)-subtype of Na+/K+-ATPase and evoking depolarization at CNS synapses. This transmembrane agrin (TM-agrin) isoform, the predominate form in neurons of the brain, induces dendritic filopodia and synapse formation in mature hippocampal neurons in large part due to the attached glycosaminoglycan chains and the action of Rho-family GTPases. Isoform 1, isoform 4, isoform 5 and isoform 6: neuron-specific (z+) isoforms that contain C-terminal insertions of 8-19 AA are potent activators of AChR clustering. Isoform 5, agrin (z+8), containing the 8-AA insert, forms a receptor complex in myotubules containing the neuronal AGRN, the muscle-specific kinase MUSK and LRP4, a member of the LDL receptor family. The splicing factors, NOVA1 and NOVA2, regulate AGRN splicing and production of the 'z' isoforms. Is involved in regulation of neurite outgrowth probably due to the presence of the glycosaminoglcan (GAG) side chains of heparan and chondroitin sulfate attached to the Ser/Thr- and Gly/Ser-rich regions. Also involved in modulation of growth factor signaling (By similarity). This released fragment is important for agrin signaling and to exert a maximal dendritic filopodia-inducing effect. All 'z' splice variants (z+) of this fragment also show an increase in the number of filopodia. Monomer. Component of the AGRN-LRP4 complex that consists of a tetramer of two AGRN-LRP4 heterodimers. Interacts (via the laminin G-like 3 domain) directly with LRP4; the interaction is required for activation of MUSK and clustering of AChR and requires the 'z8' insert present in the z(+8) isoforms. Interacts (N-terminal subunit) with TGF-beta family members, BMP2 AND BMP4; the interactions inhibit the activity of these growth factors. Interacts with TGFB1; the interaction enhances the activity of TGFB1. Interacts with DAG1; the interaction is influenced by cell surface glycosaminoglycans and by alternative splicing of AGRN. Many isoforms exist depending on the occurrence and length of inserts at the x, y or z splice site. There are 4 'z' isoforms produced with inserts of 0, 8, 11 or 19 AA. Isoforms differ in their acetylcholine receptor clustering activity and tissue specificity. In addition, a secreted isoform may be produced by alternative usage of the first exon. Embryonic nervous system and muscle. More abundant early in development. At E13, highly expressed in the developing nervous system. Isoform y(+4)z(0), containing the 'y' insert but no 'z' insert, is the most prevelant at this stage with pronounced expression in developing spinal and sympathetic ganglia. Isoforms with no 'y' insert (y0) localized to peripheral tissue. At E15, y(+4) isoform continues to be highly expressed in neural tissue predominantly in the spinal column and developing brain. The y(0) isoform is weakly expressed in capillaries and meninges and the y(0)(z0) in non-neural tissues, predominantly in epithelial cells lining the developing lung bronchioles and kidney tubules. Isoforms Z(+8) and z(+19) are highly expressed in ventral motor columns and facial nerve with weaker expression throughout spinal cord tissue. At later stages of development, isoform y(4)z(0) is the most prominent form in developing cortex, corpus striatum, hippocampus and cerebellum. Isoform y(0)z(0) expression is still detected in brain capillaries at stage P1. The z(+19) isoform is most highly expressed from E15 to E18 and declines slightly to P1. Isoforms y(1)4z(0) and y(+4)z(+8) are still expressed in adulthood, the former scattered throughout the spinal cord gray matter and, the latter, in motor neurons of the ventral spinal cord. Both laminin G-like 2 (G2) and laminin G-like 3 (G3) domains are required for alpha-dystroglycan binding. G3 domain is required for C-terminal heparin, heparan sulfate and sialic acid binding. Contains heparan and chondroitin sulfate chains and alpha-dystroglycan as well as N-linked and O-linked oligosaccharides. Glycosaminoglycans (GAGs), present in the agrin N-terminal 110 kDa fragment, are required for induction of filopodia in hippocampal neurons. The first cluster (Gly/Ser-rich) for GAG attachment contains heparan sulfate (HS) chains and the second cluster (Ser/Thr-rich), contains chondroitin sulfate (CS) chains. Heparin and heparin sulfate binding in the G3 doamin is independent of calcium ions. Binds heparin with a stoichiometry of 2:1. Binds sialic acid with a stoichiometry of 1:1 and binding requires calcium ions. At synaptic junctions, cleaved at two conserved sites, alpha and beta, by neurotrypsin. Cleavage at the alpha-site produces the agrin N-terminal 110-kDa subunit and the agrin C-terminal 110-kDa subunit. Further cleavage of agrin C-terminal 110-kDa subunit at the beta site produces the C-terminal fragments, agrin C-terminal 90 kDa fragment and agrin C-terminal 22 kDa fragment. Excessive cleavage at the beta-site releases large amounts of the agrin C-terminal 22 kDa fragment leading to destabilization at the neuromuscular junction (NMJ).