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 2 and isoform 3: these isoforms lacking the 'z' insert (z0) are muscle-specific, have no AChR clustering ability and may be involved in nervous system endothelial cell differentiation. 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 (By similarity). 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 (By similarity). 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. 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 is produced by alternative usage of the first exon. Expressed in central nervous system (CNS) synapses such as in the cerebral cortex and hippocampus. Localizes to basal lamina of hippocampal blood vessels. Both (z+) and (z-) isoforms found in kidney, heart and cerebral vasculature. All (z+), (z-), (y+) and (y-) isoforms present throughout muscle fiber basal laminae in neonatal animals. 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 (By similarity). Contains heparan and chondroitin sulfate chains and alpha-dystroglycan as well as N-linked and O-linked oligosaccharides. Heparin and heparin sulfate binding in the G3 domain 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 (By similarity). Glycosaminoglycans (GAGs), present in the 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. 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). Cleavage is developmentally regulated. In developing brain, neurotrypsin-mediated cleavage occurs mainly during late fetal days and in the first postnatal week. Z(-)/z(-) mice lacking the 'z' insert are stillborn or die immediately after birth. They did not inflate their lungs and were never seen to move spontaneously. An intramuscular nerve is formed and axons leave the nerve and branch but do not stop and arborize. AChR clusters were fewer in number, about 30% smaller in size and lower in density. Transgenic null (Tg/Agrn -/-) mice, exhibit atrophied muscle due to denervation and are smaller than normal littermates. There is impairment of locomotory behavior and half the mice die after 50 days. There is a greatly reduced number of synapses and about 30% loss of postsynaptic spines and a decrease in the length of dendrites in cortical neurons. Mice that have excessive neurotrypsin-mediated agrin cleavage, exhibit pathological symptoms characteristic of precocious sarcopenia, with fragmentation and disassembly of the neuromuscular junction (NMJ).