Agrin

Agrin (AGRN) is a large heparan sulfate proteoglycan that functions as a critical organizer of neuromuscular and neuronal synapses. As a secreted or transmembrane protein, it nucleates acetylcholine receptor (AChR) clustering and postsynaptic differentiation through the AGRN-LRP4-MUSK signaling complex, and also modulates dendritic filopodia formation, calcium homeostasis, and neurite outgrowth in the central nervous system (UniProt: O00468). Alternative splicing of AGRN generates multiple isoforms with distinct activities; neuron-specific z+ isoforms are potent AChR activators, while the transmembrane form predominates in the brain and promotes synapse formation in hippocampal neurons.

Agrin is primarily expressed in neurons and at the neuromuscular junction, where it is essential for synaptic stability and plasticity. Mutations in AGRN cause congenital myasthenic syndrome type 8 (CMS8, MIM 615120), an autosomal recessive disorder of neuromuscular transmission (UniProt: O00468). The protein also participates in positive regulation of cartilage formation through alpha-dystroglycan signaling.

In Alzheimer's disease, agrin is significantly upregulated in post-mortem AD brain tissue relative to age-matched controls (mean log2 fold-change +0.82; Chaparral AD proteomics). This upregulation suggests a potential compensatory or pathological response to synaptic dysfunction in AD pathology, though the functional consequences remain to be clarified.