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. Structure 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. A role in the retention of anionic macromolecules within the vasculature has also been suggested for agrin-linked HS at the glomerular or alveolar basement membrane. Functions Agrin may play an important role in the basement membrane of the microvasculature as well as in synaptic plasticity. Also, agrin may be involved in blood–brain barrier (BBB) formation and/or function and it influences Aβ homeostasis. Research Agrin is investigated in relation with osteoarthritis. In addition, by its ability to activate the Hippo signaling pathway, agrin is emerging as a key proteoglycan in the tumor microenvironment. Clinical significance AGRN gene mutation leads to congenital myasthenic syndromes and myasthenia gravis. A recent genome-wide association study (GWAS) has found that genetic variations in AGRN are associated with late-onset sporadic Alzheimer’s disease (LOAD). These genetic variations alter β-amyloid homeostasis contributing to its accumulation and plaque formation. References Further reading d External links Developmental neuroscience Molecular neuroscience Extracellular matrix proteins Proteoglycans