| The neuromuscular junction (NMJ) is characterized by extensive pre and postsynaptic differentiation that is essential for effective communication between the nerve and its target muscle cell. Establishment of these well-organized cellular domains is facilitated by neuronally secreted agrin, which activates the muscle specific kinase (MuSK) within the muscle membrane. MuSK activation triggers a signaling cascade whose chief effect is AChR clustering at sites directly opposite the nerve. Since agrin does not appear to bind MuSK directly, additional elements likely mediate its effect. The work presented here used a combination of biochemical, cell biological, genetic, and in vivo approaches to demonstrate that the extracellular matrix protein biglycan, a small leucine-rich repeat proteoglycan secreted by muscle at localized to the NMJ, plays important roles in both agrin-MuSK signaling and in maintenance of normal endplate architecture. This work shows that biglycan bi-directionally regulates agrin-induced AChR clustering and MuSK phosphorylation in cultured myotubes. Agrin-induced MuSK activation is compromised in biglycan null myotubes and AChR clusters in these cells are immature, but can be substantially rescind by the addition of purified biglycan. Further, biglycan binds to MuSK, specifically clusters this receptor in transfected heterologus cells, and may interact with a MuSK splice variant that is as yet largely uncharacterized. In vivo, nerve-muscle synapses of biglycan null mice exhibit abnormal functional folds, hyper-segmentation, and reduced MuSK expression. Finally, acetylcholinesterase and AChRs are focally misaligned at NMJs of biglycan mutant mice. Together, these results indicate that biglycan is an extracellular ligand for MuSK that regulates agrin signaling, MuSK localization, and synapse stability. These findings extend our understanding of NMJ synaptogenesis and raise new questions about biglycan-MuSK scaffolding to nucleate and stabilize the developing NMJ. |
