Biglycan is an extracellular MuSK binding protein important for synapse stability

Area of Science:

• Neuroscience
• Molecular Biology
• Extracellular Matrix Biology

Background:

• The receptor tyrosine kinase MuSK is essential for neuromuscular junction (NMJ) formation and maintenance.
• While proteins like Dok7, LRP4, and Wnt11r are known to be involved in NMJ development, mechanisms regulating synapse stability remain unclear.

Purpose of the Study:

• To investigate the role of the extracellular matrix protein biglycan in the stability of nerve-muscle synapses.
• To elucidate the molecular mechanisms by which biglycan influences NMJ stability.

Main Methods:

• Analysis of neuromuscular junction morphology and protein localization in biglycan null (bgn(-/o)) mice compared to wild-type controls.
• Biochemical assays to assess biglycan binding to MuSK and its effect on MuSK levels at the synapse.
• In vitro studies using myotubes to examine agrin-induced acetylcholine receptor (AChR) clustering and the effect of biglycan supplementation.

Main Results:

• Biglycan null mice exhibit abnormal nerve-muscle synapses by 5 weeks of age, characterized by perijunctional folds, increased segmentation, and misalignment of synaptic components.
• Biglycan directly binds to MuSK, and its absence leads to reduced MuSK levels at the synapse.
• While initial agrin-induced AChR clustering is normal in biglycan-deficient myotubes, the clusters are unstable and can be rescued by adding purified biglycan.

Conclusions:

• Biglycan functions as an extracellular ligand for MuSK, playing a critical role in maintaining nerve-muscle synapse stability.
• Disruption of biglycan function leads to NMJ instability, likely through altered MuSK signaling and localization.