title: Biglycan is an extracellular MuSK binding protein important for synapse stability.
abstract: The receptor tyrosine kinase MuSK is indispensable for nerve-muscle synapse formation and maintenance. MuSK is necessary for prepatterning of the endplate zone anlage and as a signaling receptor for agrin-mediated postsynaptic differentiation. MuSK-associated proteins such as Dok7, LRP4, and Wnt11r are involved in these early events in neuromuscular junction formation. However, the mechanisms regulating synapse stability are poorly understood. Here we examine a novel role for the extracellular matrix protein biglycan in synapse stability. Synaptic development in fetal and early postnatal biglycan null (bgn(-/o)) muscle is indistinguishable from wild-type controls. However, by 5 weeks after birth, nerve-muscle synapses in bgn(-/o) mice are abnormal as judged by the presence of perijunctional folds, increased segmentation, and focal misalignment of acetylcholinesterase and AChRs. These observations indicate that previously occupied presynaptic and postsynaptic territory has been vacated. Biglycan binds MuSK and the levels of this receptor tyrosine kinase are selectively reduced at bgn(-/o) synapses. In bgn(-/o) myotubes, the initial stages of agrin-induced MuSK phosphorylation and AChR clustering are normal, but the AChR clusters are unstable. This stability defect can be substantially rescued by the addition of purified biglycan. Together, these results indicate that biglycan is an extracellular ligand for MuSK that is important for synapse stability.
authors.affiliation: Department of Neuroscience, Brown University, Providence, Rhode Island 02912, USA.
authors.forename: Alison R
authors.initials: AR
authors.lastname: Amenta
authors.forename: Hilliary E
authors.initials: HE
authors.lastname: Creely
authors.forename: Mary Lynn T
authors.initials: ML
authors.lastname: Mercado
authors.forename: Hiroki
authors.initials: H
authors.lastname: Hagiwara
authors.forename: Beth A
authors.initials: BA
authors.lastname: McKechnie
authors.forename: Beatrice E
authors.initials: BE
authors.lastname: Lechner
authors.forename: Susana G
authors.initials: SG
authors.lastname: Rossi
authors.forename: Qiang
authors.initials: Q
authors.lastname: Wang
authors.forename: Rick T
authors.initials: RT
authors.lastname: Owens
authors.forename: Emilio
authors.initials: E
authors.lastname: Marrero
authors.forename: Lin
authors.initials: L
authors.lastname: Mei
authors.forename: Werner
authors.initials: W
authors.lastname: Hoch
authors.forename: Marian F
authors.initials: MF
authors.lastname: Young
authors.forename: David J
authors.initials: DJ
authors.lastname: McQuillan
authors.forename: Richard L
authors.initials: RL
authors.lastname: Rotundo
authors.forename: Justin R
authors.initials: JR
authors.lastname: Fallon
mesh_terms: D000818:Animals; D058578:Biglycan / Q000737:chemistry / Q000378:metabolism*; D019556:COS Cells; D002454:Cell Differentiation / Q000502:physiology; D002478:Cells, Cultured; D002522:Chlorocebus aethiops; D045604:Extracellular Fluid / Q000737:chemistry / Q000378:metabolism*; D008297:Male; D051379:Mice; D008809:Mice, Inbred C3H; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D011485:Protein Binding / Q000502:physiology; D055550:Protein Stability; D020794:Receptor Protein-Tyrosine Kinases / Q000737:chemistry / Q000378:metabolism*; D013569:Synapses / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure
chemical_list: D058578:Biglycan; C511695:MuSK protein, mouse; D020794:Receptor Protein-Tyrosine Kinases