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 pre-patterning 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 wks 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 pre- and post- synaptic 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.

MuSK IgG4 autoantibodies cause myasthenia gravis by inhibiting binding between MuSK and Lrp4

“…As such, antibodies that inhibit MuSK function would be expected to disrupt the architecture of the neuromuscular synapse as well as perturb neurotransmitter release and reception (9,18,19,34,(41)(42)(43)(44). Because the synaptic accumulation of acetylcholinesterase (AChE), like all other postsynaptic proteins, depends on MuSK (10,42), IgG4 antibodies to MuSK are likely to lower AChE expression at the synapse, which may explain the hypersensitivity of MuSK MG patients to AChE inhibitors.…”

Mechanisms controlling neuromuscular junction stability

“…When nerves enter transplanted muscles derived from mice lacking muscle-specific receptor tyrosine kinase (MuSK) or rapsyn, wild-type nerve terminals undergo continuous remodeling, suggesting that these muscle components are required to stabilize the immature contacts so that they can mature [60]. Interestingly, biglycans that act as ligands for Musk are proposed to stabilize synapses after P14 once they reach their mature configuration, although they are neither necessary for synapse formation in vivo, nor for the initial AChR clustering in vitro [61]. Thus, NM stabilization would involve specific cues and cellular signaling that are specific of distinct steps of synapses maturation.…”

The Neuromuscular Junction: Measuring Synapse Size, Fragmentation and Changes in Synaptic Protein Density Using Confocal Fluorescence Microscopy

“…Thresholding and synaptic areas measurements used Metamorph software by MM. 2 Imaged on a Leica DM IRE2 microscope and analyzed following current protocol by the indicated first author, blind to treatment group.…”

“…It is required for all voluntary movement. Fluorescence microscopy has long been used to study the effects of transgenes on the mouse NMJ [1][2][3] or to compare the effects of age, diet, exercise and disease upon rodent NMJs [4][5][6][7][8][9][10][11] . Such studies have taught us much about the physiology and pathophysiology of the NMJ, but the diverse parameters reported (e.g., AChR area, endplate area, perimeter length, fragmentation indices) often make it difficult to compare the findings of these studies.…”