# Mechanisms of neuromuscular junction formation

> **NIH NIH R01** · CASE WESTERN RESERVE UNIVERSITY · 2020 · $581,257

## Abstract

The neuromuscular junction (NMJ), a synapse formed between motoneurons and muscle fibers,
has contributed greatly to understanding of the general principles of synaptogenesis, as well as
of neuromuscular disorders. NMJ formation requires intimate interaction between motoneurons
and muscle fibers. For example, in antegrade signaling, motoneurons release agrin that binds
LRP4, a member of the LDL receptor family, in muscle cells to activate the receptor tyrosine
kinase MuSK, both of which are required for NMJ formation. Downstream of MuSK was not well
understood, except that AChR concentration absolutely requires the adapter protein rapsyn.
However, exactly how signals are transduced from MuSK activation to AChR concentration is not
well understood. On the other hand, skeletal muscles are known to be critical to the development
of axon terminals of motoneurons. In contrast to antegrade regulation, much less is understood
about molecular mechanisms of retrograde regulation of presynaptic differentiation by muscle
fibers. In preliminary studies, we discovered that the classic adaptor protein rapsyn is an E3
ligase. Knockin mice carrying the mutation of a single residue necessary for the enzymatic
activity are unable to form the NMJ. Our results suggest that rapsyn contributes to AChR
clustering by promoting neddylation. These observations identify a previously unappreciated
enzymatic activity of rapsyn and a role of neddylation in synapse formation. Our studies of LRP4
suggest that it could regulate developing axons by mechanisms independent of MuSK. The
proposal will test two hypotheses. First, MuSK increases rapsyn enzyme activity to promote
neddylation for AChR clustering and NMJ formation. Second, muscle regulates presynaptic
differentiation via LRP4. Results of this proposal will provide a better understanding of cellular as
well as molecular mechanisms of mammalian NMJ formation and contribute to a better
understanding of pathogenic mechanisms of neuromuscular disorders.

## Key facts

- **NIH application ID:** 9999676
- **Project number:** 5R01NS082007-09
- **Recipient organization:** CASE WESTERN RESERVE UNIVERSITY
- **Principal Investigator:** WEN-CHENG XIONG
- **Activity code:** R01 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2020
- **Award amount:** $581,257
- **Award type:** 5
- **Project period:** 2013-07-01 → 2023-06-30

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/9999676

## Citation

> US National Institutes of Health, RePORTER application 9999676, Mechanisms of neuromuscular junction formation (5R01NS082007-09). Retrieved via AI Analytics 2026-05-22 from https://api.ai-analytics.org/grant/nih/9999676. Licensed CC0.

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