Steven J. Burden, PhD

Professor; Coord Molecular Neurobiology Prog. Departments of Biochemistry and Molecular Pharmacology and Cell Biology

Burden Lab

Biochemistry and Molecular Pharmacology, Cell Biology




Contact Information

540 First Avenue
Skirball Institute of Biomolecular Medicine
Floor 5, Lab 10-11
New York, NY 10016

Office Tel: 212-263-7341
Lab Tel: 212-263-7342
Fax: 212-263-8214

Admin Contact

Edna Normand
Tel: 212-263-6354

Development and Function of Neuromuscular Synapses

Synapse formation is a multi-step process requiring a reciprocal exchange of signals between presynaptic and postsynaptic cells, leading to a high concentration of acetylcholine receptors (AChRs) in the postsynaptic membrane and their perfect registration with active zones in the presynaptic nerve terminal, insuring for fast, robust and reliable synaptic transmission.

Motor axons approach muscles that are prepatterned, as AChR expression is enhanced in the central, prospective synaptic region of muscle, prior to innervation. Muscle prepatterning requires MuSK, a receptor tyrosine kinase, and Lrp4, a member of the LDLR family that associates with MuSK and stimulates MuSK. Upon reaching the muscle, motor axons release Agrin, which stabilizes postsynaptic differentiation by binding Lrp4 and stimulating further association between Lrp4 and MuSK.These genes play critical roles in synaptic differentiation: synapses do not form in their absence, and mutations in MuSK or downstream effectors are a cause of neuromuscular disease, termed congenital myasthenia. Moreover, auto-antibodies to MuSK, Lrp4 or AChRs are responsible for myasthenia gravis.

In contrast to our understanding of mechanisms for postsynaptic differentiation, discovery of the signals and mechanisms by which muscle cells control nerve terminal differentiation has proved far more challenging. Recently, we found that Lrp4 acts in a bidirectional manner, coordinating synaptic development, as Lrp4 not only binds Agrin and regulates postsynaptic differentiation but also functions in turn as a direct, muscle-derived retrograde signal for early steps in presynaptic differentiation, demonstrating a parsimonious means for mediating reciprocal signaling between adjacent cells. Our findings suggest that Lrp4 functions as a critical check-point at three steps during synapse formation: first, prior to innervation, Lrp4 forms a complex with MuSK to establish muscle prepatterning; second, as motor axons approach muscle, Lrp4, clustered as a consequence of MuSK activation, acts as a retrograde signal to promote their differentiation; third, once motor axons establish contact with muscle, Lrp4 binds Agrin, released from motor nerve terminals, stimulating further MuSK phosphorylation and stabilizing neuromuscular synapses.

The discovery of genes critical for forming and maintaining neuromuscular synapses has provided insight into the normal mechanisms for synapse formation and led to the identification of genes that are responsible for neuromuscular disease. Further, the molecules that control neuromuscular synapse formation are likely utilized in the central nervous system as well. Our lab uses molecular genetics, biochemistry and structural biology to understand how neuromuscular synapses form during development and how synapses are maintained and stabilized in adults.