Neuromuscular junctions (NMJs) are considered by many to be the business end of our nervous system. NMJs connect nerves with muscle cells, stimulating the muscle contractions that allow you to run from the bear that you just spotted glaring at you with a fork and knife in his paws. A recent paper adds to our understanding of the signaling on both sides of the NMJ.
The two sides of a NMJ, the presynaptic and postsynaptic structures, are highly coordinated for proper development and plasticity of the junction. As with many cell and developmental processes, this coordination relies on the Wnt signaling pathway. In fruit flies, Wnt/Wingless (Wg) functions both pre- and post-synaptically in larval muscle fibers, and results from a recent paper show how this bidirectional signaling is balanced and regulated. Kamimura and colleagues found that the gene trol, which encodes the protein perlecan (a secreted heparan sulfate proteoglycan, for those down with HSPGs), regulates Wg signaling in fruit fly NMJs. trol mutations causes postsynaptic defects and an overproduction of synaptic boutons, which are button-like presynaptic hotspots of neurotransmitter-containing vesicles. In the images above, normal (top) and trol mutant larval NMJs show immunostaining for a presynaptic marker (magenta) and a postsynaptic marker (green). In mutants, some synaptic boutons lack a postsynaptic structure nearby (“ghost boutons", arrowheads) while some NMJs showed an overproduction of small synaptic boutons (“satellite boutons”, arrow).
Kamimura, K., Ueno, K., Nakagawa, J., Hamada, R., Saitoe, M., & Maeda, N. (2013). Perlecan regulates bidirectional Wnt signaling at the Drosophila neuromuscular junction originally published in the Journal of Cell Biology, 200 (2), 219-233 DOI: 10.1083/jcb.201207036