Delivering the Wnt Signal

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Science Signaling  20 Oct 2009:
Vol. 2, Issue 93, pp. ec339
DOI: 10.1126/scisignal.293ec339

At the Drosophila neuromuscular junction (NMJ), a form of Wnt signaling occurs that involves Wnt-1 (Wg in flies) released from the presynaptic motor neuron interacting with the postsynaptic receptor Frizzled-2 (dFz2), which is internalized and cleaved to release an intracellular fragment that translocates to the nucleus to regulate gene expression in the muscle. Internalization of dFz2 and intracellular signaling requires the protein dGRIP at the postsynaptic region of the muscle (see Zarnescu and Zinsmaier). This form of Wnt signaling is necessary for proper formation of synaptic boutons at the NMJ. Evenness Interrupted (Evi) is involved in Wnt secretion from epithelia, and Korkut et al. show that Evi is necessary for the transsynaptic vesicular transport of Wg from the motor neuron to the muscle and that Evi also functions within the postsynaptic muscle to properly localize dGRIP. The synaptic phenotypes of reduced numbers of synaptic boutons, increased numbers of not fully mature boutons, and misshapen boutons observed for the evi mutants resembled those of wg mutants. In evi mutants or in flies in which Evi was reduced only in motor neurons through RNA interference (RNAi), the abundance of Wg and Evi was reduced both in the presynaptic region and in the postsynaptic muscle. Expression of Evi only in the motor neuron restored Wg amounts in both pre- and postsynaptic regions. A green fluorescent protein (GFP)–tagged Evi expressed only in the motor neurons was detected both pre- and postsynaptically, consistent with transsynaptic transfer of Evi. When expressed in Schneider-2 (S2) cells, GFP-Evi was detected in both untransfected and transfected cells in the culture and in puncta in the medium. Electron microscopy revealed that Evi was present in various tubulovesicular compartments in the pre- and postsynaptic cells and that Evi was internalized into the postsynaptic cell. The decrease in bouton number required expression of Evi in both the motor neuron and the muscle, suggesting a muscle function distinct from transsynaptic delivery of Wg. Indeed, when Evi was knocked down in the muscle, internalization, movement of dFz2 away from the synaptic area, and nuclear localization of the cleaved dFz2 fragment were reduced, and dFz2 and Wg accumulated in the postsynaptic region. These phenotypes are similar to those associated with loss of dGRIP function, and in the muscles in which Evi was knocked down, the postsynaptic localization of dGRIP was impaired. Thus, Evi appears to have dual trafficking roles in Wnt signaling at the fly NMJ. In the motor neuron, Evi mediates the vesicular transsynaptic transport of Wg; in the muscle, Evi mediates the proper trafficking of the dGRIP to the postsynaptic region.

C. Korkut, B. Ataman, P. Ramachandran, J. Ashley, R. Barria, N. Gherbesi, V. Budnik, Trans-synaptic transmission of vesicular Wnt signals through Evi/Wntless. Cell 139, 393–404 (2009). [Online Journal]

D. C. Zarnescu, K. E. Zinsmaier, Ferrying Wingless across the synaptic cleft. Cell 139, 229–231 (2009). [Online Journal]

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