Wnts shape synaptic circuit through inhibiting synapse formation and determining the subcellular distribution of synapses.

The contribution of signals that negatively regulate synaptic development is not well understood. We examined synapse formation in the C. elegans cholinergic motor neuron DA9, whose en passant presynapses are restricted to a specific segment of its axon (red dots). We found that a signalling pathway composed of the Wnts lin-44 and egl-20, the Wnt receptor lin-17/Frizzled, and the cytoplasmic effector dsh-1/Dishevelled define the subcellular location of DA9 presynapses by inhibiting their assembly in regions of the axon proximal to the sources of Wnt. LIN-44/Wnt (organe gradient) is secreted from the tail hypodermis and localizes LIN-17/Frizzled (green dots) to a subdomain of the DA9 axon that is devoid of presynaptic specializations (Fig. 1-1). When this signalling pathway is compromised, DA9 synapses develop ectopically in the proximal subdomain. Conversely, the ectopic overexpression of LIN-44 in cells adjacent to DA9 is sufficient to expand LIN-17 localization in the DA9 axon and concomitantly displace the assembly of presynaptic terminals. These results demonstrate a novel role for Wnt signaling in inhibiting synapse formation and suggest that morphogenetic signals can spatially regulate the patterning of synaptic connections by subdividing regions of the axon into discrete domains1.

In order to understand how Wnt/Fz interaction inhibits the assembly of synapses, we tested downstream components of a variety of known Wnt pathways. With the exception of Dishevelled, which is universally required for all Wnt pathways, none of the other canonical Wnt components or the Planar Cell Polarity pathway components tested so far affected the distribution of synapses in DA9. We have also completed a forward visual genetic screen for mutants with a disrupted pattern of DA9 synapses, and isolated a number of lin-17/lin-44 like mutants. We are focusing on one of the mutants, which showed a lin-44-like synaptic vesicle phenotype. Importantly, Fz/LIN-17 localization in DA9 is normal in this mutant, suggesting that this molecule may be a downstream effector of Wnt signalling. Mapping and cloning of these mutants will likely elucidate molecular mechanisms of Wnt-mediated synaptic inhibition.

Fig. 1-1 Wnt/LIN-44 localizes Fz/Lin-17 to a subdomain of DA9 and locally inhibits synapse formation.