The proper functioning of the nervous system depends on precise interconnections of distinct types of neurons. Therefore, understanding how neurons specifically find and recognize their target cells is a central question in neuroscience. Involvement of specific molecular cues expressed on target cells has long been suggested. However, the nature of such cues remains poorly understood. To comprehensively address this problem, collaborating with Prof. Hiroyuki Aburatani (Research Center of Advanced Science and Technology, University of Tokyo), we conducted single-cell microarray analysis of target muscles in Drosophila (the insect fruitfly), which are innervated by distinct motor neurons. By this new method, we identified a secreted protein, Wnt4, as a novel target cue that regulates synaptic specificity in a negative manner. The results provide evidence that synaptic target specificity is generated by a repulsive signal coming from non-target cells.
The proper functioning of the nervous system depends on precise interconnections of distinct types of neurons. Specific wiring emerges during development through a series of neuronal recognition events. Neurons first recognize specific pathways and extend their axons toward the target region. They then specifically recognize their synaptic partners from an array of potential target cells and form synapses. This final matching of pre- and postsynaptic cells is thought to be mediated by specific molecular cues expressed on the target cells. While previous studies demonstrated essential roles of several target-derived attractive cues, less is known about the role of repulsion by non-target cells. In collaboration with Prof. Hiroyuki Aburatani (Research Center of Advanced Science and Technology, University of Tokyo), we conducted single-cell microarray analysis of two neighboring muscles (called M12 and M13) in Drosophila, which are innervated by distinct motor neurons, by directly isolating them from dissected embryos. We identified a number of potential target cues that are differentially expressed between the two muscles, including M13-enriched Wnt4, a secreted protein of the Wnt family. When the function of Wnt4 was inhibited, motor neurons that normally connect with M12 formed smaller synapses on M12 but instead, inappropriately connected with M13 (Figure 1). Conversely, forced expression of Wnt4 in M12 inhibited synapse formation by these motor neurons. These results suggest that Wnt4 generates target specificity by preventing synapse formation on a non-target muscle (Figure 2). In collaboration with Prof. John Thomas and Dr. Shingo Yoshikawa (Salk Institute, USA), we also found that putative receptors Frizzled 2 and Derailed-2, and a cytoplasmic protein Dishevelled may mediate the Wnt4 signal in motor neurons. These results demonstrate an important role for local repulsion in regulating cell-to-cell target specificity.
This work has been published online in the August 30th issue of Current Biology.
Mikiko Inaki, Shingo Yoshikawa, John B. Thomas, Hiroyuki Aburatani and Akinao Nose. Wnt4 is a Local Repulsive Cue That Determines Synaptic Target Specificity. Current Biology, in press.
Contact: Department of Complexity Science and Engineering, Graduate School of Frontier Sciences, University of Tokyo
Figure 1. Abnormal axon targeting in Wnt4 mutants.Motor axon targeting in a control embryo (A) and in Wnt4 mutant (B). Motor neurons that normally target muscle 12 formed smaller synapses on muscle 12 (arrows) but instead formed ectopic synapses on muscle 13 (arrowheads).
Figure 2. A model of Wnt4 function.