Postsynaptic filopodia in muscle cells interact with innervating motoneuron axons

Precise synaptogenesis is crucial to brain development, and depends on the ability of specific partner cells to locate and communicate with one another. Dynamic properties of axonal filopodia during synaptic targeting are well documented, but the cytomorphological dynamics of postsynaptic cells have received less attention. In Drosophila embryos, muscle cells bear numerous postsynaptic filopodia ('myopodia') during motoneuron targeting. Here we show that myopodia are actin-filled microprocesses, which progressively clustered at the site of motoneuron innervation while intermingling with presynaptic filopodia. In prospero mutants, which have severe delays in axon outgrowth from the CNS, myopodia were present initially but clustering behavior was not observed, demonstrating that clustering depends on innervating axons. Thus, postsynaptic filopodia are capable of intimate interaction with innervating presynaptic axons. We propose that, by contributing to direct long-distance cellular communication, they are dynamically involved in synaptic matchmaking.