Nervous system function relies on the complex branching of dendrites to cover receptive fields and process incoming stimuli. Dendrite formation begins with the development of a branchlet, a short actin-rich extension, and understanding how this structure forms is key to understanding dendrite morphology. Now, Gaia Tavosanis and colleagues uncover the molecular basis for dendrite branchlet formation in Drosophila. Focussing on the actin dynamics of ‘space-filling’ cIVda neurons, they demonstrate that RNAi knockdown of components of the actin nucleator complex Arp2/3 strongly reduces the number of dendrite branches. Supporting this, they use mosaic analysis to show that Arpc1-null neuron clones exhibit reduced dendrite branching, which can be rescued by Arpc1-GFP expression. The researchers also show that the Arp2/3 complex is locally recruited to sites about 1 min ahead of branchlet formation. They further determine that Arp2/3 activity is regulated by the cytoskeletal organiser WAVE downstream of the GTPase Rac1. Indeed, WAVE RNAi knockdown or Rac1 mutant clones have fewer dendrites, whereas ectopic activation of photoactivatable Rac1 causes rapid branchlet formation. Finally, the authors use electron microscopy to show that branched actin is a characteristic trait at dendrite branch sites. These data reveal that Arp2/3 acts downstream of WAVE and Rac1 to polymerise branched actin, which is required for early dendrite formation.
