Roundabout 2 regulates migration of sensory neurons by signaling in trans

Roundabout (Robo) receptors and their ligand Slit are important regulators of axon guidance and cell migration. The development of Drosophila embryonic sense organs provides a neuronal migration paradigm where the in vivo roles of Slit and Robo can be assayed using genetics.

Here we show that Slit-Robo signaling controls migration of Drosophila larval sensory neurons that are part of the Chordotonal (Cho) stretch receptor organs. We used live imaging to show that abdominal Cho organs normally migrate ventrally during development, whereas thoracic Cho organs do not. Robo2 overexpression in cis (in the sensory neurons) or in trans (on neighboring visceral mesoderm) transforms abdominal organs to a thoracic morphology and position by blocking migration, while loss of Slit-Robo signaling produces a reverse transformation in which thoracic organs migrate ectopically. Rescue and tissue-specific knockout experiments indicate that trans signaling by Robo2 contributes to the normal positioning of the thoracic Cho organs. The differential positioning of Cho organs between the thorax and abdomen is known to be regulated by Hox genes, and we show that the essential Hox cofactor Homothorax, represses Robo2 expression in the abdominal visceral mesoderm.

Our results suggest that segment-specific neuronal migration patterns are directed through a novel signaling complex (the "Slit sandwich") in which Robo2 on the thoracic visceral mesoderm binds to Slit and presents it to Robo receptors on Cho neurons. The differential positioning of Cho organs between thorax and abdomen may be determined by Hox gene-mediated repression of robo2.