During development, the chemokine SDF1 guides migrating neurons to different targets. However, many distinct sources of SDF1 exist and it is unclear how neurons choose the correct one. Knaut and colleagues have found that zebrafish trigeminal sensory neurons follow a dynamic track of SDF1, which depends on a shift in the SDF1 expression domain towards the final destination and on the clearance of SDF1 transcript and protein at the back of the track (J. Cell Biol. 200, 337–355; 2013 ).

The authors first observed that these particular neurons follow a narrow band of SDF1 expression that shifts forwards in the direction of their migration. They found that switching on SDF1 expression at the front of the band guides the movement of neurons, whereas two mechanisms ensure the rapid clearance of SDF1 at the back. The SDF1 mRNA band is broadened in embryos lacking the miRNA-processing enzyme DICER, resulting in guidance defects. The authors show that miR-430 targets SDF1 for degradation to ensure clearance of SDF1 transcripts. Furthermore, cells surrounding the migrating neurons express a particular SDF1 receptor, CXCR7b, which acts as a sink for the SDF1 protein, therefore reducing SDF1 action on the CXCR4b receptors on migrating neurons. How the passage of the neurons triggers a shift in SDF1 expression at the front of the migrating cells and miR-430- and CXCR7b-mediated clearance of SDF1 at the back are questions for future studies.