Temporal identity in axonal target layer recognition

Wiring the brain In the developing brain, many different nerve cells are generated, each one forming specific synaptic connections. Somehow each growing axon or dendrite reaches its target then stops growing. The mechanisms that allow connecting neurons to recognize their synaptic partners are not well understood. Two papers in this issue report on this problem. Morey et al. dissect the interplay of positive and negative gene transcription regulators that orchestrates the synthesis of specific photosensitive pigments and axon guidance molecules in a single component of the Drosophila visual system, the R7 neuron. Surprisingly, they find that a key step in the regulation of R7 layer specificity is the repression of an alternative program of targeting to another layer, one innervated by a related R8 photoreceptor neuron. Again using the Drosophila visual system as model, Milan Petrovic and Thomas Hummel show that timing is key to a neuron finding its synaptic partner. Formation of layer-specific synaptic connection is controlled by a cell-intrinsic timing mechanism that causes distinct axons to stop in different N-cadherin expressing layers, based on when and for how long they express the transcription factor Sequoia.