An architectural hallmark of neural circuits in specific brain areas of vertebrates and invertebrates is the organisation of their neuronal projections into parallel synaptic layers.
These serve as essential units for both development and function. However, our understanding of the molecular mechanisms that control the formation of layer-specific connections remains limited.
Within the adult visual system of Drosophila, the medulla is organised into 10 synaptic layers (M1-M10). The axons of the color-sensitive photoreceptor subtype R8 specifically terminate in layer M3. During early pupal development R8 axons are located in a temporary layer at the medulla neuropil border, where they pause for an extended period. During mid pupal development, they synchronously proceed to their final layer M3.
Our analysis of the Drosophila homolog of the receptor tyrosine kinase Anaplastic lymphoma kinase (Alk) and its activating secreted ligand Jelly belly (Jeb) identified their central role as an anterograde signaling pathway that controls targeting of R1-R6 axons within lamina columns and of R8 axons to their final medulla layer. R-cell axons release Jeb to activate Alk signaling in target neurons, and influence their environment through direct or indirect regulation of downstream guidance molecule expression to promote target recognition.
Our study of Netrins and their receptor Frazzled/DCC/Unc-40 provided insights into the role of interactions between afferent axons and intermediate target neurons in layer-specific targeting. We found that Netrins are locally released by axons of lamina neurons L3 and act as localised short-range chemoattractants to direct R8 axons, specifically expressing the Frazzled receptor, to their single positionally defined synaptic layer in the medulla.
Finally, ongoing work seeks to uncover the mechanisms that position R8 axons in their temporary layer, lead to their timely release and guide them to their final recipient layer. Furthermore to understand synaptic layer formation, it is essential to investigate both afferent axons and their target neurons. Using the Netrin/Frazzled guidance system as entry point, we are therefore currently addressing the question how layer-specific branch formation of partner neuron subtypes is achieved.