Migration of neuronal growth cones in the Drosophila visual system is a model system useful for understanding how axons seek their targets during development. Axons from certain photoreceptor cells in the fly's compound eye project specifically to the lamina region of the brain's optic lobe. In a genetic screen for mutations that disrupt this precision, Senti et al. identified a gene called brakeless (bks) that is necessary for these axons to terminate in the lamina. Without bks, axons fail to stop in the lamina and project onto the medulla. One of the two alternatively spliced isoforms of the gene product for bks has a zinc-finger-like domain, and both isoforms localized to cellular nuclei. Hence, in addition to target-derived cues that regulate axon outgrowth, intrinsic factors in the migrating neurons play a key role as well. It is not clear whether bks provides growth cones with the molecular machinery needed to respond to targeting signals, or acts in the nucleus to stop axon extension in response to these signals.
Senti, K.-A., Keleman, K., Eisenhaber, F., and Dickson, B.J. (2000) brakeless is required for lamina targeting of R1-R6 axons in the Drosophila visual system. Development 127: 2291-2301. [Online Journal]