A number of transcription factors are implicated in various steps of neuronal differentiation, starting with the selection of neuronal precursors to the specification of neuronal subtypes

The directed outgrowth of neuronal processes is a crucial step in neuronal development because navigation errors will, eventually, lead to wiring defects in the nervous system. Different types of neurons typically send their axons along different pathways into distinct target areas. This is achieved, in part, by the differential expression of particular axonguidance receptors, which define the spectrum of guidance cues that can be sensed (reviewed by Chisholm and TessierLavigne, 1999; Grunwald and Klein, 2002; Wadsworth, 2002; Yu and Bargmann, 2001). Ectopic expression of guidance receptors, such as UNC-5, the receptor mediating repulsive responses to netrin, in cells that normally do not express the receptor leads to a redirection of their outgrowth away from the netrin source (Hamelin et al., 1993). A number of transcription factors are implicated in various steps of neuronal differentiation, starting with the selection of neuronal precursors to the specification of neuronal subtypes (Bertrand et al., 2002; Brunet and Pattyn, 2002; Dubois and Vincent, 2001; Goulding, 1998; Lee and Pfaff, 2001; Marquardt and Pfaff, 2001; Shirasaki and Pfaff, 2002). An extensively studied family that affects aspects of neuronal differentiation are the LIM-homeodomain transcription factors, which define motorneuron-subtype identities in a combinatorial fashion (reviewed by Jacob et al., 2001; Shirasaki and Pfaff, 2002). However, expression of LIMhomeodomain transcription factors is restricted to only a few subsets of neurons, so that LIM proteins alone cannot explain how hundreds of different neuronal subtypes are specified. A few other transcription factors are known to be involved in neuronal subtype specification (Brunet and Pattyn, 2002; Dubois and Vincent, 2001; Lee and Pfaff, 2001; Marquardt and Pfaff, 2001), but a direct link between these and genes involved in axonal pathfinding has been made rarely (Erkman et al., 2000). This leaves the problem of how a particular class of neurons express the appropriate set of axon-guidance receptors and signal-transduction components largely unresolved. Neurons and their processes can be visualized in vivo in C. eleganswith GFP markers (Chalfie et al., 1994). This allows new types of genetic screens, in which mutants with axon guidance defects can be isolated directly by selecting for animals with visible axon outgrowth defects in GFP-labelled neurons. We isolated mutants with defects in the outgrowth of interneuron axons in C. elegans(H.H., unpublished). Here we describe one of these mutants defining the gene zag-1. Mutants in zag-1 show characteristic defects in the navigation of interneuron axons, ranging from fasciculation defects in the ventral cord to completely misrouted axons that extend inappropriately along the side of the animal. Furthermore, zag1-mutant animals ectopically express the glr-1::GFP marker in additional neurons, indicating that the specification of neurotransmitter subtypes, in this case expression of the glutamate receptor glr-1, is compromised. Various classes of 3795 Development 130, 3795-3805 © 2003 The Company of Biologists Ltd doi:10.1242/dev.00570