TRENDS in Neurosciences Vol.25 No.8 August 2002 http://tins.trends.com 0166-2236/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved. PII: S0166-2236(02)02206-3 423 Review In the early stages of the development of a nervous system, an orthogonal arrangement of axon tracts is established. In general, the longitudinal tracts develop at stereotyped positions along neuroepithelial cells or along glia derived from these cells, and the circumferential tracts develop across longitudinal rows of cells and often do not follow obvious cellular landmarks. At the molecular level, little is known about the extracellular cues that guide longitudinal axon migrations; however, several cues that direct circumferential migrations have been well characterized. The netrins form a family of phylogenetically conserved proteins that act as circumferential guidance cues [1–3]. The first netrin characterized, UNC-6, was identified as the protein encoded by the unc-6 gene of the nematode Caenorhabditis elegans [1]. Mutations in unc-6 cause defects in the circumferential guidance of migrating cells and axons [4]. In fact, some mutations specifically disrupt dorsal or ventral migrations, while others specifically disrupt circumferential cell or axon migrations [5,6]. Thus, in vivo the UNC-6 protein provides four distinct types of guidance information. The study of circumferential guidance in C. elegans and other model systems has led to the prediction that UNC-6/netrins are molecules that can form gradients in vivo [3,7]. Depending on the receptors that they bear, migrating cells and axons are either attracted or repelled from netrin sources (Fig. 1). UNC-5, a cell surface receptor of the immunoglobulin superfamily, is implicated in mediating repulsion [8–10], whereas UNC-40, a cell surface receptor of the same superfamily, is implicated in both attraction and repulsion from UNC-6/netrin sources [11–13]. In both vertebrates and invertebrates, ventral midline cells are a major source of netrin for early axon migrations [3,5,14–18]. Although the anatomical features of nervous systems differ, all ventral netrin expressing cells are similarly derived from the midline of the neural ectoderm [19]. Accordingly, early migrating cells and axons that have an attractive response to UNC-6/netrin are directed ventrally, whereas those that have a repulsive response are directed dorsally. The phylogenetic conservation of the UNC-6/netrin molecule and its receptors, the early ventral expression pattern, and the role of UNC-6/netrin in directing circumferential migrations both to and from the midline, are among the remarkable features of the UNC-6/netrin guidance system. The ability of guidance molecules to elicit attractive and repulsive responses from neuronal growth cones has been the subject of several reviews [7,20]. The focus of this review is the role that the C. elegans guidance molecule UNC-6 plays in directing the formation of a network of neural connections. Caenorhabditis elegans is an excellent model system to address in vivo guidance functions because its nervous system is relatively simple. In fact, each neuron and its connections are defined, and in many cases the precise temporal order in which the connections are made is known. Furthermore, the ability to perform genetic manipulations and to observe individual migrations in vivo in C. elegans allows migrations to be examined within the environment of a developing animal. UNC-6 Like most extracellular matrix and cell adhesion molecules, the UNC-6/netrin comprises multiple domains that are found in functionally divergent proteins. The UNC-6 VI and V domains are similar to the N-termini of subunits of laminin, which are phylogenetically conserved glycoprotein components How does an extracellular guidance molecule direct multiple grow th cones to different positions? The answer is important for understanding the development of complex neural connections. UNC-6 is a member of the netrin family of guidance proteins. It has phylogenetically conserved domains that mediate its different guidance and branching activities. In the Caenorhabditis elegans embryo, UNC-6 is secreted ventrally and a pattern of circumferential axon tracts develops as pioneer growth cones bearing UNC-5 and UNC-40 receptors are directed towards, or away from, the ventral sources. Following the first migrations, UNC-6 from additional sources allow s more complex migration patterns to emerge. In addition, at specific dorsoventral positions, locally restricted extracellular molecules alter growth cone responses to UNC-6, causing circumferentially migrating growth cones to turn and longitudinal nerves to develop. These observations show that extracellular guidance molecules can direct complex arrangements of migrating growth cones in vivo by eliciting different types of responses, by spatially and temporally regulating their expression and by working in concert w ith other extracellular molecules. M oving around in a w orm: netrin UNC-6 and circumferential axon guidance in C. elegans William G. Wadsworth William G. Wadsworth Dept of Pathology, Robert Wood Johnson Medical School, 675 Hoes Lane West, Piscataway, NJ 08854-5635, USA. e-mail: william.wadsworth@ umdnj.edu