We have examined the role of UNC-6 in nervous system development using the SDQR and surrounding neurons. The SDQR is a particularly good marker to study the effects of proposed UNC-6 gradients because it is born late and its axon migrates late in the first larval stage, well after initial UNC-6 expression. The SDQR axon first migrates dorsally from its cell body at the ALMR tract to the dorsal sublateral nerve, where it turns and travels anteriorly. Growth cone guidance is often describe as an attraction or repulsion. For the SDQR a simple model would be that the growth cone is initially repelled dorsally by the increasing dorsal to ventral gradient of UNC-6 which was produced earlier by ventral epidermoblast cells. Upon reaching the sublateral nerve the axon fasciculates and is guided anteriorly by cues along the nerve. We have observed SDQR axon migrations in wild type, unc-6 null mutants, and in different strains where UNC-6 is ectopically expressed to study how the growth cone responds to guidance cues in vivo. The experiments show that the SDQR growth cone does not require a nerve tract to turn anteriorly, can be redirected by ectopic UNC-6, and can migrate towards or away from an UNC-6 source. From the migrational patterns which are observed it is difficult to explain the circumferential to longitudinal SDQR growth cone migration in terms of requiring fasciculation along an established nerve, encountering new guidance cues, or expressing new receptors en route. We propose that directional information for the migrating SDQR growth cone is derived from the concurrent interpretation of dorsoventral and anteroposterior cues. Migrations in unc-6 (-) animals show that opposing an UNC-6 mediated response are the influences of another cue(s) that mediates a ventral response. According to our model, during the initial SDQR circumferential migration, the cell!s navigational program induces a greater response to the UNC-6 dorsal cue and a balanced response to anterior and posterior cues. The SDQR growth cone migrates circumferentially until the dorsal and ventral mediated responses are balanced due to the changing concentrations of extracellular dorsal and ventral cue ligands. At the same time this change influences the response to anteroposterior cues. We suspect that the dorsal signal (UNC-6) favors a response towards posterior migrations and the ventral signal towards anterior migrations so that at the dorsal sublateral region the signal for an anterior directed migration predominates. In support of this notion, circumferential growth cone migrations of animals in the unc-6 (-) background are ventrally directed and will always turn anteriorly (never posteriorly) at incorrect dorsoventral locations. However, in the ectopic UNC-6 expressing animals where the growth cone may experience levels of UNC-6 that are higher than in wild type animals, there are dorsally oblique and sometimes short posterior migrations. SDQR is not driven to the dorsal cord by ectopic UNC-6, but instead is driven anteriorly at the dorsal sublateral region (even when the dorsal sublateral nerve is missing), presumably this is a response to the increasing ventral cue levels. Finally, coupled and competing responses may explain why in some cases SDQR can paradoxically migrate towards an UNC-6 source. In brief, the pathway of the SDQR growth cone may be determined by the initial expression of receptors. These receptors subsequently mediate responses that orient the growth cone relative to the pattern of extracellular guidance cues. Because direction information is integrated, the SDQR is directed along a multi-directional route. As opposed to relying on new directional instructions during its migration, the SDQR can determine its position relative to the extracellular cues like a traveler determining position to the points of a compass. Together a system of temporally and spatially regulated patterns of guidance cues and growth cones that orient relative to the patterns, suggests a general mechanism by which axons may be positioned during embryonic scaffold formation.

Affiliation:
- Department of Pathology Robert Wood Johnson Medical School Piscataway, NJ 08854