Short- and long-range roles of UNC-6/Netrin in dorsal-ventral axon guidance in vivo in Caenorhabditis elegans

Recent studies in vertebrates and Caenorhabditis elegans have reshaped models of how the axon guidance cue UNC-6/Netrin functions in dorsal-ventral axon guidance, which was traditionally thought to form a ventral-to-dorsal concentration gradient that was actively sensed by growing axons. In the vertebrate spinal cord, floorplate Netrin1 was shown to be largely dispensable for ventral commissural growth. Rather, short range interactions with Netrin1 on the ventricular zone radial glial stem cells was shown to guide ventral commissural axon growth. In C. elegans, analysis of dorsally-migrating growth cones during outgrowth has shown that growth cone polarity of filopodial extension is separable from the extent of growth cone protrusion. Growth cones are first polarized by UNC-6/Netrin, and subsequent regulation of protrusion by UNC-6/Netrin is based on this earlier-established polarity (the Polarity/Protrusion model). In both cases, short-range or even haptotactic mechanisms are invoked: in vertebrate spinal cord, interactions of growth cones with radial glia expressing Netrin-1; and in C. elegans, a potential close-range interaction that polarizes the growth cone. To explore potential short-range and long-range functions of UNC-6/Netrin, a potentially membrane-anchored transmembrane UNC-6 (UNC-6(TM)) was generated by genome editing. unc-6(tm) was hypomorphic for dorsal VD/DD axon pathfinding, indicating that it retained some unc-6 function. Polarity of VD growth cone filopodial protrusion was initially established in unc-6(tm), but was lost as the growth cones migrated away from the unc-6(tm) source in the ventral nerve cord. In contrast, ventral guidance of the AVM and PVM axons was equally severe in unc-6(tm) and unc-6(null). Together, these results suggest that unc-6(tm) retains short-range functions but lacks long-range functions. Finally, ectopic unc-6(+) expression from non-ventral sources could rescue dorsal and ventral guidance defects in unc-6(tm) and unc-6(null). Thus, a ventral directional source of UNC-6 was not required for dorsal-ventral axon guidance, and UNC-6 can act as a permissive, not instructive, cue for dorsal-ventral axon guidance. Possibly, UNC-6 is a permissive signal that activates cell-intrinsic polarity; or UNC-6 acts with another signal that is required in a directional manner. In either case, the role of UNC-6 is to polarize the pro-protrusive activity of UNC-40/DCC in the direction of outgrowth.