The
unc-5 gene encodes a novel cell adhesion receptor of the immunoglobulin super family which is required to guide dorsal migrations of neuron growth cones on the epidermis (UNC-5 ;Neuron 4:61; Leung-Hagesteinj et al.,submitted).
unc-5 has been shown genetically to require
unc-6 ,which encodes a putative epidermal guidance cue, for its guidance functions (Neuron 4:61). The neurons mostly affected by
unc-5 mutations are the motor neurons DA, DB, DD, AS, and VD.
unc-5 has been shown to act cell autonomously in these cells (WBG 12[1]:32). We wanted to test whether
unc-5 expression in neurons that do not normally grow dorsally on the epidermis would suffice to steer them in a dorsal direction The six touch receptor neurons normally extend axons ventrally or longitudinally on the epidermis. These trajectories are not affected by
unc-5 mutations. The mec- 7 tubulin gene promoter has been shown to express lacZ specifically in the touch neurons (WBG 11[3]:40; Hamelin et al., in press). A fusion gene was made with the
mec-7 promoter and the entire
unc-5 coding region. This
mec-7 -
unc-5 construct was injected in the germline of wild type animals, along with a mec- 7-lacZ construct (WBG 11[3]:40), and pRF4 (WBG 11[1]:18). Transgenic animals were stained with X-Gal to reveal their touch neuron axons. Virtually all detectable axons (376/378) were found to be dorsally re-directed. The cell bodies were often displaced, mostly dorsally, but sometimes anteriorly (AVM, ALMs, PLMs) or posteriorly (PVM). Most transgenics were also touch insensitive. If UNC-5 induces dorsal trajectories in the touch neurons by utilizing its normal guidance functions, then these abnormal trajectories should depend on UNC-6 .In a null mutant of
unc-6 carrying the
mec-7 -
unc-5 transgene, the touch neuron axons were no longer found to be misguided Together, these results indicate that UNC-5 can act cell-autonomously, and is sufficient to steer dorsally the growth cones of neurons that normally don't.