Development of midbrain and anterior hindbrain ocular motoneurons in normal and Wnt-1 knockout mice.

The effect of homozygotic Wnt-1-/- mutations on the development of ocular motoneurons was examined with the lipophilic dye DiI and compared to control and phenotypic wild-type mouse embryos. A piece of DiI-soaked filter paper was inserted into the orbit, the midbrain, or rhombomere 5 of the hindbrain in six paraformaldehyde-fixed litters (10.5, 12.5, and 14.5 days postcoitum) containing Wnt-1, Wnt+/-, and Wnt-1+/+ individuals and three control litters. We labeled all ocular motoneurons retrogradely and all relevant nerves anterogradely in all control and phenotypic wild-type animals. In all phenotypically identified Wnt-1-/- mutants we could always label the abducens nerve and motoneurons and the optic fibers to the thalamus, but we were unable to label oculomotor or trochlear nerves or motoneurons. In addition to Wnt-1 knockout mutants, we also labeled mice from the WZT9B transgenic line carrying a lacZ reporter gene driven by the Wnt-1 gene enhancer. In these embryos we tested for co-localization of Wnt-1 expression in biotinylated dextran amine-labeled ocular motoneurons using a newly developed technique. In younger embryos we obtained evidence for co-localization of the beta-galactosidase reaction product derived from lacZ gene activity in some retrogradely filled oculomotor motoneurons and adjacent to other oculomotor and the trochlear motoneurons. Acetylcholine esterase, a marker of early differentiating cholinergic neurons, showed a similar topology with respect to the lacZ reaction product. Thus, at least some future oculomotor motoneurons express Wnt-1, whereas others and the trochlear motoneurons caudal to the ventral midbrain expression of Wnt-1 may be exposed to the short range diffusion of the Wnt-1 gene product. Thus, the Wnt-1-/- mutation precludes formation or survival of midbrain and anterior hindbrain neurons, including oculomotor and trochlear motoneurons.