Topographic restriction of TAG-1 expression in the developing retinotectalpathway and target dependent reexpression during axon regeneration

TAG-1, a glycosylphosphatidyl inositol (GPI)-anchored protein of the immuno globulin (Ig) superfamily, exhibits an unusual spatiotemporal expression pa ttern in the fish visual pathway. Using in situ hybridization and new antib odies (Abs) against fish TAG-1 we show that TAG-1 mRNA and anti-TAG-1 stain ing is restricted to nasal retinal ganglion cells (RGCs) in 24- to 72-h-old zebrafish embryos and in the adult, continuously growing goldfish retina. Anti-TAG-1 Abs selectively label nasal RGC axons in the nerve, optic tract, and tectum. Axotomized RGCs reexpress TAG-1, which occurs as late as 12 da ys after optic nerve lesion, when regenerating RGC axons arrive in the tect um, suggesting TAG-1 reexpression is target contact-dependent. Accordingly, TAG-1 reexpression ceases upon interruption of the regenerating projection by a second lesion. The topographic restriction of TAG-1 expression and it s target dependency during regeneration suggests that TAG-1 might play a ro le in the retinotopic organization and restoration of the retinotectal path way.