Ventral midline cells are required for the local control of commissural axon guidance in the mouse spinal cord

Specialized cells at the midline of the central nervous system have been im plicated in controlling axon projections in both invertebrates and vertebra tes. To address the requirement for ventral midline cells in providing cues to commissural axons in mice, we have analyzed Gli2 mouse mutants, which l ack specifically the floor plate and immediately adjacent interneurons, We show that a Dbx1 enhancer drives tau-lacZ expression in a subpopulation of commissural axons and, using a reporter line generated from this construct, as well as DiI tracing, we find that commissural axons projected to the ve ntral midline in Gli2(-/-) embryos. Netrin1 mRNA expression was detected in Gli2(-/-) embryos and, although much weaker than in wild-type embryos, was found in a dorsally decreasing gradient. This result demonstrates that whi le the door plate can serve as a source of long-range cues for C-axons in v itro, it is not required in vivo for the guidance of commissural axone to t he ventral midline in the mouse spinal cord. lifter reaching the ventral mi dline, most commissural axons remained clustered in Gli2(-/-) embryos, alth ough some were able to extend longitudinally. Interestingly, some of the lo ngitudinally projecting axons in Gli2(-/-) embryos extended caudally and ot hers rostrally at the ventral midline, in contrast to normal embryos in whi ch virtually all commissural axons turn rostrally after crossing the midlin e, This finding indicates a critical role for ventral midline cells in regu lating the rostral polarity choice made by commissural axone after they cro ss the midline, In addition, we provide evidence that interactions between commissural axons and floor plate cells are required to modulate the locali zation of Nr-CAM and TAG-1 proteins on axons at the midline, Finally, we sh ow that the floor plate is not required for the early trajectory of motoneu rons or axons of the posterior commissure, whose projections are directed a way from the ventral midline in both WT and Gli2-/- embryos, although they are less well organized in Gli2(-/-) mutants.