Early sialylation on N-CAM in splotch neural tube defect mouse embryos.

The splotch (Sp) mutant mouse is a model for neural tube defects and Waardenburg syndrome type I. The neural tube defects that arise in Sp, which include spina bifida and exencephaly, are thought to be caused by a change in the timing of the cellular events which are taking place during neurulation. Cell adhesion molecules are strongly implicated in a variety of cell-cell interactions throughout development, thus the neural cell adhesion molecule (N-CAM) may play a role in neural tube formation and closure. The N-CAM in day 9 Sp embryos is altered showing a heavy 200 kD species rather than the 180 and 140 kD isoforms which are normally present at that developmental stage [Moase and Trasler (1991) Development 113:1049-1058]. These N-CAM isoforms normally become modified post-translationally by the addition of alpha-2,8 linked polysialosyl (PSA) units beginning at gestational day 11. Sp/Sp, Sp/+, and +/+ embryos were examined by Western blot analysis with an antibody (mAb 5A5) which specifically recognizes PSA residues on N-CAM. Mutant and heterozygote embryos display a sialylated N-CAM form at 20, 14, and 12 somite-stages which is absent in controls. Enzymatic removal of PSA on N-CAM resulted in a reduction of the 200 kD PSA-free N-CAM isoforms. These results in the observed 200 kD species, and suggest that the Sp gene is involved in the regulation of expression or the post-translational modification of N-CAM.