Expression of oligo/polyalpha2,8-linked deaminoneuraminic acid and megalin during kidney development and maturation: mutually exclusive distribution with polyalpha2,8-linked N-acetylneuraminic acid of N-CAM.

The expression of homopolymers of alpha2,8-linked deaminoneuraminic acid (oligo/polyalpha2,8-KDN) and of megalin during rat kidney development was investigated using immunocytochemistry and immunoblotting, and compared to homopolymers of alpha2,8-linked N-acetylneuraminic acid (polyalpha2,8-Neu5Ac) of the neural cell adhesion molecule (N-CAM). Both, oligo/polyalpha2,8-KDN and megalin were found in early proximal tubules of embryonic day 18 kidneys. In addition, megalin, but not oligo/polyalpha2,8-KDN, was detectable in late S-shaped bodies and early capillary loop stages. Until postnatal day 7, oligo/polyalpha2,8-KDN and megalin immunoreactivity was present, not only in convoluted but also in straight proximal tubules, and then restricted to the convoluted part as in adult kidney. Immunoblotting revealed increasing megalin expression until postnatal week 3 of kidney development, when the level corresponded to adult kidney. Combined immunoprecipitation/immunoblot analyses showed a steady level of oligo/polyalpha2,8-KDN on megalin throughout development. This was in striking contrast to the expression of polyalpha2,8-Neu5Ac and N-CAM, which was highest in early embryonic kidney, undetectable in kidneys of 3-week-old rats, and mutually exclusive with oligo/polyalpha2,8-KDN in its distribution. These findings demonstrated the coincidence of oligo/polyalpha2,8-KDN and megalin expression and the first appearance of proximal tubules, and revealed the high degree of specialization of the biosynthetic machinery for protein polysialylation in kidney.