The localization of lectin binding sites during photoreceptor synaptogenesis in the chick retina.

Carbohydrate-containing macromolecules have been localized in the outer plexiform layer of the embryonic and hatchling chick retina by horseradish peroxidase-conjugated wheat germ agglutinin (WGA) and Ricinus communis agglutinin (RCA) lectins. Both WGA and RCA binding sites are present along developing photoreceptor synaptic membranes in the embryonic retina and the plasma membranes of developing neurites and glia. After photoreceptor synapse formation, RCA staining is restricted to non-synaptic membranes, but WGA staining is present on the pre- and post-synaptic membranes of receptor ribbon synapses in addition to non-synaptic membranes. These differing results between the accessibility of RCA and WGA binding sites on mature synaptic membranes in the chick retina suggests that RCA receptors on synaptic membranes are somehow masked after synapse formation and maturation, but that WGA receptors remain accessible. The effects of enzymatic digestion on WGA and RCA binding has been studied after prior treatment with neuraminidase. RCA staining of developing synaptic and non-synaptic membranes in the embryo remains the same after treatment with the enzyme, but in the hatchling RCA staining of non-synaptic membranes is enhanced, which suggests that galactosyl residues are relatively exposed on immature membranes but inaccessible to the lectin on mature membranes until neuraminidase acts to expose them by removing the terminal sialic acid residues. WGA staining on developing synaptic and non-synaptic membranes in the embryo is greatly diminished after neuraminidase pretreatment which suggests that a considerable amount of staining at this time is due to sialic acid in addition to N-acetylglucosamine. In the hatchling, photoreceptor synaptic membranes are no longer labeled with WGA and non-synaptic membrane staining is reduced after neuraminidase digestion, which suggests that after synapse formation synaptic membrane WGA labeling is primarily to sialyl residues, whereas most of the non-synaptic labeling is to N-acetylglucosamine residues.