Biosynthesis of gangliosides in the developing chick embryo retina

Chick embryo retinas were cultured with [³H]glucosamine on each day between days 6 and 12 of development. The total labeling of gangliosides decreased from day 6 to day 12. The decrease was mostly due to the decrease in the labeling of disialosyllactosylceramide (GD3), which diminished to less than 2%, while the labeling of the disialoganglioside GD1a decreased to about 30%. Labeling of endogenous gangliosides by incubating retina homogenates with CMP‐[³H]N‐acetylneuraminic acid, in addition to showing a decrease in the labeling of GD3, showed a twofold increase in the labeling of GD1a. The changes in the pattern of labeling correlated with the decrease of both the incorporation of [³H]thymidine into DNA and the activity of CMP‐NeuAc:GM3 sialosyltransferase and with the increase of both the activities of UDP‐Ga1NAc:GM3 N‐acetylgalactosaminyltransferase and choline acetyltransferase. The results suggest that the shift in the pattern of labeling of gangliosides occurs in association with the transition from the proliferative to the nonproliferative state of neuronal cells. Retinas at each age cultured with [³H]glucosamine for 22 hr maintain the radioactivity in gangliosides for at least 4 additional days in culture. This indicates that within this developmental interval there is not a period characterized by having a high turnover rate of gangliosides.