REGULATION OF GANGLIOSIDE COMPOSITION AND SYNTHESIS IS DIFFERENT IN DEVELOPING CHICK RETINAL-PIGMENT EPITHELIUM AND NEURAL RETINA

We examined the immunocytochemical expression of GM3 and GD3 in 3-day- old chick embryo retinal pigment epithelium (RPE) and neural retina (N R). We also compared the composition of gangliosides and the activitie s of key ganglioside glycosyltransferases of the RPE and NR of 8-, 12- , and 15-day old embryos. The immunocytochemical studies in 3-day-old embryos showed heavy expression of GM3 and GD3 at the inner and outer layers of the optic vesicle that are the precursors of the RPE and NR, respectively. The compositional and enzymatic studies showed pronounc ed differences between RPE and NR of 8-day and older embryos. HPTLC sh owed that at 8 days the major species were GM3 and GD3 in RPE and GD3 and GT3 in NR. As development proceeded, GD3 decreased in both tissues , GM3 became the major ganglioside in RPE, and ganglio-series ganglios ides (mainly GD1a) became the major species in NR. At 15 days the majo r species were GD1a in NR and GM3 in RPE. Enzyme determinations showed that whereas in RPE from 12-day-old embryos GM2 synthase was under th e limit of detection and GD3 synthase activity was about sixfold lower than GM3 synthase, in NR the activities of GM3 and GD3 synthases were similar and both six- to ninefold lower than GM2 synthase. These resu lts evidence a markedly different modulation of the ganglioside glycos ylating system in cells of a common origin that through distinct diffe rentiation pathways originate two closely related tissues of the optic system. In addition, they reinforce the relevance of the relative act ivities of key transferases in determining the pattern of gangliosides in different cell types.