The developmental appearance of gangliosides in mammalian brain has been studied almost exclusively at postnatal stages. These studies show the presence at birth of at least four major gangliosides, with GMI (nomenclature of Svennerholm, 1963) t h e major monosialoganglioside (Suzuki, 1965;Vanier et al., 1971;Merat and Dickerson, 1973). In recent studies on fetal brain from rats (Berra et al., 1978) and mice (Rosner, 1977), the concentration of GMI has been reported to be relatively high at the earliest developmental stages studied, and to decline as development proceeds. However, our studies of ganglioside metabolism in retina and brain of the fetal rat have led us to believe that the dominant ganglioside at early stages in development is more likely Gnn than GMl.
Ganglioside content and pattern were analyzed in the rat hippocampus and in the retina and optic tectum of the chick embryo during periods of maximum neural differentiation. Allometric plots of ganglioside sialic acid against tissue dry weight were linear on a log-log scale, in either one (hippocampus and retina) or two (optic tectum) phases and at unique rates for each tissue. Ganglioside patterns changed consistently during development with decreases in D3 and increases in M1 and D1a. The ratio of D3 to M1 + D1a was found to be a simple quantitative predictor of the extent of differentiation (particularly synaptogenesis) in each tissue. However, a full complement of gangliosides was found prior to the onset of significant synapse formation.
Qualitative aspects of protein synthesis in organelles and intact cultured cells of brain origin were compared to clarify the distinction between synaptosomal and mitochondrial protein synthesis. Brain mitochondria and synaptosomes were isolated either on a traditional Ficoll-sucrose gradient or by a new Percoll gradient procedure, and were incubated in an amino acid incorporation system containing [35S]methionine, then electrophoresed on gradient slab gels. Autoradiography of the gels revealed that in the presence of cycloheximide both mitochondria and synaptosomes synthesized at least 17 proteins in the 6,000-50,000 MW range, and that incubation with chloramphenicol reduced or eliminated these bands. With minor variation these patterns in the low-molecular-weight region also resembled patterns obtained from cycloheximide-inhibited rat liver mitochondria and intact brain cells (cultured glia, glioma, and neuroblastoma). In the higher molecular weight region of the gels (greater than 50,000) banding patterns were more complex and tended to differ between organelles and intact cells. These polypeptides probably reflect nonmitochondrial protein synthesis, and their variable response to inhibitors may account for confusion in the literature with regard to the effects of inhibitors of protein synthesis in brain mitochondria and synaptosomes.
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