Synthesis of soluble neuronal proteins in vivo. Age-dependent differences in the incorporation of leucine and phenylalanine

Abstract

[3H]Leucine and [3H]phenylalamine were injected intracerebrally into 8- and 18-day-old rats and, in addition, [3H]proline was injected into 8-day-old rats only. Neuronal perikarya were isolated from the cerebral cortex 15 min later and the rapidly labeled proteins recovered in the high-speed soluble supernatant fraction were analyzed by polyacrylamide disc gel electrophoresis.Using sodium dodecyl sulfate (SDS)-urea gels, significant differences were noted between the [3H]leucine and the [3H]phenylalanine-labeled proteins of the 8-day-old neuron, the former consisting largely of slow-moving, high molecular weight species, while the latter consisted of fast-moving, small molecular weight species. The pattern of the [3H]proline-labeled proteins conformed generally to that of [3H]-phenylalamine.The trend seen in the 8-day-old neurons was largely reserved in the 18-day-old neurons, demonstrating that the protein synthetic machinery of the cortical neuron undergoes qualitative, precursor-dependent adjustments during its development.Under the conditions of our experiments, no labeling of microtubule proteins was detected. Furthermore, no radioactivity migrating as microtubule protein was found in the vinblastine precipitate of the neuronal soluble proteins.Electrophoresis of a hypotonic extract of the neuronal soluble proteins revealed a number of additional radioactive proteins, presumed to be nascent polypeptides caught in the lumen of the cisternae of the endoplasmic reticulum, while en route to extraperikaryal destinations.