Histone H1 subtype synthesis in neurons and neuroblasts

Rat cerebral cortex neurons contain the five histone HI subtypes Hla-e and the subtype HIO present in other mammalian somatic tissues. The four subtypes Hla-d decay exponentially during postnatal development and are partially or totally replaced by Hle that becomes the major Bl subtype in adults. Hl0 accumulates in a period restricted to neuronal terminal differentiation. Here we study the synthesis of the Bl subtypes in cortical neurons and their neuroblasts by in vivo labeling with [14C]lysine. The subtype synthesis pattern of neuroblasts has been determined by labeling gravid rats during the period of proliferation of cortical neurons and synthesis in neurons has been studied by postnatal labeling. The subtype Hla is synthesized in neuroblasts but not in neurons and is therefore rapidly removed from neuronal chromatin. The synthesis of Hlb and Hld is much lower in neurons than in neuroblasts so that these subtypes are replaced to a large extent during postnatal development. Ole is synthesized at levels much higher than the other subtypes both in neurons and neuroblasts, but its very high turnover, about one order of magnitude faster than that of Hle in neurons, favors its partial replacement during postnatal development. Comparison of the synthesis rates of Hl0 in newborn and 30-day-old rats shows that the accumulation of HIO in differentiating neurons is due to an increased level of synthesis.