In vitro protein synthesis on free polyribosomes isolated from the developing mouse brain

Polysomes during early postnatal development of brain in the rat

We have attempted to show eventual modifications in the brain protein synthesis apparatus of rat during the first three weeks after birth. Through this time we noted a steady decrease (about 60%) in the free polysomes, when expressed relative to tissue weight. This decrease does not correlate with changes in the polysome profile, indicating that no loss in the efficiency of protein synthesis was involved. Translation in a reticulocyte lysate also failed to reveal differences.

Developmental studies of the first step of the initiation of brain protein synthesis, role for initiation factor 2

Developmental changes at the level of initiation step of translation in the rat brain were studied. The level of deacylated tRNAimet in rat brain was measured at two stages of postnatal development. Although the amount of tRNA was slightly lower in adult than in young (4 day old) rats, the charging capacity of initiator tRNAimet in vitro was similar at both ages. No differences during development were found in methionyl-tRNA synthetase activity, which throws doubt on its possible participation in regulation of the initiation step. When assayed in the ribosomal salt wash protein fractions, initiation factor 2 activity decreased during brain development, and increased activities were detected in the supernatant of the microsomal fractions. The decrease in eIF-2 activity paralleled the observed decrease in the rat of overall protein synthesis or initiation activity in vitro, suggesting that the regulation of the initiation step of translation during brain development may be tightly linked to changes in initiation factor 2 activity in brain tissue.

Cell-free synthesis of myelin basic proteins in normal and dysmyelinating mutant mice

Total polyribosomes were isolated from the brains of 16-20 day C57BL/6 mice, four neurological mutants (qk/qk, shi/shi, mld/mld, and jp/Y), and four heterozygote or littermate controls (qk/+, shil/+, mld, and jp littermates) and translated in a homologous, cell-free system. No differences were observed among the nine genotypes in either the yield of polysomes (32.2 +/- 0.6 A260/g brain) or in the incorporation of [35S]methionine into trichloroacetic acid-precipitable protein. However, when the four myelin basic proteins (BPs) were isolated from the translation mixtures little incorporation of [35S]methionine into the BPs was noted in those assays directed by polysomes from mld/mld or from shi/shi animals. Compared with C57BL/6 polysomes, mld littermate and shi/+ polysomes incorporated approximately half the levels of label into the four BPs while qk/+ and qk/qk incorporated normal and close-to-normal levels. Polysomes from jp littermates and jp/Y brains synthesized 66% and less than 15% of the levels of the 14K BP compared with C57BL/6 polysomes. Incorporation of label into the other three BPs was normal with jp littermate polysomes and about half the control levels with jp/Y polysomes. The data indicate that shi/shi and mld/mld mutants either produce altered BPs not recognized by our antibody or synthesize very low levels of BP. The data provide additional support for the notion that the qk/qk mutant synthesizes much higher levels of MBP than are incorporated into myelin. They also indicate that in the jimpy mutant the synthesis of the four BPs is affected to differing extents; thus, the mutant cannot be easily characterized as either an "assembly" or "synthesis" defect.

Effect of daily oral intake of manganese on free polysomal protein synthesis of rat brain

The effect of manganese on free polysomal protein synthesis of immature rat brain (3 weeks old) has been determined after 1, 2, 3, and 4 weeks of daily intake of 55 micrograms manganese/ml of drinking water. The protein synthesis was inhibited up to 35% during the first 3 weeks and returned toward the control level during the fourth week of treatment. Cross-incubation experiments with polysomes and pH 5 enzyme fractions indicated that the inhibition of protein synthesis is due to alteration of the pH 5 enzyme fraction. Furthermore, cerebral t-RNA content was reduced by 20% during the first 3 weeks and also returned to the control level after 4 weeks. The data suggest that the previously reported retardation in learning and memory of manganese treated immature rats may partly be due to alteration of cerebral RNA and protein synthesis. It was also evident that an adaptation mechanism to the observed effect of manganese developes after three weeks of daily intake of 55 micrograms manganese/ml.

Distribution and protein synthetic activities of neuronal free and membrane-bound ribosomes during postnatal development of rat cerebral cortex

The distribution and amino acid incorporative activities of free and membrane-bound ribosomes in neuronal perikarya of rat cerebral cortex at successive stages of postnatal development were investigated. The relative proportion of neuronal membrane-bound ribosomes increased significantly between 6 and 18 days of age, reaching 50% of the total ribosomal population around day 18. In contrast to the neuronal fraction, the membrane-bound ribosomes in unfractionated cerebral cortex decreased from 50% at birth to 35% in 18-day-old pups. When tested in a cell-free amino acid incorporation system the activities of both free and membrane-bound ribosomes increased up to day 10 post partum. However, whereas the activity of free ribosomes reached a constant level at this age, that of the membrane-bound fraction continued to rise until 18th day after birth. Crossover experiments employing homologous or heterologous cell sap fractions showed that the preparation from the 18-day-old animals was more efficient in supporting protein synthesis in vitro, if compared with similar preparation from the neonatal animals. This was attributed to an enhanced aminoacylation of transfer RNA in neuronal cell sap of older compared with younger rats. The observed developmental alterations in the distribution and activities of free and membrane-bound ribosomes are discussed in relation to changes that occur in morphology and function of brain during the early postnatal period.

Developmental changes in the composition of polyadenylated RNA isolated from free and membrane-bound polyribosomes of the rat forebrain, analysed by translation in vitro

Free and membrane-bound polyribosomes were isolated from the rat forebrain during its development. Polyadenylated RNA [poly(A)+ RNA] was isolated from both fractions, by using oligo(dT)-cellulose chromatography, and its composition studied by translating the poly(A)+ RNA in vitro in reticulocyte lysates. Electrophoretic analysis of the translation products showed that both free and membrane-bound polyribosomal poly(A)+ RNA gave many common components, but that there were also distinct differences in the protein composition of the products of the two fractions. Several proteins, of mol.wts. 39 000, 37 000, 31 000, 27 000 and 17 000, appeared to be products predominantly of free polyribosomal poly(A)+ RNA, whereas others, of mol.wt. 47 000, 33 000, 24 000 and 21 000 were specific to the membrane-bound polyribosomal poly(A)+ RNA fraction. More developmental changes were observed in the translational products of the membrane-bound poly(A)+ RNA fraction. Proteins of mol.wts. 33 000 and 21 000, which were predominant components of the translational products of this fraction when isolated from 10-day and older rats, were not present in translational products derived from preparations isolated from 3-day-old rats. The developmental appearance of these proteins as translational products of the membrane-bound poly(A)+ RNA suggests the appearance of new mRNA species. These transcriptional changes are discussed in relation to processes involved in brain differentiation, including myelination.

Age-dependent changes in brain protein synthesis in the rat

Brain protein synthesis was studied in vivo, in brain slices, and in cell-free systems in rats aged 1, 16, and 24 months. We observed a highly significant reduction in amino acid incorporation with advancing age. This reduction was observed in vivo, in slices, in postmitochondrial supernatant, microsomes, and membrane-bound polysomes. Free heavy polysomes showed no age-dependent decline but formed a smaller proportion of total ribosomes in older animals. These studies suggest that in the rat brain protein synthesis declines before senescence, possibly due to an impairment in the initiation process.

Thyroid hormone state and the incorporation of (14C) leucine by brain microsomes in developing rats

Cell-free incorporation of (14C) leucine into protein was 38% greater for cerebral cortical microsomes from 22-day old neonatally thyroidectomized rats compared to littermate controls. In contrast, incorporation by liver microsomes of hypothyroid rats was 33% lower compared to controls, confirming their deficient hormonal state. Incubation of cerebral microsomes from either hypothyroid or euthyroid rats with both homologous and heterologous pH 5 enzyme fractions clearly implicated the pH 5 fraction as the source of the apparent increase in protein synthetic capacity in the hypothyroid brain. Daily administration of L-thyroxine (20 microgram/100 g body wt) to hypothyroid animals between 22 and 25 days of age produced an additional increase in (14C) leucine incorporation into protein by cerebral microsomes, whereas the cell-free protein synthesis rate of euthyroid rats was unaffected by similar hormonal treatment. Liver preparations from both hypothyroid and euthyroid rats exhibited the expected increase in cell-free protein synthesis following thyroxine administration. The results support the hypothesis that the young hypothyroid brain exhibits delayed maturation and that thyroid hormones play a regulatory role in cerebral protein synthesis during a defined developmental period.

Fate of mRNA following disaggregation of brain polysomes after administration of (+)-lysergic acid diethylamide in vivo

Intravenous injection of (+)-lysergic acid diethylamide into young rabbits induced a transient brain-specific disaggregation of polysomes to monosomes. Investigation of the fate of mRNA revealed that brain poly(A+)mRNA was conserved. In particular, mRNA coding for brain-specific S100 protein was not degraded, nor was it released into free ribonucleoprotein particles. Following the (+)-lysergic acid diethylamide-induced disaggregation of polysomes, mRNA shifted from polysomes and accumulated on monosomes. Formation of a blocked monosome complex, which contained intact mRNA and 40-S plus 60-S ribosomal subunits but lacked nascent peptide chains, suggested that (+)-lysergic acid diethylamide inhibited brain protein synthesis at a specific stage of late initiation or early elongation.

Regulation of protein synthesis during postnatal maturation of mouse brain

On a DNA basis, there is higher concentration of polysomes in the brain of newborn than in the brain of adult mice, but there is no maturation-dependent decrease in tRNA content during postnatal development. The amino acid incorporating activity of cell-free systems with polysomes or mitochondria from newborn brain exceeds that of adult controls significantly in contrast to a smaller incorporating rate of labelled amino acids into synaptosomal protein. Addition of polysomes isolated from newborn brain increases the amino acid incorporation by cell-free systems with adult brain tRNA and enzymes, whereas the polysomes from adult brain decrease the incorporating activity of newborn brain systems. The loading capacity of newborn brain tRNA exceeds that of the adult controls and the velocities of its aminoacylation are four times faster. Uncharged as well as precharged newborn brain tRNA increases the amino acid incorporating activity of tRNA-dependent cell-free systems with adult brain polysomes and enzymes. In contrast to polysomes and tRNA, the newborn brain enzymes involved in protein synthesis seem to be less active in cell-free amino acid incorporation than the enzyme fractions from adult brain. These data indicate that the different protein synthesizing activity in developing and adult mouse brain is the result not only of higher amino acid incorporating activities of the newborn polysomes, but also of a stimulated acceptance and transfer function of the newborn brain tRNA.