Changes in polysomes of the developing rat brain

Requirement of Neuronal Ribosome Synthesis for Growth and Maintenance of the Dendritic Tree

The nucleolus serves as a principal site of ribosome biogenesis but is also implicated in various non-ribosomal functions, including negative regulation of the pro-apoptotic transcription factor p53. Although disruption of the nucleolus may trigger the p53-dependent neuronal death, neurotoxic consequences of a selective impairment of ribosome production are unclear. Here, we report that in rat forebrain neuronal maturation is associated with a remarkable expansion of ribosomes despite postnatal down-regulation of ribosomal biogenesis. In cultured rat hippocampal neurons, inhibition of the latter process by knockdowns of ribosomal proteins S6, S14, or L4 reduced ribosome content without disrupting nucleolar integrity, cell survival, and signaling responses to the neurotrophin brain-derived neurotrophic factor. Moreover, reduced general protein synthesis and/or formation of RNA stress granules suggested diminished ribosome recruitment to at least some mRNAs. Such a translational insufficiency was accompanied by impairment of brain-derived neurotrophic factor-mediated dendritic growth. Finally, RNA stress granules and smaller dendritic trees were also observed when ribosomal proteins were depleted from neurons with established dendrites. Thus, a robust ribosomal apparatus is required to carry out protein synthesis that supports dendritic growth and maintenance. Consequently, deficits of ribosomal biogenesis may disturb neurodevelopment by reducing neuronal connectivity. Finally, as stress granule formation and dendritic loss occur early in neurodegenerative diseases, disrupted homeostasis of ribosomes may initiate and/or amplify neurodegeneration-associated disconnection of neuronal circuitries.

Translational initiation factor eIF-2 subcellular levels and phosphorylation status in the developing rat brain

We have quantified the levels of the alpha subunit of initiation factor 2 (eIF-2) in the postmicrosomal supernatant and the ribosomal salt wash fractions from suckling and adult rat brain. The levels of eIF-2 in the ribosomal salt wash decrease in adult with respect to that present in suckling rat brain, but the total amount remains fairly constant, and a very close parallelism exists between the eIF-2 associated with ribosomes and RNA levels in the microsomal fraction in the two age groups. The phosphorylation state of eIF-2 alpha, as determined by isoelectric focusing followed by protein immunoblotting, in the same subcellular fractions, did not reveal the presence of the phosphorylated form in any of the fractions studied. These results suggest that phosphorylation of the alpha subunit is not implied in the regulation of protein synthesis initiation during brain development, and some other component regulates both the number of active ribosomes and eIF-2 levels in microsomes.

Sequence complexity of polyadenylate ribonucleic acid in El mouse brain

To elucidate the sequence complexity of polyadenylate messenger RNA [poly(A+)mRNA] regarding the seizure-susceptible El mouse brain, the proportion of complexity was measured by saturation hybridization using a single-copy DNA (scDNA) with excess RNA. At saturation, 3.81, 3.85 and 4.00% of scDNA hybridized with polysomal poly(A+)mRNAs of El(+), El(O) which had not been convulsed, and ddY (nonsusceptible) mice, respectively. The additive hybridization of a mixture of El(+) and ddY mouse mRNAs was 4.04%. The results suggest that there are considerable overlapping sets within the poly(A+)mRNA sequences between the ddY and El mice, but the complexity of ddY mice mRNA is slightly larger than that of the El mice, whereas poly(A+) heterogeneous nuclear RNA (hnRNA) hybridized with about 18% of scDNA in three strains of mice.

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.

Brain protein changes during development and sexual differentiation in the rat

Subcellular fractions were prepared from the hypothalamus-preoptic area and the 'remainder of the brain' of intact male and female rats at 0, 8, 25 and 72 days of age. Proteins associated with each fraction were subjected to SDS-PAGE chromatography and stained with Coomassie Brilliant Blue. Developmental changes were found to occur in proteins associated with the soluble (14,600, 15,000, 29,900, 38,900 and 49,000 mol. wt), nuclear (40,000-50,000 and 13,800-16,000 mol. wt.), mitochondrial-lysosomal (49,000-52,000 mol. wt.) and microsomal (14,400, 20,000, 50,100, 56,900 and 130,000 mol. wt.) fractions. In addition, soluble proteins were greater in males than in females at days 0 (53,000-56,000 mol. wt.; probably tubulin) and 25 (14,600 and 15,000 mol. wt.). These changes in brain proteins probably reflect important structural and functional changes that occur during maturation and sexual differentiation of the brain.

An in vitro amino acid incorporation method for assessing the status of in vivo protein synthesis

A quantitative in vitro amino acid incorporation assay is described which can be used to assess the status of in vivo protein synthesis. The preparation and incubation conditions employed result in constant precursor specific activity and limit amino acid incorporation to completion of nascent peptide chains. Results obtained with this method correlate well with measurements of polyribosome profiles using sucrose gradient centrifugation. The assay is easily applied to a large number of samples, and requires only a fraction of the time and tissue necessary for conventional measures of polysome aggregation. The method has been found suitable for studies of protein synthesis in mouse brain and liver, and in gerbil brain, but not in mouse kidney. Products of in vitro protein synthesis can be separated by standard electrophoretic techniques, allowing a characterization of proteins whose mRNAs are actively translated in vivo.

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.

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.

Polysome activity in relation to growth and protein starvation in brains and hearts of cultured early chick embryos

In previous studies, brains but not hearts of intact early chick embryos were found to be sensitive to protein starvation. In this study, the in vitro protein synthetic activity of polysomes isolated from brains was found to be greater than those isolated from hearts. Starvation reduced the protein synthetic activity of polysomes in vitro but the extent of the reduction was approximately the same for both brains and hearts. A reduction in the amount of ribosomes as polysomes may have contributed to the lower synthetic activity of polysomes from tissues of starved embryos but not to the differences in synthetic activities between brains and hearts. In addition, neither the stability of isolated polysomes nor ribosome-associated ribonuclease activity appeared responsible for the differences observed in polysome synthetic activities. In direct relationship to the differential sensitivity of brains and hearts to starvation observed in the intact embryo, ribosomes isolated from brains of both growing and starved embryos were more readily degraded during in vitro incubation than those from hearts.

Comparison of cell-free protein synthesis by different regions of chicken brain

The cell-free protein synthesis by the postmitochondrial supernatant from chicken cerebrum was twofold greater than protein synthesis by the cerebellum or optic lobes. Ribosomal aggregation of mRNA and ribonuclease activity of the postmitochondrial supernatant from the three brain regions was not statistically different. The higher protein synthetic activity of the cerebral postmitochondrial supernatant was associated with both the postribosomal supernatant (cell sap) and microsomal fractions. Cerebral monomeric ribosomes were more active in polyuridylic acid directed polyphenylalanine synthesis than monomeric ribosomes from either the cerebellum or optic lobes. The ability of cerebral cell sap to support polyuridylic acid directed polyphenylalanine synthesis was 1.6 to 2 times greater than cell sap from the other two regions. Cell sap factors other than tRNAphe or phenylalanyl-tRNA synthetases appear to be responsible for the higher protein synthetic activity of the cbr cell sap.

Biochemical studies of chick brain development and maturation: II. Alterations in the mechanisms of cell-free protein synthesis

The molecular mechanisms responsible for the decreased cell-free protein synthetic activity of chicken brain (cerebrum, cerebellum, and optic lobes) from the late embryonic stage to the adult stage were investigated. The changes in polyribosome content closely paralleled changes in cell-free protein synthetic activity; both increased during late embryonic development, reached a maximum around hatching,and thereafter decreased to the level found in the adult. Both cell sap and microsomal or ribosomal fractions from the adult brain tissue were less active in protein synthesis; however, the microsomal or ribosomal fractions contributed more to the decreased protein synthesis than did the cell sap. The lower activity of adult cell sap in protein synthesis was primarily due to a decreased activity in the aminoacylation of tRNA with no apparent change in the ability of the cell sap to catalyze the elongation of polyphenylalanine synthesis. Ribosomal particles (80 S) from adult and embryonic brain tissue had similar biological activities and fidelity in the translation of polyuridylic acid; however, the cell-free protein synthetic activity of the embryonic post-mitochondrial supernatant preparation was more sensitive to inhibitors of the initiation of protein synthesis (aurintricarboxylic acid and polyinosinic acid) than adult post-mitochondrial supernatant, indicating a decreased initiation capacity in adult brain post-mitochondrial supernatant compared to embryonic brain post-mitochondrial supernatant.

Mouse brain protein composition during postnatal development: an electrophoretic analysis

Changes in the concentrations of mouse brain proteins during postnatal maturation were characterized by a combination of subcellular fractionation and electrophoresis. Sodium dodecyl sulfate gel electrophoresis revealed changing protein concentrations in fractions enriched in nuclei, mitochondria plus synaptic endings, microsomes and cytosol. Postnatal maturational changes in protein concentrations were most pronounced in fractions of purified myelin membranes. The use of exponential gradient gels resulted in increased resolution of low molecular weight myelin proteins. Nuclei treated with Triton X-100 exhibited no change in relative histone concentrations during brain maturation. Nonnuclear contamination of untreated nuclear fractions was shown to be a potential source of erroneous interpretations. These findings are discussed in terms of genetic products and sodium dodecyl sulfate polyacrylamide gel electrophoresis resolution.

Polyamines and amino acid incorporation in vitro into microsomes of rat cerebral cortex

1. Polyamines were found to be associated with microsomes of rat cerebral cortex, the amount of spermine being about four times that of spermidine. Cell sap contained more spermidine than spermine. 2. Both polyamines were able to stimulate the incorporation of [(14)C]valine into microsomes in vitro with a maximum rate equal to 250% of the control. Polyamines stimulated at concentrations close to the amount of spermine and spermidine naturally present in the system. 3. Spermine (0.05mm) was used to study the mechanism of action of polyamines. The increasing of microsome and cell-sap concentration facilitated the action of spermine, but the same process was inhibited by increasing pH5-enzyme concentration. 4. Spermine did not affect the association of [(14)C]valine with tRNA in cell sap, but increased the rate of aminoacyl-tRNA formation in pH5 enzyme preparations. However, this process was not affected in any case when incorporating microsomes were present. 5. It is suggested that microsomes are the main site of action of polyamines.