Regulation of protein synthesis during postnatal maturation of the brain

Ontogenetic differences in energy metabolism and inhibition of protein synthesis in hippocampal slices during in vitro ischemia and 24 h of recovery

The present study was designed to clarify whether ontogenetic differences in the vulnerability of the brain towards hypoxic-ischemic insults are only caused by the low cerebral energy demand of immature animals or whether there are additional mechanisms, such as protein synthesis (PSR), that may be involved in this phenomenon. We therefore measured tissue levels of adenylates and PSR in hippocampal slices from immature (E40) and mature (E60) guinea pigs fetuses and from adult guinea pigs during in vitro ischemia and 24 h of recovery using a recently modified method. Hippocampal slices were incubated in a temperature controlled flow-through chamber, gassed with 95% O2/5% CO2. In vitro ischemia was induced by transferring slices to a glucose-free artificial cerebrospinal fluid (aCSF) equilibrated with 95% N2/5% CO2. The duration of ischemia ranged from 10 to 40 min. Adenylates were measured by HPLC after extraction with perchloric acid. PSR was evaluated as the incorporation rate of [14C]leucine into proteins. Under control conditions, tissue levels in adenylates did not change, whereas PSR increased slightly in hippocampal slices from mature fetuses and adult animals during a 24-h control incubation period. In slices from immature fetuses ATP levels were only maintained for 2 h. During in vitro ischemia the decline in ATP, total adenylate pool, and adenylate energy charge was much slower in slices from immature fetuses than in slices from mature fetuses or adults. After in vitro ischemia, ATP and the total adenylate pool did not completely recover in mature fetuses and adults, whereas adenylate energy charge almost returned to control values independently of the developmental stage. Two hours after in vitro ischemia PSR was undisturbed in slices from immature fetuses, but severely inhibited in slices from mature fetuses and adults. With ongoing recovery, PSR in mature fetuses returned to control values, while in adults it was still inhibited even 24 h after in vitro ischemia. From these results we conclude that hippocampal slices prepared from mature guinea pig fetuses as well as from adult guinea pigs can be held metabolically stable during long-term incubation using a recently modified technique. However, in slices from immature fetuses a stable energy state could not be maintained for more than 2 h. We further conclude that postischemic disturbances in PSR closely reflect the ontogenetic changes in the vulnerability of the brain to ischemia and that low energy metabolism is certainly not the only cause of the increased vulnerability of the fetal brain to ischemia.

Postnatal administration of two peptide solutions affects passive avoidance behaviour of young rats

The effects of two subcutaneously injected peptide solutions CERE (100 mg/kg b. wt.) and E021 (1 mg/kg b. wt.) and of 0.9% saline on passive avoidance reaction (PAR) of young rats were examined. Animals were trained and tested in a step-through avoidance task using a footshock of 0.5 mA or 1 mA. Step-through latencies were observed up to 200 s and from these data the percentage of good learners (latency = 200 s) and bad learners (latency < 200 s) was calculated. Two experimental schedules were performed (n > 6). In Expt. 1 rat pups were chronically treated with the substances within the first 7 days after birth. In Expt. 2 the 7 days of treatment started in the 4th postnatal week. In both experiments PAR acquisition was trained on the 28th day after birth (learning trial), PAR extinction testing started on the 29th day (retention trials). After applying a 0.5-mA footshock, rat pups treated with E021 within the first 7 days of life (Expt. 1) displayed significantly slower PAR extinction when compared to saline- and CERE-treated rats. In the 1 mA groups, significant differences in step-through latencies were measured between 0.9% saline- and E021-pretreated animals on retention day 11 and between saline and CERE on retention days 9 and 13. E021-treated rats of Expt. 2, receiving a footshock intensity of 0.5 mA, showed significant lower step-through latencies when compared to E021-treated rats of Expt. 1. In Expt. 2 no significant differences between treatment groups were observed.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterogeneity in the beta-subunit of translational initiation factor eIF-2 during brain development

We have detected by immunoblotting analysis of crude fractions from suckling and adult rat brain, resolved by two-dimensional isoelectric focusing-dodecyl sulfate polyacrylamide gel electrophoresis, the presence of two different forms of the beta subunit of polypeptide initiation factor 2 (eIF-2). These two forms differ in their apparent molecular weights and also in their isoelectric point values. Quantitation of both forms in the crude fractions shows that, the most basic form beta 1 (pI: 6.1, 52 kDa), is present in higher levels of the salt wash ribosomal fractions obtained from both, suckling and adult animals, than in the postmicrosomal fraction corresponding to the same animals. The most acidic form, beta 2 (pI: 5.9, 50 kDa), is present in the highest level in the postmicrosomal supernatant from adult animals. A close parallelism is found between beta 1 levels and eIF-2 activity.

Protein synthesis rates in rat brain regions and subcellular fractions during aging

In vivo protein synthesis rates in various brain regions (cerebral cortex, cerebellum, hippocampus, hypothalamus, and striatum) of 4-, 12-, and 24-month-old rats were examined after injection of a flooding dose of labeled valine. The incorporation of labeled valine into proteins of mitochondrial, microsomal, and cytosolic fractions from cerebral cortex and cerebellum was also measured. At all ages examined, the incorporation rate was 0.5% per hour in cerebral cortex, cerebellum, hippocampus, and hypothalamus and 0.4% per hour in striatum. Of the subcellular fractions examined, the microsomal proteins were synthesized at the highest rate, followed by cytosolic and mitochondrial proteins. The results obtained indicate that the average synthesis rate of proteins in the various brain regions and subcellular fractions examined is fairly constant and is not significantly altered in the 4 to 24-month period of life of rats.

Absence of age differences in protein synthesis by rat brain, measured with an initiating cell-free system

A cell-free protein synthesis system was derived from brains of young (3 month) and old (greater than 23 month) male Fischer-344 rats in order to examine brain protein synthesis in relation to age. The system was shown to be capable of reinitiating protein synthesis in vitro, and of synthesizing protein from exogenously added mRNA. Optimal ionic conditions for amino acid incorporation were 200 mM potassium ion and 5 mM magnesium ion, and amino acid incorporation depended on addition of ATP, GTP, and an energy-generating system (creatine phosphate and creatine phosphokinase). Amino acid incorporation was sensitive to the initiation inhibitors aurintricarboxylic acid and sodium fluoride. Optimal conditions were independent of the age of the rat from which the brain was taken. There was no statistically significant relation (p greater than 0.05) between capacity of amino acid incorporation and age. The aggregation state of brain polyribosomes also did not differ between young (3 month) and old (30 month) rats. The results suggest that overall brain protein synthesis capacity is age-invariant in the rat.

Stagewise decline in the activity of brain protein synthesis factors and relationship between this decline and longevity in two rodent species

The activities of brain initiation factor 2 and brain elongation factor 1, which function as rate-limiting in total protein synthesis, and estimations of brain weight were followed during postnatal life in the rat and the mouse. Both activities decreased in parallel while cumulative brain weight increased. Three exponential components were required for the mathematical expression of each of the three processes in semilogarithmic plots against time. The acceleration curves for the activities and tissue weight demonstrated a mirror image symmetry. Within the general pattern of diminution with age, the negative acceleration of the activities and the positive acceleration of the brain weight displayed repeated bursts. The activities of both factors could also be arranged into several regression lines in log/log plots against time. Significantly, in these plots, the regression line calculated for the whole set of data for each factor activity showed that the value of the ratio of the slopes (mouse to rat) was inversely related to the square root of the ratio of species longevity and was in agreement with the power law relating life spans of cells to species longevity (Röhme, Proc. Natl. Acad. Sci. U.S.A., 78 (1981) 5009).

Preparation of a cell-free extract from rat brain which can initiate protein synthesis in vitro

A cell-free protein synthesis system, derived from brains of 3 mo-old male Fischer-344 rats, has been characterized. The optimum conditions for amino acid incorporation in the system were 5 mM magnesium ion and 200 mM potassium ion. Incorporation depended on the addition of ATP, GTP, and an energy-generating system, and was sensitive to addition of the drugs aurintricarboxylic acid and sodium fluoride, inhibitors of initiation of protein synthesis. Both 40S and 80S initiation complexes were labeled in vitro, using [35S]methionine. Such labeling was sensitive to the protein synthesis inhibitors, aurintricarboxylic acid and sodium fluoride. The system, which can initiate protein synthesis, should be of use for examining mechanisms which underlie alterations in rat brain protein synthesis induced by various treatments.

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.

Protein kinase activities associated with ribosomes of developing rat brain. Identification of eukaryotic initiation factor 2 kinases

Protein kinases associated with ribosomes in the brains of suckling (4-10 days) and adult (2 months) rats were extracted from ribosomal fraction with 0.5 M KCl. The different protein kinase activities were characterized by their ability to phosphorylate three exogenous substrates: casein, histone IIs and histone IIIs in the presence of different modulators. Ribosomal salt wash fractions contain a high casein kinase activity which was partially inhibited by heparin and stimulated by calmodulin in the presence of Ca2+, indicating the presence of casein kinase I and II and calcium/calmodulin-dependent kinases. Cyclic AMP and cyclic GMP-dependent kinases and protein kinase C (calcium/phospholipids-dependent kinase) were also present. No differences were found in the casein kinase activities of suckling and adult animals, but histone kinase activities were higher in adult than in suckling animals. To identify initiation factor 2 kinases, purified factor from adult brains was used as a protein marker. In addition to the phosphorylation of both factor subunits alpha and beta by casein kinase I or II, an increased phosphorylation was detected of alpha subunit in the presence of cyclic AMP, and beta subunit, in the presence of Ca2+/calmodulin or Ca2+/phospholipids. Present results reinforce our hypothesis that, as occurs in other eukaryotic cells, the decreased rate of protein synthesis during brain development may be regulated by phosphorylation of initiation factor 2.

Utilization of tyrosine and tryptophan for protein synthesis by undernourished developing rat brain

Incorporation of tracer doses of radiolabeled tryptophan and tyrosine into brain proteins was investigated in rats malnourished during gestation and lactation. Age and time dependent increases in the radioactivity was observed in the whole homogenate and in the TCA insoluble fraction. Protein malnourished rats showed increased incorporation of tryptophan and tyrosine. However the diet restricted (Pair-fed) animals showed increased incorporation of tyrosine only. The increased incorporation may probably be due to changes in the pool size of the amino acids and effective recycling of the amino acids. The enhanced utilization in protein synthesis may also probably offer a mechanism for conservation of these amino acids.

Changes in alpha-fetoprotein and albumin synthesis rates and their levels during fetal and neonatal development of rat brain

An attempt was made to find a correlation between AFP and albumin levels in brain and their rates of synthesis in the brain cells during maturation of rat brain. Levels of alpha-fetoprotein (AFP) and albumin in the developing brain were studied by rocket immunoassay. Rate of synthesis of AFP and albumin in brain cell cultures, established from rat brain at various stages of development, were determined by incorporation of [14C]leucine into immuno-precipitable intracellular AFP and albumin. AFP and albumin levels in brain as well as rates of their synthesis by brain cells in culture registered a continuous decline during development. Synthesis of AFP and albumin in the brain is switched off after first week of postnatal life with a concomitant disappearance of these proteins from the brain. Levels of AFP and albumin in brain correlated well with rates of their synthesis by brain cells in vitro at any specific stage of brain maturation implying that levels of AFP and albumin in brain are regulated by controlling rates of their synthesis in the maturing brain cells.

The rate of protein degradation in developing brain. Methodological considerations

Recently we reported that the rate of protein breakdown decreases during development. Breakdown rates were calculated from the rates of protein synthesis and the changes in brain protein content with age. A different study, measuring breakdown by monitoring the loss of label from brain protein after an H14CO3- pulse, came to the opposite conclusion: that the rate of breakdown is low in immature brain and increases during development. We have now investigated some of the factors (the distribution of label in protein and the potential for recycling) that might introduce errors into these measurements. The specific radioactivities of both protein-bound and free amino acids were determined in the brains of young rats several days after an intraperitoneal pulse of H14CO3-. For a number of amino acids the specific radioactivity of the free amino acid is high compared with that of the protein-bound amino acid, and therefore recycling could result in an underestimate of the degradation rate. Because glutamic acid had a relatively low specific-radioactivity ratio, [1-14C]glutamic acid was used in a pulse-labelling experiment to measure degradation. The rate so obtained, 0.6% . h-1, is twice the rate found with H14CO3- labelling (based on total protein-bound radioactivity). Insofar as recycling is a possible complication, 0.6% . h-1 may be a minimum value. Although somewhat higher degradation rates are found after labelling with an intracranial pulse, which was considered as a possible route to limit recycling, there are difficulties in interpreting these data.

Role of elongation factor 1 in the translational control of rodent brain protein synthesis

The translational control of protein synthesis during early postnatal neural development and aging was examined in the mouse and the rat. The activity of brain elongation factor 1 (bEF-1) was found to decrease exponentially with age and to decline parallel to the age-dependent decrease in total protein synthesis in both rodents. This decrement in bEF-1 activity fell within the range of reported age-related decreases in protein synthesis in in vitro systems. The factor was present in multiple forms; the lighter species predominated in older animals, whereas the young light form apparently disappeared with increasing age, and was replaced by other arising from the heavy form. Elongation factor 1 derived from young brains functioned as a rate-limiting component in polypeptide synthesis in previously saturated adult systems. The data suggest that bEF-1 has an important modulatory effect on total brain protein synthesis.

Influence of prenatal hypoxia on brain development: effects on body weight, brain weight, DNA, protein, acetylcholinesterase, 3-quinuclidinyl benzilate binding, and in vivo incorporation of [14C]lysine into subcellular fractions

The influence of prenatal hypoxia on subsequent brain development in the young rat was investigated by examining body and brain weight, cerebral cortex wet weight, protein and DNA concentrations, acetylcholinesterase (AChE) activity, 3-quinuclidinyl benzilate (QNB)-binding levels, the relative amounts of protein in various subcellular fractions, and the in vivo incorporation of [14C]lysine into the protein of homogenate and subcellular fractions. Exposure of pregnant females to a mild hypoxia (9.1% O2, 10 h per day for the 9--11 days preceding birth) resulted in a reduced body weight in the pups and days 1 and 5 after birth; total cortical DNA was reduced but brain weight and protein content were unaffected, leading to a higher protein/DNA ratio in prenatally hypoxic pups. By 10 days of age these differences between prenatally hypoxic and control animals were no longer apparent. There were no differences between prenatally hypoxic and control animals in AChE and QNB binding per milligram cortex protein. The relative amount of synaptic membrane protein from the cerebral cortex was reduced at day 1 in prenatally hypoxic animals and the synaptic membrane fraction showed a higher level of incorporation of [14C]lysine on days 1, 5, and 10. The developmental profile of [14C]lysine incorporation showed a peak on day 10 which was higher in prenatally hypoxic rats. By 46 days after birth little difference could be found between prenatally hypoxic and control animals.

Development of synaptic glycoproteins: effect of postnatal age on the synthesis and concentration of synaptic membrane and synaptic junctional fucosyl and sialyl glycoproteins

Synaptic plasma membranes (SPM) and synaptic junctions (SJ) were isolated from the cortices of rats varying in age between 5 and 28 days. Gel electrophoresis of SPM and SJ indicated a marked increase in the concentration of the "PSD protein" (M. W. 52,000) with development. The biosynthesis of glycoproteins was measured following the intracranial injection of [3H]fucose or [3H]N'-acetylmannosamine. The incorporation of [3H]fucose into synaptic fractions decreased two- to threefold between 10 and 28 days whereas little change in the incorporation of [3H]N'-acetylmannosamine occurred over the same period. Gel electrophoretic analyses of labeled synaptic membranes indicated major increases in the relative incorporation of radiolabeled precursors into glycoproteins with apparent molecular weights of 74,000, 65,000, 50,000, and 40,000 with increasing age. Identification of fucosyl and sialyl glycoproteins following reaction with 125I-fucose-binding protein or labeling of sialic acid with NaIO4/NaB[3H4] demonstrated similar increases in the concentrations of these glycoproteins. Synaptic junctions contained three major glycoproteins with apparent molecular weights of 180,000, 130,000 and 110,000. The reaction of these glycoproteins with 125I-fucose-binding protein increased one- to twofold between 10 and 28 days but little variation in their relative distribution or synthesis occurred over this period. The reaction of synaptic junctional glycoproteins GP 180 and GP 110 with 125I-wheat germ agglutinin increased between 10 and 28 days. The results indicate that the molecular composition of the synapse continues to evolve after the initial synaptic contact has been formed.

Tritiated leucine incorporation in the developing hamster facial nucleus with injury: a liquid scintillation study

Tritiated leucine incorporation was examined after either crush or axotomy of the hamster facial nerve at specific stages in the maturation of the neuronal nucleolus. Changes in the neuronal metabolic response to injury in development were demonstrated with liquid scintillation examination of tritiated leucine incorporation into the trichloroacetic acid (TCA)-insoluble and TCA-soluble fractions derived from whole reactive and normal facial nuclear groups. Changes in incorporation seen in the developmental sequence were attributed to actual changes in neuronal protein metabolism, and not to changes in the amino acid pool, glial changes or hyperemic capillary changes. The ability to increase leucine incorporation over the normal as a result of injury in development coincided with the time of final nucleolar maturation in the facial motor neurons, beginning at approximately 20 days postnatal age. Thus, there is a correlation between a specific morphological event, the attainment of the mature nucleolar configuration, and the acquisition of the mature synthetic capacity as indicated by the ability to respond to injury in the mature manner.

Protein synthesis by astrocytes in primary cultures

Protein synthesis, measured as leucine incorporation into acid-precipitable proteins, was determined in astrocytes in primary cultures obtained from the cerebral hemispheres of newborn mice. As can be expected for eucaryotic, ribosomal protein synthesis, the incorporation was almost completely inhibited by cycloheximide (0.01 mM), but unaffected by chloramphenicol (0.03 mM). The rate of synthesis, measured during exposure to a high (0.8 mM) concentration of leucine was 5.4 nmol/hr/mg protein in mature (i.e., at least 4-week-old) cultures. This value is at least twice as high as the protein synthesis rates reported for the adult brain in vivo, suggesting that a very considerable part of the protein synthesis in the adult brain may take place in astrocytes. The molecular weight distribution of the synthesized proteins was determined by polyacrylamide gel electrophoresis, demonstrating synthesis of at least 50 different polypeptides, ranging in molecular weight between 190,000 and 27,000 daltons. The pattern of the synthesized proteins underwent considerable alteration with age in young cultures in which the total content of protein was still increasing, but it was remarkably stable after the age of two weeks. Exposure to dibutyryl cyclic AMP, which is known to alter morphology, content of glial fibrillary acidic protein (GFA), and activities of certain enzymes in the cultures in the cultured astrocytes, caused marked alterations in the pattern of the synthesized proteins.

Characterization of an initiating cell-free protein synthesis system derived from rabbit brain

Protein synthesis in the brain is known to be affected by a wide range of treatments. The detailed analysis of the mechanisms that are involved would be facilitated by the development of cell-free translation systems derived from brain tissue. To date, brain cell-free systems have not been fully characterized to demonstrate a capacity for initiation of translation. The following criteria were utilized to demonstrate that a cell-free protein synthesis system derived from rabbit brain was capable of initiation in vitro: (a) sensitivity of cell-free translation to the initiation inhibitor aurintricarboxylic acid (ATA); (b) binding of [35S]Met-tRNAf to 40S and 80S initiation complexes; (c) incorporation of labeled initiation methionine into high-molecular-weight proteins; and (d) the association of labeled exogenous mRNA with polysomes. The optimum conditions for amino acid incorporation in this system were 4 mM-Mg2+, 140 mM-K+, and pH 7.55. Incorporation was dependent on the addition of ATP, GTP, and an energy-generating system. Cell-free protein synthesis reflected the normal process, since a similar spectrum of proteins was synthesized in vitro and in vivo. This initiating cell-free translation system should have wide application in the analysis of the mechanisms whereby various treatments affect protein synthesis in the brain.

Analysis of messenger RNA coding for S100 protein in the mammalian brain

S100 protein is a brain-specific protein which is absent at birth and first appears in rabbit brain 2-3 days after birth. To determine how the synthesis of this brain-specific protein is regulated, mRNA was isolated from brain polysomes and assayed for S100 protein mRNA activity by in vitro translation in a heterologous cell-free system and immunoprecipitation of released polypeptides with rabbit anti-S100 protein antiserum. S100 protein mRNA was detected primarily in small polysomes containing five to eight ribosomes, and virtually no S100 protein mRNA was present in polysomes containing more than eight ribosomes. S100 protein mRNA was not detected in brain polysomes at stages prior to the induction of synthesis of S100 protein, i.e., in fetal brain or in 1-day neonates. The amount of S100 protein mRNA in polysomes of the cerebral cortex and cerebellum was measured to see if it correlated with the level of S100 protein in the two regions of adult brain. The cerebellum, which contained three to four times the level of S100 protein in the cerebral cortex, contained four times more S100 protein mRNA.

Rat brain protein synthesis declines during postdevelopmental aging

Using improved methods to measure brain protein synthesis in vivo (Dunlop et al., 1975) we have established that brain protein synthesis significantly declines in forebrain, cerebellum and brain stem when mature rats (3 months old) are compared to old rats (22.5 months old). The incorporation of (3H) L-lysine into forebrain protein is reduced 11% in 10.5 month old rats compared to 3 month old rats. A further reduction of 9% occurred between 16.5 months and 22.5 months. Our data suggest that reduced levels of protein synthesis initiation may be responsible, at least in part, for this age-related decline.

Regulation of prenatal and postnatal protein synthesis in mouse brain

Regulation of protein synthesis during prenatal and postnatal brain development was examined using postmitochondrial supernatant (PMS) fractions and isolated ribosome-pH 5 enzyme systems from fetal, neonatal, and adult neural tissue. The rate of polyuridylic acid (poly-U)-dependent protein synthetic activity was inversely proportional to the endogenous rate of protein synthesis in either the PMS fractions or ribosomal preparations. A careful analysis of the kinetics of the poly-U-dependent polypeptide synthesis revealed that there was a lag in the time at which certain of the PMS preparations could begin to utilize the poly-U template as sole source of mRNA. The lag period was dependent upon the developmental age of the neural tissue used and the Mg2+ concentration of the protein synthesis reaction. Since previous work reported that the observed developmental decrease in the rate of polypeptide synthesis utilizing a poly-U template could not be measured by several isolation techniques to determine if the purification procedure might have affected the ribosomes in some manner by removing a specific protein(s) involved in ribosome-cytosol interactions. At 6 mM-Mg2+ the rate of poly-U-dependent protein synthesis was inversely proportional to the rate of endogenous synthesis and depended upon the method used to isolate the ribosomes: microsomes congruent to Triton X-100-treated < DOC-treated < KCl-treated. However, there was no age-dependent effect with any of the ribosomal preparations. The data suggest that there is a developmental modulating effect of ribosomal activity in PMS preparations which is not found in association with the isolated ribosome-pH 5 enzyme protein synthesizing system.

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.

Compartments of protein metabolism in the developing brain

We investigated whether the higher rate of amino acid incorporation into immature than into mature brain protein is due to (a) rapid growth, (b) a small rapidly metabolized protein pool, or (c) a higher turnover rate of most of the protein. We measured net growth and the incorporation of [14C]tyrosine or [14C]valine into brain proteins in young rats and mice. The specific activity of the free amino acid pool was kept constant in the tyrosine experiments. Incorporation of tyrosine into protein was continued for up to 30 h by which time the specific activity of protein-bound amino acid reached 1/3 of that of the free (precursor) amino acid. The growth (accretion) of brain proteins was approx. 0.635% per h in mice and rats in the 1-4 day period after birth. In previous studies we found that the turnover rate of the bulk (about 96%) of adult brain proteins is below 0.3% per h. Because of the presence of a small (about 4%) active pool the average turnover rate is 0.6% per h. The present experiments show a degradation rate of 0.7-1.1% per h in the brain proteins of the young. This high metabolic rate is not due to a small rapidly degraded fraction of protein. The very rapid protein fraction previously seen in adult rats is either very small (below 1%) or absent in the young. Thus most of the proteins in the immature brain during the rapid growth phase are formed and broken down at a rate that is approximately three times higher than that of the bulk of proteins in the adult brain. The small active protein pool in the adult on the other hand has a metabolic rate higher than that of the immature brain proteins.