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

Measuring mRNA translation in neuronal processes and somata by tRNA-FRET

In neurons, the specific spatial and temporal localization of protein synthesis is of great importance for function and survival. Here, we visualized tRNA and protein synthesis events in fixed and live mouse primary cortical culture using fluorescently-labeled tRNAs. We were able to characterize the distribution and transport of tRNAs in different neuronal sub-compartments and to study their association with the ribosome. We found that tRNA mobility in neural processes is lower than in somata and corresponds to patterns of slow transport mechanisms, and that larger tRNA puncta co-localize with translational machinery components and are likely the functional fraction. Furthermore, chemical induction of long-term potentiation (LTP) in culture revealed up-regulation of mRNA translation with a similar effect in dendrites and somata, which appeared to be GluR-dependent 6 h post-activation. Importantly, measurement of protein synthesis in neurons with high resolutions offers new insights into neuronal function in health and disease states.

The influx of neutral amino acids into the porcine brain during development: a positron emission tomography study

Pigs of three different age groups (newborns, 1 week old, 6 weeks old) were used to study the transport of the large neutral amino acids 6-[18F]fluoro-L-DOPA ([18F]FDOPA) and 3-O-methyl-6-[18F]fluoro-L-DOPA ([18F]OMFD) across the blood-brain barrier (BBB) with positron emission tomography (PET). Compartmental modeling of PET data was used to calculate the blood-brain clearance (K1) and the rate constant for the brain-blood transfer (k2) of [18F]FDOPA and [18F]OMFD after i.v. injection. A 40-70% decrease of K1(OMFD), K1(FDOPA) and k2(OMFD) from newborns to juvenile pigs was found whereas k2(FDOPA) did not change. Generally, K1(OMFD) and k2(OMFD) are lower than K1(FDOPA) and k2(FDOPA) in all regions and age groups. The changes cannot be explained by differences in brain perfusion because the measured regional cerebral blood flow did not show major changes during the first 6 weeks after birth. In addition, alterations in plasma amino acids cannot account for the described transport changes. In newborn and juvenile pigs, HPLC measurements were performed. Despite significant changes of single amino acids (decrease: Met, Val, Leu; increase: Tyr), the sum of large neutral amino acids transported by LAT1 remained unchanged. Furthermore, treatment with a selective inhibitor of the LAT1 transporter (BCH) reduced the blood-brain transport of [18F]FDOPA and [18F]OMFD by 35% and 32%, respectively. Additional in-vitro studies using human LAT1 reveal a much lower affinity of FDOPA compared to OMFD or L-DOPA. The data indicate that the transport system(s) for neutral amino acids underlie(s) developmental changes after birth causing a decrease of the blood-brain barrier permeability for those amino acids during brain development. It is suggested that there is no tight coupling between brain amino acid supply and the demands of protein synthesis in the brain tissue.

Ribosomal RNA expression in the developing rat olfactory bulb

The rat olfactory bulb undergoes rapid postnatal growth, thus requiring the continuous production of new proteins. The present study examined the development of ribosomes, a key component in protein synthesis, in the bulb using an antibody specific to ribosomal RNA (Y10b). Furthermore, the potential role of afferent activity on patterns of Y10b immunoreactivity, as well as total RNA production (using incorporation of 3H-uridine) was examined by blocking an external naris on postnatal day 1 (P1). In control pups, Y10b was restricted to mitral cells at P2, with labeling expanding to include all layers by P10. Considerable increases in staining density were observed by P30. Although no differences in Y10b immunoreactive patterns were seen in naris-occluded animals 12-48 h after occlusion, or on P10 or P20, by P30, an apparent decrease in numbers of labeled profiles (presumably tufted cells) in the external plexiform layer was found. Furthermore, no changes were seen in levels of 3H-uridine incorporation. Despite the apparent insensitivity to naris closure, the results indicate that ribosome expression, as measured by Y10b immunoreactivity, undergoes rapid postnatal changes that parallel general patterns of growth in the rat olfactory bulb.

Role of microtubules in the cytoplasmic compartmentation of neurons

We have examined the role of microtubules (MTs) in the development and maintenance of the cytoplasmic compartmentation of ribosomes in cultured embryonic chick sensory neurons. Control neurons show a sharp cytoplasmic demarcation between the ribosome-rich/MT-deficient 'translational' cytoplasm of the perikaryon and the ribosome-deficient/MT-rich 'expressional' cytoplasm of the axon-like neurites. Neurites treated with 1.0 microgram/ml nocodazole for 15-20 min show in addition to the depolymerization of MTs a clear loss of the cytoplasmic demarcation between the soma and neurite. We found a decreasing density of ribosomes with increasing distance from the soma in the nocodazole treated neurites. Ribosomes, as confirmed by their diameter and sensitivity to ribonuclease treatment, populated the first 100 microns of neurites at roughly the same density as found in the soma (150-300 ribosomes per microns2 in 50 mm longitudinal thin sections). The second 100 microns contained a lower density of ribosomes than the first, 20-60 per microns2, but still higher than controls. Distances past 200 microns were devoid of unambiguous ribosomes. Neurites amputated between 50 and 150 microns from the soma then treated with nocodazole contained a dense population of ribosomes in the proximal segment still attached to the soma, but few or none in the distal segment. We interpret these findings as evidence that ribosomes are able to move from the soma down the neurite after MT disassembly.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytoplasmic RNA in nervous system tumours in children: a fluorochromic histochemical study using acridine orange

Acridine orange was used as a fluorochromic histochemical stain of nucleic acids, applied to 78 neoplasms of the central and peripheral nervous systems of 60 children. Some cases were compared with 5 adults and 4 other cases of chronic reactive gemistocytic gliosis. Opposite concentration gradients of cytoplasmic ribonucleic acid (RNA) was demonstrated in tumours of the neuronal/neuroectodermal series, and those of the glial/neuroepithelial series. Minimal AO-RNA fluorescence was seen in 8 cerebellar medulloblastomas and in a retinoblastoma; strong AO-RNA fluorescence occurred in one cerebellar medulloblastoma and in 3 primitive neuroectodemal tumours of the cerebral cortex. Intermediate intensity of fluorescence was found in neuroblastomas, and strong fluorescence was shown in well differentiated ganglioneuroma cells and in cells of chromaffin tumours. Among glial tumours, by contrast, the most anaplastic cells displayed the most RNA fluorescence, while better differentiated astrocytoma cells showed much less. Gradients also were found within some astrocytomas, corresponding to zones of relative anaplasia. Minimal or no fluorescence was detected in reactive gemistocytes or in oligodendroglioma cells. Ependymomas were weakly fluorescent and choroid plexus papillomas showed more fluorescence, similar to the findings in normal ependyma and choroid plexus. Several non-neuroepithelial tumours of the nervous system and Schwannomas also were studied. The acridine orange technique applied to either frozen or paraffin sections of nervous system tumours, has value as an adjunct in the diagnosis and grading of these neoplasms and perhaps in distinguishing reactive gliosis from benign astrocytoma.

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.

Changes in nucleolar morphology and volume of the supraoptic nucleus neurons during postnatal development of the rat

The changes in the volume and nucleolar ultrastructure of neurons of the supraoptic nucleus were examined during the postnatal period of the rat. This morphological maturation involved the gradual transformation from relatively compact nucleoli to reticulate ones which exhibited a typical nucleolonemal configuration with numerous nucleolar interstices and fibrillar centers. A progressive augmentation of the nucleolar volume was also observed during postnatal life; the average value on the first postnatal day was 2.94 +/- 0.42 micron3 while by the 40th day the mean had increased to 15.01 +/- 0.45 micron3. We suggest that this cytodifferentiation process is the morphological expression of the activation of nucleolar ribosomal RNA synthesis, which is required for the growth and maturation of the neurosecretory cells.

Thyroxine regulates the activity and the concentration of synaptic plasma membrane Na,K-ATPase in the developing rat brain cortex

The administration of thyroxine to neonatal rats stimulates the activity of synaptic membrane Na,K-ATPase in the brain cortex of euthyroid and hypothyroid animals. Thyroxine also increases NA,K-ATPase activity of isolated neuronal perikarya in neonatal rats. Binding studies employing [3H]ouabain indicate that the hormone increases the amount of Na,K-ATPase in the synaptic membranes of two-week-old rats. Thyroxine treatment however did not affect synaptic membrane Na,K-ATPase in rats aged 30 days. The results suggest that thyroid hormones are important for the maturation of the synaptic plasma membrane during the critical period of rat brain development.

Vectorial discharge and localization of nascent polypeptides in rough endoplasmic reticulum of developing neuronal perikarya of rat brain cortex

Rough endoplasmic reticulum (RER) prepared from bulk-isolated neuronal perikarya of rat brain cortex of different postnatal ages was found to be active in vectorial discharge of nascent proteins through the membrane; this activity increased with the increasing age of animals and reached maximal values in adults. RER isolated from whole cortical tissue (containing all cell types) exhibited vectorial release only up to 18 days of age; the preparation from adult animals was essentially devoid of secretory activity. Controlled proteolysis of various preparations suggested that in neuronal RER of 8-day-old rats the proportion of nascent proteins operationally retained in the intravesicular space was about twice that retained by cortical preparations. For the purpose of comparison, these parameters were studied also in liver RER.