Characterization of translation products of the polyadenylated RNA of free and membrane-bound polyribosomes of rat forebrain

Translation in the mammalian oocyte in space and time

A hallmark of oocyte development in mammals is the dependence on the translation and utilization of stored RNA and proteins rather than the de novo transcription of genes in order to sustain meiotic progression and early embryo development. In the absence of transcription, the completion of meiosis and early embryo development in mammals relies significantly on maternally synthesized RNAs. Post-transcriptional control of gene expression at the translational level has emerged as an important cellular function in normal development. Therefore, the regulation of gene expression in oocytes is controlled almost exclusively at the level of mRNA and protein stabilization and protein synthesis. This current review is focused on the recently emerged findings on RNA distribution related to the temporal and spatial translational control of the meiotic progression of the mammalian oocyte.

Method to isolate polyribosomal mRNA from scarce samples such as mammalian oocytes and early embryos

Background

Although the transcriptome of minute quantities of cells can be profiled using nucleic acid amplification techniques, it remains difficult to distinguish between active and stored messenger RNA. Transcript storage occurs at specific stages of gametogenesis and is particularly important in oogenesis as stored maternal mRNA is used to sustain de novo protein synthesis during the early developmental stages until the embryonic genome gets activated. In many cases, stored mRNA can be several times more abundant than mRNA ready for translation. In order to identify active mRNA in bovine oocytes, we sought to develop a method of isolating very small amounts of polyribosome mRNA.

Results

The proposed method is based on mixing the extracted oocyte cytoplasm with a preparation of polyribosomes obtained from a non-homologous source (Drosophila) and using sucrose density gradient ultracentrifugation to separate the polyribosomes. It involves cross-linking the non-homologous polyribosomes and neutralizing the cross-linking agent. Using this method, we show that certain stages of oocyte maturation coincide with changes in the abundance of polyribosomal mRNA but not total RNA or poly(A). We also show that the abundance of selected sequences matched changes in the corresponding protein levels.

Conclusions

We report here the successful use of a method to profile mRNA present in the polyribosomal fraction obtained from as little as 75 mammalian oocytes. Polyribosomal mRNA fractionation thus provides a new tool for studying gametogenesis and early development with better representation of the underlying physiological status.

Identification and characterization of novel developmentally regulated proteins in rat spinal cord

We previously used 2-dimensional (2-D) gel electrophoresis to identify novel proteins that may be involved in the genesis of the mammalian nervous system [1]. Several novel proteins that were up- or down-regulated coincident with neurogenesis and neuronal migration in rat neocortex were identified. To further investigate the expression of some of these developmentally regulated proteins during a comparable period in spinal cord development, 2-D electrophoresis is used to study their regulation in the crude membrane and soluble fractions of spinal cord at embryonic day 12 (E12) and embryonic day 21 (E21). This analysis indicates that 7 of the proteins that exhibited large changes in their synthesis in cerebral cortex between embryonic day 14 (E14) and embryonic day 21 (E21) demonstrate similar up- or down-regulation during spinal cord neurogenesis. However, two proteins are restricted in their expression or developmental regulation. One of these, 667-800, appears cortex-specific, while the up-regulation of protein SC.1 appears to be spinal cord specific. Several of these proteins also appear to be enriched in both the cortex and spinal cord relative to non-neural tissues (117, 162, 182, 310 [TOAD-64], 667-800) and may be neural specific. To further characterize its expression, one of these neural-specific, up-regulated proteins, TOAD-64 (protein 310) [2-4], is studied throughout embryonic and postnatal spinal cord development using peptide-specific polyclonal antibodies. As suggested by the 2-D gel analysis and the previously reported expression pattern in cerebral cortex [3], TOAD-64 is transiently expressed in postmitotic spinal cord neurons early in their development and sharply down-regulated after the second postnatal week. In the adult spinal cord, TOAD-64 expression is remarkably restricted to a subset of primary afferents to the spinal cord. This expression pattern, coupled with its recently discovered homology to two proteins implicated in axon pathfinding in the chick and nematode [5,3], suggests that TOAD-64 may have a fundamental role in axon pathfinding.

An estimation of the sensitivity of in vitro translation using two-dimensional gel analysis

Poly (A+ mRNA species, isolated from 100-day-old rat brain, were analysed by in vitro translation and two-dimensional gel electrophoresis. The synthesis of selected protein species was compared to actin on the basis of [35S]methionine incorporation. The estimated molar abundance of translation products varied from abundant species at 0.78% of the total to several are species, detectable below the 0.02% level. If these synthesis rates reflect the abundance of particular mRNAs in the mixture, this sensitivity limit compares well with accepted values using differential cDNA screening techniques. This analysis provides evidence that in vitro translation methodology is able to detect rarer mRNA species than is usually expected--these include similar abundance classes to library screening procedures.

Human carboxypeptidase E. Isolation and characterization of the cDNA, sequence conservation, expression and processing in vitro

Carboxypeptidase E (CPE), which cleaves C-terminal amino acid residues and is involved in neuropeptide processing, is itself subject to intracellular processing. Human CPE cDNA was isolated and sequence comparisons were made with those of a previously isolated brain cDNA (M1622) encoding rat CPE and of other human carboxypeptidases (M and N). Human (2.5 kb) and rat (2.1 kb) CPE cDNAs approximated to the size of their respective mRNAs; additional sequences were located in putative 5' and 3' untranslated regions of human CPE mRNA. There is 79% sequence similarity between human and rat CPE cDNAs, with greater similarity (89%) over the coding region and short sections of the non-coding sequence. The predicted 476-amino acid-residue sequences of human and rat preproCPEs are highly conserved (96% identity), with lower degree of similarity of the N-terminal signal peptide (76%). Human CPE showed 51% and 43% sequence similarity to human CPN and CPM respectively, with discrete regions of divergence dispersed between the highly conserved mechanistically implicated regions. Antiserum generated from a fusion protein, synthesized in Escherichia coli from constructs of the human cDNA, recognized an approx. 50 kDa membrane protein and a smaller soluble protein in rat and human brain preparations, corresponding to the two forms of native CPE. Human CPE mRNA transcripts directed the synthesis in reticulocyte lysate of a 54 kDa translation product, which in the presence of dog pancreas microsomal membranes was co-translationally processed with cleavage, insertion into membranes and glycosylation. Three processed forms were generated, the largest (56 kDa) and smallest (52 kDa) being equally glycosylated. The membrane association of the processed translation products and of native brain membrane CPE, detected immunologically, was resistant to moderate alkali but not pH 11.5 extraction. These results are consistent with secondary-structure predictions that CPE is a peripheral membrane protein. The dissimilar regions of human carboxypeptidases may provide information on sequences responsible for their different cellular disposition.

The human heat-shock protein family. Expression of a novel heat-inducible HSP70 (HSP70B') and isolation of its cDNA and genomic DNA

The human heat-shock protein multigene family comprises several highly conserved proteins with structural and functional properties in common, but which vary in the extent of their inducibility in response to metabolic stress. We have isolated and characterized a novel human HSP70 cDNA, HSP70B' cDNA, and its corresponding gene sequence. HSP70B' cDNA hybrid-selected an mRNA encoding a more basic 70 kDa heat-shock protein that both the major stress-inducible HSP70 and constitutively expressed HSC70 heat-shock proteins, which in common with other heat-shock 70 kDa proteins bound ATP. The complete HSP70B' gene was sequenced and, like the major inducible HSP70 gene, is devoid of introns. The HSP70B' gene has 77% sequence similarity to the HSP70 gene and 70% similarity to HSC70 cDNA, with greatest sequence divergence towards the 3'-terminus. The HSP70B' gene represents a functional gene, as indicated by Northern-blot analysis with specific oligonucleotides, hybrid-selected translation with a specific 3' cDNA sequence and S1 nuclease protection experiments. In contrast with HSP70 mRNA, which is present at low concentrations in HeLa cells and readily induced by heat or CdCl2 treatment in both fibroblasts and HeLa cells, HSP70B' mRNA was induced only at higher temperature and showed no basal expression. The differences in patterns of induction may be due to the special features of the promoter region of the HSP70B' gene.

Recovery from edema and of protein synthesis differs between the cortex and caudate following transient focal cerebral ischemia in rats

Postischemic recovery from brain edema and of protein synthesis was examined following 1 h of middle cerebral artery (MCA) occlusion in rats. Recovery from brain edema and of protein synthesis showed a good correlation until 7 days after reperfusion in each area (cerebral cortex or lateral caudate) in the occluded MCA side. However, regional differences in the above types of recovery in the cortex and in the lateral caudate were found for the first time in this experiment. A profound inhibition of protein synthesis and formation of brain edema began sooner in the lateral caudate than in the cortex and continued long after reperfusion. Grades of cerebral blood flow during ischemia and the early period of reperfusion were almost the same in the two regions. Therefore, the regional differences in the above recoveries may not be due to the difference in the blood flow during ischemia and reperfusion, but may be partly attributable to the imbalance of excitatory and inhibitory innervation in the above two areas of the brain, may be due to a distinctive response to ischemic stress, and may be caused also by the potentiative effect of free arachidonate on the excitotoxic mechanism.

Trifluoperazine activates and releases latent ATP-generating enzymes associated with the synaptic plasma membrane

Neurone-specific enolase (NSE) and the brain form of creatine phosphokinase (CPK-BB) were previously found to be present in rat synaptosomal plasma membranes (SPM) using two-dimensional gel (2-D gel) and peptide analysis; enzymatic activities of these and of pyruvate kinase (PK), all involved in ATP generation, were shown to be "cryptic" unless the SPM were treated with Triton X-100. We now show that enzymatic activation also occurs when the SPM are treated with trifluoperazine (TFP). TFP activation occurred even when the enzymes were membrane associated, showing that solubilization was not responsible for "unmasking" the enzyme activities. When TFP treatment was performed at alkaline instead of neutral pH, NSE and CPK-BB were released as well as PK, nonneuronal enolase, and aldolase which were identified by 2-D gel and tryptic peptide analysis. Other proteins released included calmodulin, actin, and the 70-kilodalton heat-shock cognate protein. Tubulin, synapsin I, and a 35-kilodalton basic protein were largely unaffected. The latter was identified as the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase on the basis of 2-D gel and peptide analyses and subsequent partial sequencing of a rat brain cDNA coding for the same protein. TFP treatment is thus useful for activating latent enzymes as well as for distinguishing enzymes that have a different disposition on the membrane.

Expression and developmental regulation of two unique mRNAs specific to brain membrane-bound polyribosomes

Translation in vitro of membrane-bound polyribosomal mRNAs from rat brain has shown several to be developmentally regulated [Hall & Lim (1981) Biochem. J. 196, 327-336]. Here we describe the isolation and characterization of cDNAs corresponding to two such brain mRNAs. One cDNA (M444) hybrid-selected a 0.95 kb mRNA directing the synthesis in vitro of a 21 kDa pI-6.3 polypeptide, which was processed in vitro by microsomal membranes. A second cDNA (M1622) hybridized to a 2.2 kb mRNA directing the synthesis of a 55 kDa pI-5.8 polypeptide. Both mRNAs were specific to membrane-bound polyribosomes. Restriction maps of the corresponding genomic DNA sequences are consistent with both being single copy. The two mRNAs were present in astrocytic and neuronal cultures, but not in liver or spleen or in neuroblastoma or glioma cells. The two mRNAs were differently regulated during brain development. In the developing forebrain there was a gradual and sustained increase in M444 mRNA during the first 3 weeks post partum, whereas M1622 mRNA appeared earlier and showed no further increase after day 10. In the cerebellum the developmental increase in M444 mRNA was biphasic. After a small initial increase there was a decrease in this mRNA at day 10, coincident with high amounts of M1622 mRNA. This was followed by a second, larger, increase in M444 mRNA, when amounts of M1622 mRNA were constant. The contrasting changes in these two mRNAs in the developing cerebellum are of particular interest, since they occur during an intensive period of cell proliferation, migration and altering neural connectivity. As these mRNAs are specific to differentiated neural tissue, they represent useful molecular markers for studying brain differentiation.

The brain 68-kilodalton microtubule-associated protein is a cognate form of the 70-kilodalton mammalian heat-shock protein and is present as a specific isoform in synaptosomal membranes

The relationship between the 68-kilodalton microtubule-associated protein (68KMAP) and the major heat-induced protein (HSP70) in rat and human cells was investigated by comparison of their heat induction properties and by tryptic and Cleveland peptide mapping procedures. HSP70 synthesis was induced by heat shock of rat and human cells, whereas 68KMAP was a major synthesised protein in the absence of heat shock, with its synthesis being only slightly increased on heat shock. Tryptic peptide mapping, however, indicated strong peptide homology between the two proteins. These data, therefore, confirm that 68KMAP represents a constitutively expressed, heat-shock cognate gene. Two-dimensional gel electrophoretic analysis of subcellular fractions of rat brain, combined with peptide mapping procedures, indicated that 68KMAP exists as at least two isoforms separable by isofocussing, the more acidic of which (alpha 68KMAP) is present in fractions enriched in microtubules, cytosol, microsomes, synaptosomal plasma membranes, and synaptic vesicles, and the more basic of which (beta 68KMAP) is present predominantly in fractions enriched in synaptic vesicles and synaptosomal plasma membranes. These two forms are distinguishable in terms of changes in Cleveland peptide maps, and we conclude that alpha- and beta 68KMAP, therefore, represent distinct forms. The significance of these findings to the molecular pathogenesis of Down's syndrome in the human brain is discussed.

Selective gene expression in focal cerebral ischemia

Regional patterns of protein synthesis were examined in rat cortex made ischemic by the occlusion of the right common carotid and middle cerebral arteries. At 2 h of ischemia, proteins were pulse labeled with intracortical injections of a mixture of [3H]leucine, [3H]isoleucine, and [3H]proline. Newly synthesized proteins were analyzed by two-dimensional gel fluorography, and the results correlated with local CBF, measured with [14C]iodoantipyrine as tracer. Small blood flow reductions (CBF = 50-80 ml 100 g-1 min-1) were accompanied by a modest inhibition in synthesis of many proteins and a marked increase in one protein (Mr 27,000). With further reduction in blood flow (CBF = 40 ml 100 g-1 min-1), synthesis became limited to a small group of proteins (Mr 27,000, 34,000, 73,000, 79,000, and actin) including two new polypeptides (Mr 55,000 and 70,000). Severe ischemia (CBF = 15-25 ml 100 g-1 min-1) caused the isoelectric modification of several proteins (Mr 44,000, 55,000, and 70,000) and induced synthesis of another protein (Mr 40,000). Two polypeptides (Mr 27,000 and 70,000) dominated residual protein synthesis in severe ischemia. The changes in protein synthesis induced by different grades of ischemia most likely comprise a variation of the so-called "heat shock" or "stress" response found in all eukaryotic cells subjected to adverse conditions. Since heat shock genes are known to confer partial protection against anoxia and a variety of other noxious insults, their induction may be a factor in limiting the extent of ischemic tissue damage.

Translational activity of mRNA coding for cytoskeletal brain proteins in newborn and adult mice: a comparative study

Translational activity of mRNA coding for cytoskeletal brain proteins was used to determine the relative abundance of the mRNA in the brains of newborn and adult mice. mRNA was translated in a cell-free system containing rabbit reticulocyte factors. The products of translation were analyzed by two-dimensional gel electrophoresis and characterized by peptide map analysis. Comparison of the products of translation from newborn and from adult brain mRNA shows a 50% decrease in actin and tubulin from newborn to the adult stage. In contrast, the 70 kd neurofilament protein and glial fibrillary acidic protein show a twofold increase in the adult stage. The heat-shock protein HSP70 increases slightly (30%) whereas the brain isozyme of creatine kinase and the heat-shock protein HSP90 are three times as high in adult subjects as in newborns.

The relationship of the rat brain 68 kDa microtubule-associated protein with synaptosomal plasma membranes and with the Drosophila 70 kDa heat-shock protein

A protein of molecular mass 68 kDa and pI5.6 is a major translation product of rat brain mRNA [Hall, Mahadevan, Whatley, Biswas & Lim (1984) Biochem. J. 219, 751-761]. In the rat brain this protein was associated with microtubule preparations and was present together with tubulin as a component of the synaptosomal plasma membranes, synaptic vesicles and post-synaptic structures. The brain mRNA for this protein was found to hybridize specifically to the Drosophila gene for the 70 kDa heat-shock protein, thus enabling its rapid isolation.

The brain isoform of a key ATP-regulating enzyme, creatine kinase, is a phosphoprotein

Two-dimensional electrophoretic analysis of crude microtubule preparations from the rat brain revealed the presence of three polypeptides in positions corresponding to those of the isovariants of purified rat brain creatine kinase (CK-BB). By the use of [gamma-32P]ATP, the two more acidic forms of these polypeptides were shown to be phosphorylated. Their identity as phosphorylated forms of CK-BB was established by using various peptide mapping techniques. Thus CK-BB is a phosphoprotein and its isoelectric variation may be attributed to phosphorylation.