Cell-free synthesis of the myelin basic proteins in a wheat germ system programmed with brain messenger RNA

Translational regulation of myelin basic protein synthesis

Synthesis of the mouse myelin basic proteins (MBPs) was studied in reticulocyte lysates programmed with brain mRNA in the presence or absence of brain factors. Addition of brain factors to the lysates increased the incorporation of [35S]methionine into total TCA-precipitable protein by a factor of 6-9, and the majority of this stimulation was found to be due to initiation factors. Although brain factors increased total protein synthesis, the percentage of MBP synthesis was reduced from 4.3% of the total counts incorporated (in the absence of brain factors) to 1.4% (in their presence). Increasing the concentration of brain mRNA in the lysates also reduced the relative levels of MBP synthesis. These results suggested the MBP mRNAs, as a group, were less efficiently initiated than most brain mRNAs. An analysis of the nucleotide sequence flanking the initiator codon of the MBP mRNAs indicates the presence of a second AUG codon 5 bases upstream, immediately followed by a termination codon, which may provide a structural explanation for the poorer initiation efficiency of the MBP mRNAs. Further analysis of the synthesis of the individual MBPs in the presence or absence of inhibitors of initiation (7-methylguanosine triphosphate and aurintricarboxylic acid) and elongation (anisomycin and emetine) indicated than the 14-kD MBP mRNA was less efficiently translated than the other MBP mRNAs. Synthesis of the 14-kD MBP was more strongly inhibited by the initiation inhibitors than the other MBP mRNAs, and synthesis of the 14-kD MBP was increased relative to the other MBPs in the presence of elongation inhibitors.(ABSTRACT TRUNCATED AT 250 WORDS)

Investigations on myelinogenesis in vitro: developmental expression of myelin basic protein mRNA and its regulation by thyroid hormone in primary cerebral cell cultures from embryonic mice

The concentration of myelin basic protein (MBP) mRNA in primary cultures of cells dissociated from embryonic mouse cerebra and grown in the presence of varying amounts of thyroid hormone was measured using a 32P-labeled cDNA probe and a dot-blot procedure. The cDNA probe contained 1.85 kilobases of the gene for MBP. The concentration of mRNA specific for MBP in control cells grown on a medium containing normal (euthyroid) calf serum increased with increasing age of culture. The greatest increase occurred between 15 and 35 days in culture (5.25-fold increase); whereas between 35 and 50 days in culture, the rate of accumulation slowed to yield a net increase of MBP mRNA of only 10%. The quantity of MBP mRNA was drastically diminished at all ages studied when the cells were grown from the sixth day onward on a medium containing hypothyroid calf serum. Although the amount of MBP mRNA in hypothyroid-treated cells did increase, the change in concentration was less (3.43-fold), and it peaked earlier (at 30 days). Unlike the euthyroid cells, after 30 days the MBP mRNA actually fell in the hypothyroid-treated cells. If hypothyroid media were supplemented with triiodothyronine (T3) on the eighth day in culture, the quantity of MBP mRNA in the cells was restored almost completely to the levels found in the control euthyroid cells at all ages. Therefore, the regulation of the synthesis of MBP by thyroid hormone is at least in part a pretranslational event; that is, thyroid hormone adjusts the concentration of the mRNA specific for MBP.

Molecular basis for heterosis for myelin basic protein content in mice

Poly(A)+ mRNA was isolated from the brains of C57BL/6J (B6), DBA/2J (D2), and F1 hybrid mice (B6 X D2) of 16-17 days of age. The yield of polysomal RNA, both poly(A)+ and poly(A)-, from the three strains of mice was comparable. When translated in vitro in a reticulocyte lysate system, the mRNA preparations had the same efficiency with respect to stimulation of amino acid incorporation into protein. However, a significant heterotic effect was seen for the production of myelin basic protein (MBP) by the mRNA from the F1 mice. That is, the fraction of protein synthesized as MBP was greater for the F1 hybrid than for either parental strain. The distribution of the form of MBPs was not different among the three strains. We therefore believe that heterosis for brain MBP content in the F1 hybrid may be regulated at the transcriptional or post-transcriptional level.

Expression of myelin basic protein genes in several dysmyelinating mouse mutants during early postnatal brain development

Northern blot and "dot" blot analyses using a myelin basic protein (MBP) specific cDNA probe and in vitro translation techniques were utilized to estimate the relative levels of myelin basic protein messenger RNA (mRNA) in the brains of C57BL/6J control mice, three dysmyelinating mutants (qk/qk, jp/Y, and shi/shi), and three heterozygote controls (qk/+, jp/+, and shi+) during early postnatal development. In general, the MBP mRNA levels measured directly by Northern blot and "dot" blot analyses correlated well with the indirect in vitro translation measurements. The Northern blots indicated that the size of MBP mRNAs in quaking and jimpy brain polysomes appeared to be similar to controls. Very low levels of MBP mRNAs were observed in shi/shi brain polyribosomes throughout early postnatal development. Compared to C57BL/6J controls, accumulation of MBP mRNAs in qk/qk and qk/+ brain polyribosomes was delayed by several days. That is, whereas MBP mRNA levels were below normal between 12 and 18 days, normal levels of message had accumulated in both qk/qk and qk/+ brain polyribosomes by 21 days. Furthermore, normal levels of MBP mRNAs were observed to be maintained until at least 27 days. MBP mRNA levels remained well below control levels in jp/Y brain polyribosomes throughout early postnatal development. The levels did, however, fluctuate slightly and peaked at 15 days in both jp/Y and jp/+ brains, 3 days earlier than in normal mice. Thus, it appears that jimpy and quaking mice exhibit developmental patterns of MBP expression different from each other and from C57BL/6J control mice.

In vitro translation of mRNA from the developing sciatic nerve

Poly(A+)RNA from sciatic nerves of embryos and chicks was translated in a rabbit reticulocyte cell-free system. The 30-K mol. wt. PO protein which is the major and specific protein constituent of peripheral nervous system myelin was used as a marker of myelin synthesis in the preparations. Immunoprecipitation of total translation products with rabbit anti-PO protein antibody showed the presence of a 29K protein band on the autoradiograms. The intensity of this specific immunoprecipitated band increased significantly from very low levels with RNA from the 15-day embryonic nerve to a maximum with extracts from 1-day-old chicks. This increase parallels that of the rapid deposition of the PO protein in the developing sciatic nerve in vivo. The experiments demonstrate for the first time, that it is possible to show synthesis of PO protein in vitro using exogenous RNA as the template.

Analysis of brain and pituitary RNA metabolism: a review of recent methodologies

Recent characterization of brain and pituitary RNA metabolism is reviewed. Relative to other tissues, the brain transcribes more of the unique, single-copy DNA. This transcriptional diversity reflects the inherent heterogeneity in organization and development of the brain. The end product of transcriptional regulation in the brain is a population of functional cytoplasmic mRNAs with multiple components, differing in complexity and abundance. Analysis of nuclear and cytoplasmic RNA provides evidence that both brain-specific synthesis and processing may determine the final mRNA population. Both polyadenylated and non-polyadenylated RNA classes contribute significantly to the total brain polysomal mRNA fraction. Characterizations of individual species of mRNA from both brain and pituitary are described. One possible transcriptional modulator in both the pituitary and brain is the presence of steroid hormone at responsive sites. Functional consequences of steroid accumulation within the brain may be (1) interactions with neurotransmitter, especially catecholamine, metabolism and function, (2) developmental interactions with neuronal systems, and (3) differentiation of glial cell function. The pleiotropic nature of steroid hormone effects (both transcriptional and non-transcriptional) within one brain region is considered by examining the biochemical effects of glucocorticoids in the hippocampus.

In vitro synthesis of polypeptides of moderately large size by poly(A)-containing messenger RNA from postmortem human brain and mouse brain

Studies were undertaken to optimize the conditions for isolation and in vitro translation of poly(A)-containing mRNA from human postmortem brain. The comparison of several methods for preparation of mRNA from frozen mouse brain indicated that although the yield of mRNA was increased using polysomes prepared in the presence of ribonucleoside vanadyl complexes and subsequently extracted with guanidinium thiocyanate, the translation products were indistinguishable from those synthesized by total cellular RNA directly extracted from tissue with guanidinium thiocyanate. The oligo d(T)-cellulose-purified poly(A)-containing mRNA preparations were translated in vitro in a rabbit reticulocyte lysate in the presence of L-[35S]methionine. Messenger RNA from frozen mouse brain stimulated protein synthesis from 9- to 20-fold over endogenous mRNA. Over 450 polypeptides were reproducibly synthesized and separated by two-dimensional polyacrylamide gel electrophoresis (PAGE); size classes up to 130,000 daltons were present. Direct extraction of RNA from frozen human cerebral cortex and cerebellum with guanidinium thiocyanate followed by oligo d(T)-cellulose chromatography yielded 1.8 micrograms/g and 2.0 micrograms/g, respectively, of poly(A)-containing mRNA; this represents a two- to fourfold increase over our earlier results. In the rabbit reticulocyte translation system human brain mRNA stimulated protein synthesis nearly threefold over endogenous mRNA. Compared with earlier studies, the number of newly synthesized polypeptides was increased by 30%. Over 300 species were separated by two-dimensional PAGE, and size classes up to 130,000 daltons were present, as compared to 70,000 in an earlier report. The polypeptides synthesized by human cerebral cortex and cerebellum were indistinguishable. However, several appeared to be uniquely human when compared with the products synthesized by mouse brain mRNA. The method described for the preparation of postmortem human brain mRNA eliminates the need to prepare polysomes, which are recovered in variable and low yield from the postmortem human brain. The procedure appears applicable to studies on the synthesis of moderately large human brain polypeptides and for investigations of brain protein polymorphism when relatively large numbers of products are required for analysis.

mRNA-directed cell-free myelin basic protein synthesis: no evidence of neuronal control

The specific mRNA of myelin basic protein (MBP) was isolated from active myelinating rat brains. The isolated mRNA was able to direct the synthesis of immunochemically detectable MBP in a wheat-germ cell free protein synthesis system. There was, however, no evidence of neuronal translational control of the in vitro MBP synthesis kinetics found within the experimental parameters. The possibility of neuronal control of MBP gene expression is therefore postulated likely to be at the transcriptional level.

The polyadenylated RNA directing the synthesis of the rat myelin basic proteins is present in both free and membrane-bound forebrain polyribosomes

Free and membrane-bound polyribosomes were isolated from the forebrain of actively myelinating 24-day-old rats. The poly(A)+ RNA (polyadenylated RNA) extracted from both fractions was translated in vitro in reticulocyte lysates [Hall & Lim (1981) Biochem. J. 196. 327-336] in the presence or absence of a heterologous microsomal membrane fraction from dog pancreas. The rat myelin basic proteins synthesized in vitro were isolated by CM-cellulose chromatography and by immunoprecipitation with purified anti-(myelin basic protein) antibody. The large (mol.wt. 18 500) and small (mol.wt. 16 000) myelin basic proteins were translational products of poly(A)+ RNA from both free and membrane-bound polyribosomes. The identity of the myelin basic proteins was verified by analysis of peptides generated by the cathepsin D digestion of the immunoprecipitated proteins synthesized in vitro, in comparison with authentic rat myelin basic proteins. Although several other translational products of membrane-bound polyribosomal poly(A)+ RNA were modified when microsomal membranes were present during translation, molecular weights of the myelin basic proteins themselves were unchanged. The myelin basic proteins synthesized in vitro also did not differ significantly in size from the authentic myelin basic proteins, indicating that these membrane proteins are unlikely to be synthesized as substantially larger precursor molecules. The presence of the specific mRNA species on both free and membrane-bound polyribosomes is compatible with the extrinsic location of the myelin basic proteins on the cytoplasmic surface of the myelin membrane.