Partial purification of the alpha-tubulin and beta-tubulin messenger RNAs from rat brain

Differential expression of several neurospecific beta-tubulin mRNAs in the mouse brain during development

The expression of alpha- and beta-tubulin mRNAs has been studied during mouse brain development by using in vitro translation and specific cDNA clone hybridization techniques. The results presented here indicate that about half of the isotubulins expressed in vivo by brain cells are directly encoded by mRNA, the other isotypes being of post-translational origin. Among the six brain beta-tubulin mRNAs detected after RNA size fractionation followed by in vitro translation, four (1.8a, 1.8c, 2.4 and 3.5 kb) appear to be uniquely expressed in brain, and very likely in neurons. These four beta-tubulin RNA transcripts are differently regulated during development. The 1.8a-kb mRNA, which codes for the neurospecific beta' 1-isotubulin, is expressed very early, whereas the 1.8c, 2.4 and 3.5-kb mRNAs, which all code for a beta 4-tubulin isotype, are all expressed later. We isolated a beta-tubulin cDNA clone which specifically hybridizes to the large, neural 3.5-kb mRNA. The two other ubiquitous tubulin mRNAs, 1.8b and 2.9 kb in length, both direct the synthesis of an electrophoretically distinct beta 3 isotype. These results support the idea that within the tubulin multigene family, distinct beta-tubulin isogenes could be specifically expressed at various steps of the neuronal differentiation, thus contributing to the isotubulin diversity and specificity observed in this cell type and possibly involved in the developmental changes of its structure and function.

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.

Posttranslational processing of alpha-tubulin during axoplasmic transport in CNS axons

Tubulin proteins in mouse retinal ganglion cell (RGC) neurons were analyzed to determine whether they undergo posttranslational processing during axoplasmic transport. Alpha- and beta-tubulin comprised heterogeneous proteins in the primary optic pathway (optic nerve and optic tract) when examined by two-dimensional (2D) PAGE. In addition, however, alpha-tubulin exhibited regional heterogeneity when consecutive 1.1-mm segments of the optic pathway were analyzed separately. In proximal segments, alpha-tubulin consisted of two predominant proteins separable by isoelectric point and several less abundant species. In more distal segments, these predominant proteins decreased progressively and the alpha-tubulin region of the gel was represented by less abundant multiple forms only; beta-tubulin region of the gel was represented by less abundant multiple forms only; beta-tubulin was the same in all segments. After intravitreal injection of [3H]proline to mice, radiolabeled alpha- and beta-tubulin heteroproteins were conveyed together at a rate of 0.1-0.2 mm/d in the slowest phase of axoplasmic transport. At 45 d postinjection, the distribution of radiolabeled heterogeneous forms a alpha- and beta-tubulin in consecutive segments of optic pathway resembled the distribution of unlabeled proteins by 2D PAGE, indicating that regional heterogeneity of tubulin arises during axonal transport. Peptide mapping studies demonstrated that the progressive alteration of alpha-tubulin revealed by PAGE analysis cannot be explained by contamination of the alpha-tubulin region by other proteins on gels. The results are consistent with the posttranslational processing of alpha-tubulin during axoplasmic transport. These observations, along with the accompanying report (J. Cell Biol., 1982, 94:150-158), provide additional evidence that CNS axons may be regionally specialized.