In vitro biosynthesis of tubulin on total, free and membrane-bound polysomes from the developing rat brain

Gene expression in cells of the central nervous system

This review summarized a part of our studies over a long period of time, relating them to the literature on the same topics. We aimed our research toward an understanding of the genetic origin of brain specific proteins, identified by B. W. Moore and of the high complexity of the nucleotide sequence of brain mRNA, originally investigated by W. E. Hahn, but have not completely achieved the projected goal. According to our studies, the reason for the high complexity in the RNA of brain nuclei might be the high complexity in neuronal nuclear RNA as described in the Introduction. Although one possible explanation is that it results from the summation of RNA complexities of several neuronal types, our saturation hybridization study with RNA from the isolated nuclei of granule cells showed an equally high sequence complexity as that of brain. It is likely that this type of neuron also contains numerous rare proteins and peptides, perhaps as many as 20,000 species which were not detectable even by two-dimensional PAGE. I was possible to gain insight into the reasons for the high sequence complexity of brain RNA by cloning the cDNA and genomic DNA of the brain-specific proteins as described in the previous sections. These data provided evidence for the long 3'-noncoding regions in the cDNA of the brain-specific proteins which caused the mRNA of brain to be larger than that from other tissues. During isolation of such large mRNAs, a molecule might be split into a 3'-poly(A)+RNA and 5'-poly(A)-RNA. In the studies on genomic DNA, genes with multiple transcription initiation sites were found in brain, such as CCK, CNP and MAG, in addition to NSE which was a housekeeping gene, and this may contribute to the high sequence complexity of brain RNA. Our studies also indicated the presence of genes with alternative splicing in brain, such as those for CNP, MAG and NGF, suggesting a further basis for greater RNA nucleotide sequence complexity. It is noteworthy that alternative splicing of the genes for MBP and PLP also produced multiple mRNAs. Such a mechanism may be a general characteristic of the genes for the myelin-specific proteins produced by oligodendrocytes. In considering the high nucleotide sequence complexity, it is interesting that MAG and S-100 beta genes etc. possess two additional sites for poly(A).(ABSTRACT TRUNCATED AT 400 WORDS)

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.

The effect of electroconvulsive shock on brain tubulin during development and aging

Effect of electroconvulsive shock on rat brain tubulin content was studied during maturation and aging. The results show that electroconvulsive shock had no effect on soluble tubulin in different brain structures of young animals (22 days) while the same treatment produced a marked decline in adult (95 days) and aged (490-511 days) animals. The same treatment produced inhibition of 3H-leucine incorporation into tubulin and decrease of 3H-colchicine binding in the proteins of synaptosomes isolated from the centricephalic structures of all the ages examined. Tubulin biosynthesis by free polysomes was not diminished to the extent which could explain the decrease of tubulin level found in the soluble or synaptosomal fraction. Thus, our results suggest that changes in soluble tubulin content in response to electroconvulsive shock could be a reflection of changes in equilibrium: tubulin dimers--microtubules--membrane-bound tubulin.

Modulation of tubulin synthesis by tri-iodothyronine in the embryonic chick brain

In organ cultures of embryonic chick brain, tri-iodothyronine is shown to elicit an age-dependent stimulation of the synthesis of tubulin. In cultures of brains from 8-day embryos where the response is maximal, a 70-100% increase in the rate of synthesis of tubulin is elicited by T3 within 2 h. The stimulatory response is dose-dependent, requires transcription, and is temporally coincident with the early phase of the normal ontogenic rise in level of tubulin and of intense neuronal differentiation in the embryonic chick brain. The overall results indicate the involvement of T3 in regulating the biogenesis of tubulin in the developing brain.

Taxol induces microtubule-rough endoplasmic reticulum complexes and microtubule-bundles in cultured chondroblasts

Taxol induces a vast increase in the number of microtubules (MTs) in functional chondroblasts. The drug also induces a marked change in MT distribution. In control cultures, anti-tubulin stains long, fine, sinuous filaments radiating from a perinuclear center. In taxol-treated cells, anti-tubulin stains stubby, straight, chevron-like structures that assume a striking antipodal distribution. Such MT-bundles are relatively stable: they persist for over 48 h after removal of taxol, and even for 16-24 h in Colcemid. Many of these supernumerary MTs bind to, and align on, the cytoplasmic face of the rough endoplasmic reticulum (RER). In binding, the MTs displace the numerous ribosomes that normally stud the surface of the cisternae of the RER. The bound MTs form a remarkably uniform layer with center-to-center spacings of 40 nm. The attached parallel arrays of MTs achieve lengths of over 10 microns. These bound MTs not only dislodge ribosomes from the RER surface, but they also zip together adjacent ER complexes, forming tiers of two to eight cisternae. Numerous cytoplasmic bundles of hexagonally-ordered MTs are also induced. When closely aligned, the MTs assume a crystalline configuration with a six-fold symmetry, a central MT being surrounded by six equidistant MTs. A single cell can have over 100 MT-bundles and the number of MTs per bundles varies from 2-30. The forces aggregating cytoplasmic MT-bundles probably differ from those that bind MTs to the RER. Taxol also fragments the prominent Golgi complex that characterizes actively secreting chondroblasts. No obvious morphologic relationship has yet been detected between these induced MTs and other organelles such as intermediate-sized filaments, microfilaments, mitochondria, Golgi cisternae, or secretory vesicles.

Blocked and methylated 5'-terminal cap structures of rat brain messenger ribonucleic acids

The presence and identity of 5'-terminal cap structures in rat brain polysomal mRNA were investigated by radiolabeling the mRNA by periodate oxidation and [3H]sodium borohydride reduction or by beta-elimination of 5'-terminal nucleoside and incorporation of 32P in the presence of polynucleotide kinase. The labeled mRNAs were digested with nucleases and the cap structures were isolated and identified by chromatographic and electrophoretic procedures The results showed that rat brain mRNAs contained cap 1 and cap 2 structures and no caps of the zero type. The proportion of cap 2 was higher than that of cap 1. Both caps had 7-methylguanosine (m7G) as the 5'-terminal nucleoside, which was linked to the next nucleoside by an inverted triphosphate bridge, as in other eukaryotic mRNAs. The most prominent nucleoside in the 5'-penultimate position was 6-methyl-2'-O-methyladenosine [m6A(m)] followed by 2'-O-methyladenosine [A(m)], which together contributed to nearly 70% of both cap 1 and cap 2 structures. 2'-O-Methylguanosine [G(m)] accounted for approximately 18%, the rest being made up of 2'-O-methylcytidine [C(m)] and 2'-O-methyluridine [U(m)].

Microtubule arrays in taxol-treated mouse dorsal root ganglion-spinal cord cultures

Exposure of organotypic dorsal root ganglion-spinal cord cultures to taxol, a potent microtubule promoting and stabilizing agent, results in an unusual abundance of microtubules in neurons, and the presence of microtubule-endoplasmic reticulum arrays in their perikarya and processes. Ordered concentric arrays of microtubules alternating with marcromolecular material are observed in dorsal root ganglion neurites. Short linear structures are discernible between some microtubules and the macromolecular material, as well as between microtubules and endoplasmic reticulum cisternae. Analyses of such unusual microtubule arrays in taxol-treated cultures may provide valuable insights into tubulin-related systems in neurons, as well as in other cells.

Tubulin synthesis in rat forebrain: studies with free and membrane-bound polysomes

Free and membrane-bound polysomes were prepared from rat forebrain and added to a cell-free system containing rabbit reticulocyte factors and L-[35S]methionine. The translation products were analyzed by two-dimensional gel electrophoresis followed by autoradiography. The free polysomes synthesized actin and at least four major tubulin subunits (alpha 1, alpha 2, beta 1, and beta 2) that are found in rat forebrain cytoplasm. The membrane-bound polysomes synthesized predominantly one protein (MB) in the tubulin region of the two-dimensional gel. MB has a molecular weight and isoelectric point similar to alpha-tubulin. Only trace amounts of alpha- and beta-tubulin and action were synthesized by the membrane-bound polysomes. MB co-purified with cytoplasmic tubulin after two cycles of aggregation and disaggregation. MB synthesized in vitro (from membrane-bound polysomes) and alpha- and beta-tubulin and actin subunits (synthesized from free polysomes) were digested with Staphylococcus aureus V8 protease, and the resulting peptides were separated by slab gel electrophoresis followed by autoradiography. The peptide pattern of MB was similar but not identical to the peptide patterns of alpha- and beta-tubulin; MB yielded peptides not found in tubulin. We conclude that membrane-bound polysomes from rat forebrain do not synthesize significant amounts of the predominant tubulin subunits synthesized by free polysomes. A major protein (MB) is synthesized by membrane-bound polysomes and is similar, but not identical, to alpha-tubulin synthesized by free polysomes on the basis of molecular weight, isoelectric point, and peptide analysis.

Association of newly synthesized tubulin with brain microsomal membranes

Tubulin has been found to be synthesized on both membrane-bound and free polyribosomes prepared from brain. Cell-free studies indicate that tubulin made on rough microsomes is incorporated into the endoplasmic reticulum membrane as it is synthesized. This tubulin remains associated with the membrane after sedimentation and washing. The tubulin is not removed from the membrane after stripping ribosomes from the membranes in KCl-puromycin, followed by repeated washing by either sedimentation or flotation in 0.05 M-KCl. The membrane tubulin is partially susceptible to proteolysis by trypsin and chymotrypsin: beta-tubulin is more accessible to the proteases than in alpha-tubulin. Nonionic detergents extract mostly beta-tubulin from the microsomal membrane. Newly synthesized tubulin which has been extracted from microsomal membranes in 0.5% Nonidet P-40, coassembles and disassembles with carrier microtubule protein. The insertion of newly synthesized tubulin into endoplasmic reticulum membrane may be the first step in the incorporation of tubulin into the plasma membrane.

An apparent paradox in the occurrence, and the in vivo turnover, of C-terminal tyrosine in membrane-bound tubulin of brain

Tubulin tyrosine ligase catalyzes the reversible addition of tyrosine to the C-terminus of tubulin alpha chains. By using ligase and carboxypeptidase A in conjunction, we have previously shown that brain cytoplasmic tubulin exists in three forms: 15-40% already has C-terminal tyrosine, another 10-30% can accept additional tyrosine, and about one-half is an uncharacterized species which is not a ligase substrate. A membrane-bound fraction of brain tubulin, purified by vinblastine precipitation from a detergent extract, has been found to differ by the complete absence of preexisting tyrosine. The membrane fraction from which tubulin was extracted also contained masked forms of both ligase and a distinct detyrosylating enzyme, which can be released by detergent extraction. The turnover of alpha-chain C-terminal tyrosine in vivo was studied by incubating brain mince with labeled tyrosine, or injecting it intracerebrally, under conditions where protein synthesis was inhibited. Tyrosine appeared to turn over to about the same extent in membrane-bound, as in soluble, tubulin. This apparently paradoxical result was not due to ATPase in the membrane fraction, which might have allowed ligase-catalyzed exchange between free and fixed tyrosine. Authentic [14C]tyrosylated tubulin added to the brain membrane fraction was not detyrosylated or subject to endoprotease digestion during subsequent procedures to isolate tubulin. The unexpected finding that tubulin tyrosylated at the C-terminal in vivo appears to be in the "non-substrate" fraction points toward a possible resolution of the paradox.

Compositional changes in soluble proteins of cerebral mantle, cerebellum, and brain stem of rat brain during development: a two-dimensional gel electrophoretic analysis

Soluble proteins from the cerebral mantle, cerebellum, and brain stem of rat brains were analyzed at various developmental stages by a two-dimensional gel electrophoresis technique. The electrophoretic technique resolved the soluble proteins into 100--150 polypeptide spots on two-dimensional gels and gave reproducible and highly resolved profiles of them. Although most of major polypeptides were commonly found in all the three brain regions, some polypeptides were shown to be unique to a specific brain region. Each brain region was different in the electrophoretic profile of soluble proteins at every developmental stage examined, although there was considerable similarity in the profiles of each of the three brain regions in fetal animals (16--17 days), indicating that soluble proteins undergo different compositional changes in each of the three brain regions during postnatal development. In addition, the number of polypeptide spots on the electrophoretic profile increased remarkably during postnatal development in all of the three brain regions, suggesting that soluble proteins become more heterogeneous during postnatal development in each of the three brain regions.

Poly(A)- and nonpoly(A)-RNA associated with rat brain microsomal fractions: in vivo labelling studies

The time course of incorporation of radiolabelled precursor into RNA associated with rat brain free polyribosomes, rough membranes, and smooth membranes was measured following a single intracranial injection of [3H] orotic acid. Polyadenylated RNAs were separated from nonpolyadenylated RNAs by affinity chromatography on oligo (dT)-cellulose columns. Poly(A)-RNA associated with each of the microsomal fractions became more rapidly labelled than did the nonpoly(A)-RNA of the same fractions. While the labelling profiles of the nonpoly(A)-RNA isolated from the polyribosomes and rough membranes are similar from one fraction to another, the specific radioactivity of the poly(A)-RNA isolated from free polyribosomes increased much more drastically than that of the poly(A)-RNA associated with rough membranes. The labelling profiles of RNA species isolated from smooth membranes were very different in this respect from the two ribosomal fractions. There was a lag of more than four hours before significant label appeared in the RNA associated with the smooth membrane fraction. These studies demonstrate that the different populations of brain microsomal RNA are labelled at different rates, perhaps reflecting differences in the turnover of these RNAs and differences in their function.