One beta-tubulin subunit accumulates during neurite outgrowth in mouse neuroblastoma cells

Patterns of tubb2b Promoter-Driven Fluorescence in the Forebrain of Larval Xenopus laevis

Microtubules are essential components of the cytoskeleton of all eukaryotic cells and consist of α- and β-tubulin heterodimers. Several tissue-specific isotypes of α- and β-tubulins, encoded by distinct genes, have been described in vertebrates. In the African clawed frog (Xenopus laevis), class II β-tubulin (tubb2b) is expressed exclusively in neurons, and its promoter is used to establish different transgenic frog lines. However, a thorough investigation of the expression pattern of tubb2b has not been carried out yet. In this study, we describe the expression of tubb2b-dependent Katushka fluorescence in the forebrain of premetamorphic Xenopus laevis at cellular resolution. To determine the exact location of Katushka-positive neurons in the forebrain nuclei and to verify the extent of neuronal Katushka expression, we used a transgenic frog line and performed several additional antibody stainings. We found tubb2b-dependent fluorescence throughout the Xenopus forebrain, but not in all neurons. In the olfactory bulb, tubb2b-dependent fluorescence is present in axonal projections from the olfactory epithelium, cells in the mitral cell layer, and fibers of the extrabulbar system, but not in interneurons. We also detected tubb2b-dependent fluorescence in parts of the basal ganglia, the amygdaloid complex, the pallium, the optic nerve, the preoptic area, and the hypothalamus. In the diencephalon, tubb2b-dependent fluorescence occurred mainly in the prethalamus and thalamus. As in the olfactory system, not all neurons of these forebrain regions exhibited tubb2b-dependent fluorescence. Together, our results present a detailed overview of the distribution of tubb2b-dependent fluorescence in neurons of the forebrain of larval Xenopus laevis and clearly show that tubb2b-dependent fluorescence cannot be used as a pan-neuronal marker.

Polyglutamylated .alpha.-tubulin can enter the tyrosination/detyrosination cycle

We have previously identified a major modification of neuronal alpha-tubulin which consists of the posttranslational addition of a varying number of glutamyl units on the gamma-carboxyl group of glutamate residue 445. This modification, called polyglutamylation, was initially found associated with detyrosinated alpha-tubulin [Eddé, B., Rossier, J., Le Caer, J.P., Desbruyères, E., Gros, F., & Denoulet, P. (1990) Science 247, 83-85]. In this report we show that a lateral chain of glutamyl units can also be present on tyrosinated alpha-tubulin. Incubation of cultured mouse brain neurons with radioactive tyrosine, in the presence of cycloheximide, resulted in a posttranslational labeling of six alpha-tubulin isoelectric variants. Because both tyrosination and polyglutamylation occur in the C-terminal region of alpha-tubulin, the structure of this region was investigated. [3H]tyrosinated tubulin was mixed with a large excess of unlabeled mouse brain tubulin and digested with thermolysin. Five peptides, detected by their radioactivity, were purified by high-performance liquid chromatography. Amino acid sequencing and mass spectrometry showed that one of these peptides corresponds to the native C-terminal part of alpha-tubulin 440VEGEGEEEGEEY451 and that the remainders bear a varying number of glutamyl units linked to glutamate residue 445, which explains the observed heterogeneity of tyrosinated alpha-tubulin. A quantitative analysis showed that the different tyrosinated forms of alpha-tubulin represent a minor (13%) fraction of the total alpha-tubulin present in the brain and that most (80%) of these tyrosinated forms are polyglutamylated.(ABSTRACT TRUNCATED AT 250 WORDS)

Coassembly of bovine and cod microtubule proteins: the ratio of the different tubulins within hybrid microtubules determines the ability to assemble at low temperatures, MAPs dependency and effects of Ca2+

Cod and bovine microtubule proteins (MTP) differ from each other in many respects, e.g., tubulin isoforms and microtubule-associated proteins (MAPs) but only cod MTP are cold-adapted. We used these differences to determine how tubulin isoform composition affects microtubule properties. Mixtures of cod and bovine MTP coassembled at 30 degrees C as shown by light scattering and immunoelectron microscopy, with no apparent preference for one set of MAPs over the other. Bovine tubulin was, in contrast to cod tubulin, unable to assemble in the absence of MAPs, while 50%/50% mixtures of bovine and cod tubulin, respectively, coassembled readily without exclusion of cod or bovine tubulin isoforms in the hybrids, as shown by two-dimensional gel electrophoresis. Alteration in MAPs dependency was also confirmed by the use of the MAPs-binding microtubule inhibitor estramustine phosphate. Addition of 10 mM Ca2+ to microtubules induced formation of spirals or rings depending on the ratio of the cod and bovine MTP, respectively. Bovine MTP were unable to assemble at low temperatures, while cod MTP are cold-adapted and assembled efficiently at 14 degrees C in the presence of MAPs. Amounts of cod MTP as low as 33% were enough to induce assembly of bovine/cod MTP hybrids. The critical concentration for assembly of a 50%/50% mixture was similar to that of 100% cod MTP. Taken together, the results show that the divergent cod and bovine MTP can coassemble, and that alterations in tubulin isotype/isoform composition above certain thresholds significantly modulate microtubule properties such as MAPs dependency, effects of Ca2+, and ability to assemble at low temperatures.

Effect of melatonin on beta-tubulin and MAP2 expression in NIE-115 cells

Physiological concentrations of the pineal hormone melatonin induce an increase of microtubules in neuroblastoma NIE-115 cells. This effect is due to an increase in the polymerization state of tubulin. Concomitantly, higher levels of soluble beta-tubulin are present in the treated cells. Unexpectedly, no significant changes in the levels of beta-tubulin or its mRNA occur in the presence of melatonin reflecting perhaps a strict control of its steady state in a physiological context. In contrast, higher amounts of microtubule-associated-protein 2 are found when the cells are exposed to melatonin. These findings support the idea that tubulin polymerization process is one of the targets of melatonin action. Furthermore, our results might explain the increase in the length and number of neurites present in these cells when they are treated with this hormone.

Biology of the congenitally hypothyroid hyt/hyt mouse

The hyt/hyt mouse has an autosomal recessive, fetal onset, characterized by severe hypothyroidism that persists throughout life and is a reliable model of human sporadic congenital hypothyroidism. The hypothyroidism in the hyt/hyt mouse reflects the hyporesponsiveness of the thyroid gland to thyrotropin (TSH). This is attributable to a point mutation of C to T at nucleotide position 1666, resulting in the replacement of a Pro with Leu at position 556 in transmembrane domain IV of the G protein-linked TSH receptor. This mutation leads to a reduction in all cAMP-regulated events, including thyroid hormone synthesis. The diminution in T3/T4 in serum and other organs, including the brain, also leads to alterations in the level and timing of expression of critical brain molecules, i.e. selected tubulin isoforms (M beta 5, M beta 2, and M alpha 1), microtubule associated proteins (MAPs), and myelin basic protein, as well as to changes in important neuronal cytoskeletal events, i.e. microtubule assembly and SCa and SCb axonal transport. In the hyt/hyt mouse, fetal hypothyroidism leads to reductions in M beta 5, M beta 2, and M alpha 1 mRNAs, important tubulin isoforms, and M beta 5 and M beta 2 proteins, which comprise the microtubules. These molecules are localized to layer V pyramidal neurons in the sensorimotor cortex, a site of differentiating neurons, as well as a site for localization of specific thyroid hormone receptors. These molecular abnormalities in specific cells and at specific times of development or maturation may contribute to the observed neuroanatomical abnormalities, i.e. altered neuronal process growth and maintenance, synaptogenesis, and myelination, in hypothyroid brain. Abnormal neuroanatomical development in selected brain regions may be the factor underlying the abnormalities in reflexive, locomotor, and adaptive behavior seen in the hyt/hyt mouse and other hypothyroid animals.

Differential distribution of glutamylated tubulin during spermatogenesis in mammalian testis

The distribution of glutamylated tubulin has been analyzed in mammalian testis using the specific mAb GT335 by immunoelectron microscopy and immunoblotting. In spermatozoa of various species, immunogold labeling showed the presence of glutamylated tubulin in all of the microtubules of axoneme and centrioles, whereas the microtubule network of the spermatid manchette was unlabeled. In earlier germ cells, centriole was the only microtubule structure to be labeled. A similar distribution was observed using the anti-acetylated tubulin antibody (6-11B-1), confirming previous results of Hermo et al. [Anat. Rec. 229:31-50, 1991]. However, among testicular somatic cells, microtubules of some Sertoli cell branches were not acetylated but glutamylated. 2-D PAGE of mouse and hamster sperm extracts showed a high level of alpha and beta-tubulin heterogeneity, comparable to that found in brain. Immunoblotting with GT335 revealed a large amount of glutamylated tubulin resolved into numerous alpha as well as beta-tubulin isoforms. This suggests that the major testis-specific tubulin isotypes (m alpha 3/7 and m beta 3) are also glutamylatable. These results show a subcellular sorting of posttranslationally modified tubulin isoforms in spermatids, glutamylation being associated with the most stable microtubule structures.

Glutamylated tubulin probed in ciliates with the monoclonal antibody GT335

Microtubular networks are extensively developed in many ciliate species. In several of them, we investigate the occurrence of the post-translational glutamylation of tubulin [Eddé et al., 1990: Science 247:82-85; Eddé et al., 1991: J. Cell. Biochem. 46:134-142] using as a probe for such modified tubulin, the monoclonal antibody GT335 [Wolff et al., 1992: Eur. J. Cell Biol. 59:425-432]. Results obtained in Paramecium strongly suggest that both axonemal and cytoplasmic tubulin are glutamylated. As in the vertebrate brain tubulin so far tested, the GT335 epitope is located at the carboxy-terminal fragment of cytoplasmic tubulin removed by subtilisin treatment. Immunoblotting and immunofluorescence experiments reveal that, unlike tubulin acetylation, glutamylation is not restricted to cold-resistant microtubules. In addition, immunofluorescence studies performed on dividing cells show that glutamylation takes place soon after the polymerization of microtubules. Finally, glutamylated tubulin is also detected in the ciliate species Euplotes, Tetrahymena, and Paraurostyla. Together with results obtained on flagellate species, this suggests that tubulin glutamylation came out early in the course of eukaryotic evolution and has been widely exploited in various cellular strategies.

Adriamycin promotes neurite outgrowth in the "neurite-minus" N1A-103 mouse neuroblastoma cell line

Adriamycin, an anticancer agent acting on topoisomerase II, promotes the arrest of cell division and neurite extension in a "neurite-minus" murine neuroblastoma cell line, N1A-103. This morphological differentiation is accompanied by a blockade in the S phase of the cell cycle, modification of the amount of peripherin, and appearance of the beta 7-tubulin isoform. Yet, adriamycin-induced N1A-103 cells fail to express other neuronal markers, such as long-lasting Ca2+ channels, synaptophysin, and the shift in the proportion of the beta'1 tubulin isoform to the beta'2 isoform, whose appearance parallels the terminal differentiation of the wild type neuroblastoma cell line N1E-115. Hence, a comparison of the behavior of these two cell lines leads to the proposal that there are two programs of neuroblastoma differentiation: one where expression is triggered by the arrest of cell division and which is observed in adriamycin-induced N1A-103 variant cells, and the other, presumably occurring further downstream, which would involve further changes in morphogenesis and acquisition of new electrophysiological properties.

Expression of the class III beta-tubulin isotype in developing neurons in culture

The expression of the class III beta-tubulin isotype was studied in cultured brain neurons by means of a monoclonal antibody (TuJ1). The results obtained indicate that during early axonal outgrowth most of the class III beta-tubulin is not incorporated into microtubules, a phenomenon which is also observed under conditions which alter the rate and extent of the neurite outgrowth response. On the other hand, a dramatic increase in its incorporation into microtubules is observed after the neurons have differentiated their neurites as axons and dendrites. In addition, the appearance of colchicine-resistant microtubules containing this isotype, a phenomenon which occurs late in neurite development, is highly coincident with the appearance of stable microtubules containing high molecular weight microtubule-associated proteins (MAPs). This pattern is different from that of the accumulation and incorporation of other beta-tubulin isotypes into microtubules. Taken collectively, our results indicate that differences exist in the in vivo utilization of tubulin isotypes in developing brain neurons and suggest that the class III beta-tubulin isotype is not a primary factor involved in the regulation of microtubule assembly during early neurite outgrowth, but that it may be important for maintaining further neurite elongation and/or determining some unique binding property of MAPs to specific microtubule subsets.

Microtubule functions

Microtubules, with intermediate filaments and microfilaments, are the components of the cell skeleton which determinates the shape of a cell. Microtubules are involved in different functions including the assembly of mitotic spindle, in dividing cells, or axon extension, in neurons. In the first case, microtubules are highly dynamic, while in the second case microtubules are quite stable, suggesting that microtubule with different physical properties (stability) are involved in different functions. Thus, to understand the mechanisms of microtubule functions it is very important to understand microtubule dynamics. Historically, tubulin, the main component of microtubules, was first characterized as the major component of the mitotic spindle that binds to colchicine. Afterwards, it was found that tubulin is particularly more abundant in brain than in other tissues. Therefore, the roles of microtubules in mitosis, and in neurons, have been more extensively analyzed and, in this review, these roles will be discussed.

Modifications of tubulin heterogeneity during embryonic and postnatal stages in a specific region of mouse brain

An auditory nucleus (the inferior colliculus of the mouse) was used to study the modifications of the heterogeneity of tubulin which occur at various stages of development and maturation. Both the cold-stable (CS) and cold-labile (CL) fractions of tubulin were analyzed by high-resolution isoelectric focusing. Our results suggest that tubulin heterogeneity is modified at critical stages of development and maturation, with specific variations of the two fractions. Stage E10 corresponding to the appearance of the first young neurons and axonal profiles is marked by the modification of the CS fraction with the emergence of isotypes alpha 5 to alpha 8 and beta 12 and beta 17. Stage E12 is characterized by the modifications of the CL fraction, particularly the beta-group; at this stage the first dendrites become visible. At birth, all isotypes increase in both the CL and CS fractions. At stages P7-P10 transient modifications of a group of both CS and CL fractions and of the beta group of CS fraction occur. These are associated with the emergence of isotypes beta 17 to beta 20 in the CL fraction. This period precludes the initial period of functional maturation of the auditory system which occurs from P10 to P20. During this period, the CL fraction (alpha and beta group) remains unmodified, whereas all isotypes of the CS fraction, except 3-4, display complex variations with an initial decrease until P12, an increase until P17, and a final decrease until P20.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential utilization of beta-tubulin isotypes in differentiating neurites

beta-Tubulin is encoded in vertebrate genomes by a family of six to seven functional genes that produce six different polypeptide isotypes. We now document that although rat PC-12 cells express five of these isotypes, only two (classes II and III) accumulate significantly as a consequence of nerve growth factor-stimulated neurite outgrowth. In contrast to previous efforts that have failed to detect in vivo distinctions among different beta-tubulin isotypes, we demonstrate using immunoblotting with isotype-specific antibodies that three beta-tubulin polypeptides (classes I, II, and IV) are used preferentially for assembly of neurite microtubules (with approximately 70% of types I and II assembled but only approximately 50% of type III in polymer). Immunofluorescence localization shows that an additional isotype (V) is partially excluded from neurites. Distinctions in in vivo localization of the neuron-specific, class III isotype have also been directly observed using immunofluorescence and immunogold electron microscopy. The sum of these efforts documents that some in vivo functional differences between tubulin isotypes do exist.

Differential axonal transport of isotubulins in the motor axons of the rat sciatic nerve

The axonal transport of the diverse isotubulins in the motor axons of the rat sciatic nerve was studied by two-dimensional polyacrylamide gel electrophoresis after intraspinal injection of [35S]methionine. 3 wk after injection, the nerve segments carrying the labeled axonal proteins of the slow components a (SCa) and b (SCb) of axonal transport were homogenized in a cytoskeleton-stabilizing buffer and two distinct fractions, cytoskeletal (pellet, insoluble) and soluble (supernatant), were obtained by centrifugation. About two-thirds of the transported-labeled tubulin moved with SCa, the remainder with SCb. In both waves, tubulin was found to be associated mainly with the cytoskeletal fraction. The same isoforms of tubulin were transported with SCa and SCb; however, the level of a neuron-specific beta-tubulin subcomponent, termed beta', composed of two related isotubulins beta'1 and beta'2, was significantly greater in SCb than in SCa, relative to the other tubulin isoforms. In addition, certain specific isotubulins were unequally distributed between the cytoskeletal and the soluble fractions. In SCa as well as in SCb, alpha''-isotubulins were completely soluble in the motor axons. By contrast, alpha''' and beta'2-isotubulins, both posttranslationally modified isoforms, were always recovered in the cytoskeletal fraction and thus may represent isotubulins restricted to microtubule polymers. The different distribution of isotubulins suggests that a recruitment of tubulin isoforms, including specific posttranslational modifications of defined isoforms (such as, at least, phosphorylation of beta' and acetylation of alpha'), might be involved in the assembly of distinct subsets of axonal microtubules displaying differential properties of stability, velocity and perhaps of function.

Analysis of tubulin proteins and peptides in neuronal and non-neuronal tissues using immobilized pH gradients

The accurate identification of the individual protein products of multi-gene families is essential to the interpretation of data from a wide range of experimental approaches including molecular biology, protein chemistry, and cell biology. We have adapted immobilized pH gradient isoelectric focusing to provide high resolution of tubulin proteins. Here we use these techniques to investigate the heterogeneity of tubulin in several neuronal and non-neuronal tissues to provide an accurate evaluation of isotubulin composition. Of the ten sources examined, the greatest number of isotubulins was found in whole adult brain. Tubulin isolated from either neonate human or rat brain consists predominantly of the more basic alpha and beta isotubulins found in adult brain. Cerebrum, cerebellum, medulla and caudate nucleus all contain the same large number of isotubulins as in whole brain, but in varying proportions. Liver, kidney and spleen isotubulin populations are all similar to each other and consist of a simpler distribution than any neuronal tissue examined. The majority of the tubulin protein in these non-neuronal tissues is composed of only the most basic alpha tubulins and intermediately-charged beta tubulins. No isotubulins were identified that were unique to these three non-neuronal tissues. Tubulin from neuroblastoma cells has an isotubulin distribution grossly similar to non-neuronal sources but additionally contains two basic beta isotubulins found in adult brain that are absent from non-neuronal tissues.

Modifications of tubulin heterogeneity during axonal growth in the embryonic nervous system

Tectal plates of mouse embryos were used to analyze the heterogeneity of tubulin with high resolution isoelectric focusing, at time of appearance of the young neurons and axons. In the cold-labile pool of tubulin, isotypes 6 and 7 appear, and isotypes 1, 3, 11, 13 increase their relative quantities. The increase of the prominence of the alpha-group with regard to the beta-group in the cold-stable pool of tubulin raises the question as to the exact role of tubulin in the cold stability. A role of other specific microtubule components also appears probable.

In vivo coassembly of a divergent beta-tubulin subunit (c beta 6) into microtubules of different function

alpha- and beta-Tubulin are encoded in vertebrate genomes by a family of approximately 6-7 functional genes whose polypeptide products differ in amino acid sequence. In the chicken, one beta-tubulin isotype (c beta 6) has previously been found to be expressed only in thrombocytes and erythroid cells, where it is assembled into a circumferential ring of marginal band microtubules. In light of its unique in vivo utilization and its divergent assembly properties in vitro, we used DNA transfection to test whether this isotype could be assembled in vivo into microtubules of divergent functions. Using an antibody specific to c beta 6, we have found that upon transfection this polypeptide is freely coassembled into an extensive array of interphase cytoplasmic microtubules and into astral and pole-to-chromosome or pole-to-pole microtubules during mitosis. Further, examination of developing chicken erythrocytes reveals that both beta-tubulins that are expressed in these cells (c beta 6 and c beta 3) are found as co-polymers of the two isoforms. These results, in conjunction with efforts that have localized various other beta-tubulin isotypes, demonstrate that to the resolution limit afforded by light microscopy in vivo microtubules in vertebrates are random copolymers of available isotypes. Although these findings are consistent with functional interchangeability of beta-tubulin isotypes, we have also found that in vivo microtubules enriched in c beta 3 polypeptides are more sensitive to cold depolymerization than those enriched in c beta 6. This differential quantitative utilization of the two endogenous isotypes documents that some in vivo functional differences between isotypes do exist.

Control of isotubulin expression during neuronal differentiation of mouse neuroblastoma and teratocarcinoma cell lines

Mouse neuroblastoma and teratocarcinoma constitute adequate cellular systems to study the expression of tubulin isoforms during early as well as later steps of neuronal differentiation. Tubulin heterogeneity is extensively analyzed using both isoelectric focusing and two-dimensional electrophoresis. Multipotential embryonal carcinoma cells express mainly one alpha-tubulin isoform (alpha 1) and three beta-tubulin isoforms: a major one (beta 3) and two minor ones (beta 4 and beta 5). Early events of neuronal differentiation are shown to induce the expression of an additional beta-tubulin isoform, beta'1, which is encoded by a specific mRNA. Neurite extension further increases tubulin heterogeneity and leads to the appearance of post-translationally modified isoforms: beta'2 in neuroblastoma and alpha 2 in teratocarcinoma cells. beta' 2 is shown to derive from the above mentioned beta'1 by phosphorylation, while alpha 2 is probably an acetylated form of the common alpha 1-tubulin. These results show that specific changes in tubulin heterogeneity are induced at different steps of neuronal differentiation and are controlled both at the transcriptional (or post-transcriptional) and post-translational levels.

An immunocytochemical analysis of the ontogeny of the microtubule-associated proteins MAP-2 and Tau in the nervous system of the rat

The developmental distribution patterns of beta-tubulin and the microtubule-associated proteins, MAP-2 and Tau, were studied by immunocytochemistry with monoclonal antibodies. The analysis of the in situ distribution of these proteins in embryonic brain tissue revealed intense immunoreactivity for beta-tubulin in proliferative and migrating neuroblasts. On the contrary, no immunoreactivity for MAP-2 or Tau was detected in this neuroepithelium; specific immunostaining for these MAPs was only present in those neuroblasts which have reached their final destination within a developing brain area, and have initiated terminal differentiation, i.e. the sprouting of axons and dendrites. During the initial stages of neuritic outgrowth both MAPs were detected in the somatodendritic compartment of developing brain neurons; Tau was also present in axons. While the distribution of MAP-2 remained essentially the same throughout development, Tau was progressively lost from cell bodies and dendrites. This pattern of compartmentation was observed in pyramidal neurons of the cerebral cortex and hippocampus, as well as in cells of other brain regions (e.g. thalamus, hypothalamus, cerebral amygdala and tectum). It was not detected in cerebellar Purkinje cells which compartmentalize Tau to axons from the outset of neuritic differentiation, and in neurons of the Gasser ganglion which transiently express MAP-2 in axons. The expression and distribution of these MAPs was also analyzed in embryonic cerebellar and hippocampal pyramidal neurons grown in culture. Both MAPs were found in these cells as soon as 6 h after plating; they were also present in all of the neurites, axons and dendrites, that these cells extend after development in vitro for several days. With subsequence development (more than 4 days in vitro) MAP-2 was lost from axons, while Tau remained homogeneously distributed in both types of neurites. Taken collectively, the present results indicate that the development of the compartmentalized distribution of MAP-2 and Tau follows a complex pattern which is specific for each of these MAPs, and which varies as a function of the neuron type and the conditions under which the cell develops. In addition, the complex variations in the distribution of both MAPs during in situ and in vitro development make it unlikely that these proteins have a role in determining the fate of a neurite as an axon or a dendrite.

Sequence and expression of the chicken beta 5- and beta 4-tubulin genes define a pair of divergent beta-tubulins with complementary patterns of expression

We have determined the nucleotide sequence of the chicken beta 5 (c beta 5)-tubulin gene. The gene displayed the coding structure common to all previously studied vertebrate beta-tubulin genes and was divided into four exon sequences interrupted by three intervening sequences (located between codons 19 and 20, within codon 56, and within codon 93). Comparison of the predicted polypeptide sequence encoded by c beta 5 with those of four other available chicken beta-tubulin sequences revealed that c beta 5 encoded a highly divergent beta-tubulin polypeptide isotype which was distinguished from previously known sequences primarily by two discrete variable sequence domains. However, c beta 5 uniquely shared identity in 16 residue positions with another divergent chicken beta-tubulin gene, c beta 4. These common sequences distinguished c beta 4 and c beta 5 from the remaining three chicken beta-tubulin genes. Analysis of the expression of c beta 5 and c beta 4 revealed a strikingly complementary pattern of gene expression: c beta 5 was expressed in a wide variety of cell and tissue types but not in neurons, whereas c beta 4 expression was detected uniquely in neuronal cells. Overall, these findings suggest the existence of two divergent families of beta-tubulin sequences in the chicken and further raise the possibility that the complementary expression of the c beta 4 and c beta 5 genes may fulfill a requirement for the presence of a divergent beta-tubulin polypeptide isotype in all cell types.

Molecular and cellular aspects of nerve regeneration

Injury of an axon leads to at least four independent events, summarized in Figure 1: first, deprivation of the nerve cell body from target-derived or mediated substances, which leads to a derepressed or a permissive state; second, disruption of anterograde transport, with a resultant accumulation of anterogradely transported molecules; third, environmental response with possible consequent changes in constituents of the extracellular matrix and substances secreted from the surrounding cells; and fourth, appearance of growth inhibitors and modified protease activity. It seems that the first three of these events are obligatory, but not sufficient, i.e., they lead to a growth state only if the cell body is able to respond to the injury-induced signals from the environment (a and b). The regenerative state is characterized by alterations in protein synthesis and axonal transport and by sprouting activity. The subsequent elongation of the growing fibers depends on a continuous supply of appropriate growth factors. These factors are presumably anchored to the appropriate extracellular matrix that serves as a substratum for elongating fibers. It should be mentioned that the proliferating nonneuronal cells have a conducive effect on regeneration by forming a scaffold for the growing fibers. Accordingly, the lack of regeneration may stem from a deficiency in the ability of glial cells to provide the appropriate soluble components or from insufficient formation of extracellular matrix. In this respect, one may consider regeneration of an injured axon as a process which involves regeneration of both the nonneuronal cells and the supported axons. The regeneration of glial cells may fulfill the rules which are applied to regeneration of any other proliferating tissue. Furthermore, the processes of regeneration in the axon and the glial cells are mutually dependent. Perhaps the triggering factors provided by the nonneuronal cells affect the nonneuronal cells themselves by modulating their postlesion gliosis and thereby inducing their appropriate activation. In such a case, regeneration of nonneuronal cells may resemble an autocrine type of regulation that exists also during ontogeny. The growth regulation is shifted back to the paracrine type upon neuronal maturation or cessation of axonal growth. When the elongating fibers reach the vicinity of the target organ, they are under the influence of the target-derived factors, which guide the fibers and eventually cease their elongation.

Sequence and expression of the chicken beta 5- and beta 4-tubulin genes define a pair of divergent beta-tubulins with complementary patterns of expression

We have determined the nucleotide sequence of the chicken beta 5 (c beta 5)-tubulin gene. The gene displayed the coding structure common to all previously studied vertebrate beta-tubulin genes and was divided into four exon sequences interrupted by three intervening sequences (located between codons 19 and 20, within codon 56, and within codon 93). Comparison of the predicted polypeptide sequence encoded by c beta 5 with those of four other available chicken beta-tubulin sequences revealed that c beta 5 encoded a highly divergent beta-tubulin polypeptide isotype which was distinguished from previously known sequences primarily by two discrete variable sequence domains. However, c beta 5 uniquely shared identity in 16 residue positions with another divergent chicken beta-tubulin gene, c beta 4. These common sequences distinguished c beta 4 and c beta 5 from the remaining three chicken beta-tubulin genes. Analysis of the expression of c beta 5 and c beta 4 revealed a strikingly complementary pattern of gene expression: c beta 5 was expressed in a wide variety of cell and tissue types but not in neurons, whereas c beta 4 expression was detected uniquely in neuronal cells. Overall, these findings suggest the existence of two divergent families of beta-tubulin sequences in the chicken and further raise the possibility that the complementary expression of the c beta 4 and c beta 5 genes may fulfill a requirement for the presence of a divergent beta-tubulin polypeptide isotype in all cell types.

Identification of conserved isotype-defining variable region sequences for four vertebrate beta tubulin polypeptide classes

We report the determination of the complete sequences for two chicken beta tubulin genes, beta 3 and beta 5. Taken with the previously published efforts, we have determined the primary structures of five of the seven beta tubulin genes in this vertebrate species. A comparison of these sequences unambiguously reveals that amino acid sequence variations among different beta tubulin gene products are distinctly clustered within an otherwise highly conserved framework of the beta tubulin molecule. To determine the extent to which this pattern of structural heterogeneity is conserved among vertebrates, we have isolated novel beta tubulin sequences from human and mouse cDNA libraries and compared these and all other known vertebrate beta tubulin sequences with the family of chicken polypeptide sequences. What emerges from such comparison is the recognition of distinct, evolutionarily conserved isotypes of beta tubulin that are distinguished primarily by their characteristic carboxyl-terminal variable region sequence and, to a lesser extent, by sequence in an amino-terminal variable domain as well. These correlations represent a convincing demonstration that multiple beta tubulin genes in vertebrates encode a family of closely related but structurally distinct beta tubulin isotypes and further serve to define the sequences of four classes of polypeptide isotypes that constitute that family.

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.

A polymer-dependent increase in phosphorylation of beta-tubulin accompanies differentiation of a mouse neuroblastoma cell line

We have examined the phosphorylation of cellular microtubule proteins during differentiation and neurite outgrowth in N115 mouse neuroblastoma cells. N115 differentiation, induced by serum withdrawal, is accompanied by a fourfold increase in phosphorylation of a 54,000-mol-wt protein identified as a specific isoform of beta-tubulin by SDS PAGE, two-dimensional isoelectric focusing/SDS PAGE, and immunoprecipitation with a specific monoclonal antiserum. Isoelectric focusing/SDS PAGE of [35S]methionine-labeled cell extracts revealed that the phosphorylated isoform of beta-tubulin, termed beta 2, is one of three isoforms detected in differentiated N115 cells, and is diminished in amounts in the undifferentiated cells. Taxol, a drug which promotes microtubule assembly, stimulates phosphorylation of beta-tubulin in both differentiated and undifferentiated N115 cells. In contrast, treatment of differentiated cells with either colcemid or nocodazole causes a rapid decrease in beta-tubulin phosphorylation. Thus, the phosphorylation of beta-tubulin in N115 cells is coupled to the levels of cellular microtubules. The observed increase in beta-tubulin phosphorylation during differentiation then reflects developmental regulation of microtubule assembly during neurite outgrowth, rather than developmental regulation of a tubulin kinase activity.

Heterogeneity of vertebrate brain tubulins

We have examined the extent of brain tubulin heterogeneity in six vertebrate species commonly used in tubulin research (rat, calf, pig, chicken, human, and lamb) using isoelectric focusing, two-dimensional electrophoresis, and peptide mapping procedures that provide higher resolution than previously available. The extent of heterogeneity is extremely similar in all of these organisms, as judged by number, range of isoelectric points, and distribution of the isotubulins. A minimum of 6 alpha and 12 beta tubulins was resolved from all sources. Even the pattern of spots on two-dimensional peptide maps is remarkably similar. These similarities suggest that the populations of tubulin in all of these brains should have similar overall physical properties. It is particularly interesting that chicken, which has only four or five beta-tubulin genes, contains approximately 12 beta tubulins. Thus, post-translational modification must generate at least some of the tubulin heterogeneity. Mammalian species, which contain 15-20 tubulin DNA sequences, do not show any more tubulin protein heterogeneity than does chicken. This suggests that expression of only a small number of the mammalian genes may be required to generate the observed tubulin heterogeneity.

Developmental and biochemical analysis of chick brain tubulin heterogeneity

Tubulin, isolated from brain tissue of chicks at different stages during late embryonic and early post-hatched development by ion-exchange chromatography and by in vitro microtubule reassembly, was analyzed by high-resolution isoelectric focusing and by two-dimensional polyacrylamide gel electrophoresis. Similar results were obtained with tubulins purified by the two methods. Sixteen isoelectric species of tubulin that differ in apparent net charge under denaturing conditions were detected by isoelectric focusing. By two-dimensional polyacrylamide gel electrophoresis, the chick brain tubulins were resolved into at least seven forms of alpha and 10 forms of beta tubulin. The number and relative proportions of the multiple brain tubulins were modulated during development. Since there are only four alpha tubulin and four beta tubulin genes in chickens, posttranslational modification of the tubulins must play a prominent role in the heterogeneity. Analysis of isotubulin distributions through cycles of microtubule assembly and disassembly indicated that the tubulins differ very little, if at all, in their capacity to assemble into microtubules. Therefore, the chemical differences that distinguish the multiple tubulins have very little structural impact on the protein surface areas involved in microtubule formation. Partial fractionation of the multiple tubulins during ion-exchange chromatography was observed, suggesting that it may be possible to isolate individual native tubulin variants for biochemical studies.

Developmental and comparative aspects of brine shrimp tubulin

Tubulin from embryos of the brine shrimp Artemia has been purified to apparent homogeneity by chromatography on phosphocellulose P11 and DEAE-cellulose, (NH4)2SO4 fractionation and assembly-disassembly of microtubules. Peptide mapping indicated that Artemia and bovine brain tubulin were very similar in spite of differences in the electrophoretic behaviour of tubulin from these two organisms. Isoelectric focusing and two-dimensional gel electrophoresis were used to resolve and identify several Artemia isotubulins . The isotubulin composition and the quantity of tubulin did not change during pre-emergence development of Artemia embryos. Formation of microtubules with tubulin purified from embryos at different stages of development did not require glycerol or microtubule-associated proteins and formation of structurally normal microtubules was actually hindered by glycerol and Mg2+. The characteristics of Artemia tubulin, in concert with the unusual life history of Artemia, suggest that this organism will be very useful for the study of tubulin gene expression and tubulin utilization during embryo development.

Regulation of mRNA levels for microtubule proteins during nerve regeneration

The molecular regulation of tubulin synthesis was investigated in the regenerating goldfish retina. Previous in vivo studies pointed to an increase in tubulin synthesis in the retina during regeneration of the injured goldfish optic nerve. Using labeled cDNA probes, we showed that this increase occurs as a result of enhanced tubulin mRNA levels. Analysis of labeled in vivo products revealed enhanced beta 2-tubulin synthesis accompanied by an increase in the level of the low-Mr microtubule-associated proteins identified as TAU factors. The results are discussed with respect to the possible involvement of these proteins in the process of nerve regeneration.