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

Tubulin targeting agents and their implications in non-cancer disease management

Microtubules act as molecular 'tracks' for the intracellular transport of accessory proteins, enabling them to assemble into various larger structures, such as spindle fibres formed during the cell cycle. Microtubules provide an organisational framework for the healthy functioning of various cellular processes that work through the process of dynamic instability, driven by the hydrolysis of GTP. In this role, tubulin proteins undergo various modifications, and in doing so modulate various healthy or pathogenic physiological processes within cells. In this review, we provide a detailed update of small molecule chemical agents that interact with tubulin, along with their implications, specifically in non-cancer disease management.

Moderate Physical Activity Increases the Expression of ADNP in Rat Brain

Activity-dependent neuroprotective protein (ADNP) is a neuroprotective protein essential for embryonic development, proper brain development, and neuronal plasticity. Its mutation causes the autism-like ADNP syndrome (also called the Helsmoortel-Van der Aa syndrome), characterized by neural developmental disorders and motor dysfunctions. Similar to the ADNP syndrome, the ADNP haploinsufficient mouse shows low synapse density, leading to motor and cognitive ability delays. Moderate physical activity (PA) has several neuroprotective and cognitive benefits, promoting neuronal survival, differentiation, neurogenesis, and plasticity. Until now, no study has investigated the effect of moderate exercise on ADNP expression and distribution in the rat brain. The aim of the current investigation was to study the effects of moderate exercise on the ADNP expression and neuronal activation measured by the microtubule protein β-Tubulin III. In pursuit of this objective, twenty-four rats were selected and evenly distributed into two categories: sedentary control rats and rats exposed to moderate physical activity on a treadmill over a span of 12 weeks. Our results showed that moderate PA increases the expression of ADNP and β-Tubulin III in the dentate gyrus (DG) hippocampal region and cerebellum. Moreover, we found a co-localization of ADNP and β-Tubulin III in both DG and cerebellum, suggesting a direct association of ADNP with adult neuronal activation induced by moderate PA.

Tubulin Complexity in Cancer and Metastasis

Tubulin plays a fundamental role in cellular function and as the subject for microtubule-active agents in the treatment of ovarian cancer. Microtubule-binding proteins (e.g., tau, MAP1/2/4, EB1, CLIP, TOG, survivin, stathmin) and posttranslational modifications (e.g., tyrosination, deglutamylation, acetylation, glycation, phosphorylation, polyamination) further diversify tubulin functionality and may permit additional opportunities to understand microtubule behavior in disease and to develop microtubule-modifying approaches to combat ovarian cancer. Tubulin-based structures that project from suspended ovarian cancer cells known as microtentacles may contribute to metastatic potential of ovarian cancer cells and could represent an exciting novel therapeutic target.

Concentration Gradient Constructions Using Inertial Microfluidics for Studying Tumor Cell-Drug Interactions

With the continuous development of cancer therapy, conventional animal models have exposed a series of shortcomings such as ethical issues, being time consuming and having an expensive cost. As an alternative method, microfluidic devices have shown advantages in drug screening, which can effectively shorten experimental time, reduce costs, improve efficiency, and achieve a large-scale, high-throughput and accurate analysis. However, most of these microfluidic technologies are established for narrow-range drug-concentration screening based on sensitive but limited flow rates. More simple, easy-to operate and wide-ranging concentration-gradient constructions for studying tumor cell–drug interactions in real-time have remained largely out of reach. Here, we proposed a simple and compact device that can quickly construct efficient and reliable drug-concentration gradients with a wide range of flow rates. The dynamic study of concentration-gradient formation based on successive spiral mixer regulations was investigated systematically and quantitatively. Accurate, stable, and controllable dual drug-concentration gradients were produced to evaluate simultaneously the efficacy of the anticancer drug against two tumor cell lines (human breast adenocarcinoma cells and human cervical carcinoma cells). Results showed that paclitaxel had dose-dependent effects on the two tumor cell lines under the same conditions, respectively. We expect this device to contribute to the development of microfluidic chips as a portable and economical product in terms of the potential of concentration gradient-related biochemical research.

The tubulin code and its role in controlling microtubule properties and functions

Microtubules are core components of the eukaryotic cytoskeleton with essential roles in cell division, shaping, motility and intracellular transport. Despite their functional heterogeneity, microtubules have a highly conserved structure made from almost identical molecular building blocks: the tubulin proteins. Alternative tubulin isotypes and a variety of post-translational modifications control the properties and functions of the microtubule cytoskeleton, a concept known as the 'tubulin code'. Here we review the current understanding of the molecular components of the tubulin code and how they impact microtubule properties and functions. We discuss how tubulin isotypes and post-translational modifications control microtubule behaviour at the molecular level and how this translates into physiological functions at the cellular and organism levels. We then go on to show how fine-tuning of microtubule function by some tubulin modifications can affect homeostasis and how perturbation of this fine-tuning can lead to a range of dysfunctions, many of which are linked to human disease.

Paclitaxel

Paclitaxel is the first microtubule-stabilizing agent identified and considered to be the most significant advance in chemotherapy of the past two decades. It is considered one of the most widely used antineoplastic agents with broad activity in several cancers including breast cancer, endometrial cancer, non-small-cell lung cancer, bladder cancer, and cervical carcinoma. It is also used for treating AIDS-related Kaposi sarcoma as a second line treatment. This comprehensive profile of paclitaxel gives overview of nomenclature, formulae, elemental analysis, appearance, application and uses. In addition, mechanism of action and resistance, different dosage forms and methods of drug preparation are elaborated. Moreover, the physicochemical properties involving X-ray powder diffraction pattern, drug solubility, melting point, differential scanning calorimetry, and stability were summarized. Furthermore, method of drug analysis including compendial, spectrophotometric, and chromatographic was discussed.

ReMAPping the microtubule landscape: How phosphorylation dictates the activities of microtubule-associated proteins

Classical microtubule-associated proteins (MAPs) were originally identified based on their co-purification with microtubules assembled from mammalian brain lysate. They have since been found to perform a range of functions involved in regulating the dynamics of the microtubule cytoskeleton. Most of these MAPs play integral roles in microtubule organization during neuronal development, microtubule remodeling during neuronal activity, and microtubule stabilization during neuronal maintenance. As a result, mutations in MAPs contribute to neurodevelopmental disorders, psychiatric conditions, and neurodegenerative diseases. MAPs are post-translationally regulated by phosphorylation depending on developmental time point and cellular context. Phosphorylation can affect the microtubule affinity, cellular localization, or overall function of a particular MAP and can thus have profound implications for neuronal health. Here we review MAP1, MAP2, MAP4, MAP6, MAP7, MAP9, tau, and DCX, and how each is regulated by phosphorylation in neuronal physiology and disease. Developmental Dynamics 247:138-155, 2018. © 2017 Wiley Periodicals, Inc.

The Altered Hepatic Tubulin Code in Alcoholic Liver Disease

The molecular mechanisms that lead to the progression of alcoholic liver disease have been actively examined for decades. Because the hepatic microtubule cytoskeleton supports innumerable cellular processes, it has been the focus of many such mechanistic studies. It has long been appreciated that α-tubulin is a major target for modification by highly reactive ethanol metabolites and reactive oxygen species. It is also now apparent that alcohol exposure induces post-translational modifications that are part of the natural repertoire, mainly acetylation. In this review, the modifications of the "tubulin code" are described as well as those adducts by ethanol metabolites. The potential cellular consequences of microtubule modification are described with a focus on alcohol-induced defects in protein trafficking and enhanced steatosis. Possible mechanisms that can explain hepatic dysfunction are described and how this relates to the onset of liver injury is discussed. Finally, we propose that agents that alter the cellular acetylation state may represent a novel therapeutic strategy for treating liver disease.

Post-translational modifications of tubulin: pathways to functional diversity of microtubules

Tubulin and microtubules are subject to a remarkable number of post-translational modifications. Understanding the roles these modifications play in determining the functions and properties of microtubules has presented a major challenge that is only now being met. Many of these modifications are found concurrently, leading to considerable diversity in cellular microtubules, which varies with development, differentiation, cell compartment, and cell cycle. We now know that post-translational modifications of tubulin affect, not only the dynamics of the microtubules, but also their organization and interaction with other cellular components. Many early suggestions of how post-translational modifications affect microtubules have been replaced with new ideas and even new modifications as our understanding of cellular microtubule diversity comes into focus.

The tubulin code: molecular components, readout mechanisms, and functions

Microtubules are cytoskeletal filaments that are dynamically assembled from α/β-tubulin heterodimers. The primary sequence and structure of the tubulin proteins and, consequently, the properties and architecture of microtubules are highly conserved in eukaryotes. Despite this conservation, tubulin is subject to heterogeneity that is generated in two ways: by the expression of different tubulin isotypes and by posttranslational modifications (PTMs). Identifying the mechanisms that generate and control tubulin heterogeneity and how this heterogeneity affects microtubule function are long-standing goals in the field. Recent work on tubulin PTMs has shed light on how these modifications could contribute to a “tubulin code” that coordinates the complex functions of microtubules in cells.

Insights into LRRK2 function and dysfunction from transgenic and knockout rodent models

Mutations in the LRRK2 (leucine-rich repeat kinase 2) gene on chromosome 12 cause autosomal dominant PD (Parkinson's disease), which is indistinguishable from sporadic forms of the disease. Numerous attempts have therefore been made to model PD in rodents via the transgenic expression of LRRK2 and its mutant variants and to elucidate the function of LRRK2 by knocking out rodent Lrrk2. Although these models often only partially recapitulate PD pathology, they have helped to elucidate both the normal and pathological function of LRRK2. In particular, LRRK2 has been suggested to play roles in cytoskeletal dynamics, synaptic machinery, dopamine homoeostasis and autophagic processes. Our understanding of how these pathways are affected, their contribution towards PD development and their interaction with one another is still incomplete, however. The present review summarizes the findings from LRRK2 rodent models and draws potential connections between the apparently disparate cellular processes altered, in order to better understand the underlying mechanisms of LRRK2 dysfunction and illuminate future therapeutic interventions.

The ADNP derived peptide, NAP modulates the tubulin pool: implication for neurotrophic and neuroprotective activities

Microtubules (MTs), key cytoskeletal elements in living cells, are critical for axonal transport, synaptic transmission, and maintenance of neuronal morphology. NAP (NAPVSIPQ) is a neuroprotective peptide derived from the essential activity-dependent neuroprotective protein (ADNP). In Alzheimer’s disease models, NAP protects against tauopathy and cognitive decline. Here, we show that NAP treatment significantly affected the alpha tubulin tyrosination cycle in the neuronal differentiation model, rat pheochromocytoma (PC12) and in rat cortical astrocytes. The effect on tubulin tyrosination/detyrosination was coupled to increased MT network area (measured in PC12 cells), which is directly related to neurite outgrowth. Tubulin beta3, a marker for neurite outgrowth/neuronal differentiation significantly increased after NAP treatment. In rat cortical neurons, NAP doubled the area of dynamic MT invasion (Tyr-tubulin) into the neuronal growth cone periphery. NAP was previously shown to protect against zinc-induced MT/neurite destruction and neuronal death, here, in PC12 cells, NAP treatment reversed zinc-decreased tau-tubulin-MT interaction and protected against death. NAP effects on the MT pool, coupled with increased tau engagement on compromised MTs imply an important role in neuronal plasticity, protecting against free tau accumulation leading to tauopathy. With tauopathy representing a major pathological hallmark in Alzheimer's disease and related disorders, the current findings provide a mechanistic basis for further development. NAP (davunetide) is in phase 2/3 clinical trial in progressive supranuclear palsy, a disease presenting MT deficiency and tau pathology.

Post-translational modifications of microtubules

Microtubules--polymers of tubulin--perform essential functions, including regulation of cell shape, intracellular transport and cell motility. How microtubules are adapted to perform multiple diverse functions is not well understood. Post-translational modifications of tubulin subunits diversify the outer and luminal surfaces of microtubules and provide a potential mechanism for their functional specialization. Recent identification of a number of tubulin-modifying and -demodifying enzymes has revealed key roles of tubulin modifications in the regulation of motors and factors that affect the organization and dynamics of microtubules.

H89 sensitive kinase regulates the translocation of Sar1 onto the ER membrane through phosphorylation of ER-coupled β-tubulin

ER-to-Golgi protein transport is carried out by transport vesicles which are formed at the ER-exit sites with recruitment of cytoplasmic coat proteins. Vesicle formation is initiated by assembly of the small G protein (Sar1) onto the ER membrane. Sar1 assembly onto the ER membrane is suppressed by protein kinase inhibitor H89, suggesting participation of H89-sensitive kinase in this process. The present study identified an effector of H89-sensitive kinase by LC-MS PMF analysis combined with 1D- and 2D-PAGE autoradiography, and examined the changes on the effector and Sar1 translocation induced by H89. H89 significantly suppressed the phosphorylation of 55 kDa protein with dosage dependency, and phosphorylation of 55 kDa, pI 5.5 protein spot in 2-D-autoradiography was drastically diminished by H89. LC-MS PMF analysis showed that the protein spot was β-tubulin. H89 significantly suppressed Sar1 translocation onto the ER. These findings indicate that β-tubulin is one of downstream effectors of H89-sensitive kinase, and that suppression of ER-coupled β-tubulin phosphorylation decreases Sar1 translocation onto the ER, suggesting that phosphorylation of β-tubulin regulates Sar1 translocation.

Systems-wide proteomic characterization of combinatorial post-translational modification patterns

Protein post-translational modifications (PTMs) have been widely shown to influence protein-protein interactions, direct subcellular location and transduce a variety of both internal and externally generated signals into cellular/phenotypic outcomes. Mass spectrometry has been a key tool for the elucidation of several types of PTMs in both qualitative and quantitative manners. As large datasets on the proteome-wide level are now being generated on a daily basis, the identification of combinatorial PTM patterns has become feasible. A survey of the recent literature in this area shows that many proteins undergo multiple modifications and that sequential or hierarchal patterns exist on many proteins; the biology of these modification patterns is only starting to be unraveled. This review will outline combinatorial PTM examples in biology, and the mass spectrometry-based techniques and applications utilized in the investigations of these combinatorial PTMs.

Recombinant mammalian tubulin polyglutamylase TTLL7 performs both initiation and elongation of polyglutamylation on beta-tubulin through a random sequential pathway

Tubulins undergo unique post-translational modifications, such as tyrosination, polyglutamylation, and polyglycylation. These modifications are performed by members of a protein family, the tubulin tyrosine ligase (TTL)-like (TTLL) family, which is characterized by the presence of a highly conserved TTL domain. We and others have recently identified tubulin polyglutamylases in the TTLL family [Janke, C., et al. (2005) Science 308, 1758-1762; Ikegami, K., et al. (2006) J. Biol. Chem. 281, 30707-30716; van Dijk, J., et al. (2007) Mol. Cell 26, 437-448]. Previously, we identified TTLL7 as a beta-tubulin-selective polyglutamylase. However, there is controversy over whether TTLL7 functions as an initiase, elongase, or both in polyglutamylation. In this report, we investigate the polyglutamylation reaction by TTLL7 by employing a recombinant enzyme and in vitro reaction. Two-dimensional electrophoresis and tandem mass spectrometry showed that TTLL7 performed both the initiation and elongation of polyglutamylation on beta-tubulin. Recombinant TTLL7 performed with a maximal and specific activity to polymerized tubulin at a neutral pH and a lower salt concentration. The initial rate and inhibitor analyses revealed that the mechanism of binding of three substrates, glutamate, ATP, and tubulin, to the enzyme was a random sequential pathway. Our findings provide evidence that mammalian TTLL7 performs both initiation and elongation in the polyglutamylation reaction on beta-tubulin through a random sequential pathway.

Arabidopsis casein kinase 1-like 6 contains a microtubule-binding domain and affects the organization of cortical microtubules,

Members of the casein kinase 1 (CK1) family are evolutionarily conserved eukaryotic protein kinases that are involved in various cellular, physiological, and developmental processes in yeast and metazoans, but the biological roles of CK1 members in plants are not well understood. Here, we report that an Arabidopsis (Arabidopsis thaliana) CK1 member named casein kinase 1-like 6 (CKL6) associates with cortical microtubules in vivo and phosphorylates tubulins in vitro. The unique C-terminal domain of CKL6 was shown to contain the signal that allows localization of CKL6 to the cortical microtubules. This domain on its own was sufficient to associate with microtubules in vivo and to bind tubulins in vitro. CKL6 was able to phosphorylate soluble tubulins as well as microtubule polymers, and its endogenous activity was found to associate with a tubulin-enriched subcellular fraction. Two major in vitro phosphorylation sites were mapped to serine-413 and serine-420 of tubulin beta. Ectopic expression of wild-type CKL6 or a kinase-inactive mutant form induced alterations in cortical microtubule organization and anisotropic cell expansion. Collectively, these results demonstrate that CKL6 is a protein kinase containing a novel tubulin-binding domain and plays a role in anisotropic cell growth and shape formation in Arabidopsis through the regulation of microtubule organization, possibly through the phosphorylation of tubulins.

Differentiation of human adult skin-derived neuronal precursors into mature neurons

The isolation of autologous neuronal precursors from skin-derived precursor cells extracted from adult human skin would be a very efficient source of neurons for the treatment of various neurodegenerative diseases. The purpose of this study was to demonstrate that these neuronal precursors were able to differentiate into mature neurons. We isolated neuronal precursors from breast skin and expanded them in vitro for over ten passages. We showed that 48% of these cells were proliferating after the first passage, while this growth rate decreased after the second passage. We demonstrated that 70% of these cells were nestin-positive after the third passage, while only 17% were neurofilament M-positive after 7 days of differentiation. These neuronal precursors expressed betaIII tubulin, the dendritic marker MAP2 and the presynaptic marker synaptophysin after 7 days of in vitro maturation. They also expressed the postsynaptic marker PSD95 and the late neuronal markers NeuN and neurofilament H after 21 days of differentiation, demonstrating they became terminally differentiated neurons. These markers were still expressed after 50 days of culture. The generation of autologous neurons from an accessible adult human source opens many potential therapeutic applications and has a great potential for the development of experimental studies on normal human neurons.

Vinblastine, a chemotherapeutic drug, inhibits palmitoylation of tubulin in human leukemic lymphocytes

BACKGROUND

We have previously shown that tubulin, the major protein of microtubules, is posttranslationally modified by palmitoylation. In addition, we demonstrated that palmitoylation of tubulin is inhibited in vitro by stoichiometric levels of the chemotherapeutic drug, vinblastine. Here, we sought to determine whether a clinically relevant dose of vinblastine inhibits palmitoylation of tubulin in vivo.

METHODS

Human CEM leukemic lymphocytes were incubated with [3H]palmitate in the presence and absence of a low, clinically relevant dose of vinblastine. [3H]palmitoylated tubulin was identified by two-dimensional PAGE and autoradiography.

RESULTS

We found, first, that tubulin was palmitoylated in CEM cells. Second, the clinically relevant dose of vinblastine inhibited palmitoylation of tubulin in vivo in CEM cells. In addition, microtubules were disassembled and cells became apoptotic.

CONCLUSION

This study identifies a previously unknown mechanism of action of vinblastine, the depalmitoylation of tubulin, and suggests that depalmitoylation of tubulin may be a target for new chemotherapeutic drugs.

The Centrosome in Normal and Transformed Cells

The centrosome is a unique organelle that functions as the microtubule organizing center in most animal cells. During cell division, the centrosomes form the poles of the bipolar mitotic spindle. In addition, the centrosomes are also needed for cytokinesis. Each mammalian somatic cell typically contains one centrosome, which is duplicated in coordination with DNA replication. Just like the chromosomes, the centrosome is precisely reproduced once and only once during each cell cycle. However, it remains a mystery how this protein-based structure undergoes accurate duplication in a semiconservative manner. Intriguingly, amplification of the centrosome has been found in numerous forms of cancers. Cells with multiple centrosomes tend to form multipolar spindles, which result in abnormal chromosome segregation during mitosis. It has therefore been postulated that centrosome aberration may compromise the fidelity of cell division and cause chromosome instability. Here we review the current understanding of how the centrosome is assembled and duplicated. We also discuss the possible mechanisms by which centrosome abnormality contributes to the development of malignant phenotype.

Class III beta-tubulin isotype: a key cytoskeletal protein at the crossroads of developmental neurobiology and tumor neuropathology

The expression of the cytoskeletal protein class III beta-tubulin isotype is reviewed in the context of human central nervous system development and neoplasia. Compared to systemic organs and tissues, class III beta-tubulin is abundant in the brain, where it is prominently expressed during fetal and postnatal development. As exemplified in cerebellar neurogenesis, the distribution of class III beta-tubulin is neuron associated, exhibiting different temporospatial gradients in the neuronal progeny of the external granule layer versus the neuroepithelial germinal matrix of the velum medullare. However, transient expression of this protein is also present in the telencephalic subventricular zones comprising putative neuronal and/or glial precursor cells. This temporospatially restricted, potentially non-neuronal expression of class III beta-tubulin may have implications in the accurate identification of presumptive neurons derived from transplanted embryonic stem cells. In the adult central nervous system, the distribution of class III beta-tubulin is almost exclusively neuron specific. Altered patterns of expression are noted in brain tumors. In "embryonal"-type neuronal/neuroblastic tumors of the central nervous system, such as the medulloblastomas, class III beta-tubulin expression is associated with neuronal differentiation and decreased cell proliferation. In contrast, the expression of class III beta-tubulin in gliomas is associated with an ascending grade of histologic malignancy and with correspondingly high proliferative indices. Thus, class III beta-tubulin expression in neuronal or neuroblastic tumors is differentiation dependent, whereas in glial tumors, it is aberrant and/or represents "dedifferentiation" associated with the acquisition of glial progenitor-like phenotype(s). From a diagnostic perspective, the detection of class III beta-tubulin immunostaining in neoplastic cells should not be construed as categorical evidence of divergent neuronal differentiation in tumors, which are otherwise phenotypically glial. Because class III beta-tubulin is present in neoplastic but not in normal differentiated glial cells, the elucidation of molecular mechanisms responsible for the altered expression of this isotype may provide critical insights into the dynamics of the microtubule cytoskeleton in the growth and progression of gliomas.

Characterisation of PGs1, a subunit of a protein complex co-purifying with tubulin polyglutamylase

Polyglutamylation is a post-translational modification initially discovered on tubulin. It has been implicated in multiple microtubule functions, including neuronal differentiation, axonemal beating and stability of the centrioles, and shown to modulate the interaction between tubulin and microtubule associated proteins. The enzymes catalysing this modification are not yet known. Starting with a partially purified fraction of mouse brain tubulin polyglutamylase, monoclonal antibodies were raised and used to further purify the enzyme by immunoprecipitation. The purified enzyme complex (Mr 360x103) displayed at least three major polypeptides of 32, 50 and 80x103, present in stochiometric amounts. We show that the 32x103 subunit is encoded by the mouse gene GTRGEO22, the mutation of which has recently been implicated in multiple defects in mice, including male sterility. We demonstrate that this subunit, called PGs1, has no catalytic activity on its own, but is implicated in the localisation of the enzyme at major sites of polyglutamylation, i.e. neurones, axonemes and centrioles.

Class III beta-tubulin in human development and cancer

The differential cellular expression of class III beta-tubulin isotype (betaIII) is reviewed in the context of human embryological development and neoplasia. As compared to somatic organs and tissues, betaIII is abundant in the central and peripheral nervous systems (CNS and PNS) where it is prominently expressed during fetal and postnatal development. As exemplified in cerebellar and sympathoadrenal neurogenesis, the distribution of betaIII is neuron-associated, exhibiting distinct temporospatial gradients according to the regional neuroepithelia of origin. However, transient expression of this protein is also present in the subventricular zones of the CNS comprising putative neuronal- and/or glial precursor cells, as well as in Kulchitsky neuroendocrine cells of the fetal respiratory epithelium. This temporally restricted, potentially non-neuronal expression may have implications in the identification of presumptive neurons derived from embryonic stem cells. In adult tissues, the distribution of betaIII is almost exclusively neuron-specific. Altered patterns of expression are noted in cancer. In "embryonal"- and "adult-type" neuronal tumors of the CNS and PNS, betaIII is associated with neuronal differentiation and decreased cell proliferation. In contrast, the presence of betaIII in gliomas and lung cancer is associated with an ascending histological grade of malignancy. Thus, betaIII expression in neuronal tumors is differentiation-dependent, while in non-neuronal tumors it is aberrant and/or represents "dedifferentiation" associated with the acquisition of progenitor-like phenotypic properties. Increased expression in various epithelial cancer cell lines is associated with chemoresistance to taxanes. Because betaIII is present in subpopulations of neoplastic, but not in normal differentiated glial or somatic epithelial cells, the elucidation of mechanisms responsible for the altered expression of this isotype may provide insights into the role of the microtubule cytoskeleton in tumorigenesis and tumor progression.

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)

Structure of tubulin C-terminal domain obtained by subtilisin treatment The major α and β tubulin isotypes from pig brain are glutamylated

Limited subtilisin digestion of the tubulin alpha, beta heterodimer has been used in this work to reduce the total number of tubulin isotypes from 20 for native to 9 for subtilisin-cleaved tubulin. This indicates that the major part of tubulin heterogeneity is located at the C-terminus of the molecule. The C-terminal peptides of both alpha and beta subunits of tubulin were purified by anion-exchange HPLC. Combined use of Edman degradation chemistry and mass spectrometry on the isolated peptides shows that subtilisin cleavage occurs at position Asp-438 and His-406 of alpha and Gln-433 and His-396 of beta tubulin chains. Quantitative analysis of our data show that cleavage at positions His-406 (alpha) and His-396 (beta) occurs with a low efficiency and indicates that the major isotypes of pig brain tubulin are modified by sequential attachment of 1 to 5 glutamic acid residues at positions Glu-445 or -435 of alpha and beta tubulin, respectively.

Review: postchaperonin tubulin folding cofactors and their role in microtubule dynamics

The microtubule cytoskeleton consists of a highly organized network of microtubule polymers bound to their accessory proteins: microtubule-associated proteins, molecular motors, and microtubule-organizing proteins. The microtubule subunits are heterodimers composed of one alpha-tubulin polypeptide and one beta-tubulin polypeptide that should undergo a complex folding processing before they achieve a quaternary structure that will allow their incorporation into the polymer. Due to the extremely high protein concentration that exists at the cell cytoplasm, there are alpha- and beta-tubulin interacting proteins that prevent the unwanted interaction of these polypeptides with the surrounding protein pool during folding, thus allowing microtubule dynamics. Several years ago, the development of a nondenaturing electrophoretic technique made it possible to identify different tubulin intermediate complexes during tubulin biogenesis in vitro. By these means, the cytosolic chaperonin containing TCP-1 (CCT or TriC) and prefoldin have been demonstrated to intervene through tubulin and actin folding. Various other cofactors also identified along the alpha- and beta-tubulin postchaperonin folding route are now known to have additional roles in tubulin biogenesis such as participating in the synthesis, transport, and storage of alpha- and beta-tubulin. The future characterization of the tubulin-binding sites to these proteins, and perhaps other still unknown proteins, will help in the development of chemicals that could interfere with tubulin folding and thus modulating microtubule dynamics. In this paper, current knowledge of the above postchaperonin folding cofactors, which are in fact chaperones involved in tubulin heterodimer quaternary structure achievement, will be reviewed.

Perinatal lethality of microtubule-associated protein 1B-deficient mice expressing alternative isoforms of the protein at low levels

Microtubule-associated protein 1B (MAP1B) has been implicated in axogenesis in cultured cells. To gain insight into the functions that MAP1B plays in vivo, we analyzed a strain of Map1B mutant mice generated by a gene trapping approach. Homozygous mice die on the first day after birth, probably due to a severe abnormal development of the nervous system. They present alterations in the structure of several brain regions. The normal Map1B gene yields different protein isoforms from alternatively spliced transcripts. The smaller isoforms were present in wild type, hetero-, and homozygous mice, but their expression was higher in the mutants than in the wild-type. Moreover, trace amounts of MAP1B protein were also observed in Map1B homozygous mutants, indicating an alternative splicing around the gene trap insertion. Thus, the Map1B gene trapped mutation reported in this work did not generated a null mutant, but a mouse with a drastic deficiency in MAP1B expression. Analyses of these mice indicate the presence of several neural defects and suggest the participation of MAP1B in neuronal migration.

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.

Mechanisms of action and resistance to tubulin-binding agents

Tubulin binding agents constitute an important class of antimitotics and are widely used for the treatment of solid tumours an haematopoietic malignancies. These compounds, currently represented by the vinca alkaloids and the taxanes, differ from most of the other clinically useful antimitotics in that their target is not nucleic acids, but the mitotic spindle, which is an essential component of the mitotic machinery. Recent data on the mechanisms of action of and mechanisms of resistance to tubulin binding agents are presented. The importance of microtubule dynamics is emphasised, in particular in relationship to the usefulness of drug combinations. Concerning the reported resistance mechanisms, an emerging body of data show that altered microtubule structure may be involved in reduced sensitivity to these compounds. Promising new molecules, including those derived from marine organisms are described.

MAP1B expression and microtubule stability in growing and regenerating axons

MAP1B is a microtubule-associated phosphoprotein that is particularly highly expressed in developing neurons. There is experimental evidence that it plays an important role in neuronal differentiation, especially the extension of axons and dendrites, but exactly what role is unclear. Recent experiments have shed light on the gene structure of MAP1B and identified some of the kinases that phosphorylate the protein. Implicit in these findings is the idea that MAP1B regulates the organisation of microtubules in neurites and is itself regulated in a complex way and at a number of levels.

Molecular interactions among protein phosphatase 2A, tau, and microtubules. Implications for the regulation of tau phosphorylation and the development of tauopathies

Hyperphosphorylated forms of the neuronal microtubule (MT)-associated protein tau are major components of Alzheimer's disease paired helical filaments. Previously, we reported that ABalphaC, the dominant brain isoform of protein phosphatase 2A (PP2A), is localized on MTs, binds directly to tau, and is a major tau phosphatase in cells. We now describe direct interactions among tau, PP2A, and MTs at the submolecular level. Using tau deletion mutants, we found that ABalphaC binds a domain on tau that is indistinguishable from its MT-binding domain. ABalphaC binds directly to MTs through a site that encompasses its catalytic subunit and is distinct from its binding site for tau, and ABalphaC and tau bind to different domains on MTs. Specific PP2A isoforms bind to MTs with distinct affinities in vitro, and these interactions differentially inhibit the ability of PP2A to dephosphorylate various substrates, including tau and tubulin. Finally, tubulin assembly decreases PP2A activity in vitro, suggesting that PP2A activity can be modulated by MT dynamics in vivo. Taken together, these findings indicate how structural interactions among ABalphaC, tau, and MTs might control the phosphorylation state of tau. Disruption of these normal interactions could contribute significantly to development of tauopathies such as Alzheimer's disease.

Microtubule polyglutamylation in Drosophila melanogaster brain and testis

The presence of glutamylated tubulin, a widespread posttranslational modification of alpha- and beta-tubulin, has been investigated in Drosophila melanogaster using the specific monoclonal antibody GT335. We show here that this modification is strongly detected in brain and testis whereas other tissues analyzed did not appear to contain any glutamylated isoforms. Neuronal microtubules are glutamylated on alpha-tubulin only whereas sperm flagella showed a strong modification of both alpha- and beta-tubulin. These results argue for an essential role for glutamylation in differentiation processes that require microtubule stabilization.

Differentiation of human colon cancer cells changes the expression of beta-tubulin isotypes and MAPs

The human colon adenocarcinoma HT29-D4 cell line is an interesting model for studies on epithelial cell differentiation. Undifferentiated cells are malignant proliferating cells, whereas differentiated cells act like epithelial polarized cells. In the present study, we first characterized the action of taxoids on the microtubular network of HT29-D4 cells according to the state of differentiation. Microtubular bundles were found in undifferentiated cells but not in differentiated cells, even with 500-fold higher taxoid concentrations for 96 h. This finding led us to study changes in microtubules according to the polarity status of the cell. E-MAP-115 was expressed only in differentiated cells; expression of beta-tubulin isotypes was altered in them relative to undifferentiated cells. Classes I, II, III, IVa and IVb isotypes were expressed in both phenotypes; however, differentiated epithelial cells displayed a specific increase in class III beta-tubulin. Thus, the increase in expression of this beta-tubulin isotype in differentiated cells is not restricted to neuronal cells. Moreover, these expression changes may reflect a higher stability of microtubular network in differentiated cells, which may explain the lower activity of anti-microtubule agents, independently of the mitotic process. These results indicate that the composition of microtubules should be considered as one of the criteria involved in the response of tumour cells to chemotherapy with anti-microtubule agents.

Mechanisms of action of and resistance to antitubulin agents: microtubule dynamics, drug transport, and cell death

PURPOSE

To analyze the available data concerning mechanisms of action of and mechanisms of resistance to the antitubulin agents, vinca alkaloids and taxanes, and more recently described compounds.

DESIGN

We conducted a review of the literature on classic and recent antitubulin agents, focusing particularly on the relationships between antitubulin agents and their intracellular target, the soluble tubulin/microtubule complex.

RESULTS AND CONCLUSION

Although it is widely accepted that antitubulin agents block cell division by inhibition of the mitotic spindle, the mechanism of action of antitubulin agents on microtubules remains to be determined. The classic approach is that vinca alkaloids depolymerize microtubules, thereby increasing the soluble tubulin pool, whereas taxanes stabilize microtubules and increase the microtubular mass. More recent data suggest that both classes of agents have a similar mechanism of action, involving the inhibition of microtubule dynamics. These data suggest that vinca alkaloids and taxanes may act synergistically as antitumor agents and may be administered as combination chemotherapy in the clinic. However, enhanced myeloid and neurologic toxicity, as well as a strong dependence on the sequence of administration, presently exclude these combinations outside the context of clinical trials. Although the multidrug resistance phenotype mediated by Pgp appears to be an important mechanism of resistance to these agents, alterations of microtubule structure resulting in altered microtubule dynamics and/or altered binding of antitubulin agents may constitute a significant mechanism of drug resistance.

Distribution of CK2, its substrate MAP1B and phosphatases in neuronal cells

Neuronal morphogenesis depends on the organization of cytoskeletal elements among which microtubules play a very important role. The organization of microtubules is controlled by the presence of microtubule-associated proteins (MAPs), the activity of which is modulated by phosphorylation and dephosphorylation. One of these MAPs is MAP1B, which is very abundant within growing axons of developing neurons where it is found phosphorylated by several protein kinases including CK2. The expression of MAP1B is notably decreased after neuronal maturation in parallel with a change in the localization of the protein, which becomes largely concentrated in neuronal cell bodies and dendrites. Interestingly, MAP1B remains highly phosphorylated at sites targeted by protein kinase CK2 in mature neurons. We have analyzed the expression and localization of CK2 catalytic subunits along neuronal development. CK2alpha subunit appears early during development whereas CK2alpha' subunit appears within mature neurons at the time of dendrite maturation and synaptogenesis, in parallel with the change in the localization of MAP1B. CK2alpha subunit is found associated with microtubule preparations obtained from either grey matter or white matter from adult bovine brain, whereas CK2alpha' subunit is highly enriched in microtubules obtained from grey matter. These results lend support to the hypothesis that CK2alpha' subunit is concentrated in neuronal cell bodies and dendrites, where it associates with microtubules, thus contributing to the increased phosphorylation of MAP1B in this localization in mature neurons.

Class III beta-tubulin isotype (beta III) in the adrenal medulla: III. Differential expression of neuronal and glial antigens identifies two distinct populations of neuronal and glial-like (sustentacular) cells in the PC12 rat pheochromocytoma cell line maintained in a Gelfoam matrix system

BACKGROUND

The rat PC12 pheochromocytoma cell line provides an established system for the study of neuronal differentiation. To our knowledge, glial differentiation has not been reported in this cell line.

METHODS

We have studied, by immunohistochemistry and immunoblotting, the presence of neuronal cytoskeletal antigens [class III beta-tubulin isotype (beta III), microtubule associated proteins MAP2, MAP1B and tau, and different neurofilament (NF) protein components], and synaptophysin in comparison with the glial fibrillary acidic protein (GFAP) and S-100 protein in the PC12 cell line. In three different experiments, PC12 cells were maintained in a three-dimensional gelatin foam (Gelfoam) matrix system for up to 34 days with and without treatment with 1 mM dibutyryl cyclic (dc)AMP. Immunohistochemistry was performed on explants ranging from 2 to 32 days-in vitro, which were fixed in either Bouin's solution, 70% ethanol, or 10% neutral-buffered formalin and embedded in paraffin. Immunoblotting was performed on Gelfoam explants with a panel of antibodies against all aforementioned neuronal and glial markers. Additional immunoblot experiments using anti-GFAP and anti-beta III monoclonal antibodies in cell suspensions and homogenates from PC12 monolayer cultures were carried out to compare growth conditions in relation to the expression of these proteins.

RESULTS

Beta III and MAP2 were demonstrated by immunohistochemistry and immunoblotting of PC12 explants maintained for up to 32 days in Gelfoam matrices with and without treatment with dcAMP. Intense filamentous and granular beta III staining of PC12 cells was observed in dcAMP-treated cultures concomitant with neuronal morphologic alterations (neuritogenesis and ganglionic phenotype). In untreated cultures, beta III staining was present in less differentiated cells, as well in cells undergoing neuritic development. The neuronal phenotype of PC12 cells was confirmed by staining for MAP2, tau, and NF proteins, as well as for synaptophysin. The presence of beta III, MAP2, MAP1B, tau, and NF proteins was confirmed by immunoblotting. Clusters of GFAP-positive and S-100 protein-positive spindle cells, phenotypically distinct from the chromaffin-like or neuronal cells, were demonstrated in Gelfoam explants at 5-30 days in vitro. In 30-day-old cultures treated with dcAMP, there was strong filamentous GFAP and diffuse S-100 protein staining in an increased number of sustentacular-like PC12 cells. GFAP staining was corroborated by immunoblotting of explants maintained under identical conditions in vitro. In contrast, immunoblots performed on homogenates from PC12 suspension and monolayer cultures were GFAP-negative.

CONCLUSIONS

Neuronal and glial-like, presumed sustentacular, phenotypes were demonstrated in PC12 cells grown in Gelfoam matrices with and without treatment with dcAMP for up to 34 days. To our knowledge, the occurrence of glial differentiation in the PC12 line is a hitherto unreported finding. Adult rat medullary sustentacular cells are known to express S-100 and GFA proteins (Suzuki and Kachi, Kaibogaku Zasshi-Anat 70(2): 130-139, 1995), and the organ culture system employed in our study may well have favored this direction of differentiation.

Assembly and colchicine binding characteristics of tubulin with maximally tyrosinated and detyrosinated alpha-tubulins

The posttranslational removal and readdition of tyrosine at the C-terminus of alpha-tubulin is associated with generation of microtubule populations that differ in intracellular distributions, turnover rates, and sensitivities to microtubule-depolymerization agents. Here, we compared the in vitro assembly and colchicine binding characteristics of tubulin dimer preparations composed of alpha-tubulin that had been maximally tyrosinated (approximately 40% tyrosinated) by tubulin-tyrosine ligase and maximally detyrosinated (100% detyrosinated) by carboxypeptidase A. Maximally tyrosinated and detyrosinated tubulins had similar critical concentrations for polymerization and similar association constants for colchicine binding. Microtubules polymerized from the two tubulins also had similar steady-state mean lengths and length distributions. The growing and shortening dynamics (dynamic instability parameters) of individual microtubules made from maximally tyrosinated or detyrosinated alpha-tubulin as determined by video-enhanced dark-field microscopy were similar, but subtle differences in the growing and shortening rates were found. On balance, however, the dynamicity and thus the overall kinetic stability of the two microtubule populations were indistinguishable. The results support the idea that detyrosination of alpha-tubulin does not by itself generate stable microtubules.

Expression of neuron-specific beta-III tubulin during olfactory neurogenesis in the embryonic and adult rat

The olfactory neuroepithelium retains the unique capacity to produce a new set of mature neurons every three to four weeks from a precursor population situated at the base of the epithelium. It is not known however, whether developing olfactory neurons in the adult rat follow the same program that is initiated embryonically. By tracking the expression of beta-III tubulin (by immunoreactivity to TuJ-1, an isoform-specific antibody) throughout embryogenesis, we have demonstrated a commitment to the olfactory neuron lineage in a subset of cells in the embryonic olfactory placode and followed their development into adulthood. We have also shown that this developmental pattern of beta-III tubulin expression is recapitulated in neurons undergoing a synchronized neurogenic response to either physical or chemical lesion in the adult neuroepithelium. The embryonic expression pattern reported here is similar to, but earlier than that reported for other markers of developing neurons, such as growth-associated protein-43 and neural cell adhesion molecule. The results of these studies suggest the retention of a conserved neurogenic program from embryonic to adult life in the olfactory neuron and, in addition, support the use of a readily accessible system such as the regenerating olfactory neuroepithelium as an alternative means of studying genes which may be crucial to normal neuronal development.

Multiple forms of tubulin: different gene products and covalent modifications

Tubulin, the subunit protein of microtubules, is an alpha/beta heterodimer. In many organisms, both alpha and beta exist in numerous isotypic forms encoded by different genes. In addition, both alpha and beta undergo a variety of posttranslational covalent modifications, including acetylation, phosphorylation, detyrosylation, polyglutamylation, and polyglycylation. In this review the distribution and possible functional significance of the various forms of tubulin are discussed. In analyzing the differences among tubulin isotypes encoded by different genes, some appear to have no functional significance, some increase the overall adaptability of the organism to environmental challenges, and some appear to perform specific functions including formation of particular organelles and interactions with specific proteins. Purified isotypes also display different properties in vitro. Although the significance of all the covalent modification of tubulin is not fully understood, some of them may influence the stability of modified microtubules in vivo as well as interactions with certain proteins and may help to determine the functional role of microtubules in the cell. The review also discusses isotypes of gamma-tubulin and puts various forms of tubulin in an evolutionary context.

Bradykinin induces tubulin phosphorylation and nuclear translocation of MAP kinase in mesangial cells

Glomerular hypertension and glomerular hypertrophy act early and synergistically to promote glomerular injury in diabetes. We have previously shown that increased renal kinin production contributes to the glomerular hemodynamic abnormalities associated with diabetes. Glomerulosclerosis, characterized by mesangial cell proliferation and matrix expansion, is the final pathway leading to renal failure. The signal(s) initiating mesangial cell proliferation is ill defined. In the present study, we utilized immunofluorescence, immunoprecipitation, and immunoblotting techniques to identify substrates that are tyrosine phosphorylated in response to bradykinin action in mesangial cells. Immunofluorescence microscopy of mesangial cells stained with anti-phosphotyrosine (anti-PY) antibodies following bradykinin treatment (10(-9)-10(-6) M) revealed a dose-dependent increase in the labeling of cytoplasmic and nuclear proteins. Immunoprecipitation with anti-PY, followed by immunoblot revealed bradykinin-induced tyrosyl phosphorylation of tubulin and mitogen-activated protein kinase (MAPK). Confocal microscopy of mesangial cells stained for MAPK indicated that bradykinin stimulation resulted in translocation of MAPK from the cytoplasm to the nucleus by 2 h. These data demonstrate that bradykinin action results in the tyrosine phosphorylation of cellular proteins in mesangial cells and suggest a role for tubulin and MAPK in the signaling cascade of bradykinin leading to altered mesangial function.

Expression of specific tubulin isotypes increases during regeneration of injured CNS neurons, but not after the application of brain-derived neurotrophic factor (BDNF)

Axonal regrowth after injury is accompanied by changes in the expression of tubulin, but the contributions of substrate molecules and neurotrophic factors in regulating these changes in vivo are not known. Adult rat retinal ganglion cells (RGCs) were examined after intraorbital axotomy, after application of a peripheral nerve (PN) graft to stimulate regeneration, and after axotomy and treatment with brain-derived neurotrophic factor (BDNF). After these treatments we used in situ hybridization to study mRNA levels for betaI, betaII, betaIII, betaIVa, and Talpha1 tubulin isotypes. Levels of mRNA for all isotypes were downregulated after intraorbital axotomy. During regrowth of injured RGC axons, mRNA levels for betaII, betaIII, and Talpha1 isotypes were upregulated specifically and dramatically, suggesting that elevated expression of these isotypes is correlated specifically with axonal regrowth. A corresponding increase in betaIII protein levels was detected by immunocytochemistry. The betaI and betaIVa mRNAs were not increased during regeneration. BDNF did not elicit a specific increase in the mRNA levels for the betaIII and Talpha1 isotypes and had only a small effect on mRNA levels for the betaII isotype. Therefore, despite the ability of BDNF to support the survival of injured RGCs and to enhance neurite outgrowth of retinal neurons in vitro, the in vivo application of BDNF alone is unable to induce the program of changes in growth-associated tubulins that accompany regeneration of RGC axons into PN grafts. We speculate that, in addition to BDNF, cooperative signaling with substrate molecules is required to allow RGCs to regenerate and exhibit tubulin isotype switching.

Expression of specific tubulin isotypes increases during regeneration of injured CNS neurons, but not after the application of brain-derived neurotrophic factor (BDNF)

Axonal regrowth after injury is accompanied by changes in the expression of tubulin, but the contributions of substrate molecules and neurotrophic factors in regulating these changes in vivo are not known. Adult rat retinal ganglion cells (RGCs) were examined after intraorbital axotomy, after application of a peripheral nerve (PN) graft to stimulate regeneration, and after axotomy and treatment with brain-derived neurotrophic factor (BDNF). After these treatments we used in situ hybridization to study mRNA levels for betaI, betaII, betaIII, betaIVa, and Talpha1 tubulin isotypes. Levels of mRNA for all isotypes were downregulated after intraorbital axotomy. During regrowth of injured RGC axons, mRNA levels for betaII, betaIII, and Talpha1 isotypes were upregulated specifically and dramatically, suggesting that elevated expression of these isotypes is correlated specifically with axonal regrowth. A corresponding increase in betaIII protein levels was detected by immunocytochemistry. The betaI and betaIVa mRNAs were not increased during regeneration. BDNF did not elicit a specific increase in the mRNA levels for the betaIII and Talpha1 isotypes and had only a small effect on mRNA levels for the betaII isotype. Therefore, despite the ability of BDNF to support the survival of injured RGCs and to enhance neurite outgrowth of retinal neurons in vitro, the in vivo application of BDNF alone is unable to induce the program of changes in growth-associated tubulins that accompany regeneration of RGC axons into PN grafts. We speculate that, in addition to BDNF, cooperative signaling with substrate molecules is required to allow RGCs to regenerate and exhibit tubulin isotype switching.

Profile of glutamylated tubulin expression during cerebellar granule cell development in vitro

The developmental regulation of tubulin and several of its posttranslational modifications was examined during the differentiation of rat cerebellar granule cells in vitro. In particular, we have noted that the glutamylation of alpha- and beta-tubulin subunits varies during development and becomes more prominent in differentiating neuronal processes. These results indicate that glutamylation of tubulin may be important in the stabilization of microtubules during the maturation of the neuronal cytoskeleton.

A brain protein (P30) that immunoreacts with a polyclonal anti-pancreatic carboxypeptidase A antibody shows properties that are shared with tubulin carboxypeptidase

A preparation of tubulin carboxypeptidase partially purified from bovine brain was found to contain a protein of molecular mass 30 kDa (P30) as determined by SDS-PAGE, that is recognized by a polyclonal anti-bovine pancreatic carboxypeptidase A. However, this protein is different from pancreatic carboxypeptidase A as judged by the isoelectric point and the pattern of peptides produced by trypsin digestion. The isoelectric point of P30 was similar to that found for tubulin carboxypeptidase (9 +/- 0.2). When the tubulin carboxypeptidase preparation was subjected to gel filtration chromatography under low salt concentration, P30 behaved as a protein of molecular mass 38 kDa whereas tubulin carboxypeptidase eluted at a position of 75 kDa molecular mass. However, when the chromatography was performed at relatively high salt concentration they behaved as proteins of 49 and 56 kDa, respectively. We considered that P30 may be an inactive monomeric form of the dimeric tubulin carboxypeptidase. However we can not rule out the possibility that it represents another carboxypeptidase not yet described.