Identification of a novel tubulin-destabilizing protein related to the chaperone cofactor E

Maintaining essential microtubule bundles in meter-long axons: a role for local tubulin biogenesis?

Axons are the narrow, up-to-meter long cellular processes of neurons that form the biological cables wiring our nervous system. Most axons must survive for an organism's lifetime, i.e. up to a century in humans. Axonal maintenance depends on loose bundles of microtubules that run without interruption all along axons. The continued turn-over and the extension of microtubule bundles during developmental, regenerative or plastic growth requires the availability of α/β-tubulin heterodimers up to a meter away from the cell body. The underlying regulation in axons is poorly understood and hardly features in past and contemporary research. Here we discuss potential mechanisms, particularly focussing on the possibility of local tubulin biogenesis in axons. Current knowledge might suggest that local translation of tubulin takes place in axons, but far less is known about the post-translational machinery of tubulin biogenesis involving three chaperone complexes: prefoldin, CCT and TBC. We discuss functional understanding of these chaperones from a range of model organisms including yeast, plants, flies and mice, and explain what is known from human diseases. Microtubules across species depend on these chaperones, and they are clearly required in the nervous system. However, most chaperones display a high degree of functional pleiotropy, partly through independent functions of individual subunits outside their complexes, thus posing a challenge to experimental studies. Notably, we found hardly any studies that investigate their presence and function particularly in axons, thus highlighting an important gap in our understanding of axon biology and pathology.

Microtubule and auditory function - an underestimated connection

The organ of Corti, located in the cochlea within the inner ear is the receptor organ for hearing. It converts auditory signals into neuronal action potentials that are transmitted to the brain for further processing. The mature organ of Corti consists of a variety of highly differentiated sensory cells that fulfil unique tasks in the processing of auditory signals. The actin and microtubule cytoskeleton play essential function in hearing, however so far, more attention has been paid to the role of actin. Microtubules play important roles in maintaining cellular structure and intracellular transport in virtually all eukaryotic cells. Their functions are controlled by interactions with a large variety of microtubule-associated proteins (MAPs) and molecular motors. Current advances show that tubulin posttranslational modifications, as well as tubulin isotypes could play key roles in modulating microtubule properties and functions in cells. These mechanisms could have various effects on the stability and functions of microtubules in the highly specialised cells of the cochlea. Here, we review the current understanding of the role of microtubule-regulating mechanisms in the function of the cochlea and their implications for hearing, which highlights the importance of microtubules in the field of hearing research.

The detyrosination/re-tyrosination cycle of tubulin and its role and dysfunction in neurons and cardiomyocytes

Among the variety of post-translational modifications to which microtubules are subjected, the detyrosination/re-tyrosination cycle is specific to tubulin. It is conserved by evolution and characterized by the enzymatic removal and re-addition of a gene-encoded tyrosine residue at the C-terminus of α-tubulin. Detyrosinated tubulin can be further converted to Δ2-tubulin by the removal of an additional C-terminal glutamate residue. Detyrosinated and Δ2-tubulin are carried by stable microtubules whereas tyrosinated microtubules are present on dynamic polymers. The cycle regulates trafficking of many cargo transporting molecular motors and is linked to the microtubule dynamics via regulation of microtubule interactions with specific cellular effectors such as kinesin-13. Here, we give an historical overview of the general features discovered for the cycle. We highlight the recent progress toward structure and functioning of the enzymes that keep the levels of tyrosinated and detyrosinated tubulin in cells, the long-known tubulin tyrosine ligase and the recently discovered vasohibin-SVBP complexes. We further describe how the cycle controls microtubule functions in healthy neurons and cardiomyocytes and how deregulations of the cycle are involved in dysfunctions of these highly differentiated cells, leading to neurodegeneration and heart failure in humans.

Regulatory Potential of Long Non-Coding RNAs (lncRNAs) in Boar Spermatozoa with Good and Poor Freezability

Long non-coding RNAs (lncRNAs) are suggested to play an important role in the sperm biological processes. We performed de novo transcriptome assembly to characterize lncRNAs in spermatozoa, and to investigate the role of the potential target genes of the differentially expressed lncRNAs (DElncRNAs) in sperm freezability. We detected approximately 4007 DElncRNAs, which were differentially expressed in spermatozoa from boars classified as having good and poor semen freezability (GSF and PSF, respectively). Most of the DElncRNAs were upregulated in boars of the PSF group and appeared to significantly affect the sperm's response to the cryopreservation conditions. Furthermore, we predicted that the potential target genes were regulated by DElncRNAs in cis or trans. It was found that DElncRNAs of both freezability groups had potential cis- and trans-regulatory effects on different protein-coding genes, such as COX7A2L, TXNDC8 and SOX-7. Gene Ontology (GO) enrichment revealed that the DElncRNA target genes are associated with numerous biological processes, including signal transduction, response to stress, cell death (apoptosis), motility and embryo development. Significant differences in the de novo assembled transcriptome expression profiles of the DElncRNAs between the freezability groups were confirmed by quantitative real-time PCR analysis. This study reveals the potential effects of protein-coding genes of DElncRNAs on sperm functions, which could contribute to further research on their relevance in semen freezability.

Tubulin-binding cofactor E-like (TBCEL), the protein product of the mulet gene, is required in the germline for the regulation of inter-flagellar microtubule dynamics during spermatid individualization

Individual sperm cells are resolved from a syncytium during late step of spermiogenesis known as individualization, which is accomplished by an Individualization Complex (IC) composed of 64 investment cones. mulet encodes Tubulin-binding cofactor E-like (TBCEL), suggesting a role for microtubule dynamics in individualization. Indeed, a population of ∼100 cytoplasmic microtubules fails to disappear in mulet mutant testes during spermatogenesis. This persistence, detected using epi-fluorescence and electron microscopy, suggests that removal of these microtubules by TBCEL is a prerequisite for individualization. Immunofluorescence reveals TBCEL expression in elongated spermatid cysts. In addition, testes from mulet mutant males were rescued to wild type using tubulin-Gal4 to drive TBCEL expression, indicating that the mutant phenotype is caused by the lack of TBCEL. Finally, RNAi driven by bam-GAL4 successfully phenocopied mulet, confirming that mulet is required in the germline for individualization. We propose a model in which the cytoplasmic microtubules serve as alternate tracks for investment cones in mulet mutant testes.This article has an associated First Person interview with the first author of the paper.

Sensory neuropathy in progressive motor neuronopathy (pmn) mice is associated with defects in microtubule polymerization and axonal transport

Motor neuron diseases such as amyotrophic lateral sclerosis (ALS) are now recognized as multi-system disorders also involving various non-motor neuronal cell types. The precise extent and mechanistic basis of non-motor neuron damage in human ALS and ALS animal models remain however unclear. To address this, we here studied progressive motor neuronopathy (pmn) mice carrying a missense loss-of-function mutation in tubulin binding cofactor E (TBCE). These mice manifest a particularly aggressive form of motor axon dying back and display a microtubule loss, similar to that induced by human ALS-linked TUBA4A mutations. Using whole nerve confocal imaging of pmn × thy1.2-YFP16 fluorescent reporter mice and electron microscopy, we demonstrate axonal discontinuities, bead-like spheroids and ovoids in pmn suralis nerves indicating prominent sensory neuropathy. The axonal alterations qualitatively resemble those in phrenic motor nerves but do not culminate in the loss of myelinated fibers. We further show that the pmn mutation decreases the level of TBCE, impedes microtubule polymerization in dorsal root ganglion (DRG) neurons and causes progressive loss of microtubules in large and small caliber suralis axons. Live imaging of axonal transport using GFP-tagged tetanus toxin C-fragment (GFP-TTC) demonstrates defects in microtubule-based transport in pmn DRG neurons, providing a potential explanation for the axonal alterations in sensory nerves. This study unravels sensory neuropathy as a pathological feature of mouse pmn, and discusses the potential contribution of cytoskeletal defects to sensory neuropathy in human motor neuron disease.

Katanin Severing and Binding Microtubules Are Inhibited by Tubulin Carboxy Tails

Microtubule dynamics in cells are regulated by associated proteins that can be either stabilizers or destabilizers. A class of destabilizers that is important in a large number of cellular activities is the microtubule-severing enzymes, yet little is known about how they function. Katanin p60 was the first ATPase associated with microtubule severing. Here, we investigate the activity of katanin severing using a GFP-labeled human version. We quantify the effect of katanin concentration on katanin binding and severing activity. We find that free tubulin can inhibit severing activity by interfering with katanin binding to microtubules. The inhibition is mediated by the sequence of the tubulin and specifically depends on the carboxy-terminal tails. We directly investigate the inhibition effect of tubulin carboxy-terminal tails using peptide sequences of α-, β-, or detyrosinated α-tubulin tails that have been covalently linked to bovine serum albumin. Our results show that β-tubulin tails are the most effective at inhibiting severing, and that detyrosinated α-tubulin tails are the least effective. These results are distinct from those for other severing enzymes and suggest a scheme for regulation of katanin activity in cells dependent on free tubulin concentration and the modification state of the tubulin.

Revisiting the tubulin folding pathway: new roles in centrosomes and cilia

Centrosomes and cilia are critical eukaryotic organelles which have been in the spotlight in recent years given their implication in a myriad of cellular and developmental processes. Despite their recognized importance and intense study, there are still many open questions about their biogenesis and function. In the present article, we review the existing data concerning members of the tubulin folding pathway and related proteins, which have been identified at centrosomes and cilia and were shown to have unexpected roles in these structures.

The structure of the complex between α-tubulin, TBCE and TBCB reveals a tubulin dimer dissociation mechanism

Tubulin proteostasis is regulated by a group of molecular chaperones termed tubulin cofactors (TBC). Whereas tubulin heterodimer formation is well-characterized biochemically, its dissociation pathway is not clearly understood. Here, we carried out biochemical assays to dissect the role of the human TBCE and TBCB chaperones in α-tubulin-β-tubulin dissociation. We used electron microscopy and image processing to determine the three-dimensional structure of the human TBCE, TBCB and α-tubulin (αEB) complex, which is formed upon α-tubulin-β-tubulin heterodimer dissociation by the two chaperones. Docking the atomic structures of domains of these proteins, including the TBCE UBL domain, as we determined by X-ray crystallography, allowed description of the molecular architecture of the αEB complex. We found that heterodimer dissociation is an energy-independent process that takes place through a disruption of the α-tubulin-β-tubulin interface that is caused by a steric interaction between β-tubulin and the TBCE cytoskeleton-associated protein glycine-rich (CAP-Gly) and leucine-rich repeat (LRR) domains. The protruding arrangement of chaperone ubiquitin-like (UBL) domains in the αEB complex suggests that there is a direct interaction of this complex with the proteasome, thus mediating α-tubulin degradation.

Effects of deuterium oxide on cell growth and vesicle speed in RBL-2H3 cells

For the first time we show the effects of deuterium oxide on cell growth and vesicle transport in rat basophilic leukemia (RBL-2H3) cells. RBL-2H3 cells cultured with 15 moles/L deuterium showed decreased cell growth which was attributed to cells not doubling their DNA content. Experimental observations also showed an increase in vesicle speed for cells cultured in deuterium oxide. This increase in vesicle speed was not observed in deuterium oxide cultures treated with a microtubule-destabilizing drug, suggesting that deuterium oxide affects microtubule-dependent vesicle transport.

Tubulin-specific chaperones: components of a molecular machine that assembles the α/β heterodimer

The tubulin heterodimer consists of one α- and one β-tubulin polypeptide. Neither protein can partition to the native state or assemble into polymerization competent heterodimers without the concerted action of a series of chaperone proteins including five tubulin-specific chaperones (TBCs) termed TBCA-TBCE. TBCA and TBCB bind to and stabilize newly synthesized quasi-native β- and α-tubulin polypeptides, respectively, following their generation via multiple rounds of ATP-dependent interaction with the cytosolic chaperonin. There is free exchange of β-tubulin between TBCA and TBCD, and of α-tubulin between TBCB and TBCE, resulting in the formation of TBCD/β and TBCE/α, respectively. The latter two complexes interact, forming a supercomplex (TBCE/α/TBCD/β). Discharge of the native α/β heterodimer occurs via interaction of the supercomplex with TBCC, which results in the triggering of TBC-bound β-tubulin (E-site) GTP hydrolysis. This reaction acts as a switch for disassembly of the supercomplex and the release of E-site GDP-bound heterodimer, which becomes polymerization competent following spontaneous exchange with GTP. The tubulin-specific chaperones thus function together as a tubulin assembly machine, marrying the α- and β-tubulin subunits into a tightly associated heterodimer. The existence of this evolutionarily conserved pathway explains why it has never proved possible to isolate α- or β-tubulin as stable independent entities in the absence of their cognate partners, and implies that each exists and is maintained in the heterodimer in a nonminimal energy state. Here, we describe methods for the purification of recombinant TBCs as biologically active proteins following their expression in a variety of host/vector systems.

Loss of MEC-17 leads to microtubule instability and axonal degeneration

Axonal degeneration arises as a consequence of neuronal injury and is a common hallmark of a number of neurodegenerative diseases. However, the genetic causes and the cellular mechanisms that trigger this process are still largely unknown. Based on forward genetic screening in C. elegans, we have identified the α-tubulin acetyltransferase gene mec-17 as causing spontaneous, adult-onset, and progressive axonal degeneration. Loss of MEC-17 leads to microtubule instability, a reduction in mitochondrial number, and disrupted axonal transport, with altered distribution of both mitochondria and synaptic components. Furthermore, mec-17-mediated axonal degeneration occurs independently from its acetyltransferase domain; is enhanced by mutation of coel-1, a tubulin-associated molecule; and correlates with the animal's body length. This study therefore identifies a critical role for the conserved microtubule-associated protein MEC-17 in preserving axon integrity and preventing axonal degeneration.

Cells determine cell density using a small protein bound to a unique tissue-specific phospholipid

Cell density is the critical parameter controlling tendon morphogenesis. Knowing its neighbors allows a cell to regulate correctly its proliferation and collagen production. A missing link to understanding this process is a molecular description of the sensing mechanism. Previously, this mechanism was shown in cell culture to rely on a diffusible factor (SNZR [sensor]) with an affinity for the cell layer. This led to purifying conditioned medium over 4 columns and analyzing the final column fractions for band intensity on SDS gels versus biological activity - a 16 kD band strongly correlated between assays. N-terminal sequencing - EPLAVVDL - identified a large gene (424 AA), extremely conserved between chicken and human. In this paper we probe whether this is the correct gene. Can the predicted large protein be cleaved to a smaller protein? EPLAVVDL occurs towards the C-terminus and cleavage would create a small 94 AA protein. This protein would run at ∼10 kD, so what modifications or cofactor binding accounts for its running at 16 kD on SDS gels? This protein has no prominent hydrophobic regions, so can it be secreted? To validate its role, the chicken cDNA for this gene was tagged with myc and his and transfected into a human osteosarcoma cell line (U2OS). U2OS cells expressed the gene but not passively: differentiating into structures resembling spongy bone and expressing alkaline phosphatase, an early bone marker. Intracellularly, two bands were observed by Western blotting: the full length protein and a smaller form (26 kD). Outside the cell, a small band (28 kD) was detected, although it was 40% larger than expected, as well as multiple larger bands. These larger forms could be converted to the predicted smaller protein (94 AA + tags) by changing salt concentrations and ultrafiltering - releasing a cofactor to the filtrate while leaving a protein factor in the retentate. Using specific degradative enzymes and mass spectrometry, the bone cofactor was identified as a lipid containing a ceramide phosphate, a single chained glycerol lipid and a linker. Tendon uses a different cofactor made up of two fatty acid chains linked directly to the phosphate yielding a molecule about half the size. Moreover, adding the tendon factor/cofactor to osteosarcoma cells causes them to stop growing, which is opposite to its role with tendon cells. Thus, the cofactor is cell type specific both in composition and in the triggered response. Further support of its proposed role came from frozen sections from 5 week old mice where an antibody to the factor stained strongly at the growing ends of the tendon as predicted. In conclusion, the molecule needed for cell density signaling is a small protein bound to a unique, tissue-specific phospholipid yielding a membrane associated but diffusible molecule. Signal transduction is postulated to occur by an increased ordering of the plasma membrane as the concentration of this protein/lipid increases with cell density.

Identification of 526 conserved metazoan genetic innovations exposes a new role for cofactor E-like in neuronal microtubule homeostasis

The evolution of metazoans from their choanoflagellate-like unicellular ancestor coincided with the acquisition of novel biological functions to support a multicellular lifestyle, and eventually, the unique cellular and physiological demands of differentiated cell types such as those forming the nervous, muscle and immune systems. In an effort to understand the molecular underpinnings of such metazoan innovations, we carried out a comparative genomics analysis for genes found exclusively in, and widely conserved across, metazoans. Using this approach, we identified a set of 526 core metazoan-specific genes (the ‘metazoanome’), approximately 10% of which are largely uncharacterized, 16% of which are associated with known human disease, and 66% of which are conserved in Trichoplax adhaerens, a basal metazoan lacking neurons and other specialized cell types. Global analyses of previously-characterized core metazoan genes suggest a prevalent property, namely that they act as partially redundant modifiers of ancient eukaryotic pathways. Our data also highlights the importance of exaptation of pre-existing genetic tools during metazoan evolution. Expression studies in C. elegans revealed that many metazoan-specific genes, including tubulin folding cofactor E-like (TBCEL/coel-1), are expressed in neurons. We used C. elegans COEL-1 as a representative to experimentally validate the metazoan-specific character of our dataset. We show that coel-1 disruption results in developmental hypersensitivity to the microtubule drug paclitaxel/taxol, and that overexpression of coel-1 has broad effects during embryonic development and perturbs specialized microtubules in the touch receptor neurons (TRNs). In addition, coel-1 influences the migration, neurite outgrowth and mechanosensory function of the TRNs, and functionally interacts with components of the tubulin acetylation/deacetylation pathway. Together, our findings unveil a conserved molecular toolbox fundamental to metazoan biology that contains a number of neuronally expressed and disease-related genes, and reveal a key role for TBCEL/coel-1 in regulating microtubule function during metazoan development and neuronal differentiation.

The evolution of multicellular animals (metazoans) from their single-celled ancestor required new molecular tools to create and coordinate the various biological functions involved in a communal, or multicellular, lifestyle. This would eventually include the unique cellular and physiological demands of specialized tissues like the nervous system. To identify and understand the genetic bases of such unique metazoan traits, we used a comparative genomics approach to identify 526 metazoan-specific genes which have been evolutionarily conserved throughout the diversification of the animal kingdom. Interestingly, we found that some of those genes are still completely uncharacterized or poorly studied. We used the metazoan model organism C. elegans to examine the expression of some poorly characterized metazoan-specific genes and found that many, including one encoding tubulin folding cofactor E-like (TBCEL; C. elegans COEL-1), are expressed in cells of the nervous system. Using COEL-1 as an example to understand the metazoan-specific character of our dataset, our studies reveal a new role for this protein in regulating the stability of the microtubule cytoskeleton during development, and function of the touch receptor neurons. In summary, our findings help define a conserved molecular toolbox important for metazoan biology, and uncover an important role for COEL-1/TBCEL during development and in the nervous system of the metazoan C. elegans.

The architecture of Trypanosoma brucei tubulin-binding cofactor B and implications for function

Tubulin-binding cofactor (TBC)-B is implicated in the presentation of α-tubulin ready to polymerize, and at the correct levels to form microtubules. Bioinformatics analyses, including secondary structure prediction, CD, and crystallography, were combined to characterize the molecular architecture of Trypanosoma brucei TBC-B. An efficient recombinant expression system was prepared, material-purified, and characterized by CD. Extensive crystallization screening, allied with the use of limited proteolysis, led to structures of the N-terminal ubiquitin-like and C-terminal cytoskeleton-associated protein with glycine-rich segment domains at 2.35-Å and 1.6-Å resolution, respectively. These are compact globular domains that appear to be linked by a flexible segment. The ubiquitin-like domain contains two lysines that are spatially conserved with residues known to participate in ubiquitinylation, and so may represent a module that, through covalent attachment, regulates the signalling and/or protein degradation associated with the control of microtubule assembly, catastrophe, or function. The TBC-B C-terminal cytoskeleton-associated protein with glycine-rich segment domain, a known tubulin-binding structure, is the only such domain encoded by the T. brucei genome. Interestingly, in the crystal structure, the peptide-binding groove of this domain forms intermolecular contacts with the C-terminus of a symmetry-related molecule, an association that may mimic interactions with the C-terminus of α-tubulin or other physiologically relevant partners. The interaction of TBC-B with the α-tubulin C-terminus may, in particular, protect from post-translational modifications, or simply assist in the shepherding of the protein into polymerization.

Gene expression changes in the motor cortex mediating motor skill learning

The primary motor cortex (M1) supports motor skill learning, yet little is known about the genes that contribute to motor cortical plasticity. Such knowledge could identify candidate molecules whose targeting might enable a new understanding of motor cortical functions, and provide new drug targets for the treatment of diseases which impair motor function, such as ischemic stroke. Here, we assess changes in the motor-cortical transcriptome across different stages of motor skill acquisition. Adult rats were trained on a gradually acquired appetitive reach and grasp task that required different strategies for successful pellet retrieval, or a sham version of the task in which the rats received pellet reward without needing to develop the reach and grasp skill. Tissue was harvested from the forelimb motor-cortical area either before training commenced, prior to the initial rise in task performance, or at peak performance. Differential classes of gene expression were observed at the time point immediately preceding motor task improvement. Functional clustering revealed that gene expression changes were related to the synapse, development, intracellular signaling, and the fibroblast growth factor (FGF) family, with many modulated genes known to regulate synaptic plasticity, synaptogenesis, and cytoskeletal dynamics. The modulated expression of synaptic genes likely reflects ongoing network reorganization from commencement of training till the point of task improvement, suggesting that motor performance improves only after sufficient modifications in the cortical circuitry have accumulated. The regulated FGF-related genes may together contribute to M1 remodeling through their roles in synaptic growth and maturation.

Mulet (mlt) encodes a tubulin-binding cofactor E-like homolog required for spermatid individualization in Drosophila melanogaster

Spermatogenesis in all animal species occurs within a syncytium. Only at the very end of spermatogenesis are individual sperm cells resolved from this syncytium in a process known as individualization. Individualization in Drosophila begins as a membrane-cytoskeletal complex known as the individualization complex (IC) assembles around the sperm heads and proceeds down the flagella, removing cytoplasm from between the sperm tails and shrink-wrapping each spermatid into its own plasma membrane as it travels. The mulet (mlt) mutation results in severely disrupted ICs, indicating that the mlt gene product is required for individualization. Inverse PCR followed by cycle sequencing maps all known P-insertion alleles of mlt to two overlapping genes, CG12214 (the Drosophila tubulin-binding cofactor E-like homolog) and KCNQ (a large voltage-gated potassium channel). However, since the alleles of mlt map to the 5'-UTR of CG12214 and since CG12214 is contained within an intron of KCNQ, it was hypothesized that mlt and CG12214 are allelic. Indeed, CG12214 mutant testes exhibited severely disrupted ICs and were indistinguishable from mlt mutant testes, thus further suggesting allelism. To test this hypothesis, alleles of mlt were crossed to CG12214 in order to generate trans-heterozygous males. Testes from all trans-heterozygous combinations revealed severely disrupted ICs and were also indistinguishable from mlt mutant testes, indicating that mlt and CG12214 fail to complement one another and are thus allelic. In addition, complementation testing against null alleles of KCNQ verified that the observed individualization defect is not caused by a disruption of KCNQ. Finally, since a population of spermatid-associated microtubules known to disappear prior to movement of the IC abnormally persists during individualization in CG12214 mutant testes, this work implicates TBCE-like in the removal of these microtubules prior to IC movement. Taken together, these results identify mlt as CG12214 and suggest that the removal of microtubules by TBCE-like is a necessary pre-requisite for proper coordinated movement of the IC.

GTP binds to α-crystallin and causes a significant conformational change

ATP was previously reported to bind to the chaperone α-crystallin resulting in a significant effect on the protein's ability to suppress the aggregation of a thermally denatured protein. Here, we have investigated the binding of GTP to α-crystallin. Unlike ATP, binding of GTP to α-crystallin did not affect its ability to suppress the aggregation of thermally denatured rhodanese. GTP binding induced a conformational change on α-crystallin, however the degree of exposed hydrophobic surfaces, which are believed to be involved in the binding of the chaperone to denaturing proteins did not change. Here, we report that GTP binds to α-crystallin and this results in a decreased stability of the chaperone as indicated by urea denaturation.

Salt stress-induced disassembly of Arabidopsis cortical microtubule arrays involves 26S proteasome-dependent degradation of SPIRAL1

The dynamic instability of cortical microtubules (MTs) (i.e., their ability to rapidly alternate between phases of growth and shrinkage) plays an essential role in plant growth and development. In addition, recent studies have revealed a pivotal role for dynamic instability in the response to salt stress conditions. The salt stress response includes a rapid depolymerization of MTs followed by the formation of a new MT network that is believed to be better suited for surviving high salinity. Although this initial depolymerization response is essential for the adaptation to salt stress, the underlying molecular mechanism has remained largely unknown. Here, we show that the MT-associated protein SPIRAL1 (SPR1) plays a key role in salt stress-induced MT disassembly. SPR1, a microtubule stabilizing protein, is degraded by the 26S proteasome, and its degradation rate is accelerated in response to high salinity. We show that accelerated SPR1 degradation is required for a fast MT disassembly response to salt stress and for salt stress tolerance.

PPARgamma Inhibitors as Novel Tubulin-Targeting Agents

The microtubule-targeting agents (MTAs) are a very successful class of cancer drugs with therapeutic benefits in both hematopoietic and solid tumors. However, resistance to these drugs is a significant problem. Current MTAs bind to microtubules, and/or to their constituent tubulin heterodimers, and affect microtubule polymerization and dynamics. The PPARγ inhibitor T0070907 can reduce tubulin levels in colorectal cancer cell lines and suppress tumor growth in a murine xenograft model. T0070907 does not alter microtubule polymerization in vitro, and does not appear to work by triggering modulation of tubulin RNA levels subsequent to decreased polymerization. This observation suggests the possible development of antimicrotubule drugs that work by a novel mechanism, and implies the presence of cancer therapeutic targets that have not yet been exploited. This review summarizes what is known about PPARγ inhibitors and cancer cell death, with emphasis on the tubulin phenotype and PPAR-dependence, and identifies potential mechanisms of action.

Faithful chaperones

This review describes the properties of some rare eukaryotic chaperones that each assist in the folding of only one target protein. In particular, we describe (1) the tubulin cofactors, (2) p47, which assists in the folding of collagen, (3) α-hemoglobin stabilizing protein (AHSP), (4) the adenovirus L4-100 K protein, which is a chaperone of the major structural viral protein, hexon, and (5) HYPK, the huntingtin-interacting protein. These various-sized proteins (102–1,190 amino acids long) are all involved in the folding of oligomeric polypeptides but are otherwise functionally unique, as they each assist only one particular client. This raises a question regarding the biosynthetic cost of the high-level production of such chaperones. As the clients of faithful chaperones are all abundant proteins that are essential cellular or viral components, it is conceivable that this necessary metabolic expenditure withstood evolutionary pressure to minimize biosynthetic costs. Nevertheless, the complexity of the folding pathways in which these chaperones are involved results in error-prone processes. Several human disorders associated with these chaperones are discussed.

TBCCD1, a new centrosomal protein, is required for centrosome and Golgi apparatus positioning

In animal cells the centrosome is positioned at the cell centre in close association with the nucleus. The mechanisms responsible for this are not completely understood. Here, we report the first characterization of human TBCC-domain containing 1 (TBCCD1), a protein related to tubulin cofactor C. TBCCD1 localizes at the centrosome and at the spindle midzone, midbody and basal bodies of primary and motile cilia. Knockdown of TBCCD1 in RPE-1 cells caused the dissociation of the centrosome from the nucleus and disorganization of the Golgi apparatus. TBCCD1-depleted cells are larger, less efficient in primary cilia assembly and their migration is slower in wound-healing assays. However, the major microtubule-nucleating activity of the centrosome is not affected by TBCCD1 silencing. We propose that TBCCD1 is a key regulator of centrosome positioning and consequently of internal cell organization.

Quality control of cytoskeletal proteins and human disease

Actins and tubulins are abundant cytoskeletal proteins that support diverse cellular processes. Owing to the unique properties of these filament-forming proteins, an intricate cellular machinery consisting minimally of the chaperonin CCT, prefoldin, phosducin-like proteins, and tubulin cofactors has evolved to facilitate their biogenesis. More recent evidence also suggests that regulated degradation pathways exist for actin (via TRIM32) and tubulin (via parkin or cofactor E-like). Collectively, these pathways maintain the quality control of cytoskeletal proteins ('proteostasis'), ensuring the appropriate function of microfilaments and microtubules. Here, we focus on the molecular mechanisms of the quality control of actin and tubulin, and discuss emerging links between cytoskeletal proteostasis and human diseases.

TBCD links centriologenesis, spindle microtubule dynamics, and midbody abscission in human cells

Microtubule-organizing centers recruit alpha- and beta-tubulin polypeptides for microtubule nucleation. Tubulin synthesis is complex, requiring five specific cofactors, designated tubulin cofactors (TBCs) A-E, which contribute to various aspects of microtubule dynamics in vivo. Here, we show that tubulin cofactor D (TBCD) is concentrated at the centrosome and midbody, where it participates in centriologenesis, spindle organization, and cell abscission. TBCD exhibits a cell-cycle-specific pattern, localizing on the daughter centriole at G1 and on procentrioles by S, and disappearing from older centrioles at telophase as the protein is recruited to the midbody. Our data show that TBCD overexpression results in microtubule release from the centrosome and G1 arrest, whereas its depletion produces mitotic aberrations and incomplete microtubule retraction at the midbody during cytokinesis. TBCD is recruited to the centriole replication site at the onset of the centrosome duplication cycle. A role in centriologenesis is further supported in differentiating ciliated cells, where TBCD is organized into "centriolar rosettes". These data suggest that TBCD participates in both canonical and de novo centriolar assembly pathways.

The microtubule-targeting agent T0070907 induces proteasomal degradation of tubulin

Current microtubule-targeting agents interfere with the regulated assembly of microtubules from alpha/beta tubulin heterodimers but do not markedly alter tubulin levels. Previously, we showed that the compound T0070907 interferes with microtubule function by reversibly decreasing alpha and beta tubulin protein levels by more than 50% in multiple CRC cell lines. Since tubulin levels are generally relatively stable, and cells lack regulatory networks to respond to decreased tubulin levels by increasing synthesis, our result suggested the possibility of cancer therapies that act directly on tubulin homeostasis. The aim of this study was to determine whether T0070907 caused tubulin loss by increasing the degradation rate, and determine the proteases responsible for any increased degradation. T0070907 increased tubulin degradation rates in HT-29 cells. The proteasomal inhibitors MG132, epoxomicin, lactacystin, and ALLN suppressed T0070907-mediated tubulin loss, although epoxomicin and lactacystin were less effective than MG132, even at concentrations that completely inhibited TNFalpha-induced IkappaBalpha degradation. Inhibitors of lysosomal, aggresomal, and calpain-mediated degradation, as well as the caspase inhibitor zVAD-fmk had no effect on tubulin loss, and the cathepsin and calpain inhibitor E64d was unable to increase epoxomicin's ability to suppress tubulin loss. We conclude that T0070907-induced tubulin degradation proceeds through a proteasome-dependent pathway.

Drosophila Tubulin-specific chaperone E functions at neuromuscular synapses and is required for microtubule network formation

Hypoparathyroidism, mental retardation and facial dysmorphism (HRD) is a fatal developmental disease caused by mutations in tubulin-specific chaperone E (TBCE). A mouse Tbce mutation causes progressive motor neuronopathy. To dissect the functions of TBCE and the pathogenesis of HRD, we generated mutations in Drosophila tbce, and manipulated its expression in a tissue-specific manner. Drosophila tbce nulls are embryonic lethal. Tissue-specific knockdown and overexpression of tbce in neuromusculature resulted in disrupted and increased microtubules, respectively. Alterations in TBCE expression also affected neuromuscular synapses. Genetic analyses revealed an antagonistic interaction between TBCE and the microtubule-severing protein Spastin. Moreover, treatment of muscles with the microtubule-depolymerizing drug nocodazole implicated TBCE as a tubulin polymerizing protein. Taken together, our results demonstrate that TBCE is required for the normal development and function of neuromuscular synapses and that it promotes microtubule formation. As defective microtubules are implicated in many neurological and developmental diseases, our work on TBCE may offer novel insights into their basis.

Gigaxonin controls vimentin organization through a tubulin chaperone-independent pathway

Gigaxonin mutations cause the fatal human neurodegenerative disorder giant axonal neuropathy (GAN). Broad deterioration of the nervous system in GAN patients is accompanied by massive disorganization of intermediate filaments (IFs) both in neurons and many non-neuronal cells. With newly developed antibodies, gigaxonin is now shown to be expressed at extremely low levels throughout the nervous system. In lymphoblast cell lines derived from severe and mild forms of GAN, mutations in gigaxonin are shown to yield highly unstable proteins, thereby permitting a rapid diagnostic test for the spectrum of GAN mutations as an alternative to invasive nerve biopsy or systematic sequencing of the GAN gene. Gigaxonin has been proposed as a substrate adaptor for an E3 ubiquitin ligase, which affects proteasome-dependent degradation of microtubule-related proteins including MAP1B, MAP8 and the tubulin folding chaperone TBCB. We demonstrate that, unlike its counterpart TBCE, TBCB only moderately destabilizes microtubules. Neither TBCB abundance nor microtubule organization or densities are altered in GAN mutant fibroblasts, thus demonstrating that altered TBCB levels are not primary determinants of IF disorganization in GAN. Characteristic GAN mutant-induced ovoid aggregates of vimentin are not produced in normal fibroblasts after disrupting microtubule assembly, either by TBCE overexpression or depolymerizing drugs. Thus, IF disorganization in GAN fibroblasts is independent of TBCB and microtubule loss and must be regulated by a yet unidentified mechanism.

Minimal plus-end tracking unit of the cytoplasmic linker protein CLIP-170

Cytoplasmic linker protein 170 (CLIP-170) is the prototype microtubule (MT) plus-end tracking protein (+TIP) and is involved in regulating MT dynamics. A comprehensive understanding of the process by which CLIP-170 tracks MT plus ends would provide insight into its function. However, the precise molecular mechanism of CLIP-170 +TIP behavior is unknown, and many potential models have been presented. Here, by separating the two CLIP-170 CAP-Gly domains and their adjacent serine-rich regions into fragments of varied size, we have characterized the minimal plus-end tracking unit of CLIP-170 in vivo. Each CLIP-170 fragment was also characterized for its tubulin polymerization activity in vitro. We found that the two CAP-Gly domains have different activities, whereas CAP-Gly-1 appears incompetent to mediate either +TIP behavior or MT nucleation, a CLIP-170 fragment consisting of the second CAP-Gly domain and its adjacent serine-rich region can both track MT plus ends in vivo and induce tubulin polymerization in vitro. These observations complement recent work on CLIP-170 fragments, demonstrate that CAP-Gly motifs do not require dimerization for +TIP and polymerization-promoting activities, and provide insight into CLIP-170 function and mechanism.

Tubulin cofactor B regulates microtubule densities during microglia transition to the reactive states

Microglia are highly dynamic cells of the CNS that continuously survey the welfare of the neural parenchyma and play key roles modulating neurogenesis and neuronal cell death. In response to injury or pathogen invasion parenchymal microglia transforms into a more active cell that proliferates, migrates and behaves as a macrophage. The acquisition of these extra skills implicates enormous modifications of the microtubule and actin cytoskeletons. Here we show that tubulin cofactor B (TBCB), which has been found to contribute to various aspects of microtubule dynamics in vivo, is also implicated in microglial cytoskeletal changes. We find that TBCB is upregulated in post-lesion reactive parenchymal microglia/macrophages, in interferon treated BV-2 microglial cells, and in neonate amoeboid microglia where the microtubule densities are remarkably low. Our data demonstrate that upon TBCB downregulation both, after microglia differentiation to the ramified phenotype in vivo and in vitro, or after TBCB gene silencing, microtubule densities are restored in these cells. Taken together these observations support the view that TBCB functions as a microtubule density regulator in microglia during activation, and provide an insight into the understanding of the complex mechanisms controlling microtubule reorganization during microglial transition between the amoeboid, ramified, and reactive phenotypes.

Multiple expressed MHC class II loci in salmonids; details of one non-classical region in Atlantic salmon (Salmo salar)

Background

In teleosts, the Major Histocompatibility Complex (MHC) class I and class II molecules reside on different linkage groups as opposed to tetrapods and shark, where the class I and class II genes reside in one genomic region. Several teleost MHC class I regions have been sequenced and show varying number of class I genes. Salmonids have one major expressed MHC class I locus (UBA) in addition to varying numbers of non-classical genes. Two other more distant lineages are also identifyed denoted L and ZE. For class II, only one major expressed class II alpha (DAA) and beta (DAB) gene has been identified in salmonids so far.

Results

We sequenced a genomic region of 211 kb encompassing divergent MHC class II alpha (Sasa-DBA) and beta (Sasa-DBB) genes in addition to NRGN, TIPRL, TBCEL and TECTA. The region was not linked to the classical class II genes and had some synteny to genomic regions from other teleosts. Two additional divergent and expressed class II sequences denoted DCA and DDA were also identified in both salmon and trout. Expression patterns and lack of polymorphism make these genes non-classical class II analogues. Sasa-DBB, Sasa-DCA and Sasa-DDA had highest expression levels in liver, hindgut and spleen respectively, suggestive of distinctive functions in these tissues. Phylogenetic studies revealed more yet undescribed divergent expressed MHC class II molecules also in other teleosts.

Conclusion

We have characterised one genomic region containing expressed non-classical MHC class II genes in addition to four other genes not involved in immune function. Salmonids contain at least two expressed MHC class II beta genes and four expressed MHC class II alpha genes with properties suggestive of new functions for MHC class II in vertebrates. Collectively, our data suggest that the class II is worthy of more elaborate studies also in other teleost species.

Op18/Stathmin counteracts the activity of overexpressed tubulin-disrupting proteins in a human leukemia cell line

Op18/stathmin (Op18) is a phosphorylation-regulated and differentially expressed microtubule-destabilizing protein in animal cells. Op18 regulates tubulin monomer-polymer partitioning of the interphase microtubule system and forms complexes with tubulin heterodimers. Recent reports have shown that specific tubulin-folding cofactors and related proteins may disrupt tubulin heterodimers. We therefore investigated whether Op18 protects unpolymerized tubulin from such disruptive activities. Our approach was based on inducible overexpression of two tubulin-disrupting proteins, namely TBCE, which is required for tubulin biogenesis, and E-like, which has been proposed to regulate tubulin turnover and microtubule stability. Expression of either of these proteins was found to cause a rapid degradation of both alpha-tubulin and beta-tubulin subunits of unpolymerized, but not polymeric, tubulin heterodimers. We found that depletion of Op18 by means of RNA interference increased the susceptibility of tubulin to TBCE or E-like mediated disruption, while overexpressed Op18 exerted a tubulin-protective effect. Tubulin protection was shown to depend on Op18 levels, binding affinity, and the partitioning between tubulin monomers and polymers. Hence, the present study reveals that Op18 at physiologically relevant levels functions to preserve the integrity of tubulin heterodimers, which may serve to regulate tubulin turnover rates.

Efficient chaperone-mediated tubulin biogenesis is essential for cell division and cell migration in C. elegans

The efficient folding of actin and tubulin in vitro and in Saccharomyces cerevisiae is known to require the molecular chaperones prefoldin and CCT, yet little is known about the functions of these chaperones in multicellular organisms. Whereas none of the six prefoldin genes are essential in yeast, where prefoldin-independent folding of actin and tubulin is sufficient for viability, we demonstrate that reducing prefoldin function by RNAi in Caenorhabditis elegans causes defects in cell division that result in embryonic lethality. Our analyses suggest that these defects result mainly from a decrease in alpha-tubulin levels and a subsequent reduction in the microtubule growth rate. Prefoldin subunit 1 (pfd-1) mutant animals with maternally contributed PFD-1 develop to the L4 larval stage with gonadogenesis defects that include aberrant distal tip cell migration. Importantly, RNAi knockdown of prefoldin, CCT or tubulin in developing animals phenocopy the pfd-1 cell migration phenotype. Furthermore, reducing CCT function causes more severe phenotypes (compared with prefoldin knockdown) in the embryo and developing gonad, consistent with a broader role for CCT in protein folding. Overall, our results suggest that efficient chaperone-mediated tubulin biogenesis is essential in C. elegans, owing to the critical role of the microtubule cytoskeleton in metazoan development.

Tubulin cofactor B plays a role in the neuronal growth cone

Tubulin cofactors, initially identified as alpha-, beta-tubulin folding proteins, are now believed to participate in the complex tubulin biogenesis and degradation routes, and thus to contribute to microtubule functional diversity and dynamics. However, a concrete role of tubulin cofactor B (TBCB) remains to be elucidated because this protein is not required for tubulin biogenesis, and it is apparently not essential for life in any of the organisms studied. In agreement with these data, here we show that TBCB localizes at the transition zone of the growth cones of growing neurites during neurogenesis where it plays a role in microtubule dynamics and plasticity. Gene silencing by means of small interfering RNA segments revealed that TBCB knockdown enhances axonal growth. In contrast, excess TBCB, a feature of giant axonal neuropathy, leads to microtubule depolymerization, growth cone retraction, and axonal damage followed by neuronal degeneration. These results provide an important insight into the understanding of the controlling mechanisms of growth cone microtubule dynamics.

Role of cofactors B (TBCB) and E (TBCE) in tubulin heterodimer dissociation

Tubulin folding cofactors B (TBCB) and E (TBCE) are alpha-tubulin binding proteins that, together with Arl2 and cofactors D (TBCD), A (TBCA or p14) and C (TBCC), participate in tubulin biogenesis. TBCD and TBCE have also been implicated in microtubule dynamics through regulation of tubulin heterodimer dissociation. Understanding the in vivo function of these proteins will shed light on the Kenny-Caffey/Sanjad-Sakati syndrome, an important human disorder associated with TBCE. Here we show that, when overexpressed, TBCB depolymerizes microtubules. We found that this function is based on the ability of TBCB to form a binary complex with TBCE that greatly enhances the efficiency of this cofactor to dissociate tubulin in vivo and in vitro. We also show that TBCE, TBCB and alpha-tubulin form a ternary complex after heterodimer dissociation, whereas the free beta-tubulin subunit is recovered by TBCA. These complexes might serve to escort alpha-tubulin towards degradation or recycling, depending on the cell requirements.

Possible regulation of microtubules through destabilization of tubulin

Chaperones and folding cofactors are known to mediate posttranslational folding of nascent tubulin, thus forming functional dimers. Based on sequence likeness, a novel protein similar to cofactor E, E-like protein (El), was identified. In overexpression experiments El, similar to a subset of folding factors (i.e. cofactors D and E), appears to disrupt functional dimers and target them for destruction by the proteasome. El apparently does not interact with microtubules directly and has no function in the tubulin folding pathway. Suppression of El expression seems to increase the cellular content of stable, posttranslationally modified microtubules by an unknown mechanism. Degradation of functional tubulin dimers as well as the alteration of the cellular content of stable microtubules through El might regulate the distribution and organization of organelles in vivo.