Gamma-tubulin: the microtubule organizer?

Recent Trends in Tubulin-Binding Combretastatin A-4 Analogs for Anticancer Drug Development

Although significant progress over several decades has been evidenced in cancer therapy, there still remains a need for the development of novel and effective therapeutic strategies to treat several relapsed and intractable cancers. In this regard, tubulin protein has become one of the efficient and major targets for anticancer drug discovery. Considering the antimitotic ability, several tubulin inhibitors have been developed to act against various cancers. Among various tubulin inhibitors available, combretastatin-A4 (CA-4), a naturally occurring lead molecule, offers exceptional cytotoxicity (including the drug-resistant cell lines) and antivascular effects. Although CA-4 offers exceptional therapeutic efficacy, several new advancements have been proposed, such as the structural modification via A and B rings, as well as cis-olefinic bridging, which provide highly efficient analogs with improved tubulin-binding efficiency to meet the anticancer drug development requirements. This review systematically emphasizes the recent trends and latest developments in the anticancer drug design & discovery, using CA-4 analogs as the tubulin inhibiting agents, highlighting their structure-activity relationships (SAR) and resultant pharmacological efficacies.

UVB Inhibits Proliferation, Cell Cycle and Induces Apoptosis via p53, E2F1 and Microtubules System in Cervical Cancer Cell Lines

Ultraviolet (UV) exposure has been linked to skin damage and carcinogenesis, but recently UVB has been proposed as a therapeutic approach for cancer. Herein, we investigated the cellular and molecular effects of UVB in immortal and tumorigenic HPV positive and negative cells. Cells were irradiated with 220.5 to 1102.5 J/m² of UVB and cell proliferation was evaluated by crystal violet, while cell cycle arrest and apoptosis analysis were performed through flow cytometry. UVB effect on cells was recorded at 661.5 J/m² and it was exacerbated at 1102.5 J/m². All cell lines were affected by proliferation inhibition, cell cycle ablation and apoptosis induction, with different degrees depending on tumorigenesis level or HPV type. Analysis of the well-known UV-responsive p53, E2F1 and microtubules system proteins was performed in SiHa cells in response to UVB through Western-blotting assays. E2F1 and the Microtubule-associated protein 2 (MAP2) expression decrease correlated with cellular processes alteration while p53 and Microtubule-associated Protein 1S (MAP1S) expression switch was observed since 882 J/m², suggesting they were required under more severe cellular damage. However, expression transition of α-Tubulin3C and β-Tubulin was abruptly noticed until 1102.5 J/m² and particularly, γ-Tubulin protein expression remained without alteration. This study provides insights into the effect of UVB in cervical cancer cell lines.

AMPKα-like proteins as LKB1 downstream targets in cell physiology and cancer

One of the key events in cancer development is the ability of tumor cells to overcome nutrient deprivation and hypoxia. Among proteins performing metabolic adaptation to the various cellular nutrient conditions, liver kinase B 1 (LKB1) and its main downstream target adenosine monophosphate (AMP)-activated protein kinase α (AMPKα) are important sensors of energy requirements within the cell. Although LKB1 was originally described as a tumor suppressor, given its role in metabolism, it potentially acts as a double-edged sword. AMPKα, a master regulator of cell energy demands, is activated when ATP level drops under a certain threshold, responding accordingly through its downstream targets. Twelve downstream kinase targets of LKB1 have been described as AMPKα-like proteins. This group is comprised of novel (nua) kinase family (NUAK) kinases (NUAK1 and 2) linked to cell cycle progression and ultraviolet (UV)-damage; microtubule affinity regulating kinases (MARKs) (MARK1, MARK2, MARK3, and MARK4) that are involved in cell polarity; salt inducible kinases (SIK) (SIK1, SIK2, also known as Qin-induced kinase or QIK and SIK3) that are implicated in cell metabolism and adipose tissue development and mitotic regulation; maternal embryonic leuzine zipper kinase (MELK) that regulate oocyte maturation; and finally brain selective kinases (BRSKs) (BRSK1 and 2), which have been mainly characterized in the brain due to their role in neuronal polarization. Thus, many efforts have been made in order to harness LKB1 kinase and its downstream targets as a possible therapeutic hub in tumor development and propagation. In this review, we describe LKB1 and its downstream target AMPK summarize major functions of various AMPK-like proteins, while focusing on biological functions of BRSK1 and 2 in different models.

Organized Neurogenic-Niche-Like Pinwheel Structures Discovered in Spinal Cord Tissue-Derived Neurospheres

The neurogenic niche of the subventricular zone (SVZ) in adult brain tissue takes the form of a pinwheel-like cytoarchitectural structure, with mono-ciliated astrocytes displaying neural stem cell (NSC) characteristics present in the core surrounded by ciliated ependymal cells. For the first time, we have demonstrated the formation of similar pinwheel structures in spinal cord and SVZ tissue-derived neurospheres cultured in vitro. To investigate whether the organization and integrity of these pinwheel structures depends on the appropriate organization of ciliated astrocytes and ependymal cells, we modified neurosphere cell arrangements via the application of the methyltransferase inhibitor 5-aza-2'-deoxycytidine (5-aza-dc) or the antiviral drug ganciclovir (GCV) in transgenic mice expressing herpes simplex virus thymidine kinase from the GFAP promoter (GFAP-TK). Treatment of neurospheres with 5-aza-dc increased FoxJ1 expression, a crucial factor for ciliogenesis, by reducing methylation of the FoxJ1 CpG island. 5-aza-dc also increased the expression of the astrocyte marker GFAP and caused aberrant accumulation of ciliated astrocytes. However, the ablation of dividing astrocytes within neurospheres by GCV treatment led to an increase in the accumulation of ciliated ependymal cells, as evidenced by the increased expression of the ependymal cell markers Vimentin or CD24. While 5-aza-dc and GCV treatment differentially affected cell arrangement, both compounds significantly diminished the number of pinwheel structures present in neurospheres. Thus, we suggest that the ratio of ciliated astrocytes to ependymal cells plays a crucial role in the correct formation of the pinwheel structures in spinal cord tissue-derived neurospheres in vitro.

Fungal Morphogenesis, from the Polarized Growth of Hyphae to Complex Reproduction and Infection Structures

Filamentous fungi constitute a large group of eukaryotic microorganisms that grow by forming simple tube-like hyphae that are capable of differentiating into more-complex morphological structures and distinct cell types. Hyphae form filamentous networks by extending at their tips while branching in subapical regions. Rapid tip elongation requires massive membrane insertion and extension of the rigid chitin-containing cell wall. This process is sustained by a continuous flow of secretory vesicles that depends on the coordinated action of the microtubule and actin cytoskeletons and the corresponding motors and associated proteins. Vesicles transport cell wall-synthesizing enzymes and accumulate in a special structure, the Spitzenkörper, before traveling further and fusing with the tip membrane. The place of vesicle fusion and growth direction are enabled and defined by the position of the Spitzenkörper, the so-called cell end markers, and other proteins involved in the exocytic process. Also important for tip extension is membrane recycling by endocytosis via early endosomes, which function as multipurpose transport vehicles for mRNA, septins, ribosomes, and peroxisomes. Cell integrity, hyphal branching, and morphogenesis are all processes that are largely dependent on vesicle and cytoskeleton dynamics. When hyphae differentiate structures for asexual or sexual reproduction or to mediate interspecies interactions, the hyphal basic cellular machinery may be reprogrammed through the synthesis of new proteins and/or the modification of protein activity. Although some transcriptional networks involved in such reprogramming of hyphae are well studied in several model filamentous fungi, clear connections between these networks and known determinants of hyphal morphogenesis are yet to be established.

Environmental Factors That Influence Stem Cell Migration: An "Electric Field"

Environmental Stimulus of Electric Fields on Stem Cell Migration. The movement of cells in response to electric potential gradients is called galvanotaxis. In vivo galvanotaxis, powered by endogenous electric fields (EFs), plays a critical role during development and wound healing. This review aims to provide a perspective on how stem cells transduce EFs into directed migration and an understanding of the current literature relating to the mechanisms by which cells sense and transduce EFs. We will comment on potential EF-based regenerative medicine therapeutics.

Development of combretastatins as potent tubulin polymerization inhibitors

The combretastatins are isolated from South African tree combretum caffrum kuntze. The lead compound combretastatin A-4 has displayed remarkable cytotoxic effect in a wide variety of preclinical tumor models and inhibits tubulin polymerization by interacting at colchicine binding site of microtubule. However, the structural simplicity of C A-4 is favorable for synthesis of various derivatives projected to induce rapid and selective vascular shutdown in tumors. Majority of the molecules have shown excellent antiproliferative activity and are able to inhibit tubulin polymerization as well as possible mechanisms of action have been investigated. In this review article, the synthesis and structure-activity relationships of C A-4 and immense number of its synthetic derivatives with various modifications on the A, B-rings, bridge carbons and their anti mitotic activities are discussed.

Inhaled vanadium pentoxide decrease gamma-tubulin of mouse testes at different exposure times

Vanadium is an important environmental and industrial pollutant whose concentrations have increased in the last decades. Due to its status as reproductive toxicant and a microtubule damaging agent, the present study investigated by immunohistochemistry the effect of the inhalation of vanadium pentoxide on gamma-tubulin within somatic and testicular germ cells. Male mice inhaled vanadium pentoxide (V2O5) (0.02 M) 1 h/twice a week for 12 weeks. Our results demonstrated that vanadium accumulates in the testes starting with the initial inhalation (24 h), and this pattern remained until the last week of treatment. In general, vanadium was capable of significantly decreasing the percentage of gamma-tubulin in all analyzed testicular cells (Sertoli, Leydig and germ cells) starting with the first week of treatment. For all cell types studied, regression analysis revealed a negative and significant relationship between the percentage of immunopositive cells to gamma-tubulin and exposure time, showing a time dependent response in all cases. Our findings suggest that alterations on this protein might imply changes in microtubule-involved function such as cell division, which in the testes might lead to damage in the spermatogenesis, leading probably to infertility.

A quick method to investigate the Drosophila Johnston's organ by confocal microscopy

Drosophila antennae is gaining attention to study the hearing molecules and its mechanism in last few decades. Various molecules required for the formation of hearing organ is conserved between Drosophila and human being. This suggests Drosophila can be used as a model organism to decipher the vertebrate hearing mechanism. In this context a protocol describing the fixation, sectioning and staining of antennae is lacking from the literature. The current paper describes various commercially available markers of the antennae to visualise it under confocal microscope.

Biciliated ependymal cell proliferation contributes to spinal cord growth

Two neurogenic regions have been described in the adult brain, the lateral ventricle subventricular zone and the dentate gyrus subgranular zone. It has been suggested that neural stem cells also line the central canal of the adult spinal cord. Using transmission and scanning electron microscopy and immunostaining, we describe here the organization and cell types of the central canal epithelium in adult mice. The identity of dividing cells was determined by 3D ultrastructural reconstructions of [(3) H]thymidine-labeled cells and confocal analysis of bromodeoxyuridine labeling. The most common cell type lining the central canal had two long motile (9+2) cilia and was vimentin+, CD24+, FoxJ1+, Sox2+, and CD133+, but nestin- and glial fibrillary acidic protein (GFAP)-. These biciliated ependymal cells of the central canal (Ecc) resembled E2 cells of the lateral ventricles, but their basal bodies were different from those of E2 or E1 cells. Interestingly, we frequently found Ecc cells with two nuclei and four cilia, suggesting they are formed by incomplete cytokinesis or cell fusion. GFAP+ astrocytes with a single cilium and an orthogonally oriented centriole were also observed. The majority of dividing cells corresponded to biciliated Ecc cells. Central canal proliferation was most common during the active period of spinal cord growth. Pairs of labeled Ecc cells were observed within the central canal in adult mice 2.5 weeks post labeling. Our work suggests that the vast majority of postnatal dividing cells in the central canal are Ecc cells and their proliferation is associated with the growth of the spinal cord.

The relative effect of citral on mitotic microtubules in wheat roots and BY2 cells

The plant volatile monoterpene citral is a highly active compound with suggested allelopathic traits. Seed germination and seedling development are inhibited in the presence of citral, and it disrupts microtubules in both plant and animal cells in interphase. We addressed the following additional questions: can citral interfere with cell division; what is the relative effect of citral on mitotic microtubules compared to interphase cortical microtubules; what is its effect on newly formed cell plates; and how does it affect the association of microtubules with γ-tubulin? In wheat seedlings, citral led to inhibition of root elongation, curvature of newly formed cell walls and deformation of microtubule arrays. Citral's effect on microtubules was both dose- and time-dependent, with mitotic microtubules appearing to be more sensitive to citral than cortical microtubules. Association of γ-tubulin with microtubules was more sensitive to citral than were the microtubules themselves. To reveal the role of disrupted mitotic microtubules in dictating aberrations in cell plates in the presence of citral, we used tobacco BY2 cells expressing GFP-Tua6. Citral disrupted mitotic microtubules, inhibited the cell cycle and increased the frequency of asymmetric cell plates in these cells. The time scale of citral's effect in BY2 cells suggested a direct influence on cell plates during their formation. Taken together, we suggest that at lower concentrations, citral interferes with cell division by disrupting mitotic microtubules and cell plates, and at higher concentrations it inhibits cell elongation by disrupting cortical microtubules.

Sas-4 proteins are required during basal body duplication in Paramecium

This study investigated the role of Sas-4 in basal body duplication in Paramecium and found that Sas-4 proteins are required to assemble and stabilize the germinative disk and cartwheel, which suggests that Sas-4 plays an essential role in basal body duplication.

Centrioles and basal bodies are structurally related organelles composed of nine microtubule (MT) triplets. Studies performed in Caenorhabditis elegans embryos have shown that centriole duplication takes place in sequential way, in which different proteins are recruited in a specific order to assemble a procentriole. ZYG-1 initiates centriole duplication by triggering the recruitment of a complex of SAS-5 and SAS-6, which then recruits the final player, SAS-4, to allow the incorporation of MT singlets. It is thought that a similar mechanism (that also involves additional proteins) is present in other animal cells, but it remains to be investigated whether the same players and their ascribed functions are conserved during basal body duplication in cells that exclusively contain basal bodies. To investigate this question, we have used the multiciliated protist Paramecium tetraurelia. Here we show that in the absence of PtSas4, two types of defects in basal body duplication can be identified. In the majority of cases, the germinative disk and cartwheel, the first structures assembled during duplication, are not detected. In addition, if daughter basal bodies were formed, they invariably had defects in MT recruitment. Our results suggest that PtSas4 has a broader function than its animal orthologues.

The polarity protein Pard3 is required for centrosome positioning during neurulation

Microtubules are essential regulators of cell polarity, architecture and motility. The organization of the microtubule network is context-specific. In non-polarized cells, microtubules are anchored to the centrosome and form radial arrays. In most epithelial cells, microtubules are noncentrosomal, align along the apico-basal axis and the centrosome templates a cilium. It follows that cells undergoing mesenchyme-to-epithelium transitions must reorganize their microtubule network extensively, yet little is understood about how this process is orchestrated. In particular, the pathways regulating the apical positioning of the centrosome are unknown, a central question given the role of cilia in fluid propulsion, sensation and signaling. In zebrafish, neural progenitors undergo progressive epithelialization during neurulation, and thus provide a convenient in vivo cellular context in which to address this question. We demonstrate here that the microtubule cytoskeleton gradually transitions from a radial to linear organization during neurulation and that microtubules function in conjunction with the polarity protein Pard3 to mediate centrosome positioning. Pard3 depletion results in hydrocephalus, a defect often associated with abnormal cerebrospinal fluid flow that has been linked to cilia defects. These findings thus bring to focus cellular events occurring during neurulation and reveal novel molecular mechanisms implicated in centrosome positioning.

FoxJ1-dependent gene expression is required for differentiation of radial glia into ependymal cells and a subset of astrocytes in the postnatal brain

Neuronal specification occurs at the periventricular surface of the embryonic central nervous system. During early postnatal periods, radial glial cells in various ventricular zones of the brain differentiate into ependymal cells and astrocytes. However, mechanisms that drive this time- and cell-specific differentiation remain largely unknown. Here, we show that expression of the forkhead transcription factor FoxJ1 in mice is required for differentiation into ependymal cells and a small subset of FoxJ1(+) astrocytes in the lateral ventricles, where these cells form a postnatal neural stem cell niche. Moreover, we show that a subset of FoxJ1(+) cells harvested from the stem cell niche can self-renew and possess neurogenic potential. Using a transcriptome comparison of FoxJ1-null and wild-type microdissected tissue, we identified candidate genes regulated by FoxJ1 during early postnatal development. The list includes a significant number of microtubule-associated proteins, some of which form a protein complex that could regulate the transport of basal bodies to the ventricular surface of differentiating ependymal cells during FoxJ1-dependent ciliogenesis. Our results suggest that time- and cell-specific expression of FoxJ1 in the brain acts on an array of target genes to regulate the differentiation of ependymal cells and a small subset of astrocytes in the adult stem cell niche.

Submembraneous microtubule cytoskeleton: regulation of microtubule assembly by heterotrimeric Gproteins

Heterotrimeric Gproteins participate in signal transduction by transferring signals from cell surface receptors to intracellular effector molecules. Gproteins also interact with microtubules and participate in microtubule-dependent centrosome/chromosome movement during cell division, as well as neuronal differentiation. In recent years, significant progress has been made in our understanding of the biochemical/functional interactions between Gprotein subunits (alpha and betagamma) and microtubules, and the molecular details emerging from these studies suggest that alpha and betagamma subunits of Gproteins interact with tubulin/microtubules to regulate the assembly/dynamics of microtubules, providing a novel mechanism for hormone- or neurotransmitter-induced rapid remodeling of cytoskeleton, regulation of the mitotic spindle for centrosome/chromosome movements in cell division, and neuronal differentiation in which structural plasticity mediated by microtubules is important for appropriate synaptic connections and signal transmission.

gammaTub23C interacts genetically with brahma chromatin-remodeling complexes in Drosophila melanogaster

The brahma gene encodes the catalytic subunit of the Drosophila melanogaster BRM chromatin-remodeling complexes. Screening for mutations that interact with brahma, we isolated the dominant-negative Pearl-2 allele of gammaTub23C. gammaTub23C encodes one of the two gamma-tubulin isoforms in Drosophila and is essential for zygotic viability and normal adult patterning. gamma-Tubulin is a subunit of microtubule organizer complexes. We show that mutations in lethal (1) discs degenerate 4, which encodes the Grip91 subunit of microtubule organizer complexes, suppress the recessive lethality and the imaginal phenotypes caused by gammaTub23C mutations. The genetic interactions between gammaTub23C and chromatin-remodeling mutations suggest that gamma-tubulin might have a role in regulating gene expression.

Effects of the herbicide isopropyl-N-phenyl carbamate on microtubules and MTOCs in lines of Nicotiana sylvestris resistant and sensitive to its action

To clarify the mechanism of isopropyl-N-phenyl carbamate (IPC) action on higher plant cells the sensitivity of microtubules (cortical network and mitotic arrays) and microtubule organizing centers to IPC treatment (30 microM) in IPC-resistant and sensitive Nicotiana sylvestris lines was studied. It was clearly demonstrated that IPC does not depolymerize plant MTs but causes the MTOC damage in cells, which results in MTOC fragmentation, splitting of the spindle poles and in abnormal division spindle formation. It was also found that IPC-resistance of mutant N. sylvestris line correlates not with tubulin resistance to IPC action but possibly with resistance of one of the proteins involved in MTOC composition.

The stumpy gene is required for mammalian ciliogenesis

Cilia are present on nearly all cell types in mammals and perform remarkably diverse functions. However, the mechanisms underlying ciliogenesis are unclear. Here, we cloned a previously uncharacterized highly conserved gene, stumpy, located on mouse chromosome 7. Stumpy was ubiquitously expressed, and conditional loss in mouse resulted in complete penetrance of perinatal hydrocephalus (HC) and severe polycystic kidney disease (PKD). We found that cilia in stumpy mutant brain and kidney cells were absent or markedly deformed, resulting in defective flow of cerebrospinal fluid. Stumpy colocalized with ciliary basal bodies, physically interacted with gamma-tubulin, and was present along ciliary axonemes, suggesting that stumpy plays a role in ciliary axoneme extension. Therefore, stumpy is essential for ciliogenesis and may be involved in the pathogenesis of human congenital malformations such as HC and PKD.

Rab11-FIP3 is critical for the structural integrity of the endosomal recycling compartment

Rab11-FIP3 is an endosomal recycling compartment (ERC) protein that is implicated in the process of membrane delivery from the ERC to sites of membrane insertion during cell division. Here we report that Rab11-FIP3 is critical for the structural integrity of the ERC during interphase. We demonstrate that knockdown of Rab11-FIP3 and expression of a mutant of Rab11-FIP3 that is Rab11-binding deficient cause loss of all ERC-marker protein staining from the pericentrosomal region of A431 cells. Furthermore, we find that fluorophore-labelled transferrin cannot access the pericentrosomal region of cells in which Rab11-FIP3 function has been perturbed. We find that this Rab11-FIP3 function appears to be specific because expression of the equivalent Rab11-binding deficient mutant of Rab-coupling protein does not perturb ERC morphology. In addition, we find that other organelles such as sorting and late endosomes are unaffected by loss of Rab11-FIP3 function. Finally, we demonstrate the presence of an extensive coiled-coil region between residues 463 and 692 of Rab11-FIP3, which exists as a dimer in solution and is critical to support its function on the ERC. Together, these data indicate that Rab11-FIP3 is necessary for the structural integrity of the pericentrosomal ERC.

Soluble tubulin complexes, γ-tubulin, and their changing distribution in the zebrafish (Danio rerio) ovary, oocyte and embryo

Tubulin dynamics, i.e., the interchange of polymeric and soluble forms, is important for microtubule (MTs) cellular functions, and thus plays essential roles in zebrafish oogenesis and embryogenesis. A novel finding in this study revealed that there were soluble pools of tubulins in zebrafish oocytes that were sequestered and maintained in a temporary "oligomeric" state, which retained assembling and disassembling potential (suggested by undetected acetylated tubulin, marker of stable tubulin), but lacked abilities to assemble into MTs spontaneously in vivo. Using differential centrifugation, gel chromatography and DM1A-probed western blot, soluble alpha-tubulin was found to be associated with large molecular weight complexes (MW range to over 2 MDa) which were reduced in amount by the blastula stage, especially in some batches of embryos, with a concomitant decrease in soluble tubulin. Complexes (MW range less than 2 MDa) then increased in the gastrula with an increase in soluble alpha-tubulin. Two different anti-gamma-tubulin monoclonal antibodies, GTU 88 and TU 30, revealed the existence of soluble gamma-tubulin in both zebrafish oocytes and embryos, which also decreased by the blastula stage and increased in the gastrula stage. Soluble alpha-tubulin and gamma-tubulin extracted from zebrafish ovaries, oocytes and embryos co-localized in fractions on three different columns: S-200 Sephacryl, DEAE and Superose-6b. The soluble tubulin complexes were competent to assemble into MTs in vitro induced by taxol, and gamma-tubulin was co-localized with assembled MTs. These soluble tubulin complexes were stable during freeze-thaw cycles and resisted high ionic interaction (up to 1.5 M NaCl). Furthermore, some ovarian soluble alpha-tubulin could be co-immunoprecipitated with gamma-tubulin, and vice versa. Two antibodies specific for Xenopus gamma-tubulin ring complex proteins (Xgrip 109 and Xgrip 195) detected single bands from ovarian extracts in western blots, suggesting the existence of Xgrip 109 and Xgrip 195 homologues in zebrafish. These findings, together with recent work on gamma-tubulin ring complexes in oocytes, eggs and embryos of other species, suggest that soluble gamma-tubulin-associated protein complexes may be involved in regulating tubulin dynamics during zebrafish oogenesis and embryogenesis.

G protein βγ subunits interact with αβ- and γ-tubulin and play a role in microtubule assembly in PC12 cells

The betagamma subunit of G proteins (Gbetagamma) is known to transfer signals from cell surface receptors to intracellular effector molecules. Recent results suggest that Gbetagamma also interacts with microtubules and is involved in the regulation of the mitotic spindle. In the current study, the anti-microtubular drug nocodazole was employed to investigate the mechanism by which Gbetagamma interacts with tubulin and its possible implications in microtubule assembly in cultured PC12 cells. Nocodazole-induced depolymerization of microtubules drastically inhibited the interaction between Gbetagamma and tubulin. Gbetagamma was preferentially bound to microtubules and treatment with nocodazole suggested that the dissociation of Gbetagamma from microtubules is an early step in the depolymerization process. When microtubules were allowed to recover after removal of nocodazole, the tubulin-Gbetagamma interaction was restored. Unlike Gbetagamma, however, the interaction between tubulin and the alpha subunit of the Gs protein (Gsalpha) was not inhibited by nocodazole, indicating that the inhibition of tubulin-Gbetagamma interactions during microtubule depolymerization is selective. We found that Gbetagamma also interacts with gamma-tubulin, colocalizes with gamma-tubulin in centrosomes, and co-sediments in centrosomal fractions. The interaction between Gbetagamma and gamma-tubulin was unaffected by nocodazole, suggesting that the Gbetagamma-gamma-tubulin interaction is not dependent on assembled microtubules. Taken together, our results suggest that Gbetagamma may play an important and definitive role in microtubule assembly and/or stability. We propose that betagamma-microtubule interaction is an important step for G protein-mediated cell activation. These results may also provide new insights into the mechanism of action of anti-microtubule drugs.

Non-centrosomal microtubule-organising centres in cold-treated cultured Drosophila cells

In this paper we describe a new type of non-centrosomal microtubule-organising centre (MTOC), which is induced by cold treatment of certain cultured Drosophila cells and allows rapid reassembly of microtubule (MT) arrays. Prolonged cooling of two types of cultured Drosophila cells, muscle cells in primary culture and a wing imaginal disc cell line Cl.8+ results in disassembly of MT arrays and induces the formation of clusters of short MTs that have not been described before. Upon rewarming, the clusters are lost and the MT array is re-established within 1 h. In Cl.8+ cells, gamma-tubulin-containing centrosomes are detected, both in cell extensions and in the expected juxtanuclear position, and gamma-tubulin co-localises with the cold-induced MT clusters. The MT plus-end-binding protein, Drosophila EB1, decorates growing tips of MTs extending from clusters. We conclude that the cold-induced MT clusters represent acentrosomal MTOCs, allowing rapid reassembly of MT arrays following exposure to cold.

G protein activation is prerequisite for functional coupling between Gα/Gβγ and tubulin/microtubules

Heterotrimeric G proteins participate in signal transduction by transferring signals from cell surface receptors to intracellular effector molecules. Interestingly, recent results suggest that G proteins also interact with microtubules and participate in cell division and differentiation. It has been shown earlier that both alpha and betagamma subunits of G proteins modulate microtubule assembly in vitro. Since G protein activation and subsequent dissociation of alpha and betagamma subunits are necessary for G proteins to participate in signaling processes, here we asked if similar activation is required for modulation of microtubule assembly by G proteins. We reconstituted Galphabetagamma heterotrimer from myristoylated-Galpha and prenylated-Gbetagamma, and found that the heterotrimer blocks Gi1alpha activation of tubulin GTPase and inhibits the ability of Gbeta1gamma2 to promote in vitro microtubule assembly. Results suggest that G protein activation is required for functional coupling between Galpha/Gbetagamma and tubulin/microtubules, and supports the notion that regulation of microtubules is an integral component of G protein mediated signaling.

Molecular processes during fat cell development revealed by gene expression profiling and functional annotation

In-depth bioinformatics analyses of expressed sequence tags found to be differentially expressed during differentiation of 3T3-L1 pre-adipocyte cells were combined with de novo functional annotation and mapping onto known pathways to generate a molecular atlas of fat-cell development.

Background

Large-scale transcription profiling of cell models and model organisms can identify novel molecular components involved in fat cell development. Detailed characterization of the sequences of identified gene products has not been done and global mechanisms have not been investigated. We evaluated the extent to which molecular processes can be revealed by expression profiling and functional annotation of genes that are differentially expressed during fat cell development.

Results

Mouse microarrays with more than 27,000 elements were developed, and transcriptional profiles of 3T3-L1 cells (pre-adipocyte cells) were monitored during differentiation. In total, 780 differentially expressed expressed sequence tags (ESTs) were subjected to in-depth bioinformatics analyses. The analysis of 3'-untranslated region sequences from 395 ESTs showed that 71% of the differentially expressed genes could be regulated by microRNAs. A molecular atlas of fat cell development was then constructed by de novo functional annotation on a sequence segment/domain-wise basis of 659 protein sequences, and subsequent mapping onto known pathways, possible cellular roles, and subcellular localizations. Key enzymes in 27 out of 36 investigated metabolic pathways were regulated at the transcriptional level, typically at the rate-limiting steps in these pathways. Also, coexpressed genes rarely shared consensus transcription-factor binding sites, and were typically not clustered in adjacent chromosomal regions, but were instead widely dispersed throughout the genome.

Conclusions

Large-scale transcription profiling in conjunction with sophisticated bioinformatics analyses can provide not only a list of novel players in a particular setting but also a global view on biological processes and molecular networks.

African swine fever virus infection disrupts centrosome assembly and function

African swine fever virus (ASFV) is a large, enveloped DNA virus that assembles in perinuclear sites located close to the centrosome. It is reported here that the microtubule network becomes disorganized soon after the onset of viral DNA replication and formation of assembly sites. ASFV infection resulted in loss of gamma-tubulin and pericentrin at the centrosome; this was due to protein relocalization, but not degradation. ASFV infection also inhibited the ability of the centrosome to nucleate microtubules. The reorganization of microtubules seen in ASFV-infected cells may therefore be mediated by gamma-tubulin and pericentrin redistribution, and consequent disruption of centrosome assembly and function.

Centrosomes and microtubules: wedded with a ring

How centrosomes nucleate microtubule growth is a question that has puzzled cell biologists for decades. It has been suspected for some time that a centrosome contains multiple copies of a basic microtubule-nucleating structure, each of which is responsible for nucleating a single microtubule. This suspicion has now been confirmed. A ring of gamma-tubulin molecules, associated with a large protein complex, apparently serves as the long-sought-after microtubule-nucleating structure.

The microtubule lattice--20 years on

In 1974, optical diffraction and image analysis indicated that tubulin dimers in the cylindrically complete A-tubule of flagellar doublet microtubules are arranged with helical symmetry, while those in the incomplete B-tubule associate differently. Recently, electron micrographs of reassembled brain microtubules decorated with kinesin heads have shown that the tubulin dimers there are arranged as in the B-tubule. The lack of symmetry of microtubules assembled in vitro prompts Linda Amos to speculate here that the assembly process in vitro may differ from that occurring in the cell.

The microtubule lattice--dynamic instability of concepts

In the February 1995 issue of trends in CELL BIOLOGY, Linda Amos presented her view of our current understanding of the lattice structure of microtubules, 20 years after publication of the original paper describing the A- and B-lattices for flagellar microtubules. However, the question of the lattices of flagellar and cytoplasmic microtubules remains a matter for debate. In this article, Eckhard Mandelkow, Young-Hwa Song and Eva-Maria Mandelkow argue that the B-lattice is predominant, implying structural asymmetry for most microtubules.

Molecular components of the centrosome

The centrosome organizes microtubules during both interphase and mitosis and therefore governs fundamental processes in the life of a eukaryotic cell. The past few years have seen a substantial increase in the identification of potential components localized at the centrosome. Although we are still far from achieving a coherent picture of the workings of the centrosome, these recent discoveries are promising first steps towards an understanding of centrosomal functions at the molecular level.

Expression of Arabidopsis gamma-tubulin in fission yeast reveals conserved and novel functions of gamma-tubulin

gamma-Tubulin localizes to microtubule-organizing centers in animal and fungal cells where it is important for microtubule nucleation. Plant cells do not have morphologically defined microtubule organizing centers, however, and gamma-tubulin is distributed in small, discrete structures along microtubules. The great difference in distribution has prompted speculation that plant gamma-tubulins function differently from animal and fungal gamma-tubulins. We tested this possibility by expressing Arabidopsis gamma-tubulin in the fission yeast Schizosaccharomyces pombe. At high temperatures, the plant gamma-tubulin was able to bind to microtubule-organizing centers, nucleate microtubule assembly, and support the growth and replication of S. pombe cells lacking endogenous gamma-tubulin. However, the distribution of microtubules was abnormal as was cell morphology, and at low temperatures, cells were arrested in mitosis. These results reveal that Arabidopsis gamma-tubulin can carry out essential functions in S. pombe and is, thus, functionally conserved. The morphological abnormalities reveal that it cannot carry out some nonessential functions, however, and they underscore the importance of gamma-tubulin in morphogenesis of fission yeast cells and in maintaining normal interphase microtubule arrays.

Mitochondria are redistributed in Drosophila photoreceptors lacking milton, a kinesin-associated protein

Photoreceptors are richly supplied with mitochondria, where they are required to meet the energetic demands, in the soma, of phototransduction and, in the terminal, of neurotransmitter release. Compromising the latter, we have made photoreceptors R1-R6 in Drosophila ommatidia homozygous for either of two alleles, milt(186) and milt(92), of milton in whole-eye mosaics. Such mutant photoreceptors fail to target mitochondria to their terminals. We show from quantitative electron microscopy (EM) that mitochondria are totally lacking at the terminal but nevertheless abundant and present throughout the soma, where their distribution differs from that of control ommatidia, however, being more heavily concentrated in the nuclear region. Mitochondria are sparse at the basalmost level of mutant ommatidia, and are lacking beneath the basement membrane, in the axons and terminals of these cells. The absence of mitochondria from R1-R6 terminals and concommitant reductions in synaptic vesicle packing density, previously reported, we show here are accompanied by reduced immunoreactivity to the photoreceptor transmitter histamine but not by any change in total head histamine content, as determined by high-performance liquid chromatography. Mutant terminals also contain vesicle profiles with a wider range of sizes. These two phenotypes suggest that the reduced availability of ATP when mutant terminals lack a mitochondrial supply compromises their ability to pump histamine into synaptic vesicles and perturbs membrane distribution within the terminal. In addition, a band of somata in the lamina cortex, at least some of which are postsynaptic neurons not homozygous for milton, also shows altered mitochondrial targeting, with abnormal clusters of mitochondria, as visualized by immunolabeling with anti-hsp and by serial EM. Within the lamina, terminals of mutant photoreceptors are penetrated by neighboring cells with invaginations that frequently contain mitochondria, suggesting that a mechanism exists for intercellular metabolic support. Our findings indicate the direct and compensatory responses in a population of neurons when mitochondria are not correctly targeted to their synaptic terminals.

Acentrosomal microtubule nucleation in higher plants

Higher plants have developed a unique pathway to control their cytoskeleton assembly and dynamics. In most other eukaryotes, microtubules are nucleated in vivo at the nucleation and organizing centers and are involved in the establishment of polarity. Although the major cytoskeletal components are common to plant and animal cells, which suggests conserved regulation mechanisms, plants do not possess centrosome-like organelles. Nevertheless, they are able to build spindles and have developed their own specific cytoskeletal arrays: the cortical arrays, the preprophase band, and the phragmoplast, which all participate in basic developmental processes, as shown by defective mutants. New approaches provide essential clues to understanding the fundamental mechanisms of microtubule nucleation. Gamma-tubulin, which is considered to be the universal nucleator, is the essential component of microtubule-nucleating complexes identified as gamma-tubulin ring complexes (gamma-TuRC) in centriolar cells. A gamma-tubulin small complex (gamma-TuSC) forms a minimal nucleating unit recruited at specific sites of activity. These components--gamma-tubulin, Spc98p, and Spc97p--are present in higher plants. They play a crucial role in microtubule nucleation at the nuclear surface, which is known as the main functional plant microtubule-organizing center, and also probably at the cell cortex and at the phragmoplast, where secondary nucleation sites may exist. Surprisingly, plant gamma-tubulin is distributed along the microtubule length. As it is not associated with Spc98p, it may not be involved in microtubule nucleation, but may preferably control microtubule dynamics. Understanding the mechanisms of microtubule nucleation is the major challenge of the current research.

Microtubule nucleation

Microtubule nucleation is the process in which several tubulin molecules interact to form a microtubule seed. Microtubule nucleation occurs spontaneously in purified tubulin solutions, and molecular intermediates between tubulin dimers and microtubules have been identified. Microtubule nucleation is enhanced in tubulin solutions by the addition of gamma-tubulin or various gamma-tubulin complexes. In vivo, microtubule assembly is usually seeded by gamma-tubulin ring complexes. Recent studies suggest, however, that microtubule nucleation can occur in the absence of gamma-tubulin, and that gamma-tubulin may have other cell functions apart from being a major component of the gamma-tubulin ring complex.

Molecular and structural characterization of the spindle pole bodies in the fission yeast Schizosaccharomyces japonicus var japonicus

The structure and localization of the microtubule organization centres (MTOCs) of the fission yeast Schizosaccharomyces japonicus var. japonicus were examined by fluorescence microscopy and electron microscopy. Spindle pole bodies (SPBs), which are the fungal equivalent of centrosomes, of Sz. japonicus were visualized by immunofluorescent staining using a monoclonal anti-gamma-tubulin antibody. The behaviour of the SPBs during the cell cycle mostly coincided with previous reports on the most widely used fission yeast Schizosaccharomyces pombe. We cloned the gamma-tubulin gene from Sz. japonicus by PCR using redundant sets of primers corresponding to conserved regions of known gamma-tubulins. The predicted amino acid sequence of Sz. japonicus gamma-tubulin was most similar to the Sz. pombe gamma-tubulin. Under the electron microscope, the SPBs of Sz. japonicus were detected as electron-dense multilayered structures located just outside the nuclear envelope. The SPBs of Sz. japonicus were composed of three electron-dense layers and were surrounded by fuzzy material. Each layer showed structural changes according to the progression of the cell cycle. In mitotic cells, the SPBs were located on the fenestrae of the nuclear envelopes through which the mitotic spindle microtubules ran into the nucleoplasm. Our results show that Sz. japonicus is a very potent and attractive organism for the investigation of the microtubule nucleation system and morphogenesis in yeasts. The Accession No. for the nucleotide sequence of the Sz. japonicus gtb1(+) gene is AF159163.

Centrosomal TACCtics

Although the centrosome was first described over 100 years ago, we still know relatively little of the molecular mechanisms responsible for its functions. Recently, members of a novel family of centrosomal proteins have been identified in a wide variety of organisms. The transforming acidic coiled-coil-containing (TACC) proteins all appear to play important roles in cell division and cellular organisation in both embryonic and somatic systems. These closely related molecules have been implicated in microtubule stabilisation, acentrosomal spindle assembly, translational regulation, haematopoietic development and cancer progression. In this review, I summarise what we already know of this protein family and will use the TACC proteins to illustrate the many facets that centrosomes have developed during the course of evolution.

A fourth component of the fission yeast gamma-tubulin complex, Alp16, is required for cytoplasmic microtubule integrity and becomes indispensable when gamma-tubulin function is compromised

gamma-Tubulin functions as a multiprotein complex, called the gamma-tubulin complex (gamma-TuC), and composes the microtubule organizing center (MTOC). Fission yeast Alp4 and Alp6 are homologues of two conserved gamma-TuC proteins, hGCP2 and hGCP3, respectively. We isolated a novel gene, alp16(+), as a multicopy suppressor of temperature-sensitive alp6-719 mutants. alp16(+) encodes a 759-amino-acid protein with two conserved regions found in all other members of gamma-TuC components. In addition, Alp16 contains an additional motif, which shows homology to hGCP6/Xgrip210. Gene disruption shows that alp16(+) is not essential for cell viability. However, alp16 deletion displays abnormally long cytoplasmic microtubules, which curve around the cell tip. Furthermore, alp16-deleted mutants are hypersensitive to microtubule-depolymerizing drugs and synthetically lethal with either temperature-sensitive alp4-225, alp4-1891, or alp6-719 mutants. Overproduction of Alp16 is lethal, with defective phenotypes very similar to loss of Alp4 or Alp6. Alp16 localizes to the spindle pole body throughout the cell cycle and to the equatorial MTOC at postanaphase. Alp16 coimmunoprecipitates with gamma-tubulin and cosediments with the gamma-TuC in a large complex (>20 S). Alp16 is, however, not required for the formation of this large complex. We discuss evolutional conservation and divergence of structure and function of the gamma-TuC between yeast and higher eukaryotes.

Alanine-scanning mutagenesis of Aspergillus gamma-tubulin yields diverse and novel phenotypes

We have created 41 clustered charged-to-alanine scanning mutations of the mipA, gamma-tubulin, gene of Aspergillus nidulans and have created strains carrying these mutations by two-step gene replacement and by a new procedure, heterokaryon gene replacement. Most mutant alleles confer a wild-type phenotype, but others are lethal or conditionally lethal. The conditionally lethal alleles exhibit a variety of phenotypes under restrictive conditions. Most have robust but highly abnormal mitotic spindles and some have abnormal cytoplasmic microtubule arrays. Two alleles appear to have reduced amounts of gamma-tubulin at the spindle pole bodies and nucleation of spindle microtubule assembly may be partially inhibited. One allele inhibits germ tube formation. The cold sensitivity of two alleles is strongly suppressed by the antimicrotubule agents benomyl and nocodazole and a third allele is essentially dependent on these compounds for growth. Together our data indicate that gamma-tubulin probably carries out functions essential to mitosis and organization of cytoplasmic microtubules in addition to its well-documented role in microtubule nucleation. We have also placed our mutations on a model of the structure of gamma-tubulin and these data give a good initial indication of the functionally important regions of the molecule.

ENDOSPERM DEVELOPMENT: Cellularization and Cell Fate Specification

The endosperm develops from the central cell of the megagametophyte after introduction of the second male gamete into the diploid central cell. Of the three forms of endosperm in angiosperms, the nuclear type is prevalent in economically important species, including the cereals. Landmarks in nuclear endosperm development are the coenocytic, cellularization, differentiation, and maturation stages. The differentiated endosperm contains four major cell types: starchy endosperm, aleurone, transfer cells, and the cells of the embryo surrounding region. Recent research has demonstrated that the first two phases of endosperm occur via mechanisms that are conserved among all groups of angiosperms, involving directed nuclear migration during the coenocytic stage and anticlinal cell wall deposition by cytoplasmic phragmoplasts formed in interzones between radial microtubular systems emanating from nuclear membranes. Complete cellularization of the endosperm coenocyte is achieved through centripetal growth of cell files, extending to the center of the endosperm cavity. Key points in cell cycle control and control of the MT (microtubular) cytoskeletal apparatus central to endosperm development are discussed. Specification of cell fates in the cereal endosperm appears to occur via positional signaling; cells in peripheral positions, except over the main vascular tissues, assume aleurone cell fate. Cells over the main vascular tissue become transfer cells and all interior cells become starchy endosperm cells. Studies in maize have implicated Crinkly4, a protein receptor kinase-like molecule, in aleurone cell fate specification.

The role of Xgrip210 in gamma-tubulin ring complex assembly and centrosome recruitment

The gamma-tubulin ring complex (gammaTuRC), purified from the cytoplasm of vertebrate and invertebrate cells, is a microtubule nucleator in vitro. Structural studies have shown that gammaTuRC is a structure shaped like a lock-washer and topped with a cap. Microtubules are thought to nucleate from the uncapped side of the gammaTuRC. Consequently, the cap structure of the gammaTuRC is distal to the base of the microtubules, giving the end of the microtubule the shape of a pointed cap. Here, we report the cloning and characterization of a new subunit of Xenopus gammaTuRC, Xgrip210. We show that Xgrip210 is a conserved centrosomal protein that is essential for the formation of gammaTuRC. Using immunogold labeling, we found that Xgrip210 is localized to the ends of microtubules nucleated by the gammaTuRC and that its localization is more distal, toward the tip of the gammaTuRC-cap structure, than that of gamma-tubulin. Immunodepletion of Xgrip210 blocks not only the assembly of the gammaTuRC, but also the recruitment of gamma-tubulin and its interacting protein, Xgrip109, to the centrosome. These results suggest that Xgrip210 is a component of the gammaTuRC cap structure that is required for the assembly of the gammaTuRC.

An abundance of tubulins

Recent data have revealed that the tubulin superfamily of proteins is much larger than was thought previously. Six distinct families within the tubulin superfamily have been discovered and more might await discovery. alpha-, beta- and gamma-tubulins are ubiquitous in eukaryotes. alpha- and beta-tubulins are the major components of microtubules, and gamma-tubulin plays a major role in the nucleation of microtubule assembly. delta- and epsilon-tubulins are widespread but not ubiquitous, and zeta-tubulin has been found so far only in kinetoplastid protozoa. delta-Tubulin has an important role in flagellar assembly in Chlamydomonas, but its role in other organisms is just beginning to be investigated, as are the functions of the recently discovered epsilon- and zeta-tubulins.