Nuclear IMPDH Filaments in Human Gliomas.

The analysis of nuclear morphology plays an important role in glioma diagnosis and grading. We previously described intranuclear rods (rods) labeled with the SDL.3D10 monoclonal antibody against class III beta-tubulin (TUBB3) in human ependymomas. In a cohort of adult diffuse gliomas, we identified nuclear rods in 71.1% of IDH mutant lower-grade gliomas and 13.7% of IDH wild-type glioblastomas (GBMs). The presence of nuclear rods was associated with significantly longer postoperative survival in younger (≤65) GBM patients. Consistent with this, nuclear rods were mutually exclusive with Ki67 staining and their prevalence in cell nuclei inversely correlated with the Ki67 proliferation index. In addition, rod-containing nuclei showed a relative depletion of lamin B1, suggesting a possible association with senescence. To gain insight into their functional significance, we addressed their antigenic properties. Using a TUBB3-null mouse model, we demonstrate that the SDL.3D10 antibody does not bind TUBB3 in rods but recognizes an unknown antigen. In the present study, we show that rods show immunoreactivity for the nucleotide synthesizing enzymes inosine monophosphate dehydrogenase (IMPDH) and cytidine triphosphate synthetase. By analogy with the IMPDH filaments that have been described previously, we postulate that rods regulate the activity of nucleotide-synthesizing enzymes in the nucleus by sequestration, with important implications for glioma behavior.

Defining the phenotypical spectrum associated with variants in TUBB2A.

BACKGROUND: Variants in genes belonging to the tubulin superfamily account for a heterogeneous spectrum of brain malformations referred to as tubulinopathies. Variants in TUBB2A have been reported in 10 patients with a broad spectrum of brain imaging features, ranging from a normal cortex to polymicrogyria, while one patient has been reported with progressive atrophy of the cerebellar vermis. METHODS: In order to further refine the phenotypical spectrum associated with TUBB2A, clinical and imaging features of 12 patients with pathogenic TUBB2A variants, recruited via the international network of the authors, were reviewed. RESULTS: We report 12 patients with eight novel and one recurrent variants spread throughout the TUBB2A gene but encoding for amino acids clustering at the protein surface. Eleven patients (91.7%) developed seizures in early life. All patients suffered from intellectual disability, and 11 patients had severe motor developmental delay, with 4 patients (36.4 %) being non-ambulatory. The cerebral cortex was normal in five individuals and showed dysgyria of variable severity in seven patients. Associated brain malformations were less frequent in TUBB2A patients compared with other tubulinopathies. None of the patients had progressive cerebellar atrophy. CONCLUSION: The imaging phenotype associated with pathogenic variants in TUBB2A is highly variable, ranging from a normal cortex to extensive dysgyria with associated brain malformations. For recurrent variants, no clear genotype-phenotype correlations could be established, suggesting the role of additional modifiers.

Auditory impairment in H-ABC tubulinopathy.

Hypomyelination with atrophy of the basal ganglia and cerebellum (H-ABC) is a neurodegenerative disease due to mutations in TUBB4A. Patients suffer from extrapyramidal movements, spasticity, ataxia, and cognitive deficits. Magnetic resonance imaging features are hypomyelination and atrophy of the striatum and cerebellum. A correlation between the mutations and their cellular, tissue and organic effects is largely missing. The effects of these mutations on sensory functions have not been described so far. We have previously reported a rat carrying a TUBB4A (A302T) mutation and sharing most of the clinical and radiological signs with H-ABC patients. Here, for the first time, we did a comparative study of the hearing function in an H-ABC patient and in this mutant model. By analyzing hearing function, we found that there are no significant differences in the auditory brainstem response (ABR) thresholds between mutant rats and WT controls. Nevertheless, ABRs show longer latencies in central waves (II-IV) that in some cases disappear when compared to WT. The patient also shows abnormal AEPs presenting only Waves I and II. Distortion product of otoacoustic emissions and immunohistochemistry in the rat show that the peripheral hearing function and morphology of the organ of Corti are normal. We conclude that the tubulin mutation severely impairs the central hearing pathway most probably by progressive central white matter degeneration. Hearing function might be affected in a significant fraction of patients with H-ABC; therefore, screening for auditory function should be done on patients with tubulinopathies to evaluate hearing support therapies.

Neuronal-Specific TUBB3 Is Not Required for Normal Neuronal Function but Is Essential for Timely Axon Regeneration.

We generated a knockout mouse for the neuronal-specific ß-tubulin isoform Tubb3 to investigate its role in nervous system formation and maintenance. Tubb3-/- mice have no detectable neurobehavioral or neuropathological deficits, and upregulation of mRNA and protein of the remaining ß-tubulin isotypes results in equivalent total ß-tubulin levels in Tubb3-/- and wild-type mice. Despite similar levels of total ß-tubulin, adult dorsal root ganglia lacking TUBB3 have decreased growth cone microtubule dynamics and a decreased neurite outgrowth rate of 22% in vitro and in vivo. The effect of the 22% slower growth rate is exacerbated for sensory recovery, where fibers must reinnervate the full volume of the skin to recover touch function. Overall, these data reveal that, while TUBB3 is not required for formation of the nervous system, it has a specific role in the rate of peripheral axon regeneration that cannot be replaced by other ß-tubulins.

Genetic defects in ß-spectrin and tau sensitize C. elegans axons to movement-induced damage via torque-tension coupling.

Our bodies are in constant motion and so are the neurons that invade each tissue. Motion-induced neuron deformation and damage are associated with several neurodegenerative conditions. Here, we investigated the question of how the neuronal cytoskeleton protects axons and dendrites from mechanical stress, exploiting mutations in UNC-70 ß-spectrin, PTL-1 tau/MAP2-like and MEC-7 ß-tubulin proteins in Caenorhabditis elegans. We found that mechanical stress induces supercoils and plectonemes in the sensory axons of spectrin and tau double mutants. Biophysical measurements, super-resolution, and electron microscopy, as well as numerical simulations of neurons as discrete, elastic rods provide evidence that a balance of torque, tension, and elasticity stabilizes neurons against mechanical deformation. We conclude that the spectrin and microtubule cytoskeletons work in combination to protect axons and dendrites from mechanical stress and propose that defects in ß-spectrin and tau may sensitize neurons to damage.

Oral delivery of dsRNA lipoplexes to German cockroach protects dsRNA from degradation and induces RNAi response.

BACKGROUND: In the past years, the concept of RNAi application for insect pest control has been proposed, considering the disruption of vital genes. However, the efficiency of RNAi is variable between different insect groups, especially by oral delivery of dsRNA. The purpose of this study is to assess the possibilities of RNAi as a tool for pest control using oral delivery of the dsRNAs encapsulated by liposome in the German cockroach Blattella germanica, which is highly sensitive to RNAi by injection of dsRNAs. RESULTS: Injecting dsRNA into the abdomen of B. germanica caused dramatic depletion of essential α-tubulin gene and mortality. In contrast, oral delivery of the naked dsRNA resulted in lower RNAi efficiency, accounting for rapid degradation of the dsRNA in the midgut of B. germanica. Notably, we have further demonstrated that continuous ingestion of dsRNA lipoplexes in which dsRNA was encapsulated with a cationic liposome carrier was sufficient to slow down the degradation of dsRNA in the midgut and to increase the mortality of the German cockroach by significantly inhibiting α-tubulin expression in the midgut. CONCLUSION: We provide empirical evidence that the formulation of dsRNA lipoplexes could be a plausible approach for insect pest control based on RNAi. © 2016 Society of Chemical Industry.

De novo TUBB2B mutation causes fetal akinesia deformation sequence with microlissencephaly: An unusual presentation of tubulinopathy.

Tubulinopathies are increasingly emerging major causes underlying complex cerebral malformations, particularly in case of microlissencephaly often associated with hypoplastic or absent corticospinal tracts. Fetal akinesia deformation sequence (FADS) refers to a clinically and genetically heterogeneous group of disorders with congenital malformations related to impaired fetal movement. We report on an early foetal case with FADS and microlissencephaly due to TUBB2B mutation. Neuropathological examination disclosed virtually absent cortical lamination, foci of neuronal overmigration into the leptomeningeal spaces, corpus callosum agenesis, cerebellar and brainstem hypoplasia and extremely severe hypoplasia of the spinal cord with no anterior and posterior horns and almost no motoneurons. At the cellular level, the p.Cys239Phe TUBB2B mutant leads to tubulin heterodimerization impairment, decreased ability to incorporate into the cytoskeleton, microtubule dynamics alteration, with an accelerated rate of depolymerization. To our knowledge, this is the first case of microlissencephaly to be reported presenting with a so severe and early form of FADS, highlighting the importance of tubulin mutation screening in the context of FADS with microlissencephaly.

Modeling chemotherapeutic neurotoxicity with human induced pluripotent stem cell-derived neuronal cells.

There are no effective agents to prevent or treat chemotherapy-induced peripheral neuropathy (CIPN), the most common non-hematologic toxicity of chemotherapy. Therefore, we sought to evaluate the utility of human neuron-like cells derived from induced pluripotent stem cells (iPSCs) as a means to study CIPN. We used high content imaging measurements of neurite outgrowth phenotypes to compare the changes that occur to iPSC-derived neuronal cells among drugs and among individuals in response to several classes of chemotherapeutics. Upon treatment of these neuronal cells with the neurotoxic drug paclitaxel, vincristine or cisplatin, we identified significant differences in five morphological phenotypes among drugs, including total outgrowth, mean/median/maximum process length, and mean outgrowth intensity (P < 0.05). The differences in damage among drugs reflect differences in their mechanisms of action and clinical CIPN manifestations. We show the potential of the model for gene perturbation studies by demonstrating decreased expression of TUBB2A results in significantly increased sensitivity of neurons to paclitaxel (0.23 ± 0.06 decrease in total neurite outgrowth, P = 0.011). The variance in several neurite outgrowth and apoptotic phenotypes upon treatment with one of the neurotoxic drugs is significantly greater between than within neurons derived from four different individuals (P < 0.05), demonstrating the potential of iPSC-derived neurons as a genetically diverse model for CIPN. The human neuron model will allow both for mechanistic studies of specific genes and genetic variants discovered in clinical studies and for screening of new drugs to prevent or treat CIPN.

RHGF-1/PDZ-RhoGEF and retrograde DLK-1 signaling drive neuronal remodeling on microtubule disassembly.

Neurons remodel their connectivity in response to various insults, including microtubule disruption. How neurons sense microtubule disassembly and mount remodeling responses by altering genetic programs in the soma are not well defined. Here we show that in response to microtubule disassembly, the Caenorhabditis elegans PLM neuron remodels by retracting its synaptic branch and overextending the primary neurite. This remodeling required RHGF-1, a PDZ-Rho guanine nucleotide exchange factor (PDZ-RhoGEF) that was associated with and inhibited by microtubules. Independent of the myosin light chain activation, RHGF-1 acted through Rho-dependent kinase LET-502/ROCK and activated a conserved, retrograde DLK-1 MAPK (DLK-1/dual leucine zipper kinase) pathway, which triggered synaptic branch retraction and overgrowth of the PLM neurite in a dose-dependent manner. Our data represent a neuronal remodeling paradigm during development that reshapes the neural circuit by the coordinated removal of the dysfunctional synaptic branch compartment and compensatory extension of the primary neurite.

[Down-regulated ßIII-tubulin expression can reverse paclitaxel resistance in A549/taxol cells lines].

BACKGROUND: Chemotherapy drug resistance is the primary causes of death in patients with pulmonary carcinoma which make tumor recurrence or metastasis. ß-tubulin is the main cell targets of anti-microtubule drug. Increased expression of ßIII-tubulin has been implicated in non-small cell lung cancer (NSCLC) cell lines. To explore the relationship among the expression level of ßIII-tubulin and the sensitivity of A549/Taxolcell lines to Taxol and cell cycles and cell apoptosis by RNA interference-mediated inhibition of ßIII-tubulin in A549/Taxol cells. METHODS: Three pairs of siRNA targetd ßIII-tubulin were designed and prepared, which were transfected into A549/Taxol cells using LipofectamineTM 2000. We detected the expression of ßIII-tubulin mRNA using Real-time fluorescence qRT-PCR. Tedhen we selected the most efficient siRNA by the expression of ßIII-tubulin mRNA in transfected group. ßIII-tubulin protein level were mesured by Western blot. The taxol sensitivity in transfected group were evaluated by MTT assay. And the cell apoptosis and cell cycles were determined by flow cytometry. RESULTS: ßIII-tubulin mRNA levels in A549/Taxol cells were significantly decreased in transfected grop by Real-time qRT-PCR than control groups. And ßIII-tubulin siRNA-1 sequence showed the highest transfection efficiency, which was (87.73±4.87)% (P<0.01); Western blot results showed that the expressional level of BIII tublin protein was significantly down-reulated in the transfectant cells than thant in the control cells. By MTT assay, we showed that the inhibition ratio of Taxol to A549/Taxol cells transfeced was higher than that of control group (51.77±4.60)% (P<0.01). The early apoptosis rate of A549/Taxol cells in transfected group were significantly higher than that of control group (P<0.01); G2-M content in taxol group obviously increased than untreated samples by the cell cycle (P<0.05). CONCLUSIONS: ßIII-tubulin down-regulated significantly sensitized NSCLC A549/Taxol cells to Paclitaxel.

γ-Tubulin 2 nucleates microtubules and is downregulated in mouse early embryogenesis.

γ-Tubulin is the key protein for microtubule nucleation. Duplication of the γ-tubulin gene occurred several times during evolution, and in mammals γ-tubulin genes encode proteins which share ∼97% sequence identity. Previous analysis of Tubg1 and Tubg2 knock-out mice has suggested that γ-tubulins are not functionally equivalent. Tubg1 knock-out mice died at the blastocyst stage, whereas Tubg2 knock-out mice developed normally and were fertile. It was proposed that γ-tubulin 1 represents ubiquitous γ-tubulin, while γ-tubulin 2 may have some specific functions and cannot substitute for γ-tubulin 1 deficiency in blastocysts. The molecular basis of the suggested functional difference between γ-tubulins remains unknown. Here we show that exogenous γ-tubulin 2 is targeted to centrosomes and interacts with γ-tubulin complex proteins 2 and 4. Depletion of γ-tubulin 1 by RNAi in U2OS cells causes impaired microtubule nucleation and metaphase arrest. Wild-type phenotype in γ-tubulin 1-depleted cells is restored by expression of exogenous mouse or human γ-tubulin 2. Further, we show at both mRNA and protein levels using RT-qPCR and 2D-PAGE, respectively, that in contrast to Tubg1, the Tubg2 expression is dramatically reduced in mouse blastocysts. This indicates that γ-tubulin 2 cannot rescue γ-tubulin 1 deficiency in knock-out blastocysts, owing to its very low amount. The combined data suggest that γ-tubulin 2 is able to nucleate microtubules and substitute for γ-tubulin 1. We propose that mammalian γ-tubulins are functionally redundant with respect to the nucleation activity.

The interplay between tubulins and P450 cytochromes during Plasmodium berghei invasion of Anopheles gambiae midgut.

BACKGROUND: Plasmodium infection increases the oxidative stress inside the mosquito, leading to a significant alteration on transcription of Anopheles gambiae detoxification genes. Among these detoxification genes several P450 cytochromes and tubulins were differently expressed, suggesting their involvement in the mosquito's response to parasite invasion. P450 cytochromes are usually involved in the metabolism and detoxification of several compounds, but are also regulated by several pathogens, including malaria parasite. Tubulins are extremely important as components of the cytoskeleton, which rearrangement functions as a response to malaria parasite invasion. METHODOLOGY/PRINCIPAL FINDINGS: Gene silencing methods were used to uncover the effects of cytochrome P450 reductase, tubulinA and tubulinB silencing on the A. gambiae response to Plasmodium berghei invasion. The role of tubulins in counter infection processes was also investigated by inhibiting their effect. Colchicine, vinblastine and paclitaxel, three different tubulin inhibitors were injected into A. gambiae mosquitoes. Twenty-four hours post injection these mosquitoes were infected with P. berghei through a blood meal from infected CD1 mice. Cytochrome P450 gene expression was measured using RT-qPCR to detect differences in cytochrome expression between silenced, inhibited and control mosquitoes. Results showed that cytochrome P450 reductase silencing, as well as tubulin (A and B) silencing and inhibition affected the efficiency of Plasmodium infection. Silencing and inhibition also affected the expression levels of cytochromes P450. CONCLUSIONS: Our results suggest the existence of a relationship between tubulins and P450 cytochromes during A. gambiae immune response to P. berghei invasion. One of the P450 cytochromes in this study, CYP6Z2, stands out as the potential link in this association. Further work is needed to fully understand the role of tubulin genes in the response to Plasmodium infection.

The multipurpose 15-protofilament microtubules in C. elegans have specific roles in mechanosensation.

Because microtubules perform many essential functions in neurons, delineating unique roles attributable to these organelles presents a formidable challenge. Microtubules endow neurons with shape and structure and are required for developmental processes including neurite outgrowth, intracellular transport, and synapse formation and plasticity; microtubules in sensory neurons may be required for the above processes in addition to a specific sensory function. In Caenorhabditis elegans, six touch receptor neurons (TRNs) sense gentle touch and uniquely contain 15-protofilament microtubules. Disruption of these microtubules by loss of either the MEC-7 beta-tubulin or MEC-12 alpha-tubulin or by growth in 1 mM colchicine causes touch insensitivity, altered distribution of the touch transduction channel, and a general reduction in protein levels. We show that the effect on touch sensitivity can be separated from the others; microtubule depolymerization in mature TRNs causes touch insensitivity but does not result in protein distribution and production defects. In addition, the mec-12(e1605) mutation selectively causes touch insensitivity without affecting microtubule formation and other cellular processes. Touching e1605 animals produces a reduced mechanoreceptor current that inactivates more rapidly than in wild-type, suggesting a specific role of the microtubules in mechanotransduction.

Tubulin-targeted drug action: functional significance of class ii and class IVb beta-tubulin in vinca alkaloid sensitivity.

Aberrant expression of beta-tubulin isotypes is frequently described in tumor tissues and tubulin-binding agent (TBA)-resistant cell lines. There is limited understanding of the role of specific beta-tubulin isotypes in cellular sensitivity to TBAs, and to gain insights into the functional role of betaII- and betaIVb-tubulin, we examined these isotypes in lung cancer cell lines NCI-H460 (H460) and Calu-6. Drug-treated clonogenic assays revealed that small interfering RNA-mediated knockdown of either betaII- or betaIVb-tubulin hypersensitized the lung cancer cell lines to Vinca alkaloids, with the effects more pronounced following betaIVb-tubulin knockdown. In contrast, there was no change in paclitaxel sensitivity following knockdown of either isotype. Cell cycle analysis revealed a greater propensity for the betaII- and betaIVb-tubulin knockdown cells to undergo G2-M cell cycle block following 5 nmol/L vincristine treatment, with the betaIVb knockdown cells being more sensitive than the betaII-tubulin knockdown cells compared with control. In contrast to betaII-tubulin knockdown, betaIVb-tubulin knockdown cells showed a significant increase in the sub-G1 population (cell death) following treatment with both 5 and 40 nmol/L of vincristine compared with controls. Importantly, betaIVb-tubulin knockdown in H460 cells caused a significant dose-dependent increase in Annexin V staining in response to vincristine but not paclitaxel. Therefore, increased sensitivity to induction of apoptosis is one mechanism underlying the Vinca alkaloid hypersensitivity. This study provides direct evidence that betaII- or betaIVb-tubulins have functionally distinct roles and expression of these isotypes may serve as strong predictors of Vinca alkaloid response and resistance.

Class III beta-tubulin mediates sensitivity to chemotherapeutic drugs in non small cell lung cancer.

First line therapy for non-small cell lung carcinoma (NSCLC) commonly includes combination therapy with a tubulin-binding agent (TBA) and a DNA-damaging agent. TBAs suppress microtubule dynamics by binding to the beta-tubulin subunit of alpha/beta-tubulin, inducing mitotic arrest and apoptosis. Up-regulation of class III beta-tubulin (betaIII-tubulin) has been implicated in clinical resistance in NSCLC, ovarian and breast tumors treated in combination with a TBA and DNA-damaging agent. To investigate the functional significance of betaIII-tubulin in resistance to both these classes of agents, small interfering RNA (siRNA) was used to silence the expression of this isotype in two NSCLC cell lines, NCI-H460 and Calu-6. Reverse transcription-PCR and immunoblotting showed that betaIII-siRNA potently inhibited the expression of betaIII-tubulin, without affecting the expression of other major beta-tubulin isotypes. Clonogenic assays showed that betaIII-siRNA cells were significantly more sensitive to TBAs, paclitaxel, vincristine, and vinorelbine, and for the first time, DNA-damaging agents, cisplatin, doxorubicin, and etoposide compared with controls. Cell cycle analysis of H460 betaIII-siRNA cells showed reduced accumulation at the G(2)-M boundary and an increase in the sub-G(1) population in response to TBA treatment compared with control cells. Importantly, betaIII-siRNA cells displayed a significant dose-dependent increase in Annexin V staining when treated with either paclitaxel or cisplatin, compared with controls. These findings have revealed a novel role for betaIII-tubulin in mediating response to both TBA and DNA-damaging agent therapy and may have important implications for improving the targeting and treatment of drug-refractory NSCLC.

Gamma-tubulin is essential for microtubule organization and development in Arabidopsis.

The process of microtubule nucleation in plant cells is still a major question in plant cell biology. gamma-Tubulin is known as one of the key molecular players for microtubule nucleation in animal and fungal cells. Here, we provide genetic evidence that in Arabidopsis thaliana, gamma-tubulin is required for the formation of spindle, phragmoplast, and cortical microtubule arrays. We used a reverse genetics approach to investigate the role of the two Arabidopsis gamma-tubulin genes in plant development and in the formation of microtubule arrays. Isolation of mutants in each gene and analysis of two combinations of gamma-tubulin double mutants showed that the two genes have redundant functions. The first combination is lethal at the gametophytic stage. Disruption of both gamma-tubulin genes causes aberrant spindle and phragmoplast structures and alters nuclear division in gametophytes. The second combination of gamma-tubulin alleles affects late seedling development, ultimately leading to lethality 3 weeks after germination. This partially viable mutant combination enabled us to follow dynamically the effects of gamma-tubulin depletion on microtubule arrays in dividing cells using a green fluorescent protein marker. These results establish the central role of gamma-tubulin in the formation and organization of microtubule arrays in Arabidopsis.

Extrinsic factors regulate partial agonist efficacy of strychnine-sensitive glycine receptors.

BACKGROUND: Strychnine-sensitive glycine receptors in many adult forebrain regions consist of alpha2 + beta heteromeric channels. This subunit composition is distinct from the alpha1 + beta channels found throughout the adult spinal cord. Unfortunately, the pharmacology of forebrain alpha2beta receptors are poorly defined compared to 'neonatal' alpha2 homomeric channels or 'spinal' alpha1beta heteromers. In addition, the pharmacologic properties of native alpha2beta glycine receptors have been generally distinct from receptors produced by heterologous expression. To identify subtype-specific pharmacologic tools for the forebrain alpha2beta receptors, it is important to identify a heterologous expression system that closely resembles these native glycine-gated chloride channels. RESULTS: While exploring pharmacological properties of alpha2beta glycine receptors compared to alpha2-homomers, we found that distinct heterologous expression systems appeared to differentially influence partial agonist pharmacology. The beta-amino acid taurine possessed 30-50% efficacy for alpha2-containing receptor isoforms when expressed in HEK 293 cells. However, taurine efficacy was dramatically reduced in L-cell fibroblasts. Similar results were obtained for beta-alanine. The efficacy of these partial agonists was also strongly reduced by the beta subunit. There were no significant differences in apparent strychnine affinity values calculated from concentration-response data between expression systems or subunit combinations. Nor did relative levels of expression correlate with partial agonist efficacy when compared within or between several different expression systems. Finally, disruption of the tubulin cytoskeleton reduced the efficacy of partial agonists in a subunit-dependent, but system-independent, fashion. CONCLUSIONS: Our results suggest that different heterologous expression systems can dramatically influence the agonist pharmacology of strychnine-sensitive glycine receptors. In the systems examine here, these effects are independent of both absolute expression level and any system-related alterations in the agonist binding site. We conclude that complex interactions between receptor composition and extrinsic factors may play a significant role in determining strychnine-sensitive glycine receptor partial agonist pharmacology.

Mechanisms and implications of platelet discoid shape.

The platelet marginal band consists of a single peripheral microtubule (MT) that is wound in 8 to 12 coils and maintains discoid cell shape. About 90% of beta-tubulin in the marginal band is of the divergent, megakaryocyte (MK)/platelet-restricted beta1 isoform. beta1-tubulin-null mice show reduced proplatelet formation, thrombocytopenia, and platelet spherocytosis. Here, we show that structural abnormalities in resting beta1-tubulin-/- platelets include frequent kinks and breaks in the marginal band. Platelets derived from mice lacking the transcription factor GATA1 show similar defects, probably as a direct consequence of absent beta1-tubulin. beta1-tubulin+/- platelets have normal ratios of beta-tubulin isotypes but the marginal band is half the normal thickness, which is sufficient to maintain elliptical cell shape. Thus, a threshold 50% or less of the normal amount of beta1-tubulin is required to preserve marginal band integrity and cell shape. beta1-tubulin-/- platelets have normal size and contents and show no defects in serotonin release or aggregation. Accordingly, the apparently isolated spherocytosis allows investigation of the role of discoid platelet shape in hemostasis. On agonist stimulation, the disorganized MTs in beta1-tubulin-/- platelets fail to condense into central rings and instead are dispersed in short bundles and linear arrays. Nevertheless, intravital microscopy and flow chamber studies demonstrate full functionality of these spherocytic platelets under physiologic shear conditions. Together, these findings highlight the essential requirements of the MK/platelet-restricted beta1-tubulin isoform in platelet structure and suggest that spherocytosis does not impair many aspects of platelet function.

The kinetically dominant assembly pathway for centrosomal asters in Caenorhabditis elegans is gamma-tubulin dependent.

gamma-Tubulin-containing complexes are thought to nucleate and anchor centrosomal microtubules (MTs). Surprisingly, a recent study (Strome, S., J. Powers, M. Dunn, K. Reese, C.J. Malone, J. White, G. Seydoux, and W. Saxton. Mol. Biol. Cell. 12:1751-1764) showed that centrosomal asters form in Caenorhabditis elegans embryos depleted of gamma-tubulin by RNA-mediated interference (RNAi). Here, we investigate the nucleation and organization of centrosomal MT asters in C. elegans embryos severely compromised for gamma-tubulin function. We characterize embryos depleted of approximately 98% centrosomal gamma-tubulin by RNAi, embryos expressing a mutant form of gamma-tubulin, and embryos depleted of a gamma-tubulin-associated protein, CeGrip-1. In all cases, centrosomal asters fail to form during interphase but assemble as embryos enter mitosis. The formation of these mitotic asters does not require ZYG-9, a centrosomal MT-associated protein, or cytoplasmic dynein, a minus end-directed motor that contributes to self-organization of mitotic asters in other organisms. By kinetically monitoring MT regrowth from cold-treated mitotic centrosomes in vivo, we show that centrosomal nucleating activity is severely compromised by gamma-tubulin depletion. Thus, although unknown mechanisms can support partial assembly of mitotic centrosomal asters, gamma-tubulin is the kinetically dominant centrosomal MT nucleator.

Cell division: The renaissance of the centriole.

Centrioles are located at the center of the cytoskeleton and duplicate exactly once per cell cycle. Recent studies suggest that centrioles are required for the organization of a functional centrosome and that centriole assembly requires both gamma- and delta-tubulin.