The essential role of LIS1, NDEL1 and Aurora-A in polarity formation and microtubule organization during neurogensis

Lissencephaly is a devastating neurological disorder caused by to defective neuronal migration. LIS1 (or PAFAH1B1), the gene mutated in lissencephaly patients and its binding protein NDEL1 were found to regulate cytoplasmic dynein function and localization. LIS1 and NDEL1 also play a pivotal role on a microtubule regulation and determination of cell polarity. For example, LIS1 is required for the precise control of mitotic spindle orientation in both neuroepithelial stem cells and radial glial progenitor cells. On the other hand, NDEL1 is essential for mitotic entry as an effector molecule of Aurora-A kinase. In addition, an atypical protein kinase C (aPKC)-Aurora-A-NDEL1 pathway is critical for the regulation of microtubule organization during neurite extension. These findings suggest that physiological functions of LIS1 and NDEL1 in neurons have been ascribed for proteins fundamentally required for cell cycle progression and control. In turn, cell cycle regulators may exert other functions during neurogenesis in a direct or an indirect fashion. Thus far, only a handful of cell cycle regulators have been shown to play physiological cell cycle-independent roles in neurons. Further identification of such proteins and elucidation of their underlying mechanisms of action will likely reveal novel concepts and/or patterns that provide a clear link between their seemingly distinct cell cycle and neuronal functions.Key words: microtubule, mitotic kinase, neurite, cell polarity, migrationDuring the development of the mammalian central nervous system, the self-renewal of neural stem cells can occur either by symmetric cell divisions, which generate two daughter cells with the same fate, or by asymmetric cell divisions, which generate one daughter cell that is identical to the mother cell and a second, different non-stem-cell progenitor (reviewed in refs. ¹ and ²). Neural non-stem-cell progenitors typically undergo symmetric, differentiating divisions, each of which generates two neurons, which are terminally differentiated, post-mitotic cells. These post-mitotic neural progenitors migrate from their birth place at the ventricular zone to their final destinations in cortical plate (reviewed in ref. ³). Coinciding with the proper positioning of post-mitotic neurons, neurons project neurite and dendrites to targets with the assistant of molecular guidance cues in the local environment. Proper navigations of neurite and dendrite processes ensure synapse formations, which are the basis of brain function. In the series of developmental steps, the determination of neuronal polarity is critically important (reviewed in refs. ⁴ and ⁵). A polarity complex of Par3, Par6, and atypical protein kinase C (aPKC) functions in various cell-polarization events including axon formation.^6,7 GTPases that regulate actin cytoskeletal dynamics have been implicated in cell polarization. Recent findings provide insights into polarization mechanisms and show intriguing crosstalk between small GTPases and members of polarity complexes in regulating cell polarization (reviewed in ref. ⁸). Thus, determination of neuronal polarity and regulation of cytoskeletal organization are intimately related.LIS1 was identified as the first gene mutated in isolated lisssencephaly sequence (ILS), a human neuronal migration defect.^9,10 LIS1 and its binding protein, NDEL1 regulate the function and localization of cytoplasmic dynein^11,13 as part of an evolutionarily conserved pathway.^14,15 Genetic analysis of fungi displaying defective nuclei migration led to the identification of a number of genes and their protein products involved in this process. For example, mutations of nudA (coding for cytoplasmic dynein heavy chain) and genes coding for other subunits of the dynein complex inhibit nuclear migration, including nudC (mammalian NudC, mNudC), nudE (Ndel1 and Nde1) and nudF (Lis1). We recently demonstrate that LIS1 suppresses the motility of cytoplasmic dynein on microtubules in vitro (Suppl. movies 1 and 2), whereas NDEL1 releases the blocking effect of LIS1 on cytoplasmic dynein.¹⁶ We demonstrated anterograde co-migration of cytoplasmic dynein and LIS1 (Suppl. movies 3 and 4). When LIS1 function was suppressed by a blocking antibody, anterograde movement of cytoplasmic dynein was severely impaired. Lis1 KO cells exhibited biased distribution around the centrosome and aberrant distribution of organelles. Our favorite model is that LIS1 fixes cytoplasmic dynein on soluble microtubules in an “idling” state, thereby creating a microtubule-LIS1-dynein complex, which could be transported by kinesin to the plus-end of microtubules.Lis1 is also essential for the precise control of mitotic spindle orientation in both neuroepithelial stem cells and radial glial progenitor cells.¹⁷ Controlled gene deletion of Lis1 in vivo in neuroepithelial stem cells, where cleavage is uniformly vertical and symmetrical, provokes rapid apoptosis of those cells, while radial glial progenitors are less affected. We believe the role of LIS1 in promoting the anterograde transport of cytoplasmic dynein on kinesin as part of a microtubule-LIS1-dynein complex, as described in the previous paragraph, is responsible for controlling spindle orientation, since when LIS1 is reduced, cortical dynein fixed on the surface of the cell is also reduced. Impaired cortical microtubule capture via loss of cortical dynein causes astral and cortical microtubules to be greatly reduced in Lis1-deficient cells.¹⁷ Thus, Lis1 is intimiately involved in the determination of cell polarity as an effector molecule, which regulates dynein localization and/or function as well as microtubule organization.Interestingly, more than half of LIS1 protein is degraded at the cell cortex after transport to the plus-end of MTs via calpain-dependent proteolysis. We recently demonstrated that inhibition or knockdown of calpain protects LIS1 from proteolysis resulting in the augmentation of LIS1 levels in Lis1^+/− mouse embryonic fibroblast (MEF) cells, which leads to rescue of the aberrant distribution of cytoplasmic dynein and intracellular components including mitochondria and β-COP positive vesicles.¹⁸ We also showed that presence of calpain inhibitors improves neuronal migration of Lis1^+/− cerebellar granular neurons.¹⁸ This study demonstrates that stabilization of proteins in disorders caused by haploinsufficiency is a potential therapeutic strategy and provides a proof-of-principle for this notion.NDEL1, a binding partner of LIS1 is also essential for the regulation of cytoplasmic dynein and microtubule organization.^12,13 In particular, NDEL1 is phosphorylated by cyclin dependent kinase1 (CDK1) in mitotic cells, or CDK5 in post-mitotic neurons, and this phosphorylation is essential for proper targeting of NDEL1 binding proteins to the centrosome.¹⁹ NDEL1 is also a substrate of the mitotic kinase Aurora-A, by which NDEL1 connects Aurora-A to other target molecules for the regulation of microtubule organization.²⁰ Interestingly, NDEL1 is differentially phosphorylated by Aurora-A and CDK1. It is possible that distinct pools of NDEL1 may be targeted by each kinase, or conversely the affects of each kinase may counteract each other within the same pool of NDEL1.Aurora-A is a one of representative mitotic kinase, whose homologues have been reported in various organisms including yeast, nematodes, fruit flies and vertebrates (reviewed in ref. ²¹). The three human homologues of Aurora kinases (A, B and C) are essential for proper execution of various mitotic events and are important for maintaining genomic integrity. Aurora-A is mainly localized at spindle poles and the mitotic spindle during mitosis, where it regulates the functions of centrosomes, spindles and kinetochores required for proper mitotic progression. In particular, Aurora-A plays a pivotal role on microtubule reorganization during remodeling from interphase microtubules to mitotic microtubules, i.e., the mitotic spindle. We recently reported molecular and cell biological data that support a unique role of aPKC-Aurora-A-NDEL1 pathway on microtubule dynamics at the neurite hillock during neurite extension.²² PKCζ phosphorylates Aurora-A at T287 and activates it, which augments interaction with TPX2 and facilitates activation of Aurora-A at the neurite hillock, followed by S251 phosphorylation of NDEL1 and recruitment. Inhibition of PKCζ/λ, depletion of Aurora-A and disruption of Ndel1 severely affected neurite extension and microtubule dynamics, suggesting that the aPKC-Aurora-A-NDEL1 pathway is an important regulatory system of microtubule oranization within neurite processes (Fig. 1A).Open in a separate windowFigure 1Models of microtubule remodeling. (A) Neurite extension: an unknown upstream cue polarity activates aPKC followed by T287 phosphorylation of Aurora-A. T287 phosphorylation of Aurora-A facilitates binding of the Aurora-A activator, TPX2 resulting in activation of Aurora-A at the neurite hillock, which leads to phosphorylation of NDEL1, one of effector molecules of Aurora-A. Finally, phosphorylation of NDEL1 triggers remodeling microtubules during neurite extension. (B) Spindle formation: NDEL1 is differentially phosphorylated at T219 and Ser251 by CDK1 and Aurora-A, respectively at the beginning of mitotic entry. NDEL1 is required for centrosome targeting of TACC3 through the interaction with TACC3. (C) Neuronal migration: during neuronal migration, NDEL1 may be differentially phosphorylated at T219 and Ser251 by CDK5 and Aurora-A, respectively. 14-3-3ɛ might negatively regulate Aurora-A kinase.Our preliminary data suggest that Aurora-A is also activated by neurons during migration, and may further link signaling components already implicated in neuronal migration. Mice deficient in Ywhae that encondes 14-3-3ɛ have defects in brain development and neuronal migration, similar to defects observed in mice heterozygous with respect to Lis1.²³ Mice heterozygous with respect to both genes have more severe migration defects than single heterozygotes. Heterozygous deletions of 17p13.3 in human result in the human neuronal migration disorders isolated lissencephaly sequence (ILS) and the more severe Miller-Dieker syndrome (MDS). Mice carrying double heterozygous mutations of Ywhae and Lis1 are therefore thought to be a mouse model of MDS. Intriguingly, 14-3-3ɛ binds to NDEL1 after phosphorylation by CDK1/CDK5, protecting phospho-NDEL1 from phosphatase attack.14-3-3 proteins mediate multiple cellular events, including scaffolding of signaling molecules, regulation of enzyme catalysis, and subcellular targeting. In the C. elegans, 14-3-3 homolog, Par5 is required for correct anterior-posterior zygote polarization.²⁴ In addition, phosphorylation-dependent interactions between 14-3-3ɛ, and the tight junction-associated protein Par3 had been reported.²⁵ Intriguingly, 14-3-3ɛ is a centrosomal protein,²⁶ suggesting that 14-3-3ɛ, Aurora-A and NDEL1 might create a complex at the centrosome, which may then be involved in the determination of polarity and neuronal migration. These findings might be the result of the known role of Aurora-A as a regulator of microtubule network. Microtubules are emanated from MTOC, and are extended into the chromosome, nucleus or the cell periphery (Fig. 1). These microtubule flows associated with the dynamic remodeling will provide enough force to maintain a neurite process, a spindle body or a leading process.Post-mitotic neurons, however, lose their mitotic competence permanently. Intuitively, once a neural progenitor differentiates into a neuron, the post-mitotic neurons have severed all ties with the cell cycle, in which the expression of cell cycle proteins are assumed to be not expressed. Emerging evidence reveals that this holds true for a handful of core cell cycle regulators, which facilitate the differentiation and maturation of neurons, suggesting that “core“ cell cycle regulators serve diverse postmitotic functions that span various developmental stages of a neuron, including neuronal migration, axonal elongation, axon pruning, dendrite morphogenesis and synaptic maturation and plasticity (reviewed in ref. ²⁷). Among the essential kinases that function in mitosis are Aurora kinases, evolutionarily conserved serine-threonine kinases that maintain genomic stability and are required for mitotic progression. Although they share conserved regions, each member (Aurora A, B and C) contributes distinctly to cell cycle progression. Aurora-A is essential for mitotic entry, centrosome maturation during late G2 and prophase, centrosome separation during bipolar spindle assembly and mitotic spindle organization (reviewed in refs. ²¹ and ²⁸). During mitotic progression, Aurora-A loss of function prevents centrosomal separation prior to mitotic spindle formation and results in monopolar spindles.²⁹ We reported an essential role of Aurora-A during neurite extension. Wirtz-Peitz et al. reported that Aurora-A phosphorylates Par-6.³⁰ This phosphorylation cascade triggered by the activation of Aurora-A is responsible for the asymmetric localization of Numb in mitosis, which provides further evidence for crosstalk of PAR proteins and Aurora-A.³⁰ Apart from neurons, the interactions between the prometastatic scaffolding protein HEF1/Cas-L/NEDD9 and the oncogenic Aurora-A kinase at the basal body of cilia had been reported.³¹ This pathway is both necessary and sufficient for ciliary resorption and constitutes an unexpected non-mitotic activity of Aurora-A in vertebrates.Aurora-A kinase, Plk1 or CDK1 has been recognized as a mitotic kinase, which regulates mitotic entry. Cells in which these genes are mutated display defective mitotic entry. Individual proteins however, have multiple functions within specific cellular context. For example, Aurora-A may participate remodeling microtubule in mitotic spindle formation and in remodeling of microtubule organization during neurite extension or neuronal migration. Apart from existing concept, elucidation of multiple functions of cell cycle regulators will provide us with a better understanding of the extent to which they exert physiological cell cycle-independent neuronal functions.     

NDEL1 phosphorylation by Aurora-A kinase is essential for centrosomal maturation, separation, and TACC3 recruitment

NDEL1 is a binding partner of LIS1 that participates in the regulation of cytoplasmic dynein function and microtubule organization during mitotic cell division and neuronal migration. NDEL1 preferentially localizes to the centrosome and is a likely target for cell cycle-activated kinases, including CDK1. In particular, NDEL1 phosphorylation by CDK1 facilitates katanin p60 recruitment to the centrosome and triggers microtubule remodeling. Here, we show that Aurora-A phosphorylates NDEL1 at Ser251 at the beginning of mitotic entry. Interestingly, NDEL1 phosphorylated by Aurora-A was rapidly downregulated thereafter by ubiquitination-mediated protein degradation. In addition, NDEL1 is required for centrosome targeting of TACC3 through the interaction with TACC3. The expression of Aurora-A phosphorylation-mimetic mutants of NDEL1 efficiently rescued the defects of centrosomal maturation and separation which are characteristic of Aurora-A-depleted cells. Our findings suggest that Aurora-A-mediated phosphorylation of NDEL1 is essential for centrosomal separation and centrosomal maturation and for mitotic entry.     

Protein phosphatase 4 catalytic subunit regulates Cdk1 activity and microtubule organization via NDEL1 dephosphorylation

Protein phosphatase 4 catalytic subunit (PP4c) is a PP2A-related protein serine/threonine phosphatase with important functions in a variety of cellular processes, including microtubule (MT) growth/organization, apoptosis, and tumor necrosis factor signaling. In this study, we report that NDEL1 is a substrate of PP4c, and PP4c selectively dephosphorylates NDEL1 at Cdk1 sites. We also demonstrate that PP4c negatively regulates Cdk1 activity at the centrosome. Targeted disruption of PP4c reveals disorganization of MTs and disorganized MT array. Loss of PP4c leads to an unscheduled activation of Cdk1 in interphase, which results in the abnormal phosphorylation of NDEL1. In addition, abnormal NDEL1 phosphorylation facilitates excessive recruitment of katanin p60 to the centrosome, suggesting that MT defects may be attributed to katanin p60 in excess. Inhibition of Cdk1, NDEL1, or katanin p60 rescues the defective MT organization caused by PP4 inhibition. Our work uncovers a unique regulatory mechanism of MT organization by PP4c through its targets Cdk1 and NDEL1 via regulation of katanin p60 distribution.     

LIS1 and NDEL1 coordinate the plus-end-directed transport of cytoplasmic dynein

LIS1 was first identified as a gene mutated in human classical lissencephaly sequence. LIS1 is required for dynein activity, but the underlying mechanism is poorly understood. Here, we demonstrate that LIS1 suppresses the motility of cytoplasmic dynein on microtubules (MTs), whereas NDEL1 releases the blocking effect of LIS1 on cytoplasmic dynein. We demonstrate that LIS1, cytoplasmic dynein and MT fragments co-migrate anterogradely. When LIS1 function was suppressed by a blocking antibody, anterograde movement of cytoplasmic dynein was severely impaired. Immunoprecipitation assay indicated that cytoplasmic dynein forms a complex with LIS1, tubulins and kinesin-1. In contrast, immunoabsorption of LIS1 resulted in disappearance of co-precipitated tubulins and kinesin. Thus, we propose a novel model of the regulation of cytoplasmic dynein by LIS1, in which LIS1 mediates anterograde transport of cytoplasmic dynein to the plus end of cytoskeletal MTs as a dynein-LIS1 complex on transportable MTs, which is a possibility supported by our data.     

Establishment of polarity during organization of the acentrosomal plant cortical microtubule array

The plant cortical microtubule array is a unique acentrosomal array that is essential for plant morphogenesis. To understand how this array is organized, we exploited the microtubule (+)-end tracking activity of two Arabidopsis EB1 proteins in combination with FRAP (fluorescence recovery after photobleaching) experiments of GFP-tubulin to examine the relationship between cortical microtubule array organization and polarity. Significantly, our observations show that the majority of cortical microtubules in ordered arrays, within a particular cell, face the same direction in both Arabidopsis plants and cultured tobacco cells. We determined that this polar microtubule coalignment is at least partially due to a selective stabilization of microtubules, and not due to a change in microtubule polymerization rates. Finally, we show that polar microtubule coalignment occurs in conjunction with parallel grouping of cortical microtubules and that cortical array polarity is progressively enhanced during array organization. These observations reveal a novel aspect of plant cortical microtubule array organization and suggest that selective stabilization of dynamic cortical microtubules plays a predominant role in the self-organization of cortical arrays.     

LIS1 at the microtubule plus end and its role in dynein-mediated nuclear migration

The cytoplasmic dynein complex and its accessory dynactin complex are involved in many cellular activities including nuclear migration in fungi (for review see Karki and Holzbaur, 1999). LIS1, the product of a causal gene for human lissencephaly (smooth brain), has also been implicated in dynein function based on studies in fungi and more recent studies in higher eukaryotic systems (for review see Gupta et al., 2002). Exactly how LIS1 may regulate the behavior of cytoplasmic dynein in various organisms is a fascinating question. In this issue, Lee et al. (2003) describe important new findings in Saccharomyces cerevisiae regarding the role of LIS1 (Pac1) in dynein-mediated nuclear migration.     

The drosophila protein asp is involved in microtubule organization during spindle formation and cytokinesis

Abnormal spindle (Asp) is a 220-kD microtubule-associated protein from Drosophila that has been suggested to be involved in microtubule nucleation from the centrosome. Here, we show that Asp is enriched at the poles of meiotic and mitotic spindles and localizes to the minus ends of central spindle microtubules. Localization to these structures is independent of a functional centrosome. Moreover, colchicine treatment disrupts Asp localization to the centrosome, indicating that Asp is not an integral centrosomal protein. In both meiotic and mitotic divisions of asp mutants, microtubule nucleation occurs from the centrosome, and gamma-tubulin localizes correctly. However, spindle pole focusing and organization are severely affected. By examining cells that carry mutations both in asp and in asterless, a gene required for centrosome function, we have determined the role of Asp in the absence of centrosomes. Phenotypic analysis of these double mutants shows that Asp is required for the aggregation of microtubules into focused spindle poles, reinforcing the conclusion that its function at the spindle poles is independent of any putative role in microtubule nucleation. Our data also suggest that Asp has a role in the formation of the central spindle. The inability of asp mutants to correctly organize the central spindle leads to disruption of the contractile ring machinery and failure in cytokinesis.     

aPKC controls microtubule organization to balance adherens junction symmetry and planar polarity during development

Tissue morphogenesis requires assembling and disassembling individual cell-cell contacts without losing epithelial integrity. This requires dynamic control of adherens junction (AJ) positioning around the apical domain, but the mechanisms involved are unclear. We show that atypical Protein Kinase C (aPKC) is required for symmetric AJ positioning during Drosophila embryogenesis. aPKC is dispensable for initial apical AJ recruitment, but without aPKC, AJs form atypical planar-polarized puncta at gastrulation. Preceding this, microtubules fail to dissociate from centrosomes, and at gastrulation abnormally persistent centrosomal microtubule asters cluster AJs into the puncta. Dynein enrichment at the puncta suggests it may draw AJs and microtubules together and microtubule disruption disperses the puncta. Through cytoskeletal disruption in wild-type embryos, we find a balance of microtubule and actin interactions controls AJ symmetry versus planar polarity during normal gastrulation. aPKC apparently regulates this balance. Without aPKC, abnormally strong microtubule interactions break AJ symmetry and epithelial structure is lost.     

Functional dissection of LIS1 and NDEL1 towards understanding the molecular mechanisms of cytoplasmic dynein regulation

LIS1 and NDEL1 are known to be essential for the activity of cytoplasmic dynein in living cells. We previously reported that LIS1 and NDEL1 directly regulated the motility of cytoplasmic dynein in an in vitro motility assay. LIS1 suppressed dynein motility and inhibited the translocation of microtubules (MTs), while NDEL1 dissociated dynein from MTs and restored dynein motility following suppression by LIS1. However, the molecular mechanisms and detailed interactions of dynein, LIS1, and NDEL1 remain unknown. In this study, we dissected the regulatory effects of LIS1 and NDEL1 on dynein motility using full-length or truncated recombinant fragments of LIS1 or NDEL1. The C-terminal fragment of NDEL1 dissociated dynein from MTs, whereas its N-terminal fragment restored dynein motility following suppression by LIS1, demonstrating that the two functions of NDEL1 localize to different parts of the NDEL1 molecule, and that restoration from LIS1 suppression is caused by the binding of NDEL1 to LIS1, rather than to dynein. The truncated monomeric form of LIS1 had little effect on dynein motility, but an artificial dimer of truncated LIS1 suppressed dynein motility, which was restored by the N-terminal fragment of NDEL1. This suggests that LIS1 dimerization is essential for its regulatory function. These results shed light on the molecular interactions between dynein, LIS1, and NDEL1, and the mechanisms of cytoplasmic dynein regulation.     

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.     

Lis1 is essential for cortical microtubule organization and desmosome stability in the epidermis

Desmosomes are cell-cell adhesion structures that integrate cytoskeletal networks. In addition to binding intermediate filaments, the desmosomal protein desmoplakin (DP) regulates microtubule reorganization in the epidermis. In this paper, we identify a specific subset of centrosomal proteins that are recruited to the cell cortex by DP upon epidermal differentiation. These include Lis1 and Ndel1, which are centrosomal proteins that regulate microtubule organization and anchoring in other cell types. This recruitment was mediated by a region of DP specific to a single isoform, DPI. Furthermore, we demonstrate that the epidermal-specific loss of Lis1 results in dramatic defects in microtubule reorganization. Lis1 ablation also causes desmosomal defects, characterized by decreased levels of desmosomal components, decreased attachment of keratin filaments, and increased turnover of desmosomal proteins at the cell cortex. This contributes to loss of epidermal barrier activity, resulting in completely penetrant perinatal lethality. This work reveals essential desmosome-associated components that control cortical microtubule organization and unexpected roles for centrosomal proteins in epidermal function.     

Requirement of Aurora-A kinase in astral microtubule polymerization and spindle microtubule flux

Mitotic Aurora-A kinase was found to be required for formation of bipolar spindle, ensuring accurate chromosome segregation in mitosis. Recently, Aurora-A was shown to promote Ran-GTP-induced spindle formation and astral microtubule development. Here, by selective immunodepletion, we showed that Aurora-A was required for centrosome- but not Ran-GTP-induced astral microtubule formation in Xenopus egg extracts. Aurora-A enhanced microtubule polymerization in both centrosome- and Ran-GTP-induced aster assemblies: shortening the timing of aster assembly and increasing the aster size. Indeed, adding of Aurora-A protein alone induced microtubule clustering, which was abrogated by Aurora kinase inhibitory small molecule ZM447439. In addition, we showed that Aurora-A was indispensable for Ran-GTP-induced bipolar spindle formation. Inhibition of Aurora-A activity by adding of kinase inactive dominant mutant led to spindle collapse and formation of monopolar spindle whereas minus-end motor protein dynein/dynactin inhibitor p50/dynamitin rescued the bipolar structure. Lastly, we revealed that Aurora-A was necessary for microtubule poleward flux and this requirement depended on kinase activity. Thus, we showed that Aurora-A promoted microtubule polymerization and maintained microtubule flux in ensuring proper bipolar spindle assembly.     

Aberrant cell plate formation in the Arabidopsis thaliana microtubule organization 1 mutant

MICROTUBULE ORGANIZATION 1 encodes a microtubule-associated protein in Arabidopsis thaliana but different alleles have contradictory phenotypes. The original mutant mor1 alleles were reported to have disrupted cortical microtubules, swollen organs and normal cytokinesis, whereas other alleles, embryo-lethal gemini pollen 1 (gem1), have defective pollen cytokinesis. To determine whether MOR1 functions generally in cytokinesis, we examined the ultrastructure of cell division in roots of the original mor1-1 allele. Cell plates are misaligned, branched and meandering; the forming cell plates remain partly vesicular, with electron-dense or lamellar content. Phragmoplast microtubules are abundant but organized aberrantly. Thus, MOR1 functions in both phragmoplast and cortical arrays.     

Role of microtubule organization in centrosome migration and mitotic spindle formation in PtK1 cells

Summary Quinacrine, an acridine derivative, has previously been shown to disrupt lateral associations between non-kinetochore microtubules (nkMTs) of opposite polarity in PtK₁ metaphase spindles such that the balance of spindle forces is significantly altered. We extended the analysis of the spatial relationship of spindle microtubules (MTs) in this study by using quinacrine to compare ATP-dependent requirements for early prometaphase centrosome separation and spindle formation. The route used for centrosome migration can take a variety of pathways in PtK₁ cells, depending on the location of the centrosomes at the time of nuclear envelope breakdown. Following quinacrine treatment centrosome separation decresased by 1.9 to 14.0 m depending on the pathway utilized. However, birefringence of the centrosomal region increased approximately 50% after quinacrine treatment. Quinacrine-treated mid-prometaphase cells, where chromosome attachment to MTs had occurred, showed a decrease in spindle length of approximately 6.0 m with only a slight increase in astral birefringence. Computer-generated reconstructions of quinacrine-treated prometaphase cells were used to confirm changes in MT reorganization. Early-prometaphase cells showed more astral MTs (aMTs) of varied length while mid-prometaphase cells showed only a few short aMTs. Late prometaphase cells again showed a large number of aMTs. Our results suggest that: (1) quinacrine treatment affects centrosome separation, (2) recruitment of nkMTs by kinetochores is quinacrine-sensitive, and (3) development of the prometaphase spindle is dependent on quinacrine-sensitive lateral interactions between nkMTs of opposite polarity. These data also suggest that lateral interactions between MTs formed during prometaphase are necessary for centrosome separation and normal spindle formation but not necessarily chromosome motion.Abbreviations aMT(s) astral microtubule(s) - DIC differential interference contrast - MT(s) microtubule(s) - kMT(s) kinetochore microtubule(s) - NEB nuclear envelope breakdown - nkMT(s) non-kinetochore microtubule(s)     

Reduction of microtubule catastrophe events by LIS1, platelet-activating factor acetylhydrolase subunit.

Forming the structure of the human brain involves extensive neuronal migration, a process dependent on cytoskeletal rearrangement. Neuronal migration is believed to be disrupted in patients exhibiting the developmental brain malformation lissencephaly. Previous studies have shown that LIS1, the defective gene found in patients with lissencephaly, is a subunit of the platelet-activating factor acetylhydrolase. Our results indicated that LIS1 has an additional function. By interacting with tubulin it suppresses microtubule dynamics. We detected LIS1 interaction with microtubules by immunostaining and co-assembly. LIS1-tubulin interactions were assayed by co-immunoprecipitation and by surface plasmon resonance changes. Microtubule dynamic measurements in vitro indicated that physiological concentrations of LIS1 indeed reduced microtubule catastrophe events, thereby resulting in a net increase in the maximum length of the microtubules. Furthermore, the LIS1 protein concentration in the brain, measured by quantitative Western blots, is high and is approximately one-fifth of the concentration of brain tubulin. Our new findings show that LIS1 is a protein exhibiting several cellular interactions, and the interaction with the cytoskeleton may prove to be the mode of transducing a signal generated by platelet-activating factor. We postulate that the LIS1-cytoskeletal interaction is important for neuronal migration, a process that is defective in lissencephaly patients.     

Centrin plays an essential role in microtubule severing during flagellar excision in Chlamydomonas reinhardtii

Previously, we reported that flagellar excision in Chlamydomonas reinhardtii is mediated by an active process whereby microtubules are severed at select sites within the flagellar-basal body transition zone (Sanders, M. A., and J. L. Salisbury. 1989. J. Cell Biol. 108:1751- 1760). At the time of flagellar excision, stellate fibers of the transition zone contract and displace the microtubule doublets of the axoneme inward. The resulting shear force and torsional load generated during inward displacement leads to microtubule severing immediately distal to the central cylinder of the transition zone. In this study, we have used a detergent-extracted cell model of Chlamydomonas that allows direct experimental access to the molecular machinery responsible for microtubule severing without the impediment of the plasma membrane. We present four independent lines of experimental evidence for the essential involvement of centrin-based stellate fibers of the transition zone in the process of flagellar excision: (a) Detergent-extracted cell models excise their flagella in response to elevated, yet physiological, levels of free calcium. (b) Extraction of cell models with buffers containing the divalent cation chelator EDTA leads to the disassembly of centrin-based fibers and to the disruption of transition zone stellate fiber structure. This treatment results in a complete loss of flagellar excision competence. (c) Three separate anti-centrin monoclonal antibody preparations, which localize to the stellate fibers of the transition zone, specifically inhibit contraction of the stellate fibers and block calcium-induced flagellar excision, while control antibodies have no inhibitory effect. Finally, (d) cells of the centrin mutant vfl-2 (Taillon, B., S. Adler, J. Suhan, and J. Jarvik. 1992. J. Cell Biol. 119:1613-1624) fail to actively excise their flagella following pH shock in living cells or calcium treatment of detergent-extracted cell models. Taken together, these observations demonstrate that centrin-based fiber contraction plays a fundamental role in microtubule severing at the time of flagellar excision in Chlamydomonas.     

Regulation of microtubule organization during interphase and M phase

Microtubule (MT) dynamics and organization change markedly during interphase-M phase transition of the cell cycle. This mini review focuses first on p220, a ubiquitous MT-associated protein of Xenopus. p220 is phosphorylated by p34cdc2 kinase and MAP kinase in M phase, and concomitantly loses its MT-binding and MT-stabilizing activities. A cDNA encoding p220 was cloned, which identified p220 as a Xenopus homolog of MAP4, and p220 was therefore termed XMAP4. To examine the physiological relevance of XMAP4 phosphorylation during mitosis, Xenopus A6 cells were transfected with cDNA encoding wild-type or various XMAP4 mutants fused with a green fluorescent protein (GFP). Mutations of serine and threonine within potential phosphorylation sites for p34cdc2 kinase to nonphosphorylatable alanine interfered with mitosis-associated reduction in MT-affinity of XMAP4 and their overexpression affected chromosome movement during anaphase A. These results indicated that phosphorylation of XMAP4 by p34cdc2 kinase is responsible for the decrease in its MT-binding and MT-stabilizing activities during mitosis which are important for chromosome movement during anaphase A. The second focus is on a novel monoclonal antibody W8C3, which recognizes alpha-tubulin. W8C3 stained spindle MTs but not interphase MTs of Xenopus A6 cells, although tubulin dimers in M phase and interphase were equally recognized by this antibody. The difference in MT staining pattern may be because the W8C3-recognition site on alpha-tubulin is sterically hidden in interphase MTs but not in spindle MTs.     

Morphogenetic plastid migration and microtubule organization during megasporogenesis inIsoetes

Summary The large megasporocytes ofIsoetes provide an exceptional system for studying microtubule dynamics in monoplastidic meiosis where plastid polarity assures coordination of plastid and nuclear division by the intimate association of MTOCs with plastids. Division and migration of the plastid in prophase establishes the tetrahedrally arranged cytoplasmic domains of the future spore tetrad and the four plastid-MTOCs serve as focal points of a unique quadripolar microtubule system (QMS). The QMS is a dynamic structure which functions in plastid deployment and contributes directly to development of both first and second division spindles. The nucleation of microtubules at discrete plastid-MTOCs is compared with centrosomal nucleation of microtubules in animal cells where growth of microtubules involves dynamic instability.Abbreviations AMS axial microtubule system - MTOC microtubule organizing center - N nucleus - QMS quadripolar microtubule system - P plastid - PPB preprophase band of microtubules     

Epithelial organization, cell polarity and tumorigenesis

Epithelial cells comprise the foundation for the majority of organs in the mammalian body, and are the source of approximately 90% of all human cancers. Characteristically, epithelial cells form intercellular adhesions, exhibit apical/basal polarity, and orient their mitotic spindles in the plane of the epithelial sheet. Defects in these attributes result in the tissue disorganization associated with cancer. Epithelia undergo self-renewal from stem cells, which might in some cases be the cell of origin for cancers. The PAR polarity proteins are master regulators of epithelial organization, and are closely linked to signaling pathways such as Hippo, which orchestrate proliferation and apoptosis to control organ size. 3D ex vivo culture systems can now faithfully recapitulate epithelial organ morphogenesis, providing a powerful approach to study both normal development and the initiating events in carcinogenesis.