CLASP Suppresses Microtubule Catastrophes through a Single TOG Domain

The GTP-tubulin cap is not the determinant of microtubule end stability in cells

Microtubules are dynamic cytoskeletal polymers essential for cell division, motility, and intracellular transport. Microtubule dynamics are characterized by dynamic instability-the ability of individual microtubules to switch between phases of growth and shrinkage. Dynamic instability can be explained by the GTP-cap model, suggesting that a "cap" of GTP-tubulin subunits at the growing microtubule end has a stabilizing effect, protecting against microtubule catastrophe-the switch from growth to shrinkage. Although the GTP-cap is thought to protect the growing microtubule end, whether the GTP-cap size affects microtubule stability in cells is not known. Notably, microtubule end-binding proteins, EBs, recognize the nucleotide state of tubulin and display comet-like localization at growing microtubule ends, which can be used as a proxy for the GTP-cap. Here, we employ high spatiotemporal resolution imaging to compare the relationship between EB comet size and microtubule dynamics in interphase LLC-PK1 cells to that measured in vitro. Our data reveal that the GTP-cap size in cells scales with the microtubule growth rate in the same way as in vitro. However, we find that microtubule ends in cells can withstand transition to catastrophe even after the EB comet is lost. Thus, our findings suggest that the presence of the GTP-cap is not the determinant of microtubule end stability in cells.

Construction and validation of co-expression vector for rice alpha tubulin and microtubule associated protein respectively fused with fluorescent proteins

Microtubule (MT) consists of α-tubulin and β-tubulin. The dynamic instability regulated by various microtubule associated proteins (MAPs) is essential for MT functions. To analyze the interaction between tubulin/MT and MAP in vivo, we usually need tubulin and MAP co-expressed. Here, we constructed a dual-transgene vector expressing rice (Oryza sativa) α-tubulin and MAP simultaneously. To construct this vector, plant expression vector pCambia1301 was used as the plasmid backbone and Gibson assembly cloning technology was used. We first fused and cloned the GFP fragment, α-tubulin open reading frame (ORF), and NOS terminator into the vector pCambia1301 to construct the p35S::GFP-α-tubulin vector that expressed GFP-α-tubulin fusion protein. Subsequently, we fused and cloned the CaMV 35S promoter, mCherry fragment, and NOS terminator into the p35S::GFP-α-tubulin vector to generate the universal dual-transgene expression vector (p35S::GFP-α-tubulin-p35S::mCherry vector). With the p35S::GFP-α-tubulin-p35S::mCherry vector, MAP ORF can be cloned into the site of 5' or 3' terminus of mCherry to co-express GFP-α-tubulin and MAP-mCherry/mCherry-MAP. To validate the availability and universality of the dual-transgene expression vector, a series of putative rice MAP genes including GL7, OsKCBP, OsCLASP, and OsMOR1 were cloned into the vector respectively, transformed into Agrobacterium tumefaciens strain, and expressed in Nicotiana benthamiana leaves. The results indicated that all of the MAPs were co-expressed with α-tubulin and localized to MTs, validating the availability and universality of the vector and that GL7, OsKCBP, OsCLASP, and OsMOR1 might be MAPs. The application of the co-expression vector constructed by us would facilitate studies on the interaction between tubulin/MT and MAP in tobacco transient expression systems or transgenic rice.

Confined migration: Microtubules control the cell rear

Cell migration through complex 3D environments relies on the interplay between actin and microtubules. A new study shows that, when cells pass through narrow constrictions, CLASP-dependent microtubule stabilisation at the cell rear controls actomyosin contractility to enable nuclear translocation and preserve cell integrity.

CLASP-mediated competitive binding in protein condensates directs microtubule growth

Microtubule organization in cells relies on targeting mechanisms. Cytoplasmic linker proteins (CLIPs) and CLIP-associated proteins (CLASPs) are key regulators of microtubule organization, yet the underlying mechanisms remain elusive. Here, we reveal that the C-terminal domain of CLASP2 interacts with a common motif found in several CLASP-binding proteins. This interaction drives the dynamic localization of CLASP2 to distinct cellular compartments, where CLASP2 accumulates in protein condensates at the cell cortex or the microtubule plus end. These condensates physically contact each other via CLASP2-mediated competitive binding, determining cortical microtubule targeting. The phosphorylation of CLASP2 modulates the dynamics of the condensate-condensate interaction and spatiotemporally navigates microtubule growth. Moreover, we identify additional CLASP-interacting proteins that are involved in condensate contacts in a CLASP2-dependent manner, uncovering a general mechanism governing microtubule targeting. Our findings not only unveil a tunable multiphase system regulating microtubule organization, but also offer general mechanistic insights into intricate protein-protein interactions at the mesoscale level.

CAMSAPs and nucleation-promoting factors control microtubule release from γ-TuRC

γ-Tubulin ring complex (γ-TuRC) is the major microtubule-nucleating factor. After nucleation, microtubules can be released from γ-TuRC and stabilized by other proteins, such as CAMSAPs, but the biochemical cross-talk between minus-end regulation pathways is poorly understood. Here we reconstituted this process in vitro using purified components. We found that all CAMSAPs could bind to the minus ends of γ-TuRC-attached microtubules. CAMSAP2 and CAMSAP3, which decorate and stabilize growing minus ends but not the minus-end tracking protein CAMSAP1, induced microtubule release from γ-TuRC. CDK5RAP2, a γ-TuRC-interactor, and CLASP2, a regulator of microtubule growth, strongly stimulated γ-TuRC-dependent microtubule nucleation, but only CDK5RAP2 suppressed CAMSAP binding to γ-TuRC-anchored minus ends and their release. CDK5RAP2 also improved selectivity of γ-tubulin-containing complexes for 13- rather than 14-protofilament microtubules in microtubule-capping assays. Knockout and overexpression experiments in cells showed that CDK5RAP2 inhibits the formation of CAMSAP2-bound microtubules detached from the microtubule-organizing centre. We conclude that CAMSAPs can release newly nucleated microtubules from γ-TuRC, whereas nucleation-promoting factors can differentially regulate this process.

Study on the Expression and Potential Function of LncRNA in Peripheral Blood of Patients with Ankylosing Spondylitis

BACKGROUND

Ankylosing spondylitis (AS) is an autoimmune disease that has the characteristics of difficult early diagnosis and a high disability rate.

OBJECTIVE

The objective of this study was to further explore the possible mechanism and potential function of lncRNA in AS.

METHODS

We used lncRNA microarray technology to detect the expression of lncRNA and mRNA in patients with active AS, stable patients, and healthy controls (HC). Afterward, bioinformatics analysis was conducted on differentially expressed genes. Seven differentially expressed lncRNAs were screened out for real-time fluorescent quantitative PCR (RT-qPCR), combined with various clinical indicators for correlation analysis, and the receiver operating characteristic (ROC) curve was used to analyze the potential of lncRNA as a diagnostic marker for AS.

RESULTS

The results showed that the expression levels of NR-037662 and ENST00000599316 in the AS subgroups were significantly higher than those in the HC group, while the expression levels of ENST00000577914 and ENST00000579003 were lower than those in the HC group. The expression levels of NR-003542 and ENST00000512051 in the ASA group were significantly higher than those in the ASS and HC groups, while NR-026756 was just the opposite. Spearman's correlation analysis showed that the expression level of NR-003542 was positively correlated with Bath Ankylosing Spondylitis Functional Index (BASFI), Erythrocyte Sedimentation Rate (ESR), and high sensitivity C-Reactive Protein (hsCRP). The expression level of NR-026756 was negatively correlated with the Bath Ankylosing Spine Inflammatory Disease Activity Index (BASDAI), BASFI, ESR, hsCRP, and globulin (GLOB). In addition, it was also found that the ROC curve analysis of the 4 lncRNAs between the AS group (ASA group and ASS group) and the HC group were statistically significant, and the area under the curve (AUC) of NR-037662, ENST00000599316, ENST00000577914, and ENST00000579003 was 0.804, 0.812, 0.706, and 0.698, respectively.

CONCLUSION

It was found that these differentially expressed lncRNAs of AS may be involved in the occurrence and development of the disease. Among them, NR-037662, ENST00000599316, ENST00000577914, and ENST00000579003 might have the potential to become AS diagnostic molecular markers. Moreover, NR -003542, ENST00000512051, and NR-026756 might have the potential to be indicators of disease activity.

More is different: Reconstituting complexity in microtubule regulation

Microtubules are dynamic cytoskeletal filaments that undergo stochastic switching between phases of polymerization and depolymerization-a behavior known as dynamic instability. Many important cellular processes, including cell motility, chromosome segregation, and intracellular transport, require complex spatiotemporal regulation of microtubule dynamics. This coordinated regulation is achieved through the interactions of numerous microtubule-associated proteins (MAPs) with microtubule ends and lattices. Here, we review the recent advances in our understanding of microtubule regulation, focusing on results arising from biochemical in vitro reconstitution approaches using purified multiprotein ensembles. We discuss how the combinatory effects of MAPs affect both the dynamics of individual microtubule ends, as well as the stability and turnover of the microtubule lattice. In addition, we highlight new results demonstrating the roles of protein condensates in microtubule regulation. Our overall intent is to showcase how lessons learned from reconstitution approaches help unravel the regulatory mechanisms at play in complex cellular environments.

Microtubule rescue control by drugs and MAPs examined with in vitro pedestal assay

Microtubules are essential cytoskeletal polymers, which exhibit stochastic transitions between assembly and disassembly, known as catastrophes and rescues. Understanding of catastrophes, rescues, and their control by drugs and microtubule associated proteins (MAPs) has been informed by in vitro reconstitutions of microtubule dynamics. In such experiments microtubules are typically observed on a flat surface of the coverslip. In contrast, we have recently proposed a modified setup in which microtubules assemble from stabilized seeds, overhanging from microfabricated pedestals, so that their dynamic extensions are fully isolated from contact with the coverslip. This assay allows to eliminate potential artifacts, which may substantially affect the frequency of microtubule rescues in vitro. Here we use the pedestal assay to study the sensitivity of microtubules to paclitaxel, one of the best-known inhibitors of microtubule dynamics. By comparing observations in the conventional and the pedestal assays, we find that microtubule dynamics are substantially more sensitive to paclitaxel when the polymers can contact the coverslip. We interpret this as a consequence of the coverslip-induced microtubule assembly perturbation, leading to formation of lattice with defects, and thereby enhancing the efficiency of paclitaxel binding to microtubules in the conventional assay. To test this idea, we use vinblastine, another small-molecule inhibitor, which had been previously shown to cause microtubule growth perturbations. We find that in the pedestal assay vinblastine sensitizes microtubules to paclitaxel to the level, observed in the conventional assay. Interestingly, a minimal fragment of MAP called CLASP2, a previously characterized rescue factor, has a strong effect on microtubule rescues, regardless of the type of assay. Overall, our study underscores the role of microtubule damage in promoting rescues and highlights the utility of the in vitro pedestal assay to study microtubule dynamics modulation by tubulin inhibitors and MAPs.

The role of intrinsic disorder in binding of plant microtubule-associated proteins to the cytoskeleton

Microtubules (MTs) represent one of the main components of the eukaryotic cytoskeleton and support numerous critical cellular functions. MTs are in principle tube-like structures that can grow and shrink in a highly dynamic manner; a process largely controlled by microtubule-associated proteins (MAPs). Plant MAPs are a phylogenetically diverse group of proteins that nonetheless share many common biophysical characteristics and often contain large stretches of intrinsic protein disorder. These intrinsically disordered regions are determinants of many MAP-MT interactions, in which structural flexibility enables low-affinity protein-protein interactions that enable a fine-tuned regulation of MT cytoskeleton dynamics. Notably, intrinsic disorder is one of the major obstacles in functional and structural studies of MAPs and represents the principal present-day challenge to decipher how MAPs interact with MTs. Here, we review plant MAPs from an intrinsic protein disorder perspective, by providing a complete and up-to-date summary of all currently known members, and address the current and future challenges in functional and structural characterization of MAPs.

Lack of chromokinesin Klp-19 creates a more rigid midzone and affects force transmission during anaphase in C. elegans

Recent studies have highlighted the significance of the spindle midzone - the region positioned between chromosomes - in ensuring proper chromosome segregation. By combining advanced 3D electron tomography and cutting-edge light microscopy we have discovered a previously unknown role of the regulation of microtubule dynamics within the spindle midzone of C. elegans. Using Fluorescence recovery after photobleaching and a combination of second harmonic generation and two-photon fluorescence microscopy, we found that the length of the antiparallel microtubule overlap zone in the spindle midzone is constant throughout anaphase, and independent of cortical pulling forces as well as the presence of the microtubule bundling protein SPD-1. Further investigations of SPD-1 and the chromokinesin KLP-19 in C. elegans suggest that KLP-19 regulates the overlap length and functions independently of SPD-1. Our data shows that KLP-19 plays an active role in regulating the length and turn-over of microtubules within the midzone as well as the size of the antiparallel overlap region throughout mitosis. Depletion of KLP-19 in mitosis leads to an increase in microtubule length in the spindle midzone, which also leads to increased microtubule - microtubule interaction, thus building up a more robust microtubule network. The spindle is globally stiffer and more stable, which has implications for the transmission of forces within the spindle affecting chromosome segregation dynamics. Our data shows that by localizing KLP-19 to the spindle midzone in anaphase microtubule dynamics can be locally controlled allowing the formation of a functional midzone.

Lateral and longitudinal compaction of PRC1 overlap zones drives stabilization of interzonal microtubules

During anaphase, antiparallel-overlapping midzone microtubules elongate and form bundles, contributing to chromosome segregation and the location of contractile ring formation. Midzone microtubules are dynamic in early but not late anaphase; however, the kinetics and mechanisms of stabilization are incompletely understood. Using photoactivation of cells expressing PA-EGFP-α-tubulin we find that immediately after anaphase onset, a single highly dynamic population of midzone microtubules is present; as anaphase progresses, both dynamic and stable populations of midzone microtubules coexist. By mid-cytokinesis, only static, non-dynamic microtubules are detected. The velocity of microtubule sliding also decreases as anaphase progresses, becoming undetectable by late anaphase. Following depletion of PRC1, midzone microtubules remain highly dynamic in anaphase and fail to form static arrays in telophase despite furrowing. Cells depleted of Kif4a contain elongated PRC1 overlap zones and fail to form static arrays in telophase. Cells blocked in cytokinesis form short PRC1 overlap zones that do not coalesce laterally; these cells also fail to form static arrays in telophase. Together, our results demonstrate that dynamic turnover and sliding of midzone microtubules is gradually reduced during anaphase and that the final transition to a static array in telophase requires both lateral and longitudinal compaction of PRC1 containing overlap zones.

An unconventional TOG domain is required for CLASP localization

Cytoplasmic linker-associated proteins (CLASPs) form a conserved family of microtubule-associated proteins (MAPs) that maintain microtubules in a growing state by promoting rescue while suppressing catastrophe.1 CLASP function involves an ordered array of tumor overexpressed gene (TOG) domains and binding to multiple protein partners via a conserved C-terminal domain (CTD).2,3 In migrating cells, CLASPs concentrate at the cortex near focal adhesions as part of cortical microtubule stabilization complexes (CMSCs), via binding of their CTD to the focal adhesion protein PHLDB2/LL5β.4,5 Cortical CLASPs also stabilize a subset of microtubules, which stimulate focal adhesion turnover and generate a polarized microtubule network toward the leading edge of migrating cells. CLASPs are also recruited to the trans-Golgi network (TGN) via an interaction between their CTD and the Golgin protein GCC185.6 This allows microtubule growth toward the leading edge of migrating cells, which is required for Golgi organization, polarized intracellular transport, and cell motility.7 In dividing cells, CLASPs are essential at kinetochores for efficient chromosome segregation and anaphase spindle integrity.8,9 Both CENP-E and ASTRIN bind and target CLASPs to kinetochores,10,11 although the CLASP domain required for this interaction is not known. Despite its high evolutionary conservation, the CTD remains structurally uncharacterized. Here, we find that the CTD can be structurally modeled as a TOG domain. We identify a surface-exposed and conserved arginine residue essential for CLASP CTD interaction with partner proteins. Together, our results provide a structural mechanism by which the CLASP CTD directs diverse sub-cellular localizations throughout the cell cycle.

Kinetochore component function in C. elegans oocytes revealed by 4D tracking of holocentric chromosomes

During cell division, chromosome congression to the spindle center, their orientation along the spindle long axis and alignment at the metaphase plate depend on interactions between spindle microtubules and kinetochores, and are pre-requisite for chromosome bi-orientation and accurate segregation. How these successive phases are controlled during oocyte meiosis remains elusive. Here we provide 4D live imaging during the first meiotic division in C. elegans oocytes with wild-type or disrupted kinetochore protein function. We show that, unlike in monocentric organisms, holocentric chromosome bi-orientation is not strictly required for accurate chromosome segregation. Instead, we propose a model in which initial kinetochore-localized BHC module (comprised of BUB-1Bub1, HCP-1/2CENP-F and CLS-2CLASP)-dependent pushing acts redundantly with Ndc80 complex-mediated pulling for accurate chromosome segregation in meiosis. In absence of both mechanisms, homologous chromosomes tend to co-segregate in anaphase, especially when initially mis-oriented. Our results highlight how different kinetochore components cooperate to promote accurate holocentric chromosome segregation in oocytes of C. elegans.

CLASPs stabilize the pre-catastrophe intermediate state between microtubule growth and shrinkage

Cytoplasmic linker-associated proteins (CLASPs) regulate microtubules in fundamental cellular processes. CLASPs stabilize dynamic microtubules by suppressing microtubule catastrophe and promoting rescue, the switch-like transitions between growth and shrinkage. How CLASPs specifically modulate microtubule transitions is not understood. Here, we investigate the effects of CLASPs on the pre-catastrophe intermediate state of microtubule dynamics, employing distinct microtubule substrates to mimic the intermediate state. Surprisingly, we find that CLASP1 promotes the depolymerization of stabilized microtubules in the presence of GTP, but not in the absence of nucleotide. This activity is also observed for CLASP2 family members and a minimal TOG2-domain construct. Conversely, we find that CLASP1 stabilizes unstable microtubules upon tubulin dilution in the presence of GTP. Strikingly, our results reveal that CLASP1 drives microtubule substrates with vastly different inherent stabilities into the same slowly depolymerizing state in a nucleotide-dependent manner. We interpret this state as the pre-catastrophe intermediate state. Therefore, we conclude that CLASPs suppress microtubule catastrophe by stabilizing the intermediate state between growth and shrinkage.

The CEP170B-KIF2A complex destabilizes microtubule minus ends to generate polarized microtubule network

Microtubule (MT) minus ends are stabilized by CAMSAP family proteins at noncentrosomal MT-organizing centers. Despite progress in identifying diverse positive regulators, knowledge on the negative regulation of the MT minus-end distribution is lacking. Here, we identify CEP170B as a MT minus-end-binding protein that colocalizes with the microtubule-stabilizing complex at the cortical patches. CEP170B depends on the scaffold protein liprin-α1 for its cortical targeting and requires liprin-α1-bound PP2A phosphatase for its MT localization. CEP170B excludes CAMSAPs-stabilized MT minus ends from the cell periphery in HeLa cells and the basal cortex in human epithelial cells and is required for directional vesicle trafficking and cyst formation in 3D culture. Reconstitution experiments demonstrate that CEP170B autonomously tracks growing MT minus ends and blocks minus-end growth. Furthermore, CEP170B in a complex with the kinesin KIF2A acts as a potent MT minus-end depolymerase capable of antagonizing the stabilizing effect of CAMSAPs. Our study uncovers an antagonistic mechanism for controlling the spatial distribution of MT minus ends, which contributes to the establishment of polarized MT network and cell polarity.

Microtubule detyrosination drives symmetry breaking to polarize cells for directed cell migration

To initiate directed movement, cells must become polarized, establishing a protrusive leading edge and a contractile trailing edge. This symmetry-breaking process involves reorganization of cytoskeleton and asymmetric distribution of regulatory molecules. However, what triggers and maintains this asymmetry during cell migration remains largely elusive. Here, we established a micropatterning-based 1D motility assay to investigate the molecular basis of symmetry breaking required for directed cell migration. We show that microtubule (MT) detyrosination drives cell polarization by directing kinesin-1-based transport of the adenomatous polyposis coli (APC) protein to cortical sites. This is essential for the formation of cell's leading edge during 1D and 3D cell migration. These data, combined with biophysical modeling, unveil a key role for MT detyrosination in the generation of a positive feedback loop linking MT dynamics and kinesin-1-based transport. Thus, symmetry breaking during cell polarization relies on a feedback loop driven by MT detyrosination that supports directed cell migration.

Microtubule remodelling as a driving force of axon guidance and pruning

The establishment of neuronal connectivity relies on the microtubule (MT) cytoskeleton, which provides mechanical support, roads for axonal transport and mediates signalling events. Fine-tuned spatiotemporal regulation of MT functions by tubulin post-translational modifications and MT-associated proteins is critical for the coarse wiring and subsequent refinement of neuronal connectivity. The defective regulation of these processes causes a wide range of neurodevelopmental disorders associated with connectivity defects. This review focuses on recent studies unravelling how MT composition, post-translational modifications and associated proteins influence MT functions in axon guidance and/or pruning to build functional neuronal circuits. We here summarise experimental evidence supporting the key role of this network as a driving force for growth cone steering and branch-specific axon elimination. We further provide a global overview of the MT-interactors that tune developing axon behaviours, with a special emphasis on their emerging versatility in the regulation of MT dynamics/structure. Recent studies establishing the key and highly selective role of the tubulin code in the regulation of MT functions in axon pathfinding are also reported. Finally, our review highlights the emerging molecular links between these MT regulation processes and guidance signals that wire the nervous system.

Preference of CAMSAP3 for expanded microtubule lattice contributes to stabilization of the minus end

CAMSAPs are proteins that show microtubule minus-end-specific localization, decoration, and stabilization. Although the mechanism for minus-end recognition via their C-terminal CKK domain has been well described in recent studies, it is unclear how CAMSAPs stabilize microtubules. Our several binding assays revealed that the D2 region of CAMSAP3 specifically binds to microtubules with the expanded lattice. To investigate the relationship between this preference and the stabilization effect of CAMSAP3, we precisely measured individual microtubule lengths and found that D2 binding expanded the microtubule lattice by ∼3%. Consistent with the notion that the expanded lattice is a common feature of stable microtubules, the presence of D2 slowed the microtubule depolymerization rate to ∼1/20, suggesting that the D2-triggered lattice expansion stabilizes microtubules. Combining these results, we propose that CAMSAP3 stabilizes microtubules by lattice expansion upon D2 binding, which further accelerates the recruitment of other CAMSAP3 molecules. Because only CAMSAP3 has D2 and the highest microtubule-stabilizing effect among mammalian CAMSAPs, our model also explains the molecular basis for the functional diversity of CAMSAP family members.

CLASP2 facilitates dynamic actin filament organization along the microtubule lattice

Coordination between the microtubule and actin networks is essential for cell motility, neuronal growth cone guidance, and wound healing. Members of the CLASP (cytoplasmic linker-associated protein) family of proteins have been implicated in the cytoskeletal cross-talk between microtubules and actin networks; however, the molecular mechanisms underlying the role of CLASP in cytoskeletal coordination are unclear. Here, we investigate CLASP2α's cross-linking function with microtubules and F-actin. Our results demonstrate that CLASP2α cross-links F-actin to the microtubule lattice in vitro. We find that the cross-linking ability is retained by L-TOG2-S, a minimal construct containing the TOG2 domain and serine-arginine-rich region of CLASP2α. Furthermore, CLASP2α promotes the accumulation of multiple actin filaments along the microtubule, supporting up to 11 F-actin landing events on a single microtubule lattice region. CLASP2α also facilitates the dynamic organization of polymerizing actin filaments templated by the microtubule network, with F-actin forming bridges between individual microtubules. Finally, we find that depletion of CLASPs in vascular smooth muscle cells results in disorganized actin fibers and reduced coalignment of actin fibers with microtubules, suggesting that CLASP and microtubules contribute to higher-order actin structures. Taken together, our results indicate that CLASP2α can directly cross-link F-actin to microtubules and that this microtubule-CLASP-actin interaction may influence overall cytoskeletal organization in cells.

Dynamic microtubules slow down during their shrinkage phase

Microtubules are dynamic polymers that undergo stochastic transitions between growing and shrinking phases. The structural and chemical properties of these phases remain poorly understood. The transition from growth to shrinkage, termed catastrophe, is not a first-order reaction but rather a multistep process whose frequency increases with the growth time: the microtubule ages as the older microtubule tip becomes more unstable. Aging shows that the growing phase is not a single state but comprises several substates of increasing instability. To investigate whether the shrinking phase is also multistate, we characterized the kinetics of microtubule shrinkage following catastrophe using an in vitro reconstitution assay with purified tubulins. We found that the shrinkage speed is highly variable across microtubules and that the shrinkage speed of individual microtubules slows down over time by as much as several fold. The shrinkage slowdown was observed in both fluorescently labeled and unlabeled microtubules as well as in microtubules polymerized from tubulin purified from different species, suggesting that the shrinkage slowdown is a general property of microtubules. These results indicate that microtubule shrinkage, like catastrophe, is time dependent and that the shrinking microtubule tip passes through a succession of states of increasing stability. We hypothesize that the shrinkage slowdown is due to destabilizing events that took place during growth, which led to multistep catastrophe. This suggests that the aging associated with growth is also manifested during shrinkage, with the older, more unstable growing tip being associated with a faster depolymerizing shrinking tip.

Synergistic stabilization of microtubules by BUB-1, HCP-1, and CLS-2 controls microtubule pausing and meiotic spindle assembly

During cell division, chromosome segregation is orchestrated by a microtubule-based spindle. Interaction between spindle microtubules and kinetochores is central to the bi-orientation of chromosomes. Initially dynamic to allow spindle assembly and kinetochore attachments, which is essential for chromosome alignment, microtubules are eventually stabilized for efficient segregation of sister chromatids and homologous chromosomes during mitosis and meiosis I, respectively. Therefore, the precise control of microtubule dynamics is of utmost importance during mitosis and meiosis. Here, we study the assembly and role of a kinetochore module, comprised of the kinase BUB-1, the two redundant CENP-F orthologs HCP-1/2, and the CLASP family member CLS-2 (hereafter termed the BHC module), in the control of microtubule dynamics in Caenorhabditis elegans oocytes. Using a combination of in vivo structure-function analyses of BHC components and in vitro microtubule-based assays, we show that BHC components stabilize microtubules, which is essential for meiotic spindle formation and accurate chromosome segregation. Overall, our results show that BUB-1 and HCP-1/2 do not only act as targeting components for CLS-2 at kinetochores, but also synergistically control kinetochore-microtubule dynamics by promoting microtubule pause. Together, our results suggest that BUB-1 and HCP-1/2 actively participate in the control of kinetochore-microtubule dynamics in the context of an intact BHC module to promote spindle assembly and accurate chromosome segregation in meiosis.

CSPP1 stabilizes growing microtubule ends and damaged lattices from the luminal side

Microtubules are dynamic cytoskeletal polymers, and their organization and stability are tightly regulated by numerous cellular factors. While regulatory proteins controlling the formation of interphase microtubule arrays and mitotic spindles have been extensively studied, the biochemical mechanisms responsible for generating stable microtubule cores of centrioles and cilia are poorly understood. Here, we used in vitro reconstitution assays to investigate microtubule-stabilizing properties of CSPP1, a centrosome and cilia-associated protein mutated in the neurodevelopmental ciliopathy Joubert syndrome. We found that CSPP1 preferentially binds to polymerizing microtubule ends that grow slowly or undergo growth perturbations and, in this way, resembles microtubule-stabilizing compounds such as taxanes. Fluorescence microscopy and cryo-electron tomography showed that CSPP1 is deposited in the microtubule lumen and inhibits microtubule growth and shortening through two separate domains. CSPP1 also specifically recognizes and stabilizes damaged microtubule lattices. These data help to explain how CSPP1 regulates the elongation and stability of ciliary axonemes and other microtubule-based structures.

CLASP2 recognizes tubulins exposed at the microtubule plus-end in a nucleotide state-sensitive manner

CLASPs (cytoplasmic linker-associated proteins) are ubiquitous stabilizers of microtubule dynamics, but their molecular targets at the microtubule plus-end are not understood. Using DNA origami-based reconstructions, we show that clusters of human CLASP2 form a load-bearing bond with terminal non-GTP tubulins at the stabilized microtubule tip. This activity relies on the unconventional TOG2 domain of CLASP2, which releases its high-affinity bond with non-GTP dimers upon their conversion into polymerization-competent GTP-tubulins. The ability of CLASP2 to recognize nucleotide-specific tubulin conformation and stabilize the catastrophe-promoting non-GTP tubulins intertwines with the previously underappreciated exchange between GDP and GTP at terminal tubulins. We propose that TOG2-dependent stabilization of sporadically occurring non-GTP tubulins represents a distinct molecular mechanism to suppress catastrophe at the freely assembling microtubule ends and to promote persistent tubulin assembly at the load-bearing tethered ends, such as at the kinetochores in dividing cells.

Beyond the GTP-cap: Elucidating the molecular mechanisms of microtubule catastrophe

Almost 40 years since the discovery of microtubule dynamic instability, the molecular mechanisms underlying microtubule dynamics remain an area of intense research interest. The "standard model" of microtubule dynamics implicates a "cap" of GTP-bound tubulin dimers at the growing microtubule end as the main determinant of microtubule stability. Loss of the GTP-cap leads to microtubule "catastrophe," a switch-like transition from microtubule growth to shrinkage. However, recent studies, using biochemical in vitro reconstitution, cryo-EM, and computational modeling approaches, challenge the simple GTP-cap model. Instead, a new perspective on the mechanisms of microtubule dynamics is emerging. In this view, highly dynamic transitions between different structural conformations of the growing microtubule end - which may or may not be directly linked to the nucleotide content at the microtubule end - ultimately drive microtubule catastrophe.

Drosophila CLASP regulates microtubule orientation and dendrite pruning by suppressing Par-1 kinase

The evolutionarily conserved CLASPs (cytoplasmic linker-associated proteins) are microtubule-associated proteins that inhibit microtubule catastrophe and promote rescue. CLASPs can regulate axonal elongation and dendrite branching in growing neurons. However, their roles in microtubule orientation and neurite pruning in remodeling neurons remain unknown. Here, we identify the Drosophila CLASP homolog Orbit/MAST, which is required for dendrite pruning in ddaC sensory neurons during metamorphosis. Orbit is important for maintenance of the minus-end-out microtubule orientation in ddaC dendrites. Our structural analysis reveals that the microtubule lattice-binding TOG2 domain is required for Orbit to regulate dendritic microtubule orientation and dendrite pruning. In a genetic modifier screen, we further identify the conserved Par-1 kinase as a suppressor of Orbit in dendritic microtubule orientation. Moreover, elevated Par-1 function impairs dendritic microtubule orientation and dendrite pruning, phenocopying orbit mutants. Overall, our study demonstrates that Drosophila CLASP governs dendritic microtubule orientation and dendrite pruning at least partly via suppressing Par-1 kinase.

Septins guide noncentrosomal microtubules to promote focal adhesion disassembly in migrating cells

Endothelial cell migration is critical for vascular angiogenesis and is compromised to facilitate tumor metastasis. The migratory process requires the coordinated assembly and disassembly of focal adhesions (FA), actin, and microtubules (MT). MT dynamics at FAs deliver vesicular cargoes and enhance actomyosin contractility to promote FA turnover and facilitate cell advance. Noncentrosomal (NC) MTs regulate FA dynamics and are sufficient to drive cell polarity, but how NC MTs target FAs to control FA turnover is not understood. Here, we show that Rac1 induces the assembly of FA-proximal septin filaments that promote NC MT growth into FAs and inhibit mitotic centromere-associated kinesin (MCAK)-associated MT disassembly, thereby maintaining intact MT plus ends proximal to FAs. Septin-associated MT rescue is coupled with accumulation of Aurora-A kinase and cytoplasmic linker-associated protein (CLASP) localization to the MT between septin and FAs. In this way, NC MTs are strategically positioned to undergo MCAK- and CLASP-regulated bouts of assembly and disassembly into FAs, thereby regulating FA turnover and cell migration.

CENPO regulated proliferation and apoptosis of colorectal cancer in a p53-dependent manner

Colorectal cancer (CRC) is considered to be a leading cause of cancer-related death. Centromere protein O (CENPO) can prevent the separation of sister chromatids and cell death after spindle injury. Nevertheless, the role of CENPO in CRC has not been reported. The expression level of CENPO in CRC was revealed by TCGA database and immunohistochemical (IHC) staining. Subsequently, the loss-of-function assays were performed to identified the role of CENPO in CRC in vitro and in vivo. Our data demonstrated that CENPO was highly expressed in CRC. The expression of CENPO was positively correlated with the deterioration of CRC. Moreover, CENPO knockdown inhibited the malignant phenotypes of CRC cells, which was characterized by slowed proliferation, cycle repression at G2, promotion of apoptosis, reduced migration and weakened tumorigenesis. Furthermore, CENPO knockdown downregulated the expression of N-cadherin, Vimentin, Snail, CCND1, PIK3CA and inhibited AKT phosphorylation in CRC cells. Moreover, the function of CENPO in regulating proliferation and apoptosis depended on p53. In summary, CENPO may play a promoting role in CRC through the epithelial mesenchymal transition (EMT) and PI3K/AKT signaling pathway, which can be regarded as a molecular therapeutic target for CRC.

Quantification of Microtubule Stutters: Dynamic Instability Behaviors that are Strongly Associated with Catastrophe

Microtubules (MTs) are cytoskeletal fibers that undergo dynamic instability (DI), a remarkable process involving phases of growth and shortening separated by stochastic transitions called catastrophe and rescue. Dissecting DI mechanism(s) requires first characterizing and quantifying these dynamics, a subjective process that often ignores complexity in MT behavior. We present a Statistical Tool for Automated Dynamic Instability Analysis (STADIA) that identifies and quantifies not only growth and shortening, but also a category of intermediate behaviors that we term “stutters.” During stutters, the rate of MT length change tends to be smaller in magnitude than during typical growth or shortening phases. Quantifying stutters and other behaviors with STADIA demonstrates that stutters precede most catastrophes in our in vitro experiments and dimer-scale MT simulations, suggesting that stutters are mechanistically involved in catastrophes. Related to this idea, we show that the anticatastrophe factor CLASP2γ works by promoting the return of stuttering MTs to growth. STADIA enables more comprehensive and data-driven analysis of MT dynamics compared with previous methods. The treatment of stutters as distinct and quantifiable DI behaviors provides new opportunities for analyzing mechanisms of MT dynamics and their regulation by binding proteins.

The mitotic spindle protein CKAP2 potently increases formation and stability of microtubules

Cells increase microtubule dynamics to make large rearrangements to their microtubule cytoskeleton during cell division. Changes in microtubule dynamics are essential for the formation and function of the mitotic spindle, and misregulation can lead to aneuploidy and cancer. Using in vitro reconstitution assays we show that the mitotic spindle protein Cytoskeleton-Associated Protein 2 (CKAP2) has a strong effect on nucleation of microtubules by lowering the critical tubulin concentration 100-fold. CKAP2 increases the apparent rate constant k_a of microtubule growth by 50-fold and increases microtubule growth rates. In addition, CKAP2 strongly suppresses catastrophes. Our results identify CKAP2 as the most potent microtubule growth factor to date. These finding help explain CKAP2's role as an important spindle protein, proliferation marker, and oncogene.

Lattice defects induced by microtubule-stabilizing agents exert a long-range effect on microtubule growth by promoting catastrophes

Microtubules are major cytoskeletal filaments important for cell division, growth, and differentiation. Microtubules can rapidly switch between phases of growth and shortening, and this dynamic behavior is essential for shaping microtubule arrays. To obtain insights into mechanisms controlling microtubule dynamics, here we used microtubule-stabilizing agents such as Taxol and their fluorescent analogs to manipulate microtubule protofilament number and generate stable defects in microtubule lattices that can be visualized using fluorescence microscopy. We show that microtubule polymerization rate increases with protofilament number and that drug-induced microtubule lattice discontinuities can promote plus-end catastrophes at a distance of several micrometers. Our data indicate that structural defects in the microtubule wall can have long-range propagating effects on microtubule tip dynamics.

Microtubules are dynamic cytoskeletal polymers that spontaneously switch between phases of growth and shrinkage. The probability of transitioning from growth to shrinkage, termed catastrophe, increases with microtubule age, but the underlying mechanisms are poorly understood. Here, we set out to test whether microtubule lattice defects formed during polymerization can affect growth at the plus end. To generate microtubules with lattice defects, we used microtubule-stabilizing agents that promote formation of polymers with different protofilament numbers. By employing different agents during nucleation of stable microtubule seeds and the subsequent polymerization phase, we could reproducibly induce switches in protofilament number and induce stable lattice defects. Such drug-induced defects led to frequent catastrophes, which were not observed when microtubules were grown in the same conditions but without a protofilament number mismatch. Microtubule severing at the site of the defect was sufficient to suppress catastrophes. We conclude that structural defects within the microtubule lattice can exert effects that can propagate over long distances and affect the dynamic state of the microtubule end.

Motor usage imprints microtubule stability along the shaft

Tubulin dimers assemble into dynamic microtubules, which are used by molecular motors as tracks for intracellular transport. Organization and dynamics of the microtubule network are commonly thought to be regulated at the polymer ends, where tubulin dimers can be added or removed. Here, we show that molecular motors running on microtubules cause exchange of dimers along the shaft in vitro and in cells. These sites of dimer exchange act as rescue sites where depolymerizing microtubules stop shrinking and start re-growing. Consequently, the average length of microtubules increases depending on how frequently they are used as motor tracks. An increase of motor activity densifies the cellular microtubule network and enhances cell polarity. Running motors leave marks in the shaft, serving as traces of microtubule usage to organize the polarity landscape of the cell.

Microtubule-associated proteins promote microtubule generation in the absence of γ-tubulin in human colon cancer cells

The γ-tubulin complex acts as the predominant microtubule (MT) nucleator that initiates MT formation and is therefore an essential factor for cell proliferation. Nonetheless, cellular MTs are formed after experimental depletion of the γ-tubulin complex, suggesting that cells possess other factors that drive MT nucleation. Here, by combining gene knockout, auxin-inducible degron, RNA interference, MT depolymerization/regrowth assay, and live microscopy, we identified four microtubule-associated proteins (MAPs), ch-TOG, CLASP1, CAMSAPs, and TPX2, which are involved in γ-tubulin-independent MT generation in human colon cancer cells. In the mitotic MT regrowth assay, nucleated MTs organized noncentriolar MT organizing centers (ncMTOCs) in the absence of γ-tubulin. Depletion of CLASP1 or TPX2 substantially delayed ncMTOC formation, suggesting that these proteins might promote MT nucleation in the absence of γ-tubulin. In contrast, depletion of ch-TOG or CAMSAPs did not affect the timing of ncMTOC appearance. CLASP1 also accelerates γ-tubulin-independent MT regrowth during interphase. Thus, MT generation can be promoted by MAPs without the γ-tubulin template.

Dynamic crotonylation of EB1 by TIP60 ensures accurate spindle positioning in mitosis

Spindle position control is essential for cell fate determination and organogenesis. Early studies indicate the essential role of the evolutionarily conserved Gαi/LGN/NuMA network in spindle positioning. However, the regulatory mechanisms that couple astral microtubules dynamics to the spindle orientation remain elusive. Here we delineated a new mitosis-specific crotonylation-regulated astral microtubule-EB1-NuMA interaction in mitosis. EB1 is a substrate of TIP60, and TIP60-dependent crotonylation of EB1 tunes accurate spindle positioning in mitosis. Mechanistically, TIP60 crotonylation of EB1 at Lys66 forms a dynamic link between accurate attachment of astral microtubules to the lateral cell cortex defined by NuMA-LGN and fine tune of spindle positioning. Real-time imaging of chromosome movements in HeLa cells expressing genetically encoded crotonylated EB1 revealed the importance of crotonylation dynamics for accurate control of spindle orientation during metaphase-anaphase transition. These findings delineate a general signaling cascade that integrates protein crotonylation with accurate spindle positioning for chromosome stability in mitosis.

TH588 and Low-Dose Nocodazole Impair Chromosome Congression by Suppressing Microtubule Turnover within the Mitotic Spindle

Simple Summary

A promising anti-cancer compound TH588 has been recently identified as a microtubule-targeting agent that inhibits tubulin polymerization in vitro and interferes with microtubule dynamics in interphase cells. Although it was shown to arrest cells in mitosis, its effect on microtubule dynamics in dividing cells remained unknown. By analyzing microtubule dynamics in living cells treated with either TH588 or low-dose nocodazole, we revealed that both of these drugs stabilize microtubules within the mitotic spindle, leading to premature formation of kinetochore-microtubule end-on attachments on uncongressed chromosomes. This causes mitotic arrest, ultimately resulting in cell death or cell division with uncongressed chromosomes. Both of these cell fates could contribute to the selective effect associated with the activity of TH588 in cancer cells.

Abstract

Microtubule-targeting agents (MTAs) have been used for decades to treat different hematologic and solid cancers. The mode of action of these drugs mainly relies on their ability to bind tubulin subunits and/or microtubules and interfere with microtubule dynamics. In addition to its MTH1-inhibiting activity, TH588 has been recently identified as an MTA, whose anticancer properties were shown to largely depend on its microtubule-targeting ability. Although TH588 inhibited tubulin polymerization in vitro and reduced microtubule plus-end mobility in interphase cells, its effect on microtubule dynamics within the mitotic spindle of dividing cells remained unknown. Here, we performed an in-depth analysis of the impact of TH588 on spindle-associated microtubules and compared it to the effect of low-dose nocodazole. We show that both treatments reduce microtubule turnover within the mitotic spindle. This microtubule-stabilizing effect leads to premature formation of kinetochore-microtubule end-on attachments on uncongressed chromosomes, which consequently cannot be transported to the cell equator, thereby delaying cell division and leading to cell death or division with uncongressed chromosomes.

Chemical Genetics Screen Identifies COPB2 Tool Compounds That Alters ER Stress Response and Induces RTK Dysregulation in Lung Cancer Cells

Activating mutations in the epidermal growth factor receptor (EGFR) are common driver mutations in non-small cell lung cancer (NSCLC). First, second and third generation EGFR tyrosine kinase inhibitors (TKIs) are effective at inhibiting mutant EGFR NSCLC, however, acquired resistance is a major issue, leading to disease relapse. Here, we characterize a small molecule, EMI66, an analog of a small molecule which we previously identified to inhibit mutant EGFR signalling via a novel mechanism of action. We show that EMI66 attenuates receptor tyrosine kinase (RTK) expression and signalling and alters the electrophoretic mobility of Coatomer Protein Complex Beta 2 (COPB2) protein in mutant EGFR NSCLC cells. Moreover, we demonstrate that EMI66 can alter the subcellular localization of EGFR and COPB2 within the early secretory pathway. Furthermore, we find that COPB2 knockdown reduces the growth of mutant EGFR lung cancer cells, alters the post-translational processing of RTKs, and alters the endoplasmic reticulum (ER) stress response pathway. Lastly, we show that EMI66 treatment also alters the ER stress response pathway and inhibits the growth of mutant EGFR lung cancer cells and organoids. Our results demonstrate that targeting of COPB2 with EMI66 presents a viable approach to attenuate mutant EGFR signalling and growth in NSCLC.

Gradual compaction of the central spindle decreases its dynamicity in PRC1 and EB1 gene-edited cells

Although different anaphase proteins bind with characteristically different strength to the central spindle, the overall central spindle dynamicity slows down as mitosis proceeds.

During mitosis, the spindle undergoes morphological and dynamic changes. It reorganizes at the onset of the anaphase when the antiparallel bundler PRC1 accumulates and recruits central spindle proteins to the midzone. Little is known about how the dynamic properties of the central spindle change during its morphological changes in human cells. Using gene editing, we generated human cells that express from their endogenous locus fluorescent PRC1 and EB1 to quantify their native spindle distribution and binding/unbinding turnover. EB1 plus end tracking revealed a general slowdown of microtubule growth, whereas PRC1, similar to its yeast orthologue Ase1, binds increasingly strongly to compacting antiparallel microtubule overlaps. KIF4A and CLASP1 bind more dynamically to the central spindle, but also show slowing down turnover. These results show that the central spindle gradually becomes more stable during mitosis, in agreement with a recent “bundling, sliding, and compaction” model of antiparallel midzone bundle formation in the central spindle during late mitosis.

Cyclical phosphorylation of LRAP35a and CLASP2 by GSK3β and CK1δ regulates EB1-dependent MT dynamics in cell migration

Mammalian cell cytoskeletal reorganization for efficient directional movement requires tight coordination of actomyosin and microtubule networks. In this study, we show that LRAP35a potentiates microtubule stabilization by promoting CLASP2/EB1 interaction besides its complex formation with MRCK/MYO18A for retrograde actin flow. The alternate regulation of these two networks by LRAP35a is tightly regulated by a series of phosphorylation events that dictated its specificity. Sequential phosphorylation of LRAP35a by Protein Kinase A (PKA) and Glycogen Synthase Kinase-3β (GSK3β) initiates the association of LRAP35a with CLASP2, while subsequent binding and further phosphorylation by Casein Kinase 1δ (CK1δ) induce their dissociation, which facilitates LRAP35a/MRCK association in driving lamellar actomyosin flow. Importantly, microtubule dynamics is directly moderated by CK1δ activity on CLASP2 to regulate GSK3β phosphorylation of the SxIP motifs that blocks EB1 binding, an event countered by LRAP35a interaction and its competition for CK1δ activity. Overall this study reveals an essential role for LRAP35a in coordinating lamellar contractility and microtubule polarization in cell migration.

Regulation of microtubule dynamics, mechanics and function through the growing tip

Microtubule dynamics and their control are essential for the normal function and division of all eukaryotic cells. This plethora of functions is, in large part, supported by dynamic microtubule tips, which can bind to various intracellular targets, generate mechanical forces and couple with actin microfilaments. Here, we review progress in the understanding of microtubule assembly and dynamics, focusing on new information about the structure of microtubule tips. First, we discuss evidence for the widely accepted GTP cap model of microtubule dynamics. Next, we address microtubule dynamic instability in the context of structural information about assembly intermediates at microtubule tips. Three currently discussed models of microtubule assembly and dynamics are reviewed. These are considered in the context of established facts and recent data, which suggest that some long-held views must be re-evaluated. Finally, we review structural observations about the tips of microtubules in cells and describe their implications for understanding the mechanisms of microtubule regulation by associated proteins, by mechanical forces and by microtubule-targeting drugs, prominently including cancer chemotherapeutics.

XMAP215 promotes microtubule catastrophe by disrupting the growing microtubule end

The GTP-tubulin cap is widely accepted to protect microtubules against catastrophe. The GTP-cap size is thought to increase with the microtubule growth rate, presumably endowing fast-growing microtubules with enhanced stability. It is unknown what GTP-cap properties permit frequent microtubule catastrophe despite fast growth. Here, we investigate microtubules growing in the presence and absence of the polymerase XMAP215. Using EB1 as a GTP-cap marker, we find that GTP-cap size increases regardless of whether growth acceleration is achieved by increasing tubulin concentration or by XMAP215. Despite increased mean GTP-cap size, microtubules grown with XMAP215 display increased catastrophe frequency, in contrast to microtubules grown with more tubulin, for which catastrophe is abolished. However, microtubules polymerized with XMAP215 have large fluctuations in growth rate; display tapered and curled ends; and undergo catastrophe at faster growth rates and with higher EB1 end-localization. Our results suggest that structural perturbations induced by XMAP215 override the protective effects of the GTP-cap, ultimately driving microtubule catastrophe.

Targeting the Microtubule EB1-CLASP2 Complex Modulates NaV1.5 at Intercalated Discs

[Figure: see text].

Differential regulation of single microtubules and bundles by a three-protein module

A remarkable feature of the microtubule cytoskeleton is co-existence of sub-populations having different dynamic properties. A prominent example is the anaphase spindle, where stable antiparallel bundles exist alongside dynamic microtubules and provide spatial cues for cytokinesis. How are dynamics of spatially proximal arrays differentially regulated? We reconstitute a minimal system of three midzone proteins: microtubule-crosslinker PRC1, and its interactors CLASP1 and Kif4A, proteins that promote and suppress microtubule elongation, respectively. We find their collective activity promotes elongation of single microtubules, while simultaneously stalling polymerization of crosslinked bundles. This differentiation arises from (i) Strong rescue activity of CLASP1, which overcomes weaker effects of Kif4A on single microtubules, (ii) Lower microtubule and PRC1-binding affinity of CLASP1, which permit dominance of Kif4A at overlaps. In addition to canonical mechanisms where antagonistic regulators set microtubule lengths, our findings illuminate design principles by which collective regulator activity creates microenvironments of arrays with distinct dynamic properties.

Anaphase A

Anaphase A is the motion of recently separated chromosomes to the spindle pole they face. It is accompanied by the shortening of kinetochore-attached microtubules. The requisite tubulin depolymerization may occur at kinetochores, at poles, or both, depending on the species and/or the time in mitosis. These depolymerization events are local and suggest that cells regulate microtubule dynamics in specific places, presumably by the localization of relevant enzymes and microtubule-associated proteins to specific loci, such as pericentriolar material and outer kinetochores. Motor enzymes can contribute to anaphase A, both by altering microtubule stability and by pushing or pulling microtubules through the cell. The generation of force on chromosomes requires couplings that can both withstand the considerable force that spindles can generate and simultaneously permit tubulin addition and loss. This chapter reviews literature on the molecules that regulate anaphase microtubule dynamics, couple dynamic microtubules to kinetochores and poles, and generate forces for microtubule and chromosome motion.

Stress-induced phosphorylation of CLIP-170 by JNK promotes microtubule rescue

The stress-induced c-Jun N-terminal kinase (JNK) controls microtubule dynamics by enhancing both microtubule growth and rescues. Here, we show that upon cell stress, JNK directly phosphorylates the microtubule rescue factor CLIP-170 in its microtubule-binding domain to increase its rescue-promoting activity. Phosphomimetic versions of CLIP-170 enhance its ability to promote rescue events in vitro and in cells. Furthermore, while phosphomimetic mutations do not alter CLIP-170’s capability to form comets at growing microtubule ends, both phosphomimetic mutations and JNK activation increase the occurrence of CLIP-170 remnants on the microtubule lattice at the rear of comets. As the CLIP-170 remnants, which are potential sites of microtubule rescue, display a shorter lifetime when CLIP-170 is phosphorylated, we propose that instead of acting at the time of rescue occurrence, CLIP-170 would rather contribute in preparing the microtubule lattice for future rescues at these predetermined sites.

Impact of the 'tubulin economy' on the formation and function of the microtubule cytoskeleton

The microtubule cytoskeleton is assembled from a finite pool of α,β-tubulin, the size of which is controlled by an autoregulation mechanism. Cells also tightly regulate the architecture and dynamic behavior of microtubule arrays. Here, we discuss progress in our understanding of how tubulin autoregulation is achieved and highlight work showing that tubulin, in its unassembled state, is relevant for regulating the formation and organization of microtubules. Emerging evidence suggests that tubulin regulates microtubule-associated proteins and kinesin motors that are critical for microtubule nucleation, dynamics, and function. These relationships create feedback loops that connect the tubulin assembly cycle to the organization and dynamics of microtubule networks. We term this concept the 'tubulin economy', which emphasizes the idea that tubulin is a resource that can be deployed for the immediate purpose of creating polymers, or alternatively as a signaling molecule that has more far-reaching consequences for the organization of microtubule arrays.

Kinesin-4 KIF21B limits microtubule growth to allow rapid centrosome polarization in T cells

When a T cell and an antigen-presenting cell form an immunological synapse, rapid dynein-driven translocation of the centrosome toward the contact site leads to reorganization of microtubules and associated organelles. Currently, little is known about how the regulation of microtubule dynamics contributes to this process. Here, we show that the knockout of KIF21B, a kinesin-4 linked to autoimmune disorders, causes microtubule overgrowth and perturbs centrosome translocation. KIF21B restricts microtubule length by inducing microtubule pausing typically followed by catastrophe. Catastrophe induction with vinblastine prevented microtubule overgrowth and was sufficient to rescue centrosome polarization in KIF21B-knockout cells. Biophysical simulations showed that a relatively small number of KIF21B molecules can restrict mirotubule length and promote an imbalance of dynein-mediated pulling forces that allows the centrosome to translocate past the nucleus. We conclude that proper control of microtubule length is important for allowing rapid remodeling of the cytoskeleton and efficient T cell polarization.

The immune system is composed of many types of cells that can recognize foreign molecules and pathogens so they can eliminate them. When cells in the body become infected with a pathogen, they can process the pathogen’s proteins and present them on their own surface. Specialized immune cells can then recognize infected cells and interact with them, forming an ‘immunological synapse’. These synapses play an important role in immune response: they activate the immune system and allow it to kill harmful cells.

To form an immunological synapse, an immune cell must reorganize its internal contents, including an aster-shaped scaffold made of tiny protein tubes called microtubules. The center of this scaffold moves towards the immunological synapse as it forms. This re-orientation of the microtubules towards the immunological synapse is known as 'polarization' and it happens very rapidly, but it is not yet clear how it works.

One molecule involved in the polarization process is called KIF21B, a protein that can walk along microtubules, building up at the ends and affecting their growth. Whether KIF21B makes microtubules grow more quickly, or more slowly, is a matter of debate, and the impact microtubule length has on immunological synapse formation is unknown.

Here, Hooikaas, Damstra et al. deleted the gene for KIF21B from human immune cells called T cells to find out how it affected their ability to form an immunological synapse. Without KIF21B, the T cells grew microtubules that were longer than normal, and had trouble forming immunological synapses. When the T cells were treated with a drug that stops microtubule growth, their ability to form immunological synapses was restored, suggesting a role for KIF21B. To explore this further, Hooikaas, Damstra et al. replaced the missing KIF21B gene with a gene that coded for a version of the protein that could be seen using microscopy. This revealed that, when KIF21B reaches the ends of microtubules, it stops their growth and triggers their disassembly. Computational modelling showed that cells find it hard to reorient their microtubule scaffolding when the individual tubes are too long. It only takes a small number of KIF21B molecules to shorten the microtubules enough to allow the center of the scaffold to move.

Research has linked the KIF21B gene to autoimmune conditions like multiple sclerosis. Microtubules also play an important role in cell division, a critical process driving all types of cancer. Drugs that affect microtubule growth are already available, and a deeper understanding of KIF21B and microtubule regulation in immune cells could help to improve treatments in the future.

Concerted action of kinesins KIF5B and KIF13B promotes efficient secretory vesicle transport to microtubule plus ends

Intracellular transport relies on multiple kinesins, but it is poorly understood which kinesins are present on particular cargos, what their contributions are and whether they act simultaneously on the same cargo. Here, we show that Rab6-positive secretory vesicles are transported from the Golgi apparatus to the cell periphery by kinesin-1 KIF5B and kinesin-3 KIF13B, which determine the location of secretion events. KIF5B plays a dominant role, whereas KIF13B helps Rab6 vesicles to reach freshly polymerized microtubule ends, to which KIF5B binds poorly, likely because its cofactors, MAP7-family proteins, are slow in populating these ends. Sub-pixel localization demonstrated that during microtubule plus-end directed transport, both kinesins localize to the vesicle front and can be engaged on the same vesicle. When vesicles reverse direction, KIF13B relocates to the middle of the vesicle, while KIF5B shifts to the back, suggesting that KIF5B but not KIF13B undergoes a tug-of-war with a minus-end directed motor.

Tubulin mutations in neurodevelopmental disorders as a tool to decipher microtubule function

Malformations of cortical development (MCDs) are a group of severe brain malformations associated with intellectual disability and refractory childhood epilepsy. Human missense heterozygous mutations in the 9 α-tubulin and 10 β-tubulin isoforms forming the heterodimers that assemble into microtubules (MTs) were found to cause MCDs. However, how a single mutated residue in a given tubulin isoform can perturb the entire microtubule population in a neuronal cell remains a crucial question. Here, we examined 85 MCD-associated tubulin mutations occurring in TUBA1A, TUBB2, and TUBB3 and their location in a three-dimensional (3D) microtubule cylinder. Mutations hitting residues exposed on the outer microtubule surface are likely to alter microtubule association with partners, while alteration of intradimer contacts may impair dimer stability and straightness. Other types of mutations are predicted to alter interdimer and lateral contacts, which are responsible for microtubule cohesion, rigidity, and dynamics. MCD-associated tubulin mutations surprisingly fall into all categories, thus providing unexpected insights into how a single mutation may impair microtubule function and elicit dominant effects in neurons.

Roles of TOG and jelly-roll domains of centrosomal protein CEP104 in its functions in cilium elongation and Hedgehog signaling

Primary cilia are generated through the extension of the microtubule-based axoneme. Centrosomal protein 104 (CEP104) localizes to the tip of the elongating axoneme, and CEP104 mutations are linked to a ciliopathy, Joubert syndrome. Thus, CEP104 has been implicated in ciliogenesis. However, the mechanism by which CEP104 regulates ciliogenesis remains elusive. We report here that CEP104 is critical for cilium elongation but not for initiating ciliogenesis. We also demonstrated that the tumor-overexpressed gene (TOG) domain of CEP104 exhibits microtubule-polymerizing activity and that this activity is essential for the cilium-elongating activity of CEP104. Knockdown/rescue experiments showed that the N-terminal jelly-roll (JR) fold partially contributes to cilium-elongating activity of CEP104, but neither the zinc-finger region nor the SXIP motif is required for this activity. CEP104 binds to a centriole-capping protein, CP110, through the zinc-finger region and to a microtubule plus-end-binding protein, EB1, through the SXIP motif, indicating that the binding of CP110 and EB1 is dispensable for the cilium-elongating activity of CEP104. Moreover, CEP104 depletion does not affect CP110 removal from the mother centriole, which suggests that CEP104 functions after the removal of CP110. Last, we also showed that CEP104 is required for the ciliary entry of Smoothened and export of GPR161 upon Hedgehog signal activation and that the TOG domain plays a critical role in this activity. Our results define the roles of the individual domains of CEP104 in its functions in cilium elongation and Hedgehog signaling and should enhance our understanding of the mechanism underlying CEP104 mutation-associated ciliopathies.

Differentially expressed transcripts and associated protein pathways in basilar artery smooth muscle cells of the high-salt intake-induced hypertensive rat

The pathology of cerebrovascular disorders, such as hypertension, is associated with genetic changes and dysfunction of basilar artery smooth muscle cells (BASMCs). Long-term high-salt diets have been associated with the development of hypertension. However, the molecular mechanisms underlying salt-sensitive hypertension-induced BASMC modifications have not been well defined, especially at the level of variations in gene transcription. Here, we utilized high-throughput sequencing and subsequent signaling pathway analyses to find a two–fold change or greater upregulated expression of 203 transcripts and downregulated expression of 165 transcripts in BASMCs derived from rats fed a high-salt diet compared with those from control rats. These differentially expressed transcripts were enriched in pathways involved in cellular, morphological, and structural plasticity, autophagy, and endocrine regulation. These transcripts changes in the BASMCs derived from high-salt intake–induced hypertensive rats may provide critical information about multiple cellular processes and biological functions that occur during the development of cerebrovascular disorders and provide potential new targets to help control or block the development of hypertension.