Doublecortin Is Excluded from Growing Microtubule Ends and Recognizes the GDP-Microtubule Lattice

Diverse microtubule-binding repeats regulate TPX2 activities at distinct locations within the spindle

TPX2 is an elongated molecule containing multiple α-helical repeats. It stabilizes microtubules (MTs), promotes MT nucleation, and is essential for spindle assembly. However, the molecular basis of how TPX2 performs these functions remains elusive. Here, we systematically characterized the MT-binding activities of all TPX2 modules individually and in combinations and investigated their respective contributions both in vitro and in cells. We show that TPX2 contains α-helical repeats with opposite preferences for "extended" and "compacted" tubulin dimer spacing, and their distinct combinations produce divergent outcomes, making TPX2 activity highly robust yet tunable. Importantly, a repeat group at the C terminus, R8-9, is the key determinant of the TPX2 function. It stabilizes MTs by promoting rescues in vitro and is critical in spindle assembly. We propose a model where TPX2 activities are spatially regulated via its diverse MT-binding repeats to accommodate its varied functions in distinct locations within the spindle. Furthermore, we reveal a synergy between TPX2 and HURP in stabilizing spindle MTs.

Doublecortin restricts neuronal branching by regulating tubulin polyglutamylation

Doublecortin is a neuronal microtubule-associated protein that regulates microtubule structure in neurons. Mutations in Doublecortin cause lissencephaly and subcortical band heterotopia by impairing neuronal migration. We use CRISPR/Cas9 to knock-out the Doublecortin gene in induced pluripotent stem cells and differentiate the cells into cortical neurons. DCX-KO neurons show reduced velocities of nuclear movements and an increased number of neurites early in neuronal development, consistent with previous findings. Neurite branching is regulated by a host of microtubule-associated proteins, as well as by microtubule polymerization dynamics. However, EB comet dynamics are unchanged in DCX-KO neurons. Rather, we observe a significant reduction in α-tubulin polyglutamylation in DCX-KO neurons. Polyglutamylation levels and neuronal branching are rescued by expression of Doublecortin or of TTLL11, an α-tubulin glutamylase. Using U2OS cells as an orthogonal model system, we show that DCX and TTLL11 act synergistically to promote polyglutamylation. We propose that Doublecortin acts as a positive regulator of α-tubulin polyglutamylation and restricts neurite branching. Our results indicate an unexpected role for Doublecortin in the homeostasis of the tubulin code.

StableMARK-decorated microtubules in cells have expanded lattices

Microtubules are crucial in cells and are regulated by various mechanisms like posttranslational modifications, microtubule-associated proteins, and tubulin isoforms. Recently, the conformation of the microtubule lattice has also emerged as a potential regulatory factor, but it has remained unclear to what extent different lattices co-exist within the cell. Using cryo-electron tomography, we find that, while most microtubules have a compacted lattice (∼41 Å monomer spacing), approximately a quarter of the microtubules displayed more expanded lattice spacings. The addition of the microtubule-stabilizing agent Taxol increased the lattice spacing of all microtubules, consistent with results on reconstituted microtubules. Furthermore, correlative cryo-light and electron microscopy revealed that the stable subset of microtubules labeled by StableMARK, a marker for stable microtubules, predominantly displayed a more expanded lattice spacing (∼41.9 Å), further suggesting a close connection between lattice expansion and microtubule stability. The coexistence of different lattices and their correlation with stability implicate lattice spacing as an important factor in establishing specific microtubule subsets.

Doublecortin reinforces microtubules to promote growth cone advance in soft environments

Doublecortin (DCX) is a microtubule (MT)-associated protein in immature neurons. DCX is essential for early brain development,1 and DCX mutations account for nearly a quarter of all cases of lissencephaly-spectrum brain malformations2,3 that arise from a neuronal migration failure through the developing cortex.4 By analyzing pathogenic DCX missense mutations in non-neuronal cells, we show that disruption of MT binding is central to DCX pathology. In human-induced pluripotent stem cell (hiPSC)-derived cortical i3Neurons, genome edited to express DCX-mEmerald from the endogenous locus, DCX-MT interactions polarize very early during neuron morphogenesis. DCX interacts with MTs through two conserved DCX domains5,6 that bind between protofilaments and adjacent tubulin dimers,7 a site that changes conformation during guanosine triphosphate (GTP) hydrolysis.8 Consequently and consistent with our previous results,5 DCX specifically binds straight growth cone MTs and is excluded from the GTP/guanosine diphosphate (GDP)-inorganic phosphate (Pi) cap recognized by end-binding proteins (EBs). Comparing MT-bound DCX fluorescence to mEmerald-tagged nanocage standards, we measure approximately one hundred DCX molecules per micrometer growth cone MT. DCX is required for i3Neuron growth cone advance in soft microenvironments that mimic the viscoelasticity of brain tissue, and using high-resolution traction force microscopy, we find that growth cones produce comparatively small and transient traction forces. Given our finding that DCX stabilizes MTs in the growth cone periphery by inhibiting MT depolymerization, we propose that DCX contributes to growth cone biomechanics and reinforces the growth cone cytoskeleton to counteract actomyosin-generated contractile forces in soft physiological environments in which weak and transient adhesion-mediated traction may be insufficient for productive growth cone advance.

Doublecortin reinforces microtubules to promote growth cone advance in soft environments

Doublecortin (DCX) is a microtubule-associated protein critical for brain development. Although most highly expressed in the developing central nervous system, the molecular function of DCX in neuron morphogenesis remains unknown and controversial. We demonstrate that DCX function is intimately linked to its microtubule-binding activity. By using human induced pluripotent stem cell (hiPSC)- derived cortical i 3 Neurons genome engineered to express mEmerald-tagged DCX from the endogenous locus, we find that DCX-MT interactions become highly polarized very early during neuron morphogenesis. DCX becomes enriched only on straight microtubules in advancing growth cones with approximately 120 DCX molecules bound per micrometer of growth cone microtubule. At a similar saturation, microtubule-bound DCX molecules begin to impede lysosome transport, and thus can potentially control growth cone organelle entry. In addition, by comparing control, DCX-mEmerald and knockout DCX -/Y i 3 Neurons, we find that DCX stabilizes microtubules in the growth cone peripheral domain by reducing the microtubule catastrophe frequency and the depolymerization rate. DCX -/Y i 3 Neuron morphogenesis was inhibited in soft microenvironments that mimic the viscoelasticity of brain tissue and DCX -/Y neurites failed to grow toward brain-derived neurotrophic factor (BDNF) gradients. Together with high resolution traction force microscopy data, we propose a model in which DCX-decorated, rigid growth cone microtubules provide intracellular mechanical resistance to actomyosin generated contractile forces in soft physiological environments in which weak and transient adhesion-mediated forces in the growth cone periphery may be insufficient for productive growth cone advance. These data provide a new mechanistic understanding of how DCX mutations cause lissencephaly-spectrum brain malformations by impacting growth cone dynamics during neuron morphogenesis in physiological environments.

Taxol acts differently on different tubulin isotypes

Taxol is a small molecule effector that allosterically locks tubulin into the microtubule lattice. We show here that taxol has different effects on different single-isotype microtubule lattices. Using in vitro reconstitution, we demonstrate that single-isotype α1β4 GDP-tubulin lattices are stabilised and expanded by 10 µM taxol, as reported by accelerated microtubule gliding in kinesin motility assays, whereas single-isotype α1β3 GDP-tubulin lattices are stabilised but not expanded. This isotype-specific action of taxol drives gliding of segmented-isotype GDP-taxol microtubules along convoluted, sinusoidal paths, because their expanded α1β4 segments try to glide faster than their compacted α1β3 segments. In GMPCPP, single-isotype α1β3 and α1β4 lattices both show accelerated gliding, indicating that both can in principle be driven to expand. We therefore propose that taxol-induced lattice expansion requires a higher taxol occupancy than taxol-induced stabilisation, and that higher taxol occupancies are accessible to α1β4 but not α1β3 single-isotype lattices.

Growth cone advance requires EB1 as revealed by genomic replacement with a light-sensitive variant

A challenge in analyzing dynamic intracellular cell biological processes is the dearth of methodologies that are sufficiently fast and specific to perturb intracellular protein activities. We previously developed a light-sensitive variant of the microtubule plus end-tracking protein EB1 by inserting a blue light-controlled protein dimerization module between functional domains. Here, we describe an advanced method to replace endogenous EB1 with this light-sensitive variant in a single genome editing step, thereby enabling this approach in human induced pluripotent stem cells (hiPSCs) and hiPSC-derived neurons. We demonstrate that acute and local optogenetic EB1 inactivation in developing cortical neurons induces microtubule depolymerization in the growth cone periphery and subsequent neurite retraction. In addition, advancing growth cones are repelled from areas of blue light exposure. These phenotypes were independent of the neuronal EB1 homolog EB3, revealing a direct dynamic role of EB1-mediated microtubule plus end interactions in neuron morphogenesis and neurite guidance.

Doublecortin and JIP3 are neural-specific counteracting regulators of dynein-mediated retrograde trafficking

Mutations in the microtubule (MT)-binding protein doublecortin (DCX) or in the MT-based molecular motor dynein result in lissencephaly. However, a functional link between DCX and dynein has not been defined. Here, we demonstrate that DCX negatively regulates dynein-mediated retrograde transport in neurons from Dcx-/y or Dcx-/y;Dclk1-/- mice by reducing dynein's association with MTs and disrupting the composition of the dynein motor complex. Previous work showed an increased binding of the adaptor protein C-Jun-amino-terminal kinase-interacting protein 3 (JIP3) to dynein in the absence of DCX. Using purified components, we demonstrate that JIP3 forms an active motor complex with dynein and its cofactor dynactin with two dyneins per complex. DCX competes with the binding of the second dynein, resulting in a velocity reduction of the complex. We conclude that DCX negatively regulates dynein-mediated retrograde transport through two critical interactions by regulating dynein binding to MTs and regulating the composition of the dynein motor complex.

Microtubule lattice spacing governs cohesive envelope formation of tau family proteins

Tau is an intrinsically disordered microtubule-associated protein (MAP) implicated in neurodegenerative disease. On microtubules, tau molecules segregate into two kinetically distinct phases, consisting of either independently diffusing molecules or interacting molecules that form cohesive 'envelopes' around microtubules. Envelopes differentially regulate lattice accessibility for other MAPs, but the mechanism of envelope formation remains unclear. Here we find that tau envelopes form cooperatively, locally altering the spacing of tubulin dimers within the microtubule lattice. Envelope formation compacted the underlying lattice, whereas lattice extension induced tau envelope disassembly. Investigating other members of the tau family, we find that MAP2 similarly forms envelopes governed by lattice spacing, whereas MAP4 cannot. Envelopes differentially biased motor protein movement, suggesting that tau family members could spatially divide the microtubule surface into functionally distinct regions. We conclude that the interdependent allostery between lattice spacing and cooperative envelope formation provides the molecular basis for spatial regulation of microtubule-based processes by tau and MAP2.

Proximity Labeling Reveals Spatial Regulation of the Anaphase-Promoting Complex/Cyclosome by a Microtubule Adaptor

The anaphase-promoting complex/cyclosome (APC/C) coordinates advancement through mitosis via temporally controlled polyubiquitination events. Despite the long-appreciated spatial organization of key events in mitosis mediated largely by cytoskeletal networks, the spatial regulation of APC/C, the major mitotic E3 ligase, is poorly understood. We describe a microtubule-resident protein, PLEKHA5, as an interactor of APC/C and spatial regulator of its activity in mitosis. Microtubule-localized proximity biotinylation tools revealed that PLEKHA5 depletion decreased APC/C association with the microtubule cytoskeleton, which prevented efficient loading of APC/C with its coactivator CDC20 and led to reduced APC/C E3 ligase activity. PLEKHA5 knockdown delayed mitotic progression, causing accumulation of APC/C substrates dependent upon the PLEKHA5-APC/C interaction in microtubules. We propose that PLEKHA5 functions as an adaptor of APC/C that promotes its subcellular localization to microtubules and facilitates its activation by CDC20, thus ensuring the timely turnover of key mitotic APC/C substrates and proper progression through mitosis.

Evidence for a HURP/EB free mixed-nucleotide zone in kinetochore-microtubules

Current models infer that the microtubule-based mitotic spindle is built from GDP-tubulin with small GTP caps at microtubule plus-ends, including those that attach to kinetochores, forming the kinetochore-fibres. Here we reveal that kinetochore-fibres additionally contain a dynamic mixed-nucleotide zone that reaches several microns in length. This zone becomes visible in cells expressing fluorescently labelled end-binding proteins, a known marker for GTP-tubulin, and endogenously-labelled HURP - a protein which we show to preferentially bind the GDP microtubule lattice in vitro and in vivo. We find that in mitotic cells HURP accumulates on the kinetochore-proximal region of depolymerising kinetochore-fibres, whilst avoiding recruitment to nascent polymerising K-fibres, giving rise to a growing "HURP-gap". The absence of end-binding proteins in the HURP-gaps leads us to postulate that they reflect a mixed-nucleotide zone. We generate a minimal quantitative model based on the preferential binding of HURP to GDP-tubulin to show that such a mixed-nucleotide zone is sufficient to recapitulate the observed in vivo dynamics of HURP-gaps.

Doublecortin engages the microtubule lattice through a cooperative binding mode involving its C-terminal domain

Doublecortin (DCX) is a microtubule (MT)-associated protein that regulates MT structure and function during neuronal development and mutations in DCX lead to a spectrum of neurological disorders. The structural properties of MT-bound DCX that explain these disorders are incompletely determined. Here, we describe the molecular architecture of the DCX–MT complex through an integrative modeling approach that combines data from X-ray crystallography, cryo-electron microscopy, and a high-fidelity chemical crosslinking method. We demonstrate that DCX interacts with MTs through its N-terminal domain and induces a lattice-dependent self-association involving the C-terminal structured domain and its disordered tail, in a conformation that favors an open, domain-swapped state. The networked state can accommodate multiple different attachment points on the MT lattice, all of which orient the C-terminal tails away from the lattice. As numerous disease mutations cluster in the C-terminus, and regulatory phosphorylations cluster in its tail, our study shows that lattice-driven self-assembly is an important property of DCX.

Visualising the cytoskeletal machinery in neuronal growth cones using cryo-electron tomography

Neurons extend axons to form the complex circuitry of the mature brain. This depends on the coordinated response and continuous remodelling of the microtubule and F-actin networks in the axonal growth cone. Growth cone architecture remains poorly understood at nanoscales. We therefore investigated mouse hippocampal neuron growth cones using cryo-electron tomography to directly visualise their three-dimensional subcellular architecture with molecular detail. Our data showed that the hexagonal arrays of actin bundles that form filopodia penetrate and terminate deep within the growth cone interior. We directly observed the modulation of these and other growth cone actin bundles by alteration of individual F-actin helical structures. Microtubules with blunt, slightly flared or gently curved ends predominated in the growth cone, frequently contained lumenal particles and exhibited lattice defects. Investigation of the effect of absence of doublecortin, a neurodevelopmental cytoskeleton regulator, on growth cone cytoskeleton showed no major anomalies in overall growth cone organisation or in F-actin subpopulations. However, our data suggested that microtubules sustained more structural defects, highlighting the importance of microtubule integrity during growth cone migration.

Summary: Cryo-electron tomographic reconstruction of neuronal growth cone subdomains reveals distinctive F-actin and microtubule cytoskeleton architectures and modulation at molecular detail.

The drebrin/EB3 pathway regulates cytoskeletal dynamics to drive neuritogenesis in embryonic cortical neurons

Co-ordinating the dynamic behaviour of actin filaments (F-actin) and microtubules in filopodia is an important underlying process in neuritogenesis, but the molecular pathways involved are ill-defined. The drebrin/end-binding protein 3 (EB3) pathway is a candidate pathway for linking F-actin to microtubules in filopodia. Drebrin binds F-actin and, simultaneously, the microtubule-binding protein EB3 when bound to microtubule plus-ends. We assessed the effect on neuritogenesis of gain- or loss-of-function of proteins in the drebrin/EB3 pathway in rat embryonic cortical neurons in culture. Loss-of-function of drebrin by gene editing or pharmacological inhibition of drebrin binding to F-actin reduced the number of dynamic microtubules in the cell periphery and simultaneously delayed the initiation of neuritogenesis, whereas over-expression of drebrin induced supernumerary neurites. Similarly, loss of EB3 inhibited neuritogenesis, whereas loss of end-binding protein 1 (EB1), a related protein that does not bind to drebrin, did not affect neuritogenesis. Over-expression of EB3, but not EB1, induced supernumerary neurites. We discovered that EB3 is more proximally located at dynamic microtubule plus-ends than EB1 in growth cone filopodia allowing for continuous microtubule elongation as the drebrin/EB3 pathway zippers microtubules to F-actin in filopodia. Finally, we showed that preventing the entry of dynamic microtubules into filopodia using a pharmacological inhibitor of microtubule dynamics is associated with a loss of EB3, but not EB1, from microtubule plus-ends and a concurrent attenuation of neuritogenesis. Collectively, these findings support the idea that neuritogenesis depends on microtubule/F-actin zippering in filopodia orchestrated by the drebrin/EB3 pathway.

Autoregulatory control of microtubule binding in doublecortin-like kinase 1

The microtubule-associated protein, doublecortin-like kinase 1 (DCLK1), is highly expressed in a range of cancers and is a prominent therapeutic target for kinase inhibitors. The physiological roles of DCLK1 kinase activity and how it is regulated remain elusive. Here, we analyze the role of mammalian DCLK1 kinase activity in regulating microtubule binding. We found that DCLK1 autophosphorylates a residue within its C-terminal tail to restrict its kinase activity and prevent aberrant hyperphosphorylation within its microtubule-binding domain. Removal of the C-terminal tail or mutation of this residue causes an increase in phosphorylation within the doublecortin domains, which abolishes microtubule binding. Therefore, autophosphorylation at specific sites within DCLK1 has diametric effects on the molecule's association with microtubules. Our results suggest a mechanism by which DCLK1 modulates its kinase activity to tune its microtubule-binding affinity. These results provide molecular insights for future therapeutic efforts related to DCLK1's role in cancer development and progression.

WDR62 regulates spindle dynamics as an adaptor protein between TPX2/Aurora A and katanin

WDR62 is a microcephaly-related, microtubule (MT)-associated protein (MAP) that localizes to the spindle pole and regulates spindle organization, but the underlying mechanisms remain elusive. Here, we show that WDR62 regulates spindle dynamics by recruiting katanin to the spindle pole and further reveal a TPX2–Aurora A–WDR62–katanin axis in cells. By combining cellular and in vitro experiments, we demonstrate that WDR62 shows preference for curved segments of dynamic GDP-MTs, as well as GMPCPP- and paclitaxel-stabilized MTs, suggesting that it recognizes extended MT lattice. Consistent with this property, WDR62 alone is inefficient in recruiting katanin to GDP-MTs, while WDR62 complexed with TPX2/Aurora A can potently promote katanin-mediated severing of GDP-MTs in vitro. In addition, the MT-binding affinity of WDR62 is autoinhibited through JNK phosphorylation-induced intramolecular interaction. We propose that WDR62 is an atypical MAP and functions as an adaptor protein between its recruiting factor TPX2/Aurora A and the effector katanin to orchestrate the regulation of spindle dynamics.

Pseudo-repeats in doublecortin make distinct mechanistic contributions to microtubule regulation

Doublecortin (DCX) is a neuronal microtubule-associated protein (MAP) indispensable for brain development. Its flexibly linked doublecortin (DC) domains-NDC and CDC-mediate microtubule (MT) nucleation and stabilization, but it is unclear how. Using high-resolution time-resolved cryo-EM, we mapped NDC and CDC interactions with tubulin at different MT polymerization stages and studied their functional effects on MT dynamics using TIRF microscopy. Although coupled, each DC repeat within DCX appears to have a distinct role in MT nucleation and stabilization: CDC is a conformationally plastic module that appears to facilitate MT nucleation and stabilize tubulin-tubulin contacts in the nascent MT lattice, while NDC appears to be favored along the mature lattice, providing MT stabilization. Our structures of MT-bound DC domains also explain in unprecedented detail the DCX mutation-related brain defects observed in the clinic. This modular composition of DCX reflects a common design principle among MAPs where pseudo-repeats of tubulin/MT binding elements chaperone or stabilize distinct conformational transitions to regulate distinct stages of MT dynamic instability.

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.

The Mechanism of Tubulin Assembly into Microtubules: Insights from Structural Studies

Microtubules are cytoskeletal components involved in pivotal eukaryotic functions such as cell division, ciliogenesis, and intracellular trafficking. They assemble from αβ-tubulin heterodimers and disassemble in a process called dynamic instability, which is driven by GTP hydrolysis. Structures of the microtubule and of soluble tubulin have been determined by cryo-EM and by X-ray crystallography, respectively. Altogether, these data define the mechanism of tubulin assembly-disassembly at atomic or near-atomic level. We review here the structural changes that occur during assembly, tubulin switching from a curved conformation in solution to a straight one in the microtubule core. We also present more subtle changes associated with GTP binding, leading to tubulin activation for assembly. Finally, we show how cryo-EM and X-ray crystallography are complementary methods to characterize the interaction of tubulin with proteins involved either in intracellular transport or in microtubule dynamics regulation.

Regulation of Neurogenesis by FGF Signaling and Neurogenin in the Invertebrate Chordate Ciona

Neurogenesis is a complex sequence of cellular processes and behaviors driven by the coordinated expression of conserved effectors. The bipolar tail neurons (BTNs) of Ciona develop according to a highly dynamic, yet highly stereotyped developmental program and thus could serve as an accessible model system for neurogenesis, including underlying cell behaviors like neuronal delamination, migration, and polarized axon outgrowth. Here we investigate both the upstream events that shape BTN neurogenesis through spatiotemporal regulation of the conserved proneural factor Neurog, spatiotemporal, and the gene expression profile of differentiating BTNs downstream of Neurog activity. We show that, although early FGF signaling is required for Neurog expression and BTN specification, Fgf8/17/18 is expressed in tail tip cells at later stages and suppresses sustained Neurog expression in the anterior BTN (aBTN) lineage, such that only one cell (the one furthest from the source of Fgf8/17/18) maintains Neurog expression and becomes a neuron. Curiously, Fgf8/17/18 might not affect neurogenesis of the posterior BTNs (pBTNs), which are in direct contact with the Fgf8/17/18-expressing cells. Finally, to profile gene expression associated with BTN neurogenesis we performed RNAseq of isolated BTN lineage cells in which BTN neurogenesis was enhanced or suppressed by perturbing Neurog function. This allowed us to identify several candidate genes that might play conserved roles in neurogenesis and neuronal migration in other animals, including mammals.

A doublecortin-domain protein of Toxoplasma and its orthologues bind to and modify the structure and organization of tubulin polymers

BACKGROUND

TgDCX is a doublecortin-domain protein associated with the conoid fibers, a set of strongly curved non-tubular tubulin-polymers in Toxoplasma. TgDCX deletion impairs conoid structure and parasite invasion. TgDCX contains two tubulin-binding domains: a partial P25α and the DCX/doublecortin domain. Orthologues are found in apicomplexans and their free-living relatives Chromera and Vitrella.

RESULTS

We report that isolated TgDCX-containing conoid fibers retain their pronounced curvature, but loss of TgDCX destabilizes the fibers. We crystallized and determined the 3D-structure of the DCX-domain, which is similar to those of human doublecortin and well-conserved among TgDCX orthologues. However, the orthologues vary widely in targeting to the conoid in Toxoplasma and in modulating microtubule organization in Xenopus cells. Several orthologues bind to microtubules in Xenopus cells, but only TgDCX generates short, strongly curved microtubule arcs. EM analysis shows microtubules decorated with TgDCX bundled into rafts, often bordered on one edge by a "C"-shaped incomplete tube. A Chromera orthologue closely mimics TgDCX targeting in Toxoplasma and binds to microtubules in Xenopus cells, but does not generate arcs or "C"-shaped tubes, and fails to rescue the defects of the TgDCX-knockout parasite.

CONCLUSIONS

These observations suggest that species-specific features of TgDCX enable it to generate strongly curved tubulin-polymers to support efficient host-cell invasion.

The model of local axon homeostasis - explaining the role and regulation of microtubule bundles in axon maintenance and pathology

Axons are the slender, cable-like, up to meter-long projections of neurons that electrically wire our brains and bodies. In spite of their challenging morphology, they usually need to be maintained for an organism's lifetime. This makes them key lesion sites in pathological processes of ageing, injury and neurodegeneration. The morphology and physiology of axons crucially depends on the parallel bundles of microtubules (MTs), running all along to serve as their structural backbones and highways for life-sustaining cargo transport and organelle dynamics. Understanding how these bundles are formed and then maintained will provide important explanations for axon biology and pathology. Currently, much is known about MTs and the proteins that bind and regulate them, but very little about how these factors functionally integrate to regulate axon biology. As an attempt to bridge between molecular mechanisms and their cellular relevance, we explain here the model of local axon homeostasis, based on our own experiments in Drosophila and published data primarily from vertebrates/mammals as well as C. elegans. The model proposes that (1) the physical forces imposed by motor protein-driven transport and dynamics in the confined axonal space, are a life-sustaining necessity, but pose a strong bias for MT bundles to become disorganised. (2) To counterbalance this risk, MT-binding and -regulating proteins of different classes work together to maintain and protect MT bundles as necessary transport highways. Loss of balance between these two fundamental processes can explain the development of axonopathies, in particular those linking to MT-regulating proteins, motors and transport defects. With this perspective in mind, we hope that more researchers incorporate MTs into their work, thus enhancing our chances of deciphering the complex regulatory networks that underpin axon biology and pathology.

Microtubules gate tau condensation to spatially regulate microtubule functions

Tau is an abundant microtubule-associated protein in neurons. Tau aggregation into insoluble fibrils is a hallmark of Alzheimer's disease and other types of dementia1, yet the physiological state of tau molecules within cells remains unclear. Using single-molecule imaging, we directly observe that the microtubule lattice regulates reversible tau self-association, leading to localized, dynamic condensation of tau molecules on the microtubule surface. Tau condensates form selectively permissible barriers, spatially regulating the activity of microtubule-severing enzymes and the movement of molecular motors through their boundaries. We propose that reversible self-association of tau molecules, gated by the microtubule lattice, is an important mechanism of the biological functions of tau, and that oligomerization of tau is a common property shared between the physiological and disease-associated forms of the molecule.

Tau repeat regions contain conserved histidine residues that modulate microtubule-binding in response to changes in pH

Tau, a member of the MAP2/tau family of microtubule-associated proteins, stabilizes and organizes axonal microtubules in healthy neurons. In neurodegenerative tauopathies, tau dissociates from microtubules and forms neurotoxic extracellular aggregates. MAP2/tau family proteins are characterized by three to five conserved, intrinsically disordered repeat regions that mediate electrostatic interactions with the microtubule surface. Here, we used molecular dynamics, microtubule-binding experiments, and live-cell microscopy, revealing that highly-conserved histidine residues near the C terminus of each microtubule-binding repeat are pH sensors that can modulate tau-microtubule interaction strength within the physiological intracellular pH range. We observed that at low pH (<7.5), these histidines are positively charged and interact with phenylalanine residues in a hydrophobic cleft between adjacent tubulin dimers. At higher pH (>7.5), tau deprotonation decreased binding to microtubules both in vitro and in cells. Electrostatic and hydrophobic characteristics of histidine were both required for tau-microtubule binding, as substitutions with constitutively and positively charged nonaromatic lysine or uncharged alanine greatly reduced or abolished tau-microtubule binding. Consistent with these findings, tau-microtubule binding was reduced in a cancer cell model with increased intracellular pH but was rapidly restored by decreasing the pH to normal levels. These results add detailed insights into the intracellular regulation of tau activity that may be relevant in both normal and pathological conditions.

Microtubule Plus End Dynamics - Do We Know How Microtubules Grow?: Cells boost microtubule growth by promoting distinct structural transitions at growing microtubule ends

Microtubules form a highly dynamic filament network in all eukaryotic cells. Individual microtubules grow by tubulin dimer subunit addition and frequently switch between phases of growth and shortening. These unique dynamics are powered by GTP hydrolysis and drive microtubule network remodeling, which is central to eukaryotic cell biology and morphogenesis. Yet, our knowledge of the molecular events at growing microtubule ends remains incomplete. Here, recent ultrastructural, biochemical and cell biological data are integrated to develop a realistic model of growing microtubule ends comprised of structurally distinct but biochemically overlapping zones. Proteins that recognize microtubule lattice conformations associated with specific tubulin guanosine nucleotide states may independently control major structural transitions at growing microtubule ends. A model is proposed in which tubulin dimer addition and subsequent closure of the MT wall are optimized in cells to achieve rapid physiological microtubule growth.

Microtubule structure by cryo-EM: snapshots of dynamic instability

The development of cryo-electron microscopy (cryo-EM) allowed microtubules to be captured in their solution-like state, enabling decades of insight into their dynamic mechanisms and interactions with binding partners. Cryo-EM micrographs provide 2D visualization of microtubules, and these 2D images can also be used to reconstruct the 3D structure of the polymer and any associated binding partners. In this way, the binding sites for numerous components of the microtubule cytoskeleton-including motor domains from many kinesin motors, and the microtubule-binding domains of dynein motors and an expanding collection of microtubule associated proteins-have been determined. The effects of various microtubule-binding drugs have also been studied. High-resolution cryo-EM structures have also been used to probe the molecular basis of microtubule dynamic instability, driven by the GTPase activity of β-tubulin. These studies have shown the conformational changes in lattice-confined tubulin dimers in response to steps in the tubulin GTPase cycle, most notably lattice compaction at the longitudinal inter-dimer interface. Although work is ongoing to define a complete structural model of dynamic instability, attention has focused on the role of gradual destabilization of lateral contacts between tubulin protofilaments, particularly at the microtubule seam. Furthermore, lower resolution cryo-electron tomography 3D structures are shedding light on the heterogeneity of microtubule ends and how their 3D organization contributes to dynamic instability. The snapshots of these polymers captured using cryo-EM will continue to provide critical insights into their dynamics, interactions with cellular components, and the way microtubules contribute to cellular functions in diverse physiological contexts.

The role of tubulin-tubulin lattice contacts in the mechanism of microtubule dynamic instability

Microtubules form from longitudinally and laterally assembling tubulin α-β dimers. The assembly induces strain in tubulin, resulting in cycles of microtubule catastrophe and regrowth. This 'dynamic instability' is governed by GTP hydrolysis that renders the microtubule lattice unstable, but it is unclear how. We used a human microtubule nucleating and stabilizing neuronal protein, doublecortin, and high-resolution cryo-EM to capture tubulin's elusive hydrolysis intermediate GDP•Pi state, alongside the prehydrolysis analog GMPCPP state and the posthydrolysis GDP state with and without an anticancer drug, Taxol. GTP hydrolysis to GDP•Pi followed by Pi release constitutes two distinct structural transitions, causing unevenly distributed compressions of tubulin dimers, thereby tightening longitudinal and loosening lateral interdimer contacts. We conclude that microtubule catastrophe is triggered because the lateral contacts can no longer counteract the strain energy stored in the lattice, while reinforcement of the longitudinal contacts may support generation of force.

Separating the effects of nucleotide and EB binding on microtubule structure

We report three high-resolution structures of microtubules in different nucleotide states—GMPCPP, GDP, and GTPγS—in the absence of any binding proteins, allowing us to separate the effects of nucleotide- and microtubule (MT)-associated protein (MAPs) binding on MT structure. End-binding (EB) proteins can bind and induce partial lattice compaction of a preformed GMPCPP-bound MT, a lattice type that is far from EBs’ ideal binding platform. We propose a model in which the MT lattice serves as a platform that integrates internal tubulin signals, such as nucleotide state, with outside signals, such as binding of MAPs. These global lattice rearrangements in turn affect the affinity of other MT partners and result in the exquisite regulation of the MT dynamics.

Microtubules (MTs) are polymers assembled from αβ-tubulin heterodimers that display the hallmark behavior of dynamic instability. MT dynamics are driven by GTP hydrolysis within the MT lattice, and are highly regulated by a number of MT-associated proteins (MAPs). How MAPs affect MTs is still not fully understood, partly due to a lack of high-resolution structural data on undecorated MTs, which need to serve as a baseline for further comparisons. Here we report three structures of MTs in different nucleotide states (GMPCPP, GDP, and GTPγS) at near-atomic resolution and in the absence of any binding proteins. These structures allowed us to differentiate the effects of nucleotide state versus MAP binding on MT structure. Kinesin binding has a small effect on the extended, GMPCPP-bound lattice, but hardly affects the compacted GDP-MT lattice, while binding of end-binding (EB) proteins can induce lattice compaction (together with lattice twist) in MTs that were initially in an extended and more stable state. We propose a MT lattice-centric model in which the MT lattice serves as a platform that integrates internal tubulin signals, such as nucleotide state, with outside signals, such as binding of MAPs or mechanical forces, resulting in global lattice rearrangements that in turn affect the affinity of other MT partners and result in the exquisite regulation of MT dynamics.

Domain swap in the C-terminal ubiquitin-like domain of human doublecortin

Doublecortin, a microtubule-associated protein that is only produced during neurogenesis, cooperatively binds to microtubules and stimulates microtubule polymerization and cross-linking by unknown mechanisms. A domain swap is observed in the crystal structure of the C-terminal domain of doublecortin. As determined by analytical ultracentrifugation, an open conformation is also present in solution. At higher concentrations, higher-order oligomers of the domain are formed. The domain swap and additional interfaces observed in the crystal lattice can explain the formation of doublecortin tetramers or multimers, in line with the analytical ultracentrifugation data. Taken together, the domain swap offers a mechanism for the observed cooperative binding of doublecortin to microtubules. Doublecortin-induced cross-linking of microtubules can be explained by the same mechanism. The effect of several mutations leading to lissencephaly and double-cortex syndrome can be traced to the domain swap and the proposed self-association of doublecortin.

Coming into Focus: Mechanisms of Microtubule Minus-End Organization

Microtubule organization has a crucial role in regulating cell architecture. The geometry of microtubule arrays strongly depends on the distribution of sites responsible for microtubule nucleation and minus-end attachment. In cycling animal cells, the centrosome often represents a dominant microtubule-organizing center (MTOC). However, even in cells with a radial microtubule system, many microtubules are not anchored at the centrosome, but are instead linked to the Golgi apparatus or other structures. Non-centrosomal microtubules predominate in many types of differentiated cell and in mitotic spindles. In this review, we discuss recent advances in understanding how the organization of centrosomal and non-centrosomal microtubule networks is controlled by proteins involved in microtubule nucleation and specific factors that recognize free microtubule minus ends and regulate their localization and dynamics.

Local control of intracellular microtubule dynamics by EB1 photodissociation

End-binding proteins, EBs, are adaptors that recruit functionally diverse +TIP proteins to growing microtubule plus ends. To test with high spatial and temporal accuracy how, when and where +TIP protein complexes contribute to dynamic cell biology, we developed a photo-inactivated EB1 variant (π-EB1) by inserting a blue light-sensitive protein-protein interaction module between the microtubule- and +TIP-binding domains of EB1. π-EB1 replaces endogenous EB1 function in the absence of blue light. In contrast, blue light-mediated π-EB1 photo-dissociation results in rapid +TIP complex disassembly, and acutely and reversibly attenuates microtubule growth independent of microtubule end association of the microtubule polymerase CKAP5 (ch-TOG, XMAP215). Local π-EB1 photo-dissociation allows subcellular microtubule dynamics control at the second and micrometre scale, and elicits aversive turning of migrating cancer cells. Importantly, light-mediated domain splitting can serve as template to optically control other intracellular protein activities.

A structural model for microtubule minus-end recognition and protection by CAMSAP proteins

CAMSAP and Patronin family members regulate microtubule minus-end stability and localization and thus organize noncentrosomal microtubule networks, which are essential for cell division, polarization and differentiation. Here, we found that the CAMSAP C-terminal CKK domain is widely present among eukaryotes and autonomously recognizes microtubule minus ends. Through a combination of structural approaches, we uncovered how mammalian CKK binds between two tubulin dimers at the interprotofilament interface on the outer microtubule surface. In vitro reconstitution assays combined with high-resolution fluorescence microscopy and cryo-electron tomography suggested that CKK preferentially associates with the transition zone between curved protofilaments and the regular microtubule lattice. We propose that minus-end-specific features of the interprotofilament interface at this site serve as the basis for CKK's minus-end preference. The steric clash between microtubule-bound CKK and kinesin motors explains how CKK protects microtubule minus ends against kinesin-13-induced depolymerization and thus controls the stability of free microtubule minus ends.

ReMAPping the microtubule landscape: How phosphorylation dictates the activities of microtubule-associated proteins

Classical microtubule-associated proteins (MAPs) were originally identified based on their co-purification with microtubules assembled from mammalian brain lysate. They have since been found to perform a range of functions involved in regulating the dynamics of the microtubule cytoskeleton. Most of these MAPs play integral roles in microtubule organization during neuronal development, microtubule remodeling during neuronal activity, and microtubule stabilization during neuronal maintenance. As a result, mutations in MAPs contribute to neurodevelopmental disorders, psychiatric conditions, and neurodegenerative diseases. MAPs are post-translationally regulated by phosphorylation depending on developmental time point and cellular context. Phosphorylation can affect the microtubule affinity, cellular localization, or overall function of a particular MAP and can thus have profound implications for neuronal health. Here we review MAP1, MAP2, MAP4, MAP6, MAP7, MAP9, tau, and DCX, and how each is regulated by phosphorylation in neuronal physiology and disease. Developmental Dynamics 247:138-155, 2018. © 2017 Wiley Periodicals, Inc.

Potential Role of Microtubule Stabilizing Agents in Neurodevelopmental Disorders

Neurodevelopmental disorders (NDDs) are characterized by neuroanatomical abnormalities indicative of corticogenesis disturbances. At the basis of NDDs cortical abnormalities, the principal developmental processes involved are cellular proliferation, migration and differentiation. NDDs are also considered “synaptic disorders” since accumulating evidence suggests that NDDs are developmental brain misconnection syndromes characterized by altered connectivity in local circuits and between brain regions. Microtubules and microtubule-associated proteins play a fundamental role in the regulation of basic neurodevelopmental processes, such as neuronal polarization and migration, neuronal branching and synaptogenesis. Here, the role of microtubule dynamics will be elucidated in regulating several neurodevelopmental steps. Furthermore, the correlation between abnormalities in microtubule dynamics and some NDDs will be described. Finally, we will discuss the potential use of microtubule stabilizing agents as a new pharmacological intervention for NDDs treatment.

Dynamic microtubule association of Doublecortin X (DCX) is regulated by its C-terminus

Doublecortin X (DCX), known to be essential for neuronal migration and cortical layering in the developing brain, is a 40 kDa microtubule (MT)-associated protein. DCX directly interacts with MTs via its two structured doublecortin (DC) domains, but the dynamics of this association and the possible regulatory roles played by the flanking unstructured regions remain poorly defined. Here, we employ quantitative fluorescence recovery after photobleaching (FRAP) protocols in living cells to reveal that DCX shows remarkably rapid and complete exchange within the MT network but that the removal of the C-terminal region significantly slows this exchange. We further probed how MT organization or external stimuli could additionally modulate DCX exchange dynamics. MT depolymerisation (nocodazole treatment) or stabilization (taxol treatment) further enhanced DCX exchange rates, however the exchange rates for the C-terminal truncated DCX protein were resistant to the impact of taxol-induced stabilization. Furthermore, in response to a hyperosmotic stress stimulus, DCX exchange dynamics were slowed, and again the C-terminal truncated DCX protein was resistant to the stimulus. Thus, the DCX dynamically associates with MTs in living cells and its C-terminal region plays important roles in the MT-DCX association.

Distinct Features of Doublecortin as a Marker of Neuronal Migration and Its Implications in Cancer Cell Mobility

Neuronal migration is a critical process in the development of the nervous system. Defects in the migration of the neurons are associated with diseases like lissencephaly, subcortical band heterotopia (SBH), and pachygyria. Doublecortin (DCX) is an essential factor in neurogenesis and mutations in this protein impairs neuronal migration leading to several pathological conditions. Although, DCX is capable of modulating and stabilizing microtubules (MTs) to ensure effective migration, the mechanisms involved in executing these functions remain poorly understood. Meanwhile, there are existing gaps regarding the processes that underlie tumor initiation and progression into cancer as well as the ability to migrate and invade normal cells. Several studies suggest that DCX is involved in cancer metastasis. Unstable interactions between DCX and MTs destabilizes cytoskeletal organization leading to disorganized movements of cells, a process which may be implicated in the uncontrolled migration of cancer cells. However, the underlying mechanism is complex and require further clarification. Therefore, exploring the importance and features known up to date about this molecule will broaden our understanding and shed light on potential therapeutic approaches for the associated neurological diseases. This review summarizes current knowledge about DCX, its features, functions, and relationships with other proteins. We also present an overview of its role in cancer cells and highlight the importance of studying its gene mutations.

Microtubule aging probed by microfluidics-assisted tubulin washout

Microtubule aging—the decrease of stability with age—is an interesting but mechanistically not understood property of microtubules. It constrains possible mechanisms of catastrophe induction and is believed to be crucial for length regulation. New in vitro experiments and model fits provide insight into the origin of microtubule aging.

Microtubules switch stochastically between phases of growth and shrinkage. The molecular mechanism responsible for the end of a growth phase, an event called catastrophe, is still not understood. The probability for a catastrophe to occur increases with microtubule age, putting constraints on the possible molecular mechanism of catastrophe induction. Here we used microfluidics-assisted fast tubulin washout experiments to induce microtubule depolymerization in a controlled manner at different times after the start of growth. We found that aging can also be observed in this assay, providing valuable new constraints against which theoretical models of catastrophe induction can be tested. We found that the data can be quantitatively well explained by a simple kinetic threshold model that assumes an age-dependent broadening of the protective cap at the microtubule end as a result of an evolving tapered end structure; this leads to a decrease of the cap density and its stability. This analysis suggests an intuitive picture of the role of morphological changes of the protective cap for the age dependence of microtubule stability.