Structural Basis of Formation of the Microtubule Minus-End-Regulating CAMSAP-Katanin Complex

Liquid-liquid phase separation of microtubule-binding proteins in the regulation of spindle assembly

Cell division is a highly regulated process essential for the accurate segregation of chromosomes. Central to this process is the assembly of a bipolar mitotic spindle, a highly dynamic microtubule (MT)-based structure responsible for chromosome movement. The nucleation and dynamics of MTs are intricately regulated by MT-binding proteins. Over the recent years, various MT-binding proteins have been reported to undergo liquid-liquid phase separation, forming either single- or multi-component condensates on MTs. Herein, we provide a comprehensive summary of the phase separation characteristics of these proteins. We underscore their critical roles in MT nucleation, spindle assembly and kinetochore-MT attachment during the cell division process. Furthermore, we discuss the current challenges and various remaining unsolved problems, highlights the ongoing research efforts aimed at a deeper understanding of the role of the phase separation process during spindle assembly and orientation. Our review aims to contribute to the collective knowledge in this area and stimulate further investigations that will enhance our comprehension of the intricate mechanisms governing cell division.

Microtubule polarity determines the lineage of embryonic neural precursor in zebrafish spinal cord

The phenomenal diversity of neuronal types in the central nervous system is achieved in part by the asymmetric division of neural precursors. In zebrafish neural precursors, asymmetric dispatch of Sara endosomes (with its Notch signaling cargo) functions as fate determinant which mediates asymmetric division. Here, we found two distinct pools of neural precursors based on Sara endosome inheritance and spindle-microtubule enrichment. Symmetric or asymmetric levels of spindle-microtubules drive differently Sara endosomes inheritance and predict neural precursor lineage. We uncover that CAMSAP2a/CAMSAP3a and KIF16Ba govern microtubule asymmetry and endosome motility, unveiling the heterogeneity of neural precursors. Using a plethora of physical and cell biological assays, we determined the physical parameters and molecular mechanisms behind microtubule asymmetries and biased endosome motility. Evolutionarily, the values of those parameters explain why all sensory organ precursor cells are asymmetric in flies while, in zebrafish spinal cord, two populations of neural precursors (symmetric vs asymmetric) are possible.

Microtubule-binding domains in Katanin p80 subunit are essential for severing activity in C. elegans

Microtubule-severing enzymes (MSEs), such as Katanin, Spastin, and Fidgetin play essential roles in cell division and neurogenesis. They damage the microtubule (MT) lattice, which can either destroy or amplify the MT cytoskeleton, depending on the cellular context. However, little is known about how they interact with their substrates. We have identified the microtubule-binding domains (MTBD) required for Katanin function in C. elegans. Katanin is a heterohexamer of dimers containing a catalytic subunit p60 and a regulatory subunit p80, both of which are essential for female meiotic spindle assembly. Here, we report that p80-like(MEI-2) dictates Katanin binding to MTs via two MTBDs composed of basic patches. Substituting these patches reduces Katanin binding to MTs, compromising its function in female meiotic-spindle assembly. Structural alignments of p80-like(MEI-2) with p80s from different species revealed that the MTBDs are evolutionarily conserved, even if the specific amino acids involved vary. Our findings highlight the critical importance of the regulatory subunit (p80) in providing MT binding to the Katanin complex.

CAMSAP2 and CAMSAP3 localize at microtubule intersections to regulate the spatial distribution of microtubules

Microtubule networks support many cellular processes and exhibit a highly ordered architecture. However, due to the limited axial resolution of conventional light microscopy, the structural features of these networks cannot be resolved in three-dimensional (3D) space. Here, we used customized ultra-high-resolution interferometric single-molecule localization microscopy to characterize the microtubule networks in Caco2 cells. We found that the calmodulin-regulated spectrin-associated proteins (CAMSAPs) localize at a portion of microtubule intersections. Further investigation showed that depletion of CAMSAP2 and CAMSAP3 leads to the narrowing of the inter-microtubule distance. Mechanistically, CAMSAPs recognize microtubule defects, which often occur near microtubule intersections, and then recruit katanin to remove the damaged microtubules. Therefore, the CAMSAP-katanin complex is a regulatory module for the distance between microtubules. Taken together, our results characterize the architecture of cellular microtubule networks in high resolution and provide molecular insights into how the 3D structure of microtubule networks is controlled.

Crystal structure of the Arabidopsis SPIRAL2 C-terminal domain reveals a p80-Katanin-like domain

Epidermal cells of dark-grown plant seedlings reorient their cortical microtubule arrays in response to blue light from a net lateral orientation to a net longitudinal orientation with respect to the long axis of cells. The molecular mechanism underlying this microtubule array reorientation involves katanin, a microtubule severing enzyme, and a plant-specific microtubule associated protein called SPIRAL2. Katanin preferentially severs longitudinal microtubules, generating seeds that amplify the longitudinal array. Upon severing, SPIRAL2 binds nascent microtubule minus ends and limits their dynamics, thereby stabilizing the longitudinal array while the lateral array undergoes net depolymerization. To date, no experimental structural information is available for SPIRAL2 to help inform its mechanism. To gain insight into SPIRAL2 structure and function, we determined a 1.8 Å resolution crystal structure of the Arabidopsis thaliana SPIRAL2 C-terminal domain. The domain is composed of seven core α-helices, arranged in an α-solenoid. Amino-acid sequence conservation maps primarily to one face of the domain involving helices α1, α3, α5, and an extended loop, the α6-α7 loop. The domain fold is similar to, yet structurally distinct from the C-terminal domain of Ge-1 (an mRNA decapping complex factor involved in P-body localization) and, surprisingly, the C-terminal domain of the katanin p80 regulatory subunit. The katanin p80 C-terminal domain heterodimerizes with the MIT domain of the katanin p60 catalytic subunit, and in metazoans, binds the microtubule minus-end factors CAMSAP3 and ASPM. Structural analysis predicts that SPIRAL2 does not engage katanin p60 in a mode homologous to katanin p80. The SPIRAL2 structure highlights an interesting evolutionary convergence of domain architecture and microtubule minus-end localization between SPIRAL2 and katanin complexes, and establishes a foundation upon which structure-function analysis can be conducted to elucidate the role of this domain in the regulation of plant microtubule arrays.

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.

CAMSAP2 localizes to the Golgi in islet β-cells and facilitates Golgi-ER trafficking

Glucose stimulation induces the remodeling of microtubules, which potentiates insulin secretion in pancreatic β-cells. CAMSAP2 binds to microtubule minus ends to stabilize microtubules in several cultured clonal cells. Here, we report that the knockdown of CAMSAP2 in primary β-cells reduces total insulin content and attenuates GSIS without affecting the releasability of insulin vesicles. Surprisingly, CAMSAP2 knockdown does not change microtubule stability. Unlike in cultured insulinoma cells, CAMSAP2 in primary β-cells predominantly localizes to the Golgi apparatus instead of microtubule minus ends. This novel localization is specific to primary β- but not α-cells and is independent of microtubule binding. Consistent with its specific localization at the Golgi, CAMSAP2 promotes efficient Golgi-ER trafficking in primary β-cells. Moreover, primary β-cells and insulinoma cells likely express different CAMSAP2 isoforms. We propose that a novel CAMSAP2 isoform in primary β-cells has a non-canonical function, which promotes Golgi-ER trafficking to support efficient production of insulin and secretion.

Antiviral signalling by a cyclic nucleotide activated CRISPR protease

CRISPR defence systems such as the well-known DNA-targeting Cas9 and the RNA-targeting type III systems are widespread in prokaryotes1,2. The latter orchestrates a complex antiviral response that is initiated through the synthesis of cyclic oligoadenylates after recognition of foreign RNA3-5. Among the large set of proteins that are linked to type III systems and predicted to bind cyclic oligoadenylates6,7, a CRISPR-associated Lon protease (CalpL) stood out to us. CalpL contains a sensor domain of the SAVED family7 fused to a Lon protease effector domain. However, the mode of action of this effector is unknown. Here we report the structure and function of CalpL and show that this soluble protein forms a stable tripartite complex with two other proteins, CalpT and CalpS, that are encoded on the same operon. After activation by cyclic tetra-adenylate (cA4), CalpL oligomerizes and specifically cleaves the MazF homologue CalpT, which releases the extracytoplasmic function σ factor CalpS from the complex. Our data provide a direct connection between CRISPR-based detection of foreign nucleic acids and transcriptional regulation. Furthermore, the presence of a SAVED domain that binds cyclic tetra-adenylate in a CRISPR effector reveals a link to the cyclic-oligonucleotide-based antiphage signalling system.

Crystal structure of the C-terminal domain of the plant-specific microtubule-associated protein Spiral2

Plant cells form microtubule arrays, called `cortical microtubules', beneath the plasma membrane which are critical for cell-wall organization and directional cell growth. Cortical microtubules are nucleated independently of centrosomes. Spiral2 is a land-plant-specific microtubule minus-end-targeting protein that stabilizes the minus ends by inhibiting depolymerization of the filament. Spiral2 possesses an N-terminal microtubule-binding domain and a conserved C-terminal domain whose function is unknown. In this study, the crystal structure of the conserved C-terminal domain of Spiral2 was determined using the single-wavelength anomalous dispersion method. Refinement of the model to a resolution of 2.2 Å revealed a helix-turn-helix fold with seven α-helices. The protein crystallized as a dimer, but SEC-MALS analysis showed the protein to be monomeric. A structural homology search revealed that the protein has similarity to the C-terminal domain of the katanin regulatory subunit p80. The structure presented here suggests that the C-terminal domain of Spiral2 represents a new class of microtubule dynamics modulator across the kingdom.

Unrestrained growth of correctly oriented microtubules instructs axonal microtubule orientation

In many eukaryotic cells, directed molecular transport occurs along microtubules. Within neuronal axons, transport over vast distances particularly relies on uniformly oriented microtubules, whose plus-ends point towards the distal axon tip (anterogradely polymerizing, or plus-end-out). However, axonal microtubules initially have mixed orientations, and how they orient during development is not yet fully understood. Using live imaging of primary Drosophila melanogaster neurons, we found that, in the distal part of the axon, catastrophe rates of plus-end-out microtubules were significantly reduced compared to those of minus-end-out microtubules. Physical modelling revealed that plus-end-out microtubules should therefore exhibit persistent long-term growth, while growth of minus-end-out microtubules should be limited, leading to a bias in overall axonal microtubule orientation. Using chemical and physical perturbations of microtubule growth and genetic perturbations of the anti -catastrophe factor p150, which was enriched in the distal axon tip, we confirmed that the enhanced growth of plus-end-out microtubules is critical for achieving uniform microtubule orientation. Computer simulations of axon development integrating the enhanced plus-end-out microtubule growth identified here with previously suggested mechanisms, that is, dynein-based microtubule sliding and augmin-mediated templating, correctly predicted the long-term evolution of axonal microtubule orientation as found in our experiments. Our study thus leads to a holistic explanation of how axonal microtubules orient uniformly, a prerequisite for efficient long-range transport essential for neuronal functioning.

For humans to be able to wiggle their toes, messages need to travel from the brain to the foot, a distance well over a meter in many adults. This is made possible by neurons, the cells that form the nervous system, which transmit electrical signals along long extensions called ‘axons’. Axons can only transmit signals if all the required molecules, which are produced in a part of the neuron known as the cell body, are ferried to the ends of the axons. This ferrying around of molecules is carried out by long, filamentous molecules called microtubules, which act as a directed carrier system, shuttling molecules along the axon, either towards or away from the cell body.

Microtubules can be thought of as asymmetrical rods. One end – known as the plus end – is dynamic and can undergo growth or shrinkage, while the other end – called the minus end – is stable. For transport along the axon to happen efficiently, microtubules in the neuron need to be oriented with their plus end pointing towards the ends of the axon. Microtubules in growing neurons develop this orientation, but how that is achieved is not fully understood.

To understand the basis of this cellular phenomenon, Jakobs, Zemel and Franze examined the behaviour of microtubules in developing neurons from fruit fly larvae. A fluorescent protein, which emits light when the microtubules are growing, helped the researchers visualise the plus end of microtubules, the microtubule orientation, and their growth in developing axons. This experiment showed that microtubules that had their plus end pointing towards the axon end shrank more slowly than those with the opposite orientation, leading them to grow longer. This resulted in a higher proportion of the correctly-oriented microtubules in the axon.

Treating the neurons with Nocodazole, a chemical that disrupts microtubule growth, or with sodium chloride, which changes the osmotic pressure, caused the microtubules that were oriented with their plus end towards the axon to grow less, and disrupted the uniform orientation of the microtubules in the axon.

The next step was to determine whether specific axonal proteins such as p150 – a protein that is enriched at the tip of the axon and decreases microtubule shrinkage rates – are involved in this process. Reducing the levels of p150 in fruit flies using molecular and genetic methods resulted in microtubules with their plus end pointing towards the axon tip shrinking faster, reducing the proportion of microtubules with this orientation in the axon. This role of proteins enriched in the axonal tip, along with previously discovered mechanisms, explains how microtubules align unidirectionally in axons.

These findings open new avenues of research into neurodegenerative diseases like Alzheimer’s and Parkinson’s, which might manifest due to a breakdown of transport along microtubules in neurons.

Opto-katanin, an optogenetic tool for localized, microtubule disassembly

Microtubules are cytoskeletal polymers that separate chromosomes during mitosis and serve as rails for intracellular transport and organelle positioning. Manipulation of microtubules is widely used in cell and developmental biology, but tools for precise subcellular spatiotemporal control of microtubules are currently lacking. Here, we describe a light-activated system for localized recruitment of the microtubule-severing enzyme katanin. This system, named opto-katanin, uses targeted illumination with blue light to induce rapid, localized, and reversible microtubule depolymerization. This tool allows precise clearing of a subcellular region of microtubules while preserving the rest of the microtubule network, demonstrating that regulation of katanin recruitment to microtubules is sufficient to control its severing activity. The tool is not toxic in the absence of blue light and can be used to disassemble both dynamic and stable microtubules in primary neurons as well as in dividing cells. We show that opto-katanin can be used to locally block vesicle transport and to clarify the dependence of organelle morphology and dynamics on microtubules. Specifically, our data indicate that microtubules are not required for the maintenance of the Golgi stacks or the tubules of the endoplasmic reticulum but are needed for the formation of new membrane tubules. Finally, we demonstrate that this tool can be applied to study the contribution of microtubules to cell mechanics by showing that microtubule bundles can exert forces constricting the nucleus.

Non-coding ribonucleic acid-mediated CAMSAP1 upregulation leads to poor prognosis with suppressed immune infiltration in liver hepatocellular carcinoma

Liver hepatocellular carcinoma (LIHC) is well-known for its unfavorable prognosis due to the lack of reliable diagnostic and prognostic biomarkers. Calmodulin-regulated spectrin-associated protein 1 (CAMSAP1) is a non-centrosomal microtubule minus-end binding protein that regulates microtubule dynamics. This study aims to investigate the specific role and mechanisms of CAMSAP1 in LIHC. We performed systematical analyses of CAMSAP1 and demonstrated that differential expression of CAMSAP1 is associated with genetic alteration and DNA methylation, and serves as a potential diagnostic and prognostic biomarker in some cancers, especially LIHC. Further evidence suggested that CAMSAP1 overexpression leads to adverse clinical outcomes in advanced LIHC. Moreover, the AC145207.5/LINC01748-miR-101–3p axis is specifically responsible for CAMSAP1 overexpression in LIHC. In addition to the previously reported functions in the cell cycle and regulation of actin cytoskeleton, CAMSAP1-related genes are enriched in cancer- and immune-associated pathways. As expected, CAMSAP1-associated LIHC is infiltrated in the suppressed immune microenvironment. Specifically, except for immune cell infiltration, it is significantly positively correlated with immune checkpoint genes, especially CD274 (PD-L1), and cancer-associated fibroblasts. Prediction of immune checkpoint blockade therapy suggests that these patients may benefit from therapy. Our study is the first to demonstrate that besides genetic alteration and DNA methylation, AC145207.5/LINC01748-miR-101-3p-mediated CAMSAP1 upregulation in advanced LIHC leads to poor prognosis with suppressed immune infiltration, representing a potential diagnostic and prognostic biomarker as well as a promising immunotherapy target for LIHC.

Comprehensive analysis of the human ESCRT-III-MIT domain interactome reveals new cofactors for cytokinetic abscission

The 12 related human ESCRT-III proteins form filaments that constrict membranes and mediate fission, including during cytokinetic abscission. The C-terminal tails of polymerized ESCRT-III subunits also bind proteins that contain Microtubule-Interacting and Trafficking (MIT) domains. MIT domains can interact with ESCRT-III tails in many different ways to create a complex binding code that is used to recruit essential cofactors to sites of ESCRT activity. Here, we have comprehensively and quantitatively mapped the interactions between all known ESCRT-III tails and 19 recombinant human MIT domains. We measured 228 pairwise interactions, quantified 60 positive interactions, and discovered 18 previously unreported interactions. We also report the crystal structure of the SPASTIN MIT domain in complex with the IST1 C-terminal tail. Three MIT enzymes were studied in detail and shown to: (1) localize to cytokinetic midbody membrane bridges through interactions with their specific ESCRT-III binding partners (SPASTIN-IST1, KATNA1-CHMP3, and CAPN7-IST1), (2) function in abscission (SPASTIN, KATNA1, and CAPN7), and (3) function in the 'NoCut' abscission checkpoint (SPASTIN and CAPN7). Our studies define the human MIT-ESCRT-III interactome, identify new factors and activities required for cytokinetic abscission and its regulation, and provide a platform for analyzing ESCRT-III and MIT cofactor interactions in all ESCRT-mediated processes.

SUMOylation of microtubule-cleaving enzyme KATNA1 promotes microtubule severing and neurite outgrowth

Katanin p60 ATPase-containing subunit A1 (KATNA1) is a microtubule-cleaving enzyme that regulates the development of neural protrusions through cytoskeletal rearrangements. However, the mechanism underlying the linkage of the small ubiquitin-like modifier (SUMO) protein to KATNA1 and how this modification regulates the development of neural protrusions is unclear. Here we discovered, using mass spectrometry analysis, that SUMO-conjugating enzyme UBC9, an enzyme necessary for the SUMOylation process, was present in the KATNA1 interactome. Moreover, GST-pull down and co-immunoprecipitation assays confirmed that KATNA1 and SUMO interact. We further demonstrated using immunofluorescence experiments that KATNA1 and the SUMO2 isoform colocalized in hippocampal neurites. We also performed a bioinformatics analysis of KATNA1 protein sequences to identify three potentially conserved SUMOylation sites (K77, K157, and K330) among vertebrates. Mutation of K330, but not K77 or K157, abolished KATNA1-induced microtubule severing and decreased the level of binding observed for KATNA1 and SUMO2. Cotransfection of SUMO2 and wildtype KATNA1 in COS7 cells increased microtubule severing, whereas no effect was observed after cotransfection with the K330R KATNA1 mutant. Furthermore, in cultured hippocampal neurons, overexpression of wildtype KATNA1 significantly promoted neurite outgrowth, whereas the K330R mutant eliminated this effect. Taken together, our results demonstrate that the K330 site in KATNA1 is modified by SUMOylation and SUMOylation of KATNA1 promotes microtubule dynamics and hippocampal neurite outgrowth.

Genetic Control of Kinetochore-Driven Microtubule Growth in Drosophila Mitosis

Centrosome-containing cells assemble their spindles exploiting three main classes of microtubules (MTs): MTs nucleated by the centrosomes, MTs generated near the chromosomes/kinetochores, and MTs nucleated within the spindle by the augmin-dependent pathway. Mammalian and Drosophila cells lacking the centrosomes generate MTs at kinetochores and eventually form functional bipolar spindles. However, the mechanisms underlying kinetochore-driven MT formation are poorly understood. One of the ways to elucidate these mechanisms is the analysis of spindle reassembly following MT depolymerization. Here, we used an RNA interference (RNAi)-based reverse genetics approach to dissect the process of kinetochore-driven MT regrowth (KDMTR) after colcemid-induced MT depolymerization. This MT depolymerization procedure allows a clear assessment of KDMTR, as colcemid disrupts centrosome-driven MT regrowth but not KDMTR. We examined KDMTR in normal Drosophila S2 cells and in S2 cells subjected to RNAi against conserved genes involved in mitotic spindle assembly: mast/orbit/chb (CLASP1), mei-38 (TPX2), mars (HURP), dgt6 (HAUS6), Eb1 (MAPRE1/EB1), Patronin (CAMSAP2), asp (ASPM), and Klp10A (KIF2A). RNAi-mediated depletion of Mast/Orbit, Mei-38, Mars, Dgt6, and Eb1 caused a significant delay in KDMTR, while loss of Patronin had a milder negative effect on this process. In contrast, Asp or Klp10A deficiency increased the rate of KDMTR. These results coupled with the analysis of GFP-tagged proteins (Mast/Orbit, Mei-38, Mars, Eb1, Patronin, and Asp) localization during KDMTR suggested a model for kinetochore-dependent spindle reassembly. We propose that kinetochores capture the plus ends of MTs nucleated in their vicinity and that these MTs elongate at kinetochores through the action of Mast/Orbit. The Asp protein binds the MT minus ends since the beginning of KDMTR, preventing excessive and disorganized MT regrowth. Mei-38, Mars, Dgt6, Eb1, and Patronin positively regulate polymerization, bundling, and stabilization of regrowing MTs until a bipolar spindle is reformed.

CAMSAP2 organizes a γ-tubulin-independent microtubule nucleation centre through phase separation

Microtubules are dynamic polymers consisting of αβ-tubulin heterodimers. The initial polymerization process, called microtubule nucleation, occurs spontaneously via αβ-tubulin. Since a large energy barrier prevents microtubule nucleation in cells, the γ-tubulin ring complex is recruited to the centrosome to overcome the nucleation barrier. However, a considerable number of microtubules can polymerize independently of the centrosome in various cell types. Here, we present evidence that the minus-end-binding calmodulin-regulated spectrin-associated protein 2 (CAMSAP2) serves as a strong nucleator for microtubule formation by significantly reducing the nucleation barrier. CAMSAP2 co-condensates with αβ-tubulin via a phase separation process, producing plenty of nucleation intermediates. Microtubules then radiate from the co-condensates, resulting in aster-like structure formation. CAMSAP2 localizes at the co-condensates and decorates the radiating microtubule lattices to some extent. Taken together, these in vitro findings suggest that CAMSAP2 supports microtubule nucleation and growth by organizing a nucleation centre as well as by stabilizing microtubule intermediates and growing microtubules.

Cells are able to hold their shape thanks to tube-like structures called microtubules that are made of hundreds of tubulin proteins. Microtubules are responsible for maintaining the uneven distribution of molecules throughout the cell, a phenomenon known as polarity that allows cells to differentiate into different types with various roles.

A protein complex called the γ-tubulin ring complex (γ-TuRC) is necessary for microtubules to form. This protein helps bind the tubulin proteins together and stabilises microtubules. However, recent research has found that in highly polarized cells such as neurons, which have highly specialised regions, microtubules can form without γ-TuRC. Searching for the proteins that could be filling in for γ-TuRC in these cells some evidence has suggested that a group known as CAMSAPs may be involved, but it is not known how.

To characterize the role of CAMSAPs, Imasaki, Kikkawa et al. studied how one of these proteins, CAMSAP2, interacts with tubulins. To do this, they reconstituted both CAMSAP2 and tubulins using recombinant biotechnology and mixed them in solution. These experiments showed that CAMSAP2 can help form microtubules by bringing together their constituent proteins so that they can bind to each other more easily. Once microtubules start to form, CAMSAP2 continues to bind to them, stabilizing them and enabling them to grow to full size.

These results shed light on how polarity is established in cells such as neurons, muscle cells, and epithelial cells. Additionally, the ability to observe intermediate structures during microtubule formation can provide insights into the processes that these structures are involved in.

Vestibular Hair Cells Require CAMSAP3, a Microtubule Minus-End Regulator, for Formation of Normal Kinocilia

Kinocilia are exceptionally long primary sensory cilia located on vestibular hair cells, which are essential for transmitting key signals that contribute to mammalian balance and overall vestibular system function. Kinocilia have a “9+2” microtubule (MT) configuration with nine doublet MTs surrounding two central singlet MTs. This is uncommon as most mammalian primary sensory cilia have a “9+0” configuration, in which the central MT pair is absent. It has yet to be determined what the function of the central MT pair is in kinocilia. Calmodulin-regulated spectrin-associated protein 3 (CAMSAP3) regulates the minus end of MTs and is essential for forming the central MT pair in motile cilia, which have the “9+2” configuration. To explore the role of the central MT pair in kinocilia, we created a conditional knockout model (cKO), Camsap3-cKO, which intended to eliminate CAMSAP3 in limited organs including the inner ear, olfactory bulb, and kidneys. Immunofluorescent staining of vestibular organs demonstrated that CAMSAP3 proteins were significantly reduced in Camsap3-cKO mice and that aged Camsap3-cKO mice had significantly shorter kinocilia than their wildtype littermates. Transmission electron microscopy showed that aged Camsap3-cKO mice were in fact missing that the central MT pair in kinocilia more often than their wildtype counterparts. In the examination of behavior, wildtype and Camsap3-cKO mice performed equally well on a swim assessment, right-reflex test, and evaluation of balance on a rotarod. However, Camsap3-cKO mice showed slightly altered gaits including reduced maximal rate of change of paw area and a smaller paw area in contact with the surface. Although Camsap3-cKO mice had no differences in olfaction from their wildtype counterparts, Camsap3-cKO mice did have kidney dysfunction that deteriorated their health. Thus, CAMSAP3 is important for establishing and/or maintaining the normal structure of kinocilia and kidney function but is not essential for normal olfaction. Our data supports our hypothesis that CAMSAP3 is critical for construction of the central MT pair in kinocilia, and that the central MT pair may be important for building long and stable axonemes in these kinocilia. Whether shorter kinocilia might lead to abnormal vestibular function and altered gaits in older Camsap3-cKO mice requires further investigation.

Finding a right place to cut: How katanin is targeted to cellular severing sites

Microtubule severing by katanin plays key roles in generating various array patterns of dynamic microtubules, while also responding to developmental and environmental stimuli. Quantitative imaging and molecular genetic analyses have uncovered that dysfunction of microtubule severing in plant cells leads to defects in anisotropic growth, division and other cell processes. Katanin is targeted to several subcellular severing sites. Intersections of two crossing cortical microtubules attract katanin, possibly by using local lattice deformation as a landmark. Cortical microtubule nucleation sites on preexisting microtubules are targeted for katanin-mediated severing. An evolutionary conserved microtubule anchoring complex not only stabilises the nucleated site, but also subsequently recruits katanin for timely release of a daughter microtubule. During cytokinesis, phragmoplast microtubules are severed at distal zones by katanin, which is tethered there by plant-specific microtubule-associated proteins. Recruitment and activation of katanin are essential for maintenance and reorganisation of plant microtubule arrays.

Microtubule-dependent subcellular organisation of pluripotent cells

With the advancement of cutting-edge live imaging technologies, microtubule remodelling has evolved as an integral regulator for the establishment of distinct differentiated cells. However, despite their fundamental role in cell structure and function, microtubules have received less attention when unravelling the regulatory circuitry of pluripotency. Here, we summarise the role of microtubule organisation and microtubule-dependent events required for the formation of pluripotent cells in vivo by deciphering the process of early embryogenesis: from fertilisation to blastocyst. Furthermore, we highlight current advances in elucidating the significance of specific microtubule arrays in in vitro culture systems of pluripotent stem cells and how the microtubule cytoskeleton serves as a highway for the precise intracellular movement of organelles. This Review provides an informed understanding of the intrinsic role of subcellular architecture of pluripotent cells and accentuates their regenerative potential in combination with innovative light-inducible microtubule techniques.

Wdr47, Camsaps, and Katanin cooperate to generate ciliary central microtubules

The axonemal central pair (CP) are non-centrosomal microtubules critical for planar ciliary beat. How they form, however, is poorly understood. Here, we show that mammalian CP formation requires Wdr47, Camsaps, and microtubule-severing activity of Katanin. Katanin severs peripheral microtubules to produce central microtubule seeds in nascent cilia. Camsaps stabilize minus ends of the seeds to facilitate microtubule outgrowth, whereas Wdr47 concentrates Camsaps into the axonemal central lumen to properly position central microtubules. Wdr47 deficiency in mouse multicilia results in complete loss of CP, rotatory beat, and primary ciliary dyskinesia. Overexpression of Camsaps or their microtubule-binding regions induces central microtubules in Wdr47^-/- ependymal cells but at the expense of low efficiency, abnormal numbers, and wrong location. Katanin levels and activity also impact the central microtubule number. We propose that Wdr47, Camsaps, and Katanin function together for the generation of non-centrosomal microtubule arrays in polarized subcellular compartments.

WDR47 protects neuronal microtubule minus ends from katanin-mediated severing

Axons and dendrites are long extensions of neurons that contain arrays of noncentrosomal microtubules. Calmodulin-regulated spectrin-associated proteins (CAMSAPs) bind to and stabilize free microtubule minus ends and are critical for proper neuronal development and function. Previous studies have shown that the microtubule-severing ATPase katanin interacts with CAMSAPs and limits the length of CAMSAP-decorated microtubule stretches. However, how CAMSAP and microtubule minus end dynamics are regulated in neurons is poorly understood. Here, we show that the neuron-enriched protein WDR47 interacts with CAMSAPs and is critical for axon and dendrite development. We find that WDR47 accumulates at CAMSAP2-decorated microtubules, is essential for maintaining CAMSAP2 stretches, and protects minus ends from katanin-mediated severing. We propose a model where WDR47 protects CAMSAP2 at microtubule minus ends from katanin activity to ensure proper stabilization of the neuronal microtubule network.

A chemical genetics approach to examine the functions of AAA proteins

The structural conservation across the AAA (ATPases associated with diverse cellular activities) protein family makes designing selective chemical inhibitors challenging. Here, we identify a triazolopyridine-based fragment that binds the AAA domain of human katanin, a microtubule-severing protein. We have developed a model for compound binding and designed ASPIR-1 (allele-specific, proximity-induced reactivity-based inhibitor-1), a cell-permeable compound that selectively inhibits katanin with an engineered cysteine mutation. Only in cells expressing mutant katanin does ASPIR-1 treatment increase the accumulation of CAMSAP2 at microtubule minus ends, confirming specific on-target cellular activity. Importantly, ASPIR-1 also selectively inhibits engineered cysteine mutants of human VPS4B and FIGL1-AAA proteins, involved in organelle dynamics and genome stability, respectively. Structural studies confirm our model for compound binding at the AAA ATPase site and the proximity-induced reactivity-based inhibition. Together, our findings suggest a chemical genetics approach to decipher AAA protein functions across essential cellular processes and to test hypotheses for developing therapeutics.

CAMSAP3 facilitates basal body polarity and the formation of the central pair of microtubules in motile cilia

Cilia are composed of hundreds of proteins whose identities and functions are far from being completely understood. In this study, we determined that calmodulin-regulated spectrin-associated protein 3 (CAMSAP3) plays an important role for the function of motile cilia in multiciliated cells (MCCs). Global knockdown of CAMSAP3 protein expression in mice resulted in defects in ciliary structures, polarity, and synchronized beating in MCCs. These animals also displayed signs and symptoms reminiscent of primary ciliary dyskinesia (PCD), including a mild form of hydrocephalus, subfertility, and impaired mucociliary clearance that leads to hyposmia, anosmia, rhinosinusitis, and otitis media. Functional characterization of CAMSAP3 enriches our understanding of the molecular mechanisms underlying the generation and function of motile cilia in MCCs.

Synchronized beating of cilia on multiciliated cells (MCCs) generates a directional flow of mucus across epithelia. This motility requires a “9 + 2” microtubule (MT) configuration in axonemes and the unidirectional array of basal bodies of cilia on the MCCs. However, it is not fully understood what components are needed for central MT-pair assembly as they are not continuous with basal bodies in contrast to the nine outer MT doublets. In this study, we discovered that a homozygous knockdown mouse model for MT minus-end regulator calmodulin-regulated spectrin-associated protein 3 (CAMSAP3), Camsap3^tm1a/tm1a, exhibited multiple phenotypes, some of which are typical of primary ciliary dyskinesia (PCD), a condition caused by motile cilia defects. Anatomical examination of Camsap3^tm1a/tm1a mice revealed severe nasal airway blockage and abnormal ciliary morphologies in nasal MCCs. MCCs from different tissues exhibited defective synchronized beating and ineffective generation of directional flow likely underlying the PCD-like phenotypes. In normal mice, CAMSAP3 localized to the base of axonemes and at the basal bodies in MCCs. However, in Camsap3^tm1a/tm1a, MCCs lacked CAMSAP3 at the ciliary base. Importantly, the central MT pairs were missing in the majority of cilia, and the polarity of the basal bodies was disorganized. These phenotypes were further confirmed in MCCs of Xenopus embryos when CAMSAP3 expression was knocked down by morpholino injection. Taken together, we identified CAMSAP3 as being important for the formation of central MT pairs, proper orientation of basal bodies, and synchronized beating of motile cilia.

A perinuclear microtubule-organizing centre controls nuclear positioning and basement membrane secretion

Non-centrosomal microtubule-organizing centres (ncMTOCs) have a variety of roles presumed to serve the diverse functions of the range of cell types in which they are found. ncMTOCs are diverse in their composition, subcellular localization, and function. Here we report a perinuclear MTOC in Drosophila fat body cells that is anchored by Msp300/Nesprin at the cytoplasmic surface of the nucleus. Msp300 recruits the MT minus-end protein Patronin/CAMSAP, which functions redundantly with Ninein to further recruit the MT polymerase Msps/XMAP215 to assemble non-centrosomal MTs and does so independently of the widespread MT nucleation factor γ-tubulin. Functionally, the fat body ncMTOC and the radial MT arrays it organizes is essential for nuclear positioning and for secretion of basement membrane components via retrograde dynein-dependent endosomal trafficking that restricts plasma membrane growth. Together, this study identifies a perinuclear ncMTOC with unique architecture and MT regulation properties that serves vital functions.

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.

The Microtubule Severing Protein Katanin Regulates Proliferation of Neuronal Progenitors in Embryonic and Adult Neurogenesis

Microtubule severing regulates cytoskeletal rearrangement underlying various cellular functions. Katanin, a heterodimer, consisting of catalytic (p60) and regulatory (p80) subunits severs dynamic microtubules to modulate several stages of cell division. The role of p60 katanin in the mammalian brain with respect to embryonic and adult neurogenesis is poorly understood. Here, we generated a Katna1 knockout mouse and found that consistent with a critical role of katanin in mitosis, constitutive homozygous Katna1 depletion is lethal. Katanin p60 haploinsufficiency induced an accumulation of neuronal progenitors in the subventricular zone during corticogenesis, and impaired their proliferation in the adult hippocampus dentate gyrus (DG) subgranular zone. This did not compromise DG plasticity or spatial and contextual learning and memory tasks employed in our study, consistent with the interpretation that adult neurogenesis may be associated with selective forms of hippocampal-dependent cognitive processes. Our data identify a critical role for the microtubule-severing protein katanin p60 in regulating neuronal progenitor proliferation in vivo during embryonic development and adult neurogenesis.

Crystal Structure of a Heterotetrameric Katanin p60:p80 Complex

Katanin is a microtubule-severing enzyme that is crucial for many cellular processes. Katanin consists of two subunits, p60 and p80, that form a stable complex. The interaction between subunits is mediated by the p60 N-terminal microtubule-interacting and -trafficking domain (p60-MIT) and the p80 C-terminal domain (p80-CTD). Here, we performed a biophysical characterization of the mouse p60-MIT:p80-CTD heterodimer and show that this complex can assemble into heterotetramers. We identified two mutations that enhance heterotetramer formation and determined the X-ray crystal structure of this mutant complex. The structure revealed a domain-swapped heterotetramer consisting of two p60-MIT:p80-CTD heterodimers. Structure-based sequence alignments suggest that heterotetramerization of katanin might be a common feature of various species. Furthermore, we show that enhanced heterotetramerization of katanin impairs its microtubule end-binding properties and increases the enzyme's microtubule lattice binding and severing activities. Therefore, our findings suggest the existence of different katanin oligomers that possess distinct functional properties.

The role of Patronin in Drosophila mitosis

Background

The calmodulin-regulated spectrin-associated proteins (CAMSAPs) belong to a conserved protein family, which includes members that bind the polymerizing mcrotubule (MT) minus ends and remain associated with the MT lattice formed by minus end polymerization. Only one of the three mammalian CAMSAPs, CAMSAP1, localizes to the mitotic spindle but its function is unclear. In Drosophila, there is only one CAMSAP, named Patronin. Previous work has shown that Patronin stabilizes the minus ends of non-mitotic MTs and is required for proper spindle elongation. However, the precise role of Patronin in mitotic spindle assembly is poorly understood.

Results

Here we have explored the role of Patronin in Drosophila mitosis using S2 tissue culture cells as a model system. We show that Patronin associates with different types of MT bundles within the Drosophila mitotic spindle, and that it is required for their stability. Imaging of living cells expressing Patronin-GFP showed that Patronin displays a dynamic behavior. In prometaphase cells, Patronin accumulates on short segments of MT bundles located near the chromosomes. These Patronin “seeds” extend towards the cell poles and stop growing just before reaching the poles. Our data also suggest that Patronin localization is largely independent of proteins acting at the MT minus ends such as Asp and Klp10A.

Conclusion

Our results suggest a working hypothesis about the mitotic role of Patronin. We propose that Patronin binds the minus ends within MT bundles, including those generated from the walls of preexisting MTs via the augmin-mediated pathway. This would help maintaining MT association within the mitotic bundles, thereby stabilizing the spindle structure. Our data also raise the intriguing possibility that the minus ends of bundled MTs can undergo a limited polymerization.

Electronic supplementary material

The online version of this article (10.1186/s12860-019-0189-0) contains supplementary material, which is available to authorized users.

Patronin governs minus-end-out orientation of dendritic microtubules to promote dendrite pruning in Drosophila

Class IV ddaC neurons specifically prune larval dendrites without affecting axons during Drosophila metamorphosis. ddaCs distribute the minus ends of microtubules (MTs) to dendrites but the plus ends to axons. However, a requirement of MT minus-end-binding proteins in dendrite-specific pruning remains completely unknown. Here, we identified Patronin, a minus-end-binding protein, for its crucial and dose-sensitive role in ddaC dendrite pruning. The CKK domain is important for Patronin’s function in dendrite pruning. Moreover, we show that both patronin knockdown and overexpression resulted in a drastic decrease of MT minus ends and a concomitant increase of plus-end-out MTs in ddaC dendrites, suggesting that Patronin stabilizes dendritic minus-end-out MTs. Consistently, attenuation of Klp10A MT depolymerase in patronin mutant neurons significantly restored minus-end-out MTs in dendrites and thereby rescued dendrite-pruning defects. Thus, our study demonstrates that Patronin orients minus-end-out MT arrays in dendrites to promote dendrite-specific pruning mainly through antagonizing Klp10A activity.

Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that minor issues remain unresolved (see decision letter).

MTOC Organization and Competition During Neuron Differentiation

Neurons are polarized cells with long branched axons and dendrites. Microtubule generation and organization machineries are crucial to grow and pattern these complex cellular extensions. Microtubule organizing centers (MTOCs) concentrate the molecular machinery for templating microtubules, stabilizing the nascent polymer, and organizing the resultant microtubules into higher-order structures. MTOC formation and function are well described at the centrosome, in the spindle, and at interphase Golgi; we review these studies and then describe recent results about how the machineries acting at these classic MTOCs are repurposed in the postmitotic neuron for axon and dendrite differentiation. We further discuss a constant tug-of-war interplay between different MTOC activities in the cell and how this process can be used as a substrate for transcription factor-mediated diversification of neuron types.

Massive cytoplasmic transport and microtubule organization in fertilized chordate eggs

Eggs have developed their own strategies for early development. Amphibian, teleost fish, and ascidian eggs show cortical rotation and an accompanying structure, a cortical parallel microtubule (MT) array, during the one-cell embryonic stage. Cortical rotation is thought to relocate maternal deposits to a certain compartment of the egg and to polarize the embryo. The common features and differences among chordate eggs as well as localized maternal proteins and mRNAs that are related to the organization of MT structures are described in this review. Furthermore, recent studies report progress in elucidating the molecular nature and functions of the noncentrosomal MT organizing center (ncMTOC). The parallel array of MT bundles is presumably organized by ncMTOCs; therefore, the mechanism of ncMTOC control is likely inevitable for these species. Thus, the molecules related to the ncMTOC provide clues for understanding the mechanisms of early developmental systems, which ultimately determine the embryonic axis.

Microtubule-severing enzymes: From cellular functions to molecular mechanism

McNally and Roll-Mecak review the molecular mechanism of microtubule-severing enzymes and their diverse roles in processes ranging from cell division to ciliogensis and morphogenesis.

Microtubule-severing enzymes generate internal breaks in microtubules. They are conserved in eukaryotes from ciliates to mammals, and their function is important in diverse cellular processes ranging from cilia biogenesis to cell division, phototropism, and neurogenesis. Their mutation leads to neurodegenerative and neurodevelopmental disorders in humans. All three known microtubule-severing enzymes, katanin, spastin, and fidgetin, are members of the meiotic subfamily of AAA ATPases that also includes VPS4, which disassembles ESCRTIII polymers. Despite their conservation and importance to cell physiology, the cellular and molecular mechanisms of action of microtubule-severing enzymes are not well understood. Here we review a subset of cellular processes that require microtubule-severing enzymes as well as recent advances in understanding their structure, biophysical mechanism, and regulation.