Microtubule Minus-End Stabilization by Polymerization-Driven CAMSAP Deposition

Structural mechanisms for centrosomal recruitment and organization of the microtubule nucleator γ-TuRC

The γ-tubulin ring complex (γ-TuRC) acts as a structural template for microtubule formation at centrosomes, associating with two main compartments: the pericentriolar material and the centriole lumen. In the pericentriolar material, the γ-TuRC is involved in microtubule organization, while the function of the centriole lumenal pool remains unclear. The conformational landscape of the γ-TuRC, which is crucial for its activity, and its centrosomal anchoring mechanisms, which determine γ-TuRC activity and turnover, are not understood. Using cryo-electron tomography, we analyze γ-TuRCs in human cells and purified centrosomes. Pericentriolar γ-TuRCs simultaneously associate with the essential adapter NEDD1 and the microcephaly protein CDK5RAP2. NEDD1 forms a tetrameric structure at the γ-TuRC base through interactions with four GCP3/MZT1 modules and GCP5/6-specific extensions, while multiple copies of CDK5RAP2 engage the γ-TuRC in two distinct binding patterns to promote γ-TuRC closure and activation. In the centriole lumen, the microtubule branching factor Augmin tethers a condensed cluster of γ-TuRCs to the centriole wall with defined directional orientation. Centriole-lumenal γ-TuRC-Augmin is protected from degradation during interphase and released in mitosis to aid chromosome alignment. This study provides a unique view on γ-TuRC structure and molecular organization at centrosomes and identifies an important cellular function of centriole-lumenal γ-TuRCs.

CAMSAP2 is required for bridging fiber assembly to ensure mitotic spindle assembly and chromosome segregation in human epithelial Caco-2 cells

In mammalian epithelial cells, cytoplasmic microtubules are mainly non-centrosomal, through the functions of the minus-end binding proteins CAMSAP2 and CAMSAP3. When cells enter mitosis, cytoplasmic microtubules are reorganized into the spindle composed of both centrosomal and non-centrosomal microtubules. The function of the CAMSAP proteins upon spindle assembly remains unknown, as these do not exhibit evident localization to spindle microtubules. Here, we demonstrate that CAMSAP2, but not CAMSAP3, is required for spindle assembly upon mitotic entry. CAMSAP2 knockout (KO) Caco-2 cells showed a delay in mitotic progression, whereas CAMSAP3 KO cells did not. The spindle in CAMSAP2 KO cells was short and displayed a reduced microtubule density, particularly around chromosomes. This indicated a loss of bridging fibers, which are known to assist alignment of sister kinetochores through interaction with kinetochore fibers. Consistent with this, live-cell imaging of CAMSAP2 KO cells captured slow elongation of the anaphase spindle and errors in chromosome segregation. Therefore, we propose that CAMSAP2 ensures efficient reorganization of cytoplasmic microtubules into the mitotic spindle through constructing bridging fibers that assist faithful segregation of sister chromatids.

Microtubule-Associated Proteins (MAPs) Are Multifunctional Cytoskeletal Proteins in the Testis That Regulate Spermatogenesis

Microtubule-associated proteins (MAPs) refer to a large superfamily of proteins that bind to microtubules (MTs) structurally, modulating the rapid transition of MTs from a stable state (polymerized) to shrinkage (or catastrophe) via depolymerization through a meta-stable state. Changes of MTs from an assembled structure as linear protofilaments that are a packed/bundled ultrastructure to disassembled subunits of heterodimers of α-/ß-tubulins (or oligomers) can take place in milliseconds within a living cell. These heterodimers can also be rapidly phosphorylated, becoming GTP-bound, or rapidly polymerized into linear protofilaments of MT again. It is such rapid cyclic changes of MTs that support cellular development, growth, and changes in cell shape in response to changes in development or other physiological phenomena, such as the series of cellular events during spermatogenesis, cell divisions, and in response to environmental toxicants to protect cellular life. In this review, we seek to give a concise update and discussion on MAPs. Particularly, we focus on a specific member of the structural MAPs, namely MAP1a, and its interaction with the microtubule affinity regulatory kinases (MARKs, including MARK1, 2, 3, and 4, all are Ser/Thr protein kinases) in particular MARK4, and how these two MAPs work together to regulate MT dynamics in Sertoli cells to support germ cell development. This information should be helpful to investigators who seek to better understand the role of MAPs in testis biology.

Brain malformation, neurodevelopmental disorder and epilepsy in a case of two rare genetic diseases: overlapping phenotype

In most cases there is a single etiological factor causing neuromotor developmental delay and epilepsy while sometimes more than one gene may be involved. These include the autosomal recessive inherited CAMSAP1 gene, which is associated with cortical developmental malformations such as pachygyria and lissencephaly and the autosomal dominant inherited NBEA gene, which plays crucial roles in vesicle trafficking as well as synapse structure and function. Loss of function of both genes together is a well-known disease mechanism. We report a 7-year-old girl with early-onset epilepsy, severe neuromotor developmental delay and brain malformation. Whole exome analysis of the patient revealed c.1153C > T p.Gln385* nonsense homozygous likely pathogenic variant in CAMSAP1 gene and c.6867G > A p.Trp2289* nonsense heterozygous pathogenic de novo variant in NBEA gene. A small number of cases associated with these genes have been reported. We report the 8th case reported in the CAMSAP1 gene and the overlapping phenotype in these two genes.

Phase separation of microtubule-binding proteins - implications for neuronal function and disease

Protein liquid-liquid phase separation (LLPS) is driven by intrinsically disordered regions and multivalent binding domains, both of which are common features of diverse microtubule (MT) regulators. Many in vitro studies have dissected the mechanisms by which MT-binding proteins (MBPs) regulate MT nucleation, stabilization and dynamics, and investigated whether LLPS plays a role in these processes. However, more recent in vivo studies have focused on how MBP LLPS affects biological functions throughout neuronal development. Dysregulation of MBP LLPS can lead to formation of aggregates - an underlying feature in many neurodegenerative diseases - such as the tau neurofibrillary tangles present in Alzheimer's disease. In this Review, we highlight progress towards understanding the regulation of MT dynamics through the lens of phase separation of MBPs and associated cytoskeletal regulators, from both in vitro and in vivo studies. We also discuss how LLPS of MBPs regulates neuronal development and maintains homeostasis in mature neurons.

Structural analysis of microtubule binding by minus-end targeting protein Spiral2

Microtubules (MTs) are dynamic cytoskeletal polymers that play a critical role in determining cell polarity and shape. In plant cells, acentrosomal MTs are localized on the cell surface and are referred to as cortical MTs. Cortical MTs nucleate in the cell cortex and detach from nucleation sites. The released MT filaments perform treadmilling, with the plus-ends of MTs polymerizing and the minus-ends depolymerizing. Minus-end targeting proteins, -TIPs, include Spiral2, which regulates the minus-end dynamics of acentrosomal MTs. Spiral2 accumulates autonomously at MT minus-ends and inhibits filament shrinkage, but the mechanism by which Spiral2 specifically recognizes minus-ends of MTs remains unknown. Here we describe the crystal structure of Spiral2's N-terminal MT-binding domain. The structural properties of this domain resemble those of the HEAT repeat structure of the tumor overexpressed gene (TOG) domain, but the number of HEAT repeats is different and the conformation is highly arched. Gel filtration and co-sedimentation analyses demonstrate that the domain binds preferentially to MT filaments rather than the tubulin dimer, and that the tubulin-binding mode of Spiral2 via the basic surface is similar to that of the TOG domain. We constructed an in silico model of the Spiral2-tubulin complex to identify residues that potentially recognize tubulin. Mutational analysis revealed that the key residues inferred in the model are involved in microtubule recognition, and provide insight into the mechanism by which end-targeting proteins stabilize MT ends.

CAMSAP3 forms dimers via its α-helix domain that directly stabilize non-centrosomal microtubule minus ends

Microtubules are vital components of the cytoskeleton. Their plus ends are dynamic and respond to changes in cell morphology, whereas the minus ends are stable and serve a crucial role in microtubule seeding and maintaining spatial organization. In mammalian cells, the calmodulin-regulated spectrin-associated proteins (CAMSAPs), play a key role in directly regulating the dynamics of non-centrosomal microtubules minus ends. However, the molecular mechanisms are not yet fully understood. Our study reveals that CAMSAP3 forms dimers through its C-terminal α-helix; this dimerization not only enhances the microtubule-binding affinity of the CKK domain but also enables the CKK domain to regulate the dynamics of microtubules. Furthermore, CAMSAP3 also specializes in decorating at the minus end of microtubules through the combined action of the microtubule-binding domain (MBD) and the C-terminal α-helix, thereby achieving dynamic regulation of the minus ends of microtubules. These findings are crucial for advancing our understanding and treatment of diseases associated with non-centrosomal microtubules.

HURP facilitates spindle assembly by stabilizing microtubules and working synergistically with TPX2

In vertebrate spindles, most microtubules are formed via branching microtubule nucleation, whereby microtubules nucleate along the side of pre-existing microtubules. Hepatoma up-regulated protein (HURP) is a microtubule-associated protein that has been implicated in spindle assembly, but its mode of action is yet to be defined. In this study, we show that HURP is necessary for RanGTP-induced branching microtubule nucleation in Xenopus egg extract. Specifically, HURP stabilizes the microtubule lattice to promote microtubule formation from γ-TuRC. This function is shifted to promote branching microtubule nucleation through enhanced localization to TPX2 condensates, which form the core of the branch site on microtubules. Lastly, we provide a high-resolution cryo-EM structure of HURP on the microtubule, revealing how HURP binding stabilizes the microtubule lattice. We propose a model in which HURP stabilizes microtubules during their formation, and TPX2 preferentially enriches HURP to microtubules to promote branching microtubule nucleation and thus spindle assembly.

Katanin regulatory subunit B1 (KATNB1) regulates BTB dynamics through changes in cytoskeletal organization

In this study, we have explored the role of the KATNB1 gene, a microtubule-severing protein, in the seminiferous epithelium of the rat testis. Our data have shown that KATNB1 expressed in rat brain, testes, and Sertoli cells. KATNB1 was found to co-localize with α-tubulin showing a unique stage-specific distribution across the seminiferous epithelium. Knockdown of KATNB1 by RNAi led to significant disruption of the tight junction (TJ) permeability barrier function in primary Sertoli cells cultured in vitro with an established functional TJ-barrier, as well as perturbations in the microtubule and actin cytoskeleton organization. The disruption in these cytoskeletal structures, in turn, led to improper distribution of TJ and basal ES proteins essential for maintaining the Sertoli TJ function. More importantly, overexpression of KATNB1 in the testis in vivo was found to block cadmium-induced blood-testis barrier (BTB) disruption and testis injury. KATNB1 exerted its promoting effects on BTB and spermatogenesis through corrective spatiotemporal expression of actin- and microtubule-based regulatory proteins by maintaining the proper organization of cytoskeletons in the testis, illustrating its plausible therapeutic implication. In summary, Katanin regulatory subunit B1 (KATNB1) plays a crucial role in BTB and spermatogenesis through its effects on the actin- and microtubule-based cytoskeletons in Sertoli cells and testis, providing important insights into male reproductive biology.

Transiently formed nucleus-to-cilium microtubule arrays mediate senescence initiation in a KIFC3-dependent manner

Despite the importance of cellular senescence in human health, how damaged cells undergo senescence remains elusive. We have previously shown that promyelocytic leukemia nuclear body (PML-NBs) translocation of the ciliary FBF1 is essential for senescence induction in stressed cells. Here we discover that an early cellular event occurring in stressed cells is the transient assembly of stress-induced nucleus-to-cilium microtubule arrays (sinc-MTs). The sinc-MTs are distinguished by unusual polyglutamylation and unique polarity, with minus-ends nucleating near the nuclear envelope and plus-ends near the ciliary base. KIFC3, a minus-end-directed kinesin, is recruited to plus-ends of sinc-MTs and interacts with the centrosomal protein CENEXIN1. In damaged cells, CENEXIN1 co-translocates with FBF1 to PML-NBs. Deficiency of KIFC3 abolishes PML-NB translocation of FBF1 and CENEXIN1, as well as senescence initiation in damaged cells. Our study reveals that KIFC3-mediated nuclear transport of FBF1 along polyglutamylated sinc-MTs is a prerequisite for senescence induction in mammalian cells.

The structure of the γ-TuRC at the microtubule minus end - not just one solution

In cells, microtubules (MTs) assemble from α/β-tubulin subunits at nucleation sites containing the γ-tubulin ring complex (γ-TuRC). Within the γ-TuRC, exposed γ-tubulin molecules act as templates for MT assembly by interacting with α/β-tubulin. The vertebrate γ-TuRC is scaffolded by γ-tubulin-interacting proteins GCP2-6 arranged in a specific order. Interestingly, the γ-tubulin molecules in the γ-TuRC deviate from the cylindrical geometry of MTs, raising the question of how the γ-TuRC structure changes during MT nucleation. Recent studies on the structure of the vertebrate γ-TuRC attached to the end of MTs came to varying conclusions. In vitro assembly of MTs, facilitated by an α-tubulin mutant, resulted in a closed, cylindrical γ-TuRC showing canonical interactions between all γ-tubulin molecules and α/β-tubulin subunits. Conversely, native MTs formed in a frog extract were capped by a partially closed γ-TuRC, with some γ-tubulin molecules failing to align with α/β-tubulin. This review discusses these outcomes, along with the broader implications.

Spermatocytes have the capacity to segregate chromosomes despite centriole duplication failure

Centrosomes are the canonical microtubule organizing centers (MTOCs) of most mammalian cells, including spermatocytes. Centrosomes comprise a centriole pair within a structurally ordered and dynamic pericentriolar matrix (PCM). Unlike in mitosis, where centrioles duplicate once per cycle, centrioles undergo two rounds of duplication during spermatogenesis. The first duplication is during early meiotic prophase I, and the second is during interkinesis. Using mouse mutants and chemical inhibition, we have blocked centriole duplication during spermatogenesis and determined that non-centrosomal MTOCs (ncMTOCs) can mediate chromosome segregation. This mechanism is different from the acentriolar MTOCs that form bipolar spindles in oocytes, which require PCM components, including gamma-tubulin and CEP192. From an in-depth analysis, we identified six microtubule-associated proteins, TPX2, KIF11, NuMA, and CAMSAP1-3, that localized to the non-centrosomal MTOC. These factors contribute to a mechanism that ensures bipolar MTOC formation and chromosome segregation during spermatogenesis when centriole duplication fails. However, despite the successful completion of meiosis and round spermatid formation, centriole inheritance and PLK4 function are required for normal spermiogenesis and flagella assembly, which are critical to ensure fertility.

CSPP1 stabilizes microtubules by capping both plus and minus ends

Although the dynamic instability of microtubules (MTs) is fundamental to many cellular functions, quiescent MTs with unattached free distal ends are commonly present and play important roles in various events to power cellular dynamics. However, how these free MT tips are stabilized remains poorly understood. Here, we report that centrosome and spindle pole protein 1 (CSPP1) caps and stabilizes both plus and minus ends of static MTs. Real-time imaging of laser-ablated MTs in live cells showed deposition of CSPP1 at the newly generated MT ends, whose dynamic instability was concomitantly suppressed. Consistently, MT ends in CSPP1-overexpressing cells were hyper-stabilized, while those in CSPP1-depleted cells were much more dynamic. This CSPP1-elicited stabilization of MTs was demonstrated to be achieved by suppressing intrinsic MT catastrophe and restricting polymerization. Importantly, CSPP1-bound MTs were resistant to mitotic centromere-associated kinesin-mediated depolymerization. These findings delineate a previously uncharacterized CSPP1 activity that integrates MT end capping to orchestrate quiescent MTs.

Loss of the polarity protein Par3 promotes dendritic spine neoteny and enhances learning and memory

The Par3 polarity protein is critical for subcellular compartmentalization in different developmental processes. Variants of PARD3, encoding PAR3, are associated with intelligence and neurodevelopmental disorders. However, the role of Par3 in glutamatergic synapse formation and cognitive functions in vivo remains unknown. Here, we show that forebrain-specific Par3 conditional knockout leads to increased long, thin dendritic spines in vivo. In addition, we observed a decrease in the amplitude of miniature excitatory postsynaptic currents. Surprisingly, loss of Par3 enhances hippocampal-dependent spatial learning and memory and repetitive behavior. Phosphoproteomic analysis revealed proteins regulating cytoskeletal dynamics are significantly dysregulated downstream of Par3. Mechanistically, we found Par3 deletion causes increased Rac1 activation and dysregulated microtubule dynamics through CAMSAP2. Together, our data reveal an unexpected role for Par3 as a molecular gatekeeper in regulating the pool of immature dendritic spines, a rate-limiting step of learning and memory, through modulating Rac1 activation and microtubule dynamics in vivo.

Deficiency of CAMSAP2 impairs olfaction and the morphogenesis of mitral cells

In developing olfactory bulb (OB), mitral cells (MCs) remodel their dendrites to establish the precise olfactory circuit, and these circuits are critical for individuals to sense odors and elicit behaviors for survival. However, how microtubules (MTs) participate in the process of dendritic remodeling remains elusive. Here, we reveal that calmodulin-regulated spectrin-associated proteins (CAMSAPs), a family of proteins that bind to the minus-end of the noncentrosomal MTs, play a crucial part in the development of MC dendrites. We observed that Camsap2 knockout (KO) males are infertile while the reproductive tract is normal. Further study showed that the infertility was due to the severe defects of mating behavior in male mice. Besides, mice with loss-of-function displayed defects in the sense of smell. Furthermore, we found that the deficiency of CAMSAP2 impairs the classical morphology of MCs, and the CAMSAP2-dependent dendritic remodeling process is responsible for this defect. Thus, our findings demonstrate that CAMSAP2 plays a vital role in regulating the development of MCs.

Roles of the tubulin-based cytoskeleton in the Toxoplasma gondii apical complex

Microtubules (MTs) play a vital role as key components of the eukaryotic cytoskeleton. The phylum Apicomplexa comprises eukaryotic unicellular parasitic organisms defined by the presence of an apical complex which consists of specialized secretory organelles and tubulin-based cytoskeletal elements. One apicomplexan parasite, Toxoplasma gondii, is an omnipresent opportunistic pathogen with significant medical and veterinary implications. To ensure successful infection and widespread dissemination, T. gondii heavily relies on the tubulin structures present in the apical complex. Recent advances in high-resolution imaging, coupled with reverse genetics, have offered deeper insights into the composition, functionality, and dynamics of these tubulin-based structures. The apicomplexan tubulins differ from those of their mammalian hosts, endowing them with unique attributes and susceptibility to specific classes of inhibitory compounds.

γ-TuRC asymmetry induces local protofilament mismatch at the RanGTP-stimulated microtubule minus end

The γ-tubulin ring complex (γ-TuRC) is a structural template for de novo microtubule assembly from α/β-tubulin units. The isolated vertebrate γ-TuRC assumes an asymmetric, open structure deviating from microtubule geometry, suggesting that γ-TuRC closure may underlie regulation of microtubule nucleation. Here, we isolate native γ-TuRC-capped microtubules from Xenopus laevis egg extract nucleated through the RanGTP-induced pathway for spindle assembly and determine their cryo-EM structure. Intriguingly, the microtubule minus end-bound γ-TuRC is only partially closed and consequently, the emanating microtubule is locally misaligned with the γ-TuRC and asymmetric. In the partially closed conformation of the γ-TuRC, the actin-containing lumenal bridge is locally destabilised, suggesting lumenal bridge modulation in microtubule nucleation. The microtubule-binding protein CAMSAP2 specifically binds the minus end of γ-TuRC-capped microtubules, indicating that the asymmetric minus end structure may underlie recruitment of microtubule-modulating factors for γ-TuRC release. Collectively, we reveal a surprisingly asymmetric microtubule minus end protofilament organisation diverging from the regular microtubule structure, with direct implications for the kinetics and regulation of nucleation and subsequent modulation of microtubules during spindle assembly.

Augmin complex activity finetunes dendrite morphology through non-centrosomal microtubule nucleation in vivo

During development, neurons achieve a stereotyped neuron type-specific morphology, which relies on dynamic support by microtubules (MTs). An important player is the augmin complex (hereafter augmin), which binds to existing MT filaments and recruits the γ-tubulin ring complex (γ-TuRC), to form branched MTs. In cultured neurons, augmin is important for neurite formation. However, little is known about the role of augmin during neurite formation in vivo. Here, we have revisited the role of mammalian augmin in culture and then turned towards the class four Drosophila dendritic arborization (c4da) neurons. We show that MT density is maintained through augmin in cooperation with the γ-TuRC in vivo. Mutant c4da neurons show a reduction of newly emerging higher-order dendritic branches and in turn also a reduced number of their characteristic space-filling higher-order branchlets. Taken together, our data reveal a cooperative function for augmin with the γ-TuRC in forming enough MTs needed for the appropriate differentiation of morphologically complex dendrites in vivo.

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.

The Shot CH1 domain recognises a distinct form of F-actin during Drosophila oocyte determination

In Drosophila, only one cell in a multicellular female germline cyst is specified as an oocyte and a similar process occurs in mammals. The symmetry-breaking cue for oocyte selection is provided by the fusome, a tubular structure connecting all cells in the cyst. The Drosophila spectraplakin Shot localises to the fusome and translates its asymmetry into a polarised microtubule network that is essential for oocyte specification, but how Shot recognises the fusome is unclear. Here, we demonstrate that the actin-binding domain (ABD) of Shot is necessary and sufficient to localise Shot to the fusome and mediates Shot function in oocyte specification together with the microtubule-binding domains. The calponin homology domain 1 of the Shot ABD recognises fusomal F-actin and requires calponin homology domain 2 to distinguish it from other forms of F-actin in the cyst. By contrast, the ABDs of utrophin, Fimbrin, Filamin, Lifeact and F-tractin do not recognise fusomal F-actin. We therefore propose that Shot propagates fusome asymmetry by recognising a specific conformational state of F-actin on the fusome.

A divergent tumor overexpressed gene domain and oligomerization contribute to SPIRAL2 function in stabilizing microtubule minus ends

The acentrosomal cortical microtubules (MTs) of higher plants dynamically assemble into specific array patterns that determine the axis of cell expansion. Recently, the Arabidopsis (Arabidopsis thaliana) SPIRAL2 (SPR2) protein was shown to regulate cortical MT length and light-induced array reorientation by stabilizing MT minus ends. SPR2 autonomously localizes to both the MT lattice and MT minus ends, where it decreases the minus end depolymerization rate. However, the structural determinants that contribute to the ability of SPR2 to target and stabilize MT minus ends remain unknown. Here, we present the crystal structure of the SPR2 N-terminal domain, which reveals a unique tumor overexpressed gene (TOG) domain architecture with 7 HEAT repeats. We demonstrate that a coiled-coil domain mediates the multimerization of SPR2, which provides avidity for MT binding, and is essential to bind soluble tubulin. In addition, we found that an SPR2 construct spanning the TOG domain, basic region, and coiled-coil domain targets and stabilizes MT minus ends similar to full-length SPR2 in plants. These results reveal how a TOG domain, which is typically found in microtubule plus-end regulators, has been appropriated in plants to regulate MT minus ends.

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

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

CAMSAP2 enhances lung cancer cell metastasis by mediating RASAL2 degradation

AIMS

Cancer metastasis significantly contributes to mortality in lung cancer patients. Calmodulin-regulated spectrin-associated protein family member 2 (CAMSAP2) plays a significant role in cancer cell migration; however, its role in lung cancer metastasis and the underlying mechanism remain largely unknown. The present study aimed to investigate the impact of CAMSAP2 on lung cancer.

MAIN METHODS

The clinical relevance of CAMSAP2 in lung cancer patients was assessed using public database. RNA interference experiments were conducted to investigate role of CAMSAP2 in cell migration through transwell and wound healing assays. Molecular mechanisms were explored by identifying the possible interacting partners and pathways using the BioGRID and KEGG pathway analyses. The impact of CAMSAP2 on Ras protein activator-like 2 (RASAL2)-mediated lung cancer metastasis was investigated through biochemical assays. Additionally, in vivo experimentation using a murine tail vein metastasis model was performed to comprehend CAMSAP2's influence on metastasis.

KEY FINDINGS

A high expression level of CAMSAP2 was associated with poor overall survival in lung cancer patients and it positively correlated with cell migration in non-small cell lung cancer (NSCLC) cell lines. Knockdown of CAMSAP2 inhibited lung cancer cell motility in vitro and metastasis in vivo. Proteomic and biochemical analyses revealed the interaction between CAMSAP2 and RASAL2, which facilitates the degradation of RASAL2 through the ubiquitin-proteasome system. These degradation processes resulted in the activation of the extracellular signal-regulated kinase (ERK) signaling pathway, thereby promoting lung cancer metastasis. Collectively, the results of this study suggest that CAMSAP2 is a crucial regulator of cancer cell migration and metastasis and a promising therapeutic target for lung cancer.

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.

Interplay between stochastic enzyme activity and microtubule stability drives detyrosination enrichment on microtubule subsets

Microtubules in cells consist of functionally diverse subpopulations carrying distinct post-translational modifications (PTMs). Akin to the histone code, the tubulin code regulates a myriad of microtubule functions, ranging from intracellular transport to chromosome segregation. However, how individual PTMs only occur on subsets of microtubules to contribute to microtubule specialization is not well understood. In particular, microtubule detyrosination, the removal of the C-terminal tyrosine on α-tubulin subunits, marks the stable population of microtubules and modifies how microtubules interact with other microtubule-associated proteins to regulate a wide range of cellular processes. Previously, we found that in certain cell types, only ∼30% of microtubules are highly enriched with the detyrosination mark and that detyrosination spans most of the length of a microtubule, often adjacent to a completely tyrosinated microtubule. How the activity of a cytosolic detyrosinase, vasohibin (VASH), leads to only a small subpopulation of highly detyrosinated microtubules is unclear. Here, using quantitative super-resolution microscopy, we visualized nascent microtubule detyrosination events in cells consisting of 1-3 detyrosinated α-tubulin subunits after nocodazole washout. Microtubule detyrosination accumulates slowly and in a dispersed pattern across the microtubule length. By visualizing single molecules of VASH in live cells, we found that VASH engages with microtubules stochastically on a short timescale, suggesting limited removal of tyrosine per interaction, consistent with the super-resolution results. Combining these quantitative imaging results with simulations incorporating parameters from our experiments, we provide evidence for a stochastic model for cells to establish a subset of detyrosinated microtubules via a detyrosination-stabilization feedback mechanism.

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.

CAMSAP1 role in orchestrating structure and dynamics of manchette microtubule minus-ends impacts male fertility during spermiogenesis

The manchette is a crucial transient structure involved in sperm development, with its composition and regulation still not fully understood. This study focused on investigating the roles of CAMSAP1 and CAMSAP2, microtubule (MT) minus-end binding proteins, in regulating manchette MTs, spermiogenesis, and male fertility. The loss of CAMSAP1, but not CAMSAP2, disrupts the well-orchestrated process of spermiogenesis, leading to abnormal manchette elongation and delayed removal, resulting in deformed sperm nuclei and tails resembling oligoasthenozoospermia symptoms. We investigated the underlying molecular mechanisms by purifying manchette assemblies and comparing them through proteomic analysis, and results showed that the absence of CAMSAP1 disrupted the proper localization of key proteins (CEP170 and KIF2A) at the manchette minus end, compromising its structural integrity and hindering MT depolymerization. These findings highlight the significance of maintaining homeostasis in manchette MT minus-ends for shaping manchette morphology during late spermiogenesis, offering insights into the molecular mechanisms underlying infertility and sperm abnormalities.

Synthetic Par polarity induces cytoskeleton asymmetry in unpolarized mammalian cells

Polarized cells rely on a polarized cytoskeleton to function. Yet, how cortical polarity cues induce cytoskeleton polarization remains elusive. Here, we capitalized on recently established designed 2D protein arrays to ectopically engineer cortical polarity of virtually any protein of interest during mitosis in various cell types. This enables direct manipulation of polarity signaling and the identification of the cortical cues sufficient for cytoskeleton polarization. Using this assay, we dissected the logic of the Par complex pathway, a key regulator of cytoskeleton polarity during asymmetric cell division. We show that cortical clustering of any Par complex subunit is sufficient to trigger complex assembly and that the primary kinetic barrier to complex assembly is the relief of Par6 autoinhibition. Further, we found that inducing cortical Par complex polarity induces two hallmarks of asymmetric cell division in unpolarized mammalian cells: spindle orientation, occurring via Par3, and central spindle asymmetry, depending on aPKC activity.

Microtubule detyrosination by VASH1/SVBP is regulated by the conformational state of tubulin in the lattice

Tubulin, a heterodimer of α- and β-tubulin, is a GTPase that assembles into microtubule (MT) polymers whose dynamic properties are intimately coupled to nucleotide hydrolysis. In cells, the organization and dynamics of MTs are further tuned by post-translational modifications (PTMs), which control the ability of MT-associated proteins (MAPs) and molecular motors to engage MTs. Detyrosination is a PTM of α-tubulin, wherein its C-terminal tyrosine residue is enzymatically removed by either the vasohibin (VASH) or MT-associated tyrosine carboxypeptidase (MATCAP) peptidases. How these enzymes generate specific patterns of MT detyrosination in cells is not known. Here, we use a novel antibody-based probe to visualize the formation of detyrosinated MTs in real time and employ single-molecule imaging of VASH1 bound to its regulatory partner small-vasohibin binding protein (SVBP) to understand the process of MT detyrosination in vitro and in cells. We demonstrate that the activity, but not binding, of VASH1/SVBP is much greater on mimics of guanosine triphosphate (GTP)-MTs than on guanosine diphosphate (GDP)-MTs. Given emerging data showing that tubulin subunits in GTP-MTs are in expanded conformation relative to tubulin subunits in GDP-MTs, we reasoned that the lattice conformation of MTs is a key factor that gates the activity of VASH1/SVBP. We show that Taxol, a drug known to expand the MT lattice, promotes MT detyrosination and that CAMSAP2 and CAMSAP3 are two MAPs that spatially regulate detyrosination in cells. Collectively, our work shows that VASH1/SVBP detyrosination is regulated by the conformational state of tubulin in the MT lattice and that this is spatially determined in cells by the activity of MAPs.

Patronin/CAMSAP promotes reactivation and regeneration of Drosophila quiescent neural stem cells

The ability of stem cells to switch between quiescent and proliferative states is crucial for maintaining tissue homeostasis and regeneration. Drosophila quiescent neural stem cells (qNSCs) extend a primary protrusion that is enriched in acentrosomal microtubules and can be regenerated upon injury. Arf1 promotes microtubule growth, reactivation (exit from quiescence), and regeneration of qNSC protrusions upon injury. However, how Arf1 is regulated in qNSCs remains elusive. Here, we show that the microtubule minus-end binding protein Patronin/CAMSAP promotes acentrosomal microtubule growth and quiescent NSC reactivation. Patronin is important for the localization of Arf1 at Golgi and physically associates with Arf1, preferentially with its GDP-bound form. Patronin is also required for the regeneration of qNSC protrusion, likely via the regulation of microtubule growth. Finally, Patronin functions upstream of Arf1 and its effector Msps/XMAP215 to target the cell adhesion molecule E-cadherin to NSC-neuropil contact sites during NSC reactivation. Our findings reveal a novel link between Patronin/CAMSAP and Arf1 in the regulation of microtubule growth and NSC reactivation. A similar mechanism might apply to various microtubule-dependent systems in mammals.

Axonal transport during injury on a theoretical axon

Neurodevelopment, plasticity, and cognition are integral with functional directional transport in neuronal axons that occurs along a unique network of discontinuous polar microtubule (MT) bundles. Axonopathies are caused by brain trauma and genetic diseases that perturb or disrupt the axon MT infrastructure and, with it, the dynamic interplay of motor proteins and cargo essential for axonal maintenance and neuronal signaling. The inability to visualize and quantify normal and altered nanoscale spatio-temporal dynamic transport events prevents a full mechanistic understanding of injury, disease progression, and recovery. To address this gap, we generated DyNAMO, a Dynamic Nanoscale Axonal MT Organization model, which is a biologically realistic theoretical axon framework. We use DyNAMO to experimentally simulate multi-kinesin traffic response to focused or distributed tractable injury parameters, which are MT network perturbations affecting MT lengths and multi-MT staggering. We track kinesins with different motility and processivity, as well as their influx rates, in-transit dissociation and reassociation from inter-MT reservoirs, progression, and quantify and spatially represent motor output ratios. DyNAMO demonstrates, in detail, the complex interplay of mixed motor types, crowding, kinesin off/on dissociation and reassociation, and injury consequences of forced intermingling. Stalled forward progression with different injury states is seen as persistent dynamicity of kinesins transiting between MTs and inter-MT reservoirs. DyNAMO analysis provides novel insights and quantification of axonal injury scenarios, including local injury-affected ATP levels, as well as relates these to influences on signaling outputs, including patterns of gating, waves, and pattern switching. The DyNAMO model significantly expands the network of heuristic and mathematical analysis of neuronal functions relevant to axonopathies, diagnostics, and treatment strategies.

Achieving the promise and avoiding the peril of chemical probes using genetics

Chemical probes can be valuable tools for studying protein targets, but addressing concerns about a probe's cellular target or its specificity can be challenging. A reliable strategy is to use a mutation that does not alter a target's function but confers resistance (or sensitizes) to the inhibitor in both cellular and biochemical assays. However, challenges remain in finding such mutations. Here, we discuss structure- and cell-based approaches to identify resistance- and sensitivity-conferring mutations. Further, we describe how resistance-conferring mutations can help with compound design, and the use of saturation mutagenesis to characterize a compound binding site. We highlight how genetic approaches can ensure the proper use of chemical inhibitors to pursue mechanistic studies and test therapeutic hypotheses.

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

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

Moving through the crowd. Where are we at understanding physiological axon growth?

Axon growth enables the rapid wiring of the central nervous system. Understanding this process is a prerequisite to retriggering it under pathological conditions, such as a spinal cord injury, to elicit axon regeneration. The last decades saw progress in understanding the mechanisms underlying axon growth. Most of these studies employed cultured neurons grown on flat surfaces. Only recently studies on axon growth were performed in 3D. In these studies, physiological environments exposed more complex and dynamic aspects of axon development. Here, we describe current views on axon growth and highlight gaps in our knowledge. We discuss how axons interact with the extracellular matrix during development and the role of the growth cone and its cytoskeleton within. Finally, we propose that the time is ripe to study axon growth in a more physiological setting. This will help us uncover the physiologically relevant mechanisms underlying axon growth, and how they can be reactivated to induce axon regeneration.

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

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

A live-cell marker to visualize the dynamics of stable microtubules throughout the cell cycle

The microtubule (MT) cytoskeleton underlies processes such as intracellular transport and cell division. Immunolabeling for posttranslational modifications of tubulin has revealed the presence of different MT subsets, which are believed to differ in stability and function. Whereas dynamic MTs can readily be studied using live-cell plus-end markers, the dynamics of stable MTs have remained obscure due to a lack of tools to directly visualize these MTs in living cells. Here, we present StableMARK (Stable Microtubule-Associated Rigor-Kinesin), a live-cell marker to visualize stable MTs with high spatiotemporal resolution. We demonstrate that a rigor mutant of Kinesin-1 selectively binds to stable MTs without affecting MT organization and organelle transport. These MTs are long-lived, undergo continuous remodeling, and often do not depolymerize upon laser-based severing. Using this marker, we could visualize the spatiotemporal regulation of MT stability before, during, and after cell division. Thus, this live-cell marker enables the exploration of different MT subsets and how they contribute to cellular organization and transport.

Microglia reactivity entails microtubule remodeling from acentrosomal to centrosomal arrays

Microglia reactivity entails a large-scale remodeling of cellular geometry, but the behavior of the microtubule cytoskeleton during these changes remains unexplored. Here we show that activated microglia provide an example of microtubule reorganization from a non-centrosomal array of parallel and stable microtubules to a radial array of more dynamic microtubules. While in the homeostatic state, microglia nucleate microtubules at Golgi outposts, and activating signaling induces recruitment of nucleating material nearby the centrosome, a process inhibited by microtubule stabilization. Our results demonstrate that a hallmark of microglia reactivity is a striking remodeling of the microtubule cytoskeleton and suggest that while pericentrosomal microtubule nucleation may serve as a distinct marker of microglia activation, inhibition of microtubule dynamics may provide a different strategy to reduce microglia reactivity in inflammatory disease.

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.

Detection and Analysis of Microtubule Nucleator γ-Tubulin Ring Complex

Golgi-derived microtubules constitute an asymmetrical microtubule network that drives polarized transport of vesicles to support cell polarization and directional migration. Golgi-based microtubule nucleation requires the γ-tubulin ring complex (γTuRC), the principal microtubule nucleator in animal cells. In this chapter, we present methods for detecting γTuRC components and associated proteins on the Golgi, examining Golgi-based microtubule nucleation, and measuring the microtubule-nucleating activity of isolated γTuRCs. These approaches have been demonstrated to be effective for assessing the microtubule-organizing function of the Golgi complex.

The diagnostic yield, candidate genes, and pitfalls for a genetic study of intellectual disability in 118 middle eastern families

Global Developmental Delay/Intellectual disability (ID) is the term used to describe various disorders caused by abnormal brain development and characterized by impairments in cognition, communication, behavior, or motor skills. In the past few years, whole-exome sequencing (WES) has been proven to be a powerful, robust, and scalable approach for candidate gene discoveries in consanguineous populations. In this study, we recruited 215 patients affected with ID from 118 Middle Eastern families. Whole-exome sequencing was completed for 188 individuals. The average age at which WES was completed was 8.5 years. Pathogenic or likely pathogenic variants were detected in 32/118 families (27%). Variants of uncertain significance were seen in 33/118 families (28%). The candidate genes with a possible association with ID were detected in 32/118 (27%) with a total number of 64 affected individuals. These genes are novel, were previously reported in a single family, or cause strikingly different phenotypes with a different mode of inheritance. These genes included: AATK, AP1G2, CAMSAP1, CCDC9B, CNTROB, DNAH14, DNAJB4, DRG1, DTNBP1, EDRF1, EEF1D, EXOC8, EXOSC4, FARSB, FBXO22, FILIP1, INPP4A, P2RX7, PRDM13, PTRHD1, SCN10A, SCYL2, SMG8, SUPV3L1, TACC2, THUMPD1, XPR1, ZFYVE28. During the 5 years of the study and through gene matching databases, several of these genes have now been confirmed as causative of ID. In conclusion, understanding the causes of ID will help understand biological mechanisms, provide precise counseling for affected families, and aid in primary prevention.

Bi-allelic CAMSAP1 variants cause a clinically recognizable neuronal migration disorder

Non-centrosomal microtubules are essential cytoskeletal filaments that are important for neurite formation, axonal transport, and neuronal migration. They require stabilization by microtubule minus-end-targeting proteins including the CAMSAP family of molecules. Using exome sequencing on samples from five unrelated families, we show that bi-allelic CAMSAP1 loss-of-function variants cause a clinically recognizable, syndromic neuronal migration disorder. The cardinal clinical features of the syndrome include a characteristic craniofacial appearance, primary microcephaly, severe neurodevelopmental delay, cortical visual impairment, and seizures. The neuroradiological phenotype comprises a highly recognizable combination of classic lissencephaly with a posterior more severe than anterior gradient similar to PAFAH1B1(LIS1)-related lissencephaly and severe hypoplasia or absence of the corpus callosum; dysplasia of the basal ganglia, hippocampus, and midbrain; and cerebellar hypodysplasia, similar to the tubulinopathies, a group of monogenic tubulin-associated disorders of cortical dysgenesis. Neural cell rosette lineages derived from affected individuals displayed findings consistent with these phenotypes, including abnormal morphology, decreased cell proliferation, and neuronal differentiation. Camsap1-null mice displayed increased perinatal mortality, and RNAScope studies identified high expression levels in the brain throughout neurogenesis and in facial structures, consistent with the mouse and human neurodevelopmental and craniofacial phenotypes. Together our findings confirm a fundamental role of CAMSAP1 in neuronal migration and brain development and define bi-allelic variants as a cause of a clinically distinct neurodevelopmental disorder in humans and mice.

PTRN-1 (CAMSAP) and NOCA-2 (NINEIN) are required for microtubule polarity in Caenorhabditis elegans dendrites

The neuronal microtubule cytoskeleton is key to establish axon-dendrite polarity. Dendrites are characterized by the presence of minus-end out microtubules. However, the mechanisms that organize these microtubules with the correct orientation are still poorly understood. Using Caenorhabditis elegans as a model system for microtubule organization, we characterized the role of 2 microtubule minus-end related proteins in this process, the microtubule minus-end stabilizing protein calmodulin-regulated spectrin-associated protein (CAMSAP/PTRN-1), and the NINEIN homologue, NOCA-2 (noncentrosomal microtubule array). We found that CAMSAP and NINEIN function in parallel to mediate microtubule organization in dendrites. During dendrite outgrowth, RAB-11-positive vesicles localized to the dendrite tip to nucleate microtubules and function as a microtubule organizing center (MTOC). In the absence of either CAMSAP or NINEIN, we observed a low penetrance MTOC vesicles mislocalization to the cell body, and a nearly fully penetrant phenotype in double mutant animals. This suggests that both proteins are important for localizing the MTOC vesicles to the growing dendrite tip to organize microtubules minus-end out. Whereas NINEIN localizes to the MTOC vesicles where it is important for the recruitment of the microtubule nucleator γ-tubulin, CAMSAP localizes around the MTOC vesicles and is cotranslocated forward with the MTOC vesicles upon dendritic growth. Together, these results indicate that microtubule nucleation from the MTOC vesicles and microtubule stabilization are both important to localize the MTOC vesicles distally to organize dendritic microtubules minus-end out.

Microtubule-severing enzymes

Stephanie Sarbanes et al. discuss microtubule-severing enzymes, highlighting their shared structure and mechanism and the diversity of processes in which they participate.

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.

CAMSAP2 promotes colorectal cancer cell migration and invasion through activation of JNK/c-Jun/MMP-1 signaling pathway

CAMSAP2 has been reported to act as an oncogene in hepatocellular carcinoma. However, the expression CAMSAP2 and its potential roles in colorectal cancer remain unclear. In this study, qRT-PCR and immunoblotting analysis were used to detect the mRNA and protein levels of CAMSAP2 in colorectal cancer tissues and cell lines. Wound-healing, transwell migration and invasion assay were performed to determine whether CAMSAP2 promotes the capabilities of migration and invasion of colorectal cancer cells. The results showed that CAMSAP2 was highly elevated in colorectal cancer tissues and cell lines. Moreover, the high CAMSAP2 expression was positively correlated with tumor invasion depth, lymph node metastasis, distant metastasis, and the poor prognosis of colorectal cancer. Additionally, ectopic expression of CAMSAP2 in colorectal cancer cells promoted the migration and invasion in vitro and enhanced the lung metastasis in nude mice. Conversely, silencing CAMSAP2 resulted in an opposite phenomenon. By gain- and loss-of function experiments, we demonstrated that MMP-1 was a substantial downstream target of CAMSAP2, and it played a crucial role in regulating the migration and invasion induced by CAMSAP2 in colorectal cancer cells. Mechanistically, CAMSAP2 promoted the activation of JNK/c-Jun signaling pathway and subsequently upregulated the transcription activity of MMP-1. Taken together, our findings demonstrated that CAMSAP2 promoted colorectal cancer cell migration, invasion and metastasis through activation of JNK/c-Jun/MMP-1 signaling pathway, indicating CAMSAP2 is a promising therapeutic target for the treatment of metastatic colorectal cancer patients.

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.

Deficiency of the bZIP transcription factors Mafg and Mafk causes misexpression of genes in distinct pathways and results in lens embryonic developmental defects

Deficiency of the small Maf proteins Mafg and Mafk cause multiple defects, namely, progressive neuronal degeneration, cataract, thrombocytopenia and mid-gestational/perinatal lethality. Previous data shows Mafg ^-/-:Mafk ^+/- compound knockout (KO) mice exhibit cataracts age 4-months onward. Strikingly, Mafg ^-/-:Mafk ^-/- double KO mice develop lens defects significantly early in life, during embryogenesis, but the pathobiology of these defects is unknown, and is addressed here. At embryonic day (E)16.5, the epithelium of lens in Mafg ^-/-:Mafk ^-/- animals appears abnormally multilayered as demonstrated by E-cadherin and nuclear staining. Additionally, Mafg ^-/-:Mafk ^-/- lenses exhibit abnormal distribution of F-actin near the "fulcrum" region where epithelial cells undergo apical constriction prior to elongation and reorientation as early differentiating fiber cells. To identify the underlying molecular changes, we performed high-throughput RNA-sequencing of E16.5 Mafg ^-/-:Mafk ^-/- lenses and identified a cohort of differentially expressed genes that were further prioritized using stringent filtering criteria and validated by RT-qPCR. Several key factors associated with the cytoskeleton, cell cycle or extracellular matrix (e.g., Cdk1, Cdkn1c, Camsap1, Col3a1, Map3k12, Sipa1l1) were mis-expressed in Mafg ^-/-:Mafk ^-/- lenses. Further, the congenital cataract-linked extracellular matrix peroxidase Pxdn was significantly overexpressed in Mafg ^-/-:Mafk ^-/- lenses, which may cause abnormal cell morphology. These data also identified the ephrin signaling receptor Epha5 to be reduced in Mafg ^-/-:Mafk ^-/- lenses. This likely contributes to the Mafg ^-/-:Mafk ^-/- multilayered lens epithelium pathology, as loss of an ephrin ligand, Efna5 (ephrin-A5), causes similar lens defects. Together, these findings uncover a novel early function of Mafg and Mafk in lens development and identify their new downstream regulatory relationships with key cellular factors.

Neurexin and frizzled intercept axonal transport at microtubule minus ends to control synapse formation

Synapse formation is locally determined by transmembrane proteins, yet synaptic material is synthesized remotely and undergoes processive transport in axons. How local synaptogenic signals intercept synaptic cargo in transport to promote its delivery and synapse formation is unknown. We found that the control of synaptic cargo delivery at microtubule (MT) minus ends mediates pro- and anti-synaptogenic activities of presynaptic neurexin and frizzled in C. elegans and identified the atypical kinesin VAB-8/KIF26 as a key molecule in this process. VAB-8/KIF26 levels at synaptic MT minus ends are controlled by frizzled and neurexin; loss of VAB-8 mimics neurexin mutants or frizzled hyperactivation, and its overexpression can rescue synapse loss in these backgrounds. VAB-8/KIF26 is required for the synaptic localization of other minus-end proteins and promotes the pausing of retrograde transport to allow delivery to synapses. Consistently, reducing retrograde transport rescues synapse loss in vab-8 and neurexin mutants. These results uncover a mechanistic link between synaptogenic signaling and axonal transport.

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.