Microtubule +TIPs at a glance

Eribulin's exclusive binding to microtubule plus ends results from discrimination between GTP and GDP forms of β-tubulin

The clinically approved anticancer agent eribulin (Halaven®) exerts cytotoxic antimitotic effects by binding to high affinity sites on exposed plus ends of growing microtubules (MT). Despite X-ray crystallographic mapping of eribulin's binding pocket within β-tubulin's vinca domain, the biophysical basis for eribulin's MT plus end binding exclusivity remains unknown. We performed surface plasmon resonance (SPR) studies of tubulin binding to biotinylated eribulin probes to ask if eribulin discriminates between GTP and GDP forms of β-tubulin, which characterize growing MT plus ends and MT sides, respectively. Tubulin binding to biotin-eribulin proceeded via a single state binding reaction, while binding to biotin-vinblastine occurred via a two-state reaction incorporating a conformational change. Biochemical approaches confirmed tubulin conformational changes induced by vinblastine but not eribulin. SPR competition studies using free eribulin and vinblastine confirmed tubulin binding specificity to cognate biotinylated probes, showing that eribulin binding within the β-tubulin vinca domain is physically and functionally distinct from vinblastine. SPR studies using tubulin containing only GTP or GDP forms of β-tubulin showed that while biotin-eribulin has only slightly higher overall affinity for GTP-tubulin, dissociation from GTP-tubulin was ∼7-fold slower than from GDP-tubulin, establishing that eribulin discriminates between GTP and GDP forms of β-tubulin. In contrast, vinblastine fails to discriminate between these two tubulin forms, consistent with its known binding to both MT ends and sides. Overall, our results establish, for the first time, the biophysical basis for eribulin's MT plus end binding exclusivity as resulting from the ability to discriminate between GTP and GDP forms of β-tubulin.

Hippocampal dendritic spines store-operated calcium entry and endoplasmic reticulum content is dynamic microtubule dependent

One of the mechanisms of calcium signalling in neurons is store-operated calcium entry (SOCE), which is activated when the calcium concentration in the smooth endoplasmic reticulum (ER) decreases and its protein-calcium sensor STIM (stromal interacting molecule) relocate to the endoplasmic reticulum and plasma membrane junctions, forms clusters and induces calcium entry. In electrically non-excitable cells, STIM1 is coupled with the positive end of a tubulin microtubule through interaction with EB1 (end-binding) protein, which controls its oligomerization, SOCE and participates in ER movement. STIM2 homologue, which is specific for mature hippocampal dendritic spines, is known to interact with EB3 protein, however, not much is known about the role of this interaction in STIM2 clustering or ER trafficking in neurons. Intriguingly, in neurons, reducing the expression of EB3 protein or disrupting the interaction of STIM2 protein with EB proteins results in decreased SOCE, in contrast to experiments with STIM1 in non-excitable cells. In this study, these two homologues are compared side-by-side in HEK-293T, and it is shown for the first time that their clustering and SOCE is oppositely regulated by dynamic tubulin microtubules. In particular, for STIM2, the interaction with dynamic microtubule cytoskeleton is required for clustering and is shown to potentiate SOCE, while for STIM1 this interaction restricts clustering, resulting in SOCE decrease. After store depletion in primary hippocampal neurons, the wild type STIM2 is redistributed from the necks to the heads of dendritic spines, while the STIM2 variant with a mutation that disrupts the interaction with EB proteins is excluded from dendritic spines. In addition, overexpression of the mutant variant leads to ER reorganization in neuronal soma and reduction of ER presence in spines. It also leads to a reduction in the number of spines containing the spine apparatus formed by ER cisternae, as well as a reduction in dendritic spines SOCE. These effects are opposite of those detected during overexpression of the wild type STIM2. Considered together, these findings underline the important role of dynamic microtubules in regulation of neuronal SOCE and ER morphology.

Hydroxyethylamine based analog targets microtubule assembly: an in silico study for anti-cancerous drug development

Microtubules are dynamic cytoskeletal structures essential for cell architecture, cellular transport, cell motility, and cell division. Due to their dynamic nature, known as dynamic instability, microtubules can spontaneously switch between phases of growth and shortening. Disruptions in microtubule functions have been implicated in several diseases, including cancer, neurodegenerative disorders such as Alzheimer's and Parkinson's disease, and birth defects. The role of microtubules during various phases of the cell cycle, particularly in cell division, makes them attractive targets for drug development against cancer. Several successful drugs currently on the market are designed to target microtubules. However, the presence of cellular toxicity and the development of multidrug resistance necessitate the search for new microtubule-targeting drugs.Here, a library of 106 biologically active compounds were screened to identify potent microtubule assembly inhibitors. Out of all the screened compounds, the hydroxyethylamine (HEA) analogues are found to be the best hit.We identified three inhibitors, BKS3031A, BKS3045A and BKS3046A, that bind at the same site as the well-known microtubule targeting agent colchicine. These inhibitors were simulated for 100 ns with tubulin complexes, and the results indicated that they remain stable within the binding pocket of α-β tubulin complexes. In addition, we estimated the binding free energy of BKS3031A, BKS3045A and BKS3046A by using molecular mechanics generalized Born surface area (MM-GBSA) calculations, and it was found to be -32.67 ± 6.01, -21.77 ± 5.12 and - 22.92 ± 5.09 kcal/mol, respectively. Our findings suggest that these novel inhibitors have potential to bind and perturb the microtubule network, positioning them as promising microtubule-targeting agents.

CLASP-mediated competitive binding in protein condensates directs microtubule growth

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

The RhoGEF Trio is transported by microtubules and affects microtubule stability in migrating neural crest cells

Directed cell migration requires a local fine-tuning of Rho GTPase activity to control protrusion formation, cell-cell contraction, and turnover of cellular adhesions. The Rho guanine nucleotide exchange factor (GEF) TRIO is ideally suited to control RhoGTPase activity because it combines two distinct catalytic domains to control Rac1 and RhoA activity in one molecule. However, at the cellular level, this molecular feature also requires a tight spatiotemporal control of TRIO activity. Here, we analyze the dynamic localization of Trio in Xenopus cranial neural crest (NC) cells, where we have recently shown that Trio is required for protrusion formation and migration. Using live cell imaging, we find that the GEF2 domain, but not the GEF1 domain of Trio, dynamically colocalizes with EB3 at microtubule plus-ends. Microtubule-mediated transport of Trio appears to be relevant for its function in NC migration, as a mutant GEF2 construct lacking the SxIP motif responsible for microtubule plus-end localization was significantly impaired in its ability to rescue the Trio loss-of-function phenotype compared to wild-type GEF2. Furthermore, by analyzing microtubule dynamics in migrating NC cells, we observed that loss of Trio function stabilized microtubules at cell-cell contact sites compared to controls, whereas they were destabilized at the leading edge of NC cells. Our data suggest that Trio is transported by microtubules to distinct subcellular locations where it has different functions in controlling microtubule stability, cell morphology, and cell-cell interaction during directed NC migration.

Microtubules provide force to promote membrane uncoating in vacuolar escape for a cyto-invasive bacterial pathogen

Intracellular bacterial pathogens gain entry to mammalian cells inside a vacuole derived from the host membrane. Some of them escape the bacteria-containing vacuole (BCV) and colonize the cytosol. Bacteria replicating within BCVs coopt the microtubule network to position it within infected cells, whereas the role of microtubules for cyto-invasive pathogens remains obscure. Here, we show that the microtubule motor cytoplasmic dynein-1 and specific activating adaptors are hijacked by the enterobacterium Shigella flexneri. These host proteins were found on infection-associated macropinosomes (IAMs) formed during Shigella internalization. We identified Rab8 and Rab13 as mediators of dynein recruitment and discovered that the Shigella effector protein IpaH7.8 promotes Rab13 retention on moving BCV membrane remnants, thereby facilitating membrane uncoating of the Shigella-containing vacuole. Moreover, the efficient unpeeling of BCV remnants contributes to a successful intercellular spread. Taken together, our work demonstrates how a bacterial pathogen subverts the intracellular transport machinery to secure a cytosolic niche.

Tubulin Complexity in Cancer and Metastasis

Tubulin plays a fundamental role in cellular function and as the subject for microtubule-active agents in the treatment of ovarian cancer. Microtubule-binding proteins (e.g., tau, MAP1/2/4, EB1, CLIP, TOG, survivin, stathmin) and posttranslational modifications (e.g., tyrosination, deglutamylation, acetylation, glycation, phosphorylation, polyamination) further diversify tubulin functionality and may permit additional opportunities to understand microtubule behavior in disease and to develop microtubule-modifying approaches to combat ovarian cancer. Tubulin-based structures that project from suspended ovarian cancer cells known as microtentacles may contribute to metastatic potential of ovarian cancer cells and could represent an exciting novel therapeutic target.

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.

Beyond uniformity: Exploring the heterogeneous and dynamic nature of the microtubule lattice

A fair amount of research on microtubules since their discovery in 1963 has focused on their dynamic tips. In contrast, the microtubule lattice was long believed to be highly regular and static, and consequently received far less attention. Yet, as it turned out, the microtubule lattice is neither as regular, nor as static as previously believed: structural studies uncovered the remarkable wealth of different conformations the lattice can accommodate. In the last decade, the microtubule lattice was shown to be labile and to spontaneously undergo renovation, a phenomenon that is intimately linked to structural defects and was called "microtubule self-repair". Following this breakthrough discovery, further recent research provided a deeper understanding of the lattice self-repair mechanism, which we review here. Instrumental to these discoveries were in vitro microtubule reconstitution assays, in which microtubules are grown from the minimal components required for their dynamics. In this review, we propose a shift from the term "lattice self-repair" to "lattice dynamics", since this phenomenon is an inherent property of microtubules and can happen without microtubule damage. We focus on how in vitro microtubule reconstitution assays helped us learn (1) which types of structural variations microtubules display, (2) how these structural variations influence lattice dynamics and microtubule damage caused by mechanical stress, (3) how lattice dynamics impact tip dynamics, and (4) how microtubule-associated proteins (MAPs) can play a role in structuring the lattice. Finally, we discuss the unanswered questions about lattice dynamics and how technical advances will help us tackle these questions.

Taxol acts differently on different tubulin isotypes

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

Characterization of human Ccser2 as a protein tracking the plus-ends of microtubules

OBJECTIVE

Microtubules, which are closely related to cell proliferation, have been the promising therapeutic target of cancer. Therefore, it is necessary to understand the intracellular control mechanisms of microtubules, the whole picture of which is still unclear though. Intracellular dynamics of microtubules are regulated by various microtubule-associated proteins, one group of which is microtubule plus-end-tracking proteins (+ TIPs), localizing to the extending tips of microtubules. Here, we report the identification and analysis of Ccser2 as a new + TIP in human breast cancer MCF-7 cells.

RESULTS

Ccser2 was found to be a member of + TIPs by microscopic observations including time-lapse imaging. The C-terminal region of Ccser2, including two SxIP motifs, was likely to be important for the tracking function. In MCF-7 cells, endogenous Ccser2 was mainly detected in the peripheral regions of microtubule fibers, suggesting that Ccser2 functions in cell projections.

Glutamylation is a negative regulator of microtubule growth

Microtubules are noncovalent polymers built from αβ-tubulin dimers. The disordered C-terminal tubulin tails are functionalized with multiple glutamate chains of variable lengths added and removed by tubulin tyrosine ligases (TTLLs) and carboxypeptidases (CCPs). Glutamylation is abundant on stable microtubule arrays such as in axonemes and axons, and its dysregulation leads to human pathologies. Despite this, the effects of glutamylation on intrinsic microtubule dynamics are unclear. Here we generate tubulin with short and long glutamate chains and show that glutamylation slows the rate of microtubule growth and increases catastrophes as a function of glutamylation levels. This implies that the higher stability of glutamylated microtubules in cells is due to effectors. Interestingly, EB1 is minimally affected by glutamylation and thus can report on the growth rates of both unmodified and glutamylated microtubules. Finally, we show that glutamate removal by CCP1 and 5 is synergistic and occurs preferentially on soluble tubulin, unlike TTLL enzymes that prefer microtubules. This substrate preference establishes an asymmetry whereby once the microtubule depolymerizes, the released tubulin is reset to a less-modified state, while polymerized tubulin accumulates the glutamylation mark. Our work shows that a modification on the disordered tubulin tails can directly affect microtubule dynamics and furthers our understanding of the mechanistic underpinnings of the tubulin code.

EB1 decoration of microtubule lattice facilitates spindle-kinetochore lateral attachment in Plasmodium male gametogenesis

Faithful chromosome segregation of 8 duplicated haploid genomes into 8 daughter gametes is essential for male gametogenesis and mosquito transmission of Plasmodium. Plasmodium undergoes endomitosis in this multinucleated cell division, which is highly reliant on proper spindle-kinetochore attachment. However, the mechanisms underlying the spindle-kinetochore attachment remain elusive. End-binding proteins (EBs) are conserved microtubule (MT) plus-end binding proteins and play an important role in regulating MT plus-end dynamics. Here, we report that the Plasmodium EB1 is an orthologue distinct from the canonical eukaryotic EB1. Both in vitro and in vivo assays reveal that the Plasmodium EB1 losses MT plus-end tracking but possesses MT-lattice affinity. This MT-binding feature of Plasmodium EB1 is contributed by both CH domain and linker region. EB1-deficient parasites produce male gametocytes that develop to the anucleated male gametes, leading to defective mosquito transmission. EB1 is localized at the nucleoplasm of male gametocytes. During the gametogenesis, EB1 decorates the full-length of spindle MTs and regulates spindle structure. The kinetochores attach to spindle MTs laterally throughout endomitosis and this attachment is EB1-dependent. Consequently, impaired spindle-kinetochore attachment is observed in EB1-deficient parasites. These results indicate that a parasite-specific EB1 with MT-lattice binding affinity fulfills the spindle-kinetochore lateral attachment in male gametogenesis.

Regulation of sertoli cell function by planar cell polarity (PCP) protein Fjx1

Four-jointed box kinase 1 (Fjx1) is a planar cell protein (PCP) and a member of the Fat (FAT atypical cadherin 1)/Dchs (Dachsous cadherin-related protein)/Fjx1 PCP complex. Fjx1 is also a non-receptor Ser/Thr protein kinase capable of phosphorylating Fat1 at is extracellular cadherin domains when it is transport across the Golgi system. As such, Fjx1 is a Golgi-based regulator of Fat1 function by determining its extracellular deposition. Herein, Fjx1 was found to localize across the Sertoli cell cytoplasm, partially co-localized with the microtubules (MTs) across the seminiferous epithelium. It was most notable at the apical ES (ectoplasmic specialization) and basal ES, displaying distinctive stage-specific expression. The apical ES and basal ES are the corresponding testis-specific cell adhesion ultrastructures at the Sertoli-elongated spermatid and Sertoli cell-cell interface, respectively, consistent with the role of Fjx1 as a Golgi-associated Ser/Thr kinase that modulates the Fat (and/or Dchs) integral membrane proteins. Its knockdown (KD) by RNAi using specific Fjx1 siRNA duplexes versus non-targeting negative control siRNA duplexes was found to perturb the Sertoli cell tight junction function, as well as perturbing the function and organization of MT and actin. While Fjx1 KD did not affect the steady-state levels of almost two dozens of BTB-associated Sertoli cell proteins, including structural and regulatory proteins, its KD was found to down-regulate Fat1 (but not Fat2, 3, and 4) and to up-regulate Dchs1 (but not Dchs2) expression. Based on results of biochemical analysis, Fjx1 KD was found to be capable of abolishing phosphorylation of its putative substrate Fat1 at its Ser/Thr sites, but not at its Tyr site, illustrating an intimate functional relationship of Fjx1 and Fat1 in Sertoli cells.

Genome-wide analysis of TPX2 gene family in Populus trichocarpa and its specific response genes under various abiotic stresses

Microtubules are essential for regulating cell morphogenesis, plant growth, and the response of plants to abiotic stresses. TPX2 proteins are the main players determining the spatiotemporally dynamic nature of the MTs. However, how TPX2 members respond to abiotic stresses in poplar remains largely unknown. Herein, 19 TPX2 family members were identified from the poplar genome and analyzed the structural characteristics as well as gene expression patterns. All TPX2 members had the conserved structural characteristics, but exhibited different expression profiles in different tissues, indicating their varying roles during plant growth. Additionally, several light, hormone, and abiotic stress responsive cis-acting regulatory elements were detected on the promoters of PtTPX2 genes. Furthermore, expression analysis in various tissues of Populus trichocarpa showed that the PtTPX2 genes responded differently to heat, drought and salt stress. In summary, these results provide a comprehensive analysis for the TPX2 gene family in poplar and an effective contribution to revealing the mechanisms of PtTPX2 in the regulatory network of abiotic stress.

Biomolecular condensation involving the cytoskeleton

Biomolecular condensation of proteins contributes to the organization of the cytoplasm and nucleoplasm. A number of condensation processes appear to be directly involved in regulating the structure, function and dynamics of the cytoskeleton. Liquid-liquid phase separation of cytoskeleton proteins, together with polymerization modulators, promotes cytoskeletal fiber nucleation and branching. Furthermore, the attachment of protein condensates to the cytoskeleton can contribute to cytoskeleton stability and organization, regulate transport, create patterns of functional reaction containers, and connect the cytoskeleton with membranes. Surface-bound condensates can exert and buffer mechanical forces that give stability and flexibility to the cytoskeleton, thus, may play a large role in cell biology. In this review, we introduce the concept and role of cellular biomolecular condensation, explain its special function on cytoskeletal fiber surfaces, and point out potential definition and experimental caveats. We review the current literature on protein condensation processes related to the actin, tubulin, and intermediate filament cytoskeleton, and discuss some of them in the context of neurobiology. In summary, we provide an overview about biomolecular condensation in relation to cytoskeleton structure and function, which offers a base for the exploration and interpretation of cytoskeletal condensates in neurobiology.

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

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

Kinetochore-microtubule attachment in human cells is regulated by the interaction of a conserved motif of Ska1 with EB1

The kinetochore establishes the linkage between chromosomes and the spindle microtubule plus ends during mitosis. In vertebrates, the spindle-kinetochore-associated (Ska1,2,3) complex stabilizes kinetochore attachment with the microtubule plus ends, but how Ska is recruited to and stabilized at the kinetochore-microtubule interface is not understood. Here, our results show that interaction of Ska1 with the general microtubule plus end-associated protein EB1 through a conserved motif regulates Ska recruitment to kinetochores in human cells. Ska1 forms a stable complex with EB1 via interaction with the motif in its N-terminal disordered loop region. Disruption of this interaction either by deleting or mutating the motif disrupts Ska complex recruitment to kinetochores and induces chromosome alignment defects, but it does not affect Ska complex assembly. Atomic-force microscopy imaging revealed that Ska1 is anchored to the C-terminal region of the EB1 dimer through its loop and thereby promotes formation of extended structures. Furthermore, our NMR data showed that the Ska1 motif binds to the residues in EB1 that are the binding sites of other plus end targeting proteins that are recruited to microtubules by EB1 through a similar conserved motif. Collectively, our results demonstrate that EB1-mediated Ska1 recruitment onto the microtubule serves as a general mechanism for the formation of vertebrate kinetochore-microtubule attachments and metaphase chromosome alignment.

Alternative Approaches to Understand Microtubule Cap Morphology and Function

Microtubules (MTs) are essential cellular machines built from concatenated αβ-tubulin heterodimers. They are responsible for two central and opposite functions from the dynamic point of view: scaffolding (static filaments) and force generation (dynamic MTs). These roles engage multiple physiological processes, including cell shape, polarization, division and movement, and intracellular long-distance transport. At the most basic level, the MT regulation is chemical because GTP binding and hydrolysis have the ability to promote assembly and disassembly in the absence of any other constraint. Due to the stochastic GTP hydrolysis, a chemical gradient from GTP-bound to GDP-bound tubulin is created at the MT growing end (GTP cap), which is translated into a cascade of structural regulatory changes known as MT maturation. This is an area of intense research, and several models have been proposed based on information mostly gathered from macromolecular crystallography and cryo-electron microscopy studies. However, these classical structural biology methods lack temporal resolution and can be complemented, as shown in this mini-review, by other approaches such as time-resolved fiber diffraction and computational modeling. Together with studies on structurally similar tubulins from the prokaryotic world, these inputs can provide novel insights on MT assembly, dynamics, and the GTP cap.

Chemical modulation of microtubule structure through the laulimalide/peloruside site

Taxanes are microtubule-stabilizing agents used in the treatment of many solid tumors, but they often involve side effects affecting the peripheral nervous system. It has been proposed that this could be related to structural modifications on the filament upon drug binding. Alternatively, laulimalide and peloruside bind to a different site also inducing stabilization, but they have not been exploited in clinics. Here, we use a combination of the parental natural compounds and derived analogs to unravel the stabilization mechanism through this site. These drugs settle lateral interactions without engaging the M loop, which is part of the key and lock involved in the inter-protofilament contacts. Importantly, these drugs can modulate the angle between protofilaments, producing microtubules of different diameters. Among the compounds studied, we have found some showing low cytotoxicity and able to induce stabilization without compromising microtubule native structure. This opens the window of new applications for microtubule-stabilizing agents beyond cancer treatment.

Decreased tubulin-binding cofactor B was involved in the formation disorder of nascent astrocyte processes by regulating microtubule plus-end growth through binding with end-binding proteins 1 and 3 after chronic alcohol exposure

Fetal alcohol syndrome (FAS) is a neurological disease caused by excessive drinking during pregnancy and characterized by congenital abnormalities in the structure and function of the fetal brain. This study was proposed to provide new insights into the pathogenesis of FAS by revealing the possible mechanisms of alcohol-induced astrocyte injury. First, a chronic alcohol exposure model of astrocytes was established, and the formation disorder was found in astrocyte processes where tubulin-binding cofactor B (TBCB) was decreased or lost, accompanied by disorganized microtubules (MT). Second, to understand the relationship between TBCB reduction and the formation disorder of astrocyte processes, TBCB was silenced or overexpressed. It caused astrocyte processes to retract or lose after silencing, while the processes increased with expending basal part and obtuse tips after overexpressing. It confirmed that TBCB was one of the critical factors for the formation of astrocyte processes through regulating MT plus-end and provided a new view on the pathogenesis of FAS. Third, to explore the mechanism of TBCB regulating MT plus-ends, we first proved end-binding proteins 1 and 3 (EB1/3) were bound at MT plus-ends in astrocytes. Then, through interference experiments, we found that both EB1 and EB3, which formed in heterodimers, were necessary to mediate TBCB binding to MT plus-ends and thus regulated the formation of astrocyte processes. Finally, the regulatory mechanism was studied and the ERK1/2 signaling pathway was found as one of the main pathways regulating the expression of TBCB in astrocytes after alcohol injury.

FTD-associated mutations in Tau result in a combination of dominant and recessive phenotypes

Although mutations in the microtubules-associated protein Tau have long been connected with several neurodegenerative diseases, the underlying molecular mechanisms causing these tauopathies are still not fully understood. Studies in various models suggested that dominant gain-of-function effects underlie the pathogenicity of these mutants; however, there is also evidence that the loss of normal physiological functions of Tau plays a role in tauopathies. Previous studies on Tau in Drosophila involved expressing the human Tau protein in the background of the endogenous Tau gene in addition to inducing high expression levels. To study Tau pathology in more physiological conditions, we recently created Drosophila knock-in models that express either wildtype human Tau (hTau^WT) or disease-associated mutant hTau (hTau^V337M and hTau^K369I) in place of the endogenous Drosophila Tau (dTau). Analyzing these flies as homozygotes, we could therefore detect recessive effects of the mutations while identifying dominant effects in heterozygotes. Using memory, locomotion and sleep assays, we found that homozygous mutant hTau flies showed deficits already when quite young whereas in heterozygous flies, disease phenotypes developed with aging. Homozygotes also revealed an increase in microtubule diameter, suggesting that changes in the cytoskeleton underlie the axonal degeneration we observed in these flies. In contrast, heterozygous mutant hTau flies showed abnormal axonal targeting and no detectable changes in microtubules. However, we previously showed that heterozygosity for hTau^V337M interfered with synaptic homeostasis in central pacemaker neurons and we now show that heterozygous hTau^K369I flies have decreased levels of proteins involved in the release of synaptic vesicles. Taken together, our results demonstrate that both mutations induce a combination of dominant and recessive disease-related phenotypes that provide behavioral and molecular insights into the etiology of Tauopathies.

Cytoskeleton saga: Its regulation in normal physiology and modulation in neurodegenerative disorders

Cells are fundamental units of life. To ensure the maintenance of homeostasis, integrity of structural and functional counterparts is needed to be essentially balanced. The cytoskeleton plays a vital role in regulating the cellular morphology, signalling and other factors involved in pathological conditions. Microtubules, actin (microfilaments), intermediate filaments (IF) and their interactions are required for these activities. Various proteins associated with these components are primary requirements for directing their functions. Disruption of this organization due to faulty genetics, oxidative stress or impaired transport mechanisms are the major causes of dysregulated signalling cascades leading to various pathological conditions like Alzheimer's (AD), Parkinson's (PD), Huntington's disease (HD) or amyotrophic lateral sclerosis (ALS), hereditary spastic paraplegia (HSP) or any traumatic injury like spinal cord injury (SCI). Novel or conventional therapeutic approaches may be specific or non-specific, targeting either three basic components of the cytoskeleton or various cascades that serve as a cue to numerous pathways like ROCK signalling or the GSK-3β pathway. An enormous number of drugs have been redirected for modulating the cytoskeletal dynamics and thereby may pave the way for inhibiting the progression of these diseases and their complications.

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

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

The microtubule plus-end tracking protein Bik1 is required for chromosome congression

During mitosis, sister chromatids congress on both sides of the spindle equator to facilitate the correct partitioning of the genomic material. Chromosome congression requires a finely tuned control of microtubule dynamics by the kinesin motor proteins. In Saccharomyces cerevisiae, the kinesin proteins Cin8, Kip1, and Kip3 have a pivotal role in chromosome congression. It has been hypothesized that additional proteins that modulate microtubule dynamics are involved. Here, we show that the microtubule plus-end tracking protein Bik1—the budding yeast ortholog of CLIP-170—is essential for chromosome congression. We find that nuclear Bik1 localizes to the kinetochores in a cell cycle–dependent manner. Disrupting the nuclear pool of Bik1 with a nuclear export signal (Bik1-NES) leads to slower cell-cycle progression characterized by a delayed metaphase–anaphase transition. Bik1-NES cells have mispositioned kinetochores along the spindle in metaphase. Furthermore, using proximity-dependent methods, we identify Cin8 as an interaction partner of Bik1. Deleting CIN8 reduces the amount of Bik1 at the spindle. In contrast, Cin8 retains its typical bilobed distribution in the Bik1-NES mutant and does not localize to the unclustered kinetochores. We propose that Bik1 functions with Cin8 to regulate kinetochore–microtubule dynamics for correct kinetochore positioning and chromosome congression.

Physical properties of the cytoplasm modulate the rates of microtubule polymerization and depolymerization

The cytoplasm is a crowded, visco-elastic environment whose physical properties change according to physiological or developmental states. How the physical properties of the cytoplasm impact cellular functions in vivo remains poorly understood. Here, we probe the effects of cytoplasmic concentration on microtubules by applying osmotic shifts to fission yeast, moss, and mammalian cells. We show that the rates of both microtubule polymerization and depolymerization scale linearly and inversely with cytoplasmic concentration; an increase in cytoplasmic concentration decreases the rates of microtubule polymerization and depolymerization proportionally, whereas a decrease in cytoplasmic concentration leads to the opposite. Numerous lines of evidence indicate that these effects are due to changes in cytoplasmic viscosity rather than cellular stress responses or macromolecular crowding per se. We reconstituted these effects on microtubules in vitro by tuning viscosity. Our findings indicate that, even in normal conditions, the viscosity of the cytoplasm modulates the reactions that underlie microtubule dynamic behaviors.

Super-Resolution Microscopy Reveals That Stromal Interaction Molecule 1 Trafficking Depends on Microtubule Dynamics

Store-operated Ca²⁺ entry (SOCE) is an essential pathway for Ca²⁺ signaling, and regulates various vital cellular functions. It is triggered by the endoplasmic reticulum Ca²⁺ sensor stromal interaction molecule 1 (STIM1). Illustration of STIM1 spatiotemporal structure at the nanometer scale during SOCE activation provides structural and functional insights into the fundamental Ca²⁺ homeostasis. In this study, we used direct stochastic optical reconstruction microscopy (dSTORM) to revisit the dynamic process of the interaction between STIM1, end-binding protein (EB), and microtubules to the ER-plasma membrane. Using dSTORM, we found that“powder-like”STIM1 aggregates into “trabecular-like” architectures toward the cell periphery during SOCE, and that an intact microtubule network and EB1 are essential for STIM1 trafficking. After thapsigargin treatment, STIM1 can interact with EB1 regardless of undergoing aggregation. We generated STIM1 variants adapted from a real-world database and introduced them into SiHa cells to clarify the impact of STIM1 mutations on cancer cell behavior. The p.D76G and p.D84Y variants locating on the Ca²⁺ binding domain of STIM1 result in inhibition of focal adhesion turnover, Ca²⁺ influx during SOCE and subsequent cell migration. Inversely, the p.R643C variant on the microtubule interacting domain of STIM1 leads to dissimilar consequence and aggravates cell migration. These findings imply that STIM1 mutational patterns have an impact on cancer metastasis, and therefore could be either a prognostic marker or a novel therapeutic target to inhibit the malignant behavior of STIM1-mediated cancer cells. Altogether, we generated novel insight into the role of STIM1 during SOCE activation, and uncovered the impact of real-world STIM1 variants on cancer cells.

HIV-1 Establishes a Sanctuary Site in the Testis by Permeating the BTB Through Changes in Cytoskeletal Organization

Studies suggest that HIV-1 invades the testis through initial permeation of the blood-testis barrier (BTB). The selectivity of the BTB to antiretroviral drugs makes this site a sanctuary for the virus. Little is known about how HIV-1 crosses the BTB and invades the testis. Herein, we used 2 approaches to examine the underlying mechanism(s) by which HIV-1 permeates the BTB and gains entry into the seminiferous epithelium. First, we examined if recombinant Tat protein was capable of perturbing the BTB and making the barrier leaky, using the primary rat Sertoli cell in vitro model that mimics the BTB in vivo. Second, we used HIV-1-infected Sup-T1 cells to investigate the activity of HIV-1 infection on cocultured Sertoli cells. Using both approaches, we found that the Sertoli cell tight junction permeability barrier was considerably perturbed and that HIV-1 effectively permeates the BTB by inducing actin-, microtubule-, vimentin-, and septin-based cytoskeletal changes in Sertoli cells. These studies suggest that HIV-1 directly perturbs BTB function, potentially through the activity of the Tat protein.

Phylogeny and structural peculiarities of the EB proteins of diatoms

The end-binding proteins are a family of microtubule-associated proteins; this family belongs to plus-end-tracking proteins (+TIPs) that regulate microtubule growth and stabilisation. Although the genes encoding EB proteins are found in all eukaryotic genomes, most studies of them have centred on one or another taxonomic group, without a broad comparative analysis. Here, we present a first phylogenetic analysis and a comparative analysis of domain structures of diatom EB proteins in comparison with other phyla of Chromista, red and green algae, as well as model organisms A. thaliana and H. sapiens. Phylogenetically, diatom EB proteins are separated into six clades, generally corresponding to the phylogeny of their respective organisms. The domain structure of this family is highly variable, but the CH and EBH domains responsible for binding tubulin and other MAPs are mostly conserved. Homologous modelling of the F. cylindrus EB protein shows that conserved motifs of the CH domain are positioned on the protein surface, which is necessary for their functioning. We hypothesise that high variance of the diatom C-terminal domain is caused by previously unknown interactions with a CAP-GLY motif of dynactin subunit p150. Our findings contribute to wider possibilities for further investigations of the cytoskeleton in diatoms.

α-tubulin tail modifications regulate microtubule stability through selective effector recruitment, not changes in intrinsic polymer dynamics

Microtubules are non-covalent polymers of αβ-tubulin dimers. Posttranslational processing of the intrinsically disordered C-terminal α-tubulin tail produces detyrosinated and Δ2-tubulin. Although these are widely employed as proxies for stable cellular microtubules, their effect (and of the α-tail) on microtubule dynamics remains uncharacterized. Using recombinant, engineered human tubulins, we now find that neither detyrosinated nor Δ2-tubulin affect microtubule dynamics, while the α-tubulin tail is an inhibitor of microtubule growth. Consistent with the latter, molecular dynamics simulations show the α-tubulin tail transiently occluding the longitudinal microtubule polymerization interface. The marked differential in vivo stabilities of the modified microtubule subpopulations, therefore, must result exclusively from selective effector recruitment. We find that tyrosination quantitatively tunes CLIP-170 density at the growing plus end and that CLIP170 and EB1 synergize to selectively upregulate the dynamicity of tyrosinated microtubules. Modification-dependent recruitment of regulators thereby results in microtubule subpopulations with distinct dynamics, a tenet of the tubulin code hypothesis.

Trim9 and Klp61F promote polymerization of new dendritic microtubules along parallel microtubules

Axons and dendrites are distinguished by microtubule polarity. In Drosophila, dendrites are dominated by minus-end-out microtubules, whereas axons contain plus-end-out microtubules. Local nucleation in dendrites generates microtubules in both orientations. To understand why dendritic nucleation does not disrupt polarity, we used live imaging to analyze the fate of microtubules generated at branch points. We found that they had different rates of success exiting the branch based on orientation: correctly oriented minus-end-out microtubules succeeded in leaving about twice as often as incorrectly oriented microtubules. Increased success relied on other microtubules in a parallel orientation. From a candidate screen, we identified Trim9 and kinesin-5 (Klp61F) as machinery that promoted growth of new microtubules. In S2 cells, Eb1 recruited Trim9 to microtubules. Klp61F promoted microtubule growth in vitro and in vivo, and could recruit Trim9 in S2 cells. In summary, the data argue that Trim9 and kinesin-5 act together at microtubule plus ends to help polymerizing microtubules parallel to pre-existing ones resist catastrophe.

Endoplasmic Reticulum-Plasma Membrane Contact Sites as an Organizing Principle for Compartmentalized Calcium and cAMP Signaling

In eukaryotic cells, ultimate specificity in activation and action-for example, by means of second messengers-of the myriad of signaling cascades is primordial. In fact, versatile and ubiquitous second messengers, such as calcium (Ca2+) and cyclic adenosine monophosphate (cAMP), regulate multiple-sometimes opposite-cellular functions in a specific spatiotemporal manner. Cells achieve this through segregation of the initiators and modulators to specific plasma membrane (PM) subdomains, such as lipid rafts and caveolae, as well as by dynamic close contacts between the endoplasmic reticulum (ER) membrane and other intracellular organelles, including the PM. Especially, these membrane contact sites (MCSs) are currently receiving a lot of attention as their large influence on cell signaling regulation and cell physiology is increasingly appreciated. Depletion of ER Ca2+ stores activates ER membrane STIM proteins, which activate PM-residing Orai and TRPC Ca2+ channels at ER-PM contact sites. Within the MCS, Ca2+ fluxes relay to cAMP signaling through highly interconnected networks. However, the precise mechanisms of MCS formation and the influence of their dynamic lipid environment on their functional maintenance are not completely understood. The current review aims to provide an overview of our current understanding and to identify open questions of the field.

Moonlighting of mitotic regulators in cilium disassembly

Correct timing of cellular processes is essential during embryological development and to maintain the balance between healthy proliferation and tumour formation. Assembly and disassembly of the primary cilium, the cell’s sensory signalling organelle, are linked to cell cycle timing in the same manner as spindle pole assembly and chromosome segregation. Mitotic processes, ciliary assembly, and ciliary disassembly depend on the centrioles as microtubule-organizing centres (MTOC) to regulate polymerizing and depolymerizing microtubules. Subsequently, other functional protein modules are gathered to potentiate specific protein–protein interactions. In this review, we show that a significant subset of key mitotic regulator proteins is moonlighting at the cilium, among which PLK1, AURKA, CDC20, and their regulators. Although ciliary assembly defects are linked to a variety of ciliopathies, ciliary disassembly defects are more often linked to brain development and tumour formation. Acquiring a better understanding of the overlap in regulators of ciliary disassembly and mitosis is essential in finding therapeutic targets for the different diseases and types of tumours associated with these regulators.

Profilin-A master coordinator of actin and microtubule organization in mammalian cells

The last two decades have witnessed a tremendous increase in cell biology data. Not least is this true for studies of the dynamic organization of the microfilament and microtubule systems in animal cells where analyses of the molecular components and their interaction patterns have deepened our understanding of these complex force-generating machineries. Previous observations of a molecular cross-talk between the two systems have now led to the realization of the existence of several intricate mechanisms operating to maintain their coordinated cellular organization. In this short review, we relate to this development by discussing new results concerning the function of the actin regulator profilin 1 as a control component of microfilament-microtubule cross-talk.

EB1 Is Essential for Spindle Formation and Chromosome Alignment During Oocyte Meiotic Maturation in Mice

The cytoskeleton plays an orchestrating role in polarized cell growth. Microtubules (MTs) not only play critical roles in chromosome alignment and segregation but also control cell shape, division, and motility. A member of the plus-end tracking proteins, end-binding protein 1 (EB1), regulates MT dynamics and plays vital roles in maintaining spindle symmetry and chromosome alignment during mitosis. However, the role of EB1 in mouse oocyte meiosis remains unknown. Here, we examined the localization patterns and expression levels of EB1 at different stages. EB1 protein level was found to be stable during meiosis. EB1 mainly localized along the spindle and had a similar localization pattern as that of α-tubulin. The EB1 protein was degraded with a Trim-Away method, and the results were further confirmed with western blotting and immunofluorescence. At 12 h of culture after EB1 knockdown (KD), a reduced number of mature MII oocytes were observed. EB1 KD led to spindle disorganization, chromosome misalignment, and missegregation; β-catenin protein binds to actin via the adherens junctional complex, which was significantly reduced in the EB1 KD oocytes. Collectively, we propose that the impairment of EB1 function manipulates spindle formation, thereby promoting chromosomal loss, which is expected to fuel aneuploidy and possibly fertilization failure.

KIF15 supports spermatogenesis via its effects on Sertoli cell microtubule, actin, vimentin, and septin cytoskeletons

Throughout spermatogenesis, cellular cargoes including haploid spermatids are required to be transported across the seminiferous epithelium, either toward the microtubule (MT) plus (+) end near the basement membrane at stage V, or to the MT minus (-) end near the tubule lumen at stages VI to VIII of the epithelial cycle. Furthermore, preleptotene spermatocytes, differentiated from type B spermatogonia, are transported across the Sertoli cell blood-testis barrier (BTB) to enter the adluminal compartment. Few studies, however, have been conducted to explore the function of MT-dependent motor proteins to support spermatid transport during spermiogenesis. Herein, we examined the role of MT-dependent and microtubule plus (+) end-directed motor protein kinesin 15 (KIF15) in the testis. KIF15 displayed a stage-specific expression across the seminiferous epithelium, associated with MTs, and appeared as aggregates on the MT tracks that aligned perpendicular to the basement membrane and laid across the entire epithelium. KIF15 also tightly associated with apical ectoplasmic specialization, displaying strict stage-specific distribution, apparently to support spermatid transport across the epithelium. We used a loss-of-function approach by RNAi to examine the role of KIF15 in Sertoli cell epithelium in vitro to examine its role in cytoskeletal-dependent Sertoli cell function. It was noted that KIF15 knockdown by RNAi that reduced KIF15 expression by ~70% in Sertoli cells with an established functional tight junction barrier impeded the barrier function. This effect was mediated through remarkable changes in the cytoskeletal organization of MTs, but also actin-, vimentin-, and septin-based cytoskeletons, illustrating that KIF15 exerts its regulatory effects well beyond microtubules.

The proline-rich domain promotes Tau liquid-liquid phase separation in cells

Tau protein in vitro can undergo liquid–liquid phase separation (LLPS); however, observations of this phase transition in living cells are limited. To investigate protein state transitions in living cells, we attached Cry2 to Tau and studied the contribution of each domain that drives the Tau cluster in living cells. Surprisingly, the proline-rich domain (PRD), not the microtubule binding domain (MTBD), drives LLPS and does so under the control of its phosphorylation state. Readily observable, PRD-derived cytoplasmic condensates underwent fusion and fluorescence recovery after photobleaching consistent with the PRD LLPS in vitro. Simulations demonstrated that the charge properties of the PRD predicted phase separation. Tau PRD formed heterotypic condensates with EB1, a regulator of plus-end microtubule dynamic instability. The specific domain properties of the MTBD and PRD serve distinct but mutually complementary roles that use LLPS in a cellular context to implement emergent functionalities that scale their relationship from binding α-beta tubulin heterodimers to the larger proportions of microtubules.

Drosophila Tubulin-Specific Chaperone E Recruits Tubulin around Chromatin to Promote Mitotic Spindle Assembly

Proper assembly of mitotic spindles requires microtubule nucleation not only at the centrosomes but also around chromatin. In this study, we found that the Drosophila tubulin-specific chaperone dTBCE is required for the enrichment of tubulin in the nuclear space after nuclear envelope breakdown and for subsequent promotion of spindle microtubule nucleation. These events depend on the CAP-Gly motif found in dTBCE and are regulated by Ran and lamin proteins. Our data suggest that during early mitosis, dTBCE and nuclear pore proteins become enriched in the nucleus, where they interact with the Ran GTPase to promote dynamic tubulin enrichment. We propose that this novel mechanism enhances microtubule nucleation around chromatin, thereby facilitating mitotic spindle assembly.

Low concentrations of vorinostat decrease EB1 expression in GBM cells and affect microtubule dynamics, cell survival and migration

Glioblastoma multiform (GBM) is the most frequent primitive brain tumor with a high recurrence and mortality. Histone deacetylase inhibitors (HDACi) have evoked great interest because they are able to change transcriptomic profiles to promote tumor cell death but also induce side effects due to the lack of selectivity. We show in this paper new anticancer properties and mechanisms of action of low concentrations of vorinostat on various GBM cells which acts by affecting microtubule cytoskeleton in a non-histone 3 (H3) manner. Indeed, vorinostat induces tubulin acetylation and detyrosination, affects EB stabilizing cap on microtubule plus ends and suppresses microtubule dynamic instability. We previously identified EB1 overexpression as a marker of bad prognostic in GBM. Interestingly, we show for the first time to our knowledge, a strong decrease of EB1 expression in GBM cells by a drug. Altogether, our results suggest that low dose vorinostat, which is more selective for HDAC6 inhibition, could therefore represent an interesting therapeutic option for GBM especially in patients with EB1 overexpressing tumor with lower expected side effects. A validation of our hypothesis is needed during future clinical trials with this drug in GBM.

Plectin dysfunction in neurons leads to tau accumulation on microtubules affecting neuritogenesis, organelle trafficking, pain sensitivity and memory

AIMS

Plectin, a universally expressed multi-functional cytolinker protein, is crucial for intermediate filament networking, including crosstalk with actomyosin and microtubules. In addition to its involvement in a number of diseases affecting skin, skeletal muscle, heart, and other stress-exposed tissues, indications for a neuropathological role of plectin have emerged. Having identified P1c as the major isoform expressed in neural tissues in previous studies, our aim for the present work was to investigate whether, and by which mechanism(s), the targeted deletion of this isoform affects neuritogenesis and proper nerve cell functioning.

METHODS

For ex vivo phenotyping, we used dorsal root ganglion and hippocampal neurons derived from isoform P1c-deficient and plectin-null mice, complemented by in vitro experiments using purified proteins and cell fractions. To assess the physiological significance of the phenotypic alterations observed in P1c-deficient neurons, P1c-deficient and wild-type littermate mice were subjected to standard behavioural tests.

RESULTS

We demonstrate that P1c affects axonal microtubule dynamics by isoform-specific interaction with tubulin. P1c deficiency in neurons leads to altered dynamics of microtubules and excessive association with tau protein, affecting neuritogenesis, neurite branching, growth cone morphology, and translocation and directionality of movement of vesicles and mitochondria. On the organismal level, we found P1c deficiency manifesting as impaired pain sensitivity, diminished learning capabilities and reduced long-term memory of mice.

CONCLUSIONS

Revealing a regulatory role of plectin scaffolds in microtubule-dependent nerve cell functions, our results have potential implications for cytoskeleton-related neuropathies.

NC1-peptide derived from collagen α3 (IV) chain is a blood-tissue barrier regulator: lesson from the testis

Collagen α3 (IV) chains are one of the major constituent components of the basement membrane in the mammalian testis. Studies have shown that biologically active fragments, such as noncollagenase domain (NC1)-peptide, can be released from the C-terminal region of collagen α3 (IV) chains, possibly through the proteolytic action of metalloproteinase 9 (MMP9). NC1-peptide was shown to promote blood–testis barrier (BTB) remodeling and fully developed spermatid (e.g., sperm) release from the seminiferous epithelium because this bioactive peptide was capable of perturbing the organization of both actin- and microtubule (MT)-based cytoskeletons at the Sertoli cell–cell and also Sertoli–spermatid interface, the ultrastructure known as the basal ectoplasmic specialization (ES) and apical ES, respectively. More importantly, recent studies have shown that this NC1-peptide-induced effects on cytoskeletal organization in the testis are mediated through an activation of mammalian target of rapamycin complex 1/ribosomal protein S6/transforming retrovirus Akt1/2 protein (mTORC1/rpS6/Akt1/2) signaling cascade, involving an activation of cell division control protein 42 homolog (Cdc42) GTPase, but not Ras homolog family member A GTPase (RhoA), and the participation of end-binding protein 1 (EB1), a microtubule plus (+) end tracking protein (+TIP), downstream. Herein, we critically evaluate these findings, providing a critical discussion by which the basement membrane modulates spermatogenesis through one of its locally generated regulatory peptides in the testis.

Regulation of cytoskeleton-associated protein activities: Linking cellular signals to plant cytoskeletal function

The plant cytoskeleton undergoes dynamic remodeling in response to diverse developmental and environmental cues. Remodeling of the cytoskeleton coordinates growth in plant cells, including trafficking and exocytosis of membrane and wall components during cell expansion, and regulation of hypocotyl elongation in response to light. Cytoskeletal remodeling also has key functions in disease resistance and abiotic stress responses. Many stimuli result in altered activity of cytoskeleton-associated proteins, microtubule-associated proteins (MAPs) and actin-binding proteins (ABPs). MAPs and ABPs are the main players determining the spatiotemporally dynamic nature of the cytoskeleton, functioning in a sensory hub that decodes signals to modulate plant cytoskeletal behavior. Moreover, MAP and ABP activities and levels are precisely regulated during development and environmental responses, but our understanding of this process remains limited. In this review, we summarize the evidence linking multiple signaling pathways, MAP and ABP activities and levels, and cytoskeletal rearrangements in plant cells. We highlight advances in elucidating the multiple mechanisms that regulate MAP and ABP activities and levels, including calcium and calmodulin signaling, ROP GTPase activity, phospholipid signaling, and post-translational modifications.

Target Molecules of STIM Proteins in the Central Nervous System

Stromal interaction molecules (STIMs), including STIM1 and STIM2, are single-pass transmembrane proteins that are located predominantly in the endoplasmic reticulum (ER). They serve as calcium ion (Ca²⁺) sensors within the ER. In the central nervous system (CNS), they are involved mainly in Orai-mediated store-operated Ca²⁺ entry (SOCE). The key molecular components of the SOCE pathway are well-characterized, but the molecular mechanisms that underlie the regulation of this pathway need further investigation. Numerous intracellular target proteins that are located in the plasma membrane, ER, cytoskeleton, and cytoplasm have been reported to play essential roles in concert with STIMs, such as conformational changes in STIMs, their translocation, the stabilization of their interactions with Orai, and the activation of other channels. The present review focuses on numerous regulators, such as Homer, SOCE-associated regulatory factor (SARAF), septin, synaptopodin, golli proteins, partner of STIM1 (POST), and transcription factors and proteasome inhibitors that regulate STIM-Orai interactions in the CNS. Further we describe novel roles of STIMs in mediating Ca²⁺ influx via other than Orai pathways, including TRPC channels, VGCCs, AMPA and NMDA receptors, and group I metabotropic glutamate receptors. This review also summarizes recent findings on additional molecular targets of STIM proteins including SERCA, IP₃Rs, end-binding proteins (EB), presenilin, and CaMKII. Dysregulation of the SOCE-associated toolkit, including STIMs, contributes to the development of neurodegenerative disorders (e.g., Alzheimer's disease, Parkinson's disease, and Huntington's disease), traumatic brain injury, epilepsy, and stroke. Emerging evidence points to the role of STIM proteins and several of their molecular effectors and regulators in neuronal and glial physiology and pathology, suggesting their potential application for future therapeutic strategies.

Microtubules enter centre stage for morphogenesis

Cell shape changes are key to observable changes at the tissue level during morphogenesis and organ formation. The major driver of cell shape changes in turn is the actin cytoskeleton, both in the form of protrusive linear or branched dynamic networks and in the form of contractile actomyosin. Over the last 20 years, actomyosin has emerged as the major cytoskeletal system that deforms cells in epithelial sheets during morphogenesis. By contrast, the second major cytoskeletal system, microtubules, have so far mostly been assumed to serve 'house-keeping' functions, such as directed transport or cell division, during morphogenetic events. Here, I will reflect on a subset of studies over the last 10 years that have clearly shown a major direct role for the microtubule cytoskeleton in epithelial morphogenesis, suggesting that our focus will need to be widened to give more attention and credit to this cytoskeletal system in playing an active morphogenetic role. This article is part of a discussion meeting issue 'Contemporary morphogenesis'.

NC1-Peptide From Collagen α3 (IV) Chains in the Basement Membrane of Testes Regulates Spermatogenesis via p-FAK-Y407

The blood-testis barrier (BTB) in the testis is an important ultrastructure to support spermatogenesis. This blood-tissue barrier undergoes remodeling at late stage VII to early stage IX of the epithelial cycle to support the transport of preleptotene spermatocytes across the BTB to prepare for meiosis I/II at the apical compartment through a mechanism that remains to be delineated. Studies have shown that NC1-peptide-derived collagen α3 (IV) chain in the basement membrane is a bioactive peptide that induces BTB remodeling. It also promotes the release of fully developed spermatids into the tubule lumen. Thus, this endogenously produced peptide coordinates these 2 cellular events across the seminiferous epithelium. Using an NC1-peptide complementary deoxyribonucleic acid (cDNA) construct to transfect adult rat testes for overexpression, NC1-peptide was found to effectively induce germ cell exfoliation and BTB remodeling, which was associated with a surge and activation of p-rpS6, the downstream signaling protein of mTORC1 and the concomitant downregulation of p-FAK-Y407 in the testis. In order to define the functional relationship between p-rpS6 and p-FAK-Y407 signaling to confer the ability of NC1-peptide to regulate testis function, a phosphomimetic (and thus constitutively active) mutant of p-FAK-Y407 (p-FAK-Y407E-MT) was used for its co-transfection, utilizing Sertoli cells cultured in vitro with a functional tight junction (TJ) barrier that mimicked the BTB in vivo. Overexpression of p-FAK-Y407E-MT blocked the effects of NC1-peptide to perturb Sertoli cell BTB function by promoting F-actin and microtubule cytoskeleton function, and downregulated the NC1-peptide-mediated induction of p-rpS6 activation. In brief, NC1-peptide is an important endogenously produced biomolecule that regulates BTB dynamics.

Microtubule Cytoskeleton and Spermatogenesis-Lesson From Studies of Toxicant Models

Studies have shown that mammalian testes, in particular the Sertoli cells, are highly susceptible to exposure of environmental toxicants, such as cadmium, perfluorooctanesulfonate, phthalates, 2,5-hexanedione and bisphenol A. However, important studies conducted by reproductive toxicologists and/or biologists in the past have been treated as toxicology reports per se. Yet, many of these studies provided important mechanistic insights on the toxicant-induced testis injury and reproductive dysfunction, relevant to the biology of the testis and spermatogenesis. Furthermore, recent studies have shown that findings obtained from toxicant models are exceedingly helpful tools to unravel the biology of testis function in particular spermatogenesis, including specific cellular events associated with spermatid transport to support spermiogenesis and spermiation. In this review, we critically evaluate some recent data, focusing primarily on the molecular structure and role of microtubules in cellular function, illustrating the importance of toxicant models to unravel the biology of microtubule cytoskeleton in supporting spermatogenesis, well beyond information on toxicology. These findings have opened up some potential areas of research which should be carefully evaluated in the years to come.

Arl4D-EB1 interaction promotes centrosomal recruitment of EB1 and microtubule growth

ADP-ribosylation factor (Arf)-like 4D (Arl4D), one of the Arf-like small GTPases, functions in the regulation of cell morphology, cell migration, and actin cytoskeleton remodeling. End-binding 1 (EB1) is a microtubule (MT) plus-end tracking protein that preferentially localizes at the tips of the plus ends of growing MTs and at the centrosome. EB1 depletion results in many centrosome-related defects. Here, we report that Arl4D promotes the recruitment of EB1 to the centrosome and regulates MT nucleation. We first showed that Arl4D interacts with EB1 in a GTP-dependent manner. This interaction is dependent on the C-terminal EB homology region of EB1 and partially dependent on an SxLP motif of Arl4D. We found that Arl4D colocalized with γ-tubulin in centrosomes and the depletion of Arl4D resulted in a centrosomal MT nucleation defect. We further demonstrated that abolishing Arl4D-EB1 interaction decreased MT nucleation rate and diminished the centrosomal recruitment of EB1 without affecting MT growth rate. In addition, Arl4D binding to EB1 increased the association between the p150 subunit of dynactin and the EB1, which is important for MT stabilization. Together, our results indicate that Arl4D modulates MT nucleation through regulation of the EB1–p150 association at the centrosome.

Intrinsic and Extrinsic Factors Affecting Microtubule Dynamics in Normal and Cancer Cells

Microtubules (MTs), highly dynamic structures composed of α- and β-tubulin heterodimers, are involved in cell movement and intracellular traffic and are essential for cell division. Within the cell, MTs are not uniform as they can be composed of different tubulin isotypes that are post-translationally modified and interact with different microtubule-associated proteins (MAPs). These diverse intrinsic factors influence the dynamics of MTs. Extrinsic factors such as microtubule-targeting agents (MTAs) can also affect MT dynamics. MTAs can be divided into two main categories: microtubule-stabilizing agents (MSAs) and microtubule-destabilizing agents (MDAs). Thus, the MT skeleton is an important target for anticancer therapy. This review discusses factors that determine the microtubule dynamics in normal and cancer cells and describes microtubule-MTA interactions, highlighting the importance of tubulin isoform diversity and post-translational modifications in MTA responses and the consequences of such a phenomenon, including drug resistance development.

Profilin choreographs actin and microtubules in cells and cancer

Actin and microtubules play essential roles in aberrant cell processes that define and converge in cancer including: signaling, morphology, motility, and division. Actin and microtubules do not directly interact, however shared regulators coordinate these polymers. While many of the individual proteins important for regulating and choreographing actin and microtubule behaviors have been identified, the way these molecules collaborate or fail in normal or disease contexts is not fully understood. Decades of research focus on Profilin as a signaling molecule, lipid-binding protein, and canonical regulator of actin assembly. Recent reports demonstrate that Profilin also regulates microtubule dynamics and polymerization. Thus, Profilin can coordinate both actin and microtubule polymer systems. Here we reconsider the biochemical and cellular roles for Profilin with a focus on the essential cytoskeletal-based cell processes that go awry in cancer. We also explore how the use of model organisms has helped to elucidate mechanisms that underlie the regulatory essence of Profilin in vivo and in the context of disease.

Plant and mouse EB1 proteins have opposite intrinsic properties on the dynamic instability of microtubules

OBJECTIVE

Most eukaryotic cells contain microtubule filaments, which play central roles in intra-cellular organization. However, microtubule networks have a wide variety of architectures from one cell type and organism to another. Nonetheless, the sequences of tubulins, of Microtubule Associated proteins (MAPs) and the structure of microtubules are usually well conserved throughout the evolution. MAPs being known to be responsible for regulating microtubule organization and dynamics, this raises the question of the conservation of their intrinsic properties. Indeed, knowing how the intrinsic properties of individual MAPs differ between organisms might enlighten our understanding of how distinct microtubule networks are built. End-Binding protein 1 (EB1), first described as a MAP in yeast, is conserved in plants and mammals. The intrinsic properties of the mammalian and the yeast EB1 proteins have been well described in the literature but, to our knowledge, the intrinsic properties of EB1 from plant and mammals have not been compared thus far.

RESULTS

Here, using an in vitro assay, we discovered that plant and mammalian EB1 purified proteins have different intrinsic properties on microtubule dynamics. Indeed, the mammalian EB1 protein increases microtubules dynamic while the plant EB1 protein stabilizes them.