Kif4 interacts with EB1 and stabilizes microtubules downstream of Rho-mDia in migrating fibroblasts

TRIM Proteins and Antiviral Microtubule Reorganization: A Novel Component in Innate Immune Responses?

TRIM proteins are a family of innate immune factors that play diverse roles in innate immunity and protect the cell against viral and bacterial aggression. As part of this special issue on TRIM proteins, we will take advantage of our findings on TRIM69, which acts by reorganizing the microtubules (MTs) in a manner that is fundamentally antiviral, to more generally discuss how host-pathogen interactions that take place for the control of the MT network represent a crucial facet of the struggle that opposes viruses to their cell environment. In this context, we will present several other TRIM proteins that are known to interact with microtubules in situations other than viral infection, and we will discuss evidence that may suggest a possible contribution to viral control. Overall, the present review will highlight the importance that the control of the microtubule network bears in host-pathogen interactions.

Dual spatio-temporal regulation of axon growth and microtubule dynamics by RhoA signaling pathways

RhoA plays a crucial role in neuronal polarization, where its action restraining axon outgrowth has been thoroughly studied. We now report that RhoA has not only an inhibitory but also a stimulatory effect on axon development depending on when and where exerts its action and the downstream effectors involved. In cultured hippocampal neurons, FRET imaging revealed that RhoA activity selectively localized in growth cones of undifferentiated neurites, whereas in developing axons it displayed a biphasic pattern, being low in nascent axons and high in elongating ones. RhoA-Rho kinase (ROCK) signaling prevented axon initiation but had no effect on elongation, whereas formin inhibition reduced axon extension without significantly altering initial outgrowth. In addition, RhoA-mDia signaling promoted axon elongation by stimulating growth cone microtubule stability and assembly, as opposed to RhoA-ROCK signaling, which restrained growth cone microtubule assembly and protrusion.

End-binding protein 1 promotes specific motor-cargo association in the cell body prior to axonal delivery of dense core vesicles

Axonal transport is key to neuronal function. Efficient transport requires specific motor-cargo association in the soma, yet the mechanisms regulating this early step remain poorly understood. We found that EBP-1, the C. elegans ortholog of the canonical-microtubule-end-binding protein EB1, promotes the specific association between kinesin-3/KIF1A/UNC-104 and dense core vesicles (DCVs) prior to their axonal delivery. Using single-neuron, in vivo labeling of endogenous cargo and EBs, we observed reduced axonal abundance and reduced secretion of DCV cargo, but not other KIF1A/UNC-104 cargoes, in ebp-1 mutants. This reduction could be traced back to fewer exit events from the cell body, where EBP-1 colocalized with the DCV sorting machinery at the trans Golgi, suggesting that this is the site of EBP-1 function. EBP-1 calponin homology (CH) domain was required for directing microtubule growth on the Golgi, and mammalian EB1 interacted with KIF1A in an EBH-domain-dependent manner. Loss- and gain-of-function experiments suggest a model in which both kinesin-3 binding and guidance of microtubule growth at the trans Golgi by EBP-1 promote motor-cargo association at sites of DCV biogenesis. In support of this model, tethering either EBP-1 or a kinesin-3/KIF1A/UNC-104-interacting domain from an unrelated protein to the Golgi restored the axonal abundance of DCV proteins in ebp-1 mutants. These results uncover an unexpected role for a microtubule-associated protein and provide insights into how specific kinesin-3 cargo is delivered to the axon.

HIV Infection: Shaping the Complex, Dynamic, and Interconnected Network of the Cytoskeleton

HIV-1 has evolved a plethora of strategies to overcome the cytoskeletal barrier (i.e., actin and intermediate filaments (AFs and IFs) and microtubules (MTs)) to achieve the viral cycle. HIV-1 modifies cytoskeletal organization and dynamics by acting on associated adaptors and molecular motors to productively fuse, enter, and infect cells and then traffic to the cell surface, where virions assemble and are released to spread infection. The HIV-1 envelope (Env) initiates the cycle by binding to and signaling through its main cell surface receptors (CD4/CCR5/CXCR4) to shape the cytoskeleton for fusion pore formation, which permits viral core entry. Then, the HIV-1 capsid is transported to the nucleus associated with cytoskeleton tracks under the control of specific adaptors/molecular motors, as well as HIV-1 accessory proteins. Furthermore, HIV-1 drives the late stages of the viral cycle by regulating cytoskeleton dynamics to assure viral Pr55Gag expression and transport to the cell surface, where it assembles and buds to mature infectious virions. In this review, we therefore analyze how HIV-1 generates a cell-permissive state to infection by regulating the cytoskeleton and associated factors. Likewise, we discuss the relevance of this knowledge to understand HIV-1 infection and pathogenesis in patients and to develop therapeutic strategies to battle HIV-1.

End-binding protein 1 regulates the metabolic fate of CD4+ T lymphoblasts and Jurkat T cells and the organization of the mitochondrial network

The organization of the mitochondrial network is relevant for the metabolic fate of T cells and their ability to respond to TCR stimulation. This arrangement depends on cytoskeleton dynamics in response to TCR and CD28 activation, which allows the polarization of the mitochondria through their change in shape, and their movement along the microtubules towards the immune synapse. This work focus on the role of End-binding protein 1 (EB1), a protein that regulates tubulin polymerization and has been previously identified as a regulator of intracellular transport of CD3-enriched vesicles. EB1-interferred cells showed defective intracellular organization and metabolic strength in activated T cells, pointing to a relevant connection of the cytoskeleton and metabolism in response to TCR stimulation, which leads to increased AICD. By unifying the organization of the tubulin cytoskeleton and mitochondria during CD4⁺ T cell activation, this work highlights the importance of this connection for critical cell asymmetry together with metabolic functions such as glycolysis, mitochondria respiration, and cell viability.

End Binding protein 1 promotes specific motor-cargo association in the cell body prior to axonal delivery of Dense Core Vesicles

Axonal transport is key to neuronal function. Efficient transport requires specific motor-cargo association in the soma, yet the mechanisms regulating this early step remain poorly understood. We found that EBP-1, the C. elegans ortholog of the canonical microtubule end binding protein EB1, promotes the specific association between kinesin-3/KIF1A/UNC-104 and Dense Core Vesicles (DCVs) prior to their axonal delivery. Using single-neuron, in vivo labelling of endogenous cargo and EBs, we observed reduced axonal abundance and reduced secretion of DCV cargo, but not other KIF1A/UNC-104 cargo, in ebp-1 mutants. This reduction could be traced back to fewer exit events from the cell body, where EBP-1 colocalized with the DCV sorting machinery at the trans Golgi, suggesting that this is the site of EBP-1 function. In addition to its microtubule binding CH domain, mammalian EB1 interacted with mammalian KIF1A in an EBH domain dependent manner, and expression of mammalian EB1 or the EBH domain was sufficient to rescue DCV transport in ebp-1 mutants. Our results suggest a model in which kinesin-3 binding and microtubule binding by EBP-1 cooperate to transiently enrich the motor near sites of DCV biogenesis to promote motor-cargo association. In support of this model, tethering either EBP-1 or a kinesin-3 KIF1A/UNC-104 interacting domain from an unrelated protein to the Golgi restored the axonal abundance of DCV proteins in ebp-1 mutants. These results uncover an unexpected role for a microtubule associated protein and provide insight into how specific kinesin-3 cargo are delivered to the axon.

Kinesin Motors in the Filamentous Basidiomycetes in Light of the Schizophyllum commune Genome

Kinesins are essential motor molecules of the microtubule cytoskeleton. All eukaryotic organisms have several genes encoding kinesin proteins, which are necessary for various cell biological functions. During the vegetative growth of filamentous basidiomycetes, the apical cells of long leading hyphae have microtubules extending toward the tip. The reciprocal exchange and migration of nuclei between haploid hyphae at mating is also dependent on cytoskeletal structures, including the microtubules and their motor molecules. In dikaryotic hyphae, resulting from a compatible mating, the nuclear location, synchronous nuclear division, and extensive nuclear separation at telophase are microtubule-dependent processes that involve unidentified molecular motors. The genome of Schizophyllum commune is analyzed as an example of a species belonging to the Basidiomycota subclass, Agaricomycetes. In this subclass, the investigation of cell biology is restricted to a few species. Instead, the whole genome sequences of several species are now available. The analyses of the mating type genes and the genes necessary for fruiting body formation or wood degrading enzymes in several genomes of Agaricomycetes have shown that they are controlled by comparable systems. This supports the idea that the genes regulating the cell biological process in a model fungus, such as the genes encoding kinesin motor molecules, are also functional in other filamentous Agaricomycetes.

Post-translational modifications and stabilization of microtubules regulate transport of viral factors during infections

Tubulin post-translational modifications (PTMs) constitute a source of diversity for microtubule (MT) functions, in addition to the different isotypes of α and β-tubulin acting as building blocks of MTs. Also, MT-associated proteins (MAPs) confer different characteristics to MTs. The combination of all these factors regulates the stability of these structures that act as rails to transport organelles within the cell, facilitating the association of motor complexes. All these functions are involved in crucial cellular processes in most cell types, ranging from spindle formation in mitosis to the defense against incoming cellular threats during phagocytosis mediated by immune cells. The regulation of MT dynamics through tubulin PTMs has evolved to depend on many different factors that act in a complex orchestrated manner. These tightly regulated processes are particularly relevant during the induction of effective immune responses against pathogens. Viruses have proved not only to hijack MTs and MAPs in order to favor an efficient infection, but also to induce certain PTMs that improve their cellular spread and lead to secondary consequences of viral processes. In this review, we offer a perspective on relevant MT-related elements exploited by viruses.

HIV-1 capsid exploitation of the host microtubule cytoskeleton during early infection

Microtubules (MTs) form a filamentous array that provide both structural support and a coordinated system for the movement and organization of macromolecular cargos within the cell. As such, they play a critical role in regulating a wide range of cellular processes, from cell shape and motility to cell polarization and division. The array is radial with filament minus-ends anchored at perinuclear MT-organizing centers and filament plus-ends continuously growing and shrinking to explore and adapt to the intracellular environment. In response to environmental cues, a small subset of these highly dynamic MTs can become stabilized, acquire post-translational modifications and act as specialized tracks for cargo trafficking. MT dynamics and stability are regulated by a subset of highly specialized MT plus-end tracking proteins, known as +TIPs. Central to this is the end-binding (EB) family of proteins which specifically recognize and track growing MT plus-ends to both regulate MT polymerization directly and to mediate the accumulation of a diverse array of other +TIPs at MT ends. Moreover, interaction of EB1 and +TIPs with actin-MT cross-linking factors coordinate changes in actin and MT dynamics at the cell periphery, as well as during the transition of cargos from one network to the other. The inherent structural polarity of MTs is sensed by specialized motor proteins. In general, dynein directs trafficking of cargos towards the minus-end while most kinesins direct movement toward the plus-end. As a pathogenic cargo, HIV-1 uses the actin cytoskeleton for short-range transport most frequently at the cell periphery during entry before transiting to MTs for long-range transport to reach the nucleus. While the fundamental importance of MT networks to HIV-1 replication has long been known, recent work has begun to reveal the underlying mechanistic details by which HIV-1 engages MTs after entry into the cell. This includes mimicry of EB1 by capsid (CA) and adaptor-mediated engagement of dynein and kinesin motors to elegantly coordinate early steps in infection that include MT stabilization, uncoating (conical CA disassembly) and virus transport toward the nucleus. This review discusses recent advances in our understanding of how MT regulators and their associated motors are exploited by incoming HIV-1 capsid during early stages of infection.

Analysis of formin functions during cytokinesis using specific inhibitor SMIFH2

The phragmoplast separates daughter cells during cytokinesis by constructing the cell plate, which depends on interaction between cytoskeleton and membrane compartments. Proteins responsible for these interactions remain unknown, but formins can link cytoskeleton with membranes and several members of formin protein family localize to the cell plate. Progress in functional characterization of formins in cytokinesis is hindered by functional redundancies within the large formin gene family. We addressed this limitation by employing Small Molecular Inhibitor of Formin Homology 2 (SMIFH2), a small-molecule inhibitor of formins. Treatment of tobacco (Nicotiana tabacum) tissue culture cells with SMIFH2 perturbed localization of actin at the cell plate; slowed down both microtubule polymerization and phragmoplast expansion; diminished association of dynamin-related proteins with the cell plate independently of actin and microtubules; and caused cell plate swelling. Another impact of SMIFH2 was shortening of the END BINDING1b (EB1b) and EB1c comets on the growing microtubule plus ends in N. tabacum tissue culture cells and Arabidopsis thaliana cotyledon epidermis cells. The shape of the EB1 comets in the SMIFH2-treated cells resembled that of the knockdown mutant of plant Xenopus Microtubule-Associated protein of 215 kDa (XMAP215) homolog MICROTUBULE ORGANIZATION 1/GEMINI 1 (MOR1/GEM1). This outcome suggests that formins promote elongation of tubulin flares on the growing plus ends. Formins AtFH1 (A. thaliana Formin Homology 1) and AtFH8 can also interact with EB1. Besides cytokinesis, formins function in the mitotic spindle assembly and metaphase to anaphase transition. Our data suggest that during cytokinesis formins function in: (1) promoting microtubule polymerization; (2) nucleating F-actin at the cell plate; (3) retaining dynamin-related proteins at the cell plate; and (4) remodeling of the cell plate membrane.

Microtubules in cell migration

Directed cell migration is critical for embryogenesis and organ development, wound healing and the immune response. Microtubules are dynamic polymers that control directional migration through a number of coordinated processes: microtubules are the tracks for long-distance intracellular transport, crucial for delivery of new membrane components and signalling molecules to the leading edge of a migrating cell and the recycling of adhesion receptors. Microtubules act as force generators and compressive elements to support sustained cell protrusions. The assembly and disassembly of microtubules is coupled to Rho GTPase signalling, thereby controlling actin polymerisation, myosin-driven contractility and the turnover of cellular adhesions locally. Cross-talk of actin and microtubule dynamics is mediated through a number of common binding proteins and regulators. Furthermore, cortical microtubule capture sites are physically linked to focal adhesions, facilitating the delivery of secretory vesicles and efficient cross-talk. Here we summarise the diverse functions of microtubules during cell migration, aiming to show how they contribute to the spatially and temporally coordinated sequence of events that permit efficient, directional and persistent migration.

Microtubules: Evolving roles and critical cellular interactions

Microtubules are cytoskeletal elements known as drivers of directed cell migration, vesicle and organelle trafficking, and mitosis. In this review, we discuss new research in the lens that has shed light into further roles for stable microtubules in the process of development and morphogenesis. In the lens, as well as other systems, distinct roles for characteristically dynamic microtubules and stabilized populations are coming to light. Understanding the mechanisms of microtubule stabilization and the associated microtubule post-translational modifications is an evolving field of study. Appropriate cellular homeostasis relies on not only one cytoskeletal element, but also rather an interaction between cytoskeletal proteins as well as other cellular regulators. Microtubules are key integrators with actin and intermediate filaments, as well as cell–cell junctional proteins and other cellular regulators including myosin and RhoGTPases to maintain this balance.

Impact statement

The role of microtubules in cellular functioning is constantly expanding. In this review, we examine new and exciting fields of discovery for microtubule’s involvement in morphogenesis, highlight our evolving understanding of differential roles for stabilized versus dynamic subpopulations, and further understanding of microtubules as a cellular integrator.

The Cytoskeleton-A Complex Interacting Meshwork

The cytoskeleton of animal cells is one of the most complicated and functionally versatile structures, involved in processes such as endocytosis, cell division, intra-cellular transport, motility, force transmission, reaction to external forces, adhesion and preservation, and adaptation of cell shape. These functions are mediated by three classical cytoskeletal filament types, as follows: Actin, microtubules, and intermediate filaments. The named filaments form a network that is highly structured and dynamic, responding to external and internal cues with a quick reorganization that is orchestrated on the time scale of minutes and has to be tightly regulated. Especially in brain tumors, the cytoskeleton plays an important role in spreading and migration of tumor cells. As the cytoskeletal organization and regulation is complex and many-faceted, this review aims to summarize the findings about cytoskeletal filament types, including substructures formed by them, such as lamellipodia, stress fibers, and interactions between intermediate filaments, microtubules and actin. Additionally, crucial regulatory aspects of the cytoskeletal filaments and the formed substructures are discussed and integrated into the concepts of cell motility. Even though little is known about the impact of cytoskeletal alterations on the progress of glioma, a final point discussed will be the impact of established cytoskeletal alterations in the cellular behavior and invasion of glioma.

Lactate Activates the E2F Pathway to Promote Cell Motility by Up-Regulating Microtubule Modulating Genes

Excess lactate production due to enhanced aerobic glycolysis is characteristic of malignant cancers, which is also intimately associated with poor cancer prognoses. Although tumor-associated lactate contributes to all major steps in carcinogenesis, its action mechanism remains obscure. To understand the molecular mechanism of the lactate-induced tumor metastatic process, we identified an array of lactate-responsive genes via transcriptome analysis of a metformin-induced hyper-glycolytic liver cancer model. Gene set enrichment analysis suggested E2F-RB pathway as the dominant regulator of the lactate-induced gene expression. We experimentally verified that lactate indeed activates E2F-mediated transcription by promoting E2F1 protein accumulation through a posttranscriptional mechanism. Literature-based analysis of target pathways potentially modulated by 136 top-ranked genes indicated that genes functioning in cell-cell or cell-matrix communications dominate the lactate-induced gene expression. Especially, those regulating microtubule functions, including a group of kinesin family members, were significantly up-regulated in lactate- and E2F1-dependent manners. Depletion of E2F1 or kinesins (KIF2C, KIF18B, KIF20A) led to deformation of microtubule structures, impairing cell motility as much as the deficit in lactate production. These results indicate that E2F pathway activation by tumor-associated lactate and subsequent transcriptional activation of microtubule functions play crucial roles in tumor metastasis, providing mechanistic clues to cell motility-directed anti-cancer strategies.

Kif18a regulates Sirt2-mediated tubulin acetylation for spindle organization during mouse oocyte meiosis

Background

During oocyte meiosis, the cytoskeleton dynamics, especially spindle organization, are critical for chromosome congression and segregation. However, the roles of the kinesin superfamily in this process are still largely unknown.

Results

In the present study, Kif18a, a member of the kinesin-8 family, regulated spindle organization through its effects on tubulin acetylation in mouse oocyte meiosis. Our results showed that Kif18a is expressed and mainly localized in the spindle region. Knock down of Kif18a caused the failure of first polar body extrusion, dramatically affecting spindle organization and resulting in severe chromosome misalignment. Further analysis showed that the disruption of Kif18a caused an increase in acetylated tubulin level, which might be the reason for the spindle organization defects after Kif18a knock down in oocyte meiosis, and the decreased expression of deacetylase Sirt2 was found after Kif18a knock down. Moreover, microinjections of tubulin K40R mRNA, which could induce tubulin deacetylation, protected the oocytes from the effects of Kif18a downregulation, resulting in normal spindle morphology in Kif18a-knock down oocytes.

Conclusions

Taken together, our results showed that Kif18a affected Sirt2-mediated tubulin acetylation level for spindle organization during mouse oocyte meiosis. Our results not only revealed the critical effect of Kif18a on microtubule stability, but also extended our understanding of kinesin activity in meiosis.

Role of Microtubules and Microtubule-Associated Proteins in HIV-1 Infection

Recent studies show that human immunodeficiency virus type 1 (HIV-1) can utilize microtubules and their associated proteins to complete key postfusion steps during infection. These include associating with both dynein and kinesin motors, as well as proteins, which enhance infection by altering microtubule dynamics during infection. In this article, we will discuss findings on how dynein and kinesin motors, as well as other microtubule-associated proteins, influence HIV-1 trafficking, viral core uncoating, and nuclear import of the viral ribonucleoprotein (RNP).

Role of kinesins in directed adenovirus transport and cytoplasmic exploration

Many viruses, including adenovirus, exhibit bidirectional transport along microtubules following cell entry. Cytoplasmic dynein is responsible for microtubule minus end transport of adenovirus capsids after endosomal escape. However, the identity and roles of the opposing plus end-directed motor(s) remain unknown. We performed an RNAi screen of 38 kinesins, which implicated Kif5B (kinesin-1 family) and additional minor kinesins in adenovirus 5 (Ad5) capsid translocation. Kif5B RNAi markedly increased centrosome accumulation of incoming Ad5 capsids in human A549 pulmonary epithelial cells within the first 30 min post infection, an effect dramatically enhanced by blocking Ad5 nuclear pore targeting using leptomycin B. The Kif5B RNAi phenotype was rescued by expression of RNAi-resistant Kif5A, B, or C, and Kif4A. Kif5B RNAi also inhibited a novel form of microtubule-based “assisted-diffusion” behavior which was apparent between 30 and 60 min p.i. We found the major capsid protein penton base (PB) to recruit kinesin-1, distinct from the hexon role we previously identified for cytoplasmic dynein binding. We propose that adenovirus uses independently recruited kinesin and dynein for directed transport and for a more random microtubule-based assisted diffusion behavior to fully explore the cytoplasm before docking at the nucleus, a mechanism of potential importance for physiological cargoes as well.

The role of plus-end directed microtubule motors in virus entry into host cells is a long-standing question. In this study, the authors identify the kinesins responsible for adenovirus plus end-directed transport along microtubules, the mechanism for kinesin recruitment, and both directed and motor-based exploratory movements involved in adenovirus’ search for the nucleus.

Differential Toxicity of mDia Formin-Directed Functional Agonists and Antagonists in Developing Zebrafish

The mammalian Diaphanous-related (mDia) formins are cytoskeletal regulators that assemble and, in some cases, bundle filamentous actin (F-actin), as well as stabilize microtubules. The development of small molecule antagonists and agonists that interrogate mDia formin function has allowed us to investigate the roles of formins in disease states. A small molecule inhibitor of FH2 domain (SMIFH2) inhibits mDia-dependent actin dynamics and abrogates tumor cell migration and cell division in vitro and ex vivo tissue explants. mDia formin activation with small molecule intramimics IMM01/02 and mDia2-DAD peptides inhibited glioblastoma motility and invasion in vitro and ex vivo rat brain slices. However, SMIFH2, IMMs, and mDia2 DAD efficacy in vivo remains largely unexplored and potential toxicity across a range of developmental phenotypes has not been thoroughly characterized. In this study, we performed an in vivo screen of early life-stage toxicity in Danio rerio zebrafish embryos 2 days post-fertilization (dpf) in response to SMIFH2, IMM01/02, and mDia2 DAD. SMIFH2 at concentrations ≥5–10 μM induced significant defects in developing zebrafish, including shorter body lengths, tail curvature and defective tail cellularity, craniofacial malformations, pericardial edema, absent and/or compromised vasculature function and flow, depressed heart rates and increased mortality. Conversely, IMM and mDia2 DAD peptides were minimally toxic at concentrations up to 10–20 and 50 μM, respectively. SMIFH2's therapeutic potential may therefore be limited by its substantial in vivo toxicity at functional concentrations. mDia formin agonism with IMMs and mDia2 DADs may therefore be a more effective and less toxic anti-invasive therapeutic approach.

Altered chemomechanical coupling causes impaired motility of the kinesin-4 motors KIF27 and KIF7

Kinesin-4 motors play important roles in cell division, microtubule organization, and signaling. Understanding how motors perform their functions requires an understanding of their mechanochemical and motility properties. We demonstrate that KIF27 can influence microtubule dynamics, suggesting a conserved function in microtubule organization across the kinesin-4 family. However, kinesin-4 motors display dramatically different motility characteristics: KIF4 and KIF21 motors are fast and processive, KIF7 and its Drosophila melanogaster homologue Costal2 (Cos2) are immotile, and KIF27 is slow and processive. Neither KIF7 nor KIF27 can cooperate for fast processive transport when working in teams. The mechanistic basis of immotile KIF7 behavior arises from an inability to release adenosine diphosphate in response to microtubule binding, whereas slow processive KIF27 behavior arises from a slow adenosine triphosphatase rate and a high affinity for both adenosine triphosphate and microtubules. We suggest that evolutionarily selected sequence differences enable immotile KIF7 and Cos2 motors to function not as transporters but as microtubule-based tethers of signaling complexes.

Microtubule Motors in Establishment of Epithelial Cell Polarity

Epithelial cells play a key role in insuring physiological homeostasis by acting as a barrier between the outside environment and internal organs. They are also responsible for the vectorial transport of ions and fluid essential to the function of many organs. To accomplish these tasks, epithelial cells must generate an asymmetrically organized plasma membrane comprised of structurally and functionally distinct apical and basolateral membranes. Adherent and occluding junctions, respectively, anchor cells within a layer and prevent lateral diffusion of proteins in the outer leaflet of the plasma membrane and restrict passage of proteins and solutes through intercellular spaces. At a fundamental level, the establishment and maintenance of epithelial polarity requires that signals initiated at cell-substratum and cell-cell adhesions are transmitted appropriately and dynamically to the cytoskeleton, to the membrane-trafficking machinery, and to the regulation of occluding and adherent junctions. Rigorous descriptive and mechanistic studies published over the last 50 years have provided great detail to our understanding of epithelial polarization. Yet still, critical early steps in morphogenesis are not yet fully appreciated. In this review, we discuss how cytoskeletal motor proteins, primarily kinesins, contribute to coordinated modification of microtubule and actin arrays, formation and remodeling of cell adhesions to targeted membrane trafficking, and to initiating the formation and expansion of an apical lumen.

NADPH oxidase mediates microtubule alterations and diaphragm dysfunction in dystrophic mice

Skeletal muscle from mdx mice is characterized by increased Nox2 ROS, altered microtubule network, increased muscle stiffness, and decreased muscle/respiratory function. While microtubule de-tyrosination has been suggested to increase stiffness and Nox2 ROS production in isolated single myofibers, its role in altering tissue stiffness and muscle function has not been established. Because Nox2 ROS production is upregulated prior to microtubule network alterations and ROS affect microtubule formation, we investigated the role of Nox2 ROS in diaphragm tissue microtubule organization, stiffness and muscle/respiratory function. Eliminating Nox2 ROS prevents microtubule disorganization and reduces fibrosis and muscle stiffness in mdx diaphragm. Fibrosis accounts for the majority of variance in diaphragm stiffness and decreased function, implicating altered extracellular matrix and not microtubule de-tyrosination as a modulator of diaphragm tissue function. Ultimately, inhibiting Nox2 ROS production increased force and respiratory function in dystrophic diaphragm, establishing Nox2 as a potential therapeutic target in Duchenne muscular dystrophy.

The Rho-specific guanine nucleotide exchange factor Plekhg5 modulates cell polarity, adhesion, migration, and podosome organization in macrophages and osteoclasts

Osteoclasts are multinucleated bone-resorbing cells that are formed by fusion of monocyte/macrophage lineage. Osteoclasts and macrophages generate podosomes that are actin-based dynamic organelles implicated in cell adhesion, spreading, migration, and degradation. However, the detailed mechanisms of podosome organization remain unknown. Here, we identified the Rho-specific guanine-nucleotide exchange factor (Rho-GEF) Plekhg5 as an up-regulated gene during differentiation of osteoclasts from macrophages. Knockdown of Plekhg5 with small interfering RNA in both macrophages and osteoclasts induced larger cell formation with impaired cell polarity and resulted in an elongated and flattened shape. In macrophages, Plekhg5 depletion enhanced random migration, but impaired directional migration, adhesion, and matrix degradation. Plekhg5 in osteoclasts affected random migration, podosome organization, and bone resorption. Plekhg5 depletion affected signaling and localization of several Rho downstream effectors. In fact, end-binding protein 1 (EB1), cofilin and vinculin were abnormally localized in Plekhg5-depleted cells, and mDia1 and LIM kinase (LIMK)1 were upregulated in Plekhg5-depleted cells compared with control cells. However, overexpression of Plekhg5 in macrophages induced an increase in its mRNA level, but failed to increase the protein level, indicating that overexpressed Plekhg5 was degraded in macrophages but not HEK293T cells. Thus, Plekhg5 affects cell polarity, migration, adhesion, degradation, and podosome organization in macrophages and osteoclasts.

Distinct functions of diaphanous-related formins regulate HIV-1 uncoating and transport

Diaphanous (Dia)-related formins (DRFs) coordinate cytoskeletal remodeling by controlling actin nucleation and microtubule (MT) stabilization to facilitate processes such as cell polarization and migration; yet the full extent of their activities remains unknown. Here, we uncover two discrete roles and functions of DRFs during early human immunodeficiency virus type 1 (HIV-1) infection. Independent of their actin regulatory activities, Dia1 and Dia2 facilitated HIV-1-induced MT stabilization and the intracellular motility of virus particles. However, DRFs also bound in vitro assembled capsid-nucleocapsid complexes and promoted the disassembly of HIV-1 capsid (CA) shell. This process, also known as "uncoating," is among the most poorly understood stages in the viral lifecycle. Domain analysis and structure modeling revealed that regions of Dia2 that bound viral CA and mediated uncoating as well as early infection contained coiled-coil domains, and that these activities were genetically separable from effects on MT stabilization. Our findings reveal that HIV-1 exploits discrete functions of DRFs to coordinate critical steps in early infection and identifies Dia family members as regulators of the poorly understood process of HIV-1 uncoating.

Molecular motors and nuclear movements in muscle

Muscle fibers have the particularity of containing numerous nuclei evenly distributed and positioned next to the plasma membrane. This unique disposition is the result of sequential events of nuclear movements that start when myoblasts fuse together and end with the clustering of few nuclei under the neuromuscular junction. Nuclei are mispositioned in multiple muscle disorders therefore the mechanisms of nuclear positioning can be novel targets for muscle disorders therapies. The 2 first nuclear movements that occur upon myoblast fusion require different microtubule motors. We performed a siRNA screen against all the microtubules motors and quantified nuclei behavior after fusion and inside the myotube. The different motors we found to be involved in the nuclear behaviors and the analysis of motors expression suggest a competition between both movement mechanisms, which potentially relies on the discrepancy between myoblast and myotube microtubules stability.

FUS inclusions disrupt RNA localization by sequestering kinesin-1 and inhibiting microtubule detyrosination

Amyotrophic lateral sclerosis–associated mutations promote the formation of cytoplasmic FUS inclusions. In this study, Yasuda et al. show in fibroblasts and neurons that kinesin-1 is sequestered in FUS inclusions, resulting in a loss of detyrosinated microtubules and mislocalization of specific RNAs.

Cytoplasmic inclusions of the RNA-binding protein fused in sarcoma (FUS) represent one type of membraneless ribonucleoprotein compartment. Formation of FUS inclusions is promoted by amyotrophic lateral sclerosis (ALS)–linked mutations, but the cellular functions affected upon inclusion formation are poorly defined. In this study, we find that FUS inclusions lead to the mislocalization of specific RNAs from fibroblast cell protrusions and neuronal axons. This is mediated by recruitment of kinesin-1 mRNA and protein within FUS inclusions, leading to a loss of detyrosinated glutamate (Glu)–microtubules (MTs; Glu-MTs) and an inability to support the localization of RNAs at protrusions. Importantly, dissolution of FUS inclusions using engineered Hsp104 disaggregases, or overexpression of kinesin-1, reverses these effects. We further provide evidence that kinesin-1 affects MT detyrosination not through changes in MT stability, but rather through targeting the tubulin carboxypeptidase enzyme onto specific MTs. Interestingly, other pathological inclusions lead to similar outcomes, but through apparently distinct mechanisms. These results reveal a novel kinesin-dependent mechanism controlling the MT cytoskeleton and identify loss of Glu-MTs and RNA mislocalization as common outcomes of ALS pathogenic mutations.

Kif4 Is Essential for Mouse Oocyte Meiosis

Progression through the meiotic cell cycle must be strictly regulated in oocytes to generate viable embryos and offspring. During mitosis, the kinesin motor protein Kif4 is indispensable for chromosome condensation and separation, midzone formation and cytokinesis. Additionally, the bioactivity of Kif4 is dependent on phosphorylation via Aurora Kinase B and Cdk1, which regulate Kif4 function throughout mitosis. Here, we examine the role of Kif4 in mammalian oocyte meiosis. Kif4 localized in the cytoplasm throughout meiosis I and II, but was also observed to have a dynamic subcellular distribution, associating with both microtubules and kinetochores at different stages of development. Co-localization and proximity ligation assays revealed that the kinetochore proteins, CENP-C and Ndc80, are potential Kif4 interacting proteins. Functional analysis of Kif4 in oocytes via antisense knock-down demonstrated that this protein was not essential for meiosis I completion. However, Kif4 depleted oocytes displayed enlarged polar bodies and abnormal metaphase II spindles, indicating an essential role for this protein for correct asymmetric cell division in meiosis I. Further investigation of the phosphoregulation of meiotic Kif4 revealed that Aurora Kinase and Cdk activity is critical for Kif4 kinetochore localization and interaction with Ndc80 and CENP-C. Finally, Kif4 protein but not gene expression was found to be upregulated with age, suggesting a role for this protein in the decline of oocyte quality with age.

Fidgetin regulates cultured astrocyte migration by severing tyrosinated microtubules at the leading edge

Fign regulates cultured astrocyte migration by severing tyrosinated microtubules (MTs). Inhibition of cellular migration induced by Fign knockdown can be rescued with concomitant knockdown of kinesin-12. A working model is given for the MT reconfiguration underlying cellular migration elicited by the cooperation of two distinct MT-related proteins.

Microtubule (MT) organization is essential for many cellular events, including mitosis, migration, and cell polarity. Fidgetin (Fign), an ATP-dependent, MT-severing protein, contributes to the regulation of MT configuration by cutting and trimming MT polymers. Functions of Fign have been indicated in neurite outgrowth, mitosis, meiosis, and cellular migration. Here we focus on migration of astrocytes. We find that Fign plays an essential role in cultured astrocyte migration by preferentially targeting MTs (or regions of MTs) that are rich in tyrosinated tubulin, a marker for especially dynamic MTs or especially dynamic regions of MTs. Inhibition of cellular migration induced by Fign knockdown can be rescued with concomitant knockdown of kinesin-12, a motor protein best known for its role in mitosis. We propose a novel working model for MT reconfiguration underlying cellular migration elicited by the functional cooperation of two distinct MT-related proteins.

An atlas of human kinase regulation

The coordinated regulation of protein kinases is a rapid mechanism that integrates diverse cues and swiftly determines appropriate cellular responses. However, our understanding of cellular decision‐making has been limited by the small number of simultaneously monitored phospho‐regulatory events. Here, we have estimated changes in activity in 215 human kinases in 399 conditions derived from a large compilation of phosphopeptide quantifications. This atlas identifies commonly regulated kinases as those that are central in the signaling network and defines the logic relationships between kinase pairs. Co‐regulation along the conditions predicts kinase–complex and kinase–substrate associations. Additionally, the kinase regulation profile acts as a molecular fingerprint to identify related and opposing signaling states. Using this atlas, we identified essential mediators of stem cell differentiation, modulators of Salmonella infection, and new targets of AKT1. This provides a global view of human phosphorylation‐based signaling and the necessary context to better understand kinase‐driven decision‐making.

The Ran Pathway in Drosophila melanogaster Mitosis

Over the last two decades, the small GTPase Ran has emerged as a central regulator of both mitosis and meiosis, particularly in the generation, maintenance, and regulation of the microtubule (MT)-based bipolar spindle. Ran-regulated pathways in mitosis bear many similarities to the well-characterized functions of Ran in nuclear transport and, as with transport, the majority of these mitotic effects are mediated through affecting the physical interaction between karyopherins and Spindle Assembly Factors (SAFs)—a loose term describing proteins or protein complexes involved in spindle assembly through promoting nucleation, stabilization, and/or depolymerization of MTs, through anchoring MTs to specific structures such as centrosomes, chromatin or kinetochores, or through sliding MTs along each other to generate the force required to achieve bipolarity. As such, the Ran-mediated pathway represents a crucial functional module within the wider spindle assembly landscape. Research into mitosis using the model organism Drosophila melanogaster has contributed substantially to our understanding of centrosome and spindle function. However, in comparison to mammalian systems, very little is known about the contribution of Ran-mediated pathways in Drosophila mitosis. This article sets out to summarize our understanding of the roles of the Ran pathway components in Drosophila mitosis, focusing on the syncytial blastoderm embryo, arguing that it can provide important insights into the conserved functions on Ran during spindle formation.

Kinesins in cell migration

Human cells express 45 kinesins, microtubule motors that transport a variety of molecules and organelles within the cell. Many kinesins also modulate the tracks they move on by either bundling or sliding or regulating the dynamic assembly and disassembly of the microtubule polymer. In migrating cells, microtubules control the asymmetry between the front and rear of the cell by differentially regulating force generation processes and substrate adhesion. Many of these functions are mediated by kinesins, transporters as well as track modulators. In this review, we summarize the current knowledge on kinesin functions in cell migration.

Small-molecule agonists of mammalian Diaphanous-related (mDia) formins reveal an effective glioblastoma anti-invasion strategy

Formin agonists impede the most dangerous aspect of glioblastoma—tumor spread into surrounding healthy tissue. Formin activation impairs a novel aspect of the transformed cell and informs the development of antitumor strategies for a cancer needing a more effective therapy.

The extensive invasive capacity of glioblastoma (GBM) makes it resistant to surgery, radiotherapy, and chemotherapy and thus makes it lethal. In vivo, GBM invasion is mediated by Rho GTPases through unidentified downstream effectors. Mammalian Diaphanous (mDia) family formins are Rho-directed effectors that regulate the F-actin cytoskeleton to support tumor cell motility. Historically, anti-invasion strategies focused upon mDia inhibition, whereas activation remained unexplored. The recent development of small molecules directly inhibiting or activating mDia-driven F-actin assembly that supports motility allows for exploration of their role in GBM. We used the formin inhibitor SMIFH2 and mDia agonists IMM-01/-02 and mDia2-DAD peptides, which disrupt autoinhibition, to examine the roles of mDia inactivation versus activation in GBM cell migration and invasion in vitro and in an ex vivo brain slice invasion model. Inhibiting mDia suppressed directional migration and spheroid invasion while preserving intrinsic random migration. mDia agonism abrogated both random intrinsic and directional migration and halted U87 spheroid invasion in ex vivo brain slices. Thus mDia agonism is a superior GBM anti-invasion strategy. We conclude that formin agonism impedes the most dangerous GBM component—tumor spread into surrounding healthy tissue. Formin activation impairs novel aspects of transformed cells and informs the development of anti-GBM invasion strategies.