Dynactin binding to tyrosinated microtubules promotes centrosome centration in C. elegans by enhancing dynein-mediated organelle transport

ZYG-12/Hook's dual role as a dynein adaptor for early endosomes and nuclei is regulated by alternative splicing of its cargo binding domain

The microtubule motor cytoplasmic dynein-1 transports and positions various organelles, but the molecular basis of this functional diversity is not fully understood. Cargo adaptors of the Hook protein family recruit dynein to early endosomes (EE) in fungi and human cells by forming the FTS-Hook-FHIP (FHF) complex. By contrast, the Caenorhabditis elegans Hook homologue ZYG-12 recruits dynein to the nuclear envelope (NE) in the meiotic gonad and mitotic early embryo by forming a Linker of Nucleoskeleton and Cytoskeleton (LINC) complex. Here, we demonstrate that ZYG-12 recruits dynein to EE in epithelia. We identify and functionally characterize the homologues of FTS (UBC-19) and FHIP (FHIP-1) that constitute the C. elegans FHF complex, validate the predicted FHIP-1-RAB-5 binding interface in vivo, and show that ZYG-12 forms FHF via a conserved segment that precedes, and is distinct from, its C-terminal NE targeting domain. Finally, we show that C-terminal ZYG-12 splice isoforms differ in their ability to target to the NE and EE. We conclude that the C. elegans Hook adaptor evolved to recruit dynein to two distinct organelles, and that cargo specificity of ZYG-12 is regulated by alternative splicing.

Cytoplasmic flow is a cell size sensor that scales anaphase

During early embryogenesis, fast mitotic cycles without interphase lead to a decrease in cell size, while scaling mechanisms must keep cellular structures proportional to cell size. For instance, as cells become smaller, if the position of nuclear envelope reformation (NER) did not adapt, NER would have to occur beyond the cell boundary. Here we found that NER position in anaphase scales with cell size via changes in chromosome motility, mediated by cytoplasmic flows that themselves scale with cell size. Flows are a consequence of friction between viscous cytoplasm and bulky cargo transported by dynein on astral microtubules. As an emerging property, confinement in cells of different sizes yields scaling of cytoplasmic flows. Thus, flows behave like a cell geometry sensor: astral microtubules approach the boundary causing flow velocity changes, which then affect the velocity of chromosome separation, thus scaling NER.

Asymmetric partitioning of persistent paternal mitochondria during cell divisions safeguards embryo development and mitochondrial inheritance

Most eukaryotes inherit only maternal mitochondria. The reasons for paternal mitochondrial elimination and the impacts of persistent paternal mitochondria on animals remain elusive. We show that undegraded paternal mitochondria in autophagy-deficient C. elegans embryos are gradually excluded from germ blastomeres through asymmetric partitioning during cell divisions. The embryonic cortical flow drives anterior-directed movements of paternal mitochondria and contributes to their asymmetric apportioning between two daughter blastomeres. By contrast, autophagosome-enclosed paternal mitochondria cluster around and segregate with centrosomes during mitosis and are rapidly degraded through lysosomes concentrated near centrosomes. Failure to exclude persistent paternal mitochondria from the germ blastomere at first cleavage causes their enrichment in the descendant endomesodermal (EMS) blastomere, leading to elevated reactive oxygen species levels, elongated EMS lineage durations, and increased embryonic lethality, which antioxidant treatments can suppress. Thus, regulated paternal mitochondrial distribution away from germ blastomeres is a fail-safe mechanism, protecting embryo development and maternal mitochondrial inheritance.

Editing of endogenous tubulins reveals varying effects of tubulin posttranslational modifications on axonal growth and regeneration

Tubulin posttranslational modifications (PTMs) modulate the dynamic properties of microtubules and their interactions with other proteins. However, the effects of tubulin PTMs were often revealed indirectly through the deletion of modifying enzymes or the overexpression of tubulin mutants. In this study, we directly edited the endogenous tubulin loci to install PTM-mimicking or -disabling mutations and studied their effects on microtubule stability, neurite outgrowth, axonal regeneration, cargo transport, and sensory functions in the touch receptor neurons of Caenorhabditis elegans. We found that the status of β-tubulin S172 phosphorylation and K252 acetylation strongly affected microtubule dynamics, neurite growth, and regeneration, whereas α-tubulin K40 acetylation had little influence. Polyglutamylation and detyrosination in the tubulin C-terminal tail had more subtle effects on microtubule stability likely by modulating the interaction with kinesin-13. Overall, our study systematically assessed and compared several tubulin PTMs for their impacts on neuronal differentiation and regeneration and established an in vivo platform to test the function of tubulin PTMs in neurons.

Dynein directs prophase centrosome migration to control the stem cell division axis in the developing Caenorhabditis elegans epidermis

The microtubule motor dynein is critical for the assembly and positioning of mitotic spindles. In Caenorhabditis elegans, these dynein functions have been extensively studied in the early embryo but remain poorly explored in other developmental contexts. Here, we use a hypomorphic dynein mutant to investigate the motor's contribution to asymmetric stem cell-like divisions in the larval epidermis. Live imaging of seam cell divisions that precede formation of the seam syncytium shows that mutant cells properly assemble but frequently misorient their spindle. Misoriented divisions misplace daughter cells from the seam cell row, generate anucleate compartments due to aberrant cytokinesis, and disrupt asymmetric cell fate inheritance. Consequently, the seam becomes disorganized and populated with extra cells that have lost seam identity, leading to fatal epidermal rupture. We show that dynein orients the spindle through the cortical GOA-1Gα-LIN-5NuMA pathway by directing the migration of prophase centrosomes along the anterior-posterior axis. Spindle misorientation in the dynein mutant can be partially rescued by elongating cells, implying that dynein-dependent force generation and cell shape jointly promote correct asymmetric division of epithelial stem cells.

The Tubulin Code, from Molecules to Health and Disease

Microtubules are essential dynamic polymers composed of α/β-tubulin heterodimers. They support intracellular trafficking, cell division, cellular motility, and other essential cellular processes. In many species, both α-tubulin and β-tubulin are encoded by multiple genes with distinct expression profiles and functionality. Microtubules are further diversified through abundant posttranslational modifications, which are added and removed by a suite of enzymes to form complex, stereotyped cellular arrays. The genetic and chemical diversity of tubulin constitute a tubulin code that regulates intrinsic microtubule properties and is read by cellular effectors, such as molecular motors and microtubule-associated proteins, to provide spatial and temporal specificity to microtubules in cells. In this review, we synthesize the rapidly expanding tubulin code literature and highlight limitations and opportunities for the field. As complex microtubule arrays underlie essential physiological processes, a better understanding of how cells employ the tubulin code has important implications for human disease ranging from cancer to neurological disorders.

Nuclear-enriched protein phosphatase 4 ensures outer kinetochore assembly prior to nuclear dissolution

A landmark event in the transition from interphase to mitosis in metazoans is nuclear envelope breakdown (NEBD). Important mitotic events occur prior to NEBD, including condensation of replicated chromosomes and assembly of kinetochores to rapidly engage spindle microtubules. Here, we show that nuclear-enriched protein phosphatase 4 (PP4) ensures robust assembly of the microtubule-coupling outer kinetochore prior to NEBD. In the absence of PP4, chromosomes exhibit extended monopolar orientation after NEBD and subsequently mis-segregate. A secondary consequence of diminished outer kinetochore assembly is defective sister chromatid resolution. After NEBD, a cytoplasmic activity compensates for PP4 loss, leading to outer kinetochore assembly and recovery of chromosomes from monopolar orientation to significant bi-orientation. The Ndc80-Ska microtubule-binding module of the outer kinetochore is required for this recovery. PP4 associates with the inner kinetochore protein CENP-C; however, disrupting the PP4-CENP-C interaction does not perturb chromosome segregation. These results establish that PP4-dependent outer kinetochore assembly prior to NEBD is critical for timely and proper engagement of chromosomes with spindle microtubules.

Dynactin 2 acts as an oncogene in hepatocellular carcinoma through promoting cell cycle progression

BACKGROUND

Dynactin (DCTN) can activate cytoplasmic dynein and drive intracellular organelle transport containing six family members (DCTN1 to DCTN6). The DCTN family has been studied as cancer-related genes or biomarkers in various cancers. Nevertheless, in hepatocellular carcinoma (HCC), the functions and prognostic roles of the DCTN family have been unexplored.

METHODS

We evaluated the diagnostic and survival effects of DCTN subunits in HCC through bioinformatics analysis and validated the results of bioinformatics by our data to address this problem.

RESULTS

The results of bioinformatics analysis found that DCTN2 was a significant prognostic factor in HCC, and high-level DCTN2 can predict poor patient survival in HCC. Cox regression analysis also suggested that DCTN2 (hazard ratio = 1.748, 95% confidence interval 1.190-2.568, P = 0.004) is an independent prognostic factor for patient survival. Western blot and quantitative reverse transcription-polymerase chain reaction assays confirmed that the protein and mRNA expression levels of DCTN2 were upregulated in HCC cell lines. The proliferation, invasion, and migration were decreased and cell apoptosis was enhanced after DCTN2 was knocked down in Huh7 and Hep3B cells. DCTN2 promoted the cell cycle progression through regulating the expression of cell cycle regulatory proteins cyclin-dependent kinase 4, Cyclin D1, and p21.

CONCLUSIONS

We propose that DCTN2 can serve as a prognostic marker for HCC. DCTN2 acts as an oncogene and promotes the cell cycle progression through the G1/S phase-related signaling pathway.

The Expression and Function of Tubulin Isotypes in Caenorhabditis elegans

Microtubules, made from the polymerization of the highly conserved α/β-tubulin heterodimers, serve as important components of the cytoskeleton in all eukaryotic cells. The existence of multiple tubulin isotypes in metazoan genomes and a dazzling variety of tubulin posttranslational modifications (PTMs) prompted the “tubulin code” hypothesis, which proposed that microtubule structure and functions are determined by the tubulin composition and PTMs. Evidence for the tubulin code has emerged from studies in several organisms with the characterization of specific tubulins for their expression and functions. The studies of tubulin PTMs are accelerated by the discovery of the enzymes that add or remove the PTMs. In tubulin research, the use of simple organisms, such as Caenorhabditis elegans, has been instrumental for understanding the expression and functional specialization of tubulin isotypes and the effects of their PTMs. In this review, we summarize the current understanding of the expression patterns and cellular functions of the nine α-tubulin and six β-tubulin isotypes. Expression studies are greatly facilitated by the CRISPR/Cas9-mediated endogenous GFP knock-in reporters and the organism-wide single cell transcriptomic studies. Meanwhile, functional studies benefit from the ease of genetic manipulation and precise gene replacement in C. elegans. These studies identified both ubiquitously expressed tubulin isotypes and tissue-specific isotypes. The isotypes showed functional redundancy, as well as functional specificity, which is likely caused by the subtle differences in their amino acid sequences. Many of these differences concentrate at the C-terminal tails that are subjected to several PTMs. Indeed, tubulin PTM, such as polyglutamylation, is shown to modulate microtubule organization and properties in both ciliated and non-ciliated neurons. Overall, studies from C. elegans support the distinct expression and function patterns of tubulin isotypes and the importance of their PTMs and offer the promise of cracking the tubulin code at the whole-genome and the whole-organism level.

Prometaphase

The establishment of a metaphase plate in which all chromosomes are attached to mitotic spindle microtubules and aligned at the cell equator is required for faithful chromosome segregation in metazoans. The achievement of this configuration relies on the precise coordination between several concurrent mechanisms that start upon nuclear envelope breakdown, mediate chromosome capture at their kinetochores during mitotic spindle assembly and culminate with the congression of all chromosomes to the spindle equator. This period is called 'prometaphase'. Because the nature of chromosome capture by mitotic spindle microtubules is error prone, the cell is provided of error correction mechanisms that sense and correct most erroneous kinetochore-microtubule attachments before committing to separate sister chromatids in anaphase. In this review, aimed for newcomers in the field, more than providing an exhaustive mechanistic coverage of each and every concurrent mechanism taking place during prometaphase, we provide an integrative overview of these processes that ultimately promote the subsequent faithful segregation of chromosomes during mitosis.

The coordination of spindle-positioning forces during the asymmetric division of the Caenorhabditis elegans zygote

In Caenorhabditis elegans zygote, astral microtubules generate forces essential to position the mitotic spindle, by pushing against and pulling from the cortex. Measuring microtubule dynamics there, we revealed the presence of two populations, corresponding to pulling and pushing events. It offers a unique opportunity to study, under physiological conditions, the variations of both spindle-positioning forces along space and time. We propose a threefold control of pulling force, by polarity, spindle position and mitotic progression. We showed that the sole anteroposterior asymmetry in dynein on-rate, encoding pulling force imbalance, is sufficient to cause posterior spindle displacement. The positional regulation, reflecting the number of microtubule contacts in the posterior-most region, reinforces this imbalance only in late anaphase. Furthermore, we exhibited the first direct proof that dynein processivity increases along mitosis. It reflects the temporal control of pulling forces, which strengthens at anaphase onset following mitotic progression and independently from chromatid separation. In contrast, the pushing force remains constant and symmetric and contributes to maintaining the spindle at the cell centre during metaphase.

Dynein-dynactin segregate meiotic chromosomes in C. elegans spermatocytes

The microtubule motor cytoplasmic dynein 1 (dynein) and its essential activator dynactin have conserved roles in spindle assembly and positioning during female meiosis and mitosis, but their contribution to male meiosis remains poorly understood. Here, we characterize the G33S mutation in the C. elegans dynactin subunit DNC-1, which corresponds to G59S in human p150^Glued that causes motor neuron disease. In spermatocytes, dnc-1(G33S) delays spindle assembly and penetrantly inhibits anaphase spindle elongation in meiosis I, which prevents the segregation of homologous chromosomes. By contrast, chromosomes segregate without errors in the early dnc-1(G33S) embryo. Deletion of the DNC-1 N-terminus shows that defective meiosis in dnc-1(G33S) spermatocytes is not due to the inability of DNC-1 to interact with microtubules. Instead, our results suggest that the DNC-1(G33S) protein, which is aggregation prone in vitro, is less stable in spermatocytes than the early embryo, resulting in different phenotypic severity in the two dividing tissues. Thus, the dnc-1(G33S) mutant reveals that dynein-dynactin drive meiotic chromosome segregation in spermatocytes and illustrates that the extent to which protein misfolding leads to loss of function can vary significantly between cell types.

Parthenolide Destabilizes Microtubules by Covalently Modifying Tubulin

Detyrosination of the α-tubulin C-terminal tail is a post-translational modification (PTM) of microtubules that is key for many biological processes.¹ Although detyrosination is the oldest known microtubule PTM,^2-7 the carboxypeptidase responsible for this modification, VASH1/2-SVBP, was identified only 3 years ago,^8,9 precluding genetic approaches to prevent detyrosination. Studies examining the cellular functions of detyrosination have therefore relied on a natural product, parthenolide, which is widely believed to block detyrosination of α-tubulin in cells, presumably by inhibiting the activity of the relevant carboxypeptidase(s).¹⁰ Parthenolide is a sesquiterpene lactone that forms covalent linkages predominantly with exposed thiol groups; e.g., on cysteine residues.^11-13 Using mass spectrometry, we show that parthenolide forms adducts on both cysteine and histidine residues on tubulin itself, in vitro and in cells. Parthenolide causes tubulin protein aggregation and prevents the formation of microtubules. In contrast to epoY, an epoxide inhibitor of VASH1/2-SVBP,⁹ parthenolide does not block VASH1-SVBP activity in vitro. Lastly, we show that epoY is an efficacious inhibitor of microtubule detyrosination in cells, providing an alternative chemical means to block detyrosination. Collectively, our work supports the notion that parthenolide is a promiscuous inhibitor of many cellular processes and suggests that its ability to block detyrosination may be an indirect consequence of reducing the polymerization-competent pool of tubulin in cells.

In Vitro Reconstitution of Dynein Force Exertion in a Bulk Viscous Medium

The forces generated by microtubules (MTs) and their associated motors orchestrate essential cellular processes ranging from vesicular trafficking to centrosome positioning [1, 2]. To date, most studies have focused on MT force exertion by motors anchored to a static surface, such as the cell cortex in vivo or glass surfaces in vitro [2-4]. However, motors also transport large cargos and endomembrane networks, whose hydrodynamic interactions with the viscous cytoplasm should generate sizable forces in bulk. Such forces may contribute to MT aster centration, organization, and orientation [5-14] but have yet to be evidenced and studied in a minimal reconstituted system. By developing a bulk motility assay, based on stabilized MTs and dynein-coated beads freely floating in a viscous medium away from any surface, we demonstrate that the motion of a cargo exerts a pulling force on the MT and propels it in opposite direction. Quantification of resulting MT movements for different motors, motor velocities, over a range of cargo sizes and medium viscosities shows that the efficiency of this mechanism is primarily determined by cargo size and MT length. Forces exerted by cargos are additive, allowing us to recapitulate tug-of-war situations or bi-dimensional motions of minimal asters. These data also reveal unappreciated effects of the nature of viscous crowders and hydrodynamic interactions between cargos and MTs, likely relevant to understand this mode of force exertion in living cells. This study reinforces the notion that endomembrane transport can exert significant forces on MTs.

The Tubulin Code in Mitosis and Cancer

The “tubulin code” combines different α/β-tubulin isotypes with several post-translational modifications (PTMs) to generate microtubule diversity in cells. During cell division, specific microtubule populations in the mitotic spindle are differentially modified, but only recently, the functional significance of the tubulin code, with particular emphasis on the role specified by tubulin PTMs, started to be elucidated. This is the case of α-tubulin detyrosination, which was shown to guide chromosomes during congression to the metaphase plate and allow the discrimination of mitotic errors, whose correction is required to prevent chromosomal instability—a hallmark of human cancers implicated in tumor evolution and metastasis. Although alterations in the expression of certain tubulin isotypes and associated PTMs have been reported in human cancers, it remains unclear whether and how the tubulin code has any functional implications for cancer cell properties. Here, we review the role of the tubulin code in chromosome segregation during mitosis and how it impacts cancer cell properties. In this context, we discuss the existence of an emerging “cancer tubulin code” and the respective implications for diagnostic, prognostic and therapeutic purposes.

The Tubulin Code in Microtubule Dynamics and Information Encoding

Microtubules are non-covalent mesoscale polymers central to the eukaryotic cytoskeleton. Microtubule structure, dynamics, and mechanics are modulated by a cell's choice of tubulin isoforms and post-translational modifications, a "tubulin code," which is thought to support the diverse morphology and dynamics of microtubule arrays across various cell types, cell cycle, and developmental stages. We give a brief historical overview of research into tubulin diversity and highlight recent progress toward uncovering the mechanistic underpinnings of the tubulin code. As a large number of essential pathways converge upon the microtubule cytoskeleton, understanding how cells utilize tubulin diversity is crucial to understanding cellular physiology and disease.

Activation and Regulation of Cytoplasmic Dynein

Cytoplasmic dynein is an AAA+ motor that drives the transport of many intracellular cargoes towards the minus end of microtubules (MTs). Previous in vitro studies characterized isolated dynein as an exceptionally weak motor that moves slowly and diffuses on an MT. Recent studies altered this view by demonstrating that dynein remains in an autoinhibited conformation on its own, and processive motility is activated when it forms a ternary complex with dynactin and a cargo adaptor. This complex assembles more efficiently in the presence of Lis1, providing an explanation for why Lis1 is a required cofactor for most cytoplasmic dynein-driven processes in cells. This review describes how dynein motility is activated and regulated by cargo adaptors and accessory proteins.

DNA methylation biomarkers for nasopharyngeal carcinoma

Background

Aberrant methylation of DNA plays an important role in the pathogenesis of nasopharyngeal carcinoma (NPC). In the current study, we aimed to integrate three cohorts profile datasets to identify abnormally methylated-differentially expressed genes and pathways associated with NPC.

Methods

Data of gene expression microarrays (GSE53819, GSE412452) and gene methylation microarrays (GSE52068) obtained from the GEO database. Aberrantly methylated differentially expressed genes (DEGs) were obtained by GEO2R. The David database was utilized to perform enrichment and functional analysis regarding selected genes. To create a protein-protein interaction (PPI), STRING and Cytoscape software were utilized. The MCODE was used for module analysis of the PPI network.

Results

In total, 181 hypomethylation-high expression genes were identified, which were enriched in the biological mechanisms involved in the differentiation of endodermal cell, mitotic nuclear division, mitotic cell cycle process, chromosome segregation and cell cycle phase transition, etc. Pathway enrichment showed ECM-receptor interaction, PI3K-Akt signaling pathway, Focal adhesion, Protein digestion and absorption and Amoebiasis, etc. The top 3 hub genes of PPI network were FANCI, POSTN, and IFIH1. Additionally, 210 hypermethylation-low expression genes were identified, and our data revealed enrichment in biological processes including axoneme assembly, micro tubular formation, assembly of axonemal dynein complex, cilium movement and cilium organization, etc. Pathway analysis indicated enrichment in B cell receptor signaling pathway, Hematopoietic cell lineage, Leukocyte transendothelial migration, Complement and coagulation cascades and Fc gamma R-mediated phagocytosis, etc. The ZMYND10, PACRG and POU2AF1 were identified as the top three hub genes of PPI network. After validation in TCGA and GEPIA database, most hub genes remained significant. Patients with high expression of POSTN found to have shorter overall survival, while in patients with high expression of ZMYND10 and POU2AF1 longer overall survival was identified.

Conclusions

The data revealed novel aberrantly methylated-differentially expressed genes and pathways in NPC by bioinformatics analysis, potentially providing novel insights for the molecular mechanisms governing NPC progression. Hub genes including FANCI, POSTN, IFIH1, ZMYND10, PACRG and POU2AF1 might serve as novel biomarkers for precision diagnosis and providing medical treatment for patient with NPC.

The Generation of Dynein Networks by Multi-Layered Regulation and Their Implication in Cell Division

Cytoplasmic dynein-1 (hereafter referred to as dynein) is a major microtubule-based motor critical for cell division. Dynein is essential for the formation and positioning of the mitotic spindle as well as the transport of various cargos in the cell. A striking feature of dynein is that, despite having a wide variety of functions, the catalytic subunit is coded in a single gene. To perform various cellular activities, there seem to be different types of dynein that share a common catalytic subunit. In this review, we will refer to the different kinds of dynein as “dyneins.” This review attempts to classify the mechanisms underlying the emergence of multiple dyneins into four layers. Inside a cell, multiple dyneins generated through the multi-layered regulations interact with each other to form a network of dyneins. These dynein networks may be responsible for the accurate regulation of cellular activities, including cell division. How these networks function inside a cell, with a focus on the early embryogenesis of Caenorhabditis elegans embryos, is discussed, as well as future directions for the integration of our understanding of molecular layering to understand the totality of dynein’s function in living cells.

Cytoskeleton Force Exertion in Bulk Cytoplasm

The microtubule and actin cytoskeletons generate forces essential to position centrosomes, nuclei, and spindles for division plane specification. While the largest body of work has documented force exertion at, or close to the cell surface, mounting evidence suggests that cytoskeletal polymers can also produce significant forces directly from within the cytoplasm. Molecular motors such as kinesin or dynein may for instance displace cargos and endomembranes in the viscous cytoplasm yielding friction forces that pull or push microtubules. Similarly, the dynamics of bulk actin assembly/disassembly or myosin-dependent contractions produce cytoplasmic forces which influence the spatial organization of cells in a variety of processes. We here review the molecular and physical mechanisms supporting bulk cytoplasmic force generation by the cytoskeleton, their limits and relevance to organelle positioning, with a particular focus on cell division.

Microtubule plus-end dynamics link wound repair to the innate immune response

The skin protects animals from infection and physical damage. In Caenorhabditis elegans, wounding the epidermis triggers an immune reaction and a repair response, but it is not clear how these are coordinated. Previous work implicated the microtubule cytoskeleton in the maintenance of epidermal integrity (Chuang et al., 2016). Here, by establishing a simple wounding system, we show that wounding provokes a reorganisation of plasma membrane subdomains. This is followed by recruitment of the microtubule plus end-binding protein EB1/EBP-2 around the wound and actin ring formation, dependent on ARP2/3 branched actin polymerisation. We show that microtubule dynamics are required for the recruitment and closure of the actin ring, and for the trafficking of the key signalling protein SLC6/SNF-12 toward the injury site. Without SNF-12 recruitment, there is an abrogation of the immune response. Our results suggest that microtubule dynamics coordinate the cytoskeletal changes required for wound repair and the concomitant activation of innate immunity.

The cortical force-generating machinery: how cortical spindle-pulling forces are generated

The cortical force-generating machinery pulls on dynamic plus-ends of astral microtubules to control spindle position and orientation, which underlie division type specification and cellular patterning in many eukaryotic cells. A prior work identified cytoplasmic dynein, a minus-end directed microtubule motor, as a key conserved unit of the cortical force-generating machinery. Here, I summarize recent structural, biophysical, and cell-biological studies that advance our understanding of how dynein is activated and organized at the mitotic cell cortex to generate functional spindle-pulling forces. In addition, I introduce recent findings of dynein-independent or parallel mechanisms for achieving oriented cell division.

A transient helix in the disordered region of dynein light intermediate chain links the motor to structurally diverse adaptors for cargo transport

All animal cells use the motor cytoplasmic dynein 1 (dynein) to transport diverse cargo toward microtubule minus ends and to organize and position microtubule arrays such as the mitotic spindle. Cargo-specific adaptors engage with dynein to recruit and activate the motor, but the molecular mechanisms remain incompletely understood. Here, we use structural and dynamic nuclear magnetic resonance (NMR) analysis to demonstrate that the C-terminal region of human dynein light intermediate chain 1 (LIC1) is intrinsically disordered and contains two short conserved segments with helical propensity. NMR titration experiments reveal that the first helical segment (helix 1) constitutes the main interaction site for the adaptors Spindly (SPDL1), bicaudal D homolog 2 (BICD2), and Hook homolog 3 (HOOK3). In vitro binding assays show that helix 1, but not helix 2, is essential in both LIC1 and LIC2 for binding to SPDL1, BICD2, HOOK3, RAB-interacting lysosomal protein (RILP), RAB11 family-interacting protein 3 (RAB11FIP3), ninein (NIN), and trafficking kinesin-binding protein 1 (TRAK1). Helix 1 is sufficient to bind RILP, whereas other adaptors require additional segments preceding helix 1 for efficient binding. Point mutations in the C-terminal helix 1 of Caenorhabditis elegans LIC, introduced by genome editing, severely affect development, locomotion, and life span of the animal and disrupt the distribution and transport kinetics of membrane cargo in axons of mechanosensory neurons, identical to what is observed when the entire LIC C-terminal region is deleted. Deletion of the C-terminal helix 2 delays dynein-dependent spindle positioning in the one-cell embryo but overall does not significantly perturb dynein function. We conclude that helix 1 in the intrinsically disordered region of LIC provides a conserved link between dynein and structurally diverse cargo adaptor families that is critical for dynein function in vivo.

A highly conserved mechanism links the microtubule minus end–directed motor dynein to structurally diverse cargo adaptors through its light intermediate chain; this interaction is crucial for dynein function in vivo.

The large size and complex organization of animal cells make the correct and efficient distribution of intracellular content a challenge. The solution is to use motor proteins, which harness energy from ATP hydrolysis to walk along actin filaments or microtubules, for directional transport of cargo. The multi-subunit motor cytoplasmic dynein 1 (dynein) is responsible for transport directed toward the minus ends of microtubules. An important question is how dynein is recruited to its diverse cargo, which includes organelles such as endosomes and mitochondria, proteins, and mRNA. In this study, we use nuclear magnetic resonance spectroscopy to show that the light intermediate chain (LIC) subunit of human dynein uses a short helix in its disordered C-terminal region to bind structurally distinct adaptor proteins that connect the motor to specific cargo. We then use genome editing in the animal model C. elegans to demonstrate the functional relevance of the C-terminal LIC helix for dynein-dependent cargo transport in neurons. Thus, dynein recruitment to cargo involves a highly conserved interaction between LIC and adaptor proteins.

The polarity-induced force imbalance in Caenorhabditis elegans embryos is caused by asymmetric binding rates of dynein to the cortex

During asymmetric cell division, the molecular motor dynein generates cortical pulling forces that position the spindle to reflect polarity and adequately distribute cell fate determinants. In Caenorhabditis elegans embryos, despite a measured anteroposterior force imbalance, antibody staining failed to reveal dynein enrichment at the posterior cortex, suggesting a transient localization there. Dynein accumulates at the microtubule plus ends, in an EBP-2EB-dependent manner. This accumulation, although not transporting dynein, contributes modestly to cortical forces. Most dyneins may instead diffuse to the cortex. Tracking of cortical dynein revealed two motions: one directed and the other diffusive-like, corresponding to force-generating events. Surprisingly, while dynein is not polarized at the plus ends or in the cytoplasm, diffusive-like tracks were more frequently found at the embryo posterior tip, where the forces are higher. This asymmetry depends on GPR-1/2LGN and LIN-5NuMA, which are enriched there. In csnk-1(RNAi) embryos, the inverse distribution of these proteins coincides with an increased frequency of diffusive-like tracks anteriorly. Importantly, dynein cortical residence time is always symmetric. We propose that the dynein-binding rate at the posterior cortex is increased, causing the polarity-reflecting force imbalance. This mechanism of control supplements the regulation of mitotic progression through the nonpolarized dynein detachment rate.

Emerging mechanisms of dynein transport in the cytoplasm versus the cilium

Two classes of dynein power long-distance cargo transport in different cellular contexts. Cytoplasmic dynein-1 is responsible for the majority of transport toward microtubule minus ends in the cell interior. Dynein-2, also known as intraflagellar transport dynein, moves cargoes along the axoneme of eukaryotic cilia and flagella. Both dyneins operate as large ATP-driven motor complexes, whose dysfunction is associated with a group of human disorders. But how similar are their mechanisms of action and regulation? To examine this question, this review focuses on recent advances in dynein-1 and -2 research, and probes to what extent the emerging principles of dynein-1 transport could apply to or differ from those of the less well-understood dynein-2 mechanoenzyme.

Physical Forces Determining the Persistency and Centering Precision of Microtubule Asters

In early embryos, microtubules form star-shaped aster structures that can measure up to hundreds of micrometres, and move at high speeds to find the geometrical centre of the cell. This process, known as aster centration, is essential for the fidelity of cell division and development, but how cells succeed in moving these large structures through their crowded and fluctuating cytoplasm remains unclear. Here, we demonstrate that the positional fluctuations of migrating sea urchin sperm asters are small, anisotropic, and associated with the stochasticity of dynein-dependent forces moving the aster. Using in vivo magnetic tweezers to directly measure aster forces inside cells, we derive a linear aster force-velocity relationship and provide evidence for a spring-like active mechanism stabilizing the transverse position of the asters. The large frictional coefficient and spring constant quantitatively account for the amplitude and growth characteristics of athermal positional fluctuations, demonstrating that aster mechanics ensure noise suppression to promote persistent and precise centration. These findings define generic biophysical regimes of active cytoskeletal mechanics underlying the accuracy of cell division and early embryonic development.

Sailing to and Docking at the Immune Synapse: Role of Tubulin Dynamics and Molecular Motors

The different cytoskeleton systems and their connecting molecular motors move vesicles and intracellular organelles to shape cells. Polarized cells with specialized functions display an exquisite spatio-temporal regulation of both cytoskeletal and organelle arrangements that support their specific tasks. In particular, T cells rapidly change their shape and cellular function through the establishment of cell surface and intracellular polarity in response to a variety of cues. This review focuses on the contribution of the microtubule-based dynein/dynactin motor complex, the tubulin and actin cytoskeletons, and different organelles to the formation of the antigen-driven immune synapse.

How cells sense their own shape - mechanisms to probe cell geometry and their implications in cellular organization and function

Cells come in a variety of shapes that most often underlie their functions. Regulation of cell morphogenesis implies that there are mechanisms for shape sensing that still remain poorly appreciated. Global and local cell geometry features, such as aspect ratio, size or membrane curvature, may be probed by intracellular modules, such as the cytoskeleton, reaction-diffusion systems or molecular complexes. In multicellular tissues, cell shape emerges as an important means to transduce tissue-inherent chemical and mechanical cues into intracellular organization. One emergent paradigm is that cell-shape sensing is most often based upon mechanisms of self-organization, rather than determinism. Here, we review relevant work that has elucidated some of the core principles of how cellular geometry may be conveyed into spatial information to guide processes, such as polarity, signaling, morphogenesis and division-plane positioning.

A CRISPR Tagging-Based Screen Reveals Localized Players in Wnt-Directed Asymmetric Cell Division

Oriented cell divisions are critical to establish and maintain cell fates and tissue organization. Diverse extracellular and intracellular cues have been shown to provide spatial information for mitotic spindle positioning; however, the molecular mechanisms by which extracellular signals communicate with cells to direct mitotic spindle positioning are largely unknown. In animal cells, oriented cell divisions are often achieved by the localization of force-generating motor protein complexes to discrete cortical domains. Disrupting either these force-generating complexes or proteins that globally affect microtubule stability results in defects in mitotic positioning, irrespective of whether these proteins function as spatial cues for spindle orientation. This poses a challenge to traditional genetic dissection of this process. Therefore, as an alternative strategy to identify key proteins that act downstream of intercellular signaling, we screened the localization of many candidate proteins by inserting fluorescent tags directly into endogenous gene loci, without overexpressing the proteins. We tagged 23 candidate proteins in Caenorhabditis elegans and examined each protein's localization in a well-characterized, oriented cell division in the four-cell-stage embryo. We used cell manipulations and genetic experiments to determine which cells harbor key localized proteins and which signals direct these localizations in vivo We found that Dishevelled and adenomatous polyposis coli homologs are polarized during this oriented cell division in response to a Wnt signal, but two proteins typically associated with mitotic spindle positioning, homologs of NuMA and Dynein, were not detectably polarized. These results suggest an unexpected mechanism for mitotic spindle positioning in this system, they pinpoint key proteins of interest, and they highlight the utility of a screening approach based on analyzing the localization of endogenously tagged proteins.

NudE regulates dynein at kinetochores but is dispensable for other dynein functions in the C. elegans early embryo

In mitosis, the molecular motor dynein is recruited to kinetochores by the Rod-Zw10-Zwilch complex (RZZ) and Spindly to control spindle assembly checkpoint (SAC) signaling and microtubule attachment. How the ubiquitous dynein co-factors Lis1 and NudE contribute to these functions remains poorly understood. Here, we show that the C. elegans NudE homolog NUD-2 is dispensable for dynein- and LIS-1-dependent mitotic spindle assembly in the zygote. This facilitates functional characterization of kinetochore-localized NUD-2, which is recruited by the CENP-F-like proteins HCP-1 and HCP-2 independently of RZZ-Spindly and dynein-LIS-1. Kinetochore dynein levels are reduced in Δnud-2 embryos, and, as occurs upon RZZ inhibition, loss of NUD-2 delays the formation of load-bearing kinetochore-microtubule attachments and causes chromatin bridges in anaphase. Survival of Δnud-2 embryos requires a functional SAC, and kinetochores without NUD-2 recruit an excess of SAC proteins. Consistent with this, SAC signaling in early Δnud-2 embryos extends mitotic duration and prevents high rates of chromosome mis-segregation. Our results reveal that both NUD-2 and RZZ-Spindly are essential for dynein function at kinetochores, and that the gain in SAC strength during early embryonic development is relevant under conditions that mildly perturb mitosis.