Formation and functions of asymmetric microtubule organization in polarized cells

RNA localization mechanisms transcend cell morphology

RNA molecules are localized to specific subcellular regions through interactions between RNA regulatory elements and RNA binding proteins (RBPs). Generally, our knowledge of the mechanistic details behind the localization of a given RNA is restricted to a particular cell type. Here, we show that RNA/RBP interactions that regulate RNA localization in one cell type predictably regulate localization in other cell types with vastly different morphologies. To determine transcriptome-wide RNA spatial distributions across the apicobasal axis of human intestinal epithelial cells, we used our recently developed RNA proximity labeling technique, Halo-seq. We found that mRNAs encoding ribosomal proteins (RP mRNAs) were strongly localized to the basal pole of these cells. Using reporter transcripts and single-molecule RNA FISH, we found that pyrimidine-rich motifs in the 5' UTRs of RP mRNAs were sufficient to drive basal RNA localization. Interestingly, the same motifs were also sufficient to drive RNA localization to the neurites of mouse neuronal cells. In both cell types, the regulatory activity of this motif was dependent on it being in the 5' UTR of the transcript, was abolished upon perturbation of the RNA-binding protein LARP1, and was reduced upon inhibition of kinesin-1. To extend these findings, we compared subcellular RNAseq data from neuronal and epithelial cells. We found that the basal compartment of epithelial cells and the projections of neuronal cells were enriched for highly similar sets of RNAs, indicating that broadly similar mechanisms may be transporting RNAs to these morphologically distinct locations. These findings identify the first RNA element known to regulate RNA localization across the apicobasal axis of epithelial cells, establish LARP1 as an RNA localization regulator, and demonstrate that RNA localization mechanisms cut across cell morphologies.

Actin-driven Golgi apparatus dispersal during collective migration of epithelial cells

As a sedentary epithelium turns motile during wound healing, morphogenesis, and metastasis, the Golgi apparatus moves from an apical position, above the nucleus, to a basal position. This apical-to-basal repositioning of Golgi is critical for epithelial cell migration. Yet the molecular mechanism underlying it remains elusive, although microtubules are believed to play a role. Using live-cell and super-resolution imaging, we show that at the onset of collective migration of epithelial cells, Golgi stacks get dispersed to create an unpolarized transitional structure, and surprisingly, this dispersal process depends not on microtubules but on actin cytoskeleton. Golgi-actin interaction involves Arp2/3-driven actin projections emanating from the actin cortex, and a Golgi-localized actin elongation factor, MENA. While in sedentary epithelial cells, actin projections intermittently interact with the apically located Golgi, and the frequency of this event increases before the dispersion of Golgi stacks, at the onset of cell migration. Preventing Golgi-actin interaction with MENA-mutants eliminates Golgi dispersion and reduces the persistence of cell migration. Taken together, we show a process of actin-driven Golgi dispersion that is mechanistically different from the well-known Golgi apparatus fragmentation during mitosis and is essential for collective migration of epithelial cells.

Tunable Cellular Localization and Extensive Cytoskeleton-Interplay of Reflectins

Reflectin proteins are natural copolymers consisting of repeated canonical domains. They are located in a biophotonic system called Bragg lamellae and manipulate the dynamic structural coloration of iridocytes. Their biological functions are intriguing, but the underlying mechanism is not fully understood. Reflectin A1, A2, B1, and C were found to present distinguished cyto-/nucleoplasmic localization preferences in the work. Comparable intracellular localization was reproduced by truncated reflectin variants, suggesting a conceivable evolutionary order among reflectin proteins. The size-dependent access of reflectin variants into the nucleus demonstrated a potential model of how reflectins get into Bragg lamellae. Moreover, RfA1 was found to extensively interact with the cytoskeleton, including its binding to actin and enrichment at the microtubule organizing center. This implied that the cytoskeleton system plays a fundamental role during the organization and transportation of reflectin proteins. The findings presented here provide evidence to get an in-depth insight into the evolutionary processes and working mechanisms of reflectins, as well as novel molecular tools to achieve tunable intracellular transportation.

Microtubules regulate pancreatic β-cell heterogeneity via spatiotemporal control of insulin secretion hot spots

Heterogeneity of glucose-stimulated insulin secretion (GSIS) in pancreatic islets is physiologically important but poorly understood. Here, we utilize mouse islets to determine how microtubules (MTs) affect secretion toward the vascular extracellular matrix at single cell and subcellular levels. Our data indicate that MT stability in the β-cell population is heterogenous, and that GSIS is suppressed in cells with highly stable MTs. Consistently, MT hyper-stabilization prevents, and MT depolymerization promotes the capacity of single β-cell for GSIS. Analysis of spatiotemporal patterns of secretion events shows that MT depolymerization activates otherwise dormant β-cells via initiation of secretion clusters (hot spots). MT depolymerization also enhances secretion from individual cells, introducing both additional clusters and scattered events. Interestingly, without MTs, the timing of clustered secretion is dysregulated, extending the first phase of GSIS and causing oversecretion. In contrast, glucose-induced Ca²⁺ influx was not affected by MT depolymerization yet required for secretion under these conditions, indicating that MT-dependent regulation of secretion hot spots acts in parallel with Ca²⁺ signaling. Our findings uncover a novel MT function in tuning insulin secretion hot spots, which leads to accurately measured and timed response to glucose stimuli and promotes functional β-cell heterogeneity.

Getting around the cell: physical transport in the intracellular world

Eukaryotic cells face the challenging task of transporting a variety of particles through the complex intracellular milieu in order to deliver, distribute, and mix the many components that support cell function. In this review, we explore the biological objectives and physical mechanisms of intracellular transport. Our focus is on cytoplasmic and intra-organelle transport at the whole-cell scale. We outline several key biological functions that depend on physically transporting components across the cell, including the delivery of secreted proteins, support of cell growth and repair, propagation of intracellular signals, establishment of organelle contacts, and spatial organization of metabolic gradients. We then review the three primary physical modes of transport in eukaryotic cells: diffusive motion, motor-driven transport, and advection by cytoplasmic flow. For each mechanism, we identify the main factors that determine speed and directionality. We also highlight the efficiency of each transport mode in fulfilling various key objectives of transport, such as particle mixing, directed delivery, and rapid target search. Taken together, the interplay of diffusion, molecular motors, and flows supports the intracellular transport needs that underlie a broad variety of biological phenomena.

Wnt ligands regulate the asymmetric divisions of neuronal progenitors in C. elegans embryos

Wnt/β-catenin signalling has been implicated in the terminal asymmetric divisions of neuronal progenitors in vertebrates and invertebrates. However, the role of Wnt ligands in this process remains poorly characterized. Here, we used the terminal divisions of the embryonic neuronal progenitors in C. elegans to characterize the role of Wnt ligands during this process, focusing on a lineage that produces the cholinergic interneuron AIY. We observed that, during interphase, the neuronal progenitor is elongated along the anteroposterior axis, then divides along its major axis, generating an anterior and a posterior daughter with different fates. Using time-controlled perturbations, we show that three Wnt ligands, which are transcribed at higher levels at the posterior of the embryo, regulate the orientation of the neuronal progenitor and its asymmetric division. We also identify a role for a Wnt receptor (MOM-5) and a cortical transducer APC (APR-1), which are, respectively, enriched at the posterior and anterior poles of the neuronal progenitor. Our study establishes a role for Wnt ligands in the regulation of the shape and terminal asymmetric divisions of neuronal progenitors, and identifies downstream components.

Cell polarization: From epithelial cells to odontoblasts

Cell polarity identifies the asymmetry of a cell. Various types of cells, including odontoblasts and epithelial cells, polarize to fulfil their destined functions. Odontoblast polarization is a prerequisite and fundamental step for tooth development and tubular dentin formation. Current knowledge of odontoblast polarization, however, is very limited, which greatly impedes the development of novel approaches for regenerative endodontics. Compared to odontoblasts, epithelial cell polarization has been extensively studied over the last several decades. The knowledge obtained from epithelia polarization has been found applicable to other cell types, which is particularly useful considering the remarkable similarities of the morphological and compositional features between polarized odontoblasts and epithelia. In this review, we first discuss the characteristics, the key regulatory factors, and the process of epithelial polarity. Next, we compare the known facts of odontoblast polarization with epithelial cells. Lastly, we clarify knowledge gaps in odontoblast polarization and propose the directions for future research to fill the gaps, leading to the advancement of regenerative endodontics.

Centrosomal and Non-Centrosomal Microtubule-Organizing Centers (MTOCs) in Drosophila melanogaster

The centrosome is the best-understood microtubule-organizing center (MTOC) and is essential in particular cell types and at specific stages during Drosophila development. The centrosome is not required zygotically for mitosis or to achieve full animal development. Nevertheless, centrosomes are essential maternally during cleavage cycles in the early embryo, for male meiotic divisions, for efficient division of epithelial cells in the imaginal wing disc, and for cilium/flagellum assembly in sensory neurons and spermatozoa. Importantly, asymmetric and polarized division of stem cells is regulated by centrosomes and by the asymmetric regulation of their microtubule (MT) assembly activity. More recently, the components and functions of a variety of non-centrosomal microtubule-organizing centers (ncMTOCs) have begun to be elucidated. Throughout Drosophila development, a wide variety of unique ncMTOCs form in epithelial and non-epithelial cell types at an assortment of subcellular locations. Some of these cell types also utilize the centrosomal MTOC, while others rely exclusively on ncMTOCs. The impressive variety of ncMTOCs being discovered provides novel insight into the diverse functions of MTOCs in cells and tissues. This review highlights our current knowledge of the composition, assembly, and functional roles of centrosomal and non-centrosomal MTOCs in Drosophila.

Local and global Cdc42 guanine nucleotide exchange factors for fission yeast cell polarity are coordinated by microtubules and the Tea1-Tea4-Pom1 axis

The conserved Rho-family GTPase Cdc42 plays a central role in eukaryotic cell polarity. The rod-shaped fission yeast Schizosaccharomyces pombe has two Cdc42 guanine nucleotide exchange factors (GEFs), Scd1 and Gef1, but little is known about how they are coordinated in polarized growth. Although the microtubule cytoskeleton is normally not required for polarity maintenance in fission yeast, we show here that when scd1 function is compromised, disruption of microtubules or the polarity landmark proteins Tea1, Tea4 or Pom1 leads to disruption of polarized growth. Instead, cells adopt an isotropic-like pattern of growth, which we term PORTLI growth. Surprisingly, PORTLI growth is caused by spatially inappropriate activity of Gef1. Although most Cdc42 GEFs are membrane associated, we find that Gef1 is a broadly distributed cytosolic protein rather than a membrane-associated protein at cell tips like Scd1. Microtubules and the Tea1–Tea4–Pom1 axis counteract inappropriate Gef1 activity by regulating the localization of the Cdc42 GTPase-activating protein Rga4. Our results suggest a new model of fission yeast cell polarity regulation, involving coordination of ‘local’ (Scd1) and ‘global’ (Gef1) Cdc42 GEFs via microtubules and microtubule-dependent polarity landmarks.

Highlighted Article: Cell polarity in fission yeast is regulated by two different Cdc42 guanine nucleotide exchange factors, coordinated by the microtubule-dependent landmark system.

SH3-domain binding protein 1 in the tumor microenvironment promotes hepatocellular carcinoma metastasis through WAVE2 pathway

SH3-domain binding protein-1 (SH3BP1) specifically inactivating Rac1 and its target WAVE2 is required for cell motility. The present study shows SH3BP1 expression patterns in human HCC tissues and cell lines were examined. The regulation of SH3BP1 on HCC cell migration and invasion related to Rac1-WAVE2 signaling was characterized using in vitro and in vivo models. SH3BP1 overexpressed in HCC tissues and highly metastatic HCC cells was significantly associated vascular invasion (VI). SH3BP1 promoted VEGF secretion via Rac1-WAVE2 signaling, so as to exert an augmentation on cell invasion and microvessel formation. In three study cohorts with a total of 516 HCC patients, high SH3BP1 expression combined with high microvessel density (MVD) was confirmed as a powerful independent predictor of HCC prognosis in both training cohorts and validation cohort. Being an important angiogenic factor of HCC through Rac1-WAVE2 signaling, SH3BP1 promotes tumor invasion and microvessel formation contributing to HCC metastasis and recurrence. SH3BP1 is a novel WAVE2 regulator, a prognostic marker and a potential therapeutic target of HCC.

Centriolar SAS-7 acts upstream of SPD-2 to regulate centriole assembly and pericentriolar material formation

The centriole/basal body is a eukaryotic organelle that plays essential roles in cell division and signaling. Among five known core centriole proteins, SPD-2/Cep192 is the first recruited to the site of daughter centriole formation and regulates the centriolar localization of the other components in C. elegans and in humans. However, the molecular basis for SPD-2 centriolar localization remains unknown. Here, we describe a new centriole component, the coiled-coil protein SAS-7, as a regulator of centriole duplication, assembly and elongation. Intriguingly, our genetic data suggest that SAS-7 is required for daughter centrioles to become competent for duplication, and for mother centrioles to maintain this competence. We also show that SAS-7 binds SPD-2 and regulates SPD-2 centriolar recruitment, while SAS-7 centriolar localization is SPD-2-independent. Furthermore, pericentriolar material (PCM) formation is abnormal in sas-7 mutants, and the PCM-dependent induction of cell polarity that defines the anterior-posterior body axis frequently fails. We conclude that SAS-7 functions at the earliest step in centriole duplication yet identified and plays important roles in the orchestration of centriole and PCM assembly.

DOI:http://dx.doi.org/10.7554/eLife.20353.001

Identification of ginkgolide targets in brain by photoaffinity labeling

Ginkgolides are terpene trilactones in Ginkgo biloba, a popular medicinal herb for memory disorders. Although ginkgolides are known for various neurobiological effects, their macromolecular target in brain is unknown. In this work, we employed benzophenone derivatives of ginkgolides to identify their binding target in brain. Photolabeling of bovine hippocampus homogenates identified a series of α-tubulin isotypes. Selective photolabeling of α-tubulin over β-tubulin, which is equally abundant in brain, suggested that ginkgolides might modulate microtubule biology differently than typical microtubule-binding agents, such as taxol. In fact, ginkgolide A did not affect microtubule polymerization or cell proliferation; instead, it inhibited detyrosination of α-tubulin and reorientation of microtubule-organizing centers. Taken together, the current findings indicate that ginkgolides constitute a new class of microtubule-binding agents with distinct effects on α-tubulin biology.

Microtubule organization is determined by the shape of epithelial cells

Interphase microtubule organization is critical for cell function and tissue architecture. In general, physical mechanisms are sufficient to drive microtubule organization in single cells, whereas cells within tissues are thought to utilize signalling mechanisms. By improving the imaging and quantitation of microtubule alignment within developing Drosophila embryos, here we demonstrate that microtubule alignment underneath the apical surface of epithelial cells follows cell shape. During development, epidermal cell elongation and microtubule alignment occur simultaneously, but by perturbing cell shape, we discover that microtubule organization responds to cell shape, rather than the converse. A simple set of microtubule behaviour rules is sufficient for a computer model to mimic the observed responses to changes in cell surface geometry. Moreover, we show that microtubules colliding with cell boundaries zip-up or depolymerize in an angle-dependent manner, as predicted by the model. Finally, we show microtubule alignment responds to cell shape in diverse epithelia.

Targeting Microtubules for Wound Repair

Significance: Fast and seamless healing is essential for both deep and chronic wounds to restore the skin and protect the body from harmful pathogens. Thus, finding new targets that can both expedite and enhance the repair process without altering the upstream signaling milieu and causing serious side effects can improve the way we treat wounds. Since cell migration is key during the different stages of wound healing, it presents an ideal process and intracellular structural machineries to target. Recent Advances and Critical Issues: The microtubule (MT) cytoskeleton is rising as an important structural and functional regulator of wound healing. MTs have been reported to play different roles in the migration of the various cell types involved in wound healing. Specific microtubule regulatory proteins (MRPs) can be targeted to alter a section or subtype of the MT cytoskeleton and boost or hinder cell motility. However, inhibiting intracellular components can be challenging in vivo, especially using unstable molecules, such as small interfering RNA. Nanoparticles can be used to protect these unstable molecules and topically deliver them to the wound. Utilizing this approach, we recently showed that fidgetin-like 2, an uncharacterized MRP, can be targeted to enhance cell migration and wound healing. Future Directions: To harness the full potential of the current MRP therapeutic targets, studies should test them with different delivery platforms, dosages, and skin models. Screening for new MT effectors that boost cell migration in vivo would also help find new targets for skin repair.

Unique and redundant β-catenin regulatory roles of two Dishevelled paralogs during C. elegans asymmetric cell division

The Wnt/β-catenin signaling pathway is utilized across metazoans. However, the mechanism of signal transduction, especially dissociation of the β-catenin destruction complex by Dishevelled proteins, remains controversial. Here, we describe the function of the Dishevelled paralogs DSH-2 and MIG-5 in the Wnt/β-catenin asymmetry (WβA) pathway in Caenorhabditis elegans, where WβA drives asymmetric cell divisions throughout development. We find that DSH-2 and MIG-5 redundantly regulate cell fate in hypodermal seam cells. Similarly, both DSH-2 and MIG-5 are required for positive regulation of SYS-1 (a C. elegans β-catenin), but MIG-5 has a stronger effect on the polarity of SYS-1 localization. We show that MIG-5 controls cortical APR-1 (the C. elegans APC) localization. DSH-2 and MIG-5 both regulate the localization of WRM-1 (another C. elegans β-catenin), acting together as negative regulators of WRM-1 nuclear localization. Finally, we demonstrate that overexpression of DSH-2 or MIG-5 in seam cells leads to stabilization of SYS-1 in the anterior seam daughter, solidifying the Dishevelled proteins as positive regulators of SYS-1. Overall, we have further defined the role of Dishevelled in the WβA signaling pathway, and demonstrated that DSH-2 and MIG-5 regulate cell fate, β-catenin nuclear levels and the polarity of β-catenin regulation.

The augmin connection in the geometry of microtubule networks

Microtubules mediate important cellular processes by forming highly ordered arrays. Organization of these networks is achieved by nucleating and anchoring microtubules at centrosomes and other structures collectively known as microtubule-organizing centers (MTOCs). However, the diverse microtubule configurations found in different cell types may not be generated and maintained by MTOCs alone. Work over the last few years has revealed a mechanism that has the capacity to generate cell-type-specific microtubule arrays independently of a specific organizer: nucleation of microtubules from the lateral surface of pre-existing microtubules. This type of nucleation requires cooperation between two different multi-subunit protein complexes, augmin and the γ-tubulin ring complex (γTuRC). Here we review recent molecular insight into microtubule-dependent nucleation and discuss the possibility that the augmin-γTuRC module, initially described in mitosis, may broadly contribute to microtubule organization also in non-mitotic cells.

Microtubule-independent secretion requires functional maturation of Golgi elements

The Golgi apparatus is responsible for processing and sorting of secretory cargos. Microtubules are known to accelerate the transport of proteins from the endoplasmic reticulum to the Golgi apparatus and from the Golgi to the plasma membrane. However, whether post-Golgi transport strictly requires microtubules is still unclear. Using the retention using selective hooks (RUSH) system to synchronize the trafficking of cargos, we show that anterograde transport of tumor necrosis factor (TNF) is strongly reduced without microtubules. We show that two populations of Golgi elements co-exist in these cells. A centrally located and giantin-positive Golgi complex sustains trafficking while newly formed peripheral Golgi mini-stacks accumulate cargos in cells without microtubules. Using a genome-edited GFP-giantin cell line, we observe that the trafficking-competent Golgi population corresponds to the pre-existing one that was present before removal of microtubules. All Golgi elements support trafficking after long-term microtubules depletion or after relocation of Golgi proteins in the endoplasmic reticulum using Brefeldin A. Our results demonstrate that functional maturation of Golgi elements is needed to ensure post-Golgi trafficking and that microtubule-driven post-Golgi transport is not strictly required.

Extracellular and Intracellular Signaling for Neuronal Polarity

Neurons are one of the highly polarized cells in the body. One of the fundamental issues in neuroscience is how neurons establish their polarity; therefore, this issue fascinates many scientists. Cultured neurons are useful tools for analyzing the mechanisms of neuronal polarization, and indeed, most of the molecules important in their polarization were identified using culture systems. However, we now know that the process of neuronal polarization in vivo differs in some respects from that in cultured neurons. One of the major differences is their surrounding microenvironment; neurons in vivo can be influenced by extrinsic factors from the microenvironment. Therefore, a major question remains: How are neurons polarized in vivo? Here, we begin by reviewing the process of neuronal polarization in culture conditions and in vivo. We also survey the molecular mechanisms underlying neuronal polarization. Finally, we introduce the theoretical basis of neuronal polarization and the possible involvement of neuronal polarity in disease and traumatic brain injury.

RFX2 Is a Major Transcriptional Regulator of Spermiogenesis

Spermatogenesis consists broadly of three phases: proliferation of diploid germ cells, meiosis, and finally extensive differentiation of the haploid cells into effective delivery vehicles for the paternal genome. Despite detailed characterization of many haploid developmental steps leading to sperm, only fragmentary information exists on the control of gene expression underlying these processes. Here we report that the RFX2 transcription factor is a master regulator of genes required for the haploid phase. A targeted mutation of Rfx2 was created in mice. Rfx2^-/- mice are perfectly viable but show complete male sterility. Spermatogenesis appears to progress unperturbed through meiosis. However, haploid cells undergo a complete arrest in spermatid development just prior to spermatid elongation. Arrested cells show altered Golgi apparatus organization, leading to a deficit in the generation of a spreading acrosomal cap from proacrosomal vesicles. Arrested cells ultimately merge to form giant multinucleated cells released to the epididymis. Spermatids also completely fail to form the flagellar axoneme. RNA-Seq analysis and ChIP-Seq analysis identified 139 genes directly controlled by RFX2 during spermiogenesis. Gene ontology analysis revealed that genes required for cilium function are specifically enriched in down- and upregulated genes showing that RFX2 allows precise temporal expression of ciliary genes. Several genes required for cell adhesion and cytoskeleton remodeling are also downregulated. Comparison of RFX2-regulated genes with those controlled by other major transcriptional regulators of spermiogenesis showed that each controls independent gene sets. Altogether, these observations show that RFX2 plays a major and specific function in spermiogenesis.

Failure of spermatogenesis, which is presumed to often result from genetic defects, is a common cause of male sterility. Although numerous genes associated with defects in male spermatogenesis have been identified, numerous cases of genetic male infertility remain unelucidated. We report here that the transcription factor RFX2 is a master regulator of gene expression programs required for progression through the haploid phase of spermatogenesis. Male RFX2-deficient mice are completely sterile. Spermatogenesis progresses through meiosis, but haploid cells undergo a complete block in development just prior to spermatid elongation. Gene expression profiling and ChIP-Seq analysis revealed that RFX2 controls key pathways implicated in cilium/flagellum formation, as well as genes implicated in microtubule and vesicle associated transport. The set of genes activated by RFX2 in spermatids exhibits virtually no overlap with those controlled by other known transcriptional regulators of spermiogenesis, establishing RFX2 as an essential new player in this developmental process. RFX2-deficient mice should therefore represent a valuable new model for deciphering the regulatory networks that direct sperm formation, and thereby contribute to the identification of causes of human male infertility.

Modulation of cell morphogenesis by tousled-like kinase in the Drosophila follicle cell

BACKGROUND

Tousled-like kinase (Tlk) is a conserved serine/threonine kinase regulating DNA replication, chromatin assembly, and DNA repair. Previous studies have suggested that Tlk is involved in cell morphogenesis in vitro. In addition, tlk genetically interact with Rho1, which encodes a key regulator of the cytoskeleton. However, whether Tlk plays a physiological role in cell morphogenesis and cytoskeleton rearrangement remains unknown.

RESULTS

In tlk mutant follicle cells, area of the apical domain was reduced. The density of microtubules was increased in tlk mutant cells. The density of actin filaments was increased in the apical region and decreased in the basal region. Because area of the apical domain was reduced, we examined the levels of proteins located in the apical region by using immunofluorescence. The fluorescence intensities of two adherens junction proteins Armadillo (Arm) and DE-cadherin (DE-cad), atypical protein kinase C (aPKC), and Notch, were all increased in tlk mutant cells. The basolateral localized Discs large (Dlg) shifted apically in tlk mutant cells.

CONCLUSIONS

Increase of protein densities in the apical region might be resulted from disruption of the cytoskeleton and shrinkage of the apical domain. Together, these data suggest a novel role of Tlk in maintaining cell morphology, possibly through modulating the cytoskeleton.

Alpha-catenin-dependent recruitment of the centrosomal protein CAP350 to adherens junctions allows epithelial cells to acquire a columnar shape

Epithelial morphogenesis involves a dramatic reorganisation of the microtubule cytoskeleton. How this complex process is controlled at the molecular level is still largely unknown. Here, we report that the centrosomal microtubule (MT)-binding protein CAP350 localises at adherens junctions in epithelial cells. By two-hybrid screening, we identified a direct interaction of CAP350 with the adhesion protein α-catenin that was further confirmed by co-immunoprecipitation experiments. Block of epithelial cadherin (E-cadherin)-mediated cell-cell adhesion or α-catenin depletion prevented CAP350 localisation at cell-cell junctions. Knocking down junction-located CAP350 inhibited the establishment of an apico-basal array of microtubules and impaired the acquisition of columnar shape in Madin-Darby canine kidney II (MDCKII) cells grown as polarised epithelia. Furthermore, MDCKII cystogenesis was also defective in junctional CAP350-depleted cells. CAP350-depleted MDCKII cysts were smaller and contained either multiple lumens or no lumen. Membrane polarity was not affected, but cortical microtubule bundles did not properly form. Our results indicate that CAP350 may act as an adaptor between adherens junctions and microtubules, thus regulating epithelial differentiation and contributing to the definition of cell architecture. We also uncover a central role of α-catenin in global cytoskeleton remodelling, in which it acts not only on actin but also on MT reorganisation during epithelial morphogenesis.

In epithelial cells, the normally centrosomal protein CAP350 binds to α-catenin at adherens junctions and helps to establish the cells' parallel apico-basal microtubule array and columnar shape.

Epithelia cover all the surfaces of and the cavities throughout the body and serve as barriers between the organism and its external environment. Epithelial differentiation requires the coordination in space and time of several mechanisms that ultimately lead to the acquisition of distinctive epithelial features, including apical-basal polarity, specialised cell-cell junctions, and columnar shape. Epithelial differentiation also induces the reorganisation of three cytoskeletal networks: actin filaments, intermediate filaments, and microtubules. In simple epithelia, cadherins and their cytoplasmic binding partners catenins play a crucial role in connecting cell-cell junctions to the actin cytoskeleton. The cadherin extracellular domain forms adhesive contacts between adjacent cells, and their cytoplasmic tail indirectly binds the actin-binding protein α-catenin, thus linking cell-cell junctions to the underlying actin cytoskeleton. We report here an additional role of α-catenin in remodelling microtubules during epithelial differentiation. In most epithelial cells, microtubules are organised as parallel bundles aligned along the apico-basal axis and as apical and basal plasma membrane-associated networks. We demonstrate that the microtubule-binding protein CAP350, which is only localised at the centrosome in most cells, is also recruited at cell–cell junctions in epithelial cells through its binding to α-catenin. In the absence of junctional CAP350, microtubules are unable to reorganise in bundles, and cells do not acquire columnar shape. Our results suggest that recruitment of centrosomal proteins to cell-cell junctions could be a general mechanism to control microtubule reorganisation in neighbour cells during epithelial differentiation.

Visualizing Microtubule Networks During Drosophila Oogenesis Using Fixed and Live Imaging

The microtubule cytoskeleton is a plastic network of polarized cables. These polymers of tubulin provide orientated routes for the dynamic transport of cytoplasmic molecules and organelles, through which cell polarity is established and maintained. The role of microtubule-mediated transport in the asymmetric localization of axis polarity determinants, in the Drosophila oocyte, has been the subject of extensive studies in the past years. However, imaging the distribution of microtubule fibers in a large cell, where vitellogenesis ensures the uptake of a thick and hazy yolk, presents a series of technical challenges. This chapter briefly reviews some of these aspects and describes two methods designed to circumvent these difficulties. We provide a detailed protocol for the visualization by immunohistochemistry of the three-dimensional organization of tubulin cables in the oocyte. Additionally, we detail the stepwise procedure for the live imaging of microtubule dynamics and network remodeling, using fluorescently labeled microtubule-associated proteins.

Integrins and epithelial cell polarity

Cell polarity is characterised by differences in structure, composition and function between at least two poles of a cell. In epithelial cells, these spatial differences allow for the formation of defined apical and basal membranes. It has been increasingly recognised that cell-matrix interactions and integrins play an essential role in creating epithelial cell polarity, although key gaps in our knowledge remain. This Commentary will discuss the mounting evidence for the role of integrins in polarising epithelial cells. We build a model in which both inside-out signals to polarise basement membrane assembly at the basal surface, and outside-in signals to control microtubule apical-basal orientation and vesicular trafficking are required for establishing and maintaining the orientation of epithelial cell polarity. Finally, we discuss the relevance of the basal integrin polarity axis to cancer. This article is part of a Minifocus on Establishing polarity.

Velocity alignment leads to high persistence in confined cells

Many cell types display random motility on two-dimensional substrates but crawl persistently in a single direction when confined in a microchannel or on an adhesive micropattern. Does this imply that the motility mechanism of confined cells is fundamentally different from that of unconfined cells? We argue that both free- and confined-cell migration may be described by a generic model of cells as "velocity-aligning" active Brownian particles previously proposed to solve a completely separate problem in collective cell migration. Our model can be mapped to a diffusive escape over a barrier and analytically solved to determine the cell's orientation distribution and repolarization rate. In quasi-one-dimensional confinement, velocity-aligning cells maintain their direction for times that can be exponentially larger than their persistence time in the absence of confinement. Our results suggest an important connection between single- and collective-cell migration: high persistence in confined cells corresponds with fast alignment of velocity to cell-cell forces.

Protein 4.1R binds to CLASP2 and regulates dynamics, organization and attachment of microtubules to the cell cortex

The microtubule (MT) cytoskeleton is essential for many cellular processes, including cell polarity and migration. Cortical platforms, formed by a subset of MT plus-end-tracking proteins, such as CLASP2, and non-MT binding proteins such as LL5β, attach distal ends of MTs to the cell cortex. However, the mechanisms involved in organizing these platforms have not yet been described in detail. Here we show that 4.1R, a FERM-domain-containing protein, interacts and colocalizes with cortical CLASP2 and is required for the correct number and dynamics of CLASP2 cortical platforms. Protein 4.1R also controls binding of CLASP2 to MTs at the cell edge by locally altering GSK3 activity. Furthermore, in 4.1R-knockdown cells MT plus-ends were maintained for longer in the vicinity of cell edges, but instead of being tethered to the cell cortex, MTs continued to grow, bending at cell margins and losing their radial distribution. Our results suggest a previously unidentified role for the scaffolding protein 4.1R in locally controlling CLASP2 behavior, CLASP2 cortical platform turnover and GSK3 activity, enabling correct MT organization and dynamics essential for cell polarity.

Cell polarity: mechanochemical patterning

Nearly every cell type exhibits some form of polarity, yet the molecular mechanisms vary widely. Here we examine what we term 'chemical systems' where cell polarization arises through biochemical interactions in signaling pathways, 'mechanical systems' where cells polarize due to forces, stresses and transport, and 'mechanochemical systems' where polarization results from interplay between mechanics and chemical signaling. To reveal potentially unifying principles, we discuss mathematical conceptualizations of several prototypical examples. We suggest that the concept of local activation and global inhibition - originally developed to explain spatial patterning in reaction-diffusion systems - provides a framework for understanding many cases of cell polarity. Importantly, we find that the core ingredients in this framework - symmetry breaking, self-amplifying feedback, and long-range inhibition - involve processes that can be chemical, mechanical, or even mechanochemical in nature.