Displacement of basolateral Bazooka/PAR-3 by regulated transport and dispersion during epithelial polarization in Drosophila

Identification of ceRNA networks in type H and L vascular endothelial cells through integrated bioinformatics methods

OBJECTIVES

Type H blood vessels are a subtype of bone-specific microvessels (CD31hiEmcnhi) that play an important regulatory role in the coupling of angiogenesis and osteogenesis. Despite reports on the distinct roles of type H and L vessels under physiological and pathological bone conditions, their genetic differences remain to be elucidated. This study aims to construct a competitive endogenous RNA (ceRNA) network of key gene for differencial expression (DE) in type H and L vascular endothelial cells (ECs) through integrated bioinformatic methods.

METHODS

We downloaded relevant raw data from the ArrayExpress and the Gene Expression Omnibus (GEO) database and used the Limma R-Bioconductor package to screen for DE lncRNAs, DE miRNAs, and DE mRNAs between type H and L vascular ECs. A total ceRNA network was constructed based on their interactions, followed by refinement using protein-protein interaction (PPI) networks to select upregulated and downregulated key genes. Enrichment analysis was performed on these key genes. Random validation was conducted using flow cytometry and real-time RT-PCR.

RESULTS

A total of 1 761 DE mRNAs, 187 DE lncRNAs, and 159 DE miRNAs were identified, and a comprehensive ceRNA network was constructed based on their interactions. Six upregulated (Itga5, Kdr, Tjp1, Pecam1, Cdh5, and Ptk2) and 2 downregulated (Csf1r and Il10) key genes were selected via PPI network to construct a subnetwork of ceRNAs related to these key genes. Upregulated key genes were mainly enriched in negative regulation of angiogenesis and vascular apoptosis. Results from flow cytometry and real-time RT-PCR were consistent with bioinformatics analysis.

CONCLUSIONS

This study proposes a ceRNA network associated with upregulated and downregulated type H and L vascular ECs based on selected key genes, providing new insights into the regulatory mechanisms of type H and L vascular ECs in bone metabolism.

Apical-basal polarity and the control of epithelial form and function

Epithelial cells are the most common cell type in all animals, forming the sheets and tubes that compose most organs and tissues. Apical-basal polarity is essential for epithelial cell form and function, as it determines the localization of the adhesion molecules that hold the cells together laterally and the occluding junctions that act as barriers to paracellular diffusion. Polarity must also target the secretion of specific cargoes to the apical, lateral or basal membranes and organize the cytoskeleton and internal architecture of the cell. Apical-basal polarity in many cells is established by conserved polarity factors that define the apical (Crumbs, Stardust/PALS1, aPKC, PAR-6 and CDC42), junctional (PAR-3) and lateral (Scribble, DLG, LGL, Yurt and RhoGAP19D) domains, although recent evidence indicates that not all epithelia polarize by the same mechanism. Research has begun to reveal the dynamic interactions between polarity factors and how they contribute to polarity establishment and maintenance. Elucidating these mechanisms is essential to better understand the roles of apical-basal polarity in morphogenesis and how defects in polarity contribute to diseases such as cancer.

Scribble and Discs-large direct initial assembly and positioning of adherens junctions during the establishment of apical-basal polarity

Apical-basal polarity is a fundamental property of animal tissues. Drosophila embryos provide an outstanding model for defining mechanisms that initiate and maintain polarity. Polarity is initiated during cellularization, when cell-cell adherens junctions are positioned at the future boundary of apical and basolateral domains. Polarity maintenance then involves complementary and antagonistic interplay between apical and basal polarity complexes. The Scribble/Dlg module is well-known for promoting basolateral identity during polarity maintenance. Here, we report a surprising role for Scribble/Dlg in polarity initiation, placing it near the top of the network-positioning adherens junctions. Scribble and Dlg are enriched in nascent adherens junctions, are essential for adherens junction positioning and supermolecular assembly, and also play a role in basal junction assembly. We test the hypotheses for the underlying mechanisms, exploring potential effects on protein trafficking, cytoskeletal polarity or Par-1 localization/function. Our data suggest that the Scribble/Dlg module plays multiple roles in polarity initiation. Different domains of Scribble contribute to these distinct roles. Together, these data reveal novel roles for Scribble/Dlg as master scaffolds regulating assembly of distinct junctional complexes at different times and places.

Par-1 controls the composition and growth of cortical actin caps during Drosophila embryo cleavage

The cell cortex is populated by various proteins, but it is unclear how they interact to change cell shape. Jiang and Harris find that the kinase Par-1 is required for Diaphanous-based actin bundles, and that these bundles intersperse with separately induced Arp2/3 networks to form an actin cap that grows into a metaphase compartment of the syncytial Drosophila embryo.

Cell structure depends on the cortex, a thin network of actin polymers and additional proteins underlying the plasma membrane. The cell polarity kinase Par-1 is required for cells to form following syncytial Drosophila embryo development. This requirement stems from Par-1 promoting cortical actin caps that grow into dome-like metaphase compartments for dividing syncytial nuclei. We find the actin caps to be a composite material of Diaphanous (Dia)-based actin bundles interspersed with independently formed, Arp2/3-based actin puncta. Par-1 and Dia colocalize along extended regions of the bundles, and both are required for the bundles and for each other’s bundle-like localization, consistent with an actin-dependent self-reinforcement mechanism. Par-1 helps establish or maintain these bundles in a cortical domain with relatively low levels of the canonical formin activator Rho1-GTP. Arp2/3 is required for displacing the bundles away from each other and toward the cap circumference, suggesting interactions between these cytoskeletal components could contribute to the growth of the cap into a metaphase compartment.

New insights into apical-basal polarization in epithelia

The establishment of an apical-basal axis of polarity is essential for the organization and functioning of epithelial cells. Polarization of epithelial cells is orchestrated by a network of conserved polarity regulators that establish opposing cortical domains through mutually antagonistic interactions and positive feedback loops. While our understanding is still far from complete, the molecular details behind these interactions continue to be worked out. Here, we highlight recent findings on the mechanisms that control the activity and localization of apical-basal polarity regulators, including oligomerization and higher-order complex formation, auto-inhibitory interactions, and electrostatic interactions with the plasma membrane.

Dynein-mediated transport and membrane trafficking control PAR3 polarised distribution

The scaffold protein PAR3 and the kinase PAR1 are essential proteins that control cell polarity. Their precise opposite localisations define plasma membrane domains with specific functions. PAR3 and PAR1 are mutually inhibited by direct or indirect phosphorylations, but their fates once phosphorylated are poorly known. Through precise spatiotemporal quantification of PAR3 localisation in the Drosophila oocyte, we identify several mechanisms responsible for its anterior cortex accumulation and its posterior exclusion. We show that PAR3 posterior plasma membrane exclusion depends on PAR1 and an endocytic mechanism relying on RAB5 and PI(4,5)P2. In a second phase, microtubules and the dynein motor, in connection with vesicular trafficking involving RAB11 and IKK-related kinase, IKKε, are required for PAR3 transport towards the anterior cortex. Altogether, our results point to a connection between membrane trafficking and dynein-mediated transport to sustain PAR3 asymmetry.

Dynamics of cortical domains in early Drosophila development

Underlying the plasma membrane of eukaryotic cells is an actin cortex that includes actin filaments and associated proteins. A special feature of all polarized and epithelial cells are cortical domains, each of which is characterized by specific sets of proteins. Typically, an epithelial cell contains apical, subapical, lateral and basal domains. The domain-specific protein sets contain evolutionarily conserved proteins, as well as cell-type-specific factors. Among the conserved proteins are, the Par proteins, Crumbs complex and the lateral proteins Scribbled and Discs large 1. Organization of the plasma membrane into cortical domains is dynamic and depends on cell type, differentiation and developmental stage. The dynamics of cortical organization is strikingly visible in early Drosophila embryos, which increase the number of distinct cortical domains from one, during the pre-blastoderm stage, to two in syncytial blastoderm embryos, before finally acquiring the four domains that are typical for epithelial cells during cellularization. In this Review, we will describe the dynamics of cortical organization in early Drosophila embryos and discuss the processes and mechanisms underlying cortical remodeling.

PAR-Complex and Crumbs Function During Photoreceptor Morphogenesis and Retinal Degeneration

The fly photoreceptor has long been used as a model to study sensory neuron morphogenesis and retinal degeneration. In particular, elucidating how these cells are built continues to help further our understanding of the mechanisms of polarized cell morphogenesis, intracellular trafficking and the causes of human retinal pathologies. The conserved PAR complex, which in flies consists of Cdc42-PAR6-aPKC-Bazooka, and the transmembrane protein Crumbs (Crb) are key players during photoreceptor morphogenesis. While the PAR complex regulates polarity in many cell types, Crb function in polarity is relatively specific to epithelial cells. Together Cdc42-PAR6-aPKC-Bazooka and Crb orchestrate the differentiation of the photoreceptor apical membrane (AM) and zonula adherens (ZA), thus allowing these cells to assemble into a neuro-epithelial lattice. In addition to its function in epithelial polarity, Crb has also been shown to protect fly photoreceptors from light-induced degeneration, a process linked to Rhodopsin expression and trafficking. Remarkably, mutations in the human Crumbs1 (CRB1) gene lead to retinal degeneration, making the fly photoreceptor a powerful disease model system.

Rap1 acts via multiple mechanisms to position Canoe and adherens junctions and mediate apical-basal polarity establishment

Epithelial apical-basal polarity drives assembly and function of most animal tissues. Polarity initiation requires cell-cell adherens junction assembly at the apical-basolateral boundary. Defining the mechanisms underlying polarity establishment remains a key issue. Drosophila embryos provide an ideal model, as 6000 polarized cells assemble simultaneously. Current data place the actin-junctional linker Canoe (fly homolog of Afadin) at the top of the polarity hierarchy, where it directs Bazooka/Par3 and adherens junction positioning. Here we define mechanisms regulating Canoe localization/function. Spatial organization of Canoe is multifaceted, involving membrane localization, recruitment to nascent junctions and macromolecular assembly at tricellular junctions. Our data suggest apical activation of the small GTPase Rap1 regulates all three events, but support multiple modes of regulation. The Rap1GEF Dizzy (PDZ-GEF) is crucial for Canoe tricellular junction enrichment but not apical retention. The Rap1-interacting RA domains of Canoe mediate adherens junction and tricellular junction recruitment but are dispensable for membrane localization. Our data also support a role for Canoe multimerization. These multifactorial inputs shape Canoe localization, correct Bazooka and adherens junction positioning, and thus apical-basal polarity. We integrate the existing data into a new polarity establishment model.

A Single-Cell Biochemistry Approach Reveals PAR Complex Dynamics during Cell Polarization

Regulated protein-protein interactions are critical for cell signaling, differentiation, and development. For the study of dynamic regulation of protein interactions in vivo, there is a need for techniques that can yield time-resolved information and probe multiple protein binding partners simultaneously, using small amounts of starting material. Here we describe a single-cell protein interaction assay. Single-cell lysates are generated at defined time points and analyzed using single-molecule pull-down, yielding information about dynamic protein complex regulation in vivo. We established the utility of this approach by studying PAR polarity proteins, which mediate polarization of many animal cell types. We uncovered striking regulation of PAR complex composition and stoichiometry during Caenorhabditis elegans zygote polarization, which takes place in less than 20 min. PAR complex dynamics are linked to the cell cycle by Polo-like kinase 1 and govern the movement of PAR proteins to establish polarity. Our results demonstrate an approach to study dynamic biochemical events in vivo.

Protein clustering for cell polarity: Par-3 as a paradigm

The scaffold protein Par-3 ( Drosophila Bazooka) is a central organizer of cell polarity across animals. This review focuses on how the clustering of Par-3 contributes to cell polarity. It begins with the Par-3 homo-oligomerization mechanism and its regulation by Par-1 phosphorylation. The role of polarized cytoskeletal networks in distributing Par-3 clusters to one end of the cell is then discussed, as is the subsequent maintenance of polarized Par-3 clusters through hindered mobility and inhibition from the opposite pole. Finally, specific roles of Par-3 clusters are reviewed, including the bundling of microtubules, the cortical docking of centrosomes, the growth and positioning of cadherin–catenin clusters, and the inhibition of the Par-6–aPKC kinase cassette. Examples are drawn from Drosophila, Caenorhabditis elegans, mammalian cell culture, and biochemical studies.

Tissue growth and tumorigenesis in Drosophila: cell polarity and the Hippo pathway

Cell polarity regulation is critical for defining membrane domains required for the establishment and maintenance of the apical-basal axis in epithelial cells (apico-basal polarity), asymmetric cell divisions, planar organization of tissues (planar cell polarity), and the formation of the front-rear axis in cell migration (front-rear polarity). In the vinegar fly, Drosophila melanogaster, cell polarity regulators also interact with the Hippo tissue growth control signaling pathway. In this review we survey the recent Drosophila literature linking cell polarity regulators with the Hippo pathway in epithelial tissue growth, neural stem cell asymmetric divisions and in cell migration in physiological and tumorigenic settings.

Polarity protein Par3/Bazooka follows myosin-dependent junction repositioning

The polarity protein Par3/Bazooka (Baz) has been established as a central component of the apical basal polarity system that determines the position of cell-cell junctions in epithelial cells. Consistent with that view, we show that shortly before gastrulation in Drosophila, Baz protein in the mesoderm is down-regulated from junctional sites in response to Snail (Sna) expression. This down-regulation leads to a specific decrease in adherens junctions without affecting other E-Cadherin pools. However, we further show that, interactions between Baz and junctions are not unidirectional. During apical constriction and the internalization of the mesoderm, down-regulation of Baz is transiently blocked as adherens junctions shift apically and are strengthened in response to tension generated by contractile actomyosin. When such junction remodeling is prevented by down-regulating myosin, Baz is lost prematurely in mesodermal epithelium. During such apical shifts, Baz is initially left behind as the junction shifts position, but then re-accumulates at the new location of the junctions. On the dorsal side of the embryo, a similar pattern of myosin activity appears to limit the basal shift in junctions normally driven by Baz that controls epithelium folding. Our results suggest a model where the sensitivity of Baz to Sna expression leads to the Sna-dependent junction disassembly required for a complete epithelium-mesenchymal transition. Meanwhile this loss of Baz-dependent junction maintenance is countered by the myosin-based mechanism which promotes an apical shift and strengthening of junctions accompanied by a transient re-positioning and maintenance of Baz proteins.

Pak4 Is Required during Epithelial Polarity Remodeling through Regulating AJ Stability and Bazooka Retention at the ZA

The ability of epithelial cells to assemble into sheets relies on their zonula adherens (ZA), a circumferential belt of adherens junction (AJ) material, which can be remodeled during development to shape organs. Here, we show that during ZA remodeling in a model neuroepithelial cell, the Cdc42 effector P21-activated kinase 4 (Pak4/Mbt) regulates AJ morphogenesis and stability through β-catenin (β-cat/Arm) phosphorylation. We find that β-catenin phosphorylation by Mbt, and associated AJ morphogenesis, is needed for the retention of the apical determinant Par3/Bazooka at the remodeling ZA. Importantly, this retention mechanism functions together with Par1-dependent lateral exclusion of Par3/Bazooka to regulate apical membrane differentiation. Our results reveal an important functional link between Pak4, AJ material morphogenesis, and polarity remodeling during organogenesis downstream of Par3.

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Pak4 regulates adherens junction accumulation at the zonula adherens

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Pak4 promotes Par3 (Bazooka) retention at the zonula adherens

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Par1 and Pak4 synergize in preventing lateral accumulation of Par3

A Par-1-Par-3-Centrosome Cell Polarity Pathway and Its Tuning for Isotropic Cell Adhesion

To form regulated barriers between body compartments, epithelial cells polarize into apical and basolateral domains and assemble adherens junctions (AJs). Despite close links with polarity networks that generate single polarized domains, AJs distribute isotropically around the cell circumference for adhesion with all neighboring cells [1-3]. How AJs avoid the influence of polarity networks to maintain their isotropy has been unclear. In established epithelia, trans cadherin interactions could maintain AJ isotropy [4], but AJs are dynamic during epithelial development and remodeling [5, 6], and thus specific mechanisms may control their isotropy. In Drosophila, aPKC prevents hyper-polarization of junctions as epithelia develop from cellularization to gastrulation [7]. Here, we show that aPKC does so by inhibiting a positive feedback loop between Bazooka (Baz)/Par-3, a junctional organizer [5, 8-10], and centrosomes. Without aPKC, Baz and centrosomes lose their isotropic distributions and recruit each other to single plasma membrane (PM) domains. Surprisingly, our loss- and gain-of-function analyses show that the Baz-centrosome positive feedback loop is driven by Par-1, a kinase known to phosphorylate Baz and inhibit its basolateral localization [8, 11, 12]. We find that Par-1 promotes the positive feedback loop through both centrosome microtubule effects and Baz phosphorylation. Normally, aPKC attenuates the circuit by expelling Par-1 from the apical domain at gastrulation. The combination of local activation and global inhibition is a common polarization strategy [13-16]. Par-1 seems to couple both effects for a potent Baz polarization mechanism that is regulated for the isotropy of Baz and AJs around the cell circumference.

A role for E-cadherin in ensuring cohesive migration of a heterogeneous population of non-epithelial cells

Collective cell migration is a key process underlying the morphogenesis of many organs as well as tumour invasion, which very often involves heterogeneous cell populations. Here we investigated how such populations can migrate cohesively in the Drosophila posterior midgut, comprised of epithelial and mesenchymal cells and show a novel role for the epithelial adhesion molecule E-cadherin (E-Cad) in mesenchymal cells. Despite a lack of junctions at the ultrastructure level, reducing E-Cad levels causes mesenchymal cells to detach from one another and from neighbouring epithelial cells; as a result, coordination between the two populations is lost. Moreover, Bazooka and recycling mechanisms are also required for E-Cad accumulation in mesenchymal cells. These results indicate an active role for E-Cad in mediating cohesive and ordered migration of non-epithelial cells, and discount the notion of E-Cad as just an epithelial feature that has to be switched off to enable migration of mesenchymal cells.

Rap1 and Canoe/afadin are essential for establishment of apical-basal polarity in the Drosophila embryo

The small GTPase Rap1 and the actin-junctional linker protein Canoe/afadin are essential for the initial establishment of polarity in Drosophila, acting upstream of Bazooka/Par3 and the adherens junctions. However, feedback and cross-regulation occur, so polarity establishment is regulated by a network of proteins rather than a linear pathway.

The establishment and maintenance of apical–basal cell polarity is critical for assembling epithelia and maintaining organ architecture. Drosophila embryos provide a superb model. In the current view, apically positioned Bazooka/Par3 is the initial polarity cue as cells form during cellularization. Bazooka then helps to position both adherens junctions and atypical protein kinase C (aPKC). Although a polarized cytoskeleton is critical for Bazooka positioning, proteins mediating this remained unknown. We found that the small GTPase Rap1 and the actin-junctional linker Canoe/afadin are essential for polarity establishment, as both adherens junctions and Bazooka are mispositioned in their absence. Rap1 and Canoe do not simply organize the cytoskeleton, as actin and microtubules become properly polarized in their absence. Canoe can recruit Bazooka when ectopically expressed, but they do not obligatorily colocalize. Rap1 and Canoe play continuing roles in Bazooka localization during gastrulation, but other polarity cues partially restore apical Bazooka in the absence of Rap1 or Canoe. We next tested the current linear model for polarity establishment. Both Bazooka and aPKC regulate Canoe localization despite being “downstream” of Canoe. Further, Rap1, Bazooka, and aPKC, but not Canoe, regulate columnar cell shape. These data reshape our view, suggesting that polarity establishment is regulated by a protein network rather than a linear pathway.