Phosphatidylinositol 3'-kinase signalling supports cell height in established epithelial monolayers

Depth correction of 3D NanoSIMS images using secondary electron pixel intensities

Strategies that do not require additional characterization to be performed on the sample or the collection of additional secondary ion signals are needed to depth correct 3D SIMS images of cells. Here, we develop a depth correction strategy that uses the pixel intensities in the secondary electron images acquired during negative-ion NanoSIMS depth profiling to reconstruct the sample morphology. This morphology reconstruction was then used to depth correct the 3D SIMS images that show the components of interest in the sample. As a proof of concept, we applied this approach to NanoSIMS depth profiling data that show the 15N-enrichment and 18O-enrichment from 15N-sphingolipids and 18O-cholesterol, respectively, within a metabolically labeled Madin-Darby canine kidney cell. Comparison of the cell morphology reconstruction to the secondary electron images collected with the NanoSIMS revealed that the assumption of a constant sputter rate produced small inaccuracies in sample morphology after approximately 0.66 μm of material was sputtered from the cell. Nonetheless, the resulting 3D renderings of the lipid-specific isotope enrichments better matched the shapes and positions of the subcellular compartments that contained 15N-sphingolipids and 18O-cholesterol than the uncorrected 3D SIMS images. This depth correction of the 3D SIMS images also facilitated the detection of spherical cholesterol-rich compartments that were surrounded by membranes containing cholesterol and sphingolipids. Thus, we expect this approach will facilitate identifying the subcellular structures that are enriched with biomolecules of interest in 3D SIMS images while eliminating the need for correlated analyses or additional secondary ion signals for the depth correction of 3D NanoSIMS images.

CD2AP links actin to PI3 kinase activity to extend epithelial cell height and constrain cell area

Epithelial cells are categorized as cuboidal versus squamous based on the height of the lateral membrane. Wang and Brieher show that CD2AP links PI3K activity to actin assembly to extend the height of the lateral membrane.

Maintaining the correct ratio of apical, basal, and lateral membrane domains is important for epithelial physiology. Here, we show that CD2AP is a critical determinant of epithelial membrane proportions. Depletion of CD2AP or phosphoinositide 3-kinase (PI3K) inhibition results in loss of F-actin and expansion of apical–basal domains, which comes at the expense of lateral membrane height in MDCK cells. We demonstrate that the SH3 domains of CD2AP bind to PI3K and are necessary for PI3K activity along lateral membranes and constraining cell area. Tethering the SH3 domains of CD2AP or p110γ to the membrane is sufficient to rescue CD2AP-knockdown phenotypes. CD2AP and PI3K are both upstream and downstream of actin polymerization. Since CD2AP binds to both actin filaments and PI3K, CD2AP might bridge actin assembly to PI3K activation to form a positive feedback loop to support lateral membrane extension. Our results provide insight into the squamous to cuboidal to columnar epithelial transitions seen in complex epithelial tissues in vivo.

Phospholipids of the Plasma Membrane - Regulators or Consequence of Cell Polarity?

Cell polarity is a key feature of many eukaryotic cells, including neurons, epithelia, endothelia and asymmetrically dividing stem cells. Apart from the specific localization of proteins to distinct domains of the plasma membrane, most of these cells exhibit an asymmetric distribution of phospholipids within the plasma membrane too. Notably, research over the last years has revealed that many known conserved regulators of apical-basal polarity in epithelial cells are capable of binding to phospholipids, which in turn regulate the localization and to some extent the function of these proteins. Conversely, phospholipid-modifying enzymes are recruited and controlled by polarity regulators, demonstrating an elaborated balance between asymmetrically localized proteins and phospholipids, which are enriched in certain (micro)domains of the plasma membrane. In this review, we will focus on our current understanding of apical-basal polarity and the implication of phospholipids within the plasma membrane during the cell polarization of epithelia and migrating cells.

Mechanisms behind the polarized distribution of lipids in epithelial cells

Epithelial cells are polarized cells and typically display distinct plasma membrane domains: basal plasma membrane domains face the underlying tissue, lateral domains contact adjacent cells and apical domains face the exterior lumen. Each membrane domain is endowed with a specific macromolecular composition that constitutes the functional identity of that domain. Defects in apical-basal plasma membrane polarity altogether or more subtle defects in the composition of either apical or basal plasma membrane domain can give rise to severe diseases. Lipids are the main component of cellular membranes and mechanisms that control their polarized distribution in epithelial cells are emerging. In particular sphingolipids and phosphatidylinositol lipids have taken center stage in the organization of the apical and basolateral plasma membrane domain. This short review article discusses mechanisms that contribute to the polarized distribution of lipids in epithelial cells.

Photoreceptor oxidative stress in hyperoxia-induced proliferative retinopathy accelerates rd8 degeneration

To investigate the impact of photoreceptor oxidative stress on photoreceptor degeneration in mice carrying the rd8 mutation (C57BL/6N). We compared the hyperoxia-induced proliferative retinopathy (HIPR) model in two mouse strains (C57BL/6J and C57BL/6N). Pups were exposed to 75% oxygen, starting at birth and continuing for 14 days (P14). Mice were euthanized at P14, or allowed to recover in room air for one day (P15), seven days (P21), or 14 days (P28). We quantified retinal thickness and the length of residual photoreceptors not affected by rosette formation. In addition we explored differences in retinal immunostaining for NADPH oxidase 4 (NOX4), Rac1, vascular endothelium, and activated Mϋller cells. We analyzed photoreceptor oxidative stress using DCF staining in cross sections and quantified NOX4 protein levels using western blotting. C57BL/6N mice in HIPR showed increased oxidative stress, NOX4, and Rac1 in the photoreceptors at P14 and P15 compared to C57BL/6J. In addition, we observed significant progression of photoreceptor degeneration, with significantly accelerated rosette formation in C57BL/6N under HIPR, compared to their room air counterparts. Furthermore, C57BL/6N under HIPR had significantly thinner central retinas than C57BL/6J in HIPR. We did not find a difference in vascular disruption or Mϋller cell activation comparing the two strains in hyperoxia. In HIPR, the C57BL/6N strain carrying the rd8 mutation showed significantly accelerated photoreceptor degeneration, mediated via exacerbated photoreceptor oxidative stress, which we believe relates to Rac1-NOX dysregulation in the setting of Crb1 loss-of-function.

Cell-cell adhesion interface: rise of the lateral membrane

The lateral membrane plays an important role in the mechanical stability of epithelial cell sheet in steady state. In addition, the lateral membrane is continuously remodeled during dynamic processes such as cell extrusion, cytokinesis, and intercellular cell movement. In wound healing, the lateral membrane must be built from flat and spread cells that had crawled into the area of the wound. Thus, forming the lateral membrane is a phenomenon that occurs not only in development but also during homeostatic maintenance and regeneration of differentiated epithelial tissues.

EPHA4-FC TREATMENT REDUCES ISCHEMIA/REPERFUSION-INDUCED INTESTINAL INJURY BY INHIBITING VASCULAR PERMEABILITY

The inflammatory response is characterized by increased endothelial permeability, which permits the passage of fluid and inflammatory cells into interstitial spaces. The Eph/ephrin receptor ligand system plays a role in inflammation through a signaling cascade, which modifies Rho-GTPase activity. We hypothesized that blocking Eph/ephrin signaling using an EphA4-Fc would result in decreased inflammation and tissue injury in a model of ischemia/reperfusion (I/R) injury. Mice undergoing intestinal I/R pretreated with the EphA4-Fc had significantly reduced intestinal injury compared to mice injected with the control Fc. This reduction in I/R injury was accompanied by significantly reduced neutrophil infiltration, but did not affect intestinal inflammatory cytokine generation. Using microdialysis, we identified that intestinal I/R induced a marked increase in systemic vascular leakage, which was completely abrogated in EphA4-Fc-treated mice. Finally, we confirmed the direct role of Eph/ephrin signaling in endothelial leakage by demonstrating that EphA4-Fc inhibited tumor necrosis factor-α-induced vascular permeability in human umbilical vein endothelial cells. This study identifies that Eph/ephrin interaction induces proinflammatory signaling in vivo by inducing vascular leak and neutrophil infiltration, which results in tissue injury in intestinal I/R. Therefore, therapeutic targeting of Eph/ephrin interaction using inhibitors, such as EphA4-Fc, may be a novel method to prevent tissue injury in acute inflammation by influencing endothelial integrity and by controlling vascular leak.

Trans-homophilic interaction of CADM1 activates PI3K by forming a complex with MAGuK-family proteins MPP3 and Dlg

CADM1 (Cell adhesion molecule 1), a cell adhesion molecule belonging to the immunoglobulin superfamily, is involved in cell-cell interaction and the formation and maintenance of epithelial structure. Expression of CADM1 is frequently down-regulated in various tumors derived from epithelial cells. However, the intracellular signaling pathways activated by CADM1-mediated cell adhesion remain unknown. Here, we established a cell-based spreading assay to analyze the signaling pathway specifically activated by the trans-homophilic interaction of CADM1. In the assay, MDCK cells expressing exogenous CADM1 were incubated on the glass coated with a recombinant extracellular fragment of CADM1, and the degree of cell spreading was quantified by measuring their surface area. Assay screening of 104 chemical inhibitors with known functions revealed that LY294002, an inhibitor of phosphoinositide 3-kinase (PI3K), efficiently suppressed cell spreading in a dose-dependent manner. Inhibitors of Akt and Rac1, downstream effectors of PI3K, also partially suppressed cell spreading, while the addition of both inhibitors blocked cell spreading to the same extent as did LY294002. Furthermore, MPP3 and Dlg, membrane-associated guanylate kinase homologs (MAGuK) proteins, connect CADM1 with p85 of PI3K by forming a multi-protein complex at the periphery of cells. These results suggest that trans-homophilic interaction mediated by CADM1 activates the PI3K pathway to reorganize the actin cytoskeleton and form epithelial cell structure.

PI3K/Akt1 signalling specifies foregut precursors by generating regionalized extra-cellular matrix

During embryonic development signalling pathways act repeatedly in different contexts to pattern the emerging germ layers. Understanding how these different responses are regulated is a central question for developmental biology. In this study, we used mouse embryonic stem cell (mESC) differentiation to uncover a new mechanism for PI3K signalling that is required for endoderm specification. We found that PI3K signalling promotes the transition from naïve endoderm precursors into committed anterior endoderm. PI3K promoted commitment via an atypical activity that delimited epithelial-to-mesenchymal transition (EMT). Akt1 transduced this activity via modifications to the extracellular matrix (ECM) and appropriate ECM could itself induce anterior endodermal identity in the absence of PI3K signalling. PI3K/Akt1-modified ECM contained low levels of Fibronectin (Fn1) and we found that Fn1 dose was key to specifying anterior endodermal identity in vivo and in vitro. Thus, localized PI3K activity affects ECM composition and ECM in turn patterns the endoderm.

DOI:http://dx.doi.org/10.7554/eLife.00806.001

From conception to birth, a single fertilised egg will multiply into trillions of cells, with each cell becoming one of the 200 or so different types of cell that are found in the human body. The development of an embryo is complex and dynamic, with cells giving up their ability to become any cell type and committing to becoming a specific cell type within a given tissue. At the same time, different groups of cells migrate to the appropriate locations within the developing embryo. Although it is challenging to decipher the roles of the individual signalling pathways that control an embryo’s development, several important components have been found.

Fibroblast growth factor (FGF) is a protein that regulates the formation of the endoderm: this is the innermost of the three layers of cells that form in the early embryo, and it gives rise to internal organs such as the gut, liver and pancreas. As well as ‘telling’ cells to become the front part, or anterior, of the endoderm, FGF also controls the migration of these cells within the embryo. However, uncoupling these two roles has been a major challenge, and the molecular mechanisms behind them are unclear.

Now, Villegas et al. have discovered that FGF activates a signalling cascade involving two enzymes called PI3K and Akt1. In lab-grown embryonic stem cells—cells that can be coaxed to become any of the cell types formed during development—this signalling cascade is essential for FGF to trigger differentiation of the cell types found in the anterior endoderm. The PI3K/Akt1 signalling cascade achieves this by reducing the level of a protein called fibronectin in the ‘extracellular matrix’ that surrounds the cells. This low level of fibronectin will in turn induce cells to stick together in an organized layer; and this rearrangement of cell-cell and cell-matrix interactions appears linked to triggering the differentiation of anterior endoderm cell types.

Villegas et al. showed that the PI3K/Akt1 pathway was also essential for endoderm formation in living mouse embryos. As a normal embryo develops, the anterior endoderm cells move into a ‘groove’ at the front the embryo, where the level of fibronectin is lower than it is at the posterior end of the embryo.

These findings highlight the importance of the extracellular matrix in the regulation of embryonic development, and should assist in the effort to turn lab-grown stem cells into the useful cell types found in internal organs.

DOI:http://dx.doi.org/10.7554/eLife.00806.002

A phosphoinositide 3-kinase (PI3K)-serum- and glucocorticoid-inducible kinase 1 (SGK1) pathway promotes Kv7.1 channel surface expression by inhibiting Nedd4-2 protein

Epithelial cell polarization involves several kinase signaling cascades that eventually divide the surface membrane into an apical and a basolateral part. One kinase, which is activated during the polarization process, is phosphoinositide 3-kinase (PI3K). In MDCK cells, the basolateral potassium channel Kv7.1 requires PI3K activity for surface-expression during the polarization process. Here, we demonstrate that Kv7.1 surface expression requires tonic PI3K activity as PI3K inhibition triggers endocytosis of these channels in polarized MDCK. Pharmacological inhibition of SGK1 gave similar results as PI3K inhibition, whereas overexpression of constitutively active SGK1 overruled it, suggesting that SGK1 is the primary downstream target of PI3K in this process. Furthermore, knockdown of the ubiquitin ligase Nedd4-2 overruled PI3K inhibition, whereas a Nedd4-2 interaction-deficient Kv7.1 mutant was resistant to both PI3K and SGK1 inhibition. Altogether, these data suggest that a PI3K-SGK1 pathway stabilizes Kv7.1 surface expression by inhibiting Nedd4-2-dependent endocytosis and thereby demonstrates that Nedd4-2 is a key regulator of Kv7.1 localization and turnover in epithelial cells.

Real-time sensing of cell morphology by infrared waveguide spectroscopy

We demonstrate that a live epithelial cell monolayer can act as a planar waveguide. Our infrared reflectivity measurements show that highly differentiated simple epithelial cells, which maintain tight intercellular connectivity, support efficient waveguiding of the infrared light in the spectral region of 1.4–2.5 µm and 3.5–4 µm. The wavelength and the magnitude of the waveguide mode resonances disclose quantitative dynamic information on cell height and cell-cell connectivity. To demonstrate this we show two experiments. In the first one we trace in real-time the kinetics of the disruption of cell-cell contacts induced by calcium depletion. In the second one we show that cell treatment with the PI3-kinase inhibitor LY294002 results in a progressive decrease in cell height without affecting intercellular connectivity. Our data suggest that infrared waveguide spectroscopy can be used as a novel bio-sensing approach for studying the morphology of epithelial cell sheets in real-time, label-free manner and with high spatial-temporal resolution.

Crumbs limits oxidase-dependent signaling to maintain epithelial integrity and prevent photoreceptor cell death

Crb fulfills a protective role during light exposure by limiting oxidative damage resulting from Rac1–NADPH oxidase complex activity.

Drosophila melanogaster Crumbs (Crb) and its mammalian orthologues (CRB1–3) share evolutionarily conserved but poorly defined roles in regulating epithelial polarity and, in photoreceptor cells, morphogenesis and stability. Elucidating the molecular mechanisms of Crb function is vital, as mutations in the human CRB1 gene cause retinal dystrophies. Here, we report that Crb restricts Rac1–NADPH oxidase-dependent superoxide production in epithelia and photoreceptor cells. Reduction of superoxide levels rescued epithelial defects in crb mutant embryos, demonstrating that limitation of superoxide production is a crucial function of Crb and that NADPH oxidase and superoxide contribute to the molecular network regulating epithelial tissue organization. We further show that reduction of Rac1 or NADPH oxidase activity or quenching of reactive oxygen species prevented degeneration of Crb-deficient retinas. Thus, Crb fulfills a protective role during light exposure by limiting oxidative damage resulting from Rac1–NADPH oxidase complex activity. Collectively, our results elucidate an important mechanism by which Crb functions in epithelial organization and the prevention of retinal degeneration.

The study of polarisation in single cells using model cell membranes

The apicobasal polarisation of epithelial cells within an epithelium is critical for its function as a selective barrier. Microenvironmental parameters, including cell-matrix and cell-cell interactions, contribute to the initiation and orientation of this polarity. However, it is often non-trivial to decipher the differential effects of these parameters in a controlled manner using traditional in vitro platforms. A reductionist platform, consisting of E-cadherin coupled onto laterally mobile supported lipid bilayers, was utilised to mimic E-cadherin presentation in the cell membrane. These functionalised bilayers were generated either on flat 2D surfaces or the interior surfaces of round microwells. This platform enabled the study of E-cadherin adhesion and the initiation of polarisation in a controlled environment, where the dimensionality of the microenvironment, type of protein coating and cell shape could be independently studied. A high proportion of single epithelial cells interacted with and clustered cellular E-cadherin in the presence of E-cadherin functionalised bilayers, which was reduced in the presence of integrin-mediated adhesion. The differential response in E-cadherin clustering correlated with the polarisation of E-cadherin and Na,K-ATPase, a reporter for the induction of basolateral polarity. Neither the three-dimensional presentation of E-cadherin nor the cell shape affected E-cadherin clustering or polarisation in single cells. Thus, the mobile presentation of E-cadherin was sufficient to mimic a cell-cell contact and induce basolateral polarisation in single cells.

Crumbs controls epithelial integrity by inhibiting Rac1 and PI3K

Drosophila Crumbs (Crb) and its mammalian ortholog CRB3 control epithelial polarity through poorly understood molecular mechanisms. Elucidating these mechanisms is crucial, because the physiology of epithelia largely depends on the polarized architecture of individual epithelial cells. In addition, loss of CRB3 favors tumor cell growth, metastasis and epithelial to mesenchymal transition (EMT). Using Drosophila embryos, we report that Rac1 sustains PI3K signaling, which is required for Rac1 activation. Crb represses this positive-feedback loop. Notably, this property confers to Crb its ability to promote epithelial integrity in vivo, because attenuation of either Rac1 or PI3K activity rescues the crb mutant phenotype. Moreover, inhibition of Rac1 or PI3K results in Crb-dependent apical membrane growth, whereas Rac1 activation restricts membrane localization of Crb and interferes with apical domain formation. This illustrates that Crb and the Rac1-PI3K module are antagonists, and that the fine balance between the activities of these proteins is crucial to maintain epithelial organization and an appropriate apical to basolateral ratio. Together, our results elucidate a mechanism that mediates Crb function and further define the role of PI3K and Rac1 in epithelial morphogenesis, allowing for a better understanding of how distinct membrane domains are regulated in polarized epithelial cells.

Phosphoinositides in cell architecture

Inositol phospholipids have been implicated in almost all aspects of cellular physiology including spatiotemporal regulation of cellular signaling, acquisition of cellular polarity, specification of membrane identity, cytoskeletal dynamics, and regulation of cellular adhesion, motility, and cytokinesis. In this review, we examine the critical role phosphoinositides play in these processes to execute the establishment and maintenance of cellular architecture. Epithelial tissues perform essential barrier and transport functions in almost all major organs. Key to their development and function is the establishment of epithelial cell polarity. We place a special emphasis on highlighting recent studies demonstrating phosphoinositide regulation of epithelial cell polarity and how individual cells use phosphoinositides to further organize into epithelial tissues.

Apical protein transport and lumen morphogenesis in polarized epithelial cells

Segregation of the apical and basolateral plasma membrane domains is the key distinguishing feature of epithelial cells. A series of interrelated cues and processes follow this primary polarization event, resulting in the morphogenesis of the mammalian epithelium. This review focuses on the role of the interactions between the extracellular matrix and neighbouring cells during the initiation and establishment of epithelial polarity, and the role that membrane transport and polarity complexes play in this process. An overview of the formation of the apical junctional complexes is given in relation to the generation of distinct membrane domains characterized by the asymmetric distribution of phosphoinositides and proteins. The mechanisms and machinery utilized by the trafficking pathways involved in the generation and maintenance of this apical-basolateral polarization are expounded, highlighting processes of apical-directed transport. Furthermore, the current proposed mechanisms for the organization of entire networks of cells into a structured, polarized three-dimensional structure are described, with an emphasis on the proposed mechanisms for the formation and expansion of the apical lumen.

In situ electrochemical imaging of membrane glycan expression on micropatterned adherent single cells

A scanning electrochemical microscopic (SECM) method for in situ imaging of four types of membrane glycan motifs on single adherent cells was proposed using BGC-823 human gastric carcinoma (BGC) cells as the model. These adherent cells were first micropatterned in the microwell of poly(dimethylsiloxane) membrane for precisely controlling the localized surface interaction, and the membrane glycans were then specifically recognized with corresponding lectins labeled with horseradish peroxidase (HRP). On the basis of the enzymatic oxidization of ferrocenylmethanol (FMA) by H(2)O(2) to yield FMA(+), the glycan expression level was detected by the reduction current of FMA(+) at the SECM tip. The cell-surface glycans could, thus, be in situ imaged by SECM at a single-cell level without peeling the cells from culture dish. Under the optimized conditions, four types of membrane glycan motifs showed statistically distinguishable expression levels. The SECM results for different glycan motifs on adherent single cells were consistent with those estimated by flow cytometric assay. This work provides a reliable approach for in situ evaluation of the characteristic glycopattern of single living cells and can be applied in cell biologic study based on cell surface carbohydrate expression.