An association between type Iγ PI4P 5-kinase and Exo70 directs E-cadherin clustering and epithelial polarization

Regulation of yeast polarized exocytosis by phosphoinositide lipids

Phosphoinositides help steer membrane trafficking routes within eukaryotic cells. In polarized exocytosis, which targets vesicular cargo to sites of polarized growth at the plasma membrane (PM), the two phosphoinositides phosphatidylinositol 4-phosphate (PI4P) and its derivative phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) pave the pathway for vesicle transport from the Golgi to the PM. PI4P is a critical regulator of mechanisms that shape late Golgi membranes for vesicle biogenesis and release. Although enriched in vesicle membranes, PI4P is inexplicably removed from post-Golgi vesicles during their transit to the PM, which drives subsequent steps in exocytosis. At the PM, PI(4,5)P2 recruits effectors that establish polarized membrane sites for targeting the vesicular delivery of secretory cargo. The budding yeast Saccharomyces cerevisiae provides an elegant model to unravel the complexities of phosphoinositide regulation during polarized exocytosis. Here, we review how PI4P and PI(4,5)P2 promote yeast vesicle biogenesis, exocyst complex assembly and vesicle docking at polarized cortical sites, and suggest how these steps might impact related mechanisms of human disease.

The exocyst complex and intracellular vesicles mediate soluble protein trafficking to the primary cilium

The efficient transport of proteins into the primary cilium is a crucial step for many signaling pathways. Dysfunction of this process can lead to the disruption of signaling cascades or cilium assembly, resulting in developmental disorders and cancer. Previous studies on the protein delivery to the cilium were mostly focused on the membrane-embedded receptors. In contrast, how soluble proteins are delivered into the cilium is poorly understood. In our work, we identify the exocyst complex as a key player in the ciliary trafficking of soluble Gli transcription factors. In line with the known function of the exocyst in intracellular vesicle transport, we demonstrate that soluble proteins, including Gli2/3 and Lkb1, can use the endosome recycling machinery for their delivery to the primary cilium. Finally, we identify GTPases: Rab14, Rab18, Rab23, and Arf4 that are involved in vesicle-mediated Gli protein ciliary trafficking. Our data pave the way for a better understanding of ciliary transport and uncover transport mechanisms inside the cell.

Null mutation of exocyst complex component 3-like does not affect vascular development in mice

Exocyst is an octameric protein complex implicated in exocytosis. The exocyst complex is highly conserved among mammalian species, but the physiological function of each subunit in exocyst remains unclear. Previously, we identified exocyst complex component 3-like (Exoc3l) as a gene abundantly expressed in embryonic endothelial cells and implicated in the process of angiogenesis in human umbilical cord endothelial cells. Here, to reveal the physiological roles of Exoc3l during development, we generated Exoc3l knockout (KO) mice by genome editing with CRISPR/Cas9. Exoc3l KO mice were viable and showed no significant phenotype in embryonic angiogenesis or postnatal retinal angiogenesis. Exoc3l KO mice also showed no significant alteration in cholesterol homeostasis or insulin secretion, although several reports suggest an association of Exoc3l with these processes. Despite the implied roles, Exoc3l KO mice exhibited no apparent phenotype in vascular development, cholesterol homeostasis, or insulin secretion.

Lipid kinases PIP5Ks and PIP4Ks: potential drug targets for breast cancer

Phosphoinositides, a small group of lipids found in all cellular membranes, have recently garnered heightened attention due to their crucial roles in diverse biological processes and different diseases. Among these, phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2), the most abundant bis-phosphorylated phosphoinositide within the signaling system, stands notably connected to breast cancer. Not only does it serve as a key activator of the frequently altered phosphatidylinositol 3-kinase (PI3K) pathway in breast cancer, but also its conversion to phosphatidylinositol-3,4,5-triphosphate (PI(3,4,5)P3) is an important direction for breast cancer research. The generation and degradation of phosphoinositides intricately involve phosphoinositide kinases. PI(4,5)P2 generation emanates from the phosphorylation of PI4P or PI5P by two lipid kinase families: Type I phosphatidylinositol-4-phosphate 5-kinases (PIP5Ks) and Type II phosphatidylinositol-5-phosphate 4-kinases (PIP4Ks). In this comprehensive review, we focus on these two lipid kinases and delineate their compositions and respective cellular localization. Moreover, we shed light on the expression patterns and functions of distinct isoforms of these kinases in breast cancer. For a deeper understanding of their functional dynamics, we expound upon various mechanisms governing the regulation of PIP5Ks and PIP4Ks activities. A summary of effective and specific small molecule inhibitors designed for PIP5Ks or PIP4Ks are also provided. These growing evidences support PIP5Ks and PIP4Ks as promising drug targets for breast cancer.

The exocyst complex is required for the trafficking and delivery of KCa3.1 to the basolateral membrane of polarized epithelia

Control of the movement of ions and water across epithelia is essential for homeostasis. Changing the number or activity of ion channels at the plasma membrane is a significant regulator of epithelial transport. In polarized epithelia, the intermediate-conductance calcium-activated potassium channel, KCa3.1 is delivered to the basolateral membrane where it generates and maintains the electrochemical gradients required for epithelial transport. The mechanisms that control the delivery of KCa3.1 to the basolateral membrane are still emerging. Herein, we investigated the role of the highly conserved tethering complex exocyst. In epithelia, exocyst is involved in the tethering of post-Golgi secretory vesicles with the basolateral membrane, which is required before membrane fusion. In our Fisher rat thyroid cell line that stably expresses KCa3.1, siRNA knockdown of either of the exocyst subunits Sec3, Sec6, or Sec8 significantly decreased KCa3.1-specific current. In addition, knockdown of exocyst complex subunits significantly reduced the basolateral membrane protein level of KCa3.1. Finally, co-immunoprecipitation experiments suggest associations between Sec6 and KCa3.1, but not between Sec8 and KCa3.1. Collectively, based on these data and our previous studies, we suggest that components of exocyst complex are crucially important in the tethering of KCa3.1 to the basolateral membrane. After which, Soluble N-ethylmaleimide-sensitive factor (SNF) Attachment Receptors (SNARE) proteins aid in the insertion of KCa3.1-containing vesicles into the basolateral membrane of polarized epithelia.NEW & NOTEWORTHY Our Ussing chamber and immunoblot experiments demonstrate that when subunits of the exocyst complex were transiently knocked down, this significantly reduced the basolateral population and functional expression of KCa3.1. These data suggest, combined with our protein association experiments, that the exocyst complex regulates the tethering of KCa3.1-containing vesicles to the basolateral membrane prior to the SNARE-dependent insertion of channels into the basolateral membrane of epithelial cells.

The exocyst complex is an essential component of the mammalian constitutive secretory pathway

Secreted proteins fulfill a vast array of functions, including immunity, signaling, and extracellular matrix remodeling. In the trans-Golgi network, proteins destined for constitutive secretion are sorted into post-Golgi carriers which fuse with the plasma membrane. The molecular machinery involved is poorly understood. Here, we have used kinetic trafficking assays and transient CRISPR-KO to study biosynthetic sorting from the Golgi to the plasma membrane. Depletion of all canonical exocyst subunits causes cargo accumulation in post-Golgi carriers. Exocyst subunits are recruited to and co-localize with carriers. Exocyst abrogation followed by kinetic trafficking assays of soluble cargoes results in intracellular cargo accumulation. Unbiased secretomics reveals impairment of soluble protein secretion after exocyst subunit knockout. Importantly, in specialized cell types, the loss of exocyst prevents constitutive secretion of antibodies in lymphocytes and of leptin in adipocytes. These data identify exocyst as the functional tether of secretory post-Golgi carriers at the plasma membrane and an essential component of the mammalian constitutive secretory pathway.

Ca2+ and Annexins - Emerging Players for Sensing and Transferring Cholesterol and Phosphoinositides via Membrane Contact Sites

Maintaining lipid composition diversity in membranes from different organelles is critical for numerous cellular processes. However, many lipids are synthesized in the endoplasmic reticulum (ER) and require delivery to other organelles. In this scenario, formation of membrane contact sites (MCS) between neighbouring organelles has emerged as a novel non-vesicular lipid transport mechanism. Dissecting the molecular composition of MCS identified phosphoinositides (PIs), cholesterol, scaffolding/tethering proteins as well as Ca²⁺ and Ca²⁺-binding proteins contributing to MCS functioning. Compelling evidence now exists for the shuttling of PIs and cholesterol across MCS, affecting their concentrations in distinct membrane domains and diverse roles in membrane trafficking. Phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) at the plasma membrane (PM) not only controls endo-/exocytic membrane dynamics but is also critical in autophagy. Cholesterol is highly concentrated at the PM and enriched in recycling endosomes and Golgi membranes. MCS-mediated cholesterol transfer is intensely researched, identifying MCS dysfunction or altered MCS partnerships to correlate with de-regulated cellular cholesterol homeostasis and pathologies. Annexins, a conserved family of Ca²⁺-dependent phospholipid binding proteins, contribute to tethering and untethering events at MCS. In this chapter, we will discuss how Ca²⁺ homeostasis and annexins in the endocytic compartment affect the sensing and transfer of cholesterol and PIs across MCS.

Listeria monocytogenes Co-Opts the Host Exocyst Complex To Promote Internalin A-Mediated Entry

The bacterial pathogen Listeria monocytogenes induces its internalization (entry) into intestinal epithelial cells through interaction of its surface protein, internalin A (InlA), with the human cell-cell adhesion molecule, E-cadherin. While InlA-mediated entry requires bacterial stimulation of actin polymerization, it remains unknown whether additional host processes are manipulated to promote internalization. Here, we show that interaction of InlA with E-cadherin induces the host membrane-trafficking process of polarized exocytosis, which augments uptake of Listeria. Imaging studies revealed that exocytosis is stimulated at sites of InlA-dependent internalization. Experiments inhibiting human N-ethylmaleimide-sensitive factor (NSF) demonstrated that exocytosis is needed for efficient InlA-mediated entry. Polarized exocytosis is mediated by the exocyst complex, which comprises eight proteins, including Sec6, Exo70, and Exo84. We found that Exo70 was recruited to sites of InlA-mediated entry. In addition, depletion of Exo70, Exo84, or Sec6 by RNA interference impaired entry without affecting surface levels of E-cadherin. Similar to binding of InlA to E-cadherin, homophilic interaction of E-cadherin molecules mobilized the exocyst and stimulated exocytosis. Collectively, these results demonstrate that ligation of E-cadherin induces exocytosis that promotes Listeria entry, and they raise the possibility that the exocyst might also control the normal function of E-cadherin in cell-cell adhesion.

The Role of Primary Cilia-Associated Phosphoinositide Signaling in Development

Primary cilia are microtube-based organelles that extend from the cell surface and function as biochemical and mechanical extracellular signal sensors. Primary cilia coordinate a series of signaling pathways during development. Cilia dysfunction leads to a pleiotropic group of developmental disorders, termed ciliopathy. Phosphoinositides (PIs), a group of signaling phospholipids, play a crucial role in development and tissue homeostasis by regulating membrane trafficking, cytoskeleton reorganization, and organelle identity. Accumulating evidence implicates the involvement of PI species in ciliary defects and ciliopathies. The abundance and localization of PIs in the cell are tightly regulated by the opposing actions of kinases and phosphatases, some of which are recently discovered in the context of primary cilia. Here, we review several cilium-associated PI kinases and phosphatases, including their localization along cilia, function in regulating the ciliary biology under normal conditions, as well as the connection of their disease-associated mutations with ciliopathies.

Essential Roles of Exocyst Complex Component 3-like 2 on Cardiovascular Development in Mice

Angiogenesis is a process to generate new blood vessels from pre-existing vessels and to maintain vessels, and plays critical roles in normal development and disease. However, the molecular mechanisms underlying angiogenesis are not fully understood. This study examined the roles of exocyst complex component (Exoc) 3-like 2 (Exoc3l2) during development in mice. We found that Exoc3l1, Exoc3l2, Exoc3l3 and Exoc3l4 are expressed abundantly in endothelial cells at embryonic day 8.5. The generation of Exoc3l2 knock-out (KO) mice showed that disruption of Exoc3l2 resulted in lethal in utero. Substantial numbers of Exoc3l2 KO embryos exhibited hemorrhaging. Deletion of Exoc3l2 using Tie2-Cre transgenic mice demonstrated that Exoc3l2 in hematopoietic and endothelial lineages was responsible for the phenotype. Taken together, these findings reveal that Exoc3l2 is essential for cardiovascular and brain development in mice.

Rho and Rab Family Small GTPases in the Regulation of Membrane Polarity in Epithelial Cells

Membrane polarity, defined as the asymmetric distribution of lipids and proteins in the plasma membrane, is a critical prerequisite for the development of multicellular tissues, such as epithelia and endothelia. Membrane polarity is regulated by polarized trafficking of membrane components to specific membrane domains and requires the presence of intramembrane diffusion barriers that prevent the intermixing of asymmetrically distributed membrane components. This intramembrane diffusion barrier is localized at the tight junctions (TJs) in these cells. Both the formation of cell-cell junctions and the polarized traffic of membrane proteins and lipids are regulated by Rho and Rab family small GTPases. In this review article, we will summarize the recent developments in the regulation of apico-basal membrane polarity by polarized membrane traffic and the formation of the intramembrane diffusion barrier in epithelial cells with a particular focus on the role of Rho and Rab family small GTPases.

A mechanism for exocyst-mediated tethering via Arf6 and PIP5K1C-driven phosphoinositide conversion

Polarized trafficking is necessary for the development of eukaryotes and is regulated by a conserved molecular machinery. Late steps of cargo delivery are mediated by the exocyst complex, which integrates lipid and protein components to tether vesicles for plasma membrane fusion. However, the molecular mechanisms of this process are poorly defined. Here, we reconstitute functional octameric human exocyst, demonstrating the basis for holocomplex coalescence and biochemically stable subcomplexes. We determine that each subcomplex independently binds to phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂), which is minimally sufficient for membrane tethering. Through reconstitution and epithelial cell biology experiments, we show that Arf6-mediated recruitment of the lipid kinase PIP5K1C rapidly converts phosphatidylinositol 4-phosphate (PI(4)P) to PI(4,5)P₂, driving exocyst recruitment and membrane tethering. These results provide a molecular mechanism of exocyst-mediated tethering and a unique functional requirement for phosphoinositide signaling on late-stage vesicles in the vicinity of the plasma membrane.

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Complete reconstitution and subunit connectivity of the human exocyst complex

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Binding to PI(4,5)P₂ in trans by each subcomplex enables membrane tethering

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PI(4)P to PI(4,5)P2 conversion is sufficient for exocyst recruitment and tethering

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Arf6 controls phosphoinositide conversion by PIP5K1C in cells and in vitro

Plasma membrane phosphatidylinositol (4,5)-bisphosphate is critical for determination of epithelial characteristics

Epithelial cells provide cell-cell adhesion that is essential to maintain the integrity of multicellular organisms. Epithelial cell-characterizing proteins, such as epithelial junctional proteins and transcription factors are well defined. However, the role of lipids in epithelial characterization remains poorly understood. Here we show that the phospholipid phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P₂] is enriched in the plasma membrane (PM) of epithelial cells. Epithelial cells lose their characteristics upon depletion of PM PI(4,5)P₂, and synthesis of PI(4,5)P₂ in the PM results in the development of epithelial-like morphology in osteosarcoma cells. PM localization of PARD3 is impaired by depletion of PM PI(4,5)P₂ in epithelial cells, whereas expression of the PM-targeting exocyst-docking region of PARD3 induces osteosarcoma cells to show epithelial-like morphological changes, suggesting that PI(4,5)P₂ regulates epithelial characteristics by recruiting PARD3 to the PM. These results indicate that a high level of PM PI(4,5)P₂ plays a crucial role in the maintenance of epithelial characteristics.

Epithelial cells provide cell-cell adhesion to maintain the integrity of multicellular organisms. Here the authors show that phospholipid phosphatidylinositol (4,5)-bisphosphate is critical for the maintenance of epithelial characteristics.

Phosphoinositides containing stearic acid are required for interaction between Rho GTPases and the exocyst to control the late steps of polarised exocytosis

Cell polarity is achieved by regulators such as small G proteins, exocyst members and phosphoinositides, with the latter playing a key role when bound to the exocyst proteins Sec3p and Exo70p, and Rho GTPases. This ensures asymmetric growth via the routing of proteins and lipids to the cell surface using actin cables. Previously, using a yeast mutant for a lysophosphatidylinositol acyl transferase encoded by the PSI1 gene, we demonstrated the role of stearic acid in the acyl chain of phosphoinositides in cytoskeletal organisation and secretion. Here, we use a genetic approach to characterise the effect on late steps of the secretory pathway. The constitutive overexpression of PSI1 in mutants affecting kinases involved in the phosphoinositide pathway demonstrated the role of molecular species containing stearic acid in bypassing a lack of phosphatidylinositol-4-phosphate PI(4)P at the plasma membrane, which is essential for the function of the Cdc42p module. Decreasing the levels of stearic acid-containing phosphoinositides modifies the environment of the actors involved in the control of late steps in the secretory pathway. This leads to decreased interactions between Exo70p and Sec3p, with Cdc42p, Rho1p and Rho3p, due to disruption of the GTP/GDP ratio of at least Rho1p and Rho3p GTPases, thereby preventing activation of the exocyst.

RASSF8-mediated transport of Echinoid via the exocyst promotes Drosophila wing elongation and epithelial ordering

Cell-cell junctions are dynamic structures that maintain cell cohesion and shape in epithelial tissues. During development, junctions undergo extensive rearrangements to drive the epithelial remodelling required for morphogenesis. This is particularly evident during axis elongation, where neighbour exchanges, cell-cell rearrangements and oriented cell divisions lead to large-scale alterations in tissue shape. Polarised vesicle trafficking of junctional components by the exocyst complex has been proposed to promote junctional rearrangements during epithelial remodelling, but the receptors that allow exocyst docking to the target membranes remain poorly understood. Here, we show that the adherens junction component Ras Association domain family 8 (RASSF8) is required for the epithelial re-ordering that occurs during Drosophila pupal wing proximo-distal elongation. We identify the exocyst component Sec15 as a RASSF8 interactor. Loss of RASSF8 elicits cytoplasmic accumulation of Sec15 and Rab11-containing vesicles. These vesicles also contain the nectin-like homophilic adhesion molecule Echinoid, the depletion of which phenocopies the wing elongation and epithelial packing defects observed in RASSF8 mutants. Thus, our results suggest that RASSF8 promotes exocyst-dependent docking of Echinoid-containing vesicles during morphogenesis.

Identification and Comprehensive Structural and Functional Analyses of the EXO70 Gene Family in Cotton

The EXO70 gene is a vital component of the exocytosis complex and participates in biological processes ranging from plant cell division to polar growth. There are many EXO70 genes in plants and their functions are extensive, but little is known about the EXO70 gene family in cotton. Here, we analyzed four cotton sequence databases, identified 165 EXO70 genes, and divided them into eight subgroups (EXO70A–EXO70H) based on their phylogenetic relationships. EXO70A had the most exons (≥11), whereas the other seven each had only one or two exons. Hence, EXO70A may have many important functions. The 84 EXO70 genes in Asian and upland cotton were expressed in the roots, stems, leaves, flowers, fibers, and/or ovules. Full-length GhEXO70A1-A cDNA was homologously cloned from upland cotton (Gossypium hirsutum, G. hirsutum). Subcellular analysis revealed that GhEXO70A1-A protein was localized to the plasma membrane. A yeast two-hybrid assay revealed that GhEXO70A1-A interacted with GhEXO84A, GhEXO84B, and GhEXO84C. GhEXO70A1-A silencing significantly altered over 4000 genes and changed several signaling pathways related to metabolism. Thus, the EXO70 gene plays critical roles in the physiological functions of cotton.

PD-L1 tumor-intrinsic signaling and its therapeutic implication in triple-negative breast cancer

Although the immune checkpoint role of programmed death ligand 1 (PD-L1) has been established and targeted in cancer immunotherapy, the tumor-intrinsic role of PD-L1 is less appreciated in tumor biology and therapeutics development, partly because of the incomplete mechanistic understanding. Here we demonstrate a potentially novel mechanism by which PD-L1 promotes the epithelial-mesenchymal transition (EMT) in triple-negative breast cancer (TNBC) cells by suppressing the destruction of the EMT transcription factor Snail. PD-L1 directly binds to and inhibits the tyrosine phosphatase PTP1B, thus preserving p38-MAPK activity that phosphorylates and inhibits glycogen synthase kinase 3β (GSK3β). Via this mechanism, PD-L1 prevents the GSK3β-mediated phosphorylation, ubiquitination, and degradation of Snail and consequently promotes the EMT and metastatic potential of TNBC. Significantly, PD-L1 antibodies that confine the tumor-intrinsic PD-L1/Snail pathway restricted TNBC progression in immunodeficient mice. More importantly, targeting both tumor-intrinsic and tumor-extrinsic functions of PD-L1 showed strong synergistic tumor suppression effect in an immunocompetent TNBC mouse model. Our findings support that PD-L1 intrinsically facilitates TNBC progression by promoting the EMT, and this potentially novel PD-L1 signaling pathway could be targeted for better clinical management of PD-L1–overexpressing TNBCs.

Control of Rab7a activity and localization through endosomal type Igamma PIP 5-kinase is required for endosome maturation and lysosome function

The small GTPase Ras-related protein Rab-7a (Rab7a) serves as a key organizer of the endosomal-lysosomal system. However, molecular mechanisms controlling Rab7a activation levels and subcellular translocation are still poorly defined. Here, we demonstrate that type Igamma phosphatidylinositol phosphate 5-kinase i5 (PIPKIγi5), an endosome-localized enzyme that produces phosphatidylinositol 4,5-bisphosphate, directly interacts with Rab7a and plays critical roles in the control of the endosomal-lysosomal system. The loss of PIPKIγi5 blocks Rab7a recruitment to early endosomes, which prevents the maturation of early to late endosomes. PIPKIγi5 loss disturbs retromer complex connection with Rab7a, which blocks the retrograde sorting of Cation-independent Mannose 6-Phosphate Receptor from late endosomes. This leads to the decreased sorting of hydrolases to lysosomes and reduces the autophagic degradation. By modulating the retromer-Rab7a connection, PIPKIγi5 is also required for the recruitment of the GTPase-activating protein TBC1 domain family member 5 to late endosomes, which controls the conversion of Rab7a from the active state to the inactive state. Thus, PIPKIγi5 is critical for the modulation of Rab7a activity, localization, and function in the endosomal-lysosomal system.

SNV discovery and functional candidate gene identification for milk composition based on whole genome resequencing of Holstein bulls with extremely high and low breeding values

We have sequenced the whole genomes of eight proven Holstein bulls from the four half-sib or full-sib families with extremely high and low estimated breeding values (EBV) for milk protein percentage (PP) and fat percentage (FP) using Illumina re-sequencing technology. Consequently, 2.3 billion raw reads were obtained with an average effective depth of 8.1×. After single nucleotide variant (SNV) calling, total 10,961,243 SNVs were identified, and 57,451 of them showed opposite fixed sites between the bulls with high and low EBVs within each family (called as common differential SNVs). Next, we annotated the common differential SNVs based on the bovine reference genome, and observed that 45,188 SNVs (78.70%) were located in the intergenic region of genes and merely 11,871 SNVs (20.67%) located within the protein-coding genes. Of them, 13,099 common differential SNVs that were within or close to protein-coding genes with less than 5 kb were chosen for identification of candidate genes for milk compositions in dairy cattle. By integrated analysis of the 2,657 genes with the GO terms and pathways related to protein and fat metabolism, and the known quantitative trait loci (QTLs) for milk protein and fat traits, we identified 17 promising candidate genes: ALG14, ATP2C1, PLD1, C3H1orf85, SNX7, MTHFD2L, CDKN2D, COL5A3, FDX1L, PIN1, FIG4, EXOC7, LASP1, PGS1, SAO, GPLD1 and MGEA5. Our findings provided an important foundation for further study and a prompt for molecular breeding of dairy cattle.

The membrane environment of cadherin adhesion receptors: a working hypothesis

Classical cadherin cell adhesion receptors are integral membrane proteins that mediate cell–cell interactions, tissue integrity and morphogenesis. Cadherins are best understood to function as membrane-spanning molecular composites that couple adhesion to the cytoskeleton. On the other hand, the membrane lipid environment of the cadherins is an under-investigated aspect of their cell biology. In this review, we discuss two lines of research that show how the membrane can directly or indirectly contribute to cadherin function. Firstly, we consider how modification of its local lipid environment can potentially influence cadherin signalling, adhesion and dynamics, focusing on a role for phosphoinositide-4,5-bisphosphate. Secondly, we discuss how caveolae may indirectly regulate cadherins by modifying either the lipid composition and/or mechanical tension of the plasma membrane. Thus, we suggest that the membrane is a frontier of cadherin biology that is ripe for re-exploration.

A conserved morphogenetic mechanism for epidermal ensheathment of nociceptive sensory neurites

Interactions between epithelial cells and neurons influence a range of sensory modalities including taste, touch, and smell. Vertebrate and invertebrate epidermal cells ensheath peripheral arbors of somatosensory neurons, including nociceptors, yet the developmental origins and functional roles of this ensheathment are largely unknown. Here, we describe an evolutionarily conserved morphogenetic mechanism for epidermal ensheathment of somatosensory neurites. We found that somatosensory neurons in Drosophila and zebrafish induce formation of epidermal sheaths, which wrap neurites of different types of neurons to different extents. Neurites induce formation of plasma membrane phosphatidylinositol 4,5-bisphosphate microdomains at nascent sheaths, followed by a filamentous actin network, and recruitment of junctional proteins that likely form autotypic junctions to seal sheaths. Finally, blocking epidermal sheath formation destabilized dendrite branches and reduced nociceptive sensitivity in Drosophila. Epidermal somatosensory neurite ensheathment is thus a deeply conserved cellular process that contributes to the morphogenesis and function of nociceptive sensory neurons.

Humans and other animals perceive and interact with the outside world through their sensory nervous system. Nerve cells, acting as the body’s ‘telegraph wires’, convey signals from sensory organs – like the eyes – to the brain, which then processes this information and tells the body how to respond. There are different kinds of sensory nerve cells that carry different types of information, but they all associate closely with the tissues and organs they connect to the brain.

Human skin contains sensory nerve cells, which underpin our senses of touch and pain. There is a highly specialized, complex connection between some of these nerve cells and cells in the skin: the skin cells wrap tightly around the nerve cells’ free ends, forming sheath-like structures. This ‘ensheathment’ process happens in a wide range of animals, including those with a backbone, like fish and humans, and those without, like insects.

Ensheathment is thought to be important for the skin’s nerve cells to work properly. Yet it remains unclear how or when these connections first appear. Jiang et al. therefore wanted to determine the developmental origins of ensheathment and to find out if these were also similar in animals with and without backbones.

Experiments using fruit fly and zebrafish embryos revealed that nerve cells, not skin cells, were responsible for forming and maintaining the sheaths. In embryos where groups of sensory nerve cells were selectively killed – either using a laser or by making the cells produce a toxin – ensheathment did not occur. Further studies, using a variety of microscopy techniques, revealed that the molecular machinery required to stabilize the sheaths was similar in both fish and flies, and therefore likely to be conserved across different groups of animals. Removing sheaths in fly embryos led to nerve cells becoming unstable; the animals were also less sensitive to touch. This confirmed that ensheathment was indeed necessary for sensory nerve cells to work properly.

By revealing how ensheathment first emerges, these findings shed new light on how the sensory nervous system develops and how its activity is controlled. In humans, skin cells ensheath the nerve cells responsible for sensing pain. A better understanding of how ensheathments first arise could therefore lead to new avenues for treating chronic pain and related conditions.

Regulation of Cell Polarity by Exocyst-Mediated Trafficking

One requirement for establishing polarity within a cell is the asymmetric trafficking of intracellular vesicles to the plasma membrane. This tightly regulated process creates spatial and temporal differences in both plasma membrane composition and the membrane-associated proteome. Asymmetric membrane trafficking is also a critical mechanism to regulate cell differentiation, signaling, and physiology. Many eukaryotic cell types use the eight-protein exocyst complex to orchestrate polarized vesicle trafficking to certain membrane locales. Members of the exocyst were originally discovered in yeast while screening for proteins required for the delivery of secretory vesicles to the budding daughter cell. The same eight exocyst genes are conserved in mammals, in which the specifics of exocyst-mediated trafficking are highly cell-type-dependent. Some exocyst members bind to certain Rab GTPases on intracellular vesicles, whereas others localize to the plasma membrane at the site of exocytosis. Assembly of the exocyst holocomplex is responsible for tethering these vesicles to the plasma membrane before their soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE)-mediated exocytosis. In this review, we will focus on the role and regulation of the exocyst complex in targeted vesicular trafficking as related to the establishment and maintenance of cellular polarity. We will contrast exocyst function in apicobasal epithelial polarity versus front-back mesenchymal polarity, and the dynamic regulation of exocyst-mediated trafficking during cell phenotype transitions.

The role of Exo70 in exocytosis and beyond

The exocyst complex mediates the tethering of secretory vesicles to the plasma membrane before SNARE-mediated membrane fusion. Recent studies have implicated the exocyst in a wide range of cellular processes. Particularly, research on the Exo70 subunit of the complex has linked the function of the exocyst in exocytosis to cell adhesion, migration and invasion. In this review, we will discuss the recent work on how Exo70 regulates these cellular processes, and how small GTPases and kinases interact with Exo70 to orchestrate its function in exocytosis and cytoskeleton organization. The study of Exo70 contributes to the understanding of many pathophysiological processes from organogenesis to cancer metastasis.

The Par3 polarity protein is an exocyst receptor essential for mammary cell survival

The exocyst is an essential component of the secretory pathway required for delivery of basolateral proteins to the plasma membranes of epithelial cells. Delivery occurs adjacent to tight junctions (TJ), suggesting that it recognizes a receptor at this location. However, no such receptor has been identified. The Par3 polarity protein associates with TJs but has no known function in membrane traffic. We now show that, unexpectedly, Par3 is essential for mammary cell survival. Par3 silencing causes apoptosis, triggered by phosphoinositide trisphosphate depletion and decreased Akt phosphorylation, resulting from failure of the exocyst to deliver basolateral proteins to the cortex. A small region of PAR3 binds directly to Exo70 and is sufficient for exocyst docking, membrane-protein delivery and cell survival. PAR3 lacking this domain can associate with the cortex but cannot support exocyst function. We conclude that Par3 is the long-sought exocyst receptor required for targeted membrane-protein delivery.

The exocyst delivers basolateral proteins from the secretory pathway to the plasma membrane of epithelial cells close to tight junctions. Here the authors show that Par3 acts as a docking site for the exocyst to regulate polarized delivery of basolateral proteins and this is essential to prevent apoptosis and promote mammary cell survival.

PIPKIγ and talin couple phosphoinositide and adhesion signaling to control the epithelial to mesenchymal transition

Epithelial cells acquire migratory/invasive and stemness traits upon conversion to the mesenchymal phenotype. The expression of E-cadherin is a key to this transition; yet precise understanding of the pathways involved in integrating E-cadherin loss to the gain of mesenchymal traits remains poorly understood. Here, we show that phosphoinositide-generating enzyme, PIPKIγ, expression is upregulated upon epithelial-mesenchymal transition (EMT) and together with the cytoskeletal protein talin assemble into a signaling complex upon E-cadherin loss. PIPKIγ and talin together control the adhesion and phosphoinositide signaling that regulates conversion to the mesenchymal phenotypes. PIPKIγ and talin regulate the stability of E-cadherin transcriptional repressors, snail and slug, induced by transforming growth factor-β1 or extracellular matrix protein. Loss of PIPKIγ or talin or their interaction impaired EMT and the acquisition of cell motility and stemness. This demonstrates a mechanism where a phosphoinositide-generating enzyme PIPKIγ couples with a cytoskeletal protein talin to control the acquisition of mesenchymal phenotypes.

PtdIns(4,5)P2 signaling regulates ATG14 and autophagy

Autophagy is a regulated self-digestion pathway with fundamental roles in cell homeostasis and diseases. Autophagy is regulated by coordinated actions of a series of autophagy-related (ATG) proteins. The Barkor/ATG14(L)-VPS34 (a class III phosphatidylinositol 3-kinase) complex and its product phosphatidylinositol 3-phosphate [PtdIns(3)P] play key roles in autophagy initiation. ATG14 contains a C-terminal Barkor/ATG14(L) autophagosome-targeting sequence (BATS) domain that senses the curvature of PtdIns(3)P-containing membrane. The BATS domain also strongly binds PtdIns(4,5)P2, but the functional significance has been unclear. Here we show that ATG14 specifically interacts with type Iγ PIP kinase isoform 5 (PIPKIγi5), an enzyme that generates PtdIns(4,5)P2 in mammalian cells. Autophagosomes have associated PIPKIγi5 and PtdIns(4,5)P2 that are colocalized with late endosomes and the endoplasmic reticulum. PtdIns(4,5)P2 generation at these sites requires PIPKIγi5. Loss of PIPKIγi5 results in a loss of ATG14, UV irradiation resistance-associated gene, and Beclin 1 and a block of autophagy. PtdIns(4,5)P2 binding to the ATG14-BATS domain regulates ATG14 interaction with VPS34 and Beclin 1, and thus plays a key role in ATG14 complex assembly and autophagy initiation. This study identifies an unexpected role for PtdIns(4,5)P2 signaling in the regulation of ATG14 complex and autophagy.

The role of Exo70 in vascular smooth muscle cell migration

Background

As a key subunit of the exocyst complex, Exo70 has highly conserved sequence and is widely found in yeast, mammals, and plants. In yeast, Exo70 mediates the process of exocytosis and promotes anchoring and integration of vesicles with the plasma membrane. In mammalian cells, Exo70 is involved in maintaining cell morphology, cell migration, cell connection, mRNA splicing, and other physiological processes, as well as participating in exocytosis. However, Exo70’s function in mammalian cells has yet to be fully recognized. In this paper, the expression of Exo70 and its role in cell migration were studied in a rat vascular smooth muscle cell line A7r5.

Methods

Immunofluorescent analysis the expression of Exo70, α-actin, and tubulin in A7r5 cells showed a co-localization of Exo70 and α-actin, we treated the cells with cytochalasin B to depolymerize α-actin, in order to further confirm the co-localization of Exo70 and α-actin. We analyzed Exo70 co-localization with actin at the edge of migrating cells by wound-healing assay to establish whether Exo70 might play a role in cell migration. Next, we analyzed the migration and invasion ability of A7r5 cells before and after RNAi silencing through the wound healing assay and transwell assay.

Results

The mechanism of interaction between Exo70 and cytoskeleton can be clarified by the immunoprecipitation techniques and wound-healing assay. The results showed that Exo70 and α-actin were co-localized at the leading edge of migrating cells. The ability of A7r5 to undergo cell migration was decreased when Exo70 expression was silenced by RNAi. Reducing Exo70 expression in RNAi treated A7r5 cells significantly lowered the invasion and migration ability of these cells compared to the normal cells. These results indicate that Exo70 participates in the process of A7r5 cell migration.

Conclusions

This research is importance for the study on the pathological process of vascular intimal hyperplasia, since it provides a new research direction for the treatment of cardiovascular diseases such as atherosclerosis and restenosis after balloon angioplasty.

The Hidden Conundrum of Phosphoinositide Signaling in Cancer

Phosphoinositide 3-kinase (PI3K) generation of PI(3,4,5)P₃ from PI(4,5)P₂ and the subsequent activation of Akt and its downstream signaling cascades (e.g. mTORC1) dominates the landscape of phosphoinositide signaling axis in cancer research. However, PI(4,5)P₂ is breaking its boundary as merely a substrate for PI3K and phospholipase C (PLC), and is now an established lipid messenger pivotal for different cellular events in cancer. Here, we review the phosphoinositide signaling axis in cancer, giving due weight to PI(4,5)P₂ and its generating enzymes, the phosphatidylinositol phosphate (PIP) kinases (PIPKs). We highlighted how PI(4,5)P₂ and PIP kinases serve as a proximal node in phosphoinositide signaling axis and how its interaction with cytoskeletal proteins regulates migratory and invasive nexus of metastasizing tumor cells.

The Exocyst Complex in Health and Disease

Exocytosis involves the fusion of intracellular secretory vesicles with the plasma membrane, thereby delivering integral membrane proteins to the cell surface and releasing material into the extracellular space. Importantly, exocytosis also provides a source of lipid moieties for membrane extension. The tethering of the secretory vesicle before docking and fusion with the plasma membrane is mediated by the exocyst complex, an evolutionary conserved octameric complex of proteins. Recent findings indicate that the exocyst complex also takes part in other intra-cellular processes besides secretion. These various functions seem to converge toward defining a direction of membrane growth in a range of systems from fungi to plants and from neurons to cilia. In this review we summarize the current knowledge of exocyst function in cell polarity, signaling and cell-cell communication and discuss implications for plant and animal health and disease.

Endosidin2 targets conserved exocyst complex subunit EXO70 to inhibit exocytosis

The exocyst complex regulates the last steps of exocytosis, which is essential to organisms across kingdoms. In humans, its dysfunction is correlated with several significant diseases, such as diabetes and cancer progression. Investigation of the dynamic regulation of the evolutionarily conserved exocyst-related processes using mutants in genetically tractable organisms such as Arabidopsis thaliana is limited by the lethality or the severity of phenotypes. We discovered that the small molecule Endosidin2 (ES2) binds to the EXO70 (exocyst component of 70 kDa) subunit of the exocyst complex, resulting in inhibition of exocytosis and endosomal recycling in both plant and human cells and enhancement of plant vacuolar trafficking. An EXO70 protein with a C-terminal truncation results in dominant ES2 resistance, uncovering possible distinct regulatory roles for the N terminus of the protein. This study not only provides a valuable tool in studying exocytosis regulation but also offers a potentially new target for drugs aimed at addressing human disease.

Emerging roles of PtdIns(4,5)P2--beyond the plasma membrane

Phosphoinositides are a collection of lipid messengers that regulate most subcellular processes. Amongst the seven phosphoinositide species, the roles for phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] at the plasma membrane, such as in endocytosis, exocytosis, actin polymerization and focal adhesion assembly, have been extensively studied. Recent studies have argued for the existence of PtdIns(4,5)P2 at multiple intracellular compartments, including the nucleus, endosomes, lysosomes, autolysosomes, autophagic precursor membranes, ER, mitochondria and the Golgi complex. Although the generation, regulation and functions of PtdIns(4,5)P2 are less well-defined in most other intracellular compartments, accumulating evidence demonstrates crucial roles for PtdIns(4,5)P2 in endolysosomal trafficking, endosomal recycling, as well as autophagosomal pathways, which are the focus of this Commentary. We summarize and discuss how phosphatidylinositol phosphate kinases, PtdIns(4,5)P2 and PtdIns(4,5)P2-effectors regulate these intracellular protein and membrane trafficking events.

The Exocyst at a Glance

The exocyst is an octameric protein complex that is implicated in the tethering of secretory vesicles to the plasma membrane prior to SNARE-mediated fusion. Spatial and temporal control of exocytosis through the exocyst has a crucial role in a number of physiological processes, such as morphogenesis, cell cycle progression, primary ciliogenesis, cell migration and tumor invasion. In this Cell Science at a Glance poster article, we summarize recent works on the molecular organization, function and regulation of the exocyst complex, as they provide rationales to the involvement of this complex in such a diverse array of cellular processes.

PIP kinases define PI4,5P₂signaling specificity by association with effectors

Phosphatidylinositol 4,5-bisphosphate (PI4,5P₂) is an essential lipid messenger with roles in all eukaryotes and most aspects of human physiology. By controlling the targeting and activity of its effectors, PI4,5P₂modulates processes, such as cell migration, vesicular trafficking, cellular morphogenesis, signaling and gene expression. In cells, PI4,5P₂has a much higher concentration than other phosphoinositide species and its total content is largely unchanged in response to extracellular stimuli. The discovery of a vast array of PI4,5P₂ binding proteins is consistent with data showing that the majority of cellular PI4,5P₂is sequestered. This supports a mechanism where PI4,5P₂functions as a localized and highly specific messenger. Further support of this mechanism comes from the de novo synthesis of PI4,5P₂which is often linked with PIP kinase interaction with PI4,5P₂effectors and is a mechanism to define specificity of PI4,5P₂signaling. The association of PI4,5P₂-generating enzymes with PI4,5P₂effectors regulate effector function both temporally and spatially in cells. In this review, the PI4,5P₂effectors whose functions are tightly regulated by associations with PI4,5P₂-generating enzymes will be discussed. This article is part of a Special Issue entitled Phosphoinositides.

Control of E-cadherin apical localisation and morphogenesis by a SOAP-1/AP-1/clathrin pathway in C. elegans epidermal cells

E-cadherin (E-cad) is the main component of epithelial junctions in multicellular organisms, where it is essential for cell-cell adhesion. The localisation of E-cad is often strongly polarised in the apico-basal axis. However, the mechanisms required for its polarised distribution are still largely unknown. We performed a systematic RNAi screen in vivo to identify genes required for the strict E-cad apical localisation in C. elegans epithelial epidermal cells. We found that the loss of clathrin, its adaptor AP-1 and the AP-1 interactor SOAP-1 induced a basolateral localisation of E-cad without affecting the apico-basal diffusion barrier. We further found that SOAP-1 controls AP-1 localisation, and that AP-1 is required for clathrin recruitment. Finally, we also show that AP-1 controls E-cad apical delivery and actin organisation during embryonic elongation, the final morphogenetic step of embryogenesis. We therefore propose that a molecular pathway, containing SOAP-1, AP-1 and clathrin, controls the apical delivery of E-cad and morphogenesis.

Targeting type Iγ phosphatidylinositol phosphate kinase inhibits breast cancer metastasis

Most deaths from breast cancer are caused by metastasis, a complex behavior of cancer cells involving migration, invasion, survival, and microenvironment manipulation. Type Iγ phosphatidylinositol phosphate kinase (PIPKIγ) regulates focal adhesion assembly, and its phosphorylation at Y639 is critical for cell migration induced by EGF. However, the role of this lipid kinase in tumor metastasis remains unclear. Here we report that PIPKIγ is vital for breast cancer metastasis. Y639 of PIPKIγ can be phosphorylated by stimulation of EGF and hepatocyte growth factor (HGF), two promoting factors for breast cancer progression. Histological analysis revealed elevated Y639-phosphorylation of PIPKIγ in invasive ductal carcinoma lesions and suggested a positive correlation with tumor grade. Orthotopically transplanted, PIPKIγ-depleted breast cancer cells showed substantially reduced growth and metastasis, as well as suppressed expression of multiple genes related to cell migration and microenvironment manipulation. Re-expression of wild-type PIPKIγ in PIPKIγ-depleted cells restored tumor growth and metastasis, reinforcing the importance of PIPKIγ in breast cancer progression. Y639-to-F or a kinase-dead mutant of PIPKIγ could not recover the diminished metastasis in PIPKIγ-depleted cancer cells, suggesting that Y639 phosphorylation and lipid kinase activity are both required for development of metastasis. Further analysis with in vitro assays indicated that depleting PIPKIγ inhibited cell proliferation, MMP9 secretion, and cell migration and invasion, lending molecular mechanisms for the eliminated cancer progression. These results suggest that PIPKIγ, downstream of EGF and/or HGF receptor, participates in breast cancer progression from multiple aspects and deserves further studies to explore its potential as a therapeutic target.

PI(4,5)P₂-binding effector proteins for vesicle exocytosis

PI(4,5)P₂participates directly in priming and possibly in fusion steps of Ca²⁺-triggered vesicle exocytosis. High concentration nanodomains of PI(4,5)P₂reside on the plasma membrane of neuroendocrine cells. A subset of vesicles that co-localize with PI(4,5)P₂ domains appear to undergo preferential exocytosis in stimulated cells. PI(4,5)P₂directly regulates vesicle exocytosis by recruiting and activating PI(4,5)P₂-binding proteins that regulate SNARE protein function including CAPS, Munc13-1/2, synaptotagmin-1, and other C2 domain-containing proteins. These PI(4,5)P₂effector proteins are coincidence detectors that engage in multiple interactions at vesicle exocytic sites. The SNARE protein syntaxin-1 also binds to PI(4,5)P₂, which promotes clustering, but an activating role for PI(4,5)P₂in syntaxin-1 function remains to be fully characterized. Similar principles underlie polarized constitutive vesicle fusion mediated in part by the PI(4,5)P₂-binding subunits of the exocyst complex (Sec3, Exo70). Overall, focal vesicle exocytosis occurs at sites landmarked by PI(4,5)P2, which serves to recruit and/or activate multifunctional PI(4,5)P₂-binding proteins. This article is part of a Special Issue entitled Phosphoinositides.

Pulmonary endoderm, second heart field and the morphogenesis of distal outflow tract in mouse embryonic heart

The second heart field (SHF), foregut endoderm and sonic hedgehog (SHH) signaling pathway are all reported to associate with normal morphogenesis and septation of outflow tract (OFT). However, the morphological relationships of the development of foregut endoderm and expression of SHH signaling pathway members with the development of surrounding SHF and OFT are seldom described. In this study, serial sections of mouse embryos from ED9 to ED13 (midgestation) were stained with a series of marker antibodies for specifically highlighting SHF (Isl-1), endoderm (Foxa2), basement membrane (Laminin), myocardium (MHC) and smooth muscle (α-SMA) respectively, or SHH receptors antibodies including patched1 (Ptc1), patched2 (Ptc2) and smoothened, to observe the spatiotemporal relationship between them and their contributions to OFT morphogenesis. Our results demonstrated that the development of an Isl-1 positive field in the splanchnic mesoderm ventral to foregut, a subset of SHF, is closely coupled with pulmonary endoderm or tracheal groove, the Isl-1 positive cells surrounding pulmonary endoderm are distributed in a special cone-shaped pattern and take part in the formation of the lateral walls of the intrapericardial aorta and pulmonary trunk and the transient aortic-pulmonary septum, and Ptc1 and Ptc2 are exclusively expressed in pulmonary endoderm during this Isl-l positive field development, suggesting special roles played in inducing the Isl-l positive field formation by pulmonary endoderm. It is indicated that pulmonary endoderm plays a role in the development and specification of SHF in midgestation, and that pulmonary endoderm-associated Isl-l positive field is involved in patterning the morphogenesis and septation of the intrapericardial arterial trunks.

PIPKIγ targets to the centrosome and restrains centriole duplication

Centriole biogenesis depends on the polo-like kinase (PLK4) and a small group of structural proteins. The spatiotemporal regulation of these proteins at pre-existing centrioles is essential to ensure that centriole duplication occurs once per cell cycle. Here, we report that phosphatidylinositol 4-phosphate 5-kinase type-1 gamma (PIP5K1C, hereafter referred to as PIPKIγ) plays an important role in centriole fidelity. PIPKIγ localized in a ring-like pattern in the intermediate pericentriolar materials around the proximal end of the centriole in G1, S and G2 phases, but not in M phase. This localization was dependent upon an association with centrosomal protein of 152 KDa (CEP152). Without detaining cells in S or M phase, the depletion of PIPKIγ led to centriole amplification in a manner that was dependent upon PLK4 and spindle assembly abnormal protein 6 homolog (SAS6). The expression of exogenous PIPKIγ reduced centriole amplification that occurred as a result of endogenous PIPKIγ depletion, hydroxyurea treatment or PLK4 overexpression, suggesting that PIPKIγ is likely to function at the PLK4 level to restrain centriole duplication. Importantly, we found that PIPKIγ bound to the cryptic polo-box domain of PLK4 and that this binding reduced the kinase activity of PLK4. Together, our findings suggest that PIPKIγ is a novel negative regulator of centriole duplication, which acts by modulating the homeostasis of PLK4 activity.

PI4KIIIα is required for cortical integrity and cell polarity during Drosophila oogenesis

Phosphoinositides regulate myriad cellular processes, acting as potent signaling molecules in conserved signaling pathways and as organelle gatekeepers that recruit effector proteins to membranes. Phosphoinositide-generating enzymes have been studied extensively in yeast and cultured cells, yet their roles in animal development are not well understood. Here, we analyze Drosophila melanogaster phosphatidylinositol 4-kinase IIIα (PI4KIIIα) during oogenesis. We demonstrate that PI4KIIIα is required for production of plasma membrane PtdIns4P and PtdIns(4,5)P2 and is crucial for actin organization, membrane trafficking and cell polarity. Female germ cells mutant for PI4KIIIα exhibit defects in cortical integrity associated with failure to recruit the cytoskeletal-membrane crosslinker Moesin and the exocyst subunit Sec5. These effects reflect a unique requirement for PI4KIIIα, as egg chambers from flies mutant for either of the other Drosophila PI4Ks, fwd or PI4KII, show Golgi but not plasma membrane phenotypes. Thus, PI4KIIIα is a vital regulator of a functionally distinct pool of PtdIns4P that is essential for PtdIns(4,5)P2-dependent processes in Drosophila development.

Dynamics and regulation of epithelial adherens junctions: recent discoveries and controversies

Adherens junctions (AJs) are evolutionarily conserved plasma-membrane structures that mediate cell-cell adhesions in multicellular organisms. They are organized by several types of adhesive integral membrane proteins, most notably cadherins and nectins that are clustered and stabilized by a number of cytoplasmic scaffolds. AJs are key regulators of tissue architecture and dynamics via control of cell proliferation, polarity, shape, motility, and survival. They are absolutely critical for normal tissue morphogenesis and their disruption results in pathological abnormalities in different tissues. Although the field of adherens-junction research dramatically progressed in recent years, a number of important questions remain controversial and poorly understood. This review outlines basic principles that regulate organization of AJs in mammalian epithelia and discusses recent advances and standing controversies in the field. A special attention is paid to the regulation of AJs by vesicle trafficking and the intracellular cytoskeleton as well as roles and mechanisms of adherens-junction disruption during tumor progression and tissue inflammation.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Phosphatidylinositol 4,5-bisphosphate: targeted production and signaling

Phosphatidylinositol 4,5-bisphosphate (PI4,5P(2)) is a key lipid signaling molecule that regulates a vast array of biological activities. PI4,5P(2) can act directly as a messenger or can be utilized as a precursor to generate other messengers: inositol trisphosphate, diacylglycerol, or phosphatidylinositol 3,4,5-trisphosphate. PI4,5P(2) interacts with hundreds of different effector proteins. The enormous diversity of PI4,5P(2) effector proteins and the spatio-temporal control of PI4,5P(2) generation allow PI4,5P(2) signaling to control a broad spectrum of cellular functions. PI4,5P(2) is synthesized by phosphatidylinositol phosphate kinases (PIPKs). The array of PIPKs in cells enables their targeting to specific subcellular compartments through interactions with targeting factors that are often PI4,5P(2) effectors. These interactions are a mechanism to define spatial and temporal PI4,5P(2) synthesis and the specificity of PI4,5P(2) signaling. In turn, the regulation of PI4,5P(2) effectors at specific cellular compartments has implications for understanding how PI4,5P(2) controls cellular processes and its role in diseases.

Role of membrane traffic in the generation of epithelial cell asymmetry

Epithelial cells have an apical-basolateral axis of polarity, which is required for epithelial functions including barrier formation, vectorial ion transport and sensory perception. Here we review what is known about the sorting signals, machineries and pathways that maintain this asymmetry, and how polarity proteins interface with membrane-trafficking pathways to generate membrane domains de novo. It is becoming apparent that membrane traffic does not simply reinforce polarity, but is critical for the generation of cortical epithelial cell asymmetry.

Comparative phenotypic analysis of the two major splice isoforms of phosphatidylinositol phosphate kinase type Iγ in vivo

Localized production of polyphosphoinositides is critical for their signaling function. To examine the biological relevance of specific pools of phosphatidylinositol 4,5-bisphosphate we compared the consequences of genetically ablating all isoforms of phosphatidylinositol phosphate (PIP) kinase type Iγ (PIPKIγ), encoded by the gene Pip5k1c, versus ablation of a specific splice isoform, PIPKIγ_i2, with respect to three reported PIPKIγ functions. Ablation of PIPKIγ_i2 caused a neuron-specific endocytosis defect similar to that found in PIPKIγ(-/-) mice, while agonist-induced calcium signaling was reduced in PIPKIγ(-/-) cells, but was not affected in the absence of PIPKIγ_i2. A reported contribution of PIPKIγ to epithelial integrity was not evident in PIPKIγ(-/-) mice. Given that mice lacking PIPKIγ_i2 live a normal lifespan whereas PIPKIγ(-/-) mice die shortly after birth, we propose that PIPKIγ-mediated metabotropic calcium signaling may represent an essential function of PIPKIγ, whereas functions specific to the PIPKIγ_i2 splice isoform are not essential for survival.

PIP2 signaling, an integrator of cell polarity and vesicle trafficking in directionally migrating cells

Cell migration is a fundamental cellular process required for embryonic development to wound healing and also plays a key role in tumor metastasis and atherosclerosis. Migration is regulated at multiple strata, from cytoskeletal reorganization to vesicle trafficking. In migrating cells, signaling pathways are integrated with vesicle trafficking machineries in a highly coordinated fashion to accomplish the recruitment and trafficking of the trans-membrane proteins toward the leading edge. Different signaling molecules regulate cell migration in different physio-pathological contexts, among them, phosphatidylinositol-4,5-biphosphate (PIP2) is an integral component of the plasma membrane and pleiotropic lipid signaling molecule modulating diverse biological processes, including actin cytoskeletal dynamics and vesicle trafficking required for cell migration. In this commentary, we provide a brief overview of our current understandings on the phosphoinositide signaling and its implication in regulation of cell polarity and vesicle trafficking in migrating cells. In addition, we highlight the coordinated role of PIPKIγi2, a focal adhesion-targeted enzyme that synthesizes PIP2, and the exocyst complex, a PIP2-effector, in the trafficking of E-cadherin in epithelial cells and integrins in migrating cancer cells.