Exocytosis of vacuolar apical compartment (VAC): a cell-cell contact controlled mechanism for the establishment of the apical plasma membrane domain in epithelial cells

Intercellular fluid dynamics in tissue morphogenesis

During embryonic development, cells shape our body, which is mostly made up of water. It is often forgotten that some of this water is found in intercellular fluid, which, for example, immerses the cells of developing embryos. Intercellular fluid contributes to the properties of tissues and influences cell behaviour, thereby participating in tissue morphogenesis. While our understanding of the role of cells in shaping tissues advances, the exploration of the contribution of intercellular fluid dynamics is just beginning. In this review, we delve into the intricate mechanisms employed by cells to control fluid movements both across and within sealed tissue compartments. These mechanisms encompass sealing by tight junctions and controlled leakage, osmotic pumping, hydraulic fracturing of cell adhesion, cell and tissue contractions, as well as beating cilia. We illustrate key concepts by drawing extensively from the early mouse embryo, which successively forms multiple lumens that play essential roles in its development. Finally, we detail experimental approaches and emerging techniques that allow for the quantitative characterization and the manipulation of intercellular fluids in vivo, as well as theoretical frameworks that are crucial for comprehending their dynamics.

Basal stem cell progeny establish their apical surface in a junctional niche during turnover of an adult barrier epithelium

Barrier epithelial organs face the constant challenge of sealing the interior body from the external environment while simultaneously replacing the cells that contact this environment. New replacement cells-the progeny of basal stem cells-are born without barrier-forming structures such as a specialized apical membrane and occluding junctions. Here, we investigate how new progeny acquire barrier structures as they integrate into the intestinal epithelium of adult Drosophila. We find they gestate their future apical membrane in a sublumenal niche created by a transitional occluding junction that envelops the differentiating cell and enables it to form a deep, microvilli-lined apical pit. The transitional junction seals the pit from the intestinal lumen until differentiation-driven, basal-to-apical remodelling of the niche opens the pit and integrates the now-mature cell into the barrier. By coordinating junctional remodelling with terminal differentiation, stem cell progeny integrate into a functional, adult epithelium without jeopardizing barrier integrity.

Multicellular rosettes link mesenchymal-epithelial transition to radial intercalation in the mouse axial mesoderm

Mesenchymal-epithelial transitions are fundamental drivers of development and disease, but how these behaviors generate epithelial structure is not well understood. Here, we show that mesenchymal-epithelial transitions promote epithelial organization in the mouse node and notochordal plate through the assembly and radial intercalation of three-dimensional rosettes. Axial mesoderm rosettes acquire junctional and apical polarity, develop a central lumen, and dynamically expand, coalesce, and radially intercalate into the surface epithelium, converting mesenchymal-epithelial transitions into higher-order tissue structure. In mouse Par3 mutants, axial mesoderm rosettes establish central tight junction polarity but fail to form an expanded apical domain and lumen. These defects are associated with altered rosette dynamics, delayed radial intercalation, and formation of a small, fragmented surface epithelial structure. These results demonstrate that three-dimensional rosette behaviors translate mesenchymal-epithelial transitions into collective radial intercalation and epithelial formation, providing a strategy for building epithelial sheets from individual self-organizing units in the mammalian embryo.

Multiscale analysis of single and double maternal-zygotic Myh9 and Myh10 mutants during mouse preimplantation development

During the first days of mammalian development, the embryo forms the blastocyst, the structure responsible for implanting the mammalian embryo. Consisting of an epithelium enveloping the pluripotent inner cell mass and a fluid-filled lumen, the blastocyst results from a series of cleavage divisions, morphogenetic movements, and lineage specification. Recent studies have identified the essential role of actomyosin contractility in driving cytokinesis, morphogenesis, and fate specification, leading to the formation of the blastocyst. However, the preimplantation development of contractility mutants has not been characterized. Here, we generated single and double maternal-zygotic mutants of non-muscle myosin II heavy chains (NMHCs) to characterize them with multiscale imaging. We found that Myh9 (NMHC II-A) is the major NMHC during preimplantation development as its maternal-zygotic loss causes failed cytokinesis, increased duration of the cell cycle, weaker embryo compaction, and reduced differentiation, whereas Myh10 (NMHC II-B) maternal-zygotic loss is much less severe. Double maternal-zygotic mutants for Myh9 and Myh10 show a much stronger phenotype, failing most of the attempts of cytokinesis. We found that morphogenesis and fate specification are affected but nevertheless carry on in a timely fashion, regardless of the impact of the mutations on cell number. Strikingly, even when all cell divisions fail, the resulting single-celled embryo can initiate trophectoderm differentiation and lumen formation by accumulating fluid in increasingly large vacuoles. Therefore, contractility mutants reveal that fluid accumulation is a cell-autonomous process and that the preimplantation program carries on independently of successful cell division.

IRSp53 controls plasma membrane shape and polarized transport at the nascent lumen in epithelial tubules

It is unclear whether the establishment of apical–basal cell polarity during the generation of epithelial lumens requires molecules acting at the plasma membrane/actin interface. Here, we show that the I-BAR-containing IRSp53 protein controls lumen formation and the positioning of the polarity determinants aPKC and podocalyxin. Molecularly, IRSp53 acts by regulating the localization and activity of the small GTPase RAB35, and by interacting with the actin capping protein EPS8. Using correlative light and electron microscopy, we further show that IRSp53 ensures the shape and continuity of the opposing plasma membrane of two daughter cells, leading to the formation of a single apical lumen. Genetic removal of IRSp53 results in abnormal renal tubulogenesis, with altered tubular polarity and architectural organization. Thus, IRSp53 acts as a membrane curvature-sensing platform for the assembly of multi-protein complexes that control the trafficking of apical determinants and the integrity of the luminal plasma membrane.

The I-BAR protein IRSp53 senses membrane curvature but its physiological role is unclear. Here, the authors show that during early lumen morphogenesis, IRSp53 controls the shape of the apical plasma membrane and polarized trafficking and ensures the correct epithelial tubular architecture and if deleted, affects renal tubules morphogenesis in various organisms.

Temporal Coordination of Collective Migration and Lumen Formation by Antagonism between Two Nuclear Receptors

During development, cells undergo multiple, distinct morphogenetic processes to form a tissue or organ, but how their temporal order and time interval are determined remain poorly understood. Here we show that the nuclear receptors E75 and DHR3 regulate the temporal order and time interval between the collective migration and lumen formation of a coherent group of cells named border cells during Drosophila oogenesis. We show that E75, in response to ecdysone signaling, antagonizes the activity of DHR3 during border cell migration, and DHR3 is necessary and sufficient for the subsequent lumen formation that is critical for micropyle morphogenesis. DHR3's lumen-inducing function is mainly mediated through βFtz-f1, another nuclear receptor and transcription factor. Furthermore, both DHR3 and βFtz-f1 are required for chitin secretion into the lumen, whereas DHR3 is sufficient for chitin secretion. Lastly, DHR3 and βFtz-f1 suppress JNK signaling in the border cells to downregulate cell adhesion during lumen formation.

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E75 antagonizes DHR3's function in inducing lumen formation of border cells (BCs)

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E75 and DHR3 temporally coordinate collective migration and lumen formation of BCs

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DHR3 is required and sufficient for chitin secretion into the lumen

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DHR3 and βFtz-f1 downregulate JNK signaling and cell adhesion in the BCs

The role of cellular interactions in the induction of hepatocyte polarity and functional maturation in stem cell-derived hepatic cells

The unique microenvironment found within the liver in vivo plays a key role in the induction of functional maturation in the developing hepatocyte. During organogenesis, hepatocytes acquire a polar phenotype that allows them to perform their functions of bile production and transport, protein synthesis, metabolism, and detoxification simultaneously, independently, and efficiently. It is thought that the induction of polarity and functional maturation in hepatocytes is dependent on the complex interplay of cell-cell and cell-extracellular matrix (ECM) interactions. While this process is highly efficient in the human liver, it has been shown that hepatocytes rapidly lose their functions when placed in cell culture. This poses a challenge for the development of a bioartificial liver (BAL) support system, which utilizes a live cellular source to perform hepatic functions in the event of acute liver failure or primary nonfunction. However, once the molecular mechanisms underlying the induction of hepatocyte polarity are fully identified, it will be possible to develop highly functional hepatic cells from human pluripotent stem cells (hPSCs). This new cell line would be an ideal cellular source for a BAL system, as it would have both the functionality and longevity to support a patient through the entire clinical course of treatment. In this review, we explore the literature that has examined the potential mechanisms that induce polarity in the developing hepatocyte and discuss the future implications of this knowledge in a clinical setting from a bioengineering perspective.

Microtubule Motors in Establishment of Epithelial Cell Polarity

Epithelial cells play a key role in insuring physiological homeostasis by acting as a barrier between the outside environment and internal organs. They are also responsible for the vectorial transport of ions and fluid essential to the function of many organs. To accomplish these tasks, epithelial cells must generate an asymmetrically organized plasma membrane comprised of structurally and functionally distinct apical and basolateral membranes. Adherent and occluding junctions, respectively, anchor cells within a layer and prevent lateral diffusion of proteins in the outer leaflet of the plasma membrane and restrict passage of proteins and solutes through intercellular spaces. At a fundamental level, the establishment and maintenance of epithelial polarity requires that signals initiated at cell-substratum and cell-cell adhesions are transmitted appropriately and dynamically to the cytoskeleton, to the membrane-trafficking machinery, and to the regulation of occluding and adherent junctions. Rigorous descriptive and mechanistic studies published over the last 50 years have provided great detail to our understanding of epithelial polarization. Yet still, critical early steps in morphogenesis are not yet fully appreciated. In this review, we discuss how cytoskeletal motor proteins, primarily kinesins, contribute to coordinated modification of microtubule and actin arrays, formation and remodeling of cell adhesions to targeted membrane trafficking, and to initiating the formation and expansion of an apical lumen.

The world of epithelial sheets

An epithelium is a layer of closely connected cells covering the body or lining a body cavity. In this review, several fundamental questions are addressed regarding the epithelium. (i) While an epithelium functions as barrier against the external environment, how is barrier function maintained during its construction? (ii) What determines the apical and basal sides of epithelial layer? (iii) Is there any relationship between the apical side of the epithelium and the apical membrane of an epithelial cell? (iv) Why are hepatocytes (liver cells) called epithelial, even though they differ completely from column-like shape of typical epithelial cells? Keeping these questions in mind, multiple shapes of epithelia were considered, extracting a few of their elemental processes, and constructing a virtual world of epithelia by combining them. Epithelial cells were also classified into several types based on the number of apical domains of each cell. In addition, an intracellular organelle was introduced within epithelial cells, the vacuolar apical compartment (VAC), which is produced within epithelial cells surrounded by external cell matrix (ECM). The VAC interacts with areas of cell-cell contact of the cell surface membrane and is converted to apical membrane. The properties of VACs enable us to answer the initial questions posed above. Finally, the genetic and molecular mechanisms of epithelial morphogenesis are discussed.

Ric-8A, an activator protein of Gαi, controls mammalian epithelial cell polarity for tight junction assembly and cystogenesis

Correct cyst morphogenesis of epithelial cells requires apical-basal polarization, which is partly regulated by mitotic spindle orientation, a process dependent on the heterotrimeric G protein subunit Gαi and its binding protein LGN. Here, we show that in three-dimensional culture of mammalian epithelial Madin-Darby canine kidney (MDCK) cells, the Gαi-activating protein Ric-8A is crucial for orientation of the mitotic spindle and formation of normal cysts that comprise a single layer of polarized cells with their apical surfaces lining an inner lumen. Consistent with the involvement of LGN, cystogenesis can be well organized by ADP-ribosylated Gαi, retaining the ability to interact with LGN, but not by the interaction-defective mutant protein Gαi2 (N150I). In monolayer culture of MDCK cells, functional tight junction (TJ) assembly, a process associated with epithelial cell polarization, is significantly delayed in Ric-8A-depleted cells as well as in Gαi-depleted cells in a mitosis-independent manner. Ric-8A knockdown results in a delayed cortical delivery of Gαi and the apical membrane protein gp135, and an increased formation of intercellular lumens surrounded by membranes rich in Gαi3 and gp135. TJ development also involves LGN and its related protein AGS3. Thus, Ric-8A regulates mammalian epithelial cell polarity for TJ assembly and cystogenesis probably in concert with Gαi and LGN/AGS3.

The serine protease-mediated increase in intestinal epithelial barrier function is dependent on occludin and requires an intact tight junction

Barrier dysfunction is a characteristic of the inflammatory bowel diseases (IBD), Crohn's disease and ulcerative colitis. Understanding how the tight junction is modified to maintain barrier function may provide avenues for treatment of IBD. We have previously shown that the apical addition of serine proteases to intestinal epithelial cell lines causes a rapid and sustained increase in transepithelial electrical resistance (TER), but the mechanisms are unknown. We hypothesized that serine proteases increase barrier function through trafficking and insertion of tight junction proteins into the membrane, and this could enhance recovery of a disrupted monolayer after calcium switch or cytokine treatment. In the canine epithelial cell line, SCBN, we showed that matriptase, an endogenous serine protease, could potently increase TER. Using detergent solubility-based cell fractionation, we found that neither trypsin nor matriptase treatment changed levels of tight junction proteins at the membrane. In a fast calcium switch assay, serine proteases did not enhance the rate of recovery of the junction. In addition, serine proteases could not reverse barrier disruption induced by IFNγ and TNFα. We knocked down occludin in our cells using siRNA and found this prevented the serine protease-induced increase in TER. Using fluorescence recovery after photobleaching (FRAP), we found serine proteases induce a greater mobile fraction of occludin in the membrane. These data suggest that a functional tight junction is needed for serine proteases to have an effect on TER, and that occludin is a crucial tight junction protein in this mechanism.

Induction of lateral lumens through disruption of a monoleucine-based basolateral-sorting motif in betacellulin

Directed delivery of EGF receptor (EGFR) ligands to the apical or basolateral surface is a crucial regulatory step in the initiation of EGFR signaling in polarized epithelial cells. Herein, we show that the EGFR ligand betacellulin (BTC) is preferentially sorted to the basolateral surface of polarized MDCK cells. By using sequential truncations and site-directed mutagenesis within the BTC cytoplasmic domain, combined with selective cell-surface biotinylation and immunofluorescence, we have uncovered a monoleucine-based basolateral-sorting motif (EExxxL, specifically (156)EEMETL(161)). Disruption of this sorting motif led to equivalent apical and basolateral localization of BTC. Unlike other EGFR ligands, BTC mistrafficking induced formation of lateral lumens in polarized MDCK cells, and this process was significantly attenuated by inhibition of EGFR. Additionally, expression of a cancer-associated somatic BTC mutation (E156K) led to BTC mistrafficking and induced lateral lumens in MDCK cells. Overexpression of BTC, especially mistrafficking forms, increased the growth of MDCK cells. These results uncover a unique role for BTC mistrafficking in promoting epithelial reorganization.

Regulation of epithelial cell polarity by PAR-3 depends on Girdin transcription and Girdin-Gαi3 signaling

Epithelial apicobasal polarity has fundamental roles in epithelial physiology and morphogenesis. The PAR complex, comprising PAR-3, PAR-6 and atypical protein kinase C (aPKC), is involved in determining cell polarity in various biological contexts, including in epithelial cells. However, it is not fully understood how the PAR complex induces apicobasal polarity. In this study, we found that PAR-3 regulates the protein expression of Girdin (also known as GIV or CCDC88A), a guanine-nucleotide-exchange factor (GEF) for heterotrimeric Gαi subunits, at the transcriptional level by cooperating with the AP-2 transcription factor. In addition, we confirmed that PAR-3 physically interacts with Girdin, and show that Girdin, together with the Gαi3 (also known as GNAI3), controls tight junction formation, apical domain development and actin organization downstream of PAR-3. Taken together, our findings suggest that transcriptional upregulation of Girdin expression and Girdin-Gαi3 signaling play crucial roles in regulating epithelial apicobasal polarity through the PAR complex.

Hepatocyte polarity

Hepatocytes, like other epithelia, are situated at the interface between the organism's exterior and the underlying internal milieu and organize the vectorial exchange of macromolecules between these two spaces. To mediate this function, epithelial cells, including hepatocytes, are polarized with distinct luminal domains that are separated by tight junctions from lateral domains engaged in cell-cell adhesion and from basal domains that interact with the underlying extracellular matrix. Despite these universal principles, hepatocytes distinguish themselves from other nonstriated epithelia by their multipolar organization. Each hepatocyte participates in multiple, narrow lumina, the bile canaliculi, and has multiple basal surfaces that face the endothelial lining. Hepatocytes also differ in the mechanism of luminal protein trafficking from other epithelia studied. They lack polarized protein secretion to the luminal domain and target single-spanning and glycosylphosphatidylinositol-anchored bile canalicular membrane proteins via transcytosis from the basolateral domain. We compare this unique hepatic polarity phenotype with that of the more common columnar epithelial organization and review our current knowledge of the signaling mechanisms and the organization of polarized protein trafficking that govern the establishment and maintenance of hepatic polarity. The serine/threonine kinase LKB1, which is activated by the bile acid taurocholate and, in turn, activates adenosine monophosphate kinase-related kinases including AMPK1/2 and Par1 paralogues has emerged as a key determinant of hepatic polarity. We propose that the absence of a hepatocyte basal lamina and differences in cell-cell adhesion signaling that determine the positioning of tight junctions are two crucial determinants for the distinct hepatic and columnar polarity phenotypes.

An overview and online registry of microvillus inclusion disease patients and their MYO5B mutations

Microvillus inclusion disease (MVID) is one of the most severe congenital intestinal disorders and is characterized by neonatal secretory diarrhea and the inability to absorb nutrients from the intestinal lumen. MVID is associated with patient-, family-, and ancestry-unique mutations in the MYO5B gene, encoding the actin-based motor protein myosin Vb. Here, we review the MYO5B gene and all currently known MYO5B mutations and for the first time methodologically categorize these with regard to functional protein domains and recurrence in MYO7A associated with Usher syndrome and other myosins. We also review animal models for MVID and the latest data on functional studies related to the myosin Vb protein. To congregate existing and future information on MVID geno-/phenotypes and facilitate its quick and easy sharing among clinicians and researchers, we have constructed an online MOLGENIS-based international patient registry (www.MVID-central.org). This easily accessible database currently contains detailed information of 137 MVID patients together with reported clinical/phenotypic details and 41 unique MYO5B mutations, of which several unpublished. The future expansion and prospective nature of this registry is expected to improve disease diagnosis, prognosis, and genetic counseling.

Vesicular sorting controls the polarity of expanding membranes in the C. elegans intestine

Biological tubes consist of polarized epithelial cells with apical membranes building the central lumen and basolateral membranes contacting adjacent cells or the extracellular matrix. Cellular polarity requires distinct inputs from outside the cell, e.g., the matrix, inside the cell, e.g., vesicular trafficking and the plasma membrane and its junctions.¹ Many highly conserved polarity cues have been identified, but their integration during the complex process of polarized tissue and organ morphogenesis is not well understood. It is assumed that plasma-membrane-associated polarity determinants, such as the partitioning-defective (PAR) complex, define plasma membrane domain identities, whereas vesicular trafficking delivers membrane components to these domains, but lacks the ability to define them. In vitro studies on lumenal membrane biogenesis in mammalian cell lines now indicate that trafficking could contribute to defining membrane domains by targeting the polarity determinants, e.g., the PARs, themselves.² This possibility suggests a mechanism for PARs’ asymmetric distribution on membranes and places vesicle-associated polarity cues upstream of membrane-associated polarity determinants. In such an upstream position, trafficking might even direct multiple membrane components, not only polarity determinants, an original concept of polarized plasma membrane biogenesis^3,4that was largely abandoned due to the failure to identify a molecularly defined intrinsic vesicular sorting mechanism. Our two recent studies on C. elegans intestinal tubulogenesis reveal that glycosphingolipids (GSLs) and the well-recognized vesicle components clathrin and its AP-1 adaptor are required for targeting multiple apical molecules, including polarity regulators, to the expanding apical/lumenal membrane.^5,6 These findings support GSLs’ long-proposed role in in vivo polarized epithelial membrane biogenesis and development and identify a novel function in apical polarity for classical post-Golgi vesicle components. They are also compatible with a vesicle-intrinsic sorting mechanism during membrane biogenesis and suggest a model for how vesicles could acquire apical directionality during the assembly of the functionally critical polarized lumenal surfaces of epithelial tubes.

Apical vacuole formation by gastric parietal cells in primary culture: effect of low extracellular Ca2+

In primary culture, the gastric parietal cell's deeply invaginated apical membrane, seen in microscopy by phalloidin binding to F-actin (concentrated in microvilli and a subapical web), is engulfed into the cell, separated from the basolateral membrane (which then becomes the complete plasma membrane), and converted, from a lacy interconnected system of canaliculi, into several separate vacuoles. In this study, vacuolar morphology was achieved by 71% of parietal cells 8 h after typical collagenase digestion of rabbit gastric mucosa, but the tight-junctional protein zonula occludens-1 (ZO-1) was completely delocalized after ∼2 h, when cells were ready for culturing. Use of low-Ca(2+) medium (4 mM EGTA) to release cells quickly from gastric glands yielded parietal cells in which ZO-1 was seen in a small spot or ring, a localization quickly lost if these cells were then cultured in normal Ca(2+) but remaining up to 20 h if they were cultured in low Ca(2+). The cells in low Ca(2+) mostly retained, at 20 h, an intermediate morphology of many bulbous canalicular expansions ("prevacuoles"), seemingly with narrow interconnections. Histamine stimulation of 20-h cells with intermediate morphology caused colocalization of proton-pumping H-K-ATPase with canaliculi and prevacuoles but little swelling of those structures, consistent with a remaining apical pore through which secreted acid could escape. Apparent canalicular interconnections, lack of stimulated swelling, and lingering ZO-1 staining indicate inhibition of membrane fission processes that separate apical from basolateral membrane and vacuoles from each other, suggesting an important role for extracellular Ca(2+) in these, and possibly other, endocytotic processes.

The KIBRA-aPKC connection: A potential regulator of membrane trafficking and cell polarity

The kidney and brain protein (KIBRA) is a scaffold or an adaptor-like protein with WW, C2-like and atypical protein kinase C (aPKC)-binding domains. Genetic studies in Drosophila revealed that KIBRA is an upstream regulator of the conserved Hippo pathway, which is implicated in organ size determination. In addition, genome-wide studies revealed an association between the single nucleotide polymorphism in the KIBRA gene locus and human episodic memory performance. However, the mechanism of action through which KIBRA is linked to these functions remains poorly understood. Recent studies on the biochemical and cellular properties of KIBRA reveal the role of KIBRA as a regulator of membrane trafficking. Further, KIBRA directly inhibits the activity of the cell polarity regulator, aPKC, which is required for apical protein exocytosis. Here, we discuss how this KIBRA-aPKC connection, a potential regulator of membrane trafficking and cell polarity, can contribute to the recently discovered functions of KIBRA.

Endothelial development taking shape

Blood vessel development is a vital process during embryonic development, during tissue growth, regeneration and disease processes in the adult. In the past decade researchers have begun to unravel basic molecular mechanisms that regulate the formation of vascular lumen, sprouting angiogenesis, fusion of vessels, and pruning of the vascular plexus. The understanding of the biology of these angiogenic processes is increasingly driven through studies on vascular development at the cellular resolution. Single cell analysis in vivo, advanced genetic tools and the widespread use of powerful animal models combined with improved imaging possibilities are delivering new insights into endothelial cell form, function and behavior angiogenesis. Moreover, the combination of in silico modeling and experimentation including dynamic imaging promotes insights into higher level cooperative behavior leading to functional patterning of vascular networks. Here we summarize recent concepts and advances in the field of vascular development, focusing in detail on the endothelial cell.

Lowe Syndrome protein OCRL1 supports maturation of polarized epithelial cells

Mutations in the inositol polyphosphate 5-phosphatase OCRL1 cause Lowe Syndrome, leading to cataracts, mental retardation and renal failure. We noted that cell types affected in Lowe Syndrome are highly polarized, and therefore we studied OCRL1 in epithelial cells as they mature from isolated individual cells into polarized sheets and cysts with extensive communication between neighbouring cells. We show that a proportion of OCRL1 targets intercellular junctions at the early stages of their formation, co-localizing both with adherens junctional components and with tight junctional components. Correlating with this distribution, OCRL1 forms complexes with junctional components α-catenin and zonula occludens (ZO)-1/2/3. Depletion of OCRL1 in epithelial cells growing as a sheet inhibits maturation; cells remain flat, fail to polarize apical markers and also show reduced proliferation. The effect on shape is reverted by re-expressed OCRL1 and requires the 5'-phosphatase domain, indicating that down-regulation of 5-phosphorylated inositides is necessary for epithelial development. The effect of OCRL1 in epithelial maturation is seen more strongly in 3-dimensional cultures, where epithelial cells lacking OCRL1 not only fail to form a central lumen, but also do not have the correct intracellular distribution of ZO-1, suggesting that OCRL1 functions early in the maturation of intercellular junctions when cells grow as cysts. A role of OCRL1 in junctions of polarized cells may explain the pattern of organs affected in Lowe Syndrome.

Tubulogenesis during blood vessel formation

The ability to form and maintain a functional system of contiguous hollow tubes is a critical feature of vascular endothelial cells (ECs). Lumen formation, or tubulogenesis, occurs in blood vessels during both vasculogenesis and angiogenesis in the embryo. Formation of vascular lumens takes place prior to the establishment of blood flow and to vascular remodeling which results in a characteristic hierarchical vessel organization. While epithelial lumen formation has received intense attention in past decades, more recent work has only just begun to elucidate the mechanisms controlling the initiation and morphogenesis of endothelial lumens. Studies using in vitro and in vivo models, including zebrafish and mammals, are beginning to paint an emerging picture of how blood vessels establish their characteristic morphology and become patent. In this article, we review and discuss the molecular and cellular mechanisms driving the formation of vascular tubes, primarily in vivo, and we compare and contrast proposed models for blood vessel lumen formation.

KIBRA suppresses apical exocytosis through inhibition of aPKC kinase activity in epithelial cells

Epithelial cells possess apical-basolateral polarity and form tight junctions (TJs) at the apical-lateral border, separating apical and basolateral membrane domains. The PAR3-aPKC-PAR6 complex plays a central role in TJ formation and apical domain development during tissue morphogenesis. Inactivation and overactivation of aPKC kinase activity disrupts membrane polarity. The mechanism that suppresses active aPKC is unknown. KIBRA, an upstream regulator of the Hippo pathway, regulates tissue size in Drosophila and can bind to aPKC. However, the relationship between KIBRA and the PAR3-aPKC-PAR6 complex remains unknown. We report that KIBRA binds to the PAR3-aPKC-PAR6 complex and localizes at TJs and apical domains in epithelial tissues and cells. The knockdown of KIBRA causes expansion of the apical domain in MDCK three-dimensional cysts and suppresses the formation of apical-containing vacuoles through enhanced de novo apical exocytosis. These phenotypes are restored by inhibition of aPKC. In addition, KIBRA directly inhibits the kinase activity of aPKC in vitro. These results strongly support the notion that KIBRA regulates epithelial cell polarity by suppressing apical exocytosis through direct inhibition of aPKC kinase activity in the PAR3-aPKC-PAR6 complex.

The serine/threonine kinase Par1b regulates epithelial lumen polarity via IRSp53-mediated cell-ECM signaling

Par1b regulates cell–ECM signaling and dictates epithelial lumenal organization by targeting IRSp53, a Rho GTPase adaptor and scaffolding protein.

The serine/threonine kinase Par1b promotes cell–cell adhesion and determines the polarity of the luminal domain in epithelial cells. In this study, we demonstrate that Par1b also regulates cell–extracellular matrix (ECM) signaling in kidney-derived Madin–Darby canine kidney (MDCK) cells and identified the rho–guanosine triphosphatase adaptor and scaffolding protein IRSp53 as a Par1b substrate involved in this pathway. Par1b overexpression inhibits basal lamina formation, cell spreading, focal adhesion, stress fiber formation, and compaction, whereas Par1b depletion has the opposite effect. IRSp53 depletion mimics Par1b overexpression on cell–ECM signaling and lumen polarity but had no effect on adherens junction formation. Par1b directly phosphorylates IRSp53 on S366 in cell lysates and stimulates phosphorylation on S453/3/5 via an indirect mechanism. A Par1b phosphorylation–deficient IRSp53 mutant but not the wild-type protein efficiently rescues both the cell spreading and the lumen polarity defects in Par1b MDCK cells. Our data suggest a model in which Par1b phosphorylation prevents recruitment of IRSp53 effector proteins to its Src homology domain 3 by promoting 14-3-3 binding in the vicinity of that domain.

Loss-of-function of MYO5B is the main cause of microvillus inclusion disease: 15 novel mutations and a CaCo-2 RNAi cell model

Autosomal recessive microvillus inclusion disease (MVID) is characterized by an intractable diarrhea starting within the first few weeks of life. The hallmarks of MVID are a lack of microvilli on the surface of villous enterocytes, occurrence of intracellular vacuoles lined by microvilli (microvillus inclusions), and the cytoplasmic accumulation of periodic acid-Schiff (PAS)-positive vesicles in enterocytes. Recently, we identified mutations in MYO5B, encoding the unconventional type Vb myosin motor protein, in a first cohort of nine MVID patients. In this study, we identified 15 novel nonsense and missense mutations in MYO5B in 11 unrelated MVID patients. Fluorescence microscopy, Western blotting, and electron microscopy were applied to analyze the effects of MYO5B siRNA knock-down in polarized, brush border possessing CaCo-2 cells. Loss of surface microvilli, increased formation of microvillus inclusions, and subapical enrichment of PAS-positive endomembrane compartments were induced in polarized, filter-grown CaCo-2 cells, following MYO5B knock-down. Our data indicate that MYO5B mutations are a major cause of microvillus inclusion disease and that MYO5B knock-down recapitulates most of the cellular phenotype in vitro, thus independently showing loss of MYO5B function as the cause of microvillus inclusion disease.

ASPP2 regulates epithelial cell polarity through the PAR complex

The PAR complex, consisting of the evolutionarily conserved PAR-3, PAR-6, and aPKC, regulates cell polarity in many cell types, including epithelial cells [1-4]. Consistently, genetic manipulation of its components affects tissue integrity in multiple biological systems [5-9]. However, the regulatory mechanisms of the PAR complex remain obscure. We report here that apoptosis-stimulating protein of p53 (ASPP2 or TP53BP2), which binds to the tumor suppressor p53 and stimulates its proapoptotic function [10-12], positively regulates epithelial cell polarity by associating with the PAR complex. ASPP2 interacts and colocalizes with PAR-3 at apical cell-cell junctions in the polarized epithelial cells. Depletion of ASPP2 in epithelial cells causes defects in cell polarity, such as the formation of tight junctions and the maintenance and development of apical membrane domains. Moreover, depletion of ASPP2 causes a defect in PAR-3 localization, as well as vice versa. Furthermore, disturbance in the interaction between ASPP2 and PAR-3 causes defects in cell polarity. We conclude that ASPP2 regulates epithelial cell polarity in cooperation with PAR-3 to form an active PAR complex. Our results, taken together with the known functions of ASPP2, suggest a close relationship between cell polarity and other cell regulatory mechanisms mediated by ASPP2.

The Cdc42 GEF Intersectin 2 controls mitotic spindle orientation to form the lumen during epithelial morphogenesis

Epithelial organs are made of tubes and cavities lined by a monolayer of polarized cells that enclose the central lumen. Lumen formation is a crucial step in the formation of epithelial organs. The Rho guanosine triphosphatase (GTPase) Cdc42, which is a master regulator of cell polarity, regulates the formation of the central lumen in epithelial morphogenesis. However, how Cdc42 is regulated during this process is still poorly understood. Guanine nucleotide exchange factors (GEFs) control the activation of small GTPases. Using the three-dimensional Madin-Darby canine kidney model, we have identified a Cdc42-specific GEF, Intersectin 2 (ITSN2), which localizes to the centrosomes and regulates Cdc42 activation during epithelial morphogenesis. Silencing of either Cdc42 or ITSN2 disrupts the correct orientation of the mitotic spindle and normal lumen formation, suggesting a direct relationship between these processes. Furthermore, we demonstrated this direct relationship using LGN, a component of the machinery for mitotic spindle positioning, whose disruption also results in lumen formation defects.

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

Cell-cell interactions influence epithelial morphogenesis through an interplay between cell adhesion, trafficking and the cytoskeleton. These cellular processes are coordinated, often by cell signals found at cell-cell contacts. One such contact-based signal is the phosphatidylinositol 3'-kinase (PI3-kinase; PI3K) pathway. PI3-kinase is best understood for its role in mitogenic signalling, where it regulates cell survival, proliferation and differentiation. Its precise morphogenetic impacts in epithelia are, in contrast, less well-understood. Using phosphoinositide-specific biosensors we confirmed that E-cadherin-based cell-cell contacts are enriched in PIP(3), the principal product of PI3-kinase. We then used pharmacologic inhibitors to assess the morphogenetic impact of PI3-kinase in MDCK and MCF7 monolayers. We found that inhibiting PI3-kinase caused a reduction in epithelial cell height that was reversible upon removal of the drugs. This was not attributable to changes in E-cadherin expression or homophilic adhesion. Nor were there detectable changes in cell polarity. While Myosin II has been implicated in regulating keratinocyte height, we found no effect of PI3-kinase inhibition on apparent Myosin II activity; nor did direct inhibition of Myosin II alter epithelial height. Instead, in pursuing signalling pathways downstream of PI3-kinase we found that blocking Rac signalling, but not mTOR, reduced epithelial cell height, as did PI3-kinase inhibition. Overall, our findings suggest that PI3-kinase exerts a major morphogenetic impact in simple cultured epithelia through preservation of cell height. This is independent of potential effects on adhesion or polarity, but may occur through PI3-kinase-stimulated Rac signaling.

Apical trafficking in epithelial cells: signals, clusters and motors

In the early days of epithelial cell biology, researchers working with kidney and/or intestinal epithelial cell lines and with hepatocytes described the biosynthetic and recycling routes followed by apical and basolateral plasma membrane (PM) proteins. They identified the trans-Golgi network and recycling endosomes as the compartments that carried out apical-basolateral sorting. They described complex apical sorting signals that promoted association with lipid rafts, and simpler basolateral sorting signals resembling clathrin-coated-pit endocytic motifs. They also noticed that different epithelial cell types routed their apical PM proteins very differently, using either a vectorial (direct) route or a transcytotic (indirect) route. Although these original observations have generally held up, recent studies have revealed interesting complexities in the routes taken by apically destined proteins and have extended our understanding of the machinery required to sustain these elaborate sorting pathways. Here, we critically review the current status of apical trafficking mechanisms and discuss a model in which clustering is required to recruit apical trafficking machineries. Uncovering the mechanisms responsible for polarized trafficking and their epithelial-specific variations will help understand how epithelial functional diversity is generated and the pathogenesis of many human diseases.

Shape meets polarity in Japan

A diverse group of developmental biologists who study cell polarity gathered in late March 2009 at the RIKEN Center for Developmental Biology in Kobe, Japan, for a symposium entitled ;Shape and Polarity'. The organizers, Masatoshi Takeichi, Fumio Matsuzaki, Hitoshi Sawa [RIKEN Center for Developmental Biology (CDB), Kobe, Japan] and Carl-Philipp Heisenberg (Max Planck Institute, Dresden, Germany), put together an engaging program that highlighted recent progress towards understanding the mechanisms of cell polarization during development, and the functions of cell polarity in shaping development.

Apical lumen formation in renal epithelia

The ability to form epithelial lumina is a central architectural characteristic of virtually all organs and indispensable for their function. Ontogenetically, the kidney is one of the best-characterized organs, but concepts of the regulated formation of its hollow epithelial structures are still emerging. Epithelial cell lines provide the opportunity to study molecular mechanisms in simplified assays modeling cyst and tube formation. In these systems, several groups have identified molecules implicated in lumen formation, and their downregulation results in either multiple-lumen or no-lumen phenotypes. On the basis of these phenotypes, we propose a working model, assigning proteins to groups with similar functions. Defects within these specific protein groups lead to distinct epithelial phenotypes. Studies of mesenchymal-to-epithelial transition underline the importance of these protein groups, but converting these basic models of lumen formation to an understanding of the mesenchymal to tubule formation during kidney development is still challenging.

Interaction between PAR-3 and the aPKC-PAR-6 complex is indispensable for apical domain development of epithelial cells

The evolutionarily conserved polarity proteins PAR-3, atypical protein kinase C (aPKC) and PAR-6 critically regulate the apical membrane development required for epithelial organ development. However, the molecular mechanisms underlying their roles remain to be clarified. We demonstrate that PAR-3 knockdown in MDCK cells retards apical protein delivery to the plasma membrane, and eventually leads to mislocalized apical domain formation at intercellular regions in both two-dimensional and three-dimensional culture systems. The defects in PAR-3 knockdown cells are efficiently rescued by wild-type PAR-3, but not by a point mutant (S827/829A) that lacks the ability to interact with aPKC, indicating that formation of the PAR-3-aPKC-PAR-6 complex is essential for apical membrane development. This is in sharp contrast with tight junction maturation, which does not necessarily depend on the aPKC-PAR-3 interaction, and indicates that the two fundamental processes essential for epithelial polarity are differentially regulated by these polarity proteins. Importantly, highly depolarized cells accumulate aPKC and PAR-6, but not PAR-3, on apical protein-containing vacuoles, which become targeted to PAR-3-positive primordial cell-cell contact sites during the initial stage of the repolarization process. Therefore, formation of the PAR-3-aPKC-PAR-6 complex might be required for targeting of not only the aPKC-PAR-6 complex but also of apical protein carrier vesicles to primordial junction structures.

ROCK-mediated contractility, tight junctions and channels contribute to the conversion of a preapical patch into apical surface during isochoric lumen initiation

Epithelial cells assemble into three-dimensional aggregates to generate lumen-containing organ substructures. Cells therein contact the extracellular matrix with their basal surface, neighbouring cells with their contact surface and the lumen with their apical surface. We investigated the development of single MDCK cells into aggregates with lumen using quantitative live-cell imaging to identify morphogenetic rules for lumen formation. In two-cell aggregates, membrane insertion into the contact surface established a preapical patch (PAP) characterized by the presence of the apical marker gp135, microvilli and the absence of E-cadherin. This PAP originated from a compartment that had hallmarks of an apical recycling endosome, and matured through Brefeldin-A-sensitive membrane trafficking and the establishment of tight junctions around itself. As a result of the activity of water and ion channels, an optically resolvable lumen formed. Initially, this lumen enlarged without changes in aggregate volume or cell number but with decreasing cell volumes. Additionally, the ROCK1/2-myosin-II pathway counteracted PAP and lumen formation. Thus, lumen formation results from PAP establishment, PAP maturation, lumen initiation and lumen enlargement. These phases correlate with distinct cell surface and volume patterns, which suggests that such morphometric parameters are regulated by trafficking, ROCK-mediated contractility and hydrostatic pressure or vice versa.

Tight junction and polarity interaction in the transporting epithelial phenotype

Development of tight junctions and cell polarity in epithelial cells requires a complex cellular machinery to execute an internal program in response to ambient cues. Tight junctions, a product of this machinery, can act as gates of the paracellular pathway, fences that keep the identity of plasma membrane domains, bridges that communicate neighboring cells. The polarization internal program and machinery are conserved in yeast, worms, flies and mammals, and in cell types as different as epithelia, neurons and lymphocytes. Polarization and tight junctions are dynamic features that change during development, in response to physiological and pharmacological challenges and in pathological situations like infection.

Stimulus-induced reorganization of tight junction structure: the role of membrane traffic

The tight junction forms a barrier that limits paracellular movement of water, ions, and macromolecules. The permeability properties of this barrier are regulated in response to both physiological and pathophysiological stimuli, and this regulation has been modeled by pharmacological agents. Although it is now known that vesicular traffic plays important roles in tight junction assembly, the molecular mechanisms by which vesicular traffic contributes to tight junction regulation remain to be defined. This review summarizes recent progress in understanding mechanisms and pathways of tight junction protein internalization and the relevance of these to tight junction regulation.

Cell polarity development and protein trafficking in hepatocytes lacking E-cadherin/beta-catenin-based adherens junctions

Using a mutant hepatocyte cell line in which E-cadherin and beta-catenin are completely depleted from the cell surface, and, consequently, fail to form adherens junctions, we have investigated adherens junction requirement for apical-basolateral polarity development and polarized membrane trafficking. It is shown that these hepatocytes retain the capacity to form functional tight junctions, develop full apical-basolateral cell polarity, and assemble a subapical cortical F-actin network, although with a noted delay and a defect in subsequent apical lumen remodeling. Interestingly, whereas hepatocytes typically target the plasma membrane protein dipeptidyl peptidase IV first to the basolateral surface, followed by its transcytosis to the apical domain, hepatocytes lacking E-cadherin-based adherens junctions target dipeptidyl peptidase IV directly to the apical surface. Basolateral surface-directed transport of other proteins or lipids tested was not visibly affected in hepatocytes lacking E-cadherin-based adherens junctions. Together, our data show that E-cadherin/beta-catenin-based adherens junctions are dispensable for tight junction formation and apical lumen biogenesis but not for apical lumen remodeling. In addition, we suggest a possible requirement for E-cadherin/beta-catenin-based adherens junctions with regard to the indirect apical trafficking of specific proteins in hepatocytes.

Par1b promotes hepatic-type lumen polarity in Madin Darby canine kidney cells via myosin II- and E-cadherin-dependent signaling

Kidney-derived Madin Darby canine kidney (MDCK) cells form lumina at their apices, and target luminal proteins to an intracellular vacuolar apical compartment (VAC) when prevented from polarizing. Hepatocytes, by contrast, organize their luminal surfaces (the bile canaliculi; BC) between their lateral membranes, and, when nonpolarized, they display an intracellular luminal compartment that is distinct from the VACs of MDCK cells. Overexpression of the serine/threonine kinase Par1b/EMK1/MARK2 induces BC-like lateral lumina and a hepatic-type intracellular luminal compartment in MDCK cells, suggesting a role for Par1b in the branching decision between kidney- and hepatic-type epithelial phenotypes. Here, we report that Par1b promotes lateral lumen polarity in MDCK cells independently of Ca(2+)-mediated cell-cell adhesion by inhibiting myosin II in a rho kinase-dependent manner. Polarization was inhibited by E-cadherin depletion but promoted by an adhesion-defective E-cadherin mutant. By contrast, apical surface formation in control MDCK cells required Ca(2+)-dependent cell-cell adhesion, but it occurred in the absence of E-cadherin. We propose that E-cadherin, when in an adhesion-incompetent state at the lateral domain, serves as targeting patch for the establishment of lateral luminal surfaces. E-cadherin depletion also reverted the hepatic-type intracellular luminal compartment in Par1b-MDCK cells to VACs characteristic of control MDCK cells, indicating a novel link between E-cadherin and luminal protein targeting.

Defects in retinal pigment epithelium cell proliferation and retinal attachment in mutant mice with p27(Kip1) gene ablation

PURPOSE

Little is known about the mechanisms that regulate cell cycle withdrawal of the retinal pigment epithelium (RPE) during development, or about the mechanisms maintaining epithelial cell quiescence in adult retinas. The present study examines the potential role of the negative cell cycle regulator p27(Kip1) in controlling RPE proliferation, using mice with targeted ablation of the p27(Kip1) gene.

METHODS

Ocular tissues were obtained from wild-type and p27(Kip1)-null mice at several postnatal ages. Following aldehyde fixation, eyes were processed intact for JB-4 histology and electron microscopy. Alternatively, tissues were removed by manual or enzymatic dissection in order to obtain flat-mounts of the RPE attached to either the choroid-sclera or neural retina, respectively. Epithelial flat-mounts were either left unlabeled, in which case melanin pigment provided internal contrast, or labeled with Alexa Fluor 488-phalloidin and propidium iodide to visualize cell boundaries and nuclei, respectively.

RESULTS

Morphometric analysis using transverse plastic sections revealed a 96% increase in nuclear density and a 12% increase in thickness (apical to basal) for mutant vs. normal epithelia at postnatal day 35 (P35). These changes were not restricted to central or peripheral regions, and were uncorrelated with focal areas of dysplasia seen in the mutant neural retina. When similar tissues were viewed as flat-mounts, an observed 100% increase in nuclear density was accompanied by only a 46% enhancement in cellular density. This resulted in a larger proportion of multinucleated cells in the nullizygous RPE as compared with the wild-type epithelium (91 versus 47%). Such a pattern was achieved relatively early in development since, at P7 when the increase in RPE nuclear density was essentially complete, cellular density was augmented by only 39%. In addition to these proliferative changes, individual epithelial cells sometimes exhibited structural abnormalities, including an altered cortical actin cytoskeleton and displacement of nuclei from their normal central position. Surprisingly, while the RPE cells of null animals were similar ultrastructurally to those of the wild-type, interdigitation of their microvillous processes with outer segments was incomplete. Quantitative analysis revealed that such areas of detachment characterize, on average, 42% of the nullizygous retina, and that there is little correlation between detachment and neural retina dysplasia from one eye to another. Together with parallel evidence demonstrating a substantial decline in the apparent adhesiveness of mutant retinas relative to the normal tissue, the data is strongly indicative of an altered epithelium-photoreceptor interaction following gene ablation.

CONCLUSIONS

The absence of a functional p27(Kip1) gene results in enhanced RPE nuclear division, without a commensurate increase in cell division. Although the mutant epithelium as a whole appears structurally normal, individual cells exhibit cytoskeletal changes and their interaction with the neural retina is compromised.

Tight junctions and cell polarity

The tight junction is an intracellular junctional structure that mediates adhesion between epithelial cells and is required for epithelial cell function. Tight junctions control paracellular permeability across epithelial cell sheets and also serve as a barrier to intramembrane diffusion of components between a cell's apical and basolateral membrane domains. Recent genetic and biochemical studies in invertebrates and vertebrates indicate that tight junction proteins play an important role in the establishment and maintenance of apico-basal polarity. Proteins involved in epithelial cell polarization form evolutionarily conserved multiprotein complexes at the tight junction, and these protein complexes regulate the architecture of epithelia throughout the polarization process. Accumulating information regarding the regulation of these polarity proteins will lead to a better understanding of the molecular mechanisms whereby cell polarity is established.

Lgl mediates apical domain disassembly by suppressing the PAR-3-aPKC-PAR-6 complex to orient apical membrane polarity

The basolateral tumor suppressor protein Lgl is important for the regulation of epithelial cell polarity and tissue morphology. Recent studies have shown a physical and functional interaction of Lgl with another polarity-regulating protein machinery, the apical PAR-3-aPKC-PAR-6 complex, in epithelial cells. However, the mechanism of Lgl-mediated regulation of epithelial cell polarity remains obscure. By an siRNA method, we here show that endogenous Lgl is required for the disassembly of apical membrane domains in depolarizing MDCK cells induced by Ca2+ depletion. Importantly, this Lgl function is mediated by the suppression of the apical PAR-3-aPKC-PAR-6 complex activity. Analysis using 2D- or 3D-cultured cells in collagen gel suggests the importance of this suppressive regulation of Lgl on the collagen-mediated re-establishment of apical membrane domains and lumen formation. These results indicate that basolateral Lgl plays a crucial role in the disassembly of apical membrane domains to induce the orientation of apical membrane polarity, which is mediated by the suppression of apical PAR-3-aPKC-PAR-6 complex activity.

Microvilli defects in retinas of ezrin knockout mice

Ezrin, a member of the ezrin/moesin/radixin (ERM) family, localizes to microvilli of epithelia in vivo, where it functions as a bridge between actin filaments and plasma membrane proteins. In the eye, ezrin has been localized to both apical microvilli of Müller cells and retinal pigment epithelium (RPE) apical microvilli and basal infoldings. In the present study, we analyze these structures in the eyes of early postnatal ezrin knockout mice. This analysis indicates that the loss of ezrin leads to substantial reductions in the apical microvilli and basal infoldings in RPE cells and in the Müller cell apical microvilli. The absence of apical microvilli in the RPE is accompanied by the presence of microvilli-like inclusions (MIs) in the RPE cytoplasm. Finally, photoreceptors in the ezrin knockout animals show substantial retardation in development as compared to their wild type littermates.

Rab11a and myosin Vb are required for bile canalicular formation in WIF-B9 cells

Hepatocytes polarize by forming functionally distinct sinusoidal (basolateral) and canalicular (apical) plasma membrane domains. Two distinct routes are used for delivery of membrane proteins to the canaliculus. Proteins having glycosylphosphatidylinositol anchors or single transmembrane domains are targeted to the sinusoidal plasma membrane from where they transcytose to the canalicular domain. In contrast, apical ATP-binding-cassette (ABC) transporters, which are required for energy-dependent biliary secretion of bile acids (ABCB11), phospholipids (ABCB4), and nonbile acid organic anions (ABCC2), lack initial residence in the basolateral plasma membrane and traffic directly from Golgi membranes to the canalicular membrane. While investigating mechanisms of apical targeting in WIF-B9 cells, a polarized hepatic epithelial cell line, we observed that rab11a is required for canalicular formation. Knockdown of rab11a or overexpression of the rab11a-GDP locked form prevented canalicular formation as did overexpression of the myosin Vb motorless tail domain. In WIF-B9 cells, which lack bile canaliculi, apical ABC transporters colocalized with transcytotic membrane proteins in rab11a-containing endosomes and, unlike the transcytotic markers, did not distribute to the plasma membrane. We propose that polarization of hepatocytes (i.e., canalicular biogenesis) requires recruitment of rab11a and myosin Vb to intracellular membranes that contain apical ABC transporters and transcytotic markers, permitting their targeting to the plasma membrane. In this model, polarization is initiated upon delivery of rab11a-myosin Vb-containing membranes to the surface, which causes plasma membrane at the site of delivery to differentiate into apical domain (bile canaliculus).

PATJ regulates tight junction formation and polarity in mammalian epithelial cells

Recent studies have revealed an important role for tight junction protein complexes in epithelial cell polarity. One of these complexes contains the apical transmembrane protein, Crumbs, and two PSD95/discs large/zonula occludens domain proteins, protein associated with Lin seven 1 (PALS1)/Stardust and PALS1-associated tight junction protein (PATJ). Although Crumbs and PALS1/Stardust are known to be important for cell polarization, recent studies have suggested that Drosophila PATJ is not essential and its function is unclear. Here, we find that PATJ is targeted to the apical region and tight junctions once cell polarization is initiated. We show using RNAi techniques that reduction in PATJ expression leads to delayed tight junction formation as well as defects in cell polarization. These effects are reversed by reintroduction of PATJ into these RNAi cells. This study provides new functional information on PATJ as a polarity protein and increases our understanding of the Crumbs–PALS1–PATJ complex function in epithelial polarity.

Organization of vesicular trafficking in epithelia

Experiments using mammalian epithelial cell lines have elucidated biosynthetic and recycling pathways for apical and basolateral plasma-membrane proteins, and have identified components that guide apical and basolateral proteins along these pathways. These components include apical and basolateral sorting signals, adaptors for basolateral signals, and docking and fusion proteins for vesicular trafficking. Recent live-cell-imaging studies provide a real-time view of sorting processes in epithelial cells, including key roles for actin, microtubules and motors in the organization of post-Golgi trafficking.