During oogenesis in Drosophila melanogaster, the cells in the follicular epithelium of the ovary undergo a transition from a cuboidal to a squamous shape. In this issue, Gomez et al. (2012. J. Cell Biol.
http://dx.doi.org/10.1083/jcb.201207150) show that the kinase Tao promotes the endocytosis of the cell adhesion molecule Fasciclin 2 from the lateral surface of the cell and is critical for the cuboidal to squamous cell shape transition. Their results indicate that Tao is rising as a regulator of cell height.
Growing evidence is pointing to the importance of multicellular bacterial structures in the interaction of pathogenic bacteria with their host. Transition from planktonic to host cell-associated multicellular structures is an essential infection step that has not been described for the opportunistic human pathogen Pseudomonas aeruginosa. In this study we show that P. aeruginosa interacts with the surface of epithelial cells mainly forming aggregates. Dynamics of aggregate formation typically follow a sigmoidal curve. First, a single bacterium attaches at cell–cell junctions. This is followed by rapid recruitment of free-swimming bacteria and association of bacterial cells resulting in the formation of an aggregate on the order of minutes. Aggregates are associated with phosphatidylinositol 3,4,5-trisphosphate (PIP3)- enriched host cell membrane protrusions. We further show that aggregates can be rapidly internalized into epithelial cells. Lyn, a member of the Src family tyrosine kinases previously implicated in P. aeruginosa infection, mediates both PIP3- enriched protrusion formation and aggregate internalization. Our results establish the first framework of principles that define P. aeruginosa transition to multicellular structures during interaction with host cells.
The asymmetric polarization of cells allows specialized functions to be performed at discrete subcellular locales. Spatiotemporal coordination of polarization between groups of cells allowed the evolution of metazoa. For instance, coordinated apical-basal polarization of epithelial and endothelial cells allows transport of nutrients and metabolites across cell barriers and tissue microenvironments. The defining feature of such tissues is the presence of a central, interconnected luminal network. Although tubular networks are present in seemingly different organ systems, such as the kidney, lung, and blood vessels, common underlying principles govern their formation. Recent studies using in vivo and in vitro models of lumen formation have shed new light on the molecular networks regulating this fundamental process. We here discuss progress in understanding common design principles underpinning de novo lumen formation and expansion.
Hepatocyte growth factor (HGF) plays central roles in tubulogenesis and metastasis[1-4]. HGF treatment of Madin-Darby canine kidney (MDCK) cells grown as cysts in three-dimensional culture induces tubulogenesis[5, 6], which like most tubulogenic processes, proceeds through distinct intermediate phases. Identification of genes associated with these phases is central to understanding the molecular mechanisms of tubulogensis; however due to inefficient, asynchronous tubule formation, isolating such genes has been unfeasible. Here we developed a synchronous, efficient tubulogenesis system and used time-course transcriptional profiling to identify genes temporally regulated in developmental intermediates. Knockdown (KD) of tensin 4 (TNS4), a particularly highly upregulated gene, leads to a decrease in formation of extensions and tubules, two sequential intermediates in tubulogensis. Exogenous expression of TNS4 marks invasive cells in an epithelial sheet. A mutation in the SH2 domain of TNS4 prevents the transition from extension formation to invasive migration during tubule formation and leads to increased basal activation of STAT3. Exogenous expression of a constitutively active STAT3 mimics the defect by the mutation. Our study highlights the role of TNS4-STAT3 axis in epithelial sheet invasion and tubulogenesis.
Apical secretion from epithelial tubes of the Drosophila embryo is mediated by apical F-actin cables generated by the formin-family protein Diaphanous (Dia). Apical localization and activity of Dia are at the core of restricting F-actin formation to the correct membrane domain. Here we identify the mechanisms that target Dia to the apical surface. PI(4,5)P2 levels at the apical membrane regulate Dia localization in both the MDCK cyst model and in Drosophila tubular epithelia. An N-terminal basic domain of Dia is crucial for apical localization, implying direct binding to PI(4,5)P2. Dia apical targeting also depends on binding to Rho1, which is critical for activation-induced conformational change, as well as physically anchoring Dia to the apical membrane. We demonstrate that binding to Rho1 facilitates interaction with PI(4,5)P2 at the plane of the membrane. Together these cues ensure efficient and distinct restriction of Dia to the apical membrane.
Many physiological processes are directional, which means that tissues and organs often need a sense of spatial orientation in order to function properly. In most tissues, this sense of direction relies on certain proteins and infrastructure components of the cell being located in specific subcellular regions, rather than being distributed in a more symmetrical fashion throughout the cell: the latter phenomenon is known as cell polarity.
Exocrine tissues (that is, glands) are composed of tubular epithelial cells organized around a central lumen: the cells in the gland secrete various products (such as enzymes) into the lumen, so that they can be carried to the target organ elsewhere in the body. Epithelial cells in these tissues are therefore polarized to enable directional transport to the lumen. An example of cell polarity is a network of actin filaments that lines the apical surface of these cells (the surface nearest the common lumen). This actin network helps to shuttle cargo to the lumen by assisting with directional, coordinated secretion, among other processes.
In fruitflies, the construction of the apical actin network depends on the presence of a protein called Diaphanous. However, the signals that lead to the localization of this protein near the apical membrane of the cells are not well understood. Now Rousso et al. report that a modified lipid, called PI(4,5)P2, is involved in this localization. However, they also show that this lipid does not govern the apical localization of Diaphanous on its own: rather, an enzyme called Rho1 must also be present to assist with the localization of Diaphanous and to ensure that actin is deposited in the correct place. Rousso et al. also demonstrate that PI(4,5)P2-mediated localization of Drosophila Diaphanous occurs in mammalian cells. Lipid-protein collaboration also targets other proteins to the apical membrane. A common mechanism may therefore underlie cell polarity in tubular organ tissues in flies and mammals.
Diaphanous; formin; tubular epithelia; apical localization; PI(4,5)P2; Rho1; D. melanogaster
Bile ducts are hepatic tubular structures that are lined by cholangiocytes, a type of liver epithelial cell. Cholangiocytes first form a single layer of cells, termed the ductal plate, surrounding the portal vein, which eventually remodels into the branching tubular network of bile ducts. The process of bile duct morphogenesis is not yet clear: a conventional model where cholangiocytes proliferate to duplicate a single layer of the ductal plate before lumen formation seems inconsistent with the observation that proliferation is dramatically reduced when hepatoblasts, liver progenitor cells, differentiate into cholangiocytes. Here, we developed a new culture system in which a liver progenitor cell line, HPPL, reorganizes from a monolayer to tubular structures in response to being overlaid with a gel containing type I collagen and Matrigel. We found that some of the HPPL in the monolayer depolarized and migrated to fold up the monolayer into a double-cell layer. These morphogenetic processes occurred without cell proliferation and required phosphatidylinositol 3-kinase and Akt activity. Later in morphogenesis, luminal space was generated between the two cell layers. This process, in particular enlargement of the apical lumen, involved transcriptional activity of HNF1β. Thus, using this sandwich culture system, we could segregate tubulogenesis of bile ducts into distinct steps and found that the PI3K/Akt pathway and HNF1β regulated different steps of the morphogenesis. Although the process of tubulogenesis in culture specifically resembled early bile duct formation, involvement of these two key players suggests that the sandwich culture might help us to find common principles of tubulogenesis in general.
The formation of epithelial tissues requires both the generation of apical-basal polarity and the co-ordination of this polarity between neighboring cells to form a central lumen. During de novo lumen formation, vectorial membrane transport contributes to formation of a singular apical membrane, resulting in contribution of each cell to only a single lumen. Here, from a functional screen for genes required for 3D epithelial architecture we identify key roles for Synaptotagmin-like proteins 2-a and 4-a (Slp2-a/4-a) in generation of a single apical surface per cell. Slp2-a localizes to the luminal membrane in a PI(4,5)P2-dependent manner, where it targets Rab27-loaded vesicles to initiate a single lumen. Vesicle tethering and fusion is controlled by Slp4-a, in conjunction with Rab27/Rab3/Rab8 and the SNARE Syntaxin-3. Together, Slp2-a/4-a co-ordinate the spatiotemporal organization of vectorial apical transport to ensure only a single apical surface, and thus formation of a single lumen, occurs per cell.
Cell polarity; epithelial morphogenesis; Rab GTPases; phosphoinositides; vesicle trafficking; lumen formation; synaptotagmin-like proteins
Cholangiocytes are cellular components of the bile duct system of the liver, which originate from hepatoblasts during embryonic liver development. Although several transcription factors and signaling molecules have been implicated in bile duct development, its molecular mechanism has not been studied in detail. Here, we applied a three-dimensional (3D) culture technique to a liver progenitor cell line, HPPL, to establish an in vitro culture system in which HPPL acquire differentiated cholangiocyte characteristics. When HPPL were grown in a gel containing Matrigel, which contains extracellular matrix components of basement membrane, HPPL developed apicobasal polarity and formed cysts, which had luminal space inside. In the cysts, F-actin bundles and atypical protein kinase C were at the apical membrane, E-cadherin was localized at the lateral membrane, and β-catenin and integrin α6 were located at the basolateral membrane. HPPL in cysts expressed cholangiocyte markers, including cytokeratin 19, integrin β4, and aquaporin-1, but not a hepatocyte marker, albumin. Furthermore, HPPL transported rhodamine 123, a substrate for multidrug resistance gene products, from the basal side to the central lumen. These data indicate that HPPL develop cholangiocyte-type epithelial polarity in 3D culture. Phosphatidylinositol 3-kinase signaling was essential for proliferation and survival of HPPL in culture, whereas laminin-1 was a crucial component of Matrigel for inducing epithelial polarization of HPPL. Because HPPL cysts display structural and functional similarities with bile ducts, the 3D culture of HPPL recapitulates in vivo cholangiocyte differentiation and is useful to study the molecular mechanism of bile duct development in vitro.
The intrahepatic biliary ducts transport bile produced by the hepatocytes out of the liver. Defects in biliary cell differentiation and biliary duct remodeling cause a variety of congenital diseases including Alagille Syndrome and polycystic liver disease. While the molecular pathways regulating biliary cell differentiation have received increasing attention (Lemaigre, 2010), less is known about the cellular behavior underlying biliary duct remodeling. Here, we have identified a novel gene, claudin 15-like b (cldn15lb), which exhibits a unique and dynamic expression pattern in the hepatocytes and biliary epithelial cells in zebrafish. Claudins are tight junction proteins that have been implicated in maintaining epithelial polarity, regulating paracellular transport, and providing barrier function. In zebrafish cldn15lb mutant livers, tight junctions are observed between hepatocytes, but these cells show polarization defects as well as canalicular malformations. Furthermore, cldn15lb mutants show abnormalities in biliary duct morphogenesis whereby biliary epithelial cells remain clustered together and form a disorganized network. Our data suggest that Cldn15lb plays an important role in the remodeling process during biliary duct morphogenesis. Thus, cldn15lb mutants provide a novel in vivo model to study the role of tight junction proteins in the remodeling of the biliary network and hereditary cholestasis.
Claudin; liver development; zebrafish; biliary duct remodeling; biliary cells; biliary duct morphogenesis; tight junctions; cholestasis
In polarized epithelial cells syntaxin 3 is at the apical plasma membrane and is involved in delivery of proteins from the trans-Golgi network to the apical surface. The highly related syntaxin 4 is at the basolateral surface. The complementary distribution of these syntaxins suggests that they play a role in the specificity of membrane traffic to the two surfaces. We constructed a chimeric syntaxin where we removed the N-terminal 29 residues of syntaxin 3 and replaced it with the corresponding portion of syntaxin 4. When expressed in polarized epithelial cells, this chimera was exclusively localized to the basolateral surface. This indicates that the N-terminal domain of syntaxin 3 contains information for its polarized localization. In contrast to the apical localization of syntaxin 3, the basolateral localization of syntaxin 4 was not dependent on its N-terminal domain. Syntaxin 3 normally binds to Munc18b, but not to the related Munc18c. Overexpression of the chimera together with overexpression of Munc18b caused membrane and secretory proteins that are normally sent primarily to the apical surface to exhibit increased delivery to the basolateral surface. We suggest that syntaxins may play a role in determining the specificity of membrane targeting by permitting fusion with only certain target membranes.
Multidrug efflux is activated at fertilization in sea urchin eggs, but it is unclear how cortical reorganization initiates transport. Using structured illumination microscopy, we found that the multidrug transporter ABCB1a translocates along polymerizing actin filaments to the microvillar tips. This short-range (micrometer scale) translocation is necessary for up-regulation of efflux activity.
Fertilization changes the structure and function of the cell surface. In sea urchins, these changes include polymerization of cortical actin and a coincident, switch-like increase in the activity of the multidrug efflux transporter ABCB1a. However, it is not clear how cortical reorganization leads to changes in membrane transport physiology. In this study, we used three-dimensional superresolution fluorescence microscopy to resolve the fine-scale movements of the transporter along polymerizing actin filaments, and we show that efflux activity is established after ABCB1a translocates to the tips of the microvilli. Inhibition of actin polymerization or bundle formation prevents tip localization, resulting in the patching of ABCB1a at the cell surface and decreased efflux activity. In contrast, enhanced actin polymerization promotes tip localization. Finally, interference with Rab11, a regulator of apical recycling, inhibits activation of efflux activity in embryos. Together our results show that actin-mediated, short-range traffic and positioning of transporters at the cell surface regulates multidrug efflux activity and highlight the multifaceted roles of microvilli in the spatial distribution of membrane proteins.
Grainyhead-like 2 (Grhl2) is a transcription factor that regulates the size of the luminal space surrounded by polarized epithelial cells. Grhl2 promotes epithelial barrier function and the formation of large lumen by up-regulating Cldn3, Cldn4, and Rab25. The results reveal a molecular network regulating epithelial lumen formation.
During development, epithelial progenitors establish intercellular junctions, including tight junctions (TJs), and form three-dimensional (3D) tissue structures, which are often associated with luminal structures. Here we identify grainyhead-like 2 (Grhl2) as a transcription factor that regulates the size of luminal space surrounded by polarized epithelial cells. We show that HPPL, a liver progenitor cell line, transfected with Grhl2 cDNA forms remarkably larger cysts than the control cells in 3D cultures. We find that Grhl2 up-regulates claudin (Cldn) 3 and Cldn4, and their functions are necessary for the formation of large cysts. Overexpression of Cldn3 alone induces the cyst expansion. In contrast, expression of Cldn4 alone does not induce expansion, as it is not localized at TJs. Of interest, Rab25, another Grhl2 target, not only increases the Cldn4 protein, but also enhances its localization at TJs. Taken together, the results indicate that Grhl2 regulates epithelial morphogenesis through transcriptional up-regulation of Cldn3 and Cldn4, as well as of Rab25, which increases the Cldn4 protein and its localization at TJs. The results reveal a molecular network regulating epithelial lumen formation organized by Grhl2.
Cyclic AMP–dependent protein kinase A (PKA) is required for MCF10A mammary epithelial acinus formation in vitro. PKA plays a dual role by facilitating polarization of cells attached to the extracellular matrix and apoptosis of detached cells.
Epithelial cells form tubular and acinar structures notable for a hollow lumen. In three-dimensional culture utilizing MCF10A mammary epithelial cells, acini form due to integrin-dependent polarization and survival of cells contacting extracellular matrix (ECM), and the apoptosis of inner cells of acini lacking contact with the ECM. In this paper, we report that cyclic AMP (cAMP)-dependent protein kinase A (PKA) promotes acinus formation via two mechanisms. First, cAMP accelerates redistribution of α6-integrin to the periphery of the acinus and thus facilitates the polarization of outer acinar cells. Blocking of α6-integrin function by inhibitory antibody prevents cAMP-dependent polarization. Second, cAMP promotes the death of inner cells occupying the lumen. In the absence of cAMP, apoptosis is delayed, resulting in perturbed luminal clearance. cAMP-dependent apoptosis is accompanied by a posttranscriptional PKA-dependent increase in the proapoptotic protein Bcl-2 interacting mediator of cell death. These data demonstrate that cAMP regulates lumen formation in mammary epithelial cells in vitro, both through acceleration of polarization of outer cells and apoptosis of inner cells of the acinus.
Structural aspects of later stages of FcRn transcytosis in neonatal rat intestinal epithelia are studied. FcRn pools are found in dilated regions of the lateral intercellular space, representing possible exit sites from the epithelium. Clathrin and multivesicular bodies play a role in the neonatal transcytotic pathway.
The neonatal Fc receptor (FcRn) transports maternal immunoglobulin (IgG) across epithelia to confer passive immunity to mammalian young. In newborn rodents, FcRn transcytoses IgG from ingested milk across the intestinal epithelium for release into the bloodstream. We used electron tomography to examine FcRn transport of Nanogold-labeled Fc (Au-Fc) in neonatal rat jejunum, focusing on later aspects of transport by chasing Au-Fc before fixation. We observed pools of Au-Fc in dilated regions of the lateral intercellular space (LIS), likely representing exit sites where Au-Fc accumulates en route to the blood. Before weaning, the jejunum functions primarily in IgG transport and exhibits unusual properties: clathrin-rich regions near/at the basolateral LIS and multivesicular bodies (MVBs) expressing early endosomal markers. To address whether these features are related to IgG transport, we examined LIS and endocytic/transcytotic structures from neonatal and weaned animals. Weaned samples showed less LIS-associated clathrin. MVBs labeled with late endosomal/lysosomal markers were smaller than their neonatal counterparts but contained 10 times more internal compartments. These results are consistent with hypotheses that clathrin-rich basolateral regions in neonatal jejunum are involved in IgG exocytosis and that MVBs function in IgG transport while FcRn is expressed but switch to degradative functions after weaning, when the jejunum does not express FcRn or transport IgG.
Epithelial cadherins are shown to have distinct functions. Using a three-dimensional culture system of epithelial kidney cells, it is shown that cadherin-6 acts as an inhibitor of tubulogenesis, whereas E-cadherin controls lumen formation.
Classic cadherins are important regulators of tissue morphogenesis. The predominant cadherin in epithelial cells, E-cadherin, has been extensively studied because of its critical role in normal epithelial development and carcinogenesis. Epithelial cells may also coexpress other cadherins, but their roles are less clear. The Madin Darby canine kidney (MDCK) cell line has been a popular mammalian model to investigate the role of E-cadherin in epithelial polarization and tubulogenesis. However, MDCK cells also express relatively high levels of cadherin-6, and it is unclear whether the functions of this cadherin are redundant to those of E-cadherin. We investigate the specific roles of both cadherins using a knockdown approach. Although we find that both cadherins are able to form adherens junctions at the basolateral surface, we show that they have specific and mutually exclusive roles in epithelial morphogenesis. Specifically, we find that cadherin-6 functions as an inhibitor of tubulogenesis, whereas E-cadherin is required for lumen formation. Ablation of cadherin-6 leads to the spontaneous formation of tubules, which depends on increased phosphoinositide 3-kinase (PI3K) activity. In contrast, loss of E-cadherin inhibits lumen formation by a mechanism independent of PI3K.
Microglial lysosomes lack ClC-7, which leads to incomplete lysosomal acidification. In quiescent microglia, ClC-7 is targeted for proteasomal degradation apparently by an endoplasmic reticulum-associated degradation (ERAD) pathway. Microglial activation recruits ClC-7 to lysosomes and causes lysosomal acidification, which leads to efficient amyloid degradation.
Incomplete lysosomal acidification in microglia inhibits the degradation of fibrillar forms of Alzheimer's amyloid β peptide (fAβ). Here we show that in primary microglia a chloride transporter, ClC-7, is not delivered efficiently to lysosomes, causing incomplete lysosomal acidification. ClC-7 protein is synthesized by microglia but it is mistargeted and appears to be degraded by an endoplasmic reticulum–associated degradation pathway. Activation of microglia with macrophage colony-stimulating factor induces trafficking of ClC-7 to lysosomes, leading to lysosomal acidification and increased fAβ degradation. ClC-7 associates with another protein, Ostm1, which plays an important role in its correct lysosomal targeting. Expression of both ClC-7 and Ostm1 is increased in activated microglia, which can account for the increased delivery of ClC-7 to lysosomes. Our findings suggest a novel mechanism of lysosomal pH regulation in activated microglia that is required for fAβ degradation.
Due to a new reconstitution system of zonula occludens (zTJ) by expressing specific claudin species, this study found that each of claudin-7, -14, and -19 could singly reconstitute zTJ, but with distinct characteristics in morphology. The molecular dynamics is one important factor for this reconstitution, as revealed by FRAP analysis.
Tight-junction strands, which are organized into the beltlike cell–cell adhesive structure called the zonula occludens (TJ), create the paracellular permselective barrier in epithelial cells. The TJ is constructed on the basis of the zonula adherens (AJ) by polymerized claudins in a process mediated by ZO-1/2, but whether the 24 individual claudin family members play different roles at the TJ is unclear. Here we established a cell system for examining the polymerization of individual claudins in the presence of ZO-1/2 using an epithelial-like cell line, SF7, which lacked endogenous TJs and expressed no claudin but claudin-12 in immunofluorescence and real-time PCR assays. In stable SF7-derived lines, exogenous claudin-7, -14, or -19, but no other claudins, individually reconstituted TJs, each with a distinct TJ-strand pattern, as revealed by freeze-fracture analyses. Fluorescence recovery after photobleaching (FRAP) analyses of the claudin dynamics in these and other epithelial cells suggested that slow FRAP-recovery dynamics of claudins play a critical role in regulating their polymerization around AJs, which are loosely coupled with ZO-1/2, to form TJs. Furthermore, the distinct claudin stabilities in different cell types may help to understand how TJs regulate paracellular permeability by altering the paracellular flux and the paracellular ion permeability.
Polymeric immunoglobulin A (pIgA) transcytosis, mediated by the polymeric immunoglobulin receptor (pIgR), is a central component of mucosal immunity and a model for regulation of polarized epithelial membrane traffic. Binding of pIgA to pIgR stimulates transcytosis in a process requiring Yes, a Src family tyrosine kinase (SFK). We show that Yes directly phosphorylates EGF receptor (EGFR) on liver endosomes. Injection of pIgA into rats induced EGFR phosphorylation. Similarly, in MDCK cells, pIgA treatment significantly increased phosphorylation of EGFR on various sites, subsequently activating extracellular signal-regulated protein kinase (ERK). Furthermore, we find that the Rab11 effector Rab11-FIP5 is a substrate of ERK. Knocking down Yes or Rab11-FIP5, or inhibition of the Yes–EGFR–ERK cascade, decreased pIgA–pIgR transcytosis. Finally, we demonstrate that Rab11-FIP5 phosphorylation by ERK controls Rab11a endosome distribution and pIgA–pIgR transcytosis. Our results reveal a novel Yes–EGFR–ERK–FIP5 signalling network for regulation of pIgA–pIgR transcytosis.
The study of epithelial morphogenesis is fundamental to increasing our
understanding of organ function and disease. Great progress has been made
through study of culture systems such as Madin-Darby canine kidney (MDCK) cells,
but many aspects of even simple morphogenesis remain unclear. For example, are
specific cell actions tightly coupled to the characteristics of the cell's
environment or are they more often cell state dependent? How does the single
lumen, single cell layer cyst consistently emerge from a variety of cell
actions? To improve insight, we instantiated in silico analogues that used
hypothesized cell behavior mechanisms to mimic MDCK cystogenesis. We tested them
through in vitro experimentation and quantitative validation. We observed novel
growth patterns, including a cell behavior shift that began around day five of
growth. We created agent-oriented analogues that used the cellular Potts model
along with an Iterative Refinement protocol. Following several refinements, we
achieved a degree of validation for two separate mechanisms. Both survived
falsification and achieved prespecified measures of similarity to cell culture
properties. In silico components and mechanisms mapped to in vitro counterparts.
In silico, the axis of cell division significantly affects lumen number without
changing cell number or cyst size. Reducing the amount of in silico luminal cell
death had limited effect on cystogenesis. Simulations provide an observable
theory for cystogenesis based on hypothesized, cell-level operating
Epithelial cells perform essential functions throughout the body, acting as both
barrier and transporter and allowing an organism to survive and thrive in varied
environments. Although the details of many processes that occur within
individual cells are well understood, we still lack a thorough understanding of
how cells coordinate their behaviors to create complex tissues. In order to
achieve deeper insight, we created a list of targeted attributes and plausible
rules for the growth of multicellular cysts formed by Madin-Darby canine kidney
(MDCK) cells grown in vitro. We then designed in silico analogues of MDCK
cystogenesis using object-oriented programming. In silico components (such as
the cells and lumens) and their behaviors directly mapped to in vitro components
and mechanisms. We conducted in vitro experiments to generate data that would
validate or falsify the in silico analogues and then iteratively refined the
analogues to mimic that data. Cells in vitro begin to stabilize at around the
fifth day even as cysts continue to expand. The in silico system mirrored that
behavior and others, achieving new insights. For example, luminal cell death is
not strictly required for cystogenesis, and cell division orientation is very
important for normal cyst growth.
The localization of polycystin (PC)1) to the plasma membrane requires coexpression with PC2 and cleavage at the PC1 G protein-coupled receptor proteolytic site. Neither the PC1 binding capacity of PC2 nor its channel function is required for this effect.
Polycystin (PC)1 and PC2 are membrane proteins implicated in autosomal dominant polycystic kidney disease. A physiologically relevant cleavage at PC1's G protein-coupled receptor proteolytic site (GPS) occurs early in the secretory pathway. Our results suggest that PC2 increases both PC1 GPS cleavage and PC1's appearance at the plasma membrane. Mutations that prevent PC1's GPS cleavage prevent its plasma membrane localization. PC2 is a member of the trp family of cation channels and is an important PC1 binding partner. The effect of PC2 on PC1 localization is independent of PC2 channel activity, as tested using channel-inhibiting PC2 mutations. PC1 and PC2 can interact through their C-terminal tails, but removing the C-terminal tail of either protein has no effect on PC1 surface localization in human embryonic kidney 293 cells. Experiments in polarized LLC-PK cells show that apical and ciliary PC1 localization requires PC2 and that this delivery is sensitive to PC2 truncation. In sum, our work shows that PC2 expression is required for the movement of PC1 to the plasma and ciliary membranes. In fibroblast cells this localization effect is independent of PC2's channel activity or PC1 binding ability but involves a stimulation of PC1's GPS cleavage before the PC1 protein's surface delivery.
Signal transducers and activators of transcription (STAT)1 is the key to the sequential control of Madin-Darby canine kidney tubulogenesis. Loss of STAT1 prevents redifferentiation. Constitutively active STAT1 is sufficient to restore cord formation but not mature lumens. These data suggest that STAT1 is necessary for the redifferentiation phase of tubulogenesis and that mature lumenogenesis requires a distinct signal.
Tubule formation in vitro using Madin-Darby canine kidney (MDCK) epithelial cells consists mainly of two processes. First, the cells undergo a partial epithelial–mesenchymal transition (pEMT), losing polarity and migrating. Second, the cells redifferentiate, forming cords and then tubules with continuous lumens. We have shown previously that extracellular signal-regulated kinase activation is required for pEMT. However, the mechanism of how the pEMT phase is turned off and the redifferentiation phase is initiated is largely unknown. To address the central question of the sequential control of these two phases, we used MDCK cells grown as cysts and treated with hepatocyte growth factor to model tubulogenesis. We show that signal transducer and activator of transcription (STAT)1 controls the sequential progression from the pEMT phase to the redifferentiation phase. Loss of STAT1 prevents redifferentiation. Constitutively active STAT1 allows redifferentiation to occur even when cells are otherwise prevented from progressing beyond the pEMT phase by exogenous activation of Raf. Moreover, tyrosine phosphorylation defective STAT1 partially restored cord formation in such cells, suggesting that STAT1 functions in part as nonnuclear protein mediating signal transduction in this process. Constitutively active or inactive forms of STAT1 did not promote lumen maturation, suggesting this requires a distinct signal.
Hypoxia can regulate many cellular processes. We show that hypoxia, via hypoxia-inducible factor (HIF)-1, blocks anoikis of epithelial cells by activating signaling pathways that inhibit expression of proapoptotic proteins Bim and Bmf. Hypoxia also disrupts mammary morphogenesis and blocks anoikis associated with lumen formation in three-dimensional in vitro model of mammary acini.
Proper adhesion to extracellular matrix is critical for epithelial cell survival. Detachment from matrix signals results in apoptosis, referred to as anoikis. Selective apoptosis of cells that become detached from matrix is associated with the formation of a lumen in three-dimensional mammary epithelial acinar structures in vitro. Because early breast cancer lesions such as carcinoma in situ, characterized by ducts exhibiting lumens filled with cells, are often associated with hypoxic markers, we sought to examine the role of hypoxia in anoikis and lumen formation in mammary epithelial cells. Here, we show that hypoxic conditions inhibit anoikis and block expression of proapoptotic BH3-only family members Bim and Bmf in epithelial cells. Hypoxia-mediated anoikis protection is associated with increased activation of the epidermal growth factor receptor–mitogen-activated protein kinase kinase–extracellular signal-regulated kinase (Erk) kinase pathway and requires the hypoxia-activated transcription factor. Consistent with these data, hypoxic conditions inhibit luminal clearing during morphogenesis in human mammary epithelial acini when grown in three-dimensional cultures and are associated with decreased expression of Bim and Bmf as well as Erk activation. We show that hypoxia regulates specific cell survival pathways that disrupt tissue architecture related to clearing of luminal space during mammary morphogenesis and suggest that hypoxia-mediated anoikis resistance may contribute to cancer progression.
A plasmid-based transfection method was used to cell-autonomously silence chloride intracellular channel 4 (CLIC4) in RPE in situ. These results show CLIC4 is critical for epithelial morphogenesis and retinal attachment. Novel candidate targets for retinal detachment therapy have also been identified.
Retinal detachment is a sight-threatening condition. The molecular mechanism underlying the adhesion between the RPE and photoreceptors is poorly understood because the intimate interactions between these two cell types are impossible to model and study in vitro. In this article, we show that chloride intracellular channel 4 (CLIC4) is enriched at apical RPE microvilli, which are interdigitated with the photoreceptor outer segment. We used a novel plasmid-based transfection method to cell-autonomously suppress CLIC4 in RPE in situ. CLIC4 silenced RPE cells exhibited a significant loss of apical microvilli and basal infoldings, reduced retinal adhesion, and epithelial-mesenchymal transition. Ectopically expressing ezrin failed to rescue the morphological changes exerted by CLIC4 silencing. Neural retinas adjacent to the CLIC4-suppressed RPE cells display severe dysplasia. Finally, a high level of aquaporin 1 unexpectedly appeared at the apical surfaces of CLIC4-suppressed RPE cells, together with a concomitant loss of basal surface expression of monocarboxylate transporter MCT3. Our results suggested that CLIC4 plays an important role in RPE-photoreceptor adhesion, perhaps by modulating the activity of cell surface channels/transporters. We propose that these changes may be attributable to subretinal fluid accumulation in our novel retinal detachment animal model.
To form epithelial organs cells must polarize and generate de novo an apical domain and lumen. Epithelial polarization is masterminded by polarity complexes, which are thought to direct downstream events such as polarized membrane traffic, though this interconnection is not well understood. We report that Rab11a regulates apical traffic and lumen formation via the Rab GEF Rabin8, and its target Rab8a. Rab8a/11a act via the exocyst to target Par3 to the apical surface, and control apical Cdc42 activation via the Cdc42 GEF, Tuba. These components assemble at a transient apical membrane initiation site to form the lumen. This Rab11a-directed network directs Cdc42-dependent apical exocytosis during lumen formation, revealing a novel interplay of the machineries of vesicular transport and polarization.
Adenomatous polyposis coli is a cytoskeletal organizer and a scaffold for mediating degradation of the Wnt effector β-catenin. We uncouple these different APC functions and show that GSK3β/CKI phosphorylation regulates APC clusters and cell migration independently of cell–cell adhesion and β-catenin transcriptional activity.
Adenomatous polyposis coli (APC), a tumor suppressor commonly mutated in cancer, is a cytoskeletal organizer for cell migration and a scaffold for GSK3β/CKI-mediated phosphorylation and degradation of the Wnt effector β-catenin. It remains unclear whether these different APC functions are coupled, or independently regulated and localized. In primary endothelial cells, we show that GSK3β/CKI-phosphorylated APC localizes to microtubule-dependent clusters at the tips of membrane extensions. Loss of GSK3β/CKI-phosphorylated APC from these clusters correlates with a decrease in cell migration. GSK3β/CKI-phosphorylated APC and β-catenin at clusters is degraded rapidly by the proteasome, but inhibition of GSK3β/CKI does not increase β-catenin–mediated transcription. GSK3β/CKI-phosphorylated and -nonphosphorylated APC also localize along adherens junctions, which requires actin and cell–cell adhesion. Significantly, inhibition of cell–cell adhesion results in loss of lateral membrane APC and a concomitant increase in GSK3β/CKI-phosphorylated APC in clusters. These results uncouple different APC functions and show that GSK3β/CKI phosphorylation regulates APC clusters and cell migration independently of cell–cell adhesion and β-catenin transcriptional activity.