Cdc42-dependent modulation of tight junctions and membrane protein traffic in polarized Madin-Darby canine kidney cells

Increased activity of epithelial Cdc42 Rho GTPase and tight junction permeability in the Cftr knockout intestine

Increased intestinal permeability is a manifestation of cystic fibrosis (CF) in people with CF (pwCF) and in CF mouse models. CF transmembrane conductance regulator knockout (Cftr KO) mouse intestine exhibits increased proliferation and Wnt/β-catenin signaling relative to wild-type mice (WT). Since the Rho GTPase Cdc42 plays a central role in intestinal epithelial proliferation and tight junction remodeling, we hypothesized that Cdc42 may be altered in the Cftr KO crypts. Immunofluorescence showed distinct tight junction localization of Cdc42 in Cftr KO fresh crypts and enteroids, the latter indicating an epithelial-autonomous feature. Quantitative PCR and immunoblots revealed similar expression of Cdc42 in the Cftr KO crypts/enteroids relative to WT, whereas pulldown assays showed increased GTP-bound (active) Cdc42 in proportion to total Cdc42 in Cftr KO enteroids. Cdc42 activity in the Cftr KO and WT enteroids could be reduced by inhibition of the Wnt transducer Disheveled. With the use of a dye permeability assay, Cftr KO enteroids exhibited increased paracellular permeability to 3 kDa dextran relative to WT. Leak permeability and Cdc42 tight junction localization were reduced to a greater extent by inhibition of Wnt/β-catenin signaling with endo-IWR1 in Cftr KO relative to WT enteroids. Increased proliferation or inhibition of Cdc42 activity with ML141 in WT enteroids had no effect on permeability. In contrast, inhibition of Cdc42 with ML141 increased permeability to both 3 kDa dextran and tight junction impermeant 500 kDa dextran in Cftr KO enteroids. These data suggest that increased constitutive Cdc42 activity may alter the stability of paracellular permeability in Cftr KO crypt epithelium.NEW & NOTEWORTHY Increased tight junction localization and GTP-bound activity of the Rho GTPase Cdc42 was identified in small intestinal crypts and enteroids of cystic fibrosis (CF) transmembrane conductance regulator knockout (Cftr KO) mice. The increase in epithelial Cdc42 activity was associated with increased Wnt signaling. Paracellular flux of an uncharged solute (3 kDa dextran) in Cftr KO enteroids indicated a moderate leak permeability under basal conditions that was strongly exacerbated by Cdc42 inhibition. These findings suggest increased activity of Cdc42 in the Cftr KO intestine underlies alterations in intestinal permeability.

Epithelial polarization in the 3D matrix requires MST3 signaling to regulate ZO-1 position

Apical-basal cell polarity must be tightly controlled for epithelial cyst and tubule formation, and these are important functional units in various epithelial organs. Polarization is achieved through the coordination of several molecules that divide cells into an apical domain and a basolateral domain, which are separated from tight and adherens junctions. Cdc42 regulates cytoskeletal organization and the tight junction protein ZO-1 at the apical margin of epithelial cell junctions. MST kinases control organ size through the regulation of cell proliferation and cell polarity. For example, MST1 relays the Rap1 signal to induce cell polarity and adhesion of lymphocytes. Our previous study showed that MST3 was involved in E-cadherin regulation and migration in MCF7 cells. In vivo, MST3 knockout mice exhibited higher ENaC expression at the apical site of renal tubules, resulting in hypertension. However, it was not clear whether MST3 was involved in cell polarity. Here, control MDCK cells, HA-MST3 and HA-MST3 kinase-dead (HA-MST3-KD) overexpressing MDCK cells were cultured in collagen or Matrigel. We found that the cysts of HA-MST3 cells were fewer and smaller than those of control MDCK cells; ZO-1 was delayed to the apical site of cysts and in cell-cell contact in the Ca2+ switch assay. However, HA-MST3-KD cells exhibited multilumen cysts. Intensive F-actin stress fibers were observed in HA-MST3 cells with higher Cdc42 activity; in contrast, HA-MST3-KD cells had lower Cdc42 activity and weaker F-actin staining. In this study, we identified a new MST3 function in the establishment of cell polarity through Cdc42 regulation.

TRPM8 as an Anti-Tumoral Target in Prostate Cancer Growth and Metastasis Dissemination

In the fight against prostate cancer (PCa), TRPM8 is one of the most promising clinical targets. Indeed, several studies have highlighted that TRPM8 involvement is key in PCa progression because of its impact on cell proliferation, viability, and migration. However, data from the literature are somewhat contradictory regarding the precise role of TRPM8 in prostatic carcinogenesis and are mostly based on in vitro studies. The purpose of this study was to clarify the role played by TRPM8 in PCa progression. We used a prostate orthotopic xenograft mouse model to show that TRPM8 overexpression dramatically limited tumor growth and metastasis dissemination in vivo. Mechanistically, our in vitro data revealed that TRPM8 inhibited tumor growth by affecting the cell proliferation and clonogenic properties of PCa cells. Moreover, TRPM8 impacted metastatic dissemination mainly by impairing cytoskeleton dynamics and focal adhesion formation through the inhibition of the Cdc42, Rac1, ERK, and FAK pathways. Lastly, we proved the in vivo efficiency of a new tool based on lipid nanocapsules containing WS12 in limiting the TRPM8-positive cells' dissemination at metastatic sites. Our work strongly supports the protective role of TRPM8 on PCa progression, providing new insights into the potential application of TRPM8 as a therapeutic target in PCa treatment.

Depletion of the apical endosome in response to viruses and bacterial toxins provides cell-autonomous host defense at mucosal surfaces

Polarized epithelial cells form an essential barrier against infection at mucosal surfaces. Many pathogens breach this barrier to cause disease, often by co-opting cellular endocytosis mechanisms to enter the cell through the lumenal (apical) cell surface. We recently discovered that the loss of the cell polarity gene PARD6B selectively diminishes apical endosome function. Here, we find that in response to the entry of certain viruses and bacterial toxins into the epithelial cells via the apical membrane, PARD6B and aPKC, two components of the PARD6B-aPKC-Cdc42 apical polarity complex, undergo rapid proteasome-dependent degradation. The perturbation of apical membrane glycosphingolipids by toxin- or virus-binding initiates degradation of PARD6B. The loss of PARD6B causes the depletion of apical endosome function and renders the cell resistant to further infection from the lumenal cell surface, thus enabling a form of cell-autonomous host defense.

CDC42 controlled apical-basal polarity regulates intestinal stem cell to transit amplifying cell fate transition via YAP-EGF-mTOR signaling

Epithelial polarity is controlled by a polarity machinery that includes Rho GTPase CDC42 and Scribble/PAR. By using intestinal stem cell (ISC)-specific deletion of CDC42 in olfactomedin-4 (Olfm4)-internal ribosome entry site (IRES)-EGFP/CreERT2;CDC42^flox/flox mice, we find that CDC42 loss initiated in the ISCs causes a drastic hyperproliferation of transit amplifying (TA) cells and disrupts epithelial polarity. CDC42-null crypts display expanded TA cell and diminished ISC populations, accompanied by elevated Hippo signaling via YAP/TAZ-Ereg (yes-associated protein/WW domain-containing transcription regulator protein 1-epiregulin) and mechanistic target of rapamycin (mTOR) activation, independent from canonical Wnt signaling. YAP/TAZ conditional knockout (KO) restores the balance of ISC/TA cell populations and crypt proliferation but does not rescue the polarity in CDC42-null small intestine. mTOR or epidermal growth factor receptor (EGFR) inhibitor treatment of CDC42 KO mice exhibits similar rescuing effects without affecting YAP/TAZ signaling. Inducible ablation of Scribble in intestinal epithelial cells mimics that of CDC42 KO defects, including crypt hyperplasia and Hippo signaling activation. Mammalian epithelial polarity regulates ISC/TA cell fate and proliferation via a Hippo-Ereg-mTOR cascade.

Zhang et al. discover that CDC42-dependent polarity signaling regulates ISC and TA cell fate and proliferation via a YAP-Ereg-mTOR cascade in the small intestine. This study shows that mammalian epithelial polarity-controlled Hippo signaling is central to cell fate balance between ISC and TA cells and intestinal crypt proliferation.

Mechanisms behind the polarized distribution of lipids in epithelial cells

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

Regulation of intestinal epithelial barrier by and dysfunction of intestinal glial cells

The enteric glia is an important component of the enteric nervous system and forms a broad network in the mucosa of the gastrointestinal tract. Enteric glial cells (EGC) are located in all layers of the intestinal wall and respond to neurotransmitters and neuromodulators through signal transduction pathways. The enteric nervous system interacts with resident glial cells in the gut, and there is increasing evidence that EGC are involved in the regulation of epithelial function. Epithelial cells have important absorption and secretion functions and are also involved in the formation of intestinal epithelial barrier. Studies have found that the enteric glia is not only involved in the regulation of gastrointestinal motility and epithelial barrier function, but also in the formation of cellular molecular bridges between intestinal neurons, enteroendocrine cells, immune cells, and epithelial cells. This article reviews the recent progress in the understanding of the role of EGC in the intestinal barrier and defense functions.

Maelstrom regulates spermatogenesis of the silkworm, Bombyx mori

The spermatogenesis of animal is essential for the reproduction and a very large number of genes participate in this procession. The Maelstrom (Mael) is identified essential for spermatogenesis in both Drosophila and mouse, though the mechanisms appear to differ. It was initially found that Mael gene is necessary for axis specification of oocytes in Drosophila, and recent studies suggested that Mael participates in the piRNA pathway. In this study, we obtained Bombyx mori Mael mutants by using a binary transgenic CRISPR/Cas9 system and analyzed the function of Mael in B. mori, a model lepidopteran insect. The results showed that BmMael is not necessary for piRNA pathway in the ovary of silkworm, whereas it might be essential for transposon elements (TEs) repression in testis. The BmMael mutation resulted in male sterility, and further analysis established that BmMael was essential for spermatogenesis. The spermatogenesis defects occurred in the elongation stage and resulted in nuclei concentration arrest. RNA-seq and qRT-PCR analyses demonstrated that spermatogenesis defects were associated with tight junctions and apoptosis. We also found that BmMael was not involved in the silkworm sex determination pathway. Our data provide insights into the biological function of BmMael in male spermatogenesis and might be useful for developing novel methods to control lepidopteron pests.

Cdc42 negatively regulates endocytosis during apical membrane maintenance in live animals

Lumen establishment and maintenance are fundamental for tubular organs physiological functions. Most of the studies investigating the mechanisms regulating this process have been carried out in cell cultures or in smaller organisms, whereas little has been done in mammalian model systems in vivo. Here we used the salivary glands of live mice to examine the role of the small GTPase Cdc42 in the regulation of the homeostasis of the intercellular canaliculi, a specialized apical domain of the acinar cells, where protein and fluid secretion occur. Depletion of Cdc42 in adult mice induced a significant expansion of the apical canaliculi, whereas depletion at late embryonic stages resulted in a complete inhibition of their postnatal formation. In addition, intravital subcellular microscopy revealed that reduced levels of Cdc42 affected membrane trafficking from and toward the plasma membrane, highlighting a novel role for Cdc42 in membrane remodeling through the negative regulation of selected endocytic pathways.

The scaffolding protein ZO-1 coordinates actomyosin and epithelial apical specializations in vitro and in vivo

Polarized epithelia assemble into sheets that compartmentalize organs and generate tissue barriers by integrating apical surfaces into a single, unified structure. This tissue organization is shared across organs, species, and developmental stages. The processes that regulate development and maintenance of apical epithelial surfaces are, however, undefined. Here, using an intestinal epithelial-specific knockout (KO) mouse and cultured epithelial cells, we show that the tight junction scaffolding protein zonula occludens-1 (ZO-1) is essential for development of unified apical surfaces in vivo and in vitro We found that U5 and GuK domains of ZO-1 are necessary for proper apical surface assembly, including organization of microvilli and cortical F-actin; however, direct interactions with F-actin through the ZO-1 actin-binding region (ABR) are not required. ZO-1 lacking the PDZ1 domain, which binds claudins, rescued apical structure in ZO-1-deficient epithelia, but not in cells lacking both ZO-1 and ZO-2, suggesting that heterodimerization with ZO-2 restores PDZ1-dependent ZO-1 interactions that are vital to apical surface organization. Pharmacologic F-actin disruption, myosin II motor inhibition, or dynamin inactivation restored apical epithelial structure in vitro and in vivo, indicating that ZO-1 directs epithelial organization by regulating actomyosin contraction and membrane traffic. We conclude that multiple ZO-1-mediated interactions contribute to coordination of epithelial actomyosin function and genesis of unified apical surfaces.

Modulating membrane fluidity corrects Batten disease phenotypes in vitro and in vivo

The neuronal ceroid lipofuscinoses are a class of inherited neurodegenerative diseases characterized by the accumulation of autofluorescent storage material. The most common neuronal ceroid lipofuscinosis has juvenile onset with rapid onset blindness and progressive degeneration of cognitive processes. The juvenile form is caused by mutations in the CLN3 gene, which encodes the protein CLN3. While mouse models of Cln3 deficiency show mild disease phenotypes, it is apparent from patient tissue- and cell-based studies that its loss impacts many cellular processes. Using Cln3 deficient mice, we previously described defects in mouse brain endothelial cells and blood-brain barrier (BBB) permeability. Here we expand on this to other components of the BBB and show that Cln3 deficient mice have increased astrocyte endfeet area. Interestingly, this phenotype is corrected by treatment with a commonly used GAP junction inhibitor, carbenoxolone (CBX). In addition to its action on GAP junctions, CBX has also been proposed to alter lipid microdomains. In this work, we show that CBX modifies lipid microdomains and corrects membrane fluidity alterations in Cln3 deficient endothelial cells, which in turn improves defects in endocytosis, caveolin-1 distribution at the plasma membrane, and Cdc42 activity. In further work using the NIH Library of Integrated Network-based Cellular Signatures (LINCS), we discovered other small molecules whose impact was similar to CBX in that they improved Cln3-deficient cell phenotypes. Moreover, Cln3 deficient mice treated orally with CBX exhibited recovery of impaired BBB responses and reduced auto-fluorescence. CBX and the compounds identified by LINCS, many of which have been used in humans or approved for other indications, may find therapeutic benefit in children suffering from CLN3 deficiency through mechanisms independent of their original intended use.

Enteropathogenic E. coli effectors EspF and Map independently disrupt tight junctions through distinct mechanisms involving transcriptional and post-transcriptional regulation

Enteropathogenic E. coli infection is characterized by rapid onset of diarrhea but the underlying mechanisms are not well defined. EPEC targets the tight junctions which selectively regulate the permeability of charged and uncharged molecules. Cooperative actions of the EPEC effectors EspF and Map have been reported to mediate tight junction disruption. To analyze the individual contributions of EspF and Map, we generated in vitro models where EspF and Map, derived from the EPEC strain E2348/69, were constitutively expressed in epithelial cells. Here we report that tight junction disruption by EspF and Map is caused by the inhibition of the junctional recruitment of proteins during tight junction assembly. Constitutive expression of EspF and Map depleted the levels of tight junction proteins. EspF down-regulated the transcript levels of claudin-1, occludin and ZO-1, while Map down-regulated only claudin-1 transcripts. Both effectors also caused lysosomal degradation of existing tight junction proteins. We also identified a novel interaction of Map with non-muscle myosin II. Consistent with earlier studies, EspF was found to interact with ZO-1 while actin was the common interacting partner for both effectors. Our data provides evidence for the distinct roles of Map and EspF in tight junction disruption through non-synergistic functions.

Cdc42 regulates junctional actin but not cell polarization in the Caenorhabditis elegans epidermis

During morphogenesis, adherens junctions (AJs) remodel to allow changes in cell shape and position while preserving adhesion. Here, we examine the function of Rho guanosine triphosphatase CDC-42 in AJ formation and regulation during Caenorhabditis elegans embryo elongation, a process driven by asymmetric epidermal cell shape changes. cdc-42 mutant embryos arrest during elongation with epidermal ruptures. Unexpectedly, we find using time-lapse fluorescence imaging that cdc-42 is not required for epidermal cell polarization or junction assembly, but rather is needed for proper junctional actin regulation during elongation. We show that the RhoGAP PAC-1/ARHGAP21 inhibits CDC-42 activity at AJs, and loss of PAC-1 or the interacting linker protein PICC-1/CCDC85A-C blocks elongation in embryos with compromised AJ function. pac-1 embryos exhibit dynamic accumulations of junctional F-actin and an increase in AJ protein levels. Our findings identify a previously unrecognized molecular mechanism for inhibiting junctional CDC-42 to control actin organization and AJ protein levels during epithelial morphogenesis.

Cdc42 activation couples fluid shear stress to apical endocytosis in proximal tubule cells

Cells lining the kidney proximal tubule (PT) respond to acute changes in glomerular filtration rate and the accompanying fluid shear stress (FSS) to regulate reabsorption of ions, glucose, and other filtered molecules and maintain glomerulotubular balance. Recently, we discovered that exposure of PT cells to FSS also stimulates an increase in apical endocytic capacity (Raghavan et al. PNAS, 111:8506–8511, 2014). We found that FSS triggered an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) that required release of extracellular ATP and the presence of primary cilia. In this study, we elucidate steps downstream of the increase in [Ca²⁺]_i that link FSS‐induced calcium increase to increased apical endocytic capacity. Using an intramolecular FRET probe, we show that activation of Cdc42 is a necessary step in the FSS‐stimulated apical endocytosis cascade. Cdc42 activation requires the primary cilia and the FSS‐mediated increase in [Ca²⁺]_i. Moreover, Cdc42 activity and FSS‐stimulated endocytosis are coordinately modulated by activators and inhibitors of calmodulin. Together, these data suggest a mechanism by which PT cell exposure to FSS is translated into enhanced endocytic uptake of filtered molecules.

Botulinum neurotoxin type B uses a distinct entry pathway mediated by CDC42 into intestinal cells versus neuronal cells

Botulinum neurotoxins (BoNTs) are responsible for severe flaccid paralysis by inhibiting the release of acetylcholine at the neuromuscular junctions. BoNT type B (BoNT/B) most often induces mild forms of botulism with predominant dysautonomic symptoms. In food borne botulism and botulism by intestinal colonisation such as infant botulism, which are the most frequent naturally acquired forms of botulism, the digestive tract is the main entry route of BoNTs into the organism. We previously showed that BoNT/B translocates through mouse intestinal barrier by an endocytosis-dependent mechanism and subsequently targets neuronal cells, mainly cholinergic neurons, in the intestinal mucosa and musculosa. Here, we investigated the entry pathway of BoNT/B using fluorescent C-terminal domain of the heavy chain (HcB), which is involved in the binding to specific receptor(s) and entry process into target cells. While the combination of gangliosides GD_1a /GD_1b /GT_1b and synaptotagmin I and to a greater extent synaptotagmin II constitutes the functional HcB receptor on NG108-15 neuronal cells, HcB only uses the gangliosides GD_1a /GD_1b /GT_1b to efficiently bind to m-IC_cl2 intestinal cells. HcB enters both cell types by a dynamin-dependent endocytosis, which is efficiently prevented by Dynasore, a dynamin inhibitor, and reaches a common early endosomal compartment labeled by early endosome antigen (EEA1). In contrast to neuronal cells, HcB uses a Cdc42-dependent pathway to enter intestinal cells. Then, HcB is transported to late endosomes in neuronal cells, whereas it exploits a nonacidified pathway from apical to basal lateral side of m-IC_cl2 cells supporting a transcytotic route in epithelial intestinal cells.

Uptake of Clostridial Neurotoxins into Cells and Dissemination

Clostridial neurotoxins, botulinum neurotoxins (BoNT) and tetanus neurotoxin (TeNT), are potent toxins, which are responsible for severe neurological diseases in man and animals. BoNTs induce a flaccid paralysis (botulism) by inhibiting acetylcholine release at the neuromuscular junctions, whereas TeNT causes a spastic paralysis (tetanus) by blocking the neurotransmitter release (glycine, GABA) in inhibitory interneurons within the central nervous system. Clostridial neurotoxins recognize specific receptor(s) on the target neuronal cells and enter via a receptor-mediated endocytosis. They transit through an acidic compartment which allows the translocation of the catalytic chain into the cytosol, a prerequisite step for the intracellular activity of the neurotoxins. TeNT migrates to the central nervous system by using a motor neuron as transport cell. TeNT enters a neutral pH compartment and undergoes a retrograde axonal transport to the spinal cord or brain, where the whole undissociated toxin is delivered and interacts with target neurons. Botulism most often results from ingestion of food contaminated with BoNT. Thus, BoNT passes through the intestinal epithelial barrier mainly via a transcytotic mechanism and then diffuses or is transported to the neuromuscular junctions by the lymph or blood circulation. Indeed, clostridial neurotoxins are specific neurotoxins which transit through a transport cell to gain access to the target neuron, and use distinct trafficking pathways in both cell types.

An emerging role for IQGAP1 in tight junction control

IQGAP1 is a scaffold protein involved in the assembly of adherens junctions. Our work has recently revealed a novel role for IQGAP1 in the regulation of tight junctions (TJ) through differential recruitment of claudins to the nascent TJ. Here, we discuss the potential mechanisms of this regulation, including IQGAP1 effects on CDC42, and IQGAP1 interactions with sorting/trafficking molecules (e.g. Exo70). Given the many roles of IQGAP1 and the large number of interacting partners, we focus our discussion of these functions in the context of junction formation, trafficking, growth factor signaling and cancer. We also propose a potential role for IQGAP1 in regulating epithelial integrity and compartmentalized signaling in epithelia.

Potential Use of Autologous Renal Cells from Diseased Kidneys for the Treatment of Renal Failure

Chronic kidney disease (CKD) occurs when certain conditions cause the kidneys to gradually lose function. For patients with CKD, renal transplantation is the only treatment option that restores kidney function. In this study, we evaluated primary renal cells obtained from diseased kidneys to determine whether their normal phenotypic and functional characteristics are retained, and could be used for cell therapy. Primary renal cells isolated from both normal kidneys (NK) and diseased kidneys (CKD) showed similar phenotypic characteristics and growth kinetics. The expression levels of renal tubular cell markers, Aquaporin-1 and E-Cadherin, and podocyte-specific markers, WT-1 and Nephrin, were similar in both NK and CKD kidney derived cells. Using fluorescence- activated cell sorting (FACS), specific renal cell populations were identified and included proximal tubular cells (83.1% from NK and 80.3% from CKD kidneys); distal tubular cells (11.03% from NK and 10.9% from CKD kidneys); and podocytes (1.91% from NK and 1.78% from CKD kidneys). Ultra-structural analysis using scanning electron microscopy (SEM) revealed microvilli on the apical surface of cultured cells from NK and CKD samples. Moreover, transmission electron microscopy (TEM) analysis showed a similar organization of tight junctions, desmosomes, and other intracellular structures. The Na⁺ uptake characteristics of NK and CKD derived renal cells were also similar (24.4 mmol/L and 25 mmol/L, respectively) and no significant differences were observed in the protein uptake and transport characteristics of these two cell isolates. These results show that primary renal cells derived from diseased kidneys such as CKD have similar structural and functional characteristics to their counterparts from a normal healthy kidney (NK) when grown in vitro. This study suggests that cells derived from diseased kidney may be used as an autologous cell source for renal cell therapy, particularly in patients with CKD or end-stage renal disease (ESRD).

An Essential Role for Cdc42 in the Functioning of the Adult Mammary Gland

The Rho family small GTPase Cdc42 has been implicated in a wide range of cellular functions including the establishment of cell polarity and the remodeling of the actin cytoskeletal architecture, resulting in the tight regulation of cell growth and survival during developmental processes. The complete knock-out of Cdc42 in the mouse is embryonic-lethal, and its targeted deletion in various tissues has been shown to disrupt tissue homeostasis. Thus far, in most studies, the targeted deletion of Cdc42 occurred during embryogenesis. Here, we have used a conditional gene deletion strategy in mice to probe the specific role of Cdc42 during adult mammary gland function. Cdc42 conditional-knock-out females were unable to adequately nourish their pups, due to a disorganized epithelial compartment within their mammary glands. A closer examination showed that their mammary epithelial cells were not able to maintain functional alveolar lumens, due to an inability to establish normal apical/basal epithelial polarity, as well as proper cell-cell contacts. Loss of these essential epithelial characteristics led to a premature sloughing off of the Cdc42-null epithelial cells. Overall our findings demonstrate that Cdc42 plays essential roles in mammary gland function post pregnancy, where it helps to establish proper epithelial cell polarity and tissue homeostasis during lactation.

Measuring receptor recycling in polarized MDCK cells

Recycling of proteins such as channels, pumps, and receptors is critical for epithelial cell function. In this chapter we present a method to measure receptor recycling in polarized Madin-Darby canine kidney cells using an iodinated ligand. We describe a technique to iodinate transferrin (Tf), we discuss how (125)I-Tf can be used to label a cohort of endocytosed Tf receptor, and then we provide methods to measure the rate of recycling of the (125)I-Tf-receptor complex. We also show how this approach, which is easily adaptable to other proteins, can be used to simultaneously measure the normally small amount of (125)I-Tf transcytosis and degradation.

Mechanical checkpoint for persistent cell polarization in adhesion-naive fibroblasts

Cell polarization is a fundamental biological process implicated in nearly every aspect of multicellular development. The role of cell-extracellular matrix contacts in the establishment and the orientation of cell polarity have been extensively studied. However, the respective contributions of substrate mechanics and biochemistry remain unclear. Here we propose a believed novel single-cell approach to assess the minimal polarization trigger. Using nonadhered round fibroblast cells, we show that stiffness sensing through single localized integrin-mediated cues are necessary and sufficient to trigger and direct a shape polarization. In addition, the traction force developed by cells has to reach a minimal threshold of 56 ± 1.6 pN for persistent polarization. The polarization kinetics increases with the stiffness of the cue. The polarized state is characterized by cortical actomyosin redistribution together with cell shape change. We develop a physical model supporting the idea that a local and persistent inhibition of actin polymerization and/or myosin activity is sufficient to trigger and sustain the polarized state. Finally, the cortical polarity propagates to an intracellular polarity, evidenced by the reorientation of the centrosome. Our results define the minimal adhesive requirements and quantify the mechanical checkpoint for persistent cell shape and organelle polarization, which are critical regulators of tissue and cell development.

The interdependence of the Rho GTPases and apicobasal cell polarity

Signaling via the Rho GTPases provides crucial regulation of numerous cell polarization events, including apicobasal (AB) polarity, polarized cell migration, polarized cell division and neuronal polarity. Here we review the relationships between the Rho family GTPases and epithelial AB polarization events, focusing on the 3 best-characterized members: Rho, Rac and Cdc42. We discuss a multitude of processes that are important for AB polarization, including lumen formation, apical membrane specification, cell-cell junction assembly and maintenance, as well as tissue polarity. Our discussions aim to highlight the immensely complex regulatory mechanisms that encompass Rho GTPase signaling during AB polarization. More specifically, in this review we discuss several emerging common themes, that include: 1) the need for Rho GTPase activities to be carefully balanced in both a spatial and temporal manner through a multitude of mechanisms; 2) the existence of signaling feedback loops and crosstalk to create robust cellular responses; and 3) the frequent multifunctionality that exists among AB polarity regulators. Regarding this latter theme, we provide further discussion of the potential plasticity of the cell polarity machinery and as a result the possible implications for human disease.

The Cdc42 Effector Kinase PAK4 Localizes to Cell-Cell Junctions and Contributes to Establishing Cell Polarity

The serine/threonine kinase PAK4 is a Cdc42 effector whose role is not well understood; overexpression of PAK4 has been associated with some cancers, and there are reports that correlate kinase level with increased cell migration in vitro. Here we report that PAK4 is primarily associated with cell-cell junctions in all the cell lines we tested, and fails to accumulate at focal adhesions or at the leading edge of migrating cells. In U2OS osteosarcoma and MCF-7 breast cancer cell lines, PAK4 depletion did not affect collective cell migration, but affected cell polarization. By contrast, Cdc42 depletion (as reported by many studies) caused a strong defect in junctional assembly in multiple cells lines. We also report that the depletion of PAK4 protein or treatment of cells with the PAK4 inhibitor PF-3758309 can lead to defects in centrosome reorientation (polarization) after cell monolayer wounding. These experiments are consistent with PAK4 forming part of a conserved cell-cell junctional polarity Cdc42 complex. We also confirm β-catenin as a target for PAK4 in these cells. Treatment of cells with PF-3758309 caused inhibition of β-catenin Ser-675 phosphorylation, which is located predominantly at cell-cell junctions.

Signalling at tight junctions during epithelial differentiation and microbial pathogenesis

Tight junctions are a component of the epithelial junctional complex, and they form the paracellular diffusion barrier that enables epithelial cells to create cellular sheets that separate compartments with different compositions. The assembly and function of tight junctions are intimately linked to the actomyosin cytoskeleton and, hence, are under the control of signalling mechanisms that regulate cytoskeletal dynamics. Tight junctions not only receive signals that guide their assembly and function, but transmit information to the cell interior to regulate cell proliferation, migration and survival. As a crucial component of the epithelial barrier, they are often targeted by pathogenic viruses and bacteria, aiding infection and the development of disease. In this Commentary, we review recent progress in the understanding of the molecular signalling mechanisms that drive junction assembly and function, and the signalling processes by which tight junctions regulate cell behaviour and survival. We also discuss the way in which junctional components are exploited by pathogenic viruses and bacteria, and how this might affect junctional signalling mechanisms.

Proteomic analysis of proteins surrounding occludin and claudin-4 reveals their proximity to signaling and trafficking networks

Tight junctions are complex membrane structures that regulate paracellular movement of material across epithelia and play a role in cell polarity, signaling and cytoskeletal organization. In order to expand knowledge of the tight junction proteome, we used biotin ligase (BioID) fused to occludin and claudin-4 to biotinylate their proximal proteins in cultured MDCK II epithelial cells. We then purified the biotinylated proteins on streptavidin resin and identified them by mass spectrometry. Proteins were ranked by relative abundance of recovery by mass spectrometry, placed in functional categories, and compared not only among the N- and C- termini of occludin and the N-terminus of claudin-4, but also with our published inventory of proteins proximal to the adherens junction protein E-cadherin and the tight junction protein ZO-1. When proteomic results were analyzed, the relative distribution among functional categories was similar between occludin and claudin-4 proximal proteins. Apart from already known tight junction- proteins, occludin and claudin-4 proximal proteins were enriched in signaling and trafficking proteins, especially endocytic trafficking proteins. However there were significant differences in the specific proteins comprising the functional categories near each of the tagging proteins, revealing spatial compartmentalization within the junction complex. Taken together, these results expand the inventory of known and unknown proteins at the tight junction to inform future studies of the organization and physiology of this complex structure.

IQGAP1 controls tight junction formation through differential regulation of claudin recruitment

IQGAP1 is a scaffolding protein previously implicated in adherens junction formation. However, its role in the establishment or maintenance of tight junctions (TJs) has not been explored. We hypothesized that IQGAP1 could regulate TJ formation by modulating the expression and/or localization of junctional proteins, and we systematically tested this hypothesis in the model Madin-Darby canine kidney (MDCK) cell line. We find that IQGAP1 silencing enhances a transient increase in transepithelial electrical resistance (TER) observed during the early stages of TJ formation (Cereijido et al., 1978). Quantitative microscopy and biochemical experiments suggest that this effect of IQGAP1 on TJ assembly is accounted for by reduced expression and TJ recruitment of claudin 2, and increased TJ recruitment of claudin 4. Furthermore, we show that IQGAP1 also regulates TJ formation through its interactor CDC42, because IQGAP1 knockdown increases the activity of the CDC42 effector JNK and dominant-negative CDC42 prevents the increase in TER caused by IQGAP1 silencing. Hence, we provide evidence that IQGAP1 modulates TJ formation by a twofold mechanism: (1) controlling the expression and recruitment of claudin 2 and recruitment of claudin 4 to the TJ, and (2) transient inhibition of the CDC42-JNK pathway.

MicroRNA-205 targets tight junction-related proteins during urothelial cellular differentiation

The mammalian bladder urothelium classified as basal, intermediate, and terminally differentiated umbrella cells offers one of the most effective permeability barrier functions known to exist in nature because of the formation of apical uroplakin plaques and tight junctions. To improve our understanding of urothelial differentiation, we analyzed the microRNA (miRNA) expression profiles of mouse urinary tissues and by TaqMan miRNA analysis of microdissected urothelial layers and in situ miRNA-specific hybridization to determine the dependence of these miRNAs on the differentiation stage. Our in situ hybridization studies revealed that miR-205 was enriched in the undifferentiated basal and intermediate cell layers. We then used a quantitative proteomics approach to identify miR-205 target genes in primary cultured urothelial cells subjected to antagomir-mediated knockdown of specific miRNAs. Twenty-four genes were reproducibly regulated by miR-205; eleven of them were annotated as cell junction- and tight junction-related molecules. Western blot analysis demonstrated that antagomir-induced silencing of miR-205 in primary cultured urothelial cells elevated the expression levels of Tjp1, Cgnl1, and Cdc42. Ectopic expression of miR-205 in MDCK cells inhibited the expression of tight junction proteins and the formation of tight junctions. miR-205- knockdown urothelial cells showed alterations in keratin synthesis and increases of uroplakin Ia and Ib, which are the urothelial differentiation products. These results suggest that miR-205 may contribute a role in regulation of urothelial differentiation by modulating the expression of tight junction-related molecules.

CLN3 deficient cells display defects in the ARF1-Cdc42 pathway and actin-dependent events

Juvenile Batten disease (juvenile neuronal ceroid lipofuscinosis, JNCL) is a devastating neurodegenerative disease caused by mutations in CLN3, a protein of undefined function. Cell lines derived from patients or mice with CLN3 deficiency have impairments in actin-regulated processes such as endocytosis, autophagy, vesicular trafficking, and cell migration. Here we demonstrate the small GTPase Cdc42 is misregulated in the absence of CLN3, and thus may be a common link to multiple cellular defects. We discover that active Cdc42 (Cdc42-GTP) is elevated in endothelial cells from CLN3 deficient mouse brain, and correlates with enhanced PAK-1 phosphorylation, LIMK membrane recruitment, and altered actin-driven events. We also demonstrate dramatically reduced plasma membrane recruitment of the Cdc42 GTPase activating protein, ARHGAP21. In line with this, GTP-loaded ARF1, an effector of ARHGAP21 recruitment, is depressed. Together these data implicate misregulated ARF1-Cdc42 signaling as a central defect in JNCL cells, which in-turn impairs various cell functions. Furthermore our findings support concerted action of ARF1, ARHGAP21, and Cdc42 to regulate fluid phase endocytosis in mammalian cells. The ARF1-Cdc42 pathway presents a promising new avenue for JNCL therapeutic development.

Rab14 regulation of claudin-2 trafficking modulates epithelial permeability and lumen morphogenesis

Epithelial permeability is regulated by targeted insertion and recycling of tight junction proteins. Rab14 regulates the lysosomal targeting of the leaky claudin, claudin-2, and depletion of Rab14 results in increased transepithelial resistance and aberrant morphogenesis in three-dimensional culture.

Regulation of epithelial barrier function requires targeted insertion of tight junction proteins that have distinct selectively permeable characteristics. The insertion of newly synthesized proteins and recycling of internalized tight junction components control both polarity and junction function. Here we show that the small GTPase Rab14 regulates tight junction structure. In Madin–Darby canine kidney (MDCK) II cells, Rab14 colocalizes with junctional proteins, and knockdown of Rab14 results in increased transepithelial resistance. In cells without Rab14, there are small changes in the trafficking of claudin-1 and occludin. In addition, there is substantial depletion of the leaky claudin, claudin-2, but not other tight junction components. The loss of claudin-2 is complemented by inhibition of lysosomal function, suggesting that Rab14 sorts claudin-2 out of the lysosome-directed pathway. MDCK I cells lack claudin-2 endogenously, and knockdown of Rab14 in these cells does not result in a change in transepithelial resistance, suggesting that the effect is specific to claudin-2 trafficking. Furthermore, leaky claudins have been shown to be required for epithelial morphogenesis, and knockdown of Rab14 results in failure to form normal single-lumen cysts in three-dimensional culture. These results implicate Rab14 in specialized trafficking of claudin-2 from the recycling endosome.

Cdc42 coordinates proliferation, polarity, migration, and differentiation of small intestinal epithelial cells in mice

BACKGROUND & AIMS

Cdc42 is a Rho GTPase that regulates diverse cellular functions, including proliferation, differentiation, migration, and polarity. In the intestinal epithelium, a balance among these events maintains homeostasis. We used genetic techniques to investigate the role of Cdc42 in intestinal homeostasis and its mechanisms.

METHODS

We disrupted Cdc42 specifically in intestinal epithelial cells by creating Cdc42flox/flox-villin-Cre+ and Cdc42flox/flox-Rosa26-CreER+ mice. We collected intestinal and other tissues, and analyzed their cellular, molecular, morphologic, and physiologic features, compared with the respective heterozygous mice.

RESULTS

In all mutant mice studied, the intestinal epithelium had gross hyperplasia, crypt enlargement, microvilli inclusion, and abnormal epithelial permeability. Cdc42 deficiency resulted in defective Paneth cell differentiation and localization without affecting the differentiation of other cell lineages. In mutant intestinal crypts, proliferating stem and progenitor cells increased, compared with control mice, resulting in increased crypt depth. Cdc42 deficiency increased migration of stem and progenitor cells along the villi, caused a mild defect in the apical junction orientation, and impaired intestinal epithelium polarity, which can contribute to the observed defective intestinal permeability. The intestinal epithelium of the Cdc42flox/flox-villin-Cre+ and Cdc42flox/flox-Rosa26-CreER+ mice appeared similar to that of patients with microvillus inclusion disease. In the digestive track, loss of Cdc42 also resulted in crypt hyperplasia in the colon, but not the stomach.

CONCLUSIONS

Cdc42 regulates proliferation, polarity, migration, and differentiation of intestinal epithelial cells in mice and maintains intestine epithelial barrier and homeostasis. Defects in Cdc42 signaling could be associated with microvillus inclusion disease.

The digestive neuronal-glial-epithelial unit: a new actor in gut health and disease

The monolayer of columnar epithelial cells lining the gastrointestinal tract--the intestinal epithelial barrier (IEB)--is the largest exchange surface between the body and the external environment. The permeability of the IEB has a central role in the regulation of fluid and nutrient intake as well as in the control of the passage of pathogens. The functions of the IEB are highly regulated by luminal as well as internal components, such as bacteria or immune cells, respectively. Evidence indicates that two cell types of the enteric nervous system (ENS), namely enteric neurons and enteric glial cells, are potent modulators of IEB functions, giving rise to the novel concept of a digestive 'neuronal-glial-epithelial unit' akin to the neuronal-glial-endothelial unit in the brain. In this Review, we summarize findings demonstrating that the ENS is a key regulator of IEB function and is actively involved in pathologies associated with altered barrier function.

PIP5KIβ selectively modulates apical endocytosis in polarized renal epithelial cells

Localized synthesis of phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂] at clathrin coated pits (CCPs) is crucial for the recruitment of adaptors and other components of the internalization machinery, as well as for regulating actin dynamics during endocytosis. PtdIns(4,5)P₂ is synthesized from phosphatidylinositol 4-phosphate by any of three phosphatidylinositol 5-kinase type I (PIP5KI) isoforms (α, β or γ). PIP5KIβ localizes almost exclusively to the apical surface in polarized mouse cortical collecting duct cells, whereas the other isoforms have a less polarized membrane distribution. We therefore investigated the role of PIP5KI isoforms in endocytosis at the apical and basolateral domains. Endocytosis at the apical surface is known to occur more slowly than at the basolateral surface. Apical endocytosis was selectively stimulated by overexpression of PIP5KIβ whereas the other isoforms had no effect on either apical or basolateral internalization. We found no difference in the affinity for PtdIns(4,5)P₂-containing liposomes of the PtdIns(4,5)P₂ binding domains of epsin and Dab2, consistent with a generic effect of elevated PtdIns(4,5)P₂ on apical endocytosis. Additionally, using apical total internal reflection fluorescence imaging and electron microscopy we found that cells overexpressing PIP5KIβ have fewer apical CCPs but more internalized coated structures than control cells, consistent with enhanced maturation of apical CCPs. Together, our results suggest that synthesis of PtdIns(4,5)P₂ mediated by PIP5KIβ is rate limiting for apical but not basolateral endocytosis in polarized kidney cells. PtdIns(4,5)P₂ may be required to overcome specific structural constraints that limit the efficiency of apical endocytosis.

CDC42 is required for structural patterning of the lung during development

The formation of highly branched epithelial structures is critical for the development of many essential organs, including lung, liver, pancreas, kidney and mammary glands. Elongation and branching of these structures require precise control of complex morphogenetic processes that are dependent upon coordinate regulation of cell shape, apical-basal polarity, proliferation, migration, and interactions among multiple cell types. Herein, we demonstrate that temporal-spatial regulation of epithelial cell polarity by the small GTPase, CDC42, is essential for branching morphogenesis of the developing lung. Epithelial cell-specific deletion of CDC42 in fetal mice disrupted epithelial cell polarity, the actin cytoskeleton, intercellular contacts, directional trafficking of proteins, proliferation and mitotic spindle orientation, impairing the organization and patterning of the developing respiratory epithelium and adjacent mesenchyme. Transition from a pseudostratified to a simple columnar epithelium was impaired, consistent with coordinate dysregulation of epithelial cell polarity, mitotic spindle orientation, and repositioning of mitotic cells within the epithelium during cell cycle progression. Expression of sonic hedgehog and its receptor, patched-1, was decreased, while fibroblast growth factor 10 expression in the mesenchyme was expanded, resulting in disruption of branching morphogenesis and bronchiolar smooth muscle formation in this model. CDC42 is required for spatial positioning of proliferating epithelial cells, as well as signaling interactions between the epithelium and mesenchyme and is, therefore, essential for formation and maintenance of the respiratory tract during morphogenesis of the fetal lung.

Tight junctions in skin: new perspectives

Tight junctions (TJs) are intercellular contacts that seal the space between the individual cells of an epithelial sheet or stratifying epithelia, such as the epidermis, so that they can collectively separate tissue compartments. Intercellular junctions, such as adherens and TJs, play a crucial role in the formation and maintenance of epithelial and endothelial barriers. A variety of components including claudins, occludin, tricellulin, zonula occluden proteins and junctional adhesion molecules have been identified in complex localization patterns in mammalian epidermis. In several skin diseases that are characterized by impaired skin barrier function, altered proliferation/differentiation of the epidermis and/or infiltration of inflammatory cells, altered expression patterns of TJ proteins have been observed. This review is aimed at providing an insight into the molecular composition, tools for identification and understanding the role of TJs in skin diseases and barrier function regulation.

C(16)-Ceramide-induced F-actin regulation stimulates mouse embryonic stem cell migration: involvement of N-WASP/Cdc42/Arp2/3 complex and cofilin-1/α-actinin

Ceramide, a major structural element in the cellular membrane, is a key regulatory factor in various cellular behaviors that are dependent on ceramide-induced association of specific proteins. However, molecular mechanisms that regulate ceramide-induced embryonic stem cell (ESC) migration are still not well understood. Thus, we investigated the effect of ceramide on migration and its related signal pathways in mouse ESCs. Among ceramide species with different fatty acid chain lengths, C(16)-Cer increased migration of mouse ESCs in a dose- (≥1μM) and time-dependent (≥8h) manners, as determined by the cell migration assay. C(16)-Cer (10μM) increased protein-kinase C (PKC) phosphorylation. Subsequently, C(16)-Cer increased focal adhesion kinase (FAK) and Paxillin phosphorylation, which were inhibited by PKC inhibitor Bisindolylmaleimide I (1μM). When we examined for the downstream signaling molecules, C(16)-Cer activated small G protein (Cdc42) and increased the formation of complex with Neural Wiskott-Aldrich Syndrome Protein (N-WASP)/Cdc42/Actin-Related Protein 2/3 (Arp2/3). This complex formation was disrupted by FAK- and Paxillin-specific siRNAs. Furthermore, C(16)-Cer-induced increase of filamentous actin (F-actin) expression was inhibited by Cdc42-, N-WASP-, and Arp2/3-specific siRNAs, respectively. Indeed, C(16)-Cer increased cofilin-1/F-actin interaction or F-actin/α-actinin-1 and α-actinin-4 interactions in the cytoskeleton compartment, which was reversed by Cdc42-specific siRNA. Finally, C(16)-Cer-induced increase of cell migration was inhibited by knocking down each signal pathway-related molecules with siRNA or inhibitors. In conclusion, C(16)-Cer enhances mouse ESC migration through the regulation of PKC and FAK/Paxillin-dependent N-WASP/Cdc42/Arp2/3 complex formation as well as through promoting the interaction between cofilin-1 or α-actinin-1/-4 and F-actin.

Epithelial junction formation requires confinement of Cdc42 activity by a novel SH3BP1 complex

Epithelial cell-cell adhesion and morphogenesis require dynamic control of actin-driven membrane remodeling. The Rho guanosine triphosphatase (GTPase) Cdc42 regulates sequential molecular processes during cell-cell junction formation; hence, mechanisms must exist that inactivate Cdc42 in a temporally and spatially controlled manner. In this paper, we identify SH3BP1, a GTPase-activating protein for Cdc42 and Rac, as a regulator of junction assembly and epithelial morphogenesis using a functional small interfering ribonucleic acid screen. Depletion of SH3BP1 resulted in loss of spatial control of Cdc42 activity, stalled membrane remodeling, and enhanced growth of filopodia. SH3BP1 formed a complex with JACOP/paracingulin, a junctional adaptor, and CD2AP, a scaffolding protein; both were required for normal Cdc42 signaling and junction formation. The filamentous actin-capping protein CapZ also associated with the SH3BP1 complex and was required for control of actin remodeling. Epithelial junction formation and morphogenesis thus require a dual activity complex, containing SH3BP1 and CapZ, that is recruited to sites of active membrane remodeling to guide Cdc42 signaling and cytoskeletal dynamics.

Nonfunction of the ECT2 gene may cause renal tubulointerstitial injury leading to focal segmental glomerulosclerosis

Background

Secondary focal segmental glomerulosclerosis (FSGS) follows congenital or acquired tubulointerstitial alterations such as in Dent’s disease, Lowe syndrome, and reflux nephropathy. Failure of adequate regeneration after tubulointerstitial injury, or abnormal tubulogenesis, can disturb intrarenal blood circulation, causing excessive glomerular filtration. The epithelial cell-transforming sequence 2 gene (ECT2) contributes to tight junction function in epithelial cells.

Methods

We encountered two patients with a nonfunctioning ECT2 genotype who later developed FSGS. Both developed proteinuria associated with acute renal failure in early childhood.

Results

Renal biopsy specimens showed marked tubulointerstitial nephritis at the onset of proteinuria, later progressing to FSGS consequent to tubulointerstitial injury. The patients did not respond to corticosteroids and attained only incomplete remission upon cyclosporine A administration. One patient received a maternal renal transplant with good function and no rejection.

Conclusions

ECT2 is important for tight junction function and maintenance of cell polarity. Nonfunction of this gene may cause renal tubulointerstitial injury, progressing to glomerular sclerosis.

Lipid polarity is maintained in absence of tight junctions

The role of tight junctions (TJs) in the establishment and maintenance of lipid polarity in epithelial cells has long been a subject of controversy. We have addressed this issue using lysenin, a toxin derived from earthworms, and an influenza virus labeled with a fluorescent lipid, octadecylrhodamine B (R18). When epithelial cells are stained with lysenin, lysenin selectively binds to their apical membranes. Using an artificial liposome, we demonstrated that lysenin recognizes the membrane domains where sphingomyelins are clustered. Interestingly, lysenin selectively stained the apical membranes of epithelial cells depleted of zonula occludens proteins (ZO-deficient cells), which completely lack TJs. Furthermore, the fluorescent lipid inserted into the apical membrane by fusion with the influenza virus did not diffuse to the lateral membrane in ZO-deficient epithelial cells. This study revealed that sphingomyelin-cluster formation occurs only in the apical membrane and that lipid polarity is maintained even in the absence of TJs.

The FGD homologue EXC-5 regulates apical trafficking in C. elegans tubules

Maintenance of the shape of biological tubules is critical for development and physiology of metazoan organisms. Loss of function of the Caenorhabditis elegans FGD protein EXC-5 allows large fluid-filled cysts to form in the lumen of the single-cell excretory canal tubules, while overexpression of exc-5 causes defects at the tubule's basolateral surface. We have examined the effects of altering expression levels of exc-5 on the distribution of fluorescently-marked subcellular organelles. In exc-5 mutants, early endosomes build up in the cell, especially in areas close to cysts, while recycling endosomes are depleted. Endosome morphology changes prior to cyst formation. Conversely, when exc-5 is overexpressed, recycling endosomes are enriched. Since FGD proteins activate the small GTPases CDC42 and Rac, these results support the hypothesis that EXC-5 acts through small GTPases to move material from apical early endosomes to recycling endosomes, and that loss of such movement is likely the cause of tubule deformation both in nematodes and in tissues affected by FGD dysfunction such as Charcot-Marie-Tooth Syndrome type 4H.

Phosphoinositides in cell architecture

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

Par6B and atypical PKC regulate mitotic spindle orientation during epithelial morphogenesis

Cdc42 plays an evolutionarily conserved role in promoting cell polarity and is indispensable during epithelial morphogenesis. To further investigate the role of Cdc42, we have used a three-dimensional matrigel model, in which single Caco-2 cells develop to form polarized cysts. Using this system, we previously reported that Cdc42 controls mitotic spindle orientation during cell division to correctly position the apical surface in a growing epithelial structure. In the present study, we have investigated the specific downstream effectors through which Cdc42 controls this process. Here, we report that Par6B and its binding partner, atypical protein kinase C (aPKC), are required to regulate Caco-2 morphogenesis. Depletion or inhibition of Par6B or aPKC phenocopies the loss of Cdc42, inducing misorientation of the mitotic spindle, mispositioning of the nascent apical surface, and ultimately, the formation of aberrant cysts with multiple lumens. Mechanistically, Par6B and aPKC function interdependently in this context. Par6B localizes to the apical surface of Caco-2 cysts and is required to recruit aPKC to this compartment. Conversely, aPKC protects Par6B from proteasomal degradation, in a kinase-independent manner. In addition, we report that depletion or inhibition of aPKC induces robust apoptotic cell death in Caco-2 cells, significantly reducing both cyst size and number. Cell survival and apical positioning depend upon different thresholds of aPKC expression, suggesting that they are controlled by distinct downstream pathways. We conclude that Par6B and aPKC control mitotic spindle orientation in polarized epithelia and, furthermore, that aPKC coordinately regulates multiple processes to promote morphogenesis.

Compartmentalized Ras proteins transform NIH 3T3 cells with different efficiencies

Ras GTPases were long thought to function exclusively from the plasma membrane (PM). However, a current model suggests that Ras proteins can compartmentalize to regulate different functions, and an oncogenic H-Ras mutant that is restricted to the endomembrane can still transform cells. In this study, we demonstrated that cells transformed by endomembrane-restricted oncogenic H-Ras formed tumors in nude mice. To define downstream targets of endomembrane Ras pathways, we analyzed Cdc42, which concentrates in the endomembrane and has been shown to act downstream of Ras in Schizosaccharomyces pombe. Our data show that cell transformation induced by endomembrane-restricted oncogenic H-Ras was blocked when Cdc42 activity was inhibited. Moreover, H-Ras formed a complex with Cdc42 on the endomembrane, and this interaction was enhanced when H-Ras was GTP bound or when cells were stimulated by growth factors. H-Ras binding evidently induced Cdc42 activation by recruiting and/or activating Cdc42 exchange factors. In contrast, when constitutively active H-Ras was restricted to the PM by fusing to a PM localization signal from the Rit GTPase, the resulting protein did not detectably activate Cdc42 although it activated Raf-1 and efficiently induced hallmarks of Ras-induced senescence in human BJ foreskin fibroblasts. Surprisingly, PM-restricted oncogenic Ras when expressed alone could only weakly transform NIH 3T3 cells; however, when constitutively active Cdc42 was coexpressed, together they transformed cells much more efficiently than either one alone. These data suggest that efficient cell transformation requires Ras proteins to interact with Cdc42 on the endomembrane and that in order for a given Ras protein to fully transform cells, multiple compartment-specific Ras pathways need to work cooperatively.

Nucleofection disrupts tight junction fence function to alter membrane polarity of renal epithelial cells

Here, we compared the effects of nucleofection and lipid-based approaches to introduce siRNA duplexes on the subsequent development of membrane polarity in kidney cells. Nucleofection of Madin-Darby canine kidney (MDCK) cells, even with control siRNA duplexes, disrupted the initial surface polarity as well as the steady-state distribution of membrane proteins. Transfection using lipofectamine yielded slightly less efficient knockdown but did not disrupt membrane polarity. Polarized secretion was unaffected by nucleofection, suggesting a selective defect in the development of membrane polarity. Cilia frequency and length were not altered by nucleofection. However, the basolateral appearance of a fluorescent lipid tracer added to the apical surface of nucleofected cells was dramatically enhanced relative to untransfected controls or lipofectamine-treated cells. In contrast, [(3)H]inulin diffusion and transepithelial electrical resistance were not altered in nucleofected cells compared with untransfected ones. We conclude that nucleofection selectively hinders development of the tight junction fence function in MDCK cells.

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.

Regulation of blood-testis barrier dynamics by TGF-beta3 is a Cdc42-dependent protein trafficking event

In the testis, the blood-testis barrier (BTB) is constituted by specialized junctions between adjacent Sertoli cells in the seminiferous epithelium near the basement membrane. Although the BTB is one of the tightest blood-tissue barriers in the mammalian body, it undergoes extensive restructuring at stage VIII of the seminiferous epithelial cycle to facilitate the transit of preleptotene spermatocytes. Thus, meiosis and postmeiotic germ cell development take place in the seminiferous epithelium behind the BTB. Cytokines (e.g., TGF-beta3) are known to regulate BTB dynamics by enhancing the endocytosis of integral membrane proteins and their intracellular degradation. This thus reduces the levels of proteins above the spermatocytes in transit at the BTB, causing its disruption after testosterone-induced new tight junction (TJ) fibrils are formed behind these cells. By using Sertoli cells cultured in vitro with an established TJ permeability barrier that mimicked the BTB in vivo, Cdc42 was shown to be a crucial regulator that mediated the TGF-beta3-induced BTB disruption. TGF-beta3 was shown to activate Cdc42 to its active GTP-bound form. However, an inactivation of Cdc42 by overexpressing its dominant-negative mutant T17N in Sertoli cell epithelium was shown to block the TGF-beta3-induced acceleration in protein endocytosis. Consequently, this prevented the disruption of Sertoli cell TJ permeability barrier and redistribution of TJ proteins (e.g., CAR and ZO-1) from the cell-cell interface to cell cytosol caused by TGF-beta3. In summary, Cdc42 is a crucial regulatory component in the TGF-beta3-mediated cascade of events that leads to the disruption of the TJ fibrils above the preleptotene spermatocytes to facilitate their transit.