Mouse Crumbs3 sustains epithelial tissue morphogenesis in vivo

Crb3 and NF2: A dynamic duo that controls assembly of the apical junctions and barrier function via Rho/ROCK signaling

The gastrointestinal epithelium serves as a critical barrier separating intestinal lumen contents from the underlying tissue environment. Structure and function of the apical junctional complex (AJC), comprising tight and adherens junctions, are essential for establishing and maintaining a polarized and functional epithelial barrier. In this study, we investigated mechanisms by which an apical polarity protein Crumbs homolog 3 (CRB3) regulates AJC assembly and barrier function in primary murine intestinal epithelial cells. Using primary colonic epithelial cells (colonoids) derived from inducible and conditional Crb3 knockdown (Crb3ERΔIEC) and control mice (Crb3fl/fl), we demonstrate that Crb3 loss leads to compromised epithelial barrier function that was associated with hypercontractile perijunctional actomyosin and defective assembly of the AJC. We identified CRB3 associates with the Band 4.1 family of cytoskeletal linker proteins, Merlin (NF2) via FERM (band4.1/ezrin/radixin/moesin) binding domain (FBD) of CRB3. Interestingly, NF2 knockdown in cultured intestinal epithelial cells phenocopied the effect of CRB3 deletion, supporting a coordinated role in AJC formation and barrier assembly. Moreover, increased active Rho was detected in assembling junctions of Crb3-null cells and inhibition of ROCKII and myosin II alleviated the hypercontractile phenotype, highlighting involvement of Rho/ROCK signaling. Additionally, increased vinculin localization at the AJC seen in Crb3-null epithelial cells indicates elevated tension at junctions. Our findings underscore the important role of Crb3 and NF2 in regulating contractility of the perijunctional actomyosin ring, mechanical tension at the AJC and barrier function via Rho/ROCK signaling during junctional assembly in intestinal epithelial cells.

PALS1-dependent modulations of mRNA profiles in MDCK II cells grown in non-confluent monolayers and three-dimensional cysts

In epithelia, apicobasal cell polarization is closely linked to cell-cell contact formation, both controlled by the conserved Crumbs (CRB) complex, which includes the transmembrane protein Crumbs (CRB3a) and adapter proteins PALS1, PATJ, and LIN7c. In MDCK II cells, a model for cell polarization, depletion of PALS1 - which binds to all CRB components - leads to defective cell polarization and improper distribution of tight junction proteins, resulting in severe epithelial barrier defects in 3D cyst models. This study investigated whether this phenotype is associated with transcriptional changes by analyzing wildtype (WT) and PALS1 knockout (KO) MDCK II cell lines grown under non-confluent conditions and in 3D cyst cultures. Our results indicate that the transition from non-confluent cells to 3D cysts involves numerous differentially expressed genes (DEGs) in both WT and KO cells. Importantly, the analyses revealed significant overlaps between WT and KO cells in their maturation processes, suggesting that most identified DEGs are linked to differentiation from non-confluent to polarized MDCK cells and likely not a result of PALS1 deficiency. Gene Ontology (GO) enrichment and over-representation analyses using REACTOME and KEGG databases confirmed these similarities. In contrast, the direct comparison of WT and KO cells at the two stages showed fewer DEGs and overlaps in associated biological processes and signaling pathways. DEGs associated with the 3D stage, in which the phenotype manifests, contain DEGs and pathways that were predominantly linked to cell cycle linked processes, centromere assembly, or DNA replication. Furthermore, the transcription of genes encoding key junction proteins, additional polarity proteins, and cell-substrate interaction proteins is less affected by the loss of PALS1, indicating that PALS1 influences the transcriptional profiles in epithelial cells as a modulating factor.

Crb3 is required to organize the apical domain of multiciliated cells

Cell shape changes mainly rely on the remodeling of the actin cytoskeleton. Multiciliated cells (MCCs) of the mucociliary epidermis of Xenopus laevis embryos, as they mature, dramatically reshape their apical domain to grow cilia, in coordination with the underlying actin cytoskeleton. Crumbs (Crb) proteins are multifaceted transmembrane apical polarity proteins known to recruit actin linkers and promote apical membrane growth. Here, we identify the homeolog Crb3.L as an important player for the migration of centrioles or basal bodies (collectively centrioles/BBs) and apical domain morphogenesis in MCCs. Crb3.L is present in cytoplasmic vesicles close to the ascending centrioles/BBs, where it partially colocalizes with Rab11a. Crb3.L morpholino-mediated depletion in MCCs caused abnormal migration of centrioles/BBs, a reduction of their apical surface, disorganization of their apical actin meshwork and defective ciliogenesis. Rab11a morpholino-mediated depletion phenocopied Crb3.L loss-of-function in MCCs. Thus, the control of centrioles/BBs migration by Crb3.L might be mediated by Rab11a-dependent apical trafficking. Furthermore, we show that both phospho-activated ERM (pERM; Ezrin-Radixin-Moesin) and Crb3.L are recruited to the growing apical domain of MCCs, where Crb3.L likely anchors pERM, allowing actin-dependent expansion of the apical membrane.

PALS1 is a key regulator of the lateral distribution of tight junction proteins in renal epithelial cells

The evolutionarily conserved apical Crumbs (CRB) complex, consisting of the core components CRB3a (an isoform of CRB3), PALS1 and PATJ, plays a key role in epithelial cell-cell contact formation and cell polarization. Recently, we observed that deletion of one Pals1 allele in mice results in functional haploinsufficiency characterized by renal cysts. Here, to address the role of PALS1 at the cellular level, we generated CRISPR/Cas9-mediated PALS1-knockout MDCKII cell lines. The loss of PALS1 resulted in increased paracellular permeability, indicating an epithelial barrier defect. This defect was associated with a redistribution of several tight junction-associated proteins from bicellular to tricellular contacts. PALS1-dependent localization of tight junction proteins at bicellular junctions required its interaction with PATJ. Importantly, reestablishment of the tight junction belt upon transient F-actin depolymerization or upon Ca2+ removal was strongly delayed in PALS1-deficient cells. Additionally, the cytoskeleton regulator RhoA was redistributed from junctions into the cytosol under PALS1 knockout. Together, our data uncover a critical role of PALS1 in the coupling of tight junction proteins to the F-actin cytoskeleton, which ensures their correct distribution along bicellular junctions and the formation of tight epithelial barrier.

Apical-basal polarity in the gut

Apical-Basal polarity is a fundamental property of all epithelial cells that underlies both their form and function. The gut is made up of a single layer of intestinal epithelial cells, with distinct apical, lateral and basal domains. Occluding junctions at the apical side of the lateral domains create a barrier between the gut lumen and the body, which is crucial for tissue homeostasis, protection against gastrointestinal pathogens and for the maintenance of the immune response. Apical-basal polarity in most epithelia is established by conserved polarity factors, but recent evidence suggests that the gut epithelium in at least some organisms polarises by novel mechanisms. In this review, we discuss the recent advances in understanding polarity factors by focussing on work in C. elegans, Drosophila, Zebrafish and Mouse.

PATJ inhibits histone deacetylase 7 to control tight junction formation and cell polarity

The conserved multiple PDZ-domain containing protein PATJ stabilizes the Crumbs-Pals1 complex to regulate apical-basal polarity and tight junction formation in epithelial cells. However, the molecular mechanism of PATJ's function in these processes is still unclear. In this study, we demonstrate that knockout of PATJ in epithelial cells results in tight junction defects as well as in a disturbed apical-basal polarity and impaired lumen formation in three-dimensional cyst assays. Mechanistically, we found PATJ to associate with and inhibit histone deacetylase 7 (HDAC7). Inhibition or downregulation of HDAC7 restores polarity and lumen formation. Gene expression analysis of PATJ-deficient cells revealed an impaired expression of genes involved in cell junction assembly and membrane organization, which is rescued by the downregulation of HDAC7. Notably, the function of PATJ regulating HDAC7-dependent cilia formation does not depend on its canonical interaction partner, Pals1, indicating a new role of PATJ, which is distinct from its function in the Crumbs complex. By contrast, polarity and lumen phenotypes observed in Pals1- and PATJ-deficient epithelial cells can be rescued by inhibition of HDAC7, suggesting that the main function of this polarity complex in this process is to modulate the transcriptional profile of epithelial cells by inhibiting HDAC7.

CRB3 navigates Rab11 trafficking vesicles to promote γTuRC assembly during ciliogenesis

The primary cilium plays important roles in regulating cell differentiation, signal transduction, and tissue organization. Dysfunction of the primary cilium can lead to ciliopathies and cancer. The formation and organization of the primary cilium are highly associated with cell polarity proteins, such as the apical polarity protein CRB3. However, the molecular mechanisms by which CRB3 regulates ciliogenesis and the location of CRB3 remain unknown. Here, we show that CRB3, as a navigator, regulates vesicle trafficking in γ-tubulin ring complex (γTuRC) assembly during ciliogenesis and cilium-related Hh and Wnt signaling pathways in tumorigenesis. Crb3 knockout mice display severe defects of the primary cilium in the mammary ductal lumen and renal tubule, while mammary epithelial-specific Crb3 knockout mice exhibit the promotion of ductal epithelial hyperplasia and tumorigenesis. CRB3 is essential for lumen formation and ciliary assembly in the mammary epithelium. We demonstrate that CRB3 localizes to the basal body and that CRB3 trafficking is mediated by Rab11-positive endosomes. Significantly, CRB3 interacts with Rab11 to navigate GCP6/Rab11 trafficking vesicles to CEP290, resulting in intact γTuRC assembly. In addition, CRB3-depleted cells are unresponsive to the activation of the Hh signaling pathway, while CRB3 regulates the Wnt signaling pathway. Therefore, our studies reveal the molecular mechanisms by which CRB3 recognizes Rab11-positive endosomes to facilitate ciliogenesis and regulates cilium-related signaling pathways in tumorigenesis.

Pals1 functions in redundancy with SMAP1 to inhibit Arf6 in order to prevent Rac1-dependent colorectal cancer cell migration and invasion

Downregulation of cell-cell adhesion and increased motility are prerequisites for the metastasis of cancer cells. We have recently shown that downregulation of the tight junction adapter protein Pals1 in colorectal cancer cells results in an increase of cell migration, invasion, and metastasis due to the enhanced activation of Arf6 and Rac1. We now reveal a redundancy between the Arf6-GAP SMAP1 and Pals1 in regulating Arf6 activity and thereby Rac1-dependent cell migration. The gene encoding SMAP1 is frequently disrupted in microsatellite instable colorectal cancer specimen and cell lines. In cells expressing SMAP1, deletion of Pals1 leads to disturbed formation of tight junctions but has no impact on Arf6 activity and cell migration. In contrast, inactivation of both SMAP1 and Pals1 results in enhanced Arf6/Rac1 activity and increased cell migration and invasion. Furthermore, analyzing patient cohorts, we found a significant decrease in patient's survival when both genes were downregulated, in contrast to cases, when expression of only one of both genes was affected. Taken together, we identified a redundancy between SMAP1 and Pals1 in the regulation of activation of Arf6/Rac1, thereby controlling cell migration, invasion, and metastasis of colorectal cancer cells.

Transcription factor HNF1β controls a transcriptional network regulating kidney cell structure and tight junction integrity

Mutations in the hepatocyte nuclear factor (HNF)1β gene (HNF1B) cause autosomal dominant tubulointerstitial kidney disease, a rare and heterogeneous disease characterized by renal cysts and/or malformation, maturity-onset diabetes of the young, hypomagnesemia, and hypokalemia. The electrolyte disturbances may develop in the distal part of the nephron, which is important for fine-tuning of Mg²⁺ and Ca²⁺ reabsorption. Therefore, we aimed to study the transcriptional network directed by HNF1β in the distal part of the nephron. We combined HNF1β chromatin immunoprecipitation-sequencing and mRNA expression data to identify direct targets of HNF1β in a renal distal convoluted tubule cell line (mpkDCT). Gene Ontology term pathway analysis demonstrated enrichment of cell polarity, cell-cell junction, and cytoskeleton pathways in the dataset. Genes directly and indirectly regulated by HNF1β within these pathways included members of the apical and basolateral polarity complexes including Crumbs protein homolog 3 (Crb3), partitioning defective 6 homolog-β (Pard6b), and LLGL Scribble cell polarity complex component 2 (Llgl2). In monolayers of mouse inner medullary collecting duct 3 cells expressing dominant negative Hnf1b, tight junction integrity was compromised, as observed by reduced transepithelial electrical resistance values and increased permeability for fluorescein (0.4 kDa) compared with wild-type cells. Expression of dominant negative Hnf1b also led to a decrease in height (30%) and an increase in surface (58.5%) of cells grown on membranes. Moreover, three-dimensional spheroids formed by cells expressing dominant negative Hnf1b were reduced in size compared with wild-type spheroids (30%). Together, these findings demonstrate that HNF1β directs a transcriptional network regulating tight junction integrity and cell structure in the distal part of the nephron.NEW & NOTEWORTHY Genetic defects in transcription factor hepatocyte nuclear factor (HNF)1β cause a heterogeneous disease characterized by electrolyte disturbances, kidney cysts, and diabetes. By combining RNA-sequencing and HNF1β chromatin immunoprecipitation-sequencing data, we identified new HNF1β targets that were enriched for cell polarity pathways. Newly discovered targets included members of polarity complexes Crb3, Pard6b, and Llgl2. Functional assays in kidney epithelial cells demonstrated decreased tight junction integrity and a loss of typical cuboidal morphology in mutant Hnf1b cells.

Polarity in respiratory development, homeostasis and disease

The respiratory system is composed of a multitude of cells that organize to form complex branched airways that end in alveoli, which respectively function to guide air flow and mediate gas exchange with the bloodstream. The organization of the respiratory sytem relies on distinct forms of cell polarity, which guide lung morphogenesis and patterning in development and provide homeostatic barrier protection from microbes and toxins. The stability of lung alveoli, the luminal secretion of surfactants and mucus in the airways, and the coordinated motion of multiciliated cells that generate proximal fluid flow, are all critical functions regulated by cell polarity, with defects in polarity contributing to respiratory disease etiology. Here, we summarize the current knowledge of cell polarity in lung development and homeostasis, highlighting key roles for polarity in alveolar and airway epithelial function and outlining relationships with microbial infections and diseases, such as cancer.

Tight Junctions, the Epithelial Barrier, and Toll-like Receptor-4 During Lung Injury

Lung epithelium is constantly exposed to the environment and is critically important for the orchestration of initial responses to infectious organisms, toxins, and allergic stimuli, and maintenance of normal gaseous exchange and pulmonary function. The integrity of lung epithelium, fluid balance, and transport of molecules is dictated by the tight junctions (TJs). The TJs are formed between adjacent cells. We have focused on the topic of the TJ structure and function in lung epithelial cells. This review includes a summary of the last twenty years of literature reports published on the disrupted TJs and epithelial barrier in various lung conditions and expression and regulation of specific TJ proteins against pathogenic stimuli. We discuss the molecular signaling and crosstalk among signaling pathways that control the TJ structure and function. The Toll-like receptor-4 (TLR4) recognizes the pathogen- and damage-associated molecular patterns released during lung injury and inflammation and coordinates cellular responses. The molecular aspects of TLR4 signaling in the context of TJs or the epithelial barrier are not fully known. We describe the current knowledge and possible networking of the TLR4-signaling with cellular and molecular mechanisms of TJs, lung epithelial barrier function, and resistance to treatment strategies.

Crumbs3 is expressed in oral squamous cell carcinomas and promotes cell migration and proliferation by affecting RhoA activity

Despite the recent progression of treatments, the 5-year survival rate of patients with oral squamous cell carcinoma (OSCC) is still poor. One of the most critical factors affecting prognosis is tumor metastasis. Developing novel molecular targeted therapies by analyzing the molecular pathway of OSCC metastasis is an urgent issue. The present study aimed to characterize the expression and function of crumbs3 (Crb3) in OSCC cell migration. Immunohistochemistry and immunoblotting revealed that Crb3 was expressed in tissues from patients with OSCC and OSCC cell lines. The motility of OSCC cell lines was decreased by knockdown of Crb3 without affecting proliferation. However, Crb3-knockout (KO) clones exhibited decreases in both cell migration and proliferation. The expression of epithelial-mesenchymal transition markers was not altered in Crb3-KO clones compared with parent cells. A xenograft mouse model of lung metastasis revealed that the metastatic potential of Crb3-KO clones was reduced. As seen with Crb3-KO clones, the motility of OSCC cells was decreased by treatment with inhibitors of RhoA activation. Serum-induced activation of RhoA in OSCC cells was evaluated by comparing the amount of GTP-bound RhoA using affinity matrices, revealing that RhoA activation was decreased in Crb3-KO clones. To the best of our knowledge, the present study was the first to demonstrate that Crb3 was expressed in squamous cell carcinoma tissues and promoted cell migration and proliferation, which was associated with RhoA activation in OSCC cells.

CD13 orients the apical-basal polarity axis necessary for lumen formation

Polarized epithelial cells can organize into complex structures with a characteristic central lumen. Lumen formation requires that cells coordinately orient their polarity axis so that the basolateral domain is on the outside and apical domain inside epithelial structures. Here we show that the transmembrane aminopeptidase, CD13, is a key determinant of epithelial polarity orientation. CD13 localizes to the apical membrane and associates with an apical complex with Par6. CD13-deficient cells display inverted polarity in which apical proteins are retained on the outer cell periphery and fail to accumulate at an intercellular apical initiation site. Here we show that CD13 is required to couple apical protein cargo to Rab11-endosomes and for capture of endosomes at the apical initiation site. This role in polarity utilizes the short intracellular domain but is independent of CD13 peptidase activity.

Multiple PDZ domain protein maintains patterning of the apical cytoskeleton in sensory hair cells

Sound transduction occurs in the hair bundle, the apical compartment of sensory hair cells in the inner ear. The hair bundle is formed of actin-based stereocilia aligned in rows of graded heights. It was previously shown that the GNAI-GPSM2 complex is part of a developmental blueprint that defines the polarized organization of the apical cytoskeleton in hair cells, including stereocilia distribution and elongation. Here, we report a role for multiple PDZ domain (MPDZ) protein during apical hair cell morphogenesis in mouse. We show that MPDZ is enriched at the hair cell apical membrane along with MAGUK p55 subfamily member 5 (MPP5/PALS1) and the Crumbs protein CRB3. MPDZ is required there to maintain the proper segregation of apical blueprint proteins, including GNAI-GPSM2. Loss of the blueprint coincides with misaligned stereocilia placement in Mpdz mutant hair cells, and results in permanently misshapen hair bundles. Graded molecular and structural defects along the cochlea can explain the profile of hearing loss in Mpdz mutants, where deficits are most severe at high frequencies.

Key Role for CRB2 in the Maintenance of Apicobasal Polarity in Retinal Pigment Epithelial Cells

Apicobasal polarity is essential for epithelial cell function, yet the roles of different proteins in its completion is not fully understood. Here, we have studied the role of the polarity protein, CRB2, in human retinal pigment epithelial (RPE) cells during polarization in vitro, and in mature murine RPE cells in vivo. After establishing a simplified protocol for the culture of human fetal RPE cells, we studied the temporal sequence of the expression and localization of polarity and cell junction proteins during polarization in these epithelial cells. We found that CRB2 plays a key role in tight junction maintenance as well as in cell cycle arrest. In addition, our studies in vivo show that the knockdown of CRB2 in the RPE affects to the distribution of different apical polarity proteins and results in perturbed retinal homeostasis, manifested by the invasion of activated microglial cells into the subretinal space. Together our results demonstrate that CRB2 is a key protein for the development and maintenance of a polarized epithelium.

Apical PAR complex proteins protect against programmed epithelial assaults to create a continuous and functional intestinal lumen

Sustained polarity and adhesion of epithelial cells is essential for the protection of our organs and bodies, and this epithelial integrity emerges during organ development amidst numerous programmed morphogenetic assaults. Using the developing Caenorhabditis elegans intestine as an in vivo model, we investigated how epithelia maintain their integrity through cell division and elongation to build a functional tube. Live imaging revealed that apical PAR complex proteins PAR-6/Par6 and PKC-3/aPkc remained apical during mitosis while apical microtubules and microtubule-organizing center (MTOC) proteins were transiently removed. Intestine-specific depletion of PAR-6, PKC-3, and the aPkc regulator CDC-42/Cdc42 caused persistent gaps in the apical MTOC as well as in other apical and junctional proteins after cell division and in non-dividing cells that elongated. Upon hatching, gaps coincided with luminal constrictions that blocked food, and larvae arrested and died. Thus, the apical PAR complex maintains apical and junctional continuity to construct a functional intestinal tube.

Recruitment of Polarity Complexes and Tight Junction Proteins to the Site of Apical Bulk Endocytosis

BACKGROUND & AIMS

The molecular motor, Myosin Vb (MYO5B), is well documented for its role in trafficking cargo to the apical membrane of epithelial cells. Despite its involvement in regulating apical proteins, the role of MYO5B in cell polarity is less clear. Inactivating mutations in MYO5B result in microvillus inclusion disease (MVID), a disorder characterized by loss of key apical transporters and the presence of intracellular inclusions in enterocytes. We previously identified that inclusions in Myo5b knockout (KO) mice form from invagination of the apical brush border via apical bulk endocytosis. Herein, we sought to elucidate the role of polarity complexes and tight junction proteins during the formation of inclusions.

METHODS

Intestinal tissue from neonatal control and Myo5b KO littermates was analyzed by immunofluorescence to determine the localization of polarity complexes and tight junction proteins.

RESULTS

Proteins that make up the apical polarity complexes-Crumbs3 and Pars complexes-were associated with inclusions in Myo5b KO mice. In addition, tight junction proteins were observed to be concentrated over inclusions that were present at the apical membrane of Myo5b-deficient enterocytes in vivo and in vitro. Our mouse findings are complemented by immunostaining in a large animal swine model of MVID genetically engineered to express a human MVID-associated mutation that shows an accumulation of Claudin-2 over forming inclusions. The findings from our swine model of MVID suggest that a similar mechanism of tight junction accumulation occurs in patients with MVID.

CONCLUSIONS

These data show that apical bulk endocytosis involves the altered localization of apical polarity proteins and tight junction proteins after loss of Myo5b.

Intestinal Morphogenesis in Development, Regeneration, and Disease: The Potential Utility of Intestinal Organoids for Studying Compartmentalization of the Crypt-Villus Structure

The morphology and structure of the intestinal epithelium are rearranged dynamically during development, tissue regeneration, and disease progression. The most important characteristic of intestinal epithelial morphogenesis is the repetitive compartmentalized structures of crypt-villus units, which are crucial for maintaining intestinal homeostasis and functions. Abnormal structures are known to be closely associated with disease development and progression. Therefore, understanding how intestinal crypt-villus structures are formed and grown is essential for elucidating the physiological and pathophysiological roles of the intestinal epithelium. However, a critical knowledge gap in understanding the compartmentalization of the crypt-villus axis remains when using animal models, due to obvious inter-species differences and difficulty in real-time monitoring. Recently, emerging technologies such as organoid culture, lineage tracing, and single cell sequencing have enabled the assessment of the intrinsic mechanisms of intestinal epithelial morphogenesis. In this review, we discuss the latest research on the regulatory factors and signaling pathways that play a central role in the formation, maintenance, and regeneration of crypt-villus structures in the intestinal epithelium. Furthermore, we discuss how these factors and pathways play a role in development, tissue regeneration, and disease. We further explore how the current technology of three-dimensional intestinal organoids has contributed to the understanding of crypt-villus compartmentalization, highlighting new findings related to the self-organizing-process-driven initiation and propagation of crypt-villus structures. We also discuss intestinal diseases featuring abnormalities of the crypt-villus structure to provide insights for the development of novel therapeutic strategies targeting intestinal morphogenesis and crypt-villus formation.

Endocytosis of Intestinal Tight Junction Proteins: In Time and Space

Eukaryotic cells take up macromolecules and particles from the surrounding milieu and also internalize membrane proteins via a precise process of endocytosis. The role of endocytosis in diverse physiological processes such as cell adhesion, cell signaling, tissue remodeling, and healing is well recognized. The epithelial tight junctions (TJs), present at the apical lateral membrane, play a key role in cell adhesion and regulation of paracellular pathway. These vital functions of the TJ are achieved through the dynamic regulation of the presence of pore and barrier-forming proteins within the TJ complex on the plasma membrane. In response to various intracellular and extracellular clues, the TJ complexes are actively regulated by intracellular trafficking. The intracellular trafficking consists of endocytosis and recycling cargos to the plasma membrane or targeting them to the lysosomes for degradation. Increased intestinal TJ permeability is a pathological factor in inflammatory bowel disease (IBD), and the TJ permeability could be increased due to the altered endocytosis or recycling of TJ proteins. This review discusses the current information on endocytosis of intestinal epithelial TJ proteins. The knowledge of the endocytic regulation of the epithelial TJ barrier will provide further understanding of pathogenesis and potential targets for IBD and a wide variety of human disease conditions.

Changes of Wnt/β-catenin signalling, BMP2, and BMP4 in the jejunum during ageing in rats

BACKGROUND AND STUDY AIMS

The renewal of intestinal epithelium is maintained by intestinal stem cells (ISCs). Studies have found an age-dependent increase of Esg⁺/Dl⁺ progenitor cells in the midgut of Drosophila. However, changes of ISCs and the molecular regulation in mammalian animals with age are yet unknown. The aim of this study was to find out the changes of ISCs and molecular regulation in mammalian animals during the process of ageing.

MATERIAL AND METHODS

Thirty Sprague-Dawley rats were divided into three groups: young (3 months old), adult (6 months old), and ageing (24 months old). Levels of PCNA, Bmi1, β-catenin and BMP4 were examined by Immunohistochemistry staining. Levels of Bmi1, GSK-3β, Dkk1 and BMP2 were determined by Western Blot.

RESULTS

Our results showed that the proliferation of ISCs was decreased and the number of intestinal stem cells declined in ageing rats. The niches of ISCs, including Wnt signalling pathway and some proteins of Bone morphogenetic protein (BMP) signalling pathway, were downregulated in the jejunum of ageing rats.

CONCLUSION

Our study indicated that age-related decreased proliferation of intestinal stem cells in the jejunum could be associated with the alleviation of niches, including Wnt signalling pathway and some proteins of BMP signalling pathway.

Retinogenesis of the Human Fetal Retina: An Apical Polarity Perspective

The Crumbs complex has prominent roles in the control of apical cell polarity, in the coupling of cell density sensing to downstream cell signaling pathways, and in regulating junctional structures and cell adhesion. The Crumbs complex acts as a conductor orchestrating multiple downstream signaling pathways in epithelial and neuronal tissue development. These pathways lead to the regulation of cell size, cell fate, cell self-renewal, proliferation, differentiation, migration, mitosis, and apoptosis. In retinogenesis, these are all pivotal processes with important roles for the Crumbs complex to maintain proper spatiotemporal cell processes. Loss of Crumbs function in the retina results in loss of the stratified appearance resulting in retinal degeneration and loss of visual function. In this review, we begin by discussing the physiology of vision. We continue by outlining the processes of retinogenesis and how well this is recapitulated between the human fetal retina and human embryonic stem cell (ESC) or induced pluripotent stem cell (iPSC)-derived retinal organoids. Additionally, we discuss the functionality of in utero and preterm human fetal retina and the current level of functionality as detected in human stem cell-derived organoids. We discuss the roles of apical-basal cell polarity in retinogenesis with a focus on Leber congenital amaurosis which leads to blindness shortly after birth. Finally, we discuss Crumbs homolog (CRB)-based gene augmentation.

Crumbs organizes the transport machinery by regulating apical levels of PI(4,5)P2 in Drosophila

An efficient vectorial intracellular transport machinery depends on a well-established apico-basal polarity and is a prerequisite for the function of secretory epithelia. Despite extensive knowledge on individual trafficking pathways, little is known about the mechanisms coordinating their temporal and spatial regulation. Here, we report that the polarity protein Crumbs is essential for apical plasma membrane phospholipid-homeostasis and efficient apical secretion. Through recruiting βHeavy-Spectrin and MyosinV to the apical membrane, Crumbs maintains the Rab6-, Rab11- and Rab30-dependent trafficking and regulates the lipid phosphatases Pten and Ocrl. Crumbs knock-down results in increased apical levels of PI(4,5)P2 and formation of a novel, Moesin- and PI(4,5)P2-enriched apical membrane sac containing microvilli-like structures. Our results identify Crumbs as an essential hub required to maintain the organization of the apical membrane and the physiological activity of the larval salivary gland.

Disruption of Robo2-Baiap2 integrated signaling drives cystic disease

Hereditary renal cystic diseases are characterized by defects in primary cilia of renal tubular epithelial cells and abnormality of tubular epithelium, which ultimately result in the development of renal cysts. However, the mechanism leading from abnormality of the tubular epithelium to cystogenesis is not well understood. In this report, we demonstrate a critical role for Robo2 in regulating epithelial development, including ciliogenesis, polarization, and differentiation. We found that Robo2 deficiency results in cystic kidneys, and the cyst cells showed defective cilia and polarity defects in tubular epithelium. The cyst cells, less than terminally differentiated, continue to proliferate. We further established that Robo2 works with p53 as well as polarity and ciliary proteins (Par3, PKCς, ZO-2, and Claudin-2) to regulate these processes. Robo2 binds to Baiap2 (also known as IRSp53) through the IRSp53/MIM homology domain in renal epithelial cells. This binding allows Robo2 to phosphorylate MDM2 at Ser166 via Baiap2 and maintain p53 homeostasis. Disruption of the Robo2-Baiap2 complex causes MDM2 to be subjected to dephosphorylation, leading to a high level of active p53, and initiated p53-mediated cellular senescence via p21 and decreased the expression of ZO-1, ZO-2, PKCς, Par3, and Claudin-2 proteins, resulting in defects in epithelial development, including ciliogenesis, polarization, and differentiation. Importantly, double knockout of Robo2 and p53 rescued all the epithelial defects in kidneys compared with those in Robo2-knockout kidneys. Taken together, the present results demonstrate that Robo2 deficiency causes renal cystic disease, which is largely dependent on defective Robo2-Baiap2 integrated signaling in kidneys.

Crumbs protein homolog 3 (CRB3) expression is associated with oestrogen and progesterone receptor positivity in breast cancer

The crumbs protein homolog 3 (CRB3) regulates the tight junction to help maintain epithelial polarity. Altered CRB3 expression was associated with carcinogenesis of epithelial cells. This study detected CRB3 expression in 192 cases of breast cancer tissues and in the Molecular Taxonomy of Breast Cancer International Consortium (Metabric) and The Cancer Genome Atlas (TCGA) datasets for association with triple negative breast cancer (TNBC) phenotypes. The in vitro experiments confirm the ex vivo data. The data showed that levels of both CRB3 mRNA and protein were associated with TNBC phenotypes, ie, 41.1% (39/95) of ER+ breast cancer was CRB3-positive, whereas 26.9% (25/93) ER- tumour was CRB3-positive (P = 0.046). Moreover, 47.6% (30/63) of PR+ breast cancer was CRB3-positive vs 28.4% (33/116) PR- tumours positive for CRB3 (P = 0.013). In addition, 40.1% (27/66) of ER+/PR+ tumour was CRB3-positive, but only 22.4% (19/85) of TNBC showed CRB3 expression (P = 0.048). Indeed, levels of CRB3 mRNA were higher in non-TNBC than TNBC in both Metabric (P = 3.682e-10) and TCGA datasets (P = 2.501e-07). The in vitro data showed that CRB3 expression was higher in luminal (MCF7 and T47D) than in HER2 (MDA-MB-453 and SK-BR-3) and basal (MDA-MB-231 and BT-549) breast cancer cell lines. More interestingly, ERα regulated expression of CRB3 protein in MCF7 and BT-549 cells and ERα expression was associated with CRB3 expression in breast cancer tissues specimens. This study demonstrated that ERα could be a novel regulator for CRB3 expression in breast cancer.

A Polarizing Issue: Diversity in the Mechanisms Underlying Apico-Basolateral Polarization In Vivo

Polarization along an apico-basolateral axis is a hallmark of epithelial cells and is essential for their selective barrier and transporter functions, as well as for their ability to provide mechanical resiliency to organs. Loss of polarity along this axis perturbs development and is associated with a wide number of diseases. We describe three steps involved in polarization: symmetry breaking, polarity establishment, and polarity maintenance. While the proteins involved in these processes are highly conserved among epithelial tissues and species, the execution of these steps varies widely and is context dependent. We review both theoretical principles underlying these steps and recent work demonstrating how apico-basolateral polarity is established in vivo in different tissues, highlighting how developmental and physiological contexts play major roles in the execution of the epithelial polarity program.

DNA methylation of CRB3 is a prognostic biomarker in clear cell renal cell carcinoma

Our previous study revealed that CRB3 protein expression was reduced in clear cell renal cell carcinoma (ccRCC) and was associated with TNM stage, pathological grade, and poor prognosis of ccRCC. This study aimed to investigate if DNA methylation of CRB3 decreases its expression, subsequently leading to the progression and poor prognosis of ccRCC. Data for DNA methylation of CRB3, CRB3 mRNA expression, and ccRCC clinicopathological parameters were extracted from the cancer genome atlas (TCGA) database. The relationships among DNA methylation of CRB3, CRB3 mRNA expression, and ccRCC clinicopathological parameters were analyzed using UALCAN, MethHC, LinkedOmics, and Wanderer. We found that CRB3 mRNA levels were lower in ccRCC compared to normal tissues. Methylation of CRB3 increased in ccRCC, with all probes showing differences between ccRCC and normal tissues. Furthermore, CRB3 DNA methylation negatively correlated with CRB3 mRNA expression. CRB3 DNA methylation was also related to pathologic stage, T stage, N stage, and M stage of ccRCC. Overall survival was shorter in ccRCC patients with high CRB3 DNA methylation compared to ccRCC patients with low CRB3 DNA methylation. Methylation of cg24798010, a CRB3 probe, was related to laterality, pathologic stage, T stage, M stage, neoplasm-histologic-grade without N stage, and race. Furthermore, treatment with the DNA methylation inhibitor Decitabine resulted in the upregulation of CRB3 mRNA in ccRCC cell lines. These results indicate that DNA methylation of CRB3 may be both a prognostic marker and therapeutic target for ccRCC.

Elevated CRB3 expression suppresses breast cancer stemness by inhibiting β-catenin signalling to restore tamoxifen sensitivity

Tamoxifen is a first‐line drug for hormone therapy (HT) in oestrogen receptor‐positive breast cancer patients. However, 20% to 30% of those patients are resistant to tamoxifen treatment. Cancer stem cells (CSCs) have been implicated as one of the mechanisms responsible for tamoxifen resistance. Our previous study indicated that decreased expression of the CRB3 gene confers stem cell characteristics to breast cancer cells. In the current investigation, we found that most of the breast cancer patient tissues resistant to tamoxifen were negative for CRB3 protein and positive for β‐catenin protein, in contrast to their matched primary tumours by immunohistochemical analysis. Furthermore, expression of CRB3 mRNA and protein was low, while expression of β‐catenin mRNA and protein was high in tamoxifen resistance cells (LCC2 and T47D TamR) contrast to their corresponding cell lines MCF7 and T47D. Similarly, CRB3 overexpression markedly restored the tamoxifen sensitivity of TamR cells by the MTT viability assay. Finally, we found that CRB3 suppressed the stemness of TamR cells by inhibiting β‐catenin signalling, which may in turn lead to a decrease in the breast cancer cell population. Furthermore, these findings indicate that CRB3 is an important regulator for breast cancer stemness, which is associated with tamoxifen resistance.

The Crumbs3 Polarity Protein

The Crumbs proteins are evolutionarily conserved apical transmembrane proteins. Drosophila Crumbs was discovered via its crucial role in epithelial polarity during fly embryogenesis. Crumbs proteins have variable extracellular domains but a highly conserved intracellular domain that can bind FERM and PDZ domain proteins. Mammals have three Crumbs genes and this review focuses on Crumbs3, the major Crumbs isoform expressed in mammalian epithelial cells. Although initial work has highlighted the role of Crumbs3 in polarity, more recent studies have found it has an important role in tissue morphogenesis functioning as a linker between the apical membrane and the actin cytoskeleton. In addition, recent publications have linked Crumbs3 to growth control via regulation of the Hippo/Yap pathway.

EPEC effector EspF promotes Crumbs3 endocytosis and disrupts epithelial cell polarity

Enteropathogenic Escherichia coli (EPEC) uses a type III secretion system to inject effector proteins into host intestinal epithelial cells causing diarrhoea. EPEC infection redistributes basolateral proteins β1-integrin and Na⁺ /K⁺ ATPase to the apical membrane of host cells. The Crumbs (Crb) polarity complex (Crb3/Pals1/Patj) is essential for epithelial cell polarisation and tight junction (TJ) assembly. Here, we demonstrate that EPEC displaces Crb3 and Pals1 from the apical membrane to the cytoplasm of cultured intestinal epithelial cells and colonocytes of infected mice. In vitro studies show that EspF, but not Map, alters Crb3, whereas both effectors modulate Pals1. EspF perturbs polarity formation in cyst morphogenesis assays and induces endocytosis and apical redistribution of Na⁺ /K⁺ ATPase. EspF binds to sorting nexin 9 (SNX9) causing membrane remodelling in host cells. Infection with ΔespF/pespFD3, a mutant strain that ablates EspF binding to SNX9, or inhibition of dynamin, attenuates Crb3 endocytosis caused by EPEC. In addition, infection with ΔespF/pespFD3 has no impact on Na⁺ /K⁺ ATPase endocytosis. These data support the hypothesis that EPEC perturbs apical-basal polarity in an EspF-dependent manner, which would contribute to EPEC-associated diarrhoea by disruption of TJ and altering the crucial positioning of membrane transporters involved in the absorption of ions and solutes.

Mechanisms of Cell Polarity-Controlled Epithelial Homeostasis and Immunity in the Intestine

Intestinal epithelial cell polarity is instrumental to maintain epithelial homeostasis and balance communications between the gut lumen and bodily tissue, thereby controlling the defense against gastrointestinal pathogens and maintenance of immune tolerance to commensal bacteria. In this review, we highlight recent advances with regard to the molecular mechanisms of cell polarity-controlled epithelial homeostasis and immunity in the human intestine.

Cross talk between the Crumbs complex and Hippo signaling in renal epithelial cells

Cell polarity has a crucial role in organizing cells into tissues and in mediating transport processes and cell-cell communication. Especially the cells of the nephron require apicobasal polarity to establish and maintain their barrier function. The Crumbs complex including the integral membrane protein Crumbs, as well as Pals1 and Patj, is essential for the establishment of apicobasal polarity. The interactions of the core proteins and the interplay with other processes have been characterized in various epithelial cell lines in detail. Notably, Crb2 and Crb3 are expressed within the kidney and play an important role in the proper function of podocytes and tubules, respectively. The interaction of polarity proteins and components of the Hippo pathway-an evolutionarily highly conserved kinase cascade regulating cell proliferation, organ size, and tissue regeneration-has been discovered recently. Here, we discuss potential molecular and physiological links between the Crumbs complex and the Hippo pathway in renal cells.

CRB3 downregulation confers breast cancer stem cell traits through TAZ/β-catenin

The cancer stem cell (CSC) theory depicts a special population within the cancer mass that self-renew and sustain the cancer, even if the other cells were eliminated by therapies. How CSCs acquire these unique traits is still unclear. Crumbs homolog 3 (CRB3), a member of the CRB polarity complex, has been reported to act as a tumor suppressor. Here, we detected significantly lower or negative CRB3 expression in human breast cancer tissues. Knockdown of CRB3 generated non-tumorigenic, immortalized breast epithelial cell line MCF 10A with CSC properties. Simultaneously, we found that CRB3 downregulation induced the epithelial–mesenchymal transition and activated TAZ (transcriptional co-activator with PDZ-binding motif) and β-catenin. Significantly, the activation of TAZ and β-catenin sufficed in conferring MCF 10A cells with CSC properties. This study demonstrates that cell polarity proteins may serve as a switch of the differentiated vs multipotent states in breast cancers.

The Crumbs_C isoform of Drosophila shows tissue- and stage-specific expression and prevents light-dependent retinal degeneration

Drosophila Crumbs (Crb) is a key regulator of epithelial polarity and fulfils a plethora of other functions, such as growth regulation, morphogenesis of photoreceptor cells and prevention of retinal degeneration. This raises the question how a single gene regulates such diverse functions, which in mammals are controlled by three different paralogs. Here, we show that in Drosophila different Crb protein isoforms are differentially expressed as a result of alternative splicing. All isoforms are transmembrane proteins that differ by just one EGF-like repeat in their extracellular portion. Unlike Crb_A, which is expressed in most embryonic epithelia from early stages onward, Crb_C is expressed later and only in a subset of embryonic epithelia. Flies specifically lacking Crb_C are homozygous viable and fertile. Strikingly, these flies undergo light-dependent photoreceptor degeneration despite the fact that the other isoforms are expressed and properly localised at the stalk membrane. This allele now provides an ideal possibility to further unravel the molecular mechanisms by which Drosophila crb protects photoreceptor cells from the detrimental consequences of light-induced cell stress.

Summary: Loss of Crb_C, one protein isoform encoded by Drosophila crumbs, results in light-dependent retinal degeneration, but does not affect any of the other crumbs-specific functions.

Pals1 Haploinsufficiency Results in Proteinuria and Cyst Formation

The nephron is the basic physiologic subunit of the mammalian kidney and is made up of several apicobasally polarized epithelial cell types. The process of apicobasal polarization in animal cells is controlled by the evolutionarily conserved Crumbs (CRB), Partitioning-defective, and Scribble protein complexes. Here, we investigated the role of protein associated with LIN-7 1 (Pals1, also known as Mpp5), a core component of the apical membrane-determining CRB complex in the nephron. Pals1 interacting proteins, including Crb3 and Wwtr1/Taz, have been linked to renal cyst formation in mice before. Immunohistologic analysis revealed Pals1 expression in renal tubular cells and podocytes of human kidneys. Mice lacking one Pals1 allele (functionally haploid for Pals1) in nephrons developed a fully penetrant phenotype, characterized by cyst formation and severe defects in renal barrier function, which led to death within 6-8 weeks. In Drosophila nephrocytes, deficiency of the Pals1 ortholog caused alterations in slit-diaphragm-like structures. Additional studies in epithelial cell culture models revealed that Pals1 functions as a dose-dependent upstream regulator of the crosstalk between Hippo- and TGF-β-mediated signaling. Furthermore, Pals1 haploinsufficiency in mouse kidneys associated with the upregulation of Hippo pathway target genes and marker genes of TGF-β signaling, including biomarkers of renal diseases. These findings support a link between apical polarity proteins and renal diseases, especially renal cyst diseases. Further investigation of the Pals1-linked networks is required to decipher the mechanisms underlying the pathogenesis of these diseases.

Drosophila Crumbs prevents ectopic Notch activation in developing wings by inhibiting ligand-independent endocytosis

Many signalling components are apically restricted in epithelial cells, and receptor localisation and abundance is key for morphogenesis and tissue homeostasis. Hence, controlling apicobasal epithelial polarity is crucial for proper signalling. Notch is a ubiquitously expressed, apically localised receptor, which performs a plethora of functions; therefore, its activity has to be tightly regulated. Here, we show that Drosophila Crumbs, an evolutionarily conserved polarity determinant, prevents Notch endocytosis in developing wings through direct interaction between the two proteins. Notch endocytosis in the absence of Crumbs results in the activation of the ligand-independent, Deltex-dependent Notch signalling pathway, and does not require the ligands Delta and Serrate or γ-secretase activity. This function of Crumbs is not due to general defects in apicobasal polarity, as localisation of other apical proteins is unaffected. Our data reveal a mechanism to explain how Crumbs directly controls localisation and trafficking of the potent Notch receptor, and adds yet another aspect of Crumbs regulation in Notch pathway activity. Furthermore, our data highlight a close link between the apical determinant Crumbs, receptor trafficking and tissue homeostasis.