Structure, regulation, and functional diversity of microvilli on the apical domain of epithelial cells.

Microvilli are actin-based structures found on the apical aspect of many epithelial cells. In this review, we discuss different types of microvilli, as well as comparisons with actin-based sensory stereocilia and filopodia. Much is known about the actin-bundling proteins of these structures; we summarize recent studies that focus on the components of the microvillar membrane. We pay special attention to mechanisms of membrane microfilament attachment by the ezrin/radixin/moesin family and regulation of this protein family. We also discuss the NHERF family of scaffolding proteins that are found in microvilli and their role in microvilli regulation. Microvilli on cultured cells are not static structures, and their dynamics and those of their components are discussed. Finally, we mention diseases related to microvilli and outline questions that our current knowledge will allow the field to address in the near future.

Membrane mucins of the intestine at a glance.

Membrane mucins cover most mucosal surfaces throughout the human body. The intestine harbors complex population of microorganisms (the microbiota) and numerous exogenous molecules that can harm the epithelium. In the colon, where the microbial burden is high, a mucus barrier forms the first line of defense by keeping bacteria away from the epithelial cells. In the small intestine where the mucus layer is less organized, microbes are kept at bay by peristalsis and antimicrobial peptides. Additionally, a dense glycocalyx consisting of extended and heavily glycosylated membrane mucins covers the surface of enterocytes. Whereas many aspects of mucosal barriers are being discovered, the function of membrane mucins remains a largely overlooked topic, mainly because we lack the necessary reagents and experimental animal models to investigate these large glycoproteins. In this Cell Science at a Glance article and accompanying poster, we highlight central concepts of membrane mucin biology and the role of membrane mucins as integral components of intestinal mucosal barriers. We also present the current consensus concerning the role of membrane mucins in host-microbe interactions. Moreover, we discuss how regulatory circuits that govern membrane mucins in the healthy gut display strong overlap with pathways that are perturbed during chronic inflammation. Finally, we review how dysregulation of intestinal membrane mucins may contribute to human diseases, such as inflammation and cancer.

Regulation of actin-based apical structures on epithelial cells.

Cells of transporting epithelia are characterized by the presence of abundant F-actin-based microvilli on their apical surfaces. Likewise, auditory hair cells have highly reproducible rows of apical stereocilia (giant microvilli) that convert mechanical sound into an electrical signal. Analysis of mutations in deaf patients has highlighted the critical components of tip links between stereocilia, and related structures that contribute to the organization of microvilli on epithelial cells have been found. Ezrin/radixin/moesin (ERM) proteins, which are activated by phosphorylation, provide a critical link between the plasma membrane and underlying actin cytoskeleton in surface structures. Here, we outline recent insights into how microvilli and stereocilia are built, and the roles of tip links. Furthermore, we highlight how ezrin is locally regulated by phosphorylation, and that this is necessary to maintain polarity. Localized phosphorylation is achieved through an intricate coincidence detection mechanism that requires the membrane lipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] and the apically localized ezrin kinase, lymphocyte-oriented kinase (LOK, also known as STK10) or Ste20-like kinase (SLK). We also discuss how ezrin-binding scaffolding proteins regulate microvilli and how, despite these significant advances, it remains to be discovered how the cell polarity program ultimately interfaces with these processes.

The human transmembrane mucin MUC17 responds to TNFα by increased presentation at the plasma membrane.

Transmembrane mucin MUC17 is an integral part of the glycocalyx as it covers the brush border membrane of small intestinal enterocytes and presents an extended O-glycosylated mucin domain to the intestinal lumen. Here, we identified two unknown phosphorylated serine residues, S4428 and S4492, in the cytoplasmic tail of human MUC17. We have previously demonstrated that MUC17 is anchored to the apical membrane domain via an interaction with the scaffolding protein PDZK1. S4492, localized in the C-terminal PDZ binding motif of MUC17, was mutated to generate phosphomimetic and phosphodeficient variants of MUC17. Using Caco-2 cells as a model system, we found that induction of an inflammatory state by long-term stimulation with the proinflammatory cytokine TNFα resulted in an increase of MUC17 protein levels and enhanced insertion of MUC17 and its two phospho-variants into apical membranes. Up-regulation and apical insertion of MUC17 was followed by shedding of MUC17-containing vesicles. Transmembrane mucins have previously been shown to play a role in the prevention of bacterial colonization by acting as sheddable decoys for encroaching bacteria. Overexpression and increased presentation at the plasma membrane of wild-type MUC17 and its phosphodeficient variant MUC17 S-4492A protected Caco-2 cells against adhesion of enteropathogenic Escherichia coli, indicating that C-terminal phosphorylation of MUC17 may play a functional role in epithelial cell protection. We propose a new function for MUC17 in inflammation, where MUC17 acts as a second line of defense by preventing attachment of bacteria to the epithelial cell glycocalyx in the small intestine.

The mucus and mucins of the goblet cells and enterocytes provide the first defense line of the gastrointestinal tract and interact with the immune system.

The gastrointestinal tract is covered by mucus that has different properties in the stomach, small intestine, and colon. The large highly glycosylated gel-forming mucins MUC2 and MUC5AC are the major components of the mucus in the intestine and stomach, respectively. In the small intestine, mucus limits the number of bacteria that can reach the epithelium and the Peyer's patches. In the large intestine, the inner mucus layer separates the commensal bacteria from the host epithelium. The outer colonic mucus layer is the natural habitat for the commensal bacteria. The intestinal goblet cells secrete not only the MUC2 mucin but also a number of typical mucus components: CLCA1, FCGBP, AGR2, ZG16, and TFF3. The goblet cells have recently been shown to have a novel gate-keeping role for the presentation of oral antigens to the immune system. Goblet cells deliver small intestinal luminal material to the lamina propria dendritic cells of the tolerogenic CD103(+) type. In addition to the gel-forming mucins, the transmembrane mucins MUC3, MUC12, and MUC17 form the enterocyte glycocalyx that can reach about a micrometer out from the brush border. The MUC17 mucin can shuttle from a surface to an intracellular vesicle localization, suggesting that enterocytes might control and report epithelial microbial challenge. There is communication not only from the epithelial cells to the immune system but also in the opposite direction. One example of this is IL10 that can affect and improve the properties of the inner colonic mucus layer. The mucus and epithelial cells of the gastrointestinal tract are the primary gate keepers and controllers of bacterial interactions with the host immune system, but our understanding of this relationship is still in its infancy.

MUC17 is an essential small intestinal glycocalyx component that is disrupted in Crohn's disease.

Crohn's disease (CD) is the chronic inflammation of the ileum and colon triggered by bacteria, but insights into molecular perturbations at the bacteria-epithelium interface are limited. We report that membrane mucin MUC17 protects small intestinal enterocytes against commensal and pathogenic bacteria. In non-inflamed CD ileum, reduced MUC17 levels correlated with a compromised glycocalyx, allowing bacterial contact with enterocytes. Muc17 deletion in mice rendered the small intestine prone to atypical infection while maintaining resistance to colitis. The loss of Muc17 resulted in spontaneous deterioration of epithelial homeostasis and extra-intestinal translocation of bacteria. Finally, Muc17-deficient mice harbored specific small intestinal bacterial taxa observed in CD. Our findings highlight MUC17 as an essential line of defense in the small intestine with relevance for early epithelial defects in CD.

BPP43_05035 is a Brachyspira pilosicoli cell surface adhesin that weakens the integrity of the epithelial barrier during infection.

The anaerobic spirochete Brachyspira causes intestinal spirochetosis, characterized by the intimate attachment of bacterial cells to the colonic mucosa, potentially leading to symptoms such as diarrhea, abdominal pain, and weight loss. Despite the clinical significance of Brachyspira infections, the mechanism of the interaction between Brachyspira and the colon epithelium is not known. We characterized the molecular mechanism of the B. pilosicoli-epithelium interaction and its impact on the epithelial barrier during infection. Through a proteomics approach, we identified BPP43_05035 as a candidate B. pilosicoli surface protein that mediates bacterial attachment to cultured human colonic epithelial cells. The crystal structure of BPP43_05035 revealed a globular lipoprotein with a six-bladed beta-propeller domain. Blocking the native BPP43_05035 on B. pilosicoli, either with a specific antibody or via competitive inhibition, abrogated its binding to epithelial cells, which required cell surface-exposed N-glycans. Proximity labeling and interaction assays revealed that BPP43_05035 bound to tight junctions, thereby increasing the permeability of the epithelial monolayer. Extending our investigation to humans, we discovered a downregulation of tight junction and brush border genes in B. pilosicoli-infected patients carrying detectable levels of epithelium-bound BPP43_05035. Collectively, our findings identify BPP43_05035 as a B. pilosicoli adhesin that weakens the colonic epithelial barrier during infection.

Neonatal microbiota colonization drives maturation of primary and secondary goblet cell mediated protection in the pre-weaning colon.

In the distal colon, mucus secreting goblet cells primarily confer protection from luminal microorganisms via generation of a sterile inner mucus layer barrier structure. Bacteria-sensing sentinel goblet cells provide a secondary defensive mechanism that orchestrates mucus secretion in response to microbes that breach the mucus barrier. Previous reports have identified mucus barrier deficiencies in adult germ-free mice, thus implicating a fundamental role for the microbiota in programming mucus barrier generation. In this study, we have investigated the natural neonatal development of the mucus barrier and sentinel goblet cell-dependent secretory responses upon postnatal colonization. Combined in vivo and ex vivo analyses of pre- and post-weaning colonic mucus barrier and sentinel goblet cell maturation demonstrated a sequential microbiota-dependent development of these primary and secondary goblet cell-intrinsic protective functions, with dynamic changes in mucus processing dependent on innate immune signalling via MyD88, and development of functional sentinel goblet cells dependent on the NADPH/Dual oxidase family member Duox2. Our findings therefore identify new mechanisms of microbiota-goblet cell regulatory interaction and highlight the critical importance of the pre-weaning period for the normal development of colonic barrier function.

Carbachol-induced MUC17 endocytosis is concomitant with NHE3 internalization and CFTR membrane recruitment in enterocytes.

We have reported that transmembrane mucin MUC17 binds PDZ protein PDZK1, which retains MUC17 apically in enterocytes. MUC17 and transmembrane mucins MUC3 and MUC12 are suggested to build the enterocyte apical glycocalyx. Carbachol (CCh) stimulation of the small intestine results in gel-forming mucin secretion from goblet cells, something that requires adjacent enterocytes to secrete chloride and bicarbonate for proper mucin formation. Surface labeling and confocal imaging demonstrated that apically expressed MUC17 in Caco-2 cells and Muc3(17) in murine enterocytes were endocytosed upon stimulation with CCh. Relocation of MUC17 in response to CCh was specific as MUC3 and MUC12 did not relocate following CCh stimulation. MUC17 colocalized with PDZK1 under basal conditions, while MUC17 relocated to the terminal web and into early endosomes after CCh stimulation. CCh stimulation concomitantly internalized the Na(+/)H(+) exchanger 3 (NHE3) and recruited cystic fibrosis transmembrane conductance regulator (CFTR) to the apical membranes, a process that was important for CFTR-mediated bicarbonate secretion necessary for proper gel-forming mucin unfolding. The reason for the specific internalization of MUC17 is not understood, but it could limit the diffusion barrier for ion secretion caused by the apical enterocyte glycocalyx or alternatively act to sample luminal bacteria. Our results reveal well-orchestrated mucus secretion and trafficking of ion channels and the MUC17 mucin.

CFTR anion channel modulates expression of human transmembrane mucin MUC3 through the PDZ protein GOPC.

The transmembrane mucins in the enterocyte are type 1 transmembrane proteins with long and rigid mucin domains, rich in proline, threonine and serine residues that carry numerous O-glycans. Three of these mucins, MUC3, MUC12 and MUC17 are unique in harboring C-terminal class I PDZ motifs, making them suitable ligands for PDZ proteins. A screening of 123 different human PDZ domains for binding to MUC3 identified a strong interaction with the PDZ protein GOPC (Golgi-associated PDZ and coiled-coil motif-containing protein). This interaction was mediated by the C-terminal PDZ motif of MUC3, binding to the single GOPC PDZ domain. GOPC is also a binding partner for cystic fibrosis transmembrane conductance regulator (CFTR) that directs CFTR for degradation. Overexpression of GOPC downregulated the total levels of MUC3, an effect that was reversed by introducing CFTR. The results suggest that CFTR and MUC3 compete for binding to GOPC, which in turn can regulate levels of these two proteins. For the first time a direct coupling between mucins and the CFTR channel is demonstrated, a finding that will shed further light on the still poorly understood relationship between cystic fibrosis and the mucus phenotype of this disease.

The MYO1B and MYO5B motor proteins and the SNX27 sorting nexin regulate membrane mucin MUC17 trafficking in enterocytes.

A dense glycocalyx, composed of the megaDalton-sized membrane mucin MUC17, coats the microvilli in the apical brush border of transporting intestinal epithelial cells, called enterocytes. The establishment of the MUC17-based glycocalyx in the mouse small intestine occurs at the critical suckling-weaning transition. The enterocytic glycocalyx extends 1 µm into the intestinal lumen and prevents the gut bacteria from directly attaching to the enterocytes. To date, the mechanism behind apical targeting of MUC17 to the brush border remains unknown. Here, we show that the actin-based motor proteins MYO1B and MYO5B, and the sorting nexin SNX27 regulate the intracellular trafficking of MUC17 in enterocytes. We demonstrate that MUC17 turnover at the brush border is slow and controlled by MYO1B and SNX27. Furthermore, we report that MYO1B regulates MUC17 protein levels in enterocytes, whereas MYO5B specifically governs MUC17 levels at the brush border. Together, our results extend our understanding of the intracellular trafficking of membrane mucins and provide mechanistic insights into how defective trafficking pathways render enterocytes sensitive to bacterial invasion.

Composition and functional role of the mucus layers in the intestine.

In discussions on intestinal protection, the protective capacity of mucus has not been very much considered. The progress in the last years in understanding the molecular nature of mucins, the main building blocks of mucus, has, however, changed this. The intestinal enterocytes have their apical surfaces covered by transmembrane mucins and the whole intestinal surface is further covered by mucus, built around the gel-forming mucin MUC2. The mucus of the small intestine has only one layer, whereas the large intestine has a two-layered mucus where the inner, attached layer has a protective function for the intestine, as it is impermeable to the luminal bacteria.

Discovery of a MUC3B gene reconstructs the membrane mucin gene cluster on human chromosome 7.

Human tissue surfaces are coated with mucins, a family of macromolecular sugar-laden proteins serving diverse functions from lubrication to the formation of selective biochemical barriers against harmful microorganisms and molecules. Membrane mucins are a distinct group of mucins that are attached to epithelial cell surfaces where they create a dense glycocalyx facing the extracellular environment. All mucin proteins carry long stretches of tandemly repeated sequences that undergo extensive O-linked glycosylation to form linear mucin domains. However, the repetitive nature of mucin domains makes them prone to recombination and renders their genetic sequences particularly difficult to read with standard sequencing technologies. As a result, human mucin genes suffer from significant sequence gaps that have hampered the investigation of gene function in health and disease. Here we leveraged a recent human genome assembly to characterize a previously unmapped MUC3B gene located at the q22 locus on chromosome 7, within a cluster of four structurally related membrane mucin genes that we name the MUC3 cluster. We found that MUC3B shares high sequence identity with the known MUC3A gene and that the two genes are governed by evolutionarily conserved regulatory elements. Furthermore, we show that MUC3A, MUC3B, MUC12, and MUC17 in the human MUC3 cluster are expressed in intestinal epithelial cells (IECs). Our results complete existing genetic gaps in the MUC3 cluster which is a conserved genetic unit in vertebrates. We anticipate our results to be the starting point for the detection of disease-associated polymorphisms in the human MUC3 cluster. Moreover, our study provides the basis for the exploration of intestinal mucin gene function in widely used experimental models such as human intestinal organoids and genetic mouse models.

IL-22 promotes the formation of a MUC17 glycocalyx barrier in the postnatal small intestine during weaning.

The intestine is under constant exposure to chemicals, antigens, and microorganisms from the external environment. Apical aspects of transporting epithelial cells (enterocytes) form a brush-border membrane (BBM), shaped by packed microvilli coated with a dense glycocalyx. We present evidence showing that the glycocalyx forms an epithelial barrier that prevents exogenous molecules and live bacteria from gaining access to BBM. We use a multi-omics approach to investigate the function and regulation of membrane mucins exposed on the BBM during postnatal development of the mouse small intestine. Muc17 is identified as a major membrane mucin in the glycocalyx that is specifically upregulated by IL-22 as part of an epithelial defense repertoire during weaning. High levels of IL-22 at time of weaning reprogram neonatal postmitotic progenitor enterocytes to differentiate into Muc17-expressing enterocytes, as found in the adult intestine during homeostasis. Our findings propose a role for Muc17 in epithelial barrier function in the small intestine.

The C-terminus of the transmembrane mucin MUC17 binds to the scaffold protein PDZK1 that stably localizes it to the enterocyte apical membrane in the small intestine.

The membrane-bound mucins have a heavily O-glycosylated extracellular domain, a single-pass membrane domain and a short cytoplasmic tail. Three of the membrane-bound mucins,MUC3, MUC12 and MUC17, are clustered on chromosome 7 and found in the gastrointestinal tract. These mucins have C-terminal sequences typical of PDZ-domain-binding proteins. To identify PDZ proteins that are able to interact with the mucins,we screened PDZ domain arrays using YFP (yellow fluorescent protein)-tagged proteins. MUC17 exhibited a strong binding to PDZK1 (PDZ domain containing 1), whereas the binding toNHERF1 (Na+/H+-exchanger regulatory factor 1) was weak.Furthermore, we showed weak binding of MUC12 to PDZK1, NHERF1 and NHERF2. GST (glutathione transferase) pull-down experiments confirmed that the C-terminal tail of MUC17 coprecipitates with the scaffold protein PDZK1 as identified byMS. This was mediated through the C-terminal PDZ-interaction site in MUC17, which was capable of binding to three of the four PDZ domains in PDZK1. Immunostaining of wild-type or Pdzk1-/- mouse jejunum with an antiserum against Muc3(17),the mouse orthologue of human MUC17, revealed strong brushborder membrane staining in the wild-type mice compared with an intracellular Muc3(17) staining in the Pdzk1-/- mice. This suggests that Pdzk1 plays a specific role in stabilizing Muc3(17)in the apical membrane of small intestinal enterocytes.

Study of mucin turnover in the small intestine by in vivo labeling.

Mucins are highly glycosylated proteins which protect the epithelium. In the small intestine, the goblet cell-secreted Muc2 mucin constitutes the main component of the loose mucus layer that traps luminal material. The transmembrane mucin Muc17 forms part of the carbohydrate-rich glycocalyx covering intestinal epithelial cells. Our study aimed at investigating the turnover of these mucins in the small intestine by using in vivo labeling of O-glycans with N-azidoacetylgalactosamine. Mice were injected intraperitoneally and sacrificed every hour up to 12 hours and at 24 hours. Samples were fixed with preservation of the mucus layer and stained for Muc2 and Muc17. Turnover of Muc2 was slower in goblet cells of the crypts compared to goblet cells along the villi. Muc17 showed stable expression over time at the plasma membrane on villi tips, in crypts and at crypt openings. In conclusion, we have identified different subtypes of goblet cells based on their rate of mucin biosynthesis and secretion. In order to protect the intestinal epithelium from chemical and bacterial hazards, fast and frequent renewal of the secreted mucus layer in the villi area is combined with massive secretion of stored Muc2 from goblet cells in the upper crypt.

Ezrin activation by LOK phosphorylation involves a PIP2-dependent wedge mechanism.

How cells specify morphologically distinct plasma membrane domains is poorly understood. Prior work has shown that restriction of microvilli to the apical aspect of epithelial cells requires the localized activation of the membrane-F-actin linking protein ezrin. Using an in vitro system, we now define a multi-step process whereby the kinase LOK specifically phosphorylates ezrin to activate it. Binding of PIP2 to ezrin induces a conformational change permitting the insertion of the LOK C-terminal domain to wedge apart the membrane and F-actin-binding domains of ezrin. The N-terminal LOK kinase domain can then access a site 40 residues distal from the consensus sequence that collectively direct phosphorylation of the appropriate threonine residue. We suggest that this elaborate mechanism ensures that ezrin is only phosphorylated at the plasma membrane, and with high specificity by the apically localized kinase LOK.

Murine Butyrophilin-Like 1 and Btnl6 Form Heteromeric Complexes in Small Intestinal Epithelial Cells and Promote Proliferation of Local T Lymphocytes.

To date, few molecular conduits mediating the cross-talk between intestinal epithelial cells and intraepithelial lymphocytes (IELs) have been described. We recently showed that butyrophilin-like (Btnl) 1 can attenuate the epithelial response to activated IELs, resulting in reduced production of proinflammatory mediators, such as IL-6 and CXCL1. We here report that like Btnl1, murine Btnl6 expression is primarily confined to the intestinal epithelium. Although Btnl1 can exist in a cell surface-expressed homomeric form, we found that it additionally forms heteromeric complexes with Btnl6, and that the engagement of Btnl1 is a prerequisite for surface expression of Btnl6 on intestinal epithelial cells. In an IEL-epithelial cell coculture system, enforced epithelial cell expression of Btnl1 significantly enhanced the proliferation of IELs in the absence of exogenous activation. The effect on proliferation was dependent on the presence of IL-2 or IL-15 and restricted to IELs upregulating CD25. In the γδ T-cell subset, the Btnl1-Btnl6 complex, but not Btnl1, specifically elevated the proliferation of IELs bearing the Vγ7Vδ4 receptor. Thus, our results show that murine epithelial cell-specific Btnl proteins can form intrafamily heterocomplexes and suggest that the interaction between Btnl proteins and IELs regulates the expansion of IELs in the intestinal mucosa.

Unfolding dynamics of the mucin SEA domain probed by force spectroscopy suggest that it acts as a cell-protective device.

MUC1 and other membrane-associated mucins harbor long, up to 1 µm, extended highly glycosylated mucin domains and sea urchin sperm protein, enterokinase and agrin (SEA) domains situated on their extracellular parts. These mucins line luminal tracts and organs, and are anchored to the apical cell membrane by a transmembrane domain. The SEA domain is highly conserved and undergoes a molecular strain-dependent autocatalytic cleavage during folding in the endoplasmic reticulum, a process required for apical plasma membrane expression. To date, no specific function has been designated for the SEA domain. Here, we constructed a recombinant protein consisting of three SEA domains in tandem and used force spectroscopy to assess the dissociation force required to unfold individual, folded SEA domains. Force-distance curves revealed three peaks, each representing unfolding of a single SEA domain. Fitting the observed unfolding events to a worm-like chain model yielded an average contour length of 32 nm per SEA domain. Analysis of forces applied on the recombinant protein revealed an average unfolding force of 168 pN for each SEA domain at a loading rate of 25 nN·s(-1). Thus, the SEA domain may act as a breaking point that can dissociate before the plasma membrane is breached when mechanical forces are applied to cell surfaces.