Chapter 7 Cell Adhesion and the Extracellular Matrix © 2020 Elsevier Inc. All rights reserved. Figure 7–1. Types of cell adhesion molecules. © 2020 Elsevier Inc. All rights reserved. 2 Figure 7–2. Structural features of selectins and PSGL-1. (Adapted from Kappelmayer, Nagy. Biomed Res Int 2017, https://doi.org/10.1155/2017/6138145, by permission with added colors.) © 2020 Elsevier Inc. All rights reserved. 3 Figure 7–3. Integrin activation and clustering. (A) Domain structure of integrin subunits, (B) activation and clustering of integrins. © 2020 Elsevier Inc. All rights reserved. 4 Figure 7–4. Hierarchical organization of integrin clusters in migrating cell. Image of a mouse embryonic fibroblast spread on fibronectin-coated substrate for 30 min. dSTORM imaging of endogenous paxillin conjugated (labeled with paxillin antibody, BD biosciences) to antibody tagged with AlexaFlour-647 (antimouse secondary antibody conjugated to AlexaFlour-647). Gray scale shows the TIRF image, whereas false color image shows the reconstructed superresolution image. Color code (top left corner, from left to right) indicates increasing intensity of molecules. Encircled in dotted lines from left- to right-cell edge, nascent adhesions, maturing adhesions, mature adhesions. Scale bar 2 μm. (Adapted from Changede, Sheetz. BioEssays 2016;39:1, 1600123, by permission.) © 2020 Elsevier Inc. All rights reserved. 5 Figure 7–5. Intercellular junctions and the junctional complex. Simple epithelial cells have a junctional complex at their apicolateral borders. The components are the tight junction, the adherens junction, and the desmosome. The tight and adherens junctions are zonular, extending right around the cells, whereas desmosomes are punctate. Desmosomes are also present beneath the junctional complex, as is another punctate junction, the gap junction. © 2020 Elsevier Inc. All rights reserved. 6 Figure 7–6. Tight junctions are anastomosing strands of multiprotein complexes. (A) The close relationship between the organization of the junctional complex, and that of the terminal web in the small intestine is illustrated in this thin-section electron micrograph. In the cytoplasm at the level of the tight junction (TJ), the core microfilaments appear as compact bundles enmeshed in a matrix of 70 Å filaments. Mats of 70 Å filaments line the membranes of the zonula adherens (ZA) or intermediate junction, and the bundles of core microfilaments became more diffuse in the adherens zone. Numerous tonofilaments course through the plaque of the spot desmosome (SD); some of these tonofilaments enter the adherens zone, while others form the basal zone. 80,000 ×. The inset image is a higher magnification of TJ region showing the “kisses.” (B) An evenly cross-linked tight junction network found between the absorptive cells of the small intestine of a Xenopus laevis, stage 57. Short ridges or grooves on the protoplasmic (PF) or extracellular (EF) faces, respectively, intersect at acute angles to form a series of adjoining, similarly shaped polygons, which display no predominant orientation relative to the cell surface. Bar denotes 0.25 μm in the electron micrographs unless marked otherwise. 60,000 ×. (C) Line drawing of the apical junctional complex of an intestinal epithelial cell. Tight junction proteins include claudins, occluding, junctional adhesion molecule (JAM), and zonula occludens-1 (ZO-1), whereas E-cadherin, α-catenin, and β-catenin interact to form the adherens junction. Myosin light chain kinase (MLCK) is associated with the perijunctional actomyosin ring. Gap junctions are tubelike connections between adjacent cells formed by assembly of connexins. Desmosomes are formed by interactions between desmoglein, desmocollin, desmoplakin, and keratin filaments. (D) Occludin has four transmembrane domains with two extracellular loops. The first loop is characterized by a high content (~ 60%) of glycine and tyrosine residues. Claudin-1 also has four transmembrane domains but shows no sequence similarity to occludin. Note that the cytoplasmic tail of claudin-1 is shorter than that of occludin. Junctional adhesion molecule (JAM) has a single transmembrane domain, and its extracellular portion bears two immunoglobulin-like loops that are formed by disulfide bonds. ([A] Adapted from Hull, Staelin. J Cell Biol 1979;81:67–82, by permission; Tsukita, Furose, Itoh. Nat Rev 2001;2:285, by permission; [B] Adapted from Hull, Staelin. J Cell Biol 1976;68:688–704, by permission; [C] From Turner JR. Intestinal mucosal barrier function in health and disease. Nat Rev Immunol 2009;9:799–809, by permission; [D] Adapted from Tsukita, Furose, Itoh. Nat Rev 2001;2:285, by permission.) © 2020 Elsevier Inc. All rights reserved. 7 Figure 7–7. Disrupted tight junction strands in intestinal disease. Freeze-fracture electron microscopy of epithelial tight junctions from the jejunum of control subject and acute celiac disease patient. The jejunum from celiac disease patient shows significant reduction in the number or horizontally oriented strands and the depth of the tight junctional strand network. Additionally, strand discontinuities appeared in celiac disease. (Adapted from Rosenthal, Barmeyer, Schulzke. Tissue Barriers 2015;3:1–2, e977176, by permission.) © 2020 Elsevier Inc. All rights reserved. 8 Figure 7–8. Fluorescence labeling of epithelial tight junctions. (A) Frozen section of mouse colon was stained for occludin (green) and ZO-1 (red) by immunofluorescence method. Nucleus was stained by a DNA-binding fluorescent dye (blue). Confocal fluorescence microscopic image shows organization of tight junctions on the apical ends of colonic epithelial cells. (B) TNF-α induces redistribution of occludin from the junctions to cytoplasm. Mice were injected with vehicle or TNF-α. Cryosections of jejunum were stained for occludin (green) by immunofluorescence method. Filamentous actin (red) was stained by binding to fluorescently labeled phalloidin, and nucleus (blue) was stained by binding to fluorescent dye. The images show the TNF-α disrupts tight junctions. In one group of mice, the jejunum was perfused with divertin (MLCK inhibitor), which shows that divertin blocks TNF-α-mediated disruption of tight junctions. ([A] From Rao et al. Unpublished; [B] Adapted from Graham et al. Nat Med 2019, https://doi.org/10.1038/s41591-019-0393-7, by permission.) © 2020 Elsevier Inc. All rights reserved. 9 Figure 7–9. Structure and function of tight junctions. (A) The “gate” function of the tight junction refers to its property of regulating the permeability of the paracellular channels, and the “fence” function maintains the separation of molecules in the apical and basolateral cell membranes. (B) The low permeability of tight junctions causes high electrical resistance across the epithelial cell layer between the apical and basal compartments. (C) Some of the molecules that contribute to the structure and function of the tight junction are shown. © 2020 Elsevier Inc. All rights reserved. 10 Figure 7–10. Fluorescence labeling of epithelial adherens junctions. Frozen section of mouse ileum was stained for E-cadherin (green) and β-catenin (red) by immunofluorescence method. Nucleus was stained by a DNA-binding fluorescent dye (blue). Confocal fluorescence microscopic image shows organization of adherens junctions along the lateral membranes of colonic epithelial cells. (Adapted from Shukla, Meena, et al. Am J Physiol (GI&Liv) 2016;310: G705, by permission.) © 2020 Elsevier Inc. All rights reserved. 11 Figure 7–11. Functions of the adherens junction. (A) Initial cell contact is made by filopodia. These interdigitate and punctate adherens junctions are formed to constitute an adhesion zipper. The adhesion is then expanded and stabilized by the formation of desmosomes. (B) Bending of epithelial cell sheets during embryonic development is accomplished by contraction of the actin filament ring underlying the apicolateral adherens junctions. This acts like a purse-string to narrow the apices of the cells. This process can result in tube formation as in generation of the neural tube. © 2020 Elsevier Inc. All rights reserved. 12 Figure 7–12. Molecular structure of the adherens junction. Two types of adhesion molecule are involved, E-cadherin and the immunoglobulin (Ig) family protein nectin. The cytoplasmic domain of E-cadherin binds β-catenin, which, in turn, binds α-catenin. This complex may link E-cadherin to the cytoskeleton, but an alternative view (inset) suggests that the whole complex cannot form simultaneously. Nectin is linked to actin by afadin. © 2020 Elsevier Inc. All rights reserved. 13 Figure 7–13. Wnt signaling pathway. In the absence of Wnt (left), cytosolic β-catenin is continually phosphorylated by casein kinase-1α (CK1) and glycogen synthase kinase-3β (GSK3β) within an APC-axin scaffold complex. This phosphorylation allows β-catenin to be ubiquitylated and rapidly degraded by proteasome. During Wnt activation (right), GSK3β activity is inhibited directly by Lrp5/6, which allows β-catenin to accumulate, enter the nucleus, interact with LEF/TCF family members, and promote transcription. (Adapted from McEwen, Escobar, Gottardi. Subcell Biochem 2012;60:171. https://doi.org/10.1007/978-94-007-4186-7_8, by permission.) © 2020 Elsevier Inc. All rights reserved. 14 Figure 7–14. The desmosome-intermediate filament complex strengthens epithelia, particularly the epidermis. (A) Fluorescence micrograph showing desmosome-intermediate filament complex in cultured epithelial cells. Desmosomes stained by