Bio202 Lecture 20: Cell-Cell Interactions Part 2 PDF

Lecture 20: Cell-cell interactions part 2 Today’s agenda: ▪ Overview: Tissue organization and ECM ▪ Connecting the cell to ECM: Integrins ▪ ECM composition (three major categories of proteins) ▪ Regulation of cell adhesion drives extravasation  Cells organize into tissues: ▪ Ce...

Lecture 20: Cell-cell interactions part 2 Today’s agenda: ▪ Overview: Tissue organization and ECM ▪ Connecting the cell to ECM: Integrins ▪ ECM composition (three major categories of proteins) ▪ Regulation of cell adhesion drives extravasation  Cells organize into tissues: ▪ Cells secrete proteins to build the ECM ▪ A tissue = cells + ECM ▪ 4 major tissue types: epithelial, connective, muscle, nervous Cross-section of intestine: Tissues are held together by cell-cell interactions, by ECM, or both Figure 20-2 Tissue organization: ▪ ECM is not abundant in epithelial tissue (is only a thin sheet of basal lamina) ▪ ECM very abundant in connective tissue (cells are more sparse and don’t necessarily contact each other) Basal lamina = thin sheet of ECM ECM in connective tissue: same basic composition as basal lamina, with some different components Figure from Alberts, Molecular Biology of the Cell Tissue organization: ▪ ECM is not abundant in epithelial tissue (is only a thin sheet of basal lamina); cell-cell contacts/cytoskeleton provide strength ▪ ECM very abundant in connective tissue (contains many collagen fibers); sparse cells so ECM bears mechanical load) Figure from Alberts, Molecular Biology of the Cell  The basal lamina is located between epithelial (or muscle) cells and connective tissue: Basal lamina = sheet-like meshwork of ECM (basal lamina is a type of ECM) Connective tissue = isolated cells, with a large amount of ECM Basal lamina Collagen fibers Figure 20-20 The basal lamina is structured differently in different tissues: Muscle: Basal lamina Epithelia: Basal lamina is sheet-like, surrounds the cells one surface of the cells rests on it Both cases: Basal lamina is adjacent to the plasma membrane, between cells and connective tissue  Aberrant cell adhesion contributes to metastasis ▪ Cells invade and migrate through barriers between tissues ▪ Cancer cells have mechanisms to break through ECM ▪ This is a contributor to metastasis because cells can invade other parts of the body Figure 20-43 Aberrant cell adhesion contributes to metastasis Tumor cell (red) migrating in blood vessels (green) Today’s topics: ▪ Overview: Tissue organization and ECM ▪ Connecting the cell to ECM: Integrins ▪ Reminders from last lecture ▪ Activation of integrins ▪ ECM composition (three major categories of proteins) ▪ Regulation of cell adhesion drives extravasation Integrins link the cell to the ECM: ▪ Integrins are transmembrane proteins on the surface of the cell ▪ Integrins cluster into adhesive structures that can mediate cell-ECM interactions Figure from Lodish, Molecular Cell Biology Reminder: Integrins are components of focal adhesions and hemidesmosomes focal adhesions Figure from Alberts, Molecular Biology of the Cell (similar to Figure 20-30)  Reminder: Integrins can link the cell to ECM Hemi-desmosomes: Figure 20-27 Note: Integrins also form the cell-ECM connection in focal adhesions  Integrins switch between inactive and active states ▪ Integrins consist of two subunits (𝝰 and β) ▪ Conformational changes (bent to extended) activate integrin ▪ Integrin activation can be triggered by ECM components outside the cell or by proteins inside the cell Binding to ECM Note: Sometimes integrins can outside the cell bind to proteins on the surface of other cells; they are not only used for cell-ECM contacts Plasma membrane Binding to linker proteins inside Figure 20-15 the cell Today’s topics: ▪ Overview: Tissue organization and ECM ▪ Connecting the cell to ECM: Integrins ▪ ECM composition (three major categories of proteins) ▪ Collagens ▪ Proteoglycans ▪ Multi-adhesive matrix proteins ▪ Regulation of cell adhesion drives extravasation Major components of the ECM: 1. Collagen: Provides structural integrity, mechanical strength 2. Proteoglycans: Proteins with attached sugar molecules; provide cushioning (they have a gel-like quality) 3. Multi-adhesive matrix proteins: Proteins that cross-link proteins and create the ECM meshwork; these proteins can also bind integrins (to link ECM to the cell) ▪ These three major components are found in both basal lamina and in the ECM of connective tissue, though the relative abundance of components can differ in these two types of ECM Structure of the basal lamina: ▪ Cross-linked networks of collagen, proteoglycans, and multi-adhesive matrix proteins ▪ Integrins (transmembrane proteins in the plasma membrane) bind to ECM components, linking the ECM to cells Plasma membrane Figure from Alberts, Molecular Biology of the Cell Major components of the ECM: 1. Collagen: Provides structural integrity, mechanical strength 2. Proteoglycans: Proteins with attached sugar molecules; provide cushioning (they have a gel-like quality) 3. Multi-adhesive matrix proteins: Proteins that cross-link proteins and create the ECM meshwork; these proteins can also bind integrins (to link ECM to the cell) There are two major types of collagen in ECM: ▪ “Sheet forming” collagen in basal lamina (collagen IV) ▪ “Fibrillar” collagen in the ECM of connective tissue ECM components: collagen in basal lamina The basal lamina has “sheet-forming” collagens (collagen IV) that assemble into a network: ▪ Three collagen IV molecules come together to form a triple subunit helix – this is the subunit ▪ These collagen IV triple helices interact via their N- and C- terminal globular domains ▪ Subunits associate into a 2D network (not into fibers) Figure from Lodish, Molecular Cell Biology ECM components: collagen in basal lamina ▪ Collagen IV proteins form a meshwork by associating through their N- and C-terminal globular domains Interacting via globular domains Figure from Alberts, Molecular Biology of the Cell ECM components: collagen in connective tissue ▪ Image shows fibroblasts surrounded by connective tissue ▪ Connective tissue has collagen fibers (not a sheet-like network) Cell Collagen fibers Collagen fibers (side view) (cross-section) Figure 20-9 ECM components: collagen in connective tissue Assembly of collagens in connective tissue: ▪ “Fibrillar collagens” form fibers and are present in connective tissue ▪ Fibrillar collagens assemble into triple-stranded molecules ▪ Subunits associate along their lengths (not via N- or C-termini) to form fibrils ▪ These assemble into the fibers found in connective tissue Figure 20-9  Collagen fibers are stronger than steel Cross-section of steel fibers Cross-section of collagen fibers ECM components: collagen in connective tissue ECM components are made through the secretory pathway… Why don’t collagen fibers form inside cells before they are secreted? ▪ Cells secrete collagen in a form that cannot Extra assemble into fibrils – this “procollagen” has domain extra domains on the ends ▪ Outside the cell, the procollagen is processed by a protease in the ECM ▪ The cleaved collagen molecules can then form fibers outside the cell Figure 20-10 Defects in processing collagen lead to hyperflexible skin: James Morris (the “elastic skin man”) 1890s Condition can arise from either: ▪ Lack of the enzyme that converts procollagen to collagen ▪ Defect in procollagen itself Figure 20-11 Major components of the ECM: 1. Collagen: Provides structural integrity, mechanical strength Sheet-forming in basal lamina Fibrillar in connective tissue 2. Proteoglycans: Proteins with attached sugar molecules; provide cushioning (they have a gel-like quality) 3. Multi-adhesive matrix proteins: Proteins that cross-link proteins and create the ECM meshwork; these proteins can also bind integrins (to link ECM to the cell) ECM components: proteoglycans Figure 20-16 GAGs are assembled on the proteoglycans in the Golgi (Do not memorize this chemical structure!) ▪ Proteoglycans have attached polysaccharide (sugar) chains called GAGs (glycosaminoglycans) ▪ GAGs are covalently-linked polysaccharide chains ▪ Proteoglycans give the ECM its gel-like properties, which resists compression on tissues Reminder: Modification of proteins occurs in the Golgi apparatus Traveling through the Golgi Ready to go to that Note - You do not need to memorize the party outside the cell! modifications. Just know that sugar residues are added to proteoglycans in the Golgi. (in the ECM) ECM components: proteoglycans ▪ Proteoglycans with GAGs can form large aggregates ▪ GAGs tend to adopt highly extended conformations, which occupy a large volume ▪ They can therefore act as “space fillers” that provide cushioning to the ECM Figure 20-17 Major components of the ECM: 1. Collagen: Provides structural integrity, mechanical strength Sheet-forming in basal lamina Fibrillar in connective tissue 2. Proteoglycans: Proteins with attached sugar molecules; provide cushioning (they have a gel-like quality) 3. Multi-adhesive matrix proteins: Proteins that cross-link proteins and create the ECM meshwork; these proteins can also bind integrins (to link ECM to the cell) ECM components: multi-adhesive matrix proteins ▪ Called “Multi-adhesive” because they bind to lots of other proteins ▪ Called “Matrix” because they help form the extracellular matrix, by crosslinking ECM proteins ▪ Also bind integrins to the link the ECM to the cell Example: Laminin ▪ Component of the basal lamina ▪ Cross-shaped protein with binding domains for other ECM proteins and for integrins ▪ Interacts with other proteins through the globular domains Figure from Alberts, Molecular Biology of the Cell ECM components: multi-adhesive matrix proteins Example: Fibronectin Figure 20-14 ▪ Fibronectin has a domain that binds ECM components (collagens) ▪ Can bind fibrillar collagens (in connective tissue) or sheet-forming collagens (in basal lamina) ▪ Also has a domain that binds integrins ▪ Fibronectin therefore helps anchor collagens in ECM to the cell via integrins Today’s topics: ▪ Overview: Tissue organization and ECM ▪ Connecting the cell to ECM: Integrins ▪ ECM composition (three major categories of proteins) ▪ Regulation of cell adhesion drives extravasation ▪ Switch from weak to strong adhesion Inner life of the Cell: Extravasation Red = red blood cells Light blue = leukocytes, which are rolling along endothelial cells that line the blood vessel Regulation of adhesion strength allows cells to have complex behaviors: ▪ Movement of leukocytes (white blood cells) to areas of infection until they ultimately invade the underlying tissue (extravasation) ▪ If there is an infection, leukocytes respond to inflammatory signals (e.g. chemokines) and start associating with the endothelial cells – this is loose adhesion ▪ Cells “roll” along the endothelial cells of the blood vessel ▪ When reach the site of infection, this switches to a strong adhesion (leukocyte) Figure 11-37 Cell-cell adhesion during extravasation: Loose adhesion: Blood vessel ▪ Cell rolling is mediated by loose adhesion endothelial cell ▪ Glycoproteins with attached sugar residues are on the surface of the leukocyte (white blood cell) ▪ Similar to the proteoglycans of the ECM, these proteins have sugar residues added in the Golgi Terminology: ▪ Glycoproteins are proteins with attached sugar residues ▪ Proteoglycans are a specific type of glycoprotein that Leukocyte contain GAGs Glycoprotein Cell-cell adhesion during extravasation: Loose adhesion: Blood vessel ▪ Cell rolling is mediated by loose adhesion endothelial cell ▪ P-selectin (a protein with a “lectin” domain that binds sugars) is on the surface of the cells lining the blood vessel ▪ Selectins bind to the sugar residues of the Lectin glycoproteins domain ▪ This binding is weak, so allows the rolling of leukocytes along the surface of the blood vessel Leukocyte Glycoprotein Cell-cell adhesion during extravasation: Integrins mediate the switch to tight adhesion: Activated integrin binds tightly to cells of the ▪ When the cell reaches the site of blood vessel (to a inflammation, there is a signal to protein called “iCAM”) switch to a tight adhesion ▪ This signal activates integrin (on the This is a leukocyte) – switches from a bent to cell-cell a straight conformation interaction ▪ Activated integrin tightly binds a protein (“iCAM”) that is on the surface of the endothelial cells ▪ This stops the rolling Leukocyte Figure 20-15 Regulation of adhesion strength allows cells to have complex behaviors: ▪ Inflammatory signals (e.g. chemokines) cause leukocytes to associate with the endothelial cells on the walls of the blood vessel ▪ Binding of selectins to glycoproteins recruits the leukocytes ▪ This is a loose adhesion – blood flow pushes the cell and it rolls ▪ Activation of integrins at site of infection triggers tight binding to the blood vessel ▪ Leukocyte stops rolling and then can crawl between the cells of the blood vessel into the infected tissue (leukocyte) Figure 11-37 Next class (Wednesday): Review for exam 4 Friday’s class: Final assignment Bring a laptop Friday so you can start working on the assignment in class! Attendance at the Friday/Monday classes is mandatory (contact us with valid excuses) Reminder: Meme / TikTok submissions due this week (check Canvas for the deadline)! ▪ Should convey a cell biology concept (something that you learned about cells) ▪ Counts for one extra percentage point on your final grade (Meme made by a former student in this class)