Plasma Membrane 3 2024 PDF

Cellular Biology & Homeostasis CELL MEMBRANE PART 3 - CELL JUNCTIONS VP 2024 Clara Camargo, DVM 1. Identify junctions along the lateral and basal surfaces of the cell 2. Describe the structure of the main types of cell-cell junctions and cell-matrix junctions 3. Describe the components of the juncti...

Cellular Biology & Homeostasis CELL MEMBRANE PART 3 - CELL JUNCTIONS VP 2024 Clara Camargo, DVM 1. Identify junctions along the lateral and basal surfaces of the cell 2. Describe the structure of the main types of cell-cell junctions and cell-matrix junctions 3. Describe the components of the junctional complex 4. Understand the physiological relevance (functions) of each junction to cells and tissues Some cell types move freely in the body  Blood cells, and some immune system cells Many others are packed into tight communities  TISSUES A tissue is an assembly of similar cells and their extracellular matrix from the same origin that work together carrying out specific functions. Cell Junctions Functions: DEFINITION Cell junctions consist of multiprotein complexes that provide contact between: 1. neighboring cells 2. between a cell and the extracellular matrix 1. Bond cells together 2. Reduce stress placed upon cells 3. Build up the paracellular barrier of epithelia 4. Control the paracellular transport 5. Enable communication between neighboring cells Intercellular space 6. Adhesion of cells to extracellular matrix Ground substance Important tissue types found in all animals where cell junctions are particularly abundant: 1. EPITHELIAL TISSUES Cells are tightly bound together into sheets called epithelia Attachments between cells are made by cell-cell junctions Cells are anchored onto the basal lamina through cell-matrix junctions i.e., Lining of the intestines, or epidermal covering of the skin 2. CONNECTIVE TISSUES Formed by the ECM produced by cells that are distributed sparsely in the interstitial matrix (mainly fibroblasts) It is the interstitial matrix (rather than the cells) that bears most of the mechanical stress to which the tissue is subjected  Direct attachments between cells are rare  Epithelia tissue cells have important attachments to the ECM (cell-matrix junctions) i.e., Bone, tendons, adipose tissue, cartilage CELL JUNCTIONS ARE ALSO PRESENT IN OTHER TISSUES i.e., muscle, nervous tissue 9 TYPES OF CELL-CELL JUNCTIONS 1. TIGHT JUNCTIONS 2. CELL-CELL ANCHORING JUNCTIONS a. ADHERENS JUNCTIONS b. DESMOSOMES 3. GAP JUNCTIONS (communicating junctions)  channel-forming junctions TYPES OF CELL-MATRIX JUNCTIONS 1. CELL-MATRIX ANCHORING JUNCTIONS a. ACTIN LINKED CELL-MATRIX JUNCTIONS b. HEMIDESMOSOMES 1. TIGHT JUNCTIONS (zonula occludens, or occluding junction) Seal gap between epithelial cells https://www.youtube.com/watch?v= vpYsjMDwsRk&t=5s Seal the intercellular space from luminal environment Seal is not absolute (diffusion barrier)  Impermeable to macromolecules  Permeability to ions and other small molecules varies  Serve as selective permeability barriers for paracellular transport Mostly at apical location Found in epithelial cells i.e.; lining of the intestinal mucosa, urinary bladder, respiratory tract “SEAM” The proteins that make up tight junctions are: claudin and occludin ELECTRON MICROGRAPH OF TIGHT JUNCTIONS 2. CELL-CELL ANCHORING JUNCTIONS (Adherens junction and desmososmes) The lipid bilayer is flimsy and cannot by itself transmit large forces from cell to cell, or cell to extracellular matrix  Anchoring junctions solve this problem by o Forming strong, membrane-spanning structures Occur in two functionally different forms: Adherens junctions and that are tethered inside the cell to the tension- Desmosomes bearing filaments of the cytoskeleton Hold cells together and are formed by o Anchoring junctions are widely distributed in animal tissues o Abundant in tissues that are subjected to severe mechanical stress, such as heart, skeletal muscle and epidermis transmembrane adhesion proteins that belong to the cadherin family (calcium-dependent adhesive proteins) CELL-CELL ANCHORING JUNCTIONS Depends on transmembrane adhesion proteins CADHERINS - "calcium-dependent adhesion“ Cadherins span the plasma membrane: One end linking to the cytoskeleton and Other end linking to structures outside the cell “FLEXIBLE SPOT WELDS” Form strong structural connections between cells A. ADHERENS JUNCTIONS Zonula adherens, intermediate junction, “belt desmosome” or adhesion belt Connect one actin filament bundle in one cell with that in the next cell o Provide anchorage site for cytoskeleton Help modeling the shape of multicellular structures o Guide the organization of developing tissues EX: construction of vertebrate nervous system o Important in tissue remodeling Located more basal than tight junctions Found in most epithelial and endothelial tissues Junctional complex B. DESMOSOMES or macula adherens Connect intermediate filaments in one cell to those in the next cell Provide anchorage site for cytoskeleton Structurally similar to adherens junctions but contain specialized proteins (desmogleins) that link with intermediate filaments instead of actin Main function is to provide mechanical strength Located in tissues that suffer a lot of stress/stretch like skin, muscles, intestines, stomach, urinary bladder… ELECTRON MICROGRAPH OF DESMOSOMES IN THE SKIN OF A BABY MOUSE 3. GAP JUNCTIONS Allow the passage of small hydrophilic molecules from cell to cell Without having to pass through the plasma membrane EX: ions, glucose, second messengers The gap is narrow (2 – 4 nm) Very important in tissues containing electrically excitable cells Action potentials can spread rapidly from cell to cell EX: cardiac contraction, peristaltic movements of intestines  Located in connective tissue, epithelial tissue, cardiac muscle, smooth muscle… “TUNNEL” CELL-MATRIX ANCHORING JUNCTIONS Depend on INTEGRINS - transmembrane proteins bridging cell-matrix interactions Sense and respond to mechanical forces acting across the junction Introduction to integrins https://www.youtube.com/watch?v=6iJVCBw4e00 Integrins: the receptors that keep it together https://www.youtube.com/watch?v=4k60P3Pnh30 INTEGRINS Are activated after binding intracellular activator proteins, and produce intracellular signals that can influence almost any aspects of cell behavior:  embryonic development      signaling proliferation survival migration blood clotting A. ACTIN-LINKED CELL-MATRIX JUNCTIONS or focal adhesion Anchor actin filaments in cell to extracellular matrix  Can be small and transient or large and durable The formation of focal adhesion https://www.youtube.com/watch?v=tQDb5qiAf7I B. HEMIDESMOSOMES Hemidesmosomes anchor intermediate filaments in a cell to their extracellular matrix They are similar in form to desmosomes, however Hemidesmosomes use INTEGRINS, instead of desmogleins Facilitate a stable adhesion of basal side of the epithelial cells to the basement membrane (ECM)