Cellular Biology & Homeostasis: Cell Membrane Part 3 - Cell Junctions PDF

Cellular Biology & Homeostasis CELL MEMBRANE PART 3 - CELL JUNCTIONS VP Summer 2023 Clara Camargo, DVM 1. Identify junctions along the lateral and basal surfaces of the cell 2. Describe the structure and functions of the main 6 types of cell-cell junctions 3. Relevance of each junction to cells a...

Cellular Biology & Homeostasis CELL MEMBRANE PART 3 - CELL JUNCTIONS VP Summer 2023 Clara Camargo, DVM 1. Identify junctions along the lateral and basal surfaces of the cell 2. Describe the structure and functions of the main 6 types of cell-cell junctions 3. Relevance of each junction to cells and tissues 4. Describe the components of the junctional complex Some cell types move freely in the body  Blood cells, and some immune system cells Many others are packed into tight communities  Tissues Fundamental social interactions between cells in a multicellular organism are those that hold cells together A tissue is an assembly of similar cells and their extracellular matrix from the same origin that work together carrying out specific functions. Organs are then formed by the functional grouping together of multiple tissues. DEFINITION Cell junctions consist of multiprotein complexes that provide contact between: 1. neighboring cells or 2. between a cell and the extracellular matrix. DIFFERENT FUNCTIONS 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 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 from an extracellular matrix produced by cells that are distributed sparsely in the matrix (fibroblasts) It is the matrix (rather than the cells) that bears most of the mechanical stress to which the tissue is subjected  Direct attachments between cells are rare  Cells (i.e., epithelial cells) have important attachments to the matrix (cell-matrix junctions) i.e., Bone or tendons, adipose tissue, cartilage Ground substance 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 • 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, bladder, respiratory tract, https://www.youtube.com/watch?v= vpYsjMDwsRk&t=5s “SEAM” The proteins that make up tight junctions are claudin and occludin ELECTRON MICROGRAPH OF TIGHT JUNCTIONS FYI CELIAC DISEASE AND CROHN’S DISEASE Tight junctions https://www.youtube.com/watch?v=Zf4C-Plk908 14 2. CELL-CELL ANCHORING JUNCTIONS 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 that are tethered inside the cell to the tension-bearing filaments of the cytoskeleton o Anchoring junctions are widely distributed in animal tissues and most abundant in tissues that are subjected to severe mechanical stress, such as heart, skeletal muscle and epidermis  Occur in two functionally different forms: • Adherens junctions and • Desmosomes Hold cells together and are formed by 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 it “FLEXIBLE SPOT WELDS” Form strong structural connections between cells https://www.youtube.com/watch?v=pBUIb5jchVo 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 epithelial cells, cardiac muscle, fibroblasts… Junctional complex https://www.youtube.com/watch?v=PXCP6QE-WXw ELECTRON MICROGRAPH OF DESMOSOMES IN THE SKIN OF A BABY MOUSE 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 that link with intermediate filaments instead of actin • Their main function is to provide mechanical strength • Located in tissues that suffer a lot of stress/stretch like skin, muscles, intestines, stomach, bladder… Desmosomes inside the tumor cells Skin tumors in Japanese flounder (X-cell tumors) Freeman et al. Parasites & Vectors 2011 3. GAP JUNCTIONS Allow the passage of small water-soluble 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, neurons… “TUNNEL” https://www.youtube.com/watch?v=K9TwazkS7nQ CELL-MATRIX ANCHORING JUNCTIONS Depend on INTEGRINS, which are transmembrane proteins like cadherins that bridge cell-cell 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 extracellular matrix They are similar in form to desmosomes • Hemidesmosomes use INTEGRINS, instead of desmogleins • Facilitate a stable adhesion of basal side of the epithelial cells to the basement membrane HAPPY STUDYING Clara Camargo, DVM [email protected] ©2021 Ross University School of Veterinary Medicine. All rights reserved.