Plasma Membrane 2 2024.pdf

Cellular Biology & Homeostasis CELL MEMBRANE PART 2 VP 2024 Clara Camargo, DVM LEARNING OBJECTIVES 1. Understand the concept of membrane flexibility and fluidity 2. Describe the movement of phospholipids 3. Describe the role of cholesterol as a “fluidity buffer” 4. Explain the extracellular matrix,...

Cellular Biology & Homeostasis CELL MEMBRANE PART 2 VP 2024 Clara Camargo, DVM LEARNING OBJECTIVES 1. Understand the concept of membrane flexibility and fluidity 2. Describe the movement of phospholipids 3. Describe the role of cholesterol as a “fluidity buffer” 4. Explain the extracellular matrix, list the main components and its function in cell physiology CELL MEMBRANE - MEMBRANE DYNAMICS Features of all biological membranes: Flexibility: Ability to change shape without losing integrity or becoming leaky Fluidity: Ability to flow How? → noncovalent interactions of lipids in the bilayer Structure and flexibility of lipid bilayer depends on:  Lipids composition  Temperature changes Membrane fluidity https://www.youtube.com/watch?v=jM_xePC70Yo CELL MEMBRANE THE LIPID BILAYER: A TWO-DIMENSIONAL FLUID Around 1970 researchers first recognised that individual lipid molecules can diffuse freely within lipid bilayers First demonstrations resulted from studies of synthetic bilayers Two types of preparations were effective for these studies: 1. Liposomes closed spherical vesicles commonly used as model membranes in experimental studies 2. Black membranes planar bilayers, formed across a hole in a partition beween two aqueous compartments used to measure the permeability properties of synthetic membranes CELL MEMBRANE - THE BILAYER Synthetic lipid bilayers form liposomes when in solution Electron micrograph of unfixed, unstained phospholipid vesiclesliposomes CELL MEMBRANE – Phospholipid movement Phospholipids can move!!! Temperature and size of hydrocarbon tail will influence movement CELL MEMBRANE - Phospholipid movement MOVEMENT OF PHOSPHOLIPIDS WITHIN THE LIPID BILAYER Catalysis of trans-bilayer movement of lipids is facilitated by: Transversal Diffusion Flip-flop: * happens rarely Flippases Floppases Scramblases (unless the process is catalysed)  special transporter proteins that move phospholipids and other lipids in the membrane O * O Lateral Diffusion diffusion in the plane: happens readily and rapidly CELL MEMBRANE - Fluidity The fluidity of a lipid bilayer must be precisely regulated  depends on its composition and temperature Membrane phase transition: between the two main phases of a lipid bilayer at a characteristic temperature  liquid-crystalline phase  Paracrystalline state (more rigid, gel phase) Some organisms can adapt to environmental temperature fluctuations, adjusting the fatty acid content of their membranes to maintain a relatively constant fluidity. Effects of temperature on the structure and metabolism of cell membranes in fish FYI https://pubmed.ncbi.nlm.nih.gov/6372513/ CELL MEMBRANE - Fluidity Lipid bilayer is stabilized by hydrophobic interactions between lipids’ fatty acid chains Fluidity depends on: 1. Phospholipid content (FA length and saturation) 2. Cholesterol content 3. Temperature  At low temperatures: less lipid movement Lipid bilayer is in paracrystalline state (more rigid, gel like)  At higher temperatures (20-40°C / 68-104°F): more lipid movement Lipid bilayer becomes more fluid, liquid-crystalline state Liquid-disordered state CELL MEMBRANE – Fluidity/FA Influence of cis-double bonds in hydrocarbon chains in membrane phospholipids:  Unsaturated FA make more difficult to pack the hydrocarbon chains together  lower melting point (liquid at colder temperatures)  Also form a thinner membrane as the phospholipids will be further spread apart Influence of shorter hydrocarbon chain in membrane phospholipids  Reduces the tendency of hydrocarbon tails to interact with one another → the membrane remains fluid at lower temperatures short-chain FA  lower melting point  Hydrocarbon chain of membrane phospholipid tails can vary from 14-24 C, most are between 18-20 C CELL MEMBRANE – Fluidity/FA Saturated fatty acids: Tend to form paracrystalline structures (rigid, gel-like state) (less space between phospholipid tails) ↑ saturated fatty acids content of a lipid bilayer ↑ Phase transition temperature of the membrane (↑ melting point) Unsaturated fatty acids: Cis-double bonds → kinks (more space between tails) Inhibits the paracrystalline conformation ↑ Unsaturated fatty acids content of a lipid bilayer ↓ Phase transition temperature of the membrane (↓ melting point) CELL MEMBRANE – Fluidity/cholesterol Eukaryotic plasma membranes can contain high cholesterol content: Cholesterol can inhibit or  up to one molecule for every phospholipid molecule delay phase transitions Cholesterol molecules improve the permeability-barrier properties of the lipid bilayer Orient themselves in bilayer with their hydroxyl groups close to the polar heads of the phospholipid molecules The rigid steroid ring can support the hydrocarbon chains and stabilize them CELL MEMBRANE - CHOLESTEROL  at high temperatures Cholesterol helps keep the membrane more stable, stiffening the bilayer and making it less fluid and less permeable  at low temperatures, acts as ‘antifreeze’ Cholesterol also helps the membrane remain fluid by preventing fatty acid tails from interacting with each other and ‘clumping up’ VIDEO: Plasma membrane fluidity https://www.jove.com/embed/player?id=10972&t=1&s=1&fpv=1 Extra Cellular Matrix (ECM) Tissues are not made solely of cells  cells are contained in a complex and intricate network of = Basal Lamina macromolecules, known as the: EXTRACELLULAR MATRIX Basement membrane and Interstitial matrix fibronectin THE CELL AND ITS “SOCIAL“ CONTEXT: THE EXTRACELLULAR MATRIX (ECM) The extracellular matrix is an intricate network of macromolecules forming the structure of tissues  Structural and biochemical support to surrounding cells  Common functions: cell adhesion, cell-to-cell communication and differentiation Animal ECM consist of:  Interstitial matrix: present in intercellular spaces  Gels of polysaccharides and fibrous proteins filling intercellular spaces, act as compression buffer to the ECM  Basement membrane/basal lamina: sheet-like depositions that lines the basal side of epithelial and endothelial tissues ECM in Connective tissue Each type of connective tissue in animals has a different type of ECM: Bone is a rigid, strong, mineralized connective tissue: consists mainly of collagen fibers and bone minerals Loose connective tissue: reticular fibers and ground substance Blood is a specialized and fluid connective tissue: blood plasma is the matrix Nature Microscope Photo Video Scaffold structures of bone tissue ECM - Fibroblasts Macromolecules in the ECM are mainly produced locally by cells (mainly fibroblasts) in the interstitial matrix, and secreted via exocytosis Fibroblasts Fibroblasts produces and secretes ECM In connective tissue → mainly collagen fibers Depending on the tissue, it can differentiate into:  Chondroblasts- cartilage  Osteoblasts- bone tissue  Myofibroblasts – muscle tissue MAJOR CLASSES OF MACROMOLECULES IN THE ECM MAMMALS HAVE > 300 MATRIX PROTEINS: 1) Glycosaminoglycans (GAGs, e.g. cartilage): large and highly charged polysaccharides GAG + proteins  proteoglycans GAG + PROTEOGLYCANS = Ground Substance (amourphous gelatinous material, fill the space between fibers and cells) 2) Fibrous proteins like collagen (e.g. skin and bone) 3) Non-collagen fibrous proteins (elastin, fibronectin, laminin) Protein in green GAG in red Structural proteins: fibrous collagen and elastin Adhesive proteins: laminin and fibronectin ECM Macromolecules - Collagen COLLAGEN are fibrous, long, stiff, triple-stranded helical proteins forming fibrils.  Rich in proline and glycine amino acids  The fibrils are glycosylated What is protein glycosylation? Where does it happen? ECM Macromolecules - Collagen  42 distinct collagen alpha chain have been identified in humans ECM Macromolecules - Elastin ELASTIN gives tissues their elasticity (skin, blood vessels, lungs are strong and elastic enough to function properly) Scanning electron micrograph of a dog‘s aorta (low magnification) Network of longitudinally oriented elastin fibers (high magnification) ECM Macromolecules - Elastin Elastin is a hydrophobic protein rich in proline and glycine (like collagen) but is not glycosylated A RUBBER BAND: The molecules are joined together by covalent bonds to generate a cross-linked network Each elastin molecule can extend and contract in a manner resembling a random coil, so that the entire assembly can stretch and recoil like a rubber band ECM Macromolecules - Fibronectin In the ECM there are also glycoproteins with multiple domains, with specific binding sites for other matrix macromolecules and for receptors of the cell surface. Fibronectin (2500 amino acids long!) is a multifunctional adhesive glycoprotein that plays an important role in:  Tissue repair  Regulation of cellular attachment and motility  Embryogenesis ECM Macromolecules - Laminin LAMININ is the primary organizer of the sheet structure of the basal lamina  Composed of three long polypeptide chains held together by disulfide bonds Multidomain protein (each chain 1500 amino acids long), asymmetric molecule https://www.nature.com/articles/s41581-020-0329-y ECM – basal lamina THE BASAL LAMINA (or basement membrane) is a specialized form of ECM Basal lamina is thin, flexible and tough  essential component of all epithelia CELL MEMBRANE - Integrins Family of transmembrane proteins synthesized in several types of cells Facilitate cell adhesion (link cytoskeleton filaments with the ECM) Mediate cellular signals (signal transduction pathways, cell recognition, cell movement) Can be receptors for certain viruses (adenovirus, hantavirus, foot and mouth disease, polio virus...) Introduction to integrins https://www.youtube.com/watch?v=6iJVCBw4e00 Integrins: the receptors that keep it together https://www.youtube.com/watch?v=4k60P3Pnh30