MED1003 Week 6 Lecture Cytoskeletal Systems PDF

Cytoskeletal systems and extracellular structures by Siu Wai (Phyllis) TSANG, PhD TUNG WAH COLLEGE email: [email protected] Office #: 3190-6713 Prepared by SWT 2024 email: [email protected]...

Cytoskeletal systems and extracellular structures by Siu Wai (Phyllis) TSANG, PhD TUNG WAH COLLEGE email: [email protected] Office #: 3190-6713 Prepared by SWT 2024 email: [email protected] 1 Cytoskeletal systems Textbook Chapter 13 Prepared by SWT 2024 email: [email protected] 2  Cytosol Introduction Region of the cytoplasm between and surrounding the organelles Gel-like substance with 20~30% proteins content Cytoskeleton cytoskeleton A complex network The cytoskeleton High level of internal organization Dynamic, changeable Interconnected filaments and tubules Extends throughout the cytosol From the nucleus to the inner surface of the plasma membrane Enables cells to assume cellular processes and maintain complex shapes cytosol Textbook Ch.13 p.376 Prepared by SWT 2024 email: [email protected] 3 Functions of cytoskeleton Maintenance of cell shape Cell movement Cell division Positioning and active movement of membrane-bounded organelles Providing matrix for enzyme attached Cell signaling Cell-cell adhesion Prepared by SWT 2024 email: [email protected] 4 Major structural elements Textbook Ch.13 1. Microtubules 2. Microfilaments 3. Intermediate filaments ▪ Each element has a characteristic size, structure and intracellular distribution ▪ Each element is formed by the immunofluorescence polymerization of a different kind of protein subunit Prepared by SWT 2024 email: [email protected] 5 Properties of Textbook Ch.13 p.377 structural (MT) (MF) (IF) elements https://d3rw207pwvlq3a.cloudfront.net/attachment Prepared by SWT 2024 email: [email protected] s/000/128/352/original/image.png?1598229107 6  (MT) (MF) (IF) Properties of structural elements Textbook Ch.13 Textbook Ch.13 p.377 Prepared by SWT 2024 email: [email protected] 7  Prepared by SWT 2024 email: [email protected] Textbook Ch.13 p. 379 8 Microtubules – the largest of the cytoskeletal elements 1. Axonemal microtubules 2. Cytosolic microtubules Highly organized microtubules Loosely organized dynamic network Associated with cellular movement Formation of mitotic and meiotic Only found in cells that have structures spindles like cilia and flagella Maintaining or altering cell shape Placement and movement of vesicles Found in all animal cells and plant cells Prepared by SWT 2024 email: [email protected] 9 Microtubules (cont’d) 1. Axonemal microtubules 2. Cytosolic microtubules flagellum https://open.oregonstate.education/app/uploads/sites/178/2023/11/Figure-06-14.jpg https://microbialnotes.com/wp-content/uploads/2023/01/eukaryotic-flagella.webp Prepared by SWT 2024 email: [email protected] 10  Textbook Ch.13 Microtubule structure Straight, hollow cylinders Vary greatly in length Consists of longitudinal arrays Protofilaments – built by heterodimers, i.e., α- & β-tubulin subunits All the dimers in the protofilaments are oriented the same way; the plus (+) and minus (-) ends Can form as singlets, doublets and triplets Prepared by SWT 2024 email: [email protected] 11 Microtubule assembly 1. Lag phase: MT formation is initially slow, due to the relatively slow process of MT nucleation → aggregation of tubulin dimers into clusters, i.e., oligomers Textbook Ch.13 p.380 2. Elongation phase: MT grows by addition of tubulin dimers at either ends (relatively fast) 3. Plateau phase: the mass of MTs increases to a point where the concentration of free tubulin becomes limiting (MT assembly is balanced by disassembly) Prepared by SWT 2024 email: [email protected] 12 Treadmilling of Microtubules MT nucleation and assembly occur at both ends, but the MTs grow much faster from one end than from the other The rapidly growing end is the plus end, the other end is the minus end The minus ends of MTs are often anchored at the Textbook Ch.13 p.381-4 centrosome The MT dynamics are confined to plus ends The different growth rates of the plus and minus ends of microtubules reflect the different critical concentrations required Prepared by SWT 2024 email: [email protected] 13 Treadmilling of Microtubules If the free tubulin concentration is higher than the critical concentration for the plus end but lower than the critical concentration for the minus end, assembly will occur at the plus end while disassembly takes place at the minus end This simultaneous assembly and disassembly produces the phenomenon known as treadmilling Textbook Ch.13 p.381 i.e., A given tubulin molecule incorporated at the plus end is displaced progressively along the MT and eventually lost by depolymerization at the opposite end Prepared by SWT 2024 email: [email protected] 14 Treadmilling of Microtubules Textbook Ch.13 p.382 Prepared by SWT 2024 email: [email protected] 15 Microtubule polarity in animal cells In the cell, the distribution of most microtubules is determined by microtubule- organizing centers (MTOCs) MT orientation in a cell (shown in orange) may vary with that cell’s function Textbook Ch.13 p.385 Prepared by SWT 2024 email: [email protected] 16 Microtubule in cell division MTs in a dividing cell are oriented with their minus ends anchored in the centrosome and their plus ends pointing away from the centrosome Cell division is preceded by the division of the centrosome Textbook Ch.13 p.385 Prepared by SWT 2024 email: [email protected] 17 Axonemal microtubules MTs are the structural components of flagella and cilia (axonemal MTs) Cilia and flagella are MT-based organelles They operate as antennae and propellers in eukaryotic cells https://micro.magnet.fsu.edu/cells/ciliaandflagella/images/ciliaandflagellafigure1.jpg Flagella are whip-like appendages that undulate to move cells; they are longer than cilia Prokaryotic and eukaryotic flagella differ greatly Cilia are hair-like structures causing the movement of unicellular paramecium Cilia are also found in specialized linings in eukaryotes Prepared by SWT 2024 email: [email protected] 18 Microfilaments – the smallest cytoskeletal elements Microfilaments (MFs), with a diameter of about 7 nm, are involved in the development and maintenance of cell shape MFs play a role in cell migration and a variety of cell movements MFs are involved in muscle contraction when interacting with myosin Actin is the protein building block of microfilaments (375 a.a., 42 kDa) Actin is an extremely abundant protein in virtually all eukaryotic cells, including those of plants, algae, and fungi Once synthesized, it folds into a roughly U-shaped globular molecule, with a central cavity that binds ATP or ADP Can be divided into muscle specific actins and non-muscle actins Prepared by SWT 2024 email: [email protected] 19 Polymerization of actins Textbook Ch.13 p.389 Muscle-specific actins (α-actins) Nonmuscle actins (β- and γ-actins) Within a microfilament, all the actin monomers are oriented in the same direction With an inherent polarity Less rigid than microtubules Plus (+) end: fast growing Minus (-) end: slow growing Monomers polymerize into a helical chain Prepared by SWT 2024 email: [email protected] 20 The architecture of MFs Textbook Ch.13 Microfilaments allow cells to adopt different shapes and perform different functions e.g., stress fibers help cells exert strong forces on their surroundings Cell cortex is crosslinked into a very loosely organized meshwork of MFs Lamellipodia and filopodia at their leading edge that allow them to move along a surface Prepared by SWT 2024 email: [email protected] 21  Textbook Ch.13 Polymerization of actins Monomeric actin binds to ATP Polymerization mechanism of MF is similar to that of MTs Upon polymerization, actin ATPase activity cleaves ATP to ADP ATP hydrolysis acts as a molecular “clock” Older actin filaments with ADP are unstable and disassemble from MF Prepared by SWT 2024 email: [email protected] 22 Actin binding proteins Textbook Ch.13 p.391 Actin binding proteins regulate the polymerization, length and organization of microfilaments Actin-binding proteins are responsible for converting actin filaments from one form to another Some proteins affect monomer availability or monomer addition Capping proteins affect severing or growth of existing filaments by binding to the end of filaments and prevent further addition or loss of subunits Crosslinking / bundling proteins affect filament organization Prepared by SWT 2024 email: [email protected] 23 “Pushing” force Localized polymerization of actins at the leading edge of the cell drives forward protrusion of the plasma membrane Intracellular movement and cell-to-cell spreading of pathogens e.g., Movement of Listeria monocytogenes A pathogenic bacterium that colonizes the epithelial lining the human gut Found in contaminated dairy products Textbook Ch.13 p.395 Infection can be lethal to newborns and immunocompromised individuals Prepared by SWT 2024 email: [email protected] 24 Myosins are actin-based motor proteins Myosins convert ATP hydrolysis into movement along actin filaments Many different classes of myosins (>30 classes in humans) Some myosins move cargoes, other myosins slide actin Myosin I can carry organelles or slide actin filaments along the membrane Actomyosin, a complex of actin filaments and myosin motor Textbook Ch.13 proteins, is responsible for force generation during muscle contraction Prepared by SWT 2024 email: [email protected] 25 Intermediate filament Intermediate filaments (IFs) have a diameter of about 8–12 nm, which makes them intermediate in size between microtubules and microfilaments IFs are the most stable and the least soluble constituents of the cytoskeleton Abundant intermediate filament protein: keratin Textbook Ch.13 p.397 An important component of structures that grow from skin in animals, including hair, appendages and the outermost layer of the skin IFs differ markedly in amino acid composition from tissue to tissue Prepared by SWT 2024 email: [email protected] 26 IF Assembly The starting point for assembly is a pair of IF polypeptides; the central domains of the two polypeptides twist around each other, with their N- and C-terminal ends aligned Two dimers align laterally to form a tetrameric protofilament Protofilaments assemble into larger filaments by end-to- end and side-to-side alignment A fully assembled IF is thought to be eight protofilaments thick at any point Textbook Ch.13 p.398 Prepared by SWT 2024 email: [email protected] 27 Mechanical strength IFs often occur in areas of the cell that are subject to mechanical stress → play a tension- bearing role IFs are less susceptible to chemical attack than are microtubules and microfilaments In humans, naturally occurring mutations of keratins give rise to a blistering skin disease called epidermolysis bullosa simplex Prepared by SWT 2024 email: [email protected] 28 Cytoskeleton ─ a mechanically integrated structure Cytoskeleton Provides an intracellular scaffolding that organizes structures and shapes of cells Microtubules are generally thought to resist bending when a cell is compressed, whereas microfilaments serve as contractile elements that generate tension Intermediate filaments are elastic and can withstand tensile forces Motility A biological term refers to the ability to move spontaneously and independently Can apply it to either tissue, cellular, and subcellular level Prepared by SWT 2024 email: [email protected] 29 Chemical agents used to perturb the cytoskeleton Textbook Ch.13 p.382 Prepared by SWT 2024 email: [email protected] 30 Extracellular structures Textbook Chapter 15 Prepared by SWT 2024 email: [email protected] 31 Introduction Textbook Ch.15 p.430 In order to understand how multicellular organisms are constructed, it is important to consider both connections between cells, i.e., cell-cell adhesions, and the extracellular structures to which cells attach Animal cells have an extracellular matrix (ECM) that takes on a variety of forms and plays important roles in cellular processes as diverse as division, motility, differentiation and adhesion The extracellular structures themselves consist mainly of macromolecules that are secreted by the cell Prepared by SWT 2024 email: [email protected] 32  Textbook Ch.15 p.431 Cell-cell junctions Multicellular organisms have specific means of joining cells in long-term associations to form tissues & organs The specialized structures where two cells come together are called cell-cell junctions In animals, the most common cell-cell junctions are 1. adhesive junctions (including adherens junctions and desmosomes), 2. tight junctions and 3. gap junctions Plant cells have special structures called plasmodesmata Prepared by SWT 2024 email: [email protected] 33 Extracellular matrix (ECM) Tissues are not simply composed of cells as cells need to interact with extracellular materials that are crucial for tissue structure and function ECM is a complex network of specific proteins & carbohydrates that fills the spaces between cells ECM plays a role in determining the shape and mechanical properties of organs and tissues Prepared by SWT 2024 email: [email protected] 34 Examples of ECM-enriched tissues Bone consists mainly of a rigid extracellular matrix that contains a small number of interspersed cells Cartilage is another tissue constructed almost entirely of matrix materials, although the matrix is much more flexible than in bone Textbook Ch.15 p.439 Connective tissue surrounding glands and blood vessels has a relatively gelatinous extracellular matrix containing numerous interspersed fibroblast cells Prepared by SWT 2024 email: [email protected] 35 Cell-cell and cell-ECM attachments Textbook Ch.15 p.431 Prepared by SWT 2024 email: [email protected] 36 ECM molecules Despite the diversity of function, ECM consists of the same three classes of molecules: 1. Structural proteins, e.g., collagens 膠原蛋白 and elastins 彈性蛋白 Prepared by SWT 2024 email: [email protected] provide the matrix its strength and flexibility 2. Protein-polysaccharide complexes, i.e., proteoglycans 蛋白聚醣 provide the matrix in which structural molecules are embedded 3. Adhesive glycoproteins, e.g., fibronectins 纖連蛋白 and lamins 層粘連蛋白 allow cells to attach to the matrix Textbook Ch.15 p.440 37 Collagen 膠原蛋白 Major component of ECM Single most abundant protein in the animal kingdom (25% of protein in body is collagen) Collagen is secreted by fibroblasts (skin, tendon…) or osteoblasts (bone) Textbook Ch.15 p.440 A fibrous protein that makes up a major part of connective tissue Provide strength and resilience Prepared by SWT 2024 email: [email protected] 38 Elastins 彈性蛋白 Collagen fibers give the ECM great tensile strength and rigid Elastins give the ECM elasticity and flexibility The elastin fiber (a) stretches to its extended form when tension is exerted on it and (b) recoils to its compact form when the tension is released Textbook Ch.15 p.442 Prepared by SWT 2024 email: [email protected] 39 Proteoglycans Proteoglycans are formed of glycosaminoglycans (GAGs) covalently attached to the core proteins GAGs are large carbohydrates characterized by repeating disaccharide units Proteoglycans are linked directly to collagen fibers to make up the fiber/network structure of the ECM Textbook Ch.15 p.442 Prepared by SWT 2024 email: [email protected] 40 Proteoglycans (cont’d) Trap extracellular fluid (water) and serve as cushion to cell resistance to forces of compression Form highly hydrated gel-like “ground substance” when interacting with glycosaminoglycans molecules Textbook Ch.15 p.442 Prepared by SWT 2024 email: [email protected] 41  42 https://pub.mdpi-res.com/polymers/polymers-14-05014/article_deploy/html/images/polymers-14-05014-g001.png?1668776149 Prepared by SWT 2024 email: [email protected] Proteoglycans Glycosaminoglycans (GAGs) They are linear polymers of repeating disaccharide units Highly negatively charged due to carboxyl and sulfate groups Strongly hydrophilic (trap extracellular fluid → resistance to forces of compression) Covalently linked to protein Interacting with proteoglycans to form gel-like structure, providing mechanical support for the tissues Prepared by SWT 2024 email: [email protected] 43 Hyaluronate 透明質酸鹽 Although most of the GAGs found in the extracellular matrix exist only as components of proteoglycans and not as free GAGs, hyaluronate is an exception It has lubricating properties and is most abundant where friction needs to be reduced, such as in joints Prepared by SWT 2024 email: [email protected] 44  45 https://www.mdpi.com/cells/cells-11-00914/article_deploy/html/images/cells-11-00914-g001.png Prepared by SWT 2024 email: [email protected] Basal lamina The basal lamina is a thin, highly specialized ECM attached to the basal surfaces of the epithelial cells (about 50 nm thick) Serves as a structural support The major adhesive glycoproteins present in the basal lamina are a family of proteins called laminins Cells can alter the properties of the basal lamina by secreting enzymes that catalyze changes in the lamina e.g., Matrix metalloproteinases (MMPs) degrade ECM locally Prepared by SWT 2024 email: [email protected] 46 Integrins Integrins are cell surface receptors They bind ECM constituents (e.g., laminins) Function Integrating the cytoskeleton with the extracellular matrix Prepared by SWT 2024 email: [email protected] 47 ECM and cells ECM serves as an inert framework that supports and surrounds cells A complex chemically and physically crosslinked network Separates one tissue space from another The bidirectional interactions between ECM and cells are complex Cells receive external information from ECM Cells frequently remodel ECM ECM promotes the formation of capillary-like structures (angiogenesis) ECM influences cell shape, fate and metabolism Prepared by SWT 2024 email: [email protected] 48 ~ The end ~ Prepared by SWT 2024 email: [email protected] 49