Lecture 9: The Cytoskeleton and Cellular Movement 2023 PDF

Cell Form and Function 2023 Lecture 9: The Cytoskeleton and Cellular Movement. Ch13/14.  Learning Outcomes List the major cytoskeletal macromolecules and compare their structures and assembly. Differentiate the role/s of the cytoskeletal protein...

Cell Form and Function 2023 Lecture 9: The Cytoskeleton and Cellular Movement. Ch13/14.  Learning Outcomes List the major cytoskeletal macromolecules and compare their structures and assembly. Differentiate the role/s of the cytoskeletal proteins Explain how the cytoskeleton contributes to cell movement Interpret human disorders in terms of cytoskeletal defects. © 2017 Pearson Education, Ltd.  Microfilaments § Microfilaments are involved in cell shape and movement § Interact with myosin - muscle contraction § Involved in cell migration, amoeboid movement, and cytoplasmic streaming § Development and maintenance of cell shape - cell cortex § Form the structural core of microvilli © 2017 Pearson Education, Ltd.  Cytoplasmic streaming – generating currents? © 2017 Pearson Education, Ltd.  Actin Is the Protein Building Block of Microfilaments § Actin found in all eukaryotic cells - folds into a globular-shaped molecule that can bind ATP or ADP (G-actin; globular actin) © 2017 Pearson Education, Ltd.  G-actin monomers self-assemble into microfilaments § G-actin monomers can assemble reversibly into filaments with a lag phase and elongation phase, similar to tubulin assembly § F-actin filaments are composed of two linear strands of polymerized G-actin wound into a helix G-actin Self-assembly F-actin © 2017 Pearson Education, Ltd.  § All the actin monomers in the filament have the same orientation (= polarity) ‘Pointed end’ ‘Barbed end’ © 2017 Pearson Education, Ltd.  Specific Drugs Affect Polymerization of Microfilaments § Cytochalasins are fungal metabolites that prevent the addition of new monomers to existing MFs § Latrunculin A is a toxin that sequesters actin monomers and prevents their addition to MFs § Phalloidin stabilizes MFs and prevents their depolymerization © 2017 Pearson Education, Ltd.  Cells Can Dynamically Assemble Actin into a Variety of Structures Bundles and Networks § The cell cortex, just beneath the plasma membrane, has actin crosslinked into a gel of microfilaments § Cells that adhere tightly to the underlying substratum have organized bundles called stress fibers © 2017 Pearson Education, Ltd.  Lamellipodia and Filopodia § Cells that crawl have lamellipodia and filopodia at their leading edge, allowing them to move along a surface § Lamellipodia have a branched network of actin § In filopodia microfilaments form highly oriented, polarized cables with the actin plus ends toward the tip of the protrusion © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd.  Actin-Binding Proteins regulate the organization of actin § Cells use actin-binding proteins to precisely control where actin assembles and the structure of the resulting network § Control occurs at the nucleation, elongation, and severing of MFs and at the association of MFs into networks © 2017 Pearson Education, Ltd.  Proteins That Bundle Actin Filaments § Actin may be bundled into tightly organized arrays, in filipodia (also focal contacts or focal adhesions) § α-Actinin is a protein that is prominent in such structures § Fascin/Fimbrin in filopodia keeps the actin tightly bundled © 2017 Pearson Education, Ltd.  Protrusions from cell surface - Microvilli § Actin bundles in microvilli are good examples of ordered actin structures § Microvilli are prominent features of intestinal mucosal cells © 2017 Pearson Education, Ltd.  e.g. fimbrin, a-actinin e.g. calmodulin and myosinI © 2017 Pearson Education, Ltd.  Forming Actin Networks Which end faces the membrane? © 2017 Pearson Education, Ltd.  Proteins That Link Actin to Membranes § MFs are connected to the plasma membrane and exert force on it § This (indirect) connection to the membrane requires one or more linking proteins – e.g.spectrin and ankyrin © 2017 Pearson Education, Ltd.  Actin – Membrane interaction Endocytosis © 2017 Pearson Education, Ltd.  Actin – Membrane interaction Actin filaments indirectly connect to extracellular surfaces via transmembrane proteins. © 2017 Pearson Education, Ltd.  Actin – membrane interactions allow cell response to mechanical forces. © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd.  Intermediate Filaments § Intermediate filaments (IFs) are not found in cytosol of plant cells but are abundant in many animal cells § IFs are the most stable and least soluble components of the cytoskeleton § They likely support the entire cytoskeleton © 2017 Pearson Education, Ltd.  Intermediate Filament Proteins Are Tissue Specific § IFs differ greatly in amino acid composition from tissue to tissue § They are grouped into six classes – I – VI An abundant IF is keratin (class I, II) an important component of structures that grow from skin in animals § Animal cells can be distinguished based on the types of IF proteins they contain—a technique known as intermediate filament typing © 2017 Pearson Education, Ltd.  § Class I: acidic keratins § Class II: basic or neutral keratins Proteins of classes I and II make up the keratins found in epithelial surfaces covering the body and lining its cavities § Class III: includes vimentin (connective tissue), desmin (muscle cells), and glial fibrillary acidic protein (GFAP) (glial cells) § Class IV: the neurofilament proteins found in neurofilaments of nerve cells © 2017 Pearson Education, Ltd. Do not need to know classes  § Class V: includes the nuclear lamins A, B, and C that form a network along the inner surface of the nuclear membrane § Class VI: nestin, the substance that makes up the neurofilaments in nerve cells of embryos © 2017 Pearson Education, Ltd.  Neurofilament (light chain) found in CSF/blood is a marker of axonal degeneration Sport Cardiovascular Ageing risk factors CNS/PNS damage Neurosurgery © 2017 Pearson Education, Ltd.  Intermediate Filaments assemble from fibrous subunits § The fundamental subunits of IF proteins are dimers § IF proteins are fibrous rather than globular § Each has a long central rodlike domain § Flanking the central helical domain are N- and C- terminal domains that differ greatly among IF proteins © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd.  Intermediate Filaments Confer Mechanical Strength on Tissues § Intermediate filaments are thought to play a tension-bearing role § IFs are less susceptible to chemical attack than are MTs and microfilaments © 2017 Pearson Education, Ltd.  The Cytoskeleton Is a Mechanically Integrated Structure § Microtubules resist bending when a cell is compressed § Microfilaments serve as contractile elements that generate tension § Intermediate filaments are elastic and can withstand tensile forces © 2017 Pearson Education, Ltd.  Integration of Cytoskeletal Elements § Linker proteins connect intermediate filaments, microfilaments, and microtubules § Create an integrated cytoskeletal network © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd.  The Cytoskeleton is important for movement § Cell motility involves § Movement of a cell through the environment § Movement of the environment past or through a cell § Movement of components in the cell § Contractility, used to describe shortening of muscle cells, is a specialized form of motility © 2017 Pearson Education, Ltd.  Two Eukaryotic Motility Systems 1. Microtubule-based motility § Examples: fast axonal transport in neurons; the sliding of MTs in cilia and flagella 2. Microfilament-based motility § Example: muscle contraction © 2017 Pearson Education, Ltd.  1. Microtubule-Based Motility: Cilia and Flagella § Microtubules are crucial for movements of cilia and flagella, the motile appendages of eukaryotic cells § Cilia are about 2–10 μm long and occur in large numbers on the surface of ciliated cells § They occur in both unicellular and multicellular eukaryotes § Cilia display an oarlike pattern of beating, generating a force parallel to the cell surface © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd.  Cilia and Flagella § Flagella move cells through a fluid environment § They are the same diameter as cilia, but usually much longer (up to 200 μm) § They are limited to one or a few per cell and move with a propagated bending motion, which generates a force parallel to the flagellum © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd.  Cilia and Flagella consist of an axoneme connected to a basal body § Cilia and flagella share a common structure, the axoneme § Axonemes have a characteristic “9 + 2” MT pattern, with 9 outer doublets and 2 MTs in the center, the central pair © 2017 Pearson Education, Ltd.  Structure of Cilia and Flagella § Each outer doublet of the axoneme consists of one complete MT (the A tubule) and one incomplete MT (the B tubule) § The tubules of the central pair are both complete § Each A tubule has a set of sidearms that project from each of the outer doublets, these contain dynein © 2017 Pearson Education, Ltd.  Doublet Sliding Within the Axoneme Causes Cilia and Flagella to Bend § Adjacent outer doublets slide relative to one another. Similar dynein arms (radial spokes) move against central pair – translates sliding motion to bending of cilia/flagella. © 2017 Pearson Education, Ltd.  Primary Cilia § Primary cilium is a long, thin organelle found on nearly all cells - common on apical surface of epithelial cells. § Primary cilia are important in development; role in embryonic patterning and organogenesis - defects in them can result in disorders such as deafness and left-right asymmetry reversals = ciliopathies § Important in sensing and responding to external stimuli – cell’s antenna. © 2017 Pearson Education, Ltd.  § Primary cilia have a “9 + 0” axoneme structure; that is, they lack the central pair – they do not move. © 2017 Pearson Education, Ltd.  Genes Genes associated associated with with function ciliogenesis Genes associated with specific ciliated cell – the photoreceptor of the retina. eg. RPE65 © 2017 Pearson Education, Ltd.  § Loss of dynein → primary cilia dyskinesia § E.g Kartagener syndrome – infertility, respiratory problems, high chance of situs inversus totalis © 2017 Pearson Education, Ltd.  2. Microfilament-Based Movement Inside Cells: Myosins § ATP-dependent motors, the large superfamily called myosins, interact with and exert force on actin microfilaments § Myosins function in a wide range of cellular events, including § Muscle contraction, Cell movement, Phagocytosis, Vesicle transport © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd.  Type II Myosins § The basic function of myosin II is to pull arrays of actin filaments together, resulting in contraction of a cell or group of cells § Resembles kinesin - both have globular domains that walk along a protein filament, and both use ATP hydrolysis to change their shape © 2017 Pearson Education, Ltd.  Kinesins Versus Myosin § Kinesins operate alone or in small numbers to transport vesicles over large differences § A single myosin II molecule slides an actin filament about 12–15 nm § Myosin II molecules move short distances but operate in large arrays, in some cases billions of motors working together to mediate muscle contraction © 2017 Pearson Education, Ltd.  A. Microfilament-Based Motility: Muscle Cells in Action § Muscle contraction is the most familiar example of mechanical work mediated by intracellular filaments § A muscle consists of parallel muscle fibers - each fiber is a long, thin, highly specialized, multinucleate cell § Each muscle fiber contains numerous myofibrils, each of which is divided along its length into repeating units called sarcomeres © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd.  + ends of actin § Myosin II moves toward the plus ends, so the thick filaments move toward the Z lines during contraction (filaments remain same length) © 2017 Pearson Education, Ltd.  The Sliding-Filament Model Explains Muscle Contraction § The sliding filament model - muscle contraction is due to thin filaments sliding past thick filaments, with no change in length of either § Cross-bridges are formed between the F-actin of thin filaments and myosin heads of thick filaments. Dissociate rapidly - requires lots of ATP. § The result is shortening of sarcomeres and muscle fiber contraction © 2017 Pearson Education, Ltd.  B. Microfilament-Based Motility in Nonmuscle Cells § Actin and myosin have been discovered in nearly all eukaryotic cells § Many cells are capable of crawling over a substrate using lamellipodia and/or filopodia § Cell crawling involves distinct events: extension of a protrusion, attachment to substrate, and generation of tension, which pulls the cell forward © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd.  Shortening of actin filaments © 2017 Pearson Education, Ltd.  Chemotaxis Is a Directional Movement in Response to a Graded Chemical Stimulus § Directional migration occurs through the formation of protrusions predominantly on one side of a cell § Diffusible molecules can act as cues for directional migration; when a cell moves in response to a chemical gradient, it is called chemotaxis § Increasing the local concentration of a chemoattractant results in dramatic changes in the actin cytoskeleton (eg. cell movement to inflammation site) © 2017 Pearson Education, Ltd. © 2017 Pearson Education, Ltd.