Cytoskeleton Slides PDF

Cytoskeleton – part 1 • Actin and cell movement LOs: 1. Understand the assembly and dynamics of actin filaments, and how they are anchored 2. Recognise the kinds of actin-based cell protrusions and describe how cell movement is powered by them Cytoskeleton overview Cell architecture is maintain...

Cytoskeleton – part 1 • Actin and cell movement LOs: 1. Understand the assembly and dynamics of actin filaments, and how they are anchored 2. Recognise the kinds of actin-based cell protrusions and describe how cell movement is powered by them Cytoskeleton overview Cell architecture is maintained by three kinds of filaments Actin (microfilaments) Thin (8nm) Cortical Dynamic Intermediate filaments Medium width (10-15 nm) Relatively stable Microtubules Wider (20-30 nm) Radial Dynamic Actin Actin filaments are made by joining actin subunits together: P minus end (ADP actin) F-Actin plus end (actin with ATP) Actin Actin filaments are typically stabilised and linked together: capping protein tropomyosin capping protein Actin network (eg at the cell cortex) Actin bundles (eg stress fibres) Actin Actin stabilizes the cell membrane: This actin mesh is anchored in the cell membrane (Svitkina et al, 1997) Actin Actin stabilizes the cell membrane: Stress fibres (bundled actin Filaments) Stress fibres are anchored in focal adhesions Focal adhesions connect actin to the ECM Actin Actin stabilizes the cell membrane: normal no talin (Brown, Gregory et al, 2002) Talin is needed to anchor the actin to the integrins - with no talin, the muscles rip the junction Focal adhesions connect actin to the ECM Cell protrusions (actin based) Cell membranes can be stretched and extended: Microvilli • lasting membrane ‘fingers’ • to increase surface area Pseudopodia, Lamellipodia and Filopodia • transient membrane extensions • allow cell movement (Sousa and Cheney, 2021) Cell protrusions Pseudopodia: (Botelho and Grinstein 2011) Immune cells extend pseudopodia to engulf bacteria (phagocytosis) Cell protrusions Filopodia and Lamellipodia: Filopodia and lamellipodia allow cell migration Cell migration Movement starts by extension of the membrane • Actin extension makes lamellipodia • Adhesion is made to the substrate • The cell contracts stress fibres • Adhesions at the rear of the cell detach (Shellard and Mayor 2020) Cytoskeleton – part 2 • Myosin and contraction LO: 3. Be able to describe the structure and function of the contractile assembly in muscle and cytokinesis and the role of myosins Myosin Myosin is a motor protein: + - It binds and moves along actin filaments Movement of myosin is ATP powered. This has massive implications! Myosin and muscle contraction Muscles contract using myosin: A muscle One muscle cell One myofibril Myosin drives contraction Muscles contract using myosin: sarcomere Actin + - Myosin bundles - + Myosin drives contraction + - Myosin moves along the actin fibres Contraction This pulls the ends of the sarcomere together (contraction) - + Contraction initiation • Muscle contraction starts with a nerve signal • This leads to Ca++ release into the cytoplasm • Ca++ causes troponin to shift tropomyosin • This exposes the myosin binding sites • Myosin moves along the actin to give contraction Cytokinesis – also via myosin Cell division occurs by the contraction of an actin/myosin ring + end + end (Pollard 2010) Cytokinesis – also via myosin Cell division occurs by the contraction of an actin/myosin ring Myosin (and DNA) in dividing brain cells (Pollard 2010) Cytoskeleton – part 3 • Microtubules LO: 4. Be able to describe microtubule assembly and organization, microtubule motors and their role in moving organelles and in mitosis Microtubules Microtubules are made of alpha- and beta-tubulin dimers Microtubules have a plus (GTP bound) and minus (GDP bound) ends. This is like actin, but GTP instead of ATP Microtubules Microtubules are anchored in the centrosome: Cortical actin Centrosome Microtubules minus ends are anchored in the centrosome (the MicroTubule Organizing Centre, MTOC) Microtubules Microtubules are not static! (Sami et al, 2019) Microtubules in mitosis In metaphase the microtubule spindle aligns the chromosomes In prophase the centrosomes are separated Microtubules in mitosis In telophase the daughter cells are ready to be separated In anaphase the DNA is separated by shortening microtubules Microtubules in mitosis (von Dassow et al, 2009) Microtubule motors Kinesin is a plus end* directed microtubule motor protein complex - end + end Dynein is a minus end directed microtubule motor protein complex (* there are exceptions) Microtubule motors in axons In neurons, kinesins transport mitochondria to the end of axons (anterograde) Dyneins transport vesicles like endosomes back toward the cell body (retrograde) (Sleigh et al 2019) Microtubule motors in mitosis + end + end - end Cytoskeleton – part 4 • Intermediate filaments and comparisons LOs: 5. Understand the structure and function of intermediate filaments 6. Recognise the similarities and differences between actin, microtubules and intermediate filaments Intermediate filaments Intermediate filaments are made of coiled dimers IFs do not have polarity – both ends are the same Hence there are no IF motor proteins Intermediate filament location Different classes of IF have different roles: Keratin Lamin Vimentin Neurofilament Anchoring intermediate filaments (Fawcett, 1966) Hemidesmosomes - joining cells to substrate keratin Desmosomes - joining cells together plectin integrins Extracellular matrix Cytoskeleton summary Actin filaments Intermediate filaments Microtubules Dynamic More stable Very dynamic Location Cell cortex, Stress fibres Cytoplasm Radial from MTOC Polarity Via ATP-Actin None Via GTP-tubulin + end motors Myosins None Kinesins - end motors None None Dyneins Desmosomes, Hemidesmosomes Centrosome (MTOC) Length stability Anchor sites Focal adhesions, Adherens junctions Key roles Cell cortex stability, Muscle contraction Cell-cell adhesion, Cell-substrate adhesion Vesicle transport, Mitosis