Cytoskeletal Elements

Cytoskeletal elements • • • • • • • Microfilaments – mainly form cortex beneath plasma membrane → 3-6nm in diameter Intermediate filaments – from perinucleus (region of cytoplasm at nucleus) to cell periphery – enriched at sites of contact → 10nm in diameter Microtubules – extend from perinucleus to cell periphery → 25nm in diameter Microfilaments Made up of actin monomers (G-actin) → globular protein Contains a nucleotide binding site (ATP or ADP) which regulates the shape of protein Actin-ATP + polymerisation = actin-ADP (ATP → ADP + Pi) Pi: inorganic phosphate (phosphate ion free in a solution) Actin filament assembly: G-actin → ATP comes along and inserts itself into actin forming ATP-actin → ATP hydrolysis occurs and the ATP becomes ADP, forming ADP actin → actin polymerisation creates a growing chain of actin-ATP (here the actin is no longer a free globular monomer called G-actin, but part of a filamentous linear polymer called F-actin) → ATP breaks down into ADP → actin depolymerisation dismantles the ADP actin end • G-actin (monomer) becomes F-actin (actin filament) – twisted helical chains, has polarity: barbed (+) and pointed end (-) • Polymerisation is aided by profilin – occurs at both ends but favours the barbed end at critical concentrations of G-actin molecules • Capping proteins prevent further polymerisation • Severing proteins: gelsolin, ADF/cofilin • Microfilaments can be cross-linked or bundled together to form many different structures Function of microfilaments • Maintenance of cell shape • Actin-myosin interaction → muscle contraction • Cell crawling Actin-myosin interaction • Myosin is form a large family of motor proteins that have varying roles in different cells • Most well-known actin-myosin interaction occurs in muscle cells to produce contraction, however also involved in cell crawling • Utilised energy from ATP hydrolysis to move along microfilament Cell crawling • Actin filaments polymerise towards the direction of crawling • Lamellipodium (protein actin projection) protrude from cell and attach to substratum (basement membrane) • Myosin II contracts pulling the cell in the direction of the newly formed actin filaments • Microfilaments are the primary regulators of cell movement → good (muscle cells, immune cells, wound healing, axon guidance – neurons) but also bad (cancer – metastasis, angiogenesis in diabetic retinopathy – epithelial cell migration, immune infiltration into CNS – multiple sclerosis, neuronal sprouting – pain) • • • 1. 2. 3. 4. 5. 6. Intermediate filaments >70 distinct intermediate filament proteins Mainly found in vertebrates Can be divided into 6 major classes Acidic Keratins (epithelia) Basic Keratins (epithelia) Desmin (muscle), GFAP (glial cells), Peripherin (peripheral neurons), Vimentin (mesenchyme) Neurofilaments (neurons) Lamins (nuclear envelope) Nestin (neural stem cells) Characteristics • Tend to form relatively permanent structures in tissues e.g. skin and hair • Involves in cell-cell (desmosomes) and cell-matrix (hemidesmosomes) adhesion • Are phosphorlylated if disassembly is required (i.e. cell division) • Structure: monomer → dimer → tetramer → protofilament → intermediate filament Cell-cell adhesion: desmosomes found in epithelial & cardiac muscle Cell-matrix adhesion: Intermediate fibres attach indirectly to adhesion proteins (integrins), which anchor the cell (mainly epithelial cells) to the basement membrane • • • • • • • • Hair: type I (acidic) filaments associate with type II (basic) filaments in the dimer Type I & II disorders: meesman corneal dystrophy Type III disorders: ALS Type IV disorders: Parkinson’s disease, Alzheimer’s disease Type V disorders (vary varied): lipodystrophy, neurological disorders Microtubules Subunits = 2 tubulin (protein) linked together (dimer complex) o 1 monomer alpha tubulin and 1 beta tubulin: bound GTP is hydrolysed during/after polymerisation o Polar: alpha tubulin at + end and beta tubulin at – end (more tubulin is added to the + end) o 13 tubulin polymers form the circumference – small tubes with internal diameter ~ 14nm All microtubules attach to a centriole in the centrosome → centrosome is an organelle near the nucleus of a cell which contains the centrioles and from which the spindle fibres develop in cell division Display dynamic instability Microtubule Associated Proteins (MAPs) • MAPs stabilise microtubules with protein interactions e.g. Tau protein • Function is tightly regulated by phosphorylation (addition of a phosphate group to a molecule – type of posttranslational modification) • Gives rise to specialised microtubule structures/functions: cilia (e.g. intestinal cilia), flagella (e.g. sperm), certrioles during cell division (organise mitotic spindle) Interaction with motor proteins • Motor proteins, dynein and kinesin ‘walk’ along microtubules o Dynein moves towards the negative pole (-) in a retrograde motion (retrograde = toward cell body) o Kinesin moves toward the positive pole (+) in an anterograde motion (away from cell body) o Process is dependent on ATP hydrolysis o Involved in many roles, including vesicular transport, flagellar/cilia motion (dynein) and transport of organelles Cilia and flagella • Both have a similar structure but cilia are short and many flagella are long and few • Cilia are expressed on many epithelial cells where they move particles e.g. epithelial cells of the respiratory passages contain cilia that dislodge and expel particles that collect in the mucous secretions • Sperm cells notably have a long and active flagellum which they use to navigate to the egg, while fallopian tubes have cilia to guide the egg • Dynein is involved in powering the beating motion of both flagella and cilia Microtubule pathologies • Neurodegeneration: Parkinson’s, Alzheimer’s and other tauopathies (tau forms a stabilising protein for microtubules) • Ciliopathies e.g. Meckel syndrome • Muscular dystrophy • Pathologies in cell division are probably lethal, fertilised egg can’t divide, possibly play a role in some cancer Drugs that target microtubules • Vincristine, vinblastine e.g. for Hodgkin’s and non-Hodgkin’s lymphomas • Colchicine: Gout disease • Taxol (taxanes) e.g. ovarian cancer