Cytoskeleton, Cell adhesion & Cell motility

• Cytoskeleton in disease and therapy:

  Defective microtubule-associated proteins

  • Mutations in the Tau protein mean it can't bind to proteins and causes "tangles" which contribute to Alzheimer's

  
  

  • Mutations in spastin can cause Hereditary Spastic Paraplegia
• Cytoskeleton in disease and therapy:

  Defective (IF) Intermediate filament-associated proteins


  • Mutations in keratin genes causes Epidermolysis bullosa symplex (EBS):

  keratin filaments in epidermis doesnt form so skin is highly sensitive to mechanical injury (blistering and sloughing)

  
  

  • Mutations in Plectin also cause EBS with associated muscular dystrophy

  
  

  • Mutations in neurofilamin IF genes can cause Amyotrophic Lateral Sclerosis (ALS, Lou Gehrig's Disease)
• Cytoskeleton in disease and therapy:

  Defective actin-associated proteins

  • Mutations in Dystrophin gene causes Duchenne and Becker Muscular Dystrophy

  
  

  • Mutations in Myosin VII causes Usher's Syndrome:

  causes deafness (stereocilia in the ear becomes disorganised which means action potential in response to sound waves travelling across so this leads to deafness) and blindness
• Cytoskeleton in disease and therapy:

  
  

  Drugs affecting the cytoskeleton

  E.G. Chemotherapeutic agents (involved in treatment of cancer)

  The chemical compounds colchicine, vinblastine and taxol are anti-cancer therapeutics that work by regulating microtubules:

  - colchicine and vinblastine destabilise microtubules and break them down

  - taxol stabilises microtubules, inhibiting new microtubules from forming

  
  

  --> The 3 chemicals inhibit the function of the mitotic spindle and thus inhibit cell division/proliferation of cancer cells.
• how do Microtubules help with anchoring and organising organelles?

  Microtubules organise the endoplasmic re7culum (E.R.)
• how do Microtubules help with maintaining cell shape and polarity?


  • stabilises mature axons but are also involved in their growth as the brain develops

  • stabilises the irregular shapes of platelets, a property related to their role in blood clotting
• what are the two motor microtubule-associated proteins?

  (these motor proteins move cargo (protein that is carried within the vesicles of a cell's secretory system) along microtubules)

  
  

  1)Kinesin:

  moves towards microtubule + ends (cell periphery)

  
  

  2)Dynein:

  moves towards - ends (near nucleus)
• Microtubules example:

  
  

  explain how microtubule rods are organised in respiratory epithelial cells and explain how the cilia move.


  orgnisation:

  Microtubule rods are organised into a "9 + 2" arrangement within respiratory epithelia: 9 pairs of microtubules rods organised at the perphiary of the cilium and two single microtubule rods at the center of the cilium.

  
  

  movement:

  Movement is initiated by the microtubule-associated

  protein= Dynein (moves towards the - end of the microtubule)

  
  

  Dynein joins one pair of microtubules to its adjacent neighbor within the periphery of the cilium causing the microtubules slide along one another, causing the cilium

  to bend. This leads to the spiraling motion of the cilia.
• Give the functions of Microtubules

  • produce the movement of cilia and flagellae

  • Separates chromosomes during cell division

  e.g. the cilia found on respiratory epithelial cells.

  
• synthesis of Microtubules

  (inc. assembly and disassembly)

  • GTP-bound alpha/beta monomers are added to the plus

  end of the microtubule with GTP being converted to GDP

  • GDP-bound monomers detach at both the - and + ends of the microtubule

  
  

  ----> The + end is more dynamic as it can extend and collapse within seconds

  (Subunits are added and lost at the plus end. They are also lost at the minus end but more slowly)
• Microtubule structure (basic)

  • Long, stiff hollow tubes

  
  

  • Rapidly assembled and disassembled

  
  

  • 13 polymer rods arranged in a circle to form a tube

  (each polymer is built from tubulin monomers)

  ---> the tubulin monomers consist of one molecule of

  alpha-tubulin and one of beta-tubulin (each of these is a

  different protein encoded by a different tubulin gene)

  
  

  • The alpha/beta monomer is asymmetric: alpha one side and beta the other.

  This means that one end of the polymer tube ends with an alpha subunit (+) and the other with a beta (-)= polarised
• how do Intermediate filaments help with anchoring and organising organelles?


  Intermediate filaments form a meshwork around the nucleus to anchor it in position ( this keeps the nucleus out the way so processes happen efficiently)
• how do Intermediate filaments help with maintaining cell shape and polarity?

  Stabilise long cell processes such as axons of nerve cells
• Give the functions of Intermediate filaments (there are 2)

  -Mechanical function of stabilising structures in a cell

  
  

  -form a network most dense around the nucleus and extending into the periphery (outer edges)

  
  

  -Anchor cells at some cell junctions (stabilisation)

  e.g. Desmosomes and hemidesmosomes

  
  

  - E.G. A paricular type of Intermediate fiber, lamins, inside the nucleus, support nuclear structure & protect chromatin
• synthesis of Intermediate filaments

  - Two intermediate filament monomers wind round each other to form a helical dimer

  - Two helical dimers combine to form a tetramer (this is the fundamental unit of the intermediate filament)

  - The tetramers link end to end and cross-link in a staggered way to form the intermediate filament

  
  

  ---->Subunit exchange is slow but can happen throughout the length of the filament

  (unlike in actin and tubulin - where monomers can only be added/removed from the filament ends)
• Intermediate filaments structure (basic)

  • Rope-like polymers from individual intermediate filament proteins

  
  

  ---> Intermediate filaments are a heterogeneous family of proteins (the composition varies due to the different cell types that can be used as monomers)

  E.g. monomer cell types:

  - Keratin monomers for epithelia

  - Lamin monomers for nuclear

  - Neurofilamin monomers for neurons

  - GFAP monomers for glia

  -vimentin monomers for mesenchymal cells

  
  

  =level of cross-linking varies across cell types
• explain the steps to actin-based motility via myosin

  • The "head region" of the myosin interacts with actin and

  binds ATP. Energy release from ATP hydrolysis forces the

  myosin tail to move, generating forces.

  • ADP is released from the myosin head and replaced by

  ATP. At this stage the head can detach from the actin

  filament.

  • The head now binds further down the actin filament

  
  

  --> repetition of the process slides the actin filaments
• what is actin-based motility?

  result of individual actin filaments being slid across each other in opposite directions using an actin associated protein called myosin ( a motor protein)
• what are the two types of protrusions during actin-based movement


  1) Filopodia: thin structures/protrusions at the leading edge

  -Sample the environment (provide sensors for which the cell can make decisions about where to go depending on what is around it|)

  -extend (via actin filaments) and withdraw

  
  

  2)Lamellipodia: wider structures/protrusions around the leading edge
• explain the steps to actin-based movement

  Actin can regulate the movement of entire cells:

  
  

  1) The cell pushes out protrusions (projections) at the front (leading edge- the direction the cell is moving)

  -> Actin filament polymerisation provides the force for

  membrane protrusion.

  
  

  2) The protrusions adhere to the surface on which the cell

  is moving through focal adhesions (holding points). F-actin connects to the focal adhesions to provide a contractile force for the cell ( this pulls the cell forwards)

  
  

  3) The rest of the cell pulls against the anchorage points to

  drag itself forward.

  
  

  4) Actin depolymerises at the rear ( end)
• how does actin help with anchoring and organising organelles?

  Actin tethers (binds) vesicles full of neurotransmitter close to the presynaptic membrane of synapses in the nervous system. So the vesicles are ready to be discharged efficiently when the presynaptic neuron is depolarised
• how does actin help with maintaining cell shape and polarity?

  -Actin filament bundles provide support

  e.g for microvilli in the gut

  
  

  -Actin filament sheets provide support:

  they are found beneath the plasma membrane, a region called the cortex of the cytoplasm.

  e.g. they maintain cell shape in red blood cells (erythrocytes)
• Give the functions of actin (there are 3)- with examples

  • Mechanical support

  e.g. stereocilia cochlea hair cells in the ear are held rigid by actin and bent as sound waves pass across them

  
  

  • Changing/maintaining cell shape:

  e.g. actin helps maintain the biconcave shape of red blood cells, which increases surface area for oxygen uptake

  
  

  • Cell motility:

  e.g. moving cells and growing nerve cell processes use actin to move.

  e.g. muscle cells use actin to contract
• Synthesis of actin

  • G-actin monomer bound to ATP can be added and removed to either end of the filament/polymer

  (BUT adds more rapidly to + end)

  
  

  • once incorporated into the filament, the ATP is hydrolysed to ADP
• Actin structure (basic)

  • G-actin (globular actin) monomers form the F-actin (filamentous actin) polymer

  
  

  (actin can be present in both globular and filamentous form in a cell)

  
  

  • The actin filament is a polarised double helix with 13 subunits for every complete turn of the helix (37nm)

  
  

  • Asymmetric shape of the actin monomer gives the filament polarity (+ and - end)

  
  

  • filament is very dynamic

  (always changing: extending/contradicting)

  
• what are the 3 protein filaments in the cytoskeleton ( include their diameter sizes in nm)?

  Actin microfilaments, 7nm diameter

  
  

  Intermediate filaments, 10nm diameter

  
  

  Microtubules, 20nm diameter
• what are associated proteins?

  proteins that bind to the protein filaments to allow it to perform different functions
• what is the role of the cytoskeleton in cell adhesion

  Binds cells to neighbors at intercellular junctions (adhesion belts and desmosomes) and to underlying extracellular matrix i.e. hemidesmosomes
• what are the functions of the cytoskeleton?

  • Cell movement

  
  

  • Cell shape & polarity

  
  

  • Cell adhesion & thereby tissue structure

  (cells attach to neighboring cells via associated processes on its surface)

  
  

  • Intracellular movement

  (e.g. vesicles & chromosomes)

  
  

  • Support of specialised structures

  (e.g. cilia, flagellae & stereocilia)
• what is the cytoskeleton?

  A network of interlinking protein filaments/polymers present in the cytoplasm of cells