WK4 - Cytoskeleton and Cell Polarity

Question 1

Define cell polarity.

Answer 1

Positional asymmetry within and between cells.

Question 2

Why is cell polarity important? Name 2 examples.

Answer 2

required for tissue structure and function
Ex. milk is secreted on the apical surface NOT basal
Ex. hepatocytes secrete endocrine, plasma proteins, carbohydrates and lipoproteins basally into sinusoids and secrete bile salts, cholesterol and bilirubin apically into canaliculi

Question 3

What determines cell polarity?

Answer 3

• cytoskeleton

- cell junctions (adhesion complexes)

Question 4

What is cytoskeleton made up of and what are the 3 major components from smallest to largest?

Answer 4

• made up of insoluble proteins within the cell arranged either as solid filaments or hollow tubules
• 3 major components include: microfilaments, intermediate filaments and microtubules

Question 5

Describe microfilaments and their functions and where they are located.

Answer 5

• (5-9nm) filament formed by double stranded helical strands of globular actin that are very dynamic (ie. they can be lengthen, shortened, severed, bundled, and/or branched)
• important in cell adhesion, generation of contractile force (by the action of force-generating motors such as myosin that bind to actin filaments) in both non-muscle cells (eg. stress fibers) and muscle cells (e.g.. sarcomeres), cell shape, surface projections (eg. microvilli)

Question 6

Describe intermediate filaments and their functions and where they are located.

Answer 6

• (10-12nm) polymers of overlapping intermediate filament monomers that are more stable than microfilaments and microtubules
• convey tensile strength within cells and across tissues when they are linked to ‘anchoring’ cell-cell or cell-ECM junctions
• located in both the cytoplasm and the nucleus where they act as architectural scaffolds to maintain cell and nuclear structure

Question 7

Name 4 examples of tissue-specific intermediate filament proteins.

Answer 7

Epithelia - keratins
muscle - desmin
connective tissue - vimentin
neurons - neurofilaments

Question 8

Describe microtubules and their functions and where they are located.

Answer 8

• (25nm) polymers of globular tubulin arranged as hollow cylinders which themselves have polarity (a fast growing ‘plus’ end where tubulin is rapidly added and a slow growing ‘minus’ end)
• highly dynamic and often held in specific positions within cell by the microtubule organizing centre which of often anchors minus ends of microtubules centrally within the cell
• motors move along tubules in either plus-end or minus-end directions
• also found in the cores of motile cell surface projections (eg. cilia, tails of spermatozoa)

Question 9

Which type of filament proteins can be used as ‘markers’ of specific tissues, which can be useful in hisopathology when attempting to determine the origin of diseased tissues?

Answer 9

intermediate filaments

Question 10

What are motors used for?

Answer 10

motors are critical for binding cargo (eg. membranes, vesicles, organelles, chromosomes) and move them around the cell

Question 11

What are the common structural theme in cell junctions (adhesion complexes)?

Answer 11

i) cell surface receptors
ii) linker proteins:
iii) cytoskeletal elements

Question 12

What are cell surface receptors?

Answer 12

transmembrane molecules that bind to other receptors on neighbouring cells or to ECM proteins outside the cell

Question 13

What are linker proteins?

Answer 13

peripheral membrane scaffolding proteins in the cytoplasm that bind to the cytoplasmic tails of the cell surface receptors and physically link the complex to the cytoskeleton inside the cell

Question 14

What are cytoskeletal elements?

Answer 14

often span from one junction/adhesion complex to another inside the same cell

Question 15

What are adherens junctions (zonula adherens)? What are its structures?

Answer 15

• form a belt (zonula) all the way around the cell
• they initiate cell-cell adhesion on the lateral aspect of cells (ie. along the side walls of neighbouring simple columnar epithelial cells)
• they are also important for tissue segregation as cells will only bind to another cell that has the same class of cell surface receptor
• Structures includes:
  1. adherens junction reeptors = “cadherins”
  2. adherens junction linker proteins = catenins
  3. cytoskeletal elements = actin
• these junctions are dynamic; they can form/release/reform along lateral cell wall depending on developmental or physiological conditions

Question 16

What are Cadherins in adherens junctions? What are their roles?

Answer 16

• adherens junction receptors
• ‘calcium dependent adherence’ molecules
• bind the same Cadherin on adjacent cells (e.g. homophilic binding); this is the basis of their role in tissue segregation during development (eg. neural N-cadherin expressing cells will bind to each other exclusively while ectodermal/epithelial E-cadherin-expressing cells will bind exclusively to each other as the neural tube separates from the overlying ectoderm during neurulation

Question 17

What are Catenins in adherens junctions? What are their functions?

Answer 17

• adherens junction linker proteins
  Functions:
• Structural: link Cadherins to Cytoskeltal elements
• signal transduction: when not in cell-cell junction can move from the cytoplasm to the nucleus to help regulate gene expression

Question 18

What are Actin in adherens junctions? What are their functions?

Answer 18

• cytoskeletal elements

Question 19

What are adherens junctions (zonula adherens)? What are its structure and functions?

Answer 19

• receptors bring membranes on adjacent cells very close together such that they form a barrier (which can be selective based on the identity of the receptors) that regulates the movement of macromolecules between cells (i.e. prevents material from ‘leaking’ freely between cells)
• as the tight junction belt/zonula is located at the apical end of the lateral cell urface it acts to delineate apical and basolateral membrane domains (i.e. prevents material that has been inserted into one membrane domain from ‘leaking’ into another membrane domain)

Question 20

What are desmosomes (Macula Densa)? What are its structure and functions?

Answer 20

• these are not belts that extend all the way around the cell; instead, they are ‘spots’ (i.e.. maculae) that are very strongly staining, and therefore appear ‘dense’ on electron micrographs
• can be located anywhere along entire lateral surface between two adjacent cells
• they are linked to stable intermediate filaments which increases their tensile strength (ie. they help hold tissues together)

Question 21

What are gap junctions (Connexons)?

Answer 21

• ‘communicating junctions’
• receptors cluster to form channels that link cells and allow for the passage of small molecules and ions between cells
• can be located along entire lateral surface between two adjacent cells

Question 22

What are hemidesmosomes? What are its structure and functions?

Answer 22

• resemble ‘half a desmosome’ by electron microscopy = ‘hemidesmosome’
• receptors are heterophilic in that they bind ECM glycoproteins rather than the same receptor protein on neighbouring cells
• cytoskeletal elements are intermediate filaments; therefore, these junctions are stable (ie. hold/anchor cells and tissues in position)
• anchor cells to ECM

Question 23

What are focal adhesions? What are its structure and functions?

Answer 23

• highly dynamic junctions which are critical for cell migration (ie. can repeatedly adhere, be pulled on by contractile elements, and released from the ECM to generate traction forces for migration)
• receptors are also heterophilic, bind to ECM
• the cytoskeletal elements are actin filament bundles/stress fibers that can bind non-muscle myosin motors that can generate contractile force
• the linkers are both structural and enzymatic (ie. kinases that can assemble and dissassemble the complex)

Question 24

How does a contractile cytoskeleton drive cell migration?

Answer 24

• new focal adhesions form anteriorly
• old focal adhesions disassemble posteriorly
• myosib based-contraction on actin stress fibers inside cell generates traction force that pulls cell forward anteriorly
• thus, in migrating cells there is an anterior/posterior polarity