Cell Junctions, Cell Adhesion and Extracellular Matrix

Question 1

What is the extracellular matrix made of and what is its function?

Answer 1

• collagen fibers (different subtypes further distinguished into type I, II…, ) in tendons and cartilage
• glycoproteins (laminin, fibronectin)
• proteoglycans (oligosacharides)
• hyaluronan
• basal lamina for endo- and epithelial cells
• connective tissue
• fibers composition in tendons…
• signals for growth, movement, expansion…

Question 2

Name all possible junctional and non-junctional cell-cell and cell-matrix adhesions and their major proteins (if applicable)

Answer 2

Cell-cell:

• > Junctional
• tight junctions (claudins)
• gap junctions (connexins)
• adhesion belt (cadherin)
• desmosomes (cadherins)
• > Non-junctional
• Ig-like CAMs
• integrins
• selectins

Cell-matrix:

• > Junctional
• focal adhesions (integrins)
• hemidesmosomes (integrings)
• > Non-junctional
• integral membrane proteoglycan
• integrins

Question 3

Characterize adhesions based on their function.

Answer 3

• > occluding: tight junction
• > communication: synapses, gap junctions, plasmodesmata
• > anchoring:
• actin filaments: adherens junctions (cell-cell) and focal adhesions (cell-matrix)
• intermediate fillaments: desmosomes (cell-cell) and hemidesmosomes (cell-matrix)

Question 4

Name examples for homotoypic and heterotypic proteins interacting in cell-cell or cell-matrix adhesions (e.g some homodimers and heterodimers).

Answer 4

• homotypic: cadherin (E-cadherin) and Ig-like CAMs (ICAM, VCAM and NCAM)
• heterotypic: integrins (alpha and beta) and selectings (P-selectins)

Question 5

Define adherens junctions.

Answer 5

• junctional cell-cell adhesion
• flexible, contractile, also called adherent belt
• cadherin homodimers
• extracellular domain = with other extracellular domain on the oposite cadherin
• intracellular domain + anchoring protein = connection to actin filaments

-anchoring p.: alpha-, beta-, gama-catenin, alpha-actinin, vinculin

Question 6

Define focal adhesions.

Answer 6

• junctional cell-matrix adhesion
• stability
• integrins (alpha+beta)
• extracellular domain = ECM connected
• intracellular domain (beta subunit) + anchoring proteins = connection to actin filaments

-anchoring p.: vinculin and either talin, filamin or alpha-actinin

Question 7

Define desmosomes.

Answer 7

• junctional cell-cell adhesions
• stability
• cadherins (desmoglein and desmocolins)
• extracellular domain = with other extracellular domain on the oposite cadherin
• intracellular domain + anchoring protein plaque = connection to intermediate fillaments like keratin or desmin

-anchoring p.: gama-catenin (plakoglobin) and desmoplakin

Question 8

Define gap junctions.

Answer 8

• junctional cell-cell adhesion
• aquaeous pore for comunication, small molecules and proteins
• hetero- or homohexamers
• 6 connexins = connexon, 1 connexon in every plasma membrane layer

Question 9

Briefly comment on the function and composition of cytoskeleton.

Answer 9

• > function: cell shape, organelles shape, movement of cells (wound healing, inflammation, embriogenesis), transport of cargo, cell division
• > composition:
• microfilaments (actin, ATP/ADP, motor protein myosin, two strand alpha-helix)
• microtubuli (tubulin, GTP/GDP, motor protein kinesin and dyenin, hollow tube with protofilaments)
• intermediate filaments (keratin, desmin…, no cleft for nucleotide binding, no motor protein, coiled coil)

Question 10

What is actin? What is its function? How is it polymerized? Elaborately describe. Describe its role in muscle cells.

Answer 10

• monomer of microfilaments
• encoded by alpha-, beta- and gama-actin gene
• 2 alpha-helical strands attached to each other
• cell movement, migration, wound healing, inflammation, lamillopodia movement

+> (barbed) (pointed)>-

• polymerizes on + end, + end elongates
• Arp2/3 complex (actin-related protein), - end embedded in the Arp complex, + end elongates, nest for better actin strand growth, Arp cannot nucleate and grow itself because it is only similar to actin (that is why it can nest the - end of actin because its + end is like actin)
• similar to Arp2/3 = WASP
• stabilization with cross-linking proteins (fimbrin monomer, alpha-actinin dimer, filamin dimer or spectrin tetramer, all have 2 actin binding sites), most stable configuration is almost perpendicular to already pre-existing actin strand, also most ideal for further Arp2/3 propagation
• when actin has to be depolymerized - ADF (actin depolymerizing factor, e.g. cofilin)
• actin pool
  
  • actin monomer can participate in polymerization = actin + profilin
  • actin monomer cannot participate in polymerizatio = actin + thymosin

Sarcomere: 2 adjacent myofibrils

Z-disc with Z-cap that has actin fibers attached to it (light area). Actin fibers overlap witn myosin II fibers (middle section, dark area), actin fibers with Z-disc repeat. Actin has on the myosin II oriented side tropomodulin cap.

Sarcoplasmic reticulum releases Ca2+ whic binds troponin and together release the tropomiosyn from actin and allows myosin to bind.
1) -<==========<+ actin
myosin with myosin head really attached to it, rigor state
2) ATP binds and myosin head is slightly released from actin
3) Myosin head moves back and ATP is hydrolysed to ADP and Pi which both stay bound to the myosin head
4) Pi is released and power stroke follows
5) ADP is released from the myosin head which returns to a rigorous state

Question 11

What is tubulin? What is its function? How is it polymerized? Elaborately describe

Answer 11

• heterodimer of alpha- and beta-tubulin
• cell division, transport of organelles and vesicles
• both subunits bind GTP/GDP but only beta subunit can hydrolyse the GTP -> dynamic, defines the stability and polymerization of the protofilament
• 13 filaments form a stiff hollow tube
• longitudal interactions: alpha with beta with alpha…
• lateral interaction: alpha with alpha and beta with beta
• beta (+) fast growing end and alpha (-) slow growing end
• the strand is nucleated on the - end and grows outwards on the + end by gama-tubulin ring complex
• growing from specialized microtubule-organizing center (MTOC) usually located in the centrosome
• GTP-bound= linear, stable, located closer to the cap
• GDP-bound= less stable, may collapse if cap is lost, located away from the cap

Catastrophe: when tubulin levels are within the critical values, cap may be lost and the tube starts to shrink and tumble
Rescue: when tubulin levels are within the critical values, cap is regained and the strand is once again elongating

Kinesins: travel to a + end
Light chain and heavy chain. Globular head binds to microtubules and ATP to facilitate the movement and tail binds cargo. Body is alpha-helical coiled coil
Dynein: travel to a - end 
Works in a similar fashion as kinesin\
(toto neviem to musim kuknut ptm)

Question 12

What are intermediate filaments? Name examples.

Answer 12

• coiled coil
• hetero- or homodimer
• plectins link intermediate filaments to F-actin and microtubules
• coiled coil paraller dimers form antiparallel tetramers with alpha-helical shape

Examples:

• keratins (epithel, hair, nails)
• neurofilaments (axons)
• vimentin-like filaments (smooth, striated and cardiac muscle cells)
• desmins (muscles)
• lamins