Histology Intro and Cytoskeleton

Question 1

Epithelial v Mesenchyal Cells

Answer 1

Epithelial Cells-

Polarized cells
Cells adhere to one another
Cells are stationary

Mesenchymal Cells-

Non-polarized cells
Cells lose adhesion property
Cells can migrate

Question 2

Purpose of Cytoskeleton (6)

Answer 2

• Controls cell shape
• Defines cell organization
• Guides transport of proteins and organelles within cell
• Moves chromosomes and splits during cell division
• Drives cell movement and attachment
• Generate force w/ motor proteins

Question 3

3 Main Classes of Cytoskeleton Proteins

Answer 3

• 1- actin
  
  • forms F-actin microfilaments; smallest width filaments
• 2- tubulin
  
  • forms microtubules; largest width
• 3- intermediate filament proteins
  
  • intermediate width

Question 4

Actin Polymerization

Answer 4

• Monomers (G-actin) assemble to form actin polymers (F-actin)
  
  • Polymerization dep on ATP (actin monomers bind ATP then added to polymer and hydrolyzed afterwards)
  • Profilin - catalyzes ADP –> ATP to “recharge” actin monomers
• Polymerization requires making an actin nucleus first (need 3 actin molecules to form nucleus)

Actin filament growth is regulated by capping proteins that bind positive end to stop polymerization

Question 5

Profilin

Answer 5

catalyzes ADP –> ATP to “recharge” actin monomers

Question 6

Treadmilling

Answer 6

• Polymer filaments are polarized; plus/barbed end and minus/pointed end
  
  • More growth on positive end b/c lower critical concentration than minus end
  • If concentration of G-actin is between the critical concentration of the plus and minus ends, G-actin is lost from minus end and added to positive end (treadmilling)

Question 7

ARP2/3 v Formins

Answer 7

• Arp 2/3 complex nucleates branches (contains 2 actin-like molecules so just need 1 more actin)
• Formins nucleate single filaments de novo

Question 8

Cofilin

Answer 8

SEVERS- creates twist in filament that promotes breaking

Question 9

Cross-binding proteins (+ 2 examples)

Answer 9

• Link 2+ actin filaments together in different ways to serve different cellular functions
  
  • Fimbrin - bundles actin to form microvilli
  • Alpha-actin - bundles actin to form stress fibers and muscle Z line

Question 10

Sarcomere (6 components)

Answer 10

• Myosin - 2 heavy chains and 4 lights chains - ea heavy chain has head N terminal and C terminal tail; forms M line; thick filament
• Actin filaments (f-actin) - linked at Z line by alpha-actin and CapZ; think filament
• Tropomyosin - tightly wound around actin to add rigidity to actin polymer (coats the actin filament)
• Titin- molecular spring that attaches Z line to M line
• Nebulin- controls F-actin length
• Accessory intermediate filaments and other proteins

Question 11

4 Steps of Muscle Contraction

Answer 11

• 1- myosin head binds actin forming cross-bridges
• 2- cross-bridges rotate toward center of sarcomere –> power stroke
• 3- myosin binds ATP so cross-bridges detach
• 4- ATP hydrolysis –> myosin re-oriented and re-energized

Question 12

Microtubule Polymerization

Answer 12

• Alpha and beta subunits form heterodimer which then polymerize –> microtubules
• Similar to actin, polarized w/ plus and minus ends
• Plus end polymerizes 2X faster than minus BUT also depolymerizes at 2X the rate of minus end
• Energy source is GTP (not ATP like actin) - need GTP-bound form to polymerize and hydrolysis to GDP-bound form to de-polymerize so if polymerization rate»> hydrolysis rate –> “GTP cap”
• Initiated by MTOC - no minus end growth because minus end anchored to MTOC

Question 13

Dynamic Instability

Answer 13

• Dynamic Instability - alteration b/n growth and rapid disassembly
• Unique to tubulin and important for its function in cell division and motility
• GTP cap favors growth but when lost (rate of hydrolysis&raquo_space;> rate of polymerization) then rapid de-polymerization occurs

Question 14

Microtubules in Mitosis

Answer 14

• Microtubules form the mitotic spindles growing out from centrioles
• Centrioles move to either pole of the cell and send out 3 types of fibers
• Process:
  
  • Dynamic instability at plus end of microtubule forces the chromosome down the microtubule
  • Microtubules polymerize and de-polymerize randomly until they “catch” a condensed chromosome

Question 15

Principal Motor Proteins (2)

Answer 15

Motion of flagella depends on molecular motors

1-Dynein-
Sliding
Minus end activity
Heavier, force-producing chains and bigger than kinesin
5x faster than kinesin
Drive golgi, rough ER and others close to nucleus

2- Kinesin- 
Plus end activity
Can take over 100 steps
Slower
Drive vesicles (further from nucleus)

Question 16

Flagellar Motion

Answer 16

• Motion of flagella depends on molecular motors
  
  • Dynein - controls sliding; heads bind to plus end but dynein moves toward minus end
    
    • Sliding is ATP-dependent
    • Process:
      
      • ATP bound: stalk detached from MT
      • ATP hydrolysis: promotes stalk-MT attachment
      • ADP+Pi release: leads to large conformational “power stroke” involving rotation of head and stalk relative to tail

Question 17

Role of Intermediate Filaments (5)

Answer 17

• Hold things together
• Connect desmosomes
• Stabilize skin
• Form structures such as hair
• Fundamental to maintaining high stress structures (ex- muscles sarcomeres)

Question 18

4 Major Types of Intermediate Filaments

Answer 18

• I- Keratins (skin, hair, nails)
• II- Vimentin (mesenchyal cells), desmin (muscle), glial fibrillary protein (in glial cells)
• III- Neurofilament proteins (in neurons)
• IV- Nuclear lamins (nuclear lamina of all cells)