unit 3 week 2 pt 3 Flashcards
What cytoskeletal element is primarily responsible for cellular motility?
Actin filaments, often working with myosin motors and actin-binding proteins.
What are some dynamic activities in non-muscle cells that rely on actin filaments?
Cytokinesis, phagocytosis, cytoplasmic streaming, vesicle trafficking, blood platelet activation, lateral movement of membrane proteins, cell-substratum interactions, cell locomotion, axonal outgrowth, and changes in cell shape.
Why is cell locomotion important in higher vertebrates?
It is required for tissue and organ development, blood vessel formation, axon development, wound healing, immune responses, and the spread of cancer.
What are the limitations of traditional cell locomotion research?
Most studies use cells moving over a flat 2D substrate, which may not fully represent how cells move in the body, where they traverse more complex 3D extracellular matrices.
How is cell locomotion similar to walking?
Like walking, cell movement involves repetitive steps: extending forward, forming temporary adhesion, generating traction, and retracting the trailing edge.
What are the four main steps of cell locomotion?
- Protrusion – A part of the cell surface extends in the direction of movement.
- Adhesion – The lower surface of the protrusion attaches to the substrate, forming temporary anchor points.
- Translocation – The bulk of the cell moves forward over these adhesion sites.
- Retraction – The cell releases its rear contacts, pulling the trailing edge forward.
What happens when a small piece of living tissue is placed in a culture dish?
Individual cells, typically fibroblasts, migrate out of the tissue onto the dish surface.
What are fibroblasts, and where are they commonly found?
Fibroblasts are the predominant cells in connective tissue.
How do fibroblasts move?
They flatten against the substrate, becoming fan-shaped with a broad front and a narrow tail. Their movement is erratic, sometimes advancing and sometimes retracting.
What is a lamellipodium, and what role does it play in fibroblast movement?
A lamellipodium is a broad, flattened, veil-like protrusion at the leading edge of the fibroblast. It provides temporary anchor points for cell movement.
What is unique about the structure of a lamellipodium?
It lacks cytoplasmic vesicles and other particulate structures, and its outer edge often exhibits an undulating, ruffled motion.
How does actin polymerization contribute to cell motility?
Actin monomers polymerize to generate force, propelling structures like lamellipodia forward, even without molecular motors.
What bacterial protein demonstrates actin-based motility, and how?
Listeria bacterium uses the ActA protein to activate actin polymerization, propelling it through the cytoplasm.
What human protein family functions similarly to ActA in mammalian cells?
The WASP/WAVE family activates the Arp2/3 complex, promoting actin polymerization at the leading edge of moving cells.
How do white blood cells move toward infection sites?
- They receive a chemical signal from the direction of infection.
- This triggers localized actin polymerization, polarizing the cell.
- The cell moves toward the stimulus, using actin-driven protrusions.
What disease results from a mutation in the WASP gene, and how does it affect immunity?
Wiskott–Aldrich syndrome – patients lack functional WASP protein, preventing their white blood cells from responding to chemotactic signals, leading to immune system deficiencies.
What initiates lamellipodium formation?
A stimulus at one end of the cell activates WASP proteins, which in turn activate the Arp2/3 complex (Step 1 & 2).
How does the Arp2/3 complex contribute to actin branching?
- Arp2/3 binds to an existing actin filament and mimics the barbed end (Step 3).
- ATP-actin monomers bind to the Arp2/3 template, forming a branched filament.
- Profilin promotes polymerization at barbed ends, extending filaments forward (Step 4 & 5).
- New Arp2/3 complexes attach to growing filaments, creating additional branches (Step 5).
- Capping proteins prevent further elongation of older filaments (Step 5).
- Cofilin binds to actin-ADP subunits, promoting disassembly at pointed ends (Step 6).
How is actin recycled?
Actin-ADP subunits released from disassembling filaments are converted back into ATP-actin by profilin, allowing continuous filament growth at the leading edge.
What does electron microscopy reveal about lamellipodia?
A branched, cross-linked actin network forms beneath the plasma membrane, with Arp2/3 complexes residing at Y-shaped junctions where new filaments branch from preexisting ones.
How does the actin network undergo treadmilling?
- Actin polymerization and branching occur at the front of the lamellipodium.
- Assembled filaments flow rearward and depolymerize at the back.
- Actin subunits are continuously recycled, allowing sustained protrusion.
How does the cell body follow the leading edge?
After lamellipodium protrusion, the bulk of the cell moves forward using traction forces at adhesion sites.
What are traction forces, and how are they studied?
They are generated at adhesion sites, where the cell grips the substrate. When cells migrate on an elastic material, their movements cause substrate deformation, which can be analyzed to calculate traction force distribution.
Where are the strongest traction forces in a migrating fibroblast?
Just behind the leading edge, where the cell adheres strongly to the substratum.
