Cell Biology Study Guide: Cytoskeleton, Actin, and Cell Migration

Q14. Discuss the structure and function of microfilaments. What motor protein is associated with microfilaments? Compare and contrast the various cellular structures formed by actin assembly and discuss their respective roles in epithelial and mesenchymal cells. What factors regulate actin polymerization and branching? Discuss the normal functions of WASP. Why does WASP loss of function result in Wiskott-Aldrich Syndrome? Discuss the initial and subsequent infection of host cells by Listeria. What role does cell signaling and the Rho family play in actin-associated processes and how are they regulated?

Background

Topic: Cytoskeleton – Microfilaments (Actin Filaments), Actin Dynamics, Cell Signaling, and Pathogen Interaction

This question tests your understanding of the structure and function of actin microfilaments, their associated proteins, regulation of actin dynamics, and the role of actin in cell structure, signaling, and infection processes.

Key Terms and Concepts

• Microfilaments (Actin Filaments): Thin, flexible protein filaments composed of actin monomers.

• Motor Protein: Myosin is the primary motor protein associated with actin filaments.

• Actin Structures: Stress fibers, lamellipodia, filopodia, cortical actin, etc.

• WASP: Wiskott-Aldrich Syndrome Protein, a regulator of actin nucleation and branching.

• Rho Family GTPases: Small GTP-binding proteins (e.g., Rho, Rac, Cdc42) that regulate actin dynamics.

Key Processes and Pathways

• Actin polymerization and branching: regulated by proteins such as Arp2/3 complex, WASP, and cofilin.

• Cell signaling pathways: Rho GTPases control actin organization and cell movement.

• Pathogen interaction: Listeria uses host actin for intracellular movement.

Step-by-Step Guidance

1. Describe the structure of microfilaments: Microfilaments are composed of actin monomers (G-actin) that polymerize to form a helical filament (F-actin). They are approximately 7 nm in diameter and are highly dynamic, allowing for rapid assembly and disassembly.

2. Identify the motor protein associated with microfilaments: The main motor protein interacting with actin filaments is myosin. Myosin uses ATP hydrolysis to generate force and movement along actin filaments.

3. Compare and contrast actin-based structures in epithelial vs. mesenchymal cells: - In epithelial cells, actin forms structures like the cortical actin network and adherens junctions, providing cell shape and stability. - In mesenchymal cells, actin forms stress fibers, lamellipodia, and filopodia, which are important for cell migration and shape changes. Consider how these structures contribute to cell adhesion, migration, and tissue organization.

4. Discuss regulation of actin polymerization and branching: Actin polymerization is regulated by nucleation-promoting factors (e.g., WASP), the Arp2/3 complex (for branching), and actin-binding proteins (e.g., profilin, cofilin). $\text{Actin polymerization: G-actin} \rightarrow F\text{-actin}$ $\text{Branching: Arp2/3 complex + WASP} \rightarrow \text{branched actin networks}$

5. Explain the normal function of WASP and consequences of its loss: WASP activates the Arp2/3 complex, promoting actin nucleation and branching. Loss of WASP impairs actin remodeling, leading to immune deficiencies as seen in Wiskott-Aldrich Syndrome.

6. Outline Listeria infection and actin-based motility: Listeria enters host cells, escapes the phagosome, and hijacks the host actin machinery to move within and between cells by inducing actin polymerization at one pole of the bacterium.

7. Discuss the role of Rho family GTPases in actin regulation: Rho, Rac, and Cdc42 regulate different aspects of actin dynamics (e.g., stress fibers, lamellipodia, filopodia). Their activity is controlled by GEFs (guanine nucleotide exchange factors) and GAPs (GTPase-activating proteins).

Try answering each part before checking the full explanation!

Final Answer:

Microfilaments are composed of actin and interact with myosin. In epithelial cells, actin forms cortical networks and adherens junctions; in mesenchymal cells, it forms structures for migration. Actin polymerization and branching are regulated by proteins like WASP and Arp2/3. WASP loss impairs immune cell function, causing Wiskott-Aldrich Syndrome. Listeria exploits actin for motility. Rho family GTPases regulate actin dynamics via signaling pathways.

Q15. Discuss the steps of cell migration/crawling and indicate the proteins involved in each of the steps. Discuss the factors that influence the direction of cell migration. Why is cell migration necessary and why should it be a controlled process?

Background

Topic: Cell Migration, Cytoskeleton Dynamics, and Regulation

This question tests your understanding of the molecular mechanisms of cell migration, the proteins involved, and the importance of regulated cell movement in physiology and disease.

Key Terms and Concepts

• Cell Migration Steps: Protrusion, adhesion, translocation, and retraction.

• Key Proteins: Actin, myosin, integrins, focal adhesion proteins, Arp2/3, cofilin, Rho GTPases.

• Chemotaxis: Directed cell movement in response to chemical gradients.

Key Processes and Pathways

• Actin polymerization drives membrane protrusion (lamellipodia, filopodia).

• Integrins and focal adhesions anchor the cell to the extracellular matrix.

• Myosin II contracts the rear of the cell for forward movement.

• Directional cues include chemokines, ECM composition, and mechanical signals.

Step-by-Step Guidance

1. List and describe the main steps of cell migration: 1. Protrusion of the leading edge (lamellipodia/filopodia formation) 2. Formation of new adhesions at the front 3. Translocation of the cell body 4. Retraction of the rear

2. Identify the proteins involved in each step: - Protrusion: Actin, Arp2/3 complex, cofilin, profilin - Adhesion: Integrins, focal adhesion kinase (FAK), talin, vinculin - Translocation: Myosin II, actin - Retraction: Myosin II, actin, focal adhesion disassembly proteins

3. Discuss factors influencing migration direction: Direction is influenced by chemotactic gradients, substrate stiffness, ECM composition, and cell polarity signals. Rho family GTPases (Rac, Rho, Cdc42) help interpret these cues.

4. Explain why cell migration is necessary and must be controlled: Cell migration is essential for development, wound healing, and immune responses. Uncontrolled migration can lead to pathological conditions such as cancer metastasis or chronic inflammation.

Try outlining the steps and proteins before checking the full explanation!

Final Answer:

Cell migration involves protrusion (actin, Arp2/3), adhesion (integrins, focal adhesion proteins), translocation (myosin II), and retraction. Direction is guided by chemotactic signals and ECM cues. Migration is vital for normal physiology but must be tightly regulated to prevent disease.