Cell Migration and ECM

  • Cell Movement Overview

    • Importance of understanding cytoskeleton involvement during cell movements.

    • Interaction between moving cells and substrates is crucial for migration.

    • Discussion on cytoskeleton and the extracellular matrix (ECM) as substrates for migration.

Stages of Cell Migration

  • Cell migration can be broken down into several distinct stages:

  • Stage 1: Protrusion of the Leading Edge

    • The leading edge extends outward in the direction of movement.

    • Components:

      • Lamellipodia: Thin sheets of cytoplasm formed by actin microfilaments.

      • Filipodia: Thin, finger-like projections also composed of actin microfilaments.

  • Stage 2: Adhesion

    • The leading edge adheres to the substrate directly beneath it.

  • Stage 3: Translocation

    • The nucleus and cell body are moved forward.

    • Actin microfilaments and myosin proteins generate contractile forces to facilitate this movement.

Mechanisms of Protrusion

  • Leading Edge Extension:

    • Actin polymerization at the leading edge drives protrusions forward.

    • Filopodia Formation: Occurs first, followed by Lamellipodia formation.

    • Lamellipodia consist of a dense meshwork of actin microfilaments, while filopodia consist of tight parallel bundles of actin microfilaments.

  • Role of Profilin:

    • Enhances actin polymerization by promoting the exchange of ADP for ATP in G-actin (globular actin).

    • Crucial for the extension of filipodia at the leading edge of the cell.

Signaling Molecules in Cell Migration

  • Chemotaxis:

    • Directional movement of cells towards or away from chemical stimuli.

    • Chemoattractants: Molecules that attract cells towards higher concentrations.

    • Chemorepellants: Molecules that repel cells, leading them away from higher concentrations.

  • Role of Hemottractin:

    • A signaling molecule detected outside the cell that triggers a signaling cascade.

    • Binding of hemottractin to G-protein coupled receptors initiates hydrolysis of PIP2 (phosphatidylinositol 4,5-bisphosphate) and releases profilin.

Focal Adhesions and Integrins

  • Focal Adhesions:

    • Structures that form complexes of molecules, allowing cells to adhere to ECM components such as collagen and laminin.

    • Integrins (transmembrane proteins) bind extracellular matrix and, together with adapter proteins, link to actin microfilaments.

  • Key Adapter Proteins:

    • Talin and Vinculin: Facilitate the binding of integrins to actin.

    • Focal adhesions must cluster together where lipid domains in the plasma membrane allow accumulation of integrins.

Actin Dynamics and Migration

  • Actin Polymerization:

    • Essential for pushing the plasma membrane forward during migration.

    • Adapter proteins must ensure integrins are bound to growing actin for proper movement.

  • Effects of Detachment:

    • Without integrin-actin attachment, actin filaments grow but cannot drive membrane protrusions, leading to retrograde flow (backward movement).

Structural Proteins in Migration

  • Cofilin:

    • Transiently binds ADP-actin, leading to dynamic remodeling of actin structures.

    • Enhances turnover of actin filaments to maintain cellular dynamics.

  • Collagen Structure:

    • Composed of three protein chains, typically containing a repeating sequence of glycine and two additional amino acids (usually proline and hydroxyproline).

    • Vitamin C is essential for hydroxylation of proline, stabilizing collagen structure.

    • Deficiency leads to diseases (e.g., scurvy) characterized by bruising and tissue breakdown.

Cellular Regulation and Interaction

  • Role of Rho GTPase:

    • Enhances actin bundling and strengthens cellular protrusions in response to chemoattractants.

    • Activates formin proteins that promote actin polymerization.

    • Antagonism Between Rho and Rac Proteins:

      • Rac promotes branching at the leading edge, while Rho promotes bundling at the trailing edge, maintaining cellular polarity during migration.

  1. Structural differences between Filipodia and Lamellipodia:

    • Filopodia: Thin, finger-like projections composed of tight parallel bundles of actin microfilaments.

    • Lamellipodia: Thin sheets of cytoplasm formed by a dense meshwork of actin microfilaments.

  2. Extracellular cues and actin reorganization:

    • Extracellular cues such as chemoattractants can signal the cell to increase actin polymerization at the leading edge, leading to protrusions like filopodia and lamellipodia.

  3. Regulation of actin meshwork assembly through Rac:

    • GTPase Rac promotes branching of actin filaments at the leading edge in response to extracellular signals, which facilitates lamellipodia formation and pushing of the membrane forward.

  4. Focal adhesion:

    • Focal adhesions are structures that form complexes of molecules that allow cells to adhere to extracellular matrix (ECM) components like collagen and laminin.

  5. Extracellular matrix (ECM):

    • The ECM is a network of proteins and carbohydrates outside of cells that provides structural and biochemical support to surrounding cells.

  6. Integrin receptor clustering and consequences:

    • Integrin receptors cluster due to engagement with ECM and adapter proteins such as talin and vinculin. This clustering activates signaling pathways that promote cell adhesion and movement.

  7. Integrins attaching the cytoskeleton to ECM:

    • Integrins bind to ECM components and connect to the actin cytoskeleton via adapter proteins like talin and vinculin, which link integrins to the actin microfilaments. Alpha-actinin helps maintain the structure and organization of actin filaments.

  8. Basic features of collagen and scurvy relation:

    • Collagen is composed of three protein chains with a repeating sequence of glycine and two other amino acids. Vitamin C is essential for hydroxylation of proline in collagen synthesis. A deficiency leads to scurvy, which is characterized by bruising and tissue breakdown.

  9. Collagen synthesis and assembly:

    • Collagen is synthesized in fibroblasts, where it is assembled into triple helices before being secreted into the extracellular space, where it forms a network that provides structural support.

  10. Stress fibers pulling the trailing end forward:

    • Stress fibers contract through myosin motor proteins, pulling the trailing edge of the cell forward by generating tension and promoting cell retraction.

  11. Myosin II and actin contraction differences:

    • In muscle cells, Myosin II works in a highly organized manner to produce powerful contractions across the entire muscle. In migrating cells, Myosin II interacts with actin to produce localized contractions that facilitate movement and shape changes rather than bulk force.

  12. Rho GTPase and trailing edge retraction:

    • Rho GTPase enhances actin bundling and promotes stress fiber formation, which contributes to the contraction and retraction of the trailing edge of migrating cells.

  13. Importance of Rac and Rho antagonism in neutrophils:

    • The antagonism between Rac and Rho is crucial for maintaining cellular polarity during migration in neutrophils, with Rac promoting forward movement and branching while Rho fosters contraction at the trailing edge, ensuring efficient migration and response to signals.