Lecture 16

• Cell motility


  The ability of cells to move
• Chemotaxis

  The movement of a cell or organism in response to a chemical stimulus
• Importance of cell motility and chemotaxis

  • Development: Directed movements during gastrulation, neural crest migration, and migration of primordial germ cells
  • Wound healing: Movement of fibroblasts to the wound site for repair
  • Infection response: Movement of immune cells to phagocytose pathogens
  • Neuronal pathfinding: Guidance of neurons to their correct positions in the nervous system
• Actin-based cell motility


  1. Lamellipodia formation

  2. Focal adhesions

  3. Translocation

  4. Retraction
• Microtubule-based cell motility

  • Provides structural integrity and directs long-range transport within cells, aiding in cell division and intracellular transport of materials
• Chemotaxis

  • Dictyostelium discoideum aggregates in response to cyclic AMP (cAMP)
  • Neutrophils chase and phagocytose bacteria, guided by chemotactic signals like the tri-peptide fMet-Leu-Phe (fMLP)
• Rho superfamily of GTPases

  • Cdc42 typically promotes filopodia formation
  • Rac encourages the formation of lamellipodia
  • Rho generally influences the formation of stress fibers
• Model systems for studying cell motility

  • Zebrafish larvae
  • Keratocytes from fish scales
• Use of Cytochalasin D disrupts actin polymerization and is used to study its impact on cell motility
• Microinjection of GTPases demonstrates the role of different Rho GTPases in actin dynamics
• GTPase regulation in chemotaxis

  • Spatial regulation: Cdc42, Rac, and Rho GTPases are spatially regulated within cells to facilitate directional movement
  • Cdc42 activation at the leading edge promotes the assembly of actin via formin and the Arp2/3 complex, leading to filopodia and lamellipodia formation
  • Rac GTPase stimulates branched actin network assembly behind the leading edge, supporting further extension
  • Rho GTPase activated at the trailing edge, promoting stress fiber formation and myosin II activation for forward movement
• In vitro wound healing assay

  1. Experimental setup: A monolayer of fibroblasts is mechanically disturbed to simulate wounding

  2. Cellular response: Cells migrate into the cleared space, a process impaired by the expression of dominant negative versions of Rac, Cdc42, or Rho GTPases, highlighting their essential roles in directed cell migration
• Cell adhesion and migration

  • Focal adhesions: Dynamic sites where integrins cluster and connect actomyosin fibers inside the cell to fibronectin in the extracellular matrix
  • Role of microtubules: Essential for the endocytic recycling of integrins and other adhesion components, facilitating the formation of new adhesion sites at the front of migrating cells
• Mesenchymal migration

  Associated with metastatic cancer cells, which lose adherence to neighboring cells, gain migratory capabilities, invade other tissues, and spread through blood vessels
• Nerve growth and neuroplasticity

  • Growth cones migrate towards each other to form new synaptic connections using processes observed in lymphocytes and fibroblasts
  • Neuroplasticity is essential for learning new patterns of thought and adapting perceptions, influenced by new experiences
• Pathogen exploitation of actin machinery

  • Listeria and Vaccinia virus hijack the actin polymerization machinery to move within host cells
• In vitro studies with ActA

  1. ActA function: Mimics WASp, initiating actin filament formation and movement in an in vitro system using Xenopus egg extract and GFP-actin

  2. Role of cofilin and CapZ: Critical for actin filament disassembly and formation, driving continuous movement through ATPase cycles
• Cilia and flagella

  Motile structures in many eukaryotic cells, from protozoa to human cells
• Cilia and flagella structure

  • Core structure: Composed of microtubules and the motor protein dynein, typically arranged in a "9+2" pattern
  • Basal body: The base of cilia and flagella, structurally similar to a centriole, anchors the axoneme to the cytoplasm of the cell
  • Length and function: Flagella are generally longer than cilia and are primarily involved in propelling the cell itself, while cilia can either move fluid and particles over the cell surface in a coordinated wave or propel smaller cells
• Dynein function in motility

  • Dynein structure: Each dynein molecule has a heavy chain with a motor domain that attaches to a microtubule via a stalk, and a tail that binds to an adjacent microtubule, forming cross-bridges
  • Sliding mechanism: The power stroke of dynein causes one microtubule to slide relative to its neighbor, facilitated by the ATP-driven conformational changes in the dynein motor domains
  • Coordination for movement: The action of dynein is highly coordinated across the axoneme to achieve the bending motions necessary for the beating of cilia and flagella
• Nexin and radial spokes

  • Role of nexin: Cross-links between the microtubule doublets, which helps to regulate the sliding movements by restricting the extent of microtubule displacement
  • Radial spokes: Extend from the microtubule doublets to the central pair, playing a critical role in the regulation of dynein activity and ensuring the structural integrity of the axoneme
  • Effect of proteolysis: Mild proteolysis, which removes nexin, transforms the bending movement into a sliding motion between microtubules when ATP is added, highlighting the mechanical interplay necessary for ciliary and flagellar motion
• Implications for cell motility and chemotaxis

  • Force generation: The movement generated by cilia and flagella is a primary means of locomotion for cells in fluid environments, crucial for chemotaxis, where cells navigate chemical gradients
  • Resistance to force: The structural components and their arrangement within cilia and flagella not only produce force but also resist mechanical stresses, ensuring effective and sustained movement
• Examples of motile cells

  • Human sperm: Utilizes a flagellum for propulsion towards the egg during fertilization
  • Mussel gill cells: Use cilia to facilitate water movement and filter feeding
  • Paramecium: A protist that uses cilia for moving through water
  • Chlamydomonas: A green algae that swims with two flagella, demonstrating a simple yet effective locomotion strategy used in many aquatic organisms