Cell Adhesion & Motility – Comprehensive Notes

Page 1 – Introduction

• Topic Focus: Cellular organelles & function with emphasis on Cell Adhesion and Motility.
• Instructor: Alex R. B. Thomsen (BSc, MSc, PhD).
• Relevance: Sets the stage for understanding how cells interact with each other and with the extracellular matrix (ECM) to maintain tissue integrity and enable movement.

Page 2 – Biological Significance of Cell Adhesion

• Anchoring Within Tissues
  • Maintains overall tissue/organ integrity.
  • Loss of adhesion can lead to pathologies such as metastasis, blistering diseases, and developmental defects.
• Barrier Formation (Epithelia)
  • Adhesion of epithelial cells creates physical boundaries between physiological compartments and between the organism and its external environment (skin, gut lining, etc.).
  • Tight adhesions regulate selective permeability.
• Requirement for Motility
  • Paradoxically, cells must stick in order to move. Cycles of attachment and detachment generate traction required for crawling, invasion, wound repair, and embryonic morphogenesis.

Page 3 – Cell Adhesion Molecules (CAMs)

• Families:
  1. Cadherins – Ca^{2+}-dependent, multiple extracellular “cadherin repeats.”
  2. Immunoglobulin (Ig) Superfamily – Ig‐like domains, typically Ca^{2+}‐independent.
  3. Selectins – Contain an N-terminal lectin-like domain that binds carbohydrates; require divalent cations for function.
  4. Integrins – Heterodimeric receptors (α + β) linking ECM to cytoskeleton, participate in bidirectional signalling.
• Domain Anatomy (generic):
  • Extracellular domain (ligand binding).
  • Single‐pass transmembrane helix.
  • Cytoplasmic tail that recruits adaptor proteins and the cytoskeleton.
• Miscellaneous Numbers Displayed:
  • “525,” “1288 12088,” etc. appeared on slide—likely domain lengths or catalogue numbers; keep in mind but biologically unelaborated.

Page 4 – Homophilic vs. Heterophilic Binding

• Homophilic Interaction
  • Same molecule on opposing cells engages in adhesion (e.g., E-cadherin ↔ E-cadherin).
  • Critical for tissue-specific sorting during embryogenesis (Steinberg’s differential adhesion hypothesis).
• Heterophilic Interaction
  • Different molecules on the two cell surfaces interact, e.g., Selectin–Carbohydrate or Integrin–ICAM.
  • Enables cell–cell recognition in immune surveillance and inflammation.

Page 5 – Integrin Basics

• Structure: Heterodimer = \alpha-chain + \beta-chain.
  • Mammals: 18 distinct \alpha, 8 distinct \beta → \ge 24 combinatorial receptors.
• Specificity:
  • Tissue-restricted expression ensures precise ligand recognition (e.g., \alpha5\beta1 binds fibronectin in fibroblasts; \alpha{IIb}\beta3 binds fibrinogen in platelets).
• Functions:
  • Adhesion to ECM and to counter-receptors on other cells.
  • Outside-in signalling: Ligand binding changes cytoplasmic tail conformation → kinase cascades (e.g., Src, FAK).
  • Inside-out signalling: Intracellular signals (Rap1, talin binding) increase ligand affinity/avidity.
  • Regulate motility, proliferation, differentiation, apoptosis (anoikis).
• Cytoskeletal Link: Bind to actin via talin, vinculin, paxillin; bind to intermediate filaments in hemidesmosomes (via BP230, plectin).

Page 6 – Vertebrate Integrin Repertoire (Selected Examples)

• \beta1 Family:
  • \alpha1\beta1 – Collagen & laminin. • \alpha3\beta1 – Fibronectin & laminin. • \alpha5\beta1 – Major fibronectin receptor. • \alphav\beta_1 – Fibronectin, vitronectin.
• \beta2 Family (Leukocyte-specific):
  • \alphaL\beta2 (LFA-1) – ICAM-1/2, crucial for immune synapse. • \alphaM\beta_2 (Mac-1) – C3b, fibrinogen; roles in phagocytosis.
• \beta3 Family:
  • \alpha{IIb}\beta3 – Platelet fibrinogen receptor (aggregation). • \alphav\beta_3 – Binds multiple RGD-containing ligands (osteopontin, vitronectin, collagen), prominent in angiogenesis.

Page 7 – Integrin–ECM Linkage Schematic

• Panel A: Triple‐helical collagen fibril (N-terminus outside) engages integrin via a collagen‐binding domain on fibronectin; fibronectin presents an RGD motif to the integrin.
• Panel B/C: Inside the cell, integrin cytoplasmic tails assemble adaptor proteins (talin, vinculin, paxillin) → connect to actin stress fibers; spacing ~50\,\text{nm} between dimers, actin filament diameter \approx 5\,\text{nm}.
• Functional Meaning: Mechanical continuity from ECM to cytoskeleton enables mechanotransduction.

Page 8 – Catalogue of Intercellular Junctions

Ethical note: Defects in any of these junctions underlie diseases (e.g., claudin mutations → deafness; connexin 26 → hereditary deafness; pemphigus vulgaris autoantibodies against desmoglein 3).

Page 9–10 – Junction Morphology Terminology

• Zona/Zonula (belt): Continuous encircling band (e.g., Zonula Occludens = tight junction; Zonula Adherens = adhesion belt).
• Macula (spot): Discrete patch (e.g., Macula Adherens = desmosome; hemidesmosome on basal surface).
• Linear vs. Punctate AJs:
  • Linear = belt along lateral membrane (ZA).
  • Punctate = dot-like, often dynamic, at free edges during wound closure; highlighted by microscopy images (10 µm scale; actin bundles directed to these sites).
  • Importance: Shows junction plasticity during morphogenesis and repair.

Page 11–12 – Tight Junctions in Detail

• Physical Barrier: Seal paracellular clefts; tracer assays demonstrate blockage of extracellular dye at TJ plane.
• Molecular Basis: Homophilic claudin/occludin strands; strands appear as “kissing points” under EM (0.5\,\mu\text{m} scale bars given).
• Fence Function: Prevent apical membrane proteins from diffusing to basolateral domain → key for epithelial polarity.
• Polarity Complexes: Par, Crumbs, and Scribble complexes align with TJs to specify apico-basal identity.

Page 13–16 – Adherens Junctions & Morphogenesis

• Cadherin‐Mediated: Ca^{2+} required; E-cadherin in epithelia, N-cadherin in mesenchyme, etc.
• Cytoskeletal Link: 𝛼-catenin connects cadherin–β-catenin complex to actin via vinculin/α-actinin, forming a contractile terminal web.
• Contractility & Morphogenesis:
  • Actomyosin tightening along adhesion belts causes epithelial sheet invagination → tube formation (illustrated neural tube example, 50\,\mu\text{m} bar).
• Clinical Tie-in: Abnormal cadherin function yields metastatic potential; neural tube closure defects (spina bifida) involve AJ/actomyosin dysregulation.

Page 17 – (Blank Slide)

• No content beyond continuation; placeholder for lecture transition.

Page 18–20 – Desmosomes

• Structure: Desmoglein + desmocollin (cadherin family) form dense adhesive spots.
• Plakoglobins (γ-catenin), Plakophilins, and Desmoplakin create cytoplasmic plaque anchoring keratin IFs.
• Mechanical Role: Many desmosomes in epithelia/cardiac muscle provide high tensile strength; EM shows keratin filaments converging on plaques.
• Pathology Connection:
  • Autoantibodies → pemphigus foliaceus/vulgaris (skin blistering).
  • Mutations (desmoplakin) → arrhythmogenic right ventricular cardiomyopathy.

Page 21–24 – Hemidesmosomes & IF Network Integrity

• Composition: \alpha6\beta4 integrin + BP180 (type XVII collagen), BP230, plectin.
• Linkage: Keratin IFs ↔ integrin ↔ laminin 332 ↔ collagen IV in basal lamina.
• Tensile Test (Slide 24):
  • With IF network → cell sheet stretches without rupture.
  • Without IFs → catastrophic tearing (epidermolysis bullosa simplex models).
  • Highlights biomechanical necessity of IF coupling.

Page 25–26 – Gap Junctions

• Connexon: Hexamer of connexin proteins; two connexons align to create aqueous channel \approx 1.5\,\text{nm} diameter.
• Permissive Size: Molecules < 1500\,\text{Da} (e.g., Ca^{2+}, IP$_3$, cAMP) pass → electrical & metabolic coupling.
• Gating: Closed by high [Ca^{2+}]_{cyt}, low pH, or specific second messengers; protects healthy cells from damaged neighbors.
• Health: Connexin 43 in heart ensures synchronous contraction; connexin 26 mutation → sensorineural deafness.

Page 27 – Focal Adhesion / Adhesion Plaque Components

• Key Players:
  • Integrins (β1, β3, etc.).
  • Talin – initial activator, binds β tail.
  • Vinculin – strengthens linkage to actin.
  • Paxillin – scaffold for signaling complexes.
  • FAK (Focal Adhesion Kinase) & Src – propagate signals regulating survival and motility.
  • Rho GTPases (Rho, Rac, Cdc42) – orchestrate actin dynamics.
• Substrate: Fibronectin on culture dish or native ECM; integrin–fibronectin interaction central to traction generation.

Page 28 & 31 – Four-Step Motility Cycle

1. Protrusion
   • Lamellipodia – broad, sheet-like actin network.
   • Filopodia – spike-like bundles.
2. New Adhesion Formation (leading edge) – nascent integrin clusters mature into focal adhesions.
3. Cell Body Translocation – Myosin II slides actin filaments generating contraction.
4. Tail Retraction – trailing adhesions disassemble via calpain, endocytosis.

• Force Balance:
  • Stationary: Actin polymerization = retrograde flow.
  • Moving: Binding of actin to substrate via integrins resists retrograde flow → net forward protrusion.

Page 29–30 – Actin Regulators in Protrusion

• Formins – nucleate & tether long, unbranched actin at membrane.
• Arp2/3 Complex – creates 70^{\circ} branched network in lamellipodia.
• Fascin & Fimbrin – bundle actin in filopodia, increasing stiffness for probing.
• Imaging: Fluorescence micrograph of lamellipodium (5 µm bar) visualizes dense actin meshwork.

Page 32 – Focal Adhesion Schematic (Repeat)

• Reinforces earlier slide: adjacency of signaling and structural molecules.

Page 33 – Cell Crawling Video Link

• Resource: Time-lapse (~YouTube) demonstrates lamellipodial dynamics; useful visual aid to consolidate theoretical steps.

Page 34–36 – Chemotaxis & Immune Example

• Chemotaxis Definition: Directed migration along extracellular chemoattractant gradient.
• Neutrophil Video: White blood cell chasing bacterium (~YouTube) highlights in vivo chemotaxis.

Page 37 – Leukocyte Extravasation Cascade

1. Rolling: Selectins (E, P on endothelium; L on leukocytes) bind sialylated mucins → transient tethers.
2. Activation: Endothelial IL-1/TNF induce chemokines; chemokine binding to GPCR on rolling cell triggers integrin activation (inside-out).
3. Firm Adhesion: Activated \beta_2 integrins (e.g., LFA-1) bind ICAM-1; cell stops.
4. Transmigration (Diapedesis): Leukocyte squeezes through junctions, guided by ECM heparan sulfate-bound chemokines.

• Clinical Correlate: Defects (LAD-I = CD18 deficiency) → recurrent infections; anti-integrin drugs (Natalizumab) modulate autoimmune inflammation.

Page 38 – Lecture Summary

• Adhesion Hierarchy:
  • Cell–cell + cell–ECM contacts maintain architecture, permeability, and signaling.
• CAM Diversity: Cadherins, IgSF, Selectins, Integrins each tailored to specific binding and signaling roles.
• Specialized Junctions: Tight (barrier/polarity), Adherens (actin‐anchored tension), Desmosomes (IF tensile strength), Hemidesmosomes (cell-ECM anchoring), Gap Junctions (communication).
• Motility: Dynamic formation and disassembly of adhesion plaques at the leading edge drive forward migration; coordinated by Rho GTPases and actin regulators.
• Broader Implications: Understanding adhesion underpins therapies for cancer metastasis, chronic inflammation, wound healing, and tissue engineering.