Cell Adhesion & Motility – Comprehensive Notes

Page 1

Topic & Presenter
• Cellular Organelles and Function – Focus on Cell Adhesion & Motility
• Lecturer: Alex R. B. Thomsen, BSc, MSc, PhD
• Context: foundation for understanding how cells remain together, establish barriers, and move within tissues.

Page 2 – Why Cell Adhesion Matters

• Anchors cells within tissues ⇒ maintains overall tissue/organ integrity.
• In epithelia, adhesion creates physical barriers separating physiological compartments (e.g., gut lumen vs. bloodstream) and protects from the external environment.
• Prerequisite for migration – cells must attach, release, and re-attach to move.

Implications: Loss of proper adhesion ➝ metastasis, chronic inflammation, developmental defects.

Page 3 – Cell-Adhesion Molecules (CAMs)

Four major families & hallmark structural motifs

Numerical note: Transcript lists domain sizes (e.g., 525 aa) – underscores diversity and modularity.

Page 4 – Binding Logic

• Homophilic binding: identical molecules on opposing cells engage (e.g., cadherin–cadherin).
• Heterophilic binding: dissimilar partners (e.g., selectin–carbohydrate ligand).
Graphical shorthand: carbohydrate ligand ⇆ selectin; cadherin ⇆ cadherin.

Page 5 – Integrin Fundamentals

• Heterodimers of $18$ known $\alpha$-subunits + $8$ $\beta$-subunits ⇒ >$24$ combinatorial receptors.
• Tissue & ligand specificity – finely tunes cell–ECM recognition.
• Cytoskeletal linkage – via talin, vinculin, paxillin → actin or intermediate filaments.
• Bidirectional signalling
– Outside-in: ECM → integrin → intracellular cascades
– Inside-out: intracellular cues (e.g., $Rap1$, $PKC$) alter extracellular affinity.
• Regulate motility, proliferation, differentiation, apoptosis.

Page 6 – Vertebrate Integrin Pairings (Selected)

Take-home: specificity is coded by $\alpha$–$\beta$ combination.

Page 7 – Fibronectin–Integrin–Actin Continuum

(A) Fibronectin dimer: cell-attachment domain (RGD motif) + collagen-binding domain.
(B) Integrin dimers insert into plasma membrane; intracellular tails recruit adaptor proteins (talin, vinculin, paxillin).
(C) Actin filaments (stress fibres) couple through these adaptors ⇒ mechanical continuity from ECM to cytoskeleton (≈$50\,\text{nm}$ extracellular / $5\,\text{nm}$ cytoplasmic distances).

Page 8 – Intercellular Junction Catalog

Page 9 – Junction Morphology Nomenclature

• Zonula (Zona) = belt-like (continuous) – applies to tight & adherens junctions.
• Macula (spot) = punctate – seen in desmosomes, hemidesmosomes, and spot AJs.

Page 10 – Fine Architecture of Adherens Junctions

• Linear (belt) AJs form the zonula adherens (ZA) encircling apical circumference and associating with parallel actin bundles.
• Punctate AJs appear as discrete cadherin clusters, especially near free edges where actin reorganises.

Page 11 – Tight Junction Specifics

• Homophilic Claudin/Occludin strands create “sealing” ridges.
• Restrict paracellular flux (FITC-dextran tracer assays).
• Prevent membrane protein diffusion, preserving apical-basolateral polarity.

Page 12 – Experimental Visualization

(A) Electron-opaque tracer halts at TJ.
(B) Freeze-fracture: network of ridges within $0.5\,\mu m$ of apex.

Page 13 – Adherens Junction Mechanics

• Cadherin–cadherin homophilic adhesion requires extracellular $Ca^{2+}$.
• Intracellular side: $\alpha$-catenin, $\beta$-catenin, vinculin link to actin (terminal web).
• Potentially contractile – myosin II may generate tension → morphogenetic movements.

Page 14–16 – Morphogenesis Example: Epithelial Folding

Sequence (Fig 14-16):

1. Microvilli, TJ, and adhesion belt (AJ) aligned.

2. Coordinated actin contraction along adhesion belt produces invagination.

3. Epithelial tube detaches → basis of neurulation (neural tube, 50 µm diameter).

Page 18–20 – Desmosomes

• Cadherin subtype: Desmoglein & Desmocollin.
• Desmoplakin + plakoglobin/ plakophilin form cytoplasmic plaque.
• Link to intermediate filaments (keratins) ⇒ distributes tensile stress (spot welds).
• EM: dense plaques ~$0.1\,\mu m$ wide; IF bundles radiate.

Page 21–24 – Hemidesmosomes & Mechanical Integrity

• Similar “half-desmosome” on basal surface; CAM is $\alpha6\beta4$ integrin plus BP180/BP230 (bullous pemphigoid antigens) & plectin.
• Anchor keratin IFs to basal lamina (laminin 332, collagen IV).
• Comparative stretch experiment: IF-coupled sheets resist rupture; IF-deficient sheets tear.

Page 25–26 – Gap Junctions

• Connexon = hexamer of connexins; two connexons (one per cell) align to form pore.
• Permeability cutoff $\approx 1.5\,\text{kDa}$ (e.g., $IP_3$, $cAMP$).
• Gating by $Ca^{2+}$, pH, or second messengers prevents damage spread during injury.

Page 27 – Focal Adhesion (Adhesion Plaque) Components

• Integrins (e.g., $\alpha5\beta1$ for fibronectin).
• Adaptor/scaffold: talin, vinculin, paxillin, focal-adhesion kinase (FAK), Src.
• Rho GTPases modulate assembly/disassembly & stress-fibre dynamics.

Page 28 & 31 – 4-Step Cell Migration Cycle

1. Protrusion – actin polymerises at leading edge forming lamellipodium; retrograde flow countered by myosin.

2. Attachment – nascent adhesions form; integrins bind ECM.

3. Traction/Translocation – actomyosin contraction pulls cell body forward.

4. Detachment – rear adhesions disassemble; tail retracts.
   Balance of polymerisation vs. retrograde flow determines net advancement.

Page 29 – Actin Regulators in Motility

• Formins: processively elongate unbranched actin – filopodial cores.
• Arp2/3 complex: nucleates branched actin network underlying lamellipodia.
• Fascin & Fimbrin: bundle actin into stiff parallel filaments (filopodia).

Page 30 – Lamellipodium Imaging

• Wide, veil-like protrusion; ~5 µm scale bar underscores rapid turnover.

Page 32 – Summary of Adhesion Plaque Signalling

Diagram reiterates Page 27; emphasises cross-talk FAK↔Src↔Rho GTPases regulating cytoskeletal tension and adhesion turnover.

Page 33 – Live-Cell Example

• Video (URL) of fibroblast cell crawling: integrates protrusion, adhesion, and retraction visually.

Page 34–36 – Chemotaxis & Immune Surveillance

• Cells sense chemoattractant gradients ⇒ biased actin polymerisation toward higher concentration.
• Neutrophil chasing bacterium video demonstrates rapid, polarised migration.

Page 37 – Leukocyte Extravasation Cascade

1. Rolling – Selectins on endothelium bind mucin-like ligands on leukocytes (slow rolling).

2. Activation – chemokines bind GPCRs → inside-out activation of integrins (e.g., $\alpha L\beta2$).

3. Firm Adhesion – high-affinity integrin binds ICAMs/VCAMs.

4. Diapedesis (Extravasation) – leukocyte migrates through junctions into tissue.
   Molecular players: DARC, IL-1/TNF receptors, heparan sulfate proteoglycans concentrate chemokines.

Page 38 – Grand Summary

• Cell–cell & cell–ECM adhesions maintain tissue integrity, create barriers, and enable migration.
• CAM families (cadherins, integrins, selectins, Ig-CAMs) mediate specific interactions.
• Specialised junctions (TJ, AJ, desmosome, hemidesmosome, GJ) integrate structure, signalling, and mechanics.
• Focal adhesions at leading edge coordinate actin dynamics with integrin signalling -> essential for directed motility & processes like chemotaxis, wound healing, immune surveillance.

Key Equations & Constants Encountered

• $Ca^{2+}$ and $Mg^{2+}$ as divalent cation cofactors.
• Molecular weight cutoff for gap-junction permeability: $< 1.5 \times 10^{3}\,\text{Da}$. • Integrin diversity: $$18{\alpha} \times 8{\beta} \Rightarrow >24$ unique receptors.

Ethical & Clinical Connections

• Loss-of-function mutations in adhesion molecules → blistering diseases (e.g., pemphigus, bullous pemphigoid).
• Over-active integrin signalling → tumour invasion; hence integrin antagonists in anti-angiogenic therapy.
• Gap-junction defects (connexin 43) → cardiac arrhythmias.

These notes encapsulate the complete set of themes, mechanisms, and examples from the lecture, serving as a standalone study guide.