Cell & Tissue Adhesion: A Cheatsheet

This cheatsheet summarizes key concepts of cell adhesion, cell junctions, and the extracellular matrix (ECM), vital for understanding multicellular organism structure and function.

Cell Adhesion & Cell Adhesion Molecules (CAMs)

Cell adhesion is the specific binding of cells to other cells or to the extracellular matrix (ECM).

• Multicellular Organization: Essential for building cells → tissues → organs.

• Mediators: All cell adhesion is facilitated by Cell Adhesion Molecules (CAMs).

Functions of CAMs

• Physiological Conditions:

  • Normal tissue development and maintenance of integrity.

  • Cell communication, differentiation, cell cycle regulation, migration, survival, and death.

• Pathological Conditions: Involvement in inflammation and tumor/metastasis development.

Structure & Working Principle of CAMs

• Structure: Transmembrane integral membrane proteins (glycoproteins) with three domains:

  • Intracellular domain: Links to the cytoskeleton.

  • Transmembrane domain.

  • Extracellular domain: Binds to other CAMs or ECM components.

• Highly Conserved: Essential for complex organism evolution.

• Working Principle: Ligand-receptor interaction; a CAM can be both ligand and receptor, initiating signaling pathways for cellular responses (e.g., kinase activation, cytoskeleton reorganization).

Classification of Cell Adhesion

• Attachment Type: Cell-cell or cell-matrix.

• Calcium Dependence: Ca²⁺-dependent or Ca²⁺-independent.

• Binding Affinity:

  • Homophilic: Between same type of molecules.

  • Heterophilic: Between different type of molecules.

• Duration: Transient or lasting junctions.

• Cytoskeletal Linkage: With or without linkage to the cytoskeleton (robust adhesion requires linkage).

Four Major Families of CAMs

1. Cadherins:

   • Ca²⁺-dependent, primarily homophilic cell-cell adhesion.

   • 180+ members in humans.

   • Structure: N-terminal extracellular domains (EC domains) separated by flexible hinge regions where Ca²⁺ binds to prevent flexing. C-terminal cytoplasmic domain associates with cytoskeleton (actin or intermediate filaments) via adapter proteins like catenins.

   • Binding: N-terminal tips bind cadherins on opposing cells, forming a zipper-like pattern. Can be cis-homophilic dimers that bind other dimers through trans-homophilic or trans-heterophilic interactions.

   • Types:

     • E-cadherin: Epithelial cells.

     • N-cadherin: Nerve, skeletal/cardiac muscle, lens cells.

     • P-cadherin: Placenta (trophoblast), epidermis.

     • VE-cadherin: Vascular endothelial cells.

     • Desmosomal cadherins: Desmocollins, desmogleins.

     • Protocadherins: "First cadherins," no cytoskeleton attachment.

   • Function & Relevance:

     • Participate in adherens junctions (actin filaments) and desmosomes (intermediate filaments).

     • Involved in intracellular signaling (catenin complex) and gene expression regulation (beta-catenin).

     • Cell Sorting: Enable cells of similar type to stick together, crucial for development (e.g., neural tube formation: neural tube cells lose E-cadherin, acquire N-cadherin).

     • Maintain epithelial layer integrity; E-cadherin removal/blocking disrupts cell-cell attachment.

     • Involved in Epithelial-Mesenchymal Transition (EMT): Loss of E-cadherin is associated with invasive behavior of tumor cells (metastasis).

2. Selectins:

   • Ca²⁺-dependent, heterophilic, transient cell-cell adhesion.

   • Bind to carbohydrates (glycoproteins, glycolipids), belonging to C-type lectins.

   • Structure: Cytoplasmic domain, large extracellular segment with a carbohydrate-recognition domain (CRD) controlled by calcium.

   • Classes: P-selectin (platelets, activated endothelial cells), E-selectin (activated endothelial cells), L-selectin (leukocytes).

   • Function: Control adhesion of circulating immune cells, lymphocyte homing, and inflammation (endothelial cells express selectins to recognize oligosaccharides on white blood cells). Mediate loose adhesion causing leukocytes to roll.

3. Integrins:

   • Mediate cell-matrix (most common) and cell-cell interactions.

   • Binding affected by Ca²⁺ and Mg²⁺ concentrations. Heterophilic interactions.

   • Structure: Heterodimers of and subunits (18 , 8 , >20 types). Both subunits span the membrane.

   • Domains: Large N-terminal extracellular domain binds specific amino acid sequence motifs (e.g., RGD sequence in fibronectin, laminin); intracellular C-terminal tail links to cytoskeleton (actin or intermediate filaments).

   • Activation: Exist in inactive (folded) and active (extended) forms. Activation involves unhooking cytoplasmic tails and unfolding external domains to reveal high-affinity binding sites.

   • Function:

     • Participate in Focal adhesions (actin filaments) and Hemidesmosomes (intermediate filaments).

     • Involved in leukocyte extravasation and lymphocyte homing (e.g., L$\beta$2 (LFA1) on leukocytes binds ICAM1 on endothelial cells). Leukocyte binding to endothelial cells is strengthened by integrins.

     • Clinical Relevance: LFA-1 (CD18) is critical for immunological processes. Defects cause Leukocyte Adhesion Deficiency (LAD) Types I-III, leading to recurrent infections.

     • Glycoprotein IIb/IIIa (integrin IIb$\beta$3) on platelets is a receptor for fibrinogen, aiding platelet activation. Inhibitors prevent blood clots. Snake venom disintegrins mimic RGD to prevent clotting.

     • Bidirectional Signaling Receptors:

       • Outside-in signaling: Conformational changes induced by ECM binding activate intracellular kinases (e.g., FAK – focal adhesion kinase), influencing cell division, differentiation, motility, growth, and survival.

       • Inside-out signaling: Cytoplasmic activation (e.g., integrin IIb$\beta$3 affinity for fibrinogen) increases integrin's affinity for extracellular ligands.

4. Immunoglobulin (Ig) Superfamily:

   • Encoded by a single gene, members differ by alternative mRNA splicing and glycosylation.

   • Mediate homophilic and heterophilic interactions.

   • Ca²⁺-independent.

   • Structure: Extracellular segment with immunoglobulin-like folded domains.

   • Function:

     • Immune Response: Involved in homing and extravasation during inflammation. ICAM-1, ICAM-2, VCAM-1 on endothelial cells mediate heterophilic binding to integrins.

     • CD4: Receptor for HIV-1 on T-helper cells.

     • Nervous System: NCAM and L1-CAM mediate homophilic interactions in nerve outgrowth, synapse formation, and nervous development. Mutations in L1-CAM can cause hydrocephalus and mental retardation.

Extracellular Matrix (ECM)

The ECM is a complex network of macromolecules (proteins, polysaccharides) secreted by local cells (fibroblasts, chondroblasts, osteoblasts) and degraded by specific enzymes.

ECM Organization

• Interstitial Matrix: Fills spaces between cells.

• Basement Membrane (Basal Lamina): Thin specialized ECM layer beneath epithelial/endothelial cells.

Key Characteristics of ECM

• Classes of macromolecules are similar across tissues but vary in amounts and organization (e.g., connective tissue, cartilage, bone).

• Active and complex role in regulating cell behavior: influences survival, development, migration, proliferation, shape, and function.

• Provides structural support, space for ligands and growth factors, and acts as a reservoir for growth factors.

• Different ECM proteins act as ligands for cell receptors.

Components of ECM

1. Fibrous Proteins:

   • Collagen:

     • Most abundant protein in mammals (25% of total protein mass).

     • Provides high tensile strength, resisting pulling forces.

     • Structure: Triple-stranded helical structure of three chains (polypeptide chains). Rich in proline and glycine, with glycine at every third amino acid position (Gly-X-Y repeats).

     • Assembly: Synthesized as pro-collagen monomers, hydroxylated (Pro, Lys, requires Vitamin C) and glycosylated, then self-assemble into trimers. After secretion, processed by proteolytic enzymes into fibrils, which cross-link into fibers.

     • Types: >20 distinct types.

       • Fibril-forming (fibrillar) (e.g., Type I, II, III): Provide tensile strength (bone, skin, tendons).

       • Fibril-associated: Decorate collagen fibrils.

       • Network-forming (Type IV): Basal lamina.

       • Transmembrane (Type XVII): Hemidesmosomes.

     • Clinical Relevance: Genetic defects lead to diseases like Ehlers-Danlos syndrome (stretchy skin, flexible joints).

   • Elastin:

     • Provides elasticity to tissues (skin, blood vessels, lungs).

     • Rich in proline and glycine, but not glycosylated.

     • Structure: Elastic fibers composed of an elastin core surrounded by a microfiber (microfibrillar) sheath (containing fibrillin-1, 2).

     • Assembly: Soluble tropoelastin is secreted and highly cross-linked to form elastic fibers and sheets.

2. Glycosaminoglycans (GAGs) & Proteoglycans:

   • GAGs: Large, highly charged polysaccharides (e.g., hyaluronan, chondroitin sulfate). Composed of repeating sugar + amino sugar units.

   • Negative charges attract cations (Na⁺), inducing osmotic water movement.

   • Provide hydration to tissues and resist compression (e.g., in joints). Impairment leads to tissue weakness.

   • Proteoglycans: GAGs covalently linked to a core protein. Secreted or membrane-attached.

   • Contain modular domains that bind ECM components, hormones, and growth factors, altering cell signaling.

   • Ex: Aggrecan (a large proteoglycan) can attach to a hyaluronan backbone to form aggregates, providing resistance to compression in cartilage.

3. Multiadhesive Glycoproteins:

   • Help organize the matrix and facilitate cell attachment. Guide cell migration and influence growth factors.

   • Fibronectin:

     • Two polypeptides linked by disulfide bonds. Binding sites for integrins, collagen, heparan sulfate, fibrin.

     • Plasma fibronectin: Secreted by hepatocytes into bloodstream.

     • Cellular fibronectin: Produced by fibroblasts, part of ECM.

   • Laminin:

     • Large family of multidomain glycoproteins.

     • Composed of three polypeptides () disulfide bonded into a cross-like structure.

     • Crucial for organizing basal laminae and anchoring them to cells.

     • Major component of basal lamina along with type IV collagen.

ECM Remodeling & Degradation

• Cells control ECM synthesis and degradation (by secreting and activating/inactivating extracellular enzymes).

• Matrix Metalloproteinases (MMPs): Zinc-containing proteases that degrade ECM.

  • Essential for processes like tissue remodeling, migration, wound healing, angiogenesis.

  • Involved in pathological conditions such as arthritis, atherosclerosis, hepatitis, and tumor progression (cells need to break down ECM to migrate during metastasis).

Cell Junctions

Cell junctions are specialized protein complexes that provide stronger stability to cell adhesions and regulate paracellular pathways.

Components of Cell Junctions

• CAMs: Specific membrane proteins.

• Intracellular Attachment Proteins: Link CAMs to cytoskeleton.

• Cytoskeleton Components: Actin filaments or intermediate filaments.

Classification of Cell Junctions

• By Shape/Extent:

  • Macula: Spot-like punctate area.

  • Zonula: Belt-like structure around the cell.

• By Connection Type:

  • Cell-cell junctions.

  • Cell-ECM junctions.

• By Function:

  1. Occluding (Tight) Junctions: Form an impenetrable seal.

  2. Anchoring Junctions: Anchor cells together or to ECM, providing mechanical stability.

  3. Communicating (Gap) Junctions: Allow direct communication between cells.

Cell-Cell Junctions

1. Tight Junction (TJ) or Zonula Occludens (ZO):

   • Belt-like band of anastomosing sealing strands encircling epithelial/endothelial cells.

   • Cell membranes come very close, forming "kissing points" or focal fusions.

   • Structure: Composed of >40 proteins, including Claudins (>20 types), Occludin, JAM proteins, ZO proteins (ZO-1, ZO-2, ZO-3), and linked to actin filaments.

   • Claudins: Confer barrier properties, form charge-selective small pores; different claudins = different permeability.

   • Functions:

     • Form selectively permeable barriers, controlling paracellular permeability (preventing leakage between cells). Permeability varies by tissue (e.g., small intestine TJs are more permeable to Na⁺ than urinary bladder TJs).

     • Separate apical and lateral-basal surfaces, maintaining cell polarity by preventing diffusion of plasma membrane molecules. They also block backflow (e.g., glucose in the gut).

     • Form bidirectional signaling platforms, regulating assembly/function and transducing signals for cell proliferation, migration, differentiation, and survival.

   • Clinical Relevance:

     • Mice lacking Claudin-1 die from rapid water loss.

     • Mutations in Claudin-14 cause hereditary deafness.

     • Mutations in Claudin-16 cause renal magnesium wasting syndrome.

     • Pathogens like cytomegalovirus and cholera toxins target ZO-1/ZO-2, causing leak-flux diarrhea and inflammatory processes.

     • Celiac disease involves disrupted tight junction strands in the jejunum.

2. Adherens Junction (AJ) or Zonula Adherens (ZA):

   • Continuous belt-like configuration (zonula adherens), usually just below tight junctions.

   • Intercellular space (20-25nm) occupied by CAMs.

   • Cytoplasmic side has a fuzzy plaque (cadherin-catenin complex) linked to actin filaments.

   • Types:

     • Zonula adherens: Hold epithelial and endothelial cells together, resist stress.

     • Punctum adherens: In non-epithelial tissues, small punctate attachments (e.g., at synapses).

     • Fascia adherens: In cardiac muscle (intercalated disks).

   • Components:

     • CAMs: Classical cadherins (E-cadherin, N-cadherin, VE-cadherin) and Nectin.

     • Intracellular attachment proteins: Catenin complex (-, -catenins, p120) and Afadin.

     • Cytoskeleton: Actin filaments, vinculin, -actinin. These form a contractile bundle containing myosin II.

   • Key Feature: Morphologic and functional integrity is calcium-dependent. Removal of Ca²⁺ leads to dissociation.

   • Functions:

     • Provides lateral adhesion between cells, essential for coordinated movement (e.g., wound repair).

     • Often the first cell junction to appear in epithelia during development, initiating tight junction formation.

     • Actin filament contraction can narrow the cell apex, playing a crucial role in animal morphogenesis (e.g., neural tube formation).

     • The E-cadherin-catenin complex is a master regulator of cell adhesion, polarity, differentiation, migration, proliferation, and survival. Disruption allows -catenin to translocate to the nucleus and activate transcription factors.

3. Desmosome (D) or Macula Adherens (MA):

   • Spot-like ("button-like") cell-cell adhesions, riveting cells together.

   • Primary function: Provide mechanical strength.

   • Structure: Dense plaque (intracellular adaptor proteins) on cytoplasmic surface. Intermediate filaments (IF) extend from plaque into cytoplasm. Intercellular space (up to 30nm) contains extracellular portions of transmembrane glycoproteins.

   • Found in tissues subject to high mechanical stress (epidermis, heart muscle, meninges).

   • Components:

     • CAMs: Non-classical cadherins (Desmoglein, Desmocollin).

     • Intracellular attachment proteins: Catenins (Desmoplakin, Plakoglobin, Plakophillin) form the electron-dense plaque.

     • Cytoskeleton: Intermediate filaments (keratin in epithelia, desmin in heart muscle, vimentin in meninges).

   • Clinical Relevance:

     • Pemphigus (e.g., Pemphigus foliaceus, Pemphigus vulgaris) is an autoimmune blistering disease where antibodies attack desmosomal cadherins (Desmoglein 1 and 3), disrupting epidermal cell adhesion.

     • Naxos disease (Plakoglobin gene mutation) affects heart (arrhythmogenic right ventricular cardiomyopathy), hair, and skin.

     • Downregulation of desmosomes is observed in some carcinomas (e.g., squamous cell carcinoma), contributing to invasion and metastasis.

4. Gap Junction (GJ) or Nexus:

   • Direct channels between adjacent cell cytoplasms.

   • Allow direct transfer of molecules and electrical signals.

   • Membranes are closely apposed with a 2-3nm gap.

   • Found in most animal tissues (connective, epithelia, heart muscle, smooth muscle, liver, kidney, lens cells).

   • Structure:

     • Formed by two connexons (hemichannels), one from each cell.

     • Each connexon consists of six membrane-spanning connexin subunits (four-pass transmembrane proteins).

     • A continuous channel (1.5nm diameter) connects the two cells.

   • Permeability: Allows molecules smaller than 1,000 Daltons to pass (small ions, nucleotides, small metabolites like Ca²⁺, cAMP, IP3, glucose). Large biomolecules (nucleic acids, proteins) are excluded. Different connexins impart different pore sizes and charge selectivity.

   • Regulation: Toggle between open and closed states. Triggered by voltage differences, pH, and Ca²⁺ concentration (high Ca²⁺ or low pH closes, low Ca²⁺ or high pH opens).

   • Functions:

     • Electrical coupling in heart muscle (synchronous contraction) and neurons (action potential propagation).

     • Metabolic coupling (e.g., cAMP transfer from follicle cells to oocytes, nutrient delivery to avascular lens cells).

     • Signal transduction via second messengers.

   • Dynamics: Highly dynamic structures, continuously assembling and disassembling. New connexons are added at the periphery, old ones removed from the center.

   • Clinical Relevance:

     • Mutations in Connexin 26 (Cx26) gene cause deafness.

     • Mutations in Cx46 and Cx50 cause congenital cataracts.

     • Mutations in Cx32 cause X-linked Charcot-Marie-Tooth demyelinating neuropathy (peripheral nerve degeneration, muscle weakness) due to defects in Schwann cells.

Cell-ECM Junctions

1. Hemidesmosome:

   • Asymmetrical structures anchoring epithelial cells to the underlying basal lamina.

   • Do not share biochemical components with desmosomes despite similar appearance.

   • Increase stability of epithelial tissues by linking intermediate filaments to basal lamina components.

   • Components:

     • Transmembrane proteins: Specialized integrin ($6" data-type="inline-math"></span><mark>\beta$4 integrin) which is a laminin receptor, and Collagen XVII (BP180).

     • Intracellular attachment proteins: Plectin and BP230.

     • Cytoskeleton: Intermediate filaments (keratin).

   • Clinical Relevance:

     • Bullous pemphigoid is an autoimmune blistering disease where antibodies target Collagen XVII or BP230, causing the epidermis to detach from the basement membrane.

     • Defects in hemidesmosome components can cause severe blistering diseases like Epidermolysis Bullosa.

2. Focal Adhesion:

   • Smaller in size than hemidesmosomes.

   • Create dynamic links between the actin cytoskeleton and ECM proteins via integrins.

   • Prominent role in dynamic processes, such as epithelial cell migration during wound repair.

   • Found in fibroblasts and smooth muscle cells, besides epithelia.

   • Components:

     • Transmembrane proteins: Integrins (in clusters).

     • Intracellular attachment proteins: Actin-binding proteins (-actinin, vinculin, talin, paxillin) and regulatory proteins like focal adhesion kinase (FAK).

     • Extracellular binding: Integrins bind to laminin and fibronectin in the ECM.

   • Functions: Act as mechanosensors, detecting and transmitting mechanical changes from the ECM into biochemical signals, affecting cell migration, differentiation, and growth.

Intercalated Discs & Endothelial Cells: Summary of Adhesion

• Intercalated Discs (Cardiac Muscle):

  • Branched cardiac muscle cells are tightly connected by specialized junctions.

  • Contain gap junctions (electrical coupling), adhering junctions (tissue architecture, cell movement), and desmosomes (strong adhesion).

• Endothelial Cells:

  • Function as gatekeepers, controlling infiltration of leukocytes and plasma proteins.

  • Express adherens and tight junctions.

  • Leukocyte diapedesis (movement through blood vessel walls) involves complex junction rearrangement during inflammation.