BIOL 251 l16-17 cell adhesion terms

Specialization

A discrete region of the plasma membrane where adhesion receptors or integral membrane proteins (IMPs) are highly concentrated rather than uniformly distributed; Establishes localized mechanical, permeability, or signaling junctions between cells.

Tight Junction (Zonula Occludens)

A fence-like cell-cell junction located at the apical end of epithelial cells that seals the paracellular space; Prevents substances from leaking between adjacent cells (paracellular barrier) and restricts the lateral diffusion of membrane lipids and proteins to maintain cell polarity.

Claudins

The primary transmembrane junctional proteins that form the sealing strands of tight junctions; Interlocks with claudins on adjacent cells to create paracellular barriers or ion-selective aqueous pores.

Paracellular Space (Intercellular Space)

The intercellular gap located between the lateral plasma membranes of adjacent cells; Serves as the pathway for solute and fluid movement between cells when not sealed by tight junctions.

Gap Junction

A cell-cell junction composed of aligned transmembrane channels connecting the cytoplasms of neighboring cells; Allows direct electrical and chemical passage of water, small solutes, inorganic ions (e.g., Ca2+), and second messengers between cells.

Connexin

An individual transmembrane protein subunit with four membrane-spanning domains; Six connexin proteins oligomerize to form a single connexon half-channel.

Connexon

A hexameric protein complex formed by six connexins in the plasma membrane; End-to-end alignment of two connexons from adjacent cells forms a functional, gated gap junction channel.

Cadherins

A major family of calcium-dependent transmembrane cell-cell adhesion proteins; Mediates homophilic cell-cell adhesion and transmits outside-in signals that regulate cell proliferation, motility, and tissue organization.

Homophilic Binding

An adhesive interaction where a cell-surface receptor binds specifically to an identical receptor type on an apposed cell; Enables cells expressing matching adhesion molecules to selectively recognize and adhere to one another.

Heterophilic Binding

An adhesive interaction where a cell-surface receptor binds to a different type of molecule on an apposed cell or ECM; Mediates diverse cell-cell and cell-matrix interactions, such as selectin binding to carbohydrates.

Adherens Junction (Zonula Adherens)

A belt-like cell-cell specialization located just below tight junctions; Connects adjacent cells through cadherins linked internally to actin microfilaments, providing mechanical support and coordinating cell sheet movements.

Desmosome (Macula Adherens)

A button-like cell-cell specialization designed to withstand severe mechanical stress; Couples specialized cadherins to internal intermediate filament networks across adjacent cells, distributing tensile forces throughout a tissue layer.

Desmoglein & Desmocollin

Specialized cadherin isoforms present in desmosomal plaques; Span the intercellular space to bind apposed desmosomal cadherins and anchor internal intermediate filaments via adapter plaques.

Immunoglobulin Superfamily (IgSF) / CAMs

Calcium-independent cell adhesion molecules containing immunoglobulin-like domains (e.g., NCAM, ICAM-1); Mediates cell-cell adhesion and signaling through homophilic or heterophilic interactions (including binding integrins).

NCAM (Neural Cell Adhesion Molecule)

A member of the immunoglobulin superfamily expressed in neural tissues; Facilitates calcium-independent cell adhesion and axon guidance.

ICAM-1 (Intercellular Adhesion Molecule 1)

An IgSF transmembrane glycoprotein expressed on endothelial cells; Serves as the ligand for activated leukocyte integrins (such as LFA-1) during firm adhesion in extravasation.

Selectins (L-, E-, P-selectin)

Carbohydrate-binding cell-surface lectins that require calcium to bind glycosylated ligands.

L-selectin

Expressed on leukocytes; mediates initial tethering.

E-selectin

Expressed on inflamed endothelial cells; binds leukocyte glycoproteins.

P-selectin

Stored and expressed on platelets and endothelial cells; mediates early leukocyte trapping.

Function

Captures passing leukocytes under shear blood flow to initiate rolling along vessel walls.

Lectin

Any protein that specifically recognizes and binds to carbohydrate groups; Mediates cell recognition, adhesion, and pathogen binding without altering the carbohydrate.

Glycoprotein

A protein covalently modified with oligosaccharide (sugar) chains; Acts as a cell-surface recognition marker and ligand for lectins/selectins.

Platelet-Activating Factor (PAF)

A potent inflammatory lipid mediator released by endothelial cells; Binds a 7-transmembrane GPCR on leukocytes to trigger intracellular signaling cascades that activate integrins.

VE-cadherin (Vascular Endothelial Cadherin)

The principal cadherin forming adherens junctions between endothelial cells; Regulates vascular permeability; its phosphorylation or endocytosis opens gaps between endothelial cells for leukocyte transmigration.

Extracellular Matrix (ECM)

An organized network of secreted proteins and polysaccharides surrounding cells; Provides physical scaffolding, withstands mechanical loads (tension/compression), and delivers essential survival, growth, and differentiation signals.

Attached Glycocalyx

A coat of cell-associated membrane glycoproteins, glycolipids, and bound proteoglycans closely associated with the plasma membrane; Protects the membrane surface and mediates cell recognition; degrades upon cell death unlike the persistent ECM.

Basement Membrane / Basal Lamina

A thin, continuous sheet of specialized ECM underlying epithelial cells and surrounding muscle fibers, fat cells, and peripheral nerves; Provides structural support, partitions tissue compartments, acts as a selective permeability filter, and guides cell migration and synaptic development.

Fibroblast

The primary connective tissue cell type; Synthesizes, secretes, and organizes the structural components of the 3D ECM, including collagen and fibronectin.

Collagen

The most abundant protein family in the ECM, forming triple-helical structures.

Fibrillar Collagen (Types I, II, III)

Forms long, staggered, rope-like fibers that provide high tensile strength to resist pulling forces.

Non-fibrillar Collagen (Type IV)

Forms flexible two-dimensional networks that constitute the core meshwork of basal laminae.

Procollagen Peptidase (Protease)

An extracellular proteolytic enzyme; Cleaves the non-helical terminal propeptides of secreted procollagen, allowing spontaneous self-assembly into mature collagen fibrils outside the cell.

Elastin

A hydrophobic, highly cross-linked ECM protein organized into expandable fibers; Confers elasticity, allowing tissues (such as arteries, skin, and lungs) to stretch under force and recoil back to their original shape.

Elastase

A specific proteolytic enzyme that cleaves elastin; Generates soluble elastin fragments during tissue injury that bind elastin receptors to stimulate cell migration and repair signaling.

Proteoglycan

A complex macromolecule comprising a central core protein covalently attached to multiple glycosaminoglycan (GAG) side chains; Hydrates the ECM to resist compressive forces and sequesters growth factors.

Glycosaminoglycans (GAGs)

Long, unbranched disaccharide polymers containing negative carboxyl and sulfate groups (e.g., chondroitin sulfate, heparan sulfate); Attracts positively charged cations (Na+) and large volumes of water to create a hydrated, gel-like matrix that resists compression.

Hyaluronic Acid (Hyaluronate)

A non-sulfated GAG of immense length; Acts as a central backbone that binds hundreds of individual proteoglycan monomers via link proteins, creating vast water-retaining complexes in cartilage.

Fibronectin

A dimeric ECM glycoprotein joined by disulfide bonds, containing distinct binding modules for collagen, heparin, fibrin, and integrins; Connects cells to connective tissue ECM, guides cell migration pathways, and relays signals that regulate motility and cell shape.

Laminin

A large, cross-shaped heterotrimeric glycoprotein composed of , , and chains; Forms the primary structural network of the basal lamina by binding type IV collagen, nidogen/entactin, perlecan, and cell-surface integrins.

Matrix Metalloproteinases (MMPs)

A family of zinc-dependent ECM-degrading enzymes; Cleaves structural ECM and basal lamina proteins to allow tissue remodeling and permit the migration of repair cells or invading cancer cells.

Integrins

Heterodimeric transmembrane cell-surface receptors composed of non-covalently linked and subunits; Serves as the primary link between the ECM (binding fibronectin, laminin, collagen) and the internal cytoskeleton, operating as bidirectional signaling transducers.

Focal Adhesion

A dynamic macromolecular complex where integrins cluster to anchor the cell to the ECM; Anchors internal actin stress fibers to extracellular substrate fibers and acts as a major hub for force transduction and chemical signaling.

Talin

A key cytosolic adapter protein localized at focal adhesions; Binds the cytoplasmic tail of -integrin subunits to link them directly to actin filaments and mediates inside-out integrin activation.

Vinculin

A structural linkage protein present in focal adhesions and adherens junctions; Strengthens and stabilizes the physical connection between talin/integrins and the actin cytoskeleton under mechanical tension.

Focal Adhesion Kinase (FAK)

A non-receptor tyrosine kinase concentrated at focal adhesions; Autophosphorylates upon integrin clustering to recruit Src and initiate downstream RAS-MAPK and PI3K-AKT signaling cascades.

Paxillin

A cytosolic docking adapter protein at focal adhesions; Serves as a scaffold to recruit FAK, Src, and structural linkers to coordinate adhesion turnover and cell migration.

Hemidesmosome

A specialized cell-ECM junction localized at the basal surface of epithelial cells; Anchors internal intermediate filaments to the basal lamina using integrin 64 and BPAG2 (BP180) linked via plectin plaques.

BPAG1 / BPAG2 (Bullous Pemphigoid Antigens)

Structural plaque (BPAG1) and transmembrane (BPAG2/BP180) proteins in hemidesmosomes; Connects intermediate filaments to the basal lamina; targeted in the autoimmune blistering disease bullous pemphigoid.

Anchorage-Dependent Growth

The absolute physiological requirement of normal non-transformed cells to adhere to an ECM substrate to survive and divide; Prevents cells from growing in inappropriate anatomical locations; bypassed by invasive cancer cells.

Glypican-4

A cell-surface, GPI-anchored heparan sulfate proteoglycan localized in lipid rafts; Functions as an accessory co-receptor that binds and concentrates extracellular signaling ligands to facilitate their binding to signal-transducing receptors.

Adhesion vs. Anchorage

Adhesion

The binding strength between an extracellular receptor domain and its external ligand (cell or ECM).

Anchorage

The strength of the internal physical linkage connecting the intracellular tail of an adhesion receptor to the cytoskeletal filament network via adapter proteins.

Outside-In Signaling

Signal transmission initiated when an extracellular ligand (ECM protein or cell-surface cadherin) binds an adhesion receptor, inducing intracellular conformational changes and kinase cascades that alter gene expression, motility, or division.

Inside-Out Signaling

Signal transmission initiated inside the cell (e.g., via GPCR or kinase activation) that alters the conformation, binding affinity, or surface clustering of adhesion receptors (such as integrins) toward their external ligands.