The Extracellular Space Epithelial tissues closely packed cells lining spaces in the body (e.g. skin, intestine, etc) Connective tissues Largely composed of non-living extracellular material (e.g. cartilage, tendon, dermis)

The Extracellular Space Proteins in plasma membrane have sugars attached Glycocalyx Mediate cell interactions Provide mechanical support Barrier to particles Binding sites for regulatory factors

The Extracellular Matrix (ECM) An organized network of materials located beyond the plasma membrane

The Extracellular Matrix (ECM) Basement membranes Thick regions of ECM Surround muscle/fat Underlie the basal surface of epithelial tissues

The Extracellular Matrix (ECM) Basement membranes Separate different tissues Provide mechanical support Barrier to macromolecule and cellular movement Substrate for cell migration Generate signals that maintain cell survival

The Extracellular Matrix (ECM) Collagens (27 different types) High tensile strength (resistant to pulling forces) Alpha-helical trimers bundle together into fibrils Types I, II, III (fibrillar) form rigid cables Adjacent collagens are strengthened by covalent cross-links Hydroxylysine - lysine Type IV (nonfibrillar) can form an interconnected lattice

The Extracellular Matrix (ECM) Collagens (27 different types) Type IV (nonfibrillar) can form an interconnected lattice Composed of helical and non-helical segments (flexibility) Globular domains at each end (lattice contact points) Collagens bind: Fibronectins Integrins (cell surface)

The Extracellular Matrix (ECM) Diseases caused by defects in collagen genes Osteogenesis imperfecta Fragile bones Ehlers-Danlos syndrome Hyperflexible joints, highly extensible skin

The Extracellular Matrix (ECM) Proteoglycans Protein core + glycosaminoglycan (GAG) polysaccharide complex Chondroitin sulfate & keratin sulfate High amount of negative charge binds cations and H2O Hydrated gel resists compressive forces Hyaluronic acid links many proteoglycans to form extremely large molecules Fill the scaffold created by collagens

The Extracellular Matrix (ECM) Fibronectins Modular domains for interactions Bind collagens, proteoglycans, integrins at cell surface Important for: linking ECM components together, cell attachment to matrix, cell migration NC cells

The Extracellular Matrix (ECM) Laminins 3 polypeptides linked by disulfide bonds Form a second lattice interwoven with Collagen IV lattice Bind to proteoglycans, integrins at cell surface PGC on laminin

ECM Remodeling Matrix metalloproteinases (MMPs) Enzymes that degrade ECM proteins Tissue remodeling Cell migration Wound healing

Steps leading to metastatic spread MMP activity

Cell - ECM Interactions Integrins Only found in animals Heterodimer of alpha and beta subunits 18 alpha and 8 beta subunits known 12 different alpha/beta combinations known Transmembrane proteins Extracellular domain, transmembrane domain, intracellular domain Inside-out signaling Post-translational alterations to cytoplasmic tail regulate conformation changes in extracellular domain Talin separates beta from alpha to open receptor to active state Plasma membrane talin

Inactive

Active

Cell - ECM Interactions Ligand binding RGD loop of Fibronectin binds to integrin receptor extracellular domain Isolated RGD Loop can be exploited to block platelet aggregation / blood clotting

Cell - ECM Interactions Integrins Two major functions Adhesion to substrate Receptors cluster increasing overall strength Signal transmission Binding of ligand (collagen) can change cytoplasmic domain Cytoplasmic domain can activate kinases such as FAK and Src Activated kinases can transmit signals to nucleus and change gene expression

Cell - ECM Interactions Structures important for adhesion to substrate Focal adhesions: Scattered, discrete, transient, dynamic, rapidly form and break Clusters of integrins bound to collagen / Fibronectin Cytoplasmic domains attach to cytoskeleton connecting exterior forces to internal signals Actin filaments Focal adhesion kinase (FAK)

Forces exerted by focal adhesions

Cell - ECM Interactions Structures important for adhesion to substrate Hemidesmosome more permanent anchor to basement membrane Integrins bound to laminin to dense collection of intermediate filaments

Cell - ECM Interactions Structures important for adhesion to substrate Hemidesmosome Disease: epidermolysis bullosa Epidermis poorly connected to basement membrane / dermis Fluid accumulates in between = blister (keratins)

Cell - ECM Interactions

Cell - Cell Interactions Cadherins: Ca2+ dependent adhesion Homophilic interactions allow self-sorting of mixed cell populations Disease role: metastasis of cancer Lose adhesion by downregulating cadherin expression Penetrate / invade barriers by upregulating MMP expression

Cell - Cell Interactions Structures important for cell-cell adhesion Adherens junctions (30nm gap between cells) Cadherin-cadherin interactions in belt-like strips holding two cells together Cytoplasmic domains link via beta-catenin and alpha-catenin to the cytoskeleton

Cell - Cell Interactions Structures important for cell-cell adhesion Desmosomes (1 um diameter disc) Resist mechanical stress Cadherin-cadherin interactions linked to cytoskeleton (intermediate filaments)

Cell - Cell Interactions Tight junctions Seal two membranes together Block paracellular movement Occludin and claudins (24 genes) Different claudins have different permeabilities #1 doesn’t allow H2O to pass, #16 is permeable to Mg2+ Important for maintaining blood-brain barrier

Cell - Cell Interactions Gap junctions Join cytoplasmic spaces between adjacent cells via a narrow pore 1.5nm diameter 1kD cutoff, small molecules freely pass (ATP, cAMP, Ca2+, etc) Subunits are connexins Open / close regulated by phosphorylation Integrates cells of a tissue into a functional unit

Cell - Cell Interactions

Plant cell-cell interactions Plasmodesmata Join adjacent plant cytoplasmic spaces Capable of dilation, 1kD cutoff can open to a 50kD cutoff Exploited by some plant viruses

Roles of the plant cell wall Cell wall functions Structural role supporting and protecting plant cells Cellulose microfibrils confer tensile strength Signaling roles Cell wall-associated transmembrane protein kinases Dynamic not static, undergoes significant remodeling