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

Basement membranes• Thick regions of ECM

– Surround muscle/fat– Underlie the basal surface of epithelial tissues

Basement membranes• Separate different tissues• Provide mechanical support• Barrier to macromolecule and cellular movement• Substrate for cell migration• Generate signals that maintain cell survival

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

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)

ECM and disease• Diseases caused by defects in collagen genes

– Osteogenesis imperfecta• Fragile bones

– Ehlers-Danlos syndrome• Hyperflexible joints,

highly extensible skin

ECM: Proteoglycans• Protein core + glycosaminoglycan (GAG) polysaccharide complex

– High amount of negative charge binds cations and H2O– Hydrated gel resists compressive forces

• Hyaluronic acid links many proteoglycans to form large molecules• Fill the scaffold created by collagens

ECM: Fibronectins• Modular domains for interactions• Bind collagens, proteoglycans, integrins at cell surface

NC cells

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)

– Degrade ECM proteins to enable:• 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: Integrins• 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: Integrins• 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: Integrins• 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: Integrins• Structures important for adhesion to substrate

– Hemidesmosome• more permanent anchor to basement membrane• Integrins bound to laminin to dense collection of intermediate filaments

ECM and disease

(keratins)

• Disease: epidermolysis bullosa– Epidermis poorly connected to basement membrane / dermis– Fluid accumulates in between = blister

Epidermolysis Bullosa Type Genes Mutated

Simplex Keratin-5; Keratin-14; plectinJunctional laminin-5; collagen XVII; a6b4 integrinDystrophic collagen VII

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 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)– 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• 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