Extracellular Matrix and Integrins

Topic 6: Extracellular Matrix and Integrins

• This module focuses on the extracellular matrix (ECM) and integrins.
• Required reading for this module includes certain pages of Chapter 19, "Cell Junctions and the Extracellular Matrix."

Learning Objectives

• Understand the classes of the ECM
• Describe collagen's structure and types.
• Describe elastin and its functionality.
• Describe proteoglycans/GAGs and their functions.
• Describe the glycoproteins found in basal lamina.
• Understand how cells attach to the ECM.
• Understand integrins and how they are activated.

The Extracellular Matrix (ECM)

• The ECM comprises a complex, organized network of polysaccharides and proteins secreted by cells.
• It dictates the shape and mechanical properties of organs and tissues.
• The ECM can hold cells and tissues together.
• It is dynamic and influences properties like tissue elasticity, cell shape, and movement.

Two Main Classes of ECM

• Basal lamina (basement membrane): a thin sheet of ECM material underlying most epithelial cells and other organized groups of cells, separating them from connective tissue.
• Connective tissue: a material forming a framework and support for body tissues/organs, with cartilage and bone as specialized forms. The majority of its volume is ECM.
• In connective tissue, cells of the fibroblast family largely secrete ECM matrix material.

Three Classes of ECM Molecules

• Despite diversity, animal cell ECMs consist of three molecule classes.
• Protein-polysaccharide complexes (proteoglycans) provide the matrix; structural proteins embed within it.
• Structural proteins (collagens and elastins) give the ECM its strength and elasticity.
• Adhesive glycoproteins (fibronectins and laminins) attach cells to the matrix.
• Variations in relative amounts/organization give rise to material diversity.

ECM Component Overview: Table 20-1

• Proteoglycans like Perlecan resist compression and manage tissue molecule movement/deposition.
• Structural proteins offer tensile strength to tissues, withstanding forces that could tear them apart.
• Adhesive glycoproteins connect cells to structural ECM components.

Proteoglycans

• These are glycoproteins with a core protein attached to one or more glycosaminoglycan (GAG) chains.
• GAGs are unbranched polysaccharide chains of repeating disaccharides, usually sulfated, where one sugar is an amino sugar and the other a uronic acid.

GAG Classes and Characteristics

• Four main GAG groups are distinguished based on their sugars.
• GAGs are highly charged and hydrophilic.
• GAGs occupy most of the extracellular space, despite being <10% of the fibrous proteins' weight.
• They attract water/cations, forming a porous, hydrated gel embedding collagen/elastin fibrils.
• High water content creates turgor pressure, enabling the matrix to resist compression.
• The class of GAGs differ by sugar type, linkage type (alpha/Beta, 1->3, 1->4, etc), and sulfation.

Hyaluronan (HA)

• Hyaluronan (HA) is the simplest GAG, serving as a space filler during tissue morphogenesis.
• HA can contain up to 25,000 disaccharide units.
• It is not sulfated.
• It is typically not linked covalently to a core protein.
• HA is made by plasma membrane-bound enzyme complex.
• Hyaluronan is a major ECM component around migrating and proliferating embryonic cells.
• It forms a viscous, lubricating gel, abundant in joints, acting as a shock absorber.
• HA is added to the backbone of complex cartilage proteoglycans.

GAG Linkage

• In proteoglycans, the core protein is synthesized on the rough ER.
• Sugar assembly occurs in the Golgi.
• Either O (Serine/Threonine) or N (Asparagine) linked to the core.
• Glycosaminoglycans may undergo additional modifications, such as sulfation.
• Differenting proteoglycans, at least one sugar side chain of a proteoglycan has to be a GAG.
• Proteoglycans have long unbranched GAG chains, and the sugars can comprise most of the weight of the molecule.

Proteoglycan Diversity

• The core proteins can be secreted or cell surface proteins; either integral membrane or GPI-anchored.
• Proteoglycans demonstrate diversity in the number, composition, and sequence of GAG chains.

Aggrecan

• Aggrecan is a major cartilage component.
• Its backbone is a long hyaluronan molecule.
• Multiple aggrecan proteins attach non-covalently.
• The protein backbone is modified by GAGs.
• Aggrecan exhibits gel-like properties and resists compression in joints.

Proteoglycan Functions

• Functions include binding to matrix proteins (collagen/adhesive glycoproteins like fibronectins/laminins).
• Modulation of secreted signaling protein activity.
• They can function as cell surface co-receptors.

Collagens

• Collagens are the most abundant proteins in mammals, making up ~25% of the total body protein.
• A primary feature is a long, stiff, triple helical stranded structure.
• They have a repeating motif: Gly-X-Y (X is commonly proline; Y is hydroxyproline).
• The human genome encodes 42 different a-chains, which are essentially different types of collagen.
• Three a-chains wind together to form a helical rod, which is a fibril (the structural collagen form).
• Collagen type properties differ in the number/length of triple helical segments, flanking segments, 3D structures and covalent modifications of the a chains.

Collagen and Fiber Structures

• Collagen fibers form long, strong structures, especially in tendons, due to high tensile strength. Tendons connect muscle to bone and must withstand significant forces.
• Minor fibrillar collagens (types V/IX) co-assemble with type I collagen into fibers and they regulate the properties of matrix components.
• Type I collagen fibrils are also used in bone construction.
• Bone composition is ~70% mineral and 30% protein, with most being collagen type 1.
• It contains significant amounts of dahllite (a crystalline calcium- and phosphate-containing mineral).
• Specialized cells (osteoblasts) secrete collagen fibrils, subsequently mineralized by small dahllite crystal deposition.
• Type IX and XII collagens are important in organizing collagen fibrils.

Elastin

• Elastin forms elastic fibers in the ECM which conveys tissue elasticity where skin, blood vessels, and lungs must be strong and elastic.
• Five times more stretchable than a rubber band of the same cross-sectional area
• Highly hydrophobic protein rich in glycine and proline
• Dominant ECM protein in arteries
• Many elastin molecules are linked together.

Basal Lamina

• Basal lamina underlies all epithelia and surrounds some nonepithelial cell types.
• Different roles are played in different tissues.
• They provide support to overlying epithelium/endothelium.
• They act as kidney filters, preventing passage of macromolecules from blood to urine.
• They serve as a barrier to cell movement, not allowing fibroblasts but allowing immune/nerve processes.
• Tissue regeneration uses the lamina to scaffold migration of regenerating cells.
• They also guide cells in embryonic development.
• Type IV collagens create a branching network in the basal lamina.
• Laminin a glycoprotein, serves as the main adhesive protein within the basal lamina.

Laminin

• It aids in anchoring cells to the lamina.
• 16 isoforms exist, composed of alpha, beta, and gamma chains.
• Laminin is the primary basal lamina ligand that binds to integrins.

Fibronectin

• Fibronectin, an adhesive glycoprotein, is an ECM protein that helps organize the matrix and attach cells.
• It attaches cells by binding to other ECM components & cell adhesion receptors such as integrins.
• Fibronectin influences cell shape, movement, and cytoskeleton organization.
• Integrins bind to the RGD sequence of fibronectin.
• Each chain contains several functional regions with unique binding specifies.

Fibronectin Fibrils

• Dimeric forms are produced by cells and circulate in blood are soluble.
• Can assemble into fibrils on the surface of cells when they bind to integrins.
• Integrins link to the actin cytoskeleton allowing fibroblast molecules to stretch.
• This exposes sites that allow for assembly of fibronectin molecules into fibrils.

Fibroblasts

• Fibroblasts secrete and exert tension on ECM proteins.
• They do this through Time-lapse confocal microscopy a of fibroblast connective tissues. Type 1 collagen (green), actin filaments (magenta), and DNA (blue).

Genetic Mutations of ECMs and Related Diseases

• Scurvy: from vitamin C deficiency, bleeding gums, bleeding under the skin, joint stiffness, poor wounds.
• Osteogenesis imperfecta: mutations in type I collagen.
• Chondrodysplasias: diseases of cartilaginous tissues due to mutations in type II collagen.
• Ehlers-Danlos syndrome: reduced or faulty type III collagen.
• Alport syndrome: is an inherited disorder the tiny blood vessels in the kidney, mutation in type IV collagen.
• Integrin a6ẞ4 mutations cause epidermolysis bullosa, which blisters of the skin/mucosa.
• Mutations in integrin subunits can also cause the disorder in which platelets fail to form blood clots, Glanzmann's disease.
• The common symptoms of Glanzmann's disease which affects platelet is excessive bleeding.
• Muscular dystrophy results in the increasing weakening and breakdown of skeletal muscles over time, due to a mutation of the protein dystrophin.

Intergins

• Integrins bind most extracellular matrix proteins in animal cells.
• They are transmembrane heterodimers of alpha and beta subunits.
• In humans, there are 8 Beta and 18 Alpha subunits, combining into heterodimers with different functions.
• The B1 are found on the surface of many vertebral cells and mediates cell matrix interactions.
• The B2 subunit in in white blood cells.
• Various subunits bind different ECM molecules like Fibronectin an laminin.

Integrins and ECMs

• Cells express various integrins, which bind to different ECMs.
• See Table 19.3 within the text for Specific Integrin Ligands.

Intergrin Linkage

• Integrins link the ECM to the cytoskeleton through small/transient or large/durable adhesions.
• Two connection types are focal adhesions (linked to actin filaments) and hemidesmosomes (linked to intermediate filaments).

Focal Adhesion

• Migratory cells, such as fibroblasts, attach to extracellular matrix molecules via focal adhesion- Note that focal adhesions are connected to large bundles of actin filaments called stress fibers.

Hemidesmosomes

• Epithelial cells attach to the laminin in the basal lamina via hemidesmosomes.

Integrin Switching

• Cells can crawl through tissue (integrins switch between active and an inactive form.
• The basis for conformational switching switching is controlled by allosteric regulation or signals coming from inside or outside.
• Inside-out signaling: stimuli talin binding causing a change in the confirmation of confirmation.
• Outside-in signaling: drive integrins to switch to an extended conformation, by to the attaching cytoskeleton.

Integrin Activation

• Integrins activated by intracellular signaling.

Intergrin Responses

• Cell-matrix adhesions respond to mechanical forces, this effects migration.

Intergrins and Cell Surface

• Cell attachment to the extracellular matrix via integrins.
• Is important for cell proliferation and survival.