Chapter 19: Cell junctions and the extracellular matrix

Know the terminology:Cadherins, integrins, tight junction, desmosome, adhesion belt, hemidesmosome, gap junction, connexin/connexan,

Basal lamina, connective tissue, fibroblast, proteoglycan, GAG (glycosaminoglycan), hyaluronin, aggrecan, collagen, fibronectin,

OverviewMulticellular organisms combine cells into tissues.

Extracellular matrix is a complex network of macromolecules (proteins, glycosaminoglycans, proteoglycans) secreted by cells

Specialized ECMs include connective tissue, basal lamina, exoskeletons, cartilage/bones,

Tissues are formed from cell-cell connections and cell-matrix connections.

Tissue composition

OutlineHow are cells organized into tissues?

I. Cell junctions-types of cell junctions-tight junctions, adherans junctions, desmosomes, focal adhesions, hemidesmosomes, gap junctions

II. Extracellular matrix proteins-types of ECM macromolecules-synthesis and properties of hyaluronan, aggrecan, collagen, fibronectin

Cell junctions

Three types of cell junctions:1. Occluding junctions: seal cells together into sheets (forming an impermeable barrier)

2. Anchoring junctions: attach cells (and their cytoskeleton) to other cells or extracellular matrix (providing mechanical support)

3. Communicating junctions: allow exchange of chemical/electrical information between cells

Example: Tight junctions of intestinal epithelium

Occluding junctions

Tight junctionEach cell possesses integral membrane proteins that bind to similar proteins in the adjacent, forming a continuous “weld”

Anchoring junctionsIntegral membrane proteins connect a cell’s cytoskeleton to another cell or extracellular matrix

Anchoring junctionsIntegral membrane proteins connect a cell’s cytoskeleton to another cell or extracellular matrix

Anchoring junctionsCytoskeletal fibers (MF, intermediate filaments)

connect to a

Membrane protein receptor

which attaches to another protein in either:-the extracellular matrix

or-another cell membrane

Cadherins and desmosomesCell to cell connectionsare mediated by cadherins.

These receptors extend out from the cell, binding to other cadherens

Cadherins participiate in adherens junctions

Under the cell membrane, contractile fibers of microfilaments connect to cell membrane proteins called cadherins

They surround the cell, forming a belt

DesmosomesCadherins can also form localized spot connections

Cadherins attach to intermediate filaments via anchoring proteins: a desmosome

Cells-to-ECM attachments: Focal adhesions and hemidesmosomes

Cytoskeletal fibers attach to transmembrane receptors (integrins) that are attached to extracellular matrix components•Focal adhesions use MF•Hemidesmosomes use IF

Gap junctions

Gap junctions allow cells to exchange electrical and/or chemical signals

Composed of proteins that form channels that allow small molecules to pass.

Subunits of these channels are connexins that are assembled together to make connexons. The connexons from 2 cells join together to make a gap junction.

Gap junctions

Regulation of connectivity

When might a cell want to alter its connections to other cells?

How do cells alter these connections?-alter the profile of cytoskeletal connections, receptors, and extracellular matrix-alter the binding affinity of receptors

-many are Ca2+ dependent-many are affected by protein kinases

Summary

Summary

Extracellular matrix

•A network of proteins, glycosoaminoglycans (GAGs) and combinations of the two (proteoglycan) found in the extracellular space•All ECM components are secreted by cells •Most cells can secrete elements of the ECM but many ECMs are built by fibroblasts

Examples of ECM:-connective tissue, basal lamina, cartilage, bone, plant/fungi cell walls, myelin sheath,

Basal lamina

Connective tissue

Connective tissue

Components of the extracellular matrix

Proteins/glycoproteins (secreted by co-translational import) differ in physical properties (e.g. size, flexibility) and are able to bind to different combinations of macromolecules on cell membranes (e.g. integrins, cadherins) and other ECM elements

Examples include:-collagen, elastin, fibronectin, laminin

Components of the extracellular matrix

Proteins can provide elastic properties in many tissues (e.g. elastin)

Components of the extracellular matrixGAGs differ in physical properties (e.g. size, flexibility, hydration).

Composed of repeating sugar + amino sugar units (e.g. N-acetylglucosamine, N-acetylgalactosamine)

They occur in long strings, often attached to proteins

Examples include:-hyaluronan, chondroitin sulfate, heparan sulfate, keratan sulfate

GAGs

GAGs attract a great deal of water. Hydroxyl groups form hydrogen bonds, and the many negative charges attract clouds of cations (Na+) that induce an osmotic movement of water. These hydrated gels resist compression (useful for joints).

Components of the extracellular matrixProteoglycans (made of both proteins and GAGs) also differ in physical properties

Synthesized in Golgi prior to secretion

In addition to structural roles, proteoglycans can also bind hormones (e.g., inflammatory chemokines, FGF, TGFβ) to alter cell signaling pathways

Examples include:-decoran, aggrecan (the main component of cartilage)

ECM proteins, GAGs, and proteoglycans

Green-proteinRed-GAG

HyaluronanVery simple GAG, consisting of 10,000+ repeats of glucuronic acid and N-acetylglucosamine

Spun directly from cell membranes by a surface enzyme complex

Attracts water and fills spaces between cells with non-compressible gel (found around joints)

Some cells secrete it to isolate themselves from other cells (e.g. myoblasts). These cells can secrete hyaluronidase to break it down, allowing contact

Hyaluronan

Very long macromolecule that hydrates and fills enormous volumes

Aggrecan

One of the largest macromolecules, consisting of a core protein with GAGs attached to form a feather-like appearance.

AggrecanAn aggrecan core protein is very large but also binds many (different) GAGs (shown in red)

Aggrecan aggregates

Each aggrecan can be attached to a hyaluronan backbone to form an aggrecan aggregate

CollagenAll multicellular animals possess collagens, often with many different collagen genes

Synthesized as pro-collagen monomers (pro−αcollagen)

Peptide modified by hydroxylation and glycosylation

Prior to secretion they self- assemble into trimers

Upon secretion the trimers are processed by proteolytic enzymes then assemble into fibrils

Collagen fibers

Collagen proteins (trimers) are then cross-linked to form collagen fibers (stiff, not very elastic)

Fibronectin

Animals have a single fibronectin gene that can be alternatively spiced into 40+ forms.

Different exons are able to bind different proteins/GAGs (e.g. integrins, collagen, etc)

Fibronectin dimerizes using 2 similar (not identical) monomers

Fibronectin

Control of ECM

Cells control the synthesis of ECM by altering gene expression and co-translational import and secretion

They also control the degradation of the ECM by secreting and activating/inactivating extracellular enzymes.

Most important are a group of serine proteases called matrix metalloproteinases

MMPs

Cells that need to migrate must first break down the connections to the ECM (e.g. tissue repair, metastasis of tumors)

MMPs can be:-secreted in active form (collagenase)-secreted as inactive form (e.g. plasminogen is inactive until it is modified by “plasminogen activators)-activated when cells stop secreting TIMPs (tissue inhibitors of metalloproteinases)