cellular interactions

chapter 20

tissues• cellular interaction

• in some organisms, cells interact to form defined tissues

• extracellular matrix• allows for cellular

interaction• extremely important in

certain tissues• connective tissue• epithelial tissue

epid

erm

isde

rmis

extracellular matrix• ECM -

• present in pretty much all animals• evolved in some ancestor of the animals

• sponges• mesohyl • basic ECM elements

• type IV collagen• spongin

• laminins• what about plants?

• special ECM• cell wall matrix

ECM fibers• collagens

• fibrous glycoproteins• most abundant protein

in humans• very strong• structure

• triple helix• arranged in

staggered array• overlap 25%

3 helical polypeptides (sinistral)

collagen molecule (dextral triple helix)

overlapping collagen fibril

mature collagen fiber

ECM fibers• collagens

• types I-XIX• I-III most common in humans• IV - not fibrillar

• spongin matrix in sponges• cuticle of nemata• bone• cartilage• type III - reticular fibers

• branching network

collagen problems• collagen-related diseases

• type I (osteogenesis imperfecta)

• type II (dwarfism)• type IV

• Alport syndrome• kidney disease

(glomerular nephritis)• blindness (ocular lesions)• hearing loss

• Ehlers-Danlos syndrome• hyperflexibility / extensible

skin

ECM fibers• elastic fibers

• elastin• cross-linked into network

• provide elastic strength• important for organs that stretch

elastic fiber

fibronectin• fibronectin structure

• glycoprotein found in nearly all animal connective tissue • two nearly identical polypeptides• can bind to cells and ECM

COOH

COOH

H2N

H2NFibrin-binding domains

heparin-binding domain

cell-binding domain

collagen-binding domain

S-S S-S

cell interaction• integrins

• receptors with α and β subunits• each with several domains• many possible

configurations of 26 known subunits

• contain relatively short transmembrane domain

• conformations• inactive - bent• active - unbent / legs apart• can adopt many different

active configurations

α subunitβ subunit

β-propellerβ-I domain

αI domain

interaction with cells• integrins

• cell-binding sites in extracellular proteins• RGD - (arginine-glycine-aspartic acid) recognized• integrin binds with RGD sequence

• two main functions• adhesion of cells to substrate• transmission of signals from external environment

• β subunits• three main classes: β1, β2, β3

• clotting• fibrinogen (RGD sequence) binds to integrin• clot busters - competitive inhibitors

proteoglycan complexes• proteoglycans

• core protein• GAGs (glycosaminoglycans)

• repeating disaccharide structure• cushion cells, help resist crushing

laminin• laminins

• glycoproteins - triple helix (like collagens)• many diverse forms (little homology)• form web-like networks

• resistant to tensile force

basement membrane• basement membrane

• basal lamina• flattened layer of ECM

• ECM secreted by fibroblasts• reticular layer

• lamina reticularis - reticular fibersepithelial cellshemidesmosome

basal lamina

reticular lamina

basement membrane• basal lamina

• 4 main functions• structural foundation for

epithelium• selective membrane• facilitate access to proteins• serve as guide for

developing neurons

basal lamina• Collagen IV • Laminins• Nidogens (entactins)• Perlecan

(proteoglycan)

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. Fre

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o. summary

collagens - (glycoprotein) - dextral triple helix of 3 sinistral

alpha helices - high tensile strength - lots of kindselastic fibers - cross-linked networks of elastin

and fibrillin - allow organs to stretch and recoilproteoglycans - - huge protein-GAG complexes - serve as cushioning - form hydrated gelsfibronectin - (glycoprotein) - two identical polypeptides -

homodimer - bind to integrins - can bind to cells and to ECM fibers - important in migration of cells

- anchors cells to proteoglycans or collagen

laminins - (glycoprotein) - triple helix of 3 linked polypeptides - high tensile strength - along with collagen IV, comprises

basis for basal laminaentactin - (glycoprotein) - connects collagen and laminin

networks - web of collagen IV and laminins

connected by entactinintegrins - - heterodimers - attached to PM - bind to various ECM elements - act as cell receptors

cell-ECM junctions• hemidesmosomes

• different class - β4

• link to intermediate filaments• dense plectin plaque

• linked to ECM by integrins • can be disassembled and

reassembled

cell-cell junctions• junction complex

• found on lateral aspect of plasma membranes

• comprise several types of junctions• tight junctions

• function• paracellular transport• creates membrane polarity:

apical, basal• membrane domains• prevent movement of proteins

between domains

cell-cell junctions• tight junctions

• complex collections of proteins• transmembrane proteins

• claudins• occludins• junctional adhesion molecule (JAM)

• polarity proteins• cytoskeletal proteins• signaling proteins

cell-cell junctions• adherens junctions

• link adjacent cells• located just basal to tight

junctions• transmembrane receptor

proteins• cadherin dimers

• bind to identical cadherins on neighboring cells

• anchor proteins (catenins)• link to actin bundles

cell-cell junctions• septate junctions

• only in invertebrates• similar in function to tight junctions• located basal to adherens junctions

cell-cell junctions• desmosomes

• cytoplasmic plaques• inner dense plaque• outer dense plaque• connected to

intermediate filaments• main function:

• link cells together• important for cells

exposed to physical stress

cell-cell junctions• junction complexes

• tight junctions• adherens junctions• desmosome• hemidesmosome• (gap junction)

cell-cell communication

hydrophilic channel

cell membrane

cell membrane

cytoplasm

cytoplasm

connexon• gap junctions• analagous to

plasmodesmata• vertebrates

• connexon (2)• connexins (6)

• invertebrates• innexon (2)

• innexins (6)

cell-cell communication• plasmodesmata

• intercellular cytoplasmic channels • desmotubule

• trapped portions of ER• passage of materials limited

• size constraints• interconnect protoplasts

• symplast cell wall

plasma membrane

Cell wall

Microfibril

Cellulose microfibrils in a plant cell wall

Cellulose molecules

β Glucose monomer

10 μm

0.5 μm

plant cell walls • comparative cell wall composition

• for bacteria - mainly peptidoglycan• for fungi - chitin• for plants cellulose

• cell wall matrix• microfibrils• proteins• pectin• hemicelluloses

cell wall matrix• microfibrils

• arranged in radial arrangement• restrict cell expansion

• cell elongation• influx of water

• increases cell volume• added by cellulose synthase

Cellulose microfibrils

Nucleus

Elongation

Vacuoles

5 µm

microtubule

cellulose synthase complex

microfibril

cell wall matrix• composition of cell wall

matrix• hemicellulose

• highly branched• crosslinked to microfibrils

• pectin• negatively charged

galacturonic acids• form a gel

• microfibrils• glycoproteins

pectin

cellulose microfibril

hemicellulose

soluble protein

• layers• middle lamella (outer layer of pectin)• primary cell wall

• more flexible, laid down first • secondary cell wall

cell wall matrix

middle lamella

plasma membrane

primary cell wall

cell wall matrix• lignin

• used to stiffen and strengthen cell walls• 2nd most abundant organic polymer• hydrophobic polymer

• p-coumaryl alcohol • coniferyl alcohol• sinapyl alcohol