2. Cell Adhesion and Communication

Learning Outcomes

Describe with examples the structure and function of different classes of cell-cell and cell-matrix interactions Give an overview of how cell adhesion molecules can interact with components of the cytoskeleton Give an overview of the role integrins play in cell signalling events

Epithelial Tissue

Covers the internal and external surface of organs  Contains a layer(s) of cells on top of basement membrane Cells adhere to each other laterally as well as to basement membrane 

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Cell adhesion overview

Lateral Cell-Cell Adhesion established via: Tight junctions (junctions are specialised modifications of cell surface in neighbouring cells) Adherens junctions  Desmosomes Gap junctions Non-junctional adhesions (proteins such as cadherins, selectins, integrins) Cell-Matrix Adhesion established via:  Hemidesmosomes Focal adhesions Non-junctional adhesions (integrins) 3 distinct classes of cell adhesion junctions: Occluding (tight) junctions= forms a barrier between epithelial cells  Attachment junctions= links cell cytoskeleton to that of its neighbouring cell or to matrix  Communication junctions= directly connect cytoplasm of adjacent cells 

Tight junctions: Structure

Structure: Proteins Seal adjacent epithelial cells in a narrow band beneath their apical surface Bands made up of interconnected (sealing) strands of transmembrane proteins that wrap around the cell 3 types of transmembrane proteins: Claudins = core proteins in tight junction fibrils Occludins = position and stabilise claudins Junctional adhesion molecules (JAMs) = position and stabilise claudins Proteins in the sealing strands attach stably to structural proteins to interconnect the sealing strands, also transiently to signalling proteins

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Caption: : Electron micrograph- network of thin fibrils where protein embedded (left) 2 plasma membranes of adjacent cells joined together by tight junctions (right)

Tight junctions: Selectivity

Tight junctions allow the passage of small ions, solutes, electric currents between epithelial layers Differing junctions allow passage of differing solutes through Dependent on size (4-40A) and charge, different tissues have different selectivities)  Selectivity determined from different claudins- humans have 24  Eg expression of claudin 16 restricted to ascending limb of loop of Henle, mutations associated with  conditions caused by Mg and Ca ions unable to pass through tight junctions

TJ: Role- regulate paracellular transport

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Transfer substances across epithelium by passing through intracellular space between cells Act as molecular sieves rather than completely impermeable ("occluding") Property of TJs which regulates this process= selective permeability 

TJ: Role- regulate transcellular transport

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Transfer substances across epithelium by passing through cells, crossing apical and basolateral membranes  Allows substances against concentration gradients eg Glu absorption Property of TJs which regulates this process=separation of apical and basolateral membrane

Transcellular transport: Glucose absorption

If absorption of glucose across intestinal epithelium relied on paracellular transport we would not gain all the glucose from our diet:  Sodium-potassium pump ATPase in basolateral membrane generates Na gradient by pumping Na out of the cell against conc gradient  Low conc of Na in cell compared to outside  Energy stored in gradient is used to bring glucose into cell from lumen of SI via Na/Glu symporter High conc Glu in cell compared to bloodstream Glu enters bloodstream via facilitated diffusion through Glu trasnporter (GLUT2 in baolateral membrane) Tight junctions play a role in maintaining separate identity of 2 membranes and membrane proteins cant cross the barrier they create:  Essential that ATPase and GLUT2 are present on basolateral membrane and not apical  Essential that Na/Glu symporter is on apical and not basololateral 

Tight Junctions: Summary

Formed by interconnected sealing strands of claudin, occludin and junctional adhesion molecules that encircle the cell  Act as molecular filters thus regulating paracellular transport across epithelial tissue  Maintain identity of apical and basolateral membranes by preventing diffusion of proteins and lipids between the two 

Attachment Junctions: Adherens

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Caption: : Electron micrograph- thick, dark bands close to PM of neighbouring cells, bridged by rod-like structures projecting into extracellular space

Serve as bridges: link actin filaments (through catenin) of adjacent cells  Zonula adherens: found immediately beneath tight junctions in epithelial tissues Other types found joining cell types   

All adherens junctions have 2 common properties: Contain cadherins: TM proteins that bind to identical cadherins on neighbouring cells Role in determining tissue organisation- enable cells to find appropriate binding partner and determine junction strength Link to actin cytoskeleton: Regulate changes in cell shape and/or to sheer stress (important in cytoskeleton dynamics) Through catenins (role in signalling : beta-catenin can act as TF)  Catenins link cadherins to MFs  eg link cytoplasmic tails of cadherins to MF in zonula adherens, mutations in catenins lead to cancer

Attachment junctions: Desmosomes

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Caption: : Electron micrograph- thick patches of electron-dense material on cytoplasmic side of PM, connected to int filaments in cytosol

Also add structural integrity to tissues by linking cytoskeleton of adjacent cells Through intermediate filaments  

2 common properties: Contain cadherins (desmocollins and desmogleins) that link adj cells together: Cadherins bridge intracellular space on one side of membrane and serve as docking sites for cytosolic proteins (desmoplakin) that bind int filaments on the other side Link to int filaments through adaptor proteins( plagoglobulins/desmoplakin): Attachment to filaments through adaptors regulated by cell signalling Can be initiated by activated growth factor for receptors (coordination of cell growth and adhesion)

Attachment junctions: Desmosomes

Attachment junctions: Desmosomes

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Electron micrograph of desmosome between 2 keratinocytes: Shows intermediate filaments linking to cytoplasmic plakoglobulin and desmoplakin that, in turn link to TM cadherins desmoglein and desmocolin Formation of bridge between plasma membrane of neighbouring cells 

Attachment junctions: Hemidesmosomes

Located on basal surface of epithelial cells  Providing structural stability to epithelial sheets  Anchor cells to underlying basement membrane through cytoplasmic 'plaques' that connect to intermediate filaments Plaques contain integrins (TM protein) which connect to int filaments through adaptor protein plectin  Extracellular region of integrins binds to components of basement membrane

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Arrangement of hemi- and desmosomes

Organises intermediate filaments into robust network  Mutations that disrupt desmosomes and/or hemidesmosomes -> severe blistering diseases Hemi and desmosomes both link intermediate filaments but are structurally very different

Integrins: general

Heterodimers receptors composed of alpha and beta subunits: 18 different alpha, 8 beta, 24 known different heterodimers Bind to ECM components: Different heteromers bind different ECM components- different cell types express different integrins so binding of a cell to its ECM is determines by the integrins present on its basolateral membrane Specificity of which ECM component an integrin binds is determined by integrin alpha subunits  Complexity: Most heterodimers can bind more than one ECM component  and each component binds more than one integrin All known binding sites on ECM proteins contain acidic amino acid (asp) and many contain sequence : Arg-Gly-Asp Also bind to components of focal adhesions- linking actin cytoskeleton to ECM      

Changes within the cell affect affinity of the integrin for its ligands Activation of adaptor proteins (eg talins) -> link integrins to actin cytoskelton focal adhesions: Integrin doesnt bind to actin cytoskleton or ECM until talin binds  Signalling via GRCP or RTK activates talin and induces dimerisation Talin binds to beta-intgrin subunit which causes conformational change in integrin extracellular binding domain, enabling ECM binding  On binding, talin also links actin filaments  Inside-out signalling: signalling pathways inside cell affect role of integrins outside

Integrins: Signalling

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Integrins: Signalling

Outside- in signalling:  transduce information outside of the cell to the inside Binding of cell to underlying matrix can cluster integrins together Makes their cytosolic domains form a docking site for signalling molecules  Integrin signalling pathways regulate many cell physiologies: Cell differentiation Inhibition of programmed cell death  Cytoskeletal rearrangements: importance in cell migration across the ECM and not sticking to it  eg during wound-healing focal adhesions promote remodelling of actin cytoskeleton to drive leading edge forward       

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Trafficking of integrins

Trafficking of integrins used to regulate cell movement: formation/disassembly of focal adhesions Endocytosis of integrins: Can break attachment to substrate  Internalised integrin can be recycled and establish new attachments in direction of movement Or integrin can be trafficked to the lysosome and degraded causing the cell to loose attachment  = dysregulation in cancers     

Communication junctions (Gap)

Gap junctions facilitate direct transfer of ions and small molecules between adjacent cells  Found in most vertebrate and invertebrate cells and contain upto thousands of channels- can form large or small junctions  Only known means of direct cell-to-cell transport in animals

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Caption: : Regions where PMs are closer together than surrounding areas with gaps bridges by channels projecting out of PM

Gap junctions

Gap junction channels consist of 2 halves (hemichannel or connexons): 2 connexons on opposing  membranes of adjacent cells dock togther to form gap junction  In intracellular gap creates 2-4nm pore for ions/small molecules to pass through Subunit of connexon= connexin 6 connexins form hetrameric connexon = channel cluster Connexin subunit = tetraspan, 4 TM alpha helices  Gap junction channels have different permeability depending on connexins present: Homomeric/homotypic, heteromeric, heterotypic  Transport: inorganic ions, sugars, amino acids, nucleotides, vitamins, cAMP and IP3, NOT macromolecules (proteins, NA, polysaccharides)

Gap junctions: Function and Regulation

Function: Used for rapid communication Reflexes reactions: in brain mediated by neurons linking gap junctions, allowing potential to be spread rapidly, avoiding delay at chemical synapses Synchronisation of cardiomycetes signal to contract: communicated through gap junction, mutations in connexins lead to arrythmia  Regulation Alternate between open and closed states  Closed- through phosphorylation of connexins when large increase in intracellular Ca ion conc and/or pH  Eg cell damage causes elevation of Ca in cell and releases damaging metabolites  Closing prevents further damage to neighbouring cells 

Summary

Connections between cells can be established via: Tight junctions Adherens junctions Desmosomes Gap Junctions Synapses Connections between cell & their underlying matrix can be established via: Hemidesmosomes Focal adhesions Non junctional adhesions