2. Cell Adhesion and Communication
Slides do conjunto (24)
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- 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
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- 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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- 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
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- 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
- Legenda: Electron micrograph- network of thin fibrils where protein embedded (left) 2 plasma membranes of adjacent cells joined together by tight junctions (right)
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- 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
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TJ: Role- regulate paracellular transport
- 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
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TJ: Role- regulate transcellular transport
- 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
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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
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- 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
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Attachment Junctions: Adherens
- Legenda: 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
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- 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
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Attachment junctions: Desmosomes
- Legenda: 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
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Attachment junctions: Desmosomes
- 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)
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Attachment junctions: Desmosomes
- 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
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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
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- 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
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- 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
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- 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 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
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- 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
- Legenda: Regions where PMs are closer together than surrounding areas with gaps bridges by channels projecting out of PM
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- 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)
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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
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- 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