Cell-Cell Anchoring junctions
adherens - junctions that connect actin filament bundle in one cell with that in the next cell
desmosome - connects intermediate filaments
Occluding Junctions
- tight junction seals gap between epithelial cells
- functions together with junctional complex -
- blocks things
Channel-Forming junctions
gap junctions that allows the passage of small water-soluble molecules from cell cell
Cell-Matrix Anchoring Junctions
actin-linked cell-matrix adhesion - anchors actin filaments in cell to ECM
hemidesmosome - anchors intermediate filaments in a cell to ECM
The Oder of the junctional complex
Occluding junction
Cell-Cell anchoring junction
Channel-Forming Junction
Cell-Matrix Anchoring junction
Apical
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Basal
Epithelial Tissue
- cells directly connected with lines cavities, surfaces, surrounds organs
- minimal ECM beneath
- mechanical stresses transmitted from cell to cell by cytoskeletal filaments anchored to cell-matrix and cell-cell adhesion sites
Connective Tissue
- ECM Bears mechanical stress directly of tension and compression
- cells dispersed when ECM which provides overall structure
Adherens junction structure
- form strong continuous adhesion belts critical for adheren cells to form epithelia
- cadherin clusters mediate the adhesion through:
1. homophillic interaction between cadherin receptors
2. links to the actin cytoskeleton - calcium keeps the connection rigid
Adherens junction function: tissue maintenance during development
- cells move, but the movement bust be coordinated
- tissue is lost in adherens junction mutants: if there are no adherens junctions to connect the cells together, then they will not stick to each other
Adhesion junction function: tumour suppression
- cadherins are tumour suppressors
- in a normal epithelium, normal cell occurs close to the basal lamina
in high grade intraepithelial neoplasia: cell division everywhere and so cadherins are lost
Importance of epithelial structure
- distinct apical surface: organ lumen or the animal surface
the basal surface: underlying tissue
epithelial polarity is critical for organ function
Epithelial polarity and control of solute diffusion between body compartments: control of glucose transport
- glucose blocked from DIFFUSING between cells by tight junctions
- must be actively transported through cells by PLASMA MEMBRANE CHANNELS
- low to high using Na+ glucose symport
- positioning of channels is determined by the SNARES
Testing the permeability barrier across an epithelia
- added dye apically: stays on apical side
added it basally: stays basal side
without tight junctions, you would get the mixing between the two
Tight junctions encircle the apical end of each cell in an epithelial sheet
- tight junctions closest to the apical side
Tight junctions are formed from strands of interacting transmembrane proteins
Core tight junction proteins: Claudin: - 4-pass transmembrane protein - essential for --> tight junction formation occludin: - 4-pass transmembrane receptor - required for barrier function not needed for maintaining overall tight junction structure
tight junctions function as
gates: prevent molecular movement across the epithelial sheet in the ECS bw cells
fences: prevent molecular movement between the apical and basolateral domains of: each cell’s plasma membrane
Cell polarity is fundamental to Cell and Developmental Biology
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How is cell polarity established?
How does asymmetry arise to make one end of the cell different from the other?
- use of landmarks to establish and elaborate polarity
- signal will cause landmarks to separate to different poles (figuratively)
Chemoattractants
- polarize cells and the cells chase prey
- chemoattractant binds to the receptor
activates Gi and G12/13 - Gi activates PIP3, which activates Rac, which activates actin polymerization, which inhibits actin myosin contraction, which inhibits PIP3
- G12/13 (front0 activates Rho, which activates actin-myosin contraction, which inhibits actin polymerization
In C. elegans, sperm entry provides a landmark for polarizing the one-cell embryo
- cytoskeletal flow goes from posterior (+) to anterior (-)
- sperm entry is on posterior
- anterior cues that have proteins that say to make this “anterior”
- anterior cues and posterior cues inhibit each other
- polarity generates two cell lineages after the cell divides
Adherin junctions (AJs) are important landmarks for epithelial polarity
- cell is all alone, and adherens junctions are looking for another cell to connect to…(once this connects to an adjacent cell, then it goes through the half mesenchymal-to-epithelial)
- there are apical protein cues that make one side apical, and prevent it from being basal
Conserved apical and basal cues controlling epithelial polarity
adherin junction links up, scribble complex comes on
Crumbs —> Par3-Park6-aPKC (tight/adhesion junction) —> scribble (adheren/septate
- whole point is that it helps elaborate pathway
The integration of polarity complexes, adhesion complexes, cytoskeletal networks and trafficking routes is critical for the structure and function of epthelia and that forms our organs
complexes -> functional epithelium –> body compartments constructed from functional epithelia
