Lecture 23: Cell Junctions and Cell Adhesions Flashcards Preview

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Flashcards in Lecture 23: Cell Junctions and Cell Adhesions Deck (61):
1

How is stress transmitted in epithelial tissue vs connective tissue?

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4 Types of cell junctions in animal cells?

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Anchoring Junctions

– Include cell-cell and cell-matrix interactions 

– Transmit external stress from across the plasma membrane to the underlying cytoskeleton 

4

Occluding Junctions

– Form a permeability barrier between cells in an epithelia sheet 

5

Channel forming Junctions

– Create cytoplasmic continuity between adjacent cells for the passage of small molecules

6

Signal Relay Junctions

– Allows extracellular signals to be passed from one cell to another 

– Some anchoring junctions and channel forming junctions also conduct signaling 

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adherens junctions and desmosomes are ___

cell-cell anchoring junctions

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actin-linked cell matrix junctions and hemidesmosomes are____

cell-matrix anchoring junctions

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tight junctions are ___

occluding junctions

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Gap junctions are ___

channel forming junctions

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tight junction

seals gap between epithelial cells

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adherens junction

connects actin filament bundle in one cell with that in the next cell

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desmosome

connects intermediate filaments in one cell to those in the next cell

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gap junction

allows the passage of small water-soluble molecules from cell to cell

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actin-linked cell-matrix junction

anchors actin filaments in cell to extracellular matrix

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hemidesmosome

anchors intermediate filaments in a cell to extracellular matrix

17

_____ link the cytoskeleton to extracellular structures 

Transmembrane proteins

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Cell-cell interactions are mediated by the ____ family of transmembrane anchors 

cadherin

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Cell-Matrix interactions are mediated by the ____ family of transmembrane anchors 

integrin

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Anchoring Junctions function

• Provide mechanical strength 

– Especially important to tissues encountering severe mechanical stress like heart, muscle, and skin 

• Connect the cytoskeleton to neighboring cells or the extracellular matrix 

21

Anchoring Junctions are composed of what two main classes of proteins?

– Intercellular anchor proteins

– Transmembrane adhesion proteins

22

Types of Anchor Junctions

– Adherens junctions 

– Desmosomes 

– Hemidesmosomes 

– Focal Adhesions 

23

Cadherins 

• Ca2+-dependent cell-cell adhesion

 • Present in adherens junctions and desmosomes 

• Found in virtually all cells and tissues 

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Cadherin superfamily

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Cadherins mediate ____ adhesion 

homophilic

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Cadherin structure and function

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How do cadherins play a role in development?

Cell with similiar subclasses of cadherins expressed on their surface stick together because of homophilic binding

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Cadherins mediate cell-cell adhesion by a homophilic mechanism 

• Tissue culture cells (that normally do not express N or E-cadherins) can be made to express either N- or Ecadherin. 

• The resulting population self selects based on homophilic interaction of the individual cadherins

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Which proteins link cadherins to actin filaments?

  • p-120 catenin
  • Beta-catenin
  • Alpha-catenin
  • vinculin

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Adherens junctions 

• Connect bundles of actin filaments between cells 

• Can form adhesion belts (zonula adherens) in epithelial layers 

• Transmembrane adhesion proteins belong to the cadherin family 

• Intracellular anchoring proteins 

– Catenins 

– Vinculin 

– α-actinin 

• Cells must be attached by adherens junctions for tight junctions to form 

32

Adherens junctions structure 

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Early formation of adherens junctions 

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Mechanotransduction in an adherens junction 

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Uses of adherens junctions in development 

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Desmosomes

• Connect intermediate filaments between cells 

• The type of intermediate filament attached is cell type specific 

– Keratin filaments in most epithelial cells such as skin 

– Desmin filaments in heart muscle 

37

Transmembrane adhesion proteins of desmosomes belong to the cadherin family:

– Desmoglein 

– Desmocollin

38

Intracellular anchoring proteins of desmosomes?

– Plakoglobin (γ-catenin) 

– Desmoplakin 

– Plakophilin

39

Desmosome Model 

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Hemidesmosomes

• “Half desmosomes” that connect intermediate filaments to the basal lamina 

• Morphologically similar to desmosomes Transmembrane adhesion proteins belong to the integrin family 

– Extracellular domain of the integrin binds to laminin (more later) 

• Intracellular anchor protein 

– Plectin

41

Desmosomes and hemidesmosomes link to the ____ network 

intermediate filament

42

____ mediate transient cell-cell adhesions in the bloodstream 

Selectins

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Selectins

– Cell surface, carbohydrate binding proteins (lectins) – Ca2+-dependent cell-cell adhesion 

– L-selectins 

• While blood cells 

– P-selectins 

• Platelets 

– E-selectins 

• Activated endothelial cells 

44

The structure and function of selectins 

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Cell Adhesion Molecules (CAMs) 

• Cell surface transmembrane proteins 

– Cell-cell adhesion 

– Cell-matrix adhesion 

• Some are Ca2+-dependent, while others are Ca2+- independent 

• Initially identified using antibodies to cell surface molecules 

• Antibodies to key CAMs were capable of inhibiting cell-cell adhesion in the test tube 

46

Ig superfamily Cell Adhesion Molecules (CAMs) structure

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Tight Junctions

• Occluding junctions in vertebrates 

• Also known as Zonula occludens (ZO) 

• Provide a barrier role in most epithelia 

• Prevent protein and lipid diffusion in the plasma membrane allowing for polarization 

– Apical vs. basolateral membrane 

• Variable permeability barrier between cells 

– Cell type dependent, bladder epithelial tight junctions are 104 X less permeable to inorganic ions like Na+ than intestinal epithelia

48

Tight junction model 

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49

What are the tight junction proteins?

claudin and occludin

50

Claudin and occludin also bind to peripheral membrane proteins called ___ proteins that anchor the sealing strands to the actin cytoskeleton 

ZO

51

ZO proteins form ____ at tight junctions 

scaffolds

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Septate Junction

Occur in invertebrates like Drosophila Functionally similar to tight junctions Different in structure and appearance 

53

Gap junctions

• Most cells in most tissues are in communication with neighboring cells by gap junctions 

• Made up of membrane-spanning channel-forming proteins called connexins 

• The channels or connexons have a maximal pore size of ~ 1.5nm 

• Allow the passage of small molecules but not proteins 

– Inorganic ions, sugars, amino acids, nucleotides, etc 

54

Anatomy of Gap junctions

• Each connexin monomer has four membranespanning segments 

• Humans have 14 different connexin genes 

• Six monomers assemble into a connexon 

• Different connexin subunit assemblies can produce different connexons 

• Connexons from adjacent cells interact to form an intercellular channel 

55

Gap junction diagram

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Structure of Gap Junction

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Gap junctions have diverse functions 

• Electrical synapse 

– Direct coupling allows the propagation of an action potential from one cell to another without the need for a chemical intermediate (much faster) 

• Sharing of metabolites helps coordinate activities 

– Glycogen breakdown in response to hormonal stimulation in the liver 

– Hepatocytes not directly stimulated by hormone (potentially several cell layers away from a capillary and the source of the hormone) detect a signal by sharing a second messenger through gap junctions 

• Communication during development 

58

Connexin turnover

Connexins are added to the edges and removed from the middle 

59

Regulators of gap junction permeability?

– pH
– Cytosolic Ca2+ concentration 
– Extracellular signals 
• Some neurotransmitters like dopamine 

60

Why regulate gap junction permeability?

– Cells need a way to close down these connection to prevent widespread damage when one participant is damaged 

61

Plasmadesmota

“Plant Gap Junctions” Since plants have a rigid cell wall, a specialized structure is required to bridge it

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