Lecture 20: Cell Junctions

Question 1

Question 2

What are the two main tight junction proteins

Answer 2

• Claudin (The main functional component)
• Occludin

Question 3

Each sealing strand of tight junction is composed of a long row of ___ transmembrane adhesion proteins embedded in each of the two interacting plasma membranes

Answer 3

4

Question 4

Functions of tight junctions

Answer 4

• Barrier function
  
  • the extracellular portions of tight-junction strands function as a zipock with diversified permeability
• Fence funciton
  
  • within plasma membranes, the strand encircle the top of individual cells to delineate the border between the apical and basolateral membrane domains, thus functions as a ‘fence’ to limit the lateral diffusion of lipids and proteins between the apical and basolateral membrane domains
• Signaling function
  
  • The cytoplasmic portions serve a magnetic bar attracting various PDZ-containing proteins
• Establish Cell polarity
  
  • Three conserved polarity proteins complexes control polarization processes and maintain polarity
    
    • Par (partitioning defective), Crumbs, and Scribble

Question 5

A connexon may be made up of the same (_____) or different (_____) subunits

Answer 5

Same (homohexameric)

Different (heterohexameric)

Question 6

What are the components of the Basal lamina (basement membrane)

Answer 6

• Lamina usually contains
• Laminin, type IV collagen, nidogen (all are glycoproteins), along with proteoglycan perlecan
• Lamina consists of two main classes of extracellular macromolecules
  
  • Fibrous protein
    
    • usually glycoproteins
  • Glycosaminoglycan (GAG), a polysaccharide chain (usually covalently linked to a specific core protein to form proteoglycans)

Question 7

How do cells form a massive and strong structure

Answer 7

• use the strength of the extracellular matrix
• use the strength of the cytoskeleton inside the cells and on cell-cell adhesions that tie the cytoskeleton of neighboring cells together

Question 8

Question 9

what play a central role in anchoring junctions

Answer 9

• Transmembrane adhesion proteins
  
  • span the membrane, with one end linking to cytoskeleton inside the cell and the other end linking to other structures outside the cell

Question 10

• What are the two families of transmembrane adhesion proteins

Answer 10

• 2 families are based on the external attachments
  
  • Cadherin:
    
    • Mediating cell-cell connection
  • Integrin:
    
    • Mediating attachment of cells to matrix
• Two other adhesion superfamilies of cell-cell adhesion molecules are selectin and immunoglobulin (Ig)-superfamily

Question 11

• Classical Cadherins cell junciton associations

Answer 11

adherens junctions, synapses

Question 12

Desmosomal cadherin cell junction associations

Answer 12

• Desmosomes

Question 13

• Adherens junciton
  
  • transmembrane adhesion protein
  • intracellular cytoskeletal attachment
  • Intracellular anchor proteins

Answer 13

• Transmembrane adhesion protein
  
  • Cadherin (classical cadherin)
• Intracellular cytoskeletal attachment
  
  • actin filaments
• Intracellular anchor proteins
  
  • alpha-catenin
  • beta-catenin
  • plakoglobin (gamma-catenin)
  • p120-catenin
  • vinculin
  • alpha-actinin

Question 14

• Desmosome
  
  • Transmembrane adhesion protein
  • Intracellular cytoskeletal attachment
  • Intracellular anchor proteins

Answer 14

• Transmembrane adhesion protein
  
  • Cadherin (desmoglein, desmocollin)
• Intracellular cytoskeletal attachment
  
  • Intermediate filaments
• Intracellular anchor proteins
  
  • Plakoglobin (gamma-catenin)
  • plakophilin
  • desmoplakin

Question 15

Classical cadherins

Answer 15

• Are the main mediators of Calcium dependent adherens junctions
  
  • form adherens juncitons
  • E-Cadherin
    
    • epithelial cells and brain
  • N-cadherin
    
    • Muscle,nerve, lens cells and fibroblast
  • VE-cadherin
    
    • Endothelial cell
  • P-cadherin
    
    • Placenta and epidermis

Question 16

Nonclassical cadherins

Answer 16

• include desmocollins and desmogleins that form desmosome junctions, and the recently discovered large subfamilyof protocadherins, which are implicated in neuronal plasticity

Question 17

• The extracellular potions and intracellular portions of the cadherin superfamily

Answer 17

• Extracellular
  
  • contain multiple copies of the cadherin domain motif
• Intracellular
  
  • are varied, thus, be able to interact with differnt intracellular ligands and components that anchor the cadherin to the cytoskeleton

Question 18

Is binding between cadherins usually homphilic or heterophilic

Answer 18

• homophilic
  
  • cadherin molecules of a specific subtype on one cell bind to other molecules of the same kind on adjacent cells

Question 19

Role of calcium in Cadherin-mediated Adhesion

Answer 19

• Extracellular portion of cadherin consists of several copies of a motif called cadherin domain
• Each cadherin domain forms a rigid unit, joined by the next cadherin domain by a hinge
• Ca²⁺ ions bind to sites near each hinge and prevent it form flexing, so that the whole string of cadherin domains behaves as rigid rod
• when Calcium is removed, the hinges can flex, and the structure becomes floppy. At the same time, the conformaiton at the N terminus changes, weakening the binding affinity and disrupting the adhesion
  
  • Cadherins then are degraded

Question 20

Characteristics of cadherin-mediated adhesion

Answer 20

• typically bind to partners with low affinity
• strong attachements result from the formation of many weak bonds in parallel
  
  • functioning like Velcro to hold cells together
• clustered side-by-side on the same cell by interactions between their N-terminal head regions, and via their attachments of their intracellular tails to other anchoring proteins

Question 21

what is sorting out in terms of Cadherin-mediated adhesion

Answer 21

• homophilic adhesions mediate highly selective recognition, enabling cells of a similar type to stick together and to stay segregated from other types of cells

Question 22

Explain how changing patterns of expressionof cadherins determines the selective assortments of cells

Answer 22

• Appearance and disappearance of cadherins correlate with major morphogenetic events in which tissues segregate from one another

Question 23

Adherens junctions

Answer 23

• Essenetial part of machinery for modeling the shape of multicellular structures
• Primarily present in epithelia
• Form continuous adhesion belt (or zonula adherens) close beneath the apical face of the epithelium, encircling each of the interacting cells in the sheet
• Within each cell, a contractile bundle of actin filaments lies adjacent to the adhesion belt, oriented parallel to the plasma membrane and tehtered to it by the cadherins and their associated intracellular anchor proteins

Question 24

Strucutre of Desmosome

Answer 24

• On the cytoplasmic surface is a dense plaque composed of different anchor proteins
• a bundle of keratin intermediate filaments is attached ot the surface of each plaque
• Proteins of cadherin family bind to the plaques and interact through their extracellular domains to hodl the adjacent membranes together by a calcium-dependent mechanism

Question 25

Molecular model of the Desmosome

Answer 25

* The desmosomal cadherins **desmoglein** and **desmocollin**undergo homophilic and heterophilic binding * Cytoplasmic tails of desmoglein and desmocollin bind plakoglobin (gamma-catenin) and plakophilin, which in turn bind to desmoplakin * Desmoplakin binds to the sides of intermediate filaments, thereby tying the desmosome to these filaments * ODP: Outer dense plaque * IDP: inner dense plaque

Question 26

Desmosomes are required in tissue with \_\_\_\_\_\_\_

Answer 26

* high mechanical stress * Missense mutations in Desmocollin-2 associated with arrhythmogenic right ventricular cardiomyopathy (ARVC) * common cause of sudden cardiac death in the young * Desmocollin not properly inserted into membrane

Question 27

The particular type of intermediate filaments attached to the desmosomes depends on the cell type: ____ filaments in most epithelial cells

Answer 27

keratin

Question 28

What are the 4 families of transmembrane adhesion proteins

Answer 28

* Cadherin * Integrins * Selectins * Immunoglobulin-superfamily

Question 29

Tight junctions

Answer 29

* Are the occluding juncitons found in epithelia and endothelial sheet * form a seal between cells * block the backflow of nutrient molecules from the basal side of the epithelium into the gut lumen * Form a fence between membrane domains * confine the transport proteins to their appropriate membrane domains * This segregation permits a vectorial transfer of nutrients across the epithelium form the gut lumen to the blood * Recruit cytoskeleton as well as signaling molecules at their cytoplasmic side

Question 30

Each sealing tstrand of tight junction is composed of a long row of ____ pass transmembrane adhesion proteins embedded in each of the two interacting plasma membranes

Answer 30

* 4 * Two main tight junction proteins * Claudin (the main functional component) * Occludin

Question 31

Transmembrane proteins of Tight Junctions

Answer 31

* Claudin is the main transmembrane protein in tight junctions * The claudin protein family has many members, which are expressed in different combinations in different epithelia * Occludin is the second major transmembrane protein * Junctional adhesion molecules (JAMs)

Question 32

Junctional Complex in mature Epithelium

Answer 32

* Includes * Tight junction * Adherens junction * Desmosome * The positioning and organization of tight junctions in relation to these other structures depends on association with intracellular scaffold protein of the Tip (tight junction protein) family, also called ZO protein * ZO anchors tight junctions to actin cytoskeleton

Question 33

Functions of tight juncitons

Answer 33

* barrier functions * Fence funcitons * delineat border between apical and basolateral membrane domains * Signaling functions * Establish cell polarity

Question 34

What are three conserved polarity protein complexes

Answer 34

* par (partitioning defective) * Crumbs * Scribble

Question 35

Gap juncitons will allow things smaller than ___ to travel through them

Answer 35

1000 daltons/MW

Question 36

Components of Basal lamina

Answer 36

* Laminin * Type IV collagen * Nidogen (all are glycoproteins) * Proteoglycan Perlecan

Question 37

\_\_\_\_ and ___ serve as linker to connect the laminin and type IV collagen networks once the laminin is in place

Answer 37

Nidogen and perlecan

Question 38

Transmembrane receptor ____ and ___ organize basal lamina assembly

Answer 38

Integrin and dystroglycan

Question 39

The intracellular linkage of integrin Beta subunit to actin is mediated by

Answer 39

talin

Question 40

Types of glycoaminoglycans

Answer 40

* Chondroitin sulfate * Dermatan sulfate * Heparan sulfate * keratan sulfate