lec 4 - cell junctions

what sheets of epithelia need

control what passes between the cells
resist shearing forces 
need to communicate 
need polarity 

types of cell junction 

communicating junctions - gap
occluding junctions - tight 
adhering junctions - adherens, desmosomes, hemidesmosomes, focal adhesions 

what part of the cytoskeleton do cell junctions adhere to 

desmosomes and hemidesmosomes link to cytokeratin 
(most) others link to actin 

top: tight junction
middle: zonula adheren (or adherens junction) 
bottom: desmosome 

junctional complex 
different junctions often form a junctional complex 

gap junctions - what 

membrane channels that bridge the gap between cells 
allow direct cytoplasmic exchange (bidirectional) 
rapid juxtacrine signalling - small molecules e.g. ions, ATP, second messengers, etc 

gap junctions - proteins and structure

6 connexins make up a connexon (or hemichannel) 
connexins have 4 transmembrane domains - linked by loops which allow the connexons from each connecting cell to dock to each other to form a gap junction 
the channel twists to open - can only happen when correct docking has occured 

location of gap junctions 

in the cell membrane - basolateral surface between 2 adjacent cells with very little room between them 
connexins synthesised in RER and oligomerize in golgi - then trafficked as connexons to plasma membrane 
in the membrane near existing junctional complexes - need ZO-1 
multiple channels cluster to form plaques

formation of gap junctions - when

after adherens and tight junctions (as needs ZO-1 proteins as anchors) 
can be before or after desmosomes and hemidesmosomes - quick turnover as they are only really inserted into membrane as needed 

tight junctions - what 

zonula occludens 
cell-cell adhesion complexes near apical membrane 
selective barrier - controls paracellular pathway, maintains cell polarity, involved in signalling pathways 
branching network of sealing strands 

tight junction - proteins and structure 

occludins

junctional adhesion molecules 

claudins 

scaffolding proteins 

occludins:

• 4 transmembrane domains

• stability and integrity 

• tightness not permeability

junctional adhesion molecules 

• transmembrane proteins 

• mediate cell-cell adhesions 

• interact with cytoskeleton and signalling proteins and scaffolding proteins 

scaffolding proteins 

• link tight junctions to actin cytoskeleton 

• inclused ZO-1, 2, 3

claudins

involved in tight junctions 
transmembrane proteins with 4 transmembrane domains 
act as selective channels 
each allows or restricts particular ions or molecules 
claudin 1 blocks most ions and water 
claudin 2 selective permeability to Na+, K+, Ca2+, water 

claudin 1 

blocks most ions and water 
part of tight junctions in skin

claudin 2 

selective permeability to Na+, K+, Ca2+, water 
in tight junctions 

tight junctions formation - how and when 

initiated by adherens junctions (form afterward) 
recruitment of polarity proteins -> claudins and occludens inserted into apical lateral memebrane -> JAMs dimerise and stabilise cell-cell contact -> ZO-1 binds claudins and occludins -> ZO-1 binds to f;actin 
forms continuous circumferential belt 

dysregulation of tight junctions 

inflammatory bowel disease: reduced claudin 1 and 2, increased claudin 3 = leaky barrier
loss of claudin 4 and 7 and occludins: linked to invasive and metastatic GI cancers 
claudin 16 and 19 mutations: impaired reabsorption of Mg2+ and Ca2+ 
salmonella: effector proteins disrupt tight junctions and allow bacterial invasion 

adherins junction what 

belt like adhesion complexes 
basal to tight junctions 
strong cell-cell adhesion 
mechanical adhesion - tensile strength 
involved in signalling anf establishing cell polarity 

adherens structure and proteins 

transmembrane proteins: E-cadherins form dimers with neighbouring cells, Ca2+ dependant 
intracellular proteins: catenins link cadherins to actin cytoskeleton, involved in regulating junction stability and signalling 

formation of adherens junctions - how and when 

cadherins on adjacent cells form homodimers (Ca2+ dependant) 
cadherin cytoplasmic tails recruit catenins which connect to actin 
junctions form a continuous circumferential belt 

dysfunction of adherins junctions 

E-cadherins essential for intestinal epithelial integrity - mutation or loss = increased intestinal permeability 
E-cadherins encoded by CDH1 gene - mutations linked to cancers

desmosomes what 

spot-like intercellular junctions - link to cytoskeleton 
strong mechanical adhesion 
abundant where high mechanical/shearing force 

desmosomes protein/structure 

desmosomal intermediate filament complex (DIFC): extracellular core, dense plaques 
transmembrane proteins: desmosomal cadherins - desmogleins and desmocollins 
intracellular proteins: plakoglobin and desmoplakin (connect the transmembrane proteins to intermediate filaments) 

desmosomes formation - how and when 

forms after adherens junctions and tight junctions have formed 
desmogleins and desmocollins on adjacent cells bind 
cytoplasmic tails recruit plakoglobin 
plakoglobin binds to desmoplakin 
desmoplakin connects to cytokeratin

hemidesmosomes what 

anchor epithelial to basement membrane

hemidesmosomes protein and structure 

transmembrane protein: integrin
intracellular protein: plectin
extracellular protein: laminin (lamina lucida)

hemidesmosomes formation - how and when 

form after desmosomes have formed 
integrin heterodimers inserted into basal membrane 
binds laminin - binds to basal lamina components 
integrin binds to plectin - links integrin to keratin 
hemidesmosomes cluster to form dense adhesion sites 

focal adhesions what

highly dynamic multi protein complexes 
mediate attachment of cell to ECM, connect to actin 
more plastic than hemidesmosomes 
involved in cell migration and signalling 

focal adhesions - structure and proteins 

transmembrane proteins: integrins 
adaptor proteins: e.g. talin 
focal adhesion kinase (FAK) regulates adhesion turnover 

focal adhesions formation 

integrins bind to ECM components (fibronectin, collagen) 
recruitment of adaptor proteins
link to actin

cell junction complex formation order 

1. initial cell to cell contact 

2. adherens junctions assemble - E-cadherin clustering, actin remodeling, polarity initiated 

3. tight junction formation - recruitment and positioning of polarity determining complexes, includes recruitment of ZO-1

4. desmosome formation 

5. hemidesmosomes and focal adhesions form 

gap junctions form after adherens and tight junctions, as needed. Rapid turnover to meet cellular needs