Organization

of

multiprotein

complexes

at

cell

cell

junctions

The

formation

of

stable

cell

cell

contacts

is

required

for

the

generation

of

barrier-forming

sheets

of

epithelial

and

endothelial

cells

During

various

physiological

processes

like

tissue

development

wound

healing

or

tumorigenesis

cellular

junctions

are

reorganized

to

allow

the

release

or

the

incorporation

of

individual

cells

Cell

cell

contact

formation

is

regulated

by

multiprotein

complexes

which

are

localized

at

specific

structures

along

the

lateral

cell

junctions

like

the

tight

junctions

and

adherens

junctions

and

which

are

targeted

to

these

site

through

their

association

with

cell

adhesion

molecules

Recent

evidence

indicates

that

several

major

protein

complexes

exist

which

have

distinct

functions

during

junction

formation

However

this

evidence

also

indicates

that

their

composition

is

dynamic

and

subject

to

changes

depending

on

the

state

of

junction

maturation

Thus

cell

cell

contact

formation

and

integrity

is

regulated

by

a

complex

network

of

protein

complexes

Imbalancing

this

network

by

oncogenic

proteins

or

pathogens

results

in

barrier

breakdown

and

eventually

in

cancer

Here

I

will

review

the

molecular

organization

of

the

major

multiprotein

complexes

at

junctions

of

epithelial

cells

and

discuss

their

function

in

cell

cell

contact

formation

and

maintenance

Introduction

In

multicellular

organisms

cell

cell

adhesion

is

involved

in

most

developmental

processes

It

is

necessary

for

example

for

the

assembly

of

coherent

sheets

of

barrier-forming

epithelial

and

endothelial

cells

which

line

the

inner

and

outer

surfaces

of

the

organism

like

those

of

the

intestine

the

skin

or

the

blood

vessels

However

also

in

adult

tissues

cell

cell

contacts

are

far

from

being

static

structures

which

maintain

the

barriers

by

simply

holding

cells

together

During

the

turnover

of

growing

tissues

such

as

the

intestine

or

the

skin

they

are

constantly

remodeled

to

allow

the

extrusion

of

old

cells

and

the

incorporation

of

young

cells

derived

from

stem

cells

without

a

concomitant

loss

of

the

barrier

function

-LRB-

Fuchs

et

al.

-RRB-

Similarly

during

leukocyte

extravasation

in

secondary

lymphoid

organs

or

at

sites

of

an

ongoing

immune

response

the

homotypic

interactions

between

endothelial

cells

must

be

altered

to

allow

the

passage

of

the

leukocytes

without

affecting

the

barrier

properties

of

the

endothelium

-LRB-

Ley

et

al.

-RRB-

Finally

during

wound

healing

and

tissue

repair

cells

undergo

a

coordinated

movement

proliferate

and

establish

new

cell

cell

contacts

once

they

encounter

cells

from

the

opposing

site

of

the

wound

-LRB-

Perez-Moreno

and

Fuchs

-RRB-

These

different

demands

in

different

cell

types

and

different

physiological

situations

require

a

sophisticated

regulatory

network

which

enables

a

partial

dismantling

and

re-establishment

of

cell

cell

contacts

while

simultaneously

preventing

the

loss

of

an

epithelial

phenotype

which

in

adult

tissues

frequently

correlates

with

tumor

progression

and

metastasis

-LRB-

Thiery

-RRB-

Not

surprisingly

the

organization

of

cell

cell

contacts

of

epithelial

has

attracted

a

great

deal

of

attention

Due

to

the

easy

accessibility

of

cultured

epithelial

cells

many

discoveries

have

been

made

in

epithelial

cells

Epithelial

cell

cell

contacts

contain

three

major

adhesive

structures

which

can

be

identified

at

the

ultrastructural

level

the

tight

junctions

-LRB-

TJs

-RRB-

the

adherens

junctions

-LRB-

AJs

-RRB-

and

the

desmosomes

In

polarized

epithelial

cells

of

certain

tissues

like

the

intestine

TJs

and

AJs

are

asymmetrically

distributed

at

the

apical

region

of

the

lateral

cell

contact

forming

the

apical

junctional

complex

-LRB-

AJC

-RRB-

which

encircles

the

apex

of

the

cells

and

demarkates

the

border

between

the

apical

and

the

basolateral

membrane

domains

-LRB-

Nelson

-RRB-

Common

to

all

three

types

of

structures

is

the

presence

of

several

adhesion

molecules

that

link

the

neighbouring

cells

through

homophilic

and

heterophilic

adhesive

interactions

and

the

presence

of

cytoplasmic

scaffolding

proteins

that

organize

signaling

complexes

and

which

provide

a

mechanical

link

to

the

actin

cytoskeleton

-LRB-

or

intermediate

filaments

in

the

case

of

desmosomes

-RRB-

The

scaffolding

proteins

might

also

link

different

protein

complexes

at

least

temporarily

and

thus

organize

supramolecular

protein

complexes

It

should

be

noted

that

these

protein

complexes

are

dynamic

and

that

their

composition

is

subject

to

regulation

depending

on

junctional

maturation

and

integrity

During

the

last

few

years

a

rapid

progress

has

been

made

in

identifying

new

proteins

at

cell

cell

contacts

and

in

particular

in

deciphering

the

molecular

composition

of

the

TJs

Among

the

most

exciting

findings

was

probably

the

discovery

of

protein

complexes

at

TJs

which

are

highly

conserved

through

evolution

and

which

regulate

cellular

polarization

in

different

organisms

and

various

cell

types

Here

I

will

review

the

major

multiprotein

complexes

present

at

cell

cell

contacts

of

vertebrate

epithelial

cells

and

highlight

the

most

recent

advances

in

the

understanding

of

their

role

in

the

organization

and

functions

of

epithelial

cell

cell

contacts

in

vertebrates

Adherens

junctions

The

function

of

adherens

junctions

A

main

function

of

AJs

is

to

connect

cells

to

regulate

tissue

formation

and

morphogenesis

during

development

as

well

as

the

maintenance

of

solid

tissues

in

the

adult

organism

-LRB-

Gumbiner

-RRB-

The

major

cell

adhesion

molecules

at

AJs

the

classical

cadherins

connect

adjacent

cells

through

homophilic

interactions

and

are

linked

to

the

cytoskeleton

through

proteins

associated

with

their

cytoplasmic

tail

the

catenins

This

link

generates

a

transcellular

network

of

actin

filaments

running

through

the

entire

sheet

of

cells

with

the

cadherin

catenin

complexes

serving

as

connectors

of

the

actin

filaments

bundles

at

the

intercellular

space

During

morphogenetic

events

for

example

during

neural

tube

formation

mechanical

forces

can

thus

be

applied

to

the

whole

cellular

sheet

In

adult

tissues

cadherin-mediated

cell

adhesion

is

absolutely

required

for

cell

cell

adhesion

to

be

maintained

as

a

loss

of

cadherin

adhesion

by

depletion

results

in

a

loss

of

cell

cell

interaction

and

rounding

up

of

the

cells

despite

a

number

of

other

adhesion

molecules

-LRB-

which

are

not

dependent

-RRB-

at

cell

cell

contacts

-LRB-

Takeichi

-RRB-

Cadherins

and

catenins

Since

the

cadherin

catenin

complex

has

been

the

subject

of

a

number

of

reviews

I

will

only

summarize

the

central

interactions

and

refer

the

reader

to

recent

comprehensive

reviews

-LRB-

Gumbiner

Halbleib

and

Nelson

Perez-Moreno

and

Fuchs

Perez-Moreno

et

al.

Pokutta

and

Weis

-RRB-

As

mentioned

above

the

cytoplasmic

tail

of

classical

cadherins

like

E-cadherin

forms

a

multiprotein

complex

with

β-catenin

γ-catenin

and

catenin

-LRB-

-RRB-

members

of

the

armadillo

repeat

domain-containing

family

of

proteins

In

this

complex

β-catenin

and

are

directly

associated

with

the

membrane-distal

and

membrane-proximal

region

respectively

-LRB-

Fig.

-RRB-

Both

regions

are

highly

conserved

among

classical

cadherins

underscoring

the

importance

of

these

interactions

Under

conditions

where

β-catenin

is

limiting

its

binding

site

in

E-cadherin

can

be

occupied

by

the

β-catenin-related

plakoglobin/γ-catenin

which

otherwise

associates

preferentially

with

desmosomal

cadherins

-LRB-

Zhurinsky

et

al.

-RRB-

The

interaction

with

β-catenin

is

required

for

the

transport

of

E-cadherin

from

the

ER

to

the

basolateral

cell

surface

-LRB-

Chen

et

al.

-RRB-

It

is

also

required

for

the

adhesive

function

of

E-cadherin

as

post-translational

modifications

of

β-catenin

which

alter

its

affinity

towards

E-cadherin

alter

the

strength

of

E-cadherin

adhesive

activity

-LRB-

Perez-Moreno

et

al.

Pokutta

and

Weis

-RRB-

The

adhesive

activity

is

thus

dependent

on

its

association

with

β-catenin

Conversely

this

association

is

also

necessary

to

stabilize

β-catenin

which

is

otherwise

rapidly

degraded

by

the

ubiquitin-proteasome

pathway

-LRB-

Nelson

and

Nusse

-RRB-

Fig.

protein

complexes

at

adherens

junctions

Two

major

protein

complexes

exist

at

AJs

of

epithelial

cells

The

cadherin

catenin

complex

consist

of

the

dependent

adhesion

molecule

E-cadherin

and

the

armadillo

repeat

proteins

and

β/γ-catenin

which

directly

bind

to

the

cytoplasmic

domain

of

E-cadherin

α-catenin

directly

associates

with

β-catenin

but

not

simultaneously

with

F-actin

The

nectin

afadin

complex

consists

of

the

independent

adhesion

molecule

nectin

and

the

PDZ

protein

afadin

Afadin

contains

a

F-actin-binding

domain

and

thus

can

link

the

nectin

afadin

system

to

the

actin

cytoskeleton

The

two

adhesion

complexes

can

be

linked

through

several

molecular

interactions

Afadin

can

directly

interact

with

α-catenin

It

also

interacts

with

which

can

interact

with

the

F-actin-binding

protein

vinculin

and

it

interacts

with

and

ADIP

which

both

can

interact

with

the

F-actin

binding

protein

α-actinin

The

nature

of

the

link

between

the

cadherin

catenin

complex

and

the

actin

cytoskeleton

is

still

unclear

Double

arrows

indicate

direct

interactions

the

question

mark

symbolizes

the

missing

link

The

association

of

E-cadherin

with

is

subject

to

a

similar

reciprocal

regulation

of

cell

cell

contact

localization

The

cadherin

molecule

is

necessary

to

localize

at

the

cell

contact

and

is

required

for

the

stable

localization

of

the

cadherin

molecule

at

AJs

-LRB-

Reynolds

and

Roczniak-Ferguson

-RRB-

In

contrast

to

β-catenin

stabilizes

cadherin

which

is

constitutively

endocytosed

-LRB-

Bryant

and

Stow

-RRB-

by

regulating

its

turnover

rate

at

the

surface

-LRB-

Davis

et

al.

-RRB-

Besides

its

additional

role

in

regulating

transcription

by

interacting

with

transcription

factors

-LRB-

van

Roy

and

McCrea

-RRB-

has

a

also

a

function

in

regulating

the

activity

of

Rho

small

GTPases

The

formation

of

early

cell

cell

contacts

correlates

with

the

activation

of

and

and

the

inhibition

of

RhoA

-LRB-

Noren

et

al.

-RRB-

and

these

changes

in

activities

of

the

small

GTPases

are

at

least

in

part

regulated

by

which

interacts

with

the

Rac

activator

-LRB-

Noren

et

al.

-RRB-

and

the

RhoA

inhibitor

-LRB-

Wildenberg

et

al.

-RRB-

The

activation

of

and

the

downregulation

of

RhoA

activity

serves

to

facilitate

new

cell

adhesion

by

increasing

the

cell

surface

interacting

with

membranes

of

neighbouring

cells

through

lamellipodia

and

filopodia

formation

and

simultaneously

inhibiting

cell

migration

by

inhibiting

stress

fiber

formation

respectively

-LRB-

Perez-Moreno

and

Fuchs

-RRB-

The

role

of

α-catenin

is

less

clear

For

a

long

time

considered

to

bridge

the

AJs

to

the

actin

cytoskeleton

through

direct

interactions

with

both

β-catenin

and

F-actin

this

role

has

been

challenged

by

the

observations

that

its

binding

to

F-actin

-LRB-

occurs

only

as

homodimer

-RRB-

and

its

heterodimeric

association

with

β-catenin

are

mutually

exclusive

-LRB-

Drees

et

al.

Yamada

et

al.

-RRB-

Therefore

the

physical

bridge

between

the

cadherin

catenin

complex

and

the

actin

cytoskeleton

remains

to

be

identified

It

is

possible

that

α-catenin

present

in

the

cadherin-associated

heterodimeric

β-catenin

α-catenin

complex

is

able

to

bind

to

other

proteins

which

associate

with

F-actin

The

putative

linker

is

required

to

fulfill

two

critera

to

bind

directly

to

α-catenin

and

simultaneously

either

indirectly

or

directly

to

F-actin

From

several

proteins

which

fulfill

these

requirements

vinculin

and

α-actinin

turned

out

not

to

mediate

actin

binding

to

the

cadherin

catenin

complex

excluding

them

from

the

list

of

possible

candidates

-LRB-

Yamada

et

al.

-RRB-

Other

candidate

proteins

including

afadin

-LRB-

Pokutta

et

al.

-RRB-

-LRB-

Itoh

et

al.

-RRB-

formin

-LRB-

Kobielak

et

al.

-RRB-

spectrin

-LRB-

Pradhan

et

al.

-RRB-

or

the

LIM

protein

ajuba

-LRB-

Marie

et

al.

-RRB-

remain

to

be

tested

Nectins

and

afadin

The

second

major

adhesive

protein

complex

at

AJs

consists

of

members

of

the

nectin

family

of

adhesion

molecules

and

a

scaffolding

protein

that

is

directly

associated

with

the

cytoplasmic

domain

of

nectins

named

afadin

-LRB-

Takai

and

Nakanishi

-RRB-

Nectins

are

immunoglobulin-like

proteins

and

comprise

a

family

consisting

of

four

members

-LRB-

to

-RRB-

which

are

localized

at

AJs

of

epithelial

cells

-LRB-

Reymond

et

al.

Sakisaka

and

Takai

-RRB-

Unlike

classical

cadherins

which

undergo

only

homophilic

interactions

in

trans

nectins

undergo

both

trans-homophilic

and

trans-heterophilic

interactions

The

major

heterophilic

binding

partners

are

other

members

of

the

nectin

family

as

well

as

members

of

nectin-related

adhesion

molecules

Nectin-like

-LRB-

Necl

-RRB-

to

-LRB-

Sakisaka

et

al.

-RRB-

Nectins

are

true

adhesion

molecules

as

they

support

cell

aggregation

when

ectopically

expressed

in

cells

-LRB-

Aoki

et

al.

Lopez

et

al.

Satoh-Horikawa

et

al.

Takahashi

et

al.

-RRB-

They

also

seem

to

influence

the

E-cadherin-mediated

adhesion

-LRB-

Martinez-Rico

et

al.

Sato

et

al.

-RRB-

suggesting

that

they

are

contributing

to

the

overall

strength

of

cell

cell

adhesion

Like

E-cadherin

appears

very

early

at

cell

cell

contacts

during

junction

formation

and

is

present

at

so-called

primordial

spot-like

AJs

-LRB-

pAJs

-RRB-

or

puncta

-LRB-

Asakura

et

al.

-RRB-

which

are

formed

at

the

tips

of

protrusions

of

two

contacting

cells

-LRB-

Perez-Moreno

and

Fuchs

-RRB-

An

important

function

of

nectins

which

is

similar

to

that

of

cadherins

is

their

ability

to

activate

and

small

GTPases

Trans-interaction

of

nectins

results

in

the

recruitment

and

activation

of

c-Src

followed

by

the

activation

of

the

two

guanine-nucleotide

exchange

factors

GEF

-LRB-

FRG

-RRB-

and

which

are

specific

for

and

respectively

-LRB-

Fukuhara

et

al.

Kawakatsu

et

al.

-RRB-

As

pointed

out

above

the

activation

of

these

small

GTPases

is

probably

required

to

facilitate

junction

formation

suggesting

that

nectins

cooperate

with

cadherins

in

the

regulation

of

the

actin

cytoskeleton

at

sites

of

cell

adhesion

However

in

addition

they

might

also

help

to

regulate

the

formation

of

tight

junctions

and

their

physical

separation

from

AJs

during

junctional

maturation

-LRB-

see

below

-RRB-

All

nectins

directly

associate

with

afadin

-LRB-

Reymond

et

al.

Takahashi

et

al.

-RRB-

L-Afadin

the

longer

version

of

two

afadin

isoforms

with

a

F-actin-binding

domain

is

a

scaffolding

protein

which

interacts

with

both

nectins

and

F-actin

through

independent

domains

suggesting

that

it

directly

links

nectin-based

adhesion

sites

to

the

actin

cytoskeleton

-LRB-

Mandai

et

al.

-RRB-

-LRB-

Fig.

-RRB-

However

through

additional

protein

interactions

it

might

also

establish

an

indirect

link

to

the

actin

cytoskeleton

as

well

as

to

the

cadherin

catenin

complex

by

its

interaction

with

classical

AJ-associated

proteins

Afadin

directly

associates

with

α-catenin

-LRB-

Pokutta

et

al.

Tachibana

et

al.

-RRB-

with

vinculin

through

its

association

with

-LRB-

Mandai

et

al.

-RRB-

and

with

α-actinin

through

its

association

with

afadin

Dil

domain-interacting

protein

-LRB-

ADIP

-RRB-

and

Lim

domain

only

-LRB-

-RRB-

-LRB-

Asada

et

al.

Ooshio

et

al.

-RRB-

-LRB-

Fig.

-RRB-

It

should

be

mentioned

that

some

of

these

protein

interactions

might

occur

specifically

in

certain

tissues

and

that

the

molecular

mechanisms

underlying

these

interactions

are

not

revealed

in

detail

Nevertheless

it

is

likely

that

through

these

multiple

interactions

the

two

major

protein

complexes

at

AJs

are

physically

linked

and

that

they

influence

each

other

in

their

localization

and

activity

-LRB-

Sakisaka

et

al.

-RRB-

Tight

junctions

The

function

of

TJs

In

polarized

epithelial

cells

the

TJs

forms

a

belt-like

structure

at

the

apical

region

of

the

cellular

junction

and

represent

a

boundary

between

the

apical

and

the

basolateral

membrane

domains

which

differ

in

the

composition

of

lipids

and

proteins

-LRB-

Tsukita

et

al.

-RRB-

At

the

TJ

area

two

structures

can

be

distinguished

by

ultrathin

electron

microscopy

sites

where

the

intercellular

space

is

basically

obliterated

and

where

the

outer

leaflets

of

the

adjacent

membranes

appear

to

be

in

direct

contact

and

regions

where

the

membranes

of

the

two

adjacent

cells

are

separated

by

intercellular

space

By

freeze-fracture

electron

microscopy

the

TJ

area

appears

as

a

branched

network

of

strands

where

the

strands

reflect

the

sites

of

direct

membrane

contacts

-LRB-

Tsukita

et

al.

-RRB-

Two

major

functions

are

attributed

to

TJs

First

the

regulation

of

the

paracellular

permeability

of

the

epithelial

sheet

for

ions

and

small

solutes

which

is

an

organ-specific

function

and

which

varies

for

different

epithelia

depending

on

the

specific

requirements

of

the

organ

-LRB-

Furuse

and

Tsukitas

Van

Itallie

and

Anderson

-RRB-

second

the

formation

of

a

physical

barrier

to

prevent

intramembrane

diffusion

of

lipids

and

proteins

a

rather

cell-autonomous

function

which

is

necessary

for

a

cell

to

maintain

an

asymmetric

distribution

of

membrane

components

and

to

develop

membrane

polarity

-LRB-

Tsukita

et

al.

-RRB-

In

the

recent

years

it

has

become

clear

that

the

molecular

basis

of

the

TJ

strands

are

claudins

-LRB-

Fig.

-RRB-

a

family

of

integral

membrane

proteins

at

TJs

which

are

constituents

of

the

TJ

strands

and

which

induce

the

formation

of

TJ

strands

upon

ectopic

expression

in

fibroblasts

-LRB-

Furuse

et

al.

-RRB-

Claudins

do

not

just

create

TJ

strands

but

within

the

strands

form

a

selective

permeability

barrier

by

forming

size

and

charge-selective

aqueous

pores

-LRB-

Tsukita

and

Furuse

Van

Itallie

and

Anderson

-RRB-

Surprisingly

the

TJ

strands

do

not

seem

to

be

the

basis

for

the

second

major

function

of

TJs

the

diffusion

barrier

for

intramembrane

particles

-LRB-

Umeda

et

al.

-RRB-

It

has

also

become

clear

that

the

TJs

harbor

peripheral

membrane

proteins

which

regulate

cell

polarity

and

membrane

asymmetry

suggesting

that

these

proteins

are

involved

in

the

role

of

TJs

in

regulating

apico-basal

polarity

Fig.

membrane

proteins

at

tight

junctions

of

epithelial

and

endothelial

can

be

grouped

into

three

classes

based

on

their

overall

organizations

The

first

class

is

characterized

by

two

extracellular

loops

four

transmembrane

regions

and

two

cytoplasmic

tails

-LRB-

occludin

claudins

tricellulin

-RRB-

The

second

class

consists

of

Ig-SF

members

which

all

contain

two

Ig-like

domains

The

third

class

-LRB-

contains

only

one

member

-RRB-

is

characterized

by

a

short

extracellular

domain

-LRB-

AA

-RRB-

a

single

transmembrane

domain

and

a

short

cytoplasmic

tail

In

contrast

to

the

other

integral

membrane

proteins

the

function

of

the

extracellular

domain

of

is

not

clear

Integral

membrane

proteins

at

TJs

Three

different

classes

of

integral

membrane

proteins

have

been

identified

at

TJs

-LRB-

Fig.

-RRB-

One

class

comprises

occludin

claudins

and

tricellulin

which

all

contain

four

transmembrane

domains

two

extracellular

loops

and

the

N-terminal

and

C-terminal

ends

are

localized

in

the

cytoplasm

Twenty-four

claudins

have

been

identified

in

humans

The

major

role

of

claudins

is

to

form

paired

strands

through

homophilic

and

heterophilic

cis

and

trans

interactions

The

large

number

of

claudins

their

ability

to

undergo

heterophilic

interactions

and

their

ion

selectivity

allows

for

the

formation

of

TJ

strands

with

specific

permeability

properties

depending

on

the

needs

of

a

specific

tissue

-LRB-

Furuse

and

Tsukita

-RRB-

Occludin

is

incorporated

into

TJ

strands

but

its

ectopic

expression

does

not

induce

strand

formation

-LRB-

Furuse

et

al.

-RRB-

suggesting

that

it

has

rather

an

accessory

role

in

TJ

strand

formation

-LRB-

Yu

et

al.

-RRB-

Tricellulin

differs

from

claudins

and

occludin

in

its

specific

enrichment

at

tricellular

contact

sites

-LRB-

Ikenouchi

et

al.

-RRB-

The

second

class

of

integral

membrane

proteins

comprises

members

of

the

CTX

subfamily

of

the

immunoglobulin

superfamily

-LRB-

Fig.

-RRB-

which

is

characterized

by

one

V-type

and

one

Ig-like

domain

-LRB-

Williams

and

Barclay

-RRB-

Based

on

sequence

homology

the

length

of

their

cytoplasmic

tails

and

the

type

of

the

C-terminal

PDZ

domain

motifs

the

Ig-SF

proteins

at

TJs

can

be

further

subdivided

into

a

group

consisting

of

JAM-A

JAM-B

and

JAM-C

and

in

a

second

group

consisting

of

CAR

CLMP

ESAM

and

-LRB-

Ebnet

et

al.

-RRB-

Except

for

JAM-A

there

is

only

little

information

on

the

role

of

these

proteins

in

the

development

or

the

function

of

tight

junctions

All

but

CLMP

interact

with

TJ-associated

scaffolding

proteins

like

-LRB-

CAR

JAM-A

B

C

-LRB-

Cohen

et

al.

Ebnet

et

al.

-RRB-

-LRB-

JAM-A

B

C

-LRB-

Ebnet

et

al.

-RRB-

or

-LRB-

ESAM

-LRB-

Hirabayashi

et

al.

Wegmann

et

al.

-RRB-

-LRB-

Fig.

-RRB-

and

in

some

cases

their

ectopic

expression

increases

the

transepithelial

electrical

resistance

-LRB-

TER

-RRB-

or

decreases

the

paracellular

permeability

suggesting

a

regulatory

role

in

TJ

formation

-LRB-

Cohen

et

al.

Hirabayashi

et

al.

Mandicourt

et

al.

Raschperger

et

al.

-RRB-

For

JAM-A

there

is

good

evidence

that

it

regulates

TJ

formation

through

its

direct

association

with

the

scaffolding

protein

-LRB-

Ebnet

et

al.

Itoh

et

al.

-RRB-

JAM-A

localizes

very

early

at

sites

of

cell

adhesion

during

cell

cell

contact

formation

-LRB-

Ebnet

et

al.

Suzuki

et

al.

-RRB-

and

this

localization

probably

serves

to

recruit

to

these

site

to

initiate

the

polarization

of

the

lateral

membrane

resulting

in

TJ

formation

-LRB-

see

below

for

details

-RRB-

Both

RNA

interference-mediated

downregulation

of

JAM-A

and

ectopic

expression

of

a

JAM-A

dominant-negative

mutant

that

mislocalizes

result

in

decreased

TER

increased

paracellular

permeability

and

a

defect

in

the

development

of

membrane

asymmetry

-LRB-

Mandell

et

al.

Rehder

et

al.

-RRB-

pointing

to

a

general

defect

in

the

formation

of

functional

TJs

The

third

class

of

integral

membrane

proteins

comprises

only

one

protein

-LRB-

-RRB-

a

homologue

of

the

Drosophila

Crumbs

protein

with

a

very

short

extracellular

domain

of

only

AA

and

a

short

cytoplasmic

domain

of

AA

-LRB-

Makarova

et

al.

Medina

et

al.

-RRB-

directly

associates

with

two

peripheral

membrane

proteins

-LRB-

Roh

et

al.

-RRB-

and

-LRB-

Lemmers

et

al.

-RRB-

which

are

components

of

the

two

major

cell

polarity

protein

complexes

localized

at

TJs

i.e.

the

PATJ

complex

and

the

aPKC

complex

-LRB-

Fig.

see

also

below

-RRB-

Its

overexpression

delays

TJ

formation

-LRB-

Lemmers

et

al.

Roh

et

al.

-RRB-

and

its

ectopic

expression

in

a

cell

line

that

expresses

only

little

endogenous

results

in

the

development

of

functional

TJs

-LRB-

Fogg

et

al.

-RRB-

These

effects

can

most

likely

be

attributed

to

its

association

with

cell

polarity

proteins

and

the

regulation

of

their

subcellular

localization

-LRB-

see

below

-RRB-

Fig.

protein

complexes

and

functional

classes

of

molecules

at

tight

junctions

The

TJs

contain

three

major

multi-protein

complexes

consisting

largely

of

scaffolding

proteins

the

ZO

protein

complex

the

PATJ

complex

and

the

aPKC

complex

Besides

these

three

protein

complexes

which

seem

to

be

constitutively

associated

at

TJs

a

number

of

proteins

with

different

functions

has

been

identified

at

TJs

These

include

additional

scaffolding

proteins

like

and

adaptor

proteins

transcription

regulators

and

RNA

processing

factors

regulatory

proteins

like

small

GTPases

and

G-proteins

kinases

and

phosphatases

and

heat

shock

proteins

Double

arrows

indicate

direct

interactions

Not

all

direct

interactions

that

have

been

identified

are

depicted

Multiprotein

complexes

at

TJs

Since

the

discovery

of

as

the

first

protein

at

TJs

and

its

molecular

cloning

-LRB-

Itoh

et

al.

Stevenson

et

al.

-RRB-

the

number

of

proteins

that

are

localized

at

TJs

has

steadily

increased

These

proteins

comprise

scaffolding

and

adapter

proteins

regulatory

proteins

like

small

GTPases

G-proteins

kinases

and

phosphatases

as

well

as

transcription

factors

or

factors

regulating

RNA

processing

-LRB-

Fig.

-RRB-

The

large

number

and

the

functional

diversity

of

these

proteins

suggest

that

TJs

are

a

focus

of

incoming

and

outgoing

signals

and

that

their

composition

is

dynamic

In

accordance

with

this

many

proteins

identified

at

TJs

are

found

at

other

compartments

of

the

cell

as

well

including

the

nucleus

or

the

cytoskeleton

and

are

actively

shuttling

between

these

compartments

and

the

TJs

-LRB-

Matter

and

Balda

-RRB-

The

organization

of

these

networks

is

regulated

by

proteins

containing

multiple

protein

protein

interaction

domains

such

as

PDZ

domains

GuK

domains

or

domains

-LRB-

Pawson

and

Nash

-RRB-

At

the

TJs

three

major

protein

complexes

exist

which

involve

one

or

several

scaffolding

proteins

the

ZO

protein

complex

the

PATJ

complex

and

the

aPKC

complex

-LRB-

Fig.

-RRB-

The

ZO

protein

complex

is

a

classical

scaffolding

protein

of

the

MAGUK

family

with

three

PDZ

domains

one

domain

and

one

GuK

domain

-LRB-

Funke

et

al.

-RRB-

It

can

directly

associate

with

several

integral

membrane

proteins

at

TJs

including

occludin

claudins

JAMs

and

CAR

through

independent

domains

-LRB-

Fig.

-RRB-

and

it

probably

serves

to

cluster

these

proteins

at

the

TJs

It

also

interacts

with

other

cytoplasmic

proteins

including

its

homologues

and

and

in

addition

with

the

actin

cytoskeleton

-LRB-

Fanning

et

al.

Fanning

et

al.

Wittchen

et

al.

-RRB-

The

exact

composition

of

the

ZO

complex

is

not

completely

understood

yet

forms

independent

complexes

with

and

-LRB-

Wittchen

et

al.

-RRB-

and

both

and

can

also

interact

with

F-actin

and

share

with

some

of

the

integral

membrane

proteins

at

TJs

like

occludin

and

claudins

-LRB-

Itoh

et

al.

Wittchen

et

al.

-RRB-

Alltogether

the

ZO

complex

provides

the

major

link

to

the

actin

cytoskeleton

at

the

TJs

-LRB-

Fig.

-RRB-

However

regarding

the

role

of

the

individual

ZO

proteins

some

redundancy

might

exist

In

agreement

with

this

the

absence

of

results

in

a

slight

delay

in

TJ

formation

but

does

not

impair

the

formation

of

functional

TJs

in

two

epithelial

cell

lines

-LRB-

McNeil

et

al.

Umeda

et

al.

-RRB-

Only

when

all

three

ZO

proteins

are

absent

the

formation

of

TJs

is

blocked

as

indicated

by

the

absence

of

TJ

strands

the

lack

of

other

TJ

proteins

like

occludin

and

JAM-A

and

the

complete

loss

of

the

barrier

function

-LRB-

Umeda

et

al.

-RRB-

Together

these

findings

indicate

a

critical

role

for

the

ZO

proteins

for

the

development

of

TJ

strands

probably

by

forming

the

physical

scaffold

for

the

strand-forming

proteins

like

claudins

and

occludin

As

will

be

discussed

below

ZO

proteins

also

form

a

platform

for

signaling

proteins

to

regulate

epithelial

proliferation

and

morphogenesis

Fig.

of

the

tight

junctional

plaque

The

major

protein

complexes

at

TJs

interact

with

specific

transmembrane

proteins

recruits

the

PATJ

complex

to

TJs

interacts

with

the

PDZ

domain

of

interacts

with

PATJ

through

a

heterodimeric

domain

interaction

is

also

localized

at

the

apical

membrane

domain

of

epithelial

cells

-LRB-

Makarova

et

al.

-RRB-

The

PATJ

complex

regulates

TJ

formation

but

the

mechanism

is

largely

unknown

The

ZO

complex

is

associated

with

the

membrane

through

multiple

interactions

of

with

various

integral

membrane

proteins

including

occludin

claudins

JAM-A

and

CAR

and

can

interact

with

both

and

also

with

claudins

The

ZO

protein

complex

probably

serves

to

link

TJs

to

the

cytoskeleton

as

all

three

ZO

proteins

directly

interact

with

F-actin

The

aPKC

complex

is

associated

with

the

membrane

through

the

interaction

of

with

JAM-A

interacts

with

aPKC

through

its

aPKC-interacting

domain

interacts

with

aPKC

through

a

domain

interaction

A

direct

interaction

between

the

PDZ

domain

of

and

PDZ

domain

of

has

also

been

described

The

PAR

complex

regulates

the

formation

of

TJs

and

apico-basal

polarity

The

JAM-A-related

Ig-SF

member

interacts

directly

with

the

role

of

this

protein

complex

is

not

clear

It

should

be

noted

that

this

drawing

is

incomplete

as

it

does

not

depict

interactions

among

the

various

protein

complexes

which

have

been

described

-LRB-

e.g.

can

also

associate

with

and

can

associate

with

PATJ

-RRB-

Also

the

multiple

PDZ

domain

protein

-LRB-

not

depicted

in

this

Figure

see

Fig.

for

a

schematic

representation

-RRB-

associates

with

claudins

JAM-A

and

CAR

as

well

as

with

angiomotin

family

members

-LRB-

Coyne

et

al.

Hamazaki

et

al.

Sugihara-Mizuno

et

al.

-RRB-

Double

arrows

with

solid

lines

indicate

direct

protein

protein

interactions

double

arrows

with

broken

lines

indicate

interactions

with

F-actin

The

PATJ

complex

The

PATJ

complex

has

originally

been

described

in

Drosophila

as

a

protein

complex

-LRB-

the

Crumbs

Stardust

Discs

lost

complex

this

complex

is

now

called

Crumbs

Stardust

-LRB-

Sdt

-RRB-

dPATJ

complex

-LRB-

Pielage

et

al.

-RRB-

-RRB-

involved

in

the

regulation

of

apico-basal

polarization

-LRB-

Tepass

et

al.

-RRB-

In

Drosophila

epithelial

cells

this

complex

localizes

to

the

apical

region

of

the

lateral

membrane

domain

called

subapical

region

-LRB-

SAR

-RRB-

or

marginal

zone

-LRB-

Knust

and

Bossinger

-RRB-

that

is

positionally

analogous

to

the

TJs

in

vertebrate

epithelial

cells

The

localization

of

the

PATJ

complex

at

the

TJ

is

mediated

through

a

direct

and

PDZ

domain-dependent

interaction

of

with

the

C-terminal

PDZ

domain

motif

in

and

a

direct

interaction

between

and

PATJ

involving

the

domain

of

and

the

domain

present

at

the

region

of

PATJ

-LRB-

Fig.

-RRB-

-LRB-

Roh

et

al.

-RRB-

RNA

interference-mediated

knockdown

of

leads

to

defects

in

the

formation

of

TJs

as

well

as

in

the

development

of

lumen-containing

epithelial

cysts

-LRB-

an

assay

system

for

apico-basal

polarity

development

-LRB-

O'Brien

et

al.

-RRB-

-RRB-

and

is

accompagnied

by

a

loss

of

PATJ

protein

expression

-LRB-

Straight

et

al.

-RRB-

Conversely

knockdown

of

PATJ

impairs

the

barrier

function

of

TJs

and

results

in

a

loss

of

at

TJs

an

internalization

of

and

a

redistribution

of

other

TJ

components

like

occludin

and

-LRB-

Michel

et

al.

Shin

et

al.

-RRB-

These

observations

strongly

suggest

that

the

PATJ

complex

is

important

for

the

development

of

functional

TJs

in

vertebrate

epithelial

cells

The

aPKC

complex

In

evolutionary

terms

the

aPKC

complex

is

the

most

ancient

among

the

three

major

protein

complexes

at

TJs

As

opposed

to

the

ZO

protein

complex

and

the

PATJ

complex

all

three

proteins

of

this

complex

exist

in

C.elegans

where

they

cooperate

to

regulate

the

development

of

membrane

asymmetry

in

the

zygote

-LRB-

Kemphues

-RRB-

The

acronym

Par

stands

for

partitioning-defective

and

reflects

the

lack

of

partition

of

cytoplasmic

P

granules

in

C.elegans

mutant

embryos

in

response

to

sperm

entry

-LRB-

Kemphues

et

al.

-RRB-

The

initial

screen

identified

six

par

genes

and

their

molecular

characterization

revealed

that

they

encoded

proteins

of

different

structures

and

functions

which

include

scaffolding/adapter

proteins

with

several

protein

protein

interaction

domains

-LRB-

-RRB-

serine/threonine

kinases

-LRB-

-RRB-

a

protein

containing

a

RING

finger

domain

typical

for

ubiquitin

ligases

-LRB-

-RRB-

and

a

member

of

the

family

of

signaling

proteins

-LRB-

-RRB-

-LRB-

reviewed

in

-LRB-

Goldstein

and

Macara

Suzuki

and

Ohno

-RRB-

-RRB-

With

the

exception

of

all

PAR

proteins

exist

in

Drosophila

and

vertebrates

Two

PAR

proteins

and

form

a

functional

unit

with

aPKC

the

aPKC

complex

-LRB-

Ohno

-RRB-

In

this

complex

and

undergo

direct

interactions

with

aPKC

-LRB-

Fig.

-RRB-

The

interaction

of

with

aPKC

is

mediated

by

a

heterotypic

domain

interaction

the

interaction

of

with

aPKC

by

the

domain

of

and

the

kinase

domain

of

aPKC

-LRB-

Ohno

-RRB-

The

interactions

of

the

two

scaffolding

proteins

and

with

aPKC

are

assumed

to

regulate

the

localization

and

the

activity

of

aPKC

respectively

Among

all

integral

membrane

proteins

tested

binds

specifically

to

JAM-A

B

and

C

-LRB-

Ebnet

et

al.

-RRB-

and

the

interaction

with

JAM-A

might

serve

to

anchor

the

PAR

aPKC

complex

to

TJs

The

interaction

with

is

assumed

to

regulate

the

activity

of

aPKC

-LRB-

Lin

et

al.

-RRB-

In

the

absence

of

small

GTPases

like

or

aPKC

is

inactive

The

binding

of

active

or

to

the

CRIB

domain

of

activates

aPKC

probably

by

inducing

a

conformational

change

of

which

allows

aPKC

to

become

active

-LRB-

Yamanaka

et

al.

-RRB-

What

is

the

function

of

the

PAR

aPKC

complex

in

TJ

physiology

A

large

body

of

evidence

indicates

a

critical

role

of

the

PAR

complex

in

TJ

formation

rather

than

in

TJ

maintenance

-LRB-

Chen

and

Macara

Gao

et

al.

Hirose

et

al.

Joberty

et

al.

Mizuno

et

al.

Nagai-Tamai

et

al.

Ooshio

et

al.

Suzuki

et

al.

Yamanaka

et

al.

-RRB-

Many

of

these

studies

applied

dominant-negative

mutants

of

either

or

or

aPKC

The

negative

effects

on

TJ

formation

were

only

observed

when

these

mutants

were

expressed

during

the

process

of

cell

cell

contact

formation

but

not

when

expressed

in

fully

polarized

epithelial

cells

where

TJ

formation

had

already

been

completed

-LRB-

Gao

et

al.

Nagai-Tamai

et

al.

Suzuki

et

al.

Yamanaka

et

al.

-RRB-

This

strongly

suggests

that

the

PAR

aPKC

complex

develops

its

polarizing

activity

at

an

early

stage

of

cell

cell

contact

formation

and

that

it

is

critical

for

the

formation

of

TJs

rather

than

for

their

maintenance

Regulation

of

membrane

asymmetry

and

TJ

formation

by

the

aPKC

complex

The

formation

of

cell

cell

contacts

and

the

development

of

intercellular

junctions

with

distinct

structures

like

AJs

and

TJs

is

a

step-wise

process

-LRB-

Fig.

-RRB-

In

the

absence

of

cell

cell

contacts

cells

form

thin

protrusions

filled

by

axial

actin

filaments

which

upon

encountering

protrusions

of

other

cells

form

multiple

transient

contacts

which

are

subsequently

stabilized

-LRB-

McNeill

et

al.

-RRB-

The

first

sites

of

cell

cell

contact

formation

are

called

primordial

spot-like

AJs

-LRB-

pAJs

-RRB-

or

puncta

-LRB-

Adams

et

al.

Yonemura

et

al.

-RRB-

The

formation

of

multiple

pAJs

between

the

protrusions

of

adjacent

cells

results

in

a

zipper-like

appearance

of

the

early

cell

contact

sites

-LRB-

McNeill

et

al.

-RRB-

-LRB-

Fig.

-RRB-

During

maturation

of

cell

cell

contacts

the

pAJs

gradually

fuse

to

form

a

linear

contact

region

the

cells

start

to

polarize

and

eventually

develop

cell

junctions

with

AJs

and

TJs

Fig.

step-wise

recruitment

of

proteins

to

cell

cell

contacts

The

earliest

sites

of

stable

physical

interaction

during

cell

cell

contact

formation

are

primordial

spot-like

AJs

-LRB-

pAJs

-RRB-

or

puncta

at

the

tips

of

cellular

protrusions

During

junctional

maturation

the

cellular

protrusions

of

adjacent

cells

interdigitate

and

multiple

puncta

are

formed

along

the

sides

of

protrusions

These

puncta

gradually

fuse

to

form

linear

arrangements

of

cell

cell

contacts

sites

thus

generating

a

zipper-like

appearance

During

further

maturation

cell

cell

contacts

are

formed

along

the

entire

lateral

cell

surface

and

the

zipper-like

cell

cell

structure

disappears

Cell

contact-associated

proteins

are

recruited

in

a

step-wise

manner

The

pAJs/puncta

are

positive

for

integral

membrane

proteins

-LRB-

E-cadherin

JAM-A

-RRB-

but

also

peripheral

membrane

proteins

-LRB-

α-catenin

afadin

-RRB-

and

contain

proteins

associated

with

AJs

as

well

as

TJs

in

polarized

cells

During

the

formation

of

zipper-like

cell

contacts

occludin

is

recruited

probably

through

its

interaction

with

Thereafter

is

recruited

by

JAM-A

and/or

and

claudins

are

recruited

probably

through

interaction

with

Indirect

evidence

suggests

that

aPKC

and

appear

slightly

later

than

The

vertical

bar

reflects

the

increase

in

the

contacting

membrane

area

during

cell

cell

contact

formation

The

pAJs

are

positive

for

typical

AJ

proteins

like

E-cadherin

α-catenin

β-catenin

afadin

and

ponsin

but

also

for

typical

TJ

proteins

like

and

JAM-A

-LRB-

Adams

et

al.

Asakura

et

al.

Ebnet

et

al.

Suzuki

et

al.

Yonemura

et

al.

-RRB-

During

maturation

occludin

is

recruited

to

these

sites

and

during

further

maturation

and

aPKC

appear

-LRB-

Suzuki

et

al.

-RRB-

-LRB-

Fig.

-RRB-

Although

direct

comparison

has

not

been

performed

yet

it

is

likely

that

aPKC

together

with

appear

slightly

later

than

-LRB-

Suzuki

et

al.

-RRB-

The

formation

of

cadherin-based

pAJs

marks

the

early

sites

of

cell

cell

adhesion

and

probably

serves

as

a

landmark

or

positional

cues

for

membrane

growth

and

for

the

recruitment

of

other

integral

and

peripheral

membrane

proteins

-LRB-

Yeaman

et

al.

-RRB-

After

the

localization

of

the

first

set

of

proteins

at

pAJs

other

proteins

can

be

recruited

through

direct

physical

interactions

with

those

already

present

For

example

α-catenin-associated

could

serve

to

recruit

occludin

and

claudins

afadin

could

serve

to

recruit

JAM-A

and

-LRB-

or

vice

versa

-RRB-

and

JAM-A

or

could

recruit

which

serves

as

scaffold

to

assemble

the

aPKC

complex

Once

the

aPKC

complex

has

been

recruited

to

nascent

cell

cell

contacts

aPKC

has

to

be

activated

as

suggested

by

the

observation

that

ectopic

expression

of

a

kinase-dead

dominant-negative

mutant

of

aPKC

prevents

the

maturation

of

pAJs

into

belt-like

AJs

and

TJs

-LRB-

Suzuki

et

al.

-RRB-

The

activation

occurs

most

likely

by

the

Rho

GTPases

and

which

bind

to

the

Crib

domain

of

thereby

inducing

a

conformational

change

which

leads

to

the

activation

of

aPKC

-LRB-

Garrard

et

al.

Ohno

Peterson

et

al.

Yamanaka

et

al.

-RRB-

Both

E-cadherin

and

could

be

responsible

for

the

activation

of

and

Whereas

E-cadherin

seems

to

activate

but

not

-LRB-

Betson

et

al.

Kovacs

et

al.

Nakagawa

et

al.

Noren

et

al.

Yamada

and

Nelson

-RRB-

induces

the

activation

of

both

and

after

ectopic

expression

in

cultured

epithelial

cells

-LRB-

Fukuhara

et

al.

Fukuyama

et

al.

Kawakatsu

et

al.

-RRB-

The

association

of

the

GEF

with

-LRB-

Chen

and

Macara

Mertens

et

al.

-RRB-

could

regulate

a

locally

restricted

activation

of

specifically

at

those

sites

where

cell

cell

adhesion

has

occured

and

where

the

activity

of

aPKC

is

required

to

promote

the

maturation

of

cell

cell

contacts

and

the

development

of

TJs

from

pAJs

The

exact

mechanism

how

the

maturation

of

cell

cell

contacts

is

regulated

by

aPKC

is

not

clear

One

could

imagine

that

aPKC

phosphorylates

components

of

TJs

and

thereby

regulates

their

specific

localization

or

their

specific

functions

at

the

TJs

Phosphorylation

of

occludin

and

by

aPKCζ

has

been

found

in

vitro

-LRB-

Nunbhakdi-Craig

et

al.

-RRB-

However

a

physiological

relevance

of

these

phosphorylations

has

not

been

demonstrated

yet

Alternatively

aPKC

could

regulate

TJ

formation

indirectly

by

regulating

the

development

of

membrane

asymmetry

along

the

lateral

cell

cell

contacts

The

following

example

might

serve

to

illustrate

an

example

for

this

activity

In

polarized

epithelial

cells

aPKC

and

another

Ser/Thr

kinase

are

separately

localized

along

the

apico-basal

axis

aPKC

localizes

to

TJs

whereas

localizes

to

the

basolateral

membrane

domain

-LRB-

Bohm

et

al.

-RRB-

is

a

substrate

for

aPKC

and

aPKC-mediated

phosphorylation

of

leads

to

its

dissociation

from

the

membrane

into

the

cytoplasm

-LRB-

Hurov

et

al.

Suzuki

et

al.

-RRB-

As

a

result

is

absent

from

aPKC-containing

membrane

domains

A

reciprocal

mechanism

has

been

described

in

Drosphila

epithelial

cells

-LRB-

Benton

and

St

Johnston

-RRB-

Drosphila

phosphorylates

Bazooka

thereby

inhibiting

its

dimerization

and

blocking

its

ability

to

assemble

a

functional

aPKC

complex

As

a

result

the

aPKC

complex

is

absent

from

membrane

domains

Through

these

reciprocal

inhibitory

interactions

two

distinct

membrane

domains

are

generated

characterized

by

the

mutual

exclusion

of

aPKC

and

Thus

by

regulating

the

formation

of

a

specific

membrane

domain

from

which

certain

proteins

are

actively

excluded

aPKC

could

indirectly

promote

the

formation

of

TJs

It

is

not

clear

yet

if

the

abilities

of

the

aPKC

complex

to

regulate

TJ

formation

and

to

regulate

membrane

asymmetry

are

mechanistically

linked

Regulation

of

TJ

maintenance

by

the

Amot

complex

Recently

a

new

protein

complex

has

been

identified

which

has

been

suggested

to

regulate

the

maintenance

rather

than

the

formation

of

TJs

The

functional

core

of

this

complex

is

a

set

of

two

proteins

which

regulate

the

activity

of

the

and

angiomotin

-LRB-

Amot

-RRB-

proteins

-LRB-

Wells

et

al.

-RRB-

-LRB-

Fig.

-RRB-

is

a

Rho

GTPase

activating

protein

-LRB-

RhoGAP

-RRB-

for

and

with

a

activity

in

epithelial

cells

it

contains

a

BAR

domain

and

a

RhoGAP

domain

-LRB-

Richnau

and

Aspenstrom

Richnau

et

al.

Wells

et

al.

-RRB-

Amot

is

a

scaffolding/adapator

protein

with

a

coiled-coil

domain

region

and

a

C-terminal

PDZ

domain-binding

motif

-LRB-

Bratt

et

al.

-RRB-

The

Amot

complex

is

targeted

to

TJs

through

a

PDZ

domain-dependent

interaction

of

Amot

with

PATJ

-LRB-

Fig.

-RRB-

Both

overexpression

of

Amot

and

partial

downregulation

of

by

RNA

interference

affect

the

barrier

function

of

TJs

More

importantly

the

loss

of

the

barrier

function

in

response

to

removal

is

accelerated

after

partial

downregulation

-LRB-

Wells

et

al.

-RRB-

suggesting

that

the

Amot

complex

is

important

for

the

maintenance

of

functional

TJs

This

function

of

the

Amot

complex

probably

resides

in

regulating

the

cycling

of

and

maintaining

the

pool

of

active

at

TJs

at

a

low

level

Fig.

Amot

complex

at

TJs

is

a

RhoGAP

with

specificity

for

in

epithelial

cells

interacts

with

Amot

through

a

reciprocal

BAR

domain-dependent

interaction

and

Amot

binding

regulates

activity

associates

with

through

its

GAP

domain

Amot

directly

interacts

with

one

-LRB-

or

several

-RRB-

of

the

PDZ

domains

of

PATJ

that

in

turn

associates

with

Surprisingly

is

directly

or

indirectly

associated

with

and

aPKC

and

this

PAR

complex

is

distinct

form

the

aPKC

complex

because

it

does

not

contain

and

because

aPKC

can

not

be

associated

with

the

kDa

isoform

of

which

lacks

the

aPKC-binding

domain

All

three

Amot-like

proteins

-LRB-

Amot

-RRB-

form

protein

complexes

with

and

PATJ

Double

arrows

with

solid

lines

indicate

direct

protein

protein

interactions

arrows

with

broken

lines

indicate

the

presence

of

the

two

proteins

in

the

same

complex

but

the

nature

of

the

interaction

has

not

been

characterized

in

detail

yet

Besides

Amot

PATJ

and

the

immunoprecipitates

contain

and

aPKC

-LRB-

Wells

et

al.

-RRB-

Surprisingly

the

immunoprecipitates

do

not

contain

and

they

contain

the

kDa

isoform

of

which

lacks

the

aPKC-interacting

domain

and

thus

can

not

directly

associate

with

aPKC

-LRB-

Lin

et

al.

-RRB-

This

suggests

that

and

aPKC

can

undergo

interactions

with

the

Amot

complex

which

are

independent

of

the

aPKC

interaction

with

the

PATJ

complex

-LRB-

Hurd

et

al.

-RRB-

-LRB-

Fig.

-RRB-

The

functional

relevance

of

this

interaction

is

not

clear

Two

Amot-like

-LRB-

Amotl

-RRB-

proteins

and

which

have

been

described

earlier

as

TJ

components

-LRB-

Nishimura

et

al.

Patrie

-RRB-

are

present

in

Amot

but

not

protein

complexes

All

three

Amot

proteins

directly

interact

with

the

scaffolding

protein

and

its

paralogue

PATJ

-LRB-

Sugihara-Mizuno

et

al.

-RRB-

-LRB-

Fig.

-RRB-

These

findings

suggest

that

additional

Amot

protein-containing

complexes

exist

with

functions

different

from

regulating

activity

Signaling

from

TJs

In

addition

to

the

relatively

stable

protein

complexes

described

so

far

-LRB-

Fig.

-RRB-

many

protein

complexes

at

TJs

assemble

only

transiently

In

addition

some

proteins

are

not

exclusively

associated

with

TJ

but

shuttle

between

the

TJ

and

other

compartments

in

the

cell

The

identification

of

such

proteins

has

revealed

that

TJ

proteins

are

engaged

in

receiving

signals

but

also

in

delivering

signals

to

the

cell

interiour

and

thereby

regulate

epithelial

proliferation

and

differentiation

-LRB-

Matter

and

Balda

-RRB-

The

mechanism

by

which

TJ

proteins

influence

for

example

gene

expression

is

most

likely

indirect

through

binding

and

sequestration

of

regulatory

molecules

at

the

TJs

as

exemplified

by

associates

with

ZONAB/DbpA

a

transcription

factor

which

promotes

proliferation

of

epithelial

cells

in

part

by

interacting

with

the

cell

division

kinase

and

also

by

regulating

the

expression

of

genes

involved

in

proliferation

like

cyclin

and

PCNA

-LRB-

Balda

et

al.

Balda

and

Matter

Sourisseau

et

al.

-RRB-

In

proliferating

cells

which

have

little

ZONAB/DbpA

expression

is

high

and

ZONAB/DbpA

protein

is

localized

in

the

nucleus

When

cells

reach

confluence

and

develop

intercellular

junctions

is

accumulating

at

cell

cell

contacts

and

recruits

ZONAB/DbpA

to

the

junctions

thus

sequestering

it

away

from

the

nucleus

-LRB-

Balda

and

Matter

-RRB-

Interestingly

during

cellular

stress

ZONAB/DbpA

associated

with

can

be

re-activated

The

heat

shock

protein

that

is

distributed

in

the

cytoplasm

under

normal

conditions

is

recruited

to

cell

cell

contacts

in

response

to

heat

shock

where

it

binds

directly

to

using

the

same

binding

interface

like

ZONAB/DbpA

i.e.

the

domain

of

-LRB-

Tsapara

et

al.

-RRB-

This

leads

to

a

loss

of

ZONAB/DbpA

from

cell

junctions

and

in

activation

of

the

transcriptional

activity

of

ZONAB/DbpA

-LRB-

Tsapara

et

al.

-RRB-

Thus

influences

gene

expression

and

cell

cycle

progression

in

a

cell

density-dependent

manner

and

this

function

can

be

regulated

during

cellular

stress

is

another

scaffolding

proteins

at

TJs

which

is

involved

in

signaling

In

contrast

to

however

seems

to

actively

shuttle

between

TJs

and

the

nucleus

contains

functional

nuclear

localization

and

nuclear

export

signals

-LRB-

Gonzalez-Mariscal

et

al.

Jaramillo

et

al.

-RRB-

and

interacts

with

various

proteins

that

have

nuclear

functions

including

the

transcription

factors

and

C/EBP

-LRB-

Betanzos

et

al.

-RRB-

the

DNA-binding

protein

SAF-B

-LRB-

Traweger

et

al.

-RRB-

and

the

family

member

ARVCF

-LRB-

Kausalya

et

al.

-RRB-

probably

inhibits

the

activity

of

the

transcription

factors

and

C/EBP

by

regulating

their

export

from

the

nucleus

which

is

consistent

with

the

predominant

nuclear

localization

of

in

sparse

cells

and

the

localization

of

as

well

as

and

C/EBP

at

TJs

in

confluent

cells

-LRB-

Betanzos

et

al.

-RRB-

In

the

case

of

ARVCF

regulates

its

nuclear

import

-LRB-

Kausalya

et

al.

-RRB-

where

it

might

regulate

transcription

similar

to

other

family

members

-LRB-

Hatzfeld

-RRB-

In

summary

the

identification

of

protein

complexes

formed

by

typical

TJ

proteins

and

typical

nuclear

proteins

involved

in

the

regulation

of

transcription

indicates

that

the

TJs

participate

in

the

regulation

of

proliferation

and

differentiation

A

protein

complex

at

the

lateral

membrane

which

regulates

TJ

formation

the

Scribble

Discs

Large

Lethal

Giant

Larvae

complex

The

Scribble

complex

comprises

the

proteins

Scribble

-LRB-

Scrib

-RRB-

Discs

Large

-LRB-

Dlg

-RRB-

and

Lethal

Giant

Larvae

-LRB-

Lgl

-RRB-

-LRB-

Fig.

-RRB-

which

have

originally

been

identified

in

Drosophila

as

tumor

suppressor

proteins

-LRB-

Bilder

-RRB-

Mutations

in

the

scrib

dlg

or

lgl

genes

result

in

overgrowth

of

certain

tissues

ultimately

leading

to

a

giant

larvae

phenotype

and

also

in

a

disruption

of

apico-basal

polarity

-LRB-

Bilder

-RRB-

All

three

proteins

are

membrane-associated

in

Drosophila

epithelial

cells

Scrib

and

Dlg

localize

to

the

Septate

Junctions

-LRB-

SJ

-RRB-

a

structure

that

is

a

functional

homologue

of

vertebrate

TJs

but

which

is

localized

basally

of

the

AJs

Lgl

localizes

along

the

lateral

membrane

domain

and

is

excluded

from

the

SAR

-LRB-

Tepass

et

al.

-RRB-

Importantly

the

Scrib

complex

genetically

interacts

with

the

aPKC

and

the

Crumbs

Sdt

PATJ

complexes

to

regulate

apico-basal

polarity

-LRB-

Bilder

et

al.

Tanentzapf

and

Tepass

-RRB-

All

three

proteins

are

conserved

and

exist

in

vertebrates

In

mammals

one

homologue

of

Drosophila

Scrib

four

homologues

of

Dlg

-LRB-

-RRB-

and

two

homologues

of

Lgl

-LRB-

-RRB-

exist

and

the

proteins

localize

to

the

basolateral

membrane

domain

of

epithelial

cells

-LRB-

Dow

and

Humbert

Humbert

et

al.

-RRB-

As

opposed

to

the

aPKC

and

the

PATJ

complexes

Scrib

Dlg

and

Lgl

do

not

seem

to

form

a

ternary

complex

through

direct

interactions

Recent

evidence

indicates

that

Scrib

exists

in

a

complex

with

-LRB-

Kallay

et

al.

-RRB-

but

it

is

not

clear

if

this

interaction

is

direct

or

indirect

as

described

in

Drosophila

-LRB-

Mathew

et

al.

-RRB-

The

role

of

the

Srib

protein

complex

in

regulating

apico-basal

polarity

and

cell

cell

contact

formation

seems

to

be

conserved

in

vertebrates

Scrib

knockdown

results

in

a

delayed

TJ

formation

-LRB-

Qin

et

al.

-RRB-

and

knockdown

of

disturbs

TJ

formation

after

switch-induced

new

cell

cell

contact

formation

-LRB-

Stucke

et

al.

-RRB-

Furthermore

Lgl

has

been

described

to

regulate

cell

cell

contact

and

apico-basal

polarity

formation

by

forming

a

complex

with

and

aPKC

-LRB-

Plant

et

al.

Yamanaka

et

al.

-RRB-

-LRB-

Fig.

-RRB-

The

interaction

of

Lgl

with

and

aPKC

precludes

the

binding

of

to

and

aPKC

Also

the

Lgl

aPKC

complex

counteracts

the

activity

of

the

aPKC

complex

by

suppressing

not

only

its

formation

but

also

its

activation

by

-LRB-

Yamanaka

et

al.

-RRB-

According

to

the

current

model

-LRB-

Fig.

-RRB-

Lgl

forms

a

complex

with

aPKC

and

early

during

cell

cell

contact

formation

and

blocks

the

formation

of

the

aPKC

complex

at

this

stage

Upon

phosphorylation

by

aPKC

Lgl

dissociates

from

the

complex

thus

allowing

for

the

formation

and

activation

of

the

aPKC

complex

and

the

development

of

apical

TJs

harboring

the

PAR

aPKC

complex

and

a

basolateral

domain

harboring

Lgl

-LRB-

Yamanaka

et

al.

-RRB-

Fig.

Scribble

-LRB-

Scrb

-RRB-

Discs

large

-LRB-

Dlg

-RRB-

and

Lethal

giant

larvae

-LRB-

Lgl

-RRB-

proteins

localize

to

the

basolateral

membrane

domain

in

polarized

epithelial

cells

Scrb

and

Lgl

exist

in

a

complex

but

it

is

not

clear

if

the

interaction

is

direct

Inset

During

the

process

of

cell

cell

contact

formation

Lgl

forms

a

transient

complex

with

aPKC

and

from

which

is

excluded

After

aPKC-induced

dissociation

of

Lgl

from

the

complex

associates

with

and

aPKC

which

promotes

TJ

formation

and

the

development

of

apical

and

basolateral

membrane

domains

Double

arrows

with

solid

lines

indicate

direct

protein

protein

interactions

double

arrows

with

broken

lines

indicate

the

presence

of

the

two

proteins

in

the

same

complex

but

the

nature

of

the

interaction

has

not

been

characterized

in

detail

yet

Protein

complexes

at

cell

junctions

and

cancer

Given

the

important

role

of

protein

complexes

localized

at

cell

junctions

of

epithelial

cells

in

regulating

cell

cell

contact

formation

cell

polarity

and

cell

proliferation

it

is

not

surprising

that

altering

the

compositions

of

these

protein

complexes

will

result

in

changes

in

cell

cell

adhesion

a

loss

in

cell

cell

contact

integrity

and

eventually

in

uncontrolled

proliferation

and

cancer

A

loss

of

cell

polarity

is

frequently

associated

with

cancer

-LRB-

Bissell

and

Radisky

Wodarz

and

Nathke

-RRB-

As

pointed

out

above

many

tumor

suppressor

genes

identified

in

Drosophila

including

dlg

scrib

and

lgl

turned

out

to

encode

proteins

which

regulate

epithelial

cell

polarity

-LRB-

Bilder

-RRB-

and

loss

of

their

expression

correlates

with

more

invasive

and

aggressive

cancers

in

mammalian

cells

-LRB-

Dow

and

Humbert

-RRB-

Recent

evidence

identified

the

aPKC

complex

at

TJs

as

a

target

for

the

oncogenic

receptor

tyrosine

kinase

When

cultured

on

reconstituted

basement

membrane

mammary

epithelial

cells

form

three-dimensional

spheroids

that

resemble

glandular

structures

-LRB-

acini

-RRB-

with

a

single-layered

polarized

epithelium

surrounding

a

luminal

space

-LRB-

Debnath

and

Brugge

-RRB-

signaling

leads

to

multiacinar

structures

as

a

result

of

hyperproliferation

and

to

filling

of

the

luminal

space

in

individual

acini

due

to

a

block

of

apoptosis

of

the

inner

cells

The

latter

of

these

two

effects

turned

out

to

be

regulated

by

the

aPKC

complex

-LRB-

Aranda

et

al.

-RRB-

Although

the

molecular

mechanism

has

not

been

revealed

in

detail

the

findings

indicate

that

in

response

to

activation

physically

interacts

with

the

aPKC

complex

which

leads

to

a

removal

of

and

the

formation

of

a

aPKC

complex

-LRB-

Fig.

-RRB-

The

newly

formed

aPKC

complex

disrupts

apico-basal

polarity

and

blocks

apoptosis

of

inner

acinar

cells

through

aPKC

activity

-LRB-

Aranda

et

al.

-RRB-

A

role

of

aPKC

in

regulating

apoptosis

of

inner

cells

has

been

observed

in

MDCK

cysts

as

well

-LRB-

Kim

et

al.

-RRB-

Thus

the

oncogene

exploits

the

PAR

aPKC

system

to

regulate

survival

of

the

cells

Fig.

oncogene

targets

the

PAR

complex

In

normal

cells

aPKC

and

form

a

stable

complex

at

TJs

of

epithelial

cells

-LRB-

left

panel

-RRB-

This

complex

is

required

for

the

fomation

of

TJs

and

the

development

of

apico-basal

polarity

activation

triggers

the

association

of

the

homodimer

with

and

aPKC

thereby

disrupting

the

aPKC

complex

As

a

consequence

the

development

of

apico-basal

polarity

is

inhibited

and

inner

cells

do

not

undergo

apoptosis

-LRB-

right

panel

-RRB-

Besides

increased

proliferation

and

reduced

apoptosis

epithelial-mesenchymal

transition

-LRB-

EMT

-RRB-

is

another

hallmark

of

tumor

progression

-LRB-

Thiery

-RRB-

During

EMT

polarized

epithelial

cells

adopt

a

mesenchymal

or

fibroblastoid

highly

motile

phenotype

and

this

is

required

during

phases

of

embryonic

development

when

epithelial

cells

leave

a

primitive

epithelium

to

migrate

to

a

distinct

site

in

the

embryo

in

order

to

induce

new

organ

formation

Not

surprisingly

typical

characteristics

of

EMT

are

transcriptional

repression

of

E-cadherin

expression

profound

changes

in

the

cytoskeleton

concomitant

with

a

loss

of

apico-basal

polarity

-LRB-

Thiery

-RRB-

When

this

developmentally

regulated

programme

is

re-activated

in

the

adult

organism

it

easily

contributes

to

tumor

progression

by

facilitating

invasion

and

metastasis

Among

the

various

physiological

inducers

of

EMT

TGFβ

signaling

turned

to

be

critically

involved

in

EMT

through

cooperation

with

receptor

tyrosine

kinases

-LRB-

RTKs

-RRB-

and

oncogenic

Ras

-LRB-

Huber

et

al.

-RRB-

Recent

evidence

indicates

that

TGFβ

signaling

affects

not

only

the

integrity

of

AJs

through

its

known

effect

on

E-cadherin

expression

but

also

the

integrity

of

TJs

by

targeting

the

aPKC

complex

TGFβ

signaling

is

mediated

by

two

TGFβ

receptors

the

Ser/Thr

kinases

TGFβ-receptor

I

-LRB-

TβRI

-RRB-

and

TβRII

In

polarized

NMuMG

cells

TGFβ

induces

EMT

by

recruiting

TβRII

to

TJs

which

results

in

TJ

dissolution

TβRI

localizes

to

TJ

through

a

direct

interaction

with

occludin

-LRB-

Barrios-Rodiles

et

al.

-RRB-

Interestingly

TβRI

interacts

also

directly

with

-LRB-

Ozdamar

et

al.

-RRB-

-LRB-

Fig.

-RRB-

A

TGFβ

signal

triggers

heterodimeric

complex

formation

between

the

two

TGFβ

receptors

bringing

TβRII

in

close

vicinity

of

the

PAR

complex

is

then

phosphorylated

by

TβRII

and

this

leads

to

the

recruitement

of

the

ubiquitin

ligase

which

in

turn

mediates

ubiquitination

of

RhoA

RhoA

however

is

critical

for

junctional

integrity

-LRB-

Sahai

and

Marshall

-RRB-

and

its

localized

degradation

might

thus

lead

to

TJ

dissolution

TGFβ

does

not

only

induce

RhoA

degradation

via

and

but

also

induces

the

downregulation

of

-LRB-

Wang

et

al.

-RRB-

-LRB-

Fig.

-RRB-

Although

the

fate

of

aPKC

during

this

process

has

not

been

clarified

these

findings

suggest

that

the

aPKC

complex

is

a

major

target

of

TGFβ

signaling

at

TJs

during

TGFβ-induced

EMT

Fig.

signaling

targets

the

PAR

complex

at

TJs

Left

panel

Under

normal

conditions

TGFβ

receptor

I

-LRB-

TβRI

-RRB-

localizes

to

TJ

through

direct

interactions

with

occludin

and

The

cells

maintain

a

polarized

morrphology

Middle

panel

TGFβ

induces

heterodimer

formation

of

the

two

TGFβ

receptors

TβRI

and

leading

to

activation

of

TβRII

folllowed

by

TβRII-mediated

phosphorylation

of

at

Right

panel

recruits

leading

to

ubiquitination

and

degradation

of

the

local

pool

of

RhoA

As

a

consequence

the

integrity

of

TJs

is

disturbed

the

polarized

morphology

can

not

be

maintained

and

the

development

of

a

fibroblastoid

morphology

is

facilitated

In

addition

to

phosphorylation

TGFβ

signaling

also

induces

downregulation

of

By

targeting

the

aPKC

complex

at

TJs

TGFβ

impairs

the

ability

of

cells

to

maintain

a

polarized

morphology

Protein

complexes

as

targets

for

pathogens

Many

pathogens

need

to

overcome

epithelial

and

endothelial

barriers

to

invade

the

host

and

establish

infection

For

this

purpose

various

strategies

have

evolved

to

disrupt

the

barrier

and

allow

the

pathogen

the

passage

into

tissues

These

strategies

include

the

release

of

proteolytic

enzymes

that

cleave

adhesion

molecules

like

occludin

E-cadherin

or

desmoglein

-LRB-

Hanakawa

et

al.

Pentecost

et

al.

Wu

et

al.

-RRB-

or

the

release

of

toxins

that

act

via

cell

surface

receptors

to

induce

intracellular

changes

-LRB-

e.g.

of

the

actin

cytoskeleton

-RRB-

which

eventually

lead

to

alterations

of

the

barrier

-LRB-

Hopkins

et

al.

Nusrat

et

al.

-RRB-

More

advanced

strategies

involve

the

delivery

of

pathogen-derived

proteins

via

secretion

systems

into

the

host

cell

cytoplasm

where

these

proteins

directly

associate

with

host

cell

proteins

to

influence

their

function

One

example

is

the

Helicobacter

pylori

-LRB-

H.pylori

-RRB-

effector

protein

Cytotoxin-associated

gene

A

antigen

-LRB-

CagA

-RRB-

H.pylori

induces

morphological

changes

of

epithelial

cells

alterations

of

the

composition

of

the

apical

junctional

complex

as

well

as

a

breakdown

of

the

epithelial

barrier

function

-LRB-

Amieva

et

al.

Bagnoli

et

al.

-RRB-

and

H.pylori

infections

can

result

in

mucosal

damage

-LRB-

ulceration

-RRB-

inflammation

-LRB-

gastritis

-RRB-

and

cancer

-LRB-

gastric

carcinoma

-RRB-

-LRB-

Peek

and

Blaser

-RRB-

The

CagA

protein

is

involved

in

many

of

these

processes

through

multiple

interactions

with

a

number

of

host

proteins

After

the

translocation

into

the

host

cell

CagA

is

phosphorylated

by

src-family

kinases

and

recruits

the

phosphotyrosine

phosphatase

-LRB-

Higashi

et

al.

-RRB-

In

addition

CagA

associates

with

several

proteins

involved

in

the

regulation

of

TJs

and

in

the

formation

of

apico-basal

polarity

First

it

recruits

and

thereby

mislocalizes

the

TJ

proteins

and

JAM-A

to

the

site

of

bacterial

attachment

-LRB-

Amieva

et

al.

-RRB-

Second

it

directly

interacts

with

the

serine/threonine

kinase

-LRB-

Saadat

et

al.

-RRB-

Under

normal

conditions

cooperates

with

the

aPKC

complex

through

reciprocal

phosphorylations

to

regulate

the

formation

of

distinct

membrane

domains

-LRB-

Hurov

et

al.

Suzuki

et

al.

-RRB-

-LRB-

see

also

above

-RRB-

The

binding

of

CagA

to

blocks

the

kinase

activity

of

thus

preventing

the

phosphorylation

of

at

the

same

time

it

prevents

the

phosphorylation

of

itself

by

aPKC

As

a

result

the

integrity

of

cell

cell

contacts

is

disturbed

and

cells

are

extruded

from

the

monolayer

-LRB-

Saadat

et

al.

-RRB-

Thus

through

its

multiple

interactions

with

signaling

molecules

scaffolding

proteins

and

cell

polarity

proteins

CagA

disregulates

critical

cellular

functions

to

enter

the

sub-epithelial

tissues

which

also

leads

to

inflammation

and

eventually

to

cancer

-LRB-

Hatakeyama

-RRB-

Conclusions

and

perspectives

The

last

decade

has

witnessed

a

steady

increase

in

the

number

of

new

proteins

localized

at

cell

cell

contacts

of

epithelial

cells

The

identification

of

claudins

at

TJs

has

strongly

increased

the

understanding

of

the

molecular

basis

of

TJ

function

The

identification

of

cell

polarity

protein

complexes

like

the

aPKC

complex

and

the

PATJ

complex

at

TJs

has

added

new

aspects

on

the

mechanisms

underlying

the

development

of

TJs

The

identification

of

the

nectin

afadin

system

provided

evidence

for

a

second

major

adhesive

system

besides

the

cadherin

catenin

system

at

AJs

It

also

became

evident

that

TJs

and

AJs

are

signaling

centers

which

are

actively

engaged

in

regulating

proliferation

and

differentiation

through

feed-back

mechanisms

with

the

cytoskeleton

and

the

nucleus

Meanwhile

many

of

the

proteins

that

regulate

formation

and

maintenance

of

cell

cell

contacts

in

epithelial

cells

have

been

found

in

other

cellular

systems

as

well

suggesting

a

general

function

in

cell

cell

contact

regulation

Claudins

nectins

and

JAMs

are

used

by

cells

of

the

male

reproductive

system

to

mediate

homotypic

Sertoli

Sertoli

cell

as

well

as

heterotypic

Sertoli

cell

spermatid

interactions

-LRB-

Gliki

et

al.

Gow

et

al.

Takai

and

Nakanishi

-RRB-

and

similar

functions

are

performed

by

these

molecules

as

those

proposed

in

epithelial

cells

i.e.

formation

of

TJ

strands

regulation

of

the

actin

cytoskeleton

and

regulation

of

cellular

polarization

respectively

-LRB-

Gliki

et

al.

Gow

et

al.

Ozaki-Kuroda

et

al.

-RRB-

In

the

peripheral

nervous

system

-LRB-

PNS

-RRB-

a

large

number

of

proteins

typically

associated

with

TJs

or

AJs

was

found

to

mediate

autotypic

Schwann

cell

interactions

within

the

myelin

sheath

as

well

as

heterotypic

Schwann

cell

axon

interactions

For

example

different

claudins

JAM-C

and

various

scaffolding

proteins

like

PATJ

and

are

localized

at

areas

of

non-compact

myelin

including

Schmidt-Lanterman-incisures

paranodal

loops

and

mesaxons

and

some

proteins

preferentially

localize

to

some

but

not

other

areas

-LRB-

Maurel

et

al.

Poliak

et

al.

Spiegel

et

al.

-RRB-

-LRB-

Fig.

-RRB-

The

absence

of

or

JAM-C

in

mice

results

in

defective

nerve

conduction

indicating

that

both

are

critically

important

for

the

proper

functioning

of

the

PNS

-LRB-

Miyamoto

et

al.

Scheiermann

et

al.

-RRB-

and

mediate

heterotypic

interactions

between

axons

and

Schwann

cells

during

myelination

-LRB-

Maurel

et

al.

Spiegel

et

al.

-RRB-

And

has

recently

been

identified

at

the

interface

between

pre-myelinating

Schwann

cells

and

axons

along

the

internodal

region

-LRB-

Chan

et

al.

-RRB-

is

expressed

by

the

Schwann

cells

not

by

the

axon

and

recruits

the

neurotrophin

receptor

to

the

glial

axon

junction

to

regulate

myelination

of

the

axon

Fig.

at

autotypic

and

heterotypic

cell

cell

contacts

in

the

peripheral

nervous

system

-LRB-

PNS

-RRB-

A

multitude

of

proteins

localized

at

TJs

of

polarized

epithelial

cells

localizes

to

autotypic

glial

glial

cell

contacts

and

heteroytpic

glial

cell

axon

contacts

Abbreviations

JXP

juxtaparanodal

region

MV

microvilli

PNL

paranodal

loops

PNJ

paranodal

junction

SLI

Schmidt-Lanterman

incisure

It

is

not

clear

if

these

proteins

form

the

same

complexes

like

in

epithelial

cells

and

it

is

likely

that

differences

exist

in

the

composition

of

protein

complexes

to

regulate

the

specific

requirements

in

the

context

of

the

given

cell

or

tissue

As

one

example

JAM-C

deficiency

in

mice

leads

to

a

mislocalization

of

aPKC

and

PATJ

bot

not

in

spermatids

-LRB-

Gliki

et

al.

-RRB-

suggesting

that

is

not

part

of

the

JAM-C-associated

polarity

complex

in

spermatids

despite

its

ability

to

interact

with

aPKC

and

also

directly

with

JAM-C

-LRB-

Ebnet

et

al.

Suzuki

and

Ohno

-RRB-

As

another

example

in

endothelial

cells

two

PAR

protein

complexes

have

been

identified

a

conventional

aPKC

complex

and

a

second

PAR

complex

in

which

and

are

independently

associated

with

VE-cadherin

and

which

lacks

aPKC

-LRB-

Iden

et

al.

-RRB-

Nevertheless

the

use

of

a

set

of

conserved

proteins

by

morphologically

diverse

cell

types

to

regulate

cell

cell

contact

formation

highlights

both

the

importance

of

the

proteins

for

cellular

function

as

well

as

their

versatility

that

allows

the

regulation

of

similar

aspects

in

different

cell

types

Many

open

questions

remain

For

example

what

is

the

molecular

nature

of

the

intramembrane

diffusion

barrier

-LRB-

fence

function

-RRB-

of

the

TJs

The

absence

of

TJ

strands

in

cells

lacking

all

three

ZO

proteins

results

in

a

complete

loss

of

the

barrier

function

of

the

epithelial

sheet

but

unexpectedly

the

fence

function

which

has

been

attributed

to

the

presence

of

TJ

strands

is

retained

-LRB-

Umeda

et

al.

-RRB-

Membrane

diffusion

barriers

exist

also

in

other

cells

even

in

the

absence

of

a

physical

cell

cell

contact

for

example

at

the

axonal

hillock

of

neurons

to

separate

somatodendritic

and

axonal

membrane

domains

-LRB-

Winckler

et

al.

-RRB-

It

has

been

suggested

that

in

these

cells

the

accumulation

of

integral

membrane

proteins

that

are

anchored

to

the

submembranous

cytoskeleton

function

as

rows

of

pickets

which

prevent

the

free

diffusion

of

even

small

molecules

-LRB-

Nakada

et

al.

-RRB-

A

second

unresolved

issue

is

if

the

activity

of

the

aPKC

complex

to

regulate

membrane

asymmetry

is

mechanistically

linked

to

its

role

in

TJ

formation

Finally

the

functional

interrelationship

of

the

three

major

polarity

complexes

at

TJs

-LRB-

depicted

in

Figs

-RRB-

has

not

been

resolved

yet

Members

of

individual

complexes

interact

with

each

other

For

example

can

also

interact

with

-LRB-

Lemmers

et

al.

-RRB-

can

also

interact

with

-LRB-

Hurd

et

al.

-RRB-

and

can

interact

with

PATJ

-LRB-

Roh

et

al.

-RRB-

Genetic

evidence

in

Drosophila

suggests

a

functional

hierarchy

among

the

protein

complexes

-LRB-

Johnson

and

Wodarz

-RRB-

but

it

is

unclear

if

a

similar

hierarchy

exists

in

vertebrate

epithelial

cells

and

which

aspects

of

cell

cell

contact

formation

are

regulated

by

these

interactions

Most

likely

many

of

these

interactions

are

dynamically

regulated

and

occur

in

a

temporally

and

spatially

restricted

manner

The

large

number

of

scaffolding

and

signaling

molecules

identified

at

cell

cell

contacts

and

the

multitude

of

physical

interactions

described

so

far

among

these

proteins

indicates

that

cell

cell

contact

formation

the

development

of

TJs

from

pAJs

and

the

aquisition

of

membrane

polarity

is

a

highly

dynamic

process

the

complexity

of

which

is

still

far

from

being

completely

understood

cell polarity
protein complexes
par proteins

adherens junction
tight junction
cell–cell adhesion
jam