The

desmosome

and

pemphigus

Desmosomes

are

patch-like

intercellular

adhering

junctions

-LRB-

maculae

adherentes

-RRB-

which

in

concert

with

the

related

adherens

junctions

provide

the

mechanical

strength

to

intercellular

adhesion

Therefore

it

is

not

surprising

that

desmosomes

are

abundant

in

tissues

subjected

to

significant

mechanical

stress

such

as

stratified

epithelia

and

myocardium

Desmosomal

adhesion

is

based

on

the

dependent

homo

and

heterophilic

transinteraction

of

cadherin-type

adhesion

molecules

Desmosomal

cadherins

are

anchored

to

the

intermediate

filament

cytoskeleton

by

adaptor

proteins

of

the

armadillo

and

plakin

families

Desmosomes

are

dynamic

structures

subjected

to

regulation

and

are

therefore

targets

of

signalling

pathways

which

control

their

molecular

composition

and

adhesive

properties

Moreover

evidence

is

emerging

that

desmosomal

components

themselves

take

part

in

outside-in

signalling

under

physiologic

and

pathologic

conditions

Disturbed

desmosomal

adhesion

contributes

to

the

pathogenesis

of

a

number

of

diseases

such

as

pemphigus

which

is

caused

by

autoantibodies

against

desmosomal

cadherins

Beside

pemphigus

desmosome-associated

diseases

are

caused

by

other

mechanisms

such

as

genetic

defects

or

bacterial

toxins

Because

most

of

these

diseases

affect

the

skin

desmosomes

are

interesting

not

only

for

cell

biologists

who

are

inspired

by

their

complex

structure

and

molecular

composition

but

also

for

clinical

physicians

who

are

confronted

with

patients

suffering

from

severe

blistering

skin

diseases

such

as

pemphigus

To

develop

disease-specific

therapeutic

approaches

more

insights

into

the

molecular

composition

and

regulation

of

desmosomes

are

required

Introduction

Desmosomes

are

intercellular

adhering

junctions

serving

to

attach

neighbouring

cells

to

each

other

They

are

most

numerous

in

tissues

subjected

to

significant

mechanical

stress

such

as

the

stratified

squamous

epithelia

of

the

skin

-LRB-

Bizzozero

-RRB-

and

of

mucous

membranes

-LRB-

Farquhar

and

Palade

-RRB-

as

well

as

the

myocardium

-LRB-

Fawcett

and

Selby

-RRB-

Moreover

desmosomes

are

found

in

simple

epithelia

and

in

non-epithelial

cells

such

as

the

meningeal

cells

of

the

arachnoidea

-LRB-

Gusek

-RRB-

and

the

follicular

dendritic

cells

of

lymph

follicles

-LRB-

Swartzendruber

-RRB-

Desmosomes

were

discovered

as

cell

contacts

in

the

middle

of

the

nineteenth

century

-LRB-

Calkins

and

Setzer

-RRB-

By

the

means

of

light

microscopy

desmosomes

were

first

described

in

the

epidermis

by

the

Italian

pathologist

Bizzozero

in

-LRB-

-RRB-

In

his

histology

text

book

the

anatomist

Josef

Schaffer

from

Vienna

introduced

the

term

desmosome

by

combining

the

greek

words

desmos

-LRB-

bond

-RRB-

and

soma

-LRB-

body

-RRB-

although

to

that

time

he

like

most

others

in

the

field

believed

that

desmosomes

were

cytoplasm-filled

intercellular

bridges

-LRB-

Schaffer

-RRB-

It

took

almost

another

century

until

Keith

Porter

using

electron

microscopy

was

able

to

confirm

the

basic

observation

of

Bizzozero

that

desmosomes

rather

are

contacts

between

adjacent

cells

and

to

allow

the

first

description

on

desmosome

ultrastructure

-LRB-

Porter

-RRB-

With

these

new

technical

advances

at

hand

several

studies

were

performed

in

the

following

years

on

the

distribution

and

organization

of

desmosomes

in

various

tissues

In

addition

starting

in

the

biochemical

approaches

and

molecular

cloning

techniques

were

applied

to

identify

the

desmosomal

components

and

to

characterize

their

interactions

-LRB-

Drochmans

et

al.

Moll

et

al.

Moll

and

Franke

Schwarz

et

al.

Skerrow

and

Matoltsy

-RRB-

Significant

insights

into

the

regulation

of

desmosomal

adhesion

also

came

from

the

field

of

dermatology

since

it

was

demonstrated

that

autoantibodies

in

patients

suffering

from

the

autoimmune

blistering

skin

diseases

pemphigus

vulgaris

-LRB-

PV

-RRB-

and

pemphigus

foliaceus

-LRB-

PF

-RRB-

are

directed

to

sensitive

cell

surface

proteins

within

desmosomes

-LRB-

Eyre

and

Stanley

Jones

et

al.

Karpati

et

al.

-RRB-

which

were

identified

as

the

desmosomal

cadherins

desmoglein

-LRB-

Dsg

-RRB-

and

Dsg

-LRB-

Amagai

et

al.

Koulu

et

al.

-RRB-

The

term

pemphigus

comes

from

the

greek

word

pemphix

-LRB-

blister

-RRB-

and

is

being

used

in

dermatology

since

-LRB-

Schmidt

et

al.

-RRB-

long

before

it

was

found

that

pemphigus

is

associated

with

autoantibodies

against

keratinocyte

surface

antigens

-LRB-

Beutner

and

Jordon

-RRB-

and

that

these

antibodies

are

sufficient

to

cause

acantholysis

i.e.

loss

of

cell

cell

adhesion

in

human

skin

in

vivo

and

in

vitro

-LRB-

Anhalt

et

al.

Schiltz

and

Michel

-RRB-

The

final

break-through

was

the

finding

that

autoantibodies

against

the

extracellular

domains

of

Dsg

and

Dsg

in

PV

and

in

PF

are

pathogenic

-LRB-

Amagai

et

al.

-RRB-

Therefore

autoantibodies

from

pemphigus

patients

have

been

used

to

characterize

the

mechanisms

involved

in

the

regulation

of

desmosomal

adhesion

Except

from

pemphigus

other

diseases

in

which

desmosomal

adhesion

is

altered

by

mutations

or

bacterial

toxins

helped

to

elucidate

the

functional

role

of

the

different

desmosomal

components

During

the

last

past

several

years

a

number

of

comprehensive

reviews

have

been

published

on

both

desmosome

structure

and

function

-LRB-

Dusek

et

al.

Garrod

et

al.

Getsios

et

al.

Green

and

Simpson

Holthofer

et

al.

Kitajima

Kottke

et

al.

Muller

et

al.

Yin

and

Green

-RRB-

and/or

on

the

mechanisms

involved

in

pemphigus

pathogenesis

-LRB-

Amagai

Hashimoto

Lanza

et

al.

Payne

et

al.

Sharma

et

al.

Sitaru

and

Zillikens

Stanley

and

Amagai

-RRB-

which

indicates

that

the

perspective

of

the

existing

model

of

the

desmosome

and

its

role

in

pemphigus

pathogenesis

are

constantly

reshaped

Moreover

because

even

textbook

knowledge

such

as

on

the

molecular

composition

of

myocardial

intercalated

discs

needs

revision

-LRB-

Borrmann

et

al.

Franke

et

al.

-RRB-

it

becomes

obvious

that

after

almost

years

of

desmosome

research

our

knowledge

is

still

far

from

complete

This

article

focuses

on

the

mechanisms

regulating

desmosomal

adhesion

which

are

compromised

in

diseases

such

as

pemphigus

The

ultrastructure

and

composition

of

desmosomes

The

first

detailed

analysis

of

desmosome

ultrastructure

was

provided

by

Odland

-LRB-

-RRB-

Desmosomes

are

discoid

junctions

with

a

diameter

of

about

μm

and

are

composed

of

two

electron-dense

plaques

in

each

of

the

two

cells

which

are

separated

by

an

intercellular

cleft

of

nm

-LRB-

Figs.

-RRB-

-LRB-

Farquhar

and

Palade

Odland

-RRB-

Within

the

plaques

an

outer

dense

plaque

can

be

separated

from

a

less

dense

inner

plaque

the

latter

of

which

is

linked

to

loops

of

intermediate

filament

bundles

-LRB-

Kelly

-RRB-

Desmosomes

contain

members

of

at

least

three

protein

families

Desmosomal

cadherins

form

the

intercellular

adhesive

interface

whereas

armadillo

and

plakin

family

proteins

built

up

the

plaques

It

is

believed

that

the

cytoplasmic

tail

of

Dsgs

and

Dscs

interact

with

plakoglobin

which

in

turn

binds

to

desmoplakin

-LRB-

Fig.

-RRB-

Desmoplakin

finally

is

anchored

to

the

intermediate

filament

cytoskeleton

-LRB-

Green

and

Simpson

-RRB-

These

interactions

seem

to

be

stabilized

laterally

by

plakophilin

-LRB-

Hatzfeld

-RRB-

Fig.

model

of

the

desmosome

The

desmosomal

cadherins

desmoglein

and

desmocollin

undergo

homophilic

and

heterophilic

binding

via

interaction

with

the

amino-terminal

extracellular

-LRB-

EC

-RRB-

domain

of

partner

molecules

on

the

same

-LRB-

cis

-RRB-

as

well

as

on

the

neighbouring

cell

-LRB-

trans

-RRB-

The

cytoplasmic

domains

are

largely

embedded

in

the

outer

dense

plaque

-LRB-

ODP

-RRB-

where

they

are

associated

with

plakoglobin

and

plakophilin

In

the

inner

dense

plaque

-LRB-

IDP

-RRB-

desmoplakin

links

these

adaptor

molecules

to

the

intermediate

filament

cytoskeletonFig

of

the

desmosome

The

electron

micrograph

of

a

keratinocyte

desmosome

shows

the

desmosomal

plaque

with

inserting

cytokeratin

intermediate

filaments

as

well

as

some

fuzzy

material

within

the

extracellular

space

likely

reflecting

the

extracellular

domains

of

desmosomal

cadherins

Desmosomes

and

desmosome-like

junctions

Adhering

junctions

are

divided

into

two

main

forms

-LRB-

-RRB-

desmosomes

which

serve

as

anchoring

structures

for

intermediate

filaments

to

desmosomal

cadherins

and

-LRB-

-RRB-

adherens

junctions

which

contain

cell-type

specific

adhesion

molecules

from

the

cadherin

super-family

that

are

linked

to

the

actin

cytoskeleton

Both

desmosomes

and

adherens

junctions

can

be

found

as

constituents

of

more

elaborated

cell

contact

complexes

Moreover

chimeric

cell

contacts

exist

which

share

features

of

both

adherens

junctions

and

desmosomes

Junctional

complexes

Polarized

epithelial

cells

display

junctional

complexes

located

at

the

uppermost

section

of

the

baso-lateral

membrane

In

apico-basal

direction

the

complex

is

composed

of

the

zonula

occludens

-LRB-

tight

junction

-RRB-

the

zonula

adherens

and

a

desmosome

-LRB-

macula

adherens

-RRB-

-LRB-

Farquhar

and

Palade

-RRB-

Accompanied

by

a

line

of

separated

desmosomes

the

zonula

occludens

and

the

zonula

adherens

span

the

entire

cell

by

forming

continuous

junction

belts

These

junctional

complexes

are

regarded

as

hallmarks

of

polarized

epithelial

cells

but

differ

in

terms

of

size

and

ultrastructure

in

cell

type-specific

manner

Area

composita

The

intercalated

discs

of

the

myocardium

also

consist

of

three

types

of

cell

junctions

i.e.

adherens

junctions

desmosomes

and

gap

junctions

-LRB-

Fig.

-RRB-

Although

transitional

forms

between

adherens

junctions

and

desmosomes

were

described

in

the

very

beginning

-LRB-

Fawcett

and

Selby

-RRB-

most

morphological

studies

regarded

the

intercalated

discs

to

be

composed

of

separated

desmosomes

and

adherens

junctions

the

former

linked

to

the

desmin

type

intermediate

filament

cytoskeleton

and

the

latter

to

actin

filaments

of

the

myofibrills

-LRB-

Fig.

-RRB-

-LRB-

Fawcett

and

McNutt

McNutt

and

Fawcett

Shimada

et

al.

-RRB-

This

view

seemed

to

be

supported

by

immuno-localization

studies

which

showed

that

desmoplakin

as

a

desmosomal

marker

and

the

myocardial

adherens

junction

protein

N-cadherin

displayed

mutually

exclusive

spatial

distribution

patterns

-LRB-

Angst

et

al.

Gutstein

et

al.

-RRB-

However

a

recent

comprehensive

set

of

studies

unequivocally

demonstrated

that

the

desmosomal

components

desmoglein

-LRB-

Dsg

-RRB-

desmocollin

-LRB-

Dsc

-RRB-

desmoplakin

and

as

well

as

the

adherens

junction

components

N-cadherin

α-catenin

and

β-catenin

afadin

vinculin

and

are

present

in

all

parts

of

intercalated

discs

irrespective

of

whether

their

ultrastructure

resembles

more

closely

typical

desmosomes

or

adherens

junctions

-LRB-

Borrmann

et

al.

Franke

et

al.

-RRB-

It

seems

that

the

two

types

of

junctions

coalesce

within

the

first-year

postpartum

and

that

plakophilin

is

of

special

importance

for

junction

integrity

-LRB-

Pieperhoff

and

Franke

Pieperhoff

et

al.

-RRB-

Therefore

the

intercalated

discs

were

now

reclassified

as

area

composita

a

mixed

type

of

adhering

junctions

Fig.

of

the

area

composita

of

a

myocardial

intercalated

disc

The

electron

micrograph

shows

an

intercalated

disc

containing

a

gap

junction

-LRB-

GJ

-RRB-

in

its

longitudinal

section

as

well

as

an

adhering

junction

with

an

extensive

electron-dense

plaque

in

the

section

perpendicular

to

the

cellular

axis

Note

that

insertion

of

actin

filaments

which

is

typical

for

adherens

junctions

is

present

in

some

parts

of

the

junction

-LRB-

asterisk

-RRB-

but

not

in

others

-LRB-

hash

key

-RRB-

Based

on

the

recent

finding

that

all

parts

of

these

adhering

junctions

contain

the

same

set

of

desmosomal

components

they

are

now

defined

as

area

composita

Complexus

adhaerentes

and

meningeal

junctions

Lymphatic

endothelial

cells

in

certain

lymphatic

vessels

and

in

the

sinus

of

lymph

nodes

form

a

kind

of

chimeric

cell

contact

which

contains

components

from

desmosomes

-LRB-

desmoplakin

plakoglobin

-RRB-

adherens

junctions

-LRB-

VE-cadherin

α-catenin

β-catenin

afadin

-RRB-

as

well

as

from

tight

junctions

-LRB-

and

-RRB-

-LRB-

Hammerling

et

al.

Schmelz

and

Franke

Schmelz

et

al.

-RRB-

The

ultrastructure

of

complexus

adhaerentes

shares

features

of

both

adherens

junctions

and

desmosomes

Complexus

adhearentes

form

continuous

belt-like

junctions

similar

to

adherens

junctions

of

vascular

endothelial

cells

whereas

their

junctional

plaques

are

more

similar

to

desmosomes

It

was

proposed

that

plakoglobin

in

these

contacts

is

responsible

for

recruitment

of

desmoplakin

-LRB-

Kowalczyk

et

al.

-RRB-

Recently

in

meningeoma

cells

a

new

type

of

adhering

junctions

was

discovered

in

which

adherens

junctions

also

contained

the

desmosomal

plaque

protein

plakophilin

-LRB-

Akat

et

al.

-RRB-

The

desmosomal

components

Desmosomal

cadherins

The

desmosomal

members

of

the

cadherin

superfamily

desmogleins

-LRB-

Dsg

-RRB-

and

desmocollins

-LRB-

Dsc

-RRB-

are

single-pass

transmembrane

glycoproteins

which

mediate

adhesion

in

dependent

manner

-LRB-

Buxton

et

al.

Garrod

et

al.

Getsios

et

al.

Nollet

et

al.

-RRB-

Desmosomal

adhesion

molecules

have

been

first

isolated

from

desmosomal

intercellular

regions

-LRB-

Gorbsky

and

Steinberg

-RRB-

Using

specific

antibodies

to

localize

proteins

at

the

cell

surface

and

to

inhibit

desmosome

formation

Dsc

-LRB-

kDa

-RRB-

and

Dsc

-LRB-

kDa

-RRB-

were

shown

to

be

directly

involved

in

cell

cell

adhesion

-LRB-

Cowin

et

al.

-RRB-

and

later

on

were

identified

to

be

cadherin

family

proteins

-LRB-

Collins

et

al.

Koch

et

al.

-RRB-

Similarly

Dsg

-LRB-

kDa

-RRB-

Dsg

-LRB-

kDa

-RRB-

Dsg

-LRB-

kDa

-RRB-

and

Dsc

-LRB-

kDa

-RRB-

were

characterized

-LRB-

Amagai

et

al.

Arnemann

et

al.

Jones

et

al.

King

et

al.

Koch

et

al.

Schafer

et

al.

Schmelz

et

al.

b

-RRB-

More

recently

Dsg

-LRB-

kDa

-RRB-

was

found

to

be

the

principal

Dsg

expressed

in

hair

follicles

-LRB-

Kljuic

et

al.

Whittock

and

Bower

-RRB-

The

genes

encoding

desmosomal

cadherins

which

share

an

amino

acid

identity

of

approximately

both

with

each

other

and

with

classical

cadherins

are

all

located

on

chromosome

in

humans

-LRB-

Cowley

et

al.

-RRB-

Mouse

models

revealed

that

Dsg

and

Dsc

are

the

most

important

desmosomal

cadherin

members

because

deficiency

caused

embryonic

lethality

-LRB-

Table

-RRB-

-LRB-

Den

et

al.

Eshkind

et

al.

-RRB-

Because

lethality

induced

by

Dsc

deficiency

occurred

before

mature

desmosmes

were

formed

and

because

Dsg

was

observed

to

be

also

localized

outside

of

desmosomes

in

embryonal

stem

cells

non-desmosmal

functions

of

Dsc

and

Dsg

seemed

to

be

responsible

in

this

context

In

contrast

ablation

of

cadherins

with

more

restrictive

expression

patterns

such

as

Dsc

led

to

localized

superficial

epidermal

acantholysis

or

mucosal

and

deep

epidermal

splitting

in

traumatized

skin

accompanied

by

hair

loss

in

the

case

of

Dsg

-LRB-

Chidgey

et

al.

Koch

et

al.

-RRB-

Dsg

mutations

are

followed

primarily

by

defective

hair

formation

-LRB-

Kljuic

et

al.

-RRB-

Table

diseases

in

humans

and

in

transgenic

mouse

modelsMouse

model

Desmosomal

componentHuman

diseaseInactivationGenetic

alteration

Dsg

palmoplantar

keratoderma

-LRB-

SPPK

-RRB-

-LRB-

Rickman

et

al.

-RRB-

Embryonic

lethalityDefective

stem

cell

proliferation

-LRB-

Eshkind

et

al.

-RRB-

Dsg

right

ventricular

cardiomyopathy

-LRB-

ARVC

-RRB-

-LRB-

Pilichou

et

al.

-RRB-

Suprabasal

epidermal

acantholysisOral

erosionsHair

loss

-LRB-

Koch

et

al.

-RRB-

Dsg

Impaired

hair

keratinisationHyperproliferation

-LRB-

Kljuic

et

al.

-RRB-

Dsg

-LRB-

Kljuic

et

al.

-RRB-

Epidermal

granular

layer

acantholysisImpaired

barrier

functionHyperproliferation

-LRB-

Chidgey

et

al.

-RRB-

Dsc

Dsc

right

ventricular

cardiomyopathy

-LRB-

ARVC

-RRB-

-LRB-

Syrris

et

al.

Heuser

et

al.

-RRB-

Embryonic

lethality

-LRB-

Den

et

al.

-RRB-

Dsc

Embryonic

lethalityMyocardial

fragilityEpidermal

subcorneal

acantholysis

-LRB-

Bierkamp

et

al.

Ruiz

et

al.

-RRB-

PlakoglobinNaxos

disease

Arrhythmogenic

right

ventricular

cardiomyopathy

-LRB-

ARVC

-RRB-

Palmoplantar

keratodermaWoolly

hair

-LRB-

McKoy

et

al.

-RRB-

Embryonic

lethalityDefects

in

heart

skin

blood

vesselsNeuroepithelium

-LRB-

Gallicano

et

al.

-RRB-

DesmoplakinArrhythmogenic

right

ventricular

cardiomyopathy

-LRB-

ARVC

-RRB-

-LRB-

Rampazzo

et

al.

-RRB-

Striate

palmoplantar

keratoderma

-LRB-

SPPK

-RRB-

-LRB-

Armstrong

et

al.

Whittock

et

al.

-RRB-

Carvajal

syndrome

Dilated

left

ventricular

cardiomyopathyStriate

palmoplantar

keratoderma

-LRB-

SPPK

-RRB-

Woolly

hair

-LRB-

Norgett

et

al.

-RRB-

Lethal

acantholytic

epidermolysis

bullosa

suprabasal

epidermal

blistering-universal

alopecia-nail

loss

-LRB-

Jonkman

et

al.

-RRB-

Plakophilin

dysplasia

and

skin

fragility

syndrome

Skin

blistering

around

mouth

on

soles

and

palms

-LRB-

McGrath

-RRB-

Embryonic

lethalityHeart

defects

-LRB-

Grossmann

et

al.

-RRB-

Plakophilin

right

ventricular

cardiomyopathy

-LRB-

ARVC

-RRB-

-LRB-

Gerull

et

al.

-RRB-

Bacterial

toxinsSuperficial

epidermal

acantholysis

-LRB-

Melish

and

Glasgow

Amagai

et

al.

b

-RRB-

Dsg

scalded

skin

syndrome

Superficial

skin

splittingFever

erythemaSkin

tenderness

-LRB-

first

described

by

Ritter

von

Rittershain

-RRB-

AutoantibodiesSuperficial

epidermal

acantholysis

-LRB-

Amagai

et

al.

-RRB-

Dsg

foliaceus

-LRB-

PF

-RRB-

Superficial

epidermal

acantholysisPemphigus

vulgaris

-LRB-

PV

mucocutaneous

-RRB-

Suprabasal

epidermal

acantholysisMucosal

erosions

-LRB-

mouth

larynx

nasal

cavity

vagina

-RRB-

-LRB-

Lever

Koulu

et

al.

Amagai

et

al.

-RRB-

Suprabasal

epidermal

acantholysisMucosal

erosions

-LRB-

Anhalt

et

al.

Amagai

et

al.

Mahoney

et

al.

-RRB-

Dsg

vulgaris

-LRB-

PV

mucosal

dominant

-RRB-

Suprabasal

epidermal

acantholysis

-LRB-

Lever

Amagai

et

al.

-RRB-

The

structure

of

desmosomal

cadherins

Desmosomal

cadherins

are

type

I

integral

membrane

proteins

The

amino-terminal

extracellular

domain

of

desmosomal

cadherins

consists

of

four

cadherin

repeats

-LRB-

-RRB-

of

about

amino

acids

followed

by

a

less

related

membrane-proximal

domain

-LRB-

EC

-RRB-

-LRB-

Dusek

et

al.

-RRB-

Based

on

the

crystal

structure

of

classical

cadherins

-LRB-

Boggon

et

al.

Overduin

et

al.

Shapiro

et

al.

-RRB-

the

EC

domains

are

thought

to

be

connected

via

flexible

linkers

which

are

rigidified

by

binding

of

up

to

three

ions

each

-LRB-

Pertz

et

al.

-RRB-

In

the

cytoplasmic

domain

a

juxtamembranuous

anchor

-LRB-

IA

-RRB-

region

is

located

which

at

least

in

the

case

of

Dsc

contains

a

desmoplakin-binding

element

-LRB-

Troyanovsky

et

al.

-RRB-

and

maybe

be

involved

in

binding

and

trafficking

of

similar

to

its

role

in

E-cadherin

-LRB-

Miranda

et

al.

-RRB-

The

following

cadherin-typical

sequence

-LRB-

ICS

-RRB-

is

required

for

binding

to

plakoglobin

-LRB-

Mathur

et

al.

Troyanovsky

et

al.

-RRB-

For

desmocollins

in

which

a

long

and

short

a

and

b

isoform

is

generated

by

alternative

splicing

-LRB-

Collins

et

al.

-RRB-

it

has

been

shown

that

the

ICS

domain

is

lacking

in

the

b

isoform

which

therefore

is

unable

to

bind

plakoglobin

but

instead

associates

with

plakophilin

-LRB-

Bonne

et

al.

Troyanovsky

et

al.

-RRB-

The

functions

of

the

C-terminal

proline-rich

linker

-LRB-

L

-RRB-

the

Dsg-specific

repeated

unit

domains

-LRB-

RUDs

-RRB-

and

the

desmoglein

terminal

domain

-LRB-

DTD

-RRB-

which

are

all

only

present

in

desmogleins

are

not

clear

-LRB-

Dusek

et

al.

-RRB-

The

dependency

of

desmosomal

cadherin-mediated

binding

It

is

well

established

that

binding

of

desmosomal

and

classical

cadherins

is

strictly

dependent

-LRB-

Chitaev

and

Troyanovsky

Heupel

et

al.

Pertz

et

al.

Waschke

et

al.

-RRB-

For

Dsg

dependency

of

homophilic

binding

has

been

characterized

in

more

detail

It

was

found

that

the

concentration

for

half-maximal

binding

activity

of

Dsg

is

mM

and

that

binding

is

highly

cooperative

with

the

Hill

coefficient

being

-LRB-

Waschke

et

al.

-RRB-

This

indicates

that

Dsg

binding

is

strong

only

at

extracellular

concentrations

higher

than

mM

Although

the

exact

extracellular

concentration

within

the

epidermis

is

unknown

it

has

been

shown

that

a

gradient

exists

with

low

concentrations

in

the

basal

layers

and

high

concentrations

in

the

superficial

epidermis

-LRB-

Elias

et

al.

Menon

and

Elias

-RRB-

Therefore

if

homophilic

binding

of

Dsg

occurs

in

vivo

it

has

to

be

considered

that

it

may

contribute

to

effective

intercellular

adhesion

only

in

the

superficial

epidermis

Transinteraction

mechanisms

of

desmosmal

cadherins

At

present

it

is

still

a

matter

of

debate

how

desmosmal

cadherins

interact

with

each

other

in

vivo

However

several

lines

of

evidence

indicate

that

the

N-terminal

EC

domain

is

important

Similar

to

classical

cadherins

desmosomal

cadherins

contain

a

cell

adhesion

recognition

-LRB-

CAR

-RRB-

site

containing

a

central

alanine

residue

-LRB-

Blaschuk

et

al.

-RRB-

However

instead

of

the

conserved

tripeptide

HAV

sequence

of

classical

cadherins

the

sequence

is

YAT

for

Dsc

and

RAL

for

Dsg

respectively

-LRB-

Tselepis

et

al.

-RRB-

Peptides

derived

from

these

sequences

were

able

to

block

homophilic

adhesion

mediated

by

Dsg

and

Dsc

and

to

inhibit

desmosomal

adhesion

in

epithelial

cells

when

the

peptides

for

Dsg

and

Dsc

were

applied

together

indicating

that

the

CAR

site

in

the

EC

domain

is

critical

for

maintenance

of

desmosmal

adhesion

-LRB-

Runswick

et

al.

Tselepis

et

al.

-RRB-

This

hypothesis

is

supported

by

studies

in

which

mechanisms

underlying

the

loss

of

keratinocyte

cohesion

in

pemphigus

were

investigated

a

monoclonal

Dsg

antibody

from

a

PV

mouse

model

directed

against

the

predicted

binding

motif

of

the

EC

has

been

shown

to

be

pathogenic

in

vivo

whereas

antibodies

targeting

other

parts

of

the

Dsg

extracellular

domain

were

not

-LRB-

Tsunoda

et

al.

-RRB-

Together

with

the

recent

finding

that

AK

similar

to

Dsg

antibodies

from

PV

patients

which

are

known

to

be

also

primarily

directed

against

the

N-terminal

part

of

EC

-LRB-

Sekiguchi

et

al.

-RRB-

is

able

to

directly

interfere

with

Dsg

binding

-LRB-

Heupel

et

al.

-RRB-

these

data

demonstrate

that

interaction

of

EC

is

crucial

for

Dsg

binding

In

addition

to

these

functional

studies

morphologic

studies

sought

to

address

the

mode

of

desmosomal

cadherin

interaction

within

desmosomes

by

using

electron

tomography

imaging

of

epidermal

tissue

-LRB-

Al-Amoudi

et

al.

He

et

al.

-RRB-

Based

on

predictions

from

the

C-cadherin

crystal

structure

He

and

colleagues

reported

that

in

mouse

epidermis

several

desmosomal

cadherins

form

knots

in

which

cadherins

display

stochastic

arrangement

In

these

knots

desmosomal

cadherins

seemed

to

interact

via

their

EC

domains

with

both

molecules

on

the

same

cell

in

cis

as

well

as

with

molecules

from

opposing

cells

in

trans

-LRB-

He

et

al.

-RRB-

Al-Amoudi

and

co-workers

refined

the

technique

by

employing

cryo-electron

microscopy

in

human

epidermis

They

confirmed

cis

and

trans-interactions

of

the

EC

domains

possibly

via

insertion

of

the

tryptophane

into

the

hydrophobic

pocket

of

the

CAR

site

-LRB-

Al-Amoudi

et

al.

-RRB-

However

they

found

that

cis-interactions

of

the

EC

regions

may

also

occur

and

that

desmosomal

cadherins

rather

show

periodically

zipper-like

arrangements

similar

to

classical

cadherins

-LRB-

Boggon

et

al.

-RRB-

Homophilic

and

heterophilic

binding

of

desmosomal

cadherins

In

contrast

to

classical

cadherins

from

adherens

junctions

which

primarily

bind

in

homophilic

manner

data

indicate

that

desmosomal

cadherins

undergo

both

homophilic

and

heterophilic

transinteraction

Using

EC

fragments

of

Dsg

and

Dsc

it

was

shown

that

homophilic

binding

occurs

in

vitro

-LRB-

Syed

et

al.

-RRB-

Similarly

homophilic

binding

of

Dsg

was

found

-LRB-

Amagai

et

al.

-RRB-

When

recombinant

proteins

consisting

of

the

complete

extracellular

domain

were

used

for

atomic

force

microscopy

-LRB-

AFM

-RRB-

measurements

it

was

demonstrated

that

the

unbinding

force

of

single

homophilically

transinteracting

molecules

was

about

pN

-LRB-

depending

on

the

retrace

velocity

nm/s

-RRB-

for

Dsg

with

a

lifetime

of

s

and

about

pN

for

Dsg

-LRB-

Heupel

et

al.

Waschke

et

al.

-RRB-

which

is

in

the

same

range

as

observed

for

other

types

of

cadherins

characterized

by

the

same

method

such

as

VE-cadherin

N-cadherin

or

LI-cadherin

-LRB-

Baumgartner

et

al.

Wendeler

et

al.

-RRB-

These

data

indicate

that

the

molecular

binding

properties

of

homophilic

adhesion

of

desmosomal

cadherins

may

be

comparable

to

other

cadherins

Heterophilic

binding

of

Dsg

to

Dsc

or

Dsc

was

also

found

on

the

molecular

level

-LRB-

Chitaev

and

Troyanovsky

Syed

et

al.

-RRB-

but

no

interaction

of

Dsg

with

Dsg

-LRB-

Heupel

et

al.

-RRB-

Aggregation

assays

of

transfected

cells

indicated

that

in

cells

heterophilic

binding

of

Dsgs

and

Dscs

might

be

of

even

greater

importance

than

homophilic

binding

to

induce

strong

intercellular

adhesion

-LRB-

Kowalczyk

et

al.

Marcozzi

et

al.

Runswick

et

al.

-RRB-

and

that

adhesion

is

strictly

dependent

on

the

ratio

of

the

respective

Dsgs

and

Dscs

-LRB-

Getsios

et

al.

-RRB-

This

view

is

supported

by

the

recent

finding

that

a

conditional

Dsc

in

mice

induced

a

severe

pemphigus-like

phenotype

with

epidermal

blistering

-LRB-

Chen

et

al.

-RRB-

Because

antibodies

in

typical

pemphigus

are

usually

not

directed

against

Dsc

but

against

Dsg

and

Dsg

it

has

to

be

considered

that

heterophilic

binding

of

these

three

molecules

is

important

for

epidermal

cohesion

in

vivo

Armadillo

family

proteins

From

the

Armadillo

family

plakoglobin

and

plakophilins

are

important

components

of

desmosomes

Plakoglobin

Plakoglobin

-LRB-

kDa

-RRB-

also

termed

γ-catenin

is

the

only

essential

desmosomal

component

which

is

also

found

in

typical

adherens

junctions

-LRB-

Cowin

et

al.

Franke

et

al.

-RRB-

The

gene

encoding

for

plakoglobin

was

mapped

to

chromosome

-LRB-

Aberle

et

al.

-RRB-

Plakoglobin

binds

to

the

cytoplasmic

cadherin-typical

sequence

of

Dsgs

and

Dscs

via

its

first

three

armadillo

repeat

domains

-LRB-

Chitaev

et

al.

-RRB-

Because

the

same

binding

site

is

required

for

interaction

of

plakoglobin

to

α-catenin

the

latter

is

excluded

from

desmosomes

Similarly

although

the

armadillo

repeat

domain

of

β-catenin

can

also

bind

to

Dsg

its

flanking

domains

inhibit

this

interaction

which

may

explain

the

absence

of

β-catenin

from

desmosomes

-LRB-

Troyanovsky

et

al.

Wahl

et

al.

-RRB-

Plakoglobin

has

been

demonstrated

to

interact

with

other

desmosomal

plaque

components

such

as

desmoplakin

plakophilins

and

also

with

cytokeratin

filaments

-LRB-

Bonne

et

al.

Chen

et

al.

Kowalczyk

et

al.

Smith

and

Fuchs

-RRB-

The

importance

of

this

linker

function

can

be

concluded

from

studies

in

which

inactivation

of

plakoglobin

led

to

embryonic

lethality

due

to

mechanical

fragility

of

the

myocardium

and

when

mice

are

viable

to

subcorneal

skin

blistering

indicating

that

plakoglobin

is

essential

for

desmosomal

stability

-LRB-

Table

-RRB-

-LRB-

Bierkamp

et

al.

Ruiz

et

al.

-RRB-

Besides

its

function

as

a

desmosomal

adaptor

protein

plakoglobin

seems

to

be

involved

in

nuclear

signalling

It

has

been

shown

that

plakoglobin

comparable

to

β-catenin

in

the

canonical

wnt

signalling

pathway

confers

transcriptional

activity

together

with

LEF

transcription

factors

-LRB-

Maeda

et

al.

-RRB-

This

mechanism

seems

to

interfere

with

β-catenin-mediated

transcription

-LRB-

Hu

et

al.

-RRB-

Because

plakoglobin

like

β-catenin

is

a

target

of

glycogen

synthase

β

which

drives

proteosomal

degradation

of

both

proteins

-LRB-

Kodama

et

al.

Muller

et

al.

Williamson

et

al.

-RRB-

a

complex

pattern

of

direct

and

indirect

transcriptional

regulation

seems

likely

A

target

gene

of

plakoglobin

signalling

is

c-Myc

the

expression

of

which

was

inhibited

in

keratinocytes

but

enhanced

in

transformed

rat

kidney

epithelial

cells

-LRB-

Kolligs

et

al.

Kolly

et

al.

Williamson

et

al.

-RRB-

This

indicates

that

the

role

of

plakoglobin

transcriptional

regulation

is

strictly

cell

type-dependent

In

keratinoytes

c-Myc

repression

by

plakoglobin

is

required

to

stop

proliferation

and

to

allow

terminal

differentiation

-LRB-

Williamson

et

al.

-RRB-

Another

potential

target

gene

is

the

anti-apoptotic

molecule

Bcl

XL

which

was

found

to

be

upregulated

in

plakoglobin-deficient

cells

leading

to

reduced

apoptosis

and

thus

might

also

be

repressed

by

plakoglobin

-LRB-

Dusek

et

al.

-RRB-

Taken

together

plakoglobin

serves

as

functional

linker

between

intercellular

adhesion

and

regulation

of

the

cell

cycle

This

might

also

be

important

for

cancer

progression

because

many

tumors

are

characterized

by

loss

of

plakoglobin

expression

Plakophilins

Plakophilin

-LRB-

kDa

-RRB-

was

first

identified

as

an

accessory

plaque

protein

because

in

contrast

to

plakoglobin

and

desmoplakin

it

was

found

in

cells

from

certain

stratified

and

complex

epithelia

only

-LRB-

Franke

et

al.

Hatzfeld

et

al.

Heid

et

al.

-RRB-

Afterwards

it

became

clear

that

plakophilin

-LRB-

kDa

-RRB-

is

ubiquitously

present

in

all

desmosomes

and

also

plakophilin

-LRB-

kDa

-RRB-

is

present

in

most

simple

and

stratified

epithelia

-LRB-

Mertens

et

al.

Schmidt

et

al.

-RRB-

The

genes

encoding

plakophilin

and

are

located

on

chromosomes

and

respectively

-LRB-

Bonne

et

al.

-RRB-

Plakophilin

and

exist

in

two

splice

variants

with

a

shorter

a

and

a

longer

b

form

-LRB-

Mertens

et

al.

Schmidt

et

al.

-RRB-

In

addition

to

their

localization

in

desmosomes

plakophilins

and

are

also

found

in

the

karyoplasm

in

a

variety

of

cells

and

plakophilin

b

is

exclusively

located

in

the

nucleus

Plakophilin

deficiency

leads

to

embryonic

death

due

to

heart

defects

indicating

that

the

presence

of

at

least

one

member

of

the

plakophilin

family

is

required

-LRB-

Table

-RRB-

-LRB-

Grossmann

et

al.

-RRB-

Under

these

conditions

cytokeratin

filaments

were

retracted

from

cell

borders

and

desmoplakin

was

localized

in

the

cytoplasm

rather

than

at

desmosomes

in

cardiomyocytes

demonstrating

the

relevance

of

plakophilin

to

desmosplakin

recruitment

Because

cardiomyocytes

in

contrast

to

epithelial

cells

express

only

plakophilin

but

not

plakophilin

and

defects

were

present

in

the

heart

only

whereas

epithelia

were

not

affected

Plakophilins

can

directly

interact

with

all

other

desmosomal

components

including

cytokeratins

via

the

aminoterminal

head

domain

-LRB-

Bonne

et

al.

Hatzfeld

Hatzfeld

et

al.

-RRB-

Plakophilin

recruits

desmosomal

components

to

the

cell

membrane

increases

size

and

number

of

desmosomes

and

therefore

seems

to

be

a

scaffolding

protein

which

induces

desmosome

assembly

-LRB-

Hatzfeld

et

al.

Kowalczyk

et

al.

Wahl

-RRB-

On

the

other

hand

because

plakophilin

is

located

in

the

dense

inner

desmosomal

plaque

whereas

cytokeratin

filaments

only

loop

into

the

outer

plaque

it

is

believed

that

plakophilin

enhances

desmoplakin

lateral

interactions

but

does

not

directly

associate

with

cytokeratin

filaments

in

vivo

-LRB-

Hatzfeld

North

et

al.

-RRB-

It

has

been

demonstrated

that

desmosome

formation

is

mediated

by

the

aminoterminal

domain

whereas

recruitment

of

plakophilin

itself

to

the

plasma

membrane

is

dependent

on

the

carboxyterminal

region

-LRB-

Sobolik-Delmaire

et

al.

-RRB-

In

addition

to

its

function

in

the

regulation

of

desmosome

assembly

plakophilins

may

also

regulate

signalling

mechanisms

both

at

cell

borders

as

well

as

in

the

nucleus

Plakophilin

associates

with

actin

filaments

at

cell

borders

and

has

been

reported

to

interact

with

a

GTP

exchange

factor

-LRB-

GEF

-RRB-

for

Rho

and

thereby

could

regulate

activity

of

Rho

GTPases

similar

to

the

closely

related

which

is

known

to

inhibit

Rho

A

and

to

activate

Rac

and

-LRB-

Anastasiadis

and

Reynolds

Hatzfeld

-RRB-

In

addition

to

their

localization

within

the

desmosomal

plaque

plakophilin

and

are

also

present

inside

nucleus

and

plakophilin

has

been

found

to

be

part

of

the

polymerase

III

complex

which

is

responsible

for

generation

of

tRNA

and

rRNA

-LRB-

Mertens

et

al.

-RRB-

By

these

two

mechanisms

it

is

possible

that

plakophilins

may

regulate

cell

adhesion

and

cell

growth

-LRB-

Hatzfeld

-RRB-

Plakin

family

proteins

Plakin

family

proteins

are

linkers

between

the

cytoskeleton

and

cell

cell

or

cell

matrix

contacts

-LRB-

Jefferson

et

al.

-RRB-

Desmoplakin

which

exists

in

two

spice

variants

of

a

protein

encoded

by

a

single

gene

on

chromosome

-LRB-

desmoplakin

I

kDa

desmoplakin

II

kDa

-RRB-

is

an

essential

component

of

the

desmosomal

plaque

and

therefore

is

regarded

as

the

prototype

of

this

family

-LRB-

Armstrong

et

al.

Hatsell

and

Cowin

Mueller

and

Franke

Sonnenberg

and

Liem

-RRB-

Other

members

such

as

plectin

envoplakin

and

periplakin

were

also

found

in

desmosomes

but

their

significance

for

the

structure

and

function

of

desmosomes

is

less

clear

Especially

plectin

is

primarily

important

in

hemidesmosmes

which

anchor

epithelia

to

the

extracellular

matrix

Desmoplakin

consists

of

an

aminoterminal

plakin

domain

which

can

interact

with

all

other

desmosomal

plaque

proteins

such

as

plakoglobin

and

plakophilins

but

also

with

Dsc

-LRB-

Kowalczyk

et

al.

Smith

and

Fuchs

Troyanovsky

et

al.

-RRB-

The

central

coiled-coil

rod

domain

which

is

important

for

dimerization

is

followed

by

the

carboxyterminal

tail

consisting

of

three

globular

subdomains

with

several

plakin-repeats

which

serve

as

linkers

for

different

intermediate

filament

types

-LRB-

Choi

et

al.

Green

et

al.

Stappenbeck

and

Green

-RRB-

It

is

well

established

that

desmoplakin

is

the

main

linker

protein

between

the

desmosomal

cadherin

plakoglobin

complex

and

the

intermediate

filament

cytoskeleton

This

has

been

shown

in

vitro

and

was

ultimately

demonstrated

in

desmoplakin-deficient

mice

which

had

a

reduced

number

of

desmosomes

and

died

at

embryonic

stage

just

after

implantation

-LRB-

Table

-RRB-

-LRB-

Bornslaeger

et

al.

Gallicano

et

al.

-RRB-

Similar

to

the

findings

in

epidermal-specific

desmoplakin-deficient

mice

which

suffered

from

skin

blistering

desmosomes

were

not

anchored

to

intermediate

filaments

-LRB-

Vasioukhin

et

al.

-RRB-

Moreover

when

desmoplakin

was

rescued

in

extraembryonic

tissues

so

that

embryos

further

developed

defects

were

present

not

only

in

the

myocardium

and

epidermis

but

also

in

the

vasculature

and

in

the

neuroepithelium

underlining

the

importance

of

desmoplakin

for

tissue

differentiation

-LRB-

Gallicano

et

al.

-RRB-

Diversity

of

desmosomes

in

different

tissues

and

specific

epithelial

layers

Although

it

was

discovered

about

years

ago

that

the

structure

of

desmosomes

is

not

identical

in

all

types

of

cells

and

tissues

-LRB-

Giudice

et

al.

Jones

et

al.

-RRB-

the

knowledge

on

the

diversity

of

desmosomes

is

still

uncomplete

and

matter

of

discussion

-LRB-

Garrod

et

al.

Getsios

et

al.

Green

and

Simpson

Hatzfeld

Holthofer

et

al.

Kottke

et

al.

Yin

and

Green

-RRB-

The

diversity

of

desmosomes

has

implications

for

tissue

differentiation

and

also

is

of

high-medical

relevance

because

diseases

caused

by

genetic

alteration

of

or

by

an

autoimmune

reponse

against

a

specific

desmosomal

component

may

affect

only

certain

but

not

all

desmosome-containing

tissues

Some

desmosomal

components

such

as

Dsg

Dsc

and

the

plaque

proteins

desmoplakin

plakoglobin

and

plakophilin

are

ubiquitously

expressed

in

all

cells

and

tissues

in

which

desmosomes

are

found

Plakophilin

is

present

in

most

simple

epithelia

except

hepatocytes

as

well

as

in

stratified

epithelia

whereas

plakophilin

is

restricted

to

stratified

and

complex

epithelia

In

epithelia

the

desmosomal

cadherins

show

typical

expression

patterns

Simple

epithelia

and

urothelium

usually

express

Dsg

and

Dsc

only

Apparently

exceptions

are

the

additional

presence

of

Dsg

in

the

mucosa

of

uterus

stomach

intestine

and

in

epithelia

of

liver

and

pancreas

as

well

as

the

expression

of

Dsc

in

intestine

and

liver

or

Dsc

in

stomach

prostate

salivary

gland

and

urothelium

Dsg

has

a

unique

tissue

distribution

in

skin

and

several

simple

epithelia

such

as

those

present

in

pancreas

salivary

glands

testis

prostate

and

hepatic

epithelium

The

Dsg

and

Dsg

pairs

are

largely

confined

to

stratified

epithelia

where

the

expression

patterns

of

the

Dsg

and

Dsc

isoforms

usually

conform

Interestingly

in

the

stratified

corneal

epithelium

only

Dsg

and

are

present

indicating

that

these

desmosomal

cadherins

in

the

absence

of

Dsg

and

Dsg

are

sufficient

to

maintain

cohesion

in

stratified

epithelia

also

In

the

epidermis

the

plaque

proteins

plakoglobin

desmoplakin

and

plakophilin

are

expressed

in

all

layers

-LRB-

Fig.

-RRB-

In

contrast

plakophilins

and

display

inverse

distribution

with

plakophilin

being

more

abundant

in

the

superficial

epidermis

These

inverse

expression

patterns

are

also

typical

for

the

Dsg

and

Dsg

pairs

Dsg

are

the

predominant

desmosomal

cadherins

in

the

superficial

epidermis

whereas

Dsg

are

primarily

expressed

in

the

lower

epidermis

In

contrast

to

Dsg

which

can

be

detected

in

some

desmosomes

in

keratinocytes

of

the

basal

layer

also

Dsc

and

Dsg

are

absent

in

the

basal

layer

-LRB-

Dusek

et

al.

Mahoney

et

al.

Spindler

et

al.

-RRB-

Because

Dsg

is

expressed

throughout

the

spinous

layers

-LRB-

Fig.

-RRB-

the

expression

patterns

of

Dsg

and

Dsg

largely

overlap

in

human

adult

epidermis

-LRB-

Mahoney

et

al.

Spindler

et

al.

-RRB-

Dsc

is

enriched

in

the

deep

epidermis

with

lower

levels

in

the

superficial

epidermis

whereas

Dsg

is

restricted

to

basal

and

suprabasal

cells

but

is

present

in

very

faint

amounts

only

-LRB-

Mahoney

et

al.

-RRB-

indicating

that

this

pair

of

proteins

may

be

primarily

important

for

cell

cohesion

in

simple

epithelia

and

myocardium

rather

than

in

complex

epithelia

However

it

is

unclear

at

present

which

desmosomal

cadherin

isoforms

are

capable

to

heretophilically

bind

to

each

other

and

thus

interpretation

of

these

distribution

patterns

with

respect

to

their

relevance

for

mechanical

adhesion

is

preliminary

In

multilayered

squamous

epithelium

of

mucous

membranes

for

instance

of

the

oral

cavity

Dsg

and

Dsg

are

strongly

expressed

throughout

all

layers

whereas

Dsg

shows

strong

expression

in

superficial

layers

but

is

missing

in

the

basal

layer

-LRB-

Mahoney

et

al.

-RRB-

Fig.

patterns

of

desmosomal

components

in

the

epidermis

The

schematic

drawing

of

the

epidermis

-LRB-

left

-RRB-

indicates

the

basal

-LRB-

BL

-RRB-

spinous

-LRB-

SL

-RRB-

granular

-LRB-

GL

-RRB-

and

corneal

-LRB-

CL

-RRB-

layer

of

the

epidermis

On

the

right

the

expression

patterns

of

desmosomal

components

in

the

specific

epidermal

layers

are

illustrated

For

instance

Dsg

and

Pkp

are

most

prominent

in

the

superficial

layers

whereas

expression

of

Dsg

and

Dsc

is

strongest

in

the

deep

epidermis

Dsg

desmoglein

Dsc

desmocollin

Pkp

plakophilin

PG

plakoglobin

DP

desmoplakinFig

of

Dsg

and

Dsg

in

human

epidermis

Intact

human

epidermis

was

immunostained

using

monoclonal

antibodies

against

Dsg

-LRB-

a

-RRB-

and

Dsg

-LRB-

b

-RRB-

A

merge

of

both

panels

is

shown

in

c.

Dsg

is

most

abundant

in

the

superficial

epidermis

but

is

also

present

in

the

basal

layer

Dsg

is

expressed

in

the

basal

layer

as

well

as

throughout

the

spinous

layer

indicating

that

in

human

epidermis

the

expression

patterns

of

these

two

proteins

broadly

overlap

Scale

bar

is

μm

It

is

important

to

note

that

the

specific

distribution

patterns

of

desmosomal

components

in

stratified

epithelia

are

important

for

epithelial

differentiation

and

function

-LRB-

Green

and

Simpson

-RRB-

It

was

shown

that

forced

overexpression

of

Dsg

in

the

suprabasal

epidermis

led

to

abnormal

differentiation

and

hyperproliferation

as

well

as

perinatal

lethality

due

to

transepidermal

water

loss

-LRB-

Elias

et

al.

Merritt

et

al.

-RRB-

Similarly

forced

suprabasal

Dsg

and

Dsc

overexpression

resulted

in

hyperproliferation

and

formation

of

papillomas

possibly

via

altered

β-catenin/wnt

signalling

-LRB-

Brennan

et

al.

Hardman

et

al.

-RRB-

Desmosome

assembly

and

disassembly

The

mechanisms

participating

in

desmosome

assembly

and

disassembly

have

been

reviewed

in

detail

elsewhere

-LRB-

Getsios

et

al.

Green

and

Simpson

Kitajima

Yin

and

Green

-RRB-

For

instance

extracellular

and

protein

kinase

C

-LRB-

PKC

-RRB-

signalling

are

well

known

to

be

involved

in

desmosome

assembly

concentrations

mM

allow

formation

of

adherens

junctions

and

desmosomes

-LRB-

Hennings

and

Holbrook

-RRB-

Desmosomal

plaques

with

inserted

cytokeratin

filaments

became

visible

as

early

as

after

min

after

the

switch

followed

by

appearance

of

assymetrical

desmosomes

after

min

and

of

symmetric

desmosomes

after

h.

Increased

extracellular

induced

incorporation

of

desmosomal

components

such

as

Dsgs

plakoglobin

and

desmoplakin

into

the

desmosomal

plaque

-LRB-

Hennings

and

Holbrook

Pasdar

et

al.

Pasdar

and

Nelson

-RRB-

Activation

of

PKC

is

required

for

translocation

of

desmosomal

components

to

the

cell

membrane

and

for

desmosome

assembly

-LRB-

Sheu

et

al.

-RRB-

but

also

was

found

to

reduce

desmosomal

adhesion

and

to

increase

dependence

of

desmosomes

-LRB-

Kimura

et

al.

-RRB-

indicating

that

regulation

of

desmosomal

adhesion

by

PKC

is

complex

Before

desmosome

assembly

adhesion

zippers

of

E-cadherin-containing

puncta

form

on

filopodial

processes

of

neighbouring

cells

an

event

that

requires

both

α-catenin

and

VASP-driven

actin

reorganization

-LRB-

Vasioukhin

et

al.

-RRB-

Afterwards

these

intermediate

junctions

mature

to

adherens

junctions

and

desmosomes

are

assembled

at

regions

where

membranes

are

brought

together

It

appears

that

Dscs

initiate

the

formation

of

desmosomes

This

is

based

on

the

observations

that

Dsc

is

the

first

desmosomal

component

at

the

cell

surface

followed

by

Dsg

in

MDCK

cells

-LRB-

Burdett

and

Sullivan

-RRB-

and

that

in

keratinocytes

N-terminally

deleted

Dsc

which

compromised

desmosome

formation

was

still

able

to

bind

to

β-catenin

Therefore

it

can

be

speculated

that

Dsc

could

localize

to

pre-existing

adherens

junctions

to

induce

desmosome

formation

-LRB-

Hanakawa

et

al.

-RRB-

Desmosomal

cadherins

seem

to

be

transported

in

vesicles

from

the

Golgi

along

microtubules

whereas

non-membranous

cytoplasmic

particles

containing

desmoplakin

and

plakophilin

are

associated

with

intermediate

filaments

and

move

towards

cell-junctions

by

actin-based

motility

-LRB-

Godsel

et

al.

Green

and

Simpson

-RRB-

Desmoplakin

trafficking

seems

to

be

dependent

on

intracellular

levels

because

patients

with

Darier

's

disease

which

results

from

mutations

in

a

sarcoplasmic

reticulum

pump

show

desmoplakin

retention

in

the

cytoplasm

and

altered

desmosome

structure

-LRB-

Dhitavat

et

al.

Dhitavat

et

al.

Sakuntabhai

et

al.

-RRB-

On

the

cell

surface

the

desmosomal

cadherins

together

with

plakoglobin

and

desmoplakin

are

sufficient

to

nucleate

a

desmosomal

plaque

-LRB-

Kowalczyk

et

al.

-RRB-

Further

plaque

enlargement

and

desmoglein

clustering

are

dependent

on

plakoglobin

together

with

plakophilin

-LRB-

Bornslaeger

et

al.

Koeser

et

al.

-RRB-

Therefore

keratinocytes

deficient

for

either

plakoglobin

or

desmoplakin

display

reduced

numbers

of

desmosomes

disturbed

plaque

formation

and

reduced

anchorage

of

cytokeratin

filaments

-LRB-

Bierkamp

et

al.

Vasioukhin

et

al.

-RRB-

It

appears

that

during

desmosome

assembly

clusters

are

formed

in

the

beginning

which

upon

attachment

to

cytokeratin

filaments

become

integrated

in

desmosomes

-LRB-

Sato

et

al.

-RRB-

Once

they

are

formed

desmosomes

are

stable

throughout

the

cell

cycle

and

are

not

disrupted

during

mitosis

although

the

desmosomal

components

are

subjected

to

a

significant

turnover

with

a

half-life

of

about

min

like

it

was

shown

for

Dsc

-LRB-

Windoffer

et

al.

-RRB-

Finally

it

has

to

be

emphasized

that

a

reciprocal

dependence

of

desmosomes

and

adherens

junctions

seems

to

exist

This

can

be

concluded

from

experiments

in

which

expression

of

N-terminally

deleted

Dsc

or

desmoplakin

deficiency

resulted

in

impaired

formation

of

both

desmosomes

and

adherens

junctions

-LRB-

Hanakawa

et

al.

Vasioukhin

et

al.

-RRB-

Regulation

of

keratinocyte

proliferation

by

desmosomal

cadherins

Evidence

is

accumulating

that

desmosomal

cadherins

such

as

Dsg

regulate

keratinocyte

proliferation

-LRB-

Muller

et

al.

-RRB-

It

has

been

shown

that

autoantibodies

from

pemphigus

vulgaris

patients

induce

continuing

keratinocyte

proliferation

by

impaired

Dsg

signalling

which

finally

leads

to

c-Myc

overexpression

-LRB-

Muller

et

al.

Williamson

et

al.

-RRB-

According

to

this

concept

in

healthy

epidermis

Dsg

binding

results

in

inhibition

of

glycogen

synthase

kinase

-LRB-

-RRB-

via

activation

of

phosphatidylinositol

trisphosphate

kinase

-LRB-

-RRB-

and

Akt

In

consequence

phosphorylation-dependent

degradation

of

plakoglobin

is

abolished

which

allows

plakoglobin

to

translocate

into

the

nucleus

and

to

induce

growth

arrest

via

suppression

of

c-Myc

-LRB-

Muller

et

al.

Williamson

et

al.

-RRB-

Desmosome-associated

diseases

Several

diseases

have

been

found

in

which

impaired

desmosomal

adhesion

contributes

to

pathogenesis

Inactivation

of

desmosomal

function

may

be

reduced

by

completely

different

mechanisms

including

genetic

defects

of

desmosomal

components

cleavage

of

desmosomal

cadherins

by

bacterial

toxins

and

binding

of

autoantibodies

to

desmogleins

the

latter

of

which

is

the

cause

of

the

autoimmune

disease

pemphigus

Although

altered

expression

of

desmosomal

cadherins

such

as

Dsg

and

Dsg

have

been

observed

in

human

carcinomas

such

as

squamous

cell

carcinoma

as

well

as

gastric

colorectal

and

breast

carcinomas

mutations

are

usually

absent

-LRB-

Bazzi

and

Christiano

-RRB-

Therefore

the

role

of

desmosomal

cadherins

in

cancer

is

unclear

at

present

Genetic

diseases

Mutations

in

desmosomal

plaque

components

in

humans

affect

the

myocardium

as

well

as

the

epidermis

with

its

appendages

-LRB-

Table

-RRB-

Mutations

in

genes

for

the

essential

desmosomal

plaque

components

desmoplakin

and

plakoglobin

result

in

heart

skin

and

hair

defects

-LRB-

Bazzi

and

Christiano

Green

and

Simpson

McGrath

-RRB-

In

contrast

genetic

alterations

of

Dsg

Dsc

and

plakophilin

selectively

lead

to

heart

defects

because

these

are

the

only

isoforms

of

their

protein

families

in

the

myocardium

On

the

other

hand

mutations

in

Dsg

and

plakophilin

which

are

primarily

expressed

in

the

epidermis

cause

skin

defects

whereas

loss

of

Dsg

in

hair

follicles

results

in

hair

loss

Genetic

heart

defects

Interestingly

all

defects

of

desmosomal

components

causing

heart

defects

such

as

desmoplakin

plakoglobin

plakophilin

Dsg

and

Dsc

lead

to

the

phenotype

of

arrhythmogenic

right

ventricular

cardiomyopathy

-LRB-

ARVC

-RRB-

which

is

clinically

characterized

by

right

bundle

block

and

arrhythmia

and

histologically

by

fibrofatty

replacement

of

cardiomyocytes

possibly

due

to

impaired

cell

adhesion

caused

by

loss

and

alterations

of

desmosomes

-LRB-

Asimaki

et

al.

Gerull

et

al.

McKoy

et

al.

Pilichou

et

al.

Rampazzo

et

al.

Syrris

et

al.

Heuser

et

al.

-RRB-

This

is

in

line

with

embryonic

lethality

due

to

myocardial

rupture

in

mice

models

deficient

in

these

proteins

Therefore

the

thinnest

parts

of

the

right

ventricle

are

the

most

severely

affected

but

left

ventricle

involvement

also

occurs

-LRB-

van

Tintelen

et

al.

-RRB-

However

fibrofatty

transdifferentiation

of

cardiomyocytes

can

not

be

simply

explained

by

impaired

desmosomal

adhesion

but

rather

seems

to

be

caused

by

altered

wnt

β-catenin

signalling

in

response

to

nuclear

translocation

of

plakoglobin

-LRB-

Garcia-Gras

et

al.

-RRB-

Genetic

defects

of

skin

and

its

appendages

Haploinsufficiency

of

the

gene

encoding

Dsg

results

in

the

autosomal

dominant

skin

disease

striate

palmoplantar

keratoderma

-LRB-

SPPK

-RRB-

which

is

characterized

by

linearly

arranged

thickening

of

the

stratum

corneum

on

the

palms

soles

knees

ankles

and

finger

knuckles

-LRB-

Milingou

et

al.

Rickman

et

al.

-RRB-

However

blisters

are

absent

indicating

that

disturbed

differentiation

is

the

primary

mechanism

underyling

this

entity

rather

than

a

loss

of

keratinocyte

adhesive

function

Similarly

mutated

Dsg

leads

to

autosomal

recessive

inherited

hypotrichosis

due

to

defective

hair

follicle

differentiation

a

phenotype

related

to

the

lanceolate

hair

mouse

-LRB-

Kljuic

et

al.

-RRB-

In

contrast

ablation

of

plakophilin

results

in

the

recessive

skin-fragility

ectodermal

dysplasia

syndrome

which

in

was

the

first

genetic

desmosome-associated

disease

to

be

described

-LRB-

McGrath

-RRB-

Here

both

loss

and

alterations

of

desmosomes

and

lacking

insertion

of

cytokeratin

filaments

due

to

inability

to

recruit

desmoplakin

cause

skin

blistering

around

the

mouth

as

well

as

on

palms

and

soles

accompanied

by

dystrophic

hair

and

nails

-LRB-

McGrath

et

al.

McMillan

and

Shimizu

-RRB-

Mutations

in

plaque

proteins

with

involvement

of

various

tissues

Mutations

in

plakoglobin

are

the

cause

of

Naxos

disease

in

which

ARVC

and

palmoplantar

keratoderma

are

associated

with

woolly

hair

-LRB-

McKoy

et

al.

-RRB-

Interestingly

in

contrast

to

plakoglobin-deficient

mice

-LRB-

Bierkamp

et

al.

-RRB-

acantholysis

is

absent

indicating

that

some

mechanical

functions

of

plakoglobin

are

maintained

in

these

patients

The

most

variable

phenotypes

are

the

consequence

of

desmoplakin

alterations

An

autosomal

recessive

disorder

with

dilated

cardiomyopathy

keratoderma

and

woolly

hair

called

Carvajal

syndrome

is

comparable

to

Naxos

disease

-LRB-

Norgett

et

al.

-RRB-

Haploinsufficiency

leads

to

SPPK

whereas

non-sense

mutations

are

accompanied

by

skin

fragility

leading

to

blisters

in

the

face

as

well

as

on

extremities

and

trunk

and

also

with

wolly

hair

-LRB-

Armstrong

et

al.

Whittock

et

al.

-RRB-

However

the

most

severe

disorder

is

lethal

acantholytic

epidermolyis

bullosa

which

is

caused

by

C-terminally

truncated

desmoplakin

and

was

fatal

in

a

hair

and

nailless

newborn

due

to

extensive

blistering

leading

to

transcutaneous

fluid

loss

-LRB-

Jonkman

et

al.

-RRB-

On

the

ultra-structural

level

desmosomes

were

reduced

in

desmoplakin-related

SPPK

similar

to

SPPK

caused

by

mutations

in

Dsg

-LRB-

Wan

et

al.

-RRB-

but

not

in

lethal

acantholytic

epidermolysis

bullosa

However

desmosome

shedding

alterations

of

desmosomal

plaques

and

impaired

cytokeratin

insertion

were

typically

associated

with

desmoplakin

mutations

-LRB-

Jonkman

et

al.

Norgett

et

al.

Wan

et

al.

-RRB-

At

present

it

is

unclear

why

different

mutations

affect

different

tissues

Infectious

diseases

Staphylococcal

scalded

skin

syndrome

-LRB-

SSSS

-RRB-

which

was

first

described

by

Ritter

von

Rittershain

in

is

the

systemic

variant

of

epidemic

pemphigus

neonatorum

or

sporadic

bullous

impetigo

and

is

characterized

by

superficial

epidermal

splitting

accompanied

by

fever

erythema

and

skin

tenderness

-LRB-

Farrell

Lyell

Stanley

and

Amagai

-RRB-

Most

cases

are

caused

by

staphylococcal

exfoliative

toxin

-LRB-

ET

-RRB-

a

serine

protease

which

has

been

shown

to

selectively

cleave

Dsg

between

EC

and

in

conformation-dependent

manner

but

not

Dsg

or

E-cadherin

-LRB-

Table

-RRB-

-LRB-

Amagai

et

al.

Hanakawa

et

al.

Melish

and

Glasgow

-RRB-

Assuming

that

ET

does

not

cleave

other

superficially

expressed

desmosomal

cadherins

such

as

Dsg

or

Dsc

SSSS

is

a

good

example

that

extensive

epidermal

blistering

can

be

induced

by

proteolytic

cleavage

of

a

single

adhesion

molecule

According

to

its

bacterial

pathogenesis

SSSS

can

be

effectively

treated

with

antibiotics

-LRB-

Stanley

and

Amagai

-RRB-

Pemphigus

Pemphigus

is

an

autoimmune

blistering

skin

disease

which

is

characterized

by

intraepidermal

blistering

-LRB-

Lever

-RRB-

The

two

major

types

of

pemphigus

are

the

more

severe

pemphigus

vulgaris

-LRB-

PV

-RRB-

which

accounts

for

of

cases

and

pemphigus

foliaceus

-LRB-

PF

-RRB-

-LRB-

Bystryn

and

Rudolph

Schmidt

et

al.

Stanley

and

Amagai

-RRB-

Pemphigus

is

a

rare

disease

with

a

yearly

incidence

of

cases

per

million

and

apart

from

being

present

in

humans

is

also

found

in

horses

dogs

and

cats

In

contrast

to

other

autoimmune

diseases

which

primarily

affect

women

pemphigus

is

equally

distributed

between

both

genders

and

is

diagnosed

mostly

between

the

fourth

and

sixth

decades

In

PV

there

are

two

main

forms

the

mucosal-dominant

and

the

mucocutaneous

type

In

both

cases

the

disease

most

commonly

begins

with

painful

non-healing

ulcerations

not

only

in

the

mucous

membranes

of

the

mouth

but

also

in

the

larynx

nose

and

vagina

Later

on

flaccid

blisters

may

occur

on

the

scalp

trunk

groin

and

axillae

which

easily

rupture

leaving

sharply

outlined

erosions

and

heal

without

scarring

-LRB-

Fig.

-RRB-

In

contrast

to

PV

PF

only

affects

the

epidermis

and

because

epidermal

splitting

is

restricted

to

the

superficial

epidermis

lesions

appear

as

crusted

erosions

on

the

upper

torso

face

and

scalp

It

has

to

be

mentioned

that

pemphigus

can

be

induced

by

drugs

such

as

penicillamine

penicillin

captopril

and

β-blockers

and

also

can

occur

as

a

paraneoplastic

entity

accompanying

or

preceding

lymphoma

and

lung

carcinoma

-LRB-

Yeh

et

al.

-RRB-

Moreover

in

South

America

an

endemic

form

of

PF

called

Fogo

selvagem

exists

which

is

thought

to

be

transmitted

by

insect

vectors

-LRB-

Aoki

et

al.

Diaz

et

al.

-RRB-

Currently

the

therapy

of

pemphigus

is

based

on

immunosupression

and

reduction

of

autoantibody

load

Conventional

therapy

includes

high-dose

corticosteroids

intravenous

immune

globulin

and

cytotoxic

agents

Before

systemic

corticosteroids

were

available

of

PV

patients

died

within

a

year

Second-line

therapies

for

refractory

PV

include

rituximab

an

antibody

directed

against

B

cell

which

reduces

autoantibody-producing

B

cells

as

well

as

plasmapheresis

to

physically

remove

autoantibodies

-LRB-

Ahmed

et

al.

Shimanovich

et

al.

-RRB-

Fig.

phenotype

of

Pemphigus

vulgaris

and

Pemphigus

foliaceus

Patients

suffering

from

the

mucocutaneous

form

of

pemphigus

vulgaris

-LRB-

PV

-RRB-

usually

have

flaccid

blisters

and

erosions

on

the

trunk

-LRB-

a

-RRB-

accompanied

by

mucosal

ulcerations

in

the

mouth

-LRB-

b

-RRB-

In

contrast

Pemphigus

foliaceus

patients

are

characterized

by

crusted

epidermal

erosions

-LRB-

c

-RRB-

whereas

involvement

of

mucous

membranes

is

absent

Histology

and

autoantibody

profile

in

pemphigus

Besides

the

clinical

phenotype

diagnosis

of

pemphigus

is

based

on

histology

and

the

patients

autoantibody

profile

-LRB-

Bystryn

and

Rudolph

-RRB-

The

histologic

hallmark

of

pemphigus

is

acantholysis

i.e.

loss

of

cell

cell

adhesion

between

keratinocytes

In

PV

the

epidermal

cleavage

plane

is

located

in

the

deep

epidermis

usually

right

above

the

basal

layer

-LRB-

Fig.

-RRB-

In

contrast

in

PF

epidermal

splitting

occurs

between

granular

layers

Skin

blisters

can

be

induced

by

rubbing

on

healthy-appearing

epidermis

a

phenomenon

referred

to

as

Nikolsky

sign

Fig.

histology

of

epidermal

lesions

from

pemphigus

patients

Hematoxylin

eosin-stained

paraffin

sections

from

PV

-LRB-

a

-RRB-

and

PF

-LRB-

b

-RRB-

patients

showed

suprabasal

epidermal

cleavage

in

the

PV

and

superficial

granular

blistering

in

PF

Scale

bar

is

μm

Autoantibodies

in

pemphigus

are

sufficient

to

cause

blistering

in

human

skin

in

vivo

and

in

vitro

-LRB-

Anhalt

et

al.

Schiltz

and

Michel

-RRB-

In

contrast

to

other

autoimmune

blistering

skin

diseases

such

as

bullous

pemphigoid

or

epidermolysis

bullosa

acquisita

-LRB-

Sitaru

and

Zillikens

Yancey

-RRB-

pemphigus

antibodies

do

not

require

the

complement

system

or

leukocytes

to

induce

blisters

in

vivo

-LRB-

Anhalt

et

al.

-RRB-

It

is

generally

accepted

that

PV

and

PF

are

characterized

by

different

autoantibody

profiles

which

generally

correlate

with

disease

activity

-LRB-

Bystryn

and

Rudolph

Harman

et

al.

Ishii

et

al.

Stanley

and

Amagai

Stanley

et

al.

Yeh

et

al.

-RRB-

Patients

suffering

from

mucosal-dominant

PV

usually

have

antibodies

directed

against

Dsg

but

not

Dsg

whereas

mucocutaneous

PV

is

characterized

by

both

Dsg

and

Dsg

autoantibodies

-LRB-

Amagai

et

al.

Ding

et

al.

Jamora

et

al.

Miyagawa

et

al.

-RRB-

In

contrast

in

PF

patients

usually

antibodies

against

Dsg

but

not

Dsg

are

detected

-LRB-

Amagai

et

al.

-RRB-

However

it

is

also

known

that

in

some

cases

this

correlation

between

the

clinical

phenotype

and

the

autoantibody

profile

was

not

found

-LRB-

Baykal

et

al.

Jamora

et

al.

Yoshida

et

al.

Zagorodniuk

et

al.

-RRB-

Over

the

last

decade

there

is

a

debate

whether

these

autoantibodies

against

desmosomal

cadherins

are

pathogenic

-LRB-

Amagai

et

al.

-RRB-

It

has

been

shown

by

passive

transfer

of

affinity-purified

Dsg

antibody

fractions

as

well

as

by

depletion

of

pathogenic

activity

by

absorption

against

desmoglein

extracellular

domains

that

Dsg

antibodies

in

PF

and

the

combination

of

Dsg

and

Dsg

autoantibodies

in

PV

as

well

as

in

paraneoplastic

pemphigus

are

sufficient

to

induce

skin

blistering

-LRB-

Amagai

et

al.

Koulu

et

al.

Mahoney

et

al.

-RRB-

An

active

PV

mouse

model

in

which

Dsg

mice

were

immunized

with

Dsg

before

splenocytes

from

these

animals

were

transferred

to

lymphopenic

mice

supported

the

notion

that

Dsg

antibodies

alone

can

cause

mucosal

erosions

-LRB-

Amagai

et

al.

-RRB-

Similar

in

keratinocyte

cultures

depletion

of

Dsg

antibodies

from

PF-IgG

by

preincubation

with

recombinant

Dsg

but

not

after

preincubation

with

VE-cadherin

completely

abolished

keratinocyte

dissociation

-LRB-

Waschke

et

al.

-RRB-

Pemphigus

IgG

were

found

to

include

a

plethora

of

more

than

different

autoantibodies

against

keratinocyte

antigens

such

as

antibodies

against

Dsg

Dsg

Dsc

desmoplakin

plakoglobin

and

E-cadherin

and

several

other

proteins

not

associated

with

cell

junctions

-LRB-

Amagai

et

al.

Evangelista

et

al.

Kljuic

et

al.

Korman

et

al.

Nguyen

et

al.

-RRB-

For

instance

in

all

PF

sera

as

well

as

in

of

mucocutaneous

PV

sera

autoantibody

activities

against

E-cadherin

were

detected

most

of

which

were

due

to

Dsg

autoantibodies

cross-reacting

with

E-cadherin

-LRB-

Evangelista

et

al.

-RRB-

Some

of

the

different

autoantibodies

have

clearly

been

shown

not

to

be

pathogenic

such

as

the

Dsg

Dsg

antibodies

in

PF

-LRB-

Nagasaka

et

al.

-RRB-

Therefore

similar

to

other

autoimmune

diseases

the

pathogenetic

relevance

of

autoantibodies

against

a

specific

protein

in

pemphigus

has

to

be

challenged

until

it

has

been

convincingly

demonstrated

-LRB-

Amagai

et

al.

-RRB-

However

it

has

been

reported

that

antibodies

others

than

those

directed

to

desmogleins

also

contribute

to

epidermal

blistering

because

PV-IgG

not

containing

Dsg

antibodies

were

effective

to

cause

blistering

in

Dsg

mice

-LRB-

Nguyen

et

al.

-RRB-

It

is

possible

that

these

antibodies

include

antibodies

to

cholinergic

receptors

and

to

pemphaxin

which

have

both

been

detected

in

of

PV

and

PF

sera

-LRB-

Grando

Nguyen

et

al.

-RRB-

The

pathogenic

relevance

of

antibodies

against

cholinergic

receptors

was

concluded

from

experiments

where

preincubation

of

monkey

oesophagus

with

PV-IgG

blocked

staining

by

a

rabbit

acetylcholine

receptor

antibody

and

the

fact

that

this

antibody

induced

keratinocyte

dissociation

in

culture

-LRB-

Nguyen

et

al.

-RRB-

However

antibodies

against

pemphaxin

alone

were

not

sufficient

to

induce

skin

blistering

-LRB-

Nguyen

et

al.

-RRB-

Moreover

it

has

not

been

demonstrated

so

far

that

autoantibodies

from

pemphigus

patients

which

target

cholinergic

receptors

are

capable

to

induce

acantholysis

Therefore

at

present

it

is

safe

to

believe

that

epidermal

blistering

in

pemphigus

is

primarily

caused

by

antibodies

against

Dsg

and

Dsg

These

pathogenic

antibodies

in

PV

and

PF

mainly

belong

to

the

IgG

and

IgG

subclasses

-LRB-

Bhol

et

al.

Rock

et

al.

Spaeth

et

al.

-RRB-

The

knowledge

that

autoantibodies

against

desmosomal

cadherins

are

sufficient

to

induce

acantholysis

in

complement

and

leukocyte-independent

manner

makes

pemphigus

one

of

the

best-characterized

models

to

study

the

direct

mechanisms

underlying

autoimmune

diseases

Besides

the

role

of

autoantibodies

the

contribution

of

Dsg

autoreactive

T

helper

-LRB-

Th

-RRB-

cells

has

also

been

characterized

for

PV

and

endemic

PF

-LRB-

Hertl

et

al.

-RRB-

and

cells

in

PV

recognize

the

extracellular

domain

of

Dsg

when

presented

on

the

HLA

class

II

alleles

and

whereas

in

Fogo

selvagem

patients

and

were

most

common

In

PV

patients

as

well

as

in

healthy

carriers

of

the

PV-associated

HLA

II

alleles

Dsg

and

Dsg

T

cells

were

found

In

healthy

individuals

cells

with

characteristics

of

regulatory

T

-LRB-

-RRB-

cells

which

inhibit

T

cell

activation

were

most

prevalent

In

contrast

in

PV

patients

the

levels

of

cells

were

reduced

while

cells

were

increased

-LRB-

Veldman

et

al.

-RRB-

Therefore

it

is

possible

that

an

imbalance

of

autoreactive

and

Th

cells

plays

a

role

in

the

induction

of

PV

by

promoting

the

proliferation

of

anti-Dsg

producing

B

cells

The

mechanisms

underlying

pemphigus

skin

blistering

As

a

first

concept

it

was

proposed

that

proteolytic

cleavage

of

molecules

responsible

for

intercellular

adhesion

was

the

mechanism

underlying

pemphigus

skin

blistering

Later

on

with

the

identification

of

desmosomal

cadherins

as

the

target

antigens

of

pemphigus

autoantibodies

and

with

more

sophisticated

cell

biologic

tools

at

hand

the

ideas

of

direct

antibody-mediated

inhibition

and

of

indirect

signalling-mediated

reduction

of

desmoglein

binding

were

developed

-LRB-

Fig.

-RRB-

Fig.

two

principal

mechanisms

underlying

pemphigus

skin

blistering

Two

principal

mechanisms

have

been

proposed

by

which

autoantibodies

specific

for

Dsg

and

Dsg

could

impair

desmosomal

adhesion

First

antibodies

could

directly

interfere

with

desmoglein

transinteraction

-LRB-

a

-RRB-

Second

antibody

binding

has

been

shown

to

trigger

intracellular

signalling

pathways

which

indirectly

results

in

loss

of

desmoglein-mediated

binding

-LRB-

b

-RRB-

Proteolytic

cleavage

of

desmosomal

cadherins

Proteolytic

cleavage

of

cell

adhesion

molecules

has

first

been

suggested

to

be

involved

in

pemphigus

acantholysis

because

protease

inhibitors

blocked

pemphigus

IgG-induced

cell

detachment

in

culture

-LRB-

Farb

et

al.

-RRB-

Moreover

plasminogen

activator

activity

and

expression

of

the

urokinase-type

plasminogen

activator

receptor

-LRB-

uPAR

-RRB-

system

were

found

to

be

increased

following

treatment

with

PV

and

PF-IgG/serum

in

keratinocytes

in

vitro

as

well

as

in

PV

patients

skin

-LRB-

Feliciani

et

al.

Hashimoto

et

al.

Lo

Muzio

et

al.

Schaefer

et

al.

Seishima

et

al.

Yamamoto

et

al.

-RRB-

possibly

via

phospholipase

C

-LRB-

PLC

-RRB-

mediated

signalling

-LRB-

Esaki

et

al.

-RRB-

Anti-uPA

antibodies

and

a

PA

inhibitor

were

sufficient

to

block

acantholysis

induced

by

PV

or

PF-IgG

in

several

studies

-LRB-

Feliciani

et

al.

Hashimoto

et

al.

Morioka

et

al.

-RRB-

but

not

in

that

by

Schuh

et

al.

-LRB-

-RRB-

However

a

definitive

evaluation

of

the

uPAR

system

for

pemphigus

acantholysis

became

possible

since

a

study

using

uPA

and

tissue

PA-deficient

mice

showed

extensive

skin

blistering

in

response

to

PV

and

PF-IgG

-LRB-

Mahoney

et

al.

-RRB-

Thus

the

plasminogen

activator

system

does

not

appear

to

be

essential

for

pemphigus

skin

blistering

but

may

aggravate

the

phenotype

especially

when

secondary

inflammatory

mediators

such

as

and

TNF-α

are

released

-LRB-

Feliciani

et

al.

-RRB-

The

same

may

hold

true

for

other

proteases

such

as

matrix

metalloproteinases

-LRB-

MMP

-RRB-

or

proteases

of

the

ADAM

-LRB-

a

disintegrin

and

metalloproteinase

-RRB-

family

which

was

overexpressed

but

not

activated

following

treatment

with

PV

serum

was

reported

to

specifically

cleave

Dsg

during

apoptosis

-LRB-

Cirillo

et

al.

d

-RRB-

on

the

other

hand

was

upregulated

by

activation

of

the

epidermal

growth

factor

receptor

-LRB-

EGFR

-RRB-

and

caused

shedding

of

Dsg

-LRB-

Bech-Serra

et

al.

Santiago-Josefat

et

al.

-RRB-

These

results

may

be

important

for

PV

because

EGFR

activation

was

observed

following

treatment

with

PV-IgG

-LRB-

Chernyavsky

et

al.

Frusic-Zlotkin

et

al.

-RRB-

The

presence

of

proteolytic

enzymes

in

PV

sera

may

also

explain

why

IgG-depleted

PV

sera

were

found

to

be

pathogenic

in

culture

-LRB-

Cirillo

et

al.

-RRB-

However

because

no

direct

evidence

was

provided

that

or

or

any

other

proteinase

cleaves

members

of

the

Dsg

family

in

pemphigus

the

significance

of

these

findings

for

acantholysis

in

pemphigus

is

unclear

and

the

specific

proteolysis

hypothesis

proposed

for

pemphigus

requires

further

experimental

substantiation

-LRB-

Cirillo

et

al.

-RRB-

Nevertheless

the

fact

that

specific

cleavage

of

Dsg

by

staphylococcal

exfoliative

toxin

in

bullous

impetigo

is

sufficient

to

cause

a

histologic

phenotype

comparable

to

PF

-LRB-

Amagai

et

al.

Hanakawa

et

al.

-RRB-

indicates

that

in

principle

specific

proteolysis

could

be

an

effective

mechanism

in

pemphigus

Direct

inhibition

of

desmoglein

binding

Since

it

was

discovered

that

autoantibodies

in

pemphigus

are

directed

to

desmosomal

adhesion

molecules

it

was

believed

that

these

autoantibodies

might

directly

interfere

with

desmoglein

binding

-LRB-

Fig.

-RRB-

-LRB-

Amagai

et

al.

Jones

et

al.

Koulu

et

al.

-RRB-

a

mechanism

also

refered

to

as

steric

hindrance

-LRB-

Sharma

et

al.

-RRB-

This

model

is

attractive

because

it

has

been

shown

that

autoantibodies

against

Dsg

and

Dsg

in

PV

and

PF

patients

primarily

target

the

aminoterminal

part

of

the

EC

domain

-LRB-

Futei

et

al.

Hacker-Foegen

et

al.

Ishii

et

al.

Muller

et

al.

Sekiguchi

et

al.

-RRB-

The

EC

domain

according

to

morphologic

studies

on

desmoglein

transinteraction

in

desmosomes

is

increasingly

recognized

as

the

part

of

the

desmosomal

cadherin

ectodomain

responsible

for

trans-interaction

-LRB-

Al-Amoudi

et

al.

He

et

al.

-RRB-

and

may

harbour

the

putative

transadhesive

interface

based

on

data

from

the

crystal

structure

of

classical

cadherins

-LRB-

Boggon

et

al.

Overduin

et

al.

Shapiro

et

al.

-RRB-

Moreover

it

seems

that

autoantibody

reactivity

to

the

aminoterminal

parts

-LRB-

EC

-RRB-

of

the

Dsg

ectodomain

correlates

with

high

disease

activity

as

well

as

epidermal

or

mucosal

involvement

in

PV

although

the

titers

of

these

antibodies

do

not

show

this

correlation

-LRB-

Amagai

et

al.

Muller

et

al.

Salato

et

al.

-RRB-

First

functional

data

that

anti-Dsg

antibodies

in

PV

may

directly

interfere

with

Dsg

binding

were

provided

using

monoclonal

antibodies

derived

from

the

active

PV

mouse

model

-LRB-

Amagai

et

al.

-RRB-

AK

which

was

directed

against

the

aminoterminal

part

of

EC

was

found

to

be

pathogenic

and

capable

to

induce

epidermal

blistering

in

vivo

at

least

when

PF-IgG

or

exfoliative

toxin

A

was

added

to

inactivate

Dsg

-LRB-

Shimizu

et

al.

Tsunoda

et

al.

-RRB-

Antibodies

to

other

parts

of

the

Dsg

extracellular

domain

such

as

AK

and

AK

were

ineffective

to

induce

blisters

Recently

by

using

single-molecule

atomic

force

microscopy

-LRB-

AFM

-RRB-

it

was

shown

that

PV-IgG

as

well

as

AK

directly

interfered

with

homophilic

Dsg

binding

under

cell

free

conditions

-LRB-

Heupel

et

al.

-RRB-

which

supports

the

hypothesis

of

direct

inhibition

of

Dsg

binding

in

PV

-LRB-

Stanley

and

Amagai

-RRB-

However

no

direct

inhibition

of

Dsg

binding

by

PV-IgG

and

PF-IgG

was

detected

by

AFM

These

autoantibodies

induced

keratinocyte

dissociation

and

reduced

binding

of

both

Dsg

and

Dsg

microbeads

to

the

surface

of

cultured

keratinocytes

as

revealed

by

laser

trapping

-LRB-

Heupel

et

al.

Waschke

et

al.

-RRB-

These

data

suggest

that

autoantobodies

interfere

with

Dsg

binding

rather

by

indirect

cell-dependent

mechanisms

Finally

it

has

to

be

noted

that

if

direct

inhibition

occurs

it

is

not

possible

to

discriminate

at

present

whether

interference

with

Dsg

binding

in

PV

was

mediated

by

steric

hindrance

i.e.

by

blocking

trans-interaction

of

desmoglein

molecules

by

the

bound

autoantibody

or

rather

by

allosteric

effects

i.e.

autoantibody-induced

conformational

changes

of

the

Dsg

ectodomain

which

in

turn

interfere

with

Dsg

transinteraction

An

antibody

directed

against

the

putative

transadhesive

interface

may

directly

induce

steric

hindrance

whereas

antibodies

directed

against

other

parts

of

the

desmoglein

ectodomain

could

indirectly

inhibit

desmoglein

binding

by

allosteric

mechanisms

The

fact

that

AK

and

AK

which

were

directed

to

the

middle

and

the

carboxyterminal

parts

of

the

Dsg

ectodomain

were

not

pathogenic

and

did

not

directly

interfere

with

Dsg

binding

suggests

that

these

specific

antibodies

were

not

capable

of

causing

allosteric

hindrance

-LRB-

Heupel

et

al.

Tsunoda

et

al.

-RRB-

On

the

other

hand

an

antibody

directed

against

the

domain

although

this

part

of

the

molecule

may

not

be

involved

in

transinteraction

might

be

large

enough

to

cause

steric

hindrance

of

Dsg

transinteraction

Therefore

direct

inhibition

instead

of

steric

hindrance

of

desmoglein

binding

should

be

used

until

discrimination

between

steric

and

allosteric

effects

is

possible

Desmoglein

compensation

in

pemphigus

The

desmoglein

compensation

hypothesis

was

proposed

to

explain

the

different

clinical

phenotypes

of

PV

and

PF

on

the

basis

of

their

different

autoantibody

profiles

-LRB-

Amagai

Payne

et

al.

Sharma

et

al.

Shirakata

et

al.

Stanley

and

Amagai

Udey

and

Stanley

-RRB-

According

to

this

concept

in

the

deep

epidermis

which

contains

both

Dsg

and

Dsg

Dsg

compensates

for

the

functional

loss

of

Dsg

induced

by

Dsg

autoantibodies

resulting

in

more

superficial

blistering

in

PF

-LRB-

Fig.

-RRB-

In

PV

when

only

Dsg

antibodies

are

present

no

epidermal

blistering

would

occur

because

Dsg

is

considered

to

compensate

for

autoantibody-induced

loss

of

Dsg

binding

However

acantholysis

occurs

in

mucous

membranes

where

Dsg

is

assumed

to

be

the

predominantly

expressed

Dsg

isoform

leading

to

the

phenotype

of

mucosal-dominant

PV

When

autoantibodies

to

Dsg

are

also

produced

in

PV

epidermal

blistering

occurs

However

it

is

unclear

why

the

cleavage

plane

is

restricted

to

the

deep

epidermis

in

PV

since

in

PF

Dsg

autoantibodies

cause

superficial

blistering

-LRB-

Fig.

-RRB-

For

this

reason

and

other

reasons

such

as

the

cases

of

pemphigus

in

which

the

autoantibody

profiles

do

not

correlate

with

the

clinical

phenotype

or

the

presence

of

other

desmosomal

cadherin

isoforms

in

the

epidermis

this

concept

has

been

challenged

-LRB-

Amagai

et

al.

Bystryn

and

Rudolph

Muller

et

al.

Spindler

et

al.

-RRB-

Fig.

desmoglein

compensation

hypothesis

Based

on

the

different

autoantibody

profiles

in

PV

and

PF

together

with

the

findings

that

Dsg

is

present

in

the

deep

epidermis

only

whereas

Dsg

is

primarily

expressed

in

the

superficial

epidermis

the

desmoglein

compensation

hypothesis

has

been

proposed

to

explain

the

epidermal

cleavage

planes

typical

for

PV

and

PF

According

to

this

model

blistering

in

PF

affects

the

superficial

epidermis

because

Dsg

is

present

in

the

deep

epidermis

to

compensate

for

the

autoantibody-induced

loss

of

Dsg

binding

In

PV

epidermal

involvement

would

occur

only

when

autoantibodies

against

both

Dsg

and

Dsg

are

present

because

Dsg

is

found

in

all

epidermal

layers

and

could

compensate

for

loss

of

Dsg

binding

when

antibodies

to

Dsg

are

solely

presentFig

of

Dsg

and

Dsg

in

PV

lesional

epidermis

Epidermis

from

a

patient

with

mucocutaneous

PV

was

stained

for

Dsg

-LRB-

a

-RRB-

and

Dsg

-LRB-

b

-RRB-

A

merge

of

both

panels

is

shown

in

c.

Both

Dsg

and

Dsg

are

expressed

in

the

basal

layer

underneath

the

blister

as

well

as

in

keratinocytes

in

the

blister

roof

However

Dsg

staining

appears

to

be

fragmented

throughout

the

epidermis

whereas

Dsg

staining

is

more

continuous

Note

that

in

the

level

of

the

cleavage

plane

the

apical

membrane

of

basal

cells

shows

strong

immunostaining

for

Dsg

and

Dsg

-LRB-

arrows

-RRB-

Therefore

based

on

the

desmoglein

compensation

hypothesis

the

expression

patterns

of

Dsg

and

Dsg

can

not

explain

why

the

cleavage

plane

is

located

suprabasally

in

PV

but

not

in

other

epidermal

layers

Scale

bar

is

μm

Experimental

support

for

the

desmoglein

compensation

hypothesis

in

vivo

was

obtained

in

mice

It

was

shown

that

PF-IgG

were

sufficient

to

cause

skin

blistering

in

Dsg

mice

but

not

in

normal

mice

-LRB-

Mahoney

et

al.

-RRB-

In

skin

layers

where

Dsg

and

Dsg

were

found

autoantibodies

against

both

desmogleins

were

required

for

blistering

In

line

with

these

findings

forced

expression

of

Dsg

in

the

superficial

epidermis

abolished

the

ability

of

PF-IgG

to

induce

acantholysis

in

mice

-LRB-

Wu

et

al.

-RRB-

In

contrast

in

human

skin

and

in

cultured

human

keratinocytes

in

vitro

PF-IgG

were

effective

to

induce

acantholysis

despite

of

the

presence

of

both

Dsg

and

Dsg

-LRB-

Spindler

et

al.

-RRB-

The

discrepancy

between

these

conflicting

findings

may

be

explained

in

part

by

the

notion

that

the

desmoglein

compensation

hypothesis

is

based

on

the

following

two

assumptions

-LRB-

-RRB-

the

expression

pattern

of

Dsg

and

Dsg

do

not

substantially

overlap

in

epidermal

and

mucosal

layers

where

the

cleavage

plane

in

PV

and

PF

is

located

-LRB-

-RRB-

Dsg

and

Dsg

autoantibodies

only

lead

to

inactivation

of

either

Dsg

or

Dsg

respectively

Because

of

the

latter

the

desmoglein

compensation

hypothesis

has

been

used

to

promote

the

idea

that

autoantibodies

reduce

Dsg

binding

by

direct

inhibition

rather

than

by

unspecific

proteolysis

-LRB-

Mahoney

et

al.

-RRB-

Regarding

the

distribution

of

Dsg

and

Dsg

it

is

important

to

note

that

Dsg

expression

patterns

in

specific

epidermal

layers

are

different

in

mice

and

men

In

mice

expression

of

Dsg

is

restricted

to

the

basal

and

immediately

suprabasal

epidermal

layer

-LRB-

Mahoney

et

al.

-RRB-

In

human

skin

when

PV

and

PF

were

used

for

staining

a

similar

staining

pattern

was

revealed

-LRB-

Amagai

et

al.

Shimizu

et

al.

-RRB-

In

contrast

when

specific

antibodies

or

in

situ

hybridisation

were

used

for

Dsg

mapping

in

human

epidermis

it

was

demonstrated

that

Dsg

is

present

throughout

the

spinous

layers

and

thus

Dsg

distribution

showed

substantial

overlap

with

expression

of

Dsg

-LRB-

Arnemann

et

al.

Mahoney

et

al.

Spindler

et

al.

-RRB-

However

immunostaining

of

human

epidermis

using

another

monoclonal

antibody

detected

expression

of

Dsg

in

the

lower

epidermis

only

-LRB-

Wu

et

al.

-RRB-

In

oral

mucosa

equally

strong

expression

of

Dsg

and

Dsg

was

found

throughout

the

epithelium

when

specific

antibodies

were

used

-LRB-

Mahoney

et

al.

-RRB-

whereas

Dsg

staining

intensity

was

found

to

be

much

lower

when

PV-IgG

were

used

for

immunstaining

-LRB-

Shirakata

et

al.

-RRB-

Taken

together

the

expression

patterns

of

Dsg

and

Dsg

broadly

overlap

in

human

epidermis

and

appear

to

be

identical

in

oral

mucosa

With

respect

to

the

second

assumption

the

desmoglein

compensation

is

based

on

i.e.

selective

inactivation

of

Dsg

but

not

of

Dsg

by

Dsg

antibodies

it

was

shown

recently

that

both

PF-IgG

-LRB-

only

containing

Dsg

antibodies

-RRB-

and

PV-IgG

from

patients

with

only

Dsg

antibodies

were

equally

effective

to

reduce

binding

of

Dsg

and

Dsg

beads

to

the

surface

of

cultured

keratinocytes

-LRB-

Heupel

et

al.

Spindler

et

al.

-RRB-

These

data

indicate

that

PV-IgG

and

PF-IgG

can

reduce

binding

of

Dsg

and

Dsg

at

least

on

the

keratinocyte

cell

surface

Taken

together

the

relevance

of

desmoglein

compensation

for

pemphigus

pathogenesis

in

humans

can

not

be

concluded

from

experiments

in

mice

especially

because

distribution

patterns

of

Dsg

and

Dsg

substantially

differ

in

the

two

species

Therefore

alternative

models

have

to

be

worked

out

to

explain

the

different

epidermal

cleavage

planes

in

PV

and

PF

These

may

involve

different

signalling

pathways

required

for

maintenance

of

desmosomal

adhesion

in

the

specific

epidermal

layers

as

outlined

below

Signalling

pathways

in

pemphigus

and

in

desmosome

disassembly

Since

it

has

been

shown

that

PV-IgG

upon

binding

to

keratinocytes

induced

a

rapid

transient

increase

of

intracellular

-LRB-

Seishima

et

al.

-RRB-

several

signalling

pathways

have

been

shown

to

be

involved

in

pemphigus

pathogenesis

-LRB-

Fig.

-RRB-

-LRB-

Kitajima

Lanza

et

al.

Sharma

et

al.

Sitaru

and

Zillikens

-RRB-

Interestingly

evidence

has

been

provided

that

transadhering

non-desmosomal

cadherins

for

instance

Dsg

are

involved

in

outside-in

signalling

and

that

binding

of

pemphigus

IgG

interferes

with

this

function

-LRB-

Muller

et

al.

-RRB-

This

can

be

concluded

from

experiments

which

showed

that

autoantibody

binding

as

well

as

keratinocyte

separation

started

between

desmosomes

-LRB-

Sato

et

al.

Takahashi

et

al.

-RRB-

and

that

non-junctional

Dsg

and

plakoglobin

were

depleted

first

before

changes

in

the

desmosomal

fractions

were

present

-LRB-

Aoyama

and

Kitajima

Williamson

et

al.

Yamamoto

et

al.

-RRB-

Interestingly

to

trigger

Dsg-induced

signalling

autoantibody-mediated

cross-linking

of

Dsg

and

Dsg

seems

not

to

be

required

because

monovalent

Fab

fragments

and

single-chain

variants

of

PV

and

PF-IgG

were

effective

to

cause

skin

blistering

in

vivo

and

to

disrupt

the

desmosomal

plaque

in

vitro

-LRB-

de

Bruin

et

al.

Ishii

et

al.

Payne

et

al.

Rock

et

al.

-RRB-

PLC

and

PKC

It

was

shown

that

PV-IgG

caused

a

rapid

transient

phospholipase

C

-LRB-

PLC

-RRB-

dependent

increase

of

inositol

trisphosphate

and

of

intracellular

leading

to

activation

of

both

PKC

and

plasminogen

activator

-LRB-

PA

-RRB-

-LRB-

Esaki

et

al.

Kitajima

et

al.

Memar

et

al.

Osada

et

al.

Seishima

et

al.

-RRB-

Because

a

chelator

of

intracellular

free

blocked

keratinocyte

dissociation

in

vitro

and

inhibitors

of

calmodulin

PLC

and

PKC

were

effective

to

block

PV-IgG-induced

acantholysis

in

vivo

-LRB-

Arredondo

et

al.

Sanchez-Carpintero

et

al.

-RRB-

it

is

possible

that

this

signalling

pathway

may

be

involved

in

PV

acantholysis

However

because

the

PA

system

is

not

believed

to

be

crucial

in

this

process

PKC

signalling

may

contribute

to

PV

acantholysis

by

other

pathways

such

as

phosphorylation

of

β-catenin

in

adherens

junctions

-LRB-

Chernyavsky

et

al.

-RRB-

This

hypothesis

is

supported

by

experiments

which

showed

that

keratinocyte

adhesion

was

negatively

regulated

by

pharmacologic

PKC

activation

-LRB-

Kimura

et

al.

-RRB-

Activation

of

at

present

is

the

most

promising

signalling

mechanism

to

be

responsible

for

acantholysis

in

pemphigus

It

has

been

demonstrated

in

vivo

that

and

one

of

its

downstream

targets

heat

shock

protein

-LRB-

HSP

-RRB-

were

phosphorylated

in

response

to

PV-IgG

and

PF-IgG

and

that

pharmacologic

inhibition

of

abolished

blister

formation

-LRB-

Berkowitz

et

al.

-RRB-

Similarly

and

the

human

homolog

HSP

were

found

to

be

phosphorylated

in

skin

lesions

of

PV

and

PF

patients

-LRB-

Berkowitz

et

al.

-RRB-

In

cultured

human

keratinocytes

phosphorylation

of

and

occured

after

min

of

exposure

to

PV-IgG

-LRB-

Berkowitz

et

al.

Kawasaki

et

al.

-RRB-

However

another

study

found

that

activity

of

was

not

increased

before

min

and

that

activity

peaked

after

min

of

PV-IgG

treatment

-LRB-

Chernyavsky

et

al.

-RRB-

In

the

latter

study

it

was

shown

that

activation

was

mediated

at

least

in

part

by

Dsg

and/or

Dsg

antibodies

because

Dsg

depletion

by

siRNA

reduced

activation

by

Inhibition

of

blocked

autoantibody-induced

keratinocyte

dissociation

Rho

A

inactivation

cytokeratin

retraction

and

reorganization

of

the

actin

cytoskeleton

-LRB-

Berkowitz

et

al.

Chernyavsky

et

al.

Waschke

et

al.

-RRB-

These

results

demonstrate

that

is

involved

in

the

mechanisms

leading

to

acantholysis

and

that

inhibition

of

could

be

a

beneficial

approach

to

treat

pemphigus

patients

Nevertheless

based

on

the

finding

that

activation

of

can

also

be

a

consequence

of

cell

detachment

in

rat

intestinal

epithelium

-LRB-

Rosen

et

al.

-RRB-

it

was

proposed

that

activation

is

a

consequence

of

cell

dissociation

-LRB-

Sharma

et

al.

-RRB-

However

this

is

unlikely

because

significant

pemphigus-IgG-induced

acantholysis

usually

takes

h

to

occur

-LRB-

Berkowitz

et

al.

Caldelari

et

al.

Lanza

et

al.

Mahoney

et

al.

Nagasaka

et

al.

Nguyen

et

al.

Shu

et

al.

Spindler

et

al.

Takahashi

et

al.

Tsunoda

et

al.

Waschke

et

al.

Waschke

et

al.

Williamson

et

al.

-RRB-

The

mechanisms

by

which

and

lead

to

keratinocyte

dissociation

are

largely

unclear

but

may

involve

reorganization

of

the

keratinocyte

cytoskeleton

It

has

been

shown

recently

that

serine

phosphorylation

of

cytokeratin

by

induced

cytokeratin

network

disassembly

-LRB-

Woll

et

al.

-RRB-

Similarly

was

found

to

associate

with

cytokeratin

filaments

and

to

inhibit

assembly

of

glial

fibrillary

acidic

protein

-LRB-

GFAP

-RRB-

-LRB-

Perng

et

al.

-RRB-

Similarly

was

found

to

interact

with

actin

filaments

and

to

prevent

formation

of

aggregates

of

thermally

denatured

actin

-LRB-

Panasenko

et

al.

-RRB-

However

because

phosphorylation

of

or

HSP

promoted

actin

assembly

and

stabilized

F-actin

against

depolymerization

in

response

to

cytochalasin

D

or

heat

shock

-LRB-

Benndorf

et

al.

Geum

et

al.

Guay

et

al.

-RRB-

one

would

expect

that

PV-IgG-induced

phosphorylation

also

would

stabilize

actin

filaments

Therefore

it

is

more

likely

that

is

part

of

a

salvage

pathway

in

response

to

PV-IgG

and

does

not

directly

contribute

to

PV-IgG-induced

actin

reorganization

Rho

A

GTPase

Rho

GTPases

are

important

regulators

of

the

cytoskeleton

and

of

cell

adhesion

-LRB-

Braga

and

Yap

Bustelo

et

al.

Fukata

et

al.

Jaffe

and

Hall

-RRB-

In

the

epidermis

Rac

was

found

not

to

be

crucial

for

maintaining

epithelial

integrity

but

for

stem

cell

differentiation

presumably

via

negative

regulation

of

c-Myc

expression

-LRB-

Benitah

et

al.

Chrostek

et

al.

-RRB-

Recently

it

has

been

shown

that

PV

and

PF-IgG-induced

epidermal

splitting

keratinocyte

dissociation

as

well

as

loss

of

Dsg

and

Dsg

binding

in

vitro

were

accompanied

by

inactivation

of

Rho

A

and

that

specific

activation

of

Rho

A

by

the

bacterial

toxin

cytotoxic

necrotizing

factor

y

-LRB-

CNFy

-RRB-

abolished

these

effects

-LRB-

Spindler

et

al.

Waschke

et

al.

-RRB-

Moreover

toxin-mediated

inactivation

of

Rho

A

and

Rac

resulted

in

epidermal

splitting

keratinocyte

dissociation

and

actin

reorganization

similar

to

treatment

with

pemphigus

IgG

Especially

inactivation

of

Rho

GTPases

by

toxin

B

caused

deep

epidermal

acantholysis

comparable

to

the

effects

of

PV-IgG

Based

on

this

observation

it

is

tempting

to

speculate

that

the

cleavage

plane

in

pemphigus

may

in

part

be

explained

by

different

layer-specific

signalling

pathways

important

for

keratinocyte

cohesion

Because

cytokeratin

retraction

and

actin

reorganization

in

response

to

pemphigus

IgG

were

also

abrogated

by

activation

of

Rho

A

these

results

suggested

that

Rho

A

could

be

involved

in

the

regulation

of

desmoglein

cytoskeletal

anchorage

-LRB-

Waschke

et

al.

-RRB-

This

idea

was

supported

by

Triton

extraction

experiments

which

showed

that

Rho

A

activation

reduced

Dsg

in

the

non-cytoskeleton

bound

fraction

Alternatively

Rho

GTPases

may

regulate

endocytosis

of

desmosomal

cadherins

similar

to

their

role

in

E-cadherin

turnover

-LRB-

Akhtar

and

Hotchin

Izumi

et

al.

Kamei

et

al.

-RRB-

Interestingly

endocytosis

and

depletion

of

are

increasingly

recognized

to

contribute

to

pemphigus

acantholysis

-LRB-

Calkins

et

al.

Yamamoto

et

al.

-RRB-

Taken

together

Rho

GTPases

seem

to

be

important

for

maintenance

of

desmosomes

and

activation

of

Rho

A

could

be

used

to

develop

new

strategies

in

pemphigus

treatment

These

data

are

in

contrast

to

previous

findings

which

suggested

that

Rho

GTPases

are

not

involved

in

the

regulation

of

desmosomal

adhesion

This

was

concluded

from

experiments

in

keratinocytes

which

showed

that

Rac

inactivation

by

transfection

with

a

dominant-inactive

mutant

as

well

as

inactivation

of

Rho

A

by

toxin

for

min

was

sufficient

to

displace

E-cadherin

but

not

desmoplakin

from

cell

junctions

-LRB-

Braga

et

al.

-RRB-

These

studies

led

to

the

hypothesis

that

Rho

GTPases

are

important

regulators

of

keratinocyte

adherens

junctions

but

not

desmosomes

-LRB-

Braga

and

Yap

-RRB-

However

because

inactivation

of

Rho

A

by

a

cell-permeable

fusion

toxin

was

sufficient

to

displace

Dsg

from

cell

borders

after

min

the

negative

results

from

the

early

study

might

be

caused

by

the

shorter

incubation

period

-LRB-

Waschke

et

al.

-RRB-

Plakoglobin

Besides

its

involvement

in

PV

pathogenesis

plakoglobin

is

important

for

several

aspects

of

desmosomal

adhesion

Plakoglobin

deficiency

resulted

in

subcorneal

acantholysis

loss

of

desmosomes

and

impaired

cytoskeletal

anchorage

of

desmoplakin

and

desmogleins

in

vivo

and

in

vitro

supporting

the

notion

that

plakoglobin

is

an

important

cytoskeletal

linker

and

is

crucial

for

desmosome

assembly

-LRB-

Bierkamp

et

al.

Yin

et

al.

-RRB-

However

at

least

in

part

plakoglobin

functions

seem

to

be

compensated

by

other

desmosomal

plaque

proteins

such

as

plakophilin

because

a

plakoglobin-deficient

keratinocyte

cell

line

displayed

Dsg

cytoskeletal

anchorage

and

keratinocyte

aggregation

similar

to

wild-type

cells

-LRB-

Caldelari

et

al.

-RRB-

The

linker

function

of

plakoglobin

seems

to

be

regulated

by

tyrosine

phosphorylation

of

plakoglobin

because

following

EGFR

activation

phosphorylated

plakoglobin

remained

associated

with

Dsg

but

not

with

desmoplakin

-LRB-

Gaudry

et

al.

-RRB-

Moreover

plakoglobin

phosphorylation

was

shown

to

be

required

for

EGFR-induced

loss

of

cell

adhesion

and

to

regulate

binding

of

plakoglobin

to

desmoplakin

-LRB-

Miravet

et

al.

Yin

et

al.

-RRB-

These

data

indicate

that

plakoglobin

is

required

to

integrate

the

effects

from

extracellular

cues

and

from

different

signalling

pathways

in

order

to

allow

coordinated

modulation

of

desmosomal

adhesion

In

addition

plakoglobin

regulates

the

turn

over

of

desmosomal

components

because

the

protein

levels

of

desmogleins

as

well

as

of

plakoglobin

and

desmoplakin

were

decreased

in

plakoglobin-deficient

cells

-LRB-

Yin

et

al.

-RRB-

With

respect

to

pemphigus

it

has

been

shown

that

plakoglobin

is

critical

for

PV

pathogenesis

because

keratinocytes

from

plakoglobin-deficient

mice

were

resistant

to

PV-IgG-induced

keratinocyte

dissociation

cytokeratin

retraction

and

disruption

of

the

desmosomal

plaque

-LRB-

Caldelari

et

al.

de

Bruin

et

al.

-RRB-

These

studies

provided

first

evidence

that

direct

inhibition

of

Dsg

binding

alone

can

not

account

for

acantholysis

in

PV

and

raised

the

hypothesis

that

plakoglobin

could

be

part

of

a

receptor

complex

required

to

transfer

the

signal

from

autoantibody-bound

Dsg

into

the

cell

a

phenomenon

referred

to

as

outside-in

signalling

-LRB-

Muller

et

al.

-RRB-

As

outlined

above

it

has

been

shown

that

plakoglobin

is

involved

in

c-Myc

repression

which

is

required

for

keratinocyte

differentiation

and

that

PV-IgG

by

depleting

noncytokeratin-anchored

plakoglobin

on

the

cell

surface

also

led

to

c-Myc

overexpression

-LRB-

Williamson

et

al.

-RRB-

However

as

discussed

below

the

role

of

c-Myc

signalling

in

acantholysis

is

unclear

at

present

EGFR

The

involvement

of

the

EGFR

in

PV

pathogenesis

has

been

reported

by

showing

that

EGFR

activity

increased

after

min

and

peaked

after

min

of

PV-IgG

incubation

This

was

followed

by

activation

of

c-Jun

phosphorylation

and

finally

by

keratinocyte

apoptosis

-LRB-

Chernyavsky

et

al.

Frusic-Zlotkin

et

al.

-RRB-

Inhibition

of

EGFR

reduced

EGFR

signalling

as

well

as

apoptosis

and

acantholysis

-LRB-

Frusic-Zlotkin

et

al.

-RRB-

This

finding

was

surprising

because

EGFR

signalling

usually

is

considered

to

promote

cell

survival

and

proliferation

-LRB-

Bazley

and

Gullick

Muller

et

al.

-RRB-

However

because

h

were

required

to

induce

acantholysis

very

high

concentrations

of

IgG

-LRB-

mg/ml

-RRB-

were

used

and

activation

of

was

not

detected

in

this

study

the

significance

of

these

results

is

unclear

EGFR

could

contribute

to

PV

pathogenesis

by

stimulating

desmosome

disassembly

It

has

been

shown

that

EGF-induced

loss

of

keratinocyte

aggregation

was

mediated

by

plakoglobin

phosphorylation

which

led

to

uncoupling

of

the

desmoglein

complex

from

desmoplakin

and

thus

from

the

cytoskeleton

-LRB-

Gaudry

et

al.

Yin

et

al.

-RRB-

Moreover

EGFR

seems

to

negatively

regulate

the

protein

levels

of

desmosomal

cadherins

in

part

by

promoting

their

metalloproteinase-mediated

degradation

as

well

as

the

incorporation

of

desmosomal

cadherins

into

the

desmosomal

plaque

-LRB-

Bech-Serra

et

al.

Lorch

et

al.

Santiago-Josefat

et

al.

-RRB-

Src

The

tyrosine

kinase

Src

is

activated

by

PV-IgG

within

min

and

seems

to

contribute

to

EGFR

and

activation

because

inhibition

of

Src

reduced

EGFR

and

phosphorylation

by

and

respectively

-LRB-

Chernyavsky

et

al.

-RRB-

Because

Src

inhibition

reduced

PV-IgG-induced

loss

of

keratinocyte

cohesion

cytokeratin

retraction

and

apoptosis

in

vitro

and

inhition

of

tyrosin

kinases

blocked

PV-IgG-induced

acantholysis

in

vivo

-LRB-

Chernyavsky

et

al.

Sanchez-Carpintero

et

al.

-RRB-

these

results

indicate

that

Src

is

significantly

involved

in

PV

acantholysis

Besides

activation

of

and

EGFR

a

recent

study

provided

another

possible

underlying

mechanism

Src

was

shown

to

directly

phosphorylate

in

response

to

PV-IgG

which

correlated

with

the

degree

of

acantholysis

-LRB-

Chernyavsky

et

al.

-RRB-

Because

is

involved

in

stabilization

of

classical

cadherins

such

as

E-cadherin

on

the

cell

surface

as

well

as

in

cadherin-dependent

regulation

of

Rho

A

and

Rac

activity

-LRB-

Alema

and

Salvatore

Kowalczyk

and

Reynolds

-RRB-

Src-mediated

phosphorylation

of

may

be

important

for

PV

pathogenesis

Cholinergic

receptors

The

cholinergic

system

of

the

human

epidermis

involves

two

classes

of

cholinergig

receptors

the

nicotinic

and

the

muscarinic

acetylcholine

receptors

-LRB-

nAChR

and

mAChR

-RRB-

which

in

keratinocytes

are

involved

in

the

regulation

of

cell

cell

and

cell

matrix

adhesion

as

well

as

in

cell

migration

-LRB-

Grando

-RRB-

As

outlined

above

it

is

unclear

whether

autoantibodies

against

cholinergic

receptors

in

pemphigus

directly

contribute

to

acantholysis

However

it

was

shown

that

cholinergic

agonists

can

ameliorate

PV

acantholysis

in

vivo

and

in

vitro

-LRB-

Nguyen

et

al.

-RRB-

and

that

local

application

of

cholinergic

agonists

such

as

pilocarpine

had

therapeutic

effect

on

oral

and

skin

lesions

in

PV

patients

-LRB-

Iraji

and

Yoosefi

Namazi

-RRB-

Cholinergic

agonists

increased

protein

levels

of

Dsg

Dsg

and

E-cadherin

and

antagonists

to

cholinergic

receptors

resulted

in

keratinocyte

dissociation

which

was

paralleled

by

phosphorylation

of

these

molecules

-LRB-

Grando

-RRB-

Recently

a

mechanism

by

which

signalling

from

cholinergic

receptors

could

directly

interfere

with

PV-IgG-induced

effects

was

demonstrated

The

mAChR

agonist

pilocarpine

inhibited

PV-IgG-induced

acantholysis

by

reducing

serine

phosphorylation

of

β-catenin

and

Src-mediated

tyrosine

phosphorylation

of

by

activation

of

the

specific

protein

phosphatases

-LRB-

Chernyavsky

et

al.

-RRB-

Similarly

an

agonist

of

the

α

nAChR

reduced

phosphorylation

both

on

the

level

of

Src

activation

as

well

as

by

activation

of

the

tyrosine

phosphatase

Moreover

it

was

shown

that

and

receptors

can

lead

to

activation

of

Rho

A

and

that

carbachol

which

activates

α

and

α

nAChR

activates

Rho

A

Rac

and

-LRB-

Chernyavsky

et

al.

b

Ruiz-Velasco

et

al.

-RRB-

which

also

might

be

involved

in

the

inhibition

of

PV-IgG-induced

acantholysis

Taken

together

these

data

indicate

that

activation

of

cholinergic

receptors

could

provide

a

promising

strategy

to

treat

PV

patients

Regulation

of

cell

cycle

and

gene

expression

Recent

data

support

the

hypothesis

that

PV-IgG

change

expression

patterns

of

molecules

involved

in

cell

cycle

regulation

and

that

these

molecules

might

be

involved

in

acantholysis

As

outlined

above

PV-IgG-induced

plakoglobin

depletion

resulted

in

c-Myc

overexpression

in

cultured

keratinocytes

-LRB-

Muller

et

al.

Williamson

et

al.

-RRB-

Accordingly

c-Myc

overexpression

was

found

in

the

epidermis

of

PV

patients

but

interestingly

not

in

PF

skin

indicating

that

this

mechanism

may

only

be

important

for

PV

-LRB-

Williamson

et

al.

-RRB-

Because

pharmacologic

inhibition

of

c-Myc

as

well

as

of

plakoglobin

degradation

by

blocking

GSK

abrogated

pemphigus-IgG-induced

skin

blistering

these

results

indicate

that

this

mechanism

could

also

be

important

to

induce

acantholysis

However

the

findings

that

c-Myc

overexpression

was

found

after

h

whereas

PV-IgG-induced

loss

of

cell

aggregation

was

observed

as

early

as

after

h

and

that

c-Myc

overexpression

was

absent

in

PF

skin

indicate

that

c-Myc

overexpression

likely

contributes

to

ongoing

proliferation

of

keratinocytes

in

PV

whereas

it

seems

not

to

be

essential

for

acantholysis

Therefore

other

mechanisms

must

exist

in

addition

to

induce

keratinocyte

dissociation

These

mechanisms

may

amongst

others

involve

expression

of

genes

different

from

c-Myc

which

is

suggested

by

the

fact

that

plakoglobin

deficiency

resulted

in

protection

of

keratinocytes

from

apoptosis

possibly

via

overexpression

of

the

anti-apoptotic

molecule

Bcl-XL

-LRB-

Dusek

et

al.

-RRB-

Similar

to

c-Myc

cyclin-dependent

kinase

-LRB-

-RRB-

another

kinase

involved

in

the

regulation

of

cell

cycle

progression

seems

to

be

involved

in

keratinocyte

proliferation

and

acantholysis

in

pemphigus

-LRB-

Lanza

et

al.

-RRB-

PV

serum

increased

protein

levels

of

and

PV

serum-induced

acantholysis

in

vivo

was

abolished

by

pharmacologic

inhibition

of

Moreover

siRNA-mediated

down-regulation

of

blocked

cell

dissociation

in

cultured

keratinocytes

indicating

that

may

be

important

for

PV

acantholysis

in

vivo

and

in

vitro

However

at

present

it

is

unclear

how

continuing

keratinocyte

proliferation

in

consequence

to

increased

c-Myc

and

signalling

contributes

to

acantholysis

in

PV

One

explanation

is

that

reduced

expression

of

desmosomal

components

in

hyperproliferating

cells

may

foster

the

loss

of

keratinocyte

cohesion

in

pemphigus

PV-IgG

do

not

only

induce

direct

signalling

effects

on

cell

junctions

and

the

cytoskeleton

but

also

change

gene

expression

patterns

which

may

also

contribute

to

pemphigus

pathogenesis

It

has

been

shown

by

DNA

microarray

that

PV-IgG

within

h

down-regulated

transcription

of

genes

including

genes

encoding

for

adhesion

molecules

including

Dsg

and

desmoplakin

cytoskeletal

proteins

such

as

different

cytokeratins

as

well

as

molecules

involved

in

cell

cycle

regulation

such

as

and

cyclin

in

cultured

keratinocytes

and

that

expression

of

some

of

these

genes

was

antagonistically

regulated

by

methylprednisolone

-LRB-

Nguyen

et

al.

-RRB-

When

PV

serum

was

used

for

h

the

expression

of

even

more

genes

was

altered

In

cultured

human

keratinocytes

transcription

of

genes

was

decreased

including

Dsc

and

plakophilin

whereas

transcription

of

genes

was

increased

In

vivo

expression

of

genes

was

reduced

whereas

transcription

of

other

genes

was

inceased

-LRB-

Lanza

et

al.

-RRB-

Pharmacologic

inhibition

of

blunted

the

effect

of

PV-IgG

on

the

expression

of

most

of

the

genes

including

desmoplakin

and

also

blocked

PV-IgG-induced

acantholysis

Thus

it

is

conceivable

that

altered

desmosome

formation

may

be

involved

in

the

mechanisms

underlying

keratinocyte

dissociation

in

pemphigus

Apoptosis

Apoptosis

has

been

detected

in

skin

lesions

and

in

perilesional

skin

of

PV

and

PF

patients

-LRB-

Gniadecki

et

al.

Puviani

et

al.

Wang

et

al.

-RRB-

and

hallmarks

of

apoptosis

such

as

DNA

fragmentation

increased

expression

of

pro-apoptotic

molecules

Fas

FasL

Bax

depetion

of

anti-apoptotic

and

FLIPL

as

well

as

activation

of

caspases

and

have

been

observed

following

treatment

of

cultured

keratinocytes

with

PV-IgG

or

PV

serum

-LRB-

Arredondo

et

al.

Baroni

et

al.

Chernyavsky

et

al.

Frusic-Zlotkin

et

al.

Pelacho

et

al.

Puviani

et

al.

Wang

et

al.

b

-RRB-

Hence

compelling

evidence

has

been

provided

for

the

presence

of

programmed

cell

death

in

PV

although

the

phenotype

of

acantholytic

cells

is

different

from

cells

undergoing

apoptosis

-LRB-

Arredondo

et

al.

-RRB-

However

for

several

experiments

prolonged

incubation

times

of

h

-LRB-

Arredondo

et

al.

Baroni

et

al.

Frusic-Zlotkin

et

al.

Wang

et

al.

b

-RRB-

were

required

whereas

in

most

studies

from

the

literature

acantholysis

was

clearly

present

after

h

-LRB-

Berkowitz

et

al.

Caldelari

et

al.

Lanza

et

al.

Mahoney

et

al.

Nagasaka

et

al.

Nguyen

et

al.

Shu

et

al.

Spindler

et

al.

Takahashi

et

al.

Tsunoda

et

al.

Waschke

et

al.

Waschke

et

al.

Williamson

et

al.

-RRB-

In

another

study

apoptosis

was

detected

by

TUNEL

reactivity

starting

after

h

of

PV-IgG

treatment

-LRB-

Chernyavsky

et

al.

-RRB-

However

in

this

study

loss

of

intercellular

adhesion

was

present

after

min

and

thus

before

onset

of

apoptosis

Therefore

it

is

conceivable

that

apoptosis

may

be

an

event

parallel

to

or

in

consequence

of

acantholysis

On

the

other

hand

it

was

reported

that

activated

caspase

cleaves

Dsg

-LRB-

Weiske

et

al.

-RRB-

and

that

caspase

and

calpain

inhibitors

can

block

PV-IgG-induced

acantholysis

in

keratinocyte

monolayers

and

in

skin

organ

culture

-LRB-

Arredondo

et

al.

Wang

et

al.

b

Weiske

et

al.

-RRB-

Taken

together

it

is

unclear

at

present

to

which

extent

PV-IgG-induced

acantholysis

is

caused

by

apoptosis

Targets

of

signalling

pathways

in

pemphigus

Pemphigus

is

a

desmosomal

disease

because

pathogenic

autoantibodies

are

directed

against

Dsg

and

Dsg

Moreover

these

antibodies

result

in

the

depletion

of

desmosomal

components

from

the

cell

surface

alterations

of

desmosomal

plaques

and

a

loss

of

desmosomes

-LRB-

Aoyama

and

Kitajima

Aoyama

et

al.

Calkins

et

al.

de

Bruin

et

al.

Sato

et

al.

Shu

et

al.

Waschke

et

al.

Williamson

et

al.

Yamamoto

et

al.

-RRB-

However

it

is

unclear

at

present

whether

desmosomes

are

the

primary

targets

of

autoantibodies

or

whether

components

outside

of

desmosomes

cause

disassembly

of

desmosomes

via

mechanisms

involving

adherens

junctions

or

the

cytoskeleton

which

finally

results

in

acantholysis

The

latter

hypothesis

is

supported

by

the

observation

that

intercellular

spaces

between

desmosomes

widen

before

desmosomes

separate

-LRB-

Takahashi

et

al.

-RRB-

Effects

on

desmosomes

and

adherens

junctions

It

is

well

established

that

pemphigus

autoantibodies

lead

to

depletion

of

desmosomal

components

from

desmosomes

as

well

as

from

the

cell

surface

Within

min

PV-IgG

induced

internalization

and

lysosomal

degradation

of

cytoskeleton-unbound

Dsg

together

with

plakoglobin

-LRB-

Aoyama

and

Kitajima

Aoyama

et

al.

Calkins

et

al.

Sato

et

al.

Williamson

et

al.

Yamamoto

et

al.

-RRB-

After

h

Dsg

but

not

Dsg

plakoglobin

or

desmoplakin

was

also

depleted

from

the

cytoskeletal

fractions

leading

to

reduced

total

cellular

Dsg

levels

-LRB-

Calkins

et

al.

Yamamoto

et

al.

-RRB-

This

effect

was

mediated

by

Dsg

antibodies

because

monoclonal

Dsg

antibodies

such

as

were

effective

to

induce

Dsg

depletion

and

this

effect

correlated

with

the

pathogenic

activity

of

these

autoantibodies

in

vivo

and

in

vitro

-LRB-

Shu

et

al.

Yamamoto

et

al.

-RRB-

However

no

strong

correlation

between

Dsg

depletion

and

keratinocyte

dissociation

was

observed

Dsg

depletion

was

observed

in

cells

at

confluence

as

well

as

in

normal

human

keratinocytes

In

contrast

in

confluent

HaCaT

monolayers

a

reduction

of

Dsg

half-life

but

no

depletion

of

total

cellular

Dsg

was

detected

-LRB-

Cirillo

et

al.

Waschke

et

al.

-RRB-

Because

depletion

of

Dsg

was

first

found

in

parallel

with

serine

phosphorylation

of

Dsg

it

was

suggested

that

phosphorylation

might

be

required

for

Dsg

degradation

-LRB-

Aoyama

and

Kitajima

Aoyama

et

al.

Bystryn

and

Rodriguez

Rodriguez

and

Bystryn

-RRB-

However

phosphorylation

of

Dsg

was

observed

in

some

studies

but

not

in

others

which

might

be

due

to

different

cell

culture

models

indicating

that

Dsg

phosphorylation

is

not

necessary

for

acantholysis

-LRB-

Chernyavsky

et

al.

Nguyen

et

al.

-RRB-

Some

evidence

exists

that

depletion

of

Dsg

may

also

occur

in

pemphigus

However

studies

to

systematically

investigate

Dsg

turnover

in

pemphigus

are

lacking

Dsg

internalization

and

decreased

immunostaining

of

Dsg

have

been

observed

following

treatment

with

PF-IgG

serum

and

PV

serum

-LRB-

Cirillo

et

al.

Waschke

et

al.

-RRB-

This

is

in

line

with

previous

findings

from

the

literature

which

demonstrated

that

PF-IgG

did

not

bind

to

keratinocytes

in

the

lower

epidermis

of

PF

patients

but

strongly

labelled

basal

keratinocytes

in

the

epidermis

of

healthy

subjects

suggesting

that

in

PF

the

lower

epidermis

was

depleted

from

Dsg

-LRB-

Bystryn

and

Rodriguez

Rodriguez

and

Bystryn

-RRB-

Taken

together

it

is

unclear

at

present

to

which

extent

Dsg

depletion

from

desmosomes

causes

antibody-mediated

acantholysis

However

even

if

depletion

of

desmosomal

components

occured

secondary

to

keratinocyte

dissociation

it

likely

would

further

aggravate

the

loss

of

keratinocyte

cohesion

The

involvement

of

adherens

junctions

in

pemphigus

pathogenesis

is

unclear

at

present

It

has

been

shown

that

formation

of

adherens

junctions

is

a

prerequisite

for

desmosome

assembly

and

α-catenin-deficient

keratinocytes

had

reduced

numbers

of

desmosmes

-LRB-

Vasioukhin

et

al.

-RRB-

Because

Rho

GTPases

are

known

to

primarily

regulate

adhesion

mediated

by

classical

cadherins

in

adherens

junctions

whereas

the

requirement

of

Rho

A

and

Rac

activity

for

maintenance

of

desmosomal

adhesion

was

reported

just

recently

-LRB-

Braga

and

Yap

Fukata

et

al.

Spindler

et

al.

Waschke

et

al.

-RRB-

it

was

suggested

that

pemphigus

IgG-induced

inactivation

of

Rho

A

causes

disassembly

of

desmosomes

via

destabilization

of

adherens

junctions

-LRB-

Sharma

et

al.

-RRB-

Although

alterations

of

adherens

junctions

in

response

to

PV-IgG

were

negligible

compared

to

the

effects

on

desmosomes

-LRB-

Calkins

et

al.

de

Bruin

et

al.

Muller

et

al.

Waschke

et

al.

-RRB-

this

scenario

can

not

be

completely

ruled

out

at

present

In

fact

it

was

reported

recently

that

PV-IgG

lead

to

phosphorylation

of

β-catenin

and

by

PKC

and

Src

respectively

-LRB-

Chernyavsky

et

al.

-RRB-

However

more

studies

are

required

to

address

the

question

how

catenin

phosphorylation

in

adherens

junctions

is

associated

with

keratinocyte

dissociation

in

pemphigus

Effects

on

the

cytoskeleton

Alterations

of

the

cytoskeleton

have

been

described

both

in

pemphigus

skin

lesions

as

well

as

in

keratinocytes

exposed

to

pemphigus

autoantibodies

These

effects

include

dysorganization

of

cytokeratin

filaments

the

actin

cytoskeleton

as

well

as

of

microtubules

First

it

has

been

described

that

cytokeratin

filaments

become

detached

from

cell

junctions

and

accumulate

in

the

perinuclear

region

a

phenomenon

commonly

referred

to

as

keratin

retraction

-LRB-

Berkowitz

et

al.

Caldelari

et

al.

Calkins

et

al.

Chernyavsky

et

al.

Jinbu

et

al.

Kitajima

et

al.

Waschke

et

al.

Wilgram

et

al.

Williamson

et

al.

-RRB-

It

was

shown

that

cytokeratin

retraction

could

be

detected

as

early

as

h

after

addition

of

PV-IgG

i.e.

when

first

alterations

of

desmosomes

became

visible

and

was

fully

established

after

h

-LRB-

Berkowitz

et

al.

Calkins

et

al.

Chernyavsky

et

al.

-RRB-

However

because

no

cytokeratin

filament

reorganization

was

observed

before

cell

detachment

and

the

desmosomes

of

acantholytic

cells

remained

anchored

to

cytokeratin

filament

bundles

-LRB-

Kitajima

et

al.

Shimizu

et

al.

Spindler

et

al.

-RRB-

it

appears

that

cytokeratin

retraction

is

not

required

for

acantholysis

but

may

be

a

secondary

event

Recent

studies

demonstrated

that

acantholysis

in

pemphigus

was

paralleled

by

alterations

of

actin

filaments

including

increased

stress

fiber

formation

as

well

as

fragmentation

of

actin

filament

bundles

Moreover

pharmacological

inhibition

of

pemphigus

IgG-induced

acantholysis

also

was

effective

to

reverse

the

effects

on

the

cytoskeleton

-LRB-

Berkowitz

et

al.

Spindler

et

al.

Waschke

et

al.

Waschke

et

al.

-RRB-

Similarly

microtubule

distribution

was

found

to

be

altered

by

pemphigus

IgG

-LRB-

Kitajima

et

al.

-RRB-

Taken

together

pemphigus

IgG

cause

profound

cytoskeletal

reorganization

but

it

is

unclear

at

present

whether

these

effects

contribute

to

acantholysis

or

are

triggered

consequent

to

keratinocyte

dissociation

Nevertheless

it

is

likely

that

alterations

in

the

cytoskeleton

may

account

for

the

changes

in

cell

shape

during

keratinocyte

dissociation

which

seem

to

start

between

existing

desmosomes

-LRB-

Bystryn

and

Grando

Takahashi

et

al.

-RRB-

The

role

of

the

different

mechanisms

in

blister

formation

As

outlined

above

both

direct

and

indirect

mechanisms

finally

leading

to

a

loss

of

desmoglein-mediated

adhesion

have

been

found

to

be

involved

in

pemphigus

acantholysis

At

present

compelling

evidence

indicates

that

acantholysis

in

PV

and

in

PF

is

initiated

by

cellular

signalling

pathways

rather

than

by

direct

inhibition

of

Dsg

binding

-LRB-

Fig.

-RRB-

This

was

first

shown

using

plakoglobin-deficient

keratinocytes

which

were

resistant

against

PV-IgG-induced

acantholysis

and

was

further

supported

by

the

findings

that

inhibition

of

and

activation

of

Rho

A

is

sufficient

to

abolish

PV-IgG

and

PF-IgG-induced

skin

blistering

-LRB-

Berkowitz

et

al.

Caldelari

et

al.

Waschke

et

al.

-RRB-

Moreover

low

temperature-abrogated

keratinocyte

dissociation

but

not

antibody

binding

to

the

keratinocyte

cell

surface

indicating

that

direct

inhibition

of

cadherin

transinteraction

by

autoantibody

binding

is

not

sufficient

to

cause

acantholysis

-LRB-

Calkins

et

al.

-RRB-

If

extracellular

autoantibody-mediated

direct

inhibition

of

Dsg

binding

would

be

the

primary

cause

of

acantholysis

it

is

hard

to

imagine

how

this

process

should

be

blocked

by

modified

intracellular

signalling

or

low

temperature

It

is

possible

that

antibody-mediated

direct

inhibition

of

Dsg

binding

may

contribute

to

trigger

cellular

signalling

events

-LRB-

Muller

et

al.

Sharma

et

al.

Tsunoda

et

al.

-RRB-

However

because

PF-IgG

were

shown

to

induce

cellular

signalling

events

but

not

to

directly

reduce

Dsg

binding

it

seems

that

direct

inhibition

of

Dsg

transinteraction

is

not

essential

to

alter

Dsg-mediated

signalling

-LRB-

Heupel

et

al.

Waschke

et

al.

-RRB-

Fig.

mechanisms

involved

in

pemphigus

acantholysis

Accumulating

evidence

indicates

that

PV-IgG

and

PF-IgG

initiate

keratinocyte

dissociation

via

intracellular

signalling

pathways

including

MAPK

Rho

A

and

plakoglobin

-LRB-

PV

only

-RRB-

In

addition

other

mechanisms

such

as

direct

inhibition

of

Dsg

binding

and

Dsg

depletion

from

desmosomes

as

well

as

other

signalling

events

seem

to

contribute

to

PV

pathogenesis

whereas

their

role

for

acantholysis

in

PF

is

unclear

These

mechanisms

may

account

for

the

more

severe

clinical

phenotype

of

PV

compared

to

PF

PLC

phospholipase

C

PKC

protein

kinase

C

cdk

cyclin-dependent

kinase

EGFR

epidermal

growth

factor

receptor

It

is

tempting

to

speculate

that

the

clinical

phenotype

of

PV

may

be

more

severe

compared

to

PF

because

additional

mechanisms

could

be

involved

in

PV

-LRB-

Fig.

-RRB-

Indeed

some

mechanisms

have

only

been

found

to

contribute

to

acantholysis

in

PV

but

not

in

PF

For

instance

direct

inhibition

of

Dsg

transinteraction

was

found

in

PV

but

not

in

PF

under

conditions

where

antibodies

caused

acantholysis

-LRB-

Heupel

et

al.

-RRB-

Therefore

it

is

possible

that

direct

inhibition

as

a

second

pathway

to

reduce

Dsg

binding

in

PV

may

explain

the

more

severe

phenotype

of

PV

Similarly

depletion

of

Dsg

has

been

convincingly

shown

in

PV

whereas

depletion

of

Dsg

in

both

PV

and

PF

is

less

clear

-LRB-

Yamamoto

et

al.

-RRB-

Moreover

altered

plakoglobin

signalling

leading

to

c-Myc

overexpression

was

found

in

PV

patients

's

skin

but

not

in

PF

-LRB-

Williamson

et

al.

-RRB-

Other

pathways

have

been

investigated

for

PV

only

whereas

data

for

PF

are

lacking

As

outlined

above

inhibitors

of

calmodulin

PKC

EGFR

as

well

as

of

tyrosin

kinases

have

been

shown

to

inhibit

PV-IgG-induced

acantholysis

Finally

it

has

to

be

emphasized

that

desmoglein

proteolysis

mechanical

stress

as

well

as

secondary

changes

for

example

in

the

extracellular

concentration

might

contribute

to

skin

blistering

For

instance

acantholysis

may

foster

skin

blistering

by

derangement

of

the

epidermal

gradient

which

in

turn

would

result

in

loss

of

desmosomal

cadherin

binding

especially

when

the

tight

junctions

of

the

granular

layer

are

affected

Concluding

remarks

The

major

goal

for

the

future

is

to

elucidate

the

primary

signalling

pathways

responsible

for

the

diverse

effects

of

pemphigus

IgG

such

as

inhibition

of

desmoglein

binding

depletion

of

desmosomal

components

loss

of

desmosomes

reorganization

of

the

cytoskeleton

and

finally

the

induction

of

acantholysis

Other

pathways

will

be

identified

to

represent

salvage

mechanisms

to

rescue

keratinocyte

cohesion

-LRB-

Grando

-RRB-

Moreover

future

experiments

may

reveal

how

the

different

signalling

pathways

are

involved

in

the

regulation

of

desmosomal

adhesion

in

the

specific

epidermal

layers

This

knowledge

may

elucidate

why

the

cleavage

planes

in

PV

and

PF

usually

are

different

The

second

goal

is

to

characterize

whether

acantholysis

in

pemphigus

is

caused

by

mechanisms

directly

targeting

desmosomes

or

whether

cellular

signalling

pathways

regulate

desmosomal

adhesion

via

reorganisation

of

the

cytoskeleton

or

via

other

types

of

cell

contacts

such

as

adherens

junctions

desmosomes
desmogleins
pemphigus
autoantibodies
steric hindrance
desmoglein compensation