basement membrane

cell-matrixinteraction sites

cell-cellinteraction sites

Cell-Cell Interaction

Cell-cell communication via cell adhesion molecules is critical for assemblingcells into tissues, controlling cell shape and cell function (together with cell-matrix interactions

Cell-Cell InteractionCell-Cell Interaction

Major cell-cell adhesion sites can be visualized by electron microscopy

Tight junctionsAdherens junctionsDesmosomes

Hemidesmosom

Desmosom

Gapjunction

Adherensjunction

Tightjunction

Mikrovili

Basallamina

Cells can also interact without forming visible adhesion sites

most important cell adhesion molecules:

Cadherins

Ig-superfamily

Selectins

(all cells)

(ubiquitous, enriched in the nervous system)

(interaction of leukocytes with endothelial cells

Cell-Cell InteractionCell-Cell Interaction

Major cell-cell adhesion sites can be visualized by electron microscopy

Tight junctionsAdherens junctionsDesmosomes

Cells can also interact without forming visible adhesion sites

most important cell adhesion molecules:

Cadherins

Ig-superfamily

Selectins

(all cells)

(ubiquitous, enriched in the nervous system)

(interaction of leukocytes with endothelial cells

Cell-Cell InteractionCell-Cell Interaction

Major cell-cell adhesion sites can be visualized by electron microscopy

Tight junctionsAdherens junctionsDesmosomes

Cadherins

• Ca2+-dependent cell-cell adhesion proteins,

epressed in almost all cells of vertebrates and invertebrates

• Transmembrane proteins

• characteristic structural feature:

- tandem repeats of homologous domains (CAD-domains): length of about 110 amino acids

-

β-sheet structure, distantly related- to immunoglobulin-fold

Leckband, Prakasam (2006) 8, 259Annu Rev Biomed Eng

The structure of cadherins

crystal structure of theextracellular domain fromC-Cadherin ( )Xenopus

EC1-EC5: CAD-domains

classic cadherins

actin

E-, N-, P-Cadherin

desmocollin

desmoglein

desmosomal cadherins

intermediate-filaments

linked tocytoskeleton

Cadherins

large superfamily(> 100 different genes identified in the vertebrate genome)

protocadherins

7TM-cadherin (Flamingo)

Fat-family

�-Pcdh(CNR-Cadherin)

EGF-RepeatsLaminin-G-Domäne-Repeats

in many cases notassociated with the

cytoskeleton

Cadherins

large superfamily(> 100 different genes identified in the vertebrate genome)

Classical Cadherins (Examples)

Name Main location Junction Association Phenotype when Inactivated in Mice

E-cadherin epithelia epithelial adherens Junctions (AJ)

embryonic lethal, mice die at blastocyst stage

N-cadherin neurons, heart, muscle, eye lens

AJ-like structures, synapses

embryonic lethal, heart defects

VE-cadherin Endothelzellen adherens junctions

embryonic lethal, apoptosis of endothelial cells

cadherin-11

mesenchymal cells, neural crest cells, several cancer types

bone and behavioral abnormalities, resistant to rheumatic arthritis

Features of classical cadherins

five extracellular CAD-domains

engaged in homophilic interactions

specificity determined by the most N-terminal domain

associated with the cytoskeletonvia various linker proteins:

various anchor proteins connect -Catenein

to the actin filaments (previously

�

-cateninhad been considered to be the major linking protein)

�

�-Catenin is required to link Cadherinto other anchor proteins

p120-Catenin is required for stable location of Cadherinat the plasma membrane (inhibits endocytosis)

adapterproteins

actinfilaments

F-actin

cell 1 cell 2

intercellular-space

cadherins

�-Catenin

vinculin VASP

�-actinin

p120-catenin

�-Catenin

(prevalent structural model)

adapterproteins

actinfilaments

Cadherin-mediated cell cell adhesion

Major functions of classical cadherins :

cell shape, tissue architecture cell polarity

cell-sorting

signaling

classical cadherins (E-cadherin) form theadherens junctions (AJs) between epithelial cells

Major functions of classical cadherins :

cell shape, tissue architecture cell polarity

cell-sorting

signaling

Cell-Sorting

Ektoderm

neural crestcells

infolding of theneural tube

expression of E-Cadherin

expression of N-Cadherin

expression of Cadherin-6B

expression of Cadherin-7

changes of cadherin expression during development of theneural tube and neural crest cells

neural tubeclosure

cell adhesion by cadherins is strictly regulated examples:

- migration of neural crest cells: E-cadherin expression is down-regulated (→ disassembly of AJs) and substituted by other cadherins (→ allowing cell contact during migration)

- gastrulation:

inhibited, if E-cadherin is not down-regulated (Cells cannot leave the ectoderm)

- pathological conditions: Carcinomas down-regulate the expression of E-Cadherin, which is often substituted by N-cadherin

Major functions of classical cadherins :

cell shape, tissue architecture cell polarity

cell-sorting

signaling

Signaling via Cadherins / Catenins

examples:

1. Modulation of kinase/ phosphatase activities by binding to cadherin/catenin complexes

- N-cadherin activates FGF-receptors - E-cadherin inhibits FGF- and EGF-receptors - VE-cadherin: Co-receptor for VEGF,

loss of VE-cadherin leads to apoptosis of endothelilal cells

2. Wnt-signaling

Wnt

Dsh

GSK

Fzd

active inactive

proteasomaldegradation

GSK

P-Catenin-Catenin

LEF1/TCF-Cat

cadherin

�-Catenin has two functions:

1)

2) component of Wnt-pathway

component of cadherin-mediatedcell-cell contacts

Non-Classical Cadherins (Examples)

Name Main Location Functions

Desmosomal Cadherins (they are associated with intermediate filaments)

Desmoglein, Desmocollin

skin (desmosomes) inactivation results in detachment of the skin, hyperproliferation, abnormal differentiation

Protocadherins PAPS (paraxial protocadherin)

paraxial mesoderm cell movements during gastrulation

-, -,- Pcdh´s (clustered protocadherins)

neurons (synapses) three gene clusters encoding more than 50 cadherins. All members of one cluster contain the same C-terminal domain. Not connected to cytoskeleton

Cadherin 23 Protocadherin 15

inner ear deafness, if inactivated

Others Flamingo-cadherins

epithelia, associated with junctions

planar cell polarity

FAT, Dachsous epithelia, associated with junctions

inactivation in Dros. results in overgrowth of imaginal discs, defects in differentiation and morphogenesis, formation of tumors

Desmosomes

intermediatefilaments

cadherin-familyadhesion proteins

dense plaque ofanchor proteins

Desmosomes

intermediatefilaments

cadherin-familyadhesion proteins

desmoplakin

plakoglobin

plakophilin

desmoglein

desmocollin

plasma membrane

cadherin-familyadhesion proteins

intermediatefilaments

(= -catenin)�

dense plaque ofanchor proteins

Planar cell polarity (PCP)

Polarization of cells of a cell sheetwithin the plane

PCP is best investigated in Drosophila wing epithelium

flamingo-cadherin

Seifert, Mlodzik (2007)8, 126Nat Rev Genet

mutant clone(marker: wing hairs)

wing epithelial cells

Cell-Cell Interaction the most important cell adhesion molecules:

• Cadherins (all cells)

• Ig-superfamily (ubiquitous, enriched in the nervous system)

• Selectins ( interaction of leukocytes with endothelial cells)

Cell Adhesion Molecules of theImmunoglobulin (Ig)-Superfamily:

Large, ancient superfamily

All members are characterized by the presence of (multiple) Ig-like domains that were

first identified in antibodies.

Two layers of antiparallel -sheets are foldedon top of each other. The two layers areconnected by one disulfide bond.

� schematic representation ofimmunoglobulin G

Cell Adhesion Molecules of the Immunoglobulin (Ig)-Superfamily: In addition to cell adhesion molecules and antibodies, Ig-like domains are found in

many proteins. Ig-like domains are encoded in 765 human genes!

Examples are: - T-cell receptor, CD4, MHC I, MHC.II - FGF-receptor

Cell Adhesion Molecules of the Immunoglobulin (Ig)-Superfamily: Cell adhesion is not Ca2+ -dependent (in contrast to the cadherins). Cell adhesion can be

- homophilic (between like molecules) or - heterophilic (between different molecules)

Binding is more stable than binding between cadherins!

plasmamembrane

plasmamembrane

intercellularspace

Possible binding pattern between two Ig-CAMs

Some Types of Ig-Superfamily Cell Adhesion Molecules

Name Main Location Typical Function

neuronal CAM´s

NCAM L1 (NgCAM)

nerves cell adhesion, -migration, axonal growth and guidance

non-neuronal CAM´s

ICAM´s (intercellular adhesion molecules)

leukocytes, lymphocytes, endothelial cells

VCAM´s (vascular cell adhesion molecules) endothelial cells

PECAM (platelet endothelial cell adhesion Molecule)

platlets; endothelial cells

adhesion of lymphocytes and leukocytes to endothelial and epithelial cells (inflammation response)

Nectins adherens junctions together with cadherins essential for adherens junctions assembly

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Schematic Structure of Some Ig-CAMs

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NCAM

L1

ICAM-1

ICAM-2

Nectin-1

F11/Contactin

Ig-domain

FNIII-domain

GPI-anchor

Biological functions of neuronal CAM´s (examples): cell-cell adhesion (neuron-neuron, neuron-glia)

regulation of signal transduction processes (e.g. activation of FGF-receptors via interaction with NCAM)

cell migration

neurite outgrowth

- axon fasciculation - axon guidance and sorting along nerve fascicles - branching of axon bundles

Cell-substrate interactions(integrins, laminins)

receptors for soluble factors(growth factors, chemoattractants)

Neurite outgrowth

cell-cell interactions(CAMs, cadherins)

Cell-substrate interactions(integrins, laminins)

changes in fasciclin-II expression during axonal growth

(NCAM equivalent in insects)

three grasshopperneurons

fasciclin-II

NCAM (neural cell adhesion molecule)

• Ig-superfamiliy protein with

- 5 Ig-domains - 2 Fibronectin-typeIII domains

• multiple forms by alternative spicing (120 - 180 kDa):

- transmembrane proteins - lipid-bound proteins (GPI-anchor)

• homophilic interaction •

in the adult mainly expresseed in the nervous system (and neuromuscular during embryogenesis expression in many tissues,

vertebrate NCAMs:

junctions)

• modification by polysialic acids (PSA)

OH

OHNH

2

COOHH

OH

H

OH

CH2

OH

NCAM-modification with polysialic acid (PSA)

sialic acid

2 different polysialyltransferases attach PSA-chains to domain Ig5

O

Biological Functions of Polysialic Acid

modulation of cell adhesion strength

modulation of fasciculation and branching of axons

olfactory bulb

trajectory of motor axons in the embryonic chicken hindlimb

-

-

neuraminidasetreatedcontrol

wild type NCAM-mutant

normal

L1-mutant

Neural Cell Recognition molecule L1

corticospinal tract (CST) at the levelof the pyramids in the medulla

many mutations in the L1 gene have beendescribed in humans leading to a broadspectrum of diseases:

- mental retardation (IQ < 20 - 50)

- hydrocephalus

- corpus callosum hypoplasia

- absence or diminution of thecorticospinal tract

- paralysis of lower extremities