microbiology

Epithelial to Mesenchymal

Transition

Epithelial vs Mesenchymal cells (A) Epithelial cells are inter-

connected through tight junctions

(gray), E-cadherin-based junctions

(red), which are connected to the

actin cytoskeleton, gap junctions

(red/blue), and hemidesmosomes

(cyan), which are connected to

the cytokeratin-based intermediate filament cytoskeleton.

Epithelial cells also have specialized cell-ECM interactions

for adhesion to the laminin-rich basement membrane.

CSHL collection

Epithelial vs Mesenchymal cells

(B) Mesenchymal cells show a shift

to a vimentin-based intermediate

filament cytoskeleton and altered

composition of cell-ECM

interactions optimized for adhesion

to the collagen-rich interstitial matrix.

Mesenchymal cells also produce abundant TGFb, growth

factors (GF), and matrix metalloproteinases (MMPs), as

well as components of the extracellular matrix.

CSHL collection

Epithelial vs Mesenchymal cells

Use this slide to practice describing the differences between

these two types of cells

Cell phenotypes in early mouse embryo. (A) Gastrulating mouse embryo. (C') Scanning electronic micrograph of a transverse section through a mouse embryo showing the different cell phenotypes: columnar epithelium, mesenchyme and squamous epithelium. (B) Diagram of the embryonic part of a mouse embryo. Dashed box outlines the three germ layers. (C) Scanning micrograph shown in C' color-coded for the three germ layers.

EMT and its reverse MET

.

Diagrammatic representation

of EMT, its reverse MET

(mesenchymal to Epithelial

Transition), egression and

ingression. (A) Epithelial cells

undergo EMT to assume

mesenchymal characteristics.

Conversely, mesenchymal

cells undergo MET to epithelialize. (B) In an ingression, cells

undergo EMT and leave an epithelium, while in an egression

cells undergo MET and join a preexisting epithelium as in

endoderm formation in birds and mammals.

Rear-front to apico-basal polarity

Categories of EMT

Three types of EMT can

be discerned depending

on the physiological tissue

context.

Type1 EMT occurs in

embryogenesis and organ development,

type2 EMT is important for tissue regeneration and organ

fibrosis, and type 3 EMT is associated with cancer

progression and cancer stem cell properties.

EMT and MET The process of MET

(mesenchymal–epithelial

transition) enables cells that

have undergone EMT to revert

to the epithelial state. MET also

contributes to development and the generation of metastatic

carcinomas. Consecutive rounds of EMT and MET occur

during development. In 1ary EMT, epithelial cells without

prior history of EMT differentiate into mesenchymal cells. In

2ary EMT, cells that have already undergone EMT and MET

initiate a new EMT process, e.g. following dissemination,

cancer cells can revert through MET to an epithelial state.

1ary and 2ary EMT

Steps during EMT and ingression

A conserved series of steps underlie EMT and

ingression process for several developmental

events.

Steps during EMT and ingression

Changes in cell adhesion during EMT

The first cell (left) presents

a cell with epithelial

characteristics and apico-

basal polarity. The two other

cells are transitioning to

mesenchymal status. An

external cue (Growth Factors) starts an STP leading to

cytoskeleton remodeling and loss of apico-basal polarity.

From the review paper EMT-pathways

External cues involved in EMT Signaling and STP from several different signal

molecules (TGF, Wnt, Shh, and Delta/Notch), as well

as changes in integrin/ECM and injury (inflammation,

hypoxia) can lead to EMT.

Cellular events during EMT a) The first steps of EMT are

the disassembly of epithelial

cell–cell contacts (tight

junctions, adherens junctions,

desmosomes and gap

junctions) and the loss of cell

polarity through the

disruption of the Crumbs

(Crb), partitioning defective

(PAR) and Scribble (SCRIB)

polarity complexes.

Cellular events during EMT b| Next, the epithelial actin

architecture reorganizes, and

cells acquire motility and

invasive capacities by

forming lamellipodia,

filopodia and invadopodia,

and by expressing matrix

metalloproteinases (MMPs)

that can degrade extracellular

matrix (ECM) proteins.

Changes in cell adhesion during EMT

The DDR1 complex

activates RhoE, which

weakens actomyosin

contractility at points of

cell-cell contacts (Adherens

junctions, AJ).

TGF receptors located next to tight junctions (TJ) trigger

ubiquitylation and degradation of RhoA and the

destabilization of cortical actin filaments associated with TJ.

From the review paper EMT-pathways

Changes in cell adhesion during EMT

Activation of transcriptional

repressor, such as Snail and

Serpent (Srp/GATA)

downregulates genes

encoding junctional proteins,

including E-cadherins, claudins

and occludin, thus compromising further AJ and TJ. EPB4-

1L5 sequesters p120-catenin weakening AJ attachment to

microfilaments and promotes integrin-ECM adhesion.

Integrin also leads to the activity of Srp, which represses the

apical polarity gene Crumbs (Crb). From the review paper

TFs during EMT

EMT is driven by SNAIL, zinc-finger E-box-binding (ZEB)

and basic helix–loop–helix (bHLH) transcription factors that

repress epithelial marker genes and activate genes

associated with the mesenchymal phenotype. These TFs are

themselves regulated through their cellular localization and

stability.

Snail regulation

Post-translational modifications

(PTM) of the different TFs regulate

their activities, subcellular

localization and stability. For

example phosphorylation (PTM)

of Snail by GSK3β leads to its export from the nucleus (and

therefore its inactivity as a TF).

Snail phosphorylation by PAK1 on

the other hand promotes the nuclear

retention of SNAIL1 and enhances its activity.

Twist and ZEB regulation TWIST is phosphorylated by the

MAPK p38, JNK and ERK, which

protects it from degradation, and

thus promotes its nuclear import

and functions. ZEB2 undergoes

another PTM ( sumoylation,

Sumo by Polycomb repressive

complex 2 (PRC2) and is

subsequently exported from the

nucleus, which reduces its activity

as a transcription factor.

EMT during mouse gastrulation (A) Scheme of the embryonic part of

a early mouse embryo. Dashed box

outlines the primitive streak.

(B') Scanning EM showing a

transverse section through a

mouse primitive streak. (C) Cells

undergo an EMT event at the

primitive streak. (D) Signaling

pathways that regulate the different

EMT steps at the murine primitive

streak.

EMT during mouse gastrulation (C) Cells undergo an EMT event

at the primitive streak.

(1) First intercellular spaces

appear between cells and

(2) basal lamina breaks down.

(3) Cells acquire a bottle shape,

(4) round up as they travel

through the streak, and

(5) finally acquire a stellate morphology and

migrate away from the streak.

EMT during mouse gastrulation

EMT during mouse gastrulation Signaling in cells located in the

epiblast (left) ensures

maintenance of the epithelial

phenotype via the

expression of Pofut2 and

Sox3. Upon activation by

Wnt, TGF and FGF

proteins, cells in the primitive streak (middle) undergo

apical constriction (indicated by green ring of contracting

actin) to allow invagination before ingression.

EMT during mouse gastrulation Blue cells in the primitive

streak express Eomes, Mesp1

and Mesp2, which are

responsible for ingression.

RhoA downregulation by

Net1 destabilizes basal

microtubules and

contributes to the breakdown of the basement membrane.

Snail represses Sox3 and E-cadherin, which is also

downregulated post-transcriptionally (PTM). In the

migratory cells (right), Rac1 mediates cell protrusion.

Another EMT example Signal molecules such as Wnt,

BMP4 retinoic acid (RA) and

FGF lead to EMT. Try to

follow the arrows and

recognize some of the players

from previous slides. The point

here is not to memorize this

pathway, but to understand

common themes and players.