CSB331 Lecture 1 Notes (January 11, 2012) – Cell-Cell Adhesion: Adherens Junctions (Cadherins)
5 – Two basic tissue types: epithelial and mesenchymal
Tissues are made up of cells (aqueous bags), but how do they give strength?
o Connective tissue: Cells make a lot of the ECM that bears the stress (for tissues like
bone, tendons, dermis)
o Epithelial tissue: Cells themselves through their cytoskeletal filaments that bear the
stress and those cytoskeletal filaments must be anchored between cells so they cross
the whole epithelia (their mechanical role is one of many roles though)
6 – Cytoskeleton and cell junctions in polarized epithelial cells
Three types of junctions:
o Tight junctions – regulate what can pass through epithelia
Cell-cell anchoring junctions anchors between cells
o Adherens junction – link actin filaments between cells
o Desmosomal junction – link IF between cells
Cell-matrix anchoring junctions
o Hemidesmosomes – anchor IF
o Integrin – anchor actin filaments
Note that hemidesmosomes are a special kind of integrin.
7 – Resolving power for light versus electron microscopy
Every window is 10 fold more magnified.
20 mm is the size of thumb.
20 μm is the size of average animal cell (1000x smaller than thumb).
20 nm is the size of cell-cell junction (1000x smaller than animal cell).
Can see down to a certain size with LM (subcellular organelles, mitochondria, bacterium)
because the resolution is limited due to the wavelength of light.
Cannot see cell-junctions with LM, you need EM.
8 – Cell junctions can be visualized with electron microscopy
Desmosomal junctions seen well in epithelium because dense and distinct.
In some cells, junctions aren't seen because they are smaller but it doesn't mean they are not
important. Must use molecular biology and biochemistry techniques to study functions.
9 – Cell junctions in vertebrate epithelial cells
Cell-cell junctions are symmetrical meaning that in one cell, the actin cytoskeleton of one cell
connects to the acting cytoskeleton in the next cell. This is the same for intermediate filaments.
You never see intermediate filaments on one side and actin on the other side. This is because of the specificity of the transmembrane proteins for the cytoskeletal filaments and what they are
binding outside the cell.
10 – Four types of anchoring junctions
The cadherin family looks after both types of cell-cell junctions. The cadherin family is a big
family. We are looking at the members of the cadherin superfamily that mediate adherens
junctions called the classical cadherins.
11 – Schematic representation of the cadherin superfamily
The two cadherins we are doing are not the only members of this family. The two cadherins are
well studied and important in cell-cell adhesions. A lot of the work that has been done on these
adherens junctions have been done in Drosophila because they have adherens junctions and can
be visualized with fluorescence microscopy. The classical cadherins mediate adherens
12 – Some members of the cadherin superfamily
E-cadherin is a hallmark of epithelial tissues that strongly regulates the epithelial phenotype.
N-cadherin originally found in neurons but expressed in many cell types.
13 – E-cadherin expression confers the polarized epithelial phenotype
E-cadherin gives epithelial cells their characteristics (apical and basal polarity). Because E-
cadherin is so important for the epithelial polarity and the formation of other junctions, if you
remove E-cad then it can cause EMT. If you express E-cad in mesenchymal cells by
overexpression then they can adopt epithelial phenotype.
14 – Structural organization of the ectodomain of classical cadherins
For all classical cadherins, they all have 5 extracellular cadherin repeats.
Each have Ca2+ binding domains where several Ca2+ bind.
The region where they bind is called the hinge region.
The Ca2+ bind between E-cad repeats in hinge region.
15 – Classical cadherins show homophilic binding
The classical cadherins mediate homotypic adhesion.
They are homophilic.
T each type only wants to bind its own kind (E-cad only binds E-cad, not N-cad or any other
16 – Knob and pocket model for cadherin interaction
How is homotypic adhesion possible?
EC1 is the furthest out cadherin repeat. EC1 mediates selective binding.
Each outer domain new the N-terminal have a knob and pocket. They are arranged so that when
they interact with another classical cadherin in trans, the knob of one fits into the pocket of the
The hinge region is shown in detail where the Ca2+ is binding.
Ca2+ binding in hinge region is thought to give a structure that subtly affects the structure of
EC1 domain. If you remove Ca2+, the structure of outermost domains change and lose affinity
for each other.
17 – Cadherin-mediated adhesion is calcium dependent
If you have less than 0.05 mM Ca2+, it will collapse. Normally, we know Ca2+ is higher
extracellularly at mM. Ordinarily, the rigid structure is maintained because there is plenty of
Ca2+ outside the cell.
Intracellularly, however, Ca2+ is lower because cells use Ca2+ as a signalling molecule. They
regulate how much is in the cell and keep it low until a signal needs to happen, they let let Ca2+
flood in to get intracellular signalling. This does not happen outside the cell. Cells get rid of Ca2+
by sending it outside the cell. Extracellularly, Ca2+ concentration is high enough for Ca2+ always
to be bound.
Cells growing on a dish and you want to lift them off the dish. If you artificially remove Ca2+ by
adding chelator, then cells will be released from each other. When cadherins are collapsed over,
because it is not their natural structure, they are also more susceptible to being cleaved by
proteases which helps lift the cells off the dish.
If you remove Ca2+, the hinge regions collapse and fold over. The whole molecular which used
to be a rigid rod ready to interact) collapses and unable to make interactions.
18 – Lateral (cis) interactions stabilize cadherin associations
Before we were looking at a monomer. This is a model for how cadherins are believed to make
their interactions. They interact in trans for sure. It has been proposed that to be more stable,
they first interact laterally (cis interactions) and upon clustering, they are more stable and able
to interact in trans.
More than just the EC1 domain is interacting here. In this model, EC1 domain is needed for
selectivity because of the knob and pocket model. It decides whether the interactions are made
in the first place. It is possible that later on, they settle into more overlapping interacting
19 – A typical junction, many cadherins are arranged in parallel, functioning like Velcro to hold cells
Each one of the interactions is not very strong. You need lots of cadherins to come together and
make bridges between cells to get a strong adhesive structure. It has been compared to Velco.
Cells are well tethered together and has a strong structure if you have lots of cadherin
molecules together making the adherens junction.
The distance between the plasma membranes is approx. 20 nm. 20 – Transmembrane adhesion proteins use intracellular anchor proteins to link the cell cytoskeleton to
We just talked about the ectodomain (outside the cell) of classical cadherins.
For most cell-cell adhesion processes, the intracellular or endodomain does not interact directly
with actin or IF. They have intracellular anchor proteins that connect cytoplasmic tail of
transmembrane receptors (classical cadherins) or endodomain to cytoskeletal filaments.
21 – The linkage of classical cadherins to the actin cytoskeleton
What does the tail associate with?
For classical cadherins to associate with actin cytoskeleton, they need to have linker proteins.
They need β-catenin which links to another anchor protein called α-catenin. It goes from tail to
β-catenin to α-catenin and other anchor proteins to the actin cytoskeleton.
22 – The cadherin-catenin complex
Showing the ectodomains interacting.
β-catenin links to α-catenin linked to actin.
Technically, α-catenin can interact directly with actin, but when it is bound in an adherens
complex to β-catenin, α-catenin binds to actin through the other proteins. Vinculin is thought to
be involved in mechanosensing.
When a cell's adhesion molecule like E-cad or when it is associated with another molecule, the
stress is sensed. It can tell whether it is attached and transmit that information to the
cytoskeleton on the inside and vice versa.
23 – The linkage of classical cadherins to the actin cytoskeleton
If force is exerted on these cells, another molecule gets activated and phosphorylates β-catenin
that causes it to dissociate and go to the nucleus and turn on genes involved in making colon
cancer cells more invasive. It also goes backwards as well. When epithelia are remodelling, there
are actin rearrangements. That information by one cell rearranging its actin and pulling on the
junction will affect the cell beside it and that is how you get coordinated behaviour between
When forces from the outside are sense, they are transmitted to the endodomain. There are
signalling events that cause actin rearrangements so the cytoskeleton remodels. The
information is transmitted inside the cell. Whatever actin conformation is on the inside
(actinomyosin contraction) and that from the inside will be transmitted to the outside to the
other E-cad molecule bound. When cells tighten their actin cytoskeleton, that information is
transmitted across epithelial layer. There is communication between the endodomain and
In general, β-catenin not only plays a role in part of adherens junction by tethering α-catenin to
E-cad, but also plays a role in the Wnt signalling pathway. If β-catenin is released from the tail of
cadherin, it is free to go to the nucleus and acts as a transcriptional co-activator for binding
partner in the nucleus. Signals can get transduced, including mechanosensing.
If there is a lot of force or other signalling, β-catenin can be caused to dissociate from E-cad
where it goes to the nucleus and causes alterations in gene transcription. Outside the cell, that information can be transmitted and used to change gene expression. β-catenin goes between
those two processes.
β-catenin is involved in EMT.
24 – Adherens junctions connect to bundles of cortical actin filaments in polarized epithelia (e.g.,
epithelial cells in small intestine)
Adherens junction is shown where E-cadherin would be. There are a lot of actin filaments
bundled that connect to cadherins from one cell to the next cell.