Cell junctions

Why is it especially important in epithelium they are connected by cell junctions?

To allow barriers to form

Transmembrane proteins/domains

Aspects of the cell that span across the cell membrane and then interact with neighbouring epithelial cells

Membrane spanning domain has a hydrophilic and hydrophobic domain

What are the 4 main types of transmembrane proteins?

• immunoglobulin superfamily

• Cadherins

• Selectins

• Intergrins

What part of the transmembrane protein interacts with the neighbouring epithelial cells?

Extracellular domain

What are the two main types of adhesion molecules?

• homophilic

• Heterophilic

  -According to which cell adhesion molecules are present in neighbouring cells

What family are Cell adhesion molecules part of?

• immunoglobulin super family - transmembrane domain and EC domain of the cell adhesion molecule

N-CAM

First to be identified - found in neuronal tissue

V-CAM

found in vascular tissue

How do cell adhesion molecules make sure the right type interact with each other?

• only interact in homophilic nature

Cadherins

• adhesive glycoprotein

• Transmembrane and EC domain of cell adhesion molecules

How do Cadherins differ from CAMs (cell adhesion molecules)?

• require Ca - if this is not present Cadherins break down and junctions break apart

Describe the structure of Cadherins?

• Cadherins bind to intracellular protein beta-catenin

• Which in turn binds to alpha-catenin and associates with actin filaments of the cytoskeleton for mechanical structure

What are the types of cell junctions?

• Tight junctions

• Adherens junction

• Desmosomes

• Hesidesmosomes

• Gap junctions

Tight junctions

• weblike strands of transmembrane proteins = fuse together the outer surfaces of adjacent plasma membranes to seal off passageways between adjacent cells

• Eg; cells of epithelial tissue lining the stomach, intestines and urinary bladder

• Inhibition of substance passage between cells = prevent contents of organs from leaking into blood or surrounding tissue

Adherens junctions

• contains plaque (dense layer of proteins on the inside of plasma membranes that attaches to both membrane proteins and to the microfilaments of the cytoskeleton)

• TM proteins called Cadherins joins the cell = each insert into a plaque from the opposite side of the plasma membrane= partially crossing the intracellular space and connects to Cadherins on the adjacent cell

• In epithelial cells = form extensive zones called adhesion belts = help to resist separation during various contactile activities

Desmosomes

• Spot weld - like junctions - common across epidermis (stop cells separating under tension)

• like Adherens they contain plaques and have the TM glycoprotein Cadherins (connect cells the same way)

• HOWEVER- they do not attach to microfillaments.

• Instead the Desmosomes plaque attaches to elements of the cytoskeleton known as intermediate filaments (containing keratin) = extend from Desmosomes on one side of the cell, across the cytosol to Desmosomes on the other side of the cell

• This structural arrangement = contributes to stability of cells and tissue

Hemidesmosomes

• resemble Desmosomes, but do not link adjacent cells

• Look like half a desmosome- hence the name

• TM glycoproteins = intergrins rather than Cadherins

• On the inside of plasma membranes = intergrins attach to intermediate filaments made from the protein keratin

• On the the outside of the plasma membrane the intergrins attach to the protein laminin on the basement membrane

• Hemidesmosomes = ANCHORS for cells to basement membrane

Gap junctions

• membrane proteins called connexins form tiny fluid filled tunnels called connexons that connect neighbouring cells

• Plasma membranes and gap junctions = not fused together and are separated by a very narrow intracellular gap

• Connexons allow ions and small molecules to diffuse from the cytosol of one cell to another

• Transfer of nutrients and waste occurs at gap junctions

• Allow cells to communicate with each other

• Allow nerve/muscle impulse to spread rapidly between cells = crucial in normal operation of parts of the nervous system and for contraction of the muscle of the heart, GI tract and uterus