Cell-cell junctions

What are the 4 types of cell junctions?

• tight junction

• cell-cell anchoring junctions

• channel-forming junctions

• cell-matrix anchoring junctions

Which type of cell junction is found on the apical membrane?

tight junction

Which type of cell junction is found on the basal membrane? What are the 2 sub-types?

cell-matrix anchoring junctions

hemidesmosomes and actin-linked cell-matrix adhesion

What do tight junctions seal the gap between?

epithelial cells

How do adherens junctions connect a cell with the neighbouring cell?

connect actin filament bundle in one cell with that in the next cell

How do desmosomes connect a cell with the neighbouring cell?

connect intermediate filaments in one cell to those in the next cell

What do gap junctions allow from one cell to the next?

passage of small water-soluble molecules from cell to cell

How do actin-linked cell-matrix adhesion anchor a cell to the extracellular matrix?

through actin filaments

How do hemidesmosomes anchor a cell to the extracellular matrix?

through intermediate filaments

What are cadherins and their role? In which type of cell junction are they found?

transmembrane proteins that make a strong link between cells

adherens junctions

Are adherens junctions homophilic or homophobic interactions? Which ion is the interaction dependent on?

homophilic

calcium

When under tension, What structure forms within the cytosol of a cell being joined to the next? (4 molecules should be named)

• cadherin attaches to

• extended alpha catenin which is joined to

• actin filaments/cytoskeletons

• (by a vinculin molecule)

• that are pulled on by myosin II

How do cell-cell junctions respond to sensory information that tension is increasing between two linked cells? What is tension sensing dependent on?

strengthen actin linkage

alpha catenin

Which two types of cell junctions are actin-linked junctions?

adherens junction and focal adhesion

What’s different between adherens junction and focal adhesion? What do they both have?

adherens: cell-cell adhesion + dependent on cadherins adhesion molecules

focal: cell-ECM + dependent on integrins adhesion molecules - has subunits anchored in the CM

actin link to cytoskeleton + vinculin

What are the 2 functions of claudins?

• barrier (e.g. in the ascending limb of loop of Henle, selective for water and ions)

• allow mvt - become leaky (e.g. in the proximal tubule of a nephron).

What causes morphogenesis in sheets of epithelial cells?

cadherins in epithelial membrane bind to catenins that bind to actin allowing contraction with myosin - when adhesion belts tighten in an organised manner, cells pinch off to form round sheets of epithelia to make up epithelial tubes

What distinguishes E and N Cadherins? And their similarity?

• E in epithelia = cell-cell adhesion

• N in neural tissue = cell migration/neuronal connectivity

both cadherin proteins involved in cellular adhesion

Describe an adhesion belt

continuous ring of adherens junctions that encircle epithelial cells, contributing to tissue cohesion

What distinguishes hemidesmosomes from desmosomes? Their similarity?

cell to basal lamina with integrins // cell-cell junction with cadherins

intermediate filaments

In what type of tissue are desmosomes most prominent?

heart muscle and epidermis

What type of junction involves integrins and actin? Cadherins and actin?

focal adhesions

adherens junction

What type of junction involves intermediate filaments and integrins? Intermediate filaments and cadherins?

hemidesmosome

desmosome

What type of filaments make up desmosome junctions? What are they anchored to?

keratin intermediate filaments

cytoplasmic plaque

What type of disease is pemphigus? What does it target?

autoimmune skin blistering

desmogleins, key proteins in desmosomes that usually hold keratinocytes together in epidermis, the disruption = blisters

What type of mvt do tight junctions allow? What are they also called? Are they found on apical or basolateral membranes of the epithelia?

selective barrier, transcellular and paracellular transport

occluding junctions

apical

How do occludin junctions differ from tight junctions? And zonula occludens?

occludin is a tight junction - simply describes the sealing of the tight junctions

zonula occludins are the same - specific role in creating a belt-like structure around epithelial cells

Do tight junctions require a lot of energy?

when mediating transcellular transport and ion movement : YES LOTS

What connections make up tight junctions? Which proteins allow these?

focal connections binding the plasma membrane together

sealing strands of claudin, occludin, actin

How many different types of tight junctions exist? What differentiates them? What can they join?

25 with different properties including different affinities, leaking or blocking of specific molecules

areas within cells or within membranes

What allows the specialisation of membrane regions? How does the apical membrane differ drom the basolateral one?

lipids and proteins are segregated

apical outer: glycolipid and cholesterol

basolateral: phosphatidylcholine

What do gap junctions allow, through what mediators?

regulated and direct cell-cell communication through water, inorganic ions, sugars, amino acids, ATP, cAMP, IP₃

How long can gap junctions be? What diameter pores can they form? Where are they found?

2-4 nm

1.5 nm

Connective tissue, epithelia, neurons, heart muscle

By observing mediators that can cross through gap junctions, what size of mediators can cross? What can therefore not cross?

anything up to 1000 MW

lipids, carbs, …

What unit are gap junctions made up of? What do these two units form and what subtypes exist?

connexins that form connexons that make up intercellular channels (with 2, 6 connexons for eg) - either homotypic or heterotypic

What do cell-cell junctions allow for cells?

• Integrity (flexible + rigid)

• Cytoskeleton

• Compartments

• Communication

What do gap junctions regulate in terms of cell function? How do they limit cell damage?

Membrane potential, pH, Ca²⁺ and cell signals

stop propagation of calcium which is toxic when leaked = good control

Describe a selectin

• adhesion molecules found on endothelial cells, platelets, and leukocytes that mediate the rolling and adhesion of immune cells like WBC to blood vessels during inflammation and immune responses

• help coordinate mvt of immune cells to sites where they are needed for fighting infection or tissue repair.