cell junctions Flashcards
what are cell junctions
they are multi-protein complexes that link two neighboring cells, or a cell to the ECM within a tissue of a multicellular organism.
cell junctions are abundant in
epithelial tissue
What do cell junctions do ?
allow cells within a tissue to withstand external forces that try to pull them apart
Classification of Cell Junctions:
By location:
• Cell – cell
• Cell – matrix
• By function:
• Anchoring
• Occluding
• Communicating (AKA channel-forming)
Occluding junctions =
Tight junctions
What are tight junctions ?
they are complex structures that seal the gap between adjacent epithelial cells
tight junction’s location
apicolateral borders of adjacent epithelial cells
Tight junction’s structure
They have a branching network of sealing strands that encircles the apical ends of epithelial cells
Each strand is composed of a long row of four- span transmembrane proteins:
1) Claudin
• The main structural component (backbone) of TJs
• Forms & regulates aqueous channels used for paracellular diffusion
2) Occludin
• Not essential for the assembly of TJs, but important for limiting junctional permeability
how do the extracellular domains of claudins & occludins on neighboring cell membranes bind ?
homophilically, they act as a zipper to seal the intercellular space.
what is localized in specific areas of TJs at tricellular contacts (corners where three epithelial cells meet) ?
the protein (tricellulin)
What is the functions of tight junctions
1) Gates or selective permeability barriers that regulate paracellular transport of ions and small molecules.
• All TJs are impermeable to macromolecules, but their permeability to ions and small molecules varies between tissues.
• Targeting TJ proteins by pathogens increases the permeability of TJ, allowing the pathogens to invade the cell.
2) Fences that confine apical & basolateral lipids and proteins to their respective domains, thus play a role in polarity & transcellular transport of solutes.
• i.e., glucose transport across intestinal epithelial cells:
• Na+-driven glucose symports in the apical domain actively transport glucose from the lumen into the cell.
• Glucose uniporters in the basolateral domain passively transport glucose out & into the ECF.
• Na+ gradient is established & maintained by Na+/K+ pump
• Intercellular spaces are tightly sealed by TJs to prevent leakage of transported glucose back into the lumen.
Communicating junctions =
Gap junctions
What are gap junctions
It is an array of aqueous channels that interconnect the cytosols of adjacent cells:
• Allows ions & small hydrophilic molecules (< 1kDa) to pass by diffusion.
- i.e., amino acids, sugars, nucleotides, second messengers, metabolites.
• Excludes macromolecules.
Where does gap junctions express ?
In most tissues (i.e., connective, epithelia, heart muscle & neurons).
Gap Junction’s Structure
• Gap junction channel = 2 connexons (hemichannels)
• Single connexon = 6 connexins
Gap Junction’s Functions
1) Electrical coupling of excitable cells:
• Rapid,simultaneous activation,hence synchronized contraction of heart muscle cells & smooth muscle cells of intestine, uterus & bladder.
2 ) Electrical synapses in the brain:
• Allow conduction of action potential at the synapse without the delay associated with neurotransmitter release.
• Allow for bidirectional signal transmission which increases neuronal sensitivity and promotes synchronous firing of a group of interconnected neurons.
3 ) Metabolic coupling of non-excitable cells:
• Coordination of non-excitable cells activities by bidirectional transport of ions, small metabolites & second messengers.
4 ) Provide a form of cell-cell adhesion. I
what are the two anchoring junctions ?
1) Cell – Cell Anchoring Junctions:
• Link the cytoskeletons of two neighboring cells.
• 2 types:
a) Adherens junctions
b) Desmosomes
2) Cell – Matrix Anchoring Junctions:
• Link the cytoskeleton of a cell to ECM.
• 2 types:
a) Actin linked cell – matrix junctions (AKA focal adhesions)
b) Hemidesmosomes
Anchoring Junctions Composition
I. Transmembrane adhesion proteins:
• 2 superfamilies:
— Cadherins (mediate cell – cell adhesion)
— Integrins (mediate cell – matrix adhesion)
II. Intracellular adaptor protein:
• Indirectly links adhesion proteins to cytoskeletal filaments
There are two types of Cell – Cell Anchoring Junctions
Adherens Junctions & Desmosomes
What are Adherens Junctions ?
they are multiprotein complexes that mediate cell-cell adhesion in all types of tissue.
• especially common in epithelial cells.
Adherens Junction’s Structure
• AJs are built primarily from classical cadherins (Ca2+ -dependent, homophilic):
a ) Extracellular domains of cadherins on neighboring cells bind to each other.
b ) Intracellular domains of cadherins are linked to actin filaments via adaptor proteins (not shown).
c ) Cadherins bind to their partners with a relatively low affinity. Strong attachments result from the formation of many such weak bonds in parallel.
• AJs form a continuous belt that encircles entire epithelial cells.
Adherens Junction’s Functions :
1 ) Promote adhesion of identical cells & further organization and separation of tissues during embryonic development (How?)
• Homophilic binding of cadherins leads to cell sorting, which is a crucial process in development.
— i.e., development of vertebrate nervous system
2 ) Enable tissues to withstand mechanical stress, thus maintain their integrity.
— i.e., physical trauma, cardiac muscle contraction, & flow within blood and lymphatic vessels.
What are Desmosomes
they are structurally similar to adherens junctions, except they are composed of non-classical cadherins that are linked to intermediate filaments (IF) via adaptor proteins.
Desmosomes Function
Maintain tissue integrity by providing mechanical strength.
