Lecture 22 - Cell adhesion Flashcards
Epithelia
Sheets of tightly bound cells in epithelial tissues
What does apical-basal polarity result from?
the differential distribution of phospholipids, protein complexes, and cytoskeletal components
Junctions connect cells at the ______ region
lateral
Cells in the epithelium
attached to each other directly by cell-cell junctions, where cytoskeletal filaments are anchored, transmitting stresses across the interiors of the cells
What are the three types of junctions that are vey important?
- tight junctions
- adherens junctions
- gap junctions
Adherens junctions associate with the ______ ________
actin cytoskeleton
- junctions must be anchored to actin
What is the main component of the adherent junctions?
cadherin/catenin
Where are cadherin and catenin located in the cell?
cadherin: transmembrane protein
catenin: cytoplasmic protein
Cadherin-cadherin binding is _____ dependent
Ca2+
*each domain of cadherin (5) binds to Ca
What do all of the cadherin superfamily members contain?
they all have extracellular portions containing multiple copies of the extracellular cadherin domain
How do cadherin domains bind to other cadherin domains?
each cadherin domain forms a more-or-less rigid unit, joined to the next cadherin domain by a hinge
- Ca2+ ions bind to each hinge and prevents it from flexing
- When Ca2+ is removed, the hinges flex, and the structure becomes floppy => folds in on itself
Typical cadherins vs atypical cadherins
typical: homophilic (bind to same cadherins)
atypical: heterophilic (bind to different cadherins)
What do cadherins mediate? What does this enable?
they mediate highly selective recognition which enables cells of a similar type to stick together and to stay segregated from other types of cells
How do cadherins play a role in sorting cells?
cells are sorted by the type ofo cadherin and amounts of cadherins
Assembly of strong cell-cell adhesions requires….
changes in the actin cytoskeleton
How do cadherins change the actin cytoskeleton?
they generate local signals to inhibit the GTPase Rho and activate the GTPase Rac
What do catenins do?
they link classical cadherins to the actin cytoskeleton
What are 2 examples of atypical cadherins? Why are they heterophilic?
Fat and Dachsous => heterophilic because of their role in planar cell polarity
What does the barrier function of epithelia require?
that adjacent cells be sealed together by tight junctions so molecules cannot leak freely across the cell sheet
How can tight junctions be visualized? What do they look like?
by freeze-fracture electron microscopy; seen as a branching network of sealing strands that completely encircles the apical end of each cell
What are the main transmembrane proteins in tight junctions?
claudins and related proteins occludins
What do claudins do?
they form selective channels allowing specific ions to cross the tight-junctional barrier, from one extracellular space to another
- they form SMALL pores
Septate junctions
‘tight junctions’ in insects
- organized differently, but have the same function
- also made of claudins
What are gap junctions and what is their structure?
- they are a pore that connects 2 cells, allowing transport of molecules between the cells
- they are clusters of channels that join 2 cells together and consist of building blocks of 2 connexons or hemichannels (one coming from each cell)
What are connexons/ hemichannels formed from?
each one is formed from a complex of 6 connexin proteins
Vertebrate vs invertebrate gap junctions
vertebrate gap junctions = connexins
invertebrate gap junctions = innexins
What do the pores in gap junctions allow to pass?
exchange of inorganic ions and small molecules, but not of macromolecules
connexin and innexin form similar sized pores
