L10 Cell Junctions and Adhesions Flashcards
(31 cards)
two main ways in which animal cells are bound together
either to each other or to the extracellular matrix
four main types of cell junctions
anchoring junctions
occluding junctions
channel forming junctions
signal relaying junctions
types of anchoring junctions
adherens junctions:
- use cadherins (adhesion proteins) to connect to cadherins on neighbouring cells
- cadherins anchored inside cell to actin cytoskeleton via anchor proteins like alpha catenin, beta catenin and p120-catenin
desmosomes:
- cells use desmoglein and desmocollin to connect to same proteins on nearby cells
- connect to intermediate filaments inside cells via anchor proteins like plakoglobin, plakophilin, desmoplakin
types of cadherins
E cadherin: found in epithelial cells
N cadherin: found in nerve cells and muscles
P cadherin: found in placenta
Ve cadherin: found in vascular endothelial cells
they all require Ca2+ for binding
what type of binding does cadherin prefer
homophilic binding: E cadherin only bind to E cadherin and N cadherin only bind to N cadherin etc
this property explains the assembly of cells to form tissues
changing patterns of cadherin expression during construction of the nervous system
cells on surface express E cadherin > keeps them stuck together as epithelial sheet
cells at border between neural and non neural tissue switch to cadherin 6B > help define the edges where neural crest cells will form and eventually migrate
cells that become neural tissue switch to N cadherin
as neural crest cells leave the neural tube and start migrating > express cadherin 7 (mobile type of adherin) > allow cells to move but still stay connected in small groups
how does cadherin-catenin complex help maintain correct spindle orientation
cadherins at cell junctions stick together while catenins link cadherins to actin cytoskeleton inside cell > form stable junction
allows mitotic spindle to be parallel to the ECM > cell divides side by side and not top to bottom
without this complex > spindles randomly orientated > misplaced daughter cells and tissue disorganisation
roles of cadherins and actin cytoskeleton
at cellular levels: actin assembly at adherens junction contributes to integrity and mechanical homeostasis of the cell-cell junctions
assembly and disassembly of cell-cell adhesions are important in embryonic morphogenesis
myogenesis: muscle formation
nervous system development
what is epithelial mesenchymal transition (EMT)
process where cells change from being structured and stuck to being loose and mobile
epithelial cells: tightly stuck to each other; have clear top-bottom (apical-basal) polarity; express lots of E-cadherin
mesenchymal cells (after EMT): loosely attached or not attached; no clear shape or direction (lost polarity); can move around; express N cadherin instead of E cadherin
when can EMT be seen
during normal embryo development and wound healing (cells migrate to close the wound)
abnormal: cancer progression and metastasis
what are desmosomes
“super glue dots” that are linked to intermediate filaments
main function: provide mechanical strength
what are desmoglein, desmocollin, plakoglobin, plakophilin, desmoplakin and how do they link to intermediate filaments
desmosomal cadherins (outside cell):
- desmoglein: binds to desmogleins on nearby cells > strong adhesions
- desmocollin: also binds across cells with desmoglein
anchor proteins:
-plakoglobin: connects desmoglein/desmocollin to desmoplakin
- plakophilin: stabilises desmosome and recruit other components
- desmoplakin: directly connects the complex to intermediate filaments
what type of interaction does desmosomal cadherin prefer
do not only prefer homophilic interaction, but heterophilic is the norm
what are occluding junctions
tight junctions form at apical side to create a tight barrier between cells > prevent solutes from leaking through
role of tight junctions
act like barriers or fences between top and bottom of cell (formation of apical basolateral diffusion barrier)
stop transport proteins from moving to the wrong side and prevent solutes (eg glucose) from leaking back if its diffused against potential gradient (regulation of paracellular permeability)
what is claudin and occludin
proteins in tight junctions
claudin: the main sealing protein that forms actual barrier that controls what can pass between cells
occludin: supporting protein to help stabilise the tight junction
what are channel forming junctions
tiny tunnels connecting neighbouring cells > share small solutes
known as gap junctions for animal cells and plasmodesmata for plant cells
function of channel forming junctions
allow neighbouring cells to exchange small molecules but not macromolecules
gap junctions allow electric current to pass through
gap junctions are dynamic structures which can readily assemble and disassemble
structure of gap junction
formed by assemblies of proteins called connexons
each connexon made of 6 connexin subunits
what are neurexin and neuroligin
proteins that help form and stabilise synapses
neurexin: on presynaptic neuron
neuroligin: on postsynaptic neuron
bind to each other across synapse > help two neurons stick together and communicate properly
what is extracellular matrix (ECM)
network of proteins and sugars outside cells that provides support and structure
what is basal lamina
a type of ECM found in epithelial tissues
thin sheet
wraps around other cell types like muscle cells
major components of basal lamina
fibrous proteins: glycoproteins such as laminin, type IV collagen and nidogen
polysaccharide chains covalently linked to core proteins to form proteoglycans
components of ECM of animal connective tissue
glycosaminoglycan polysaccharide chains
non proteoglycan polysaccharides
fibrous proteins
