S2W5 - Interactions Between Cells and their Environment Flashcards
how do epithelial cells interact with each other and the extracellular matrix?
through junctions to form tissues
what are the 5 types of junctions?
tight junctions
adherens junction
desmosome
gap junction
hemidesmosome
draw a diagram labelling the positioning of the 5 different junctions
what types of junctions are present in epithelial cells?
all junctions
function of tight junction
- help polarise cells
- act as fences in the membrane, preventing mixing of apical and basolateral membrane proteins
tight junctions form
sealing strands (a tight junction belt)
tight junctions are composed of two transmembrane proteins:
Claudin and occludin
- required in both cells
- extracellular domain in one cell interacts with the extracellular domain in the neighbouring cells
- homophilic interactions (occludin attracted to occludin, claudin attracted to claudin)
adherens junction
joins an actin bundle in one cell to a similar bundle in a neighbouring cell, thus sticking 2 cells together
desmosome
joins the intermediate filaments in one cell to those in a neighbour cell, thus sticking 2 cells together
adherens junctions, desmosomes, and hemidesmosomes are also termed
anchoring junctions
function of anchoring junctions
provide mechanical strength to the epithelium
function of cell-cell anchoring junctions
link cytoskeletons of neighbouring cells
function of cell-ECM anchoring junctions
link cytoskeleton to basal lamina
two types of proteins involved in anchoring junctions
adhesion proteins and linker proteins
transmembrane adhesion proteins
- transmembrane proteins
- extracellular domains interact with adhesion proteins of neighbouring cell (side) or extracellular matrix (bottom)
- intracellular domains interact with linker proteins
intracellular linker proteins
- cytosolic proteins
- link transmembrane adhesion proteins to cytoskeletal filaments
adherens junctions
- form an adhesion belt that encircles the inside of the plasma membrane
- transmembrane adhesion proteins = classical cadherins
- cadherin proteins from neighbouring cells interact with each other via homophilic interactions (eg e-cadherin/e-cadherin)
- intracellular linker proteins link cadherin proteins to actin filaments
- cadherin proteins become concentrated at sites of cell-cell interactions, forming adherens junctions
both desmosomes and hemidesmosomes link to —-; why?
intermediate filaments eg keratin filaments. intermediate filaments provide the most structural strength
distinguish between desmosomes and hemidesmosomes
- desmosomes are linked to keratin filaments and connect to a neighbouring cell
- hemidesmosomes anchor keratin filaments to the basal lamina
desmosomes
- transmembrane adhesion proteins = nonclassical cadherin proteins (desmoglein, desmocollin)
- these undergo homophilic and heterophilic binding
- intracellular linker proteins link desmoglein and desmocollin to keratin filaments inside the cell
hemidesmosomes
- transmembrane adhesion proteins = integrins that bind to laminin in the basal lamina (ECM)
- intracellular linker proteins link integrins to keratin filaments inside cell
gap junction
allow for communication between cells:
- couple cells electrically and metabolically
- allow passage of ions and metabolites (<1000 daltons)
- not very selective as to what passes through
passes through gap junctions:
cAMP, nucleotides, glucose, amino acids
does not pass through gap junctions:
macromolecules, proteins, nucleic acids
describe the gated nature of gap junctions
can be in an open or closed state by extracellular or intracellular signals
eg treatment with dopamine causes close gap junctions
dramatic increase in cytosolic Ca2+ ->
close gap junction
membrane damage ->
- Ca2+ leaks into the damaged cell
- gap junctions close
- prevents loss of metabolites from adjacent cells
describe the structure of a gap junction
1 subunit = connexin
6 connexins = form connexon (hemichannel), which by itself is closed
2 connexons = form intracellular channel (open)
how does a hemidesmosome differ from an adherens junction and a desmosome?
it is a cell-ECM anchoring junction
why are mature epithelial cells polarised?
junctions are arranged in a specific order (ie ends are different
give an example of the polarity of epithelial cells
sealing strand (tight junction belt) above the adhesion belt
intercellular junctions in plant cells
- plant cells lack cell junctions found in animal cells
- they are surrounded by cell walls (hold cells together, provide mechanical strength)
- plasmodesmata are intercellular junctions that allow for communication between cells
- need to cross cell wall, so have different structure from gap junctions
describe the structure and functioning of the plasmodesmata
- cytoplasmic channels which lead to a continuous plasma membrane and ER across plasmodesmata
- intercellular free movement of soluble molecules (<1000 daltons), like sugars, ions, other essential nutrients
- controlled trafficking of larger soluble molecules via gating, like proteins or regulatory RNAs
callose deposition in cell wall
- callose is a plant polysaccharide
- permeability control through reversible callose deposition
animal tissues are composed of
cells and extracellular matrix
compare epithelial tissue and connective tissue
epithelial tissue (epithelium):
- eg intestinal lining, skin epidermis
- cells closely associated and attached to each other
- limited ECM (a thin basal lamina)
- cytoskeletal filaments provide resistance to mechanical stress
connective tissues:
- eg skin dermis, bone, tendon, cartilate
- cells are rarely connected and are attached to the matrix
- plentiful ECM
- ECM provides resistance to mechanical stress
what gives different tissues different properties?
different compositions of ECM
what is the primary component in connective tissues?
ECM
3 major classes of macromolecules in the extracellular matrix:
- glycosaminoglycans (GAGs) and proteoglycans
- fibrous proteins (collagens, elastin)
- glycoproteins (eg laminin, fibronectin)
connective tissue ECM: glycosaminoglycans (GAGs)
- long, linear, chains of a repeating disaccharide
- highly negatively charged (attract Na+ and water)
- form hydrated gels, resist compression
- space filling
- most GAGs synthesised inside cell and released by exocytosis
hyaluronan
- simple GAG
- long chain of repeating disaccharide subunits (up to 25,000)
- hyaluronan is spun directly from cell surface by a plasma membrane enzyme complex
connective tissue ECM: proteoglycans
- subclass of glycoproteins
- protein with at least one sugar side chain which must be a glycosaminoglycan (GAG)
- typically, more extensive addition of sugars (up to 95% of total weight)
connective tissue ECM: collagen
- fibrous protein
- provides tensile strength
- resists stretching
structure of typical collagen (fibril-forming collagen)
- three chains wound around each other in a triple helix
- assemble into ordered polymers to form collagen fibrils, which can then pack together into collagen fibres
collagen is secreted as —– by —–
procollagen by fibroblasts (skin, tendon, other connective tissue) and osteoblasts (bone)
once procollagen is secreted outside,
it is processed to collagen and assembled into large structures (collagen fibrils)
how do cells interact with collagen in the ECM of connective tissues?
- connective tissue cells that secrete collagen also organise collagen in the ECM
- they bind to collagen in ECM through integrin (cell surface adhesion receptor) and fibronectin (glycoprotein)
fibronectin
- binds collagen
- binds integrin
integrin
- binds fibronectin (extracellular domain)
- binds adaptor proteins - actin filaments (intracellular domain)
connective tissue ECM: elastin
- elastin is a fibrous protein
- networks of elastin give tissues elasticity, allowing it to stretch and relax like a rubber band (resilience)
epithelial tissue ECM: basal lamina
the basal lamina is a basement membrane
- specialised type of ECM
- underlies all epithelia
- thin (40-120nm thick)
- ECM is secreted by the epithelial cells
- influences cell polarity (apical - basal)
how does the basal lamina separate the epithelia from underlying tissue?
- prevents fibroblasts in underlying connective tissue from interacting with epithelial cells
- yet allows passage of macrophages and lymphocytes
basal lamina contains a lot of
- laminin (glycoprotein)
- type 4 collagen (fibrous protein)
- integrin (transmembrane adhesion protein)
basal lamina is attachment site for
epithelia
basal lamina is anchored by
hemidesmosomes
basal lamina is organised by
laminin:
- interacts with other components of ECM
- links integrin to type IV collagen
describe the structure and contents of the plant cell wall
- more rigid than the ECM of animal tissues
- main components: cellulose, pectin (polysaccharides)
- cellulose microfibrils provide tensile strength
- pectin is space filling and provides resistance to compression
how is the plant cell wall made?
- plant cells synthesise cellulose chains at the plasma membrane using a cellulose synthase complex
- other cell wall components are synthesised in the Golgi and exported by exocytosis
