15 - the extracellular matrix Flashcards
what is the extracellular matrix?
the collection of macromolecules which surround cells and fill the volume of connective tissue.
how are macromolecules produced and organised?
by cells typically of the fibroblast family.
they form point of interaction between the cell and its environment. the ECM can modify cell behaviour such as proliferation, motility and survival.
…
specific ECM macromolecules and their interactions vary drastically between tissues.
how to ECM properties define tissue properties?
calcified ECM forms hard structures such as bone and teeth.
transparent ECM forms the cornea.
rope like fibrous structures provide tensile strength for tendons.
what are fibroblasts responsible for in connective tissues?
fibroblasts are present in most connective tissues. they are responsible for producing ECM macromolecules.
once fibroblasts are secreted, what do they do?
once secreted they shape and remodel macromolecules such as collagen to fulfil specific mechanical purposes, e.g. for tensile strenght.
within certain tissues, specialised cells from fibroblast family are responsible, what are two of these?
chondroblasts in cartilage
osteoblasts in bone.
what are the two classes of macromolecules the ECM consists of?
glycosaminoglycan (GAG) polysaccharide chains
fibrous proteins (e.g. collagen)
how are proteoglycans formed?
GAGs are typically covalently bound to core proteins to form proteoglycans.
what do proteoglycans form?
Proteoglycans form a hydrated gel substance which has fibrous proteins embedded within.
in unbranched polysaccharide chains with repeating disaccharide units, what is there always and what is there usually present?
one unit is always an amino sugar (e.g. N-acetylglucosamine)
one is usually a uronic acid (e.g. glucouronic acid)
as a result of GAGs containing one amino sugar and usually a uronic acid, what can be said about GAGs?
they have sulphate or carboxyl groups on their sugars.
they are highly negatively charged.
they are produced by the cell and typically expelled by exocytosis.
polysaccharide chains do not fold, what do they do instead?
they form extended hydrated gel structures.
what does it mean if the GAGs are low density?
Low density - typically <10% of the ECM weight with the rest being fibrous proteins.
what does it mean if the GAGs are high volume?
High volume - negative charges attract cations (e.g. Na+) along with large amounts of water.
this swelling makes the gels resistive to compressive forces.
what are GAGs replaced with in plants, fungi and insects?
other polysaccharides (e.g. cellulose, chitin).
what is hyaluronan?
Hyaluronan (hyaluronic acid) is the simplest of GAGs and found in all tissues/fluids from embryo to adult.
repeating disaccharide of up to 25,000 repeats.
what are three of the usual features of hyaluronan?
- it has no sulphate sugars
- does not typically bind to any core proteins
- produced at the cell surface rather than via exocytosis.
what is the function of hyaluronan?
functions stem largely from interactions with cell or ECM proteins and also proteoglycans.
what are some specific functions of hyaluronan?
- resists compressive forces (e.g. in joints)
- produced during wound healing
- lubricant within joint fluid
- fills space during embryonic development.
what are proteoglycans?
a form of glycoprotein.
to be a proteoglycan at least one sugar side chain must be a GAG.
all GAGs, except _____, covalently bind with core proteins to form proteoglycans.
hyaluronan.
why is there such a big variability in proteoglycans?
huge variability as single proteins can carry many different GAGs with varied level of sulphate groups and negative charge.
how are proteoglycans assembled?
core proteins (polypeptide chains) are made on ribosomes and threaded into the lumen of the endoplasmic reticulum.
protein and polysaccharide are combined within the Golgi apparatus before exocytosis to the cellular environment.
linkage tetrasacharide forms the bridge between the protein and the GAG.
what is aggrecan?
a specific proteoglycan helping to form cartilage.
massive proteoglycan with other 100 GAG chains.
assembles with hyaluronan in cartilage to form micron size aggregates.
what is decorin?
smaller proteoglycan with 1 GAG chain.
secreted by fibroblasts
regulates collagen fibril assembly
what do the functions of proteoglycans depend on?
many diverse functions depending on interactions with other proteins (e.g. collagen, fibronectin)
what does the mesh and charged densities formed by gels do?
act as a ‘sieve’ - regulated the diffusion of molecules and cells.
….
binding of secreated molecules (e.g. growth factors) controlling their diffusion and cell interactions.
- e.g. heparin sulphate GAG chains bind fibroblast growth factors and stimulate cell proliferation.
- can also bind secreted chemotactic attractants (chemokines)
- single molecules can bind to either the GAG chain or the core protein.
why is elastin needed?
many tissues require the ability to stretch and return to their original shape.
how extensible are elastic fibres within the ECM?
at least 5 times more extensible than a rubber band due to a random coil structure.
what is the main protein of elastic fibres?
elastin
where is elastin secreted where is assembles into a dense crosslinked mesh of fibres and sheets?
elastin in secreted into the extracellular space near cellular membranes where it assembles into a dense crosslinked mesh of fibres and sheets.
how is tearing of tissues prevented?
elastic fibres are counterbalanced by collagen fibres to prevent tearing of tissues.
what is elastin scaffold by during assembly?
elastic microfibrils
Microfibrils include a number of glycoproteins such as fibrillin.
what can mutations in the fibrillin gene lead to?
conditions such as Merfan’s syndrome. in severe cases the aorta has increased chance to rupture.
what is a family of proteins which appears in all multicellular animals. in mammals they constitute 25% of total protein mass?
Collagen
what is collagen secreted by?
cells within the connective tissues (e.g. skin and bone)
what form does collagen take?
long stiff triple helix with three collagen polypeptide α chains.
Helical structure is stabilised by abundant proline and glycine amino acids.
The alignment/organisation of collagen is partly controlled by fibroblasts pulling and exerting forces on the fibres.
how many types of collagen molecule are there?
42 potential α chains allow roughly 40 types of collagen molecule.
what are fibrillar collagens?
Fibrillar collagens (e.g. collagen type I, II, III) are most common and aggregate into long fibrils.
- 10-300 nm diameter, 10s to 100s of microns in length
- Can aggregate further into collagen fibres with diameters of microns
what are examples of collagens with other functions?
Collagens with other functions include types IX and XII which are fibril-associated collagens - linking fibrils together or to other ECM components.
GAGs resist compressive force whilst collagen fibrils resist tensile forces.
what are examples in the body?
in skin a woven structure resists stretch in multiple directions.
in tendons aligned fibrils provide tensional strength along that axis.
in mature bone (and cornea) layers of collagen alternate in direction similar to plywood.
what is the alignment/organisation of collagen fibrils partly controlled by?
fibroblasts pulling and exerting forces on the fibres.
what is fibronectin?
an adhesive glycoprotein found within the ECM that allows cells to attach to the wider ECM.
it is dimer assembled from two very large subunits via disulphide bonds.
what do the specific domains along fibronectin length allow?
- collagen binding
- fibronectin binding
- cell binding - via integrins
- heparin binding - which also allows the binding of growth factors.
what is laminin-1?
a key ECM protein which assembles from three polypeptide chains (α, β and γ) held together via disulphide bonds.
multiple isoforms of the α, β and γ chains exist, what do these allow for?
different assemblies with specific functions within the ECM.
specific domains of laminin allow cells or other ECM proteins to bind.
what can individual laminin heterotrimers assemble with each other to form?
extensive structures such as the basal lamina.
what is the basal lamina?
ECM macromolecules can be organised into sheets or matts providing structural support.
these sheets are thin and flexible (40-120 nm).
what are the key components of the basal lamina?
many molecules form the basal lamina but key components include:
- laminin
- collagen (type IV)
- nidogen
- the protroglucan perlecan
what are the three layers of the basal lamina?
- Lamina lucida(or rara) -an electron-lucent later, adjacent to basal plasma membrane
- Lamina densa-an electron-dense layer, just below
- Lamina (fibro)reticularis -connects basal lamina to underlying connective tissue
in kidneys how is the basal lamina used?
in kidneys a thick basal lamina filters blood macromolecules before passing as urine.
what do basal lamina do?
They can provide a basement membrane for epithelial cells or can surround individual cells (e.g. muscle, fat or Schwann cells).
what are some other functions of the basal lamina?
- determining cell polarity
- influencing cell metabolism
- organising proteins in adjacent plasma membranes
- inducing cell differentiation
- acting as specific highways for cell migration.
how do cells connect with their environment?
through the use of specific adhesion proteins.
this can include cell-cell binding and cell-ECM binding.
for cell-ECM binding this involves the family of integral proteins.
what are hemidesmsomes?
Hemidesmosomes are anchoring junctions which connect the intermediate filaments of the cytoskeleton to the basal lamina.
These attachment complexes provide rigidity to epithelial cell layers by mechanically integrating the call with the basal lamina.
Transmembrane proteins span the cell membrane connecting the cytoskeleton to the ECM. For hemidesmosomes this involves integrin α6β4 or collagen type XVII.
what are integrins?
A family of membrane spanning proteins responsible for cell adhesion to the ECM.
what are integrins comprised of?
Comprised of α and β subunits. These are 8 β-chain variants and 18 α-chain variants providing 24 types of integrin in humans.
different combinations are specialised for binding to specific ECM proteins.
what are three examples?
- Integrin α5β1 binds to fibronectin via it’s RGD (Arginine, Glycine, and Aspartate) motif.
- Integrin α 3 β1 binds to laminin via it’s IKVAV motif.
- Integrin α11 β1 binds to collagen via it’s GFOGER motif.
what are focal adhesions?
They are core components of hemidesmosomes
FAs are clusters of integrins and linkage proteins (e.g. vinculin, talin and paxillin
Signalling proteins such as focal adhesion kinase (FAK) are also recruited to FAs
Integrins mediate cell-matrix interactions including the transfer of mechanical signals to the cell and cytoskeleton.
Stimulate the formation of actin stress fibres as integrins engage ECM ligands
…
ECM distribution controls integrin spacing, cell adhesion, cell spreading, proliferation, survival and other cell functions.
what has microfabrication of synthetic ECM patterns allowed for?
Microfabrication of synthetic ECM patterns has allowed up to study the effects of integrin spacing on cellular function.
…
The pattern of adhesion sites and the mechanical properties of the ECM can stimulate cells and reorganise the tissue.
how do cells typically receive signals?
Generally, cells typically receive signals in chemical form via various signalling molecules.
what happens when a signalling molecule binds with an appropriate cell surface receptor?
When a signalling molecule binds with an appropriate cell surface receptor, the binding triggers a chain of events that carries and amplifies the signal to the cell interior
Cells can also send signalling molecules to other cells over both short and long distances.
In most cases signalling molecules bind to the GAG chains of proteoglycans.
In the ECM, proteoglycans can bind many growth factors and regulate them by doing what?
In the ECM, proteoglycans can bind many growth factors and regulate them by immobilising, blocking or protecting them.
what is chemotaxis?
the process of cell movement in the direction of an extracellular gradient of chemicals.
what are the keys to numerous cellular processes?
- Neutrophils chasing methionine released by bacteria
- Neuronal axons extending towards sources of soluble netrin
- Movement of spermatozoa towards the ovum
what are two diseases altered chemotaxis is associated with?
decreased chemotaxis in cases of AIDS and multiple sclerosis.
increased chemotaxis during inflammation and tumour metastasis.
what are chemokines?
During chemotaxis, cell receptors bind chemical signalling molecules – chemokines.
chemokines are chemoattractant cytokines.
they interact with G-protein-coupled receptors (GPCRs) which regulate cellular signal transduction pathways.
chemokines can be delivered in soluble form or bound to the ECM (haptotaxis)
this receptor activation drives branching actin filaments to assemble the cell’s leading edge to orientate towards the chemokine target.
how are chemokines produced?
Most are produced under pathological conditions by infiltrating leukocytes or cells endogenous to the affected tissue.
how are some chemokines involved in ‘housekeeping functions’?
- Maturation of leukocytes in bone marrow
- Trafficking and homing of lymphocytes
- Regeneration of circulating leukocytes
what are inflammatory cytokines?
alert the immune system to the presence of potential infection or danger
what are homeostatic cytokines?
instruct the homeostasis and function of immune cells.
