L13: Extracellular matrix part 2 Flashcards
Elastin?
tropoelastin and Elastin in the ECM
Tropoelastin:
Precursor: Tropoelastin is the soluble precursor of elastin, the protein responsible for the elasticity of tissues.
Synthesis: It is synthesized through transcription of the elastin gene and translated into a soluble monomer (~65 kDa).
Post-Translational Modifications: Tropoelastin undergoes hydroxylation on proline residues during translation, which is crucial for its assembly and function.
Secretion and Formation: After translation, tropoelastin is secreted by cells and further processed by tropoelastin protease to form smaller tropoelastin filaments.
Elastin Assembly and Cross-Linking:
Microfibrils: Tropoelastin assembles into microfibrils, which are initial structures that contribute to the formation of mature elastin fibers.
Cross-Linking: The microfibrils are then acted upon by lysyl oxidase, an enzyme that facilitates the cross-linking of tropoelastin molecules, leading to the formation of insoluble elastin fibers.
Mature Elastin: The cross-linked elastin fibers form a highly durable and flexible matrix that is resistant to deformation.
Functions of Elastin:
Structural Integrity and Flexibility: Elastin, in combination with collagen, provides structural integrity to tissues, while also enabling the reversible changes in the matrix. This allows tissues to stretch and return to their original shape.
Extensibility and Deformability: Elastin allows tissues to be highly extensible and deformable, which is important for tissues that undergo repetitive stretching and relaxation, such as the lungs and blood vessels.
Key Tissue Locations:
Elastin is highly expressed in tissues that require significant flexibility and resilience, including:
Lungs
Skin
Major blood vessels (e.g., aorta, where elastin forms about 50% of the dry weight)
The ability of elastin to stretch and recoil is critical for the proper function of these tissues, such as the expansion and contraction of the lungs during breathing or the elasticity of the arteries as blood is pumped through them.
GAGs and proteoglycans?
Proteoglycans are heavily
glycosylated protein cores.
Form a ‘gel’ that acts to:
-link to fibrillar ECM network
-bind soluble factors (eg: cytokines)
-resist pressure and lubricate e.g: joins and synovial fluid?
-act as signalling molecules for cells
GAG disaccharides (form glyosylated part?) are added in Golgi
to protein cores to form
proteoglycans - exported in secretory
vesicles (exception is Hyaluronan
which has no protein core and is
‘built’ at the plasma membrane)
Occupy 10% of ECM by weight but fill
large volume - due to its ability to absorb lots of water
Unbranched polysaccharide chains
with disaccharide subunits
Repeasting disacharide subunit- glucuronic acid and N-acetylglucosamine
Can be at various lengths
Attached to serine residues in the core protein via this link in the tetrasacharide consisting of xylose, galactose, , galactose and glucuronic acid
glycosaminoglycans (GAGs)
5 main types:
Hyaluronan : main GAG in connective tissue
- high molecular weight chains (up to ~ >1,000 kDa), length ~ 2.5-10 μm
- “backbone” for the assembly of other GAGs, no protein core, highly
negatively charged - effective in hydration. Binds adhesion receptors.
- major component of the synovial fluid (joint cavities) and vitreous body
of eye.
Other 4 major GAGs attach through protein cores to form proteoglycans:
- chondroitin sulphate (associates with eg: aggrecan, versican)
- dermatan sulphate (associates with eg: decorin, versican, biglycan)
- keratan sulphate (associates with eg: lumican, aggrecan)
- heparan sulphate (associates with eg: perlecan, agrin)
Complicated network of interactions between proteins to provide strength and elasticity- is what he said
Functions:
- trap water and cations
- resist compression and retain shape
- occupy space - to keep bones apart so they dont rub against eachother?
- link to collagen fibers to form network
- combined with calcium hydroxyapatite and carbonate in bones
ECM turnover?
Localized Degradation of ECM Components:
During processes like wound healing, tissue remodeling, or cell migration, there is often a need for the breakdown of extracellular matrix (ECM) components. This is necessary for cell movement and to replace damaged ECM proteins that result from stretching and compression forces.
Matrix Metalloproteinases (MMPs):
MMPs are enzymes that degrade ECM proteins, allowing for remodeling.
They depend on Ca²⁺ or Zn²⁺ ions for their activity.
The MMP family consists of about 25 members, which can be either soluble or membrane-associated.
MMP-2 (gelatinase) is one of the most widely distributed MMPs and is involved in breaking down gelatin (a denatured form of collagen).
Clarification on Gelatin:
Gelatin is the breakdown product of collagen, not coagulation. It’s a form of denatured collagen that results when collagen is broken down.
While MMP-2 does degrade gelatin, it is not a coagulation enzyme. Coagulation refers to the process of blood clotting, which involves different enzymes and pathways.
Serine Proteases:
Serine Proteases are enzymes that have a highly reactive serine residue in their active site, which allows them to cleave peptide bonds in proteins.
There are about 14 different subtypes of serine proteases.
Elastin is one of the proteins that can be cleaved by serine proteases.
Protease Inhibitors:
Proteases need to be regulated by inhibitors to ensure proper balance in ECM remodeling. If not regulated, excessive degradation of ECM components can cause tissue damage or diseases like cancer metastasis.
Tissue Inhibitors of Metalloproteinases (TIMPs):
TIMPs are specific inhibitors of MMPs, helping to regulate MMP activity and prevent excessive ECM degradation.
Serine Protease Inhibitors (SERPINS):
SERPINS inhibit serine proteases, regulating their activity.
Role of Protease Inhibitors:
These inhibitors work together to maintain the architecture of the ECM and ensure the local levels of key ECM proteins (like collagen, fibronectin, and laminin) are controlled.
This regulation is crucial for cell migration, which is essential in development (e.g., tissue formation, wound healing) and disease (e.g., cancer metastasis, tissue repair).
ECM degradation?
MMPs
Initially synthesized as inactive zymogens with a pro-peptide
domain that must be removed before the enzyme is active. The
pro-peptide domain forms part of the “cysteine switch”:
conserved cysteine residue that interacts with the zinc in the
active site and prevents binding and cleavage of the substrate,
keeping the enzyme in an inactive form
Can be dysregulated in diseases eg: metastasis, rheumatoid and
osteo-arthritis
cell ECM adhesion molecules?
- Cell interactions with ECM proteins are essential in
numerous contexts. - Mediated by different classes of adhesion molecules:
integrins, cadherins, selectins, Ig superfamily (CAMs)
Integrins- bind to ecm. Heterophillic interactions, protein binding to is diff from receptor itself.
Integrins can bind to other cell surface receptors of ig superfam, heterophillic with vcam-1 for instance on endothelial surface
Homophillic interactions such as N- cadherins and PECAM-1
characteristics of GAGs/PGs?
GAG: hyaluronan: localisation- synovial fluid, vitreous humor, ecm of loose connective tissue. non sulphated, large polymers, hydrating, shock absorbing.
chondroitin sulfate: localisation- cartilage, bone, heart valves. most abundance GAG
heparan sulfate: localisation- basement membranes, components of cell surface. conains higher acetylated glucosamine than heparin, major anticoagulant
heparin: localisation- component of intracellular granules of mast cells lining the arteries of the lungs, liver and skin. highly sulphate, potentially acts as an anti-infective
dermatan sulfate: localisation- skin, blood vessels, heart valves
keratan sulfate: localisation- cornea, bone, cartilage aggregated with chondroitin sulfates
