L13: Extracellular matrix Flashcards
Overview of ECM components?
Extracellular Matrix Components and Their Roles:
Basement Membrane/Basal Lamina (Laminins):
Primarily found in epithelial and endothelial tissues.
Helps define tissue polarity and serves as a barrier to macromolecules, bacteria, and other harmful entities.
Collagens:
Most abundant protein in the body, and ubiquitous across tissues.
Forms fibers that provide tensile strength to tissues.
Fibronectin (Tissue and Plasma Fibronectin):
Abundant in the extracellular matrix.
Forms fibers and is flexible, playing a role in tissue structure and cell adhesion.
Elastin:
Forms fibers that provide elasticity within tissues, allowing them to stretch and return to their original shape.
Proteoglycans/Glycosaminoglycans (GAGs):
Provide hydration and protect against compression (e.g., in joints).
Assist in packing and maintaining the structure of tissues.
All of these components work co-operatively in organism development, and contribute to cell adhesion, polarity, proliferation, differentiation, and migration.
Basement membrane?
basement membrane: The basement membrane is made up of two sublayers (as seen at the electron micrograph level):
Basal Laminae (nearest epithelial cells):
~120 nm thick.
Composed mainly of laminin and collagen IV.
Laminin connects to itself, forming a network.
Collagen IV also forms a network and is integral to the structure.
Reticular Laminae (nearest connective tissue):
Connects the basal laminae to connective tissue.
Primarily made up of collagen I.
Additional Smaller Proteins:
Perlican and nidogen:
These proteins connect to both cytosolic collagen and laminin, facilitating interaction between the components of the extracellular matrix.
Integrin Receptors:
Amino terminal parts of integrin receptors are involved in linking the extracellular matrix to the cell cytoskeleton, playing an important role in cell adhesion and signaling.
laminins?
Laminins:
Primary Component of Basal Laminae: Laminins are the main proteins that make up the basal laminae, a key part of the basement membrane.
Structure:
Cross-shaped trimeric glycoprotein consisting of three polypeptides: alpha, beta, and gamma.
Heterotrimeric structure formed by the combination of these polypeptides.
Laminins have 5 alpha, 4 beta, and 3 gamma isoforms, which combine to form 15 different types.
Size: Laminins can be as large as ~900 kDa.
Binding Domains:
Laminins have specific binding sites for:
Nidogen
Perlecan
Cell surface receptors such as integrins and dystroglycan (which binds to the C-terminus of laminin I heterodimer).
Functional Structure:
The “short” arms of the cross-shaped structure bind other laminins, contributing to the formation of a sheet-like network.
Role in Tissue Integrity:
Laminins are essential for the structural integrity of epithelial and endothelial tissues. Their organization in the basal lamina supports the tissue’s stability and functionality.
collagens?
Ubiquitous: makes up to half of body protein by weight
Provides tensile strength and elasticity in:
Tendons, Skin, Cartilage, Bone
3 polypeptide () chains formed of left handed helix, which forms
right-handed fibres
Enriched in hydroxy-proline and hydroxy-lysine. Hydroxy- covalent posttrans modification
Different (vertebrate) collagens by different combinations of -
chains. E.g.:
Type I, II, III = main fibres, flexible
Type I = bone, skin, tendons (90% of all collagen)
Type II = cartilage
Type IV = basement membrane
fibroblasts and collagen in connective tissues?
Fibroblsts and collagen in connective tissues
Fiboblasts embedded in dense collagen meshwork. Migrate through collagen, pulling so see collagen moving. Helps immune system sense movement of cells in the body.
Fibril-forming: type 1. polymerised form: fibril. tissue distribution: bone, skin, tendon, ligaments, cornea, internal organs (account for 90% of body collagen)
fibril-associated: type IX. polymerised form: lateral association. tissue distribution cartilage
network forming: type IV. polymerised form: sheet-like network. tissue distribution: basal laminae.
type V: polymerised form- fibril (with type 1). tissue distribution: as for type 1.
type 2: polymerised form- fibril. tissue distribution: cartilage, intervertebral disc, notochord, vitreous humor of eye
type 3: polymerised form- fibril. tissue distribution: skin, blood vessels, internal organs.
type XI: polymerised form- fibril (with type 2). tissue distribution: as for type 2.
fibronectin (FN)
Fibronectin (FN) Types:
Soluble (plasma) Fibronectin: Found in blood plasma, secreted as a soluble protein dimer (~230 kDa monomers linked by disulfide bonds, ~60 nm long, ~2 nm thick).
Insoluble Fibronectin: Assembled into fibrillar extracellular matrix (ECM) at the cell membrane. This form is involved in various tissue structures and has important roles in cell adhesion.
Properties of Insoluble Fibronectin:
Abundance: Insoluble fibronectin is most abundant in tissues like the heart, lungs, and liver.
Secretion: Primarily made by fibroblasts, epithelial cells, and hepatocytes (for plasma).
Assembly: The soluble form of fibronectin can undergo mechanical unfolding when cells generate forces, exposing new assembly sites for fibrillogenesis (the formation of fibrils). This process contributes to ECM assembly.
Binding: Insoluble fibronectin can bind to collagens, fibrin, and proteoglycans, helping to structure the ECM.
Fibroblast Movement and Metastasis:
Fibroblasts use mechanical forces to move through the collagen matrix, a process that involves dynamic interaction with the ECM.
In cancer, the fibronectin matrix and fibroblasts undergo changes that can facilitate metastasis. This transition involves a switch from a fibroblast phenotype to an ameboid phenotype, allowing the cells to more easily invade tissues and spread (metastasis).
The forces generated by fibroblasts distorting the collagen matrix are important in processes like tissue remodeling, wound healing, and cancer progression.
collagen synthesis?
Endoplasmic Reticulum:
ribosomes attached to ER; form rough er
protein synthesized at surface into ER lumen;
co-translational and post-translational
modifications (hydroxylation);
3 proto-alpha-chains form fibres (triple helix) -
soluble procollagen moved to Golgi
apparatus
Golgi Apparatus:
Soluble pro collagen
packed into secretion vesicles;
fuse with plasma membrane
Pro collagen secreted and protease cleavage in c termini prod tropocollagen
Outside Cell
Procollagen processed by enzymes outside
cell (eg: Procollagen C proteinase);
Tropocollagen self-assemble into collagen
fibrils and crosslinked (eg: lysyl oxidase)
collagens in disease?
Fibrosis:
eg: lung, liver, skin. Characterised by excessive deposition of Collagens I or III
usually by fibroblasts. Can result in loss of tissue function - eg: in lung loss of
oxygen exchange due to decreased mechanical response
Scurvy:
Lack of Vitamin C (ascorbic acid) - essential co-factor for hydroxylases which add
hydroxyl groups to proline & lysine for stable triple-helix formation. Vitamin C
deficiency leads to formation of unstable Procollagen - vessels, tendons and skin
are fragile
Osteogenesis imperfecta (brittle bone disease):
Autosomal dominant - mutations in genes encoding 1(1) or 2(1) chains (Type
I collagen). Glycine mutated (critical to triple helix formation) resulting in helices
that are poorly formed and unstable.
fibronectin’s structure + mechanical force unfolding model?
Fibronectin and Its Structure:
Fibronectin is a highly modular glycoprotein involved in ECM assembly, tissue repair, and cell adhesion. It has two major domains:
Type 1 (Yellow Domain):
Function: Responsible for binding to other fibronectin molecules, fibrin, and contributing to the assembly of fibrillar structures (important for ECM organization).
Type 2 (Blue Domain):
Function: Binds to collagen, helping to connect fibronectin with other components of the ECM.
Additional Binding Sites:
Fibronectin also has several important binding motifs for other molecules:
Heparin and Syndecan Binding: Heparin and syndecan (a type of proteoglycan) binding sites are present in fibronectin, which are crucial for its interactions with other ECM components.
Integrin Receptor Binding Motif: This allows fibronectin to interact with integrin receptors on the surface of cells, facilitating cell adhesion and signaling.
Fibrin Binding: A binding site for fibrin, which is important for clot formation and tissue repair.
Disulfide Bridges and Unfolding Mechanism:
Cysteine Residues: Fibronectin molecules are linked by disulfide bridges between two cysteine residues. This is essential for the stability of the fibronectin dimer.
Unfolding Model:
Globular Form (No Force): When there is no mechanical force applied, fibronectin exists in a compact, globular form. This is a stable, folded state.
Fibrillar Formation (Under Force): As fibronectin begins to assemble into fibrillar structures, it experiences mechanical force. When the tissue is strained or subjected to mechanical stress, fibronectin partially unfolds, stretching its protein domains. This unfolding helps to generate the elasticity required for the tissue to adapt to stress and strain.
Elasticity: As the tissue is stretched, the fibronectin molecules unfold, which allows the tissue to be more elastic. When the force is relieved (i.e., during compression relief), fibronectin returns to a more extended, backfolded state, ready to handle future mechanical stresses.
Fibronectin’s Role in ECM and Cell Interaction:
Fibronectin’s ability to undergo mechanical unfolding and folding is crucial for the formation of ECM networks and its ability to provide flexibility and elasticity to tissues. It serves as a connectivity hub in the ECM, linking various proteins, cells, and other matrix components, which is essential for maintaining tissue integrity and function.
This mechanistic understanding of fibronectin helps explain its role in processes such as tissue remodeling, wound healing, and cancer metastasis, where ECM deformation and the interaction with fibroblasts and other cells are critical.
fibronectin functions?
Essential for mammalian development and embryogenesis -
required during neuritogenesis, vascular development and cell
migration – signalling molecule and scaffold
Required for efficient wound healing response - high levels of
plasma FN found at wound sites that act to cluster platelets and
recruit fibroblasts to the site. Fibroblasts then assemble new
matrix to close the wound
FN levels often seen to be increased in tumours - proposed to act
to promote cell survival, resistance to apoptosis and invasion.
