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what is the ecm?
The extracellular matrix (ECM) is the acellular component found throughout all tissues and organs. It not only provides critical structural scaffolding for resident cells but also delivers biochemical and biomechanical signals that regulate morphogenesis, cellular differentiation, and tissue homeostasis.
The extracellular matrix (ECM) is a complex 3-dimensional (3D) meshwork of proteins that provides structural support to cells, tissues, organs, and organisms [1]. In addition, the ECM provides physical, mechanical, and chemical cues that regulate proliferation, migration, differentiation, and function during development, tissue morphogenesis, and homeostasis [2]. Thus, alterations in ECM composition results in the perturbation of its overall architectural properties and associated signaling functions, leading to various pathologies including developmental defects, fibrosis, and cancer [3, 4, 5].
The basement membrane
The basement membrane is a specialized structure within the extracellular matrix that provides both mechanical support and functional connectivity between epithelial tissues and the underlying connective tissue. It is composed of two distinct sublayers, distinguishable at the level of electron microscopy: the basal lamina and the reticular lamina.
The basal lamina, which lies closest to the epithelial cells, is approximately 120 nm thick and consists primarily of laminin and collagen type IV. Laminin molecules self-associate to form a network, while collagen IV forms a separate, yet structurally critical, network that contributes to the overall integrity of the basement membrane. These two networks are essential for maintaining the selective permeability and organization of epithelial layers.
Beneath the basal lamina is the reticular lamina, which anchors the epithelial layer to the underlying connective tissue. This layer is mainly composed of collagen type I, a fibrillar collagen that integrates with the extracellular matrix of the connective tissue, reinforcing structural cohesion between tissue compartments.
In addition to these primary components, smaller yet functionally important molecules such as perlecan and nidogen are present within the basement membrane. These proteins serve as molecular bridges, binding to both laminin and collagen IV, and thereby enhancing the connectivity and stability of the matrix.
Furthermore, integrin receptors play a key role in linking the basement membrane to the intracellular environment. Specifically, the amino-terminal domains of integrins bind to components of the extracellular matrix, facilitating cell adhesion and contributing to signal transduction pathways that influence cell behavior, including migration, proliferation, and differentiation.
laminins
Laminins are a primary structural component of the basal lamina, a crucial sublayer of the basement membrane. These large, cross-shaped trimeric glycoproteins play a central role in organizing the extracellular matrix and maintaining tissue integrity, particularly in epithelial and endothelial tissues.
Structurally, laminins are heterotrimers, composed of three distinct polypeptide chains: α (alpha), β (beta), and γ (gamma). The diversity of laminin molecules arises from the existence of five alpha, four beta, and three gamma isoforms, which can combine to form at least 15 different laminin types. Individual laminin molecules can be quite large, with a molecular weight approaching 900 kDa.
Laminins contain multiple binding domains that allow them to interact with various components of the extracellular matrix. These include specific sites for nidogen and perlecan, as well as for cell surface receptors such as integrins and dystroglycan. The latter binds specifically to the C-terminal region of the laminin-1 heterotrimer, facilitating anchorage to the cell surface and linking the basement membrane to the cytoskeleton.
Functionally, the “short arms” of the laminin’s cross-shaped structure mediate lateral interactions with other laminin molecules, promoting the assembly of a sheet-like network that is characteristic of the basal lamina. This network provides not only mechanical support but also contributes to signaling pathways that regulate cell adhesion, migration, and differentiation.
collagen
Collagen is a ubiquitous structural protein, constituting up to 50% of the total protein content in the human body by weight. It is critical for providing tensile strength and elasticity to various connective tissues, including tendons, skin, cartilage, and bone.
Structurally, collagen is composed of three polypeptide chains, each forming a left-handed helix. These chains intertwine to form a right-handed triple helix, giving collagen its characteristic strength and stability. The protein is particularly enriched in the amino acids hydroxyproline and hydroxylysine, which result from post-translational hydroxylation—a crucial modification that enhances helix stability.
There are multiple types of collagen, each with distinct roles depending on their polypeptide chain composition. Among vertebrates, the most abundant are Type I, II, and III. These types primarily form flexible fibres. Type I collagen, which comprises around 90% of all collagen, is dominant in bone, skin, and tendons. Type II is mainly found in cartilage, while Type IV is a key structural component of the basement membrane, forming a more sheet-like network.
The biosynthesis of collagen begins in the rough endoplasmic reticulum (ER), where ribosomes attached to the ER membrane synthesize the initial polypeptide chains. These chains are translocated into the ER lumen, where they undergo co-translational and post-translational modifications, such as hydroxylation of proline and lysine residues. The modified proto-α-chains then associate to form soluble procollagen triple helices.
Procollagen is transported to the Golgi apparatus, where it is packaged into secretory vesicles. These vesicles fuse with the plasma membrane, releasing the procollagen into the extracellular space. Outside the cell, procollagen is cleaved by specific enzymes, such as procollagen C proteinase, at its terminal regions to form tropocollagen. Tropocollagen molecules then self-assemble into collagen fibrils, which are subsequently stabilized through enzymatic cross-linking, for example by lysyl oxidase. This process is essential for forming the robust collagen fibres that maintain tissue integrity.
fibronectin
Fibronectin is a highly modular glycoprotein that plays a crucial role in extracellular matrix (ECM) assembly, tissue repair, and cell adhesion. It is composed of multiple domains that enable it to interact with various ECM components and cellular receptors, making it vital for tissue organization and function.
Structure of Fibronectin
Fibronectin consists of two major domains:
Type 1 (Yellow Domain): This domain is primarily responsible for binding to other fibronectin molecules, fibrin, and contributing to the assembly of fibrillar structures. These interactions are essential for the organization of the ECM.
Type 2 (Blue Domain): This domain binds to collagen, which helps connect fibronectin with other ECM components, contributing to the structural integrity of tissues.
Additional Binding Sites
Fibronectin also contains several other important binding motifs that facilitate its interactions with various molecules:
Heparin and Syndecan Binding: Fibronectin has binding sites for heparin and syndecan (a proteoglycan). These interactions are vital for the protein’s function in ECM organization.
Integrin Receptor Binding Motif: Fibronectin interacts with integrin receptors on the surface of cells, which is crucial for cell adhesion and signaling.
Fibrin Binding: A site for fibrin binding enables fibronectin’s involvement in clot formation and tissue repair.
Disulfide Bridges and Unfolding Mechanism
Fibronectin molecules are held together by disulfide bridges between cysteine residues, which contribute to the stability of the fibronectin dimer. This is essential for maintaining its structure.
The unfolding mechanism of fibronectin is particularly interesting, as it allows the protein to adapt to mechanical stress:
Globular Form (No Force): In the absence of mechanical force, fibronectin exists in a compact, globular form, which is stable and folded.
Fibrillar Formation (Under Force): When fibronectin assembles into fibrillar structures under mechanical stress, its protein domains partially unfold. This unfolding helps generate elasticity, allowing the tissue to stretch and adapt to force. When the mechanical stress is relieved, fibronectin returns to a more extended, backfolded state, ready to handle future forces.
This ability to unfold and refold is crucial for maintaining the elasticity and flexibility of tissues under mechanical strain.
Fibronectin’s Role in ECM and Cell Interaction
Fibronectin’s structural and mechanical properties are key to its role in ECM formation and its ability to provide flexibility and elasticity to tissues. As a connectivity hub, fibronectin links various proteins, cells, and other matrix components, thereby playing a vital role in maintaining tissue integrity and function. Its dynamic unfolding and folding mechanism also make it an essential protein in processes such as tissue remodeling, wound healing, and cancer metastasis, where ECM deformation and interactions with cells like fibroblasts are critical.
Fibronectin Functions
Fibronectin is essential for several biological processes:
Embryogenesis and Development: It is required for neuritogenesis, vascular development, and cell migration, acting both as a signaling molecule and as a scaffold for cells.
Wound Healing: High levels of fibronectin are found at wound sites, where it helps to cluster platelets and recruit fibroblasts, which in turn assemble a new matrix to close the wound.
Tumor Progression: In tumors, fibronectin levels are often elevated, where it is thought to promote cell survival, resistance to apoptosis, and invasion. This makes fibronectin a key player in cancer metastasis.
Types of Fibronectin
Fibronectin exists in two primary forms:
Soluble (Plasma) Fibronectin: Found in blood plasma, this soluble protein exists as a dimer (~230 kDa) linked by disulfide bonds and is secreted into the bloodstream by various cells.
Insoluble Fibronectin: This form assembles into fibrillar ECM structures at the cell membrane. It is involved in maintaining the structure of various tissues and is particularly abundant in organs like the heart, lungs, and liver.
Properties of Insoluble Fibronectin
Abundance: Insoluble fibronectin is abundant in tissues like the heart, lungs, and liver.
Secretion: It is mainly secreted by fibroblasts, epithelial cells, and hepatocytes.
Assembly: The soluble form of fibronectin can undergo mechanical unfolding when cells generate forces, exposing new sites for fibrillogenesis, the formation of fibrils that contribute to ECM structure.
Fibroblast Movement and Cancer Metastasis
Fibroblasts play an important role in moving through the collagen matrix by generating mechanical forces that interact dynamically with the ECM. In cancer, the fibronectin matrix and the behavior of fibroblasts change to facilitate metastasis. Fibroblasts undergo a phenotypic switch to an ameboid phenotype, enabling them to invade tissues and spread throughout the body, contributing to cancer progression. The forces exerted by fibroblasts on the ECM are critical in processes such as tissue remodeling, wound healing, and cancer metastasis.
gags and pgs
GAGs and Proteoglycans in Connective Tissue
Connective tissue contains five main types of glycosaminoglycans (GAGs). The most important one is hyaluronan. It has a high molecular weight (over 1,000 kDa) and can be up to 10 μm long. It has no protein core and is made at the plasma membrane. Because it’s highly negatively charged, it attracts water and helps with hydration. It forms a backbone for other GAGs to attach. It’s found in synovial fluid, vitreous humor, and loose connective tissue. It also binds to adhesion receptors and helps absorb shock.
The other four GAGs attach to protein cores to form proteoglycans:
Chondroitin sulfate: found in cartilage, bone, and heart valves; associates with aggrecan and versican.
Dermatan sulfate: found in skin, blood vessels, heart valves; binds decorin, biglycan, versican.
Keratan sulfate: found in cornea, bone, cartilage; partners with lumican and aggrecan.
Heparan sulfate: found in basement membranes and cell surfaces; associates with perlecan and agrin.
Heparin, similar to heparan sulfate, is highly sulfated and stored in mast cells in places like the lungs and skin. It can act as an anticoagulant and possibly help fight infections.
Functions of GAGs and proteoglycans:
Trap water and cations
Resist compression and help tissues retain shape
Fill space and prevent bones from rubbing together
Form networks by linking to collagen
Help with signaling and binding cytokines
Found in synovial fluid to lubricate joints
Combine with calcium salts in bone
Proteoglycans are made in the Golgi (except hyaluronan), where GAG chains are added to the protein core. Despite being only 10% of ECM by weight, they take up a lot of space due to water absorption. Their structure is made of repeating disaccharides like glucuronic acid and N-acetylglucosamine. The chains are unbranched and attach to serine on the protein core via a special tetrasaccharide linker (xylose–galactose–galactose–glucuronic acid).
elastin
Tropoelastin is the soluble precursor of elastin, the protein that gives tissues elasticity. It is about 65 kDa and is made from the elastin gene. During translation, some proline residues are hydroxylated, which helps it assemble properly. Once made, it is secreted and cut by tropoelastin protease into smaller filaments.
These filaments assemble into microfibrils, which are the early form of elastin fibers. Then, lysyl oxidase cross-links the tropoelastin molecules, forming mature, insoluble elastin. This final form is stretchy, durable, and resists deformation.
Functions of elastin:
Provides stretch and recoil to tissues
Helps tissues maintain shape after deformation
Works with collagen to give both strength and flexibility
Where elastin is found:
Lungs (helps with breathing by expanding and recoiling)
Skin (gives flexibility)
Large blood vessels like the aorta (about 50% of its dry weight is elastin, helping with pressure changes)
Elastin is essential in tissues that repeatedly stretch and return to shape, making it crucial for normal function in the lungs, arteries, and skin.
localised degradation of ecm components
Localized Degradation of ECM Components
ECM degradation is essential for processes like wound healing, tissue remodeling, and cell migration. It allows cells to move, clears damaged matrix proteins, and reshapes the ECM.
Matrix Metalloproteinases (MMPs) are key enzymes that break down ECM proteins. They need calcium or zinc ions to function and are made as inactive zymogens, which become active after the pro-peptide domain (part of a “cysteine switch”) is removed. This domain blocks the active site by binding zinc. MMPs can be soluble or membrane-bound; there are ~25 types.
MMP-2, also called gelatinase, breaks down gelatin—a denatured form of collagen—not involved in coagulation.
Serine Proteases are another group of ECM-degrading enzymes. They use a reactive serine residue in their active site to cleave proteins. Around 14 subtypes exist, and they can degrade proteins like elastin.
Since ECM degradation must be tightly controlled, protease inhibitors are crucial.
TIMPs (Tissue Inhibitors of Metalloproteinases) block MMPs.
SERPINS (Serine Protease Inhibitors) block serine proteases.
These inhibitors maintain ECM balance, prevent over-degradation, and protect against tissue damage. Dysregulation of MMPs or proteases can lead to diseases like cancer metastasis, rheumatoid arthritis, or osteoarthritis.
