10 Extracellular Matrix, Cell Adhesion, and Cell Fate

Extracellular Matrix (ECM) Components and Functions

  • Fibronectin

    • Large, dimeric adhesion protein found in the extracellular matrix of many tissues.

    • Crucial during development for cell migration (e.g., germ cells to gonads, heart cells to midline, somite/neural crest cells, somatogenesis).

    • Contains the tripeptide RGD (Arginine-Glycine-Aspartate).

  • Integrins

    • Transmembrane proteins that act as receptors for ECM components.

    • They consist of two subunits: an α\alpha subunit and a β\beta subunit. There are many types, leading to numerous combinations (2424 α\alpha units and 99 β\beta units in mammals, resulting in 24×924 \times 9 possible integrins).

    • Possess an RGD-binding domain on their extracellular side, allowing them to bind to fibronectin, collagen, and laminin (which also contain RGD motifs).

    • Function as attachment sites to the ECM.

    • Intracellular Signaling: Integrins transduce extracellular changes into intracellular signals by interacting with intracellular proteins, often actin-binding proteins like α\alpha-actinin, connecting the ECM to the cytoskeleton (actin).

    • Signaling Pathways: Involved in various signaling pathways, including PI, FAK, Pax, and SHC pathways.

    • Anoikis: A type of programmed cell death (apoptosis) that occurs when epithelial cells lose their proper integrin-mediated attachments to the ECM. This is essentially "death by detachment."

  • Collagen & Laminin

    • Other important ECM proteins.

    • There are six types of collagen; Type IV collagen is particularly important.

    • Basal Lamina: A specialized subtype of ECM formed by the combined interaction of laminin and Type IV collagen. It creates a sheet-like structure that underlies epithelial tissues, providing a stable substrate for cells to reside on.

The Impact of ECM on Cell Fate: Mammary Gland Experiment

This experiment demonstrates the profound influence of the ECM on cell behavior and gene expression.

  • Scenario 1: Cells cultured on plastic

    • Mammary gland tissue cells from mice are isolated and cultured on plastic.

    • Morphology: They maintain a nice epithelial, single-layer (monolayer) structure.

    • Gene Expression: Genes associated with cell division like GTEC, C MYC, and Cylin b1 are turned on, leading to proliferation.

    • Functionality: However, genes for milk components (e.g., lactoferrin, casein, whey protein) are turned off. The cells are identifiable as mammary gland cells by origin but fail to perform their key function.

  • Scenario 2: Cells cultured on ECM (Basal Lamina)

    • The same mammary gland cells are transferred to a plastic surface coated with extracellular matrix, specifically a basal lamina (containing Type IV collagen and laminin).

    • Gene Expression Shifts:

      • Cell cycle genes (c MYC, cyclin genes) are downregulated.

      • Regulatory genes like p21 (a cell cycle brake) and lactoferrin (involved in milk production) are turned on.

      • Integrin genes are also turned on, promoting stronger interaction with the applied ECM.

    • Morphological Changes: The cells change from a flat monolayer to a three-dimensional colonial structure, stacking on top of each other.

    • Self-Organization: Over time, these cell colonies self-organize, forming tight junctions (expressing proteins like ZO-1), and the ECM itself is pulled around to cover the cells, not just remain at the base.

    • Duct Formation: The cells continue to organize until they form duct-like structures, replicating their in-vivo function of producing and releasing milk.

    • Important Note: These cells stop dividing after forming the ducts. Proliferation is halted, demonstrating a shift from growth to differentiation and function.

  • Conclusion: The presence of the basal lamina ECM alone is sufficient to induce significant changes in gene expression, morphology, and restore the functional phenotype of mammary gland cells, highlighting the ECM's critical role in cell fate regulation.

Epithelial-Mesenchymal Transition (EMT)

EMT is a fundamental biological process where epithelial cells lose their cell-cell adhesions and apical-basal polarity, become more motile, and acquire a mesenchymal phenotype.

  • Process:

    1. Loss of Adhesions: Cell-cell adhesion junctions (e.g., adherens junctions) break down.

    2. ECM Degradation: Cells release enzymes called MMPs (Matrix MetalloProteinases) through their basal membrane. These proteases break down the surrounding extracellular matrix, creating pathways for escape.

    3. Detachment & Reprogramming: The cell detaches from its neighbors and the ECM. Loss of integrin-ECM and adherens junction signaling leads to a complete change in gene expression.

    4. Phenotype Shift: The epithelial cell transforms into a mesenchymal cell, characterized by increased motility and invasive potential.

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