Comprehensive Study Notes on Cell Adhesion and the Matrix

Introduction to Cell Adhesion

Cell adhesion is a fundamental biological process defined as the ability of cells to adhere to one another or to their environment. This process is critical for the organization of multicellular structures including tissues and organs.
Key functions and importance of cell adhesion include: - Enabling populations of cells to stick together to form coherent tissues. - Facilitating cell migration to ensure tissues form in the correct anatomical locations. - Governing how organs are constructed from different tissue types. - Determining the specific locations where organs form within an organism.
Modes of cell adhesion are broadly categorized into two types: - Junctional adhesion: Involves specialized, structured complexes (junctions) between cells. - Non-junctional adhesion: Involves less structured or transient interactions.
Adhesion interactions can be further distinguished by the target: - Cell-to-extracellular matrix (Cell-ECM) interactions. - Cell-to-cell (Cell-Cell) interactions.
Cell adhesion serves as a critical component of cell communication and tissue formation.

The Extracellular Matrix (ECM)

The Extracellular Matrix (ECM) consists of macromolecules secreted by cells into their immediate environment, known as the extracellular space.
Characterization of the ECM: - It forms a region of non-cellular material located in the interstices (spaces) between cells. - It is produced, modified, and physically oriented by the cells residing within it. - Glycoproteins are the primary molecules responsible for organizing the matrix and the cells into ordered, functional structures.
Primary Composition of the ECM: - Collagen: Plays a central role in assembling the ECM, promoting cell adhesion and growth, altering cell shape, and permitting cell migration. - Fibronectin: A very large adhesive molecule that acts as a linker. It connects cells to one another and links them to collagen and proteoglycans. Interaction with fibronectin provides proper alignment of cells within the ECM. - Laminin: A major component of the basal lamina. It facilitates strong adhesion of cells to the basal lamina.

Cell-ECM Attachment Dynamics

Attachment behavior and function vary depending on the cell type and tissue context: - Blood Vessel Epithelium: Attachments are strong and stable, serving to form a robust barrier. - Cell Migration: Attachments are transient, characterized by being rapidly made and broken to allow movement.
Biological Signaling: Cell-to-ECM attachment provides essential signals for various processes, such as cell movement and proliferation during the development and formation of tissues.

Integrins: The Primary ECM Receptors

Integrins are membrane receptors that enable a cell to bind to adhesive glycoproteins in the ECM.
Historical Context and Discovery: - In $1985$ , Chen et al. and Knudsen et al. used antibodies to fibronectin to identify a fibronectin receptor complex. - This complex was found to bind fibronectin on the outside of the cell and the actin cytoskeleton on the inside. - In $1986$ , Horwitz et al. and Tamkum et al. named this family of receptors "Integrins."
Structural Binding Properties: - Outside the cell: Integrins bind to the Arginine-Glycine-Aspartate ( $RGD$ ) sequence found in proteins like fibronectin, vitronectin, and laminin. - Inside the cell: Integrins bind to proteins such as talin and $\alpha\text{-actinin}$ , which connect to actin microfilaments.
Processes Regulated by Integrin-Matrix Adhesion: - Cell signaling - Gene expression - Mitosis - Apoptosis - Cell differentiation
Pathways and Effects of Activated Integrins: - Signaling cascades involving kinases such as $ERK$ , $Akt$ , and $p38$ . - Ion channel and $Ca^{2+}$ regulation. - Mechanotransduction and growth/hypertrophy. - Focal adhesion and costamere formation. - Ischemic protection. - Stem cell growth, homing, differentiation, and engraftment. - Potential for viral uptake (e.g., in myocarditis).

Focal Adhesions

Focal adhesions are large macromolecular structures consisting of more than $50$ different proteins.
Functional Roles: - Anchorage of the cell to the ECM. - Integrin-dependent signaling. - Regulation of actin dynamics.
Subcellular Communication: Many proteins within focal adhesions can shuttle between the adhesion site and the nucleus, allowing communication between different subcellular domains.
Compositional Diversity: - Scaffolding molecules. - GTPases. - Enzymes, including kinases, phosphatases, proteases, and lipases.
Signaling Hubs: Focal adhesions act as central hubs for cell signaling. Formation is dependent on internal signaling events and integrin-ligand interactions.
Key Kinases in Dynamics: - Focal Adhesion Kinase ( $FAK$ ) and $Src$ are prominent kinases that perform tyrosine phosphorylation of proteins like paxillin and vinculin. This regulation governs focal adhesion dynamics and subsequent cell behavior.
Temporal Dynamics: In live cells, there is a constant state of formation and disassembly of focal adhesions (e.g., paxillin dynamics).

Cell Migration Mechanism

Cell migration is a vital contributor to cell fate during development. In $1940$ , Mary Rawles demonstrated that pigment cells in chick development originate in the neural crest and migrate to the epidermis and feathers.
The Mechanics of Movement: - Migration occurs through dual binding that allows the cell to contract actin microfilaments against a fixed ECM. - Leading Edge: Membrane protrusions (lamellipodia/filopodia) are stabilized by nascent adhesive foci that mature into focal adhesions. - Trailing Edge: Focal adhesions disassemble, allowing the rear of the cell to retract. - Necessary for translocation of focal adhesions toward the center of the cell and the release of the trailing edge.
Regulation of Migration Proteins: - Assembly: Regulated by tyrosine phosphorylation, $FAK$ , and $Rho$ . - Turnover/Disassembly: Regulated by the kinase activity of $Src$ and cleavage by the calcium-dependent protease, Calpain.

Force Regulation in Adhesion

Intracellular Force: The $Rho$ / $ROCK$ pathway increases focal adhesion and stress fiber formation.
Extracellular Force: The rigidity of the ECM strengthens the link between integrins and the cytoskeleton.
Forces play a recursive role in regulating cell locomotion and migration.

Cell-to-Cell Adhesion: Cadherins

Cadherins are calcium-dependent adhesion molecules that facilitate interactions between adjacent cells. The cadherin-cadherin bond is remarkably strong.
Structure and Anchoring: - Cadherins bind cells by interacting with cadherins on the neighboring cell. - The actin cytoskeleton is crucial for organizing cadherins into stable linkages. - Cadherins are anchored in place by proteins called catenins, which bind directly to the cytoskeleton.
Surface Tension: The surface tension of cell aggregates is linearly related to the level of cadherin expression.
Cadherin Subtypes and Tissue Expression: - $E\text{-cadherin}$ : Found in early mammalian embryonic cells and epithelial cells. - $P\text{-cadherin}$ : Localized in the placenta. - $N\text{-cadherin}$ : Found in the central nervous system. - $R\text{-cadherin}$ : Found in the retina. - $B\text{-cadherin}$ : Found in various neural structures.

Selective Cell Adhesion and Sorting

Histological Evidence: In $1955$ , Townes and Holtfreter dissociated cells from the three germ layers of amphibian embryos. When mixed, the cells reaggregated and spatially segregated into regions reflecting their original embryonic positions, demonstrating the selective affinity of germ layers.
Differential Adhesion Hypothesis (Malcolm Steinberg, $1964$ ): - Cell sorting is based on thermodynamic stability. - Strength of adhesion (denoted as $D$ ) determines relative positioning: - Separate Aggregates: Occur if cell types show little to no mutual adhesion. - Randomly Mixed: Occurs if the strength of type I-I adhesion is weaker or equal to type I-II or II-II adhesion. - Central Localization: If type I-I adhesion is stronger than I-II or II-II, type I cells will migrate to the center of the aggregate (e.g., $P\text{-cad} > E\text{-cad}$ or vice versa).
Determinants of Sorting: Both the type and the quantity of cadherins expressed determine how cells sort. For example, $R\text{-cadherin}$ and $N\text{-cadherin}$ do not bind well to each other, making expression levels less relevant for their specific sorting compared to $E$ and $P$ cadherins.

Specialized Adhesion Complexes

Desmosomes: - Act as "rivets" providing strong links between cells. - Essential for mechanical strength and resilience in high-stress tissues like the skin and heart. - Connect the intracellular intermediate filament cytoskeleton of adjacent cells via transmembrane desmosomal cadherins. - Plaque proteins (e.g., plakoglobin and desmoplakin) link the cadherins to the intermediate filaments.
Adherens Junctions and Adhesion Belts: - Adherens Junction: A protein complex connecting the actin cytoskeleton to cadherins. - Adhesion Belt: A continuous, belt-like structure of adherens junctions. - Role in Structural Integrity: Maintains the intensity and structure of tissues/organs via a continuous network. - Role in Mechanotransduction: Senses forces from neighbors to inform behaviors like response to tissue stiffness. - Role in Tissue Development: Actin network contraction provides forces for migration and morphogenesis.

Clinical Implications of Adhesion Defects

Cadherin/Desmosome Defects: - Neurodevelopmental disorders. - Cardiomyopathies (e.g., Arrhythmogenic cardiomyopathy). - Skin and hair disorders (e.g., Pemphigus/skin blistering). - Sensory deficits (e.g., hereditary deafness).
Integrin/Focal Adhesion Defects: - Impaired wound healing. - Vascular disease. - Cancer progression/metastasis. - Blood clotting disorders (e.g., Glanzmann’s thrombasthenia).