L14: Better phrasing for an essay Flashcards
cadherins
Cadherins are a family of highly conserved transmembrane proteins that mediate calcium-dependent cell-cell adhesion, playing a crucial role in tissue architecture and integrity. They are widely expressed across different tissue types and are essential components of adherens junctions in epithelial and other tissue systems.
Structurally, cadherins consist of extracellular cadherin (EC) domains—typically labeled EC1 to EC5—that mediate homophilic interactions with cadherins on adjacent cells. These domains require calcium ions (Ca²⁺) to maintain a rigid, adhesive conformation. In the absence of calcium, cadherins become structurally unstable and lose their adhesive capabilities. When calcium is present, cadherins undergo a conformational change that promotes their clustering at the plasma membrane and the formation of stable adherens junctions.
The cytoplasmic tail of cadherins includes two key regions: the juxta-membrane domain and the catenin-binding site. These domains are crucial for linking cadherins to the cytoskeleton and intracellular signaling machinery. α-Catenin, in particular, plays a pivotal role in anchoring cadherins to the actin cytoskeleton. It binds to filamentous actin (F-actin) and contributes to the mechanical stability of the junctions by regulating tension and cytoskeletal dynamics.
Beyond their primary role in adherens junctions, cadherins also contribute to the organization of other junctional complexes. Once adherens junctions are established, they assist in the recruitment and stabilization of tight junctions, further strengthening the epithelial barrier and regulating paracellular permeability.
Cadherins and Cell-Cell Adhesion (Especially in Adherens Junctions – AJs)
1. Homophilic Binding
Definition: Cadherin molecules on one cell bind to identical cadherins on adjacent cells (homotypic interaction).
This specificity helps maintain tissue integrity and organization.
- Structure
Cadherins are transmembrane proteins made primarily of extracellular cadherin (EC) domains or repeats.
These domains form the adhesive interface between cells.
- Calcium-Dependence
Cadherins require calcium ions (Ca²⁺) to stabilize the structure of their extracellular domains.
Without Ca²⁺, the cadherin extracellular domain becomes floppy and non-functional, leading to loss of adhesion.
Functional Role in Adherens Junctions (AJs)
4. Key Functions
Mediate strong cell-cell adhesion in epithelial tissues.
Maintain tissue architecture and signal transduction.
- Main Cadherin Types in AJs
E-cadherin (epithelial cadherin) is the most studied and is a hallmark of epithelial cell junctions.
- Link to Actin Cytoskeleton
Inside the cell, cadherins are linked to the actin cytoskeleton via anchor proteins, which include:
Catenins (especially β-catenin and α-catenin)
α-actinin
Vinculin
These links stabilize the junction and allow mechanical force transmission between cells.
E-cadherin: stage E4: phenotype: pre-implantation lethality
N-cadherin: E10, mycoytes dissociate, severe cardiovascular defect.
P-cadherin: V,F. precocious differentiation of mammary gland.
VE-cadherin: E10: vascular defects.
alpha catenin: E4- Pre-implantation lethality
beta catenin: E8. Epiblast dissociates, no mesoderm formation.
gamma catenin (plakoglobin): E12-17. Cardiac rupture, desmosomal defects, epidermal blistering.
tight junctions?
Functions of Tight Junctions:
✅ Permeability barrier: Regulate paracellular transport (movement of molecules between cells).
✅ Cell polarity: Separate the apical and basolateral surfaces, helping cells maintain function.
✅ Macromolecular restriction: Prevent unwanted passage of proteins, ions, and pathogens.
Key Components:
Claudins (main structural proteins)
Homophilic interactions form the tight seal.
Different claudins determine selective permeability.
Interact with cytosolic scaffolding proteins (ZO-1, ZO-2, MAGI).
Occludins
Help stabilize the junction.
Contribute to regulation of tight junction permeability.
Junctional Adhesion Molecules (JAMs)
Interact with cingulin, which links to the actin cytoskeleton.
Important in cell signaling and immune response.
found mainly in epithelia and endothelial cells.
anchoring adhesions
Anchoring Adhesions: Connecting Cells to Their Environment
Anchoring adhesions are critical cellular structures that maintain the mechanical integrity of tissues by linking the cytoskeleton to either neighboring cells or the extracellular matrix (ECM). These adhesions can be broadly categorized into cell-cell junctions and cell-matrix junctions, each relying on specific types of receptors and cytoskeletal elements.
Cell-cell junctions are responsible for mediating adhesion between adjacent cells. Among these, two primary types of anchoring junctions are found: adherens junctions and desmosomes. Adherens junctions connect the actin cytoskeleton of one cell to that of another through transmembrane proteins known as cadherins. These cadherins engage in homophilic interactions, binding identical cadherin molecules on neighboring cells, and are linked intracellularly to actin filaments via adaptor proteins such as catenins. In contrast, desmosomes anchor the cytoskeleton via intermediate filaments, providing tissues with tensile strength. Like adherens junctions, desmosomes also use cadherins—specifically desmogleins and desmocollins—as their primary adhesion molecules.
On the other hand, cell-matrix junctions link cells to the extracellular matrix through integrin receptors. These junctions include focal adhesions, which connect the actin cytoskeleton to the ECM, and hemidesmosomes, which connect intermediate filaments to the ECM. Focal adhesions are dynamic structures involved in signal transduction, cell migration, and mechanosensing. They utilize integrins to bind ECM proteins such as fibronectin, laminin, and collagen, transmitting signals that influence cell behavior. Hemidesmosomes, in contrast, are more stable structures that provide strong, anchoring connections between epithelial cells and the basement membrane. These junctions play a vital role in maintaining tissue stability, particularly in skin and other epithelia.
Together, these anchoring adhesions integrate mechanical and chemical cues from the environment, coordinating cellular structure and function. They not only maintain physical connectivity but also facilitate intracellular signaling pathways essential for development, repair, and homeostasis.
catenins
Catenins and the Regulation of Cadherin-Mediated Cell Adhesion
Cadherin-mediated cell-cell adhesion is critically dependent on a group of intracellular proteins called catenins, which serve both structural and regulatory functions. These proteins link cadherins to the cytoskeleton and modulate adhesion strength and cadherin turnover. Most of the key players in this system belong to the armadillo protein family, characterized by repeating armadillo domains that mediate protein-protein interactions.
β-Catenin and γ-Catenin (also known as plakoglobin) are core members of the armadillo family and bind directly to the cytoplasmic tail of classical cadherins. Their binding is essential for the formation and maintenance of adherens junctions. Once attached, they recruit α-catenin, a crucial actin-binding protein, thereby connecting cadherins to the actin cytoskeleton. This linkage stabilizes the adhesive complex and allows it to resist mechanical stress.
α-Catenin, (vinculin family), is essential for cadherin function. It binds to β-catenin or plakoglobin and mediates direct interaction with F-actin, facilitating both actin binding and bundling. α-Catenin plays a structural role in strengthening cell-cell adhesion and shares homology with vinculin, a protein known for linking integrins to actin at focal adhesions.
p120^ctn (p120-catenin) and δ-Catenin (delta-catenin) also belong to the armadillo family and bind to the juxtamembrane domain of cadherins. These proteins have more regulatory functions compared to β- and γ-catenin. They can influence cadherin stability at the plasma membrane, having both positive and negative effects on adhesion. p120^ctn is widely expressed and is known to regulate cadherin endocytosis and recycling, which affects the overall level of surface cadherins. In contrast, δ-catenin is more neuron-specific and has been observed to negatively affect epithelial cell adhesion, possibly by disrupting cadherin clustering or retention at junctions.
All together, these proteins orchestrate the assembly and maintenance of adherens junctions, enabling cadherins to function effectively as adhesion molecules. By forming complexes and linking to the actin cytoskeleton, they ensure mechanical integrity, dynamic responsiveness, and signal transduction at cell-cell junctions.
desmosomes
Desmosomes are specialized cell-cell junctions that provide strong mechanical adhesion between cells, particularly in tissues subject to high mechanical stress, such as the heart and skin. A key example is the intercalated discs between cardiomyocytes, where desmosomes maintain structural integrity during repeated contractions. The main adhesion molecules in desmosomes are members of the desmosomal cadherin family, specifically desmoglein and desmocollin. These cadherins span the plasma membrane and mediate adhesion through homophilic and heterophilic interactions with cadherins on adjacent cells. On the intracellular side, cadherins are anchored to plakophilin and plakoglobin (both part of the armadillo family), which then bind to desmoplakin. Desmoplakin links the entire complex to the intermediate filament cytoskeleton, creating a robust and resilient mechanical linkage.
focal adhesions
Focal adhesions are key cell-matrix anchoring junctions that mediate the connection between the extracellular matrix (ECM) and the intracellular actin cytoskeleton. The primary receptors involved in focal adhesions are integrins, which are heterodimeric transmembrane proteins composed of α and β subunits. In mammals, 18 α-subunits and 8 β-subunits combine to form 24 distinct integrin heterodimers, each recognizing specific motifs in ECM proteins such as the RGD (Arg-Gly-Asp) motif found in fibronectin. Integrin activation is regulated by four cation-binding sites, particularly on the β-subunit, which contains metal ion coordination sites that influence ligand binding and conformational states. Binding specificity depends on the combination of α and β subunits. Internally, integrins link to the actin cytoskeleton through a network of adaptor proteins including talin, α-actinin, and vinculin, which stabilize and reinforce the connection. Additionally, Focal Adhesion Kinase (FAK), a tyrosine kinase, plays a crucial role in focal adhesion turnover, cell migration, and mechanotransduction. Other regulatory elements include PIP5 kinase and Rho GTPases—with PIP5K being regulated by Rho. Rho GTPases further influence the recruitment and activity of α-actinin and vinculin, thereby modulating actin cytoskeletal dynamics.
integrins: chatgpt
General Properties of Integrins
Ubiquitously expressed across different cell types.
24 heterodimers in vertebrates (18 α + 8 β subunits).
Large proteins (each subunit is >1000 amino acids).
Weak individual interactions, but strong adhesion occurs when integrins cluster.
Often inactive until activated by specific signals.
Key Integrin Subtypes & Functions
β1 Integrins
The most versatile β-subunit.
α5β1 binds fibronectin (via the RGD motif).
α1β1 & α2β1 bind collagen.
β2 Integrins (Leukocyte Integrins)
Allow leukocytes to leave blood vessels and migrate into tissues.
Example: αLβ2 (LFA-1) binds to ICAMs on endothelial cells.
α6β4 Integrin (Special Case)
Found in hemidesmosomes (not desmosomes!).
Links to intermediate filaments (keratins) instead of actin.
Has a much longer cytoplasmic tail (~1000 aa vs. ~50 aa in other integrins) → important for signaling.
Activation Mechanism (Ca²⁺ vs. Mg²⁺ Binding)
When bound to Ca²⁺, integrins are inactive (bent conformation, binding site near plasma membrane).
When Mg²⁺ replaces Ca²⁺, the integrin straightens → activates binding to ECM proteins like fibronectin or vitronectin.
The RGD motif (Arginine-Glycine-Aspartic acid) is the key sequence in ECM proteins that integrins recognize.
Integrin signaling is a complex and dynamic process that regulates cell adhesion, migration, and communication with the extracellular matrix (ECM). There are two main types of integrin regulation: outside-in and inside-out.
Outside-In Regulation refers to the process where an “active” integrin binds to an ECM ligand, such as fibronectin or collagen, and transmits an intracellular signal. This mechanism is similar to how growth factor receptors, like growth factor receptor tyrosine kinases (GF-RTKs), interact with their ligands. Upon ligand binding, integrins undergo conformational changes that allow them to relay signals into the cell, initiating various downstream signaling cascades.
Inside-Out Regulation occurs when extracellular agonists, such as growth factors or cytokines, stimulate intracellular signaling pathways that “activate” integrins, enhancing their extracellular adhesion functions. This regulation ensures that integrins are not always active on the cell surface but are rather in an inactive state until external signals trigger their activation. In most cases, integrins are either located on the surface of the cell or are “hidden” inside the cell in an inactive state, ready to be activated upon receiving the right signals.
Several factors contribute to integrin activation, including:
pH: Changes in pH can influence integrin conformation and its ability to bind ECM components.
Calcium concentration: Calcium ions are essential for maintaining the structure and function of integrins, with low or high concentrations affecting their activity.
Inositol lipid turnover: Phosphoinositides, involved in cellular signaling, can regulate integrin activation and its interaction with the cytoskeleton.
Phosphorylation and dephosphorylation: The addition or removal of phosphate groups on integrin-associated proteins can control integrin activation and its downstream effects.
Proteolysis: The degradation of integrin subunits or ECM components can impact integrin function and the integrity of focal adhesions.
chatgpt check: α6β4 is NOT in desmosomes; it is in hemidesmosomes.
It connects epithelial cells to the basement membrane, linking keratin intermediate filaments to laminin in the ECM.
Desmosomes use desmogleins and desmocollins, not integrins.
hemidesmosomes
Hemidesmosomes are specialized cell-matrix adhesion structures that link the extracellular matrix (ECM) to intermediate filaments, primarily keratin in epithelial cells. These junctions play a critical role in maintaining the structural integrity of tissues by anchoring the basal surface of epithelial cells to the underlying basement membrane.
Key Components of Hemidesmosomes:
Integrins: The central receptors in hemidesmosomes are integrins, which are heterodimeric transmembrane proteins consisting of α and β subunits. These integrins bind to specific ECM components, such as laminin, in the basement membrane. This binding connects the extracellular matrix to the cytoskeleton within the cell.
Intracellular Domain of Integrins: The intracellular portion of integrins is linked to the keratin filaments (intermediate filaments) via plectins, which act as anchor proteins. Plectins are multifunctional linker proteins that connect cytoskeletal elements, like intermediate filaments, to the integrin complex at the cell membrane. This interaction is crucial for stabilizing the cell’s attachment to the ECM.
Function: Hemidesmosomes function as mechanical anchors, resisting shearing forces and promoting cellular stability, particularly in tissues exposed to high mechanical stress, such as the skin. These junctions also play a role in regulating cell signaling and maintaining tissue organization.
