Retta - L20

Cell-Cell Junctions: Cadherins

• Cadherins are adhesive proteins crucial for cell-cell adhesion, forming dimers on the plasma membrane.
• Interaction with neighboring cell cadherins and actin filaments is mediated by adaptor proteins known as catenins (α, β, p120).
• Cadherins not only provide structural support but also participate in signaling transduction linked to growth factor receptors.
• These proteins can regulate signaling pathways through tyrosine kinases, facilitating molecular crosstalk between cadherins and growth factor receptors.

Integrins: Structure and Function

• Integrins are α and β heterodimers functioning as transmembrane receptors that connect to extracellular matrix (ECM) proteins.
• Their cytoplasmic domains interact with various adaptor proteins that link them to the actin cytoskeleton.
• Integrins have a key role in signaling pathways, including PI3Kinase, Src, and focal adhesion kinase (FAK), influencing processes such as cell proliferation and migration.
• Anoikis refers to apoptosis occurring in adherent cells that detach from the extracellular matrix, highlighting the importance of integrins for cell survival.

Molecular Crosstalk Between Cadherins and Integrins

• Rap1, a small GTPase, mediates the interaction between cadherins and integrins, strengthening cell-matrix adhesions when cell-cell junctions are compromised.
• This cross-talk is influenced by growth factor receptors, allowing complex regulatory pathways that affect gene expression and signaling.

Reactive Oxygen Species and Crosstalk

• Rho GTPase modulates cytoskeleton dynamics, participating in the cadherin-integrin signaling nexus.
• Reactive oxygen species (ROS) play a vital role in this crosstalk, being sensitive to oxidative-reductive changes, thus coordinating multiple regulatory proteins.

Cadherins: A Diverse Family of Molecules

• Cadherins require calcium ions for proper function and folding of their extracellular domain, facilitating homophilic interactions.
• Classical cadherins encompass E-cadherin (epithelial), N-cadherin (neuronal), and P-cadherin (placental).
• Non-classical cadherins include desmosomal cadherins and protocadherins, with various tissue specificities.

Functional Roles of Cadherins

• E-cadherin is vital for embryo implantation; absence leads to developmental failure.
• N-cadherin is important for neuronal connections and muscle cell adherence; mutations can result in embryonic lethality.
• P-cadherin is linked to epidermal and placental development; its mutation affects mammary gland structure but is less lethal than E- or N-cadherin.

Cadherin-Mediated Homophilic Adhesion

• Cadherins mediate homophilic adhesion, meaning they bind to identical proteins on adjacent cells.
• Heterophilic adhesion refers to the interaction between different protein types, such as selectins.

Cadherins and Calcium Dependency

• The structure of cadherins changes without sufficient calcium, compromising cell-cell junction integrity.
• Calcium chelators can disrupt junctions and are useful in experimental studies to analyze adhesion dynamics.

Developmental Role of Cadherins

• During embryogenesis, cadherins drive cell differentiation and organization, such as the transition from E-cadherin to N-cadherin during neural tube formation.
• Cadherin expression changes coincide with morphogenetic events, influencing cell migration and tissue formation.

Cadherin-Dependent Cell Sorting

• Cadherins facilitate sorting of cells based on adhesive specificity through reassociation of cells expressing similar cadherins.
• Experimental manipulation of cadherin levels can be employed to study tissue organization and adhesion properties.

Monitoring Cadherin Dynamics

• Fluorescent tagging (e.g., GFP cadherin) enables visualization of cadherin dynamics and interactions in live cell studies.
• Video microscopy can be used to track protein distribution and behavior at cell junctions, providing insights into the adhesion process.### Cadherins and Epithelial Cell Behavior
• Cadherins localize at the plasma membrane, forming cell-cell junctions when cells are closely packed.
• Diffused cadherins in the cytoplasm are targeted for degradation in the proteasome.
• Synthesis and localization of new cadherins to the plasma membrane are essential when cells are not attached.

Epithelial-Mesenchymal Transition (EMT)

• EMT is significant in cancer, particularly in carcinoma development from epithelial tissues.
• Normal epithelial cells can undergo morphological changes to acquire a mesenchymal phenotype.
• Characterized by the loss of tight cell-cell contacts and the ability to interact with the extracellular matrix via integrins.
• Transition is marked by a shift from E-cadherin in epithelial cells to N-cadherin in mesenchymal cells.
• EMT occurs during embryonic development and is also a hallmark of tumor genesis.

Role of the Cytoskeleton in EMT

• Cytoskeletal reorganization is crucial for morphological changes during EMT.
• Actin stress fibers and intermediate filaments form connections that influence cell shape and adhesion.
• Signaling events mediate the transition, altering cadherin expression and cellular behavior.

Cadherin Functions and Interactions

• Cadherins, along with catenins (p120, β-catenin, and α-catenin), connect to the actin cytoskeleton, playing roles in adhesion and signaling.
• Rho GTPase is involved in actin cytoskeleton regulation, connecting to cadherins' adhesive functions.
• VE-cadherin, specific to endothelial cells, functions similarly to cadherins in epithelial cells, mediating cell adhesion and signal transduction.

Adherent Junctions and Mechanical Forces

• Initial cell-cell contact through few cadherins expands to stronger adhesion by recruiting more cadherins and catenins.
• Tension in cell-cell junctions strengthens adhesion, promoting enhanced interaction through actin filament dynamics.
• Adjustments in junction strength are tied to cellular movement and tension.

Desmosomes and Hemidesmosomes

• Desmosomes connect cells with non-classical cadherins and provide mechanical strength through intermediate filaments.
• Hemidesmosomes, mediated by integrins, anchor cells to the extracellular matrix, interacting with laminin.
• Both junction types play crucial roles in maintaining tissue integrity by anchoring epithelial cells.

Tight Junctions

• Tight junctions act as barriers, preventing solute passage between cells; they require transport systems to move solutes.
• Composed of claudins and occludins, tight junctions form molecular seals through transmembrane protein interactions.
• Zonula occludens (ZO) proteins anchor tight junctions to the actin cytoskeleton.

Gap Junctions

• Gap junctions facilitate metabolic and electrical coupling between cells, allowing small molecules (under 1 kDa) to pass.
• Formed by connexins, the channels (connexons) enable communication and coordination among adjacent cells.

Pathogen Transmigration

• Pathogens may utilize paracellular or transcellular routes to traverse epithelial barriers.
• Paracellular route involves breaching cell junctions, while transcellular route requires entering one cell and exiting through the base.### Intracellular Mediators and Cell Communication
• c-AMP and I3P are important intracellular mediators that facilitate rapid metabolic and signaling connections between cells.
• Animal cells communicate predominantly through gap junctions, which are specialized channels that allow the passage of small molecules and ions.

Gap Junction Structure

• Gap junctions consist of connexons, each formed by six connexin subunits.
• Connexons can form heteromeric or homomeric structures, leading to either homotypic or heterotypic intercellular channels.
• In humans, there are 14 distinct connexins available for forming these channels.

Functions of Gap Junctions

• Gap junctions enable electrical coupling in nerve cells, allowing rapid propagation of action potentials compared to chemical synapses.
• They are critical for the synchronization of contractions in heart muscle cells and smooth muscle cells responsible for intestinal peristalsis.
• Major roles include regulating vascular tone and mediating communication during embryogenesis and in normal ovarian follicle development.

Regulation of Gap Junctions

• The opening and closing of gap junctions can be modulated by various signals, such as neurotransmitters.
• Dopamine decreases gap-junction communication in retinal neurons by reducing c-AMP diffusion among them.

Cell Junction Types Overview

• Tight junctions: Seal neighboring cells in epithelial layers to prevent molecule leakage.
• Adhesion junctions: Connect actin bundles between adjacent cells.
• Desmosomes: Link intermediate filaments between cells.
• Gap junctions: Permit the passage of ions and small water-soluble molecules.
• Hemidesmosomes: Anchor cells to the basal lamina.
• Focal adhesions: Mediate interactions between cells and the extracellular matrix, composed primarily of integrins.

Selectins and Transient Cell Adhesion

• Selectins are adhesion proteins that facilitate transient interactions in the bloodstream, particularly during inflammation.
• They assist in the extravasation process of white blood cells exiting the bloodstream to reach damaged tissues.
• Different types of selectins include L-selectin (on white blood cells), P-selectin (on platelets and activated endothelial cells), and E-selectin (on activated endothelial cells).

Mechanism of White Blood Cell Extravasation

• Initial adhesion between white blood cells and endothelial cells is weak and mediated by selectins.
• This weak adhesion allows leukocytes to roll along the endothelial surface, leading to stronger adhesion through integrins once activated.
• Integrins then mediate firm adhesion to endothelial cells, allowing white blood cells to migrate through the endothelial layer into tissues.

Overall Summary of Leukocyte Adhesion

• The process unfolds in a sequence: weak interactions by selectins, triggering integrin activation, and concluding with strong adhesion facilitating leukocyte passage.
• Chemotaxis from inflamed tissues directs leukocyte activation and migration, highlighting the role of selectins in immune response dynamics.

Cell-Cell Junctions: Cadherins

• Cadherins are adhesive proteins crucial for cell-cell adhesion, forming dimers on the plasma membrane.
• Interaction with neighboring cell cadherins and actin filaments is mediated by adaptor proteins known as catenins (α, β, p120).
• Cadherins not only provide structural support but also participate in signaling transduction linked to growth factor receptors.
• These proteins can regulate signaling pathways through tyrosine kinases, facilitating molecular crosstalk between cadherins and growth factor receptors.

Integrins: Structure and Function

• Integrins are α and β heterodimers functioning as transmembrane receptors that connect to extracellular matrix (ECM) proteins.
• Their cytoplasmic domains interact with various adaptor proteins that link them to the actin cytoskeleton.
• Integrins have a key role in signaling pathways, including PI3Kinase, Src, and focal adhesion kinase (FAK), influencing processes such as cell proliferation and migration.
• Anoikis refers to apoptosis occurring in adherent cells that detach from the extracellular matrix, highlighting the importance of integrins for cell survival.

Molecular Crosstalk Between Cadherins and Integrins

• Rap1, a small GTPase, mediates the interaction between cadherins and integrins, strengthening cell-matrix adhesions when cell-cell junctions are compromised.
• This cross-talk is influenced by growth factor receptors, allowing complex regulatory pathways that affect gene expression and signaling.

Reactive Oxygen Species and Crosstalk

• Rho GTPase modulates cytoskeleton dynamics, participating in the cadherin-integrin signaling nexus.
• Reactive oxygen species (ROS) play a vital role in this crosstalk, being sensitive to oxidative-reductive changes, thus coordinating multiple regulatory proteins.

Cadherins: A Diverse Family of Molecules

• Cadherins require calcium ions for proper function and folding of their extracellular domain, facilitating homophilic interactions.
• Classical cadherins encompass E-cadherin (epithelial), N-cadherin (neuronal), and P-cadherin (placental).
• Non-classical cadherins include desmosomal cadherins and protocadherins, with various tissue specificities.

Functional Roles of Cadherins

• E-cadherin is vital for embryo implantation; absence leads to developmental failure.
• N-cadherin is important for neuronal connections and muscle cell adherence; mutations can result in embryonic lethality.
• P-cadherin is linked to epidermal and placental development; its mutation affects mammary gland structure but is less lethal than E- or N-cadherin.

Cadherin-Mediated Homophilic Adhesion

• Cadherins mediate homophilic adhesion, meaning they bind to identical proteins on adjacent cells.
• Heterophilic adhesion refers to the interaction between different protein types, such as selectins.

Cadherins and Calcium Dependency

• The structure of cadherins changes without sufficient calcium, compromising cell-cell junction integrity.
• Calcium chelators can disrupt junctions and are useful in experimental studies to analyze adhesion dynamics.

Developmental Role of Cadherins

• During embryogenesis, cadherins drive cell differentiation and organization, such as the transition from E-cadherin to N-cadherin during neural tube formation.
• Cadherin expression changes coincide with morphogenetic events, influencing cell migration and tissue formation.

Cadherin-Dependent Cell Sorting

• Cadherins facilitate sorting of cells based on adhesive specificity through reassociation of cells expressing similar cadherins.
• Experimental manipulation of cadherin levels can be employed to study tissue organization and adhesion properties.

Monitoring Cadherin Dynamics

• Fluorescent tagging (e.g., GFP cadherin) enables visualization of cadherin dynamics and interactions in live cell studies.
• Video microscopy can be used to track protein distribution and behavior at cell junctions, providing insights into the adhesion process.### Cadherins and Epithelial Cell Behavior
• Cadherins localize at the plasma membrane, forming cell-cell junctions when cells are closely packed.
• Diffused cadherins in the cytoplasm are targeted for degradation in the proteasome.
• Synthesis and localization of new cadherins to the plasma membrane are essential when cells are not attached.

Epithelial-Mesenchymal Transition (EMT)

• EMT is significant in cancer, particularly in carcinoma development from epithelial tissues.
• Normal epithelial cells can undergo morphological changes to acquire a mesenchymal phenotype.
• Characterized by the loss of tight cell-cell contacts and the ability to interact with the extracellular matrix via integrins.
• Transition is marked by a shift from E-cadherin in epithelial cells to N-cadherin in mesenchymal cells.
• EMT occurs during embryonic development and is also a hallmark of tumor genesis.

Role of the Cytoskeleton in EMT

• Cytoskeletal reorganization is crucial for morphological changes during EMT.
• Actin stress fibers and intermediate filaments form connections that influence cell shape and adhesion.
• Signaling events mediate the transition, altering cadherin expression and cellular behavior.

Cadherin Functions and Interactions

• Cadherins, along with catenins (p120, β-catenin, and α-catenin), connect to the actin cytoskeleton, playing roles in adhesion and signaling.
• Rho GTPase is involved in actin cytoskeleton regulation, connecting to cadherins' adhesive functions.
• VE-cadherin, specific to endothelial cells, functions similarly to cadherins in epithelial cells, mediating cell adhesion and signal transduction.

Adherent Junctions and Mechanical Forces

• Initial cell-cell contact through few cadherins expands to stronger adhesion by recruiting more cadherins and catenins.
• Tension in cell-cell junctions strengthens adhesion, promoting enhanced interaction through actin filament dynamics.
• Adjustments in junction strength are tied to cellular movement and tension.

Desmosomes and Hemidesmosomes

• Desmosomes connect cells with non-classical cadherins and provide mechanical strength through intermediate filaments.
• Hemidesmosomes, mediated by integrins, anchor cells to the extracellular matrix, interacting with laminin.
• Both junction types play crucial roles in maintaining tissue integrity by anchoring epithelial cells.

Tight Junctions

• Tight junctions act as barriers, preventing solute passage between cells; they require transport systems to move solutes.
• Composed of claudins and occludins, tight junctions form molecular seals through transmembrane protein interactions.
• Zonula occludens (ZO) proteins anchor tight junctions to the actin cytoskeleton.

Gap Junctions

• Gap junctions facilitate metabolic and electrical coupling between cells, allowing small molecules (under 1 kDa) to pass.
• Formed by connexins, the channels (connexons) enable communication and coordination among adjacent cells.

Pathogen Transmigration

• Pathogens may utilize paracellular or transcellular routes to traverse epithelial barriers.
• Paracellular route involves breaching cell junctions, while transcellular route requires entering one cell and exiting through the base.### Intracellular Mediators and Cell Communication
• c-AMP and I3P are important intracellular mediators that facilitate rapid metabolic and signaling connections between cells.
• Animal cells communicate predominantly through gap junctions, which are specialized channels that allow the passage of small molecules and ions.

Gap Junction Structure

• Gap junctions consist of connexons, each formed by six connexin subunits.
• Connexons can form heteromeric or homomeric structures, leading to either homotypic or heterotypic intercellular channels.
• In humans, there are 14 distinct connexins available for forming these channels.

Functions of Gap Junctions

• Gap junctions enable electrical coupling in nerve cells, allowing rapid propagation of action potentials compared to chemical synapses.
• They are critical for the synchronization of contractions in heart muscle cells and smooth muscle cells responsible for intestinal peristalsis.
• Major roles include regulating vascular tone and mediating communication during embryogenesis and in normal ovarian follicle development.

Regulation of Gap Junctions

• The opening and closing of gap junctions can be modulated by various signals, such as neurotransmitters.
• Dopamine decreases gap-junction communication in retinal neurons by reducing c-AMP diffusion among them.

Cell Junction Types Overview

• Tight junctions: Seal neighboring cells in epithelial layers to prevent molecule leakage.
• Adhesion junctions: Connect actin bundles between adjacent cells.
• Desmosomes: Link intermediate filaments between cells.
• Gap junctions: Permit the passage of ions and small water-soluble molecules.
• Hemidesmosomes: Anchor cells to the basal lamina.
• Focal adhesions: Mediate interactions between cells and the extracellular matrix, composed primarily of integrins.

Selectins and Transient Cell Adhesion

• Selectins are adhesion proteins that facilitate transient interactions in the bloodstream, particularly during inflammation.
• They assist in the extravasation process of white blood cells exiting the bloodstream to reach damaged tissues.
• Different types of selectins include L-selectin (on white blood cells), P-selectin (on platelets and activated endothelial cells), and E-selectin (on activated endothelial cells).

Mechanism of White Blood Cell Extravasation

• Initial adhesion between white blood cells and endothelial cells is weak and mediated by selectins.
• This weak adhesion allows leukocytes to roll along the endothelial surface, leading to stronger adhesion through integrins once activated.
• Integrins then mediate firm adhesion to endothelial cells, allowing white blood cells to migrate through the endothelial layer into tissues.

Overall Summary of Leukocyte Adhesion

• The process unfolds in a sequence: weak interactions by selectins, triggering integrin activation, and concluding with strong adhesion facilitating leukocyte passage.
• Chemotaxis from inflamed tissues directs leukocyte activation and migration, highlighting the role of selectins in immune response dynamics.