Cell Adhesion & Communication (ECM 3)

Introduction to Cell Adhesion and Communication

• Focus on two main topics:

  • Extracellular Matrix (ECM)

  • Cell Adhesion & Communication

  • Using model systems to study cell biology

Learning Outcomes

• By the end of the session, students should be able to:

  • Describe the major superfamilies of proteins important in cell adhesion:

  • Cadherins

  • Integrins

  • Selectins

  • Immunoglobulin Superfamily Members

  • Compare cell adhesion methods in animal and plant cells

Common Features of Junction Complexes

• Components:

  • Transmembrane adhesion proteins

  • Intracellular link to cytoskeleton

  • Extracellular link to outside structures

• Functions of Specific Proteins:

  • Cadherins: Mediate cell-cell attachments

  • Integrins: Mediate cell-matrix attachments

• Cadherin and Integrin superfamilies vary in their structures:

  • Some cadherins link to actin to form adherens junctions

  • Others link to intermediate filaments forming desmosome junctions

Tight Junctions (TJs)

• Claudin: A four-pass transmembrane protein constituting TJ strands

• Junctional Adhesion Molecules (JAMs): Class of cell-cell adhesion molecules with two immunoglobulin repeats localized to TJs

• Occludin: A four-pass transmembrane protein at TJs

• ZO Proteins: Zonula-occluding family proteins serve as TJ undercoating scaffolding

Cadherins

• General Characteristics:

  • Found in all multicellular animals and Choanoflagellates

  • Not present in plants, fungi, bacteria, or archaea

  • Can exist as free-living or in colonies

  • Thought to be linked to the evolution of multicellularity

• Definition:

  • Name derived from requirement of Ca2+ to mediate cell-cell adhesion

  • Demonstration of Function:

  • During embryogenesis, cells loosely adhere until the 8-cell stage, followed by compaction (formation of tight cell junctions)

  • Adding EDTA chelating agent removes Ca2+, leading to cell separation; adding Ca2+ back re-establishes adhesion

Types of Cadherins

• Naming Based on Cell Type:

  • N-cadherin: Found in nerve cells

  • E-cadherin: Found in epithelial cells

  • P-cadherin: Found in placental cells

• Diversity within Tissues:

  • Cadherins can be present in multiple cell types and exhibit tissue-specific diversity

Cadherin Structure

• Domain Structure:

  • Extracellular region has multiple cadherin domains

  • Intracellular region displays greater diversity

  • Cadherins typically form homodimers, with over 180 cadherins identified in humans

• Non-classical Cadherins:

  • Examples include Desmocollin (found in desmosome junctions)

  • Identified in Drosophila, regulates epithelial growth and cell polarity

  • Lacks a transmembrane domain, attached via GPI anchor

Cadherin Defects

• Phenotypes of Cadherin Defects:

  • Table of cadherins and their associations, showing main locations and phenotypes when inactivated

• Key Classical Cadherins:

  • E-cadherin: Loss leads to embryonic compaction failure

  • N-cadherin: Abnormal heart development in mice

  • P-cadherin: Impairs mammary gland formation

• Non-Classical Cadherins:

  • Impact on skin integrity and other developmental processes

Cadherin Function

• Importance of Ca2+:

  • Ca2+ binding stabilizes flexible hinge regions between cadherin repeats, preventing flexing and promoting binding

  • Removal of Ca2+ leads to weakened cadherin interaction and increased proteolytic degradation

• Homophilic Attachments:

  • Cadherins play crucial roles in tissue organization, demonstrated by classic experiments where early embryonic cells rearranged based on cadherin types

Cadherin Expression Changes

• Changes in cadherin expression influence neural tube development:

  • Transition from E-cadherin to N-cadherin during neural tube formation in chick embryos

Catenins and Cytoskeletal Links

• Catenins:

  • Link intracellular cadherin domains to actin filaments, critical for adhesion strength

• Role of β-catenin:

  • Acts as both an anchor at adherens junctions and as a transcriptional regulator in Wnt signaling

  • Presence or absence of Wnt signaling affects β-catenin levels, linking signaling pathways to cell adhesion

Integrins

• General Characteristics:

  • Comprised of two non-covalently associated glycoprotein subunits

  • Transmembrane proteins with short intracellular C-terminal and large extracellular N-terminal domains

• Functions and Signaling:

  • Bind to extracellular matrix proteins and link to actin cytoskeleton via Talin

  • Dynamic regulation of attachments essential for cell migration

• Outside-In and Inside-Out Activation:

  • Ligand binding causes integrin conformation change affecting cytoskeletal interaction

  • Intracellular signals activate Talin binding to β-integrin chains, promoting ligand interaction

Selectins

• Types:

  • L-selectin: Found on white blood cells

  • P-selectin: Activated on platelets and endothelial cells during inflammatory responses

  • E-selectin: Present on activated endothelial cells

• Functions:

  • Control white blood cell movement between blood and tissues by regulating cell adhesion

  • Binds to specific oligosaccharides during immune responses

Immunoglobulin Superfamily Members

• Types:

  • ICAMs (Intercellular Cell Adhesion Molecules): Characteristic antibody extracellular domains

  • VCAMs (Vascular Cell Adhesion Molecules): Heterophilic binding to integrins

  • NCAMs (Neural Cell Adhesion Molecules): Homophilic interventions in neuronal adhesion

• Variability in NCAM due to alternative splicing and negative charges affecting adhesion properties

Plant Cell Adhesion

• Cell Wall Composition:

  • Composed of cellulose, hemicellulose, pectins, and wall proteins

  • Support, shape maintenance, resist turgor pressure, and control growth

  • Pectin-rich middle lamella is vital for cell adhesion, exhibiting a hydrated gel form

• Pectin Structures:

  • Homogalacturonan (HGA), Rhamnogalacturonan I & II, Xylogalacturonan (XGA)

  • Insights into pectin synthesis are still being unveiled

Implications of Defects in Plant Cell Adhesion

• Defects in polysaccharide synthesis lead to loss of cell adhesion, elucidating the critical role of cell wall components in plant integrity

Lecture Summary

• Comparison of Cell Adhesion in Plants and Animals:

  • Animals: Predominantly through proteins (Cadherins, Integrins, Selectins, Ig superfamily)

  • Plants: Predominantly through polysaccharides (pectins and other wall components)

Common Features of Junctional Complexes

Junctional complexes are specialized regions of the plasma membrane where adhesion molecules are concentrated to provide structural integrity.

1. Core Components:

   • Transmembrane Adhesion Proteins: Cross the lipid bilayer; their extracellular domains interact with other cells or the ECM, while their intracellular domains provide anchoring sites.

   • Intracellular Linkers: Adapter proteins (e.g., catenins, talin, vinculin) that bridge the adhesion protein to the cytoskeleton.

   • Cytoskeletal Attachment: Provides mechanical stability. Cadherins typically link to actin filaments (adherens junctions) or intermediate filaments (desmosomes).

2. Functional Differentiation:

   • Cadherins: Primarily manage cell-cell attachments.

   • Integrins: Primarily manage cell-matrix attachments.

Tight Junctions (TJs)

Tight junctions act as selective permeability barriers, regulating the paracellular pathway.

• Claudin: A crucial $4$-pass transmembrane protein that forms the primary seal of TJ strands. It determines the ion selectivity of the junction.

• Occludin: Another $4$-pass transmembrane protein that contributes to junction stability and barrier function.

• Junctional Adhesion Molecules (JAMs): Belong to the Ig superfamily; they possess $2$ immunoglobulin repeats and are involved in the assembly and stabilization of TJs.

• ZO Proteins (Zonula-occludens): Scaffolding proteins (e.g., $ZO-1$, $ZO-2$, $ZO-3$) that contain multiple binding domains to link claudins/occludins to the actin cytoskeleton.

Cadherins: The Calcium-Dependent Glue

Cadherins are the most significant mediators of tissue morphogenesis and maintenance.

• General Characteristics:

  • Ubiquitous in multicellular animals and present in unicellular Choanoflagellates, suggesting their role in the evolutionary transition to multicellularity.

  • Absent in plants, fungi, and prokaryotes.

• Mechanism of Adhesion:

  • Adhesion is calcium-dependent. Extracellular cadherin domains (usually $5$ repeats) require $Ca^{2+}$ to remain rigid. In the absence of $Ca^{2+}$, the domains become flexible and "floppy," leading to loss of adhesion and increased susceptibility to proteolysis.

  • EDTA Experiment: During the $8$-cell stage of embryogenesis (compaction), adding EDTA (a calcium chelator) causes cells to dissociate. Reintroducing $Ca^{2+}$ restores cell-cell contact.

• Structural Varieties:

  • Classical Cadherins: Include E-cadherin (epithelial), N-cadherin (neural/muscle), and P-cadherin (placental/epidermal).

  • Non-classical Cadherins: Examples include Desmocollin and Desmoglein (found in desmosomes) and Protocadherins found in the brain.

  • Drosophila Fat Cadherin: Regulates epithelial growth and polarity; uniquely lacks a transmembrane domain and is attached by a GPI anchor.

Cadherin Function and Tissue Organization

• Homophilic Binding: Cadherins of a specific type preferentially bind to identical cadherins on neighboring cells. This allows like-cells to "sort" themselves out during development.

• Neural Tube Development: A primary example of expression switching. The precursor to the neural tube initially expresses E-cadherin. As it invaginates, it switches to N-cadherin expression, allowing it to detach from the E-cadherin-expressing ectoderm.

• Catenins and the Cytoskeleton:

  • The intracellular tail of cadherins binds to eta-catenin or $ext{γ}$-catenin.

  • eta-catenin serves a dual role: it anchors the junction to $ext{α}$-catenin (which links to actin) and acts as a central signal transducer in the $Wnt$ signaling pathway.

Integrins: The Matrix Linkers

Integrins serve as the primary bridge between the ECM and the cell's interior, facilitating bidirectional signaling.

• Structure: Heterodimers composed of one $ext{α}$ and one $ext{β}$ subunit. Humans have $18$ $ext{α}$ and $8$ $ext{β}$ subunits, forming roughly $24$ unique integrin types.

• Activation States:

  • Outside-In Signaling: Binding to an ECM ligand (like laminin or fibronectin) induces a conformational change that propagates to the cytoplasmic tail, recruiting signaling molecules like Focal Adhesion Kinase (FAK).

  • Inside-Out Signaling: Intracellular signals trigger Talin to bind to the $ext{β}$ chain, causing the extracellular domains to extend and increase their affinity for ECM ligands.

Selectins and Immunoglobulin Superfamily

These proteins regulate more dynamic or transient interactions, specifically in the immune system.

• Selectins: Calcium-dependent lectins that bind to specific surface carbohydrates (oligosaccharides).

  • L-selectin: On leukocytes.

  • P-selectin: On platelets and endothelium (rapidly mobilized).

  • E-selectin: On activated endothelial cells.

  • Function: Mediate the "rolling" of white blood cells along vessel walls during inflammation.

• Ig Superfamily (CAMs):

  • NCAM (Neural CAM): Mediates calcium-independent homophilic adhesion; highly regulated by polysialic acid chains which can decrease adhesion strength through negative charge repulsion.

  • ICAM/VCAM: Mediate heterophilic binding to integrins on leukocytes to provide strong adhesion for extravasation.

Plant Cell Adhesion

Plant adhesion is fundamentally different because it relies on the cell wall rather than transmembrane proteins.

• Cell Wall: A composite material of cellulose microfibrils, hemicellulose, and pectins.

• Middle Lamella: The pectin-rich layer between adjacent cells. Pectins like Homogalacturonan (HGA) can be cross-linked by calcium to form a rigid hydrated gel, acting as the primary adhesive.

• Defects: Mutations affecting pectin biosynthesis or secretion result in a complete loss of cell-cell adhesion, causing plant tissues to fall apart.