Cell Junctions and Extracellular Matrix

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Questions and Answers

What is the primary role of tight junctions in epithelial cells?

Providing structural support

Connecting to the cytoskeleton

Preventing passage of molecules between cells (correct)

Facilitating cell communication

Which protein is most critical for the assembly and structure of sealing strands in tight junctions?

Claudin (correct)

Occludin

Cadherin

Zonula occludens

What are the main extracellular components of sealing strands in tight junctions?

Transmembrane proteins (correct)

Phospholipids

Protein kinases

Glycoproteins

Which of the following accurately describes the appearance of tight junctions in a freeze-fracture electron micrograph?

Band of branching sealing strands Signup and view all the answers

What role do cadherins play in the sorting of cells expressing different types?

They allow for self-sorting based on type and level. Signup and view all the answers

What type of junctions are mentioned but not shown for clarity alongside tight junctions?

Adherens junctions Signup and view all the answers

What is the result of cortical tension in unattached cells?

It creates a pulling effect to draw the surface inward. Signup and view all the answers

Which scaffold proteins are concentrated beneath the plasma membrane at tight junctions?

ZO-1, ZO-2, and ZO-3 Signup and view all the answers

Which molecular mechanism is involved in the initial interaction of epithelial cell precursors?

Assembly of small cadherin clusters at the contact site. Signup and view all the answers

What domains are present in the scaffold proteins at tight junctions?

Three PDZ domains, an SH3 domain, a GK domain, and a proline-rich domain Signup and view all the answers

What is the effect of large adherens junctions on Rho and Rac activity over time?

They inhibit Rac and stimulate Rho. Signup and view all the answers

What happens to a small extracellular tracer molecule when added to either side of an epithelium with tight junctions?

It is blocked from crossing the epithelium Signup and view all the answers

Which protein complex is involved in linking cadherins to actin filaments?

p120-catenin and a specific catenin complex. Signup and view all the answers

What triggers the localized disassembly of actin myosin fibers at the cell junction?

Inhibition of Rho and activation of Rac. Signup and view all the answers

What occurs when cells expressing high levels of E-cadherin interact with those expressing low levels?

High E-cadherin cells adhere more strongly. Signup and view all the answers

What is mechanotransduction in the context of adherens junctions?

The transmission of mechanical signals into cellular responses. Signup and view all the answers

What role does thrombin play in platelet activation?

It initiates a signaling pathway through a G-protein coupled receptor. Signup and view all the answers

How does talin become functional in the context of integrin activation?

It unfolds to expose its binding sites after being recruited to the plasma membrane. Signup and view all the answers

What do the colored stains in the experiment involving fibroblasts indicate?

A concentration of phosphotyrosine proteins at focal adhesions. Signup and view all the answers

What is the primary function of vinculin in the context of talin and integrins?

It forms actin linkages through binding to talin. Signup and view all the answers

What activates integrin signaling in cells according to the content?

Extracellular signals binding G-protein coupled receptors. Signup and view all the answers

How does talin respond to mechanical tension at cell matrix junctions?

It promotes local recruitment of vinculin and actin. Signup and view all the answers

What characterizes the structure of the talin protein?

It features hidden vinculin-binding sites that become accessible upon activation. Signup and view all the answers

What cellular processes are regulated by signals generated at adhesion sites?

Cell division, growth, and survival. Signup and view all the answers

What role does vinculin play in the strengthening of cell junctions?

It enhances the binding of actin filaments to adherens junctions. Signup and view all the answers

What occurs to the actin bundles when epithelial cells narrow at their apical surfaces?

They contract, aiding in the shaping of the epithelial tube. Signup and view all the answers

How do adherens junctions connect adjacent epithelial cells?

By linking cadherins with cadherins on neighboring cells. Signup and view all the answers

What developmental process involves the narrowing of epithelial cells to form a tube?

Neural tube formation. Signup and view all the answers

During germ-band extension in Drosophila, what happens to the distance between cells?

Cells converge and decrease in distance on the dorsal-ventral axis. Signup and view all the answers

What is a key characteristic of the actin filament bundles in adherens junctions?

They are anchored via adaptor proteins to cadherins. Signup and view all the answers

What causes the contraction of actin myosin bundles at the vertical cell boundaries in Drosophila epithelium?

Reduction of E-cadherin and localized actin contraction. Signup and view all the answers

What is the primary structural feature of adherens junctions in epithelial cells?

A contractile belt of actin filaments. Signup and view all the answers

What is one of the primary roles of agrin in the junctional basal lamina?

To regulate the assembly of acetylcholine receptors Signup and view all the answers

Which protein is best known for binding actin and helping to link integrins to the actin cytoskeleton?

Talin Signup and view all the answers

What role do hemidesmosomes play in epithelial cells?

They anchor epithelial cells to the basal lamina. Signup and view all the answers

Which integrin family component is involved in the connection of keratin filaments inside epithelial cells?

Integrin α6β4 Signup and view all the answers

What can result from defects in the components involved in hemidesmosomes?

Blistering diseases such as bullous pemphigoid Signup and view all the answers

Which molecule is likely to interact with specific ion channels on the presynaptic membrane?

Laminin isoforms Signup and view all the answers

What kind of autoimmune disease is bullous pemphigoid?

A disease with antibodies against collagen XVII or BP230 Signup and view all the answers

How do talin and kindlin interact with integrin molecules?

They bind to different sites on the integrin tail Signup and view all the answers

What role do integrin molecules play in cell dynamics?

They link extracellular fibronectin to actin filaments inside the cell. Signup and view all the answers

In what manner is fibronectin organized at the leading edge of a migrating fibroblast?

It exists as small dotlike aggregates that later form fibrils. Signup and view all the answers

What is the primary function of the basal lamina in relation to epithelial cells?

It supports the structure of epithelial cells. Signup and view all the answers

What is unique about the structure of laminin-111?

It has multiple domains enabling it to bind to various molecules. Signup and view all the answers

How does the basal lamina contribute to the kidney glomerulus function?

It supports the glomerular capillaries while filtering molecules. Signup and view all the answers

What happens to fibronectin when tension is applied through integrins?

It stretches and reveals additional binding sites. Signup and view all the answers

What characterizes the organization of the basal lamina in skeletal muscle cells?

It creates sheets surrounding the muscle cells. Signup and view all the answers

What kind of cell structures might intersect with the basal lamina in the kidney?

Slit diaphragms that span gaps between epithelial sheets. Signup and view all the answers

Study Notes

Cell Junctions and the Extracellular Matrix

Animal cells are bound together in two major tissue types: connective tissue and epithelial tissue.
In connective tissue, the extracellular matrix is the primary stress-bearing component.
In epithelial tissue, the cell cytoskeleton is the major stress-bearing component, with cytoskeletons interconnected by junctions transmitting mechanical stress. Cell-matrix attachments connect epithelial tissue to underlying connective tissue.

Cell Junctions

Various cell junctions are found in vertebrate epithelial cells, with different types in different regions.
Tight junctions form a seal between epithelial cells.
Adherens junctions connect actin filaments in adjacent cells.
Desmosomes connect intermediate filaments in adjacent cells.
Gap junctions allow passage of small molecules between cells.
Cell-matrix anchoring junctions, including actin-linked cell-matrix junctions and hemidesmosomes, attach cells to the extracellular matrix.

Cadherins

Cadherins are present in all multicellular animals.
They depend on Ca2+ ions to mediate cell adhesion.
Classical cadherins include E-cadherin, N-cadherin, and P-cadherin.
Nonclassical cadherins are more diverse, including protocadherins, desmocollins, and desmogleins.
Cadherins have extracellular domains containing multiple copies of the cadherin domain and more varied intracellular domains.

Cell-Cell Junctions

Cell-cell junctions come in varied forms.
Anchoring junctions employ cadherins to connect the cytoskeleton of one cell with its neighbor.
They resist forces that pull cells apart.
Cell-cell anchoring junctions must be dynamic and adaptable.

Cadherins Mediate Homophilic Adhesion

Cadherins generally bind homophilically (to the same type of cadherin).
Some other cell adhesion molecules may bind heterophilically.

Transmembrane Adhesion Proteins

Transmembrane adhesion proteins link the cytoskeleton to extracellular structures.
These linkages can be either to other cells or to the extracellular matrix.
Internal linkage to the cytoskeleton is often indirect, via intracellular adaptor proteins.

Anchoring Junctions

Various anchoring junctions, include adherens junctions, desmosomes, cell-matrix junctions.
Distinct proteins are associated with each type
The arrangement and composition of junctions vary according to the cell type.
These junctions are vital for cell-to-cell or cell-matrix integrity.

Desmosomes

Desmosomes are specialized cell junctions that link intermediate filaments in adjacent cells.
They are important in tissues subjected to considerable mechanical stress (e.g., skin, heart).
Nonclassical cadherins, desmogleins, and desmocollins, are key components of desmosomes.
Desmoplakin, plakoglobin, and plakophilin are intracellular adaptor proteins that connect intermediate filaments to the desmosome, tying the desmosome to the filaments.

Tight Junctions

Tight junctions seal gaps between epithelial cells, forming a barrier.
They regulate the movement of molecules across the epithelium.
Tight junctions consist of transmembrane adhesion proteins, such as claudins, occludins.
Scaffolding proteins (ZO-1, ZO-2, ZO-3) organize the tight-junction proteins.

Gap Junctions

Gap junctions form channels between adjacent cells.
They're vital for electrical and metabolic coupling between cells.
Connexins, in a complex called connexons, form the gap-junction channels.

Plasmodesmata

Plant cells have channels called plasmodesmata that connect their cytoplasm.
Plasmodesmata help in the transport and communication between cells.
The channels are lined with plasma membrane, and usually contain a desmotubule.

Selectins

Selectins mediate transient cell-cell adhesion in the bloodstream.
They bind to oligosaccharides on the surface of other cells, creating a weak adhesion allowing cell rolling.
These junctions are important during immune responses and cell migration into tissues.

Immunoglobulin Superfamily

These proteins mediate Ca2+ independent cell-cell adhesion.
Some members participate in cell-cell interactions (e.g., NCAM, ICAM, Nectin), that are found in various tissues.

Extracellular Matrix of Animals

The extracellular matrix is a complex network of proteins and polysaccharides outside the cells.
It provides structural support, regulates cell behavior and cell signaling.
Fibroblasts are cells that produce and maintain the extracellular matrix.
The extracellular matrix is composed of fibrous proteins (e.g., collagen, elastin) and proteoglycans.

Proteoglycans and GAGs

Proteoglycans are proteins with covalently attached glycosaminoglycan (GAG) chains, giving a large, hydrated gel-like structure.
Hyaluronan is the primary GAG that forms a gel.
The proteoglycans and GAGs are important in structural support and regulation of cellular interactions.

Collagen

Collagen is the most abundant protein in the animal extracellular matrix.
It forms bundles, providing tensile strength, flexibility, and resistance to stretching.
Specialized types of collagen, have distinct roles in specific tissues.
Post-translational modifications are essential, and involve the addition of hydroxyproline and hydroxylysine.
Collagen molecules form cross-links to increase strength.

Elastin

Elastin provides elasticity to tissues.
It's a protein that can stretch and recoil.
Elastin molecules are joined together forming a flexible network.

Fibronectin

The glycoprotein fibronectin helps organize the extracellular matrix.
Fibronectin has multiple domains, enabling it to bind with other proteins and molecules.
It often associates with integrins on the cell surface to create cell-matrix linkages.
Tension exerted by cells regulates assembly of fibronectin fibrils

Basal Lamina

The structure of the lamina depends on the type of tissue
Important in cell-cell interactions, as it supports cell sheets and functions in filtration in certain tissues (e.g., kidney).
The basal lamina consists of laminin, type IV collagen, and other components such as proteoglycans.
It helps organize the structure of certain tissue.

Integrins

Integrins are transmembrane heterodimers that link the extracellular matrix to the cytoskeleton.
Specific sites on the integrin interact with proteins in the extracellular matrix.
Integrins are involved in transmitting information about mechanical forces to the cell's interior.
This interaction recruits signaling proteins like talin and kindlin to the cell surface.
Integrins can switch between active and inactive conformations, impacting cellular adhesion and signaling.

Hemidesmosomes

They're localized between cells and the extracellular matrix, providing mechanical attachments.
The structure includes transmembrane proteins (e.g., integrins), and intracellular proteins (e.g., BP230 and plectin) link these to the keratin filaments.
Hemidesmosomes are important for epidermal attachment to the basal lamina.

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Description

Explore the critical functions of cell junctions and the extracellular matrix in animal tissues. This quiz covers the different types of junctions found in epithelial cells, their connections, and the role of the extracellular matrix in connective tissue. Test your knowledge about how these structures contribute to tissue integrity and function.