Cell adhesion/junction

In what type of tissues are cells tightly bound together into sheets called epithelia?

Epithelial tissues, such as the lining of the gut or the epidermis of the skin.

What term describes the differential distribution of components that gives epithelial cells a 'top' and 'bottom'?

Apical-basal polarity.

What structure is composed of the basal lamina and the reticular lamina?

The basement membrane.

How are mechanical stresses transmitted across an epithelial sheet?

From cell to cell via cytoskeletal filaments anchored to cell-matrix and cell-cell adhesion sites.

The _ junction associates with the actin cytoskeleton to provide strong cell-cell adhesion.

adherens

What is the main molecular component of adherens junctions?

The Cadherin/Catenin complex.

In the Cadherin/Catenin complex, what is the role of catenins?

Catenins link cadherins to the actin cytoskeleton.

What type of binding do classical cadherins, like E-cadherin, exhibit?

Homophilic binding, where they bind to other cadherins of the same type.

Which component of the adherens junction is a transmembrane protein?

Cadherin.

Which component of the adherens junction is a cytoplasmic protein?

Catenin.

The binding between two cadherin molecules is dependent on the presence of what ion?

Calcium ions ($Ca^{2+}$).

What happens to the structure of the extracellular portion of a cadherin molecule when calcium is removed?

The hinges between cadherin domains flex, and the structure becomes floppy.

How do calcium ions affect the hinge regions between cadherin domains?

Calcium ions bind to the hinge regions and prevent them from flexing, making the structure more rigid.

Cadherins mediate highly selective recognition, allowing cells of a _ type to stick together.

similar

What happens when a mixed population of cells expressing E-cadherin and N-cadherin are cultured together?

The cells sort out, forming separate aggregates of E-cadherin-expressing cells and N-cadherin-expressing cells.

What is the outcome when cells expressing high levels of E-cadherin are mixed with cells expressing low levels of E-cadherin?

The cells with higher E-cadherin levels sort to the inside of the aggregate, while the low-level cells form an outer layer.

The assembly of strong cell-cell adhesions via cadherins leads to the activation of the GTPase and the inhibition of the GTPase .

Rac; Rho

What term is used for cadherins that exhibit heterophilic binding?

Atypical cadherins.

What are the two examples of atypical (heterophilic) cadherins mentioned in the lecture?

Fat (Ft) and Dachsous (Ds).

What experimental evidence demonstrates the heterophilic binding of Fat (Ft) and Dachsous (Ds) cadherins?

When cells expressing Ft are mixed with cells expressing Ds, they clump together, which does not happen when they are cultured separately.

What biological phenomenon involves the collective alignment of cell polarity across a tissue plane, influenced by Ft and Ds?

Planar cell polarity.

Which type of junction is responsible for creating a seal to prevent molecules from leaking freely across an epithelial sheet?

Tight junctions.

In the gut epithelium, tight junctions prevent glucose from the lumen from leaking into the extracellular space, ensuring it is transported through which structures?

Glucose transporters (apical Na+-driven transporter and basolateral passive transporter).

How do tight junctions appear when visualized by freeze-fracture electron microscopy?

As a branching network of sealing strands that encircles the apical end of each cell.

What are the two main types of transmembrane proteins that form the sealing strands of tight junctions?

Claudins and occludins.

Besides forming a seal, what other function can claudins perform within a tight junction?

They can form selective channels that allow specific ions to cross the tight-junctional barrier.

What is the name of the functional equivalent of tight junctions in insects?

Septate junctions.

What was demonstrated by the 'Smurf fly' experiment, where mutant flies without septate junctions turned blue when fed blue dye?

It provided evidence that septate junctions form a crucial permeability barrier in the insect gut.

Which type of cell junction directly connects the cytoplasm of adjacent cells, allowing for intercellular communication?

Gap junctions.

A gap junction intercellular channel is formed by two _, with one contributed by each cell.

connexons (or hemichannels)

How many connexin protein subunits are required to form a single connexon?

Six.

What family of proteins forms gap junctions in vertebrates?

Connexins.

What family of proteins forms gap junctions in invertebrates?

Innexins.

What is the approximate pore size of a gap junction, which dictates what can pass between cells?

About 1.4 nm in diameter.

What types of substances are able to pass through gap junctions?

Inorganic ions and other small water-soluble molecules.

What type of molecules are too large to pass through gap junctions?

Macromolecules, such as proteins and nucleic acids.

What is a 'homotypic' intercellular channel in a gap junction?

A channel formed by two identical connexons.

What is a 'heterotypic' intercellular channel in a gap junction?

A channel formed by two different types of connexons.

What is a 'homomeric' connexon?

A connexon composed of six identical connexin subunits.

What is a 'heteromeric' connexon?

A connexon composed of a mix of different connexin subunits.

In the diagram of epithelial cell polarity, the surface facing the lumen or outside environment is called the _ region.

apical

In the diagram of epithelial cell polarity, the surface attached to the basement membrane and adjacent cells is called the _ region.

basolateral

Which junction type, depicted in a detailed diagram, is located most apically in an epithelial cell?

Tight junctions.

Which junction type anchors cells to the basal lamina of the basement membrane?

Hemidesmosomes.

Summarize the primary function of tight junctions.

They create a semi-permeable seal between adjacent epithelial cells.

Summarize the primary function of adherens junctions.

They provide strong cell-cell adhesion by linking the actin cytoskeletons of adjacent cells.

Summarize the primary function of gap junctions.

They connect the cytoplasms of adjacent cells, allowing for direct passage of small molecules and ions.