Topic 36: Cell-Cell Junctions

What is the main structural difference between epithelial and connective tissue in resisting mechanical stress?

Epithelial tissue relies on cell-cell and cell-matrix junctions while connective tissue relies mainly on the extracellular matrix

What is the cell density of epithelial tissue?

High cell density with tightly packed cells

What is the cell density of connective tissue?

Low cell density with abundant extracellular matrix

How is the ECM organized in epithelial tissue?

Thin

How is the ECM organized in connective tissue?

Dense

What are the main epithelial cell-cell junctions?

Tight junctions

What are the main epithelial cell-matrix junctions?

Hemidesmosomes and focal adhesions

What are the three basic components of all cell junctions?

Cell surface adhesion receptors

What are cadherins?

Calcium-dependent cell-cell adhesion proteins

How do cadherins mediate adhesion?

Through homophilic binding between cadherins on adjacent cells

What is the role of Ca2+ in cadherin function?

It stabilizes cadherin structure and enables adhesion

What happens to cadherins when Ca2+ is removed?

They collapse and cell-cell adhesion is lost

What cytoskeleton is linked to adherens junctions?

Actin filaments

What adaptor proteins link cadherins to actin?

Beta-catenin

What is the mechanical nature of adherens junctions?

Strong but dynamic and remodelable "Velcro-like" adhesion

What is cadherin-mediated cell sorting?

Cells segregate based on matching cadherin expression

What type of adhesion do cadherins promote in sorting?

Homotypic adhesion

What happens when different cadherin-expressing cells are mixed?

They self-sort into separate clusters

What developmental example shows cadherin switching?

Neural ectoderm switching from E-cadherin to N-cadherin

What experiment demonstrates cadherin-based sorting?

Dissociated amphibian embryonic cells re-aggregate into correct layers

What triggers adherens junction formation?

Calcium-dependent cadherin binding between cells

What links cadherins to the cytoskeleton?

Catenins connecting to actin

What role does Rac play in adherens junction formation?

Promotes actin remodeling and junction expansion

What role does Rho play in adherens junctions?

Promotes contractile actin bundle formation

What is the mechanosensor in adherens junctions?

Alpha-catenin

What are integrins?

Cell-matrix adhesion receptors

What do integrins bind to?

Extracellular matrix proteins like fibronectin and collagen

What cytoskeleton is linked to focal adhesions?

Actin filaments

What are the main adaptor proteins in focal adhesions?

Talin

What is mechanotransduction in junctions?

Conversion of mechanical force into biochemical signaling responses

What are desmosomes?

Strong cell-cell junctions linking intermediate filaments

What adhesion proteins are in desmosomes?

Desmoglein and desmocollin

What adaptor proteins are in desmosomes?

Plakoglobin

What cytoskeleton is linked to desmosomes?

Intermediate filaments (keratin)

How do desmosomes differ from adherens junctions?

Desmosomes provide strong mechanical resistance while adherens junctions are dynamic and actin-based

What are hemidesmosomes?

Cell-matrix junctions anchoring epithelial cells to ECM via intermediate filaments

What is the function of desmosomes and hemidesmosomes together?

They provide mechanical strength to epithelial tissues

What are tight junctions?

Sealing junctions that control paracellular permeability

What are the main tight junction proteins?

Claudins

What adaptor proteins are in tight junctions?

ZO proteins

What cytoskeleton is linked to tight junctions?

Actin filaments

Where are tight junctions located in epithelial cells?

At the apical-most region above adherens junctions and desmosomes

What is the barrier function of tight junctions?

Prevents diffusion between cells

What is the fence function of tight junctions?

Prevents mixing of apical and basolateral membrane domains

Why are tight junctions important?

They maintain epithelial polarity

What are gap junctions?

Channels that allow direct communication between cells

What is the basic unit of gap junctions?

Connexons

What are connexons made of?

Six connexin proteins

How do gap junctions form?

Two connexons align to form a channel between cells

What is the size limit for molecules passing through gap junctions?

Small molecules and ions typically under ~1 kDa

What is the main function of gap junctions?

Electrical and chemical coupling between cells

Why must gap junction permeability be regulated?

To control signaling and prevent uncontrolled spread of signals

What is a key function of epithelial junction organization?

Maintaining polarity