Cell Junction & EMC

What are the two main superfamilies of proteins responsible for cell adhesion?

• Cadherins → mediate cell-to-cell attachment

• Integrins → mediate cell-to-matrix attachment

How do cadherins mediate adhesion between cells?

They mediate homophilic adhesion—cadherin molecules on one cell bind to the same type on another cell, forming calcium-dependent links.

What does the extracellular region of a classical cadherin contain?

Five extracellular cadherin (EC) domains that bind Ca²⁺ ions between hinges, making the molecule rigid and preventing flexing.

Why is calcium important for cadherin function?

Without Ca²⁺, cadherins lose rigidity and adhesion weakens, causing cell–cell junctions to disassemble.

What is the role of cadherin-dependent adhesion in tissue development?

It guides organization of developing tissues—for example, different cadherins expressed during nervous system development determine how cells sort into layers

How are cadherins linked to the cytoskeleton?

Through catenins (β-catenin, α-catenin, and p120-catenin), which anchor cadherins to actin filaments inside the cell.

What is the function of adherens junctions?

They connect actin filaments between neighboring cells and respond to tension, maintaining tissue integrity during movement or deformation.

How does tissue remodeling depend on actin and adhesion?

It requires coordinated actin-mediated contraction and cell–cell adhesion to allow tissue reshaping while maintaining connections

What are desmosomes and what filaments do they connect to?

Desmosomes link intermediate filaments between cells.

• Keratin in epithelial cells

• Desmin in heart muscle cells

How do tight junctions regulate permeability?

They form a selective seal between epithelial cells. While impermeable to macromolecules, permeability to small ions varies by tissue.

What experiment demonstrates tight junction function?

Adding a low molecular-mass tracer to one side of epithelium—none passes beyond tight junctions, confirming barrier function.

What is the function of selectins?

They mediate transient cell–cell adhesion in the bloodstream (e.g., leukocyte rolling along endothelium).

Name three additional classes of cell adhesion molecules besides cadherins.

• Integrins

• Selectins

• Immunoglobulin superfamily (Ig-CAMs)

What are the two major forms of extracellular matrix (ECM)?

• Basement membranes (basal lamina)

• Interstitial matrices

How does ECM influence development and morphogenesis?

It affects cell adhesion, migration, morphology, and differentiation—guiding tissue shape and structure.

What are the three major classes of macromolecules in the ECM?

• Glycosaminoglycans (GAGs) – large, charged polysaccharides

• Fibrous proteins – mainly collagens and elastins

• Non-collagen glycoproteins – e.g., fibronectin, laminin

Describe the diversity of the ECM across tissues.

ECM composition varies: it can be calcified (bone/teeth), gel-like (cartilage), or transparent (cornea) depending on functional needs.

What are glycosaminoglycans (GAGs) and proteoglycans?

• GAGs: Negatively charged polysaccharides forming hydrated gels.

• Proteoglycans: GAG chains covalently attached to a core protein, occupying large hydrated spaces in ECM.

What is collagen’s role in ECM?

It is the major structural protein providing tensile strength and framework for tissue architecture.

What is the basal lamina and its functions?

A thin (40–120 nm) specialized ECM layer beneath epithelial cells and around muscle, fat, and nerve cells.
Functions include:

• Determining cell polarity

• Influencing metabolism

• Promoting survival, proliferation, and migration

• Organizing membrane proteins

What experiment demonstrates the role of the junctional basal lamina?

Regeneration experiments at neuromuscular junctions show the basal lamina guides axon regrowth to original synaptic sites.

How do cells influence the ECM?

Cells both synthesize and orient ECM components and sense mechanical properties like stiffness, adjusting behavior accordingly.

What protein gives tissues their elasticity?

Elastin, which allows tissues to stretch and recoil (e.g., lungs, arteries, skin).

How do fibronectin and integrins interact?

Fibronectin binds integrins on cell surfaces, connecting ECM to cytoskeleton. Tension from cells regulates fibronectin fibril assembly.

What is the significance of ECM stiffness in different organs?

Organ-specific stiffness affects cell shape, differentiation, and function (e.g., soft brain tissue vs. stiff bone).

ummarize the major ECM components and their roles.

• GAGs/proteoglycans: Hydration, compression resistance

• Collagens: Tensile strength

• Elastin: Elasticity

• Fibronectin/laminin: Cell adhesion and matrix organization

Summarize key junction types and their cytoskeletal connections:

Junction Type	Main Proteins	Cytoskeletal Link	Function
Adherens	Cadherin + catenin	Actin	Shape & tension
Desmosome	Cadherin-like	Intermediate filaments	Strength
Tight Junction	Claudins + occludins	Actin (indirect)	Barrier
Gap Junction	Connexins	None	Communication