Neubüser Cell Adhesion

From apical to basal:

1. tight junctions: seal gap between epithelial cells

2. adherens junctions: connect actin filament bundles of one cell with that in the next cell

3. desmosome: connect intermediate filaments in one cell with those in the next cell

4. gap junctions (channel forming junctions): allows passage of small water-soluble molecules from cell to cell

5. actin linked cell-matrix junction: anchors actin filaments in cell to extracellular matrix

6. hemidesmosome: anchors intermediate filaments of the cell to ecm

Common:

• cell-cell adherens via Cadherins (but not the same ones)

• Ca2+ dependent

Differences:

• Anchorage: desmosomes linked to intermediate filaments in the cell, adherens junctions with actin

• structure of connectors:

  • Desmosome: IF - desmoplakin - plakoglobin / plakophilin —-> [through membrane] —> desmoglein / desmocollin (=non classical cadherins)

  • AJ: actin - vinculin (tension) - alpha/beta catenin - p120 catenin - classical cadherin [E-Cad] [trough membrane]

• function:

  • Desmosome: mechanical strength, protection from ripping

  • AJ: epithelial folding, tubes, pits, spheres / remodelling, Adherence junctions respond to forces generated by the actin cytoskeleton (myosin II pull too)

• claudin and occludin

• purpuse: sealing the intercellular space and apical / basal polarity

  • The Par3/Par6/aPKC complex assembles at tight junctions and initiates the formation of a basal lamina on the opposite end of the cell

• thin, tough, flexible sheet of extracellular matrix molecules

• 40-120 nm thick

• below all epithelia

• surrounds muscle, fat and Schwann cells

• multiple functions: structural stability, filtration; influences cell metabolism, cell survival, cell proliferation, cell differentiation; organizes protein distribution in the plasma membrane, highway for cell migration

• The basal lamina is synthesized by the cells on each side of it.

• Collagen (type IV) (gives tensile strengh)

• laminin = main organizere (3 intercoiled chains (a/ß/y)

• perlecan

• nidogen (additional connector of collagen and laminin)

• integrins (through the plasma membrane to anchor the cells) (connect to laminin)

1. actin linked cell matrix adherens

   • via activated integrins (ligand binding)

   • laminin - integrin (a/ß subunit) - kindlin (bud at end) - talin - vinculin - actin

2. hemidesmosomes

   • clusters for strong adherens

   • collagen XII and collagen I / II under basal lamina with collagen IV - laminin - integrin + collagen 17 - plectin + bp230 - intermediate filaments (kreatin)

Common:

• linked to intermediate filaments

• give the cell mechanical strengh

Difference:

• adhesion molecule —> cadherins vs. integrins

• cell-cell vs. cell-matrix adhesion

  
  

ß-catenin is a component of the canonical WNT pathway, upon activation the ß-catenin translocates to the nucleus and associates with Lef/Tcf proteins to form transcriptional activator

The use of Fab fragments in cell aggregation assays allows for increased specificity, reduced potential for crosslinking (IgG has two connected binding sites), improved steric hindrance (smaller size), and enhanced flexibility (small and better accessibility) in blocking the interactions between cell adhesion molecules.

19–1 Given the numerous processes inside cells that are regulated by changes in Ca2+ concentration, it seems likely that Ca2+-dependent cell–cell adhesions are also regulated by changes in Ca2+ concentration.

True. Changes in intracellular Ca2+ concentration play a crucial role in regulating various cellular processes, including cell–cell adhesions. For example, in cadherin-based cell–cell adhesions, Ca2+ is involved in modulating the binding affinity of cadherin molecules. When Ca2+ levels are low, cadherins do not bind strongly, allowing cells to detach. Increased Ca2+ levels strengthen cadherin binding, promoting cell–cell adhesion.

19–2 Tight junctions perform two distinct functions: they seal the space between cells to restrict paracellular flow and they fence off plasma membrane domains to prevent the mixing of apical and basolateral membrane proteins.

True. Tight junctions are complex structures that serve as both a barrier to paracellular transport (preventing the passage of ions and molecules between cells) and as a boundary between apical and basolateral membrane domains in epithelial cells. This helps maintain cell polarity and the separation of distinct membrane protein populations.

19–3 The elasticity of elastin derives from its high content of α helices, which act as molecular springs.

False. Elastin's elasticity arises from a unique structure composed of cross-linked tropoelastin units, not α helices. Elastin is rich in glycine and proline, and it forms a rubber-like protein that provides elasticity to tissues like skin, blood vessels, and lungs.

19–4 Integrins can convert mechanical signals into intracellular molecular signals.

True. Integrins are cell surface receptors that link the extracellular matrix to the cytoskeleton. They can transmit signals bidirectionally, converting mechanical signals from the extracellular environment into intracellular signaling events. This process, known as mechanotransduction, is essential for various cellular functions, including cell adhesion, migration, and response to mechanical forces in the microenvironment.

1. adherens junction

1. desmosome

1. actin linked cell-matrix junction

1. hemidesmosome