Cell Biology Junctions Quiz

Keywords from Lecture 7

• Collagen and elastin
• Fibronectin
• RGD domain
• Integrins
• Outside-in and inside-out signaling
• FAK
• Velcro Principle
• Anchorage dependence

Hemichannels

• Are hemichannels usually kept in the closed or open conformation?
• Many integrin connections (focal adhesions) need to be able to make and break.
• FAK is responsible for breaking them.

Lecture 8

• Cell junctions, cell adhesion, and the ECM

Cell Interactions

• Interactions between cells in multicellular organisms are fundamental.
• Cells connect to each other through cell-cell junctions or the ECM.
• Attachments to other cells and ECM control cell orientation.
• Defects in cell junctions, cell adhesion, and ECM underlie many diseases.

Cell Structure

• Small motile cells filled with aqueous medium and enclosed in plasma membrane.
• Two basic building strategies:
  • Strength of the ECM
  • Strength of the cytoskeleton within the cell along with cell-cell adhesions that link cytoskeletons of neighboring cells.
  • Cytoskeleton of one cell is linked to the cytoskeleton of the neighboring cell which provides strength.

Animal Tissues

• Animal tissues are varied but can be classified into:
  • Connective tissues (e.g., bone, tendon)
    • Abundant ECM, sparse cells
    • Matrix (rich in fibrous polymers like collagen) bears mechanical stress.
  • Epithelial tissues (e.g., lining of gut, skin)
    • Cells closely bound together into sheets.
    • Scarce ECM, mainly a thin basal lamina.
    • Cells linked via cell-cell adhesion; stress is dispersed this way.

Cell Junctions

• Physical attachment is crucial.
• Junctions between cell-cell or cell-matrix are structurally diverse.
• Four main functions can be distinguished, each on a molecular basis.

Types of Cell Junctions

• Anchoring junctions:
  • Both cell-cell and cell-matrix adhesions
  • Transmit stresses, linked to cytoskeletal filaments
• Occluding junctions:
  • Seal gaps in epithelia, creating impermeable or selectively permeable barriers.
• Channel-forming junctions:
  • Create passageways connecting adjacent cell cytoplasms.
• Signal-relaying junctions:
  • Allow signal relay across plasma membranes at cell-cell contact sites.

Anchoring Junctions (Details)

• Anchor cells to each other and the ECM.
• Intermediate filaments, oriented toward the basal side, connect to basal lamina or other adjacent cells.
• Tight junctions: Located primarily at the apical surface of cells. Prevent leakage through the epithelial layer.
• Channel-forming junction (e.g., gap junctions): Found usually closer to the basal side of cells or beneath tight junctions. Allow transfer of small molecules like ions between cells.
• Signal relaying junctions (e.g., Notch/Delta pathway): Ligand on one cell interacts with a receptor on another cell, transmitting a signal to both cells.

Cadherins

• Cadherins mediate Ca2+-dependent cell-cell adhesion.
• Present in all multicellular animals but absent in fungi, plants, bacteria, and archaea.
• Their adhesive functions are complemented by signaling functions to relay information internally to the cell.
• Several types, named according to the tissue where they're found (E-cadherin, N-cadherin, P-cadherin).
• Their affinity for partners is relatively low but strength is high due to many weak bonds.
• Homophilic binding (like binds to like): This specificity is essential for tissue organization.
• Calcium (Ca2+) is critical to the binding of cadherins.
• Cell-cell adhesions are mediated by cadherins at junctions - these are called adherens junctions.
• Actin cytoskeleton linked between the two cells

Cell Segregation

• Cadherins are essential for cell sorting/segregation.
• Dissociated cells from different embryonic vertebrate organs (like liver, retina) will re-assemble into tissues of origin.
• Selective expression and regulation of different cadherins leads to different cell types

Cell-Cell Junctions (Summary)

• Critical for modeling the shapes of multicellular structures.
• Example: Adherens junctions form a continuous adhesion belt in epithelia.
• Junctions send signals, affecting cell differentiation (making large changes to their internal affairs).
• Complex crosstalk between adhesion machinery and chemical signaling pathways (e.g., β-catenin signaling, Wnt signaling).
• Desomosomes give epithelia mechanical strength. Structurally similar to adherens junctions but link to intermediate filaments (keratin in epithelial cells, desmin in heart muscle cells).