Module 7: Interactions Between Cells and Their Environment

Question 1

What are integrins?

Answer 1

• Family of membrane proteins unique to animals.
• Composed of two membrane-spanning polypeptide chains, an alpha and beta chains.

Question 2

What are the two conformations of integrins?

Answer 2

• Bent conformation: It corresponds to its inactive state,
• Upright conformation: Integrins with a bound ligand are found in an upright conformation.
  - Activated integrins become clustered -> strengthens the cell-ECM interactions.

Question 3

What are the major activities of integrins?

Answer 3

1) adhesion of cells to ECM (or to other cells).
2) transmission of signals between the external environment and the cell interior.

Question 4

What do integrins bind to intracellularly?

Answer 4

Bind ligands such as Talin.

Question 5

What do integrins bind to extracellularly?

Answer 5

• Different integrins bind
  different ECM components such as collagen, laminin, and fibronectin (causing “Outside-in” signaling).

Question 6

How does the outside-in-signals induce a conformational change in talin?

Answer 6

• Actin filament polymerization
• Cytoplasmic protein kinases (e.g. FAK and Src) activation to
  phosphorylate other proteins à life saving signals.

Question 7

What can the outside-in-signals by integrins influence?

Answer 7

Cell differentiation, motility, growth, and cell survival.

Question 8

What is a focal adhesion?

Answer 8

Cultured cells are anchored to the surface of the dish only at scattered, discrete sites, called focal adhesions.
- They play a role in cell adhesion and locomotion.

Question 9

What is the relation between focal adhesions and mechanical force?

Answer 9

Focal adhesions create mechanical forces or respond to such forces from the environment.
- Actin filaments are the source of these forces.

Question 10

What are hemidesmosomes?

Answer 10

• Cell-ECM attachment in vivo is seen at the basal surface of epithelial cells, anchored to the underlying basement membrane.
• Hemidesmosomes contain a dense cytoplasmic plaque with keratin filaments.
• Keratin filaments are linked to the ECM by integrins.

Question 11

What are cell-cell adhesions mediated by?

Answer 11

• Selectins
• Members of the IgSF,
• Members of the integrin family,
• Cadherins.

Question 12

What are selectins?

Answer 12

• They are a family of membrane glycoproteins that bind to specific oligosaccharides.
• They are present in platelets, endothelial cells, leukocytes.
• They have a small cytoplasmic segment, a single membrane-spanning domain, and a large extracellular portion.

Question 13

What are selectins involved in?

Answer 13

• Cell-cell adhesion, and
  transmembrane signal transduction.
• Signaling: cell growth, migration, differentiation, or survival.

Question 14

What is the term “Lectin”?

Answer 14

It is a term used for a compound that binds to specific carbohydrate groups.

Question 15

What are the Immunoglobulin superfamily?

Answer 15

The human genome encodes 765 different Ig domains. These are members of the immunoglobulin superfamily.
- Involved in immune functions.
- Original function likely cell-
adhesion mediators (Ca2+
independent).
- Developmental roles in neuronal growth and circuitry.

Question 16

Where are cadherins found?

Answer 16

In adherens junctions and desmosome.
- Linkages are calcium-dependent.

Question 17

What are cadherins?

Answer 17

Membrane glycoprotein family.
- Cadherins typically join cells of similar type to one another
(Ca2+ dependent).
- Possibly the single most
important factor in molding
cells into cohesive tissues in the embryo and holding them
together in the adult.
- Cadherin loss is associated with malignancy.

Question 18

What are tight junctions and how do they occur?

Answer 18

• They seal the extracellular space.
• they occur between neighboring epithelial cells.
• The prevent solute distribution where different solute concentrations are in
  adjacent compartments.
• The points of cell–cell
  contact are sites where
  integral proteins of two
  adjacent membranes meet
  within the extracellular
  space.
• Major proteins are claudins

Question 19

What are gap junctions?

Answer 19

• They are sites between animal cells that are specialized for intercellular communication.
• Plasma membranes of a gap
  junction contain channels that
  connect the cytoplasm of one cell with the cytoplasm of the
  adjoining cell.
• Plasma membranes come very close to one another but do not make direct contact at gap junction.

Question 20

What are gap junctions composed of?

Answer 20

They are composed of several integral membrane proteins known as connexin, and
organized into multi-subunit
complexes called connexons, that span the membrane.