Chapter 7: Interactions between cells and their environment

Question 1

What is glycocalyx?

Answer 1

it is a protective cell coat formed by carbohydrate chains of glycoproteins and glycolipids projecting out of the plasma membrane.

Question 2

What are the functions of the glycocalyx?

Answer 2

• provide mechanical protection for cell
• serve as a barrier to particles moving toward the cell membrane
• Bind to regulatory factors
• Mediate cell-cell and cell-substratum interactions

Question 3

Describe the ECM (Extracellular Matrix). Why is it significant?

Answer 3

• its an organized network of extracellular materials beyond the immediate vicinity of the plasma membrane
• It plays a key regulatory role in determining the shape and activities of the cell

Question 4

What is the basal Lima (Basement Membrane)? How common is it?

Answer 4

• one of the best defined ECM
• shows up in many multiple tissues
  1) surrounds nerve fibers, muscles, and fat cells
  2) underlies the basal surface of epithelial tissues
  3) underlies the inner endothelial lining of blood vessels

Question 5

What is the function of the basal membrane?

Answer 5

 Provide mechanical support to cells attached
 Generate signals that maintain cell survival
 Serve as a substratum for cell migration
 Separate adjacent tissues within an organ
 Act as a barrier to the passage of
macromolecules
- Proteins
- Cancer cells

Question 6

Describe Collagen.

Answer 6

• A fibrous glycoprotein
• Present ONLY in ECM
• Found in all animals
• has highly resistant pulling force: Tensile strength
• the single most abundant protein in the human body: 25% of all proteins
• produced primarily by fibroblast, also can be made smooth muscles and epithelial cells

Question 7

How many types of collagen proteins exist in the human body?

Answer 7

 28 subtypes in the human body
- Each collagen is restricted to particular locations in
the body

Question 8

What are common features among collagen proteins?

Answer 8

Common features:
o Most collagen molecules are trimers
consisting of 3 α chains
o The 3 α chains of a collagen molecule are
wound around each other to form a unique,
rod-like triple helix along at least part of the
length
- Type I, II, III and some others are fibrillar
collagens

Question 9

Describe what a collagen molecule looks like.

Answer 9

-triple helix of three
helical alpha chains

-Hydrogen bonds
between proline and
serine hold the triple
helix together

Question 10

What is a Type IV Collagen?

Answer 10

Type IV is nonfibrillar restricted to the basement

membranes

Question 11

What is the function of collagen?

Answer 11

• Provides the insoluble framework that determines many of the mechanical properties of the ECM
  
  • Tendon: resist pulling forces
  • Cornea: plywood-like structure

Question 12

What can abnormalities in collagen lead to?

Answer 12

It can lead to serious diseases. 
 Fibrosis
• Excessive collagen scar tissue substitutes the
normal tissue
• Lung: pulmonary fibrosis
• Liver: Cirrhosis

Question 13

What are proteoglycans?

Answer 13

• they are protein-polysaccharide complexes, made of
  1) a core protein
  2) multiple glycosaminoglycans (GAGs)
  * these are repeating chains of disaccharides:
  - A-B-A-B-A
  * they’re highly acidic and negatively charged
• the core proteins of mulitple proteoglycans can bind to hyaluronic (a nonsulfated GAG) to form gigantic complexes

Question 14

What is the function of proteoglycans?

Answer 14

• They provide resistance to crushing (compression) forces

 Negative charged GAGs attract a huge
amount of cations binding to proteoglycans ,
which in turn attract a large amount of H2O

 GAG + Cations + H2O form a porous gel

 Gel fills the extracellular space with help of
scaffold formed by collagen

Question 15

What does Fibronectin do for the ECM?

Answer 15

• provides binging sites for numerous components of the ECM, including collagens, protoglycans, and other fibronectin molecules
• it facilitates the establishment of a stable, interconnected network of ECM
• provides binding sites for receptors on the cell surface
• holds ECM in a stable attachment to the cell
• Influence cell’s potential for migration, growth and differentiation

Question 16

What are Laminins? What are their function?

Answer 16

• Extracellular glycoproteins
• More than 15 types have been found
• Help cell migration, growth and differentiation during
  embryonic development
• Components of basement membranes lining tissues.
• Bind to cell membrane receptors and other ECM proteins

Question 17

Explain the Dynamic Properties of the ECM.

Answer 17

Spatially, the ECM can be stretched
during tension

Temporally, the ECM undergoes
constant remodeling by degradation
and reconstruction

Question 18

How does the cell interact with extracellular materials?

Answer 18

• receptors on the plasma membrane help cells attach to extracellular materials
• these receptors are known as Integrins

Question 19

What are Integrins?

Answer 19

• a family of integral membrane proteins
• ONLY found in ANIMALS
• made of HETERODIMERS with ALPHA and BETA subunits
• Plays a MAJOR role in:
  1) Adhesion of cells to their substratum or
  other cells
  2)Transmission of signals between the
  external environment and the cell interior

Question 20

How are Integrins inactivated/activated?

Answer 20

• Integrins are inactivated in its “bent” confirmation
• Integrins are activated in its “upright” confirmation
• Signaling of integrin is bi-directional, meaning:
  
  • INSIDE-OUT SIGNALING: Talin binding to cytoplasmic tails separates a and b chains, inducing a conformational change that allows Integrin to bind
    extracellular matrix ligands
  • OUTSIDE-IN SIGNALING: Ligand binds to the extracellular domain of integrin inducing a conformational change that initiates polymerization of actin filaments

Question 21

Explain the role of integrins in platelet aggregation?

Answer 21

• Blood clots form when platelets adhere to one another through fibrinogen bridges that bind to the platelet-integrins
• Integrin binding protein possesses the RGD (arginine-glycine-aspartic acid) domain

Question 22

In terms of connecting the ECM with the cytoskeleton, what two VERY IMPORTANT specialized structures are Integrins involved in forming?

Answer 22

• Focal Adhesion

- Hemidesmosomes

Question 23

What are Focal Adhesions? What are its components?

Answer 23

• sub-cellular structures where cells adhere to their substratum and transmit signals in both directions across the plasma membrane
• Components:
   Extracellular ligands
  • Collagen, Fibronectin

 Large clusters of integrins

 Intracellular adaptor
proteins
-Talin, α-actinin, and vinculin

 Actin filaments

Question 24

What are the properties of Focal Adhesions?

Answer 24

• SCATTERED, DISCRETE SITES
• DYNAMIC: Rapidly assemble and disassemble responding to stimuli or enter mitosis
• may act as a SENSORY STRUCTURE: Collecting extracellular information and transmitting to the cell interior
• function in CELL LOCOMOTION

Question 25

What are Hemidesmosomes?

Answer 25

- they are differentiated sites at the basal surface of epithelial cells where the cells are attached to the underlying basement membrane

Question 26

What are some characteristics of Hemidesmosomes?

Answer 26

 Contain a dense plaque with Keratin filaments on the inner surface of the plasma membrane - Thicker - Intermediate filaments - Play a supportive function - Keratin filaments are linked to the ECM by membrane-spanning integrins

Question 27

What are some properties of Hemidesmosomes?

Answer 27

 The tightest attachment between a cell and its extracellular matrix  Transmit signals from the ECM to the attached epithelial cells

Question 28

How is cell to cell interaction/adhesion SELECTIVE?

Answer 28

Cells have surface-recognition sites that organize cell-cell interaction/adhesion - Selectins - Certain members of the immunoglobulin superfamily (IgSF) - Certain members of the integrin family - Cadherins

Question 29

What are the three types of SELECTINS?

Answer 29

 E-selectin: endothelial cells  P-selectin: platelets and endothelial cells  L-selectin: leukocytes

Question 30

What is the function of SELECTINS?

Answer 30

- mediate transient interactions between circulating leukocytes and vessel walls at sites of inflammation and clotting - Ca2+-dependent

Question 31

What is the functions of the Immunoglobin Superfamily?

Answer 31

- Functions in: cell to cell interaction and immune response - Mediating interactions of lymphocytes with cells required for an immune response (e.g., macrophages, lymphocytes, and target cells) - Ca2+-independent

Question 32

What are CADHERINS?

Answer 32

- Glycoproteins that mediate Ca2+-dependent cell to cell adhesion.

Question 33

How do CADHERINS work?

Answer 33

- Join cells of similar types to one another by preferential binding to the same cadherin present at the surface of the neighboring cell

Question 34

How many family of CADHERINS are there? Why is this important?

Answer 34

 Family of at least 30 related glycoproteins for cell adhesion  E-cadherin (epithelial)  N-cadherin (neural)  P-cadherin (placental)  The greater the number of interacting cadherins in a cluster, the greater the strength of adhesion between apposing cells

Question 35

Describe the cell to cell interactions.

Answer 35

Cell to cell adhesive junctions - Adherens Junctions - Desmosomes Epithelial specialized cell to cell junctions - Tight Junctions - Gap Junctions

Question 36

Describe the characteristics of Adheren Junctions?

Answer 36

- Commonly found in the body, especially in epithelia - Form "belts" to encircle each cell near it apical surface - Cadherin molecules are responsible for the establishment of AJ (Adheren Junctions) - The cytoplasmic domain of the cadherin molecule is connected to the actin filaments of the cytoskeleton by linking proteins

Question 37

What are the functions of AJs?

Answer 37

 Anchor cells to other cells  Connect the external environment to the actin cytoskeleton  Provide a signaling pathway for signals from extracellular space to the cytoplasm

Question 38

Describe Desmosomes.

Answer 38

 Disk-shaped adhesive junctions between cells  Found in a variety of tissues (areas of mechanical stress, e.g. skin, gum, cervix).  Cadherins link the two cells across a narrow gap  Intermediate filaments are linked to the cytoplasmic domain of cadherin

Question 39

What are Tight Junctions? What are their purpose?

Answer 39

- they're specialized contacts between epithelial cells - exists at sites where two compartments containing different solute concentrations (I.E. Urinary bladder, skin) - Located at very apical surface between adjacent cells - Serves as a barrier to free diffusion of water and solutes from the extracellular matrix compartment - Some are permeable to specific ions or solutes

Question 40

What are Gap Junctions? What are their purpose?

Answer 40

- sites between animal cells that are specialized for intercellular communication - "Pipelines" pass through adjoining cell membranes and open into the cytoplasm of adjoining cells - made entirely out of integral membrane proteins connexin - Connexin are organized into a complex called Connexon

Question 41

How do Gap Junctions mediate intercellular interactions?

Answer 41

Gap-junction intercellular communication (GJIC) allows the passage of low-weight molecules or ions Gap junctions can allow integration of activities of individual cells into a functional unit  Highly active regulatory substances, such as cAMP  Key metabolites, such as ATP, amino acids, small co-enzymes

Question 42

What is a Junctional Complex?

Answer 42

 A specialized array of junctions for cell-cell interaction lining on the lateral surface of epithelial cells  Composed of - Tight junctions - Adherens junction - Desmosomes