Cell Junctions Flashcards
What are the four types of cellular junctions?
Tight (or occluding) junctions
Cell-Cell Anchoring junctions
Gap (or communicating) junctions
Cell-Matrix Anchoring junctions
What is the general function of tight junctions?
Keep things out of the cell.
Form a permeability barrier.
What is the purpose of tight junctions in the blood epithelium and intestinal lumen?
Blood vessel epithelium, esp. in the brain. - to prevent substances in blood from leaking out
Intestinal lumen - to prevent water from outside the intestines from leaking in
What is the importance of transcellular transport across cells linked by tight junctions?
Transcellular transport enables the cell to move nutrients through the cell, establishing a gradient. Otherwise, cell couldn’t absorb anything.
Explain the process of paracellular transport.
Substances cannot cross the apical or basolateral membrane. Instead, they are passed through intercellular space between the cells.
What are the two general functions of anchoring junctions?
To attach cells and their cytoskeletons either to other cells or to the cell matrix.
To stabilize the cell against mechanical stress.
What are the different types of anchoring junctions?
Desmosomes
Adherens junctions
Actin-linked cell-matrix junction
Hemidesmosomes
What is the transmembrane protein found in adherens junctions
A) Nonclassical cadherins
B) Classical cadherins
C) Integrin
D) alpha-beta-integrin, type 17 collagen
B
What is the transmembrane protein found in desmosomes?
A) alpha-beta-integrin, type 17 collagen
B) Classical cadherins
C) Nonclassical cadherins
D) Integrin
C
What is the transmembrane protein found in actin-linked cell-matrix junctions?
A) Nonclassical cadherins
B) Classical cadherins
C) alpha-beta-integrin, type 17 collagen
D) Integrin
D
What is the transmembrane protein found in hemidesmosomes?
A) Nonclassical cadherins
B) alpha-beta-integrin, type 17 collagen
C) Integrin
D) Classical cadherins
B
What cytoskelelal element do desmosomes and hemidesmosomes bind?
A) Actin
B) Myosin
C) Intermediate filaments
D) Microtubules
C
What cytoskelelal element do adherens and actin-linked cell matrix junctions bind?
A) Actin
B) Myosin
C) Intermediate filaments
D) Microtubules
A
What role do selectins play in the interaction between white blood cells and endothelial cells?
WBC rolls along blood vessel. When its selectin contacts a specific sugar signal, it is allowed to leave the blood vessel.
Explain the relationship between desmosomes and pemphigus?
Antibodies against desmosomal cadherin are produced. Cells lose cell-cell adhesion, resulting in blistering.
What is the general function of gap junctions?
To allow small molecules and electric signals to pass through interacting cells
What are the transmembrane proteins that form gap junctions?
Connexins
4 transmembrane proteins
6 together make a pore
What are the 2 types of transmembrane proteins that form tight junctions?
Claudin and occludin
What can pass through connexons?
Molecules less that 1000 Da in size
Explain the relationship of GAGs to proteoglycans.
Proteoglycans are GAGs covalently attached to a protein. They are synth. on core proteins and have sugars attached to then in the Golgi.
Describe the characteristics of glycosaminoglycans (GAGs).
One sulfated sugar, one uronic acid.
Pattern is repeated in a chain.
Absorbs lots of water to occupy space
Which cells secrete the most matrix molecules of connective tissue?
Fibroblasts, chondroblasts (cartilage), osteoblasts (bone) and epithelium (basal lamina)
What are the major components of the extracellular matrix
Glycoaminoglycans, proteoglycans, fibrous proteins
Purpose of proteoglycans
Form specialized gel. Bind growth factors, protease + protease inhibitors.
What are the special features of collagens that provide strength to the matrix?
Long stiff triple helix alpha chains
Gly- smallest aa, hydrophobic ixns
Hydroxyproline+hydroxyserine interchain bonds stabilize the triple helix
What are three principal types of collagens?
Collagen I
Collagen II
Collagen V
Explain how a deficiency in vitamin C reduces the stability of the triple helix and the stability of collagen.
A lack of vitamin C inhibits the hydroxylation of serines and prolines, destabilizing the collagen triple helices
Identify the characteristic of elastin fibers that gives them their elasticity.
Kinked and cross-linked nature of fibers
Explain the consequences of a mutation of the fibrillin gene, i.e. Marfan’s syndrome
Excessive growth of long bones
Tall stature
Weakened blood vessels, heart valves
Aortic dilation and rupture
Malformed cartilage and ligaments
Skeletal abnormalities
Identify the tripeptide sequence in the fibronectin repeat that serves as a binding site to integrins.
Arg-Gly-Asp (RGD) sequence binds integrins.
Identify the major glycoprotein and proteoglycan components of basal lamina.
Laminin is a major adhesive protein that binds cells to the basal lamina
Type IV collagen, Perlecan, Nigoden, Heparin Sulfate are also abundant
Describe the functions of the basal lamina.
a. Molecular filter: Kidney glomerulus
b. Barrier for cell migration: Cancer
c. Cell migration: Neuromuscular Junction
d. Cell orientation is reciprocally regulated by fibronectin fibrils and cytoskeleton
e. Cell survival: Anchorage Dependence is mediated by integrins
Describe the basic structure of integrins.
Transmembrane heterodimer of alpha- and beta- glycoproteins (24 types, 9 types). Ligand binding is Ca2+ or Mg2+-dependent. Focal contact.
Identify the intracellular proteins to which integrins attach and predict their importance to the strength of the association.
Fibronectin
Laminin
Important, as these are the most abundant proteins in the ECM. These allow the cells to remain attached to ECM
Distinguish between inside-out signaling and outside-in signaling
Inside-Out Signaling
Inactive integrins are activated by the cells
a. Platelets activate 3 integrin to bind fibrinogen and cause aggregation
b. T lymphocytes active 2 integrins to bind antigen presenting cells
Outside-In Signaling
a. Clustering of integrins at ECM contact sites activates signaling
b. Focal Adhesion Kinase (FAK) is recruited and activated.
c. Similar to conventional growth factor receptors
