Outline: Epithelial Pharmacology
Lecture 1: Structure and function of epithelia Dr Lesley MacVinish
Lecture 2: Structure and function of CFTR
Lecture 3: Cystic fibrosis
Lecture 4: Therapies for CF
Essays
2018 (Paper 4) Discuss current and future therapies for cystic fibrosis and whether a cure is
on the horizon.
2017 (Paper 3) Homeostasis of airway surface liquid in cystic fibrosis: pathophysiology and
therapeutic targets.
2016 (Paper 3) Discuss the pathogenesis and therapy of inflammation in cystic fibrosis lung
disease.
2015 (paper 3) Infection in patients with cystic fibrosis: Discuss cause and effect.
2014 (paper 3) Discuss strategies for correcting the ΔF508 CFTR protein-folding defect.
2013 (paper 3) Discuss the challenges and promises for treatment of cystic fibrosis.
2012 (paper 3) CFTR: more than simply a chloride channel. Discuss.
2011 (paper 4) Discuss problems associated with Cystic Fibrosis gene therapy.
2010 (paper 4) Discuss CFTR function and its significance as a target for therapy.
2009 (paper 4) A cure for cystic fibrosis may never be found. Discuss.
Lecture 1: Structure and Function of Epithelia
Epithelium
It is a tissue that lines the internal cavities (lung, bladder, stomach) and surfaces of canals
and tracts (alimentary, respiratory, circulatory, genitourinary) throughout the body. It is
one of the four basic types of animal tissue, along with connective tissue, muscle tissue and
nervous tissue.
Functions
The functions of epithelium include:
1) protection of tissues and organs from external milieu
2) allowing movement of water, nutrients and waste, i.e. homeostasis
3) may contain glands for secretion of hormones and other products
The functions will be different for different tissues and organs:
protection for the skin
exchange for respiratory and urinary systems by regulating fluid volumes, secreting
and absorbing electrolytes (Na+, K+, Cl- and HCO3-) and non-electrolytes (sugars, amino
acids)
secretion for digestive system via exocrine glands and endocrine system via
endocrine glands
Classification
There are two classifications of the types of epithelia - by shape and by the number of layers
of cells.
,by shape:
squamous - very flat cells
cuboidal - taller than squamous, equilateral
columnar - cell length greater than width
by number of layers:
simple - single layer of cells, all cells sit on the basement membrane
stratified - several cells stacked upon one another, not all cells touch the basement membrane
pseudostratified - simple but gives an illusion of stratification, nuclei at various levels but all cells sit on the
basement membrane
Simple Squamous Epithelium:
permits exchange of nutrients, wastes or respiratory gases occurs
in alveoli and capillaries of lungs, kidney glomerulus and tubules, capillaries
Simple Cuboidal Epithelium:
cells are larger in volume that in squamous epithelium and undertake absorption and
secretion
in tubules of kidneys and ducts of exocrine glands
Simple Columnar Epithelium:
adapted for absorption and secretion due to the largest cytoplasmic volumes
large organelle density and energy reserves
,produces mucus from goblet cells
in the small intestine, covered with villi and microvilli to increase surface area for absorption
ciliated in small bronchioles to aid mucus movement and in the fallopian tubes of the female
reproductive tract to move the eggs to the uterus
Stratified Squamous Epithelium:
outer layers of skin, mouth, vagina
protects against abrasion, drying out and infection
Stratified Cuboidal Epithelium:
lines ducts of sweat glands
secretes water and ions
Stratified Columnar Epithelium:
secretes mucus
lines epididymus, mammary glands, larynx
Pseudostratified Epithelium:
columnar, tall and thin cells
all cells rest on the basement membrane
nuclei appear at various levels but no distinct layering
in upper and lower respiratory tracts, ciliated
cilia beat in a rhythmic manner to propel mucus along the surfaces of cells
enable mucociliary clearance - important in cystic fibrosis
Cell Polarity
The epithelial cells are polarised, so apical and basolateral membranes differ in protein
composition. It is not clear how polarity is maintained. The apical surface is that which faces
the lumen or outside of the organ. The basolateral surface faces the basal lamina -
extracellular material secreted by the epithelial cells - and the blood vessels. The basement
membrane provides rigidity and separation from neighbouring cell types. The surface area
of basolateral membrane is increased by lateral indignations to enhance water and solute
transport across the membrane. The membrane transporters are distributed unevenly
between apical and basolateral surfaces.
Electroneutrality
Maintenance of cell volume and composition for homeostasis requires electroneutrality,
which implies no accumulation of charge in the cell. Thus, net transport of cations must be
balanced by net movement of anions in parallel or cations in the opposite direction. It is
maintained by electrochemical gradients that are created by the activity of membrane
pumps.
Transport Across Epithelium
There are 3 types of transport across the epithelium:
1) intercellular - via tight junctions that separate apical and basolateral membranes
2) paracellular - via intercellular spaces between the cells
3) transcellular - through the cells - pass both apical and basolateral membranes
, Transcellular: Ion Transport Across Epithelial Cells
- [K+] is maintained at ~100 mM - fairly constant
Na+ is the main ion for absorption - at low concentration inside the cell ~10-25 mM
Cl- is the main ion for secretion - at high concentration inside the cell ~121 mM
Apical surface contains:
ENaC (absorption of Na+)
CFTR (secretion of Cl-)
CAC (secretion of Cl-)
Basolateral surfaces contain:
Na+/K+ ATPase (3 Na+ out vs 2K+ in)
Na+/K+/2Cl- symporter (absorption of Na+, K+, Cl-)
different types of K+ channels (secretion of K+)
The activities of ion transporters are regulated by cAMP and Ca2+ signalling pathways and
can be regulated by different agonist and antagonist upstream. PKA and PKC phosphorylate
CFTR, PKA also activates Na+/K+/2Cl- symporter, Ca2+ stimulates opening of CAC and both
PKA and Ca2+ stimulate K+ channels (different types).
As can be seen, absence of one transporter would have a knock-on effect on the activity of
other transporters in epithelium and will disrupt ionic balance, which is why absent CFTR in
CF has a severe disease phenotype and it depends on how much of the expected numbers
of the transporter is expressed.
What drugs inhibit ion secretion and absorption?
Amiloride blocks ENaC at low mM concentration
Ouabin inhibits Na+/K+ pump
Loop diuretics, e.g. furosemide, block Na+/K+/2Cl- co-transporter (NKCC)
XE991/293B block cAMP-activated K+ channels experimentally
-Charybdotoxin blocks Ca2+-activated K+ channels
Ba2+ causes a non-specific blockade of any K+ channels
No specific blockers for Cl- efflux channels
What types of junctions connect epithelial cells?
Lecture 1: Structure and function of epithelia Dr Lesley MacVinish
Lecture 2: Structure and function of CFTR
Lecture 3: Cystic fibrosis
Lecture 4: Therapies for CF
Essays
2018 (Paper 4) Discuss current and future therapies for cystic fibrosis and whether a cure is
on the horizon.
2017 (Paper 3) Homeostasis of airway surface liquid in cystic fibrosis: pathophysiology and
therapeutic targets.
2016 (Paper 3) Discuss the pathogenesis and therapy of inflammation in cystic fibrosis lung
disease.
2015 (paper 3) Infection in patients with cystic fibrosis: Discuss cause and effect.
2014 (paper 3) Discuss strategies for correcting the ΔF508 CFTR protein-folding defect.
2013 (paper 3) Discuss the challenges and promises for treatment of cystic fibrosis.
2012 (paper 3) CFTR: more than simply a chloride channel. Discuss.
2011 (paper 4) Discuss problems associated with Cystic Fibrosis gene therapy.
2010 (paper 4) Discuss CFTR function and its significance as a target for therapy.
2009 (paper 4) A cure for cystic fibrosis may never be found. Discuss.
Lecture 1: Structure and Function of Epithelia
Epithelium
It is a tissue that lines the internal cavities (lung, bladder, stomach) and surfaces of canals
and tracts (alimentary, respiratory, circulatory, genitourinary) throughout the body. It is
one of the four basic types of animal tissue, along with connective tissue, muscle tissue and
nervous tissue.
Functions
The functions of epithelium include:
1) protection of tissues and organs from external milieu
2) allowing movement of water, nutrients and waste, i.e. homeostasis
3) may contain glands for secretion of hormones and other products
The functions will be different for different tissues and organs:
protection for the skin
exchange for respiratory and urinary systems by regulating fluid volumes, secreting
and absorbing electrolytes (Na+, K+, Cl- and HCO3-) and non-electrolytes (sugars, amino
acids)
secretion for digestive system via exocrine glands and endocrine system via
endocrine glands
Classification
There are two classifications of the types of epithelia - by shape and by the number of layers
of cells.
,by shape:
squamous - very flat cells
cuboidal - taller than squamous, equilateral
columnar - cell length greater than width
by number of layers:
simple - single layer of cells, all cells sit on the basement membrane
stratified - several cells stacked upon one another, not all cells touch the basement membrane
pseudostratified - simple but gives an illusion of stratification, nuclei at various levels but all cells sit on the
basement membrane
Simple Squamous Epithelium:
permits exchange of nutrients, wastes or respiratory gases occurs
in alveoli and capillaries of lungs, kidney glomerulus and tubules, capillaries
Simple Cuboidal Epithelium:
cells are larger in volume that in squamous epithelium and undertake absorption and
secretion
in tubules of kidneys and ducts of exocrine glands
Simple Columnar Epithelium:
adapted for absorption and secretion due to the largest cytoplasmic volumes
large organelle density and energy reserves
,produces mucus from goblet cells
in the small intestine, covered with villi and microvilli to increase surface area for absorption
ciliated in small bronchioles to aid mucus movement and in the fallopian tubes of the female
reproductive tract to move the eggs to the uterus
Stratified Squamous Epithelium:
outer layers of skin, mouth, vagina
protects against abrasion, drying out and infection
Stratified Cuboidal Epithelium:
lines ducts of sweat glands
secretes water and ions
Stratified Columnar Epithelium:
secretes mucus
lines epididymus, mammary glands, larynx
Pseudostratified Epithelium:
columnar, tall and thin cells
all cells rest on the basement membrane
nuclei appear at various levels but no distinct layering
in upper and lower respiratory tracts, ciliated
cilia beat in a rhythmic manner to propel mucus along the surfaces of cells
enable mucociliary clearance - important in cystic fibrosis
Cell Polarity
The epithelial cells are polarised, so apical and basolateral membranes differ in protein
composition. It is not clear how polarity is maintained. The apical surface is that which faces
the lumen or outside of the organ. The basolateral surface faces the basal lamina -
extracellular material secreted by the epithelial cells - and the blood vessels. The basement
membrane provides rigidity and separation from neighbouring cell types. The surface area
of basolateral membrane is increased by lateral indignations to enhance water and solute
transport across the membrane. The membrane transporters are distributed unevenly
between apical and basolateral surfaces.
Electroneutrality
Maintenance of cell volume and composition for homeostasis requires electroneutrality,
which implies no accumulation of charge in the cell. Thus, net transport of cations must be
balanced by net movement of anions in parallel or cations in the opposite direction. It is
maintained by electrochemical gradients that are created by the activity of membrane
pumps.
Transport Across Epithelium
There are 3 types of transport across the epithelium:
1) intercellular - via tight junctions that separate apical and basolateral membranes
2) paracellular - via intercellular spaces between the cells
3) transcellular - through the cells - pass both apical and basolateral membranes
, Transcellular: Ion Transport Across Epithelial Cells
- [K+] is maintained at ~100 mM - fairly constant
Na+ is the main ion for absorption - at low concentration inside the cell ~10-25 mM
Cl- is the main ion for secretion - at high concentration inside the cell ~121 mM
Apical surface contains:
ENaC (absorption of Na+)
CFTR (secretion of Cl-)
CAC (secretion of Cl-)
Basolateral surfaces contain:
Na+/K+ ATPase (3 Na+ out vs 2K+ in)
Na+/K+/2Cl- symporter (absorption of Na+, K+, Cl-)
different types of K+ channels (secretion of K+)
The activities of ion transporters are regulated by cAMP and Ca2+ signalling pathways and
can be regulated by different agonist and antagonist upstream. PKA and PKC phosphorylate
CFTR, PKA also activates Na+/K+/2Cl- symporter, Ca2+ stimulates opening of CAC and both
PKA and Ca2+ stimulate K+ channels (different types).
As can be seen, absence of one transporter would have a knock-on effect on the activity of
other transporters in epithelium and will disrupt ionic balance, which is why absent CFTR in
CF has a severe disease phenotype and it depends on how much of the expected numbers
of the transporter is expressed.
What drugs inhibit ion secretion and absorption?
Amiloride blocks ENaC at low mM concentration
Ouabin inhibits Na+/K+ pump
Loop diuretics, e.g. furosemide, block Na+/K+/2Cl- co-transporter (NKCC)
XE991/293B block cAMP-activated K+ channels experimentally
-Charybdotoxin blocks Ca2+-activated K+ channels
Ba2+ causes a non-specific blockade of any K+ channels
No specific blockers for Cl- efflux channels
What types of junctions connect epithelial cells?
