function of epithelia Flashcards
why is water movement across the epithelia important
body temp regulation
mucus movement - pathogen clearance from the lung
renal fluid balance
digestion and nutrient absorption
reproduction
diarrhoea - pathogen clearance from the gut
the transepithelial potential arises from ion movements which are determined by:
ionic valency
concentration gradience
ionic permeability
what is the flux of an ion described as
ficks law of diffusion
what drives water movement across a “tight” epithelial monolayer
ion movement
what determines direction and magnitude of water movement
polarised organisation of channels, pumps, exchangers and junction proteins
what are the 2 routes of transepithelial water movement
intracellular and paracellular
what is required for water to move across membranes
an osmotic graduient (Na+ drives inwardly to move H2O)
how does intracellular movement of water work
occurs within cells and is regulated by water channels known as aquaporins
how does paracellular movement of water work
occurs between cells and is regulated by tight junction permeability
what does adrenaline do to water uptake
increased adrenaline = increased water uptake
what is H2O secretion dependant on
Chloride ions
what keeps the epithelium at a basal moisture level
H2O outtake from Cl-
what drives Na+ uptake
antihistamines
what is fluid uptake dependant on
Na+ (ENaC)
Cl- channels =
fluid secretion (basolateral to apical transport)
Na+ channels =
fluid absorption (apical to basolateral transport)
what do swelling-activated Cl- channels do
activated transiently by osmotic shock - sustained opening does not occur
what do calcium-activated Cl- channels do
activated by release of intracellular Ca2+ stores - activity is transient and therefore unlikely to be sustained in development
what do outwardly rectifiying Cl- channels do
regulated by release of intracellular ATP - maintains cell membrane potential by regulated depolarisation to physiological set point
what do CFTRs do
best characterised channels due to role in CF disease - long presumed to be the channel regulating fluid secretion in adult lungs
what do voltage-dependant Cl- channels do
recently characterised in lung - expression pattern follows the process of lung development
what does CFTR stand for
the cystic fibrosis transmembrane conductance regulator
what is the CFTR a member of
the ATP binding cassette (ABC) glycoprotein superfamily
what is the CFTR
a 170kDa glycoprotein Cl- channel composed of:
2, 6-span transmembrane domains
2 nucleotide binding domains (NBD! and 2)
a single R domain of highly charged amino acids
what is the R domain within the CFTR
a regulatory site containng several phosphorylation sites for PKA and PKC. activation of CFTR Cl- conductance requires ATP binding to the NBD domains and phosphorylation of the R domains
what are the steps of the CFTR cycle
- channel inactive
- cAMP activation of PKA phosphorylates R domain
- ATP binds to NBD1&2
- ATP is hydrolysed
- channel opens and conducts Cl-
- dephosphorylation of the R domain inactivates the channel
what mutation causes CF
delta F508 (loss of phenylalanine)
where are voltage-dependant Cl- ion channels (CLCN1-7) located
widely distributed (found in epithelia, muscle, nerve tissue) (also found in plants)
how are CLCNs opened
the channel openings are gated by membrane potential
how is CLCN2 that is expressed in epithelium activated
at negative (hyperpolarised) cell membrane potentials
what is the structure of CLCNs
10 transmembrane domains which dimerise to form 2 pores (tandem pore-domain) - each pore is voltage gated
what happens when CLCN1 mutates
myotonia (failure of muscles to relax after contraction as cells remain depolarised)
what happens when CLCN5 mutates
dent’s disease (fluid transport problems in kidney resulting in kidney stones, calcium and protein loss in urine)
where are CLCN2&3 developmentally expressed and what do they control during development
in the fetal lung - they control lung fluid volume
what is ENaC
a Na+ selective ion channel found in all secretory epithelia (eg lung, kidney, gut, salivary duct, sweat duct)
what is ENaC composed of
3 subunits (a,b,y)
what happens when a subunit of ENaC is knocked out
it is lethal at birth due to flooding of the lungs
what happens when b or y subunits of ENaC are knocked out
not lethal but is associated with a makred reduction in the rate of Na+ transport
what is the dominant ENaC pore forming subunit
aENaC
what is required to form a tetramer and confer Na+ selectivity
association of a with b and/or y
what are the common structural features of ENaC subunits
extracellular domain, transmembrane domain and intracellular domain
where is the cysteine rich region located on ENaC
the extracellular loops
what do the cysteien rich regions on ENaC do
they are rich in CSSC sulphur cross linking which determines tertiary structure
what do histidine glycine residue rich regions on ENaC do
they are involved in channel opening and closinv
what do proline tyrosine residue rich regions on ENaC do
they serve as a binding motif for NEDD4 - a ubiquitin ligase which targets the subunit for membrane removal and proteolytic degradation
what is ENaC inhibited by
amiloride
what is ENaC activates by
b2 adrenergic agnoists
what is ENaC conductance induced by
catecholamines (adrenaline)
what level is airway surface liquid (ASLL) controlled at
depth of 7um
what is needed to homeostatically control ASL death
both CFTR and ENaC
what increases ENaC activity
channel activating proteases (CAP)
what is the mechanism of action of CAPs
secreted CAP inhibitors block CAP activity in concentrated ASL, 3 extracellular CAPs found in ASL can cleave ENaC, so increasing its activity
what is the theory of regulated fluid balance in the lung (when its too high
high ASL dilutes secreted protease inhibitors and secreted CFTR agonists - CFTR inactivates - CAPs are free to cleave and activate ENaC - apical to basolateral Na+ transport increased, Cl- follows by paracellular routes - inwardly directed osmotic gradient lowers ASL height
what is the theory of regulated fluid balance in the lung (when its too low)
low ASL concentrates secreted CAP protease inhibitors and secreted CFTR agonists - CFTR active - CAPs are inhibited, ENaC activity ceases as C-terminus of b-ENaC cleaved - basolateral to apical Cl- increased, Na+ follows via a paracellular route - outwardly directed osmotic gradient raises ASL height
