lecture 6 Flashcards
what are the two types of Cl- channels?
CFTR and Ca2+ activated Cl- channel (CaCC)
what is channel activity regulated by?
PKA and ATP
how does the CFTR channel open and close?
- PKA phosphorylation of RD induces ATP binding and dimerisation of NBDs
- conformational change in NBDs terminal transmitted to MSDs- leading to pore opening
- ATP hydrolysed and pore closes
- dephosphorylation of RD by protein phosphotases closes the channel, even in prescence of ATP
what does CFTR require to open?
ATP binding and PKA phosphorylation
where are Calcium activated Cl- channels (CaCC) found?
-present in apical membrane of most epithelial cells that express CFTR
-apical Membrane of gland secretory acinar cells
-endocrine cells, smooth & skeletal muscle, neurones
how are CaCC activated?
-by rise in cytosolic Ca2+
what regulates CaCC?
calmodulin and calmodulin dependent kinase
what is the structure of TMEM16A?
-10 TMDs
-pore region is TMD 6-9
-Ca2+ ions bind to glutamate residues in one fo the two alpha helices of intracellular loop 3 (opens pore, enables Cl- transport)
what are the two mechanisms in which HCO3- is secreted?
- directly through Cl- channel itself
- indirectly, via coupling the Cl- channel with an apical Cl-/HCO3- exchanger
what does cAMP/PKA activation in CFTR switch on?
anion epithelial activity
-requires physical interaction of 2 proteins aided by scaffold protein CAP70 and CFTR RD phosphorylation
what happens in CF with anion exchangers?
-anion exchanger activity is inhibited
-reduces net HCO3-
-reduces fluid secretion
what do acinar cells do?
-secrete digestive enzyme
-produce low volume NaCl rich fluid into ducts using TMEM16A channels
what do duct cells do?
-transport digestive enzymes to small intestine
-produce high volume NaHCO3 rich secretion using CFTR and SLC26A6
what is the CF treatment?
pancreatic enzyme replacement therapy (PERT)
what is fluid secretion driven by?
NaCl and NaHCO3- secretion
with CFTR the dominant anion channel
what is fluid absorption driven by?
multiple absorptive mechanisms depending on site in GI tract
what does failure in fluid absorption or excessive fluid secretion lead to?
rapid dehydration, electrolyte imbalance, death
where does ENaC mediated fluid absorption occur?
-colon
what are the other two types of absorption?
- Na linked absorptive ion transporters (NaCl)
- nutrient absorptive transporters
what condition occurs with dysregulation of fluid homeostasis?
secretory diarrhoeas (SD)
what is secretory diarrhoea?
-2nd leading cause of death in under 5s
-caused by dysregulation in cell signalling
how can SD be prevented?
by safe drinking water, adequate hygiene and sanitation
what occurs in SD with cholera toxin infection?
-cholera toxin inhibits NaCl and fluid absorption
-stimulates CFTR mediated Cl-/HCO3- and fluid secretion
how does this infection occur?
-cholera toxin causes ADP-ribosylation of G protein that blocks GTP hydrolysis
-anion channel becomes permenantly active- lots of cAMP
-leads to uncontrolled overstimulation of CFTR and inhibits NaCl absorption
= excessive salt and water loss into intestinal lumen = severe diarrhoea
