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human epithelial biology > function of epithelia > Flashcards

function of epithelia Flashcards

1
Q

why is water movement across the epithelia important

A

body temp regulation
mucus movement - pathogen clearance from the lung
renal fluid balance
digestion and nutrient absorption
reproduction
diarrhoea - pathogen clearance from the gut

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2
Q

the transepithelial potential arises from ion movements which are determined by:

A

ionic valency
concentration gradience
ionic permeability

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3
Q

what is the flux of an ion described as

A

ficks law of diffusion

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4
Q

what drives water movement across a “tight” epithelial monolayer

A

ion movement

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5
Q

what determines direction and magnitude of water movement

A

polarised organisation of channels, pumps, exchangers and junction proteins

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6
Q

what are the 2 routes of transepithelial water movement

A

intracellular and paracellular

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7
Q

what is required for water to move across membranes

A

an osmotic graduient (Na+ drives inwardly to move H2O)

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8
Q

how does intracellular movement of water work

A

occurs within cells and is regulated by water channels known as aquaporins

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9
Q

how does paracellular movement of water work

A

occurs between cells and is regulated by tight junction permeability

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10
Q

what does adrenaline do to water uptake

A

increased adrenaline = increased water uptake

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11
Q

what is H2O secretion dependant on

A

Chloride ions

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12
Q

what keeps the epithelium at a basal moisture level

A

H2O outtake from Cl-

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13
Q

what drives Na+ uptake

A

antihistamines

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14
Q

what is fluid uptake dependant on

A

Na+ (ENaC)

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15
Q

Cl- channels =

A

fluid secretion (basolateral to apical transport)

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16
Q

Na+ channels =

A

fluid absorption (apical to basolateral transport)

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17
Q

what do swelling-activated Cl- channels do

A

activated transiently by osmotic shock - sustained opening does not occur

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18
Q

what do calcium-activated Cl- channels do

A

activated by release of intracellular Ca2+ stores - activity is transient and therefore unlikely to be sustained in development

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19
Q

what do outwardly rectifiying Cl- channels do

A

regulated by release of intracellular ATP - maintains cell membrane potential by regulated depolarisation to physiological set point

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20
Q

what do CFTRs do

A

best characterised channels due to role in CF disease - long presumed to be the channel regulating fluid secretion in adult lungs

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21
Q

what do voltage-dependant Cl- channels do

A

recently characterised in lung - expression pattern follows the process of lung development

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22
Q

what does CFTR stand for

A

the cystic fibrosis transmembrane conductance regulator

23
Q

what is the CFTR a member of

A

the ATP binding cassette (ABC) glycoprotein superfamily

24
Q

what is the CFTR

A

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

25
Q

what is the R domain within the CFTR

A

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

26
Q

what are the steps of the CFTR cycle

A
  1. channel inactive
  2. cAMP activation of PKA phosphorylates R domain
  3. ATP binds to NBD1&2
  4. ATP is hydrolysed
  5. channel opens and conducts Cl-
  6. dephosphorylation of the R domain inactivates the channel
27
Q

what mutation causes CF

A

delta F508 (loss of phenylalanine)

28
Q

where are voltage-dependant Cl- ion channels (CLCN1-7) located

A

widely distributed (found in epithelia, muscle, nerve tissue) (also found in plants)

29
Q

how are CLCNs opened

A

the channel openings are gated by membrane potential

30
Q

how is CLCN2 that is expressed in epithelium activated

A

at negative (hyperpolarised) cell membrane potentials

31
Q

what is the structure of CLCNs

A

10 transmembrane domains which dimerise to form 2 pores (tandem pore-domain) - each pore is voltage gated

32
Q

what happens when CLCN1 mutates

A

myotonia (failure of muscles to relax after contraction as cells remain depolarised)

33
Q

what happens when CLCN5 mutates

A

dent’s disease (fluid transport problems in kidney resulting in kidney stones, calcium and protein loss in urine)

34
Q

where are CLCN2&3 developmentally expressed and what do they control during development

A

in the fetal lung - they control lung fluid volume

35
Q

what is ENaC

A

a Na+ selective ion channel found in all secretory epithelia (eg lung, kidney, gut, salivary duct, sweat duct)

36
Q

what is ENaC composed of

A

3 subunits (a,b,y)

37
Q

what happens when a subunit of ENaC is knocked out

A

it is lethal at birth due to flooding of the lungs

38
Q

what happens when b or y subunits of ENaC are knocked out

A

not lethal but is associated with a makred reduction in the rate of Na+ transport

39
Q

what is the dominant ENaC pore forming subunit

A

aENaC

40
Q

what is required to form a tetramer and confer Na+ selectivity

A

association of a with b and/or y

41
Q

what are the common structural features of ENaC subunits

A

extracellular domain, transmembrane domain and intracellular domain

42
Q

where is the cysteine rich region located on ENaC

A

the extracellular loops

43
Q

what do the cysteien rich regions on ENaC do

A

they are rich in CSSC sulphur cross linking which determines tertiary structure

44
Q

what do histidine glycine residue rich regions on ENaC do

A

they are involved in channel opening and closinv

45
Q

what do proline tyrosine residue rich regions on ENaC do

A

they serve as a binding motif for NEDD4 - a ubiquitin ligase which targets the subunit for membrane removal and proteolytic degradation

46
Q

what is ENaC inhibited by

A

amiloride

47
Q

what is ENaC activates by

A

b2 adrenergic agnoists

48
Q

what is ENaC conductance induced by

A

catecholamines (adrenaline)

49
Q

what level is airway surface liquid (ASLL) controlled at

A

depth of 7um

50
Q

what is needed to homeostatically control ASL death

A

both CFTR and ENaC

51
Q

what increases ENaC activity

A

channel activating proteases (CAP)

52
Q

what is the mechanism of action of CAPs

A

secreted CAP inhibitors block CAP activity in concentrated ASL, 3 extracellular CAPs found in ASL can cleave ENaC, so increasing its activity

53
Q

what is the theory of regulated fluid balance in the lung (when its too high

A

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

54
Q

what is the theory of regulated fluid balance in the lung (when its too low)

A

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

human epithelial biology (5 decks)

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