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Volker Endeward, Fabian Itel, Samer Al-Samir, Mohamed Chami, Fredrik Öberg, Kristina Hedfalk, Gerolf Gros
Intrinsic CO2 permeability of cell membranes and role of
CO2 channels
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co
ntr
ol
10
µM
DID
S
Fy N
ull
Fy N
ull +
10
µM
DID
S
JK
Nu
ll
JK
Nu
ll +
10
µM
DID
S
Mc
leo
d
Me
cL
eo
d 1
0 µ
M D
IDS
Ke
ll N
ull
Ke
ll N
ull +
10
µM
DID
S
Rh
po
s.
Rh
ne
g.
Rh
Nu
ll
Rh
Nu
ll +
1
0 µ
M D
IDS
Co
lto
n N
ull
Co
lto
n N
ull +
10
µM
DID
S
PC
O 2 (c
m/s
)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
* *
* **
* * * *
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Intrinsic CO2 permeability of a red cell membrane
PCO2 (cm/s)
Red cell 0.15 ± 0.08
Red cell AQP1,
functional Rh0.015 ± 0.003
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Gas permeability of synthetic phospholipid bilayers
Alberts et al.Molecular Biology Of The Cell, 4th Edition
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?CO2
1. What are the intrinsic CO2 permeabilities of cell membranes?
2. Which mechanisms are responsible for the given intrinsic permeabilities of cell membranes?
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1. What are the intrinsic CO2 permeabilities of cell membranes?
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HC 18O 16 O 2- + H +
H 216O + C 18O 16 O
H 218O + C 16 O 2
Cell
H C 18 O 16O 2- + H +
Mass spectrometer
CAH 2
18 O + C 16 O 2
H 216 O + C 18 O 16O
PHCO3 -
PCO2
PH2O
2/ 3
1/ 3
1/3
2/3
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Cell membranes show CO2 permeabilities lower then synthetic lipid bilayer
PCO2 (cm/s) ± S.D.
Synthetic lipid bilayer 0.35 - 3.2
Red cell, functional gas channel 0.015 ± 0.003
MDCK 0.017 ± 0.004
tsA201 0.007 ± 0.003
Basolateral membrane ofproximal colon epithelium ~ 0.022
Apical membrane ofproximal colon epithelium 0.0015 ± 0.0006
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2. Which mechanisms are responsible for the given intrinsic permeabilities of cell membranes?
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Parameter
studied
Cholesterol fraction of
total bilayer lipids
(mol %)
Ratio of
parameter
w over w/o
cholesterol
PNH3 30 % 0.31 Antonenko et al. 1997
PNH3 52 % 0.012 Hill & Zeidel 2000
PH2O (f) 40 % 0.18 Lande et al. 1995
PH2O (f) 52 % 0.026 Hill & Zeidel 2000
PH2O (d) 66 % (L+Chol) 0.26 Finkelstein 1976
" 66 % (SM + Chol) 0.037 "
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PC = Phosphatidylcholine
PS = Phosphatidylserine
Chol = Cholesterol (0 – 70%)
Ø = ~ 150 nm
PC:PS:Chol – vesicles with different cholesterol content
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time (s)
0 20 40 60 80 100 120 140
3
4
5
6[C
18O
16O
] - [C
18O
16O
] ∞ (
µM
)
70% chol.
30% chol.
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Effect of cholesterol on lipid vesicle CO2 permeability
% Cholesterol
0 20 40 60 80 100
PC
O2 (
cm/s
)
0.0001
0.001
0.01
0.1
>0.16 cm/sPC:PS = 8:2
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Comparison of cell membranes and cholesterol-containing vesicles
PCO2 (cm/s) ±
S.D
Cholesterolcontent (Mol%)
PCO2 predicted from cholesterol effect in vesicles
(cm/s)
Lipid bilayer 0.35 / 3.2 -
Red cell: AQP1, functional Rh 0.015 ± 0.003 45 0.010
MDCK 0.017 ± 0.004 37 0.015
tsA201 0.007 ± 0.003 - -
Basolateral membraneprox colon epithelium
~ 0.022 42 0.011
Apical membrane ofprox colon epithelium
0.0015 ± 0.0006 77 0.0016
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CO2 permeabilities of cell membranes appear to be
essentially determined by their cholesterol content
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cholesterol depletionwith β-cyclodextrin
cholesterol enrichmentwith β-cyclodextrin
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Is cholesterol the cause of the low CO2 permeability ofMDCK cells?
MD
CK
-Cho
l
MD
CK
con
trol
MD
CK
+C
hol
PC
O2 (
cm/s
)
0.000
0.025
0.050
0.700
*
> 0.75
Reduction of cholesterol with cyclodextrin raises PCO2.
Enrichment with cholesterol lowers PCO2 compared to normal cells.
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• We show that cell membranes possess a low intrinsic CO2 permeability,
often in the range of 0.01 cm/s.
• This permeability is 2, and in one case 3, orders of magnitude lower than
the CO2 permeability of pure artificial phospholipid bilayers.
• The main cause of this low CO2 permeability is the cholesterol content of
the cell membrane. With increasing cholesterol content PCO2 decreases
in artificial vesicles as well as in intact cells.
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1. Consequences of the extremely low CO2 permeability of the
apical membrane of colon epithelium
2. Effect of low CO2 membrane permeability on red blood cell
gas exchange
Physiological consequences of low
CO2 membrane permeabilities
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Consequences of low apical CO2 permeability in colonocytes
40
100
pCO 2(mmHg )
diffusion pathway (µm)0 20
apical membrane
colon lumen
basal membrane
capillary
PCO2 = 0.0015 cm/s
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PCO2
(cm/s)
t95
(ms)
transit time
lung capillary
(ms)
transit time
heavy exercise
(ms)
normal membrane resistance
0.15 110 700 350
permeability
functional gas channel
0.01 1000 700 350
Example of a cell with a high gas exchange: red blood cell
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• From these considerations we can see that gas exchange of cells with
a low CO2 permeability is limited.
• Hypothesis: cell membranes with normal cholesterol and low intrinsic PCO2
adapt their CO2 permeabilities to their needs by incorporating gas chanels
in the membrane.
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AqpZ
AQP1
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Aquaporin 1 as a CO2 channel in cholesterol-containing lipid vesicles
hAQ
P1
LPR
400
hAQ
P1
LPR
230
hAQ
P1
LPR
200
hAQ
P1
LPR
140
AQ
Pz
LPR
200
P
CO
2 (cm
/s)
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
******
***
**
ns
Incorporation of AQP1 into vesicles causes a rise in PCO2
Change of PCO2 in vesicles with decreasing Lipid- Protein-Ratios (LPR)
PC:PS:Chol 8:2:10
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cont
rol P
C:P
S:C
hol (
8:2:
10)
cont
rol +
DID
S
hAQ
P L
PR
200
hAQ
P L
PR
200
+ D
IDS
hAQ
P1
LPR
140
hAQ
P1
LPR
140
+ D
IDS
PC
O2 (
cm/s
)
0.00
0.01
0.02
0.03
0.04
* *
DIDS reduces the CO2 permeability of AQP1 containing vesicles
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Aquaporin 1 as a CO2 channel in MDCK cells
Expression of AQP1 in MDCK cells raises PCO2
cont
rol
cont
rol +
DID
S
hAQ
P1
hAQ
P1+
DID
S
PC
O2 (
cm/s
)
0,000
0,005
0,010
0,015
0,020
0,025
0,030
**
$$
n.s.
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• We conclude that in a membrane of normal cholesterol content and low
CO2 permeability, incorporation of AQP1 into the membrane significantly
increases the CO2 permeability in a concentration dependent manner.
• AQP1 acts as a DIDS-sensitive CO2 channel.
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Gas CO2 O2 NO N2
Lipid-water partition
coefficient0.95 2.9 3.8 4.1
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CO2 O2
Lipid-water partition coefficient 0.95 2.9
Reduction of membrane permeability by cholesterol 1/100 (1/100) ?
Membrane permeability 0.01 cm/s (0.03 cm/s) ?
Heart muscle under heavy exercise:partial pressure difference across
the membrane ΔP5 mmHg (40 mmHg) ?
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Summary
With rising cholesterol content the CO2 permeability ( PCO2) of lipid vesicles decreases drastically.
The intrinsic PCO2 of cell membranes is low due to their cholesterol content:
1) cell membranes and lipid vesicles with identical cholesterol content exhibit identical CO2 permeability
2) cholesterol-depleted cell membranes have an increased CO2
permeability, cholesterol-enriched cell membranes a reduced permeability
Cell membranes with normal cholesterol raise their CO2 permeability, when functionally required, by incorporation of CO2 channels:
1) AQP1 incorporated in lipid vesicles raises CO2 permeability in a concentration-dependent manner
2) AQP1 expression in MDCK cells increases membrane PCO2.
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Medizinische Hochschule Hannover Vegetative Physiologie
Samer Al-Samir
Timo Meine
Werner Zingel
Gerolf Gros
Universität Basel Dept. Chemie / Biozentrum
Fabian Itel
Mohamed Chami
University of Gothenburg Dept. Chemistry/Biochemistry
Frederic Öberg
Kristina Hedfalk