Metabolism:
Inhibition of Mitochondrial Pyruvate
Transport by Zaprinast Causes Massive
Accumulation of Aspartate at the Expense
of Glutamate in the Retina
J. Biol. Chem. 2013, 288:36129-36140.
doi: 10.1074/jbc.M113.507285 originally published online November 1, 2013
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This article cites 49 references, 18 of which can be accessed free at
http://www.jbc.org/content/288/50/36129.full.html#ref-list-1
Downloaded from http://www.jbc.org/ at University of Washington on March 23, 2014
Jianhai Du, Whitney M. Cleghorn, Laura
Contreras, Ken Lindsay, Austin M. Rountree,
Andrei O. Chertov, Sally J. Turner, Ayse
Sahaboglu, Jonathan Linton, Martin Sadilek,
Jorgina Satrústegui, Ian R. Sweet, François
Paquet-Durand and James B. Hurley
THE JOURNAL OF BIOLOGICAL CHEMISTRY VOL. 288, NO. 50, pp. 36129 –36140, December 13, 2013
© 2013 by The American Society for Biochemistry and Molecular Biology, Inc. Published in the U.S.A.
Inhibition of Mitochondrial Pyruvate Transport by Zaprinast
Causes Massive Accumulation of Aspartate at the Expense of
Glutamate in the Retina*□
S
Received for publication, August 2, 2013, and in revised form, October 30, 2013 Published, JBC Papers in Press, November 1, 2013, DOI 10.1074/jbc.M113.507285
Background: Pyruvate transport into mitochondria is a key step in energy metabolism. Zaprinast is a well known phosphodiesterase inhibitor.
Results: Zaprinast has a strong influence on pyruvate transport into mitochondria.
Conclusion: Inhibition of the mitochondrial pyruvate carrier by Zaprinast causes accumulation of aspartate at the expense of
glutamate.
Significance: Maintenance of normal amino acid levels in the retina relies on pyruvate transport into mitochondria.
Transport of pyruvate into mitochondria by the mitochondrial pyruvate carrier is crucial for complete oxidation of glucose and for biosynthesis of amino acids and lipids. Zaprinast is
a well known phosphodiesterase inhibitor and lead compound
for sildenafil. We found Zaprinast alters the metabolomic profile of mitochondrial intermediates and amino acids in retina
and brain. This metabolic effect of Zaprinast does not depend on
inhibition of phosphodiesterase activity. By providing 13C-labeled glucose and glutamine as fuels, we found that the metabolic profile of the Zaprinast effect is nearly identical to that of
inhibitors of the mitochondrial pyruvate carrier. Both stimulate
oxidation of glutamate and massive accumulation of aspartate.
Moreover, Zaprinast inhibits pyruvate-driven O2 consumption
in brain mitochondria and blocks mitochondrial pyruvate carrier in liver mitochondria. Inactivation of the aspartate glutamate carrier in retina does not attenuate the metabolic effect of
Zaprinast. Our results show that Zaprinast is a potent inhibitor
of mitochondrial pyruvate carrier activity, and this action causes
aspartate to accumulate at the expense of glutamate. Our findings show that Zaprinast is a specific mitochondrial pyruvate
* This work was supported, in whole or in part, by National Institutes of Health
Grants EY06641 and EY017863 (to J. B. H.). This work was also supported by
grants from the Deutsche Forschungsgemeinschaft (Pa1751/4-1), EU
(DRUGSFORD: HEALTH-F2-2012-304963), and the Kerstan Foundation (to
F. P.-D.) and the Ministerio de Economı́ay Competitividad (BFU201130456-C02-01/BMC) and Coumunidad Autónoma de Madrid (S2010/BMD2402) (to J. S.). This work was also funded by the CIBERER, an initiative from
the Instituto de Salud Carlos III, and an institutional grant from the Fundación Ramón Areces to the Centro de Biología Molecular Severo Ochoa.
□
S
This article contains supplemental Methods and Figs. 1–9.
1
Recipient of a Junta De Ampliación de Estudios postdoctoral contract from
Consejo Superior de Investigaciones Científicas.
2
To whom correspondence should be addressed: Depts. of Biochemistry and
Ophthalmology, University of Washington, Seattle, Washington 98195.
Tel.: 206-543-2871; Fax: 206-685-2320; E-mail: jbhhh@u.washington.edu.
DECEMBER 13, 2013 • VOLUME 288 • NUMBER 50
carrier (MPC) inhibitor and may help to elucidate the roles of
MPC in amino acid metabolism and hypoglycemia.
Pyruvate is a critical metabolite that links glycolysis with the
mitochondrial tricarboxylic acid (TCA) cycle, and it is a hub for
synthesis of amino acids, carbohydrates, and fatty acids. Pyruvate enters mitochondria through a recently identified mitochondrial pyruvate carrier (MPC),3 a 150-kDa complex of
MPC1 and MPC2 located on the inner mitochondrial membrane (1, 2). Knock-out of MPC1 in Drosophila elevates pyruvate and decreases mitochondrial TCA intermediates (1). Children with mutations in a conserved region of human MPC1
have symptoms of lactic acidosis and hyperpyruvatemia (1, 3).
MPC inhibitors have been identified in the past 40 years including the classical ␣-cyanocinnamate analogs, UK5099, ␣-cyano4-hydroxycinnamic acid (4), and insulin sensitizer thiazolidinedione (5–7).
Zaprinast, a lead compound used for the development of
sildenafil (Viagra), is a well established inhibitor of cGMP-specific phosphodiesterase (PDE). PDEs hydrolyze the cyclic phosphate bond in cAMP and cGMP, and they function to inactivate
cyclic nucleotide signaling pathways. Zaprinast has been used
as a tool to study PDE5 and PDE6 (8 –11). PDE5, PDE6, and
PDE9 are cGMP-specific, PDE4, PDE7, and PDE8 are cAMPspecific, and the rest of the members of the PDE family have
dual specificity (12). Zaprinast is the only drug that inhibits
PDE6 more potently than PDE5 (13). PDE6 is primarily
3
The abbreviations used are: MPC, mitochondrial pyruvate carrier; PDE, phosphodiesterase; PDH, pyruvate dehydrogenase activity; ␣KG, ␣-ketoglutarate; GPR35, G protein-coupled receptor 35; AGC1, aspartate glutamate
carrier.
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Jianhai Du‡, Whitney M. Cleghorn‡, Laura Contreras§1, Ken Lindsay‡, Austin M. Rountree¶, Andrei O. Chertov‡,
Sally J. Turner‡, Ayse Sahaboglu储, Jonathan Linton‡, Martin Sadilek**, Jorgina Satrústegui§, Ian R. Sweet¶,
François Paquet-Durand储, and James B. Hurley‡ ‡‡2
From the ‡Department of Biochemistry, ¶Diabetes and Obesity Center of Excellence, **Department of Chemistry, and
‡‡
Department of Ophthalmology, University of Washington, Seattle, Washington 98195, §Department of Molecular Biology,
Centre for Molecular Biology Severo Ochoa, Universidad Autonoma de Madrid-Consejo Superior de Investigaciones Científicas,
CIBER of Rare Diseases (CIBERER), and Health Research Institute Jimenez Diaz Foundation, Autonomous University of Madrid,
28049 Madrid, Spain, and the 储Division for Experimental Ophthalmology, Institute for Ophthalmic Research, Centre for
Ophthalmology, 72076 Tuebingen, Germany
Pyruvate Transport Balances Aspartate and Glutamate
expressed in retinal photoreceptors where it is responsible for
light-dependent signal transduction. Mutations in the Pde6a, b,
or c genes all cause retinal degeneration in humans (14 –16) and
in mouse models (17, 18). We performed a study intended to
use Zaprinast to simulate the effects of PDE6 dysfunction in an
ex vivo retinal degeneration model. We treated cultured mouse
retinal explants with Zaprinast, but we found unexpectedly that
Zaprinast is a potent inhibitor of mitochondrial pyruvate transport. This led to a novel and important finding, the focus of this
report, that inhibition of mitochondrial pyruvate transport triggers dramatic metabolomic changes in neuronal tissues that
severely alter the concentrations of glutamate and aspartate.
36130 JOURNAL OF BIOLOGICAL CHEMISTRY
VOLUME 288 • NUMBER 50 • DECEMBER 13, 2013
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EXPERIMENTAL PROCEDURES
Reagents—Zaprinast was obtained from EMD Millipore
Corp (Billerica, MA). [13C6]Glucose was obtained from Cambridge Isotope Laboratories, Inc. (Andover, MA). [2-14C]Pyruvate was purchased from PerkinElmer Life Sciences. Other 13C
tracers and reagents were purchased from Sigma unless otherwise specified.
Animals—C57BL/6 mice (6 – 8 weeks old) and C3Sn.BLiAPde6b⫹/DnJ (PDE6b⫹) mice were purchased from The Jackson Laboratory (Bar Harbor, ME). C3H retinal degeneration
(rd1) mice, which bear a Pde6b mutation, were provided by Dr.
Thomas A Reh, University of Washington. Pde6b⫹ mice were
used as the background control for rd1 mice. Cyclic nucleotide
gated channel 1 knock-out (Cngb1⫺/⫺) mice on C57BL/6
background were obtained from the Institute for Ophthalmic
Research, Tuebingen, Germany (19, 20). Aralar/AGC1⫹/⫺
mice (21) were crossed to produce Aralar/AGC1⫺/⫺ and
Aralar/AGC1⫹/⫹ control littermates in Sv129/C57BL6 background at Dr. Jorgina Satrústegui’s laboratory (Madrid, Spain).
Experiments were performed in accordance with the Institutional Animal Care and Use Committee (IACUC) recommendations at the University of Washington guidelines after
IACUC approval and with procedures approved in the Directive 86/609/EEC of the European Union and the Ethics Committee of the Universidad Autónoma de Madrid.
Retina Isolation and Culture—Mice were euthanized using
CO2 and cervical dislocation. The eyes were removed, and retinas were separated from retinal pigment epithelium in cold
HBSS. The retina was cultured in Krebs-Ringer/HEPES/bicarbonate buffer (22) with glucose (5 mM) or other 13C-labeled
substrate (5 mM) in a 37 °C 5% CO2 incubator.
cGMP Assay—Two mouse retinas were homogenized in 300
l of 0.1 N HCL. After centrifugation, 100 l of the supernatant
was assayed for cGMP by using a Direct cGMP ELISA kit from
Enzo Life Sciences, Inc. (Farmingdale, NY).
Gas Chromatography/Mass Spectrometry (GC/MS) Analysis
of Metabolites—Individual mouse retina was rinsed in cold
0.9% NaCl and snap-frozen in liquid nitrogen. The retina was
homogenized in a 700:200:50 cold mixture of methanol/chloroform/water with 5 nmol of internal standard methyl succinate. After centrifugation, the supernatant was dried under
vacuum, derivatized, and analyzed by GC-MS (Agilent 7890/
5975C) (22). The peaks were analyzed using Agilent data analysis software. The measured distribution of mass isotopomers
was corrected for natural abundance of 13C metabolite intensity
defined with standards and verified by mass after each experiment. Enrichment was calculated by dividing the labeled ions
with total ion intensity. The natural abundance from the tracers
and derivatization reagents were corrected using IsoCor software (23, 24).
O2 Consumption Measurements of Isolated Mitochondria—
Brain mitochondria were isolated as previously reported (22).
O2 consumption was measured in an Oroboros Oxygraph-2K.
The respiration buffer consisted of 150 mM KCl, 10 mM
KH2PO4, 1 mM MgCl2, pH 7.4. 25 l of 5 mg/ml mitochondria
was injected into the chamber for each assay. Respiration was
measured in response to serial additions of pyruvate/malate,
ADP, drug (DMSO, Zaprinast, or UK5099), glutamate/malate,
and succinate. The final concentrations of substrates were 100
M EGTA, 50 M CaCl2, 1.5 mM pyruvate, 0.5 mM malate, 2.5
mM ADP, 1 mM glutamate/0.5 mM malate, and 5 mM succinate.
After the addition of each component, the O2 consumption rate
was allowed to stabilize for 4 –5 min before the slope was
quantified.
MPC Activity Assay—Mouse liver mitochondria were prepared as described before (25). Mitochondrial pyruvate transport was measured as reported (4, 26) with modifications. The
liver mitochondria were suspended in medium containing
sucrose (250 mM), Tris-HCl (5 mM), and EGTA (2 mM) with pH
7.6. (⬃5 mg of mitochondrial protein/ml of medium). An aliquot of the mitochondrial suspension (50 l or 0.5 mg of mitochondrial protein) was added to 150 l of medium containing
KCl (125 mM) and Tris-HCl (20 mM), pH 6.8, with or without
Zaprinast. After incubation for 5 min at room temperature,
[2-14C]pyruvate (15 M or 0.0225 Ci) was spiked into the
200-l solution to start the assay. After 1 min, the mitochondria-associated radioactivity was separated from the free
medium by transferring the mitochondrial suspension to a
0.4-ml centrifuge tube containing a layer of oil consisting of
1:37.5 n-dodecane:bromododecane (Sigma) and spinning at
12,535 ⫻ g for 8 s. The portion of the tube containing the mitochondrial pellet was cut off with a razor blade, placed into a
scintillation vial, and then counted for radioactivity (26).
Measurement of Mitochondrial Metabolites—Freshly isolated mitochondria had intact mitochondrial membranes. To
disrupt them we treated mitochondria with 0.1% Triton and
freeze-thawed 5 times. Both membrane-intact and -disrupted
mitochondria from separate isolations were incubated with 1
mM [13C3]pyruvate in the buffer used for respiration studies on
isolated mitochondria at 30 °C for 10 min. The metabolites
were analyzed by GC/MS. To test shuttling of metabolites into
and out of mitochondria, we incubated freshly isolated mitochondria with 100 M EGTA, 1 mM glutamate, 0.5 mM malate,
2.5 mM ADP, 50 M CaCl2, and 1 mM [13C3]pyruvate in respiration buffer at 30 °C for 10 min. After centrifugation, the mitochondrial pellet and incubation medium were collected for
GC/MS analysis.
Glucose Concentration Assay—Glucose in the culture medium
was measured using an Amplex威 Red Glucose/Glucose Oxidase
Assay kit (Invitrogen).
Pyruvate Dehydrogenase Activity (PDH) Activity Assay—
PDH activity was measured using an NADH cycling assay in the
presence of diaphorase to reduce the generated NADH and
Pyruvate Transport Balances Aspartate and Glutamate
8
7
*
6
5
4
*
*
3
*
2
1
0
*
*
*
*
*
*
3PG
Py PE
ru P
v
La ate
ct
a
C te
Su itra
cc te
Fu i na
m te
ar
at
e
AK
M G
al
a
Al t e
an
G ine
ly
cin
Va e
Le l ine
Is uc
ol i n
eu e
ci
M Pr ne
et ol i
hi ne
on
in
Se e
Th ri
re ne
o
C ni n
y
G ste e
lu i n
ta e
As ma
p te
G art
lu at
ta e
m
in
As GA e
pa B
ra A
gi
Ly ne
s
Ta i ne
ur
in
e
N
AA
U
re
2- a
H
G
Metabolites (Zap/DMSO)
A
TCA
C
3
*
2
1
0
te
Others
1.5
NS
1.0
0.5
e
te
e
0.0
ine
i n e al i n
at
ala
Zaprinast
DMSO
er
an
V
l
cin
S
M
c
A
P
Su
FIGURE 1. Zaprinast significantly changes the metabolomic profile in the retina. A, Zaprinast changes metabolites in mouse retina. The retina was cultured
for 1 h with 200 M Zaprinast or with DMSO as a control. Metabolites were extracted, quantified by GC-MS, and normalized to the DMSO alone controls (n ⫽
10). 3-PG, 3-phosphoglyceric acid; PEP, phosphoenolpyruvate; AKG, ␣-ketoglutarate; GABA, ␥-aminobutyric acid; 2-HG, 2-hydroxyglutarate, NAA, N-acetyl
aspartate. B, Zaprinast increases pyruvate release from the retina. The medium from A was assayed for metabolites. The data are -fold changes over DMSO
control. C, Zaprinast does not change glucose consumption. The medium from A was tested for glucose concentration. NS, no significant difference. * indicates
p ⬍ 0.05 versus DMSO-treated.
cta
La
te
va
u
yr
MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium
bromide) was as an electron acceptor (27). Purified PDH protein from pig heart was incubated with 1 mM MgCl2, 5 mM
CaCl2, 5 mM l-carnitine, 0.2 mM thiamine pyrophosphate, 0.1
mM CoA, 2.5 mM NAD⫹, 1 mM MTT, and 1 unit/ml diaphorase
in 50 mM HEPES/KOH buffer, pH 7.5, at 30 °C. 5 mM pyruvate
was added into the mixture to start the reaction, and the plate
was read at 570 nm every 10 s for 10 min. The ⌬570-nm with
maximum linear rate was used for calculating the enzyme
activity.
Statistical Analysis—Data are expressed as the mean ⫾ S.E.
The significance of differences between means was determined
by unpaired two-tailed t tests or analysis of variance with an
appropriate post hoc test. A p value ⬍ 0.05 was considered to be
significant.
RESULTS
Zaprinast Causes Depletion of Glutamate and Accumulation
of Aspartate—We began the experiments with the intention of
identifying the metabolomic signature of PDE6 inhibition. We
isolated mouse retinas and incubated them with or without
Zaprinast in the presence of 5 mM glucose and then extracted
metabolites for GC/MS analysis. We then determined the concentration of Zaprinast required to inhibit PDE activity in
mouse retinas. The concentration of Zaprinast required to
cause accumulation of cGMP in the retina is in the range of
20⬃200 M (supplemental Fig. 1A). We also found large
DECEMBER 13, 2013 • VOLUME 288 • NUMBER 50
changes in several metabolite levels at 200 M (Fig. 1A). Glutamate and aspartate had the largest changes: about a 5-fold
decrease and increase, respectively. The effects on both glutamate and aspartate were first detectable at 30 min after the
addition of Zaprinast, became more apparent at 60 min, and
remained constant from 2 to 6 h (supplemental Fig. 1, B and C).
We reasoned that the change of these two metabolites might be
primarily responsible for the overall change of metabolic profile
as aspartate-derived metabolites, malate, methionine, cysteine,
and N-acetyl-aspartate increased, whereas glutamate-derived
metabolites, glutamine and ␣-ketoglutarate (␣KG), decreased.
We considered that glutamate in the retina might decrease
because it is released into the medium. However, both glutamate and aspartate were undetectable in the incubation
medium. Pyruvate increased by about 2–3-fold in response to
Zaprinast in both retina and medium, but lactate did not
change (Fig. 1, A and B). Glucose deprivation (22, 28) causes
similar changes of glutamate and aspartate levels in retina and
brain slices. Therefore, we tested the effect of Zaprinast on glucose consumption. The retina consumed about 1.2 mol/h/
retina. Zaprinast did not affect the rate of glucose consumption
(Fig. 1C).
The Effect of Zaprinast on Glutamate and Aspartate Is Independent of PDE Inhibition—To investigate the relationship
between PDE6 inhibition and the metabolic effects of Zaprinast, we compared the concentrations of Zaprinast required to
JOURNAL OF BIOLOGICAL CHEMISTRY
36131
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Medium Metabolites
(Zap/DMSO)
B
Amino Acids
Glucose Consumption
(μmol/h/Retina)
Glycolysis
Pyruvate Transport Balances Aspartate and Glutamate
B
*
20
100
*
4
2
0
20 50 100 200
Zaprinast (μM)
*
*
40
20
0
*
*
DMSO Zap DMSO Zap
WT
Cngb1 -/-
cGMP
(pmol/ml)
*
*
*
50
25
0
0.2
*
*
*
*
50
2
20
Zaprinast (μM)
200
30
*
20
*
10
0
*
DMSO Zap
PDE6b+
*
DMSO
Zap
rd1
F
60
*
40
*
20
0
0
*
*
0.1
1
10
Sildenafil (μM)
50
Metabolites (nmol)
E
Metabolites (nmol)
D
80
60
75
0
*
Glutamate
Aspartate
100
75
50
25
0
Glutamate
Aspartate
0
0.1
1
Sildenafil (μM)
50
FIGURE 2. PDE6 is not involved in the effect of Zaprinast on glutamate and aspartate. A, Zaprinast increases cGMP at 50 M and cAMP at 200 M (n ⫽ 6).
B, Zaprinast dose-dependently decreases glutamate and increases aspartate. Glutamate is shown as white bars, and aspartate is shown as black bars in this and
in the next two panels. The retina was treated for 1 h. The decrease of glutamate starts at concentrations of Zaprinast as low as 2 M (n ⫽ 5). C, Cngb1 deficiency
does not block the effect of 200 M Zaprinast (Zap) on glutamate and aspartate. The retina was incubated for 1 h (n ⫽ 5). D, Zaprinast at 200 M decreases
glutamate and increases aspartate in rd1 mice. Retinas from 3-month-old PDE6b⫹ and rd1 mice were incubated with Zaprinast for 1 h (n ⫽ 4). E, sildenafil
increases cGMP in retinas (n ⫽ 3). F, glutamate and aspartate do not change in retinas treated with sildenafil (n ⫽ 3). * indicates p ⬍ 0.05 versus DMSO-treated.
inhibit PDE in the retina with the concentration required to
cause glutamate to decrease. We found that Zaprinast increases
cGMP at 50 and 100 M and increases both cGMP and cAMP at
200 M. This indicates that Zaprinast inhibits PDE6 specifically
between 50 and 100 M, but it inhibits other PDEs at 200 M
(Fig. 2A). Surprisingly, Zaprinast decreased glutamate and
increased aspartate at much lower concentrations beginning at
2 M (Fig. 2B).
To test whether cyclic nucleotide-gated channels and Ca2⫹
are involved in the decrease of glutamate, we used the retina
from Cngb1⫺/⫺ mice in which the rising cGMP could not activate cyclic nucleotide gated channels to cause depolarization
and elevation of intracellular free Ca2⫹ (19, 29). Remarkably,
Zaprinast decreased glutamate and increased aspartate in
Cngb1⫺/⫺ retina (Fig. 2C). Furthermore, neither of the cellpermeable cGMP analogues 8-Br-cGMP and 8-(4-chlorophenylthio)-cGMP nor inhibitors of Protein kinase G and PKA
changed the levels of glutamate and aspartate in the retina (supplemental Fig. 2). These findings suggested that Zaprinast
increases aspartate and lowers glutamate by a different mechanism than we expected, i.e. in a PDE6-independent fashion.
As further confirmation that the metabolic effects of Zaprinast occur independently of PDE6 inhibition, we applied Zapri-
36132 JOURNAL OF BIOLOGICAL CHEMISTRY
nast to the retina from rd1 mice that were 2 months old. The
rd1 mutation causes almost all PDE6-expressing photoreceptors to die and disappear by 1 month of age and thus abolishes
PDE6 activity. We found that glutamate and aspartate levels in
rd1 retina are lower than normal, consistent with photoreceptors being a major source of these amino acids in normal retinas. We also found that Zaprinast potently decreases glutamate
and increases aspartate in rd1 retina, suggesting that Zaprinast
acted at a site distinct from PDE6 (Fig. 2D). As further confirmation that the effect of Zaprinast on glutamate and aspartate
was not through PDE, we evaluated the effect of sildenafil,
another PDE5/6 inhibitor. Sildenafil between 0.1–50 M
robustly increased cGMP in WT retina but did not have a significant effect on glutamate or aspartate (Fig. 2, E and F). Taken
altogether, these results show unambiguously that the effect of
Zaprinast on glutamate and aspartate occurs independently of
PDE inhibition.
The Effect of Zaprinast on Glutamate and Aspartate Occurs
Independently of GPR35 Activation—Zaprinast also has been
identified as a potent agonist of G protein-coupled receptor 35
(GPR35) (30, 31). GPR35 is expressed predominantly in spleen,
immune cells, and gastrointestinal tissues, but expression in eye
also has been reported (32, 33). To test whether GPR35 contribVOLUME 288 • NUMBER 50 • DECEMBER 13, 2013
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Metabolites (nmol)
*
0
6
0
C
*
Metabolites (nmol)
40
cAMP
(pmol/ml)
cGMP
(pmol/ml)
A
Pyruvate Transport Balances Aspartate and Glutamate
B
Pyruvate
CO2
Acetyl-CoA
Citrate
*
*
*
*
5
0
M1 M2 M3
La
ct
E
2
1
10
*
* *
M1 M2 M3 M4 M5
800
400
0
M1 M2 M3
*
*
80
40
0
M1 M2 M3 M4 M5M6
I
800
600
400
200
0
*
M1 M2 M3 M4
Malate (pmol)
0
120
αKG (pmol)
Citrate (pmol)
*
3
F
1200
* * *
M1M2 M3 M4 M5
J
300
200
100
0
*
*
M1 M2 M3 M4
Aspartate (nmol)
4
H
0
*
4
3
2
1
0
*
*
M1 M2 M3 M4
FIGURE 3. Zaprinast blocks synthesis of citrate, ␣KG, and glutamate from glucose-derived pyruvate. A, schematic of carbon labeling (black circles for 13C)
from [13C6]glucose after the first TCA cycle. Pyruvate and lactate are fully labeled on all three carbons. After PDH removes one labeled carbon, the remaining
two labeled carbons in acetyl-CoA incorporate into mitochondrial intermediates in the first cycle. OAA, oxaloacetate. B, Zaprinast decreased the enrichment of
TCA cycle intermediates from [13C6]glucose. The retina was incubated with 5 mM [13C6]glucose for 15 min. C–J, mass isotopomer distributions of intermediates
from B. The fraction of each isotopomer was multiplied by the total metabolite concentration and normalized by protein concentration (pmol or nmol/mg of
protein). Zaprinast increases the total incorporation of labeled carbon into pyruvate, but it decreases labeling of TCA intermediates. * indicates p ⬍ 0.05 versus
DMSO-treated. (n ⫽ 3).
utes to the metabolic effect of Zaprinast, we preincubated the
retina with a GPR35 antagonist, CID 2745687, for 30 min followed by an additional 30 min of Zaprinast treatment. CID
2745687 did not block the effect of Zaprinast on glutamate and
aspartate. Furthermore, the GPR35 agonist, pamoic acid, did
not change glutamate or aspartate levels (supplemental Fig. 3).
These results show that GPR35 was not responsible for the
observed Zaprinast effects.
Zaprinast Blocks Entry of Glucose-derived Pyruvate into the
TCA Cycle—To identify the mechanism behind the metabolic
effect of Zaprinast, we incubated the retina with [13C6]glucose
for 15 min with/without Zaprinast and then analyzed the isotopomer distributions of the intermediates. Under normal conditions, pyruvate and lactate become fully labeled by glycolysis
of [13C6]glucose. Pyruvate loses 1 13C to synthesize acetyl-CoA,
and the two-labeled carbons appear in most TCA intermediates
in the first round of the TCA cycle (Fig. 3A). In the second cycle
citrate gains a total of four labeled carbons, and the rest of the
intermediates have either three or four carbons labeled due to
the loss of CO2 at one of two different positions (supplemental
Fig. 4A). Zaprinast did not change the 13C enrichment in pyruvate and lactate. However, the enrichment of glutamate, aspartate, and TCA intermediates was much lower after Zaprinast
treatment (Fig. 3B). Zaprinast increased the concentration of
M3 (three labeled carbons) pyruvate but not lactate, and it significantly decreased both enrichment and concentration of the
M2 isotopomer of citrate (the form with two labeled carbons
from the first cycle), M4 (four labeled carbons from the second
DECEMBER 13, 2013 • VOLUME 288 • NUMBER 50
cycle), and M5/M6 (after three or more cycles) (Fig. 3, C–E;
supplemental Fig. 4B). These results suggest that Zaprinast
inhibits the formation of citrate from pyruvate. Accordingly,
the downstream intermediates ␣KG and glutamate had fewer
M2 and M3/4 isotopomers in Zaprinast-treated retinas. Zaprinast decreased M2 in both succinate and malate but increased
both M2 and M3 in aspartate (Fig. 3, F--J). Zaprinast increased
the total amount of labeled aspartate, but it decreased the overall enrichment of aspartate because Zaprinast causes a very
large increase in the amount of unlabeled aspartate. Overall,
these results indicate that Zaprinast inhibits the incorporation
of pyruvate-derived carbons into the TCA cycle. This causes
accumulation of oxaloacetate and aspartate. We also noted that
the excess oxaloacetate “spills over” into the M3 isotopomers of
malate and succinate by the reversible malate dehydrogenase
and fumarate hydratase reactions.
Oxidized Glutamate Is the Source of the Aspartate That Accumulates in Response to Zaprinast Treatment—The decrease of
glutamate that occurs with Zaprinast treatment always occurs
in parallel with an increase of aspartate. We hypothesized that
the increased carbons of aspartate come from oxidation of glutamate through ␣KG dehydrogenase. To test this hypothesis,
we labeled the retina with [13C5]glutamine for 5 min and then
replaced it with unlabeled glucose for 10 min (Fig. 4). The short
time pulse makes it possible to trace the fate of glutamate as
TCA intermediates are labeled in the first cycle and then start to
lose the labeled carbons in the second cycle. During the first
round of the TCA cycle, glutamate and ␣KG are fully labeled as
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Glutamate (nmol)
Pyruvate (nmol)
D
15
5
*
0%
CO2
Succinate
G
20%
αKG
Malate Glutamate
10
40%
CO2
Aspartate
DMSO
Zaprinast
at
e
ru
va
te
Ci
tra
te
αK
G
lu G
ta
m
Su ate
cc
in
at
e
M
al
As ate
pa
rta
te
OAA
15
Py
OAA
C
DMSO
Zaprinast
60%
Succinate (pmol)
Lactate
13C
12C
Lactate (nmol)
[13C6]Glucose
80%
Enrichment (%)
A
B
Citrate
CO 2
Malate
[ 13 C 5 ]Glutamine
αKG
Aspartate
Glutamate
Fumarate
ta
lu
200
*
*
M1M2M3 M4 M5M6
Succinate (pmol)
M1M2 M3 M4 M5
500
400
300
200
100
0
E
30
αKG (pmol)
200
G
100
D
400
0
*
*
*
0%
G
DMSO
Zaprinast
Glutamate (nmol)
600
CO 2
300
0
40%
20
10
0
*
M1 M2 M3 M4 M5
*
M1 M2 M3 M4
300
200
100
0
H
*
M1M2 M3 M4 M5
I
500
400
300
200
100
0
*
M1 M2 M3 M4
10
8
6
4
2
0
*
M1 M2 M3 M4
FIGURE 4. Carbons used to increase aspartate come from oxidation of glutamate. A, schematic of carbon labeling (black circles represent 13C) from
[13C5]glutamine after the first TCA cycle. Most of the 13C from [13C5]glutamine labels five carbons in ␣KG and four carbons for the rest of the intermediates in
the first cycle. However, a small fraction (⬍10%) of the [13C5]glutamine also labels five carbons in citrate by reversal of the isocitrate dehydrogenase reaction.
B, Zaprinast increases enrichment of aspartate and malate from [13C5]glutamine. The retina was pulsed with 5 mM [13C5]glutamine for 5 min and then chased
with unlabeled glucose for 10 min. C–I, mass isotopomer distribution of intermediates from B. The metabolite concentration was normalized by protein
concentration (pmol or nmol/mg protein). Zaprinast increases M4 succinate, malate, and aspartate from [13C5]glutamine. * indicates p ⬍ 0.05 versus DMSO
treated (n ⫽ 3).
M5, and the subsequent intermediates have four 13C carbons. A
small amount of M5 citrate also accumulates. This must result
from the reversal of the isocitrate dehydrogenase reaction (34)
(Fig. 4A). For the second cycle, ␣KG and glutamate begin with
three labeled carbons. After oxidative decarboxylation of ␣KG,
the downstream intermediates have two labeled carbons, and
citrate ends up having either two or three labeled carbons (supplemental Fig. 5A). Enrichment of citrate decreased with Zaprinast treatment, whereas enrichment of malate and aspartate
increased (Fig. 4B). In the first TCA cycle, Zaprinast decreases
the labeled citrate (M4) but significantly increases labeled succinate, malate, and especially aspartate (Fig. 4, C–I). These
results show that Zaprinast prevents glutamate-derived carbons from passing through citrate to the second cycle. Consistent with this, M2 citrate, M3 glutamate, and M3 ␣KG
decreased in response to Zaprinast. Zaprinast blocked citrate
formation and thereby caused accumulation of oxaloacetate
and aspartate produced from oxidized glutamate. To confirm
this we incubated the retina with and without glutamine for 2 h.
Both glutamate and aspartate increased ⬃5-fold in response to
glutamine (relative to glucose alone), and Zaprinast shifted the
glutamate-to-aspartate ratio by decreasing glutamate about
5-fold and increasing aspartate about 10-fold (supplemental
Fig. 5B).
Zaprinast Inhibits O2 Consumption in Intact Retina—To test
the effects of Zaprinast and UK5099 on mitochondrial function
in ex vivo retina, we measured O2 consumption from small
pieces of intact mouse retina. Zaprinast caused a rapid increase
followed by sustained inhibition of O2 consumption. This
biphasic effect can be explained by a mixed effect of Zaprinast
36134 JOURNAL OF BIOLOGICAL CHEMISTRY
on PDE and MPC activities. PDE inhibition increases O2 consumption by stimulating energy consumption whereas MPC
inhibition decreases availability of fuel to mitochondria. Consistent with this we found that sildenafil increases O2 consumption whereas UK5099 inhibits it (Supplemental Fig. 6). UK5099
causes a slow but strong inhibition, probably due to its ability to
inhibit complex II (Results not shown). Consistent with this we
found that UK5099 at 100 M but not 10 nM inhibits succinatedriven O2 consumption.
Zaprinast Inhibits Pyruvate-driven O2 Consumption in Brain
Mitochondria—Pyruvate enters via the MPC into the mitochondrial matrix where PDH transforms it into acetyl-CoA.
PDH and the dehydrogenases of the TCA cycle produce
NADH that provides reducing power for O2 consumption.
To test the hypothesis that Zaprinast inhibits pyruvate oxidation, we isolated brain mitochondria, and measured rates of O2
consumption fueled either by pyruvate, glutamate, or succinate. Zaprinast almost completely blocked pyruvate-driven O2
consumption (Fig. 5, A--D). However, Zaprinast did not block
O2 consumption fueled by glutamate or succinate. This is consistent with our isotopic labeling experiment in which Zaprinast did not inhibit glutamate oxidation. We next tested the
hypothesis that Zaprinast blocks pyruvate transport by inhibiting MPC. Similar to Zaprinast, MPC inhibitor UK5099 blocked
pyruvate-driven but increased glutamate and succinate-driven
oxygen consumption (Fig. 5, E and F, supplemental Fig. 7, A and
B). There was no significant difference in the rate of oxygen
consumption between Zaprinast and UK5099 (supplemental
Fig. 7, C and D).
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Citrate (pmol)
Glutamine (nmol)
C
F
80%
αKG
Succinate
DMSO
Zaprinast
Aspartate (nmol)
CO2
120%
Malate (pmol)
OAA
m
in
La e
ct
Py at
ru e
va
Ci te
tra
te
G αK
lu
ta G
m
Su at
cc e
in
a
M te
al
As at
pa e
rta
te
13C
12C
Acetyl-CoA
A
Enrichment (%)
Pyruvate Transport Balances Aspartate and Glutamate
Pyruvate Transport Balances Aspartate and Glutamate
Zaprinast Inhibits MPC but Not PDH—To confirm that
Zaprinast targets the MPC, we measured MPC activity directly
using isolated mouse liver mitochondria. The MPC kinetics we
measured are similar to previous reports (2, 4). We measured
flux of [2-14C]pyruvate transport into isolated liver mitochondria. We confirmed the specificity of the assay by showing that
10 mM unlabeled pyruvate inhibits uptake of the labeled pyruvate (supplemental Fig. 8, A and B). Zaprinast inhibits pyruvate
uptake (Fig. 6A) at a dose (Fig. 6B) that is similar to the dose
required for its effect on glutamate and aspartate in the retina
(Fig. 2A).
To exclude the possibility that Zaprinast might inhibit PDH,
we incubated pure PDH enzyme with Zaprinast or the known
PDH inhibitor 3-fluropyruvate. Zaprinast did not affect the
activity of PDH at any concentration tested. In contrast, 3fluropyruvate effectively inhibited PDH activities (Fig. 6C).
To confirm that the metabolic effect of Zaprinast is caused by
MPC inhibition and not by inhibition of PDH activity, we
bypassed MPC by adding [13C3]pyruvate to mitochondria with
disrupted membranes (see “Experimental Procedures”). In this
experiment pyruvate is accessible directly to PDH. Both Zaprinast and UK5099 decrease the amount of labeled citrate produced by mitochondria with intact membranes, but they have
no effect on citrate formation when mitochondrial membranes
are disrupted (Fig. 6D). As expected, disruption of the mitoDECEMBER 13, 2013 • VOLUME 288 • NUMBER 50
chondrial membrane blocks the ability of either Zaprinast or
UK5099 to stimulate depletion of glutamate (Fig. 6D).
Known Inhibitors of MPC Replicate the Effects of Zaprinast—
We then asked whether other known MPC inhibitors cause metabolic effects similar to those of Zaprinast. Two known MPC
inhibitors, ␣-cyano-4-hydroxycinnamic acid and UK5099, produce effects nearly identical to the effects of Zaprinast; they are
increased aspartate and decreased glutamate, increased pyruvate, and suppressed pyruvate-driven O2 consumption (Fig. 6,
F–G). Taken together, these results are consistent with our
other data showing that Zaprinast blocks pyruvate transport
into mitochondria.
Inactivation of Aspartate Glutamate Carrier (AGC1) Does
Not Prevent the Decrease of Glutamate Caused by Zaprinast—
Aspartate normally is transported out of mitochondria via
AGC1 in exchange for glutamate from cytosol (Fig. 7A). AGC1
allows the carbons from aspartate to bypass the conventional
“complete” version of the TCA cycle. In the absence of AGC1,
the carbons that would normally be used to make aspartate are
forced into the complete TCA cycle, thus decreasing the levels
of the amino acid in AGC1⫺/⫺ retina. In the absence of AGC1,
the malate aspartate shuttle becomes impaired, and pyruvate is
diverted to produce lactate and not acetyl-CoA. Consistently,
the retina from AGC1⫺/⫺ mice had lower levels of aspartate
and pyruvate than WT mice (Fig. 7B) (35).
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FIGURE 5. Zaprinast inhibits pyruvate-driven oxygen consumption. Zaprinast inhibits O2 consumption in brain mitochondria when fueled by pyruvate but
not when fueled by glutamate or succinate (Suc) in conditions starting with either glutamate/malate (Glu/Mal, A and B) or pyruvate/malate (Pyr/Mal, C and D).
The large dip upon the addition of both Zaprinast (200 M) and DMSO is an artifact caused by differences in O2 solubility between water and DMSO. The
substrate was added at the time point indicated by arrows. B and D are increments (⌬OCR) over the previous rate by subtracting OCR initiated by the substrate
added before. OCR, oxygen consumption rate (n ⫽ 3). E and F, MPC inhibitor UK5099 inhibits pyruvate-driven oxygen consumption. Like Zaprinast, UK5099 at
10 nM inhibits oxygen consumption fueled by pyruvate but not glutamate and succinate. E is a representative trace from F, the normalized data (n ⫽ 3).
* indicates p ⬍ 0.05 versus DMSO treated. Drug indicates the adding of DMSO, Zaprinast, or UK5099. * indicates p ⬍ 0.05 versus DMSO treated (n ⫽ 3).
Pyruvate Transport Balances Aspartate and Glutamate
[2- C] Pyruvate
Liver Mitochondria
0.4
B
0.3
0.2
*
0.1
DMSO Zap Cold Pyr
13
C Citrate
(Zap/DMSO)
1.5
0.3
0.2
0.1
0.0
0.01 0.1 1
10 100
Zaprinast (μM)
[ 13 C3] Pyruvate
Brain Mitochondria
E
1.0
0.5
*
0.0
2.0
C Glutamate
(Zap/DMSO)
D
0.4
*
75
*
50
*
25
0
*
Retina
*
*
DMSO
*
4CIN UK5099 Zap
G
0.4
0.2
*
0.0
DMSO 3-FP 2 20 200
Zaprinast (µM)
[ 13 C3] Pyruvate
Brain Mitochondria
1.0
0.5
Pyruvate
(nmol/mg Protein)
Metabolites
(nmol/mg Protein)
Glutamate
Aspartate
0.6
12
*
*
DMSO Zap UK DMSO Zap UK
Intact Mem
Disrupted Mem
Retina
*
8
*
*
4
0
DMSO 4CIN UK5099 Zap
FIGURE 6. Zaprinast inhibits transport of pyruvate into mitochondria but does not inhibit PDH. A and B, Zaprinast (Zap) inhibits pyruvate influx into liver
mitochondria. A, isolated liver mitochondria was preincubated with DMSO or Zaprinast at 100 M for 5 min before adding 15 M [2-14C]pyruvate for 1 min. As
a positive control, 10 mM unlabeled pyruvate (Cold Pyr) was added into mitochondria instantly after [2-14C]pyruvate addition (n ⫽ 5). B, Zaprinast dose-dependently inhibits mitochondrial pyruvate transport (n ⫽ 5). C, Zaprinast does not affect PDH activity. PDH protein was incubated with either Zaprinast at different
concentrations or PDH inhibitor 3-fluropyruvate (3-FP) at 5 mM for 10 min. PDH activity was measured by cycling assay (n ⫽ 4). D–E, both Zaprinast and UK5099
inhibit citrate and glutamate synthesis from [13C3]pyruvate in brain mitochondria with intact membranes but not when mitochondrial membranes are
disrupted. Membrane intact and disrupted mitochondria were incubated with 1 mM [13C3]pyruvate and Zaprinast (100 M) or UK5099 (UK, 100 M) for 10 min
at 30 °C. The 13C citrate and glutamate were measured by GC-MS. Mem, membrane (n ⫽ 3). F–G, MPC inhibitors increase pyruvate and aspartate but decrease
glutamate in the retina. Retinas were treated with ␣-cyano-4-hydroxycinnamic acid (4CIN, 100 M), UK5099 (100 M), and Zaprinast (100 M) for 1 h. (n ⫽ 3).
* indicates p ⬍ 0.05 versus DMSO treated.
We investigated whether AGC1 plays a role in the Zaprinast
effect and found that it does not (Fig. 7B). When incubated with
[13C6]glucose, AGC1⫹/⫺ or AGC1⫺/⫺ retinas have 13C enrichment similar to WT except that the overall levels of pyruvate
and aspartate are lower than normal (supplemental Fig. 9).
AGC1 deficiency impaired production of pyruvate, citrate, glutamate, and especially aspartate, consistent with a previous
study of AGC1-deficient neurons (35). Nevertheless, Zaprinast
still decreased labeling of glutamate from glucose and increased
labeling of aspartate and pyruvate in both AGC1⫹/⫺ and
AGC1⫺/⫺ retinas (Fig. 7, C–F).
Zaprinast Causes Accumulation of Aspartate in Mitochondria and Prevents Generation of Glutamate from Aspartate—
To examine the distribution of accumulated aspartate in or
outside of mitochondria, we incubated isolated intact brain
mitochondria with EGTA, malate, glutamate, Ca2⫹, ADP, and
[13C3]pyruvate for 10 min and then measured the metabolites
from the mitochondria and from the incubation medium. Consistent with previous experiments, Zaprinast blocked incorporation of 13C from pyruvate into citrate both in and outside of
mitochondria. Similarly, Zaprinast also inhibited incorporation
of 13C into aspartate. At the same time, Zaprinast stimulated
the accumulation of unlabeled aspartate in mitochondria (Fig.
8, A–C). Some glutamate may enter the mitochondria by a glutamate carrier (36), contributing to aspartate increase in mito-
36136 JOURNAL OF BIOLOGICAL CHEMISTRY
chondria, but not the medium, within 10 min of incubation.
Each of these findings is consistent with inhibition by Zaprinast
of MPC-catalyzed uptake of pyruvate into mitochondria.
Unlabeled aspartate accumulates in mitochondria during
Zaprinast treatment (Fig. 8, A–C). To confirm that Zaprinast
decreases utilization of aspartate, we incubated the retina with
labeled aspartate in the presence of glucose. During 1 h without
Zaprinast, aspartate contributed 13C to ⬃30% of glutamate and
other TCA cycle intermediates (Fig. 8, D–E). However, Zaprinast blocked the incorporation of 13C from aspartate into citrate and glutamate (Fig. 8, F–H). This reflects diminished
acetyl-CoA synthesis in mitochondria, consistent with inhibition of pyruvate carrier activity by Zaprinast. In addition, the
decreased lactate/pyruvate ratio (Fig. 8I) might lower cytosolic
reducing power, which causes accumulation of aspartate by
reversing the malate dehydrogenase reaction in the cytoplasm
(Fig. 9). Consistent with this, we found that Zaprinast decreases
NADH levels in the retina (Fig. 8J).
DISCUSSION
We demonstrated that the well known PDE5/6 inhibitor
Zaprinast blocks pyruvate transport into mitochondria by a
mechanism that is independent of PDE inhibition. Inhibition of
pyruvate transport either by Zaprinast or by known MPC inhibitors decreases de novo synthesis of glutamate from glucose and
VOLUME 288 • NUMBER 50 • DECEMBER 13, 2013
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100
1.5
0.0
DMSO Zap UK DMSO Zap UK
Intact Mem
Disrupted Mem
F
C
14
[2- C] Pyruvate
Liver Mitochondria
13
0.0
*
0.5
PDH activity
(Units/mg Portein)
14
0.5
Pyruvate Uptake
(nmol/mg protein/min)
Pyruvate uptake
(nmol/mg protein/min)
A
Pyruvate Transport Balances Aspartate and Glutamate
NAD+ NADH
A
Glutamate
Aspartate
OAA
αKG
Malate
OGC
AGC1
Aspartate
Glutamate
αKG
Malate
OAA
NAD+ NADH
Matrix
Cytoplasm
*
*
Aspartate
E
DMSO
Zaprinast
1
0
*
*
*
WT AGC1+/- AGC1-/-
C Glutamate (Fold)
Glutamate
#
*
13
2
* *
* *
#
#
*
13
*
2
0
C Pyruvate (Fold)
C
F
2
1
0
4
3
*
*
*
WT AGC1+/- AGC1-/-
DMSO
Zaprinast
*
*
2
*
1
0
Pyruvate
Aspartate (Fold)
4
WT DMSO
WT Zap
AGC1+/- DMSO
AGC1 +/- Zap
AGC1 -/- DMSO
AGC1 -/- Zap
13 C
6
13
C Citrate (Fold)
D
8
WT
AGC1+/- AGC1-/-
4
3
*
2
1
0
WT
*
*
*
AGC1+/- AGC1-/-
FIGURE 7. Disruption of AGC1 activity does not prevent Zaprinast from stimulating oxidation of glutamate into aspartate. A, schematic of malateaspartate shuttle. OAA, oxaloacetate; OGC, 2-oxoglutarate carrier. B, the retina from AGC1⫺/⫺ mice has lower aspartate and pyruvate, but Zaprinast (Zap)
induces similar metabolic changes. The retina was incubated with DMSO or Zaprinast for 1 h in the presence of glucose. C–F, pyruvate, citrate, glutamate, and
aspartate labeled from [13C6]glucose. Retinas from WT, AGC1⫹/⫺, and AGC1⫺/⫺ mice were incubated with 5 mM [13C6]glucose for 15 min. Data were expressed
as -fold increase of 13C label over the DMSO control. * indicates p ⬍ 0.05 versus DMSO treated within groups, and # indicates versus DMSO or Zaprinast treated
in the WT group. (n ⫽ 5).
aspartate, and it increases the net oxidation of glutamate into
aspartate. The result is severe depletion of glutamate and severalfold accumulation of aspartate. These findings are significant because glutamate is important as a fuel for energy
production, as a neurotransmitter, and as a substrate for
glutathione synthesis.
The Effect of Zaprinast on Glutamate and Aspartate Occurs
Independently of Its Previously Known Effects on PDE—The
ability of Zaprinast to inhibit PDE has been characterized (9,
10). Here we report four lines of evidence that Zaprinast also
has an independent activity that causes severe depletion of glutamate and accumulation of aspartate. 1) The concentration at
which Zaprinast decreases glutamate is about 20 times lower
than that required to inhibit PDE. 2) Other PDE inhibitors,
sildenafil or cell-permeable cGMP analogues, do not influence
glutamate or aspartate. 3) Inactivation of cyclic nucleotide
gated channels and inhibition of PKG or PKA do not rescue
Zaprinast-induced glutamate depletion. 4) Zaprinast causes
depletion of glutamate and accumulation of aspartate even in
the retina, which has no PDE6.
Previous studies found that depletion of glutamate correlates
with cell death in the retina (22, 28). Interestingly, in long term
organotypic retinal explant cultures, at concentrations of up to
200 M, Zaprinast induces selective photoreceptor death without affecting other retinal neurons (37). Why does Zaprinast
DECEMBER 13, 2013 • VOLUME 288 • NUMBER 50
not cause generalized cell death and instead appears to affect
only PDE6 under culture conditions (37– 40)? Tissue culture
medium, such as R16 (41), used for organotypic retinal culture
contains both fatty acids and glutamine. This provides two
opportunities to bypass the consequences of MPC inhibition,
either via direct acetyl-CoA uptake from fatty acid oxidation or
via glutamine to glutamate conversion. Hence, the composition
of the culture medium may help to discriminate between PDE6
and MPC effects of Zaprinast in cultured retina.
Zaprinast Inhibits Transport of Pyruvate into Mitochondria—
MPC transports pyruvate into mitochondria. UK5099 or
␣-cyano-4-hydroxycinnamic acid are known inhibitors of MPC
activity (1, 4). In this study we showed that MPC can also be
inhibited by Zaprinast. Inhibition or knockdown of MPC prevents pyruvate-dependent acetyl-CoA formation and O2 consumption (5, 6, 42) and decreases the lactate/pyruvate ratio
(43). Our study shows that Zaprinast and UK5099 not only
block pyruvate-dependent mitochondrial O2 consumption and
citrate synthesis in brain mitochondria but also cause increases
in pyruvate and aspartate and decrease in glutamate in brain
mitochondria and the retina.
We confirmed by direct measurements of pyruvate uptake
that Zaprinast dose-dependently inhibits MPC activity in isolated mitochondria. MPC is an integral membrane protein in
the mitochondrial inner membrane. To rule out any effect on
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Metabolites (Zap/DMSO)
B
0.4
*
0.0
0.8
0.0
E
[ 13 C4]Aspartate
Glutamate
OAA
Citrate
H
1.0
0.5
*
0.0
DMSO Zap
M3 Glutamate
M4 Citrate
CO 2
1.5
DMSO Zap DMSO Zap
Mitochondria Medium
3
2
1
0
DMSO Zap DMSO Zap
Mitochondria Medium
F
80%
60%
40%
20%
0%
I
1.2
0.8
0.4
0.0
*
DMSO Zap
J
60
20
0
*
3
2
1
0
80
40
4
DMSO
15%
10%
*
*
*
0 20 50 200
Zaprinast (μM)
Zap
*
*
*
5%
0%
0 20 50 200
Zaprinast (μM)
FIGURE 8. Zaprinast causes accumulation of aspartate inside mitochondria and decreases aspartate utilization. A–C, most of the aspartate increase
induced by Zaprinast (Zap) does not come from [13C3]pyruvate. Brain mitochondria were incubated with 1 mM [13C3]pyruvate and 100 M Zaprinast for 10 min
supplemented with EGTA 100 M, 1 mM glutamate, 0.5 mM malate, 2.5 mM ADP, and 50 M Ca2⫹. Metabolites from both mitochondria and medium were
analyzed (n ⫽ 3). OAA, oxaloacetate. D, schematic of carbon labeling (black circles represent 13C) from [13C4]aspartate. E, [13C4]aspartate increased glutamate
enrichment and TCA cycle intermediates. The retina was incubated with 250 M [13C4]aspartate and unlabeled glucose for 1 h (n ⫽ 3). F–H, M4 aspartate
increased, but M4 citrate and M3 glutamate decreased in response to Zaprinast (100 M). I, Zaprinast decreased the lactate/pyruvate ratio (n ⫽ 3). J,
Zaprinast decreased the level of NADH in the retina. Retinas were treated with Zaprinast for 1 h. * indicates p ⬍ 0.05 versus DMSO treated or without
Zaprinast (n ⫽ 3).
FIGURE 9. MPC inhibition causes accumulation of aspartate at the
expense of glutamate. Inhibition of MPC by Zaprinast or UK5099 blocks
entry of pyruvate into mitochondria, which decreases production of acetylCoA, citrate, ␣KG, glutamate, and NADH and increases oxaloacetate (OAA)
and aspartate. Aspartate exits mitochondria in exchange for glutamate via
AGC1 to oxidize more glutamate into aspartate. In the cytosol the glutamate
may be transaminated into ␣KG, which exchanges with malate or enters the
mitochondria through either AGC1 or glutamate carrier (GC). Metabolites
with red represent increases, blue represents decrease, and black represents
unchanged or untested. Thick lines represent more flux in that direction.
GOT1, cytosolic glutamate oxoglutarate transaminase; GOT2, mitochondrial
glutamate oxoglutarate transaminase.
36138 JOURNAL OF BIOLOGICAL CHEMISTRY
PDH activity we measured the effect of Zaprinast on mitochondria whose membrane structure was disrupted by detergent.
There was no effect of Zaprinast on these preparations. This
shows that Zaprinast does not target PDH or citrate synthase.
We also confirmed directly that Zaprinast does not affect the
activity of purified PDH. In our MPC activity assay some
labeled pyruvate may have been metabolized. We did not correct for this or for labeled pyruvate that may have been at the
surface of the pellets. This may explain why the pyruvate uptake
continued for more than 1 min and neither cold pyruvate nor
Zaprinast completely eliminated the labeled pyruvate in mitochondrial pellet. Future studies should address the structural
basis for Zaprinast inhibition of MPC and whether Zaprinast
directly binds MPC.
Zaprinast Causes Oxidation of Glutamate and Accumulation of Aspartate—Hypoglycemia or inhibition of glycolysis
leads to decreased glutamate and cellular accumulation or
release of aspartate in brain (44 – 46), synaptosomes (47), and
the retina (22, 28). During glucose deprivation, neurons turn to
glutamine and glutamate for energy (48). Zaprinast does not
affect glucose uptake or glycolysis (as assessed by lactate production), but its block of pyruvate transport mimics hypoglycemic conditions because less glucose-derived pyruvate is available to mitochondria. The lack of two carbons from pyruvate
causes oxaloacetate to accumulate at the expense of glutamate.
Some pyruvate may enter mitochondria before Zaprinast takes
effect or there might be some residual MPC activity, so some
citrate, ␣KG, glutamate, and succinate can be labeled by 13C
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G
*
4
As
pa
rta
La te
c
Py tate
ru
va
C te
itr
at
e
G αK
lu
ta G
m
Fu at
m e
ar
a
M te
al
at
e
αKG
Enrichment (%)
DMSO Zap DMSO Zap
Mitochondria Medium
D
*
0.4
*
5
M4 Asp
*
C
NADH (% of Total)
Asp (Fold)
0.8
13 C
C Citrate (Fold)
13
B 1.2
1.2
Lactate/Pyruvate
A
Unlabeled Asp (Fold)
Pyruvate Transport Balances Aspartate and Glutamate
Pyruvate Transport Balances Aspartate and Glutamate
Acknowledgments—The Cngb1⫺/⫺ mice were kindly provided by
Stylianos Michalakis and Martin Biel, Center for Integrated Protein
Science CIPS-M and Department of Pharmacy, Ludwig-Maximilians
Universität München. Oxygen consumption and pyruvate uptake experiments were carried out by Diabetes Research Center Cell Function Analysis Core (National Institutes of Health Grant P30DK017047).
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