Misfolded mutant SOD1 directly inhibits VDAC1 conductance in a mouse model of inherited ALS.
ABSTRACT Mutations in superoxide dismutase (SOD1) cause amyotrophic lateral sclerosis (ALS), a neurodegenerative disease characterized by loss of motor neurons. With conformation-specific antibodies, we now demonstrate that misfolded mutant SOD1 binds directly to the voltage-dependent anion channel (VDAC1), an integral membrane protein imbedded in the outer mitochondrial membrane. This interaction is found on isolated spinal cord mitochondria and can be reconstituted with purified components in vitro. ADP passage through the outer membrane is diminished in spinal mitochondria from mutant SOD1-expressing ALS rats. Direct binding of mutant SOD1 to VDAC1 inhibits conductance of individual channels when reconstituted in a lipid bilayer. Reduction of VDAC1 activity with targeted gene disruption is shown to diminish survival by accelerating onset of fatal paralysis in mice expressing the ALS-causing mutation SOD1(G37R). Taken together, our results establish a direct link between misfolded mutant SOD1 and mitochondrial dysfunction in this form of inherited ALS.
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ABSTRACT: Oxidative stress and mitochondrial damage have been implicated in the pathogenesis of several neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. Oxidative stress is characterized by the overproduction of reactive oxygen species, which can induce mitochondrial DNA mutations, damage the mitochondrial respiratory chain, alter membrane permeability, and influence Ca(2+) homeostasis and mitochondrial defense systems. All these changes are implicated in the development of these neurodegenerative diseases, mediating or amplifying neuronal dysfunction and triggering neurodegeneration. This paper summarizes the contribution of oxidative stress and mitochondrial damage to the onset of neurodegenerative eases and discusses strategies to modify mitochondrial dysfunction that may be attractive therapeutic interventions for the treatment of various neurodegenerative diseases.Neural Regeneration Research 07/2013; 8(21):2003-14. · 0.23 Impact Factor
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ABSTRACT: Lipid rafts are envisaged as lateral assemblies of specific lipids and proteins that dissociate and associate rapidly and form functional clusters in cell membranes. These structural platforms are not confined to the plasma membrane; indeed lipid microdomains are similarly formed at subcellular organelles, which include endoplasmic reticulum, Golgi and mitochondria, named raft-like microdomains. In addition, some components of raft-like microdomains are present within ER-mitochondria associated membranes. This review is focused on the role of mitochondrial raft-like microdomains in the regulation of cell apoptosis, since these microdomains may represent preferential sites where key reactions take place, regulating mitochondria hyperpolarization, fission-associated changes, megapore formation and release of apoptogenic factors. These structural platforms appear to modulate cytoplasmic pathways switching cell fate towards cell survival or death. Main insights on this issue derive from some pathological conditions in which alterations of microdomains structure or function can lead to severe alterations of cell activity and life span. In the light of the role played by raft-like microdomains to integrate apoptotic signals and in regulating mitochondrial dynamics, it is conceivable that these membrane structures may play a role in the mitochondrial alterations observed in some of the most common human neurodegenerative diseases, such as Amyotrophic lateral sclerosis, Huntington's chorea and prion-related diseases. These findings introduce an additional task for identifying new molecular target(s) of pharmacological agents in these pathologies.APOPTOSIS 02/2015; · 3.61 Impact Factor
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ABSTRACT: Mutations in superoxide dismutase 1 (SOD1) cause familial amyotrophic lateral sclerosis (FALS), while wild-type SOD1 has been implicated in sporadic ALS (SALS). SOD1 mutants are now recognized to acquire one or more toxicities that include their association with mitochondrial and endoplasmic reticulum membranes but the underlying structural mechanism remains unknown. Here we determine NMR conformations of both wild-type and a truncation mutant (L126Z) of SOD1 in aqueous solution and a membrane environment. The truncation mutant (which causes FALS at very low levels, indicating its elevated toxicity) is highly unstructured in solution, failing to adopt the β-barrel SOD1 native structure. Wild-type SOD1 is also highly unstructured upon reduction of disulfides and depletion of zinc. Most remarkably, both mutant and wild type adopt similar, highly-helical conformations in a membrane environment. Thus, either truncation or depletion of zinc is sufficient to eliminate the native β-barrel structure, and transform cytosolic SOD1 into membrane proteins energetically driven by forming amphiphilic helices in membranes. That zinc-deficiency is sufficient to produce a similar transformation in wild-type SOD1 implies that the wild-type and FALS-linked SOD1 mutants may trigger ALS by a common mechanism.Biochimica et Biophysica Acta (BBA) - Biomembranes 10/2014; · 3.43 Impact Factor
Misfolded Mutant SOD1 Directly Inhibits VDAC1
Conductance in a Mouse Model of Inherited ALS
Adrian Israelson,1Nir Arbel,2Sandrine Da Cruz,1Hristelina Ilieva,1Koji Yamanaka,3Varda Shoshan-Barmatz,2
and Don W. Cleveland1,*
1Ludwig Institute for Cancer Research and Departments of Cellular and Molecular Medicine and Neurosciences, University of California at
San Diego, La Jolla, CA 92093-0670, USA
2Department of Life Sciences and the National Institute for Biotechnology in the Negev, Ben-Gurion University of the Negev,
Beer-Sheva 84105, Israel
3Laboratory of Motor Neuron Diseases, RIKEN Brain Science Institute, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan
Mutations in superoxide dismutase (SOD1) cause
amyotrophic lateral sclerosis (ALS), a neurodegen-
erative disease characterized by loss of motor
neurons. With conformation-specific antibodies, we
now demonstrate that misfolded mutant SOD1 binds
directly to the voltage-dependent anion channel
(VDAC1), an integral membrane protein imbedded
in the outer mitochondrial membrane. This interac-
tion is found on isolated spinal cord mitochondria
and can be reconstituted with purified components
in vitro. ADP passage through the outer membrane
is diminished in spinal mitochondria from mutant
SOD1-expressing ALS rats. Direct binding of mutant
SOD1 to VDAC1 inhibits conductance of individual
tion of VDAC1 activity with targeted gene disruption
is shown to diminish survival by accelerating onset
of fatal paralysis in mice expressing the ALS-causing
mutation SOD1G37R. Taken together, our results
establish a direct link between misfolded mutant
SOD1 and mitochondrial dysfunction in this form of
Amyotrophic lateral sclerosis (ALS) is a progressive adult-onset
neurodegenerative disorder characterized by the selective loss
of upper and lower motor neurons in the brain and spinal cord
(Cleveland and Rothstein, 2001). The typical age of onset is
between 50 to 60 years, followed by paralysis and ultimately
death within 2–5 years after onset (Mulder et al., 1986). Most
instances of ALS are sporadic lacking any apparent genetic
linkage, but 10% are inherited in a dominant manner. Twenty
percent of thesefamilial cases havebeen attributed to mutations
in the gene encoding cytoplasmic Cu/Zn superoxide dismutase
(SOD1) (Rosen et al., 1993). Although multiple hypotheses have
been proposed to explain mutant SOD1-mediated toxicity (Ilieva
et al., 2009), the exact mechanism(s) responsible for motor
neuron degeneration remains unsettled.
Mitochondrial dysfunction has been proposed to contribute
to disease pathogenesis. Histopathological observations of dis-
turbed mitochondrial structure have been reported in muscle
of both sporadic and familial ALS patients (Hirano et al., 1984a,
models expressing dismutase active (Dal Canto and Gurney,
1994; Higgins et al., 2003; Kong and Xu, 1998; Wong et al.,
1995), but not inactive mutants (Bruijn et al., 1997). Moreover,
functionality of mitochondria has been reported to be affected
in spinal cord and skeletal muscles of human sporadic ALS or
familial ALS patients (Dupuis et al., 2003; Echaniz-Laguna et al.,
2002; Vielhaber et al., 1999; Wiedemann et al., 2002), as well as
in some ALS mouse models (Damiano et al., 2006; Mattiazzi
et al., 2002; Nguyen et al., 2009).
A proportion of the predominantly cytosolic SOD1 has been
reported to localize to mitochondria in certain contexts. In both
rodent models and patient samples, mutant SOD1 is present in
fractions enriched for mitochondria derived from affected, but
not unaffected, tissues (Bergemalm et al., 2006; Deng et al.,
2006; Liu et al., 2004; Mattiazzi et al., 2002; Vande Velde et al.,
2008; Vijayvergiya et al., 2005) and a clear temporal correlation
between mitochondrial association and disease progression
was shown for multiple mutant SOD1s (Liu et al., 2004). Purifica-
tion of mitochondria, including floatation steps that eliminate
protein only aggregates, coupled with protease accessibility has
demonstrated mutant SOD1 deposition on the cytoplasmic-
facing surface of spinal cord mitochondria (Liu et al., 2004;
Vande Velde et al., 2008). Sensitivity to proteolysis and immuno-
precipitation with an antibody specific for misfolded SOD1
further indicated that misfolded forms of dismutase active and
inactive SOD1 are deposited onto the cytoplasmic face of the
outer membrane of spinal cord mitochondria (Vande Velde
et al., 2008). This is accompanied by altered accumulated levels
of a few mitochondrial proteins, reduced import of multiple mito-
chondrial proteins, and reduced complex I activity (T. Miller, C.
Vande Velde, and D.W.C., unpublished data).
Oxidative phosphorylation requires the transport of metabo-
lites, including ADP, ATP, and inorganic phosphate across both
mitochondrial membranes. Located in the outer mitochondrial
membrane, the voltage-dependent anion channel (VDAC),
Neuron 67, 575–587, August 26, 2010 ª2010 Elsevier Inc. 575
known as mitochondrial porin, assumes a crucial position in the
cell, controlling metabolic cross-talk between the mitochondrion
and the rest of the cell, thus regulating the metabolic and ener-
getic functions of mitochondria (Shoshan-Barmatz et al., 2006,
2008). Of the three VDAC isoforms (VDAC1–3), VDAC1 is the
most abundant in most cells. VDAC1 is a primary contributor
to ATP/ADP flux across the outer mitochondrial membrane
(Colombini, 2004;Lemasters andHolmuhamedov, 2006). Initially
named somewhat misleadingly as a channel for anions, it is also
responsible for import/export of Ca2+(Gincel et al., 2001) and
other cations (Benz, 1994; Colombini, 2004), adenine nucleo-
tides (Rostovtseva and Colombini, 1997; Rostovtseva and
Bezrukov, 1998) and other metabolites (Hodge and Colombini,
1997). Indeed, it has been demonstrated that silencing VDAC1
VDAC1 is also a key player in mitochondria-mediated apopto-
sis. VDAC1 has been implicated in apoptotic-relevant events,
due to serving as the target for members of the pro- and anti-
apoptotic Bcl2-family of proteins (Arbel and Shoshan-Barmatz,
2010; Shimizu et al., 1999) and due to its function in the release
of apoptotic proteins from the intermitochondrial membrane
space (Abu-Hamad et al., 2009; Shoshan-Barmatz et al., 2006,
2008; Tajeddine et al., 2008). VDAC1 has also been implicated
in Parkinson’s disease as a direct target for Parkin-mediated
poly-ubiquitylation and mitophagy (Geisler et al., 2010).
Starting from recognition that a proportion of misfolded,
mutant SOD1 is bound to the cytoplasmic face of the outer
membrane of mitochondria in affected tissues (Liu et al., 2004;
Rakhit et al., 2007; Vande Velde et al., 2008), we now identify
damage to spinal cord mitochondria to arise through direct
binding of misfolded SOD1 onto the cytoplasmic-facing domain
of VDAC1, thereby inhibiting its conductance.
Mutant SOD1 and VDAC1 Interact In Vivo in Spinal
Cord of Transgenic SOD1 Rats
To investigate potential interactions between mutant SOD1 and
VDAC1, mitochondria from rats expressing wild-type human
SOD1 (hSOD1wt) or either of two different ALS-linked SOD1
mutants, a dismutase active hSOD1G93Aand a dismutase inac-
tive hSOD1H46R, were highly purified by repeated centrifugation
steps(summarized inFigure 1A)including afinaldensitygradient
flotation step to eliminate any contaminating protein only aggre-
gates (proteins sediment downward in theseconditions because
of their higher density), as previously described (Vande Velde
et al., 2008). Immunoblotting of immunoprecipitates generated
after addition of an SOD1 antibody to solubilized mitochondrial
lysates revealed that a proportion of VDAC1 was coprecipitated
with dismutase active and inactive mutant SOD1, but not wild-
type SOD1 (Figure 1B). Parallel immunoprecipitations with a
VDAC1 antibody confirmed coprecipitation of both hSOD1G93A
and hSOD1H46Rwith VDAC1 (Figure 1D). Binding to VDAC1 was
a property only of spinal cord mitochondria, as no association
of mutant SOD1 was seen with purified brain mitochondria
from the same animals using immunoprecipitation with SOD1
(Figure 1C) or VDAC1 (Figure 1E) antibodies. This latter finding
is consistent with prior efforts that had demonstrated that
mutant SOD1 associates with the cytoplasmic face of the outer
membrane of mitochondria in spinal cord, but not other tissue
types (Liu et al., 2004; Vande Velde et al., 2008). Moreover,
mutant SOD1 binding to VDAC1 is inversely correlated with the
level of hexokinase-I, a known partner that binds to VDAC1
exposed on the cytoplasmic mitochondrial surface (Abu-Hamad
et al., 2008; Azoulay-Zohar et al., 2004; Zaid et al., 2005), with
hexokinase accumulating to much higher level in brain than
spinal cord mitochondria (Figure 1F).
Misfolded Mutant SOD1 Specifically Interacts with
VDAC1 In Vivo in Spinal Cord of Transgenic SOD1 Rats
To test the nature of the interaction between mutant SOD1 and
VDAC1, immunoprecipitation was performed with a SOD1 anti-
body that recognizes a ‘‘disease-specific epitope’’ (DSE) that is
unavailable on correctly folded SOD1 (Cashman and Caughey,
2004; Paramithiotis et al., 2003; Urushitani et al., 2007), but is
present on misfolded mutant SOD1s in inherited ALS (Rakhit
et al., 2007). Using one such antibody (DSE2), age-dependent
deposition of mutant SOD1 onto the cytoplasmic face of spinal
cord mitochondria has been shown to reflect association of
misfolded SOD1 (Vande Velde et al., 2008). We exploited this
antibody to examine if the SOD1 associated with VDAC1 is
bound through misfolded SOD1. Liver, brain, and spinal cord
cytosolic and mitochondrial fractions purified from symptomatic
rats expressing mutant hSOD1G93Awere immunoprecipitated
(see schematic in Figure 2A) with the DSE2 antibody, which
recognizes an epitope in the electrostatic loop of hSOD1
(between residues 125–142) that is buried in normally folded
SOD1. Misfolded mutant SOD1G93Awas not detectable in the
soluble fraction of any tissue, but was immunoprecipitated from
the spinal cord, but not liver or brain, mitochondrial fractions
Solubilized spinal cord mitochondria purified from presymp-
tomatic and symptomatic rats expressing either of two different
SOD1 mutants, dismutase active hSOD1G93Aand dismutase
inactive hSOD1H46R, as well as hSOD1wtwere immunoprecipi-
tated with the DSE2 antibody and coimmunoprecipitated
components identified by immunoblotting. An age-dependent
increase in misfolded SOD1 was seen for both mutants, with
a significantly higher proportion of the dismutase inactive
SOD1H46Rin a misfolded conformation. In samples from symp-
tomatic animals, VDAC1 coprecipitated together with the mis-
folded mutantSOD1, asrevealed byimmunoblotting ofimmuno-
precipitates (Figure 2C). This association was selective for
VDAC1, as misfolded mutant SOD1 did not coimmunoprecipi-
tate with any of three other mitochondrial proteins examined
(Figure 2C), including two additional outer mitochondrial
membrane proteins with domains facing the cytoplasm: TOM40,
the 40 kDa component of transport across the outer membrane
(TOM) complex mediating all protein import from the cytoplasm
to the mitochondria, and VDAC2, a second voltage-dependent
anion channel isoform that has been estimated to represent
7% (kidney) to 25% (brain) of accumulated VDAC (Yamamoto
Mutant SOD1 Directly Inhibits VDAC1 Conductance
576 Neuron 67, 575–587, August 26, 2010 ª2010 Elsevier Inc.
et al., 2006). It also did not coprecipitate cyclophilin-D, an impor-
tant component of the permeability transition pore.
Furthermore, in order to determine which cells accumulate the
misfolded form of SOD1, we performed immunostaining using
the DSE2 antibody. Spinal cords from loxSOD1G37Rmice at
different stages of the disease were subjected to immunostain-
ing with DSE2 antibody (Figure 2D). The accumulation of mis-
folded SOD1 dramatically increased with disease progression.
Although little accumulation of misfolded SOD1 is found by
disease onset, it was preferentially found within motor neurons.
During disease progression, a dramatic increase of misfolded
SOD1 was apparently accumulated in other cells as well and
probably also extracellularly. Throughout disease a proportion
of the misfolded SOD1 was colocalized with mitochondria of
motor neurons and other cells, starting at onset and increasing
with disease progression (Figure 2D).
Binding of Mutant SOD1 Directly Inhibits VDAC1
To test if binding of mutant SOD1 affects VDAC1 function,
VDAC1 was purified from spinal cords of nontransgenic rats
using conditions previously demonstrated to yield polarized
VDAC1 membrane insertion such that the VDAC1 surface
exposed on the cis side is the surface exposed to the cytosol
when inserted into the mitochondrial outer membrane (Azou-
lay-Zohar et al., 2004; Israelson et al., 2005; Arbel and
Shoshan-Barmatz, 2010). Activity of individual channels was
measured as a function of time by the ions passing across the
bilayer in response to an applied voltage gradient. This revealed
that in the absence of SOD1, VDAC1 was stably in a fully open
state (4 nS at 1 M KCl [Shoshan-Barmatz et al., 2006]) and re-
mained so for extended periods.
Mutant SOD1 proteins hSOD1G93A, hSOD1G85R, as well as
hSOD1wt, were expressed using baculovirus and purified
(Figure 3C; Hayward et al., 2002). Wild-type SOD1, even at the
highest added concentration (8 mg/ml), had no effect on VDAC1
conductance when added on either cis or trans sides of the
membrane (Figures 3E and 3I). However, addition of purified
recombinant hSOD1G93Aor hSOD1G85R(Figure 3C) substantially
reduced VDAC1 channel conductance (Figures 3F and 3G). Both
mutant SOD1s modified VDAC1 conductance only when added
to the cis side (Figures 3F and 3G), but not the trans side (Figures
3J and 3K) of the bilayer, indicating that mutant SOD1 interacts
Figure 1. A Complex Containing Mutant SOD1 and VDAC1 from Spinal Cord Mitochondria
(A) Schematic outlining the different purification steps used. Floated isolated mitochondria from (B and D) hSOD1wt, hSOD1G93A, and hSOD1H46Rrat spinal cords
or (C and E) brain were immunoprecipitated with (B and C) an SOD1 antibody or (D and E) VDAC1 antibody.
(B) Immunoblot of the SOD1 immunoprecipitates using VDAC1 antibody indicates that mutant SOD1 proteins hSODG93Aand hSOD1H46Rcoprecipitate VDAC1
(top). SOD1 immunoprecipitation was confirmed by reprobing the membrane with anti-SOD1 antibody (bottom).
(C) Immunoblots of SOD1 immunoprecipitates as in (B) except with brain mitochondria.
(D) Immunoprecipitation using VDAC1 antibody immunoblotted with SOD1 antibody (top). The membrane was then reprobed for VDAC1 (bottom).
(E) Immunoblots of VDAC1 immunoprecipitates as in (D), except with brain mitochondria. Abbreviations: U, unbound fraction (20%); B, bound fraction.
(F) Reduced hexokinase-I levels in spinal cord mitochondria. Polyacrylamide gel analysis of extracts of floated brain and spinal cord mitochondria. (Left)
Coomassie stain; (right) immunoblot for VDAC1, hexokinase I (HK-I), VDAC2, cytochrome c (Cyt. C), and cyclophilin D (Cyp-D).
Mutant SOD1 Directly Inhibits VDAC1 Conductance
Neuron 67, 575–587, August 26, 2010 ª2010 Elsevier Inc. 577
with what would correspond to the cytosolic face of VDAC1 in-
serted into the outer mitochondrial membrane. Use of multi-
channel recordings revealed that not only did mutant SOD1
significantly lower the maximum voltage gated conductance of
individual channels, it also provoked a stable, reduced level of
VDAC1 conductance at all applied voltages (Figures 3L–3N). In
order to determine if this interaction is specific for mutant
SOD1, the effect of another aggregating protein (a-synuclein)
was tested on bilayers containing reconstituted VDAC1. Even
when added to levels 25 times greater than an amount of mutant
SOD1 that markedly affected VDAC1 conductance (Figures 3F
and 3G), neither wild-type nor mutant a-synuclein affected
VDAC1 channel activity at any voltage (Figure 1S).
ADP Transport across the Outer Mitochondrial
Membrane Is Reduced in Spinal Cords of Mutant
Since both dismutase active and inactive SOD1 mutant
proteins reduced VDAC1 channel conductance for K+and
Cl?(Figure 3), we next tested whether mitochondrial conduc-
tance across the outer mitochondrial membrane was affected
in animals chronically expressing mutant SOD1. To do this,
we examined the uptake into mitochondria of adenine nucle-
otides (Figure 4A) which are known to be transported by
VDAC1 (Lemasters and Holmuhamedov, 2006; Rostovtseva
and Colombini, 1997). Freshly isolated spinal cord and liver
mitochondria from SOD1G93Arats were incubated (for 1 min)
Figure 2. The Misfolded Mutant SOD1 Specifically Coprecipitates with VDAC1 in Spinal Cord Mitochondria
(A) Schematic showing the isolation of cytosolic and mitochondrial fractions.
(B) Liver, brain, and spinal cord cytosolic, and mitochondrial fractions were purified from symptomatic rats expressing hSOD1G93Aand the fractions were sub-
jected to immunoprecipitation using DSE2 (3H1), a monoclonal antibody only recognizing misfolded SOD1 (Vande Velde et al., 2008). The immunoprecipitates
were immunoblotted using an SOD1 antibody.
(C) Isolated floated mitochondria from hSOD1wt, hSOD1G93A, and hSOD1H46Rrat spinal cords (from presymptomatic and symptomatic animals) were immuno-
precipitated with DSE2 (3H1), and the immunoprecipitates were immunoblotted using VDAC1, VDAC2, TOM-40, and cyclophilin-D antibodies. SOD1 immuno-
precipitation was confirmed by reprobing the membrane with an SOD1 antibody (top).
(D) Immunohistochemical detection of misfolded SOD1 using DSE2 antibody shows that misfolded SOD1 (green) colocalizes with TOM20 (red), a mitochondrial
outer membrane protein in a subset of spinal cord neurons assessed using NeuN (blue), a neuronal marker as highlighted by filled arrows. DSE2 positive staining
can be detected in some neurons at onset and significantly increases with the appearance of disease symptoms.
Of note DSE2 staining is not restricted to neuronal mitochondria but is also detected in nonneuronal cells and the extracellular space as shown with thin arrows.
No DSE2 staining was detected in neurons of 1 year old nontransgenic control mice (Non Tg). Scale bar: 10 mm. Abbreviation: U, unbound fraction (20%); B,
bound fraction; Pre, presymptomatic; Sym, symptomatic.
Mutant SOD1 Directly Inhibits VDAC1 Conductance
578 Neuron 67, 575–587, August 26, 2010 ª2010 Elsevier Inc.
Figure 3. Mutant, but Not Wild-Type, SOD1 Interacts with Bilayer-Reconstituted VDAC1 to Reduce Its Channel Conductance
(A) Coomassie Blue staining and immunoblot of purified VDAC1 purified from rat spinal cord.
(B)Schematic presentation showing theplanar lipid bilayer reconstitution and channel conductance assaysystem. Purifiedspinalcord VDAC1 was reconstituted
into a planar lipid bilayer, and channel currents through VDAC1 were recorded.
(C) Coomassie Blue staining and immunoblot of purified recombinant hSOD1wt, hSOD1G93A, and hSOD1G85Rexpressed in insect cells using baculovirus.
(D–G) Currents through VDAC1 in response to a voltage step from 0 to ?10 mV were recorded before and 2 min after the addition (to 2 mg/ml final) of purified
recombinant (E) hSOD1wt, (F) hSOD1G93A, or (G) hSOD1G85Rto the cis side of the bilayer.
(H–K) Currents through VDAC1 as in (D)–(G), except after SOD1 addition to the trans side of the bilayer. The dotted lines indicate current levels in the maximal and
zero conductance states. These examples are representative of the results from 3–4 independent reconstitution experiments.
(L–N)MutantSOD1 effect on VDAC1 channel activity at different voltages. Averagesteady-state conductance of VDAC1 before and after addition of (L) hSOD1wt,
(M) hSOD1G93A, or (N) hSOD1G85R, determined as a function of voltage with a multichannel recording.
Mutant SOD1 Directly Inhibits VDAC1 Conductance
Neuron 67, 575–587, August 26, 2010 ª2010 Elsevier Inc. 579
with radio-labeled [3H]ADP and the amount of imported ADP
was measured by scintillation counting after rapid filtration
to remove the unincorporated ADP. Coincubation with 1 mM
of the VDAC1 inhibitor DIDS (4, 40-diisothiocyanostilbene-2,
20-disulfonic acid) demonstrated that ?2/3 of the ADP uptake
was through VDAC1 (Figures 4B and 4C). Compared to mito-
chondria from non-transgenic animals, uptake of ADP by
spinal cord mitochondria from SOD1 mutant expressing
animals was selectively and progressively inhibited, yielding
?40% inhibition of VDAC1-dependent uptake (?25% overall
inhibition of ADP uptake) by a symptomatic stage (Figure 4B).
Inhibition of ADP uptake was selective to spinal mitochondria
as liver mitochondria from the same hSOD1G93Aanimals re-
tained normal ADP import at all ages examined (Figure 4C).
Mutant SOD1 Binding to Mitochondria In Vitro
Diminishes ADP but Not Ca2+Uptake
To test if inhibition of ADP import seen in spinal cord mito-
chondria from mutant SOD1 animals could be generated
solely from mutant SOD1 binding to the cytoplasmic face of
those mitochondria, purified recombinant SOD1 proteins
(hSOD1wt, hSOD1G93A, and hSOD1G85R) (Figure 3C) were
added to mitochondria purified from spinal cords or livers
of non transgenic rats (Figure 5A). Although a proportion of
each of the recombinant SOD1s associated with both spinal
cord and liver mitochondria (Figure 5D), accumulation of
radio-labeled Ca2+(presumably through the action of the
mitochondrial calcium uniporter) into spinal cord or liver mito-
chondria was not affected by the addition of wild-type or
mutant SOD1 (Figure 5C). On the other hand, VDAC1-medi-
ated ADP accumulation into the same spinal cord or liver
mitochondria wasinhibitedbyboth hSOD1G93A
Figure 4. ADP Transport across the Outer Mito-
chondrial Membrane Is Reduced in Mitochondria
from Spinal Cord of SOD1G93AALS Rats
(A) Schematic presentation of method for measuring ADP
accumulation into isolated mitochondria as measured
using radio-labeled [3H]ADP.
(B and C) Mitochondria were isolated from (B) spinal cord
and (C) liver of nontransgenic, hSOD1wt, hSOD1G93A
presymptomatic, and hSOD1G93A
Student’s t test was used and p < 0.001 (marked by three
asterisks) and p < 0.01 (marked by two asterisks) were
considered statistically significant. Values represent the
means ± SEM of three to four independent experiments.
(Figure 5B). This inhibition corresponded to
a proportion of misfolded SOD1 associated
with those mitochondria after incubation with
either mutant, but not wild-type SOD1, as
intact mitochondria with the DSE2 antibody
to misfolded SOD1 (Figure 5E). In contrast,
wild-type SOD1 associated with the same mito-
chondria was not recognized by this misfolded
SOD1 antibody (Figure 5E), consistent with its
Reduced VDAC1 Activity Diminishes Survival of Mutant
SOD1G37RMice by Accelerating Disease Onset
with VDAC1 thereby inhibiting VDAC1 conductance (Figure 3),
(2) spinal cord mitochondria from SOD1 mutant animals have
progressive loss of ADP uptake, and (3) misfolded mutant
SOD1 binds to normal mitochondria in vitro accompanied by
selective loss of ADP conductance (Figure 5), we examined how
reduced level and activity of VDAC1 affect disease course in
SOD1G37Rmutant mice. To do this, we exploited mice heterozy-
gous for disruption of the VDAC1 gene (producing what is effec-
tively a null allele [Weeber et al., 2002]). These mice accumulate
about half the normal level of VDAC1 protein (Figure 2S), while
overall ADP conductance of spinal mitochondrial isolated from
VDAC1+/?mice is reduced by ?25% (Figure 3S) relative to
wild-type mice. After mating with SOD1G37Rmice, sex matched
cohorts of mice and their littermates carrying the SOD1G37R
transgene and zero, one, or two active VDAC1 alleles were ob-
tained and followed for disease onset, progression and survival.
Measurement of ADP conductance of spinal mitochondria
from SOD1G37R/VDAC1+/?mice revealed a reduction to a level
comparable to that corresponding to complete deletion of
VDAC1 (Figure 3S).
A simple and objective measure of disease onset and early
disease progression was applied by initiation of weight loss,
reflecting denervation-induced muscle atrophy. While timing of
progression from onset through either early (Figure 6E) or late
tion of VDAC1 levels, disease onset (Figures 6A and 6D) and
Mutant SOD1 Directly Inhibits VDAC1 Conductance
580 Neuron 67, 575–587, August 26, 2010 ª2010 Elsevier Inc.
progression to an early disease point (Figure 6B) were acceler-
ated by 41 and 45 days, respectively, in SOD1G37R/VDAC1+/?
mice (183 ± 22 and 230 ± 28 days) compared with their
SOD1G37Rlittermates (224 ± 19 and 275 ± 25 days). Moreover,
age at which end stage disease was reached was also reduced
by an average of 59 days (Figure 6C; SOD1G37R/VDAC1+/?mice
[310 ± 42 days] compared with their SOD1G37Rlittermates
Figure 5. Mutant SOD1 Proteins Affect ADP
but Not Ca2+Accumulation into Mitochon-
(A) ADP or Ca2+accumulation into isolated mito-
chondria was measured using a filter trap assay
with radio-labeled45CaCl2or [3H]ADP. Mitochon-
dria were isolated from fresh spinal cords and
livers of nontransgenic rats.
(B) ADP and(C)Ca2+
measured before and after the addition of 3 mM
(50 mg/ml) hSOD1wt, hSOD1G93A, or hSOD1G85R
purified proteins. Student’s t test was used and
p < 0.001 (marked by three asterisks) and
p < 0.01 (marked by two asterisks) were consid-
ered statistically significant. Values represent the
means ± SEM of three independent experiments.
(D) Purified hSOD1wt, hSOD1G93A, or hSOD1G85R
were incubated with liver or spinal cord mitochon-
drial fractions purified from a nontransgenic rat for
20 min at 37?C. The samples were then washed
three times and the mitochondrial pellet was sub-
jected to immunoblot using an SOD1 antibody.
(E) Purified hSOD1wt, hSOD1G93A, or hSOD1G85R
was incubated for 20 min at 37?C with spinal
cord mitochondria purified from nontransgenic
rats. The samples were then washed three times
and the mitochondrial pellet was subjected to
monoclonal antibody only recognizing misfolded
SOD1. The immunoprecipitates were immuno-
blotted using an SOD1 antibody.
[369 ± 32 days]). A similar reduction in
age of onset and life span was also
observed for SOD1G37R/VDAC1?/?mice
(Figure 4S), demonstrating that reduction
in VDAC1 activity does affect SOD1
primarily by accelerating an early step in
disease onset or spread.
We have demonstrated here in floated
spinal cord mitochondria from mutant
SOD1 expressing animals that both mis-
SOD1 mutants bind directly and selec-
tively to the cytoplasmically exposed
face of VDAC1. Both dismutase active
and dismutase inactive, but not wild-
type, SOD1 binding to VDAC1 reduces channel conductance,
as demonstrated for K+and Cl?ions by electrophysiological
recording and for ADP by inhibition of normal ADP accumulation
into mitochondria. Channel conductance was not affected in
liver mitochondria (where misfolded SOD1 does not accumu-
late). Mutant association and conductance inhibition is repli-
cated in spinal cord mitochondria purified from mutant
Mutant SOD1 Directly Inhibits VDAC1 Conductance
Neuron 67, 575–587, August 26, 2010 ª2010 Elsevier Inc. 581
increasing in severity during disease progression contempora-
neous with increased accumulation of misfolded mutant SOD1.
The clear implication from this is that only the misfolded portion
of SOD1 is able to affect the channel, thereby partially blocking
metabolite flux across the outer mitochondrial membrane.
Reduced conductance by VDAC1 will decrease ATP synthesis,
increase the ADP/ATP ratio in the cytosol and reduce membrane
potential (as outlined in Figure 7). Chronic mitochondrial
dysfunction can in turn drive generation of damaging reactive
chemical damage to it, as has been previously documented
selectively in spinal cords from mutant SOD1 animals (Liu
et al., 2004; Vande Velde et al., 2008). Thus, our evidence
demonstrates that reduced VDAC1 conductance, and corre-
spondingly reduced respiration rate (Lemasters and Holmuha-
medov, 2006), are direct components of intracellular damage
from mutant SOD1.
Survival in the hSOD1G37RMouse Model of
Ages of (A) disease onset (determined as the time
when mice reached peak body weight), (B) early
disease (determined as the time when mice lost
10% of maximal weight), and (C) disease end
stage (determined as the time when the animal
could not right itself within 20 s when placed on
its side) of SOD1G37R-VDAC1+/?
ages ± SD is provided.
(D, E, and F) Mean onset (D), mean duration of
early disease (from onset to 10% weight loss, E)
and mean duration of late disease (from 10%
weightloss toend-stage,F). Errorbars denote SD.
See also Figure S4.
SOD1 lower VDAC1-dependent ADP
conductance by half as much as does
complete VDAC1 deletion (Figure 3S),
further reduction in conductance (by
VDAC1 gene inactivation) significantly
progression), reducing survival by more
heterozygous and homozygous mice.
Intracellular targets for SOD1 damage
beyond VDAC1 have been proposed
(Ilieva et al., 2009), including aberrant
synaptic glutamate recovery by astro-
cytes (Rothstein et al., 1995), mutant
chromogranin (Urushitani et al., 2006),
not onlydoes mutant
onset (but not
inhibition of the ERAD pathway by mutant SOD1 binding to
the integral membrane protein derlin (Nishitoh et al., 2008),
and excessive production by microglia of extracellular super-
oxide following mutant SOD1 binding to the small G protein
Rac1 and its subsequent stimulation of NAPDH oxidase (Har-
raz et al., 2008). Moreover, it was recently proposed that
misfolded SOD1 damage to mitochondria can induce morpho-
logical changes and cytochrome c release in the presence of
Bcl-2 (Pedrini et al., 2010). To those hypotheses, we propose
that the partial blockage of the VDAC1 channel by direct asso-
ciation with misfolded SOD1 would make motor neurons more
vulnerable to any of these additional stresses derived either
from mutant SOD1 acting within motor neurons, astrocytes,
microglia, and possibly additional neighboring nonneuronal
cells. Indeed, in the presence of reduced VDAC1 conductance
such pathways must play roles in pathogenesis, as we have
shown that mutant SOD1-mediated disease still ensues in
VDAC1 null mice.
Mutant SOD1 Directly Inhibits VDAC1 Conductance
582 Neuron 67, 575–587, August 26, 2010 ª2010 Elsevier Inc.
Surprisingly, in the absence of VDAC1, we have found a 60%
residual ADP conductance which seems most likely to be
contributed by compensatory VDACs or VDAC-like activity(ies).
Although no other VDAC isoform is known to be overexpressed
that differ in conductance and selectivity (Xu et al., 1999). It is
plausible that in the absence of VDAC1, VDAC2 exists predom-
inantly in a high conductance state, as a compensatory mecha-
nism. This mechanism should now be tested by purifying VDAC2
from VDAC1 knockout mouse, and testing its channel properties
in lipid bilayers.
The compromise in mutant SOD1-mediated VDAC1 conduc-
tance that we have found offers a mechanistic explanation for
alteration in mitochondrial electron transfer chain complexes
and the capacity to consume oxygen and synthesize ATP previ-
ously reported in one mutant SOD1 expressing mouse line (Jung
et al., 2002; Kirkinezos et al., 2005; Mattiazzi et al., 2002). The
recent report that association of hSOD1G93Aand hSOD1G85R
transfer chain to limit Ca2+-induced Jm depolarization (Nguyen
et al., 2009) is also fully compatible with altered adenine nucleo-
tide transport across the outer mitochondrial membrane as the
initiating deficit. So too is the report of reduced ability of mito-
Ca2+addition (Damiano et al., 2006).
VDAC1has beenproposed to bethe mediator forROSrelease
from the intermitochondrial spaces to the cytosol (Han et al.,
2003; Madesh and Hajno ´czky, 2001). Moreover, hexokinase
(known to interact with VDAC1) has been shown in cell culture
Binding to VDAC1
Model showing the effects of misfolded SOD1
binding to VDAC1. Misfolded SOD1 is proposed
to inhibit VDAC1 conductance and suppress
both uptake and release of mitochondrial metabo-
This reduction in metabolites flux would result in
reduced energy production and oxidative stress
leading to mitochondrial dysfunction.
7. EffectsofMisfolded SOD1
to decrease ROS release when overex-
pressed, thereby reducing intracellular
levels of ROS (Ahmad et al., 2002; da-
Silva et al., 2004). The relatively low level
to that in brain (Figure 1F) may therefore
be a component of selective vulnerability.
This is also consistent with the selective
association of misfolded mutant SOD1
with VDAC1 on the cytoplasmic face of
mitochondria from spinal cord, but not
liver or brain. Although both tissues accu-
mulate high levels of mutant SOD1 (Liu
et al., 2004; Vande Velde et al., 2008),
SOD1 is bound to the cytoplasmic face of
spinal cord mitochondria, while apparently imported into the
intermembrane space of mitochondria from cortex of the same
animals and not associated with liver mitochondria at all (Vande
Velde et al., 2008). Another factor likely underlying the differ-
encesin mutantSOD1 association with mitochondria, andthere-
fore potentially factors underlying selective vulnerability, is that
mitochondria from different tissues (and which retain different
functionalproperties) havedifferent proteincompositions (Bailey
et al., 2007; Mootha et al., 2003), including hexokinase levels.
This is accompanied by intrinsic differences in O2?$production,
lipid peroxidation, DNA oxidation and Ca2+accumulation
capacity (Sullivan et al., 2004).
Our finding that VDAC1 is one of the targets for misfolded
SOD1 within the nervous system raises substantial implications
for the mechanism underlying premature degeneration and
death of motor neurons. A variety of apoptotic stimuli are known
to trigger cell death by modulation of VDAC1 (Abu-Hamad et al.,
2008; Shoshan-Barmatz et al., 2006; Tsujimoto and Shimizu,
2002; Yagoda et al., 2007; Zaid et al., 2005; Zamzami and
Kroemer, 2003; Zheng et al., 2004), implicating VDAC1 as
a component of the apoptotic machinery. Although VDAC1
proteins have been reported to be dispensable for Ca2+and
oxidative stress-induced permeability transition pore (PTP)
opening (Baines et al., 2007), siRNA-mediated reduction in
VDAC1 has supported VDAC1 as an indispensable protein for
endostatin-, cisplatin-, and selenite-induced oxidative stress
induced PTP opening and apoptosis (Tajeddine et al., 2008;
Tomasello et al., 2009; Yuan et al., 2008). Moreover, VDAC1
was recently shown to be involved in staurosporine- and
Mutant SOD1 Directly Inhibits VDAC1 Conductance
Neuron 67, 575–587, August 26, 2010 ª2010 Elsevier Inc. 583
ceramide-induced cell death downstream of BAD and BCL-XL
(Roy et al., 2009) and curcumin induced apoptosis by cooperat-
ing with Bax in the release of AIF from mitochondria (Scharstuhl
et al., 2009). Since VDAC1 is one of several targets for a choles-
terol-like small molecule (TRO19622) that can protect motor
neurons from SOD1 mutant-mediated death in culture and
modestly delay disease onset in SOD1 mutant mice (Bordet
et al., 2007), it now seems likely that its efficacy may be through
direct effect on VDAC1.
Finally, it is well established that although motor neurons are
the final targets in ALS, mutant damage within astrocytes and
microglia contributes to driving rapiddisease progression (Beers
et al., 2006; Boille ´e et al., 2006a, 2006b; Clement et al., 2003;
Yamanaka et al., 2008a, 2008b). In this context, we show here
that little accumulation of misfolded SOD1 is found by disease
onset, but it is preferentially within motor neurons. However,
during disease progression a dramatic increase of misfolded
SOD1 is observed accumulated in other cells as well and prob-
ably extracellularly. Interestingly, mitochondrial dysfunction(s)
within mutant astrocytes has been reported to cause acute
motor neuron death in astrocyte-motor neuron cocultures (Cas-
sina et al., 2008) and astrocytes expressing mutant SOD1 have
been reported to induce mitochondrial dysfunction within motor
neurons (Bilsland et al., 2008). Coupling these findings with the
appearance of aberrant mitochondria within motor neurons in
multiple animal models of SOD1 mutant mediated ALS (Bendotti
et al., 2001; Jaarsma et al., 2001; Kong and Xu, 1998; Wong
et al., 1995) and the association of mutant SOD1 with mitochon-
dria within affected tissues, we propose that misfolded SOD1
association directly with VDAC1 represents a primary event of
damage within motor neurons.
Transgenic Rats and Mice
Transgenic rats expressing hSOD1wt(Chan et al., 1998), hSOD1G93A(Howland
et al., 2002), and hSOD1H46R(Nagai et al., 2001) were as originally described.
All animal procedures were consistent with the requirements of the Animal
Care and Use Committee of the University of California.
Mice heterozygousforthe mutant
(LoxSOD1G37R) (Boille ´e et al., 2006b) were crossed with mice heterozygous
for a VDAC1 gene disruption (Weeber et al., 2002). Mice were genotyped by
PCR for the presence of the mutant SOD1 transgene (Williamson and Cleve-
land, 1999) and using a four-primer multiplex PCR for the presence of
VDAC1 (Weeber et al., 2002), as previously described.
For survival experiments, SOD1G37R, VDAC1+/?
compared with their contemporaneously produced SOD1G37R, VDAC1+/+
littermates. Time of disease onset was retrospectively determined as the
time when mice reached peak body weight, early disease was defined at the
time when denervation-induced muscle atrophy had produced a 10% loss
of maximal weight, and end-stage was determined by paralysis so severe
that the animal could not right itself within 20 s when placed on its side, an
endpoint frequently used for SOD1 mutant mice and one that was consistent
with the requirements of the Animal Care and Use Committee of the University
mice were always
Mitochondria were purified as previously described (Vande Velde et al., 2008).
Tissues were homogenized on ice in 5 volumes of ice-cold homogenization
buffer (HB) composed of 210 mM mannitol, 70 mM sucrose, 1 mM EDTA-(Tris)
and 10 mM Tris-HCl (pH 7.2). Homogenates were centrifuged at 1,000 3 g for
10min.Supernatantswererecovered,and pelletswerewashed with½volume
HB and centrifuged at 1,000 3 g. Supernatants were pooled and centrifuged
at 12,000 3 g for 15 min to yield a crude mitochondrial pellet. The supernatant
was used to make cytosolic fractions by further centrifugation at 100,000 3 g
for 1 hr. The mitochondria were gently resuspended in HB and then adjusted
tube. Mitochondria were overlaid with an equal volume of 1.175 g/ml and
1.079 g/ml Optiprep and centrifuged at 50,000 3 g for 4 hr (SW-55; Beckman).
Mitochondria were collected at the 1.079/1.175 g/ml interface and washed
once to remove the Optiprep. Optiprep stock solution was diluted in 250 mM
sucrose, 120 mM Tris-HCl (pH 7.4), 6 mM EDTA plus protease inhibitors.
For activity assays, spinal cords were homogenized in 5 volumes of ice-cold
homogenization buffer (HB) on ice. Homogenates were centrifuged at
1,000 3 g for 5 min. Supernatants were recovered and centrifuged again at
1,000 3 g for 5 min. Supernatants were centrifuged at 12,000 3 g for 10 min
to yield crude mitochondrial pellets. These mitochondria were gently resus-
pended in HB and then adjusted to 12% Optiprep (iodixanol) and centrifuged
at 17,000 3 g for 10 min (SW-55; Beckman). The majority of the myelin (at the
top of the sample) was removed and the mitochondria were washed once with
HB (without EDTA) to remove the Optiprep.
Liver was homogenized in 5 volumes of ice-cold homogenization buffer (HB)
on ice. Homogenates were centrifuged at 1,000 3 g for 5 min. Supernatants
were recovered, and centrifuged again at 1,000 3 g for 5 min. Supernatant
was centrifuged at 12,000 3 g for 10 min to yield a crude mitochondrial pellet.
These mitochondria were resuspended in HB (without EDTA) and centrifuged
again at 12,000 3 g for 10 min. The pellet was resuspended in a small volume
of HB without EDTA.
VDAC Channel Recording and Analysis
Reconstitution of VDAC into a planar lipid bilayer (PLB), single channel current
recording, and data analysis were carried out as previously described (Gincel
et al., 2001). Briefly, PLB were prepared from soybean asolectin dissolved in
n-decane (50 mg/ml). Only PLB with a resistance greater than 100 GU, were
used. Purified protein (about 1 ng) was added to the cis chamber. After one
or a few channels were inserted into the PLB, the excess protein was removed
by perfusion of thecis chamber with20volumes of asolution toprevent further
incorporation. Currents were recorded under voltage-clamp using a Bilayer
Clamp BC-525B amplifier (Warner Instrument Corp.). The currents were
measured with respect to the trans side of the membrane (ground). The
currents were low-pass, filtered at 1 kHz and digitized online using a Digidata
1200 interface board and pCLAMP 6 software (Axon Instruments, Inc.). Sigma
Plot 6.0 scientific software (Jandel Scientific) was used for curve fitting. All
experiments were performed at room temperature.
Please see Supplemental Information for the following experimental proce-
dures: Protein Purification, Immunoprecipitation, DSE2 antibodies, Immunos-
taining, Ca2+and ADP Accumulation by Mitochondria, and Immunoblotting.
Supplemental Information includes four figures and Supplemental Experi-
mental Procedures and can be found with this article online at doi:10.1016/j.
William Craigen (Baylor College of Medicine) for VDAC1 knockout mice, and
Larry Hayward (UMass Medical School) for wild type and mutant SOD1 bacu-
lovirus stock. This work has been supported by a grant from the NIH (R37
NS27036). A.I. has been supported by EMBO Long-Term Fellowship and by
a postdoctoral fellowship from IsrALS. D.W.C. receives salary support from
the Ludwig Institute for Cancer Research.
Accepted: July 22, 2010
Published: August 25, 2010
Mutant SOD1 Directly Inhibits VDAC1 Conductance
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