Expert consensus document: Mitochondrial function as a therapeutic target in heart failure

TLDR: In this article, insights into the mechanisms of mitochondrial dysfunction in heart failure are presented, along with an overview of emerging treatments with the potential to improve the function of the failing heart by targeting mitochondria.

Abstract: Heart failure is a pressing worldwide public-health problem with millions of patients having worsening heart failure. Despite all the available therapies, the condition carries a very poor prognosis. Existing therapies provide symptomatic and clinical benefit, but do not fully address molecular abnormalities that occur in cardiomyocytes. This shortcoming is particularly important given that most patients with heart failure have viable dysfunctional myocardium, in which an improvement or normalization of function might be possible. Although the pathophysiology of heart failure is complex, mitochondrial dysfunction seems to be an important target for therapy to improve cardiac function directly. Mitochondrial abnormalities include impaired mitochondrial electron transport chain activity, increased formation of reactive oxygen species, shifted metabolic substrate utilization, aberrant mitochondrial dynamics, and altered ion homeostasis. In this Consensus Statement, insights into the mechanisms of mitochondrial dysfunction in heart failure are presented, along with an overview of emerging treatments with the potential to improve the function of the failing heart by targeting mitochondria.

Full text

University of Groningen
Mitochondrial function as a therapeutic target in heart failure
Brown, David A.; Perry, Justin B.; Allen, Mitchell E.; Sabbah, Hani N.; Stauffer, Brian L.;
Shaikh, Saame Raza; Cleland, John G. F.; Colucci, Wilson S.; Butler, Javed; Voors, Adriaan
A.
Published in:
Nature reviews cardiology
DOI:
10.1038/nrcardio.2016.203
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
2017
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Brown, D. A., Perry, J. B., Allen, M. E., Sabbah, H. N., Stauffer, B. L., Shaikh, S. R., Cleland, J. G. F.,
Colucci, W. S., Butler, J., Voors, A. A., Anker, S. D., Pitt, B., Pieske, B., Filippatos, G., Greene, S. J., &
Gheorghiade, M. (2017). Mitochondrial function as a therapeutic target in heart failure.
Nature reviews
cardiology
,
14
(4), 238-250. https://doi.org/10.1038/nrcardio.2016.203
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the
author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.
More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-
amendment.
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the
number of authors shown on this cover page is limited to 10 maximum.

Heart failure (HF) is associated with substantial clin-
ical burden and economic costs worldwide. The dis-
ease is particularly prevalent in elderly individuals, in
whom the incidence and associated costs are projected
to double over the next 20years
1,2
. Economic costs
associated with the management of patients with HF
is estimated at >US$30billion annually in the USA
alone, and accounts for roughly 2–3% of total health-
care spending globally
3,4
. Despite these enormous costs,
mortality from HF remains high. Death from HF within
5years of diagnosis is common despite current opti-
mal medical therapy. Mortality and rehospitalization
within 60–90days after discharge from hospital can
be as high as 15% and 35%, respectively
5
. These event
rates have largely not changed over the past 15years,
despite implementation of evidence-based therapy
5
.
HF rehospitalization rates also remain high, with care
typically focused on symptomatic relief. Patients with
HF are often designated as having either reduced ejec-
tion fraction (HFrEF), or preserved ejection fraction
(HFpEF). Patients with HFpEF also have poor prognosis
after the first diagnosis
6
. Regardless of the HF aetiology,
novel treatments that improve intrinsic cardiac function
remain elusive.
Advances in the treatment of ischaemic and valvu-
lar heart disease have clearly improved patient sur-
vival. The residual cardiac dysfunction and associated
comorbid ities, however, have led, in the long-term, to
the development of HF with attendant poor quality of
life. Commonly prescribed HF medications, although
bene ficial in promoting some symptom relief, often do
not fully address the underlying causes of progressive
left ventricu lar dysfunction
7
. Most standard-of-care
pharmaco logical approaches to HF act by reducing
workload on the failing heart and, in doing so, attempt to
rebalance energy supply and energy demand, albeit to a
lower level (FIG.1). Hallmarks of current therapies include
modulation of neuro hormonal abnormalities, unloading
the heart (thatis, vasodilatation), and/or reducing the
heart rate — all important determinants of reducing
myo cardial oxygen consumption
8
. β-Blockers, ivabradine,
and antago nism of the renin–angiotensin–aldosterone
system all act in concert to reduce myocardial energy
requirements and attenuate or prevent further adverse
cardiac remodel ling. Although these therapies have
improved survival in patients with chronic ambulatory
HFrEF over the past 2–3decades, death and poor qual-
ity of life continue to adversely affect this ever-increasing
Correspondence to M.G.
Center for Cardiovascular
Innovation, Northwestern
University Feinberg School of
Medicine, 201 East Huron,
Galter 3–150, Chicago,
Illinois60611, USA.
mgheorgh@nm.org
doi:10.1038/nrcardio.2016.203
Published online 22 Dec 2016
EXPERT CONSENSUS DOCUMENT
Mitochondrial function as a
therapeutic target in heart failure
David A.Brown
1
, Justin B.Perry
1
, Mitchell E.Allen
1
, Hani N.Sabbah
2
, Brian L.Stauffer
3
,
Saame Raza Shaikh
4
, John G.F.Cleland
5
, Wilson S.Colucci
6
, Javed Butler
7
,
Adriaan A.Voors
8
, Stefan D.Anker
9
, Bertram Pitt
10
, Burkert Pieske
11
,
Gerasimos Filippatos
12
, Stephen J.Greene
13
and Mihai Gheorghiade
14
Abstract
|
Heart failure is a pressing worldwide public-health problem with millions of patients
having worsening heart failure. Despite all the available therapies, the condition carries a very
poor prognosis. Existing therapies provide symptomatic and clinical benefit, but do not fully
address molecular abnormalities that occur in cardiomyocytes. This shortcoming is particularly
important given that most patients with heart failure have viable dysfunctional myocardium,
inwhich an improvement or normalization of function might be possible. Although the
pathophysiology of heart failure is complex, mitochondrial dysfunction seems to be an important
target for therapy to improve cardiac function directly. Mitochondrial abnormalities include
impaired mitochondrial electron transport chain activity, increased formation of reactive oxygen
species, shifted metabolic substrate utilization, aberrant mitochondrial dynamics, and altered ion
homeostasis. In this Consensus Statement, insights into the mechanisms of mitochondrial
dysfunction in heart failure are presented, along with an overview of emerging treatments
withthe potential to improve the function of the failing heart by targeting mitochondria.
238
|
APRIL 2017
|
VOLUME 14 www.nature.com/nrcardio
CONSENSUS
STATEMENT
©
2
0
1
7
M
a
c
m
i
l
l
a
n
P
u
b
l
i
s
h
e
r
s
L
i
m
i
t
e
d
,
p
a
r
t
o
f
S
p
r
i
n
g
e
r
N
a
t
u
r
e
.
A
l
l
r
i
g
h
t
s
r
e
s
e
r
v
e
d
.
©
2
0
1
7
M
a
c
m
i
l
l
a
n
P
u
b
l
i
s
h
e
r
s
L
i
m
i
t
e
d
,
p
a
r
t
o
f
S
p
r
i
n
g
e
r
N
a
t
u
r
e
.
A
l
l
r
i
g
h
t
s
r
e
s
e
r
v
e
d
.

patientpopulation. This unmet need is probably not
going to be met by drugs that modulate neurohormonal
abnormalities and lower heart rates, because further inter-
vention along these axes is likely to be counterproductive
as hypotension and bradycardia become limiting factors.
The search for more effective and complementary therapy
for this patient population must be focused on improv-
ing the intrinsic function of the viable, but dysfunctional,
cardiac unit — the cardiomyocytes
3,9
. The novel therapy
must be haemodynamically neutral (no decrease in blood
pressure or heart rate) and must target the myocardium as
the centrepiece of the therapeutic intervention
10
.
The vast majority of phaseIII trials in patients with
HF conducted in the past decade have been negative,
arguably for the same reasons discussed above
11,12
.
Furthermore, a relative underinvestment in cardiovascu-
lar drug development, as well as strategic abandonment
by pharmaceutical companies of new therapies for which
the risks are perceived to be higher than the rewards, have
also contributed to slow development of drugs for HF
13
.
Moreover, the development of effective therapies for
HFpEF is imperative to treat this patient population, but
the variability in HFpEF phenotypes (such as age, andthe
presence of diabetes mellitus or hypertension),and
thedifficulty in establishing reliable preclinical models
of HFpEF, also hinder progress. Despite these obstacles,
ample opportunity exists to improve HF treatments,
prov ided the focus is directed towards cardiomyocytes
and their intrinsic function.
A roundtable meeting was held in Stresa, Italy on
23October2015 to discuss the multifaceted problem
of insufficient energy production in HF, and the role
it has in progressive left ventricular dysfunction. This
meeting was attended by academics, clinicians, and
representatives from the pharmaceutical industry. The
meeting focused on mitochondrial dysfunction as the
source of energy deprivation in HF, and how correction
of mitochondrial dysfunction using emerging novel
therapies might lead to functional improvement of the
HF phenotype. This Consensus Statement summarizes
the findings from that roundtable discussion.
Bioenergetics of the beating heart
Aristotle considered the heart to be the body’s fur-
nace, radiating energy in the form of heat
14
. Given the
astounding energetic cost of cardiac function, this con-
cept is not so far from the truth. Humans produce and
consume roughly their body weight in ATP (about 65 kg)
every single day
15
. The heart accounts for only ~0.5% of
body weight, but is responsible for roughly 8% of ATP
consumption. This high energy flux is dynamic: the
heart stores only enough energy to support pumping
for a few heart beats, turning over the entire metabo-
lite pool approximately every 10 s even at resting heart
rates
16
. As the most metabolically active organ in the
body, the heart possesses the highest content of mito-
chondria of any tissue. Mitochondria comprise 25–30%
of cell volume across mammalian species
17,18
, with only
the myofilaments being more densely packed within
cardiac myocytes. The high mitochondrial content of
cardiomyocytes is needed to meet the enormous energy
requirement for contraction and relaxation (whichis
also an active process). About 90% of cellular ATP
isutil ized to support the contraction–relaxation cycle
within the myocardium
19
. ATP-dependent release of
actin from myosin is required for both contraction
(asmyosin heads cycle through cross-bridges with actin)
and relaxation. Cellular sequestration of calcium back
into the sarcoplasmic reticulum during diastole also
requires a tremendous amount of ATP. Cells sustain
the energy requirements necessary to support cardiac
function through remarkable metabolic supply–demand
matching
20,21
(FIG.1). Bioenergetic homeostasis is accom-
plished almost exclusively through an ‘energy grid’ com-
prised of a mitochondrial network and their associated
phosphate- transfer couples. Cardiac mitochondria
must operate at high efficiency levels to respond instan-
taneously to the energetic needs of contractile units,
ademand that is ever-changing and necessitated by the
body’s dynamic requirements for oxygen-bearing blood.
Myocardial energy requirements are more pro-
nounced during physical activity, when demands for
energy increase to maintain cardiac function commensur-
ate with the needs of the body. However, other mitochon-
drial abnormalities besides energy deprivation during
physical activity can contribute to the pathologies seen
in patients with HF. Mitochondrial abnormalities in HF
are not only a question of reduced capacity to generate
ATP (even though that capacity is reduced at rest in HF
compared with resting normal), but can also be directly
Author addresses
1
Department of Human Nutrition, Foods, and Exercise, Virginia Tech, 1035 Integrated
Life Sciences Building, 1981 Kraft Drive, Blacksburg, Virginia 24060, USA.
2
Division of Cardiovascular Medicine, Department of Medicine, Henry Ford Hospital,
2799 West Grand Boulevard, Detroit, Michigan 48202, USA.
3
Division of Cardiology, Department of Medicine, University of Colorado Denver,
12700East 19th Avenue, B139, Aurora, Colorado 80045, USA.
4
Department of Biochemistry and Molecular Biology, East Carolina Diabetes and Obesity
Institute, Brody School of Medicine, East Carolina University, 115 Heart Drive, Greenville,
North Carolina 27834, USA.
5
National Heart & Lung Institute, National Institute of Health Research Cardiovascular
Biomedical Research Unit, Royal Brompton & Harefield Hospitals, Imperial College,
London, UK.
6
Cardiovascular Medicine Section, Boston University School of Medicine and Boston
Medical Center, 88 East Newton Street, C-8, Boston, Massachusetts 02118, USA.
7
Division of Cardiology, Health Sciences Center, T-16 Room 080, SUNY at Stony Brook,
New York 11794, USA.
8
University of Groningen, Department of Cardiology, University Medical Center
Groningen, Groningen 9713 GZ, Netherlands.
9
Department of Innovative Clinical Trials, University Medical Centre Göttingen (UMG),
Robert-Koch-Straße, D-37075, Göttingen, Germany.
10
University of Michigan School of Medicine, 1500 East Medical Center Drive, Ann Arbor,
Michigan 48109, USA.
11
Department of Cardiology, Charité University Medicine, Campus Virchow Klinikum,
and German Heart Center Berlin, Augustenburger Platz1, 13353 Berlin, Germany.
12
National and Kopodistrian University of Athens, School of Medicine, Heart Failure Unit,
Department of Cardiology, Athens University Hospital Attikon, Rimini1, Athens 12462,
Greece.
13
Division of Cardiology, Duke University Medical Center, 2301 Erwin Road Suite 7400,
Durham, North Carolina 27705, USA.
14
Center for Cardiovascular Innovation, Northwestern University Feinberg School of
Medicine, 201 East Huron, Galter 3–150, Chicago, Illinois60611, USA.
CONSENSUS STATEMENT
NATURE REVIEWS
|
CARDIOLOGY VOLUME 14
|
APRIL 2017
|
239
©
2
0
1
7
M
a
c
m
i
l
l
a
n
P
u
b
l
i
s
h
e
r
s
L
i
m
i
t
e
d
,
p
a
r
t
o
f
S
p
r
i
n
g
e
r
N
a
t
u
r
e
.
A
l
l
r
i
g
h
t
s
r
e
s
e
r
v
e
d
.
©
2
0
1
7
M
a
c
m
i
l
l
a
n
P
u
b
l
i
s
h
e
r
s
L
i
m
i
t
e
d
,
p
a
r
t
o
f
S
p
r
i
n
g
e
r
N
a
t
u
r
e
.
A
l
l
r
i
g
h
t
s
r
e
s
e
r
v
e
d
.

linked to cardiomyocyte injury and death and, therefore,
to disease progression. Abnormal mitochondria are a
major source of reactive oxygen species (ROS) produc-
tion, which can induce cellular damage. Abnormal mito-
chondria canpromote programmed cell death through
the release of cytochromec into the cytosolic compart-
ment and activation of caspases. Therefore, mitochon-
dria directly influence ongoing cell injury and death.
Mitochondrial abnormalities have also been implicated
in aberrant cellular calcium homeostasis, vascular smooth
muscle pathology, myofibrillar disruption, and altered
cell differentiation, all important issues in cardiovascular
disease, including HF.
Mitochondria in cardiomyocytes
Mitochondria are primarily located within subsarco-
lemmal, perinuclear, and intrafibrillar regions of the
cardio myocyte. Although they are symbiotic partners
with the other cellular compartments, mitochondria are
in many ways discrete entities. Mitochondrial dynamics
in the form of fission, fusion, and autophagy are highly
regulated processes that are essential for energy produc-
tion and structural integrity of the organelles
22–29
. Altered
mitochondrial biogenesis, fragmentation, and hyper-
plasia have been observed in studies of human
30
and
animal models
31,32
of HF. These effects seem to be caused
by altered expression of proteins that regulate mitochon-
drial dynamics
33
. As many of these factors are ‘master
regulators’ of mitochondrial metabolism, these changes
might be directly related to the decreased capacity to
oxidize fatty acid substrates often seen in HF
34,35
.
Mitochondria have their own DNA (mtDNA) and a
genetic code that is distinct from the host-cell nuclear
DNA. mtDNA is circular in shape, analogous to DNA
found in lower organisms, and a primitive fingerprint
leftover from bacterial origin. Evolutionary selection
pressures have led to mitochondria ‘outsourcing’ almost
all their protein-making needs to their cellular hosts. The
overwhelming majority (>99%) of mitochondrial pro-
teins come from nuclear-encoded DNA. Theseproteins
are synthesized via cellular protein synthesis machinery,
and are actively imported into mitochondria through
mitochondrial membrane transporters
36
. mtDNA
encodes 13 protein subunits found within threeof the
electron transport protein complexes, and a handful
of ribosomal and transfer RNAs
37
. These proteins are
Figure 1
|
Energy supply–demand matching in health and heart failure. The
delicate balance between cardiac demands for energy and supply of energy
is tipped in heart failure, in which energy supply cannot match demand.
Next-generation therapeutics can improve on existing standard-of-care
therapies by bolstering mitochondrial energy production. ACE, angioten-
sin-converting enzyme; ARB, angiotensinII-receptor blocker; ETC, electron
transport chain; HFpEF, heart failure with preserved ejection fraction; HFrEF,
heart failure with reduced ejection fraction; ROS, reactive oxygen species.
Energy (ATP) demand ↓ Energy (ATP) supply
Energy (ATP) demand
Energy (ATP) supply
Preserved cardiac function ↑ Energy supply
Restored mitochondrial
function
↑ Cardiolipin
↑ Redox-buffering capacity
↑ ETC (super) complexes
↑ Biogenesis
↓ Mitochondrial capacity
↑ ROS
↓ ETC (super) complexes
↓ Cardiolipin
Altered fission/fusion
Abnormal Ca
2+
/Na
+
/Fe
2+
↓ Membrane fluidity
↓ Redox buffering
Mitochondrial dysfunction
Mitochondrial dysfunction
↓ Mitochondrial capacity
↑ ROS
↓ ETC (super) complexes
Altered fission/fusion
Abnormal Ca
2+
/Na
+
/Fe
2+
↓ Membrane fluidity
↓ Redox buffering
HFpEF
Diastolic failure
HFrEF
Systolic failure
Glycolysis Mitochondria
(oxidative phosphorylation)
ContractionRelaxation
Synthesis, transport,
phosphorylation, pumps
Heart rate
Contributors to energy supplyContributors to energy demand
Health Heart failure
Current treatment paradigms Next-generation heart failure therapeutics
↓ Energy supply
(no reserve,
exercise intolerance)
↓ Energy demand
β-Blockers
ACE/ARB
Hydralazine/
nitrates
Ivabradine
CONSENSUS STATEMENT
240
|
APRIL 2017
|
VOLUME 14 www.nature.com/nrcardio
©
2
0
1
7
M
a
c
m
i
l
l
a
n
P
u
b
l
i
s
h
e
r
s
L
i
m
i
t
e
d
,
p
a
r
t
o
f
S
p
r
i
n
g
e
r
N
a
t
u
r
e
.
A
l
l
r
i
g
h
t
s
r
e
s
e
r
v
e
d
.
©
2
0
1
7
M
a
c
m
i
l
l
a
n
P
u
b
l
i
s
h
e
r
s
L
i
m
i
t
e
d
,
p
a
r
t
o
f
S
p
r
i
n
g
e
r
N
a
t
u
r
e
.
A
l
l
r
i
g
h
t
s
r
e
s
e
r
v
e
d
.

made in specialized ribosomes or ‘mitoribosomes’,
which are physically attached to the mitochondrial
innermembrane
38
.
Many inherited familial cardiomyopathies (both
adult and paediatric) are associated with mtDNA
mutations
39
. In humans, mitochondria are maternally
inherited
40
, owing to high mitochondrial density in the
egg and the active degradation of mitochondria in the
sperm during fertilization
41
. The proximity of mtDNA
to sites of mitochondrial ROS generation, poor repair
mechanisms, and a lack of protective histones combine
to make mtDNA particularly susceptible to oxidative
injury andmutation.
Mitochondrial genetics contribute to cardiomyo-
pathies by expressing mutant proteins that influence
energy homeostasis. With 1,000–10,000 genes per mito-
chondria (polyploidy), mitochondrial genetics operate
on population-based (instead of Mendelian) princi-
ples
37
. Mutated mtDNA is found alongside nonmutated
copies, leading to mitochondrial ‘heteroplasmy’. The
extent of heteroplasmy in mutated mtDNA influences
the susceptibility to inherited mitochondrial disease
42
.
Mutated mtDNA can be found in 1in200 individuals,
a frequency that is 20-fold higher than the incidence
of mitochondrial disease. This mismatch indicates that
healthy individ uals often harbour mutated mtDNA
that has no observable phenotypic consequences until
a certain mutation threshold is reached
37
. Although
very early in preclinical development, various innova-
tive approaches to reduce the extent of heteroplasmy
using genome editing might ultimately lead to effective
therapy for HF caused by genetic mitochondrial dis-
ease
43–45
. Given that mitochondrial abnormalities, such
as increased ROS production, altered mitochondrial
energetics, and impaired mitochondrial ion homeo-
stasis, are observed in genetic mitochondrial diseases
as well as HF, innovative approaches that target mito-
chondrial dysfunction might share efficacy across
thesediseases.
Heart failure is a bioenergetic disease
The ‘myocardial power grid’ consists of mitochondrial
ATP supply that transfers energy throughout the cell
along intracellular phosphotransfer buffering systems
(FIG.2). Mitochondria utilize carbon sources from food
substrates, which are catabolized and passed through the
Krebs cycle and are then channelled through a series of
redox reactions along the inner mitochondrial mem-
brane. The oxidation of these substrates creates a pro-
ton electrochemical gradient, predominantly in the form
of mitochondrial membrane potential (ΔΨ
m
)
46
. Protons
that re-enter the mitochondrial matrix through com-
plexV (mitochondrial ATP synthase) liberate energy
that phosphorylates ADP, regenerating ATP. Newly
synthesized ATP is rapidly transferred out of mitochon-
dria and energy is subsequently distributed throughout
the cell via reversible phosphate exchange networks,
primarily catalysed by creatine kinase and adenylate
kinase-associated reactions
16,47
.
The evidence that HF involves impaired cellular
energy production and transfer is considerable (TABLE1).
Among studies that have directly examined energetics
in human HF, all but three noted some form of bioener-
getic impairment in the failing heart. This decrement in
bioenergetics is reflected by a decrease in cellular ATP,
phosphocreatine (PCr), or the PCr/ATP ratio. Impaired
bioenergetics affect patients with HFrEF and those with
HFpEF (TABLE1).
Although it is difficult to tell from the heterogeneous
patient population included in TABLE1, the progression
to HF is likely to be associated with a gradual decline in
bioenergetic reserve capacity that ultimately reaches a
critical threshold, after which endogenous mechanisms
can no longer compensate for faltering energy supply
48
.
Attempts to improve bioenergetics in HF tend to focus
on mitochondrial energy production as a target, because
direct augmentation of myocardial creatine with oral
creatine supplementation is thwarted by a decreased
capacity to transport creatine into the failing cardio-
myocytes
49
. Skeletal muscles also show mitochondrial
dysfunction in HF, contributing to the exercise intoler-
ance that characterizes the HF state
50
. Abnormal mito-
chondrial function has also been reported in patients
with renal insufficiency
51
, and in patients with insulin
resistance
52
. Given that patients with HF often manifest
both renal insufficiency and insulin resistance, treating
Figure 2
|
Impaired mitochondrial capacity and function in heart failure. Decreased capacity of mitochondria to
generate and transfer energy within heart cells results in energy deficits, which influences all cellular processes that
require energy, most notably the processes of contraction and relaxation.
Mitochondria
↓ Contraction
↓ Relaxation
↓ Mitochondrial capacity
Contraction
Relaxation
Myofibril
ATP
ATP
Myocyte
Healthy
Heart
failure
Myofibre
CONSENSUS STATEMENT
NATURE REVIEWS
|
CARDIOLOGY VOLUME 14
|
APRIL 2017
|
241
©
2
0
1
7
M
a
c
m
i
l
l
a
n
P
u
b
l
i
s
h
e
r
s
L
i
m
i
t
e
d
,
p
a
r
t
o
f
S
p
r
i
n
g
e
r
N
a
t
u
r
e
.
A
l
l
r
i
g
h
t
s
r
e
s
e
r
v
e
d
.
©
2
0
1
7
M
a
c
m
i
l
l
a
n
P
u
b
l
i
s
h
e
r
s
L
i
m
i
t
e
d
,
p
a
r
t
o
f
S
p
r
i
n
g
e
r
N
a
t
u
r
e
.
A
l
l
r
i
g
h
t
s
r
e
s
e
r
v
e
d
.