DR MICHAEL MURPHY
Dysfunction drives disease
Dr Michael Murphy is taking a novel approach to treating the most burdensome non-communicable
diseases of modern society. Here, he discusses how and why his passion for this important work developed
Could you outline how you became involved
in mitochondrial pharmacology?
The focus of my undergraduate studies in
Dublin, Ireland, was chemistry, but I always
saw this as a path to research in biochemistry.
I was unsure of the area in which I wanted to
focus my PhD, so I visited the University of
Cambridge, UK, to meet potential supervisors.
I thought the work of Professor Martin
Brand was the most interesting so, under his
supervision, I concentrated my attention on
basic mitochondrial bioenergetics and have
continued to work on mitochondria ever since.
When I was an academic at the University
of Otago, New Zealand, in the 1990s it
was becoming obvious that mitochondrial
dysfunction was at the heart of a range
of human diseases. This included primary
mitochondrial diseases – those due to the
mutations in genes encoding mitochondrial
proteins or RNA molecules – and, more
importantly, secondary mitochondrial diseases.
These include most of the important disorders
that concern society today, such as diabetes,
neurodegenerative diseases, metabolic
syndrome and inflammatory disorders. This
realisation led me to collaborate with a
colleague in the Department of Chemistry at
the University of Otago, Professor Robin Smith,
to develop methods for targeting therapeutic
molecules to mitochondria.
What are your primary research objectives?
My primary objectives are to understand how
mitochondrial dysfunction contributes to
human diseases and to develop new therapies.
These involve pioneering new methods to
assess mitochondrial dysfunction in both
animal models of human diseases and patients.
This is an important parallel strand to our work
that supports the mitochondrial pharmacology
research; the development of bioactive
mitochondria-targeted molecules as therapies.
Why is it so important to study mitochondria?
What happens when they dysfunction?
Mitochondria are essential for energy
metabolism; breaking down the food we eat
and using the oxygen we breathe to provide
energy for the cell. More recently, it has become
clear that mitochondria are also central to
many other aspects of cell function and fate
determination, including apoptosis, necrosis,
calcium homeostasis and inflammation.
Consequently, if anything goes wrong with
mitochondria, the cells have trouble functioning
correctly, leading to disease.
Mitochondrial dysfunction contributes to many
different pathologies. The interesting aspect of
these secondary diseases is that, even though
20
INTERNATIONAL INNOVATION
DR MICHAEL MURPHY
the primary damage is not mitochondrial, the
progression of the pathology requires their
participation. Furthermore, there are many
common aspects to mitochondrial dysfunction,
notably reactive oxygen species (ROS).
Importantly, this means that mitochondria can
be targeted to treat a wide range of diseases.
What are the principal challenges that
need to be overcome for mitochondrial
pharmacology to achieve its full potential?
In many ways the major scientific challenge
has been addressed. We now have methods
to routinely and robustly deliver bioactives
and pharmacophores to mitochondria in
patients to treat both acute disorders, such
as ischaemia reperfusion injury, and also
chronic disorders such as the metabolic
syndrome. Of course, there is huge potential
to develop better drugs but the principle that
mitochondria are a viable drug target is now
established. The pharmaceutical industry has
previously regarded mitochondrial diseases
as a relatively niche market, but the growing
realisation that their dysfunction contributes
to so many secondary mitochondrial
disorders, and that this class of diseases
includes many of the economically and
socially important afflictions that we face,
is gradually taking hold. What is needed
now is more investment in clinical trials for
mitochondria-targeted drugs and for a therapy
to be licensed.
Looking forward, what are your hopes
for the emerging field of mitochondrial
pharmacology?
My hope is that there is continued realisation
in the medical research community that
many of the most challenging disorders that
industrialised countries face – metabolic
syndrome, neurodegenerative diseases, heart
attack, stroke and inflammatory disorders –
are secondary mitochondrial diseases. The
pharmaceutical industry should then be
motivated to invest more in mitochondrial
pharmacology and target processes and
cell compartments that they have hitherto
ignored. In 10-15 years, I fully anticipate a
range of mitochondrial therapies to target
diseases that have not traditionally been
thought of as mitochondrial.
Pharmaceutical
powerhouses
Researchers at the MRC Mitochondrial Biology Unit, Cambridge, are
developing novel drugs to prevent the mitochondrial oxidative damage
implicated in the pathology of a wide range of diseases
MITOCHONDRIA ARE THE powerhouses
of cells that provide the energy we need to
live. However, these organelles are involved
in many other metabolic processes, as well as
inflammation and cell death, which means
that the health of the whole cell is threatened
when mitochondria fail to function properly.
Mutations in genes that control this organelle’s
activity lead to primary mitochondrial
dysfunction, which typically results in a range
of diseases associated with poor growth
and development. Secondary mitochondrial
dysfunction is more complex; the primary
condition is not caused by mitochondria, but
their function is disrupted by the progression
of the disease, leading to exacerbated or
additional symptoms. There are a wide range
of prevalent and important diseases associated
with secondary mitochondrial dysfunction,
including diabetes, inflammatory disorders,
neurodegeneration, stroke and heart disease.
Mitochondrial diseases typically share three
common traits: disrupted energy production,
oxidative damage by reactive oxygen species
(ROS) and deregulated calcium homeostasis.
During respiration, sugars and fats are broken
down and used to produce a form of energy
the cell can use to do work, with oxygen
being reduced to water as a by-product. If this
reduction is incomplete or premature, ROS
are formed, which can detrimentally oxidise
important cellular components, such as
proteins, lipids and DNA.
PREVENTING OXIDATIVE DAMAGE
Mitochondria are a major source of ROS in the
cell, and yet they are particularly vulnerable to
their damaging effects, being densely packed
with fragile proteins and mitochondrial DNA.
Oxidative damage leads to mitochondrial
dysfunction and subsequent cell death, and
is a major cause of pathology in both primary
and secondary mitochondrial diseases. Its
prevention is therefore an appealing potential
drug target for the mitigation of a wide range
of diseases.
Disappointingly, extensive trials of many
antioxidants, such as vitamins E and C, have
not been successful in preventing mitochondrial
oxidative damage – potentially because the
antioxidants are diluted throughout the body
and cannot build up in the mitochondria where
they are most needed. Dr Michael Murphy,
a group leader investigating mitochondrial
dysfunction at the Medical Research Council
Mitochondrial Biology Unit in Cambridge, UK,
is developing a novel solution to this problem:
WWW.RESEARCHMEDIA.EU
21
DR MICHAEL MURPHY
bioactive mitochondria-targeted compounds
as therapeutics to reduce or eliminate oxidative
damage in these organelles.
MITOCHONDRIA-TARGETED
ANTIOXIDANT
Murphy and colleagues from the University
of Otago, New Zealand, have developed
MitoQ, a mitochondria-targeted antioxidant.
It consists of ubiquinone – the functional
group of mitochondrial CoenzymeQ10 – linked
to triphenylphosphonium (TPP), a positive
ion known to be preferentially taken up by
mitochondria because they are the most
negatively charged organelles in the cell. This
allows the antioxidant to be directly targeted
to the mitochondria and accumulate at
concentrations several thousand times higher
than in the extracellular environment, increasing
its efficacy while reducing any potential
extracellular inactivation or toxic side-effects.
MitoQ is particularly attractive as an
antioxidant because it is continually recycled;
the ubiquinone moiety is rapidly reduced
by the enzyme succinate dehydrogenase of
the respiratory chain to ubiquinol, the active
antioxidant form. When ubiquinol reduces ROS
to more stable molecules, such as oxygen and
water, it becomes re-oxidised to the ubiquinone
form, which may itself react with superoxide, an
important ROS.
While MitoQ adheres to the mitochondrial
inner membrane, the 10 carbon aliphatic
chain linking TPP to ubiquinone enables the
antioxidant to penetrate deeply into the core
of the organelle, preventing lipid peroxidation
that would damage its function. “The length
of the carbon chain turns out to be critical,”
expounds Murphy. “Work in my lab by Andrew
James showed that a length of 10 carbon atoms
enabled the ubiquinone moiety to reach the
active site of succinate dehydrogenase so that it
could be reduced to the active ubiquinol.”
A CURE FOR MANY DISEASES
Murphy used mouse models to establish the
effectiveness of MitoQ in vivo, showing that
substantial quantities can be administered, both
intravenously and orally, without any adverse
toxic effects. The compound is rapidly taken
up by tissues with large energy requirements
that typically have a high concentration of
mitochondria, specifically the heart, liver,
kidney, muscle and, to a lesser extent, the brain.
Furthermore, long term administration leads
to a substantial accumulation of MitoQ in the
liver and heart without any changes in physical
activity, mass or hormone balance.
The protective effects of MitoQ were first seen
using a murine heart attack model. Ischaemia
reperfusion (IR) injury involves a blockage
to the movement of blood to the heart and
subsequent re-establishment of flow. This leads
to a burst of ROS that can cause significant
secondary mitochondrial dysfunction in the
heart. MitoQ protected the mouse hearts
against tissue damage and mitochondrial
dysfunction compared with either a methyl-TPP
or short-chain ubiquinol control, showing that
the complete MitoQ structure is vital for its
beneficial effects.
Studies in mice have since demonstrated the
protective role of MitoQ in a wide range of
disease models, including Alzheimer’s disease,
hypertension, sepsis and multiple sclerosis, as
well as reduction of toxicity from alcohol and
cocaine abuse. Murphy explains why MitoQ
is effective against so many pathologies:
“These are secondary mitochondrial diseases
and consequently their mechanisms overlap.
In particular, there is likely to be a large
component of inflammation and, therefore,
the wide applicability of mitochondria-targeted
antioxidants is due to their impact on pathways
common to a range of pathologies”.
SAFETY
The combination of rapid mitochondrial
uptake and antioxidant recycling makes
MitoQ
a
promising
antioxidant
for
mitochondrial diseases in humans. Antipodean
Pharmaceuticals developed MitoQ into a drug
for oral consumption, which has now passed
toxicity tests and been approved for human
clinical trials.
Since then it has been trialled in a double-blind
study comparing two dosage levels with a
placebo given to patients newly diagnosed with
Parkinson’s disease. The symptoms of this have
been shown to be worsened by mitochondrial
oxidative stress. Despite the fact that the results
failed to show significant therapeutic results, this
study showed that long-term administration of
MitoQ was safe in humans, as in mice.
PROMISING CLINICAL RESULTS
Hepatitis C is a virus that infects the liver,
causing increased oxidative stress and
mitochondrial damage believed to play an
important role in subsequent liver injury.
Hepatitis C patients who were unresponsive to
conventional antiviral treatments were chosen
for a MitoQ trial to assess its ability to reduce
liver damage. For four weeks, the participants
were given either MitoQ or a placebo, after
which time those given the active drug showed
significant decreases in serum levels of alanine
transaminase, an enzyme indicative of liver
damage. There was no change in the amount of
virus in the patients, showing that MitoQ was
preventing liver damage by reducing oxidative
damage, rather than affecting the ability of the
virus to replicate.
This study was the first to demonstrate that
mitochondria-targeted
antioxidants
can
provide a clinical benefit to humans. Further
trials are being planned to test MitoQ’s efficacy
in metabolic disorders.
PREVENTING ROS PRODUCTION
Murphy and his colleagues are also working
on strategies to prevent the initial production
of ROS before they have the chance to
cause problems. The researchers began
by investigating the acute production of
mitochondrial ROS that causes IR injury in
heart attacks and found that the main source
was enzyme complex I – the entry point for
electrons into the respiratory chain. When
blood flow to the heart stopped, enzyme
complex I entered an inactive state. However,
when the blood returned to the mitochondria,
the complex was rapidly reactivated, leading to
incomplete reduction of oxygen and elevated
production of ROS.
Nitric oxide is known to protect heart tissue
from IR injury during heart attacks and this
was thought to act by S-nitrosation, a type of
protein modification, although it was unclear
exactly how. Murphy collaborated with
Professor Robin Smith from the University of
Otago, and with Drs Thomas Krieg and Edward
Chouchani at the University of Cambridge, to
develop MitoSNO – an S-nitrosating agent
bound to TPP – to examine its cardioprotective
mechanism. Using the IR injury mouse
model, Murphy and colleagues found that
administration of MitoSNO, just prior to
reperfusion, led to S-nitrosation of complex I of
the mitochondrial respiratory chain, preventing
oxidative damage and cell death in the heart
when blood flow was restored.
MitoSNO acts during the reperfusion of
ischaemic tissue to slow the reactivation of
complex I, and can therefore only protect
the heart when injected during this process.
A specific amino acid, cysteine 39 of the
The combination of rapid mitochondrial uptake and antioxidant recycling makes MitoQ a
promising antioxidant for mitochondrial diseases in humans
22
INTERNATIONAL INNOVATION
INTELLIGENCE
MITOCHONDRIA:
POWERHOUSE IN MEDICINE
ND3 complex I subunit, becomes exposed
by conditions of hypoxia during restricted
blood flow, and it is the S-nitrosation of this
residue that produces the reversible inhibition
of the complex by MitoSNO, according to
Murphy: “MitoSNO could lock complex I into
its inactive form for 5-10 minutes, allowing it
to slowly wake up without leading to a burst
of ROS”.
PROTECTING HEART TISSUE
Murphy is particularly enthusiastic about
MitoSNO’s potential use for limiting damage
in patients having a heart attack. “The
main advantage of using MitoSNO over
current approaches to treating IR injury is
that it acts at the proximal step to decrease
mitochondrial ROS,” he enthuses. While it
cannot stop the cause of the heart attack,
MitoSNO could be administered alongside
interventions designed to restart the blood
flow and would help to reduce the associated
oxidative cell damage and reduce the amount
of tissue necrosis after the event. The
treatment could also be used during coronary
artery repair. Murphy hopes that MitoSNO
will be approved for clinical use: “The
development of MitoSNO as a treatment for
cardiac IR injury is developing rapidly through
collaboration with Krieg, and is now moving
on to large animal studies in pigs. If they are
promising there is a clear path onto assessing
this compound in phase I/II studies in humans
within three to five years”.
THE OUTLOOK
In addition to the promising pharmaceutical
trials of MitoQ and MitoSNO, Murphy and
his collaborators are continuing to investigate
the basics of mitochondrial dysfunction to
explore new ways of preventing their associated
pathologies. The inner and outer membranes,
matrix and inter-membrane spaces of
mitochondria all perform different functions,
and the Cambridge researchers are working
with Dr Richard Hartley of the University of
Glasgow, UK, on targeting different drug types
to each individual area. This could yield a
diverse range of alternative pharmaceuticals for
mitochondria-associated diseases.
Mitochondria have proven to be a viable drug
target against a wide range of important diseases
and, as a consequence of Murphy’s work, are finally
being recognised as pharmacologically vital.
OBJECTIVES
To determine the mechanisms of mitochondrial
dysfunction in pathology and develop new
therapeutic strategies. These objectives will be
achieved by addressing three research questions:
• Is it possible to determine the mechanisms
of disruption to mitochondrial function in
pathology by targeting probes to measure
reactive species in vivo and by assessing
reversible redox modifications to proteins?
• Can pathological disruption to mitochondrial
function be prevented by targeting bioactive
molecules to mitochondria in vivo?
• Can in vivo assessment of mitochondrial
reactive species and redox changes in animal
models inform our understanding of specific
pathologies and drive the development of
rational, translatable therapies for patients?
KEY COLLABORATORS
Professor Robin Smith, Universtiy of Otago,
New Zealand • Dr Lorraine Work; Dr Richard
Hartley, University of Glasgow, UK • Dr
Kourosh Saeb-Parsy; Dr Thomas Krieg,
University of Cambridge, UK • Dr Edward
Chouchani, Medical Research Council
Mitochondrial Biology Unit (MRC-MBU), UK
FUNDING
Medical Research Council (MRC)
Biotechnology and Biological Sciences Research
Council (BBSRC)
CONTACT
Dr Michael Murphy
Group Leader
MRC Mitochondrial Biology Unit
Wellcome Trust / MRC Building
Hills Road
Cambridge
CB2 0XY
UK
T +44 1223 252 900
E [email protected]
MICHAEL MURPHY received his BA in
Chemistry at Trinity College, Dublin, Ireland,
in 1984 and his PhD in Biochemistry at the
University of Cambridge, UK, in 1987. After
stints in the USA, Zimbabwe and Ireland, he
took up a faculty position in the Biochemistry
Department at the University of Otago, New
Zealand, in 1992. In 2001, he moved to the MRC
Mitochondrial Biology Unit in Cambridge, UK,
(then called the MRC Dunn Human Nutrition
Unit) where he is a group leader.
WWW.RESEARCHMEDIA.EU
23