Download Of mice and molecules - Neuroscience Syracuse University

PROFESSOR SANDRA HEWETT
Of mice and molecules
With expertise in neuroscience and biology, Professor Sandra
Hewett discusses how her ambitious research is helping to unveil the
mechanisms that underlie cell death in the central nervous system and
explain the complex interplay between excitotoxicity and inflammation
Can you provide an insight into your
background and outline what sparked
your current research interests?
It was during my postdoctoral studies at the
Washington University School of Medicine
that I began studying the mechanisms that
underlie acute neuronal injury – particularly
regarding stroke, which is caused by cerebral
ischaemia, or the loss of blood flow to the
brain. Although the laboratory in which I
was based was predominantly focused on
understanding the mechanisms by which
neurons die in an autonomous fashion, I
was fascinated by the potential that stroke
could result in non-autonomous neuronal
cell death – namely, that other cells mediate
the injury. I therefore started to explore how
astrocytes respond to the injured environment
and whether these cells contribute to injury.
As for my current research interests, they are
an extension of this work. As a postdoctoral
researcher, I was very lucky to have a
wonderful mentor who allowed me to study
the potential for inflammatory astrocyte
signalling to contribute neuronal injury and
who encouraged me to continue this initial
work in my own laboratory.
What are the core aims and objectives
of your laboratory’s work?
The work in my laboratory focuses on
elucidating the molecular and biochemical
mechanisms by which inflammatory factors
upregulated during and following acute injury
– for example, in stroke, trauma and epilepsy –
can either promote or protect neurons (that is,
nerve cells). In particular, we are concentrating
on the interactions between neurons
and astrocytes.
How is the combined usage of in
vitro and in vivo experimentation
helping to facilitate your work?
As both in vitro and in vivo models of injury
are employed in my research, I am fond of
saying that we take a ‘molecules to mouse’
and ‘mouse to molecules’ approach to the
essential questions posed. Combining these
two approaches is a very powerful method.
Our cell culture models allow us to look at
the cellular interactions that follow injury
with specificity and detail, leading us to make
insights into potential therapeutic targets. In
turn, we are then able to test these potential
therapies in animals as part of an
applied research strategy. This
is appropriate since mice and
humans share many of the same
neurobiological properties.
In what ways does cyst(e)ine/
glutamate antiporter (system
xc) influence hypoglycaemic
neuronal cell death?
We found that the excitotoxic
neuronal injury that follows
glucose deprivation – aglycaemia
– is initiated by glutamate
extruded from astrocytes via
system xc-, an amino acid transporter that
imports L-cystine and exports L-glutamate.
Thus, the release of astrocyte glutamate
appears to be a primary contributing factor
to hypoglycaemic neuronal injury, at least in
our cell culture model. Our next steps are to
confirm this in an animal model.
Could you discuss your lab’s most
exciting findings to date?
Something that is truly unique and exciting
is our work on the bimodal actions of IL1β – intriguingly, it can either contribute
to or protect from neural injury via what
is essentially the same mechanism: the
upregulation of astrocyte system xc-. The
concept that IL-1β and system xc- are at the
crossroads of injury and protection is the
factor that is especially original and intriguing
about the work in our lab. Indeed, while our
published data predict the ability of IL-1β
enhanced cyst(e)ine/glutamate antiporter
activity to contribute to injury in cerebral
ischaemia and hypoglycaemic injury, we have
recently generated new, as-yet unpublished
results that indicate this activity could also
be potentially protective in direct models of
oxidative stress. Hence we posit that IL-1βmediated upregulation of astrocyte system
xc- represents a protective mechanism,
which under certain conditions, will ultimately
go awry. For these reasons, understanding
the regulation of system xc- by IL-1β at the
molecular level is of utmost importance, so
that we may use this information to devise
strategies that harness the beneficial effects
and, when appropriate, employ strategies that
reduce its activity in order to decrease the
probability of neuronal injury.
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Astrocyte-neuron interactions
Based in the Department of Biology at Syracuse University, USA, researchers at the Sandra Hewett
Lab are making inroads into understanding the molecular and biochemical mechanisms by which
upregulated inflammatory factors fuel the progression of acute neuronal injury
PRIMARILY THE RESULT of trauma, stroke
or epilepsy, acute brain injury represents
a prominent cause of death and disability
worldwide. Yet, despite its prevalence, the
complexity of the pathophysiological processes
involved in such injuries means that they are
not fully understood. For instance, in addition
to the damage inflicted on the brain at the
moment of injury, further damage is incurred
as a result of a series of cellular events that
occur minutes or even days afterward. The
outcomes of these dangerous secondary
processes are capable of inflicting as much (or
more) harm as the initial moment of trauma.
Importantly, if viable therapeutic targets for
acute brain injury are to be identified, it is
imperative to forge a detailed and robust
understanding of the cellular interactions that
occur following the initial injury, in order to
determine their contribution to subsequent
brain damage or their potential to participate
in cellular protection and/or repair.
In response, researchers at the Sandra
Hewett Lab in the Department of Biology
at Syracuse University are conducting
rigorous investigations into the molecular
and biomechanical mechanisms that drive
the progression of neuronal injury. Using a
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INTERNATIONAL INNOVATION
combination of in vitro and in vivo models,
Professor Sandra Hewett and her team are
making innovative insights into the upregulation
of inflammatory factors in the brain following
injury. Notably, their studies have pinpointed
an intriguing dichotomy: namely, the proinflammatory cytokine interleukin-1β (IL1β) appears to have a dual role in which
it both advances the pathophysiological
process under certain contexts, yet in other
paradigms could also facilitate repair. Moving
beyond the traditional view – which holds
that brain damage associated with acute
injury is mediated by the overstimulation of
inflammatory factors and excitatory amino
acid receptors – Hewett’s team has found
compelling evidence that demonstrates
inflammatory genes expressed in parenchymal
cells in the central nervous system may also
play a key role in protection and repair.
EXPLORING CEREBRAL ISCHAEMIA
In their studies, the researchers at the
Sandra Hewett Lab are focusing on the
processes that underpin cerebral ischaemia
and hypoglycaemia. Firstly, as a subtype of
stroke, cerebral ischaemia occurs when
the blood flow to the brain is interrupted
and consequently fails to meet metabolic
demand. This limits oxygen and nutrient supply
to the brain, in turn triggering a series of
biochemical reactions that result in the death
of brain cells, or cerebral infarction. Causes
of cerebral ischaemia can range from sickle
cell anaemia to congenital heart defects,
while symptoms can include impairments in
vision, speech and movement, paralysis and
unconsciousness. When tissue cell death
occurs – the symptoms become permanent.
Importantly, Hewett’s research on cerebral
ischaemia has demonstrated that the brain
damage arising from loss of blood flow
involving IL-1β occurs via signalling through
interleukin 1 receptor, type I (IL-1RI). The
team’s results additionally emphasise the
significance of astrocytes – namely, the starshaped glial cells found in abundance in the
brain and spinal cord – in the mechanisms that
underpin the pathology of cerebral ischaemia.
Indeed, their work on the regulation of the
cysteine-glutamate antiporter (system xc-)
via IL-1β has flagged up that IL-1β-potentiated
hypoxic neuronal injury is associated with the
upregulation of system xc- in astrocytes. “As
a result of this discovery, we have now begun
to work on understanding the molecular
mechanisms that occur in this cell type,”
Hewett’s team has found compelling evidence linking the inflammatory genes expressed in
parenchymal cells in the central nervous system to both injury and protection
Hewett outlines. “We have elucidated that IL-1β
works to increase transcription of the gene for
system xc- (increased xCT mRNA) and stabilises
xCT mRNA. Overall, the effect is to increase the
number of transporters on the cell’s surface.”
COMBATING HYPOGLYCAEMIA
In addition, the team is also exploring the
cellular and molecular mechanisms involved
in severe hypoglycaemia, the process
whereby brain glucose levels can reach zero.
Hypoglycaemia occurs during strokes but is
also an event most commonly connected to
diabetes – for instance, it occurs in diabetic
patients who may take too much insulin, who
might not eat enough or who exercise too
intensively. It is a serious medical emergency
that causes cognitive impairment, seizures,
unconsciousness, coma and neuronal cell
death. Additionally, it is known that individuals
who experience one or more episodes of
severe hypoglycaemia are at increased
risk of dementia. Hence, it is imperative to
understand the cell and molecular processes
initiated in the brain by hypoglycaemia.
In spite of solid evidence from previous in
vitro and in vivo models that hypoglycaemic
neuronal cell death is induced as a result of
glutamate excitotoxicity, the cellular source
from which glutamate is released – as well as
the molecular mechanisms that underpin this
process – were incompletely defined prior to the
work of the Hewett Lab. “Previous work from
my laboratory determined that the astrocyte
system xc- contributed to hypoxic neuronal
FASCINATING FINDINGS
•
Hewett and her team discovered that
IL-1β fuels the activity and expression
of the cyst(e)ine-glutamate antiporter
(system xc-) in astrocytes
•
Under conditions of energy deprivation,
system xc- induces excitotoxic
neuronal cell death; however, the same
transporter also drives the synthesis of
the antioxidant molecule glutathione
•
Their findings suggest that in addition
to its pathogenic role, IL-1β also
upregulates processes that protect
the brain against oxidative stress
injury via a glutamate-mediated mechanism,
thus leading us to attempt to determine
whether a similar mechanism might be in
play during hypoglycaemia,” Hewett reveals.
Excitingly, their results demonstrated that
glutamate efflux from astrocytes – via system
xc- – contributes to glucose deprivationinduced neuronal cell death in vitro.
SPOTLIGHT ON EPILEPSY
In a further line of research, Hewett is also
investigating the cellular and molecular
mechanisms that cause seizures in epilepsy.
Here, she is collaborating with Dr James
Hewett, an associate professor who is also
based in the Department of Biology at Syracuse
University. Their work in this area stemmed
from a finding reported in several studies
that the supplementation of polyunsaturated
fatty acids (PUFA) – which accumulate in
the brain following seizure – can increase
seizure threshold in some animal models
of epilepsy. “Because nearly 30 per cent of
individuals diagnosed with epilepsy do not
respond to current anti-epileptic drugs,
there is a pressing need to understand the
cellular and molecular mechanisms that
underlie seizure genesis so that new therapies
can be developed,” Hewett emphasises.
“Using our mice models, we found that
seizure threshold is regulated by the PUFAmetabolising enzyme L-12/15 Lipoxygenase.
It could be that PUFAs accumulate in the
animals that lack L-12/15 Lipoxygenase and
that this is what underlies this effect – but that
remains to be experimentally determined.”
MOVING FORWARDS
To date, Hewett and her team
have made important strides in
carving a clearer knowledge about
the molecular and biochemical
processes that underlie
pathophysiological processes
in the brain. Looking ahead, it
is possible that as a result of
their findings the astrocyte will
become a more viable therapeutic
target to address ischaemic
and hypoglycaemic injury than
the inhibition of downstream
neuronal effectors. However, in
order to design the most effective
therapies, the researchers are
currently planning to focus on
forging a deeper understanding
of the timing and duration of
the dual inflammatory-repair
response mediated by IL-1β.
INTELLIGENCE
THE ROLE OF INFLAMMATORY FACTORS
IN ACUTE NEURONAL INJURY
OBJECTIVES
• To understand the cellular and
molecular mechanisms that drive the
progression of acute neuronal injury
• To clarify the role of interleukin1β in pathology and repair
KEY COLLABORATOR
Dr James Hewett, Syracuse University, USA
FUNDING
US National Institutes of Health
CONTACT
Dr Sandra Hewett
Executive Director of Neuroscience Studies and The
Beverly Petterson Bishop Professor of Neuroscience
Syracuse University
362 Life Sciences Complex
Syracuse
New York
13244
USA
T +1 860 212 1378
E [email protected]
http://bit.ly/SandraHewett
@NeuroscienceSU
SANDRA HEWETT completed a
BSc in Biology before receiving her
PhD in Pharmacology/Toxicology
in 1992 from Michigan State
University, USA. After completing
postdoctoral research at Washington University
School of Medicine, she worked at the University of
Connecticut Health Center. In 2011, Hewett became
the inaugural Beverly Petterson Bishop Professor of
Neuroscience and Executive Director of Neuroscience
Studies at Syracuse University, New York, USA.
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