Download mechanisms of neurotransmitter receptor biogenesis and trafficking

Dr Nancy J Leidenheimer hopes to understand the processes of
neurotransmitter receptor formation and trafficking. Here, she discusses her
interests, her research and the promise of its therapeutic application
understand brain function. Computers will be
the key to storing, processing and integrating
vast amounts of information, as well as to
modelling and mapping the brain’s circuitry.
Despite incredible advances in neuroscience,
these findings have yet to translate into
effective treatments for many neurological
disorders and psychiatric conditions. Thus,
while unravelling the complexities of the
brain and its dysfunction will be a key to
treating devastating brain disorders, there
will likely be a long time lag between
understanding these disorders and the
application of clinically effective treatments.
What key challenges do you face in
your work?
What excites you most about cellular
neuroscience?
The unimaginable complexity of nerve
cells is fascinating. Neurons have
elaborate morphologies, dynamic function
and unbelievably complex regulatory
mechanisms. We are still at the point where
we understand only a small fraction of what
is knowable; the brain has roughly 100 billion
neurons that are interconnected by 100
trillion synapses. One of the most tantalising
questions is how cellular complexity
translates into thought and mood. The
answer to this mystery is beginning to be
revealed thanks in part to technological
advancements such as those in the
neuroimaging field. It has been a privilege to
watch the field of neuroscience unfold.
Do you believe the complex mysteries of the
brain will ever be completely unravelled?
The brain is the last frontier in understanding
human physiology. Whether the mind
is capable of understanding itself is
an intriguing question. Hopefully, the
technological advances of the past few
decades will allow us to go beyond the
brain’s capacity, permitting us to thoroughly
Oddly, the biggest challenge we face is the
lack of cell biology knowledge pertinent for
understanding how γ-aminobutyric acid
(GABA) chaperones the GABAA receptor. It
turns out that the chaperoning story is an
elaborate one that likely involves a number
of other proteins and processes about which
little is known. For example, we now know
that GABA is found in the endoplasmic
reticulum. We do not know how it gets
inside this cell organelle but assume it is
transported there by a protein. Because it
has been known for decades that GABA is
present inside mitochondria, a cell organelle
involved in energy metabolism, we looked
to the scientific literature to see how GABA
enters mitochondria. Surprisingly, this
mechanism remains unknown in animal cells,
but was recently discovered for plant cell
mitochondria. Thus, the clues we gather, and
hypotheses we formulate, are being pieced
together from limited scientific information
that has us delving into diverse scientific
literature. While this substantially broadens
the scope of our research and slows progress,
it keeps things interesting.
Another challenge we face is scepticism.
It is difficult for some to consider that a
neurotransmitter may have a function beyond
the synapse, especially as a chaperone within
DR NANCY J LEIDENHEIMER
Regulating
receptors
the endoplasmic reticulum. A certain amount
of scepticism, however, is healthy for keeping
science in check. Lastly, a challenge facing
biomedical science is the contraction of
National Institutes of Health (NIH)-funded
research, which could have a long-term
negative impact on the scientific enterprise.
Could you discuss the promise of
pharmacological chaperones as an
approach for the treatment of disorders
that result from protein misfolding?
My colleague Henry Lester (Cal Tech) and
I recently guest-edited a special issue of
the journal Pharmacological Research.
This edition, entitled Pharmacological
Chaperones: On the Frontier of 21st Century
Therapeutics highlights the numerous
proteins that undergo pharmacological
chaperoning, and the potential of
pharmacological chaperones as therapeutic
agents for a spectrum of diseases. While
gene therapy has been widely considered
for the correction of genetic disorders,
including those diseases resulting from
protein misfolding, the road has been long
and uncertain. Pharmacological chaperones
appear to be a promising and, in some
respects, more practical approach for the
treatment of genetic disorders in which
mutated misfolded proteins can be coaxed
to fold correctly. One of our currently funded
projects focuses on using pharmacological
chaperones to ‘rescue’ epilepsy-associated
GABAA receptor mutants that are not
properly folded.
Looking ahead, what direction will your
research take?
For now, our finding that the neurotransmitter
GABA acts as chaperone to facilitate GABAA
receptor biogenesis raises numerous questions
that will take years to address. We are also
beginning a drug discovery programme.
It is possible that, if successful, this new
aspect of our programme may lead us in a
translational direction.
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DR NANCY J LEIDENHEIMER
Unfolding the mysteries of
neurotransmitter receptor biogenesis
Ongoing research at Louisiana State University Health Sciences Center Shreveport aims to
better understand the formation of neurotransmitter receptors and is providing new insights into
previously underappreciated mechanisms of receptor biogenesis. These insights may translate into the
development of a novel treatment avenue for certain neurological and psychiatric disorders
NEURONS ARE NOT static entities, but instead
highly dynamic transducers of chemical signalling
molecules called neurotransmitters. The process
whereby neurons exchange information between
each other is called neurotransmission. This
remarkable feat is achieved through membrane
bound neurotransmitter receptors that bind
neurotransmitters with high specificity. The
brain utilises a variety of neurotransmitter/
neurotransmitter receptor partners. The
activation of neurotransmitter receptors by
their neurotransmitters leads to either brain
excitation or inhibition.
For example, the predominant inhibitory
neurotransmitter receptor of the brain, the
GABAA receptor, is highly specific for its ligand,
γ-aminobutyric acid (GABA), from which it
derives its name. Neurotransmitter receptors
are mostly found at the synapse – the specialised
structure, at which neurotransmitter is released
from one neuron onto the neurotransmitter
receptor of an adjacent neuron. When the
receptor is activated by a neurotransmitter,
biochemical and/or electrical changes occur
in the receiving neuron leading to either
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INTERNATIONAL INNOVATION
inhibition or excitation. In the case of the GABAA
receptor, activation leads to an increase in
chloride conductance, hyperpolarisation and
neuronal inhibition.
changes in neurotransmission at the synapse,
becoming either stronger or weaker depending
on the context.
RECEPTOR BIOGENESIS
The GABAA receptor represents
an extremely important
therapeutic target due to its
central role in maintaining
neuronal inhibition
At the level of neurons and networks, the precise
consequences of neurotransmission depend
on the type of receptor present, the number of
receptors and their spatial location. Principally
because of this, changes at the receptor level can
significantly impact the function of neurons and
the networks they are part of. A good example
of this are the mechanisms underlying learning
and memory, which fundamentally depend upon
Receptors are proteins that need to be folded
into very specific shapes, or conformations.
Since the number of theoretically possible
conformations a given protein can form can
be impossibly large, active mechanisms are
required for achieving the correct conformation.
According to Levinthal’s paradox, if even a very
small protein needed to sequentially sample
all possible folding shapes before achieving its
proper conformation, and each folding event
occurred on the femtosecond timescale, it
would take longer than the age of the Universe
to achieve proper folding.
The full mechanisms for protein folding have
remained surprisingly elusive, although it is known
that drug molecules called pharmacological
chaperones can play a crucial role in facilitating
the process. Finding out how proteins are folded
into their proper conformation is an important
INTELLIGENCE
endeavour of biological research since a protein’s
conformation determines its function and
misfolded proteins can cause widespread cellular
damage or even cell death. It is worth noting
that it has been estimated that 40 per cent of
human diseases involve protein misfolding as
part of their pathology.
evidence for this mechanism in neurons as
well. As she explains: “The biogenesis of both
inhibitory and excitatory neurotransmitter
receptors appear to be subject to this type
of proteostatic regulation. At this point our
research has generated many more questions
than answers”.
THE RIGHT BALANCE
COLLABORATIVE EFFORTS
Dr Nancy J Leidenheimer is working to better
understand the mechanisms that underlie
neurotransmitter
receptor
biogenesis,
specifically that of GABAA, an inhibitory
receptor important for ionotropic chloride ion
conductance. She seeks to provide information
critical for elucidating brain function by first
understanding the biochemistry of receptors,
particularly how their formation is regulated in a
cell organelle called the endoplasmic reticulum.
The GABAA receptor represents an extremely
important therapeutic target due to its central
role in maintaining neuronal inhibition.
Perturbations of this inhibitory tone in favour of
excitation can lead to a wide range of disorders;
most notably epilepsy, anxiety and insomnia.
Many current treatments for such disorders
increase GABAA receptor activity.
Leidenheimer’s work relies predominantly on
two distinct cell culture model systems. The
first is rat brain neuronal cultures which allow
GABAA receptor biogenesis to be assessed in
a native neuronal environment. In the other
model, GABAA receptor DNA is introduced into
a non-neuronal cell, resulting in the induced
expression of the GABAA receptor: “This
reductionist approach allows us to study the
receptor by isolating it from the complexities of
neurotransmission,” Leidenheimer reveals.
For many proteins, folding in the endoplasmic
reticulum appears to be an inefficient process
with inefficiently folded proteins often destined
for degradation without use. Thus, in many
cases the properly folded protein only forms a
fraction of its total protein present in a cell due
to ‘inefficient’ protein folding processes. “This
inefficiency has long been a mystery given the
general efficiency of biological mechanisms,”
Leidenheimer explains.
However, it may not actually be the case
that folding is wastefully inefficient. This
could instead merely reflect an incomplete
understanding of proteostatic mechanisms.
Leidenheimer’s research supports this latter
view, positing that protein folding ‘inefficiency’
serves a useful purpose, performing a regulatory
role that allows the amount of properly
folded, functionally active protein to be
tightly controlled.
A major arm of Leidenheimer’s research stems
from this view and she is now exploring the
possibility that neurotransmitters may enter
the endoplasmic reticulum and exert a direct
effect on their receptor biogenesis. Thus, she
proposes that the neurotransmitter GABA acts
as a ‘cognate ligand chaperone’ for its receptor,
the GABAA receptor. This is particularly exciting
because it provides an additional mechanism
by which a neurotransmitter can regulate
the amount of active receptor, through the
proteostatic control of receptor biogenesis.
Leidenheimer has shown that GABA performs
the role of chaperone for its receptor in a
recombinant expression system and has strong
In both systems she employs fluorescent
microscopy and a wide range of immunological
and biochemistry techniques to determine the
localisation of receptors and the interaction
of the receptor with other proteins during the
‘quality control’ phase of receptor biogenesis.
Through her collaborative efforts with Dr Sheryl
Smith of the SUNY Downstate Medical Center,
Leidenheimer has used electrophysiological
recordings to show that GABAA receptors that
are chaperoned by GABA retain their functional
capacity at the cell surface. In another
collaboration with Dr Charles Meshul of the
Oregon Health Sciences University, GABA has
been detected in the endoplasmic reticulum of
neurons using electron microscopy.
WIDER CONTEXT
This work is not simply transforming our basic
understanding of biogenesis and receptor
expression, but will hopefully also have farreaching consequences that translate to the
bedside. An understanding of the factors
controlling GABAA receptor formation opens
the possibility of developing novel drugs for
the treatment of a variety of brain disorders,
particularly those that centre on GABAA receptor
dysfunction or an imbalance between excitation
and inhibition in the central nervous system.
Pharmacological chaperones also represent
a growing field of study, and are viewed with
optimism as a potential source of many future
clinical breakthroughs in relation to the treatment
of protein folding disorders. Leidenheimer
hopes pharmacological chaperones will provide
some effective alternatives to current GABAA
targeting drugs, which are problematic due
to the tolerance and addiction that often
occur during treatment: “Currently our lab is
developing a high throughput, cell based drug
screen for the discovery of GABAA receptor
pharmacological chaperones”.
MECHANISMS OF
NEUROTRANSMITTER RECEPTOR
BIOGENESIS AND TRAFFICKING
OBJECTIVE
To unravel the mechanisms that underlie
neurotransmitter receptor biogenesis.
KEY COLLABORATORS
Charles K Meshul, PhD, Research
Biologist and Director, Electron
Microscopy Facility, Portland Veterans
Administration Medical Center and
Professor, Department of Behavioral
Neuroscience and Pathology, Oregon
Health & Science University
Sheryl S Smith, PhD, Professor of
Physiology and Pharmacology, SUNY
Downstate Medical Center
FUNDING
National Institutes of Health (NIH) grants
2R01MH602640-07; 5R03NS075526-02
CONTACT
Dr Nancy J Leidenheimer
Professor, Department of Biochemistry
and Molecular Biology
Louisiana State University Health Sciences
Center Shreveport
1501 Kings Highway
Shreveport
Louisiana, 71130
USA
T +1 318 675 7855
E [email protected]
NANCY LEIDENHEIMER, PHD is
a Professor in the Department of
Biochemistry and Molecular Biology and
an adjunct professor in the Department
of Pharmacology, Toxicology and
Neuroscience at Louisiana State University
Health Sciences Center in Shreveport.
She has served on National Institutes
of Health grant review panels and on
the Editorial Board of the Journal of
Biological Chemistry. She most recently
guest-edited a focus issue of the journal
Pharmacological Research entitled
Pharmacological Chaperones: On the
Frontier of 21st Century Therapeutics.
Leidenheimer’s work is supported by the
National Institutes of Health, specifically
the National Institute of Mental Health
and National Institute of Neurological
Disorders and Stroke.
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