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All Aboard the SCP-2 Submarine
The Role of Sterol Carrier Protein 2 in the Endocannabinoid System And Anxiety Disorders
Messmer SMART Team: Briana Miller, David Gonzalez, Anwuri Osademe, Ngozi Osademe, Sonia Sosa-Gonzalez, Gabriella Leachmen, and Michaun Cobb
Advisor: Mrs. Carol Johnson
Mentors: Elizabeth Sabens Liedhegner, Ph.D. and Cecilia J. Hillard, Ph.D., Medical College of Wisconsin
Abstract
The endocannabinoid system (ECS) plays a role in diverse disorders such as anxiety, addiction, eating
and memory disorders. The ECS is found throughout the body and consists of two lipid signaling
molecules, N-arachidonylethanolamine (AEA) and 2-arachidonoylglycerol (2-AG), and their target
receptor, CB1R. In the brain, these ligands bind to CB1R and modulate the release of
neurotransmitters from nerve cells resulting in changes in synaptic transmission. Disorders result when
levels of AEA and 2-AG are either too high or too low. While 2-AG is synthesized at the plasma
membrane (PM) AEA is produced in the ER and may require an intracellular, carrier protein to move
through the cytoplasm to the PM where it is released. The lipid binding protein, sterol carrier protein 2
(SCP-2) is hypothesized to transport AEA due to its ability to bind to membranes and its nonspecific
hydrophobic binding pocket. This proposed binding pocket is composed of 2 alpha helices, 3 beta sheets
and the core hydrophobic amino acid residues F13, F27, F35, F37 and F80. The Messmer SMART Team
(Students Modeling A Research Topic) created a model of SCP-2 using 3D printing technology.
Understanding the structure of SCP2 and how this protein might regulate AEA and 2-AG levels could
lead to possible new treatments for debilitating mood, appetite, memory and anxiety disorders.
Introduction
•
•
Fig. 4
Presynaptic Neuron
•
ECS in the brain acts as a dimmer
switch for the communication between
nerve cells.
The endocannabinoids inhibit
neurotransmitter release, slowing down
nerve cell communication and
contributing to the regulation of
synaptic activity.
Decreases in activity in the ECS in the
brain have been hypothesize to result in
anxiety disorders (see Figure 4).
Fig. 6 – Cells transfected to
overexpress the lipid carrier
protein, SCP2, take up more
AEA into the cell than cells
that do not have SCP2,
although both cells do take
AEA into the cell. This
research suggests that there is
a correlation between AEA and
SCP2 levels in the cell which
could be important in
understanding how to regulate
AEA levels in people who
have anxiety.
Neurotransmitter
Fig. 1
E - AEA binds to it’s receptor,
CB1, and decreases
neurotransmitter release
Vesicle
CB1 Receptors
http://www.anxiety-symptoms-relief-secrets.com/anxiety/symptoms-of-anxiety-and-stress
Neurotransmitter Receptor
• Symptoms of anxiety include, but are not limited to, difficulty sleeping,
irritability, headaches, trembling, nausea, sweating and muscle tension.
• Although some treatments for anxiety are available, they are not effective
for everyone and many suffer without treatment.
• Approximately 40 million adult Americans suffer from various anxiety
disorders during the course of a year.
D – When AEA/SCP-2
complex arrives at the
membrane, AEA is
released and diffuses
through the membrane
into the synapse
B – Neurotransmitter binds to
receptors on the post-synaptic cell,
resulting in depolarization and an
increase in Calcium (Ca2+)
Fig. 2
Fig. 7
2+
Ca
http://www.nimh.nih.gov/statistics/1ANYANX_ADULT.shtml
Endocannabinoid System
• The endocannabinoid system (ECS) is a system found in various parts of the body
and is involved in many biological functions such as anxiety, memory, and appetite.
• The ECS system consists of 2 main lipid signaling molecules,
arachidonylethanolamine, AEA, or anandamide and 2-arachidonoylglycerol, 2AG
(figure 3A & 3B below), and their target receptor CB1.
Fig. 3A
C – In response to Ca2+,
AEA is synthesized in the
ER. AEA slips into the
binding pocket of SCP-2
and is transported to the
cell membrane
1QND.pdb
ER
Fig. 3B
http://www.jbc.org/content/282/17/13098/F1.expansion.html
F – SCP-2 can
also pick up AEA
at the membrane
and transport it to
the ER where it is
degraded by a
hydrolase
•
As shown in Fig. 3A-B, AEA is a lipid, and may require a transport protein to carry
it from the endoplasmic reticulum (ER), where it is synthesized to the cell
membrane, where it is released.
• It is hypothesized that the transport protein, Sterol Carrier Protein, SCP-2 could be
an ideal candidate.
Hill et al, 2008
Fig. 5 Serum AEA levels and feelings
of anxiety as scored on the Hamilton
Anxiety Test. The graph shows that as
AEA increases, anxiety decreases.
Fig. 6
Hillard and Liedhegner, unpublished data
A Physical Model of SCP-2
SCP-2
Figure 2: shows the
demographics for anxiety
disaggregated by sex,
race, and age.
Fig. 5
AEA and SCP-2 Relationship
A– Neurotransmitter is released from the
pre-synaptic cell and diffuses across the
synapse to the post-synaptic cell
• Anxiety is a disorder characterized by feelings of worry, nervousness, and
unease that can be debilitating and life changing for sufferers.
Figure 1:
Causes for
anxiety
AEA and Anxiety
The Transportation of AEA by SCP-2
Postsynaptic Neuron
A SMART Team project supported by the National Institutes of Health Science Education Partnership Award (NIH-SEPA 1R25RR022749) and an NIH CTSA Award (UL1RR031973).
Fig. 7 SCP-2 functions to transport lipids through the
cytoplasm. Since AEA is a lipid, it needs a protein to be
transported through the watery cell cytoplasm. The common
sterol carrier protein, SCP-2, the focus of our research this
year, is hypothesized to be that transport protein. It has a
hydrophobic pocket (shown in yellow) where the AEA fits
into the protein and then the SCP-2/AEA complex moves to
the cell membrane. Understanding the structure of SCP-2
may lead to a new understanding of how AEA is regulated
and to new, more effective treatments for these disorders.
Conclusion
A better understanding of how the ECS is regulated could give us more effective
ways to treat anxiety. SCP-2 transport may be one way that AEA is regulated.
Therefore, studying this transporter protein could lead to medical breakthroughs
for dealing with many physiological functions that pertain to the ECS.
References:
Hill MN, et al. (2008). Pharmacopsychiatry 41: 48-53.
Lopez-Garcia F, et al. (2000). Journal of Molecular Biology 295:595.