Download Cannabinoid Signaling and Lipid Rafts

Cannabinoid Signaling and Lipid Rafts
More evidence is showing that CB1 receptor binding, signaling, anandamide transport, and 2-AG
synthesis is influenced by lipid rafts. The plasma membrane is a highly disordered phospholipid
by-layer that consists of compartmentalized microdomains. These microdomains that are
enriched in cholesterol, sphingolipids, and arachidonic acid are referred to as lipid rafts. Rafts
can be conceived as platforms that collect different components, such as receptors, effectors, and
enzymes that make for more efficient cellular signaling (Daienes, E. et al., 2007). Rafts are able
to modulate the activity of a number of G-protein coupled receptors, for many GPCR’s have
been found to be sequestered in lipid rafts. It was first suggested that endocannabinoid signaling
was lipid raft dependent when studies showed that depleting membrane cholesterol induced
apoptosis in a variety of cell lines. These studies showed that reduced membrane cholesterol
reduced AEA binding to the cells. This finding supports other studies which have found that lipid
raft perturbation enhanced CB1 receptor binding to CB1 specific antibodies, and changed CB1
receptor trafficking (Bari et al, 2008) Also, it has been observed that CB1-dependent signaling is
significantly reduced by cholesterol enrichment, which increases the rigidity of lipid rafts thus
reducing the rafts’ stability (Simons, R., Ehehalt, R., 2002; Mitter, D. et al., 2003). On the
contrary, no effects of raft disruptions on CB2 binding and signaling has been found (Bari, M.,
2006). This prompts us to ask the questions: What structural components of CB1 receptors
specifically interact with lipid rafts; and how can raft disruption influence CB1 binding and
signaling? Evidence leads us to believe that the juxtamembrane segment of the receptor may be a
critical component for the CB1 receptor to interact with lipid rafts, as it mediates interaction with
phospholipid molecules with the lipid raft itself (Pike, L., 2003). Furthermore, omega-3 fatty
acids and polyunsaturated acids seem to be important in the elastic properties of lipid rafts that
elicit tighter interactions with regions of the CB1 receptor (Feller, S.E., Gawrich, K., 2005).
AEA is a very flexible molecule that may be influenced by the surrounding environment, thus
suggesting the influence of lipid rafts on CB1 receptor responsiveness to AEA.
The involvement of lipid rafts in the dynamics of AEA and 2-AG synthesis, degradation and
metabolism have been explored in cultured glioma cells, yet few studies have examined lipid raft
function in neuronal cell types (Bari et al., 2005, 2006). A recent study found that the disruption
of lipid rafts in the striatum negatively affected 2-AG metabolism but not AEA (Maccarrone, M.,
2009). Namely, disrupting the cholesterol content in striatal slices of mice failed to change the
activity of NAPE-PLD or FAAH. Depletion of cholesterol in lipid rafts negatively altered the
activity of the AEA transporter, AMT. On the other hand, the activity of the 2-AG synthesizing
enzyme, DAGL was significantly increased after treating striatal slices, yet the activity of
MAGL was not altered. The transport of 2-AG across the plasma membrane was downregulated,
which coincides with the finding the cholesterol depletion downregulates AEA transport.
Another major finding of this study is that disruption of cholesterol in lipid rafts enhances
binding and activity of CB1 receptors in the striatum. Other studies, as well, have found that
lipid raft perturbation enhanced CB1 receptor binding to CB1 specific antibodies and changed
CB1 receptor trafficking (Bari et al., 2008). Lipid rafts can also negatively affect receptors
through sequestration or internalization. Typically, the desensitization of CB1 receptors is
mediated by the phosphorylation of serine residues in the C-terminus and third intracellular loop
by means of specific kinases. Phosphorylation targets the receptor for internalization and
uncouples the receptor for the G-protein by recruiting beta arrestin 2. The kinases and beta
arrestin 2 are dependent of lipid rafts therefore it is conceivable that disruption of lipid rafts
could influence receptor desensitization (Dainese, E. et al., 2007). Further studies have been able
to show that changes in diet have an effect on membrane composition, and that obese rats fed a
diet with a high amount of fish oil (high in omega 3 polyunsaturated fat) had significantly higher
amounts of lineolic acid, EPA and DPA, and a lower proportion of arachidonic acid and ratio of
omega 6/omega 3polyunsaturate fatty acids in lipid rafts isolated from spenocytes (Ruth, M., et
al, 2009). The next section of this review, will take a closer look in the normal function of the
endocannabinoid system in appetite and energy metabolism, as well as, the disruption of the
endocannabinoid system in obesity and its potential systemic effects.