Chapter : The endocannabinoid system in Fragile X Syndrome
Henry G. S. Martin, Daniela Neuhofer and Olivier J.J. Manzoni
Fragile X Syndrome : From Genetics to Targeted Treatment, 2017, Pages 241-259
INSERM U901, INMED and UMR S901, Aix-Marseille University Marseille 13009, France.
Endocannabinoid signaling is a finely tuned system of molecules and lipid messengers that modulate
neurotransmission in a rapid and sustained manner at many central synapses. The endocannabinoid
system is important for many brain activities, but is also implicated in several neuropsychiatric disorders.
In this chapter we discuss emerging evidence of dysfunction in endocannabinoid signaling in Fragile X
Syndrome from the Fmr1 knockout mouse model. Synthesis, receptor activation and degradation of
endocannabinoids are all highly regulated and tailored to the needs of the particular synapse. In the
Fmr1 knockout mouse contrasting deficits in endocannabinoid signaling have been reported. However
there may be means of reconciling some of the confusing data. Finally, a better understanding of the
changes in endocannbinoid signaling occurring in Fragile X Syndrome will hopefully open new treatment
Keywords : Endocannabinoids, 2-AG, anandamide, Fmr1 KO, CB1 receptor, mGlu5 receptor, DGLα, MAGL, long-term depression, Homer, JZL184, Rimonabant.
The endocannabinoid system
The endocannabinoid (eCB) system, named after that notorious group of compounds, is a neuromodulatory hub. Unique among modulatory molecules, the eCB system does not rely on afferent
innervation to generate signaling, but instead is a locally driven signaling module present at the majority
of both inhibitory and excitatory central synapses. eCB neuromodulation functions through the local
synthesis of bioactive lipid derivatives in or near the synapse and the activation of neighbouring eCB
receptors. Stimulation of eCB synthesis is an active process, dependent on neuronal state and synaptic
input. In the most common scenario, eCB molecules function as retrograde messengers; synthesized in
the post-synapse and acting as ligands to receptors localized in the presynaptic bouton, where invariably their function is to inhibit neurotransmitter release. This allows the eCB system to act as both a
homeostatic and phasic modulator of neurotransmission. The dynamic nature of the eCB system makes
it a powerful mechanism in the integration of neuronal inputs and network modulation. Unsurprisingly
therefore the eCB system is thought to have an important role in cognition, as well as learning and
memory. Dysfunction of the eCB system profoundly affects neuronal function and is associated with
many neuropsychiatric disorders notably anxiety and depression (Mechoulam & Parker, 2013), but also
addiction and genetic disorders (Chakrabarti, Persico, Battista, & Maccarrone, 2015). This list has
recently expanded to include Fragile X Syndrome (FXS), where changes in the eCB system are being
actively explored in the Fmr1 KO mouse model. What is becoming clear is that the eCB system is
profoundly affected in FXS. However due to the modular nature of the eCB system, this also opens
opportunities for pharmacological treatments (Ligresti, Petrosino, & Di Marzo, 2009).
In the CNS two principal eCBs are thought to be responsible for most neuromodulatory function: 2- arachildonoylglycerol (2-AG) and anandamide (István Katona & Freund, 2012). Both molecules are
synthesized on-demand and can signal to a variety of receptors. Most prominent in neuronal function is
the CB1 receptor, found concentrated at many presynaptic specializations. Upon ligand binding, CB1
receptors couple to Gi/0 linked G-protein complexes and act to inhibit neurotransmitter release (Fig. 1).
The strength and duration of inhibition depends on the expression and coupling of the CB1 receptor, but
also critically on the local concentration of eCBs. Since eCBs rely on passive diffusion the local
concentration reflects the distance from the source, however both 2-AG and anandamide are party to
rapid degradation by local selective lipases; notably monoacyglycerol Lipase (MAGL) and ABHD6 for 2-AG and fatty acid amide hydrolase (FAAH) for anandamide. Thus the activation of CB1 receptors is tightly controlled by the turnover of eCB ligand. Other eCB receptors are present in the CNS, but their
localization and importance are debated. CB2 receptors are principally associated with immune system
cells, but are also reported in the CNS although their function and coupling are unclear. Orphan Gprotein coupled receptors (GPCRs) notably GPR55 may too be of importance. Finally it also appears that eCBs may directly couple to ion channels, particularly the TRPV1 receptor of the vanilloid family (Castillo, Younts, Chávez, & Hashimotodani, 2012). In this mode, a non-retrograde mechanism is proposed wherein anandamide production leads to postsynaptic depolarization due to TRPV1 channel opening. Functionally, this too may lead to depression of synaptic transmission only via a postsynaptic
mechanism. The pathways leading to the biosynthesis of 2-AG and anandamide are complex and in the case of anandamide incompletely described (István Katona & Freund, 2012). Specific molecules and enzymes in the context of FXS are described below; otherwise the reader is directed to some excellent reviews (Castillo et al., 2012; Kano, Ohno-Shosaku, Hashimotodani, Uchigashima, & Watanabe, 2009). However it is worth noting that two distinct forms of 2-AG synthesis exist (Takako Ohno-Shosaku & Kano, 2014). The first, in response to increased neuronal depolarization / firing, leads to a calcium dependent activation of phospholipase C β (PLCβ) and the release of the 2-AG from its precursor diacylglycerol (DAG) by DAG lipase. Generally this leads to a transitory global decrease in synaptic activity. The second mechanism depends on the activation of Gq coupled G-proteins, most prominently through Group I mGlu GPCRs in the post-synapse, again coupling to PLCβ and DAG lipase. Synaptic activation of group I mGlu receptors leads to a synaptic specific depression of neurotransmission that either alone or coupled with postsynaptic depolarization may lead to prolonged depression of synaptic activity (Heifets & Castillo, 2009; Robbe, Kopf, Remaury, Bockaert, & Manzoni, 2002). This long-term depression (LTD) is widely expressed in the CNS and is important in learning and memory functions and ultimately behavior.