Parsing the players: 2-arachidonoylglycerol synthesis and degradation in the CNS

Abstract

The endogenous cannabinoid signalling system, composed of endogenous cannabinoids, cannabinoid receptors and the enzymes that synthesize and degrade the endogenous cannabinoids, is much more complex than initially conceptualized. 2-Arachidonoylglycerol (2-AG) is the most abundant endocannabinoid and plays a major role in CNS development and synaptic plasticity. Over the past decade, many key players in 2-AG synthesis and degradation have been identified and characterized. Most 2-AG is synthesized from membrane phospholipids via sequential activation of a phospholipase Cβ and a diacylglycerol lipase, although other pathways may contribute in specialized settings. 2-AG breakdown is more complicated with at least eight different enzymes participating. These enzymes can either degrade 2-AG into its components, arachidonic acid and glycerol, or transform 2-AG into highly bioactive signal molecules. The implications of the precise temporal and spatial control of the expression and function of these pleiotropic metabolizing enzymes have only recently come to be appreciated. In this review, we will focus on the primary organization of the synthetic and degradative pathways of 2-AG and then discuss more recent findings and their implications, with an eye towards the biological and therapeutic implications of manipulating 2-AG synthesis and metabolism.

Linked articles: This article is part of a themed section on Cannabinoids 2013. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2014.171.issue-6.

Keywords: 2-AG; arachidonic acid; cannabinoid; diacylglycerol; hydrolysis; metabolism; monoacylglycerol lipase; synaptic plasticity; synthesis.

Figure 1

2-AG trafficking and its action at the synapse. (A) 2-AG synthesis and breakdown. Schematic representation of an excitatory terminal and post-synaptic spine. Conventional action potential-induced neurotransmitter release (e.g. glutamate) occurs via activation of calcium channels adjacent to transmitter-filled vesicles, which fuse with the membrane to release their contents. 2-AG can be produced either following synaptic depolarization (e.g. DSE) or by activation of G_q/11-coupled GPCRs, such as group I mGlu receptors, which then activate PLCβ (PLCβ, cleaving phosphatidyl bisphosphate (PIP₂) into DAG and inositol trisphosphate (IP₃). DAG is hydrolysed by DAG lipase, yielding 2-AG. Rather than being released from vesicles, lipophilic endocannabinoids cross the membrane, perhaps utilizing facilitated transport. The mechanism of subsequent passage across the synapse is unknown but may involve carrier proteins. Activation of pre-synaptic CB₁ receptors inhibits transmitter release by inhibiting Ca²⁺ channels. On the post-synaptic side, 2-AG can be broken down into glycerol and AA by the enzyme ABHD6, embedded in the membrane. On the pre-synaptic side, 2-AG can be broken down by MAGL, loosely associated with the plasma membrane, or, in principle, by ABHD12, a transmembrane protein, into glycerol and AA. (B) ABHD12 localization on Golgi. Emerging evidence suggests that ABHD12 is embedded in the Golgi membrane, with its active site facing the lumen.

Figure 2

Primary routes of 2-AG synthesis and degradation. 2-AG synthesis can potentially occur from three precursors: (1) DAG (via DAGL), in turn, DAG can be synthesized from PIP₂ via PL C-β (PLCβ); (2) 2-Arachidonoyl-LPA via 2-LPA phosphatase (MAG kinase reverses the actions of 2-LPA phosphatase); and (3) 2-Arachidonoyl-LPI via a lyso-PLC, 2-Arachidonoyl-LPI can, in turn, be synthesized from PI via the actions of PLA1 (PIP₂ phosphatases produce PI from PIP₂). 2-AG metabolism can potentially occur via five different routes: (1) MAGL, FAAH, ABHD6 and ABHD12 hydrolyse 2-AG into AA and glycerol; (2) COX-2 converts 2-AG into PGE₂-G; (3) MAG kinase converts 2-AG into 2-arachidonoyl-LPA (2-LPA phosphatase reverses the actions of MAG kinase); (4) cytochrome p450 converts 2-AG into 11,12-EET and (5) 12-lipoxygenase (12-LOX) can oxygenate 2-AG into 12-HETE-G.