Regulated endosomal trafficking of Diacylglycerol lipase alpha (DAGLα) generates distinct cellular pools; implications for endocannabinoid signaling.

Diacylglycerol lipase alpha (DAGLα) generates the endocannabinoid (eCB) 2-arachidonylglycerol (2-AG) that regulates the proliferation and differentiation of neural stem cells and serves as a retrograde signaling lipid at synapses. Nothing is known about the dynamics of DAGLα expression in cells and this is important as it will govern where 2-AG can be made and released. We have developed a new construct to label DAGLα at the surface of live cells and follow its trafficking. In hippocampal neurons a cell surface pool of DAGLα co-localizes with Homer, a postsynaptic density marker. This surface pool of DAGLα is dynamic, undergoing endocytosis and recycling back to the postsynaptic membrane. A similar cycling is seen in COS-7 cells with the internalized DAGLα initially transported to EEA1 and Rab5-positive early endosomes via a clathrin-independent pathway before being transported back to the cell surface. The internalized DAGLα is present on reticular structures that co-localize with microtubules. Importantly, DAGLα cycling is a regulated process as inhibiting PKC results in a significant reduction in endocytosis. This is the first description of DAGLα cycling between the cell surface and an intracellular endosomal compartment in a manner that can regulate the level of the enzyme at the cell surface.

Assay and Inhibition of the Purified Catalytic Domain of Diacylglycerol Lipase Beta.

The diacylglycerol lipases (DAGLα and DAGLß) hydrolyze DAG to generate 2-arachidonoylglycerol (2-AG), the principal endocannabinoid and main precursor of arachidonic acid (AA). The DAGLs make distinct tissue specific contributions toward 2-AG and AA levels, and therefore, selective modulators for these enzymes could play crucial roles toward harnessing their therapeutic potential. Relatively high-throughput assays have recently been reported for DAGLα and have proven useful toward the characterization of inhibitors of this enzyme. Similar assays are also warranted for DAGLß which was the aim of this study. We first adapted previously reported DAGLα membrane assays (using PNPB and DiFMUO as substrates) to measure recombinant DAGLß activity in membranes. In contrast to results with DAGLα, both substrates provided a relatively limited signal window for measuring DAGLß activity, however, an improved window was obtained when employing a third commercially available substrate, EnzChek. In order to further improve on the assay parameters, we successfully purified the glutathione S-transferase (GST) tagged catalytic domain of DAGLß. Activity of the enzyme was confirmed using EnzChek as well as two DAGL inhibitors (THL and OMDM-188). The purified DAGLß catalytic domain assay described here provides the basis for a relatively clean and convenient assay with the potential to be adapted for high-throughput drug discovery efforts.

A novel live cell assay to measure diacylglycerol lipase α activity.

Diacylglycerol lipase α (DAGLα) hydrolyses DAG to generate the principal endocannabinoid (eCB) 2-arachidonoylglycerol (2-AG) in the central nervous system. DAGLα dependent cannabinoid (CB) signalling has been implicated in numerous processes including axonal growth and guidance, adult neurogenesis and retrograde signalling at the synapse. Recent studies have implicated DAGLα as an emerging drug target for several conditions including pain and obesity. Activity assays are critical to the drug discovery process; however, measurement of diacylglycerol lipase (DAGL) activity using its native substrate generally involves low-throughput MS techniques. Some relatively high-throughput membrane based assays utilizing surrogate substrates have been reported, but these do not take into account the rate-limiting effects often associated with the ability of a drug to cross the cell membrane. In the present study, we report the development of a live cell assay to measure DAGLα activity. Two previously reported DAGLα surrogate substrates, p-nitrophenyl butyrate (PNPB) and 6,8-difluoro-4-methylumbelliferyl octanoate (DiFMUO), were evaluated for their ability to detect DAGLα activity in live cell assays using a human cell line stably expressing the human DAGLα transgene. Following optimization, the small molecule chromogenic substrate PNPB proved to be superior by providing lower background activity along with a larger signal window between transfected and parental cells when compared with the fluorogenic substrate DiFMUO. The assay was further validated using established DAGL inhibitors. In summary, the live cell DAGLα assay reported here offers an economical and convenient format to screen for novel inhibitors as part of drug discovery programmes and compliments previously reported high-throughput membrane based DAGL assays.

Loss of retrograde endocannabinoid signaling and reduced adult neurogenesis in diacylglycerol lipase knock-out mice.

Endocannabinoids (eCBs) function as retrograde signaling molecules at synapses throughout the brain, regulate axonal growth and guidance during development, and drive adult neurogenesis. There remains a lack of genetic evidence as to the identity of the enzyme(s) responsible for the synthesis of eCBs in the brain. Diacylglycerol lipase-alpha (DAGLalpha) and -beta (DAGLbeta) synthesize 2-arachidonoyl-glycerol (2-AG), the most abundant eCB in the brain. However, their respective contribution to this and to eCB signaling has not been tested. In the present study, we show approximately 80% reductions in 2-AG levels in the brain and spinal cord in DAGLalpha(-/-) mice and a 50% reduction in the brain in DAGLbeta(-/-) mice. In contrast, DAGLbeta plays a more important role than DAGLalpha in regulating 2-AG levels in the liver, with a 90% reduction seen in DAGLbeta(-/-) mice. Levels of arachidonic acid decrease in parallel with 2-AG, suggesting that DAGL activity controls the steady-state levels of both lipids. In the hippocampus, the postsynaptic release of an eCB results in the transient suppression of GABA-mediated transmission at inhibitory synapses; we now show that this form of synaptic plasticity is completely lost in DAGLalpha(-/-) animals and relatively unaffected in DAGLbeta(-/-) animals. Finally, we show that the control of adult neurogenesis in the hippocampus and subventricular zone is compromised in the DAGLalpha(-/-) and/or DAGLbeta(-/-) mice. These findings provide the first evidence that DAGLalpha is the major biosynthetic enzyme for 2-AG in the nervous system and reveal an essential role for this enzyme in regulating retrograde synaptic plasticity and adult neurogenesis.

The fibroblast growth factor receptor acid box is essential for interactions with N-cadherin and all of the major isoforms of neural cell adhesion molecule.

Interactions between the neural cell adhesion molecules NCAM and N-cadherin with the fibroblast growth factor receptor (FGFR) are important for a number of developmental events and have also been implicated in tumor progression. The factors regulating these interactions are not known. We have used co-immunoprecipitation and co-clustering paradigms to show that both adhesion molecules can interact with the 3Ig IIIC isoform of the FGFR1 in a number of cell types. Interestingly, whereas the interaction can be seen over most of the cell surface, it is not seen at points of cell-cell contact where the adhesion molecules accumulate at stable junctions. We also demonstrate for the first time that all of the major isoforms of NCAM can interact with the FGFR. Using deletion mutagenesis we have found that the adhesion molecule/FGFR interaction can withstand the removal of most of any one of the FGFR immunoglobulin-like domains (D1-D3). In contrast, the FGFR interaction with N-cadherin and NCAM (but not FGF) is absolutely dependant on the presence of the acid box motif that can be found in the linker region between D1 and D2. As this motif can be spliced out of all four FGFRs, it suggests that this is one mechanism that can regulate the interaction of the receptor with different ligand classes.