Differential Subcellular Recruitment of Monoacylglycerol Lipase Generates Spatial Specificity of 2-Arachidonoyl Glycerol Signaling during Axonal Pathfinding

Abstract

Endocannabinoids, particularly 2-arachidonoyl glycerol (2-AG), impact the directional turning and motility of a developing axon by activating CB 1 cannabinoid receptors (CB 1 Rs) in its growth cone. Recent findings posit that sn-1-diacylglycerol lipases (DAGL␣/␤) synthesize 2-AG in the motile axon segment of developing pyramidal cells. Coincident axonal targeting of CB 1 Rs and DAGLs prompts the hypothesis that autocrine 2-AG signaling facilitates axonal outgrowth. However, DAGLs alone are insufficient to account for the spatial specificity and dynamics of 2-AG signaling. Therefore, we hypothesized that local 2-AG degradation by monoacylglycerol lipase (MGL) must play a role. We determined how subcellular recruitment of MGL is temporally and spatially restricted to establish the signaling competence of 2-AG during axonal growth. MGL is expressed in central and peripheral axons of the fetal nervous system by embryonic day 12.5. MGL coexists with DAGL␣ and CB 1 Rs in corticofugal axons of pyramidal cells. Here, MGL and DAGL␣ undergo differential axonal targeting with MGL being excluded from the motile neurite tip. Thus, spatially confined MGL activity generates a 2-AG-sensing microdomain and configures 2-AG signaling to promote axonal growth. Once synaptogenesis commences, MGL disperses in stationary growth cones. The axonal polarity of MGL is maintained by differential proteasomal degradation because inhibiting the ubiquitin proteasome system also induces axonal MGL redistribution. Because MGL inactivation drives a CB 1 R-dependent axonal growth response, we conclude that 2-AG may act as a focal protrusive signal for developing neurons and whose regulated metabolism is critical for attaining correct axonal complexity.

Fetal endocannabinoids orchestrate the organization of pancreatic islet microarchitecture

“…If 2-AG signaling is significant in modulating pancreatic islet microstructure (that is, either the size of α and β cell pools or their spatial positions), then genetic disruption of 2-AG metabolism might impose permanent phenotypes. We have previously found that MAGL-mediated degradation is rate-limiting for fetal 2-AG signaling in brain (23). Therefore, we first tested the analogous hypothesis by determining pancreatic islet morphology in adult MAGL −/− mice.…”

“…formation of the endocrine pancreas, even more so because eCB signaling so far has been documented only in the fetal brain and musculature (10,22) and has been shown to progress from a predominantly autocrine (prenatal) toward a paracrine (postnatal) mode of action (23). Similar changes in the molecular configuration of eCB signaling in the fetal pancreas would have a notable impact on both reconstructing signaling events and interpreting functional outcome and disease relevance.…”

Nerve growth factor scales endocannabinoid signaling by regulating monoacylglycerol lipase turnover in developing cholinergic neurons

“…Loss of CB 1 R function disrupts neuronal morphogenesis (2,16). Therefore, we asked whether neurite complexity of cholinergic neurons routing normally to the fetal basal forebrain is impaired upon manipulating CB 1 Rs.…”

“…By taking advantage of BRCA1's cisplatin sensitivity (15), we rescued MGL in cholinergic growth cones, introducing a "stop" signal for NGF-induced growth responses. We have recently shown that endocannabinoid-mediated axonal growth and guidance requires the precisely ordered molecular assembly of 2-AG signaling networks during corticogenesis (2,16). Here, we defined, by in situ hybridization, that CB 1 R mRNA was present in cholinergic territories, particularly the medial septum (MS) (Fig.…”

“…2-AG liberated from postsynaptic neurons serves as a retrograde messenger to limit neurotransmitter release from many mature presynapses (1). However, 2-AG signaling also emerges as a molecular determinant of cortical development (2)(3)(4). Despite recent advances, the contribution of endocannabinoids (specifically 2-AG) to neuronal diversification and the development of synaptic connectivity in subcortical territories remains unknown.…”