Interplay Between n-3 and n-6 Long-Chain Polyunsaturated Fatty Acids and the Endocannabinoid System in Brain Protection and Repair

Abstract

The brain is enriched in arachidonic acid (ARA) and docosahexaenoic acid (DHA), long-chain polyunsaturated fatty acids (LCPUFAs) of the n-6 and n-3 series, respectively. Both are essential for optimal brain development and function. Dietary enrichment with DHA and other long-chain n-3 PUFA, such as eicosapentaenoic acid (EPA), has shown beneficial effects on learning and memory, neuroinflammatory processes, and synaptic plasticity and neurogenesis. ARA, DHA and EPA are precursors to a diverse repertoire of bioactive lipid mediators, including endocannabinoids. The endocannabinoid system comprises cannabinoid receptors, their endogenous ligands, the endocannabinoids, and their biosynthetic and degradation enzymes. Anandamide (AEA) and 2-arachidonoylglycerol (2-AG) are the most widely studied endocannabinoids and are both derived from phospholipid-bound ARA. The endocannabinoid system also has well-established roles in neuroinflammation, synaptic plasticity and neurogenesis, suggesting an overlap in the neuroprotective effects observed with these different classes of lipids. Indeed, growing evidence suggests a complex interplay between n-3 and n-6 LCPUFA and the endocannabinoid system. For example, long-term DHA and EPA supplementation reduces AEA and 2-AG levels, with reciprocal increases in levels of the analogous endocannabinoid-like DHA and EPA-derived molecules. This review summarises current evidence of this interplay and discusses the therapeutic potential for brain protection and repair.

Keywords: Endocannabinoid system; Neurogenesis; Neuroinflammation; Omega-3 fatty acids; Omega-6 fatty acids.

Fig. 1

N-6 and n-3 PUFA metabolism and lipid mediators produced from ARA, DHA and EPA. Synthesis of n-6 and n-3 LCPUFA begins with desaturation of LA and ALA to γ-linolenic acid (GLA, 18:3n-6) and stearidonic acid (18:4n-4), respectively, catalysed by Δ6 desaturase (FADS2 gene). GLA is elongated to dihomo-γ-linolenic acid (DGLA, 20:3n-6) and SDA to eicosatetraenoic acid (20:4n-3) (ELOVL1 gene). Δ5-Desaturase (FADS1 gene) converts DGLA to ARA (20:4n-6) and 20:4n-3 to EPA (timnodonic acid, 20:5n-3). Two cycles of elongation (elongase-2, ELOVL2 gene) convert ARA to adrenic acid (AdA, 22:4n-6) and then tetracosatetraenoic acid (24:4n-6), and EPA to docosapentaenoic acid (DPAn-3, clupanodonic acid, 22:5n-3) and then tetracosapentaenoic acid (24:5n-3). A second desaturation by Δ6 desaturase produces tetracosapentaenoic acid (24:5n-6) and tetracosahexaenoic acid (nisinic acid, 24:6n-3), respectively. These are translocated to the peroxisome for β-oxidation by acyl-coenzyme-A oxidase (ACOX1 gene) and d-bifunctional enzyme (HSD1784 gene) and peroxisomal thiolases to produce docosapentaenoic acid (DPAn-6, osbond acid, 22:5n-6) and DHA (cervonic acid, 22:6n-3), which are translocated back to the endoplasmic reticulum

Fig. 2

Main lipid mediators produced from ARA, DHA and EPA. ARA, DHA and EPA are precursors to multiple metabolites, including oxylipins produced by cyclooxygenase (COX) and acetylated COX-2 (A-COX), lipoxygenase (LOX) and cytochrome P450 (CYP) enzymes and the endocannabinoids (eCB). The major pathways in the synthesis of ARA, DHA and EPA-derived endocannabinoids are shown in Fig. 3. 2-AG 2-arachidonoylglycerol, 2-DHG 2-docosahexaenoylglycerol, 2-EET-EG 2-epoxy-eicosatrienoic acid glycerol, 2-EPG 2-eicosapentaenoylglycerol, ABHD6/12 α/β-Hydrolase domain containing 6 or 12, AEA N-arachidonoylethanolamide (anandamide), AT aspirin-triggered, DHEA N-docosahexanoylethanolamine (synaptamide), DiHDoHE dihydroxy-docosahexaenoic acid, DiHDPE dihydroxy-docosapentaenoic acid, DiHEPE dihydroxy-eicosapentaenoic acid, DiHETE dihydroxy-eicosatetraenoic acid, DiHETrE dihydroxy-eicosatrienoic acid, EDP epoxy-docosapentaenoic acids, EET epoxy-eicosatrienoic acid, EET-EA epoxy-eicosatrienoic acid ethanolamide, EETeTr epoxy-eicosatetraenoic acids, EFOX electrophilic fatty acid oxo-derivatives, EpDPE epoxy-docosapentaenoic acid, EPEA N-eicosapentaenoylethanolamine, EpETE epoxy-eicosapentaenoic acid, EpETrE epoxy-eicosatrienoic acid, Epo epoxygenase, FAAH fatty acid amide hydrolase, HDoHE hydroxy-docosahexaenoic acid, HEDPEA hydroxy-epoxy-docosapentaenoyl ethanolamide, HEET-EA hydroxyepoxy-eicosatrienoic acid ethanolamide, HEPE hydroxy-eicosapentaenoic acid, HETE hydroxy-eicosatetraenoic acid, HETE-EA hydroxy-eicosatetraenoic acid ethanolamide, HHTrE hydroxy-heptadecatrienoic acid, HpDoHE hydroperoxy-docosahexaenoic acid, HpEPE hydroperoxy-eicosapentaenoic acid, HpETE hydroperoxy-eicosatetraenoic acid, Hx hepoxilin, Lt leukotriene, Lx lipoxin, MAGL monoacylglycerol lipase, MaR maresin, (N)PD1 (neuro)protection D1, oxo-EET oxo-eicosatetraenoic acid, PGD prostaglandin D metabolite, PGE prostaglandin E metabolite, PGF prostaglandin F metabolite, PGI prostacyclin, PGS prostaglandin E, D or F or prostacyclin synthase, PD protectin, RvD resolvin D series, RvE resolvin E series, Tx thromboxane, TxS thromboxane synthase, Trx trioxilin, from DHA and hydroxy-eicosapentaenoic ϖ-hydrolase

Fig. 3

Interplay in the synthesis and actions of the 2-acylglycerols and ethanolamides derived from ARA, DHA and EPA. The major pathway for AEA production begins with N-acyltransferase (NAT) transferring ARA from phosphatidylcholine (ARA-PC) to phosphatidylethanolamine (PE) to generate N-arachidonoyl phosphatidylethanolamine (NArPE), which is followed by hydrolysis by N-acyl phosphatidylethanolamine-selective phospholipase D (NAPE-PLD) to produce AEA. Further pathways include NAPE deacylation by the α/β-hydrolase domain containing 4 (ABHD4) and either the glycerophosphoarachidonoylethanolamide produced (GP-NAPE) cleaved by phosphodiesterase (PDE) to produce AEA or lyso-NAPE is hydrolysed by lyso-NAPE-phospholipase D (PLD) directly to AEA. NAPE can also be hydrolysed by phospholipase C (NAPE-PLC) to generate phospho-anandamide (PAEA), which is dephosphorylated to AEA by phosphatases such as protein tyrosine phosphatase (PTPN22). DHEA and EPEA production from phospholipid bound DHA and EPA appears to share the same pathways. Synthesis of 2-AG occurs from phosphatidylinositol-bound ARA (ARA-PI) via phospholipase C-β (PLCβ) and production of an ARA-diacylglycerol (DAG), which is hydrolysed by diacylglycerol lipases-α to produce 2-AG. Further pathways include dephosphorylation of 2-AG-lysophosphatidic acid (2-AG-LPA) by LPA phosphatase (2-LPA-P) or via phospholipase A₁ (PLA₁) converting PI to 2-arachidonoyl-lyso PI (2-AG-LPI) and then to 2-AG by lyso phospholipase C (lyso-PLC). The pathways of 2-DPG and 2-EPG production are currently unknown. 2-AG and AEA act at CB1 and CB2 receptors, GPR55 and PPAR, with AEA additionally acting at TRPV-1 (shown in grey). Dietary DHA and EPA enrichment decreases phospholipid ARA and increases phospholipid DHA and EPA, and favours production of DHA and EPA-derived endocannabinoids, whereas acute DHA and EPA treatment in vitro increases 2-AG. DHA and EPA also regulate CB1, CB2 TRPV-1 and PPAR receptor activity and levels. For detailed explanations, refer to the text