N-3 Polyunsaturated Fatty Acids and the Resolution of Neuroinflammation

1Inflammation is a critical process in host defense, facilitating tissue repair, regeneration and maintenance of homeostasis. However, if uncontrolled, it becomes a chronic low-grade inflammation that is characterized by the production of pro-inflammatory cytokines and adipokines leading to tissue damage and loss of function. This chronic low-grade inflammation is observed in many chronic pathologies including neurodegenerative diseases and also in aging. Hence it becomes a significant public health concern and constitutes a financial burden that impacts millions worldwide. It is thus of importance to find treatments that ensure resolution of inflammation in a specific time-limited manner. Nutrition has gained importance in recent years since nutriments such as n-3 polyunsaturated fatty acids (PUFAs) have anti-inflammatory properties. They exert their effect in part through their conversion into bioactive lipid mediators called specialized pro-resolving mediators (SPMs) that underlie most of the beneficial effects attributed to their precursors. Recently, attention has been paid to these derivatives in the regulation of neuroinflammation. In this review, we present an overview of brain n-3 PUFAs and their effect on neuroinflammation and of the formation and mechanisms of the action of SPMs in the brain.

2The brain contains high levels of PUFAs (25–30%) that are mainly docosahexaenoic acid (DHA, n-3 PUFA) (12–14% of total fatty acids) and arachidonic acid (AA, n-6 PUFA) (8–10% of total fatty acids). There are regional differences: the hippocampus and prefrontal cortex contain the highest DHA content whereas the hypothalamus has the lowest. There are also cellular differences: astrocytes, oligodendrocytes, and microglial cells (representing respectively 70%, 10–15% and 10–15% of brain glial cells; ) contain DHA in different proportions. DHA represents 10–12% in astrocytes, 5% in the oligodendrocytes and up to 2% in microglial cells. Neurons cannot synthesize long-chain PUFAs (LC-PUFAs) but can incorporate them in their membranes: DHA represents 8.3% of the total fatty acids in neurons.

3Numerous studies have discussed the transport of DHA through the blood–brain barrier (BBB). DHA enters the brain as unesterified DHA that is the major pool supplying the brain with DHA. However, the precise mechanisms of entry are still not fully described. Some transporters facilitate the uptake of DHA into the brain: fatty acid transport proteins (FATPs), fatty acid translocase (CD36) and major facilitator superfamily domain containing 2A (MFSD2A). recently reported that a member of the long-chain acyl-CoA synthetase (ACSL) family, ACSL6, is also involved in brain DHA uptake but its role has yet to be determined.

4This brain fatty acid composition can be affected by environmental factors such as nutrition, something to which individuals are continuously exposed. Indeed, the PUFA content in all brain structures is strongly impacted by the PUFAs present in the diet. A diet rich in n-3 PUFAs (DHA found in fish or its precursor, alpha-linolenic acid, found in vegetable oil) increases brain DHA in rodents. However, DHA supplementation is more effective than alpha-linolenic supplementation in increasing the DHA content in the brain. In rodents, DHA supplementation from 16 weeks to 16 months or from 20 to 22 months of age compensates a DHA decrease due to aging. On the other hand, a diet deficient in n-3 PUFAs decreases brain DHA in all brain structures, with the hippocampus, containing most DHA, being the most affected and the hypothalamus the least affected.

5Studies from Broadhurst and Crawford suggest that the amount of DHA incorporated into the brain depends on the complexity of the brain structure and on behavior development. A decrease in brain DHA induced by a deficient diet during gestation and lactation can be reversed by 2-month DHA supplementation at weaning. The organism also adapts its metabolism to an n-3 PUFA deficiency condition: the half-life of DHA increases in the brain to reduce its loss and the activity of the enzymes responsible for DHA conversion, the ∆6 desaturase and elongase, is increased in the liver. Brain cells are also impacted by dietary PUFA supply. An n-3 PUFA deficient diet decreases DHA in astrocytes, neurons and oligodendrocytes whereas n-3 PUFA supplementation increases DHA levels in glial cells. Moreover, we recently showed that the fatty acid composition of the microglial cells is also modulated by n-3 PUFA dietary intake during the gestation/lactation period. Maternal n-3 LC-PUFA dietary supplementation during gestation and lactation increases the DHA level in the offspring’s microglia at P21 as compared with a maternal diet that contains equilibrated levels of n-6 and n-3 PUFA precursors. Interestingly, maternal n-3 PUFA deficiency does not impact the DHA level suggesting that microglial cells are protected from n-3 PUFA deficiency.

6n-3 PUFAs have powerful anti-inflammatory properties. They play an important role in the regulation of the synthesis and release of pro-inflammatory mediators. Pro-inflammatory factors include interleukin-1 beta (IL-1β), interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α) and play a role in neuronal plasticity. If sustained, uncontrolled inflammation can lead to neuronal damage that is involved in many neuronal pathologies. Hence, limiting the inflammation and enhancing the resolution of inflammation is of great interest.

7In humans, the anti-inflammatory properties of n-3 PUFAs were first identified in epidemiological studies in Eskimos that consume a lot of n-3 LC-PUFAs from eating fish. Clinical studies have highlighted the beneficial effect of n-3 LC-PUFAs in chronic inflammatory and autoimmune diseases. Indeed, fish oil supplementation decreases pro-inflammatory cytokine expression, such as IL-1β in blood monocytes, and improves the symptoms of patients suffering from rheumatoid arthritis or multiple sclerosis. Moreover, DHA supply significantly decreases the circulating inflammatory markers and the oxidative stress. DHA supply for several months also improves the working and long-term memories in patients with moderate cognitive alterations.

8In animals, numerous studies have demonstrated the anti-inflammatory properties of n-3 PUFAs in the brain. In lipopolysaccharide (LPS)-, or IL-1β-, induced inflammation models, dietary n-3 LC-PUFA supplementation in adulthood prevents LPS-induced hippocampal increase of pro-inflammatory cytokines IL-1β and TNF-α in rats and mice. A dietary supply in eicosapentaenoic acid (EPA) decreases the production of LPS-induced pro-inflammatory cytokines IL-1β, IL-6 and TNF-α in the hippocampus and increases the production of anti-inflammatory cytokines IL-10 and IL-4 in both rats and mice. This modification is associated with a decrease in the phosphorylation of c-Jun and c-Jun N-terminal kinase proteins and in nuclear factor-kappa B (NFκB) that regulate inflammation. EPA dietary supply attenuates the activation of microglial cells and astrocytes triggered by an intracerebral IL-1β administration and increases the production of IL-10 in the hippocampus in rats. Following dietary DHA supplementation, there is an increase of DHA in the phospholipid and free fatty acid fractions. However, only the unesterified DHA is necessary to attenuate neuroinflammation. Nevertheless, the DHA phospholipid pool is an important source of unesterified DHA. On the contrary, in mice, an n-3 PUFA deficiency from the first day of gestation to weaning increases the expression of pro-inflammatory markers in the hippocampus and an alteration in the motility and the phenotype of microglial cells and alters synaptic plasticity. Using the model of intraperitoneal (ip) injection of LPS, we showed that IL-6 expression is strongly induced in n-3 PUFA-deficient mice whereas sickness behavior is down-regulated via the impairment of IL-6 signaling in the brain. Moreover, when the deficiency continues in adulthood, it alters GABAergic, dopaminergic and cholinergic neurotransmission and anxiety-related, depressive-like behaviors that can be compared with autistic behavior in adults. Following ip injection of LPS, n-3 PUFA-deficient mice display altered hippocampal synaptic plasticity that likely contributes to spatial memory impairment and higher glucocorticoid levels. On the contrary, an n-3 PUFA dietary supply improves emotional behavior alteration and memory deficit in rats, restores social and memory performance altered in autism models and improves depressive symptoms in both mice and rats. Of note, n-3 LC-PUFA dietary supplementation attenuates depressive symptoms in depressed patients presenting inflammation. This protective effect seems to be linked to EPA which could target microglia to reduce neuroinflammation. n-3 LC-PUFAs could also modulate neuroinflammation through their effect on the HPA axis. With Alzheimer’s disease, β-amyloid triggers depressive symptoms through neuroinflammatory processes that could be improved by dietary n-3 PUFAs. Altogether these data reinforce the importance of n-3 PUFAs as regulators of inflammation and associated depressive symptoms.

9Another way to modulate neuroinflammation is to administer n-3 PUFAs directly into the brain or peripherally. Indeed, a 24-hour intracerebroventriculary (icv) DHA brain infusion attenuates hippocampal neuroinflammation initiated by icv LPS in mice. In a mouse model of cerebral ischemia, DHA icv administration inhibits NFκB activation and cyclooxygenase-2 (COX-2) expression in the hippocampus. Moreover, intrathecal injection of DHA decreases microglial activation, mitogen-activated protein kinase (MAPK) phosphorylation and the production of pro-inflammatory cytokines in the spinal cord of mice. In addition, in a model of traumatic brain injury in rats, n-3 PUFA ip injection attenuates microglial-induced inflammation by inhibiting the NFκB pathway.

10Dietary n-3 LC-PUFA supplementation requires the use of fish oil. However, fish oil may provide confusing factors such as vitamins, for example. Thus, the use of Fat-1 transgenic mice that convert n-6 to n-3 PUFAs through a desaturase from C. elegans is a model that enables the elimination of confusing factors provided by the diet. Such mice have higher DHA levels in the hippocampus and cortex. They express less COX-2 (involved in the production of lipid mediators) in the cortex than wild type mice. They are protected against cognitive deficits induced by ip LPS injection through a decrease in neuroinflammation. This is associated with a decrease in microglial activation.

11In vitro studies have shown that n-3 PUFAs have an inhibitory effect on the production of pro-inflammatory cytokines in microglial cells. Indeed, in the microglial cell line or primary culture microglial cells, DHA prevents LPS-induced NKκB activation and then cytokine production by inhibiting LPS receptor presentation and decreases the oxidative stress, nitric oxide (NO) production and inducible nitric oxide synthase (iNOS) expression. Moreover, DHA prevents LPS-induced MAPK phosphorylation (MAPK pathway playing essential roles in the expression of inflammatory molecules) and induces peroxisome proliferator-activated receptor (PPAR)-γ nuclear translocation that exerts anti-inflammatory effects. Of interest, EPA also affects the production of TNF-α, IL-6 and NO by inhibiting NFκB phosphorylation via sirtuin-1 (SIRT-1). It is likely that DHA attenuates the inflammatory response in LPS-activated microglia by remodeling lipid bodies that are dynamic organelles in which DHA is incorporated and by altering their interplay with mitochondria and other associated organelles. In addition, DHA and EPA are able to enhance myelin or amyloid β (Aβ) peptide phagocytosis that is associated with a shift in microglial polarization toward the beneficial M2 phenotype and to a decrease in pro-inflammatory cytokine production.

12This modulation of neuroinflammation induced by n-3 LC-PUFA supply is attributed, in part, to SPM synthesis. In the various phases of inflammatory response, prostaglandins, leukotrienes and thromboxane are synthesized first (Figure 1). They permit the propagation of inflammation. They also stimulate the synthesis of SPMs with pro-resolutive properties.

13Many of the n-3 PUFA-derived immunomodulators that orchestrate the inflammatory response are lipids. Some of them are involved in the regulation of inflammation (prostaglandins, thromboxanes, leukotrienes, etc.) and others are implicated in the resolution of inflammation (resolvins, protectins and maresins). There is a temporal shift in the lipid mediator synthesis from the initiation of inflammation to the resolution that allows the formation of different lipid mediators at different times (Figure 1). Here we have focused on SPMs, which are of great interest since they permit a return to homeostasis. Charles N. Serhan first identified these SPMs at the periphery and characterized their anti-inflammatory and pro-resolutive properties. SPMs actively orchestrate and finely tune the inflammatory response. They decrease pro-inflammatory cytokines and increase anti-inflammatory cytokines, accelerate the phagocytosis of cellular debris and dead cells without immune suppression. A failure in the resolution of inflammation is detrimental for tissue and contributes to a chronic inflammatory response.

14Free (unesterified) n-3 LC-PUFAs are released from membrane phospholipids through the action of phospholipases A2 (PLA2) in response to stimulation. DHA is hydrolyzed by calcium independent PLA2 (iPLA2) from phospholipids and plasmenylethanolamine-PLA2 from plasmalogens. After this step, n-3 LC-PUFAs undergo an enzymatic conversion to generate SPMs (Figures 2 and 3).

15n-3 PUFA-derived SPMs are synthesized mainly from DHA and EPA via COX-2, lipoxygenases (LOX) and CYP450 monoxygenases (CYP450). In the brain, 15-LOX, 12/15-LOX and 5-LOX are the most abundant LOX and are widely distributed, suggesting the potential production of SPMs. They are expressed in the second step of inflammation in the hippocampus. 15-LOX is involved in neurodegeneration and neurotoxicity due to the increased oxidative stress it generates in models of Alzheimer’s disease and brain ischemia. However, it is also implicated in neuroprotection. Indeed, it increases the production of 12-HETE and 15-HETE that promote the activation of PPARγ that is neuroprotective through its anti-inflammatory properties. The inhibition of 15-LOX induces hippocampus-dependent cognitive alterations. Indeed, it prevented long term potentiation, a major molecular mechanism that describes the strengthening of synapses and underlies learning and memory. Moreover, 15-LOX deletion drastically decreases SPM production in the brain while SPMs also modulate synaptic plasticity. In the brain, the inducible COX-2 is activated via an NFκB pathway. COX-2 catalyzes the first step of the synthesis of prostaglandins and thromboxanes derived from n-6 PUFAs that contribute to the initiation of inflammation. COX-2 also produces hydroxyl fatty acids from DHA in both an aspirin-dependent and aspirin-independent manner. In response to LPS, COX-2 is rapidly expressed in the hippocampus. Inhibition of COX-2 delays resolution of acute inflammation. CYP450s play a complex role in inflammation by producing n-6 PUFA-derived bioactive mediators such as epoxides that are anti-inflammatory in monocytes and macrophages.

16DHA is the precursor of resolvins D1-6 (RvD1-6), neuroprotectin D1 (NPD1) and maresins 1–2 (Mar1-2) which all have pro-resolutive and anti-inflammatory properties. RvD1-6 are synthesized from DHA but RvD1 is the most studied because it has powerful anti-inflammatory and pro-resolutive properties. DHA is converted into monohydroxy DHA, 17-hydroxy docosahexaenoic acid (17-HDHA) by acetylated COX-2, CYP450 and 15-LOX and then into RvD1 by 5-LOX. DHA is also converted into di-hydroxy-DHA, termed protectin D1 (PD1) or neuroprotectin D1 (NPD1) when produced in the central nervous system by 5- and 15-LOX. Acetylated COX-2 permits the synthesis of aspirin-triggered PD1 (AT-PD1) which has powerful protective effects. DHA is transformed into Mar 1-2 by 12/15-LOX via the synthesis of 14-HDHA.

17EPA is the precursor of resolvins E1 (RvE1), E2 and E3 that have many biological roles. It is converted by aspirin-triggered acetylated COX-2 or CYP450 into 18R-hydroxyeicosapentaenoic acid (18R-HEPE), that is transformed into RvE1 or E2 by 5-LOX or into RvE3 by 15-LOX.

18Little is known concerning the pharmacokinetics and dynamics of oxylipins. They are synthesized in situ, reinforcing the importance of the fatty acid composition of the brain. They are not stored, but produced on demand and they are unstable, being rapidly metabolized.

19The structure of all derivatives is highly preserved in the evolution from fish to humans suggesting their great bioactive role in all organ systems. Dysfunction of SPM production can be due to insufficient EPA and DHA supply leading to inadequate production of SPMs or to the polymorphism of the enzymes involved in their synthesis or to a defect in the binding of SPMs to their receptors. In humans, reduced SPM production is associated with chronicity and the magnitude of persistent inflammation. SPMs have multiple biological roles in the maintenance of homeostasis.

20In human serum, the DHA-derivatives represent 30.7% of the identified SPMs. RvD1, PD1 and Mar1 were detected at 30.9 pg/mL, 5.6 pg/mL and 21.2 pg/mL, respectively. They act in the nanomolar or picomolar range as compared with DHA and EPA acting in the micromolar range.

21RvDs have been identified in mice peritoneal exudates. They have many properties at the periphery: protection against bacterial infection, prevention of leucocyte infiltration, inhibition of the production of cytokines, etc.. RvD1 and its precursor metabolites have been detected in the brain.

22In vitro, 17-HDHA has been found in glial cells after DHA and inflammatory stimulus. In vivo, RvD1 has been identified in mouse brain following cerebral ischemia. Intravenous (iv) DHA injection increases RvD1 levels in rats suggesting the conversion of DHA into RvD1. RvD1 acts through the lipoxin A4 receptor/formyl peptide receptor 2 (ALX/Fpr2) in rodents and the G protein coupling receptor 32 (GPR32) in humans in the picomolar range but induces biological effects in the nanomolar range. An overexpression of ALX/Fpr2 or GPR32 increases in phagocytosis by macrophages whereas their deletion decreases the phagocytosis response. RvD1 regulates the expression of specific micro-RNA (miR) to control the intensity and the length of inflammation via the regulation of target genes such as inflammatory cytokine genes. RvD1 increases the expression of miR-21, miR-146b and miR-219 and decreases the expression of miR-208a in macrophages and peritoneal exudates in mice. RvD1 modulates miR-155, miR-146, miR-21 and miR-219 in microglial cells. These miRs have different biological roles: miR-21 is essential to the production of anti-inflammatory cytokine IL-10, miR-146 regulates the transcription of cytokines, chemokines and their receptor, miR-219 decreases the transcription of TNF-α and miR-208a regulates the activation of NFkB.

23RvD1 controls the inflammatory response in many animal models via its anti-inflammatory and pro-resolutive properties. In rats, endogenous RvD1 levels decrease at the beginning of inflammation and then increase during the resolution phase. Recent studies describe the anti-inflammatory properties of RvD1 in microglia and astrocyte cell cultures. Indeed, in BV2 microglia cell culture, RvD1 promotes the IL-4-induced M2 phenotype and inhibits LPS-induced pro-inflammatory cytokines. In astrocyte cell culture, RvD1 attenuates LPS-induced TNF-α. In vivo, in a model of remote damage, Bisicchia et al. recently show in rats that RvD1 promotes functional recovery and reduces neuroinflammation via miRs.

24The effect of RvD1 has been studied in patients suffering from Alzheimer’s disease. This pathology is characterized by an increase in microglial activation and in pro-inflammatory cytokine production in the brain. Interestingly, RvD1 levels in the cerebrospinal fluid are positively correlated with the enhancement of cognitive functions of patients with dementia. Indeed, RvD1 may be involved in Aβ phagocytosis. This has been shown in vitro in macrophages isolated from Alzheimer’s patients. Thus the decrease in RvD1 levels in Alzheimer patients’ brains could contribute to the evolution of the disease.

25RvD1 attenuates the pro-inflammatory status in the central nervous system. Indeed, an intrathecal injection of 17R-HDHA decreases TNF-α release in the spinal cord in rats. Orr et al. show that icv injection of 17S-HDHA into mice decreases the expression of hippocampal pro-inflammatory cytokines IL-1β and TNF-α induced by LPS acute icv injection. However, these authors detected NPD1 but did not detect RvD1, suggesting that the effect of 17S-HDHA was rather due to the conversion of 17S-HDHA into NPD1. RvD1 is also able to stimulate phagocytosis in macrophages. Indeed, DHA and RvD1 induce the polarization of macrophages toward an M2 phagocytic phenotype in mice.

26Studies have highlighted the protective role of RvD1 in the occurrence of cognitive deficits. Terrando et al. showed that an ip injection of 17-HDHA restores transmission and synaptic plasticity and prevents astrogliosis and cognitive decline in a systemic inflammation model in mice. Conversely, an inhibition of 15-LOX, associated with a decrease in RvD1, alters synaptic plasticity and working memory as demonstrated in rats. RvD1 plays also a role during the recovery phase following cerebral ischemia. Indeed, the precursor of 17-HDHA, 17-hydroperoxy docosahexaenoic acid (17-HpDHA), accumulates in the hippocampus of mice. An ip chronic administration of 17R-HDHA synthesized by acetylated COX-2, prevents cognitive deficits and attenuates motor disorders but doesn’t ameliorate microglial activation and sleep quality in mice. Moreover, Fat-1 mice that have more brain n-3 LC-PUFAs, have higher hippocampus RvD1 levels, associated with less cognitive deficits, a better neuronal survival, a decrease in astrocyte and microglial activation and a reduction in pro-inflammatory status following brain ischemia.

27Studies have also highlighted the protective role of resolvins in the depressive-like behavior in rodents. Some of them have been recently reported by Furuyashiki et al.. An icv injection of RvD1, D2, E1, E2, or E3 significantly decreases LPS-induced depressive-like behavior in mice. Moreover, the occurrence of depressive-like behavior associated with pain can also be prevented by an intrathecal injection of 17R-HDHA that is associated with the decrease of pain perception and a restoration of dopamine and glutamate levels in the brain both in rats and mice. RvD1 and D2 have also positive effects in chronic mild stress-induced depression and in post-myocardial infarct depression in rats and mice.

28The effects of RvD1 were tested on different brain cells. In microglial cells, RvD1 potentiates the effect of the anti-inflammatory cytokine IL-4 in the activation of M2 phenotype of microglia. Moreover, we showed that RvD1 decreases LPS-induced pro-inflammatory cytokines (TNF-α, IL-6 and IL-1β) gene expression in microglial BV2 cells via the modulation of miRs. This suggests its pro-resolutive activity in microglia. In astrocytes, RvD1 decreases TNF-α release induced by LPS injection. In neurons from spinal nods, RvD1 increases neurite outgrowth. All these studies suggest that RvD1 can play a central role in the regulation of neuroinflammatory pathologies.

29Other D resolvins have been identified in rodent brain: RvD2, RvD4 and RvD5. RvD2 limits the activation of microglial cells and inhibits the TLR4/NFκB pathway. Moreover, chronic intrathecal RvD2 injection prevents the behavioral alterations induced by LPS central injection. It also prevents depressive-like behavior induced by ip LPS injection by regulating mammalian target of rapamycin complex 1 (mTORC1) complex. RvD5 level is decreased in the brain of Alzheimer’s disease patients but its role has not yet been identified.

30Di-hydroxy-DHA termed protection D1 (PD1) has been identified in blood, peritoneal neutrophils and neuroprotectin D1 (NPD1) in the brain in response to zymosan in mice. Marcheselli et al. have measured NPD1 production in mice hippocampus following brain ischemia. One receptor for NPD1 has recently been identified as GPR37. NPD1 inhibits the oxidative stress in retinal epithelial cells and stimulates their proliferation. NPD1 is also able to inhibit neovascularization via microglial cell ramification in mice. In the central nervous system of mice, the NPD1 level greatly increases in the hippocampus following brain ischemia or acute central LPS injection. Hence, NPD1 limits neutrophil infiltration, inhibits NFκB and then decreases pro-inflammatory gene expression. NPD1 levels decrease in the hippocampus of Alzheimer’s disease patients. It plays a role in cellular survival via anti-apoptotic protein induction and attenuates pro-inflammatory responses following Aβ exposure via NFκB regulation.

31Mar1 has been identified in mice peritoneal macrophages. Its receptor has not been identified yet. Mar1 is involved in the resolution of inflammation, prevents neutrophil infiltration, and increases the phagocytosis of apoptotic neutrophils by macrophages in a peritonitis murine model. In a murine model of colitis, Mar1 decreases the expression of pro-inflammatory cytokines by inhibiting the NFκB pathway and activating the M2 phenotype in macrophages. Mar1 and its precursor 14-HDHA have recently been identified in the hippocampus of mice. In post-mortem brain of Alzheimer’s disease patients, the Mar1 level is decreased. In this pathology, its role is to stimulate Aβ plaque phagocytosis by microglial cells and to decrease inflammatory marker levels. Hence, Mar1 may play an important role in the pathogenesis of Alzheimer’s disease. In cerebral ischemia in mice, Mar1 icv injection decreases inflammation and mitochondrial damage and also reduces neurological deficits via activation of SIRT-1 signaling. After spinal cord injury in mice, iv injection of Mar1 promotes resolution of inflammation (reducing pro-inflammatory cytokines, silencing pro-inflammatory signaling cascades and enhancing the M2 repair macrophage phenotype) and functional recovery. Mar1 also decreases in vitro neuronal death.

32RvE1, and its precursor 18-HEPE, have been detected in the hippocampus of rats and mice. In human serum, the EPA-derivatives represent 25.9% of the identified SPMs.

33RvE1 has been initially identified in mouse exudates. RvE1 directly binds to its receptor ChemR23 or CMKLR1 (chemokine-like receptor 1). It is also a partial agonist of LTB4 receptor (BLT1). In the central nervous system, ChemR23 has been identified in the prefrontal cortex, hippocampus and brainstem, in microglial cells and in neurons. It is highly expressed in neurons, in microglial cells and in astrocytes in the post-mortem hippocampus of Alzheimer’s patient brain. This increase could be due to a compensatory mechanism to counter-balance the decrease in RvE1 in such patients. In vitro, RvE1 plays also a direct role in microglial cells by inhibiting microglial activation and pro-inflammatory cytokine release. RvE1 and its precursor, 18R-HEPE, exert anti-inflammatory and anti-apoptotic properties. Indeed, they restore mitochondrial dysfunction induced by inflammation in mononuclear blood cells. In vivo, RvE1 modulates the inflammatory profile and microglial activation in mice. Intraperitoneal injection of RvE1 also modulates inflammation (by reducing Il-1β, Il-6 and IL-10 levels in the prefrontal cortex) and decreases the measures of Aβ pathology in a murine model of Alzheimer’s disease. RvE1 and RvE2 centrally administered also reduce the LPS-induced depressive-like behavior through ChemR23 in mice. RvE1, a total agonist of ChemR23, is more effective than RvE2, which is only a partial agonist of this receptor.

34Resolution of inflammation is an active process involving the regulation of the synthesis of numerous mediators in a tightly coordinated manner. The balance between n-3 PUFA-derived SPMs and the pro-inflammatory mediators determines the duration of the inflammatory response and the timing of resolution.

35We have previously shown that PUFA consumption leads to modifications in PUFA levels in the brain. The PUFA derivative levels and their biosynthetic enzyme expression also depend on dietary PUFAs.

36SPM levels in peripheral organs and the brain are modulated by different dietary supplies of PUFAs (Figure 4). In humans supplemented with 2.4g/day of n-3 LC-PUFAs for 3 weeks (EPA+DHA), SPM levels are generally found to be between 20 and 200 pg/mL. A shorter supplementation period (5 days) also increased DHA- and EPA- derived SPMs in plasma, especially 18-HEPE, RvE1, 17- and 14-HDHA. In another study, supplementing patients with a higher fish oil supplement (17.6g/day EPA+DHA) for 24h, only PD1 is detected in the plasma and this increases from 1.0–1.2 pg/mL to 2.7–4.1 pg/mL. These differences in SPM levels in human plasma could be due to differences in study design, dose, duration of n-3 fatty acid supplementation and patient characteristics, etc.. Moreover, dietary supplementation in DHA (1.7g/j) and EPA (0.6g/j) for 6 months increased the RvD1 released by blood mononuclear cells in Alzheimer’s disease patients as compared with controls. Results from intervention studies show that EPA and DHA dietary supplementation increases EPA- and DHA-derived oxylipins although with high inter-individual variances.

37In animals, a 3-week fish oil supplementation in arthritic mice increases synthesis of n-3 PUFA-derived SPMs associated with a diminished production of pro-inflammatory mediators. n-3 PUFA dietary supplementation (140 mg EPA+110 mg DHA/kg animal) in aged rats for 10 weeks enhances the production of lipid mediators derived from DHA and EPA in the cerebral cortex. DHA and EPA supply also decreases the n-6 PUFA-derived metabolites (which are mainly pro-inflammatory) and increases the n-3 PUFA-derived metabolites in the plasma and at the periphery both in mice and rats. A diet enriched with fish oil (containing 9% EPA + 9% DHA) for 8 weeks in mice increases plasmatic RvD1. In Fat-1 mice, brain EPA increase is accompanied by an increase in EPA-derived HEPEs. Conversely, an n-6 PUFA dietary supply from weaning to 15 weeks of age increases the n-6 PUFA metabolites and decreases the EPA-derived metabolites in the cortex and the plasma of rats. Recently, we showed that, in mice, n-3 LC-PUFA supplementation from weaning for 2 months induces n-3 LC-PUFA enrichment in the hippocampus and subsequently an increase in n-3 PUFA-derived SPMs and a decrease in n-6 PUFA-derived mediators. Moreover, in response to LPS, n-3 LC-PUFA-deficient mice present a pro-inflammatory lipid mediator profile whereas n-3 LC-PUFA-supplemented mice display an anti-inflammatory profile in the hippocampus. Indeed, the consumption of n-3 LC-PUFAs increases the EPA and DHA derivatives. Hashimoto et al. found the same in the cortex of aged rats. These results differ from those of Trépanier et al. who found that, in a model of Fat-1 mice, a greater increase in brain DHA induced by dietary supply has no effect on the resolution of inflammation after an icv LPS injection.

38SPM production is also finely tuned by the regulation of the enzymes in their biosynthesis pathway. A 15-week n-3 PUFA-deficient diet increases the COX-2 expression in the prefrontal cortex of rats, suggesting an increase in AA-derived pro-inflammatory mediator levels. Conversely, a 15-week n-6 PUFA-deficient diet decreases brain COX-2 expression in rats. Interestingly, Taha et al. showed that rats fed on an n-6 PUFA-deficient diet for 15 weeks are protected from an increase in COX-2 observed in LPS-treated n-3 PUFA-deficient animals. However, we showed that 2 months’ n-3 LC-PUFA dietary supplementation was not sufficient to modulate COX-2 expression in the brain of mice. An n-3 LC-PUFA diet increases 15-LOX expression in the hippocampus without changes in the 5-LOX expression. The increase in 15-LOX in the brain has also been described following a 15-week n-6 PUFA-deficient diet in rats. 15-LOX is responsible for many n-6 and n-3 PUFA derivatives. Those synthesized from AA drastically decrease, and those produced from DHA increase, following a diet enriched in n-3 PUFAs in both mice and rats.

39Results on 15-LOX functions on inflammation regulation are conflicting. 15-LOX was initially described as deleterious in neurodegenerative pathologies because it increased the oxidative stress and neuronal degeneration. It alters the mitochondrial function of neurons and then induces neuronal death. However, recent studies demonstrate the protective role of 15-LOX, particularly in cognition. An increase in its expression is associated with a better working memory. Moreover, an increase in 15-LOX expression during cerebral ischemia is essential for the recovery of neurological functions after the ischemic event. These results suggest that 15-LOX has a beneficial role in acute inflammation (ischemia, LPS) and is deleterious in chronic inflammation (neurodegenerative pathologies). SPM production is also partly regulated by the availability of free PUFAs that are their precursors. Indeed, PLA2 activity also depends on PUFA dietary supply. An n-6 PUFA-deficient diet increases the iPLA2 that is responsible for the release of DHA from the membranes and a decrease in calcium sensitive cytosolic PLA2 (cPLA2) associated with the hydrolysis of AA from the membranes.

40Lipid nutrition, to which people are exposed throughout their lives, seems to play a major role in the synthesis of bioactive SPMs.

41Numerous studies have highlighted that inflammation modulates lipid mediator synthesis at the periphery and in the brain (Figure 4).

42In humans, Wang et al. showed that the RvD1 level in the cerebrospinal fluid (CSF) of Alzheimer’s disease patients is positively correlated with cognitive function. Moreover, 15-LOX was decreased in the CSF suggesting an alteration of the resolution of inflammation in Alzheimer’s patients.

43In rats, brain ischemia increases the production of 5 of mono-, di- and tri-hydroxy-DHA derivatives. However, LPS icv injection does not impact the RvD1 level. We have recently shown that, in mice, ip LPS injection modifies the n-6 PUFA derivative profile but not n-3 LC-PUFA derivatives.

44In vitro, most studies have been performed on peripheral immune cells. The lipid mediator profile in rodent neutrophils and macrophages changes with macrophage phenotype M1 or M2, or with the inflammatory stimulus in neutrophils. Indeed, M2 macrophages produce more D resolvins, protectins and maresins and less AA-derived pro-inflammatory mediators than M1 macrophages.

45Inflammation more drastically alters the expression of biosynthetic enzymes than an n-3 LC-PUFA dietary supply. In rats, traumatic brain injury increases the COX-2, 5-LOX, 15-LOX and CYP450 expression in the hippocampus and cortex, suggesting an alteration of all lipid mediator biosynthesis pathways. LPS also regulates the expression of these enzymes. An icv LPS injection increases COX-2 expression and activity in the brain in mice and rats. Moreover, in in vitro human monocytes and dendritic cells, anti-inflammatory cytokines IL-4 or IL-13 increase the production of 15-LOX and decrease the production of 5-LOX.

46Neuroinflammation also modifies SPM receptor expression. Indeed, we show that LPS increases significantly the expression of RvD1 and RvE1 receptors (ALX/Fpr2 and ChemR23, respectively) in BV-2 microglial cells. The increase in ALX/Fpr2 expression was also detected in monocytes, in the hippocampus and in the cortex and in microglial cells in response to an inflammatory stimulus. Then, inflammation activates the SPM signaling pathway in the brain to regulate the inflammatory response.

47Several possibilities can be considered to translate the findings described above and then attenuate the inflammatory tone, amplitude and duration of inflammation. The first one is to increase the local production of n-3 LC-PUFA-derived SPMs. We see that dietary means is a good way to modulate the level of the fatty acids from which they are synthesized and then to modify their synthesis. The SPM profile synthesized by each individual could be responsible for the differences in the effects of n-3 PUFA obtained in humans. SPM profiles should be established in patients with different acute and chronic inflammatory pathologies, and in mice under the same conditions to find markers of neuroinflammation in the plasma that can be transposed to humans. Thanks to new technologies in liquid chromatography mass spectrometry (LC-MS/MS), specific mediators produced during physiological and non-physiological conditions should be identified, allowing patient stratification according to disease severity. It could be interesting to determine an individual metabolomic profile to define personalized nutrition (n-3 PUFAs and doses) allowing an increase of n-3 PUFA-derived SPMs in the target tissue. Indeed, there are individual differences in diets and in n-3 PUFA supplementation and also in nutrient metabolism and biological responses to food/nutrients. The aim of personalized nutrition is to increase health using nutrition by delivering specific personalized intervention suited to each individual based on the individual’s nutritional phenotype, metabolic profile, and environmental factors in order to prevent and treat chronic disease. Personalized nutrition can also be applied to healthy people. It is nowadays accessible because of a better understanding of the mechanisms of the effect of nutrition on health and also because of the progress in technologies enabling the identification of specific markers. Personalized nutrition has already shown its efficacy, especially in the Food4Me study involving >1600 participants from 7 European countries and in a systematic review and meta-analysis showing a greater efficacy of personalized nutrition in changing diet than a conventional approach.

48The second possibility for taking advantage of research on the resolution of inflammation is to administer exogenous SPMs. Serhan defines a new concept of resolutive pharmacology targeting the development of SPM analogs, resistant to local inactivation, to stimulate natural circuits of resolution. The objective of this new therapeutic pathway is to administer these analogs in association with classical therapy in order to decrease the doses, thus limiting the secondary effects. A clinical trial has reported for the first time the efficacy of an RvE1 analog in patients with dry eye symptoms (Resolvyx Pharmaceuticals). These encouraging results should be extended to the use of such molecules to treat other inflammatory diseases.

49In the investigation of new anti-inflammatory treatments without the secondary effects of traditional therapy, SPMs are promising therapeutic compounds: they are of natural origin and are active at low concentrations (nM) as compared with their precursor (µM). SPMs are detectable in the brain and their level can be modulated by dietary supplementation. They have potent anti-inflammatory and pro-resolutive properties and we confirm the main role of nutrition as an environmental factor that greatly influences the inflammatory response. It is important to determine if inflammatory pathologies are due to unresolved inflammation, attributed to a decrease in n-3 LC-PUFA dietary intake leading to a decrease in SPM levels, to an enzyme or receptor polymorphism, to a de-regulation of SPM receptors or to a decrease in their expression. We should also determine if these mechanisms could be restored by n-3 LC-PUFA or SPM analog administration.

50All authors (CJ, CR, SL) contributed to the writing of the manuscript.

51The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.