Dietary polyunsaturated fatty acids (PUFAs) have gained more importance these last decades since they regulate the level of long-chain PUFAs (LC-PUFAs) in all cells and especially in brain cells. Because LC-PUFAs, especially those of the n-3 family, display both anti-inflammatory and pro-resolution properties, they play an essential role in neuroinflammation. Neuroinflammation is a hallmark of neurological disorders and requires to be tightly controlled or at least limited otherwise it can have functional consequences and negatively impact the quality of life and well-being of patients. LC-PUFAs exert these beneficial properties in part through the synthesis of specialized pro-resolving mediators (SPMs) that are involved in the resolution of inflammation and to the return of homeostasis. SPMs are promising relevant candidates to resolve brain inflammation and to contribute to neuroprotective functions and lead to novel therapeutics for brain inflammatory diseases. Here we present an overview of the origin and accumulation of PUFAs in the brain and brain cells and their conversion into SPMs that are involved in neuroinflammation and how nutrition induces variations in LC-PUFA and SPM levels in the brain and in brain cells.
- long-chain polyunsaturated fatty acids (LC-PUFAs)
- docosahexaenoic acid (DHA)
- eicosapentaenoic acid (EPA)
- specialized pro-resolving mediators (SPMs)
- brain cells
Polyunsaturated fatty acids (PUFA) are essential fatty acids including precursors and long-chain PUFAs (LC-PUFAs). Precursors have to be provided by the diet because they cannot be produced by mammals . They can be converted into LC-PUFAs. However, as the conversion rate is very low in human [2, 3], it is recommended to consume also LC-PUFAs that modulate LC-PUFA composition of brain and brain cells. Altered dietary intake and/or PUFA metabolism has been reported to be involved in a number of neurological disorders
2. PUFAs in the brain and brain cells
2.1 Origin of PUFAs in the brain
2.1.1 Metabolism of PUFAs
PUFAs are fatty acids containing more than one double bond on their carbon chain. They are classified into two main series, the n-6 PUFAs and the n-3 PUFAs depending on the position of the first double bond from the methyl terminal end. N-6 PUFAs have the first double bond at the 6th carbon and n-3 PUFAs at the 3rd. Of these two series, linoleic acid (LA) and alpha-linolenic acid (ALA) are the precursors and are essential fatty acids because mammals cannot synthesize them.
2.1.2 Dietary origin
The precursors LA and ALA are found mainly in vegetables, oils, and seeds (60% of LA in sunflower oil and 10% of ALA in rapeseed oil, for example) (Figure 1) [16, 17]. Although human can synthesize LC-PUFAs from these precursors, the conversion efficiency is very low (<5%) even in healthy adults [2, 3]. Hence, the main part of LC-PUFAs comes from the diet. AA is found in meats (5–10%) and eggs (15%) [18, 19] and DHA and EPA are found in fatty fishes (18.7% EPA + DHA in salmon, 32.9% EPA + DHA in tuna, for example) (Figure 1) . However, lean fishes (sole, codfish, etc.) contain also appreciable amounts of DHA and EPA. Therefore, LC-PUFAs dietary intakes are crucial to maintain adequate levels of LC-PUFAs in membranes. That is why there are dietary recommendations for PUFAs. Dietary intakes recommend ~500 mg/day in EPA and DHA (2 portions of fish/week) and a ratio LA/ALA close to 4–5 to meet all the needs of the body into DHA and to protect against cardiovascular disease risk [21, 22]. Preclinical and clinical studies indicate that increasing dietary ALA and reducing LA are beneficial in increasing n-3 LC-PUFA bioavailability [23, 24]. Despite these recommendations, dietary n-3 PUFA intake is insufficient, both for the precursor ALA and the LC-PUFAs DHA and EPA. Indeed, in the western diet, there is an imbalance between n-6 and n-3 PUFAs leading to an n-3 PUFA consumption 12–20 times lower than n-6 PUFA consumption [10, 25]. This is due to the increased industrialization in the developed nations accompanied by changes in dietary habits. It is particularly characterized by an increase in LA and AA together with a decrease in ALA and DHA. A high intake of LA associated with a low intake of ALA leads to the accumulation of n-6 PUFAs, including AA. In case of severe n-3 PUFA deficiency, the expression of desaturases and elongases are upregulated in the liver in order to compensate and provide DHA to the brain [26, 27]. In addition, under dietary n-3 PUFA deficiency, the half-life of brain DHA is increased by twofold as under balanced diet . Dietary lipids, representing 35–40% of total energy intake, are essentially found (90–95%) in the form of triglycerides (a glycerol backbone with three fatty acids). They are also found in the form of phospholipids (in which the 3-position on the glycerol is replaced by a phosphorylated alcohol function). There is still a debate concerning the better form to enhance EPA/DHA bioavailability, krill oil as a source of phospholipids or fish oil as a source of triglycerides [29, 30]. More studies have to be performed.
2.2 PUFA content in the brain
The brain contains high levels of PUFAs (25–30%) that are mainly DHA (n-3 PUFA) (12–14% of total fatty acids) and AA (n-6 PUFA) (8–10% of total fatty acids) [12, 31, 32, 33, 34, 35]. Most LC-PUFAs accumulate during brain development, especially during the perinatal period: in humans between the beginning of the third trimester of gestation and 2 years and in rodents between the 7th and the 21st postnatal day [36, 37, 38]. These periods correspond to the rapid neuronal maturation, synaptogenesis, and gray matter expansion [39, 40]. The brain LC-PUFA content differs in brain structures [12, 31, 35, 41, 42], for example, in the adult C57Bl6/J mice, AA is higher in hippocampus (10.2%), followed by the prefrontal cortex (9.7%), the hypothalamus (8.5%), the cortex (7.7%), the cerebellum (6.5%), and the brain stem (5.5%) . DHA is higher in the prefrontal cortex (14.3%) and in the hippocampus (13.7%), followed by cerebellum (12.2%) and cortex (11.9%), hypothalamus (10.1%), and brain stem (8.2%) . Then the AA/DHA ratio varies from 0.75 to 0.85 in the hypothalamus and hippocampus to 0.54 in the cerebellum. These variations may be due to different LC-PUFA entry mechanisms into the brain or to different incorporation into membranes of cells composing the structure considered. These levels are comparable in human: prefrontal cortex contains between 12.3 and 15.9% of DHA in rats and mice and between 14.1 and 15.9% [12, 35, 43, 44].
2.3 PUFA content in brain cells
Brain cells comprise neurons and glial cells: 70% astrocytes, 10–15% oligodendrocytes, and 10–15% microglial cells . Very few studies reported the fatty acid composition of the individual cells. Bourre et al. determined the fatty acid composition in neurons, astrocytes, and oligodendrocytes in 15- or 60-days rats and confirmed previous results obtained in 1973 and 1981 [46, 47, 48, 49]. We recently described the fatty acid composition of microglial cells in 21-days mice [46, 50].
Neurons cannot synthesize LC-PUFAs but can incorporate them in their membranes. They contain 8.2–8.3% DHA and 2.2–2.8% n-3 DPA (22:5 n-3) for n-3 LC-PUFAs, 10.3–15.1% AA, 2.2% n-6 DPA, and 1.0–2.1% adrenic acid (22: 4 n-6) for n-6 LC-PUFAs . They contain 3.1–6.9% LA. Then the ratio n-3/n-6 is 0.46–0.50.
Astrocytes are supportive glial cells that play many roles including synaptic transmission and energy metabolite furniture to different neural elements. They respond to all forms of central nervous system (CNS) insults through a process referred to as reactive astrogliosis. Dysfunctions of astrocytes result in pathological changes in the CNS. Astrocytes contain 10.6–12.1% DHA and 0.7–1.3% of n-3 DPA for n-3 LC-PUFAs and 10.1–10.3% of AA, 2.5–2.7% of n-6 DPA and 2.4–2.7% adrenic acid (22:4 n-6) . They contain few PUFA precursors: only 1.2–1.4% of LA and no ALA. The ratio n-3/n-6 is 0.72–0.76.
Oligodendrocytes provide a supporting role for neurons and are involved in the formation of myelin sheaths of nerve cell axons. They are highly dynamic and can respond to environmental influences and neuronal activity. They can also regenerate myelin spontaneously after CNS injury. Any disturbances in their functioning are associated with major diseases of the nervous system. They contain mainly 5.1% DHA for n-3 LC-PUFAs and 9.3% AA and 3.5% n-6 DPA for n-6 LC-PUFAs . They contain not as much as LA: only 2.7%. The ratio n-3/n-6 is 0.33.
Microglial cells are the innate immune cells of the brain. They play a major role in synaptogenesis, synapse structure and function, and neuroinflammation. They perpetually scan and control their environment and once activated, they deliver pro-inflammatory and pro-regeneration responses. Their fatty acid composition differs from that of the other brain cells. In all these cells, DHA is the main fatty acid. Microglial cells are characterized by few DHA (<1%) and n-3 DPA (0.1%) but high content of EPA (3.7%) . They contain few AA (1.6%). They contain PUFA precursors: 8.0% LA and 1.3% ALA. The ratio n-3/n-6 is 0.42. This microglial fatty acid composition also differs from the whole brain hippocampus that contains higher DHA than EPA . Then, it seems that EPA metabolism is different in microglial cells than in other brain cells and the whole brain structure. It is not highly β-oxidized as in the whole brain . More studies have to be performed to elucidate the role of EPA in microglial cells.
2.4 Nutrition as a major factor of variation of brain and brain cell PUFA content
Nutrition is an environmental factor to which individuals are continuously exposed throughout life. And it is an environmental factor that changed a lot these last decades. Indeed, there was a dramatic reduction in the dietary supply of n-3 PUFAs in western societies associated with a drastic increase in the n-6 PUFAs, leading to an imbalanced n-6/n-3 PUFA ratio estimated at 12–20 in developed countries instead of five recommended .
This is particularly important considering that brain fatty acid composition varies with the fatty acids of the dietary supply . Indeed, PUFA content is strongly impacted by the dietary PUFAs in all brain structures [12, 54]. A diet deficient in n-3 PUFA precursor during development and/or adulthood decreases brain DHA in all brain structures; the prefrontal cortex and the hippocampus that contain the highest DHA content are the most sensitive whereas the hypothalamus that contains the lowest DHA, is the least sensitive [12, 31, 55, 56, 57, 58]. These differences may be attributed to the evolution of brain performance [59, 60]. In such case of n-3 PUFA deficiency, changes in metabolism occur: the half-life of DHA increases in the brain to reduce its loss  and the activity of DHA synthesis enzymes (Δ6 destaurase and elongase) is increased in the liver [26, 62, 63]. In contrast to the deficiency, the supplementation in n-3 LC-PUFAs increases brain DHA [64, 65, 66, 67]. DHA supplementation is more efficient than ALA supplementation to increase brain DHA [68, 69]. A DHA supplementation is also efficient to reverse brain DHA decrease due to an n-3 PUFA deficiency or to aging [33, 70, 71, 72]. Also, genetic models of n-3 PUFA enrichment such as Fat-1 mice possess higher brain DHA content [12, 73, 74, 75, 76, 77].
Brain cells are also impacted by dietary PUFA supply. An n-3 PUFA precursor-deficient diet decreases DHA in neurons (4.6%
All these results suggest that brain DHA levels are highly variables, depending on the brain structures or brain cells considered and on the dietary fatty acid intake. This may have consequences on inflammatory processes since n-3 LC-PUFAs have immunomodulatory properties .
3. Bioactive PUFA derivatives
3.1 Bioactive PUFA derivative metabolism
3.1.1 PUFA derivative synthesis pathways
Some of the immunomodulatory properties of LC-PUFAs are attributed to the synthesis of bioactive lipid mediators. Different lipid mediators are synthesized: those involved in the regulation of inflammation such as the eicosanoids (prostaglandins, leukotrienes, and thromboxanes) and those implicated in the resolution of inflammation called specialized pro-resolving mediators (SPMs, resolvins, protectins, and maresins) (Figure 2). Among the eicosanoids, those synthesized from n-3 PUFAs are less potent inflammatory than those synthesized from n-6 PUFAs  highlighting the interest to increase n-3 PUFA and decrease n-6 PUFA contents in the membranes. Then, when co-present, EPA-derived eicosanoids antagonize those synthesized from AA. The main EPA-derived mediators include 3-series prostaglandin (PG), 5-series leukotriene (LT), and 3-series thromboxane (TX), reported to be nonactive (Figure 2). DHA is also converted into 3-series PG (Figure 2). In addition, eicosanoids synthesized from AA and EPA act in competition as they share the same G-protein-coupled receptors. Moreover, EPA is a competitive inhibitor to AA. Indeed, it reduces the production of AA by inhibiting the activity of Δ5 desaturase converting dihomo-gamma-linolenic acid (dGLA) into AA . EPA also reduces
3.1.2 Bioactive lipid mediators
DHA is converted into monohydroxy DHA (17-HDHA) by acetylated COX-2, CYP450, and 15-LOX [107, 108] and then into RvD1 by 5-LOX [109, 110]. RvD1 and its precursors have mostly been described at the periphery but have also been detected in the brain. RvD1 was measured in mouse brain following cerebral ischemia. Its level is increased following a DHA intravenous injection  and modulated during inflammation: it decreases at the beginning and then increases during the resolution phase . RvD1 acts through the regulation of micro-RNAs (miRNAs) that modulate the expression of target genes such as inflammatory genes [113, 114, 115, 116, 117]. DHA can also be converted into di-hydroxy-DHA termed protectin D1 (PD1) or neuroprotectin D1 (NPD1) when produced in the CNS by 5- and 15-LOX [118, 119, 120, 121]. NPD1 was measured in hippocampus. Its level greatly is increased following brain ischemia or acute central LPS injection [70, 122] and decreased in the hippocampus of Alzheimer’s disease patients . NPD1 acts through NFkB and then decreases pro-inflammatory gene expression [122, 124, 125]. At last, DHA can also be converted into 14-HDHA and then in Mar1–2 by 12/15-LOX [107, 108, 126]. Mar1 and its precursor 14-HDHA have recently been identified in the hippocampus of mice . Its level is decreased in post-mortem Alzheimer’s disease patients contributing to the progression of this pathology . Mar1 promotes the resolution of inflammation, reducing pro-inflammatory cytokines, silencing pro-inflammatory signaling cascades, and enhancing M2 repair macrophage phenotype after cerebral ischemia or spinal cord injury [128, 129, 130] (Figure 3).
EPA is converted into resolvins E1, E2, and E3 by acetylated COX-2 or CYP450
3.1.3 SPM receptors
SPMs act through specific receptors, some but not all of them have recently been identified. RvD1 acts through lipoxin A4 receptor/formyl peptide receptor 2 (ALX/Fpr2) in rodents and G protein coupling receptor 32 (GPR32) in human  at picomolar range but induces biological effects at nanomolar range [110, 135]. RvE1 directly binds to its receptor G protein coupling receptor ChemR23 or chemokine like receptor 1 (CMKLR1) . It is also a partial agonist of a leukotriene B4 receptor (BLT1) . In the CNS, ALX/Fpr2 has been identified in the brainstem, spinal cord, hypothalamus, cortex, hippocampus, cerebellum, and striatum  and ChemR23 in the prefrontal cortex, hippocampus, and brainstem . At the cellular levels, these two receptors have been detected in microglial cells [117, 139], neurons [137, 140] and astrocytes [96, 113] (Figure 3).
In the next sections, we will focus on the role of these SPMs to better understand the beneficial effects of the n-3 PUFAs.
3.2 Role of bioactive lipid derivatives in neuroinflammation
SPMs have multiple biological roles, focusing to the return to homeostasis. In human serum, DHA- and EPA-derivatives represent 30.7 and 25.9% of the identified SPMs, respectively [142, 143]. The most SPMs studied are RvD1 and RvE1 because they have powerful anti-inflammatory and pro-resolution properties. We will then detail the biological roles for these two bioactive mediators.
3.2.1 Biological role of RvD1 and RvE1 in humans
The effect of RvD1 was mainly studied in patients suffering from Alzheimer’s disease. Interestingly, RvD1 levels in cerebrospinal fluid are positively correlated with the enhancement of cognitive functions of patients with dementia . Moreover, it was suggested
3.2.2 Biological roles of RvD1 and RvE1 in rodents
RvD1 and RvE1 are active in reducing the pro-inflammatory status in the CNS. Indeed, the precursors of RvD1, 17R-HDHA, and 17S-HDHA decrease the production of pro-inflammatory cytokines TNF-α in the spinal cord and IL-1β and TNF-α in the hippocampus [70, 149]. Moreover, RvD1 is able to induce the polarization of macrophages and microglia toward an M2 phagocytic phenotype [150, 151, 152]. In addition, RvD1 reduces neuroinflammation
RvD1 is also involved in the prevention of cognitive deficits. In a systemic inflammation model, cognitive decline is prevented by an intraperitoneal (ip) injection of the precursor of RvD1, 17R-HDHA, and is associated with the restoration of transmission and synaptic plasticity and to the prevention of astrogliosis [154, 156]. Moreover, in a model of traumatic brain injury, cognitive deficits are also prevented by an ip chronic administration of 17R-HDHA . Of note, Fat-1 mice that have more brain n-3 LC-PUFAs have higher hippocampus RvD1 that is 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 [77, 157]. Inversely, an inhibition of 15-LOX associated with a decrease in RvD1 induces alterations in synaptic plasticity and working memory .
Additionally, RvD and E are also associated with the prevention of depressive-like behaviors . An intracerebroventricular (icv) injection of RvD1, D2, E1, E2, or E3 significantly decreases LPS-induced depressive-like behaviors [159, 160, 161]. Moreover, an intrathecal injection of 17R-HDHA prevents the occurrence of depressive-like behaviors and is associated with the decrease of pain perception and a restoration of dopamine and glutamate levels in the brain [149, 162]. RvD1 and D2 have also positive effects in chronic mild stress-induced depression and in post-myocardial infarct depression [163, 164].
3.2.3 Biological roles of RvD1 and RvE1 in
in vitrobrain cell models
The effects of RvD1 were tested on different brain cells. In microglial cells, RvD1 potentiates the activation of the anti-inflammatory M2 phenotype of microglia, enhancing the effect of the anti-inflammatory cytokine IL-4, Arg1, and Ym1 expression and decreasing CD11b expression [152, 155, 165]. Moreover, we showed that RvD1 decreases LPS-induced proinflammatory cytokine (TNF-α, IL-6 and IL-1β) gene expression in microglial BV2 cells
All these studies point out the central role of n-3 LC-PUFA and their bioactive mediators in the regulation of inflammation in the brain, especially through their effect on microglia.
3.3 Nutrition as a factor of variation of SPM levels
The level of these lipid derivatives is modulated by the diet. Indeed, we recently show that a dietary n-3 LC-PUFA supplementation induces an n-3 LC-PUFA enrichment in the hippocampus associated with an increase in n-3 PUFA-derived SPMs and a decrease in n-6 PUFA-derived SPMs . Our results confirm previous ones reporting that oral administration of EPA and DHA results in the generation of EPA-and DHA-derived mediators in the cortex of aged rats  and in the down-regulation of the production of n-6 PUFA-derived mediators [169, 170]. The cellular origin of these bioactive lipid derivatives is still unknown. As described in the paragraph above, we know that dietary PUFA supplementation affects PUFA composition in brain cells that potentially could impact brain cell PUFA lipid derivatives. In response to LPS, n-3 LC-PUFA-supplemented mice display an anti-inflammatory SPM profile whereas n-3 LC-PUFA-deficient mice exhibit a pro-inflammatory SPM profile . These results corroborate previous ones
The level of SPMs is also dependent on the regulation of their biosynthesis enzymes. 15-LOX mRNA expression increases in n-3 LC-PUFA supplemented group and decreases in n-3 LC-PUFA deficient diet [27, 69, 182]. 15-LOX has beneficial properties such as neuroprotective properties
Changes in SPM level and composition induced by the diet can have a great influence on the pro- and anti-inflammatory status of hippocampus and brain cells and reinforce the recommendation of n-3 PUFA-rich diet.
These data highlight that n-3 LC-PUFA and their bioactive lipid derivatives are important regulators of neuroinflammation. SPMs are promising therapeutic compounds: they are of natural origin and act in physiologic dose ranges (nanomolar) as compared to EPA and DHA that act at micromolar ranges, and this confers the main advantage to use SPMs. Both brain n-3 LC-PUFA and SPMs are modulated by the diet in the brain and in brain cells confirming the notable role of nutrition in the regulation of inflammation. Alteration in dietary n-3 PUFAs should have dramatic consequences in brain and brain cell PUFA metabolism and finally in the response to neuroinflammation. The use of SPMs to treat neuroinflammation is still in emergence since some data are missing such as the affinity and function of SPM receptors. This field has to be completed. The instability of SPMs may be bypassed by the use of SPM analogues or by their encapsulation. The clinical form and the way of administration should also be defined.
Corinne Joffre was sponsored by the Fondation de France and the Fondation pour la Recherche Médicale.