Brain Fatty Acid Composition and Inflammation in Mice Fed with High-Carbohydrate Diet or High-Fat Diet

Male Swiss mice (total = 72 animals) were maintained in standard laboratory conditions in a photoperiod (12 h light/12 h darkness), temperature (22 ± 1 °C), and humidity-controlled environment. Food and water were available ad libitum.

All experiments were carried out in accordance with the international guidelines for the use and care of laboratory animals approved by the Scientific Advisory Committee on Animal Care of State University of Maringá (protocol 002/2014).

Mice (six weeks of age) were fed with standard rodent chow (Nuvilab^®, Curitiba, PR, Brazil) before the initiation of the experimental protocol.

After three days of acclimatization, the animals (weighing about 35 g) were divided into two groups: HFD and HCD.

The amounts of protein, carbohydrate, and total fat were 20.3, 36.5, and 35.2 g/100 g for the high fat diet, and 14.2, 73.8, and 4.0 g/100 g for the high carbohydrate diet, respectively. Highly refined ingredients (Rhoster Company, Araçoiaba da Serra, SP, Brazil) were used to prepare diets. The diets composition were based on purified diets for maintenance of laboratory adult rodents proposed by the American Institute of Nutrition (AIN-93-M) [13]. Details about the composition of the diets can be found in our previous work [12].

Mice fed with HFD or HCD for 0 (before starting the diets), 7, 14, 28, or 56 days (n = 8 for each time of treatment with HCD or HFD) were fasted (from 5:00 p.m. to 8:00 a.m.), before being sacrifice by decapitation.

The brains were quickly and carefully removed immediately prior to the liver that was used in our previous study [12], frozen in liquid nitrogen, and stored at −80 °C until analysis being performed.

A method in reduced scale was used to extract the total lipid content of the brain samples. For this purpose, 1.000 ± 0.001 g of homogenized brain samples were used. FA methyl esters (FAME) of brain homogenates were prepared by ultrasound to assist total lipid methylation as described by Santos et al. [14]. FAME separation was performed by gas chromatography in a Thermo Scientific™TRACE™Ultra Gas Chromatographer (Thermo Scientific™, Waltham, MA, USA), fitted with a flame ionization detector (FID) and a fused-silica capillary column. For identification of the FAs, the retention times were compared to those of standard methyl esters. The results of FA contents in the brain were expressed as mg/100 mg sample. More details about this methodology can be found in our previous study [12].

Total RNA was extracted using TRIzol reagent (Invitrogen Life Technologies, Waltham, MA, USA) and reverse transcribed to cDNA (High-Capacity cDNA kit, Applied Biosystems, Foster City, CA, USA). Gene expression was evaluated by real-time PCR using SYBR Green as the fluorescent dye (Invitrogen Life Technologies, Waltham, MA, USA). Primer sequences are in Table 1. Analysis of gene expression was performed according to a previously described method [15], using ribosomal protein lateral stalk subunit P0 gene (Rplp0) as the internal control.

The IMI was calculated by the sum of expressions of F4/80 + IL-6 + IL-1β + TNFα + iNOS + COX-2 + Itgam + Aif1 (pro-inflammatory factors) divided by IL-10 (anti-inflammatory factor), as previously described [12].

Results are reported as the mean ± standard deviation of the mean and analyzed by Student’s t-test or ANOVA, followed by the post-test of Tukey using the Graph-Pad Prism Version 5.0 software (Graph Pad Software Inc., San Diego, CA, USA) to assess differences between means. p-values < 0.05 were used to indicate statistical significances.

Brains from the HFD and HCD groups had higher content of palmitic acid (16:0; Figure 1), stearic acid (18:0; Figure 1), oleic acid (18:1n-9; Figure 2), AA (20:4n-6; Figure 3), and DHA (22:6n-3; Figure 4), as compared with other FA.

We observed an increase (p < 0.05) in palmitic acid (16:0), stearic acid (18:0), heneicosanoic acid (21:0), and tetracosanoic acid (24:0) content between day 0 and day 56, in both HCD and HFD groups. Except for tetracosanoic acid (24:0), the brain of the HCD or HFD group exhibited a similar SFA content on day 56 (Figure 1).

The content of 7-hexadecanoic acid (16:1n-9) was decreased (p < 0.05) from day 7 until day 56 in the brains of both groups. Brains of the HFD group had lower (p < 0.05) palmitoleic acid (16:1n-7), oleic acid (18:1n-9), and vaccenic acid (18:1n-7) after 56 days (Figure 2).

The content of linoleic acid (18:2n-6) was decreased (p < 0.05) and increased (p < 0.05) in the brain of HCD and HFD, respectively (Figure 3).

γ-linolenic acid (18:3n-6), AA (20:4n-6), docosatetraenoic acid (22:4n-6), and docosapentaenoic acid (22:5n-6) were increased between day 0 and day 56 (HFD or HCD). The increase was more pronounced for brain docosapentaenoic acid (22:5n-6) content from day 56 in HCD compared to HFD (Figure 3).

EPA (20:5n-3) and DHA (22:6n-3) levels were increased (p < 0.05) whereas α-linolenic acid (ALA, 18:3n-3) levels were decreased (p < 0.05) during the 56-day period, either for HFD or HCD (Figure 4).

SFA represents approximately 45% of the total brain FA, either in HCD or HFD mice whereas MUFA represents approximately 25% of the total brain FA, either in HCD or HFD (Table 2).

PUFA represent approximately 35% of the total brain FA, being n-3 PUFA a half of this percentage, either in HCD or HFD (Table 2).

The brains exhibited similar PUFA/SFA, MUFA/SFA, and n-6/n-3 ratio throughout the 56-day period regardless of diet given (Table 2).

Lipid accumulation, calculated by the sum of all FA of each family (SFA, MUFA, and PUFA) was faster in the brains of HFD mice. The HFD group reached the maximum FA accumulation on day 28, whereas HCD mice reached maximum value on day 56 only. After 56 days, however, the sum of all FA evaluated, i.e., SFA plus MUFA plus PUFA (HFD group vs. HCD group) was similar (Table 2).

Brains from the HFD group exhibited lower (p < 0.05) mRNA expressions of the F4/80 and Itgam (Table 3) on day 56. The IMI was higher (46%) in the brain of HFD mice.

Brain FA profile is tightly regulated and exhibits only a lower response to diet composition changes in comparison with other tissues like liver, skeletal muscle, and heart [16,17,18,19].

In agreement with other studies [20,21], a predominance of palmitic acid (16:0), stearic acid (18:0), oleic acid (18:1n-9), AA (20:4n-6), and DHA (22:6n-3) was described in the brains of both groups during the experimental period.

SFA can activate transcription factors of glial cells and the innate immune system, triggering expression of pro-inflammatory genes such as cytokines, chemokines, iNOS, and COX. SFA also activates the nuclear factor-kappa B (NF-κB) that raises the expressions of inflammatory genes [22]. As a consequence, the inflammatory state is induced in the neurons by these fatty acids [23,24].

As we previously quantified [12] the amount of SFA in the high fat diet is more than five times higher than that found in the high carbohydrate diet. Nevertheless, except for tetracosanoic acid (24:0), either HFD or HCD mice exhibited similar brain SFA composition on day 56 (Figure 1). The tetracosanoic acid (24:0), found in higher quantities in brains of the HFD group is reported to be toxic for oligodendrocytes and astrocytes [25]. Reduced myristic acid content (14:0) was reported either in HFD or HCD mice between day 0 and day 56. This FA is an important cellular component of several proteins that require myristoylation to exert their biological effects [26].

The amount of MUFA in the HFD is more than five times higher if compared with HCD [12]. Notwithstanding, except for 7-hexadecanoic acid (16:1n-9), HFD and HCD mice exhibited similar brain MUFA composition on day 56.

Oleic acid (18:1n-9) was increased (p < 0.05) in both groups at day 56, being more pronounced in HCD. This FA has anti-inflammatory properties in the brain by inhibiting activation of NF-kB signaling pathways in neurons and astrocytes. Oleic acid also promotes axonogenesis in the striatum during brain development and it is used as a brain energy source during the decreased availability of glucose [27,28].

ALA (18:3n-3) and LA (18:2n-6), the main precursors of PUFA in the brain may be converted into DHA (22:6n-3) and AA (20:4n-6), respectively, by desaturation and elongation. n-3 PUFA and n-6 PUFA compete for the same desaturases and elongases [29]. Since n-6/n-3 ratios in the diets are similar [12], the competition for these enzymes is probably equivalent for both groups.

Brain LA (18:2n-6), was increased (p < 0.05) and decreased (p < 0.05) in HFD and HCD, respectively. This difference might be explained by the fact that HFD mice had a higher (p < 0.05) content of palmitic acid (16:0) in the diet [12] and the entry of LA (18:2n-6) into the brain is proportional to the blood concentration of palmitic acid (16:0) [30].

Circulating ALA (18:3n-3) and LA (18:2n-6) are able to cross the blood-brain barrier being then converted into DHA (22:6n-3) and AA (20:4n-6) in the brain [31]. ALA (18:3n-3) and LA (18:2n-6) from the diet can also be converted to DHA (22:6n-3) and AA (20:4n-6) in the liver and then released to bloodstream [24]. DHA is taken up by the brain in preference to other fatty acids [32].

Brain is rich in PUFA, which represent about 20% of the brain dry weight [29]. PUFA levels in this study agree with previous reports in which DHA (22:6n-3) and AA (20:4n-6) are the main n-3 and n-6 PUFA, respectively [33,34]. Between day 0 and day 56 period, there was an increase (p < 0.05) of DHA and AA in brains of either HCD or HFD mice (Figure 3 and Figure 4).

EPA (20:5n-3) content was only about 1% of that of DHA (22:6n-3). Brain EPA (20:5n-3) content is very low probably because of its intense metabolism. This FA is rapidly converted through β-oxidation, elongation, and desaturation to docosapentaenoic acid (22:5n-3) and DHA (22:6n-3) [35].

HCD mice had a more intense increase (p < 0.05) of docosapentaenoic acid (22:5n-6) content, a metabolite of LA (18:2n-6). Ghosh et al. [36] reported a reduction of this FA in cell membranes of prefrontal white matter in postmortem patients with bipolar disorder and schizophrenia.

The content of the docosatetraenoic acid (22:4n-6), a product of elongase activity [37], was increased (p < 0.05) in the brain of the HFD and HCD groups, respectively.

PUFA, which are abundant in cell membrane phospholipids of neural tissues [38], have a pivotal role for maintaining membrane fluidity, permeability, lipid–protein and lipid–lipid interactions for brain neurogenesis and modulation of inflammation [37,39].

The pro-inflammatory mediators derived from AA (20:4n-6)—i.e., prostaglandins, thromboxanes, leukotrienes, and lipoxins—intensify neuroinflammation [11]. In contrast, DHA (22:6n-3) and EPA (20:5n-3) have anti-inflammatory, antiapoptotic, and antioxidant properties [40].

HFD mice had a faster brain deposition of FA, reaching the maximum FA accumulation on day 14 whereas HCD reached maximum FA deposition on day 14 or 28. However, on day 56, the sum of SFA, MUFA, and PUFA was similar (HFD vs. HCD). A summary of all findings is in Figure 5.

Neuroinflammation is characterized by increased expression of inflammatory genes [22,41]. There was higher (p < 0.05) mRNA expression of F4/80 and Itgam on day 56 in HCD, that are markers of microglia infiltration [42,43]. The IMI was increased (46%) in HFD brain mainly because IL-10 was poorly expressed in this group (Table 3).