The interaction between brain and liver regulates lipid metabolism in the TBI pathology

Abstract

To shed light on the impact of systemic physiology on the pathology of traumatic brain injury (TBI), we examine the effects of TBI (concussive injury) and dietary fructose on critical aspects of lipid homeostasis in the brain and liver of young-adult rats. Lipids are integral components of brain structure and function, and the liver has a role on the synthesis and metabolism of lipids. Fructose is mainly metabolized in the liver with potential implications for brain function. Lipidomic analysis accompanied by unbiased sparse partial least squares discriminant analysis (sPLS-DA) identified lysophosphatidylcholine (LPC) and cholesterol ester (CE) as the top lipid families impacted by TBI and fructose in the hippocampus, and only LPC (16:0) was associated with hippocampal-dependent memory performance. Fructose and TBI elevated liver pro-inflammatory markers, interleukin-1α (IL-1α), Interferon-γ (IFN-γ) that correlated with hippocampal-dependent memory dysfunction, and monocyte chemoattractant protein-1 (MCP-1) positively correlated with LPC levels in the hippocampus. The effects of fructose were more pronounced in the liver, in agreement with the role of liver on fructose metabolism and suggest that fructose could exacerbate liver inflammation caused by TBI. The overall results indicate that TBI and fructose interact to influence systemic and central inflammation by engaging liver lipids. The impact of TBI and fructose diet on the periphery provides a therapeutic target to counteract the TBI pathogenesis.

Keywords: Cognition; Fructose; Liver; Lysophosphatidylcholine; Traumatic brain injury; cPLA2.

Figure 1.. Lipidomic analysis showing the effects of TBI and fructose consumption in the hippocampus.

(A) Score plot of the sparse partial least squares analysis (sPLS-DA) showing the lipid profile of each experimental group. (B) Top 30 lipid species according to loading 2 values of the sPLS-DA. (C) shows concentrations of each lipid class in the different experimental groups. 2-way ANOVA with Fisher’s LSD post-test (*p<0.05 vs SW-group), Abbreviations: LPC, Lysophosphatidylcholine, LPE, Lysophosphatidylethanolamine, PC, Phosphatidylcholine, PE, Phosphatidylethanolamine, SM, Sphingomyelin, CE, Cholesterol ester, CER, Ceramides, DAG, Diacylglycerol, DCER, Dihydroceramides,FFA, Free fatty acids, HCER, Hexosyl ceramides, LCER, Lactosyl ceramides, TAG, Triglyceride. Animal groups: SW, Sham-Water; SF, Sham-Fructose; TW, TBI-Water; TF, TBI-Fructose.

Figure 2.. Altered concentrations of Lysophosphatidylcholine (LPC, A) and Cholesteryl esters (CE, B) species in the hippocampus in response to TBI and fructose.

(A) Concentrations of LPC (A) and CE (B) species according to fatty acyl composition. Statistical significance was evaluated by 2-way ANOVA with Fisher’s LSD post-test (*p<0.05 vs SW group and SF group, # p<0.05 vs TW group).

Figure 3.. LPC (16:0) levels change in proportion to BM performance.

(A) Latency to find the escape hole during memory test in BM. 2-way ANOVA with Fisher’s LSD posttest (*p≤0.05 vs SW) († p = 0.09 vs SW). (B) Correlation analysis using the latency as a variable versus the concentration of LPC (16:0) (Pearson correlation, R=0.44, p=0.03). (C) Correlation analysis using the latency as a variable versus the concentration of CE (20:4) (Pearson correlation, R=0.22, p=0.32)

Figure 4.. Effects of TBI and fructose on pro-inflammatory cytokines (IFN-γ, IL-1α, MCP-1, TNF-α) in the liver

(A); 2-way ANOVA with Fisher’s LSD post-test (*p<0.05 vs SW group). (B) Correlation analysis using the latency as a variable shows association between IL-1α (R=0.51, p=0.015) and TNF-α (C, R=0.40, p=0.06) levels with memory performance. (D) Correlation analysis shows association between MCP-1 and LPC (16:0) levels (R= 0.48, p=0.024). Pearson correlation, n=5-6 per group.

Figure 5.. Altered activation of cPLA2 in the hippocampus and liver in response to TBI and fructose.

(A) Quantification of ratio from p-cPLA2/cPLA2 in the hippocampus for each experimental group. (B) Quantification of ratio P-cPLA2/cPLA2 in the liver for each group. n=6 per group. 2-way ANOVA with Fisher LSD posttest, *p<0.05, **p<0.01, ***p<0.001 vs SW-group. The uncropped western blots are shown in the supplementary information (Supplementary Figure 1).

Figure 6.. Lipid metabolism in the liver in response to TBI and fructose.

(A) Quantification of Fatty acid synthase (FAS; A), ApoB-100 (B), (C) ApoB-48 (C), and ABCA1 (D) in the liver. n=6 per group. 2-way ANOVA with Fisher’s LSD posttest, (*p<0.05, **p<0.01, *** p<0.001 vs SW-group, #p<0.05, ### p<0.001 vs TW-group). The uncropped western blots are shown in the supplementary information (Supplementary Figure 2). (E) Correlation analysis shows association between ratio ABCA1/actin and LPC 16:0 levels (R= 0.48, p=0.017).

Figure 7.. Proposed mechanisms by which brain interacts with liver in response to the actions of TBI and fructose.

The effects of TBI on brain function and plasticity involve the action of liver on lipid metabolism. Lysophosphatidylcholine (LPC) is a lipid inflammatory mediator that it is derived from the turnover of phosphatidylcholine (PC) involving cPLA2 and secreted as component of very low-density lipoprotein (VLDL). TBI in combination with fructose diet alter lipid species in the brain such as LPC (16:00) that uses precursor molecules synthesized in the liver. The liver is a main target for the action of fructose and then fructose byproducts can influence TBI pathogenesis. Fructose increases the secretion of VLDL to the blood and could promote the conversion of PC to LPC by cPLA2. The hepatic ABCA1 transporter exports cholesterol and phospholipids such as LPC from liver to the bloodstream. Thereby, fructose can help LPC to reach the brain and to amplify brain degeneration started by TBI with subsequent effects on cognitive performance. TBI and fructose induce systemic inflammation that can affects the brain, i.e., TBI stimulates the release of pro-inflammatory cytokines from liver which are transported into brain, taking advantage of a breakdown of the blood brain barrier (BBB). Brain inflammation can enhance degenerative events resulting in cognitive dysfunction, demyelination, and cell degeneration.