Plasma Lipid Profiles Change with Increasing Numbers of Mild Traumatic Brain Injuries in Rats

Abstract

Mild traumatic brain injury (mTBI) causes structural, cellular and biochemical alterations which are difficult to detect in the brain and may persist chronically following single or repeated injury. Lipids are abundant in the brain and readily cross the blood-brain barrier, suggesting that lipidomic analysis of blood samples may provide valuable insight into the neuropathological state. This study used liquid chromatography-mass spectrometry (LC-MS) to examine plasma lipid concentrations at 11 days following sham (no injury), one (1×) or two (2×) mTBI in rats. Eighteen lipid species were identified that distinguished between sham, 1× and 2× mTBI. Three distinct patterns were found: (1) lipids that were altered significantly in concentration after either 1× or 2× F mTBI: cholesterol ester CE (14:0) (increased), phosphoserine PS (14:0/18:2) and hexosylceramide HCER (d18:0/26:0) (decreased), phosphoinositol PI(16:0/18:2) (increased with 1×, decreased with 2× mTBI); (2) lipids that were altered in response to 1× mTBI only: free fatty acid FFA (18:3 and 20:3) (increased); (3) lipids that were altered in response to 2× mTBI only: HCER (22:0), phosphoethanolamine PE (P-18:1/20:4 and P-18:0/20:1) (increased), lysophosphatidylethanolamine LPE (20:1), phosphocholine PC (20:0/22:4), PI (18:1/18:2 and 20:0/18:2) (decreased). These findings suggest that increasing numbers of mTBI induce a range of changes dependent upon the lipid species, which likely reflect a balance of damage and reparative responses.

Keywords: lipid; lipidomics; liquid chromatography–mass spectrometry; mild traumatic brain injury; repeated mild traumatic brain injury.

Figure 1

Orthogonal projection to latent structure-discriminant analysis (OPLS-DA) of plasma lipids following sham, 1× or 2× mTBI. (A) OPLS-DA visualisations of control samples vs. 1× mTBI samples. (B) OPLS-DA visualisations of 2× mTBI vs. 1× mTBI. The model was scaled proportionally to R2x, R2x(1) = 0.143, R2Xo(1) = 0.415. (C), OPLS-DA visualisations of control vs. 2× mTBI. (D) singular mTBI intervention (1× mTBI) was projected onto the final model for validation. The model was scaled proportionally to R2x, R2x(1) = 0.0886, R2Xo(1) = 0.548. mTBI, mild traumatic brain injury.

Figure 2

Lipids that change with one and two injuries vs. sham control. (A), increase after 1× or 2× mTBI; (B), decrease after 1× or 2× mTBI; (C), increase with 1× mTBI and decrease with 2× mTBI. Boxplots represent the minimum, first quartile, median, third quartile, and maximum, for the control, 1× mTBI and 2× mTBI groups. Significant differences between groups are indicated above the boxplots by * (p < 0.05) or ** (p < 0.01). CE, cholesterol ester; HCER, hexosylceramide; mTBI, mild traumatic brain injury; PI, phosphoinositol; PS, phosphoserine.

Figure 3

Lipids that change with one injury vs. sham control. Boxplots represent the minimum, first quartile, median, third quartile, and maximum, for the control, 1× mTBI and 2× mTBI groups. Significant differences between groups are indicated above the boxplots by ** (p < 0.05) or ** (p < 0.01). FFA, free fatty acid; mTBI, mild traumatic brain injury.

Figure 4

Lipids that change with two injuries vs. sham control. (A), Increase after 2× mTBI; (B), decrease after 2× mTBI. Boxplots represent the minimum, first quartile, median, third quartile, and maximum, for the control, 1× mTBI and 2× mTBI groups. Significant differences between groups are indicated above the boxplots by * (p < 0.05) or ** (p < 0.01). HCER, hexosylceramide; LPE, lysophosphatidylethanolamine; mTBI, mild traumatic brain injury; PC, phosphocholine; PE, phosphoethanolamine; PI, phosphoinositol.