Association of sub-acute changes in plasma amino acid levels with long-term brain pathologies in a rat model of moderate-severe traumatic brain injury

Abstract

Introduction: Traumatic brain injury (TBI) induces a cascade of cellular alterations that are responsible for evolving secondary brain injuries. Changes in brain structure and function after TBI may occur in concert with dysbiosis and altered amino acid fermentation in the gut. Therefore, we hypothesized that subacute plasma amino acid levels could predict long-term microstructural outcomes as quantified using neurite orientation dispersion and density imaging (NODDI).

Methods: Fourteen 8-10-week-old male rats were randomly assigned either to sham (n = 6) or a single moderate-severe TBI (n = 8) procedure targeting the primary somatosensory cortex. Venous blood samples were collected at days one, three, seven, and 60 post-procedure and NODDI imaging were carried out at day 60. Principal Component Regression analysis was used to identify time dependent plasma amino acid concentrations after in the subacute phase post-injury that predicted NODDI metric outcomes at day 60.

Results: The TBI group had significantly increased plasma levels of glutamine, arginine, alanine, proline, tyrosine, valine, isoleucine, leucine, and phenylalanine at days three-seven post-injury. Higher levels of several neuroprotective amino acids, especially the branched-chain amino acids (valine, isoleucine, leucine) and phenylalanine, as well as serine, arginine, and asparagine at days three-seven post-injury were also associated with lower isotropic diffusion volume fraction measures in the ventricles and thus lesser ventricular dilation at day 60.

Discussion: In the first such study, we examined the relationship between the long-term post-TBI microstructural outcomes across whole brain and the subacute changes in plasma amino acid concentrations. At days three to seven post-injury, we observed that increased plasma levels of several amino acids, particularly the branched-chain amino acids and phenylalanine, were associated with lesser degrees of ventriculomegaly and hydrocephalus TBI neuropathology at day 60 post-injury. The results imply that altered amino acid fermentation in the gut may mediate neuroprotection in the aftermath of TBI.

Keywords: amino acids; brain microstructure; diffusion magnetic resonance imaging; neurite orientation dispersion and density imaging (NODDI); traumatic brain injury.

FIGURE 1

Plasma levels of amino acids: histidine, asparagine, serine, glutamine, arginine, glycine, aspartic acid, glutamic acid, and threonine of sham and traumatic brain injured (TBI) animals at day one, three, seven, and sixty post-procedure. Asterisks (*) next to sham/TBI legends, post-injury time axis label, and interaction effect indicated statistically significant group, post-injury time, and group × post-injury time effects, respectively, in a two-way repeated measures analysis of variance (ANOVA). *On the graph at each timepoint indicated significant difference between TBI and sham animals at each timepoint, Fisher’s Least Squared Difference post-hoc test. *P-value < 0.05, ^**P-value < 0.01, ^***P-value < 0.001, ^****P-value < 0.0001.

FIGURE 2

Plasma levels of amino acids: alanine, proline, cystine, lysine, tyrosine, methionine, valine, isoleucine, and leucine of sham and TBI animals at day one, three, seven, and sixty post-procedure. Asterisks (*) next to sham/TBI legends, post-injury time axis label, and interaction effect indicated statistically significant group, post-injury time, and group × post-injury time effects, respectively, in ANOVA. *On the graph at each timepoint indicated significant difference between TBI and sham animals at each timepoint, Fisher’s Least Squared Difference post-hoc test. *P-value < 0.05, ^**P-value < 0.01, ^***P-value < 0.001, ^****P-value < 0.0001.

FIGURE 3

Plasma levels of amino acids: phenylalanine and tryptophan of sham and TBI animals at day one, three, seven, and sixty post-procedure. Asterisks (*) next to sham/TBI legends, post-injury time axis label, and interaction effect indicated statistically significant group, post-injury time, and group × post-injury time effects, respectively, in ANOVA. *On the graph at each timepoint indicated significant difference between TBI and sham animals at each timepoint, Fisher’s Least Squared Difference post-hoc test. *P-value < 0.05, ^**P-value < 0.01, ^***P-value < 0.001.