Dietary branched chain amino acids ameliorate injury-induced cognitive impairment

Abstract

Neurological dysfunction caused by traumatic brain injury results in profound changes in net synaptic efficacy, leading to impaired cognition. Because excitability is directly controlled by the balance of excitatory and inhibitory activity, underlying mechanisms causing these changes were investigated using lateral fluid percussion brain injury in mice. Although injury-induced shifts in net synaptic efficacy were not accompanied by changes in hippocampal glutamate and GABA levels, significant reductions were seen in the concentration of branched chain amino acids (BCAAs), which are key precursors to de novo glutamate synthesis. Dietary consumption of BCAAs restored hippocampal BCAA concentrations to normal, reversed injury-induced shifts in net synaptic efficacy, and led to reinstatement of cognitive performance after concussive brain injury. All brain-injured mice that consumed BCAAs demonstrated cognitive improvement with a simultaneous restoration in net synaptic efficacy. Posttraumatic changes in the expression of cytosolic branched chain aminotransferase, branched chain ketoacid dehydrogenase, glutamate dehydrogenase, and glutamic acid decarboxylase support a perturbation of BCAA and neurotransmitter metabolism. Ex vivo application of BCAAs to hippocampal slices from injured animals restored posttraumatic regional shifts in net synaptic efficacy as measured by field excitatory postsynaptic potentials. These results suggest that dietary BCAA intervention could promote cognitive improvement by restoring hippocampal function after a traumatic brain injury.

Fig. 1.

Consumption of BCAAs restored cognitive performance by correcting injury-induced alterations in net synaptic efficacy. (A) Consumption of 100 mM each of leucine, isoleucine, and valine (LIV) for 5 days restored hippocampal BCAA levels. (B) Consumption of the LIV treatment significantly increased both alanine and glutamine concentrations (P < 0.05). Both retrograde (C) and an anterograde (D) contextual fear conditioning tests demonstrated significant cognitive impairment (P < 0.05) after LFPI. This was completely reversed by the consumption of the LIV treatment. In contrast, phenylalanine consumption (100 mM; LFPI-Phe) had no effect on cognitive performance (P > 0.05). Bars indicate the average percentage of “fear behaviors” per 5 min. Insets show a representative sample of the percentage of “fear behaviors” on a per-minute basis. In area CA1 (E) and dentate gyrus (F), consumption of the LIV treatment restored net synaptic efficacy (P < 0.05), whereas consumption of phenylalanine (100 mM) had no effect (P > 0.05). Insets depict representative waveforms collected at maximal stimulation. Scale bar, 0.5 mV/10 ms. (A and B) n = 6 samples. (C) Sham, LFPI and LFPI-LIV had n = 12, 11, and 9, respectively. (D) Sham, LFPI. LFPI-LIV, and LFPI-Phe had n = 14, 11, 10, and 10, respectively. (E and F) n = 8 samples. For all panels, data are means ± SEM *Means differ significantly from sham values (P < 0.05). ^†Means of data collected from LFPI-Phe mice differ significantly from sham values (P < 0.05).

Fig. 2.

Branched chain amino acids restore net synaptic efficacy. In both area CA1 (A) and the dentate gyrus (B), input/output curves demonstrate that FPI significantly alters net synaptic efficacy. BCAA administration (100 μM each of valine, isoleucine, and leucine) completely restores net synaptic efficacy. Insets depict representative waveforms at maximal stimuli. Scale bar, 0.5 mV/10 ms. In area CA1 (C) and dentate gyrus (D), individual application of leucine, isoleucine, and valine (100 μM) completely restored net synaptic efficacy when compared with either slices from sham or LFPI animals alone. For each panel, n = 8, and data are means ± SEM *Significance when compared with sham values (P < 0.05).

Fig. 3.

Brain injury leads to diminished BCAA transamination and alterations in BCAA and glutamate metabolizing enzymes. (A) After incubation with [¹⁵N]-leucine (100 μM), LFPI significantly reduced the production of ¹⁵N-labeled GABA, aspartate, and glutamate. n = 6, with bars representing means ± SEM. (B) Densitometry analysis of the expression of cytosolic branched chain aminotransferase (BCATc; 49 kDa) reveals a significant reduction (P < 0.05) in area CA1 of injured animals. (C) Branched chain keto acid dehydrogenase (BCKD; 51 kDa) expression significantly decreased (P < 0.05) in the dentate gyrus of injured animals. (D) Glutamate dehydrogenase (GDH; 61 kDa) expression significantly increased (P < 0.05) in the dentate gyrus of injured mice. (E) Glutamic acid decarboxylase (GAD; 65 kDa) was significantly reduced (P < 0.05) in area CA1. (F) Aspartate aminotransferase 1 (AAT1; 46 kDa) was significantly reduced (P < 0.05) in the dentate gyrus. (G) Expression of aspartate aminotransferase 2 (AAT2; 47 kDa) was significantly reduced (P < 0.05) in both the dentate gyrus and area CA1 from injured mice. For each Western blot, n = 4 samples, with densitometry representing means ± SEM *Denotes significance at the P < 0.05 confidence level when compared to sham values.

Fig. 4.

In area CA1 (A) and dentate gyrus (B), slices were exposed to 2-amino-2-norbornane-carboxylic acid (BCH; 100 μM), a nonhydrolyzable BCAA analog, glutamine (100 μM), or a combination of all three branched chain ketoacids (BCKAs; 100 μM). Neither BCKAs nor BCH had a salutary effect in either region, whereas only in the dentate gyrus did glutamine restore net synaptic efficacy. In area CA1 (C) and dentate gyrus (D), the glutaminase inhibitor 6-diazo-5-oxo-l-norleucine (DON; 50 μM) did not block the restorative effect of the combination of BCAAs. In the dentate gyrus (D) but not in area CA1 (C), DON completely blocked the glutamine-mediated restoration of net synaptic efficacy in this region. Solid line represents sham value at maximal stimulation; dotted line represents LFPI value at maximal stimulation. For each panel, n = 8, and data are means ± SEM *Significance when compared with sham values (P < 0.05).