The catabolism of the essential amino acid tryptophan to kynurenine is emerging as a potential key pathway involved in post-cardiac arrest brain injury. The aim of this study was to evaluate the effects of the modulation of kynurenine pathway on cardiac arrest outcome, through genetic deletion of the rate-limiting enzyme of the pathway, indoleamine-2,3-dyoxygenase(IDO).
Wild-type (WT) and IDO-deleted (IDO -/-) mice were subjected to 8 min cardiac arrest. Survival, neurological outcome, and locomotor activity were evaluated following resuscitation. Brain magnetic resonance imaging with diffusion tensor and diffusion-weighted imaging sequences was performed, together with microglia/macrophage activation and neurofilament light chain measurements.
IDO-/- mice showed higher survival compared to WT mice (IDO-/- 11/16, WT 6/16, log-rank p=0.036). Neurological function was higher in IDO-/- mice than in WT mice following cardiac arrest (IDO-/- 9±1, WT 7±1, p=0.012, n=16). IDO-deletion preserved locomotor function while maintaining physiologic circadian rhythm after cardiac arrest. Brain magnetic resonance imaging with diffusion tensor imaging showed an increase in mean fractional anisotropy in the corpus callosum (IDO-/- 0.68±0.01, WT 0.65±0.01, p=0.010, n=5-4) and in the external capsule (IDO -/- 0.47±0.01, WT 0.45±0.01, p=0.006, n=5-4) in IDO-/- mice compared to WT ones. Increased release of neurofilament light chain was observed in WT mice compared to IDO-/- (median concentrations (IQR), pg/mL: WT 1138 (678–1384), IDO-/- 267 (157–550), p<0.001, n=3-4).
Brain magnetic resonance imaging with diffusion-weighted imaging revealed restriction of water diffusivity 24 hours after cardiac arrest in WT mice, IDO-deletion prevented water diffusion abnormalities, which was reverted in IDO-/- mice receiving L-kynurenine (Apparent Diffusion Coefficient, μm2/ms: WT 0.48±0.07, IDO-/- 0.59±0.02, IDO-/- +L-Kynurenine 0.47±0.08, p=0.007, n=6).
Kynurenine pathway represents a novel target to prevent post-cardiac arrest brain injury. The neuroprotective effects of IDO-deletion were associated with preservation of brain white matter microintegrity and with reduction of cerebral cytotoxic edema.