A mouse without kynurenine aminotransferase II: Investigating the kynurenine pathway

Abstract

Kynurenic acid (KYNA) is a neuroprotective metabolite of tryptophan present at sub-micromolar concentrations in mammalian brain. KYNA is an antagonist of excitatory neurotransmitter receptors with particularly high affinity for the glycine site of the NMDA receptor and the alpha7 nicotinic acetylcholine receptor. In order to study KYNA's role in brain function, this dissertation details the characterization of a null mutant mouse for kynurenine aminotransferase II (KAT II) designated mkat-2-/- . While both Western Blot analysis and PCR verified the absence of KAT II in various tissues including brain, mkat-2-/- animals had lower KAT activity in brain only at postnatal day (PND) 7 and 14, but not later in life. Brain KYNA levels were 40-60% of wild-type levels in mkat-2-/- animals before PND 28 while adult (2 to 18 months) levels were not different between genotypes. In liver, KYNA levels were over 90% lower in mkat-2 -/- animals throughout life. Brain and liver kynurenine, 3-hydroxykynurenine and quinolinic acid (QUIN) were present at levels similar to wild-type at all ages tested. mkat-2-/- were viable, developed normally, and had normal litter sizes and longevity. Behavioral deficits were observed in young but not adult mkat-2-/- mice, specifically increased locomotor activity and decreased latency to fall from a rotarod apparatus. In order to determine the relevance of endogenous KYNA to excitotoxic processes, we infused various excitotoxins into the striatum and quantified cell death. Intrastriatal injection of QUIN resulted in significantly larger lesion in mkat-2-/- compared to wild-type mice at PND 14, i.e. when brain KYNA levels are reduced ∼60%. This heightened vulnerability was not seen at 2 months, when brain KYNA concentrations in mutant mice were normal. Moreover, when a kynurenine 3-hydroxylase inhibitor, UPF 648, was used to restore KYNA levels in PND 14 mkat-2-/- mice, the increased QUIN toxicity was blocked. The influence of KYNA on excitotoxic vulnerability was specific to QUIN, as the lesion caused by an intrastriatal kainic acid or NMDA injection was not different in the two genotypes, regardless of age. Therefore, brain KYNA influences both behavior and neurotoxic processes in mouse. Further, pharmacological up-regulation of brain KYNA may offer a new venue for the treatment of human brain diseases.