Metabolites of the kynurenine pathway (KP) of tryptophan (TRP) degradation have been closely linked to the pathogenesis of several neurodegenerative disorders. Recent work has highlighted the therapeutic potential of inhibiting two critical regulatory enzymes in this pathway-kynurenine-3-monooxygenase (KMO) and tryptophan-2,3-dioxygenase (TDO). Much evidence indicates that the efficacy of KMO inhibition arises from normalizing an imbalance between neurotoxic [3-hydroxykynurenine (3-HK); quinolinic acid (QUIN)] and neuroprotective [kynurenic acid (KYNA)] KP metabolites. However, it is not clear if TDO inhibition is protective via a similar mechanism or if this is instead due to increased levels of TRP-the substrate of TDO. Here, we find that increased levels of KYNA relative to 3-HK are likely central to the protection conferred by TDO inhibition in a fruit fly model of Huntington's disease and that TRP treatment strongly reduces neurodegeneration by shifting KP flux toward KYNA synthesis. In fly models of Alzheimer's and Parkinson's disease, we provide genetic evidence that inhibition of TDO or KMO improves locomotor performance and ameliorates shortened life span, as well as reducing neurodegeneration in Alzheimer's model flies. Critically, we find that treatment with a chemical TDO inhibitor is robustly protective in these models. Consequently, our work strongly supports targeting of the KP as a potential treatment strategy for several major neurodegenerative disorders and suggests that alterations in the levels of neuroactive KP metabolites could underlie several therapeutic benefits.T he kynurenine pathway (KP), the major catabolic route of tryptophan (TRP) metabolism in mammals ( Fig. 1), has been closely linked to the pathogenesis of several brain disorders (1). This pathway contains several neuroactive metabolites, including 3-hydroxykynurenine (3-HK), quinolinic acid (QUIN) and kynurenic acid (KYNA) (2). QUIN is a well-characterized endogenous neurotoxin that specifically activates N-methyl-D-aspartate (NMDA) receptors, thereby inducing excitotoxicity (3, 4). The metabolites 3-HK and QUIN are also neurotoxic via the generation of free radicals and oxidative stress (5, 6). Conversely, KYNA-synthesized by kynurenine aminotransferases (KATs)-is neuroprotective through its antioxidant properties and antagonism of both the α7 nicotinic acetylcholine receptor and the glycine coagonist site of the NMDA receptor (7-13). Levels of these metabolites are regulated at two critical points in the KP: (i) the initial, rate-limiting conversion of TRP into N-formylkynurenine by either tryptophan-2,3-dioxygenase (TDO) or indoleamine-2,3-dioxygenase 1 and 2 (IDO1 and IDO2); and (ii) synthesis of 3-HK from kynurenine by the flavoprotein kynurenine-3-monoxygenase (KMO) (1).Alterations in levels of the KP metabolites have been observed in a broad range of brain disorders, including both neurodegenerative and psychiatric conditions (14). In neurodegenerative diseases such as Huntington's (HD), Parkinson's (PD), and Alzheimer's...

BackgroundThe kynurenine pathway (KP), the major catabolic route of tryptophan (TRP) metabolism, has been closely linked to the pathogenesis of several brain disorders. This pathway produces several neuroactive metabolites, including 3-hydroxykynurenine (3-HK), quinolinic acid (QUIN) and kynurenic acid (KYNA). A shift towards the synthesis of neurotoxic 3-HK and QUIN relative to neuroprotective KYNA has been found in Huntington’s disease (HD) patients and models. In a Drosophila model of HD, genetic inhibition of two pivotal KP enzymes – kynurenine 3-monooxygenase (KMO) and tryptophan 2,3-dioxygenase (TDO) – normalises KP imbalances and rescues neurodegeneration. Genetic down-regulation of TDO has also been found to improve phenotypes in worm models of neurodegeneration, including a polyglutamine model.AimsOur goal was to characterise the neuroprotection conferred by TDO inhibition in HD.Methods/techniquesWe employed Drosophila melanogaster expressing a mutant HTT exon 1 fragment (HTT93Q) for elucidating phenotypic and metabolitic effects arising from genetic and pharmacological manipulation of the KP.Results/outcomeAs TDO inhibition results in increased levels of TRP, we explored the neuroprotective potential of TRP in HD flies, which was previously found to improve phenotypes in a worm model of Parkinson’s disease. Animals fed with TRP showed a dose-dependent reduction of neurodegeneration and increased levels of KYNA relative to 3-HK. Similarly, TDO-/- HD flies displayed a decreased 3-HK/KYNA ratio. When KYNA synthesis was pharmacologically blocked in TDO-/- HD flies, a dramatic reversal of the neuroprotection conferred by TDO mutation was observed. Moreover, restoration of physiological 3-HK levels in TDO-/- HD flies was not sufficient for inducing neurodegeneration, suggesting that increased KYNA levels is central to the protection arising from TDO inhibition in this model. Interestingly, QUIN feeding enhanced neurodegeneration in HD flies, and abolished the protective effects of KMO inhibition. In addition, a transgenic Drosophila line encoding human kynurenine aminotransferase (hKAT) – the enzyme which synthesises KYNA - reduced neurodegeneration in HD flies, further supporting that increased KYNA levels is neuroprotective in HD flies. Finally, we found that treatment with the TDO inhibitor 680C91 was protective in HD flies.ConclusionsThis study supports the concept that the targeted manipulation of TDO and KMO may constitute a viable therapeutic strategy in HD.

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