Targeting Kynurenine 3-Monooxygenase (KMO): Implications for Therapy in Huntingtons Disease

Model of tryptophan metabolism, readily scalable using tissue-specific gene expression data

Tryptophan is utilized in various metabolic routes including protein synthesis, serotonin, and melatonin synthesis and the kynurenine pathway. Perturbations in these pathways have been associated with neurodegenerative diseases and cancer. Here we present a comprehensive kinetic model of the complex network of human tryptophan metabolism based upon existing kinetic data for all enzymatic conversions and transporters. By integrating tissue-specific expression data, modeling tryptophan metabolism in liver and brain returned intermediate metabolite concentrations in the physiological range. Sensitivity and metabolic control analyses identified expected key enzymes to govern fluxes in the branches of the network. Combining tissue-specific models revealed a considerable impact of the kynurenine pathway in liver on the concentrations of neuroactive derivatives in the brain. Moreover, using expression data from a cancer study predicted metabolite changes that resembled the experimental observations. We conclude that the combination of the kinetic model with expression data represents a powerful diagnostic tool to predict alterations in tryptophan metabolism. The model is readily scalable to include more tissues, thereby enabling assessment of organismal tryptophan metabolism in health and disease.

Development of LC/MS/MS, high-throughput enzymatic and cellular assays for the characterization of compounds that inhibit kynurenine monooxygenase (KMO)

Kynurenine monooxygenase (KMO) catalyzes the conversion of kynurenine to 3-hydroxykynurenine. Modulation of KMO activity has been implicated in several neurodegenerative diseases, including Huntington disease. Our goal is to develop potent and selective small-molecule KMO inhibitors with suitable pharmacokinetic characteristics for in vivo proof-of-concept studies and subsequent clinical development. We developed a comprehensive panel of biochemical and cell-based assays that use liquid chromatography/tandem mass spectrometry to quantify unlabeled kynurenine and 3-hydroxykynurenine. We describe assays to measure KMO inhibition in cell and tissue extracts, as well as cellular assays including heterologous cell lines and primary rat microglia and human peripheral blood mononuclear cells.

The causative role and therapeutic potential of the kynurenine pathway in neurodegenerative disease

Metabolites of the kynurenine pathway (KP), which arise from the degradation of tryptophan, have been studied in detail for over a century and garnered the interest of the neuroscience community in the late 1970s and early 1980s with work uncovering the neuromodulatory potential of this pathway. Much research in the following decades has found that perturbations in the levels of KP metabolites likely contribute to the pathogenesis of several neurodegenerative diseases. More recently, it has become apparent that targeting KP enzymes, in particular kynurenine 3-monooxygenase (KMO), may hold substantial therapeutic potential for these disorders. Here we provide an overview of the KP, the neuroactive properties of KP metabolites and their role in neurodegeneration. We also discuss KMO as a therapeutic target for these disorders, and our recent resolution of the crystallographic structure of KMO, which will permit the development of new and improved KMO inhibitors which may ultimately expedite clinical application of these compounds.

Potential roles of zinc in the pathophysiology and treatment of major depressive disorder

Incomplete response to monoaminergic antidepressants in major depressive disorder (MDD), and the phenomenon of neuroprogression, suggests a need for additional pathophysiological markers and pharmacological targets. Neuronal zinc is concentrated exclusively within glutamatergic neurons, acting as an allosteric modulator of the N-methyl D-aspartate and other receptors that regulate excitatory neurotransmission and neuroplasticity. Zinc-containing neurons form extensive associational circuitry throughout the cortex, amygdala and hippocampus, which subserve mood regulation and cognitive functions. In animal models of depression, zinc is reduced in these circuits, zinc treatment has antidepressant-like effects and dietary zinc insufficiency induces depressive behaviors. Clinically, serum zinc is lower in MDD, which may constitute a state-marker of illness and a risk factor for treatment-resistance. Marginal zinc deficiency in MDD may relate to multiple putative mechanisms underlying core symptomatology and neuroprogression (e.g. immune dysfunction, monoamine metabolism, stress response dysregulation, oxidative/nitrosative stress, neurotrophic deficits, transcriptional/epigenetic regulation of neural networks). Initial randomized trials suggest a benefit of zinc supplementation. In summary, molecular and animal behavioral data support the clinical significance of zinc in the setting of MDD.

Mouse models of polyglutamine diseases in therapeutic approaches: review and data table. Part II

Mouse models of human diseases are created both to understand the pathogenesis of the disorders and to find successful therapies for them. This work is the second part in a series of reviews of mouse models of polyglutamine (polyQ) hereditary disorders and focuses on in vivo experimental therapeutic approaches. Like part I of the polyQ mouse model review, this work is supplemented with a table that contains data from experimental studies of therapeutic approaches in polyQ mouse models. The aim of this review was to characterize the benefits and outcomes of various therapeutic strategies in mouse models. We examine whether the therapeutic strategies are specific to a single disease or are applicable to more than one polyQ disorder in mouse models. In addition, we discuss the suitability of mouse models in therapeutic approaches. Although the majority of therapeutic studies were performed in mouse models of Huntington disease, similar strategies were also used in other disease models.

The kynurenine pathway in neurodegenerative diseases: mechanistic and therapeutic considerations

The kynurenine pathway (KP), the primary route of tryptophan degradation in mammalian cells, consists of many metabolites including kynurenic acid (KYNA), quinolinic acid (QUIN), 3-hydroxykynurenine (3-HK) and picolinic acid (PIC). The former two are neuroactive, while the latter two are molecules with pro-oxidants and antioxidants properties. These agents are considered to be involved in aging and numerous neurodegenerative diseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD) and amyotrophic lateral sclerosis (ALS). Several studies have demonstrated that altered kynurenine metabolism plays an important role in the pathogenesis of this group of diseases. The important metabolites and key enzymes show significant importance in those disorders. Both analogs of the neuroprotective metabolites and small molecule enzyme inhibitors preventing the formation of neurotoxic compounds may have potential therapeutic significance. In this review we discuss the mechanistic and therapeutic considerations of KP in aging and the main neurodegenerative diseases and review the updated knowledge in this therapeutic field.

Protein oxidation in Huntington disease

Huntington disease (HD) is an inherited neurodegenerative disorder caused by expansion of CAG repeats in the huntingtin gene, affecting initially the striatum and progressively the cortex. Oxidative stress, and consequent protein oxidation, has been described as important to disease progression. This review focuses on recent advances in the field, with a particular emphasis on the identified target proteins and the role that their oxidation has or might have in the pathophysiology of HD. Oxidation and the resulting inactivation and/or degradation of important proteins can explain the impairment of several metabolic pathways in HD. Oxidation of enzymes involved in ATP synthesis can account for the energy deficiency observed. Impairment of protein folding and degradation can be due to oxidation of several heat shock proteins and Valosin-containing protein. Oxidation of two enzymes involved in the vitamin B6 metabolism could result in decreased availability of pyridoxal phosphate, which is a necessary cofactor in transaminations, the kynurenine pathway and the synthesis of glutathione, GABA, dopamine and serotonin, all of which have a key role in HD pathology. In addition, protein oxidation often contributes to oxidative stress, aggravating the molecular damage inside the cell.

Preparation and photophysical properties of a caged kynurenine

We have prepared l-kyurenine 4-hydroxyphenacyl ester, a caged derivative of L-kynurenine. N(α)-tBOC-L-tryptophan was reacted with 4-hydroxyphenacyl bromide in DMF with K(2)CO(3) as the base to give the N(α)-tBOC 4-hydroxyphenacyl ester. The ester was then treated with O(3) in MeOH at -20°C, followed by trifluoroacetic acid in CH(2)Cl(2), then aqueous HCl to obtain the caged kynurenine as the dihydrochloride salt. The caged kynurenine is stable as a dry solid in the dark at -78°C, but in aqueous solutions in phosphate buffer at pH 7-8 hydrolyzes rapidly (t(1/2) ∼5 min). Solutions in Tris at pH 7 are more stable (t(1/2) >30 min), and solutions in 1mM HCl are stable for several hours. As expected, the ester is cleaved in microseconds with laser pulses at 355 nm. The caged kynurenine may be useful for preparation of substrate complexes for crystallography or in biological studies on kynurenine.

The kynurenine pathway modulates neurodegeneration in a Drosophila model of Huntington's disease

Neuroactive metabolites of the kynurenine pathway (KP) of tryptophan degradation have been implicated in the pathophysiology of neurodegenerative disorders, including Huntington's disease (HD) [1]. A central hallmark of HD is neurodegeneration caused by a polyglutamine expansion in the huntingtin (htt) protein [2]. Here we exploit a transgenic Drosophila melanogaster model of HD to interrogate the therapeutic potential of KP manipulation. We observe that genetic and pharmacological inhibition of kynurenine 3-monooxygenase (KMO) increases levels of the neuroprotective metabolite kynurenic acid (KYNA) relative to the neurotoxic metabolite 3-hydroxykynurenine (3-HK) and ameliorates neurodegeneration. We also find that genetic inhibition of tryptophan 2,3-dioxygenase (TDO), the first and rate-limiting step in the pathway, leads to a similar neuroprotective shift toward KYNA synthesis. Importantly, we demonstrate that the feeding of KYNA and 3-HK to HD model flies directly modulates neurodegeneration, underscoring the causative nature of these metabolites. This study provides the first genetic evidence that inhibition of KMO and TDO activity protects against neurodegenerative disease in an animal model, indicating that strategies targeted at two key points within the KP may have therapeutic relevance in HD, and possibly other neurodegenerative disorders.

Modeling Huntington disease in yeast: perspectives and future directions

Yeast have been extensively used to model aspects of protein folding diseases, yielding novel mechanistic insights and identifying promising candidate therapeutic targets. In particular, the neurodegenerative disorder Huntington disease (HD), which is caused by the abnormal expansion of a polyglutamine tract in the huntingtin (htt) protein, has been widely studied in yeast. This work has led to the identification of several promising therapeutic targets and compounds that have been validated in mammalian cells, Drosophila and rodent models of HD. Here we discuss the development of yeast models of mutant htt toxicity and misfolding, as well as the mechanistic insights gleaned from this simple model. The role of yeast prions in the toxicity/misfolding of mutant htt is also highlighted. Furthermore, we provide an overview of the application of HD yeast models in both genetic and chemical screens, and the fruitful results obtained from these approaches. Finally, we discuss the future of yeast in neurodegenerative research, in the context of HD and other diseases.

Modeling Huntington disease in yeast: perspectives and future directions

Yeast have been extensively used to model aspects of protein folding diseases, yielding novel mechanistic insights and identifying promising candidate therapeutic targets. In particular, the neurodegenerative disorder Huntington disease (HD), which is caused by the abnormal expansion of a polyglutamine tract in the huntingtin (htt) protein, has been widely studied in yeast. This work has led to the identification of several promising therapeutic targets and compounds that have been validated in mammalian cells, Drosophila and rodent models of HD. Here we discuss the development of yeast models of mutant htt toxicity and misfolding, as well as the mechanistic insights gleaned from this simple model. The role of yeast prions in the toxicity/misfolding of mutant htt is also highlighted. Furthermore, we provide an overview of the application of HD yeast models in both genetic and chemical screens, and the fruitful results obtained from these approaches. Finally, we discuss the future of yeast in neurodegenerative research, in the context of HD and other diseases.