Of mice, rats and men: Revisiting the quinolinic acid hypothesis of Huntington's disease

Abstract

The neurodegenerative disease Huntington's Disease (HD) is caused by an expanded polyglutamine (polyQ) tract in the protein huntingtin (htt). Although the gene encoding htt was identified and cloned more than 15 years ago, and in spite of impressive efforts to unravel the mechanism(s) by which mutant htt induces nerve cell death, these studies have so far not led to a good understanding of pathophysiology or an effective therapy. Set against a historical background, we review data supporting the idea that metabolites of the kynurenine pathway (KP) of tryptophan degradation provide a critical link between mutant htt and the pathophysiology of HD. New studies in HD brain and genetic model organisms suggest that the disease may in fact be causally related to early abnormalities in KP metabolism, favoring the formation of two neurotoxic metabolites, 3-hydroxykynurenine and quinolinic acid, over the related neuroprotective agent kynurenic acid. These findings not only link the excitotoxic hypothesis of HD pathology to an impairment of the KP but also define new drug targets and therefore have direct therapeutic implications. Thus, pharmacological normalization of the imbalance in brain KP metabolism may provide clinical benefits, which could be especially effective in the early stages of the disease. KeywordsExcitotoxicity; 3-Hydroxykynurenine; Kynurenines; Microglia; Neuroprotection Excitotoxins and Huntington's disease: the beginningHuntington's disease (HD), originally termed Huntington's chorea because of the characteristic involuntary movements shown by affected individuals, is a chronic neurodegenerative disorder, which is inherited in an autosomal dominant fashion. Overt motor symptoms usually develop in mid-life, and patients succumb to the disease after another 15-20 years on average. Long believed to be an exclusive basal ganglia disease because of the substantial and progressive neostriatal shrinkage and the massive dilation of the adjacent lateral ventricles, the neuropathology of HD is now known to involve a large number of brain regions and neuronal populations (Jackson et al., 1995;Rosas et al., 2008;Vonsattel et al., 1985).* We dedicate this article to the memory of our friend Paolo Guidetti, who passed away, prematurely on December 28, 2007.Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. NIH Public Access NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptFor a century following the first description of the disease by George Huntington in 1872 (Huntington, 1872), speculations about the cause of neuronal loss in HD were sporadic a...

The Tryptophan Metabolite 3-Hydroxyanthranilic Acid Plays Anti-Inflammatory and Neuroprotective Roles During Inflammation

“…Because neuroinflammation is thought to contribute to neurodegeneration in several different types of human CNS diseases, our results have implications for the therapeutic use of 3-HAA or related compounds. Conversely, they also suggest that therapies aimed at blockade of microglial kynurenine production 17 could have unintended consequences on inflammation and neuronal survival.…”

“…27 There is evidence that 3-HAA (and related KP intermediates) may have toxic effects on neurons. [15][16][17][18][19] However, these experiments were performed in the absence of added cytokines and/or in highly enriched neuronal cultures. In light of the cytokine-suppressive effects that we see in our culture, we next examined whether 3-HAA modulated neuronal survival in mixed CNS cultures treated with cytokines or TLR ligands, which induce neuronal death via glial inflammatory mechanisms.…”

“…[11][12][13][14] Microglial kynurenines, such as 3-HK, 3-HAA, and quinolinic acid (Figure 1), have been implicated as neurotoxins in a number of neurologic diseases due to their free radical-generating and N-methyl-D-aspartic acid (NMDA) agonist activities. [15][16][17][18][19] Because of the reported neurotoxic properties of microglial kynurenines, KP enzymes such as KMO are currently being targeted for drug development for neurodegenerative diseases, such as Huntington's disease. 17,20 Microglial response to a number of brain insults could also promote injury by establishing a cytokine cascade in the CNS through positive feedback mechanisms (IL-1 induces itself) and activation of astrocytes.…”

“…[15][16][17][18][19] Because of the reported neurotoxic properties of microglial kynurenines, KP enzymes such as KMO are currently being targeted for drug development for neurodegenerative diseases, such as Huntington's disease. 17,20 Microglial response to a number of brain insults could also promote injury by establishing a cytokine cascade in the CNS through positive feedback mechanisms (IL-1 induces itself) and activation of astrocytes. [21][22][23][24][25] IL-1 is required for human astrocyte inducible nitric oxide synthase (iNOS) and tumor necrosis factor (TNF)-␣ release 26 and is critical for neurotoxicity.…”

BDNF regulation under GFAP promoter provides engineered astrocytes as a new approach for long-term protection in Huntington's disease

“…[14][15][16][17] BDNF can also be relevant in the regulation of cognitive alterations observed in HD. 18,19 Thus, due to its prosurvival effects in striatal and cortical neuropathology, BDNF is the main candidate for neuroprotective therapies 20,21 as it has been tested after intrastriatal administration of quinolinate (QUIN), an N-methyl-Daspartic acid receptor agonist used as an acute model of HD 22 , and in transgenic mouse models. 5 However, the main problem to use neurotrophic factors as therapeutic agents for neurodegenerative disorders is the chronic delivery system.…”