Neostriatal and cortical quinolinate levels are increased in early grade Huntington's disease

Huntington's disease (HD), an inherited neurodegenerative disorder, is caused by an abnormal polyglutamine expansion in the huntingtin protein. This genetic defect may result in heightened neuronal susceptibility to excitotoxic injury, a mechanism that has been postulated to play a critical role in HD. Quinolinate (QUIN) and kynurenate (KYNA), two endogenous neuroactive metabolites of the kynurenine pathway of tryptophan degradation, have been proposed to modulate excitotoxic neuronal death in HD. A third kynurenine pathway metabolite, the free radical generator 3-hydroxykynurenine (3-HK), has also been hypothesized to play a causal role in the pathogenesis of HD. We show here that the brain levels of both 3-HK and QUIN are increased three to four-fold in low-grade (grade 0/1) HD brain. These changes were seen in the neocortex and in the neostriatum, but not in the cerebellum. In contrast, brain 3-HK and QUIN levels were either unchanged or tended to decrease in grade 2 and advanced grade (grades 3–4) HD brain. Brain kynurenine and KYNA levels fluctuated only modestly as the illness progressed. These results support a possible involvement of 3-HK and QUIN in the early phases of HD pathophysiology and indicate novel therapeutic strategies against the disease.

Introduction

Huntington's disease (HD) is an inherited neurodegenerative disease that is caused by an expansion in the number of CAG (glutamine) repeats in the huntingtin protein (The Huntington's Disease Collaboration Group, 1993). Several mechanisms have been invoked to explain how the genetic mutation results in the characteristic pattern of nerve cell death observed in HD. The initial cascade of pathophysiological events appears to involve interference with DNA transcription (Luthi-Carter and Cha, 2003) and inhibition of fast axonal transport (Feany and La Spada, 2003). Eventually, these impairments result in abnormal intracellular protein aggregation, dysregulation of calcium homeostasis, and finally neuronal collapse (MacDonald et al., 2003, Panov et al., 2002).

The most dramatic neuronal damage in HD is seen in the neostriatum, which undergoes progressive deterioration and substantial shrinkage during the disease and in the neocortex. Neuronal loss in many other brain areas such as the cerebellum is conspicuously absent or comparatively minor (Dunlap, 1927, Rosas et al., 2003). Many of the distinct neuropathological features of the HD neostriatum, including the survival of large cholinergic and medium-sized aspiny interneurons, have been shown to be duplicated in experimental animals by an intrastriatal injection of quinolinate (QUIN), a product of tryptophan degradation along the kynurenine pathway. This suggested that QUIN, a selective NMDA receptor agonist and potent excitotoxin present in the mammalian brain, might be causally involved in HD pathology (Schwarcz and Albin, 2002). Subsequent studies indicated that two other metabolic products of the kynurenine pathway (collectively termed “kynurenines”), the free radical generator and QUIN precursor 3-hydroxykynurenine (3-HK) and the neuroinhibitory compound kynurenic acid (KYNA), may also play a role in the pathophysiology of HD. 3-HK induces “HD-like” neuronal death in cultured striatal neurons (Okuda et al., 1996) and potentiates QUIN excitotoxicity in vivo and in vitro (Chiarugi et al., 2001, Guidetti and Schwarcz, 1999). In contrast, KYNA, which preferentially antagonizes the function of NMDA and α7 nicotinic acetylcholine receptors at low concentrations, provides anti-excitotoxic neuroprotection, and is particularly effective in preventing QUIN-induced injury (Foster et al., 1984, Schwarcz and Pellicciari, 2002).

Several studies, mostly conducted using tissue from HD patients who died after a prolonged illness and whose brains showed severe striatal atrophy, have described abnormalities of cerebral kynurenine pathway metabolism in HD. This included enzymatic changes, reductions in the tissue levels of QUIN (Heyes et al., 1991) and KYNA (Beal et al., 1990, Beal et al., 1992, Jauch et al., 1995), and elevated 3-HK levels (Pearson and Reynolds, 1992). Notably, the increase in brain 3-HK was found to be more pronounced in early stage (grade; cf. Vonsattel et al., 1985) HD cases, which also showed unexpected, moderate elevations of KYNA in cortex and neostriatum (Guidetti et al., 2000, Pearson and Reynolds, 1992). Taken together, these reports suggested that qualitatively different changes in kynurenine pathway metabolism occur at various stages of the neurodegenerative process. These results, which were duplicated in mice transgenic for full-length mutant huntingtin (Guidetti et al., 2000), suggested that a disproportionately increased flux through the 3-HK/QUIN branch of the pathway might contribute to neuronal degeneration in the early phases of HD.

The present study was designed to test this hypothesis further, comparing the levels of 3-HK, QUIN and KYNA, and their common bioprecursor l-kynurenine, in neostriatum, frontal cortex, and cerebellum in various grades of the disease. Because data from later, advanced grades of the disease have been reported previously by us and others, samples from grade 3/4 cases were only included in small numbers and for comparative purposes. Our data, which have been communicated in abstract form (Guidetti et al., 2003), provide evidence for a regionally selective increase in brain 3-HK and QUIN levels in grade 0/1 HD brains and suggest novel strategies to attenuate or prevent neurodegeneration in HD.