George C.-T. Jiang

Inhibition of tryptophan hydroxylase activity and decreased 5‐HT1A receptor binding in a mouse model of Huntington's disease

The pathogenic mechanisms of the mutant huntingtin protein that cause Huntingtons disease (HD) are unknown. Previous studies have reported significant decreases in the levels of serotonin (5‐HT) and its metabolite 5‐hydroxyindoleacetic acid (5‐HIAA) in the brains of the R6/2 transgenic mouse model of HD. In an attempt to elucidate the cause of these neurochemical perturbations in HD, the protein levels and enzymatic activity of tryptophan hydroxylase (TPH), the rate‐limiting enzyme in 5‐HT biosynthesis, were determined. Enzyme activity was measured in brainstem homogenates from 4‐, 8‐, and 12‐week‐old R6/2 mice and compared with aged‐matched wild‐type control mice. We observed a 62% decrease in brainstem TPH activity (p = 0.009) in 4‐week‐old R6/2 mice, well before the onset of behavioral symptoms. In addition, significant decreases in TPH activity were also observed at 8 and 12 weeks of age (61%, p = 0.02 and 86%, p = 0.005, respectively). In the 12‐week‐old‐mice, no change in immunoreactive TPH was observed. In vitro binding showed that TPH does not bind to exon 1 of huntingtin in a polyglutamine‐dependent manner. Specifically, glutathione‐S‐transferase huntingtin exon 1 proteins with 20, 32 or 53 polyglutamines did not interact with radiolabeled tryptophan hydroxylase. Therefore, the inhibition of TPH activity does not appear to result from a direct huntingtin/TPH interaction. Receptor binding analyses for the 5‐HT1A receptor in 12‐week‐old R6/2 mice revealed significant reductions in 8‐OH‐[3H]DPAT binding in several hippocampal and cortical regions. These results demonstrate that the serotonergic system in the R6/2 mice is severely disrupted in both presymptomatic and symptomatic mice. The presymptomatic inhibition of TPH activity in the R6/2 mice may help explain the functional consequences of HD and provide insights into new targets for pharmacotherapy.

Neurotoxicity of depleted uranium : Reasons for increased concern

Depleted uranium (DU) is a byproduct of the enrichment process of uranium for its more radioactive isotopes to be used in nuclear energy. Because DU is pyrophoric and a dense metal with unique features when combined in alloys, it is used by the military in armor and ammunitions. There has been significant public concern regarding the use of DU by such armed forces, and it has been hypothesized to play a role in Gulf War syndrome. In light of experimental evidence from cell cultures, rats, and humans, there is justification for such concern. However, there are limited data on the neurotoxicity of DU. This review reports on uranium uses and its published health effects, with a major focus on in vitro and in vivo studies that escalate concerns that exposure to DU might be associated with neurotoxic health sequelae.

Toxicity Studies on Depleted Uranium in Primary Rat Cortical Neurons and in Caenorhabditis Elegans: What Have We Learned?

Depleted uranium (DU) is the major by-product of the uranium enrichment process for its more radioactive isotopes, retaining approximately 60% of its natural radioactivity. Given its properties as a pyrophoric and dense metal, it has been extensively used in armor and ammunitions. Questions have been raised regarding the possible neurotoxic effects of DU in humans based on follow-up studies in Gulf War veterans, where a decrease in neurocognitive behavior in a small population was noted. Additional studies in rodents indicated that DU readily traverses the blood–brain barrier, accumulates in specific brain regions, and results in increased oxidative stress, altered electrophysiological profiles, and sensorimotor deficits. This review summarizes the toxic potential of DU with emphasis on studies on thiol metabolite levels, high-energy phosphate levels, and isoprostane levels in primary rat cortical neurons. Studies in Caenorhabditis elegans detail the role of metallothioneins, small thiol-rich proteins, in protecting against DU exposure. In addition, recent studies also demonstrate that only one of the two forms, metallothionein-1, is important in the accumulation of uranium in worms.

Intersubunit binding domains within tyrosine hydroxylase and tryptophan hydroxylase

Tryptophan hydroxylase (TPH), the rate‐limiting enzyme in the biosynthesis of the neurotransmitter serotonin (5‐HT) belongs to the aromatic amino acid hydroxylase superfamily, which includes phenylalanine hydroxylase (PAH) and tyrosine hydroxylase (TH). The crystal structures for both PAH and TH have been reported, but a crystallographic model of TPH remains elusive. For this reason, we have utilized the information presented in the TH crystal structure in combination with primary sequence alignments to design point mutations in potential structural domains of the TPH protein. Mutation of a TH salt bridge (K170E) was sufficient to alter enzyme macromolecular assembly. We found that the disruption of the cognate intersubunit dimerization salt bridge (K111–E223) in TPH, however, did not affect the macromolecular assembly of TPH. Enzyme peaks representing only tetramers were observed with size exclusion chromatography. By contrast, a single‐point mutation within the tetramerization domain of TPH (L435A) was sufficient to disrupt the normal homotetrameric assembly of TPH. These studies indicate that, although the proposed salt bridge dimerization interface of TH is conserved in TPH, this hypothetical TPH intersubunit binding domain, K111–E223, is not required for the proper macromolecular assembly of the protein. However, leucine 435 within the tetramerization domain is necessary for the proper macromolecular assembly of TPH. J. Neurosci. Res. 61:313–320, 2000.

CHAPTER 29 – Depleted Uranium

Publisher Summary This chapter describes depleted uranium (DU) and its applications in weapons of mass destruction. It also highlights the DU exposure pathways, pharmacokinetics, health effects, toxicity, and available treatments. The major use of DU is by the military as an alloy in armor and ammunition. These applications take advantage of the unique metallic properties of DU, specifically the density and pyrophoric properties. Uranium is the heaviest naturally occurring element and is extremely dense. Uranium has a density 1.7 times the density of lead, and rods made of uranium are resistant to deformation. Uranium shielding is therefore used in the armor of military armored vehicles, allowing the deflection of enemy projectiles. The chemical toxicity of DU is only an issue if the metal is internalized. The three traditional pathways of exposure are inhalation, ingestion, and dermal contact. Typically, in nonmilitary situations, the main routes of natural uranium uptake are by inhalation and ingestion. Because of the use of DU in ammunitions and armor by the military, the more important routes of exposure are inhalation and internalization of the DU. DU can cause oxidative DNA damage by catalyzing hydrogen peroxide and ascorbate reactions, resulting in single strand breaks in plasmid DNA in vitro. The treatments for uranium exposure are limited. Chelation therapy is used to prevent acute toxicity of high doses of uranium in the systemic circulation, typically resulting from some sort of ingestion.

Chapter 33 – Depleted Uranium

This chapter describes depleted uranium (DU) and its applications in weapons of mass destruction. It also highlights the DU exposure pathways, pharmacokinetics, health effects, toxicity, and available treatments. The major use of DU is by the military as an alloy in armor and ammunition. These applications take advantage of the unique metallic properties of DU, specifically the density and pyrophoric properties. Uranium is the heaviest naturally occurring element and is extremely dense. Uranium has a density 1.7-times the density of lead, and rods made of uranium are resistant to deformation. Uranium shielding is therefore used in the armor of military armored vehicles, allowing the deflection of enemy projectiles. The chemical toxicity of DU is only an issue if the metal is internalized. The three traditional pathways of exposure are inhalation, ingestion, and dermal contact. Typically, in nonmilitary situations, the main routes of natural uranium uptake are by inhalation and ingestion. Because of the use of DU in ammunitions and armor by the military, the more important routes of exposure are inhalation and internalization of the DU. DU can cause oxidative DNA damage by catalyzing hydrogen peroxide and ascorbate reactions, resulting in single strand breaks in plasmid DNA in vitro. The treatments for uranium exposure are limited. Chelation therapy is used to prevent acute toxicity of high doses of uranium in the systemic circulation, typically resulting from some sort of ingestion.