Rebekah J. Jakel

Nrf2, a multi-organ protector?

NF‐E2‐related factor 2 (Nrf2) is a basic leucine zipper transcription factor that binds to the promoter sequence “antioxidant responsive element (ARE)” leading to coordinated up‐regulation of ARE‐driven detoxification and antioxidant genes. Since the expression of a wide array of antioxidant and detoxification genes are positively regulated by the ARE sequence, Nrf2 may serve as a master regulator of the ARE‐driven cellular defense system against oxidative stress. In support of this, numerous studies have shown that Nrf2 protects many cell types and organ systems from a broad spectrum of toxic insults and disease pathogenesis. This Nrf2‐conferred, multi‐organ protection phenomenon raises an interesting question about how a single protein can protect many different organs from various toxic insults. A possible molecular mechanism explaining this phenomenon is that Nrf2 protects many different cell types by coordinately up‐regulating classic ARE‐driven genes as well as cell type‐specific target genes that are required for the defense system of each cell type in its unique environment. This hypothesis is supported by microarray data indicating the protective role of Nrf2 is conveyed through both known ARE‐driven genes and novel cell type‐specific genes. The widespread nature of Nrf2 may have an important therapeutic potential, allowing prevention of carcinogenesis and neurodegenerative diseases. Lee, J.‐M., Li, J., Johnson, D. A., Stein, T. D., Kraft, A. D., Calkins, M. J., Jakel, R. J., Johnson, J. A. Nrf2, a multi‐organ protector? FASEB J. 19, 1061–1066 (2005)

The Nrf2–ARE Pathway

Transcriptional activation of protective genes is mediated by a cis‐acting element called the antioxidant responsive element (ARE). The transcription factor Nrf2 (NF–E2‐related factor 2) binds to the ARE. Activation of this pathway protects cells from oxidative stress‐induced cell death. Increased oxidative stress is associated with neuronal cell death during the pathogenesis of multiple chronic neurodegenerative diseases, including Alzheimers disease, Parkinsons disease, Huntingtons disease, and amyotrophic lateral sclerosis. We hypothesize that Nrf2–ARE activation is a novel neuroprotective pathway that confers resistance to a variety of oxidative, stress‐related, neurodegenerative insults. In recent studies, primary neuronal cultures treated with chemical activators of the Nrf2–ARE pathway displayed significantly greater resistance to oxidative stress‐induced neurotoxicity. Similar cultures generated from ARE–hPAP reporter mice demonstrated selective activation of the Nrf2–ARE pathway in astrocytes, suggesting that Nrf2 activation in astrocytes somehow confers resistance to naive neurons. Further, in chemical models of neurodegeneration, Nrf2 knockout mice are significantly more sensitive to mitochondrial complex I and II inhibitors. Combining these observations with the results implying that the astrocyte is central to Nrf2–ARE‐mediated neuroprotection, we transplanted Nrf2‐overexpressing astrocytes into the mouse striatum prior to lesioning with malonate. This procedure led to dramatic protection against malonate‐induced neurotoxicity. Translating this to other chemical and genetic models of neurodegeneration will be discussed.

Human neural stem cell transplants improve motor function in a rat model of Huntington's disease

The present study investigated the neuroanatomical and behavioral effects of human stem cell transplants into the striatum of quinolinic acid (QA)‐lesioned rats. Twenty‐four rats received unilateral QA (200 nM/μl) injections into the striatum. One week later, rats were transplanted with stem cells derived from human fetal cortex (12 weeks postconception) that were either 1) pretreated in culture media with the differentiating cytokine ciliary neurotrophic factor (CNTF; n = 9) or 2) allowed to grow in culture media alone (n = 7). Each rat was injected with a total of 200,000 cells. A third group of rats (n = 8) was given a sham injection of vehicle. Rats transplanted with human stem cells performed significantly better over the 8 weeks of testing on the cylinder test compared with those treated with vehicle (P ≤ 0.001). Stereological striatal volume analyses performed on Nissl‐stained sections revealed that rats transplanted with CNTF‐treated neurospheres had a 22% greater striatal volume on the lesioned side compared with those receiving transplants of untreated neurospheres (P = 0.0003) and a 26% greater striatal volume compared with rats injected with vehicle (P ≤ 0.0001). Numerous human nuclei‐positive cells were visualized in the striatum in both transplantation groups. Grafted cells were also observed in the globus pallidus, entopeduncular nucleus, and substantia nigra pars reticulata, areas of the basal ganglia receiving striatal projections. Some of the human nuclei‐positive cells coexpressed glial fibrillary acidic protein and NeuN, suggesting that they had differentiated into neurons and astrocytes. Taken together, these data demonstrate that striatal transplants of human fetal stem cells elicit behavioral and anatomical recovery in a rodent model of Huntingtons disease. J. Comp. Neurol. 475:211–219, 2004.

Neuronal cell death in Huntington’s disease: a potential role for dopamine

Huntingtons disease is an inherited neurodegenerative disorder, the cause of which is unknown. Excitotoxicity, mitochondrial dysfunction and oxidative stress are all likely to contribute to the striatal cell death that occurs in this disorder. There are accumulating data indicating that under specific circumstances, dopamine, which occurs in high concentrations in the basal ganglia, might be neurotoxic. In this article, the current models used to study Huntingtons disease are reviewed and the recent findings that implicate dopamine in the pathophysiology of this progressive disorder are discussed. Although many questions remain unanswered, the dopaminergic system could contribute to striatal vulnerability in Huntingtons disease and provide a novel avenue for the development of new therapies.

Nrf2-mediated protection against 6-hydroxydopamine

Parkinsons disease (PD) is a neurodegenerative movement disorder characterized by cell loss in the substantia nigra resulting in striatal dopamine depletion. Although the cause of sporadic PD is unknown, oxidative stress is thought to contribute to disease pathogenesis. One mechanism by which cells defend themselves against oxidative stress is through the transcriptional upregulation of cytoprotective genes. Under oxidative stress conditions, the transcription factor NF-E2-related factor (Nrf2) binds to the antioxidant response element (ARE) to induce antioxidant and phase II detoxification enzymes. Here we show that loss of Nrf2-mediated transcription exacerbates vulnerability to the neurotoxin 6-hydroxydopamine (6-OHDA) both in vitro and in vivo. We further demonstrate that activation of the Nrf2-ARE pathway by the known chemical inducer tert-butylhydroquinone can protect against 6-OHDA in vitro. Induction of this pathway by transplantation of astrocytes overexpressing Nrf2 can protect against 6-OHDA-induced damage in the living mouse. This suggests that the Nrf2-ARE pathway is a promising target for therapeutics aimed at reducing or preventing cell death in PD.

USING HUMAN NEURAL STEM CELLS TO MODEL NEUROLOGICAL DISEASE

Although many common neurological diseases can be modelled in rodents, in many cases these animal models do not faithfully reproduce the human syndrome at either the molecular or anatomical levels — perhaps owing to important species differences. The study of diseased human brain tissue is therefore crucial for understanding how mutant proteins or toxins might lead to neuronal dysfunction. Unfortunately, this tissue is both scarce and difficult to manipulate. Human stem cells represent a renewable source of tissue that can generate both neurons and glia. Studies that use human stem cells from diseased tissues or stem cells that have been engineered to express specific mutant proteins promise to provide new insights into the mechanisms that underlie neurological diseases.

Evidence that Par-4 Participates in the Pathogenesis of HIV Encephalitis

Progressive neuronal degeneration in brain regions involved in learning and memory processes is a common occurrence in patients infected with human immunodeficiency virus type 1 (HIV-1). We now report that levels of Par-4, a protein recently linked to neuronal apoptosis in Alzheimers disease, are increased in neurons in hippocampus of human patients with HIV encephalitis and in monkeys infected with a chimeric strain of HIV-1 and simian immunodeficiency virus. Par-4 levels increased rapidly in cultured hippocampal neurons following exposure to the neurotoxic HIV-1 protein Tat, and treatment of the cultures with a Par-4 antisense oligonucleotide protected the neurons against Tat-induced apoptosis. Additional findings show that Par-4 participates at an early stage of Tat-induced neuronal apoptosis before caspase activation, oxidative stress, and mitochondrial dysfunction. Our data suggest that Par-4 may be a mediator of neuronal apoptosis in HIV encephalitis and that therapeutic approaches targeting the Par-4 apoptotic cascade may prove beneficial in preventing neuronal degeneration and associated dementia in patients infected with HIV-1.

6-Hydroxydopamine injections into the nigrostriatal pathway attenuate striatal malonate and 3-nitropropionic acid lesions

The mitochondrial inhibitors malonate and 3-nitropropionic (3NP) acid are potent neurotoxins in vivo. Administration of these compounds results in neuronal loss similar to that seen in Huntingtons disease. Although the mechanism of cell death produced by these compounds likely involves activation of N-methyl-D-aspartate receptors, it remains unclear why the striatum demonstrates regional susceptibility to the toxicity of these and other mitochondrial poisons. We hypothesized that dopamine, a weak neurotoxin that occurs in high concentrations in the striatum, may contribute to the neuronal damage caused by mitochondrial inhibition. We investigated whether depletion of striatal dopamine using the catecholaminergic toxin 6-hydroxydopamine would attenuate lesions induced by mitochondrial inhibition. We found that dopamine depletion reduced significantly the extent of histological damage in the striatum elicited by both intraparenchymal injections of 0.8 micromol malonate and 20 mg/kg systemic administration of 3NP. These data suggest that dopamine or one of its metabolites may contribute to mitochondrial toxin-induced cell death.

Methamphetamine toxicity is attenuated in mice that overexpress human manganese superoxide dismutase.

We have investigated methamphetamine (MA) toxicity in transgenic mice that overexpress the human form of mitochondrial manganese superoxide dismutase (MnSOD). Our results reveal a significant reduction in the long-term depletion of striatal dopamine and protein oxidation following repeated administration of MA in transgenic vs. non-transgenic littermates. These findings support the notion that ROS contribute to MA-induced brain damage and suggest that mitochondria may play an important role in this form of neurodegeneration.

Serotonin syndrome after a massive overdose of controlled-release paroxetine.

Background Serotonin syndrome is a condition resulting from an overabundance of serotonin at postsynaptic receptors. The syndrome usually responds to cyproheptadine and benzodiazepines. However, some patients do not respond to conventional treatment. Objective The authors report on the history and management of a patient, a 55-year-old man, who had taken a massive overdose of paroxetine, controlled-release. Method The authors present a case report of history and treatment. Paroxetine levels were measured on Days 3 and 7 after admission, with rising values of the drug. The patient received a course of cyproheptadine and lorazepam, to which he was unresponsive, and he was transferred to the Medical Intensive Care Unit for heavy sedation. Results The patient had initially presented with minor symptoms of serotonin syndrome, but these quickly progressed in severity, and he eventually died from resulting complications: a pulmonary embolism 9 days after admission, despite appropriate prophylaxis with enoxaparin. Conclusion The authors are the first to report on a paroxetine overdose of this magnitude, and they provide one of the few reports on a prolonged course of serotonin syndrome that was unresponsive to standard treatment. (Psychosomatics 2010; 51:437–442)