Maria João Gama

Inflammatory signalling pathways involved in astroglial activation by unconjugated bilirubin

During neonatal hyperbilirubinaemia, astrocytes activated by unconjugated bilirubin (UCB) may contibute to brain toxicity through the production of cytokines. As a first step in addressing the signal transduction cascades involved in the UCB‐induced astroglial immunological response, we tested whether tumour necrosis factor (TNF)‐α receptor 1 (TNFR1), mitogen‐activated protein kinase (MAPK) and nuclear factor κB (NF‐κB) would be activated in astrocytes exposed to UCB, and examined the profile of cytokine production. Astrocyte cultures stimulated with UCB showed a rapid rise in TNFR1 protein levels, followed by activation of the MAPKs p38, Jun N‐terminal kinase1/2 and extracellular signal‐regulated kinase1/2, and NF‐κB. Interestingly, the induction of these signal effectors preceded the early up‐regulation of TNF‐α and interleukin (IL)‐1β mRNAs, and later secretion of TNF‐α, IL‐1β and IL‐6. Treatment of astrocytes with UCB also induced cell death, with levels comparable to those obtained after exposure of astrocytes to recombinant TNF‐α and IL‐1β. Moreover, loss of cell viability and cytokine secretion were reduced when the NF‐κB signal transduction pathway was inhibited, suggesting a key role for NF‐κB in the astroglial response to UCB. These results demonstrate the complexity of the molecular mechanisms involved in cell injury by UCB during hyperbilirubinaemia and provide a basis for the development of novel therapeutic strategies.

Tauroursodeoxycholic Acid Prevents MPTP-Induced Dopaminergic Cell Death in a Mouse Model of Parkinson’s Disease

Mitochondrial dysfunction and oxidative stress are implicated in the neurodegenerative process in Parkinson’s disease (PD). Moreover, c-Jun N-terminal kinase (JNK) plays an important role in dopaminergic neuronal death in substantia nigra pars compacta. Tauroursodeoxycholic acid (TUDCA) acts as a mitochondrial stabilizer and anti-apoptotic agent in several models of neurodegenerative diseases. Here, we investigated the role of TUDCA in preventing 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced neurodegeneration in a mouse model of PD. We evaluated whether TUDCA modulates MPTP-induced degeneration of dopaminergic neurons in the nigrostriatal axis, and if that can be explained by regulation of JNK phosphorylation, reactive oxygen species (ROS) production, glutathione S-transferase (GST) catalytic activation, and Akt signaling, using C57BL/6 glutathione S-transferase pi (GSTP) null mice. TUDCA efficiently protected against MPTP-induced dopaminergic degeneration. We have previously demonstrated that exacerbated JNK activation in GSTP null mice resulted in increased susceptibility to MPTP neurotoxicity. Interestingly, pre-treatment with TUDCA prevented MPTP-induced JNK phosphorylation in mouse midbrain and striatum. Moreover, the anti-oxidative role of TUDCA was demonstrated in vivo by impairment of ROS production in the presence of MPTP. Finally, results herein suggest that the survival pathway activated by TUDCA involves Akt signaling, including downstream Bad phosphorylation and NF-κB activation. We conclude that TUDCA is neuroprotective in an in vivo model of PD, acting mainly by modulation of JNK activity and cellular redox thresholds, together with activation of the Akt pro-survival pathway. These results open new perspectives for the pharmacological use of TUDCA, as a modulator of neurodegeneration in PD.

Transcriptional regulation of the human CYP46A1 brain-specific expression by Sp transcription factors

Brain defective cholesterol homeostasis has been associated with neurologic diseases, such as Alzheimer’s and Huntington’s disease. The elimination of cholesterol from the brain involves its conversion into 24(S)‐hydroxycholesterol by CYP46A1, and the efflux of this oxysterol across the blood–brain barrier. Herein, we identified the regulatory elements and factors involved the human CYP46A1 expression. Functional 5′deletion analysis mapped a region spanning from nucleotides ‐236/‐64 that is indispensable for basal expression of this TATA‐less gene. Treatment of SH‐SY5Y cells with mithramycin A resulted in a significant reduction of promoter activity, suggesting a role of Sp family of transcription factors in CYP46A1 regulation. Combination of Sp1, Sp3, and Sp4 over‐expression studies in Drosophila SL‐2 cells, and systematic promoter mutagenesis identified Sp3 and Sp4 binding to four GC‐boxes as required and sufficient for high levels of promoter activity. Moreover, Sp3 and Sp4 were demonstrated to be the major components of the protein‐DNA complexes observed in primary rat cortical extracts. Our results suggest that the cell‐type specific expression of Sp transcription factors – substitution of Sp1 by Sp4 in neurons – is responsible for the basal expression of the CYP46A1 gene. This study delineates for the first time the mechanisms underlying the human CYP46A1 transcription and thereby elucidates potential pathways underlying cholesterol homeostasis in the brain.

Sp proteins play a critical role in histone deacetylase inhibitor‐mediated derepression of CYP46A1 gene transcription

J. Neurochem. (2010) 113, 418–431.

Glutathione S-Transferase pi Mediates MPTP-Induced c-Jun N-Terminal Kinase Activation in the Nigrostriatal Pathway

Parkinson’s disease (PD) is a progressive movement disorder resulting from the death of dopaminergic neurons in the substantia nigra. Neurotoxin-based models of PD using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) recapitulate the neurological features of the disease, triggering a cascade of deleterious events through the activation of the c-Jun N-terminal kinase (JNK). The molecular mechanisms underlying the regulation of JNK activity under cellular stress conditions involve the activation of several upstream kinases along with the fine-tuning of different endogenous JNK repressors. Glutathione S-transferase pi (GSTP), a phase II detoxifying enzyme, has been shown to inhibit JNK-activated signaling by protein–protein interactions, preventing c-Jun phosphorylation and the subsequent trigger of the cell death cascade. Here, we use C57BL/6 wild-type and GSTP knockout mice treated with MPTP to evaluate the regulation of JNK signaling by GSTP in both the substantia nigra and the striatum. The results presented herein show that GSTP knockout mice are more susceptible to the neurotoxic effects of MPTP than their wild-type counterparts. Indeed, the administration of MPTP induces a progressive demise of nigral dopaminergic neurons together with the degeneration of striatal fibers at an earlier time-point in the GSTP knockout mice when compared to the wild-type mice. Also, MPTP treatment leads to increased p-JNK levels and JNK catalytic activity in both wild-type and GSTP knockout mice midbrain and striatum. Moreover, our results demonstrate that in vivo GSTP acts as an endogenous regulator of the MPTP-induced cellular stress response by controlling JNK activity through protein–protein interactions.

GSTpi expression in MPTP-induced dopaminergic neurodegeneration of C57BL/6 mouse midbrain and striatum.

MPTP-induced dopaminergic neurotoxicity involves major biochemical processes such as oxidative stress and impaired energy metabolism, leading to a significant reduction in the number of nigrostriatal dopaminergic neurons. Glutathione S-transferase pi (GSTpi) is a phase II detoxifying enzyme that provides protection of cells from injury by toxic chemicals and products of oxidative stress. In humans, polymorphisms of GSTP1 affect substrate selectivity and stability increasing the susceptibility to parkinsonism-inducing effects of environmental toxins. Given the ability of MPTP to increase the levels of reactive oxygen species and the link between altered redox potential and the expression and activity of GSTpi, we investigated the effect of MPTP on GSTpi cellular concentration in an in vivo model of Parkinson’s disease. The present study demonstrates that GSTpi is actively expressed in both substantia nigra pars compacta and striatum of C57BL/6 mice brain, mostly in oligodendrocytes and astrocytes. After systemic administration of MPTP, GSTpi expression is significantly increased in glial cells in the vicinity of dopaminergic neurons cell bodies and fibers. The results suggest that GSTpi expression may be part of the mechanism underlying the ability of glial cells to elicit protection against the mechanisms involved in MPTP-induced neuronal death.

Oxidative stress and antioxidants in neurological diseases: is there still hope?

Oxidative stress is a pathological feature common to a multitude of neurological diseases. The production of reactive oxygen species (ROS) is the main mechanism underlying this cellular redox imbalance. Antioxidants protect biological targets against ROS, therefore, they have been considered as attractive potential therapeutic agents to counteract ROS-mediated neuronal damage. However, despite encouraging in vitro and preclinical in vivo data, the clinical efficacy of antioxidant treatment strategies is marginal and most clinical trials using antioxidants as therapeutic agents in neurodegenerative diseases have yielded disappointing outcomes. This might in part be due to the need of adjustment in concentrations and time parameters between preclinical studies and clinical settings. Moreover new efficient delivery methods need to be investigated, particularly taking into account that a successful therapeutic agent for neurological diseases should readily cross the blood-brain barrier (BBB). In that sense, the use of compounds that cross the BBB and boost the endogenous antioxidant defense machinery, by activating for instance the Nrf2 pathway, or compounds that are able to modulate ROS production, such as NOX enzyme inhibitors, seems to represent a more promising approach to combat oxidative stress in the central nervous system (CNS). Here we present a brief overview of the main players in oxidative stress and outline evidences of their involvement in Parkinsons disease, Alzheimers disease, Huntingtons disease and multiple sclerosis. Finally, we review and critically discuss the potential of antioxidants as therapeutics for central nervous system disorders with a special focus on emerging novel therapeutic strategies.

S‐Glutathionylation of Keap1: a new role for glutathione S‐transferase pi in neuronal protection

Oxidative stress is a key pathological feature of Parkinsons disease (PD). Glutathione S‐transferase pi (GSTP) is a neuroprotective antioxidant enzyme regulated at the transcriptional level by the antioxidant master regulator nuclear factor‐erythroid 2‐related factor 2 (Nrf2). Here, we show for the first time that upon MPTP‐induced oxidative stress, GSTP potentiates S‐glutathionylation of Kelch‐like ECH‐associated protein 1 (Keap1), an endogenous repressor of Nrf2, in vivo. S‐glutathionylation of Keap1 leads to Nrf2 activation and subsequently increases expression of GSTP. This positive feedback regulatory loop represents a novel mechanism by which GSTP elicits antioxidant protection in the brain.

Glutathione S-transferase pi regulates UV-induced JNK signaling in SH-SY5Y neuroblastoma cells.

Activation of c-Jun N-terminal kinase (JNK) signaling pathway is a key event in apoptosis. The cellular mechanisms underlying the control of JNK catalytic activity before and immediately after stress in neuronal cells are still not completely understood. Under resting conditions the basal activity of JNK is low, since JNK is kept inactive by the presence of one or more endogenous repressors, including glutathione S-transferase pi (GSTpi). The aim of this study was to investigate the control of JNK signaling by GSTpi. We examined the modifications of GSTpi protein expression and oligomerization after UV irradiation-induced stress in human SH-SY5Y neuroblastoma cells. In parallel, we investigated the effect of UV irradiation on JNK activation and c-Jun phosphorylation, and whether apoptosis represents a functional consequence triggered by this signaling pathway. We show that in SH-SY5Y cells JNK phosphorylation and activation precedes c-Jun phosphorylation and caspase-3 cleavage. Importantly, the increase of JNK enzymatic activity correlates with the dissociation of GSTpi-JNK complexes and the increased concentration of GSTpi multimer forms. Results presented herein show for the first time direct interaction between JNK and GSTpi in SH-SY5Y neuroblastoma cells, and suggest that in these cells GSTpi may serve as a regulator of JNK catalytic activity. This work contributes to further elucidate the mechanisms underlying the regulation of JNK activity under stress conditions.

Ubiquitin–Proteasome System Impairment and MPTP-Induced Oxidative Stress in the Brain of C57BL/6 Wild-type and GSTP Knockout Mice

The ubiquitin–proteasome system (UPS) is the primary proteolytic complex responsible for the elimination of damaged and misfolded intracellular proteins, often formed upon oxidative stress. Parkinson’s disease (PD) is neuropathologically characterized by selective death of dopaminergic neurons in the substantia nigra (SN) and accumulation of intracytoplasmic inclusions of aggregated proteins. Along with mitochondrial dysfunction and oxidative stress, defects in the UPS have been implicated in PD. Glutathione S-transferase pi (GSTP) is a phase II detoxifying enzyme displaying important defensive roles against the accumulation of reactive metabolites that potentiate the aggression of SN neuronal cells, by regulating several processes including S-glutathionylation, modulation of glutathione levels and control of kinase-catalytic activities. In this work we used C57BL/6 wild-type and GSTP knockout mice to elucidate the effect of both MPTP and MG132 in the UPS function and to clarify if the absence of GSTP alters the response of this pathway to the neurotoxin and proteasome inhibitor insults. Our results demonstrate that different components of the UPS have different susceptibilities to oxidative stress. Importantly, when compared to the wild-type, GSTP knockout mice display decreased ubiquitination capacity and overall increased susceptibility to UPS damage and inactivation upon MPTP-induced oxidative stress.