Estrogen and neuroprotection: higher constitutive expression of glutaredoxin in female mice offers protection against MPTP-mediated neurodegeneration

Knockdown of cytosolic glutaredoxin 1 leads to loss of mitochondrial membrane potential: implication in neurodegenerative diseases

Mitochondrial dysfunction including that caused by oxidative stress has been implicated in the pathogenesis of neurodegenerative diseases. Glutaredoxin 1 (Grx1), a cytosolic thiol disulfide oxido-reductase, reduces glutathionylated proteins to protein thiols and helps maintain redox status of proteins during oxidative stress. Grx1 downregulation aggravates mitochondrial dysfunction in animal models of neurodegenerative diseases, such as Parkinson's and motor neuron disease. We examined the mechanism underlying the regulation of mitochondrial function by Grx1. Downregulation of Grx1 by shRNA results in loss of mitochondrial membrane potential (MMP), which is prevented by the thiol antioxidant, α-lipoic acid, or by cyclosporine A, an inhibitor of mitochondrial permeability transition. The thiol groups of voltage dependent anion channel (VDAC), an outer membrane protein in mitochondria but not adenosine nucleotide translocase (ANT), an inner membrane protein, are oxidized when Grx1 is downregulated. We then examined the effect of β-N-oxalyl amino-L-alanine (L-BOAA), an excitatory amino acid implicated in neurolathyrism (a type of motor neuron disease), that causes mitochondrial dysfunction. Exposure of cells to L-BOAA resulted in loss of MMP, which was prevented by overexpression of Grx1. Grx1 expression is regulated by estrogen in the CNS and treatment of SH-SY5Y cells with estrogen upregulated Grx1 and protected from L-BOAA mediated MMP loss. Our studies demonstrate that Grx1, a cytosolic oxido-reductase, helps maintain mitochondrial integrity and prevents MMP loss caused by oxidative insult. Further, downregulation of Grx1 leads to mitochondrial dysfunction through oxidative modification of the outer membrane protein, VDAC, providing support for the critical role of Grx1 in maintenance of MMP.

Oestrogen regulates the expression and function of dopamine transporters in astrocytes of the nigrostriatal system

Dopamine is actively and specifically eliminated from the extracellular space by astrocytes and neurones through dopamine transporters (DAT) and, afterwards, either recycled into vesicles or metabolised. The availability of dopamine reflects a critical point in the regulation of dopamine activity within the nigrostriatal circuit under normal and pathological conditions. From previous studies, we know that oestrogen regulates the efficacy of dopaminergic neurones at the synaptic level and improves dopamine function during Parkinson's disease. Accordingly, we investigated the contribution of local astroglial for extracellular dopamine elimination and the impact of oestrogen on DAT expression and activity. Using neonatal striatal and midbrain astrocyte cultures, we could demonstrate that astrocytes possess a specific dopamine uptake machinery and express DAT at considerable levels. The application of 17beta-oestradiol decreased the expression of DAT by 80% and 60% in midbrain and striatal astroglia cultures, respectively. The unspecific dopamine transporters (OCT3, VMAT2) were not detected in astroglia. Functionally, oestrogen exposure inhibited the clearance of dopamine from the extracellular space by 45% and 35% compared to controls in midbrain and striatal astroglia, respectively. The effect on DAT expression and activity was completely antagonised by the oestrogen receptor antagonist ICI 182 780. In conclusion, our data suggest that the positive reinforcement of dopamine transmission under physiological conditions and the alleviative impact of oestrogen under pathological conditions may be the result of a decline in DAT expression and therefore delayed dopamine uptake by astroglia.

Downregulation of glutaredoxin but not glutathione loss leads to mitochondrial dysfunction in female mice CNS: Implications in excitotoxicity

Oxidative stress, excitotoxicity and mitochondrial dysfunction play synergistic roles in neurodegeneration. Maintenance of thiol homeostasis is important for normal mitochondrial function and dysregulation of protein thiol homeostasis by oxidative stress leads to mitochondrial dysfunction and neurodegeneration. We examined the critical roles played by the antioxidant, non-protein thiol, glutathione and related enzyme, glutaredoxin in maintaining mitochondrial function during excitotoxicity caused by beta-N-oxalyl amino-L-alanine (L-BOAA), the causative factor of neurolathyrism, a motor neuron disease involving the pyramidal system. L-BOAA causes loss of GSH and inhibition of mitochondrial complex I in lumbosacral cord of male mice through oxidation of thiol groups, while female mice are resistant. Reducing GSH levels in female mice CNS by pretreatment with diethyl maleate or L-propargyl glycine did not result in inhibition of complex I activity, unlike male mice. Further, treatment of female mice depleted of GSH with L-BOAA did not induce inhibition of complex I indicating that GSH levels were not critical for maintaining complex I activity in female mice unlike their male counterpart. Glutaredoxin, a thiol disulfide oxidoreductase helps maintain redox status of proteins and downregulation of glutaredoxin results in loss of mitochondrial complex I activity. Female mice express higher levels of glutaredoxin in certain CNS regions and downregulation of glutaredoxin using antisense oligonucleotides sensitizes them to L-BOAA toxicity seen as mitochondrial complex I loss. Ovariectomy downregulates glutaredoxin and renders female mice vulnerable to L-BOAA toxicity as evidenced by activation of AP1, loss of GSH and complex I activity indicating the important role of glutaredoxin in neuroprotection. Estrogen protects against mitochondrial dysfunction caused by excitotoxicity by maintaining cellular redox status through higher constitutive expression of glutaredoxin in the CNS. Therapeutic interventions designed to upregulate glutaredoxin may offer neuroprotection against excitotoxicity in motor neurons.

Estrogen and the development and protection of nigrostriatal dopaminergic neurons: concerted action of a multitude of signals, protective molecules, and growth factors

The nigrostriatal dopamine system comprises the dopaminergic neurons located in the ventral midbrain, their axonal connections to the forebrain, and their direct cellular target cells in the striatal complex, i.e. GABAergic neurons. The major function of the nigrostriatal dopaminergic unit is the coordination and fine tuning of motor functions at the extrapyramidal level. Numerous biologically active factors including different types of growth factors (neurotrophins, members of the TGFbeta family, IGFs) and peptide/steroid hormones have been identified in the past to be implicated in the regulation of developmental aspects of this neural system. Some of these developmentally active determinants have in addition been found to play a crucial role in the mediation of neuroprotection concerning dopaminergic neurons. Estrogen was identified as such a compound interfering with embryonic neuronal differentiation and cell survival. The physiological mechanisms underlying these effects are very complex and include interactions with other developmental signals (growth factors), inflammatory processes as well as apoptotic events, but also require the activation of nonneuronal cells such as astrocytes. It appears that estrogen is assuming control over or at least influences a multitude of developmental and protective cellular mechanisms rather than taking over the part of a singular protagonist.

Estrogen treatment of spinal cord injury attenuates calpain activation and apoptosis

Spinal cord injury (SCI) is a devastating neurologic injury, and currently, the only recommended pharmacotherapy is high-dose methylprednisolone, which has limited efficacy. Estrogen is a multi-active steroid with anti-oxidant and anti-apoptotic effects. Estrogen may modulate intracellular Ca2+ and prevent inflammation. For this study, male rats were divided into three groups. Sham-group animals received a laminectomy at T12. Injured rats received both laminectomy and 40 gram centimeter force SCI. Estrogen-group rats received 4 mg/kg 17beta-estradiol (estrogen) at 15 min and 24 hr post-injury, and vehicle-group rats received equal volumes of dimethyl sulfoxide. Animals were sacrificed at 48 hr post-injury, and 1-cm segments of the lesion, rostral penumbra, and caudal penumbra were excised. The degradation of 68 kD neurofilament protein (NFP) and estrogen receptors (ER) was examined by Western blot analysis. Protein levels of calpain and the activities of calpain and caspase-3 were also examined. Levels of cytochrome c were determined in both cytosolic and mitochondrial fractions. Cell death with DNA fragmentation was examined using the TUNEL assay. At the lesion, samples from both vehicle and estrogen treated animals showed increased levels of 68 kD NFP degradation, calpain content, calpain activity, cytochrome c release, and degradation of ERalpha and ERbeta, as compared to sham. In the caudal penumbra, estrogen treatment significantly attenuated 68 kD NFP degradation, calpain content, calpain activity, levels of cytosolic cytochrome c, and ERbeta degradation. At the lesion, vehicle-treated animals displayed more TUNEL+ cells, and estrogen treatment significantly attenuated this cell death marker. We conclude that estrogen may inhibit cell death in SCI through calpain inhibition.

Modulation of glutaredoxin-1 expression in a mouse model of allergic airway disease

Glutaredoxins (GRX) are antioxidant enzymes that preferentially catalyze the reduction of protein-glutathione mixed disulfides. The formation of mixed disulfides with GSH is known as S-glutathionylation, a post-translational modification that is emerging as an important mode of redox signaling. Since asthma is a disease that is associated with increased oxidative stress and altered antioxidant defenses, we investigated the expression of GRX in a murine model of allergic airway disease. Sensitization and challenge of C57BL/6 mice with ovalbumin resulted in increased expression of GRX1 mRNA, as well as increased amounts of GRX1 protein and total GRX activity in the lung. Because GRX1 expression is prominent in bronchial epithelium, we isolated primary epithelial cells from mouse trachea to investigate the presence of GRX. Primary tracheal epithelial cells were found to express both GRX1 and 2 mRNA and detectable GRX activity. Treatment with IFN-gamma increased the expression of GRX1 and overall GRX activity, resulting in attenuation of protein S-glutathionylation. In contrast, TGF-beta1 caused decreased GRX1 expression and overall GRX activity, leading to markedly enhanced protein S-glutathionylation. GRX1 joins the cadre of antioxidant defenses known to be modulated during allergic airway inflammation.

Gender disparities in Parkinson's disease

Parkinson's disease is a chronic neurodegenerative disorder of unknown etiology. There are sparse data on gender differences in this disorder, but it is clear that there are gender discrepancies in incidence, symptoms, medication effects and treatments. There also appear to be lifecycle fluctuations in the disease course of female Parkinson's disease patients. The effect of estrogen in this disorder is multifold and its role in the development and treatment of PD will be discussed.

17Beta-estradiol protects against oxidative stress-induced cell death through the glutathione/glutaredoxin-dependent redox regulation of Akt in myocardiac H9c2 cells

The GSH/glutaredoxin (GRX) system is involved in the redox regulation of certain enzyme activities, and this system protects cells from H2O2-induced apoptosis by regulating the redox state of Akt (Murata, H., Ihara, Y., Nakamura, H., Yodoi, J., Sumikawa, K., and Kondo, T. (2003) J. Biol. Chem. 278, 50226-50233). Estrogens, such as 17beta-estradiol (E2), play an important role in development, growth, and differentiation and appear to have protective effects on oxidative stress mediated by estrogen receptor alpha (ERalpha). However, the role of the ERbeta-mediated pathway in this cytoprotection and the involvement of E2 in the redox regulation are not well understood. In the present study, we demonstrated that E2 protected cardiac H9c2 cells, expressing ERbeta from H2O2-induced apoptosis concomitant with an increase in the activity of Akt. E2 induced the expression of glutaredoxin (GRX) as well as gamma-glutamylcysteine synthetase, a rate-limiting enzyme for the synthesis of GSH. Inhibitors for both gamma-glutamylcysteine synthetase and GRX and ICI182,780, a specific inhibitor of ERs, abolished the protective effect of E2 on cell survival as well as the activity of Akt, suggesting that ERbeta is involved in the cytoprotection and redox regulation by E2. Transcription of the GRX gene was enhanced by E2. The promoter activity of GRX was up-regulated by an ERbeta-dependent element. These results suggest that the GRX/GSH system is involved in the cytoprotective and genomic effects of E2 on the redox state of Akt, a pathway that is mediated, at least in part, by ERbeta. This mechanism may also play an antiapoptotic role in cancer cells during carcinogenesis or chemotherapy.

Regulation of mitochondrial respiratory chain structure and function by estrogens/estrogen receptors and potential physiological/pathophysiological implications

It is well known that the biological and carcinogenic effects of 17beta-estradiol (E2) are mediated via nuclear estrogen receptors (ERs) by regulating nuclear gene expression. Several rapid, non-nuclear genomic effects of E2 are mediated via plasma membrane-bound ERs. In addition, there is accumulating evidence suggesting that mitochondria are also important targets for the action of estrogens and ERs. This review summarized the studies on the effects of estrogens via ERs on mitochondrial structure and function. The potential physiological and pathophysiological implications of deficiency and/or overabundance of these E2/ER-mediated mitochondrial effects in stimulation of cell proliferation, inhibition of apoptosis, E2-mediated cardiovascular and neuroprotective effects in target cells are also discussed.

Loss of wild-type huntingtin influences motor dysfunction and survival in the YAC128 mouse model of Huntington disease

Huntington disease (HD) is an adult-onset neurodegenerative disease caused by a toxic gain of function in the huntingtin (htt) protein. The contribution of wild-type htt function to the pathogenesis of HD is currently uncertain. To assess the role of wild-type htt in HD, we generated YAC128 mice that do not express wild-type htt (YAC128-/-) but express the same amount of mutant htt as normal YAC128 mice (YAC128+/+). YAC128-/- mice perform worse than YAC128+/+ mice in the rotarod test of motor coordination (P = 0.001) and are hypoactive compared with YAC128+/+ mice at 2 months (P = 0.003). Striatal neuropathology was not clearly worse in YAC128-/- mice compared with YAC128+/+ mice. There was no significant effect of decreased wild-type htt on striatal volume, neuronal counts or DARPP-32 expression but a modest worsening of striatal neuronal atrophy was evident (6%, P = 0.03). The testis of YAC128+/+ mice showed atrophy and degeneration, which was markedly worsened in the absence of wild-type htt (P = 0.001). YAC128+/+ mice also showed a male specific deficit in survival compared with WT mice which was exacerbated by the loss of wild-type htt (12-month-male survival, P < 0.001). Overall, we demonstrate that the loss of wild-type htt influences motor dysfunction, hyperkinesia, testicular degeneration and impaired lifespan in YAC128 mice. The mild effect of wild-type htt on striatal phenotypes in YAC128 mice suggests that the characteristic striatal neuropathology in HD is caused primarily by the toxicity of mutant htt and that replacement of wild-type htt will not be an adequate treatment for HD.