An exploratory investigation of brain-selective estrogen treatment in males using a mouse model of Alzheimer's disease

Estrogens are neuroprotective, and studies suggest that they may mitigate the pathology and symptoms of Alzheimer's disease (AD) in female models. However, central estrogen effects have not been examined in males in the context of AD. The purpose of this follow-up study was to assess the benefits of a brain-selective 17β-estradiol estrogen prodrug, 10β,17β-hydroxyestra-1,4-dien-3-one (DHED), also in the male APPswe/PS1dE9 double-transgenic mouse model of the disease. After continuously exposing 6-month old animals to DHED for two months, their brains showed decreased amyloid precursor and amyloid-β protein levels. The DHED-treated APPswe/PS1dE9 double transgenic subjects also exhibited enhanced performance in a cognitive task, while 17β-estradiol treatment did not reach statistical significance. Taken together, data presented here suggest that DHED may also have therapeutic benefit in males and warrant further investigations to fully elucidate the potential of targeted estrogen therapy for a gender-independent treatment of early-stage AD.

A comparative evaluation of treatments with 17β-estradiol and its brain-selective prodrug in a double-transgenic mouse model of Alzheimer's disease

Estrogens are neuroprotective and, thus, potentially useful for the therapy of Alzheimer's disease; however, clinical use of hormone therapy remains controversial due to adverse peripheral effects. The goal of this study was to investigate the benefits of treatment with 10β,17β-dihydroxyestra-1,4-dien-3-one (DHED), a brain-selective prodrug of 17β-estradiol, in comparison with the parent hormone using APPswe/PS1dE9 double transgenic mice to model the pathology of the disease. Ovariectomized and intact females were continuously treated with vehicle, 17β-estradiol, or DHED via subcutaneous osmotic pumps from 6 to 8months of age. We confirmed that this prolonged treatment with DHED did not stimulate uterine tissue, whereas 17β-estradiol treatment increased uterine weight. Amyloid precursor protein decreased in both treatment groups of intact, but not in ovariectomized double transgenic females in which ovariectomy already decreased the expression of this protein significantly. However, reduced brain amyloid-β peptide levels could be observed for both treatments. Consequently, double-transgenic ovariectomized and intact mice had higher cognitive performance compared to untreated control animals in response to both estradiol and DHED administrations. Overall, the tested brain-selective 17β-estradiol prodrug proved to be an effective early-stage intervention in an Alzheimer's disease-relevant mouse model without showing systemic impact and, thus, warrants further evaluation as a potential therapeutic candidate.

Sex-dependent mitochondrial respiratory impairment and oxidative stress in a rat model of neonatal hypoxic-ischemic encephalopathy

Increased male susceptibility to long-term cognitive deficits is well described in clinical and experimental studies of neonatal hypoxic-ischemic encephalopathy. While cell death signaling pathways are known to be sexually dimorphic, a sex-dependent pathophysiological mechanism preceding the majority of secondary cell death has yet to be described. Mitochondrial dysfunction contributes to cell death following cerebral hypoxic-ischemia (HI). Several lines of evidence suggest that there are sex differences in the mitochondrial metabolism of adult mammals. Therefore, this study tested the hypothesis that brain mitochondrial respiratory impairment and associated oxidative stress is more severe in males than females following HI. Maximal brain mitochondrial respiration during oxidative phosphorylation was two-fold more impaired in males following HI. The endogenous antioxidant glutathione was 30% higher in the brain of sham females compared to males. Females also exhibited increased glutathione peroxidase (GPx) activity following HI injury. Conversely, males displayed a reduction in mitochondrial GPx4 protein levels and mitochondrial GPx activity. Moreover, a 3-4-fold increase in oxidative protein carbonylation was observed in the cortex, perirhinal cortex, and hippocampus of injured males, but not females. These data provide the first evidence for sex-dependent mitochondrial respiratory dysfunction and oxidative damage, which may contribute to the relative male susceptibility to adverse long-term outcomes following HI. Lower basal GSH levels, lower post-hypoxic mitochondrial glutathione peroxidase (mtGPx) activity, and mitochondrial glutathione peroxidase 4 (mtGPx4) protein levels may contribute to the susceptibility of the male brain to oxidative damage and mitochondrial dysfunction following neonatal hypoxic-ischemia (HI). Treatment of male pups with acetyl-L-carnitine (ALCAR) protects against the loss of mtGPx activity, mtGPx4 protein, and increases in protein carbonylation after HI. These findings provide novel insight into the pathophysiology of sexually dimorphic outcomes following HI.

BRCA1 is a novel regulator of metabolic function in skeletal muscle

Breast cancer type 1 (BRCA1) susceptibility protein is expressed across multiple tissues including skeletal muscle. The overall objective of this investigation was to define a functional role for BRCA1 in skeletal muscle using a translational approach. For the first time in both mice and humans, we identified the presence of multiple isoforms of BRCA1 in skeletal muscle. In response to an acute bout of exercise, we found increases in the interaction between the native forms of BRCA1 and the phosphorylated form of acetyl-CoA carboxylase. Decreasing BRCA1 content using a shRNA approach in cultured primary human myotubes resulted in decreased oxygen consumption by the mitochondria and increased reactive oxygen species production. The decreased BRCA1 content also resulted in increased storage of intracellular lipid and reduced insulin signaling. These results indicate that BRCA1 plays a critical role in the regulation of metabolic function in skeletal muscle. Collectively, these data reveal BRCA1 as a novel target to consider in our understanding of metabolic function and risk for development of metabolic-based diseases.

AICAR inhibits oxygen consumption by intact skeletal muscle cells in culture

Activation of 5' adenosine monophosphate-activated protein kinase (AMPK) with aminoimidazole carboxamide ribonucleotide (AICAR) increases skeletal muscle glucose uptake and fatty acid oxidation. The purpose of these experiments was to utilize AICAR to enhance palmitate consumption by mitochondria in cultured skeletal muscle cells. In these experiments, we treated C2C12 myotubes or adult single skeletal muscle fibers with varying concentrations of AICAR for different lengths of time. Surprisingly, acute AICAR exposure at most concentrations (0.25-1.5 mM), but not all (0.1 mM), modestly inhibited oxygen consumption even though AICAR increased AMPK phosphorylation. The data suggest that AICAR inhibited oxygen consumption by the cultured muscle in a non-specific manner. The results of these experiments are expected to provide valuable information to investigators interested in using AICAR in cell culture studies.

Ectopic lipid deposition and the metabolic profile of skeletal muscle in ovariectomized mice

Disruptions of ovarian function in women are associated with increased risk of metabolic disease due to dysregulation of peripheral glucose homeostasis in skeletal muscle. Our previous evidence suggests that alterations in skeletal muscle lipid metabolism coupled with altered mitochondrial function may also develop. The objective of this study was to use an integrative metabolic approach to identify potential areas of dysfunction that develop in skeletal muscle from ovariectomized (OVX) female mice compared with age-matched ovary-intact adult female mice (sham). The OVX mice exhibited significant increases in body weight, visceral, and inguinal fat mass compared with sham mice. OVX mice also had significant increases in skeletal muscle intramyocellular lipids (IMCL) compared with the sham animals, which corresponded to significant increases in the protein content of the fatty acid transporters CD36/FAT and FABPpm. A targeted metabolic profiling approach identified significantly lower levels of specific acyl carnitine species and various amino acids in skeletal muscle from OVX mice compared with the sham animals, suggesting a potential dysfunction in lipid and amino acid metabolism, respectively. Basal and maximal mitochondrial oxygen consumption rates were significantly impaired in skeletal muscle fibers from OVX mice compared with sham animals. Collectively, these data indicate that loss of ovarian function results in increased IMCL storage that is coupled with alterations in mitochondrial function and changes in the skeletal muscle metabolic profile.

Measuring mitochondrial respiration in intact single muscle fibers

Measurement of mitochondrial function in skeletal muscle is a vital tool for understanding regulation of cellular bioenergetics. Currently, a number of different experimental approaches are employed to quantify mitochondrial function, with each involving either mechanically or chemically induced disruption of cellular membranes. Here, we describe a novel approach that allows for the quantification of substrate-induced mitochondria-driven oxygen consumption in intact single skeletal muscle fibers isolated from adult mice. Specifically, we isolated intact muscle fibers from the flexor digitorum brevis muscle and placed the fibers in culture conditions overnight. We then quantified oxygen consumption rates using a highly sensitive microplate format. Peak oxygen consumption rates were significantly increased by 3.4-fold and 2.9-fold by simultaneous stimulation with the uncoupling agent, carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP), and/or pyruvate or palmitate exposure, respectively. However, when calculating the total oxygen consumed over the entire treatment, palmitate exposure resulted in significantly more oxygen consumption compared with pyruvate. Further, as proof of principle for the procedure, we isolated fibers from the mdx mouse model, which has known mitochondrial deficits. We found significant reductions in initial and peak oxygen consumption of 51% and 61% compared with fibers isolated from the wild-type (WT) animals, respectively. In addition, we determined that fibers isolated from mdx mice exhibited less total oxygen consumption in response to the FCCP + pyruvate stimulation compared with the WT mice. This novel approach allows the user to make mitochondria-specific measures in a nondisrupted muscle fiber that has been isolated from a whole muscle.

Adaptation of microplate-based respirometry for hippocampal slices and analysis of respiratory capacity

Multiple neurodegenerative disorders are associated with altered mitochondrial bioenergetics. Although mitochondrial O(2) consumption is frequently measured in isolated mitochondria, isolated synaptic nerve terminals (synaptosomes), or cultured cells, the absence of mature brain circuitry is a remaining limitation. Here we describe the development of a method that adapts the Seahorse Extracellular Flux Analyzer (XF24) for the microplate-based measurement of hippocampal slice O(2) consumption. As a first evaluation of the technique, we compared whole-slice bioenergetics with previous measurements made with synaptosomes or cultured neurons. We found that mitochondrial respiratory capacity and O(2) consumption coupled to ATP synthesis could be estimated in cultured or acute hippocampal slices with preserved neural architecture. Mouse organotypic hippocampal slices oxidizing glucose displayed mitochondrial O(2) consumption that was well coupled, as determined by the sensitivity to the ATP synthase inhibitor oligomycin. However, stimulation of respiration by uncoupler was modest (<120% of basal respiration) compared with previous measurements in cells or synaptosomes, though enhanced slightly (to ∼150% of basal respiration) by acute addition of the mitochondrial complex I-linked substrate pyruvate. These findings suggest a high basal utilization of respiratory capacity in slices and a limitation of glucose-derived substrate for maximal respiration. The improved throughput of microplate-based hippocampal respirometry over traditional O(2) electrode-based methods is conducive to neuroprotective drug screening. When coupled with cell type-specific pharmacology or genetic manipulations, the ability to measure O(2) consumption efficiently from whole slices should advance our understanding of mitochondrial roles in physiology and neuropathology.

Effects of the organochlorine pesticide methoxychlor on dopamine metabolites and transporters in the mouse brain

Pesticide exposure has been suggested as a risk factor in developing Parkinson's disease (PD). While the molecular mechanism underlying this association is not clear, several studies have demonstrated a role for mitochondrial dysfunction and oxidative damage in PD. Although data on specific pesticides associated with PD are often lacking, several lines of evidence point to the potential involvement of the organochlorine class of pesticides. Previously, we have found that the organochlorine pesticide methoxychlor (mxc) causes mitochondrial dysfunction and oxidative stress in isolated mitochondria. Here, we sought to determine whether mxc-induced mitochondrial dysfunction results in oxidative damage and dysfunction of the dopamine system. Adult female CD1 mice were dosed with either vehicle (sesame oil) or mxc (16, 32, or 64 mg/kg/day) for 20 consecutive days. Following treatment, we observed a dose-related increase in protein carbonyl levels in non-synaptic mitochondria, indicating oxidative modification of mitochondrial proteins which may lead to mitochondrial dysfunction. Mxc exposure also caused a dose-related decrease in striatal levels of dopamine (16-31%), which were accompanied by decreased levels of the dopamine transporter (DAT; 35-48%) and the vesicular monoamine transporter 2 (VMAT2; 21-44%). Because mitochondrial dysfunction, oxidative damage, and decreased levels of DAT and VMAT2 are found in PD patients, our data suggest that mxc should be investigated as a possible candidate involved in the association of pesticides with increased risk for PD, particularly in highly exposed populations.

Interaction of mitochondrial respiratory inhibitors and excitotoxins potentiates cell death in hippocampal slice cultures

The broad-spectrum insecticide rotenone, an inhibitor of complex I of the mitochondrial electron transport chain (ETC), gives rise to oxidative stress and bioenergetic failure. Pesticides including rotenone have been implicated in human neurodegenerative diseases, including Parkinson's disease. Another intensively investigated hypothesis of neurodegenerative disease involves the toxic action of the excitatory neurotransmitter glutamate. In the present study, we determined whether concomitant exposure of rotenone plus tetraethylammonium chloride (TEA) or the specific glutamate receptor agonists N-methyl-D-aspartate (NMDA) or alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) would cause greater cell death in organotypic hippocampal slice cultures than when given separately. Low, sublethal rotenone (100 nM), TEA (0.5-2.0 mM), NMDA (1.0-10 microM), and AMPA (1.0-10 microM) alone resulted in little cell death as determined by propidium iodide fluorescence. However, cell death was significantly to dramatically potentiated when the hippocampal slices were coincubated with comparable concentrations of rotenone plus TEA, NMDA, or AMPA. Similarly, in the presence of 10 microM NMDA, ETC inhibitors blocking other mitochondrial complexes also potentiated cell death. Immunohistochemical analysis using glial fibrillary acidic protein antibody determined that the cell death was preferentially neuronal. These results demonstrate that two different classes of toxicants can interact, resulting in potentiation of neurotoxicity, and further suggest that a combinatorial therapeutic approach may be required to ameliorate the potentiated cell death.

Postnatal developmental regulation of Bcl-2 family proteins in brain mitochondria

Although it has been long recognized that the relative balance of pro- and antiapoptotic Bcl-2 proteins is critical in determining the susceptibility to apoptotic death, only a few studies have examined the level of these proteins specifically at mitochondria during postnatal brain development. In this study, we examined the age-dependent regulation of Bcl-2 family proteins using rat brain mitochondria isolated at various postnatal ages and from the adult. The results indicate that a general down-regulation of most of the proapoptotic Bcl-2 proteins present in mitochondria occurs during postnatal brain development. The multidomain proapoptotic Bax, Bak, and Bok are all expressed at high levels in mitochondria early postnatally but decline in the adult. Multiple BH3-only proteins, including direct activators (Bid, Bim, and Puma) and the derepressor BH3-only protein Bad, are also present in immature brain mitochondria and are down-regulated in the adult brain. Antiapoptotic Bcl-2 family members are differentially regulated, with a shift from high Bcl-2 expression in immature mitochondria to predominant Bcl-x(L) expression in the adult. These results support the concept that developmental differences in upstream regulators of the mitochondrial apoptotic pathway are responsible for the increased susceptibility of cells in the immature brain to apoptosis following injury.

Methoxychlor causes mitochondrial dysfunction and oxidative damage in the mouse ovary☆

Methoxychlor (MXC) is an organochlorine pesticide that reduces fertility in female rodents by causing ovarian atrophy, persistent estrous cyclicity, and antral follicle atresia (apoptotic cell death). Oxidative damage resulting from reactive oxygen species (ROS) generation has been demonstrated to lead to toxicant-induced cell death. Thus, this work tested the hypothesis that MXC causes oxidative damage to the mouse ovary and affects mitochondrial respiration in a manner that stimulates ROS production. For the in vitro experiments, mitochondria were collected from adult cycling mouse ovaries, treated with vehicle (dimethyl sulfoxide; DMSO) or MXC, and subjected to polarographic measurements of respiration. For the in vivo experiments, adult cycling CD-1 mice were dosed with either vehicle (sesame oil) or MXC for 20 days. After treatment, ovarian mitochondria were isolated and subjected to measurements of respiration and fluorimetric measurements of H2O2 production. Some ovaries were also fixed and processed for immunohistochemistry using antibodies for ROS production markers: nitrotyrosine and 8-hydroxy-2'-deoxyguanosine (8-OHG). Ovaries from in vivo experiments were also used to measure the mRNA expression and activity of antioxidants such as Cu/Zn superoxide dismutase (SOD1), glutathione peroxidase (GPX), and catalase (CAT). The results indicate that MXC significantly impairs mitochondrial respiration, increases production of H2O2, causes more staining for nitrotyrosine and 8-OHG in antral follicles, and decreases the expression and activity of SOD1, GPX, and CAT as compared to controls. Collectively, these data indicate that MXC inhibits mitochondrial respiration, causes ROS production, and decreases antioxidant expression and activity in the ovary, specifically in the antral follicles. Therefore, it is possible that MXC causes atresia of ovarian antral follicles by inducing oxidative stress through mitochondrial production of ROS.

Methoxychlor inhibits brain mitochondrial respiration and increases hydrogen peroxide production and CREB phosphorylation

The organochlorine insecticide methoxychlor (mxc) is an established reproductive toxicant that affects other systems including the central nervous system (CNS), possibly by mechanisms involving oxidative stress. This study tested the hypothesis that mxc inhibits brain mitochondrial respiration, resulting in increased production of reactive oxygen species (ROS). Oxygen electrode measurements of mitochondrial respiration and Amplex Red measurements of H(2)O(2) production were performed with rat brain mitochondria exposed in vitro to mxc (0-10 microg/ml) and with brain mitochondria from mice chronically exposed in vivo to mxc (0-64 mg/kg/day) for 20 days by intraperitoneal injection. In vitro mxc exposure inhibited ADP-dependent respiration (state 3) using both complex I- and II-supported substrates. Similarly, state 3 respiration was inhibited following in vivo mxc exposure using complex I substrates. H(2)O(2) production was stimulated after in vitro mxc treatment in the presence of complex I substrates, but not in mitochondria isolated from in vivo mxc-treated mice. Because previous studies demonstrated a relationship between oxidative stress and CREB phosphorylation, we also tested the hypothesis that mxc elevates phosphorylated CREB (pCREB) in mitochondria. Enzyme-linked immunosorbent assay (ELISA) measurements demonstrated that pCREB immunoreactivity was elevated by in vitro mxc exposure in the presence or absence of respiratory substrates, indicating that stimulation of H(2)O(2) production is not necessary for this effect. These multiple effects of mxc on mitochondria may play an important role in its toxicity, particularly in the CNS.

Calcium-dependent dephosphorylation of brain mitochondrial calcium/cAMP response element binding protein (CREB)

Calcium-mediated signaling regulates nuclear gene transcription by calcium/cAMP response element binding protein (CREB) via calcium-dependent kinases and phosphatases. This study tested the hypothesis that CREB is also present in mitochondria and subject to dynamic calcium-dependent modulation of its phosphorylation state. Antibodies to CREB and phosphorylated CREB (pCREB) were used to demonstrate the presence of both forms in isolated mitochondria and mitoplasts from rat brain. When energized mitochondria were exposed to increasing concentrations of Ca2+ in the physiological range, pCREB was lost while total CREB remained constant. In the presence of Ru360, an inhibitor of the mitochondrial Ca2+ uptake uniporter, calcium-dependent loss of pCREB levels was attenuated, suggesting that intramitochondrial calcium plays an important role in pCREB dephosphorylation. pCREB dephosphorylation was not, however, inhibited by the phosphatase inhibitors okadaic acid and Tacrolimus. In the absence of Ca2+, CREB phosphorylation was elevated by the addition of ATP to the mitochondrial suspension. Exposure of mitochondria to the pore-forming molecule alamethicin that causes osmotic swelling and release of intermembrane proteins enriched mitochondrial pCREB immunoreactivity. These results further suggest that mitochondrial CREB is located in the matrix or inner membrane and that a kinase and a calcium-dependent phosphatase regulate its phosphorylation state.

Noncholinesterase mechanisms of chlorpyrifos neurotoxicity: altered phosphorylation of Ca2+/cAMP response element binding protein in cultured neurons

Previous studies suggest that low doses of the organophosphate insecticide chlorpyrifos (CPF) disrupt brain development and cognitive function by mechanisms that do not involve the inhibition of acetylcholinesterase (AChE). In the present study we tested the hypothesis that CPF and its metabolites alter the Ca2+/cAMP response element binding protein (CREB), a critical molecule in brain development and cognitive function. We further tested the hypothesis that changes in CREB occur independent of AChE inhibition. Western blot analysis of lysates from primary cultures of cortical neurons exposed to CPF, CPF-oxon, or trichloropyridinol (TCP) for 1 h and cultures exposed to trichloropyridinol (TCP) for 7 days indicated that all exposures increased the level of the phosphorylated (activated) form of CREB (pCREB), without significant changes in total CREB or alpha-tubulin. Remarkably, pCREB in cortical neurons was elevated by 300-400% of control levels with estimated EC50s of 60 pM, <30 fM, and <30 pM for CPF, CPF-oxon, and TCP, respectively. AChE activity and cell viability were not affected by organophosphate concentrations that caused significant increases in pCREB (up to 100 nM, 100 pM, and 10 microM of CPF, CPF-oxon, and TCP, respectively). The level of pCREB in hippocampal neurons was also elevated after exposure to CPF, but pCREB in cultured astrocytes was not affected. Inclusion of the cytochrome P-450 inhibitor SKF-525A did not inhibit the effects of CPF on pCREB levels, indicating that metabolism of CPF to CPF-oxon was not necessary to cause the increase in pCREB. The increases in neuronal pCREB observed in this study provide biochemical evidence that CPF and its metabolites are active at critical sites within the nervous system at levels far below those required to inhibit AChE, which could explain many of the reported neurodevelopmental and behavioral changes attributed to CPF toxicity.