Regulation of cytokine-induced neuron death by ovarian hormones: involvement of antiapoptotic protein expression and c-JUN N-terminal kinase-mediated proapoptotic signaling

Neurosteroids: A potential target for neuropsychiatric disorders

Neurosteroids are steroids produced by endocrine glands and subsequently entering the brain, and also include steroids synthesis in the brain. It has been widely known that neurosteroids influence many neurological functions, including neuronal signaling, synaptic adaptations, and neuroprotective effects. In addition, abnormality in the synthesis and function of neurosteroids has been closely linked to neuropsychiatric disorders, such as Alzheimer's disease (AD), schizophrenia (SZ), and epilepsy. Given their important role in brain pathophysiology and disorders, neurosteroids offer potential therapeutic targets for a variety of neuropsychiatric diseases, and that therapeutic strategies targeting neurosteroids probably exert beneficial effects. We therefore summarized the role of neurosteroids in brain physiology and neuropsychiatric disorders, and introduced the recent findings of synthetic neurosteroid analogues for potential treatment of neuropsychiatric disorders, thereby providing insights for further research in the future.

Steroid hormones: risk and resilience in women's Alzheimer disease

More women have Alzheimer disease (AD) than men, but the reasons for this phenomenon are still unknown. Including women in clinical research and studying their biology is key to understand not just their increased risk but also their resilience against the disease. In this sense, women are more affected by AD than men, but their reserve or resilience mechanisms might delay symptom onset. The aim of this review was to explore what is known about mechanisms underlying women's risk and resilience in AD and identify emerging themes in this area that merit further research. We conducted a review of studies analyzing molecular mechanisms that may induce neuroplasticity in women, as well as cognitive and brain reserve. We also analyzed how the loss of steroid hormones in aging may be linked to AD. We included empirical studies with human and animal models, literature reviews as well as meta-analyses. Our search identified the importance of 17-b-estradiol (E2) as a mechanism driving cognitive and brain reserve in women. More broadly, our analysis revealed the following emerging perspectives: (1) the importance of steroid hormones and their effects on both neurons and glia for the study of risk and resilience in AD, (2) E2's crucial role in women's brain reserve, (3) women's verbal memory advantage as a cognitive reserve factor, and (4) E2's potential role in linguistic experiences such as multilingualism and hearing loss. Future directions for research include analyzing the reserve mechanisms of steroid hormones on neuronal and glial plasticity, as well as identifying the links between steroid hormone loss in aging and risk for AD.

Pregnancy, postpartum and parity: Resilience and vulnerability in brain health and disease

Risk and resilience in brain health and disease can be influenced by a variety of factors. While there is a growing appreciation to consider sex as one of these factors, far less attention has been paid to sex-specific variables that may differentially impact females such as pregnancy and reproductive history. In this review, we focus on nervous system disorders which show a female bias and for which there is data from basic research and clinical studies pointing to modification in disease risk and progression during pregnancy, postpartum and/or as a result of parity: multiple sclerosis (MS), depression, stroke, and Alzheimer's disease (AD). In doing so, we join others (Shors, 2016; Galea et al., 2018a) in aiming to illustrate the importance of looking beyond sex in neuroscience research.

Using the NGF/IL-6 ratio as a reliable criterion to show the beneficial effects of progesterone after experimental diffuse brain injury

Acute progesterone injection has been shown to reduce brain edema following traumatic brain injury (TBI) due to its neuroprotective effect. We investigated the effects of sustained release of progesterone through implantation of subcutaneous capsules on rat's brain edema and alteration of cerebrospinal fluid (CSF), and serum ratio of NGF/IL-6 after TBI. This experiment was performed on ovariectomized (OVX) rats and the brain injury was induced by Marmarou's method. A high and a low dose of progesterone (HP and LP) was injected intraperitoneally two h after the brain injury. In addition, in the capsule progesterone-treated group (CP), the intervention was implemented 6 h after the brain injury. Brain edema, NGF and IL-6 biomarkers in serum and cerebrospinal fluid (CSF) were measured 48 h after the TBI in injection groups and one week after the TBI in the CP group. No significant difference was found in the two groups or in the admonition methods. After TBI, the NGF level increased and IL-6 level decreased by injection of both doses, as well as by taking the capsule. Ratio of NGF/IL-6 in CSF increased significantly by all forms of progesterone administration. The increase in the level of NGF and IL-6 after TBI was higher in CSF than in serum. These results indicated that effects of progesterone in capsule form were better than the injection form. Progesterone probably works by increasing NGF and reducing IL-6. Future studies should investigate the ratio of these biomarkers as a variable to determine the neuroprotective effects of another drug.

WWOX Phosphorylation, Signaling, and Role in Neurodegeneration

Homozygous null mutation of tumor suppressor WWOX/Wwox gene leads to severe neural diseases, metabolic disorders and early death in the newborns of humans, mice and rats. WWOX is frequently downregulated in the hippocampi of patients with Alzheimer’s disease (AD). In vitro analysis revealed that knockdown of WWOX protein in neuroblastoma cells results in aggregation of TRAPPC6AΔ, TIAF1, amyloid β, and Tau in a sequential manner. Indeed, TRAPPC6AΔ and TIAF1, but not tau and amyloid β, aggregates are present in the brains of healthy mid-aged individuals. It is reasonable to assume that very slow activation of a protein aggregation cascade starts sequentially with TRAPPC6AΔ and TIAF1 aggregation at mid-ages, then caspase activation and APP de-phosphorylation and degradation, and final accumulation of amyloid β and Tau aggregates in the brains at greater than 70 years old. WWOX binds Tau-hyperphosphorylating enzymes (e.g., GSK-3β) and blocks their functions, thereby supporting neuronal survival and differentiation. As a neuronal protective hormone, 17β-estradiol (E2) binds WWOX at an NSYK motif in the C-terminal SDR (short-chain alcohol dehydrogenase/reductase) domain. In this review, we discuss how WWOX and E2 block protein aggregation during neurodegeneration, and how a 31-amino-acid zinc finger-like Zfra peptide restores memory loss in mice.

The neuroprotective actions of oestradiol and oestrogen receptors

Hormones regulate homeostasis by communicating through the bloodstream to the body's organs, including the brain. As homeostatic regulators of brain function, some hormones exert neuroprotective actions. This is the case for the ovarian hormone 17β-oestradiol, which signals through oestrogen receptors (ERs) that are widely distributed in the male and female brain. Recent discoveries have shown that oestradiol is not only a reproductive hormone but also a brain-derived neuroprotective factor in males and females and that ERs coordinate multiple signalling mechanisms that protect the brain from neurodegenerative diseases, affective disorders and cognitive decline.

Low-frequency high-magnitude mechanical strain of articular chondrocytes activates p38 MAPK and induces phenotypic changes associated with osteoarthritis and pain

Osteoarthritis (OA) is a debilitating joint disorder resulting from an incompletely understood combination of mechanical, biological, and biochemical processes. OA is often accompanied by inflammation and pain, whereby cytokines associated with chronic OA can up-regulate expression of neurotrophic factors such as nerve growth factor (NGF). Several studies suggest a role for cytokines and NGF in OA pain, however the effects of changing mechanical properties in OA tissue on chondrocyte metabolism remain unclear. Here, we used high-extension silicone rubber membranes to examine if high mechanical strain (HMS) of primary articular chondrocytes increases inflammatory gene expression and promotes neurotrophic factor release. HMS cultured chondrocytes displayed up-regulated NGF, TNFα and ADAMTS4 gene expression while decreasing TLR2 expression, as compared to static controls. HMS culture increased p38 MAPK activity compared to static controls. Conditioned medium from HMS dynamic cultures, but not static cultures, induced significant neurite sprouting in PC12 cells. The increased neurite sprouting was accompanied by consistent increases in PC12 cell death. Low-frequency high-magnitude mechanical strain of primary articular chondrocytes in vitro drives factor secretion associated with degenerative joint disease and joint pain. This study provides evidence for a direct link between cellular strain, secretory factors, neo-innervation, and pain in OA pathology.

Apoptosis-related proteins and proliferation markers in the orbitofrontal cortex in major depressive disorder

BACKGROUND

In major depressive disorder (MDD), lowered neural activity and significant reductions of markers of cell resiliency to degeneration occur in the prefrontal cortex (PFC). It is still unclear whether changes in other relevant markers of cell vulnerability to degeneration and markers of cell proliferation are associated with MDD.

METHODS

Levels of caspase 8 (C8), X-linked inhibitor of apoptosis protein (XIAP), direct IAP binding protein with low pI (DIABLO), proliferating cell nuclear antigen (PCNA) and density of cells immunoreactive (-IR) for proliferation marker Ki-67 were measured in postmortem samples of the left orbitofrontal cortex (OFC) of subjects with MDD, and psychiatrically-normal comparison subjects.

RESULTS

There was significant increase in C8, a higher ratio of DIABLO to XIAP, lower packing density of Ki-67-IR cells, and an unexpected age-dependent increase in PCNA in subjects with MDD vs. controls. PCNA levels were significantly higher in MDD subjects unresponsive to antidepressants or untreated with antidepressants. The DIABLO/XIAP ratio was higher in MDD subjects without antidepressants than in comparison subjects.

LIMITATIONS

Qualitative nature of responsiveness assessments; definition of resistance to antidepressant treatment is still controversial; and unclear role of PCNA.

CONCLUSIONS

Markers of cell vulnerability to degeneration are increased and density of Ki67-positive cells is low MDD, but accompanied by normal XIAP levels. The results suggest increased vulnerability to cell pathology in depression that is insufficient to cause morphologically conspicuous cell death. Persistent but low-grade vulnerability to cell degeneration coexisting with reduced proliferation readiness may explain age-dependent reductions in neuronal densities in the OFC of depressed subjects.

High mechanical strain of primary intervertebral disc cells promotes secretion of inflammatory factors associated with disc degeneration and pain

Introduction

Excessive mechanical loading of intervertebral discs (IVDs) is thought to alter matrix properties and influence disc cell metabolism, contributing to degenerative disc disease and development of discogenic pain. However, little is known about how mechanical strain induces these changes. This study investigated the cellular and molecular changes as well as which inflammatory receptors and cytokines were upregulated in human intervertebral disc cells exposed to high mechanical strain (HMS) at low frequency. The impact of these metabolic changes on neuronal differentiation was also explored to determine a role in the development of disc degeneration and discogenic pain.

Methods

Isolated human annulus fibrosus (AF) and nucleus pulposus (NP) cells were exposed to HMS (20% cyclical stretch at 0.001 Hz) on high-extension silicone rubber dishes coupled to a mechanical stretching apparatus and compared to static control cultures. Gene expression of Toll-like receptors (TLRs), neuronal growth factor (NGF) and tumour necrosis factor α (TNFα) was assessed. Collected conditioned media were analysed for cytokine content and applied to rat pheocromocytoma PC12 cells for neuronal differentiation assessment.

Results

HMS caused upregulation of TLR2, TLR4, NGF and TNFα gene expression in IVD cells. Medium from HMS cultures contained elevated levels of growth-related oncogene, interleukin 6 (IL-6), IL-8, IL-15, monocyte chemoattractant protein 1 (MCP-1), MCP-3, monokine induced by γ interferon, transforming growth factor β1, TNFα and NGF. Exposure of PC12 cells to HMS-conditioned media resulted in both increased neurite sprouting and cell death.

Conclusions

HMS culture of IVD cells in vitro drives cytokine and inflammatory responses associated with degenerative disc disease and low-back pain. This study provides evidence for a direct link between cellular strain, secretory factors, neoinnervation and potential degeneration and discogenic pain in vivo.

The role of estrogen and progesterone, administered alone and in combination, in modulating cytokine concentration following traumatic brain injury

Cytokines play an important role in the pathophysiology of traumatic brain injury (TBI). This study was designed to determine the effects of administering progesterone (P) and estrogen (E), alone and in combination, on brain water content, blood-brain barrier (BBB) disturbance, and brain level of cytokines following diffuse TBI. Ovariectomized rats were divided into 9 groups, treated with vehicle, E1, E2, P1, P2, E1+P1, E1+P2, E2+P1, and E2+P2. Levels of BBB disruption (5 h), cytokines, and water content (24 h) were evaluated after TBI induced by the Marmarou method. Physiological (E1 and P1) and pharmacological (E2 and P2) doses of estrogen and progesterone were administered 30 min after TBI. Water content in the E1+P2-treated group was higher than in the E1-treated group. The inhibitory effect of E2 on water content was reduced by adding progesterone. The inhibitory effect of E1 and E2 on Evans blue content was reduced by treatment with E1+P1 and E2+P2, respectively. The brain level of IL-1β was reduced in E1 and E2, after TBI. In the E2+P2-treated group, this level was higher than in the E2-treated group. The brain level of TGF-β was also elevated by the administration of progesterone and estrogen alone, and reduced when the hormones were administered in combination. In conclusion, a combined administration of progesterone and estrogen inhibited the decreasing effects of administration of progesterone and estrogen alone on water content and BBB disruption that mediated to change the proinflammatory cytokines.

Protective actions of ovarian hormones in the serotonin system of macaques

The serotonin neurons of the dorsal and medial raphe nuclei project to all areas of the forebrain and play a key role in mood disorders. Hence, any loss or degeneration of serotonin neurons could have profound ramifications. In a monkey model of surgical menopause with hormone replacement and no neural injury, E and P decreased gene expression in the dorsal raphe nucleus of c-jun n-terminal kinase (JNK1) and kynurenine mono-oxygenase (KMO) that promote cell death. In concert, E and P increased gene expression of superoxide dismutase (SOD1), VEGF, and caspase inhibitory proteins that promote cellular resilience in the dorsal raphe nucleus. Subsequently, we showed that ovarian steroids inhibit pivotal genes in the caspase-dependent and caspase-independent pathways in laser-captured serotonin neurons including apoptosis activating factor (Apaf1), apoptosis-inducing factor (AIF) and second mitochondria-derived activator of caspases (Smac/Diablo). SOD1 was also increased specifically in laser-captured serotonin neurons. Examination of protein expression in the dorsal raphe block revealed that JNK1, phosphoJNK1, AIF and the translocation of AIF from the mitochondria to the nucleus decreased with hormone therapy, whereas pivotal execution proteins in the caspase pathway were unchanged. In addition, cyclins A, B, D1 and E were inhibited, which would prevent re-entry into the cell cycle and catastrophic death. These data indicated that in the absence of gross injury to the midbrain, ovarian steroids inhibit the caspase-independent pathway and cell cycle initiation in serotonin neurons. To determine if these molecular actions prevented cellular vulnerability or death, we examined DNA fragmentation in the dorsal raphe nucleus with the TUNEL assay (terminal deoxynucleotidyl transferase nick end labeling). Ovarian steroids significantly decreased the number of TUNEL-positive cells in the dorsal raphe. Moreover, TUNEL staining prominently colocalized with TPH immunostaining, a marker for serotonin neurons. In summary, ovarian steroids increase the cellular resilience of serotonin neurons and may prevent serotonin neuron death in women facing decades of life after menopause. The survival of serotonin neurons would support cognition and mental health.

Protective actions of sex steroid hormones in Alzheimer's disease

Risk for Alzheimer's disease (AD) is associated with age-related loss of sex steroid hormones in both women and men. In post-menopausal women, the precipitous depletion of estrogens and progestogens is hypothesized to increase susceptibility to AD pathogenesis, a concept largely supported by epidemiological evidence but refuted by some clinical findings. Experimental evidence suggests that estrogens have numerous neuroprotective actions relevant to prevention of AD, in particular promotion of neuron viability and reduction of beta-amyloid accumulation, a critical factor in the initiation and progression of AD. Recent findings suggest neural responsiveness to estrogen can diminish with age, reducing neuroprotective actions of estrogen and, consequently, potentially limiting the utility of hormone therapies in aged women. In addition, estrogen neuroprotective actions are also modulated by progestogens. Specifically, continuous progestogen exposure is associated with inhibition of estrogen actions whereas cyclic delivery of progestogens may enhance neural benefits of estrogen. In recent years, emerging literature has begun to elucidate a parallel relationship of sex steroid hormones and AD risk in men. Normal age-related testosterone loss in men is associated with increased risk to several diseases including AD. Like estrogen, testosterone has been established as an endogenous neuroprotective factor that not only increases neuronal resilience against AD-related insults, but also reduces beta-amyloid accumulation. Androgen neuroprotective effects are mediated both directly by activation of androgen pathways and indirectly by aromatization to estradiol and initiation of protective estrogen signaling mechanisms. The successful use of hormone therapies in aging men and women to delay, prevent, and or treat AD will require additional research to optimize key parameters of hormone therapy and may benefit from the continuing development of selective estrogen and androgen receptor modulators.

Progesterone receptors: form and function in brain

Emerging data indicate that progesterone has multiple non-reproductive functions in the central nervous system to regulate cognition, mood, inflammation, mitochondrial function, neurogenesis and regeneration, myelination and recovery from traumatic brain injury. Progesterone-regulated neural responses are mediated by an array of progesterone receptors (PR) that include the classic nuclear PRA and PRB receptors and splice variants of each, the seven transmembrane domain 7TMPRbeta and the membrane-associated 25-Dx PR (PGRMC1). These PRs induce classic regulation of gene expression while also transducing signaling cascades that originate at the cell membrane and ultimately activate transcription factors. Remarkably, PRs are broadly expressed throughout the brain and can be detected in every neural cell type. The distribution of PRs beyond hypothalamic borders, suggests a much broader role of progesterone in regulating neural function. Despite the large body of evidence regarding progesterone regulation of reproductive behaviors and estrogen-inducible responses as well as effects of progesterone metabolite neurosteroids, much remains to be discovered regarding the functional outcomes resulting from activation of the complex array of PRs in brain by gonadally and/or glial derived progesterone. Moreover, the impact of clinically used progestogens and developing selective PR modulators for targeted outcomes in brain is a critical avenue of investigation as the non-reproductive functions of PRs have far-reaching implications for hormone therapy to maintain neurological health and function throughout menopausal aging.

Neuroprotective actions of ovarian hormones without insult in the raphe region of rhesus macaques

Using a nonhuman primate model of surgical menopause, our laboratory has shown that ovarian hormone treatment (HT) improves 5-HT neural function in the dorsal raphe nucleus (DRN). We further hypothesize that HT may increase 5-HT neuronal resilience. Recent data from microarray analysis indicated that HT regulates gene expression in pathways that lead to apoptosis. In this study, we questioned whether HT alters protein expression in caspase-dependent and independent pathways. Ovariectomized monkeys received Silastic implants containing placebo (empty), estrogen (E) or E+ progesterone (P). A small block of the midbrain containing the DRN was dissected and subjected to subcellular fractionation, yielding cytosolic, nuclear and mitochondrial fractions (n=4/group). The pro-apoptotic protein, c-jun n-terminal kinase (JNK1) and its phosphorylation were decreased by E+P treatment in the cytosolic fraction. Downstream of JNK are proteins in the caspase-dependent and -independent pathways. First, in the caspase-dependent pathway, cytoplasmic and mitochondrial fractions were immunoblotted for Bcl-2 family members, cytochrome c, Apaf1 and XIAP. However, the expression of these proteins did not differ among treatments. Pro-caspase 3 was decreased by E+P, but there was no evidence of active caspase in any group. Then, we examined the involvement of a protein in the caspase-independent pathway, called apoptosis-inducing factor (AIF). AIF mRNA (n=3/group) and AIF mitochondrial protein tended to decrease with hormone treatment. However, AIF protein in the nuclear fraction in E+P treated monkeys was significantly reduced. This indicates that HT is reducing the translocation of AIF from mitochondria to nucleus, thus inhibiting AIF-mediated apoptosis. AIF was immunocytochemically localized to large 5-HT-like neurons of the dorsal raphe. These data suggest that in the absence of global trauma or ischemia, HT may act through the caspase-independent pathway to promote neuroprotection in the 5-HT system.

A high soy diet enhances neurotropin receptor and Bcl-XL gene expression in the brains of ovariectomized female rats

Estrogen is a powerful neuroprotective agent with the ability to induce trophic and antiapoptotic genes. However, concerns about negative overall health consequences of estrogen replacement after menopause have led to the adoption of other strategies to obtain estrogen's benefits in the brain, including the use of selective estrogen receptor modulators, high soy diets, or isoflavone supplements. This study sought to determine the ability of a high soy diet to induce neuroprotective gene expression in the female rat brain and compare the actions of soy with estrogen. Adult ovariectomized female rats were treated with 3 days of high dose estrogen or 2 weeks of a soy-free diet, a high soy diet, or chronic low dose estrogen. Different brain regions were microdissected and subjected to real time RT-PCR for neuroprotective genes previously shown to be estrogen-regulated. The principle findings are that a high soy diet led to the widespread increase in the mRNA for neurotropin receptors TrkA and p75-NTR, and the antiapoptotic Bcl-2 family member Bcl-X(L). Immunohistochemistry confirmed increases in both TrkA and Bcl-X(L). Chronic low dose estrogen mimicked some of these effects, but acute high dose estrogen did not. The effects of a high soy diet were particularly evident in the parietal cortex and hippocampus, two regions protected by estrogen in animal models of neurological disease and injury. These results suggest that a high soy diet may provide beneficial effects to the brain similar to low dose chronic estrogen treatment such as that used for postmenopausal hormone replacement.

Estrogen regulates Bcl-w and Bim expression: role in protection against beta-amyloid peptide-induced neuronal death

Estrogen is neuroprotective against a variety of insults, including beta-amyloid peptide (Abeta); however, the underlying mechanism(s) is not fully understood. Here, we report that 17beta-estradiol (E2) selectively regulates neuronal expression of the Bcl-2 family (bcl-2, bcl-x, bcl-w, bax, bak, bad, bik, bnip3, bid, and bim). In primary cerebrocortical neuron cultures under basal conditions, we observe that E2 upregulates expression of antiapoptotic Bcl-w and downregulates expression of proapoptotic Bim in an estrogen receptor (ER)-dependent manner. In the presence of toxic levels of Abeta, we observe that E2 attenuates indices of neuronal apoptosis: c-Jun N-terminal kinase (JNK)-dependent downregulation of Bcl-w and upregulation of Bim, mitochondrial release of cytochrome c and Smac, and cell death. These neuroprotective effects of E2 against Abeta-induced apoptosis are mimicked by the JNK inhibitor SP600125 (anthra[1,9-cd]pyrazol-6(2H)-one). In addition, E2 attenuates Abeta-induced JNK phosphorylation in an ER-dependent manner, but does not affect basal levels of JNK phosphorylation. These results suggest that E2 may reduce Abeta-induced neuronal apoptosis at least in part by two complementary pathways: (1) ER-dependent, JNK-independent upregulation of Bcl-w and downregulation of Bim under basal conditions, and (2) ER-dependent inhibition of Abeta-induced JNK activation and subsequent JNK-dependent downregulation of Bcl-w and upregulation of Bim, resulting in mitochondrial release of cytochrome c and Smac and eventual cell death. These data provide new understanding into the mechanisms contributing to estrogen neuroprotection, a neural function with potential therapeutic relevance to Alzheimer's disease.

The endogenous estrogen status regulates microglia reactivity in animal models of neuroinflammation

It has been previously demonstrated that 17beta-estradiol (E(2)) inhibits the response of microglia, the resident brain macrophages, to acute injuries in specific brain regions. We here show that the effect of E(2) in acute brain inflammation is widespread and that the hormone reduces the expression of inflammatory mediators, such as monocyte chemoattractant protein-1, macrophage inflammatory protein-2, and TNF-alpha, induced by lipopolysaccharide, demonstrating that microglia are a direct target of estrogen action in brain. Using the APP23 mice, an animal model of Alzheimer's disease reproducing chronic neuroinflammation, we demonstrate that ovary ablation increases microglia activation at beta-amyloid (Abeta) deposits and facilitates the progression of these cells toward a highly reactive state. Long-term administration of E(2) reverts the effects of ovariectomy and decreases microglia reactivity compared with control animals. In this animal model, these events do not correlate with a reduced number of Abeta deposits. Finally, we show that E(2) inhibits Abeta-induced expression of scavenger receptor-A in macrophage cells, providing a mechanism for the effect of E(2) on Abeta signaling observed in the APP23 mice. Altogether, our observations reveal a substantial involvement of endogenous estrogen in neuroinflammatory processes and provide novel mechanisms for hormone action in the brain.

Multiple pathways transmit neuroprotective effects of gonadal steroids

Numerous preclinical studies suggest that gonadal steroids, particularly estrogen, may be neuroprotective against insult or disease progression. This paper reviews the mechanisms contributing to estrogen-mediated neuroprotection. Rapid signaling pathways, such as MAPK, PI3K, Akt, and PKC, are required for estrogen's ability to provide neuroprotection. These rapid signaling pathways converge on genomic pathways to modulate transcription of E2-responsive genes via ERE-dependent and ERE-independent mechanisms. It is clear that both rapid signaling and transcription are important for estrogen's neuroprotective effects. A mechanistic understanding of estrogen-mediated neuroprotection is crucial for the development of therapeutic interventions that enhance quality of life without deleterious side effects.

Neuroprotection by ovarian hormones in animal models of neurological disease

Ovarian hormones can protect against brain injury, neurodegeneration, and cognitive decline. Most attention has focused on estrogens and accumulating data demonstrate that estrogen seems to specifically protect cortical and hippocampal neurons from ischemic injury and from damage due to severe seizures. Although multiple studies demonstrate protection by estrogen, in only a few instances is the issue of how the steroid confers protection known. Here, we first review data evaluating the neuroprotective effects of estrogens, a selective estrogen receptor modulator (SERM), and estrogen receptor alpha- and beta-selective ligands in animal models of focal and global ischemia. Using focal ischemia in ovariectomized ERalphaKO, ERbetaKO, and wild-type mice, we clearly established that the ERalpha subtype is the critical ER mediating neuroprotection in mouse focal ischemia. In rats and mice, the middle cerebral artery occlusion (MCAO) model was used to represent cerebrovascular stroke, while in gerbils the two-vessel occlusion model, representing global ischemia, was used. The gerbil global ischemia model was used to evaluate the neuroprotective effects of estrogen, SERMs, and ERalpha- and ERbeta-selective compounds in the hippocampus. Analysis of neurogranin mRNA, a marker of viability of hippocampal neurons, with in situ hybridization, revealed that estrogen treatment protected the dorsal CA1 regions not only when administered before, but also when given 1 h after occlusion. Estrogen rarely is secreted alone and studies of neuroprotection have been less extensive for a second key ovarian hormone progesterone. In the second half of this review, we present data on neuroprotection by estrogen and progesterone in animal model of epilepsy followed by exploration into ovarian steroid effects on neuronal damage in models of multiple sclerosis and traumatic brain injury.

Dramatic decrease of innervation density in bone after ovariectomy

Recent studies have demonstrated that bone is highly innervated and contains neuromediators that have functional receptors on bone cells. However, no data exist concerning the quantitative changes of innervation during bone loss associated with estrogen withdrawal. To study the involvement of nerve fibers in the regulation of bone remodeling, we have evaluated the modifications of innervation in a classical in vivo model of osteopenia in rats, ovariectomy (OVX). Skeletal innervation was studied by immunocytochemistry using antibodies directed against specific neuronal markers, neurofilament 200 and synaptophysin, and the neuromediator glutamate. Sciatic neurectomy, another model of bone loss due to limb denervation and paralysis, was used to validate our quantitative image analysis technique of immunostaining for nerve markers. Female Wistar rats at 12 wk of age were sham-operated (SHAM) or ovariectomized (OVX). Bone mineral density measurement and bone histomorphometry analysis of tibiae 14 d after surgery demonstrated a significant bone loss in OVX compared with SHAM. We observed an important reduction of nerve profile density in tibiae of OVX animals compared with SHAM animals, whereas innervation density in skin and muscles was similar for OVX and control rats. Quantitative image analysis of immunostainings demonstrated a significant decrease of the percentage of immunolabeling per total bone volume of neurofilament 200, synaptophysin, and glutamate in both the primary and secondary spongiosa of OVX rats compared with SHAM. These data indicate for the first time that OVX-induced bone loss in rat tibiae is associated with a reduction in nerve profile density, suggesting a functional link between the nervous system and the bone loss after ovariectomy.

The diabetes-linked transcription factor PAX4 promotes {beta}-cell proliferation and survival in rat and human islets

The mechanism by which the β-cell transcription factor Pax4 influences cell function/mass was studied in rat and human islets of Langerhans. Pax4 transcripts were detected in adult rat islets, and levels were induced by the mitogens activin A and betacellulin. Wortmannin suppressed betacellulin-induced Pax4 expression, implicating the phosphatidylinositol 3-kinase signaling pathway. Adenoviral overexpression of Pax4 caused a 3.5-fold increase in β-cell proliferation with a concomitant 1.9-, 4-, and 5-fold increase in Bcl-xL (antiapoptotic), c-myc, and Id2 mRNA levels, respectively. Accordingly, Pax4 transactivated the Bcl-xL and c-myc promoters, whereas its diabetes-linked mutant was less efficient. Bcl-xL activity resulted in altered mitochondrial calcium levels and ATP production, explaining impaired glucose-induced insulin secretion in transduced islets. Infection of human islets with an inducible adenoviral Pax4 construct caused proliferation and protection against cytokine-evoked apoptosis, whereas the mutant was less effective. We propose that Pax4 is implicated in β-cell plasticity through the activation of c-myc and potentially protected from apoptosis through Bcl-xL gene expression.

Estrogens sensitize anterior pituitary gland to apoptosis

Tissue homeostasis results from a balance between cell proliferation and cell death by apoptosis. Estradiol affects proliferation as well as apoptosis in hormone-dependent tissues. In the present study, we investigated the apoptotic response of the anterior pituitary gland to lipopolysaccharide (LPS) in cycling female rats, and the influence of estradiol in this response in ovariectomized (OVX) rats. The OVX rats were chronically estrogenized with implanted Silastic capsules containing 1 mg of 17beta-estradiol (E2). Cycling or OVX and E2-treated rats were injected with LPS (250 microg/rat ip). Apoptosis was determined by the terminal deoxynucleotidyl-mediated dUTP nick-end labeling (TUNEL) method in sections of the anterior pituitary gland and spleen. Chronic estrogenization induced apoptosis in the anterior pituitary gland. Acute endotoxemia triggered apoptosis of cells in the anterior pituitary gland of E2-treated rats but not of OVX rats. No differences were observed in the apoptotic response to LPS in spleen between OVX and E2-treated rats. The apoptotic response of the anterior pituitary to LPS was variable along the estrous cycle, being higher at proestrus than at estrus or diestrus I. Approximately 75% of the apoptotic cells were identified as lactotropes by immunofluorescence. In conclusion, our results indicate that estradiol induces apoptosis and enables the proapoptotic action of LPS in the anterior pituitary gland. Also, our study suggests that estrogens may be involved in anterior pituitary cell renewal during the estrous cycle, sensitizing lactotropes to proapoptotic stimuli.