Alison Berent-Spillson

Protection against glucose-induced neuronal death by NAAG and GCP II inhibition is regulated by mGluR3

Glutamate carboxypeptidase II (GCP II) inhibition has previously been shown to be protective against long‐term neuropathy in diabetic animals. In the current study, we have determined that the GCP II inhibitor 2‐(phosphonomethyl) pentanedioic acid (2‐PMPA) is protective against glucose‐induced programmed cell death (PCD) and neurite degeneration in dorsal root ganglion (DRG) neurons in a cell culture model of diabetic neuropathy. In this model, inhibition of caspase activation is mediated through the group II metabotropic glutamate receptor, mGluR3. 2‐PMPA neuroprotection is completely reversed by the mGluR3 antagonist (S)‐α‐ethylglutamic acid (EGLU). In contrast, group I and III mGluR inhibitors have no effect on 2‐PMPA neuroprotection. Furthermore, we show that two mGluR3 agonists, the direct agonist (2R,4R)‐4‐aminopyrrolidine‐2, 4‐dicarboxylate (APDC) and N‐acetyl‐aspartyl‐glutamate (NAAG) provide protection to neurons exposed to high glucose conditions, consistent with the concept that 2‐PMPA neuroprotection is mediated by increased NAAG activity. Inhibition of GCP II or mGluR3 may represent a novel mechanism to treat neuronal degeneration under high‐glucose conditions.

Early menopausal hormone use influences brain regions used for visual working memory.

Objective:The cognitive benefit of postmenopausal hormone use is controversial; however, timing of treatment close to menopause may increase the likelihood of preserving cognitive function. We examined the effects of early-initiation hormone use on visual working memory, hypothesizing that long-term hormone use is associated with greater brain activation during visual working memory. Methods:This was a cross-sectional comparison of long-term early hormone users-current (n = 13) and past (n = 24; 2.1 ± 1.0 years off hormones)-with never users (n = 18), using a visual memory task and functional magnetic resonance imaging (MRI). We evaluated 55 women older than 60 years at the University of Michigans General Clinical Research Center. Hormone users had completed at least 10 continuous years of conjugated equine estrogens with or without medroxyprogesterone acetate, begun within 2 years of menopause. Women were excluded for illness, medication, intermittent estrogen use, phytoestrogen use, recent smoking, and MRI contraindications. The primary outcome was functional MRI-detected brain activity during the visual memory task. Results:Compared with never users, both groups of hormone users had increased activation in the frontal and parietal cortices, insula, hippocampus, and cingulate; combined hormone users also had increased activation in the putamen and raphe (corrected P < 0.05 or uncorrected P < 0.001 with a priori hypothesis). Across the entire sample, the medial temporal cortex (P < 0.0001 right; P < 0.018 left) and right hippocampus (P < 0.0001) positively correlated with task performance. Conclusions:Hormone use was associated with increased brain activation during the visual memory task, in regions used for visual working memory. A positive correlation between activation and task performance suggests that early-initiation, long-term postmenopausal hormone use may benefit visual working memory.

Hormonal Environment Affects Cognition Independent of Age during the Menopause Transition

CONTEXT Cognitive decline is prevalent in aging populations, and cognitive complaints are common during menopause. However, the extent of hormonal influence is unclear, particularly when considered independent of the aging process. OBJECTIVE We sought to determine differences in cognitive function attributable to menopause, hypothesizing that differences would be associated with reproductive rather than chronological age. DESIGN AND SETTING In this cross-sectional study at a university hospital, we combined neuropsychological measures with functional magnetic resonance imaging to comprehensively assess cognitive function. PARTICIPANTS Sixty-seven menopausal women, aged 42-61 yr, recruited from a population-based menopause study, grouped into menopause stages based on hormonal and cycle criteria (premenopause, perimenopause, and postmenopause), participated in the study. MAIN OUTCOME MEASURES Neuropsychological and functional magnetic resonance imaging measures of verbal, visual, and executive cognitive function. RESULTS We found age-independent menopause effects on verbal function. Menopause groups differed in phonemic verbal fluency (F = 3.58, P < 0.019) and regional brain activation (inferior frontal cortex: corrected P < 0.000 right, P < 0.036 left; left prefrontal cortex: P < 0.012); left temporal pole: P < 0.001). Verbal measures correlated with estradiol and FSH (phonemic fluency: R = 0.249, P < 0.047 estradiol, R = -0.275, P < 0.029 FSH; semantic fluency: R = 0.318, P < 0.011 estradiol, R = -0.321, P < 0.010 FSH; right inferior frontal cortex: R = 0.364, P < 0.008 FSH; left inferior frontal cortex: R = -0.431, P < 0.001 estradiol, left prefrontal cortex: R = 0.279, P < 0.045 FSH; left temporal pole: R = -0.310, P < 0.024 estradiol, R = 0.451, P < 0.001 FSH; left parahippocampal gyrus: R = -0.278, P < 0.044 estradiol; left parietal cortex: R = -0.326, P < 0.017 estradiol). CONCLUSIONS Results suggest that verbal fluency mechanisms are vulnerable during the menopausal transition. Targeted intervention may preserve function of this critical cognitive domain.

Metabotropic glutamate receptor 3 protects neurons from glucose‐induced oxidative injury by increasing intracellular glutathione concentration

High glucose concentrations cause oxidative injury and programmed cell death in neurons, and can lead to diabetic neuropathy. Activating the type 3 metabotropic glutamate receptor (mGluR3) prevents glucose‐induced oxidative injury in dorsal root ganglion neurons co‐cultured with Schwann cells. To determine the mechanisms of protection, studies were performed in rat dorsal root ganglion neuron–Schwann cell co‐cultures. The mGluR3 agonist 2R,4R‐4‐aminopyrrolidine‐2,4‐dicarboxylate prevented glucose‐induced inner mitochondrial membrane depolarization, reactive oxygen species accumulation, and programmed cell death, and increased glutathione (GSH) concentration in co‐cultured neurons and Schwann cells, but not in neurons cultured without Schwann cells. Protection was diminished in neurons treated with the GSH synthesis inhibitor l‐buthionine‐sulfoximine, suggesting that mGluR‐mediated protection requires GSH synthesis. GSH precursors and the GSH precursor GSH‐ethyl ester also protected neurons from glucose‐induced injury, indicating that GSH synthesis in Schwann cells, and transport of reaction precursors to neurons, may underlie mGluR‐mediated neuroprotection. These results support the conclusions that activating glial mGluR3 protects neurons from glucose‐induced oxidative injury by increasing free radical scavenging and stabilizing mitochondrial function, through increased GSH antioxidant defense.

Transforming Growth Factor-β Induces Cellular Injury in Experimental Diabetic Neuropathy

The mechanism/s leading to diabetic neuropathy are complex. Transforming growth factor-beta1 (TGF-beta1) has been associated with diabetic nephropathy and retinopathy but not neuropathy. In this study, changes in TGF-beta isoforms were examined in vivo and in vitro. Two groups of animals, streptozotocin diabetic with neuropathy and non-diabetic controls were examined at 4 weeks (n=10/group) and 12 weeks (n=8/group). In diabetic DRG using quantitative real-time PCR (QRT-PCR), TGF-beta1 and TGF-beta2 mRNA, but not TGF-beta3, was increased at 4 and 12 weeks. In sciatic nerve TGF-beta3 mRNA was primarily increased. Immunohistochemistry (DRG) and immunoblotting (sciatic nerve) showed similar differential protein expression. In sciatic nerve TGF-beta formed homo- and hetero-dimers, of which beta(2)/beta(3), beta(1)/beta(1), and beta(1)/beta(3) were significantly increased, while that of the TGF-beta(2)/beta(2) homodimer was decreased, in diabetic compared to non-diabetic rats. In vitro, pretreatment of embryonic DRG with TGF-beta neutralizing antibody prevents the increase in total TGF-beta protein observed with high glucose using immunoblotting. In high glucose conditions, combination with TGF-beta2>beta1 increases the percent of cleaved caspase-3 compared to high glucose alone and TGF-beta neutralizing antibody inhibits this increase. Furthermore, consistent with the findings in diabetic DRG and nerve, TGF-beta isoforms applied directly in vitro reduce neurite outgrowth, and this effect is partially reversed by TGF-beta neutralizing antibody. These findings implicate upregulation of TGF-beta in experimental diabetic peripheral neuropathy and indicate a novel mechanism of cellular injury related to elevated glucose levels. In combination, these findings indicate a potential new target for treatment of diabetic peripheral neuropathy.

Postmenopausal hormone use impact on emotion processing circuitry

Despite considerable evidence for potential effects of estrogen on emotional processing, several studies of postmenopausal women who began hormone therapy (HT) remote from menopause report no effects of HT on emotional measures. As early HT initiation may preserve brain mechanisms, we examined effects of HT on emotional processing in postmenopausal women who started HT early after menopause. We performed a cross-sectional comparison of 52 postmenopausal women 66±5 years old, including 15 users of conjugated equine estrogen, 20 users of conjugated equine estrogen plus medroxyprogesterone acetate, and 17 who never used hormones (NT). All hormone users started therapy within two years of menopause, and received at least 10 years of continuous therapy. Outcomes were fMRI-detected brain activity and behavioral measures during an emotional processing picture rating task. During processing of positive pictures, NT women had greater activation than estrogen treated women in medial prefrontal cortex extending to the anterior cingulate, and more activation than estrogen plus progestin treated women in the insula. During processing of negative pictures, estrogen treated women had higher activation than NT women in the entorhinal cortex. Current compared to past HT users showed greater activation in the hippocampus and higher emotion recognition accuracy of neutral stimuli. Estrogen plus progestin treated women had slower response time than NT women when rating all pictures. In conclusion, hormone use was associated with differences in brain functional responses during emotional processing. These fMRI effects were more prominent than those observed for behavioral measures and involved brain regions implicated in cognitive-emotional integration.

Early Initiation of Hormone Therapy in Menopausal Women Is Associated with Increased Hippocampal and Posterior Cingulate Cholinergic Activity

CONTEXT The role of ovarian hormones in maintaining neuronal integrity and cognitive function is still debated. This study was undertaken to clarify the potential relationship between postmenopausal hormone use and the cholinergic system. OBJECTIVE We hypothesized that early initiated hormone therapy (HT) preserves the cholinergic system and that estrogen therapy (ET) would be associated with higher levels of acetylcholinesterase activity in the posterior cingulate cortex and hippocampus compared to estrogen plus progestin therapy (EPT) or no HT. DESIGN AND SETTING We conducted a cross-sectional study at a university teaching hospital. PATIENTS Fifty postmenopausal women (age, 65.2 ± 0.7 yr) with early long-term HT (n = 34; 13 ET and 21 EPT) or no HT (n = 16) participated in the study. INTERVENTIONS There were no interventions. MAIN OUTCOME MEASURE We measured cholinergic activity (acetylcholinesterase) in the hippocampus and posterior cingulate brain regions as measured by N-[(11)C]methylpiperidin-4-yl propionate and positron emission tomography as a marker of cholinergic function. RESULTS Significant effects of treatment on cholinergic activity measures were obtained in the left hippocampus (F = 3.56; P = 0.04), right hippocampus (F = 3.42; P = 0.04), and posterior cingulate (F = 3.76; P = 0.03). No significant effects were observed in a cortical control region. Post hoc testing identified greater cholinergic activity in the EPT group compared to the no-HT group in the left hippocampus (P = 0.048) and posterior cingulate (P = 0.045), with a nonstatistically significant trend in the right hippocampus (P = 0.073). CONCLUSIONS A differential effect of postmenopausal ET and EPT on cholinergic neuronal integrity was identified in postmenopausal women. The findings are consistent with a preservation of cholinergic neuronal integrity in the EPT group.

Distinct cognitive effects of estrogen and progesterone in menopausal women

The effects of postmenopausal hormone treatment on cognitive outcomes are inconsistent in the literature. Emerging evidence suggests that cognitive effects are influenced by specific hormone formulations, and that progesterone is more likely to be associated with positive outcomes than synthetic progestin. There are very few studies of unopposed progesterone in postmenopausal women, and none that use functional neuroimaging, a sensitive measure of neurobiological function. In this study of 29 recently postmenopausal women, we used functional MRI and neuropsychological measures to separately assess the effects of estrogen or progesterone treatment on visual and verbal cognitive function. Women were randomized to receive 90 days of either estradiol or progesterone counterbalanced with placebo. After each treatment arm, women were given a battery of verbal and visual cognitive function and working memory tests, and underwent functional MRI including verbal processing and visual working memory tasks. We found that both estradiol and progesterone were associated with changes in activation patterns during verbal processing. Compared to placebo, women receiving estradiol treatment had greater activation in the left prefrontal cortex, a region associated with verbal processing and encoding. Progesterone was associated with changes in regional brain activation patterns during a visual memory task, with greater activation in the left prefrontal cortex and right hippocampus compared to placebo. Both treatments were associated with a statistically non-significant increase in number of words remembered following the verbal task performed during the fMRI scanning session, while only progesterone was associated with improved neuropsychological measures of verbal working memory compared to placebo. These results point to potential cognitive benefits of both estrogen and progesterone.

Functional neuroimaging of emotional processing in women with polycystic ovary syndrome: a case-control pilot study.

OBJECTIVE To evaluate emotional processing in women with insulin-resistant polycystic ovary syndrome (IR-PCOS) and its relationship to glucose regulation and the mu-opioid system. DESIGN Case-control pilot. SETTING Tertiary referring medical center. PATIENT(S) Seven women with IR-PCOS and five non-insulin-resistant controls, aged 21-40 years, recruited from the general population. INTERVENTION(S) Sixteen weeks of metformin (1,500 mg/day) in women with IR-PCOS. MAIN OUTCOME MEASURE(S) Assessment of mood, metabolic function, and neuronal activation during an emotional task using functional magnetic resonance imaging (fMRI), and mu-opioid receptor availability using positive emission tomography (PET). RESULT(S) We found that insulin-resistant PCOS patients [1] had greater limbic activation during an emotion task than controls (n = 5); [2] trended toward decreased positive affect and increased trait anxiety; [3] after metformin treatment, had limbic activation that no longer differed from controls; and [4] had positive correlations between fMRI limbic activation during emotional processing and mu-opioid binding potential. CONCLUSION(S) Patients with IR-PCOS had greater regional activation during an emotion task than the controls, although this resolved with metformin therapy. Alterations in mu-opioid neurotransmission may underlie limbic system activity and mood disorders in IR-PCOS.

Metabotropic Glutamate Receptors (mGluRs) and Diabetic Neuropathy

Multiple in vivo and in vitro studies show that excessive release of glutamate, and subsequent activation of ionotropic glutamate receptors (iGluRs) and some metabotropic glutamate receptors (mGluRs) cause neuronal cell death through either necrosis or apoptosis. However, recently alternative evidence has shown that mGluRs have modulatory effects on excitotoxicity and neuronal cell death. Metabotropic glutamate receptors form a family of eight subtypes (mGluR1-8), subdivided into three groups (I-III) that initiate their biological effects by G protein-linked intracellular signal transduction. Their expression throughout the mammalian nervous system implicates these receptors as essential mediators of a cells fate during injury to the nervous system. Activation of group-II (mGluR2 and -3) or group-III metabotropic glutamate receptors (mGluR4, -6, -7 and -8) has been established to be neuroprotective in vitro and in vivo. In contrast, group-I mGluRs (mGluR1 and -5) need to be antagonized in order to evoke protection. The pathological signaling pathways associated with diabetic neuropathy are complex and this influences development of appropriate therapies. The Group II mGluRs target several signaling pathways affected in diabetic neuropathy, prevent cellular injury in the peripheral nervous system, and may provide a novel mechanism for treatment of diabetic neuropathy. Direct or indirect activation of mGluR2/3 in animal models protects against development of diabetic neuropathy. The potential mechanisms and role of mGluRs in protection against diabetic neuropathy will be reviewed.