Recent studies describing the seemingly contradictory actions of estrogens in ischemic stroke injury have led us to reevaluate the circumstances under which estrogen therapy (ET) provides benefits against cerebral stroke and decipher its mechanisms of action. One prominent feature that follows stroke injury is massive central and peripheral inflammatory responses. Evidence now suggests that postischemic inflammatory responses strongly contribute to the extent of brain injury, and 17β-estradiol (E ₂ ) may protect the ischemic brain by exerting antiinflammatory actions. In an attempt to explain recently reported dichotomous effects of E ₂ in stroke injury, we tested the hypothesis that an extended period of hypoestrogenicity both prevents E ₂ from protecting the brain against ischemia and simultaneously suppresses its antiinflammatory actions. We report that E ₂ exerts profound neuroprotective action when administered immediately upon ovariectomy, but not when administered after 10 weeks of hypoestrogenicity. Consistently, E ₂ treatment given immediately at the time of ovariectomy attenuated central and peripheral production of proinflammatory cytokines after ischemic stroke. In contrast, E ₂ did not suppress production of proinflammatory molecules when it was administered after 10 weeks postovariectomy. These results demonstrate that a prolonged period of hypoestrogenicity disrupts both neuroprotective and antiinflammatory actions of E ₂ . Our findings may help to explain the results of the Women's Health initiative that reported no beneficial effect of ET against stroke because the majority of the subjects initiated ET after an extended period of hypoestrogenicity.

The pathogenesis of Alzheimer's disease (AD) is a critical unsolved question; and although recent studies have demonstrated a strong association between altered brain immune responses and disease progression, the mechanistic cause of neuronal dysfunction and death is unknown. We have previously described the unique CVN-AD mouse model of AD, in which immune-mediated nitric oxide is lowered to mimic human levels, resulting in a mouse model that demonstrates the cardinal features of AD, including amyloid deposition, hyperphosphorylated and aggregated tau, behavioral changes, and age-dependent hippocampal neuronal loss. Using this mouse model, we studied longitudinal changes in brain immunity in relation to neuronal loss and, contrary to the predominant view that AD pathology is driven by proinflammatory factors, we find that the pathology in CVN-AD mice is driven by local immune suppression. Areas of hippocampal neuronal death are associated with the presence of immunosuppressive CD11c⁺microglia and extracellular arginase, resulting in arginine catabolism and reduced levels of total brain arginine. Pharmacologic disruption of the arginine utilization pathway by an inhibitor of arginase and ornithine decarboxylase protected the mice from AD-like pathology and significantly decreased CD11c expression. Our findings strongly implicate local immune-mediated amino acid catabolism as a novel and potentially critical mechanism mediating the age-dependent and regional loss of neurons in humans with AD.

Neuroinflammation is a common feature of many neurological disorders, and it is often accompanied by the release of proinflammatory cytokines and chemokines. Estradiol-17β (E₂) exhibits antiinflammatory properties, including the suppression of proinflammatory cytokines, in the central nervous system. However, the mechanisms employed by E₂ and the role(s) of estrogen receptors (ERs) ERα and ERβ are unclear. To investigate these mechanisms, we employed an in vivo lipopolysaccharide (LPS) model of systemic inflammation in ovariectomized (OVX) and OVX and E₂-treated (OVX+E₂) mice. Brain levels of proinflammatory cytokines (IL-1β, IL-6, and IL-12p40) and chemokines (CCL2/MCP-1, CCL3/MIP-1α, CCL5/RANTES, and CXCL1/KC) were quantified in mice at 0 (sham), 3, 6, 12, and 24 h after infection using multiplex protein analysis. E₂ treatment inhibited LPS-induced increases in all cytokines. In contrast, E₂ treatment only suppressed CCL/RANTES chemokine concentrations. To determine whether ERα and ERβ regulate brain cytokine and chemokine levels, parallel experiments were conducted using ERα knockout and ERβ knockout mice. Our results revealed that both ERα and ERβ regulated proinflammatory cytokine and chemokine production through E₂-dependent and E₂-independent mechanisms. To assess whether breakdown of the blood-brain barrier is an additional target of E₂ against LPS-induced neuroinflammation, we measured Evan’s blue extravasation and identified distinct roles for ERα and ERβ. Taken together, these studies identify a dramatic cytokine- and chemokine-mediated neuroinflammatory response that is regulated through ERα- and ERβ-mediated ligand-dependent and ligand-independent mechanisms.