Erin L. Scott

Critical Role of NADPH Oxidase in Neuronal Oxidative Damage and Microglia Activation following Traumatic Brain Injury

Background Oxidative stress is known to play an important role in the pathology of traumatic brain injury. Mitochondria are thought to be the major source of the damaging reactive oxygen species (ROS) following TBI. However, recent work has revealed that the membrane, via the enzyme NADPH oxidase can also generate the superoxide radical (O2 −), and thereby potentially contribute to the oxidative stress following TBI. The current study thus addressed the potential role of NADPH oxidase in TBI. Methodology/Principal Findings The results revealed that NADPH oxidase activity in the cerebral cortex and hippocampal CA1 region increases rapidly following controlled cortical impact in male mice, with an early peak at 1 h, followed by a secondary peak from 24–96 h after TBI. In situ localization using oxidized hydroethidine and the neuronal marker, NeuN, revealed that the O2 − induction occurred in neurons at 1 h after TBI. Pre- or post-treatment with the NADPH oxidase inhibitor, apocynin markedly inhibited microglial activation and oxidative stress damage. Apocynin also attenuated TBI-induction of the Alzheimers disease proteins β-amyloid and amyloid precursor protein. Finally, both pre- and post-treatment of apocynin was also shown to induce significant neuroprotection against TBI. In addition, a NOX2-specific inhibitor, gp91ds-tat was also shown to exert neuroprotection against TBI. Conclusions/Significance As a whole, the study demonstrates that NADPH oxidase activity and superoxide production exhibit a biphasic elevation in the hippocampus and cortex following TBI, which contributes significantly to the pathology of TBI via mediation of oxidative stress damage, microglial activation, and AD protein induction in the brain following TBI.

Estrogen regulation of Dkk1 and Wnt/β-Catenin signaling in neurodegenerative disease.

17β-estradiol (E2 or estrogen) is an endogenous steroid hormone that is well known to exert neuroprotection. Along these lines, one mechanism through which E2 protects the hippocampus from cerebral ischemia is by preventing the post-ischemic elevation of Dkk1, a neurodegenerative factor that serves as an antagonist of the canonical Wnt signaling pathway, and simultaneously inducing pro-survival Wnt/β-Catenin signaling in hippocampal neurons. Intriguingly, while expression of Dkk1 is required for proper neural development, overexpression of Dkk1 is characteristic of many neurodegenerative diseases, such as stroke, Alzheimers disease, Parkinsons disease, and temporal lobe epilepsy. In this review, we will briefly summarize the canonical Wnt signaling pathway, highlight the current literature linking alterations of Dkk1 and Wnt/β-Catenin signaling with neurological disease, and discuss E2s role in maintaining the delicate balance of Dkk1 and Wnt/β-Catenin signaling in the adult brain. Finally, we will consider the implications of long-term E2 deprivation and hormone therapy on this crucial neural pathway. This article is part of a Special Issue entitled Hormone Therapy.

Proline-, glutamic acid-, and leucine-rich protein 1 mediates estrogen rapid signaling and neuroprotection in the brain

Significance Ever since the rapid extranuclear signaling effects of 17β-estradiol (E2) were first identified in the brain decades ago, it has remained an enigma as to how these nonclassical effects are achieved. Using a forebrain-specific knockout animal model, the current study demonstrates that a recently cloned estrogen receptor coregulator protein, Proline-, glutamic acid-, and leucine-rich protein 1 (PELP1), is critical for mediating E2 regulation of rapid extranuclear signaling, as well as E2-induced neuroprotection and cognitive function in the hippocampus after ischemic injury. Our studies also identified PELP1 as a novel interacting protein and a substrate of glycogen synthase kinase-3β (GSK3β). Finally, PELP1 was also shown to mediate E2 genomic effects to regulate genes involved in inflammation, metabolism, and survival after ischemic injury. 17-β estradiol (E2) has been implicated as neuroprotective in a variety of neurodegenerative disorders. However, the underlying mechanism remains unknown. Here, we provide genetic evidence, using forebrain-specific knockout (FBKO) mice, that proline-, glutamic acid-, and leucine-rich protein 1 (PELP1), an estrogen receptor coregulator protein, is essential for the extranuclear signaling and neuroprotective actions of E2 in the hippocampal CA1 region after global cerebral ischemia (GCI). E2-mediated extranuclear signaling (including activation of extracellular signal-regulated kinase and Akt) and antiapoptotic effects [such as attenuation of JNK signaling and increase in phosphorylation of glycogen synthase kinase-3β (GSK3β)] after GCI were compromised in PELP1 FBKO mice. Mechanistic studies revealed that PELP1 interacts with GSK3β, E2 modulates interaction of PELP1 with GSK3β, and PELP1 is a novel substrate for GSK3β. RNA-seq analysis of control and PELP1 FBKO mice after ischemia demonstrated alterations in several genes related to inflammation, metabolism, and survival in PELP1 FBKO mice, as well as a significant reduction in the activation of the Wnt/β-catenin signaling pathway. In addition, PELP1 FBKO studies revealed that PELP1 is required for E2-mediated neuroprotection and for E2-mediated preservation of cognitive function after GCI. Collectively, our data provide the first direct in vivo evidence, to our knowledge, of an essential role for PELP1 in E2-mediated rapid extranuclear signaling, neuroprotection, and cognitive function in the brain.

Attenuation of mitochondrial and nuclear p38α signaling: A novel mechanism of estrogen neuroprotection in cerebral ischemia

P38 mitogen-activated protein kinase (MAPK) is a pro-apoptotic and pro-inflammatory protein that is activated in response to cellular stress. While p38 is known to be activated in response to cerebral ischemia, the precise role of p38 and its isoforms in ischemia-induced neuronal apoptosis remains unclear. In the current study, we examined the differential activation and functional roles of p38α and p38β MAPK isoforms in short-term ovariectomized female rats treated with either the neuroprotective ovarian hormone 17beta-estradiol (E2) or placebo in a model of global cerebral ischemia (GCI). GCI induced biphasic activation of total p38 in the hippocampal CA1, with peaks at 30 min and 1 day after 10-min ischemia-reperfusion. Further study demonstrated that activated p38α, but not p38β, translocated to the nucleus 30 min and 3 h post reperfusion, and that this event coincided with increased phosphorylation of activating transcription factor 2 (ATF2), a p38 target protein. Intriguingly, activated p38α was also enhanced in mitochondrial fractions of CA1 neurons 1 day after GCI, and there was loss of mitochondrial membrane potential, as well as enhanced cytochrome c release and caspase-3 cleavage at 2 days post GCI. Importantly, E2 prevented the biphasic activation of p38, as well as both nuclear and mitochondrial translocation of p38α after GCI, and these findings correlated with attenuation of mitochondrial dysfunction and delayed neuronal cell death in the hippocampal CA1. Furthermore, administration of a p38 inhibitor was able to mimic the neuroprotective effects of E2 in the hippocampal CA1 region by preventing nuclear and mitochondrial translocation of p38α, loss of mitochondrial membrane potential, and neuronal apoptosis. As a whole, this study suggests that changes in subcellular localization of the activated p38α isoform are required for neuronal apoptosis following GCI, and that E2 exerts robust neuroprotection, in part, through dual inhibition of activation and subcellular trafficking of p38α.

Intranasal Delivery of a Caspase-1 Inhibitor in the Treatment of Global Cerebral Ischemia

Caspase-1 is an enzyme implicated in neuroinflammation, a critical component of many diseases that affect neuronal degeneration. However, it is unknown whether a caspase-1 inhibitor can modify apoptotic neuronal damage incurred during transient global cerebral ischemia (GCI) and whether intranasal administration of a caspase-1 inhibitor is an effective treatment following GCI. The present study was conducted to examine the potential efficiency of post-ischemic intranasal administration of the caspase-1 inhibitor Boc-D-CMK in a 4-vessel occlusion model of GCI in the rat. Herein, we show that intranasal Boc-D-CMK readily penetrated the central nervous system, subsequently inhibiting caspase-1 activity, decreasing mitochondrial dysfunction, and attenuating caspase-3-dependent apoptotic pathway in ischemia-vulnerable hippocampal CA1 region. Further investigation regarding the mechanisms underlying Boc-D-CMK’s neuroprotective effects revealed marked inhibition of reactive gliosis, as well as reduction of the neuroinflammatory response via inhibition of the downstream pro-inflammatory cytokine production. Intranasal Boc-D-CMK post-treatment also significantly enhanced the numbers of NeuN-positive cells while simultaneously decreasing the numbers of TUNEL-positive and PARP1-positive cells in hippocampal CA1. Correspondingly, behavioral tests showed that deteriorations in spatial learning and memory performance, and long-term recognition memory following GCI were significantly improved in the Boc-D-CMK post-treated animals. In summary, the current study demonstrates that the caspase-1 inhibitor Boc-D-CMK coordinates anti-inflammatory and anti-apoptotic actions to attenuate neuronal death in the hippocampal CA1 region following GCI. Furthermore, our data suggest that pharmacological inhibition of caspase-1 is a promising neuroprotective strategy to target ischemic neuronal injury and functional deficits following transient GCI.