X chromosome dosage and the response to cerebral ischemia

Out of the core: the impact of focal ischemia in regions beyond the penumbra

The changes in the necrotic core and the penumbra following induction of focal ischemia have been the focus of attention for some time. However, evidence shows, that ischemic injury is not confined to the primarily affected structures and may influence the remote areas as well. Yet many studies fail to probe into the structures beyond the penumbra, and possibly do not even find any significant results due to their short-term design, as secondary damage occurs later. This slower reaction can be perceived as a therapeutic opportunity, in contrast to the ischemic core defined as irreversibly damaged tissue, where the window for salvation is comparatively short. The pathologies in remote structures occur relatively frequently and are clearly linked to the post-stroke neurological outcome. In order to develop efficient therapies, a deeper understanding of what exactly happens in the exo-focal regions is necessary. The mechanisms of glia contribution to the ischemic damage in core/penumbra are relatively well described and include impaired ion homeostasis, excessive cell swelling, glutamate excitotoxic mechanism, release of pro-inflammatory cytokines and phagocytosis or damage propagation via astrocytic syncytia. However, little is known about glia involvement in post-ischemic processes in remote areas. In this literature review, we discuss the definitions of the terms "ischemic core", "penumbra" and "remote areas." Furthermore, we present evidence showing the array of structural and functional changes in the more remote regions from the primary site of focal ischemia, with a special focus on glia and the extracellular matrix. The collected information is compared with the processes commonly occurring in the ischemic core or in the penumbra. Moreover, the possible causes of this phenomenon and the approaches for investigation are described, and finally, we evaluate the efficacy of therapies, which have been studied for their anti-ischemic effect in remote areas in recent years.

Immunomodulatory role of estrogen in ischemic stroke: neuroinflammation and effect of sex

Although estrogen is predominantly related to the maintenance of reproductive functioning in females, it mediates various physiological effects in nearly all tissues, especially the central nervous system. Clinical trials have revealed that estrogen, especially 17β-estradiol, can attenuate cerebral damage caused by an ischemic stroke. One mechanism underlying this effect of 17β-estradiol is by modulating the responses of immune cells, indicating its utility as a novel therapeutic strategy for ischemic stroke. The present review summarizes the effect of sex on ischemic stroke progression, the role of estrogen as an immunomodulator in immune reactions, and the potential clinical value of estrogen replacement therapy. The data presented here will help better understand the immunomodulatory function of estrogen and may provide a basis for its novel therapeutic use in ischemic stroke.

Sex Differences in Poststroke Inflammation: a Focus on Microglia Across the Lifespan

Stroke is one of the leading causes of death worldwide and currently only few therapeutic options are available. Stroke is a sexually dimorphic disease contributing to the difficulty in finding efficient treatments. Poststroke neuroinflammation is geared largely by brain microglia and infiltrating peripheral immune cells and largely contributes to sex differences in the outcome of stroke. Microglia, since very early in the development, are sexually divergent, imprinting specific sex-related features. The diversity in terms of microglial density, morphology, and transcriptomic and proteomic profiles between sexes remains in the adulthood and is likely to contribute to the observed sex-differences on the postischemic inflammation. The impact of sexual hormones is fundamental: changes in terms of risk and severity have been observed for females before and after menopause underlining the importance of altered circulating sexual hormones. Moreover, aging is a driving force for changes that interact with sex, shifting the inflammatory response in a sex-dependent manner. This review summarizes the present literature on sex differences in stroke-induced inflammatory responses, with the focus on different microglial responses along lifespan.

Age and sex differences in the pathophysiology of acute CNS injury

Despite the immeasurable burden on patients and families, no effective therapies to protect the CNS after an acute injury are available yet. Furthermore, the underlying mechanisms that promote neuronal death and functional deficits after injury remain to be poorly understood. The prevalence, age of onset, pathophysiology, and symptomatology of many CNS insults differ significantly between males and females. In the case of stroke, younger males tend to show a higher risk than younger females, while this trend reverses with age. Accumulating evidence from preclinical studies have shown that sex hormones play a crucial role in providing neuroprotection following ischemic stroke and other acute CNS injuries. Estrogen, in particular, exerts a neuroprotective effect by modulating the immune responses after injury. In addition, there exists a sexual dimorphism in cell death pathways between males and females that are independent of hormones. Meanwhile, recent studies suggest that microRNAs are critically involved in the sex-specific mechanisms of cell death. This review discusses the current knowledge on the contribution of sex and age to outcome after stroke. Implication of the interplay between these two factors on other CNS injuries (spinal cord injury and traumatic brain injury) from the experimental evidence were also discussed.

Stroke sensitivity in the aged: sex chromosome complement vs. gonadal hormones

Stroke is a sexually dimorphic disease. Elderly women not only have higher stroke incidence than age-matched men, but also have poorer recovery and higher morbidity and mortality after stroke. In older, post-menopausal women, gonadal hormone levels are similar to that of men. This suggests that tissue damage and functional outcomes are influenced by biologic sex (XX vs. XY) rather than the hormonal milieu at older ages. We employed the Four Core Genotype (FCG) mouse model to study the contribution of sex chromosome complement and gonadal hormones to stroke sensitivity in aged mice in which the testis determining gene (Sry) is removed from the Y chromosome, allowing for the generation of XX males and XY females. XXF, XXM, XYF, XYM and XYwt aged mice were subjected to middle cerebral artery occlusion (MCAO). XXF and XXM mice had significantly larger infarct volumes than XYF and XYM cohorts respectively. There was no significant difference in hormone levels among aged FCG mice. XXF/XXM mice also had more robust microglial activation and higher serum levels of pro-inflammatory cytokines than XYF/XYM cohort respectively. We concluded that the sex chromosome complement contributes to ischemic sensitivity in aged animals and leads to sex differences in innate immune responses.

Sex differences in stroke across the lifespan: The role of T lymphocytes

Stroke is a sexually dimorphic disease. Ischemic sensitivity changes throughout the lifespan and outcomes depend largely on variables like age, sex, hormonal status, inflammation, and other existing risk factors. Immune responses after stroke play a central role in how these factors interact. Although the post-stroke immune response has been extensively studied, the contribution of lymphocytes to stroke is still not well understood. T cells participate in both innate and adaptive immune responses at both acute and chronic stages of stroke. T cell responses also change at different ages and are modulated by hormones and sex chromosome complement. T cells have also been implicated in the development of hypertension, one of the most important risk factors for vascular disease. In this review, we highlight recent literature on the lymphocytic responses to stroke in the context of age and sex, with a focus on T cell response and the interaction with important stroke risk factors.

Mir363-3p improves ischemic stroke outcomes in female but not male rats

With age, stroke prevalence is higher, and stroke outcome, worse, in women. Thus there is an urgent need to identify stroke neuroprotectants for this population. Using a preclinical stroke model, our studies focused on microRNAs (miRNAs), a class of translational repressors, as neuroprotectants. Analysis of circulating miRNA in the acute phase of stroke indicated potential neuroprotective capacity for miR363. Specifically, mir363 is elevated in serum of adult female rats that typically have small infarct volumes, but is deficient in age-matched males or middle-aged males and females, groups that have greater stroke-associated impairment. To directly test the effect of mir363 on stroke outcomes, first, adult females were treated with antagomirs to mir363 post stroke and next, middle-aged females were treated with mimic to mir363-3p post stroke. Antagomir treatment to adult females significantly increased infarct volume and impaired sensory motor performance. Reciprocally, mir363 mimic to middle-aged females reduced infarct volume, preserved forebrain microvessels and improved sensory motor performance. In the early acute stroke phase, mir363-3p mimic reduced the expression and functional activity of caspase-3, a critical component of the apoptotic cell cascade. In contrast, mir363-3p mimic treatment had no effect on stroke outcomes or caspase regulation in young males. Collectively, these studies show that mir363 is neuroprotective for stroke in females and implicates caspase-3 as a sex-specific miRNA-sensitive node for recovery from ischemic stroke.

Method parameters' impact on mortality and variability in mouse stroke experiments: a meta-analysis

Although hundreds of promising substances have been tested in clinical trials, thrombolysis currently remains the only specific pharmacological treatment for ischemic stroke. Poor quality, e.g. low statistical power, in the preclinical studies has been suggested to play an important role in these failures. Therefore, it would be attractive to use animal models optimized to minimize unnecessary mortality and outcome variability, or at least to be able to power studies more exactly by predicting variability and mortality given a certain experimental setup. The possible combinations of methodological parameters are innumerous, and an experimental comparison of them all is therefore not feasible. As an alternative approach, we extracted data from 334 experimental mouse stroke articles and, using a hypothesis-driven meta-analysis, investigated the method parameters’ impact on infarct size variability and mortality. The use of Swiss and C57BL6 mice as well as permanent occlusion of the middle cerebral artery rendered the lowest variability of the infarct size while the emboli methods increased variability. The use of Swiss mice increased mortality. Our study offers guidance for researchers striving to optimize mouse stroke models.

Sex differences in stroke: the contribution of coagulation

Stroke is now the leading cause of adult disability in the United States. Women are disproportionately affected by stroke. Women increasingly outnumber men in the elderly population, the period of highest risk for stroke. However, there is also a growing recognition that fundamental sex differences are present that contribute to differential ischemic sensitivity. In addition, gonadal hormone exposure can impact coagulation and fibrinolysis, key factors in the initiation of thrombosis. In this review we will discuss sex differences in stroke, with a focus on platelets, vascular reactivity and coagulation.

PPARγ activation rescues mitochondrial function from inhibition of complex I and loss of PINK1

Parkinson's disease has long been associated with impaired mitochondrial complex I activity, while several gene defects associated with familial Parkinson's involve defects in mitochondrial function or 'quality control' pathways, causing an imbalance between mitochondrial biogenesis and removal of dysfunctional mitochondria by autophagy. Amongst these are mutations of the gene for PTEN-induced kinase 1 (PINK1) in which mitochondrial function is abnormal. Peroxisome proliferator-activated receptor gamma (PPARγ), a nuclear receptor and ligand-dependent transcription factor, regulates pathways of inflammation, lipid and carbohydrate metabolism, antioxidant defences and mitochondrial biogenesis. We have found that inhibition of complex I in human differentiated SHSY-5Y cells by the complex I inhibitor rotenone irreversibly decrease mitochondrial mass, membrane potential and oxygen consumption, while increasing free radical generation and autophagy. Similar changes are seen in PINK1 knockdown cells, in which potential, oxygen consumption and mitochondrial mass are all decreased. In both models, all these changes were reversed by pre-treatment of the cells with the PPARγ agonist, rosiglitazone, which increased mitochondrial biogenesis, increased oxygen consumption and suppressed free radical generation and autophagy. Thus, rosiglitazone is neuroprotective in two different models of mitochondrial dysfunction associated with Parkinson's disease through a direct impact on mitochondrial function.

Multiple interacting cell death mechanisms in the mediation of excitotoxicity and ischemic brain damage: a challenge for neuroprotection

There is currently no approved neuroprotective pharmacotherapy for acute conditions such as stroke and cerebral asphyxia. One of the reasons for this may be the multiplicity of cell death mechanisms, because inhibition of a particular mechanism leaves the brain vulnerable to alternative ones. It is therefore essential to understand the different cell death mechanisms and their interactions. We here review the multiple signaling pathways underlying each of the three main morphological types of cell death--apoptosis, autophagic cell death and necrosis--emphasizing their importance in the neuronal death that occurs during cerebral ischemia and hypoxia-ischemia, and we analyze the interactions between the different mechanisms. Finally, we discuss the implications of the multiplicity of cell death mechanisms for the design of neuroprotective strategies.

Sex, stroke, and inflammation: the potential for estrogen-mediated immunoprotection in stroke

Stroke is the third leading cause of death and the primary cause of disability in the developed world. Experimental and clinical data indicate that stroke is a sexually dimorphic disease, with males demonstrating an enhanced intrinsic sensitivity to ischemic damage throughout most of their lifespan. The neuroprotective role of estrogen in the female brain is well established, however, estrogen exposure can also be deleterious, especially in older women. The mechanisms for this remain unclear. Our current understanding is based on studies examining estrogen as it relates to neuronal injury, yet cerebral ischemia also induces a robust sterile inflammatory response involving local and systemic immune cells. Despite the potent anti-inflammatory effects of estrogen, few studies have investigated the contribution of estrogen to sex differences in the inflammatory response to stroke. This review examines the potential role for estrogen-mediated immunoprotection in ischemic injury.

The effects of estrogen in ischemic stroke

Stroke is a leading cause of death and the most common cause of long-term disability in the USA. Women have a lower incidence of stroke compared with men throughout most of the lifespan which has been ascribed to protective effects of gonadal steroids, most notably estrogen. Due to the lower stroke incidence observed in pre-menopausal women and robust preclinical evidence of neuroprotective and anti-inflammatory properties of estrogen, researchers have focused on the potential benefits of hormones to reduce ischemic brain injury. However, as women age, they are disproportionately affected by stroke, coincident with the loss of estrogen with menopause. The risk of stroke in elderly women exceeds that of men and it is clear that in some settings estrogen can have pro-inflammatory effects. This review will focus on estrogen and inflammation and its interaction with aging.

Interactions between age, sex, and hormones in experimental ischemic stroke

Age, sex, and gonadal hormones have profound effects on ischemic stroke outcomes, although how these factors impact basic stroke pathophysiology remains unclear. There is a plethora of inconsistent data reported throughout the literature, primarily due to differences in the species examined, the timing and methods used to evaluate injury, the models used, and confusion regarding differences in stroke incidence as seen in clinical populations vs. effects on acute neuroprotection or neurorepair in experimental stroke models. Sex and gonadal hormone exposure have considerable independent impact on stroke outcome, but these factors also interact with each other, and the contribution of each differs throughout the lifespan. The contribution of sex and hormones to experimental stroke will be the focus of this review. Recent advances and our current understanding of age, sex, and hormone interactions in ischemic stroke with a focus on inflammation will be discussed.

Transgenic overexpression of peroxiredoxin-2 attenuates ischemic neuronal injury via suppression of a redox-sensitive pro-death signaling pathway

AIMS

Peroxiredoxins (PRXs) are a newly characterized family of peroxide scavenging enzymes that not only help maintain cellular redox homeostasis but also may directly engage in a variety of intracellular signaling pathways. PRX2 is a neuronal-specific PRX believed to participate in cerebral antioxidant responses in several neurodegenerative diseases. This study investigates the potential neuroprotective effect and the underlying mechanism of PRX2 in models of ischemic neuronal injury.

RESULTS

Transgenic mice overexpressing PRX2 showed reduced brain injury and improved neurological recovery up to 3 weeks after transient focal cerebral ischemia compared to wild-type littermates. In primary cultures of cortical neurons, transfection of PRX2 but not the loss-of-catalytic-site PRX2 mutant conferred neuroprotection against cell death induced by oxygen glucose deprivation. PRX2 exhibited potent pro-survival effects in ischemic neurons by maintaining thioredoxin (Trx) in its reduced state, thereby preventing oxidative stress-mediated activation of apoptosis signal-regulating kinase 1 (ASK1) and the downstream MKK/JNK pro-death signaling pathway. PRX2 failed to provide additional neuroprotection against ischemic injury in Trx- or ASK1-knockdown neuron cultures and in mice treated with a JNK inhibitor.

INNOVATION

This study provides evidence that neuronal overexpression of PRX2 confers prolonged neuroprotection against ischemic/reperfusion brain injury. Moreover, the results suggest a signaling pathway by which PRX2 suppresses ischemia-induced neuronal apoptosis.

CONCLUSIONS

Enhanced neuronal expression and activity of PRX2 protect against ischemic neuronal injury by directly modulating the redox-sensitive Trx-ASK1 signaling complex.