Protection of Oligodendrocytes Through Neuronal Overexpression of the Small GTPase Ras in Hyperoxia-Induced Neonatal Brain Injury

Mechanistic advances of hyperoxia-induced immature brain injury

The impact of hyperoxia-induced brain injury in preterm infants is being increasingly investigated. However, the parameters and protocols used to study this condition in animal models lack consistency. Research is further hampered by the fact that hyperoxia exerts both direct and indirect effects on oligodendrocytes and neurons, with the precise underlying mechanisms remaining unclear. In this article, we aim to provide a comprehensive overview of the conditions used to induce hyperoxia in animal models of immature brain injury. We discuss what is known regarding the mechanisms underlying hyperoxia-induced immature brain injury, focusing on the effects on oligodendrocytes and neurons, and briefly describe therapies that may counteract the effects of hyperoxia. We also identify further studies required to fully elucidate the effects of hyperoxia on the immature brain as well as discuss the leading therapeutic options.

Neuronal Protection by Ha-RAS-GTPase Signaling through Selective Downregulation of Plasmalemmal Voltage-Dependent Anion Channel-1

The small GTPase RAS acts as a plasma membrane-anchored intracellular neurotrophin counteracting neuronal degeneration in the brain, but the underlying molecular mechanisms are largely unknown. In transgenic mice expressing constitutively activated V12-Ha-RAS selectively in neurons, proteome analysis uncovered a 70% decrease in voltage-dependent anion channel-1 (VDAC-1) in the cortex and hippocampus. We observed a corresponding reduction in the levels of mRNA splicing variant coding for plasma membrane-targeted VDAC-1 (pl-VDAC-1) while mRNA levels for mitochondrial membrane VDAC-1 (mt-VDAC-1) remained constant. In primary cortical neurons derived from V12-Ha-RAS animals, a decrease in pl-VDAC-1 mRNA levels was observed, accompanied by a concomitant reduction in the ferricyanide reductase activity associated with VDAC-1 protein. Application of MEK inhibitor U0126 to transgenic cortical neurons reconstituted pl-VDAC-1 mRNA to reach wild-type levels. Excitotoxic glutamate-induced cell death was strongly attenuated in transgenic V12-Ha-RAS overexpressing cortical cultures. Consistently, a neuroprotective effect could also be achieved in wild-type cortical cultures by the extracellular application of channel-blocking antibody targeting the N-terminus of VDAC-1. These results may encourage novel therapeutic approaches toward blocking pl-VDAC-1 by monoclonal antibody targeting for complementary treatments in transplantation and neurodegenerative disease.

Key roles of glial cells in the encephalopathy of prematurity

Across the globe, approximately one in 10 babies are born preterm, that is, before 37 weeks of a typical 40 weeks of gestation. Up to 50% of preterm born infants develop brain injury, encephalopathy of prematurity (EoP), that substantially increases their risk for developing lifelong defects in motor skills and domains of learning, memory, emotional regulation, and cognition. We are still severely limited in our abilities to prevent or predict preterm birth. No longer just the "support cells," we now clearly understand that during development glia are key for building a healthy brain. Glial dysfunction is a hallmark of EoP, notably, microgliosis, astrogliosis, and oligodendrocyte injury. Our knowledge of glial biology during development is exponentially expanding but hasn't developed sufficiently for development of effective neuroregenerative therapies. This review summarizes the current state of knowledge for the roles of glia in infants with EoP and its animal models, and a description of known glial-cell interactions in the context of EoP, such as the roles for border-associated macrophages. The field of perinatal medicine is relatively small but has worked passionately to improve our understanding of the etiology of EoP coupled with detailed mechanistic studies of pre-clinical and human cohorts. A primary finding from this review is that expanding our collaborations with computational biologists, working together to understand the complexity of glial subtypes, glial maturation, and the impacts of EoP in the short and long term will be key to the design of therapies that improve outcomes.

A review of the effects of early postnatal hyperoxia exposure on the immature brain

Preterm birth is a public health priority worldwide, with approximately 15 million premature babies born each year. Oxygen supplementation is one of the most common interventions for preterm infants. However, prolonged oxygen inhalation at supraphysiological concentrations can lead to the development of bronchopulmonary dysplasia (BPD). In addition to lifelong pulmonary sequelae, clinical evidence suggests that BPD is associated with adverse neurodevelopmental outcomes, such as motor impairment, cognitive impairment, and behavioral deficits, severely affecting the quality of life of preterm infants. However, the mechanisms underlying the combination of neurodevelopmental impairment with BPD remain unclear. Therefore, in recent years, attention has also been focused on the effects of hyperoxia on brain development in preterm infants. In this review, we outline the pathophysiological mechanisms of brain injury caused by developmental hyperoxia exposure in current animal models and briefly describe the pharmacological therapies that may be applicable to the associated brain injury. Overall, more studies are needed to assess the effects of hyperoxia on the immature brain, particularly combined analyses of the lungs and brain in the same experimental setting, to elucidate the potential causes of combined neurodevelopmental impairment in BPD.

How lung injury and therapeutic oxygen could alter white matter development

Developmental brain injury describes a spectrum of neurological pathologies resulting from either antenatal or perinatal injury. This includes both cognitive and motor defects that affect patients for their entire lives. Developmental brain injury can be caused by a spectrum of conditions including stroke, perinatal hypoxia-ischemia, and intracranial hemorrhage. Additional risk factors have been identified including very low birth weight, mechanical ventilation, and oxygen (O2 ) supplementation. In fact, infants with bronchopulmonary dysplasia, an inflammatory disease associated with disrupted lung development, have been shown to have decreased cerebral white matter and decreased intracranial volumes. Thus, there appears to be a developmental link between the lung, O2 , and the brain that leads to proper myelination. Here, we will discuss what is currently known about the link between O2 and myelination and how scientists are exploring mechanisms through which supplemental O2 and/or lung injury can affect brain development. Consideration of a link between the diseased lung and developing brain will allow clinicians to fine tune their approaches in managing preterm lung disease in order to optimize brain health.

Neonatal Oxidative Stress Impairs Cortical Synapse Formation and GABA Homeostasis in Parvalbumin-Expressing Interneurons

Neonatal brain injury is often caused by preterm birth. Brain development is vulnerable to increased environmental stress, including oxidative stress challenges. Due to a premature change of the fetal living environment from low oxygen in utero into postnatal high-oxygen room air conditions ex utero, the immature preterm brain is exposed to a relative hyperoxia, which can induce oxidative stress and impair neuronal cell development. To simulate the drastic increase of oxygen exposure in the immature brain, 5-day-old C57BL/6 mice were exposed to hyperoxia (80% oxygen) for 48 hours or kept in room air (normoxia, 21% oxygen) and mice were analyzed for maturational alterations of cortical GABAergic interneurons. As a result, oxidative stress was indicated by elevated tyrosine nitration of proteins. We found perturbation of perineuronal net formation in line with decreased density of parvalbumin-expressing (PVALB) cortical interneurons in hyperoxic mice. Moreover, maturational deficits of cortical PVALB+ interneurons were obtained by decreased glutamate decarboxylase 67 (GAD67) protein expression in Western blot analysis and lower gamma-aminobutyric acid (GABA) fluorescence intensity in immunostaining. Hyperoxia-induced oxidative stress affected cortical synaptogenesis by decreasing synapsin 1, synapsin 2, and synaptophysin expression. Developmental delay of synaptic marker expression was demonstrated together with decreased PI3K-signaling as a pathway being involved in synaptogenesis. These results elucidate that neonatal oxidative stress caused by increased oxygen exposure can lead to GABAergic interneuron damage which may serve as an explanation for the high incidence of psychiatric and behavioral alterations found in preterm infants.

Perinatal Hyperoxia and Developmental Consequences on the Lung-Brain Axis

Approximately 11.1% of all newborns worldwide are born preterm. Improved neonatal intensive care significantly increased survival rates over the last decades but failed to reduce the risk for the development of chronic lung disease (i.e., bronchopulmonary dysplasia (BPD)) and impaired neurodevelopment (i.e., encephalopathy of prematurity (EoP)), two major long-term sequelae of prematurity. Premature infants are exposed to relative hyperoxia, when compared to physiological in-utero conditions and, if needed to additional therapeutic oxygen supplementation. Both are associated with an increased risk for impaired organ development. Since the detrimental effects of hyperoxia on the immature retina are known for many years, lung and brain have come into focus in the last decade. Hyperoxia-induced excessive production of reactive oxygen species leading to oxidative stress and inflammation contribute to pulmonary growth restriction and abnormal neurodevelopment, including myelination deficits. Despite a large body of studies, which unraveled important pathophysiological mechanisms for both organs at risk, the majority focused exclusively either on lung or on brain injury. However, considering that preterm infants suffering from BPD are at higher risk for poor neurodevelopmental outcome, an interaction between both organs seems plausible. This review summarizes recent findings regarding mechanisms of hyperoxia-induced neonatal lung and brain injury. We will discuss common pathophysiological pathways, which potentially link both injured organ systems. Furthermore, promises and needs of currently suggested therapies, including pharmacological and regenerative cell-based treatments for BPD and EoP, will be emphasized. Limited therapeutic approaches highlight the urgent need for a better understanding of the mechanisms underlying detrimental effects of hyperoxia on the lung-brain axis in order to pave the way for the development of novel multimodal therapies, ideally targeting both severe preterm birth-associated complications.

Neonatal hyperoxia induces gut dysbiosis and behavioral changes in adolescent mice

BACKGROUND

Supplemental oxygen is often required to treat preterm infants with respiratory disorders. Experimental studies have demonstrated that hyperoxia results in the disruption of intestinal and neuronal plasticity and myelination of the brain. The association between the neonatal hyperoxia and changes of phenotypes in gut microbiota and in behaviors is not clear to date.

METHODS

We designed an animal experiment that C57BL/6 mice pups were reared in either room air (RA) or hyperoxia (85% O2) from postnatal days 1 to 7. From postnatal days 8 to 42, the mice were reared in RA. Intestinal microbiota was sampled from the lower gastrointestinal tract on postnatal days 7 and 42, and behavioral tests were performed and brain tissues were collected on postnatal day 42.

RESULTS

Neonatal hyperoxia decreased intestinal tight junction protein expression and altered intestinal bacterial composition and diversity on postnatal day 7. Among the concrete discriminative features, Proteobacteria and Epsilonbacteraeota were significantly elevated in hyperoxia-reared mice on postnatal days 7 and 42, respectively. Hyperoxia-reared mice exhibited significantly reduced sociability and interest in social novelty and impaired motor coordination compared with RA-reared mice on postnatal day 42. Hyperoxia-reared mice also exhibited significantly reduced myelination and a significantly higher number of apoptotic cells in the brain compared with RA-reared mice on postnatal day 42.

CONCLUSION

Neonatal hyperoxia during the first week of life altered gut microbiota and reduced brain myelination that might associate with the deficits of social interaction and motor coordination in adolescent mice.

Hyperoxia Leads to Transient Endocrine Alterations in the Neonatal Rat During Postnatal Development

Introduction: High oxygen concentrations have been identified as one factor contributing to the pathogenesis of the retinopathia of prematurity, chronic lung disease of the preterm infant and preterm brain injury. Preterm infants also show short- and long-term alterations of the endocrine system. If hyperoxia is one pathogenetic factor has not been investigated yet. With regard to the high prevalence of neurodevelopmental impairments in preterm infants, the hypothalamus-pituitary-thyroid (HPT) axis, the hypothalamus-pituitary-adrenal (HPA) axis and the hypothalamus-pituitary-somatotropic (HPS) axis are of special interest due to their important role in neurodevelopment. Objective: The aim of this study was to investigate the effect of hyperoxia on the endocrine system in the neonatal rat by analyzing the activities of the HPT, HPA and HPS axes, respectively. Methods: Three-days old Wistar rats were exposed to hyperoxia (oxygen 80%, 48 h). On postnatal day 5 (P5) and P11, transcript levels of thyroid-stimulating hormone (TSH), proopiomelanocortin and growth hormone (GH) were analyzed in pituitary sections by in situ hybridization. Serologic quantification of TSH and thyroxine (T4), adrenocorticotropic hormone and GH were performed by Multiplex analysis and Enzyme-linked Immunosorbent Assay. Results: At P5, significantly lower GH levels were observed in pituitaries (mRNA) and in sera of rats exposed to hyperoxia. Serum TSH was significantly elevated without changes in T4. Conclusion: This is the first study demonstrating transient endocrine alterations following hyperoxia in the neonatal rat making oxygen a possible contributor to the pathogenesis of endocrine alterations seen in preterm infants. Considering the detrimental multi-organ effects of hyperoxia on the immature organism, a rational use of therapeutic oxygen in the treatrnent of preterm infants is of utmost importance.

Repetitive Erythropoietin Treatment Improves Long-Term Neurocognitive Outcome by Attenuating Hyperoxia-Induced Hypomyelination in the Developing Brain

Introduction: Preterm infants born before 28 weeks of gestation are at high risk of neurodevelopmental impairment in later life. Cerebral white and gray matter injury is associated with adverse outcomes. High oxygen levels, often unavoidable in neonatal intensive care, have been identified as one of the main contributing factors to preterm brain injury. Thus, preventive and therapeutic strategies against hyperoxia-induced brain injury are needed. Erythropoietin (Epo) is a promising and also neuroprotective candidate due to its clinical use in infants as erythropoiesis-stimulating agent. Objective: The objective of this study was to investigate the effects of repetitive Epo treatment on the cerebral white matter and long-term motor-cognitive outcome in a neonatal rodent model of hyperoxia-induced brain injury. Methods: Three-day old Wistar rats were exposed to hyperoxia (48 h, 80% oxygen). Four doses of Epo (5,000 IU/kg body weight per day) were applied intraperitoneally from P3-P6 with the first dose at the onset of hyperoxia. Oligodendrocyte maturation and myelination were evaluated via immunohistochemistry and Western blot on P11. Motor-cognitive deficits were assessed in a battery of complex behavior tests (Open Field, Novel Object Recognition, Barnes maze) in adolescent and fully adult animals. Following behavior tests animals underwent post-mortem diffusion tensor imaging to investigate long-lasting microstructural alterations of the white matter. Results: Repetitive treatment with Epo significantly improved myelination deficits following neonatal hyperoxia at P11. Behavioral testing revealed attenuated hyperoxia-induced cognitive deficits in Epo-treated adolescent and adult rats. Conclusion: A multiple Epo dosage regimen protects the developing brain against hyperoxia-induced brain injury by improving myelination and long-term cognitive outcome. Though current clinical studies on short-term outcome of Epo-treated prematurely born children contradict our findings, long-term effects up to adulthood are still lacking. Our data support the essential need for long-term follow-up of preterm infants in current clinical trials.

Clobetasol Attenuates White Matter Injury by Promoting Oligodendrocyte Precursor Cell Differentiation

INTRODUCTION

White matter injury (WMI) is the most common brain injury in preterm infants and can result in life-long neurological deficits. The main cause of WMI is damage to the oligodendrocyte precursor cells (OPC) in the brain that results in delayed myelin sheath formation, or the destruction of existing myelin sheaths. OPC undergo highly regulated and strictly timed developmental changes that result in their transformation to mature oligodendrocytes capable of myelin production.

OBJECTIVE

Studies have shown that clobetasol strongly promotes differentiation of OPC into myelin sheaths. Therefore, we hypothesized that clobetasol may be a therapeutic option for the treatment of preterm WMI.

METHODS

We induced a WMI rat model and observed white matter damage under an optical microscope. Rats subjected to WMI were injected intraperitoneally with clobetasol (2 or 5 mg/kg daily) from day 1 to day 5 in the early treatment groups, or from day 6 to day 10 in the late treatment groups. After 17 days, the rats were sacrificed and the expression of myelin basic protein (MBP) was visualized using immunofluorescence. In addition, we evaluated myelin sheath formation using electron microscopy. The rats were also subjected to the suspension test, ramp test, and open field test to evaluate neurobehavioral functions.

RESULTS

A rat model of WMI was successfully induced. It was found that clobetasol significantly induced MBP expression and myelin sheath formation and improved neurobehavioral function in the rats subjected to WMI.

CONCLUSIONS

Our results indicate that clobetasol attenuates WMI by promoting OPC differentiation, and it may be an effective therapeutic agent for the treatment of preterm WMI.

Adverse neuropsychiatric development following perinatal brain injury: from a preclinical perspective

Perinatal brain injury is a leading cause of death and disability in young children. Recent advances in obstetrics, reproductive medicine and neonatal intensive care have resulted in significantly higher survival rates of preterm or sick born neonates, at the price of increased prevalence of neurological, behavioural and psychiatric problems in later life. Therefore, the current focus of experimental research shifts from immediate injury processes to the consequences for brain function in later life. The aetiology of perinatal brain injury is multi-factorial involving maternal and also labour-associated factors, including not only placental insufficiency and hypoxia-ischaemia but also exposure to high oxygen concentrations, maternal infection yielding excess inflammation, genetic factors and stress as important players, all of them associated with adverse long-term neurological outcome. Several animal models addressing these noxious stimuli have been established in the past to unravel the underlying molecular and cellular mechanisms of altered brain development. In spite of substantial efforts to investigate short-term consequences, preclinical evaluation of the long-term sequelae for the development of cognitive and neuropsychiatric disorders have rarely been addressed. This review will summarise and discuss not only current evidence but also requirements for experimental research providing a causal link between insults to the developing brain and long-lasting neurodevelopmental disorders.

Perspectives of RAS and RHEB GTPase Signaling Pathways in Regenerating Brain Neurons

Cellular activation of RAS GTPases into the GTP-binding “ON” state is a key switch for regulating brain functions. Molecular protein structural elements of rat sarcoma (RAS) and RAS homolog protein enriched in brain (RHEB) GTPases involved in this switch are discussed including their subcellular membrane localization for triggering specific signaling pathways resulting in regulation of synaptic connectivity, axonal growth, differentiation, migration, cytoskeletal dynamics, neural protection, and apoptosis. A beneficial role of neuronal H-RAS activity is suggested from cellular and animal models of neurodegenerative diseases. Recent experiments on optogenetic regulation offer insights into the spatiotemporal aspects controlling RAS/mitogen activated protein kinase (MAPK) or phosphoinositide-3 kinase (PI3K) pathways. As optogenetic manipulation of cellular signaling in deep brain regions critically requires penetration of light through large distances of absorbing tissue, we discuss magnetic guidance of re-growing axons as a complementary approach. In Parkinson’s disease, dopaminergic neuronal cell bodies degenerate in the substantia nigra. Current human trials of stem cell-derived dopaminergic neurons must take into account the inability of neuronal axons navigating over a large distance from the grafted site into striatal target regions. Grafting dopaminergic precursor neurons directly into the degenerating substantia nigra is discussed as a novel concept aiming to guide axonal growth by activating GTPase signaling through protein-functionalized intracellular magnetic nanoparticles responding to external magnets.