Strain-related brain injury in neonatal mice subjected to hypoxia-ischemia

Pipeline to Neonatal Clinical Transformation: The Importance of Preclinical Data

Historically, neonatal neuroscience boasted a robust and successful preclinical pipeline for therapeutic interventions, in particular for the treatment of hypoxic-ischemic encephalopathy (HIE). However, since the successful translation of therapeutic hypothermia (TH), several high-profile failures of promising adjunctive therapies, in addition to the lack of benefit of TH in lower resource settings, have brought to light critical issues in that same pipeline. Using recent data from clinical trials of erythropoietin as an example, the authors highlight several key challenges facing preclinical neonatal neuroscience for HIE therapeutic development and propose key areas where model development and collaboration across the field in general can ensure ongoing success in treatment development for HIE worldwide.

Pretreatment with oleuropein protects the neonatal brain from hypoxia-ischemia by inhibiting apoptosis and neuroinflammation

Hypoxic-ischemic (HI) encephalopathy is a cerebrovascular injury caused by oxygen deprivation to the brain and remains a major cause of neonatal mortality and morbidity worldwide. Therapeutic hypothermia is the current standard of care but it does not provide complete neuroprotection. Our aim was to investigate the neuroprotective effect of oleuropein (Ole) in a neonatal (seven-day-old) mouse model of HI. Ole, a secoiridoid found in olive leaves, has previously shown to reduce damage against cerebral and other ischemia/reperfusion injuries. Here, we administered Ole as a pretreatment prior to HI induction at 20 or 100 mg/kg. A week after HI, Ole significantly reduced the infarct area and the histological damage as well as white matter injury, by preserving myelination, microglial activation and the astroglial reactive response. Twenty-four hours after HI, Ole reduced the overexpression of caspase-3 and the proinflammatory cytokines IL-6 and TNF-α. Moreover, using UPLC-MS/MS we found that maternal supplementation with Ole during pregnancy and/or lactation led to the accumulation of its metabolite hydroxytyrosol in the brains of the offspring. Overall, our results indicate that pretreatment with Ole confers neuroprotection and can prevent HI-induced brain damage by modulating apoptosis and neuroinflammation.

ATP-P2X7 signaling mediates brain pathology while contributing to viral control in perinatal Zika virus infection

Zika virus (ZIKV), the causative agent of Zika fever, is a flavivirus transmitted by mosquitoes of the Aedes genus. Zika virus infection has become an international concern due to its association with severe neurological complications such as fetal microcephaly. Viral infection can induce the release of ATP in the extracellular environment, activating receptors sensitized by extracellular nucleotides, such as the P2X7 receptor. This receptor is the primary purinergic receptor involved in neuroinflammation, neurodegeneration, and immunity. In this work, we investigated the role of ATP-P2X7 receptor signaling in Zika-related brain abnormalities. Wild-type mice (WT) and P2X7 receptor-deficient (P2X7-/-) C57BL/6 newborn mice were subcutaneously inoculated with 5 × 106plaque-forming units of ZIKV or mock solution. P2X7 receptor expression increased in the brain of Zika virus-infected mice compared to the mock group. Comparative analyses of the hippocampi from WT and P2X7-/-mice revealed that the P2X7 receptor increased hippocampal damage in CA1/CA2 and CA3 regions. Doublecortin expression decreased significantly in the brains of ZIKV-infected mice. WT ZIKV-infected mice showed impaired motor performance compared to P2X7-/- infected mice. WT ZIKV-infected animals showed increased expression of glial markers GFAP (astrocytes) and IBA-1 (microglia) compared to P2X7-/- infected mice. Although the P2X7 receptor contributes to neuronal loss and neuroinflammation, WT mice were more efficient in controlling the viral load in the brain than P2X7 receptor-deficient mice. This result was associated with higher induction of TNF-α, IFN-β, and increased interferon-stimulated gene expression in WT mice than P2X7-/-ZIKV-infected. Finally, we found that the P2X7 receptor contributes to inhibiting the neuroprotective signaling pathway AKT/mTOR while stimulating the caspase-3 activation, possibly two distinct pathways contributing to neurodegeneration. These findings suggest that ATP-P2X7 receptor signaling contributes to the antiviral response in the brain of ZIKV-infected mice while increasing neuronal loss, neuroinflammation, and related brain abnormalities.

Enhancing 7-dehydrocholesterol suppresses brain ferroptosis and tissue injury after neonatal hypoxia-ischemia

Neonatal hypoxic-ischemic brain injury (HIBI) results in part from excess reactive oxygen species and iron-dependent lipid peroxidation (i.e. ferroptosis). The vitamin D precursor 7-dehydrocholesterol (7-DHC) may inhibit iron-dependent lipid peroxidation. Primary neurons underwent oxygen and glucose deprivation (OGD) injury and treatment with 7-DHC-elevating medications such as cariprazine (CAR) or vehicle. Postnatal day 9 mice underwent sham surgery or carotid artery ligation and hypoxia and received intraperitoneal CAR. In neurons, CAR administration resulted in significantly increased cell survival compared to vehicle controls, whether administered 48 h prior to or 30 min after OGD, and was associated with increased 7-DHC. In the mouse model, malondialdehyde and infarct area significantly increased after HIBI in the vehicle group, which were attenuated by post-treatment with CAR and were negatively correlated with tissue 7-DHC concentrations. Elevating 7-DHC concentrations with CAR was associated with improved cellular and tissue viability after hypoxic-ischemic injury, suggesting a novel therapeutic avenue.

Brain organoids for hypoxic-ischemic studies: from bench to bedside

Our current knowledge regarding the development of the human brain mostly derives from experimental studies on non-human primates, sheep, and rodents. However, these studies may not completely simulate all the features of human brain development as a result of species differences and variations in pre- and postnatal brain maturation. Therefore, it is important to supplement the in vivo animal models to increase the possibility that preclinical studies have appropriate relevance for potential future human trials. Three-dimensional brain organoid culture technology could complement in vivo animal studies to enhance the translatability of the preclinical animal studies and the understanding of brain-related disorders. In this review, we focus on the development of a model of hypoxic-ischemic (HI) brain injury using human brain organoids to complement the translation from animal experiments to human pathophysiology. We also discuss how the development of these tools provides potential opportunities to study fundamental aspects of the pathophysiology of HI-related brain injury including differences in the responses between males and females.

Elevated antioxidant defence in the brain of deep-diving pinnipeds

While foraging, marine mammals undertake repetitive diving bouts. When the animal surfaces, reperfusion makes oxygen readily available for the electron transport chain, which leads to increased production of reactive oxygen species and risk of oxidative damage. In blood and several tissues, such as heart, lung, muscle and kidney, marine mammals generally exhibit an elevated antioxidant defence. However, the brain, whose functional integrity is critical to survival, has received little attention. We previously observed an enhanced expression of several antioxidant genes in cortical neurons of hooded seals (Cystophora cristata). Here, we studied antioxidant gene expression and enzymatic activity in the visual cortex, cerebellum and hippocampus of harp seals (Pagophilus groenlandicus) and hooded seals. Moreover, we tested several genes for positive selection. We found that antioxidants in the first line of defence, such as superoxide dismutase (SOD), glutathione peroxidase (GPX) and glutathione (GSH) were constitutively enhanced in the seal brain compared to mice (Mus musculus), whereas the glutaredoxin and thioredoxin systems were not. Possibly, the activity of the latter systems is stress-induced rather than constitutively elevated. Further, some, but not all members, of the glutathione-s-transferase (GST) family appear more highly expressed. We found no signatures of positive selection, indicating that sequence and function of the studied antioxidants are conserved in pinnipeds.

A Novel Murine Multi-Hit Model of Perinatal Acute Diffuse White Matter Injury Recapitulates Major Features of Human Disease

The selection of an appropriate animal model is key to the production of results with optimal relevance to human disease. Particularly in the case of perinatal brain injury, a dearth of affected human neonatal tissue available for research purposes increases the reliance on animal models for insight into disease mechanisms. Improvements in obstetric and neonatal care in the past 20 years have caused the pathologic hallmarks of perinatal white matter injury (WMI) to evolve away from cystic necrotic lesions and toward diffuse regions of reactive gliosis and persistent myelin disruption. Therefore, updated animal models are needed that recapitulate the key features of contemporary disease. Here, we report a murine model of acute diffuse perinatal WMI induced through a two-hit inflammatory–hypoxic injury paradigm. Consistent with diffuse human perinatal white matter injury (dWMI), our model did not show the formation of cystic lesions. Corresponding to cellular outcomes of dWMI, our injury protocol produced reactive microgliosis and astrogliosis, disrupted oligodendrocyte maturation, and disrupted myelination.. Functionally, we observed sensorimotor and cognitive deficits in affected mice. In conclusion, we report a novel murine model of dWMI that induces a pattern of brain injury mirroring multiple key aspects of the contemporary human clinical disease scenario.

Basal forebrain magnocellular cholinergic systems are damaged in mice following neonatal hypoxia-ischemia

Neonatal hypoxic-ischemic encephalopathy (HIE) causes lifelong neurologic disability. Despite the use of therapeutic hypothermia, memory deficits and executive functions remain severely affected. Cholinergic neurotransmission from the basal forebrain to neocortex and hippocampus is central to higher cortical functions. We examined the basal forebrain by light microscopy and reported loss of choline acetyltransferase-positive (ChAT)+ neurons, at postnatal day (P) 40, in the ipsilateral medial septal nucleus (MSN) after neonatal hypoxia-ischemia (HI) in mice. There was no loss of ChAT+ neurons in the ipsilateral nucleus basalis of Meynert (nbM) and striatum. Ipsilateral striatal and nbM ChAT+ neurons were abnormal with altered immunoreactivity for ChAT, shrunken and crenated somas, and dysmorphic appearing dendrites. Using confocal images with 3D reconstruction, nbM ChAT+ dendrites in HI mice were shorter than sham (p = .0001). Loss of ChAT+ neurons in the MSN directly correlated with loss of ipsilateral hippocampal area. In the nbM and striatum, percentage of abnormal ChAT+ neurons correlated with loss of ipsilateral cerebral cortical and striatal area, respectively. Acetylcholinesterase (AChE) activity increased in adjacent ipsilateral cerebral cortex and hippocampus and the increase was linearly related to loss of cortical and hippocampal area. Numbers and size of cathepsin D+ lysosomes increased in large neurons in the ipsilateral nbM. After neonatal HI, abnormalities were found throughout the major cholinergic systems in relationship to amount of forebrain area loss. There was also an upregulation of cathepsin D+ particles within the nbM. Cholinergic neuropathology may underlie the permanent dysfunction in learning, memory, and executive function after neonatal brain injury.

The Vannucci Model of Hypoxic-Ischemic Injury in the Neonatal Rodent: 40 years later

Perinatal hypoxic-ischemic (HI) brain damage has long been a major cause of acute mortality and chronic neurologic morbidity in infants and children. Experimental animal models are essential to gain insights into the pathogenesis and management of perinatal HI brain damage. Prior to 1980 only large animal models were available. The first small animal model was developed in the postnatal 7 (P7) rat in 1981, now known as the Vannucci model. This model combines unilateral carotid artery ligation with subsequent hypoxia to produce transient hemispheric hypoxia-ischemia in the hemisphere ipsilateral to the ligation while the contralateral hemisphere is exposed to hypoxia only. This model has been characterized with studies of cerebral hemodynamics, cerebral metabolic changes, and acute and chronic neuropathology. Over the past 40 year this animal model has been utilized in numerous laboratories around the world, has been adapted to the immature mouse, as well as to immature rodents at various stages of development. This brief review describes the validation and characterization studies of the original model and some of the adaptations. A discussion of all of the studies focused on specific cell types is beyond the scope of this review. Rather, we present the application of the model to the study of a specific cell type, the pre-oligodendrocyte, and the role this cell plays in the development of white matter injury in the preterm brain.

Early Brain microRNA/mRNA Expression is Region-Specific After Neonatal Hypoxic-Ischemic Injury in a Mouse Model

Background: MicroRNAs (miRNAs) may be promising therapeutic targets for neonatal hypoxic-ischemic brain injury (HIBI) but targeting miRNA-based therapy will require more precise understanding of endogenous brain miRNA expression. Methods: Postnatal day 9 mouse pups underwent HIBI by unilateral carotid ligation + hypoxia or sham surgery. Next-generation miRNA sequencing and mRNA Neuroinflammation panels were performed on ipsilateral cortex, striatum/thalamus, and cerebellum of each group at 30 min after injury. Targeted canonical pathways were predicted by KEGG analysis. Results: Sixty-one unique miRNAs showed differential expression (DE) in at least one region; nine in more than one region, including miR-410-5p, -1264-3p, 1298-5p, -5,126, and -34b-3p. Forty-four mRNAs showed DE in at least one region; 16 in more than one region. MiRNAs showing DE primarily targeted metabolic pathways, while mRNAs targeted inflammatory and cell death pathways. Minimal miRNA-mRNA interactions were seen at 30 min after HIBI. Conclusion: This study identified miRNAs that deserve future study to assess their potential as therapeutic targets in neonatal HIBI. Additionally, the differences in miRNA expression between regions suggest that future studies assessing brain miRNA expression to guide therapy development should consider evaluating individual brain regions rather than whole brain to ensure the sensitivity needed for the development of targeted therapies.

C3a Receptor Signaling Inhibits Neurodegeneration Induced by Neonatal Hypoxic-Ischemic Brain Injury

Hypoxic-ischemic neonatal encephalopathy due to perinatal asphyxia is the leading cause of brain injury in newborns. Clinical data suggest that brain inflammation induced by perinatal insults can persist for years. We previously showed that signaling through the receptor for complement peptide C3a (C3aR) protects against cognitive impairment induced by experimental perinatal asphyxia. To investigate the long-term neuropathological effects of hypoxic-ischemic injury to the developing brain and the role of C3aR signaling therein, we subjected wildtype mice, C3aR deficient mice, and mice expressing biologically active C3a in the CNS to mild hypoxic-ischemic brain injury on postnatal day 9. We found that such injury triggers neurodegeneration and pronounced reactive gliosis in the ipsilesional hippocampus both of which persist long into adulthood. Transgenic expression of C3a in reactive astrocytes reduced hippocampal neurodegeneration and reactive gliosis. In contrast, neurodegeneration and microglial cell density increased in mice lacking C3aR. Intranasal administration of C3a for 3 days starting 1 h after induction of hypoxia-ischemia reduced neurodegeneration and reactive gliosis in the hippocampus of wildtype mice. We conclude that neonatal hypoxic-ischemic brain injury leads to long-lasting neurodegeneration. This neurodegeneration is substantially reduced by treatment with C3aR agonists, conceivably through modulation of reactive gliosis.

Temporal brain microRNA expression changes in a mouse model of neonatal hypoxic-ischemic injury

BACKGROUND

Neonatal hypoxic-ischemic brain injury (HIBI) results in significant morbidity and mortality despite current standard therapies. MicroRNAs (miRNAs) are a promising therapeutic target; however, there is a paucity of data on endogenous miRNA expression of the brain after HIBI during the primary therapeutic window (6-72 h after injury).

METHODS

Postnatal day 9 mouse pups underwent unilateral carotid ligation+hypoxia (HIBI), sham surgery+hypoxia, or sham surgery+normoxia (controls). miRNA sequencing was performed on the ipsilateral brain of each of the three groups plus the contralateral HIBI brain at 24 and 72 h after injury. Findings were validated in eight key miRNAs by quantitative polymerase chain reaction.

RESULTS

Hypoxia resulted in significant differential expression of 38 miRNAs at both time points. Mir-2137, -335, -137, and -376c were significantly altered by neonatal HIBI at 24 and 72 h, with 3 of the 4 demonstrating multiphasic expression (different direction of differential expression at 24 versus 72 h).

CONCLUSIONS

Our global assessment of subacute changes in brain miRNA expression after hypoxia or HIBI will advance research into targeted miRNA-based interventions. It will be important to consider the multiphasic miRNA expression patterns after HIBI to identify optimal timing for individual interventions.

IMPACT

This study is the first to comprehensively define endogenous brain microRNA expression changes outside of the first hours after neonatal hypoxic-ischemic brain injury (HIBI). Mir-2137, -335, -137, and -376c were significantly altered by neonatal HIBI and therefore deserve further investigation as possible therapeutic targets. The expression profiles described will support the design of future studies attempting to develop miRNA-based interventions for infants with HIBI. At 24 h after injury, contralateral HIBI miRNA expression patterns were more similar to ipsilateral HIBI than to controls, suggesting that the contralateral brain is not an appropriate "internal control" for miRNA studies in this model.

Hypothermia modulates myeloid cell polarization in neonatal hypoxic-ischemic brain injury

Background

Neonatal encephalopathy due to hypoxia–ischemia (HI) is a leading cause of death and disability in term newborns. Therapeutic hypothermia (HT) is the only recommended therapy. However, 30% still suffer from neurological deficits. Inflammation is a major hallmark of HI pathophysiology with myeloid cells being key players, participating either in progression or in resolution of injury-induced inflammation. In the present study, we investigated the impact of HT on the temporal and spatial dynamics of microglia/macrophage polarization after neonatal HI in newborn mice.

Methods

Nine-day-old C57BL/6 mice were exposed to HI through occlusion of the right common carotid artery followed by 1 h hypoxia. Immediately after HI, animals were cooled for 4 h or kept at physiological body core temperature. Analyses were performed at 1, 3 and 7 days post HI. Brain injury, neuronal cell loss, apoptosis and microglia activation were assessed by immunohistochemistry. A broad set of typical genes associated with classical (M1) and alternative (M2) myeloid cell activation was analyzed by real time PCR in ex vivo isolated CD11b⁺ microglia/macrophages. Purity and composition of isolated cells was determined by flow cytometry.

Results

Immediate HT significantly reduced HI-induced brain injury and neuronal loss 7 days post HI, whereas only mild non-significant protection from HI-induced apoptosis and neuronal loss were observed 1 and 3 days after HI. Microglia activation, i.e., Iba-1 immunoreactivity peaked 3 days after HI and was not modulated by HT. However, ex vivo isolated CD11b⁺ cells revealed a strong upregulation of the majority of M1 but also M2 marker genes at day 1, which was significantly reduced by HT and rapidly declined at day 3. HI induced a significant increase in the frequency of peripheral macrophages in sorted CD11b⁺ cells at day 1, which deteriorated until day 7 and was significantly decreased by HT.

Conclusion

Our data demonstrate that HT-induced neuroprotection is preceded by acute suppression of HI-induced upregulation of inflammatory genes in myeloid cells and decreased infiltration of peripheral macrophages, both representing potential important effector mechanisms of HT.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02314-9.

Neuronal deficiency of hypoxia-inducible factor 2α increases hypoxic-ischemic brain injury in neonatal mice

The cellular responses to hypoxia or hypoxia-ischemia (HI) are governed largely by the hypoxia-inducible factor (HIF) family of transcription factors. Our previous studies show that HIF-1α induction is an important factor that mediates protective effects in the brain after neonatal HI. In the present study, we investigated the contribution of another closely related HIF α isoform, HIF-2α, specifically the neuronal HIF-2α, to brain HI injury. Homozygous transgenic mice with a floxed exon 2 of HIF-2α were bred with CaMKIIα-Cre mice to generate a mouse line with selective deletion of HIF-2α in forebrain neurons. These mice, along with their wildtype littermates, were subjected to HI at postnatal day 9. Brain injury at different ages was evaluated by the levels of cleaved caspase-3 and spectrin breakdown products at 24 hr; and histologically at 6 days or 3 months after HI. Multiple behavioral tests were performed at 3 months, prior to sacrifice. Loss of neuronal HIF-2α exacerbated brain injury during the acute (24 hr) and subacute phases (6 days), with a trend toward more severe volume loss in the adult brain. The long-term brain function for coordinated movement and recognition memory, however, were not impacted in the neuronal HIF-2α deficient mice. Our data suggest that, similar to HIF-1α, neuronal HIF-2α promotes cell survival in the immature mouse brain. The two HIF alpha isoforms may act through partially overlapping or distinct transcriptional targets to mediate their intrinsic protective responses against neonatal HI brain injury.

Current Therapies for Neonatal Hypoxic-Ischaemic and Infection-Sensitised Hypoxic-Ischaemic Brain Damage

Neonatal hypoxic-ischaemic brain damage is a leading cause of child mortality and morbidity, including cerebral palsy, epilepsy, and cognitive disabilities. The majority of neonatal hypoxic-ischaemic cases arise as a result of impaired cerebral perfusion to the foetus attributed to uterine, placental, or umbilical cord compromise prior to or during delivery. Bacterial infection is a factor contributing to the damage and is recorded in more than half of preterm births. Exposure to infection exacerbates neuronal hypoxic-ischaemic damage thus leading to a phenomenon called infection-sensitised hypoxic-ischaemic brain injury. Models of neonatal hypoxia-ischaemia (HI) have been developed in different animals. Both human and animal studies show that the developmental stage and the severity of the HI insult affect the selective regional vulnerability of the brain to damage, as well as the subsequent clinical manifestations. Therapeutic hypothermia (TH) is the only clinically approved treatment for neonatal HI. However, the number of HI infants needed to treat with TH for one to be saved from death or disability at age of 18-22 months, is approximately 6-7, which highlights the need for additional or alternative treatments to replace TH or increase its efficiency. In this review we discuss the mechanisms of HI injury to the immature brain and the new experimental treatments studied for neonatal HI and infection-sensitised neonatal HI.

Endonuclease V Regulates Atherosclerosis Through C-C Motif Chemokine Ligand 2-Mediated Monocyte Infiltration

Background

In cardiovascular diseases, atherosclerotic disorder are the most frequent and important with respect to morbidity and mortality. Inflammation mediated by immune cells is central in all parts of the atherosclerotic progress, and further understanding of the underlying mechanisms is needed. Growing evidence suggests that deamination of adenosine‐to‐inosine in RNA is crucial for a correct immune response; nevertheless, the role of adenosine‐to‐inosine RNA editing in atherogenesis has barely been studied. Several proteins have affinity for inosines in RNA, one being ENDOV (endonuclease V), which binds and cleaves RNA at inosines. Data on ENDOV in atherosclerosis are lacking.

Methods and Results

Quantitative polymerase chain reaction on ENDOV mRNA showed an increased level in human carotid atherosclerotic plaques compared with control veins. Inosine‐ribonuclease activity as measured by an enzyme activity assay is detected in immune cells relevant for the atherosclerotic process. Abolishing EndoV in atherogenic apolipoprotein E‐deficient (ApoE^−/−) mice reduces the atherosclerotic plaque burden, both in size and lipid content. In addition, in a brain stroke model, mice without ENDOV suffer less damage than control mice. Finally, lack of EndoV reduces the recruitment of monocytes to atherosclerotic lesions in atherogenic ApoE^−/− mice.

Conclusions

ENDOV is upregulated in human atherosclerotic lesions, and data from mice suggest that ENDOV promotes atherogenesis by enhancing the monocyte recruitment into the atherosclerotic lesion, potentially by increasing the effect of CCL2 activation on these cells.

Micro-Actuated Tunable Hierarchical Silver Nanostructures to Measure Tensile Force for Biomedical Wearable Sensing Applications

Commercially available biomedical wearable sensors to measure tensile force/strain still struggle with miniaturization in terms of weight, size, and conformability. Flexible and epidermal electronic devices have been utilized in these applications to overcome these issues. However, current sensors still require a power supply and some form of powered data transfer, which present challenges to miniaturization and to applications. Here, we report on the development of flexible, passive (thus zero power consumption), and biocompatible nanostructured photonic devices that can measure tensile strain in real time by providing an optical readout instead of an electronic readout. Hierarchical silver (Ag) nanostructures in various thicknesses of 20–60 nm were fabricated and embedded on a stretchable substrate using e-beam lithography and a low-temperature dewetting process. The hierarchical Ag nanostructures offer more design flexibility through a two-level design approach. A tensional force applied in one lateral (x- or y-) direction of the stretchable substrate causes a Poisson contraction in the other, and as a result, a shift in the reflected light of the nanostructures. A clear blue shift of more than 100 nm in peak reflectance in the visible spectrum was observed in the reflected color, making the devices applicable in a variety of biomedical photonic sensing applications.

Drug delivery platforms for neonatal brain injury

Hypoxic-ischemic encephalopathy (HIE), initiated by the interruption of oxygenated blood supply to the brain, is a leading cause of death and lifelong disability in newborns. The pathogenesis of HIE involves a complex interplay of excitotoxicity, inflammation, and oxidative stress that results in acute to long term brain damage and functional impairments. Therapeutic hypothermia is the only approved treatment for HIE but has limited effectiveness for moderate to severe brain damage; thus, pharmacological intervention is explored as an adjunct therapy to hypothermia to further promote recovery. However, the limited bioavailability and the side-effects of systemic administration are factors that hinder the use of the candidate pharmacological agents. To overcome these barriers, therapeutic molecules may be packaged into nanoscale constructs to enable their delivery. Yet, the application of nanotechnology in infants is not well examined, and the neonatal brain presents unique challenges. Novel drug delivery platforms have the potential to magnify therapeutic effects in the damaged brain, mitigate side-effects associated with high systemic doses, and evade mechanisms that remove the drugs from circulation. Encouraging pre-clinical data demonstrates an attenuation of brain damage and increased structural and functional recovery. This review surveys the current progress in drug delivery for treating neonatal brain injury.

Neonatal hypoxic-ischemic brain injury leads to sex-specific deficits in rearing and climbing in adult mice

The study examined the morphological and long-term behavioral impacts of neonatal hypoxic-ischemic brain injury in a mouse model. We investigated the modification of different behavioral domains, such as spontaneous climbing, which represents fine motor skills. We also focused on sex-dependent differences during hypoxic-ischemic encephalopathy. The Rice-Vannucci model of hypoxia-ischemia was used, adjusted and adapted to 7-day-old C57BL/6NTac mice. The effects of induced hypoxia and ischemia were also studied separately. At postnatal day 60, mice underwent behavioral testing using the LABORAS apparatus. The perfusion for histological evaluation was performed one day after the behavioral analyses. In groups with separately induced hypoxia or ischemia, the observed alterations in behavior were not accompanied by morphological changes in the cortex or hippocampal formation. Female mice naturally climbed significantly more and hypoxic females reared less than hypoxic males (p<0.05). Male mice postnatally exposed to hypoxia-ischemia exhibited significantly lower vertical activity and higher horizontal activity (p<0.05). Mild hypoxic damage may not be morphologically detectable but may induce substantial behavioral changes in adult mice. There were significant differences between horizontal and vertical activity in reaction to hypoxia-ischemia. Our study indicates that the importance of behavioral testing is irreplaceable and may be reflected in neonatal medicine.

Brain damage caused by neonatal hypoxia-ischemia and the effects of hypothermia in severe combined immunodeficient (SCID) mice

Neonatal hypoxic-ischemic encephalopathy (HIE) is a major cause of brain damage in newborns. Although therapeutic hypothermia has been shown to be neuroprotective against neonatal HIE in clinical trials, its effect is not satisfactory. Cell-based therapies have attracted much attention as novel treatments for HIE. Preclinical studies on a variety of human cell transplantation methods have been performed in immunodeficient/immunosuppressed animals, such as severe combined immunodeficient (SCID) mice, which lack functional T and B lymphocytes. The detailed characteristics of neonatal HIE in SCID mice, however, have not been delineated. In preclinical studies, novel therapies for neonatal HIE should be evaluated in combination with hypothermia, which has become a standard treatment for neonatal HIE. However, the effects of hypothermia in SCID mice have not been delineated. In the present study, we compared neonatal hypoxic-ischemic (HI) brain damage in SCID mice and wild-type mice treated with or without hypothermia. Male and female mouse pups were subjected to HI insult induced by unilateral common carotid artery ligation combined with systemic hypoxia on postnatal day 12. In the first 4 h after HI insult, body temperature was maintained at 36 °C for the normothermia groups or 32 °C for the hypothermia groups. The severity of brain damage in SCID mice did not differ from that in wild-type mice based on most evaluations, i.e., cerebral blood flow, hemiparesis, muscle strength, spontaneous activity, cerebral hemispheric volume, neuropathological injury, and serum cytokine levels, although spleen weight, brain weight, leukocyte counts and the levels of some cytokines in the peripheral blood were different between genotypes. The effects of hypothermia in SCID mice were comparable to those in wild-type mice based on most evaluations. Taken together, these findings indicate that SCID mice can be used as an appropriate preclinical model for cell therapies for neonatal HIE.

Mesenchymal Stromal Cell-Derived Extracellular Vesicles Reduce Neuroinflammation, Promote Neural Cell Proliferation and Improve Oligodendrocyte Maturation in Neonatal Hypoxic-Ischemic Brain Injury

Background: Neonatal encephalopathy caused by hypoxia-ischemia (HI) is a major cause of childhood mortality and disability. Stem cell-based regenerative therapies seem promising to prevent long-term neurological deficits. Our previous work in neonatal HI revealed an unexpected interaction between mesenchymal stem/stromal cells (MSCs) and the brains' microenvironment leading to an altered therapeutic efficiency. MSCs are supposed to mediate most of their therapeutic effects in a paracrine mode via extracellular vesicles (EVs), which might be an alternative to cell therapy. In the present study, we investigated the impact of MSC-EVs on neonatal HI-induced brain injury.

Methods: Nine-day-old C57BL/6 mice were exposed to HI through ligation of the right common carotid artery followed by 1 h hypoxia (10% oxygen). MSC-EVs were injected intraperitoneally 1, 3, and 5 days after HI. One week after HI, brain injury was evaluated by regional neuropathological scoring, atrophy measurements and immunohistochemistry to assess effects on neuronal, oligodendrocyte and vessel densities, proliferation, oligodendrocyte maturation, myelination, astro-, and microglia activation. Immunohistochemistry analyses were complemented by mRNA expression analyses for a broad set of M1/M2- and A1/A2-associated molecules and neural growth factors.

Results: While total neuropathological scores and tissue atrophy were not changed, MSC-EVs significantly protected from HI-induced striatal tissue loss and decreased micro- and astroglia activation. MSC-EVs lead to a significant downregulation of the pro-inflammatory cytokine TNFa, accompanied by a significant upregulation of the M2 marker YM-1 and the anti-inflammatory cytokine TGFb. MSC-EVs significantly decreased astrocytic expression of the A1 marker C3, concomitant with an increased expression of neural growth factors (i.e., BDNF, VEGF, and EGF). These alterations were associated with an increased neuronal and vessel density, coinciding with a significant increase of proliferating cells in the neurogenic sub-ventricular zone juxtaposed to the striatum. MSC-EV-mediated neuroprotection went along with a significant improvement of oligodendrocyte maturation and myelination.

Conclusion: The present study demonstrates that MSC-EVs mediate anti-inflammatory effects, promote regenerative responses and improve key developmental processes in the injured neonatal brain. The present results suggest different cellular target mechanisms of MSC-EVs, preventing secondary HI-induced brain injury. MSC-EV treatment may be a promising alternative to risk-associated cell therapies in neonatal brain injury.

Parp3 promotes astrocytic differentiation through a tight regulation of Nox4-induced ROS and mTorc2 activation

Parp3 is a member of the Poly(ADP-ribose) polymerase (Parp) family that has been characterized for its functions in strand break repair, chromosomal rearrangements, mitotic segregation and tumor aggressiveness. Yet its physiological implications remain unknown. Here we report a central function of Parp3 in the regulation of redox homeostasis in continuous neurogenesis in mice. We show that the absence of Parp3 provokes Nox4-induced oxidative stress and defective mTorc2 activation leading to inefficient differentiation of post-natal neural stem/progenitor cells to astrocytes. The accumulation of ROS contributes to the decreased activity of mTorc2 as a result of an oxidation-induced and Fbxw7-mediated ubiquitination and degradation of Rictor. In vivo, mTorc2 signaling is compromised in the striatum of naïve post-natal Parp3-deficient mice and 6 h after acute hypoxia-ischemia. These findings reveal a physiological function of Parp3 in the tight regulation of striatal oxidative stress and mTorc2 during astrocytic differentiation and in the acute phase of hypoxia-ischemia.

How to Improve the Antioxidant Defense in Asphyxiated Newborns-Lessons from Animal Models

Oxygen free radicals have been implicated in brain damage after neonatal asphyxia. In the early phase of asphyxia/reoxygenation, changes in antioxidant enzyme activity play a pivotal role in switching on and off the cascade of events that can kill the neurons. Hypoxia/ischemia (H/I) forces the brain to activate endogenous mechanisms (e.g., antioxidant enzymes) to compensate for the lost or broken neural circuits. It is important to evaluate therapies to enhance the self-protective capacity of the brain. In animal models, decreased body temperature during neonatal asphyxia has been shown to increase cerebral antioxidant capacity. However, in preterm or severely asphyxiated newborns this therapy, rather than beneficial seems to be harmful. Thus, seeking new therapeutic approaches to prevent anoxia-induced complications is crucial. Pharmacotherapy with deferoxamine (DFO) is commonly recognized as a beneficial regimen for H/I insult. DFO, via iron chelation, reduces oxidative stress. It also assures an optimal antioxidant protection minimizing depletion of the antioxidant enzymes as well as low molecular antioxidants. In the present review, some aspects of recently acquired insight into the therapeutic effects of hypothermia and DFO in promoting neuronal survival after H/I are discussed.

Animal models for neonatal brain injury induced by hypoxic ischemic conditions in rodents

Neonatal hypoxia-ischemia and resulting encephalopathies are of significant concern. Intrapartum asphyxia is a leading cause of neonatal death globally. Among surviving infants, there remains a high incidence of hypoxic-ischemic encephalopathy due to neonatal hypoxic-ischemic brain injury, manifesting as mild conditions including attention deficit hyperactivity disorder, and debilitating disorders such as cerebral palsy. Various animal models of neonatal hypoxic brain injury have been implemented to explore cellular and molecular mechanisms, assess the potential of novel therapeutic strategies, and characterize the functional and behavioural correlates of injury. Each of the animal models has individual advantages and limitations. The present review looks at several widely-used and alternative rodent models of neonatal hypoxia and hypoxia-ischemia; it highlights their strengths and limitations, and their potential for continued and improved use.

TrkB agonists prevent postischemic emergence of refractory neonatal seizures in mice

Refractory neonatal seizures do not respond to first-line antiseizure medications like phenobarbital (PB), a positive allosteric modulator for GABAA receptors. GABAA receptor-mediated inhibition is dependent upon electroneutral cation-chloride transporter KCC2, which mediates neuronal chloride extrusion and its age-dependent increase and postnatally shifts GABAergic signaling from depolarizing to hyperpolarizing. Brain-derived neurotropic factor-tyrosine receptor kinase B activation (BDNF-TrkB activation) after excitotoxic injury recruits downstream targets like PLCγ1, leading to KCC2 hypofunction. Here, the antiseizure efficacy of TrkB agonists LM22A-4, HIOC, and deoxygedunin (DG) on PB-refractory seizures and postischemic TrkB pathway activation was investigated in a mouse model (CD-1, P7) of refractory neonatal seizures. LM, a BDNF loop II mimetic, rescued PB-refractory seizures in a sexually dimorphic manner. Efficacy was associated with a substantial reduction in the postischemic phosphorylation of TrkB at Y816, a site known to mediate postischemic KCC2 hypofunction via PLCγ1 activation. LM rescued ischemia-induced phospho-KCC2-S940 dephosphorylation, preserving its membrane stability. Full TrkB agonists HIOC and DG similarly rescued PB refractoriness. Chemogenetic inactivation of TrkB substantially reduced postischemic neonatal seizure burdens at P7. Sex differences identified in developmental expression profiles of TrkB and KCC2 may underlie the sexually dimorphic efficacy of LM. These results support a potentially novel role for the TrkB receptor in the emergence of age-dependent refractory neonatal seizures.

Vancomycin Is Protective in a Neonatal Mouse Model of Staphylococcus epidermidis-Potentiated Hypoxic-Ischemic Brain Injury

Infection is correlated with increased risk of neurodevelopmental sequelae in preterm infants. In modeling neonatal brain injury, Toll-like receptor agonists have often been used to mimic infections and induce inflammation. Using the most common cause of bacteremia in preterm infants, Staphylococcus epidermidis, we present a more clinically relevant neonatal mouse model that addresses the combined effects of bacterial infection together with subsequent hypoxic-ischemic brain insult. Currently, there is no neuroprotective treatment for the preterm population. Hence, we tested the neuroprotective effects of vancomycin with and without adjunct therapy using the anti-inflammatory agent pentoxifylline. We characterized the effects of S. epidermidis infection on the inflammatory response in the periphery and the brain, as well as the physiological changes in the central nervous system that might affect neurodevelopmental outcomes. Intraperitoneal injection of postnatal day 4 mice with a live clinical isolate of S. epidermidis led to bacteremia and induction of proinflammatory cytokines in the blood, as well as transient elevations of neutrophil and monocyte chemotactic cytokines and caspase 3 activity in the brain. When hypoxia-ischemia was induced postinfection, more severe brain damage was observed in infected animals than in saline-injected controls. This infection-induced inflammation and potentiated brain injury was inoculum dose dependent and was alleviated by the antibiotic vancomycin. Pentoxifylline did not provide any additional neuroprotective effect. Thus, we show for the first time that live S. epidermidis potentiates hypoxic-ischemic preterm brain injury and that peripheral inhibition of inflammation with antibiotics, such as vancomycin, reduces the extent of brain injury.

Combined Treatment with Insulin-Like Growth Factor 1 and AMD3100 Improves Motor Outcome in a Murine Model of Neonatal Hypoxic-Ischemic Encephalopathy

Stem cell transplantation is a promising intervention for neonatal hypoxic-ischemic encephalopathy (HIE); however, universal feasibility and safety have not been thoroughly evaluated. AMD3100 and insulin-like growth factor 1 (IGF1) mobilize progenitor cells into peripheral circulation. The objective of this study was to assess the short-term efficacy of inducing endogenous stem cell mobilization after injury in a model of neonatal HIE. Postnatal day 9 CD1 pups received sham surgery or unilateral carotid artery ligation and 30 min of hypoxia followed by saline, AMD3100, IGF1, or both agents. Intraperitoneal injections of 5-ethynyl-2'-deoxy-uridine (EdU) and 5-bromo-2'-deoxyuridine were used to -label replicating progenitor cells. At P14, animals underwent rotarod testing, and the brains were sectioned for area measurements and immunofluorescence staining. Comparisons were made using one-way analysis of variance. Spearman's rho was calculated to assess correlation between rotarod results and markers of brain injury. Pups treated with both agents had improved rotarod performance (p = 0.02) and increased EdU+ progenitor cells in the subgranular zone (SGZ) compared to injured controls (p = 0.10). An increase in active cells within the SGZ was correlated with improved rotarod performance (r = 0.84, p = 0.04). There were no differences in overall injury score or in brain area or number of activated cells in the subventricular zone between the treatment groups. Combined treatment with AMD3100 and IGF1 shows promise for decreasing brain injury and improving motor function in pups after HIE which correlated with changes in the number of active progenitor cells in the SGZ.

Properdin: A Novel Target for Neuroprotection in Neonatal Hypoxic-Ischemic Brain Injury

Background: Hypoxic-ischemic (HI) encephalopathy is a major cause of neonatal mortality and morbidity, with a global incidence of 3 per 1,000 live births. Intrauterine or perinatal complications, including maternal infection, constitute a major risk for the development of neonatal HI brain damage. During HI, inflammatory response and oxidative stress occur, causing subsequent cell death. The presence of an infection sensitizes the neonatal brain, making it more vulnerable to the HI damage. Currently, therapeutic hypothermia is the only clinically approved treatment available for HI encephalopathy, however it is only partially effective in HI alone and its application in infection-sensitized HI is debatable. Therefore, there is an unmet clinical need for the development of novel therapeutic interventions for the treatment of HI. Such an alternative is targeting the complement system. Properdin, which is involved in stabilization of the alternative pathway convertases, is the only known positive regulator of alternative complement activation. Absence of the classical pathway in the neonatal HI brain is neuroprotective. However, there is a paucity of data on the participation of the alternative pathway and in particular the role of properdin in HI brain damage.

Objectives: Our study aimed to validate the effect of global properdin deletion in two mouse models: HI alone and LPS-sensitized HI, thus addressing two different clinical scenarios.

Results: Our results indicate that global properdin deletion in a Rice-Vannucci model of neonatal HI and LPS-sensitized HI brain damage, in the short term, clearly reduced forebrain cell death and microglial activation, as well as tissue loss. In HI alone, deletion of properdin reduced TUNEL+ cell death and microglial post-HI response at 48 h post insult. Under the conditions of LPS-sensitized HI, properdin deletion diminished TUNEL+ cell death, tissue loss and microglial activation at 48 h post-HI.

Conclusion: Overall, our data suggests a critical role for properdin, and possibly also a contribution in neonatal HI alone and in infection-sensitized HI brain damage. Thus, properdin can be considered a novel target for treatment of neonatal HI brain damage.

Animal models of developmental motor disorders: parallels to human motor dysfunction in cerebral palsy

Cerebral palsy (CP) is the most common motor disability in children. Much of the previous research on CP has focused on reducing the severity of brain injuries, whereas very few researchers have investigated the cause and amelioration of motor symptoms. This research focus has had an impact on the choice of animal models. Many of the commonly used animal models do not display a prominent CP-like motor phenotype. In general, rodent models show anatomically severe injuries in the central nervous system (CNS) in response to insults associated with CP, including hypoxia, ischemia, and neuroinflammation. Unfortunately, most rodent models do not display a prominent motor phenotype that includes the hallmarks of spasticity (muscle stiffness and hyperreflexia) and weakness. To study motor dysfunction related to developmental injuries, a larger animal model is needed, such as rabbit, pig, or nonhuman primate. In this work, we describe and compare various animal models of CP and their potential for translation to the human condition.

The optimal choices of animal models of white matter injury

White matter injury, the most common neurological injury in preterm infants, is a major cause of chronic neurological morbidity, including cerebral palsy. Although there has been great progress in the study of the mechanism of white matter injury in newborn infants, its pathogenesis is not entirely clear, and further treatment approaches are required. Animal models are the basis of study in pathogenesis, treatment, and prognosis of white matter injury in preterm infants. Various species have been used to establish white matter injury models, including rodents, rabbits, sheep, and non-human primates. Small animal models allow cost-effective investigation of molecular and cellular mechanisms, while large animal models are particularly attractive for pathophysiological and clinical-translational studies. This review focuses on the features of commonly used white matter injury animal models, including their modelling methods, advantages, and limitations, and addresses some clinically relevant animal models that allow reproduction of the insults associated with clinical conditions that contribute to white matter injury in human infants.

Assessing Cerebral Metabolism in the Immature Rodent: From Extracts to Real-Time Assessments

Brain development is an energy-expensive process. Although glucose is irreplaceable, the developing brain utilizes a variety of substrates such as lactate and the ketone bodies, β-hydroxybutyrate and acetoacetate, to produce energy and synthesize the structural components necessary for cerebral maturation. When oxygen and nutrient supplies to the brain are restricted, as in neonatal hypoxia-ischemia (HI), cerebral energy metabolism undergoes alterations in substrate use to preserve the production of adenosine triphosphate. These changes have been studied by in situ biochemical methods that yielded valuable quantitative information about high-energy and glycolytic metabolites and established a temporal profile of the cerebral metabolic response to hypoxia and HI. However, these analyses relied on terminal experiments and averaging values from several animals at each time point as well as challenging requirements for accurate tissue processing.More recent methodologies have focused on in vivo longitudinal analyses in individual animals. The emerging field of metabolomics provides a new investigative tool for studying cerebral metabolism. Magnetic resonance spectroscopy (MRS) has enabled the acquisition of a snapshot of the metabolic status of the brain as quantifiable spectra of various intracellular metabolites. Proton (1H) MRS has been used extensively as an experimental and diagnostic tool of HI in the pursuit of markers of long-term neurodevelopmental outcomes. Still, the interpretation of the metabolite spectra acquired with 1H MRS has proven challenging, due to discrepancies among studies, regarding calculations and timing of measurements. As a result, the predictive utility of such studies is not clear. 13C MRS is methodologically more challenging, but it provides a unique window on living tissue metabolism via measurements of the incorporation of 13C label from substrates into brain metabolites and the localized determination of various metabolic fluxes. The newly developed hyperpolarized 13C MRS is an exciting method for assessing cerebral metabolism in vivo, that bears the advantages of conventional 13C MRS but with a huge gain in signal intensity and much shorter acquisition times. The first part of this review article provides a brief description of the findings of biochemical and imaging methods over the years as well as a discussion of their associated strengths and pitfalls. The second part summarizes the current knowledge on cerebral metabolism during development and HI brain injury.

Short-, Mid-, and Long-Term Effect of Granulocyte Colony-Stimulating Factor/Stem Cell Factor and Fms-Related Tyrosine Kinase 3 Ligand Evaluated in an In Vivo Model of Hypoxic-Hyperoxic Ischemic Neonatal Brain Injury

Hematopoietic growth factors are considered to bear neuroprotective potential. We have previously shown that delayed treatment with granulocyte colony-stimulating factor (G-CSF)/stem cell factor (SCF) and Fms-related tyrosine kinase 3 ligand (FL) ameliorates excitotoxic neonatal brain injury. The effect of these substances in combined-stressor neonatal brain injury models more closely mimicking clinical conditions has not been investigated. The aim of this study was to assess the short-, mid-, and long-term neuroprotective potential of G-CSF/SCF and FL in a neonatal model of hypoxic-hyperoxic ischemic brain injury. Five-day-old (P5) CD-1 mice were subjected to unilateral common carotid artery ligation and subsequent alternating periods of hypoxia and hyperoxia for 65 minutes. Sixty hours after injury, pups were randomly assigned to intraperitoneal treatment with (i) G-CSF (200 μg/kg)/SCF (50 μg/kg), (ii) FL (100 μg/kg), or (iii) vehicle every 24 hours for three or five consecutive days. Histopathological and functional outcomes were evaluated on P10, P18, and P90. Baseline outcome parameters were established in sham-treated and healthy control animals. Gross brain injury did not significantly differ between treatment groups at any time point. On P10, caspase-3 activation and caspase-independent apoptosis were similar between treatment groups; cell proliferation and the number of BrdU-positive vessels did not differ on P18 or P90. Neurobehavioral assessment did not reveal significant differences between treatment groups in accelerod performance, open field behavior, or novel object recognition capacity on P90. Turning behavior was more frequently observed in G-CSF/SCF- and FL-treated animals. No sex-specific differences were detected in any outcome parameter evaluated. In hypoxic-hyperoxic ischemic neonatal brain injury, G-CSF/SCF and FL treatment does not convey neuroprotection. Prior to potential clinical use, meticulous assessment of these hematopoietic growth factors is mandated.

An Inhibitor of the Mitochondrial Permeability Transition Pore Lacks Therapeutic Efficacy Following Neonatal Hypoxia Ischemia in Mice

Neonatal hypoxic ischemic (HI) brain injury causes lifelong neurologic disability. Therapeutic hypothermia (TH) is the only approved therapy that partially mitigates mortality and morbidity. Therapies specifically targeting HI-induced brain cell death are currently lacking. Intracellular calcium dysregulation, oxidative stress, and mitochondrial dysfunction through the formation of the mitochondrial permeability transition pore (mPTP) are drivers of HI cellular injury. GNX-4728, a small molecule direct inhibitor of the mPTP that increases mitochondrial calcium retention capacity, is highly effective in adult neurodegenerative disease models and could have potential as a therapy in neonatal HI. A dose of GNX-4728, equivalent to that used in animal models, 300 mg/kg, IP was highly toxic in p10 mice. We then tested the hypothesis that acute administration of 30 mg/kg, IP of GNX-4728 immediately after HI in a neonatal mouse model would provide neuroprotection. This non-lethal lower dose of GNX-4728 (30 mg/kg, IP) improved the respiratory control ratio of neonatal female HI brain tissue but not in males. Brain injury, assessed histologically with a novel metric approach at 1 and 30 days after HI, was not mitigated by GNX-4728. Our work demonstrates that a small molecule inhibitor of the mPTP has i) an age related toxicity, ii) a sex-related brain mitoprotective profile after HI but iii) this is not sufficient to attenuate forebrain HI neuropathology.

HIF1α Signaling in the Endogenous Protective Responses after Neonatal Brain Hypoxia-Ischemia

Hypoxia-inducible factor 1α (HIF1α) is a key regulator of oxygen homeostasis, and its target genes mediate adaptive, protective, and pathological processes. The role of HIF1α in neuronal survival is controversial and the brain maturation stage is important in determining its function in brain ischemia or hypoxia-ischemia (HI). In this study, we used neuron-specific HIF1α knockout mice at postnatal day 9 (P9), and immature cortical neurons (days 7-8 in vitro) treated with the HIF1α inhibitor 2-methoxyestradiol (2ME2) or stabilizer dimethyloxalylglycine (DMOG), to examine the function of neuronal HIF1α in neonatal HI in vivo (Vannucci model) and in vitro (oxygen glucose deprivation, OGD). Inhibition of HIF1α with 2ME2 in primary neurons or deletion of neuronal HIF1α in P9 mice increased both necrotic and apoptotic cell death following HI, as evaluated by the protein levels of 145/150-kDa and 120-kDa spectrin breakdown products 24 h after HI. DMOG attenuated neuronal death right after OGD. Acute pharmacological manipulation of HIF1α synchronously regulated the expression of its targets, vascular endothelial growth factor (VEGF) and erythropoietin (Epo), in the same manner. The in vivo findings agree with our previous data using the same HIF1α-deficient mice at an earlier age. This study confirms the role of neuronal HIF1α signaling in the endogenous protective responses following HI in the developing brain.

Strain-Related Differences in Mouse Neonatal Hypoxia-Ischemia

Neonatal hypoxic-ischemic brain injury is commonly studied by means of the Vannucci procedure in mice or rats (unilateral common carotid artery occlusion followed by hypoxia). Previously, we modified the postnatal day 7 (P7) rat procedure for use in mice, and later demonstrated that genetic strain strongly influences the degree of brain injury in the P7 mouse model of hypoxia-ischemia (HI). Recently, the P9 or P10 mouse brain was recognized as the developmental equivalent of a term neonatal human brain, rather than P7. Consequently, the Vannucci procedure has again been modified, and a commonly used protocol employs 10% oxygen for 50 min in C57Bl/6 mice. Strain differences have yet to be described for the P9/P10 mouse model. In order to determine if the strain differences we previously reported in the P7 mouse model are present in the P9 model, we compared 2 commonly used strains, CD1 and C57Bl/6J, in both the P7 (carotid ligation [in this case, right] followed by exposure to 8% oxygen for 30 min) and P9 (carotid ligation [in this case left] followed by exposure to 10% oxygen) models of HI. Experiments using the P7 model were performed in 2001-2012 and those using the P9 model were performed in 2012-2016. Five to seven days after the HI procedure, mice were perfused with 4% paraformaldehyde, their brains were sectioned on a Vibratome (50 µm) and alternate sections were stained with Perl's iron stain or cresyl violet. Brain sections were examined microscopically and scored for the degree of injury. Since brains in the P7 group had been scored previously with a slightly different system, they were reanalyzed using our current scoring system which scores injury in 11 regions: the anterior, middle, and posterior cortex; the anterior, middle, and posterior striatum; CA1, CA2, CA3, and the dentate gyrus of the hippocampus and thalamus, on a scale from 0 (none) to 3 (cystic infarct) for a total score of 0-33. Brains in the P9 group were scored with the same system. Given the same insult, the P7 CD1 mice had greater injury than the C57Bl/6J mice, which agrees with our previous findings. The P9 CD1 mice also had greater injury than the C57Bl/6J mice. This study confirms that CD1 mice are more susceptible to injury than C57Bl/6J mice and that strain selection is important when using mouse models of HI.

Severe Perinatal Hypoxic-Ischemic Brain Injury Induces Long-Term Sensorimotor Deficits, Anxiety-Like Behaviors and Cognitive Impairment in a Sex-, Age- and Task-Selective Manner in C57BL/6 Mice but Can Be Modulated by Neonatal Handling

Perinatal brain injury (PBI) leads to neurological disabilities throughout life, from motor deficits, cognitive limitations to severe cerebral palsy. Yet, perinatal brain damage has limited therapeutic outcomes. Besides, the immature brain of premature children is at increased risk of hypoxic/ischemic (HI) injury, with males being more susceptible to it and less responsive to protective/therapeutical interventions. Here, we model in male and female C57BL/6 mice, the impact of neonatal HI and the protective effects of neonatal handling (NH), an early life tactile and proprioceptive sensory stimulation. From postnatal day 1 (PND1, modeling pre-term) to PND21 randomized litters received either NH or left undisturbed. HI brain damage occurred by permanent left carotid occlusion followed by hypoxia at PND7 (modeling full-term) in half of the animals. The behavioral and functional screening of the pups at weaning (PND23) and their long-term outcomes (adulthood, PND70) were evaluated in a longitudinal study, as follows: somatic development (weight), sensorimotor functions (reflexes, rods and hanger tests), exploration [activity (ACT) and open-field (OF) test], emotional and anxiety-like behaviors [corner, open-field and dark-light box (DLB) tests], learning and memory [T-maze (TM) and Morris Water-Maze (MWM)]. HI induced similar brain damage in both sexes but affected motor development, sensorimotor functions, induced hyperactivity at weaning, and anxiety-like behaviors and cognitive deficits at adulthood, in a sex- and age-dependent manner. Thus, during ontogeny, HI affected equilibrium especially in females and prehensility in males, but only reflexes at adulthood. Hyperactivity of HI males was normalized at adulthood. HI increased neophobia and other anxiety-like behaviors in males but emotionality in females. Both sexes showed worse short/long-term learning, but memory was more affected in males. Striking neuroprotective effects of NH were found, with significantly lower injury scores, mostly in HI males. At the functional level, NH reversed the impaired reflex responses and improved memory performances in hippocampal-dependent spatial-learning tasks, especially in males. Finally, neuropathological correlates referred to atrophy, neuronal densities and cellularity in the affected areas [hippocampal-CA, caudate/putamen, thalamus, neocortex and corpus callosum (CC)] point out distinct neuronal substrates underlying the sex- and age- functional impacts of these risk/protection interventions on sensorimotor, behavioral and cognitive outcomes from ontogeny to adulthood.

Bestrophin-3 Expression in a Subpopulation of Astrocytes in the Neonatal Brain After Hypoxic-Ischemic Injury

Bestrophin-3, a potential candidate for a calcium-activated chloride channel, recently was suggested to have cell-protective functions. We studied the expression and alternative splicing of bestrophin-3 in neonatal mouse brain and after hypoxic-ischemic (HI) injury and in human neonatal brain samples. HI brain injury was induced in 9-day old mice by unilateral permanent common carotid artery occlusion in combination with exposure to 10% oxygen for 50 min. Endoplasmic reticulum stress was induced by thapsigargin treatment in primary culture of mouse brain astrocytes. We also investigated expression of bestrophin-3 protein in a sample of human neonatal brain tissue. Bestrophin-3 protein expression was detected with immunohistochemical methods and western blot; mRNA expression and splicing were analyzed by RT-PCR. HI induced a brain tissue infarct and a pronounced increase in the endoplasmic reticulum-associated marker CHOP. Three days after HI a population of astrocytes co-expressed bestrophin-3 and nestin in a penumbra-like area of the injured hemisphere. However, total levels of Bestrophin-3 protein in mouse cortex were reduced after injury. Mouse astrocytes in primary culture also expressed bestrophin-3 protein, the amount of which was reduced by endoplasmic reticulum stress. Bestrophin-3 protein was detected in astrocytes in the hippocampal region of the human neonatal brain which had patchy white matter gliosis and neuronal loss in the Sommer’s sector of the Ammon’s horn (CA1). Analysis of bestrophin-3 mRNA in mouse brain with and without injury showed the presence of two truncated spliced variants, but no full-length mRNA. Total amount of bestrophin-3 mRNA increased after HI, but showed only minor injury-related change. However, the splice variants of bestrophin-3 mRNA were differentially regulated after HI depending on the presence of tissue injury. Our results show that bestrophin-3 is expressed in neonatal mouse brain after injury and in the human neonatal brain with pathology. In mouse brain bestrophin-3 protein is upregulated in a specific astrocyte population after injury and is co-expressed with nestin. Splice variants of bestrophin-3 mRNA respond differently to HI, which might indicate their different roles in tissue injury.

Severe-combined immunodeficient rats can be used to generate a model of perinatal hypoxic-ischemic brain injury to facilitate studies of engrafted human neural stem cells

Cerebral palsy (CP) encompasses a group of non-progressive brain disorders that are often acquired through perinatal hypoxic-ischemic (HI) brain injury. Injury leads to a cascade of cell death events, resulting in lifetime motor and cognitive deficits. There are currently no treatments that can repair the resulting brain damage and improve functional outcomes. To date, preclinical research using neural precursor cell (NPC) transplantation as a therapy for HI brain injury has shown promise. To translate this treatment to the clinic, it is essential that human-derived NPCs also be tested in animal models, however, a major limitation is the high risk of xenograft rejection. A solution is to transplant the cells into immune-deficient rodents, but there are currently no models of HI brain injury established in such a cohort of animals. Here, we demonstrate that a model of HI brain injury can be generated in immune-deficient Prkdc knockout (KO) rats. Long-term deficits in sensorimotor function were similar between KO and wildtype (WT) rats. Interestingly, some aspects of the injury were more severe in KO rats. Additionally, human induced pluripotent stem cell derived (hiPSC)-NPCs had higher survival at 10 weeks post-transplant in KO rats when compared to their WT counterparts. This work establishes a reliable model of neonatal HI brain injury in Prkdc KO rats that will allow for future transplantation, survival, and long-term evaluation of the safety and efficacy of hiPSC-NPCs for neonatal brain damage. This model will enable critical preclinical translational research using human NPCs.

Peripheral T Cell Depletion by FTY720 Exacerbates Hypoxic-Ischemic Brain Injury in Neonatal Mice

Hypoxic-ischemic injury to the developing brain remains a major cause of significant long-term morbidity and mortality. Emerging evidence from neonatal brain injury models suggests a detrimental role for peripheral lymphocytes. The immunomodulatory substance FTY720, a sphingosine-1-phosphate receptor agonist, was shown to reduce adult ischemia-induced neurodegeneration through its lymphopenic mode of action. In the present study, we hypothesized that FTY720 promotes neuroprotection by reducing peripheral lymphocytes and their infiltration into the injured neonatal brain. Term-born equivalent postnatal day 9 C57BL/6 mice were exposed to hypoxia ischemia (HI) followed by a single injection of 1 mg/kg FTY720 or vehicle (0.9% sodium chloride). Brain injury, microglia, and endothelial activation were assessed 7 days post HI using histology and western blot. Peripheral and cerebral leukocyte subsets were analyzed by multichannel flow cytometry. Whether FTY720s’ effects could be attributed to its lymphopenic mode of action was determined in T cell-depleted mice. In contrast to our hypothesis, FTY720 exacerbated HI-induced neuropathology including loss of gray and white matter structures. While microglia and endothelial activation remained unchanged, FTY720 induced a strong and sustained depletion of peripheral T cells resulting in significantly reduced cerebral infiltration of CD4 T cells. CD4 T cell subset analysis revealed that circulating regulatory and effector T cells counts were similarly decreased after FTY720 treatment. However, since neonatal HI per se induces a selective infiltration of Foxp3 positive regulatory T cells compared to Foxp3 negative effector T cells effects of FTY720 on cerebral regulatory T cell infiltration were more pronounced than on effector T cells. Reductions in T lymphocytes, and particularly regulatory T cells coincided with an increased infiltration of innate immune cells, mainly neutrophils and inflammatory macrophages. Importantly anti-CD3-mediated T cell depletion resulted in a similar exacerbation of brain injury, which was not further enhanced by an additional FTY720 treatment. In summary, peripheral T cell depletion by FTY720 resulted in increased infiltration of innate immune cells concomitant to reduced T cell infiltration and exacerbation HI-induced brain injury. This study indicates that neonatal T cells may promote endogenous neuroprotection in the term-born equivalent hypoxic-ischemic brain potentially providing new opportunities for therapeutic intervention.

The duration of hypothermia affects short-term neuroprotection in a mouse model of neonatal hypoxic ischaemic injury

Neonatal hypoxic-ischaemic encephalopathy (HIE) is major cause of neonatal mortality and morbidity. Therapeutic hypothermia is standard clinical care for moderate hypoxic-ischaemic (HI) brain injury, however it reduces the risk of death and disability only by 11% and 40% of the treated infants still develop disabilities. Thus it is necessary to develop supplementary therapies to complement therapeutic hypothermia in the treatment of neonatal HIE. The modified Rice-Vannucci model of HI in the neonatal mouse is well developed and widely applied with different periods of hypothermia used as neuroprotective strategy in combination with other agents. However, different studies use different periods, time of initiation and duration of hypothermia following HI, with subsequent varying degrees of neuroprotection. So far most rodent data is obtained using exposure to 5-6h of therapeutic hypothermia. Our aim was to compare the effect of exposure to three different short periods of hypothermia (1h, 1.5h and 2h) following HI insult in the postnatal day 7 C57/Bl6 mouse, and to determine the shortest period providing neuroprotection. Our data suggests that 1h and 1.5h of hypothermia delayed by 20min following a 60min exposure to 8%O2 do not prove neuroprotective. However, 2h of hypothermia significantly reduced tissue loss, TUNEL+ cell death and microglia and astroglia activation. We also observed improved functional outcome 7 days after HI. We suggest that the minimal period of cooling necessary to provide moderate short term neuroprotection and appropriate for the development and testing of combined treatment is 2h.

CK2 inhibition confers functional protection to young and aging axons against ischemia by differentially regulating the CDK5 and AKT signaling pathways

White matter (WM) is injured in most strokes, which contributes to functional deficits during recovery. Casein kinase 2 (CK2) is a protein kinase that is expressed in brain, including WM. To assess the impact of CK2 inhibition on axon recovery following oxygen glucose deprivation (OGD), mouse optic nerves (MONs), which are pure WM tracts, were subjected to OGD with or without the selective CK2 inhibitor CX-4945. CX-4945 application preserved axon function during OGD and promoted axon function recovery when applied before or after OGD. This protective effect of CK2 inhibition correlated with preservation of oligodendrocytes and conservation of axon structure and axonal mitochondria. To investigate the pertinent downstream signaling pathways, siRNA targeting the CK2α subunit identified CDK5 and AKT as downstream molecules. Consequently, MK-2206 and roscovitine, which are selective AKT and CDK5 inhibitors, respectively, protected young and aging WM function only when applied before OGD. However, a novel pan-AKT allosteric inhibitor, ARQ-092, which targets both the inactive and active conformations of AKT, conferred protection to young and aging axons when applied before or after OGD. These results suggest that AKT and CDK5 signaling contribute to the WM functional protection conferred by CK2 inhibition during ischemia, while inhibition of activated AKT signaling plays the primary role in post-ischemic protection conferred by CK2 inhibition in WM independent of age. CK2 inhibitors are currently being used in clinical trials for cancer patients; therefore, our results will provide rationale for repurposing these drugs as therapeutic options for stroke patients by adding novel targets.

Attenuation of Retinal Vascular Development in Neonatal Mice Subjected to Hypoxic-Ischemic Encephalopathy

A significant proportion of children that survive hypoxic-ischemic encephalopathy (HIE) develop visual impairment. These visual deficits are generally attributed to injuries that occur in the primary visual cortex and other visual processing systems. Recent studies suggested that neuronal damage might also occur in the retina. An important structure affecting the viability of retinal neurons is the vasculature. However, the effects of HIE on the retinal neurovasculature have not been systemically evaluated. Here we investigated whether exposure of postnatal day 9 (P9) neonatal mice to HIE is sufficient to induce neurovascular damage in the retina. We demonstrate that the blood vessels on the surface of the retina, from mice subjected to HIE, were abnormally enlarged with signs of degeneration. The intermediate and deep vascular layers in these retinas failed to form normally, particularly in the periphery. All the vascular damages observed here were irreversible in nature up to 100 days post HIE. We also observed loss of retinal neurons, together with changes in both astrocytes and Müller cells mainly in the inner retina at the periphery. Collectively, our findings suggest that HIE results in profound alterations in the retinal vasculature, indicating the importance of developing therapeutic strategies to protect neurovascular dysfunction not only in the brain but also in the retina for infants exposed to HIE.

Interaction between hypothermia and delayed mesenchymal stem cell therapy in neonatal hypoxic-ischemic brain injury

Acute hypothermia treatment (HT) is the only clinically established intervention following neonatal hypoxic-ischemic brain injury. However, almost half of all cooled infants still die or suffer from long-lasting neurological impairments. Regenerative therapies, such as mesenchymal stem cells (MSC) appear promising as adjuvant therapy. In the present study, we hypothesized that HT combined with delayed MSC therapy results in augmented protection, improving long-term neurological outcome. Postnatal day 9 (P9) C57BL/6 mice were exposed to hypoxia-ischemia followed by 4 h HT. Murine bone marrow-derived MSC (1 × 10⁶ cells/animal) were administered intranasally at P12. Cytokine and growth factor levels were assessed by ELISA and Luminex® multiplex assay 24 h following MSC delivery. One week after HI, tissue injury and neuroinflammatory responses were determined by immunohistochemistry and western blot. Long-term motor-cognitive outcome was assessed 5 weeks post injury. MSC responses to the brains' environment were evaluated by gene expression analysis in MSC, co-cultured with brain homogenates isolated at P12. Both, MSC and HT improved motor deficits, while cognitive function could only be restored by MSC. Compared to each single therapy, combined treatment led to increased long-lasting motor-cognitive deficits and exacerbated brain injury, accompanied by enhanced endothelial activation and peripheral immune cell infiltration. MSC co-cultured with brain extracts of HT-treated animals revealed increased pro-inflammatory cytokine and decreased growth factor expression. In vivo protein analysis showed higher pro-inflammatory cytokine levels after combined treatment compared to single therapy. Furthermore, HI-induced increase in growth factors was normalized to control levels by HT and MSC single therapy, while the combination induced a further decline below control levels. Our results suggest that alteration of the brains' microenvironment by acute HT modulates MSC function resulting in a pro-inflammatory environment combined with alteration of the homeostatic growth factor milieu in the neonatal hypoxic-ischemic brain. This study delineates potential unexpected side effects of cell-based therapies as add-on therapy for acute hypothermia treatment.

TWEAK Receptor Deficiency Has Opposite Effects on Female and Male Mice Subjected to Neonatal Hypoxia-Ischemia

Tumor necrosis factor (TNF)-like weak inducer of apoptosis (TWEAK) is a multifunctional cytokine member of the TNF family. TWEAK binds to its only known receptor, Fn14, enabling it to activate downstream signaling processes in response to tissue injury. The aim of this study was to investigate the role of TWEAK signaling in neonatal hypoxia–ischemia (HI). We found that after neonatal HI, both TWEAK and Fn14 expression were increased to a greater extent in male compared with female mice. To assess the role of TWEAK signaling after HI, the size of the injury was measured in neonatal mice genetically deficient in Fn14 and compared with their wild-type and heterozygote littermates. A significant sex difference in the Fn14 knockout (KO) animals was observed. Fn14 gene KO was beneficial in females; conversely, reducing Fn14 expression exacerbated the brain injury in male mice. Our findings indicate that the TWEAK/Fn14 pathway is critical for development of hypoxic–ischemic brain injury in immature animals. However, as the responses are different in males and females, clinical implementation depends on development of sex-specific therapies.

Cerebral Artery Diameter in Inbred Mice Varies as a Function of Strain

Many strains of mice are utilized in mouse models of cerebrovascular diseases. Variations in vascular anatomy between these strains has been documented and may influence the phenotype in stroke models. To address inter-strain variations in the circle of Willis anatomy, the diameters of internal carotid, posterior communicating, anterior cerebral, and middle cerebral arteries in 144 mice from 32 inbred strains were measured. Arterial diameters were analyzed as a function of animal weight, age, and strain. Variations in the structure of the circle of Willis across strains were observed and noted. While right-sided anterior cerebral arteries were significantly greater in diameter than their left-sided counterparts across most strains, variations in arterial diameter are strain specific. Adult mouse weight was not found to be associated with arterial diameter across strains, suggesting that cerebral artery size is associated with strain independently of weight. This study demonstrates strain dependent variations in the murine circle of Willis, which should be taken into consideration when studying mouse models of cerebrovascular diseases.