Characteristic neuropathology of leukomalacia in extremely low birth weight infants

Consequences of oxygen deprivation on myelination and sex-dependent alterations

Oxygen deprivation is one of the main causes of morbidity and mortality in newborns, occurring with a higher prevalence in preterm infants, reaching 20 % to 50 % mortality in newborns in the perinatal period. When they survive, 25 % exhibit neuropsychological pathologies, such as learning difficulties, epilepsy, and cerebral palsy. White matter injury is one of the main features found in oxygen deprivation injury, which can lead to long-term functional impairments, including cognitive delay and motor deficits. The myelin sheath accounts for much of the white matter in the brain by surrounding axons and enabling the efficient conduction of action potentials. Mature oligodendrocytes, which synthesize and maintain myelination, also comprise a significant proportion of the brain's white matter. In recent years, oligodendrocytes and the myelination process have become potential therapeutic targets to minimize the effects of oxygen deprivation on the central nervous system. Moreover, evidence indicate that neuroinflammation and apoptotic pathways activated during oxygen deprivation may be influenced by sexual dimorphism. To summarize the most recent research about the impact of sexual dimorphism on the neuroinflammatory state and white matter injury after oxygen deprivation, this review presents an overview of the oligodendrocyte lineage development and myelination, the impact of oxygen deprivation and neuroinflammation on oligodendrocytes in neurodevelopmental disorders, and recent reports about sexual dimorphism regarding the neuroinflammation and white matter injury after neonatal oxygen deprivation.

Human neocortical development as a basis to understand mechanisms underlying neurodevelopmental disabilities in extremely preterm infants

AIM

Recent advances in perinatal and neonatal medicine have resulted in marked improvements in the survival rates of extremely preterm infants (born before 28 gestational weeks) around the world, and Japan is among the countries with the highest reported survival rates of extremely preterm infants. However, it remains a major concern that many survivors develop neurodevelopmental disabilities, including cognitive dysfunctions and neurodevelopmental disorders later in life. In order to understand the pathophysiological mechanisms underlying the neurodevelopmental disabilities observed in the survivors of extremely preterm births, we reviewed recently reported findings about the development of the human neocortex.

METHODS

First, we have summarized the current knowledge about the development of the neocortex, including recently reported human- and/or primate-specific developmental events. Next, we discussed the possible causal mechanisms underlying the development of neurodevelopmental disabilities in extremely preterm infants.

RESULTS

Around the birth of extremely preterm infants, neurogenesis and succeeding neuronal migrations are ongoing in the neocortex of human brain. Expansion and maturation of the subplate, which is thought to reflect the axonal wiring in the neocortex, is also prominent at this time.

CONCLUSION

Brain injuries that occur around the birth of extremely preterm infants are presumed to affect the dynamic developmental events in the neocortex, such as neurogenesis, neuronal migrations and maturation of the subplate, which could underlie the neurodevelopmental disabilities that often develop subsequently in extremely preterm infants. These possibilities should be borne in mind while considering maternal and neonatal care to further improve the long-term outcomes of extremely preterm infants.

Early Prediction of Periventricular Leukomalacia Using Quantitative Texture Analysis of Serial Cranial Ultrasound Scans in Very Preterm Infants

We compared texture parameters of serial cranial ultrasound (cUS) images of periventricular leukomalacia (PVL) and normal periventricular echogenicity (PVE) in very preterm infants and evaluated the early predictive values of texture analysis (TA) for PVL. Ten individuals with PVL and 10 control individuals with PVE assessed with an initial cUS within 1 wk of birth and follow-up cUS at 2-3 and 4-6 wk of life were included. TA was performed on the region of interest of PVE at the parieto-occipital area on serial cUS. Opposite changes in variance were obtained between the first two cUS sessions in both groups (p = 0.017 in PVL and p = 0.005 in PVE). The variance-to-mean ratio (VMR) between the second and first cUS sessions differed (p = 0.016) and reliably stratified the groups (area under the receiver operating characteristic curve: 0.820, 95% confidence interval: 0.587-1.000, sensitivity: 100%, specificity: 60%). TA of serial cUS helps to predict PVL within 3 wk of life.

Dysmaturation of Premature Brain: Importance, Cellular Mechanisms, and Potential Interventions

Prematurity, especially preterm birth (less than 32 weeks' gestation), is common and associated with high rates of both survival and neurodevelopmental disability, especially apparent in cognitive spheres. The neuropathological substrate of this disability is now recognized to be related to a variety of dysmaturational disturbances of the brain. These disturbances follow initial brain injury, particularly cerebral white matter injury, and involve many of the extraordinary array of developmental events active in cerebral white and gray matter structures during the premature period. This review delineates these developmental events and the dysmaturational disturbances that occur in premature infants. The cellular mechanisms involved in the genesis of the dysmaturation are emphasized, with particular focus on the preoligodendrocyte. A central role for the diffusely distributed activated microglia and reactive astrocytes in the dysmaturation is now apparent. As these dysmaturational cellular mechanisms appear to occur over a relatively long time window, interventions to prevent or ameliorate the dysmaturation, that is, neurorestorative interventions, seem possible. Such interventions include pharmacologic agents, especially erythropoietin, and particular attention has also been paid to such nutritional factors as quality and source of milk, breastfeeding, polyunsaturated fatty acids, iron, and zinc. Recent studies also suggest a potent role for interventions directed at various experiential factors in the neonatal period and infancy, i.e., provision of optimal auditory and visual exposures, minimization of pain and stress, and a variety of other means of environmental behavioral enrichment, in enhancing brain development.

White matter injury in the preterm infant: pathology and mechanisms

The human preterm brain is particularly susceptible to cerebral white matter injury (WMI) that disrupts the normal progression of developmental myelination. Advances in the care of preterm infants have resulted in a sustained reduction in the severity of WMI that has shifted from more severe focal necrotic lesions to milder diffuse WMI. Nevertheless, WMI remains a global health problem and the most common cause of chronic neurological morbidity from cerebral palsy and diverse neurobehavioral disabilities. Diffuse WMI involves maturation-dependent vulnerability of the oligodendrocyte (OL) lineage with selective degeneration of late oligodendrocyte progenitors (preOLs) triggered by oxidative stress and other insults. The magnitude and distribution of diffuse WMI are related to both the timing of appearance and regional distribution of susceptible preOLs. Diffuse WMI disrupts the normal progression of OL lineage maturation and myelination through aberrant mechanisms of regeneration and repair. PreOL degeneration is accompanied by early robust proliferation of OL progenitors that regenerate and augment the preOL pool available to generate myelinating OLs. However, newly generated preOLs fail to differentiate and initiate myelination along their normal developmental trajectory despite the presence of numerous intact-appearing axons. Disrupted preOL maturation is accompanied by diffuse gliosis and disturbances in the composition of the extracellular matrix and is mediated in part by inhibitory factors derived from reactive astrocytes. Signaling pathways implicated in disrupted myelination include those mediated by Notch, WNT-beta catenin, and hyaluronan. Hence, there exists a potentially broad but still poorly defined developmental window for interventions to promote white matter repair and myelination and potentially reverses the widespread disturbances in cerebral gray matter growth that accompanies WMI.

Potential New Non-Invasive Therapy Using Artificial Oxygen Carriers for Pre-Eclampsia

The molecular mechanisms of pre-eclampsia are being increasingly clarified in animals and humans. With the uncovering of these mechanisms, preventive therapy strategies using chronic infusion of adrenomedullin, vascular endothelial growth factor-121 (VEGF-121), losartan, and sildenafil have been proposed to block narrow spiral artery formation in the placenta by suppressing related possible factors for pre-eclampsia. However, although such preventive treatments have been partly successful, they have failed in ameliorating fetal growth restriction and carry the risk of possible side-effects of drugs on pregnant mothers. In this study, we attempted to develop a new symptomatic treatment for pre-eclampsia by directly rescuing placental ischemia with artificial oxygen carriers (hemoglobin vesicles: HbV) since previous data indicate that placental ischemia/hypoxia may alone be sufficient to lead to pre-eclampsia through up-regulation of sFlt-1, one of the main candidate molecules for the cause of pre-eclampsia. Using a rat model, the present study demonstrated that a simple treatment using hemoglobin vesicles for placental ischemia rescues placental and fetal hypoxia, leading to appropriate fetal growth. The present study is the first to demonstrate hemoglobin vesicles successfully decreasing maternal plasma levels of sFlt-1 and ameliorating fetal growth restriction in the pre-eclampsia rat model (p < 0.05, one-way ANOVA). In future, chronic infusion of hemoglobin vesicles could be a potential effective and noninvasive therapy for delaying or even alleviating the need for Caesarean sections in pre-eclampsia.

Association of impaired neuronal migration with cognitive deficits in extremely preterm infants

Many extremely preterm infants (born before 28 gestational weeks [GWs]) develop cognitive impairment in later life, although the underlying pathogenesis is not yet completely understood. Our examinations of the developing human neocortex confirmed that neuronal migration continues beyond 23 GWs, the gestational week at which extremely preterm infants have live births. We observed larger numbers of ectopic neurons in the white matter of the neocortex in human extremely preterm infants with brain injury and hypothesized that altered neuronal migration may be associated with cognitive impairment in later life. To confirm whether preterm brain injury affects neuronal migration, we produced brain damage in mouse embryos by occluding the maternal uterine arteries. The mice showed delayed neuronal migration, ectopic neurons in the white matter, altered neuronal alignment, and abnormal corticocortical axonal wiring. Similar to human extremely preterm infants with brain injury, the surviving mice exhibited cognitive deficits. Activation of the affected medial prefrontal cortices of the surviving mice improved working memory deficits, indicating that decreased neuronal activity caused the cognitive deficits. These findings suggest that altered neuronal migration altered by brain injury might contribute to the subsequent development of cognitive impairment in extremely preterm infants.

Cerebral Palsy-Trends in Epidemiology and Recent Development in Prenatal Mechanisms of Disease, Treatment, and Prevention

Cerebral palsy (CP) is the most common motor disability in childhood. This syndrome is the manifestation of intrauterine pathologies, intrapartum complications, and the postnatal sequel, especially among preterm neonates. A double hit model theory is proposed suggesting that an intrauterine condition along with intrapartum or postnatal insult lead to the development of CP. Recent reports demonstrated that treatment during the process of preterm birth such as magnesium sulfate and postnatal modalities such as cooling may prevent or reduce the prevalence of this syndrome. Moreover, animal models demonstrated that postnatal treatment with anti-inflammatory drugs coupled with nanoparticles may affect the course of the disease in pups with neuroinflammation. This review will describe the changes in the epidemiology of this disease, the underlying prenatal mechanisms, and possible treatments that may reduce the prevalence of CP and alter the course of the disease.

Artificial oxygen carriers rescue placental hypoxia and improve fetal development in the rat pre-eclampsia model

Pre-eclampsia affects approximately 5% of all pregnant women and remains a major cause of maternal and fetal morbidity and mortality. The hypertension associated with pre-eclampsia develops during pregnancy and remits after delivery, suggesting that the placenta is the most likely origin of this disease. The pathophysiology involves insufficient trophoblast invasion, resulting in incomplete narrow placental spiral artery remodeling. Placental insufficiency, which limits the maternal-fetal exchange of gas and nutrients, leads to fetal intrauterine growth restriction. In this study, in our attempt to develop a new therapy for pre-eclampsia, we directly rescued placental and fetal hypoxia with nano-scale size artificial oxygen carriers (hemoglobin vesicles). The present study is the first to demonstrate that artificial oxygen carriers successfully treat placental hypoxia, decrease maternal plasma levels of anti-angiogenic proteins and ameliorate fetal growth restriction in the pre-eclampsia rat model.

Considerations for the Optimization of Induced White Matter Injury Preclinical Models

White matter (WM) injury in relation to acute neurologic conditions, especially stroke, has remained obscure until recently. Current advances in imaging technologies in the field of stroke have confirmed that WM injury plays an important role in the prognosis of stroke and suggest that WM protection is essential for functional recovery and post-stroke rehabilitation. However, due to the lack of a reproducible animal model of WM injury, the pathophysiology and mechanisms of this injury are not well studied. Moreover, producing selective WM injury in animals, especially in rodents, has proven to be challenging. Problems associated with inducing selective WM ischemic injury in the rodent derive from differences in the architecture of the brain, most particularly, the ratio of WM to gray matter in rodents compared to humans, the agents used to induce the injury, and the location of the injury. Aging, gender differences, and comorbidities further add to this complexity. This review provides a brief account of the techniques commonly used to induce general WM injury in animal models (stroke and non-stroke related) and highlights relevance, optimization issues, and translational potentials associated with this particular form of injury.

Stromal Cell-Derived Factor-1α Plays a Crucial Role Based on Neuroprotective Role in Neonatal Brain Injury in Rats

Owing to progress in perinatal medicine, the survival of preterm newborns has markedly increased. However, the incidence of cerebral palsy has risen in association with increased preterm birth. Cerebral palsy is largely caused by cerebral hypoxic ischemia (HI), for which there are no effective medical treatments. We evaluated the effects of stromal cell-derived factor-1α (SDF-1α) on neonatal brain damage in rats. Left common carotid (LCC) arteries of seven-day-old Wistar rat pups were ligated, and animals were exposed to hypoxic gas to cause cerebral HI. Behavioral tests revealed that the memory and spatial perception abilities were disturbed in HI animals, and that SDF-1α treatment improved these cognitive functions. Motor coordination was also impaired after HI but was unimproved by SDF-1α treatment. SDF-1α reduced intracranial inflammation and induced cerebral remyelination, as indicated by the immunohistochemistry results. These data suggest that SDF-1α specifically influences spatial perception abilities in neonatal HI encephalopathy.

Neutrophil and monocyte toll-like receptor 4, CD11b and reactive oxygen intermediates, and neuroimaging outcomes in preterm infants

BACKGROUND

Activated leukocytes and infection are implicated in neonatal brain injury. Leukocyte surface receptors are increased in stroke models and may be targets for future adjunctive therapies.

METHODS

Serial blood samples were analyzed from preterm infants (n = 51; <32 wk gestation) on days 0, 1, 2, and 7 of life. Monocyte and neutrophil activation were evaluated via flow cytometry at baseline and following endotoxin stimulation ex vivo by measuring CD11b (activation), toll-like receptor 4 (TLR-4; endotoxin recognition) expression, and intracellular reactive oxygen intermediate (ROI) production (function).

RESULTS

Control preterm infants with normal neuroimaging had elevated baseline CD11b and TLR-4 expression and ROI production compared with adults as well as a robust immune response following endotoxin stimulation. Preterm infants with abnormal neuroimaging had increased neutrophil TLR-4 and ROI compared with all controls.

CONCLUSION

Preterm infants have a robust immune response compared with adults. Increased TLR-4 expression in preterm infants with abnormal neuroimaging is similar to findings in adult stroke. In addition, ROI production may cause tissue injury. The modulation of these responses may be beneficial in preterm inflammatory disorders.

The role of inflammation in perinatal brain injury

Inflammation is increasingly recognized as being a critical contributor to both normal development and injury outcome in the immature brain. The focus of this Review is to highlight important differences in innate and adaptive immunity in immature versus adult brain, which support the notion that the consequences of inflammation will be entirely different depending on context and stage of CNS development. Perinatal brain injury can result from neonatal encephalopathy and perinatal arterial ischaemic stroke, usually at term, but also in preterm infants. Inflammation occurs before, during and after brain injury at term, and modulates vulnerability to and development of brain injury. Preterm birth, on the other hand, is often a result of exposure to inflammation at a very early developmental phase, which affects the brain not only during fetal life, but also over a protracted period of postnatal life in a neonatal intensive care setting, influencing critical phases of myelination and cortical plasticity. Neuroinflammation during the perinatal period can increase the risk of neurological and neuropsychiatric disease throughout childhood and adulthood, and is, therefore, of concern to the broader group of physicians who care for these individuals.

Brain injury in premature neonates: A primary cerebral dysmaturation disorder?

With advances in neonatal care, preterm neonates are surviving with an evolving constellation of motor and cognitive disabilities that appear to be related to widespread cellular maturational disturbances that target cerebral gray and white matter. Whereas preterm infants were previously at high risk for destructive brain lesions that resulted in cystic white matter injury and secondary cortical and subcortical gray matter degeneration, contemporary cohorts of preterm survivors commonly display less severe injury that does not appear to involve pronounced glial or neuronal loss. Nevertheless, these milder forms of injury are also associated with reduced cerebral growth. Recent human and experimental studies support that impaired cerebral growth is related to disparate responses in gray and white matter. Myelination disturbances in cerebral white matter are related to aberrant regeneration and repair responses to acute death of premyelinating late oligodendrocyte progenitors (preOLs). In response to preOL death, early oligodendrocyte progenitors rapidly proliferate and differentiate, but the regenerated preOLs fail to normally mature to myelinating cells required for white matter growth. Although immature neurons appear to be more resistant to cell death from hypoxia-ischemia than glia, they display widespread disturbances in maturation of their dendritic arbors, which further contribute to impaired cerebral growth. These complex and disparate responses of neurons and preOLs thus result in large numbers of cells that fail to fully mature during a critical window in development of neural circuitry. These recently recognized forms of cerebral gray and white matter dysmaturation raise new diagnostic challenges and suggest new therapeutic directions centered on reversal of the processes that promote dysmaturation.

Pathophysiology of glia in perinatal white matter injury

Injury to the preterm brain has a particular predilection for cerebral white matter. White matter injury (WMI) is the most common cause of brain injury in preterm infants and a major cause of chronic neurological morbidity including cerebral palsy. Factors that predispose to WMI include cerebral oxygenation disturbances and maternal-fetal infection. During the acute phase of WMI, pronounced oxidative damage occurs that targets late oligodendrocyte progenitors (pre-OLs). The developmental predilection for WMI to occur during prematurity appears to be related to both the timing of appearance and regional distribution of susceptible pre-OLs that are vulnerable to a variety of chemical mediators including reactive oxygen species, glutamate, cytokines, and adenosine. During the chronic phase of WMI, the white matter displays abberant regeneration and repair responses. Early OL progenitors respond to WMI with a rapid robust proliferative response that results in a several fold regeneration of pre-OLs that fail to terminally differentiate along their normal developmental time course. Pre-OL maturation arrest appears to be related in part to inhibitory factors that derive from reactive astrocytes in chronic lesions. Recent high field magnetic resonance imaging (MRI) data support that three distinct forms of chronic WMI exist, each of which displays unique MRI and histopathological features. These findings suggest the possibility that therapies directed at myelin regeneration and repair could be initiated early after WMI and monitored over time. These new mechanisms of acute and chronic WMI provide access to a variety of new strategies to prevent or promote repair of WMI in premature infants.

Effects of endurance exercise on expressions of glial fibrillary acidic protein and myelin basic protein in developing rats with maternal infection-induced cerebral palsy

Periventricular leukomalacia (PVL) is a common white matter lesion affecting the neonatal brain. PVL is closely associated with cerebral palsy (CP) and characterized by increase in the number of astrocytes, which can be detected by positivity for glial fibrillary acidic protein (GFAP). Change in myelin basic protein (MBP) is an early sign of white matter abnormality. Maternal or placental infection can damage the neonatal brain. In the present study, we investigated the effects of treadmill walking exercise on GFAP and MBP expressions in rats with maternal lipopolysaccharide (LPS)-induced PVL. Immunohistochemistry was performed for the detection of GFAP and MBP. The present results showed that intracervical maternal LPS injection during pregnancy increased GFAP expression in the striatum and decreased MBP expression in the corpus callosum of rats. The results also showed that treadmill walking exercise suppressed GFAP expression and enhanced MBP expression in the brains of rats with maternal LPS-induced PVL. The present study revealed that treadmill walking exercise is effective for the suppressing astrogliosis and hypomyelination associated with PVL. Here in this study, we showed that treadmill walking exercise may be effective therapeutic strategy for alleviating the detrimental effects of CP.

Astrocyte-derived proinflammatory cytokines induce hypomyelination in the periventricular white matter in the hypoxic neonatal brain

Hypoxic exposure in the perinatal period causes periventricular white matter damage (PWMD), a condition associated with myelination abnormalities. Under hypoxic conditions, glial cells were activated and released a large number of inflammatory mediators in the PWM in neonatal brain, which may result in oligodendrocyte (OL) loss and axonal injury. This study aims to determine if astrocytes are activated and generate proinflammatory cytokines that may be coupled with the oligodendroglial loss and hypomyelination observed in hypoxic PWMD. Twenty-four 1-day-old Wistar rats were exposed to hypoxia for 2 h. The rats were then allowed to recover under normoxic conditions for 7 or 28 days before being killed. Another group of 24 rats kept outside the chamber was used as age-matched controls. Upregulated expression of TNF-α and IL-1β was observed in astrocytes in the PWM of P7 hypoxic rats by double immunofluorescence, western blotting and real time RT-PCR. This was linked to apoptosis and enhanced expression of TNF-R₁ and IL-1R₁ in APC⁺ OLs. PLP expression was decreased significantly in the PWM of P28d hypoxic rats. The proportion of myelinated axons was markedly reduced by electron microscopy (EM) and the average g-ratios were higher in P28d hypoxic rats. Upregulated expression of TNF-α and IL-1β in primary cultured astrocytes as well as their corresponding receptors in primary culture APC⁺ oligodendrocytes were detected under hypoxic conditions. Our results suggest that following a hypoxic insult, astrocytes in the PWM of neonatal rats produce inflammatory cytokines such as TNF-α and IL-1β, which induce apoptosis of OLs via their corresponding receptors associated with them. This results in hypomyelination in the PWM of hypoxic rats.

STAT3-mediated astrogliosis protects myelin development in neonatal brain injury

OBJECTIVE

Pathological findings in neonatal brain injury associated with preterm birth include focal and/or diffuse white matter injury (WMI). Despite the heterogeneous nature of this condition, reactive astrogliosis and microgliosis are frequently observed. Thus, molecular mechanisms by which glia activation contribute to WMI were investigated.

METHODS

Postmortem brains of neonatal brain injury were investigated to identify molecular features of reactive astrocytes. The contribution of astrogliosis to WMI was further tested in a mouse model in genetically engineered mice.

RESULTS

Activated STAT3 signaling in reactive astrocytes was found to be a common feature in postmortem brains of neonatal brain injury. In a mouse model of neonatal WMI, conditional deletion of STAT3 in astrocytes resulted in exacerbated WMI, which was associated with delayed maturation of oligodendrocytes. Mechanistically, the delay occurred in association with overexpression of transforming growth factor (TGF)β-1 in microglia, which in healthy controls decreased with myelin maturation in an age-dependent manner. TGFβ-1 directly and dose-dependently inhibited the maturation of purified oligodendrocyte progenitors, and pharmacological inhibition of TGFβ-1 signaling in vivo reversed the delay in myelin development. Factors secreted from STAT3-deficient astrocytes promoted elevated TGFβ-1 production in cultured microglia compared to wild-type astrocytes.

INTERPRETATION

These results suggest that myelin development is regulated by a mechanism involving crosstalk between microglia and oligodendrocyte progenitors. Reactive astrocytes may modify this signaling in a STAT3-dependent manner, preventing the pathological expression of TGFβ-1 in microglia and the impairment of oligodendrocyte maturation.

Intracerebral lipopolysaccharide induces neuroinflammatory change and augmented brain injury in growth-restricted neonatal rats

INTRODUCTION

Intrauterine growth restriction (IUGR) alters fetal development and is associated with neurodevelopmental abnormalities. We hypothesized that growth restriction from reduced intrauterine perfusion would predispose neonatal rats to subsequent inflammatory brain injury.

METHODS

In this study, IUGR was achieved by induced placental insufficiency in pregnant rats at 14 days of gestation. IUGR offspring and sham-operated control pups were subsequently injected with intracerebral lipopolysaccharide (LPS) as a model of periventricular leukomalacia (PVL).

RESULTS

LPS similarly elevates proinflammatory cytokines in the brains of both IUGR and control rat pups. However, the chemokines cytokine-induced neutrophil chemoattractant-1 (CINC-1) and macrophage chemoattractant protein-1 (MCP-1), as well as microglia activation, were significantly higher in LPS-treated IUGR rat pups as compared with LPS-treated controls. In addition to the unique brain inflammatory response, IUGR rat pups demonstrated increased brain damage with an increased number of apoptotic cells, larger lateral ventricular size, and more severe impairment of myelination.

DISCUSSION

This study provides evidence that placental insufficiency may sensitize the innate immune system in the immature brain and reveals a possible link between brain inflammation and injury.

Microglial reaction in axonal crossroads is a hallmark of noncystic periventricular white matter injury in very preterm infants

Disabilities after brain injury in very preterm infants have mainly been attributed to noncystic periventricular white matter injury (PWMI). We analyzed spatiotemporal patterns of PWMI in the brains of 18 very preterm infants (25-29 postconceptional weeks [pcw]), 7 preterm infants (30-34 pcw), and 10 preterm controls without PWMI. In very preterm infants, we examined PWMI in detail in 2 axonal crossroad areas in the frontal lobe: C1 (lateral to the lateral angle of the anterior horn of the lateral ventricle, at the exit of the internal capsule radiations) and C2 (above the corpus callosum and dorsal angle of the anterior horn). These brains had greater microglia-macrophage densities and activation but lesser astroglial reaction (glial fibrillary acidic protein and monocarboxylate transporter 1 expression) than in preterm cases with PWMI. In preterm infants, scattered necrotic foci were rimmed by axonal spheroids and ionized calcium binding adaptor molecule 1-positive macrophages. Diffuse lesions near these foci consisted primarily of hypertrophic and reactive astrocytes associated with fewer microglia. No differences in Olig2-positive preoligodendrocytes between noncystic PWMI and control cases were found. These data show that the growing axonal crossroad areas are highly vulnerable to PWMI in very preterm infants and highlight differences in glial activation patterns between very preterm and preterm infants.

Models of fetal brain injury, intrauterine inflammation, and preterm birth

Intrauterine infection and inflammation are known risk factors for brain damage in the neonate irrespective of the gestational age. Infection-induced maternal immune activation leads to a fetal inflammatory response mediated by cytokines that has been implicated in the development of not only periventricular leukomalacia and cerebral palsy but also a spectrum of neurodevelopmental disorders such as autism and schizophrenia (Behav Brain Res 2009; 204:313, Ann Neurol 2005; 57:67, Am J Obstet Gynecol 2000; 182:675). A common link among the neurobehavioral disorders associated with intrauterine inflammation appears to be the evidence for immune dysregulation in the developing brain (Behav Brain Res 2009; 204:313). The timing of the immune challenge with respect to the gestational age and neurologic development of the fetus may be crucial in the elicited response (J Neurosci 2006; 26:4752). Studies involving animal models of maternal inflammation serve a key role in elucidation of mechanisms involved in fetal brain injury associated with exposure to the maternal milieu. These animal models have been shown to result in fetal microglial activation, neurotoxicity as well motor deficits and behavioral abnormalities in the offspring (J Neurosci 2006; 26:4752, J Neurosci Res 2010; 88:172, Am J Obstet Gynecol 2009; 201:279, Am J Obstet Gynecol 2008; 199:651). A better understanding of the mechanisms of perinatal brain injury will allow discoveries of novel neuroprotective agents, better outcomes following preterm birth and stratification of fetuses and neonates for therapies in cases of preterm birth, preterm premature rupture of membranes, and chorioamnionitis.

Differential susceptibility to axonopathy in necrotic and non-necrotic perinatal white matter injury

BACKGROUND AND PURPOSE

White matter injury (WMI) is the leading cause of brain injury in preterm survivors and results in myelination failure. Although axonal degeneration occurs in necrotic lesions, the role of axonopathy in myelination failure remains controversial for diffuse non-necrotic WMI, which is currently the major form of WMI. We determined the burden of axonopathy in diffuse lesions.

METHODS

We analyzed WMI in a preterm fetal sheep model of global cerebral ischemia that replicates the relative burden of necrotic and non-necrotic human WMI. WMI was analyzed at 1 or 2 weeks after ischemia and identified by ex vivo high-field (11.7 Tesla) magnetic resonance imaging of fixed brain tissue. Axonal integrity was analyzed by immunohistochemical detection of axon injury markers and by transmission electron microscopy to quantify axon loss and degeneration in magnetic resonance imaging-defined lesions.

RESULTS

Axonal degeneration, defined by staining for neurofilament protein and β-amyloid precursor protein, was restricted to discrete necrotic foci with robust microglial activation. Unexpectedly, axonal degeneration was not visualized in the major form of WMI, which comprised large non-necrotic lesions with diffuse reactive astrogliosis. In these major lesions, quantitative electron microscopy studies confirmed no significant differences in the density of intact and degenerating axons or in the distribution of axon diameters relative to controls.

CONCLUSIONS

The mechanism of myelination failure differs significantly in perinatal WMI dependent on the burden of necrosis. Axonopathy is associated with focal necrotic injury but not with primary diffuse non-necrotic lesions, which supports that intact axons in the primary lesions are potential targets for myelination.

The yin and yang of microglia

Microglia, the resident immune cells of the mammalian central nervous system (CNS), play a pivotal role in both physiological and pathological conditions such as the restoration of CNS integrity and the progression of neurodegenerative disorders. Extensive data have been published that describe neuroinflammation by microglial activation to have detrimental consequences on the developing and mature brain. On the other hand, a properly directed and limited inflammatory response is known to be a natural healing process after an insult in several other tissues. Thus, it is not surprising that research results illustrating benefits of neuroinflammation have been emerging over the past decade. Inflammation-mediated benefits for CNS outcomes include mechanisms such as neuroprotection, mobilization of neural precursors for repair, remyelination and axonal regeneration. Here, we review data that highlight the dual aspects of microglia with a focus on the developing brain, i.e. as aggressors potentiating damage and as helpers in the recovery process following CNS damage.

Animal models of periventricular leukomalacia

Periventricular leukomalacia, specifically characterized as white matter injury, in neonates is strongly associated with the damage of pre-myelinating oligodendrocytes. Clinical data suggest that hypoxia-ischemia during delivery and intrauterine or neonatal infection-inflammation are important factors in the etiology of periventricular leukomalacia including cerebral palsy, a serious case exhibiting neurobehavioral deficits of periventricular leukomalacia. In order to explore the pathophysiological mechanisms of white matter injury and to better understand how infectious agents may affect the vulnerability of the immature brain to injury, novel animal models have been developed using hypoperfusion, microbes or bacterial products (lipopolysaccharide) and excitotoxins. Such efforts have developed rat models that produce predominantly white matter lesions by adopting combined hypoxia-ischemia technique on postnatal days 1-7, in which unilateral or bilateral carotid arteries of animals are occluded (ischemia) followed by 1-2 hour exposure to 6-8% oxygen environment (hypoxia). Furthermore, low doses of lipopolysaccharide that by themselves have no adverse-effects in 7-day-old rats, dramatically increase brain injury to hypoxic-ischemic challenge, implying that inflammation sensitizes the immature central nervous system. Therefore, among numerous models of periventricular leukomalacia, combination of hypoxia-ischemia-lipopolysaccharide might be one of the most-acceptable rodent models to induce extensive white matter injury and ensuing neurobehavioral deficits for the evaluation of candidate therapeutics.

Actualities on molecular pathogenesis and repairing processes of cerebral damage in perinatal hypoxic-ischemic encephalopathy

Hypoxic-ischemic encephalopathy (HIE) is the most important cause of cerebral damage and long-term neurological sequelae in the perinatal period both in term and preterm infant.

Hypoxic-ischemic (H-I) injuries develop in two phases: the ischemic phase, dominated by necrotic processes, and the reperfusion phase, dominated by apoptotic processes extending beyond ischemic areas. Due to selective ischemic vulnerability, cerebral damage affects gray matter in term newborns and white matter in preterm newborns with the typical neuropathological aspects of laminar cortical necrosis in the former and periventricular leukomalacia in the latter.

This article summarises the principal physiopathological and biochemical processes leading to necrosis and/or apoptosis of neuronal and glial cells and reports recent insights into some endogenous and exogenous cellular and molecular mechanisms aimed at repairing H-I cerebral damage.

In vivo MRI analysis of an inflammatory injury in the developing brain

Cerebral periventricular white matter injury stands as a leading cause of cognitive, behavioral and motor impairment in preterm infants. There is epidemiological and histopathological evidence demonstrating the role of prenatal or neonatal inflammation in brain injury in preterm infants. In order to define the effect of an inflammatory insult in the developing brain on magnetic resonance (MR) imaging, we obtained high resolution conventional and diffusion MR images of the brain of rat pups after an inflammatory injury. Rat pups were subjected on postnatal day 5 (P5) to a stereotaxic injection of lipopolysaccharide in the corpus callosum and then imaged at 11.7 T on days 0, 2 and 4 following the injury. They were subsequently sacrificed for immunohistochemistry. Diffusion tensor imaging (DTI) acquired at high spatial resolution showed an initial reduction of the apparent diffusion coefficient (ADC) in the white matter. This was followed by an increase in ADC value and in T2 relaxation time constant in the white matter, with an associated increase of radial diffusivity of the corpus callosum, and a 10-fold increase in ventricular size. On histology, these MR changes corresponded to widespread astrogliosis, and decreased proportion of the section areas containing cresyl violet positive stain. The increase in radial diffusivity, typically attributed to myelin loss, occurred in this case despite the absence of myelin at this developmental stage.

A history of our understanding of cerebral vascular development and pathogenesis of perinatal brain damage over the past 30 years

This article reviews our studies focusing on cerebral vascular development, the pathogenesis of subependymal/intraventricular hemorrhage (SEH/IVH), periventricular leukomalacia (PVL), and pontosubicular neuron necrosis (PSN). Their pathogenesis consists of predisposing developmental and causal factors. SEH/IVH may be caused by reperfusion or overperfusion following ischemia in the subependymal germinal matrix with characteristic vasculature. The cause of PVL is multifactorial (ie, ischemia and inflammation), predisposed by the maturational status of the vasculature and oligodendroglia in the white matter. Focal PVL is ischemic necrosis, and diffuse PVL or white matter injury may include cytotoxic damage. PSN has an apoptotic character, and may be induced by ischemic and oxidative stress on specific immature neurons. Further studies on preventive and therapeutic measures are necessary in clinical, pathologic, and experimental fields. The monitoring and control methods of brain hemodynamics and cellular stability should be more developed to prevent brain damages.

The encephalopathy of prematurity--brain injury and impaired brain development inextricably intertwined

The field of neonatal neurology, and specifically its focus on the premature infant, had its inception in neuropathologic studies. Since then, the development of advanced imaging techniques has guided our developing understanding of the etiology and nature of neonatal brain injury. This review promotes the concept that neonatal brain injury has serious and diverse effects on subsequent brain development, and that these effects likely are more important than simple tissue loss in determining neurologic outcome. Brain injury in the premature infant is best illustrative of this concept. This "encephalopathy of prematurity" is reviewed in the context of the remarkable array of developmental events actively proceeding during the last 16-20 weeks of human gestation. Recent insights into the brain abnormalities in survivors of preterm birth obtained by both advanced magnetic resonance imaging and neuropathologic techniques suggest that this encephalopathy is a complex amalgam of destructive and developmental disturbances. The interrelations between destructive and developmental mechanisms in the genesis of the encephalopathy are emphasized. In the future, advances in neonatal neurology will likely reiterate the dependence of this field on neuropathologic studies, including new cellular and molecular approaches in developmental neurobiology.

Interleukin-1beta-induced brain injury in the neonatal rat can be ameliorated by alpha-phenyl-n-tert-butyl-nitrone

To examine the possible role of inflammatory cytokines in mediating perinatal brain injury, we investigated effects of intracerebral injection of interleukin-1beta (IL-1beta) on brain injury in the neonatal rat and the mechanisms involved. Intracerebral administration of IL-1beta (1 microg/kg) resulted in acute brain injury, as indicated by enlargement of ventricles bilaterally, apoptotic death of oligodendrocytes (OLs) and loss of OL immunoreactivity in the neonatal rat brain. IL-1beta also induced axonal and neuronal injury in the cerebral cortex as indicated by elevated expression of beta-amyloid precursor protein, short beaded axons and dendrites, and loss of tyrosine hydroxylase-positive neurons in the substantia nigra and the ventral tegmental areas. Administration of alpha-phenyl-n-tert-butyl-nitrone (PBN, 100 mg/kg i.p.) immediately after the IL-1beta injection protected the brain from IL-1beta-induced injury. Protection of PBN was linked with the attenuated oxidative stress induced by IL-1beta, as indicated by decreased elevation of 8-isoprostane content and by the reduced number of 4-hydroxynonenal or malondialdehyde or nitrotyrosine-positive cells following IL-1beta exposure. PBN also attenuated IL-1beta-stimulated inflammatory responses as indicated by the reduced activation of microglia. The finding that IL-1beta induced perinatal brain injury was very similar to that induced by lipopolysaccharide (LPS), as we previously reported and that PBN was capable to attenuate the injury induced by either LPS or IL-1beta suggests that IL-1beta may play a critical role in mediating brain injury associated with perinatal infection/inflammation.

Molecular mechanisms involved in injury to the preterm brain

Injury to the premature brain is a major contributor to infant mortality and morbidity, often leading to mental retardation and sensory-motor impairment. The disease process is believed to be caused, sustained, and aggravated by multiple perinatal factors that team up in a multi-hit fashion. Clinical, epidemiological, and experimental studies have revealed that key factors such as inflammation, excitotoxicity, and oxidative stress contribute considerably to white- and gray-matter injury in premature infants, whose brains are particularly susceptible to damage. Depending on the timing, lesions of the immature brain may influence developmental events in their natural sequence and redirect subsequent development. We review current concepts on molecular mechanisms underlying injury to the premature brain.

Neonatal chimerization with human glial progenitor cells can both remyelinate and rescue the otherwise lethally hypomyelinated shiverer mouse

Congenitally hypomyelinated shiverer mice fail to generate compact myelin and die by 18-21 weeks of age. Using multifocal anterior and posterior fossa delivery of sorted fetal human glial progenitor cells into neonatal shiverer x rag2(-/-) mice, we achieved whole neuraxis myelination of the engrafted hosts, which in a significant fraction of cases rescued this otherwise lethal phenotype. The transplanted mice exhibited greatly prolonged survival with progressive resolution of their neurological deficits. Substantial myelination in multiple regions was accompanied by the acquisition of normal nodes of Ranvier and transcallosal conduction velocities, ultrastructurally normal and complete myelination of most axons, and a restoration of a substantially normal neurological phenotype. Notably, the resultant mice were cerebral chimeras, with murine gray matter but a predominantly human white matter glial composition. These data demonstrate that the neonatal transplantation of human glial progenitor cells can effectively treat disorders of congenital and perinatal hypomyelination.

Role of perinatal inflammation in cerebral palsy

Inflammatory molecules are promptly upregulated in the fetal environment and postnatally in brain-damaged subjects. Intrauterine infections and inflammation are often associated with asphyxia. This double-hit effect by combined infection or inflammation and hypoxia is therefore a frequent concomitant in neonatal brain damage. Animal models combining hypoxia and infection were recently designed to explore the mechanisms underlying brain damage in such circumstances and to look for possible neuroprotective strategies. Proinflammatory cytokines are thought to be major mediators in brain injury in neonates with perinatal asphyxia, bacterial infection, or both. Cytokines, however, could also have neuroprotective properties. The critical point in the balance between neurodamaging and neuroprotective effects of cytokines has yet to be unraveled. This understanding might help to develop new therapeutic approaches to counteract the inflammatory disequilibrium observed in the pathophysiologic mechanisms associated with brain injury.

Brain injury in premature infants: a complex amalgam of destructive and developmental disturbances

Brain injury in premature infants is of enormous public health importance because of the large number of such infants who survive with serious neurodevelopmental disability, including major cognitive deficits and motor disability. This type of brain injury is generally thought to consist primarily of periventricular leukomalacia (PVL), a distinctive form of cerebral white matter injury. Important new work shows that PVL is frequently accompanied by neuronal/axonal disease, affecting the cerebral white matter, thalamus, basal ganglia, cerebral cortex, brain stem, and cerebellum. This constellation of PVL and neuronal/axonal disease is sufficiently distinctive to be termed "encephalopathy of prematurity". The thesis of this Review is that the encephalopathy of prematurity is a complex amalgam of primary destructive disease and secondary maturational and trophic disturbances. This Review integrates the fascinating confluence of new insights into both brain injury and brain development during the human premature period.

The diagnosis, management, and postnatal prevention of intraventricular hemorrhage in the preterm neonate

Intraventricular hemorrhage (IVH) occurs in 20% to 25% of very low birthweight preterm neonates and may be associated with significant sequelae. Infants who have IVH are at risk for posthemorrhagic hydrocephalus and periventricular leukomalacia; as many as 75% of those who have parenchymal involvement of hemorrhage suffer significant neurodevelopmental disability. Because of the prevalence of IVH and the medical and societal impact of this disease, many postnatal pharmacologic prevention strategies have been explored. Randomized clinical prevention trials should provide long-term neurodevelopmental follow-up to assess the impact of preterm birth, injury, and pharmacologic intervention on the developing brain.

Diffuse axonal injury in periventricular leukomalacia as determined by apoptotic marker fractin

Periventricular leukomalacia (PVL), the major substrate of neurologic deficits in premature infants, is associated with reduced white matter volume. Using immunomarkers of axonal pathology [beta-amyloid precursor protein (beta-APP) and apoptotic marker fractin], we tested the hypothesis that widespread (diffuse) axonal injury occurs in the gliotic white matter beyond the foci of necrosis in PVL, thus contributing to the white matter volume reduction. In a cohort of 17 control cases and 13 PVL cases with lesions of different chronological ages, diffuse axonal damage in PVL was detected by fractin in white matter sites surrounding and distant from acute and organizing foci of necrosis. Using beta-APP, axonal spheroids were detected within necrotic foci in the acute and organizing (subacute) stages, a finding consistent with others. Interestingly, GAP-43 expression was also detected in spheroids in the necrotic foci, suggesting attempts at axonal regeneration. Thirty-one percent of the PVL cases had thalamic damage and 15% neuronal injury in the cerebral cortex overlying PVL. We conclude that diffuse axonal injury, as determined by apoptotic marker fractin, occurs in PVL and that its cause likely includes primary ischemia and trophic degeneration secondary to corticothalamic neuronal damage.

Lipopolysaccharide-induced peroxisomal dysfunction exacerbates cerebral white matter injury: attenuation by N-acetyl cysteine

Cerebral white matter injury during prenatal maternal infection characterized as periventricular leukomalacia is the main substrate for cerebral palsy (CP) in premature infants. Previously, we reported that maternal LPS exposure causes oligodendrocyte (OL)-injury/hypomyelination in the developing brain which can be attenuated by an antioxidant agent, N-acetyl cysteine (NAC). Herein, we elucidated the role of peroxisomes in LPS-induced neuroinflammation and cerebral white matter injury. Peroxisomes are important for detoxification of reactive oxidative species (ROS) and metabolism of myelin-lipids in OLs. Maternal LPS exposure induced selective depletion of developing OLs in the fetal brain which was associated with ROS generation, glutathione depletion and peroxisomal dysfunction. Likewise, hypomyelination in the postnatal brain was associated with decrease in peroxisomes and OLs after maternal LPS exposure. Conversely, NAC abolished these LPS-induced effects in the developing brain. CP brains imitated these observed changes in peroxisomal/myelin proteins in the postnatal brain after maternal LPS exposure. In vitro studies revealed that pro-inflammatory cytokines cause OL-injury via peroxisomal dysfunction and ROS generation. NAC or WY14643 (peroxisome proliferators activated receptor (PPAR)-alpha agonist) reverses these effects of pro-inflammatory cytokines in the wild-type OLs, but not in PPAR-alpha(-/-) OLs. Similarly treated B12 oligodenroglial cells co-transfected with PPAR-alpha siRNAs/pTK-PPREx3-Luc, and LPS exposed PPAR-alpha(-/-) pregnant mice treated with NAC or WY14643 further suggested that PPAR-alpha activity mediates NAC-induced protective effects. Collectively, these data provide unprecedented evidence that LPS-induced peroxisomal dysfunction exacerbates cerebral white matter injury and its attenuation by NAC via a PPAR-alpha dependent mechanism expands therapeutic avenues for CP and related demyelinating diseases.

The fetal inflammatory response syndrome

The fetal inflammatory response syndrome (FIRS) is a condition characterized by systemic inflammation and an elevation of fetal plasma interleukin-6. This syndrome has been observed in fetuses with preterm labor with intact membranes, preterm prelabor rupture of the membranes, and also fetal viral infections such as cytomegalovirus. FIRS is a risk factor for short-term perinatal morbidity and mortality after adjustment for gestational age at delivery and also for the development of long-term sequelae such as bronchopulmonary dysplasia and brain injury. Multiorgan involvement in FIRS has been demonstrated in the hematopoietic system, thymus, adrenal glands, skin, kidneys, heart, lung, and brain. This article reviews the fetal systemic inflammatory response as a mechanism of disease. Potential interventions to control an exaggerated inflammatory response in utero are also described.

Activated protein C reduces endotoxin-induced white matter injury in the developing rat brain

Periventricular leukomalacia (PVL), the dominant form of brain injury in premature infants, is characterized by white matter injury (WMI) and is associated with cerebral palsy. The pathogenesis of PVL is complex and likely involves ischemia/reperfusion, free radical formation, excitotoxicity, impaired regulation of cerebral blood flow, a procoagulant state, and inflammatory mechanisms associated with maternal and/or fetal infection. Using an established animal model of human PVL, we investigated whether activated protein C (APC), an anti-coagulant factor with anti-inflammatory, anti-apoptotic, anti-oxidant, and cytoprotective activities, could reduce endotoxin-induced WMI in the developing rat brain. Intraperitoneal injections of lipopolysaccharide (LPS) (0.5 mg/kg body weight) were given at embryonic days 18 (E18) and 19 (E19) to pregnant Sprague-Dawley rats; control rats were injected with sterile saline. A single intravenous injection of recombinant human (rh) APC (0.2 mg /kg body weight) was given to pregnant rats following the second LPS dose on embryonic day 19 (E19). Reduced cell death in white matter and hypomyelination were shown on TUNEL and myelin basic protein (MBP) staining, respectively, on late postnatal days (P7) in APC-treated groups. There were significantly fewer TUNEL+nuclei in the periventricular WM in the APC+LPS group than in the untreated LPS group. Compared to the APC+LPS and control group, MBP expression was weak in the LPS group on P7, indicating endotoxin-induced hypomyelination in the developing rat brain. APC attenuated the LPS-induced protein expression of inflammatory cytokines, tumor necrosis factor-alpha, and interleukin-6, as evaluated by ELISA in neonatal rat brains. A single intraperitoneal injection of rhAPC (0.2 mg/kg body weight) to neonatal rats on P1 also had similar protective and anti-inflammatory effects against maternally administered LPS. Collectively, these data support the hypothesis that APC may provide protection against an endotoxin-evoked inflammatory response and WMI in the developing rat brain. Moreover, our results suggest that the possible use of APC in treatment of preterm infants and pregnant women with maternal or placental infection may minimize the risk of PVL and cerebral palsy.

Hypoxic-ischemic injury in the immature brain--key vascular and cellular players

Over the past decade, much has been learned about the cellular and molecular mechanisms underlying hypoxic-ischemic (H-I) injury in the preterm human brain. The pathogenesis of H-I brain injury is now understood to be multifactorial and quite complex, depending on (i) the severity, intensity and timing of asphyxia, (ii) selective ischemic vulnerability, (iii) the degree of maturity of the brain, and (iv) the characteristics of the ensuing reoxygenation/reperfusion phase. Each of these factors has differential effects on the distinct cell populations in the brain, with certain specific cell types being particularly vulnerable in the developing brain. In this review, we discuss the role of the blood vessels and the distinct cell populations, which are the mayor constitutive elements of the immature brain, in the pathophysiology of H-I lesion. The presence of fragile and poorly anastomosed blood vessels and the existence of disturbances in the blood-brain barrier alter blood flow, vascular tone and nutrient delivery. Brain cells are sensitive to the overstimulation of neurotransmitter receptors, particularly glutamate receptors, which can provoke excitotoxicity leading to the death of neurons and other cells such as astrocytes and oligodendrocyte progenitors. Microglial activation by means of excitatory amino acids and by leukocyte migration initiates the inflammatory response giving rise to an increase in regional cerebral blood flow and promoting astrocyte and oligodendrocyte injuries. A better understanding of these aspects of H-I injury will contribute to more efficient strategies for the management of the associated damage.

Astrocytes and developmental white matter disorders

There is an increasing awareness that the astrocytes in the immature periventricular white matter are vulnerable to ischemia and respond to inflammation. Here we provide a synopsis of the articles that have evaluated the causes and consequences of developmental brain injuries to white matter astrocytes as well as the consequences of several genetic mutations that result in abnormal astrocyte development. Emerging data suggest that the astrocytes are not simply responding to the injury but are likely victims as well as culprits. Given the important roles that astrocytes play in maintaining ionic, neurotransmitter, and metabolic homeostasis in the brain, a more thorough understanding of the mechanisms that lead to their incapacitation, demise, or reactions as well as a better understanding of the stimuli that regulate their neuroprotective and regenerative properties will enable these cells to be manipulated to preserve the integrity of white matter and to potentially provide therapeutics to enhance neonatal regeneration and recovery from brain injury.

Inflammation, brain damage and visual dysfunction in preterm infants

Antenatal intrauterine infection and the fetal inflammatory response appear to be important pathogenetic factors in preterm birth and subsequent neonatal disorders of the lung and brain. In this paper, we expand this concept to include visual dysfunction. Although present data tend to support our notion, we suggest that more experimental and epidemiological research is needed to elucidate mechanisms of infection/inflammation-induced damage to the eye and visual brain pathways of preterm infants.

Neonatal infection and long-term neurodevelopmental outcome in the preterm infant

PURPOSE OF REVIEW

The relationship between infection, the inflammatory response and adverse neurodevelopmental outcome in preterm infants is slowly being elucidated. The developing brain, particularly the periventricular white matter, is vulnerable to cytotoxic and hypoxic/ischemic injury, which places these infants at increased risk for abnormal cognitive and motor functioning. This review summarizes current data evaluating associations between infection and neurodevelopmental outcome in the preterm infant.

RECENT FINDINGS

Preterm infants are at risk for intrauterine and postnatal infections. Recent studies have linked infection/inflammation associated with chorioamnionitis, sepsis and necrotizing enterocolitis with adverse neurodevelopmental outcome and impaired growth in preterm infants. Investigators have also shown associations between infection and brain injury, including severe intraventricular hemorrhage and periventricular leukomalacia. Very-low-birth-weight preterm infants are at substantial risk for neonatal infection, with associated morbidity and mortality. It is postulated that exposure of the preterm brain to inflammatory mediators during infectious episodes contributes to brain injury and poor developmental outcome.

SUMMARY

Enhanced understanding of the interaction of infection, inflammation and brain injury will be critical to developing strategies to improve neurodevelopmental outcome in preterm infants.

Role of interleukin-6 in lipopolysaccharide-induced brain injury and behavioral dysfunction in neonatal rats

There are increasing data in support of the hypothesis that inflammatory cytokines are involved in neonatal white matter damage. Despite extensive study of the proinflammatory cytokines tumor necrosis factor-alpha and interleukin-1beta, the role of interleukin-6 in the development of white matter damage is largely unknown. In the present study, the role(s) of interleukin-6 in mediating lipopolysaccharide-induced brain injury and behavioral changes was investigated by the intracerebral injection of lipopolysaccharide with interleukin-6 neutralizing antibody in the 5-day-old rat brain. Brain injury was examined in brain sections at postnatal day 8 and postnatal day 21. Behavioral tests including righting reflex, wire hanging maneuver, cliff avoidance, locomotor activity, gait analysis, responses in the elevated plus-maze and passive avoidance were performed from postnatal day 3 to postnatal day 21. Changes in astroglia, microglia and oligodendrocytes were studied using immunohistochemistry in the postnatal day 21 rat brain. Our results show that interleukin-6 antibody attenuated lipopolysaccharide-induced brain lateral ventricle dilation and improved neurobehavioral performance. Interleukin-6 antibody also suppressed lipopolysaccharide-induced astrogliosis and microglial activation, and increased the number of oligodendrocytes in white matter. However, no changes of tumor necrosis factor-alpha and interleukin-1beta were detected. In contrast, no histopathological changes and glial activation were observed in rats injected with only interleukin-6. The present study indicates that the contribution to brain injury by interleukin-6 depends on its interaction with other lipopolysaccharide-induced agents and not on interleukin-6 alone.

Perinatal subplate neuron injury: implications for cortical development and plasticity

Perinatal brain injury may result in widespread deficits in visual, motor and cognitive systems suggesting disrupted brain development. Neurosensory and cognitive impairment are observed at increasing frequency with decreasing gestational ages, suggesting a unique vulnerability of the developing brain. The peak of human subplate neuron development coincides with the gestational ages of highest vulnerability to perinatal brain injury in the premature infant. At the same time, human thalamocortical connections are forming and being refined by activity-dependent mechanisms during critical periods. Subplate neurons are the first cortical neurons to mature and are selectively vulnerable to early hypoxic-ischemic brain injury in animal models. Timing of subplate neuron death determines the resulting defect in thalamocortical development: very early excitotoxic subplate neuron death results in failure of thalamocortical innervation, while later subplate neuron death interferes with the refinement of thalamocortical connections into mature circuits. We suggest that subplate neuron injury may be a central component of perinatal brain injury resulting in specific neurodevelopmental consequences.

Effect of tumor necrosis factor-alpha on developing optic nerve oligodendrocytes in culture

There is increasing evidence that proinflammatory cytokines are involved in the development of periventricular leukomalacia (PVL), a condition in which developing oliodendrocytes (OLs) are preferentially injured. In the present study, we utilized an in vitro assay to demonstrate that the A2B5+ OL progenitors as well as the O4+ prooligodendrocytes (pro-OLs) were more susceptible to tumor necrosis factor-alpha (TNF-alpha) cytotoxicity than the O4+/O1+ immature OLs. OL progenitors were isolated from optic nerves of 7-day-old rat pups and cultured in chemically defined medium supplemented with platelet-derived growth factor and basic fibroblast growth factor. OL progenitors were allowed to differentiate into pro-OLs and immature OLs under special cultural conditions. Cells at three different developmental stages were subjected to TNF-alpha treatment. Cell death, presumably by apoptosis as evidenced by TUNEL staining and caspase-3 activation, was observed following TNF-alpha treatment. Corresponding to TNF-alpha-induced apoptosis, cell survival rate decreased in a time- and dose-dependent manner. The sensitivity of different OL developmental stages to TNF-alpha decreased with the progression of cell maturation. However, this differential response was not related to differentially expressed TNF-alpha receptors. Consistent with reports that progenitor cells are preferentially injured in PVL, our results may further support the role of TNF-alpha as a potential mediator of PVL.