Neurobiology of periventricular leukomalacia in the premature infant

Myelin transcription factor 1 (MyT1) immunoreactivity in infants with periventricular leukomalacia

Myelin transcription factor 1 (MyT1) is a zinc-dependent, DNA-binding protein, and is known to be expressed in early progenitors of oligodendrocytes. We examined the immunoreactivity of MyT1 in developing human brains and brains with periventricular leukomalacia (PVL) to understand the relationship between the expression of MyT1 and myelination in PVL brains. MyT1-positive glial cells were first detected at 19 gestational weeks (GWs) and then gradually increased until 26-29 GWs in the control group. Then they decreased and became very rare at 1 year of age. The expression of MyT1 immunoreactivity shifted from the nucleus to the cytoplasm of the glial cells in the developmental time course. In the chronic stage of PVL, MyT1-positive cells were significantly increased around necrotic foci and some of the regions were coincident with increasing MBP and PLP immunoreactivity. These results may reflect myelin repair on dysmyelination around PVL areas. Therefore, MyT1 may play an important role in the myelin repair in PVL regions.

Protection of mature oligodendrocytes by inhibitors of caspases and calpains

Mature mouse oligodendrocytes (OLs) are susceptible to death in demyelinating diseases such as multiple sclerosis and in brain injury following neurotrauma, ischemia, or stroke. To understand mechanisms leading to death of mature OLs and develop strategies for protection, we utilized cultures of mature mouse OLs to investigate the role of caspases and calpains in OL cell death mediated by different mechanisms. The agents used were (i) staurosporine, which induces apoptotic death via inhibition of protein kinases; (ii) kainate, which activates non-NMDA glutamate receptors; (iii) thapsigargin, which releases intracellular calcium stores; and (iv) SNAP, which releases active NO species and causes necrotic cell death. Inhibitors blocking primary effector caspases (including caspase 3), the FAS (death receptor)-mediated initiator caspases (including caspase 8), and stress-induced caspases (including caspase 9), were tested for their protective effects. Inhibition of caspases 3, 8, and 9 each robustly protected OLs following insult with staurosporine, thapsigargin, or kainate when added at optimal times. The time of addition of the inhibitors for maximal protection varied with the agent, from 1 h of preincubation before addition of staurosporine to 6 h after addition of kainate. Much less protection was seen for the NO generator SNAP under any condition. The role of calcium in OL death in each model was investigated by chelating extracellular Ca++ with EGTA, and by inhibiting the Ca++-activated calpain proteases. Calcium chelation did not protect against staurosporine, but decreased OL death initiated by kainate, thapsigargin, or NO. The calpain inhibitors PD150606 and calpain inhibitor I protected from cell death initiated by staurosporine, kainate, and thapsigargin, but not from cell death initiated by the NO donor SNAP.

Cerebral infarctions in the fetus and neonate: maternal-placental-fetal considerations

Historical data, clinical examination findings, and laboratory information must be integrated along a variable timeline that includes antepartum, intrapartum, and postnatal time periods when cerebral infarction can occur, in the context of the neonates genetic endowment. Genetic susceptibility or prenatal acquired vulnerabilities regarding stroke syndromes may set in motion a cascade of molecular pathways that ultimately cause or exacerbate brain injury when the vulnerable child experiences adverse medical conditions. The clinician must consider maternal, placental, and fetal conditions on which a stroke syndrome may be superimposed, with or without additional brain injury from other pathogenic mechanisms. Evaluation of fetal and neonatal cerebral infarction requires knowledge of mechanisms of brain injury that cross medical disciplines and may involve consultation with maternal/fetal specialists, placental and pediatric pathologists, neonatologists, geneticists, and other pediatric subspecialties. Comprehensive evaluations of survivors of cerebral infarction are needed to better understand structural and functional plasticity of the developing brain after a cerebrovascular event in the fetal and neonatal periods.

Advances in postnatal neuroimaging: relevance to pathogenesis and treatment of brain injury

The human brain is susceptible to a wide variety of insults. The permanent residua of these abnormalities are represented in dysfunction of one or more areas of neurodevelopment. A full understanding of normal brain development, mechanisms of brain injury, and consequences for subsequent brain development is required to determine which infants are at risk for neurodevelopmental handicap, and to monitor the effects of new treatments and management regimens designed to prevent these disabilities. Advanced magnetic resonance techniques, such as quantitative morphometric magnetic resonance techniques, diffusion-weighted magnetic resonance techniques, and magnetic resonance spectroscopy applied to the study of early human brain development have given us a better understanding of the pathophysiologic mechanisms of brain injury and its effects on subsequent brain development. Magnetic resonance imaging has provided an invaluable tool for the study of the fetal and newborn brain in vivo.

Effects of intrauterine inflammation on developing rat brain

Damage to the white matter in the brain during development can lead to cerebral palsy (CP), a heterogeneous group of clinical syndromes that results in life-long disorders of movement and posture. Periventricular leukomalacia (PVL) is a pathological process within the white matter characterized by oligodendrocyte loss and is associated with the development of CP. Clinically, CP and PVL are associated with intrauterine infection and inflammation, but mechanisms involved are not well understood. We developed a model of intrauterine inflammation in Lewis and Fischer 344 rats to study the effects of intrauterine inflammation on developing glia. Pregnant rats were intracervically injected with lipopolysaccharide (LPS) at 15 days of gestation (E15) and a dose of LPS that caused low fetal mortality was determined. At E20, treated fetuses had increased TUNEL(+) nuclei and tumor necrosis factor (TNF)-alpha-immunoreactive areas within the brains. In a second series of animals allowed to survive until postnatal day 21 (PND 21), immunostaining was performed against several glial markers. Staining for the oligodendrocyte-specific proteins 2', 3'-cyclic nucleotide phosphodiesterase (CNP) and myelin proteolipid protein (PLP) was decreased in treated pups compared to shams within the corpus callosum, a white matter structure used as a representative area of developing white matter. Treated pups had activated astrocytes lining cerebral blood vessels, as observed by glial fibrillary acidic protein (GFAP) staining, while sham pups did not. Activated microglia were not detected using OX42 as a cell marker. Our model of intrauterine inflammation causes increased TUNEL and TNF-alpha staining early after injury, suggesting increased apoptotic cell death, possibly by cytokine-related mechanisms.

Educational neuroimaging: a proposed neuropsychological application of near-infrared spectroscopy (nIRS)

OBJECTIVE

To provide a description of an emerging neuroimaging methodology, near-infrared spectroscopy (nIRS), and a potential educational application of the unique aspects of this technology.

SUMMARY

nIRS is documented for its potential as a personal, portable, brain imaging system that may prove useful for cerebral monitoring in applied settings such as home, school, and work. The basis of nIRS brain imaging is reviewed, with summary descriptions of optical and neurovascular issues as well as a brief comparison to other brain imaging methodologies. Recent developments in nIRS technology are discussed, including ongoing validation efforts and potential applications for neuropsychologists. We describe one potential application of nIRS (i.e., educational neuroimaging) as an illustration of the use of nIRS technology and the potential expansion of the neuropsychologist's role in the educational setting.

CONCLUSION

nIRS holds the potential of opening new clinical questions and opportunities for neuropsychologists, and may provide a low-cost means of repeatable, neurovascular monitoring in nonmedical settings.

Vulnerability of the developing brain. Neuronal mechanisms

Despite the improved survival of tiny preterm neonates, their neurodevelopmental outcomes remain a cause for grave concern. The authors propose two primary mechanisms leading to enhanced neuronal cell death in the immature brain: (1) NMDA-mediated excitotoxicity resulting from repetitive or prolonged pain, and (2) enhanced naturally occurring neuronal apoptosis during early development due to multiple metabolic stresses or lack of social stimulation. The pattern and magnitude of abnormalities will depend on genetic variability as well as the timing, intensity, and duration of adverse environmental experiences. Thus, cumulative brain damage during infancy will finally lead to reductions in brain volume, abnormal behavioral and neuroendocrine regulation, and poor cognitive outcomes during childhood and adolescence. The public health and economic importance of preventing or ameliorating the subtle brain damage caused by these mechanisms cannot be overestimated. This certainly justifies concerted efforts by neuroscientists and clinicians to investigate the mechanisms underlying early neuronal injury, to minimize the impact of adverse experiences and environmental factors in neonates, and to develop novel therapeutic strategies for improving the cognitive and behavioral outcomes of ex-preterm neonates.

Periventricular leukomalacia, inflammation and white matter lesions within the developing nervous system

Periventricular leukomalacia (PVL) occurring in premature infants, represents a major precursor for neurological and intellectual impairment, and cerebral palsy in later life. The disorder is characterized by multifocal areas of necrosis found deep in the cortical white matter, which are often symmetrical and occur adjacent to the lateral ventricles. There is no known cure for PVL. Factors predisposing to PVL include birth trauma, asphyxia and respiratory failure, cardiopulmonary defects, premature birth/low birthweight, associated immature cerebrovascular development and lack of appropriate autoregulation of cerebral blood flow in response to hypoxic-ischemic insults. The intrinsic vulnerability of oligodendrocyte precursors is considered as central to the pathogenesis of PVL. These cells are susceptible to a variety of injurious stimuli including free radicals and excitotoxicity induced by hypoxic-ischemic injury (resulting from cerebral hypoperfusion), lack of trophic stimuli, as well as secondary associated events involving microglial and astrocytic activation and the release of pro-inflammatory cytokines TNF-alpha and IL-6. It is yet unclear whether activated astrocytes and microglia act as principal participants in the development of PVL lesions, or whether they are representatives of an incidental pathological response directed towards repair of tissue injury in PVL. Nevertheless, the accumulated evidence points to a pathological contribution of microglia towards damage. The topography of lesions in PVL most likely reflects a combination of the relatively immature cerebrovasculature together with a failure in perfusion and/or hypoxia during the greatest period of vulnerability occurring around mid-to-late gestation. Mechanisms underlying the pathogenesis of PVL have so far been related to prenatal ischemic injury to the brain initiated within the third trimester, which result in global cognitive and developmental delay and motor disturbances. Over the past few years, several epidemiological and experimental studies have implicated intrauterine infection and chorioamnionitis as causative in the pathogenesis of PVL. In particular, recent investigations have shown that inflammatory responses in the fetus and neonate can contribute towards neonatal brain injury and development-related disabilities including cerebral palsy. This review presents current concepts on the pathogenesis of PVL and emphasizes the increasing evidence for an inflammatory pathogenic component to this disorder, either resulting from hypoxic-ischemic injury or from infection. These findings provide the basis for clinical approaches targeted at protecting the premature brain from inflammatory damage, which may prove beneficial for treating PVL, if identified early in pathogenesis.

Perinatal brain injury in the preterm and term newborn

Major advances in understanding the cellular mechanisms of brain injury have presented a host of potential targets for intervention. This is particularly true of hypoxic-ischemic injury, the most important form of perinatal brain injury. As the window for effective clinical intervention may be particularly narrow in the fetus and newborn because of the often-delayed and subtle presentation of the onset of the insult, recent focus has been on defining and countering the more delayed mechanisms of brain injury. Recent insights into the mechanisms of oligodendrocyte injury and the role of inflammatory substances in perinatal brain injury are also discussed.