Cerebellar development in a baboon model of preterm delivery: impact of specific ventilatory regimes

Cerebellar development after preterm birth

Preterm birth and its complications and the associated adverse factors, including brain hemorrhage, inflammation, and the side effects of medical treatments, are the leading causes of neurodevelopmental disability. Growing evidence suggests that preterm birth affects the cerebellum, which is the brain region involved in motor coordination, cognition, learning, memory, and social communication. The cerebellum is particularly vulnerable to the adverse effects of preterm birth because key cerebellar developmental processes, including the proliferation of neural progenitors, and differentiation and migration of neurons, occur in the third trimester of a human pregnancy. This review discusses the negative impacts of preterm birth and its associated factors on cerebellar development, focusing on the cellular and molecular mechanisms that mediate cerebellar pathology. A better understanding of the cerebellar developmental mechanisms affected by preterm birth is necessary for developing novel treatment and neuroprotective strategies to ameliorate the cognitive, behavioral, and motor deficits experienced by preterm subjects.

SCN2A contributes to oligodendroglia excitability and development in the mammalian brain

Spiking immature oligodendrocytes (OLs), referred to as spiking OLs, express voltage-activated Na⁺ channels (Na_v) and K⁺ (K_v) channels, endowing a subpopulation of OLs with the ability to generate Na_v-driven spikes. In this study, we investigate the molecular profile of spiking OLs, using single-cell transcriptomics paired with whole-cell patch-clamp recordings. SCN2A, which encodes the channel Na_v1.2, is specifically expressed in spiking OLs in the brainstem and cerebellum, both in mice and in Olive baboons. Spiking OLs express lineage markers of OL progenitor cells (OPCs) and pre-myelinating OLs, indicating they belong to a transitional stage during differentiation. Deletion of SCN2A reduces the Na_v current-expressing OL population and eliminates spiking OLs, indicating that SCN2A is essential for spiking in OLs. Deletion of SCN2A does not impact global OL proliferation but disrupts maturation of a subpopulation of OLs, suggesting that Na_v1.2 is involved in heterogeneity in OL lineage cells and their development.

Hippocampal epigenetic and insulin-like growth factor alterations in noninvasive versus invasive mechanical ventilation in preterm lambs

BACKGROUND

The brain of chronically ventilated preterm human infants is vulnerable to collateral damage during invasive mechanical ventilation (IMV). Damage is manifest, in part, by learning and memory impairments, which are hippocampal functions. A molecular regulator of hippocampal development is insulin-like growth factor 1 (IGF1). A gentler ventilation strategy is noninvasive respiratory support (NRS). We tested the hypotheses that NRS leads to greater levels of IGF1 messenger RNA (mRNA) variants and distinct epigenetic profile along the IGF1 gene locus in the hippocampus compared to IMV.

METHODS

Preterm lambs were managed by NRS or IMV for 3 or 21 days. Isolated hippocampi were analyzed for IGF1 mRNA levels and splice variants for promoter 1 (P1), P2, and IGF1A and 1B, DNA methylation in P1 region, and histone covalent modifications along the gene locus.

RESULTS

NRS had significantly greater levels of IGF1 P1 (predominant transcript), and 1A and 1B mRNA variants compared to IMV at 3 or 21 days. NRS also led to more DNA methylation and greater occupancy of activating mark H3K4 trimethylation (H3K4me³), repressive mark H3K27me³, and elongation mark H3K36me³ compared to IMV.

CONCLUSIONS

NRS leads to distinct IGF1 mRNA variant levels and epigenetic profile in the hippocampus compared to IMV.

IMPACT

Our study shows that 3 or 21 days of NRS of preterm lambs leads to distinct IGF1 mRNA variant levels and epigenetic profile in the hippocampus compared to IMV. Preterm infant studies suggest that NRS leads to better neurodevelopmental outcomes later in life versus IMV. Also, duration of IMV is directly related to hippocampal damage; however, molecular players remain unknown. NRS, as a gentler mode of respiratory management of preterm neonates, may reduce damage to the immature hippocampus through an epigenetic mechanism.

Preterm Birth Impedes Structural and Functional Development of Cerebellar Purkinje Cells in the Developing Baboon Cerebellum

Human cerebellar development occurs late in gestation and is hindered by preterm birth. The fetal development of Purkinje cells, the primary output cells of the cerebellar cortex, is crucial for the structure and function of the cerebellum. However, morphological and electrophysiological features in Purkinje cells at different gestational ages, and the effects of neonatal intensive care unit (NICU) experience on cerebellar development are unexplored. Utilizing the non-human primate baboon cerebellum, we investigated Purkinje cell development during the last trimester of pregnancy and the effect of NICU experience following premature birth on developmental features of Purkinje cells. Immunostaining and whole-cell patch clamp recordings of Purkinje cells in the baboon cerebellum at different gestational ages revealed that molecular layer width, driven by Purkinje dendrite extension, drastically increased and refinement of action potential waveform properties occurred throughout the last trimester of pregnancy. Preterm birth followed by NICU experience for 2 weeks impeded development of Purkinje cells, including action potential waveform properties, synaptic input, and dendrite extension compared with age-matched controls. In addition, these alterations impact Purkinje cell output, reducing the spontaneous firing frequency in deep cerebellar nucleus (DCN) neurons. Taken together, the primate cerebellum undergoes developmental refinements during late gestation, and NICU experience following extreme preterm birth influences morphological and physiological features in the cerebellum that can lead to functional deficits.

Respiratory Support of the Preterm Neonate: Lessons About Ventilation-Induced Brain Injury From Large Animal Models

Many preterm neonates require mechanical ventilation which increases the risk of cerebral inflammation and white matter injury in the immature brain. In this review, we discuss the links between ventilation and brain injury with a focus on the immediate period after birth, incorporating respiratory support in the delivery room and subsequent mechanical ventilation in the neonatal intensive care unit. This review collates insight from large animal models in which acute injurious ventilation and prolonged periods of ventilation have been used to create clinically relevant brain injury patterns. These models are valuable resources in investigating the pathophysiology of ventilation-induced brain injury and have important translational implications. We discuss the challenges of reconciling lung and brain maturation in commonly used large animal models. A comprehensive understanding of ventilation-induced brain injury is necessary to guide the way we care for preterm neonates, with the goal to improve their neurodevelopmental outcomes.

Preterm birth disrupts cerebellar development by affecting granule cell proliferation program and Bergmann glia

Preterm birth is a leading cause of long-term motor and cognitive deficits. Clinical studies suggest that some of these deficits result from disruption of cerebellar development, but the mechanisms that mediate cerebellar abnormalities in preterm infants are largely unknown. Furthermore, it remains unclear whether preterm birth and precocious exposure to the ex-utero environment directly disrupt cerebellar development or indirectly by increasing the probability of cerebellar injury, including that resulting from clinical interventions and protocols associated with the care of preterm infants. In this study, we analyzed the cerebellum of preterm pigs delivered via c-section at 91% term and raised for 10 days, until term-equivalent age. The pigs did not receive any treatments known or suspected to affect cerebellar development and had no evidence of brain damage. Term pigs sacrificed at birth were used as controls. Immunohistochemical analysis revealed that preterm birth did not affect either size or numbers of Purkinje cells or molecular layer interneurons at term-equivalent age. The number of granule cell precursors and Bergmann glial fibers, however, were reduced in preterm pigs. Preterm pigs had reduced proliferation but not differentiation of granule cells. qRT-PCR analysis of laser capture microdissected external granule cell layer showed that preterm pigs had a reduced expression of Ccnd1 (Cyclin D1), Ccnb1 (Cyclin B1), granule cell master regulatory transcription factor Atoh1, and signaling molecule Jag1. In vitro rescue experiments identified Jag1 as a central granule cell gene affected by preterm birth. Thus, preterm birth and precocious exposure to the ex-utero environment disrupt cerebellum by modulating expression of key cerebellar developmental genes, predominantly affecting development of granule precursors and Bergmann glia.

Beyond the uterine environment: a nonhuman primate model to investigate maternal-fetal and neonatal outcomes following chronic intrauterine infection

BackgroundIntrauterine infection is a significant cause of early preterm birth. We have developed a fetal-neonatal model in the rhesus macaque to determine the impact of chronic intrauterine infection with Ureaplasma parvum on early neonatal reflexes and brain development.MethodsTime-mated, pregnant rhesus macaques were randomized to be inoculated with U. parvum (serovar 1; 10⁵c.f.u.) or control media at ~120 days' gestational age (dGA). Neonates were delivered by elective hysterotomy at 135-147 dGA (term=167d), stabilized, and cared for in our nonhuman primate neonatal intensive care unit. Neonatal reflex behaviors were assessed from birth, and fetal and postnatal brain magnetic resonance imaging (MRI) was performed.ResultsA total of 13 preterm and 5 term macaque infants were included in the study. Ten preterm infants survived to 6 months of age. U. parvum-infected preterm neonates required more intensive respiratory support than did control infants. MRI studies suggested a potential perturbation of brain growth and white matter maturation with exposure to intra-amniotic infection.ConclusionWe have demonstrated the feasibility of longitudinal fetal-neonatal studies in the preterm rhesus macaque after chronic intrauterine infection. Future studies will examine long-term neurobehavioral outcomes, cognitive development, neuropathology, and in vivo brain imaging to determine the safety of antenatal antibiotic treatment for intrauterine infection.

Effect of postnatal progesterone therapy following preterm birth on neurosteroid concentrations and cerebellar myelination in guinea pigs

Allopregnanolone protects the fetal brain and promotes normal development including myelination. Preterm birth results in the early separation of the infant from the placenta and consequently a decline in blood and brain allopregnanolone concentrations. Progesterone therapy may increase allopregnanolone and lead to improved oligodendrocyte maturation. The objectives of this study were to examine the efficacy of progesterone replacement in augmenting allopregnanolone concentrations during the postnatal period and to assess the effect on cerebellar myelination – a region with significant postnatal development. Preterm guinea pig neonates delivered at 62 days of gestation by caesarean section received daily s.c. injections of vehicle (2-Hydroxypropyl-β-cyclodextrin) or progesterone (16 mg/kg) for 8 days until term-equivalent age (TEA). Term delivered controls (PND1) received vehicle. Neonatal condition/wellbeing was scored, and salivary progesterone was sampled over the postnatal period. Brain and plasma allopregnanolone concentrations were measured by radioimmunoassay; cortisol and progesterone concentrations were determined by enzyme immunoassay; and myelin basic protein (MBP), proteolipid protein (PLP), oligodendroctye transcription factor 2 (OLIG2) and platelet-derived growth factor receptor-α (PDGFRα) were quantified by immunohistochemistry and western blot. Brain allopregnanolone concentrations were increased in progesterone-treated neonates. Plasma progesterone and cortisol concentrations were elevated in progesterone-treated male neonates. Progesterone treatment decreased MBP and PLP in lobule X of the cerebellum and total cerebellar OLIG2 and PDGFRα in males but not females at TEA compared with term animals. We conclude that progesterone treatment increases brain allopregnanolone concentrations, but also increases cortisol levels in males, which may disrupt developmental processes. Consideration should be given to the use of non-metabolizable neurosteroid agonists.

Animal models of bronchopulmonary dysplasia. The preterm baboon models

Much of the progress in improved neonatal care, particularly management of underdeveloped preterm lungs, has been aided by investigations of multiple animal models, including the neonatal baboon (Papio species). In this article we highlight how the preterm baboon model at both 140 and 125 days gestation (term equivalent 185 days) has advanced our understanding and management of the immature human infant with neonatal lung disease. Not only is the 125-day baboon model extremely relevant to the condition of bronchopulmonary dysplasia but there are also critical neurodevelopmental and other end-organ pathological features associated with this model not fully discussed in this limited forum. We also describe efforts to incorporate perinatal infection into these preterm models, both fetal and neonatal, and particularly associated with Ureaplasma/Mycoplasma organisms. Efforts to rekindle the preterm primate model for future evaluations of therapies such as stem cell replacement, early lung recruitment interventions coupled with noninvasive surfactant and high-frequency nasal ventilation, and surfactant therapy coupled with antioxidant or anti-inflammatory medications, to name a few, should be undertaken.

Brain MRI measurements at a term-equivalent age and their relationship to neurodevelopmental outcomes

BACKGROUND AND PURPOSE

An increased prevalence of disabilities is being observed as more preterm infants survive. This study was conducted to evaluate correlations between brain MR imaging measurements taken at a term-equivalent age and neurodevelopmental outcome at 2 years' corrected age among very low-birth-weight infants.

MATERIALS AND METHODS

Of the various brain MR imaging measurements obtained at term-equivalent ages, reproducible measurements of the transcerebellar diameter and anteroposterior length of the corpus callosum on sagittal images were compared with neurodevelopmental outcomes evaluated by the Bayley Scales of Infant Development (II) at 2 years' corrected age (mean ± standard deviation, 16.1 ± 6.4 months of age).

RESULTS

Ninety infants were enrolled. The mean gestational age at birth was 27 weeks and the mean birth weight was 805.5 g. A short corpus callosal length was associated with a Mental Developmental Index <70 (P = .047) and high-risk or diagnosed cerebral palsy (P = .049). A small transcerebellar diameter was associated with a Psychomotor Developmental Index <70 (P = .003), Mental Developmental Index <70 (P = .004), and major neurologic disability (P = .006).

CONCLUSIONS

A small transcerebellar diameter and short corpus callosal length on brain MR imaging at a term-equivalent age are related to adverse neurodevelopmental outcomes at a corrected age of 2 years and could be a useful adjunctive tool for counseling parents about future developmental outcomes.

Hedgehog rushes to the rescue of the developing cerebellum

Premature birth often is associated with neurodevelopmental disabilities. In this issue of Science Translational Medicine, a pair of papers investigate--in patients and in a reliable animal model--the effects of glucocorticoids on cerebellar development after premature birth.

[Cerebellar injury in premature infants less than 30 weeks of gestation]

Traditionally, the cerebellum has been regarded as a central component of the motor system. Recent studies suggest an important role played by the cerebellum in the development of cognitive and social functions. The objective of this study was to evaluate the incidence of cerebellar injury and to define the obstetrical, neonatal, and radiologic characteristics, as well as the functional outcomes in a population of very preterm infants.

METHODS

This retrospective study included neonates born before 30 weeks of gestational age between March 2004 and July 2007. Infants underwent MRI studies at a term-adjusted age; for each preterm infant with cerebellar injury, we identified two infants for the control group with normal MRI, matched on the basis of gestational age. We collected pertinent demographic, prenatal, and acute postnatal data for all infants. Follow-up assessment was performed at 2 years, using the Brunet-Lezine scale.

RESULTS

A total of 148 ex-preterm infants were studied. Cerebellar injury was present in 14 (9 %) cases and associated with supratentorial parenchymal injury in 90 %. Duration of ventilation was longer in children with cerebellar injury, compared to controls (19.5 days vs 16.5 days; P=0.03). The other neonatal criteria analyzed were comparable between the two groups. Global developmental, functional, and social-behavioral deficits were more common and profound in preterm infants with cerebellar injury, with no significant difference.

CONCLUSION

This study confirms the high incidence of cerebellar injury in very preterm infants and the importance of a specific neurobehavioral follow-up.

The effects of postnatal estrogen therapy on brain development in preterm baboons

OBJECTIVE

Estrogen receptors are present within the fetal brain, suggesting that estrogens may exert an influence on cerebral development. Loss of placentally derived estrogen in preterm birth may impair development.

STUDY DESIGN

Baboons were delivered at 125 days of gestation (term approximately 185 days), randomly allocated to receive estradiol (n = 10) or placebo (n = 8), and ventilated for 14 days. Brains were assessed for developmental and neuropathological parameters.

RESULTS

Body and brain weights were not different between groups, but the brain/body weight ratio was increased (P < .05) in estradiol-treated animals. There were no differences (P > .05) between groups in any neuropathological measure in either the forebrain or cerebellum. There were no intraventricular hemorrhages; 1 estradiol animal displayed ectactic vessels in the subarachnoid space.

CONCLUSION

Brief postnatal estradiol administration to primates does not pose an increased risk of injury or impaired brain development.

Ibuprofen treatment for closure of patent ductus arteriosus is not associated with increased risk of neuropathology

Ibuprofen is an effective pharmacological intervention for closure of a patent ductus arteriosus (PDA) in preterm infants and is an alternative to surgical ligation; however, it is not certain whether ibuprofen treatment is associated with adverse effects on the brain. Therefore, this study examined neuropathological outcomes of ibuprofen therapy for a PDA. Fetal baboons were delivered at 125 d of gestation (dg; term ∼185 dg) by caesarean section, given surfactant, and ventilated for 14 d with positive pressure ventilation (PPV). Baboons were randomly allocated to receive either ibuprofen (PPV+ ibuprofen, n = 8) or no therapy (PPV, n = 5). Animals were killed on day 14 and brains assessed for cerebral growth, development, and neuropathology. Body and brain weights, the total volume of the brain, and the surface folding index (measure of brain growth) were not different (p > 0.05) between PPV+ ibuprofen-treated and PPV animals. There was no difference (p > 0.05) in the number of myelin basic protein-immunoreactive (IR) oligodendrocytes, glial fibrillary acid protein-IR astrocytes, or Iba1-IR macrophages/microglia in the forebrain. No overt cerebellar alterations were observed in either group. Ibuprofen treatment for PDA closure in the preterm baboon neonate is not associated with any increased risk of neuropathology or alterations to brain growth and development.

Neuronal damage in the preterm baboon: impact of the mode of ventilatory support

We evaluated the impact of randomized ventilatory strategies on specific neuronal populations of the cerebral cortex of preterm baboons. In the first series, baboons (n = 5) were delivered at 125 days of gestation (dg; term, 185 days) and exposed to 14 days of positive pressure ventilation (PPV) and compared with 140 dg controls (n = 6). In the second series, baboons were delivered at 125 dg and ventilated by either i) PPV for 1 day, followed by 27 days of nasal continuous positive airway pressure (early [EnCPAP]; n = 6) or ii) PPV for 5 days, followed by 23 days of CPAP (delayed [DnCPAP]; n = 4). Gestational controls were delivered at 153 dg (n = 3). The density of immunoreactive neurons for calretinin and somatostatin was assessed in the primary and secondary visual cortices, cingulate and parietal cortices, and subiculum in paraffin sections. Compared with gestational controls, PPV for 14 days resulted in a reduction in the density of calretinin-positive cells in the visual cortex (Areas 17 and 18) but not in the other cortical areas. No effect of PPV was observed on somatostatin-positive cells. DnCPAP, but not EnCPAP, was associated with a reduction in the density of calretinin and somatostatin-positive cells in the visual cortical areas but not in the other cortical areas compared with gestational controls. Taken together, these data demonstrate that ventilatory strategies involving greater than 5 days of PPV have a regionally selective impact on cortical neuronal subpopulations within the visual area but not in areas of association cortex in a nonhuman primate model of prematurity.

Erythropoietin ameliorates damage to the placenta and fetal liver induced by exposure to lipopolysaccharide

Intrauterine infection and inflammation have been causally linked to preterm birth and fetal brain injury. Using an ovine model of endotoxin-induced brain injury we have recently shown that recombinant human erythropoietin (rhEPO) reduces brain injury and protects against damage to myelination in major myelinated axon tracts. Our present objective was to determine whether rhEPO is also protective of the placenta and the fetal liver, organs which could influence fetal well-being. At 107 +/- 1 days of gestational age (DGA) chronically catheterized fetal sheep were randomly assigned to receive, on 3 consecutive days, either: 1) an i.v. bolus dose of lipopolysaccharide (LPS; approximately 0.9 microg/kg; n = 8); 2) i.v. bolus dose of LPS, followed at 1 h by 5000 IU/kg of rhEPO (LPS + rhEPO, n = 8); 3) rhEPO (n = 3). Seven untreated fetuses served as controls (n = 7). The placenta and fetal liver were examined histologically at 116 +/- 1 DGA; a placental injury index was formulated comprising measures of placental area, apoptosis, tissue injury and the size of the intervillous space. In LPS-exposed fetuses this index was greater than in control or rhEPO alone fetuses (p < 0.02). Treatment of LPS-exposed fetuses with rhEPO resulted in a reduction in the index (p < 0.05) and in the extent of liver necrosis. We conclude that rhEPO offers protection to the placenta and fetal liver in the presence of acute inflammation.

High-frequency oscillatory ventilation is not associated with increased risk of neuropathology compared with positive pressure ventilation: a preterm primate model

High-frequency oscillatory ventilation (HFOV) may improve pulmonary outcome in very preterm infants, but the effects on the brain are largely unknown. We hypothesized that early prolonged HFOV compared with low volume positive pressure ventilation (LV-PPV) would not increase the risk of delayed brain growth or injury in a primate model of neonatal chronic lung disease. Baboons were delivered at 127 +/- 1 d gestation (dg; term approximately 185 dg), ventilated for 22-29 d with either LV-PPV (n = 6) or HFOV (n = 5). Gestational controls were delivered at 153 dg (n = 4). Brains were assessed using quantitative histology. Body, brain, and cerebellar weights were lower in both groups of prematurely delivered animals compared with controls; the brain to body weight ratio was higher in HFOV compared with LV-PPV, and the surface folding index was lower in the LV-PPV compared with controls. In both ventilated groups compared with controls, there was an increase in astrocytes and microglia and a decrease in oligodendrocytes (p < 0.05) in the forebrain and a decrease in cerebellar granule cell proliferation (p < 0.01); there was no difference between ventilated groups. LV-PPV and HFOV ventilation in prematurely delivered animals is associated with decreased brain growth and an increase in subtle neuropathologies; HFOV may minimize adverse effects on brain growth.