Transient Hypoxemia Chronically Disrupts Maturation of Preterm Fetal Ovine Subplate Neuron Arborization and Activity.

Preterm infants are at risk for a broad spectrum of neurobehavioral disabilities associated with diffuse disturbances in cortical growth and development. During brain development, subplate neurons (SPNs) are a largely transient population that serves a critical role to establish functional cortical circuits. By dynamically integrating into developing cortical circuits, they assist in consolidation of intracortical and extracortical circuits. Although SPNs reside in close proximity to cerebral white matter, which is particularly vulnerable to oxidative stress, the susceptibility of SPNs remains controversial. We determined SPN responses to two common insults to the preterm brain: hypoxia-ischemia and hypoxia. We used a preterm fetal sheep model using both sexes that reproduces the spectrum of human cerebral injury and abnormal cortical growth. Unlike oligodendrocyte progenitors, SPNs displayed pronounced resistance to early or delayed cell death from hypoxia or hypoxia-ischemia. We thus explored an alternative hypothesis that these insults alter the maturational trajectory of SPNs. We used DiOlistic labeling to visualize the dendrites of SPNs selectively labeled for complexin-3. SPNs displayed reduced basal dendritic arbor complexity that was accompanied by chronic disturbances in SPN excitability and synaptic activity. SPN dysmaturation was significantly associated with the level of fetal hypoxemia and metabolic stress. Hence, despite the resistance of SPNs to insults that trigger white matter injury, transient hypoxemia disrupted SPN arborization and functional maturation during a critical window in cortical development. Strategies directed at limiting the duration or severity of hypoxemia during brain development may mitigate disturbances in cerebral growth and maturation related to SPN dysmaturation.SIGNIFICANCE STATEMENT The human preterm brain commonly sustains blood flow and oxygenation disturbances that impair cerebral cortex growth and cause life-long cognitive and learning disabilities. We investigated the fate of subplate neurons (SPNs), which are a master regulator of brain development that plays critical roles in establishing cortical connections to other brain regions. We used a preterm fetal sheep model that reproduces key features of brain injury in human preterm survivors. We analyzed the responses of fetal SPNs to transient disturbances in fetal oxygenation. We discovered that SPNs are surprisingly resistant to cell death from low oxygen states but acquire chronic structural and functional changes that suggest new strategies to prevent learning problems in children and adults that survive preterm birth.

Prenatal cerebral ischemia triggers dysmaturation of caudate projection neurons.

OBJECTIVE: Recently, we reported that the neocortex displays impaired growth after transient cerebral hypoxia-ischemia (HI) at preterm gestation that is unrelated to neuronal death but is associated with decreased dendritic arbor complexity of cortical projection neurons. We hypothesized that these morphological changes constituted part of a more widespread neuronal dysmaturation response to HI in the caudate nucleus (CN), which contributes to motor and cognitive disability in preterm survivors. METHODS: Ex vivo magnetic resonance imaging (MRI), immunohistochemistry, and Golgi staining defined CN growth, cell death, proliferation, and dendritic maturation in preterm fetal sheep 4 weeks after HI. Patch-clamp recording was used to analyze glutamatergic synaptic currents in CN neurons. RESULTS: MRI-defined growth of the CN was reduced after ischemia compared to controls. However, no significant acute or delayed neuronal death was seen in the CN or white matter. Nor was there significant loss of calbindin-positive medium spiny projection neurons (MSNs) or CN interneurons expressing somatostatin, calretinin, parvalbumin, or tyrosine hydroxylase. Morphologically, ischemic MSNs showed a markedly immature dendritic arbor, with fewer dendritic branches, nodes, endings, and spines. The magnitude and kinetics of synaptic currents, and the relative contribution of glutamate receptor subtypes in the CN were significantly altered. INTERPRETATION: The marked MSN dendritic and functional abnormalities after preterm cerebral HI, despite the marked resistance of immature CN neurons to cell death, are consistent with widespread susceptibility of projection neurons to HI-induced dysmaturation. These global disturbances in dendritic maturation and glutamatergic synaptic transmission suggest a new mechanism for long-term motor and behavioral disabilities in preterm survivors via widespread disruption of neuronal connectivity.