Mechanisms of hypoxia-mediated memory impairment in an animal model of acute respiratory failure in preterm survivors

Summary Due to advances in neonatal care, most preterm infants no longer develop cerebral palsy from hypoxic-ischemic white matter injury (WMI). However, up to 50% develop gray matter associated cognitive and learning disabilities by school age that persist throughout life. These developmental disabilities are commonly associated with a systemic hypoxia (Hx) episode arising from lung immaturity, acute respiratory failure or other common complications of prematurity. We have developed a preterm-equivalent mouse model which delivers a single mild clinically relevant episode of Hx at P2. Our preliminary findings support that Hx impairs learning and memory by disrupting hippocampal: (1) neuronal structural complexity; (2) gene expression; (3) synaptic plasticity/intrinsic excitability; and (4) fear-conditioning responses in juvenile mice exposed to Hx. We hypothesize that neonatal Hx is sufficient to persistently disrupt neuronal maturation without causing significant neuronal loss or white matter injury (WMI). In aim 1, we will determine if Hx disrupts hippocampal maturation independent of acute or delayed cerebral inflammation, neuronal degeneration or WMI. We will determine how Hx disrupts CA1 neuronal dendritic arbor maturation and spine density using innovative super-resolution light microscopy integrated with analysis of synaptic transmission, long-term potentiation (LTP) and intrinsic excitability. We will determine if these changes occur independently of hippocampal neuronal or glial cell death. Since WMI is common in preterm infants, we will determine if CA1 neuronal dysmaturation occurs independently of WMI. Aim 2 will build upon preliminary data showing that Hx causes persistent gene transcriptional changes in mouse hippocampus at P16 and P30. The hippocampal response to Hx involved regulators of excitatory and inhibitory synaptic transmission, synaptic plasticity and epigenetic regulators, all integral to learning and memory. To define novel molecular mechanisms of hippocampal dysmaturation, we will take an unbiased approach using single nucleus RNAseq to determine cell type-specific early and late gene expression changes arising from Hx at P2. In aim 3, we hypothesize that disturbances in synaptic transmission at CA3-CA1 synapses disrupt hippocampus-dependent memory mechanisms in young adult mice exposed to Hx as neonates at P2. We will determine the effects of Hx on excitatory and inhibitory synaptic activity and on the subunit composition of glutamate receptors that regulate learning and memory in neonates. We have identified that Hx disrupts the action of several key modulators of excitatory synaptic activity. We will focus on the synaptic potassium channel SK2, which acts as a negative feedback regulator to limit synaptic depolarization by NMDAR and AMPAR. To further define a mechanistic role for glutamatergic synaptic activity in disrupted LTP, we will determine if an allosteric AMPA receptor agonist (ampakine) delivered in vivo will strengthen synaptic transmission/LTP in vitro. Neurobehavioral testing using hippocampal memory paradigms will facilitate our long-term objective to develop rational therapies to prevent or reverse the chronic effects of Hx on maturation of hippocampal learning and memory mechanisms.