Self-Regulation of Cerebral Metabolism and Its Neuroprotective Effect After Hypoxic-Ischemic Injury: Evidence From 1H-MRS

The dynamic transcriptomic response of the goldfish brain under chronic hypoxia

Oxygen is essential to fuel aerobic metabolism. Some species evolved mechanisms to tolerate periods of severe hypoxia and even anoxia in their environment. Among them, goldfish (Carassius auratus) are unique, in that they do not enter a comatose state under severely hypoxic conditions. There is thus significant interest in the field of comparative physiology to uncover the mechanistic basis underlying hypoxia tolerance in goldfish, with a particular focus on the brain. Taking advantage of the recently published and annotated goldfish genome, we profile the transcriptomic response of the goldfish brain under normoxic (21 kPa oxygen saturation) and, following gradual reduction, constant hypoxic conditions after 1 and 4 weeks (2.1 kPa oxygen saturation). In addition to analyzing differentially expressed protein-coding genes and enriched pathways, we also profile differentially expressed microRNAs (miRs). Using in silico approaches, we identify possible miR-mRNA relationships. Differentially expressed transcripts compared to normoxia were either common to both timepoints of hypoxia exposure (n = 174 mRNAs; n = 6 miRs), or exclusive to 1-week (n = 441 mRNAs; n = 23 miRs) or 4-week hypoxia exposure (n = 491 mRNAs; n = 34 miRs). Under chronic hypoxia, an increasing number of transcripts, including those of paralogous genes, was downregulated over time, suggesting a decrease in transcription. GO-terms related to the vascular system, oxidative stress, stress signalling, oxidoreductase activity, nucleotide- and intermediary metabolism, and mRNA posttranscriptional regulation were found to be enriched under chronic hypoxia. Known 'hypoxamiRs', such as miR-210-3p/5p, and miRs such as miR-29b-3p likely contribute to posttranscriptional regulation of these pathways under chronic hypoxia in the goldfish brain.

The Notch pathway regulates autophagy after hypoxic-ischemic injury and affects synaptic plasticity

Following neonatal hypoxic-ischemia (HI) injury, it is crucial factor to reconstruct neural circuit and maintain neural network homeostasis for neurological recovery. A dynamic balance between the synthesis and degradation of synaptic protein is required for maintaining synaptic plasticity. Protein degradation is facilitated by autophagy. This study aimed to investigate the regulation of synaptic structural plasticity by the Notch pathway, by assessing changes in Notch pathway activation and their effects on synaptic proteins and autophagy after HI injury. The study involved 48 male newborn Yorkshire piglets, each weighing 1.0-1.5 kg and 3 days old. They were randomly assigned to two groups: the HI group and the Notch pathway inhibitor + HI group (n = 24 per group). Each group was further divided into six subgroups according to HI duration (n = 4 per group): a control subgroup, and 0-6, 6-12, 12-24, 24-48, and 48-72 h subgroups. The expression of Notch pathway-related proteins, including Notch1, Hes1, and Notch intracellular domains, increased following HI injury. The expression of autophagy proteins increased at 0-6 h and 6-12 h post-HI. The expression of synaptic proteins, such as postsynaptic density protein 95 (PSD95) and synaptophysin, increased 6-12 h and 12-24 h after HI, respectively. Notably, the increased expression of these proteins was reversed by a Notch pathway inhibitor. Transmission electron microscopy revealed the presence of autophagosome structures in synapses. These findings shed light on the underlying mechanisms of neurological recovery after HI injury and may provide insights into potential therapeutic targets for promoting neural circuit reconstruction and maintaining neural network homeostasis.

N-Acetyl-Aspartyl-Glutamate in Brain Health and Disease

N-acetyl-aspartyl-glutamate (NAAG) is the most abundant dipeptide in the brain, where it acts as a neuromodulator of glutamatergic synapses by activating presynaptic metabotropic glutamate receptor 3 (mGluR3). Recent data suggest that NAAG is selectively localized to postsynaptic dendrites in glutamatergic synapses and that it works as a retrograde neurotransmitter. NAAG is released in response to glutamate and provides the postsynaptic neuron with a feedback mechanisms to inhibit excessive glutamate signaling. A key regulator of synaptically available NAAG is rapid degradation by the extracellular enzyme glutamate carboxypeptidase II (GCPII). Increasing endogenous NAAG—for instance by inhibiting GCPII—is a promising treatment option for many brain disorders where glutamatergic excitotoxicity plays a role. The main effect of NAAG occurs through increased mGluR3 activation and thereby reduced glutamate release. In the present review, we summarize the transmitter role of NAAG and discuss the involvement of NAAG in normal brain physiology. We further present the suggested roles of NAAG in various neurological and psychiatric diseases and discuss the therapeutic potential of strategies aiming to enhance NAAG levels.

The Potential Relationship Between HIF-1α and Amino Acid Metabolism After Hypoxic Ischemia and Dual Effects on Neurons

Hypoxia inducible factor (HIF) is one of the major transcription factors through which cells and tissues adapt to hypoxic-ischemic injury. However, the specific mechanism by which HIF regulates amino acid metabolism and its effect on neurons during hypoxic ischemia (HI) have remained unclear. This study analyzed the changes in cerebral metabolism of amino acids after HI by using ¹H-MRS and investigated the relationship between the changes in cerebral metabolism of amino acids and HIF-1α as well as the potential effects on neurons. Newborn pigs were used as an HI model in this study. Twenty-eight newborn Yorkshire pigs (male, 1.0-1.5 kg) aged 3-5 days were selected and randomly divided into experimental groups tested at 0-2 h (n = 4), 2-6 h (n = 4), 6-12 h (n = 4), 12-24 h (n = 4), 24-48 h (n = 4), and 48-72 h (n = 4) after HI, and a control group (n = 4). After the modeling was completed, ¹H-MRS imaging was conducted, followed by immunohistochemical staining of HIF-1α, NeuN, and doublecortin (DCX), and immunofluorescence of glutamic oxaloacetic transaminase (GOT)-1, GOT2, glutathione synthase (GS), glutamate-cysteine ligase catalytic subunit (GCLC), and glutamate-cysteine ligase modifier subunit (GCLM) in brain tissues. The expression of HIF-1α exhibited two increases after HI injury. The first time was opposite to the trends of change of GOT2, aspartic acid, and the number of neurons, while the second was consistent with these trends, suggesting that HIF-1α may have a two-way induction effect on neurons by regulating GOT2 after HI. HIF-1α was closely related to GCLM expression, and GSH level was correlated with the number of hippocampal neurons, indicating that HIF-1α may regulate GCLM to promote GSH synthesis and additionally play a neuroprotective role.