Neonatal hypoxic-ischemia (HI), also known as hypoxic-ischemic encephalopathy (HIE),is a major public health problem with a high morbidity and mortality rate[
17]. In the hours after glucose-oxygen deprivation, astrocyte gene expression is altered in response to hypoxia, neuronal cell death, neurotransmitter release, and cytotoxic substance production. Astrocyte activation and transition to type A2 in a transient middle cerebral artery occlusion stroke (tMACO) model, exerting neurotrophic or anti-inflammatory effects[
18]. A growing number of studies in recent years have demonstrated that non-coding RNAs play an important role in molecular changes and phenotypic shifts in reactive astrocytes.
2.1. MiRNAs
Astrocyte swelling is one of the earliest responses to cerebral hypoxia-ischemia, which is regulated by water channel protein aquaporin 4 (AQP4). AQP4 is highly expressed in astrocytes and is enriched in astrocyte end-feet and gathered at cell synapses and edges[
19]. It mediates water exchange through the blood-brain barrier (BBB), and controls cell volume, extracellular space volume, and astrocyte migration[
20]. It was found that cellular volume during neonatal HI in AQP4−/− astrocytes was significantly less than that in AQP4+/+ astrocytes, and demonstrated that modulation of astrocyte AQP4 at different stages of HIE (HI and reoxygenation) may be beneficial for the treatment[
21]. MiR-320a directly targets AQP4[
22]. In a neonatal mouse model of epilepsy (intraperitoneal injection of lithium chloride-pilocarpine), brain miR-320a expression levels were decreased and negatively correlated with AQP4 expression[
23]. Anti-miR-320a administration benefits ischemic rats and reduces cerebral infarct volume[
22]. In cerebral ischemia/reperfusion injury (I/RI), miR-130b regulates AQP4 expression at the post-transcriptional level and produces neuroprotection[
24]. Additionally, miR-145 protects astrocytes from post-OGD injury by inhibiting AQP4 expression[
25]. With the established belief that AQP4 inhibitors may be a therapeutic option in the absence of cytotoxic edema after HIE, targeting relevant miRNAs provides possible new directions for drug development.
AQP4 co-localizes extensively with glutamate transporter 1 (GLT-1) and has a regulatory role for GLT-1 in astrocytes[
26]. One of the basic functions of astrocytes is to encapsulate and take up most synaptically released glutamate, which optimizes neuronal function and prevents glutamate excitotoxicity[
27]. The major transporters that function include the glutamate-aspartate transporter (GLAST, human homologs EAAT1) and glutamate transporter-1 (GLT-1, human homologs EAAT2)[
28]. Interestingly, glutamate transport from neurons to astrocytes is inefficient in the neonatal brain, which may be related to the low expression of astrocytic glutamate transporters[
29]. It has also demonstrated that GLAST predominates in the neonatal neocortex and hippocampus[
30], which could explain the particular vulnerability to excitotoxicity of the neonatal brain[
31]. Upregulation of astrocyte GLT-1 could be a potential therapeutic target for HIE. MiR-124 expression is significantly downregulated in a neonatal HIE model and an in vitro OGD model. It was found that overexpression of miR-124 was neuroprotective and anti-apoptotic and ameliorated long-term neurological deficits[
32]. It also regulates GLT-1 expression in astrocytes after OGD/reperfusion via Akt and mTOR pathways[
33]. In addition, of the neurologic disorders that cause brain injury in adults, miR-218 is abundantly expressed in motor neurons, and in studies of ALS, it was found that motor neuron-derived miR-218 can be taken up by peripheral astrocytes and is sufficient to down-regulate EAAT2 in astrocytes and affect glutamate uptake[
34]. Interestingly, miR-218-5p promotes endovascular trophoblast differentiation and spiral artery remodeling and is down-regulated in preeclamptic (PE) placentas[
35], which are associated with preterm labor, neonatal asphyxia and even neonatal death[
36]. Targeting the specific mechanisms of miR-218 and its role in the placenta may increase mechanistic understanding of miR-218 within neonatal encephalopathy.
During cerebral ischemia, astrocytes undergo autophagy, and attempts to cope with stress in this way may protect astrocytes from apoptosis in response to ischemic injury[
37]. Autophagy is a lysosomal degradation pathway in which the cell's own cytoplasmic proteins or organelles are phagocytosed and encapsulated into vesicles, which fuse with lysosomes and degrade their contents[
38]. Melatonin administration attenuates brain damage in neonatal HI rats by inhibiting cellular autophagy and decreasing apoptosis, obtaining a neuroprotective effect[
39]. The microRNA-30 family consists of six different miRNAs, including miR-30a, miR-30b, miR-30c-1, miR-30c-2, miR-30d, and miR-30e, which are important regulators of tissue and organ development as well as the pathogenesis of related diseases[
40,
41]. In related studies, miR-30d inhibited the expression of Beclin1 in rat primary astrocytes[
42,
43]. Beclin1 is closely associated with autophagosome formation and fusion of autophagosomes, and its interaction with Bcl-2 family proteins mediates cellular autophagy and apoptosis[
44]. Thus miR-30d can promote apoptosis by inhibiting OGD-induced autophagy in astrocytes[
43]. What’s more, miR-30d-5p was found to be involved in neuronal apoptosis and autophagy in neonatal HI rats and may also potentially target brain-derived neurotrophic factor (BDNF)[
45]. Targeted modulation of the miR-30 family in neonatal encephalopathy could serve as a potential therapeutic strategy. In addition, astrocytes undergo pyroptosis during HIBD. It is found that miR-148-3p inhibits pyroptosis of rat hippocampal astrocytes by blocking STAT/JMJD3 axis, and dexmedetomidine (DEX), a potent α
2-adrenoceptor agonist, attenuates brain injury in neonatal rats through upregulation of miR-148a-3p[
46].
As one of the brain-specific microRNAs, miR-134 promotes the proliferation of neural precursor cells and embryonic neuronal maturation in neuronal development[
47]. miR-134 localizes to the synaptic-dendritic spacer and regulates excitatory synaptic propagation at postsynaptic sites in rat hippocampal neurons thereby modulating synaptic development, maturation, and/or plasticity[
48]. Overexpression of miR-134 negatively affects and impairs neuronal formation in the postnatal mouse brain[
49]. The study demonstrates that miR-134-5 expression is increased and mediates hypoxic-ischemic brain injury and neuronal death via the C/EBPα/miR-134-5p/KPNA3 axis in pheochromocytoma (PC12) cells[
50] and inhibition of NLRX1 expression[
51] in a neonatal rat model of HIE. However, overexpression of miR-134 did not affect astrocyte survival but decreased extracellular glutamate concentration and promoted astrocyte maturation by increasing GLT-1[
52]. This may be involved in the future synergistic treatment of neonatal HIE.
2.2. LncRNAs and circRNAs
Metastasis-associated lung adenocarcinoma transcript 1 (MALAT1) is a long noncoding RNA that was initially identified to be associated with non-small cell lung cancer as well as lung cancer metastasis[
53]. Recent studies have revealed that lncRNA MALAT1 is also involved in the regulation of endothelial cell function and vascular growth[
54]. In vivo studies confirmed that lncRNA MALAT1 is significantly increased under hypoxia and controls phenotypic switch in endothelial cells. Genetic ablation of lncRNA MALAT1 inhibited endothelial cell proliferation and reduced neonatal retinal vascularization[
55]. Of attention is the fact that lncRNA MALAT1 also exerts its regulatory function in astrocytes. It was demonstrated that lncRNA MALAT1 affects AQP4 expression in astrocytes by competitively binding miR-145, increases the number of apoptotic cells and infarct size, and promotes cerebral ischemia-reperfusion injury[
56]. Knockdown or silencing of lncRNA MALAT1 protects neonatal bronchopulmonary dysplasia (BPD) and ischemic cardiomyopathy[
57,
58]. Correspondingly, the role of lncRNA MALAT1 in ischemic stroke provides a new therapeutic strategy for neonatal ischemic encephalopathy.
LncRNA H19 (H19) was originally identified as a fetal transcript, which is widely expressed in mouse fetal liver, intestine, cardiac muscle, and skeletal muscle[
59]. At present, this transcript has been recognized as an oncogenic lncRNA, which is involved in human cancer development through different mechanisms[
60]. H19 was upregulated in HIE newborns and rat models. It was proved that H19 alleviated nerve damage of neonatal HIE rats by upregulating the Akt3/mTOR pathway as a miR-29b sponge[
61]. It is confirmed that H19 is transported from neurons to astrocytes by exosomes to induce downregulation of insulin-like growth factor-1 (IGF-1) through the H19/let-7a/IGF-1 receptor axis[
62]. A further study elucidated that H19 could sponge miR-130a-3p and competitively regulate the downstream target DAPK1, which in turn worsen brain injury in neonatal HIE[
63]. Inhibition of H19 may benefit neonatal brain injury, but further studies are needed regarding the specific role played by H19 in astrocytes after neonatal HI.
A study comparing three-month-old and eight-year-old pigs by cerebral cortex transcriptomic analysis found that circRNA expression in the brain depended on developmental age, while >80% of mRNAs, miRNAs, and lncRNAs but <20% of circRNAs were expressed at both ages[
64]. Interestingly, circRNAs can determine developmental and aging processes by altering the expression of specific mRNAs[
65]. The study identified an altered expression pattern of circRNAs in neonatal hypoxic-ischemic brain damage (HIBD) rats by high-throughput sequencing analysis and predicted circRNA/ miRNA interactions[
66], further demonstrating that dysregulated circRNAs mechanistically behave similarly to those in adults during the physiopathological process of HIBD.
Additionally, circRNAs play a role in astrocytes in adult stroke, for example: circRNA HECTD1 (circHectd1) expression level is significantly increased in tMCAO mice model. Mechanistically, circHectd1 can function as an endogenous miR-142 sponge to inhibit the activity of miR-142/TIPARP axis, which in turn inhibits astrocyte activation by promoting cell autophagy[
67]. Furthermore, circ_0025984 acts as a sponge for miR-143-3p to counteract its effects on autophagy and apoptosis in astrocytes under ischemic conditions to achieve protection of astrocytes[
68]. (
Table 1)