Our findings showed that the expression of miRNA-181a, miRNA-181b, miRNA-223, miRNA-137, miRNA-125b, and miRNA-107 was considerably upregulated in first-episode SCZ, indicating that these six miRNAs are linked to the early onset of schizophrenia. There are many previous findings in the literature that support the results of this study. MiR-137 is highly conserved in mammals. MiR-137 regulates the maturation of synapses and the formation of neural cells(25). A previous study found that miR-137 was upregulated in schizophrenia(37). Prior studies suggested an association between miRNAs and psychiatric symptoms. Song et al.(20) found significantly elevated plasma levels of miRNA-181b, miRNA-30e, miRNA-34a, and miRNA-7 in patients with first-episode SCZ, and the level of miRNA-181b declined with improvement of psychotic symptoms after six weeks of treatment with antipsychotics. Prior findings also indicated a positive correlation between the improvement of negative symptoms and a decrease level of miR-181b (which was previously elevated). The level of miR-219-2-3p was found significantly correlated with the severity of psychiatric symptoms in schizophrenia(45). The interaction of hsa-miR-219, CAKM2G, GRIN2B and GRIN3A polymorphisms may predispose Chinese to schizophrenia(46). MiR-107 has been found significantly upregulated in the prefrontal cortex of patients with SCZ(31). A previous study found that miR-181a was upregulated in brain post-mortem tissues of schizophrenia(47). Schizophrenia patients had considerably higher serum levels of miR-223. And in miR-223 overexpression cells, they discovered that the mRNA levels of INPP5B, RHOB, SKIL, and SYNE1 associated with the cytoskeleton or cell movement were drastically downregulated(24).
The hyperactive dopaminergic function related to SCZ may also be caused by dysregulated NMDA receptors neurotransmission(48). It has been hypothesized that the dysfunction of NMDA receptors is a contributing factor for SCZ(49, 50). All the seven miRNAs with aberrant expression were involved in the glutamate pathway. Through highly conserved binding to mouse, rat, and human 3 'UTR complementary mRNA sites, miR-223(51) can directly inhibit the expression of the AMPAR subunit GluR2 and the NMDAR subunit NR2B in the central nervous system, thereby specifically regulating neuronal excitability in response to glutamate. Produced by lymphocytes, miR-223 can cross the blood-brain barrier to affect the expression of genes in the brain. In patients with SCZ, the upregulation of miR-223 may lead to dysfunction of NMDA receptors and decreased synaptic activity, which might be related to the development of SCZ(52). Beveridge et al.(42) investigated the expression of miRNAs in the cerebral cortex of individuals with schizophrenia, which showed that miR-181b was overexpressed in the superior temporal gyrus. MiR-181b, which is preferentially expressed in lymphoid cells and is involved in the regulation of the inotropic glutamate receptor, is encoded by two genetic loci on chromosomes 1 and 9. The upregulation of miR-181b was associated with the downregulation of SCZ-related genes, including VSNL1 and GRIA2(53). According to recent studies, overexpression of miR-181a could significantly and negatively regulate the expression of GRIA2 and inhibit the GRIA2 protein activity(54). MiR-125b can also directly and specifically regulate the mRNA expression of glutamate NMDA receptor NR2A and affect the function of NMDA receptors (38). As the miRNAs inhibit the expression of glutamate receptors, the abnormally expressed miRNAs might be associated with hypofunction of NMDA receptors. MiR-107 inhibits the expression of GRIN3A and has been found significantly upregulated in the prefrontal cortex of patients with SCZ(31). In vivo, Increased miR-137 expression in vivo can lead to changes in the distribution of synaptic vesicles, nerve fiber damage, dementia, hippocampus-dependent learning, and memory deficits. The dysregulation of miR-137 may impair synaptic plasticity in the hippocampus(55). MiR-137 modulates the glutamatergic synaptic transmission, and its effect on glutamate signaling may lead to the pathogenesis of SCZ. A variety of glutamatergic receptors, including AMPA and the NMDA receptor subunits GluA1 and GluN2A, as well as a number of presynaptic targets involved in neurotransmitter release, are regulated by miR-137(56, 57). Thus, overexpression of miR-137 may impact synaptogenesis, pre-synaptic micro-structure, and function of synapsis, thereby affecting the density and function of synapses, which might also be a potential central disruption related to schizophrenia(25). Studies indicated that the disruption of NMDA receptor signaling by dizocilpine could reduce the level of miR-219 (a miRNA specifically in the brain) in the mouse prefrontal cortex(45). MiR-219(45) inhibits the expression of GRIN1. It was found that, in the hippocampus and amygdala of patients with epilepsy, the expression level of miR-219 was negatively associated with the expression of NMDA-NR1, suggesting that miR-219 might regulate NMDA activity in this patient population(35). Based on the above findings, it could be inferred that the abnormal elevation of the above miRNAs may affect the glutamatergic pathway by inhibiting the expression of NMDA receptors, which is associated with psychotic symptoms in SCZ.
Although the present study is not the first one to examine the expression of miRNA in PBMCs in SCZ, it has a broader scope and yielded findings that the level of circulating miRNAs might be associated with SCZ and other related syndromes. Although the cortical miRNA-related pathophysiology of SCZ still remains unclear, it is possible that the alteration of expression is present outside the brain, indicating that peripheral miRNAs might be biomarkers for SCZ. Furthermore, studies on peripheral miRNA suggested that miRNA dysregulation occurs in the early stage of SCZ. Our results indicated that some miRNAs could inhibit the expression of glutamate receptor subunits(45, 51, 56), affect the function of glutamate receptors, and result in the hypofunction of NMDA receptors. Despite the increasing number of studies, biomarkers of SCZ are still underexplored(58), with often inconsistent and even conflicting findings yielded in existing studies. There are also several limitations to this study, with the most important ones being the small sample size and lack of validation using human brain tissues; thus, the present findings are only preliminary and should be interpreted with caution.
There are some limitations to this study. This study was a cross-sectional study and did not examine the longitudinal study of indicator changes. The sample size of this experiment is insufficient. There is a gender imbalance and age difference in the study population of this experiment, which may have an impact on the results of the experiment. Although the majority of the participants in this experimental investigation were from Hunan Province, regional disparities may still exist. Given the similarity between neuronal cells and PBL in the mechanisms of receptor expression and transduction, it has been demonstrated that changes in miRNA expression in lymphocytes or leukocytes are correlated, but it has yet to be established whether blood RNA can be used in place of RNA in brain tissue.
In conclusion, the present study provides preliminary evidence that six miRNAs are implicated in the pathogenesis of SCZ and have significant diagnostic value for this mental disorder. These miRNAs regulate the glutamate signaling pathway, which is one of the most important mechanisms for the development of psychotic symptoms and one of the most prominent factors affecting the development of the cerebral cortex in people at a high risk of SCZ. Future studies may focus on miRNAs as epigenetic regulators of expression of genes related to mental disorders. Hopefully, microRNAs may serve as state biomarkers for the diagnosis and personalized management of SCZ.