It appears that this study focused on identifying differentially expressed lncRNAs and mRNAs in the plasma exosomes of adolescents with MDD and HC. We used bioinformatic analysis to identify miRNAs that were matched with the differentially expressed lncRNAs, and then constructed lncRNA-miRNA-mRNA regulatory networks to identify candidate genes. These genes were then validated in a separate validation set. The results showed that the AC156455.1-miR126-AAK1 and CCDC18AS1-miR6835-CCND2 regulatory axes were differentially expressed between the MDD and HC groups, or before and after treatment in the MDD group. These genes were also able to distinguish the MDD group from the HC group, though with poorer ability than a combined indicator. The levels of AAK1, CCDC18AS1, and miR6835-5p expression were also different between the response and nonresponse groups. Furthermore, the CCDC18-AS1, miR-6835-5p, and CCND2 expression levels at baseline were able to predict the therapeutic effect of sertraline, possibly mediated through changes in suicidal ideation and cognitive function. Overall, these findings suggest that these genes may play a role in the development and treatment of MDD.
Adolescence is a special period of whole life with the transformation of brain development, sleep patterns, mental health, nutrition, education and schooling, parenting programmes for parents(39). Some researcher considered adolescence is a necessary and typical time of emotional disequilibrium and unpredictable behavior(40). The neurocognitive maturation continuing past 20 years of age, thus the common definition of adolescence of 10 ~ 18 years is overly restricted, 10 ~ 24 years are supposed to better fit with the development of adolescents(41). Therefore, we set the inclusion criteria of 10 ~ 23 years in this study may provide a more comprehensive understanding of the issues being studied.
Exosomes are small vesicles that are released by cells under both normal and pathological conditions, they carry nucleic acids and proteins that can provide information about the health of the cells they come from(42). Exosomal lncRNAs and miRNAs can be stably detected in circulating plasma and serum, and are not affected by RNase-dependent degradation(43, 44). Exosomes can also be easily isolated from various biofluids, such as plasma, serum, and urine(45). These makes them great candidates for biomarker discovery in clinical diagnostics. Previous research has shown that serum exosome-derived miR-139-5p could be a potential diagnostic biomarker for major depressive disorder (MDD), and that serum exosomal let-7e, miR-145, and miR-146a can predict antidepressant response. Overall, exosomes have great potential for the diagnosis and treatment of MDD(46, 47).
Starbase is a database that is commonly used to decode miRNA-lncRNA, miRNA-mRNA, miRNA-ceRNA, miRNA-circRNA, miRNA-pseudogene, and protein-RNA interaction networks from CLIP-Seq data(48). This database focus on tumor-related non-coding RNAs, there is a lack of databases related to neuropsychiatric disorders. But starbase v.20 includes approximately 10,000 miRNA-lncRNA interactions, which can provide comprehensive matched-miRNAs for differentially expressed lncRNAs(48). The target mRNAs of these matched-miRNAs were predicted using the datasets from miRDB and TargetScan, and only those that were also differentially expressed in the microarray analysis were selected. miRDB uses machine learning methods to predict miRNA targets by analyzing thousands of miRNA-target interactions from high-throughput sequencing experiments(49). TargetScan, on the other hand, predicts miRNA targets by searching for the presence of conserved 8mer, 7mer, and 6mer sites that match the seed region of each miRNA(50). By using both databases with different prediction mechanisms, the researchers were able to screen for target mRNAs in a more reliable and comprehensive way.
AC156455.1 is an m6A and immune-related lncRNA and can be diagnostic biomarker of colorectal cancer, even predicting prognosis(51). There have no published studies associated with neurological or psychiatric disorders. The function of miR-126 are mainly associated with angiogenesis, erythropoiesis, endothelial/leukocyte interaction, inflammation and immune response(52). Recently study indicated that miR-126 was downregulated in treatment-resistant depressed patients after 8-week psychotherapy (53). Another study also displayed that plasma exosomal miR-126 was downregulated in bipolar disorder patients compared to HCs(54). Adaptor-associated kinase 1 gene was abbreviated to AAK1. A twin study of genome-wide profiling of DNA methylome and transcriptome in peripheral blood monocytes for MDD showed that AAK1 was one of the significant differentially methylated regions associated with lifetime history of MDD(55). The AAK1 gene is contributed to neurotransmitter release and recycling of synaptic vesicle proteins by involving in intracellular vesicle trafficking(56). The differential expressed AAK1 protein were detected in hippocampus of CUMS mice(57). Nowadays, AAK1 inhibitors were widely researched for the treatment of diverse neurological and psychiatric disorders, such as schizophrenia, bipolar disorder, Parkinson's disease and neuropathic pain(58). In our study, it is the first time to detected AC156455.1, miR-126-5p and AAK1 in plasma exosome of adolescents with MDD, and the expression trend of them was consistent with previous studies.
The previous studies showed that CCDC18-AS1 was associated with immune microenvironment, and was positively correlated with activated memory CD4 T cell, activated myeloid dendritic cell, neutrophils, macrophage M1, and T follicular helper cells, while negatively correlated with T regulatory cell(59) (60). The miR-6835 could bind to Insulin-like growth factor 2 (IGF2) (61). IGF2 belongs to the insulin growth factor family, which can modulate the proliferation, survival, and differentiation of many types of neuronal systems, including cholinergic, dopaminergic, and serotonergic neurons(62). The level of IGF2 was reduced in the hippocampus of CUMS mice, and increasing the level of IGF2 can alleviate depression-like behaviors in CUMS mice(63). D-type cyclins (D1, D2, and D3), as the regulatory partners for cyclin-dependent kinases 4 and 6 (CDK4 and CDK6), are core components of the machinery that drives cell cycle progression(64). A previous study implied that CCND2 is linked to the etiology of schizophrenia(65). The CCND2 knocked-out mouse model displayed a pattern of hippocampal over-activity shared similarities with that seen in prodromal schizophrenia patients and novelty-induced hyperlocomotion (a rodent correlate of positive symptoms of schizophrenia), decreased sucrose preference and executive function, impaired working memory(66). The study of Ahmed, et al also illustrated that CCND2 knockout mice exhibited increased activity of dopaminergic neuronal in the ventral tegmental area, behavioral hyperresponsiveness to amphetamine, and impaired hippocampus-dependent cognition(67). Moreover, GSK3β could inhibit the expression of CCND2(68). Our previous studies have shown that GSK3β level was reduced in MDD patients(69). Based on this result, perhaps CCND2 may be elevated in MDD patients, consistent with our current results.
The top 5 pathway of CCND2 enriched was FoxO signaling pathway, Cell cycle, p53 signaling pathway, PI3K-Akt signaling pathway, and cellular senescence. MiR-6835 was associated with LPS-induced inflammation and resulted in the dysregulated expression of FOXO3a(70). FOXOs are core genes in the FOX signaling pathway. FOXOs may be regulated by serotonin or norepinephrine signaling, the hypothalamic-pituitary-adrenal (HPA) axis and brain-derived neurotrophic factor (BDNF), which are all associated with the occurrence and development of MDD(71). A previous study implied that activation of PI3K/Akt/FoxO3a and PKA/CREB signaling pathways are closely related to the pathogenesis of depression and may be downstream targets of fluoxetine(72).
Our study also has a few limitations. Firstly, the sample size of validation set was small, especially the HC group. Small sample sizes may be produced false negatives, for example, the P-value of the miR-126-5p expression levels between MDD group and HC group was 0.77. if validation could be carried out in a larger sample, more accurate results may be obtained. Secondly, the interaction among lncRNAs, miRNAs and mRNAs were predicted by databases, there have no experiments to validate. We will perform cell and animal experiments in the future to explore the practical regulatory relationship among them.