We used an approach of meta-analysis and identified 1400 DEGs in MDD by integrating 461 independent samples from four microarray gene expression datasets; only 671 DEGs were included in all four datasets. It’s possible that there are potential interesting genes which haven’t been identified because they are not being measured in each previous study. Among the top 30 DEGs, 11 are not included in all datasets. A recent meta-analysis using GWAS data of 135,458 major depression cases and 344,901 controls identified 44 significantly associated genomic regions for MDD[41]. Only 5 of the genes in or near the 4 SNP were identified in the 1400 DEGs of MDD in our study, including ZNF445(chr_44287760_I), RSRC1(rs7430565), MLF1(rs7430565), LINC01231(rs1354115) and CCDC68(rs1833288). However, none of the GWAS genes were present in the 198 over-lapping DEGs of AD and MDD. It’s probably because the expression data from seven MDD cohorts used in the meta-analysis of GWAS study were all from blood but not brain tissue.
Compared to previous study of AD[31], 198 DEGs were over-lapping in prefrontal cortex of both MDD and AD, 109 DEGs are consistently upregulated or downregulated. This result was different from the study of human blood microRNA expression comparison[32]. In the study of microRNA expression, all seven common differentially expressed microRNAs were up-regulated in patients with MDD and down-regulated in patients with AD. When comparing the 198 DEGs and the 43 DEGs regulated by the 7 microRNAs based on the DIANA database, only 3 DEGs (ANKS1A, PKN2, PRKCQ) were shared in common. As for AD patients, ANKS1A and PKN2 were both upregulated in prefrontal cortex and blood, while PRKCQ was downregulated in prefrontal cortex but upregulated in blood. PRKCQ is involved in platelet activation[42]. Inyushin et al. revealed that Aβ peptide is massively released to the blood upon the activation/aggregation of platelets, so platelet hyper-activation has been conclusively shown to be an aspect of AD, which may explain the increasing of PRKCQ in blood sample of AD cases[43]. PRKCQ is also associated with axon guidance[44]. Zhang et al. proved that axon-guidance molecules play an important role in guiding growth cones to form synapses and are involved in the pathogenesis of AD by regulating the levels of Aβ and hyperphosphorylation of tau through various signaling pathways[45], which may explain the decreasing of PRKCQ in brain sample of AD cases.
However, these 3 DEGs were all upregulated in prefrontal cortex but downregulated in blood in MDD patients. To better understand MDD, various pathophysiological mechanisms have been proposed, including changes in monoaminergic neurotransmission, imbalance of excitatory and inhibitory signaling in the brain, hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis, and abnormalities in normal neurogenesis, which indicated the significant role of signal transduction in MDD [44]. PKN2[46] and PRKCQ[47] have been proved to be involved in the pathway of signal transduction. Moreover, PRKCQ is related to axon guidance[44] and ANKS1A is associated with neuron remodeling[48], which are specific for brain. So it maybe account for the different expression between brain and blood.
Another possible explanation is that human prefrontal cortex has different regions and expression pattern is different in different regions (including rostral prefrontal cortex, dorsolateral prefrontal cortex, ventrolateral prefrontal cortex, medial prefrontal cortex and orbitofrontal prefrontal cortex). Here we combined them together, so it’s likely that the 198 DEGs don’t show specific expression pattern (for instance all genes upregulated in AD and downregulated in MDD) in AD and MDD.
Functional enrichment analysis identified 67 GO terms and 209 KEGG terms associated to the 198 common DEGs in both AD and MDD. And the most significantly enriched GO term is protein binding (one of the molecular function) which may be corresponded with the highly enriched KEGG terms named Signal Transduction, Metabolism of proteins and Immune System. A recent study found that [49] the inflammasome becomes activated when the microglia cells sense the aggregated amyloid-β, and the activation of inflammasome and subsequent caspase 1 cleavage contribute to disease development and progression. It builds up a connection between Immune System and protein binding. Similarly, another study[50] indicated that systemic inflammation can activate microglial TLR4, NLRP3 inflammasome, and complement in the brain, leading to neuroinflammation, Aβ accumulation, synapse loss and neurodegeneration, and targeting the molecular mechanisms underlying the TLR-complement-NLRP3 inflammasome signaling pathways can be a preventive and therapeutic approach for AD. It combined the signal transduction, immune system and metabolism of proteins together. That’s to say, no matter MDD is the risk factor or prodrome of AD, MDD may affect Immune system and signal transduction like the studies implied above and contribute to aggregation of amyloid-β (One of the important molecular pathology of AD).
Our results show that the Reactome term named Rho GTPase cycle significantly enriched in AD and MDD. On the one hand, protein aggregation is a hallmark of diverse neurodegenerative diseases like AD, and a study has implied that signaling from the RHO GTPase and the ROCK1 and LIMK1 kinases controls cofilin-1 activity to remodel actin and modulate aggregate entry[51]. On the other hand, depression induces structural and functional synaptic plasticity in brain reward circuits, and Rho GTPase-related genes, which are known regulators of synaptic structure), revealed a sustained reduction in RAS-related C3 botulinum toxin substrate 1 (Rac1, one of Rho GTPase-related genes) expression in the nucleus accumbens after chronic social defeat stress[52]. These findings reveal great value of Rho GTPase in AD and MDD. The mechanical property of extracellular matrix and cell-supporting substrates is known to modulate neuronal growth, differentiation, extension and branching[53], and AD is associated with reduced brain tissue stiffness[54]. We can see that the term named Extracellular matrix organization is also upregulated in AD and MDD, which may suggest that MDD may change the hardness of local brain tissue and finally evolve into AD. Extracellular vesicles, specifically exosomes, have been demonstrated to participate in mediating inflammatory response[55] and Tau propagation[56] in brain. And Vesicle-mediated transport is downregulated in both two, which provides possible insights between AD and MDD in another aspects. In protein-protein interaction network, CDC42 is the most important one which is also proved to be participated in 9 of top 10 significantly enriched pathways. In previous study, CDC42 has been proven to involve in dendritic cell migration[57] and dendritic spine morphogenesis[58]. The dendritic spine could pave the way to the identification of novel biomarkers to monitor synaptic loss in AD[59]. Moreover, CDC42 is associated with ephrin receptor signaling pathway. Adam et al. found that the Eph/ephrin system has been implicated in pathological settings of Alzheimer's disease[60], and Vargas et al. suggested that the activation of the Ephrin-A4/c-Abl axis would explain the synaptic spine alterations found in AD[61]. These findings above indicate the important role of CDC42 in the process of AD.
A limitation of this study is that MDD is probably not a unitary disease, but rather a heterogeneous syndrome which means different MDD patients could have different neurophysiological changes[62], but the expression data of MDD we could acquire didn’t take this issue into account, so it may explain the relatively lack of connections among the 198 DEGs. In other words, different types of MDD may be associated with AD in different pathways. That’s why we’ve got some DEGs that are not closely related. And some potential connections between AD and MDD are stated above. What’s more, in this study we only analyzed gene expression data in prefrontal cortex, it’s necessary to include more areas of brain and compare the different expression patterns among different areas, which may help us understand this question in a more general view.
In conclusion, our meta-analysis is the first study to make connections between MDD and AD with the expression data of brain tissue. Our results add some new potential perspectives to the comprehensive neurobiological model between MDD and the development of AD, mainly including the “trilogy” (change of signal transduction, the binding of amyloid β, inflammatory response),the reducing stiffness of brain tissue, and Vesicle-mediated transport disorder.