The mTOR signaling pathway exists in almost all the peripheral tissues and the central nervous system, and is involved in regulating protein synthesis, mitochondrial biogenesis, cell proliferation, cell survival, cell death [20, 21], and synaptic plasticity [12]. The abnormality of the mTOR pathway has attracted more and more attention, and has been found in many diseases, including cancer [22], obesity [23], type II diabetes mellitus [24], neurological and psychiatric diseases [25], neurodegeneration, and brain tumors. Recent studies have shown that many psychiatric drugs, including mood stabilizers and neurorelaxants, which are also autophagy-inducible factors, can regulate autophagy and play a therapeutic role in the mTOR pathway [26, 27].
To date, there have been relatively few studies on the relationship between expression levels of the mTOR pathway and the pathogenesis of schizophrenia. They have mostly been pre-clinical studies, with few studies involving schizophrenia patients. In this study, acute schizophrenic patients were recruited to study the expression levels of the MTOR, DEPTOR, RPTOR, and RICTOR genes of the mTOR pathway before and after olanzapine treatment and to explore their feasibility to be the biomarker of schizophrenia and olanzapine effect.
Previous studies have shown that mTOR catalytic subunits are essential for normal brain physiology and development. MTOR deficiency mice may exhibit telencephalon deficiency or early embryonic death [28, 29]. Overactivation of MTOR can also lead to cortical atrophy in the early embryonic stage and to cortical hypertrophy and severe epileptic seizures in late embryonic or postpartum mitotic neurons [30]. Merenlender-Wagner et al. have confirmed that MTOR-dependent autophagic dysfunction is accompanied by changes in the gene expression and protein level of microtubule-associated protein 6 (MAP6) in brain samples of postmortem schizophrenic patients and in schizophrenic mouse models [31, 32]. Zhou et al. found that the downregulation of DISC1 in the dentate gyrus of adult mice resulted in abnormal morphology and excitability of neural networks and a schizophrenia-like behavior phenotype, which could be rescued by rapamycin injection [33].
Few studies have researched MTOR gene expression in patients with mental disorders. Mostaid et al. found that mTOR mRNA expression levels were negatively correlated with the duration of illness in treatment-resistant schizophrenia patients, and clozapine exposure could decrease mTOR mRNA expression levels in an in vitro culture of PBMC cells from treatment-resistant schizophrenia patients [34]. Machado-Vieira et al. found decreased mTOR mRNA expression levels in 25 unmedicated depressed individuals with bipolar disorder, which showed no significant change after 6 weeks of lithium therapy [35]. Dong et al. suggested that prenatal stress induces decreased mTOR mRNA levels, which may be associated with anxiety-like and alcohol drinking behaviors in adulthood [36]. These above studies suggested that there may be abnormal expression of the MTOR gene in schizophrenia and other psychiatric disorders. Our results found that MTOR gene expression levels were significantly lower in acute schizophrenia patients before treatment than in healthy controls and did not change significantly after olanzapine treatment.
DEPTOR has been reported to be an endogenous regulator of the mechanistic target of rapamycin complex 1 (mTORC1) and mTORC2. DEPTOR is widely expressed from the forebrain to the hindbrain, including the hippocampus, the mediobasal hypothalamus, and the circumventricular organs (CVOs) [37]. There have been relatively few studies on DEPTOR in mental disorders and no reports in schizophrenia. Fabbri and Serretti used the Systematic Treatment Enhancement Program for Bipolar Disorder (STEP-BD) genome-wide dataset to investigate the genetic predictors of long-term treatment outcomes and found that the DEPTOR gene, which is susceptible to antidepressant action, may affect the long-term treatment outcome of BD [38]. Davies et al. demonstrated the reduction of DEPTOR protein level in the precentral gyrus, postcentral gyrus, and occipital lobe of Alzheimer’s disease (AD) patients, as well as a reduction of DEPTOR expression in late-onset AD compared to early-onset familial AD [39]. In our study, DEPTOR gene expression did not differ significantly between healthy controls and acute schizophrenia patients before treatment, but after 4 weeks of olanzapine treatment, the DEPTOR expression level significantly increased in schizophrenic patients.
RPTOR is an important component of mTORC1 and a regulatory protein of mTOR. RPTOR knockout mice showed decreased body weight, brain weight, and cortical thickness compared to other 7-week-old wild-type mice [40]. The research on RPTOR in mental disorders has focused on the prediction function of mTOR pathway-related genes in antipsychotic (AP)-induced extrapyramidal symptoms (EPS). Mas et al. analyzed gene–gene interactions in nine genes related to the mTOR pathway in order to develop genetic predictors of the appearance of EPS and identified a four-way interaction among rs1130214 (AKT1), rs456998 (FCHSD1), rs7211818 (Raptor), and rs1053639 (DDIT4) that correctly predicted AP-induced EPS in 97 of the 114 patients (85% accuracy). Then, they validated the predictive power of the four-way interaction in two independent cohorts and reported 86% and 88% accuracy, respectively [19]. Boloc et al. developed a pharmacogenetic predictor of AP-induced EPS based on two SNPs in the AKT1 gene (rs33925946 and rs1130214) and two SNPs in the RPTOR gene (rs3476568 and rs9915667) in 131 schizophrenia inpatients treated with risperidone. Their prediction model achieved 66% accuracy of AP-induced EPS in the discovery cohort and showed similar performance in replications of schizophrenia cohort treatment with risperidone or other Aps [41]. In this study, RPTOR gene expression levels were significantly lower in acute schizophrenia patients before treatment than in health controls and did not change significantly after olanzapine treatment.
RICTOR is a component of mTORC2. RICTOR KO animal experiments have shown that it plays an important role in the pathogenesis of schizophrenia. Dadalko et al. found that neuron-specific Rictor knockout mice exhibited altered striatal DA-dependent behaviors, such as increased basal locomotion and stereotypic counts and exaggerated response to the psychomotor effects of amphetamine [42]. Siuta et al. demonstrated that neuronal Rictor knockout mice showed impairments in neuronal Akt Ser473 phosphorylation, prepulse inhibition (PPI) deficits, hypodopaminergia in the rostral cortex, an increase in NE transporter (NET) expression and function, and schizophrenia-like behaviors [43]. Moreover, RICTOR is also associated with the pathological mechanisms of other mental disorders. Miyata et al. used ovariectomized (OVX) mice exposed to chronic mild stress to simulate depression during menopause to conduct studies of genome-wide gene expression in both the medial prefrontal cortex and blood cells. They found that RICTOR was the top-ranked regulator associated with the production of OVX-induced gene expression alterations in both tissues [44]. Eriguchi et al. used exome sequencing to identify novel risk loci of sporadic Tourette syndrome (TS) cases and found that rs140964083 (RICTOR) was a novel candidate factor for TS etiology [45]. In this study, the RICTOR gene expression level was significantly lower in acute schizophrenia patients before treatment than in healthy controls and did not change significantly after olanzapine treatment.
This study also found that MTOR, DEPTOR, RPTOR, and RICTOR gene expression levels were significantly pairwise correlated in acute schizophrenia patients and the normal control group, and MTOR pathway genes might interact and coordinate as a whole to play the biological role. However, after 4 weeks of olanzapine treatment, the pairwise correlations of gene expression levels between DEPTOR and MTOR and between DEPTOR and RICTOR disappeared. MTOR, DEPTOR, and RICTOR are the key components of the mTORC2 complex. These findings suggest that olanzapine may influence the mRNA expression of DEPTOR and the formation of the mTOR complex.