The mTOR signaling pathway is present in almost all peripheral tissues and the central nervous system and is involved in regulating protein synthesis, mitochondrial biogenesis, cell proliferation, cell survival, cell death [
25,
26], and synaptic plasticity [
15]. Abnormalities in the mTOR pathway have attracted increasing attention and have been identified in many diseases, including cancer [
27], obesity [
28], type II diabetes mellitus [
29], neurological and psychiatric diseases [
30], neurodegeneration, and brain tumors. Recent studies have shown that many psychiatric drugs, including mood stabilizers and neurorelaxants, which are also autophagy-inducing factors, regulate autophagy and exert a therapeutic effect on the mTOR pathway [
31,
32].
Alterations in the expression of mTOR pathway genes in patients with schizophrenia treated with olanzapine
To date, relatively few studies have examined the relationship between the expression levels of the mTOR pathway and the pathogenesis of schizophrenia. They have mostly been preclinical studies, with few studies involving patients with schizophrenia. In the present study, patients with acute schizophrenic were recruited to study the expression levels of the MTOR, DEPTOR, RAPTOR, and RICTOR genes in the mTOR pathway before and after olanzapine treatment. Before olanzapine treatment, the MTOR, RAPTOR, and RICTOR mRNAs were expressed at significantly lower levels in the case group than in the control group, while DEPTOR mRNA expression levels showed no significant differences between the two groups. After olanzapine treatment, the DEPTOR mRNA expression levels significantly increased in the case group, with no significant differences detected in the other three target genes.
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 patients with treatment-resistant schizophrenia, and clozapine exposure decreased mTOR mRNA expression levels in an in vitro culture of PBMCs from patients with treatment-resistant schizophrenia [
33]. 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 [
34]. 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 [
35]. Based on these studies, the MTOR gene may be abnormally expressed in patients with schizophrenia and other psychiatric disorders. Our results revealed significantly lower MTOR mRNA expression levels in patients with acute schizophrenia before treatment than in healthy controls, and the levels did not change significantly after olanzapine treatment.
DEPTOR has been reported to be an endogenous regulator of mechanistic target of rapamycin complex 1 (mTORC1) and mTORC2. DEPTOR is widely expressed in regions ranging from the forebrain to the hindbrain, including the hippocampus, the mediobasal hypothalamus, and the circumventricular organs (CVOs) [
36]. Relatively few studies have assessed DEPTOR expression in patients with mental disorders and no reports have documented its expression in patients with 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 patients with bipolar disorder [
37]. Davies et al. documented a reduction in DEPTOR protein levels in the precentral gyrus, postcentral gyrus, and occipital lobe of patients with Alzheimer’s disease (AD), as well as a reduction in DEPTOR expression in patients with late-onset AD compared to individuals with early-onset familial AD [
38]. In our study, DEPTOR mRNA expression did not differ significantly between healthy controls and patients with acute schizophrenia before treatment, but after 4 weeks of olanzapine treatment, the DEPTOR expression level increased significantly in patients with schizophrenia.
RAPTOR is an important component of mTORC1 and a regulator of mTOR. RAPTOR knockout mice show decreased body weight, brain weight, and cortical thickness compared to 7-week-old wild-type mice [
39]. Research on RAPTOR in mental disorders has focused on the predictive function of mTOR pathway-related genes in antipsychotic-induced extrapyramidal symptoms (EPS). Mas et al. analyzed gene–gene interactions among nine genes related to the mTOR pathway 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, the authors validated the predictive power of the four-way interaction in two independent cohorts and reported 86 and 88% accuracy, respectively [
22]. Boloc et al. developed a pharmacogenetic predictor of antipsychotic-induced EPS based on two SNPs in the AKT1 gene (rs33925946 and rs1130214) and two SNPs in the RAPTOR gene (rs3476568 and rs9915667) in 131 inpatients with schizophrenia treated with risperidone. Their prediction model achieved 66% accuracy for antipsychotic-induced EPS in the discovery cohort and showed similar performance in replications of the schizophrenia cohort treatment with risperidone or other antipsychotics [
40]. In the present study, significantly lower RAPTOR mRNA expression levels were detected in patients with acute schizophrenia before treatment than in healthy controls, and the levels did not change significantly after olanzapine treatment.
RICTOR is a component of mTORC2. Experiments with RICTOR KO animals have identified an important role for this gene 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 stereotypy counts and an exaggerated response to the psychomotor effects of amphetamine [
19]. According to Siuta et al., neuronal Rictor knockout mice show impairments in neuronal Akt Ser473 phosphorylation, prepulse inhibition deficits, hypodopaminergia in the rostral cortex, an increase in NE transporter expression and function, and schizophrenia-like behaviors [
41]. 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 and conducted studies of genome-wide gene expression in both the medial prefrontal cortex and blood cells. RICTOR was the top-ranked regulator associated with the OVX-induced alterations in gene expression in both tissues [
42]. Eriguchi et al. used exome sequencing to identify novel risk loci for sporadic Tourette syndrome cases and found that rs140964083 (RICTOR) was a novel candidate factor for Tourette syndrome etiology [
43]. In the present study, the RICTOR mRNA expression level was significantly lower in patients with acute schizophrenia before treatment than in healthy controls and did not change significantly after olanzapine treatment.
Alterations in correlations between mTOR pathway gene expression levels indicated the dysfunction of DEPTOR and the mTORC2 complex
As shown in the present study, MTOR, DEPTOR, RAPTOR, and RICTOR mRNA expression levels exhibited significant pairwise correlations in patients with acute schizophrenia and the normal control group, and MTOR pathway genes might interact and coordinate as a whole to exert their biological functions. However, after 4 weeks of olanzapine treatment, the pairwise correlations between DEPTOR and MTOR mRNA expression and between DEPTOR and RICTOR mRNA expression disappeared. MTOR, DEPTOR, and RICTOR are the key components of the mTORC2 complex. mTORC1 is a key inhibitor of autophagy, yet the function of mTORC2 in autophagy is controversial. However, more papers have recently begun to focus on the role of mTORC2 in autophagy mechanisms. Bernard et al. identified that reactive oxygen species (ROS) are one of the central inducers of mTORC2 activation during chronic autophagy [
44]. Aspernig et al. found that the inactivation of mTORC2-SGK-1 (serum/glucocorticoid regulated kinase 1, SGK-1) signaling impairs mitochondrial homeostasis and triggers the increased release of mitochondria-derived reactive oxygen species (mtROS) to induce autophagy [
45]. In the study by Lampada et al., genetic inhibition of mTORC2 and pharmacological inhibition of both mTORC1/2 led to decreased phosphorylation of c-MET, one of the receptor tyrosine kinases, in autophagy-proficient but not autophagy-impaired cells [
46]. Olanzapine, an autophagy activator, protects neurons from fatal mitochondrial damage [
47]. Therefore, these findings might suggest that olanzapine may modulation the expression of the DEPTOR mRNA, the formation of the mTOR complex, and even neuronal autophagy.