Recently, we conducted a metabolomics study to determine whether the baseline metabolic profile (that is, metabotype) of a patient with MDD would allow a prediction of patient's response to treatment with sertraline or placebo.32
In that study, utilizing an electrochemistry-based analytical platform that enables quantification of redox active compounds, we found that metabolites from phenylalanine, tryptophan, purine and tocopherol pathways enabled separation of responders and non-responders to treatment with the SSRI or placebo. The current study extends on these findings by (1) analyzing different classes of metabolites using GC-TOF mass spectrometry analytical platform; (2) identifying the biochemical changes that occur in MDD patients 1 week and 4 weeks after treatment with sertraline or placebo, and (3) identifying which changes are associated with a reduction in depressive symptoms after 1 and 4 weeks of treatment with sertraline or placebo.
Several metabolites from multiple pathways were changed in sertraline- and placebo-treated subjects. In both sertraline- and placebo-treated patients, the larger effects were observed after 4 weeks of treatment compared with 1 week. Multiple mechanisms could account for the biochemical changes observed in this study. Progression of the disease itself, as well as behavioral changes (for example, changes in diet) associated with improvement of the symptoms after treatment, could explain some of the effects such as changes in fatty acids and indices of energy metabolism. Thus, it was suggested that plasma levels of linoleic acid may serve as a possible biomarker for residual depression and anxiety in older people with previous depression;40
it was also shown earlier that increased dietary intake of oleic acid was associated with reduced risk of severe depressed mood among women while increased dietary intake of linoleic acid was associated with increased risk of severe depressed mood among men.41
It was recently found that omega-3 augmentation of SSRI citalopram treatment produced a significantly greater reduction in HAM-D scores compared with citalopram treatment alone.42
In a yeast model, sertraline has been shown to inhibit phospholipases A1 and D and activates phospholipase A2.43
It is thus possible, if the effect on phospholipases is similar in mammalian systems, that the changes in fatty acids observed in this study could be the result of sertraline inhibition and/or activation of specific phospholipases. It was also shown in primary cultures of mouse astrocytes that chronic SSRIs treatment upregulates calcium-dependent phospholipase A2 (PLA2), which releases arachidonic acid from the membrane-bound phospholipids.44, 45
Additional mechanism, involving PLA2 activation and increased release of arachidonic acid following SSRIs could involve membrane phospholipids remodeling and changes in signal transduction pathways.46, 47
The changes noted in fatty acids in the placebo group are of interest and could reflect changes in fatty acid biosynthesis or degradation as disease progresses. It can also be common to what was observed in the sertraline group.
Several studies implicated impairment of energy metabolism pathways in both pathogenesis of depression and effects of antidepressants. In an animal model of depression, levels of aconitate hydratase and pyruvate dehydrogenase were found to be regulated in opposite directions by stress and antidepressant drug treatment, thus representing molecular correlates of both vulnerability to depression and response to drug.48
Chronic administration of different antidepressants, including SSRIs, was associated with increased levels of biomarkers for energy metabolism, such as activity of brain creatine kinase, of mitochondrial respiratory chain complexes II and IV, and of citrate synthase and succinate dehydrogenase.17, 49, 50
Recently in an animal model significant changes in amino-acid metabolism related to energy metabolism via TCA cycle, and increased levels of glucose, glucose-6-phosphate and fructose-6-phosphate following chronic paroxetine treatment were found.18
Additional mechanisms related to the observed changes in energy metabolism following sertraline treatment could be related to the effects of 5-HT on glucose metabolism and glycolysis. It was demonstrated that 5-HT stimulates glucose uptake in skeletal muscle and activates skeletal muscle 6-phosphofructo-1-kinase activity in a dose-response through 5-HT2A receptors.51
Recently, it was found that 5-HT through 5-HT2A receptors controls hepatic glycolysis in mice,52
and 5-HT had stimulatory and inhibitory effects on glycogen synthesis in hepatocytes mediated by 5-HT1/2A and 5-HT2B receptors, respectively.53
We also analyzed associations between changes in individual metabolites and treatment outcome. After 1 week of sertraline treatment, the decrease in 5-methoxytryptamine and increases in arachidonic acid and α-ketoglutarate levels were the most significant in association with symptoms reduction. Association of decrease in 5-MTPM with symptoms reduction suggests potential role of methoxyindole branch of the tryptophan pathway in recovery form depressed state; currently we are investigating different branches of the pathway in the effects of SSRIs. Changes in the levels of arachidonic levels most likely reflect increase in the activity in phospholipase A2, resulting from increased 5-HT levels at earlier time point.44, 45
Association of increased levels of α-ketoglutarate with symptoms reduction is not clear; however, it may be related to the activation of energy metabolism following SSRI treatment. In support of this mechanism is the observation that improvement of symptoms at 4 weeks of treatment with sertraline was associated with decreased levels of lactic acid. The most significant changes associated with symptoms reduction after 4 weeks of treatment with sertraline were decreases in the levels of BCAAs, valine, leucine and isoleucine. In our previous study, we have demonstrated that higher levels of leucine and isoleucine were associated with depression.29
BCAAs have well-established anabolic effects on protein metabolism that involves activation of mTOR pathway. In this respect, it is particularly interesting that antidepressant effect of NMDA receptor antagonist ketamine results from activation of mTOR pathway involving BCAAs.54
In a recent study, Webhofer et al.18
found that in mice valine, leucine and isoleucine levels were increased after chronic treatment with SSRI paroxetine. Detailed mapping of effects of SSRIs, as well as of other antidepressants, on metabolism in animals and in humans should be compared at a more detailed level to enable translational research.
Of interest, there were several metabolites that closely correlated with changes in BCAAs and these included methionine and tyrosine. Methionine and the methionine cycle are closely linked to the folate cycle, to one carbon metabolism and to methylation processes that have been implicated in depression.
While there is strong evidence that MDD is influenced by inheritance,55
GWA studies for possible genetic factors that contribute to MDD risk, as well as GWA studies for genes associated with SSRI response in MDD patients, have provided somewhat disappointing results.56, 57
However, it was demonstrated that the SNP in the ABCB1
gene encoding ATP-binding cassette, subfamily B (MDR/TAP), member 1 (ABCB1) transporter protein expressed in the endothelial cells of the blood–brain barrier, is associated with severity of MDD and its response to citalopram and escitaloppram.58, 59
Recently, in GWA study Kohli et al.60
identified the SNP in a gene encoding a neuron-specific neutral amino-acid transporter, SLC6A15
, as an MDD risk gene. While the exact function of the SLC6A15
SNP is not well understood, from both human and animal studies, it is known that SLC6A15
belongs to the solute carrier 6 (SLC6) gene family, which also includes the transporters for monoamines (including serotonin transporter, the primary target for SSRIs) and neurotransmitter amino acids, as well as a sodium-dependent transporter for neutral amino acids, including BCAAs.61
Earlier it was shown that in the SLC6A15
knockout mice, leucine uptake into cortical synaptosomes is significantly reduced.62
Zhou et al.63
demonstrated that SSRIs sertraline and fluoxetine have high affinity for the prokaryotic leucine transporter homolog (LeuT) of SLC6A15
. Interestingly, SSRIs also bind to the homologous norepinephrine and dopamine transporters, although with much lower affinity than to their principal target serotonin transporter,64, 65
and only one or two different functional group substitutions are sufficient to convert an SSRI into a norepinephrine reuptake inhibitor with higher affinity to norepinephrine transporter.66, 67
Therefore, it is not unlikely that observed effect of sertraline on BCAAs could result from the SLC6A15
SNP in depressed patients. All these findings and the association between the improvement of depression and changes in BCAAs levels observed in our study strongly suggest potential role of BCAA in depression and in the effects of SSRI; further studies are needed to examine potential role of BCAAs in depression and the effects of different antidepressants, in both patients and animal models.
One of the limitations of this study is that despite comprehensive biochemical analysis it included relatively small number of subjects. However, this study should be viewed as hypotheses-generating to test whether metabolomics approach could provide insights into pathways implicated in SSRI and placebo response and mechanism of delayed response. Replication and validation of these findings in other clinical trials is important for future studies, however, this is outside the scope of this first report. Whether and to what extent peripheral biochemical changes reflect CNS (central nervous system) changes related to mechanisms of depression and drug effects also should be addressed in future studies. Depression is a disease that affects both brain and also the rest of the body, as is well known from the cortisol data;68, 69
therefore, it is possible that the peripheral changes reflect both direct CNS and additional CNS-linked peripheral effects and maybe direct peripheral effects also. Irrespective of the exact mechanism that leads to the changes and to response, the presence of consistent changes can be an important signature that could be the basis of further iterative understanding from both animal and human studies.
In conclusion, our data for the first time illustrate that biochemical changes following SSRI treatment are progressive and some of the pathways that are modified in the first week might lead to subsequent changes noted in the following weeks. The presence of consistent changes could provide an opportunity for developing biomarkers that can predict response or no response to antidepressant treatment.