This study examined serum metabolite differences between ESRD/ ESRD with depression and healthy controls, and between ESRD and ESRD with depression. Based on the pattern recognition method and the recognition model (PLS-DA, OPLS-DA), metabolite changes between groups were distinguished, and satisfactory model parameters were obtained. Through multivariate and univariate statistical analyses, the unique metabolic patterns related to ESRD were also obtained. The differential metabolites in ESRD group and healthy controls were involved in metabolic pathways such as alanine, aspartate and glutamate metabolism, phenylalanine metabolism, glutathione metabolism, and cysteine and methionine metabolism. ESRD with depression group was significantly different from ESRD without depression group in metabolic pathways, such as energy metabolism, glycerolipid metabolism and glutamate-centered metabolism (Figure 6). Additionally, differential metabolites with high diagnostic performance may serve as potential diagnostic markers for distinguishing ESRD and ESRD with depression patients.
Metabolic disorders in ESRD patients
Clinical biochemical characteristics of ESRD patients
Compared with the healthy control group, the SBP, DBP, TG, Cr, BUN, K+, phosphorus and depression scores of the ESRD without depression group were increased, while HB and albumin were significantly decreased. These biochemical changes are consistent with hypertension, azotemia, and anemia in ESRD patients, and suggest a high inflammatory response in ESRD patients.
Reduction of antioxidants
Glutathione, taurine and hypotaurine metabolism, cysteine and methionine metabolic pathways are abnormal in ESRD patients, and metabolites in these pathways such as: S-adenosyl methionine (SAM), glutathione (GSH) and taurocholic acid, etc. antioxidants were lower than that of the healthy control group. The GSH structure contains an active sulfhydryl group (-SH) that is easily oxo-dehydrogenated; studies have shown that SAM inhibits the strong inflammatory and oxidative stress processes that occur in patients[14, 15]. Therefore, ESRD patients have reduced antioxidant capacity and may have oxidative stress damage in vivo, which is consistent with the findings of Kalender B et al. [16].
Metabolic disorders of aromatic amino acids
Phenylalanine, tyrosine and tryptophan belong to aromatic amino acids, in which phenylalanine is catalyzed by phenylalanine hydroxylase to form tyrosine, and tyrosine is further metabolized to produce catecholamine (dopamine, norepinephrine and adrenaline). Compared with the healthy control group, the tyrosine content of the ESRD without depression group was decreased, which was consistent with previous studies[17, 18], and the decrease of tyrosine was also observed in patients with renal failure and diabetes[19]. In addition, patients with ESRD had a lower Kynurenine (KYN) and a higher 3-Hydroxyanthranilic acid (3-HANA) than the healthy controls. Tryptophan is mainly metabolized by the kynurenine pathway (KP) and the 5-TH metabolic pathway, the former being more than 95% in mammals; KYN can inhibit antigen presentation, suppress immune response, and ultimately reduce inflammation[20]. However, 3-HANA is neurotoxic and induces the formation of free radicals such as hydroxyl radicals and hydrogen peroxide, and raises the level of oxidative stress[21]. It is concluded that patients with ESRD may be in a state of high inflammatory response and oxidative stress.
ESRD with metabolic disorders in patients with depression
Clinical biochemical characteristics of ESRD patients with depression
In our study, the neutrophil percentage of the ESRD with depression group was higher than that of the ESRD without depression group (p < 0.05). The occurrence of depression is highly related to inflammation[22], which is shown obviously in ESRD patients, with neutrophils as the indicator of inflammatory response[23]. Turkmen et al.[24] reported that the presence of inflammatory factors (such as TNF-α, IL-4, IL-6) in ESRD patients may be effectively regulated by the hypothalamic-pituitary-adrenal axis (HPA); Inflammatory factors can also directly stimulate HPA to cause abnormalities. Studies have shown that HPA abnormalities are one of the main causes of depression. Therefore, high inflammatory response in ESRD patients with depression may be the pathological basis of depression.
Energy metabolism
Compared with the ESRD without depression group, the ESRD with depression group had higher N-acetyl-L-aspartic acid (NAA) and gentisic acid, and lower 5-HT and thiamine pyrophosphate (TPP). KEGG analysis showed that these metabolites were involved in the tricarboxylic acid cycle (TCA cycle). TCA cycle is the ultimate and hub metabolic pathway of three major nutrients (sugars, lipids and amino acids). TCA metabolic abnormality has been reported in ESRD with depression[13]. Disorder of nutrient metabolism is common in ESRD patients, leading to insufficient energy supply for biochemical reactions.
NAA as a biomarker of neuronal damage severity, only exists in neurons, which is one of the most concentrated metabolites in the human brain, and is not detected in the blood[25, 26]. This study revealed that the NAA level in the ESRD with depression group was higher than that in the ESRD without depression group, which may be due to neuronal apoptosis and necrosis, indicating that neuronal activity was reduced or functional damage in ESRD patients with depression. 5-HT is an important neurotransmitter, and the lack of 5-HT in the central nervous system can result in depression. Reduction of 5-HT function and activity is closely related to depression, loss of appetite, and endocrine dysfunction[27]. This phenomenon can be observed in patients with major depression [29]. In accordance with the findings above, our result showed that the 5-HT level in ESRD patients with depression was lower than that in ESRD patients. We speculated that the factors affecting the metabolism of tryptophan to 5-HT and further metabolism to melatonin or acetyl-CoA in ESRD with depression patients may be one of the causes of depression.
KP in the inflammatory hypothesis of depression [30]: the emergence of inflammatory depression is caused by immune function and neurotransmitter changes in the activation of Indoleamine 2,3-dioxygenase (IDO), which not only leads to tyrosine failure also causes an increase in neurotoxic products through KP, resulting depression. There have been KP disorders in ESRD patients in this study, which may be one of the causes of depression with the prolongation of ESRD.
Thiamine, the vitamin B1, whose biologically active form is TPP[28]. TPP is an important cofactor for pyruvate dehydrogenase complex (PDHC), α-ketoglutarate dehydrogenase complex (KGDHC) and branched-chain a-keto acid dehydrogenase (BCKDH) [29]. PDHC and KGDHC are important components of cells using glucose to produce adenosine triphosphate (ATP) pathways, and BCKDH is a key enzyme for gluconeogenesis [30]. Consequently, thiamine plays an important role in maintaining the balance of oxidative metabolism in the brain. In patients with ESRD, the accumulation of toxins results in gastrointestinal reactions such as nausea, vomiting, and loss of appetite, leading to insufficient intake of thiamine, meanwhile, water-soluble thiamine is lost during dialysis, which together lead to thiamine deficiency. Deficiency of TPP can lead to metabolic disorders of sugar, lipids and amino acids, resulting in the reduction of ATP synthesis in the brain; moreover, TPP deficiency leads to cytotoxic edema, exacerbation of excitotoxic damage, oxidative stress damage, induction of inflammatory reaction, and destruction of blood brain barrier; all above considered to be potential mechanisms of depression [30, 31]. Specially, TPP catalyzes the conversion of tryptamine to 5-HT by increasing the activity of decarboxylation enzyme, thereby affecting the secretion of 5-HT[32]. In this study, ESRD patients with depression presented simultaneous descending of 5-HT and TPP, which may jointly promote the occurrence and deterioration of depression. Therefore, we speculated that serotonin reuptake inhibitors combined with thiamine may have a good anti-depressant effect on ESRD patients with depression.
Glycerolipid metabolism
In this study, we also found abnormalities in the glycerolipid metabolism, with elevated LysoPC (18:1 (9Z)), PG (18:0/18:1 (11Z)) and PA (16:0/18: 2 (9Z, 12Z)) in ESRD with depression compared to the ESRD without depression group. Phospholipids that account for 60% of the brain weight, is critical for brain neuronal structures, especially synaptic structures. The three phospholipids of PA, PG and LysoPC play important roles in signal transduction of dopamine, serotonin, glutamate and acetylcholine[33]. Study has reported that PA, PG and LysoPC are important signaling molecules with various biological functions involved in cell proliferation and inflammatory processes[34], but the specific mechanism remains to be further studied.
Glutamate-centered metabolism
The glutamate-centered metabolism was also a main pathway enriched by the differential metabolites between the ESRD with depression and ESRD without depression groups, such as L-glutamic acid (decreased in the ESRD with depression group), urocanic acid and creatine (elevated in the ESRD with depression group). Glutamate is an excitatory neurotransmitter with the highest content, the widest distribution and the strongest effect in central nervous system[35]. Glutamate can be recycled in brain cells by two conversion mechanisms. First, glutamate is the starting material for GABA biosynthesis, catalyzed by Glutamic acid decarboxylase (GAD), which is thought to be associated with mood disorders and schizophrenia[36]. In addition, glutamate is also the starting material for the synthesis of glutamine by Glutamine synthetase (GS) in astrocytes and neutralizes ammonia during biochemical metabolism[37]. Cryan et al.[38] have reported the reduction in glutamate-glutamine cycle in plasma and cerebrospinal fluid in ESRD patients with depression. In the present study, L-glutamate deficiency may be one of the causes of depression in patients. L-arginine is a downstream metabolite of L-glutamic acid as well as the sole provider of guanyl that in turn synthesizes creatine. Creatine can rapidly re-synthesize ATP for energy supply. In this study, the creatine may be increase in response to energy metabolism disorder.
Biomarkers for diagnosis
Based on the ROC curve analysis, 20 differential compounds have high diagnostic value (AUC ≥ 0.9) and can be used as a biomarker for predicting ESRD with depression. Besides, the combined analysis of biomarkers with high diagnostic potency obtained an ROC curve (AUC = 0.945), which may be of diagnostic value superior to a single differential compound.