Kynurenine or tryptophan catabolite (TRYCAT) pathway contributes to the pathophysiology of major depression disorder (MDD) and major depressive episodes (MDE) in bipolar disorder and suicidal behaviors. The consequences of the overactivation of this pathway large reduced tryptophan (TRP) levels in peripheral blood and the CNS and increased levels of neurotoxic TRYCATs including kynurenine (KYN), 3-hydroxy kynurenine (3HK), quinolinic acid (QA), xanthurenic acid (XA), and picolinic acid (PA). However, other TRYCATs are protective, such as kynurenic acid (KA) and anthranilic acid (AA). Inflammation and cell-mediated immune activation along with oxidative and nitrosative stress (O&NS) may stimulate the first and rate-limiting enzyme of this pathway, namely indoleamine-2,3-dioxygenase (IDO). Therefore, during depression, balancing neuroprotective versus neurotoxic TRYCATs and balancing activation of the immune response system (IRS) versus the compensatory immune response system is crucial for achieving better treatment outcomes. Furthermore, targeting the causes of TRYCAT pathway activation (immune activation and O&NS) is probably the most effective strategy to treat depression. In the present review, we aim to provide a comprehensive explanation of the impact of TRYCATs in terms of pathophysiology and treatment of MDD and MDE.

In Schizophrenia, pathway-genotypes may be constructed by combining interrelated immune biomarkers with changes in specific neurocognitive functions that represent aberrations in brain neuronal circuits. These constructs provide insight on the phenome of schizophrenia and show how pathway-phenotypes mediate the effects of genome X environmentome interactions on the symptomatology/phenomenology of schizophrenia. Nevertheless, there is a lack of knowledge how to construct pathway-phenotypes using Partial Least Squares (PLS) path modeling and Soft Independent Modeling of Class Analogy (SIMCA). This paper aims to provide a step-by-step utilization guide for the construction of pathway-phenotypes that reflect aberrations in the neuroimmune - brain circuit axis (NIBCA) in deficit schizophrenia. This NIBCA index is constructed using immune biomarkers (CCL-2, CCL-11, IL-1β, sIL-1RA, TNF-α, sTNFR1, sTNFR2) and neurocognitive tests (Brief Assessment of Cognition in Schizophrenia) predicting overall severity of schizophrenia (OSOS) in 120 deficit SCZ and 54 healthy participants. Using SmartPLS path analysis, a latent vector is extracted from those biomarkers and cognitive tests, which shows a good construct reliability (Cronbach alpha and composite reliability) and replicability and which is reflectively measured through its NIBCA manifestations. This NIBCA pathway-phenotype explains 75.0% of the variance in PHEMN (psychotic, hostility, excitation, mannerism and negative) symptoms. Using SIMCA, we constructed a NIBCA pathway-class that defines deficit schizophrenia as a qualitatively distinct nosological entity and which allows patients with deficit schizophrenia to be authenticated as belonging to the deficit schizophrenia class. In conclusion, our nomothetic approach to develop a nomological network combining neuro-immune and neurocognitive phenome markers to predict OSOS and cross-validate a diagnostic class generated replicable models reflecting the key phenome of the illness, which may mediate the effects of genome X environmentome interactions on the final outcome phenome features, namely symptomatology and phenomenology.

Objective: A previous study showed that schizophrenia is accompanied by lowered levels of trace/metal elements including cesium. There are no data whether changes in cesium, rubidium and rhenium are associated with activated immune-inflammatory pathways, cognitive impairments, and the symptomatology of schizophrenia. Methods: This study measured cesium, rubidium, and rhenium, cognitive impairments (using the Brief Assessment of Cognition in Schizophrenia) and the cytokines/chemokines interleukin (IL)-1β, tumor necrosis factor (TNF)-α, and CCL11 (eotaxin) in 120 schizophrenia patients and 54 healthy controls. Severity of illness was assessed using the Brief Psychiatric Rating Scale (BPRS), the Scale for the Assessment of Negative Symptoms (SANS), the Fibromyalgia and Chronic Fatigue Syndrome Rating (FF) Scale and the Hamilton Depression Rating Scale (HAM-D). Results: Serum cesium was significantly lower in schizophrenia patients as compared with controls. Serum cesium was significantly and inversely associated with CCL11 and TNF-α, but not IL-1β. Moreover, there were significant inverse associations between serum cesium levels and the BPRS, FF, HAM-D and SANS scores and positive correlations between cesium and neurocognitive probe results including the Tower of London, Symbol Coding, Controlled Word Association, Category Instances, Digit Sequencing Task, and List Learning tests. Conclusion: The results suggest that lowered serum cesium levels may play a role in the pathophysiology of SCZ, specific symptom domains including negative, depressive and fatigue symptoms, neurocognitive impairments (spatial working, episodic and semantic memory and executive functions) and neuro-immune pathways as well.

The tryptophan catabolite (TRYCAT) pathway is implicated in the pathophysiology of schizophrenia (SCZ) since the rate-limiting enzyme indoleamine-dioxygenase (IDO) may be induced by inflammatory and oxidative stress mediators. This systematic review searched PubMed, Web of Science, and Google Scholar for papers published from inception until August 2021 and meta-analyzed the association between SCZ and TRYCATs in the central nervous system (CNS) and peripheral blood. We included 61 studies comprising 2813 patients and 2948 healthy controls. In the CNS we found a significant (p<0.001) increase in the kynurenine/tryptophan (KYN/TRP) (standardized mean difference, SMD=0.769, 95% confidence interval, CI: 0.456; 1.082) and kynurenic acid (KA)/KYN+TRP (SMD=0.697, CI:0.478-0.917) ratios, KA (SMD=0.646, CI: 0.422; 0.909) and KYN (SMD=1.238; CI: 0.590; 1.886), while the 3OH-kynurenine (3HK) + KYN-3-monooxygenase (KMO)/KYN ratio was significantly reduced (SMD=-1.089, CI: -1.682; -0.496). There were significant differences between KYN/TRP, (KYN+KA)/TRP, (3HK+KMO)/KYN, KA, and KYN levels among the CNS and peripheral blood, and among serum and plasma KYN. The only useful peripheral marker of CNS TRYCATs findings was the increased KYN/TRP ratio in serum (SMD=0.211, CI: 0.056; 0.366, p=0.007), but not in plasma. There was no significant increase in a neurotoxic composite score based on KYN, 3HK, and picolinic, xanthurenic, and quinolinic acid. SCZ is accompanied by increased IDO activity in the CNS and serum, and reduced KMO activity and a shift towards KA production in the CNS. This CNS TRYCATs profile indicates neuroprotective, negative immunoregulatory and anti-inflammatory effects. Peripheral blood levels of TRYCATs are dissociated from CNS findings except for a modest increase in serum IDO activity.