The role of branched chain amino acid and tryptophan metabolism in rat's behavioral diversity: Intertwined peripheral and brain effects

Regulatory mechanisms of amino acids in ferroptosis

Ferroptosis, an iron-dependent non-apoptotic regulated cell death process, is associated with the pathogenesis of various diseases. Amino acids, which are indispensable substrates of vital activities, significantly regulate ferroptosis. Amino acid metabolism is involved in maintaining iron and lipid homeostasis and redox balance. The regulatory effects of amino acids on ferroptosis are complex. An amino acid may exert contrasting effects on ferroptosis depending on the context. This review systematically and comprehensively summarized the distinct roles of amino acids in regulating ferroptosis and highlighted the emerging opportunities to develop clinical therapeutic strategies targeting amino acid-mediated ferroptosis.

Endothelial LAT1 (SLC7A5) Mediates S-Nitrosothiol Import and Modulates Respiratory Sequelae of Red Blood Cell Transfusion In Vivo

BACKGROUND

 Increased adhesivity of red blood cells (RBCs) to endothelial cells (ECs) may contribute to organ dysfunction in malaria, sickle cell disease, and diabetes. RBCs normally export nitric oxide (NO)-derived vascular signals, facilitating blood flow. S-nitrosothiols (SNOs) are thiol adducts formed in RBCs from precursor NO upon the oxygenation-linked allosteric transition in hemoglobin. RBCs export these vasoregulatory SNOs on demand, thereby regulating regional blood flow and preventing RBC-EC adhesion, and the large (system L) neutral amino acid transporter 1 (LAT1; SLC7A5) appears to mediate SNO export by RBCs.

METHODS

 To determine the role of LAT1-mediated SNO import by ECs generally and of LAT1-mediated SNO import by ECs in RBC SNO-dependent modulation of RBC sequestration and blood oxygenation in vivo, we engineered LAT1fl/fl; Cdh5-Cre+ mice, in which the putative SNO transporter LAT1 can be inducibly depleted (knocked down, KD) specifically in ECs ("LAT1ECKD").

RESULTS

 We show that LAT1 in mouse lung ECs mediates cellular SNO uptake. ECs from LAT1ECKD mice (tamoxifen-induced LAT1fl/fl; Cdh5-Cre+) import SNOs poorly ex vivo compared with ECs from wild-type (tamoxifen-treated LAT1fl/fl; Cdh5-Cre-) mice. In vivo, endothelial depletion of LAT1 increased RBC sequestration in the lung and decreased blood oxygenation after RBC transfusion.

CONCLUSION

 This is the first study showing a role for SNO transport by LAT1 in ECs in a genetic mouse model. We provide the first direct evidence for the coordination of RBC SNO export with EC SNO import via LAT1. SNO flux via LAT1 modulates RBC-EC sequestration in lungs after transfusion, and its disruption impairs blood oxygenation by the lung.

Chemical Approaches for Studying the Biology and Pharmacology of Membrane Transporters: The Histidine/Large Amino Acid Transporter SLC7A5 as a Benchmark

The localization of membrane transporters at the forefront of natural barriers makes these proteins very interesting due to their involvement in the absorption and distribution of nutrients and xenobiotics, including drugs. Over the years, structure/function relationship studies have been performed employing several strategies, including chemical modification of exposed amino acid residues. These approaches are very meaningful when applied to membrane transporters, given that these proteins are characterized by both hydrophobic and hydrophilic domains with a different degree of accessibility to employed chemicals. Besides basic features, the chemical targeting approaches can disclose information useful for pharmacological applications as well. An eminent example of this picture is the histidine/large amino acid transporter SLC7A5, known as LAT1 (Large Amino Acid Transporter 1). This protein is crucial in cell life because it is responsible for mediating the absorption and distribution of essential amino acids in peculiar body districts, such as the blood brain barrier and placenta. Furthermore, LAT1 can recognize a large variety of molecules of pharmacological interest and is also considered a hot target for drugs due to its over-expression in virtually all human cancers. Therefore, it is not surprising that the chemical targeting approach, coupled with bioinformatics, site-directed mutagenesis and transport assays, proved fundamental in describing features of LAT1 such as the substrate binding site, regulatory domains and interactions with drugs that will be discussed in this review. The results on LAT1 can be considered to have general applicability to other transporters linked with human diseases.

Differential Abundance of Brain Mitochondrial Proteins in Yak and Cattle: A Proteomics-Based Study

The plateau adaptability and stress resistance of yaks are widely known based on their capacity to survive under severe habitat conditions. However, a few studies on brain mitochondria have characterized these adaptations at the protein level. We identified and quantified the brain mitochondrial proteins using isobaric tags for relative and absolute quantification (iTRAQ) and Proteomics. Western blotting was used to verify changes in the expression of target proteins. A total of 57 differentially abundant proteins (DAPs) were identified in the yak brain tissue. Gene Ontology (GO) analysis showed molecular functions of these DAPs including downregulated oxidoreductase activity but upregulated coenzyme binding. Significantly enriched biological processes were oxidation-reduction process (downregulated) and small molecule metabolic processes (upregulated). STRING protein interaction analysis indicated a complex interaction between dehydrogenase, transaminase, and ATP synthetase families. Reactome pathway analysis highlighted that the majority of DAPs participated in aerobic metabolic pathways such as metabolism, citric acid cycle, and respiratory electron transport. Immunoblotting confirmed that changes in FKBP4 and ATPAF2 expression were consistent with the results of mass spectrometry. We performed a high-throughput screening to identify DAPs in brain mitochondria between yak and cattle, which could explain the plateau adaptability of yaks.

Review of the Correlation of LAT1 With Diseases: Mechanism and Treatment

LAT1 is a member of the system L transporter family. The main role of the LAT1 is to transport specific amino acids through cell membranes to provide nutrients to cells and participate in several metabolic pathways. It also contributes to the transport of hormones and some drugs, which are essential for the development and treatment of some diseases. In recent years, many studies have shown that LAT1 is related to cancer, obesity, diabetes, and other diseases. However, the specific mechanism underlying the influence of LAT1 on such conditions remains unclear. Through the increasing number of studies on LAT1, we have obtained a preliminary understanding on the function of LAT1 in diseases. These studies also provide a theoretical basis for finding treatments for LAT1-related diseases, such as cancer. This review summarizes the function and mechanism of LAT1 in different diseases and the treatment of LAT1-related diseases. It also provides support for the development of novel and reliable disease treatments.

Metformin Promotes Anxiolytic and Antidepressant-Like Responses in Insulin-Resistant Mice by Decreasing Circulating Branched-Chain Amino Acids

Epidemiological studies indicate that insulin resistance (IR), a hallmark of type 2 diabetes, is associated with an increased risk of major depression. Here, we demonstrated that male mice fed a high-fat diet (HFD) exhibited peripheral metabolic impairments reminiscent of IR accompanied by elevated circulating levels of branched-chain amino acids (BCAAs), whereas both parameters were normalized by chronic treatment with metformin (Met). Given the role of BCAAs in the regulation of tryptophan influx into the brain, we then explored the activity of the serotonin (5-HT) system. Our results indicated that HFD-fed mice displayed impairment in the electrical activity of dorsal raphe 5-HT neurons, attenuated hippocampal extracellular 5-HT concentrations and anxiety, one of the most visible and early symptoms of depression. On the contrary, Met stimulated 5-HT neurons excitability and 5-HT neurotransmission while hindering HFD-induced anxiety. Met also promoted antidepressant-like activities as observed with fluoxetine. In light of these data, we designed a modified HFD in which BCAA dietary supply was reduced by half. Deficiency in BCAAs failed to reverse HFD-induced metabolic impairments while producing antidepressant-like activity and enhancing the behavioral response to fluoxetine. Our results suggest that Met may act by decreasing circulating BCAAs levels to favor serotonergic neurotransmission in the hippocampus and promote antidepressant-like effects in mice fed an HFD. These findings also lead us to envision that a diet poor in BCAAs, provided either alone or as add-on therapy to conventional antidepressant drugs, could help to relieve depressive symptoms in patients with metabolic comorbidities.SIGNIFICANCE STATEMENT Insulin resistance in humans is associated with increased risk of anxiodepressive disorders. Such a relationship has been also found in rodents fed a high-fat diet (HFD). To determine whether insulin-sensitizing strategies induce anxiolytic- and/or antidepressant-like activities and to investigate the underlying mechanisms, we tested the effects of metformin, an oral antidiabetic drug, in mice fed an HFD. Metformin reduced levels of circulating branched-chain amino acids, which regulate tryptophan uptake within the brain. Moreover, metformin increased hippocampal serotonergic neurotransmission while promoting anxiolytic- and antidepressant-like effects. Moreover, a diet poor in these amino acids produced similar beneficial behavioral property. Collectively, these results suggest that metformin could be used as add-on therapy to a conventional antidepressant for the comorbidity between metabolic and mental disorders.

The Human SLC7A5 (LAT1): The Intriguing Histidine/Large Neutral Amino Acid Transporter and Its Relevance to Human Health

SLC7A5, known as LAT1, belongs to the APC superfamily and forms a heterodimeric amino acid transporter interacting with the glycoprotein CD98 (SLC3A2) through a conserved disulfide. The complex is responsible for uptake of essential amino acids in crucial body districts such as placenta and blood brain barrier. LAT1/CD98 heterodimer has been studied over the years to unravel the transport mechanism and the role of each subunit. Studies conducted in intact cells demonstrated that LAT1/CD98 mediates a Na⁺ and pH independent antiport of amino acids. Some novel insights into the function of LAT1 derived from studies conducted in proteoliposomes reconstituted with the recombinant human LAT1. Using this experimental tool, it has been demonstrated that the preferred substrate is histidine and that CD98 is not required for transport being, plausibly, involved in routing LAT1 to the plasma membrane. Since a 3D structure of LAT1 is not available, homology models have been built on the basis of the AdiC transporter from E.coli. Crucial residues for substrate recognition and gating have been identified using a combined approach of bioinformatics and site-directed mutagenesis coupled to functional assays. Over the years, the interest around LAT1 increased because this transporter is involved in important human diseases such as neurological disorders and cancer. Therefore, LAT1 became an important pharmacological target together with other nutrient membrane transporters. Moving from knowledge on structure/function relationships, two cysteine residues, lying on the substrate binding site, have been exploited for designing thiol reacting covalent inhibitors. Some lead compounds have been characterized whose efficacy has been tested in a cancer cell line.

High concentration of branched-chain amino acids promotes oxidative stress, inflammation and migration of human peripheral blood mononuclear cells via mTORC1 activation

Leucine, isoleucine and valine are essential aminoacids termed branched-chain amino acids (BCAA) due to its aliphatic side-chain. In several pathological and physiological conditions increased BCAA plasma concentrations have been described. Elevated BCAA levels predict insulin resistance development. Moreover, BCAA levels higher than 2mmol/L are neurotoxic by inducing microglial activation in maple syrup urine disease. However, there are no studies about the direct effects of BCAA in circulating cells. We have explored whether BCAA could promote oxidative stress and pro-inflammatory status in peripheral blood mononuclear cells (PBMCs) obtained from healthy donors. In cultured PBMCs, 10mmol/L BCAA increased the production of reactive oxygen species (ROS) via both NADPH oxidase and the mitochondria, and activated Akt-mTOR signalling. By using several inhibitors and activators of these molecular pathways we have described that mTOR activation by BCAA is linked to ROS production and mitochondrial dysfunction. BCAA stimulated the activation of the redox-sensitive transcription factor NF-κB, which resulted in the release of pro-inflammatory molecules, such as interleukin-6, tumor necrosis factor-α, intracellular adhesion molecule-1 or CD40L, and the migration of PBMCs. In conclusion, elevated BCAA blood levels can promote the activation of circulating PBMCs, by a mechanism that involving ROS production and NF-κB pathway activation. These data suggest that high concentrations of BCAA could exert deleterious effects on circulating blood cells and therefore contribute to the pro-inflammatory and oxidative status observed in several pathophysiological conditions.

Gene environment interaction in periphery and brain converge to modulate behavioral outcomes: Insights from the SP1 transient early in life interference rat model

It is generally assumed that behavior results from an interaction between susceptible genes and environmental stimuli during critical life stages. The present article reviews the main theoretical and practical concepts in the research of gene environment interaction, emphasizing the need for models simulating real life complexity. We review a novel approach to study gene environment interaction in which a brief post-natal interference with the expression of multiple genes, by hindering the activity of the ubiquitous transcription factor specificity protein 1 (Sp1) is followed by later-in-life exposure of rats to stress. Finally, this review discusses the role of peripheral processes in behavioral responses, with the Sp1 model as one example demonstrating how specific behavioral patterns are linked to modulations in both peripheral and central physiological processes. We suggest that models, which take into account the tripartite reciprocal interaction between the central nervous system, peripheral systems and environmental stimuli will advance our understanding of the complexity of behavior.

Hypotensive effect of S-adenosyl-L-methionine in hypertensive rats is reduced by autonomic ganglia and KATP channel blockers

S-adenosyl-L-methionine (SAM) is an amino acid involved in a number of physiological processes in the nervous system. Some evidence suggests a therapeutic potential of SAM in hypertension. In this study we investigated the effect of intracerebroventricular (ICV) infusions of SAM on arterial blood pressure in rats. Mean arterial blood pressure (MABP) and heart rate (HR) were measured at baseline and during ICV infusion of either SAM or vehicle (aCSF; controls) in conscious, male normotensive Wistar Kyoto rats (WKY) and Spontaneously Hypertensive Rats (SHR). MABP and HR were not affected by the vehicle. WKY rats infused with SAM (10 μM, 100 μM and 1 mM) showed a biphasic hemodynamic response i.e., mild hypotension and bradycardia followed by a significant increase in MABP and HR. On the contrary, SHR infused with SAM showed a dose-dependent hypotensive response. In separate series of experiments, pretreatment with hexamethonium, a ganglionic blocker as well as pretreatment with glibenclamide, a KATP channel blocker reduced the hemodynamic effects of SAM. SAM may affect the nervous control of arterial blood pressure via the autonomic nervous system and KATP channel-dependent mechanisms.

Inflammatory and oxidative stress-related effects associated with neurotoxicity are maintained after exclusively prenatal trichloroethylene exposure

Trichloroethylene (TCE) is a widespread environmental toxicant with immunotoxic and neurotoxic potential. Previous studies have shown that continuous developmental exposure to TCE encompassing gestation and early life as well as postnatal only exposure in the drinking water of MRL+/+ mice promoted CD4⁺ T cell immunotoxicity, glutathione depletion and oxidative stress in the cerebellum, as well increased locomotor activity in male offspring. The purpose of this study was to characterize the effects of exclusively prenatal exposure on these parameters. Another goal was to investigate potential plasma oxidative stress/inflammatory biomarkers to possibly be used as predictors of TCE-mediated neurotoxicity. In the current study, 6 week old male offspring of dams exposed gestationally to 0, 0.01, and 0.1mg/ml TCE in the drinking water were evaluated. Our results confirmed that the oxidized phenotype in plasma and cerebellum was maintained after exclusively prenatal exposure. A Phenotypic analysis by flow cytometry revealed that TCE exposure expanded the effector/memory subset of peripheral CD4⁺ T cells in association with increased production of pro-inflammatory cytokines IFN-γ and IL-17. Serum biomarkers of oxidative stress and inflammation were also elevated in plasma suggesting that systemic effects are important and may be used to predict neurotoxicity in our model. These results suggested that the prenatal period is a critical stage of life by which the developing CNS and immune system are susceptible to long-lasting changes mediated by TCE.