Glycine modulates membrane potential, cell volume, and phagocytosis in murine microglia

Antibiotics treatment promotes vasculogenesis in the brain of glioma-bearing mice

In recent years, several studies described the close relationship between the composition of gut microbiota and brain functions, highlighting the importance of gut-derived metabolites in mediating neuronal and glial cells cross-talk in physiological and pathological condition. Gut dysbiosis may affects cerebral tumors growth and progression, but the specific metabolites involved in this modulation have not been identified yet. Using a syngeneic mouse model of glioma, we have investigated the role of dysbiosis induced by the administration of non-absorbable antibiotics on mouse metabolome and on tumor microenvironment. We report that antibiotics treatment induced: (1) alteration of the gut and brain metabolome profiles; (2) modeling of tumor microenvironment toward a pro-angiogenic phenotype in which microglia and glioma cells are actively involved; (3) increased glioma stemness; (4) trans-differentiation of glioma cells into endothelial precursor cells, thus increasing vasculogenesis. We propose glycine as a metabolite that, in ABX-induced dysbiosis, shapes brain microenvironment and contributes to glioma growth and progression.

Microglia and GABA: Diverse functions of microglia beyond GABA-receiving cells

Neurotransmitters modulate intracellular signaling not only in neurons but also in glial cells such as astrocytes, which form tripartite synapses, and oligodendrocytes, which produce the myelin sheath on axons. Another major glial cell type, microglia, which are often referred to as brain-resident immune cells, also express receptors for neurotransmitters. Recent studies have mainly focused on excitatory neurotransmitters such as glutamate, and few have examined microglial responses to the inhibitory neurotransmitter GABA. Microglia can also structurally and functionally modulate inhibitory neuronal circuits, but the underlying molecular mechanisms remain largely unknown. Since the well-regulated balance of excitatory/inhibitory (E/I) neurotransmission is believed to be the basis of proper brain function, understanding how microglia regulate and respond to inhibitory neurotransmission will help us deepen our knowledge of neuron-glia interactions. In this review, we discuss the mechanisms by which GABA alters microglial behavior and the possibility that microglia are more than just GABA-receiving cells.

Taurine Promotes Neurite Outgrowth and Synapse Development of Both Vertebrate and Invertebrate Central Neurons

Taurine is a sulfur-containing amino acid that is widely expressed throughout the human brain, heart, retina, and muscle tissues. Taurine deficiency is associated with cardiomyopathy, renal dysfunction, abnormalities of the developing nervous system, and epilepsy which suggests a role specific to excitable tissues. Like vertebrates, invertebrates maintain high levels of taurine during embryonic and larval development, which decline during aging, indicating a potential developmental role. Notwithstanding its extensive presence throughout, taurine’s precise role/s during early brain development, function, and repair remains largely unknown in both vertebrate and invertebrate. Here, we investigated whether taurine affects neurite outgrowth, synapse formation, and synaptic transmission between postnatal day 0 rat cortical neurons in vitro, whereas its synaptogenic role was tested more directly using the Lymnaea soma-soma synapse model. We provide direct evidence that when applied at physiological concentrations, taurine exerts a significant neurotrophic effect on neuritic outgrowth and thickness of neurites as well as the expression of synaptic puncta as revealed by immunostaining of presynaptic synaptophysin and postsynaptic PSD95 proteins in rat cortical neurons, indicating direct involvement in synapse development. To demonstrate taurine’s direct effects on neurons in the absence of glia and other confounding factors, we next exploited individually identified pre- and postsynaptic neurons from the mollusk Lymnaea stagnalis. We found that taurine increased both the incidence of synapse formation (percent of cells that form synapses) and the efficacy of synaptic transmission between the paired neurons. This effect was comparable, but not additive, to Lymnaea trophic factor-induced synaptogenesis. This study thus provides direct morphological and functional evidence that taurine plays an important role in neurite outgrowth, synaptogenesis, and synaptic transmission during the early stages of brain development and that this role is conserved across both vertebrate and invertebrate species.

Glutamine Cooperatively Upregulates Lipopolysaccharide-Induced Nitric Oxide Production in BV2 Microglial Cells through the ERK and Nrf-2/HO-1 Signaling Pathway

Glutamine (Gln) is a nonessential α-amino acid for protein biosynthesis. However, the mechanism through which Gln regulates NO production in microglial cells is still unclear. In this study, we investigated whether the presence or absence of Gln affects NO production in lipopolysaccharide (LPS)-stimulated BV2 microglial cells. Our data revealed that Gln depletion decreased cell viability accompanied by mild cytotoxicity, and blocked LPS-induced NO production concomitant with a significant decrease in inducible NO synthase (iNOS) expression. Additionally, Gln depletion for 24 h blocked the restoration of LPS-mediated NO production in the presence of Gln, suggesting that Gln depletion caused long-term immune deprivation. In particular, sodium-coupled amino acid transporter 1 and 2 (SNAT1 and SNAT2), which are the main Gln transporters, were highly upregulated in LPS-stimulated BV2 microglial cells, in the presence of Gln accompanied by NO production. Regardless of the presence of Gln, LPS positively stimulated nuclear factor erythroid 2-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1) expression, and transient Nrf2 knockdown and HO-1 inhibition stimulated LPS-induced NO production and iNOS expression; however, transient Nrf2 knockdown did not affect SNAT1 and SNAT2 expression, indicating that Gln transporters, SNAT1 and SNAT2, were not regulated by Nrf2, which downregulated the HO-1-mediated NO production. Moreover, Gln depletion significantly reduced LPS-induced extracellular signal-regulated kinase (ERK) phosphorylation; furthermore, a specific ERK inhibitor, PD98059, and transient ERK knockdown attenuated LPS-stimulated NO production and iNOS expression, in the presence of Gln, accompanied by downregulation of SNAT1 and SNAT2, suggesting that the ERK signaling pathway was related to LPS-mediated NO production via SNAT1 and SNAT2. Altogether, our data indicated that extracellular Gln is vital for NO production from microglia in inflammatory conditions.

Effect of Glycine on BV-2 Microglial Cells Treated with Interferon-γ and Lipopolysaccharide

Microglia are first-line defense antigen-presenting phagocytes in the central nervous system. Activated microglial cells release pro-inflammatory cytokines and can trigger an oxidative burst. The amino acid glycine exerts anti-inflammatory, immunomodulatory and cytoprotective effects and influences cell volume regulation. This study aimed to investigate the role of glycine in the modulation of inflammatory processes in mouse BV-2 microglial cells. Inflammatory stress was induced by lipopolysaccharide/interferon-γ (LPS/IFN-γ) treatment for 24 h in the absence or presence of 1 or 5 mM glycine. Cells were analyzed by flow cytometry for cell volume, side scatter, apoptosis/necrosis and expression of activation-specific surface markers. Apoptosis progression was monitored by life cell imaging. Reduced glutathione/oxidized glutathione (GSH/GSSG) ratios and release of the pro-inflammatory cytokines IL-6 and TNF-α were measured using luminescence-based assays and ELISA, respectively. We found that LPS/IFN-γ-induced apoptosis was decreased and the fraction of living cells was increased by glycine. Expression of the surface markers CD11b, CD54 and CD80 was dose-dependently increased, while IL-6 and TNF-α release was not altered compared to LPS/IFN-γ-treated cells. We showed that in BV-2 microglial cells glycine improves viability and counteracts deleterious responses to LPS/IFN-γ, which might be relevant in neurodegenerative processes associated with inflammation, like Alzheimer’s or Parkinson’s disease.

Acid- and Volume-Sensitive Chloride Currents in Microglial Cells

Many cell types express an acid-sensitive outwardly rectifying (ASOR) anion current of an unknown function. We characterized such a current in BV-2 microglial cells and then studied its interrelation with the volume-sensitive outwardly rectifying (VSOR) Cl⁻ current and the effect of acidosis on cell volume regulation. We used patch clamp, the Coulter method, and the pH-sensitive dye BCECF to measure Cl⁻ currents and cell membrane potentials, mean cell volume, and intracellular pH, respectively. The ASOR current activated at pH ≤ 5.0 and displayed an I⁻ > Cl⁻ > gluconate⁻ permeability sequence. When compared to the VSOR current, it was similarly sensitive to DIDS, but less sensitive to DCPIB, and insensitive to tamoxifen. Under acidic conditions, the ASOR current was the dominating Cl⁻ conductance, while the VSOR current was apparently inactivated. Acidification caused cell swelling under isotonic conditions and prevented the regulatory volume decrease under hypotonicity. We conclude that acidification, associated with activation of the ASOR- and inactivation of the VSOR current, massively impairs cell volume homeostasis. ASOR current activation could affect microglial function under acidotoxic conditions, since acidosis is a hallmark of pathophysiological events like inflammation, stroke or ischemia and migration and phagocytosis in microglial cells are closely related to cell volume regulation.

The role of glycine in regulated cell death

The cytoprotective effects of glycine against cell death have been recognized for over 28 years. They are expressed in multiple cell types and injury settings that lead to necrosis, but are still not widely appreciated or considered in the conceptualization of cell death pathways. In this paper, we review the available data on the expression of this phenomenon, its relationship to major pathophysiologic pathways that lead to cell death and immunomodulatory effects, the hypothesis that it involves suppression by glycine of the development of a hydrophilic death channel of molecular dimensions in the plasma membrane, and evidence for its impact on disease processes in vivo.

Myo-inositol improves the host's ability to eliminate balofloxacin-resistant Escherichia coli

Antibiotic-resistant mechanisms are associated with fitness costs. However, why antibiotic-resistant bacteria usually show increasing adaptation to hosts is largely unknown, especially from the host's perspective. The present study reveals the host's varied response to balofloxacin-resistant Escherichia coli (BLFX-R) using an integrated proteome and metabolome approach and identifies myo-inositol and phagocytosis-related proteins as crucial biomarkers. Originally, macrophages have an optimal attractive preference to BLFX-S due to more polarization of BLFX-S than BLFX-R, which renders faster elimination to BLFX-S than BLFX-R. The slower elimination to BLFX-R may be reversed by exogenous myo-inositol. Primarily, myo-inositol depolarizes macrophages, elevating adherence to both BLFX-S and BLFX-R. Since the altered adherence is equal to both strains, the myo-inositol-treated macrophages are free of the barrier to BLFX-R and thereby promote phagocytosis of BLFX-R. This work provides a novel strategy based on metabolic modulation for eliminating antibiotic-resistant bacteria with a high degree of host adaptation.