Potential Roles of Redox Dysregulation in the Development of Schizophrenia

Abstract

Converging evidence implicates redox dysregulation as a pathological mechanism driving the emergence of psychosis. Increased oxidative damage and decreased capacity of intracellular redox modulatory systems are consistent findings in persons with schizophrenia as well as in persons at clinical high risk who subsequently developed frank psychosis. Levels of glutathione, a key regulator of cellular redox status, are reduced in the medial prefrontal cortex, striatum, and thalamus in schizophrenia. In humans with schizophrenia and in rodent models recapitulating various features of schizophrenia, redox dysregulation is linked to reductions of parvalbumin containing gamma-aminobutyric acid (GABA) interneurons and volumes of their perineuronal nets, white matter abnormalities, and microglia activation. Importantly, the activity of transcription factors, kinases, and phosphatases regulating diverse aspects of neurodevelopment and synaptic plasticity varies according to cellular redox state. Molecules regulating interneuron function under redox control include NMDA receptor subunits GluN1 and GluN2A as well as KEAP1 (regulator of transcription factor NRF2). In a rodent schizophrenia model characterized by impaired glutathione synthesis, the Gclm knockout mouse, oxidative stress activated MMP9 (matrix metalloprotease 9) via its redox-responsive regulatory sites, causing a cascade of molecular events leading to microglia activation, perineural net degradation, and impaired NMDA receptor function. Molecular pathways under redox control are implicated in the etiopathology of schizophrenia and are attractive drug targets for individualized drug therapy trials in the contexts of prevention and treatment of psychosis.

Keywords: Clinical high risk; Gclm KO; Glutathione; Grin2A KO; MMP9; Oxidative stress; Psychosis; Redox; Schizophrenia.

Figure 1.. Central role of the glutathione system in regulating cellular redox state, protein activation, and oxidative damage.

(1) In animals, reactive oxygen species (ROS) are produced mostly by mitochondria but to a limited extend enzymatically. Superoxide is a reactive oxygen species generated by the mitochondrial electron transfer chain in the conversion of glucose and oxygen to ATP (the major energy currency in animals). Necessarily, cells are equipped with enzymatic antioxidant systems that efficiently neutralize ROS through controlled redox reactions. Reduction of mitochondrial-produced ROS begins with the conversion of superoxide to another oxidant, hydrogen peroxide, by superoxide dismutase enzymes with the highly reactive hydroxyl radical an intermediate. Three molecules with critical roles in cell signaling are generated enzymatically: hydrogen peroxide, nitric oxide, and S-nitrosoglutathione. Nitric oxide is produced by nitric oxide synthase. Nitric oxide combines with glutathione to produce S-nitrosoglutathione. Hydrogen peroxide is reduced by glutathione in a reversible reaction catalyzed by glutathione peroxidase, producing glutathione disulfide. Glutathione disulfide is reduced back to glutathione in a reversible reaction catalyzed by glutathione reductase, with NADPH serving as the electron donor. (2) Hydrogen peroxide may react with metals (the Fenton reaction) to form the highly reactive peroxide radical. The peroxide radical irreversibly oxidizes lipids, proteins, and nucleic acids, producing toxic nucleophiles. A second role of glutathione is to reduce toxic nucleophiles in a reaction catalyzed by glutathione transferase; the resulting glutathione:nucleophile conjugate is then exported from the cell, potentially depleting glutathione/cysteine stores. (3) A major focus of this paper is the critical role glutathione plays in controlling redox-mediated cell signaling (7). Thiol groups on proteins readily participate in reversible redox reactions that change the structure of the protein and hence its reactivity. Reversible redox reactions involving thiol groups is a ubiquitous strategy controlling numerous transcription factors, kinases, and phosphatases. One such reaction, protein S-glutathionylation, is shown here; others include the formation of disulfide bonds, sulfenic acid, and S-nitrosoglutathione. (4) The rate-limiting step in glutathione synthesis is the combination of the amino acids cysteine and glutamate forming gamma-glutamylcysteine in a reaction catalyzed by glutamate-cysteine ligase. The availability of cysteine and glutatmate-cysteine ligase is rate-limiting in the synthesis of glutathione. The amino acid glycine is then added in a reaction catalyzed by glutathione synthase, to produce glutathione. Abbreviations: reactive oxygen species (ROS); superoxide (O ⁻₂); hydrogen peroxide (H₂O₂); peroxide radical (^•OH); nitric oxide (NO); nitric oxide molecular oxygen (O₂); S-nitrosoglutathione (GSNO); thiol (SH);Superoxide dismutase (SOD); glutathione (GSH); glutathione disulfide (GSSGS); glutathione peroxidase (GPx); glutathione reductase (GR); glutathione-S-transferase (GST); glutamate-cysteine ligase (GCL); glutathione synthase (GS)

Figure 2.. Redox regulation via oxidative stress sensitive targets in various cellular compartments:

Various redox sensitive targets are presented: the consequences of a redox imbalance leading to oxidation of the “redox-sensing” thiols, conformational and functional changes of target proteins are depicted in red. (1) The Keap1-Nrf2 pathway is the major regulator of cytoprotective responses to endogenous and exogenous stresses caused by ROS and electrophiles (77). Through Nrf2 activation of ARE located at its promoter site, the transcription of glutamate-cysteine ligase is under the control of the Keap1-Nrf2 pathway. Under normal conditions Nrf2 is repressed by Keap1. Under oxidative stress, Nrf2 is derepressed and translocates to the nucleus where it binds to ARE and activates transcription of ARE-responsive genes, leading to enhanced glutathione synthesis through increased expression of glutathione-cysteine ligase (77). Interestingly, Keap1 is a very cysteine-rich protein and the thiol switches at C151, C273, and C288 (146) may play a functional role by altering its conformation. (2) There is a tight interaction between NMDAR (subunits NR1, NR2A) hypofunction and redox dysregulation: a) Activation of synaptic NMDAR boosts intrinsic antioxidant defenses, through direct transcriptional control of the glutathione system, promoting its synthesis, recycling, and utilization (80); NMDAR hypoactivity during development thus leads to deleterious loss of antioxidant control and increased oxidative stress; NMDAR blockade by ketamine in adults disrupts excitatory/inhibitory balance in cortical circuits, affecting parvalbumin interneurons through NADPH oxidase-induced ROS generation (81). b) Vice-versa, NMDAR is a target of redox regulation through the NR1 and NR2A subunits which possess extracellular redox-sensitive sites (78) within the M3-S2 and S2-M4 linkers (C726 and C780 of the ligand binding domain) (147, 148). Deletion of NR1 subunit of NMDARs in parvalbumin interneurons leads to parvalbumin and another marker of parvalbumin interneurons, GAD67, expression deficits (149). (3) Mitochondria: Redox reactions are involved in regulating mitochondrial function via redox modification of specific redox sensing thiols in subunits of mitochondrial respiratory chain complexes. Oxidative thiol-modifications of specific cysteine thiols located in the 51 kDa- and 75 kDa-subunits of complex I result in a reduction of its catalytic activity (150). Emerging evidence points to the involvement of mitochondrial dysfunction with alteration of network morphology and activity of complex I in schizophrenia (64). (4) There is feedforward potentiation loop between oxidative stress and neuroinflammation involving the following steps: activation of MMP9 through its redox thiol switch by oxidative stress, leading to shedding of the extracellular domain of RAGE, sRAGE and the translocation of the intracellular domain of RAGE to the nucleus, followed by activation of the transcription factor Nfkb, secretion of pro-inflammatory cytokines, microglia activation, and further ROS production and oxidative stress during juvenile postnatal development. Blocking MMP9 activation prevented this sequence of alterations and rescued the normal maturation of parvalbumin interneurons/perineuronal nets, even if performed after an additional insult that exacerbated the long-term interneurons/perineuronal net impairments. MMP9 inhibition at early developmental stages prevented the interneurons/perineuronal nets deficit in adulthood (135). Abbreviations: Kelch-like ECH-associated protein (KEAP1); nuclear factor, erythroid 2 like 2 (NRF2); antioxidant response elements (ARE); reactive oxygen species (ROS); glutathione (GSH); glutamate-cysteine ligase (GCL); n-methyl-d-aspartate receptor (NMDAR); NMDA receptor subunit 1 isoform (NR1); NMDA receptor subunit 2a isoform (NR2A); (parvalbumin interneurons (PVI); oxidized thiols (S-S); reduced thiols (SH-SH); matrix metalloproteinase 9 (MMP9); receptor for advanced glycosylation end product (RAGE); extracellular soluble domain of RAGE (sRAGE); intracellular domain of RAGE (intraRAGE); nuclear factor NF-kappa-B (Nfkb); interleukin 6 (IL6); interleukin 1b (IL1b); tumor necrosis factor alpha (TNF); perineuronal nets (PNN)