GR activity prevents GSSG overaccumulation by reducing this

GR activity prevents GSSG overaccumulation by reducing this molecule to GSH (Kranner et al., 2006). High GR activity seems to be essential for undisturbed seed germination as GR and other enzymatic antioxidants maintain sunflower (Helianthus annuus L.) seeds vigour (Bailly et al., 2002). Germination of apple embryos shortly pre-treated with HCN (Bogatek et al., 2003; Krasuska and Gniazdowska, 2012) or with NO (Krasuska and Gniazdowska, 2012) was accompanied with stimulation of GR activity. High GR activity was observed in germinating non-dormant barley caryopsis (Ma et al., 2016) and in embryonic axes of lupine (Garnczarska and Wojtyla, 2008). Therefore we were not surprised, that in axes of non-dormant apple embryos transcript levels of GR were higher as compared to the control (dormant) (Table 2). This points to a pivotal role of GR in seed dormancy release and in glutathione-NO regulation.
Conclusion Our data have confirmed the importance of glutathione pool and its redox state, GSNOR and GSNO in seed dormancy removal. Increased GSH level and total glutathione pool in embryonic axes positively correlated with seed transition from dormant to ready-to-germinate state. GSNOR being a natural eliminator of GSNO seemed to be crucial for maintenance of cellular RNS level, although we suggested that GSNOR enzymatic activity (its decline) rather than alteration in the amount of the protein was linked to dormancy alleviation. These data matched well to elevated GSNO content in axes of stratified apple embryos. Formation of GSNO prevents over-accumulation of NO and GSNO participates in posttranslational protein modification (S-nitrosation), that may probably affect GSNOR activity. Therefore, alteration in GSNO and GSH content occurred during apple seed dormancy release and they were related to higher GR activity and gene transcription, regulating pool of these vital metabolites. Consequently, both enzymes GSNOR and GR could be considered as elements of biochemical mechanism for adjustment of intracellular NO level.
Conflicts of interest
Authors contributions
Introduction As resident effector cells of the central nervous system (CNS), microglia actively monitor the surrounding microenvironment (Nimmerjahn et al., 2005) and are the first to respond to inflammation, infection or trauma by undergoing a series of morphologic, phenotypic and functional changes (Gertig and Hanisch, 2014). Activation is marked by increased production of immune-related proteins (Gehrmann et al., 1995, Ransohoff and Perry, 2009), ability to migrate (Ullrich et al., 2001) and to enter the AP1903 and to proliferate (Kim and de Vellis, 2005). In addition to playing an important role in innate immunity, microglia are also involved in adaptive immunity by becoming antigen-presenting cells (Shrikant et al., 1996). Once activated, microglia express high levels of inducible nitric oxide (NO) synthase (iNOS, NOS2) and release mediators that include pro-inflammatory cytokines, prostaglandins and NO (Ransohoff and Perry, 2009). NO easily crosses cell membranes and diffuses readily in tissue and fluids (Coleman, 2001). While chemically reactive because of its unpaired electron, for a radical, NO is relatively stable and has a physiologically-relevant half-life (Stamler, 1994). Production of NO is dependent on arginine, NADPH and O2 with NO and citrulline as its products. iNOS is one of three distinct enzymes that belong to the NOS family (Butler and Nicholson, 2003). Although very little iNOS is present in the absence of stimulation, once synthesized it releases a high flux of NO that does not require an increase in intracellular calcium because the Ca2+/calmodulin complex is bound with high affinity to iNOS even at basal calcium levels (Butler and Nicholson, 2003). Once released NO can produce cellular signaling through a wide variety of mechanisms including nitrosylation of metal and thiol moieties and nitration of lipids and proteins (Foster et al., 2003). The protective or damaging effects of NO release depend on its flux rate, local environmental conditions, and cell type (Gow and Ischiropoulos, 2001). Many of the biological effects of NO are mediated by the S-nitrosylation of low-mass and protein thiols to form S-nitrosothiols (SNO) (Foster et al., 2009). S-nitrosoglutathione (GSNO), an endogenous S-nitrosothiol, is considered to be the primary source of bioactive NO in the body and to be critical to NO signaling (Hogg, 2002). GSNO reductase (GSNOR) regulates GSNO concentration by degrading it through consumption of NADH. ADH Class III, a glutathione-dependent formaldehyde dehydrogenase, is the only GSNOR present in the brain and, as such, it plays a critical role in modulating NO chemical reactivity (Westerlund et al., 2005). GSNOR is differentially distributed in the brain where it is expressed at high levels in the hippocampus and cerebellum and more modestly in the deeper layers of the cortex (Galter et al., 2003). Degradation of GSNO results in the regulation of protein S-nitrosylation-directed signaling. Many neurodegenerative diseases, including Alzheimer’s, Parkinson’s and amyotrophic lateral sclerosis, are characterized by activated microglia and dysregulation of NO metabolisms (Graeber et al., 2011). The high content of polyunsaturated lipids and a high flux of reactive oxygen species produced during neurochemical reactions contribute to CNS vulnerability to oxidative stress (Ozcelik and Uzun, 2009).