Extracellular ATP induces graded reactive response of astrocytes and strengthens their antioxidative defense in vitro

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Adenosine triphosphate (ATP) plays a fundamental role inside the cells as the energy transfer molecule and phosphate group donor (Lipmann, 1941). Outside the cells, ATP acts as a versatile signaling molecule, acting at two classes of purinoceptors, ligand‐gated P2X and G‐protein–coupled P2Y receptors. Seven P2XR subunits assemble to form homomeric or heteromeric receptor channels activated by ATP (Khakh and North, 2012). Eight P2Y receptors have been identified exhibiting differential sensitivity to ATP (P2Y2,4,11), ADP (P2Y1,12,13), and other nucleotides (Abbracchio et al., 2006). ATP‐induced signaling is short‐lived and terminated by an enzymatic degradation of ATP, catalyzed by the ectonucleotidase enzyme chain. Ectonucleoside triphosphate diphosphohydrolases (E‐NTPDases) degrade ATP and ADP, while ecto‐5′‐nucleotidase hydrolyzes AMP to adenosine. Adenosine acts at its own P1 receptors (Burnstock et al., 2011; Fredholm et al., 2011) to induce cellular responses.
ATP is released from neurons (White, 1977; Pankratov et al., 2006; Fields, 2011), astrocytes (Koizumi, 2010), and microglia (Imura et al., 2013; George et al., 2015) through multiple pathways (Bodin and Burnstock, 2001). Since P2 receptors display widespread cellular distribution in the brain (Burnstock et al., 2011), multiple roles have been attributed to the extracellular ATP, including neurotransmission, neuromodulation, and trophic actions. Moreover, there is growing evidence for a rapid increase in the extracellular ATP levels upon noxious brain conditions, such as trauma (Wang et al., 2004; Davalos et al., 2005; Franke et al., 2006) and hypoxia/ischemia (Juranyi et al., 1999; Melani et al., 2005). In such conditions, ATP acts as a danger‐associated molecular pattern (DAMP) (Bours et al., 2006; Rodrigues et al., 2015), able to sustain its own release and preserve the enhanced levels by acting at P2X7R and P2Y1R at neurons, microglia, and astrocytes (Pankratov et al., 2006; Kim et al., 2007; Bennett et al., 2012). By acting at low‐affinity P2X7R, ATP controls and directs the neuroinflammatory response of overactivated microglia (Koizumi et al., 2013; Idzko et al., 2014) and induces release of IL‐1β, essential for morphological and functional activation of astrocytes (Liu et al., 2000; Dunn et al., 2002; Cahill and Rogers, 2008; Silverman et al., 2009; He et al., 2015). The actions of ATP through P2Y1R promote reactive astrogliosis upon mechanical injury (Franke et al., 2001), ischemic conditions (Sun et al., 2008), or Alzheimer disease (Delekate et al., 2014).
Reactive astrogliosis is a common response of astrocytes to noxious stimuli, characterized by hypertrophy and hyperplasia of astrocytes (Sofroniew, 2009) and their migration to the site of injury, resulting in loss of nonoverlapping domain organization and formation of a glial scar (Oberheim et al., 2008). While in physiological conditions astrocytes produce growth factors and antioxidative molecules, which support neuronal stability (Shih et al., 2003), reactive astrocytes release proinflammatory mediators and reactive oxygen (ROS) and nitrogen species (RNS), which promote inflammation (Farina et al., 2007; Kim et al., 2010; Steele et al., 2013).
The large body of evidence obtained in different experimental models and human specimens indicates that the process of reactive astrogliosis is not an all‐or‐none response leading to the formation of a glial scar, but a broad continuum of alterations in the cells' morphology and function (Sofroniew, 2009). Given that ATP acts both as a trophic factor and DAMP molecule, the aim of our study was to assess responses of astrocytes to a range of ATP concentrations intended to mimic graded intensity of the stimulus. We demonstrate that ATP induced the graded response of astrocytes in terms of proliferation and shape remodeling. On the other hand, it did not promote IL‐1β release and migration of activated astrocytes but instead strengthened their antioxidative capacity by inducing superoxide dismutase (SOD) activity and increasing their glutathione (GSH) content.
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