Adenosine is a ubiquitous homeostatic purine

Adenosine is a ubiquitous homeostatic purine nucleoside that accumulates extracellularly in response to metabolic stresses such as hypoxia and inflammation. Activation of either G protein-coupled adenosine receptors (ARs; A1R, A2AR, A2BR, and A3R) by extracellular adenosine can modulate cell signaling. However, A2A receptor activation significantly modulates neuronal integrity and neuroprotection by adenosine receptor modulation has been demonstrated in several model systems (Lusardi, 2009). In accordance, we have demonstrated that A2AAR signaling had a protective effect in traumatic optic neuropathy by attenuating microglia induced inflammatory response (Ahmad et al., 2013). ARs agonists have limited therapeutic use due to systemic side effects (Fredholm et al., 2005). However, a promising alternative might be the augmentation of the adenosine levels by targeting enzymes or nucleoside transporters that regulate the extracellular levels of adenosine (Shen et al., 2012). Metabolic clearance of adenosine occurs through key enzyme adenosine kinase (ADK) and evidence shows that the inhibition of this enzyme increases extracellular adenosine levels in cell and tissues (Boison and Shen, 2010). Indeed, the inhibition of ADK has been proven to possess potential therapeutic usefulness in a wide range of neurological disorders (Boison, 2008). In this context, we previously reported that pharmacologic inhibition of ADK augments adenosine and exerts activity in retina of diabetic mice (Elsherbiny et al., 2013). Here, we seek to investigate the retinal protective role of ABT-702, a selective adenosine kinase inhibitor against marked TON-induced retinal inflammation and damage.
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Results
Discussion Inflammation plays a key role in many CNS diseases, including neural injury, infections and other diseases (Zheng et al., 2012). In case of optic nerve injury, inflammatory responses are immediately activated followed by activation of glial Ciclopirox ethanolamine along with release of inflammatory molecules. In TON, influx of activated microglia play key role in retinal damage by secreting pro-inflammatory cytokines and cytotoxic molecules in response to oxidative stress. We previously demonstrated that extracellular adenosine has an anti-inflammatory effect in the retinal microglial cells near RGC mediated by adenosine receptor A2A (A2AAR) signaling (Ibrahim et al., 2011). Therapeutically, adenosine and its agonists have protective effect in various animal models of inflammation, hypoxia and ischemia but are limited there by systemic side effects such as hypotension, bradycardia, and sedation (Williams, 1996). In addition, physiological or inflammatory conditions limits adenosine availability because of its rapid reuptake via nucleoside transporters (NTs) and subsequently metabolized intracellularly (Moser et al., 1989). Adenosine kinase (ADK) is thought to be the principal enzyme responsible for regulating the level of adenosine under physiological conditions. Thus, use of ADK inhibitors represents an effective alternative for greater therapeutic effects of extracellular adenosine at particular site and event along with lower hemodynamic toxicity. Pharmacologic inhibition of ADK has been reported to exert beneficial effects in different disease models (Ugarkar et al., 2000, Vlajkovic et al., 2011, Annes et al., 2012). In our earlier study, we also demonstrated that ABT-702, a selective ADK inhibitor had a protective role in diabetic retina due to its potential to amplify therapeutic effects at site of injury (Elsherbiny et al., 2013). The activation of microglia plays an important role in inflammatory response in TON (Zheng et al., 2012). We previously reported that TON milieu caused microglia activation as indicated by increased Iba-1 expression (Ahmad et al., 2013). In the present study, ABT-702 treatment inhibited TON- induced increase of retinal Iba-1 levels. Further, activated microglia released inflammatory molecules such as IL-6, and TNF-α, which may be toxic to neurons and other glial cells (Smith et al., 2012). Here, we found that ABT-702 reduced retinal increase of mRNA levels of TNF-α and IL-6 in TON. These finding suggest that ABT-702 exerted its protective effect by augmenting the anti-inflammatory mechanism of adenosine mediated by attenuation of microglia activation.