Abstract
Mounting evidence suggests that the guanine-based purines stand out as key player in cell metabolism and in several models of neurodegenerative disorders, such as Parkinson’s and Alzheimer’s diseases. Guanosine (GUO) and guanine (GUA) are extracellular signaling molecules derived from the breakdown of the correspondent nucleotide, GTP, and their intracellular and extracellular levels are regulated by the fine-tuned activity of two major enzymes, purine nucleoside phosphorylase (PNP) and guanine deaminase (GDA). Noteworthy, GUO and GUA, seem to play opposite roles in the modulation of cognitive functions, such as learning and memory. Indeed GUO, despite exerting neuroprotective, anti-apoptotic and neurotrophic effects, causes a decay of cognitive activities, whereas GUA administration in rats results in working memory improvement (prevented by L-NAME pre-treatment). This study was designed to investigate, in a model of SH-SY5Y neuroblastoma cell line, the signal transduction pathway activated by extracellular GUA. Altogether, our results showed that: (i) in addition to an enhanced phosphorylation of ASK1, p38 and JNK, likely linked to a non-massive and transient ROS production, the PKB/NO/sGC/cGMP/PKG/ERK cascade seems to be the main signaling pathway elicited by extracellular GUA; (ii) the activation of this pathway occurs in a pertussis-toxin sensitive manner, thus suggesting the involvement of a putative G protein coupled receptor; (iii) the GUA-induced NO production, strongly reduced by cell pre-treatment with L-NAME, is negatively modulated by the EPAC-cAMP-CaMKII pathway, which causes the over-expression of GDA that, in turn, reduces the levels of GUA. These molecular mechanisms activated by GUA may be useful to support our previous observation showing that GUA improves learning and memory functions through the stimulation of NO signaling pathway, and underscore the therapeutic potential of oral administration of guanine for treating memory-related disorders.
Highlights
Guanine-based purines are known to play crucial role in the modulation of neurotransmission and neuropathologies (Ciccarelli et al, 2001; Boison, 2011; Bettio et al, 2016; Di Liberto et al, 2016)
In the SH-SH5Y lysates, the levels of Guanine-based nucleotides prevailed over the correspondent nucleobases and the inhibition of the uptake, performed by treating cell culture with a cocktail of inhibitors of nucleoside and nucleobase transmembrane transporters (100 μM propentofylline, 10 μM NBTI, 10 μM dypiridamole) did not caused any significant effect (Figure 1A)
In an attempt to investigate whether the activation of collateral signaling pathway was able to affect the nitric oxide (NO)-soluble guanylyl cyclase (sGC)-cGMPPKG-ERK cascade, we examined the possible role of cAMPEpac-CaMKII pathway
Summary
Guanine-based purines are known to play crucial role in the modulation of neurotransmission and neuropathologies (Ciccarelli et al, 2001; Boison, 2011; Bettio et al, 2016; Di Liberto et al, 2016). The purine nucleoside Guanosine (GUO), which is mostly released from astrocytes under pathological conditions (i.e., hypoxic or hypoglycemic stress), is thought to exert both neurotrophic and neuroprotective effects (Di Iorio et al, 2001, 2004; Giuliani et al, 2012, 2015; Lanznaster et al, 2016); it oversees neuronal development and synaptic activity, and protects neuronal and glial cells against oxidative stress and excitotoxicity (Neary, 1996; Schmidt et al, 2007; Tarozzi et al, 2010; Quincozes-Santos et al, 2014; Bellaver et al, 2015; Ribeiro, 2016; Thomaz et al, 2016). When intracellular levels of GUA are excessive, it may be transported outside the cells by specific transmembrane nucleobases transporters, most of the extracellular GUA derives from the breakdown of the released GTP and it is generated by GUO in a reaction catalyzed by the purine nucleoside phosphorylase (PNP) (Rathbone et al, 2008; Giuliani et al, 2016, 2017; Peña-Altamira et al, 2017).
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