A holistic view of adenosine in the control of intestinal neuromuscular functions: the enteric ‘purinome’ concept

Abstract

Adenosine is involved in the modulation of enteric neuromuscular functions, operating a fine tuning of smooth muscle contractility, peristaltic reflex and transit. In this issue of the BJP, Zizzo et al. report novel findings on the expression of adenosine receptors in mouse duodenum, extending our knowledge of their involvement in the control of spontaneous and neurogenic intestinal motility. In this study, particular attention was paid to the differential activation of adenosine receptors, as a result of their interplay with regulatory systems, modulating the availability of endogenous adenosine in a compartmentalised manner. This evidence will contribute to the holistic evaluation of the role played by adenosine in the regulation of intestinal motility, in accordance with the novel concept of the enteric ‘purinome’. This commentary discusses the role of the ‘purinome’ in the modulation of enteric neuromuscular activity, pointing out its involvement in the intestinal neuroplasticity associated with bowel dysmotility.

LINKED ARTICLE

This article is a commentary on Zizzo et al., pp. 1580–1589 of this issue. To view this paper visit http://dx.doi.org/10.1111/j.1476-5381.2011.01498.x

Intestinal functions are the resultant of integrated and complex interplays between the enteric nervous system (ENS), smooth muscle and the mucosal immune system, aimed at maintaining body homeostasis through finely regulated and coordinated motor, secretory and absorptive activities. In particular, the ENS is endowed with a wide array of restorative, maintenance and adaptive functions, which can be activated or modulated in response to different physiological or noxious stimuli. In this regard, the tight interactions between the neuronal elements of ENS and the enteric immune cells, carrying out an active patrolling of gut lumen, ensure effective motor and secretory adaptive changes, with the purpose of counteracting pathogenic assaults, as well as normalizing abnormal intestinal activities (Antonioli et al., 2008).

This complex enteric framework is carefully controlled by several mediators and increasing evidence points to a prominent role played by adenosine in the modulation of gastrointestinal neuromuscular functions, with an array of fine tuning processes operated by this nucleoside at the level of contractility, peristaltic reflex and transit. Consistent with this view, several investigations have demonstrated wide and heterogeneous distributions of receptors for adenosine throughout the gut as well as the presence of adenosine synthetic and catabolic enzymes and transporters in the neuromuscular compartment and in the mucosa and submucosal layer of the digestive tract in both humans and rodents, supporting the involvement of adenosine in the modulation of gastrointestinal functions (Antonioli et al., 2008).

In this issue of the British Journal of Pharmacology, the article by Zizzo et al. (2011) reports their findings on the expression of adenosine receptor subtypes in mouse duodenum and their involvement in the modulation of spontaneous and neurogenic contractile activity in this section of the gut. First, the authors showed that transcripts coding for all four adenosine receptors were expressed in the mouse duodenum, and that A_2B receptors were confined to the intestinal mucosa, in line with the role that has been postulated for this receptor subtype in the autocrine and paracrine regulation of enteric secretion (Antonioli et al., 2008). Then, by means of functional experiments, they went on to demonstrate that adenosine exerted an inhibitory control on duodenal motility, either by activation of A₁ inhibitory receptors, located on smooth muscle, or by modulation of neurotransmitter release, via A₁ or A_2A receptors located on enteric nerves. In particular, A₁ receptors were found to act via direct inhibition of enteric cholinergic nerves, while A_2A receptors were shown to exert indirect modulating actions on cholinergic excitatory pathways through the recruitment of inhibitory nitrergic nerves. Notably, these authors obtained solid evidence that, although both A₁ and A_2A receptors were fully available for pharmacological activation by specific ligands, exogenously applied adenosine relaxed the duodenal preparations through a preferential activation of A₁ receptors, while the actions of endogenous adenosine, as elicited by electrical stimuli, were restricted to the preferential activation of neuronal A_2A receptors. Of interest, Zizzo et al. highlighted a pivotal role played by nucleoside transporters in this selective activation of A_2A receptors by endogenous adenosine, through the segregation of purine bioavailability in discrete microenvironments within the duodenal neuromuscular layer.

The evidence presented by Zizzo et al. is in full agreement with the novel concept of the ‘purinome’, consisting of a dynamic molecular network, involving adenosine synthetic and catabolic enzymes, transporters and receptors, aimed at triggering, maintaining and terminating the purinergic signalling in specific locations under different pathophysiological conditions. Indeed, it is being increasingly appreciated that adenosine receptor activation by the endogenous mediator is the resultant of a fine regulation operated by synthetic or catabolic enzymes and transporters, aimed at channelling the production of endogenous adenosine into discrete compartments, in order to selectively target specific receptor subtypes (Volonté and D'Ambrosi, 2009).

In the last few years, the intriguing hypothesis of the ‘purinome’ has broadened our understanding of the variety of roles played by adenosine in the intestinal neuromuscular compartment. In line with this concept, several authors are advocating a holistic evaluation of the ‘enteric purinome’ in an attempt to gain detailed information on the involvement of the purinergic system in the regulation of intestinal motility. In this regard, Duarte-Araújo et al. (2004) provided the initial evidence supporting the coexistence of both inhibitory A₁ and facilitatory A_2A adenosine receptors in the cholinergic nerves of rat ileum, thus raising the question of how endogenous adenosine could activate each receptor subtype to differentially regulate intestinal neurotransmission. These authors found the answer by a contextual evaluation of the role played by A₁ and A_2A adenosine receptors in relationship with two major determinants of extracellular adenosine levels, adenosine deaminase and the nucleoside transporters. Owing to the efficiency of these inactivating systems, Duarte-Araújo et al. (2004), in line with the recent findings reported by Zizzo et al., proposed that the selective activation of prejunctional facilitatory A_2A receptors could be a consequence of endogenous adenosine release or production restricted to a discrete region of the myenteric nerve synapse.

In line with the concept of the enteric ‘purinome’, it is noteworthy that the specific activation of purinergic receptors in the modulation of the myenteric plexus can be influenced also by synthetic enzymes, which, because of their differential tissue distribution and expression, together with differential substrate availability, may contribute differently to the modulation of enteric purinergic signalling. In this respect, an interesting study by Duarte-Araújo et al. (2009) focused its attention on the enzymes involved in the extracellular conversion of nucleotides into nucleosides, such as the ecto-nucleotidases. The main concept arising from this study was that ATP, released from activated smooth muscle cells, as well as from stimulated myenteric neurons, can be converted preferentially into AMP, and then dephosphorylated to adenosine by ecto-5′-nucleotidase. Although, under physiological conditions, this extracellular sequential catabolism of ATP, operated by ecto-nucleotidases, seems to contribute marginally to the overall interstitial adenosine concentration within the myenteric plexus, the authors claimed that this enzymatic pathway could become more important under pathological conditions, such as intestinal ischaemia and inflammation, characterized by increased extracellular ATP levels.

The evaluation of how and to what extent the enteric ‘purinome’ is involved in the pathophysiology of intestinal motor disorders represents an interesting point of investigation and discussion. De Man et al. (2003) provided the first evidence about a significant rearrangement of intestinal purinergic signalling in the presence of inflammatory disorders. These authors demonstrated that, in a model of chronic intestinal inflammation induced by Schistosoma mansoni in mice, the physiological inhibitory control exerted by A₁ receptors on contractions of the small intestine was no longer evident. The mechanism proposed to explain this loss of modulating action was the desensitization of A₁ receptors caused by their prolonged exposure to high concentrations of endogenous adenosine, released as a consequence of the infectious disorder. Recently, our research group reported a functional rearrangement of high affinity A₁ and A_2A receptors in the presence of experimental colitis (Antonioli et al., 2011). In particular, we observed a preferential activation of A_2A receptors over A₁ receptors in the presence of bowel inflammation. Such a differential receptor activation was ascribed to a channelled production of endogenous adenosine towards A_2A receptors operated by the synthetic enzyme, ecto-5′-nucleotidase, to which this receptor pathway seems to be functionally linked.

The presence of functional interplays between adenosine receptors and enzymatic pathways, critically involved in controlling the magnitude of purinergic responses to pathological events, is consistent with previous findings from our laboratory (Antonioli et al., 2010). In that study, we observed an impairment of A₃ receptor-mediated tonic inhibitory control by endogenous adenosine in rat colonic motility during bowel inflammation, despite obtaining evidence of an increased density of functioning and pharmacologically recruitable A₃ receptors. In an attempt to explain this intriguing discrepancy, we investigated the mechanisms underlying the regulation of adenosine concentration, focusing our attention on adenosine deaminase, the main enzyme responsible for adenosine catabolism. As, in the presence of colitis, a marked increase of adenosine deaminase was observed in the neuromuscular layer, in accordance with the ‘purinome’ concept we proposed that such a mechanism could represent a regulating system, aimed at preventing A₃ receptors from being activated by endogenous adenosine. In this framework, the enhanced A₃ receptor expression was interpreted as the consequence of a compensatory response to a reduced availability of endogenous adenosine in the compartment of the A₃ receptor biophase.

In conclusion, the interesting findings reported by Zizzo et al. represent a step forward in the better understanding of how adenosine can influence the ENS functions under both normal and pathological conditions. Although these authors have previously highlighted the involvement of adenosine receptor subtypes in the regulation of mouse intestinal motility (Zizzo et al., 2006; 2007;), with the present study they have made a first attempt of providing a comprehensive evaluation of the roles played by adenosine in the regulation of duodenal motility. In particular, consistent with the emerging concept of the enteric ‘purinome’, they propose the evaluation of the mechanisms involved in nucleoside metabolism, together with the activation of adenosine receptor subtypes, as an integrated unit aimed at modulating the activity of myenteric neurons, through a fine tuning of purinergic signalling. Accordingly, future investigations, addressing the possible role of the enteric ‘purinome’ in the processes of intestinal neuroplasticity, could be of pivotal importance in elucidating the pathophysiology of various pathological conditions associated with functional bowel disorders, including intestinal infection, inflammatory bowel diseases, irritable bowel syndrome, intestinal ischaemia and post-operative ileus, in order to channel investigative efforts towards novel and targeted therapeutic interventions in the adenosine system.

Glossary

Abbreviation

ENS

enteric nervous system

Conflict of interest

The authors declare no conflicts of interest.