S-nitrosothiol signals in the enteric nervous system: lessons learnt from big brother

Abstract

Nitric oxide (NO) is a functionally important neurotransmitter signaling molecule generated by mammalian and bacterial nitric oxide synthases (NOS), and by chemical conversion of dietary nitrite in the gastrointestinal (GI) tract. Neuronal NOS (nNOS) is the most abundant isoenzyme in the enteric nervous system, and targeted deletion in transgenic mice has clearly demonstrated its importance in normal gut function. Enteric neuropathy is also often associated with abnormal NO production, for example in achalasia and diabetic gastroparesis. Not surprisingly therefore, aberrant nNOS activity is widely implicated in enteric disease, and represents a potential molecular target for therapeutic intervention. One physiological signaling mechanism of NO bioactivity is through chemical reaction with the heme center of guanylyl cyclase, resulting in the conversion of cGMP from GTP. This second messenger nucleotide signal activates cGMP-dependent protein kinases, phosphodiesterases, and ion channels, and is implicated in the neuronal control of GI function. However, few studies in the GI tract have fully related NO bioactivity with specific molecular targets of NO-derived signals. In the central nervous system (CNS), it is now increasingly appreciated that NO bioactivity is often actively transduced via S-nitrosothiol (SNO) signals rather than via activation of guanylyl cyclase. Moreover, aberrant S-nitrosylation of specific molecular targets is implicated in CNS pathology. S-nitrosylation refers to the post-translational modification of a protein cysteine thiol by NO, forming an endogenous SNO. Because cysteine residues are often key regulators of protein function, S-nitrosylation represents a physiologically important signaling mechanism analogous to other post-translational modifications, such as O-phosphorylation. This article provides an overview of how neurotransmitter NO is produced by nNOS as this represents the most prominent and well defined source of SNO production in the enteric nervous system. Further, it provides a perspective of how S-nitrosylation signals derived from multiple diverse sources may potentially transduce NO bioactivity in the GI tract. Possible lessons that might be learnt from the CNS, such as SNO mediated auto-inhibition of nNOS activity and modulation of neuronal cell death, are also explored as these may have pathophysiological relevance in enteric neuropathy. Thus, S-nitrosylation may mediate previously underappreciated NO-derived signals in the enteric nervous system that regulate homeostatic gut functions and disease susceptibility.

Keywords: S-nitrosoglutathione; S-nitrosothiol; enteric glia and neurons; enteric neuropathy; nNOS; nitric oxide.

Figure 1

Schematic representation (top) of nNOS activity generating SNO from l-arginine or from exogenous nitrite. This synthesis step may involve direct protein–protein interactions with nNOS or another SNO-synthase, transfer of a coordinated NO from a transition metal center, or by chemical reaction with a reactive nitrosium ion intermediate. (Bottom) shows anti-SNO immunoreactivity in the mouse small intestinal myenteric plexus (scale bar = 40 μm).

Figure 2

Schematic representation (top) of nNOS_α PDZ domain-interactions with PSD-95 and the NMDA receptor in the CNS. Calcium ion entry activates nNOS production of NO. Excessive NO production results in the auto-S-nitrosylation and inhibition of nNOS and the NMDA receptor. Although auto-S-nitrosylation of nNOS represents a likely inhibitory mechanism in the GI tract, a physiological role for NMDA receptor signaling remains controversial. (Bottom) Shows the various nNOS protein isoforms; nNOS_α contains a NH₂-terminus PDZ/GLGF-domain (∼100 amino acids), that modulates interactions with other PDZ-containing proteins. nNOS_β and nNOS_γ lack a PDZ domain; H, heme consensus site; Ca, calmodulin consensus site; FMN, flavin mononucleotide consensus site; and FAD, flavin adenine dinucleotide consensus site.