Breaking it down: the metabolism of neurotransmitters in the colonic wall

Prior studies suggest that β-nicotinamide adenine dinucleotide (β-NAD) is an important inhibitory motor neurotransmitter in the enteric nervous system. Metabolism of β-NAD at the neuroeffector junction (NEJ) is likely to be necessary for terminating inhibitory neurotransmission and may also produce bioactive metabolites. The enteric NEJ consists of enteric neurons and postjunctional cells of the SIP syncytium, smooth muscle cells (SMCs), interstitial cells of Cajal (ICC), and cells expressing platelet-derived growth factor receptor α (PDGFRα(+) cells). We examined possible specialized functions of the NEJ in β-NAD metabolism by determining the degradation of 1,N(6)-etheno-NAD (eNAD) in colonic tunica muscularis of wild-type, Cd38(−/−), Nt5e(−/−), Enpp1(−/−), and Cd38(−/−)/Nt5e(−/−) mice and in SIP cells from mice expressing cell-specific fluorescent reporters purified by FACS. We measured eNAD and its metabolites eADP-ribose, eAMP and e-adenosine (eADO) from tissues and sorted SIP cells using liquid chromatography. eNAD exposed to colonic muscularis of wild-type mice produced eADPR, eAMP, and eADO. CD38 mediated the conversion of eNAD to eADPR, whereas ENPP1 mediated degradation of eNAD and eADPR to eAMP. NT5E (aka CD73) was the primary enzyme forming eADO from eAMP. PDGFRα(+) cells and SMCs were involved in production of eADO from eNAD, and ICC were not involved in extracellular metabolism of eNAD. CD38 mediated the eNAD metabolism in whole tissues, but CD38 did not appear to be functionally expressed by SMCs or ICC. NT5E was expressed in SMCs > PDGFRα(+) cells. Our data show that extracellular metabolism of β-NAD in the colon is mediated by multiple enzymes with cell-specific expression.