RET Activation by Glial Cell Line-Derived Neurotrophic Factor Protects Myenteric Neurons Against Selectively Lethal Challenge by Inflammation
Intestinal functions are predominantly controlled by the enteric nervous system (ENS), which is comprised of the inner submucosal and outer myenteric plexuses. The myenteric plexus innervates the intestinal smooth muscle layers to coordinate intestinal motility. Alterations in the structure of the myenteric plexus can result in dysmotility, which is seen in severe intestinal inflammation, such as the transmural inflammation of Crohn’s disease, one of the two major types of Inflammatory Bowel Disease (IBD). As well, animal models of IBD have displayed sustained and seemingly irreversible myenteric neuron loss. GDNF, the neurotrophin responsible for development of the ENS, displays neuroprotective effects in adult systems, but this has not been studied in the post-natal ENS. We proposed that GDNF could protect myenteric neurons from inflammatory damage and tested this in a co-culture model system as well as in the TNBS-induced model of colitis in rats. Two distinct neurotoxic challenges that are present in intestinal inflammation are metabolic inhibition and high levels of nitric oxide, and these were modelled in vitro using chemical agents. Quantification of neuronal structure using immunocytochemistry and image analysis showed that either exogenous or cell-mediated GDNF delivery were equally successful in preventing myenteric neuron loss from these challenges. Since only the mature isoform of GDNF has been shown to influence myenteric neurons, the processing of GDNF in the intestine during physiological and inflammatory states was investigated. The matrix metalloproteinase MMP-9 was shown to be an essential protease in GDNF processing to yield the mature isoform as well as functional outcomes. Both MMP-9 and GDNF were expressed in intestinal smooth muscle cells, and these showed parallel upregulation with pro-inflammatory stimuli. We conclude that exogenous or cell-mediated GDNF delivery is effective in preserving myenteric neurons during inflammatory challenge.