Intestinal
inflammation challenges both function and structure of the enteric nervous system (ENS). In the animal model of TNBS-induced
colitis, an influx of immune cells causes early neuron death in the neuromuscular layers, followed by axonal outgrowth from surviving neurons associated with upregulation of the
neurotrophin GDNF (
glial cell line-derived neurotrophic factor).
Inflammation could involve
ischemia and metabolic inhibition leading to neuronal damage, which might be countered by a protective action of
GDNF. This was examined in a primary co-culture model of rat myenteric neurons and smooth muscle, where metabolic challenge was caused by dinitrophenol (DNP), O-
methyl glucose (OMG) or
hypoxia. These caused the specific loss of 50% of neurons by 24 h that was blocked by
GDNF both in vitro and in whole mounts. Neuroprotection was lost with RET inhibition by
vandetanib or GSK3179106, which also caused neuron loss in untreated controls. Thus, both basal and upregulated
GDNF levels signal via RET for neuronal survival. This includes a key role for upregulation of HIF-1α, which was detected in neurons in
colitis, since the inhibitor
chetomin blocked rescue by
GDNF or ischemic pre-conditioning in vitro. In DNP-treated co-cultures, neuron death was not inhibited by zVAD,
necrosulfonamide or
GSK872, and cleaved
caspase-3 or - 8 were undetectable. However, combinations of inhibitors or the RIP1kinase inhibitor Nec-1 prevented neuronal death, evidence for RIPK1-dependent necroptosis. Therefore,
inflammation challenges enteric neurons via
ischemia, while
GDNF is neuroprotective, activating RET and HIF-1α to limit programmed cell death. This may support novel strategies to address recurrent
inflammation in IBD.