Neural interactions with the microbiome and immune system that produce inflammation in Hirschsprung disease models

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ABSTRACT The importance of the enteric (ENS) and autonomic nervous systems (ANS) in maintaining intestinal health is made evident in Hirschsprung disease (HSCR), defined by the absence of the ENS in distal bowel and caused by a number of mutations (e.g., Ednrb). HSCR is managed with surgery to remove the ‘abnormal’ bowel, but many patients suffer from persistent functional and inflammatory bowel complications (e.g., Hirschsprung Associated Enterocolitis, HAEC), indicating that the ENS in proximal bowel is also affected by HSCR-related mutations despite appearing anatomically normal. Our lab generated a novel HSCR mouse model with Ednrb mutations that expresses GCaMP, a genetically-encoded calcium indicator, to measure synaptic connectivity in proximal bowel where the ENS is present. By using in vivo and ex vivo colon preparations that keep intrinsic and extrinsic nerve pathways intact, we have previously shown how ENS, ANS (sympathetic and parasympathetic), and sensory inputs normally influence colon function. Ednrb-/- (null)-GCaMP mice had specific changes in myenteric neuron activity (spontaneous and synaptically-evoked) in proximal colon at post-natal ages that likely contribute to the chronic issues in HSCR. Interestingly, age-matched Ednrb+/- (het)-GCaMP mice displayed a similar functional phenotype, despite having ENS innervation in the entire bowel, and they continued to be significantly different from wild-type littermates as adults. These mice exhibited dysmotility, had altered immune cell counts in the lamina propria, and appeared to be more vulnerable to experimentally-induced inflammation. Therefore, ‘subclinical’ mutations in HSCR-related genes (i.e., those that would not cause a HSCR diagnosis) appear to be sufficient to impair aspects of ENS circuit development. Importantly, Ednrb-/- and Ednrb+/- GCaMP mice model different aspects of HSCR: ‘clinical’ HSCR complications in newborns and ‘subclinical’ HSCR issues that persist into childhood and adulthood. Several recent HSCR studies have identified shifts in the microbiome and impaired immunity that may underlie increased inflammation and HAEC, but the causal role of the microbiome in producing HSCR-related dysfunction is unknown. Based on what is known about neuro- immune-microbiota interactions, I hypothesize that the ENS/ANS contributes to immune dysfunction indirectly via microbiome changes due to dysmotility, but also potentially through changes in direct neuro-immune communication. To test this hypothesis, experiments in Aim 1 (K99) will define the immune cell profile and microbiome in ‘clinical’ and ‘subclinical’ HSCR mouse models at post-natal ages, and determine whether the ‘HSCR microbiome’ is necessary and/or sufficient to produce ENS, ANS, or immune dysfunction. Experiments in Aim 2 (R00) will test whether HSCR-related dysfunction in the ENS/ANS impairs the ability to respond to inflammatory immune challenge, and determine the role of the ‘HSCR microbiome’ in resolving inflammation. As numerous digestive disorders exhibit neural, microbiome and/or immune dysregulation, the proposed research program will broadly impact human health.