The mammalian enteric nervous system (ENS) regulates diverse intestinal functions in response to changes in the content of the intestinal lumen. The ENS consists of enteric neurons and glia localized within the wall of the gastrointestinal tract and extending processes into the gut lamina propria. Recent studies suggest that ENS-derived peptides or neurotransmitters can modulate immune cells and affect intestinal immune homeostasis. Indeed, in the lamina propria, neural projections and glia are in close proximity to intestinal immune cells and epithelial cells, providing an anatomical basis for the existence of neuro-immune circuits and their modulation of physiological intestinal functions. Conversely, the ENS can be modulated by the immune system and intestinal microbiota, but we have very little understanding of the multidirectional crosstalk between these systems in maintenance of intestinal homeostasis and in disease processes.
In our transcriptomic studies of intestinal innate lymphoid cells, we found that CCR6+ ILC3 selectively express multiple genes implicated in nervous system functions (e.g. pathfinding, signaling, neurotransmission). Among these, VIPR2, a receptor for vasoactive intestinal peptide (VIP), stood out, because we found that VIP-producing neurons specifically extended into ILC3-containing cryptopatches within the small intestine lamina propria. These aggregates of RORgt-dependent CCR6+ lymphocytes, also referred to as lymphoid tissue inducer (LTi) cells, receive signals from bacteria in the gut lumen and seed the formation of B cell-containing isolated lymphoid follicles. In response to the microbiota, the LTi cells also produce high levels of IL-22, which acts on intestinal epithelial cells to promote expression of antimicrobial peptides and enforce barrier functions. We employed chemogenetics to demonstrate that activation of VIP-producing neurons results in inhibition of IL-22 production by the LTi cells, which renders the mice highly susceptible to infection with Citrobacter rodentium, an invasive enteropathic bacterium. In contrast, inhibition of the VIPergic neurons resulted in enhanced IL-22 production by LTi cells and greater resistance to the pathogenic bacterium. The VIP neurons are rapidly activated following food intake, and that results in VIPR2-dependent reduction of IL-22 production. As a consequence, there is an increase in growth of luminal bacteria, particularly SFB, but there is also an increase in lipid absorption across the epithelium, since IL-22 inhibits expression of lipid-binding proteins and transporters. Thus, there is a circadian tradeoff between barrier function and nutrient uptake, regulated by a VIP- and IL-22-dependent neuroimmune circuit. We are currently asking how food is sensed and collaborating with Rob Froemke’s lab to determine the role of the vagus nerve and of central nervous system circuitry in the response of the VIPergic enteric neurons.
We are also investigating the diversity of enteric neurons and glia, using a combination of single nuclear RNAseq and spatial transcriptomics, and the roles of other neuronal genes expressed in ILC3, particularly in the context of formation of innervated cryptopatches and of the VIP-dependent neuroimmune circuit. Other questions include a) how are enteric neurons modulated by the intestinal microbiota and by immune cells during homeostasis and during microbial/immune disturbances? b) how do specific components of the ENS modulate innate and adaptive immune cell responses? c) does the ENS “sense” pathogens and promote integration of the immune response to eliminate the threat? d) can we modulate neuronal circuits of the ENS and the CNS to influence immune responses and change the outcome/recovery of intestinal infections? With this research we aim to perform a comprehensive description of the components of different neuroimmune circuits, how they are activated and how they maintain intestinal immune homeostasis.
From a translational standpoint, we are applying our understanding of neuronal-immune cell interaction and the subsequent modulation of epithelial barrier integrity to study the gut-liver axis. Non-alcoholic fatty liver disease (NAFLD) has been increasing in prevalence over the last 20 plus years, with no approved medical therapies to date. This unmet medical need underlines our incomplete understanding of the pathogenic factors contributing to NAFLD development. The microbiota has emerged as a potential variable mediating both the development and progression of NAFLD. As such, we are interested in evaluating the outcomes of manipulating VIP neurons in animal models of NAFLD and assessing whether the resultant change in microbiota dependent gut permeability can affect the exposure of the liver microenvironment to microbes and/or microbial metabolites. Our aims are to investigate whether these changes can occur and if so whether the gut enteric system can be therapeutically targeted to ameliorate downstream liver disease.
Related Publications:
Talbot, J., Hahn, P., Kroehling, L., Nguyen, H. Li, D. & Littman, D.R. (2019) VIP-producing enteric neurons interact with innate lymphoid cells to regulate feeding-dependent intestinal epithelial barrier functions. Submitted. bioRxiv doi: https://doi.org/10.1101/721464
Pokrovskii, M., Hall, J.A., Ochayon, D.E., Yi, R., Chaimowitz, N.S., Seelamneni, H., Carriero, N., Watters, A., Waggoner, S.N., Littman, D.R.*, Bonneau, R.* & Miraldi, E.R. (2018) Transcriptional regulatory networks that promote and restrict identities and functions of intestinal innate lymphoid cells. Immunity, 51, 185-97. PMID: 31278058
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