In the past 5 years, we have studied in depth the intestinal macrophage population with a special interest in the subpopulation that closely interacts with the enteric nervous system, both in mice and humans. In mice, we introduced the concept that these so-called neuron-associated macrophages (NA-MF) closely resemble microglia, the resident macrophages of the brain. We showed that NA-MF have a similar gene signature, morphology and above all a similar function as brain microglia. An unexpected but exciting finding was that resident macrophages in the gut, like microglia, play a key role in shaping the enteric nervous system early in life. Before weaning, enteric neurons are rather disorganized with too many neurons and too many interconnecting nerve tracts compared to the adult and mature enteric nervous system. We showed that around the time of weaning, intestinal macrophages start to engulf neurons and synapses and transform the immature enteric nervous system into its mature state with well-organized ganglia elegantly interconnected with nerve tracts. Notably, we demonstrated that once macrophages have finalized this vital task, they completely switch their function and start protecting and nourishing the enteric neurons. Surprisingly, this switch is instructed by the enteric neurons themselves via the release of TGF-beta, which interestingly imprints macrophages to become neuroprotective and obtain a microglia-like transcriptome. Finally, we were able to identify a membrane marker characteristic for NA-MF, i.e. F11r, which allowed us to investigate the NA-MF in more detail in intestinal disease models, but also in a variety of other organs. Not only in the gut, but also in lung, liver, pancreas, peripheral nerves, etc we were able to demonstrate the presence of NA-MF, again characterized by the expression of microglia-like genes and closely interacting with nerve fibres. Also in the human gut, we identified the NA-MF population and demonstrated the role of TGF-beta to obtain the microglia-like phenotype. So taken together, the concept of NA-MF is not restricted to the gut but is biologically conserved and consistent throughout the entire nervous system, both in mice and humans.
In the second part of the project, the potential role of NA-MF in intestinal neurodegenerative disorders such as ageing, diabetes and obesity was evaluated. We introduced and optimized preclinical models of these three conditions and developed new methods, such as a novel method to measure gastric emptying in mice using MRI. We showed that the number of enteric neurons indeed decreases (=neurodegeneration) in all three models, a finding that was associated with impaired intestinal function. In aged mice, we identified a gene potentially contributing to the loss of enteric neurons, a finding that is currently further explored. In diabetic mice, we have identified two macrophage populations which protect diabetic mice from developing delayed gastric emptying, a finding of which the therapeutic potential is further explored.
Finally, we have succeeded to develop a co-culture platform derived from human inducible pluripotent stem cells (iPSCs). iPSC-derived enteric neurons are co-cultured with iPSC-derived macrophages up to 60 days revealing enteric ganglia interconnected with nerve tracts and covered with mature macrophages. This co-culture is unique and will allow us to further study in great depth how macrophages interact with enteric neurons and will serve as a platform to validate identified therapeutic targets, opening new avenues for the treatment of intestinal neurodegenerative disorders.
