Neuro-immune interaction in the enteric nervous system from early development to aging

Tissue resident macrophages have long been known for their role in host defense and pathogen clearance, but only in recent years have we come to appreciate that these cells can develop a surprising degree of functional specialisation, thereby fulfilling tissue-specific functions in the organ in which they reside. The instruction of such specific functions is largely imprinted via signalling from surrounding cells, the so-called macrophages niche. Thanks to the signals derived from the surrounding niche, macrophages can adopt specific transcriptomes and functions, and fulfil tissue-specific functions including the recycling of surfactant in the lung, the regulation of electric conduction in the heart, and regulation of arterial stiffness in the aorta. In the gut, tissue resident macrophages can be found throughout the distinct layers from the lumen to the serosa, where they perform functions specific to their niche. Beneath the epithelial cells and in close proximity to luminal content, lamina propria macrophages participate in host defense by the clearance of invading pathogens; deeper within the tissue and located close to blood vessels, blood vessel-associated macrophages maintain vascular integrity; finally, muscularis macrophages located close to the neurons of the myenteric plexus maintain neuronal health and regulate peristalsis. Neuron-associated macrophages are thus critical for the correct functioning of the GI tract, and for the support and survival of enteric neurons. The enteric nervous system (ENS) derives from precursors that colonise the GI tract prior to birth, and continues to undergo maturation during early postnatal life. Indeed, the spontaneous contractions that occur throughout the length of the intestine, and that are required to propel intestinal content, only mature after the time of weaning. This maturation reflects the needs of the intestine, as it needs to adapt to the transition from a liquid diet to a solid diet after the time of weaning. In Chapter 3, we demonstrate that early in life, macrophages located in the muscularis externa participate in the refinement of enteric nervous system, and that the absence of such refinement leads to accelerated intestinal transit and hyperganglionosis. At the time of weaning, these macrophages undergo a transcriptional and functional transition, with a reduction of phagocytic Lyve1+ macrophages, and the expansion of a neuron-associated macrophage subset. The phenotype of neuron-associated macrophages was shown to be imprinted by transforming growth factor beta (TGFβ), that is produced by the enteric nervous system itself. Loss of TGFβ signalling led to a reduction in neuron-associated macrophages, loss of enteric neurons and delay in intestinal transit. Taken together, these findings highlight the developmental plasticity of muscularis macrophages, and shed light on how neurons can imprint a local, neurosupportive macrophage population. Once the neuron-associated macrophage population has been established after weaning, these macrophages provide support and protection to enteric neurons throughout life. Considering their vital role in sustaining neuronal survival, it is of key importance to investigate the role these cells may play in neurodegenerative diseases of the GI tract. It has been shown that during aging, there is a progressive loss of enteric neurons which may underlie age-associated gastro-intestinal complaints, including constipation. In Chapter 4, we demonstrate that in mice, females develop a delay in gastro-intestinal transit and neurodegeneration with aging, while male mice are protected from age-related gastro-intestinal dysfunction. To investigate whether loss or dysfunction of muscularis macrophages may underlie aging-associated neurodegeneration, we performed single-cell RNA sequencing on muscularis macrophages on young and old, male and female mice. Single-cell RNA sequencing revealed that significant transcriptional shifts occur between young and old female macrophages, while the transcriptome of old male macrophages remains relatively similar to that of young macrophages. Furthermore, we detected a significant loss of neuron-associated macrophages in aging females mice, that may underlie neurodegeneration in aging female mice. Of interest, neuron-associated macrophages were preserved in aging male mice, an observation that was associated with the expansion of a subset of neuron-associated macrophages upregulating homeostatic genes, which may provide support to neuronal regeneration in aging male mice. Finally, we detected a reduction of phagocytosis by aging female macrophages, coupled with downregulation of key genes including Itgb5. To what extent this may be implicated in the clearance of neuronal debris and insoluble protein aggregates remains to be studied. In line, we observed accumulation of such aggregates in the aging muscularis externa. While further studies are warranted to validate these findings, our results suggest that dysfunction of loss of muscularis macrophages may play a critical role in neurodegeneration and constipation in the aging female muscularis externa. Taken together, the findings presented in this thesis highlight the exceptional plasticity of muscularis macrophages, that adapt throughout life to the evolving needs of the tissue. Furthermore, we uncover a hitherto unknown role for muscularis macrophages in shaping the enteric nervous system during development, and shed light on the instruction of neuro—supportive macrophages in adulthood. These findings pave the way towards harnessing the therapeutic potential of muscularis macrophages, and may, in the future, enable the scientific community to tackle neurodegenerative diseases that affect the GI tract, which currently lack effective therapeutic strategies.

Jaar van publicatie:2023

Toegankelijkheid:Open