Homeostasis of Glial Cells in the Mammalian Gut

The gastrointestinal tract is a vital organ that converts our diet into useful digestible nutrients, contributes to the maintenance of water balance and protects our body from pathogenic microorganisms that are present within the lumen of the gut, along with large numbers of beneficial bacteria. In order for the gut to carry out its essential functions, it contains exquisitely specialised cells, including epithelial cells, immune cells, nerve cells and muscle cells. Intestinal epithelial cells are tightly connected to each other to form a sophisticated gatekeeping system that allows the selective transport of nutrients and water but keeps away harmful toxins or pathogenic bacteria. Immune cells constantly monitor the lumen and the wall of the gut and respond in case the essential intestinal barrier is breached. Finally, complex networks of nerve cells within the gut wall are responsible for generating intestinal movements that are essential for proper digestive function by activating the musculature of the gut wall. Since the intestinal epithelium is constantly exposed to harmful substances and pathogenic microorganisms, it is quite vulnerable and is often damaged. Normally this does not have detrimental consequences for an organism since all cells of the intestinal epithelium are continuously replenished by stem cells that are dedicated to producing constantly fresh epithelial cells. Although the continuous regeneration of the intestinal epithelium is essential for maintaining it in good working order, other cell types play a major role in keeping them healthy. In particular, glial cells, which normally accompany and support nerve cells in all parts of the nervous system, are also found in the vicinity of intestinal epithelial cells and release substances that are essential for maintaining the intestinal epithelial barrier; if these enteric glial cells are eliminated in experimental conditions, the barrier breaks down and animals die from acute inflammation of the small intestine. In addition, several studies have suggested that the inflammation that accompanies common gut diseases, such as Crohn's disease or ulcerative colitis, may also involve the abnormal interaction of glial cells with intestinal epithelial cells and immune cells. These observations support the idea that despite their specialised functions, the different cell types that make up the gut wall (and indeed any organ) need to work in concert in order to support its physiological roles.

Despite the important roles of the intestinal glial cells in supporting the critical functions of the nerve cells and the epithelium of the gut, very little is known about their biology in healthy individuals and in disease situations. In this proposal we will aim at filling this knowledge gap by building on some of our own recent observations. In particular, we will identify and characterise the properties of the gliogenic stem cells which generate new glial cells throughout life. We will also identify conditions and signals that modulate the behaviour of intestinal glial cells. Finally, we plan to characterise molecules which are located within the nucleus and are important for these cells to maintain their properties and continue to generate new glial cells throughout adult life.

Normal digestive function depends on the fine balance between the loss of old and the production of new cells in the different gut tissues and the optimal cross talk between the different cell types. Breakdown of such an equilibrium results in uncontrolled growth of cells (cancer), severe inflammation of the gut wall (inflammatory bowel disease-IBD) or inability of the gut wall to protect the internal environment of an organisms from toxic substances or pathogenic bacteria. Understanding how local glial cells contribute to the integrity and normal function of gut tissues, we can ultimately use these cells as a means to alter the course of common debilitating gastrointestinal disorders.

The overall objective of this project is to elucidate the cellular and molecular mechanisms that underpin homeostasis of the enteric nervous system. By employing inducible lineage tracing in the gut of adult mice we have obtained evidence that the population of mucosal glial cells, which is critical for epithelial barrier function, is transitory and continuously replenished by new cells originating in the myenteric ganglia. In addition, we have demonstrated that mucosal colonisation by glial cells is initiated during the early postnatal period under the control of the gut microbiome. In this project, we will test the hypothesis that the homeostatic population of mucosal glia is maintained by unipotent self-renewing progenitora that reside within the myenteric ganglia. We propose experiments to characterise the properties of these gliogenic progenitors and determine the spatial distribution of their progeny. In addition, we will examine the role of members of the Sox family of transcriptional regulators in maintaining the population of mucosal glial cells and uncover gene regulatory networks that control glial cell homeostasis in the adult gut. Finally, we will test the prediction that microbiome-innate immune cell axis controls the homeostasis of mucosal glial cells via cytokines of the TNF family.

Our studies will provide critical insight into the cellular and molecular mechanisms that integrate the population of enteric glial cells into the dynamic and often pernicious microenvironment of the gut in adult animals. Understanding such mechanisms is essential for elucidating the regulatory processes that maintain gastrointestinal function and internal homeostasis under physiological conditions. Ultimately, it provides us with the opportunity to harness enteric glia to influence the outcome of pathogenetic mechanisms of common gastrointestinal disorders, including inflammatory bowel disease.

The most immediate impact of this project will be in the fields of tissue maintenance and repair and the primary beneficiaries will be researchers in the tissue homeostasis and gastrointestinal biology. We will rely on existing proactive strategies to reach these scientists and will ensure that ideas and reagents resulting from this project will reach workers in relevant fields. In particular, information regarding the properties of gliogenic progenitors, the transcriptional mechanisms that maintain enteric glia homeostasis and their response to the microbiome-cytokine axis, as well as potential new experimental approaches that will be established in the course of our work will be particularly useful for advancing the understanding self-renewal of stem and progenitor cells and how they are influenced by cell intrinsic mechanisms and environmental signals.

We will disseminate information by contacting directly our collaborators in this project and other prominent scientists in the field of tissue homeostasis and gastrointestinal biology. We will also invite colleagues from around the world to present research seminars, so that we learn from their experience and expose them to our work. Active participation in national and international meeting that are relevant to our work will be essential in order to maximise interactions with researchers in other fields. We will host researchers and trainees in their labs in order to be exposed to ideas and techniques that are being used routinely in our group or will be developed further in the context of the present project.

We will seek to publish our findings in high profile journals with broad readership. This is particularly important for the field of enteric neuroscience since, despite the central role of the ENS in regulating gastrointestinal physiology and its implication in a variety of common digestive disorders, most of the research in this area is published in highly specialised journals and consequently addresses a relatively narrow audience. We will give particular emphasis on the publication of review articles or commentaries which in addition to summarising existing information, will put forward ideas and hypotheses that help advancing our understanding of the ENS in the context of the complex microenvironment of the gut.

One of our main responsibilities is to engage with the wider public. We will engage with local high schools and give the opportunity to students to spend the summer months shadowing members of our lab. In addition, we will continue our contributions to the University of the Third Age, which holds an annual event at NIMR.

Although the main goal of our research project is to uncover fundamental mechanism relating to the homeostasis of the ENS, it is possible that emerging results will have implications for the pathogenesis and treatment of gastrointestinal disorders and/or lead to a better understanding of the role of microflora on digestive function. On this occasion we will take action by contacting our Technology Transfer Officer at NIMR (http://www.nimr.mrc.ac.uk/research-facilities/technology-transfer/) and the Business Manager at MRC Technology (http://www.mrctechnology.org/) who have extensive expertise in patenting and licensing to consider the potential commercial exploitation of our findings.