Abstract
In this thesis, we have described organoids as a model to study developmental cues underpinning homeostatic turnover with a focus on the intestine and
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its hormone-producing enteroendocrine cells (EECs). EECs are extremely rare in vivo (less than 1% of the intestinal epithelium), producing more than twenty different hormones. Due to their rarity and diversity, the development and regulation of these cells have remained enigmatic. Their study is further complicated by the lack of good in vitro models and the fact that human EECs are different from their murine counterparts. EECs sense luminal components such as microbiota-secreted products and nutrients, although the specific stimuli regulating hormone secretion are poorly described. Since EECs control key processes related to food intake and processing, these would be attractive targets for the treatment of metabolic diseases. Organoids have allowed us to assess what peptides human EECs produce, and we found new signals where these cells respond to. With such signals, we could influence hormone secretion, that would potentially be interesting for obesity or anorexia treatment.
A second hard-to-study cell type is the venom-producing cell of snake venom glands. Snakebite is responsible for more than 100.000 lethal cases worldwide every year, while the same venom also contains therapeutically interesting molecules. An example are substances that lower the blood pressure. To study cells that make this venom, we have established an organoid system of the snake venom gland. These first organoids from reptiles evidence that snakes have stem cells in their adult stage, just like mammals have. We have generated more insights into the diversity of venom-producing cells in snakes, and hope that these cultures could be used for the production of anti-venom.
Finally, we have discovered in using our organoid models that different functional cell types (enterocytes, goblet cells and EECs) of the intestine can alter their activities over their lifetime, and that this is regulated by a BMP morphogen signaling gradient.
I believe the work in this thesis highlights the utility of organoids to identify developmental cues regulating homeostatic turnover. Organoids allow for a reductionist approach, in which the role of individual signaling components acting on the epithelium in the absence of mesenchyme can be studied. It also stresses the importance of understanding the maturation level of the cell types in organoids. These cultures are widely used to study the functioning of cell types, for example lipid uptake in the intestine. A lack of BMP exposure in this scenario would severely hamper definite conclusions, as enterocytes in such cultures would lack expression of essential players in lipid handling. We secondly demonstrated organoids as a model to study the functioning of hard-to-study cell types, in particular the intestinal EECs.
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