Human intestinal nutrient sensors
A multi-institutional group of researchers has developed strategies to identify regulators of intestinal hormone secretion. In response to incoming food, these hormones are secreted by rare hormone producing cells in the gut and play key roles in managing digestion and appetite. The team has developed new tools to identify potential ‘nutrient sensors’ on these hormone producing cells and study their function. This could result in new strategies to interfere with the release of these hormones and provide avenues for the treatment of a variety of metabolic or gut motility disorders. The work will be presented in an article in Science.
The intestine acts as a vital barrier. It protects the body from harmful bacteria and highly dynamic pH levels, while allowing nutrients and vitamins to enter the bloodstream. The gut is also home to endocrine cells, which secrete many hormones that regulate bodily functions. These enteroendocrine cells (EECs, endocrine cells of the gut) are very rare cells that release hormones in response to various triggers, such as stretching of the stomach, energy levels and nutrients from food. These hormones in turn regulate key aspects of physiology in response to the incoming food, such as digestion and appetite. Thus, EECs are the body’s first responders to incoming food, and instruct and prepare the rest of the body for what is coming.
Medications that mimic gut hormones, most famously GLP-1, are very promising for the treatment of multiple metabolic diseases. Directly manipulating EECs to adjust hormone secretion could open up new therapeutic options. However, it has been challenging to understand how gut hormone release can be influenced effectively. Researchers have had trouble identifying the sensors on EECs, because EECs themselves represent less than 1% of cells in the intestinal epithelium, and in addition the sensors on these EECs are expressed in low amounts. Current studies mainly rely on mouse models, even though the signals to which mouse EECs respond are likely different compared to those to which human EECs respond. Therefore, new models and approaches were required to study these signals.
The team has previously developed methods to derive large quantities of EECs in human organoids. Organoids contain the same cell types of the organ they are derived from, and are therefore useful to explore the development and function of cells such as EECs. Using a special protein Neurogenin-3, the researchers could generate high numbers of EECs.
In the past, the researchers developed a way to increase the number of EECs in organoids of the intestine. Considering that EECs have different sensors and hormone profiles in different regions of the gut, studying these rare cells requires that the researchers make EEC enriched organoids of all these different regions. In the current study, the team managed to enrich EECs in organoids of other parts of the digestive system, including the stomach. Like the real stomach, these stomach organoids respond to known inducers of hormone release and secrete large amounts of the hormone Ghrelin, which is also called the ‘hunger hormone’ because it plays a key role in signaling hunger to the brain. This confirms that these organoids can be used to study hormone secretion in EECs.
Since EECs are rare, researchers have struggled to profile many EECs. In the current study, the team identified a so-called surface marker, called CD200, on human EECs. The researchers used this surface marker to isolate a large number of human EECs from organoids and study their sensors. This revealed numerous receptor proteins that had not yet been identified in EECs. The team then stimulated the organoids with molecules that would activate these receptors and identified multiple new sensory receptors that control hormone release. When these receptors were inactivated using CRISPR-based gene editing, hormone secretion was often blocked.
With these data, the researchers can now predict how human EECs respond when certain sensory receptors are activated. Their findings thus pave the way for additional studies to explore the effects of these receptor activations. The EEC enriched organoids will allow the team to perform larger, unbiased studies to identify new regulators of hormone secretion. These studies may eventually lead to therapies for metabolic diseases and gut motility disorders.
