Signaling between the gut and the brain is important for the regulation of many different behavioral and physiological processes. In the intestine, a highly heterogenous group of sensory secretory cells named enteroendocrine cells (EECs) is found embedded within the intestinal epithelium and capable of signaling through a variety of different peptide hormones and neurotransmitters. Due to their prime physical location, EECs are exposed to the contents of the gut lumen, sense changes in this environment, and transmit this information to the body and neurons of the enteric nervous system and those signaling to the brain. EECs are therefore considered integral components of the microbiota-gut-brain-axis and EEC dysfunction or abnormal signaling has been linked to many different disorders including obesity, diabetes, and inflammatory bowel diseases. Serotonergic enterochromaffin (EC) cells form the largest EEC subclass and these cells are found in all the different regions of the gut while comprising a rather heterogeneous group themselves. In general, EC cells function as chemo- and mechanoreceptors, which means that they must integrate multimodal sensory information on the cellular level and respond in an activated state by secreting the classical neurotransmitter serotonin. Gastrointestinal serotonin mediates diverse physiological processes in the body, most notably the regulation of gut motility. While there has been a growing understanding of the molecular mechanisms and sensory stimuli by which EC cells are activated, the process(es) via which excitation-secretion coupling is achieved and the molecular mechanisms that control serotonin release from these cells remain incompletely understood. Interestingly, several studies have proposed that some EECs, including EC cells, adopt morphological features (i.e. long axon-like processes termed ‘neuropods’) and exhibit molecular properties (i.e. expression of components of the synaptic vesicle fusion machinery) that determine fast cell-to-cell signaling in neurons of the brain. It has therefore been hypothesized that EC cells may communicate with neurons via fast, synapse-like mechanisms. However, as outlined above, evidence supporting this notion has largely been circumstantial.
The overall goal of this project was therefore to answer the question whether mouse EC cells signal in a synaptic-like fashion with the ENS and sensory afferents to mediate gut-brain-communication and to determine the molecular release machinery that mediates serotonin release from these cells. To achieve this, three specific objectives were formulated:
1. To define functional properties and molecular requirements of vesicle fusion in EC cells.
2. To dissect the functional organization of EC cell release sites and EC connectivity.
3. To probe consequences of defective serotonin release.
The project results achieved so far are described in detail below. In summary, we found that despite the expression of key components of the neuronal presynaptic neurotransmitter release machinery in EC cells, cultured cells release the majority of serotonin with relatively slow kinetics from large secretory vesicles, unlike fast synaptic transmission, but similar to the signaling mode of other endocrine cell types.
