Enterochromaffin Cell: Friend or Foe for Human Health?

Enterochromaffin (EC) cells are a type of enteroendocrine cells that reside in the epithelial lining of the gastrointestinal tract. They function as sensory transducers of stimuli present in the intestinal lumen, as afferent nerves do not protrude into the lumen, but have been shown to act as a friend or a foe depending on various conditions. It has been well-described that EC cells play their role by releasing 5-HT to activate neighboring afferent nerves in a paracrine fashion, while emerging evidence suggests the existence of synaptic-like connections between enteroendocrine cells and adjacent afferent nerves based on in vitro co-culture and in vivo viral tracing studies [1]. However, the lack of electron microscopic evidence makes the existence of such synapses less conclusive; in fact, recent anterograde tracing from dorsal root ganglia labelled spinal afferent nerve endings in the mucosa of mouse colon but they did not form any close synaptic association with EC cells [2]. In addition to the regulation of peristaltic and secretory reflexes, evidence accumulated in the past two decades suggests that EC cells and associated serotonergic signaling are implicated in gastrointestinal discomfort in functional bowel disorders, such as irritable bowel syndrome (IBS). Increased density of EC cells with enhanced spontaneous release of 5-HT has been reported in both IBS-D and IBS-C patients, and 5-HT release is positively correlated with the severity of abdominal pain [3]. Furthermore, the 5-HT₃ receptor antagonist alosetron and the 5-HT₄ receptor agonist tegaserod have been used clinically to mitigate symptoms in different subpopulations of IBS patients [4]. However, as 5-HT participates in a great many physiological activities, the exact mechanisms underlying the contribution of EC cells and 5-HT to visceral pain and its related negative emotions remain incompletely understood.

In a recent paper published in Nature, Bayrer and Castro et al. [5] elucidated the role of EC cell-mucosal afferent circuits in the regulation of visceral pain and anxiety (Fig. 1). The authors first revealed a sex-dependent sensitization of pelvic mucosal afferents in ex vivo colonic preparations and in vivo visceral pain responses by isovalerate (ISV), one of the bacteria-derived short-chain fatty acids. This sensitizing effect of ISV was present in male mice only, perhaps due to a higher basal sensitivity of mucosal afferents in females. To investigate if this sensitizing effect was mediated by EC cells, the authors used transgenic mice to either selectively inhibit 5-HT release from EC cells or chemogenetically activate EC cells. Silencing EC cells abrogated the sensitization of mucosal afferent mechanosensitivity and visceral pain responses by ISV in male mice, and on the other hand, chemogenetic activation of EC cells enhanced mucosal afferent mechanosensitivity and visceral pain responses in males only via a 5-HT₃-dependent mechanism, with prolonged activation inducing long-term hypersensitivity. The authors concluded that activation of EC cells by microbiota metabolites sensitizes neighboring mucosal afferents to drive both acute and persistent visceral hypersensitivity in a sex-dependent manner.

Fig. 1.

Schematic showing the sex-dependent regulation of visceral pain and anxiety by the EC cell-mucosal afferent circuit in mice. Activation of EC cells by gut microbiota-derived metabolites, such as isovalerate, results in the release of 5-HT, which sensitizes adjacent mucosal afferent nerves and drives visceral pain and anxiety in male mice but not females. Notably, anxiety was evaluated on combined male and female animals in this study, and thus it is unknown if there is a sex difference in EC cell-modulated anxiety.

This study expands on one of this group’s earlier reports that have characterized the biophysical, pharmacological, and genetic properties of EC cells as a chemosensor in the gut epithelium [6]. The present study also provides further insight into the specific role of EC cell-mucosal afferent circuits in the development of visceral hyperalgesia by combining traditional electrophysiological techniques with advanced optogenetic and chemogenetic methods. The findings in this paper help to understand the complex pathogenesis of some disorders featuring visceral pain, such as IBS, and offer promising therapeutic strategies to manage visceral pain clinically. This study is consistent with accumulated evidence supporting the pivotal role of EC cells as the chief sentinel for sensing gut microbiota and its metabolites and mediating hyperalgesia through paracrine or neural pathways [7], including a recent study revealing that EC cell density is increased in the caecum of IBS-D patients, and the abundance of a certain gut bacterial genus is positively correlated with EC cell number, the IBS-severity score, and the degree and frequency of abdominal pain [8]. A recent study from our group has revealed that folic acid attenuates visceral pain by reducing microbiota-derived hydrogen sulfide [9], and EC cells might participate in this signaling pathway by the detection of hydrogen sulfide.

It is acknowledged that IBS is more common in women than men. Although there have been some independent lines of evidence suggesting some contributing factors, such as sex hormones, gender-related differences in immune functions, and intestinal permeability, the exact pathophysiological mechanisms remain largely unknown [10]. This study highlights the sex difference in the contribution of EC cell-mucosal afferent signaling to visceral hyperalgesia, i.e., the microbial or chemogenetic activation of EC cells induces visceral hyperalgesia in male mice rather than females, suggesting that the EC cell-mucosal afferent circuit may be tonically engaged in visceral pain regulation in females with a narrower dynamic range and elevated basal sensitivity, which may predispose females to a higher incidence of visceral pain. However, the fact that activation of this circuit does not induce hyperalgesia in female mice seems to contradict the higher prevalence of IBS in women, which undermines the proposal of this circuit as a key contributor to the pathogenesis of IBS. The underlying molecular mechanisms, e.g., whether estrogen affects this circuit, require further investigation. It has been thought that alosetron (a 5-HT₃ receptor antagonist approved for the treatment of IBS with diarrhea in women) exerts its analgesic effect centrally or by inhibition of high threshold nociceptors in the gut wall. This paper sheds light on an alternative or further mechanism of the sex-biased application of this drug.

Intestinal afferents are classified into mucosal, muscular, serosal, mesenteric, or muscular/mucosal afferents based on the location of their receptive fields on flat-sheet preparations [11], while alternatively, they can be classified into high threshold, low threshold, or wide dynamic range afferents based on their sensitivity to distension pressure. The authors concluded that it is the mucosal afferents that are coupled with EC cell signals by assuming that mucosal afferents were not activated in their distension paradigm. Although mucosal afferents are not sensitive to distension directly, they may be indirectly activated by 5-HT released from EC cells during distension. As early as the 1950s, it was shown that distension of guinea-pig ileum increases 5-HT release independent of peristalsis [12], and another study suggested that contraction of the smooth muscle and subsequent deformation of the mucosa leads to 5-HT release [13]. Therefore, the distension set-up in this paper may not distinguish mucosal afferents from other distension-sensitive afferents. Based on previous work from one of the corresponding authors of this paper, mucosal afferents only account for 4% of lumbar splanchnic afferents and 23% of pelvic afferents [11]. It is arguable that such a small subpopulation can drive overall hypersensitivity, as their signals may be submerged in the total large-volume peripheral neural signal traffic, although it is possible that mucosal afferents project to specific neural circuits in the central nervous system to play a key role in the regulation of visceral pain.

Interestingly, the authors demonstrated that both activation and inhibition of EC cells increased anxiety-like behaviors, and it is puzzling that the authors evaluated anxiety in mixed male and female animals. Considering the sex differences in IBS as well as in the contribution of EC cell-mucosal afferent signaling to visceral hyperalgesia, anxiety behavior should be examined in each sex individually. A lack of mechanistic understanding of these changes suggests that studies restricted to the periphery may not be adequate to address mechanisms underlying this kind of integrative behavioral state. Ongoing work in our group has revealed differential roles of the paraventricular thalamic nucleus and connected neural circuits in regulating comorbid visceral pain and anxiety. A combination of studies targeting the peripheral and central nervous systems will be a trending strategy to achieve a better understanding of the intact neural signaling pathways underlying complex pathophysiological conditions.

In summary, this paper proposes a critical role for an EC cell-mucosal afferent signaling circuit in acute and persistent visceral pain, which will inspire therapeutic strategies for ameliorating visceral pain clinically. In addition, this study highlights the sex difference in the contribution of this circuit to visceral pain, providing a mechanistic insight into the sex disparity in visceral pain-related disorders. However, some questions remain unresolved, e.g., why microbial or chemogenetic activation of EC cells did not induce visceral hyperalgesia in female mice when IBS is more prevalent in females, and whether there is any sex difference in anxiety behavior driven by this circuit. Although further investigations are required to address these questions, there is no doubt that EC cells and 5-HT play essential roles in orchestrating physiological activities to facilitate digestion and absorption, whereas they may drive pathophysiological processes upon abnormal stimulation.

Conflict of interest

The funders had no role in the study design, data collection, and analysis, decision to publish, or preparation of the manuscript.