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LEtS set the tone

The lower oesophageal sphincter (LES), a specialised thick region of smooth muscle, is located at the gastro-oesophageal junction. The LES plays an important role in controlling reflux of gastric contents into the oesophagus by virtue of its ability to generate contractile tone whilst allowing passage of food into the stomach (Sidhu & Triadafilopoulos, 2008). The LES walks a tightrope between the two functions. Reduced resting tone or too frequent spontaneous relaxations results in gastro-oesophageal reflex disease, which is a common diagnosis, affecting up to 18–28% of people (Mittal & Vaezi, 2020). On the other hand, increased tone and reduced relaxation are defining features of achalasia, a rare but devastating condition that prevents bolus propulsion into the stomach. Hence, maintaining a tight control of LES tone is important for normal gastrointestinal motility. LES tone is maintained by engaging multiple cell types like neurons, smooth muscle cells (SMCs) and interstitial cells of Cajal (ICC) (Mittal & Vaezi, 2020; Sidhu & Triadafilopoulos, 2008). LES SMCs are relatively depolarized, facilitating the activation of L-type Ca2+ channels, which is an important Ca2+ influx pathway, critical to maintain the contractile tone. However, the mechanism of depolarizing current generation was not clear. The study by Drumm et al. (2022) in this issue of The Journal of Physiology highlights the important roles of intramuscular ICC (ICC-IM) in generating and maintaining the LES tone in mouse and monkey. While there are several ICC classes (intramuscular, myenteric and sub-mucosal) in the gastrointestinal tract, in the LES there is only ICC-IM (Drumm et al., 2022). Drumm et al. (2022) demonstrate in rodents that ICC-IM generates spontaneous, sustained Ca2+ transients from multiple intracellular sites. They found that inositol 1,4,5-trisphosphate receptor type 1 (IP3R1) is the primary source of Ca2+ release and Ca2+ transients are sustained through store operated Ca2+ entry mechanisms, a process in which emptying of endoplasmic reticulum Ca2+ stores triggers Ca2+ influx to maintain the Ca2+ level inside the cell (Prakriya & Lewis, 2015). The localised spontaneous Ca2+ transients in ICC-IM lead to the activation of the Ca2+-activated chloride channel Ano1. While the study did not directly record it, the resulting chloride flux presumably depolarizes the electrically coupled SMCs, which leads to opening of the voltage-gated L-type Ca2+ channels, further increasing Ca2+ entry into SMCs, maintaining the LES tone via excitation–contraction coupling. Since the rodent model does not fully mimic the physiology of humans, the authors extrapolated the critical findings in the rodent model to primates using muscle strip experiments, highlighting the potential translational importance. Drumm et al. (2022) support previous findings, such as expression of Ano1 in LES ICC, which generates a Ca2+-dependent chloride conductance, and the relatively depolarized state of LES SMCs. The current study uses cutting edge techniques to further explore the LES tone mechanisms, providing several important and novel findings. Previously, LES tone regulation was attributed mainly to SMCs through intracellular Ca2+ release. Using genetically encoded Ca2+ indicators to image Ca2+ transients in single ICC, the current study characterised the dynamics of spontaneous Ca2+ transients in LES ICCwith remarkable precision and resolution, and when combined with targeted pharmacological inhibition, revealed a novel role of ICC-IM in generation of tone in the LES. This provides an intriguing mechanism for the maintenance of the relatively depolarized state of LES SMCs through ICC-IM. This mechanism explains why, for example, the LES is hypotensive in the mice lacking ICC-IM (W/WV mutant mice). The findings in the mouse LES are further extended to monkeys, a non-human primate. Like other exciting findings that yield novel and innovative mechanisms, these results also provide further intriguing questions. Studies that link Ano1 activation following spontaneous Ca2+ transients in ICC-IM provide an interesting opportunity to further understand the mechanism of LES tone regulation. Though the study provides possible explanation for hypotensive LES tone in W/WV mice, likely abnormalities in SMCs in the W/WV model (Goyal & Chaudhury, 2010) encourage further studies to understand ICC-IM-related neuro-muscular regulation. The findings also need to be put into a wider context, since previous studies have demonstrated that NO-sensitive guanylyl cyclase expressed in both SMCs and ICC is responsible for the regulation of basal LES tone (Groneberg et al., 2015). In addition, stretch-activated LES relaxation and contraction were also observed in W/WV mice and are mediated through mechanosensitive neurons located in the myenteric plexus (Jiang et al., 2009). How mechanical stretch modulates LES tone through ICC-IM is of interest since ICC are also mechanosensitive (Won et al., 2005). Thus, further studies will need to delineate the role of different cell types in generating and maintaining the basal LES tone. In conclusion, the current study demonstrates a novel role of LES ICC-IM through which tonic contraction is generated and maintained. The finding that Ano1 antagonists block monkey LES tone suggests that similar mechanisms might be responsible for regulating LES tone in humans. Given the critical role of LES tone in human health and that disruptions lead to disease, these findings provide a mechanistic basis for altered LES tone in these diseases and potential for selective modulation of LES tone through regulating Ca2+ signalling in ICC-IM.