LEtS set the tone

The lower esophageal sphincter (LES), a specialized thick region of smooth muscle, is located at the gastro-esophageal junction. The LES plays an important role in controlling reflux of gastric contents into the esophagus by virtue of its ability to generate contractile tone whilst allowing passage of food into the stomach (Sidhu & Triadafilopoulos, 2008). The LES walks a tight rope between the two functions. Reduced resting tone or too frequent spontaneous relaxations result in gastroesophageal reflex disease (GERD), which is a common diagnosis, affecting up to 18 to 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 (GI) motility.

LES tone is maintained by engaging multiple cell types like neurons, smooth muscle cells (SMCs) and interstitial cells of Cajal (ICC) (Sidhu & Triadafilopoulos, 2008; Mittal & Vaezi, 2020). LES SMCs are relatively depolarized facilitating the activation of L-type calcium (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., (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 GI tract, in the LES there are only intramuscular ICC (ICC-IM) (Drumm et al., 2022). Drumm et al., 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 trigger Ca²⁺ influx to maintain the Ca²⁺ level inside the cell (Prakriya & Lewis, 2015). The localized 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., support previous findings, such as expression of Ano1 in LES ICCs, which generate Ca²⁺ dependent chloride conductance, and the relatively depolarized state of LES SMCs (Drumm et al., 2022). The current study uses cutting edge techniques to further build 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 (GECI) to image Ca²⁺ transients in single ICC, the current study characterized the dynamics of spontaneous Ca²⁺ transients in LES ICC with remarkable precision and resolution, and when combined with targeted pharmacological inhibition, reveal 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/W^V mutant mice). The findings in the mouse LES is further extended to monkeys, a non-human primate.

Like other novel and innovative mechanisms that yield exciting findings, they also provide further intriguing questions. Studies that link Ano1 activation following spontaneous Ca²⁺ transients in ICC-IM provides an interesting opportunity to further understand the mechanism of LES tone regulation. Though the study provides possible explanation for hypotensive LES tone in W/W^V mice, likely abnormalities in SMCs in the W/W^V model (Goyal & Chaudhury, 2010) encourages further studies to understand ICC-IM related neuro-muscular regulation. The exciting findings also need to be put into a wider context, since previous studies have demonstrated that NO-sensitive guanylyl cyclase (NO-GC) expressed in both SMC 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/W^V mice which 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 humans health and disruptions lead to disease, these exciting findings provide a mechanistic basis for altered LES tone in these diseases and potential for selective modulation of LES tone through regulating Ca²⁺ signaling in ICC-IM.