Published online Nov 28, 2018. doi: 10.3748/wjg.v24.i44.4989
Peer-review started: August 24, 2018
First decision: October 5, 2018
Revised: November 7, 2018
Accepted: November 7, 2018
Article in press: November 7, 2018
Published online: November 28, 2018
Interstitial cells of Cajal (ICCs) and PDGFRα+ cells are mainly located in the smooth muscle layer of the colon, which is widely believed to play a critical role in the generation of colonic transit motility. Large numbers of studies have shown that ICCs are mainly responsible for the contraction response of the smooth muscle, while PDGFRα+ cells mainly regulate the relaxation of the smooth muscle. Inhibitory neuronal transmitters [nitric oxide (NO) and purine] can respectively act on ICCs and PDGFRα+ cells to mediate colonic transit, thus propelling feces into the rectum and the anus. However, the distributions of ICC and PDGFRα+ in the colon remain unclear. Therefore, it is an important scientific problem to study the transit mechanism of the proximal and distal colon.
In recent years, the incidence of colonic dysmotility diseases has been increasing year by year, such as inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), and complications of some diseases (e.g., diabetes-induced slow transit constipation), whose main clinical symptoms are diarrhea or constipation. The smooth muscle layer, as the main source of colon power, consists of ICCs, PDGFRα+ cells, and smooth muscle cells (SMCs) contacted by the gap junction. Therefore, it is very significant to study the differential distributions of ICCs and PDGFRα+ cells in the two ends of the colon.
In this study, we for the first time researched the distributions and functions of ICCs and PDGFRα+ cells in the proximal and distal colon. Then, we studied the roles of inhibitory neurotransmitters NO for ICCs and purine for P2Y1 receptor on PDGFRα+ cells in colonic motility transit. This study may represent a future strategy for therapeutic intervention in disorders of colonic motility, such as IBD and IBS, by understanding the distributions and function of NO-ICCs and purine-PDGFRα+ cells at both ends of the colon.
First, we compared the isolated colonic transit differences between the proximal and distal colon using colonic migrating motor complexes (CMMCs). Then, we used the smooth muscle contraction experiment, which is the major source of colon motility, to compare the drug differences between the proximal and distal colon by adding the blockers and agonists of anoctamin-1 (ANO1) channels on ICCs and small conductance calcium-activated potassium channel 3 (SK3) channels on PDGFRα+ cells. Subsequently, we compared the membrane potentials of the proximal and distal colon by intracellular recordings. Later, Western blot analysis was used to detect the protein expression of c-Kit, ANO1, PDGFRα, and SK3 in the colon. Finally, we added immunofluorescence methods to visually describe the distributions of ICC and PDGFRα+ cells in the proximal and distal colon.
Treatment with tetrodotoxin (TTX) to block the enteric nervous system (ENS) in the CMMC experiment almost completely blocked colonic transit. However, in the smooth muscle contraction experiment, when the ENS was blocked, the contraction of the colon was enhanced, suggesting that inhibitory nerve regulation plays critical roles in the transmission of the colon. In addition, when the ANO1 channel on ICCs was blocked by NPPB, the proximal colon showed a more obvious inhibitory role. While the SK3 channels on PDGFRα+ cells were blocked by apamin, there was a more obvious drug effect in the distal colon, indicating that the proximal colon might distribute more ICCs, and the distal colon has more PDGFRα+ cells. Intracellular electrical recording experiments indicated that slow inhibitory junction potentials (sIJP) mediated by the NO-ICC-ANO1 signal pathway was more obvious in the proximal colon, while fast inhibitory junction potentials (fIJP) mediated by purine-PDGFRα+-SK3 was more prominent in the distal colon, indicating that there are more ICCs in the proximal colon and more PDGFRα+ cells in the distal colon from the membrane potential level.
In this study, we demonstrated that NO has a more obvious effect on the ICCs in the proximal colon, while purine has a more prominent effect on the distal PDGFRα+ cells, indicating that there are more ICC cells in the proximal colon and more PDGFRα+ cells in the distal colon. In addition, NO and purine acting on the SMC/ICC/PDGFRα+ cell (SIP) syncytium (consisting of SMCs, ICCs, and PDGFRα+ cells) are both inhibitory neurotransmitters, suggesting that colonic transit is mainly dominated by inhibitory neuromodulation. Although the stomach and small intestine are dominated by excitatory neuromodulation, this feature of the colon may contribute to the adequate absorption of nutrients and the formation of feces.
This is the first study to report that the differential distributions of ICCs and PDGFRα+ cells at the two ends of the colon. Our findings have highlighted the effects of inhibitory neuromodulation NO-ICC-ANO1 and purine-PDGFRα+ cells-SK3 on colonic transit. In order to maintain regular colonic transit, the perfect cooperation of NO-ICC and purine-PDGFRα+ cells is required. Therefore, in the future study of colon dynamic disorder such as IBD or IBS, we can start from whether the distribution and function of ICC and PDGFRα+ cells have changed, and then extend to the effect of the ENS, especially inhibitory neuromodulation, on colonic transmission disorder to find the pathogenesis of colonic transit dysfunction. These findings in the present study may provide new insights and strategies for the diagnosis and treatment of gastrointestinal motility disorders.