Interstitial Cells: Their Role in the Physiology and Pathophysiology of the Gastrointestinal Tract
AuthorBlair, Peter James
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Interstitial cells are fundamental to control of gastrointestinal (GI) tract motility under physiological conditions. There are two types of interstitial cell: interstitial cells of Cajal (ICC) and platelet-derived growth factor receptor alpha-positive cells (PDGFRα+). Microscopy studies have resulted in ICC being divided into several subpopulations based on their anatomical locations in the gut wall. For instance, spindle-shaped ICC found within the muscle layers are termed intramuscular ICC (ICC-IM) and ICC that form an anastomosing network at the level of the myenteric plexus are termed myenteric ICC (ICC-MY). These subpopulations have been identified primarily from studies of rodents, while less is known about the subpopulations of ICC in larger animals, such as humans and non-human primates. Thus, in chapter 2 of this thesis, we utilized immunohistochemistry and confocal microscopy to carefully examine ICC throughout the GI tract of cynomolgus monkeys. In general we found that ICC subpopulations identified in rodents were also present in monkeys. However, one interesting difference was noted in the taenia coli of monkeys. Taenia coli, which are not present in common rodent models such as mice and rats, are thick bands of longitudinal muscle that run along the length of the colon and within these bands we found a dense population of ICC-IM. Furthermore, we also identified a population of subserosal ICC (ICC-SS) in the colon. ICC-SS were observed in both taenia and non-taenia regions of the colon, although their morphology was different.Different subpopulations of ICC are now known to have distinct functions. For example, ICC-IM are widely acknowledged to be critical mediators of enteric neurotransmission. One line of evidence for this is the close association between ICC and enteric neurons. Again, this was elucidated by studies on rodent models. Therefore, in chapter 3 of this thesis, the relationship between ICC and enteric neurons was investigated in cynomolgus monkeys. Double label immunohistochemistry revealed that ICC-IM throughout the monkey GI tract are closely apposed to enteric neurons in a similar manner to rodents. Interestingly, ICC-IM and ICC-SS found within the taenia of monkeys were also observed to exhibit close associations with enteric neurons. Furthermore, PDGFRα+ cells have recently been implicated as mediators of enteric neurotransmission and have also been shown to form close relationships with enteric neurons in rodents. The results of this study demonstrate that this is also the case in monkeys.ICC-MY are known to be the pacemakers of the GI tract. They generate spontaneous, rhythmic depolarizations, known as slow waves, which conduct into neighboring smooth muscle cells via gap junctions and activate L-type calcium channels, thereby triggering the phasic contractions of the gut. Over the years, numerous ion channels have been postulated to contribute to the generation of pacemaker activity. Anoctamin 1 (ANO1; previously termed DOG1) was recently discovered to be expressed by gastrointestinal stromal tumors (i.e., tumors of ICC) and was subsequently found to be a calcium-activated chloride channel. This led to the hypothesis that ANO1 may be involved in producing slow wave activity. This theory was tested in chapter 4 of this thesis. Firstly, normal populations of ICC in mice, monkey and humans were found to express ANO1. Secondly, calcium-activated chloride channel blockers and ANO1 knockout mice were employed to determine whether they are involved in pacemaker activity. The results demonstrate that ANO1 calcium-activated chloride channels play a crucial role in the generation of pacemaker activity. In addition to the important role of interstitial cells under physiological conditions, alterations in interstitial cells (especially ICC) have been implicated in a variety of GI pathophysiological conditions. For instance, multiple studies have observed reduced numbers of ICC throughout the GI tract in diabetes. However, the majority of these studies have been concerned with type I diabetes and, in addition, little is known about the functional changes that may occur. Thus, in chapter 5 of this thesis, an animal model of type II diabetes was employed to investigate functional changes relating to interstitial cells. One of the most prominent GI complications of diabetes is diabetic gastroparesis. Therefore, we focused on the gastric antrum, and using techniques such as video imaging and intracellular microelectrode recording, discovered that both contractile and electrical pacemaker activities were altered in type II diabetes. In particular, slow waves were decreased in amplitude and increased in frequency and this appeared to be a consequence of enhanced prostaglandin synthesis due to upregulation of prostaglandin E synthase. Immunohistochemical studies revealed that the density of networks of ICC, PDGFRα+ cells and enteric neurons were unchanged in diabetes. In conclusion, the studies in this thesis provide novel information regarding ICC in physiological and pathophysiological conditions. For example, ICC subpopulations and their relationship to neurons have been shown to be remarkably similar in rodents and non-human primates. Also, ANO1 calcium-activated chloride channels have been demonstrated for the first time to be critical for generation of slow waves. In relation to pathophysiology, functional changes in gastric pacemaker activity have been discovered in an animal model of type II diabetes and these changes have been linked to excess prostaglandin production.