Interstitial cells of Cajal in the gut: what makes them tick?

Every area of science has its topics of controversy, and in the field of enteric neuroscience and motility, interstitial cells of Cajal (ICC) have been the focus of lively debate for a number of years. Where ICC come from, what they do, how they work, whether they are involved in disease processes, and whether they are important elements of the neuromuscular circuitry of the gut are all questions that have been extensively studied and often disputed.

Ramon y Cajal is credited with first describing these interstitial cells in 1899, and he speculated that they could modify smooth muscle contraction by serving as an interface between autonomic neurons and smooth muscle cells. In the 1980s, these cells gained the attention of gastrointestinal physiologists and clinicians when Thuneberg and Faussone-Pellegrini proposed that ICC generate the rhythmic electrical and contractile activity that exists from the stomach to the rectum. Furthermore, they discovered unique ultrastructural features that distinguish ICC from smooth muscle cells and fibroblasts. They observed that ICC were linked to one another and to smooth muscle cells by gap junctions, and that nerve terminals appeared to target these cells. However, efforts to definitively determine the source and functions of these cells were hampered by an inability to easily identify and isolate them.

A major breakthrough in our ability to study and understand ICC was provided in 1992 by Maeda, Nishi and colleagues (Maeda et al. 1992). They proposed that ICC express the receptor tyrosine kinase, kit, that kit is essential for the formation of ICC networks, and that mutations in the kit gene or kit immunoneutralization result in disrupted rhythmicity in the gut. The knowledge that ICC express kit provided a tool to easily identify these cells under healthy and pathophysiological conditions, it made possible the use of patch clamp recording techniques to elucidate the ionic currents in these cells, and it led to the use of mutant strains of mice with disrupted ICC networks. This set the stage for a number of advances in our efforts to understand ICC, including early work by Huizinga and colleagues at McMasters University, and by Sanders, Ward and their Reno colleagues, that in the absence of ICC, no rhythmic electrical activity is present.

Agreement has ultimately been reached with regard to many features of ICC. Developmental studies have demonstrated that ICC are not derived from the neural crest, but are of mesodermal origin. Voltage clamp studies of isolated ICC have conclusively shown that these cells, unlike gastrointestinal smooth muscle cells, exhibit spontaneous, rhythmic inward currents, and are therefore likely to serve as the pacemaker cells in the muscularis of the gut. Furthermore, studies using immunostaining for kit have demonstrated that the density of ICC networks is dramatically reduced in gastrointestinal disorders such as slow transit constipation, and diabetic gastroparesis.

The importance of ICC as mediators of neuromuscular transmission is still debated, but the greatest enigma regarding ICC in the gut is the question of what makes them tick. In other words, what cellular mechanisms are responsible for their pacemaker activity? Studies of ionic currents in ICC have led to the descriptions of several potential players, including Cl⁻ channels, non-selective cation channels, and Na⁺ channels. Unfortunately, differences in experimental preparations and interspecies variations have prevented a consensus model from emerging, although a single common mechanism may not exist. For example, Cl⁻ channel integrity appears to depend on culture conditions, but they have been recently demonstrated in situ (Wang et al. 2008). Also, Na⁺ channels are expressed in ICC from human and canine gut, but have not been detected in ICC of rodents.

Recently, another breakthrough has allowed for more specific identification of ICC. Farrugia and colleagues demonstrated that enteric ICC express the Tmem16a gene product, anoctamin 1 (ANO1), but unlike kit, ANO1 is not expressed by other cell types in the gut such as mast cells (Gomez-Pinilla et al. 2009). Expression studies have shown that ANO1 functions as a Ca²⁺-activated Cl⁻ channel. This issue of The Journal of Physiology contains a pair of papers by Sanders, Ward and colleagues that supports the concept that ANO1 is important in ICC pacemaker activity (Hwang et al. 2009; Zhu et al. 2009). They show that ICC express a current with the same properties as ANO1, that inhibition of the current is associated with a disruption of the slow wave currents and contractions, and that Tmem16a null mice fail to develop slow wave activity. These studies were facilitated, in part, by a new transgenic mouse with copGFP constitutively expressed in ICC, which allows for the identification of ICC in mixed cell dispersions, and should be an important tool for future studies of ICC function and integrity.

The knowledge that ANO1 Cl⁻ currents exist in ICC adds an important piece to the puzzle of pacemaker mechanisms in ICC. However, a clear image of all the key players and the exact sequence of intracellular messengers and membrane channel events that generates the slow wave in ICC has yet to emerge. Since motor patterns in the different regions of the gut have different functions, the associated pacemaker activities may not be identical and hence one single model of pacemaking that applies to all regions of the gut and all species might not exist.