A Fundamental Investigation into the Presence of Interstitial Cells of Cajal within the Gastrointestinal Tract of Dania rerio

Abstract

Coordinated gastrointestinal (GI) motility results from the complex interactions between interstitial cells of Cajal (ICC), enteric neurons, and smooth muscle cells. Kit positive immunoreactivity has been extensively utilized as a marker for ICC; these cells function to generate rhythmic depolarization in the GI smooth muscle termed the electrical slow wave. Coordinated GI motility is regulated by the electrical slow wave. Directly lesioning ICC populations with neutralizing antibodies results in GI dysmotility and loss of the electrical slow wave. Furthermore, GI dysmotility symptoms and other human pathologies have been correlated with ICC deficiencies within the GI tract. Currently, there are few treatments and therapeutic interventions for GI motility disorders, Irritable Bowl Syndrome alone affects up to 20% of the world population. 1- 5 The high prevalence of GI motility disorders combined with the lack of effective interventions, suggest new treatments are needed. Rational drug development and new treatment design will benefit from the development of new model systems. Greater understanding of the regulation of GI motility, pathologies that affect GI motility, and the development of new therapeutics will all benefit from a new model of GI motility. This thesis provides support to the use of Danio rerio, the zebrafish, as a model of human GI motility. Danio rerio is an excellent model system for several human physiological systems or diseases including macular degeneration, neural degenerative diseases, cardiovascular disease, cholesterol and lipid metabolism, and cardiovascular disease. 6 - 20 The zebrafish model system offers the opportunity for forward genetic analysis; novel genes, genetic elements, and pathologies can be characterized with reverse genetic screens through the evaluation of interesting phenotypes. Zebrafish reach sexual maturity quickly (three months), produce large numbers of offspring (100 per week), and are more cost efficient to house compared to mammalian models. Most seductive of zebrafish traits, optical transparency of larvae, enables direct visualization in-vivo of organ function (including the GI tract) under physiological settings, without perturbing the organism. This thesis critically examines the hypothesis that GI motility in the zebrafish involves ICC. Prior to the data presented within this thesis, there have not been any published reports on the presence of ICC within the zebrafish GI tract. Results from this investigation show Kit-like immunoreactivity within the muscular layers of the zebrafish GI tract. Additionally, orthologs for the human receptor tyrosine kinase Kit, kita and kitb, and orthologs for the human Kit ligand, kitla and kitlb, are shown to be expressed within the zebrafish GI tract. Kit receptor and ligand are necessary for ICC development and maintenance in mammals. Expression of kit receptors and ligands within the zebrafish GI tract are consistent with the hypothesis that ICC support spontaneous rhythmic contractions in the zebrafish-- this suggests that the zebrafish may be a suitable model for GI motility.