Nothobranchius as a model for anorexia of aging research: an evolutionary, anatomical, histological, immunohistochemical, and molecular study

Anorexia of aging, defined as a decrease in appetite and a preponderant loss of body weight occurring in late life, is one of the most common diseases affecting older people. The peptide hormone cholecystokinin (Cck) is known to play a key role in regulating food intake and satiety in higher vertebrates. In humans as well as in rats, an increased concentration of Cck was described as the basis of appetite loss in elderly. However, the role of increased plasma Cck concentrations in mediating the age-related decrease in appetite remains to be established. Although in vitro studies are an excellent resource for investigating aging, the use of a model organism that shares and imitates the human physiological processes guarantees a better understanding of the in vivo mechanisms. African annual fishes from the genus Nothobranchius are emerging as a prominent model organism in biogerontology and developmental biology due to their short captive lifespan. Therefore, in the current study, we aimed to investigate the possibility of using the genus Nothobranchius to model the anorexia of aging and their potential contribution to better understanding the pathway by which Cck induce appetite loss in older people providing a comparative/evolutionary localization of the current study model among the aging canonicals models, the morphology of its gastrointestinal tract and its Cck expression pattern. The comparative/evolutionary investigation was conducted using the NCBI blastp (protein-protein BLAST) and NCBI Tree Viewer. The macroscopic morphology, histological features, ultrastructural organization of Nothobranchius rachovii gastrointestinal tract were investigated using stereomicroscope, Masson's trichrome and alcian blue–PAS staining, and transmission electron microscopy, respectively. The cck expression pattern was studied through immunofluorescence labeling, western blotting, and quantitative RT-PCR. The intestine was folded into different segments divided into an anterior intestine made of a rostral intestinal bulb and an intestinal annex of lower diameter, mid and posterior intestine. The gradual transition from the rostral intestinal bulb to the posterior intestine sections's epithelium is characterized by a gradual reduction in the striated muscular bundles, villi height, and goblet mucous cells count. The lining epithelium of the intestinal villi was characterized by a typical brush border enterocytes full of mitochondria. Moreover, Cck expression was detected