Analysis of the accumulation and transit along the zebrafish gut lumen of quantum rod/glycyl-L-sarcosine conjugates

The zebrafish (Danio rerio) model is increasingly gaining relevance in research as a model for vertebrate gastrointestinal (GI) physiology, mainly for the ease with which it is possible to analyse basic functions at the gut level. Also, it is commonly used to test the fate of drugs, novel pharmacological compounds and/or xenobiotics interacting with the epithelial barrier along the GI tract. In vertebrates, the gastrointestinal homeostasis and functionality depend on proper physicochemical, mechanical and enzymatic processes. Alterations of such processes directly elicit pathophysiological onsets. For studying and monitoring GI processes e.g. ingestion, transit, absorption, the zebrafish ‘toolbox’ can be easily implemented by up-to-date nanotechnology tools i.e. colloidal semiconductor nanocrystals aiming at elucidating the gut dynamics. Here, to detect and monitor digestive and absorptive processes along the functional gut of the zebrafish we exploit PEG-functionalized Quantum Rods (QRs) with high photostability and biocompatibility. More in detail, QRs were conjugated with glycyl-L-sarcosine (QRs-GlySar) dipeptide molecules, as potential substrates of transport/sensing systems functionally expressed on the luminal plasma membranes of the enterocytes constituting the intestinal epithelial barrier. QRs conjugated with GlySar were administered to 5 dpf (days post fertilization) zebrafish larvae for 48-72 hours, to investigate their differential accumulation, transit, and interaction with the gut lumen and the epithelial barrier. Control experiments were carried out with QRs not conjugated. By tracing the signal fluorescence QRs-GlySar, we easily detected a time- and dose-dependent trend of accumulation and peristaltic progression of the QRs bioconjugates along the gut lumen; at the same time, no QRs were detected in gills or other tissues/organs. Interestingly, the accumulation, transit, and local interactions appeared to have differential trends based on the presence or absence of the conjugated dipeptide. Moreover, when larvae were treated with QRs-GlySar, fluorescent signals were detected in the circulatory blood flow, indicating the occurrence of physiological absorptive processes which might drive QRs across transepithelial pathways, remarkably. Overall, these preliminary results give hints on the effectiveness of the combination of QRs (as imaging probes) with specific dipeptides or dipeptide-like molecules (as physiologically active substrates) both in investigating functional processes of the zebrafish GI tracts, as well as in disclosing mechanisms potentially triggered by the interactions of dipeptides at epithelial level.