Microplastics (MPs) in fish can cross the intestinal barrier and are often bioaccumulated in several tissues, causing adverse effects. While the impacts of MPs on fish are well documented, the mechanisms of their cellular internalization remain unclear. A rainbow-trout (Oncorhynchus mykiss) intestinal platform, comprising proximal and distal intestinal epithelial cells cultured on an Alvetex scaffold, was exposed to 50 mg/L of MPs (size 1–5 µm) for 2, 4, and 6 h. MP uptake was faster in RTpi-MI compared to RTdi-MI. Exposure to microplastics compromised the cellular barrier integrity by disrupting the tight-junction protein zonula occludens-1, inducing significant decreases in the transepithelial-electrical-resistance (TEER) values. Consequently, MPs were internalized by cultured epithelial cells and fibroblasts. The expression of genes related to endocytosis (cltca, cav1), macropinocytosis (rac1), and tight junctions’ formation (oclna, cldn3a, ZO-1) was analyzed. No significant differences were observed in cltca, oclna, and cldn3a expression, while an upregulation of cav1, rac1, and ZO-1 genes was detected, suggesting macropinocytosis as the route of internalization, since also cav1 and ZO-1 are indirectly related to this mechanism. The obtained results are consistent with data previously reported in vivo, confirming its validity for identifying MP internalization pathways. This could help to develop strategies to mitigate MP absorption through ingestion.

The ever-increasing number and variation of raw materials utilized to provide alternative feed formulations continues to allow for a more sustainable and flexible approach. Testing all these options in vivo is still the most robust and reliable manner to pick the best raw material candidates, but it requires the use of large numbers of animals and is time-consuming and expensive. Therefore, we are developing an in vitro platform that can provide a reliable evaluation of new ingredients. The main aim of this work was to combine an in vitro digestion protocol of extruded, commercially relevant aquafeeds with the exposure of intestinal epithelial cells to the extracted bio-available fraction (BAF). The results show that 250,000 cells/cm² represents the optimal seeding density and that up to 50% BAF concentration for up to 24 h had no negative effects on the epithelial barrier morphology and function. It is possible to determine amino acid digestibility and bioavailability in all the experimental conditions (with and without BSA, at 25% and 50% dilution) and at all time points (0, 6, and 24 h). However, BAF concentration, the medium used for its dilution, and the length of exposure to the different epithelial cell lines can all influence the results and, therefore, must be selected according to the final aim of the experiment.

In vitro organotypic cell-based intestinal platforms, able to faithfully recapitulate the complex functions of the organ in vivo, would be a great support to search for more sustainable feed ingredients in aquaculture. We previously demonstrated that proliferation or differentiation of rainbow trout intestinal cell lines is dictated by the culture environment. The aim of the present work was to develop a culture platform that can efficiently promote cell differentiation into mature enterocytes. We compared four options, seeding the RTpiMI cell line derived from the proximal intestine on (1) polyethylene terephthalate (PET) culture inserts ThinCert™ (TC), (2) TC coated with the solubilized basement membrane matrix Matrigel^® (MM), (3) TC with the rainbow trout fibroblast cell line RTskin01 embedded within the Matrigel^® matrix (MMfb), or (4) the highly porous polystyrene scaffold Alvetex^® populated with the abovementioned fibroblast cell line (AV). We evaluated the presence of columnar cells with a clear polarization of brush border enzymes, the formation of an efficient barrier with a significant increase in transepithelial electrical resistance (TEER), and its ability to prevent the paracellular flux of large molecules but allow the transit of small compounds (proline and glucose) from the apical to the basolateral compartment. All parameters improved moving from the simplest (TC) through the more complex platforms. The presence of fibroblasts was particularly effective in enhancing epithelial cell differentiation within the AV platform recreating more closely the complexity of the intestinal mucosa, including the presence of extracellular vesicles between fibroblasts and epithelial cells.