The 3D Pattern of the Rainbow Trout (Oncorhynchus mykiss) Enterocytes and Intestinal Stem Cells

Daily rhythm in feeding behavior and digestive processes in totoaba (Totoaba macdonaldi) under commercial farming conditions

To identify daily changes in the digestive physiology of Totoaba macdonaldi, the feed intake, activity (pepsin, trypsin, chymotrypsin, lipase, amylase, and L-aminopeptidase), and gene expression (aminopeptidase and maltase-glucoamylase) of key digestive enzymes were measured in the intestine and the pyloric caeca. Fish were fed for three weeks every four hours during the light period to apparent satiation, and samples were taken every four hours throughout a 24-h cycle under a 12:12 L:D photoperiod. The feed consumption steadily increased until the third feeding (16:00 h, ZT-8) and decreased significantly towards the end of the day. The activity of pepsin and alkaline enzymes (trypsin, chymotrypsin, lipase, amylase, and L-aminopeptidase) exhibited a pattern dependent on the presence of feed, showing a significant reduction during the hours of darkness (ZT-12 to ZT-24). Expression of the intestinal brush border enzyme (L-aminopeptidase) increased during the darkness period in anticipation of the feed ingestion associated with the subsequent light period. The cosinor analysis used to estimate the feed rhythms for all tested enzymes showed that activity in the intestine and pyloric caeca exhibited significant rhythmicity (p < 0.05). However, no rhythmicity was observed in the intestinal expression of maltase-glucoamylase. Our results demonstrate that some of the behavioral and digestive physiology features of totoaba directly respond to rhythmicity in feeding, a finding that should be considered when establishing optimized feeding protocols.

An in-depth characterisation of European seabass intestinal segments for assessing the impact of an algae-based functional diet on intestinal health

Sustainable farming of fish species depends on emerging new feed ingredients, which can alter the features of the digestive tract and influence animals' overall health. Recent research has shown that functional feeds hold great potential for enhancing fish robustness by evoking appropriate responses at the intestine level. However, there is a lack of extensive and accurate descriptions of the morphology of the gastrointestinal tract of most farmed fish. We have characterised the intestine of European seabass thoroughly, by targeting four segments - anterior, mid, posterior and rectum. Results indicated that the anterior segment is mostly associated with absorption-related features; this segment has the largest absorptive area, the longest villi, and the highest number of neutral goblet cells (GC). The posterior segment and rectum have distinct histomorphometric features, but both seem to be important for immunity, displaying the highest count of acid GC and the highest expression of immune-related genes. The strongest proliferating cell nuclear antigen (PCNA) signal was observed in the anterior intestine and rectum, with PCNA⁺ cells appearing at the base of the villi and the corresponding villi branches. We have also evaluated the impact of a novel feed supplemented with a macro- and microalgae blend and found that there were no differences in terms of growth. However, the alterations observed in the mid intestine of fish fed the blend, such as thickening of the submucosa and lamina propria, an increased number of leucocytes, and higher expression of immune- and oxidative stress-related genes, suggest that algae may have an immunomodulatory effect. In the current article, we have described the morphology and expression patterns of the intestine segments of European seabass in detail and have presented a comprehensive report of the indices and methods used for the semi-quantitative and quantitative histomorphometric assessments, thereby providing useful information for future studies that aim to maintain intestinal health through dietary interventions.

Distinct Organotypic Platforms Modulate Rainbow Trout (Oncorhynchus mykiss) Intestinal Cell Differentiation In Vitro

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.

Establishment of an In Vitro Model to Study Viral Infections of the Fish Intestinal Epithelium

Viral infections are still a major concern for the aquaculture industry. For salmonid fish, even though breeding strategies and vaccine development have reduced disease outbreaks, viral diseases remain among the main challenges having a negative impact on the welfare of fish and causing massive economic losses for the industry. The main entry port for viruses into the fish is through mucosal surfaces including that of the gastrointestinal tract. The contradictory functions of this surface, both creating a barrier towards the external environment and at the same time being responsible for the uptake of nutrients and ion/water regulation make it particularly vulnerable. The connection between dietary components and viral infections in fish has been poorly investigated and until now, a fish intestinal in vitro model to investigate virus-host interactions has been lacking. Here, we established the permissiveness of the rainbow trout intestinal cell line RTgutGC towards the important salmonid viruses-infectious pancreatic necrosis virus (IPNV), salmonid alphavirus (subtype 3, SAV3) and infectious salmon anemia virus (ISAV)-and explored the infection mechanisms of the three different viruses in these cells at different virus to cell ratios. Cytopathic effect (CPE), virus replication in the RTgutGC cells, antiviral cell responses and viral effects on the barrier permeability of polarized cells were investigated. We found that all virus species infected and replicated in RTgutGC cells, although with different replication kinetics and ability to induce CPE and host responses. The onset and progression of CPE was more rapid at high multiplicity of infection (MOI) for IPNV and SAV3 while the opposite was true of ISAV. A positive correlation between the MOI used and the induction of antiviral responses was observed for IPNV while a negative correlation was detected for SAV3. Viral infections compromised barrier integrity at early time points prior to observations of CPE microscopically. Further, the replication of IPNV and ISAV had a more pronounced effect on barrier function than SAV3. The in vitro infection model established herein can thus provide a novel tool to generate knowledge about the infection pathways and mechanisms used to surpass the intestinal epithelium in salmonid fish, and to study how a virus can potentially compromise gut epithelial barrier functions.

Omics and imaging combinatorial approach reveals butyrate-induced inflammatory effects in the zebrafish gut

BACKGROUND

Prebiotic feed additives aim to improve gut health by influencing the microbiota and the gut barrier. Most studies on feed additives concentrate on one or two (monodisciplinary) outcome parameters, such as immunity, growth, microbiota or intestinal architecture. A combinatorial and comprehensive approach to disclose the complex and multifaceted effects of feed additives is needed to understand their underlying mechanisms before making health benefit claims. Here, we used juvenile zebrafish as a model species to study effects of feed additives by integrating gut microbiota composition data and host gut transcriptomics with high-throughput quantitative histological analysis. Zebrafish received either control, sodium butyrate or saponin-supplemented feed. Butyrate-derived components such as butyric acid or sodium butyrate have been widely used in animal feeds due to their immunostimulant properties, thereby supporting intestinal health. Soy saponin is an antinutritional factor from soybean meal that promotes inflammation due to its amphipathic nature.

RESULTS

We observed distinct microbial profiles associated with each diet, discovering that butyrate (and saponin to a lesser extent) affected gut microbial composition by reducing the degree of community-structure (co-occurrence network analysis) compared to controls. Analogously, butyrate and saponin supplementation impacted the transcription of numerous canonical pathways compared to control-fed fish. For example, both butyrate and saponin increased the expression of genes associated with immune response and inflammatory response, as well as oxidoreductase activity, compared to controls. Furthermore, butyrate decreased the expression of genes associated with histone modification, mitotic processes and G-coupled receptor activity. High-throughput quantitative histological analysis depicted an increase of eosinophils and rodlet cells in the gut tissue of fish receiving butyrate after one week of feeding and a depletion of mucus-producing cells after 3 weeks of feeding this diet. Combination of all datasets indicated that in juvenile zebrafish, butyrate supplementation increases the immune and the inflammatory response to a greater extent than the established inflammation-inducing anti-nutritional factor saponin. Such comprehensive analysis was supplemented by in vivo imaging of neutrophil and macrophage transgenic reporter zebrafish (mpeg1:mCherry/mpx:eGFPi¹¹⁴) larvae. Upon exposure to butyrate and saponin, these larvae displayed a dose-dependent increase of neutrophils and macrophages in the gut area.

CONCLUSION

The omics and imaging combinatorial approach provided an integrated evaluation of the effect of butyrate on fish gut health and unraveled inflammatory-like features not previously reported that question the usage of butyrate supplementation to enhance fish gut health under basal conditions. The zebrafish model, due to its unique advantages, provides researchers with an invaluable tool to investigate effects of feed components on fish gut health throughout life.

Towards the Identification of a Suitable Commercial Diet for Carpione (Salmo carpio, Linnaeus 1758): A Multidisciplinary Study on Fish Performances, Animal Welfare and Quality Traits

Simple Summary

Carpione (Salmo carpio, Linnaeus 1758) is an endangered precious endemism of Lake Garda (Northern Italy), the largest Italian lake. To date, several bottlenecks about its culture remain unsolved, including the identification of a proper growth-out diet. In the present study, four different grossly isolipidic, isoproteic, and isoenergetic diets containing ingredients from different origins were used for S. carpio culture. Specifically, a diet largely based on marine ingredients, and currently used for carpione farming, was used as control. Three other diets were formulated in order to include relevant percentages of vegetable ingredients or processed animal proteins (at two different inclusion levels). After a three-month feeding trial, fish zootechnical performances, welfare, and flesh quality were evaluated through a multidisciplinary approach, including histology, gene expression, chemical analysis, and Fourier transform infrared spectroscopy (FTIR). This study provided the first insights on carpione physiological responses to different commercial dietary formulations.

Abstract

Carpione (Salmo carpio, Linnaeus 1758) is an endangered precious endemism of Lake Garda (Northern Italy), the largest Italian lake. To date, several bottlenecks about its culture remain unsolved, including the identification of a proper growth-out diet. The aim of the present study was to test four different grossly isolipidic, isoproteic, and isoenergetic diets in which the main ingredients had a different origin. Specifically, a diet currently used by local farmers for carpione culture, largely based on marine ingredients, was used as control (CTRL), while the other three diets were formulated by partially replacing marine ingredients with plant ones (VEG) or with different percentages of processed animal proteins (PAP1 and PAP2). The feeding trial was run in triplicate, over a three-month period. No significant differences in growth performance among the experimental groups were observed. However, remarkable histological alterations and inflammatory markers upregulation were observed in VEG group, while PAP inclusion played a role in attenuating inflammation and improving nutrient uptake. Fillet analyses highlighted significant differences in marketable traits and flesh fatty acid composition among the experimental groups, including the reduction of polyunsaturated fatty acids related to PAPs inclusion. In conclusion, PAPs used in the present study promoted S. carpio gut health and absorption capacity, while further studies are required to maintain proper quality traits of the final product.

Telocytes: Active Players in the Rainbow Trout (Oncorhynchus mykiss) Intestinal Stem-Cell Niche

In order to improve the sustainability of trout farming, it is essential to develop alternatives to fish-based meals that prevent intestinal disorders and support growth performances. Therefore, an accurate knowledge of intestinal morphology and physiology is desirable. We previously described the epithelial component of the intestinal stem-cell (ISC) niche in rainbow trout (Oncorhynchus mykiss), which is one of the most successfully farmed species and a representative model of the salmonids family. This work aims to expand that knowledge by investigating the niche stromal components that contribute to intestinal homeostasis. We analyzed samples belonging to five individuals collected from a local commercial farm. Histological and ultrastructural studies revealed peculiar mesenchymal cells adjacent to the epithelium that generated an intricate mesh spanning from the folds' base to their apex. Their voluminous nuclei, limited cytoplasm and long cytoplasmic projections characterized them as telocytes (TCs). TEM analysis showed the secretion of extracellular vesicles, suggesting their functional implication in cell-to-cell communication. Furthermore, we evaluated the localization of well-defined mouse TC markers (pdgfrα and foxl1) and their relationship with the epithelial component of the niche. TCs establish a direct connection with ISCs and provide short-range signaling, which also indicates their key role as the mesenchymal component of the stem-cell niche in this species. Interestingly, the TC distribution and gene-expression pattern in rainbow trout closely overlapped with those observed in mice, indicating that they have the same functions in both species. These results substantially improve our understanding of the mechanisms regulating intestinal homeostasis and will enable a more detailed evaluation of innovative feed effects.

New Stable Cell Lines Derived from the Proximal and Distal Intestine of Rainbow Trout (Oncorhynchus mykiss) Retain Several Properties Observed In Vivo

We derived two novel cell lines from rainbow trout (RT) proximal (RTpi-MI) and distal intestine (RTdi-MI) and compared them with the previously established continuous cell line RTgutGC. Intestinal stem cells, differentiating and differentiated epithelial cells, and connective cells were found in all cell lines. The cell lines formed a polarized barrier, which was not permeable to large molecules and absorbed proline and glucose. High seeding density induced their differentiation into more mature phenotypes, as indicated by the downregulation of intestinal stem cell-related genes (i.e., sox9, hopx and lgr5), whereas alkaline phosphatase activity was upregulated. Other enterocyte markers (i.e., sglt1 and pept1), however, were not regulated as expected. In all cell lines, the presence of a mixed population of epithelial and stromal cells was characterized for the first time. The expression by the stromal component of lgr5, a stem cell niche regulatory molecule, may explain why these lines proliferate stably in vitro. Although most parameters were conserved among the three cell lines, some significant differences were observed, suggesting that characteristics typical of each tract are partly conserved in vitro as well.