Immunohistochemical characterization of the hepatic progenitor cell compartment in medaka (Oryzias latipes) following hepatic injury

Comparative Study in Zebrafish and Medaka Unravels the Mechanisms of Tissue Regeneration

Tissue regeneration has been in the spotlight of research for its fascinating nature and potential applications in human diseases. The trait of regenerative capacity occurs diversely across species and tissue contexts, while it seems to decline over evolution. Organisms with variable regenerative capacity are usually distinct in phylogeny, anatomy, and physiology. This phenomenon hinders the feasibility of studying tissue regeneration by directly comparing regenerative with non-regenerative animals, such as zebrafish (Danio rerio) and mice (Mus musculus). Medaka (Oryzias latipes) is a fish model with a complete reference genome and shares a common ancestor with zebrafish approximately 110–200 million years ago (compared to 650 million years with mice). Medaka shares similar features with zebrafish, including size, diet, organ system, gross anatomy, and living environment. However, while zebrafish regenerate almost every organ upon experimental injury, medaka shows uneven regenerative capacity. Their common and distinct biological features make them a unique platform for reciprocal analyses to understand the mechanisms of tissue regeneration. Here we summarize current knowledge about tissue regeneration in these fish models in terms of injured tissues, repairing mechanisms, available materials, and established technologies. We further highlight the concept of inter-species and inter-organ comparisons, which may reveal mechanistic insights and hint at therapeutic strategies for human diseases.

Establishment of an Adult Medaka Fatty Liver Model by Administration of a Gubra-Amylin-Nonalcoholic Steatohepatitis Diet Containing High Levels of Palmitic Acid and Fructose

Among lifestyle-related diseases, fatty liver is the most common liver disease. To date, mammalian models have been used to develop methods for inhibiting fatty liver progression; however, new, more efficient models are expected. This study investigated the creation of a new model to produce fatty liver more efficiently than the high-fat diet medaka model that has been used to date. We compared the GAN (Gubra-Amylin nonalcoholic steatohepatitis) diet, which has been used in recent years to induce fatty liver in mice, and the high-fat diet (HFD). Following administration of the diets for three months, enlarged livers and pronounced fat accumulation was noted. The GAN group had large fat vacuoles and lesions, including ballooning, compared to the HFD group. The GAN group had a higher incidence of lesions. When fenofibrate was administered to the fatty liver model created via GAN administration and liver steatosis was assessed, a reduction in liver fat deposition was observed, and this model was shown to be useful in drug evaluations involving fatty liver. The medaka fatty liver model administered with GAN will be useful in future fatty liver research.

Development and regeneration dynamics of the Medaka notochord

The notochord is an embryonic tissue that acts as a hydrostatic skeleton until ossification begins in vertebrates. It is composed of outer sheath cells and inner vacuolated cells, which are generated from a common pool of disc-shaped precursors. Notochord extension during early embryogenesis is driven by the growth of vacuolated cells, reflecting in turn the expansion of their inner vacuole. Here we use desmogon, a novel desmosomal cadherin, to follow notochord development and regeneration in medaka (Oryzias latipes). We trace desmogon ​+ disc-shaped precursors at the single cell level to demonstrate that they operate as unipotent progenitors, giving rise to either sheath or vacuolated cells. We reveal that once specified, vacuolated cells grow asynchronously and drive notochord expansion bi-directionally. Additionally, we uncover distinct regenerative responses in the notochord, which depend on the nature of the injury sustained. By generating a desmogon CRISPR mutant we demonstrate that this cadherin is essential for proper vacuolated cell shape and therefore correct notochord and spine morphology. Our work expands the repertoire of model systems to study dynamic aspects of the notochord in vivo, and provides new insights in its development and regeneration properties.

A critical review of histopathological findings associated with endocrine and non-endocrine hepatic toxicity in fish models

Although frequently examined as a target organ for non-endocrine toxicity, histopathological evaluation of the liver is becoming a routine component of endocrine disruption studies that utilize various fish species as test subjects. However, the interpretation of microscopic liver findings can be challenging, especially when attempting to distinguish adverse changes associated with endocrine disrupting substances from those caused by systemic or direct hepatic toxicity. The purpose of this project was to conduct a critical assessment of the available peer-reviewed and grey literature concerning the histopathologic effects of reproductive endocrine active substances (EAS) and non-endocrine acting substances in the livers of fish models, and to determine if liver histopathology can be used to reliably distinguish endocrine from non-endocrine etiologies. The results of this review suggest that few compound-specific histopathologic liver effects have been identified, among which are estrogen agonist-induced increases in hepatocyte basophilia and proteinaceous intravascular fluid in adult male teleosts, and potentially, decreased hepatocyte basophilia in female fish exposed to substances that possess androgenic, anti-estrogenic, or aromatase inhibitory activity. This review also used published standardized methodology to assess the credibility of the histopathology data in each of the 117 articles that reported liver effects of treatment, and consequently it was determined that in only 37% of those papers were the data considered either highly credible or credible. The outcome of this work highlights the value of histopathologic liver evaluation as an investigative tool for EAS studies, and provides information that may have implications for EAS hazard assessment.

Intraperitoneal exposure of whitefish to microcystin-LR induces rapid liver injury followed by regeneration and resilience to subsequent exposures

To date, there has been no systematic approach comprehensively describing the sequence of pathological changes in fish during prolonged exposure to microcystin-LR (MC-LR). Towards this aim, juvenile whitefish individuals received an intraperitoneal injection with pure MC-LR, and the injection was repeated every week to maintain continuous exposure for 28days. During the exposure period, growth and condition of the fish were assessed based on biometric measurements. Additionally, selected biochemical markers were analysed in the fishes' blood, and their livers were carefully examined for morphological, ultrastructural, and molecular changes. The higher dose of MC-LR (100μg·kg^-1) caused severe liver injury at the beginning of the exposure period, whereas the lower dose (10μg·kg^-1) caused less, probably reversible injury, and its effects began to be observed later in the exposure period. These marked changes were accompanied by substantial MC-LR uptake by the liver. However, starting on the 7th day of exposure, cell debris began to be removed by phagocytes, then by 14th day, proliferation of liver cells had markedly increased, which led to reconstruction of the liver parenchyma at the end of the treatment. Surprisingly, despite weekly-repeated intraperitoneal injections, MC-LR did not accumulate over time of exposure which suggests its limited uptake in the later phase of exposure. In support, mRNA expression of the membrane transport protein oatp1d was decreased at the same time as the regenerative processes were observed. Our study shows that closing of active membrane transport may serve as one defence mechanism against further MC-LR intoxication.

Phenotypic characterization of transgenic Japanese medaka (Oryzias latipes) that express a red fluorescent protein in hepatocytes

Transgenic organisms that express fluorescent proteins are used frequently for in vivo visualization of proteins and cells. The phenotype of a transgenic medaka (Oryzias latipes) strain that expresses a red fluorescent protein (RFP) in hepatocytes was characterized using light and fluorescence microscopy, immunohistochemistry, and transmission electron microscopy (TEM). Expression of RFP was first detected by confocal fluorescence microscopy in the location of the liver bud of live medaka embryos at 60 hr postfertilization (developmental stage 27). Subsequently, RFP signal was observed exclusively in hepatocytes throughout life using fluorescence microscopy in live fish and immunohistochemistry in formalin-fixed, paraffin-embedded liver sections. As the fish aged, prominent intracytoplasmic eosinophilic inclusions immunoreactive for RFP were observed by light microscopy and were correlated with membrane-bound electron dense inclusions on TEM. These results define the onset and location of RFP expression in the Tg(zf.L-fabp:DsRed) medaka and characterize a histologic phenotype that results from RFP expression in hepatocytes.