Ultrastructural, immunocytochemical and morphometric characterization of liver peroxisomes in gray mullet, Mugil cephalus

Molecular characterization of novel mitochondrial peroxiredoxins from the Antarctic emerald rockcod and their gene expression in response to environmental warming

In the present study we describe the molecular characterization of the two paralogous mitochondrial peroxiredoxins from Trematomus bernacchii, a teleost that plays a pivotal role in the Antarctic food chain. The two putative amino acid sequences were compared with orthologs from other fish, highlighting a high percentage of identity and similarity with the respective variant, in particular for the residues that are essential for the characteristic peroxidase activity of these enzymes. The temporal expression of Prdx3 and Prdx5 mRNAs in response to short-term thermal stress showed a general upregulation of prdx3, suggesting that this isoform is the most affected by temperature increase. These data, together with the peculiar differences between the molecular structures of the two mitochondrial Prdxs in T. bernacchii as well as in the tropical species Stegastes partitus, suggest an adaptation that allowed these poikilothermic aquatic vertebrates to colonize very different environments, characterized by different temperature ranges.

Seasonal and Morphological Variations of Brown Trout (Salmo trutta f. fario) Kidney Peroxisomes: A Stereological Study

Literature about fish kidney peroxisomes is scarce. To tackle this caveat, a stereological approach on renal peroxisome morphological parameters was performed for the first time in a fish, establishing correlations with maturation stages as it was previously done in brown trout liver. Three-year-old brown trout males and females were collected at the major seasons of their reproductive cycle. Trunk kidney was fixed and processed for catalase cytochemistry. Classical stereological methods were applied to electromicrographs to quantitate morphological parameters. Different seasonal variation patterns were observed between genders, and between renal proximal tubule segments I and II. In males, peroxisomes from proximal tubule segment II had a relatively higher volume and number in May, being individually bigger in February. Females presented similar trends, though with less marked variations. Overall, males and females did not show exactly the same seasonal patterns for most peroxisomal parameters, and no correlations were found between the latter and the gonado-somatic index (GSI). Hence, and despite the variations, the morphology of renal peroxisomes is not strictly correlated with gonad maturation kinetics, therefore suggesting that kidney peroxisome morphology is not seasonally modulated by sex steroids, like estradiol, as it seems to happen in liver peroxisomes.

Estimation of volume densities of hepatocytic peroxisomes in a model fish: catalase conventional immunofluorescence versus cytochemistry for electron microscopy

Accurately accessing changes in the intracellular volumes (or numbers) of peroxisomes within a cell can be a lengthy task, because unbiased estimations can be made only by studies conducted under transmission electron microscopy. Yet, such information is often required, namely for correlations with functional data. The optimization and applicability of a fast and new technical proceeding based on catalase immunofluorescence was implemented herein by using primary hepatocytes from brown trout (Salmo trutta f. fario), exposed during 96 h to two distinct treatments (0.1% ethanol and 50 µM of 17α-ethynylestradiol). The time and cost efficiency, together with the results obtained by stereological analyses, specifically directed to the volume densities of peroxisomes, and additionally of the nucleus in relation to the hepatocyte, were compared with the well-established 3,3'-diaminobenzidine cytochemistry for electron microscopy. With the immuno technique it was possible to correctly distinguish punctate peroxisomal profiles, allowing the selection of the marked organelles for quantification. By both methodologies, a significant reduction in the volume density of the peroxisome within the hepatocyte was obtained after an estrogenic input. The most interesting point here was that the volume density ratios were quite correlated between both techniques. Overall, the immunofluorescence protocol for catalase was evidently faster, cheaper and provided reliable quantitative data that discriminated in the same way the compared groups. After this validation study, we recommend the use of catalase immunofluorescence as the first option for rapid screening of changes of the amount of hepatocytic peroxisomes, using their volume density as an indicator.

Assessment of lysosomal membrane stability and peroxisome proliferation in the head kidney of Atlantic cod (Gadus morhua) following long-term exposure to produced water components

There is a need for sensitive biological effect methods by which to detect impacts of chronic exposure to low concentrations of contaminants. Two methods shown to be potentially useful for monitoring purposes in fish include lysosomal membrane stability and peroxisome proliferation. These biological endpoints were assessed in Atlantic cod (Gadus morhua) head kidney following exposure to a mixture of produced water components including polycyclic aromatic hydrocarbons, phenol, and alkylphenols. Lysosomal damage of head kidney cells occurred within the first two weeks and did not recover during the entire exposure period (32 weeks). Lysosomal membrane stability was not affected by gender and was responsive at low concentrations of contamination, indicating that lysosomal membrane stability measured in the head kidney could be a useful biomarker for effects of offshore pollution. Peroxisome proliferation, measured as acyl-CoA oxidase activity in the head kidney, appeared to be a potential biomarker in male cod exposed less than 16 weeks.

Cloning and expression pattern of peroxisome proliferator-activated receptors, estrogen receptor alpha and retinoid X receptor alpha in the thicklip grey mullet Chelon labrosus

Aquatic organisms are exposed to diverse xenobiotics that cause peroxisome proliferation and/or endocrine disruption, both modulated in vertebrates by transcription factors of the nuclear receptor (NR) superfamily. Peroxisome proliferators are agonists of peroxisome proliferator-activated receptors (PPARs) that heterodimerize with the retinoid X receptor (RXR). Many xenoestrogens activate the estrogen receptor (ER). Here, 1090 bp of PPARalpha, 1255 bp of PPARgamma, 278 bp of RXRalpha, and 578 bp of ERalpha of thicklip grey mullet Chelon labrosus were cloned. Sequences were highly conserved, although relevant changes with respect to mammalian homologs were identified in PPARgamma and ERalpha. Semi-quantitative RT-PCR was used to determine if these NRs were expressed in different tissues of male, female and undifferentiated mullets captured in January and June. Expression of PPARs was highest in liver and lowest in muscle. RXRalpha expression was homogeneous excepting a low expression in male and female gill in January and brain and heart of undifferentiated fish in January and June. ERalpha expression predominated in liver and female gonad in June. The expression level of PPARs and ERalpha was significantly higher in liver in January than in gills in January or June. The present results show tissue-dependent modulation of expression of NRs in mullets.

Flow cytometric assessment of peroxisome proliferation from frozen liver of fibrate-treated monkeys

Multiple methods currently exist for the assessment of peroxisome proliferation, including gene expression, enzyme activity, immunolabeling coupled with image analysis, and electron microscopy. This study describes a novel flow cytometric method to efficiently quantify peroxisome proliferation in cells from frozen livers. Frozen livers from cynomolgus monkeys treated with ciprofibrate at doses of 0, 3, 30, 150, and 400 mg/kg/day for 15 days were mechanically disaggregated using an automated dispersion method. The resulting cell suspensions were labeled using an allophycocyanin (APC)-conjugated antibody directed against peroxisomal membrane protein 70 (PMP70). Statistically significant increases in mean fluorescence intensity were observed from animals dosed at 30, 150, and 400 mg/kg/day compared to control. Parallel comparisons using electron microscopy and immunofluorescence microscopy suggest that flow cytometry may be an alternative to electron microscopy in determinations of peroxisome proliferation. Flow cytometric analysis of freshly isolated hepatocytes and frozen liver from rats treated with fenofibrate at 200 mg/kg/day for 10 days showed the flow cytometric method could detect peroxisome proliferation in both species. The research described here demonstrates the feasibility of applying flow cytometry for the detection of peroxisome proliferation.

Comparative Histological Study of Teleost Livers in Relation to Phylogeny

This report presents a detailed description of hepatic architecture in 200 teleost livers by light microscopy and extensively discusses the phylogenetic viewpoint. The 200 teleost livers showed a great variety of histological images, but not the same image, as in mammalian livers. The hepatocyte-sinusoidal structures of the fish livers were classified into three different types: (a) cord-like form, (b) tubular form, (c) solid form. Biliary tract structures were classified into four types: (a) isolated type, (b) biliary-arteriolar tract (BAT) type, (c) biliary-venous tract (BVT) type, and (d) portal tract type. As phylogenic advancement is graded from low to high, the parenchymal arrangement progressed from solid or tubular form to cord-like form, but the biliary tract structures were not involved. We demonstrate that this study is the first to investigate teleost livers phylogenically, and their architectural differences are shown in the route of hepatic ontogenesis. In hepatic ontogenesis, the formation of the parenchymal arrangement is acquired phylogenically, but the biliary pathway may be adapted in the ecological and behavioral patterns.

Expression of peroxisome proliferator-activated receptors in the liver of gray mullet (Mugil cephalus)

During the last decade, peroxisome proliferation has emerged as a novel biomarker of exposure to certain organic chemical pollutants in aquatic organisms. Peroxisome proliferation is mediated by nuclear receptors, peroxisome proliferator-activated receptors (PPARs). Three PPAR subtypes have been described in mammals: PPAR alpha, PPAR beta and PPAR gamma. PPARs have also been discovered in several fish species. The aim of the present study was to investigate the expression of PPAR subtypes and their cellular distribution patterns in the liver of gray mullet Mugil cephalus, a fish species widely distributed in estuaries and coastal areas in Europe and used as sentinel of environmental pollution. For this purpose, antibodies were generated against the three subtypes of mouse PPARs and different protocols of antigen retrieval were used. In western blots, main bands were detected of approximately 44 kDa for PPAR alpha, two bands of 44 and 58 kDa for PPAR beta and a single band of 56 kDa for PPAR gamma. Similar results were obtained in mouse liver and may indicate antibody recognition of two forms of the protein in certain cases. PPAR alpha was the subtype most markedly expressed in gray mullet liver, being expressed mainly in melanomacrophages, nuclei of hepatocytes and sinusoidal cells and connective tissue surrounding bile ducts. PPAR beta was expressed in the same cell types but immunolabeling was generally weaker than for PPAR alpha. PPAR gamma showed very weak expression; positivity was mainly found in melanomacrophages and connective tissue surrounding bile ducts. Our results demonstrate that all the three PPAR subtypes are expressed in gray mullet liver but in different intensities. The cellular distribution patterns of PPAR subtypes in gray mullet liver resembled partly those found in mouse liver with PPAR alpha as the main subtype expressed in hepatocytes. The fact that melanomacrophages, cells of the immune system in fish, show strong expression of both PPAR alpha and PPAR beta whereas PPAR gamma expression is almost restricted to this cell type suggest a significant role of PPAR-mediated regulation of cell function in melanomacrophages.

Peroxisome proliferation as a biomarker in environmental pollution assessment

Peroxisome proliferators comprise a heterogeneous group of compounds known for their ability to cause massive proliferation of peroxisomes and liver carcinogenesis in rodents. In recent years it has become evident that other animals may be threatened by peroxisome proliferators, in particular aquatic organisms living in coastal and estuarine areas. These animals are exposed to a variety of pollutants of industrial, agricultural and urban origin which are potential peroxisome proliferators. Both laboratory and field studies have shown that phthalate ester plasticizers, PAHs and oil derivatives, PCBs, certain pesticides, bleached kraft pulp and paper mill effluents, alkylphenols and estrogens provoke peroxisome proliferation in different fish or bivalve mollusc species. The response appears to be mediated by peroxisome-proliferator activated receptors, members of the nuclear receptor family, recently cloned in fish. Based on these results it is proposed that peroxisome proliferation could be used as a biomarker of exposure to a variety of pollutants in environmental pollution assessment. This is illustrated by a case study in which mussels, used worldwide as sentinels of environmental pollution, were transplanted from reference to contaminated areas and vice versa. In mussels native to an area polluted with PAHs and PCBs, peroxisomal acyl-CoA oxidase (AOX) activity and peroxisomal volume density were 2-3 fold and 5-fold higher, respectively, compared to the reference site. When animals were transplanted to the polluted station, with increased concentration of organic xenobiotics, a concomitant significant increase of AOX was recorded. Conversely, in animals transplanted to the cleaner station, AOX activity and peroxisomal volume density decreased significantly. These results indicate that peroxisome proliferation is a rapid (i.e., two days) and reversible response to pollution in mussels. Before peroxisome proliferation can be implemented as a biomarker in biomonitoring programs, a well-defined protocol should be established and validated in intercalibration and quality assurance programmes. Furthermore, the influence of biotic and abiotic factors, some of which are known to affect peroxisome proliferation (season, tide level, interpopulation and interindividual variability), should be taken into consideration. The possible hepatocarcinogenic effects as well as the potential adverse effects on reproduction, development, and growth of peroxisome proliferators are unknown in aquatic organisms, thus providing a challenge for future investigations.

Measurement of peroxisomal enzyme activities in the liver of brown trout (Salmo trutta), using spectrophotometric methods

BACKGROUND

This study was aimed primarily at testing in the liver of brown trout (Salmo trutta) spectrophotometric methods previously used to measure the activities of catalase and hydrogen peroxide producing oxidases in mammals. To evaluate the influence of temperature on the activities of those peroxisomal enzymes was the second objective. A third goal of this work was the study of enzyme distribution in crude cell fractions of brown trout liver.

RESULTS

The assays revealed a linear increase in the activity of all peroxisomal enzymes as the temperature rose from 10 degrees to 37 degrees C. However, while the activities of hydrogen peroxide producing oxidases were strongly influenced by temperature, catalase activity was only slightly affected. A crude fraction enriched with peroxisomes was obtained by differential centrifugation of liver homogenates, and the contamination by other organelles was evaluated by the activities of marker enzymes for mitochondria (succinate dehydrogenase), lysosomes (aryl sulphatase) and microsomes (NADPH cytochrome c reductase). For peroxisomal enzymes, the activities per mg of protein (specific activity) in liver homogenates were strongly correlated with the activities per g of liver and with the total activities per liver. These correlations were not obtained with crude peroxisomal fractions.

CONCLUSIONS

The spectrophotometric protocols originally used to quantify the activity of mammalian peroxisomal enzymes can be successfully applied to the study of those enzymes in brown trout. Because the activity of all studied peroxisomal enzymes rose in a linear mode with temperature, their activities can be correctly measured between 10 degrees and 37 degrees C. Probably due to contamination by other organelles and losses of soluble matrix enzymes during homogenisation, enzyme activities in crude peroxisomal fractions do not correlate with the activities in liver homogenates. Thus, total homogenates will be used in future seasonal and toxicological studies of brown trout peroxisomes.

Antioxidant enzymes and peroxisome proliferation in relation to contaminant body burdens of PAHs and PCBs in bivalve molluscs, crabs and fish from the Urdaibai and Plentzia estuaries (Bay of Biscay)

With the aim of studying levels of antioxidant and peroxisomal enzymes and the structure of peroxisomes in relation to body burdens of polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), mussels Mytilus galloprovincialis, oysters Crassostrea sp., crabs Carcinus maenas and mullets Mugil cephalus were sampled in two Basque estuaries (Bay of Biscay): Urdaibai (Laida, Txatxarramendi, Arteaga, and downstream a sewage treatment plant-STP) and Plentzia. In general, animals showed higher concentrations of contaminants in winter than in summer and no relevant differences were detected among locations. Conversely, antioxidant enzyme activities were higher in summer. Enzyme expression was studied in mullets using immunochemical methods. By immunoblotting season-dependent differences were detected for Mn-superoxide dismutase (Mn-SOD). As for the immunohistochemical staining, mullets sampled in summer in Plentzia showed significantly higher optical densities for acyl-CoA oxidase and lower for both Cu,Zn-SOD and Mn-SOD than those collected downstream a STP as well as higher catalase immunostaining than those collected in winter. Peroxisomal volume density (V(vp)) of mussels sampled in Laida and Txatxarramendi did not show seasonal variations, while for oysters collected in Laida and Arteaga V(vp) was higher in summer. Crab and mullet V(vp) were also higher in summer. In conclusion, the estuaries of Urdaibai and Plentzia can be considered as low to moderately polluted areas and levels of PAHs and PCBs do not show marked variations apart from seasonal variations. Animals can be adapted to low pollution conditions and, under these circumstances, seasonal factors might affect biomarker responses to a greater extent than pollution variations.

Interactive effects of benzo(a)pyrene and cadmium and effects of di(2-ethylhexyl) phthalate on antioxidant and peroxisomal enzymes and peroxisomal volume density in the digestive gland of mussel Mytilus galloprovincialis Lmk

Exposure of marine animals to certain organic and metal pollutants is thought to enhance reactive oxygen species (ROS) production with concomitant alterations of antioxidant defence mechanisms. Some of these organic pollutants cause peroxisome proliferation, a process resulting also in possible enhanced production of ROS. The aim of this study was to investigate the effects of two organic xenobiotics, benzo(a)pyrene (B(a)P) and di(2-ethylhexyl)phthalate (DEHP), as well as the effects of cadmium (Cd), on antioxidant and peroxisomal enzymes and on peroxisomal volume density in the digestive gland of mussel, Mytilus galloprovincialis Lmk., experimentally exposed for 21 days. Special attention was paid to the interactive effects of organic and metal compounds by exposing one group of mussels to a mixture of B(a)P and Cd. Exposure of mussels to Cd caused a decrease in superoxide dismutase (SOD) activity, in Mn-SOD protein levels and in volume density of peroxisomes. B(a)P exposure significantly increased catalase and glutathione peroxidase (GPX) and inhibited Mn-SOD after 21 days of exposure. B(a)P also caused a slight increase in acyl-CoA oxidase (AOX) activity and peroxisomal volume density after 21 days of exposure. Cd tended to inhibit changes provoked by B(a)P, indicating that responses to organic xenobiotics can be modulated by concomitant exposure to metal contaminants. Exposure to DEHP increased catalase and AOX and inhibited SOD activity and Mn-SOD protein levels. In conclusion, peroxisome proliferation, measured as an increase of the peroxisomal enzymes catalase and AOX (up to 1.53-fold for AOX), is a specific response to organic contaminants such as B(a)P and DEHP, whereas Cd does not cause peroxisome proliferation. Thus, peroxisome proliferation may be a specific biomarker of organic pollutants in mussels. Both organic and metal pollutants inhibited SOD activity and protein levels (up to 0.21-fold for Mn-SOD protein levels), the latter offering potential as general marker of pollution.

Cell biology of peroxisomes and their characteristics in aquatic organisms

The general characteristics of peroxisomes in different organisms, including aquatic organisms such as fish, crustaceans, and mollusks, are reviewed, with special emphasis on different aspects of the organelle biogenesis and mechanistic aspects of peroxisome proliferation. Peroxisome proliferation and peroxisomal enzyme inductions elicited by xenobiotics or physiological conditions have become useful tools to study the mechanisms of peroxisome biogenesis. During peroxisome proliferation, the induction of peroxisomal proteins is heterogeneous, enzymes that show increased activity being involved in different aspects of lipid homeostasis. The process of peroxisome biogenesis is coordinately triggered by a whole array of structurally dissimilar compounds known as peroxisome proliferators, and investigating the effect of some of these compounds that commonly appear as pollutants in the environment on the peroxisomes of aquatic animals inhabiting marine and estuarine habitats seems interesting. It is also important to determine whether peroxisome proliferation in these animals is a phenomenon that might occur under normal physiological or season-related conditions and plays a metabolic or functional role. This would help set the basis for understanding the process of peroxisome biogenesis in aquatic animals.