The fine structure of the epidermis of two species of salmonid fish, the Atlantic salmon (Salmo salar L.). and the brown trout (Salmo trutta L.)

ITRAQ Proteomic Analysis of Yellow and Black Skin in Jinbian Carp (Cyprinus carpio)

Colors are important phenotypic traits for fitness under natural conditions in vertebrates. Previous studies have reported several functional genes and genetic variations of pigmentation, but the formation mechanisms of various skin coloration remained ambiguous in fish. Jinbian carp, a common carp variant, displays two colors (yellow and black) in the skin, thus, it is a good model for investigating the genetic basis of pigmentation. In the present study, using the Jinbian carp as model, isobaric tags for relative and absolute quantification (ITRAQ) proteomics analysis was performed for yellow and black skin, respectively. The results showed that 467 differentially expressed proteins (DEPs) were identified between the yellow skin and the black skin. Similar to mammals, the up-regulated DEPs in black skin included UV excision repair protein RAD23 homolog A (Rad23a), melanoregulin (mreg), 5,6-dihydroxyindole-2-carboxylic acid oxidase5 (tyrp1) and melanocyte protein PMEL (PMEL), which were mainly grouped into melanogenesis pathway. However, several up-regulated DEPs in yellow skin were mainly enriched in nucleotide metabolism, such as GTPase IMAP family member 5 (GIMAP5), AMP deaminase 1 (AMPD1), adenosylhomocysteinase b (ahcy-b), and pyruvate kinase (PKM). In summary, several candidate proteins and their enrichment pathways for color variation in Jinbian carp were identified, which may be responsible for the formation of different colorations.

Proteomic profiling of salmon skin mucus for the comparison of sampling methods

The epidermal mucus protects fish against harmful environmental factors and the loss of physiological metabolites and water. It is an efficient barrier between the fish and the biosphere. The integrity of the skin mucus is thus of vital importance for the welfare and survival of the fish. Since excreted proteins and small molecules in the mucus can mirror the health status of the fish, it is a valuable matrix for monitoring stress, pathogen exposure, and nutritional effects. Several methods for sampling epidermal mucus from different fish species have previously been described, but information about their efficiency or the comparability of mucus analyses is lacking. In the present study, skin mucus from farmed Atlantic salmon was therefore sampled by three methods, including absorption or wiping with tissue paper, and scraping with a blunt blade, and the mucus proteome was analyzed by ultra-high pressure liquid chromatography high-resolution mass spectrometry. The measured protein contents, numbers, compositions and the observed data quality were compared between sampling methods. Furthermore, functional annotation and classification of the identified proteins was performed. The results showed that the three skin mucus sample types differed qualitatively as well as quantitatively. The absorbed mucus was the least tainted by proteins resulting from damage inflicted to the fish epidermis by the sampling procedure. Wiped mucus showed a better protein yield than absorbed and delivered a larger proteome of identifiable proteins, with less contamination from epithelial proteins than observed for scraped mucus. We recommend that future research of mucus should use the absorption method in cases, where it is important that the mucus is devoid of proteins from the underlying epithelium, and the wiping method, when protein yield is crucial or when the proteome of the outer epithelium is of interest.

Morphological study of the gastrointestinal tract of the snow trout, Schizothorax esocinus (Actinopterygii: Cypriniformes)

The present study aimed to investigate the macroscopic structure of the gastrointestinal tract (GIT) of Schizothorax esocinus Heckel, 1838. The surface architecture of the buccopharynx, oesophagus and the entire intestinal tract of S. esocinus has been examined under scanning electron microscope (SEM) after fixing in 2.5% glutaraldehyde buffered with 0.1 M sodium cacodylate at pH 7.3 for 18–48 hours and post-fixation for two hours at room temperature in 1% osmium tetra oxide buffered at pH 7.3 with 0.1 M cacodylate. The mucosal surface of buccopharynx, esophagus, intestinal bulb, and intestine reveal prominent longitudinal major or primary mucosal folds which are further subdivided into the series of irregular and well-circumscribed folds called minor or secondary folds. However, in the intestinal bulb and intestine, the longitudinal major or primary folds themselves form wavy or zigzagging patterns along the mucosal surface. The fine structure of the surface epithelium further shows that the apical surfaces of the epithelial cells are ped with finger-print like microridges, arranged in various patterns and regularly spaced. The rectal mucosa, on the other hand, displays a highly irregular type of major mucosal folds. The separation can’t be seen between major mucosal folds. A thin film of mucous spread over the mucosal folds and the numerous pores through which mucous cells release their content has also been noted along the rectal mucosa. This investigation suggests the possible role of different digestive organs in relation to feeding, digestion, storage, absorption, and various other physiological processes, thereby providing a knowledge necessary to the understanding of pathological or physiological alterations in both aquaculture and natural environment.

Structural and functional maturation of skin during metamorphosis in the Atlantic halibut (Hippoglossus hippoglossus)

To establish if the developmental changes in the primary barrier and osmoregulatory capacity of Atlantic halibut skin are modified during metamorphosis, histological, histochemical, gene expression and electrophysiological measurements were made. The morphology of the ocular and abocular skin started to diverge during the metamorphic climax and ocular skin appeared thicker and more stratified. Neutral mucins were the main glycoproteins produced by the goblet cells in skin during metamorphosis. Moreover, the number of goblet cells producing neutral mucins increased during metamorphosis and asymmetry in their abundance was observed between ocular and abocular skin. The increase in goblet cell number and their asymmetric abundance in skin was concomitant with the period that thyroid hormones (THs) increase and suggests that they may be under the control of these hormones. Several mucin transcripts were identified in metamorphosing halibut transcriptomes and Muc18 and Muc5AC were characteristic of the body skin. Na⁺, K⁺-ATPase positive (NKA) cells were observed in skin of all metamorphic stages but their number significantly decreased with the onset of metamorphosis. No asymmetry was observed between ocular and abocular skin in NKA cells. The morphological changes observed were linked to modified skin barrier function as revealed by modifications in its electrophysiological properties. However, the maturation of the skin functional characteristics preceded structural maturation and occurred at stage 8 prior to the metamorphic climax. Treatment of Atlantic halibut with the THs disrupter methimazole (MMI) affected the number of goblet cells producing neutral mucins and the NKA cells. The present study reveals that the asymmetric development of the skin in Atlantic halibut is TH sensitive and is associated with metamorphosis and that this barrier's functional properties mature earlier and are independent of metamorphosis.

Epidermis structure in the brook trout (Salvelinus fontinalis) and its Arctic char (Salvelinus alpinus) hybrid

Brook trout (Salvelinus fontinalis) is a species of fish native to North-East America. Brook trout are also commercially raised in large numbers for food production. Skin infection and/or parasite outbreaks can have a serious economics effect on aquaculture businesses. For this reason, it has been hybridized with the more resistant Arctic char (Salvelinus alpinus). The aim of this study was an examination of the epidermal structure and dynamic in brook trout and its Arctic char hybrid which is less sensitive to skin infection. The samples of fish (72 brook trout, 72 brook trout × Arctic char hybrid) from fish farm in Pravíkov (49°19’10”N, 15°5’40”E) were collected five times during the year 2011. Absolute and relative epidermal thickness (in relation to body size) and relative proportion of secretory cells in a given volume of epidermis were measured. The epidermis structure of brook trout and brook trout × Arctic char hybrid both display similar seasonal dynamics, with a decrease in absolute and relative epidermal thickness and a reduction in the relative percentage of mucous secretory cells over the summer. On the other hand, the lower absolute (mean 103 µm (range 84–146 µm) in brook trout; 88 µm (range 68–115 µm) in hybrids) and relative epidermal thickness (mean 4.8 (range 3.6–6.8) in brook trout; 4.4 (range 2.9–6.4) in hybrids) and lower volume of secretory cells was observed to the hybrid (mean 28% (range 19–33%) in brook trout; 23% (range 10–30%) in hybrids). It can interrelate with their higher resistance to infection and/or parasite outbreaks.

Probiotics-pathogen interactions elicit differential regulation of cutaneous immune responses in epidermal cells of Atlantic cod Gadus morhua

Little is known on the cutaneous immune responses during probiotics-pathogen interactions in fish. Thus, this study employed Atlantic cod primary epidermal (EP) cell cultures as a model to understand this interaction. The probiotics-pathogen interactions in the EP cell cultures were elucidated using Vibrio anguillarum 2133 (VA) as the pathogen and two host-derived bacteria (GP21 and GP12) as the probiotics. There was a regional size difference on the EP cells; i.e., EP cells from the dorsal region were significantly larger than the EP cells at the ventral side. VA significantly decreased viability of EP cells. In the presence of probiotics, this inhibition was mitigated. The probiotics reduced VA-induced cellular apoptosis and the probiotics-pathogen interactions influenced cellular myeloperoxidase activity during the latter stage of co-incubation. The probiotics-pathogen interactions triggered differential regulation of immune-related genes and the effects of the interaction were dependent on the region where the cells were isolated and the length of the co-incubation period. In most cases, the presence of probiotics alone showed no significant change on the mRNA level of immune genes in the EP cells but triggered immunostimulatory activity when incubated with VA. This study showed that the virulence of VA in EP cells could be modulated by host-derived probiotics and the immunomodulatory characteristics of the two candidate probionts advanced their immune-related probiotic potential.

Exploring the transcriptome of Atlantic salmon (Salmo salar) skin, a major defense organ

The skin of fish is the first line of defense against pathogens and parasites. The skin transcriptome of the Atlantic salmon is poorly characterized, and currently only 2,089 expressed sequence tags (ESTs) out of a total of half a million sequences are generated from skin-derived cDNA libraries. The primary aim of this study was to enhance the transcriptomic knowledge of salmon skin by using next-generation sequencing (NGS) technology, namely the Roche-454 platform. An equimolar mixture of high-quality RNA from skin and epidermal samples of salmon reared in either freshwater or seawater was used for 454-sequencing. This technique yielded over 600,000 reads, which were assembled into 34,696 isotigs using Newbler. Of these isotigs, 12 % had not been sequenced in Atlantic salmon, hence representing previously unreported salmon mRNAs that can potentially be skin-specific. Many full-length genes have been acquired, representing numerous biological processes. Mucin proteins are the main structural component of mucus and we examined in greater detail the sequences we obtained for these genes. Several isotigs exhibited homology to mammalian mucins (MUC2, MUC5AC and MUC5B). Mucin mRNAs are generally >10 kbp and contain large repetitive units, which pose a challenge towards full-length sequence discovery. To date, we have not unearthed any full-length salmon mucin genes with this dataset, but have both N- and C-terminal regions of a mucin type 5. This highlights the fact that, while NGS is indeed a formidable tool for sequence data mining of non-model species, it must be complemented with additional experimental and bioinformatic work to characterize some mRNA sequences with complex features.

Factors influencing the structure, function and biota of the salmonid epidermis

The present contribution shows the extent and the limitations of our knowledge of the structure, functions and biota of the epidermis of just one family of teleost fish, the Salmonidae. Even within this group it is clear that the epidermis can respond, with remarkable rapidity in some cases, to changes in the environment and to internal stimuli, presumably mediated via the fish's endocrine system. Against this background of constantly changing structure it is believed that the emphasis of the multiple functions of the epidermis must also be changing. Thus, future studies on the skin offish must take into account the possibility of such variations and relate the biology of the skin and its inhabitants to broader aspects of the biology of the fish.

Vibrio salmonicida pathogenesis analyzed by experimental challenge of Atlantic salmon (Salmo salar)

Cold-water vibriosis (CV) is a bacterial septicemia of farmed salmonid fish and cod caused by the Gram-negative bacterium Vibrio (Aliivibrio) salmonicida. To study the pathogenesis of this marine pathogen, Atlantic salmon was experimentally infected by immersion challenge with wild type V. salmonicida and the bacterial distribution in different organs was investigated at different time points. V. salmonicida was identified in the blood as early as 2 h after challenge demonstrating a rapid establishment of bacteremia without an initial period of colonization of the host. Two days after immersion challenge, only a few V. salmonicida were identified in the intestines, but the amount increased with time. In prolonged CV cases, V. salmonicida was the dominating bacterium of the gut microbiota causing a release of the pathogen to the water. We hypothesize that V. salmonicida uses the blood volume for proliferation during the infection of the fish and the salmonid intestine as a reservoir that favors survival and transmission. In addition, a motility-deficient V. salmonicida strain led us to investigate the impact of motility in the CV pathogenesis by comparing the virulence properties of the mutant with the wild type LFI1238 strain in both i.p. and immersion challenge experiments. V. salmonicida was shown to be highly dependent on motility to gain access to the fish host. After invasion, motility was no longer required for virulence, but the absence of normal flagellation delayed the disease development.

Neural network detected in a presumed vestigial trait: ultrastructure of the salmonid adipose fin

A wide variety of rudimentary and apparently non-functional traits have persisted over extended evolutionary time. Recent evidence has shown that some of these traits may be maintained as a result of developmental constraints or neutral energetic cost, but for others their true function was not recognized. The adipose fin is small, fleshy, non-rayed and located between the dorsal and caudal fins on eight orders of basal teleosts and has traditionally been regarded as vestigial without clear function. We describe here the ultrastructure of the adipose fin and for the first time, to our knowledge, present evidence of extensive nervous tissue, as well as an unusual subdermal complex of interconnected astrocyte-like cells equipped with primary cilia. The fin contains neither adipose tissue nor fin rays. Many fusiform actinotrichia, comprising dense striated macrofibrils, support the free edge and connect with collagen cables that link the two sides. These results are consistent with a recent hypothesis that the adipose fin may act as a precaudal flow sensor, where its removal can be detrimental to swimming efficiency in turbulent water. Our findings provide insight to the broader themes of function versus constraints in evolutionary biology and may have significance for fisheries science, as the adipose fin is routinely removed from millions of salmonids each year.

Cellular responses in primary epidermal cultures from rainbow trout exposed to zinc chloride

In this study, we examined the effects of zinc chloride (ZnCl(2)) (0-200mg/L) on primary epidermal cultures from Oncorhynchus mykiss. Increases in the rate and amount of mucus released were detected post-exposure, as was a dose-dependent increase in the synthesis of acidic glycoproteins. The cytotoxicity of ZnCl(2) to the cultures was significantly increased (P< or =0.05) when exposures were conducted in serum-free medium as opposed to medium containing serum. Significant increases in the levels of apoptosis and necrosis were observed with increasing exposure concentration, but these were seen to decrease over time. The in vitro cytological and pathological changes observed in this study were found to be in accordance with previously published in vivo studies on the effects of heavy metals on the integument. This model system may help to further elucidate the effects of ecotoxicants on the external innate immune system of fish.

Immunocytochemical localization of keratins, associated proteins and uptake of histidine in the epidermis of fish and amphibians

Keratinization and the role of histidine in some species of fish and amphibians have been analyzed by immunocytochemistry and autoradiography. In cartilaginous and bony fishes, staining of acidic (AE1-positive) and basic (AE3-positive) keratins was strong and their distribution patterns were uniform in all epidermal layers. The AE2 antibody (for keratins K1 and K10 that are typical for keratinization) did not produce any positivity. This was also observed in lungfish epidermis but the AE2 antibody often produced some positivity in the more keratinized layers. In the axolotl (urodele), that is adapted to aquatic conditions, as well as in other species of urodele (newts) that are more adapted to terrestrial conditions, the same pattern was present as in fish. In the latter, the AE2 antibody non-specifically stained all epidermal layers. In more terrestrially-adapted anurans (frog and toad) AE1 immunopositivity was mainly found in basal layers, the AE3 antibody stained the entire epidermis, and AE2 immunopositivity was often localized in the external layers of the epidermis. This pattern resembled that in the epidermis of amniotes. Administration of tritiated histidine to goldfish epidermis showed that at 1, 4 and 24 h after injection, labelling was low and uniformely distributed in all epidermal layers. In newt and toad epidermis, histidine labelling increased from 1 to 4 h after injection but tended to remain evenly distributed throughout the epidermis. However, from 4 up to 24 h after injection, labelling became concentrated in the upper intermediate and replacement layers, suggesting that turnover proteins were produced. Histidine was probably converted into other metabolites at 4-24 h after injection. Whether the newly synthetized proteins were a form of keratin or a specific histidine-rich protein remains to be determined biochemically. Uptake of tritiated thymidine in newt epidermis indicated that keratinocytes move into the uppermost stratum intermedium within 4 days, and reach the replacement layer in approximately 6 days. Taken together, the data obtained with tritiated histidine and thymidine suggest that most histidine is taken up in the upper intermedium and replacement layer at 4-24 h after injection. Neither a granular layer nor crossreaction with filaggrin and loricrin were observed in fish and amphibian epidermis. Although the cell membrane of superficial corneous cells of amphibian epidermis became thicker, the absence of loricrine immunolabelling suggests that a cell corneous envelope containing this protein is not present or undetectable.

Keratinization of the epidermis of the Australian lungfish Neoceratodus forsteri (dipnoi)

The differentiation of the epidermis in sarcopterigian fish may reveal some trend of keratinization followed by amphibian ancestors to adapt their epidermis to land. Therefore, the process of keratinization of the epidermis of the Australian lungfish Neoceratodus forsteri was studied by histochemistry, electron microscopy, and keratin immunocytochemistry. The epidermis is tri-stratified in a 2-3-month-old tadpole but becomes 6-8 stratified in young adults. Keratin filaments increase from basal to external cells where loose tonofilament bundles are present. This is shown also by the comparison of positivity to sulfhydryl groups and increasing immunoreactivity to alpha-keratins in more external layers of the epidermis. Two broad-spectrum anti alpha-keratin monoclonal antibodies (AE1 and AE3) stain all epidermal layers as they do in actinopterigian fish. In the adult epidermis, but not in that of the larva, the AE2 antibody (a marker of keratinization in mammalian epidermis) often immunolabels more heavily the external keratinized layers where sulfhydryl groups are more abundant. Mucous granules are numerous and concentrate on the external surface of the epidermis to be discharged and contribute to cuticle formation. Keratin is therefore embedded in a mucus matrix, but neither compact keratin masses nor cell corneous envelope were seen in external cells. It is not known whether specific matrix proteins are associated with mucus. There was no immunolocalization of the keratin-associated proteins, filaggrin and loricrin, which suggests that the epidermis of this species lacks the matrix and cell corneus envelope proteins characteristic of that of amniotes. In conclusion, while specific keratins (AE2 positive) are probably produced in the uppermost layers as in amphibian epidermis, no interkeratin, matrix proteins seem to be present in external keratinocytes of the lungfish other than mucus.

Heat shock protein (hsp70) in brown trout epidermis after sudden temperature rise

So far, hsp70 has not yet been studied in the fish skin. This organ has a potential as an indicator organ and we investigate whether hsp70 could be used as a biomarker. In this study, we examined whether and how the epidermis reacts to a temperature rise. Brown trout, Salmo trutta fario, were exposed to higher temperature for 2 h and were allowed to recover subsequently. Samples were taken from controls, after heat shock, as well as after 24 and 48 h of recovery. The occurrence of hsp70 in trout skin was examined by Western blot. The amount of hsp70 was higher after 2-h heat shock and was rising until the end of the experiment. Immunocytochemically, hsp70 was detected in epidermal filament cells. After 2-h heat shock, hsp70 was predominantly located in the nucleus. At this time, light and electron microscopy revealed several features known to occur under a variety of stressors. Ultrastructurally, the appearance of compact filament aggregates in pavement cells was remarkable. After 24 h of recovery, filament compaction was lacking and after 48 h aspects of regeneration were obvious. However, an increased amount of apoptotic cells in the epidermis was prominent at this time only.

Localization of calmodulin positive immunoreactivity in the surface epidermis of the brown trout, Salmo trutta

Calmodulin is a Ca2+-dependent modulatory protein which is required in the general regulation of a large number of key processes of cellular metabolism. In the present study, the localization of calmodulin positive immunoreactivity in the epidermis of the brown trout, Salmo trutta was investigated using a specific mouse monoclonal antibody to calmodulin of IgG2 class. The immunoreaction was found only in the superficial epithelial cells that constitute the main histological site for the production of calmodulin positive substances. Because of its distribution, this protein might have a physiological significance in the activation of the microvillar skeleton and in the control of the permeability of the skin epithelium.

Localization of cytokeratins in tissues of the rainbow trout: fundamental differences in expression pattern between fish and higher vertebrates

Using a panel of antibodies against different cytokeratins in immunofluorescence microscopy on frozen tissue sections and two-dimensional gel electrophoresis of cytoskeletal proteins from these tissues, we have studied the tissue distribution of cytokeratins in a fish, the rainbow trout Salmo gairdneri. We have distinguished at least 14 different cytokeratin polypeptides in only a limited number of tissues, thus demonstrating the great complexity of the cytokeratin pattern in a fish species. The simplest cytokeratin pattern was that present in hepatocytes, comprising one type-II (L1) and two type-I (L2, L3) polypeptides that appear to be related to mammalian cytokeratins 8 and 18, respectively. Two or all three cytokeratins of this group were also identified in several other epithelial tissues, such as kidney. Epithelia associated with the digestive tract contained, in addition, other major tissue-specific cytokeratins, such as components D1-D3 (stomach, intestine and swim bladder) and B1 and B2 (biliary tract). With the exception of D1, all these polypeptides were also found in a cultured cell line (RTG-2). Epidermal keratinocytes contained D1 and six other major cytokeratins, termed E1-E6. The most complex cytokeratin pattern was that found in the gill epithelium. Surprisingly, antibodies specific for cytokeratins of the L1-L3 group also reacted with certain cell-sheet-forming tissues that are not considered typical epithelia and in higher vertebrates express primarily, if not exclusively, vimentin. Such tissues were (a) endothelia, including the pillar cells of the "gill filaments", (b) scale-associated cells, and (c) the ocular lens epithelium, and also several nonepithelial cell types, such as (d) fibroblasts and other mesenchymal cells, (e) chondrocytes, (f) certain vascular smooth muscle cells, and (g) astroglial cells of the optic nerve. The differences between the patterns of cytokeratin expression in this fish species and those of higher vertebrates are discussed. It is concluded that the diversity of cytokeratins has already been established in lower vertebrates such as fish, but that the tissue-expression pattern of certain cytokeratins has been restricted during vertebrate evolution. We discuss the value of antibodies specific for individual cytokeratin polypeptides as marker molecules indicating cell and tissue differentiation in fish histology, embryology, and pathology.

Effects of external osmolality, calcium and prolactin on growth and differentiation of the epidermal cells of the cichlid teleost Sarotherodon mossambicus

Osmolality and concentrations of divalent cations--calcium, and to a lesser extent magnesium--of the water are the main environmental factors that determine development and degree of mucification of the skin epithelium of Sarotherodon mossambicus. Epithelial thickness and number of mucocytes in fish exposed to low (freshwater level) concentrations of calcium and magnesium are directly related to the height of the osmotic gradient between water and blood plasma. No such relationship is found in fish exposed to a high (seawater level) concentration of calcium in the water, irrespective of the height of the osmotic gradient. The results strongly indicate that the effects of osmolality and divalent cations are indirect, and mediated by prolactin, since administration of ovine or fish prolactin stimulates growth and multiplication of the cells of the basal layer of the epidermis, and promotes the differentiation of the mucocytes.

A proposed function for microridges on epithelial cells

Microridges (MR), also called microvillar ridges, microplicae and cytoplasmic folds, have been noted on many epithelial surfaces. Several functions have been proposed for these structures. In the present study we examine the mechanical role that microridges may play in holding mucus to the lumenal surface to the esophagus in the trout Salmo gairdneri. Our findings support the hypothesis that the microridges help hold a protective coat of mucus to the epithelium. In addition, the complex curved or whorled arrangement of microridges appears to facilitate the spread of mucus away from goblet cells.