Surface carbohydrates of Eudiplozoon nipponicum pre- and post-fusion

Eudiplozoon nipponicum (Monogenea, Diplozoidae) and its adaptation to haematophagy as revealed by transcriptome and secretome profiling

Background

Ectoparasites from the family Diplozoidae (Platyhelminthes, Monogenea) belong to obligate haematophagous helminths of cyprinid fish. Current knowledge of these worms is for the most part limited to their morphological, phylogenetic, and population features. Information concerning the biochemical and molecular nature of physiological processes involved in host–parasite interaction, such as evasion of the immune system and its regulation, digestion of macromolecules, suppression of blood coagulation and inflammation, and effect on host tissue and physiology, is lacking. In this study, we report for the first time a comprehensive transcriptomic/secretome description of expressed genes and proteins secreted by the adult stage of Eudiplozoon nipponicum (Goto, 1891) Khotenovsky, 1985, an obligate sanguivorous monogenean which parasitises the gills of the common carp (Cyprinus carpio).

Results

RNA-seq raw reads (324,941 Roche 454 and 149,697,864 Illumina) were generated, de novo assembled, and filtered into 37,062 protein-coding transcripts. For 19,644 (53.0%) of them, we determined their sequential homologues. In silico functional analysis of E. nipponicum RNA-seq data revealed numerous transcripts, pathways, and GO terms responsible for immunomodulation (inhibitors of proteolytic enzymes, CD59-like proteins, fatty acid binding proteins), feeding (proteolytic enzymes cathepsins B, D, L1, and L3), and development (fructose 1,6-bisphosphatase, ferritin, and annexin). LC-MS/MS spectrometry analysis identified 721 proteins secreted by E. nipponicum with predominantly immunomodulatory and anti-inflammatory functions (peptidyl-prolyl cis-trans isomerase, homolog to SmKK7, tetraspanin) and ability to digest host macromolecules (cathepsins B, D, L1).

Conclusions

In this study, we integrated two high-throughput sequencing techniques, mass spectrometry analysis, and comprehensive bioinformatics approach in order to arrive at the first comprehensive description of monogenean transcriptome and secretome. Exploration of E. nipponicum transcriptome-related nucleotide sequences and translated and secreted proteins offer a better understanding of molecular biology and biochemistry of these, often neglected, organisms. It enabled us to report the essential physiological pathways and protein molecules involved in their interactions with the fish hosts.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07589-z.

Morphological and molecular studies of Eudiplozoon nipponicum (Goto, 1891) and Eudiplozoon kamegaii sp. n. (Monogenea; Diplozoidae)

Eudiplozoon nipponicum (Goto, 1891) Khotenovsky, 1985 (Monogenea: Diplozoidae), is known to parasitise Cyprinus carpio Linnaeus and species of Carassius. In this study, we conducted a taxonomic re-examination of E. nipponicum using genetic analysis and morphological comparisons from different host species from a single water system. rDNA nucleotide sequences of the internal transcription spacer 2 (ITS-2) region (645 bp) showed interspecific-level genetic differences among diplozoids from species of Carassius and C. carpio (p-distance: 3.1-4.0%) but no difference among those from different species of Carassius (0-0.4%) or between those from C. carpio collected in Asia and Europe (0-1.1%). Large variation was observed among 346 bp cytochrome c oxidase subunit I (COI) sequences (0.3-16.0 %); the topology of the phylogenetic tree showed no relationship to host genera or geographical regions of origin. Morphological observation showed that average clamp size of diplozoids from C. carpio was larger than those from Carassius spp. The number of folds on the hindbody was 10-25 for diplozoids from C. carpio and 12-19 for those from Carassius spp. Thus, our ITS-2 sequence and morphological comparison results indicate that diplozoids from C. carpio and species of Carassius belong to different species. The scientific name E. nipponicum should be applied to the species infected to the type host, Carassius sp. of Nakabo (2013) (Japanese name ginbuna). The diplozoid infecting C. carpio (Eurasian type) should be established as a new species: Eudiplozoon kamegaii sp. n. A neotype of E. nipponicum is designated in this report because the original E. nipponicum specimens are thought to have been lost.

Architecture of Paradiplozoon homoion: A diplozoid monogenean exhibiting highly-developed equipment for ectoparasitism

Diplozoidae (Monogenea) are blood-feeding freshwater fish gill ectoparasites with extraordinary body architecture and a unique sexual behaviour in which two larval worms fuse and transform into one functioning individual. In this study, we describe the body organisation of Paradiplozoon homoion adult stage using a combined approach of confocal laser scanning and electron microscopy, with emphasis on the forebody and hindbody. Special attention is given to structures involved in functional adaptation to ectoparasitism, i.e. host searching, attachment and feeding/metabolism. Our observations indicate clear adaptations for blood sucking, with a well-innervated mouth opening surrounded by sensory structures, prominent muscular buccal suckers and a pharynx. The buccal cavity surface is covered with numerous tegumentary digitations that increase the area in contact with host tissue and, subsequently, with its blood. The buccal suckers and the well-innervated haptor (with sclerotised clamps controlled by noticeable musculature) cooperate in attaching to and moving over the host. Putative gland cells accumulate in the region of apical circular structures, pharynx area and in the haptor middle region. Paired club-shaped sacs lying laterally to the pharynx might serve as secretory reservoirs. Furthermore, we were able to visualise the body wall musculature, including peripheral innervation, the distribution of uniciliated sensory structures essential for reception of external environmental information, and flame cells involved in excretion. Our results confirm in detail that P. homoion displays a range of sophisticated adaptations to an ectoparasitic life style, characteristic for diplozoid monogeneans.

The surface topography of Eudiplozoon nipponicum (Monogenea) developmental stages parasitizing carp (Cyprinus carpio L.)

Using scanning electron microscopy (SEM), the external morphology of all developmental stages (egg, oncomiracidium, diporpa, just fused juvenile and adult) of the parasite, Eudiplozoon nipponicum (Monogenea, Diplozoidae), from the gills of carp was studied. During the ontogeny, the tegument, tegumentary and sensory structures are subsequently developed. The tegument of free swimming oncomiracidium occurs in two types — the ciliated and non-ciliated with numerous uniciliated sensory structures. An attachment apparatus starts to form during the oncomiracidium stage. Further developmental stages are adapted to the environment of the gills. Tegumentary folds become more apparent later in development and assist to the parasite’s attachment. In connection with its reproductive strategy, the two morphological structures of diporpa (ventral sucker and dorsal papilla) appear to play important role. On the gills, two individuals need to meet and these structures mediate the fusion between two diporpae. The hindbody of adult parasite is highly modified for attachment. The haptor, folds and lobular extensions are most developed. The forebody is flexible and able to interact with host gill tissue via the mouth and associated mouth structures. The process of food intake of the parasite was discussed.

Three-dimensional imaging of monogenoidean sclerites by laser scanning confocal fluorescence microscopy

A nondestructive protocol for preparing specimens of Monogenoidea for both alpha-taxonomic studies and reconstruction of 3-dimensional structure is presented. Gomori's trichrome, a stain commonly used to prepare whole-mount specimens of monogenoids for taxonomic purposes, is used to provide fluorescence of genital spines, the copulatory organ, accessory piece, squamodisc, anchors, hooks, bars, and clamps under laser scanning confocal microscopy.

Characterisation of carbohydrate-binding sites in developmental stages of Myxobolus cerebralis

Glycans and lectins (carbohydrate-binding molecules) form a mutual recognition system, which enables parasitic organisms to attach themselves to the host cells and/or take part in the migration of their developmental stages into the target tissue. The aim of the present study was to identify and characterise the potential binding activity of glycoconjugates in different developmental stages of Myxobolus cerebralis, the causative agent of whirling disease in salmonids. The binding patterns of 13 biotinylated neoglycoconjugates were histochemically examined in thin-sections of infected rainbow trout (Oncorhynchus mykiss) and oligochaetes (Tubifex tubifex), as well as isolated waterborne triactinomyxon spores. A distinct structure-selective and developmental stage-regulated expression of certain classes of carbohydrate binding was observed. In triactinomyxon spores, the expression of carbohydrate binding activity for alpha-l-Fuc-BSA-biotin, alpha-d-GalNAc-BSA-biotin, beta-d-GlcNAc-BSA-biotin, Lac-BSA-biotin and ASF-biotin was up-regulated in the polar capsules; the shell valves showed no activity. In the gut of T. tubifex, polar capsules of the parasite showed strong positive reaction only for beta-d-GlcNAc-BSA-biotin. In fish cartilage, polar capsules were negative, but the spore shell valves showed a broad range of carbohydrate binding activity. No activity was detected for either alpha6- or alpha3-linked N-acetyl-d-neuraminic acid to galactose. An adhesion assay was performed on GlycoWell plates and Myxobolus spores were found to specifically adhere to matrices containing residues of lactose, fucose, galactose, N-acetyl-d-galactosamine and N-acetyl-d-glucosamine. This is the first study to identify lectin activity in a myxozoan parasite; activity that is likely to play a role in the recognition systems involved in host specificity and the processes of spore attachment and invasion.