Fasciola gigantica: Comparison of the tegumental ultrastructure in newly excysted metacercariae and in vitro penetrated juvenile flukes indicates intracellular sources of molecules with vaccinal and immunomodulatory potential

Fasciola gigantica: Ultrastructural localisation of neoblast recruitment in somatic tissues during growth and development in the hepatic parenchyma of experimentally infected mice

Application of 'omics' technology, and advances in in vitro methods for studying the growth of Fasciola hepatica, have highlighted the central role of migrating neoblasts in driving forward development and differentiation towards the adult-like form. Neoblast populations present molecular heterogeneity, morphological variation and changes associated with recruitment of these stem cells into their final tissue locations. However, terminal differentiation towards function, has received much less attention than has been the case for the free-living Platyhelminths. An actively replicating neoblast population, comprising cells with heterochromatic nuclei consistent with regulation of gene expression, has been identified in the parenchyma of juvenile Fasciola gigantica migrating in the liver of experimentally infected mice. In some of these cells, early cytoplasmic differentiation towards myocyte function was noted. Neoblasts have also been identified close to, and incorporated in, the subtegumental zone, the gastrodermis and the excretory ducts. In these locations, progressive morphological differentiation towards terminal function has been described. This includes the appearance of specific progenitors of type-1, type-2 and type-3 tegumental cells, the latter possibly contributing to tegumental spine development. 'Cryptic' surface molecular differentiation is postulated to account for recognition and 'docking' of migrating neoblasts with their final site for terminal differentiation.

Form and Function in the Digenea, with an Emphasis on Host-Parasite and Parasite-Bacteria Interactions

This review covers the general aspects of the anatomy and physiology of the major body systems in digenetic trematodes, with an emphasis on new knowledge of the area acquired since the publication of the second edition of this book in 2019. In addition to reporting on key recent advances in the morphology and physiology of tegumentary, sensory, neuromuscular, digestive, excretory, and reproductive systems, and their roles in host-parasite interactions, this edition includes a section discussing the known and putative roles of bacteria in digenean biology and physiology. Furthermore, a brief discussion of current trends in the development of novel treatment and control strategies based on a better understanding of the trematode body systems and associated bacteria is provided.

Role of Fasciola hepatica Small RNAs in the Interaction With the Mammalian Host

MicroRNAs (miRNAs) are important post-transcriptional regulators of gene expression being involved in many different biological processes and play a key role in developmental timing. Additionally, recent studies have shown that miRNAs released from parasites are capable of regulating the expression of host genes. In the present work, we studied the expression patterns of ncRNAs of various intra-mammalian life-cycle stages of the liver fluke, Fasciola hepatica, as well as those packaged into extracellular vesicles and shed by the adult fluke. The miRNA expression profile of the intra-mammalian stages shows important variations, despite a set of predominant miRNAs that are highly expressed across all stages. No substantial variations in miRNA expression between dormant and activated metacercariae were detected, suggesting that they might not be central players in regulating fluke gene expression during this crucial step in the invasion of the definitive host. We generated a curated pipeline for the prediction of putative target genes that reports only sites conserved between three different prediction approaches. This pipeline was tested against an iso-seq curated database of the 3’ UTR regions of F. hepatica genes to detect miRNA regulation networks within liver fluke. Several functions related to the host immune response or modulation were enriched among the targets of the most highly expressed parasite miRNAs, stressing that they might be key players during the establishment and maintenance of infection. Additionally, we detected fragments derived from the processing of tRNAs, in all developmental stages analyzed, and documented the presence of novel long tRNA fragments enriched in vesicles. We confirmed the presence of at least 5 putative vault RNAs (vtRNAs), that are expressed across different stages and enriched in vesicles. The presence of tRNA fragments and vtRNAs in vesicles raise the possibility that they could be involved in the host-parasite interaction.

Insights into Fasciola hepatica Juveniles: Crossing the Fasciolosis Rubicon

Unraveling the molecular interactions governing the first contact between parasite and host tissues is of paramount importance to the development of effective control strategies against parasites. In fasciolosis, a foodborne trematodiasis caused mainly by Fasciola hepatica, these early interactions occur between the juvenile worm and the host intestinal wall a few hours after ingestion of metacercariae, the infectious stage of the parasite. However, research on these early events is still scarce and the majority of studies have focused on the adult worm. Here, we review current knowledge on the biology and biochemistry of F. hepatica juveniles and their molecular relationships with the host tissues and identify the research needs and gaps to be covered in the future.

Diversity of extracellular vesicles from different developmental stages of Fasciola hepatica

The secretion of extracellular vesicles (EVs) in Fasciola hepatica adult worms was described by our group in 2012. Since then, EVs have been found in other helminths, thus providing a new paradigm for the complete understanding of host-parasite communication. However, information was lacking regarding the possible existence and role of EVs from other developmental stages of the parasite. In this study, we confirm the secretion of EVs by F. hepatica eggs and juvenile forms. EVs were isolated by size exclusion chromatography and characterised by nanoparticle tracking analysis and electron microscopy. We observed a large diversity in the morphologies of these EVs, suggesting specific functions for different subpopulations, as has been proposed in other model systems. The identification of these populations of morphologically diverse EVs will facilitate future studies aimed at biochemically characterising the different classes of these vesicles as a first step in deciphering their role in host-parasite communication.

Fasciola gigantica: Ultrastructural cytochemistry of the tegumental surface in newly- excysted metacercariae and in vitro-penetrated juvenile flukes informs a concept of parasite defence at the interface with the host

Cytochemical staining techniques were carried out en bloc with in vitro excysted and gut-penetrated Fasciola gigantica larvae in order to visualise the glycocalyx of the tegument, a structure which comprises the parasite component of the host-parasite interface, yet is incompletely preserved by conventional fixation and preparation techniques for electron microscopy. Positive reactivity with ruthenium red and periodic acid-thiocarbohydrazine-osmium (PATCO) techniques revealed that the glycocalyx is polyanionic and carbohydrate-rich throughout its depth. It comprises a trilaminate arrangement, with a thin dense zone and fibrillar layer closely apposed to the outer aspect of the apical plasma membrane, invested by an irregular thick mucopolysaccharide capsule. The latter, not recorded in adult flukes, may represent a specific adaptation to facilitate invasion in the face of host immunity, and may also protect the parasite surface from the action of host- and parasite-derived proteases. Early in the invasion of a naïve host, the glycocalyx may be partly responsible for triggering the responses of innate immunity, while later in infection, or when an anamnestic response is initiated in an immunocompetent host, the antibodies and activated lymphocytes of specific acquired immunity are invoked to interact with the parasite surface. The cytochemical properties of the glycocalyx, together with its potential for dynamic turnover due to exocytosis of the T0 tegumental secretory bodies, are likely to aid neutralisation of potentially damaging immune effectors and ensure their removal from the vicinity of the parasite by sloughing in complex with glycocalyx components.