Navigation within host tissues: Schistosoma mansoni and Trichobilharzia ocellata schistosomula respond to chemical gradients

Human growth factor-mediated signalling through lipid rafts regulates stem cell proliferation, development and survival of Schistosoma mansoni

Although the mechanisms by which schistosomes grow and develop in humans are poorly defined, their unique outer tegument layer, which interfaces with host blood, is considered vital to homeostasis of the parasite. Here, we investigated the importance of tegument lipid rafts to the biology of Schistosoma mansoni in the context of host-parasite interactions. We demonstrate the temporal clustering of lipid rafts in response to human epidermal growth factor (EGF) during early somule development, concomitant with the localization of anteriorly orientated EGF receptors (EGFRs) and insulin receptors, mapped using fluorescent EGF/insulin ligand. Methyl-β-cyclodextrin (MβCD)-mediated depletion of cholesterol from lipid rafts abrogated the EGFR/IR binding at the parasite surface and led to modulation of protein kinase C, extracellular signal-regulated kinase, p38 mitogen-activated protein kinase and Akt signalling pathways within the parasite. Furthermore, MβCD-mediated lipid raft disruption, and blockade of EGFRs using canertinib, profoundly reduced somule motility and survival, and attenuated stem cell proliferation and somule growth and development particularly to the fast-growing liver stage. These findings provide a novel paradigm for schistosome development and vitality in the host, driven through host-parasite interactions at the tegument, that might be exploitable for developing innovative therapeutic approaches to combat human schistosomiasis.

Making sense of sensory behaviors in vector-borne helminths

Migrations performed by helminths are impressive and diverse, and accumulating evidence shows that many are controlled by sophisticated sensory programs. The migrations of vector-borne helminths are particularly complex, requiring precise, stage-specific regulation. We review the contrasting states of knowledge on snail-borne schistosomes and mosquito-borne filarial nematodes. Rich observational data exist for the chemosensory behaviors of schistosomes, while the molecular sensory pathways in nematodes are well described. Recent investigations on the molecular mechanisms of sensation in schistosomes and filarial nematodes have revealed some features conserved within their respective phyla, but adaptations correlated with parasitism are pronounced. Technological developments are likely to extend these advances, and we forecast how these technologies may be applied.

Insights on the Gut-Mesentery-Lung Axis in Pulmonary Arterial Hypertension: A Poorly Investigated Crossroad

Pulmonary arterial hypertension (PAH) is a life-threatening disease characterized by the hyperproliferation of vascular cells, including smooth muscle and endothelial cells. Hyperproliferative cells eventually obstruct the lung vasculature, leading to irreversible lesions that collectively drive pulmonary pressure to life-threatening levels. Although the primary cause of PAH is not fully understood, several studies have indicated it results from chronic pulmonary inflammation, such as observed in response to pathogens' infection. Curiously, infection by the intravascular parasite Schistosoma mansoni recapitulates several aspects of the widespread pulmonary inflammation that leads to development of chronic PAH. Globally, >200 million people are currently infected by Schistosoma spp., with about 5% developing PAH (Sch-PAH) in response to the parasite egg-induced obliteration and remodeling of the lung vasculature. Before their settling into the lungs, Schistosoma eggs are released inside the mesenteric veins, where they either cross the intestinal wall and disturb the gut microbiome or migrate to other organs, including the lungs and liver, increasing pressure. Spontaneous or surgical liver bypass via collateral circulation alleviates the pressure in the portal system; however, it also allows the translocation of pathogens, toxins, and antigens into the lungs, ultimately causing PAH. This brief review provides an overview of the gut-mesentery-lung axis during PAH, with a particular focus on Sch-PAH, and attempts to delineate the mechanism by which pathogen translocation might contribute to the onset of chronic pulmonary vascular diseases.

Kinins and Their Receptors in Infectious Diseases

Kinins and their receptors have been implicated in a series of pathological alterations, representing attractive pharmacological targets for several diseases. The present review article aims to discuss the role of the kinin system in infectious diseases. Literature data provides compelling evidence about the participation of kinins in infections caused by diverse agents, including viral, bacterial, fungal, protozoan, and helminth-related ills. It is tempting to propose that modulation of kinin actions and production might be an adjuvant strategy for management of infection-related complications.

Dopaminergic antagonists inhibit bile chemotaxis of adult Clonorchis sinensis and its egg production

Human clonorchiasis, caused by Clonorchis sinensis, is endemic in East Asian countries. C. sinensis metacercariae excyst in the duodenum of mammalian hosts, migrate to the intrahepatic bile duct, and mature into adults in the milieu of bile. We have previously shown that newly excysted juvenile C. sinensis move chemotactically toward bile and bile acids. Here, the chemotactic behavior of adult C. sinensis (CsAd) toward bile and bile acids was investigated. CsAds moved toward 0.05-5% bile and were most attracted to 0.5% bile but moved away from 10% bile. Upon exposure to 1-10% bile, CsAds eventually stopped moving and then died quickly. Among bile acids, CsAds showed strong chemotaxis toward cholic acid (CA) and deoxycholic acid. On the contrary, CsAds repelled from lithocholic acid (LCA). Moreover, at higher than 10 mM LCA, CsAds became sluggish and eventually died. Dopamine D1 receptor antagonists (LE-300 and SKF-83566), D2/3 receptor antagonists (raclopride and its derivative CS-49612), and a dopamine re-uptake inhibitor inhibited CA-induced chemotaxis of CsAds almost completely. Clinically used antipsychotic drugs, namely chlorpromazine, haloperidol, and clozapine, are dopaminergic antagonists and are secreted into bile. They completely inhibited chemotaxis of CsAds toward CA. At the maximum doses used to treat patients, the three tested medicines only expelled 2-12% of CsAds from the experimentally infected rabbits, but reduced egg production by 64-79%. Thus, antipsychotic medicines with dopaminergic antagonism could be considered as new anthelmintic candidates for human C. sinensis infections.

Bile acids drive chemotaxis of Clonorchis sinensis juveniles to the bile duct

Clonorchiasis is a neglected tropical disease caused by Chinese liver fluke, Clonorchis sinensis infection. C. sinensis is a biological carcinogen causing cholangiocarcinoma in humans. In the mammalian host, C. sinensis newly excysted juveniles (CsNEJs) migrate from the duodenum into the bile duct. Bile drives the chemotactic behavior of CsNEJs. Little is known about which components of bile induce the chemotaxis. We designed a chemotaxis assay panel and measured the chemotactic behavior of CsNEJs in response to bile or bile acids. The CsNEJs migrated toward 0.1–1% bile but away from 5–10% bile. The CsNEJs showed strong chemoattraction to cholic acid ≥25 mM, but chemorepulsion to lithocholic acid ≥0.25 mM. To the CsNEJs, mixture of cholic acid and lithocholic acid was chemoattractive at a ratio greater than 25:1 but chemorepulsive at one smaller than that. Regarding migration in the mammalian hosts, high concentration of lithocholic acid in the gallbladder bile may repel CsNEJs from entering it. However, bile in the hepatic bile duct has a chemoattractive strength of cholic acid but a trace amount of lithocholic acid. Collectively, our results explain why the CsNEJs migrate principally to the hepatic bile ducts, bypassing the gallbladder.

We previously reported that Clonorchis sinensis newly excysted juveniles (CsNEJs) were chemotactically attracted to bile. However, there is still a paucity of information regarding which components and what concentration of bile induce the chemotactic behavior. Here, we show, among various bile components tested, two have opposing chemotactic influences on the CsNEJs; cholic acid was characterized as a chemoattractant and lithocholic acid as a chemorepellent. Chemorepulsive migration was dependent on the concentration of lithocholic acid. Notably, the ratio (25:1) of cholic acid and lithocholic acid plays a critical role in defining chemotactic preferences of CsNEJs. We suspect that this bile acid ratio directs the parasites in the mammalian host, i.e. the high concentration of lithocholic acid in the gallbladder bile may repel CsNEJs from entering it. Bile in the hepatic bile duct has a chemoattractive level of cholic acid but a trace amount of lithocholic acid. These findings may explain why the CsNEJs preferentially migrate to the common and hepatic bile ducts rather than the gallbladder. Deeper understanding on the parasitism of the liver fluke is likely to have major implications for the studies on other parasites.

The distribution of digenean metacercariae within bream (Abramis brama) gill apparatus: preferences, co-occurrence and interactions of parasites

Species-specific microenvironmental preferences and interactions between parasite species have been the focus of many ecological studies. Here, we studied the distribution of ectoparasite species within the gill apparatus of bream (Abramis brama) from Lake Lubāns (Latvia) to establish whether digenean metacercariae: (1) prefer specific patches within the gill apparatus; (2) co-occur in the same patches with monogeneans and copepods within a host individual; and (3) interact with monogeneans and copepods. We recorded all parasites on gill arches of the same host species and used null models to analyse co-occurrences of digenean metacercariae, monogeneans and copepods. Zero-inflated mixture models were used to define the preferred patches of parasites. We found that digenean metacercariae (Bucephalus polymorphus) prefer specific patches of the gill apparatus to encyst, and shared these preferences with monogeneans and copepods, but did not interact with them. We concluded that digenean metacercariae have a species-specific microenvironmental preference to encyst in the gill apparatus and their occurrence (even in high numbers) does not reduce the success of attachment of monogeneans and copepods in the same gill patches.

Purinergic signaling in schistosomal infection

Human schistosomiasis is a chronic inflammatory disease caused by blood fluke worms belonging to the genus Schistosoma. Health metrics indicate that the disease is related to an elevated number of years lost-to-disability and years lost-to-life. Schistosomiasis is an intravascular disease that is related to a Th1 and Th2 immune response polarization, and the degree of polarization affects the outcome of the disease. The purinergic system is composed of adenosine and nucleotides acting as key messenger molecules. Moreover, nucleotide-transforming enzymes and cell-surface purinergic receptors are obligatory partners of this purinergic signaling. In mammalian cells, purinergic signaling modulates innate immune responses and inflammation among other functions; conversely purinergic signaling may also be modulated by inflammatory mediators. Moreover, schistosomes also express some enzymes of the purinergic system, and it is possible that worms modulate host purinergic signaling. Current data obtained in murine models of schistosomiasis support the notion that the host purinergic system is altered by the disease. The dysfunction of adenosine receptors, metabotropic P2Y and ionotropic P2X₇ receptors, and NTPDases likely contributes to disease morbidity.

TRP channels in schistosomes

Praziquantel (PZQ) is effectively the only drug currently available for treatment and control of schistosomiasis, a disease affecting hundreds of millions of people worldwide. Many anthelmintics, likely including PZQ, target ion channels, membrane protein complexes essential for normal functioning of the neuromusculature and other tissues. Despite this fact, only a few classes of parasitic helminth ion channels have been assessed for their pharmacological properties or for their roles in parasite physiology. One such overlooked group of ion channels is the transient receptor potential (TRP) channel superfamily. TRP channels share a common core structure, but are widely diverse in their activation mechanisms and ion selectivity. They are critical to transducing sensory signals, responding to a wide range of external stimuli. They are also involved in other functions, such as regulating intracellular calcium and organellar ion homeostasis and trafficking. Here, we review current literature on parasitic helminth TRP channels, focusing on those in schistosomes. We discuss the likely roles of these channels in sensory and locomotor activity, including the possible significance of a class of TRP channels (TRPV) that is absent in schistosomes. We also focus on evidence indicating that at least one schistosome TRP channel (SmTRPA) has atypical, TRPV1-like pharmacological sensitivities that could potentially be exploited for future therapeutic targeting.

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We provide an overview of transient receptor potential (TRP) channels in schistosomes and other parasitic helminths.

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TRP channels are important for sensory signaling, ion homeostasis, organellar trafficking, and a host of other functions.

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Very little work has been done on TRP channels in parasitic helminths.

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TRPV channels, found throughout the Metazoa, appear not to be present in parasitic platyhelminths.

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TRP channels in schistosomes appear to have atypical pharmacology, perhaps an entrée for therapeutic targeting.

Protein kinase A signalling in Schistosoma mansoni cercariae and schistosomules

Cyclic AMP (cAMP)-dependent protein kinase/protein kinase A regulates multiple processes in eukaryotes by phosphorylating diverse cellular substrates, including metabolic and signalling enzymes, ion channels and transcription factors. Here we provide insight into protein kinase A signalling in cercariae and 24h in vitro cultured somules of the blood parasite, Schistosoma mansoni, which causes human intestinal schistosomiasis. Functional mapping of activated protein kinase A using anti-phospho protein kinase A antibodies and confocal laser scanning microscopy revealed activated protein kinase A in the central and peripheral nervous system, oral-tip sensory papillae, oesophagus and excretory system of intact cercariae. Cultured 24h somules, which biologically represent the skin-resident stage of the parasite, exhibited similar activation patterns in oesophageal and nerve tissues but also displayed striking activation at the tegument and activation in a region resembling the germinal 'stem' cell cluster. The adenylyl cyclase activator, forskolin, stimulated somule protein kinase A activation and produced a hyperkinesia phenotype. The biogenic amines, serotonin and dopamine known to be present in skin also induced protein kinase A activation in somules, whereas neuropeptide Y or [Leu(31),Pro(34)]-neuropeptide Y attenuated protein kinase A activation. However, neuropeptide Y did not block the forskolin-induced somule hyperkinesia. Bioinformatic investigation of potential protein associations revealed 193 medium confidence and 59 high confidence protein kinase A interacting partners in S. mansoni, many of which possess putative protein kinase A phosphorylation sites. These data provide valuable insight into the intricacies of protein kinase A signalling in S. mansoni and a framework for further physiological investigations into the roles of protein kinase A in schistosomes, particularly in the context of interactions between the parasite and the host.

Sensory Protein Kinase Signaling in Schistosoma mansoni Cercariae: Host Location and Invasion

Schistosoma mansoni cercariae display specific behavioral responses to abiotic/biotic stimuli enabling them to locate and infect the definitive human host. Here we report the effect of such stimulants on signaling pathways of cercariae in relation to host finding and invasion. Cercariae exposed to various light/temperature regimens displayed modulated protein kinase C (PKC), extracellular signal–regulated kinase (ERK) and p38 mitogen-activated protein kinase (p38 MAPK) activities, with distinct responses at 37°C and intense light/dark, when compared to 24°C under normal light. Kinase activities were localized to regions including the oral sensory papillae, acetabular ducts, tegument, acetabular glands, and nervous system. Furthermore, linoleic acid modulated PKC and ERK activities concurrent with the temporal release of acetabular gland components. Attenuation of PKC, ERK, and p38 MAPK activities significantly reduced gland component release, particularly in response to linoleic acid, demonstrating the importance of these signaling pathways to host penetration mechanisms.

Avian schistosomes and outbreaks of cercarial dermatitis

Cercarial dermatitis (swimmer's itch) is a condition caused by infective larvae (cercariae) of a species-rich group of mammalian and avian schistosomes. Over the last decade, it has been reported in areas that previously had few or no cases of dermatitis and is thus considered an emerging disease. It is obvious that avian schistosomes are responsible for the majority of reported dermatitis outbreaks around the world, and thus they are the primary focus of this review. Although they infect humans, they do not mature and usually die in the skin. Experimental infections of avian schistosomes in mice show that in previously exposed hosts, there is a strong skin immune reaction that kills the schistosome. However, penetration of larvae into naive mice can result in temporary migration from the skin. This is of particular interest because the worms are able to migrate to different organs, for example, the lungs in the case of visceral schistosomes and the central nervous system in the case of nasal schistosomes. The risk of such migration and accompanying disorders needs to be clarified for humans and animals of interest (e.g., dogs). Herein we compiled the most comprehensive review of the diversity, immunology, and epidemiology of avian schistosomes causing cercarial dermatitis.

Cercarial dermatitis, a neglected allergic disease

Cercarial dermatitis (swimmer's itch) is a common non-communicable water-borne disease. It is caused by penetration of the skin by larvae (cercariae) of schistosomatid flukes and develops as a maculopapular skin eruption after repeated contacts with the parasites. The number of outbreaks of the disease is increasing, and cercarial dermatitis can therefore be considered as an emerging problem. Swimmer's itch is mostly associated with larvae of the bird schistosomes of Trichobilharzia spp. Recent results have shown that mammalian infections (including man) manifest themselves as an allergic reaction which is able to trap and eliminate parasites in the skin. Studies on mammals experimentally infected by bird schistosome cercariae revealed, however, that during primary infection, parasites are able to escape from the skin to the lungs or central nervous system. This review covers basic information on detection of the infectious agents in the field and the clinical course of the disease, including other pathologies which may develop after infection by cercariae, and diagnosis of the disease.

Pathogenicity of Trichobilharzia spp. for Vertebrates

Bird schistosomes, besides being responsible for bird schistosomiasis, are known as causative agents of cercarial dermatitis. Cercarial dermatitis develops after repeated contact with cercariae, mainly of the genus Trichobilharzia, and was described as a type I, immediate hypersensitivity response, followed by a late phase reaction. The immune response is Th2 polarized. Primary infection leads to an inflammatory reaction that is insufficient to eliminate the schistosomes and schistosomula may continue its migration through the body of avian as well as mammalian hosts. However, reinfections of experimental mice revealed an immune reaction leading to destruction of the majority of schistosomula in the skin. Infection with the nasal schistosome Trichobilharzia regenti probably represents a higher health risk than infections with visceral schistosomes. After the skin penetration by the cercariae, parasites migrate via the peripheral nerves, spinal cord to the brain, and terminate their life cycle in the nasal mucosa of waterfowl where they lay eggs. T. regenti can also get over skin barrier and migrate to CNS of experimental mice. During heavy infections, neuroinfections of both birds and mammals lead to the development of a cellular immune response and axonal damage in the vicinity of the schistosomulum. Such infections are manifest by neuromotor disorders.

Chemosensory behaviors of parasites

Many multicellular parasites seek out hosts by following trails of host-emitted chemicals. Host seeking is a characteristic of endoparasites such as parasitic worms as well as of ectoparasites such as mosquitoes and ticks. For host location, many of these parasites use CO(2), a respiration byproduct, in combination with host-specific chemicals. Recent work has begun to elucidate the behavioral responses of parasites to CO(2) and other host chemicals, and to unravel the mechanisms of these responses. Here we discuss recent findings that have greatly advanced our understanding of the chemosensory behaviors of host-seeking parasites. We focus primarily on well-studied parasites such as nematodes and insects, but also note broadly relevant findings in a few less well studied parasites.

The schistosome excretory system: a key to regulation of metabolism, drug excretion and host interaction

There is a gulf between the enormous information content of the various genome projects and the understanding of the life of the parasite in the host. In vitro studies with adult Schistosoma mansoni using several substrates suggest that the excretory system contains both P-glycoproteins and multiresistance proteins. If both these families of protein were active in vivo, they could regulate parasite metabolism and be responsible for the excretion of drugs. During skin penetration, membrane-impermeant molecules of a wide range of molecular weights can be taken into the cercaria and schistosomulum through the nephridiopore, through the surface membrane or through both. We speculate that this uptake process might stimulate novel signalling pathways involved in growth and development.

Penetration of cercariae into the living human skin: Schistosoma mansoni vs. Trichobilharzia szidati

We studied the skin invasion of Schistosoma mansoni cercariae by placing gamma-irradiated and nonirradiated cercariae onto the living human skin and timing the behavior of 53 individuals. The skin invasion of S. mansoni was less efficient compared to the bird schistosome Trichobilharzia szidati. S. mansoni cercariae crept longer on the skin after attachment until they started penetration movements (median of 43 s [range of 15 s-6.58 min]; T. szidati, median of 8 s [range of 0-80 s]). Subsequent to this longer exploratory phase, 74% penetrated into wrinkles (T. szidati 84%), 22% into the smooth skin surface (T. szidati 0%), and 4% into hair follicles (T. szidati 16%). The S. mansoni cercariae needed, on average, 6.58 min (range of 1.57-13.13 min) for full entry, while T. szidati needed 4.0 min (range of 1.38-13.34 min); the fastest S. mansoni cercaria entered the skin within 94 s, while T. szidati entered within 83 s. Sixty percent of the S. mansoni cercariae had the tails still attached when the bodies disappeared in the skin whereas all T. szidati cercariae shed their tails within 0-105 s after the onset of penetration movements. The faster invasion of T. szidati may result from the more sophisticated host-finding mechanisms of this species. Regarding S. mansoni, cercarial dermatitis, as immediate skin response, developed after a sensitization period of 19 days.

Navigation within host tissues: cues for orientation of Diplostomum spathaceum (Trematoda) in fish towards veins, head and eye

Cercariae of Diplostomum spathaceum penetrate the skin of fish, and then migrate along blood vessels and tissues towards the head and the eye-lens. We studied their orientation behaviour in tail fins of guppies and in chemical concentration gradients within agar-filled choice chambers. In fins, they entered veins and orientated cranially, independent of the blood flow and living cells. In choice chambers, they were attracted by a small molecular fraction of fish serum, D-glucose (at 1, 10, and 1000 μm), D-mannose, D-maltotriose and Cl-ions, whereas D-glucosamine repelled them (even at 1·0 nm). Amino acids were not attractive, but arginine in tetrapeptides attracted at concentrations as low as 1 μm and melatonin at 0·4–4·3 pm. We suggest a preliminary model for the behaviour of diplostomula in fish fins and attracting (+) or repelling (−) host cues: (1) migration towards deeper skin layers and avoidance of skin surface, cues: Cl-ions (+ and −), glucose (+), glucosamine (−), light radiation (−); (2) orientation in cranial direction, cue: Cl-ions (+); (3) localization of blood vessels, cues: glucose (+), arginine-residues (+); (4) localization of the retina, cue: melatonin (+). A comparison with the navigation mechanisms of tissue-migrating schistosomules and hookworm larvae reveals an enormous diversity of strategies.

Molluscan and vertebrate immune responses to bird schistosomes

There is a growing understanding of risks posed by human contact with the cercariae of bird schistosomes. In general, there are no fundamental biological differences between human and bird schistosomes in terms of their interactions with snail and vertebrate hosts. The penetration of host surfaces is accompanied by the release of penetration gland products and the shedding of highly antigenic surface components (miracidial ciliated plates and cercarial glycocalyx) which trigger host immune reactions. New surface structures are formed during transformation: the tegument of mother sporocysts and the tegumental double membrane of schistosomula. These surfaces apparently serve as protection against the host immune response. Certain parasite excretory-secretory products may contribute to immunosuppression or, on the other hand, stimulation of host immune reactions. Discovery of new species and their life cycles, the characterization of host-parasite interactions (including at the molecular level), the determination of parasite pathogenicity towards the host, the development of tools for differential diagnosis and the application of protective measures are all topical research streams of the future. Regularly updated information on bird schistosomes and cercarial dermatitis can be found at http://www.schistosomes.cz (web pages of Schistosome Group Prague).

Behavioural strategies used by the hookworms Necator americanus and Ancylostoma duodenale to find, recognize and invade the human host

The infective third-stage larvae of the hookworms Necator americanus and Ancylostoma duodenale infect their human hosts by active skin invasion, but A. duodenale is in addition capable of oral infection. The behaviour of the larvae when crawling on surfaces has already been described. Here we analyse in various in vitro systems the other behavioural invasion phases: activation, penetration, and orientation within the host. The larvae normally remained in a motionless, energy-saving, resting posture. An activation to sinusoidal locomotion was stimulated in both species by similar cues such as touch, vibration, water currents, heat, light, and chemicals. Human breath in addition stimulated searching and waving ("nictating") behaviour, which facilitates a change-over to the host. Activating cues in air streams were warmth and moisture; CO2 activated only in combination with warmth and/or moisture. Penetration behaviour in both species was stimulated by warmth and skin extracts. The stimulating components of skin extracts were fatty acids, but their stimulating characteristics differed from those inducing schistosome cercarial skin penetration. After penetration into agar substrates, both species showed thermo-orientation, but only A. duodenale followed gradients of serum. The directing serum cues were not amino acids and glucose (the supposed cues for schistosome blood vessel localization), but Ringer's solution attracted the larvae. The host-finding and host-invasion behaviour of both hookworm species is well adapted to the invasion of the human skin, and there seems to be no particular adaptation of A. duodenale behaviour to the oral infection mode. Hookworm host-finding behaviour is not as complex as that of schistosome cercariae but seems well adapted to the ecological conditions in the transmission sites.