Assessing the in vivo toxicity of titanium dioxide nanoparticles in Schmidtea mediterranea: uptake pathways and (neuro)developmental outcomes

Titanium dioxide nanoparticles (TiO2-NPs) in aquatic environments, originating from urban run-off, product use and post-consumer degradation, interact with aquatic organisms through water and sediments. Thorough toxicity assessment requires comprehensive data across all ecosystem compartments especially the benthic zone, which is currently lacking. Moreover, a proper physicochemical characterization of the particles is needed before and during toxicity assessment. In the present work, we used the planarian Schmidtea mediterranea to investigate the effects of TiO2-NPs (5 mg/L and 50 mg/L). Planarians are benthic organisms that play an important role in the food chain as predators. Our study integrated particle characterization with toxicokinetic and toxicodynamic parameters and showed that the uptake of TiO2-NPs of 21 nm occurred through the epidermis and intestine. Epidermal irritation and mucus production occurred immediately after exposure, and TiO2-NPs induced stronger effects in regenerating organisms. More specifically, TiO2-NPs interfered with neuroregeneration, inducing behavioral effects. A delay in the formation of the anterior commissure between the two brain lobes after seven and nine days of exposure to 50 mg/L was observed, probably as a result of a decrease in stem cell proliferation. Our findings underscore the need to incorporate multiple exposure routes in toxicity screenings. Additionally, we highlight the vulnerability of developing organisms and recommend their inclusion in future risk assessment strategies.

A Spatiotemporal Characterisation of Redox Molecules in Planarians, with a Focus on the Role of Glutathione during Regeneration

A strict coordination between pro- and antioxidative molecules is needed for normal animal physiology, although their exact function and dynamics during regeneration and development remains largely unknown. Via in vivo imaging, we were able to locate and discriminate between reactive oxygen species (ROS) in real-time during different physiological stages of the highly regenerative planarian Schmidtea mediterranea. All ROS signals were strong enough to overcome the detected autofluorescence. Combined with an in situ characterisation and quantification of the transcription of several antioxidant genes, our data showed that the planarian gut and epidermis have a well-equipped redox system. Pharmacological inhibition or RNA interference of either side of the redox balance resulted in alterations in the regeneration process, characterised by decreased blastema sizes and delayed neurodevelopment, thereby affecting tails more than heads. Focusing on glutathione, a central component in the redox balance, we found that it is highly present in planarians and that a significant reduction in glutathione content led to regenerative failure with tissue lesions, characterised by underlying stem cell alterations. This exploratory study indicates that ROS and antioxidants are tightly intertwined and should be studied as a whole to fully comprehend the function of the redox balance in animal physiology.

Differential effect of silver nanoparticles on the microbiome of adult and developing planaria

Silver nanoparticles (AgNPs) are widely incorporated in household, consumer and medical products. Their unintentional release via wastewaters raises concerns on their environmental impact, particularly for aquatic organisms and their associated bacterial communities. It is known that the microbiome plays an important role in its host's health and physiology, e.g. by producing essential nutrients and providing protection against pathogens. A thorough understanding of the effects of AgNPs on bacterial communities and on their interactions with the host is crucial to fully assess AgNP toxicity on aquatic organisms. Our results indicate that the microbiome of the invertebrate Schmidtea mediterranea, a freshwater planarian, is affected by AgNP exposure at the tested 10 μg/ml concentration. Using targeted amplification of the bacterial 16S rRNA gene V3-V4 region, two independent experiments on the microbiomes of adult worms revealed a consistent decrease in Betaproteobacteriales after AgNP exposure, mainly attributed to a decrease in Curvibacter and Undibacterium. Although developing tissues and organisms are known to be more sensitive to toxic compounds, three independent experiments in regenerating worms showed a less pronounced effect of AgNP exposure on the microbiome, possibly because underlying bacterial community changes during development mask the AgNP induced effect. The presence of a polyvinyl-pyrrolidone (PVP) coating did not significantly alter the outcome of the experiments compared to those with uncoated particles. The observed variation between the different experiments underlines the highly variable nature of microbiomes and emphasises the need to repeat microbiome experiments, within and between physiological states of the animal.

Do you have the nerves to regenerate? The importance of neural signalling in the regeneration process

The importance of nerve-derived signalling for correct regeneration has been the topic of research for more than a hundred years, but we are just beginning to identify the underlying molecular pathways of this process. Within the current review, we attempt to provide an extensive overview of the neural influences during early and late phases of both vertebrate and invertebrate regeneration. In general, denervation impairs limb regeneration, but the presence of nerves is not essential for the regeneration of aneurogenic extremities. This observation led to the "neurotrophic factor(s) hypothesis", which states that certain trophic factors produced by the nerves are necessary for proper regeneration. Possible neuron-derived factors which regulate regeneration as well as the denervation-affected processes are discussed.