Octopamine signaling in the metazoan pathogen Schistosoma mansoni: localization, small-molecule screening and opportunities for drug development

ABSTRACT Schistosomiasis is a tropical disease caused by a flatworm trematode parasite that infects over 200 million people worldwide. Treatment and control of the disease rely on just one drug, praziquantel. The possibility of drug resistance coupled with praziquantel's variable efficacy encourages the identification of new drugs and drug targets. Disruption of neuromuscular homeostasis in parasitic worms is a validated strategy for drug development. In schistosomes, however, much remains to be understood about the organization of the nervous system, its component neurotransmitters and potential for drug discovery. Using synapsin as a neuronal marker, we map the central and peripheral nervous systems in the Schistosoma mansoni adult and schistosomulum (post-infective larva). We discover the widespread presence of octopamine (OA), a tyrosine-derived and invertebrate-specific neurotransmitter involved in neuromuscular coordination. OA labeling facilitated the discovery of two pairs of ganglia in the brain of the adult schistosome, rather than the one pair thus far reported for this and other trematodes. In quantitative phenotypic assays, OA and the structurally related tyrosine-derived phenolamine and catecholamine neurotransmitters differentially modulated schistosomulum motility and length. Similarly, from a screen of 28 drug agonists and antagonists of tyrosine-derivative signaling, certain drugs that act on OA and dopamine receptors induced robust and sometimes complex concentration-dependent effects on schistosome motility and length; in some cases, these effects occurred at concentrations achievable in vivo. The present data advance our knowledge of the organization of the nervous system in this globally important pathogen and identify a number of drugs that interfere with tyrosine-derivative signaling, one or more of which might provide the basis for a new chemotherapeutic approach to treat schistosomiasis. This article has an associated First Person interview with the first author of the paper.

We discovered that the parasitic flatworm, Schistosoma, has a neurotransmitter, octopamine, which plays an important role in controlling its movement. We tested different neurotransmitters and modulators of these neurotransmitter signaling pathways on parasites. Octopamine and related neurotransmitters all caused a pronounced increase in movement. We also used laser microscopy to map the nervous system, localize octopamine throughout the nervous system and to determine that the flatworm brain is made up of four distinct lobes, rather than two lobes as was previously reported.
What are the potential implications of these results for your field of research?
Schistosoma is a globally important, yet understudied pathogen. There is only one drug used in its treatment, praziquantel, raising concerns for the emergence of resistance. It is important to seek out new drug targets and develop novel treatments for the disease. Other successful anthelmintics like ivermectin and levamisole demonstrate that interfering with worm motor control is a viable treatment that does not require killing the parasite, but allows for clearance from the host. The importance of octopamine in Schistosoma motor control, therefore, suggests strong potential for drug design to treat schistosomiasis.
What are the main advantages and drawbacks of the model system you have used as it relates to the disease you are investigating?
The disadvantage of this model system is that generating flatworms takes much time and effort: the life cycle requires a snail and rodent host to generate the flatworm and typically takes three months. However, the advantage of working with the flatworms is that it allows direct testing of compounds on the parasites and observation of schistosome neurobiology. These aspects have measurable outputs, including detecting motor control changes and neuronal localization.
What has surprised you the most while conducting your research?
What surprised me the most was the findings of the confocal immunolocalization studies. Namely, the four lobes in the brain and the abundance of octopamine throughout the nervous system. These findings suggest that octopamine is an important schistosome neurotransmitter and that the brain is, perhaps, more complex than previously thought.

"[…] it is primarily a disease of the developing world, […] which means, unfortunately, that the field is underfunded and understudied."
Describe what you think is the most significant challenge impacting your research at this time and how will this be addressed over the next 10 years?
The most significant challenge impacting my research is the attention given to the field of neglected tropical disease (NTD) research, including schistosomiasis. These diseases are 'neglected', in large part, because there is a lack of resources dedicated to NTD research. Despite schistosomes infecting hundreds of millions of people globally, it is primarily a disease of the developing world, as are other NTDs, which means, unfortunately, that the field is underfunded and understudied. Despite this neglect, advances in NTDs continue to be made, in large part due to funding from both governmental and non-profit organizations. For this manuscript we received funding from Canadian, American and British funding agencies, including The Company of Biologists, who helped make this research possible. In the next 10 years, NTD research must continue to receive financial support, to continue making advancements in understanding parasite biology and in drug discovery.
What changes do you think could improve the professional lives of early-career scientists?
Changes in academia that can help improve the professional lives of early-career scientists include introducing more structure into a scientist's training. Much of the learning process is informal and varies greatly depending on the lab environment and the trainee's supervisor/s. Standardizing the training process nationally, for both supervisor and trainee, in the form of courses, goal-setting and check-points could help to facilitate the training/learning process and ensure success.
What's next for you?
My plans are to complete my postdoc in the chemical and geneexpression profiling of the schistosome parasite at UC San Diego and to seek employment. I want to continue performing pre-clinical drug discovery to develop tools to treat globally important diseases. I am also interested in translational research for the betterment of human and animal health.