Kinetic profiling an abundantly expressed planarian serotonergic GPCR identifies bromocriptine as a perdurant antagonist

Regulatory roles of dopamine D2 receptor in milk protein production and apoptosis in mammary epithelial cells

Dopamine D2 receptors (D2Rs) play crucial roles in regulating diverse physiological functions of the central nervous system and peripheral organs. D2Rs are also expressed in mammary glands. However, which cell types express D2Rs and whether they are involved in milk production remains unclear. The present findings revealed that D2Rs are expressed in the apical regions of the lateral membranes of mammary epithelial cells (MECs) in lactating mice. We also investigated the effects of the D2R agonist bromocriptine and/or antagonist domperidone on intracellular cAMP levels, milk protein production, and apoptosis in a lactation culture model of MECs that produce major milk components like lactating MECs in vivo. We found that bromocriptine decreased intracellular cAMP levels, whereas domperidone dose-dependently neutralized this effect. Bromocriptine also inhibited casein and lactoferrin production and suppressed activities of STAT5 and glucocorticoid receptors (GRs). Domperidone neutralized the inhibition of casein production as well as STAT5 and GR inactivation induced by bromocriptine. Furthermore, D2R activation by bromocriptine induced apoptosis and inactivated ERK, a signaling molecule responsible for promoting cell proliferation and survival. Domperidone attenuated ERK inactivation and apoptosis induced by bromocriptine. These findings suggest that D2Rs play regulatory roles in milk protein production and apoptosis in MECs.

Adult and regenerating planarians respond differentially to chronic drug exposure

There is a lack of data on the effects of chronic exposure to common drugs and stimulants on the developing nervous system. Freshwater planarians have emerged as a useful invertebrate model amenable to high-throughput behavioral phenotyping to assay chemical safety in adult and developing brains. Here, we leverage the unique strength of the system to test in parallel for effects on the adult and developing nervous system, by screening ten common drugs and stimulants (forskolin, clenbuterol, LRE-1, MDL-12,330A, adenosine, caffeine, histamine, mianserin, fluoxetine and sertraline) using the asexual freshwater planarian Dugesia japonica. The compounds were tested up to 100 μM nominal concentration for their effects on planarian morphology and behavior. Quantitative phenotypic assessments were performed on days 7 and 12 of exposure using an automated screening platform. The antidepressants sertraline and fluoxetine were the most potent to induce lethality, with significant lethality observed at 10 μM. All ten compounds caused sublethal morphological and/or behavioral effects, with the most effects, in terms of potency and breadth of endpoints affected, seen with mianserin and fluoxetine. Four of the compounds (forskolin, clenbuterol, mianserin, and fluoxetine) were developmentally selective, causing effects at lower concentrations in regenerating planarians. Of these, fluoxetine showed the greatest differences between the two developmental stages, inducing many behavioral endpoints in regenerating planarians but only a few in adult planarians. While some of these behavioral effects may be due to neuroefficacy, these results substantiate the need for better evaluation of the safety of these common drugs on the developing nervous system.

Identification of novel modulators of a schistosome transient receptor potential channel targeted by praziquantel

Given the worldwide burden of neglected tropical diseases, there is ongoing need to develop novel anthelmintic agents to strengthen the pipeline of drugs to combat these burdensome infections. Many diseases caused by parasitic flatworms are treated using the anthelmintic drug praziquantel (PZQ), employed for decades as the key clinical agent to treat schistosomiasis. PZQ activates a flatworm transient receptor potential (TRP) channel within the melastatin family (TRPMPZQ) to mediate sustained Ca2+ influx and worm paralysis. As a druggable target present in many parasitic flatworms, TRPMPZQ is a promising target for a target-based screening campaign with the goal of discovering novel regulators of this channel complex. Here, we have optimized methods to miniaturize a Ca2+-based reporter assay for Schistosoma mansoni TRPMPZQ (Sm.TRPMPZQ) activity enabling a high throughput screening (HTS) approach. This methodology will enable further HTS efforts against Sm.TRPMPZQ as well as other flatworm ion channels. A pilot screen of ~16,000 compounds yielded a novel activator of Sm.TRPMPZQ, and numerous potential blockers. The new activator of Sm.TRPMPZQ represented a distinct chemotype to PZQ, but is a known chemical entity previously identified by phenotypic screening. The fact that a compound prioritized from a phenotypic screening campaign is revealed to act, like PZQ, as an Sm.TRPMPZQ agonist underscores the validity of TRPMPZQ as a druggable target for antischistosomal ligands.

The Journey to Discovering a Flatworm Target of Praziquantel: A Long TRP

Infections caused by parasitic flatworms impose a considerable worldwide health burden. One of the most impactful is schistosomiasis, a disease caused by parasitic blood flukes. Treatment of schistosomiasis has relied on a single drug - praziquantel (PZQ) - for decades. The utility of PZQ as an essential medication is, however, intertwined with a stark gap in our knowledge as to how this drug works. No flatworm target has been identified that readily explains how PZQ paralyzes and damages schistosomes. Recently, a schistosome ion channel was discovered that is activated by PZQ and displays characteristics which mirror key features of PZQ action on schistosomes. Here, the journey to discovery of this target, properties of this ion channel, and remaining questions are reviewed.

Regenerative Adaptation to Electrochemical Perturbation in Planaria: A Molecular Analysis of Physiological Plasticity

Anatomical homeostasis results from dynamic interactions between gene expression, physiology, and the external environment. Owing to its complexity, this cellular and organism-level phenotypic plasticity is still poorly understood. We establish planarian regeneration as a model for acquired tolerance to environments that alter endogenous physiology. Exposure to barium chloride (BaCl₂) results in a rapid degeneration of anterior tissue in Dugesia japonica. Remarkably, continued exposure to fresh solution of BaCl₂ results in regeneration of heads that are insensitive to BaCl₂. RNA-seq revealed transcriptional changes in BaCl₂-adapted heads that suggests a model of adaptation to excitotoxicity. Loss-of-function experiments confirmed several predictions: blockage of chloride and calcium channels allowed heads to survive initial BaCl₂ exposure, inducing adaptation without prior exposure, whereas blockade of TRPM channels reversed adaptation. Such highly adaptive plasticity may represent an attractive target for biomedical strategies in a wide range of applications beyond its immediate relevance to excitotoxicity preconditioning.

•

Exposure to BaCl₂ causes the heads of Dugesia japonica to degenerate

•

Prolonged exposure to BaCl₂ results in regeneration of a BaCl₂-insensitive head

•

Ion channel expression is altered in the head to compensate for excitotoxic stress

•

TRPMa is upregulated in BaCl₂-treated animals; blocking TRPM prevents adaptation

Neural control of body-plan axis in regenerating planaria

Control of axial polarity during regeneration is a crucial open question. We developed a quantitative model of regenerating planaria, which elucidates self-assembly mechanisms of morphogen gradients required for robust body-plan control. The computational model has been developed to predict the fraction of heteromorphoses expected in a population of regenerating planaria fragments subjected to different treatments, and for fragments originating from different regions along the anterior-posterior and medio-lateral axis. This allows for a direct comparison between computational and experimental regeneration outcomes. Vector transport of morphogens was identified as a fundamental requirement to account for virtually scale-free self-assembly of the morphogen gradients observed in planarian homeostasis and regeneration. The model correctly describes altered body-plans following many known experimental manipulations, and accurately predicts outcomes of novel cutting scenarios, which we tested. We show that the vector transport field coincides with the alignment of nerve axons distributed throughout the planarian tissue, and demonstrate that the head-tail axis is controlled by the net polarity of neurons in a regenerating fragment. This model provides a comprehensive framework for mechanistically understanding fundamental aspects of body-plan regulation, and sheds new light on the role of the nervous system in directing growth and form.

Planarian regeneration as a model of anatomical homeostasis: Recent progress in biophysical and computational approaches

Planarian behavior, physiology, and pattern control offer profound lessons for regenerative medicine, evolutionary biology, morphogenetic engineering, robotics, and unconventional computation. Despite recent advances in the molecular genetics of stem cell differentiation, this model organism's remarkable anatomical homeostasis provokes us with truly fundamental puzzles about the origin of large-scale shape and its relationship to the genome. In this review article, we first highlight several deep mysteries about planarian regeneration in the context of the current paradigm in this field. We then review recent progress in understanding of the physiological control of an endogenous, bioelectric pattern memory that guides regeneration, and how modulating this memory can permanently alter the flatworm's target morphology. Finally, we focus on computational approaches that complement reductive pathway analysis with synthetic, systems-level understanding of morphological decision-making. We analyze existing models of planarian pattern control and highlight recent successes and remaining knowledge gaps in this interdisciplinary frontier field.

Invertebrate serotonin receptors: a molecular perspective on classification and pharmacology

Invertebrate receptors for the neurotransmitter serotonin (5-HT) have been identified in numerous species from diverse phyla, including Arthropoda, Mollusca, Nematoda and Platyhelminthes. For many receptors, cloning and characterization in heterologous systems have contributed data on molecular structure and function across both closely and distantly related species. This article provides an overview of heterologously expressed receptors, and considers evolutionary relationships among them, classification based on these relationships and nomenclature that reflects classification. In addition, transduction pathways and pharmacological profiles are compared across receptor subtypes and species. Previous work has shown that transduction mechanisms are well conserved within receptor subtypes, but responses to drugs are complex. A few ligands display specificity for different receptors within a single species; however, none acts with high specificity in receptors across different species. Two non-selective vertebrate ligands, the agonist 5-methoxytryptamine and antagonist methiothepin, are active in most receptor subtypes in multiple species and hence bind very generally to invertebrate 5-HT receptors. Future challenges for the field include determining how pharmacological profiles are affected by differences in species and receptor subtype, and how function in heterologous receptors can be used to better understand 5-HT activity in intact organisms.

Structure-activity profiling of alkaloid natural product pharmacophores against a Schistosoma serotonin receptor

Serotonin (5-HT) is an important regulator of numerous aspects of flatworm biology, ranging from neuromuscular function to sexual maturation and egg laying. In the parasitic blood fluke Schistosoma mansoni, 5-HT targets several G-protein coupled receptors (GPCRs), one of which has been demonstrated to couple to cAMP and regulate parasite movement. This receptor, Sm.5HTR_L, has been successfully co-expressed in mammalian cells alongside a luminescent cAMP-biosensor, enabling pharmacological profiling for candidate anti-schistosomal drugs. Here, we have utilized this assay to perform structure-activity investigations of 143 compounds containing previously identified alkaloid natural product pharmacophores (tryptamines, aporphines and protoberberines) shown to regulate Sm.5HTR_L. These experiments mapped regions of the tryptamine pharmacophore amenable and intolerant to substitution, highlighting differences relative to orthologous mammalian 5-HT receptors. Potent Sm.5HTR_L antagonists were identified, and the efficacy of these compounds were evaluated against live adult parasites cultured ex vivo. Such structure-activity profiling, characterizing the effect of various modifications to these core ring systems on Sm.5HTR_L responses, provides greater understanding of pharmacophores selective for this target to aid future drug development efforts.

•

Various alkaloids were screened against a schistosome serotonin receptor, Sm.5HTR_L.

•

Compounds with a tryptamine core displayed agonist activity at Sm.5HTR_L.

•

Aporphine and protoberberine compounds displayed antagonist activity at Sm.5HTR_L.

•

Compound activity at Sm.5HTR_L is broadly mirrored by motility effects on adult worms.

Coalescing beneficial host and deleterious antiparasitic actions as an antischistosomal strategy

Conventional approaches for antiparasitic drug discovery center upon discovering selective agents that adversely impact parasites with minimal host side effects. Here, we show that agents with a broad polypharmacology, often considered ‘dirtier’ drugs, can have unique efficacy if they combine deleterious effects on the parasite with beneficial actions in the host. This principle is evidenced through a screen for drugs to treat schistosomiasis, a parasitic flatworm disease that impacts over 230 million people. A target-based screen of a Schistosoma serotoninergic G protein coupled receptor yielded the potent agonist, ergotamine, which disrupted worm movement. In vivo, ergotamine decreased mortality, parasite load and intestinal egg counts but also uniquely reduced organ pathology through engagement of host GPCRs that repressed hepatic stellate cell activation, inflammatory damage and fibrosis. The unique ability of ergotamine to engage both host and parasite GPCRs evidences a future strategy for anthelmintic drug design that coalesces deleterious antiparasitic activity with beneficial host effects.

More than 200 million people worldwide are infected with parasitic worms that cause the disease schistosomiasis. Most cases occur in sub-Saharan Africa. Long-term infections can damage organs, and children who are affected may suffer delayed growth and learning difficulties. Despite its significant health and economic impact, schistosomiasis is still considered a ‘neglected’ tropical disease. This means there has not been adequate investment into developing new treatments or cures.

A drug called praziquantel is currently the only treatment for schistosomiasis. However, the drug has unpleasant side effects, cannot cure all infected individuals, and there is a concern that worms may develop resistance to its effects. This means there is an urgent need to develop new therapies. One possible approach would be to develop drugs that interfere with the worm’s ability to move.

Chan et al. screened thousands of existing chemicals for interactions with a protein that is known to control how the worms move. A drug called ergotamine, which is currently used to treat migraines, strongly interacted with the protein. Treating infected mice with ergotamine eliminated the parasites and reduced the organ damage caused by the infection. Praziquantel also reduced the number of parasites in the mice but it did not prevent organ damage.

The results presented by Chan et al. show that a single drug can interact with targets in both the worm and the animals it infects. Searching for drugs that have this dual effect may help to develop more effective treatments for schistosomiasis and other diseases caused by parasites. Ergotamine itself is unlikely to be used to treat people for schistosomiasis because of the side effects produced when using it repeatedly. However, these findings will help researchers identify and develop safer drugs with similar benefits.

Unique pharmacological properties of serotoninergic G-protein coupled receptors from cestodes

Background

Cestodes are a diverse group of parasites, some of them being agents of neglected diseases. In cestodes, little is known about the functional properties of G protein coupled receptors (GPCRs) which have proved to be highly druggable targets in other organisms. Notably, serotoninergic G-protein coupled receptors (5-HT GPCRs) play major roles in key functions like movement, development and reproduction in parasites.

Methodology/Principal findings

Three 5-HT GPCRs from Echinococcus granulosus and Mesocestoides corti were cloned, sequenced, bioinformatically analyzed and functionally characterized. Multiple sequence alignment with other GPCRs showed the presence of seven transmembrane segments and conserved motifs but interesting differences were also observed. Phylogenetic analysis grouped these new sequences within the 5-HT7 clade of GPCRs. Molecular modeling showed a striking resemblance in the spatial localization of key residues with their mammalian counterparts. Expression analysis using available RNAseq data showed that both E. granulosus sequences are expressed in larval and adult stages. Localization studies performed in E. granulosus larvae with a fluorescent probe produced a punctiform pattern concentrated in suckers. E. granulosus and M. corti larvae showed an increase in motility in response to serotonin. Heterologous expression revealed elevated levels of cAMP production in response to 5-HT and two of the GPCRs showed extremely high sensitivity to 5-HT (picomolar range). While each of these GPCRs was activated by 5-HT, they exhibit distinct pharmacological properties (5-HT sensitivity, differential responsiveness to ligands).

Conclusions/Significance

These data provide the first functional report of GPCRs in parasitic cestodes. The serotoninergic GPCRs characterized here may represent novel druggable targets for antiparasitic intervention.

Cestode parasites are flatworms with the ability to parasitize almost every vertebrate species. Several of these parasites are etiological agents of neglected diseases prioritized by WHO, such as hydatid disease, or hydatidosis, a zoonosis caused by species of the genus Echinococcus that affects millions of people worldwide. Due to the scarcity of anthelmintic drugs available and the emergence of resistant parasites, the discovery of new anthelmintic drugs is mandatory. Neuromuscular function has been the target of commonly used drugs against parasitic diseases to impact movement, parasite development and reproduction. Here we describe three new proteins, some of them highly expressed in cestodes which could be relevant for motility. Using different approaches, the three proteins were identified as G protein coupled receptors for serotonin, an important neurotransmitter and a known modulator of cestode motility. These new receptors exhibit unique characteristics including a particular sensitivity to serotonin as well as a distinctive pharmacology, which will assist their targeting for chemotherapeutic intervention.

Reasons to Be Nervous about Flukicide Discovery

The majority of anthelmintics dysregulate neuromuscular function, a fact most prominent for drugs against nematode parasites. In contrast to the strong knowledge base for nematode neurobiology, resource and tool deficits have prevented similar advances in flatworm parasites since those driven by bioimaging, immunocytochemistry, and neuropeptide biochemistry 20-30 years ago. However, recent developments are encouraging a renaissance in liver fluke neurobiology that can now support flukicide discovery. Emerging data promote neuromuscular signalling components, and especially G protein-coupled receptors (GPCRs), as next-generation targets. Here, we summarise these data and expose some of the new opportunities to accelerate progress towards GPCR-targeted flukicides for Fasciola hepatica.

Utilizing the planarian voltage-gated ion channel transcriptome to resolve a role for a Ca2+ channel in neuromuscular function and regeneration

The robust regenerative capacity of planarian flatworms depends on the orchestration of signaling events from early wounding responses through the stem cell enacted differentiative outcomes that restore appropriate tissue types. Acute signaling events in excitable cells play an important role in determining regenerative polarity, rationalized by the discovery that sub-epidermal muscle cells express critical patterning genes known to control regenerative outcomes. These data imply a dual conductive (neuromuscular signaling) and instructive (anterior-posterior patterning) role for Ca²⁺ signaling in planarian regeneration. Here, to facilitate study of acute signaling events in the excitable cell niche, we provide a de novo transcriptome assembly from the planarian Dugesia japonica allowing characterization of the diverse ionotropic portfolio of this model organism. We demonstrate the utility of this resource by proceeding to characterize the individual role of each of the planarian voltage-operated Ca²⁺ channels during regeneration, and demonstrate that knockdown of a specific voltage operated Ca²⁺ channel (Ca_v1B) that impairs muscle function uniquely creates an environment permissive for anteriorization. Provision of the full transcriptomic dataset should facilitate further investigations of molecules within the planarian voltage-gated channel portfolio to explore the role of excitable cell physiology on regenerative outcomes. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Anthelmintics: From discovery to resistance II (San Diego, 2016)

The second scientific meeting in the series: “Anthelmintics: From Discovery to Resistance” was held in San Diego in February, 2016. The focus topics of the meeting, related to anthelmintic discovery and resistance, were novel technologies, bioinformatics, commercial interests, anthelmintic modes of action and anthelmintic resistance. Basic scientific, human and veterinary interests were addressed in oral and poster presentations. The delegates were from universities and industries in the US, Europe, Australia and New Zealand. The papers were a great representation of the field, and included the use of C. elegans for lead discovery, mechanisms of anthelmintic resistance, nematode neuropeptides, proteases, B. thuringiensis crystal protein, nicotinic receptors, emodepside, benzimidazoles, P-glycoproteins, natural products, microfluidic techniques and bioinformatics approaches. The NIH also presented NIAID-specific parasite genomic priorities and initiatives. From these papers we introduce below selected papers with a focus on anthelmintic drug screening and development.

•

Special Issue from the “Anthelmintics: From Discovery to Resistance II” meeting, San Diego, February 2016.

•

Meeting themes: drug discovery, modes of action and resistance.

•

Human and veterinary parasites covered.

•

Academic and industrial attendees.

Pharmacological profiling an abundantly expressed schistosome serotonergic GPCR identifies nuciferine as a potent antagonist

5-hydroxytryptamine (5-HT) is a key regulator of muscle contraction in parasitic flatworms. In Schistosoma mansoni, the myoexcitatory action of 5-HT is effected through activation of a serotonergic GPCR (Sm.5HTR_L), prioritizing pharmacological characterization of this target for anthelmintic drug discovery. Here, we have examined the effects of several aporphine alkaloids on the signaling activity of a heterologously expressed Sm.5HTR_L construct using a cAMP biosensor assay. Four structurally related natural products - nuciferine, D-glaucine, boldine and bulbocapnine - were demonstrated to block Sm.5HTR_L evoked cAMP generation with the potency of GPCR blockade correlating well with the ability of each drug to inhibit contractility of schistosomule larvae. Nuciferine was also effective at inhibiting both basal and 5-HT evoked motility of adult schistosomes. These data advance our understanding of structure-affinity relationships at Sm.5HTR_L, and demonstrate the effectiveness of Sm.5HTR_L antagonists as hypomotility-evoking drugs across different parasite life cycle stages.