Abstract
Novel chemical controls are needed that selectively target human, animal, and plant parasitic nematodes with reduced adverse effects on the host or the environment. We hypothesize that the phosphodiesterase (PDE) enzyme family represents a potential target for development of novel nematicides and anthelmintics. To test this, we identified six PDE families present in the nematode phylum that are orthologous to six of the eleven human PDE families. We characterized the binding interactions of family-selective PDE inhibitors with human and C. elegans PDE4 in conjunction with molecular dynamics (MD) simulations to evaluate differences in binding interactions of these inhibitors within the PDE4 catalytic domain. We observed that roflumilast (human PDE4-selective inhibitor) and zardaverine (selective for human PDE3 and PDE4) were 159- and 77-fold less potent, respectively, in inhibiting C. elegans PDE4. The pan-specific PDE inhibitor isobutyl methyl xanthine (IBMX) had similar affinity for nematode and human PDE4. Of 32 residues within 5 Å of the ligand binding site, five revealed significant differences in non-bonded interaction energies (van der Waals and electrostatic interaction energies) that could account for the differential binding affinities of roflumilast and zardaverine. One site (Phe506 in the human PDE4D3 amino acid sequence corresponding to Tyr253 in C. elegans PDE4) is predicted to alter the binding conformation of roflumilast and zardaverine (but not IBMX) into a less energetically favorable state for the nematode enzyme. The pharmacological differences in sensitivity to PDE4 inhibitors in conjunction with differences in the amino acids comprising the inhibitor binding sites of human and C. elegans PDE4 catalytic domains together support the feasibility of designing the next generation of anthelmintics/nematicides that could selectively bind to nematode PDEs.
Highlights
The efficacy of—and resistance to—anthelmintic/nematicidal compounds for controlling parasitic nematodes is a growing concern in the fields of medicine, veterinary medicine, and agriculture
We found that C. elegans PDE4 has a Km for cAMP of 1.7 μM (n = 2), identical to that of human PDE4 measured in our lab, and very similar to published values for human PDE4D2 [56]
The observation of 82 conserved residues in all nematode PDE4 catalytic domain sequences at sites that are variable in non-nematode PDE4 orthologs may be indicative of nematode-specific structural differences in the catalytic domain that underlie the observed differences in binding affinity of PDE4-selective inhibitor compounds for human and C. elegans PDE4 (Table 2)
Summary
The efficacy of—and resistance to—anthelmintic/nematicidal compounds for controlling parasitic nematodes is a growing concern in the fields of medicine, veterinary medicine, and agriculture. Widespread administration of current drugs to treat human diseases in impoverished regions of the world may result in increasing levels of resistance in human parasitic nematodes [1]. Reduced livestock health and profitability as a result of anthelmintic resistance poses growing challenges to this industry [2]. In the United States, plant-parasitic nematodes account for an estimated 8–15% of all crop losses and cause approximately $100 billion in annual damages [3,4,5]; the historical use of organophosphates or carbamates have been greatly restricted due to their health and environmental hazards [6, 7]. An ideal anthelmintic/nematicide would disrupt the parasitic nematode lifecycle while leaving the host and other organisms unaffected
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