Structure of the trehalose-6-phosphate phosphatase from Brugia malayi reveals key design principles for anthelmintic drugs.

Parasitic nematodes are responsible for devastating illnesses that plague many of the world's poorest populations indigenous to the tropical areas of developing nations. Among these diseases is lymphatic filariasis, a major cause of permanent and long-term disability. Proteins essential to nematodes that do not have mammalian counterparts represent targets for therapeutic inhibitor discovery. One promising target is trehalose-6-phosphate phosphatase (T6PP) from Brugia malayi. In the model nematode Caenorhabditis elegans, T6PP is essential for survival due to the toxic effect(s) of the accumulation of trehalose 6-phosphate. T6PP has also been shown to be essential in Mycobacterium tuberculosis. We determined the X-ray crystal structure of T6PP from B. malayi. The protein structure revealed a stabilizing N-terminal MIT-like domain and a catalytic C-terminal C2B-type HAD phosphatase fold. Structure-guided mutagenesis, combined with kinetic analyses using a designed competitive inhibitor, trehalose 6-sulfate, identified five residues important for binding and catalysis. This structure-function analysis along with computational mapping provided the basis for the proposed model of the T6PP-trehalose 6-phosphate complex. The model indicates a substrate-binding mode wherein shape complementarity and van der Waals interactions drive recognition. The mode of binding is in sharp contrast to the homolog sucrose-6-phosphate phosphatase where extensive hydrogen-bond interactions are made to the substrate. Together these results suggest that high-affinity inhibitors will be bi-dentate, taking advantage of substrate-like binding to the phosphoryl-binding pocket while simultaneously utilizing non-native binding to the trehalose pocket. The conservation of the key residues that enforce the shape of the substrate pocket in T6PP enzymes suggest that development of broad-range anthelmintic and antibacterial therapeutics employing this platform may be possible.

SPK-1, an SR protein kinase, inhibits programmed cell death in Caenorhabditis elegans.

To identify genes involved in protecting cells from programmed cell death in Caenorhabditis elegans, we performed a genetic screen to isolate mutations that cause an increase in the number of programmed cell deaths. We screened for suppressors of the cell-death defect caused by a partial loss-of-function mutation in ced-4, which encodes an Apaf-1 homolog that promotes programmed cell death by activating the caspase CED-3. We identified one extragenic ced-4 suppressor, which has a mutation in the gene spk-1. The spk-1 gene encodes a protein homologous to serine-arginine-rich (SR) protein kinases, which are thought to regulate splicing. Previous work suggests that ced-4 can be alternatively spliced and that the splice variants function oppositely, with the longer transcript (ced-4L) inhibiting programmed cell death. spk-1 might promote cell survival by increasing the amount of the protective ced-4L splice variant. We conclude that programmed cell death in C. elegans is regulated by an alternative splicing event controlled by the SR protein kinase SPK-1.

Six and Eya promote apoptosis through direct transcriptional activation of the proapoptotic BH3-only gene egl-1 in Caenorhabditis elegans.

The decision of a cell to undergo programmed cell death is tightly regulated during animal development and tissue homeostasis. Here, we show that the Caenorhabditis elegans Six family homeodomain protein C. elegans homeobox (CEH-34) and the Eyes absent ortholog EYA-1 promote the programmed cell death of a specific pharyngeal neuron, the sister of the M4 motor neuron. Loss of either ceh-34 or eya-1 function causes survival of the M4 sister cell, which normally undergoes programmed cell death. CEH-34 physically interacts with the conserved EYA domain of EYA-1 in vitro. We identify an egl-1 5' cis-regulatory element that controls the programmed cell death of the M4 sister cell and show that CEH-34 binds directly to this site. Expression of the proapoptotic gene egl-1 in the M4 sister cell requires ceh-34 and eya-1 function. We conclude that an evolutionarily conserved complex that includes CEH-34 and EYA-1 directly activates egl-1 expression through a 5' cis-regulatory element to promote the programmed cell death of the M4 sister cell. We suggest that the regulation of apoptosis by Six and Eya family members is conserved in mammals and involved in human diseases caused by mutations in Six and Eya.

Caenorhabditis elegans genes required for the engulfment of apoptotic corpses function in the cytotoxic cell deaths induced by mutations in lin-24 and lin-33.

Two types of cell death have been studied extensively in Caenorhabditis elegans, programmed cell death and necrosis. We describe a novel type of cell death that occurs in animals containing mutations in either of two genes, lin-24 and lin-33. Gain-of-function mutations in lin-24 and lin-33 cause the inappropriate deaths of many of the Pn.p hypodermal blast cells and prevent the surviving Pn.p cells from expressing their normal developmental fates. The abnormal Pn.p cells in lin-24 and lin-33 mutant animals are morphologically distinct from the dying cells characteristic of C. elegans programmed cell deaths and necrotic cell deaths. lin-24 encodes a protein with homology to bacterial toxins. lin-33 encodes a novel protein. The cytotoxicity caused by mutation of either gene requires the function of the other. An evolutionarily conserved set of genes required for the efficient engulfment and removal of both apoptotic and necrotic cell corpses is required for the full cell-killing effect of mutant lin-24 and lin-33 genes, suggesting that engulfment promotes these cytotoxic cell deaths.

A High-Quality De novo Genome Assembly from a Single Mosquito Using PacBio Sequencing.

A high-quality reference genome is a fundamental resource for functional genetics, comparative genomics, and population genomics, and is increasingly important for conservation biology. PacBio Single Molecule, Real-Time (SMRT) sequencing generates long reads with uniform coverage and high consensus accuracy, making it a powerful technology for de novo genome assembly. Improvements in throughput and concomitant reductions in cost have made PacBio an attractive core technology for many large genome initiatives, however, relatively high DNA input requirements (~5 µg for standard library protocol) have placed PacBio out of reach for many projects on small organisms that have lower DNA content, or on projects with limited input DNA for other reasons. Here we present a high-quality de novo genome assembly from a single Anopheles coluzzii mosquito. A modified SMRTbell library construction protocol without DNA shearing and size selection was used to generate a SMRTbell library from just 100 ng of starting genomic DNA. The sample was run on the Sequel System with chemistry 3.0 and software v6.0, generating, on average, 25 Gb of sequence per SMRT Cell with 20 h movies, followed by diploid de novo genome assembly with FALCON-Unzip. The resulting curated assembly had high contiguity (contig N50 3.5 Mb) and completeness (more than 98% of conserved genes were present and full-length). In addition, this single-insect assembly now places 667 (>90%) of formerly unplaced genes into their appropriate chromosomal contexts in the AgamP4 PEST reference. We were also able to resolve maternal and paternal haplotypes for over 1/3 of the genome. By sequencing and assembling material from a single diploid individual, only two haplotypes were present, simplifying the assembly process compared to samples from multiple pooled individuals. The method presented here can be applied to samples with starting DNA amounts as low as 100 ng per 1 Gb genome size. This new low-input approach puts PacBio-based assemblies in reach for small highly heterozygous organisms that comprise much of the diversity of life.

A target repurposing approach identifies N-myristoyltransferase as a new candidate drug target in filarial nematodes.

Myristoylation is a lipid modification involving the addition of a 14-carbon unsaturated fatty acid, myristic acid, to the N-terminal glycine of a subset of proteins, a modification that promotes their binding to cell membranes for varied biological functions. The process is catalyzed by myristoyl-CoA:protein N-myristoyltransferase (NMT), an enzyme which has been validated as a drug target in human cancers, and for infectious diseases caused by fungi, viruses and protozoan parasites. We purified Caenorhabditis elegans and Brugia malayi NMTs as active recombinant proteins and carried out kinetic analyses with their essential fatty acid donor, myristoyl-CoA and peptide substrates. Biochemical and structural analyses both revealed that the nematode enzymes are canonical NMTs, sharing a high degree of conservation with protozoan NMT enzymes. Inhibitory compounds that target NMT in protozoan species inhibited the nematode NMTs with IC50 values of 2.5-10 nM, and were active against B. malayi microfilariae and adult worms at 12.5 µM and 50 µM respectively, and C. elegans (25 µM) in culture. RNA interference and gene deletion in C. elegans further showed that NMT is essential for nematode viability. The effects observed are likely due to disruption of the function of several downstream target proteins. Potential substrates of NMT in B. malayi are predicted using bioinformatic analysis. Our genetic and chemical studies highlight the importance of myristoylation in the synthesis of functional proteins in nematodes and have shown for the first time that NMT is required for viability in parasitic nematodes. These results suggest that targeting NMT could be a valid approach for the development of chemotherapeutic agents against nematode diseases including filariasis.

Identification and characterization of the cofactor-independent phosphoglycerate mutases of Dirofilaria immitis and its Wolbachia endosymbiont.

Drug treatments for heartworm disease have not changed significantly in the last decade. Due to concerns about possible drug resistance and their lower efficacy against adult worms, there is a need for the development of new antifilarial drug therapies. The recent availability of genomic sequences for the related filarial parasite Brugia malayi and its Wolbachia endosymbiont enables genome-wide searching for new drug targets. Phosphoglycerate mutase (PGM) enzymes catalyze the critical isomerization of 3-phosphoglycerate (3-PG) and 2-phosphoglycerate (2-PG) in glycolytic and gluconeogenic metabolic pathways. There are two unrelated PGM enzymes, which are structurally distinct and possess different mechanisms of action. The mammalian enzyme requires 2,3-bisphosphoglycerate as a cofactor (dependent PGM or dPGM), while the other type of PGM does not (independent PGM or iPGM). In the present study, we have determined that Dirofilaria immitis and its Wolbachia endosymbiont both possess active iPGM. We describe the molecular characterization and catalytic properties of each enzyme. Our results will facilitate the discovery of selective inhibitors of these iPGMs as potentially novel drug treatments for heartworm disease.