Identification, Baculoviral Expression, and Biochemical Characterization of a Novel Cholinesterase of Amblyomma americanum (Acari: Ixodidae)

A cDNA encoding a novel cholinesterase (ChE, EC 3.1.1.8) from the larvae of Amblyomma americanum (Linnaeus) was identified, sequenced, and expressed in Sf21 insect cell culture using the baculoviral expression vector pBlueBac4.5/V5-His. The open reading frame (1746 nucleotides) of the cDNA encoded 581 amino acids beginning with the initiation codon. Identical cDNA sequences were amplified from the total RNA of adult tick synganglion and salivary gland, strongly suggesting expression in both tick synganglion and saliva. The recombinant enzyme (rAaChE1) was highly sensitive to eserine and BW284c51, relatively insensitive to tetraisopropyl pyrophosphoramide (iso-OMPA) and ethopropazine, and hydrolyzed butyrylthiocholine (BuTCh) 5.7 times as fast as acetylthiocholine (ATCh) at 120 µM, with calculated K_M values for acetylthiocholine (ATCh) and butyrylthiocholine of 6.39 µM and 14.18 µM, respectively. The recombinant enzyme was highly sensitive to inhibition by malaoxon, paraoxon, and coroxon in either substrate. Western blots using polyclonal rabbit antibody produced by immunization with a peptide specific for rAaChE1 exhibited reactivity in salivary and synganglial extract blots, indicating the presence of AaChE1 antigenic protein. Total cholinesterase activities of synganglial or salivary gland extracts from adult ticks exhibited biochemical properties very different from the expressed rAaACh1 enzyme, evidencing the substantial presence of additional cholinesterase activities in tick synganglion and saliva. The biological function of AaChE1 remains to be elucidated, but its presence in tick saliva is suggestive of functions in hydrolysis of cholinergic substrates present in the large blood mean and potential involvement in the modulation of host immune responses to tick feeding and introduced pathogens.

Periviscerokinin (Cap2b; CAPA) receptor silencing in females of Rhipicephalus microplus reduces survival, weight and reproductive output

BACKGROUND

The cattle fever tick, Rhipicephalus (Boophilus) microplus, is a vector of pathogens causative of babesiosis and anaplasmosis, both highly lethal bovine diseases that affect cattle worldwide. In Ecdysozoa, neuropeptides and their G-protein-coupled receptors play a critical integrative role in the regulation of all physiological processes. However, the physiological activity of many neuropeptides is still unknown in ticks. Periviscerokinins (CAP2b/PVKs) are neuropeptides associated with myotropic and diuretic activities in insects. These peptides have been identified only in a few tick species, such as Ixodes ricinus, Ixodes scapularis and R. microplus, and their cognate receptor only characterized for the last two.

METHODS

Expression of the periviscerokinin receptor (Rhimi-CAP2bR) was investigated throughout the developmental stages of R. microplus and silenced by RNA interference (RNAi) in the females. In a first experiment, three double-stranded (ds) RNAs, named ds680-805, ds956-1109 and ds1102-1200, respectively, were tested in vivo. All three caused phenotypic effects, but only the last one was chosen for subsequent experiments. Resulting RNAi phenotypic variables were compared to those of negative controls, both non-injected and dsRNA beta-lactamase-injected ticks, and to positive controls injected with beta-actin dsRNA. Rhimi-CAP2bR silencing was verified by quantitative reverse-transcriptase PCR in whole females and dissected tissues.

RESULTS

Rhimi-CAP2bR transcript expression was detected throughout all developmental stages. Rhimi-CAP2bR silencing was associated with increased female mortality, decreased weight of surviving females and of egg masses, a delayed egg incubation period and decreased egg hatching (P < 0.05).

CONCLUSIONS

CAP2b/PVKs appear to be associated with the regulation of female feeding, reproduction and survival. Since the Rhimi-CAP2bR loss of function was detrimental to females, the discovery of antagonistic molecules of the CAP2b/PVK signaling system should cause similar effects. Our results point to this signaling system as a promising target for tick control.

Pyrokinin receptor silencing in females of the southern cattle tick Rhipicephalus (Boophilus) microplus is associated with a reproductive fitness cost

Background

Rhipicephalusmicroplus is the vector of deadly cattle pathogens, especially Babesia spp., for which a recombinant vaccine is not available. Therefore, disease control depends on tick vector control. However, R.microplus populations worldwide have developed resistance to available acaricides, prompting the search for novel acaricide targets. G protein-coupled receptors (GPCRs) are involved in the regulation of many physiological processes and have been suggested as druggable targets for the control of arthropod vectors. Arthropod-specific signaling systems of small neuropeptides are being investigated for this purpose. The pyrokinin receptor (PKR) is a GPCR previously characterized in ticks. Myotropic activity of pyrokinins in feeding-related tissues of Rhipicephalussanguineus and Ixodesscapularis was recently reported.

Methods

The R.microplus pyrokinin receptor (Rhimi-PKR) was silenced through RNA interference (RNAi) in female ticks. To optimize RNAi, a dual-luciferase assay was applied to determine the silencing efficiency of two Rhimi-PKR double-stranded RNAs (dsRNA) prior to injecting dsRNA in ticks to be placed on cattle. Phenotypic variables of female ticks obtained at the endpoint of the RNAi experiment were compared to those of control female ticks (non-injected and beta-lactamase dsRNA-injected). Rhimi-PKR silencing was verified by quantitative reverse-transcriptase PCR in whole females and dissected tissues.

Results

The Rhimi-PKR transcript was expressed in all developmental stages. Rhimi-PKR silencing was confirmed in whole ticks 4 days after injection, and in the tick carcass, ovary and synganglion 6 days after injection. Rhimi-PKR silencing was associated with an increased mortality and decreased weight of both surviving females and egg masses (P < 0.05). Delays in repletion, pre-oviposition and incubation periods were observed (P < 0.05).

Conclusions

Rhimi-PKR silencing negatively affected female reproductive fitness. The PKR appears to be directly or indirectly associated with the regulation of female feeding and/or reproductive output in R.microplus. Antagonists of the pyrokinin signaling system could be explored for tick control.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05349-w.

Association of Salivary Cholinesterase With Arthropod Vectors of Disease

Acetylcholinesterase (AChE) was previously reported to be present in saliva of the southern cattle tick, Rhipicephalus (Boophilus) microplus (Canestrini), with proposed potential functions to 1) reduce acetylcholine toxicity during rapid engorgement, 2) modulate host immune responses, and 3) to influence pathogen transmission and establishment in the host. Potential modulation of host immune responses might include participation in salivary-assisted transmission and establishment of pathogens in the host as has been reported for a number of arthropod vector-borne diseases. If the hypothesis that tick salivary AChE may alter host immune responses is correct, we reasoned that similar cholinesterase activities might be present in saliva of additional arthropod vectors. Here, we report the presence of AChE-like activity in the saliva of southern cattle ticks, Rhipicephalus (Boophilus) microplus; the lone star tick, Amblyomma americanum (Linnaeus); Asian tiger mosquitoes, Aedes albopictus (Skuse); sand flies, Phlebotomus papatasi (Scopoli); and biting midges, Culicoides sonorensis Wirth and Jones. Salivary AChE-like activity was not detected for horn flies Haematobia irritans (L.), stable flies Stomoxys calcitrans (L.), and house flies Musca domestica L. Salivary cholinesterase (ChE) activities of arthropod vectors of disease-causing agents exhibited various Michaelis-Menten KM values that were each lower than the KM value of bovine serum AChE. A lower KM value is indicative of higher affinity for substrate and is consistent with a hypothesized role in localized depletion of host tissue acetylcholine potentially modulating host immune responses at the arthropod bite site that may favor ectoparasite blood-feeding and alter host defensive responses against pathogen transmission and establishment.

Sand Fly Colony Crash Tentatively Attributed to Nematode Infestation

Maintenance of laboratory colonies of insects and other arthropod pests offers significant research advantages. The availability, age, sex, housing conditions, nutrition, and relative uniformity over time of biological material for research facilitate comparison of results between experiments that would otherwise be difficult or impossible. A laboratory research colony of Phlebotomus papatasi (Scopoli), old world sand flies, was maintained with high-colony productivity for a number of years, but within a relatively short (4-6 mo) time period, colony productivity declined from over 10,000 flies per week to less than 100 per week. Mites and nematodes were both visible in the larval medium; however, the mites had been present throughout high productivity periods; therefore, it seemed reasonable to investigate the nematodes. PCR amplification of 18S rRNA yielded a clean cDNA sequence identified by BLAST search as Procephalobus sp. 1 WB-2008 (Rhabditida: Panagrolaimidae) small subunit ribosomal RNA gene, GenBank EU543179.1, with 475/477 nucleotide identities. Nematode samples were collected and identified as Tricephalobus steineri, (Andrássy, 1952) Rühm, 1956 (Rhabditida: Panagrolaimidae) based on morphological characteristics of the esophagus and the male copulatory apparatus. Mites (Tyrophagus putrescentiae [Acariformes: Acaridae]) may have played an additional predatory role in the loss of sand fly colony productivity. We hypothesized that the origin of the nematode infestation was rabbit dung from a local rabbitry used in preparation of the larval medium. Colony productivity was fully restored within 3 mo (two sand fly generational periods) by replacement of the rabbit dung from a clean source for use to prepare sand fly larval medium.

Baculoviral Expression of Presumptive OP-Resistance Mutations in BmAChE1 of Rhipicephalus (Boophilus) microplus (Ixodida: Ixodidae) and Biochemical Resistance to OP Inhibition

The southern cattle tick, Rhipicephalus (Boophilus) microplus (Canestrini), transmits bovine babesiosis and anaplasmosis, and is endemic to Mexico, Latin and South America. Rhipicephalus (B.) microplus infestations within the United States are a continuing threat to U.S. cattle producers. An importation barrier between Texas and Mexico keeps the ticks from re-entering the United States. All cattle imported into the United States are dipped in an organophosphate (OP) acaricide and hand inspected for presence of ticks. Tick resistance has developed to most available acaricides, including coumaphos, the OP used in the cattle dip vats. OP-resistance can result from one or more mutations in the gene encoding the enzyme, acetylcholinesterase (AChE), resulting in production of an altered AChE resistant to OP inhibition. Previous research reported a large number of BmAChE1 mutations associated with OP resistance. We report baculovirus expression of recombinant tick BmAChE1 (rBmAChE) enzymes containing a single resistance-associated mutation, to assess their contribution to OP inhibition resistance. Surprisingly, of the naturally occurring BmAChE1 resistance-associated mutations, only D188G resulted in markedly reduced sensitivity to OP-inhibition suggesting that OP-insensitivity in BmAChE1 may result from the D188G mutation, or may possibly result from multiple mutations, each contributing a small decrease in OP sensitivity. Furthermore, an OP-insensitivity mutation (G119S) found in mosquitoes was expressed in rBmAChE1, resulting in 500-2000-fold decreased sensitivity to OP inhibition. Recombinant BmAChE1 with the G119S mutation demonstrated the lack of any structural prohibition to broad and high-level OP-insensitivity, suggesting potential increases in tick OP-resistance that would threaten the U.S. importation barrier to ticks.

Acetylcholinesterase of Haematobia irritans (Diptera: Muscidae): baculovirus expression, biochemical properties, and organophosphate insensitivity of the G262A mutant

This study reports the baculovirus expression and biochemical characterization of recombinant acetylcholinesterase from Haematobia irritans (L.) (rHiAChE) and the effect of the previously described G262A mutation on enzyme activity and sensitivity to selected organophosphates. The rHiAChE was confirmed to be an insect AChE2-type enzyme with substrate preference for acetylthiocholine (Km 31.3 microM) over butyrylthiocholine (Km 63.4 microM) and inhibition at high substrate concentration. Enzyme activity was strongly inhibited by eserine (2.3 x 10(-10) M), BW284c51 (3.4 x 10(-8) M), malaoxon (3.6 x 10(-9) M), and paraoxon (1.8 x 10(-7) M), and was less sensitive to the butyrylcholinesterase inhibitors ethopropazine (1.1 x 10(-6) M) and iso-OMPA (4.1 x 10(-4) M). rHiAChE containing the G262A substitution exhibited decreased substrate affinity for both acetylthiocholine (Km 40.9 microM) and butyrylthiocholine (Km 96.3 microM), and exhibited eight-fold decreased sensitivity to paraoxon, and approximately 1.5- to 3-fold decreased sensitivity to other inhibitors. The biochemical kinetics are consistent with previously reported bioassay analysis, suggesting that the G262A mutation contributes to, but is not solely responsible for observed phenotypic resistance to diazinon or other organophosphates.

Activation of the human immunodeficiency virus long terminal repeat in THP-1 cells by a staphylococcal extracellular product

Staphylococcal strains can release a factor that strongly activates the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR) in THP-1 cells transfected with the HIV-1 LTR-driven luciferase reporter gene (THP-1 LTRluc). The factor is present in the overnight culture fluid and is readily released from the organisms into aqueous medium by vigorous mixing. Staphylococcal extracellular material is a complex mixture of polysaccharide and protein containing peptidoglycan and teichoic acid, released in part by cell wall turnover. The importance of the carbohydrate component is emphasized by concanavalin A (Con A) inhibition of staphylococcal product-induced LTR activation but not of activation by phorbol 12-myristate 13-acetate or tumor necrosis factor. The effect of Con A was decreased or abolished by sugars in the order methyl alpha-D-mannopyranoside > methyl alpha-D-glucopyranoside > mannose > glucose = fructose > N-acetylglucosamine. Wheat germ agglutinin was less inhibitory than Con A; in this instance N-acetylglucosamine decreased inhibition, whereas methyl alpha-D-mannopyranoside or methyl alpha-D-glucopyranoside did not. The induction of luciferase activity in THP-1 LTRluc by the staphylococcal extracellular product also was inhibited by fetal bovine and normal human serum. A comparison of 31 staphylococcal isolates (9 Staphylococcus aureus, 11 Staphylococcus epidermidis, 2 Staphylococcus haemolyticus, 4 Staphylococcus hominis, 2 Staphylococcus capitis, 2 Staphylococcus warneri, 1 Staphylococcus saprophyticus) revealed wide variation in LTR activating activity that did not correlate closely with slime production. Our findings, using induction of luciferase in THP-1 LTRluc as a model for upregulation of HIV infection, raise the possibility that staphylococci, as well as certain other microorganisms, release carbohydrate-containing exopolymers, which can activate the HIV-1 LTR, thus influencing progression of HIV infection.

Stimulation of the bactericidal activity of polymorphonuclear leukocytes by manganese

An antioxidant effect of manganese (Mn) complexes due to the scavenging of the superoxide anion (O2-.) or to the decomposition of hydrogen peroxide (H2O2) has been described. We report here that Mn also can exert a prooxidant effect under certain experimental conditions. Thus Mn2+ in phosphate buffer increased the bactericidal effect of phorbol myristate acetate-stimulated polymorphonuclear leukocytes (PMNs) on extracellular Escherichia coli. This effect was inhibited by azide, catalase, and a decrease in chloride concentration and was not observed when normal PMNs were replaced by those of patients with chronic granulomatous disease or myeloperoxidase (MPO) deficiency. Mn2+ could be replaced by Mn3+ or by superoxide dismutase (SOD). These findings suggest that Mn (or SOD), by increasing the conversion of O2-. to H2O2, can increase the activity of the MPO-H2O2-chloride antimicrobial system released by stimulated PMNs.

Effects of gamma-interferon on human neutrophils: protection from deterioration on storage

Human polymorphonuclear leukocytes (PMN) preincubated overnight with 100 U/mL gamma-interferon (IFN-gamma) had an increased metabolic response, as measured by iodination and/or superoxide production, to stimulation by tumor necrosis factor (TNF), granulocyte-macrophage colony-stimulating factor (GM-CSF), formylmethionyl-leucyl-phenylalanine (FMLP), opsonized zymosan, and lipopolysaccharide (LPS), as compared with cells comparably preincubated in the absence of IFN-gamma. The decline in the staphylocidal activity of the stored PMN was also prevented in part by IFN-gamma, as was the depressed adherence of PMN stimulated with phorbol myristate acetate (PMA), FMLP, TNF, GM-CSF, and LPS. This protective effect of IFN-gamma on PMN function was associated with the prolonged surface expression of the complement receptor three (CR3) alpha-chain (CD11b), CR3 beta-chain (CD18), FcRII (CD32), and FcRIII (CD16), and the appearance of surface FcRI (CD64). The polymerase chain reaction (PCR) was used to amplify neutrophil RNA-derived cDNA recognized by synthetic oliogonucleotides designed from published nucleotide sequences for specific proteins. Using this procedure, mRNA for gp91-phox, p67-phox, p47-phox, CD64, two forms of CD32, CD16, CD11b, CD18, and actin were found to be depressed after overnight storage of neutrophils, and this decrease in steady-state mRNA levels was in part or totally prevented by IFN-gamma. CD64 and gp91-phox mRNA were generally increased by IFN-gamma to a level greater than that of freshly isolated neutrophils. Northern analysis of CD64 and p47 phox mRNAs confirmed the findings with the PCR method. These findings suggest that storage of PMN in a functionally active state is favored by the presence of IFN-gamma.