The construction of a cDNA expression library for the sheep scab mite Psoroptes ovis

The Construction of Full-Length cDNA Library for Otodectes cynotis

BACKGROUND

Otodectes cynotis (Hering, 1838) is the pathogen of otodectic mange distributed worldwide. The mite mainly infests carnivores and, sometimes, humans. However, due to the lack of cDNA library, research on its pathogenesis has been challenging.

METHODS

To solve this problem, the present study first sampled O. cynotis mites from an infested cat from Xi'an, China, for RNA extraction. Then, the full-length cDNA library was constructed using the SMART technique. Finally, positive clones > 500 bp and Hsc70-5 gene fragment specifically amplified from the cDNA library were sequenced and analyzed to verify the library's reliability.

RESULTS

Results showed that RNA extracted from 300 mites had good quality with a concentration of 149 ng/μl and OD260/OD280 of 1.99. The library satisfied the quality standard of a good library with a titer of 5.02 × 10⁵ PFU/ml and a combination rate of 97.61%. In addition, clone 4 and Hsc70-5 showed 98.38% and 99.72% identity with Ef1-α and Hsc70-5 gene sequences of O. cynotis in GenBank, respectively.

CONCLUSION

The cDNA library of O. cynotis constructed here was successful and reliable, creating the basis for research on RNA sequencing and functional genes of O. cynotis.

Improvement on the extraction method of RNA in mites and its quality test

To solve the long-existing difficult problems in extracting RNA and constructing a complementary DNA (cDNA) library for trace mites, we conducted a further comparative experiment among three RNA extraction methods (TRIzol method, Omega method, and Azanno method) based on our previous attempts at the construction of cDNA library of mites, with Psoroptes cuniculi still used as the experimental subject. By subsequently decreasing the number of mites, the least number of mites needed for RNA extraction of each method were found by criteria of completeness, concentration, and purity of the extracted RNA. Specific primers were designed according to the allergen Pso c1, Pso c2, and Actin gene sequences of Psoroptes to test the reliability of cDNA library. The results showed that Azanno method needed only 10 mites with sensitivity 204 times higher than previously used TRIzol method and 20 times higher than Omega method; clear RNA band was detected by agarose gel electrophoresis; and ultraviolet spectrophotometer determination showed that RNA concentration, 260/280, and 260/230 were in the range of 102 to 166 ng/μl, 1.83 to 1.99, and 1.49 to 1.72, respectively. Finally, specific primers detection showed that the amplified sequences had 98.33, 98.19, and 99.52% identities with those of P. cuniculi or Psoroptes ovis in GenBank, respectively, indicating that the cDNA library constructed using 10 mites was successful and it could meet the requirements for molecular biology research. Therefore, we concluded that Azanno method was more effective than TRIzol method and Omega method in RNA extraction and cDNA library construction of trace mites.

Constructing and detecting a cDNA library for mites

RNA extraction and construction of complementary DNA (cDNA) library for mites have been quite challenging due to difficulties in acquiring tiny living mites and breaking their hard chitin. The present study is to explore a better method to construct cDNA library for mites that will lay the foundation on transcriptome and molecular pathogenesis research. We selected Psoroptes cuniculi as an experimental subject and took the following steps to construct and verify cDNA library. First, we combined liquid nitrogen grinding with TRIzol for total RNA extraction. Then, switching mechanism at 5' end of the RNA transcript (SMART) technique was used to construct full-length cDNA library. To evaluate the quality of cDNA library, the library titer and recombination rate were calculated. The reliability of cDNA library was detected by sequencing and analyzing positive clones and genes amplified by specific primers. The results showed that the RNA concentration was 836 ng/μl and the absorbance ratio at 260/280 nm was 1.82. The library titer was 5.31 × 10(5) plaque-forming unit (PFU)/ml and the recombination rate was 98.21%, indicating that the library was of good quality. In the 33 expressed sequence tags (ESTs) of P. cuniculi, two clones of 1656 and 1658 bp were almost identical with only three variable sites detected, which had an identity of 99.63% with that of Psoroptes ovis, indicating that the cDNA library was reliable. Further detection by specific primers demonstrated that the 553-bp Pso c II gene sequences of P. cuniculi had an identity of 98.56% with those of P. ovis, confirming that the cDNA library was not only reliable but also feasible.

Scabies: New Future for a Neglected Disease

Scabies is a disease of global proportions in both human and animal populations, resulting from infestation of the skin with the "itch" mite Sarcoptes scabiei. Despite the availability of effective chemotherapy the intensely itching lesions engender significant morbidity primarily due to secondary sepsis and post-infective complications. Some patients experience an extreme form of the disease, crusted scabies, in which many hundreds of mites may infest the skin causin severe crusting and hyperkeratosis. Overcrowded living conditions and poverty have been identified as significant confounding factors in transmission of the mite in humans. Control is hindered by difficulties with diagnosis, the cost of treatment, evidence for emerging resistance and lack of effective vaccines. Historically research on scabies has been extremely limited because of the difficulty in obtaining sufficient quantities of the organism. Recent molecular approaches have enabled considerable advances in the study of population genetics and transmission dynamics of S. scabiei. However, the most exciting and promising development is the potential exploitation of newly available data from S. scabiei cDNA libraries and EST projects. Ultimately this knowledge may aid early identification of disease, novel forms of chemotherapy, vaccine development and new treatment possibilities for this important but neglected parasite.

Characterisation of aminopeptidase activity in scab mites (Psoroptes spp.)

Soluble and membrane-bound aminopeptidase activities were demonstrated in extracts of P. cuniculi (Delafond). Leucine aminopeptidase (LAP) activity in the soluble fraction of P. cuniculi extracts displayed substrate preference for amino acid derivatives with terminal leucine and methionine over those with acidic, basic or heterocyclic groups. P. cuniculi LAP was inhibited by leucinethiol (IC(50) = 1.4 +/- 0.4 nM), bestatin (IC(50) = 3.9 +/- 1.7 microM), Arphamenine A (IC(50) = 0.37 +/- 0.03 mM) the chelating agent 1,10-phenanthroline (IC(50) = 2.3 +/- 0.5 mM), Zn(2+), Cu(2+) Ni(2+), and Co(2+), and activated by Mn(2+) and Mg(2+). The LAP activity was visualised as a single major band after electrophoresis on native gels and eluted from a size exclusion column as a single major peak representing a molecular mass range of 85-116 kDa. Degenerate oligonucleotide primers were used to amplify short fragments of genomic DNA containing nucleotide sequence coding for the cation-binding motifs of the co-catalytic Zn(2+) binding domains of dizinc leucine aminopeptidases in both P. cuniculi and P.ovis (Hering). The major soluble aminopeptidase from these mites therefore displays most of the characteristics associated with typical cytosolic leucine aminopeptidases belonging to the M17 family of metalloproteinases.

Identification of an antigen from the sheep scab mite, Psoroptes ovis, homologous with house dust mite group I allergens

Infestation of sheep with the ectoparasitic mite Psoroptes ovis, results in a severe allergic dermatitis. Currently, little is known about the allergens/antigens that stimulate the allergic response. We have isolated an 836-bp cDNA from a P. ovis cDNA library which displays strong homology to cysteine proteases and, in particular, to the group I house dust mite allergens Der p 1, Der f 1 and Eur m 1. The cDNA was expressed in Escherchia coli, fused to a hexahistidine tag and the recombinant protein (Pso o 1) purified using a nickel-affinity column. The recombinant Pso o 1 was tested for recognition by immunoglobulin (Ig)G and IgE in serum from P. ovis naïve and P. ovis infested sheep. Using Western blots, both classes of antibody to Pso o 1 were detected in postinfestation serum. In enzyme-linked immunosorbent assays, a pronounced IgG-antibody response to Pso o 1 was detected in five of five sheep 3 weeks postinfestation. The IgE-antibody response to whole mite extract was poor in four of five animals. However, a marked IgE response occurred in the fifth animal, and IgE anti Pso o 1 was detected in the serum.

Expression and characterisation of a Psoroptes ovis glutathione S-transferase

The astigmatid mite Psoroptes ovis is the causative agent of sheep scab, a highly contagious parasitic disease of sheep. Infection causes severe allergic dermatitis, resulting in damage to the fleece and hide, loss of condition and occasional mortality. Interest in the P. ovis allergens led us to characterise a glutathione S-transferase (GST) which displays homology to GST allergens isolated from the house dust mite, Dermatophagoides pteronyssinus and the cockroach, Blatella germanica. A cDNA encoding a mu-class GST from P. ovis was expressed in Escherichia coli and the recombinant protein purified for biochemical analysis. SDS-PAGE analysis indicated that the purified product was homogeneous and had an apparent molecular weight of 30 kDa. The recombinant GST (rGST) is active towards the substrate 1-chloro-2,4-dinitrobenzene (CDNB), whereas 1,2-dichloro-4-nitrobenzene (DCNB) is a poor substrate. The recombinant protein was also tested for recognition by IgE and IgG antibodies in serum from P. ovis naïve and P. ovis infested sheep. Neither IgE nor IgG antibodies were detected to the rGST. Prausnitz--Küstner testing with rGST did not provoke a characteristic weal and flare response. Biopsies collected at the PK test sites were stained for eosinophils, neutrophils, mast cells and basophils. Neutrophil, mast cell and basophil counts were not significantly different to the controls. Eosinophil numbers were significantly higher than controls, but were not due to an IgE response.