Submitted to: Experimental and Applied Acarology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/16/2010
Publication Date: 2/1/2011
Citation: Olafson, P.U., Pruett Jr, J.H., Temeyer, K.B. 2011. Multiple transcripts encode glucose 6-phosphate dehydrogenase in the southern cattle tick, Rhipicephalus (Boophilus) microplus. Experimental and Applied Acarology. 53(2):147-165.
Interpretive Summary: The southern cattle tick is a blood-feeding arthropod that vectors the causative agents of bovine babesiosis and anaplasmosis. While the tick has been eradicated from the United States, it remains endemic to Mexico. In fact, Boophilus ticks are regularly introduced into South Texas on tick-infested cattle, horses, and ungulate wildlife that cross the Rio Grande from Mexico. Reports of ticks in Mexico developing resistance to coumaphos, the organophosphate acaricide used to treat imported cattle, has thus prompted the evaluation of alternative control technologies. A more comprehensive understanding of tick biology would be beneficial to the design of such strategies. Ticks ingest large volumes of blood resulting in the buildup of a byproduct (heme) that can cause oxidative damage to the organism. Here, we describe the characterization of four gene products that encode four unique variants of the glucose 6-phosphate dehydrogenase (G6PDH) enzyme. We have named these G6PDH-A, -B, -C, and -D. The G6PDH enzyme plays a pivotal role in the production of NAPDH, a cofactor that enables the activity of enzymes needed to protect cells from oxidative damage. Interestingly, while two of the products are detectable at various lifestages of the cattle tick and in salivary glands of fed and unfed cattle ticks of both sexes (G6PDH-A and -C), the other two (G6PDH-B and -D) are preferentially expressed in unfed and fed, adult females with a near absence of the products in fed and unfed adult males. We postulate that the tick produces these additional variants as a mechanism for coping with the oxidative stress that is induced upon ingesting large volumes of blood, and that the near absence of the products in adult males is a reflection of the reduced amount of time and smaller bloodmeal uptake of adult male ticks relative to adult female ticks. Although additional studies are required to define the specific roles of these variants, the female-specific products are of interest as possible targets of control since they may aid in successful feeding of the adult female tick.
Technical Abstract: Glucose 6-phosphate dehydrogenase (G6PDH) is an enzyme that plays a critical role in the production of NADPH. Here we describe the identification of four transcripts (G6PDH-A, -B, -C, and -D) that putatively encode the enzyme in the southern cattle tick, Rhipicephalus (Boophilus) microplus. The genomic DNA that is spliced to produce G6PDH-A and -B is 8,600-9,000 bases in length and comprises 12 exons. Comparison of the R. microplus G6PDH gene structure with those available from insects and mammals revealed that the tick gene is most like that of humans. Detection of the four transcripts was evaluated by quantitative RT-PCR using template from larvae, unfed adult females and males, salivary gland tissues from 2- to 3-day-fed adult females and males, and salivary gland tissue of 4- to 5-day-fed adult females. The G6PDH-A and -C transcripts were present in all templates, and both displayed induced expression in salivary gland tissue of fed, adult females but not matched males. The G6PDH-D transcript was detected only in unfed adults and in larvae, a stage in which it was most abundant relative to the other three transcripts. The G6PDH-B transcript, while detectable in all templates, was of low copy number suggesting it is a rare transcript. Induced expression of G6PDH-A and G6PDH-C in fed females may play a role in the tolerance of oxidative stress that is induced upon feeding, and the transcript abundance in fed females may be a function of bloodmeal volume and the time adult females spend on the host relative to adult males.