Submitted to: Gene
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 16, 2010
Publication Date: N/A
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 characterization 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 sequence encoding G6PDH-A, -B, and -D in R. microplus is approximately 13733 bases in length and comprises 14 exons (exons Id, Ia or Ib thru exon XII). While G6PDH-C is predominantly encoded by exons II thru XII of this genomic sequence, we have yet to identify the exon encoding the start of this transcript and it remains inconclusive whether it is encoded by the same or another gene. G6PDH-A and -C are expressed in larvae, unfed adult females and males, salivary gland tissues from fed adult females and males, and midguts of fed adult females. The G6PDH-B transcript is present at a much greater intensity in salivary gland tissue of fed, adult females than in the other life stages and tissues analyzed, and G6PDH-D was only detectable in larvae, unfed adult females, and salivary gland tissues from fed adult females. We postulate that the differential expression of G6PDH-B and -D may play a role in tolerance of oxidative stress that is induced upon feeding, and that the transcript abundance in fed females is a function of bloodmeal volume and the time adult females spend on the host relative to adult males. Comparison of the R. microplus G6PDH structure with those available from insects and mammals revealed that the tick gene is most like that of human and rat. The large size of this tick gene compared with that from arthropods, particularly of intron sequences, may provide insight into the expansive genome size previously reported for R. microplus (Ullman et al., Insect Mol. Biol. 2005; 14: 217-222).