Presence of Calreticulin in Vector Fleas (Siphonaptera)

Calreticulin has been defined in the cat flea, Ctenophalides felis (Bouche), and oriental rat flea, Xenopsylla cheopis (Rothschild). Calreticulin, a major endoplasmic reticulum protein, was previously identified as a component of ixodid tick saliva. Using a riboprobe generated from the tick calreticulin complementary DNA (cDNA), we distinguished 2 transcripts for calreticulin in cat fleas by Northern blot analysis. Increased expression of calreticulin was not evident in fed versus unfed adult fleas. We were able to amplify a calreticulin flea product from fed female messenger RNA (mRNA) using primers designed from the tick calreticulin gene. One of these products hybridized to the tick riboprobe. Localization of specific antibody to cat flea tissues showed calreticulin in the midgut with no detection in the salivary glands. We also observed specific labeling of calreticulin with antibody in the ovaries of fed females. Several cat flea polypeptides appear to crossreact with anticalreticulin antibody in Western blots. We did not detect a calreticulin using antibody to the tick-secreted protein in cat flea salivary glands. This antibody did recognize a protein in the rat flea salivary glands. Our results show that fleas have calreticulin and, possibly, several isoforms. It appears that the salivary glands of the cat and oriental rat flea differ in detectable levels of calreticulin. The specific antibody labeling of the ovaries is interesting and remains to be understood. Calreticulin's appearance in the midgut suggests a possible source of calreticulin as a flea secretion. Further studies are in progress to complete the sequencing of the flea polymerase chain reaction (PCR) product to compare to tick-secreted calreticulin. Comparisons to other blood-feeding arthropods at the protein and gene level are also being done. We hope to define further the expression of calreticulin in fleas, and in general, blood-feeding arthropods, with respect to its role in feeding and pathogen transmission.