Identification and functional characterization of a pyrokinin neuropeptide receptor in the Lyme disease vector,Ixodes scapularis

Pyrokinin-related peptides are pleiotropic factors that are defined by their conserved C-terminal sequence FXPRL-NH₂. The pyrokinin nomenclature derives from their originally identified myotropic actions and, as seen in some family members, a blocked amino terminus with pyroglutamate. The black-legged tick, Ixodes scapularis, is well known as a vector of Lyme disease and various other illnesses; however, in comparison to blood-feeding insects, knowledge on its physiology (along with other Ixodid ticks) is rather limited. In this study, we have isolated, examined the expression profile, and functionally deorphanized the pyrokinin peptide receptor in the medically important tick, I. scapularis. Phylogenetic analysis supports that the cloned receptor is indeed a bona fide member of the pyrokinin-related peptide receptor family. The tick pyrokinin receptor transcript expression is most abundant in the central nervous system (i.e. synganglion), but is also detected in trachea, female reproductive tissues, and in a pooled sample comprised of Malpighian (renal) tubules and the hindgut. Finally, functional characterization of the identified receptor confirmed it as a pyrokinin peptide receptor as it was activated equally by four endogenous pyrokinin-related peptides. The receptor was slightly promiscuous as it was also activated by a peptide sharing some structural similarity, namely the CAPA-periviserokinin (CAPA-PVK) peptide. Nonetheless, the I. scapularis pyrokinin receptor required a CAPA-PVK peptide concentration of well over three orders of magnitude to achieve a comparable receptor activation response, which indicates it is quite selective for its native pyrokinin peptide ligands. This study sets the stage for future research to examine the prospective tissue targets identified in order to resolve the physiological roles of this family of peptides in Ixodid ticks.