Exploring the role of glycoprotein hormone GPA2/GPB5 in the kiss of death bug, Rhodnius prolixus

The most complex molecules with hormonal function are glycoprotein hormones (GPHs). They consist of three hormones produced by the pituitary; two gonadotropins known as follitropin and lutropin, and a thyroid-stimulating hormone (thyrotropin). Both vertebrates and invertebrates contain the GPA2 and GPB5 genes which code for proteins linked to the GPH α and β subunits. These genes are regarded as the molecular ancestors of GPH subunits. The N-terminal signal peptide included in the sequences of all GPH subunits and those of GPA2 and GPB5 indicates that these molecules are all secreted as glycoproteins. Heterodimeric glycoprotein hormones regulate various physiological processes and behaviors including reproduction, energy metabolism, growth, and development. They are formed by noncovalent interactions between two cystine-knot glycoprotein subunits, an alpha subunit (GPA) and a beta subunit (GPB).

Rhodnius prolixus is a hemimetabolous blood-feeding insect (AKA the kissing bug) native to Central and South America. It has a five-stage nymphal life cycle (or instar), followed by a final moult to become an adult. These R. prolixus bugs can grow to be up to 34 mm long. Each bug consumes significant amounts of blood for energy demanding activities that have epidemiological relevance, such as reproduction, seeking food sources, and dispersion. Immediately after ingestion a blood meal triggers diuresis (excretion of urine) to remove excess water and salt. During diuresis, R. prolixus transmits the protozoan parasite Trypanosoma cruzi, the causative agent of Chagas disease, to a human host, resulting in cardiac irregularities, gastrointestinal malfunction, and often death.

In a new research paper published in Peptides, Canadian scientists: Areej Al-Dailami, Jimena Leyria, Ian Orchard, and led by Professor Angela Lange from the Department of Biology at the University of Toronto investigated the glycoprotein hormone GPA2/GPB5 and its receptor LGR1 in the fifth instar of R. prolixus and looked at their distribution. Additionally, the authors provided molecular and physiological evidence pointing to potential functions for this glycoprotein hormone-signaling pathway, particularly in connection with prolonged fasting and feeding-related events.

The GPA2/GPB5 subunits in R. prolixus have structural similarities to those seen in humans, Drosophila melanogaster, and Aedes aegypti, including the ten conserved cysteine residues crucial for the production of cystine-knots and disulfide bridges. R. prolixus GPA2 and GPB5 subunits share 10 conserved cysteine residues with the classic human GPA1, GPA2 and GPB5 subunits. This is in contrast to the classic GPB1-4 subunits, which are comprised of 12 cysteine residues that form an additional disulfide bridge yielding the seatbelt structure encircling the GPA1 subunit to reinforce the dimer structure. The central nervous system (CNS) is the primary tissue producing GPA2 and GPB5. Compared to insects that received control double stranded ampicilin resistance gene (dsARG) injections, unfed insects with their LGR1 transcript downregulated lose more weight and experience higher mortality rates. This shows that during extended periods of starvation, insects with knocked-down LGR1 may be less resistant to desiccation.

R. prolixus LGR1 transcript expression in Malpighian tubules (MTs) and the CNS changes post blood meal, indicating that this signaling pathway may also influence endocrinological events related to feeding. After the LGR1 transcript was knocked down in R. prolixus, 82% of them fed less than eight times their initial body weight. This significant reduction in the amount of LGR1 transcript in the CNS and MTs therefore led to a significant reduction in the overall size of the blood meal consumed. Interestingly, although having a much smaller blood meal, the LGR1 knocked down insects lost body weight at a similar pace to the control insects. This suggests that GPA2/GPB5 may tightly regulate and limit water loss while the insect is in a starvation phase, but it is possible that it is not directly engaged in rapid post-feeding diuresis. The fact that death rates increased when LGR1 was downregulated may indicate that this hormone also plays a role in subsequent feeding-related activities, such as the excretion of toxins from a blood meal.

This Canadian study demonstrated for the first time the potential physiological functions of GPA2/GPB5 in R. prolixus, where glycoprotein hormone signaling would seem to be relevant during a protracted fast and encourage blood-gorging. It will be fascinating to see how future research on LGR1 receptor expression, downstream signaling mechanisms, and impacts on the physiology of prospective target tissues develops. By pinpointing targets to change feeding, diuresis, and the spread of this disease vector, the glycoprotein hormone system and mechanisms of action can be used to control these vectors.