Vampire diaries: Protein evolution in bat venom

Bat out of hell

Image: CDC: Photo: Daniel Streicker

Animal venoms are proving a rich source of novel compounds which have the potential to be transformed into new drugs for human treatments. One such compound is captopril, which was found in the venom of the Brazilian pit viper and is currently used successfully to treat hypertension. Other snake venom components are being used to treat heart attacks and the venom of a cone snail is a candidate for relieving persistent pain.

Scorpions, snakes, jellyfish and spiders have all been targeted but one species that has been largely neglected is the vampire bat. Vampires are prevalent in popular culture but the venom of the blood-sucking vampire bat, which lives in Central and South America, is likely to have more positive effects than converting bitten victims into vampires. It could also be an important source of new drugs.

In order to be able to feed freely on the blood of their prey, vampire bats inject a complex mixture that includes an anticoagulant protein, with the thematic name of draculin. It prevents the blood from clotting so that the bats can feed for up to several hours if they wish. However, if the same animal is targeted regularly, it can build up a resistance to draculin.

A second series of compounds from the venom that dissolves clots are the salivary plasminogen activators referred to as DSPAs, which are structurally similar to a human tissue-type plasminogen activator and are used to treat strokes. But apart from these two proteins, there is little knowledge of the make up of the venom of the most common vampire bat, Desmodus rotundus.

This lack of knowledge has now been rectified by an international team of scientists. Senior reporter Bryan Fry and colleagues from the University of Queensland, with collaborators from the University of Porto, Portugal, the University of Karachi, Pakistan, and the National Autonomous University of Mexico, Cuernavaca, undertook a proteomic and transcriptomic approach to the venom of D. rotundus. Their key aims were to expand the range of known proteins and to probe the way that they have evolved.

Venom composition

The venom is injected into the prey from glands in the lower jaw of the bat which consist of two distinct lobes. RNA was each extracted from both lobes and subjected to classical transcriptome analysis followed by a detailed phylogenetic analysis to reveal the evolutionary history of each toxin type that was represented. The proteins were also extracted from the glands and separated by 2D gel electrophoresis before being identified by shotgun sequencing using mass spectrometry.

The first key observation was that the two lobes of the venom glands contained an identical set of proteins, countering the view by some venom researchers that the lobes had followed different evolutionary routes. That path would inevitably have led to variations in the protein compositions.

A whole range of proteins were found in the bat venom, far more than previously reported. They included different forms of DSPA, as well as a novel isoform, and a large sequence of draculin, far longer than previously known. This identified draculin as a lactotransferrin.

Many other proteins had not been reported in vampire bat venom before, such as Kunitz peptides, secretoglobulin, lacritin, kallikrein and pituitary adenylate cyclase activating polypeptide (PACAP). Their functions in other isoforms that are unrelated to venom include anticoagulation, secretion promotion, and vasodilation, properties which would help a bat to envenomate its prey.

Evolutionary pattern

The evolutionary examination showed that many of the proteins evolved under positive Darwinian selection, mutating to afford stronger properties. For instance, Kunitz domains I and II, desmallipins and plasminogen activator genes mutated to introduce amino acids that gave different biochemical and structural properties. However, many mutations in other proteins had no effect on the structure and function of the toxins.

The researcher suggested that these regular mutations were required to keep the toxins of the vampire bat fit for purpose, to outmanoeuvre changes in their prey, like the development of resistance to the venom. This is not an unknown phenomenon, having been associated with the venom of other mammals including snakes, lizards and certain cephalopods like octopus and cuttlefish.

The increased knowledge of the composition of vampire bat venom will increase understanding of its evolutionary history. In addition, it will help to understand the mode of action of the venom in prey at the molecular level, a process which opens up the possibility of new lead compounds for drug development.