Snake venom—it’s so precious, yet so hard to come by. Not only do scientists need it to make antivenom for treating snake bites, which kill some 100,000 people each year, but the substance also could be the source of potent molecules for making life-saving medicines. Yet until now, obtaining snake venom has involved “milking” a snake—massaging its venom gland to expel a cocktail of toxins in tiny amounts. The lack of a more accessible method to get larger amounts of these toxin mixtures means that only venom from a handful of poisonous snakes has been studied.
Now, scientists in a lab known for growing mini-organs from mouse or human stem cells have figured out how to make mini-venom glands in a dish that can pump the stuff out in bulk quantities (Cell 2020, DOI: 10.1016/j.cell.2019.11.038). The method could allow researchers to study the fascinating molecules that make up venom and to potentially make antivenom for any snake species more easily.
“The technique they’ve got here looks absolutely fantastic,” says Stephen Mackessy, a biologist at the University of Northern Colorado who studies snake venom and was not involved in the work.
Making mini-organs, or organoids, generally requires stem cells, but nobody knows what a reptilian stem cell looks like. So the researchers—students in molecular biologist Hans Clevers’s lab at the Hubrecht Institute in the Netherlands—obtained eggs of a Cape coral snake from a breeder, chopped up a bit of the embryo’s venom gland, and bathed it in the same mixture of chemicals that makes mammalian mini-organs grow. The only change they made to the protocol was lowering the temperature at which the tissue was incubated from 37 °C, the body temperature of mice and humans, to 28–32 °C. Snakes don’t have a set body temperature, but they run cooler than mammals, and higher temperatures killed the cells.
“Much to their surprise, it immediately started growing, and it made the structures that we know are mini-organs from what we had seen in humans before,” Clevers says. The venom that the organoids produced was at least as concentrated as naturally-produced venom, he adds.
The researchers compared the DNA and RNA sequences from individual cells in the organoids to that of venom glands from the snake and confirmed they were extremely similar. Next, they compared the toxin proteins from organoids to those from actual venom glands. Their characterization of about 20 different protein components in the venom demonstrated that they had similar functions as the gland-excreted proteins—for example, ratcheting up or tamping down how neurons fire, accelerating or interfering with blood clotting, and interfering with muscle cell activation. When the researchers examined the cellular organization of the organoids, they found that distinct cells within the structures made specific proteins, and that these cells were grouped together much as they are in venom glands.
They also found cells in the organoids that carry the same gene expression markers as human stem cells. “That was very surprising,” Clevers says. The finding suggests that stem cell biology is conserved over 300 million years of evolution. “It also means that this technology will probably work for all vertebrates—reptiles, amphibians, birds and mammals,” he says.
Clevers thinks that this method will make it easier to design antibodies against venom proteins using current techniques.
But Mackessy notes that researchers may still be a long way off from adapting this organoid technology to making antivenom because scaling up the system to produce the amounts needed may be laborious and costly. However, he adds that the system has enormous potential for understanding basic cellular processes like secretion. Cells in all organisms secrete different substances, but one fascinating thing about venom toxins is that the cells that produce them don’t succumb to their cytotoxic effects. Snakes thus make a unique model system for studying how cellular substances are produced and packaged.
The students who started the project had done so on a lark. They wanted to create organoids from the most iconic organ they could think of. “It was what we here call a ‘Friday afternoon experiment,’ “ Clevers says. “But when we started reading up on snake venom and observed the science, it turned out to be a very interesting project.”
The group has now grown venom-producing organoids from about 10 other snake species and a lizard, Clevers says. They are now working with a snake-wrangling collaborator to make a priority list of 50 species to create a venom-molecule biobank. “This will be a bioprospecting library with immense biological activity,” he says.