Happy St. Patrick’s Day to all who are celebrating!

St. Patrick is the primary patron saint of Ireland who lived during the fifth century (1). His existence and missionary work in Ireland are celebrated on the alleged day of his death, March 17^th. Legend has it St. Patrick was once attacked by a group of snakes, and in response he dramatically banished all snakes from Ireland (see an artist’s rendering of this event below). Although research suggests that snakes never lived in Ireland in the first place, we decided to take this opportunity to learn more about snakes and the venom some of them produce, since the field of biochemistry has worked hard to find the pot of gold at the end of this venomous rainbow (2).

Before we dive into snake venom, let us start by noting that only 10-15% of snakes are known to be venomous (3). Despite their bad reputation, many species of snakes contribute positively to human life in many different ways (e.g. pest control) (4). Furthermore, biochemists and other scientists of the modern era have begun to revive traditional and ancient uses of snake venoms in treatments for human diseases. In fact, you may already know someone who has taken a medication inspired by molecules found in snake venom! Several different drugs used as blood thinners in heart attack patients are derived from snake venom (5 6 7 8).

As we hinted earlier, the use of snake venom in medicine and healing is not a new idea. In fact, snakes are symbols of healing in many cultures (e.g. ancient Greek and Roman depictions of two snakes winding around the staff of the god of medicine, Apollo) (9). There is evidence from thousands of years ago of venom being used to treat smallpox and leprosy. There’s also evidence snake venom was applied to wounds to stop bleeding (10 11). Snake venom has also been long-used in Ayurvedic medicine (12). (note – the development of antidotes and medicines in the ancient world was not without its victims. History indicates that prisoners were experimented on to develop some antivenom treatments (13)). Modern science is now trying to dissect how snake venom can be broken apart and developed into drugs to treat human disease.

According to the historical anecdotes above, snake venom can be used to prevent blood clotting, increase blood flow, induce clotting, and act as an antibacterial. How can snake venom have all these different and sometimes opposing functions?

The answer to this question likely lies in the hands of evolution. Snakes probably evolved over 100 million years ago (14), and have been locked in an evolutionary battle for survival with their prey ever since. The ability to make venom was a huge technological advance in this battle (one small slither for snake, one large leap for snake-kind). Without strong limbs, shredding teeth or sharp claws, snakes don’t have a lot of options for immobilizing their prey. For snakes without venom, the best options are to only eat things that were smaller than them or to use their long snake body to squeeze their prey until they lose consciousness from a lack of oxygen. The ability to produce venom to immobilize prey much larger than an individual snake suddenly opened up a huge range of prey options. Different species of snake took different slices of this metaphorical prey pie, resulting in a lot of different kinds of venom that work in lots of different ways (imagine what you might need to immobilize and then eat (with no hands) a bird compared to a rodent).

So how do snakes make their venom, and what is venom made of?

Venomous snakes have special venom glands in their heads that produce all the chemicals that make up their venom (not unlike our own salivary glands). These glands are surrounded by special muscles that squeeze the gland when a venomous snake bites, and this pushes the venom out of the gland and through their hollow fangs.

In terms of what chemicals and molecules those glands actually synthesize to create venom is a bit more complicated. As we discussed above, the hundreds of different species of venomous snake all make different types of venom that are highly specialized to the lifestyle and typical food choices of each snake (15 16). For most venomous snakes, the liquid secreted by the venom glands contains metal ions, amino acids, nucleic acids, carbohydrates and lipids, but the primary active components of venom are proteins and peptides (very short proteins) (17). The makeup and relative volume of these proteins and peptides is the main driver of the astounding variability in venom composition both between and within snake species.

What are some of the different ways snakes use venom to immobilize their prey, and how do they work?

There are four main ways venom can immobilize and incapacitate prey: the venom can shut down the nervous system and paralyze the animal; the venom can disrupt normal blood flow and render the animal unconscious; the venom can kill cells in different tissues, causing complete or partial paralysis; and the venom can cause extreme pain that incapacitates the animal. Venoms that affect the nervous system are called neurotoxic venoms; those that affect blood flow are called hemotoxic venoms; those that directly cause cell death are called cytotoxic venoms; and those that cause damage at the venom injection site are referred to as proteolytic venoms.

Lucky (shamrock emoji) for snakes, an individual’s venom is usually not restricted to a single venom type. A lot of venomous snakes make venom that can immobilize their prey using multiple methods of toxicity. Furthermore, the proteins and peptides that cause these different kinds of toxicity can also do so in many different ways.

In general, proteins that cause neurotoxicity disrupt the movement of ions (salts like potassium and sodium) in and out of nerve cells. Since nerve cells use these ion movements to talk to each other, the disruption of this process by venom proteins essentially cuts off communication from nerve cells to their nerve cell or muscle cell neighbors. This communication failure is what results in paralysis. Other proteins can also target certain ion channels and instead stimulate them to produce intense phantom pain at the bite site that can also in capacitate the animal (18).

Venom proteins that disrupt blood flow can do so in two different ways: they can either drastically decrease blood pressure so there is no longer a sufficient supply of oxygen to the brain, leading to a loss of consciousness; or they can increase blood pressure and promote the formation of blood clots that can lead to partial paralysis or a loss of consciousness. Both strategies hit the same cell signaling pathways that regulate blood clot formation and blood vessel shape. In the venoms that decrease blood pressure and prevent the formation of blood clots, blood vessels are stimulated to expand (increase in vessel size but no change in blood volume = decrease in blood pressure) and blood clots are unable to form and existing clots are dissolved, resulting in internal bleeding and loss of consciousness. In venoms that promote the formation of blood clots, the venom proteins instead activate platelets and help produce more of the protein webbing (fibrin) that helps hold it all together.

Venom proteins can cause cells to die in many different ways. Some venom proteins force cells to eat themselves, some stimulate cells to burst like a balloon, and others induce controlled cell death processes that do relatively little damage to their surroundings. These proteins do this in many different ways, but in general they increase stress signals inside cells (some venom proteins and peptides can actually get inside cells to wreak this havoc) and cause these cells to go into panic mode and die.

Finally, all venoms contain proteins and peptides that destroy proteins they encounter in their prey. This damage promotes pain and blocks the activity of the immune system as it tries to triage the bite area, but these proteins can also play an important role in digestion. Essentially, this type of venom starts melting the prey from the inside out, making it easier to break down once it is in the snake’s digestive tract.

But as they say, too much of a good thing is too much of a good thing, but there is an amount of snake venom that can be juuuuuust right to actually help human health rather than hurt it.

Many of the proteins and peptides found in snake venom have powerful potential applications in the clinic. Here are a few examples from some of the most common proteins found across snake venoms:

• Secreted phospholipase A2’s (PLA2) are enzymes that destroy muscle cells and cause paralysis at high concentrations. At low concentrations, PLA2’s can act as antibacterial agents, antiparasitic agents or antiviral agents against viruses like HIV-1 and dengue virus.

• Three-finger toxins (3FT) are peptides that cause paralysis, cardiac arrest and muscle twitching at high concentrations. At low concentrations, 3FT’s can help regulate blood pressure, and function as an analgesic (pain-reliever). In fact, 3FT’s can be a stronger pain reliever than morphine, with the added bonus that 3FT’s don’t cause respiratory distress like morphine does.

• Snake venom serine proteases (SVSP) are enzymes that induce blood clotting at high concentrations. At low concentrations, SVSP’s can be used in surgeries as a sealant to prevent excessive bleeding.

The future of snake-inspired medicine is an exciting one. As showcased by the few examples above, snake venom-derived compounds present a wide range of therapeutic applications. Thanks to evolution, there is tremendous diversity in proteins and peptides that are specific to many different targets. This diversity of compounds has great potential for use as therapeutics, as their evolutionarily-honed specificity limits off-target effects thus reducing potential side-effects.

There are already several drugs derived from snake venoms that are undergoing clinical trials. These new drugs are being tested as treatments for cancer, heart disease, stroke, and pain (among other things). Some snake venom-derived compounds are even currently being tested as treatments for SARS-CoV2 infection, since the PLA2 class of proteins are known to have antiviral and antimicrobial activities. Other proteins found in snake venoms directly regulate the ACE signaling pathway that SARS-CoV2 uses to both break into host cells and cause clotting disorders.

We hope you enjoyed learning a bit about snakes on this St. Patrick’s day. If these kinds of scientific questions and adventures interest you, please get in touch with us! Check out our research! Join us! For questions about how to get involved with our department or praise for this article, please email Haley.Barlow@umassmed.edu.