Why the cobra is resistant to its own venom? - Dr. Zoltan Takacs

Why the cobra is resistant to its own venom?

1. Cobra venom is no danger for cobra
2. Cobra DNA is different from mouse
3. Cobra DNA makes mouse resistant
4. Mouse is bare naked to the toxin
5. Cobra masks its receptor by a sugar
6. Cobra and mongoose does the same
7 .Scorpions, fish, newts, frogs...

One of our research project is to understand why venomous animals are resistant to their own venoms. Here we detail one of our work which aimed to explore why the cobra is resistant to its own venom.

1. Lethal cobra venom is no danger to the cobra

Snakes that produce lethal neurotoxins (L to R):
- Forest cobra
- Sea krait
- King cobra

The principal components of the cobra venom are neurotoxins. These neurotoxins are very powerful, they can kill a mouse or a human within minutes. They act by binding to a target, a receptor, on the surface of the muscle cell, therefore preventing the communication between the nerve and muscle that results in a paralysis and may easily lead to death.

However, the cobras also have this receptor on the surface of their own muscle cells, but still they are completely resistant to their own neurotoxins. Why the cobra is resistant to its own neurotoxin?

2. The explanation is encoded in the cobra's DNA

yellow numbers	-	position number
green letters	-	cobra receptor DNA
green letters	-	cobra receptor protein
red letters	-	mouse receptor protein

Vipers, like rattlesnakes, are enjoy a protection against their own venom because of their blood contains elements that inactivates their venom. However, this is not the case in cobras. Instead, according to the cobra DNA sequence, the receptor that serves as a target for the toxin has a structure that is different from the one present in mammals.

This difference may mean that the neurotoxin can not recognize the receptor in cobras, as it can recognize in mammals. Like a key only opens one lock, it will not work in a lock that is different. Are these differences in the mouse and cobra DNA causing resistance to the cobra?

3. Cobra DNA makes mouse receptor neurotoxin resistant

LEFT: Mouse receptor is inhibited after exposure to neurotoxin, as there is no current. Cobra receptor is resistant, as the current remains following neurotoxin exposure.

RIGHT: Dose-response relationship shows that mouse receptor is inhibited by, while the cobra is resistant to neurotoxin.

We inserted a big part of the cobra receptor DNA into the homologous position of the mouse receptor to test if it makes the mouse receptor resistant to cobra neurotoxin. The graphs above shows that the normal mouse receptor is inhibited by the neurotoxin,

but insertion of the cobra's DNA into the mouse gene converts the mouse receptor resistant to cobra neurotoxin. What is the difference between the cobra and the mouse receptor that is responsible for this effect?

4. Mouse receptor is bare naked for cobra neurotoxin

a-BTX	-	neurotoxin
ACh	-	natural transmitter
purple star	-	transmitter binding site
letters on yellow	-	mouse's receptor
letters on red	-	neurotoxin binding site

The mouse (and humans) is sensitive to cobra neurotoxin (a-BTX) because its receptor contains special residues (marked in red boxes) that bind the neurotoxin. Once the neurotoxin bound to the receptor, it prevents the binding of the natural transmitter (ACh),

that in turn would bind to some of the very same residues (purple stars) as the neurotoxin. This result in a receptor inhibition that can cause death. What is happening at the same place in the cobra receptor?

5. Cobra also has the neurotoxin binding site - hidden under a sugar

a-BTX	-	neurotoxin
ACh	-	natural transmitter
green box	-	sugar
purple star	-	transmitter binding site
letters on yellow	-	cobra's receptor
letters on red	-	neurotoxin binding site

The cobra receptor is very similar to the mouse receptor. Of course, it has the binding site (purple stars) for the natural transmitter (ACh), but interestingly it has a sugar molecule (green box) on the receptor that prevents the binding of neurotoxin. The sugar molecule has no effect on the binding of the much smaller natural transmitter, it only prevents the binding of the neurotoxin.

However, beneath the sugar molecule, the cobra also has the same binding site (marked in red boxes) for the cobra neurotoxin (a-BTX) as the mouse has. If the sugar is removed from the cobra, then it will be sensitive to its own neurotoxin. Is this resistance unique to the cobras?

6. DNA reveals that mongoose and cobra has the same strategy

green box	-	DNA codes a sugar
yellow letters	-	conserved nucleotide
blue letters	-	variable nucleotide
green animal	-	neurotoxin resistant
red animal	-	neurotoxin sensitive

Comparison of DNA sequences from different species shows that only two groups of animals have sugar in their receptors: snakes and mongooses. Cobras produce neurotoxins, mongooses are eating cobras: a good reason to be protected in both cases, - and an excellent example for convergent evolution at the molecular level. However, all of the residues that are responsible for neurotoxin binding are conserved across different animal species, including the cobra,

meaning that snake neurotoxins are targeted against evolutionarily conserved motifs and these motifs are essential for structure/function of the receptor. In plain language: "It makes sense: a toxin is targeted against something which is common in animals and is important for their lives". What is the case in other poisonous and venomous animals?

7. Scorpions, fish, newts, and frogs - are they doing the same?

Scorpion, and poison arrow frog are likely to have the a receptor that has a different structure from the one in mammals

Scorpions, puffer fish, newts, frogs all produce lethal poisons or venoms but of course, they are also resistant against their own poison or venom. In many case, it has been shown that the blood-free tissues of these animals are still resistant, indicating that they receptor also has a unique structure -	a small modification that makes it impossible for the toxin from their own poison or venom to recognize it. Again..., a key opens only one lock - if the lock has small change inside the key will not work.
Takacs Z, Wilhelmsen KC and Sorota S (2004) Cobra (Naja spp.) nicotinic acetylcholine receptor exhibits resistance to Erabu sea snake (Laticauda semifasciata) short-chain a-neurotoxin. Journal of Molecular Evolution 58, 516-526.
Takacs Z, Wilhelmsen KC and Sorota S (2001) Snake a-neurotoxin binding site on the Egyptian cobra (Naja haje) nicotinic acetylcholine receptor is conserved. Molecular Biology and Evolution 18, 1800-1809.